Ferroelectrics could pave way for ultra-low power computing

Minimum capacity

Engineers at the University of California, Berkeley, have shown that it is possible to reduce the minimum voltage necessary to store charge in a capacitor, an achievement that could reduce the power draw and heat generation of today’s electronics.

 

“Just like a Formula One car, the faster you run your computer, the hotter it gets. So the key to having a fast microprocessor is to make its building block, the transistor, more energy efficient,” said Asif Khan, UC Berkeley graduate student in electrical engineering and computer sciences. “Unfortunately, a transistor’s power supply voltage, analogous to a car’s fuel, has been stuck at 1 volt for about 10 years due to the fundamental physics of its operation. Transistors have not become as ‘fuel-efficient’ as they need to be to keep up with the market’s thirst for more computing speed, resulting in a cumulative and unsustainable increase in the power draw of microprocessors. We think we can change that.”

 

Khan, working in the lab of Sayeef Salahuddin, UC Berkeley assistant professor of electrical engineering and computer sciences, has been leading a project since 2008 to improve the efficiency of transistors. 

The researchers took advantage of the exotic characteristics of ferroelectrics, a class of material that holds both positive and negative electrical charges. Ferroelectrics hold electrical charge even when you don’t apply a voltage to it. What’s more, the electrical polarization in ferroelectrics can be reversed with the application of an external electrical field.

The engineers demonstrated for the first time that in a capacitor made with a ferroelectric material paired with a dielectric – an electrical insulator – the charge accumulated for a given voltage can, in effect, be amplified, a phenomenon called negative capacitance.

 

The team report their results in the Sept. 12 issue of the journal Applied Physics Letters. The experiment sets the stage for a major upgrade to transistors, the on-off switch that generate the zeros and ones of a computer’s binary language.

 

“This work is the proof-of-principle we have needed to pursue negative capacitance as a viable strategy to overcome the power draw of today’s transistors,” said Salahuddin, who first theorized the existence of negative capacitance in ferroelectric materials as a graduate student with engineering professor Supriyo Datta at Purdue University. “If we can use this to create low-power transistors without compromising performance and the speed at which they work, it could change the whole computing industry.”

 

The researchers paired a ferroelectric material, lead zirconate titanate (PZT), with an insulating dielectric, strontium titanate (STO), to create a bilayer stack. They applied voltage to this PZT-STO structure, as well as to a layer of STO alone, and compared the amount of charge stored in both devices.

 

“There was an expected voltage drop to obtain a specific charge with the dielectric material,” said Salahuddin. “But with the ferroelectric structure, we demonstrated a two-fold voltage enhancement in the charge for the same voltage, and that increase could potentially go significantly higher.”

Since the first commercial microprocessors came onto the scene in the early 1970s, the number of transistors squeezed onto a computer chip has doubled every two years, a progression predicted by Intel co-founder Gordon Moore and popularly known as Moore’s Law. Integrated circuits that once held thousands of transistors decades ago now boast billions of the components.

 

But the reduced size has not led to a proportional decrease in the overall power required to operate a computer chip. At room temperature, a minimum of 60 millivolts is required to increase by tenfold the amount of electrical current flowing through a transistor. Since the difference between a transistor’s on and off states must be significant, it can take at least 1 volt to operate a transistor, the researchers said. “We’ve hit a bottleneck,” said Khan. “The clock speed of microprocessors has plateaued since 2005, and shrinking transistors further has become difficult.”

 

The researchers noted that it becomes increasingly difficult to dissipate heat efficiently from smaller spaces, so reducing transistor size much more would come at the risk of frying the circuit board. The solution proposed by Salahuddin and his team is to modify current transistors so that they incorporate ferroelectric materials in their design, a change that could potentially generate a larger charge from a smaller voltage. This would allow engineers to make a transistor that dissipates less heat, and the shrinking of this key computer component could continue.

 

Notably, the material system the UC Berkeley researchers reported shows this effect at above 200 degrees Celsius, much hotter than the 85 degrees Celsius (185 degrees Fahrenheit) at which a current day microprocessor works. The researchers are now exploring new ferroelectric materials for room temperature negative capacitance in addition to incorporating the materials into a new transistor.

 

This story is reprinted from material from UC Berkeley, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier.

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Better lithium-ion batteries are on the way

A revolutionary conducting polymer enables the use of low-cost, high-energy silicon for the next generation of lithium-ion battery anodes

 Lithium-ion batteries are everywhere, in smart phones, laptops, an array of other consumer electronics, and the newest electric cars. Good as they are, they could be much better, especially when it comes to lowering the cost and extending the range of electric cars. To do that, batteries need to store a lot more energy.

 

The anode is a critical component for storing energy in lithium-ion batteries. A team of scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has designed a new kind of anode that can absorb eight times the lithium of current designs, and has maintained its greatly increased energy capacity after over a year of testing and many hundreds of charge-discharge cycles.

 

The secret is a tailored polymer that conducts electricity and binds closely to lithium-storing silicon particles, even as they expand to more than three times their volume during charging and then shrink again during discharge. The new anodes are made from low-cost materials, compatible with standard lithium-battery manufacturing technologies. The research team reports its findings inAdvanced Materials, now available online.

 

 “High-capacity lithium-ion anode materials have always confronted the challenge of volume change – swelling – when electrodes absorb lithium,” says Gao Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD), a member of the BATT program (Batteries for Advanced Transportation Technologies) managed by the Lab and supported by DOE’s Office of Vehicle Technologies.

 

Says Liu, “Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more – it has by far the highest capacity among lithium-ion storage materials – but it swells to more than three times its volume when fully charged.”

 

This kind of swelling quickly breaks the electrical contacts in the anode, so researchers have concentrated on finding other ways to use silicon while maintaining anode conductivity. Many approaches have been proposed; some are prohibitively costly.

 

One less-expensive approach has been to mix silicon particles in a flexible polymer binder, with carbon black added to the mix to conduct electricity. Unfortunately, the repeated swelling and shrinking of the silicon particles as they acquire and release lithium ions eventually push away the added carbon particles. What’s needed is a flexible binder that can conduct electricity by itself, without the added carbon.

 

“Conducting polymers aren’t a new idea,” says Liu, “but previous efforts haven’t worked well, because they haven’t taken into account the severe reducing environment on the anode side of a lithium-ion battery, which renders most conducting polymers insulators.”

 

One such experimental polymer, called PAN (polyaniline), has positive charges; it starts out as a conductor but quickly loses conductivity. An ideal conducting polymer should readily acquire electrons, rendering it conducting in the anode’s reducing environment.

 

The signature of a promising polymer would be one with a low value of the state called the “lowest unoccupied molecular orbital,” where electrons can easily reside and move freely. Ideally, electrons would be acquired from the lithium atoms during the initial charging process. Liu and his postdoctoral fellow Shidi Xun in EETD designed a series of such polyfluorene-based conducting polymers – PFs for short.

 

Compared with the electronic structure of PAN, the absorption spectra Yang obtained for the PFs stood out immediately. The differences were greatest in PFs incorporating a carbon-oxygen functional group (carbonyl).

 

The icing on the anode cake is that the new PF-based anode is not only superior but economical. “Using commercial silicon particles and without any conductive additive, our composite anode exhibits the best performance so far,” says Gao Liu. “The whole manufacturing process is low cost and compatible with established manufacturing technologies. The commercial value of the polymer has already been recognized by major companies, and its possible applications extend beyond silicon anodes.”

 

This story is reprinted from material from Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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Proton-based transistor

Protonic parallel to electronic circuitry

Human devices, from light bulbs to iPods, send information using electrons. Human bodies and all other living things, on the other hand, send signals and perform work using ions or protons.

 

Materials scientists at the University of Washington have built a novel transistor that uses protons, creating a key piece for devices that can communicate directly with living things. The study is published online this week in the interdisciplinary journalNature Communications.

 

Devices that connect with the human body’s processes are being explored for biological sensing or for prosthetics, but they typically communicate using electrons, which are negatively charged particles, rather than protons, which are positively charged hydrogen atoms, or ions, which are atoms with positive or negative charge.

 

“So there’s always this issue, a challenge, at the interface – how does an electronic signal translate into an ionic signal, or vice versa?” said lead author Marco Rolandi, a UW assistant professor of materials science and engineering. “We found a biomaterial that is very good at conducting protons, and allows the potential to interface with living systems.”

 

In the body, protons activate “on” and “off” switches and are key players in biological energy transfer. Ions open and close channels in the cell membrane to pump things in and out of the cell. Animals including humans use ions to flex their muscles and transmit brain signals. A machine that was compatible with a living system in this way could, in the short term, monitor such processes. Someday it could generate proton currents to control certain functions directly.

 

A first step toward this type of control is a transistor that can send pulses of proton current. The prototype device is a field-effect transistor, a basic type of transistor that includes a gate, a drain and a source terminal for the current. The UW prototype is the first such device to use protons. It measures about 5 microns wide, roughly a twentieth the width of a human hair.

 

 

“In our device large bioinspired molecules can move protons, and a proton current can be switched on and off, in a way that’s completely analogous to an electronic current in any other field effect transistor,” Rolandi said.

 

The device uses a modified form of the compound chitosan originally extracted from squid pen, a structure that survives from when squids had shells. The material is compatible with living things, is easily manufactured, and can be recycled from crab shells and squid pen discarded by the food industry.

 

First author Chao Zhong, a UW postdoctoral researcher, and second author Yingxin Deng, a UW graduate student, discovered that this form of chitosan works remarkably well at moving protons. The chitosan absorbs water and forms many hydrogen bonds; protons are then able to hop from one hydrogen bond to the next.

 

Computer models of charge transport developed by co-authors M. P. Anantram, a UW professor of electrical engineering, and Anita Fadavi Roudsari at Canada’s University of Waterloo, were a good match for the experimental results.

 

“So we now have a protonic parallel to electronic circuitry that we actually start to understand rather well,” Rolandi said.

 

Applications in the next decade or so, Rolandi said, would likely be for direct sensing of cells in a laboratory. The current prototype has a silicon base and could not be used in a human body. Longer term, however, a biocompatible version could be implanted directly in living things to monitor, or even control, certain biological processes directly.

 

This story is reprinted from material from the University of Washington, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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NEWS FROM THE WORLD OF MATERIALS

Materials in Focus

High piezoelectric properties discovered in non-polar block copolymer system

Oak Ridge National Laboratory (ORNL). See also the press release by Ron Walli of ORNL.

Image credit: ORNL. Click image to enlarge.

Image caption: Schematic of the piezoelectric effect in a non-polar block copolymer system.

The discovery of piezoelectric behavior in non-polar block copolymers that is an order of magnitude higher than that found in more traditional inorganic ceramic piezoelectric materials caught researchers from Oak Ridge National Laboratory (ORNL) in Tennessee and Aachen University in Germany off guard.  “People did not expect that non-polar block copolymers would show any strong response to electric fields, so that was quite a surprise,” says Volker Urban of ORNL. “When we observed how large the effect was at first we were not sure whether we should even call this a piezoelectric effect.” However, common characteristics with conventional piezoelectric ceramics, such as linear dependence on the electric field strength, convinced them that they were observing a new form of piezoelectric material having different physics at the molecular level.

Most piezoelectric materials found to date have been perovskite ceramics, like lead zirconate titanate (PZT), whose crystalline structure is responsible for piezoelectricity. As reported recently in Advanced Materials, Urban and his colleagues experimented with the non-polar block copolymer poly(styrene-b-isoprene) in solution with toluene. The solvent makes the block copolymers more mobile, but the system retains its underlying phase morphology, such as nanoscale lamellar structures. “So you retain this phase morphology but you make the system more flexible and then you can align the lamellae much more easily in an electric field,” Urban says. “That’s really unique to our research.”

Crystalline piezoelectric materials lose their anisotropy when heated above the Curie temperature, resulting in a loss of their piezoelectric properties. The block copolymer system, however, shows an increase in piezoelectricity when crossing the order-disorder transition temperature. The researchers explain this contrasting behavior by discussing the thermodynamics of the block copolymer system, specifically the entropic gain and the enthalpic penalty as the polymer chain reaches a more Gaussian conformation.

Urban sees potential long term applications for this discovery in the areas of batteries, capacitors, and fuel cells. Polymers have already been used as membranes in fuel cells. “But what has been overlooked until now is what effect electric fields, which of course are present in these electric storage devices, may have on the structure of the polymers that are involved in the system,” Urban says. “This has been neglected completely until now.” He acknowledges that fuel cell membranes have been made from polymers that are simpler than block copolymers, so the effects will be different. But he believes that this research provides a unique perspective that may open up new avenues for the improvement of such devices in the future. [Advanced Materials]

Nano Focus

Antimagnet proposed using superconductor-metamaterial hybrid

Universitat Autònoma de Barcelona, Spain. 

Image credit: Alvaro Sanchez, Universitat Autònoma de Barcelona, Spain. Click image to enlarge.

Image caption: (a) a magnet and its field; ((b) two adjacent magnets whose fields interfere; (c) antimagnetic cylinder (yellow) enclosing one of the magnets, preventing the internal magnetic field from leaking out, and preventing interference with the magnetic field of the other magnet.

Building on recent revelations about metamaterials that can cloak a region in space from electromagnetic waves, researchers at the Universitat Autònoma de Barcelona have determined through simulations that it is possible to design an antimagnet using two materials that are within the realm of possibility: metamaterials and superconductors. According to a paper published recently in the New Journal of Physics, Alvaro Sanchez and his colleagues have outlined a method of designing an antimagnet that would “switch off the magnetic interaction of a magnetic material with existing magnetic fields without modifying them.” Such a device could have applications in medical MRI or in reducing the magnetic signatures of planes or ships. 

To be precise about what the researchers are proposing, they define an antimagnet as “a material forming a shell that encloses a given region in space while fulfilling the following two conditions: (i) the magnetic field created by any magnetic element inside the inner region—e.g., a permanent magnet—should not leak outside the region enclosed by the shell; and (ii) the system formed by the enclosed region plus the shell should be magnetically undetectable from outside (no interaction—e.g., no magnetic force—with any external magnetic sources).”

Sanchez and his co-authors propose a cylindrical shell for their antimagnet, although they contend that other shapes are possible. The use of a superconductor with magnetic permeability μ = 0 on the inside of the cylinder satisfies condition (i). To satisfy condition (ii), the outer shell of the cylinder would have to be made of a magnetic material with homogeneous radial and angular magnetic permeabilities. Since no known material has these properties, they propose using alternating layers of available materials. A superparamagnet made by embedding ferromagnetic nanoparticles in a non-magnetic material could be used for the first type of layer, and arrays of superconducting plates could be used for the second type, according to the researchers. A ten-layer cylinder with carefully defined permeabilities could produce the desired antimagnetic properties, they contend. The researchers recognize the difficulty of producing a practical antimagnetic device at this time, but offer the results of their simulations as a significant step in the development process. [New Journal of Physics]

Bio Focus

Proton-based transistor could provide viable bio-interface

University of Washington. See also the press release by Hannah Hickey of the University of Washington Office of News and Information.

Image credit: Uniiversity of Washington. Click image to enlarge.

Image caption: On the left is a colored photo of the University of Washington device overlaid on a graphic of the other components. On the right is a magnified image of the chitosan fibers. The white scale bar is 200 nanometers.

Researchers at the University of Washington in Seattle and collaborators at the University of Waterloo in Canada have developed a protonic field effect transistor (H⁺-FET) that controls the flow of protons instead of electrons, making it a good potential starting point for bio-interface devices. In biology, it’s generally the movement of ions (H⁺, Na⁺, K⁺, and Ca²⁺), instead of electrons, that controls processes such as ATP synthesis, neuronal signaling, and cell communication. Protons specifically play a key role in biological energy transduction mediated by ATP. 

Lead author Marco Rolandi of the University of Washington coined the word “bionanoprotonics” in a recent paper in Nature Communications to describe this novel field, complementing research being done in bionanoelectronics.  “We had to learn and, at times, make up all new terminology because all of a sudden there are no electrodes, there are ‘protodes’ for contacts, and there’s no electronic current, there’s protonic current,” he says.

  

The H⁺-FET consists of maleic chitosan nanofibers bridging the source and drain of the transistor, which are made of proton-conducting PdH_x. The prototype is built on a traditional Si/SiO₂substrate, which would have to be replaced with a biocompatible and flexible material if these devices are ever used in biological systems. Maleic chitosan is a biodegradable, non-toxic polysaccharide chitin derivative that forms many hydrogen bonds when hydrated. When an electrostatic potential is applied between source and drain, the protons dissociated from the maleic acid groups “hop” along the hydrogen bond network as described by the Grotthus mechanism. This hopping results in a protonic current from source to drain, which can be modulated by a voltage applied to the gate. The measured mobility of this current is 4.9 x 10^-3 cm²V^-1 s^-1. “We think it is actually a molecular level process rather than protons [as hydronium ions] just diffusing between the water molecules, pushing them around,” Rolandi says.  Because the mechanism is specific to protons, this device will not be suitable for controlling Na⁺, K⁺, or Ca²⁺.  “We wish we could work with those ions, but we’re happy with protons for now,” he says. 

Future work will attempt to make a truly nanoscale device; the prototype is a microscale device with nanoscale fibers. Rolandi would like to bridge the source and drain contacts with a single nanofiber of maleic chitosan to see whether that improves the on/off ratio of the H⁺-FET, which is currently low compared to traditional semiconductors. A further goal is to interface these transistors with cell cultures. Ultimately, in the distant future, the goal is to optimize the materials and performance of H⁺-FETs in physiological conditions so that in vivo sensing and stimulation of proton-selective ion channels could become possible. [Nature Communications]

Energy Focus

MATERIALS FOR ENERGY BLOG

Join the conversation! Read the latest blog entries and let us know what you think by adding a comment.

Add your comments, or e-mail materialsforenergy@mrs.org to suggest future topics and contributors.

Energy Quartely in MRS Bulletin, September 2011

By Dr. Russell Chianelli, The University of Texas at El Paso, M.R.T.I

Opening the grid across continents: Desert visions.

The September issue of Energy Quarterly in MRS Bulletin reports on the Desertec initiative that promises to distribute solar power from the Middle East and North Africa (MENA) for distribution to Europe. Corinna Wu describes Desertec as a centerpiece of Europe’s plans to dramatically increase renewable sources in its energy supply mix. There are many obstacles to developing the system, some political and some technical. However, one issue that arises in centralized solar with power transmission is the transmission grids that must be constructed and the materials used. In some areas, for example in the Southwestern United States, distributed energy systems involving individual people, businesses, and institutions installing solar power devices on local buildings eliminates the need for long transmission systems. However, in this case, energy storage is the issue. We look forward with interest to the competition between these two approaches.

Solid-state lighting: The future looks bright.

Also, in the September issue is an article by Prachi Patel in which she describes the progress in making LEDs (light-emitting diodes) for practical solid-state lighting devices. LEDs promise to reduce energy use by 75% and increase lifetime by a factor of ten. But technical challenges remain. LEDs are made using semiconductor materials (e.g., InGaN or GaN). However, cost and spectral properties need improvement. These issues are covered in detail in this article.

Batteries for energy: generation and storage

An interview with Yet-Ming Chiang of MIT covers a crucial issue for efficient use of energy in electrical vehicles and solar installations: batteries. Chiang is an entrepreneurial leader in next-generation nano-phosphate lithium batteries, having formed companies such as A123 Systems, which now supplies these batteries to industry, and 24M Systems, which produces flow batteries. Flow batteries are preferred for solar-produced-energy storage. The interview also discusses Chiang’s experience as an entrepreneur and the process that he followed from laboratory research to marketable products.

Image in Focus

Anode Feathers

SEM image of an anode used in lithium ion batteries. This particular sample consists of 95% pure semiconducting single-walled carbon nanotubes.

Credit: Laila Jaber Ansari, Northwestern University 

(Click image to enlarge.)

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