Each year, motor vehicles in the United States release about 350 million tons of carbon into the atmosphere in the form of carbon monoxide and carbon dioxide. To reduce these emissions significantly, a radically different propulsion system is needed.
Scientists and engineers working with the PNGV see the hybrid vehicle as the most likely track for the car of the future. Energy for the hybrid vehicle would be stored by advanced batteries, flywheels, or ultracapacitors. Power source candidates for the hybrid include small diesel engines, fuel cells similar to those that power spacecraft, and gas turbines.
Technologies that would enable the hybrid to run efficiently and reliably are largely dependent on how successful science and industry are at developing new materials, including ceramic materials. A hybrid electric vehicle with a gas turbine engine, compared with spark ignition engines widely used today, would weigh less, last longer, be more fuel-efficient, could burn a variety of fuels, and would have lower emissions of nitrogen oxides and carbon gases.
Today, however, gas turbines fit into few hybrid car schemes because of other technical problems. For one thing, small gas turbines that might be used in a hybrid vehicle must operate at elevated temperatures to achieve the needed high efficiency. It is not practical to cool small turbines because the components would have to be very small compared to the size of the cooling passages. Even the most advanced superalloys do not have sufficient high-temperature strength and fatigue resistance at the required temperatures. The gas turbine is a good example of the need to develop reliable, low-cost ceramic or fiber-reinforced composite components, including turbine blades, vanes, scrolls, and combustors.
In 1983 Department of Energy Energy Efficiency and Renewable Energy's Office of Transportation Technologies started the Ceramic Technology Project. ORNL was asked to carry out technical management of the project, whose goal was to involve industry in developing technology to make reliable ceramics, advanced processing technology to minimize the number and size of flaws in ceramics, and new methods to test the mechanical behavior of ceramics to more accurately predict how long they will hold up before failing under conditions typical of high-temperature engines.
Since that time, the ceramics and automobile industries have been developing and testing ceramic parts. Nissan introduced silicon nitride ceramic turbochargers in 1985, suggesting that it is feasible to economically mass produce gas turbine engines. In the DOE-NASA Advanced Turbine Technology Applications Program, the General Motors Allison Turbine Division ran silicon nitride turbine rotors for 1000 hours in a simulated driving cycle that went up to full speed at 1370°C (2500°F).
What were the achievements of the Ceramic Technology Project? According to D. Ray Johnson, project leader in ORNL's Metals and Ceramics Division: "In assessing the needs of the automobile industry, we found that increased reliability of ceramic materials was necessary. So the project developed silicon nitride ceramics that were highly reliable, including manufacturing processes to eliminate the sources of flaws that had limited the strength of silicon nitride. For example, it was found that growing elongated silicon nitride grains that behave like whiskers in the ceramic itself greatly improved its toughness and reduced its tendency to form cracks even if flaws were present. Our researchers also developed better ways to test the mechanical strength of ceramics and accurately predict the lifetime of ceramic components under a given set of conditions."
Allied Signal Ceramic Components fabricated the blades of this turbine from silicon nitride using ORNL's gelcasting technology.
After the first ten years of the Ceramic Technology Project, it was time to shift gears. "During our recent visits to automobile and engine companies," Johnson says, "we were told that ceramics can now run reliably in conventional automotive engines, but they `cost too much' to be used for production." Realizing that ceramic components must be competitive in price with metal components in the cost-conscious auto industry, the direction of the Ceramic Technology Project shifted toward reducing the cost of ceramics and finding ways to mass produce reliable ceramic parts for the auto industry economically.
Johnson says that in 1993 DOE introduced a ceramic manufacturing initiative to bring the automotive and ceramic industries together with government researchers to solve manufacturing problems. One result of the cost-effective ceramics initiative was the development of a Ceramic Manufacturability Center at ORNL's High Temperature Materials Laboratory for users from government and industry.
ORNL researchers have made invaluable contributions to the program through their research and development work, as well as through their guidance of research performed in industrial and university laboratories. "We are using the computer to model the costs of ceramic manufacturing to determine ways to bring down the cost," Johnson says. "It turns out that ceramic machining can contribute more than half the total cost of a ceramic component. So, we are looking at alternatives to diamond machining and other processes for cutting, shaping, and finishing ceramics to reduce costs. We are trying to find more economic ways of synthesizing silicon nitride powders and forming and densifying ceramics. Gelcasting is a candidate method for forming ceramics because it improves precision of shapes and minimizes the need for finish machining."
Gelcasting, an advanced forming method for ceramics, was invented in 1987 by Mark A. Janney and Ogbemi O. Omatete, both of ORNL's Metals and Ceramics Division. It allows a ceramic part to be machined before, not after, the part is heated to make it hard. AlliedSignal Ceramic Components and other companies are interested in using the technology for manufacturing airplane and automobile engine parts that are light and resistant to corrosion and high temperatures. Gelcasting has been patented and licensed to two companies. In addition, with contributions by Stephen Nunn and Claudia Walls, gelcasting received an R&D 100 award in 1995.
Other ceramic technologies developed at ORNL include self-aligning grips for tensile testing of ceramics, invented by Kenneth C. Liu, licensed to Instron Corporation, and sold to laboratories all over the world. A method of producing low-cost silicon nitride ceramics by microwave processing has been developed by Terry N. Tiegs, James O. Kiggans, and colleagues. That technology, which produces ceramics that resist fracturing and thermal shock, also does the job at a price competitive with metal components, making it very promising as a method of mass-producing automotive ceramic components.
ORNL researchers have made substantial contributions to the development of tougher ceramics, protective coatings for ceramics, and low-expansion ceramics; joining of ceramics to metals; ceramic machining; analytical electron microscopy of ceramics; nondestructive evaluation of ceramics; characterization of mechanical behavior of ceramics; development of a computer database of mechanical properties, and participation in an international effort to develop standards for ceramics.
"Another goal is to pave the way for an automated, intelligent manufacturing plant," Johnson says. "Industrial researchers are developing intelligent, computer-controlled processes that use robotics to handle materials and sensors to determine product quality on-line. The expected results are an increase in the reliability of structural ceramics and lowered manufacturing costs for ceramic auto parts."
If the cars of the future are made of these advanced materials, advances will likely be made in reducing use of fuel and the release of carbon dioxide to the air.
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