Archive for January 2009
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Ceramics as alternative to conventional laser generation
Kudos to Akio Ikesue and Yan Lin Aung for their recent article in Nature Photonics. Ikesue (bias confession: he is an ACerS member) and Yan report on recent developments in the field. From their summary:
The opaque and translucent cement and clay often used in tableware are not appropriate for optical applications because of the high content of optical scattering sources, that is, defects. Recently, scientists have shown that by eliminating the defects, a new, refined ceramic material — polycrystalline ceramic — can be produced. This advanced ceramic material offers practical laser generation and is anticipated to be a highly attractive alternative to conventional glass and single-crystal laser technologies in the future. Here we review the history of the development of ceramic lasers, the principle of laser generation based on this material, some typical results achieved with ceramic lasers so far, and discuss the potential future outlook for the field.
Clearly, they think the potential is pretty bright, so the story is a good read.
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Video(s) of the week - The hard and “soft” of ceramic armor
Today’s videos are something of an introduction to ceramic armor. The first video uses the suit worn in the most recent Batman movie (The Dark Knight) as a jumping off point for explaining the production and composition of ceramic armor plates manufactured by Ceradyne, one of the leading advanced technical ceramics, for military and police situations. Besides an overview of the manufacturing process, the video contains remarkable footage of a soldier picking himself up after being shot in the chest by a sniper.
The second video is meant to show there is more than one way to skin the ceramics-as-armor cat. This video is about a “liquid” armor (actually ceramic powders sprayed on Kevlar) that can take advantage of some of the same material characteristics displayed in our previous videos about materials that can allow one to “walk on water.” This material was developed by a team led by Norman Walker at the University of Delaware’s Center for Composite Materials and the Army Research Lab. The group calls the product Shear Thickening Fluid Fabric.
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Solar refrigeration coming to off-grid areas
Artist's rendition of Promethean Power’s solar-refrigeration solution.
A cool idea for solar-powered cold-storage is about to make refrigeration a lot more affordable, practical and environmentally friendly in off-the-grid and partially-electrified areas of the world.
Birth of a ‘cool’ concept
The system is an energy-efficient hybrid refrigerator that combines conventional compressor-created cold air with clean, quiet cooling generated by thermoelectric technology and photovoltaic solar panels. The concept is the brainchild of MIT graduate Sorin Grama and Sam White, an entrepreneur with business-and-funding savvy. In 2007, the pair founded Promethean Power Systems, Inc. According to its website, the objective of the Cambridge, Mass.-based firm is to “develop a complete, stand-alone rural refrigeration system that stimulates businesses, reduces dependency on fossil fuels and increases the quality of life in emerging markets by enabling its users to reliably store food, vaccines and other perishable items.
Overcoming challenges
“Thermoelectric cooling is basically a semiconductor chip that creates heat on one side and runs cold enough to create ice on the other,” White explains in a Nov. 7, 2008, interview with a newspaper called India New England. The problem with thermoelectric technology is its inefficiency, White says, noting that it uses twice the amount of power to produce the same amount of cooling as a conventional refrigerator. To date, that inefficiency has limited the use of thermoelectric technology to small consumer-electronic applications. However, Grama and White say they have overcome this obstacle by developing proprietary “smart” controls that enable their cooling system’s compressor, thermoelectric modules and solar panels to work together synergistically.
System how-to’s
Reporter Chris Nelson reports the “how-to’s” of the Promethean system in his India New England article: “Early in the morning and late in the afternoon, when the sun’s rays are less intense, the solar panels can’t produce enough electricity to run the compressor. But they can generate just enough juice to run the thermoelectric modules, which produce cold air until the compressor kicks in,” the article explains. “That usually happens around midday,” it continues, “when the sun is at its highest point in the sky and the solar panels are cranking out plenty of electricity. But even then the compressor won’t use all of the power generated, so the thermoelectric modules will use the excess to provide extra cooling.” White adds that, during periods of extended cloudiness, the system uses a small generator to backup its solar panels. The result he says, is a cooling system that utilizes 20 percent less power while creating the same amount of refrigeration as a compressor alone.
Commercialization
White and Grama were first rewarded for this cool concept in 2007, when it captured second prize and $10,000 in MIT’s annual graduate-level business-plan competition. The pair followed that victory with a trip to India where they cased out and confirmed the need for such a product. Following that trip, they built and tested a laboratory-scale, 60-liter chilling unit. In September 2008, they reportedly began constructing a 500-liter system with funding from Quercus Trust, a private investment firm located in Newport Beach, Calif. The larger system will be tested in India sometime during 2009, White indicates. The pair say they’ll sell their system for about $17,000. That’s nearly $5,000 more than the cost of a conventional diesel-powered unit but - unlike diesel systems - Promethean technology requires no additional fuel charges, Grama says. Their system also features extremely low maintenance costs, he says. Because it involves no moving parts, there’s little to break down and a remote-monitoring unit that enables anywhere-any time remote diagnosis, further reduces maintenance expense, he claims.
“As a result,” Grama says on the firm’s website, “our system can provide cooling power at an operating cost that is 66 percent lower than the operating cost of conventional units.”
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Air Force cuts costs, boosts performance of aerospace mirrors
NASA's James Webb Space Telescope features mirror segments fabricated under MMD’s Advance Mirror System Demonstrator Program. (Credit: NASA/E. Given)
Air Force researchers are investigating ways to boost the performance of aerospace mirrors, while cutting in half the cost and time required to manufacture them. No wonder! It takes about two years and nearly one million dollars to produce a one-meter, lightweight glass aerospace mirror, according to the AF Research Lab’s Materials and Manufacturing Directorate. The MMD has responsibility for developing, producing and maintaining materials used in AF aircraft, missiles, rockets and ground-based military systems.
Aerospace mirrors are key components of everything from surveillance-and-reconnaissance systems to transformational communications networks, directed-energy technology, laser-radar devices and large, high-powered telescopes. Hence MMD’s concern with manufacturing these mirrors in the most efficient and cost-effective way. Until recently, “state-of-the-art” mirror manufacturing meant using monolithic glass. Appealing because of its ability to be bent into diverse shapes, precisely ground and “polished to an angstrom-level surface finish,” monolithic glass also possesses another important asset - a “coefficient of thermal expansion that can be chemically tailored to be near zero ppm/°C,” say MMD’s Lawrence Matson and Pete Meltzer Jr. This coefficient of thermal expansion “minimizes distortion of the optical surface caused by thermal excursions during service,” Matson and Meltzer explain. This advantage has enabled MMD to form mirrors with an areal density of about 15 kg/m2, an accomplishment that took place during MMD’s Advance Mirror System Demonstrator Program.
During this program, MMD was able to achieve a 50 percent reduction in mirror weight and fabrication costs compared with those required for construction of the Hubble Space Telescope. Today, however, it appears MMD researches have maxed-out the benefits of monolithic glass. It’s unlikely any more weight or cost reductions can be squeezed from monolithic glass because its structure offers low-elastic modulus, strength and fracture toughness,” say Matson and Meltzer. “Continued lightweighting would result in very fragile structures that would be difficult to polish and would fail catastrophically with a single handling mishap or during launch.” And, so, like fickle fans abandoning an aging superstar, AF researchers are abandoning monolithic glass in favor of new material up-and-comers.
To date, the most promising of these have been ceramic, metal and polymer hybrids and composite materials produced in one of two ways - by using either replicated nanolaminate foil technology pioneered by government labs or by sol-polymer-spinning technology. In either case, Matson and Meltzer say CTE matching is mandatory for success. “Both of these approaches require materials that are CTE-matched in order to obtain and keep the correct contour and smoothness of the optical surface during fabrication and operation,” they advise, indicating MMD’s focus on finding new foil chemistries with CTEs in the zero to 3ppm/°C range. MMD researchers also have discovered that replicated nanolaminate hybrid/composite mirror systems require “CTE-tailored bonding agents” to connect the replicated foil to unpolished structural substrates. To solve this problem, MMD researchers are creating “nanosized, negative CTE particles that can be uniformly dispersed in potential bonding medium, such as organic and inorganic polymers, aero gels and glass sols [and] could also be used to spin an optical surface of visible quality onto unpolished structural substrates,” the researchers said. The two have found they must use nano-sized powders to assure “uniform dispersion and to minimize print-through distortions on the optical surface.” With material solutions in the works, the MMD research team is now turning its attention to finding uniform, stress-free, reflective coatings and dielectric stacks for large mirror systems. It believes the result will be lighter, better, cheaper and faster-to-make aerospace mirrors suitable for any DOD or NASA application.
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Creating alternating current with piezoelectrics
Flexible charge pumps that produce alternating current by stretching and relaxing zinc oxide wires. (Credit: GTI, Gary Meek)
Researchers at the Georgia Institute of Technology have created a micro-scale “flexible charge pump” that produces alternating current by utilizing the piezoelectric properties of cyclically stretched and released zinc oxide wires.
“The flexible charge pump offers yet another option for converting mechanical energy into electrical energy,” says project leader Zhong-Lin Wang, director of GIT’s Center for Nanostructure Characterization. Wang reports details of the project in the November issue of Nature Nanotechnology. “This adds to our family of very small-scale generators able to power devices used in medical sensing, environmental monitoring, defense technology and personal electronics,” he says, defining the pump’s significance.
Wang reports that the generator can produce up to 45 millivolts of electricity by converting about seven percent of the mechanical energy applied to the zinc-oxide wires. He notes that arrays of such generators could be used to charge low-power devices like sensors.
On GIT’s website, Wang explains that earlier nanowire generators and microfiber nanogenerators developed by his team required “intermittent contact between vertically-grown zinc oxide nanowires and an electrode or the mechanical scrubbing of nanowire-covered fibers.” Such pumps were difficult to build and had a short lifetime because their need for mechanical contact created wear that eventually wore them out. Additionally, because zinc oxide is soluble in water, so they also had to be protected from moisture.
The design of the new flexible charge pump resolves all of the earlier pumps’ shortcomings, according to Wang. In the new pump, he says, when the zinc-oxide wires are mechanically stretched and released, their piezo properties cause the material to create a piezoelectric potential that grows and diminishes.
On GIT’s website, he explains that a “Schottky barrier” controls the electrons’ alternating flow, and that the driving force is an electric potential. “The electrons flow in and out, just like AC current,” Wang describes. “The alternating flow of electrons is the power output process.”
The newly developed generator is not comprised of nanometer-scale structures. For ease of fabrication, Wang has chosen to construct it with zinc-oxide piezoelectric fine wires that measure three to five microns in diameter and 200 to 300 microns in length. He notes, however, that the same piezoelectric principles “would apply at the nanoscale.”
The GIT research team used physical vapor deposition to grow the wires at about 600°C. Then they used an optical microscope to bond the wires onto a polyimide film and closed both ends (which serve as electrodes) with silver paste. Polyimide was then used to encapsulate both wires and electrodes to prevent them from becoming worn.
During the testing phase, a motor-driven mechanical arm was used to repeatedly bend the encased wires to measure the electric energy generated. The bending supplied the tensile strain that generated the piezoelectric potential field along the wires. “This, in turn, [drives] a flow of electrons into an external circuit, creating the alternating charge-and-discharge cycle and corresponding current flow,” Wang explains on GIT’s website. He notes that the team controls the amount of electricity produced in both voltage and current by increasing or decreasing the strain rate.
To confirm that they were measuring current produced by the generator, Wang’s team repeated the test under the same conditions with stretched carbon filters and Kevlar fibers coated with polycrystalline zinc oxide. They did not see current flow in these instances.
What does the future hold for such small-scale generators. Wang says he sees them being used in self-powered wireless sensing systems that gather, store and transmit data. “Self-powered nanotechnology could be the basis for a new industry. That’s really the only way to build independent systems,” he concludes.
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