The Study On High-pressure Sintering Of AlN Ceramics

Aluminum nitride (AlN) is a ceramic material with high thermal conductivity and good dielectric properties. It has a thermal expansion coefficient which closes to that of silicon. Compared with conventional alumina substrates, the high thermal conductivity of AlN provides a higher energy dissipation that allows the development of electronic packages with higher density of components. In addition, AlN finds its usage as a structural ceramic because of its good mechanical properties. However, due to its strong covalent bonding (Al-N), and the small self-diffusion coefficients of its constituent elements, full densification of AlN ceramics is difficult to attain, which constrains its application. In order to lower the sintering temperature and remove the oxygen atoms from the grain boundaries and lattices, some additives such as CaO, Y₂O₃ and other rare-earth oxides or alkaline earth oxides have been tried to improving the sintering process. AlN ceramics, which room-temperature thermal conductivities range from 70 to 260 W/(m·K), can be fabricated by pressureless sintering with post heat-treatment, hot pressing, and hot isostatic pressing. Recently, there has been a growing interest in low-temperature sintering of AlN ceramics. Troceynski and Nicholson successfully prepared dense AlN ceramics using Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂ as sintering additives at 1600℃for 1h. However, the amount of additives used was very high: 9%. Streicher et al. sintered AlN at 1650℃in the presence of 0.5 wt% CaCO₃, or 3CaO·3SiO₂Al₂O₃. Jarrige et al. obtained a dense material by heating AlN at 1600℃using Y₂O₃-12CaO·7Al₂O₃ (abbreviated C₁₂A₇) or CaYAlO₄-C₁₂A₇ mixtures as sintering additives. Some multicomponent additive systems have also been proposed and sintering temperature as low as 1550℃has been reported. The High Pressure Sintering (HPS), which is a novel densification method and has unique advantage, has potential application in not only facilitating densification of AlN but also advancing property. In this paper, HPS of AlN ceramics was experimental studied by the numbers in a cubic anvil high-pressure and high-temperature apparatus. And the high pressure sintering property, microstructure, thermal conductivity, high pressure annealing, residual stress and sintering mechanism were discussed in detail.The experimental results confirm that HPS of AlN ceramics can effectively reduce sintering temperature, shorten sintering time, accelerate densification and reduce pollution and improving property. It has the characteristic of speediness, neatness and high densification, which is a new approach of AlN ceramics.The relative density of pure AlN ceramics sintered at 1300℃and 5.0 GPa for 50 min is 94.9%. The microstructure of the pure AlN ceramics sintered at high pressure is obviously different from the ceramics sintered at normal pressure. The pure AlN ceramics prepared at high pressure have fully dense, fine crystal grain, indistinct boundary and symmetrical microstructures. Controlling fracture mode was intragranular when the sintering temperature was as low as 1400℃under 5.0 GPa. The microstructure of single-phase equiaxed polycrystal in AlN body materials was formed at 1800℃and 5.0 GPa for 50 min. High pressure sintering is in favor of improvement of density and the enhancement of the thermal conductivity of AlN ceramics. The thermal conductivity of the AlN ceramics sintered at 5.0 GPa and 1800℃for 50min could reach 115 W/(m·K). The lattice parameters of AlN ceramics prepared by high-pressure sintering decrease slightly, unlike those of the AlN starting powder. The lattice parameters of the sample sintered at 5.0 GPa and 1700℃for 125 min were about 0.09 % lower than those of the AlN powder. Its prime reason is high pressure sintering.Because high pressure is applied and the high pressure cell is blocked out when the sample are sintered, it is unfit that Li₂CO₃, CaCO₃ etc are used as sintering additive which can be decompounded and give out gas at high temperature. Because some reaction that can give birth to gases don't carry through at high pressure, AlN ceramic grain boundary phase sintered at high pressure is different from one sintered at atmospheric pressure. The optimization content of La₂O₃ and the grain boundary phase is LaAlO₃. The relative density of the AlN bulk material with La₂O₃ as a sintering aid sintered at 1300℃and 5.0 GPa and for 50min is 96.3 %. The relative density of the AlN ceramics sintered at 1200℃and 5.0 GPa and for 50min with Y₂O₃ as a sintering aid is 95.3 %. Until now we have not found that the relative density of AlN ceramics sintered by above conventional methods can reach the value as low as the temperature 1200℃. The grain boundary phase of AlN ceramics with Y₂O₃ as a sintering aid is Y₄Al₂O₉ and YAlO₃ when the sintering temperature is low, but the one is Y₃Al₅O₁₂ when the sintering temperature is above 1600℃. The thermal conductivity of the A1N bulk material with Y₂O₃ as a sintering aid sintered at 1400℃and 5.0 GPa and for 75min and 1700℃is 45.6 W/(m·K) and 105.9 W/(m·K), respectively. The thermal conductivity of the samples increases with the increase of sintering time and temperature. It is well recognized that the oxygen in the A1N lattice, porosity, secondary phase composition and amount, impurities and defects in the AlN lattice all affect the thermal conductivity of the lattice.The HPS temperatures of AlN ceramics with Y₂O₃-Li₂O-CaO, Y₂O₃-Li₂O-CaF₂, Y₂O₃-CaO, Y₂O₃-CaF₂ etc. compound sintering aid are lower and its thermal conductivity is higher than the ones with single sintering aid. The thermal conductivity of the AlN bulk material with Y₂O₃-CaF₂-Li₂O as a sintering aid sintered at 1600℃and 5.0 GPa and for 75 min is 145.3 W/(m·K).The structural adjustment and performance improvement were put up through high pressure annealing in the first time. The density of the AlN samples increases with the increase of annealing time when the high pressure annealing time is less than 2 h. But the density of the AlN samples decreases with the increase of annealing time and anti-densification is come forth when the high pressure annealing time is above 2 h. The thermal conductivity of AlN ceramics annealed for 2h under high pressure is 173.2 W/(m·K) which is 2.2 times more than the one of the samples which didn't treat at high pressure. But The thermal conductivity of AlN ceramics annealed for 4h under high pressure cut down to 80.9 W/(m·K). The structural integrity and size of AlN grain increases with the increases of annealing time when the high pressure annealing time is less than 2 h. The pore become bigger and bigger along with the increases of annealing time and it can go against densification of AlN samples. In the process of high pressure annealing, the kinetics index of crystal growth is 3, the activation energy is Q=(50.6±9.6) kJ/mol and the mostly mechanism of matter migratory is pressure-enhanced volume diffusion.The residual stresses have been evaluated by Micro-Raman Spectroscopy in AlN ceramics sintered at high pressure at first. The residual compression stress increased according to the extension of the sintering time. Residual compression stress of the AlN ceramics sintered at 5.0 GPa and 1700℃for 125 min is 2.0 GPa. It is important to point out that the stress arises from lattice mismatch and distortion. And high pressure sintering is considered to be the main factor that would cause the lattice distortion of the ceramics. Annealing is an effective method which removes residual stresses in AlN ceramics.The relative density of AlN micron powder pressed bar at 5.0 GPa for 5min is 88.72 %. In other words, there are 'cold sintering' phenomenon when AlN micron powder were pressed under high pressure. The relative density of AlN pressed bar increases, but open pore decreases with the increase of high pressure. Obvious pressure-induced grain refinement is observed during the compacting of AlN powder under high pressure. The average particle diameter drops down to 1.47μm from 2.10μm when high pressure is added to 6.0 GPa from atmospheric pressure.In this paper, high pressure sintering mechanism has been studied. In the early stage of high pressure sintering, the sintering mechanism is mostly particle rearrangement, which consists of crystal grain refinement, grain boundary sliding, grain fragmentation and grain plastic deformation and flow. In the final-stage of high pressure sintering, the mechanism of densification was analyzed in terms of Nabarro-Herring vacancy creep model. In other words, the final stage of densification in high pressure sintering of AlN ceramics occurs by stress-directed vacancy diffusion.