Graphene ballistic rectifier. Credit: University of Manchester
A graphene-based electrical nano-device has been created which could substantially increase the energy efficiency of fossil fuel-powered cars.
The nano-device, known as a ‘ballistic rectifier’, is able to convert heat which would otherwise be wasted from the car exhaust and engine body into a useable electrical current.
Parts of car exhausts can reach temperatures of 600 degrees Celsius. The recovered energy can then be used to power additional automotive features such as air conditioning and power steering, or be stored in the car battery.
The nano-rectifier was built by a team at The University of Manchester led by Professor Aimin Song and Dr. Ernie Hill, with a team at Shandong University. The device utilises graphene’s phenomenally high electron mobility, a property which determines how fast an electron can travel in a material and how fast electronic devices can operate.
The resulting device is the most sensitive room-temperature rectifier ever made. Conventional devices with similar conversion efficiencies require cryogenically low temperatures.
Even today’s most efficient internal combustion engines can only convert about 70% of energy burned from fossil fuels into the energy required to power a car. The rest of the energy created is often wasted through exhaust heat or cooling systems.
Greg Auton, who performed most of the experiment said: “Graphene has exceptional properties; it possesses the longest carrier mean free path of any electronic material at room temperature.
“Despite this, even the most promising applications to commercialise graphene in the electronics industry do not take advantage of this property. Instead they often try to tackle the the problem that graphene has no band gap.”
Professor Song who invented the concept of the ballistic rectifier said: “The working principle of the ballistic rectifier means that it does not require any band gap. Meanwhile, it has a single-layered planar device structure which is perfect to take the advantage of the high electron-mobility to achieve an extremely high operating speed.
“Unlike conventional rectifiers or diodes, the ballistic rectifier does not have any threshold voltage either, making it perfect for energy harvest as well as microwave and infrared detection”.
Graphene was the world’s first two-dimensional material, isolated in 2004 at The University of Manchester, since then a whole family of other 2D materials have been discovered.
The advantage of a graphene-based nano-rectifier is its high conversion efficiency from an alternating current to a direct current at room temperature, enabled by the extremely high electron mobility achieved in this work.
The Manchester-based group is now looking to scale up the research by using large wafer-sized graphene and perform high-frequency experiments. The resulting technology can also be applied to harvesting wasted heat energy in power plants.
Harvesting heat produced by a car’s engine which would otherwise be wasted and using it to recharge the car’s batteries or powering the air-conditioning system could be a significant feature in the next generation of hybrid cars.
Prof Ian Kinloch, Professor of Materials Science The average car currently loses around 70% of energy generated through fuel consumption to heat. Utilising that lost energy requires a thermoelectric material which can generate an electrical current from the application of heat.
Thermoelectric materials convert heat to electricity or vice-versa, such as with refrigerators. The challenge with these devices is to use a material that is a good conductor of electricity but also dissipates heat well.
Currently, materials which exhibit these properties are often toxic and operate at very high temperatures – higher than that produced by car engines. By adding graphene, a new generation of composite materials could reduce carbon emissions globally from car use.
Scientists from The University of Manchester working with European Thermodynamics Ltd have increased the potential for low cost thermoelectric materials to be used more widely in the automotive industry.
The team, led by Prof Ian Kinloch, Prof Robert Freer and Yue Lin, added a small amount of graphene to strontium titanium oxide.
The resulting composite was able to convert heat which would otherwise be lost as waste into an electric current over a broad temperature range, going down to room temperature.
Prof Freer said: “Current oxide thermoelectric materials are limited by their operating temperatures which can be around 700 degrees Celsius. This has been a problem which has hampered efforts to improve efficiency by utilising heat energy waste for some time.
“Our findings show that by introducing a small amount of graphene to the base material can reduce the thermal operating window to room temperature which offers a huge range of potential for applications.
“The new material will convert 3-5% of the heat into electricity. That is not much but, given that the average vehicle loses roughly 70% of the energy supplied to it by its fuel to waste heat and friction, recovering even a small percentage of this with thermoelectric technology would be worthwhile.”
Improving fuel efficiency, whilst retaining performance, has long been a driving force for car manufacturers. Graphene could also aid fuel economy and safety when used as a composite material in the chassis or bodywork to reduce weight compared to traditional materials used.
Scientists have moved graphene—the incredibly strong and conductive single-atom-thick sheet of carbon—a significant step along the path from lab bench novelty to commercially viable material for new electronic applications.
Researchers from the University of Manchester, together with BGT Materials Limited, a graphene manufacturer in the United Kingdom, have printed a radio frequency antenna using compressed graphene ink. The antenna performed well enough to make it practical for use in radio-frequency identification (RFID) tags and wireless sensors, the researchers said. Even better, the antenna is flexible, environmentally friendly and could be cheaply mass-produced. The researchers present their results in the journal Applied Physics Letters, from AIP Publishing.
The study demonstrates that printable graphene is now ready for commercial use in low-cost radio frequency applications, said Zhirun Hu, a researcher in the School of Electrical and Electronic Engineering at the University of Manchester.
“The point is that graphene is no longer just a scientific wonder. It will bring many new applications to our daily life very soon,” added Kostya S. Novoselov, from the School of Physics and Astronomy at the University of Manchester, who coordinated the project.
Graphene Gets Inked
Since graphene was first isolated and tested in 2004, researchers have striven to make practical use of its amazing electrical and mechanical properties. One of the first commercial products manufactured from graphene was conductive ink, which can be used to print circuits and other electronic components.
Graphene ink is generally low cost and mechanically flexible, advantages it has over other types of conductive ink, such as solutions made from metal nanoparticles.
To make the ink, graphene flakes are mixed with a solvent, and sometimes a binder like ethyl cellulose is added to help the ink stick. Graphene ink with binders usually conducts electricity better than binder-free ink, but only after the binder material, which is an insulator, is broken down in a high-heat process called annealing. Annealing, however, limits the surfaces onto which graphene ink can be printed because the high temperatures destroy materials like paper or plastic.
The University of Manchester research team, together with BGT Materials Limited, found a way to increase the conductivity of graphene ink without resorting to a binder. They accomplished this by first printing and drying the ink, and then compressing it with a roller, similar to the way new pavement is compressed with a road roller.
Compressing the ink increased its conductivity by more than 50 times, and the resulting “graphene laminate” was also almost two times more conductive than previous graphene ink made with a binder.
The high conductivity of the compressed ink, which enabled efficient radio frequency radiation, was one of the most exciting aspects of the experiment, Hu said.
Paving the Way to Antennas, Wireless Sensors, and More
The researchers tested their compressed graphene laminate by printing a graphene antenna onto a piece of paper. The antenna measured approximately 14 centimeters long, and 3.5 millimeter across and radiated radio frequency power effectively, said Xianjun Huang, who is the first author of the paper and a PhD candidate in the Microwave and Communcations Group in the School of Electrical and Electronic Engineering.
Printing electronics onto cheap, flexible materials like paper and plastic could mean that wireless technology, like RFID tags that currently transmit identifying info on everything from cattle to car parts, could become even more ubiquitous.
Most commercial RFID tags are made from metals like aluminium and copper, Huang said, expensive materials with complicated fabrication processes that increase the cost.
“Graphene based RFID tags can significantly reduce the cost thanks to a much simpler process and lower material cost,” Huang said. The University of Manchester and BGT Materials Limited team has plans to further develop graphene enabled RFID tags, as well as sensors and wearable electronics.
A graphene lightbulb with lower energy emissions, longer lifetime and lower manufacturing costs has been launched thanks to a University of Manchester research and innovation partnership.
Graphene Lighting PLC is a spin-out based on a strategic partnership with the National Graphene Institute (NGI) at The University of Manchester to create graphene applications.
The UK-registered company will produce the lightbulb, which is expected to perform significantly better and last longer than traditional LED bulbs.
It is expected that the graphene lightbulbs will be on the shelves in a matter of months, at a competitive cost.
Chancellor George Osborne and Sir Kostya Novoselov with the graphene lightbulb.
The University of Manchester has a stake in Graphene Lighting PLC to ensure that the University benefits from commercial applications coming out of the NGI.
The graphene lightbulb is believed to be the first commercial application of graphene to emerge from the UK, and is the first application from the £61m NGI, which only opened last week.
Graphene was isolated at The University of Manchester in 2004 by Sir Andre Geim and Sir Kostya Novoselov, earning them the Nobel prize for Physics in 2010. The University is the home of graphene, with more than 200 researchers and an unrivalled breadth of graphene and 2D material research projects.
The NGI will see academic and commercial partners working side by side on graphene applications of the future. It is funded by £38m from the Engineering and Physical Sciences Research Council (EPSRC) and £23m from the European Regional Development Fund (ERDF).
There are currently more than 35 companies partnering with the NGI. In 2017, the University will open the Graphene Engineering Innovation Centre (GEIC), which will accelerate the process of bringing products to market.
Professor Colin Bailey, Deputy President and Deputy Vice-Chancellor of The University of Manchester said: “This lightbulb shows that graphene products are becoming a reality, just a little more than a decade after it was first isolated – a very short time in scientific terms.
“This is just the start. Our partners are looking at a range of exciting applications, all of which started right here in Manchester. It is very exciting that the NGI has launched its first product despite barely opening its doors yet.”
James Baker, Graphene Business Director, added: “The graphene lightbulb is proof of how partnering with the NGI can deliver real-life products which could be used by millions of people.
“This shows how The University of Manchester is leading the way not only in world-class graphene research but in commercialisation as well.”
According to the World Water Management Institute, over one-third of the human population is affected by water scarcity. If nothing is done to prevent it, an estimated 1.8 billion people will be living in countries or regions with absolute water scarcity by 2025. Thankfully, due to bio-mimicry and advancements in physics, water filtration and desalination technologies have been growing and improving.
Graphene is a material possessing a very unique structure and properties, giving it a wide range of implications and uses, such as improved water filtration. Graphene is a one-atom thick sheet of carbon atoms. It is nearly transparent, very light, an excellent conductor of both heat and electricity, hydrophobic, and extremely strong. It is so strong that James Hone, professor of mechanical engineering at Columbia University, claims, “It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap.”
But how can graphene be such a great water filter if it is also hydrophobic? Single-atom-wide holes (called capillaries or defects) are made in the graphene sheet by bombarding it with gallium ions. This allows water to be vigorously sucked through the holes in the material structure. Not everything can fit through these tiny holes and, like a sieve, whatever is too big will be filtered out.
University of Manchester researchers discovered that graphene is impermeable to all gases and vapors, except for water; even helium, the hardest gas to separate out, cannot pass through, along with any salts nine Angstroms or larger.
This process is most efficient when the water layer being filtered is only one atom thick (the same thickness as graphene), but filtration will still occur when the graphene is submerged in water. Due to graphene’s increased permeability – fifty times greater than that of conventional membranes – the filtration is ultrafast and has even been compared to the speed of an ordinary coffee filter. The University of Manchester team’s ultimate goal is to “make a filter device that allows a glass of drinkable water made from seawater after a few minutes of hand pumping.” A scientific advance like this would have game changing implications for water supply and policy around the world.
Due to its interesting properties, scientists have been trying to create and implement graphene more effectively since its discovery a few years ago. Columbia University engineering researchers have experimentally demonstrated for the first time that it is possible to electrically contact a two-dimensional material like graphene along its one-dimensional edge rather than contacting it from the top, which has been the conventional approach.
Through this approach, a new assembly technique has been developed that prevents contamination within layered materials (including layered graphene). Kenneth Shepard, Professor of Electrical Engineering at Columbia University, says that this “novel edge-contact geometry provides more efficient contact than the conventional geometry,” opening up possibilities in device applications and fundamental physics exploration.
So keep your eyes peeled; smart phones and other portable devices using graphene “could potentially be commercially available within the next 5-10 years.” Graphene could also be used for more efficient and economically viable biofuel creation or lithium ion batteries found in electrically powered vehicles.
Scientists are also actively fighting water scarcity by taking inspiration from the creatures that handle it best. The Namibian Beetle (Stenocara gracilipes) is native to the southwest coast of Africa, one of the driest deserts in the world. The Namib Desert is known for its high temperatures, strong winds, and negligible rainfall, although it does experience fogs that move in from the Atlantic Ocean early in the morning and late at night.
The Namibian Beetle capitalizes on this windborne dew and gains an average of about twelve percent of its body weight through a technique known as fog-basking. When fog-basking, the beetle points its back at the oncoming breeze carrying the tiny dewdrops and waits. The back of the beetle is hydrophobic, but spotted with small hydrophilic bumps. When the dew-carrying breeze blows by, tiny water droplets are attracted to the hydrophilic bumps and condense, accumulating on the beetle’s back.
When the drops grow to a substantial size, the weight of the droplets and the force of the wind exceed the hydrophilic forces and the drops fall down the hydrophobic back, finally sliding into the beetle’s mouth. Products like fog nets have been enlisted to help solve human water scarcity, but mimicking the beetle’s perfectly efficient biology can help scientists confront the water issue more effectively.
The challenge now is to create passive devices to collect water in desiccated environments for local consumption, particularly in poor countries. One example of a bio-mimicry product that seems to take inspiration from the Namibian Beetle is the Dew Bank Bottle. More bio-mimicry in the future, in many scientific fields, could help scientists discover more efficient, natural ways of solving some of our greatest issues.
Advances in physical understanding, its applications, and the study of our environment and bio-mimicry help us develop more effective ways to fight freshwater scarcity around the world. Graphene has proven to be an incredible material with a vast range of unique, useful properties, but taking a step outside the lab to examine how life naturally overcomes different problems can be just as informative. Hopefully, humble creatures like the Namibian Beetle will help usher in a day when the lack of clean drinking water is no longer an issue. In the words of Dr. Irina Grigorieva, “We are not there yet but this is no longer science fiction.”
Researchers from the University of Cambridge have developed advanced molecular ‘sieves’ which could be used to filter carbon dioxide and other greenhouse gases from the atmosphere.
Newly-developed synthetic membranes provide a greener and more energy-efficient method of separating gases, and can remove carbon dioxide and other greenhouse gases from the atmosphere, potentially reducing the cost of capturing carbon dioxide significantly.
The synthetic membranes, made of materials known as polymers of intrinsic microporosity (PIMs), mimic the hourglass-shaped protein channels found in biological membranes in cells. The tiny openings in these molecular ‘sieves’ – just a few billionths of a metre in size – can be adjusted so that only certain molecules can pass through. Details are published in the journal Nature Communications.
Polymer molecular sieves with interconnected pores (in green) for rapid and selective transport of molecules. Credit: Qilei Song
Current methods for separating gases are complex, expensive and energy-intensive. Additionally, conventional polymers, while reliable and inexpensive, are not suitable for large scale applications, as there is a trade-off between low permeability levels and a high degree of selective molecular separation.
Researchers are attempting to develop new methods of energy-efficient and environmental-friendly membrane-separation technology, which is an essential process in everything from water purification to controlling gas emissions.
The team from the University’s Cavendish Laboratory, working with researchers from Kyoto University, has developed an alternative approach to generating polymer membranes, ‘baking’ them in the presence of oxygen, a process known as thermal oxidation.
Inducing a thermal oxidation reaction in the PIMs causes the loosely-packed long chains of polymer molecules to form into a cross-linked network structure, with hourglass-shaped cavities throughout. This structure not only results in a membrane which is more selective to gas molecules, but also the size of necks and cavities can be tuned according to what temperature the PIMs are ‘baked’ at.
“The secret is that we introduce stronger forces between polymer chains,” said Dr Qilei Song of the Cavendish Laboratory, the paper’s lead author. “Heating microporous polymers using low levels of oxygen produces a tougher and far more selective membrane which is still relatively flexible, with a gas permeability that is 100 to 1,000 times higher than conventional polymer membranes.”
The cross-linked structure also makes these membranes more stable than conventional solution-processed PIMs, which have a twisted and rigid structure – like dried pasta – that makes them unable to pack efficiently. Thermal oxidation and crosslinking reinforces the strength of channels while controlling the size of the openings leading into the cavities, which allows for higher selectivity.
The new membrane is twice as selective for separation carbon dioxide as conventional polymer membranes, but allows carbon dioxide to pass through it a few hundred times faster. These thermally modified PIMs membranes are among molecular sieves with the highest combinations of gas permeability and selectivity. In addition to possible uses for separating carbon dioxide from flue gas emitted from coal-fired power plants, the membranes could also be used in air separation, natural gas processing, hydrogen gas production, or could help make more efficient combustion of fossil fuels and power generation with much lower emissions of air pollutants.
“Basically, we developed a method for making a polymer that can truly contribute to a sustainable environment,” said Professor Easan Sivaniah from Kyoto University’s Institute for Integrated Cell-Material Sciences (iCeMS).
“This new way of modifying PIMs brings the prospect of large-scale, energy-efficient gas separation a step closer,” said Professor Peter Budd, from the University of Manchester, one of the inventors of PIMs materials.
The properties of a material were once thought to be defined only by its chemical composition. But size matters too, especially for semiconductors. Make crystals of silicon small enough – less than 10 nanometres – and their tiny dimensions can start to dictate how the atoms behave and react in the presence of other things.
These tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable, novel properties that scientists think they will soon be used in everything from light bulbs to imaging of cancer cells or in the design of ultra-efficient solar cells.
Semiconductors such as silicon or indium arsenide are chosen to build electronic circuits because of the discrete energy levels at which they can give off electrons or photons. This makes them useful in building switches, transistors and other devices. It was once thought these energy levels – known as band gaps – were fixed. But shrinking the physical size of the semiconductor material to quantum-dot level seems able to change the band gaps, altering the wavelengths of light the material can emit or changing the energy it takes to change a material from an insulator to a conductor.
Instead of looking for brand new materials to build different devices, then, quantum dots make it possible to use a single type of semiconductor to produce a range of different characteristics. Researchers could tune dots made from silicon to emit a range of different colours in different situations, for example, instead of having to use a range of materials with different chemical
“The main application for quantum dots at the moment is biological tagging of cells,” says Paul O’Brien, a professor of inorganic materials at the University of Manchester and co-founder of Nanoco Technologies a quantum dot manufacturer also based in Manchester. They are used in the same way as fluorescent dyes, to label agents, he says, but with the advantage that a single laser source can be used to illuminate many different tags each with a specific wavelength.
By attaching different types of quantum dots to proteins that target and attach to specific cell types in the body, these bits of semiconductor can be used by doctors to monitor different kinds of cells. When a laser is then directed on to tagged cells, doctors can see what colour they glow.
The ability to shine also makes quantum dots well suited to produce white light. Existing white bulbs based on low energy light emitting diode (LED) technology tend to produce a garish and bluish form of light that notoriously feels cold, says O’Brien. This is because these LEDs use a phosphor that produces an artificial white light that contains less red wavelengths than natural white light. By embedding quantum dots into a film that is placed over a bulb containing blue LEDs, it is possible to get a much warmer colour of white light. The blue light from the LED stimulates the quantum dots which, in turn, emit light in a range of colours. Provided you have chosen your dots carefully, these will combine to form white light.
The first of these quantum dot lights hit the market in 2010, a partnership between QD Vision, an MIT spinout in Lexington, Massachusetts, and Nexxus Lighting of Charlotte, North Carolina.
Backlights for laptops, tablets and mobile devices are next in line, and they should appear in products before the end of 2012 says VJ Sahi, head of corporate development at materials design company Nanosys of Palo Alto, California. Besides the colour advantages, quantum-dot-based backlights can be three times more efficient than traditional backlights.
Eventually, says Sahi, quantum dots will do more than just light up displays. The long-term aim is use them to create each red, green and blue sub-pixel that makes up a coloured display. This should produce much brighter colours and consume less power than LCD or even the latest state-of-the-art organic LED (OLED) displays. They should also have no problems with viewing angles, he adds.
The interesting properties of quantum dots come from the fact that they behave like tuning forks for photons, a result of a phenomenon called confinement. At less than 10 nanometres in size – about 50 atoms – they fall within the dimensions of a critical quantum characteristic of the material known as the exciton Bohr radius. The energy levels of electrons within the material’s atoms are constrained and, when a photon or electron hits an atom and excites it, the atom re-emits the energy as a photon of a very specific energy level.
Quantum dots also have another trick up their sleeve. Besides converting photons of one energy into photons of another, they can also be used to release electrons and create electrical currents: in other words they can be used to make solar cells. Arthur Nozik at the National Renewable Energy Laboratory in Boulder, Colorado, says that quantum-dot solar cells would be much more efficient at converting the energy from photons and therefore boost the amount of power they can produce.
Such applications are many years from becoming commercial reality. But they serve to demonstrate that no material technology stands still; sometimes all you have to do is cut it down to size.