Accurate heat capacity data at phase transitions from relaxation calorimetry

Extracting accurate heat capacities by conventional relaxation calorimetry at first-order or very sharp second-order phase transitions is extremely difficult. The so-called “scanning method” provides a key to overcome this challenge. Here, we introduce new corrections in the data analysis of this method. Critical examinations of the improvements are made experimentally by investigating the well-studied first-order ferroelectric phase transitions of KH₂PO₄ and BaTiO₃ using a commercial relaxation calorimeter Physical Property Measurement System (PPMS) supplied by Quantum Design. The results for KH₂PO₄ are shown to be excellent; a very sharp peak in heat capacity is obtained and the absolute values are shown to agree well with the previous results obtained by adiabatic calorimetry on much larger samples. The critical behavior of the heat capacity in the vicinity of the transition temperature, as well as the thermodynamic quantities such as the transition enthalpy and entropy, also agrees very well with the previous results. For BaTiO₃, clear hysteretic behavior of the transition is observed for heating and cooling curves.     

Enthalpy relaxation studies of two structurally related amorphous drugs and their binary dispersions

Objective: The purpose of the current study was to evaluate the enthalpy relaxation behavior of valdecoxib (VLB) and etoricoxib (ETB) and their binary dispersions to derive relaxation constants and to understand their molecular mobilities. Methods: Solid dispersions of VLB and ETB were prepared with 1%, 2%, 5%, 10%, 15%, and 20% (w/w) concentrations of polyvinylpyrrolidone (PVP) in situ using differential scanning calorimetry (DSC). Enthalpy relaxation studies were carried out with isothermal storage periods of 1, 2, 4, 6, 16, and 24 hours at 40°C and 0% relative humidity (RH). Results: PVP increased the glass transition temperature (T_g) and decreased the enthalpy relaxation. Significant differences between two drugs were observed with respect to their relaxation behavior which may be due to differences in intermolecular interactions as predicted by Couchman–Karasz equation and molecular mobility. Kohlrausch–Williams–Watts equation was found to be inadequate in describing complex molecular relaxations in binary dispersions. The enthalpy relaxation behavior of VLB and ETB was found to be significantly different. PVP stabilized VLB significantly; however, its effect on ETB was negligible. The extent of enthalpy relaxation was found to correlate with hydrogen bonding tendency of the drug molecules. Conclusion: The outcome can help in rational designing of amorphous systems with optimal performance.     

Fractional polarization,a new technique of the thermally stimulated current method to study the physical and morphological structure of materials

Segregation phenomena in copolymers are studied using thermally stimulated current method. The origin of the peaks observed with the thermally stimulated current technique can also be investigated; but molecular parameters such as the relaxation times, the free activation energy, the migration enthalpy and entropy, and the degree of disorder number in polymers are calculated analysing the relaxation map obtained by the fractional polarization method. This method represents an important technological advance with regard to the study of inner responses and characterization of the amorphous states of polymers.     

Thermal behavior and phase identification of Valsartan by standard and temperature-modulated differential scanning calorimetry

Thermal behavior of angiotensin II type 1 (AT₁) receptor antagonist, Valsartan (VAL), was examined employing thermogravimetric analysis (TGA), standard differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC). The stability of VAL was measured by TGA from 25 to 600°C. Decomposition of Valsartan starts around 160°C. The DSC curve shows two endotherms, occurring around 80°C and 100°C, related to evaporation of water and enthalpy relaxation, respectively. Valsartan was identified by DSC as an amorphous material and it was confirmed by X-ray powder diffraction. The glass transition of fresh Valsartan appears around 76°C (fictive temperature). TMDSC allows separation of the total heat flow rate into reversing and nonreversing parts. The nonreversing curve corresponds to the enthalpy relaxation and the reversing curve shows changes of heat capacity around 94°C. In the second run, TMDSC curve shows the glass transition process occurring at around 74°C. Results from standard DSC and TMDSC of Valsartan were compared over the whole range of temperature.     

Adam-Gibbs Formulation of Enthalpy Relaxation Near the Glass Transition

The entropically based nonlinear Adam-Gibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional Tool-Narayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Δμ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above T_g.     

聚醚酰亚胺热焓松弛的分子量依赖关系

采用ＤＳＣ对不同分子量的聚醚酰亚胺的等温及动态热焓松弛行为进行了研究，并以ＫＷＷ方程对实验数据进行了非线性拟合。结果表明，分子量对热焓松弛的影响是由高分子端基引起自由体积的差异所致。不同分子量的聚醚酰亚胺分子具有不同数量的端基，淬冷时端基数目的差异将产生不同数量的过剩自由体积，从而影响松弛热焓及松弛速率、低分子量的聚醚酰亚胺松弛相对较快，且起始热焓高，松弛的表观活化能亦较小。     

Measurement of high temperature phase stability and thermophysical properties of alloy 740

In the present study, the characterisation of phase stability and measurement of different thermophysical properties of alloy 740 has been carried out. The transformation temperatures including liquidus/solidus and corresponding enthalpy of transformation have been measured for different phase changes up to melting using dynamic calorimetry. Further, the enthalpy increment data have been measured in the temperature range of 473–1473?K to obtain the heat capacity using static calorimetry. The present calorimetric data have been analysed in corroboration with the results obtained using JMatPro and Thermo-Calc simulation. In addition, the temperature dependence of other thermophysical properties such as thermal expansivity, density, thermal diffusivity and thermal conductivity are also measured in the range of 300–1473?K using thermomechanical analyser and laser flash method.     

聚对苯二甲酸乙二醇酯熔体等温结晶的焓松弛

利用微分扫描热分析（DSC）获得了聚对苯二甲酸乙二醇酯（PET）熔体等温结晶过程焓松弛速率曲线,并对结晶焓松弛速率曲线进行微分从而获得等温结晶焓松弛加速度曲线。文中通过定义相对结晶速率R和相对结晶加速度A,确定了相对结晶速率R和相对结晶加速度A与结晶焓松弛速率和结晶焓松弛加速度之间对应相差的一个常数值,然而其所对应的变...     

Characterization of amorphous alloys by thermal analysis techniques

Various thermal analysis studies were conducted to characterize metallic glasses. The methods used, namely differential scanning calorimetry, dilatometry, dynamic mechanical analysis and thermomagnetometry, are shown to be useful for studying thermally induced phenomena. Examples covered demonstrate the usefulness of thermal techniques in evaluating structural relaxation. Curie temperature, glass transition, and devitrification processes. A basis of understanding structural relaxation and the glass transition is discussed in terms of a diagram of enthalpy against temperature.     

Experimental investigation and thermodynamic modeling of the U-Nb system

The phase equilibria and thermodynamic properties of the U-Nb system were investigated through experiments and thermodynamic modeling.In the experiments,a series of samples with different con-tent of Nb were prepared by arc melting.The Nb content covers the entire composition range of the U-Nb binary system.After solidification,the samples were annealed and then analyzed by X-ray diffraction,scanning electron microscopy,energy dispersive X-ray spectroscopy,differential scanning calorime-ter and neutron diffraction.The equilibrium phases,including γ1 (U-rich bcc),αU,and γ2 (U-depleted bcc),were determined in detail from the experimental characterization.The occurrence of the invariant reaction γ1 → αU + γ2 was confirmed,and the reaction temperature and composition were accurately identified.Moreover,the experimental findings clarify the dispute between the Dwight and Terekhov theories regarding the phase region (i.e.,whether the phase region is αU + γ1 or βU + γ'2).Furthermore,the U-Nb phase diagram was reassessed by incorporating the present experimental data and previously published reliable experimental data from the literature.A set of self-consistent thermodynamic param-eters were developed using CALPHAD (Calculation of phase diagrams),and the calculations reasonably agree with the experimental observations.     

Calorimetric techniques for the evaluation of thermal efficiencies of shape memory alloys

The latent heat and entropy changes of NiTi shape memory effect (SME) alloys have been evaluated by three different calorimetric techniques; adiabatic calorimetry, differential scanning calorimetry and a Clapeyron analysis of isothermal stress-strain data. It is found that these techniques provide consistent estimates for the enthalpy and entropy to within 20% for NiTi and noble metal SME alloys. From published thermodynamic data for SME alloys, thermal efficiencies were calculated based on an ideal SME heat engine cycle. It was found that NiTi provides the maximum thermal efficiency with the highest temperature transformation range.     

AsNCa3 at high pressure

A constant pressure optimization scheme is applied to the study of ternary calcium nitrides under pressure. The enthalpy is minimized with respect to the electronic configuration, the positions of the atoms and the cell metric (i.e. the lattice parameters). Symmetry corrections can be performed during the relaxation towards the equilibrium structure in order to be able to investigate the compound in a certain proposed symmetry. We obtain excellent agreement with experiment for the zero-pressure structural parameters of the cubic anti-perovskite structure BiNCa₃ and of the distorted anti-perovskite structures AsNCa₃ and PNCa₃. For AsNCa₃ the structural parameters, band gap energies, etc. are investigated as a function of the pressure. A new cubic phase is predicted to have a lower enthalpy than the orthorhombic phase for pressures above 59 GPa.     

Characterization of molecular orientation in hot stretched polystyrene

Differential scanning calorimetry (DSC) and birefringence was used to study molecular orientation of polystyrene. Models were developed relating the orientation and stress levels to the temperature, strain rate and amount of stretch. The number of relaxations observed on the DSC was influenced most by the temperature of stretch. At a temperature of 1.03T _g K, maximum relaxation peaks and maximum tensile properties were developed over a range of molecular weights and molecular weight distributions. At given process conditions, birefringence was found to be influenced by {ie335-01}, while DSC enthalpy was influenced by {ie335-02}.     

分子量对聚丙烯拉伸微孔膜结构的影响

以重均分子量和分子量分布不同的三种聚丙烯树脂为原料,采用熔融挤出-退火-室温拉伸-热拉伸的方法制备了聚丙烯微孔膜.采用凝胶渗透色谱(GPC)、松弛时间谱、差示扫描量热(DSC)、扫描电镜(SEM)等测试手段表征了分子量对最终微孔膜结构的影响.结果表明,重均分子量为587.0 kg/mol、特征松弛时间最长的T30S粒料...     

The activation–relaxation technique: an efficient algorithm for sampling energy landscapes

Activated processes, i.e., rare events requiring thermal fluctuations many times larger than the average thermal energy, play a central role in controlling the relaxation and diffusion mechanisms of disordered materials such as amorphous and glassy solids, polymers and bio-molecules. As the time scales involved are much longer than those associated with thermal vibrations, these processes cannot be studied efficiently with standard real-space methods such as molecular dynamics (MD). They can be investigated much more efficiently by working in the potential energy space. Instead of defining moves in terms of atomic displacements, the activation–relaxation technique (ART) follows paths directly in the energy landscape, from local minima to adjacent saddle points, giving full freedom for the system to create events of any complexity. In this paper, we review the technique in detail and present some recent applications to amorphous semiconductors and glasses.     

聚乙二醇-纤维素接枝物固态相变材料的贮热性能

用化学偶联法,将聚乙二醇(PEG)接枝到纤维素分子链上,制备聚乙二醇-纤维素接枝物。用差示扫描量热(DSC)研究了接枝物的热力学性质。结果表明,PEG-CELL接枝物的相变焓、相变温度与PEG的分子量、PEG的质量百分比有关。当PEG的分子量在2000以下时,制备的接枝物相变焓很低;当PEG的分子量大于4000时。同等分子量情况下,相变焓、相变温度随PEG的质量百分含量减少而下降．所制备的PEG-CELL接枝物为固态相变材料,热滞后性降低,具有很好的热稳定性。     

Thermodynamics of lutetium uranosilicate

The standard enthalpy of formation of crystalline Lu(HSiUO₆)₃ · 10H₂O at 298.15 K, ?10668.0±16.0 kJ mol^?1, and the standard enthalpy of its dehydration were determined by reaction calorimetry. The heat capacity of this compound in the range 80–300 K was measured by adiabatic vacuum calorimetry, and its thermodynamic functions were calculated. The standard entropy of formation at 298.15 K is ?3812.9±1.2 J mol^?1 K^?1, and the standard Gibbs energy of formation at 298.15 K, ?9531.0±16.5 kJ mol^?1. The standard thermodynamic functions of the reactions of the synthesis of lutetium uranosilicate were calculated and discussed.