Electrical property requirements

Tetrabromobisphenol A is used in epoxy resins especially for glass fiber reinforced used in printed circuit board. Nonreactive compounds such as tetrabromophatalate ester, bis(tribromophenoxy) ethane, and decabromodiphenyl ether are also used. The use of synergists, such as antimony oxide, reduces the quantity of brominated flame retardant necessary but decreases the electrical properties required. 

Although the operating concept of SOFCs is rather simple, the selection of materials for the individual components presents enormous challenges. Each material must have the electrical properties required to perform its function in the cell. There must be enough chemical and structural stability to endure fabrication and operation at high temperatures. The fuel cell needs to run at high temperatures in order to achieve sufficiently high current densities and power output ... 

Semiconductor materials have had to meet progressively more stringent requirements as the density and performance of semiconductor devices has increased. This trend will continue. The purity of the matoial, the dimensions of the devices, and the electrical properties require higher precision in their measurement and the ability to determine the device parameters to a resolution and sensitivity that pushes measurement techniques to their very limit. Semiconductor measurements cover a broad range of techniques and disciplines. After a brief listing of optical and physicall chemical characterization methods we give in this chapter a discussion of the general trend in electrical characterization and present a few examples of the charactmzation techniques used today. 

Without exception, devices that utilize electrical properties require electrolytes to be used in contact with other materials (electrodes) in which some part of the conduction process involves electronic carriers. This means that charge transfer interfaces are formed between the electrolyte and the electrodes and it is at these interfaces that the full subtleties of the electrochemical processes occur. By comparison, ionic transport across the polymer electrolyte is simple. 

The details of electrical properties require some basic information about electric fields. Fields are usually thought of as Cartesian vectors. They arise from or may be defined from a scalar function, the electric potential, V. The x-component of the electric field is dV/dx, which we will d gnate as Vx. If the potential V is independent of x, then there is no x-component of die field. If V has only a linear dependence on x, the field is uniform in die x-direction. Of course, the elearic potential may depend on the spatial coordinates in some... 

The end-of-life of a material is based upon the assumption that at least a 2 1 factor of safety exists in the applicable physical and electrical property requirements. The end-of-life of a material is the time at each aging temperature, when a property value has decreased 50 percent of its unaged level. A 50 percent loss of property due to thermal degradation is not expected to result in premature, unsafe failure. 

The term electrical properties requires clarification. As we shall see, the dielectric permittivity of a material is, in general, a complex quantity when measured in the frequency domain. Its real part is usually called the permittivity and decreases with increasing frequency. Its imaginary part is usually called the dielectric loss factor and may increase or decrease with increasing frequency. A true dc conduction in a material contributes only to the imaginary part of the complex permittivity and leads to a loss factor which is inversely proportional to frequency, while other dielectric processes... 

Metallization. Integrated circuits require conductive layers to form electrical connections between contacts on a device, between devices on a chip, between metal layers on a chip, and between chips and higher levels of interconnections needed for packaging the chips. It is critical to the success of IC fabrication that the metallization be stable throughout the process sequence in order to maintain the correct physical and electrical properties of the circuit. It must also be possible to pattern the blanket deposition. 

Grade XXXPC is similar in electrical properties to Grade XXXP and suitable for punching at lower temperatures than Grade XXXP. This grade is recommended for apphcations requiring high insulation resistance and low dielectric losses under severe humidity conditions. 

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

Relatively few processible polyimides, particularly at a reasonable cost and iu rehable supply, are available commercially. Users of polyimides may have to produce iutractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions iu order to meet property requirements such as thermal and thermoxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the stmcture—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

Liquid Level. The most widely used devices for measuring Hquid levels involve detecting the buoyant force on an object or the pressure differential created by the height of Hquid between two taps on the vessel. Consequently, care is required in locating the tap. Other less widely used techniques utilize concepts such as the attenuation of radiation changes in electrical properties, eg, capacitance and impedance and ultrasonic wave attenuation. 

The main electroceramic apphcations of titanium dioxide derive from its high dielectric constant (see Table 6). Rutile itself can be used as a dielectric iu multilayer capacitors, but it is much more common to use Ti02 for the manufacture of alkaline-earth titanates, eg, by the cocalciuation of barium carbonate and anatase. The electrical properties of these dielectrics are extremely sensitive to the presence of small (<20 ppm) quantities of impurities, and high performance titanates require consistently pure (eg, >99.9%) Ti02- Typical products are made by the hydrolysis of high purity titanium tetrachloride. 

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers loss of flavor and development of rancidity ia foods and an iacrease ia the viscosity, acidity, and formation of iasolubles ia lubricants. The need for antioxidants depends upon the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants are used to stabilize polymers such as natural mbber and polyunsaturated oils. Saturated polymers have greater oxidative stabiUty and require relatively low concentrations of stabilizers. Specialized antioxidants which have been commercialized meet the needs of the iadustry by extending the useflil Hves of the many substrates produced under anticipated conditions of exposure. The sales of antioxidants ia the United States were approximately 730 million ia 1990 (1,2). 

Material property specifications must be written by design and material engineers to control engineering requirements and to control incoming raw material quahty. Material property requirements depend on various ia-use functional needs ia terms of electrical, mechanical, thermal, chemical, optical, and magnetic properties. 

---