Synthesis of finely dispersed chromium diboride from nanofibrous carbon

摘要

© 2018, National University of Science and Technology MISIS. All rights reserved. The paper presents experimental data on synthesis of finely dispersed powder of chromium diboride. Chromium diboride was prepared by reduction of chromium oxide Cr 2 O 3 with nanofibrous carbon (NFC) and boron carbide in the induction furnace under argon atmosphere. NFC is a product of catalytic decomposition of light hydrocarbons. The main characteristic of a NFC is high specific surface area (~150,000 m 2 /kg), which is significantly higher than that of soot (~50,000 m 2 /kg). The content of impurities in NFC is about 1 wt . Boron carbide used as a reagent is characterized by high dis-persity (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 Cr 2 O 3 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, in-ductively 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, sedimentation analysis, synchronous thermogravimetry and differential scanning calorimetry (TG/DSC). The material obtained at optimal parameters is represented by a single phase – chromium diboride CrB 2 . 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 m 2 /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 stoichio-metry 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 Cr 2 O 3 is almost completely reduced to diboride CrB. 2 .