Effect of pore structure on the compressive property of porous Ti produced by powder metallurgy technique

Porous Ti with an average macro-pore size of 200–400 μm and porosity in the range of 10–65% has been manufactured using polymethyl methacrylate (PMMA) powders as spacer particles. The compressive strength and elastic modulus of resultant porous Ti are observed in the range of 32–530 MPa and 0.7–23.3 GPa, respectively. With the increasing of the porosity and macro-pore size, the compressive strength and modulus decrease as described by Gibson–Ashby model. The failure due to cracking (complete fracture) of the struts on porous Ti is controlled primarily by macro-pores. Fractography shows evidence of the brittle cleavage fracture mainly, but containing a few fine shallow dimples and a small amount of transcrystalline fracture of similarly oriented laths. The failure mechanism has been discussed by taking the intrinsic microstructural features into consideration.     

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si₃N₄ ceramics were prepared by freeze casting using liquid N₂ as refrigerant. The pore structure, porosity, α → β-Si₃N₄ transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α → β-Si₃N₄ phase transformation, but was beneficial to the mechanical properties of the obtained porous Si₃N₄ due to its unique pore structure.     

Monodisperse porous pod-like hematite: Hydrothermal formation, optical absorbance, and magnetic properties

Uniform monodisperse porous silk-worm pod-like hematite (α-Fe₂O₃) superstructures were efficiently obtained via a facile hydrothermal route. This was realized via the co-precipitation of FeCl₃ and NaOH solutions at room temperature in the presence of H₃BO₃, followed by a mild hydrothermal treatment (150 °C, 12.0 h). The UV-vis band is mainly located in the far-UV region, with one intense broadband from 260 to 290 nm. The porous pod-like superstructures exhibit special magnetic properties with a remnant magnetization of 0.22 emu g^− 1 and a high coercivity of 3315.5 Oe at room temperature, indicating the as-obtained porous pod-like superstructures of a promising candidate for gas sensors, lithium ion battery, photocatalysis, and water treatment.     

Effects of local thermal non-equilibrium on the pore pressure and thermal stresses around a spherical cavity in a porous medium

In this paper, we use a local thermal non-equilibrium (LTNE) thermo-poroelasticity theory to investigate temperatures, pore pressure and thermal stresses around a spherical cavity in an infinite fluid saturated porous medium. In the LTNE theory, the solid and fluid phases undergo different temperature variations which induce additional pore pressure and thermal stresses. The asymptotic short time solutions of temperature, pore pressure and thermal stresses are obtained using the Laplace transform technique. Numerical results for two porous materials (clay and sandstone) are presented to examine the effects of LTNE on the temperature, pore pressure and thermal stresses around the spherical cavity. The results show that for the clay, the LTNE radial stress has a significantly larger peak value (magnitude) than that of the classical radial stress. The influence of LTNE on the pore pressure and tangential stress, however, are marginal. For the sandstone, both the radial stress (magnitude) and pore pressure are significantly increased by the LTNE effect.     

无机多孔基复合相变材料的热湿性能研究进展

无机多孔基复合相变材料兼具调温调湿性能,能够减少室内温度波动,调节室内湿度平衡,既提高了环境的舒适度,又减少了建筑能耗,是近几年建筑材料领域的研究热点。介绍了相变材料的分类及优缺点,并总结了不同种类相变材料的性能优化措施;简述了常用的无机多孔材料,分析比较了其对相变材料的强化导热效果和定形效果;分析了无机多孔基复合相变材料的热湿综合性能,并讨论了材料的不足之处。最后,提出了优化材料性能的措施以及研究方向,为深入研究调温调湿材料提供帮助。     

Improving strength, drying shrinkage, and pore structure of concrete using metakaolin

This paper presents the results of an investigation on the use of metakaolin (MK) as a supplementary cementing material to improve the performance of concrete. Two MK replacement levels were employed in the study: 10% and 20% by weight of the Portland cement used. Plain and PC-MK concretes were designed at two water–cementitious materials (w/cm) ratios of 0.35 and 0.55. The performance characteristics of the concretes were evaluated by measuring compressive and splitting tensile strengths, water absorption, drying shrinkage, and weight loss due to the corresponding drying. The porosity and pore size distribution of the concretes were also examined by using mercury intrusion porosimetry (MIP). Tests were conducted at different ages up to 120 days. The results revealed that the inclusion of MK remarkably reduced the drying shrinkage strain, but increased the strengths of the concretes in varying magnitudes, depending mainly on the replacement level of MK, w/cm ratio, and age of testing. It was also found that the ultrafine MK enhanced substantially the pore structure of the concretes and reduced the content of the harmful large pores, hence made concrete more impervious, especially at a replacement level of 20%.     

Control of molecular packing structure of a derivative of vanadyl-phthalocyanine using pore wall of porous alumina and/or magnetic field

We filled one-dimensional alumina pores with a molten derivative of vanadyl-phthalocyanine (VOPcHt) and evaluated the effect of the surface of pore wall on the molecular packing structure of VOPcHt. We also evaluated the effect of a magnetic field of 5.0 T on the packing structure of VOPcHt that was solidified on a flat substrate. X-ray diffraction measurements revealed that both the surface effect as well as the magnetic field controlled the packing structure of VOPcHt. When both the pore wall and the magnetic field existed, the surface effect was dominant in the control of the packing structure. These results provide practical methods to fabricate nanostructures of organic molecules with a controlled molecular packing structure.     

Critical WIP loops: a mechanism for material flow control in flow lines

Critical WIP loops (CWIPL) is a proposed material flow control mechanism for a balanced flow line environment aiming at improving throughput and lead time. The mechanism establishes critical loops which their WIP identifies the time of releasing raw material to the line. So, through control of WIP level of critical loops the material flow is managed. The proposed mechanism releases the raw material to the line if the ‘total WIP of the line’ or ‘the WIP of the last machine’ is less than the limit. Besides the aforementioned condition, the necessary condition for releasing the raw material to the line is ‘idleness of the first machine’. Simulation is used to compare the performance of the CWIPL, CONWIP and G-MaxWIP. Different line characteristics such as number of machines, processing time distributions and the maximum WIP level of the line are considered in numerical examples. The results show that CWIPL improves both throughput and lead time compared with CONWIP, while CWIPL has better results than G-MaxWIP with respect to both throughput and lead time in the flow line that has less than nine machines.     

Effects of scan line spacing on pore characteristics and mechanical properties of porous Ti6Al4V implants fabricated by selective laser melting

The use of porous structures is gaining popularity in biomedical implant manufacture fields due to its ability to promote increased osseointegration and cell proliferation. Selective laser melting (SLM) is a metal additive manufacturing (MAM) technique capable of producing the porous structure. Adjusting the parameter of scan line spacing is a simple and fast way to gain porous structures in SLM process. By using the medical alloy of Ti6Al4V, we systematically study the influence of the scan line spacing on pore characteristics and mechanical properties of porous implant for the first time. The scanning electron microscope (SEM) results show that the porous Ti6Al4V implants with interconnected pore sizes which ranges from 250 to 450 μm is appropriate for compact bone. The compression strength and modulus of the porous Ti6Al4V implants decrease with the increase of the scan line spacing, and two equations by fitting the data have been established to predict their compression properties. The compressive deformation of the porous Ti6Al4V implants presented an adiabatic shear band (ASB) fracture, which is similar to dense Ti6Al4V owing to the dense thin wall structures. The ability to create both high porosity and strong mechanical properties implants opens a new avenue for fabricating porous implants which is used for load-bearing bone defect repair and regeneration.     

Space-holder effect on designing pore structure and determining mechanical properties in porous titanium

The present work studied the effect of space holders on pore structure and mechanical properties in porous titanium. Four types of space holders (sodium chloride, starch, and urea with different size and morphology) were utilized to fabricate porous titanium. The space holders played a key role in the pore structure as confirmed by microstructural observations and three-dimensional computed tomography technique. Mechanical properties of each sample were investigated and discussed on the basis of the tomography results and finite element method. It is concluded that the porosity determines the elastic modulus regardless of the type and morphology of space holders, whereas both porosity and type of space holder affected strength.     

A review of Fe3O4 thin films: Synthesis,modification and applications

Magnetite (Fe₃O₄) has been used for thousands of years as one of the important magnetic materials. The rapid developments of thin film technology in the past few decades attract the attention of material scientists on the fabrication of magnetite thin films. In this article, we present an overview of recent progress on Fe₃O₄ thin films. The widely used preparation methods are surveyed, and the effect of substrates is discussed. Specifically the modified Fe₃O₄ thin films exhibit excellent electrical and magnetic properties compared with the pure films. It is noteworthy that modified Fe₃O₄ thin films can be put into two categories: (1) doped films, where foreign metal ions substitute iron ions at A or B sites; and (2) hybrid films, where magnetite phases are mixed with other materials. Notably, Fe₃O₄ thin films show great potentials in many applications such as sensors and batteries. It is expected that the investigations of Fe₃O₄ thin films will give us some breakthroughs in materials science and technology.     

New ternary intermetallics RE5Ru3Al2 (RE = La, Ce, Pr): Synthesis, crystal structures, magnetic and electric properties

Intermetallics RE₅Ru₃Al₂ (RE = La, Ce, Pr) were prepared by arc melting of the elemental components with subsequent annealing at 820 K. The crystal structures were determined from single-crystal (Ce₅Ru₃Al₂, Pr₅Ru₃Al₂) and powder (La₅Ru₃Al₂) X-ray diffraction at room temperature. The compounds belong to new structural types: space group R3, Z = 6, a = 13.9270(3) Å, c = 8.3260(2) Å for Ce₅Ru₃Al₂ and space group I2₁3, Z = 4, a = 9.95419(6) Å and 9.8084(3) Å for La₅Ru₃Al₂ and Pr₅Ru₃Al₂, respectively. The trigonal structure of Ce₅Ru₃Al₂ is a distorted variant of the cubic La₅Ru₃Al₂ and Pr₅Ru₃Al₂ structures. An interesting feature of the Ce₅Ru₃Al₂ are the short (2.5299(16) Å and 2.5969(15) Å) Ce-Ru distances. Magnetic measurements revealed the Pauli paramagnetic behavior in La₅Ru₃Al₂ and the Curie-Weiss paramagnetism in Pr₅Ru₃Al₂. The latter compound likely exhibits a kind of magnetically ordered state below 24 K. In turn, Ce₅Ru₃Al₂ remains paramagnetic down to 4.2 K and shows signs of mixed valence states of Ce ions. Electrical resistivity measurements indicated simple metallic conductivity in La₅Ru₃Al₂ and Pr₅Ru₃Al₂, and a more complex metallic behavior in Ce₅Ru₃Al₂.     

层次孔炭材料的设计制备及其在储能领域的应用

层次孔炭材料呈合理的微孔-中孔/大孔结构及孔径分布,具有高的电化学活性表面、极短的扩散距离和较高的传质速率,在用作储能器件电极材料时,表现出优异的功率特性.通过综述近来年层次孔炭材料的设计制备及其在储能领域的应用进展,重点介绍了本课题组自2008年以来的研究成果,进而展望了层次孔炭材料的发展方向.指出:层次孔炭材料主要通过模板法或模板-活化联合法制备.这两种方法可以实现炭材料纳米结构的精确调控.最近,开发出来的更简易的免模板法展现出较好的应用前景.     

Recent progress in patterned silicon nanowire arrays: fabrication, properties and applications

Currently there is great interest in patterned silicon nanowire arrays and applications. The accurately controlled fabrication of patterned silicon nanowire arrays with the desirable axial crystallographic orientation using simpler and quicker ways is very desirable and of great importance to material synthesis and future nanoscale optoelectronic devices that employ silicon. The recent advances in manipulating patterned silicon nanowire arrays and patents are reviewed with a focus on the progress of nanowire fabrication and applications.     

Recent progress in the development and properties of novel metal matrix nanocomposites reinforced with carbon nanotubes and graphene nanosheets

One-dimensional carbon nanotubes and two-dimensional graphene nanosheets with unique electrical, mechanical and thermal properties are attractive reinforcements for fabricating light weight, high strength and high performance metal-matrix composites. Rapid advances of nanotechnology in recent years enable the development of advanced metal matrix nanocomposites for structural engineering and functional device applications. This review focuses on the recent development in the synthesis, property characterization and application of aluminum, magnesium, and transition metal-based composites reinforced with carbon nanotubes and graphene nanosheets. These include processing strategies of carbonaceous nanomaterials and their composites, mechanical and tribological responses, corrosion, electrical and thermal properties as well as hydrogen storage and electrocatalytic behaviors. The effects of nanomaterial dispersion in the metal matrix and the formation of interfacial precipitates on these properties are also addressed. Particular attention is paid to the fundamentals and the structure–property relationships of such novel nanocomposites.     

Synthesis of metallic Zn microprisms, their growth mechanism and PL properties

Novel metallic Zn hexagonal hollow microprisms have been synthesized by a simple thermal evaporation technique using NH₃ as a carrier gas under atmospheric pressure. As-prepared hollow microprisms were characterized using X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. The hollow microprisms collected on silicon substrate are found to be 3-7 μm long with diameter in the range 900-950 nm. A vapour-solid (VS) process based growth mechanism has been proposed for the formation of hexagonal Zn microprisms grown along (0001) basal plane in [0001] direction. Photoluminescence (PL) emission spectrum of zinc microprisms at room temperature exhibited a very prominent peak at 384 nm owing to the radiative recombination of electrons in s, p conduction band near Fermi surface and the holes in the d bands generated by optical excitation.     

Correlation of microstructure, electrical properties and electrochemical phenomena in reinforced mortar. Breakdown to multi-phase interface structures. Part II: Pore network, electrical properties and electrochemical response

Since reinforced mortar is a multi-phase composite material at different levels of aggregation, a combination of techniques, namely electrochemical impedance spectroscopy (EIS) (for investigating the electrochemical phenomena on the steel reinforcement) and microstructure analysis (for qualifying and quantifying the composite bulk material), was used to provide insight into the macro- and micro-level interactions, involved in conditions of corrosion and cathodic protection for the here investigated reinforced mortar specimens.

After 120days of exposure to the relevant conditions of chloride-induced corrosion and impressed current cathodic protection (CP), it was found that the accumulation, volume expansion and propagation of corrosion products bring about significant structural alterations in the cement matrix. Further, the current flow involved in CP applications, along with the protection itself, contributes to additional changes in the bulk material. In this study, the elements of the equivalent electrical circuit from EIS measurements are discussed in correlation to the evolution of porosity, pore size distribution and pore interconnectivity of the bulk matrix, during corrosion and CP application. The results indicate that different parameters in the EIS modeling concept correspond well to specific interface microstructures.

The outcomes of this combination of techniques will possibly provide implications for computer simulation of the corrosion process and CP applications as well as modeling of concrete performance in aggressive environments.