Search Results

You are looking at 81 - 90 of 632 items for :

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

It is shown that the heat of transition of the phase change II → I at 129° on heating KNO3 is dependent on the thermal history of the sample, since it involves two steps, viz., II→ III and III→ I at 2° interval. During cooling, the latter step is fast and truly reversible, though with a temperature hysteresis. The former step is sluggish and is dependent both on temperature and time. Our results indicate that KNO3 can be used for calibration purpose only if the material has not been heated beyond 128° in the immediately preceding three hours.

Restricted access

Abstract  

Al2O3-Cr2O3 solid solutions with 0, 4, 7, 10 and 20 mol% of corundum were synthesized using a high-pressure/high-temperature apparatus and characterized by X-ray powder diffraction. Calorimetric measurements were carried out using DSC-111 (Setaram). Heat capacity was measured by the enthalpy method in a temperature range of 260–340 K, near magnetic phase transition in pure Cr2O3 (305 K). Magnetic contribution into the heat capacity was derived and found to change irregularly with the composition. Heat capacity of solid solutions remains constant in a relatively wide range of composition, while the C p values of the end members differ significantly. This phenomenon is very important for the modeling of the thermodynamic functions of intermediate solid solutions.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: E. Grell, E. Lewitzki, R. Schneider, G. Ilgenfritz, I. Grillo, and M. von Raumer

Abstract  

Differential scanning calorimetry (DSC) studies of micellar, 60 mM solutions of the octaethyleneglycol alkylethers C14E8 and C16E8 provide evidence for a narrow endothermic transition at 41 and 32C,respectively, characterized by an enthalpy change of 2 kJ mol−1 for both detergents. The observed thermal transition is indicative of a concerted transition of the surfactant molecules, as illustrated on the basis of a simple molecular model. The effect of co-solvents such as different alcohols on the thermal transition is investigated. Glycerol markedly lowers the transition temperature whereas the transition is absent in the presence of at least 10% ethanol. The calorimetric transition correlates with the temperature dependent increase of viscosity and static light scattering as well as with changes observed by small-angle neutron scattering (SANS). The SANS results provide clear evidence for a distinct structural change occurring at the transition temperature, which is interpreted as a sphere-to-rod transition of the detergent micelles. Moreover, the rod length increases with increasing temperature. We suggest that the process causing the thermal transition acts as the prerequisite of the growth process.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: G. Wolf, J. Lerchner, H. Schmidt, H. Gamsjäger, E. Königsberger, and P. Schmidt
Restricted access

The height of DSC phase transition peaks

II. Some applications to liquid crystals

Journal of Thermal Analysis and Calorimetry
Authors: P. Navard and J. M. Haudin

Liquid crystal → liquid crystal and liquid crystal → isotropic liquid transitions of several materials are studied using a numberN, whereN is defined as the ratioh′/h, h andh′ being the heights of the transition peaks at heating rates p and2† p respectively.N is found close to two, which shows that a heat capacity increase occurs near the transition temperature, in agreement with the current theories describing these transitions.

Restricted access

Abstract  

La2Mo2O9 (LMO) was synthesized at lower temperature 973 K (LT-phase) by ceramic route. Differential thermal analysis (DTA) scan of LT-phase of LMO showed α→β transition at 843 K during heating and β→α conversion via a metastable γ-phase during cooling. This was also confirmed by thermo-dilatometry and impedance spectroscopy. La2Mo1.95V0.05O9-δ (LMVO), La1.96Sr0.04Mo2O9-δ (LSMO) and La1.96Sr0.04Mo1.95V0.05O9-δ (LSMVO) were prepared in a similar way. These compounds exhibited α→β transition on heating with shift in transition temperature, but the existence of γ-phase during cooling disappeared. Substitution increased the ionic conductivity of α-phase and reduced that of β-phase.

Restricted access

Abstract  

Heat capacities, electrical conductivities and phase transition temperature of hafnium hydrides, HfHx (0.99≤x≤1.83), were studied using a direct heating pulse calorimeter and a differential scanning calorimeter from room temperature to above 500 K. The heat capacity of HfH1.83 was larger than that of pure hafnium and showed no anomaly of heat capacity. In contrast, there were λ-type peaks for the heat capacity and DSC curves for HfHx (1.1≤x≤1.6) near 385 and 356 K. The anomalies of heat capacity and electrical conductivity of HfHx (1.1≤x≤1.6) were considered the result of phase transition and order-disorder phase transition for hydrogen in the hafnium hydride lattice for HfHx (1.1≤x≤1.3).

Restricted access

Abstract  

Phase transitions of hexatriacontane (C36) and octacosane (C28), both as the solution grown single crystal (SGC) and polycrystalline aggregates (MCC) prepared by cooling at 1 K min–1 from the isotropic liquid state, were measured by the simultaneous DSC-FTIR method. MCC of C36 showed the freezing of the high temperature stable phase, which had a slight lower order of the lattice vibration mode comparing with the room temperature stable phase. MCC of C28 demonstrated thermo-reversible phase transition, however, had a binomial distribution of crystal stability of the monoclinic phase.

Restricted access

Abstract  

The modeling of magnetocaloric effect (MCE) based on quasiparticle formalism was used to show a possibility to increase our knowledge of diffusion and phase transitions by a transdisciplinary scientific approach. Generalized understanding of diffusion and of some phase transitions was suggested. New relations between MCE and elastic parameters of materials, obtained from modeling procedure, were presented. The paper contains also the short discussion of a necessity to use hypercomplex mathematics in modeling of magnetic processes and specifically in a modeling of MCE. Some remarks concerning nanomaterials are added.

Restricted access

Abstract  

The phase transition of a fluid - in particular water - confined in the pores of silicon during drying is studied. The influence of this process on surface size and porosity is discussed. Methods of air drying, supercritical drying and freeze drying are considered.

Restricted access