Effect of porous NiTi alloy on bone formation: A comparative investigation with bulk NiTi alloy for 15 weeks in vivo

The purpose of this study is to investigate the effect of porous NiTi alloy on bone formation with a bulk NiTi alloy as a contrast. The porous NiTi alloy prepared by element powder sintering under Ar protection has a porosity of 45% and a mean pore size of 130 μm, and the pores are highly interconnected. The porous and bulk NiTi alloys were bilaterally implanted into the femurs of rabbits for 15 weeks. The bone-implant interface and bone ingrowth were evaluated by undecalcified histological examination under light and fluorescent microscope as well as environmental scanning electron microscope (ESEM). The results show: osteoblasts are very active with fast proliferation and no adverse tissue reaction occurs for the porous NiTi alloy after 15 weeks implantation; porous NiTi alloy has better osteoconductivity and osteointegration than the bulk one; the osteoblasts can ingrow the pores of porous NiTi implant, and direct bone-implant interface can be observed by fluorescent light microscope.     

Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties

Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 μm s⁻¹. Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young's modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen.     

Pore structures of high-porosity NiTi alloys made from elemental powders with NaCl temporary space-holders

The unusual pseudo-elasticity and shape memory effect make NiTi alloys promising energy absorption materials. In the present study, powders of Ti, Ni and NaCl particles were mixed and cold-pressed into green ingots and green ingots were then desalted and sintered in vacuum to form high-porosity NiTi alloy specimens with porosity up to 90%. Microstructure observation shows that two kinds of pores with sizes of 200-400 μm and 10-50 μm respectively are well-distributed in these high-porosity NiTi alloys. Characteristics of pores were studied and formation mechanism was discussed.     

High-porosity NiTi superelastic alloys fabricated by low-pressure sintering using titanium hydride as pore-forming agent

Porous NiTi shape memory alloys (SMAs) were successfully fabricated by low-pressure sintering (LPS), and the pore features have been controlled by adjusting the processing parameters. The porous NiTi SMAs with high porosity (45%) and large pore size (200–350 μm) can be prepared by LPS using TiH_1.5 as pore-forming agent. These alloys exhibit isotropic pore structure with three-dimensional interconnected pores. The porous NiTi SMA produced by LPS exhibits superelasticity and mechanical properties superior to that by conventional sintering.     

Characteristics of the membrane utilized in a compact absorber for lithium bromide–water absorption chillers

This study aims at investigating experimentally and analytically the characteristics and properties of a membrane utilized to design compact absorbers for lithium bromide–water absorption chillers. The main focus of this study are the factors that influence the water vapor transfer flux into a lithium bromide–water solution in confined narrow channels under vacuum conditions, as well as the properties limits for utilization in compact absorber design. The results indicate that the desired membrane characteristics for this application are as follows: high permeability to water vapor, hydrophobic to the aqueous solution with high liquid entry pressure (LEP) to avoid wettability of the membrane pores and no capillary condensation of water vapor to avoid blocking of the pores. For practical use, this membrane should have a thin hydrophobic microporous active layer with a thickness up to 60 μm, mean pore sizes around 0.45 μm and a porosity of up to 80%. The active layer should be attached to a porous support layer to meet the mechanical strength requirements needed for practical use in the absorber of lithium bromide water absorption chillers application.     

Processing and microstructure of particle stabilized silica foams

Silica foams containing ~ 85% porosity and with different shapes and sizes have been prepared by air entrainment in suspensions of hydrophobized silica + alumina (5 wt.%) powder mixture in aqueous solution of isopropanol and binders, followed by casting, drying and sintering. The silica powder with surfaces modified by presence of long chain amphiphilic molecules has been used successfully for stabilization of air bubbles in suspensions, so that their disproportionation and collapse could not be observed even after 4 weeks. Microstructural examination using optical and scanning electron microscopy as well as measurements by mercury porosimetry has shown trimodal pore size distribution with fine (4–10 μm), medium (50–100 μm) and coarse (~ 680 μm) pores.     

Effect of Porosity on Thermal Conductivity of Al–Si–Fe–X Alloy Powder Compacts

The thermal conductivity and thermal diffusivity of hot- and cold-pressed Al–17Si–5Fe–3.5Cu–1.1Mg–0.6Zr (mass%) alloy powder compacts were investigated as a function of the porosity volume fraction. Samples with a very low degree of porosity were produced by hot-pressing air atomized alloy powder with a particle size of 45–100 m. The same powder was used to produce highly porous compacts by cold compaction using a manual press. The thermal diffusivity of the powder compacts was measured using a sinusoidal modulation photopyroelectric technique in a configuration that is similar to the laser flash method. The thermal diffusivity of the material decreases by a factor of about 13 with an increasing porosity of 25 vol% and a factor of about 300 at 60 vol % porosity. Since the calculated specific heat (weighted average of mass specific heat values of major alloy compounds) is much less porosity dependent, the porosity dependence of the thermal conductivity is similar to the thermal diffusivity and decreases exponentially with increasing porosity. Microstructural characterization of high porosity samples prepared by cold compaction indicated that the distribution of pores is not uniform over the cross-section. An interconnecting network of open and closed pores in the form of channels created pockets of porosity,clearpage 2.3pc which are largely responsible for a drastic reduction of thermal conductivity 4pc with increasing porosity.     

Rotating bending fatigue response of laser processed porous NiTi alloy

Porous implants are known to promote cell adhesion and have low elastic modulus, a combination that can significantly increase the life of an implant. However, porosity can significantly reduce the fatigue life of porous implants. Very little work has been reported on the fatigue behavior of bulk porous metals, specifically on porous nitinol (NiTi) alloy. In this article, we report high-cycle rotating bending fatigue response of porous NiTi alloys fabricated using Laser Engineered Net Shaping (LENS?). Samples were characterized in terms of monotonic mechanical properties and microstructural features. Rotating bending fatigue results showed that the presence of 10% porosity in NiTi alloys can decrease the actual fatigue failure stress, at 10⁶ cycles, up to 54% and single reversal failure stress by ~ 30%. From fractographic analysis, it is clear that the effect of surface porosity dominates the rotating bending fatigue failure of porous NiTi samples.     

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.     

Thermophysical properties of porous SiC ceramics fabricated by pressureless sintering

Highly porous SiC with approximately 30–41% porosity was fabricated by pressureless sintering without sintering additives at temperatures in the range 1700–2000 °C. Thermal diffusivities, specific heats, thermal conductivities and thermal resistivities of sintered samples are reported for temperatures from room temperature to 1000 °C. The thermal diffusivities and thermal conductivities of all samples decreased significantly with increasing temperature over this range, whereas specific heats and thermal resistivities increased. At any given temperature, the greater the porosity of the SiC, the lower the thermal conductivity.     

The resistance of a dense moving layer through which a gas stream is injected from below

For a tall and narrow vessel with an opening in the bottom — through which granular material is discharged under conditions of a gas countercurrent — we have experimentally confirmed the equality of the resistance factors for a moving and a nonmoving layer. We have established the practical equality for the porosity of a dense layer — at the threshold of fluidization — and the porosity of a moving bed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 3, pp. 423–428, March, 1969.     

Microstructural development in copper-interlayer transient liquid phase bonds between martensitic NiAl and NiTi

Transient liquid phase (TLP) bonding of Ni–24 at% Al–16 at%Cr and Ni–30 at% Al with a tetragonal L10 type martensitic microstructure to nominally stoichiometric NiTi with a monoclinic (distorted B19 type) martensitic matrix is investigated in this paper. The TLP bonds described in this article employed 50 m thick copper interlayers. Bonds were prepared using holding times of between 20min and 2 h at a bonding temperature of 1150°C. A holding time of 1 h at 1150°C was sufficient to remove the liquid-phase from the bond-line, however a layer of Ni2AlTi (L21 type Heusler phase) was left at the bond line. This layer remained present with further holding up to 2 h at 1150°C. With 50 m thick copper interlayers, 2 h holding at 1150°C resulted in significant titanium diffusion, from the NiTi substrate, to the joint. Depletion of titanium from the NiTi substrate, in turn, reduced the solidus temperature of the NiTi substrate to the point where localized melting occurred on the surface of the NiTi substrate. Extensive microstructural changes were observed in the NiTi substrate after TLP bonding and these are discussed in detail. In contrast, the NiAl substrate was largely unchanged after TLP bonding.     

The sintering and morphology of interconnected porosity in UO2 powder compacts

Uranium dioxide powder compacts of 46% green density were sintered in flowing hydrogen at temperatures between 1500 and 1700° C. On annealing, the compacts readily formed an interconnected system of pores stabilized by grain boundaries. The volume of open porosity decreased with an activation energy of 4.6 J mol^–1 at a rate controlled by grain growth. The grain-boundary migration removed the restraint on the porosity allowing shrinkage to commence. The compact surface area decreased with a higher activation energy of 6.0 J mol^–1. The mechanism proposed for the diminishing area was the smoothing of the faceted powder grains. Nucleation of atomic layers on the facets was shown to account for the high activation energy. The equilibrium shapes that may be adopted by interconnected porosity were calculated using a model in which simpler geometry was substituted for the real anticlastic surface curvature. The model demonstrated the stabilizing effect of increasing grain-boundary energy and the formation of closed pores.     

New powellite type oxides in Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi)—Their synthesis, structure and dielectric properties

New complex oxides having powellite (CaMoO₄) type structure in the Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi) were prepared employing the method of solid state reaction between the component oxides at high temperature (1000–1100 °C). The new compounds, CaRNbMoO₈ (R = Y, La, Nd, Sm, Bi) are colorless and electrical insulators. The dielectric constants (K at 1 MHz) of these compounds are in the range 14–33 and K shows very little variation in the temperature range 30–100 °C. Their temperature coefficient of dielectric constant (TCK) is negative, which varies from − 21 to − 220 ppm/°C.     

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.     

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.     

Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

Superelastic property of shape memory alloys (SMAs) is becoming increasingly important for impact applications due to their large recoverable strains and high capacity to dissipate energy. In this work, tensile behavior of superelastic NiTi SMA wires at impact strain rates was studied by instrumented tensile-impact technique, which allows to obtain material properties on the order of 1–10² s⁻¹. The results show that even at impact strain rates, martensite can be induced by tension in NiTi. At impact, a plateau stress appears during transformations similar to that at quasi-static strain rates, but 100–150 MPa higher in stress. This is due to the higher temperatures achieved during the deformation due to the close to adiabatic nature of the impact event. The influence of the strain rate over the mechanical behavior of NiTi was spread to the quasi-static strain rates so that the evolution of several parameters was also studied on the range 10⁻⁵–10² s⁻¹. Therefore, forward stress-induced martensitic (SIM) transformation stresses (σ^Ms and σ^Mf) and deformation energy (E_d) increase with strain rate, but they are strain rate independent from 10⁻¹ s⁻¹ at least until 10² s⁻¹. Reverse SIM transformation stresses (σ^As and σ^Af), recoverable strain energy (E_r), and dissipated energy (W_d) depend mainly on maximum strain achieved during the deformation, but for strains corresponding to a load–unload cycle with complete SIM transformation, σ^As, σ^Af and E_r are higher at impact than at quasi-static strain rates, and W_d shows similar values at very low strain rates and at impact.     

Influence of starch content and sintering temperature on the microstructure of porous yttria-stabilized zirconia tapes

Porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% were developed using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. Two kinds of pores were observed in the sintered tapes: large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The top surface of the sintered tapes had a greater porosity than the bottom one for all the starch contents examined. The total porosity and the percentage of open porosity in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. The open pores between the YSZ particles were removed by sintering at 1600 °C. As the volume fraction of starch increased from 17.6 to 37.8 vol.%, there was a gradual increase in the interconnectivity of the pore structure.     

Structure Formation in B2Sr2CaCu2O8 Films Prepared from Metal Methacrylate Solutions

Structure formation in Bi–Sr–Ca–Cu–O films produced by spray pyrolysis using methacrylate solutions is studied. The process is shown to involve liquid-phase, amorphous–nanocrystalline, and high-crystallinity states. At annealing temperatures in the range 120–150°C, the surface morphology of the films is governed by the deposition and drying conditions. The films are in a high-porosity, amorphous state, with dome-shaped hillocks on their surfaces. Annealing at 250–500°C leads to the formation of oxides and intermediate products of solid-state reactions, release of gaseous products, and formation of nanocrystalline 2212 nuclei and intermediate phases. Annealing at 600–850°C increases the crystallite size to several microns. The resulting films are free of pores and dome-shaped hillocks and consist of scaly 2212 crystallites, with [001] preferential orientation, and small amounts of weakly textured, high-T _c 2223.