High‐temperature ternary oxide phases in Ta/Nb‐Alumina composite materials

Coarse-grained composites of refractory ceramics and refractory metals are a novel approach for materials at application temperatures up to 1500 °C. Al$_{2}$O$_{3}$ and the refractory metals Nb and Ta are suitable candidates for enhanced thermal shock capability, as they show similar thermal expansion. During fabrication, a key aspect to consider is the possible formation of additional phases upon interaction of the constituent phases as well as through reaction with the environment. X-Ray diffraction (XRD) and investigations of the microstructure with scanning electron microscopy methods unveil Al$_{2}$O$_{3}$–Nb composite to form NbO, whereas for Al$_{2}$O$_{3}$–Ta the ternary compound aluminum tantalate (AlTaO$_{4}$) is found. Thermodynamic calculations show that the changing oxygen solubility in Nb accounts for the formation of NbO, and explain the absence of a corresponding niobate (AlNbO4) phase. AlTaO$_{4}$ is identified as the disordered tetragonal high-temperature modification

Coarse‐Grained Refractory Composite Castables Based on Alumina and Niobium

Niobium-alumina composite aggregates with 60 vol% metal content and with particle sizes up to 3150 μm are produced using castable technology followed by sintering, and a crushing and sieving process. X-Ray diffraction (XRD) analysis reveals phase separation during crushing as the niobium:corundum volume ratios is between 37:57 and 64:31 among the 4 produced aggregate classes 0–45, 45–500, 500–1000, and 1000–3150 μm. The synthesized aggregates are used to produce coarse-grained refractory composites in a second casting and sintering step. The fine- and coarse-grained material shows porosities between 32% and 36% with a determined cold modulus of rupture of 20 and 12 MPa, and E-moduli of 37 and 46 GPa, respectively. The synthesized fine-grained composites reached true strain values between 0.08 at 1100 °C and 0.18 at 1500 °C and the coarse-grained ones values between 0.02 and 0.09. The electrical conductivity for the fine-grained and the coarse-grained material is 448±66 and 111±25  S cm$^{−1}$, respectively