Materials science

Definition:Materials science examines the structure and properties of materials in order to create materials with useful and novel properties. To this end, materials scientists use principles from chemistry, physics, various subfields of engineering, and applied mathematics. Depending on the material being studied and its intended applications, other fields, such as biology or even planetary science, may be relevant. Materials science has applications in every imaginable industry in which synthetic materials are used and is responsible for countless innovations, among them lighter and stronger automotive parts, bacteria-resistant medical supplies, superior toothpastes, and plastic bags that keep fruit fresher. The key element that distinguishes materials science is the field’s focus on the means of creating or developing materials rather than the basic science of the underlying principles.

Basic Principles

The advent of materials science occurred when humans first began to create tools and build structures. Its importance is exemplified by the fact that many time periods, including the Bronze Age, the Iron Age, and the Silicon Age, are named for the materials that contributed greatly to human development during those periods. Similarly, the Industrial Revolution, while not named for a specific material, refers to a period of significant development in the means of creating materials and incorporating them into devices. For much of history, knowledge of what would now be considered materials science was passed from parent to child, leading to the use of terms such as “smith” and “carpenter” as family names in addition to job titles. Later, apprenticeship became the dominant form of education, eventually evolving into the higher education system of the twenty-first century. Materials science is a common major or academic department at larger or more science-oriented colleges and universities, and the fields of chemistry, physics, biology, and engineering also encompass elements of the science.

Materials science and materials engineering often overlap; however, science and engineering are distinct fields. Scientists attempt to understand phenomena, whereas engineers seek to apply scientific knowledge to problems. Although these definitions are not mutually exclusive, scientists and engineers approach their work differently: A scientist might design a device as a proof of concept, whereas an engineer would experiment with different designs to optimize the performance of a given device. Thus, materials scientists are primarily concerned with understanding why materials behave as they do and using that knowledge to devise new materials, while materials engineers focus on optimizing the application of these concepts and materials for particular uses.

Core Concepts

Metals. Metals, as materials, are composed of metallic elements that exist as positively charged ions embedded in a sea of electrons, known as “delocalized electrons” because they are not tightly bound to their source atoms. These bonds contrast with covalent and ionic bonds, in which electrons are localized between their source atom and the bonded atom. Metals conduct electricity and heat well. Metal materials may be pure metals or alloys, which are mixtures of metals. Brass (composed of copper and zinc) and bronze (copper and tin) are examples of alloys.

Ceramics. Ceramics are solids that are held together by covalent or ionic bonds. Because the nature (strength, length, orientation) of these bonds depends on the identity of the constituent compounds, the properties of these materials vary much more dramatically than those of metals. Ceramics may be semiconductors (conductors of electricity at high temperatures) or superconductors (perfect conductors of electricity) or may be piezoelectric (the application of pressure influences its mechanical properties and vice versa) and pyroelectric (the application of heat influences its mechanical properties and vice versa).

Semiconductors. Semiconductors, the basis for electronics, conduct electricity at high temperatures. The conductivity of semiconductors can be fine-tuned through a process called “doping,” which is the intentional introduction of a given impurity into an otherwise perfectly ordered material. This introduced material has a different number of electrons than the host material, which affects how readily electricity flows through the material. Doping produces either n-type or p-type semiconductors, depending on whether the impurity has more or fewer electrons than the host atoms, respectively.

Polymers. Polymers are solids composed of chains of repeating units, or monomers. For example, silk is composed of repeating units of fifty-nine amino acids, and polyethylene is composed of ethylene monomers. The properties of polymers can vary dramatically: Silk is a polymer, but amber and rubber are as well. This variation can be ascribed to the fact that both the identity of the monomer and its microstructure (how it is organized into a chain) determine how a polymer behaves. Examples of different microstructures include chains with branches, comb polymers (many shorter chains descending from a central chain, forming a comb shape), and star polymers (numerous polymers all extending from the same central point, forming a star shape). Polymers also differ in chain length.

Plastics. Plastics are polymers that retain whatever shape they are formed into, whereas nonplastic polymers, such as rubber, return to their original shape after the deforming force is removed. This property is known as plasticity. Plastics may also contain additives used as fillers or to fine-tune their properties. Like all polymers, plastics come in a wide variety of types, as defined by the constituent monomers and microstructure, and these types have different properties. Plastics are also categorized by how they respond to heating: Thermoplastics maintain their chemical structures when melted, while thermosetting plastics do not. Thus, thermoplastics can be molded repeatedly, whereas thermosets can only be molded once.

Composites. Composite materials contain numerous components with substantially different chemical and physical properties that retain these individual properties when combined. One example of a composite is concrete, which is composed of cement and an aggregate, such as sand or gravel. When combined to form concrete, neither the cement nor the aggregate undergoes physical or chemical changes.

Biomaterials. Biomaterials are materials with biological applications and biological materials that can serve other uses. In the former case, materials are designed to be incorporated into living things and must therefore exhibit biocompatibility, which refers to a material’s ability to be accepted by a biological system, without toxicity in the original material or any of the degradation products that might be produced by exposure to physiological conditions. In the latter case, biological materials are altered to serve human needs, such as when wood is pressurized to create a building material that is much more resistant to decay and deformation than the original material.

Nanomaterials. Nanomaterials are materials on the nanoscale, that is, on the order of nanometers, or 10^-9 meters. These materials are particularly interesting because the properties of nanoscale components are at times dictated by quantum mechanics rather than classical (Newtonian) mechanics. An example of nanomaterials is quantum dots, which are semiconductor particles with diameters of 2 to 10 nanometers. The color that these particles emit can be adjusted by changing their size, with smaller particles emitting colors toward the blue end of the spectrum and larger particles emitting colors toward the red end of the spectrum.

Applications Past and Present

Medicine. The applications of materials science in medicine range from new drug delivery systems to improved prostheses and artificial organs. The discovery of new materials for use in medical equipment has led to improvements in durability, cost, and practicality. For example, dental fillings were made of gold for centuries. More recently, ceramic materials were used as cheaper alternatives, with the added advantage of being less noticeable due to their subtler, off-white color. However, ceramic fillings eventually degrade. To address this issue, researchers worked to develop new filling materials that are cheap, strong, biocompatible, and stable. Titanium, which can be implanted into the jawbone itself in a biocompatible fashion, emerged as a suitable candidate material. Materials science has similarly been key to the development of scaffolding used to grow tissue artificially. When tissues are grown in the laboratory, they require scaffolding for support, much like a vine requires a trellis. Materials scientists work to create biocompatible, effective scaffolding for this purpose.

Transportation. Materials scientists are responsible for scientific advancements that affect nearly every category of vehicle. The materials in the framework of these vehicles must be both lightweight for fuel efficiency and strong for safety. Furthermore, a wide range of materials is needed to produce everything from flame-retardant upholstery to heat-resistant engine parts. One example of a promising category of material for use in automobiles and other engine-propelled vehicles is piezoelectric ceramics, which are ceramics that respond to physical deformation with an electrical response and electrical stimulation with a physical deformation. Lead zirconate titanate is an example of a piezoelectric ceramic. In cars, piezoelectrics are used as passive sensors (for instance, in accelerometers or airbag impact sensors) and generators (spark plugs) due to their ability to convert movement or impact into electricity. They also function as active sensors (such as fuel level sensors) and actuators (such as those used to position mirrors) thanks to their ability to respond to electricity mechanically.

Electronics. The electronics industry was born of the development of new materials that gave humans great control over the flow of electrons and the ability to create circuits, allowing scientists to build devices to serve their needs. One of the most important classes of materials in the electronics industry is semiconductors. Silicon is an example of a semiconductor, and its prevalence in electronics has given rise to terms such as “Silicon Valley” and “the silicon revolution.” The purity of silicon in electronics applications is crucial; one area of continuing research focuses on the development of better methods for creating thin films of pure silicon based on an understanding of its structure and properties.

Food and Drink. Packaging is crucial to keeping food fresh, especially considering how far most food must travel to reach the consumer’s home. Containers for food such as produce must typically be transparent, allowing the buyer to check the contents for damage, and maintain the optimal levels of water vapor, oxygen, and carbon dioxide to maintain freshness and discourage rot or drying. For any food or drink container, it is crucial that the material not degrade or leach into the product, as such materials can harm human health. For example, bisphenol A (BPA) is present in some plastics but can disrupt the endocrine system when ingested. Studies suggested that BPA is present in most people in detectable quantities, which raised particular concerns about pregnant women and the potential effects of fetal exposure to BPA. Companies thus began to phase out the use of BPA in food packaging both voluntarily and in response to new regulations. In response to such concerns, materials scientists worked to develop better materials and methods for packaging foodstuffs.

Energy. Materials science plays an important role in the attempt to meet ever-increasing energy demands across the globe as supplies of nonrenewable energy sources dwindle. For example, research into fuel cells, which turn fuel into electricity by means of a chemical reaction, relies on materials science to devise better anode, cathode, and electrolyte materials to improve the efficiency of these cells and best accommodate the specific fuels being used. Similarly, materials scientists are continually striving to create more efficient, sturdier, and more versatile photovoltaic systems, which convert solar energy into electricity, as well as to determine the materials that will create the cleanest, most efficient biofuels.

Sensors. Sensors are used in a wide range of fields to detect specific targets, which may include tumors, pollutants in wastewater, or physical imperfections in a crucial device component. Ideally, a sensor responds with high selectivity and specificity, meaning that it responds nearly every time it encounters the target and rarely responds to anything other than the target. The advantage of a sensor is that it responds to something that is difficult to detect, such as the presence of a contaminant in parts-per-million concentrations, in a way that is much easier to detect, such as by emitting light or changing color. One sensor design involves the use of self-assembled monolayers (that is, single layers of a material) on gold, glass, or another substrate to detect biologically relevant molecules. The binding of the target molecule improves the fluorescence of the self-assembled monolayer, and this change can be easily detected.

Social Context and Future Prospects

Materials science will likely continue to contribute to significant advances in industry and science as a whole and particularly in the medical field. For example, much of the work in the fields of cancer detection and treatment involves the creation of biocompatible materials that can target tumors, such as tumor-targeting quantum dots that fluoresce after reaching their targets. Other materials are being designed to deliver chemotherapy drugs directly to tumors, minimizing the damage done to healthy cells and allowing the patient to maintain better overall health during the treatment. As these technologies mature, the identification and treatment of cancer will become more successful and less invasive. On a broader scale, materials science offers potential treatments for a wide range of human ailments, promising to improve the overall quality of life.

Another prominent example of the social significance of materials science is its relevance in addressing climate change. As the negative effects of human activity on the environment become increasingly clear, pressure is mounting to find more economical and efficient ways to use natural resources. To this end, materials science has sought to find ways to reduce dependence on nonrenewable resources and reuse “waste” material in an economically viable fashion. For example, a key area of research in materials science is the improvement of solar cells. Future solar cells will be more efficient, more versatile, and less expensive to make. Another popular research area is the use of waste material as a substitute for freshly generated material in various production and manufacturing processes. For example, materials scientists seek to incorporate agricultural waste, such as coconut husks and palm fronds, into new materials and use waste materials to generate energy to offset the energy consumed by processing raw agricultural products.

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