Pressureless sintering of aluminium nitride

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What is aluminium nitride ceramics
Aluminum nitride ceramics are ceramics with AlN powder as the main crystalline phase.

What is Aluminum Nitride
Aluminum nitride is a refractory compound with hexagonal wurtzite structure
Pure aluminium nitride is blue-white, usually gray or gray-white
Aluminum nitride crystals are covalent compounds with Warzite structure and tetrahedron [AIN4] as structural unit
Density is 3.26g/cm3
Lattice constants a = 3.11, C = 4.980
The decomposition temperature at atmospheric pressure is 2480°C

Characteristics Of Aluminium Nitride Ceramic Materials
1. The thermal conductivity is 5-10 times higher than that of alumina ceramics, which is equivalent to highly toxic beryllium oxide (Be0).
2. The thermal expansion coefficient (4.3×10^-6/℃) matches the semiconductor silicon material (3.5-4.0×10^-6/℃).
3. Good mechanical properties, higher bending strength than Be0 ceramics, close to alumina.
4. Excellent electrical performance, high insulation resistance and low dielectric loss.
5. Circuit materials have good compatibility and can be multi-layer wiring to achieve high density and miniaturization of packaging.
6. Non-toxic, conducive to environmental protection.

Application of Aluminum Nitride Ceramics
Aluminum nitride ceramic substrates have high thermal conductivity, low expansion coefficient, high strength, high temperature resistance, chemical corrosion resistance, high resistivity and low dielectric loss. They are ideal heat dissipation substrates and packaging materials for large scale integrated circuits.

Aluminum nitride is a new type of wear-resistant ceramic material with high hardness, which surpasses traditional alumina.

GaAs crystal crucible, AI evaporating dish, magnetohydrodynamic power generation device and corrosion resistant parts of high temperature turbine can be made by using the heat resistance, melt erosion resistance and thermal shock resistance of aluminium nitride ceramics.

Aluminum nitride ceramics have good metallization properties and can replace poisonous beryllium oxide ceramics widely used in the electronic industry.

Aluminum nitride is a covalent compound with high melting point and low atomic self-diffusion coefficient. Therefore, it is difficult for pure AIN powder to sinter densification at normal sintering temperature, and it is difficult for materials with low densification to have high thermal conductivity. In addition to density, another factor affecting thermal conductivity of AIN ceramics is impurity content, especially oxygen content. Because of the strong affinity of aluminum nitride to oxygen, some oxygen will dissolve into the AIN lattice and form aluminum vacancies. The scattering of phonons by aluminium vacancies reduces the average free path of phonons, which leads to the decrease of thermal conductivity.

Therefore, in order to prepare high thermal conductivity aluminium nitride ceramics, two problems must be solved in the sintering process. The first is to reduce the sintering temperature, and the second is to avoid oxygen atoms melting into the aluminium nitride lattice as far as possible when sintering at high temperature.

The usual sintering methods are SPS sintering, microwave sintering and pressureless sintering.

Pressureless sintering of aluminium nitride ceramics
Pressure-free sintering is the most suitable technology to realize large-scale sintering, and is also the current general method of sintering aluminum nitride substrate materials. The technology uses pressureless sintering equipment, filling 0.1Mpa of nitrogen pressure in the sintering process, and sintering at 1750-2000℃.

Characteristics of pressureless sintering equipment
1. Using resistance heating, setting temperature compensation or multi-zone temperature control, the temperature uniformity is good.
2. It can be used stably for a long time under 2400 C condition by using the technology of super-high temperature and high current.
3. Using special high temperature infrared measurement technology, the temperature control is accurate and the error is small.