Tape-cast ceramic membranes for microfiltration application

Alumina membrane filters in the form of thin (0.3–0.8 mm) discs of 25–30 mm diameter suitable for microfiltration application, have been fabricated by the tape-casting technique. Their pore size could be varied in the range 0.1–0.7 m and porosity in the range 25%–55% through optimization of experimental parameters. The most important factor which determines the pore size, is the initial particle size of ceramic powders used for this purpose. Temperature of firing, and also the soaking time are crucial parameters which determine the porosity. Water permeability under suction conditions varies in the range 110–900 lm^–2h^–1 depending on porosity, pore size and thickness of the membrane. Most of these membranes, particularly those with pore sizes less than 0.5 m, are found to be suitable for complete removal of bacteria from water and are also reusable after cleaning by acid or heat sterilization.     

3D characterization of open porous vacuum plasma sprayed titanium coatings by means of high resolution micro computer tomography

Open porous Ti coatings were applied by vacuum plasma spraying. The overall porosity was determined by light microscopy on cross-section cuts as well as micro computer tomography (μ-CT). μ-CT was additionally used on detached coatings to characterize pore size and interconnectivity as well as the sinter necks between individual Ti particles, as these are important factors for biomedical applications (tissue ingrowth) and coating integrity. The μ-CT measurements with a resolution of 5 μm voxels and the subsequent data treatment showed an excellent pore connectivity and yielded an average pore size of ~ 80–140 µm, pore connection diameter of ~ 50 µm and Ti sinter neck diameter of ~ 30–40 µm.     

Low cost porous mullite-corundum ceramics by gelcasting

A gelcasting process has been successfully employed to fabricate porous mullite-corundum ceramic composites from natural clay and corundum powders. The specimens based on the mullite composition are found with an open porosity of 45–47.9%, mean pore size of 1.28–2.55 m, and nitrogen permeability of 965–5038 m³· m^–2· bar^–1· hr^–1by reactively sintering the gelled mixture of kaolinite and -alumina at 1100–1500°C. The open porosity (_o), mean pore size (d _p), pore size distribution and gas permeability can be controlled by adjusting raw material ratios and sintering temperatures. The gas permeability of the specimens is found to be more dependent on the pore size distribution as well as d _pthan on _o. In addition, the gas transportation mechanism in porous mullite-corundum ceramic composites is dominated by viscous flow.     

Isotropic forming of porous structures via metal injection molding

Metal injection molding (MIM) is a near net-shape process that offers the unique ability to manufacture porous components with homogeneous porosity, pore structure and permeability. MIM is a process that can significantly reduce production cost when large quantities of components with complex shape need to be delivered. In this study, MIM is used to produce porous 316L stainless steel structure from both water and gas atomized powders. The porous components made by MIM were characterized to evaluate their suitability for small pore structure applications. The porous structures were analyzed for porosity, pore size, permeability, and thermal conductivity as a function of powder type and processing conditions. A typical MIM powder (<20 μm) processed at 50 vol% loading in a binder system produced a uniform pore structure with a permeability of less than 1⋅10^{− 13} m² and a maximum pore radius of less than 5 μm. Water-atomized powder proved to be better suited for low-solids-loading metal injection molding (<50 vol% loading) since its irregular shape provided greater strength and fewer defects during the molding and debinding process steps. Measurements of thermal conductivity show that the water-atomized powder had less thermal conductivity (∼2 W/m-K) than the gas-atomized powder (∼3 W/m-K). This study shows that MIM is a suitable process that can be used to manufacture functional porous structures that require isotropic pore size and complex shape.     

Permeability and high temperature strength of porous mullite-alumina ceramics for hot gas filtration

The gas permeability and mechanical properties of mullite-alumina ceramics for potential use as filters in hot gas separation environments are examined. The mullite-alumina ceramics with different levels of induced porosity and pores sizes were fabricated by slip casting and characterised in terms of microstructure and strength properties at ambient and elevated temperatures. Permeability to nitrogen gas flow of the porous structures at ambient temperature was investigated over a range of flow velocities to quantify and assess the permeability. The strength at high temperatures is equivalent to ambient data signifying no discernible degradation. Nitrogen gas permeability tests reveal dramatic reductions in the pressure drop–gas velocity curves with increasing porosity. It is shown that the gas permeability increases with the level of porosity and pore size, with maximum Darcian permeability constant of k = 2.5×10⁻¹⁴ m² for a porosity of 71%.     

The influence of molybdenum concentration on the magnetic permeability and kinetics of order formation in molybdenum permalloys

The initial magnetic permeabilities of molybdenum permalloys with a range of molybdenum concentrations have been measured as a function of isothermal annealing time in the temperature range 400 to 600° C. Annealing in the range from 400 to 460° C produces short-range ordering, leading to a maximum initial permeability when the magnetostriction is reduced to zero. From 460 to approximately 490 to 500° C it is suggested that microdomains of long-range order nucleate and grow in a short-range ordered matrix and these reduce the maximum initial permeability which can be obtained. Above about 490 to 500° C, long-range order does not form, but equilibrium values of short-range order are too small to produce initial magnetic permeabilities as large as in the lower temperature region. The rate of short-range order formation has been found to increase sharply at molybdenum concentrations of about 4 wt % (2.5 at. %) and the activation energy for short-range order formation falls sharply from about 180 kJ mol^–1 to 140 kJ mol^–1 at this molybdenum concentration. It is thought that different ordered structures and mechanisms of ordering occur below and above 4 wt% molybdenum. Quenching experiments suggest that quenched-in vacancies can increase the rates of short-range order production.     

Strontium modification of aluminium alloy castings in the expendable pattern casting process

The influence of strontium modification on microstructure, pore characteristics and tensile properties of Al-Si-Cu castings produced by the expendable pattern casting (EPC) process has been investigated. Test castings have been produced at strontium concentrations varying from 0.0004%–0.03%. The castings have been analysed by microscopic and image analysis techniques. The results indicate that optimum strontium concentrations are between 0.01% and 0.015%. A coarsening of silicon particles is observed at strontium concentrations greater than 0.015%–0.02%. The addition of strontium to the melt also refines the iron- and copper-bearing phases in the casting. The amount of bulk porosity, average pore size and tensile properties of modified EPC castings are comparable to green sand castings. EPC castings are prone to surface porosity. Strontium modification has a significant effect on the pore distributions in EPC castings. The overall porosity and the maximum pore size increase with strontium modification.     

Comparison between the process–structure–property relationships of silica foams prepared through two different processing routes

Cellular silica with improved framework, crosslinking, and stability properties are desirable for applications in thermal insulation. A process for the preparation of cellular silica foam with interconnected cells with tailored porosity and pore size distribution has been attempted. The silica foams have been prepared through two different methods; surfactant- and particle-based stabilization. The silica foams prepared through two different processes namely surfactant-stabilized foams (SSF) and particle-stabilized foams (PSF) have exhibited a wide range of differences in their structure which in turn have shown to affect the final properties of the foam. The cell size distributions in SSF (89 vol% porosity) and PSF (85 vol% porosity) have been found in the range of 50–250 μm (monomodal) and 4–10 μm and 50–100 μm (bimodal), respectively, whereas the cell counts of both have been found in close proximity. The microstructure of both the sintered SSF as well as PSF samples foams have shown an open and interconnected porosity with the permeability of both in the region of ~10⁻⁸ m². The mechanical (compressive) strength and Young’s modulus of the PSF are a third of that in SSF. The structure–property relationship of both the SSF and PSF and their comparison have been discussed.     

Influence of pore size on tensile strength, permeability and porosity of hyaluronan-collagen scaffolds

Recent investigations have shown the importance of scaffold pore size on the realisation of tissue engineered cartilage which promotes cell adhesion, proliferation and differentiation. The objective of this study was to investigate the influence of pore size on the mechanical properties, the permeability and the porosity of hyaluronan-collagen scaffolds. Hyaluronan-collagen scaffolds with three different mean pore sizes (302.5, 402.5 and 525 microm) have been produced according to a standardised protocol. The maximum stress at rupture, the Young's Moduli, permeability and porosity of the scaffolds were investigated. The permeability was determined both empirically and mathematically. Increased pore sizes indicated a larger stress at rupture as well as increased Young's Moduli. Porosity and permeability were raised by increasing pore sizes. The mathematically calculated permeability showed the same trend. The results indicate a higher mechanical stability for scaffolds with larger pores. The experimental and mathematical experiments both show increased permeability and fluid mobility for larger pores in scaffolds. Morphological changes resulting from the alteration of pore size led to non-correlation between the calculated and the experimental permeability.     

多孔青铜过滤片的制备工艺及结构特性探究

开发一种工艺简单、重复性好、孔形孔径易控制、制取成本低的铜基多孔材料制备工艺是当前的研究热点之一.本文以青铜粉为原料,K_2CO_3为造孔剂,采用烧结溶解法制备多孔青铜过滤片,研究了造孔剂、烧结温度对样品孔隙率的影响,分析了烧结温度、压制压力对样品最大孔径和透气系数的影响,以及孔隙率与抗压强度的关系.研究结果表明:当造孔剂体积分数为20%~40%时,所制备样品的孔隙率为22.8%~44.4%,开孔孔隙率为18.5%~37.2%;随着烧结温度的升高,样品孔隙率和透气系数下降;随着压制压力增加,最大孔径和透气系数均减小;随着样品孔隙率增大,抗压强度减小.当选择造孔剂体积分数30%、压制压力150 MPa、烧结温度800℃的工艺参数下,制备出孔隙率32.2%、最大孔径4.6μm、透气系数9.27 m~3/(h·k Pa·m~2)、压缩强度27.9 MPa的多孔青铜过滤片.     

Gas permeability of thin polyimide foils prepared by in-situ polymerisation

The entrance windows to the gas detector chambers as well as to the target containers used in high-energy and high-intensity accelerators must be as thin as possible to minimise energy losses of the particles used in astrophysics and nuclear physics studies. Because of their good physical properties, polyimide foils are often considered as suitable material for such windows, but commercially available foils, having a thickness greater than 7–8 μm (>1 mg/cm²), would cause energy losses of particles significant for some nuclear reactions studied. Foils prepared by in-situ polymerisation can, however, be as thin as 0.07 μm (10 μg/cm²). The permeability of 4 μm foils produced by in-situ polymerisation has been measured at room temperature for He and Ar. For He measurements were performed in the pressure range of 4–70 mbar and for Ar in the range of 20–140 mbar and the permeability was found to be in good agreement with the values published for the thicker commercial foils.     

Complex susceptibility analysis of magneto-fluids: Optical band gap and surface studies on the nanomagnetite-based particles

An aqueous stable magnetic fluid containing Fe₃O₄ nano-particles with a mean diameter of 4–7 nm, which is in the range of super-paramagnetism, is prepared. The particles are synthesized via co-precipitation method from ferrous and ferric solutions. X-ray diffraction, transmission electron microscopy, transformer method are used to study the physical properties of the magnetic fluids and powders. A method is given to analyze and resolve the real and imaginary parts of the measured complex susceptibility of magnetic fluids. The band gap parameters of the magneto-nanopowders such as the direct-, indirect-band gap energies, Fermi energy and Urbach energy are determined. A comparative study between the different techniques used to calculate the powder particle size is presented. Adsorption of nitrogen gas is used to identify and determine the particles mean diameter and to study their microstructure, the magnetic properties and surface porosity. The study showed that the total pore system of the magnetic nano-powders consists mainly of mesopores.     

Permeability, porosity and pore geometry of hot-pressed calcite

Permeability may be altered in the Earth by plastic flow of the rock matrix. In order to better understand the relation between plastic flow and pore geometry, we measured the permeability of a suite of hot-pressed calcite samples with differing porosities. We found that the permeability dramatically decreased with decreasing porosity, particularly in the range of 10 to 4% total porosity. These results agree with a model for pore geometry changes during hot-pressing as previously developed for ceramics. Measurements of unconnected and interconnected porosity showed that the interconnected porosity virtually disappeared in samples with a total porosity of 4% or less. Scanning electron microscope observations showed that the porosity of samples above 10% total porosity were composed of large ‘spheroidal’ pores which were often connected by ‘tubular’ pores. During the last stage of hot-pressing, these ‘tubes’ are thought to collapse making the pore network disconnected.     

Reaction Sintering with Negative Volume Changes

The preparation of high-porosity sialon and SiC materials by reaction sintering accompanied by weight loss and shrinkage is considered. -Sialon ceramics with a 60% porosity, pore size of 0.7 m, and gas permeability K = 0.04 m² are obtained via carbothermal reduction and simultaneous nitriding of kaolin in kaolin + graphite green compacts. The SiC materials prepared from mixtures of SiO₂ and graphite have a porosity of 83% and a pore size of 1 m. The most promising technique is elemental synthesis. In the SiC materials prepared by milling silicon + graphite mixtures to a specific surface area of 80 m²/g, pressing at 100–200 MPa, and sintering in argon at 1600°C for 15 min, the pore size is as small as 0.2–0.3 m.     

LaB6 and TiB2 electrodes for the alkali metal thermoelectric converter

LaB₆ and TiB₂ electrodes for the alkali metal thermoelectric converter (AMTEC) were prepared from the respective powders by a screen-printing method and their electrode properties were investigated. Optimum values were obtained for particle size, thickness and porosity of electrode. When the vacuum level of the low-temperature side of the AMTEC increased above 10 Pa, the power density decreased remarkably. These results can be interpreted as different electrode processes in the AMTEC: (1) a charge transfer process, (2) surface diffusion on the electrode, (3) desorption from electrode particles, and (4) vapour-phase diffusion in the electrode pore. The maximum power density was 0.54 W cm^–2 (LaB₆) and 0.24 W cm^–2 (TiB₂ at 800 °C.     

Dependence of magnetic properties on atomic order in 77% Ni-14% Fe-5% Cu-4wt% Mo alloys

We have studied the effect of atomic order on the lattice parameter,a, Curie temperature,T , and the initial permeability, _i, of a series of molybdenum permalloys with varied Ni-Fe ratios and molybdenum concentrations. The ordering temperature range was between 380 and 600° C. The results of the effect of short-range order and long-range order on the lattice parameter after annealing for about 5 h between 420 and 470° C indicated a decrease of between 0.15 and 0.22 pm (0.04 to 0.06%) in the lattice parameter. No superlattice lines were detected. This may probably be due to the similarity in the atomic scattering factor of nickel and iron. It was also noted that both long-range and short-range ordering increased the Curie temperature of the ordered materials by about 1.4 to 6.6% due to the production of stronger and shorter Ni-Fe bonds. The number of Ni-Fe bonds, which controls the exchange force, was found to depend on the amount of order and molybdenum content in each material. The Curie temperature, which is a measure of the exchange force, is also an indirect means of measuring the degree of lattice ordering because the exchange integral is affected by metallurgical variables such as atomic ordering, composition, etc. Results of the isothermal annealing time on the initial permeability in the temperature range (380 to 600° C) indicated that between 380 and 460°C, maximum permeability was obtained at a critical degree of short-range ordering which is thought to correspond to a state when both the magnetostriction and anisotropy constants are close to zero. The maximum permeability was independent of ordering temperature in this range, although the time to reach this maximum decreases with increasing temperature. On annealing in the temperature range 460 to 500° C, the permeability reaches a maximum, the maximum permeability in this temperature range decreases with increasing temperature. As in the lower temperature range, the time to reach this maximum decreases with increasing temperature. Annealing between 500 and 600° C produced no maximum permeability. The permeability levelled off after an initial gradual increase. The activation energies for short-range order formation were found to be smaller (between 135 and 142 kJ mol^-1) in alloys with molybdenum concentrations above 4 wt% Mo (>2.5 at.% Mo) and higher activation energies (between 196 and 210 kJ mol^–1) in alloys with molybdenum concentrations below 4 wt% Mo (<2.5 at.% Mo) which suggests the formation of different ordered structures and mechanisms below and above 4 wt% Mo.     

Type ÉM2-15 vibration magnetometer for measuring the magnetic saturation moment of ferromagnetic specimens

Conclusion The type ÉM2-15 vibration magnetometer that has been developed makes it possible to measure the magnetic saturation moment of ferromagnetic specimens in a magnetic field having a strength of 480 kA/m over a temperature range from –150 to +300° C. The maximum measuring error of the magnetic moments in the ranges from 10·10^–6 to 2000·10^–6 A·m² is no more than 3%. The temperature measuring error is ±2° C and the temperature is maintained within ±1° C at any point in the range.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 61–63, May, 1975.     

Early detection and monitoring of fatigue in high strength steels with MWM-Arrays

Fatigue monitoring of cyclically loaded shot peened high-strength steel components can be accomplished via magnetic permeability measurements during laboratory tests or in service. These measurements can be performed either continuously using permanently mounted Meandering Winding Magnetometer Arrays (MWM^®-Arrays) or intermittently with scanning MWM-Arrays. The results obtained to date suggest that MWM-Array permeability measurements can provide early detection of fatigue damage in steels before conventional methods can detect any changes. This has been demonstrated to be particularly significant in the presence of high compressive stresses introduced by shot peening. One of the fatigue tests was suspended when accelerating changes in local permeability were detected. Examination of the fatigue specimen in a scanning electron microscope detected only a few relatively small cracks, e.g. 50–200 μm long at the surface. Fractography, however, revealed significantly longer cracks. For the same specimen, conventional eddy current and ultrasonic testing failed to provide any indications of cracks, and fluorescent liquid penetrant detected only an inconclusive spot indication. This paper provides a comparison of the permeability changes and fractography data with a fatigue crack growth curve based on a FASTRAN analysis accounting for residual stresses from shot peening. A comparison of the experimental data and crack growth analysis results suggested that MWM-Array magnetic permeability measurements may detect cracks in the compressive stress field when they are about 50 μm deep.     

Studies on the processing of nickel base porous wicks for capillary pumped loop for thermal management of spacecrafts

Nickel base sintered porous wicks have potential application as capillary structure in two-phase heat transfer loops of a heat dissipation system, like capillary pumped loop (CPL) and loop heat pipe (LHP). A porous wick is located inside the evaporator of the CPL system and it transports working fluid in the loop by capillary action.In the present work, experimental trials were carried out to achieve porous wick having high porosity with interconnected pores of average size less than 5 μm, higher aspect ratio (L/D >10) and permeability better than 10 m-Darcy (10^?14 m²). A carbonyl nickel powder (2–7 μm) was used as raw material. Loose carbonyl nickel powders (2–7 μm) were sintered in graphite mould under hydrogen atmosphere at different temperatures in order to optimize porosity, pore size and permeability of the sintered wick. For mechanical and physical properties characterization, samples were cut from sintered rod using EDM wire cut to avoid pore closure. Profile making on the sintered rod is also done by wire EDM. Microstructural characterization as well as the effect of W-EDM on the surface pores was done using Scanning Electron Microscopy (SEM). Surface profile making through W-EDM had shown encouraging result. After optimization of process parameters cylindrical wick (L/D ratio: 10) with porosity of 64%, average pore size of 5 μm and a permeability of 70 m-Darcy could be realized.The present paper explain the details of processing of cylindrical shaped porous wicks through sintering technique and effect of EDM on surface pores characteristic.     

Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process

Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm³ and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m²/g. And there is an obvious Si–CH₃ group on the surface of the silica aerogel.