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.     

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.     

Low temperature co-fired AlN multilayer substrates

A process for low temperature co-fired AlN multilayer substrates is introduced. Some key factors about this technology are delineated and discussed. A two-step burnout process may solve the contradiction between tungsten oxidation and carbon removal. Sintering with additives appears to improve densification at low temperature. DyN was found as a second phase in AlN ceramics, which suggests that Dy₂O₃ efficiently removes oxygen from the AlN lattice. The microstructure of AlN ceramics is ideal for achieving high thermal conductivity. Analysis of the AlN-W interface showed there were no second phases, but there was probably an intricate interlocking structure between the grains of tungsten and AlN. Co-firing at 1650°C for 4 h produced an AlN multilayer substrate with a thermal conductivity of up to 130 W m⁻¹ K⁻¹. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

稀土氧化物对SPS烧结AlN陶瓷电性能的影响

以AlN粉末为原料, 添加稀土氧化物(Sm₂O₃、Y₂O₃), 在氮气气氛下, 采用SPS烧结方法制备AlN陶瓷, 研究稀土氧化物的掺杂对AlN烧结试样相组成、微观结构和电性能的影响。实验表明: Sm₂O₃、Y₂O₃与Al₂O₃反应生成的液相稀土金属铝酸盐会提高AlN陶瓷致密度, 且在晶界处形成导电通路降低了AlN陶瓷电阻率。随着Sm₂O₃掺杂量的增加, 晶界相逐渐由Sm₄Al₂O₉过渡到SmAlO₃, 且Sm₄Al₂O₉对电阻率贡献最大。其中, 3wt% Sm₂O₃掺杂AlN陶瓷电阻率最低, 为      

A comparative study of Spark Plasma Sintering (SPS), Hot Isostatic Pressing (HIP) and microwaves sintering techniques on p-type Bi2Te3 thermoelectric properties

The sintering of a synthesized p-type Bi₂Te₃ nano-powder has been investigated by three different techniques. Sintering techniques such as Hot Isostatic Pressing (HIP), microwave sintering and Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters i.e. temperature, pressure, and power, microstructure and thermoelectric properties of the prepared ceramics. This study demonstrates that the highest figure of merit ZT has been obtained using microwaves or SPS techniques. Ceramics observations reveal differences in microstructure as well as the presence of intra-granular precipitates in the pellets sintered by the three techniques. We finally conclude about the relationship between properties and microstructure to get optimum thermoelectric materials.     

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).     

烧结助剂及Mo含量对AlN-Mo复合陶瓷热导率的影响

以CaF2、CaCO3为烧结助剂,采用热压烧结法制备了AlN-Mo复合材料。利用XRD和SEM分析了AlN-Mo复合陶瓷的相组成及其微观形貌,并讨论了烧结助剂和Mo含量对该材料热导率的影响。结果表明,CaF2和CaCO3烧结助剂的添加量在1%～3%(质量分数)范围内,AlN-Mo复合材料的热导率随着CaF2含量的增加而升高,随着CaCO3含量的增加先升高后降低。在烧结助剂的种类和含量一定时,含20%(体积分数)Mo的AlN-Mo复合陶瓷的热导率高于含18%(体积分数)Mo的AlN-Mo复合陶瓷的热导率。     

Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

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.     

Spark Plasma Sintering of ZrB2-SiC-ZrC ultra-high temperature ceramics at 1800 °C

The compositional effects in ZrB₂-SiC-ZrC ultra high temperature composites with four different compositions were investigated via Spark Plasma Sintering (SPS) at a maximum temperature of 1800 °C. Density, Rockwell hardness, and thermal conductivity were measured, along with structural X-ray diffraction (XRD) and microstructural characterization. The relative amounts of SiC and ZrC had an influence on the composites’ density, mechanical and thermal properties.     

Fabrication of transparent Nd:YAG ceramics by vacuum sintering with CaF2 and tetraethoxysilane additives

Abstract

Nd:YAG ceramics with CaF₂ and tetraethoxysilane (TEOS) as sintering additives were fabricated by vacuum sintering at 1750°C for 5 h and the microstructures were characterised using X-ray diffraction and scanning electron microscopy. Scanning electron microscopy result shows that the sintered bulk doped with TEOS contains many pores in the grains. The bulk doped with CaF₂ displays uniform microstructure. The average size of the grains is 10 mm and few pores can be detected in this sintered bulk. The maximum transmittance of the sintered Nd:YAG ceramics were 44% doped with CaF₂ and 14% doped with TEOS.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

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.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

Mg-Si-Sn基热电器件电极材料的优化选择与连接工艺

研究了采用不同放电等离子烧结(SPS)工艺获得的单质金属(Ni、Cu、Ag、Al)电极与Mg-Si-Sn基热电材料结合界面的微观形貌和成分分布特征, 测试了合金(Ni-Al、Cu-Al)、金属/合金复合电极材料的热膨胀系数、电导率和热导率等物性参数。实验结果表明: 通过SPS烧结可以有效实现电极材料与Mg-Si-Sn基材料的连接, 复合电极材料Ni-Al/Al(60:40)和Cu-Al/Cu(45:55)具有高的电导率和热导率, 并且热膨胀系数与Mg-Si-Sn基热电材料相匹配, 有可能成为Mg-Si-Sn基材料的较理想电极材料。     

The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites

The sintering and grain growth behaviour of alumina + 2, 3.5 and 5 wt.% carbon nanotubes (CNTs) and alumina + 2 wt.% carbon black nanocomposites prepared by Spark Plasma Sintering (SPS) were studied. The addition of CNTs to ceramics produces a large reduction in the sintering temperature required for their complete densification and a significant grain size refinement by a previously unreported mechanism. The CNTs form a strong entangled network around the grains, which constrains the normal and abnormal grain growth. An alumina/alumina + 2 wt.% CNT/alumina laminate structure was prepared to demonstrate directly the large grain-growth retardation effect of CNTs. These effects open up the possibility of using CNTs as a sintering aid to control the sintering behaviour and microstructures of ceramics in bulk, laminate and functionally gradient (FGM) form.     

Effect of additives on some properties of silicon oxynitride ceramics

Silicon oxynitride ceramics are formed by reaction sintering of silicon nitride and silica with certain metal oxide additives. The reaction rate during sintering and the subsequent properties of silicon oxynitride are affected by the quantity and kinds of additives. The reaction rate increased for addition of equal molar amounts of ZrO₂, ZrO₂ (+2.8 mol % Y₂O₃), AlO_1.5, LnO_1.5, CeO₂, MgO, in that order (where Ln=Nd, Sm, Gd, Dy, Er, Yb and Y). The lanthanide oxide (1.5 mol %)-doped silicon oxynitride ceramics had a high fracture toughness, because crack deflection occurred due to the precipitation of an intergranular crystalline phase with a high thermal expansion coefficient compared with silicon oxynitride. The oxidation rate was higher with an increasing quantity of additive. In samples containing an intergranular crystalline phase, stability of the crystalline phase is an important factor and could impair the oxidation resistance of silicon oxynitride ceramics.     

(YCa)F3助烧AlN陶瓷的显微结构和热导率

采用（ＣａＹ）Ｆ_３为助烧结剂，低温烧结（１６５０℃， ６ｈ）制备出热导率为２０８Ｗ／ｍ·Ｋ的ＡＩＮ陶瓷，在烧结过程中，热导率随保温时间的变化服从方程：λ（ｔ）＝λ∞－△λ（０）·ｅ~（－ｔ／ｒ）·用ＳＥＭ、 ＳＴｈＭ、 ＴＥＭ和 ＨＲＥＭ对 ＡＩＮ陶瓷的显微结构及其对热导率的影响进行了研究，结果表明，晶粒尺寸对ＡＩＮ陶瓷热导率的影响可以忽略，而分隔在ＡＩＮ晶粒之间的晶界相会降低热导率。     

Microstructure and thermal conductivity of AIN ceramics containing additives

The effect on AIN ceramic of the addition of Y₂O₃, Yb₂O₃, Er₂O₃ and CaO were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal conductivity measurements. The effect of grain boundary segregation and second phase distribution on the thermal conductivity are discussed. The Er₂O₃-CaO-and the Yb₂O₃-CaO-AIN ceramics have a higher thermal conductivity than the CaO-and the Y₂O₃-CaO-AIN ceramics. This is explained on the basis of the free energy of formation (G°), the vaporization of the sintering additives and the microstructural development. Oxidation of freshly cleaned surfaces of those AIN ceramics was studied.