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

Abstract

Ceramic-matrix and metal-matrix composites have been studied for many years. Traditionally, ceramic-matrix composites consist of ceramic (or carbon) fibers embedded in a ceramic matrix. The role of these fibers is to enhance the fracture toughness of the combined material system while still taking advantage of the inherent high strength and Young's modulus of the ceramic matrix. [1][2][3] On the other hand, metalmatrix composites are made by dispersing a reinforcing material into a metal matrix. [4][5][6][7] The reinforcement usually serves as a structural task, but can be also used to improve physical properties such as wear resistance and thermal conductivity. [8] In general, ceramic-metal composites combine high melting point, hardness and chemical stability of ceramics, and high toughness and ductility of metals. In case of high temperature and wear applications, the combination of ceramics with refractory metals (Nb, Mo, Ta, W, and Re) with melting point above 2000 °C is of high interest. [4][5][6][7][9][10][11][12][13] Such materials are especially attractive for the metallurgical industry, where special functional components are exposed to extreme conditions. Examples are electrodes with nonwetting behavior, [14] ladle sliding gate plates with enhanced thermal shock properties, casting nozzles, or integrated heating elements. Such components with high electrical conductivity can also be applied to replace carbon-bonded parts (e.g., graphite electrodes), in cases where the emission of carbon dioxide should be minimized.Most works report the use of fine-grained powders to produce dense sintered composites with high strength and toughness at the same time. [15] However, dense refractories usually show high shrinkage on sintering, with a pure brittle behavior accompanied by a high susceptibility to cracking and subsequent spalling. To improve the thermal shock resistance, a low Young's modulus and high porosity are beneficial. Recently, the concept of coarse-grained refractory composites based on tantalum-alumina and niobium-alumina was introduced. [16] By combining powder metallurgy with castable technology, low shrinkage values and good mechanical properties up to 1500 °C were obtained. The composites showed plastic deformation behavior between 1300 and 1500 °C. [17] Furthermore, the technology allows to produce large refractory components with low residual stresses. In a novel study, a two-step castable procedure was explored to produce refractory niobium-alumina composites. [18] The first step consisted in the synthesis of composite aggregates, while the second involved the production of coarse-grained refractory castables from the presynthesized composite aggregates. Such a fractal design is required in order to ensure a coherent composition at different aggregate size scales and hence to achieve good electrical

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

“…Tantalum carbide formation was indeed more pronounced in case of the tantalum‐alumina composite beads with porosity values up to as recently shown. [ 30 ] Besides, the decomposition of Ta–H‐based phases, the lattice parameters of tantalum did not substantially change during the heat treatments for synthesing the fine‐grained and the coarse‐grained 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.…”

Characterization of Sintered Niobium–Alumina Refractory Composite Granules Synthesized by Castable Technology

“…In previous works, we investigated the synthesis of niobiumalumina and tantalum-alumina composite aggregates with circular shape [12,13] and irregularly shaped ones based on a crushing process. [14] In the study by Zienert et al, [15] coarse-grained refractory castables were produced using crushed fine-grained castable-based powder and aggregates.…”