Ceramics control measures

Later, Viniegra et al. (1960) carried out the first study of saturnism in a small town near Mexico City, where ceramics are made according to the traditional system. These authors found high blood lead concentrations in 48 persons (male and female) of various ages living there. After this study, many more have been carried out on this metal and its presence in the Mexican environment and people. However, no effective control measures, which are urgently needed in this country, have been implemented. 

In flexible vinyl compounds, gloss is usually controlled with 1-3 phr of fine particle silica or talc, occasionally with ceramic microspheres. Measured gloss (ASTM D523) is, however, often lower in plasticized compounds if 2-5 percent acrylic impact modifier is used instead. This approach provides lower haze, generally better physical properties, and reduced equipment wear. The use of fine-pailicle fillers is preferable in cases where it is also operating to reduce plateout. 

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This propei ty is useful in process control for polymers, ceramic materials, and semiconduc tors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). TEe value for water is higher than for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

As we shall see, there are two ways of improving the strength of ceramics decreasing by careful quality control, and increasing Ki - by alloying, or by making the ceramic into a composite. But first, we must examine how strength is measured. 

Ceramic boards are currently widely used in high-performance electronic modules as interconnection substrates. They are processed from conventional ceramic precursors and refractory metal precursors and are subsequently fired to the final shape. This is largely an art a much better fundamental understanding of the materials and chemical processes will be required if low-cost, high-yield production is to be realized (see Chapter 5). A good example of ceramic interconnection boards are the multilayer ceramic (MLC) stractures used in large IBM computers (Figure 4.11). These boards measure up to 100 cm in area and contain up to 33 layers. They can interconnect as many as 133 chips. Their fabrication involves hundreds of complex chemical processes that must be precisely controlled. 

The temperature changes of the bulk chip upon microhotplate heating were assessed. The chip was mounted in a standard ceramic DIL package. The discrepancy between ambient temperature and the bulk-silicon chip temperature was measured as a function of the microhotplate temperature and is shown in Fig. 5.20. The measurement was done at room temperature, and the control voltage was increased in steps of 25 mV thus heating the membrane from room temperature to 500 °C. The maximum discrepancy between bulk chip temperature and ambient temperature was less than 4 °C, which demonstrates the excellent thermal isolation between the microhotplate on the dielectric membrane and the bulk substrate. 

Materials in a colloidal state are frequently preferred in industrial processing operations because their large surface areas per unit volume enhance chemical reactivity, adsorptive capacity, heat transfer rates, and so on. Therefore, one cannot overlook the importance of the flow behavior and properties of colloids since they exert a significant influence on the performance, efficiency, and economy of the process. Note that some examples of this (e.g., ceramic processing, electrophoretic display devices, and food colloids) were mentioned in the vignettes presented in Chapter 1. In addition, one often uses the flow properties and behavior of the products as measures of the microstructure (or, morphology ) of the products and as a means of quality control (e.g., printing inks, toners, paints, skin creams, blood substitutes,... 

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