Course of porosity in layered beds of spherical particles

Measurements of area porosity of layered beds in the ?interfacial“ region of contact between layers of spheres of unequal diameter. Definition, computational determination and correlation of the ?interfacial excess pressure drop“ associated with the flow across the region of contact.     

Comparison of the porosity of hand-mixed and capsulated glass-ionomer luting cements

The strength of dental glass-ionomer cements will be influenced by defects present within its structure. This study measured the surface area porosity, percentage surface area porosity, and mean surface area of small bubbles (<0.01 mm2) and the surface area porosity, percentage surface area porosity and diameter of large bubbles within 40-microm-thick layers of four cements, using image analysis software. Two hand-mixed cements (Fuji I and KetacCem) and two capsulated cements (Fuji Cap I and KetacCem Maxicap) were viewed under transmitted light at x117.6 magnification. For each selected area (64.75 mm2) of each cement sample, five independent measurements were made of each of these parameters. Analysis of variance (ANOVA) indicated that there were no significant differences between the four cements in the small bubble parameters measured, whilst there were significant differences in the surface area porosity, percentage surface area porosity and diameter of the large bubbles. It was concluded that the hand-mixed cements tested had a greater number of larger diameter bubbles compared with the capsulated cements.     

金属纤维毡形变机理与孔隙特性关系研究

以直径6μm和22μm的不锈钢纤网为原料,通过叠配、高温烧结一定厚度的金属纤维毡,测试不同压缩强度下,金属纤维毡应变量与孔隙特性的变化,对比变形后纤维毡截面的微观形貌,研究金属纤维毡变形机理与孔隙特性的关系。结果表明：压缩初期,22μm粗纤维层形变量大,承担整体变形的主要部分,其孔隙变化对过滤性能影响较小。压缩中期,22μm粗纤维层致密化,整体变形承担部分转为6μm细纤维层,孔隙变化对过滤性能影响更大,整个压缩过程表现为粗纤维层—细纤维层—粗纤维层交替的变形机理。     

A model for prediction of pressure and redistribution of gas-forming elements in multicomponent casting alloys

A finite element model for simulating macrosegregation in multicomponent alloys is extended to include the calculation of pressure and redistribution of gas-forming elements during solidification. The model solves the conservation equations of mass, momentum, energy, and alloy components, including gas-forming elements such as hydrogen and nitrogen. The results of transport calculations are contrasted with thermodynamic equilibrium conditions to establish the possible formation of pores, assuming that there is no barrier to nucleation of the pores. By solving the transport of gaseous solutes and comparing their Sievert’s pressure with the local pressure, the new mode can predict regions of possible formation of intergranular porosity. Simulations were performed for a nickel-base alloy (INCONEL 718) in plate castings with equiaxed structure, and the evolution of microporosity for different initial concentrations of hydrogen and nitrogen was analyzed. The simulations showed that during solidification and cooling, a large fraction of the hydrogen escapes and a smaller fraction of nitrogen escapes from the casting. The initial gas concentration is an important factor in porosity formation, but the pressure drop due to shrinkage flow is not very significant. The resulting gas porosity is rather insensitive to initial nitrogen concentration, but sensitive to the concentration of hydrogen.     

The organization of visual corticocortical connections in early postnatal kittens

We tested the hypothesis that, in newborn kittens, superficial layers of the extrastriate cortex receive more specific patterns of corticocortical innervation from the striate cortex than deep layers. First, we injected retrogradely transported tract-tracers at a range of depths in area 18 to label area 17. All injections were of similar tangential diameter and were in the same region of rostral area 18, where the visual field 10-20 degrees below the horizontal meridian is represented. Injections that involved only the superficial layers of area 18 labelled cells mainly in the superficial layers (future layers 2-4) of area 17, across a region that was 2-3 mm wider than the diameter of the injection site in the rostrocaudal direction. Injections that involved all layers of area 18 labelled cells in both superficial and deep layers (5 and 6) of area 17, across a region that was 6-9 mm wider than the diameter of the injection site in the rostrocaudal direction. These values demonstrate that, in neonates, the convergence of projections from area 17 to the superficial layers of area 18 is less than that to the deep layers of area 18. The lower values for convergence obtained by injecting only the superficial layers of area 18 in kittens were similar to those obtained by injecting all layers of area 18 in adult cats; the values obtained by injecting all layers of area 18 in kittens were much higher. Second, we injected the full depth of area 17 in newborn kittens with labels that travel anterogradely and retrogradely. Confirming the conclusions from the use of retrograde tracers, these focal injections produced very widespread labelling of the deep layers of area 18, but much more localized and topographically organized labelling of its superficial layers. These results indicate that there is a considerable postnatal improvement in the accuracy with which corticocortical cells in striate visual cortex target appropriate regions in extrastriate cortex, in agreement with previous findings. They also demonstrate that this change occurs mainly among those striate cortical neurons that innervate a wide region of the deep layers of extrastriate cortex at birth. The innervation of the superficial layers of extrastriate cortex is much more accurate from the outset.     

Laboratory extrusion of Nb3Sn

A study of laboratory direct extrusion of HIP processed Nb₃Sn has been undertaken to assess the effect of hydrostatic pressure on the ductile fracture process that is observed in simple compression above 1400 °C. (Remanent porosity and low melting point microconstituents provide fracture centers; and void growth, linkage, and gross fracture can proceed rather rapidly, with pore linkage being obvious at the specimen surface after a 15 pct reduction in simple compression.) A rather porous, 1200 °C HIP processed material (5 pct porosity and 2.4 pct of low melting point phases) has been successfully extruded at 1650 °C in round bar form to a 51 pct area reduction with neither extensive pore linkage nor gross fracture. The results imply that little porosity development occurs in the presence of hydrostatic pressure of about twice the flow stress. Moreover, in the absence of free surface conditions, effective strains of the order of 0.7 will produce no more than about 10 pct porosity development under hydrostatic pressures of about one-half of the flow stress. Gross porosity development is associated with free surface deformation. Powdered graphite proves to be a poor lubricant, with the bulk of the extrusion pressure reflecting sticking friction.     

Effect of hydrostatic pressure on deformation,damage evolution,and fracture of iron with various initial porosities

This study evaluated the effects of superimposed hydrostatic pressure (138 to 1104 MPa) on densification and plastic flow behavior of porous iron (0.3 to 11.1 % porosity). Pressurization alone caused densification of the porous iron with the effect being most pronounced when the porosity was greater than 3.7% and the pressure above 276 MPa. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization or deformation under pressure. The effect of hydrostatic pressure on the flow stress of porous iron was small when densification resulting from pressurization was not a factor. The ductility was found to increase linearly with pressure and the effect of pressure on fracture strain increased with the initial porosity of the iron. Evaluation of the effect of hydrostatic pressure on development of porosity and growth during tensile deformation was limited to hydrostatic pressures of 138 and 276 MPa and iron compacts with initial porosities of 0.3, 1.5, and 3.7% because of the pressurization effects. It appeared that the porosity at fracture was similar in these compacts at both pressures but it was much larger than that observed at 0.1 MPa. The greater ductility of the iron compacts tested under hydrostatic pressure results from a decrease in the growth of pores with deformation and from a greater damage tolerance prior to fracture. As observed for porosity, the average maximum pore diameters at fracture for the compacts tested under pressure were similar but larger than those observed at 0.1 MPa. It appears that a general model of ductile fracture for porous materials cannot be based solely on a critical degree of dilation or on maximum pore extension as a fracture criterion.     

Fabrication of High-Porosity Lotus-Type Porous Aluminum in Vacuum

Lotus-type porous aluminums with porosities from 10 to 26 pct were fabricated with the Bridgman-type directional solidification method (Gasar). A vacuum atmosphere is critical to obtain high-porosity lotus-type porous aluminum by the Gasar process. The lotus-type porous aluminum was directionally solidified under a pure hydrogen pressure of 0.2 to 16 kPa. The influence of hydrogen pressure on the porosity and pore size in vacuum was investigated. The porosity and pore size increase with decreasing hydrogen pressure, but there exists a maximum porosity at some critical hydrogen pressure. Since a low hydrogen pressure is adopted, the effect of capillary pressure and hydrostatic pressure on the porosity becomes important. With the decreasing of hydrogen pressure, the influence of hydrostatic pressure and capillary pressure on porosity becomes stronger and stronger. The influence of melt height, which is proportional hydrostatic pressure, on porosity and pore size was investigated. The calculated porosities considering capillary pressure and hydrostatic pressure are in good agreement with experimental results.

     

Description and Phylogenetic Position of Dinosaur Eggshells from the Luanchuan Area of Western Henan Province, China

Here, we examine 145 dinosaur eggshells from a new fossil locality in the Qiupa Formation (Upper Cretaceous) of the Luanchuan area in western Henan Province, China. The eggshells display elongatoolithid macro- and microstructures, including ridges and nodes on the outer surface, two ultrastrncturai layers, and an undulatory boundary between the mammiilary and continuous layers.A phylogenic analysis shows that the Luanchuan eggshells belong to the ooclade Eiongatoolithidae.Within Elongatoolithidae, the thickness ratio of the mammillary layer to the entire eggshell, and the porosity of the Luanchuan eggshells are comparable to the oogenera Macroolithus and Elongatoolithus. There is no direct evidence for the taxonomic identity of the Luanchuan eggshells;however, they were likely laid by oviraptorids based on their phyiogenetic position and their similarities with known oviraptorid eggs.     

Fundamental studies of copper anode passivation during electrorefining: Part II. Surface morphology

Passivation of commercial copper anodes and pure copper has been previously analyzed by performing electrochemical measurements. Chronopotentiometry results revealed four characteristic regions involving I—active dissolution, II—prepassivation, III—passivation onset, and IV—passivation, for commercial copper anodes, while only active dissolution was observed for pure copper under the conditions employed. In order to establish the relationship between surface morphology and passivation response, scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS) were applied to characterize morphology of the product layers formed on a commercial copper anode surface for the distinctive electrochemical regions. The morphology studies suggested that the formation and stability of copper oxide surface films are critical to the onset and development of passivation. The structure and porosity of the slimes layer present in the outer layer of the anode influence the stability of copper oxide surface films which dominantly control the passivation response.     

Utility of NMR T2 distributions, connection with capillary pressure, clay effect, and determination of the surface relaxivity parameter rho 2

Nuclear magnetic resonance magnetization decays are now used routinely to determine reservoir properties of economic significance to the oil and gas industry. Although individual features of relaxation time distributions are subject to degradation in the presence of measurement noise, integrated and average measures of those distributions are remarkably stable. Properties such as porosity, producible porosity, hydraulic permeability, and the capillary pressure curve can be measured rapidly and continuously. Laboratory comparisons of NMR T2 distributions with two different capillary pressure measurements enable the surface relaxivity parameter rho 2 to be determined for sandstones. Comparison with relaxivities of synthetic materials having well-defined surface areas strongly suggests that the NMR-effective internal surface area of rocks is closely connected with the hydraulic radius, and, surprisingly, is not greatly affected by the presence of clay, which dominates the pore space surface are as measured by gas adsorption.     

Chemico-Osmotic Efficiency of a Geosynthetic Clay Liner

Chemico-osmotic efficiency coefficients, ω, are determined from measured differential pressures across specimens of a geosynthetic clay liner (GCL) containing granular bentonite in response to applied differences in potassium chloride (KCl) concentrations under no-flow conditions. The results show that the GCL acts as a semipermeable membrane with ω values at steady state, ωss, ranging from 0.08 to 0.69 for KCl concentration differences ranging from 0.0039 to 0.047 M. The chemico-osmotic efficiency of the GCL decreases with increasing porosity and increasing KCl concentration. The decrease in ωss with increasing porosity is consistent with an increase in pore size reflected by an increase in measured hydraulic conductivity. The decrease in ωss with increasing KCl concentration is consistent with compression of the diffuse double layers surrounding the clay particles, and is reflected by a time-dependent decrease in the induced differential pressure as well as an increase in the hydraulic conductivity of the specimen. The results of this study are potentially significant with respect to the evaluation of the hydraulic and contaminant transport performance of GCLs used in waste containment applications.     

Deformation of porous materials during hot forging combined with extrusion

Conclusions A study has been made of the character of the redistribution of volumes of a porous blank during hot forging with elements of extrusion; the feasibility is shown of controlling the process with the aim of ensuring that certain volumes of material occupy the required positions. It has been confirmed by experiment that the process of hot forging of porous materials with elements of extrusion involves more than one stage. The character of pressing pressure variation in various stages has been determined. In pressing in conical dies densification of central volumes of material lags slightly behind that of peripheral regions. For specimens of the geometric parameters investigated, the width of the peripheral blank zone, which in the steady-state extrusion stages moves parallel to the compressing part of the die, is (0.15–0.20)D_c. The degree of deformation and the redistribution of volumes of material in peripheral layers are affected mainly by the temperature conditions of the pressing process and to a much smaller extent by the starting blank porosity.Translated from Poroshkovaya Metallurgiya, No. 6(282), pp. 15–19, June, 1986.     

电热法制取铝硅合金用团块的性能（Ⅰ）——气孔率

介绍了一种用于测试电热法制取铝硅合金用团块气孔率的新方法,并在此基础上考察了压力、烧结条件、粒度对气孔率的影响。结果表明,团块气孔率随着压力的增大和物料粒度的减小而减小,而随烧结温度的升高及烧结时间的延长而增大。在压力、烧结条件、物料粒度三者之中,烧结条件对团块气孔率的影响最为明显。     

Effect of pressure difference on the quality of titanium casting

In casting titanium using a two-compartment casting machine, Her? et al. (1993) reported that the pressure difference between the melting chamber and the mold chamber affected the soundness of the castings. This study tested the hypothesis that differences in pressure produce castings with various amounts of porosity and different mechanical properties values. Plastic dumbbell-shaped patterns were invested with an alumina-based, phosphate-bonded investment material. Both chambers of the casting machine were evacuated to 6 x 10(-2) torr; the argon pressure difference was then adjusted to either 50, 150, 300, or 450 torr. The porosity of the cast specimens was determined by x-ray radiography and quantitative image analysis. Tensile strength and elongation were measured by means of a universal testing machine at a strain rate of 1.7 x 10(-4)/s. The fractured surfaces were examined by SEM. Changes in Vickers hardness with depth from the cast surface were measured on polished cross-sections of the specimens. Raising the argon pressure difference to 300 and 450 torr caused a significant increase in internal porosity and a resultant decrease in the engineering tensile strength and elongation. The highest tensile strength (approximately 540 MPa), elongation (approximately 10%), bulk hardness (HV50g 209), and lowest porosity level (approximately 0.8%) occurred in the specimens cast at 150 torr. Turbulence of the metal during casting was thought to be responsible for the increase in porosity levels with the increase in argon pressure difference. By choosing an argon pressure difference (around 150 torr) suitable for this geometry, we could produce castings which have adequate mechanical properties and low porosity levels.     

Comminution behavior of ASM diamond micropowder during cold pressing

Conclusions ASM powder comminuted under a pressure of 5 GPa can be represented as a two-fraction mixture of grains. This means that, at certain ratios between the linear dimensions characterizing the two fractions forming as a result of cold pressing, viz., X₂/X₁ > 8, a densification phenomenon can be expected to manifest itself. The phenomenon is particularly pronounced at an optimum ratio between the weights of the fractions. Because of their specific character, grain size distribution histograms of such powders comminuted under the above-mentioned pressure can be employed for choosing a suitable grain size for attaining closer grain packing and, after subsequent sintering, obtaining minimum porosity in dielectric heat-conducting layers and decreasing the amount of the conducting phase in them.Translated from Poroshkovaya Metallurgiya, No. 8 (272), pp. 23–27, August, 1985.     

金相磨抛工艺对碳化钨涂层组织评定的影响

碳化钨涂层内部为复合相组织,硬度高脆性大,金相制备易导致涂层内部出现裂纹、分层、拔出等缺陷。本文利用超音速火焰喷涂制备WC-10Co-4Cr涂层,采用不同的金相研磨和抛光工艺,分别评定WC-10Co-4Cr涂层的组织。结果表明:不同的金相研磨和抛光工艺对WC-10Co-4Cr涂层的孔隙率的评定结果有明显影响。随着磨抛压力的上升,涂层内部孔隙的尺寸明显增大,涂层原有孔隙周边被压裂、脱落,且脱落碳化钨颗粒进一步增强了对涂层表面的磨削效果,使得涂层表面的局部区域出现大范围脱落,从而导致随着研磨压力的上升,涂层的孔隙率升高了12.5%至30%,随着抛光压力的上升,涂层的孔隙率升高了近一倍。因此,为降低磨抛工艺对涂层组织评定的影响,建议使用15N的磨抛压力开展碳化钨涂层的金相制备,以获得更加准确的金相评定结果。     

Effect of low hydrostatic pressures on the diffusional phase transformation in dispersed multilayered systems

The effect of low hydrostatic pressures on the growth kinetics of phases in two-layer and multilayer Ni-Al diffusion specimen was studied by x-ray diffractometric, microprobe, optical and electron microscopic methods at temperatures T=0.4–0.6 T_m (T_m=melting temperature of the principal Ni₂Al₃ phase). Two types of specimens were investigated: massive two-layer, consisting of ∽3·10^?3 m thick sheets of nickel and aluminum presintered together, and finely dispersed multilayered, consisting of alternating layers of 10^?7-5·10^?7 m thick nickel and aluminum sheets. It was found that theapplication of pressure accelerated the formation of the Ni₂Al₃ and NiAl phases in both types of diffusion specimens. This effect is attributed to the action of pressure in preventing the formation of diffusional porosity at phase interfaces, and improving diffusional contact between adjoining layers.     

Densification of iron compacts with various initial porosities under hydrostatic pressure

This study evaluated the effects of hydrostatic pressures up to 1104 MPa on densification of porous iron containing 0.3–11.1% porosity. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization up to 1104MPa. Current deformation models of ductile porous materials based on Gurson's yield criterion and rigid-plastic FEM analysis predicted much faster densification with pressurization than observed for porosity contents of 6.2% or less. A reason proposed for this behavior is the omission in the models of an internal pressure which builds up in the pores during the compaction process. A modification is introduced to Gurson's model to take account of internal pressure effects. The modified model exhibited excellent agreement with the experimental observations and provides a constitutive relation for the simulation of compaction and forming processes of P/M parts.