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Atmospheric Pressure and Vacuum Sintering of High Purity Dense MgO Ceramics

Features of Magnesium Oxide:

Magnesia ceramics have a large coefficient of thermal expansion, a melting point of 2800°C, and good insulation.

High-purity magnesia ceramics have strong corrosion resistance to metals and alkaline solutions, and are used as crucibles for melting high-purity iron and alloys, nickel, cobalt, uranium, tin, lead, zinc, copper and their alloys; as crucibles for fused alumina and aluminum salt containers; thermocouple protection tubes made of magnesium oxide are used to measure high temperatures above 2000C. Magnesia ceramics can also be used as lining for high temperature furnaces.

Compared with the current mainstream semiconductor memory, the new generation of non-volatile high-density magnetic memory (MRAM) has the advantages of lower power consumption, faster read and write speed, and longer life, and is widely used in shift tunnel junction (MTJ) It is a key component of MRAM, and its isolation layer is an oxide film with a thickness of 0.8^-2 nm. MgO thin film has the advantages of huge magnetoresistance effect, good non-volatility, high dielectric and low dielectric loss, and is widely used as the isolation layer of MTJ. The preparation of MgO film is usually based on the MgO target as the cathode source, and the MgO thin film is deposited on the base material by magnetron sputtering. Therefore, preparing high-purity, dense, fine and uniform targets is one of the key conditions for obtaining high-performance and defect-free films.

At present, high-density MgO ceramics are basically sintered by hot pressing and hot isostatic pressing, but carburization pollution is prone to occur during sintering, and it consumes a lot of energy, so it cannot be mass-produced. Therefore, pressureless sintering with low energy consumption and mass production has become a research hotspot. In order to overcome the shortcomings of high pressureless sintering temperature and low density of MgO, and obtain a high-purity and dense MgO target with a fine and uniform microstructure, this study used high-purity MgO powder as raw material, and compared normal pressure and vacuum atmosphere sintering, and explored the sintering temperature. , holding time and the effect of sintering atmosphere on the densification of MgO.

MgO Densification and Grain Growth Kinetic Analysis

1.In the middle and later stages of sintering, when the closed pores are just formed, the densification process of the MgO sample is the spheroidization and disappearance of pores, and the disappearance of pores is closely related to the surface energy γsv of the pores and the pore pressure Pg

close. The effective driving force for the densification of the sintered body is: ΔP = γsv/ρ – Pg (1) where ρ is the radius of curvature. At this time, the air pressure Pg in the pores is approximately the pressure of the sintering atmosphere, which is 0.1 MPa for normal pressure sintering and 9×10^-7 MPa for vacuum sintering; the surface energy of MgO is 1 J m^-2. According to the measurement statistics of the pore diameter, if the radius of curvature does not exceed 0.1-0.001 μm, it can be estimated that the magnitude of the surface tension during sintering is about 10 MPa. The driving force for sintering is similar. Thereafter, with the progress of sintering, the closed pores gradually become isolated isolated pores, and the spheroidization becomes smaller. Assuming that the pores in this process are all spherical, and the gas does not diffuse, according to the gas mass conservation in the pores: P1 R13 = Pt Rt3 (2) In the formula, P1 and R1 are the pores when closed pores (ρ = 90%) are formed Air pressure and radius; Pt, Rt are pore air pressure and radius when the holding time is t. Assuming that the normal pressure and vacuum sintering conditions are the same before the formation of closed pores, substituting γsv = 1 J m^{– 2}and ρ = Rt /2 into the equation (1), (2) gives the normal pressure and vacuum sintering kinetics at the end of sintering as:

ΔP1 = 2 /Rt – 0.1R13 /Rt3 ( 3)

ΔP2 = 2 /Rt – 9×10^-7R13 /Rt3 ( 4)

From equations (3) and (4), it is known that the shrinkage of pores at the end of sintering increases the surface tension, but the pore pressure in equation (3) increases at a faster rate. When ΔP1 is zero, the densification process stops, and thereafter Raising the temperature will only cause the grains to coarsen, and the density will basically change; while the pore pressure in equation (4) is almost zero, theoretically prolonging the holding time can obtain a MgO sample close to the theoretical density, but the actual sintering is more complicated, and only A sintered body with a relative density of 99.12%.

2.Optimizing the parameters in the sintering process, vacuum sintering at 1500℃ for 4 h, the relative density is 99. 12%, the average grain size is 11. 71 μm high-purity MgO ceramic.

3.Analyzing the sintering kinetics in the normal pressure and vacuum sintering process, it can be known that the vacuum sintering kinetics is higher than the normal pressure sintering kinetics, especially after most of the pores in the sintered body form isolated pores; due to the higher vacuum sintering kinetics, the small grains in the sample It can also grow rapidly, so the grain size in the vacuum sintered sample is more uniform, the size standard deviation is smaller, and the proportion of abnormally grown grains is small.

Effect of sintering parameters on density and grain growth

1.The relative density and average grain size of MgO ceramics sintered at atmospheric pressure vary with temperature. It can be seen that as the sintering temperature rises from 1380°C to 1500°C, the relative density of MgO ceramics gradually increases from 92.68% to 1500°C. 98.1%, after which the relative density decreases slightly with the increase of sintering temperature. The average grain size increases from 5．88μm grows slowly to 12．75μm, and then the grain size increases sharply with the increase of temperature. If the sintering temperature is too high, the MgO powder will gradually lose its activity. The decrease in the relative density of the sintered sample at high temperature should be due to the over-fired expansion of MgO ceramics at high temperature.

2.The results of the comparative experiments of normal pressure and vacuum sintering show that the overall vacuum sintering atmosphere can significantly increase the relative density of MgO ceramics without causing significant grain coarsening. At a lower sintering temperature and a lower holding time (such as 1500 ℃ for 2 h), there is only a slight difference in the relative density and grain size of the vacuum and atmospheric pressure sintered samples; while at a high sintering temperature and a high holding time (such as At 1550℃ for 4-6 h), the relative density and average grain size of the vacuum sintered samples were significantly increased compared with the atmospheric pressure sintered samples. It can be found that the optimal sintering condition is vacuum sintering at 1500 ℃ for 4 h. Under this sintering condition, the highest density of MgO ceramics obtained is 99.12%, and the average grain size is 11.71 μm.

Conclusions of normal pressure and vacuum sintering of magnesia ceramics

1.At the end of atmospheric pressure sintering, a certain amount of isolation pores will always remain in the atmospheric pressure sintered MgO sample, which is difficult to eliminate; vacuum sintering eliminates the resistance to densification at the end of sintering by eliminating the air pressure in the isolation pores, thereby significantly improving the density of the sintered body.

2.The parameters in the sintering process were optimized and vacuum sintered at 1500°C for 4 h to obtain high-purity MgO ceramics with a relative density of 99.12% and an average grain size of 11.71μm.