Preparation And Properties Study Of AlN Ceramics Sintered Under Low-Temperature

With the rapid development of electronics and information technology, the problem of heat elimination serious restrict the progress of electronic components to the high-density, multi-function, high-speed and high-power. Aluminum nitride is a promising substrate and package material for high power integrated circuits due to a unique combination of properties including high theoretical thermal conductivity, thermal expansion coefficient which is close to the silicon, high electrical resistivity, and low dielectric constant and loss.As covalent compound, AlN has a high melting point and a low self-diffusivity. It always requires high temperature for sintering. The high cost limits its application as a package substrate. In this paper, using AlN nano-powder as raw material, by adding different sintering aids and additives and control of sintering process reasonably, AlN ceramics with high densification was prepared under low-temperature(1600℃) and pressureless sintering process, which shows high thermal conductivity and a low relative dielectric constant and dielectric loss.In this paper, oxidation phenomena in the sintering process of AlN ceramic was researched, in addition, the experiment of air oxidation, the atmosphere, the buried powders, temperature and others were carried out. It is found that when the sintering temperature was as low as 600℃, it was not oxidized. It became amorphous when the temperature reached to 800℃and until 1200℃pure Al2O3 phase was obtained. Under the N2 atmosphere, using of AlN+4wt% amorphous carbon powder as mixed buried powders, no oxidation occurred, it avoid the high cost of using high-purity N2 and the risk of using hydrogen atmosphere, such as CO and H2.According to the thermodynamic analysis and phase diagram analysis, the whole experimental scheme using different sintering aids was established. The results show that: adding Y2O3, CaF2 as single sintering aids, dense AlN ceramics could not be obtained at 1600℃, adding 3wt%Y2O3-2wt%CaF2 as sintering aids, AlN ceramics with the theoretical density were prepared at 1600℃for 4h, the relative density reached to 99.4%. Too long or too short of the holding time did not contribute to the densification of the AlN ceramics. XRD analysis displayed that the main crystalline phase is AlN, and the grain boundary phase is compounds created by Al2O3 and the sintering aids. It was manifest from SEM photos of cross-section that the grain grew well with little pore in the structure. Thermal conductivity is 83.62W/m?K. To prolong the holding time can make the grain continue to grow and improve the thermal conductivity. Vickers hardness were 13.8GPa. It has low relative dielectric constant and dielectric loss.This article investigated the influence of carbon nanotubes as additives to the sintering process of AlN ceramic. The experimental results showed that: adding multi-wall carbon nanotubes is helpful for the densification of AlN ceramic. However, with the addition increase, the effect would reduce. 1wt% MWNTs is the best amount. The role of MWNTs are believed to remove the oxygen atoms from the grain boundaries and lattices dissolved oxygen, which is known to be deleterious to thermal conductivity. The relative dielectric constant and electric resistivity decreases gradually with the increase of multi-walled carbon nanotubes containing. While 1wt% of multi-wall carbon nanotubes were added, another 3wt% Y2O3-2wt%CaF2 were also added as sintering aids, more dense AlN ceramics were prepared at 1550℃for 4h. The thermal conductivity is 90.57W/m·K, Vickers hardness is 12.4 GPa, it also shows relative low dielectric constant and dielectric loss. It was observed that all peaks were attributed to AlN phase and no MWNTs phase was found because of so little content. However, it was estimated from the thermal conductivity and dielectric properties test that C element must be existed in it. Untill now, it does not have enough evidence to explain the presence of the MWNTs structure.By the select of different additives and the optimize of sintering process, AlN ceramics with high thermal conductivity and relative low dielectric constant and dielectric loss were prepared at a low temperature of 1600℃. It reduced sintering temperature greatly. Moreover, it will be of great value for the industrial production of AlN substrates.