Granular microstructure

Barnes and co-workers have studied mixed-monolayer systems [278,281,283,284] and found some striking nonidealities. Mixed films of octadecanol and cholesterol, for example, show little evaporation resistance if only 10% cholesterol is present [278] apparently due to an uneven granular microstructure in films with cholesterol [284]. Another study of cellulose decanoate films showed no correlation between holes in the monolayer and permeation rate [285]. Polymerized surfactants make relatively poor water evaporation retarders when compared to octadecanol [286]. There are problems in obtaining reproducible values for r [287] due to impurities in the monolayer material or in the spreading solvent. 

M. Ostoja-Starzewski Mechanics of damage in a random granular microstructure Percolation of inelastic phases. Int. J. Eng. Sci. 27, 315-326 (1989)... 

Regardless of the material combination, departures from conventional sputter deposition, such as cluster beam methods with particle size selection, shaping and heating capabilities independent of the substrate, may be necessary to achieve the desired granular microstructure [50-53]. 

In summary, the difiusion of K into C o films produces a minimum resistivity phase that is barely metallic with p = 2.2 milliohm-cm at composition K3.oo o.o5C6o, and a high resistivity (max) phase at composition Ks.ooio.osCjo. Intermediate compositions yield activated conduction with x-dependent activation energy, which can be explained by a simple model with immiscible phases where the metallic phase maximally avoids percolation. The recognition that the normal state has a granular microstructure will also be important in understanding superconductivity in thin films. 

In this paper, we study the fundamental fragmentation problem of a specimen with an inherent granular microstructure that is subjected to a rapid expansion. Of special interest are the effects of the average grain size, initial strain rate and cohesive failure model parameters (strength and toughness) on the fragmentation process. 

The key elements of the numerical scheme used in this study are its ability to incorporate the granular microstructure of the ceramic material, to simulate the spontaneous initiation, propagation and branching of intergranular cracks and subsequent fragmentation of the body, to account for inertial and finite kinematics effects and to capture the complex contact events taking place between the fragments. 

Figure 4. Granular microstructure of the sample with an average grain size of (a) 40 fx.m, (b d) 60 fim and (e) 80 pm. For figures (b), (c) and (d), the average grain size is the same but the distributions are different.

Mueth, D. et al. 2000. Signatures of granular microstructure in dense shear flows. Nature 406 385-389. 

On the other hand, Xiao et al. [215] reported that smooth, dense, and erystalline PbTe films with nearly stoichiometric composition could be obtained by an optimized electrodeposition process from highly acidic (pH 0) tellurite solutions of uncomplexed Pb(II), on Au-coated silicon wafers. The results from electroanalyti-cal studies on Te, Pb, and PbTe deposition with a Pt rde at various temperatures and solution compositions supported the induced co-deposition scheme. The microstructure and preferred orientation of PbTe films was found to change significantly with the deposition potential and electrolyte concentration. At -0.12 V vs. Ag/AgCl(sat. KCl), the film was granular and oriented preferentially in the [100] direction. At potentials more negative than -0.15 V, the film was dendritic and oriented preferentially in the [211] direction (Pig. 3.13). 

Obviously, typical composite propellants are highly filled polymers and, especially in the case of the rubber-based systems, are more nearly granular media than merely filled rubbers. Certain specific differences between the double-base binder and the elastomeric binder lead to some differences in behavior, but many of the resulting properties show similarities. The relationships between bulk properties and microstructure in the rubber-based systems are much better understood than those of the double-base propellants and, therefore, the following discussions focus primarily on the former category. 

The morphology and microstructure of as-polymerized polytetrafluoro-ethylenes is a study in itself. We observe that fibrils are common in some lots of granular PTFE while other specimens consist of beadlike particles, the surfaces of which bear markings suggesting lamellar crystals. Of special note is the (rare) occurrence of shish-kebab structures in as-polymerized PTFE (Figure 1.3). 

In [9] Passman, Nunziato and Walsh presented a multiphase mixture in which each constituent had a simple geometrical structure characterized by a scalar kinematic parameter, its volume fraction. But when the kinematical describer is more complex and takes value on a manifold, it is necessary to consider the more general microstructure introduced, e.g., by Capriz in the essay [3], where materials as liquid crystals, granular and porous media, Cosserat and micromorphic continua are studied. 

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