The Graphene Corner
Graphene is the lightest known material. Produced in single sheets, it’s composed of carbon atoms bonded together in a honeycomb-shaped lattice that’s just one atom thick. Hyper conductive and 100 to 300 times stronger than steel, it’s also incredibly cheap.
Discovered in 2003, up until a short time ago, no one knew what what to do with it. Now, it’s a bleeding-edge material that’s being combined all the time with all sort of other substances to develop varieties of inventions ranging from electronics to medical devices to energy efficiency. That’s why we’re introducing this mini-feature, “The Graphene Corner”, to keep up with all the innovations.
Abandoned 1950s Space Craft Power Upgraded by Graphene
We see thermionic emissions all the time. For example, when metal is heated, such as an incandescent lamp filament, its electrons take on LOTS of energy until they take on so much that they break free of the nuclear orbits and flow from the heated surface towards a cooler one. This is also known as the Edison Effect.
Thermionic energy converters (TEC) were developed in the 1950s and came to be used for space craft power supplies. TECs change heat into electricity by taking advantage of that process. Thermionic converters have two electrodes, a hot one (which is the cathode or emitter) and a cold one (which is the anode or plate). The electrodes are separated by a gap and kept in a vacuum tube. When the hot electrode heats up to over 2240°F, the electrons flow as current towards to gap. The number of electrons being emitted from the hot electrode depends on the temperature and the amount of energy they have absorbed (called the “work function”). Because of the heat, plasma forms in the gap between the two electrodes. The amount of current might be only .5 to 1 volt but at several amperes of power, making the range of efficiency between 5 and 20%. Because of the amount of heat involved, it was difficult to make them work efficiently with fossil fuel burning power stations and so the technology was essentially mothballed.
But that might be changing. Recently, Stanford University researchers led by Prof. Roger Howe made some changes to the design of TECs. By using a graphene-based anode as the collector instead of the traditional tungsten and making the gap between the electrodes adjustable, they made a TEC that was 6.7 times more efficient at converting heat into electricity at a much cooler 1832°F.
Though currently in an experimental form, the goal is to incorporate TECs in thermal plants (such as fossil fuel and geothermal plants) to convert even more energy from their heat and use less fuel.
Graphene Transistors — A Work in Progress
Invented at Bell Labs in the 1950s, transistors can work as signal amplifiers and on/off switches. Basic transistors have three connectors and come in two semiconductor sandwich flavors: negative-positive-negative (NPN) and positive-negative-positive (PNP). Due to their low energy use and switching speed, transistors have long been known as the basic component of the computer chip. The first Intel chip in 1971 had 2,300 transistors and ran at 740 Khz. A fourth-gen Intel Core processor has 1.7 billion transistors and runs at 3 GHz. The problem that’s been lurking for years has been that silicon semiconductors can only switch so fast.
While graphene’s hyper-conductivity sounds like a boon for the computer industry, there’s one problem — there’s no band gap anywhere in the stuff.
What’s a band gap? In the classic model of an atom, you have a nucleus which has a swarm of electrons orbiting it. The orbits are thought to be arranged in bands. The bands closest to the nucleus that have electrons in them are known as core levels. The bands that are the furthest out with electrons in them are called valence bands. Both bands of electrons are held tightly in place by the nucleus. The next band out is the conduction band and this is where traveling electrons move in and out. If there’s wide gap between the valence band and the conduction band, electrons aren’t able to leave very easily, which makes that material an insulator.
For metals, the the valence and conduction bands overlap. When an electron in the valence band gets excited —by heat, for example, it can jump to the conduction level when it gets enough energy. But in materials that are semiconductors, there’s a gap between the valence and conduction bands, so sometimes it will act like a metal, other times like an insulator.
To use graphene as a semiconductor, a single sheet can either be doped with nitrogen so it will act like it has a band gap. Or, two graphene sheets are used with one stacked atop the other, called bilayer graphene . Applying a strong electric field perpendicular to the sheets induces a band gap. This makes graphene so tempting to use because it’s only one atom thick and millions (if not billions) of low power transistors could be used for a single processor very cheaply. Unfortunately, bilayering is extremely hard to make presently because the layers must be precisely aligned.
Once the secrete to making affordable graphene transistors is unlocked, look to see a hundred fold increase in computer processor speeds.
While graphene transistors may be a few years away, graphene is being used to print transistors. Researchers in AMBER, funded by the Science Foundation Ireland and hosted in Trinity College Dublin, announced they had printed —like ink-jet printed— 2D nano-transistors using graphene nanosheets as the electrodes and two other nanomaterials, tungsten diselenide (semiconductor) and boron nitride (insulator). The big innovation is creating the layered materials ink that can reliably print the nanomaterials to paper or fabric and be long-lasting. Graphene, in this case, was sheared into flakes ( exfoliation) just a few nanometers thick by hundreds of nanometers wide and mixed into an ink that was reliable and scaleable for industry use.
One future possibility is that printed electronic circuitry could allow consumer products to transmit information to your smart phone. But before you cringe at the thought of your groceries texting you all the time, this technology can be used for security, identification, drug labeling, and health monitoring when applied to clothing.