AlN SiC Solid Solution Research Articles

Growth of SiC and SiC-AlN solid solution by container-free liquid phase epitaxy

SiC and SiC-AlN solid solution have been grown by container-free liquid phase epitaxy (CFLPE) from the silicon melt suspended in a high frequency electromagnetic field. Substrate temperature was 1450–1700°C. The substrates were 6H-SiC crystals with a {0001} basal-plane orientation. Aluminum (acceptor) and nitrogen (donor) have been used as impurities. Growth rate of 6H-SiC layers was controlled in the range of 0.02 to 2 μ / min. Layers were single crystal. The concentration N d − N a was varied in the range of 8 × 10 15 to 1 × 10 19 cm −3. For p-type layers, aluminum concentration was controlled from 1 × 10 18 to 2 × 10 20 cm −3. Heteroepitaxial 3C-SiC layers have been grown on 6H-SiC substrates from the liquid state. Minimum half-width of the X-ray rocking curve of 3C-SiC layer was 11.5 arc sec. A red-light-emitting diode has been fabricated based on a 3C-SiC/ 6H-SiC p-n heterojunction. Single crystal SiC-AlN solid solution layers with AlN concentration up to 10 mol% have been grown by CFLPE.

Microstructure and Mechanical Properties of SiC-AlN Ceramics after Phase Separation Treatment

The microstructure and mechanical properties of SiC-AlN solid solution ceramics and the ceramics after phase separation treatment were evaluated in relation to the temperature and time of annealing. The SiC-AlN (SiC/AlN=70/30) solid solution ceramics with the addition of Y2O3 and Al2O3 were prepared by hot-pressing at 2200°C under 22MPa for 8h in Ar, which were followed by a subsequent heat-treatment in the miscibility gap at a temperature of 1600°C or 1800°C up to 120h in Ar. The modulated structure characteristic of the spinodal decomposition was developed within crystalline grains by the phase separation treatment. The fracture toughness was enhanced by the phase separation at both 1600°C and 1800°C for 8h. The hardness also increased with the phase separation treatment. The changes of these properties are considered to be dependent on the introduction of a modulated structure in the crystalline grains. Both the solid solution ceramics and the ceramics with phase separation exhibited an increase in crack resistance with crack extension. This phenomenon suggests that the crystalline structure of solid solution or the modulated structure contributes effectively to the shielding of crack extension. Although the modulated structures which consist of phases with different Young's modulus and linear thermal expansion coefficient were introduced to the crystalline grains by the phase separation treatment, these two properties of the ceramics were independent of the phase separation.

Mechanical and Thermal Properties of SiC-AlN Ceramics with Modulated Texture

The mechanical and thermal properties of SiC-AlN ceramics were examined in terms of the development of the modulated texture. The SiC-AlN solid solution ceramics with the addition of Y2O3 and Al2O3 were prepared by hot-pressing method, which were followed by the subsequent heat treatment to develop the modulated texture in the miscibility gap at the temperatures of 1600°C or 1800°C. It was found that the flexural strength and Weibull modulus of the ceramics (nominal composition: 70mol% SiC/30mol% AlN) increase from 589 to 674MPa and from 7.6 to 10.5 with the phase separation treatment at 1600°C for 8h. Thermal shock resistance is also enhanced by the phase separation. The addition of AlN to SiC causes a decrease in thermal conductivity. The phase separation does not influence the thermal conductivity of the SiC-AlN solid solution ceramics. The mass gain of the SiC-AlN ceramics due to the oxidation under 1400°C in air is lower than that of SiC ceramics.

Development of a tough cemented carbide cutting tool with an iron based binder: M.V. Deshpande et al, (Sandvik Asia Ltd, Poona, India)

Two hafnium diboride based composites, respectively, containing 20 vol%SiC and 20 vol%SiC–10 vol%AlN, were hot-pressed. Microstructures and mechanical properties were investigated and the thermal stability over 2000 °C was evaluated by oxyacetylene torch. Results indicated that the addition of AlN greatly improved the powder sinterability by eliminating the oxygen contamination. The mechanical properties for HfB2–SiC–AlN composite, especially the flexural strength were enhanced remarkably through the improvement in density and the formation of AlN–SiC solid solution. Oxyacetylene ablation results indicated that the addition of AlN also improved the thermal stability of composite under severe oxidation conditions. The ablation behavior was investigated and the ablation mechanism was discussed.

Cutting performance of cermets and relevant material properties: M. Fahrmann et al, (Frauenhofer Institution for Ceramic Technology, Dresden, Germany)

Two hafnium diboride based composites, respectively, containing 20 vol%SiC and 20 vol%SiC–10 vol%AlN, were hot-pressed. Microstructures and mechanical properties were investigated and the thermal stability over 2000 °C was evaluated by oxyacetylene torch. Results indicated that the addition of AlN greatly improved the powder sinterability by eliminating the oxygen contamination. The mechanical properties for HfB2–SiC–AlN composite, especially the flexural strength were enhanced remarkably through the improvement in density and the formation of AlN–SiC solid solution. Oxyacetylene ablation results indicated that the addition of AlN also improved the thermal stability of composite under severe oxidation conditions. The ablation behavior was investigated and the ablation mechanism was discussed.

小型パンチ試験法によるSiC-AlN系固溶体と複合体の高温強度の評価

小型パンチ試験法によるSiC-AlN系固溶体と複合体の高温強度の評価

ＳｉＣ‐ＡｌＮ系単相アロイと２相コンポジットの機械的性質

In an attempt to strengthen and toughen silicon carbide ceramics, fully dense SiC-AlN alloys containing up to 25mol%AIN were fabricated by hot-isostatic pressing of the reaction-synthesized SiC-AlN solid solution powders. It was found that the solution of AlN yields much higher strength (up to 1000MPa at 5mol%AlN), and increases fracture toughness of SiC. In the SiC-AlN composites prepared by hot-isostatic pressing of the SiC and AlN elememtal powder mixtures, the addition of AlN particales improves fracture toughness more effectively in comparison with the single phase alloys.

Mesure des capacites calorifiques et des enthalpies de la solution solide AIN-SiC

The AlN-SiC solid solution was prepared by a reaction of powdered silicon, carbon and aluminium nitride at 2000°C. In the molar ratio range from 1 AlN -0 SiC to about 0.5 AlN -0.5 SiC high purity phases with wurtzite structure were obtained.

The formation of SiC-AlN solid solution by the carbothermal reduction process of montmorillonite

It is well known that a solid solution forms in the a-SiC (2H)-AIN system over a wide composition range, because both of them have wurtzite (2H) structures [1]. The lattice parameters of the solid solution vary with the composition. It has been reported that their behaviour approximately follows Vegard's law [2]. Although SiC-A1N solid solutions are prepared by hot-pressing of both powders [3], they can also be obtained directly by the carbothermal reduction of oxides having the SiO2-A1203 compositions in nitrogen. In the carbothermal reduction process, the intimate mixing of SiO2, A1203, and carbon is desirable. So far, two attempts at the preparation of solid solutions by carbothermal reduction have been reported; SiC-A1N from the mixture of fumed silica, AI(OH)3, and starch [4, 5], and SiC-A1N-A12OC from acidtreated kaolinite and starch [5]. Aluminosilicate materials, for example kaolinite and zeolite, consist of silicon, aluminium and oxygen, which are mixed at the atomic level. Of these, montmorillonite shows a layered structure and takes up a number of organics in its interlayer space to form "intercalation compounds" [6]. In montmorilloniteorganic intercalation compounds, silicate and organic layers are interstratified alternately. Hence, the intimate mixing between SiO2, A1203, and carbon is expected after their thermal transformation into a nonoxidative atmosphere. We prepared n-alkylammoniummontmorillonite-polyacrylonitrile (PAN) intercalation compounds and applied these to the carbothermal reduction processes for fl-sialon [7-9] and SiC production [10]. In such processes, the intimate mixing and the two-dimensional ordering in the intercalation compounds affected the reactions to a great extent. In the present study, the preparation of a SiC-A1N solid solution from the montmorillonitePAN intercalation compound by carbothermal reduction was attempted. To investigate the effect of the mixing condition on solid solution formation, montmorillonite-carbon mixtures were also converted. The amount of carbon in the mixtures was varied to study its influence on solid solution formation. The method for the preparation of the intercalation compound was described elsewhere [7]. The compound after thermal pre-treatment at 220°C was characterized by X-ray powder diffraction (XRD) analysis (Rigaku, RAD II-A, Mn filtered FeK~ radiation) and thermogravimetry (TG) (Shimadzu, TGA-20); the basal spacing was 1.75nm and the amount of carbon from PAN approximately corresponded to that in the mixture containing 53 wt % carbon. The procedure for the preparation of the mixtures was the same as described elsewhere [10]. The carbon contents were 40, 50, and 60 wt %. These indicate that the calculated C/SiO2 molar ratios were 5.8, 8.2, and 12.0, respectively; the C/A1203 ratios were 21, 37, and 55, respectively. Both kinds of starting materials were placed in a graphite boat, and heated in a nitrogen flow (flow rate: 900 ml min~) for 0 to 10 h. Crystalline phases were identified by XRD analysis after decarbonization at 650 ° C for 6 h. The characterization of 2H compounds was based on the profile and the position of their (1 0 0) lines. The position was precisely measured by using NaC1 as an internal standard and compared with those calculated from the reported d values for a-SiC (2H) [11] and A1N [12] (a-SiC (2H), 42.56°; A1N, 42.13°). The composition was approximately estimated from the lattice parameter, a, which was calculated from the position of the (1 00) lines on the basis of the reported relationship [2]. The Si/A1 ratio in the products was obtained by inductively-coupled plasma emission spectroscopy (ICP) (Nippon Jarrell Ash, ICAP-575 II). The products were dissolved in water by alkali fusion. Details were described elsewhere [8]. The reaction temperature was fixed at 1670 ° C in the present study on the basis of our primary results. The presence of the boundary temperature above which SiC formed instead of Si3N4 was reported at ~ 1400 to 1600°C in the SiO2-C-N2 system [13, 14]. A similar one was observed in the carbothermal reduction process of kaolinite [15]. Hence, the reaction temperature was initially set at 1700°C in our primary runs. When the mixture containing 40% carbon was heated, a large amount of/~-SiC and smaller amounts of the 2H compounds were detected. Two (1 0 0) lines of the 2H compounds were observed and one was ascribed to almost pure A1N (detailed discussion for the assignment was similar to that shown below). As heating time increased, the peak intensity of the (1 0 0) line of A1N remarkably decreased; A1N seemed to decompose at 1700 ° C. Thus, the~eaction temperature was lowered to 1670°C. In the reactions at 1670°C, the peak intensity of the (1 00) line of A1N did not remarkably decrease. In the XRD patterns of the products obtained by heating the intercalation compound for 0 to 10h,

Solid solutions and composites in the SiCAlN and SiCBN systems

The effects of AlN and BN additives on hot-pressed SiC were investigated. SiCAlN diffusion couples were also studied. Analytical scanning transmission electron microscopy was used to determine the phases in contact, the local compositions and the composition gradients. Approximate diffusion depths range from 0.5 μm at 1950°C to 15 μm at 2300°C (both for 1 h), and the corresponding diffusion coefficients are of the order of from 1.5 × 10 −12 to 6 × 10 −10 cm 2s −1. The slow diffusion rates and the added β-to-α nucleation barrier cause considerable inhomogeneities in the high temperature hot-pressed SiCAlN solid solutions. The diffusion rates at the lower temperatures, although small, may lead to dissolution of small particles in an SiCAlN composite. The solubility of BN in βSiC under conventional hot-pressing conditions is small and was not directly measured. BN stabilizes βSiC and low hexagonality polytypes.