Modeling of Effective Thermal Conductivity of Dunite Rocks as a Function of Temperature

The thermal conductivity, thermal diffusivity, and heat capacity per unit volume of dunite rocks taken from Chillas near Gilgit, Pakistan, have been measured simultaneously using the transient plane source technique. The temperature dependence of the thermal transport properties was studied in the temperature range from 303 K to 483 K. Different relations for the estimation of the thermal conductivity are applied. A proposed model for the prediction of the thermal conductivity as a function of temperature is also given. It is observed that the values of the effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 9%.     

Locating the normal to relaxor phase boundary in Ba(Ti1−xHfx)O3 ceramics

In this article, we report our studies on the relaxor behavior of Ba(Ti_1−xHf_x)O₃ ceramics, made with close compositions between 0.20 ≤ x ≤ 0.30, to locate the hafnium concentration boundary for the normal to relaxor crossover. X-ray diffraction followed by Rietveld refinement shows the occurrence of single-phase cubic structure for the synthesized Ba(Ti_1−xHf_x)O₃ ceramics. Temperature and frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric permittivity has been studied in the temperature range of 90-350 K at frequencies of 0.1, 1, 10, and 100 kHz. A diffuse phase transition accompanying frequency dispersion is observed in the permittivity versus temperature plots revealing the occurrence of relaxor ferroelectric behavior. The T_m verses Hf concentration plot shows a discontinuous jump and change in the slope at x = 0.23. Quantitative characterization based on phenomenological models has also been presented. The plausible mechanism of the relaxor behavior has been discussed. Substitution of Hf⁴⁺ for Ti⁴⁺ in BaTiO₃ reduces the long-range polar ordering yielding a diffuse ferroelectric phase transition.