Low Temperature Combustion Synthesis In Ti-B System Induced By PTFE

TiB2and TiC ceramics present series of excellent properties such as high melting point, highhardness, excellent chemical resistance and good thermal and electrical conductivities, which makethem have the wide application, as cutting tools, wear resistant parts, metal melting crucibles andelectrode materials. However, the methods to produce TiB2and TiC ceramics are high energy consumingas the results of the synthesis temperatures higher than1000°C, in addition, the synthetic time islong.For saving energy consuming, the research on the low-temperature synthesis routes of TiB2andTiC ceramics became more and more environmentally important in recent years.In present thesis, TiB2-TiC composite powders were prepared at lower temperature in the Ti-2Bsystem with the PTFE as a chemical activator. The process and reaction mechanisms wereinvestigated by differential thermal analysis (DTA), thermal gravimetric analysis (TG), X-raydiffraction (XRD) methods and field emission scanning electron microscopy (FESEM). The principlefindings of the present study are as follows:(1) According to the thermodynamic calculation of PTFE-Ti-B system, the standard formationenthalpy of the reaction between Ti and PTFE is as high as1030.7kJ/mol, solid-state reaction betweenTi and B could be induced theoretically.The changes of the standard Gibbs free energy in thePTFE-Ti-B system show that TiB2and TiC phases are stable as the final product, while TiB phase ismetastable. According to the theoretical estimation of adiabatic temperatures (Ta d), the value ofTadof PTFE-Ti-B system is higher than3100K for those with the PTFE content less than41wt%, so thecombustion reaction in the PTFE-Ti-B system could be self-sustainable once it is initiated.(2) According to the TG-DTA results, when the content of PTFE is less than10wt%, there aretwo exothermic peaks in the DTA curves, one is in the range of500-570℃, it corresponds to thereaction between Ti and PTFE, the other one is in the range of850-1050℃which caused by thereaction between Ti and B. For the DTA curves of Ti-B system with more than10wt%PTFE, there isonly one extremely sharp exothermic reaction peak in the range of510-550°C, it means the reactionbetween Ti and B was induced.(3) The optimum amount of PTFE is10wt%and the Ti-B system can be induced after heating to550℃by adding this amount of PTFE. The formation mechanisms of TiC and TiB2in the TE reactioncan be described as follow: titanium reacting with PTFE conduct at first, resulting in large amounts ofheat, subsequently, the combustion reaction between titanium and boron particles to form TiB2take place, TiC is the product of the reaction between Ti and PTFE.(4) The particle sizes of Ti significantly effects on the phase compositions of the TE products ofPTFE-Ti-B system. The solid-state reaction between Ti and B could not be induced effectively byusing ultra-fine Ti powders. Whereas, the particles size of B only has little effect on the phasecompositions of the TE products, but has significant effects on the size of TiB2-TiC compositespowders. TiB2-TiC composite powder in an average particle size less than0.4μm can be synthesizedvia low-temperature combustion synthesis at550°C by adding10wt%1μm PTFE Ti-B system of45μm Ti particle and1μm B particle.