Search Results

You are looking at 61 - 70 of 632 items for :

  • "phase transition" x
  • Chemistry and Chemical Engineering x
  • Refine by Access: All Content x
Clear All

Abstract  

This work reports a non-isothermal kinetics of the melting and the nematic to isotropic (N–I) phase transitions of the pentylcyanobiphenyl (5CB) liquid crystal compared with octylcyanobiphenyl (8CB) liquid crystal using calorimetric technique. Temperature scans and heating rate scans were performed for 5CB and 8CB from 280 to 333 K at various rates using differential scanning calorimetry from 0.5 to 20 K min−1. Double activation was observed for 5CB for two heating rate regimes whereas 8CB indicated single activation only. The 5CB has smaller enthalpy and entropy of the transitions and needs larger activation than 8CB. This kinetic change can be explained in terms of the length scale and mobility of the liquid crystal molecules.

Restricted access

The phase transition in KHSO4 crystals of varying deutrium content and different Cu2+ concentration is studied. The specific heat at constant pressure,C p, and DTA measurements are investigated. The measurements are carried out in the vicinity of phase transitionT c=468 K. Multiple peaks are observed in the temperature dependence ofC p and a noticeable change due to Cu2+ dopant. This behaviour can be attributed to the fact of order disappearance in partial melting of proton in sublattice.

Restricted access

Summary  

Temperature induced magnetic phase transition in the interstitial compound LaFe11.6Si1.4C0.2 was studied by zero-field Mössbauer spectroscopy. A paramagnetic doublet and a magnetically split sextet co-existed in the zero-field spectra in the temperature region of 213-218 K, and a thermal hysteresis was observed in the heating and cooling cycles. The spectra indicate that the magnetic phase transition induced by temperature is of first order for this sample.

Restricted access

Abstract  

New cubic leucite-type compounds, CsMSi2O6 (M=B0.2Al0.8,Al0.2Fe0.8), Cs2MSi5O12 (M=Cd, Mg, Ni, Zn) have been synthesized by the two-stage heat treatment of the solid-state reaction. The thermal expansion properties of the synthesized leucite-type compounds have been studied with HTXRD and LTXRD in the temperature range of 123 to 1273 K. The thermal expansion rate of CsB0.2Al0.8Si2O6 was found to be considerably smaller than that of CsAlSi2O6, while the thermal expansion property of Cs2MSi5O12 (M=Mg, Zn, Cd) was found to have a linear relationship in the temperature range of 298 to 1273 K. By using Rietveld analysis it was found that the thermal expansion rate decreased with increasing the Si—O—M(Si) angle for cubic leucite-type compounds at 298 K, and that the phase transitions of CsAlSi2O6 and Cs0.9Al0.9Si2.1O6 were due to the relationship between the bond angle of Si—O—M(Si) of the three-dimensional framework structure and the space ratio in the unit cell at 298 K.

Restricted access

Heat capacities of the thiourea clathrate compound of 1,1,2,2-tetrachloroethane, {(NH2)2CS}3(CHCl2)2, were measured at temperatures between 13 and 330 K. Two phase transitions were found. The enthalpy and entropy changes of the transition are 5940 J·mol−1 and 28.1 JK−1· mol−1 for the one occurring at 224 K and 2756 J·mol−1 and 11.3 JK−1·mol−1 for the other at 248 K. It is concluded from the transition entropy values that the guest molecules are orientionally disordered nearly to the same extent as in the neat liquid.

Restricted access

Abstract  

The phase transitions of Ba2-xSrxIn2O5 were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba2-xSrxIn2O5 with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba2-xSrxIn2O5 was established and existence of tricritical point at ~1100C with x=0.4~0.5 was suggested.

Restricted access

Abstract  

Magnetic and thermodynamic properties of the LiCoO2 positive-electrode material used in lithium-ion battery were first examined. Partially deintercalated LiCoO2 that is Li0.75CoO2, showed definite anomaly in the magnetic susceptibility at T=ca. 175 K probably related to magnetic phase transition which was supported by observation of a weak anomaly in heat capacity. On the other hand, LiCoO2 did not show such magnetic phase transition as expected, whereas Li0.5CoO2 a weak one in the similar temperature range. These behaviors are discussed in association with the mixing of Co3+ and Co4+ electronic structures.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: Christoph Berthold, Volker Presser, Nadja Huber, and Klaus G. Nickel

experiment, the sample was 4 mm in diameter, and it was possible to reduce the measurement time for each diffraction pattern to 3 s covering a theta range of 30°2θ. 17 XRD patterns were recorded during the 96 s of rapid sample heating. The phase transition

Restricted access

Abstract  

The complex (C11H18NO)2CuCl4(s) was synthesized. Chemical analysis, elemental analysis, and X-ray crystallography were used to characterize the structure and composition of the complex. Low-temperature heat-capacities of the compound were measured by an adiabatic calorimeter in the temperature range from 77 to 400 K. A phase transition of the compound took place in the region of 297–368 K. Experimental molar heat-capacities were fitted to two polynomial equations of heat-capacities as a function of the reduced temperature by least square method. The peak temperature, molar enthalpy, and entropy of phase transition of the compound were calculated to be T trs = 354.214 ± 0.298 K, Δtrs H m = 76.327 ± 0.328 kJ mol−1, and Δtrs S m = 51.340 ± 0.164 J K−1 mol−1.

Restricted access

phase transition reversed when the process was performed at 500 °C or higher temperatures. Of course, a part of the ceramic reacted to produce lithium carbonate. These findings are all in good agreement with the literature, where it has been shown that α

Restricted access