Oxygen Transport in Aluminum Nitride Substrates

The diffusion of oxygen in commercially available aluminum nitride substrates has been studied by performing interdiffusion-type experiments. The diffusion of oxygen both within AlN grains and along grain boundaries was investigated. By using as-received AlN substrates and an electron microprobe as an analytical tool, it was found that the rate of oxygen diffusion along grain boundaries was strongly influenced by the presence of impurities and/or other phases at these boundaries. The diffusion of oxygen within AlN grains was studied between 1600° and 1900°C in a flowing nitrogen atmosphere by using secondary ion mass spectrometry (SIMS) for determining oxygen concentration profiles. The chemical diffusion coefficient of oxygen in the AlN lattice as a function of temperature is described by the equation The oxygen concentration profiles determined by SIMS also show a contribution from diffusion along grain boundaries. Therefore, it was possible to determine values for the product δD'_o (δ= width of grain boundaries, D'_o= grain boundary diffusion coefficient of oxygen).     

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.     

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.     

Self-Propagating High-Temperature Synthesis of Aluminum Nitride under Lower Nitrogen Pressures

The synthesis of aluminum nitride (AlN) via self-propagating high-temperature synthesis (SHS) was attempted, using aluminum powder that was mixed with AlN powder as a diluent. The AlN content in the reactant was varied over a range of 30%–70%, and the nitrogen pressure was varied over a range of 0.1–1.0 MPa. The SHS reaction that was performed using a reactant that contained 50% AlN diluent, under a nitrogen-gas pressure of 0.8 MPa, yielded the highest conversion ratio of aluminum powder to AlN powder. A mechanism for the reaction of aluminum with nitrogen gas during the SHS process was discussed, based on observations of the microstructures of the reaction zone and products.     

Evolution of Carbon during Burnout and Sintering of Tape-Cast Aluminum Nitride

The surface nature and the composition of AlN powder, as-received and exposed to binder burnout, were studied using XPS and TEM. The surface of as-received AlN powder was covered by a thin layer of aluminum oxynitride and oxide mixture. A small portion of residual carbon from binder burnout was bound to oxygen atoms on the AlN powder surface, and the majority of the carbon was amorphous graphitoid carbon which covered the AlN powder surface uniformly. AlN samples were made using tape casting and pressureless sintering. Surface-carbon-to-oxygen ratio of AlN powder after binder burnout was evaluated using XPS. The surface C/O atomic ratios were observed to correlate with the sintering behavior, the composition of the second phase, the second phases distribution, and grain-boundary composition, as well as thermal conductivity of AlN samples.     

Conversion of Alumina to Aluminum Nitride Using Polymeric Carbon Nitride as a Nitridation Reagent

Polymeric carbon nitride, which was synthesized by polymerization of dicyandiamide at 500°C, was used as a nitridation reagent in the conversion of δ‐alumina (δ‐Al₂O₃) to aluminum nitride (AlN). The products obtained at various reaction temperatures were characterized by powder X‐ray diffraction, ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). δ‐Al₂O₃ began to convert to AlN at 900°C, which is the lowest temperature reported for the formation of AlN from Al₂O₃, and completely converted to AlN at 1400°C. The occurrence of reaction intermediates during nitridation was confirmed by ²⁷Al MAS NMR and XPS. The change in Raman spectra with reaction temperatures indicated that lattice defects in AlN were reduced by calcining at higher reaction temperatures.     

Chemical Analysis of Aluminum as a Propellant Ingredient and Determination of Aluminum and Aluminum Nitride in Condensed Combustion Products

A brief survey of typical problems in the analysis of aluminum powder in aluminized solid propellants and in analysis of condensed combustion products of these propellants was carried out. Recommendations for applying the versions developed by the authors of the known methods are given. The permanganatometric variant of the titrimetric method was found suitable for most tasks concerning the measuring of the metallic/unburned aluminum. The determination of aluminum nitride in combustion products using the combination of chemical and X‐ray diffraction methods was described and illustrated by results obtained from condensed combustion products of propellant formulations containing highly active ultrafine aluminum powder. Even for this formulation the content of aluminum nitride in the final condensed combustion products was found to be negligibly small independently of the nature of the gas (argon or nitrogen) used for bomb pressurization.     

Carbon–Nitrogen Pyrolyzates: Attempted Preparation of Carbon Nitride

Precursors for the preparation of carbon nitride were selected from a number of relatively high nitrogen content organic materials. The precursors were pyrolyzed in a closed system at 700°C under 225 MPa. The resulting residues showed little nitrogen loss, but were amorphous with relatively low densities. Raman spectroscopy and ¹³C NMR indicate the presence of trigonal carbon.     

Cluster Size Determination in the Chemical Vapor Deposition of Aluminum Nitride

Aluminum nitride was prepared in a laminar-flow, tubular reactor using an atmospheric pressure chemical vapor deposition (APCVD) method at a reaction temperature ranging from 700° to 1100°C and AlCl₃ and NH₃ concentrations of 0.4 and 8 mol%, respectively. Films grew on the reactor wall and particles formed in the gas phase. The production rates of films and particles were independently determined. A comprehensive model was constructed to estimate the molecular size of the growth species in the APCVD process to simultaneously form films and particles of AIN from AlCl₃, and NH₃. Transport equations of a dominant growth species used in growing films on the reactor wall and particles in the gas phase in a laminar-flow, tubular reactor were formulated and solved. An assumption made in the model was to use the surface reaction rate constant measured for the film surface for the particle surface. Comparing the film and particle growth data measured experimentally with those obtained from model prediction allows us to conclude that the growth species are clusters ranging in size from 0.8 nm at 700°C, equivalent to 8 units of AIN, to 0.5 nm at 1100°C, equivalent to 1 unit of AIN.     

氧掺杂石墨相碳化氮的制备研究进展

石墨相碳化氮是一种非金属光催化剂,近年来已成为重要的研究热点。但是,在可见光下其催化性能较差,严重地制约着其应用。在石墨相碳化氮基础上掺杂氧,可以显著地改变其结构和性能。目前为止,关于氧掺杂石墨相碳化氮的研究,主要体现在它的制备方法上。综述了氧掺杂石墨相碳化氮的制备方法情况,重点指出了其制备方法的优缺点,展望了其未来发展的方向。     

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.     

Determination of the Oxygen Deficiency in Vacuum-Hot-Pressed Ferroelectric Ceramics Using a Thermogravimetric Technique

An attempt to measure the oxygen deficiency in vacuum-hot-pressed ferroelectric target materials was made, using a thermogravimetric method. The approach was intended to measure the relative mass increase of oxygen-deficient sheet samples, with respect to an oxidized reference that was annealed in air. A saturation of mass change was achieved, which indicated a saturated oxidation state and provided a basis for further calculation of the oxygen deficiency in the samples. Results were obtained for barium strontium titanate and lead lanthanum zirconate titanate compositions, which shows the feasibility of applying the thermogravimetric method to ferroelectric materials.     

氧和氮在炭分子筛上的吸附与扩散

用重量法研究了氧、氮在两种空分用炭分子筛上的吸附与扩散。结果表明,氧和氮在炭分子筛中的扩散是活化扩散,该过程可用双孔模型进行描述。求得了氧、氮的扩散系数。     

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.     

煤中碳、氢和氮含量测定的操作要点

煤是我国重要的能源物质,关于煤类型的研究对提高煤炭利用率,节能环保,合理使用资源具有重要意义。其中,通过技术性手段对煤的各项指标进行测定,是研究煤的种类及加工途径的重要方式。基于此,主要通过碳氢氮分析仪对煤中的碳氢氮进行测定,重点阐述测定原理以及在测定过程中需要注意的问题。     

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.     

Simultaneous Chemical Vapor Deposition of Boron Nitride and Aluminum Nitride

Two chemically different phases, hexagonal BN and AIN, were simultaneously produced by chemical vapor deposition (CVD) using an impinging jet reactor and the BCl₃─AlCl₃─NH₃─Ar reagent system. The microstructure of the BN + AIN composite coatings was strongly dependent on temperature, pressure, and BCl₃ and AlCl₃ concentrations. The growth characteristics of BN and AIN in the codeposition system were similar to those expected from the single-phase deposition processes (i.e., BN-CVD and AIN-CVD), except the growth of AIN whiskers was accentuated, and competition between BN and AIN deposition in the composites was suspected to be the cause of less-crystalline deposits. In both BN + AIN-CVD and AIN-CVD, the growth of AIN whiskers became more apparent with increasing pressure or temperature. The codeposition behavior observed experimentally was compared with thermodynamic predictions.