Microstructure factor

It was pointed out by Nieh and Wadsworth [5] that fine grain size is a necessary but insufficient condition for HSRS. This conclusion resulted from the observation that many fine-grained composites are not superplastic at high strain rates. Evidently, in addition to grain size, microstructural factors, such as detailed structure and chemical composition at the reinforcement-matrix interfaces and grain boundaries, may play important roles. 

In addition to the alloy compositions being of importance with regard to susceptibility to stress-corrosion cracking, the resistance of the alloy can be altered by microstructural factors. Hanninen has reviewed the available literature quite thoroughly and has concluded that a fine grain size is likely to be beneficial. Strain imposed prior to use tends to be deleterious because deformed material usually acts anodic with respect to unstrained material and because the introduction of plastic deformation may also... 

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

In order to distinguish between isolated silicon-hydrogen bonds in a dense network and other bonding configurations, such as clustered monohydride and dihydride bonds, bonds on internal void surfaces, and isolated dihydride bonds, Mahan et al. [60] have defined the microstructure factor R as... 

The variation of deposition temperature has similar effects on the material properties to those on PECVD-deposited material. With increasing temperature (125-650°C), the material becomes more dense (the refractive index extrapolated to 0 eV increases from 3.05 to 3.65). and the hydrogen content is decreased (15 to 0.3 at.%), as well as the microstructure factor (0.4 to 0). The activation energy is 0.83 eV up to a deposition temperature of 500°C. The dark conductivity and AM 1.5 photoconductivity are about 5 x 10 " and 5 x 10 cm , respec-... 

The HWCVD deposition process is more or less the same as for PECVD, and was described in Section 1.7. Important differences between the two is the absence of ions, and the limited number of different species present in the gas phase, in the former. At low pressure atomic Si is the main precursor. This yields void-rich material with a high microstructure factor. Increasing the pressure allows gas phase reactions with Si and H to create more mobile deposition precursors (SiH3), which improves the material quality. A further increase leads to the formation of higher silanes, and consequently to a less dense film. 

An important question now arises what microstructural factor(s) is (are) responsible for the observed decrease of the DSC desorption peak temperature with increasing... 

In passing, we should also mention one additional microstructural factor potentially impacting the overall electrode performance constriction of the ionic current in the electrolyte near the electrode/electrolyte interface. To better understand this effect, consider a circular disk electrode of diameter d immersed in a semi-infinite electrolyte of conductivity... 

Here, vmech is the mechanically effective chain density specified, e.g., in [168], Ac 0.67 [170] is a microstructure factor which describes the fluctuations of network junctions, Na the Avogadro number, p mass density, Ms and Zs molar mass and length of a statistic segment, respectively, kB the Boltzmann constant, and T absolute temperature. 

The mechanical properties of rapidly polymerizing acrylic dispersions, in simulated bioconditions, were directly related to microstructural characteristics. The volume fraction of matrix, the crosslinker volume in the matrix, the particle size distribution of the dispersed phase, and polymeric additives in the matrix or dispersed phase were important microstructural factors. The mechanical properties were most sensitive to volume fraction of crosslinker. Ten percent (vol) of ethylene dimethacrylate produced a significant improvement in flexural strength and impact resistance. Qualitative dynamic impact studies provided some insight into the fracture mechanics of the system. A time scale for the elastic, plastic, and failure phenomena in Izod impact specimens was qualitatively established. The time scale and rate sensitivity of the phenomena were correlated with the fracture surface topography and fracture geometry in impact and flexural samples. 

Table 17.7 summarizes the effects of the microstructural factors on the microscopy fractal dimensions, Dj, y, and Zlpr- Different fractal dimensions reflect different aspects of the microstructure of the fat crystal networks and thus have different meanings. It is necessary to define which structural characteristic is most closely related to the macroscopic physical property of interest (mechanical strength, permeability, diffusion) and then use the fractal dimension that is most closely related to the particular structural characteristic in the modeling of that physical property. 

Table 17.7. The effect of the microstructural factors on the microscopy fractal dimensions, D >, Df, and Dfj.

The maximum ionic conductivity in ZrO2-based systems is observed when the content of acceptor-type dopant cations with the smallest radii (Sc, Yb, Y) is close to the minimum necessary to completely stabilize the cubic fiuorite-type phase in the operating temperature range [9,11,16, 32-35]. This concentration (often referred to as the low stabilization limit) and the conductivity of the ceramic electrolytes are dependent, to a finite extent, on the pre-history and various micro structural features. In addition to the metastable states discussed above, critical microstructural factors... 

Microstructural factors also play important roles in determining the electrochemical and physical properties of semiconductor-electrolyte systems. For example, semiconductor electronic properties are usually interpreted in terms of ideal band models for perfect crystals—i.e., for systems that exhibit absolute long-range order. For many systems, however, this is a gross oversimplification and, in the extreme of the amorphous state, it may be appropriate to abandon band models altogether... 

H. Kokawa, Microstructural Factors Governing Hardness in Friction-Stir Welds of Solid Solution Hardened A1 Alloys, Met-all. Mater. Trans. A, Vol 32, Dec 2001, p 3033-3042... 

The microstructural factors that have the greatest effect on mechanical properties are the per cent crystallinity and the preferred orientation of the crystals (if any). Composite mechanics concepts will be needed to explain the mechanical properties of spherulitic polymers hence we return to them at the end of the next chapter. 

An old point of controversy in rubber elasticity theory deals with the value of the so-called front factor g = Ap which was introduced first in the phantom chain models to connect the number of elastically effective network chains per unit volume and the shear modulus by G = Ar kTv. We use the notation of Rehage who clearly distinguishes between A andp. The factor A is often called the microstructure factor. One obtains A = 1 in the case of affine networks and A = 1 — 2/f (f = functionality) in the opposite case of free-fluctuation networks. The quantity is called the memory factor and is equal to the ratio of the mean square end-to-end distance of chains in the undeformed network to the same quantity for the system with junction points removed. The concept of the memory factor permits proper allowance for changes of the modulus caused by changes of experimental conditions (e.g. temperature, solvent) and the reduction of the modulus to a reference state However, in a number of cases a clear distinction between the two contributions to the front factor is not unambiguous. Contradictory results were obtained even in the classical studies. 

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