The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B4C led to a decrease in the ZrB2 grain size from 22 µm for nominally pure ZrB2 to 5.4 µm for ZrB2 containing 15 vol% B4C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB2 to 80 W/m·K for ZrB2 containing 15 vol% B4C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB2, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB2 containing 15 vol% B4C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB2 and less than 6 W/m·K when 15 vol% B4C was added.

Zirconium diboride ceramics were densified by hot pressing and spark plasma sintering with heating rates varying from 5 to 300celcius/min. Slower heating rates produced larger grains due to the longer times spent at temperatures between 1500 and 1900celcius, which is the temperature range in which ZrB2 grains coarsen. Heating rates above 50celcius/min resulted in rapid densification, but this led to the retention of up to 3.3 vol.% of ZrO2 particles in the ceramics. After densification, changes to the microstructure were evaluated to interpret the effects of heating rate on thermal and mechanical properties. The flexure strength of ceramics processed by hot pressing up to 80celcius/min was proportional to the inverse square root of the maximum grain size based on the Griffith criteria. Conversely, densification by spark plasma sintering, which had heating rates of up to 300celcius/min, resulted in microcracks that decreased the elastic modulus from >500 GPa for pristine specimens to 20celcius/min also reduced the thermal conductivity due to the presence of retained ZrO2, but improved the strength by reducing the maximum grain size.

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB2. The resulting (Zr,Ti)B2 ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB2 resulted in grain size decreasing from 22 μm for nominally pure ZrB2 to 7 μm for ZrB2–50 vol.% TiB2. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB2 to 58 W/m⋅K for ZrB2–50 vol.% TiB2. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB2 at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB2. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB2 decreasing to 57 W/m⋅K when 50 vol.% TiB2 was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB2. Additions of ≥25 vol.% TiB2 reduced the phonon contribution to nearly zero for all temperatures.

The thermal properties of zirconium diboride (ZrB2) ceramics with carbon additions of up to 3 wt% were characterized up to 2000°C. Carbon contents were selected to produce ZrB2 that was nominally pure, contained dissolved carbon, or contained carbon inclusions. The microstructure and density changes that resulted from the carbon additions affected the thermal behavior of ZrB2 at room and elevated temperatures. Thermal diffusivity at 200°C increased from 0.150 cm2/s for nominally pure ZrB2 to 0.175 cm2/s for ZrB2 with 3 wt% carbon. The thermal diffusivity decreased with increasing temperature, reaching a value of 0.143 cm2/s at 2000°C for ZrB2 with 3 wt% carbon. In addition, thermal diffusivity changed irreversibly during the first thermal cycle after densification due to changes in the microstructure that started between 1550°C and 1650°C. Heating resulted in the formation of a new phase, growth of ZrB2 grains, changes in the morphology of carbon inclusions, and migration of W impurities from the ZrB2 matrix into the new phase. Heat capacity, unlike thermal diffusivity, did not change during thermal cycling. Thermal conductivity, which was calculated from thermal diffusivity, heat capacity, and density, was as high as 64.2 W·(m·K)−1 at 2000°C for ZrB2 with 3 wt% carbon. The phonon contribution to thermal conductivity decreased to nearly zero with the addition of 3 wt% carbon due to the presence of elongated carbon inclusions around ZrB2 grains.

Zirconium diboride (ZrB2) ceramics were densified by pressureless sintering (PS), hot pressing, or spark plasma sintering (SPS) of powders with a range of starting particle sizes and oxygen contents. Microstructural analysis of the ZrB2 ceramics revealed a wide range of final grain sizes. SPS resulted in an average grain size as small as 1.6 μm after densification at 1900°C, while the largest grains, 31 μm, were produced by PS at 2100°C. Oxygen impurities in boride ceramics caused grain coarsening in all densification techniques, but inhibited full densification only for PS. Carbon was added to react with and remove oxygen impurities, which promoted densification, reduced ZrB2 grain size, and led to increased room‐temperature flexure strengths. The highest strength was 527 MPa for SPS ZrB2, while the lowest strength was measured for pressurelessly sintered ZrB2, 300 MPa. Overall, SPS was the superior technique for providing the highest strength and greatest ability to remove oxygen.