Aluminum Nitride Silicon Carbide Research Articles

RF and Thermal Analysis of Beam Compression Unit for a $W$ -Band Gyrotron

A four-section beam compression unit (BCU) with copper and aluminum nitride–silicon carbide (AlN-SiC) annular disks was proposed for a $W$ -band gyrotron. Various dimensional parameters were calculated, based on which the BCU was modeled in a high-frequency structure simulator (HFSS), and the transmission characteristics were plotted for various sets of dimensional parameters. The dimensional parameters were optimized for achieving the attenuation of more than 60 dB over 85–105 GHz. It was further examined that the hollow electron beam radius over various sections of the BCU should not lead to beam–wave interaction, and if so, it should not propagate toward the interaction cavity. For this purpose, azimuthal electric field intensities in various azimuthally symmetric modes below the operating mode were plotted against the radial coordinate in various sections of the BCU and checked for the azimuthal electric field intensity values with the range of beam radius in the corresponding section. The electric field intensity maxima should be avoided within the range of beam radius in the corresponding section. Furthermore, the transient thermal analysis of the BCU was carried out considering temperature varying thermal conductivity in two proposed models: 1) with radially constant and 2) radially varying heat loads within AlN-SiC disks to predict the maximum temperature attained in the ceramic as 76.8 °C and 88.5 °C, respectively, taking 8-L/min flow rate of deionized water and ten heat load pulses.

Microwave-assisted combustion synthesis of AlN–SiC composites using a solid source of nitrogen

In this paper, the synthesis of Aluminum Nitride–Silicon Carbide composites through a combustion reaction among aluminum, silicon nitride (as the solid source of nitrogen) and carbon powders was investigated by X-ray diffraction and scanning electron microscopy. The combustion reaction was initiated through the ignition of the powder mixture by microwave heating in a domestic oven, thereby led to the synthesis of the desired ceramic. To achieve a higher-quality product, some parameters were altered during the synthesis. These included the mixing time, the amount of carbon in the reactant mixture and the weight of the reactant compacts. The product was not affected dramatically by the alteration in the first two parameters; however, the increase in the weight of the samples made it possible to achieve a more complete combustion and a higher degree of conversion to the products. It can be concluded that the procedure used in this work successfully leads to the synthesis of the product with no need to apply an electric field or utilize a nitrogen atmosphere.

The impact of film thickness and substrate surface roughness on the thermal resistance of aluminum nitride nucleation layers

Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1 ± 2.8 m2K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). Atomic resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR.

Structure, mechanical and functional properties of aluminum nitride-silicon carbide ceramic material

Two-phase ceramic composites of the dielectric-semiconductor type having different semiconducting phase content (aluminum nitride ceramics with uniformly distributed inclusions of silicon carbide of a certain size) have been produced by pressureless sintering. These composites are characterized by Vickers hardness HV (150 N) 9.5–15.8 GPa, Palmqvist fracture toughness 3.0–4.2 MPa m0.5, bending strength 132–209 MPa, thermal conductivity 37–82 W/(m K), and by a coefficient of the microwave electromagnetic energy attenuation to 36.3 dB/cm. It has been found that as the size of silicon carbide grains in aluminum nitride-based ceramics increases, the thermal conductivity increases and microwave energy attenuation decreases, which is indicative of the decisive role of grain boundaries in scattering both phonons and microwave radiation.

Use of Post‐heat Treatment to Obtain a 2H Solid Solution in Spark Plasma Sintering‐Processed AlN–SiC Mixtures

Dense aluminum nitride–silicon carbide (AlN–SiC) solid solutions with a 2H structure were fabricated by the post‐heat treatment of dense AlN–SiC composites fabricated by spark plasma sintering (SPS). The changes in the relative density, microstructure, and phases present were investigated for the compositions of 25 mol% AlN–75 mol% SiC (AlN 25), 50 mol% AlN–50 mol% SiC (AlN 50), and 75 mol% AlN–25 mol% SiC (AlN 75). The AlN–SiC composites fabricated by the SPS were dense ceramics with fine microstructures and composed of 2H and 6H AlN–SiC solid solutions. The AlN 50 samples heat treated at 2200°C and the AlN 75 samples heat treated at 2100°C were also dense ceramics and composed of only the 2H AlN–SiC solid solution. In contrast, the AlN 25 samples heat treated at 2200°C were porous ceramics and composed of several AlN–SiC solid solutions (2H, 4H, 6H, and 15R), and the AlN 75 samples heat treated at 2200°C were decomposed into an Al melt. Dense AlN–SiC solid solutions composed of only the 2H phase can be obtained by controlling the heat‐treatment temperatures, except for the AlN 25 sample.

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two‐phase AlN–SiC composites were prepared using Al4C3 and either Si (N2 atmosphere) or Si3N4 (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al4C3 under N2 atmosphere was shown to be a simple processing route for the preparation of porous two‐phase AlN–SiC materials. The homogeneous two‐phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum‐silicon carbide (Al4SiC4) with the specific surface area of 15.5 m2·g−1. The powders nitrided at and above 1400°C for 3 h contained the 2H‐phases which consisted of AlN‐rich and SiC‐rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m2·g−1, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β‐SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self‐sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al‐Si‐C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.

Reactive Synthesis and Phase Stability Investigations in the Aluminum Nitride–Silicon Carbide System

The effect of AlN on the structure formation of SiC was investigated. SiC was synthesized in the presence of AlN under vacuum at 1500°C, and the result was cubic SiC. The synthesis of AlN–SiC composites through the reaction Si3N4+ 4Al + 3C = 3SiC + 4AlN was also investigated and compared with synthesis via field‐activated self‐propagating combustion (FASHS). Reactants were heated in a vacuum furnace at temperatures ranging from 1130° to 1650°C. Below 1650°C, the reaction is not complete and at this temperature the product phases are AlN and cubic SiC. At 1650°C, the product contained an outer layer which contained β‐SiC only and an inner region which contained AlN and cubic SiC. 2H‐SiC and AlN composites synthesized via field‐activated self‐propagating combustion were annealed at 1700°C under vacuum. The AlN dissociated and evaporated and the 2H‐SiC transformed to the cubic β phase. Reasons for the differences in products of furnace heating and FASHS are discussed.

Platinum–aluminum nitride–silicon carbide diodes as combustible gas sensors

In this article we report on the novel use of a Pt/AlN/SiC structure as a combustible gas sensor. This device structure was fabricated by depositing a 2000 Å thick layer of AlN on the SiC substrate at 900 °C by laser ablation. Catalytic Pt gates were deposited onto the AlN at room temperature either by laser ablation or by rf sputtering. The electrical characteristic of the resultant devices from room temperature to 650 °C revealed current rectifying behavior. Most importantly, their electrical characteristic changed in response to propane, propylene, and CO introduced into their ambient at temperatures as low as 250 °C. They responded to changing gas composition over a wide range of combustible and oxygen concentrations from lean to fuel rich conditions. The response was a function of the ratio of combustible/oxygen concentration rather than to the absolute combustible concentration. However, this relationship exhibited some degree of combustible specificity.

Room temperature tensile and flexural strength of ceramics in AlN-SiC system

Tensile and bend strength were obtained for four compositions in the aluminum nitride-silicon carbide system: AlN, 75%AlN-25%SiC particulate, 50%AlN-50%SiC solid solution and SiC. Room temperature tensile strength, four-point bend strength, and the Weibull parameters for each of the tested materials are presented. Optical and SEM fractography were employed to identify failure initiating flaws for the fractured tensile specimens. The four-point bend strength is predicted using the Weibull tensile parameters and compared to measured values. Accuracy of the predicted results are rationalized based on observed microstructures and failure initiating flaws.

Study on the Heat–Treatment Reinforcement of Aluminum Nitride–Silicon Carbide Composites

AbstractThe effects of heat – treatment on the mechanical properties of AIN – SiC whisker composites with Y2O3 and SiO2 additives has been studied. When the sample containing 10 wt% Y2O3 +SiO2 (Y2O3/SiO2= 1/0. 66) was treated at 1300°C in air for 140 hours the strength of compositqs was raised from 481 MPa to 784 MPa.The phase composition, microstructure and grain boundary phase structure have been characterized by combining XRD, SEM, TEM/EDXA and HREM techniques. The reinforcement mechanism of the composites results from crystallization of the glass phase in the grain boundary at the high temperature oxidizing atmosphere to form the crossing structure of 2Hδ Sialon fibrous phase and SiC whisker.