Field‐Assisted Sintering of Nb–Al₂O₃ Composite Materials and Investigation of Electrical Conductivity

Abstract

Field‐assisted sintering technique (FAST) is used for the preparation of Nb–Al2O3 composite materials. The electrical conductivity is investigated depending on the particle size of the used starting powders and under varying volume contents of the refractory metal in the starting powder mixture. The percolation threshold is investigated and found to be influenced not only by the metal fraction but also by the particle size of the alumina used for sample preparation. For the fine‐ and coarse‐grained alumina, a percolation threshold of 17.5 and 10 vol% Nb is estimated, respectively. Furthermore, the microstructure is investigated to gain a basic understanding of the dependency between microstructural features and the resulting material properties on the macroscopic scale. Also, the influence of the sintering process and the resulting microstructure–properties relationship is considered. It could be shown that the electrical properties are anisotropic because of anisotropy effects caused by the FAST process.

Coarse‐Grained Refractory Composite Castables Based on Alumina and Niobium

“…The mechanical strength of coarse‐grained material will be further improved by using more dense aggregates, which will be synthesized by cold isostatic pressing and sintering or using field‐assisted sintering technique that resulted in our material in porosity values of 25% and 5%, [ 19 ] respectively. However, in contrast, the porous material can be used to reduce internal stress due to its ability to show strain values up to 25%.…”

Nanoscale Oxide Formation at α‐Al2O3–Nb Interfaces

“…The development of composites based on refractory ceramics and refractory metals (RM), [ 1–5 ] thus, aims at taking advantage of the benefits of ceramics, but also at exploiting the ductility and electrical conductivity of the RM. [ 5–8 ] The electrical conductivity offers a path to strongly reduce the thermal gradients through resistive heating of the parts before service. Among ceramics and metals, the combination of body‐centered‐cubic Nb (Nb bcc ) and (hexagonal) α‐Al 2 O 3 is particularly promising due to their similar thermal expansion behavior ( α Nb = 8.0 to 10.3 × 10 −6 K −1 [ 9 ] and = 9.3 to 11.2 × 10 −6 K −1 [ 10 ] from 527 to 1827 °C) and, thus, low‐thermal misfit stresses during fabrication and service.…”

High‐Temperature Compressive Behavior of Refractory Alumina–Niobium Composite Material

“…[ 8,27,30,31 ] The high cost of niobium as a refractory metal is known; however, in composites, it provides sufficient electrical conductivity even with a small amount of Nb even above 20 vol%. [ 32 ] Therefore, the combination of the properties of Nb and alumina makes coarse‐grained Nb–Al 2 O 3 composites possible candidates for the applications of sandwich/core–shell structures, which are partially heated by resistance heating. These sandwich/core–shell structures consist of a composite core material and outside with a refractory ceramic shell.…”

“…Several studies were conducted to investigate the synthesis of Nb–Al 2 O 3 composites and the shrinkage behavior during sintering, [ 24,32 ] metal–ceramic interfaces, [ 26,33–35 ] as well as the mechanical behavior at room [ 15,27 ] and high temperatures, [ 15,24,36 ] the wear behavior, [ 30,37 ] mechanical alloying, [ 38 ] the fracture behavior, [ 15,27,37,39 ] the high‐temperature oxidation, [ 40 ] creep resistance, and thermal shock resistance. [ 8,15,24,36,39,41 ]…”