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Nanostructuring greatly improves thermoelectric material
RESEARCH NEWS
Nanostructuring greatly improves thermoelectric material ENERGY
A nanocrystalline form of a standard thermoelectric material shows significantly improved performance, report US and Chinese researchers, setting the scene for low-cost cooling and power-generation devices based on the thermoelectric effect [Poudel et al., Sciencexpress (2008), doi: 10.1126/ science.1156446]. The prospect of generating electrical power from waste heat is a tantalizing one. Similarly, the thermoelectric effect could be used for new airconditioning products, refrigeration, or cooling electronics. For such devices to be competitive, it is generally agreed that the dimensionless figure of merit, ZT, of the thermoelectric materials should be >1. Yet peak ZT values of commonly used Bi2Te3-based materials have remained stubbornly at ~1 for decades. Although other materials have been developed for hightemperature applications, Bi2Te3 and its alloys still dominate near room temperature. The team from Boston College, GMZ Energy, Massachusetts Institute of Technology (MIT), and Nanjing University tried a new tack. They synthesized a p-type nanocrystalline BixSb2–xTe3 alloy by ball milling the bulk material and hot
False-color transmission electron micrograph showing the random orientation of the grains in the nanocrystalline thermoelectric material. (Courtesy of Boston College, MIT, GMZ Energy, and Nanjing University, China.)
pressing the resulting nanoparticles into ingots. This results in a nanostructured material with highly crystalline, randomly oriented grains. Strong phonon scattering at the interfaces gives a significant reduction in the thermal conductivity compared with the bulk alloy, and is largely responsible for the high peak ZT value of 1.4 at 100°C. “We have found a way to improve an old material by breaking it up and then rebuilding
it in a composite of nanostructures in bulk form,” explains Zhifeng Ren of Boston College. The method of manufacture is simple, can be scaled up for mass production, and most importantly, is cheap. The researchers have demonstrated the potential of the new materials by constructing cooling devices that are able to produce temperature differences of 86°C, 106°C, and 119°C with the temperature of the hot side set at 50°C, 100°C, and 150°C, respectively. “The amazing part is that the process they introduce is a combination of ‘low-tech’, scalable, and cost-effective engineering,” says Qiang Li of Brookhaven National Laboratory. “Yet it produces a bulk nanocomposite of a well-known industrial material with superior thermoelectric properties.” The team will work on further improvements in the materials’ properties, Ren told Materials Today. “At the same time, we will push efficient cooling devices and power-generation systems to market as soon as possible through GMZ Energy, a company we founded in 2007.” Jonathan Wood
Si nanophotonic switch brings on-chip optical networks closer OPTICAL MATERIALS Scientists at IBM’s T. J. Watson Research Center have built the world’s smallest nanophotonic switch, an essential component if signals between microprocessors on a chip are to be routed using light pulses and optical networks [Vlasov et al., Nat. Photonics (2008) 2, 242]. Greater computing performance can be achieved by increasing the number of microprocessor cores on a single chip. Current multicore microprocessors use Cu wires to transmit information between them. But electrical interconnects use too much power and cannot handle the necessary amount of information to be able to put tens or even hundreds of cores on a chip. “Using photons instead of electrons to connect different cores of a computer processor provides enormous opportunities in performance improvement,” says coauthor Fengnian Xia. “There are also lots of challenges, such as development of ultracompact Si photonic devices, dense integration of photonics and electronics, power management, and innovative network designs.”
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One necessary component is an optical switch that can pass input optical signals into one waveguide or deflect them into a different output as required. Not only does the switch need to be a compact device to take up little room on the chip, it should be able to route a huge amount of data in different wavelength channels simultaneously and deal with the temperature fluctuations that occur in chips. The IBM team lead by Yurii Vlasov has demonstrated a Si optical switch that ticks all of these boxes. Their 40 µm x 12 µm device consists of five Si microring resonators with coupling coefficients that have been carefully controlled. Switching is achieved by tuning one of the central rings out of resonance, disrupting propagation of the optical signal through the device and redirecting it. The device can cope with temperature fluctuations of ±15°C and switch up to nine 40 Gbit/s optical channels of different wavelengths. Vlasov and colleagues expect that 25 more closely spaced channels would allow a total switch throughput of 1 Tbit/s.
MAY 2008 | VOLUME 11 | NUMBER 5
Si optical switches could ‘direct traffic’ within an on-chip optical network, routing optical messages between each processor core. (Courtesy of IBM Research.) “This new development is a critical addition in the quest to build an on-chip optical network,” says Vlasov. “In view of all the progress that this field has seen in the last few years, it looks that our vision for on-chip optical networks is becoming more and more realistic.”
Jonathan Wood
Nanostructuring greatly improves thermoelectric material ENERGY
A nanocrystalline form of a standard thermoelectric material shows significantly improved performance, report US and Chinese researchers, setting the scene for low-cost cooling and power-generation devices based on the thermoelectric effect [Poudel et al., Sciencexpress (2008), doi: 10.1126/ science.1156446]. The prospect of generating electrical power from waste heat is a tantalizing one. Similarly, the thermoelectric effect could be used for new airconditioning products, refrigeration, or cooling electronics. For such devices to be competitive, it is generally agreed that the dimensionless figure of merit, ZT, of the thermoelectric materials should be >1. Yet peak ZT values of commonly used Bi2Te3-based materials have remained stubbornly at ~1 for decades. Although other materials have been developed for hightemperature applications, Bi2Te3 and its alloys still dominate near room temperature. The team from Boston College, GMZ Energy, Massachusetts Institute of Technology (MIT), and Nanjing University tried a new tack. They synthesized a p-type nanocrystalline BixSb2–xTe3 alloy by ball milling the bulk material and hot
False-color transmission electron micrograph showing the random orientation of the grains in the nanocrystalline thermoelectric material. (Courtesy of Boston College, MIT, GMZ Energy, and Nanjing University, China.)
pressing the resulting nanoparticles into ingots. This results in a nanostructured material with highly crystalline, randomly oriented grains. Strong phonon scattering at the interfaces gives a significant reduction in the thermal conductivity compared with the bulk alloy, and is largely responsible for the high peak ZT value of 1.4 at 100°C. “We have found a way to improve an old material by breaking it up and then rebuilding
it in a composite of nanostructures in bulk form,” explains Zhifeng Ren of Boston College. The method of manufacture is simple, can be scaled up for mass production, and most importantly, is cheap. The researchers have demonstrated the potential of the new materials by constructing cooling devices that are able to produce temperature differences of 86°C, 106°C, and 119°C with the temperature of the hot side set at 50°C, 100°C, and 150°C, respectively. “The amazing part is that the process they introduce is a combination of ‘low-tech’, scalable, and cost-effective engineering,” says Qiang Li of Brookhaven National Laboratory. “Yet it produces a bulk nanocomposite of a well-known industrial material with superior thermoelectric properties.” The team will work on further improvements in the materials’ properties, Ren told Materials Today. “At the same time, we will push efficient cooling devices and power-generation systems to market as soon as possible through GMZ Energy, a company we founded in 2007.” Jonathan Wood
Si nanophotonic switch brings on-chip optical networks closer OPTICAL MATERIALS Scientists at IBM’s T. J. Watson Research Center have built the world’s smallest nanophotonic switch, an essential component if signals between microprocessors on a chip are to be routed using light pulses and optical networks [Vlasov et al., Nat. Photonics (2008) 2, 242]. Greater computing performance can be achieved by increasing the number of microprocessor cores on a single chip. Current multicore microprocessors use Cu wires to transmit information between them. But electrical interconnects use too much power and cannot handle the necessary amount of information to be able to put tens or even hundreds of cores on a chip. “Using photons instead of electrons to connect different cores of a computer processor provides enormous opportunities in performance improvement,” says coauthor Fengnian Xia. “There are also lots of challenges, such as development of ultracompact Si photonic devices, dense integration of photonics and electronics, power management, and innovative network designs.”
10
One necessary component is an optical switch that can pass input optical signals into one waveguide or deflect them into a different output as required. Not only does the switch need to be a compact device to take up little room on the chip, it should be able to route a huge amount of data in different wavelength channels simultaneously and deal with the temperature fluctuations that occur in chips. The IBM team lead by Yurii Vlasov has demonstrated a Si optical switch that ticks all of these boxes. Their 40 µm x 12 µm device consists of five Si microring resonators with coupling coefficients that have been carefully controlled. Switching is achieved by tuning one of the central rings out of resonance, disrupting propagation of the optical signal through the device and redirecting it. The device can cope with temperature fluctuations of ±15°C and switch up to nine 40 Gbit/s optical channels of different wavelengths. Vlasov and colleagues expect that 25 more closely spaced channels would allow a total switch throughput of 1 Tbit/s.
MAY 2008 | VOLUME 11 | NUMBER 5
Si optical switches could ‘direct traffic’ within an on-chip optical network, routing optical messages between each processor core. (Courtesy of IBM Research.) “This new development is a critical addition in the quest to build an on-chip optical network,” says Vlasov. “In view of all the progress that this field has seen in the last few years, it looks that our vision for on-chip optical networks is becoming more and more realistic.”
Jonathan Wood