Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.     

Directional Solidification of TiAl‐based Alloys and Properties of Directionally Solidified Ingots

The mechanical properties of TiAl‐based alloys with lamellar microstructure are extremely anisotropic. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of mechanical properties. Unfortunately, simple casting operations often lead to a solidification texture with the lamellar boundaries perpendicular to the heat flow direction. This difficulty can be overcome by directionally solidifying TiAl‐based alloys. We have been performing directional solidification experiments with and without using a seeding technique. The current status of directional solidification of TiAl‐based alloys is reviewed.     

Porous ceramics mimicking nature—preparation and properties of microstructures with unidirectionally oriented pores

Abstract

Porous ceramics with unidirectionally oriented pores have been prepared by various methods such as anodic oxidation, templating using wood, unidirectional solidification, extrusion, etc. The templating method directly replicates the porous microstructure of wood to prepare porous ceramics, whereas the extrusion method mimics the microstructures of tracheids and xylems in trees. These two methods are therefore the main focus of this review as they provide good examples of the preparation of functional porous ceramics with properties replicating nature. The well-oriented cylindrical through-hole pores prepared by the extrusion method using fibers as the pore formers provide excellent permeability together with high mechanical strength. Examples of applications of these porous ceramics are given, including their excellent capillary lift of over 1 m height which could be used to counteract urban heat island phenomena, and other interesting properties arising from anisotropic unidirectional porous structures.     

Electrical and flexural anisotropy in freeze tape cast stainless steel porous substrates

Engineered metal foams have strong potential in applications such as fuel cell electrodes, sensors, variable springs, filtering media, and compositionally graded composite structures. In this study, the variation in mechanical and electrical properties was characterized in engineered porous metal foams with aligned porosity to ascertain the degree of anisotropy that can be induced in these substrates. Porous substrates were prepared by freeze tape casting of powdered ferritic stainless steel. After directional solidification of the slurry, the solvent was sublimed from ~ 1 mm thick tapes, yielding porous green metal compacts which were sintered in a protective atmosphere. The resulting disks exhibit long range ordered acicular pores with substantial anisotropy in the mechanical and electrical properties that is related to the cross sectional pore morphology and connectivity. DC conductivity testing revealed up to 61% variation depending on direction of measurement relative to the alignment of pores. Also, an 89% variation in flexural rigidity in relation to pore orientation was observed in identical disks.     

Porous ceramics mimicking nature—preparation and properties of microstructures with unidirectionally oriented pores

Porous ceramics with unidirectionally oriented pores have been prepared by various methods such as anodic oxidation, templating using wood, unidirectional solidification, extrusion, etc. The templating method directly replicates the porous microstructure of wood to prepare porous ceramics, whereas the extrusion method mimics the microstructures of tracheids and xylems in trees. These two methods are therefore the main focus of this review as they provide good examples of the preparation of functional porous ceramics with properties replicating nature. The well-oriented cylindrical through-hole pores prepared by the extrusion method using fibers as the pore formers provide excellent permeability together with high mechanical strength. Examples of applications of these porous ceramics are given, including their excellent capillary lift of over 1 m height which could be used to counteract urban heat island phenomena, and other interesting properties arising from anisotropic unidirectional porous structures.     

Anisotropic compressive properties of porous copper produced by unidirectional solidification

Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The compressive yield strength of the porous copper with the cylindrical pores orientated parallel to the compression direction decreases linearly with increasing porosity. For the porous copper whose pore axes are perpendicular to the compressive direction, the compressive yield strength slightly decreases in the porosity range up to 30% and then decreases significantly with increasing porosity. The compressive stress–strain curves depend on the compressive direction with respect to the pore direction, which are due to the stress concentration around the pores and the buckling of the copper between the pores. From two different types of stress–strain curve, the energy absorption capacity of the porous copper with the pores parallel to the compressive direction is higher than that perpendicular to the compressive direction at a given porosity.     

Fabrication of Lotus‐Structured Porous Iron by Unidirectional Solidification under Nitrogen Gas

A novel technique has been developed to fabricate lotus‐structured porous iron in which long cylindrical pores are aligned in one direction. The iron is melted and unidirectionally solidified in a pressurized gas mixture of nitrogen and argon. The process involves the dissolution of nitrogen in molten iron and the evolution of nitrogen pores due to the decrease in solubility of nitrogen during solidification. The porosity is controlled by adjusting the partial pressures of nitrogen and argon during melting and solidification. The nitrogen concentration in solid iron increases with increasing partial pressure of nitrogen at a given total pressure, leading to improvement of the mechanical properties of the porous iron.     

金属/气体共晶定向凝固规则多孔金属的研究进展

金属／气体共晶定向凝固是一种制备规则多孔金属的新工艺。评述了该工艺的发展历程、原理和最新进展，同时介绍了由该工艺制备的规则多孔金属的结构和性能特点及其应用。     

Effects of rheological properties on ice-templated porous hydroxyapatite ceramics

Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.     

凝固速率对藕状多孔金属结构的影响

使用自制的Gasar装置制备出Cu、Mg和Ni三种藕状多孔金属, 将理论计算与实验结果相结合研究了材料的凝固速率对多孔结构的影响。结果表明, 对于三种多孔金属, 随着气泡半径的增大气泡的上浮速率逐渐增大。仅当金属凝固速率最大值在气泡上浮速率的范围内, 才能制备出有大量规则气孔的藕状多孔结构。     

Characterisation of pore structures in nanoporous materials for advanced bionanotechnology

Porous materials are potential candidates for applications in various fields, such as bionanotechnology, gas separation, catalysts and micro-electronics. In particular, their applications in bionanotechnology include biosensors, biomedical implants and microdevices, biosupporters, bio-encapsules, biomolecule separations and biomedical therapy. All these bionanotechnology applications utilise the shape, size and size distribution of pores in porous materials. Therefore the controlled creation of pores with desired shape, size and size distribution is most important in the development of nanoporous materials. Accordingly, the accurate evaluation of pore structure is necessary in the development of nanoporous materials and their applications. This article reviews recent developments in analytical techniques to characterise the pore structures of nanoporous materials.     

Porous alumina/zirconia layered composites with unidirectional pore channels processed using a tertiary-butyl alcohol-based freeze casting

Symmetric three-layer porous alumina/zirconia composites with controlled “designer” pore structure have been prepared by a tertiary-butyl alcohol-based freeze casting and sintering at 1400–1500 °C in air. Unidirectional aligned macropore channels were developed over a long range by controlling the solidification direction of tertiary-butyl alcohol solvent; in this case, they were surrounded by more dense structured walls. In layered composite, the bottom layer consisted of small sized pore channels (∼11 μm in diameter) compared with the middle and the top layer, due to the comparatively rapid velocity of the TBA crystal growth during solidification. The compressive strength (63–376 MPa) of the sintered porous layered composite remarkably increased as the porosity decreased (64–32%).     

铸造多晶硅制备技术的研究进展

近年来,由于低成本、低耗能和少污染的优势,铸造多晶硅成为主要的光伏材料之一,越来越受到人们的广泛关注.系统论述了太阳能级多晶硅制备技术的研究进展,重点介绍了目前铸造多晶硅制备技术,如浇注法(Ingot casting)、定向凝固法及电磁感应加热连续铸造法(EMCP).另外,着重阐述了铸造多晶硅中磷和硼的提纯、多晶硅晶粒组织中晶界和位错的形成与控制以及定向凝固的数值模拟技术,讨论了铸造多晶硅材料的研究现状和存在的问题,展望了今后的发展方向.     

多孔钛的研究进展

作为结构功能一体化材料,多孔钛在众多工业领域具有广泛的应用前景,已成为近年来十分活跃的研究方向。简要回顾了多孔钛的研究历史,重点介绍了多孔钛的制备方法与孔结构,并对多孔钛的渗透性能、力学性能和耐腐蚀性能以及主要的商业应用进行了介绍。众多研究和应用表明,多孔钛的性能与功能强烈依赖于孔结构,不同方法制备多孔钛的孔结构可以归纳为均一孔结构、双峰孔结构、梯度孔结构、蜂窝结构和闭孔结构5种类型。除孔结构外,与致密钛合金一样,多孔钛的力学性能和耐腐蚀性能还对间隙元素C,N,O敏感,制备过程中应加以控制。与基于粉末固态扩散机制的传统制备技术相比,增材制造技术由于可以获得任意形式的孔结构,在多孔钛未来的发展和应用中,将呈现出越来越重要的作用。     

Highly porous metals and ceramics

Abstract

Cellular solids are a class of highly porous materials that covers a wide range of structures, such as foamed materials, replicated porous or additive manufactured structures, honeycombs, lattice structures, interconnected fibres or hollow spheres and syntactic foams. The pores can be more or less closed or completely open. Depending on their morphology and composition, these materials possess unique properties and are used as components in special and advanced engineering applications. These include filtering liquids and particles in gas streams, porous burners, biomedical devices, lightweight stiff structures, impact absorbers, sound damping modules, thermal management components, etc. The present paper gives an overview on the main fabrication methods and the resulting architectures, in relation to applications of metals and ceramics with >50 vol.-% porosity.     

Anisotropic Wetting Surfaces with One‐Dimesional and Directional Structures: Fabrication Approaches,Wetting Properties and Potential Applications

This review article provides a brief summary of recent research progress on anisotropic wetting on one‐dimensional (1D) and directionally patterned surfaces, as well as the technical importance in various applications. Inspiration from natural structures exhibiting anisotropic wetting behavior is first discussed. Development of fabrication techniques for topographically and chemically 1D patterned surfaces and directional nanomaterials are then reviewed, with emphasis on anisotropic behavior with topographically (structurally) patterned surfaces. The basic investigation of anisotropic wetting behavior and theoretical simulations for anisotropic wetting are also further reviewed. Perspectives concerning future direction of anisotropic wetting research and its potential applications in microfluidic devices, lab‐on‐a‐chip, sensor, microreactor and self‐cleaning are presented.     

对GASARITE脱合金制备微-纳复合多孔金属的研究

Gasar工艺是一种制备规则排列微米多孔金属的定向凝固工艺,脱合金工艺是一种通过选择性溶解固溶体合金而制备无序纳米多孔金属的工艺方法,将Gasar和脱合金工艺相结合可以制备一种有序-无序结合、微米-纳米复合的特殊结构多孔金属.选择Cu-Mn二元合金为研究对象,研究了Gasar工艺参数及合金成分对定向凝固多孔Cu-Mn合金结构的影响.脱合金工艺在Gasar工艺制备的定向凝固多孔Cu-Mn合金基础上进行,分析了腐蚀温度等脱合金工艺参数对纳米多孔结构的影响.在优化的Gasar和脱合金工艺参数下,制备得到了一种特殊结构的微-纳复合多孔金属.     

利用合金低温生长晶体硅的研究进展

基于光伏行业对低成本晶体硅材料的巨大需求,晶体硅低温生长技术逐渐受到人们的青睐。该技术是在低温下将硅与低熔点金属进行共熔化处理,结合液相外延、合金定向凝固等技术生长、制备晶体硅。首先阐述了晶体硅低温制备原理,并对其生长行为、受控因素和应用现状进行综述,指出了目前各自存在的问题与难点,最后对下一阶段晶体硅生长技术的研究重点和发展前景进行了展望。     

规则多孔铜热膨胀性能的研究

在氢气或氢气和氩气的混合高压气氛中,采用定向凝固技术制备了规则多孔铜材料。测试了不同气孔率的规则多孔铜在平行和垂直气孔方向上的热膨胀系数;研究了气孔、气孔方向和气孔率对其热膨胀系数的影响规律,并对其规律做了理论预测。结果表明,规则多孔铜的热膨胀系数随着温度升高先急剧增大到一定值后趋于平稳;温度在40~130℃,气孔中存在闭孔时,规则多孔铜的CTE值随气孔率的增大而缓慢增大,且比纯铜时略大;当气孔主要以通孔形式存在时,气孔率与孔径的比值越大,规则多孔铜的CTE值越低。温度>130℃时,规则多孔铜的热膨胀系数与纯铜的几乎相同,气孔的存在对铜的膨胀无明显影响。     

Gasar—A new Class of Porous Materials

The paper summarizes published data and also deals with technology, structure, applications, and properties of gasars – new porous materials based on original findings obtained by authors. The method consists of melting a material in a gas atmosphere to saturate it with hydrogen and directional solidifying under strictly controlled thermodynamic and kinetic conditions. The materials produced by this method, have a monolithic matrix and pores of proper geometric shapes, providing to gasars higher strength, plasticity, thermal and electrical conductivities as compared with those of other porous materials. Gasar is recommended for prospective application as filters, bearings, metal‐matrix composites.