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.     

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.     

Low-Temperature Sintering and High Thermal Conductivity of YLiO2-Doped AlN Ceramics

The present work indicates through thermodynamic considerations that YLiO₂ additive is beneficial for low-temperature sintering of AlN ceramics. Pressureless sintering of commercially available AIN powders with simultaneous additions of YLiO₂ and CaO resulted in materials with high thermal conductivity (170 W·m^–1·K^–1 after sintering at 1600°C for 6 h). It is demonstrated that improvement of thermal conductivity is possible at low firing temperature by use of sintering aids.     

Effect of Silicon Dioxide on the Thermal Diffusivity of Aluminum Nitride Ceramics

The thermal diffusivity of AlN ceramics was significantly decreased by the addition of SiO₂. The AlN ceramics with 4 wt% SiO₂ could not be densified by pressureless sintering in the temperature range 1400° to 1800°C. The thermal diffusivity of these samples was very low because of their porous structure. The AlN ceramics containing 2, 4, and 8 wt% SiO₂ were densified by hot-pressing and also had low thermal diffusivity. In these samples, the grains of the 27R polytype that resulted from the reaction between AlN and SiO₂ were dispersed, obstructing the conduction of heat. The relation between the amount of 27R polytype and the thermal diffusivity of the AlN ceramics was determined.     

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.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.     

Electrically Conductive CNT-Dispersed Silicon Nitride Ceramics

Carbon nanotube (CNT)-dispersed Si₃N₄ ceramics with electrical conductivity were developed based on the lower temperature densification technique, in which the key point is the addition of both TiO₂ and AlN as well as Y₂O₃ and Al₂O₃ as sintering aids. This new ceramic with a small amount of CNTs exhibits very high electrical conductivity in addition to high strength and toughness. Since Si₃N₄ ceramics with Y₂O₃–Al₂O₃–TiO₂–AlN were originally used as a wear material, electrically conductive Si₃N₄ ceramics are expected to be applied for high-performance static-electricity-free bearings for aerospace and other high-performance components.     

Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF₂ and YF₃ as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂. Liquid CaYAlO₄ mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F₂ at a relatively low temperature results in homogeneous YF₃ distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity.     

Rare-Earth Zirconate Ceramics with Fluorite Structure for Thermal Barrier Coatings

A series of rare-earth zirconate Ln₂Zr₂O₇ ceramics (Ln=Dy, Er, and Yb) with a fluorite structure (F-Ln₂Zr₂O₇) were prepared by pressureless sintering from zirconia and rare-earth oxide powders at 1600°C for 10 h in air. The microstructure experiments were performed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The thermal conductivity and thermal expansion of these ceramics were evaluated using a steady-state laser heat-flux technique and high-temperature dilatometry, respectively. The XRD and SEM results demonstrate that Ln₂Zr₂O₇ ceramics with a single fluorite phase are synthesized and no other phases are found. The results of thermal conductivity show that their thermal conductivities (1.3–1.9 W/(m·K), 20°–800°C) are as low as those of the referenced Ln₂Zr₂O₇ ceramics (Ln=La, Nd, Sm, and Gd) with pyrochlore structure (P-Ln₂Zr₂O₇). It is concluded that rare-earth zirconate ceramics with a fluorite structure can be considered as candidate materials for future thermal barrier coatings.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Densification, Structure, and Thermophysical Properties of Ytterbium–Gadolinium Zirconate Ceramics

(Yb _x Gd_{1− x} )₂Zr₂O₇ (0≤ x ≤1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550–1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr₂O₇ ceramics have the lowest thermal conductivity among all the composition combinations studied.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Effects of Samarium Oxide Addition on the Phase Composition, Microstructure, and Electrical Resistivity of Aluminum Nitride Ceramics

The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm₂O₃ additive were investigated. The phase composition was approximately consistent with that estimated from the Sm₂O₃–Al₂O₃ phase diagram using the amount of added Sm₂O₃ and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm₂O₃ additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10¹⁰–10¹²Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.     

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β-Silicon Nitride

Si₃N₄ powders with the concurrent addition of Yb₂O₃ and MgSiN₂ were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si₃N₄, Yb₂O₃, and MgO addition. MgSiN₂ addition was effective for improving the thermal conductivity of Si₃N₄ ceramics, and a material with high thermal conductivity over 140 W·(m·K)⁻¹ could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN₂-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.     

Hf3AlN: A Novel Layered Ternary Ceramic with Excellent Damage Tolerance

In this work, bulk Hf₃AlN ceramic was synthesized by an in situ reaction/hot pressing method using Hf and AlN as initial materials. The reaction path during the synthesis process was investigated. Hf₃AlN was found to form via the reaction of Hf and AlN above 1000°C. Furthermore, physical and mechanical properties of Hf₃AlN, such as electrical conductivity, flexural strength, and elastic moduli were also characterized. Similar to typical layered ternary ceramics Ti₃SiC₂ and Ti₃AlC₂, Hf₃AlN possesses metallic conductivity and excellent damage tolerance, which is also the first one of this type that has ever been reported to crystallize in an orthorhombic structure. It is believed that a typical layered crystal structure and weak interlayer bondings contribute to the damage tolerance of Hf₃AlN. Moreover, the stiffness of Hf₃AlN can sustain a temperature as high as 1450°C, being 250°C higher than that of Ti₃AlC₂, which renders it a promising high-temperature structural material.     

Fabrication of Si3N4 Ceramics with Metal Nitride Additives by Isostatic Hot-Pressing

High-density Si₃N₄ ceramics with nitride additives such as TiN, AlN, ZrN, VN, NbN, YN, TUN, Mg₃N₂, and HfN were fabricated by isostatic hot-pressing. The temperature dependence of Vickers microhardness, fracture toughness, and thermal conductivity were measured; these properties, as well as microstructure, were strongly influenced by the additive used.     

High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y₂O₃) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al₂O₃, reacts with Y₂O₃ to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al₂O₃–Y₂O₃ was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al₂O₃–Y₂O₃. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.     

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.     

Effects of Palladium Dispersions on Gas-Sensitive Conductivity of Semiconducting Ga2O3 Thin-Film Ceramics

The gas sensitivity of Ga₂O₃ thin-film n -type conductors was investigated at temperatures of 500–1000°C. Palladium dispersions whose particle sizes are dependent on the preceding annealing processes were deposited by a wet-chemical technique onto Ga₂O₃ thin-film ceramics. The palladium clusters and their temperature-dependent growth were detected using scanning electron microscopy micrographs and X-ray photoemission spectroscopy measurements. The effect of the palladium dispersions on the gas-sensitive behavior of the Ga₂O₃ ceramics was investigated in various O₂/H₂ mixtures in the N₂ carrier gas at 700°C. The conductivity of the ceramics treated in this way was dependent on the O₂ partial pressure, as well as on the H₂ partial pressure of the surrounding gas atmosphere. The ceramic conductivity can be described as a function of the O₂:H₂ ratio, in accordance with the relation σ( p O₂/ p H₂/)^−1/3.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.