How Heat Flows
Through a combination of atomic-scale materials design and ultrafast measurements, researchers at the University of Illinois have revealed new insights about how heat flows across an interface between two materials. This is important because improved control of heat exchange is a key element to enhancing the performance of current technologies such as ICs and combustion engines as well as emerging technologies such as thermoelectric devices, which harvest renewable energy from waste heat. Achieving control, however, is hampered by an incomplete understanding of how heat is conducted through and between materials. The researchers demonstrated that a single layer of atoms could disrupt or enhance heat flow across an interface.
David Cahill, a Willett Professor and the head of materials science and engineering at Illinois and co-author of the paper said, “Heat travels through electrically insulating material via ‘phonons,’ which are collective vibrations of atoms that travel like waves through a material. Compared to our knowledge of how electricity and light travel through materials, scientists’ knowledge of heat flow is rather rudimentary.”
One reason such knowledge remains elusive is the difficulty of accurately measuring temperatures, especially at small-length scales and over short time periods – the parameters that many micro- and nano-devices operate under. Over the past decade, Cahill’s group has refined a measurement technique using very short laser pulses, lasting only one trillionth of a second, to probe heat flow accurately with nanometer-depth resolution. He teamed with Paul Braun, the Racheff Professor of Materials Science and Engineering at the University of Illinois and a leader in nanoscale materials synthesis, to apply the technique to understanding how atomic-scale features affect heat transport.
Braun said the experiments used a ‘molecular sandwich’ that allowed them to manipulate and study the effect that chemistry at the interface has on heat flow, at an atomic scale. The researchers assembled their molecular sandwich by first depositing a single layer of molecules on a quartz surface. Next, through a technique known as transfer-printing, they placed a very thin gold film on top of these molecules. Then they applied a heat pulse to the gold layer and measured how it traveled through the sandwich to the quartz at the bottom. By adjusting just the composition of the molecules in contact with the gold layer, the group observed a change in heat transfer depending on how strongly the molecule bonded to the gold. They demonstrated that stronger bonding produced a twofold increase in heat flow.
The Illinois group is already working toward a deeper fundamental understanding of heat transfer by refining measurement methods for quantifying interfacial bonding stiffness, as well as investigating temperature dependence, which will reveal a better fundamental picture of how the changes in interface chemistry are disrupting or enhancing the flow of heat across the interface.
Growing Computer Components From Scratch
Researchers at the University of Leeds have used a type of bacterium which ‘eats’ iron to create a surface of magnets, similar to those found in traditional hard drives and wiring and creates tiny magnets within itself as it ingests the iron. The team has also begun to understand how the proteins inside these bacteria collect, shape and position these “nanomagnets” inside their cells and can now replicate this behavior outside the bacteria. The research team hopes to develop a bottom up approach to creating cheaper, more environmentally friendly electronic devices.
Dr. Sarah Staniland from the University of Leed’s School of Physics and Astronomy, who led the project as part of a longstanding collaboration with the Tokyo University of Agriculture and Technology said, “We are quickly reaching the limits of traditional electronic manufacturing as computer components get smaller. The machines we’ve traditionally used to build them are clumsy at such small scales. Nature has provided us with the perfect tool to circumvent this problem.”
The magnetic array was created by Leeds PhD student Johanna Galloway using a protein that creates perfect nanocrystals of magnetite inside the bacterium Magnetospirilllum magneticum. Using a process akin to potato-printing on a much smaller scale, the protein was attached to a gold surface in a checkerboard pattern and placed in a solution containing iron. At a temperature of 80°C, similarly-sized crystals of magnetite form on the sections of the surface covered by the protein and the team are currently working to reduce the size of these islands of magnets in order to make arrays of single nanomagnets. They also plan to vary the magnetic materials that this protein can control. These next steps would allow each of these nanomagnets to hold one bit of information allowing the construction of better hard drives.
Galloway explained that using today’s top-down method that sculpts tiny magnets out of a big magnet it is increasingly difficult to produce the small magnets of the same size and shape which are needed to store data but with this new method the proteins do all the hard work; they gather the iron, create the most magnetic compound, and arrange it into regularly-sized cubes.
A different protein was used to create tiny electrical wires by Dr Masayoshi Tanaka, during a secondment to Leeds from Tokyo University of Agriculture and Technology: these ‘nanowires’ are made of ‘quantum dots’ – particles of copper indium sulphide and zinc sulphide which glow and conduct electricity – and are encased by fat molecules, or lipids. The magnetic bacteria contain a protein that moulds mini compartments for the nanomagnets to be formed in using the cell membrane lipids. Dr. Tanaka used a similar protein to make tubes of fat containing quantum dots – biological-based wiring.
“It is possible to tune these biological wires to have a particular electrical resistance. In the future, they could be grown connected to other components as part of an entirely biological computer,” said Dr Tanaka.
The research group and the team at Tokyo University of Agriculture and Technology plan to examine the biological processes behind the behavior of these proteins in order to develop a toolkit of proteins and chemicals that could be used to grow computer components from scratch.
—Ann Steffora Mutschler