Search Results

You are looking at 101 - 110 of 668 items for :

  • "phase transition" x
  • Refine by Access: All Content x
Clear All

Abstract  

[NiL3]X2 (where L=N-phenylethane-1,2-diamine and X=I and ClO4 ), [NiL2X2] (X is Cl, Br, NCS, 0.5SO4 2− or 0.5SeO4 2−) and [NiL2(H2O)2](NO3)2 have been synthesized from solution and their thermal study has been carried out in the solid phase. [NiL2Cl2] upon heating undergoes irreversible endothermic phase transition (142–152C, ΔH=0.35 kJ mol−1) without showing any visual colour change. This phase transition is assumed to be due to conformation changes of the diamine chelate rings. NiLCl2 and NiL2.5I2 have been prepared pyrolytically from [NiL2Cl2] and [NiL3]I2 respectively in the solid state. [NiL2(H2O)2](NO3)2 upon heating undergoes deaquation-anation reaction without showing any visual colour change. [NiL2X2] (X is Cl, Br, NCS), [NiL2(H2O)2](NO3)2 and [NiL2(NO3)2] possess trans-octahedral configuration, whereas, [NiL2X2] (X is 0.5SO4 2− or 0.5SeO4 2−) are having cis-octahedral configuration. Amongst the complexes, only NiLCl2 shows unusually high (5.1 BM at 27C) magnetic susceptibility value.

Restricted access

Thermodynamic considerations of the grain size dependency of material properties

A new approach to explain the variation of the dielectric permittivity of BaTiO3 with grain size

Journal of Thermal Analysis and Calorimetry
Authors: P. Perriat, J. Niepce, and G. Caboche

Abstract  

Phase transitions which depend on grain size induce very interesting properties in materials such as zirconia or barium titanate. A new and rigorous thermodynamic treatment of this type of phase transition is proposed with consideration of the surface phenomena. An interpretation is given of the observed differences when the material—particularly BaTiO3—under consideration is a fine grain powder or is a fine grain ceramic.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: K. Gavrichev, A. Bolshakov, D. Kondakov, A. Khoroshilov, and S. Denisov

Abstract  

Differential scanning calorimetry, differential thermogravimetry, X-ray analysis and electronic microscopic studies of thermal transformations of PbO2 were carried out. Formation of fine dispersed (less than 100 nm) particles of α-PbO was observed at PbO2 thermal decomposition at heating to 580°C. Reverse reaction of Pb3O4 formation from PbO was found at cooling and annealing at 400°C in air. At heating of α-PbO to 650°C the particle growth to 1 μm with formation of β-PbO took place. Thermal decomposition with formation of β-PbO particles with size from 0.3 to 1 μm at PbO2 heating to 650°C was observed. Transition from PbO to Pb3O4 at cooling of sample heated to 650°C was not detected. Interpretation of observed phenomena from the point of view of particle size influences on the shift of α-PbO↔β-PbO phase transition temperature and on the chemical activity of phases are presented.

Restricted access

The influence of stoichiometric ratio on the phase transition of sodium potassium sulphate (NaxK1−x)2SO4 crystals is investigated in the temperature range 300–500 K. The stoichiometric ratiox is chosen to bex=0.2, 0.3, 0.4, 0.8 and 0.9. The study has been carried out by using differential scanning calorimetry, DSC technique. The value of the specific heat,C p, at the transition temperature,T c, increases asx is increased up tox=0.4 and then decreases for higher values ofx. The change in the specific heat, ΔC p, is following a relation of the form ΔC p ΔC p α(T−T c/T c)−α withα = 0.12 which is in a fair agreement with the 3d Ising model. Insertion of Cu2+ ions into the lattice of (NaxK1−x)2SO4 crystals leads to a multiple peak in the temperature dependence ofC p. The results are discussed from thermodynamical point of view.

Restricted access

Modelling of a high pressure calorimeter

Application to the measurement of the latent heat of a model food (tylose)

Journal of Thermal Analysis and Calorimetry
Authors: A. Ousegui, S. Zhu, H. Ramaswamy, and A. Le Bail

Abstract  

The evaluation of the latent heat and enthalpy of fusion of food systems in the case of high pressure–low temperature processing is important for modelling purposes as well as for technical applications. A high pressure calorimeter has been designed for this purpose. The high pressure calorimeter was used to evaluate the latent heat during a pressure scan at constant temperature. It permits to measure the heat of phase transitions and to obtain the relationship between the initial freezing temperature T ifp and the average pressure while the phase transition is going on. This work presents a modelling of results obtained from an experimental approach using a high pressure calorimeter and from a mathematical model developed from existing data on the phase change of pure water. The modelling work consisted in evaluating the latent heat measured in previous tests from computations taking into account the dependence of the latent heat of fusion of water on pressure. Models predicting the amount of frozen water in a food matrix under atmospheric conditions were used to determine the initial amount of frozen water in the sample. Then a stepwise procedure was operated in a program to reproduce the pressure rise occurring during a high pressure calorimeter test. The amount of melted ice at each pressure step was calculated using conventional ice fraction models, which were adapted to pressure dependence of the initial freezing temperature and the dependence of the latent heat pressure. The comparison was satisfactory, especially at low temperatures.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: T. Shibasaki, T. Furuya, J. Kuwahara, Y. Takahashi, H. Takahashi, and T. Hashimoto

Summary Structural phase transitions upon the application of high pressure in LaGaO3 and LaCrO3, which were prospected from diffraction and thermal analyses of phase transition under ambient pressure, were discovered by using high-pressure X-ray diffraction. It was revealed that the crystal structures of LaCrO3 and LaGaO3 changed completely from that of orthorhombic distorted perovskite to that of a rhombohedral distorted one upon the application of pressure higher than 5.4 and 3.0 GPa, respectively, at room temperature. The variation of lattice constants with pressure was investigated for the high-pressure rhombohedral phases of LaCrO3 and LaGaO3 and isothermal compressibility was estimated. The variation of lattice constants with pressure at room temperature in the high-pressure rhombohedral phase was compared with that with temperature at ambient pressure in high-temperature rhombohedral phase. It was found that the application of pressure decreased the crystal symmetry, which was opposite to the result in the case of increasing the temperature.

Restricted access

Abstract  

Thermal analysis in the form of electrical resistivity measurement is reviewed. It is useful for studying phase transitions and electrical conduction mechanisms. The resistivity can be the volume resistivity or the contact resistivity, as illustrated for the case of continuous carbon fiber polymer-matrix composites.

Restricted access

Summary {\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs20 Principle and technical development of low temperature calorimetry are described. Typical experimental results obtained by our group at Osaka University over the four decades are given. These include phase transitions in equilibrium crystals and glass transitions in non-equilibrium frozen-in disordered solids including crystals. It can be concluded that the glass transitions observed exclusively in liquids so far are just one example of transitions that must be of wide occurrence in solids arising from freezing of relevant degrees of freedom. Interplay between the phase and glass transitions in crystals is discussed in relation to useful dopant that may accelerate some molecular motions that had failed to maintain equilibrium at low temperatures. \par }

Restricted access

Abstract  

Present work is a part of thermodynamic research program on the MX−LnX3 system (M=alkali metal,X=Cl, Br andLn=lanthanide). Molar enthalpies of mixing in the LiBr−NdBr3, NaBr−NdBr3 and KBr−NdBr3 liquid binary systems have been determined at temperature 1063 K by direct calorimetry in the whole range of composition. Investigated systems are generally characterized by negative enthalpies of mixing with minimum atX NdBr3≈0.3–0.4. These enthalpies decrease with decrease of ionic radii of alkali metals. Molar enthalpies of solid-solid and solid-liquid phase transitions of K3NdBr6 and Cs3NdBr6 have been also determined by differential scanning calorimetry (DSC). K3NdBr6 is formed at 689 K from KBr and K2NdBr5 with enthalpy of 44.0 kJ·mol−1 whereas Cs3NdBr6 is stable at ambient temperature and undergoes phase transition in the solid state at 731 K with enthalpy of 8.8 kJ·mol−1. Enthalpies of melting have been also determined.

Restricted access

A DSC investigation of severaln-alkylammonium salts is reported. The results of this investigation can be used to help decide which of several crystalline forms should be studied by X-ray diffraction for the best correlation between the liquid and solid states.

Restricted access