The Preparation And The Formation Mechanism Of Ultrafine CaB₆、Tungsten Boride、TiAl、TiC Powders By Salt-asssited Combustion Synthesis

Boride material and Titanium compound material with its superior performance has been widely used in the contemporary era. This paper reviews the CaB₆, boride tungsten, Ti Al and TiC powders performance, application and preparation methods. The function of NaCl diluent in the process of combustion synthesis was studied by introducing NaCl to the systems of Ca O- B₂O₃-Mg、W- B₂O₃-Mg、Ti-Al and Ti-C. A formation mechnasim of the four systems in the reaction media of molten NaCl was proposed. Finding out the NaCl on the microstructure, content and particle size of product of the rule. The mainpoints are summarized as follows.1.The Ca O-B₂O₃-Mg system, NaCl as the molten salt, the ultrafine CaB₆ powder were successfully prepared by Salt-assisted Combustion Synthesis. Combustion synthesis product including CaB₆ and by-product magnesium oxide. Combustion products have burnt and reunion, the reunion phenomenon was obvious With the increase of the proportion of NaCl. Combustion products with HCl and deionized water washing and after washing is contained in the products of the CaB₆ and very small amounts of NaCl. Due to the fine grain products, and storage condition was not vacuum, so will be in the product particles exist between gaseous oxygen. The CaB₆ leacheate square structure, good appearance, And reunion is much less before leaching, smaller particles of product, Product particles of the average particle size decreases with the increase of NaCl content, When the content of NaCl to 20 wt. %, the average particle size of CaB₆ was 288 nm. However, the trend of average particle size decreases began to grow slowly, when NaCl content of 15 wt%.2. The boride tungsten materials were successfully prepared by Salt-assisted Combustion Synthesis through W-B₂O₃-Mg as reaction system under the action of NaCl. Combustion products including Mg O, WB2 and WB, etc, particles on the combustion synthesis product is bigger, and presents some irregular shape. In NaCl content of k = 0 wt. %, produces mainly WB2, only has very small amounts of WB. But with the addition of NaCl product in WB gradually increased, when the content of NaCl k = 20 wt. %, the main phase into WB leacheate. Leacheate average particle size decreases with the increase of NaCl content of k value, and the particle size of the minimum 0.85 microns when k = 20 wt. %3. KClO₃ as exothermic agent, Ti-Al as reaction system, Ti Al metallic compound were successfully prepared by Salt-assisted Combustion Synthesis. The main products for Ti Al、Ti3Al、Ti O2、Al2O3. The reaction product particles are irregular topography with wider range of particle size distribution. The average particle size will gradually become smaller with the increase of NaCl. As a result of NaCl in Ti Al crystals can inhibit Ti Al grain in the process of growing up, so the average grain size of Ti Al is gradually decreases with the increase of NaCl.4. With NaCl as diluent, Ti-C as reaction system, Through Salt-assisted Combustion Synthesis TiC powders was successfully achieved. The purity of product is higher, The product of five components of NaCl have reached more than 98% purity. TiC powders of grain size gradually decreases along with the increase in the amount of NaCl. Before not to join in NaCl, the grains of combustion synthesis product particles under no 1 microns. The average particle size of particles of 3.8 microns. When the product after adding NaCl began to appear in the nano-scale particles. In NaCl content 20 wt %, the average particle size of TiC powders down to 1.15 microns.5. NaCl can reduce system reaction temperature, absorb the reaction emits heat. NaCl in the early stages of the reaction in the form of liquid, increasing the diffusion effect, promote the products of nucleation. But because NaCl absorbs heat response, speed up the rate of reaction system in the process of temperature drop of nucleation grain couldn’t continue to grow and precipitation from the liquid phase, maintained a smaller particles morphology. Part in the liquid phase after the grain will be affected by NaCl block and parcels, unable to continue to receive diffusion atoms and affect grain grew up, eventually lead to the overall grain size reduced.