Chromium diboride powder production via self-propagating high temperature synthesis

In this study, Self-Propagating High-Temperature Synthesis (SHS) method was conducted for production of chromium diboride with a high energy efficient, fast and low-cost powder production technique. During the experimental studies, the effects of the amount of initial mixture and the reaction parameters on the reaction efficiency and the compositions of SHS alloys were investigated. Experimental studies that is done for production of chromium diboride powder via SHS can be divided into four sections: Thermochemical Investigations, SHS Reactions, Refinement Studies and Characterization of Products. Self-Propagating High-Temperature Synthesis is one of the important methods to synthesize advanced materials such as ceramics; abrasives, cutting tools and polishing powders, resistive heating elements, shape-memory alloys; high-temperature structural alloys; master alloys; neutron attenuators as well as conventional metals and their alloys. Although the discovery of metallothermic reactions (Beketov 1865; Goldschmidt 1895) is earlier, combining with flame propagation theories and the first gasless metallothermic combustion experiments were conducted by Merzhanov et al. in the middle of 1960s. Self-Propagating High-Temperature Synthesis reactions are highly exothermic. Thus, the propagation of reactions and the yield of reaction products continue in self-sustaining mode without requiring additional heat or energy. Before Self-Propagating High-Temperature Synthesis experiments, thermochemical investigations were performed to estimate the possible product compositions, required mixtures of the raw materials, generated energy and maximum adiabatic temperatures of SHS reactions by using FactSageTM 6.3 thermochemical databases software. Thermodynamic calculations were made by using the advanced "Phase Diagram", "Reaction" and "Equilib" module of FactSageTM 6.3 with Fact, FS, SGTE, BINS databases. Binary and ternary phase stability diagrams of selected alloy groups were plotted by using "Phase Diagram" module with SGTE (Scientific Group Thermodata Europe) database and estimated alloy compositions were selected due to the solidification graphs of these alloy systems. Amount of the raw materials used to obtain the estimated alloy compositions were calculated and "Reaction" and "Equilib" modules of FactSageTM 6.3 were used to determine the reaction characteristics. Gibbs free energy (ΔG), enthalpy (ΔH), entropy (ΔS), specific heat (Cp) and equilibrium constant (Keq) of SHS reactions were calculated by using Factsage. Specific heat is the main process parameter for Self-Propagating High-Temperature Synthesis reactions; it shows whether a Self-Propagating High-Temperature Synthesis reaction is self sustaining or not. The specific heat is the amount of heat required to change a unit mass of a substance by one degree temperature. If specific heat is between 2250-4500 J/g, then it can be said Self-Propagating High-Temperature Synthesis reaction is available for the reactants. Under this range the reaction will not occur and over these values SHS reaction would exploidable.Furthermore, for the main reaction in the present study, specific heat was calculated and it is between the value of 2250-4500 J/g. Another main process parameter is adiabatic temperature and it also shows if the reaction is self-sustaining or not. According to adiabatic temperature of the CrB2 production reaction, it can be said it is high enough to occur in a self-sustaining way. Adiabatic temperature was calculated by using FactSage Thermodynamic Databases. Self-Propagating High-Temperature Synthesis experiments were carried out sequential synthesis process. The formation ofCrB2 was conducted using the raw materials (Cr2O3, B2O3, Mg). Initial molar composition ratios for each mixture were calculated and themixtures were mixed throughly for 15 minutes in turbula mixer and poured into Self-Propagating High-Temperature Synthesiscrucible. Cr- Ni wire was placed at the top of crucible and the reactionrealized by passing current through the wire. After triggering occurs, a highly exothermic reaction takes place. Self-sustaining reaction propagatesthroughout the Self-Propagating High-Temperature Synthesis mixture, yielding the desired product. Then, Self-Propagating High-Temperature Synthesis product wasdischarged, crushed and groundedvia ring mill. X-ray diffraction analysis showed that SHS products contained mostly MgO together with Mg3B2O6(kotoite phase) that was formed by the reaction between non reduced B2O3 and MgO. The powder mixture produced from Self-Propagating High-Temperature Synthesis by using different amounts of B2O3, Cr2O3 and Mg was firstly leached with37 % HNO3acid and for the following specimens65 % HClacid was used for leaching, to remove MgO and Mg3B2O6 in the second stage of the experiments. During the Self-Propagating High-Temperature Synthesis experiments, while all experiments were carried out under air atmosphere, the last experiment (11th experiment) was carried out under Argon-atmosphere, in order to see whether there's a difference from air and Argon atmosphere in terms of final composition product. The results showed that there's not much difference between air and Argon atmosphere. The temperature of the leaching solution was increased up to 80°C within two minutes just after the addition of powder. While there was no significant effect of temperature on the dissolution of MgO, hot acid provided complete dissolution of Mg3B2O6. No change in the colour of powder was observed after the removal of impurities. All the leaching experiments were investigated at 400 rpm stirring rate with 10. After the S/L separation CrB2 based filter cakes were obtained. Leaching experiments that was carried out has showed that HNO3 was not able to remove the Mg-including phases from the products. Consequently, the production of chromium diboride powder has been successfully carried out by Self-Propagating High-Temperature Synthesis following by HCl leaching.After the leaching experiments carried out with HCl, Mg inclusions were removed up to 0,12 %. The results of chemical analyses which were conducted to leaching solution and XRD analyses of leaching cake showed that CrB2 powder was formed and MgO and Mg-borates phases mostly eliminated.