Determination of porosity of reduced hematite by stereologic methods

The stereologic analysis of a solid grain has been applied for determination of its porosity. The investigations were carried out on a natural hematite grain reduced to magnetite at 723 K. Stereologic analysis enabled the porosity of this material to be determined on the basis of independently measured grain contours and volume fractions of grain components: hematite, magnetite and pores. A conformity criterion has been formulated, regarding determination of grain volume on the basis of known densities of components and their measured volume fractions in the grain, and on the basis of weight and apparent grain density measurements. The volume increase accompanying reduction of hematite to magnetite, determined from these independent measurements, was in the range 7–19%. By measuring Ferets diameter, the volume of the convex envelope of the grain increased as a result of phase transformation by about 11%. Simultaneously, the volume of the solid phase was found to decrease by 1.5%. The total porosity of the reduced grain, determined by stereologic analysis amounted to 17.5%; 7.7% of which was related to cracks and large pores, and 9.8% to small pores in the magnetite phase. The porosity value determined by the classical mercury method was approximately the same.     

Use of normalized porosity in models for the porosity dependence of mechanical properties

The use of normalized porosity in models for the porosity dependence of mechanical properties is addressed first for the frequently used power law expression for such dependence, i.e., E/E₀ = (1−P)ⁿ where E is the property of interest at any volume fraction porosity (P) and E₀ is the value of E at P = 0. Normalizing P by P_C, the value of P at which the property of interest inherently goes to zero, giving E/E₀ = (1−P/P_C)ⁿ, clearly calls attention to the importance of P_C values < 1 (e.g., potentially as low as ∼ 0.2), a fact long known but inadequately recognized. Serious problems from the arbitrary use of both n and P_C as fitting parameters with little or no guidance as to the dependence that n and P_C (which is microstructurally sensitive) have on the type of porosity are shown. Further, porosity normalization of the power law model indicates at best limited compression of different porosity dependences into a single universal porosity dependence function and little distinguishing of property dependences as a function of the type of porosity. However, normalized porosity of minimum solid area (MSA) models gives a single universal porosity dependence. The difference in responses to P normalization of the two modeling approaches is attributed to their being based respectfully on little or no pore character and on detailed pore character. Thus, P normalization may be a valuable tool for evaluating porosity models, but must be applied in a more rigorous fashion, i.e., P_C determined primarily by measurement and correlation with the type of porosity (as with MSA models) and not as an arbitrary fitting parameter as used in the evaluations of the power law model.     

Calculation of friction at permeable surface

The laws of friction between a stream of fluid and a permeable surface are generalized for bodies of various shapes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 5, pp. 719–722, November, 1984.     

Hydraulic resistance of channels with permeable walls

We present the results from an experimental study of hydraulic resistance in turbulent flow for an incompressible liquid in channels (collectors) of constant cross section, with a run off and influx through a permeable (perforated) wall.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 3, pp. 405–412, March, 1969.     

Application of permeable collectors in deep-bed filtration

A study on the possible use of media composed of permeable spheres in deep-bed filtration was conducted. The permeable spheres were made of thin fibers and have different porosities. With such media, particle retention takes place at the outside surfaces as well as throughout the entire body of the spheres. Theoretical analysis based on trajectory calculations using cell models to characterize both the interphere and intrasphere flow fields was carried out for estimating the particle collection efficiency. A parametric study was made to examine the effect of the various operating variables. In contrast to conventional deep-bed filtration using impermeable grains, filtration using permeable spheres performs better in particle collection. This conclusion was confirmed by experiments presented in the second part of this study.     

Introducing porosity into polyimide nanoparticles

Novel porous polyimides (PIs) having diameters of several hundred nanometers have been fabricated successfully from precursor poly(amic acid) (PAA) derivatives with poly(acrylic acid) (PAS) as the porogen, using a reprecipitation method and subsequent imidization. The superficial high porosity with deep pores was introduced when using a more compatible combination of PAA and the porogen, i.e., PI (BPDA-PDA) and PAS rather than PI (10FEDA-4FMPD and PAS); the pore sizes ranged from 20 to 100 nm. The resulting porous PI nanoparticles had thermally stabilities (determined from their 5% weight loss temperatures at 400 degrees C) similar to those of corresponding PI nanoparticles lacking porous structures. Microphase separation within the PAA nanoparticles after reprecipitation induced the porous surface structure, the properties of which were influenced by the molecular weight of PAS and the chemical structure of PAA. These unique porous PI nanoparticles have great potential for application as low-k materials in next-generation technologies.     

Hartmann flow over a permeable bed

Hartmann flow past a permeable bed is investigated in the presence of a transverse magnetic field with an interface at the surface of the permeable bed. The flow above the bed, called Zone 1, is governed by the Hartmann equation and that below the bed, called Zone 2, is governed by the modified Darcy law. Solutions for these two zones are separately obtained and are matched at the interface using suitable boundary conditions. The mass efflux, change in volume rate of flow, the induced magnetic field and current density are evaluated to determine whether a magnetic method might be potentially useful in the study of pore-size distribution.We find that, for mercury flowing over a long permeable channel of width 0.7 cm, the effect of magnetic field (of strength 0.25 Web/m², i.e. the Hartmann number M = 8.8) in the presence of a porous wall (of porosity k = 5 × 10^?6 Darcy) is to retard the mass flow and to increase the friction factor relative to the corresponding quantities for non-magnetic flow. The amount of retardation is 22 per cent more than that obtained by Wallace et al. [12] in the experiments on the flow of mercury in porous media in a transverse magnetic field. Since the magnitude of retardation is related to the pore size distribution, our model is potentially useful to study the pore size distribution than that of Wallace et al. [12]. In addition, transition to turbulence occurs at a higher Reynolds number owing to the presence of a magnetic field.     

Gravity waves over a permeable bed

The attenuation of Waves on the free surface of a viscous fluid which is bounded below by a porous bed of finite depth, is studied. It is assumed that the viscosity, permeability of the porous bed and the bottom friction, according to the quadratic law, contribute to the dissipation of the wave energy. By linearizing the nonlinear frictional force, the complete linear problem is solved for the damping coefficients and the results are compared with the case when the turbulence is absent.     

Surface friction at a permeable plate

An approximate solution is proposed for the problem of determining the friction at a permeable plate. Expressions are given for the tangential stresses with continuous uniform injection through a transverse slot.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 2, pp. 342–345, August, 1969.     

透水沥青混凝土单轴冲击压缩特性研究

为研究透水沥青混凝土的动态力学特性,采用74 mm钢质分离式霍普金森压杆装置对不掺纤维和掺0.3%聚酯纤维的透水沥青混凝土进行了不同应变率的单轴冲击压缩试验。研究表明,透水沥青混凝土具有明显的应变率效应,试件压缩率随着应变率的增大而提高,掺聚酯纤维透水沥青混凝土的压缩率是不掺纤维的1.2倍左右;透水沥青混凝土的动态应力-应变曲线可分为弹性变形阶段、塑性变形阶段和破坏阶段。从试件的破坏形态可以看出,集料的断裂是透水沥青混凝土破坏的主要原因。在透水沥青混凝土中掺加聚酯纤维能够延缓裂缝的出现和开展,提高材料的冲击抗压强度,增幅最大为45.1%。     

Features of turbulent heat transfer on permeable surfaces

The well-known fact of intensification of heat transfer in strongly accelerated flows because of destruction of the viscous sublayer by substance injected through a porous surface is analyzed experimentally.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 3, pp. 388–392, September, 1984.     

Dependence of ceramic fracture properties on porosity

A connected-grain model developed earlier to study the modulus of elasticity as a power-law of density was extended to study the dependence of the flexural strength of polycrystalline ceramics on porosity. Relations were derived for specific surface fracture energy, fracture toughness and flexural strength as power laws of (1 ?p), wherep is porosity. Model validity was confirmed with data on α-alumina, UO₂, Si₃N₄, and the YBa₂Cu₃O_7?δ superconductor.