Aman Haque likes a challenge, and understanding thermal transport at the nanoscale is unexplored territory with plenty of room for experimental discovery. The professor of mechanical engineering is one of only a handful of researchers around the world with the capability to study the thermal and electrical properties of a material in real time under an electron microscope. He does it with a laboratory small enough to sit on a fingertip.
Haque shrinks the test facilities that fit into a full scale lab onto a microchip – including sensors, actuators, heaters and cooling fans, thermal and electronic measurement devices, a tensile testing tool, and the microelectronics to make them all work. This laboratory is small enough to fit into the specimen holder of a transmission electron microscope such as the state of the art Titan, housed in the Materials Research Institute’s microscopy facilities.
“My expertise is developing experimental set-ups,” said Haque. “I am interested in doing very fundamental scientific experiments at the length scale where these experiments are difficult to do.”
Haque and his students look at how heat transfers in ultrathin films.
“People are trying to capture the awesome properties of nanoscale materials so that we can use them at the bulk level,” said Haque, whose name is pronounced like the bird of prey. “But that application of nanoscale materials is still at the laboratory level.”
There is, however, one place where nanoscale materials are already in use: in the microelectronics industry and the shrinking world of cell phones and computers. Haque collaborates with Intel, the giant computer chip manufacturer, to study how heat moves in the materials, such as low-K oxides, that Intel uses in its computer chips. His group also works on a phenomenon called interfacial thermal resistance (ITR), which is one of the dominant factors affecting thermal transport when materials shrink to the nanoscale. In microelectronics, the ability to dissipate heat is crucial, and materials with high ITR are poor conductors of heat due to scattering of phonons and electrons at the point that two different materials meet. This phenomenon is one of the most difficult properties to measure at the nanoscale, Haque said, so his group is developing new techniques to observe ITR under the microscope.
“In the 1970s, nobody thought that heat transfer would be a problem for computer engineers. Even in the eighties and nineties we could have a fan in a computer and it would cool well enough. But these days, if you are looking at high performance computer chips, you cannot cool computers with fans anymore,” Haque explained. This is where a nanoscale material could have immediate applications, Haque said. You could think of a nanoscale material that could go into nanoscale devices for applications like cooling transistors.
In fact, that may be just the kind of material Haque has developed. Although details are still confidential due to intellectual property concerns, he believes that he has discovered a material with ballistic transport properties. That means that the heat conductors, packets of heat energy called phonons, pass at high speed through the material without scattering or heating up the material. The material remains at room temperature as it dissipates heat from a source, in this case a heater but potentially a computer chip, into a heat sink. Think of it as room temperature superconductivity, but with phonons instead of electrons.
“This is a very old idea, but people have been doing it at very low temperature, as low as 4 Kelvin. At that low temperature there is very little scattering. We are seeing ballistic heat transfer not at 4 K but at 300 K, room temperature. Because there is no scattering, the material will transfer huge amounts of heat without getting heated at all. Batteries are also sensitive to temperature. People are trying to get very high energy density batteries, and it’s the same with electronics. Anywhere people are dealing with energy they will want either a good conductor or a poor conductor of thermal energy.”
What can you do with a lab on a chip?
A miniaturized lab that can be inserted into the sample holder of a TEM opens up possibilities that previously could only be understood through simulation and mathematical modeling. In the TEM, he can see defects in a material that can affect the material’s thermal, electronic, and mechanical properties. Those properties can be tested when particular dopants are added. He can watch in real time as a crystalline material becomes amorphous as he adds heat.
“These are unique research capabilities where you can see what is happening inside a material as you pass current, as you pass heat, as you transmit load. There is no guessing. This automatically connects experiment to theory. Because the theory people see what is happening, the theory they come up with is accurate and fast. I believe that is the most unique thing we are doing here at Penn State,” Haque said.
Recently, the Materials Research Institute hired senior scientist Bernd Kabius, an expert at devising experiments for high-end electron microscopy, to help Penn State advance the microscopy field using the new Titan double aberration corrected TEM. Kabius and Haque have already begun what they hope will be a brilliant partnership.
“Bernd is a catalyzing agent for Penn State,” Haque stated. “He and I are looking to do something revolutionary in the sense that we are designing ways to map temperatures inside the TEM. Right now I have some sensors that tell me what the temperature is at various points, but the technique we are going to develop is going to do temperature mapping in very high resolution at nanoscale.”
Haque predicts that this will be like the difference between taking an image of a material with a conventional camera and the same image with an infrared camera. Whereas current TEM techniques show the structure and chemistry of materials, their proposed technique would provide a map of temperatures throughout the structure. They are also planning to make similar maps of electrical fields.
“Imagine you could map mechanical, electrical, and thermal fields right inside the TEM. Right now we have nothing. All we can do is see. But with fantastic TEM capabilities, if we add fantastic in situ capabilities, it will make sense of everything,” Hague exclaimed.
A new thermal microscope could put Penn State on the map
Haque believes there is a big opportunity for Penn State in the field of micro and nanoscale thermal management, mostly because so few universities are well equipped with the instrumentation to do thermal work. One piece of equipment that is necessary is called a thermal reflectance microscope, a tool with the capability of measuring temperatures at very high resolution. With such an instrument, Haque suggested, he could immediately begin rounding up collaborators and training their groups to do their own measurements.
“When you have it (the TRM), everything goes fast. If we could get 10 collaborators, then definitely we could have something here at Penn State,” he said. A few days ago, Haque was told by the College of Engineering that he was approved to purchase the TRM he had proposed and that it would be housed in the Materials Characterization Laboratory in the Millennium Science Complex. The instrument should be installed in 2016.
Aman Haque is a professor in the Department of Mechanical and Nuclear Engineering. He can be contacted at email@example.com.