The below article presents the results of spectroscopic research, theoretical ), microbiological, and antioxidative calculations for three compounds from the group of 1,3,4-thiadiazoles: 2-amino-5-phenyl-1,3,4-thiadiazole (TB), 2-amino-5-(2-hydroxyphenyl)-1,3,4-thiadiazole (TS), 2-amino-5-(2-hydroxy-5-sulfobenzoyl)-1,3,4-thiadiazole (TSF). In the fluorescence emission spectra (TS) of solutions with varying concentrations of hydrogen ions, a particularly interesting effect of dual fluorescence was observed. The aforementioned effect was observed even more clearly in the environment of butan-1-ol, relative to the compound’s concentration. Depending on the modification of the resorcylic substituent (TS and TSF), we observed the emergence of two separate, partially overlapping, fluorescence emission spectra or a single emission spectrum. Interpretation of the obtained spectra using stationary and time-resolved spectroscopy allowed the correlation of the effect’s emergence with the phenomenon of molecular aggregation (of a particular type) as well as, above all, the structure of the substituent system. The overlap of said effects most likely induces the processes related to the phenomenon of charge transfer (in TS) and is responsible for the observed fluorescence effects. Also, the position of the –OH group (in the resorcylic ring) is significant and can facilitate the charge transfer (CT). The determinations of the changes in the dipole moment and TD-DFT calculations further corroborate the above assumption. The following paper presents the analysis  of the fluorescence effects relative to the changes in the structure of the resorcylic group combined with pH effects. The results of biological studies also indicate the highest pharmacological potential of the analogue in the case where the effects of dual fluorescence emission are observed, which predisposes this particular group of fluorophores as effective fluorescence probes or potential pharmaceuticals with antimycotic properties. Contemporary medicine related to the treatment of conditions such as neoplastic, neurodegenerative, or mycotic diseases requires the application of scientific methods relying on the techniques of molecular spectroscopy and photophysics/photochemistry to provide an in-depth description of the physicochemical properties of new compounds offering the aforementioned properties. Therefore, both in the context of medicine and pharmacology, researchers employing the methods of molecular spectroscopy are faced with the very difficult and highly engaging task  of identifying new compounds displaying very particular molecular properties, oftentimes providing a multi-directional range of specific pharmacological effects. The selected molecules need to be very thoroughly analyzed in terms of their photophysical and photochemical properties to allow the identification of the analogues with the highest potential for actual medical applications.Particular promise in the fight against the aforementioned diseases is shown by the compounds from the group of 1,3,4-thiadiazoles substituted with a resorcylic group A–F.1,3,4-Thiadiazoles are a group of new potential pharmaceuticals showing neuroprotective , antibacFor the performed study on the mechanisms of molecular interactions in aqueous solutions of varying pH as well as in selected organic solvents (of varying concentration), the following compounds were selected: 2-amino-5-phenyl-1,3,4-thiadiazole (TB), 2-amino-5-(2-hydroxyphenyl)-1,3,4-thiadiazole (TS), and 2-amino-5-(2-hydroxy-5-sulfobenzoyl)-1,3,4-thiadiazole (TSF) A–F.It is also noteworthy that the aforementioned 1,3,4-thiadiazole analogues show, apart from promising biological properties, also rather interesting spectroscopic properties. The same include the atypical effect of the tautomeric shift induced by changes in medium polarizability (changes of the keto/enol equilibrium ), effectHowever, the primary focus of the presented study was on spectroscopic analyses and quantum-mechanical calculations aimed at understanding the compounds’ unique fluorescence properties related to the effect of dual fluorescence emission. The analogues selected for the study A–F were intermolecular charge transfer (CT) inducing the emergence of specific CT states , where temergence of the so-called excited-state intramolecular proton transfer (ESIPT) ,17 in thin the case of additional concentration-induced effects, we are dealing with excimers ;one should also mention processes related to breaking the Kasha rule ;a very interesting explanation of these types of effects provided in the literature also involves the processes related to molecular aggregation ) ,21.It is also noteworthy that in the literature , the pheHowever, it should be noted and emphasized that modeling the activity of fluorophores relative to the changes in the spectroscopic properties of the analyzed systems, both under conditions of varying pH and polarity, becomes particularly significant also in terms of designing specific fluorescence probes which are always in high demand in the context of molecular biology.In the 1,3,4-thiadiazoles selected for the study, the emergence of fluorescence effects related to the dual nature of the same is also closely correlated with the structure of the substituent group used and modified (in our case for the first time) on the side of the resorcylic system A,C,E. WeThe primary goal of this work is to attempt to explain and give a brief description of the dual fluorescence effect observed in the selected analogues from 1,3,4-thiadiazoles including an aqueous environment with a different concentration of hydrogen ions, and their dependence on the structure of the substituent group on the resorcyl system side.The presented paper marks out significant novelties in comparison to our previous work where selected analogues from 1,3,4-thiadiazoles were investigated mainly towards specific solvent effect and basic oxidative properties (thin-layer chromatography (TLC)-DPPH method with “dot-blot” test). In this paper, the authors intend to investigate if similar fluorescence effects will be observed in a more physiological environment and compare it with oxidative as well as more important antifungal activities of these molecules.Uncovering the exact details of the changes in the molecular organization of the selected compounds in media with varying concentrations of hydrogen ions and varying polarity is significant in determining their potential for pharmacological and medical applications as well as the directions of further model studies in media such as micellar o liposomal systems. Such molecules may also be used as molecular probes very sensitive to environmental changes in systems studies in the context of molecular biology . For thi2 group. 2-Amino-5-(2-hydroxyphenyl)-1,3,4-thiadiazole (TS) contains, in its benzene ring, a substituted hydroxylic group in the ortho position. In the case of 2-amino-5-(2-hydroxy-5-sulfobenzoyl)-1,3,4-thiadiazole (TSF), the benzene ring includes an –OH group substituted in the ortho position . The remaining elements of the molecular structure are the same in all the analogues.The selected compounds from the 1,3,4-thiadiazole group were synthesized in such a way so as to obtain significant differences in the structure of the substituent group in the resorcylic system . In the −1 for TSF and 303 cm−1 for TS in the case of spectra observed at pH 12, and bathochromic shift by 1016 cm−1 for TSF and 684 cm−1 for TS in the case of spectra observed at pH 2 (as well as pH 1) relative to the spectra of the same compounds at pH 7. For TB, +-H group is formed in all the studied analogues. The effect was most evident in the case of TB but was also present for TS and TSF, although the latter two analogues containing an –OH group may additionally be subject to head to tail aggregation as well. This was reflected in the absorption spectra where the long wavelength spectra registered for this analogue revealed a clear broadening of the bands. In the case of TSF, we observed what most likely was a greater equilibrium between the respective types of aggregation together with molecule ionization, whereas for TS, we recorded a band with the maximum at approx. 350 nm, evidencing the possible presence of aggregated structures even larger than dimers.The obtained results indicate very clear changes in the shape of the spectra, particularly in the region significant to physiological values. For the solvents used in the study, the positions of the absorption and fluorescence maxima as well as the respective Stokes shifts are presented in Moreover, in the case of TS a (fairly) clear isosbestic point could be observed at approx. 345 nm, which may suggest that the transfer between the respective aggregation forms in the compound is highly fluid (and the molecules are susceptible to such interactions). For the band located in the long wavelength side, this points to a stronger aggregation (head to tail) whose level is most likely constant at a low pH. The intensity of bands evidencing the aggregation effects occurring in the selected analogues is also significantly correlated with the structure of the very substituent group in the resorcylic ring bound to the 1,3,4-thiadiazole ring.+-H group.− group was, respectively, 7.35 and 7.02, and for the –N+-H group, the pK for all the compounds was approx. 4.85. For pK higher than the larger of the above, we observed for the respective analogues (TS and TSF) the prevalence of forms with the –OH group, whereas for pK below the lower of the above, for each analogue the prevalence of forms with the –N+-H group was observed. − and –N+-H). The most significant change was observed for TSF (for which we could probably observe the greatest equilibrium/transfer between the respective types of ionized forms that could undergo the given types of aggregation) and the changes were also relatively clearly manifested for TS. In the case of TB, the changes of this type were the least extensive, as confirmed by the observations in In the case of TS and TSF, the pK for the –OThe excitation wavelength for all the analyzed samples corresponds to the absorption spectra maximum A,B. In tHowever, the most interesting effects observed in the presented study were those presented in In order to provide a further, more in-depth insight into the character of the molecular interactions occurring for TS in aqueous solutions of varying pH  and head to tail aggregation (characteristic for long wavelength excitation) of the analyzed molecules. Furthermore, by using the spectral shift in the absorption spectra in combination with the mechanism provided by the exciton splitting theory, we were also able to calculate the distances between adjacent chromophores of the analyzed molecules ,26. So, To recapitulate at this point, one should strongly emphasize the impact on the observed spectral effects (in particular in the emission spectra of TS and TSF) of such factors as, above all, the phenomenon of molecular aggregation (of two types) and difference in the substituent structure of the respective compounds .We can therefore preliminarily posit that the fluorescence emission spectra of the analyzed compounds under high pH conditions are dominated by the emission from monomeric forms; various ionized forms, and at low pH, in the case of TS/TSF, form the two aforementioned aggregated forms. For TS, at pH lower than 5, the aggregation processes may, given the prevalence of card pack aggregative interactions, also trigger effects related to intermolecular charge transfer (CT), which influence the dual character of emission spectra, as confirmed in the conducted TD-DFT calculations as well as, most importantly, calculations of the dipole moment in the ground and excited state (described below).Next, Emission in the case of excitation was registered at the wavelengths corresponding to the maxima of the respective fluorescence emission spectra in For TS, 0, which induces the respective changes in the fluorescence emission spectra. In the pH ranges where the effects of dual fluorescence or multiple bands significantly shifted relative to each other are not observed, the excitation spectra at wavelengths corresponding to the respective forms of the analyzed compounds are fairly consistently overlapping within practically the entire wavelength range.Within these pH ranges, the spectra show a significantly increased half-value width relative to other pH levels, which clearly suggests the presence of other than monomeric forms of the compound. Thus, for TS  we can observe the strongest effects related to exciton splitting at the main energetic level STo emphasize the influence of the type of aggregation on the observed fluorescence effects, one needs only to carefully analyze the results presented in The figure illustrates the overlap between the normalized excitation spectra  factors influencing the effects observed in the fluorescence spectra , spectral RLS measurements were conducted (∆λ = 0) for the selected derivatives in the entire pH range. In summary, it is noteworthy at this point that the RLS spectra related to chromophoric aggregation of the analyzed molecules are very characteristic and clearly correlate the observed fluorescence effects with various types of said aggregation. However, it should also be emphasized that the same is not the only factor determining the observed fluorescence effects as very intensive RLS bands were also observed for TB where, as discussed in the context of previous experiments, we observed no significant changes in the fluorescence emission spectra relative to the emergence of dual fluorescence or presence of multiple fluorescence emission bands partially shifted relative to each other. The oscillative structure of the observed RLS bands evidences the abundance of aggregated structures of varying sizes in the analyzed compounds  and may Nonetheless, it can be concluded with considerable certainty that in the case of the observed fluorescence effects, aggregation is a factor necessary for their emergence. The above hypothesis is corroborated by the results presented in F which describe changes in the polarity/polarizability of the solvent used.In order to better understand the mechanism of the spectral shifts described above  as well Firstly, as can be observed for all selected compounds , the sloMost notably, however, one should take into account the results of dipole moment calculations performed for the analyzed molecules in ground and excited states based on their respective absorption and fluorescence emission spectra in correlation with Reichardt’s  and KawsAs we can see, the most significant changes of the dipole moment of the analyzed molecules (between the ground and excited states) were observed for TS molecules and TSF, the compounds whose fluorescence emission spectra revealed the most interesting fluorescence effects . The above calculations confirmed in this case that CT occurred between the analyzed molecules. In the case of TB and TS we observed the ratio in the dipole moment between the excited and ground states of 1.50 whereas in the case of TSF the difference increased to 1.62 (D). Results of the above calculations confirm the significant influence of the structure of the substituent system on the observed fluorescence effects. They also suggest that an important effect of solvatochromic shifts entails the intersection of states with varying distributions of electronic density dictated by the polarity of the respective medium. In polar solvents, electronic states with high dipole moments (often in fact the CT states) are stabilized relative to the states with low dipole moments. If such states are characterized by similar energies, a change in the polarity/polarizability of the medium may lead to a reversal of the states’ order. Since photochemical properties are determined by the character of the lowest excited state of the given multiplicity (Kasha’s rule), they can be (as observed in the present study) subject to very significant changes with the solvent’s polarity.In the subsequent part of the study on the mechanism of molecular interactions inducing the respective fluorescence effects in the emission spectra, as presented above in The methodology is presented and described in detail in The correlation between fluorescence decay and pH observed for TS is similar to that obtained for TSF. For pH <4, we recorded a significant reduction of the fluorescence lifetime. Moreover, for this analogue we observed two exponential fluorescence decay at pH = 2, which suggests the possible presence of more than one molecular form in the solution. The measured fluorescence lifetime for the main component, 1.64 ns, fits in well with the observed decrease of the parameter at low pH levels, with the share of this component estimated at 72%. The fluorescence lifetime recorded for the second component was considerably shorter (0.29 ns). At pH within the range of 4–12, the fluorescence lifetime for the TS analogue was decisively shorter than in the case of TSF but similarly to the latter, it remained at a roughly constant level.In the entire range of pH values, the fluorescence lifetime recorded for TB was the shortest among all three analogues (0.1–0.18). In the case of this analogue, the second constituent, with the fluorescence decay time of 1.2 ns and relative share in total fluorescence estimated as 11%–20%, was observed both at low (pH = 2) and high  pH levels. Clearly, aggregation which triggers the effects associated with dual fluorescence takes place in TS and TSF in practically the whole range of pH values (the lifetime remains at a fairly constant level). For TB, at pH lower than 7, we observed a far more prominent presence of the second constituent in the overall lifetime, which indicates the existence of spectral forms other than aggregates. The reasonability of using fluorescence lifetime measurements in this paper is worth emphasizing. In particular, due to the process related CT, a significant increase in the average fluorescence lifetime is observed. Hence, the effect of dual fluorescence reported in this work, allowed for the discovery of noticeable changes in the fluorescence lifetime of the fluorophore, which together with noticeable aggregation processes, fully confirms our hypotheses.To recapitulate the deliberations so far, it should be emphasized that  effects observed in the case of TS and TSF molecules (in high concentrations) are triggered primarily by the intermolecular phenomenon of charge transfer (CT) enforced (or facilitated) in this particular case by the phenomenon of molecular aggregation of a given type . Also, the structure of the substituent group on the side of the resorcylic system significantly influences the described overlap of two phenomena.− molecule, TSH molecule, TSH2+ and sample TS dimer as optimized using B3LYP/aug-cc-pVDZ and polarizable continuum model (PCM) with water used as the solvent. −, TSH, and TSH2+ has a single, low-lying and intensive excited state responsible for the shape of its absorption and emission spectra , therefore the results should be treated as only semi-quantitative . Nonetheless, the calculations offer a certain insight into the experimentally analyzed photophysics. As shown in The main computational findings are as follows. Firstly, the TS molecule in the form of TS spectra A–C. HoweAlso, it is worth mentioning that the dual fluorescence observed in the selected 1,3,4-thiadiazole analogues may occur via a pathway other than that proposed in this work (two independent modes of action may be possible). Based on the DFT calculations, the investigated compounds in their ground state occur solely as enol tautomers. Excitation of the enol form would likely enable an additional and independent process resulting in dual fluorescence emission. Once in the excited state either cis or trans-enol the enol form may tautomerize to the excited keto-form, which in turn may emit the long wavelength fluorescence.Since the high concentrations are characteristic of aggregation-related dual fluorescence effects, this second and independent process may be dominant at low concentrations. The possibility of such mechanism is justified by the lack of clear differences between absorption spectra of the investigated derivatives recorded in series of various organic solvents. Moreover, according to our preliminary calculations, the energy differences between the excited enol and excited keto form are relatively small, which is consistent with the proposed relaxation pathway.In a conclusion, the hypothetical second mechanism is proposed on the basis of preliminary calculations and relies upon an excitation of the neutral TS molecule, which in its ground state occurs as enol tautomer. The excitation of the enolic TS results either in straightforward fluorescence emission from the excited enol tautomer (higher energy emission band) or the excited keto tautomer is formed as result of proton (hydrogen) transfer from the phenolic –OH to the nearest thiadiazole nitrogen atom. The excited state of such keto tautomer is energetically similar to that of the excited enol and may emit photons manifesting as the second (lower energy) emission band, with subsequent return to the enolic form characteristic of the ground state.The proton transfer into one of the nitrogen atoms is favored by the conformation in which the phenolic –OH resides near the N rather than S atom of the thiadiazole ring. Based on crystal structures reported to date , such co• radicals ..40].The TB, TS, and TSF (about 1 mg) samples were dissolved in 2 mL of distilled water, and then appropriate aliquots of this solution were added to 50 mL of distilled water. Due to a sparing solubility of the thiadiazoles tested, the concentrations of samples were not determined. In order to obtain pH 12, 0.1 M of NaOH was added to each sample. Next, 0.1 M of HCl acid was gradually added resulting in a desirable pH of aqueous TB, TS, and TSF solutions. The respective titration curve plots are given in The electronic absorption spectra of the selected compounds in selected organic solvents were recorded using a double-beam UV–Vis spectrophotometer Cary 300 Bio Varian  equipped with a thermostatted cuvette holder with a 6 × 6 multicell Peltier block. The temperature was controlled with a thermocouple probe (Cary Series II from Varian) placed directly in the sample.The UV–Vis spectra were recorded on Cary 300 Bio (Varian) instrument. All measurements were carried out in a thermostatted cuvette holder with a 6 × 6 multicell Peltier block. The thermocouple probe (Cary Series II from Varian) was applied for the temperature control.Cary Eclipse spectrofluorometer (Varian) was applied for the recording of fluorescence excitation, emission, and synchronous spectra. All measurements were carried out at 22 °C. All the fluorescence spectra were recorded with 0.5 nm resolution together with the lamp and photomultiplier spectral characteristics corrections. Resonance light scattering (RLS) measurements were carried out according to the previously reported protocol ,37 with Analytical grade methanol, DMSO, dimethylformamide  were made up in 1 mL of each solvent. The absorbance intensity was adjusted by the addition of an appropriate aliquot of TB, TS, and TSF into 2 mL of solvent. The concentration in several samples remained undetermined due to a poor solubility of the thiadiazoles . The molar concentrations of TB, TS, and TSF dissolved in DMSO and DMF were as follows: C1 = 0.056 mM, C2 = 0.055 mM, C3 = 0.038 mM, C4 = 0.062 mM, C5 = 0.056 mM, C6 = 0.036 mM.In order to estimate the dipole moments of TB, TS, and TSF molecules, two methods were used depending on the internal electric field effect.T(30) value, specifically the microscopic solvent polarity  and The change in dipole moment can be determined by Equation (2):In method 2 the ground and excited state dipole moments were calculated based on Kawski and Bilot equations ,29. By eF and F + 2gV=41 and m2 were calculated by plotting Stokes shifts and va + vf against the bulk solvent polarity functions F and F + 2g):F, and F + 2g is the fluorescence intensity, iα is the preexponential factor, and if is the fractional contribution of each fluorescence lifetime component. Minimization of the reduced χ2 value together with the residual distribution of the experimental data allowed for achieving the best-fit parameters.Multifrequency cross-correlation phase and modulation K2 fluorometer  was applied for the fluorescence lifetime measurements. A continuous wave 280 nm light emitting diode  was applied for the excitation and the decays were recorded in a 10 × 10 mm quartz cuvette. The fluorescence signal was recorded using a monochromator set at the maximum emission (±10 nm). These emissions were of 405 nm for TSF, 382 nm for TB, and 415 nm for TS. Measurements were performed for 15 modulation frequencies ranging from 2 to 200 MHz. The phase shift and demodulation ratios were referenced to the aqueous solution of Ludox@ from Aldrich  without the emitter filter. ISS Vinci 2.0 software was used for the data analysis utilizing a multiexponential decay model with discrete fluorescence lifetime components (Equation (12))The presented results were calculated as an average of three to five repetitions of fluorescence lifetime measurements.The DFT calculations were made using the Gaussian 09 package , B3LYP e• radicals, so that their final concentration in each well was 200 μM. The plate was shaken on the internal shaker of the Tecan Infinite 200 microplate reader  for 10 s to ensure thorough mixing of reagents in all wells, and spectrophotometric measurements started in the range of 500–550 nm with a 1 nm wavelength step at 25 °C for 30 min. The results obtained were the average of five exposures of each sample to a beam of light. All experiments were repeated three times.Increasing concentrations of ethanolic solutions of tested thiadiazol derivatives  were applied to the following wells of the 96-well plate, so that their final concentration in 200 μL of the total volume of each sample was within the range of 0–60 μM. Samples of reference compounds, namely quercetin (QUER) and TROLOX (TROL), were prepared in the same way and their final concentrations in 200 μL of the total volume of each sample was within the range of 0–75 μM. Immediately before the measurement, all wells were supplemented with 1 mM of an ethanolic solution of DPPHThe antifungal agents  were dissolved in 100% DMSO, at a final concentration of 1000 μg/mL. Ten 2-fold serial dilutions were prepared in Roswell Park Memorial Institute (RPMI)-1640 medium , with the final drug concentrations ranging from 256 to 0.125 μg/mL of examined agents, in accordance with CLSI guidelines .Candida crusei (Polish isolate), C. fructus (JCM 1513), C. fragicola (JCM 1589), C. butyri (JCM 1501), C. tropicalis (ATCC 1369), C. shehatae (ATCC 22984), C. fluviatilis (CBS 6776), C. freyschussi (CBS 3562), and C. parapsilopsis (DSM 70125) strains were used in this study. The strains were inoculated onto Sabouraud dextrose agar  plates from glycerol stocks and incubated at 30 °C for 24 h. They were then subcultured on the same medium for a further 24 h at 30 °C. The yeast inocula were prepared by diluting the overnight culture with 0.9% NaCl to 1–5 × 106 a colony-forming unit CFU/mL compared to the isolate density standard corresponding to 0.5 in the McFarland scale.Thiadiazole derivative MICs were determined using the broth microdilution method described in CLSI M27-A2 for yeasts. The MIC was defined as the lowest concentration of antifungal agent that produced a prominent decrease in optical turbidity when compared with the control. The antimicrobial activity of thiadiazole derivatives was interpreted as strong/good activity (MIC: <100 μg/mL), moderate activity (MIC: 100–500 μg/mL), weak activity (MIC: 500–1000 μg/mL), and inactive/no antimicrobial effect (MIC: >1000 μg/mL) according to the criteria proposed by Morales et al. .Fungal inocula (30 μL) were added to each well on a sterile, 100-well flat-bottomed microtiter plate containing the test concentration of thiadiazole derivatives (350 μL/well). Each concentration was tested in triplicate for each organism. Two wells containing a fungal suspension with no antifungal agent (compound-free growth control) and two wells containing only growth media (background control) were included in this plate. ODs  were measured for 24 h at 30 °C using a multi-detection microplate reader  at 600 nm and were automatically recorded every 2 h for each well. Turbidimetric growth curves were obtained depending on the changes in the OD of fungal growth for each antifungal agent concentration and the compound-free growth control. Growth curve parameters  were determined using the PYTHON script according to Hoeflinger et al. .The fluorescence spectroscopy experiments presented in this paper indicate the emergence in the fluorescence emission spectra of the selected 1,3,4-thiadiazole analogues (TS) of the effect of dual fluorescence in varying pH media or for varying concentrations of the analyzed compounds. The effect studied with the use of fluorescence spectroscopy (with the RLS technique and excitation spectra) and by measuring the respective fluorescence lifetimes revealed a strong dependence on the phenomenon of molecular aggregation of the analyzed analogues. A very important role in the context of the observed fluorescence effects was also played by the structure of the substituent system (in the resorcylic group) which determined the type of aggregative interactions. The studies of TS fluorescence spectra indicated the emergence of the dual fluorescence effect for this compound within the pH range of 1 to ~4/5. Based on the exciton splitting theory, the effects can be associated with the prevalence of card pack aggregation. Moreover, the calculations pertaining to the changes in dipole moment between the ground and excited states of the analyzed molecules, performed with two different methods, as well as quantum-mechanical TD-DFT calculations very clearly suggest that the possibility of intermolecular CT may in fact be the main cause of the effects related to dual fluorescence in the case of TS as well as highly concentrated TSF. The effects of fluorescence emission quenching or the noticeable  reduction of the mean fluorescence lifetime, confirm the participation of aggregation effects as a factor necessary for CT to occur. Charge transfers seemed to be particularly evident in the TS molecule, it was also in this compound that the UV–Vis, fluorescence excitation spectra within the pH range of 1–4/5, revealed the strongest aggregation effects. Moreover, CT effects must be associated with the correct conformation of the molecule , positioned on the side of the nitrogen atom in the thiadiazole ring, to allow the formation of correct hydrogen bonds.In case of the investigated 1,3,4-thiadiazole analogues a second mechanism for their dual fluorescence cannot be excluded. In particular an excitation of the enolic tautomer may lead to the formation of its excited keto tautomer, and the subsequent fluorescence emission from such excited keto form of the compound. In order to verify this hypothesis more in-depth studies will be carried out in the future.The preliminary microbiological studies also clearly revealed the influence of the substituent system on the antimycotic properties of the analyzed analogues. The most promising results were obtained for TS and TSF in which the effects of dual fluorescence were also observed. It is noteworthy that the studies discussed above may facilitate a quick analysis of the structure of the studied molecule in any biological or crystalline system, which may allow the whole group of analogues to be used as excellent fluorescence probes, highly sensitive to changing conditions in the given environment.Furthermore, one will also be able to exactly determine which molecular form and which particular conformation is responsible for a given aspect of the biological activity displayed by this group of analogues. At this point it is also worth emphasizing that due to their interesting photophysical properties the thiadiazoles selected in this research represent a group of remarkably useful molecular probes, whose properties easily change depending on environmental conditions.


Self arranged Cactis", all varying between 115 nm and 8 μm. The molecular structure of goethite was characterized by FTIR and elemental analysis of EDX converged with the obtained data. The average corrosion rate for 1020 carbon steel in the weathering was 1.7592 mpy. The data obtained in this work will contribute to the understanding of the corrosion process of 1020 carbon steel, one of the most used in civil construction, as well as in material sciences, where iron oxides are widely used in metallurgy, catalysis and adsorption, and the domain of morphology is fundamental for each application.A new morphology of goethite aggregates (α-FeOOH) obtained through the natural corrosion process of 1020 carbon steel parts exposed to weathering was found. Micrographies obtained by SEM reveal micro and nanostructures with forms of nanosquares, microparticles, nanowires inside microparticles and the unpublished structure of " Electrochemistry; Inorganic chemistry; Materials chemistry; Natural corrosion; Electron microscopy; Morphology; Goethite; Carbon steel ISeveral authors report the synthesis of nano and microparticles of iron oxides using different precursors with the intention of modeling their forms and influencing their morphologies, especially nano rods, wires, cubes, rings and spidles , 9, 10. In the present work, a way for obtaining solid oxide-hydroxide of iron in the form of Goethite α-FeOOH with new morphology was employed from samples of 1020 carbon steel exposed to weathering that underwent a natural corrosion process. This iron corrosion product was characterized by infrared spectroscopy - FTIR, the morphology elucidated by scanning electron microscopy - SEM and elemental analysis performed by X - ray dispersive energy - EDX microanalysis.22.12.Fifteen 1020 carbon steel test samples were used for atmospheric weathering tests with dimensions of 100 × 70 mm, with a total area of 7000 mm2.23) and 50.0 g of tin(II) chloride (SnCl2)). Every 5 min the samples were removed from the bath and washed in running water. This process was repeated repeatedly until the variation of the sample weight was no more significant in relation to the total weight of the sample. The corrosion rate was calculated by following equation derivate from Faraday equation:4K = proportionality constant to adequate the unities = 8,76 x 10W = loss of mass = final mass (g) - initial mass (g)2)A = sample exposure area (cmt = exposure time (h)3)d = sample density (g cmThe technique of weathering (ABNT NBR 6209)  was used2.3The morphologies of each fragment were obtained by micrography using a scanning electron microscope model Phenom Prox-X with 1000–22000 of magnification. The EDS spectra were collected in a spectrometer by dispersive energy of X-rays connected to the scanning electron microscope using a 15 kV accelerating voltage.2.4−1 resolution was used. The samples were homogenized in KBr pastilles in which approximately 3 mg of each for analysis, and all spectra were collected with 64 scans.The infrared spectra were collected in the tree samples; the spectrometer JASCO, a model FTIR-4100 with 4 cm33.1Observing the tendency of mass loss of the SAE 1020 carbon steel sample exposed to weathering, it is possible to note that the corrosion product is soluble, but little adherent to the surface of the samples. Thus, the accumulation of corrosion products leads to a decrease in the exposed area in the corrosive medium over time. This fact is in agreement with the expected, because iron oxides are removed from the surface of the metal because of the action of rain, wind and dew. However, the large amount of corrosion products formed along the exposure of the specimens causes the products to accumulate on the surface of the samples and the decrease of the exposure area occurs.2 (7000 mm2) it was possible to calculate the corrosion rate as a function of time loss and subsequent classification in terms of atmospheric corrosivity found at the Port of São Francisco do Sul, according to the NACE RP-07-75 standard [Using an exposure area of 70 cmstandard . The valIt was observed with the weathering test in the port environment that initially the corrosive process is intense, and its average speed decreased with the time of exposure. After the corrosion product has formed, the material will corrode at a rate which depends on the composition of the material and the corrosion product, in this case the iron oxides, which over time agglomerate on the surface of the metal material. However, the iron oxides layer is porous, and this makes it difficult to adhere to the steel surface, and the marine environment is classified as medium to high atmospheric corrosivity.3.2−1 assigned to (δ Fe–O–H) bending vibrations and 625cm−1 attributed to (ν Fe–O) stretching vibrations [−1 [−1 (δ O–H) and (γ O–H) are observed.The FTIR analyzes are widely used in the characterization of iron oxides and hydroxides, the FTIR spectra of three different patina sites observed in the exposed samples can be observed in brations , 16. Theions [−1 , in our −1 can be characterized by stretching of hydroxyl group. The described stretches are characteristic of the vibrational modes associated with the goethite structure (α-FeOOH), confirming the molecular structure of the major corrosion product, in this way the oxidation product of the 1020 weathered carbon steel samples is formed by goethite.The bands observed at 2900-3500 cmThe formation of the goethite structure (α-FeOOH) under the described experimental conditions can be explained by the electrochemical reactions describe in 4- ion is the predecessor of the iron(III) oxide-hydroxide.In this propose, The semi-reaction 4 shows the reduction of the iron specie from (VI) to (III), leading to the formation of the species of interest (FeOOH). Until this stage, only the iron species are mentioned, and Global Reaction 1 indicates the partial balance of the equations involving the iron species. Semi-reaction 5 indicates the participation of oxygen in the overall process, which Global Reaction 2 was obtained for the corrosion process of iron alloy exposed to weathering (presence of water and oxygen).3.3The morphologies of inorganic materials in micro and nanometric scale are great importance for several areas in the materials science and technology fields. In this sense, knowing the microscope of oxidation products from iron base products is extremely important to understand the processes of corrosion of base materials of this metal, that is great interest for an industry as well as for a conservation of art works and cultural goods that use this support - many cultural artifacts are submerged today and the understanding of morphology is essential for the diagnosis of their conservation status as well as for characterization of materials exposed in the experimental conditions of this work. Several works approach the experimental methods and the different types of morphology obtanied from different iron precursors , 23, 24.self-organized Cactis", nanosquas and nanowires within microparticles of 1020 carbon steel oxidation products can be observed [Through SEM analysis, observed . Note thRecently, Sayed and Polshettiwar  reportedThe SEM- EDS technique is also widely used in the elemental characterization of iron oxides from various sources, whether synthetic or natural, and found extensively in recent works , 31, 32.4Self arranged Cactis", was found. This discovery will help researchers and professionals from different areas in the application of these materials, as well as in the identification of patinas in structures and objects based on carbon steel and encourage studies on oxides and different materials based on iron, searching for new structures and new applications.In this work, natural corrosion products of 1020 carbon steel were obtained by natural weathering, characterized by spectroscopic methods and their micro and nanometric structure elucidated by SEM. The major chemical composition obtained in the final products was goethite (α-FeOOH), an oxide-hydroxide of iron. In relation to morphology, an unpublished form of organization of these aggregates, called "Thiago G. Costa: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.Vanessa Wandersee Cunha Ostroski: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.Fernando S. de Souza: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.The authors declare no conflict of interest.No additional information is available for this paper.No additional information is available for this paper.


The low-velocity impact behavior of Shape Memory Alloy (SMA) reinforced resin matrix polymers is investigated and the influence of the SMA volume fraction on the impact performance of polymer composites is considered for the first time, which are the highlights in this paper. Firstly, 12 kinds of polymer composite specimens with different SMA volume fractions are fabricated in terms of the SMA layup spacing, SMA diameter, and the interaction between the two. Secondly, a low-velocity impact test is carried out in order to study the impact performances of the above polymer composites. Finally, the damage morphology of the specimen after impact is observed by the visualization method and the low-velocity impact performance of the 12 kinds of polymer composites is analyzed on the basis of the force and energy history curve. Recently, composite materials are used extensively as a result of their inherently high specific mechanical properties as well as low weight. However, due to the lack of a thorough thickness reinforcement or weak interfaces of the composites, which often produce some visible or invisible damage in structures subjected to external loads, these damages will greatly affect the mechanical performance, and reduce the integrity and stability of composite structures, and then reduce the lifetime of composite materials ,6,7,8,9.Over the past years, a considerable amount of research has been devoted to evaluating the mechanical and impact performances of SMA and fiber hybrid reinforced composites. Zhang et al.  studied 2O3 nanoparticles.Although the mechanical and impact performances of SMA and fiber hybrid reinforced composites have been an active research for many years, there are only few studies in the literatures on SMA separately reinforced resin matrix composites, especially that of impact behavior. Raghavan et al.  studied This paper focuses on the low-velocity impact behavior of SMA separately reinforced resin matrix polymer composites based on the above research. More importantly, the influence of the SMA volume fraction on the impact performance of polymer composites is studied for the first time. For these goals, 12 kinds of polymer composites with different SMA volume fractions are fabricated and a low-velocity impact test is conducted.The polymer matrix is synthesized by an epoxy vinyl ester resin of bisphenol A , methyl ethyl ketone peroxide  as the hardener and Dimethylaniline  as the accelerator in the weight ratio of 100:1.5:0.15. All the chemical products are commercially available and used as received without further purification. The shape memory alloy (SMA) wire is a super-elastic 55.71%, 55.71%, and 56.09% Ni balanced with Ti alloy with 0.3-, 0.4-, and 0.5-mm diameters, respectively, and their basic material parameters  are shown in All composites specimens are divided into 12 groups, with the schematics and codes shown in (i)The treated SMA wire is arranged neatly according to the above design scheme , and the(ii)The epoxy resin is mixed, the curing agent and the accelerator in a ratio of 100:1.5:0.15 is added, then they are uniformly stirred, and placed in a vacuum drying oven for about 4 min to extract the air. Note: The vacuum time should not be too long, otherwise the mixture will easily become gelled, resulting in the failure of the preparation.(iii)The air-free mixtures are poured into the mold with SMA wires, and the whole system is cured at room temperature for 12 h.The overall preparation process of SMA-reinforced polymer composites is shown in Besides, we also give the marketing cost of the various types of polymer composites, as seen in Low-velocity impact tests are performed by using an Instron CEAST 9350HV drop weight impact testing machine   accordint during the impact process can be calculated by Equation (1)m is the mass of impactor, v0 is the initial velocity of the impactor, the gravitational acceleration g is 9.81 m/s2, and v(t) and δ(t) are the velocity and displacement of the impactor at time t, respectively.The impact behavior of composites subjected to the impact energy of 5 and 14 J is tested. The velocity-time and displacement-time curves of the impactor were calculated in terms of ASTM D5420-2010. The force–time curve of the impactor can be captured by a high accuracy sensor. The absorbed energy In order to better observe the damage morphology of the 12 group composites under low-velocity impact, the damage graph of the specimen after impact  and the It can be seen from As can be seen from However, the overall damage trend of different types of test specimens is basically the same under the three impact energies of 5 and 14 J. The impact resistance of the test specimens with SMA layup spacing of 2, 3, and 4 mm is better, the SMA layup spacing of 1 mm is good, and the SMA layup spacing of 5 mm is poor.It can be seen from As can be seen from In summary, from the overall damage of the test specimen, the diameter of the SMA has a certain influence on the overall damage state of the test specimen. The damage of the test specimen embedded with an SMA diameter of 0.5 mm is less than that of embedded SMA diameters of 0.3 and 0.4 mm.It can be seen from As can be seen from In summary, the SMA layup spacing has a greater influence on the overall damage state of the test specimen than that of the SMA diameter when 14 J of impact energy is applied.F-t) history curves and energy–time (E-t) history curves of SMA reinforced resin matrix composites embedded with an SMA diameter of 0.5 mm and different SMA layup spacing values of 1, 2, 3, 4 and 5 mm respectively under two impact energies.F-t curves of E-t curves of From F-t curves of E-t curves of From the F-t curves of From the In order to more clearly illustrate the specific value of the impact parameters of the 6 groups composite specimens, the maximum contact force, the average contact force, and the ultimate absorbed energy of the composites with different SMA layup spacing values are shown in F-t and E-t curves of SMA reinforced resin matrix composites with an SMA layup spacing of 3 mm and different SMA diameters of 0.3, 0.4, and 0.5 mm respectively under the two impact energies.F-t curves of E-t curves of From the F-t curves of E-t curves of From the Similarly, to more accurately study the effect of the SMA diameters on the impact behavior of the composites, the comparison of the maximum contact force and average contact force of the specimen are shown in In order to more clearly illustrate the specific value of the impact parameters of the 3 group composite specimens, the maximum contact force, the average contact force, and the ultimate absorbed energy of composites with different SMA diameters are shown in F-t and E-t curves of the SMA reinforced resin matrix composites with different SMA diameters of 0.3, 0.4, and 0.5 mm and SMA layup spacing values of 2, 3.7, and 6 mm, respectively, under the two impact energies.F-t curves of E-t curves of From the F-t curves of E-t curves of From the Similarly, to more accurately study the impact behavior of composites under different SMA diameters and layup spacing values, the comparison of the maximum contact force and average contact force value of specimens are shown in In order to more clearly illustrate the specific value of impact parameters of the 3 groups of composite specimens, the maximum contact force, the average contact force, and the ultimate absorbed energy of the composites embedded with different SMA diameters and layup spacing values are shown in This paper mainly studies the low-velocity impact performance of SMA-reinforced polymer composites embedded in different layup spacing  and diameter  values. The influence of alloy spacing and diameter on the damage morphology and impact parameters of the specimens was compared with the impact properties of pure polymer composites. In addition, this chapter extends the difference in the spacing and diameter of the alloy to the influence of the difference in volume fraction on the impact characteristics of the specimen, and further analyzes the impact of the different layup spacing and diameter on the impact characteristics of the specimen based on the volume fraction.(1)Embedding the SMA wire into the pure polymer composites can effectively improve the bearing capacity and energy absorption characteristics of the entire test specimen and reduce the damage of the test specimen.(2)The SMA layup spacing has a great influence on the impact performance of the specimen. The specimen (V3) with a layup spacing of 3 mm has the best impact performance, the largest load carrying capacity and the smallest damage among six groups polymer composites. Here, the impact performance is the best when the volume fraction of the SMA wire in the test specimen is 0.49% in the 3 groups of polymer composites.(3)SMA diameter also has an effect on the impact properties of the specimen, which is the best for the specimen (DA3) with a diameter of 0.5 mm among the 3 group composite specimens. Here, the impact performance is the best when the volume fraction of the SMA wire in the test specimen is 0.49% in the 3 groups of polymer composites.(4)When the two factors of the SMA layup spacing and diameter are changed at the same time, the overall damage state of the test specimen, as well as the bearing capacity and energy absorption characteristics are also affected. Among the three test specimens, the impact resistance of the test specimen (DB2) with an SMA layup spacing of 3.7 mm and a diameter of 0.4 mm is the best, and the SMA layup spacing had a greater influence on the impact characteristics of the test specimen than the SMA diameter. Here, the impact performance is best when the volume fraction of the SMA wire in the test specimen is 0.28% in the 3 groups of polymer composites.The following conclusions can be drawn:


A novel method for simultaneously directly measuring six-degrees-of-freedom (6DOF) geometric motion errors of CNC machine tools was proposed, and a corresponding measurement system was developed. This method can not only be applied for measuring a linear axis, but also for a rotary axis. A single-mode fiber was used to separate the measuring unit from the laser source in order to ensure system thermal stability and measurement accuracy. The method has the advantages of high efficiency and good accuracy, and requires no complicated decoupling calculation. The positioning error of the linear axis and radial motion error of the rotary axis are measured by laser interferometry and other 5DOF geometric motion errors by laser collimation. A series of experiments were performed to verify the feasibility and effectiveness of the developed measurement system. The linear and rotary axes are the key components in many mechanical machines, especially for computer numerical control (CNC) machine tools. The linear axis is the basis for linear motion, and the rotary axis is the basis for angular motion. They play a critical role in machining accuracy of machine tools. It has been verified that the machining accuracy can be greatly improved by error compensation ,2,3. HowFor a five-axis CNC machine tool, 42 geometric motion errors need to be measured ,6,7. In Laser interferometry is the most effective and precise method for measuring length with a large measurement range, while laser collimation is the right choice for measuring microangles. Based on a combination of the two, we proposed a series of methods for simultaneously measuring 6DOF geometric motion errors of a linear axis ,27,28. BAs shown in There are 42 geometric motion errors for a five-axis CNC machine tool to be measured, including 21 errors for the three linear axes, five errors for the spindle, and 16 errors for the two rotary axes.As shown in In As shown in In The configuration for measuring 6DOF geometric motion errors of the rotary axis is illustrated in In According to the principle for simultaneously measuring 6DOF geometric motion errors of both linear and rotary axes shown in Measurements of the positioning error of the linear axis and radial motion error in the X-axis direction of the rotary axis are based on laser interferometry. As shown in The radial motion error RR3 is the error-sensitive element for straightness errors of the linear axis, and radial motion error in the Y-axis direction and axial motion error of the rotary axis. As shown in Similarly, the radial motion error BS5 is used as the error-sensitive element for the yaw and pitch of the linear axis, and angular positioning error and tilt motion error around the Y-axis of the rotary axis. As shown in For a rotary axis measurement, theoretically, if the reverse rotation angle of the reference axis is the same as the rotation angle of the axis under test, the reading in the Y-axis direction on the detector PSD1 is zero. In fact, the reverse rotation angle of the reference axis is different from the rotation angle of the axis under test because of the angular positioning error of the rotary axis, and the angular positioning error θ is the nominal rotation angle of the rotary axis given by the machine tool.Here, 1 and P2 are the incident spots of the beams 1 and 3 on RR2 and RR3, respectively; Q1 and Q2 are the emergent spots of the reflected light; O1 and O2 are the centers of the incident planes of RR2 and RR3, respectively. Q1 and Q2 are in the same position in Z-axis direction without roll error. If there is a roll error, the positions of Q1 and Q2 change to Q1’ and Q2’, respectively. Thus, according to positional changes of the light spot in the Z-axis direction on two QDs, the roll error RR2 and RR3 are the error-sensitive components for measuring the roll of the linear axis and tilt motion error around the X-axis of the rotary axis. As shown in Similarly, the tilt motion error Except for the positioning error of the linear axis and radial motion error in the X-axis direction of the rotary axis, the other 5DOF geometric motion error measurements are based on laser collimation. Beam drift, which is mainly caused by slow deformation of the mechanical structure and air turbulence, influences the accuracy of 5DOF geometric motion error measurements. Therefore, a common-path compensation method is integrated into the proposed system . As showIn order to compensate for the measurement error caused by the laser drift, Equations (2a) and (3a) for computing the straightness errors, the pitch and yaw of the linear axis can be corrected asSimilarly, Equations (2b) and (3b) for the rotary axis measurements can be corrected asTheoretically, laser beam drift has no influence on the roll measurement of the linear axis and the tilt motion error measurement around the X-axis of the rotary axis. Thus, Equations (4a) and (4b) require no correction.All the equations from Equations (1a)–(6b) are applicable for other axes of machine tools as well, including Y- and Z-axis, and the A-axis. Based on the measurement principles mentioned above, an integrated system was developed, which consisted of two QDs , two PSDs , two RRs , and other optical elements. Additionally, a commercial laser interferometer , which can measure up to 800 mm/s at a range of 5 m, was combined into the system. A reference rotary axis, which is consisted of a step-rotary table  and a circular-grating angle-encoder , was used for a rotary axis servo tracking measurement at a speed up to approximately 0.8 degrees per second.2) is up to 0.9997 and the standard deviation is 0.81 μm. A grating ruler  was used to calibrate the linear errors in Y- and Z-axis directions, and an autocollimator  was used to calibrate the angular errors around Y- and Z-axes. The results are listed in the “Calibration” column in A piezo nanopositioner  was used to obtain the real measurement resolution of the linear errors in Y- and Z-axis directions of the proposed system. As shown in The stability experiment was performed within 30 min, and the temperature changed 0.2 degrees. The error-sensitive unit was placed 300 mm away from the measurement unit. The experimental results were automatically recorded and the beam angular drift was compensated in real time by common-path compensation. The standard deviations of stability are listed in the “Stability” column in As shown in It took about 140 min to perform three repeated measurements using the proposed system and the standard instruments. The environmental parameters, including temperature, pressure, and humidity, were recorded by an environmental compensator . It shows that the temperature of the laboratory was 24 ± 0.2 degrees, the relative humidity was 23.5 ± 1%, and the pressure was 1014.5 mbar. The results of the repeatability tests and comparison experiments are presented in The experimental results in There are some comparison deviations between the proposed system and standard instruments. For instance, for the positioning error measurements, the maximum deviation between the mean of three measurements from the proposed system and that from the standard instrument is 0.54 μm. For comparison deviations between the proposed system and the standard instruments, the main reason is that their error-sensitive units are located at a different position, and it is almost impossible for them to measure the same point. Additionally, manufacturing and installation errors of optical elements and error crosstalk between 6DOF geometric motion errors also contribute to comparison deviation, especially for roll measurement. Theoretically, such deviation can be decreased by mathematical analysis and error modeling.As shown in The measurement was repeated three times within 45 min, and the results of the repeatability tests and comparison experiments are presented in As shown in The repeatability errors of 6DOF geometric motion errors for the rotary axis are larger than those of the linear axis. This is because the target rotary axis itself demonstrates poor repeatability according to the results from the autocollimator. It should be noted that laser beam drift has a great influence on the linear axis measurements, and the manufacturing and installation accuracy of the reference axis has great influence on the rotary axis measurements. Generally speaking, the errors of the proposed 6DOF system mainly include the random error, the manufacturing and installation errors, and the error crosstalk, they are very complex and cannot be given in detail in this paper due to the limitation of length. We will analyze these errors and establish a compensation model in future works.It is the primary approach and developmental direction to improve and maintain the machining accuracy of CNC machine tools by simultaneously measuring the multi-DOF geometric motion errors of the linear and rotary axes, and subsequently establishing an error compensation model. In this study, based on a combination of laser interferometry and laser collimation, we proposed a direct measurement method for simultaneously measuring the 6DOF geometric motion errors for linear and rotary axes, and developed a corresponding measurement system. The feasibility and effectiveness of the method and system were verified through a series of experiments. Future work will focus on error modeling and accuracy improvement.The authors also published a USA patent No. US 9857161 B2 resulting from the work reported in this manuscript.


The manufacturing process of ultra-high-performance concrete (UHPC) leads to a considerable amount of greenhouse gas emissions, which contribute to global warming. Using recycled concrete aggregates (RCA) to replace natural sand helps to reduce natural resources and energy consumption. In this study, the feasibility of manufacturing UHPC with fine RCA was investigated for the sustainable development of construction materials industry. We aimed to study the rheological properties, autogenous shrinkage, mechanical properties, and pore structure of UHPC with different amounts of RCA. The natural aggregate content was replaced with fine RCA at rates of 0, 20, 40, 60, 80, and 100 wt.%, and the packing density of the mixed fine aggregates in this study was estimated using a linear packing model. It was found that (1) the workability, mechanical properties, and deformation behaviour of UHPC with fine RCA were comparable to or even higher than those of UHPC made of high-quality aggregates; (2) the optimal replacement rate of fine RCA was in the range of 40–60 wt.%, considering the mechanical properties and deformation behaviour of UHPC; (3) the tensile strength, flexural strength, and Young’s modulus of UHPC increased by 6.18%, 12.82%, and 3.40%, respectively, when the replacement rate of fine RCA was 60 wt.%; (4) the maximum packing density of mixed fine aggregates presented a monotonic decreasing trend as the replacement percentage of fine RCA increased. These findings help to encourage and further promote the utilisation of RCA to produce UHPC. Ultra-high-performance concrete (UHPC) exhibits excellent mechanical properties and good durability, which have attracted considerable attention from scientific researchers and engineers. To achieve such high mechanical performance and good durability, manufacturers specifically design the mixture of UHPC to have a low water-to-binder ratio, a minimum pore size and steel fibres incorporated. Moreover, the raw materials used to produce UHPC are also of high standard, which is necessary to achieve its aforementioned excellent properties. However, the large amount of cement, steel fibre and high-quality aggregates (such as silica sand) not only raises the costs of UHPC but also depletes natural resources and causes environmental burden. There are generally three ways for UHPC to reach sustainable development: 1) reduce the cement content in it by using more supplementary cementitious materials, (2) reduce the natural aggregate content in it by using alternative aggregates, and (3) consume less cement in it by developing new construction techniques [ reduce tNatural aggregates (river sand and gravel), as important components of UHPC, take thousands of years to form, yet the consumption rate of these materials is much higher than their renewal rate. Natural aggregates are mostly used to produce construction and building materials . In factCurrently, there are many previous studies that have assessed the effect of the different types and proportions of recycled concrete aggregates (RCA) obtained from C&D waste on the overall performance of recycled aggregate concrete (RAC). Brand et al.  indicateIn general, the durability of RAC is poorer than that of regular concrete without RCA ,24,25,26Thus far, both coarse and fine RCA can be used to produce regular concrete. In general, the specific surface area of fine RCA is larger than that of coarse RCA, and thus, the addition of fine RCA can result in a decrease in workability and mechanical strength for regular concrete . As for In this study, we focused on investigating whether it is feasible to manufacture UHPC with fine RCA. On this basis, the rheological properties, autogenous shrinkage, mechanical properties  and pore structure of UHPC with different amounts of RCA were comprehensively investigated. The natural aggregate content was replaced with fine RCA at rates of 0, 20, 40, 60, 80 and 100 wt.%, and the packing density of the mixed fine aggregates was estimated through a linear packing model. The findings of this study provide an insight into the utility of fine RCA to manufacture UHPC.The cementitious materials used in the study contained P·II 52.5 cement, class I  fly ash and silica fume. The chemical compositions of these cementitious materials were characterised by X-ray fluorescence  and are presented in We are aiming to study the effects of the fine RCA content on the properties of UHPC, and thus, six different UHPC mixtures were designed in the work. The mixtures of UHPC used in the paper are shown in n items of size xd , we can order similar particles from the largest size to the smallest size or vice versa, with a known independent monosized packing density for each particle. The maximum packing density xth component in the mixture, respectively, and the functions The packing density of the aggregates plays an important role in the mix design of cement-based materials, and the overall performance of cement-based materials can be improved by optimising the packing density of the particles. The packing density of the aggregates can be calculated by some modelling methods. The packing density of the mixed fine aggregates in this study was estimated using a linear packing model. There are two basic assumptions in the model: 1) the aggregates are non-deformable and 2) the interactions among the aggregates are linear [The maximum packing density of the aggregates can be obtained via Equations (1)–(3).According to the above mix proportions, specimens of UHPC were cast without any mechanical vibration. The specimens were covered with plastic sheets after casting, cured for 48 h at room temperature and then demoulded. Subsequently, they were cured in a standard curing room with a temperature range of 20 ± 2 °C and a relative humidity of >95% for 28 days. The initial slump flow of the mortar of UHPC was maintained at 260 ± 5 mm to fulfil the requirement of a self-consolidating mortar. Hence, different amounts of high-range water reducer were used in the study. The slump flow was measured according to ASTM C 230/C 230M . To studth day.According to ASTM C1698 , the aut3 cubic specimen with a constant loading of 0.8 MPa/s. The flexural strength of UHPC was determined with a four-point bending loading configuration by using a 100 × 100 × 400 prism specimen, and the space between the loading points was one-third of the clear span, where there is no shear stress in the zone. In the flexural strength test, the specimens endured pure bending between two loading points at a distance of 100 mm, and the specimens were supported by two metal rollers, with the distance between them being 300 mm. The direct tensile strength of UHPC was measured on dog-bone specimens in accordance with literature procedures [3 for the measurement of the Young’s modulus.According to the Chinese standard GB/T 50081-2002 , the comocedures , and theocedures . The strocedures . Moreove3, which was split from the 100 × 100 × 100 mm3 cubic specimen. The hydration reaction of the UHPC sample used for MIP was terminated at 28th day by immersing the specimens in absolute alcohol. It should be pointed out that the samples were dried at 60 °C in a vacuum drying chamber for 72 h before the examination, and three replicated measurements were conducted for each UHPC mixture. In addition, an X-ray computed tomography  system and its corresponding analysis software (VG Studio MAX 3.0) were also used to investigate the pore structure of UHPC.The porosity and pore size distribution of UHPC were tested by mercury intrusion porosimetry , and the size of the sample used for MIP was about 5 × 5 × 5 mmIt should be pointed out that at least three repeated measurements were undertaken on the rheological properties, autogenous shrinkage, mechanical properties  and porosity of UHPC, to improve the accuracy of the experimental results. Moreover, only the mean values are reported in this paper.The amount of high-range water reducer in UHPC mixtures could be used to assess the fluidity of UHPC since all the slump flow of the UHPC mixtures was maintained at 260 ± 5 mm in the study. The amount of high-range water reducer in different UHPC mixtures is given in According to the Bingham model, the yield stress and plastic viscosity of UHPC with different amounts of fine RCA were fitted, and the results are presented in th day is presented in th day was 461.06 and 546.14 μm/m, respectively, which indicates that the former had a 15.58% lower autogenous shrinkage than that of the latter. What is more, the autogenous shrinkage of UHPC with NRS or fine RCA only (N100R0 and N0R100) was always higher than that of UHPC using mixed NRS and fine RCA , which suggests that the autogenous shrinkage of UHPC could be decreased by using mixed fine aggregates. This could be mainly due to the increased porosity and pore size of UHPC owing to the use of mixed fine aggregates [The autogenous shrinkage of UHPC on the 28gregates , which wThe evolution of the compressive strength of the investigated UHPC is shown in In general, the compressive strength of UHPC is determined by three types of phases: mortar phase, aggregate phase, and the interface between these two. Compared to the other two phases, the aggregate phase is the strongest for UHPC with RCA, and thus, the quality of the aggregate has an important impact on the compressive strength of UHPC. Because there is some old mortar on the surface of RCA, the ITZ is weak in UHPC. The load is transmitted either via the ITZ or via the contacting points of the aggregates. There are three types of ITZ in UHPC with RCA: (1) the ITZ between the aggregate and the new cementitious paste, (2) the ITZ between the old cementitious paste and the new cementitious paste, and (3) the ITZ between the RCA and the old adhered mortar. Moreover, the third ITZ is the weakest because there are inherent micro-cracks in it and the stress concentration at the tips of the cracks is very high. The compressive strength of UHPC with fine RCA decreased owing to the appearance of a weak ITZ.The direct tensile strength of UHPC is presented TsF and In addition, the relationship between the direct tensile strength and the compressive strength of UHPC was investigated in the study and the results are presented in R-squared of the Weibull distribution fitting was 0.96942, which indicates that the fitting results agreed very well with the experimental results and that Equation (4) could be used to evaluate the direct tensile strength of UHPC by its compressive strength. Taken altogether, the Weibull distribution model established in the work was accurate enough to express the relationship between the direct tensile strength and the compressive strength of UHPC. In practice, the model could provide a simple way of characterising the tensile strength of UHPC via its compressive strength since the direct tensile strength of UHPC is generally much more difficult to obtain than its compressive strength. However, it should be noted that the empirical model might not accurately assess the direct tensile strength of UHPC with different mixtures .The The variations in the four-point flexural strength of UHPC are illustrated in The Young’s modulus of UHPC was measured before the cracks initiated in it, and thus, the inherent micro-cracks in the ITZ are the key elements to the Young’s modulus of UHPC. The stiffness of UHPC with RCA decreased owing to the inherent micro-cracks during the preparation process of RCA and to the weak ITZ between the aggregate and the old adhered mortar, which led to the decrease in the Young’s modulus of UHPC with RCA. In addition, the stiffness of the mortar phase in UHPC with RCA could be reduced by the extra voids if there was more water during the hydration process, which also resulted in the decrease in the Young’s modulus of UHPC with RCA.According to Equations (1)–(3), the maximum packing density of mixed fine aggregates could be determined and the results are shown in The porosity and pore size distribution of UHPC are summarised in As for the pore size distributions of N100R0, N80R20, N60R40, N40R60, N20R80 and N0R100, there were multi-peaks in the different types of UHPC, as shown in 3, respectively, which was in good agreement with the result of their porosity. Furthermore, it can also be seen from In addition, the pore size and steel fibre distributions of UHPC were also studied by X-CT, and the results are presented in th day, with an autogenous shrinkage of 461.06 μm/m, compressive strength of 180.32 MPa, tensile strength of 8.94 MPa, flexural strength of 19.97 MPa, Young’s modulus of 49.94 GPa, and porosity of 2.39%. The findings of N40R60 mentioned above indicated that the comprehensive performance of UHPC was improved due the addition of fine RCA and that using fine RCA to produce UHPC contributed to reducing natural resource and energy consumption. It should be pointed out that the other important properties  of UHPC with fine RCA should be investigated comprehensively before using them in engineering applications. These findings help to encourage and further promote the use of RCA to produce UHPC.In summary, the workability, mechanical properties and deformation behaviour of UHPC with fine RCA were comparable to or even higher than those of UHPC made of high-quality aggregates. Considering the mechanical properties and deformation behaviour of UHPC, we found the optimal replacement rate of fine RCA to be in the range of 40–60 wt.%. When the replacement rate of fine RCA was 60 wt.%, the UHPC containing fine RCA exhibited better performance on the 28Both the yield stress and the plastic viscosity of UHPC increased with increasing amounts of fine RCA. When the natural aggregate content was replaced with fine RCA at a rate of 60 wt.%, the yield stress and the plastic viscosity of UHPC increased by 54.66% and 30.70%, respectively.The autogenous shrinkage of UHPC generally decreased because of the addition of fine RCA and it was lowest when the replacement rate of fine RCA was 60 wt.%. The autogenous shrinkage of N40R60 and N100R0 after 28 days was 461.06 and 546.14 μm/m, respectively, with the former being 15.58% lower than the latter.The compressive strength of UHPC with fine RCA was between 173.05 and 184.26 MPa, and it reached the maximum value when the replacement rate of fine RCA was 40 wt.%.The tensile strength of UHPC with fine RCA was in the range of 7.14–9.13 MPa. When the replacement rate of fine RCA was 40 wt.%, the tensile strength of UHPC (N60R40) increased to the maximum value, which suggests that N60R40 had a 8.43% higher tensile strength than that of N100R0.The relationship between the tensile strength and the compressive strength of UHPC could be described by a Weibull distribution model, and the model established in the work provided a simple way of characterising the tensile strength of UHPC via its compressive strength.The four-point flexural strength of UHPC with fine RCA was in the range of 16.64–19.97 MPa. When the replacement rate of fine RCA was 60 wt.%, the four-point flexural strength of UHPC (N40R60) reached the maximum value, which suggests that N40R60 had a 12.82% higher four-point flexural strength than that of N100R0.The Young’s modulus of UHPC with fine RCA was in the range of 43.66–49.94 GPa. When the replacement rate of fine RCA was 60 wt.%, the Young’s modulus of UHPC (N40R60) reached the maximum value, which indicates that N40R60 had a 3.40% higher Young’s modulus than that of N100R0.According to the linear packing model established in the work, the maximum packing density of mixed fine aggregates displayed a monotonic decreasing trend when the replacement percentage of fine RCA was increased.There were no steel fibre clusters detected in the specimens of UHPC, and an isotropic and homogenous distribution of steel fibres was obtained through the special mixing procedure in this work.The findings of N40R60 concluded in the paper indicated that the comprehensive performance of UHPC was improved due the addition of fine RCA and that using fine RCA to produce UHPC contributed to reducing natural resource and energy consumption.The feasibility of manufacturing UHPC with fine RCA was investigated in this work. On this basis, the workability, deformation behaviour, mechanical properties and pore structure of UHPC with different amounts of RCA were systemically studied. In addition, the packing density of the mixed fine aggregates was estimated via a linear packing model. From the experimental results, the following important conclusions are drawn:


Within the proposed analytical model, we derived expressions of the explosion probability and the duration of the induction period as a function of the initiation energy (exposure). An experimental verification of the model was performed by exploring the initiation of pentaerythritol tetranitrate (PETN) with the first harmonic of YAG: Nd laser excitation , which has confirmed the adequacy of the model. This validation allowed us to make a few quantitative assessments and predictions. For example, there must be a few dozen optically excited molecules produced by the initial fluctuations for the explosive decomposition reaction to occur and the life-time of an isolated cell before the micro-explosion must be of the order of microseconds.We propose a fluctuation model of the photochemical initiation of an explosive chain reaction in energetic materials. In accordance with the developed model, density fluctuations of photo-excited molecules serve as reaction nucleation sites due to the stochastic character of interactions between photons and energetic molecules. A further development of the reaction is determined by a competition of two processes. The first process is growth in size of the isolated reaction cell, leading to a micro-explosion and release of the material from the cell towards the sample surface. The second process is the overlap of reaction cells due to an increase in their size, leading to the formation of a continuous reaction zone and culminating in a macro-explosion,  Laser initiation of high energy density materials has long attracted the attention of researchers ,5,6,7,8 2H4N4O4, also known as FOX-7) [3H6N6O6) [5H8N4O12) [Contrary to the popular opinion, there is both theoretical and experimental evidence that electronically excited states play a crucial role in triggering decomposition chemistry in molecular materials. For example, quantum-chemical calculations of gas-phase decomposition of 1,1-diamino-2,2-dinitroethylene (DADNE) molecules (Cs FOX-7)  establiss FOX-7) ,19. Expe3H6N6O6) , pentaerH8N4O12) ,22,23, aH8N4O12) ,25. In pH8N4O12) ,22. ReceH8N4O12)  ruled ouH8N4O12) ,22. The H8N4O12)  as well H8N4O12) ,29,30,31H8N4O12) ,22. ThusH8N4O12) .Due to its stochastic nature, this process is inevitably accompanied by fluctuations that result in the emergence of microscopic regions with high density of excited molecules. A similar effect is well known in radiation chemistry, where cells with high ionization density are formed in the tracks of nuclear particles due to the stochastic nature of the ionization process . Similar3 m/s, which causes their overlap in a time of the order of 10−7 s and leads to the explosion [It was established that the pulsed laser and electron beam initiation of silver azide results in the occurrence of pre-detonation luminescence, accompanying the initial stage of the explosive decomposition ,35,36. Ixplosion ,35,36. Wxplosion  are implDespite existing experimental data and vast interest in gaining a better understanding of ultra-fast and chain processes in materials, fundamentals of the chain reaction of explosive decomposition remained practically unexplored in the field of laser initiation of detonation. The role of density fluctuations of photo-excited molecules in triggering the chemical reactions in energetic materials is yet to be established. In this study, aimed at making the first step towards the development of the phenomenological model of photochemical initiation in explosives, we analyze the process of photo-initiation of chemical decomposition of energetic materials from a point of view of stochastic interactions between photons and energetic molecules. We suggest an analytical fluctuation model, in which local splashes of molecular photo-excitation density serve as reaction nucleation sites, and the reaction propagation depends on the interplay of increasing size of these sites and their overlap. Further, we validate the model with experiments on the initiation of PETN with the first harmonic of YAG: Nd laser excitation  and check our predictions. Although we generally follow the ignition and growth concept in our study, there are significantly different aspects in our model in comparison to the widely used Lee-Tarver model, which describes the shock initiation of heterogeneous solid explosives by Lagrangian hydrodynamic code ,39. In tThree well-established provisions laid the groundwork for the proposed model. First, the stochastic nature of the interaction of photons with the energetic molecules at the photochemical initiation leads to fluctuations of the density of the excited molecules. In the case of fairly large fluctuations, conditions for the emergence of a self-sustaining chain reaction can be satisfied; in other words, those fluctuations are beginning to act as nucleation of microcells of the explosive decomposition reaction. With the reaction progressing across the sample, the sizes of these reaction cells are growing, leading to their overlap, which forms the explosion zone covering the whole sample or, at least, a significant part of it. An increase of the initiating radiation intensity leads to an increase in the concentration of reaction cells and consequently reduces the time required for their overlap. The growth rate of the linear sizes of the cells does not depend on the concentration  exhibit a lower initiation threshold ,9,10 thai.e., macro-explosion).Within this context, it seems that a reasonable, well-grounded scenario of the photochemical initiation of explosive decomposition is the following. The chain reaction begins in the micro-regions of the fluctuation of increasing density of the excited molecules. At low concentrations of these regions, corresponding to low exposures of initiation, any significant overlap of these cells does not have sufficient time to occur prior to the ejection of matter from the reaction cells (a micro-explosion cell), and the reaction dies out in the formation of cavities and/or pits. At high concentrations of cells, their overlap occurs before the micro-explosion of an individual cell, and we observe an explosion of a significant part of the sample k/ is p = 1mk− = (Nb03/k)k−1. The concentration of such cells that contain k excited molecules, n = p/b03 will be: We now turn to a quantitative analysis of the situation. Assuming that N value is proportional to initiation exposure H  with η as a coefficient:At low absorption, the By combining Equations (1) and (2), one obtains:Each of these cells originates the chain reaction of explosive decomposition, and the size of each cell grows as the reaction progresses ,35,36.e; this is also the time of the formation of the explosion, covering the entire sample. As a simple estimate of the τe we take the time needed for the reaction front to propagate at a distance equal to the average distance between cells, 1/3n, , where υ is a velocity of the reaction front (or the growth rate of the linear dimensions of the reaction cell). Then, taking into account Equation (3), we derive an estimate of eτ:Next, we estimate the time required for individual cells to overlap, τ.0τ for mathematical convenience. An additional benefit here is that this way brings a clear physical meaning to the constant C, which can be attributed to the initiation exposition, corresponding to the explosion probability of 50%.Equation (4) is deliberately multiplied and divided by 5 cm/s (ref. [In the case of a chain character of the process (and this is the case discussed in the article), the propagation of the reaction zone front is defined by a diffusion of active radicals . In our /s From Equations (4) and (5), we derive the probability of explosion as:W, and the delay time with respect to the initiating impulse, τe (that is the duration of the induction period) by using expressions Equations (4) and (6).The adequacy of the proposed model can be tested in a relatively simple and effective way. We will describe the experimental data on the probability of explosion as a function of initiation exposure, 5H8N4O12) samples for our experiments were synthesized by the nitration of pentaerythritol with concentrated nitric acid. They were purified by recrystallization from acetone and repeated recrystallization in hexane to obtain a powder of PETN with a grain size of 100 microns. The samples under study consisted of tablets having a diameter of 3 mm and a thickness of 0.5 mm, pressed into a stainless steel shell . A schematic representation of the explosion chamber is shown in The pentaerythritol tetranitrate  YAG: Nd laser LDPL10M. Initiation exposure was determined using the pyroelectric head PE50BF-DIF-C  and was constantly monitored by a signal of the calibrated photodiode. Special care was taken to ensure a fairly homogeneous distribution of the excitation intensity over the sample surface, and only the central part of the laser beam was used for initiation. The beam diameter is 6 mm and the tablets are 3 mm in diameter. The energy deviation of the initiating pulse did not exceed 3%.eτ. Examples of the obtained oscillograms are given in The acoustic sensor in the experimental set up enabled a reliable timing of the start of the explosive decomposition of the sample with respect to the initiating pulse as the acoustic sensor detects two pulses, the first pulse appears as the result of the impact of the initiating laser pulse adjacent to the back side of the sample surface of the explosion chamber, the second pulse is the result of the explosion of the sample. The time interval between the leading edges of the pulses (at the level of the amplitude of 0.1) was taken as the duration of the induction period, W, and the duration of the induction period, eτ, on exposure initiation. At a glance, the experimental data are well approximated by the expressions Equations (4) and (6), which provides a solid argument in favor of the adequacy of the proposed model. More specifically, the values of fitting parameters k and 0τ,  were obtained from the experimentally measured dependencies, W(H) and e(H)τ and turned out to be k = 35 and τ0 = 1.7 µs. The expressions Equations (4) and (6) yield the same values of k and τ0.k = 35 excited molecules and τ0 = 1.7 µs, although appear quite reasonable, refer to the conditions of the experiment. Obviously, the reduction in strength of the metal shell surrounding the sample may lead to a decrease of τ0 and increase of k. Therefore the results obtained in this experiment and the values of k and τ0 are determined, generally speaking, only at the level of the order of magnitude in such a way that k is of the order of ten molecules and τ0 is of the order of a µs.We note that both estimates, C, which corresponds to the initiation exposure that provides 50% probability of explosion (see Equation (6)), is calculated as C = 3.5 J/cm2, also happens to be the same when approximated by both dependences  serve as reaction nucleation sites. Unlike Lee-Tarver nucleation and growth model  in its s(i) the experimental data are well approximated by theoretically derived functions of the induction period, τe, and the probability of explosion, W, depending on the initiation exposure, H, as illustrated in (ii) the values of the constants that are included in the different dependencies, coincide, and the estimated micro parameters  yield reasonable values; and finally, (iii) the seemingly disparate facts: the fluctuation-triggered origin of the chain reaction, the formation of pits and cavities on the surface of the non-exploded samples, and the role of the sample’s confinement on the initiation threshold of high explosive materials, can be explained now from the common point of view.We suggest that several important arguments evidence for adequacy of the proposed model. Specifically: We believe that this phenomenological model signifies a breakthrough that will serve as yet another critical building block towards design of a united microscopic theory of initiation of detonation. In accordance with the proposed model, the first step in the explosive decomposition reaction is attributed to the optical absorption of a photon by an individual molecule. This perturbation drives the system (the molecule) to the excited state and triggers a chain of processes. The absorption therefore is the earliest elementary act of a photochemical reaction. Contrary to this, the first step of the thermochemical initiation is the absorption of a group of photons by a “hot spot” , locally heating the material up to a critical temperature to trigger a thermal decomposition, which is not described by the developed model. As was demonstrated in this research, the concept of photo-excitation density fluctuations laid a foundation to design the experiments. The theoretical model based on the concept was able to accurately describe and interpret the performed measurement, which evidences in favor of photo- (rather than thermo-) activation of chemistry of the laser excitation explored here. Hence, the developed model provides a new and attractive tool to study photo-activated processes relevant to stability/decomposition and chain reactions in materials as it clearly discriminates actions and consequences of photo- and thermochemical mechanisms, which was not attainable before.i.e., photo-excitation of PETN molecules, actually takes place. It is also shown that the photo-initiation leads to the formation of active radicals, providing the further degradation of the material by an explosive chain decomposition reaction, or an autocatalytic process. Thus, the offered conditions are ideal not only for the development of an appropriate model, but also for the experimental verification of its adequacy. Other mechanisms of photo-initiation, especially the most popular photo-thermal initiation mechanism, fall short in providing a similarly appealing situation and require a lot of preliminary work that goes far beyond the scope of this article.There are reasons that prompted us to limit our consideration in this article only to the photo-chemical mechanism, without involving other initiation scenarios, in particular the photo-thermal initiation mechanism of hot spots ,21,22,32