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Condensed-Matter and Materials Physics: The Science of the World Around Us 3 How Will the Energy Demands of Future Generations Be Met? The availability of affordable and renewable energy sources represents one of the greatest challenges that will face humankind in the 21st century. In the United States, there is a pressing need to develop energy sources in order to reduce dependency on fossil fuels while minimizing carbon emissions and other sources of harm to the environment. Promising technologies for solar energy, hydrogen fuel cells, solid-state lighting, supercapacitors, rechargeable batteries, and improved nuclear power will play critical roles, but fundamentally new scientific approaches are also needed. To meet U.S. needs, many profound scientific challenges must be addressed with urgency. Condensed-matter and materials physics (CMMP) is uniquely positioned to address these challenges, which require better understanding of energy conversion and storage as well as new technologies for increasing end-use energy efficiency. How can sunlight be converted to usable energy more efficiently? In what new ways can hydrogen be generated and stored? Can renewable, affordable, and benign fuels be developed? How can new approaches and new materials be used to create better light-emitting diodes (LEDs) and light-conversion materials? Discovering and understanding new materials will be key. No single strategy will provide all the answers, and some approaches may take decades to come to fruition, so research investment over a broad front is needed to meet this immense challenge. What is certainly clear, however, is that new materials, nanoscience, and new theoretical approaches will play a critical role in overcoming many of the technical barriers to achieving energy security.
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Condensed-Matter and Materials Physics: The Science of the World Around Us SETTING THE CONTEXT For the past half century, U.S. economic growth has been based partly on the availability of plentiful and cheap energy. The United States, with only 5 percent of the world’s population, is now consuming 25 percent of the world’s oil production.1 However, with oil production in many countries now in decline and demand predicted to double during the next two or three decades, the era of cheap oil is over. The same is true of natural gas. Coal could provide a solution, since the United States has 25 percent of the world’s proven supplies, but increasing U.S. dependence on coal for electricity generation could have unfavorable environmental consequences unless it is accompanied by carbon capture and storage. Indeed, there is compelling evidence that increasing emissions of anthropogenic greenhouse gases are leading to global warming and concomitant stress on the environment. As a consequence, whereas science in the second half of the 20th century focused its attention on launching the information age and in employing the revolution in molecular biology to design new classes of medical therapeutics and to solve the human genome, it is expected that during the first half of the 21st century, science will be called on to help address the massive task of transforming the global energy supply from fossil fuels to renewable sources. Many of the issues are political and/or economic in nature and therefore lie outside the scope of this report, but for the transition to renewable energy sources to occur on the required timescale, many scientific and technological breakthroughs will be necessary. These are global challenges, of course, but as the world’s most prolific energy consumer, the United States bears a responsibility to demonstrate leadership. Furthermore, with a growing energy deficit (Figure 3.1), the U.S. economy and quality of life are highly vulnerable to these inexorable trends. The role of CMMP scientists in the energy challenge is to create more technical options for lawmakers to consider for the U.S. energy portfolio. The CMMP community is already contributing strongly to the technologies surrounding the energy sector, and it is well positioned to do so even more in the future. For example, CMMP will be able to exploit some of the exciting developments in nanotechnology in order to solve some of the most pressing problems (see Chapter 6). This can only happen, however, if there is a national commitment to meeting this energy challenge and if sufficient funding is made available. Both the urgent need to address this challenge in the coming decade and the scientific opportunity for CMMP to contribute to the solution motivate the choice of energy as a grand challenge by the Committee on CMMP 2010. Further, energy research offers an opportunity for answering basic questions about materials and advancing a fun- 1 For more information on international petroleum (oil) consumption, see http://www.eia.doe.gov/emeu/international/oilconsumption.html; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 3.1 Total U.S. energy flow, 2006 (quadrillion British thermal units [Btu]), showing input energy sources (production) at left (including imports at lower left) and output (end-use) sectors at right. SOURCE: Energy Information Administration, Annual Energy Review 2006, available at http://www.eia.doe.gov/emeu/aer/pdf/aer.pdf.
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Condensed-Matter and Materials Physics: The Science of the World Around Us damental understanding of solids. Two examples of crosscutting opportunities for the coming decade include the development of new materials by design: (1) the promotion of specific energy-related technologies such as photonic crystals to enhance photovoltaic efficiency and broaden the photosensitive response range and (2) the endowment of nanomaterials with properties not found in their bulk counterparts through the use of theoretical modeling, experimental synthesis, materials characterization, and properties measurements. The latter, for example, could address the need for independent control of increased hydrogen adsorption and faster kinetics for hydrogen release in the hydrogen storage needs of a hydrogen economy. In the sections below, the committee highlights some of the ways in which the CMMP community can develop cutting-edge science that will strongly influence future developments in energy conversion, energy storage, and end-use energy efficiency. ENERGY CONVERSION For the foreseeable future, electricity will play a central role in the U.S. energy strategy on account of the extensiveness of the existing distribution network and the fact that electricity can be produced from a variety of sources, including gas, coal, oil, hydropower, and nuclear energy. The coming decades will see a greater emphasis on localized electricity generation from solar energy, biomass, thermoelectric devices, and fuel cells for both automotive and stationary use. In addition, there is already clear evidence that hybrid power systems, such as those now used increasingly in automotive applications, will become much more prevalent. In the subsections that follow, the vital role that CMMP is expected to play in energy conversion is discussed. There are several relevant energy-conversion technologies, including sunlight to electricity (photovoltaic cells), sunlight to chemical energy (the photocatalytic splitting of water to make hydrogen or the photochemical production of other chemical fuels), chemical energy to electricity (fuel cells), heat to electricity (thermoelectric devices), biomass to chemical energy (biofuels), and nuclear energy to electricity (nuclear fission and fusion). Solar Cells The ultimate source of all energy is the Sun: there are 120,000 terawatts (TW) of solar energy potential globally, in comparison with the total global energy consumption of 12.8 TW in 1998.2 To capitalize on this important renewable resource, many researchers are working toward cheaper, more efficient solar cells 2 Department of Energy, Basic Research Needs for Solar Energy Utilization, Washington, D.C., 2005. Available at http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us that are manufacturable at large scale. There is now a hierarchy of solar cells with different costs and efficiencies, ranging from very expensive triple-junction cells, with efficiencies approaching 40 percent, to the low-cost flexible systems based on conducting polymers, with power efficiencies of up to 5 percent. All of the different solar technologies pose difficult materials and design challenges. Triple-junction systems, for example, could be further improved by incorporating narrower band gap semiconductor materials that can harvest solar energy in the infrared range. Polymer-based cells are typically based on polythiophene derivatives, which act as the hole carriers, and fullerenes, which provide the electron transport. Charge separation can be achieved quite efficiently in the polymer cells, but charge transport is relatively slow, especially for the electrons, and carrier recombination prior to reaching the electrodes is a problem. Device architectures that use nanomaterials in order to reduce the likelihood of recombination are being explored, and new polymers with greater carrier mobility are being sought. An additional challenge is that many conducting polymers are unstable in the presence of light and oxygen owing to the formation of reactive oxygen radicals, so strategies for minimizing this problem are urgently needed, especially through the design of new materials. One approach for improving the cost-effectiveness of the more expensive solar cells is to use them in combination with concentrators that can harvest the sunlight from a large area and focus the incident energy onto a small area of active cells. This approach has the potential to double the efficiency of flat panel photovoltaic cells and to reduce the cost of producing solar electricity by over 50 percent. Solar concentrators normally have to be used in combination with a tracking system, however, which is not required with conventional flat panels. An even more dramatic enhancement in efficiency may emerge from a recent CMMP-related scientific breakthrough based on the production of multiple electron-hole pairs from a single solar photon within nanoparticles of PbSe. Hydrogen Generation by Photocatalysis The energy required to split water into hydrogen and oxygen is too high to be supplied by simple heating. Hydrogen can be extracted by electrolysis, although this is expensive and certainly less attractive than the direct splitting of water to generate H2 and O2, which could then be used in a fuel cell, as described below. Sunlight, however, can be harnessed to split water into hydrogen and oxygen by means of photocatalysis. Photocatalysts are typically based on semiconductors with band gaps in excess of ~1.7 electronvolts (eV); this is sufficient to drive the chemical reaction and to overcome the energy loss at the electrodes. In this context, TiO2 has been widely studied, especially as a photocatalyst for decomposing organic molecules, but it has poor efficiency for direct water splitting (<0.5 percent), in part because less than 2 percent of the solar photon flux has an energy greater than the band gap
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Condensed-Matter and Materials Physics: The Science of the World Around Us of 3.0 eV. Even the best materials at the present time, which include mixed oxides based on Bi2O3 and WO3 with band gaps of about 2.6 eV, have efficiencies of only about 2 percent. Further effort is urgently needed to develop new materials and nanoscale structures that meet the technical requirements for good photocatalysts; these requirements include excellent chemical stability, resistance to photocorrosion in the presence of light and water, and low electron-hole recombination rates. As with the solar cell developments, the use of nanomaterials may offer significant advantages in addressing some of these issues, with CMMP providing modeling and design tools for the development of advanced photocatalysts. An alternative strategy, based on the production of hydrogen and oxygen from water by using biocatalysts such as hydrogenase enzymes, is under development at the pilot-plant scale and may make its mark in the longer term. This exciting biotechnology approach will be especially attractive in the event that the hydrogen economy becomes a reality. One of the major obstacles in this area is the fact that the natural enzymes that catalyze the dissociation of water are destroyed by the very oxygen that they produce. However, progress is being made to address this problem using a combination of molecular biology approaches and computer modeling of protein/protein interactions, structure-function relationships, intramolecular gas diffusion, and metallocluster electronic structure. Fuel Cells Fuel cells, which are an integral part of the concept of renewable chemical fuels, already have an advantage over the internal combustion engine in terms of their efficiency, but to make this technology viable, substantial further increases in fuel cell efficiency are needed, as well as dramatic improvements in hydrogen generation and storage capabilities. Fuel cells present a number of difficult technical challenges to the CMMP community. There are several fuel cell formats, the most common ones being the high-temperature (>800°C) solid oxide fuel cell (SOFC) and the low-temperature (~75°C) polymer electrolyte membrane (PEM) fuel cell. SOFCs are ideally suited for large-scale electricity generation in the megawatt range, whereas the PEM systems are appropriate for 1 kilowatt (kW) to 100 kW applications, such as in automotive power. Both types of fuel cell present challenges, including mechanical breakdown of SOFCs under extended high-temperature operation and the durability of the electrolytes in PEM cells. Nafion, a sulfonated tetrafluoroethylene copolymer, is widely used as the proton conductor, but its proton transport kinetics is impaired at higher operating temperatures owing to dehydration. A recently developed photochemically cured PEM coming out of CMMP basic research, however, appears to offer greatly enhanced performance at higher temperatures (Figure 3.2).
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 3.2 (Left) New photochemically cured liquid polymer electrolyte membrane (PEM) having higher acid content and therefore higher electrical conductivity (σ) and greater power output. (Right) Lithographically patterned “liquid PEMs” lead to higher surface area of the membrane and the catalyst and therefore to higher power output. SOURCE: Joseph M. DeSimone, University of North Carolina at Chapel Hill. Further advances in membrane materials development could be expected from CMMP efforts. One of the cost barriers stemming from low operating temperatures arises from the need to use platinum coatings at the electrodes in order to help dissociate hydrogen or to reform hydrocarbons to produce hydrogen in situ. The development of inexpensive catalysts to replace platinum would have a significant impact, and further CMMP research is needed in this area. Theoretical models have recently shown that catalysts made from Pt-Ni alloys, for example, require less platinum and achieve improved performance. Thermoelectrics The physics behind thermoelectricity is the basis for the interconversion between thermal and electrical energy and for the controlled cooling and heating of materials. Several fundamental breakthroughs in this field starting in the early 1990s led to the development of bulk materials with nanostructured constituents, as well as to an intellectual framework for the control of the nanostructures themselves to enhance their thermoelectric performance. Achievements using these two different approaches led to a doubling of the thermoelectric performance over the
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Condensed-Matter and Materials Physics: The Science of the World Around Us next decade, as the two approaches came together. Bulk thermoelectric materials have now been made using Si and Ge nanoparticles, which may be compacted into centimeter-size nanocomposites whose properties may be controlled by the processing parameters and guided by theoretical modeling considerations. The introduction of size as another system parameter facilitates the enhancement of thermoelectric performance through a simultaneous decrease in the thermal conductivity, which is phonon- and interface-dominated, and an increase in the power factor, which is dominated by electrons and holes and their interaction with phonons in the interconversion process of thermal and electrical energy. A significant remaining challenge is to develop a suite of nanothermoelectric materials that can operate efficiently over the wide temperature ranges desired for power conversion for energy-related industries, cooling (and heating) for the optoelectronics industry, power generation applications for space vehicle propulsion, energy harvesting from the waste heat in cars, and the utilization of geothermal energy. Biofuels The production of so-called biofuels from renewable biomaterials, such as palm oil and sugar, is attractive from many perspectives. There are two important strategies in this area. In the first, vegetable oils that are based on the triglyceride esters of long-chain fatty acids can be transesterified to yield the corresponding methanol esters and glycerol. These methanol esters are sufficiently similar to conventional diesel fuel that they can be mixed with oil-derived diesel in virtually any ratio and burned in a conventional diesel engine. This biodiesel approach is becoming popular in Europe, where many countries have no oil or gas reserves and diesel is already widely used as an automotive fuel. Countries such as Indonesia and Malaysia, producers of vegetable oils for use in foods and personal care products, are gearing up to increase their output, while several oil companies are constructing plants for biodiesel production. The second strategy involves the production of ethanol from sugarcane or other agricultural products. The fermentation technology for converting sugar into ethanol has been extensively exploited in Brazil, where ethanol accounts for about 40 percent of automotive fuel, and where many automobiles have flexible engines that can run on gasoline-ethanol mixtures in any ratio.3 In the United States, ethanol is used as a fuel additive at the 10 to 15 percent level, and its use is likely to increase sharply in the near future. Most of the ethanol in the United States is made from corn rather than sugar, which has directly impacted the price of animal feed. Many of the issues in the biofuels area (such as the evaluation of alternative crops, the impact of biofuels on food prices, and the question of land use and its 3 The Aspen Institute, A High Growth Strategy for Ethanol, Washington, D.C., 2006.
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Condensed-Matter and Materials Physics: The Science of the World Around Us environmental ramifications) lie outside the domain of this report. In terms of the technologies that are required for the development of biofuels, however, CMMP researchers can play an important role in a number of materials issues. These include the development of improved processes for converting less-expensive cellulosic biomass into ethanol or another liquid fuel, the development of better catalysts for the remediation of emissions from diesel engines (thus enabling the wider use of biodiesel fuels), and the structural characterization with synchrotron x-rays of the enzymes that are used for enzymatic hydrogen production. Furthermore, the development of biofuels presents many excellent opportunities for collaborations between CMMP and the biosciences. Nuclear Energy Conversion Concerns over energy resource availability, climate change, air quality, and energy security suggest that nuclear power technologies may play an important role in the future. The CMMP community has an opportunity to contribute to the development of advanced materials for use in the extreme environments found in fission and fusion reactors. Although the technology for electricity production from nuclear energy is already well developed, the United States has joined a multinational initiative aimed at designing the next generation of power reactors, the so-called Generation IV systems. The technology roadmap for this initiative is aimed at designing systems that address a number of societal concerns, including sustainability, economics, safety, and reliability, as well as issues relating to security and proliferation. The materials-related challenges associated with the various reactor designs under consideration are discussed in A Technology Roadmap for Generation IV Nuclear Energy Systems.4 Research areas of great opportunity include the transmutation of intermediate-lived actinides into shorter-lived or stable nuclides by means of irradiation, the development of materials with good temperature stability combined with superior resistance to fast neutron fluxes and fluences, the design of novel refractory fuel concepts with enhanced fission product retention capability, and the development of advanced glassy materials for nuclear waste disposal applications. Some of the proposed designs are able to produce either electricity or chemical fuels such as hydrogen or both. The projected limitations on the availability of uranium ores are a consideration that will stimulate CMMP research in the area of partially or fully closed fuel-cycle systems that are enabled by the recovery 4 U.S. Department of Energy Nuclear Energy Research Advisory Committee and the Generation IV International Forum, 2002. Available at http://nuclear.energy.gov/genIV/documents/gen_iv_roadmap.pdf; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us of the heavy, long-lived radioactive elements. The closed-fuel-cycle approach also reduces the problem of radioactive waste disposal. Energy from nuclear fusion is also an important technology option for the future, and the international ITER project will provide a number of important challenges in the CMMP area. In particular, materials in the vicinity of the plasma will be subjected to extreme conditions of temperature and neutron irradiation that can lead to mechanical degradation and activation. Tritium contamination could also be a problem. Materials that minimize these challenges will have to be developed, and there is a likely need for a dedicated materials-testing facility to assist with the development of such advanced materials. ENERGY STORAGE In addition to energy generation, energy storage represents an important and challenging element of the overall energy strategy. The principal means of storing energy at the present time include the use of batteries; chemical storage in high-energy-density synfuels such as hydrogen, ethanol, and methane, ideally produced in a renewable manner as discussed in the previous subsections; and the use of supercapacitors. Batteries Rechargeable lithium batteries have had a major impact on consumer electronics, such as cellular telephones and laptop computers. They typically employ oxide cathodes (such as LiCoO2), carbon-based anodes, and electrolytes that afford efficient lithium transport. Energy density is a critical parameter, since it determines both the storage capacity and the weight of a battery. The output voltage is another critical parameter. Rechargeable lithium batteries typically deliver about 3.7 V, compared with ~1.2 V for conventional nickel-based batteries. Among the many materials challenges is the need to develop superior cathodes with a combination of good thermodynamic properties, which give high energy densities and sufficiently fast kinetics at the desired temperature. Such materials must exhibit good electronic and high ionic conductivities in combination with a sufficiently high redox couple. A variety of electrolytes have been developed, including liquids, solid polymers, and solid inorganic materials that are suited for various purposes. Recent work has shown, rather surprisingly, that crystalline polymer electrolytes can be superior to non-crystalline ones in certain instances. Major improvements in kinetics, however, are most likely to be achieved through nanostructuring. For example, the resiliency of nanostructured, multiwalled carbon nanotubes, when used as a filler, prevents the structural degradation of graphitic crystallites during the expansion/contraction processes that occur in the graphitic electrodes during
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Condensed-Matter and Materials Physics: The Science of the World Around Us the charging/discharging cycles. The high electrical conductivity, resilient elastic properties, and high surface-to-volume ratios improve the battery performance and significantly increase battery lifetimes. Here CMMP scientists can play a major role in both the modeling and the materials design of advanced battery systems, especially through collaborative efforts with materials chemists. Hydrogen Storage The efficient storage of hydrogen for fuel cell applications represents one of the major challenges for the hydrogen economy. The goal of the Department of Energy for the year 2015 is to achieve 9.0 percent (by weight) hydrogen storage at reasonable pressures and ambient temperatures.5 The ideal medium for automotive applications would be a storage material, probably a solid or a liquid, that could reversibly adsorb and desorb hydrogen without the physical removal of the storage medium from the vehicle. The most common storage media are porous solids that adsorb hydrogen through a physisorption mechanism and metal hydrides or chemical hydrides that bind hydrogen through a chemisorption mechanism. Porous materials that store hydrogen by physisorption include inexpensive activated carbons, aluminosilicate zeolites, and hybrid metal-organic framework materials. Physisorptive host-hydrogen interactions, however, are intrinsically weak (~5 kilojoules per mole [kJ/mol]), so these materials will only store significant amounts of hydrogen at cryogenic temperatures. The hydrides, by contrast, will bind hydrogen strongly but in some cases have too high a mass density for automotive applications or suffer from poor release kinetics at ambient temperatures. CMMP researchers who have recently entered this field are already impacting the modeling and design of new materials and the development of advanced measurement techniques to study and control hydrogen adsorption and desorption mechanisms. There is much current interest in chemical hydrides, such as borohydrides (e.g., NaBH4); these have excellent gravimetric and volumetric storage capacities, but many chemical hydrides suffer from poor reversibility. It is very expensive to regenerate borohydride from its oxidation products, so fuel cells using liquid storage media may be good alternatives to rechargeable lithium batteries for portable applications, but they are unlikely to be economical for large-scale applications such as automotive use. A series of recent developments have stimulated considerable interest in storage media that can bind hydrogen more strongly by chemisorbing dihydrogen, without dissociation, at under-coordinated metal sites in porous inorganic or hybrid metal- 5 Department of Energy, Basic Research Needs for the Hydrogen Economy, Washington, D.C., 2003. Available at http://www.sc.doe.gov/bes/reports/files/NHE_rpt.pdf; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us organic framework materials. These materials have heats of adsorption of ~10 kJ/mol, approximately double those associated with conventional physisorption, and can therefore retain hydrogen at much higher temperatures. Further work is needed in order to create new materials that contain large numbers of such binding sites and that can operate in the desirable temperature range for specific applications. CMMP researchers are collaborating strongly with materials chemists and materials scientists in the modeling and design of new metal-organic framework materials suitable for hydrogen storage. Many other gas storage and separation technologies are closely related to the hydrogen storage problem. Important examples include methane storage for fuel applications and carbon dioxide disposal in the context of carbon sequestration and storage. The latter will become extremely important as increasing amounts of coal are used to produce electricity. Supercapacitors In the past decade, supercapacitors in the form of electric double layer capacitors (EDLCs) with high charge-storage capacity have been actively studied as an attractive energy storage device to replace batteries for some applications. In particular, EDLCs are able to store and deliver energy rapidly and efficiently. The EDLCs have a long life cycle because of their simple charge separation mechanism, which uses highly porous carbons as the electrode material. Their widely scalable storage capabilities make it possible to hybridize EDLCs with other energy storage devices, such as batteries and fuel cells. Even though EDLCs are now being used in various types of electronic device applications, such as for memory backup in vehicle computers, their intrinsically low energy density limits their impact on the energy storage market. The present challenge is to increase energy storage density and to lower fabrication costs through the optimization of the cell design and the development of improved electrode materials. CMMP has the opportunity to make key contributions to the development of advanced nanocarbon materials for model systems and for specific applications to fuel cells, hybrid electrical vehicles, and portable electronics. It is expected that the market for supercapacitors with enhanced performance will expand rapidly in the next decade. END-USE ENERGY EFFICIENCY In addition to recognizing the role of energy conversion and storage, it is important not to underestimate the role of conservation in the global energy challenge. As discussed in Chapter 8, the per capita consumption of energy in the United States is approximately double that in most other advanced countries. While there are good reasons why consumption in the United States may be some-
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Condensed-Matter and Materials Physics: The Science of the World Around Us what higher than that in, say, Europe, this fact also points to the largely untapped potential of conservation in meeting the nation’s energy needs. The following subsections address some of the specific CMMP-related technology areas where this might be achieved. Solid-State Lighting Among the newly emerging end-use energy-efficiency technologies, solid-state lighting deserves special attention because it was in its infancy at the time of the last decadal review of CMMP and remarkable progress has been made since then. The discovery of blue light-emitting diodes in 1994 facilitated a large number of developments in the lighting and displays area. White lighting systems based on blue InGaN LEDs operating at about 460 nanometers (nm) in combination with yellow phosphors using Ce3+-garnets give a reasonable quality of white light that is good enough for off-grid lighting applications, flashlights, traffic lights, and automotive lighting (Figure 3.3). CMMP has played an important role in establishing the science base for LEDs from their discovery until the present time. FIGURE 3.3 Emerging applications of solid-state lighting: (a) landscape lighting, (b) roadway lighting, and (c) traffic signals. SOURCE: Philips Lighting.
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Condensed-Matter and Materials Physics: The Science of the World Around Us The efficiency of the blue LEDs has improved to the point that solid-state lighting is comparable to fluorescent lighting and has the potential to be two to three times superior in converting electrical power (watts, W) into light (lumens, lm) (see Figure 1.2 in Chapter 1). The theoretical maximum for solid-state lighting is about 300 lm/W, but 200 lm/W represents a more realistic goal for practical applications. Lighting consumes approximately 22 percent of the electricity that is generated in the United States, and solid-state lighting has the potential to reduce energy consumption for lighting requirements by about 60 percent by 2025.6 Success in this area would therefore reduce electricity consumption in the United States overall by around 13 percent over the next 20 years, making it a very important element of the nation’s energy strategy and virtually guaranteeing that solid-state lighting will emerge as a multibillion-dollar industry worldwide. In addition, when combined with solar panels, the solid-state lighting approach can provide lighting for homes and villages in remote, off-grid locations in developing countries (Figure 3.4). Among the technical challenges for CMMP, improvement in LED efficiency is, of course, critical, and depends on factors such as the reduction of the defect densities in the semiconductors. The external quantum efficiencies of the InGaN-based blue LEDs have improved dramatically in the past decade, from less than 5 percent when they were first developed in the mid-1990s to around 66 percent today.7 Green LEDs, however, which are required for lighting applications using red, green, and blue LEDs, are much less efficient than the blue and red ones, so further effort is required in this area. Other challenges in solid-state lighting are numerous and varied. Systems based on blue LEDs in combination with yellow phosphors are very simple and cost-effective, but the quality of the light is insufficient for residential and commercial lighting applications owing to poor color rendering. The light quality will be improved in the next generation of white LEDs by using a blue LED in combination with both a green and a red phosphor, a configuration that fills the visible spectrum in a more pleasing manner. However, the blue to red down-conversion with the currently available materials is relatively inefficient, in part due to the large downshift in frequency from coupling to lattice vibrations, so new materials are currently needed. Other obstacles include the poor performance of the polymer or glass encapsulants that are used to disperse and protect the phosphors and the losses associated with poor light extraction. While new polymer or glass compositions are required to solve the encapsulation problem, nanostructured optical band gap materials may solve the latter. Other challenges are likely to stem from 6 Department of Energy, Basic Research Needs for Solid-State Lighting, Washington, D.C., 2006. Available at http://www.sc.doe.gov/bes/reports/files/SSL_rpt.pdf; last accessed September 17, 2007. 7 Department of Energy, Basic Research Needs for Solid-State Lighting, Washington, D.C., 2006. Available at http://www.sc.doe.gov/bes/reports/files/SSL_rpt.pdf; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 3.4 Applications of solid-state lighting in developing countries. The system comprises two modular 1-watt white light-emitting diode lamps; a 12-volt, 7-ampere-hour sealed lead acid battery (rechargeable and maintenance free); and a 5-watt solar panel. The system has a 100,000-hour lifetime. SOURCE: Courtesy of Light Up the World Foundation. a shift toward ultraviolet rather than blue LEDs, since this will require yet a new generation of phosphors. In parallel with the InGaN-based lighting development, there will be progress in making lighting and display systems using organic and polymer-based LEDs. While these may not match the lifetimes of the inorganic materials, which are aiming for 100,000 hours of continuous operation, they are likely to be much cheaper and will be preferred for area illumination requiring large emissive surfaces. Smart Windows The development of smart windows based on novel nanocomposites represents an entirely different opportunity for end-use energy efficiency and an area in which CMMP is already playing an important role. For example, inorganic nanomaterials
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Condensed-Matter and Materials Physics: The Science of the World Around Us with band gaps in the near infrared (IR) could be added to polymer or glass windows in order to reduce the transmission of heat. Even more attractive, the use of switchable IR filters—based, for example, on thermochromic materials that show semiconductor-to-metal transitions when heated above room temperature—would enable the development of passive, smart windows for energy-efficiency applications. Nanoparticles of vanadium dioxide would be suitable for this application, since they would be optically transparent in the visible range and have an insulator-to-metal transition at 67°C. Other Energy Conservation Opportunities There are many other areas in which CMMP can contribute to energy conservation. One of the most intriguing but elusive of these is the use of high-temperature superconductors to reduce losses from power transmission and increase the grid capacity. A recent report from the Department of Energy8 argues that the electric power grid is in a critical condition and is likely to worsen as the demand increases. Transmission losses in the grid are now almost 10 percent. Superconductivity offers powerful new opportunities for restoring the reliability of the power grid and improving its capacity and efficiency. This will require the discovery of a new generation of ductile superconductors that have higher critical temperatures, sustain higher currents, and are cheaper to fabricate. The search for a room-temperature superconductor is certainly one of the elusive “holy grails” for CMMP, as is the quest for a better understanding of the mechanism of high Tc superconductivity. Other, more immediate possibilities include improving the power efficiency of computers by inventing new materials for the current computing paradigms and by developing new computing paradigms based on optical signaling and low-power computing. Improvements in power supplies would also have an impact. For example, present power supplies run at 60 to 70 percent efficiency levels in converting alternating current to low-voltage direct current. It is estimated that increasing the efficiency to 80 percent would save more than 1 percent of the electricity used in the United States.9 In the area of new materials, CMMP researchers are contributing to the development of plant-based plastics (bioplastics) for potential use in markets such as the car industry; however, the low strength and durability of bioplastics have proven their application to be challenging. Additionally, CMMP researchers are working on new materials for buildings that incorporate embedded sensors, self- 8 Department of Energy, Basic Research Needs for Superconductivity, Washington, D.C., 2006. Available at http://www.sc.doe.gov/bes/reports/files/SC_rpt.pdf; last accessed September 17, 2007. 9 S. Ashley, “Power-Thrifty PCs,” Sci. Am. 290, 31-32 (2004).
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Condensed-Matter and Materials Physics: The Science of the World Around Us healing composites, and responsive materials for inexpensive monitoring and self-diagnostic capabilities. CONCLUSIONS While it is hard to exaggerate the seriousness of the global energy challenge, the required sense of urgency in U.S. society with respect to this challenge is not yet apparent. There are no quick or singular solutions to meeting the growing global energy requirements; the cost-effectiveness of solar energy remains unresolved, the efficiency of the photochemical splitting of water is still very poor, fuel cells suffer from longevity problems, bioethanol and biodiesel fuels compete with the food chain, and society has still not embraced nuclear energy. A sustained effort on a broad range of options is therefore required. The aggressive development of renewable energy sources must be accompanied by an equally determined effort to reduce energy consumption and waste across all sectors. The Committee on CMMP 2010 believes that a substantial increase in funding for energy-related education and research would set the United States on the path to solving this technological challenge. Priority research areas should include photovoltaic cells, fuel cells, hydrogen generation and storage, thermoelectrics, catalysis, nuclear power, solid-state lighting, and batteries. Crosscutting areas benefiting a variety of energy-related options include new materials development, catalysis science, membrane design, nanoscience advances, and advances in materials design and the modeling of materials properties. New materials will be particularly important in meeting this challenge, and the committee looks forward to the recommendations of the current National Research Council study on “Assessment of and Outlook for New Materials Synthesis and Crystal Growth” for realizing this need.