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Materials for Aperture Grills for CRTsIntroductionThe case study in this section illustrates how CES can be used for selecting materials. The underlying methodology for selection, used here, is described in more detail in references [1, 2].Two types of cathode ray tube (CRT) currently dominate the computer monitor and television marketplace. In the older technology, colour separation is achieved by using a shadow mask: a thin metal plate with a grid of holes that allow only the correct beam to strike a red, green or blue phosphor. A shadow mask can heat up and distort at high brightness levels, causing the beams to miss their targets, and giving a blotchy image (called 'doming'). To avoid this, the newest shadow masks are made of Invar, a nickel alloy with a near-zero expansion coefficient between room temperature and 150°C. It is a consequence of shadow-mask technology that the glass screen of the CRT curves inward on all four sides, increasing the probability of reflected glare. Sony's 'Trinitron' technology overcomes this problem and allows greater brightness by replacing the shadow mask by an aperture grill of fine vertical wires, each about 200 mm in thickness, which allows the intended beam to strike either the red, the green or the blue phosphor to create the image (Figure 1). The glass face of the Trinitron tube is curved in one plane only, reducing glare.
Figure 1 An aperture grill consisting of taught, vertical wiresThe wires of the aperture grill are tightly stretched, so that they remain taut even when hot. It is this tension which allows the greater brightness. What is the best material from which to make them? The table below summarises the requirements. Design requirements
Table 2The ModelA thin, taut wire slackens and sags when the strain eT due to thermal expansion,eT = aDT (1) exceeds the elastic strain eE caused by the pre-tension, eE=s / E (2) Here a is the thermal expansion coefficient of the wire, DT the temperature rise caused by the electron beams which strike it, s is the tensile stress in the wire and E its modulus. We wish to maximise the brightness, and thus DT. The pre-tension is limited by the elastic limit of the wire, sel. Inserting this and equating the two strains (equations 1 and 2) gives
The result could hardly be simpler. To maximise the brightness, maximise
There are further requirements. The material must be available as wire. The wires must conduct, otherwise they would charge up, distorting the image, but all metals conduct, so we have no problem here. Cost is another matter: it is desirable that the wire is not too expensive. The SelectionFigure 1 is a chart of elastic limit, sel, plotted against the product E a using the CMS Non-ferrous Metals database. The selection line for M1 is marked on it. Materials above the line have the best ability to stay taut when heated. A second stage, Figure 2, isolates materials which are available as wire and have a maximum operating temperature above 400°C (673 K). The final stage, Figure 3, captures materials with good conductivity (the reciprocal of the resistivity); and which cost less than $100/kg. The materials which passed all three stages are listed in Table 2.The best materials for this application are refractory metal wires, among which Tungsten, Zirconium and Invar stand out as attractive, though expensive, choices.
Figure 1 A chart of, elastic limit, sel, plotted against the product Ea, using the Non-ferrous Metals database and showing the index M1
Figure 2 Maximum service temperatures, Tmax, for materials which are available in the form of wire
Figure 3 A 'protective' stage in which electrical conductivity, 1/r e, is plotted against cost CmResults
Table 2 Selection results: Materials for Aperture GrillsReferences
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