In practical applications, in addition to high thermal conductivity and high electrical insulation performance, aluminum nitride ceramic substrates are also required to have high bending strength in many fields. The three-point bending strength of aluminum nitride currently circulating in the market is usually 400~500MPa, which seriously limits the promotion and application of aluminum nitride ceramic substrates, especially in the field of IGBT power devices with high-reliability requirements. Due to the complex manufacturing process of AlN materials and high production costs, most AlN materials currently do not meet the application requirements of high thermal conductivity and high strength.
In the preparation of aluminum nitride ceramic substrates, choosing the right sintering method and sintering aids often achieves twice the result with half the effort. Introducing sintering aids in the preparation is currently a commonly used method for sintering aluminum nitride ceramics. On the one hand, it forms a low-temperature eutectic phase, realizes liquid phase sintering, and promotes densification of the green body; on the other hand, it removes oxygen impurities in aluminum nitride, improves the lattice, and improves thermal conductivity. At present, the sintering aids used in sintering AlN ceramics mainly include Y2O3, CaO, Yb2O3, Sm2O3, Li2O3, B2O3, CaF2, YF3, CaC2, etc. or their mixtures. The study found an aluminum nitride ceramic sintered body, which uses a composite sintering aid of Li2O, CaF2, and Y2O3 as an aid for the aluminum nitride ceramic sintered body, which can promote the densification of the aluminum nitride ceramic sintering process and improve the bending strength; by reducing the introduction of oxygen elements and consolidating the Al2O3 on the grain boundary, the sintering of the ceramic is finally promoted and the thermal conductivity is improved, so that the prepared aluminum nitride ceramic substrate has both high thermal conductivity and high bending strength. In the formula system of aluminum nitride ceramic sintered body, when Y2O3 is higher than 3.5wt%, the Y-Al-O content increases significantly during the sintering process and forms agglomerates. Since the thermal conductivity of Y3Al5O12 is low (about 9 W/(m·K)), the thermal conductivity of the aluminum nitride ceramic product after sintering is seriously affected. When the content of CaF2 and Li2O is higher than 1.33wt%, due to the volatilization of fluoride and lithium-containing compounds, the porosity of the aluminum nitride ceramic sintered body is increased during the sintering process, and the density of the ceramic is reduced, resulting in a sharp decrease in the flexural strength of the aluminum nitride ceramic product after sintering. When each additive is less than the minimum value, it cannot enhance the mechanical properties or the effect is minimal.
It has been proposed that the preparation method of the new research has been verified by experiments. When the mass fraction of the Y compound is 1.33 ~ 3.5wt%, the mass fraction of the Ca compound is 0.4 ~ 1.33wt%, and the mass fraction of the Li compound is 0.1 ~ 1.33wt%, based on pure aluminum nitride powder, the bending strength and thermal conductivity of the prepared aluminum nitride ceramic products can be rapidly improved. Then, microwave sintering can significantly reduce the sintering temperature of aluminum nitride ceramics, improve the density, reduce porosity, and solve the problem that the bending strength of the existing aluminum nitride ceramic sintered body is not high and it is difficult to apply to the high-strength field requirements so that the prepared aluminum nitride ceramic substrate has both high thermal conductivity and high bending strength.
As an ideal material for ceramic substrates, AlN has a broad market, and different product types meet the needs of different application scenarios, among which AMB, DBC, DPC, HTCC, and structural parts are the main product types. DPC is favored by the high-power LED market, and AMB and DBC are developing rapidly with the IGBT market and the new energy and electric vehicle fields; HTCC is driven by the growth of demand in the radio frequency and military fields; the demand for aluminum nitride will continue to benefit from the fast-growing semiconductor and new energy markets.
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