Material difference

Materials science is rapidly transforming the way that everything from cars to light bulbs is made, says Paul Markillie

“I DO not depend on figures at all,” said Thomas Edison. “I try an experiment and reason out the result, somehow, by methods which I could not explain.” And so it was that by testing 1,600 different materials, from coconut fibre to fishing line and even a hair from a colleague’s beard, Edison finally found a particular type of bamboo which could be used, in carbonised form, as the filament in the first proper incandescent light bulb. He demonstrated it on New Year’s Eve 1879 at his laboratory in Menlo Park, New Jersey.

The details of all this painstaking trial and error filled more than 40,000 pages of Edison’s notebooks, but his solution was soon superseded. By the start of the 20th century filaments were being made from tungsten, which burned brighter and lasted longer. For over 100 years the world was illuminated by light bulbs with tungsten filaments, and the light bulb became the cartoonist’s fixed shorthand for innovation of all sorts.

Now light bulbs are being replaced by light-emitting diodes (LEDs), which are more efficient at turning electricity into light than filaments are, and far longer-lived. LEDs first appeared in the 1960s as indicator lights on electrical equipment. Today they provide powerful illumination for buildings, streets and cars. In poor parts of the world they are bringing light to people who have never seen an old-fashioned bulb.

Both Edison’s light bulb and the LED are inventions of materials science, the process of turning matter into new and useful forms. But in the years between them both the materials and the science became much more complex. The semiconducting materials, such as germanium or silicon, from which LEDs are made, often with the careful addition of atoms of some other substance, require a different approach from that at Menlo Park. The sort of light they produce is fine-tuned by microscopic structures and the details of those extra atoms. Pace Edison, this sort of thing depends on a lot of figures—not to mention quantum theory.

The ability to understand the properties of materials at the tiniest scales not only lets people do old things better; it lets them do new things. In Edison’s day, using light to send messages was the province of the Aldis lamps that flashed messages in morse code from ship to ship. Laser diodes—semiconductor devices engineered to produce a much purer light than LEDs—can flicker on and off in a controlled way billions of times a second. In an astounding number of applications where information has to get from A to B—be those end points a DVD and a speaker, a bar code and a supermarket checkout or the two ends of a transatlantic fibre-optic cable—laser diodes are doing the work. For all its seeming abstraction, the virtual world is built on very real, very well-understood materials.

This is what some scientists describe as a “golden age” for materials. New, high-performing substances such as exotic alloys and superstrong composites are emerging; “smart” materials can remember their shape, repair themselves or assemble themselves into components. Little structures that change the way something responds to light or sound can be used to turn a material into a “metamaterial” with very different properties. Advocates of nanotechnology talk of building things atom by atom. The result is a flood of new substances and new ideas for ways of using them to make old things better—and new things which have never been made before.

University materials departments are flourishing, spawning a vibrant entrepreneurial culture and producing a spate of innovations (see box below). Many of these discoveries will fail to scale up from laboratory demonstration to commercial proposition. But some just might change the world, as light bulbs did.

Faster, higher, stronger

The understanding of the material world provided by a century of physics and chemistry accounts for much of the ever-accelerating progress. But this is not a simple triumph of theory. Instruments matter too. Machines such as electron microscopes, atomic-force microscopes and X-ray synchrotrons allow scientists to measure and probe materials in much greater detail than has ever been possible before.

A project at the International Centre for Advanced Materials at the University of Manchester shows such advances in action. In one of its labs scientists are using secondary ion mass spectrometry (SIMS) to study the way that hydrogen atoms—the smallest atoms there are—diffuse into materials such as steel, a process that can cause tiny cracks. SIMS works by bombarding a sample with a beam of charged particles, which causes secondary particles to be ejected from the surface. These are measured by an array of detectors to create an image with a resolution down to 50 nanometres (billionths of a metre). It does not just reveal the crystalline structure of the metal—and any flaws in it—but also determines chemical impurities, such as the presence of hydrogen. “We can now do in an afternoon what we once did in months,” says Paul O’Brien, a professor at the university. The hope is that BP, the oil company which is sponsoring the centre, will get better steels for its offshore and processing work as a result.

As well as having ever better instruments, the researchers are also benefiting from a massive increase in available computing power. This allows them to explore in detail the properties of virtual materials before deciding whether to try and make something out of them.

“We are coming out of an age where we were blind,” says Gerbrand Ceder, a battery expert at the University of California, Berkeley. Together with Kristin Persson, of the Lawrence Berkeley National Laboratory, Mr Ceder founded the Materials Project, an open-access venture using a cluster of supercomputers to compile the properties of all known and predicted compounds. The idea is that, instead of setting out to find a substance with the desired properties for a particular job, researchers will soon be able to define the properties they require and their computers will provide them with a list of suitable candidates.

Their starting point is that all materials are made of atoms. How each atom behaves depends on which chemical element it belongs to. The elements all have distinct chemical properties that depend on the structure of the clouds of electrons that make up the outer layers of their atoms. Sometimes an atom will pair off one of its electron with an electron from a neighbouring atom to form a “chemical bond”. These are the kind of connections that give structure to molecules and to some sorts of crystalline material, such as semiconductors. Other sorts of atom like to share their electrons more widely. In a metal the atoms share lots of electrons; there are no bonds (which makes metals malleable) and electric currents can run free.

When it comes to making chemical bonds, one element, carbon, is in a league of its own; a more or less infinite number of distinct molecules can be made from it. Chemists call these carbon-based molecules organic, and have devoted a whole branch of their subject—inorganic chemistry—to ignoring them. Mr Ceder’s Materials Project sits in that inorganic domain. It has simulated some 60,000 materials, and five years from now should reach 100,000. This will provide what the people working on the project call the “materials genome”: a list of the basic properties—conductivity, hardness, elasticity, ability to absorb other chemicals and so on—of all the compounds anyone might think of. “In ten years someone doing materials design will have all these numbers available to them, and information about how materials will interact,” says Mr Ceder. “Before, none of this really existed. It was all trial and error.”

Latest discoveries Curiouser and curiouser IN the month of November 2015 alone, materials scientists alerted the media to more than 100 significant discoveries. Here is a small selection from the professional journals: • A type of crystal called a perovskite can be used to make light-emitting diodes that glow exceptionally bright. These could be used in lighting and displays. Hanwei Gao and Biwu Ma of Florida State University. Advanced Materials.

• Experiments with an exotic form of electronics called “valleytronics”, named after one of the ways in which electrons can move through a semiconductor, shows that the technology might be used to make ultra-low-power computers. Seigo Tarucha and colleagues at the University of Tokyo. Nature Physics.

• Quantum dots made from nanoparticles of iron pyrite, commonly known as fool’s gold, could help batteries charge up much faster. Cary Pint, Anna Douglas and colleagues at Vanderbilt University, Nashville. ACS Nano.

• Biosensors made from graphene can provide high levels of sensitivity to help speed up the development of new drugs. Aleksey Arsenin and Yury Stebunov of the Moscow Institute of Physics and Technology. ACS Applied Materials and Interfaces.

• Materials called microwave absorbers are used to make detection by radar of objects such as stealth fighters more difficult. A new lightweight material with arrays of patterned conductors would greatly improve cloaking properties. Wenhua Xu and colleagues of Huazhong University of Science and Technology, China. Journal of Applied Physics.

• A new class of “porous liquid”, which features permanent holes at the molecular level, could provide a number of practical applications, including capturing carbon-dioxide emissions from factories. Stuart James and colleagues at Queen’s University Belfast. Nature.

• Voltage-sensitive nanomaterials could be inserted into human tissue to gather information about how the brain functions and help diagnose injury and disease. James Delehanty and colleagues at the US Naval Research Laboratory, Washington, DC. NANO Letters.

A walk through the labs of General Electric (GE)—the firm into which Edison’s trial-and-error-based businesses were merged in 1892—shows similar approaches already in practice. Michael Idelchik, the head of GE Research, points to new artificial garnets developed for use in body scanners. The scanners have to turn X-rays into visible light to create images, and the better they do so the lower the dose of X-rays the patient is exposed to. The company looked at 150,000 subtly different types of crystal that scintillate when subjected to X-rays before settling on a specific type of garnet which, it hopes, will make scans much faster—safer and more pleasant for the patient, more cost-effective for the hospital.

On top of the possibilities offered by single materials comes the potentially even richer world of combining them. Elsewhere in Mr Idelchik’s empire work focuses on replacing nickel-alloy parts for jet engines with parts made from ceramic-matrix composites (CMCs). Their strong chemical bonds mean ceramics can endure more heat than metals; at the same time, and for related reasons, they are more brittle. A CMC that combines a metal with a ceramic—GE is using silicon carbide—can get you the best of both worlds. The company hopes CMCs that need less cooling will mean more efficient engines that emit less carbon dioxide.

Computing power helps create such hybrids. It also helps designers understand how such novel materials can best be used. Many prototypes are now produced in virtual form long before a physical item is made, using software from companies such as Altair, a Michigan firm, Autodesk, a Californian one (see the “Brain scan” interview later in this report), and Dassault Systèmes, a French group. Engineers can model a chemicals plant, architects can “walk” clients through a digital representation of a building, and cars can be virtually test-driven on different roads and parked alongside rivals’ vehicles in street scenes.

All this greatly speeds up product development. The software is powerful enough to take the properties of the materials used into account, allowing it to calculate things such as loads, stresses, fluid dynamics, aerodynamics, thermal conditions and much more.

Manufacturers are only just beginning to realise the potential this offers, says Jeff Kowalski, chief technology officer of Autodesk. Many firms simply adapt parts to use new materials, expecting to produce them with the same tools and processes as before. That gives “substandard results”, he reckons. It is when new materials are used to redefine production processes and enable wholly new types of product that things get really innovative, and cartoonists get to draw light bulbs over people’s heads.

Just the thing

Business is heading towards a world of “generative design”, says Mr Kowalski: engineers will set out what they want to achieve and the computer will provide designs to fit that purpose. As materials knowledge grows, computers will also find materials to meet the properties specified by a designer. The properties of materials may even vary throughout their length and breadth, because it is becoming easier to tinker with the microstructure. Some companies are already well on their way to offering such Savile Row tailoring of materials.