1、Unit 4 Applications and Processing of Ceramics,Types and Applications of Ceramics,Glasses The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2
2、O, K2O, and Al2O3, which influence the glass properties. A typical sodalime glass consists of approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime). Possibly the two prime assets of these materials are their optical transparency and the relative ease with which they may be f
3、abricated.,Glass-ceramics Most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high-temperature heat treatment. This process is called crystallization, and the product is a fine-grained polycrystalline material which is often called a g
4、lassceramic. The formation of these small glass-ceramic grains is, in a sense, a phase transformation, which involves nucleation and crystal growth stages. As a consequence, the kinetics (i.e., the rate) of crystallization may be described using the same principles that were applied to phase transfo
5、rmations for metal systems.,The dependence of degree of transformation on temperature and time may be expressed using isothermal transformation and continuous cooling transformation diagrams. The continuous cooling transformation diagram for the crystallization of a lunar glass is presented.,The beg
6、in- and end-transformation curves on this plot have the same general shape as those for an ironcarbon alloy of eutectoid composition. Also included are two continuous cooling curves, which are labeled “1” and “2”; the cooling rate represented by curve 2 is much greater than that for curve 1.,As also
7、 noted on this plot, for the continuous cooling path represented by curve 1, crystallization begins at its intersection with the upper curve, and progresses as time increases and temperature continues to decrease; upon crossing the lower curve, all of the original glass has crystallized.,The other c
8、ooling curve (curve 2) just misses the nose of the crystallization start curve. It represents a critical cooling rate (for this glass, 100C/min) that is, the minimum cooling rate for which the final room-temperature product is 100% glass;,for cooling rates less than this, some glass-ceramic material
9、 will form. A nucleating agent (frequently titanium dioxide) is often added to the glass to promote crystallization. The presence of a nucleating agent shifts the begin and end transformation curves to shorter times. Glass-ceramic materials have been designed to have the following characteristics: r
10、elatively high mechanical strengths; low coefficients of thermal expansion (to avoid thermal shock); relatively high temperature capabilities; good dielectric (介电的) properties (for electronic packaging applications); and good biological compatibility.,Some glassceramics may be made optically transpa
11、rent; others are opaque. Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware. The most common uses for these materials are as ov
12、enware, tableware, oven windows, and rangetops (灶台)primarily because of their strength and excellent resistance to thermal shock. They also serve as electrical insulators and as substrates for printed circuit boards, and are used for architectural cladding (骨架外墙), and for heat exchangers and regener
13、ators (蓄热器).,Clay Products One of the most widely used ceramic raw materials is clay. This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. Another reason for its popularity lies in the ease with which clay products may be formed; w
14、hen mixed in the proper proportions, clay and water form a plastic mass that is very amenable (易控制的) to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength.,Most of the clay-based products fall withi
15、n two broad classifications: the structural clay products (建筑粘土制品) and the whitewares (白色陶瓷). Structural clay products include building bricks, tiles, and sewer pipesapplications in which structural integrity is important. The whiteware ceramics become white after the high-temperature firing. Includ
16、ed in this group are porcelain, pottery, tableware, china, and plumbing fixtures (管子附件) (sanitary ware). In addition to clay, many of these products also contain nonplastic ingredients, which influence the changes that take place during the drying and firing processes, and the characteristics of the
17、 finished piece.,Refractory ceramics Another important class of ceramics that are utilized in large tonnages is the refractory ceramics. The salient (突出的) properties of these materials include the capacity to withstand high temperatures without melting or decomposing, and the capacity to remain unre
18、active and inert when exposed to severe environments. In addition, the ability to provide thermal insulation is often an important consideration. Refractory materials are marketed in a variety of forms, but bricks are the most common. Typical applications include furnace linings (衬里、内衬) for metal re
19、fining, glass manufacturing, metallurgical (冶金的) heat treatment, and power generation.,Abrasive ceramics Abrasive ceramics are used to wear, grind, or cut away other material, which necessarily is softer. Therefore, the prime requisite for this group of materials is hardness or wear resistance; in a
20、ddition, a high degree of toughness is essential to ensure that the abrasive (磨料) particles do not easily fracture. Furthermore, high temperatures may be produced from abrasive frictional forces, so some refractoriness (耐火度、耐火性) is also desirable. Diamonds, both natural and synthetic, are utilized a
21、s abrasives; however, they are relatively expensive. The more common ceramic abrasives include silicon carbide, tungsten carbide (WC), aluminum oxide (or corundum), and silica sand.,Cements Several familiar ceramic materials are classified as inorganic cements: cement, plaster of paris (熟石膏), and li
22、me, which, as a group, are produced in extremely large quantities. The characteristic feature of these materials is that when mixed with water, they form a paste that subsequently sets and hardens. This trait is especially useful in that solid and rigid structures having just about any shape may be
23、expeditiously (迅速地) formed. Also, some of these materials act as a bonding phase that chemically binds particulate aggregates into a single cohesive structure. Under these circumstances, the role of the cement is similar to that of the glassy bonding phase that forms when clay products and some refr
24、actory bricks are fired. One important difference, however, is that the cementitious (似水泥的、胶结的）bond develops at room temperature.,Of this group of materials, portland cement is consumed in the largest tonnages. It is produced by grinding and intimately mixing clay and lime-bearing minerals in the pr
25、oper proportions, and then heating the mixture to about 1400C (2550F) in a rotary kiln; this process, sometimes called calcination (煅烧), produces physical and chemical changes in the raw materials. The resulting “clinker”(灰渣) product is then ground into a very fine powder to which is added a small a
26、mount of gypsum (石膏)(CaSO42H2O) to retard the setting process. This product is portland cement. The properties of portland cement, including setting time and final strength, to a large degree depend on its composition.,Advanced ceramics (1) Microelectromechanical Systems (MEMS) Microelectromechanica
27、l systems (abbreviated MEMS) are miniature (微型的) “smart” systems consisting of a multitude of mechanical devices that are integrated with large numbers of electrical elements on a substrate of silicon. The mechanical components are microsensors and microactuators (微型促动器). Microsensors collect enviro
28、nmental information by measuring mechanical, thermal, chemical, optical, and/or magnetic phenomena. The microelectronic components then process this sensory input, and subsequently render decisions that direct responses from the microactuator devices-devices that perform such responses as positionin
29、g, moving, pumping (快速摇动), regulating, and filtering.,These actuating devices include beams, gears, motors, and membranes, which are of microscopic dimensions, on the order of microns in size.,There are some limitations to the use of silicon in MEMS. Silicon has a low fracture toughness, a relativel
30、y low softening temperature (600C) and is highly active to the presence of water and oxygen. Consequently, research is currently being conducted into using ceramic materialswhich are tougher, more refractory, and more inertfor some MEMS components, especially high-speed devices and nanoturbines. Tho
31、se ceramic materials being considered are amorphous silicon carbonitrides (silicon carbidesilicon nitride alloys), which may be produced using metal organic precursors.,(2) Optical Fibers One new and advanced ceramic material that is a critical component in our modern optical communications systems
32、is the optical fiber. The optical fiber is made of extremely high-purity silica, which must be free of even minute levels of contaminants and other defects that absorb, scatter, and attenuate (减弱) a light beam. Very advanced and sophisticated processing techniques have been developed to produce fibe
33、rs that meet the rigid restrictions required for this application.,(3) Piezoelectric ceramics A few ceramic materials (as well as some polymers) exhibit the unusual phenomenon of piezoelectricityelectric polarization (i.e., an electric field or voltage) is induced in the ceramic crystal when a mecha
34、nical strain (dimensional change) is imposed on it. The inverse piezoelectric effect is also displayed by this group of materials; that is, a mechanical strain results from the imposition of an electrical field. Piezoelectric materials may be utilized as transducers (转换器、传感器) between electrical and
35、mechanical energies. One of the early uses of piezoelectric ceramics was in sonar (声纳), wherein underwater objects (e.g., submarines) are detected and their positions determined using an ultrasonic emitting and receiving system.,A piezoelectric crystal is caused to oscillate (摆动) by an electrical si
36、gnal, which produces high-frequency mechanical vibrations that are transmitted through the water. Upon encountering an object, these signals are reflected back, and another piezoelectric material receives this reflected vibrational energy, which it then converts back into an electrical signal. Dista
37、nce from the ultrasonic source and reflecting body is determined from the elapsed time between sending and receiving events.,Tube piezoelectric ceramic is used for ultrasonic sonar, hydrophone, underwater speaker, oil well dredge (挖泥机), etc.,More recently, the utilization of piezoelectric devices ha
38、s grown dramatically as a consequence of increases in automatization and consumer attraction to modern sophisticated gadgets (小机械). Applications that employ piezoelectric devices are found in the automotive, computer, commercial/consumer, and medical sectors. Some of these applications are as follow
39、s: automotivewheel balances, seat belt buzzers, tread-wear indicators, keyless door entry, and airbag sensors; computer microactuators for hard disks and notebook transformers. Commonly used piezoelectric ceramics include barium titanate (BaTiO3), lead titanate (PbTiO3), lead zirconatetitanate (PZT)
40、 Pb(Zr,Ti)O3, and potassium niobate (KNbO3).,keyless door entry system,airbag sensors,microactuators for hard disks,Fabrication and Processing of Glass and Glass-Ceramics,Before we discuss specific glass-forming techniques, some of the temperature-sensitive properties of glass materials must be pres
41、ented. Glassy, or noncrystalline, materials do not solidify in the same sense as do those that are crystalline. Upon cooling, a glass becomes more and more viscous in a continuous manner with decreasing temperature; there is no definite temperature at which the liquid transforms to a solid as with c
42、rystalline materials. In fact, one of the distinctions between crystalline and noncrystalline materials lies in the dependence of specific volume (比容) (or volume per unit mass, the reciprocal (倒数) of density) on temperature.,For crystalline materials, there is a discontinuous decrease in volume at t
43、he melting temperature However, for glassy materials, volume decreases continuously with temperature reduction; a slight decrease in slope of the curve occurs at what is called the glass transition temperature, or fictive temperature (假想温度), Below this temperature, the material is considered to be a
44、 glass; above, it is first a supercooled liquid, and finally a liquid.,Also important in glass-forming operations are the viscositytemperature characteristics of the glass.,1. The melting point corresponds to the temperature at which the viscosity is 10 Pa-s ; the glass is fluid enough to be conside
45、red a liquid. 2. The working point represents the temperature at which the viscosity is 103Pa-s; the glass is easily deformed at this viscosity.,3. The softening point, the temperature at which the viscosity is 4106Pa-s, is the maximum temperature at which a glass piece may be handled without causin
46、g significant dimensional alterations.,4. The annealing point (退火点) is the temperature at which the viscosity is 1012Pa-s; at this temperature, atomic diffusion is sufficiently rapid that any residual stresses may be removed within about 15 min.,5. The strain point corresponds to the temperature at
47、which the viscosity becomes 31013Pa-s; for temperatures below the strain point, fracture will occur before the onset of plastic deformation. The glass transition temperature will be above the strain point. Most glass-forming operations are carried out within the working range between the working and
48、 softening temperatures. Of course, the temperature at which each of these points occurs depends on glass composition. For example, the softening points for sodalime and 96% silica glasses are about 700 and 1550C, respectively. That is, forming operations may be carried out at significantly lower te
49、mperatures for the sodalime glass. The formability of a glass is tailored to a large degree by its composition.,Glass forming Glass is produced by heating the raw materials to an elevated temperature above which melting occurs. Most commercial glasses are of the silicasodalime variety; the silica is
50、 usually supplied as common quartz sand, whereas Na2O and CaO are added as soda ash (Na2CO3) and limestone (CaCO3). For most applications, especially when optical transparency is important, it is essential that the glass product be homogeneous and pore free. Homogeneity is achieved by complete melting and mixing of the raw ingredients. Porosity results from small gas bubbles that are produced; these must be absorbed into the melt or otherwise eliminated, which requires proper adjustment of the viscosity of the molten material.,
