Unit 8 Electronic Ceramics: Electrical Insulators and Conductors
There are literally hundreds of applications of advanced ceramics that depend primarily on the reaction of the material to applied electric or magnetic fields. Some of these are enumerated here, along with a brief description of the special characteristics that make these materials useful for particular applications. In many cases, while the electronic properties are paramount, for many of these applications there are also strigent mechanical and thermal property requirements that must be met.
Many ceramic materials are electrical insulators and, consequently ceramics have been used for years for dc and lowfrequency ac electrical insulator shapes ranging from large, highvoltage suspension insulators for power transmission lines to simple, lowvoltage shapes for lamp and switch bases. These shapes have traditionally been made from clay based porcelains and are not usually included in the advanced ceramics category. On the other hand, the utilization of advanced ceramic electrical insulator materials suitable for more exotic applications is growing very rapidly. The materials most often used are alumina ceramics, beryllia ceramics, aluminum nitride, and a variety of special glasses, including those that can be converted into crystalline form after shaping (glass ceramics). The most important electrical properties of such insulation materials are very low electrical conductivity, low dielectric constant (a low tendency to polarize or store charge), a high dielectric strength (resistance to breakdown under large voltage drops), and, for highfrequency applications, low dielectric losses (low propensity to convert energy in the altering field into heat).
A wide variety of shapes are made from advanced ceramic insulator materials, many of which are so intricate that they must be made by injection molding or isostatic pressing followed by machining and finishing. An especially important ceramic insulation application is as smooth substrates for thick film and thin film deposition of circuitry. Substrates arre usually made as thin (a few mm), flat rectangular sheets utilizing tape casting technology. Most frequently, discrete electronic devices such as silicon chips or discrete capacitors will be attached to the film circuitry on the substrate to form what are known as hybrid circuits. It is not at all unusual for multilayer ceramic substrates to be employed. Multilayer substrates are made by thick film printing of circuitry onto unfired ceramic tapes using metal inks, then stacking and laminating the green tapes together to form a sandwich structure, and then "cofiring" the ceramic and metal inks to form a single mutilayer substrate. The circuits on different layers of the multilayer structure are connected at appropriate points by metal filled holes called vias in the intervening ceramic layers. Substrates not only support the circuitry, but they also provide for dissipation of heat generated in the circuitry, either by absorbing it themselves or by conveying it to an attached heat sink. When substrates and their associated circuitry are fitted with external leads and are encapsulated to protect the circuitry from the environment, the entire assembly is usually called an electronic package.
Ceramic insulator materials are also commonoly used as capacitor dielectrics, that is, as the material placed between the plates of a capacitor to serve as the charge storage medium. While any insulating material can be used for such an application, it is usually desirable to use materials that will allow the maximum amount of charge storage (capacitance) in the smallest possible device. This consideration means that materials with very high dielectric constants should be used. In addition to high dielectric constant, a capacitor dielectric should have high dielectric strength and low dielectric losses and should exhibit minimal variations in these properties with temperature or voltage changes. The most important group of advanced ceramic capacitor dielectric materials consists of combinations of barium titanate (BaTiO3) with a variety of other oxides used to modify its fundamental properties. There are hundreds of titanate based materials in use. Ceramic capacitor dielectric are often made in the form of small, thin discs or thin walled hollow tubes, with the plates being deposited onto each side by thick film techniques.
A very important and rapidly growing form of high rating ceramic capacitor, called a ceramic chip capacitor, is made by a process similar to that used for multilayer ceramic substrates. Very thin sheets of titanate dielectric are produced by tape casting, and a pattern of metal electrodes is thick film printed onto one side. Many layers of tape are then stacked on top of one another and laminated together. Individual "chips" are diced out of this laminate and are fired to mature the ceramic metal sandwich. These tiny chip capacitors can be soldered directly onto printed circuitry.
A number of ceramic materials are electrical insulators with respect to the movement of electrons, nevertheless they exhibit measurable electrical conductivities because of the ability of certain ions to move through the material when an electric field is applied. Such materials are called ionic conductors. If the conductivity is relatively high, they are called fast ion conductors or solid electrolytes. The most important fast ion conductors are AgI (Ag is the conducting ion), CaF2 (F is the conducting ion), the so called beta aluminas (having roughly the formula MAl11O17, where M is silver or an alkali such as sodium, the M ion being the one responsible for conduction), zirconia (ZrO2) doped with lime or yttrium oxide (with O being the conducting ion), and a number of special glasses (usually with alkali ions imparting conduction). Generally, the conductivity of ionic conductors increases rapidly with an increase in temperature, so they are almost always utilized at temperatures above room temperature, and sometimes at quite high temperatures. Their behavior as purely ionic conductors allows their use as solid electrolytes in high temperature batteries and fuel cells, and the fact that only one particular type of ion moves in an electric field makes them useful as ion specific sensor materials (an example is the use of stabililzed zirconia as an oxygen sensor in automobile exhaust systems to sense the efficiency of the combustion process and activate changes in fuel to air ratios).
Although silicon, germanium, and gallium arsenide are the most utilized semiconductor materials, a number of other ceramic materils also are employed for semiconductor applications. Among the most used for these applications are various doped or slightly reduced oxides (especially ZnO) and doped silicon carbide. Such materials are commonly used as varistors (resistance changes with applied voltage) and thermistors (resistance changes with temperature). Varistors are commonly used to protect devices from damage when the varistor becomes highly conductive due to a voltage spike. Thermistors can be used as temperature measurement devices, and, if they are doped so as to have a positive temperature coefficient of resistance (their resistance increases with increasing temperature), they can be used as self limitinig heater elements in a variety of applications, including to rapidly heat automatic chock elements in automobile engines so that the chock quickly closes after startup. When fabricated into single crystal form, ceramic semiconducting materials can be used to form pn junction diodes, and these can be used as power transistors, as light emitting diodes (LED), and even as semiconductor laser diodes.
Selected from:" Ceramics: industrial processing and testing", J.T.Jones and M.F.Berard, Iowa State University Press, Ames, Iowa, 1993
Words and Expressions
enumerate 数，列举
paramount 优于
stringent 严格的
exotic 外来的
beryllia 氧化铍
dielectric constant 介电常数
polarize 极化
circuitry 线路
chip 薄片
hybrid 杂化
laminate 层叠
dissipation 消散
heat sink 散热
encapsulate 封装
capacitor 电容器
titanate 钛酸盐
dice 切割
mature 成熟
sodium 钠
zirconia 氧化锆
lime 石灰
impart 分给
germanium 锗
gallium 镓
arsenide 砷化物
varistor 压敏电阻
thermistor 热敏电阻
diode 二极管
