Microscopic characterization of a TiB 2 -Carbon material composite: Raw materials and composite characterization
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IT is the aluminum industry that is most interested in titanium diboride (TiB2) and TiB2-carbon composites (TCC). The interesting properties of TiB2 were recognized as early as the 1950s.[1] It is hard [2,3] and a good electrical conductor,[4,5,6] it has low solubility in molten aluminum,[7,8] and it is readily wetted by molten aluminum.[9,10,11] Scientists and engineers have been searching for such a material to replace carbonaceous materials (anthracite and pitch binder) used in the traditional fabrication of cathode blocks because carbon is not wetted by molten aluminum. This means that a significant depth of metal (⬎5 cm)[12,13,14] has to be kept within the cell to create a uniform and flat cathode surface for the electrolysis. The presence of this liquid metal pool became bothersome as the electrolyte-metal interface (a liquid-liquid interface) was set in motion by the ever-increasing electromagnetic forces generated by the electromagnetic fields and the cell current itself. An unstable and wavy interface requires that the anode-to-cathode (molten Al surface) distance (ACD) be kept larger than otherwise desirable. In this context, a wettable material would be of great interest as one would be able to eliminate the unstable metal pool and operate at lower ACD, thus improving the energy efficiency of the process. Since the first characterization of TiB2 and the fabrication of TiB2-graphite/carbon composites, many concepts of the drained cathode cell design have been published.[15] However none, as yet, is in full MARTIN DIONNE, R&D Scientist, and AMIR MIRCHI, Internal Consultant, are with the Arvida Research and Development Centre, Alcan Inc., Jonquiere, PQ, Canada G7S 4K8. GILLES L’ESPE´RANCE, Professor, is with the Centre for Characterization and Microscopy of Materials, Department of Engineering Physics and Materials Engineering, Ecole Polytechnique de Montreal, Montreal, PQ, Canada H3C 3A9. Manuscript submitted April 26, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
industrial application, as the ideal material has not yet been developed. The objective of this article is to present pertinent results obtained from the complete characterization of the raw materials (anthracite, TiB2 powder, and tar pitch) and their interactions during baking of the TCC material. These results are intended as a complement to previously published work.[16]
II. EXPERIMENTAL PROCEDURE A. Materials The TiB2-carbon composite material, provided by Alcan, consists of an electrically calcined mixture of anthracite, TiB2 powder, and a pitch binder. Porosity measurements were carried out according to ASTM C 830-93. The mean open porosity obtained from four different TCC samples was 18 ⫾ 1 pct and the mean bulk density was 2.2 ⫾ 0.1 g/cm3. A total of three different size fractions of anthracite were used to fabricate the TCC. The typical real density of the anthracite aggregates was 1.91g/cm3 and the typical ash content was 4 to 5 wt pct (values specified by Alcan). A commercial TiB2 powder elaborated using the carbothermic
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