Materials science in context

Materials science in context

Junjie Chen

Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China

Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com

Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure matter. Materials can be classified on the basis of their physical and chemical properties. Materials science is the study of materials and their applications. Raw materials can be processed in different ways to influence their properties, by purification, shaping or the introduction of other materials. New materials can be produced from raw materials by synthesis. In industry, materials are inputs to manufacturing processes to produce products or more complex materials [1]. The relevant structure of materials has a different length scale depending on the material. The structure and composition of a material can be determined by microscopy or spectroscopy.

In engineering, materials can be categorized according to their microscopic structure:

• Ceramics: non-metal, inorganic solids,
• Glasses: amorphous solids,
• Metals: pure or combined chemical elements with specific chemical bonding behavior,
• Polymers: materials based on long carbon or silicon chains,
• Hybrids: combinations of multiple materials, for example composites.

A metamaterial is any material engineered to have a property that is not found in naturally occurring materials, usually by combining several materials to form a composite and tuning the shape, geometry, size, orientation and arrangement to achieve the desired property [2]. In foams and textiles, the chemical structure is less relevant to immediately observable properties than larger-scale material features: the holes in foams, and the weave in textiles.

Materials can be compared and classified by their large-scale physical properties.

Mechanical properties determine how a material responds to applied forces.

Examples include:

• Stiffness
• Strength
• Toughness
• Hardness

Materials may degrade or undergo changes of properties at different temperatures. Thermal properties also include the material's thermal conductivity and heat capacity, relating to the transfer and storage of thermal energy by the material. Materials can be compared and categorized by any quantitative measure of their behavior under various conditions. Notable additional properties include the optical, electrical, and magnetic behavior of materials.

The interdisciplinary field of materials science covers the design and discovery of new materials, particularly solids [3]. The field is also commonly termed materials science and engineering emphasizing engineering aspects of building useful items, and materials physics, which emphasizes the use of physics to describe material properties. The intellectual origins of materials science stem from the Age of Enlightenment, when researchers began to use analytical thinking from chemistry, physics, and engineering to understand ancient, phenomenological observations in metallurgy and mineralogy. Materials science still incorporates elements of physics, chemistry, and engineering. As such, the field was long considered by academic institutions as a sub-field of these related fields. Beginning in the 1940s, materials science began to be more widely recognized as a specific and distinct field of science and engineering.

Materials scientists emphasize understanding how the history of a material (processing) influences its structure, and thus the material's properties and performance [4]. The understanding of processing-structure-properties relationships is called the materials paradigm. This paradigm is used to advance understanding in a variety of research areas, including nanotechnology and metallurgy.

A material is defined as a substance (most often a solid, but other condensed phases can be included) that is intended to be used for certain applications. There is a myriad of materials around us; they can be found in anything from buildings and cars to spacecraft. The main classes of materials are metals, semiconductors, ceramics and polymers. New and advanced materials that are being developed include nanomaterials and energy materials to name a few [5]. The basis of materials science is studying the interplay between the structure of materials, the processing methods to make that material, and the resulting material properties. The complex combination of these produces the performance of a material in a specific application. Many features across many length-scales impact material performance, from the constituent chemical elements, its microstructure, and macroscopic features from processing. Together with the laws of thermodynamics and kinetics materials scientists aim to understand and improve materials.

Structure is one of the most important components of the field of materials science. The very definition of the field holds that it is concerned with the investigation of "the relationships that exist between the structures and properties of materials". Materials science examines the structure of materials from the atomic scale, all the way up to the macro scale. Characterization is the way materials scientists examine the structure of a material. This involves methods such as diffraction with X-rays, electrons or neutrons, and various forms of spectroscopy and chemical analysis such as Raman spectroscopy, energy-dispersive spectroscopy, chromatography, thermal analysis, and electron microscope analysis.

Structure is studied in the following levels.

Atomic structure deals with the atoms of the materials, and how they are arranged to give rise to molecules, crystals, and the like [6]. Much of the electrical, magnetic and chemical properties of materials arise from this level of structure. The length scales involved are in angstroms [6]. The chemical bonding and atomic arrangement (crystallography) are fundamental to studying the properties and behavior of any material.

To obtain a full understanding of the material structure and how it relates to its properties, the materials scientist must study how the different atoms, ions and molecules are arranged and bonded to each other [6]. This involves the study and use of quantum chemistry or quantum physics [6]. Solid-state physics, solid-state chemistry and physical chemistry are also involved in the study of bonding and structure.

Crystallography is the science that examines the arrangement of atoms in crystalline solids [6]. Crystallography is a useful tool for materials scientists. In single crystals, the effects of the crystalline arrangement of atoms are often easy to see macroscopically, because the natural shapes of crystals reflect the atomic structure. Further, physical properties are often controlled by crystalline defects. The understanding of crystal structures is an important prerequisite for understanding crystallographic defects. Mostly, materials do not occur as a single crystal, but in polycrystalline form, as an aggregate of small crystals or grains with different orientations. Because of this, the powder diffraction method, which uses diffraction patterns of polycrystalline samples with a large number of crystals, plays an important role in structural determination. Most materials have a crystalline structure, but some important materials do not exhibit regular crystal structure. Polymers display varying degrees of crystallinity, and many are completely non-crystalline. Glass, some ceramics, and many natural materials are amorphous, not possessing any long-range order in their atomic arrangements. The study of polymers combines elements of chemical and statistical thermodynamics to give thermodynamic and mechanical descriptions of physical properties.

Materials, which atoms and molecules form constituents in the nanoscale (namely they form nanostructure) are called nanomaterials. Nanomaterials are subject of intense research in the materials science community due to the unique properties that they exhibit [6]. In many materials, atoms or molecules agglomerate together to form objects at the nanoscale. This causes many interesting electrical, magnetic, optical, and mechanical properties. In describing nanostructures, it is necessary to differentiate between the number of dimensions on the nanoscale.

Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope [6]. The microstructure of a material (which can be broadly classified into metallic, polymeric, ceramic and composite) can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high and low temperature behavior, wear resistance, and so on. Most of the traditional materials (such as metals and ceramics) are micro-structured. The manufacture of a perfect crystal of a material is physically impossible. For example, any crystalline material will contain defects such as precipitates, grain boundaries (Hall-Petch relationship), vacancies, interstitial atoms or substitutional atoms. The microstructure of materials reveals these larger defects and advances in simulation have allowed an increased understanding of how defects can be used to enhance material properties.

Materials science is the study of the properties of solid materials and how those properties are determined by a material's composition and structure [7]. It grew out of an amalgam of solid-state physics, metallurgy, and chemistry, since the rich variety of materials properties cannot be understood within the context of any single classical discipline. With a basic understanding of the origins of properties, materials can be selected or designed for an enormous variety of applications, ranging from structural steels to computer microchips [7]. Materials science is therefore important to engineering activities such as electronics, aerospace, telecommunications, information processing, nuclear power, and energy conversion.

The many materials studied and applied in materials science are usually divided into four categories: metals, polymers, semiconductors, and ceramics [7]. An industrially advanced society uses energy and materials in large amounts. Transportation, heating and cooling, industrial processes, communications, in fact, all the physical characteristics of modern life, depend on the flow and transformation of energy and materials through the techno-economic system. These two flows are inseparably intertwined. The relationship of materials science to energy usage is pervasive and complex. At every stage of energy production, distribution, conversion, and utilization, materials play an essential role, and often special materials properties are needed [7]. Remarkable growth in the understanding of the properties and structures of materials enables new materials, as well as improvements of old ones, to be developed on a scientific basis, thereby contributing to greater efficiency and lower costs.

Energy materials can be classified in a variety of ways [7]. For example, they can be divided into materials that are passive or active [7]. Those in the passive group do not take part in the actual energy-conversion process but act as containers, tools, or structures such as reactor vessels, pipelines, and turbine blades. Active materials are those that take part directly in energy conversion, such as batteries, catalysts, and superconducting magnets.

Another way of classifying energy materials is by their use in conventional, advanced, and possible future energy systems [7]. In conventional energy systems such as fossil fuels, hydroelectric generation, and nuclear reactors, the materials problems are well understood and are usually associated with structural mechanical properties or long-standing chemical effects such as corrosion [7]. Advanced energy systems are in the development stage and are in actual use in limited markets. These include photovoltaics, geothermal energy, and wind power [7]. Possible future energy systems are not yet commercially deployed to any significant extent and require much more research before they can be used [7]. These include hydrogen fuel and fast-breeder reactors, and superconducting magnets for storing electricity [7]. Classifying energy materials as passive or active or in relation to conventional, advanced, or future energy systems is useful because it provides a picture of the nature and degree of urgency of the associated materials requirements [7]. But the most illuminating framework for understanding the relation of energy to materials is in the materials properties that are essential for various energy applications.

In order to extract useful work from a fuel, it must first be burned so as to bring some fluid (usually steam) to high temperatures [7]. Thermodynamics indicates that the higher the temperature, the greater the efficiency of the conversion of heat to work; therefore, the development of materials for combustion chambers, pistons, valves, rotors, and turbine blades that can function at ever-higher temperatures is of critical importance [7]. The first steam engines had an efficiency of less than one percent, while modern steam turbines achieve efficiencies of 35 percent or more [7]. Part of this improvement has come from improved design and metalworking accuracy, but a large portion is the result of using improved high-temperature materials [7]. The early engines were made of cast iron and then ordinary steels [7]. Later, high-temperature alloys containing nickel, molybdenum, chromium, and silicon were developed that did not melt or fail at temperatures above 540 °C [7]. But modern combustion processes are nearing the useful temperature limits that can be achieved with metals, and so new materials that can function at higher temperatures, particularly intermetallic compounds and ceramics, are being developed.

The structural features that limit the use of metals at high temperatures are both atomic and electronic. All materials contain dislocations [7]. The simplest of these are the result of planes of atoms that do not extend all through the crystal, so that there is a line where the plane ends that has fewer atoms than normal. In metals, the outer electrons are free to move. This gives a delocalized cohesion so that, when a stress is applied, dislocations can move to relieve the stress. The result is that metals are ductile: not only can they be easily worked into desired shapes, but when stressed they will gradually yield plastically rather than breaking immediately [7]. This is a desirable feature, but the higher the temperature, the greater the plastic flow under stress, and, if the temperature is too high, the material will become useless. In order to get around this, materials are being studied in which the motion of dislocations is inhibited. Ceramics such as silicon nitride or silicon carbide and intermetallics such as nickel aluminide hold promise because the electrons that hold them together are highly localized in the form of valence or ionic bonds. It is as if metals were held together by a slippery glue while in nonmetals the atoms were connected by rigid rods. Dislocations thus find it much harder to move in nonmetals; raising the temperature does not increase dislocation motion, and the stress needed to make them yield is much higher. Furthermore, their melting points are significantly higher than those of metals, and they are much more resistant to chemical attack. But these desirable features come at a price. The very structure that makes them attractive also makes them brittle; that is, they do not flow when subject to a high stress and are prone to failure by cracking [7]. Modern research is aimed at overcoming this lack of ductility by modification of the material and how it is made [7]. Hot pressing of ceramic powders, for example, minimizes the number of defects at which cracks can start, and the addition of small amounts of certain metals to intermetallics strengthens the cohesion among crystal grains at which fractures normally develop. Such advances, along with intelligent design, hold the promise of being able to build heat engines of much higher efficiency than those now available.

Diamond drill bits are an excellent example of how an old material can be improved [7]. Diamond is the hardest known substance and would make an excellent drill bit except that it is expensive and has weak planes in its crystal structure. Because natural diamonds are single crystals, the planes extend throughout the material, and they cleave easily [7]. Such cleavage planes allow a diamond cutter to produce beautiful gems, but they are a disaster for drilling through rock. This limitation was overcome by Stratapax, a sintered diamond material developed by the General Electric Company of the United States [7]. This consists of synthetic diamond powder that is formed into a thin plate and bonded to tungsten-carbide studs by sintering (fusing by heating the material below the melting point). Because the diamond plate is polycrystalline, cleavage cannot propagate through the material [7]. The result is a very hard bit that does not fail by cleavage when it is used to drill through rock to get at natural gas.

Photovoltaic systems are an attractive alternative to fossil or nuclear fuels for the generation of electricity [7]. Sunlight is free, it does not use up an irreplaceable resource, and its conversion to electricity is nonpolluting. In fact, photovoltaics is now in use where power lines from utility grids are either not possible or do not exist, as in outer space or remote, nonurban locations. The barrier to widespread use of sunlight to generate electricity is the cost of photovoltaic systems. The application of materials science is essential in efforts to lower the cost to levels that can compete with those for fossil or nuclear fuels. The conversion of light to electricity depends on the electronic structure of two or more layers of semiconductor material that can absorb photons, the primary energy packets of light. The photons raise the energy level of the electrons in the semiconductor, exciting some to jump from the lower-energy valence band to the higher-energy conduction band. The electrons in the conduction band and the holes they have left behind in the valence band are both mobile and can be induced to move by a voltage. The electron motion, and the movement of holes in the opposite direction, constitute an electric current. The force that drives electrons and holes through a circuit is created by the junction of two dissimilar semiconducting materials, one of which has a tendency to give up electrons and acquire holes (thereby becoming the positive, or p-type, charge carrier) while the other accepts electrons (becoming the negative, or n-type, carrier). The electronic structure that permits this is the band gap; it is equivalent to the energy required to move an electron from the lower band to the higher. The magnitude of this gap is important. Only photons with energy greater than that of the band gap can excite electrons from the valence band to the conduction band; therefore, the smaller the gap, the more efficiently light will be converted to electricity, since there is a greater range of light frequencies with sufficiently high energies [7]. On the other hand, the gap cannot be too small, because the electrons and holes then find it easy to recombine, and a sizable current cannot be maintained.

The band gap defines the theoretical maximum efficiency, but this cannot be attained because of other materials factors [7]. For each material there is an intrinsic rate of recombination of electrons and holes that removes their contribution to electric current. This recombination is enhanced by surfaces, interfaces, and crystal defects such as grain boundaries, dislocations, and impurities [7]. Also, a fraction of the light is reflected by the surface rather than being absorbed, and some can pass through the cell without exciting electrons to the conduction band.

Improvements in the trade-off between efficiency and cost are well illustrated by the preparation of silicon that is the basic material [7]. Initially, high-purity silicon was grown from a silicon melt by slowly pulling out a seed crystal that grew by the accretion and slow solidification of the molten material. Known as the Czochralski process, this resulted in a high-purity, single-crystal ingot that was then sliced into wafers about one millimeter thick [7]. Each wafer's surface was then doped with impurities to create p-type and n-type materials with a junction between them. Metal was then deposited to provide electrical leads, and the wafer was encapsulated. This was an expensive and time-consuming process; it has been much improved in a variety of ways. For example, high-purity silicon can be made at drastically reduced cost by chemically converting ordinary silicon to silane or trichlorosilane and then reducing it back to silicon [7]. This silane process is capable of continuous operation at a high production rate and with low energy input. In order to avoid the cost and waste associated with sawing silicon into wafers, methods of directly drawing molten silicon into thin sheets or ribbons have been developed; these can produce crystalline, polycrystalline, or amorphous material. Another alternative is the manufacture of thin films on ceramic substrates, a process that uses much less silicon than other methods. Single-crystal silicon has a higher efficiency than other forms, but it is also much more expensive [7]. The materials challenge is to find a combination of cost and efficiency that makes photovoltaic electricity economically possible.

The global effort to improve the efficiency of ground transportation vehicles, such as automobiles, buses, trucks, and trains, and thereby reduce the massive amounts of pollutants they emit, provides an excellent context within which to illustrate how materials science functions to develop new or better materials in response to critical needs [7]. For the automobile industry in particular, the story is a fascinating one in which the desire for lower vehicle weight, reduced emissions, and improved fuel economy has led to intense competition among aluminum, plastics, and steel companies for shares in the enormous markets involved (40 million to 50 million cars and trucks per year worldwide) [7]. In this battle, materials scientists have a key role to play because the success of their efforts to develop improved materials will determine the shape and viability of future automobiles.

Just how seriously suppliers to the industry view the need either to protect or to increase their share of these enormous markets is demonstrated by their establishing of special programs, consortia, or centers that are specifically designed to develop better alloys, plastics, or ceramics for automotive applications [7]. For example, in the United States a program at the Aluminum Company of America (Alcoa) called the aluminum intensive vehicle (AIV), and a similar one at Reynolds Metals, were established to develop materials and processes for making automobile "space frames" consisting of aluminum-alloy rods and die-cast connectors joined by welding and adhesive bonding [7]. Not to be outdone, another aluminum company, Alcan Aluminium Limited of Canada, in a program entitled aluminum structured vehicle technology (ASVT), began to investigate the construction of automobile unibodies from adhesively bonded aluminum sheet [7]. The plastics industry, of course, has a powerful interest in replacing as many metal automobile components as possible, and in order to help bring this about a center called D&S Plastics International was formed in the Detroit, Mich., area of the United States by three corporations [7]. The specific aim of this center was to develop materials and a process suitable for forming several connected panels or components (for example, body panels and bumper fascias) simultaneously out of different types of plastics [7]. The centerpiece of the operation was a 4,000-ton co-injection press that could lead to cost reductions as great as 50 percent and thereby make the use of plastics for automotive applications more attractive.

In programs such as these, and in many more carried out by vendors and within the automobile companies themselves, materials scientists with specialized training in advanced metals, plastics, and ceramics have been leading a revolution in the automotive industry [7]. The following sections describe specific needs that have been identified for improving the performance of automobiles and other ground-transportation vehicles, as well as approaches that materials scientists have taken in response to those needs. Since aluminum has about one-third the density of steel, its substitution for steel in automobiles would seem to be a sensible approach to reducing weight and thereby increasing fuel economy and reducing harmful emissions. Such substitutions cannot be made, however, without due consideration of significant differences in other properties of the two materials. This is one important facet of the materials scientist's job, to help evaluate the suitability of a material for a given application based on how its properties balance against load and performance requirements specified by the design engineer [7]. In this case (aluminum versus steel), it is instructive to consider the materials scientist's approach to evaluating the use of aluminum in automotive panels, such components as doors, hoods, trunk decks, and roofs that can make up more than 60 percent of a vehicle's weight.

Two primary properties of any metal are its yield strength, defined as its ability to resist permanent deformation (such as a fender dent), and its elastic modulus, defined as its ability to resist elastic or springy deflection like a drum head [7]. By alloying, aluminum can be made to have a yield strength equal to a moderately strong steel and therefore to exhibit similar resistance to denting in an automobile panel [7]. On the other hand, alloying does not normally affect the elastic modulus of metals significantly, so that automotive door panels or hoods made from aluminum alloys, all of which have approximately one-third the modulus of steel, would be floppy and suffer large deflections when buffeted by the wind, for example. From this point of view, aluminum would appear to be a marginal choice for body panels.

One might attempt to overcome this deficiency by increasing the thickness of the aluminum sheet stock to three times the thickness of the steel it is intended to replace [7]. This, however, would simply increase the weight to roughly that of an equivalent steel structure and thus defeat the purpose of the exercise. Fortunately, as was elegantly demonstrated in 1980 by two British materials scientists, Michael Ashby and David Jones, when proper account is taken of the way an actual door panel deflects, constrained as it is by the door edges, it is possible to use aluminum sheet only slightly thicker than the steel it would replace and still achieve equivalent performance [7]. The net result would be a weight savings of almost two-thirds by the substitution of aluminum for steel on such body components. This suggests that understanding the interrelationship between materials properties and structural design is an important factor in the successful application of materials science.

Another important activity of the materials scientist is that of alloy development, which in some cases involves designing alloys for very specific applications [7]. For example, in Alcoa's AIV effort, materials scientists and engineers developed a special casting alloy for use as cast aluminum nodes (connecters) in their space frame design [7]. Ordinarily, metal castings exhibit very little toughness, or ductility, and they are therefore prone to brittle fracture followed by catastrophic failure. Since the integrity of an automobile would be limited by having relatively brittle body components, a proprietary casting alloy and processing procedure were developed that provide a material of much greater ductility than is normally available in a casting alloy.

Many other advances in aluminum technology, brought about by materials scientists and design engineers, have led to a greater acceptance of aluminum in automobiles, trucks, buses, and even light rail vehicles [7]. Among these are alloys for air-conditioner components that are designed to be chemically compatible with environmentally safer refrigerants and to withstand the higher pressures required by them. Also, alloys have been developed that combine good formability and corrosion resistance with the ability to achieve maximum strength without heat treating; these alloys develop their strength during the forming operation [7]. As a consequence, the list of vehicles that contain significant quantities of aluminum substituted for steel has steadily grown. A milestone was reached in 1992 with a limited-edition Jaguar sports car that was virtually all aluminum, including the engine, adhesively bonded chassis, and skin [7]. Somewhat less expensive and in full production were Honda's Acura NSX, containing more than 400 kilograms (900 pounds) of aluminum compared with about 70 kilograms for the average automobile, and General Motors' Saturn, with an aluminum engine block and cylinder heads [7]. These vehicles and others took their place alongside the British Land Rover, which was built with all-aluminum body panels beginning in 1948, a choice dictated by a shortage of steel during World War II and continued by the manufacturer ever since.

Ceramics play an important role in engine efficiency and pollution abatement in automobiles and trucks [7]. For example, one type of ceramic, cordierite (a magnesium aluminosilicate), is used as a substrate and support for catalysts in catalytic converters. It was chosen for this purpose because, along with many ceramics, it is lightweight, can operate at very high temperatures without melting, and conducts heat poorly (helping to retain exhaust heat for improved catalytic efficiency). In a novel application of ceramics, a cylinder wall was made of transparent sapphire (aluminum oxide) by General Motors' researchers in order to examine visually the internal workings of a gasoline engine combustion chamber [7]. The intention was to arrive at improved understanding of combustion control, leading to greater efficiency of internal-combustion engines. Another application of ceramics to automotive needs is a ceramic sensor that is used to measure the oxygen content of exhaust gases [7]. The ceramic, usually zirconium oxide to which a small amount of yttrium has been added, has the property of producing a voltage whose magnitude depends on the partial pressure of oxygen surrounding the material [7]. The electrical signal obtained from such a sensor is then used to control the fuel-to-air ratio in the engine in order to obtain the most efficient operation.

Because of their brittleness, ceramics have not been used as load-bearing components in ground-transportation vehicles to any great extent [8]. The problem remains a challenge to be solved by materials scientists of the future.

Declaration of competing interest

The author declares that there is no conflict of interest.

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