Dependency of thermal conductivity on the temperature and composition of d-camphor in the neopentylglycol–d-camphor alloys

Abstract

Dependencies of thermal conductivity of solid phases for neopentylglycol (NPG)–[x] wt.% d-camphor (DC), x = 0, 3, 30, 46, 70 and 96 alloys on temperature and composition have been investigated by using radial heat flow apparatus. From graphs of the solid phases thermal conductivity variations versus temperature, the thermal conductivities of the solid phases at their melting temperature and temperature coefficients for NPG–[x] wt.% DC, x = 0, 3, 30, 46, 70 and 96 alloys were found to be 0.29, 0.28, 0.27, 0.26, 0.25 and 0.24 W/Km and 0.002996, 0.002694, 0.005690, 0.010626, 0.003520 and 0.001404 K⁻¹, respectively. The ratios of thermal conductivity of liquid phase to thermal conductivity of solid phase for NPG–[x] wt.% DC, x = 0, 3, 30, 46, 70 and 96 alloys at their melting temperature were found with a Bridgman type directional solidification apparatus to be 1.07, 0.93, 0.43, 0.50, 0.40 and 0.35, respectively.

Introduction

The thermal conductivity (K) plays a critical role in controlling the performance and stability of materials and it is one of the main fundamental properties of materials such as density, melting point, entropy, resistance, and crystal structure parameters. Thermal properties of materials are crucial in many industrial applications. The investigation of thermal conductivity is a valuable tool for the study of transport mechanisms of alloys. Thermal conductivity is a very important physical quantity that characterize the heat performance of heat storage material. Thermal conductivity is not only important physical quantity characterizing thermophysical property of heat storage material but also essential engineering parameters determining the design of energy storage system. Therefore, it is quite necessary to strengthen the study on thermophysical property and find the change rule especially for binary systems of polyalcohol, which are more widely accepted for solar energy storage owing to more phase change temperature choice.

The thermal conductivity of pure materials changes with the temperature but in alloys, the thermal conductivity also changes with the composition. In the experimental determination of the thermal conductivity of solids, a number of different methods of measurements are required for different ranges of temperature and for various classes of materials having different ranges of thermal conductivity values. A particular method may thus be preferable over the others for a given material and temperature range.

The methods for the measurement of thermal conductivity fall in two categories: the stationary and dynamic states. In the steady state methods of measurement, the specimen subjected to a temperature profile that is time invariant, and the thermal conductivity is determined directly by measuring the rate of heat flow per unit area and temperature gradient after equilibrium has been reached. In the none steady state methods, the temperature distribution in the specimen varies with time, and the measurement of the rate of temperature change, which normally determines the thermal diffusivity, replaces the measurement of the rate of heat flow. The thermal conductivity is then calculated from the thermal diffusivity with a further knowledge of the density and specific heat of the material.

Many attempts have been made to determine the thermal conductivity values of solid and liquid phases in various materials by using different methods. One of the common techniques for measuring the thermal conductivity of the solids is the radial heat flow method. There are several different types of apparatus employing radial heat flow [1]. Radial heat flow methods are steady state methods and the classification is mainly based upon specimen geometry; cylindrical or spherical. For example the cylindrical heat flow method uses a specimen in the form of a right circular cylinder with a coaxial central hole which contains either a heater or a sink depending on whether the described heat flow direction is to be radially outward or inward. The temperatures within the specimen are measured by thermocouples. This method was widely used for measuring the thermal conductivity of solids for pure and binary alloys [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

Energy storage of materials has become important in the fields of new energy application and energy-saving technology. NPG (neopentylglycol) as well as some of its derivatives will experience crystal structure phase transition at certain temperature. They will change from an ordered crystal phase to another disordered crystal phase. The latter has higher symmetry. Because of the higher enthalpy and smaller volume change experienced in the course of phase change, NPG and some of its derivatives have been considered as better heat storage materials than paraffin et al., which can storage energy when changes from liquid phase to solid phase. NPG is being considered as potantiel candidates for the thermal storoge of energy. Owing to the considerable enthalpy of a solid–solid transition in the temperature range from 20 to 200 °C, these substances are attractive to both chemists and engineers. Murrill and Breed [18] reported on the transition parameters in the compounds CR¹R²R³R⁴, where R^s were methyl, methylol, amino and carboxy, by differential scanning calorimetry. Zhang and Yang [19], [20], [21], [22] measured the heat capacities and transition parameters for a series of polyalcohols having solid–solid transition, by an adiabatic calorimeter. Although the neopentylglycol (NPG) is being considered as potential candidate for the thermal storage of energy, the values of thermal conductivities of NPG and DC alloys have not been reported yet.

Therefore, the propose of the present work is to determine experimentally the thermal conductivity of solid and liquid phases for NPG–[x] wt.% DC, x = 0, 3, 30, 46, 70 and 96 alloys obtained with the radial heat flow apparatus and the Bridgman type directional growth apparatus at their melting temperatures. Firstly, the variations of thermal conductivity of solid phases with temperature for different compositions of DC in the NPG–DC alloys have been measured and the values of thermal conductivity at their melting temperature have been found from graphs of thermal conductivity variations versus temperature. Secondly, the ratios of thermal conductivity of liquid phases to thermal conductivity of solid phases for the same alloys have been measured. Finally, the thermal conductivity of liquid phases at their melting temperature have been determined by using the measured values of thermal conductivity ratios and thermal conductivity of solid phases at their melting temperatures for the same alloys.