Special Report - Advanced Nanoscale Materials: Putting Science at Your Fingertips
As the U.S. attempts to move away from dependence on foreign oil and toward less-polluting forms of energy, converting wasted heat into useful energy is increasingly important. In our country today, as much as 30 percent of the energy involved in large-scale industrial processes is lost through smokestacks. Gasoline and diesel engines lose 35 to 40 percent of their fuel energy in waste heat, primarily in the exhaust.
Scientists at Pacific Northwest National Laboratory are using nanoscale materials to create a thermoelectric device to harvest and recover waste heat from diesel and gasoline engines, exhaust systems and industrial manufacturing processes, including glass, aluminum and chemical.
And they're seeing promising results—the thin- film thermoelectric devices they are constructing with two-dimensional thin films show high conductivities. Their goal is to achieve 20 percent efficiency—an accomplishment that will make thermoelectric power generation using waste heat economically feasible. This energy would be useful in powering electrical accessories on vehicles, which reduces the load on the main engine, as well as for many stationary applications.
The best thermoelectric devices are semiconductors because they have a combination of good electrical conductivity, capacity for high voltage generation with temperature difference and low thermal conductivity.
PNNL scientists are taking semiconductors a step further with nanoscale thin films that have essentially two dimensions—length and width. These films are too thin to have a perceptible thickness. Researchers predict 15 percent conversion efficiency from heat to electricity with multilayer thin-film materials, nearly double the efficiency of semi-conductors made from non- nanoscale, or bulk, materials.
"Because we're taking electrons that move in three dimensions and squishing them down to two dimensions, the electrons can only move along one plane so they move faster and with more mobility," said PNNL Laboratory Fellow Pete Martin. "That leads to more power output and efficiency—a key issue in semiconductors."
Scientists layer as many as 3,000 to 10,000 layers of compounds, such as silicon and silicon germanium, to create the semiconductors. "You need all those layers to generate sufficient power," said Larry Olsen, a PNNL scientist who also works with thermoelectrics. "If you just had one thin layer, you'd get a current, but it would be weak."
The thermoelectric device relies on the heat flow created by temperature differences to produce electrical energy. For example, in an automotive exhaust system operating at 600-700 degrees Centigrade, the hot exhaust gases naturally flow to cooler ambient temperatures outside the exhaust. As they pass through the thermoelectric device, it converts the heat into electrical energy, which can be used to generate electrical power.
One of the main challenges in making thin-film materials is using the right materials to form a high quality film. The materials must be deposited on the substrate in a controlled manner when they are formed so they demonstrate certain properties. In addition to high efficiency, the desired semiconductors must be stable at high operating temperatures—they should not decompose, oxidize, melt or change composition.
Two-dimensional thin films also can be deposited over large surface areas readily. One of the goals of the PNNL program is to scale up the technology to manufacturing size. "If you're going to cover a large exhaust pipe, flues in process industries, or some part of a smokestack, you're going to have to cover a large surface area with these films." Martin said. "We have one deposition system that is three meters in diameter so we can deposit these films over a large area."