Thermophysical properties of composite materials in the Si3N4 - BN system

Abstract

The results of a study of the thermophysical properties (thermal diffusivity, heat capacity, heat conductivity, and coefficient of linear thermal expansion) of Si 3 N 4 -BN hot-pressed composite (with BN concentration varying from 10 to 60 wt.%) in the temperature range of 20 -900°C are reported.Ceramic materials intended for service under heavy-duty conditions -high temperatures (up to 1300 -1500°C) and high mechanical loads -must necessarily be tested for thermophysical properties. Knowledge of heat conductivity, heat capacity, thermal diffusivity, and coefficient of linear thermal expansion (CLTE) is of utmost importance for conducting analysis of the structural design and chemical composition of the material under specific conditions of thermal and mechanical loading [1].Our goal in this work was to study the thermophysical properties of Si 3 N 4 -BN composite ceramic of variable composition which is prepared by hot pressing technology and is intended for operation under heavy-duty conditions. These composites combine advantageously superior properties of ceramic materials such as high strength, thermal stability, and good fabricability [2, 3]. The ceramic matrix was prepared using ultradisperse powders of composition Si 3 N 4 -Y 2 O 3 (MgO); the filler was hexagonal BN. The mass fraction of BN varied within 10 -60% [4].Si 3 N 4 -BN composite specimens containing 10, 30, 40, and 60% BN were tested to determine thermal diffusivity, heat capacity, and heat conductivity. Different methods were used to measure heat capacity and thermal diffusivity for ceramic materials: the former was measured in air within the temperature range of 20 -900°C, and the latter was measured using a rapid method. In this way, the accuracy of determination could be improved considering that materials of this class are insulators at room temperature, while at temperatures above 500°C they develop semiconducting properties. Since the heat capacity and thermal diffusivity are known from independent measurements, one can construct a temperature curve for heat conductivity:where l is heat conductivity, W/(m × K); a is thermal diffusivity, m 2 /sec; C p is heat capacity, kJ/(kg × K); r is density, kg/m 3 . We consider materials whose density does not change with temperature and which undergo no phase transition on heating. Otherwise, the use of formula (1) may lead to erroneous results. The method by which the thermal diffusivity

Influence of YAG-Based Composite Sintering Aids on Gas Pressure Sintered BN_w/Si₃N₄ Composites

“…However, the addition of h-BN content might have negative effects on the bending strength and fracture toughness due to uneven pores distribution in the composites [4,5]. The appropriate adding of sintering additives such as Y 2 O 3 [6], Al 2 O 3 [7] and some other rare-earth oxides [8] could accelerate the sintering process of BN/Si 3 N 4 composites. For example, Li Yongfeng et al [9] studied the effects of the Yb 2 O 3 content from 2wt.% to 15wt.% on the mechanical properties of Si 3 N 4 /BN composites and found that 4wt.% Yb 2 O 3 addition was most beneficial to the flexural strength which reached the value of 264.3 MPa.…”