High‐Temperature Ternary Oxide Phases in Tantalum/Niobium–Alumina Composite Materials

Abstract

Refractory metal (molybdenum, niobium, and tantalum) matrix composite materials in combination with refractory ceramics (alumina, zirconia, and mullite) are of particular interest for high-temperature applications, e.g., in refractory linings in the casting industry. [1] However, besides the high-temperature applicability, other aspects need consideration as well for the choice of material combination, such as: 1) the reaction between the metal and ceramic particles, 2) the chemical interaction with the environment, and 3) the thermal mismatch between the ceramic and the metallic phases. [2] The majority of the past publications dealt with so-called fine-grained refractory composites. [3][4][5][6][7] However, their disadvantages are high shrinkage on sintering, thus resulting in limited thermal shock ability. The refractory metals Nb and Ta are promising candidates for application with Al 2 O 3 due to their similar thermal expansion behavior. Wang et al. [8] and White et al. [9] showed for temperatures between 1000 and 2000 K a similarly linear increase of the linear thermal expansion coefficient for the three elements, with 8.8 to 11.2 Â 10 À6 K À1 for α-Al 2 O 3 , [8] 8.3 to 10.4 Â 10 À6 K À1 for Nb, and 7.1 to 8.4 Â 10 À6 K À1 for Ta. [9] Recently, Zienert et al. [10] demonstrated the fabrication of coarse-grained refractory composites based on alumina with niobium or tantalum, while Weidner et al. [11] reported first mechanical properties at high temperatures (1300-1500 °C) under compressive load. In these coarse-grained refractory composites, alumina with different particle classifications from 0-20 μm up to 2-5 mm (with different volume

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

“…Moreover, in Nb, both along grain boundaries and subgrain boundaries, the formation of small precipitates was observed (see Figure 6d). In addition, the fracture of niobium grains occurred starting from regions where NbO (see the study by Eusterholz et al) [ 35 ] was formed during the sintering process.…”

“…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 ]…”

Full and Hollow Metal–Ceramic Beads Based on Tantalum and Alumina Produced by Alginate Gelation

“…Sodium alginate was also responsible for some residual carbon that reacted with Ta generating the tantalum carbide phase. Finally, the aluminum tantalate was formed as a metastable phase from the reaction between the main raw materials, as discussed by Eusterholz et al [ 35 ] Moreover, these phases might have also originated from reactions with CO and gases from the atmosphere. Among all the different newly formed phases, has likely the lowest temperature stability: depending on its amount (which could not be determined in this work), the refractoriness under load of the material may be negatively affected.…”

Low Shrinkage, Coarse‐Grained Tantalum–Alumina Refractory Composites via Cold Isostatic Pressing

“…Oxygen is then dissolved in the tantalum lattice and forms also the ternary oxide phase where the tantalum–oxygen solid solution and corundum are in contact. [ 29 ] The free carbon leads to the formation of tantalum carbide. It can be expected that the amount of dissociated gas species correlates with the open porosity of the composite material.…”

“…In contrast to in the Al–Nb–O system, the formation of is preferred in the Ta(O)–alumina system as recently shown. [ 29 ] During the second heat treatment, the amount of the tantalum carbide increased, which shows an exchange with the furnace atmosphere. It seems that in the case of the tantalum–alumina composites, the corresponding oxygen lead to the formation of the phase and did not further dissolve into the tantalum lattice.…”