Random Surface Corrugations

The question which kind of antireflection surface relief is the most efiicient, regular or random, is still open to debate [188]. Some of the most eflicient detectors in the solar cells field actually have random surface profiles— stochastically distributed pyramids made in transparent conductive oxide (zinc oxide) using chemical vapor deposition [197]. However, ordered and random profiles both effectively behave as gradient refractive index profile starting Irom the effective value nearly equal to that of the environment index and ending with that of the substrate. In this manner, the question reduces to the determination of an optimal gradient-index profile. 

A problem with random profiles is their repeatability from sample to sample, since it depends on a number of process parameters and their very nature is stochastic. Some solutions that could be applied to overcome this is to use advanced replication technologies like, e.g., nanoimprint to replicate the optimized random profile. Another is to apply top-down planar fabrication to pattern quasiperiodic stmctures that behave as pseudorandom, although they are actually deterministic. 

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Capacity and efficiency. Figure 8.12a compares random to corrugated sheet structured pacHng in terms of specific surface areas and pack ing factors. For random packings, the lines shown were taken from... 

Pressure drop has been reported for a number of rotor internals, including corrugated structured packing (28), foam metal (26,40,42), rectangular and elliptical cylinder plastic grains randomly packed (41), wire screen (43), and glass beads (17). In spite of the variation in porosity from 0.38 to 0.95 and in volumetric surface area from 500 to 4000 m2/m3, all of these studies showed similarities of increased pressure drop as rotor speed increased and gas rates increased. 

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