Volume Fraction Of Liquid Phase Research Articles

Mathematical modeling of mixed‐culture biofilms

About 10 years ago a set of mass balance equations for mathematical modeling of mixed-culture biofilms (MCBs) was presented. That model was able to describe the progression of the biofilm thickness and the spatial distribution and development in time of particulate and dissolved components in the biofilm as a function of transport and transformation processes. Experimental observations made in the past years have shown that some of the assumptions made in that MCB model were too simple. Therefore, an extended MCB model with additional processes has been developed. This model includes a more flexible description of transport of dissolved components in the biofilm and considers diffusive transport of particulate components in the biofilm solid matrix, changes of the biofilm liquid phase volume fraction (porosity), and simultaneous detachment and attachment of cells and particles at the biofilm surface. The extended MCB model is implemented in AQUASIM, a new computer program designed for the analysis of aquatic systems, which is used here to illustrate and discuss the effect of the additional processes on MCB behavior. © 1996 John Wiley & Sons, Inc.

New experimental findings and biofilm modelling concepts

About ten years ago a mathematical model was presented which describes the spatial distribution and development in time of microbial species in mixed-culture biofilms. The model was based on the continuum approach and was one-dimensional in space. These two concepts still are the basis of practically all biofilm models used today. On the experimental side some remarkable new findings have been made in the past years: transport of dissolved components in the biofilm is not always due to molecular diffusion only, transport of particulate components can not be exclusively related to the net growth rates of the microbial species in the biofilm, the liquid phase volume fraction (porosity) in the biofilm is not a constant, and simultaneous attachment and detachment of cells and particles at the biofilm surface is an essential process. These experimental findings had a significant impact on our notion of biofilm systems and called for the integration of new processes in the original mixed-culture biofilm model. The new processes can reproduce most of the experimental observations, however, they are described by empirical mathematical functions. Their mechanisms and significance for biofilm behavior have not been completely elucidated yet. Thus, the extended mixed-culture biofilm model represents primarily a tool for research on biofilm processes.

Processing and properties of aluminium-silicon carbide composites made by PM : J.L. Rossi et al (Inst. Pesquisas Energiticas e Nucleares, Sao Paulo, Brazil)

The effects of vacuum hot pressing temperature and pressure on tensile properties and microstructures of 20 vol.% SiC whisker (SiCw)-2124 Al composites were investigated. The tensile strength of SiCw-2124 Al composite increased rapidly with increasing vacuum hot pressing temperature up to 570 °C and became saturated above 570 °C. The increase in tensile strength with increasing vacuum hot pressing temperature is due to the enhanced densification of composite and reduced damage of whiskers from the softening of 2124 Al matrix with increasing volume fraction of liquid phase. The vacuum hot pressing pressure needed to be higher than 70 MPa to achieve densification of SiCw-2124 Al composite above 99%. A vacuum hot pressing pressure above 70 MPa was not helpful to enhance the tensile strength, since the aspect ratio of whiskers decreased owing to the damage during vacuum hot pressing. Combining the effects of the aspect ratio of whiskers and the density of composite, which are dependent on the vacuum hot pressing parameters, an equation is proposed to estimate the tensile strength of SiCw-2124 Al composite. The effect of porosity on tensile strength of SiCw-2124 Al composite was found to be more sensitive compared with powder metallurgy alloys or ceramics, since the most pores were located at whisker-matrix interfaces and reduced the load transfer efficiency between SiC whiskers and 2124 Al matrix.

Effects of vacuum hot pressing parameters on the tensile properties and microstructures of SiC-2124 Al composites

The effects of vacuum hot pressing temperature and pressure on tensile properties and microstructures of 20 vol.% SiC whisker (SiC w)-2124 Al composites were investigated. The tensile strength of SiC w-2124 Al composite increased rapidly with increasing vacuum hot pressing temperature up to 570 °C and became saturated above 570 °C. The increase in tensile strength with increasing vacuum hot pressing temperature is due to the enhanced densification of composite and reduced damage of whiskers from the softening of 2124 Al matrix with increasing volume fraction of liquid phase. The vacuum hot pressing pressure needed to be higher than 70 MPa to achieve densification of SiC w-2124 Al composite above 99%. A vacuum hot pressing pressure above 70 MPa was not helpful to enhance the tensile strength, since the aspect ratio of whiskers decreased owing to the damage during vacuum hot pressing. Combining the effects of the aspect ratio of whiskers and the density of composite, which are dependent on the vacuum hot pressing parameters, an equation is proposed to estimate the tensile strength of SiC w-2124 Al composite. The effect of porosity on tensile strength of SiC w-2124 Al composite was found to be more sensitive compared with powder metallurgy alloys or ceramics, since the most pores were located at whisker-matrix interfaces and reduced the load transfer efficiency between SiC whiskers and 2124 Al matrix.

Permeability in a State of Solidification of a Porous Medium Saturated with Aqueous Solution. Influence of Initial Concentration and Supercooling Degree of Aqueous Solution.

Permeability of the mushy region formed by solidification of a porous medium Saturated with an aqueous solution was measured by a transient method. A packed bed of glass beads with 2.56 mm mean diameter saturated with NaCl aqueous solution was used as the porous medium. Measurements were carried out with various volume fractions of liquid phase and with various initial concentrations and supercooling degrees of the solution. From the measured results, it was found that the permeability was proportional to the n th power of the volume fraction and decreased with increase of the supercooling degree, decreasing slightly with increase of the initial concentration. An experimental equation of the permeability was obtained as a function of the volume fraction, supercooling degree, and initial concentration.

New experimental findings and biofilm modelling concepts

About ten years ago a mathematical model was presented which describes the spatial distribution and development in time of microbial species in mixed-culture biofilms. The model was based on the continuum approach and was one-dimensional in space. These two concepts still are the basis of practically all biofilm models used today. On the experimental side some remarkable new findings have been made in the past years: transport of dissolved components in the biofilm is not always due to molecular diffusion only, transport of particulate components can not be exclusively related to the net growth rates of the microbial species in the biofilm, the liquid phase volume fraction (porosity) in the biofilm is not a constant, and simultaneous attachment and detachment of cells and particles at the biofilm surface is an essential process. These experimental findings had a significant impact on our notion of biofilm systems and called for the integration of new processes in the original mixed-culture biofilm model. The new processes can reproduce most of the experimental observations, however, they are described by empirical mathematical functions. Their mechanisms and significance for biofilm behavior have not been completely elucidated yet. Thus, the extended mixed-culture biofilm model represents primarily a tool for research on biofilm processes.

Transient liquid-phase sintering of ceramic-reinforced Fe-based composites

The microstructural development of ceramic-reinforced iron-based composites has been studied. The composites were fabricated via powder metallurgy and liquid-phase sintering, a processing route which achieves near-net-shape with good ceramic particulate dispersion. Two matrix alloys were used, Fe-1 wt% C-1 wt% Si and Fe-2 wt% Cu; up to 30 wt% (≈36 vol%) yttria-stabilized zirconia in the form of ∼20 μm particles was added to these alloys. The microstructural evolution of these composite materials was studied by examining the densification rate and volume fraction of liquid phase as a function of time. Different particle/matrix interfaces developed in the two composites. A glassy silicon-rich layer formed in the Fe-1C-1Si-YSZ composites and a more limited crystalline layer was found in the Fe-2Cu-YSZ composites.

鉄粉焼結体の緻密化と機械的性質におよぼすBの影響

The effect of addition of master alloy powders containing B on the densification, microstructure and mechanical properties of atomized iron powder compacts has been studied.The results are summarized as follows.(1) The Ni-B master alloys activated the sintering process through the formation of eutectic liquid. However, there was limitation to sintered density that was attained by those alloys.(2) Densities of the order 99% theoretical were readily achieved with addition of the Fe- Ni-B master alloy powders. Volume fraction of liquid phase was increased during the sintering process by addition of these additives.(3) Rapid densification occured at 1403 K in iron powder compacts from 16 μm and at 1423 K from 80 μm, but in compacts from 45 μm having large pore size distribution occurred above 1433 K.(4) Sintered strength increased with the amount of master alloy powders. With 5% additive, the tensile strength of 400 MPa was obtained. Maximum elongation of 30% occurred at 2% addition.(5) Elongation was closely related to the forming process of the eutectic phase and decreased as it became continuous since a fracture occurred along the eutectic structure.

On a relation between foam and surface dilatational viscosities

The significance of surface rheological properties in macroscopic foam rheological behavior is illustrated within the context of a pure foam dilatation. By application of the rheological theory of spatially periodic suspensions to a spatially periodic foam, the leading-order contribution to the macroscopic foam pressure tensor is shown to depend directly upon the rheological response of individual foam surfaces to globally induced deformation. Focusing upon a “wet” foam geometry within a pure foam dilatational motion enables us to determine a foam dilatational viscosity to leading order in the liquid-phase volume fraction as a function of surface dilatational viscosity as well as of thin foam film parameters such as the disjoining pressure.

Liquid Redistribution during Liquid‐Phase Sintering

A simple two‐dimensional packing model, consisting of arrays of circular particles, was used to calculate the free energy changes associated with the filling of pores of different coordinations with liquid. The calculations were used to determine the equilibrium distributions of a liquid in different packing arrangements of particles. The effect of both the volume fraction of liquid and shrinkage on liquid distribution was examined. It was found that, when liquid redistribution can easily take place, the volume fraction of liquid phase, the pore size distribution of a powder compact, and the amount of densification that has occurred all influence the homogeneity of the distribution of the liquid phase. In addition, the model predicts that, as shrinkage occurs or as the volume fraction of liquid phase increases, the pores will try to fill sequentially in order of increasing size. A consequence of this sequential filling of the pores is that the radius of curvature of the liquid meniscus, and hence the driving force for liquid‐phase sintering, changes systematically as shrinkage occurs. The modeling suggests that the driving force for sintering changes in a way that depends on the initial overall pore size distribution of the particle arrangement and the way the pore size distribution changes during sintering.