材料科学与工程专业英语

1、2.6 semiconductorFollowing the discussion of intrinsic ,elemental semiconductors we note that the fermi function indicates that the number of charge carriers increases exponentially with temperature. This effect so dominates the conductivity of semiconductors that conductivity also follows an expone

2、ntial increase with temperature(an example of an arrhenius equation ).This increase is in sharp contrast to the behavior of metals. We consider the effect of impurities in extrinsic,elemental semiconductors.Doping a group IV a material(such as Si) with a group V a impurity (such as P)produces an n-t

3、ype semiconductor in which negative charge carriers(conduction electrons)dominate.The “extea”electron from the group V A addition produces a donor level in the energy band structure of the semiconductor.As with instrinsic semiconductors,extrinsic semiconduction exhibits arrhenius behavior.in n-type

4、material, the temperature span between the regions of extrinsic and insrinsic behavior is called the exhaustion range .A p-type semiconductor is produced by doping a group IV a material with a group III a impurity(such as Al).The group III A element has a “missing” electron producing an acceptor lev

5、el in the band stucture and leading to formation of positive charge carriers (electron holes). The region between extrinsic and instrinsic behavior for p-type semiconductors is called the saturation range . Hall effect measurements can distinguish between n-type and p-type conduction. Compound semic

6、onductors usually have an MX composition with an average of four valence electrons per atom .The III-V and II-VI compounds are the common examples .amorphous semiconductors are the non-crystalline materials with semiconducting behavior.Elemental and compound material are both found in this category

7、.To appreciate the applications of semiconductors,we review a few decades.the solid state rectifier (or diode) contains a single p-n junction .Current flows readily when this junction is forward biased but is almost completely choked off when reverse biased.the transistor is a device consisting of a

8、 pair of nearby pn junctions.The net result is a solid state amplifier. Replacing vacuum tubes with solid state elements such as these produced substantial miniaturization of electrical circuits.Further miniaturization has resulted by the production of microcircuis consisting of precise parrerns of

9、n-type and p-type regions on a single crystal chip. The major electrical properties needed to specify an intrinsic semiconductor are band gap,electron mobility,hole mobility,and conduction electron density (=electron hole density ) at room temperature.For extrinsic semiconductors,one needs to specif

10、y either the donor level (for n-type material) or the acceptor level (for p-type material).2.7 compositesOne category of structural engineering material is that of composites .These materials involve some combination of two or more components from the “fundamental” materal types .A key philosophy in

11、 selecting composite materials is that they provide the “best of both worlds” that is ,attrative properties from each component. A classic example is fiberglass.The strength of small diameter glass fibers is combined with the ductility of the polymetric matrix.The combination of these two components

12、 provides a product superior to either component alone .Many composites,such as fiberglass,involve combinations that cross over the boundaries of different kinds of materials. Others,such as concrete,involve different component from within a single material type.In general,we shall use a fairly narr

13、ow definition of composites.We shall consider only thode material thata combine different components on the microscopic(rather than macroscopic )scale .We shall noot include multiphase alloys and ceramics ,which are the result of routine processing.Similarly,the microcircuits be discussed later are

14、not include because each component retains its distinctive character in these material systems. In spite of these restrictions,we shall find this category to include a tremendously diverse collection of materials,from the common to some of most sophisticated. We shall consider three categories of co

15、mposites mateials. Conveninently ,these categories are demonstrated by three of our most common structural material ,fiberglass ,wood,and concrete .Fiberglass(or glass fiber reinforced polymer ) is an excellent example of a human made fiber reinforced composite.The glass-polymer system is just one o

16、f many important example .The fiber reinforcement is generally found in one of three primary configutations: aligned in a single direction ,randomly chopped,or woven in a fabric that is laminated with the matrix.Wood is a stuctural analog of fiberglass ,that is ,a natural fiber reinforced composite.

17、The fibers of wood are elongated,biological cells. The matrix corresponds to lignin and hemicellulose deposits.concrete is our best example of an aggregate composite, in which particles rather than fibers reinforce amatrix common concrete is rock and sand in a calcium aluminosilicate (cement)matrix.

18、While concrete has been a construction material for centuries ,these are numerous c composites developed in recent decades that use a similar particulate reinforcement concept. The concept of property averaging is central to understanding the utility of composite material.an important example is the

19、 elastic modulus of a composite .The modulus is a sensitive function of the gemetry of the reinforcing component.similarly important is the srength of the interface betweeen the reinforcing component and the matrix .We sahll concentrate on these mechanical properties of composites in regard for thei

20、r wide use as structural materials.So caaled “advanced” composites have provided some unusually attractive features,such as high strrenth to weight ratios.Some care is required in citing these properties,as they can be highly directional in nature. 2.6there are numerous uses of piezoelectrics. for i

21、nstance, plates cut from a single crystal can exhibit a specific natural resonance frequency(i.e., the frequency of an electromagnetic wave that causes it to vibrate mechanically at the same frequency); these can be used as a frequency standard in highly stable crystal controlled clocks and in fixed

22、 frequency communications devices. other resonant applications include selective wave filters and transducers(e.g., for ultrasonic cleaning and drilling) and non-resonant devices(e.g., accelerometers, pressure gauges, microphone pickups) are dominated by ceramic piezoelectrics. fiberglass was a conv

23、enient and familiar example of fiber reinforced composites. Similarly ,concrete is an excellent example of an aggregate composite. As with wood,this common construction material is used in staggering quantities. The weight of concrete used annually exceeds that of all metals combined.For concrete, t

24、he term “aggregate” refers to a combination of sand(fine aggregate) and gravel(coarse aggregate). This component of concrete is a “natual” material in the same sense as wood. Ordinarily ,these materials are chosen for their relatively high density and strength. A table of aggregate compositions woul

25、d be complex and largely meaningless. In general, aggregate materials are geological silicates chosen from locally available deposits. As such, these materials are complex and relatively impure examples of the crystalline silicates. Igneous rocks are common examples. “igneous” means solidified from

26、a molten state. For quickly cooled igneous rocks ,some fraction of the resulting material may be non-crystalline, corresponding to the glassy silicates. The relative particle sizes of sand and gravel are measured(and controlled) by passing these materials through standard screens(or sieves). The rea

27、son for a combination of fine and coarse aggregate in a given concrete mix is that the space is more efficiently filled by a range of particle sizes. The combination of fine and coarse aggregate accounts for 60 to 75 percent of the total volume of the final concrete.Modern concrete uses portland cem

28、ent,which is a calcium aluminosilicate. There are five common types of portland cement. They vary in the relative concentrations of four calcium containing minerals. The matrix is formed by the addition of water to the appropriate cement powder. The particle sizes for the cement powers are relativel

29、y small compared to the finest of the aggregates. Variation in cement particle size can strongly affect the rate at which the cement hydrates. As one might expect from inspecting the complex compositions of portland cement, the chemistry of the hydration process is equally complex.In ploymer technol

30、ogy, we noted several “additives” which provided certain desirable features to the end product. In cement technology , there are a numble of admixtures,which are additions providing certain features. Any component of concrete other than aggregate,cement,or water is, by definition ,an admixture. One

31、of the admixtures is an “air entrainer” which reminds us that air can be thought of as a fourth component of concrete. The air entrainer admixture increases the concentration of entrapped air bubbles, usually for the purpose of workability(during forming) and increased resistance to freeze thaw cycl

32、es.Why concrete is an important engineering material, a large numble of other composite systems are based on particle reinforcement. Particulate composites refer specifically to systems with relatively large size dispersed particles(at least several micrometers in diameter),and the particles are in

33、relatively high concentration(greater than 25 and frequently between 60 and 90) of small diameter oxide particles. The oxide particles strengthen the metal by serving as obstacles to dislocation motion. like so many accomplishments of human beings, those fiber reinforced composites imitate nature. C

34、ommon wood is such a composite, which serves as an excellent structural material. In fact, the weight of wood used each year in the Uited Sates exceeds the combined total for steel and concrete. We find two categories , softwoods and hardwoods. These are relative terms, although softwoods generally

35、have lower strengths. The fundamental difference between the categories is their seasonal nature. Softwoods are “evergreens” with needlelike leaves and exposed seeds. Hardwoods are deciduous( i.e., lose their leaves annually)with covered seeds( i.e.,nuts)The microstructure of wood illustrates its co

36、mmonality with the human-made composites. The dominant feature of the microstructure is the large number of tubelike cells oriented vertically. These longgitudinal cells are aligned with the vertical axis of the tree. There are some radial cells perpendicular to the longitudinal ones. As the name im

37、plies, the radial cells extend from the center of the tree trunk out radically toward the surface. The longitudinal cells carry sap and other fluids critical to the growth process. Early seaon cells are of larger diameter than later season cells. This growth pattern leads to the characteristic “ring

38、 structure” which indicates the trees age. The radial cells store food for the growing tree. The cell walls are composed of cellulose. The strength of the cells in the longitudinal direction is a function of fiber alignment in that direction. The cells are held together by a matrix of lignin and hem

39、icellulose. Lignin is a phenol propane network ploymer, and hemicellulose is ploymeric cellulose with a relatively low degree of ploymerization.Related to this ,the dimensions as well as the proper ties of wood vary significantly with atmospheric moisture levels. Care will be required in specifying

40、the atmospheric conditions for which mechanical property data apply. let us begin by concentrating on fiberglass, or glass fiber reinforced ploymer. This is a classic example of a modern composite system. A typical fracture surface of a composite shows fibers embedded in the ploymeric matrix, such f

41、ibers may have different composition since each is the result of substantial development that has led to optimal suitability for specific applications. For example, the most generally used glass fiber composition is E glass, in which E stands for its especially low electrical conductivity and its at

42、tractiveness as a dielectric. Its popularity in structural composites is related to the chemical durability of the borosilicate composition. We should note that optimal strengh is achieved by the aligned, continuous fiber reinforcement. In other words, the strength is highly anisotropic.The fiber re

43、inforced composites include some of the most sophisticated materials developed by man for some of the most demanding engineering applications. Important examples include boron reinforced aluminum, graphite epoxy, and al reinforced aluminum. Metal fibers are frequently small diameter wires. Especiall

44、y high strength reinforcement come from “whiskers” which are small, single crystal fibers that can be grown with a nearly perfect crystalline structure. Unfortunately , whiskers cannot be grown as continuous filaments in the manner of glass fibers or metal wires.2.5polymerpolymers are chemical compo

45、unds that consist of long,chainlike molecules made up of multiple repeatinf units.The term polyner was coined in 1832 by the Swedish chemist Jins Jacob Berzelius from the Greek pols,or "many" and meros,or "parts."Polymers are also referred to as macromolecules,or "gaint mole

46、cules"-a term introduced by the German chemist Hermann Staudinger in 1992.Some gaint molecules occur maturally.Proteins ,for example ,are natural polymers of amino acids that make up much of the structural material of animals;and the polymers deoxyribonucleic acid(DNA) and ribonucleic acid(RNA)

47、 are liner strands of nucleotides that define the genetic make up of living organisms.Other examples of natural polmers are silk ,wool.natural rubber,cellulose ,and shellac.There materials have been known and exploited since ancient times.Indeed,people in what is now Switzerland cultivates flax,a so

48、urce of polymeric cellulose fibres,during the Neolithic Period,or New Stone Age,some 10 000 years ,while other ancients collected proteinaceous wool fibers from sheep and silk fibers from silkworms.About five millennia ago,tanners produced leather through the cross linking of proteins by gallic acid

49、 forming the basis of the oldest industry in continuous production.Even embalming,the art for which ancient Egypt is famous is based on the condensation and cross linking of proteins with form aldehyde. Early developments in polymer technology,taking place in the 19th century,involved the conversion

50、 of natural polymers to more useful products-for example,the conversion of cellulose,obtained from cotton or wood,into celluloid,one of the first plastic.Before the 1930s only a small number of synthetically produced polymers were available commercially,but after that period and especially after Wor

51、ld War II,synthetic compounds came to dominance.Derived principally from the refining of petroleum and natural gas,synthetic polymers are made into the plastics,rubbers,man-made fibres,adhesives,and surface coatings that have become so ubiquitous in modern life.As an important materials,the polymers

52、 are available in a wide variety of commercial forms:fibers,thin films and sheets,foams and in bulk.A common synonym for polymers is "plastic",a name derived from the deformability associated with the fabrication of most polymeric products.To some critics,"plastic" is a synonym f

53、or modern culture.Accurate or not,it represents the impact that this complex family of engineering materials has had on our society. Polymers are distinguished from our previous types of materials by chemistry.Metal,ceramics.and glasses are inorganic materials.The polymers discussed here are organic

54、.The student should not be concerned about a lack of background in organic chenistry.This passage is intended to provide any of the fundamentals of organic chemistry needed to appreciate the unique nature of polymeric materials.We begin our discussion of polymers by investigating polymerization,the

55、process by which long chain or network molecules are made from relatively small organic molecules.The structural features of the resulting polymers are rather unique compared to the inorganic materials.Ingeneral,the ,elting point and rigidity of polymers increase with the extent of plymerization and

56、 with the complexity of the molecular structure. We shall find that polymers fall into one of two main categories.Thermoplastic polymers are materials that become less rigid upon heating,and thermosetting polymers become more rigid upon heating.For both categories,it is important to appreciate the r

57、oles played by additives,which provide important features such as color and resistance to combustion.As with ceramics and glasses,we shall discuss important mechanical and optical properties of polymers.Mechanically,polymers exhibit behavior associated with their long chain molecular structure.Examp

58、les include viscoelastic and elastomeric deformation .Optical properties such as transparency and color,so important in ceramic technology,are also significant in the selection of polymers.olymerization The term polymer simply means "many mers" where mer is the building block of the long c

59、hain or network molecule.There are two distinct ways in which a poly merization reaction can take place.Chain growth(also known as addition polymerization)involves a rapid "chain reaction" of chemically activated monomers.Step growth(also known as condensation polymerization)involves individual chemical reactions between pairs of reactive monomers and is a much slower process.In either case,the critical feature of a monomer,which permits it to join with similar molecules and form a polymer,is the presence of reactive sites,that is double bonds(chain growth) or reactive funct