Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

一种低温共烧AlN陶瓷基片的排胶技术

介绍一种由高热导率AlN陶瓷和金属W共烧制备低温AlN陶瓷基片的排胶技术．研究了排胶过程中残余碳对AlN陶瓷基片相组成、烧结特性和微观结构的影响．结果表明：两步排胶法可以较好地解决W氧化及AlN陶瓷颗粒表面吸附残余碳的问题．     

Thermal conductivity of Ca1 ? xHoxF2 + x optical ceramics

The thermal conductivity of Ca_{1 − x} Ho _x F_{2 + x} (x ≤ 0.03) optical ceramics has been studied experimentally in the temperature range 50–300 K. With increasing holmium content, the thermal conductivity of the ceramics decreases, especially at low temperatures: from 10.2 to 2.3 W/(m K) at 300 K and from 250 to 4.5 W/(m K) at 50 K.     

Thermal Conductivity of a Simulated Fuel with Dissolved Fission Products

The thermal diffusivity of a simulated fuel with fission products forming a solid solution was measured using the laser-flash method in the temperature range from room temperature to 1673 K. The density and the grain size of the simulated fuel with the solid solutions used in the measurement were 10.49 g · cm⁻³ (96.9% of theoretical density) at room temperature and 9.5 μm, respectively. The diameter and thickness of the specimens were 10 and 1 mm, respectively. The thermal diffusivity decreased from 2.108 m² · s⁻¹ at room temperature to 0.626 m² · s⁻¹ at 1673 K. The thermal conductivity was calculated by combining the thermal diffusivity with the specific heat and density. The thermal conductivity of the simulated fuel with the dissolved fission products decreased from 4.973 W · m⁻¹ · K⁻¹ at 300 K to 2.02 W · m⁻¹ · K⁻¹ at 1673 K. The thermal conductivity of the simulated fuel was lower than that of UO₂ by 34.36% at 300 K and by 15.05% at 1673 K. The difference in the thermal conductivity between the simulated fuel and UO₂ was large at room temperature, and decreased with an increase in temperature. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Temperature-stable and low loss Fe-containing dielectrics in BaO-Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Polycrystalline samples of Ba₄Ln₂Fe₂Ta₈O₃₀ (Ln = La and Nd) were prepared by a high temperature solid-state reaction technique. The formation, structure, dielectric and ferroelectric properties of the compounds were studied. Both compounds are found to be paraelectrics with filled tetragonal tungsten bronze (TB) structure at room temperature. Dielectric measurements revealed that the present ceramics have exceptional temperature stability, a relatively small temperature coefficient of dielectric constant (τ _ε ) of −25 and −58 ppm/°C, with a high dielectric constant of 118 and 96 together with a low dielectric loss of 1.2 × 10⁻³ and 2.8 × 10⁻³ (at 1 MHz) for Ba₄La₂Fe₂Ta₈O₃₀ and Ba₄Nd₂Fe₂Ta₈O₃₀, respectively. The measured dielectric properties indicate that both materials are possible candidates for the fabrication of discrete multilayer capacitors in microelectronic technology.     

Preparation and thermophysical properties of (NdxGd1?x)2Zr2O7 ceramics

Ceramic powders of (Nd _x Gd_1−x )₂Zr₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were synthesized by chemical-coprecipitation followed by calcination method, and were then pressureless-sintered at 1,600 °C for 10 h in air. Phase constituents and morphologies of the synthesized powders and sintered ceramics were identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A high-temperature dilatometer and a laser-flash method were used to analyze the thermal expansion coefficient and thermal diffusion coefficient of different ceramics from room temperature up to 1,400 °C. Thermal conductivity was calculated from thermal diffusivity, density, and specific heat. (Nd _x Gd_1−x )₂Zr₂O₇ (0.1 ≤ x ≤ 1.0) ceramics are with a pyrochlore-type structure; however, pure Gd₂Zr₂O₇ exhibits a defective fluorite-type structure. The average linear thermal expansion coefficients of different (Nd _x Gd_1−x )₂Zr₂O₇ ceramics decrease with increasing the value of x from 0 to 1.0 in the temperature range of 25–1,400 °C. The thermal conductivities of (Nd _x Gd_1−x )₂Zr₂O₇ ceramics are located within the range of 1.33 to 2.04 W m⁻¹ K⁻¹ from room temperature to 1,400 °C.     

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

Structure and thermal conductivity of powder injection molded AlN ceramic

AlN powders doped with Y₂O₃ (5 wt.%) were compacted by employing powder injection molding (PIM) technique. The binder consisted of paraffin wax (PW, 60 wt.%), polypropylene (PP, 35 wt.%) and stearic acid (SA, 5 wt.%). The feedstock was prepared with a solid loading of 62 vol.%. The binder was removed through debinding process in two steps, solvent debinding followed by thermal debinding. At last, the debound samples were sintered in flowing nitrogen gas at atmospheric pressure. The result reveals that thermal debinding atmosphere has significant effect on the thermal conductivity and structure of AlN ceramics. The thermal conductivity of injection molded AlN ceramics thermal debound in flowing nitrogen gas is 231 W m^?1 K^?1.     

Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Preparation and properties of ALCON (Al28C6O21N6) ceramics

The synthesis of Al₂₈C₆O₂₁N₆ powder (ALCON), starting from the binary compounds is described. The powder is resistant to oxidation in air up to 760°C. From the prepared powder, fully dense ceramics have successfully been prepared using hot pressing. The as-prepared ceramics had a thermal conductivity of 20 W m^–1 K^–1. Experiments showed that it is also possible to prepare ALCON ceramics by reactive hot-pressing, starting from Al₂O₃, AlN and Al₄C₃. Further optimization is expected to raise the thermal conductivity significantly. The strength, about 300 MPa, is similar to that of AlN. The thermal expansion coefficient of 4.8 × 10^–6K^–1 closely matches that of silicon, making application of ALCON ceramics as heat sinks an interesting possibility.     

Density and Thermal Conductivity Measurements for Silicon Melt by Electromagnetic Levitation under a Static Magnetic Field

The density and thermal conductivity of a high-purity silicon melt were measured over a wide temperature range including the undercooled regime by non-contact techniques accompanied with electromagnetic levitation (EML) under a homogeneous and static magnetic field. The maximum undercooling of 320 K for silicon was controlled by the residual impurity in the specimen, not by the melt motion or by contamination of the material. The temperature dependence of the measured density showed a linear relation for temperature as: ρ(T) = 2.51 × 10³−0.271(T−T _m) kg · m⁻³ for 1367 K < T < 1767 K, where T _m is the melting point of silicon. A periodic heating method with a CO₂ laser was adopted for the thermal conductivity measurement of the silicon melt. The measured thermal conductivity of the melt agreed roughly with values estimated by a Wiedemann–Franz law.     

Study on microstructure and thermal conductivity of Spark Plasma Sintering AlN ceramics

Dense aluminum nitride (AlN) ceramics were prepared by Spark Plasma Sintering with rare-earth oxide and CaF₂ as sintering additives. The effect of sintering additives on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that those sintering additives not only promoted densification through liquid-phase sintering but also improved thermal conductivity by decreasing oxygen impurities. Thermal conductivities of samples sintered with optimum proportion of rare-earth oxide and CaF₂ were higher than those of other samples. During the Spark Plasma Sintering process, the microstructures, especially the content and distribution of secondary phases, played important roles on the thermal conductivity of AlN ceramics.     

High dielectric AlN/Al2O3 composite powder using a plasma spray approach for microelectronics application

Increasing demand for higher performance dielectric material for multi-layer ceramics packaging has led to the use of the AlN system due to its very high thermal conductivity and coefficient of expansion compatibility with silicon. This paper reports on a novel process method used to produce an AlN/Al₂O₃ composite powder system which can be subsequently tape cast as a dielectric substrate. The mixture of both Al₂O₃ and AlN was first mechanically alloyed and then spray-dried to obtain a suitable agglomerated powder that was subsequently plasma-sprayed, resulting in a fine micrometer level integrated composite powder. The two main criteria used to ascertain the optimal process parameters during plasma spraying were a high gamma/alpha Al₂O₃ phase ratio, which ensured that all the Al₂O₃ phase had melted during plasma spraying, and a minimal reduction in the AlN/Al₂O₃ ratio to ensure minimal change in the AlN during processing. For the plasma-sprayed composite powders, fully sintered ceramic tapes were produced attaining>99.0% of the theoretical density after sintering at 1650°C for 6 h, which yielded a thermal conductivity value of 32.0 W m^–1 K^–1.     

Electrical conduction in La<Subscript>0.9</Subscript>Ba<Subscript>0.1</Subscript>Er<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−α</Subscript> ceramics

Doubly doped LaErO₃ ceramics, La_0.9Ba_0.1Er_1−x Mg _x O_3−α (x = 0.05, 0.10, 0.15, 0.20), were synthesized by solid-state reaction method and characterized by X-ray diffraction (XRD). The samples have a single orthorhombic perovskite-type structure. The conduction behavior was investigated using various electrochemical methods including AC impedance spectroscopy, gas concentration cell, isotope effect of hydrogen, and hydrogen electrochemical permeation (pumping) in the temperature range of 500–1000 °C. The results indicated that specimens were pure ionic conductors under low oxygen partial pressure (about 10⁻⁷–10⁻²⁰ atm) and mixed conductors of proton, oxide ion, and electron hole under high oxygen partial pressure (about 10⁻⁵–1 atm). The pure ion conduction of the ceramics in hydrogen atmosphere was confirmed by electromotive force method of hydrogen concentration cell, and the observed emf values coincided well with the theoretical ones. The conductivity in H₂O–Ar atmosphere was higher than that in D₂O–Ar atmosphere, exhibiting an obvious isotope effect and proton conduction in water vapor containing atmosphere. It has been confirmed by electrochemical hydrogen permeation (hydrogen pumping) experiment that the ceramics were mainly proton conductors in hydrogen containing atmosphere. Whereas in dry oxygen-containing atmosphere, observed emf values of the oxygen concentration cell were far lower than the theoretical ones, indicating that the ceramics were mixed conductors of electron hole and oxide ion.     

Compositionally graded multilayer Ba<Subscript>x</Subscript>Sr<Subscript>0.95−x</Subscript>Ca<Subscript>0.05</Subscript>TiO<Subscript>3</Subscript> ceramics prepared by tape casting for use as dielectric materials

Compositionally graded multilayer Ba_xSr_0.95−xCa_0.05TiO₃ (BSCT) ceramics were prepared via tape casting method using nanometer powders from co-precipitation. Microstructures and dielectric properties of the BSCT system were investigated. The powders were characterized by using transmission electron microscope and BET surface area measurement. Surface morphologies of the sintered samples and multilayer structure were examined by scanning electron microscopy. BSCT particles were of spherical shape with diameters in the range of 73–93 nm. Their specific surface areas were in the range of 11.7–14.6 m²/g. The graded BSCT ceramics with nine layers laminated in vertical way exhibited a higher sintered density, with an average grain size of 0.4 μm, after sintered at 1,200 °C. Dielectric constant, dielectric loss and tunability of the graded ceramics were 2223.94, 1.5 × 10⁻³ at 2 MHz and 42.9% at 3.0 kV/mm, with good dielectric temperature and frequency stability, which made it a promising candidate used for tunable ceramic capacitors and phase shifters.     

Improved thermoelectric properties of La-doped Bi2Sr2Co2O9-layered misfit oxides

Bi₂Sr_2−x La _x Co₂O₉ (x = 0, 0.02, 0.04, and 0.08) polycrystalline-layered misfit oxides have been prepared by solid-state reactions. Electrical property measurements indicated that all the samples are p-type semiconductors. The electrical conductivity decreased and the Seebeck coefficient increased with increasing temperature. The thermal conductivities were very low, only 0.6–0.7 W m⁻¹ K⁻¹ at room temperature. La doping was effective in increasing the Seebeck coefficient, reducing the thermal conductivity, and hence improving the thermoelectric performance. A highest dimensionless figure of merit ZT of 0.147 was obtained for Bi₂Sr_1.96La_0.04Co₂O₉ sample at 737 K, about two times higher than that of the sample without La doping.     

The study of AlN ceramics sintered at superhigh pressure with belt-type press technology

The effects of Y₂O₃ content, sintering time, sintering temperature, sintering pressure on thermal conductivity of AlN ceramics had been studied. X-ray diffraction (XRD), scanning electron microscope (SEM), laser conductometer and laser granularity dimension analysis measurer were respectively used to measure the phases, microstructure, thermal conductivity and particle size distribution of the samples. These studies reveal that the Y₂O₃ is an effective sintering addtive, and the best conditions of sintering are that the pressure is 5.15× 10⁹ Pa, the temperature is 1700∘C and the sintering time is 115 min. Under these conditions, the sintered body has reasonable structure and its thermal conductivity is 200 w/(m⋅k).     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Modeling Thermal Conductivity of Electrolyte Mixtures in Wide Temperature and Pressure Ranges: Seawater and Its Main Components

A model has been established for calculating the thermal conductivity of aqueous electrolyte solutions containing the Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, and Br⁻ ions. The model is based on a previously developed computational framework for the thermal conductivity of mixed-solvent electrolyte systems, which has been expanded by explicitly accounting for pressure effects in addition to temperature and electrolyte composition effects. The model consists of a contribution of the solvent, a contribution of individual species expressed using modified Riedel coefficients, and an ionic strength-dependent term that is due to interactions between species. The model accurately represents the thermal conductivity of solutions containing single and multiple salts at temperatures ranging from 273 K to 573 K, pressures up to at least 1400 bar, and concentrations up to the limit of solid saturation. Further, the model has been applied to seawater and used to elucidate the discrepancies between the experimental data for seawater and those for Na–K–Mg–Ca–Cl–SO₄ salt solutions. With parameters evaluated on the basis of data for binary and multicomponent salt solutions, the model provides reliable predictions of the thermal conductivity of seawater.