Preparation and thermal conductivity characterization of ZrB2 porous ceramics fabricated by spark plasma sintering

Abstract Zirconium diboride (ZrB2) porous ceramics were prepared by two‐step sintering (TSS) method using spark plasma sintering (SPS). The ZrB2 porous ceramics were first synthesized using zirconium oxide (ZrO2) and boron carbide (B4C) as precursors, and then sintered to ZrB2 porous ceramics. The porosity between 0.72 and 0.54 for ZrB2 ceramics could be controlled by changing the ratio of ZrB2 from ZrO2 and B4C reduction reaction at 1600 °C under vacuum. The porosity was further reduced by the second sintering step from 0.38 to 0.21. The shape and size of pores changed with the varying ZrB2 contents from the boron carbide reduction reaction. The effect of the synthesized ZrB2 phase on the microstructure was investigated by measuring the average diameter (max, min, mean) and area of the pores. For all samples, the average max diameter was about 3 μm, whereas the average areas increased from 12.7 to 15.7 μm2 and then decreased to 13.3 μm2 as the porosity increased from 0.21 to 0.38. The pore distribution on the ZrB2 matrix was homogenous. Thermal conductivities were calculated using measured thermal diffusivities, heat capacities, and densities of materials. As the temperature increased, the thermal conductivity decreased slightly between room temperature and 473 K and then increased from 473 K to 773 K. The thermal conductivity of the ZrB2 ceramic with 0.21 porosity decreased first sharply between room temperature and 373 K and then increased between 373 K and 673 K, and again decreased between 673 K and 773 K. The similar tendency was observed for the other three samples with porosity of 0.28, 0.29 and 0.38. The calculated thermal conductivity for the samples with porosity values of 0.21, 0.28, 0.29 and 0.38 at room temperature were 56.5 W·m− 1·K− 1, 48.0 W·m− 1·K− 1, 39.1 W·m− 1·K− 1, and 28.9 W·m− 1·K− 1, respectively, which were lower than the previously reported values. The thermal conductivity of all ZrB2 porous ceramics decreased with the increase in the volume fractions of pores.