Grain resistance across

Case (i) Ogt Cgi,. Conduction along the grain boundaries is negligible, and conduction through the grains and across grain boundaries dominates. The behavior can be described in terms of complex resistivity ... 

In the region 11 above the Curie temperature, resistance across the grain boundary increases exponentially with increasing temperature. 

Wood chipboard is free from grain and is thus essentially isotropic in its behaviour. The mechanical properties are approximately the same as the average of the properties of the original wood measured along and across the grain. The water resistance of chipboard is poor but, being isotropic, it does not warp as long as it is able to swell freely in all directions. 

Figure 50 shows three examples for (different) kinetic situations in which bulk diffusion, surface reaction, and transport across a grain boundary are the sluggish steps. Nonetheless, the other parameters can also be evaluated. This becomes especially clear from the top figure, where the nonunity intercepts reveal surface effects. Similarly, the nonzero bending of the profiles in the other two figures indicates transport resistances. 

Electrical moisture meters provide a quick and reasonably accurate nondestructive alternative. The direct-current resistance of the timber is measured or either the alternating-current capacitance or power loss can be measured. Direct-current resistance moisture meters are more common a pair of needles, a fixed distance apart, is driven into the wood across or along the grain (depending on the manufacturer s instructions) and the electrical resistance measured. The procedure is reasonably accurate between the fibre saturation point (defined later) at 30% and about 6% moisture content (at which point the resistance becomes too great to measure with reasonable accuracy). In this moisture content range, the relationship between electrical resistance and moisture content is represented by a log-log plot. 

The effect of the two types of boundary on the measurements of resistivity vs. temperature are shown in Fig. 12.21. None of the leads, the c-axis material and the a-axis material measured in the [010] and [001] directions, shows ideal behavior. This is, at least in part because of the necessity of some compromise in the deposition temperature between ideal a-axis and c-axis conditions. However, the results for the twist grain boundary are consistent with that found previously for (103) films measured across many such grain boundaries there... 

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