Low Temperature Solid-state Synthesis Of TiC Induced By Fluoropolymer And Graphite Fluoride

TiC is one of the most typical and important transition metal carbides with excellent properties, such as high melting temperature, hardness, strength, wear resistance, corrosion resistance, electrical conductivity and thermal conductivity. Consequently, TiC is widely applied in the manufacture of ceramic metal, heat-resistance alloy, and cemented carbide. Conventionally, TiC is synthesized by methods such as carbothermal reduction of titanium oxide using carbon, direct chemical reaction between element Ti and C, mechanical alloying, chemical vaporous deposition, sol-gel method and microwave synthesis. However, there are some disadvantages on these methods such as high reaction temperatures, long holding times, complicated technological process and so on. In order to significantly reduce the synthesis temperature of TiC from titanium and carbon, polytetrafluoroethylene and polyvinylidene fluoride were employed to induce the solid state synthesis at a low temperature. On the other hand, we take graphite fluoride as a carburizing agent to reaction with titanium. The main content were summarized as follows:(1) Thermodynamics analyses for combustion synthesis process of TiC from Ti/C/PTFE, Ti/C/PVDF and Ti/(CF1.0)n three kinds of system were calculated. It is revealed that the Gibbs free energy for all the reactions are negative at the ignition temperature 500 °C. The energy released from the reaction between titanium and PTFE or titanium and PVDF is about 850 k J/mol which is higher than the activation energy 364 k J/mol between the reaction titniuam and carbon. All the adiabatic combustion temperatures are higher than 1800 °C and the self-sustaining temperature 1527 °C. The released energy for Ti/(CF1.0)n system is 2100 k J/mol which is enough to activate the reaction between titanium and carbon. The adiabatic combustion temperature 1810 °C is high enough to sustain the self-propagating combustion of this system.(2) Nano-TiC was prepared at 530 °C, using PTFE as the reaction promotor between titanium and carbon. The analysis results reveals that the reaction temperature raised to a higher temperature with the increasing of PTFE adding in the Ti/C mixture. When the adding amount of PTFE up to 3 wt%, the heat released from the reaction between PTFE and titanium is enough to activate the reaction on Ti/C/PTFE. The reaction of the system finished in an instant and the average size of the product is about 81 nm. The particles mainly with spherical and a small number were converted to polyhedral particles when grew up. When the adding amount of PTFE is up to 4 wt%, the intermediate product Ti F3 residues in the product. In the process, the reaction between titanium and PTFE occurs first, then the heat releases from the reaction activate the reaction between titanium and carbon. There are two kinds of carbon reacted with titanium, one is high active carbon from the pyrolysis of PTFE and another is from the raw material.(3) The results about Ti/C/PTFE system reveal that the ignition temperature of Ti/C/PVDF system is much higher than that of Ti/PVDF system. When the adding amount of PVDF is up to 20 wt%, the ignition temperature is 499 °C which is 50 °C higher than that of Ti/PVDF system and 20 °C lower than that of 3wt%PTFE/Ti/C system. But too much by-products Ti F3 and Ti H1.5 lowered the concentration of titanium and reduced the reaction area between titanium and carbon, so only a small amount of TiC0.625 in the product.(4) The powder of TiC with different sizes were synthesized by using graphite fluoride as carburizing agent reacted with titanium. When the F/C atomic ratio is lower than 0.5, the reaction between graphite fluoride and titanium contain two weak reaction peaks in the DSC curve. There is reside carbon which from the pyrolysis of graphite fluoride in the product. When the atomic ratio of F/C is equal or higher than 0.5, the reaction between graphite fluoride and titanium is strong at 610 °C and the reaction rate is fast. With the increasing of F/C atomic ratio, the size of TiC powder prepared by the reaction between graphite fluoride and titanium turned from nanoscale to micron level. The reaction process is that with the increasing of temperature the fluorine break away from graphite fluoride at first, then the fluorine reacts with titanium and releases a large amount of heat. Finally, the heat from the reaction between fluorine and titanium prompts the reaction between carbon with titanium and Ti F3, respectively.