Micromesh

Micromesh Sieving machines (Air jet, sonic wet and dry) Biickbee Mears, Veco, Endecottes Alpine, ATM, Gilson, Gradex, Hosokawa, Retsch, Seishin 5-500 -1- im 1-5... 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

The common sieves are made of woven wire cloth and have square apertures. The sizes of the sieve openings have been standardized, and currently two different sets of standard series, the Tyler Standard and the U.S. Series ASTM Standard, are used in the United States. The mesh number of a sieve is normally defined as the number of apertures per unit area (square inch). Thus, the higher the mesh number the smaller the aperture. Typical mesh numbers, aperture sizes, and wire diameters are given for the lyier sieves and the U.S. ASTM sieves in Table 1.3. Sieve analysis covers the approximate size range of 37 pm to 5,660 pm using standard woven wire sieves. Electroformed micromesh sieves extend the range down to 5 pm or less while punched plate sieves extend the upper limit. 

Flow electrodes. Rather than move the electrode past the solution, the sample solution can be flowed past a stationary electrode. The tubular platinum electrode (Figure 5.34) and the gold micromesh flow-through electrode (Figure 5.35) are both ingenious attempts to produce electrodes that are useful for the measurement of electroactive materials in a continuously flowing stream. Ap-... 

Figure 5.35 Gold micromesh flow-through electrode (a) cell system (b) detail of gold micromesh screen.

Electroformed micromesh sieves extend the range down to 5 pm or less, and punched plate sieves extend the upper range. 

With micromesh sieves, near-monodisperse powders can be generated in the 1 to 10 pm size range [3]. An overview of recent developments in fine sieving in Japan has been presented in two reviews [5,6]. 

Complete instructions and procedures on the use and calibration of testing sieves are contained in ASTM STP447B [7]. Contents include analytical methods, information relating to wire mesh, perforated plate and micromesh sieves, dry and wet testing and other methods. This publication also contains a list of all published ASTM standards on sieve analysis... 

Micromesh sieves [11] were first described by Daescher et. al. [35] and are available in standard sizes ranging from 500 pm down to 3 pm. Etched sieves usually have round or square apertures, ranging in size from 500 to 1200 pm, but other aperture shapes are available. They are available in 3, 8 and 12 in diameter frames, as well as custom-made sizes. Other aperture sizes are also available and Zwicker reports using a 1 pm sieve [36]. 

The tolerances with micromesh sieves are much better than those for woven-wire sieves the apertures being guaranteed to 2 pm of nominal size apertures except for the smaller aperture sieves. Each type of sieve has advantages and disadvantages [38] e.g. sieves having a large percentage open area are structurally weaker but measurement time is reduced. 

Several manual methods of wet sieving using micromesh sieves have been described. Mullion [84] uses an 80 kHz, 40 W ultrasonic bath in which the micromesh sieve rests on a support, which, in turn, rests on a beaker in the bath. Sieving intervals are 5 min. with an initial load of 1 g. and the operation is deemed complete when no further powder can be seen passing through the sieve. 

Burt used 0.5 to 1.0 g samples for the micromesh sieves and found that the efficiency increased with decreasing sample size and then only when three felvation units were used. Separation efficiency increased if the fluid flow was pulsed two or three times a second so a pulsator was added to the equipment. The technique was found to be unsuitable with micromesh sieves, which were too fragile, but was more successful with woven wire sieves. The technique was not proposed as an alternative to standard sieving methods, but may be useful if only small quantities are available, or with hazardous materials, where small samples are desirable for safety reasons. The technique has also been used to grade 5 kg samples [109] in the size range 45 to 64 pm. 

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