| The zirconium diboride has been considered as promising candidates for structural applications due to their unique combinations of high melting point, high hardness, high thermal conductivity, tailored electronic properties, high corrosion and wear resistance. Furthermore, they show excellent chemical stability, good oxidation resistance, and low electrical resistivity. However, the poor thermal shock resistance limited it using. The laminated ZrB2-SiC ceramics were prepared to improve the physical properties as thermal protection system.ZrB2was synthesized from ZrO2and B4C by SPS at1300,1400,1500and1600℃for3min and10min under vacuum condition, respectively. The analyzed reaction temperature was consistent with the thermodynamic calculations. An oxygen content of1.04wt%was detected in the ZrB2porous ceramics sintered at1600℃for10min holding and the results revealed that reaction was completed in10min at1600℃. The B2O3evaporation as a function of temperature during heating was further discussed to explain the oxygen impurity and why the reaction is complete at1600℃.ZrB2porous ceramics were prepared by two-step sintering (TSS) method using Spark Plasma Sintering (SPS). The ZrB2porous ceramic were first synthesized using zirconium oxide (ZrO2) and boron carbide (B4C) as precursors, and then sintered to ZrB2porous ceramic. The porosity between0.72and0.54for ZrB2ceramics could be controlled by changing the ratio of ZrB2from ZrO2and B4C reduction reaction at1600℃under vacuum. The porosity was further reduced by the second sintering step from0.38to0.21. The shape and size of pores changed with the verifying of ZrB2content from boron carbide reduction reaction. The effect of synthesized ZrB2phase 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 about3μm, whereas the average areas increased from12.7to15.7p.m2and then decreased to13.3μm2as the porosity increased from0.21to0.38. The pore distribution on ZrB2matrix was homogenous. Thermal conductivities were calculated using measured thermal diffusivities, heat capacities, and densities of materials. As the temperature increased, the thermal conductivity for ZrB2ceramic with0.21porosity decreased first sharply between from room temperature and373K and then increased between373K and673K, again decreased between from673K and773K. The similar tendency was observed for the other three samples with porosity of0.28,0.29and0.38. The calculated thermal conductivity for samples with porosity values of0.21,0.28,0.29and0.38at room temperature were56.5W·m-1·K-1,48.0W·m-1.K-1,39.1W·m-1·K-1, and28.9W·m-1·K-1, respectively, which were lower than the reported values previously. The thermal conductivity of all ZrB2porous ceramics decreased with the increase in the volume fractions of pores. The△Tc is400℃～425℃for porous ceramic with porosity of0.185.ZrO2and B4C were added to ZrB2raw powders to prepare ZrB2porous ceramics by Reactive Spark Plasma Sintering (RSPS), and the gas escape (such as CO and B2O3) resulted in higher porosity. X-Ray Diffraction results indicated that the reduction reaction was complecated after the RSPS process. The porosity could be controlled by changing the ratio of synthesized ZrB2to raw ZrB2powders. The porosity of porous ceramics with20wt%and40wt%synthsized ZrB2are0.185and0.222, respectivly, and the dense ZrB2-SiC with a prosity of0.057was prepared for comparison. The thermal conductivity for ZrB2-SiC was reduced as the temperature increased throughout the measured temperature range. In contrast, the thermal conductivity values of both porous ceramics increased from room temperature to373K, and then reduced from373K to773K. The thermal conductivity values at room-temperature were83.4W·m-1·K-1for ZrB2-SiC,68.5W·m-1·K-1and55.7W·m-1K-1for ceramics with prosity of0.185and0.222, respectivly. The porosity exhibited great influence on the thermal conductivity by reducing the thermal diffusion. The thermal conductivity of ceramic with a porosity of0.222decreased-22%, compared with ceramic with a porosity of0.185. The△Tc is380℃～420℃for ZrB2-5wt%SiC with porosity of0.185.Laminated ZrB2-SiC ceramics with ZrB2porous interlayers were prepared using ZrB2-SiC flakes and ZrO2-B4C casting films by RSPS at1600℃. The thickness and relative density of porous were controlled by the pressure. The relative densities of porous layer varies from27%～83%. The energy to fracture the laminated ZrB2-SiC ceramic with interlayers of0.42%relative density is1.27J, which is～30%higher that dense ZrB2-SiC ceramic. The fracture toughness of laminated ZrB2-SiC ceramics with83%relative density porous layers is4.78MPa·m1/2, which is3%higher than ZrB2-SiC dense ceramic.
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