Thermal anomaly in LiKSO4 crystals in the temperature range 300–800 K

Thermal analysis, DTA, TG, TMA and DSC were performed on LiKSO₄ in the temperature range 300 to 800 K, using a Heraeus TA 500 thermal analyser. The thermal expansion coefficients measured along the three fundamental crystallogrpahic axes, together with the specific heatC _p, show anomalous behaviour around the phase transition temperature of these crystals atT _c=705 K. Anisotropy in the thermal expansion coefficient as well as endothermic peaks aroundT _c were also observed, while no loss of weight was detected. The temperature-dependence of the thermal conductivity of LiKSO₄ crystals has been estimated, using the temperature-variation of the thermal diffusivity.     

Synthesis of the titanium phosphide telluride Ti2PTe2: A thermochemical approach

The phosphide telluride Ti₂PTe₂ can be synthesised from the elements or from oxides in a thermite type reaction. Both ways have been optimised by consideration of the thermodynamic behaviour of the compound. Hence, the investigation of phase equilibria in the ternary system Ti/P/Te and of the thermal decomposition of Ti₂PTe₂ was necessary. This investigation was performed by using different experimental approaches as total pressure measurements, thermal analysis and mass spectrometry. The results were supported and further analysed by thermodynamic modelling of the ternary system. It was shown that Ti₂PTe_2(s) decomposes to Ti₂P_(s) and Te_2(g) in six consecutive steps. The growth of single crystals of Ti₂PTe₂ is thermodynamically described as a chemical vapour transport with TiCl_4(g) acting as the transport agent.     

Thermodynamic studies on the ferroelectric phase transition in neutron irradiated (LixK1−x)2SO4 crystals at high temperature

Thermodynamic studies of (Li_xK_1?x)₂SO₄, LKS, mixed crystals have been made in the concentration range (x=0.1, 0.2, ...,x=0.5). The thermal behaviour has been investigated by differential thermal analysis, DTA, and differential scanning calorimeter, DSC, in the vicinity of high temperature phases. Also, the effect of the thermal neutron irradiations on the thermal properties of mixed crystals was studied. The results showed a change in the transition temperatureT _c, as well as the value of specific heatC _p at transition temperature, due to the change of stoichiometric ratio and radiation doses. The change of enthalpy and entropy of mixed crystals have been estimated numerically.     

Photopyroelectric method

High resolution measurements have been performed with a photopyroelectric (PPE) technique to study the static and dynamic thermal parameters behaviour in the critical region of different liquid and solid samples. In particular the specific heat (c), the thermal diffusivity (D) and the thermal conductivity (k) have been simultaneously determined in the critical region around the antiferromagnetic/paramagnetic (AP) phase transition occurring in FeF₂ and around the Smectic-A/Nematic (AN) phase transition occurring in the 8S5 liquid crystal.The high resolution of the measuring technique has allowed the determination of thec andD critical exponents and amplitude ratio of their critical terms.     

Effect of mechanical activation on the hydrogen reduction kinetics of magnetite concentrate

The combined thermal analysis techniques thermal expansion: and differential scanning calorimetry were used to characterize various phase transitions that exist in the solid solutions of PbZr_1?xSn_xO₃. Using thermodynamic quantities, i.e., thermal expansion and specific heat to distinguish first-order transitions from second-order ones, we demonstrate that some perovskite antiferroelectrics can exhibit continuous transition at their Curie temperature T _C. We observed such a transition in antiferroelectric crystals of solid solutions based on PbZrO₃. Although pure PbZrO₃ is a classical example of antiferroelectric crystal with a first-order transition at T _C, the solid solutions of PbZr_1?xSn_xO₃ in the range of composition of x > 0.25 seem to exhibit a second-order phase transition.     

Photothermal applications to the thermal analysis of solids

Major application of optically-induced thermal waves to the thermal and thermodynamic analysis of solids are reviewed. The spectrum of available techniques,from the conventional photoacoustic detection to novel photothermal laser probing and frequency multiplexing is discussed, and their utilization for the measurement of thermophysical thermal transport-related parameters of solids is presented. These include the thermal diffusivity, effusivity, conductivity and specific heat. The ability of photothermal methods to perform thermal analysis on large classes of solids, including conducting and insulating bulk materials, crystals, layered porous and coated structures, thin films and inhomogeneous thermal profiles is highlighted. Finally, special capabilities of photothermal analysis, such as the monitoring of surface thermodynamic phenomena and phase transition studies, including high-T _c superconductors, are described in order to give a complete overview of the rich potential of photothermal-based methodologies.     

The low‐temperature phase transition of 9‐methylfluoren‐9‐ol: comparison of the crystal structures at 100 and 200 K

Crystals of 9‐methyl­fluoren‐9‐ol, C₁₄H₁₂O, undergo a reversible phase transition at 176 (2) K. The structure of the high‐temperature α form at 200 K is compared with that of the low‐temperature β form at 100 K. Both polymorphs crystallize in space group P with Z = 4 and contain discrete hydrogen‐bonded R(8) ring tetramers arranged around crystallographic inversion centres. The most obvious changes observed on cooling the crystals to below 176 K are an abrupt increase of ca 0.5 Å in the shortest lattice translation, and a thermal transition with ΔH = 1 kJ mol^?1.     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

镉基杂化晶体的介电相变和蓝白光致发光双重特性

通过采用容易无序的胺,我们合成了2种有机无机杂化晶体,分别为基于bempy （bempy=1-甲基-1-溴乙基吡咯烷阳离子）的溴盐化合物（bempy） Br （1）和镉基溴化物（bempy）₂CdBr₄（2）,并对其结构相变、介电相变和蓝白荧光进行了详细的表征分析。化合物1在测试温度范围内未观察到可逆相变,化合物2为高温介电相变,介电和差示扫描量热法测试表征其相变温度为357 K。同时,化合物1和2均具备蓝白光致发光特性,荧光测试表明,化合物1和2分别在538 nm和547、750 nm处存在发射峰。化合物2具备介电相变和蓝白光致发光的双重特性。     

Thermal behaviour of (NH4)3VO2F4 and Na(NH4)2VO2F4

The thermal behaviour of (NH₄)₃VO₂F₄ and Na(NH₄)₂VO₂F₄ was investigated using TG, DTA and DSC techniques. The occurrence of a first order phase transition with the onset of decomposition in both the compounds is confirmed. The temperature, energetics and hysteresis of the transition are obtained. A possible path for the thermal degradation is given for both the compounds, and the residues are identified.     

Thermal decomposition of lead titanyl oxalate tetrahydrate

The thermal behaviour of PbTiO(C₂O₄)₂·4H₂O (PTO) has been investigated, employing TG, quantitative DTA, infrared spectroscopy and (high temperature) X-ray powder diffraction.The decomposition involves four main steps. The first is the dehydration of the tetrahydrate (30–180°C), followed by a small endothermic (270–310°C) and a large exothermic decomposition of the oxalate. The main (exothermic) oxalate decomposition (310–390°C) results in a stable oxide-carbonate PbTiO₂₅.(CO₃)_0.5. In the last step a phase transition, release of CO₂ and ordering of the crystalline cubic PbTiO₃ lattice can be detected (460–530_C).It can be argued that for thermodynamic reasons the presence of lead-oxo- carbonates in the oxide-carbonate intermediate is not possible.No differences could be found in thermal behaviour of two crystallographically different synthetic forms of PTO, of which one has an orthorhombic lattice.     

The effect of ethanol on the phase behaviour and micro-rheology of liquid crystals

The effect of ethanol on the phase behaviour and micro-rheology of lyotropic liquid crystals (LC) has been studied using a binary mixture of monoglyceride (MG) and aqueous ethanol. The phase behaviour study reveals the structural modulation of surfactant aggregates with increasing ethanol concentration, namely a bicontinuous cubic phase (Ia3d) transitions to the lamellar phase (L_α), at a fixed MG concentration. This behaviour is explained by considering the critical packing parameter (CPP) of the surfactant molecule. Because ethanol dehydrates the surfactant head group (a _s), the CPP values increase (decreasing a _s) and thus the formation of larger CPP aggregates is favoured (i.e., the Ia3d–L_α transition occurs). Cross-polarised images and X-ray scattering data support this conclusion. The structural modulation of the LC has further been investigated using a diffusing wave spectroscopy technique. The correlation and relaxation times, determined from the intersection point at short and long time scales of the mean square displacement (MSD), decrease with increasing concentrations of ethanol, indicating structural modulation of the LC. The micro-viscoelastic moduli (G′ and G′′) derived from the Laplace transformation of the MSD decrease with increasing ethanol concentrations, due to the LC modulation. The thermal effects on the micro-rheology of the LC have also been studied.     

Isolation, structural, spectral, and thermal studies of imperatorin micro-crystals from Prangos pabularia

Imperatorin (mol. formula = C₁₆H₁₄O₄, mol. mass = 270) micro-crystals were isolated from ethyl acetate extract of the root parts of the plant Prangos pabularia. The composition related structural, spectral, and thermal properties were investigated. All the crystals were found to have single phase with orthorhombic structure. UV–Visible spectrum showed both direct forbidden transition and indirect allowed transition. The calculated optical band gaps (E _g) were 3.64 and 3.70 eV for direct forbidden and indirect allowed transitions, respectively. From the thermal studies of these crystals, melting point, stability, and other relevant information was obtained, ensuring the suitability of the material for application in any device where the crystals may have to withstand this range of temperature.     

Thermodynamic insights into n-alkanes phase change materials for thermal energy storage

n-Alkanes have been widely used as phase change materials (PCMs) for thermal energy storage applications because of their exceptional phase transition performance, high chemical stability, long term cyclic stability and non-toxicity. However, the thermodynamic properties, especially heat capacity, of n-alkanes have rarely been comprehensively investigated in a wide temperature range, which would be insufficient for design and utilization of n-alkanes-based thermal energy storage techniques. In this study, the thermal properties of n-alkanes (C₁₈H₃₈-C₂₂H₄₆), such as thermal stability, thermal conductivity, phase transition temperature and enthalpy were systematically studied by different thermal analysis and calorimetry methods, and compared with previous results. Thermodynamic property of these n-alkanes was studied in a wide temperature range from 1.9 K to 370 K using a combined relaxation (Physical Property Measurement System, PPMS), differential scanning and adiabatic calorimetry method, and the corresponding thermodynamic functions, such as entropy and enthalpy, were calculated based on the heat capacity curve fitting. Most importantly, the heat capacities and related thermodynamic functions of n-heneicosane and n-docosane were reported for the first time in this work, as far as we know. This research work would provide accurate and reliable thermodynamic properties for further study of n-alkanes-based PCMs for thermal energy storage applications.     

Thermotriggered Domino-like Single-Crystal-to-Single-Crystal Phase Transition from Face-to-Edge to Face-to-Face Packing of Anthracenes

Stimuli-triggered crystal-to-crystal and single-crystal-to-single-crystal (SCSC) transformations have received significant attention in the scientific community. To visualize such phenomenon, controlling the optical properties and the thermodynamic stability of the molecular crystals is a very important research subject. In this report, the selective growth of photoluminescent (PL) 1,8-bisphenylanthracene polymorphic ( cI and cII ) and 1,2-dichloroethane-inclusion crystals ( iC ) under various optimized conditions is described. These crystals exhibited unique mechano- and thermoresponsive disordering, crystal-to-crystal phase transition, and SCSC phase transition. In particular, rapid thermostimulus SCSC occurred from blue-PL cI into greenish-blue-PL cII . Interestingly, the SCSC phase transition of cI into cII was triggered by thermal stimuli and propagated spontaneously. Thermotriggered domino-like SCSC phase transition was observed on a fully visible timescale (ca. 125 μm min⁻¹).     

The Non-Isothermal Devitrification of Potassium Germanate Glasses

The devitrification behaviour of the glasses K₂O·xGeO₂ with x=4, 7 or 8 was examined by means of differential thermal analysis (DTA), the Fourier transformation infrared (FTIR) transmittance spectra and X-ray diffraction (XRD). The glass transition temperatures were related to the molar ratio GeO₄/GeO₆. For the glass with x=4, metastable K₄Ge₉O₂₀> crystals are initially formed and then converted at higher temperatures into stable K₂Ge₄O₉ crystals. The glasses with x=7 or 8 both devitrify into K₂Ge₇O₁₅> crystals. The effects of the specific surface area of the samples on the devitrification mechanisms were established. Bulk nucleation predominates in the glass with x=4, while the glasses with x=7 or 8 crystallize from the surface. The activation energies for crystal growth were evaluated from the DTA curves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and characterisation of imidazolium-based ionic liquid crystals bearing a cholesteryl mesogenic group

A series of cholesteryl-containing imidazolium chlorides and imidazolium tetrachloroaluminates were synthesised, and the chemical structure, liquid crystalline behaviour and ionic conductivity were characterised by several technical methods. Whereas the imidazolium chlorides show chiral smectic A (SmA*) phase on heating and cooling cycles, the imidazolium tetrachloroaluminates display chiral nematic (N*) phase, which is uncommon for ionic liquid crystals (ILCs). The imidazolium chlorides display similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than the different alkyl substituent groups. The imidazolium tetrachloroaluminates show lower melting point temperatures and lower clear point temperature than the imidazolium chlorides. The mesophases exist at rather moderate temperatures. Non-mesomorphic imidazolium tetrachloroaluminate(III) salts with short alkyl substituents have been known for a long time, and the synthesised imidazolium tetrachloroaluminates are the first examples of tetrahalogenoaluminate(III)-containing ILCs. For the imidazolium tetrachloroaluminates, imidazolium cations combine loosely with AlCl₄^? ions because AlCl₄^? ions are large and occupy more space in spite of the hydrogen bond and electrostatic attraction interaction, indicating that the layer structure can be destroyed easily to form N* phase on heating.     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Gel synthesis and crystallization of lithium-germanate glass powders

Lithium germanate gels, whose compositions are expressed by the general formula Li₂O·xGeO₂ with x=3; 4; 7, were synthesized by hydrolytic polycondensation of germanium ethoxide with lithium methoxide (x=3 and 7) or lithium hydroxide monohydrate (x=4) in alcoholic medium. The values of glass transition temperature of the gels exhibit a maximum at x=4. Crystallization behaviour of the gels, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. The terms x=4 and 7 crystallize in two steps. Microcrystallites of the same composition of the gel are initially formed in an amorphous matrix and are then converted at higher temperatures into well shaped crystals. In term x=3, Li₂GeO₃ and Li₂Ge₄O₉ crystals are directly formed. The values of activation energy for each crystallization step are consistent with the crystallization mechanisms and comparable with those reported with conventional melt glasses from oxides.