Mechanical enhancement of the carbothermic formation of TiB 2
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I. INTRODUCTION
TITANIUM diboride has become an increasingly important material with its adoption as a high-temperature coating for electrodes in highly aggressive environments such as aluminium smelting. It is generally prepared in two ways, via direct combination of the elements or high-temperature carbothermic reduction of a mixture of rutile and boric oxide. The elements undergo a violent self-propagating hightemperature synthesis unless heated with care.[1] However, the elements are expensive compared to their oxides, and direct reaction is uneconomical on a large scale. Consequently, a carbothermic reduction route is used; however, the overall reaction [1] is only thermodynamically favorable above 1315 8C, and the heat of reaction is highly endothermic at 11250 kJ. TiO2 1 B2O3 1 5C ⇒ 5CO 1 TiB2
[1]
The carbothermic reduction of TiO2 has been examined on numerous occasions,[2–17] primarily as part of a study of the commercially important Becher process, where FeTiO3 is reduced to iron and TiO2, which then undergoes further reduction to phases of the general formula TinO2n21. This reduction initiates below 900 8C and leads to a decreasing value of n with increasing temperature.[11,13–15] Thus, although reaction [1] is unfavorable below 1300 8C, the actual titanium species present changes with temperature, leading to a different, but unknown, reaction. Other work on milled powder has shown that, above 1000 8C, the main titanium oxide phase is Ti3O5, although, in the presence of sufficient carbon, TiC is the major phase.[11] In industrial practice, a stoichiometric mixture of the feed powders is placed into a crucible in an induction furnace and heated to .1500 8C, where the reaction starts.[1] The
furnace is maintained at temperature for ,15 hours to allow the reaction to go to completion and is then allowed to cool to ambient temperature. Both heating and cooling stages are performed under argon to minimize carbon loss prior to the onset of the reaction and to prevent oxidation of the TiB2 during cooling. Once the reaction has started, the inert gas is no longer required, as the CO given off during the reaction provides a protective atmosphere. The main advantage of this process is that the unwanted product (i.e., CO) is volatile, thus producing a boride which requires little post-treatment, other than size reduction and sintering to final form. However, the extremely endothermic nature of the reaction makes it energetically expensive, and a lower temperature and/or faster route for the systhesis of TiB2 would be expected to provide major energy savings. Previous work[18] has indicated that more-powerful reductants, such as boron, can be used to form TiB2 from TiO2 at ,1100 8C. A further decrease in temperature to 800 8C was achieved using FeTiO3 (the industrial precursor of TiO2) as the titanium source, suggesting that iron has a beneficial role.[19] However, both of these reactions use elemental boron, which is expensive, and the product requires a subsequent leaching step to separate the TiB2 from the bo
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