MATERIALS SCIENCE - The Handy Chemistry Answer Book (2014)

The Handy Chemistry Answer Book (2014)

MATERIALS SCIENCE

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What is materials science?

Materials science is a field at the intersection of the basic sciences and engineering with a focus on the relationship between the microscopic (atomic or molecular) structure of a material and its macroscopic properties. Many techniques relevant to chemistry are used to characterize materials, and the descriptions of the underlying microscopic structure of the material are discussed with regard to solid-state chemistry. Thus many components of materials science represent applications of chemistry. In this section we will provide a brief introduction to materials science with a focus on some of the topics that are more relevant to chemistry.

What are some of the different classes of materials?

Biomaterials—materials involving various types of biological molecules

Carbon—materials built from networks of carbon atoms, such as graphite, graphene, diamond, or carbon nanotubes

Ceramics—inorganic (nonmetallic) solids, these are typically prepared by heating and cooling

Composite materials—materials made from two or more components with distinct physical properties

Functionally graded materials—any material that varies gradually in structure or properties throughout its volume

Glass—amorphous solids, typically appearing to have the properties of a solid at the macroscopic level

Metals—composed of metallic elements, good conductors of electricity and heat, typically malleable and ductile

Nanomaterials—materials whose structural features are observable on the nanoscale (typically length scales of less than a tenth of a micrometer)

Polymers—materials/compounds consisting of multiple repeating structural units

Refractory—materials that retain their strength even when they are heated to very high temperatures

Semiconductors—materials with conductivity properties intermediate to those of metals and nonmetals

Thin films—materials that are used in very thin layers, typically ranging from a single layer of molecules to layers that may be several micrometers in thickness

Why do we study materials science?

People study materials science so that others are to be able to choose an appropriate material for an application based on considerations of performance and cost. We want to be able to understand the capabilities and limitations of various materials as well as how, if at all, their properties change after repeated use. By studying materials science, we also become better able to design new materials with the characteristics we desire.

What macroscopic properties of materials are typically studied?

Some of the most commonly studied properties of materials include:

· Thermal conductivity (how well they transmit heat)

· Electrical conductivity (how well they transmit electrons)

· Heat capacity (how their temperature changes with added heat)

· Optical absorption, transmission, and scattering properties

· Stability toward mechanical wear and chemical corrosion

What properties are being targeted for optimization in modern materials design?

Below is a short list of current goals in materials science engineering. This is by no means a comprehensive list, but it is just meant to give you an idea of what is going on in the field of materials science research today.

· Develop structural materials with high temperature stability to increase engine efficiency at high temperatures

· Develop strong, chemically stable, rust- and corrosion-resistant materials for use in construction

· Develop lightweight, mechanically strong materials for high-speed flight

· Develop strong, cost-efficient types of glass to make unbreakable windows increasingly available to the general public

· Develop materials to facilitate the processing of nuclear waste

· Develop fibers with extremely low light absorption for use in optical communication cables

What is an atomic packing factor?

The atomic packing factor is the fraction of the volume of a crystal that is filled up by its atoms. In other words, the higher the atomic packing factor, the less empty space there is in the material.

How are ceramics made?

Ceramics are nonmetallic materials that are made of a mixture of metallic and non-metallic elements. A ceramic is made by taking an inorganic material, heating it to a high temperature such that the (atomic/molecular) components can rearrange easily, and then allowing the material to cool to room temperature. The resulting materials are typically strong, hard, brittle, and are poor conductors of heat and electricity.

What is tribology?

Tribology is a subfield of materials science dedicated to studying the wear of materials. This may include the effects of friction on a material as well as how to better engineer surfaces or lubricate interfaces between surfaces to extend their lifetime.

What is a fullerene?

A fullerene is any molecule made up of only carbon atoms that has a shape of a sphere, ellipsoid (a distorted sphere), or a tube. The name fullerene comes from Richard Buckminster Fuller, an architect who designed the geodesic dome (Spaceship Earth at Epcot Center is a geodesic dome). The U.S. Post Office recently commemorated Fuller and and his geodesic dome on a stamp.

What are buckyballs?

Buckyballs, or buckminsterfullerenes, are sphere-shaped fullerenes. The most common is C60, which is a sphere composed of alternating five- and six-membered rings of carbon atoms like a soccer ball. This molecule can actually be found in common soot, but don’t think you can start selling the remains of your bonfire for cutting-edge fullerene research—it’s very, very difficult to purify C60.

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Buckminsterfullerenes are sphere-shaped molecules made only of carbon atoms.

What are carbon nanotubes?

Cylindrical fullerenes are known as carbon nanotubes. While C60 is a mix of five- and six-membered rings, nanotubes are usually arrays of only six-membered rings. They are just a few nanometers wide, but can be up to several millimeters long. The properties of this form of matter are almost unique in the world, and as a result carbon nan-otubes have caught the attention of many chemists. Nanotubes conduct heat and electricity very well, but are also extremely strong (specifically in tests of tensile strength).

What is scanning electron microscopy?

Scanning electron microscopy (SEM) is a technique used to capture a picture of a sample by focusing a beam of electrons onto the sample, scanning the beam around the surface of the sample, and then detecting the electrons after they have been scattered off of the sample. The scattered electrons are then analyzed to produce an image of the sample. In general, imaging methods that make use of electrons can offer higher resolution than those based on light due to the shorter wavelengths associated with electrons (as opposed to photons). SEM can be used to obtain very high-resolution images of a sample on length scales as short as one nanometer. The downside to electron-based methods (again, as opposed to using light) is that the electron-based methods are often damaging to the sample (especially to live samples), whereas shining a beam of light on a sample doesn’t typically cause a lot of damage. SEM has been useful for characterizing materials as well as a wide range of other kinds of samples.

What is transmission electron microscopy?

Transmission electron microscopy (TEM) is similar to SEM in that it uses a beam of electrons to study the sample, but in this case the beam of electrons passes directly through the sample to reach the electron detector. TEM images are able to provide higher resolution than that attainable using a light microscope. The first transmission electron microscope with a resolution greater than that attainable with a light microscope was built in 1933, and there have been commercial TEMs available since 1939. So while it may seem like a very advanced technology, TEM is, in fact, quite an old technique.

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Graphene is a material made of carbon arranged in a flattened buckyball pattern. It has many potential uses in electronics and other mechanical and engineering applications.

What is graphene?

In one sense graphene is an unrolled carbon nanotube, or a flattened buckyball. Graphene is a material made of carbon atoms arranged in a hexagonal, “honeycomb” lattice. It is similar to graphite, except that it is only one sheet of atoms thick! A square meter of graphene weighs less than a milligram. Since its discovery, graphene has garnered significant attention for its electronic, thermal, optical, mechanical, and other properties. There is currently a huge amount of research into the properties and applications of graphene, and one Nobel Prize has been awarded (the 2010 Prize in Physics) for research into its properties.

How do photovoltaics convert light into energy?

Photovoltaic cells are the materials responsible for converting the energy in photons of light from the Sun into energy that can be stored or used. Individual cells usually range in size from areas of roughly one square inch to one square foot, and thousands of cells can be used simultaneously to harvest large amounts of energy. When photons of light strike the photovoltaic material, they excite electrons from a piece of silicon that has been treated such that the excited electrons will gather on one side. This creates a potential difference within the photovoltaic cell such that there is now a positive and negative side (similar to a battery). At this point, the photovoltaic cell has now converted (some of) the energy from the photons into electrical potential energy. This potential difference can be discharged to transfer the energy for immediate use or to be stored while the photovoltaic cell continues to collect additional photons.

How is hydrogen stored for use as a fuel?

It would be nice if we could store hydrogen as a liquid; however, it has a very low boiling point (−252.9 °C), which makes this rather inefficient. Due to the strong tendency to evaporate at room temperature, significant energy must actually be expended just to keep hydrogen in its liquid phase. To store hydrogen gas, one possibility is to just compress it inside a metallic container similar to what is typically done with other gases. There have been several other approaches used, however, to attempt to store hydrogen for use as a fuel. These include both chemical and physical storage methods. Some of the chemical storage systems that have been investigated include metal hydrides (like NaAlH4, LiAlH4, or TiFeH2), aqueous carbohydrate solutions (which release H2 via an enzymatic reaction), synthesized hydrocarbons, ammonia, formic acid, ionic liquids, carbonite compounds, and others as well. These methods generally rely on chemical reactions to make H2 available for use as an energy source. Physical storage methods include cryogenic compression (involving a combination of low temperatures and high pressures) and a variety of materials, such as metal-organic frameworks, carbon nan-otubes, clathrate hydrates, capillary arrays, and others as well. Unfortunately, few of these physical storage methods have thus far been able to demonstrate strongly promising results in working toward a practically useful method of storing hydrogen as a fuel.

What are some applications of functionally graded materials?

Recall from above that a functionally graded material is one that varies in one or more properties throughout its dimensions. These constitute a relatively young class of materials with promising applications in a variety of areas. For example, the living tissues in your body, including your bones, are classified as (natural) functionally graded materials, so if scientists want to develop materials capable of replacing these, they are looking to develop a functionally graded material. They are also useful in aerospace applications, where materials that can withstand a large thermal (temperature) gradient are needed. Functionally graded materials are commonly found in energy conversion devices and have also been used in gas turbine engines. They can also be good at preventing the propagation of cracks through the volume of a material, which makes them promising candidates for defense applications like developing bullet-resistant materials to create armors for humans and vehicles.

Why are scientists so interested in semiconductors?

Recall that semiconductors are a class of materials defined by their conductivity properties. Specifically, they have intermediate conductivity properties between those of things that conduct extremely well (like metals) and things that don’t tend to conduct well at all (insulators); this is what makes them so useful. Scientists are able to use semiconductors to control the flow of electricity in circuits, which has been crucial for the development of all of the complicated electronic devices you’re familiar with. Semiconductors can be “doped” with materials containing extra electrons, or with materials that are electron deficient, to control the direction of electron flow through the material. Semiconductors have also played a big role in developing solar energy capture devices. The amount of energy a semiconductor needs to absorb to “release” an electron such that electricity can flow can be finely tuned, allowing scientists to develop materials capable of storing solar energy (from photons of light) in the form of electricity.

What are some applications of thin films?

Thin films are layers of material ranging in thickness from nanometers (10−9 m) to micrometers (10−6 m). They are used commonly for coating optical surfaces and also for coatings on semiconductors. Thin films are used to make mirrors, and these can be finely tuned (in terms of their composition and thickness) to obtain optical surfaces with a wide variety of specific reflection properties, such as wavelength specific mirrors and two-way mirrors (the ones that are transparent from one side but reflective from the other). Thin films are also very useful for coating semiconductors to tune their conductive properties for different applications.

How does materials science help to keep your home warm in the winter?

By designing effective ways to insulate your home, of course! In total, 48% of the energy used annually in the U.S. is spent on heating buildings during the colder seasons and keeping them cooled during the warmer seasons. Insulation materials help to maintain the temperature differences inside and outside of buildings. In addition to polystyrene and other types of insulation, materials to seal window panes and other potential leaks can significantly reduce the amount of energy we need to spend on heating and cooling.

How does materials science help to improve the fuel efficiency of your car?

One way is through better tires; there are currently tires in development made of rubber that rolls along with less resistance, which has the potential to improve gas mileage by as much as 10 percent just through the tires! Another way involves using special lubricants that work well at a wider range of temperatures, allowing for better fuel efficiency even when the engine is cold as well as when it’s warm. This also has the potential to improve fuel efficiency by about 6 percent. Additionally, materials science is also the field responsible for developing lightweight materials to construct all of the components of a vehicle, which further contributes to improving vehicle fuel efficiency.

What are a few of the big challenges materials science researchers are working on now?

One challenge is to make materials that will allow us to start making smaller, lighter cars that can more easily be powered by electricity. LED lights are another big area in materials science right now. We need to develop materials that can use energy-efficient LEDs to produce the kinds of light we need at low costs. One last area to mention is reducing the amount of waste we produce in general. There are many approaches to reducing waste, and from a materials science perspective we would like to make products out of long-lasting, durable materials that can be repurposed or reused at the end of a product’s lifetime.

What is electrical resistivity?

Electrical resistivity is a measure of how well a material resists the flow of an electric current. A material with a high electrical resistivity is a poor conductor of electricity, while a material with a low electrical resistivity is a good conductor of electricity. This property is typically expressed in units of ohms × meters (Ω × m). Recall that ohms (Ω) are a unit of resistance (see “Physical and Theoretical Chemistry”).

What is magnetic permeability?

The magnetic permeability of a material describes the extent to which it is able to support a magnetic field (inside itself). It describes the amount of magnetization that takes place in a material when an external magnetic field is applied. Materials with a high magnetic permeability are able to support a stronger magnetic field within themselves.

What is “heat treating” a material?

Heat treating can be used to achieve different purposes for different materials, but it generally involves heating or cooling a material to a relatively extreme temperature with the goal of changing its properties. Most often, this is done to make a material harder or softer. Heating or cooling the material allows the internal/microscopic structure to change in a way that is preserved when the material returns to ambient temperature.

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Lonsdaleite is made up of carbon atoms, just like diamonds, but the hexagonal structure makes it even harder than the sparkling gem.

What makes a fabric “waterproof”?

One type of waterproof fabric is waterproof simply because the threads are woven so tightly that water cannot easily get inside or through. Other waterproof materials are waterproof because they have been treated with a rubber (or some other) coating that keeps water out. Some coatings will only temporarily waterproof a material such that the coating or treatment wears away over time and needs to be reapplied.

What is the hardest material that has been discovered?

At some point you may have heard that diamond is the hardest material known to humankind. While diamond is extremely hard, there are actually a few materials that are even harder still! Years ago, some synthetically produced nanomaterials were discovered that are even harder than diamonds. Even more recently, two additional naturally occurring materials, both of which are even harder yet, were discovered. These materials are wurtzite boron nitride and a mineral called lonsdaleite. Wurtzite boron nitride has its atoms arranged in a very similar structure to the arrangement in diamond, but they are just different atoms (boron and nitrogen, rather than carbon). The other material, lonsdaleite, is actually also made from carbon atoms, but these are arranged dif ferently from those of diamond. Lonsdaleite is also sometimes called hexagonal diamond and can be formed when meteorites, which contain graphite, hit the Earth at very high speeds. Wurtzite boron nitride is produced naturally at high temperatures and pressures during volcanic eruptions. To date, there are only small amounts of either of these materials that have ever been found or synthesized.

What happens when you “fire” a wet clay pot in a kiln?

Before the clay is placed in the kiln, it is usually dried in the air for at least several days. This first step has already removed the majority of the water, but there will still be some trapped inside the clay. As it is heated in the kiln, the remaining water will turn to steam as it evaporates from the clay. If it is heated too fast, it may turn to steam while still trapped in the clay and cause the pot to explode! As the pot continues to heat some of the organic materials in the clay will burn off, which is necessary for the clay to form a strong final structure.

The next stage is an interesting one, and to understand it we need to consider the chemical composition of clay. Clay consists of a unit of alumina (Al2O3) and two units of silica (SiO2) complexed with two molecules of water. So even after all of the “excess” water has evaporated away, there is still a significant quantity of water that remains chemically bonded within the clay (at this point water accounts for about 14% of the mass of the clay). As the temperature continues to increase those remaining water molecules begin to be released, and they too evaporate away. This is another step where the heating must be done slowly, otherwise the water can create steam pockets within the clay that will expand and eventually explode.

Other changes occur as well, such as changes in the crystalline structure of the silica that will occur multiple times as the pot is heated. Eventually the glass-making oxides within the clay melt, and the clay will fuse into a ceramic material. The materials that melt relatively easily will tend to fill in remaining empty spaces, strengthening the final product. One final note is that changes in the crystalline structure of the silica will also occur upon cooling, and one must take care to cool the pot sufficiently slowly so that these changes don’t cause cracks to develop during cooling.

What kind of glass is used in your iPhone (or other smartphone) screen?

The glass in most iPhone screens to date, along with that in many other smartphones, is trademarked with the name Gorilla® Glass. This is an alkali-aluminosilicate glass that has been used in over one billion devices! It is lightweight, thin, and resists scratching and cracking significantly better than many other types of everyday glass.

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Pyrex® baking dishes are a common sight in many kitchens. The material is a soda-lime glass that is resistant to breaking and cracking.

What is a Pyrex® baking dish made of?

Pyrex® is a type of glass that was originally introduced in the year 1915 to be used in laboratory glassware and home kitchenware as well. It is a borosilicate glass composed of approximately 51% oxygen, 38% silicon, 14% boron, 1% aluminum, 1% potassium, and 0.3% sodium (by mass). Since its original introduction in the early 1900s, a different company has now become responsible for making Pyrex® glassware and they now make it from a soda-lime glass, which is different from the original formula. This new formula is cheaper to produce than the original and is more resistant to breaking when dropped, but has poorer heat resistance.

How do OLED screens work, and what are they made of?

OLEDs (organic light-emitting diodes) are a class of LEDs (light-emitting diodes) in which an organic material emits light when an electric current is applied. These can be used to create television screens, computer monitors, cellular phone screens, etc. An OLED may employ either small organic molecules or polymers. One advantage of OLED screens is that they do not need a backlight, which allows them to be thin and lightweight and also to display deeper black image levels than backlit screens.

What is the sticky stuff that you lick to seal an envelope?

The glue that you lick on the seal of an envelope is typically a substance called gum arabic, which is made of polysaccharides and glycoproteins. This gum can be found in the sap of acacia trees.

What is a gel?

Gels are solid materials that have flexible properties but do not actually flow in the same way that liquids do. They are made from a crosslinked bonding network of atoms, which actually contains a majority of liquid-like molecules interspersed by weight, but it still behaves as a solid. The crosslinked network within the gel gives it its solid-like properties, while the fluid component gives the gel its stickiness.

What are metamaterials?

A metamaterial is a type of artificially engineered material that typically features a pattern or periodic arrangement of a material. Metamaterials are characterized by the fact that they take on specific macroscopic physical properties based on their structure or the pattern in which the material is arranged, but not necessarily the composition of the material. Another way to say this is that the elemental makeup of a metamaterial is not as important as the internal structure of the metamaterial. By relying on the structure of the material to influence its properties, metamaterials have been able to achieve properties that haven’t been achieved in other types of materials. One example includes materials with a negative refractive index (see “Physical and Theoretical Chemistry”), which have been able to achieve the first demonstrations of “invisibility cloaking” over certain wavelength ranges, and this technology will hopefully continue to progress as time goes on.

What is Aerogel?

Aerogels are materials that are very similar to normal gels, except for the key detail that the liquid component has been replaced with a gas! Since liquids were such a large component of gels by weight, Aerogels are very lightweight. This type of material is translucent and has been given nicknames like “solid smoke” or “solid air.” It is produced by extracting the liquid component from a gel using a process called supercritical drying, which allows the liquid to be evaporated away without causing the solid network of chemical bonds to collapse. Aerogels have been produced from a variety of materials including alumina, silica, chromia, and tin dioxide.

What is a superalloy?

Superalloys are alloys that display a particularly excellent ability to resist deformation under stress at high temperatures along with good resistance to corrosion and great surface stability. Most often, a superalloy involves nickel, cobalt, or nickel-iron as the base alloying element. Superalloys have been used primarily in turbines and in the aerospace industry.

What are auxetic materials?

Auxetic materials have a unique property—when they are stretched, they actually become thicker in the directions perpendicular to the applied force! Think about this in comparison to anything else you stretch; it is really quite a strange phenomenon. This occurs as a result of hinged arrangements within the material that flex apart when a force is applied to stretch it.