Dielectric Materials for Wireless Transmission and Energy Storage
Inside the electronic equipment we use every day are the materials that perform the miracles of modern technology. Among the most important categories of electronic materials are dielectrics, substances that are poor conductors of electricity, but do a very good job of maintaining electrostatic fields that can store energy. These dielectric materials are created, studied, and optimized in the Center for Dielectric Studies (CDS) in the Materials Research Institute at Penn State, as well as in other labs across the campus.
Founded in 1983, the CDS is the nations longest running federally sponsored joint university/industry collaboration, with over 20 industrial partners sponsoring research and helping to move discoveries from the lab to the marketplace. Since 2001, the CDS has partnered with the University of Missouri-Rolla to increase its industry membership and expand its research programs. In 2005, the Center for Dielectric Studies was awarded a $5 million/five-year research grant, a MURI, by the Department of Defense to study dielectrics for pulsed power.
This MURI is related to dielectric materials that store energy. Across the Penn State campus there are many faculty working on other ways to store energy: chemical storage in batteries; mechanical storage, such as in a flywheel; as well as the electrical methods that the CDS is working on.
Prof. Lanagan gives a simple explanation of their approach: "Take a positive and negative charge and pull them apart. As you close an external circuit, the charge runs around. That’s a very fast way to deliver energy, and fast means high power. So there are two things to think about: the first is how much energy can be stored in a volume, that is the storage density (because everyone wants more in less volume); and the second thing people want is power density ‾ how fast can you get that energy out of that material.
"There are certain times you need a fast way to deliver high energy. One of them is in heart defibrillators, where a sensor detects an arrhythmia and actuates a dielectric material capacitor to deliver an electric shock to the heart. Another is in electromagnetic launch systems for projectiles, something the navy is very interested in."
The next generation of aircraft carriers and electronic warships will require the equivalent of a small electric power plant, enough energy to power 80,000 homes. The navy would like to eliminate powder explosives entirely on shipboard, turning instead to electromagnetic aircraft launch, electromagnetic weapons, pulsed energy and laser weapons, all of which require large amounts of energy stored in a much smaller volume than is currently feasible. "A capacitor large enough to jump start your heart is smaller than the size of my fist," Lanagan says, holding out his closed hand. "But one to launch a missile is probably a quarter the size of this building."
Capacitors of that size are already in existence; they are used to create nuclear fusion. In the fusion process, banks of room-sized capacitors release large pulses of energy into a system that focuses the energy on a small pellet, creating a reaction similar to the inside of the Sun. To get such large amounts of energy into a small enough volume to power all the shipboard demands, Lanagan and his coworkers must deal with fundamental materials issues.
Hydrogen Cars and Shrinking Cell Phones
The same material that can deliver the energy to launch a missile can be used to help power a hybrid automobile or allow for smaller, more functionalized cell phones. Hybrid electric vehicles have taken on tremendous appeal with gasoline holding steady at well over $2 a gallon in the U.S. and close to $7 a gallon in Europe. In general hybrid vehicles use capacitors to equal out the engine’s energy demand and raise fuel mileage. The energy from gas or batteries is stored in relatively large capacitors that can deliver a steady electric charge. Current capacitors take up too much of the vehicle’s storage space, says Lanagan. "If we can shrink the capacitors, it will take less room for equipment and leave more room for passengers. Part of that requires shrinking these energy storage devices."
Like shrinking parts for hybrid vehicles, cell phones, too, are putting more and more components into a smaller package. In order to make this happen, researchers must develop new materials with unusual properties. Mike Lanagan is collaborating with Clive Randall, director of the CDS and professor of materials science and engineering, and Tom Shrout, professor of materials, on materials with higher permittivity, which allow for more charge storage. The higher the permittivity, the smaller the component you can make. "Every integrated circuit requires a cascade of capacitors," Clive Randall explains. "For every integrated circuit in a cell phone, there are 30 to 50 capacitors, which are very cheap to make but have extraordinary technology involved. This is particularly true of ceramic multilayer technology, where massive improvements are pushing other capacitor technologies out of the market."
"We are working on making finer and finer layers of metal and metal insulators, like a sandwich. We are dealing with one micron layers, 1/25 of 1/1000 of an inch," Lanagan says. "The thinner the layer, the higher the capacitance. As we drive this down, we are getting into the realm of nanotechnology."
Understanding these materials requires looking closely at surface interfaces, how materials interact when they are in close contact, and phase changes, how their properties change when subjected to temperature and pressure. "Electronic components that break down are like a disease in the body," Randall says. "We want to learn how to prevent the disease of components."
To study the material physics at a small scale, the Center for Dielectric Studies uses a $1.4 million transmission electron microscope with "all the bells and whistles," located in the Materials Characterization Lab. This is the kind of equipment that not many companies can afford, Prof. Randall says. With it they are able to get new insights into what phases are found in the material production process. "It allows for nanometer scale chemical analysis where we are finding unanticipated behavior in materials. We can use the TEM in novel ways to find out unusual and unexpected things. This is really new and important stuff," he says.
In that micron-thick layer created in the CDS lab, small particles of other materials are added to change the electrical properties of the dielectric material. These particles need to be made smaller as well. There are new methods for making smaller particles and new methods of deposition being studied. Many Penn State faculty are working on this problem, including professors Randall, Elizabeth Dickey, Jim Adair, Tom Shrout, and Susan Trolier-McKinstry, among others.
Smart Materials
In the W. M. Keck Smart Materials Integration Laboratory in the Materials Research Institute, scientists are developing two methods of shrinking materials for wireless electronics. The first of these is LTCC ‾low temperature co-fired ceramic. "This system is interesting because it allows for more functions. You can build them all right into the same system," says Randall.
LTCCs are ceramics, but not a typical hard material. In their unfired state, the material looks like a piece of flexible blue photographic film that is easily bent and snaps back into place when released. Although it is a blue color, this phase of the material is called green, because it has yet to be heat treated. This material is made of 50 percent ceramic by volume and 50 percent polymer. At this stage it acts like a flexible polymer and can be easily shaped. One way it is shaped is with a punch, a simple device that cuts a pattern into the ceramic. The one they use in the Keck lab is a laboratory punch that shapes one piece at a time, but there are commercial models used in industry that can make 20,000 punches per second. Once the holes are made they be can filled with a conductor, creating a connection path from bottom to top, making an integrated package with 3D circuitry.
Patterns can also be made on LTCC material in the same way patterns are made on t‾shirts, by screen printing. Only in the LTCC process, the screens are a finer mesh. A patterned screen is laid down on a substrate of LTCC material, and a squeegee comes down and lays a metallic conducting paste across the pattern. Once the flexible LTCC is fired, it becomes like a hard ceramic with a circuit inked on the surface.
Although LTCCs have been studied for 25 years, it is still a developing industry, Randall says. They were first used for computers, but more recently are being used in wireless telephones. Although a few U.S. companies have quality LTCC products, most of the activity involving LTCC manufacturing is taking place in the Far East, he says.
Susan Trolier-McKinstry, professor of ceramic science and engineering and director of the W. M. Keck Smart Materials Integration Laboratory, is using mold replication and thin film technology to develop high frequency ultrasound arrays of piezoelectric transducers. "When the piezoelectric transducer is pinged with an electric field, it vibrates, sending out a wave." The ultrasound transducers are made with lead zirconate titanate (PZT), the same material as Randall and Lanagan’s capacitors. She is working to produce an array of very small transducers that can show the image of a cell. "In order to image at the cellular level, you have to go to a higher frequency than with the typical ultrasound," she says. "As you scale down, you achieve higher frequencies."
Prof. Trolier-McKinstry’s current transducers are made with a mold replication process. With the mold technique she can create tube structures a few microns in diameter that can focus sound like a beam of light. "It’s hard to make those structures," she said in a talk at Crossover 2005, a meeting of biomaterials scientists and medical clinicians held at Penn State in October. "The mold is made of silicon with holes a few microns in diameter. You can put the electrode materials into the mold, and when you remove the silicon, a freestanding array of tubes is left."
The future of high frequency ultrasound is in cellular imaging on a time lapse scale that would allow 4D images -- images in three dimensions plus a fourth dimension of time. Because of the small size -- an entire display will fit on a 2mm x 2mm block -- the high frequency ultrasound will have a useful depth of only around 7mm. That makes this ultrasound a good candidate for imaging the eye or examining skin cancers. On the tip of a probe, the ultrasound device could be used to explore inside a surgical incision, searching for cancer cells that were missed during surgery.
Smart materials such as piezoelectrics sense a change in the environment and respond to that change in a useful way. MRI and the scientists in the Center for Dielectric Studies have been world leaders in smart materials research since the Center’s founding over 20 years ago. Working with the U.S. Navy and other federal agencies, and with industrial partners in the U.S. and abroad, the CDS has developed a range of dielectric materials that are used in medical research, defense technologies, computers and electronics, digital cameras, and in automobile airbags and electronically controlled shock absorbers. Smart materials studied and created at Penn State are performing miracles in the modern age of electronics.
Thomas R. Shrout, Professor of Materials, can be contacted at Validate to view address.
Clive A. Randall, Professor of Materials Science and Engineering, can be contacted at Validate to view address.
Michael T. Lanagan, Associate Professor of Engineering Science and Mechanics, can be contacted at Validate to view address.
Susan Troiler-McKinstry, Professor of Ceramic Science and Engineering, can be contacted at Validate to view address.
