Zirconium carbide (ZrC) ceramic material with high melting point, high hardness, excellent mechanical properties, and high conductive (hot) rate and excellent antioxidant ablation performance, as one of ultra high temperature ceramic material system, can be used as heat materials applied in spaceflight and propulsion systems, such as the shuttle's wing leading edge, a hypersonic scramjet, etc.
The lattice structure of ZrC ceramic material is shown in Figure 1. Zr atoms form a compact cubic lattice, and C atoms are located in the octahedral gap of the lattice, so the crystal structure of ZrC belongs to the typical NaCl type face-centered cubic structure. ZrC lattice constant A =0.46930nm, C atom to Zr atom radius ratio 0.481.
Arc furnace carbonthermal reduction method
At present, arc furnace carbonthermal reduction is the most effective method for industrial preparation. The method uses zircon sand or zircon as the precursor, and then generates ZrC powder through carbonthermal reduction reaction under high temperature and high pressure. The reaction mechanism is as follows:
During the reaction process, the temperature of the arc furnace should be strictly controlled. If the reaction temperature is too low, less SiO will be excluded, which will lead to the formation of ZrC powder containing more impurity phases Si and C, thus affecting the purity of ZrC powder.
The preparation of ZrC powder by arc furnace carbonthermal reduction method has the advantages of simple equipment structure and simple operation, but its cost is high and the ZrC powder size is large.
Figure 2 shows ZrC ceramic powder prepared by carbothermic reduction reaction using ZrO2 as raw material, with uniform particle size and size less than 200nm.
Self-propagating high temperature synthesis (SHS)
Self-propagating high temperature synthesis is a new technology that synthesizes materials in a very short time by utilizing the high heat of reaction between reactants. The process flow of ZrC powder preparation by self-propagating high-temperature synthesis technology is shown in Figure 3:
The ceramic powder prepared by self-propagating high-temperature synthesis has the following characteristics:
①The reaction process uses the chemical reaction itself heat, without external heat source.
② The products of desired composition and structure are obtained through the self-sustaining reaction of rapid automatic wave combustion.
③ By changing the heat release and transmission speed to control the process speed, temperature, conversion rate and product composition and structure.
Figure 4 shows nano-zRC powder prepared by self-propagating high-temperature synthesis technology with uniform particle size.
Sol-gel method has become a new field of powder preparation technology, which is a kind of powder preparation method with the help of colloidal dispersion system.
The basic principle of the method is: the metal alkyd, water, alcohol and necessary catalyst are stirred evenly to prepare uniform solution, hydrolytic polycondensation reaction to form wet gel, wet gel after drying and heat treatment to form massive powder, and then mechanical grinding or grinding to form ultrafine ceramic powder. Sol-gel method has the advantages of uniform particle size and composition distribution and small particle size.
The advantages of sol-gel method for preparing ceramic powder are as follows:
① The raw materials used are high purity inorganic salts or alcohols, to avoid the influence of impurity elements, so the purity of the ceramic powder prepared is high.
② The reaction mainly occurs in the liquid phase, which can achieve the precise control of the chemical ratio of the material in a short time, thus ensuring the uniformity of the particle size of the product.
③ The reaction synthesis temperature of this method is low and the cost effect is low. However, this method also has the characteristics of long preparation period and relatively complex operation.
Figure 5 shows ZrC powder prepared by sol-gel method.
Laser gas reaction method
Using Zr(OC4H9)4 as precursor, Zr/O/C nano powders were prepared by laser gas reaction method. Then, the powders were placed in argon and treated at 1500℃ to produce ZrC powders of about 40nm.
The advantages of laser gas reaction are less pollution in the process of powder preparation, and the particle size and stoichiometric ratio can be easily controlled. The distribution range of powder particle size obtained is very narrow and the production efficiency is high. Figure 6 shows ZrC powder prepared by laser gas reaction method.
The preparation principle of high frequency plasma method is as follows: using high frequency induction coil heating, using ZrCl4 carbon black and nano active Mg as raw materials, high purity Ar as carrier gas, loading the raw materials into high temperature plasma region and heating rapidly to the reaction temperature, through gas reaction to get nano ZrC powder.
High frequency induction thermal plasma is electroless heating, which can avoid electrode pollution, high energy density, high temperature in the reactor, and large temperature difference, no need for high temperature treatment in the preparation process, can effectively prevent particle agglomeration, is conducive to the preparation of uniformly dispersed particles of ultrafine powder, has a good application prospect. But this process is a new powder preparation technology, the theory and technology is not mature, industrial production still need a period of time.
High energy ball milling
High energy mechanical ball grinding method is to use the rotation or vibration of the ball mill to make the hard ball of raw materials for strong impact, grinding and mixing, the mixed powder grinding into fine particles.
A large number of defects produced in the ball milling process lead to the reduction of activation energy, reducing grain size and increasing temperature can effectively improve the diffusion rate, so that the non-diffusible alloying elements can be diffused through mechanical alloying to achieve the purpose of alloying.