Relative permittivities ranging from 12 to 100 were obtained from porous titania. Relative permittivities from 8 to 30 were obtained from porous calcium magnesium titanates. Porosity was introduced as microporosity by partial sintering, as 15 lm diameter spherical pores with porogens, and by fabrication of a microcellular structure with cylindrical pores. The relative permittivity as a function of porosity was compared with effective media models. Permittivity could be fitted to the Bruggeman 1/3 power law over a porosity range from 5% to 78%.

Spark plasma joining is used to join ZrB2–SiC composites with seamless microstructures at the joint that results in retention of high‐temperature mechanical and oxidation properties after joining. Our approach uses a spark plasma sintering furnace and Zr–B powder filler layers to join the parts together. The joining processing parameters used to optimize the joint microstructure were filler composition, target temperature, hold time, and volume of filler. A filler of 1 mm3 and spark plasma joining conditions at 1800°C for 300 s resulted in the formation of a joint region that was indistinguishable from the bulk substrates. Room and high‐temperature (1350°C) shear strengths of joined substrates measured equal to baseline substrates and oxidation behavior for joined and baseline substrates were equivalent after static air oxidation at 1700°C. X‐Ray diffraction measurements show the joint is composed of ZrB2 and ZrC. We found the joining mechanism to be solid‐state bonding of ZrB2 that formed from the Zr–B filler and reaction bonding by the formation of ZrC. Spark plasma joining rapidly joins ZrB2–SiC and probably other conductive ultra high‐temperature ceramic composites, and has the potential to impact the rapid assembly and joining of complex thermal protection material systems.

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