The Taxonomy of Materials

As we begin our study of MSE, it will be valuable to make broad classifications and generalizations regarding the behavior and applicability of materials systems. Before we continue, it's important to recognize that there are always numerous ways to classify things - for example: a tomato is botanically a fruit (a berry, in fact). It is produced from the ovary of a flowering plant (A tomato is considered a fruit legally, as well (Nix v. Hedden, 149 U.S. 304 (1893)). In culinary practice, however, it is vegetable because of its relative low sugar content and applicability in salads or main courses instead of in desserts. This concept is important whenever we classify anything, including materials - classification depends on perspective and intent. So, we'll consider materials' physical characteristics (their constituent elements, their assembly) when classifying them, but we'll also consider how we use them.

In MSE, we typically consider three main aspects when classifying a material: bonding, structure, and properties. We'll use a traditional and broad classification scheme here because it is useful in facilitating discussion. The four traditional materials classes defined in MSE are metals, ceramics, polymers, and composites. We'll include a fifth classification, molecular solids because students encounter them frequently in chemistry classes, and they exhibit behaviors that do not fall neatly within the other classifications.

Further, we sometimes use application as a sub-classification. If you ask a materials scientist which field they work on, they'll sometimes say "composites" or "metals", but it is much more common for them to say something "structural materials", "electronic materials", "biological materials", or "two-dimensional nanomaterials". All these classifications tell you about the types of materials they may work with.

Below, we've outlined general characteristics of the different traditional materials classes. These descriptions are broad strokes - we're generalizing hundreds of thousands of materials into a few classifications. There will of course be exceptions and outliers. Understand that some of the structural descriptions are a bit difficult to understand at this point - no worries, we'll go into those in detail later in the course.

Metals

• Everyday Examples: Iron, aluminum, gold, copper, tin, and their alloys.
• Bonding: Materials that are classified as metals are dominated by metallic bonding --- in which the electrons are "shared" between all atoms. The electrons are "delocalized" across the metal itself, leaving metal ionic cores behind. The reason this happens with metals is complex - a topic for a solid state physics course - but the metal atoms reach a lower energy configuration by freeing their electrons to be shared across the material. This is called the "free electron model", and it happens with elements that have low-ish electronegativities (ones that are happy to give up their electrons). The free electron model is powerful in predicting many of the behaviors of metals (i.e. electrical and thermal conductivity), and is useful in the mental model it provides: that the electrons in metals are free from their metal cores. This allows us to make predictions about how metals behave in different situations. We'll use this model frequently.
• Structure: Because metal atoms like to share their electrons, they frequently pack together closely into regular (crystalline) arrangement. We'll explore this more in later learning modules. You can have an elemental metal, which is a "pure" metal containing only one type of atom (e.g., elemental gold, elemental iron). However, you can also have metal alloys which can contain many different types of atoms (e.g., bronze (Cu and another element such as As or Sn), electrum (Au-Ag alloy), etc). We often denote alloys with a dash between the constituent elements to distinguish it from a compound. Metals alloys typically have the metal atoms as the major alloy component, but often also include non-metals like carbon or silicon in the minority.
• Properties: Metals are typically good conductors of heat and electricity, they are often strong, but ductile (not brittle). They are optically opaque and have a lustrous appearance. Metals are often chemically reactive and will react with oxygen in air, with notable exception of the noble metals.
• Applications: Metals are used in many applications due to their combination of properties. They're used in electrical wires due to their unparalleled electrical conductivity. Strong metals (ones that require lots of applied force to yield permanent deformation) like steels and aluminums are used in structural applications. Metals are also used extensively in jewelry-making and other art because of their ability to be shaped (malleability) or cast (poured into a mold), and their aesthetic.

Ceramics

• Everyday Examples: Traditional ceramics include porcelain, brick, and earthenware. Technical ceramics are high-performance ceramics used in industry and technology and include materials like boron nitride, aluminum oxide, diamond, tungsten carbide, etc.
• Bonding: Ceramics possess either predominantly ionic or covalent bonding. This means that materials that have a pairing of a metal and electronegative element will form a ceramic. This includes oxides, nitrides, silicides, and carbides. Many also define various covalently-bonded materials like silicon, diamond, or gallium arsenide.
• Structure: Ceramics may have regular crystalline structures in which atoms are arranged uniformly on a lattice. Most traditional "ceramists" consider ceramics to be crystalline. However, many materials that share the properties of ceramics (see below) can have glassy, amorphous (non-regular) structures. Here, we will consider ceramics to have both crystalline and non-crystalline structures because they share many properties.
• Properties: Ceramics are characterized by their brittleness (propensity to fracture) and hardness (resistance to scratching). They are typically resistant to corrosion and stable at higher temperatures. They can be opaque, transparent, or translucent, depending on a variety of structural factors. Ceramics are not usually good electrical conductors, but can have very high or very low thermal conductivities. A special and very important subclass of ceramics are semiconductors, which are useful because their electrical conductivity can be tightly controlled to create semiconducting devices - the foundation of modern computing.
• Applications: Traditional ceramics are used in traditional applications like pottery (the word ceramic is Greek for "of pottery") and construction. Technical ceramics have properties that are so useful that they've found applications in almost every field: biomedical implants, disk breaks, sensors, lenses, cutting tools, temperature-resistant aerospace components, and high-performance electronics.

Polymers

• Everyday Examples: Polymers are everywhere. Common synthetic polymer products you'd encounter on a daily basis are polyethylenes, acrylics, silicones, various resins, polystyrene, polyvinyl cholride, nylon, and neoprene. Many natural materials are polymers as well! This includes collagen (like in your skin and hair), lignin and cellulose (in wood), polysacchrides (sugars), and even the nucleaic acids that constitute your DNA.
• Bonding: Polymer materials are comprised of long chain-like molecules which has repeating units (mers) along the chain. Typically, when writing a formula for a polymer we write something like $\ce{(C2H4)_n}$, where $n$ represents the number of repeat units. Along the chain (intrachain), these repeat units are typically bonded via covalent interactions. These long molecules also interact with other molecules within the assembly - they have interchain interactions as well. These interchain interactions can be via secondary interactions (van der Waals or hydrogen bonding) but may also be primary interactions (covalent or ionic bonding).
• Structure: Polymer structure is complex and hierarchical. At the level of the polymer chain, we observing the chain twisting and turning in space, becoming entangled with their neighbors, or threading back-and-forth to form platelet-like structures. Often there is no regular, periodic structure in these assemblies like we observe for metals and ceramics. In other circumstances, the chains may pack together closely to create partially or fully crystalline polymer assemblies.
• Properties: Polymers have tremendously varying properties that are often highly tunable by controlling polymer chemistry and structure. In general, polymers are often mechanical ductile (can be stretched) and are less stiff (can be reversibly deformed) and less strong (can be permanently deformed) than their metal and ceramic counterparts. With some interesting and useful exceptions, they're electrically and thermally insulating. They're low density because they're mostly made up of carbon and oxygen (although there are inorganic polymers that are made of silicon or other non-carbon backbones). They're also lightweight and inexpensive, which has lead to their applicability in many areas.
• Applications: Since their advent, synthetic polymers have served to replace other materials in construction, packaging, textiles, medical devices, and consumer products. They're now ubiquitous in nearly every aspect of our society. Polymer science has also revolutionized technologies such as the pneumatic tires and shatter-resisent windows.

Molecular Solids

• Everyday Examples: Molecular solids show up in all sorts of interesting places (the environment/geology, food, medicine), as many natural or biological systems contain small molecules. Examples include ice water (H2O), nicotine, sucrose, caffeine, and dry ice. There are also rarer examples that only form at low temperature or high pressures such as solid noble gases (argon, krypton, xenon) and interesting examples of solid elements like white phosphorous and some selenium and sulfer crystals. Solid halogens like iodine are molecular crystals of halogen dimers (I2, F2, Cl2). Organic Light Emitting Diodes (OLEDs) - used in fancy flat screen televisions - utilize molecular solids.
• Bonding: In molecular solids, the dominant interatomic or intermolecular interactions are secondary - e.g., van der Waals, dipole-dipole, and hydrogen bonding. There are typically no delocalized electrons between atoms or molecules, and interactions are generally weak.
• Structure: When scientists discuss molecular solids they usually refer to their molecular crystals, in which there are regular arrays of molecules in space. There are very few references to molecular solids which lack crystal structure.
• Properties: The weak interatomic and intermolecular interactions in molecular solids mean that they will be soft and weak materials (solid argon behaves a bit like butter). They also have low melting temperatures in general when compared to materials which have metallic, ionic, or covalent bonding. Since electrons are bound to the atomic or molecular unit, they are most typically electrical and thermal insulators (although some interesting organic molecular solids behave as semiconductors).
• Applications: Because of their poor mechanical properties and low melting, few molecular solids appear in structural applications (although ice is an interesting exception, in some ways). Molecular solids do appear frequently in medicine, as drugs themselves are often molecular solids. These materials show up in the natural world in many places, however, and while they may not be useful in engineering application, they're very interesting on their scientific merit.

Composites

• Definition Composite materials are a bit different than the other classes because they are comprised of the other materials classes. Composites are combinations of two or more materials (not elements) that yield a material that possess distinctive properties without dissolving into one another. The constituent materials must have distinctive properties, and the combination must be a material that is unlike the individual components.
• Everyday Examples: These include plywood (glue and wood veneer), fiberglass-reinforced plastic (glass fiber and polymer), carbon fiber reinforce polymer (carbon fiber and plastic), concrete (rock aggregate and cement), straw-strengthened mud bricks, and dental composites (glass-strengthened resins). Each of these have at least two distinct components, and the resulting material has different properties than the individual constituents alone.
• Bonding: Since composite materials may be combinations of any other materials classes: metal-ceramic, metal-polymer, ceramic-ceramic, etc, the bonding depends on the nature of those materials. We usually subclassify composites based on these components: e.g., metal-matrix or polymer-matrix composites.
• Structure: Composites often have one material (i.e. a reinforcement) dispersed and held together in a matrix or binder. Composites can have distinct structural features that range from the nanoscale to the macroscale. Composites are materials in which we start with the components separately and mix them together. Materials engineers can control the shape and distribution of the reinforcement phases to control the properties.
• Properties: The intention of creating composite materials is to acquire the "best of both worlds" from their constitutive components. For example, carbon-fiber reinforced polymers (CFRP) have high strength from the carbon fiber, but the carbon fiber alone does not have the solidity to serve as a structural material bearing complex loads. The polymer matrix serves to bind the carbon fiber together and maintain the carbon fiber's shape. Composites can have superlative properties due to this combination of materials. This includes excellent strength- or stiffness-for-weight behavior. Improved thermal, electrical, optical and other properties are possible as well.
• Applications: Composite materials have found great application in the aerospace industry, reducing weight while maintaining mechanical performance of airliners. Athletes have also adopted composites due to their unique properties - and now these materials appear in tennis racquets, hockey sticks, fishing poles, lacrosse sticks, track shoes, and so on. Wind turbines have blades made from fiberglass-reinforced plastics (FGRP). Composites are, however, typically more expensive and difficult to fabricate than materials of other classes, which limits their application.

There are many other sub-classes or alternative classifications, but the traditional classification scheme above should serve as we introduce the basics of the field. There are also some materials that may not fall within any of these core classifications, or ones that may fall between classifications (e.g., semi-metals or metalloids). For the most part, classification is intuitive. Practice on the materials around you!