Heat conduction in ionic polycrystalline ?nanocomposites?

Abstract

Alumina, which is a typical ionic crystal, has thermal conduction primarily controlled by phonons. With the introduction of 10 vol% of Cr3C 2 phase, the thermal conductivity of the composite decreased with decreasing size of the Cr3C2 particles and reached a value well below the theoretical prediction at 0.5 #m Cr3C 2. No apparent impurity phase and microcracking were observed at the interface of the composite. Discovery of the entrapment of fine Cr3C2 grains by AlzO 3 grains was suggested to be a significant defect for phonon transportation, regardless of the minor electron contribution of the Cr3C2 phase. Taking advantage of this finding, a novel composite material with superior mechanical properties and lower thermal conductivity for preventing energy dissipation in energy-conversion systems, particularly for engine components, is proposed. Thermal conduction is a basic thermo-physical property primarily tracable to the contribution of phonons and electrons in most solid materials. For dielectric materials such as A1203 phonon is the only contribution to thermal conduction. In view of the thermo-physical characteristics of a composite material, Hasselman and Johnson [11 proposed a relation considering an interfacial resistance factor in relation to Rayleigh-Maxwell theory, which led to a somewhat more realistic approach in predicting the thermal conduction behaviour of brittle composite materials. However, microcracks, mostly resulting from the mismatch in coefficients of thermal expansion (CTE) between the components, or thermal expansion anisotropy of individual components, usually makes these properties unpredictable. Recently, Fu et al. [2] developed a composite material with superior mechanical properties and good oxidation resistance [3], containing A1203 and volume fractions of Cr3C 2. With the incorporation of 10 vol % fine Cr3C2 particle, they found that some of the Cr3C2 particles were entrapped in the A1203 grains, resembling the "nanocomposite" claimed by Niihara [4]. This letter describes the thermal conduction properties of the composite containing 10 vol % Cr3C 2 with various particle sizes. A powder mixture containing fine A1203 powder (purity > 99.5%, mean particle size 0.4 ktm, Alcoa, A16-SG) and 10 vol % Cr3C2 powder (purity > 99.0%, mean particle size 0.5/~m, 1.5 #m and 7.5 #m, H. C. Stark, grade A) was prepared by ball-milling for 24 h with deionized water as a medium solution. After drying, the mixed powder was sintered by means of hot-pressing at 1400 °C for 1 h in Ar atmosphere under an uniaxial pressure of 30 MPa. Thermal diffusivity and thermal conductiv-