AlN is one of the promising materials for ceramic substrate and packaging because of its remarkable properties ?? high thermal conductivity, high electrical resistivity, high dielectrical constant, similar thermal conductivity to silicon and so on. But there are some problems to take advantage of it. One of the problems is low sinterability because of its high covalent bonding ratio. Thus the special sintering conditions - high pressure, high temperature, sintering additive - are needed to sinter AlN. But high pressure and high temperature cause a cost problem in industries. So, the studies of pressureless sintering with sintering additives which are utilized to increase sinterability of AlN are conducted with active. Another problem is thermal conductivity which is inferior to its theoretical value, 320 W/mK. The main factors of this phenomenon are impurities and defects in grain, grain boundary etc. Using high purity raw powder, increasing the grain size and getting rid of the defects in grain are the solution. Thermal conduction in crystalline materials is considered as diffusion of phonon. Phonon is the main thermal carrier in crystalline materials like a ceramics. And understanding of phonon diffusion and scattering mechanism is essential to make sense of its thermal conductivity. Temperature difference between two sides, front and real is the driving force of diffusion of phonon which is understood with Fick’s law. One of the constant in Fick’s law is Diffusivity which is also one important factor of the equation of thermal conductivity. Three factors, density, thermal diffusivity, and specific heat are needed to define thermal conductivity. The phonon scattering in crystalline reduces its mean free paths which, results in decrease of thermal diffusivity. Consequently, increase of phonon scattering in crystalline lead to decrease of thermal conductivity. there are three phonon scattering mechanism, phonon-defect, phonon-grain boundary, and phonon-phonon. The last one is related to thermal vibration of lattice. So, it could be ignored in room temperature. In this study, control the remaining two factors are attempt with sintering additives and co-additive even at low temperature. In detail, to archive the low temperature and pressureless sintering with sintering additive, $Y_2O_3$, CaO, BaO, $La_2O_3$ were used as sintering additives. And co-sintering additive, $M_2O_3$, was added to decrease the eutectic temperature of each composition. The stability of $2^{nd}$ phase of each composition and decreasing of eutectic temperature were assured with thermodynamics view. All additive systems in this study were confirmed having low temperature below 1550 $\circ C$ and making stable $2^{nd}$ phases during the sintering.
Archimedes method was used to define the relative density of sintered body at various sintering temperature and times. It was confirmed that full density was archived at 1500 $\circ C$, and after then it was sustained in all conducted experiment condition based on the results. So, the additive systems were effective to archive low temperature sintering around 1500 $\circ C$ which is below 100 $\circ C$ than recently reported data. Grain growth and $2^{nd}$ phase distribution were confirmed with SEM Images. Grain growth was observed at all composition which means that the mass transport was active during the sintering. And distribution of $2^{nd}$ phases was not same but homogeneous. Trapped in triple junction and isolated each other are the best case to increase thermal conductivity. And the $2^{nd}$ one is interconnected channel throun grain edges. These two cases were observed with SEM Images and analyzed the relation with thermal conductivity.
The thermal diffusivity was measured with laser flash method. And thermal conductivity was calculated. Thermal conductivity, over 100 W/mK, was attained at 1500 $\circ C$, 3 hour sintering condition. And very high thermal conductivity, 172 W/mK, was attained at 1500 $\circ C$, 7 hour sintering condition. The Raman peak width has relation with defect concentration. Thermal conductivity also has relation with defect concentration. So, there are some reports about the relation between Raman peak width and thermal conductivity. In this study, Raman spectroscopy was used to characterize the concentration of oxygen-related defect which has relation with thermal conductivity. Based on the results, there are close relation between thermal conductivity and oxygen-related defect. And the thermal conductivity was resulted in decrease of concentration of oxygen-related defect.
Electrical resistivity was characterized with Impedance spectroscopy. Ceramics, like an AlN has high resistivity and constantly decrease with increasing temperature. AlN has too high electrical resistivity to measure. So the measurement should have conducted in high temperature. And electrical resistivity at room temperature was expected based on its nature. In this study, the electrical resistivity of grain and grain boundary could be analyzed. And it was characterized that all considered composition have enough electrical resistivity for application, over $10^{-7} \Omega cm)^-1$ at room temperature.
질화 알루미늄은 높은 열전도성과 전기절연성, 높은 유전상수, 그리고 실리콘과 유사한 열팽창계수 등의 특성을 바탕으로 세라믹 기판 소재로의 응용을 위한 연구가 활발히 진행되고 있다. 하지만 질화 알루미늄을 실제로 응용하는데 있어서는 몇 가지 대표적인 어려움이 있다. 첫째로 강한 공유결합성 물질로서 소결이 어렵다. 이를 해결하기 위해 고온, 고압 및 소결 첨가제 등의 소결 조건이 필요하다. 하지만, 고온, 고압의 소결 분위기를 유지할 경우 생산원가가 비싸지기 때문에 상업적으로 응용하는데는 어려움이 있다. 1981년 K. Komeya의 논문을 통해 다양한 소결 조제를 이용한 질화 알루미늄의 소결 가능성이 보고된 이 후 소결 조제를 이용한 액상 소결에 대한 연구가 활발히 진행되었다. 다음으로 소결체의 열전도도가 이론값인 320 W/mK에 크게 못 미치는 문제가 있다. 이는 소결체 내부에 존재하는 불순물과 입계, 이차상 등의 영향에 의한 것으로 알려져 있으며 고순도의 초기 분말을 사용함으로써 개선시킬 수 있는 것으로 알려져 있다. 특히 입내에 존재하는 산소 불순물이 질화 알루미늄의 열전도도를 떨어뜨리는 주된 원인으로 알려져 있으며, 소결 중에 형성된 액상을 통한 물질 이동으로 이 산소 불순물의 농도를 낮출 수 있는 것으로 선행 연구들을 통해 보고 되고 있다.
본 연구에서는 열역학적인 고찰과 선행 연구 결과들을 통해 소결 조제를 선정하고 보조 소결 조제를 이용하여 이들의 액상이 형성되는 온도를 낮추어 저온 소결을 시도하였다. 소결체의 열전도도를 측정하기 위해 Laser flash method를 이용해 열확산계수를 구하여 밀도와 비열의 곱으로 열전도도를 계산하였으며, 형성된 액상을 통해 입내의 산소 불순물이 효과적으로 제거되었는지를 확인하기 위해 Raman spectroscopy를 이용하여 분석하였다. 그리고 소결체가 충분한 전기 절연성을 갖는지 확인하기 위해 Impedance spectroscopy를 이용해 전기전도도 및 비저항을 측정하였다.
