AlN Ceramics Research Articles

Quantitative study of the enhancement of the thermal conductivity of AlN ceramics bynanoscale processing

We have theoretically studied and quantitatively analysed the enhancement of thethermal conductivity of AlN microceramics (i.e. commercially available powder)by the insertion of AlN nanosized particles of high-specific surface area, and ofY2O3 and CaO as sintering additives. We have also studied the enhancementof the thermal conductivity of AlN nanoceramics by usingY2O3 as the sintering additive. Thermal conductivity calculations have been carried out byapplying the Callaway theory in its full form and by incorporating a detailed and accurateaccount of three-phonon processes. The role of densification of the AlN ceramic samples inenhancing the thermal conductivity has been quantified at low, intermediate and hightemperatures. In addition to explaining the experimentally observed room-temperatureconductivity results, the conductivity variation has been predicted over a large temperaturerange.

Optimum sintering conditions for optical properties of translucent aluminum nitride ceramics

High translucency is one of the excellent properties of AlN ceramics because of its wide optical band gap energy of 6.2 eV. We have achieved success in producing AlN ceramic tube of 98% total visible light transmittance at 0.6 mm thick tube wall by applying an improved sintering technique. This AlN ceramic was produced by sintering at 1880°C using Ca3Al2O6 as a sintering additive and in reduction atmosphere to remove the sintering additive from the final sintered material. After the sintering, the annealing with Al2O3 was carried out to improve the transmittance of AlN ceramics. The amount of metal impurities and carbon were 27ppm and 100ppm, respectively. It is noteworthy that the amount of oxygen in AlN ceramics after the annealing was 0.36wt% which was higher than the 0.07wt% of the as-sintered AlN ceramics. Therefore, total transmittance was improved by the oxygen doping. On the other hand, thermal conductivity of AlN after the annealing was lowered by 35% compared with that of as-sintered AlN. Positron Annihilation Lifetime (PAL) measurement was applied to investigation of a relationship between the total transmittance and lattice defects. The present PAL results confirms the presence of negatively charged atomic-size vacancies that is aluminum site vacancies and also suggests the increase in the concentration of the vacancies after the annealing compared with that of the as-sintered material. However, it can be concluded that there is no relation between aluminum site vacancies and the light transmittance of AlN ceramics.

Sintering, high temperature strength and oxidation resistance of liquid-phase-pressureless-sintered SiC–AlN ceramics with addition of rare-earth oxides

SiC–AlN composites can be pressureless sintered with the addition of rare-earth oxides (RE = Y, Yb, Er, Lu, Ho, Sm, Ce) and without protective powder bed. Sintered bodies showed high density (≥97% T.D.) and were mainly composed by 2H SiC–AlN solid solution with additive containing grain boundary phases. The high temperature strength and oxidation resistance of the composites were also investigated and correlated with the cationic radii of the oxide additives. The flexural strength at 1500 °C increased from 269 MPa to 587 MPa as direct consequence of the rare-earth cationic radius decrease. The specific weight gain during oxidation at 1500 °C for 200 h showed the same behaviour, decreasing from 12.60 mg/cm 2 to 3.13 mg/cm 2.

Phosphoric acid treated AlN powder for aqueous processing of net-shape dense AlN and β-SiAlON parts

Due to fast hydrolysis of AlN when in contact with water, AlN based ceramics are processed in organic solvents, which are volatile, expensive, harmful to the health and environment. The present work intends to protect AlN powder against hydrolysis in order to enable aqueous processing of AlN based ceramics. A commercial AlN powder was treated in an ethanol solution of H3PO4 and Al(H2PO4)3 kept at 80°C for 24 h to protect it from hydrolysis. The dispersing behaviour of the treated AlN powder in water and the ability to consolidate defect free parts of AlN and β-Si4Al2O2N6 ceramics from aqueous suspensions by slip casting and gelcasting techniques were studied. The consolidated parts were then sintered in a graphite furnace and the sintered bodies exhibited properties [bulk density (BD), apparent porosity (AP), crystalline phase composition and microstructural features] similar to those reported in literature for the same materials processed from organic media.

Fabrication and Properties of Aluminum Nitide Ceramics Doped Carbon Nanotubes

Aluminum nitride ceramics doped carbon nanotubes were fabricated by dispersed multi-walled carbon nanotubes (MWNTs) into AlN powders homogenously. The densification, electric resistance, thermal conductivity and relative dielectric constant of AlN ceramics were investigated. It was found that MWNTs additives could purify the grain boundaries. Furthermore, it induced the increase of the thermal conductivity and the decrease of the relative dielectric constant. However, the linear shrinkage and electric resistance decreased with increasing MWNTs amount. AlN ceramics with high thermal conductivity and low relative dielectric constant were obtained without apparent decrease of electric resistivity.

高熱伝導性AlNセラミックスのミリ波焼結

高熱伝導性AlNセラミックスのミリ波焼結

Ca-Y-Al-O系焼結助剤を用いたAlNの低温焼結

Ca-Y-Al-O系焼結助剤を用いたAlNの低温焼結

Y2O3첨가가 AlN 세라믹스의 방전 플라즈마 소결 거동 및 열전도도에 미치는 영향

Spark plasma sintering (SPS) of AlN ceramics were carried out with Y₂O₃ as sintering additive at a sintering temperature 1,550~1,700℃. The effect of Y₂O₃ addition on sintering behavior and thermal conductivity of AlN ceramics was studied. Y₂O₃ added AlN showed higher densification rate than pure AlN noticeably, but the formation of yttrium aluminates phases by the solid-state reaction of Y₂O₃ and Al₂O₃ existed on AlN surface could delay the densification during the sintering process. The thermal conductivity of AlN specimens was promoted by the addition of Y₂O₃ up to 3 wt% in spite of the formation of YAG secondary phase in AlN grain boundaries because Y₂O₃ addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. However, the thermal conductivity rather decreased over 3 wt% addition because an immoderate formation of YAG phases in grain boundary could decrease thermal conductivity by a phonon scattering surpassing the contribution of Y₂O₃ addition.

Investigation of isothermal oxidation of AlN ceramics using different kinetic model

The oxidation kinetics of AlN ceramics under the conditions of different rate-controlling steps existing has been discussed using two kinds of models (the model used in the literature and Chou model) based on the results reported in literatures. The analysis shows that though both models can be used to describe the oxidation behavior of AlN ceramics, Chou model is preferred. It is because, not only Chou model can give a clear physical meaning in expressing all parameters that will be convenient for theoretical discussion, but also it will perform a simple calculation. The most important thing is that the new model will lead to a more accurate calculation result.

Fabrication of AlN Ceramics Using AlN and Nano-Y<sub>2</sub>O<sub>3</sub> Composite Particles Prepared by Mechanical Treatment

AlN ceramics used in electronic substrates and packages are fabricated by the densification of green bodies with sintering aids such as rare-earth oxides. The homogeneous dispersion of the sintering aids, achieved with the help of the mechanochemical bonding of several types of fine powders, is the key process for obtaining a good sinterability and high performance. In this study, we fabricated the AlN ceramics using AlN and nano-Y2O3 composite particles prepared by mechanical treatment. The AlN powder and nano-Y2O3 powder were ball-milled with Al2O3 balls and a dispersant in ethanol in a plastic pot. The powder mixture of AlN and Y2O3 was composited by mechanical treatment. The composite powder was granulated and pressed to obtain a green body. After dewaxing, the AlN green body was fired at 1800°C in 0.6 MPa N2. The sintered body possessed a fracture toughness of 3.6 Pa•m1/2, higher than that, 3.1 Pa•m1/2, of the AlN ceramics fabricated without the mechanical treatment. An observation of the fractured surface revealed that grain boundary reinforcement enhances the fracture toughness of the AlN ceramics made of composite particles.

Chemical compatibility of a TiAl-Nb melt with oxygen-free crucible ceramics made of aluminum nitride

The problem of uncontrolled oxygen contamination of intermetallic TiAl ingots is considered for the application of crucibles and molds based on traditional oxide ceramics. A synthesized Ti-45.9Al-8Nb (at %) alloy is solidified in alternative oxygen-free crucibles made of high-purity aluminum nitride (99.99% AlN) upon holding at 1670°C for 5, 12, and 25 min and subsequent quenching in a high-purity argon atmosphere. The initial material and the solidified ingots are studied by scanning electron microscopy, optical microscopy, X-ray diffraction, electron-probe microanalysis, and gas-content chemical analysis. The key features of the interaction of the TiAl-Nb melt with AlN ceramics are revealed. Partial thermal dissociation of the crucible material according to the reaction AlN → Al + N and the reaction of atomic nitrogen with the melt lead to the formation of a solid 6.4-μm-thick TiN coating on the ingot surface and provide perfect wettability of the crucible by the melt and easy removal of solidified casting items from the mold. The TiN coating serves as a diffusion barrier that hinders the diffusion of nitrogen and residual oxygen from the pores in the crucible toward the melt. As a result, no oxide particles are detected in the ingots. However, few single microprecipitates of two nitride phases ((Ti,Al)xNy, NbN) are detected in the near-bottom region, 300 μm thick, in the alloy after holding at 1670°C for 25 min. The total oxygen contamination in a two-phase α2 + γ ingot does not exceed 1100 wt ppm, which is 1.5–2 times lower than that obtained in the experiments performed with modern advanced oxide crucibles made of yttrium ceramics Y2O3. AlN is shown to be a promising crucible material that can be considered as an alternative to oxide ceramics in the metallurgy of TiAl intermetallics.

Effect of Second Phase After-Heat Treatment on the Thermal Conductivity of AlN Ceramics

The high thermal conductivity of Aluminum nitride, coupled with its high electrical resistivity and nontoxic nature, makes it a very promising material for electronic substrate. In this study, microstructural characterization on the thermal conductivity of AlN ceramics was investigated. An AlN ceramic was prepared with a dopant Y2O3 under a reducing nitrogen atmosphere with carbon. In order to obtain high thermal conductivity, cooling rate control and after-heat treatment was carried out. Morphology of the second phase was characterized using scanning electron microscopy (SEM). SEM studies showed that the microstructural change caused by after-heat treatment have a major influence on the thermal conductivity.

Interactions between AlN and SiAlON Ceramics

In the present study, interactions between AlN and SiAlON laminated couples were investigated after gas pressure (GPS) and spark plasma sintering (SPS) by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) with the aim to produce laminated composites. In the laminated couples sintered by GPS, a significant reaction zone (~100-150 μm), containing a high aspect ratio of elongated polytypoid grains, was observed at the interface. However, in the case of laminated couples sintered by SPS, a considerably thin reaction region (~2-3 μm) was observed, elongated polytypoid grain formations were also detected.

Thermal Conductivity Measurement of the AlN Ceramics at the Grain Scale Using Thermoreflectance Technique

The thermal conductivity at the micrometer-scale of AlN ceramics with eliminated grain boundary phase was measured by the thermoreflectance technique with periodic heating. Thermal conductivities were ranged from 160 to 260 W/m•K and an average value of 201 W/m•K was obtained from a 22 m2 area. The variation in the thermal conductivities was related to the individual AlN grains and grain boundary characteristics.

Nitride & Oxy-Nitride Ceramics for High Temperature and Engineering Applications

Nitrides and oxynitrides of silicon and aluminum occupy prominent positions among other non-oxide members of ceramics on account of their inherent unique combination of properties arising out of their directional covalent bonding in the condensed state. Since many reviews could be found in the literature on the materials either individually or combining some of them, the present article is devoted to the key areas of the present research on the developments of these materials. For example, α’ – SiAlON is now attracting significant attention and the article highlights how compositional variations could lead to easy densification and improvement of its mechanical properties. Experimental results from various authors show that the mechanism of stabilization of α’ – SiAlON is very complex and cannot be explained on the basis of the size of the metal cations and solubility in the transient glassy phase. Y, Yb, Dy and to some extent Ca are good for stabilization of the phase. Evolution of different phases during temperature rise and how these phases affect sintering of the α’ material has been presented in this article. In-situ formation of elongated β’ grains have been shown to increase the toughness of the material. Simultaneously, the development of AlN ceramics in regard to application of this ceramic in different areas apart from that of microelectronics has been discussed. +Improved mechanical and electrical properties of AlN and its composites have shown their promise in diverse applications such as armors, electron tube components and dielectric devices.

Role of additives in enhancing the thermal conductivity of AlN ceramics

The thermal conductivity of AlN ceramics with different amounts of Y2O3 as sintering additive has been calculated at low, intermediate and high temperatures by applying the Callaway theory and using a detailed account of three-phonon scattering processes. By making a quantitative analysis of the results, it is found that the increase in the room-temperature conductivity due to the addition of Y2O3 is largely attributed to the decrease in the oxygen impurities level in the AlN crystal. In addition to matching the experimental measurements (available in a limited temperature range) our theoretical work predicts the resulting thermal conductivity over a large temperature range.

Electron Spin Resonance of Defects Related to Thermal Conductivity in AlN Ceramics

Defects of AlN ceramics with a thermal conductivity from 102 to 216 W m-1 K-1 are investigated using electron spin resonance (ESR). Two ESR signals at g=2.006 and 1.999 are related to thermal conductivity. The g=1.999 signal intensity increases with a decrease in thermal conductivity, an increase in oxygen concentration, and an increase in Mg concentration. The g=1.999 signal is tentatively supposed to be caused by an electron-trapped center. The g=2.006 signal intensity is increased with an increase in thermal conductivity, a decrease in oxygen concentration, and an increase in Ca concentration. A defect responsible for the g=2.006 signal is assigned as a hole trapped in an ON–VAl complex.

Structure and properties of AlN ceramics prepared with spark plasma sintering of ultra-fine powders

The AlN powders with grain size about 100 nm were synthesized by low-temperature carbon thermal reduction method. The synthesized powders without any additives could be completely densified through spark plasma sintering (SPS) technique above 1470 °C in vacuum under the pressure of 40 MPa. The properties of AlN ceramics sintered at different temperature and pressure were investigated. The results indicated that the density of SPS sintered AlN ceramics decreased with the reduction of sintering pressure and temperature. The grain size of the sintered samples reduced from about 1 μm to 200 nm as the sintering pressure and temperature decreased from 40 MPa and 1600 °C to 30 MPa and 1500 °C. No inter-granular phase was found in the microstructure of sintered samples, which resulted in a strong grain bond and consequently improved the mechanical properties especially the fracture toughness. Moreover, the effects of the grain size and porosity on the mechanical properties of the sintered samples were also discussed.

Effects of Additives of CaO and Rare-Earth Oxides on the Sintering Behavior of AlN Ceramics

Effects of Additives of CaO and Rare-Earth Oxides on the Sintering Behavior of AlN Ceramics

Thermal conductivity of single crystal and ceramic AlN

We have applied the Callaway theory and used a detailed account of three-phonon scattering processes to calculate the thermal conductivity of three AlN single crystal samples containing different amounts of oxygen and two AlN ceramic samples with different grain sizes and oxygen contamination levels. The N-drift contribution to the total conductivity has been quantified. The influence on the thermal conductivity of oxygen-related defects, and grain boundaries in ceramic samples, has been investigated. The theoretical results obtained from this work are in good agreement with available experimental data. Our calculations suggest that the “effective” boundary length is greater than the reported grain size for each of the two ceramic samples studied by Watari et al. [J. Mater. Res. 17, 2940 (2002)].