High temperature and pressure preparation of chromium nitride

Nitrogen compounds have a wide range of applications in modern society due to their unusual and even unique properties. For example, C-BN, Si3N4 has high hardness as a cutting material, GaN is the most widely used semiconductor materials, corrosion resistant transitional metal nitride TiN, hydrogen storage material Li3N, with superconducting properties of MoN and so on… CrN is a kind of dilute magnetic semiconductor material with oxidation resistance, corrosion resistance, high hardness (HV1750), high temperature resistance (700℃) and good thermal conductivity. Using it to make cutting tools, stamping and drawing die, powder forming die, folding parts, die casting die, etc., has excellent compression resistance, scratch resistance and mirror. As coating, additive and membrane material, it is widely used in modern industry and process production. It is an indispensable material for modern technology production.

CrN powder crystals are traditionally synthesized by ammonolysis. It usually goes through the process of mixing and sintering in the reactor. Reactor sintering needs to be synthesized under the high temperature environment of flowing ammonia gas, and the pressure of oxygen and nitrogen in the reactor needs to be strictly controlled. The reaction cycle is long (2-20h in heat preservation), the production cost is high, and the oxygen content of the CrN generated by the reaction is high. Therefore, it is of great significance to invent a low cost, high efficiency and high safety method to prepare CrN and popularize it. High temperature and high pressure synthesis methods can generally significantly reduce the reaction period and improve production efficiency. Chromium nitride (CrN) was prepared by ion exchange reaction under high pressure proposed by Professor He Duanwei in 2012. The reactants are sodium chromate (Na2CrO4) and hexagonal boron nitride (H-BN), the reaction temperature is 800-1400℃, the reaction pressure is 0-5GPa. But the industrial synthesis cost of hexagonal boron nitride (H -BN) is higher. Therefore, the search for new reactants and reaction mechanism is very beneficial to the promotion of this efficient and safe high temperature and high pressure preparation technology.

CrCl3 and NaN3 were used as raw materials for the preparation of chromium nitride at high temperature and high pressure, and the reaction rate was reduced by adding NaCl to ensure the safety of the experiment and the stability of the cavity. For the consideration of possible gas release and synthesis safety, the molar ratio of NaCl to NaN3 is added at least about 2 times to reduce the reaction rate, that is, the molar ratio of N2 to NaCl is at least 1 times to fully release the gas. The invention synthesizes the initial block sample by adjusting the reaction temperature and reaction time, and the reaction products are CrN and NaCl, and the high purity CrN is obtained by washing

Chemical formula of reaction:

CrCl3 + NaN3 – CrN + NaCl

The specific technical scheme of the invention is described below

The invention relates to a preparation method of high temperature and high pressure chromium nitride, which is synthesized by using a press; The process includes raw material mixing, cold pressing molding, assembly, high temperature and high pressure synthesis, cooling unloading to compress the block products; The cold pressing molding is the size of the pressing machine synthesis cavity to press the mixed raw materials into a cylindrical shape; The assembly is to put the mixed raw materials into the container, and then into the heating container, and into the heat preservation sealed container, and then into the synthetic cavity; The cooling pressure relief, after the high temperature and high pressure synthesis products, stop the electric heating to make the assembly block natural cooling to normal temperature, and then pressure relief; It is characterized in that the raw material mixing is to mix chromium trichloride (CrCl3), sodium azide (NaN3) and sodium chloride (NaCl) according to the mole ratio of 1:2:4-5; The high temperature and high pressure synthesis is in the pressure of 5GPa, the temperature of 1700 ~ 2000℃ holding pressure for 15 ~ 60min; Finally, the bulk product was crushed, ground, washed and dried to obtain high purity CrN powder.

The block product is a mixture of CrN and NaCl. CrN with high purity can be obtained by grinding, grinding and washing the bulk product.

The addition of sodium chloride (NaCl) in the high temperature and high pressure synthesis process is mainly used to reduce the reaction rate, ensure the safety of the experiment and the stability of the cavity, and is conducive to the formation of CrN.

The experiment of the invention can be completed on a domestic SPD 6×600T type six-sided press. The experimental results show that temperature and the ratio of raw materials are the important factors affecting the synthesis of CrN. It can be seen from Example 1 ~ 6 that under the conditions of synthesis of the present invention, namely, the molar ratio of chromium trichloride, sodium azide and sodium chloride is 1∶2∶4 ~ 5, the synthesis pressure is 5.0GPa, the synthesis temperature range is 1700 ~ 2000℃, the heat holding pressure is 15 ~ 60 minutes, and the products are crushed and ground, and the products are washing, the high purity CrN can be obtained. The optimal raw material mixture is chromium trichloride, sodium azide and sodium chloride at the molar ratio of 1∶2∶5. The optimal combination of high temperature and high pressure is heat preservation at 1700℃ ~ 2000℃ under pressure of 5GPa

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