The effect of carbon coating of AIN powder on sintering behavior and thermal conductivity

High thermal conductive AlN ceramics doped with Y₂O₃ were produced by sintering the powders obtained after applying a carbon coating to the surface of AlN powder grains. During sintering at 1800°C for 1 hour, the carbon reacts with the surface of the AlN grains by carbothermal-reduction of Al₂O₃, and also with the Al₂Y₄O₉ intermediate phase to form AlN, Y₂O₃ and CO. By adding 0.56 mass% of carbon, almost all the Al₂Y₄O₉ is reacted and the thermal conductivity increases from 184 W/(m · K) to 224 W/(m · K). Further carbon addition decreases the thermal conductivity and also the final sintered density.     

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.     

The use of yttrium (III) isopropoxide to improve thermal conductivity of polycrystalline aluminum nitride (AlN) ceramics

Instead of Y₂O₃ powders, yittrium isopropoxide (YIP) was used as a sintering additive to sinter high thermal conductivity polycrystalline aluminum nitride (AlN). The reasons for using sintering additive in sol-gel form are due to the fact that the particle sizes are uniform in the nano scale and also they promote a better coating of AlN grains, being more effective during sintering process. The binder burn out was carried in two different atmospheres, N₂ (N₂ BBO) and air (air BBO). The thermal conductivity of dense polycrystalline aluminum nitride samples with the addition of Y₂O₃ (YIP formulation) ranging from 1.0 to 10.0 wt% with N₂ BBO and air BBO was measured by the laser-flash technique. The results of measured thermal conductivity exhibited higher values than those reported for samples of same yttria formulation (Y₂O₃ powder) and sintered conditions.     

Low temperature sintering of aluminum nitride with millimeter-wave heating

Rapid sintering of Yb₂O₃-added AlN was performed by applying the 28 GHz millimeter-wave heating method. It was found that full densification over 97%T.D. was attained by sintering at 1600°C for 20 min. The densification temperature was decreased by about 300°C, compared with those in the conventional method by an electric furnace. A high thermal conductivity over 180 W/(m · K) was obtained in the sample sintered at 1700°C for 40 min, even under non-reducing atmosphere. The main factor resulting in the rapid and low temperature sintering was attributed to efficient selective absorption of the millimeter-wave into Yb₂O₃ additive.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

Chemical reactions to increase thermal conductivity by decreasing oxygen contents during AlN sintering with an Y₂O₃ additive in a reducing nitrogen atmosphere with carbon were investigated. They were: Al₂O₃ + N₂ + 3CO ⇋ 2AlN + 3CO₂, Al₂Y₄O₉ + N₂ + 3CO ⇋ 2AlN + 2Y₂O₃ + 3CO₂ and Y₂O₃ + N₂ + 3CO ⇋ 2YN + 3CO₂. Some of the CO₂ gas reduced to CO gas in the presence of carbon by a chemical reaction: CO₂ + C ⇋ 2CO. These reactions were confirmed by examining oxygen contents, the grain boundary phases of the sintered AlN, and the trapped CO and CO₂ gases in the sintered bodies. These reducing reactions proceed with increasing sintering temperature and periods, and hence the thermal conductivity is increased.     

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).     

The influence of microstructure on the thermal conductivity of aluminium nitride

Starting from three different commercial powders, AIN materials were densified by pressureless sintering under various temperature and time values in order to investigate the influence of microstructure on thermal conductivity. The influence of the sintering aids (3 wt% Y₂O₃ and 2 wt% CaC₂) and of the forming processes (cold isostatic pressing and thermocompression of tape cast pieces) were also been evaluated. Thermal conductivity increased with the purity level of the starting powder and with an increasing the sintering temperature and soaking time. The highest thermal conductivity values (196 Wm^–1 K^–1) were obtained with the purest powder and high temperature (1800 °C) sintering over long periods (6 h). No influence on thermal conductivity was detected from the forming technique.     

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.     

Fabrication of particulate aluminium-matrix composites by liquid metal infiltration

Aluminium-matrix composites were fabricated by liquid metal infiltration of porous particulate reinforcement preforms, using AlN, SiC and Al₂O₃ as the particles. The quality of the composites depended on the preform fabrication technology. In this work, this technology was developed for high-volume fraction (up to 75%) particulate preforms, which are more sensitive to the preform fabrication process than lower volume fraction whisker/fibre preforms as their porosity and pore size are much lower. The technology developed used an acid phosphate binder (with P/Al molar ratio=23) in the amount of 0.1 wt% of the preform, in contrast to the much larger binder amount used for whisker preforms. The preforms were made by filtration of a slurry consisting of the reinforcement particles, the binder and carrier (preferably acetone), and subsequent baking (preferably at 200 °C) for the purpose of drying. Baking in air at 500 °C instead of 200 °C caused the AlN preforms to oxidize, thereby decreasing the thermal conductivity of the resulting Al/AlN composites. The reinforcement-binder reactivity was larger for AlN than SiC, but this reactivity did not affect the composite properties due to the small binder amount used. The Al/AlN composites were superior to the Al/SiC composites in the thermal conductivity and tensile ductility. The Al/Al₂O₃ composites were the poorest due to Al₂O₃ particle clustering.     

Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

CaO-Y2O3添加剂对AlN陶瓷显微结构及性能的影响

研究了掺杂ＣａＯ－Ｙ２Ｏ３热压烧结和常压烧结ＡｌＮ陶瓷的性能和显微结构．结果表明：热压烧结ＡｌＮ陶瓷的第二相为Ｙ３Ａｌ５Ｏ１２，常压烧结ＡｌＮ陶瓷的第二相为Ｙ３Ａｌ５Ｏ１２和Ｃａ３Ｙ２Ｏ６；热压烧结ＡｌＮ的第二相体积百分数和晶格氧含量均低于常压烧结；热压烧结ＡｌＮ陶瓷的微观结构良好，其热导率达到２００Ｗ／ｍ·Ｋ．     

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.     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Sintering behaviour of slip-cast Al2O3 – Y-TZP composites

The sintering behaviour of alumina–Y-TZP composites prepared by slip-casting technique were studied. Slip-cast samples containing varying amounts of Y-TZP ranging up to 90 vol% were prepared and evaluated. Sintering studies were carried out at 1450°C to 1600°C. Sintered samples were characterised where appropriate to determine phases present, grain sizes, bulk density and mechanical properties. Good correlation was obtained between the calculated prepared powder density and experimental results. The sintered bulk density of the composites was observed to increase with increasing Y-TZP content and sintering temperature up to 1550°C. Maximum hardness values (>14 GPa) were obtained for all samples containing <60 vol% Y-TZP and when sintered at 1550°C. It has been found that the additions of up to 50 vol% Y-TZP was effective in suppressing Al₂O₃ grain growth.     

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si₃N₄ and either 60 vol% TiN or 50 vol% TiB₂, while three were monolithic materials, namely Si₃N₄ with 2.5 wt% Y₂O₃ as a sintering additive, Si₃N₄ without additives, and Si₂ N₂O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.