Metal-Ceramic Beads Based on Niobium and Alumina Produced by Alginate Gelation

Abstract

Full metal-ceramic composite beads containing different amounts of niobium and alumina, particularly 100 vol% alumina, 100 vol% niobium, and 95/5 vol% niobium/alumina, were produced by the alginate gelation process. The suspension for bead fabrication contained sodium alginate as gelling agent and was added dropwise into a calcium chloride solution to trigger the consolidation process. After debinding in air, sintering of the composite beads was performed under inert atmosphere. Samples in green and sintered state were analyzed by digital light microscopy and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Investigations by mercury intrusion porosimetry revealed that pure alumina beads featured smaller pores compared to composite beads, although the open porosities were comparable. The fracture strength was evaluated on single beads. Contrary to the pure alumina, the composite beads showed a clear plastic deformation. Pure niobium beads showed a ductile behavior with very large deformations. XRD analyses revealed the presence of calcium hexaluminate and beta-alumina as minor phases in the alumina beads, while the composite ones contained about 25 wt% of impurities. The impurities comprised NbO arising from the oxidation, and -Nb2C, from the reaction with the residual sodium alginate.

Synthesis of Niobium-Alumina Composite Aggregates and Their Application in Coarse-Grained Refractory Ceramic-Metal Castables

“…Mechanical and functional properties of coarse-grained refractory composite are mainly depending on the properties of the used aggregates. Despite the presented splinteryshaped particles produced by crushing, circular-shaped, porous aggregates based on alginate gelation [38] or almost dense particles based on pelletising can improve thermo-mechanical properties of coarse-grained refractory composites. It is a challenging task to find the best-fitting material solution for high temperature application simultaneously offering a good thermal-shock behaviour and sufficient mechanical properties [39].…”

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

“…In case of Nb‐based composite beads, very high displacements and a clear plastic behavior were observed. [ 19 ] This was not the case for Ta‐based beads, probably due to the insufficient sintering obtained during the thermal treatment. Figure presents the Weibull diagrams for all batches.…”

“…Green samples from the Ta_60 slurry showed a good spherical shape and were mostly uniform in size. As in our previous work, [ 19 ] the average diameter was simply controlled through nozzles of different gauge. It can be also noted that the shape and size of the beads became more regular with decreasing nozzle size (for the L samples, the slurry was poured directly from the syringe).…”

“…All batches showed an open porosity above 40% after sintering, which is high in comparison to common refractory ceramics but in line with our previous results, using Nb instead of Ta. [ 19 ] Larger beads have a smaller surface‐to‐volume ratio, which might result in less effective debinding and sintering steps. The shrinkage was smaller for the L batches, which were freeze‐dried according to the schedule shown in Figure 1.…”

“…Thirty beads per batch were loaded to fracture in a diametral compression mode between two parallel steel plates at a displacement rate of 0.05 mm s À1 . As explained already in our previous work, [19] the force-displacement data were converted into a fracture stress according to the method reported by Kschinka et al, who investigated the mechanical behavior of glass spheres [29] σ…”