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Gong L.,Materials Science Center | Young R.J.,Materials Science Center | Kinloch I.A.,Materials Science Center | Riaz I.,University of Manchester | And 2 more authors.
ACS Nano | Year: 2012

The stress transfer between the internal layers of multilayer graphene within polymer-based nanocomposites has been investigated from the stress-induced shifts of the 2D Raman band. This has been undertaken through the study of the deformation of an ideal composite system where the graphene flakes were placed upon the surface of a polymer beam and then coated with an epoxy polymer. It is found that the rate of band shift per unit strain for a monolayer graphene flake is virtually independent of whether it has one or two polymer interfaces (i.e., with or without an epoxy top coating). In contrast, the rate of band shift is lower for an uncoated bilayer specimen than a coated one, indicating relatively poor stress transfer between the graphene layers. Mapping of the strain in the coated bilayer regions has shown that there is strain continuity between adjacent monolayer and bilayer regions, indicating that they give rise to similar levels of reinforcement. Strain-induced Raman band shifts have also been evaluated for separate flakes of graphene with different numbers of layers, and it is found that the band shift rate tends to decrease with an increase in the number of layers, indicating poor stress transfer between the inner graphene layers. This behavior has been modeled in terms of the efficiency of stress transfer between the inner graphene layers. Taking into account the packing geometry of polymer-based graphene nanocomposites and the need to accommodate the polymer coils, these findings enable the optimum number of graphene layers for the best reinforcement to be determined. It is demonstrated that, in general, multilayer graphene will give rise to higher levels of reinforcement than monolayer material, with the optimum number of layers depending upon the separation of the graphene flakes in the nanocomposite. © 2012 American Chemical Society.


News Article | December 22, 2015
Site: phys.org

NREL's scientists took a different approach to the PEC process, which uses solar energy to split water into hydrogen and oxygen. The process requires special semiconductors, the PEC materials and catalysts to split the water. Previous work used precious metals such as platinum, ruthenium and iridium as catalysts attached to the semiconductors. A large-scale commercial effort using those precious metals wouldn't be cost-effective, however. The use of cheaper molecular catalysts instead of precious metals has been proposed, but these have encountered issues with stability, and were found to have a lifespan shorter than the metal-based catalysts. Instead, the NREL researchers decided to examine molecular catalysts outside of the liquid solution they are normally studied in to see if they could attach the catalyst directly onto the surface of the semiconductor. They were able to put a layer of titanium dioxide (TiO2) on the surface of the semiconductor and bond the molecular catalyst to the TiO2. Their work showed molecular catalysts can be as highly active as the precious metal-based catalysts. Their research, "Water Reduction by a p-GaInP2 Photoelectrode Stabilized by an Amorphous TiO2 Coating and a Molecular Cobalt Catalyst," has been published in Nature Materials. Jing Gu and Yong Yan are lead authors of the paper. Contributors James Young, Nathan Neale and John Turner are all with NREL's Chemistry and Nanoscience Center. Contributor K. Xerxes Steirer is with NREL's Materials Science Center. Turner points out that although the molecular catalysts aren't as stable as the metal-based catalysts, PEC systems are shut down each evening as the sun sets. That leaves time to regenerate a molecular catalyst. "Hopefully you would not have to do that every day, but it does point to the fact that low stability but highly active catalysts could be viable candidates as a long-term solution to the scalability issue for PEC water splitting systems," Turner said. Explore further: New nanomaterials will boost renewable energy More information: Jing Gu et al. Water reduction by a p-GaInP2 photoelectrode stabilized by an amorphous TiO2 coating and a molecular cobalt catalyst, Nature Materials (2015). DOI: 10.1038/nmat4511


News Article | April 20, 2016
Site: cleantechnica.com

There may be a remarkable potential energy future for nanotubes. Researchers at the Energy Department’s National Renewable Energy Laboratory (NREL) believe finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator which captures and uses waste heat. According to press information, this research might help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as a potentially important part of renewable energy portfolios. Then there is the emerging and expanding field of energy efficiency. “There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that’s what we feel this paper does,” said Andrew Ferguson, a research scientist in NREL’s Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn. The research, “Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties,” appears in the journal Nature Energy. The research represents a collaboration between : As  reported in EurekAlert, nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. “Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible. ‘How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well.” Blackburn, a senior scientist and manager of NREL’s Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones.  His methods were critical to the research, Ferguson said. “We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes,” he said. Further information on this valuable research endeavor is available at this AAAS story.   Drive an electric car? Complete one of our short surveys for our next electric car report.   Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.  


Sureeyatanapas P.,Materials Science Center | Hejda M.,Materials Science Center | Eichhorn S.J.,Materials Science Center | Young R.J.,Materials Science Center
Composites Science and Technology | Year: 2010

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm2O3, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre-matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm3+ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm3+ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites. © 2009 Elsevier Ltd. All rights reserved.


News Article | April 5, 2016
Site: phys.org

The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios. "There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn. The research, "Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties," appears in the journal Nature Energy, and is a collaboration between NREL, Professor Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot. Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible. How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well. Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson said. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," he said. To generate highly enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were prepared on a glass substrate to create a film, which was then soaked in a solution of oxidant, triethyloxonium hexachloroantimonate (OA), a process known as "doping." Doping increases the density of charge carriers, which flow through the film to conduct electricity. The researchers found the samples that performed the best were exposed to a higher concentration of OA, but not at the highest doping levels. They also discovered an optimum diameter for a carbon nanotube to achieve the best thermoelectric performance. When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity because thermopower decreases with increasing conductivity. The researchers discovered, however, that with carbon nanotubes you can retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result is another benefit of carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity. More information: Azure D. Avery et al. Tailored semiconducting carbon nanotube networks with enhanced thermoelectric properties, Nature Energy (2016). DOI: 10.1038/nenergy.2016.33


News Article | April 5, 2016
Site: cleantechnica.com

A new National Renewable Energy Laboratory (NREL) study found that a “finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat.” This is significant because “more than half of the energy consumed worldwide is rejected primarily as waste heat.” Andrew Ferguson, a research scientist in NREL’s Chemical and Materials Science Center and a co-lead author of the paper, described some of what thermoelectric power generators must overcome. “A big challenge for organic thermoelectrics in general, including SWCNT ( Single Wall Carbon Nanotubes) based thermoelectrics, is the n-type leg, which is required for typical thermoelectric generator designs. N-type doped organic semiconductors are typically less stable than their p-type counterparts. This challenge also exists for any electronic applications that may use organic semiconductors, including applications like field-effect transistors for digital logic. “Another big challenge is rigorously comparing the transport properties within one plane of the material and providing ZT (the typical thermoelectric performance figure of merit) values where the thermal transport is measured in the same plane as the electrical conductivity and thermopower. We have done that in this study, and we are now working on ways to further decrease the in-plane thermal conductivity. “The thermoelectric generator device architecture is another challenge for organic thermoelectrics. SWCNT-based devices may be even more challenging in this regard, since they have highly anisotropic carrier transport. There will be a lot of great research in the coming years on how to engineer the appropriate architecture for different types of real-world organic thermoelectric devices. Within this overarching challenge, there are a number of separate aspects to consider: “organic thermoelectric materials are typically fabricated as thin films, requiring novel solutions to afford efficient device architectures, “the unusual device architectures require elegant strategies to fabricate the multiple n- and p-type legs required in a thermoelectric generator, and to deposit the electrical contacts to these legs, “generating high-efficiency devices in unique form factors. The latter goal represents one of the advantages of organic semiconductors, in terms of making flexible devices in unique form factors that aren’t really achievable for rigid inorganic thermoelectrics, but it is also an aspect that is still relatively uncharted territory.” There are already prototype thermoelectric generators using single wall nanotubes. “The majority of these, except for one example, use SWCNT films that have both semiconducting and metallic SWCNTs, and have power factors that are at least an order of magnitude lower than we find in our study. The one example using pure semiconducting SWCNTs has films that have power factors that are roughly 1/3 of our values. So, the technology is still at the prototype stage, but our fundamental results can help to inform strategies for dramatically improving the performance of these prototypes.” He did not want to “put an exact time frame on when devices will be in the market,” but did say they are “working on a number of strategies to further improve the thermoelectric figure of merit by increasing the power factor and decreasing the thermal conductivity.” There are still many discoveries ahead about the potential of both single wall carbon nanotubes and organic semiconductors. Andrew Ferguson and Jeffrey Blackburn, a senior scientist and manager of NREL’s Spectroscopy and Photoscience group, were the lead authors of  “Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties,” which appears in the journal Nature Energy. Their work is a collaboration between NREL, Professor Yong-Hyun Kim’s group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink’s group at the University of Denver. Image: Solutions containing carbon nanotubes of different thermoelectric materials. Photo by Dennis Schroeder / NREL   Drive an electric car? Complete one of our short surveys for our next electric car report.   Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.  


News Article | April 5, 2016
Site: www.cemag.us

​A finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat, according to researchers at the Energy Department's National Renewable Energy Laboratory (NREL). The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios. "There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," says Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn. The research, "Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties," appears in the journal Nature Energy, and is a collaboration between NREL, Professor Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot. Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible. How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well. Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson says. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," he says. To generate highly enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were prepared on a glass substrate to create a film, which was then soaked in a solution of oxidant, triethyloxonium hexachloroantimonate (OA), a process known as "doping." Doping increases the density of charge carriers, which flow through the film to conduct electricity. The researchers found the samples that performed the best were exposed to a higher concentration of OA, but not at the highest doping levels. They also discovered an optimum diameter for a carbon nanotube to achieve the best thermoelectric performance. When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity because thermopower decreases with increasing conductivity. The researchers discovered, however, that with carbon nanotubes you can retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result is another benefit of carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity. The investigation conducted by the NREL researchers was funded by a grant from its Laboratory Directed Research and Development program, while the development of the semiconducting SWCNT separations was funded by the Solar Photochemistry Program within the Energy Department's Office of Basic Energy Sciences. NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC. Source: National Renewable Energy Laboratory


News Article | April 5, 2016
Site: www.rdmag.com

The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios. "There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn. The research, "Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties," appears in the journal Nature Energy, and is a collaboration between NREL, Prof.Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot. Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible. How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well. Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson said. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," he said. To generate highly enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were prepared on a glass substrate to create a film, which was then soaked in a solution of oxidant, triethyloxonium hexachloroantimonate (OA), a process known as "doping." Doping increases the density of charge carriers, which flow through the film to conduct electricity. The researchers found the samples that performed the best were exposed to a higher concentration of OA, but not at the highest doping levels. They also discovered an optimum diameter for a carbon nanotube to achieve the best thermoelectric performance. When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity because thermopower decreases with increasing conductivity. The researchers discovered, however, that with carbon nanotubes you can retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result is another benefit of carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity.


News Article | April 6, 2016
Site: www.nanotech-now.com

Abstract: A finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat, according to researchers at the Energy Department's National Renewable Energy Laboratory (NREL). The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios. "There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn. The research, "Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties," appears in the journal Nature Energy, and is a collaboration between NREL, Professor Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot. Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible. How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well. Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson said. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," he said. To generate highly enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were prepared on a glass substrate to create a film, which was then soaked in a solution of oxidant, triethyloxonium hexachloroantimonate (OA), a process known as "doping." Doping increases the density of charge carriers, which flow through the film to conduct electricity. The researchers found the samples that performed the best were exposed to a higher concentration of OA, but not at the highest doping levels. They also discovered an optimum diameter for a carbon nanotube to achieve the best thermoelectric performance. When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity because thermopower decreases with increasing conductivity. The researchers discovered, however, that with carbon nanotubes you can retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result is another benefit of carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity. ### The investigation conducted by the NREL researchers was funded by a grant from its Laboratory Directed Research and Development program, while the development of the semiconducting SWCNT separations was funded by the Solar Photochemistry Program within the Energy Department's Office of Basic Energy Sciences. About National Renewable Energy Laboratory NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | April 8, 2016
Site: www.materialstoday.com

A finely-tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat to generate electricity, according to researchers at the US Department of Energy's National Renewable Energy Laboratory (NREL). This research could help guide the development of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is ultimately lost as waste heat, thermoelectric power generation is emerging as a potentially important future renewable energy technology. "There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author with Jeffrey Blackburn of a paper in Nature Energy. This work is a collaboration between NREL, Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology and Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry and Sarah Guillot. Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. But inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible or irregularly shaped, because they are often heavy and lack the required flexibility. By contrast, carbon nanotubes, which are organic, are lighter and more flexible. How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or semiconducting, and both types are produced simultaneously in current SWCNT synthesis processes. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT affects the thermoelectric performance as well. Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has built up quite a bit of expertise in separating semiconducting nanotubes from metallic ones. and his methods were critical to the research. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," said Ferguson. To generate highly-enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were then prepared on a glass substrate to create a film, which was soaked in a solution of the oxidant triethyloxonium hexachloroantimonate (OA) as a doping step. Doping increases the density of charge carriers that flow through the film to conduct electricity. The researchers found the samples that performed best were exposed to higher concentrations of OA, but not the highest. They also discovered an optimum diameter for the carbon nanotubes that ensured the best thermoelectric performance. When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity, because thermopower decreases with increasing conductivity. The researchers discovered, however, that the carbon nanotube films could retain large thermopowers even at very high electrical conductivities. Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result represents another benefit of using carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity. This story is adapted from material from NREL, 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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