Mass Transport Properties of Materials

Substituting values from above and solving for gives 

Recall from the beginning of this chapter that the transport of mass at steady state is governed by Pick s Law, given in one-dimension by Equation (4.4)  

1 The Molecular Origins of Mass Diffusivity. In a manner directly analogous to the derivations of Eq. (4.6) for viscosity and Eq. (4.34) for thermal conductivity, the diffusion coefficient, or mass diffusivity, D, in units of m /s, can be derived from the kinetic theory of gases for rigid-sphere molecules. By means of summary, we present all three expressions for transport coefficients here to further illustrate their similarities. 

As before, M is the molecular weight of the rigid sphere, T is the absolute temperature, and a and are the same Lennard-Jones parameters used in Eq. (4.6) (note that 

2 Mass Diffusivity in Liquid Metais and Ailoys. The hard-sphere model of gases works relatively well for self-diffusion in monatomic liquid metals. Several models based on hard-sphere theory exist for predicting the self-diffusivity in liquid metals. One such model utilizes the hard-sphere packing fraction, PF, to determine D (in cm /s)  

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The sensitivity of Magnetic Resonance (MR) to the local concentration, molecular dynamics and molecular environment of these nuclei make it well suited for the study of deterioration processes in concrete materials. Hydrogen (water), lithium, sodium, chlorine and potassium are all MR sensitive nuclei and play an important role in cement chemistry. The ability of MRI to spatially resolve and non-destructively examine test samples as a function of treatment or exposure has the potential to provide new insight to better understand deterioration mechanisms and mass transport properties of concrete materials. 

The gas response of the field-effect devices is determined by the catalytic properties of the contact material, which includes both the catalytic layer and the underlying material. The temperature plays a dominant role in the detection process because the origin of the gas response is found in the chemical reactions that take place on the sensor surface, and it is furthermore also influenced by the mass transport properties of the molecules in the gas phase. This permits arrays of sensors of a common design to be tailor-made for detection of a range of gases and for use in a range of applications... 

Little is known about the mass transport properties of reinforced-composite materials. Certainly, there are no new relations or concepts that govern estimations of diffusiv-ities that have not already been discussed. In most polymer-matrix composites, the transport properties of the polymer play an important role in diffusion through the composite. For example, hydrophilic polymers such as epoxy readily absorb water from the atmosphere. Thermoplastic polymers absorb relatively little moisture since they are more hydrophobic, but are more susceptible to uptake of organic solvents. 

It is useful to examine reticulated foams in the context of their suitability as scaffolds for artificial organ development. Reticulated foams are made in a postprocessing step after polyurethane foams are made. Polyether and polyester polyurethanes are made for reticulation. The major use of the materials is in air filtration where resistance to flow (a mass transport property) is an important requirement. 2005 by CRC Press LLC... 

In the previous section, we put forth evidence for the claim that the presence of point defects can completely alter the observed properties of materials. However, this discussion was set forth without entering into the question of how materials are brought to a state in which there is a given distribution of point defects. Many processes in materials are mediated by diffusion in which mass is transported from one part of the material to another. When viewed from the atomic scale, these diffusive processes may be seen as a conspiratorial effect resulting from the repeated microscopic hopping of particles over time. Our aim in the present section is to examine several different perspectives on diffusion, and to illustrate how a fundamental solution may be constructed for diffusion problems in much the same way we constructed the fundamental solution for elasticity problems in section 2.5.2. 

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