高气孔率及低热导率多孔莫来石陶瓷的制备(英文)

采用叔丁醇基凝胶注模工艺制备多孔莫来石陶瓷,研究了固含量和烧结工艺对多孔莫来石陶瓷显微结构、气孔率、气孔尺寸及分布、压缩强度和室温热导率的影响。结果表明,固含量相同时,随着烧结温度的升高,多孔莫来石陶瓷的气孔率不断降低,而抗压强度则不断增加;当固含量为15%、烧结温度为1 350℃时,多孔莫来石陶瓷的气孔率最高为71.7%,平均气孔孔径为3.49μm,而热导率则低至0.103W/(m·K)。通过改变烧结温度和初始固含量可调整多孔莫来石陶瓷的微观结构和性能。     

热处理对AlN陶瓷的热导率及介电性能的影响

采用放电等离子烧结技术制备Al N陶瓷,并利用真空无压烧结方法进行热处理。研究了不同烧结助剂在高温热处理过程中对Al N陶瓷的物相组成、显微结构、导热性能及介电性能的影响。结果表明,Al N陶瓷经过热处理后,Al N晶粒进一步发育长大,尽可能实现面接触,材料中的晶间相分布更加合理。与只引入单一稀土氧化物为烧结助剂的Al N陶瓷相比,引入复合烧结助剂稀土氧化物及氟化钙(Ca F2)的Al N陶瓷经过热处理后,在有效除氧及生成具有挥发性晶间相双重效果作用下,材料热导率及介电损耗改善效果明显。     

耐火材料热导率的数值推演方法

为了解决目前通过GB/T 5990—2006测定耐火材料热导率时存在的成本高、周期长、试样制作困难等问题,基于微观结构特征和数值分析技术,提出了一种耐火材料热导率推演方法,并运用该方法推演了高铝砖的热导率。结果表明,数值推演结果与理论计算结果相吻合。推演方法实现了对耐火材料热导率的预测,解决了试验方法测定热导率存在的问题,克服了多孔材料气孔率与其热导率的理论计算公式适用范围有限且不精确的缺点,提供了对热导率推演和设计的高效方法。     

烧结助剂对氮化硅陶瓷热导率影响的研究进展

随着大规模、超大规模集成电路的发展,以及集成电路在通讯、交通等领域的运用。对于基板材料的要求日益严苛,氮化硅陶瓷因为有着优异的力学性能、介电性能和导热性,是作为基板材料的重要候选材料之一。氮化硅陶瓷的理论热导率高达200-320 W/(m·K),但是实际上高热导率的氮化硅难以制成。随着科研者将精力投入到氮化硅上,近年来氮化硅陶瓷的实际热导率得到提高,但是与理论热导率还有着不少差距。据文献记载,选择合适的烧结助剂能够有效的提高氮化硅陶瓷的热导率。本文综述了不同种类的烧结助剂对氮化硅陶瓷热导率的影响。     

高热导率氮化铝陶瓷研究进展

氮化铝(AIN)陶瓷具有热导率高、热膨胀系数低、电阻率高等特性以及良好的力学性能,被认为是新一代高性能陶瓷基片和封装的首选材料.本文简要介绍了氮化铝陶瓷的基本特性,重点总结了氮化铝陶瓷的国内外研究现状及其制备工艺,并列举了一些氮化铝陶瓷的应用实例.     

采用HOTDISK测量材料热导率的实验研究

使用HotDisk热分析仪进行材料热导率测量的实验研究。通过对不锈钢标准件,冰和多孔介质的热导率测量,证明该分析仪器能够适应不同材料的热物性测试。对冰的测量结果与文献值的最大相对误差仅为0.324％,对不锈钢标准件测量结果与标准值一致,在室温(22℃)左右为14.07W·m~(-1)·K~(-1)。对建筑用砂与空气、水和冰组成的饱和多孔介质的有效热导率测量结果表明:砂的颗粒粒径大小对材料的有效热导率影响很大,这种影响主要是接触热阻和连续相相对体积大小相互作用的结果。     

高热导率氮化硅散热基板材料的研究进展

针对越来越明显的大功率电子元器件的散热问题,主要综述了目前氮化硅陶瓷作为散热基板材料的研究进展。对影响氮化硅陶瓷热导率的因素、制备高热导率氮化硅陶瓷的方法、烧结助剂的选择、以及氮化硅陶瓷机械性能和介电性能等方面的最新研究进展作了详细论述,最后总结了高热导率氮化硅作为散热基板材料的发展趋势。     

β-Si3N4陶瓷热导率的研究现状

β-Si3N4陶瓷具有较高的热导率(200～320 W·m-1·K-1),在高速电路和大功率器件散热及封装材料等领域展现了良好的应用前景,并引起广泛关注.基于氮化硅陶瓷导热机理,本文阐述了影响β-Si3 N4陶瓷热导率的因素,并从原料的选取、烧结助剂的选择、晶种的引入和工艺控制四个方面,介绍了国内外提高其热导率的研究进展.     

探针法测量微细颗粒固定床有效热导率

针对微小反应器内多孔介质涉及的复杂热质传递问题,设计并搭建热导率测试装置,经标定,测量误差控制在2.97%内,可重复性良好。利用探针法测量了微小反应器固定床内微细树脂颗粒有效热导率,对其随粒径、孔隙率、流速、有无离子交换等工况的变化进行了研究。结果表明:干燥状态下有效热导率随颗粒粒径与孔隙率的增大而减小,低流量(0.8mL/min、1.6mL/min、3.2mL/min)下有效热导率随粒径与孔隙率增大而增大,经交换离子处理前后有效热导率基本不变,说明探针法测量热导率的应用不受颗粒中离子变化和相关化学处理的影响。不同区域细沙和土壤的实验结果进一步验证了上述结论,表明探针法测量微细颗粒固定床有效热导率是一种可靠、有效的在线测量方法。     

多孔氧化铝陶瓷制备技术研究进展

多孔氧化铝陶瓷同时具有氧化铝陶瓷耐高温、耐腐蚀性好和多孔材料比表面积大、热导率低等优良特点,近年来被广泛应用于节能、环保、生物、化工等众多领域。介绍了现阶段常见的多孔氧化铝陶瓷制备技术,总结了各种制备技术的特点,并对各种不同工艺的研究现状进行了简要综述。最后,展望了多孔氧化铝陶瓷未来可能的发展方向。     

Thermal Conductivity: VIII,A Theory of Thermal Conductivity of Porous Materials

The effective thermal conductivity of a porous material is due to both conduction and radiation processes. A theory is presented relating the effective conductivity to the conductivity of the solid material, to the emissivity of the surface of the pores, and to the size, shape, and distribution of the pores. By means of an anisotropic distribution and orientation of pores, materials can be prepared having different thermal conductivities in different directions.     

Multiscale Genome Modeling for Predicting the Thermal Conductivity of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics have been widely used in industry due to its high thermal conductivity. Understanding the relations between the microstructure and the thermal conductivity of SiC ceramics is critical for improving the efficiency of heat removal in heat sink applications. In this paper, a multiscale model is proposed to predict the thermal conductivity of SiC ceramics by bridging atomistic simulations and continuum model via a materials genome model. Interatomic potentials are developed using ab initio calculations to achieve more accurate molecular dynamics (MD) simulations. Interfacial thermal conductivities with various additive compositions are predicted by nonequilibrium MD simulations. A homogenized materials genome model with the calculated interfacial thermal properties is used in a continuum model to predict the effective thermal conductivity of SiC ceramics. The effects of grain size, additive compositions, and temperature are also studied. The good agreement found between prediction results and experimental measurements validates the capabilities of the proposed multiscale genome model in understanding and improving the thermal transport characteristics of SiC ceramics.     

Thermal Insulation Materials of Alumina Based Porous Ceramics

Significant energy saving effects can be made through the improvement of furnace refractories,especially the thermal insulation refractories. In this study,the preparation and the application of different alumina based porous ceramics were briefly introduced. Alumina based porous ceramics were prepared combined foaming method with gelcasting,sol- gel process or cement curing process. The influences of different preparation methods on the sintering shrinkage, porosity, phase composition, microstructure, compressive strength and thermal conductivity were discussed. Alumina based porous ceramics with relatively high strength and low thermal conductivity could be obtained through the above mentioned methods. Compared with the traditional lining materials,about 40% energy could be saved when they were used as the furnace wall.     

Effect of Grain Thermal Expansion Mismatch on Thermal Conductivity of Porous Ceramics

Many ceramics contain microcracks, which are often situated between sintered grains. These microcracks constitute thermal resistances, which may affect heat transfer through the material and its effective thermophysical properties. The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grains on opposite sides of the microcracks. This physical mechanism affects changes of the material's thermal conductivity, k , with temperature. The above mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature-induced changes of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly via the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for several industrial refractories. In this paper, the influence of the above physical mechanism is discussed relative to other heat transfer mechanisms described in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficients, is developed. This model allows calculation of the contact area and the average microcrack opening between the grains as functions of the temperature, the characteristic grains sizes and their thermal expansion coefficients, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials containing microcracks that appear as a result of thermal contraction of grains. The calculated thermal conductivity satisfactorily correlates with the experimental data collected for several chrome-magnesite refractories over a wide range of temperatures and gas pressures.     

蜂窝陶瓷蓄热材料的研究现状

蜂窝陶瓷蓄热材料应该具有热膨胀系数低、比热容大、比表面积大、导热性能好、抗热震性好等特性。本文详细介绍了几种多孔陶瓷材料的优缺点,指出堇青石质复相材料是目前研究最广泛的蜂窝陶瓷材料。堇青石与多种催化剂匹配性好,比表面积大、热膨胀系数小,但耐热性稍差,于是通过添加一些添加剂来提高堇青石作为蜂窝陶瓷蓄热体的性能。这些添加剂与堇青石结合形成复相材料,可以降低热膨胀系数、提高抗热震性等。     

预测聚合物基复合材料导热系数方法综述

由聚合物与高导热填料共混制得的导热聚合物基复合材料,被应用于防腐和节能要求较高的换热场合,符合换热设备新材料的要求;而聚合物基复合材料的等效导热系数预测比较复杂。总结了预测聚合物基复合材料等效导热系数的多种方法,包括最小热阻力法、热阻网络法、傅里叶定律法、均匀化方法和逾渗理论方法,归纳了这些模型和方法的特点,对应用这些模型和方法提出了建议。     

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Effective thermophysical properties of ceramic materials (mainly insulating materials) with porosity (II) >30% are reviewed. Nonmonotonic pressure and temperature dependences of the effective thermal conductivity (X) are analyzed, based on the ceramic microstructure (pores, cracks, and grain boundaries present in many industrial refractories) and several heat-transfer mechanisms in composite multiphase materials. These mechanisms include heat conduction in solid and gas phases, thermal radiation, gas convection, and the mechanism originating from intrapore chemical conversion processes accompanied by gas emission. For high temperatures, λ of porous insulations is governed by thermal radiation. Contact-heat-barrier resistances play a less-important role in highly porous ceramics than in their dense counterparts. This underlies a weaker pressure dependence at low temperatures (<500°C) of λ of the majority of industrial insulating materials than in dense materials possessing microcracks and small pores in the grain-boundary region. For high gas pressure, λ of porous insulating materials is governed by free convective-gas motion. For low gas pressures (normally <1 kPa), where heat transfer in pores occurs in the free-molecular regime, X is controlled by the pressure-dependent mean free path of gas molecules in pores. A classification of the porous material structure and thermophysical properties is proposed, based on the geometric model described in Part 1 of this series.     

Sintered Reaction-Bonded Silicon Nitride with High Thermal Conductivity and High Strength

Sintered reaction-bonded silicon nitride (SRBSN) materials were prepared from a high-purity Si powder doped with Y₂O₃ and MgO as sintering additives by nitriding at 1400°C for 8 h and subsequently postsintering at 1900°C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four-point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high-thermal conductivity silicon nitride ceramics that were prepared by sintering of Si₃N₄ powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength.     

Thermal Conductivity Characterization of Hafnium Diboride-Based Ultra-High-Temperature Ceramics

We evaluated the thermal conductivity of HfB₂-based ultra-high-temperature ceramics from laser flash diffusivity measurements in the 25°–600°C temperature range. Commercially available powders were used to prepare HfB₂ composites containing 20 vol% SiC, some including TaSi₂ (5 vol%) and Ir (0.5 or 2 vol%) additions. Samples were consolidated via conventional hot pressing or spark plasma sintering. Processing differences were shown to lead to differences in magnitude and temperature dependence of effective thermal conductivity. We compared results with measured values from heritage materials and analyzed trends using a network model of effective thermal conductivity, incorporating the effects of porosity, grain size, Kapitza resistance, and individual constituent thermal conductivities.     

Low Thermal Conductivity in Garnets

The thermal conductivity of dense, polycrystalline garnet ceramics with compositions of Y₃Al_xFe_(5−x)O₁₂(x = 0.0, 0.7, 1.4, and 5.0) was measured in the temperature range 23° to 1000°C. The high-temperature thermal conductivity of some of these garnets was found to be as low as 2.4 W·m^-1K^-1. The effects of temperature and composition on the observed thermal conductivity are discussed with reference to established theories of thermal conduction. The potential use of yttrium aluminum garnet or YAG (Y₃Al₅O₁₂), in particular, for a specific application of advanced thermal barrier coatings (TBCs) with improved durability is considered.