Impurity Distribution in Polycrystalline Aluminum Nitride Ceramics

Convergent-beam electron diffraction and diffuse darkfield imaging of transmission electron microscopy were used to obtain qualitative information regarding the distribution of impurities in polycrystalline AIN. Impurities are distributed homogeneously within the grains of a given ceramic, but an amorphous grain-boundary phase on the order of 1 to 2 nm in thickness is observed between the AIN matrix grains.     

High-Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot-pressing. Bar-shaped samples were subjected to creep in four-point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady-state creep rate, dɛ/d t was proportional to σ ⁿ d ^{− m} where the stress exponent, n , was between 1.27 and 1.43 and grain-size exponent, m , between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.     

A New Compound with Ultra Low Dielectric Loss at Microwave Frequencies

A new ultra low loss microwave dielectric ceramic, Mg(Sn_0.05Ti_0.95)O₃ (MSnT), was found and investigated. The compounds were prepared by the conventional solid-state route, and sintered at 1360°–1480°C for 2–6 h. The investigations show that the MgTi₂O₅ secondary phase was observed. Moreover, the dielectric properties were correlated with the formation second phase. The excellent microwave dielectric properties of Q × f =322 000 (GHz), ɛ_r=17.4, and τ_f=−54 ppm/°C were obtained from the new MSnT ceramics sintered at 1390°C for 4 h.     

Dielectric Dispersion in Polycrystalline Ferrites: Random Network Model

A new approach to the problem of the dielectric permittivity and losses observed in some NiZn polycrystalline ferrites which strongly differs from the Debye form is presented. Because the ferrite sample is considered as a disordered structure where the well-conducting grains of various shape and size are separated by boundaries of low conductivity, its electric model is in the form of a large three-dimensional network composed of either resistors with the probability p or capacitors with the probability I–p. These networks are studied by computer simulation. The resulting equivalent admittance and its standard deviation coincide very well with the experimental results obtained for a series of NiZn ferrites. It is shown that a random network model explains the "universal'dielectric behavior of ferrites at constant temperature (e.g., high dielectric permittivity at the low frequency, dielectric dispersion in the wide frequency range from 0.01 Hz to l MHz, and statistical fluctuations of the tangent of losses) due to their inhomogeneous granular structure. Random network models can be applied also to the large class of inhomogeneous dielectrics whose polarization mechanism may be explained by many-body interactions.     

Microstructural Effects on the Thermal Conductivity of Polycrystalline Aluminum Nitride

Polycrystalline AIN bodies were made with a range of porosities from various AIN powders by sintering or hot-pressing. Thermal conductivity data for material produced without sintering aids showed a gradual, yet definite, porosity dependence with scatter similar to other property-porosity studies. The thermal conductivity-porosity data for ALN with sintering aids showed the existence of two distinct regions: (1) a higher-porosity region (greater than approximately 2%) which was similar to the data for material without sintering aids, and (2) a low-porosity region where grain boundaries were seen to dominate thermal conductivity. Auger spectroscopy was used to investigate fracture surfaces for dense CaO-doped materials. Thermal conductivity was observed to correlate to effective grain-boundary thickness, which was calculated from quantitative analysis of the Auger data. A model consisting of cubic grains in a continuous grain-boundary phase accurately describes these data.     

Electron Spin Resonance Analysis of Lattice Defects in Polycrystalline Aluminum Nitride

Electron spin resonance was used in the present study to detect lattice defects in an aluminum nitride lattice. The ESR spectra were obtained from polycrystalline AlN with various thermal conductivities. Measurement of the AlN g values clearly indicated that the obtained ESR signals arose from electrons trapped by nitrogen vacancies. The ESR study revealed that thermal conductivity increases with an increase in the number of electrons trapped by nitrogen vacancies. The explanation for that phenomenon is that the thermal conductivity of AlN increases with a decreasing concentration of oxygen impurities incorporated into the AlN lattice, and the concentration of nitrogen vacancies changes inversely with the concentration of oxygen impurities.     

Transient Effects of Nuclear Radiation on the Dielectric Properties of Refractory Low-Loss Ceramics at Microwave Frequencies

Selected ceramic materials were exposed to an intense burst of nuclear radiation to determine the transient effects of such an environment on their dielectric properties at frequencies in the L, S, and X bands. The radiation source was a pulse reactor of the swimming pool type which delivers a maximum dose of 2 × 10¹⁴/cm² and 5 × 10⁵ rd gamma during a 13 ms pulse. The dielectric constant and loss of the materials under test were measured before, during, and after irradiation using a resonant cavity technique successfully adapted to the unusual conditions of the experiment. The time resolution in the dielectric measurements was 10 ms. Results are reported for polycrystalline alumina, beryllia, magnesia, boron nitride, clear and opaque fused silica, and, for comparison, four organic dielectrics. Among the materials which exhibited radiation effects, changes in dielectric constant ranged from 0.001 to 0.008 and in tan δ from a factor of 2 to several orders of magnitude.     

Effects of Annealing on Dielectric Loss and Microstructure of Aluminum Nitride Ceramics

The effect of annealing on tan δ and microstructures of aluminum nitride (AlN) ceramics were explored. Yttria was added as a sintering additive to AlN powders, and the powders were pressureless-sintered at 1900°C for 2 h in a nitrogen flow atmosphere. In succession to sintering, AlN samples were annealed at 720, 970 and 1210°C for 2 and 4 h. Very low tan δ values between 2.6 and 6.0 × 10⁻⁴ at 28 GHz were obtained when the AlN samples were annealed for 4 h at all the annealing temperatures.     

Oxidation of Sintered Aluminum Nitride at Near-Ambient Temperatures

Oxidation of sintered aluminum nitride at low temperatures (20°–200°C) was studied using transmission electron microscopy (TEM). Particles of α-Al₂O₃, about 20–30 Å in size, were found to form within minutes on freshly cleaned surfaces of AlN at room temperature. The oxide was found to grow nearly epitaxially on AlN when the {0001}_AlN planes were exposed to the surface. Limited nonepitaxial oxidation was also observed when the basal planes were inclined to the TEM foil surface. After 10 h in air at 75°C, the particles coarsened to about 50 Å, while after 150 h at 200°C, an oxide film, about 500 Å thick, was observed on some grains.     

Effects of Organotitanate Additions on the Dispersion of Aluminum Nitride in Nonpolar Solvents

Organotitanate additives are known to reduce the viscosity of many filled polymer systems, including formulations of interest for ceramic powder injection molding. Mineral oil was used as a model solvent for these systems to examine the effects of Organotitanate additions on the stability and rheology of A1N dispersions. Addition of the titanate coupling agents neoalkoxy tri(neodecanoyl) titanate and neoalkoxy tri(dioctylphosphato) titanate resulted in better dispersion as measured by sedimentation tests and rheological characterizations. Excellent correlation of the sediment heights and measured viscosities with adsorption isotherms for the organotitanates was shown, with both lower sediment height and lower viscosity obtained when a monolayer of the titanate molecules was present on the powder surface. Comparison of the behavior of the mineral oil based systems with the viscosity of corresponding polypropylene based binder systems indicates that the titanate additions are effective in reducing particle-particle interaction effects in both systems.     

Effects of Dopants on the Complex Impedance and Dielectric Properties of Aluminum Nitride

Complex impedance patterns of aluminum nitride (AlN) doped with various ions of different ionic sizes and valences have been examined at temperatures between 400° and 950°C. The patterns showed distinct differences between samples doped with ions of different sizes. Grain and grain boundary contributions to the resistance and capacitance were compared. It was found that the grain boundary: grain resistance ratio increases with density. Samples doped with large cations have higher effective dielectric constants at the grain boundary than samples doped with small cations or undoped samples, indicating a thinner grain-boundary layer.     

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.     

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.     

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.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Method for Measuring the Dielectric Constant of Ferroelectric Ceramics at S-Band Frequencies

A method is presented for measuring the complex dielectric constant of ferroelectric ceramics at microwave frequencies. The method employs a disk-shaped sample in a radial-line configuration at the end of a coaxial-line structure. An equivalent circuit, established by admittance measurements, is used to relate the radial admittance at the sample face to the measured admittance on the slotted line preceding the coaxial sample holder. The nonuniform variation of electric field over the face of the sample is taken into account by means of a continued fraction expansion of x J _t( x )/ J ₀( x ) in terms of x = k'a , where k' is the wave number in the ferroelectric sample and a is its radius. Representative data on the variation of the complex dielectric constant with temperature at 3 kmc. are given for two samples of BaTiO₃ fired under atmospheric pressure and 5000 lb. per sq. in. The frequency spectrum of hot-pressed Cd₂Nb₂O₇ in the range 1.8 to 4.0 kmc. is also presented.     

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.     

Microwave Properties of Polycrystalline Hybrid Garnets

The microwave properties of a number of polycrystalline mixed garnets have been studied. The compositions investigated included solid solutions of yttrium iron garnet with gadolinium iron, samarium iron, yttrium aluminum, yttrium gallium, and yttrium chromium garnets. Ball-mill mixing techniques were employed in specimen preparation. Line width, saturation magnetization, and effective g-factor measurements were made as a function of composition and temperature for all specimens studied. Curie temperatures were also measured for each specimen. The merits of using these garnet compositions for microwave component applications are amplified by the fact that some of their properties may be independently controlled. Data on temperature compensation and high-power effects are presented as well as a discussion of the applicability of these materials to microwave solid-state devices.