EELS Study of Oxygen Diffusion in Aluminum Nitride

The oxygen/nitrogen interdiffusion in AIN ceramics was investigated by electron energy-loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM). AL₂O₃/AIN diffusion couples were prepared by oxidation of an AIN ceramic. The samples were annealed for different times in the temperature range from 1500° to 1900°C. The diffusion couples were encapsulated in a tantalum ampoule to guarantee an inert atmosphere. Oxygen concentration profiles across the oxide/nitride interface were measured by EELS. The oxygen/nitrogen interdiffusion coefficient in AIN is given by D = 1.5_–1.2^+6.0× 10^–8 exp[–240 ± 40 (kJ/mol)/( RT )] (cm²/s). The magnitude and temperature dependence of the N/O interdiffusion is comparable to diffusion coefficients reported in the literature for other non-oxide ceramic materials. At standard AIN sintering conditions the O/N interdiffusion is so slow that it cannot provide an effective means for oxygen removal from the AIN grains.     

Interfacial Reaction between Titanium Thin Films and Aluminum Nitride Substrates

Interdiffusion and reaction at the interface between titanium thin films and AlN have been studied by using Rutherford backscattering spectrometry and transmission electron microscopy. Ti₂AlN is formed as a result of reaction with titanium and AlN at temperatures of 800°-950°C. The activation energy for Ti₂AlN formation in the temperature range of 800°-850°C is 224 kJ/mol, which is similar to that of nitrogen diffusion in titanium. Therefore, the formation of Ti₂AlN is believed to be controlled by the diffusion of nitrogen in titanium.     

High Thermal Conductivity Aluminum Nitride Substrates Prepared by Aqueous Tape Casting

High thermal conductivity aluminum nitride (AlN) substrates were prepared by aqueous tape casting. The characteristics of surface-treated AlN powder were studied in aqueous ball-milling media. The oxygen content of AlN powder with the dispersant was lower than that of AlN powder without the dispersant at the same ball-milling time. The isoelectric points of the surface-treated AlN with and without a dispersant were, respectively, at pH∼3.35 and pH∼3.90. The atmosphere had considerable effects on organic additive burnout of aqueous AlN green sheets. The composition of grain boundaries changed with increasing holding time at 1850°C. A translucent AlN substrate with a uniform microstructure and a thermal conductivity of 263 W·(m·K)⁻¹ was obtained by pressureless sintering at 1850°C for 6 h in a nitrogen atmosphere.     

Comparison of Aqueous and Non-Aqueous Tape Casting of Aluminum Nitride Substrates

A direct comparison of aqueous and non-aqueous tape casting was investigated. Aqueous and non-aqueous formulations were developed for tape casting of aluminum nitride (AlN) powder. The oxygen content of the AlN powder in aqueous ball-milling media had a slight increase, but it did not almost influence the thermal conductivity of AlN substrate. The solid loading of aqueous AlN slurry was higher than that of the non-aqueous one, but its viscosity was lower than that of non-aqueous AlN slurry. Under the same burnout program, the residual carbon content in aqueous AlN green sheet was lower than that of the non-aqueous one. The thermal conductivity of the aqueous AlN substrate sintered at 1850°C for 3 h was 138 W·(m·K)⁻¹, which was close to 142 W·(m·K)⁻¹ of the non-aqueous AlN substrate.     

High Temperature Crystallization Process of a-Si Thin-films on Aluminum Nitride Substrates

Crystallization of silicon (Si) from amorphous silicon (a-Si) on foreign substrates has been studied by various research institutes. Crystallization of silicon thin-films on foreign substrates acts as an active layer in silicon thinfilm solar cells. In this research, due to the compatibility of thermal stability and expansion coefficient with Si, we used an aluminum nitride (AIN) substrate as an alternative candidate to glass and other ceramic substrates. P-type amorphous Si 5 μm thin-film was deposited using an ebeam evaporator directly on AIN substrates. The deposited layer was annealed at high temperature (°C) with N2 environment in a conventional tube furnace. Optical characterization was done using an optical microscope to investigate the surface morphology of as-grown and annealed samples. A smoother surface with an average grain size of about 3–4 μm was formed after annealing. Reflectance parameters were measured by UV-vis spectrometry. UV-vis-NIR was studied on as-grown and annealed samples to calculate the quality factor of the Si thin-film which was about 84.4 %. X-ray diffraction (XRD) was used to determine the phase direction of the Si thin-film before and after thermal annealing. It was observed that FWHM varied from 7.73 to 9.30 cm?1, Raman shift was from 520 to 522 cm^?1, and the stressed level also changed from 180 to 540 Mpa after annealing at high temperature for a long time. Interestingly, a crystallinity fraction was achieved of about 90 % at 1100 °C.     

Thermogravimetric Determination of Carbon, Nitrogen, and Oxygen in Aluminum Nitride

A thermogravimetric method for the measurement of carbon, nitrogen, and oxygen in aluminum nitride is described. Since carbon oxidation occurs at lower temperatures than the oxidation of the aluminum compounds, thermogravimetric analyses in the respective temperature regimes can yield measurement of the oxygen, nitrogen, and carbon content. Thermogravimetric analyses were performed on mixtures of aluminum nitride and carbon with various concentrations in order to evaluate the accuracy of this method. The results were compared with those obtained by the hot gas extraction method. Errors produced by overlapping of the temperature ranges of the reactions were evaluated and experimental conditions were optimized to minimize error. This method can produce reliable results at elemental concentrations higher than a few percent. The absolute errors of the C/AI, N/Al, and O/Al atomic ratios were less then 0.02. The detection limit was evaluated to 0.2 wt% of each element. The applicability of the method to the study of the kinetics of the nitridation reaction was demonstrated.     

Inversion Domains in Aluminum Nitride

Flat and curved, basal and nonbasal planar defects are frequently observed in sintered AIN ceramics. To prove the existence of inversion domains, TEM techniques sensitive to crystal polarity were used. Dark-field imaging in certain multiple-beam situations with g = (0002) (where g is operating reflection) yielded strong domain contrast giving clear evidence of an inverted crystal structure within the domains. This was confirmed by the dynamic contrast in the disks of convergent-beam electron diffraction. All planar defects observed were found to be associated with inversion domains. The formation of inversion twins may be nucleated by accidental growth or by nucleation and growth within the bulk of a preexisting AIN grain. Both mechanisms are supposed to be associated with the accommodation of impurities.     

Electrical Conduction in Aluminum Nitride

Electrical conductivities of aluminum nitride (AIN) doped with various ions of different sizes and valences have been measured at elevated temperatures. Grain conductivities of samples doped with large cations, obtained from complex impedance patterns, were much higher than those doped with small cations. The temperature dependence of electrical conductivity showed two regions having different activation energies. It was proposed that the precipitated impurities play a decisive role in the electrical conduction of AIN.     

Intrinsic Disorder in Aluminum Nitride

An interatomic potential model is developed for aluminum nitride and is used in conjunction with atomistic computer simulation methods to obtain the energies of intrinsic point defects. These energies are found to be very high, indicating no significant intrinsic disorder, even close to the melting point. Interstitial formation is particularly unfavorable, suggesting that these species will play no part in defect-controlled processes in AlN.     

Composition and Microstructure of Chemically Vapor-Deposited Boron Nitride, Aluminum Nitride, and Boron Nitride + Aluminum Nitride Composites

The composition and microstructure of dispersed-phase ceramic composites containing BN and AIN as well as BN and AIN single-phase ceramics prepared by chemical vapor deposition have been characterized using X-ray diffraction, scanning electron microscopy, electron microprobe, and transmission electron microscopy techniques. Under certain processing conditions, the codeposited coating microstructure consists of small single-crystal AIN fibers (whiskers) surrounded by a turbostratic BN matrix. Other processing conditions resulted in single-phase films of AIN with a fibrous structure. The compositions of the codeposits range from 2 to 50 mol% BN, 50 to 80 mol% AIN with 7% to 25% oxygen impurity as determined by electron microprobe analysis.     

Thermodynamic and Kinetic Effects of Oxygen Removal on the Thermal Conductivity of Aluminum Nitride

High thermal conductivity, low dielectric constant, high electrical resistivity, low density, and a thermal expansion coefficient that matches well with that of silicon are the principal attributes of AIN that have attracted much attention over the past decade. It is also now well established that oxygen as an impurity lowers the thermal conductivity of AIN. Processing techniques have been developed which not only facilitate pressureless densification of AIN but also enhance its thermal conductivity. The present work explores the thermodynamics and the kinetics of oxygen removal and the resultant enhancement of thermal conductivity. Polycrystalline AIN ceramics were fabricated with Y₂O₃, Dy₂O₃, Yb₂O₃, CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AIN grain size of sintered samples ranged between 2 and 9 μm. The samples typically contained two or three phases with the predominant phase being AIN. Secondary phases in Y₂O₃-doped AIN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y₃Al₅O₁₂, YAIO₃, and Y₄Al₂O₉, as well as Y₂O₃, depending upon the Y₂O₃/Al₂O₃ ratio, was revealed by X-ray diffraction. Thermal conductivity increased with the amount of additive and annealing time. Thermal conductivity also depended on the type of additive. Samples with thermal conductivity up to 200 W/(m · K) were fabricated. The variation in thermal conductivity with the type and the amount of the additive is explained on the basis of the thermodynamics of oxygen removal. In particular, the higher thermal conductivity of CaO-doped, in comparison with MgO-doped, samples is rationalized on the basis that the free energy of formation, ΔG°, of CaAl₂O₄ is less than that of MgAl₂O₄. It is proposed that the higher the |ΔG°|, with ΔG° < 0, the higher is the resultant thermal conductivity. An increase in the thermal conductivity with annealing time is attributed to the kinetics of oxygen removal from AIN grains.     

Inversion Domain Boundaries in Aluminum Nitride

Inversion domain boundaries (IDBs) have been identified in undoped, hot-pressed AIN. The microstructure and micro-chemistry of the IDBs have been studied using conventional transmission electron microscopy, convergent-beam electron diffraction, and analytical electron microscopy. Two distinct IDB morphologies are present: a planar variant which lies on the basal plane (0001), and a curved variant which does not possess a particular habit plane, but portions of the boundary are often seen lying on one of the {1011} planes. The boundaries exhibit α-like fringe contrast, indicating that a translation exists across the boundary. The displacement vectors R_F for the planar and curved boundaries have been investigated using both two-beam and multiple-beam techniques. Microchemical analysis has revealed oxygen segregation to the planar IDB; when present on the curved IDB, oxygen is at a lower concentration than in the planar case. Lattice fringe imaging and long-exposure selected area electron diffraction patterns have indicated the presence of thin, platelike precipitates at the planar IDBs. Sintering and annealing studies indicate that oxygen is necessary for the formation of the planar IDBs and that oxygen is not uniformly distributed along the curved IDBs.     

Measurement of the Oxygen and Impurity Distribution in Polycrystalline Aluminum Nitride with Secondary Ion Mass Spectrometry

The lattice oxygen content in sintered polycrystalline aluminum nitride substrates was measured via secondary ion mass spectrometry (SIMS). This was achieved by quantitative analysis of spatial and depth-resolved ion images of polished specimens using the solid-state standard addition method. The thermal conductivity of the polycrystalline material, measured with the laser flash technique, was found to be strongly correlated with the oxygen content in the AIN grains. This dependence is similar to that observed in single-crystal studies and is consistent with the phonon scattering model of AIN thermal conductivity. Scanning electron microscopy and SIMS images of a variety of other species (C, F, Cl, Y, Si, and Ca) were also obtained. In general, impurities were localized within second-phase regions although calcium was also found to be distributed uniformly along AIN grain boundaries. Other impurity constituents are also discussed.     

Preparation of Highly Oriented Aluminum Nitride Thin Films on Polycrystalline Substrates by Helicon Plasma Sputtering and Annealing

Highly c -axis-oriented aluminum nitride (AlN) thin films were prepared on polycrystalline Si₃N₄ and SiC substrates by helicon plasma sputtering. The difference in the substrate materials scarcely influenced the crystal structures of the films. The full width at half-maximum (FWHM) of the X-ray rocking curves of the films was 3.2°, which is the smallest value for AlN thin films deposited on polycrystalline substrates to our knowledge. Annealing at 800°C in vacuum decreased the FWHM from 3.2° to 2.8°. The structure of the films was densely packed and consisted of many fibrous grains. The films developed c -axis orientation on the polycrystalline substrates, because the interaction between the films and substrate surfaces was small, and (100) and (110) AlN planes grew preferentially. The films were piezoelectric and generated electrical signals in response to mechanical stress.     

Synthesis of Aluminum Nitride/Boron Nitride Composite Materials

Aluminum nitride/boron nitride composite was synthesized by using boric acid, urea, and aluminum chloride (or aluminum lactate) as the starting compounds. The starting materials were dissolved in water and mixed homogeneously. Ammonolysis of this aqueous solution resulted in the formation of a precomposite gel, which converted into the aluminum nitride/boron nitride composite on further heat treatment. Characterization of both the precomposite and the composite powders included powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Analysis of the composite revealed that the aluminum nitride phase had a hexagonal structure, and the boron nitride phase a turbostratic structures.     

Ceramics Based on Aluminum Nitride

The results of studying the effect of Y₂O₃and CaO additives introduced into ceramic mixtures in the form of oxides and salts on sintering and properties of AlN-based ceramics are discussed. The effect of high-temperature annealing of ceramics in a mixture of nitrogen and hydrogen on its properties is specified.     

氮化铝陶瓷的研制及应用

氮化铝 (AlN)因具有高热导率、低介电常数、与硅相匹配的热膨胀系数及其他优良的物理特性 ,在新材料领域越来越引起人们的关注。此文主要介绍并分析了AlN粉体合成、烧结、性能结构、AlN陶瓷的应用与前景     

Oxidation Kinetics of Aluminum Nitride

Thermal oxidation kinetics of aluminum nitride (AlN) powders, having fine- and coarse-particle-size distributions, were studied using thermogravimetric analysis (TGA). The kinetics showed dependence on the particle-size parameter, and the experimental TGA data were curve fitted using empirical mass relations employing both linear and parabolic models. The simulations predicted mixed kinetics in AlN oxidation.