Abstract
The study presents a possible mechanism to produce carbides and diborides of transition metals, such as titanium, vanadium, chromium and zirconium. The carbothermal synthesis of transition metal carbides has defined the direct dependence between the thermodynamic stability of oxides and the temperature range of the reduction onset (the stronger the oxide, the higher the value of the temperature is). It reaches 2000-2100, 1500-1600, 1300-1400 and 2100-2200°C for such carbides as TiC, VC0,88, Cr3C2 and ZrC respectively. The same dependence has not been found for the diborides of these metals. Optimum synthesis temperatures for all these compounds lie in the range of 1600-1700 °C. This viable method to produce transition metal carbides consists in the transfer of vaporous higher and lower oxides. Diborides preparation involves the transfer of oxides and boron vapors onto the surface of the carbon material with the subsequent chemical interaction. In the case of carbide-boron reduction of zirconium oxide in excess of boron carbide, the reaction product will be a composite material (B4C – ZrB2). The ceramics based on this composite possesses high performance properties.
Highlights
Carbides and diborides of certain transition metals find wide application in state-of-the art technologies both in the form of powder and compact products owing to their unique properties, such as refractoriness, high hardness, chemical inertness, etc
2 Experimental To do experiments on the synthesis of carbides and diborides of titanium, vanadium, chromium and zirconium, nanofibrous carbon (NFC) was used as a carbon material characterized by a high specific surface area (~150 m2/g) and a low content of impurities
The synthesis of titanium, vanadium, chromium, and zirconium carbides has revealed that the values of the optimum production temperatures depend directly on the thermodynamic stability of the oxides used as reagents: ZrO2 > TiO2 > V2O3> Cr2O3 (Fig. 1)
Summary
Carbides and diborides of certain transition metals (titanium, vanadium, chromium, and zirconium) find wide application in state-of-the art technologies both in the form of powder and compact products owing to their unique properties, such as refractoriness, high hardness, chemical inertness, etc. The most promising techniques applied in production of carbides and diborides are considered to be carbothermal and boron carbide reduction with carbon respectively. The objective of the study is to explain the processes occurring during the synthesis of transition metals carbides by the carbothermal method and diborides by the carbide-boron method. To do experiments on the synthesis of carbides and diborides of titanium, vanadium, chromium and zirconium, nanofibrous carbon (NFC) was used as a carbon material characterized by a high specific surface area (~150 m2/g) and a low content of impurities (at the level of 1 wt.%). The synthesis of titanium, vanadium, chromium and zirconium diborides was carried out in an argon protective atmosphere, which prevented undesirable nitriding of the boron-containing reagent – boron carbide [1]
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