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.     

Preparation of aluminum nitride green sheets by aqueous tape casting

Aluminum nitride green sheets were prepared by aqueous tape casting. The characteristics of a treated AlN were studied in aqueous ball-milling media. The oxygen content picked up with the increase of ball-milling time. It was noted that the oxygen content of AlN powder with the dispersant DP270 was lower than that of AlN powder without the dispersant DP270. The isoelectric points of the treated AlN with and without DP270 were, respectively, at pH 3.35 and pH 3.90. The dispersant DP270 not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder. The tape casting slurry exhibited a typical shear-thinning behavior. Aqueous AlN green tape had a smooth surface and a narrow pore size distribution. Its relative density was 52.6%. No other crystalline phase was detected by XRD except for AlN and sintering aid yttria in AlN green sheet.     

High-Thermal-Conductivity AlN Filler for Polymer/Ceramics Composites

To increase the thermal conductivity of polymer/ceramic composites, aluminum nitride (AlN) granules were added as a ceramic filler. Granules, sintered at 1850°C for 24 h, showed a very high conductivity of 266±26 W (m·°C)⁻¹, as measured by a thermal microscope equipped with thermoreflectant and periodic heating techniques. This conductivity exceeds 80% of the theoretical value of AlN. Ceramic fillers consisting of the obtained AlN granules and commercially available hexagonal boron nitride particles (h-BN) powder plus polyimide resins were mixed and then molded at 100 MPa and 420°C in a vacuum. The resultant composite showed a high conductivity of 9.3 W (m·°C)⁻¹. This study demonstrates that a high-thermal-conductivity filler effectively enhances the conductivity of polymer/ceramic composites.     

Enhancement of Densification and Thermal Conductivity in AlN Ceramics by Addition of Nano-Sized Particles

The effect of addition of nano-sized particles on densification and thermal conductivity of AlN ceramics was investigated. The commercially available AlN powder (∼0.9 μm) was mixed with 1.89 mass% nano-sized AlN particles (<0.1 μm), 3.53 mass% Y₂O₃, and 2.0 mass% CaO as sintering aid. The mixture was fired at 1500° and 1600°C in a tungsten resistance furnace under flowing N₂ atmosphere. The results showed that a fully densified specimen was obtained at the lower temperature of 1600°C by addition of nano-sized particles. The thermal conductivity of the resulting product was 133 W/m°C. The value is much higher than the 52 W/m°C for the sample prepared without adding the nano-sized AlN powder. This study indicates a strong potential for the use of nano-sized particles as additives in the densification of AlN ceramics.     

Hot Isostatically Pressed SiC-AIN Powder Mixtures: Effect of Milling on Solid-Solution Formation and Related Properties

Powder mixtures with nominal compositions of 75 mol% SiC and 25 mol% AlN as well as 50 mol% SiC and 50 mol% AlN were prepared by ball milling for 1-300 h and subsequently sintered to full density by hot isostatic pressing (HIPing) at 1850°C under a pressure of 200 MPa. Microstructures and properties of the HIP-sintered SiC-AlN powder mixtures were investigated with special attention to the effect of ball milling on the formation of SiC-AlN solid solutions. Extensive ball milling facilitated effectively the formation of solid solutions by improvement in mixing homogeneity and pulverization of the SiC and AlN powders. Thermal conductivity of the HIP-sintered SiC-AlN powder mixtures decreased remarkably with ball-milling time, mainly because of the enhanced formation of solid solutions. Mechanical properties-such as strength, fracture toughness, and microhardness-were dependent on the microstructural homogeneity and the grain refinement resulting from the formation of solid solutions.     

Effect of Heat Treatment on the Microstructure and Properties of Dense AIN Sintered with Y2O3 Additions

The secondary phase constitution in two sintered AIN ceramics (1.8% and 4.2% Y₂O₃ additions) was studied as a function of heat treatment temperatures between 1750° and 1900°C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such as density, thermal conductivity ( K ), and lattice constants, and on the mechanical properties in three-point bending, was also investigated. Y₃Al₅O₁₂ was found to getter dissolved oxygen from the AIN lattice below 1850°C, but evaporated at 1850°C and above. Y₄Al₂O₉ appeared to sublimate below 1850°C in the atmosphere used in this study. Depending on the secondary phase constitution, heat treatment affected thermal conductivity favorably or adversely. Occasionally, samples with similar lattice oxygen contents were found to have different thermal conductivities, suggesting that factors besides dissolved oxygen can also influence K . Lattice parameter measurements indicated that, within the small range of lattice oxygen concentrations in the AIN samples studied, the c-axis was more sensitive than the a -axis to oxygen content.     

Ion-Exchange Loading of Yttrium Acetate as a Sintering Aid on Aluminum Nitride Powder via Aqueous Processing

A novel fabrication process of AlN ceramics via aqueous colloidal processing and pressureless sintering has been presented. The chemical stability of AlN powder in water was improved by the surface chemical modification with sebacic acid, while maintaining a hydrophilic surface. The treatment of the sebacic acid-modified powder with yttrium acetate tetrahydrate resulted in strong immobilization of Y³⁺ ions, as a sintering aid, at a highly dispersive level on the AlN powder surface through ion exchange with the free carboxyl groups of the sebacic acid molecules attached to the AlN surface. By selecting slip compositions for a well-deflocculated condition and firing conditions to burn out organic components in the slip cast compacts, a thermal conductivity of about 250 W/(m·K) could be attained by the pressureless sintering at 1900°C for 5 h.     

Preparation and Thermal Ablation Behavior of HfB2–SiC-Based Ultra-High-Temperature Ceramics Under Severe Heat Conditions

HfB₂–SiC-based ultra-high-temperature ceramics with aluminum nitride (AlN) as a sintering aid were hot pressed at 1850°C. The sinterability and mechanical properties were investigated and compared with the composite without a sintering aid. It was shown that the addition of AlN greatly improved the powder sinterability and enabled the production of a nearly full-dense composite. The mechanical properties, especially the flexural strength, were enhanced remarkably through the improvement in the sinterability and microstructure. The oxidation resistance of a composite doped with 10 vol% AlN was evaluated by a plasma arc heater and the ablation mechanism was discussed.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

Effect of Grain Contiguity on the Thermal Diffusivity of Aluminum Nitride

Thermal diffusivity of AlN-based ceramics was studied as a function of second-phase amount and heat-treatment time. The Y₂O₃·Al₂O₃ contents varied over the range of 13-31 vol%. The thermal diffusivity decreased as the amount of second phase increased. After sintering at 1850°C, the AlN ceramics consisted of rounded, largely isolated grains. Heat treatment of these samples for 5-50 h at 1800°C resulted in microstructures that consisted of largely contiguous AlN grains. There was a substantial increase in the thermal diffusivity after the heat-treatment step, and the incremental improvement was essentially constant for the three compositions that have been studied. The amount of second phase was unchanged during heat treatment; therefore, the increase in thermal diffusivity is assumed to be a direct result of the enhanced contiguity of AlN grains.     

Reactive Laser Ablation Synthesis of Nanosize Aluminum Nitride

An aluminum (Al) target was laser ablated in a nitrogen (N₂) atmosphere, producing aluminum nitride (AlN) powder. These powders were calcined at 900°C for 2 h. Powders were produced at various nitrogen pressures, and the calcined powders were tested for unreacted aluminum content, using differential thermal analysis (DTA). The AlN powder, produced at a laser fluence of 12 J/cm² and a nitrogen pressure of 10.0 kPa (75 torr), showed no evidence of unreacted aluminum by DTA and was phase-pure AlN by X-ray diffraction (XRD). The surface area of this powder is 82 m²/g, corresponding to a particle size of ∼11 nm, which is in good agreement with TEM observations.     

Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramic

An aluminum nitride (AlN) ceramic with a thermal conductivity value of 272 W·(m·K)⁻¹, which is as high as the experimentally measured thermal conductivity of an AlN single crystal, was successfully fabricated by firing at 1900°C with a sintering aid of 1 mol% Y₂O₃ under a reducing N₂ atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AlN grains.     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Preparation of AlN ceramic bonded carbon by gelcasting and spark plasma sintering

To obtain light, strong materials with high thermal conductivity, a new carbon-based material, AlN ceramic bonded carbon (AlN/CBC), was fabricated by combining gelcasting and spark plasma sintering techniques. The results showed that AlN/CBC (20 vol% AlN) has a unique microstructure containing carbon grains of 15 μm in size and an AlN grain-boundary layer of 0.5-3 μm in thickness. Continuous AlN ceramic networks bonded the carbon grains together. Compared with the conventional AlN/carbon (AlN/C) material made by a ball-milling method, AlN/CBC showed a higher strength and a higher thermal conductivity by two and four times, respectively.     

Corrosion of Aluminum Nitride (AlN) in Aqueous Cleaning Solutions

The corrosion of aluminum nitride (AlN) in aqueous solutions has been evaluated in situ , using an ammonium/ammonia ion-selective electrode. The corrosion behavior of AlN over a pH range of 5.5–12 indicates that the corrosion products that are formed act as a protective barrier layer in the pH regime where they have the lowest stability. An insoluble barrier layer is formed via the oxidation of the AlN surface in air at a temperature of 750°C for 10 min. This oxynitride layer, which is 200 Å thick, prevents the corrosion of the AlN in a pH 9.5 aqueous solution; however, the thermal conductivity is reduced by 6.6%.     

High-Thermal-Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al₂O₃), as is clearly demonstrated in the aluminum nitride-yttria (AlN-Y₂O₃) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln₂O₃: Al₂O₃, where Ln is a lanthanide element) ratios, with samaria (Sm₂O₃) and lutetia (Lu₂O₃) being the dopants that give the highest- and lowest-thermal-conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution-reprecipitation and bulk diffusion. Removal of grain-boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase-sintered AlN ceramics.     

Thermomechanical Properties of β-Sialon Synthesized from Kaolin

β-Sialon powder was synthesized by the simultaneous reduction and nitridation of Hadong kaolin at 1350°C in an N₂–H₂ atmosphere, using graphite as a reducing agent. The average particle size of β-sialon powder was about 4.5 μm. The synthesized β-sialon powder was pressureless sintered from 1450° to 1850°C under a N₂ atmosphere. The relative density, modulus of rupture, fracture toughness, and microhardness of β-sialon ceramics sintered at 1800°C for 1 h were 92%, 248 MPa, 2.8 MN/m^3/2, and 13.3 GN/m², respectively. The critical temperature difference (ΔT_c) in water-quench thermal-shock behavior was about 375°C for the synthesized β-sialon ceramics.     

Ambient-Temperature Mechanochemical Formation of Titanium Nitride-Alumina Composites from TiO2 and FeTiO3

The fabrication of a homogeneous submicrometer-sized powder composed of nanocrystalline (<10 nm) alumina and titanium nitride during high-energy ball-milling is reported in this paper. The starting materials were rutile (TiO₂) and aluminum powder. A similar composite with iron was also produced using the mineral ilmenite (FeTiO₃) as the starting material. The powders were ball-milled together under a nitrogen atmosphere for 100 h in a laboratory-scale mill and subjected to thermal analysis and isothermal annealing at up to 1200°C. X-ray diffraction showed that all of the phases formed within the milling step and underwent grain growth on annealing. Differential thermal analysis indicated no residual elemental aluminum, confirming that the reaction was completed during the milling operation.     

Influence of the de-waxing atmosphere on the properties of AlN ceramics processed from aqueous media

The influence of binder burnout atmosphere (air or N₂) on surface chemistry of thermo-chemically treated AlN powders processed in aqueous media, and on the final properties of AlN ceramics was studied. The surface chemistry after de-waxing was accessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD), SEM, high-resolution transmission electron microscopy (HR-TEM), were used to identify the phase assemblage and for microstructural analysis. The effects of the residual carbon and oxygen at the surface on the thermal conductivity and sintered density of AlN samples were investigated. The surface C/O ratios were observed to correlate with the sintering behaviour, the composition and distribution of secondary phases, and grain-boundary composition, as well as thermal conductivity of AlN samples. Thermal conductivities of about 140 W/mK were obtained for the aqueous processed AlN samples de-waxed in nitrogen atmosphere and sintered for 2 h at 1750 °C in the presence of 4 wt.% YF₃ + 2 wt.% CaF₂ as sintering additives.     

Morphological Effect of Second Phase on the Thermal Conductivity of AIN Ceramics

The effect of the morphology of second phases in sintered microstructure on the thermal conductivity of AIN ceramics was investigated. When an Y₂O₃-doped AIN specimen was cooled down slowly at a rate of 3°C/min after sintering at 1850° or 1900°C, the second phases were concentrated in the corners of the AIN grains by an increase of dihedral angle during cooling. On the other hand, the fast-cooled specimen at a rate of 60°C/min showed a different structure of the second phases interconnected through the triple-grain junctions. The specimen with isolated second-phase morphology showed a higher thermal conductivity than those with interconnected second-phase morphology. The measured thermal conductivity of the specimens with different morphologies of the second phases agreed well with the calculated one derived from modeled microstructures. From the comparison of the measured and calculated thermal conductivity, it was shown that the thermal conductivity of the specimen with interconnected second-phase morphology decreased steeply with an increase of the amount of the second phases, assuming the content of lattice oxygen to be constant. However, the thermal conductivity of the specimen with isolated second-phase morphology was rather insensitive to an increase of the amount of the second phases.