Oxidation of aluminium nitride substrates

The growth of oxide films on two types of aluminium nitride substrates of different origin has been studied as a function of temperature. At a given set of oxidation reaction parameters, the oxide layers grown on substrates with a relatively large grain size and high concentrations of Y-Al-O-based liquid sintering aid phases (type I substrates) were observed to be thicker and more diffuse than those obtained on substrates with an average particle size of approximately 3 m and low liquid sintering aid concentrations (type II substrates). The effects of the oxygen partial pressure variation on the oxide film growth have been investigated for the oxidation of type II AIN substrates. The kinetics of the growth of oxide films on such substrates were analysed and determined to fit best to a linear rate law. This type of rate law indicates that the rate-limiting step in the growth of oxide films on high-quality type II aluminium nitride substrates is an interface reaction-controlled process.     

Hydrophobing of aluminium nitride powders

The hydrophobing of AIN powders through adsorption of capric acid, stearic acid and cetyl alcohol on the particle surface was investigated by statistical analysis. Stearic acid as surface adsorbent and cyclohexane as solvent were identified as the best combination for achieving highly effective hydrophobicity of AIN. The adsorption data obtained for this combination indicated a Langmuir chemisorption isotherm. Even after 96 h leaching in water, no crystalline phase other than AIN could be detected by X-ray diffraction (XRD).     

Wettability of aluminium nitride by tin–aluminium melts

The wettability of aluminium nitride by Sn–Al melts was studied by the sessile drop method in a vaccum of 2 × 10^–3 Pa at 1100 °C over the whole concentration region. The minimum interval on the contact-angle concentration dependence curve was observed at intermediate composition. For comparison, experiments were also performed on porous AlN. Wetting of porous nitride is worse than the dense nitride. The results have been analysed on the basis of the relation between wettability and the chemical interface reactivity in solid–liquid metal systems.     

Evaporated aluminium nitride encapsulating films

AIN films were deposited onto GaAs and vitreous carbon substrates held at room temperature, by reactive evaporation of aluminium in the presence of nitrogen and/or NH₃ gas mixture. These films and their combination with very thin layers of Si₃N₄ were successfully used as encapsulants for GaAs and were found to withstand annealing temperatures of up to 1100°C. Films grown by this novel method were analysed by Rutherford backscattering spectrometry and reflection high energy electron diffraction. Oxygen, nitrogen and aluminium were the only elements detected in the encapsulants. However, the best encapsulants were found to have the lowest oxygen content. The deposition conditions were found to be very important in preventing the reaction of the films with the surface of GaAs during heat treatment.     

Novel growth of aluminium nitride nanowires

This work describes novel growth of aluminium nitride (AIN) nanowires by nitridation of a mixture consists of aluminium and ammonium chloride powders (Al:NH4Cl = 1.5:1 weight ratio) at 1000 degrees C for 1 h in flowing nitrogen gas (1 l/min). XRD analysis of the product showed the formation of pure hexagonal AIN. SEM micrographs of as-synthesized product revealed the growth of homogeneous AIN nanowires (phi 40-150 nm). No droplets were observed at the tips of obtained nanowires which suggests that they were grown mainly by a vapor-phase reactions mechanism. Thermodynamic analysis of possible intermediate reactions in the operating temperatures range illustrates that these nanowires could be grown via spontaneous vapor-phase chlorination-nitridation sequences.     

Synthesis of oxygen-free aluminium nitride ceramics

The aluminum nitride raw material in the form of powder was synthesized using the Self propagating High temperature Synthesis (SHS) method which provides no oxygen impurities. Then AIN powder was sintered to the full density without sintering additives and under a high pressure in a belt apparatus. For the AIN ceramics obtained the temperature dependences of the thermal diffusivity were measured with the laser-flash method. Finally we produced oxygen-free aluminium nitride ceramics with parameters comparable with theoretical data.     

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 10² to 10⁵ Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.     

Oxidation of sintered silicon nitride

The influence of oxidation at 1200 °C in air for up to 1000 h on the mechanical properties of two Si₃N₄-Y₂O₃-Al₂O₃ materials with different Y₂O₃/Al₂O₃ ratios, Material A (Si₃N₄-13.9 wt% Y₂O₃-4.5 wt% Al₂O₃) and Material B (Si₃N₄-6.0 wt% Y₂O₃-12.4 wt% Al₂O₃), was investigated. The oxidation significantly improves the high-temperature strength and fracture toughness of both materials, but more for Material A. After oxidation, Material A at 1300 °C retains 93% of its room-temperature strength and 87% higher than that before the oxidation. The oxidation has a different effect on the room-temperature K _IC for the two materials. The room-temperature Weibull modulus of Material A decreased by more than half while the 1200 °C Weibull modulus decreased slightly after oxidation. The annealing treatment prior to oxidation had no effect on the high-temperature strengths of the materials after oxidation. The effect of oxidation on mechanical properties is discussed in terms of the microstructure change of the materials.     

Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

Thermal conduction mechanism of aluminium nitride ceramics

Extremely large grain size AIN ceramics were produced by HIP sintering at an ultra-high temperature of 2773 K without reducing the oxygen content in order to determine experimentally whether the factor controlling thermal conductivity is either grain boundaries or the internal structure of the grains. The room-temperature thermal conductivity of the HIPed AIN with a grain size of 40 m was 155 Wm^–1 K^–1, and was almost equal to that of the normally sintered AIN with a grain size of 4 m. Therefore, thermal conductivity at room temperature is independent of AIN grain size, or the number and amount of grain-boundary phase for reasonably well-sintered AIN ceramics. The calculated phonon mean free path of sintered bodies was 10–30 nm at room temperature, which is too small to compare with the AIN grain size. Consequently, it is shown that the thermal conductivity of sintered AIN is controlled by the internal structure of the grains, such as oxygen solute atoms.     

Oxidation mechanism of porous silicon nitride

The oxidation of reaction-bonded silicon nitride (RBSN) has been studied in air between 800 and 1500° C. The extent of internal oxidation is governed by the radius of the pore channels allowing the oxygen to penetrate the specimen. The velocity of oxygen transport into narrow channels is very low compared to the reaction rate of oxygen with Si₃N₄. Because of these two concurrent processes an oxygen gradient is built up along the channel axis leading to SiO₂ formation mainly at the channel mouth. The typical oxidation isotherm of RBSN is represented by an asymptotic law. The mass gain and the penetration depth of oxidation is calculated, based on reaction-rates of Si₃N₄-powder, oxygen-diffusion-data and the pore-characteristics of the RBSN-materials, and compared with the experimental results. The results clearly indicate, that high quality RBSN may wel be used in oxidizing atmosphere without extensive internal oxidation.     

Kinetics of densification during hot-pressing of aluminium nitride

Aluminium nitride powder has been hot-pressed at 1700° C under pressures in the range 5 to 30 MPa, and with additions of small weight percentages of alumina. Analysis of the densification kinetics shows two regimes. For pressures below 24 MPa the stress exponent of the densification rate is approximately 1.0 and the grain-size exponent is approximately 3, suggesting that grain-boundary diffusion is rate controlling. For pressures above 24 MPa the stress exponent is approximately 10, indicative of dislocation creep. At low pressures the existence of a threshold stress is apparent, values of which vary with the second-phase volume.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Morphologies and growth mechanisms of aluminium nitride whiskers

Morphologies of AlN whiskers grown by the vapour–liquid–solid mechanism (VLS) were investigated. Several types of whisker structures, such as growth hill, wavy, crossed and stack structures, were found due to the variation of growth conditions. Growth mechanisms and orientations of AlN whiskers were also studied. Besides preferential crystallographic planes, several other planes were found to be growth layers due to the perturbation of the AlN lattice change caused by dissolution of oxygen. A screw dislocation growth mechanism was clearly confirmed. An oblique growth mechanism was found in this work, which may be the result of two processes: the vapour–liquid–solid process and dissolution of oxygen. This revised version was published online in November 2006 with corrections to the Cover Date.     

Glass-ceramic interactions and thick-film metallization of aluminium nitride

A frit-bonded thick-film metallization for aluminium nitride (AIN) ceramic substrates has been developed. The glass system is thermodynamically stable with respect to AIN at the temperatures employed during thick-film processing. The model glass, a lithium borate, has comparable physical properties to that of the standard lead borosilicate glasses used in fritbonded films designed for oxide ceramics. The wetting properties of the glass on AIN, in both air and nitrogen atmospheres, have been determined by use of a hot-stage microscope. The low-temperature oxidation of the AIN surface was found to be a significant factor in the glass spreading rate. The glass was formulated into a palladium-silver thick-film metallization and the performance of this material on three types of AIN substrate was determined. Examination of the film morphology and the fracture surfaces was carried out using scanning electron microscopy.     

Growth of aluminium nitride whiskers by sublimation-recrystallization method

Aluminium nitride (AlN) with various morphologies was fabricated through a sublimation-recrystallization method. The influences of type of reactor and temperature gradient were explored, as well as the orientations and growth mechanism of the obtained AlN whiskers. In the early stage of preparation, a vapour-liquid-solid (VLS) mechanism dominated, producing AlN pillars, whiskers and noncrystalline fibres. In the later stage, as the catalyst liquid was removed by volatilization, the pillars and noncrystalline fibres stopped growing, but the growth of AlN whiskers continued through a vapour-solid (VS) mechanism. By Laue method and rotating-crystal method of x-ray diffraction, together with electron diffraction, most of the AlN whiskers were discovered to grow on planes , or , where l=0, 1, 2, 3, along crystal axes , or , where w=0, 1, 2, 3. Oblique grown whiskers also appeared, with a growth direction at an angle of about 54° to the growth plane, .     

Oxide film formation on aluminium nitride substrates covered with thin aluminium layers

The growth of oxide films on aluminium nitride substrates covered by vapour-deposited aluminium films of 1.5 and 4 m thickness has been studied in air at atmospheric pressure as a function of temperature. Oxide films were grown by oxidation in air at temperatures between 800 and 1300°C. The kinetics of the growth of oxide films on such substrates was observed to be complex. In particular, there are three subsequent periods of observed oxide growth: (1) an initial period of rapid oxide growth, (2) an incubation period with very slow oxide growth, and (3) a second period of relatively fast oxide growth.