Hot pressing and degradation of TiB inert cathodes
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Primary aluminium production is an energy intensive process, and the aluminiumindustry strives to lower the energy consumption. However, with the current technology a considerable reduction in the energy consumption is difficult to achieve. Implementation of an inert wettable drained cathode may reduce the energyconsumption for the process considerably. TiB2 is the inert cathode material which has been considered the most promising and has received most attention. Although TiB2 does not react with aluminium and has a low solubility in aluminium, limited lifetime during operation has been reported. TiB2 cathodes have therefore not been introducedon an industrial scale. Degradation during operation is caused by penetration of aluminium along the grain boundaries, and has been attributed to the reaction between impurity phases in the material and aluminium. This work has focused on the effect ofoxide impurity phases on the microstructural evolution during the sintering process andthe effect of these impurities on the chemical and mechanical stability in an electrolysis cell environment. Moreover, the effect of sintering aids in the form of C, and C combined with Cr and B has also been investigated. The samples were hot pressed at temperatures between 1300 and 1900 °C at 50 MPa for 1 hour in Ar atmosphere. It was first shown that preferential orientation of the TiB2 grains occurred during hot pressing. Densities around 93 % were achieved for TiB2 without sintering additions at 1800 °C, and a TiO1-xCx solid solution was observed atthe grain boundaries. C was added in order to carbothermally reduce the oxideimpurities present in the TiB2 powder during hot pressing. Reduction of the oxideimpurities by the addition of C resulted in an increase in the density above 97 %, and the amount of the secondary phase TiO1-xCx(ss) was considerably reduced. Excess Caddition relative to the amount necessary for carbothermal reduction of the oxideimpurities resulted in residual C and increased porosity in the materials, whereas toolow C addition resulted in residual TiO1-xCx(ss). A method to avoid residual C in the materials was developed. TiB2 materials without oxide impurities or residual C can be achieved by reaction of the excess carbon with Crand B. It was demonstrated that the desired reactions removing residual oxides and excess carbon occurred, resulting in TiB2 materials containing the secondary phases(Ti,Cr)C(ss), CrB and (Ti,Cr)B2(ss). The degradation resistance of the hot pressed TiB2 and three commercially available materials was investigated by immersion in liquid aluminium and cryolitic melts, and/or by electrolytic deposition of aluminium on the sample surface. The degradation resistance was dependent on the secondary phases present along the grain boundariesand at triple junctions. Reduction of the mechanical strength and change in fracturemode from transgranular to intergranular was observed after immersion in liquid aluminium. Materials containing oxides or metallic phases along the grain boundaries suffered from severe degradation upon exposure towards liquid aluminium. The volume expansion associated with the reaction between the oxide impurities and aluminium resulted in severe cracking of the material. The metallic sintering aids dissolved readily in liquid aluminium resulting in rapid penetration of aluminium along the grain boundaries. Carbothermal reduction of the oxide impurities increased the degradation resistance in liquid aluminium. The degradation resistance for the materials with C, Cr and B additions was also investigated, and the degradation resistance was dependent on the carbide and boride secondary phases in the material. The materials containing (Ti,Cr)C(ss) and CrB suffered from severe cracking after exposure towards aluminium, and it was proposed that this is due to reactions between these secondary phases and aluminium upon formation of Al4C3 and AlB2. The(Ti,Cr)B2(ss) did, however, not react with aluminium, and the material with this phase along the grain boundaries showed an improved degradation resistance. All thematerials immersed in cryolitic melts suffered from penetration of cryolite along thegrain boundaries. No significant differences in degradation resistance were observedfor the materials with a relatively high amount of oxide impurities and the materials which were stable towards liquid aluminium. The mechanism for the unexpected rapid penetration of the fluoride melt in the materials is still to be investigated. After electrolytic deposition of aluminium, penetration of aluminum was observed at the surface of the materials which were shown to be stable in molten aluminium. This was attributed to the effect of the exposure and penetration of cryolitic melt before the deposition of aluminium occurred.