Synthesis of finely dispersed chromium diboride from nanofibrous carbon

The paper presents experimental data on synthesis of finely  dispersed powder of chromium diboride. Chromium diboride was  prepared by reduction of chromium oxide Cr₂O₃ with nanofibrous  carbon (NFC) and boron carbide in the induction furnace under argon atmosphere. NFC is a product of catalytic decomposition of light  hydro carbons. The main characteristic of a NFC is high specific surface area (~150,000 m²/kg), which is significantly higher than that  of soot (~50,000  m²/kg). The content of impurities in NFC is about  1  wt  %. Boron carbide used as a reagent is characterized by high dispersity (at the level of ~2  μm) and insignificant content of impurities – no more than 1.5  wt  %. Based on analysis of state diagram of  the Cr – B system, composition of the charge and upper temperature  limit of diboride formation reaction were determined for obtaining  chromium diboride in powder state. According to the results of thermodynamic analysis, the temperature of beginning of reaction for  chromium oxide Cr₂O₃ reduction by carbon and boron carbide was  determined at various CO pressures. Composition and characteristics  of chromium diboride were studied using X-ray phase analysis, inductively coupled plasma atomic emission spectrometry (AES-ISP),  scanning electron microscopy using local energy-dispersive X-ray  microanalysis (EDX), low-temperature adsorption of nitrogen, followed by determination of specific surface area by BET method,  sedi mentation analysis, synchronous thermogravimetry and differential scanning calorimetry (TG/DSC). The material obtained at optimal parameters is represented by a single phase – chromium diboride  CrB₂ . The content of impurities in chromium diboride does not exceed 2.5  wt  %. The powder particles were predominantly aggregated.  The average size of the particles and aggregates is equal to 7.95  μm  within a wide range of size distribution. The specific surface area of a  single-phase sample is 3600  m²/kg. Oxidation of chromium diboride  begins at a temperature of 430  °C and when the temperature reaches  1000  °C, the degree of oxidation is approximately 25  %. Optimum synthesis parameters are the ratio of reagents according to stoichiometry to obtain chromium diboride at a temperature of 1700  °C and  holding time of 20  min. It is shown that for this process nanofibrous  carbon is an effective reducing agent and that chromium oxide Cr₂O₃  is almost completely reduced to diboride CrB₂ .

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