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Scientists Confirm Existence of Superheavy Element 114

admin — Mon, 05 Oct 2009 03:23:43 +0000

Berkeley, CA – Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have been able to confirm the production of the superheavy element 114, ten years after a group in Russia, at the Joint Institute for Nuclear Research in Dubna, first claimed to have made it. The search for 114 has long been a key part of the quest for nuclear science’s hoped-for Island of Stability.

Heino Nitsche, head of the Heavy Element Nuclear and Radiochemistry Group in Berkeley Lab’s Nuclear Science Division (NSD) and a professor of chemistry at the University of California at Berkeley, and Ken Gregorich, a senior staff scientist in NSD, led the team that independently confirmed the production of the new element, which was first published by the Dubna Gas Filled Recoil Separator group.

Using an instrument called the Berkeley Gas-filled Separator (BGS) at Berkeley Lab’s 88-Inch Cyclotron, the researchers were able to confirm the creation of two individual nuclei of element 114, each a separate isotope having 114 protons but different numbers of neutrons, and each decaying by a separate pathway.

“By verifying the production of element 114, we have removed any doubts about the validity of the Dubna group’s claims,” says Nitsche. “This proves that the most interesting superheavy elements can in fact be made in the laboratory.”

The realm of the superheavy

Elements heavier than uranium, element 92 – the atomic number refers to the number of protons in the nucleus – are radioactive and decay in a time shorter than the age of Earth; thus they are not found in nature (although traces of transient neptunium and plutonium can sometimes be found in uranium ore). Elements up to 111 and the recently confirmed 112 have been made artificially – those with lower atomic numbers in nuclear reactors and nuclear explosions, the higher ones in accelerators – and typically decay very rapidly, within a few seconds or fractions of a second.

Beginning in the late 1950s, scientists including Gertrude Scharff-Goldhaber at Brookhaven and theorist Wladyslaw Swiatecki, who had recently moved to Berkeley and is a retired member of Berkeley Lab’s NSD, calculated that superheavy elements with certain combinations of protons and neutrons arranged in shells in the nucleus would be relatively stable, eventually reaching an “Island of Stability” where their lifetimes could be measured in minutes or days – or even, some optimists think, in millions of years. Early models suggested that an element with 114 protons and 184 neutrons might be such a stable element. Longtime Berkeley Lab nuclear chemist Glenn Seaborg, then Chairman of the Atomic Energy Commission, encouraged searches for superheavy elements with the necessary “magic numbers” of nucleons.

“People have been dreaming of superheavy elements since the 1960s,” says Gregorich. “But it’s unusual for important results like the Dubna group’s claim to have produced 114 to go unconfirmed for so long. Scientists were beginning to wonder if superheavy elements were real.”

To create a superheavy nucleus requires shooting one kind of atom at a target made of another kind; the total protons in both projectile and target nuclei must at least equal that of the quarry. Confirming the Dubna results meant aiming a beam of 48Ca ions – calcium whose nuclei have 20 protons and 28 neutrons – at a target containing 242Pu, the plutonium isotope with 94 protons and 148 neutrons. The 88-Inch Cyclotron’s versatile Advanced Electron Cyclotron Resonance ion source readily created a beam of highly charged calcium ions, atoms lacking 11 electrons, which the 88-Inch Cyclotron then accelerated to the desired energy.

Four plutonium oxide target segments were mounted on a wheel 9.5 centimeters (about 4 inches) in diameter, which spun 12 to 14 times a second to dissipate heat under the bombardment of the cyclotron beam.

“Plutonium is notoriously difficult to manage,” says Nitsche, “and every group makes their targets differently, but long experience has given us at Berkeley a thorough understanding of the process.” (Experience is especially long at Berkeley Lab and UC Berkeley – not least because Glenn Seaborg discovered plutonium here early in 1941.)

When projectile and target nuclei interact in the target, many different kinds of nuclear reaction products fly out the back. Because nuclei of superheavy elements are rare and short-lived, both the Dubna group and the Berkeley group use gas-filled separators, in which dilute gas and tuned magnetic fields sweep the copious debris of beam-target collisions out of the way, ideally leaving only compound nuclei with the desired mass to reach the detector. The Berkeley Gas-filled Separator had to be modified for radioactive containment before radioactive targets could be used.

In sum, says Gregorich, “The high beam intensities from the 88-Inch Cyclotron, together with the efficient background suppression of the BGS, allow us to look for nuclear reaction products with very small cross-sections – that is, very low probabilities of being produced. In the case of element 114, that turned out to be just two nuclei in eight days of running the experiment almost continuously.”

Tracking the isotopes of 114

The researchers identified the two isotopes as 286114 (114 protons and 172 neutrons) and 287114 (114 protons and 173 neutrons). The former, 286114, decayed in about a tenth of a second by emitting an alpha particle (2 protons and 2 neutrons, a helium nucleus) – thus becoming a “daughter” nucleus of element 112 – which subsequently spontaneously fissioned into smaller nuclei. The latter,287114, decayed in about half a second by emitting an alpha particle to form 112, which also then emitted an alpha particle to form daughter element 110, before spontaneously fissioning into smaller nuclei.

The Berkeley Group’s success in finding these two 114 nuclei and tracking their decay depended on sophisticated methods of detection, data collection, and concurrent data analysis. After passing through the BGS, the candidate nucleus enters a detector chamber. If a candidate element 114 atom is detected, and is subsequently seen to decay by alpha-particle emission, the cyclotron beam instantly shuts off so further decay events can be recorded without background interference.

In addition to such automatic methods of enhancing data collection, the data was analyzed by completely independent software programs, one written by Gregorich and refined by team member Liv Stavsetra, another written by team member Jan Dvořák.

“One surprise was that the 114 nuclei had much smaller cross sections – were much less likely to form – than the Dubna group reported,” Nitsche says. “We expected to get about six in our eight-day experiment but only got two. Nevertheless, the decay modes, lifetimes, and energies were all consistent with the Dubna reports and amply confirm their achievement.”

Says Gregorich, “Based on the ideas of the 1960s, we thought when we got to element 114 we would have reached the Island of Stability. More recent theories suggest enhanced stability at other proton numbers, perhaps 120, perhaps 126. The work we’re doing now will help us decide which theories are correct and how we should modify our models.”

Nitsche adds, “During the last 20 years, many relatively stable isotopes have been discovered that lie between the known heavy element isotopes and the Island of Stability – essentially they can be considered as ‘stepping stones’ to this island. The question is, how far does the Island extend – from 114 to perhaps 120 or 126? And how high does it rise out the Sea of Instability.”

The accumulated expertise in Berkeley Lab’s Nuclear Science Division; the recently upgraded Berkeley Gas-filled Separator that can use radioactive targets; the more powerful and versatile VENUS ion source that will soon come online under the direction of operations program head Daniela Leitner – all add up to Berkeley Lab’s 88-Inch Cyclotron remaining highly competitive in the ongoing search for a stable island in the sea of nuclear instability.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at http://www.lbl.gov.

Solution Grown Nano Crystal Contacts for Solar PV Cells

admin — Wed, 23 Sep 2009 20:55:25 +0000

Berkeley, CA – In a development that holds much promise for the future of solar cells made from nanocrystals, and the use of solar energy to produce clean and renewable liquid transportation fuels, researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have reported a technique by which the electrical conductivity of nanorod crystals of the semiconductor cadmium-selenide was increased 100,000 times.

“The key to our success is the fabrication of gold electrical contacts on the ends of cadmium-selenide rods via direct solution phase-growth of the gold tips,” says Paul Alivisatos, interim-Director of Berkeley Lab, who led this research. “Solution-grown contacts provide an intimate, abrupt nanocrystal-metal contact free of surfactant, which means that unlike previous techniques for adding metal contacts, ours preserves the intrinsic semiconductor character of the starting nanocrystal.”

Image (a) is a transmission electron micrograph of a cadmium-selenide nanocrystal before gold tip growth in solution and image (b) is after tips have been added. Image (c) is a scanning electron micrograph of a single nanocrystal two-terminal device.

Berkeley

Alivisatos is a chemist who holds joint appointments with Berkeley Lab’s Materials Sciences Division, and with the University of California-Berkeley where he is the Larry and Diane Bock professor of Nanotechnology. He is an internationally-recognized authority on nanocrystal growth and the corresponding author of a paper published in the on-line edition of NanoLetters entitled: “Enhanced Semiconductor Nanocrystal Conductance via Solution Grown Contacts.”

Co-authoring the paper with Alivisatos were Matthew Sheldon and Paul-Emile Trudeau, members of Alivisatos’ research group; Taleb Mokari, of Berkeley Lab’s Molecular Foundry; and Lin-Wang Wang, in Berkeley Lab’s Computational Research Division.

With the world demand for energy projected to more than double by 2050 and more than triple by the end of the 21st century, it is imperative that sustainable and carbon-neutral energy technologies be developed. The use of sunlight to generate electricity as well as to split water molecules for the production of fuels is envisioned as an ideal energy source, and nanocrystals could be pivotal to the success of this vision. Electrical conductance in semiconductor nanocrystals is a critical element for both solar electricity and solar fuel technologies.

“Standard contacting procedures that deposit metal onto semiconductor nanocrystals directly, such as those used in commercial wafer-scale chip fabrication, cause alloying and chemical reactions at the metal-semiconductor interface,” says Sheldon, who was the lead author on the NanoLetters paper. “This means that the finished electrical device is actually made of a different material than the starting nanocrystal.”

Sheldon notes that while chemical treatments, such as etching off surfactant, have been shown to enhance the conductivity of thin film nanocrystal solids, these treatments will often alter the semiconductor’s electrical properties, for example switching the material from n-type to p-type or altering the density of surface states. Furthermore, he says, previous studies have not explained why electrical conductance was enhanced, other than acknowledging the removal of surfactant coverage.

In this new study, Sheldon, Alivisatos and their co-authors used single nanostructure electrical measurements to make systematic comparisons between cadmium-selenide nanorods with and without gold tips. The solution-grown tipping process started with the addition of gold salt to a solution of toluene and cadmium-selenide nanorods, which resulted in gold metal being selectively deposited on the nanorod tips. A silicon wafer test chip was then dipped in this nanorod solution. After submersion, the evaporation of the toulene solvent oriented individual cadmium-selenide nanorods across pre-defined gold electrodes, which were fabricated through electron beam lithography. The results were gold-tipped cadmium-selenide heterostructure devices whose electrical conductance was characterized in a two-terminal geometry as a function of source-drain voltage and temperature.

Matthew Sheldon, a member of the Paul Alivisatos research group, was part of a Berkeley Lab research team that developed a technique by which the electrical conductivity of nanorod crystals of the semiconductor cadmium-selenide was increased 100,000 times. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

Says Alivisatos, “Our study shows that the superior performance of gold-tipped cadmium-selenide heterostructures results from a lower Schottky barrier and that solution grown contacts do not alter the chemical composition of the semiconductor. Further, our work demonstrates the increasing sophistication of high-quality electrical devices that can be achieved through self-assembly and verifies this process as an excellent route to the next generation of electronic and optoelectronic devices utilizing colloidal nanocrystals.”

Adds Sheldon, “We believe our approach is an ideal strategy for making future devices from nanocrystals because it preserves the semiconductor character of the nanocrystal as synthesized with the precise control of their synthesis developed over the past decades.”

Sheldon says the next step in this work will be to determine if the dramatic improvements in electrical behavior can translate to improvements in nanocrystal-based energy production. Initially, the group plans to investigate the use of solution grown contacts in photovoltaic applications.

This research was primarily funded by the DOE Office of Science through Berkeley Lab’s Helios Solar Energy Research Center.

Phase inhomogeneity phenomenon may lead to break throughs in Superconductivity

admin — Sat, 19 Sep 2009 23:14:16 +0000

In finally answering an elusive scientific question, researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that the selective placement of strain can alter the electronic phase and its spatial arrangement in correlated electron materials.

This unique class of materials is commanding much attention now because they can display properties such as colossal magnetoresistance and high-temperature superconductivity, which are highly coveted by the high-tech industry.

Junqiao Wu, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California-Berkeley’s Department of Materials Science and Engineering, led the study in which it was demonstrated that irregularities in the micro-domain structure of correlated electron materials – a phenomenon known as “phase inhomogeneity” – can be generated by external stimuli and could be engineered at the sub-micron scale to achieve desired properties.

These optical images of a multiple-domain vanadium oxide microwire taken at various temperatures show pure insulating (top) and pure metallic (bottom) phases and co-existing metallic/insulating phases (middle) as a result of strain engineering. (Image from Junqiao Wu)

“By continuously tuning strain over a wide range in single-crystal vanadium oxide micro- and nano-scale wires, we were able to engineer phase inhomogeneity along the wires,” says Wu. “Our results shed light on the origin of phase inhomogeneity in correlated electron materials in general, and open opportunities for designing and controlling phase inhomogeneity of correlated electron materials for future devices.”

Wu is the corresponding author of a paper describing this work which was published in the journal Nature Nanotechnology and is entitled: “Strain engineering and one-dimensional organization of metal-insulator domains in single crystal VO2 beams.” Co-authoring the paper with Wu were Jinbo Cao, Elif Ertekin, Varadharajan Srinivasan, Wen Fan, Simon Huang, Haimei Zheng, Joanne Yim, Devesh Khanal, Frank Ogletree and Jeffrey Grossman.

Whereas in conventional materials, the motion of one electron is relatively independent of any other, in “correlated electron materials” quantum effects enable electrons to act collectively, like dancers in a chorus line. Emerging from this collective electronic behavior are properties such as colossal magnetoresistance, where the presence of a magnetic field increases electrical resistance by orders of magnitude, or high-temperature superconductivity, in which the materials lose all electrical resistance at temperatures much higher than conventional superconductors.

Frequently observed spatial phase inhomogeneities are believed to be critical to the collective electronic behavior of correlated electron materials. However, despite decades of investigation, the question of whether such phase inhomogeneities are intrinsic to correlated electron materials or caused by external stimuli has remained largely unanswered.

“This question is not only important for our understanding of the physics behind correlated electron materials,” says Wu, “it also directly determines the spatial scale of correlated electron material device applications.”

Junqiao Wu (sitting) led a team that included Jinbo Cao (standing) which demonstrated that phase inhomogeneity in correlated electron materials can be created through the application of external strain. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

To determine if phase inhomogeneity can be caused by external effects, Wu and his colleagues worked with vanadium oxide, a representative correlated electron material that features a metal-nonmetal transition, where in the nonmetal state its electrons can no longer carry an electrical current. After synthesizing the vanadium oxide into flexible single-crystal micro- and nanowires, the research team subjected the wires to strain by bending them to different curvatures. Different curvatures yielded different strains, and the phase transitions were measured in each of the strained areas.

“The metal-nonmetal domain structure was determined by competition between elastic deformation, thermodynamic and domain wall energies in this coherently strained system,” says Wu. “A uniaxial compressive strain of approximately 1.9-percent was able to drive the metal-nonmetal transition at room temperature.”

The ability to fabricate single-crystal micro- and nanowires of vanadium oxide that were free of structural defects made it possible to apply such high strain without plastic deformation or fracturing of the material, Wu says. Bulk and even thin film samples of vanadium oxide cannot tolerate a strain of even one-percent without suffering dislocations.

Wu says that in the future strain engineering might be achieved by interfacing a correlated electron material such as vanadium oxide with a piezoelectric – a non-conducting material that creates a stress or strain in response to an electric field.

“By applying an electric field, the piezoelectric material would strain the correlated electron material to achieve a phase transition that would give us the desired functionality,” says Wu. ”To reach this capability, however, we will first need to design and synthesize such integrated structures with good material quality.”

This work was supported in part by Berkeley Lab through its Laboratory Directed Research and Development Program, and in part by a grant from the National Science Foundation.

Researchers Discover Non-toxic Nanocrystals that May Lead to a Method of CO2 Storage

admin — Tue, 28 Jul 2009 13:56:43 +0000

Lawrence Berkeley

Berkeley Lab researchers have produced non-toxic magnesium oxide nanocrystals that efficiently emit blue light and could also play a role in long-term storage of carbon dioxide, a potential means of tempering the effects of global warming.

In its bulk form, magnesium oxide is a cheap, white mineral used in applications ranging from insulating cables and crucibles to preventing sweaty-palmed rock climbers from losing their grip. Using an organometallic chemical synthesis route, scientists at the Molecular Foundry have created nanocrystals of magnesium oxide whose size can be adjusted within just a few nanometers.

And unlike their bulk counterpart, the nanocrystals glow blue when exposed to ultraviolet light.

A high resolution electron micrograph image of magnesium oxide nanocrystals; the inset shows a single nanocrystal.

Current routes for generating these alkaline earth metal oxide nanocrystals require processing at high temperatures, which causes uncontrolled growth or fusing of particles to one another-not a desirable outcome when the properties you seek are size-dependent. On the other hand, vapor phase techniques, which provide size precision, are time and cost intensive, and leave the nanocrystals attached to a substrate.

Delia Milliron, Facility Director of the Inorganic Nanostructures Facility at Berkeley Lab’s nanoscience research center, the Molecular Foundry commented;

We’ve discovered a fundamentally new, unconventional mechanism for nicely controlling the size of these nanocrystals, and realized we had an intriguing and surprising candidate for optical applications. This efficient, bright blue luminescence could be an inexpensive, attractive alternative in applications such as bio-imaging or solid-state lighting.

Unlike conventional incandescent or fluorescent bulbs, solid-state lighting makes use of light-emitting semiconductor materials-in general, red, green and blue emitting materials are combined to create white light. However, efficient blue light emitters are difficult to produce, suggesting these magnesium oxide nanocrystals could be a bright candidate for lighting that consumes less energy and has a longer lifespan.

These minute materials do more than glow, however. Along with their promising optical behavior, these magnesium oxide nanocrystals will be a subject of study in an entirely different field of research: Berkeley Labs’ Energy Frontier Research Center (EFRC) for Nanoscale Control of Geologic CO₂, designed to “establish the scientific foundations for the geological storage of carbon dioxide.”

Experts say carbon dioxide capture and storage will be vital to achieving significant cuts in greenhouse gas emissions, but the success of this technology hinges on sealing geochemical reservoirs deep below the earth’s surface without allowing gases or fluids to escape. If properly stored, the captured carbon dioxide pumped underground forms carbonate minerals with the surrounding rock by reacting with nanoparticles of magnesium oxide and other mineral oxides.

“These nanocrystals will serve as a test system for modeling the kinetics of dissolution and mineralization in a simulated fluid-rock reservoir, allowing us to probe a key pathway in carbon dioxide sequestration,” said Jeff Urban, a staff scientist in the Inorganic Nanostructures Facility at the Molecular Foundry who is also a member of the EFRC research team. “The geological minerals that fix magnesium into a stable carbonate are compositionally complex, but our nanocrystals will provide a simple model to mimic this intricate process.”

Molecular Foundry post-doctoral scholar Hoi Ri Moon, staff scientist Jeff Urban and Facility Director Delia Milliron demonstrate magnesium oxide nanocrystals that could be a bright candidate for solid-state lighting. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

Hoi Ri Moon, a post-doctoral researcher at the Foundry working with Milliron and Urban, noted her team’s direct synthesis method could also be helpful for already-established purposes. “As a user facility that provides support to nanoscience researchers around the world, we would like to pursue studies with other scientists who could use our nanocrystals as ‘feedstock’ for catalysis, another application for which magnesium oxide thin films are commonly used,” said Moon.

“Size-controlled synthesis and optical properties of monodisperse colloidal magnesium oxide nanocrystals,” by Hoi Ri Moon, Jeffrey J. Urban and Delia J. Milliron, appears in Angewandte Chemie International Edition and is available in Angewandte Chemie International Edition online.

Work at the Molecular Foundry was supported by the Office of Basic Energy Sciences within the DOE Office of Science.

The Molecular Foundry at Lawrence Berkeley National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://nano.energy.gov.

New Berkeley Laboratory Report on the U.S. Wind Energy Industry

admin — Tue, 28 Jul 2009 10:10:58 +0000

For the fourth consecutive year, the U.S. was home to the fastest-growing wind power market in the world in 2008, according to a report released by the U.S. Department of Energy and prepared by Lawrence Berkeley National Laboratory (Berkeley Lab). Specifically, U.S. wind power capacity additions increased by 60 percent in 2008, representing a $16 billion investment in new wind projects.

“At this pace, wind is on a path to becoming a significant contributor to the U.S. power mix,” notes report author Ryan Wiser, of Berkeley Lab.

Wind projects accounted for 42% of all new electric generating capacity added in the U.S. in 2008, and wind now delivers nearly 2% of the nation’s electricity supply.

The 2008 edition of the “Wind Technologies Market Report” provides a comprehensive overview of developments in the rapidly evolving U.S. wind power market. The need for such a report has become apparent in the past few years, as the wind power industry has entered an era of unprecedented growth, both globally and in the United States. At the same time, the last year has been one of upheaval, with the global financial crisis impacting near-term growth prospects for the wind industry, and with federal policy changes enacted to push the industry towards continued aggressive expansion.

“With the market evolving at such a rapid pace, keeping up with trends in the marketplace has become increasingly difficult,” notes report co-author Mark Bolinger. “Yet, the need for timely, objective information on the industry and its progress has never been greater…this report seeks to fill that need.”

Drawing from a variety of sources, this report analyzes trends in wind power capacity growth, turbine size, turbine prices, installed project costs, project performance, wind power prices, and how wind prices compare to the price of conventional generation. It also describes developer consolidation trends, current ownership and financing structures, and trends among major wind power purchasers. Finally, the report examines other factors impacting the domestic wind power market, including grid integration, transmission issues, and policy drivers. The report concludes with a preview of possible near- to medium-term market developments.

Some of the key findings from the just-released 2008 edition include:

•The U.S. is the fastest-growing wind market worldwide. The U.S. has led the world in new wind capacity for four straight years, and overtook Germany to take the lead in cumulative wind capacity installations.

•Growth is distributed across much of the U.S. Texas leads the nation with 7,118 MW of new wind capacity, but 13 states had more than 500 MW of wind capacity as of the end of 2008, with seven topping 1,000 MW, and three topping 2,000 MW. Over 10% of the electricity generation in Iowa and Minnesota now comes from wind power.•Market growth is spurring manufacturing investments in the U.S. Several major foreign wind turbine manufacturers either opened or announced new U.S. wind turbine manufacturing plants in 2008. Likewise, new and existing U.S.-based manufacturers either initiated or scaled-up production. The number of utility-scale wind turbine manufacturers assembling turbines in the U.S. increased from just one in 2004 (GE) to five in 2008 (GE, Gamesa, Clipper, Acciona, CTC/DeWind).

•Wind turbine prices and installed project costs continued to increase into 2008. Near the end of 2008 and into 2009, however, turbine prices have weakened in response to reduced demand for wind due to the financial crisis.

•Wind project performance has improved over time, but has leveled off in recent years. The longer-term improvement in project performance has been driven in part by taller towers and larger rotors, enhanced project siting, and technological advancements.

•Wind remained economically competitive in 2008. Despite rising project costs, in recent years wind has consistently been priced at or below the price of conventional electricity, as reflected in wholesale power prices. With wholesale prices plummeting in recent months, however, the economic position of wind in the near-term has become more challenging.

•Expectations are for a slower year in 2009, in large part due to the global recession. Projections among industry prognosticators range from 4,400 MW to 6,800 MW of wind likely to be installed in the U.S. in 2009. After a slower 2009, most predictions show market resurgence in 2010 and continuing for the immediate future.

Berkeley Lab’s contributions to this report were funded by the Wind & Hydropower Technologies Program, Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy.