USE OF CARBON MATERIAL WITH DEVELOPED SURFACE FOR SYNTHESIS OF HIGHER CHROMIUM CARBIDE

The paper presents experimental data on synthesis of finely dispersed powder of chromium carbide Cr3C2 . Chromium carbide was prepared by reduction of chromium oxide Cr2O3with nanofibrous carbon (NFC) in induction furnace in argon atmosphere. NFC is a product of catalytic decomposition of light hydrocarbons. The main characteristic of NFC is high specific surface area (~150,000 m2/kg), which is significantly higher than that of carbon black (~50,000 m2/kg). Content of impurities in NFC is at the level of 1 wt %. Based on analysis of state diagram of Cr – C system, composition of charge and the upper temperature limit of carbide formation reaction for obtaining chromium carbide in powder state are determined. Based on thermodynamic analysis, temperature of the onset of carbothermic reduction reaction of chromium oxide Cr2O3was determined at various CO pressures. Characteristics of chromium carbide were studied using X-ray diffraction analysis, pycnometric analysis, scanning electron microscopy using local energy dispersive X-ray microanalysis (EDX), lowtemperature nitrogen adsorption followed by determination of specific surface area by means of 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 carbide Cr3C2 . Powder particles were predominantly aggregated. Average size of particles and aggregates equaled 6.5 μm within a wide range of size distribution. Specific surface value of the obtained samples was 2200 m2/kg. Oxidation of chromium carbide began at temperature of ~640 °C and practically ends at ~1000 °C. Optimum parameters of synthesis are provided by ratio of reagents according to carbide of Cr3C2composition stoichiometry at temperature of 1300 °С and holding time of 20 minutes. It is shown that for this process nanofibrous carbon is an effective reducing agent and that chromium oxide Cr2O3is almost completely reduced to carbide Cr3C2 .

Polarity-assisted formation of hollow-frame sheathed nitrogen-doped nanofibrous carbon for supercapacitors

The polar groups of polymer nanofibers induce uniform decoration of MOFs and the subsequent formation of hollow carbon-frames during pyrolysis.

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 .