Heat is the great enemy of modern electronics—it can spawn errors and fry components. But now scientists have turned heat to their advantage by creating devices that run on heat instead of electricity. The advance could lead to thermal computers that run off of body heat or other waste heat from our surroundings.
A heat current is simply the flow of energy from a hotter object to a colder object. Imagine heating a metal pipe at one end: Heat flows from the hot end to the cold end, and at every point along the pipe the temperature diminishes. In physics terms, there's a "uniform temperature gradient." The heat takes this simple path because the pipe conducts the same amount of heat in every place and in every direction.
Yet materials don't have to conduct heat so simply. If you stacked alternating sheets of a material that conducts heat and another that insulates it, the heat would be conducted more freely sideways than in the top-to-bottom direction. Electrical engineers are familiar with this principle: It's the same one that makes resistors, one of the most common electrical components, conduct more when wired in parallel than when wired in series. The breakthrough of the new research is to tailor composite materials so that their thermal conduction is not just side to side or top to bottom, but in a direction that changes throughout. "Heat current, like electric current, should be viewed as a medium that can be manipulated, controlled, and processed," says study author Yuki Sato, a physicist at Harvard University.
Sato and Harvard colleague Supradeep Narayana's simplest demonstration is of a thermal shield: a device that excludes a heat current from a certain region. It consists of a cylinder, roughly as big as a C-size battery, containing 40 alternating, concentric layers of natural rubber and boron nitride-infused silicone elastomer. The researchers cast the device in a block of conductive jelly and kept one side warm and the other cold. With no shield in place, the heat would have flowed through the jelly in a uniform temperature gradient, as in the pipe example. However, when the researchers viewed the temperature gradient from above with an infrared camera, they found that their shield almost totally excluded the heat current from the region inside without affecting its flow in the jelly outside.
The Harvard pair performed a couple more tricks. In one, they swapped the shield's concentric structure for larger, alternating layers that splayed outward like a vacuum-cleaner brush. This device concentrated the heat current so that the entire warm-to-cold temperature gradient, instead of being spread uniformly across the jelly, was focused into the small region inside. Next, Sato and Narayana modified the device so that its layers wrapped around like a twisted mop. Amazingly, this "inverter" turned around the heat current inside by 180°, so that it flowed in the opposite way than the jelly outside. The findings are due to be published in Physical Review Letters.
"This is an interesting subject," says Alex Zettl, a physicist at the University of California, Berkeley. "The manipulation of heat currents has not been studied much or exploited. … Nevertheless, there are likely situations where heat currents can be successfully harnessed for computation and other applications."
Computation, which requires devices such as transistors to switch based on different inputs, would be a big step. But not too big a step, explains Sato. Materials already exist whose conductivity depends on temperature; if such materials were used in the researchers' inverter, the device would switch around the heat current only if the surroundings were warm enough. This, says Sato, would be the basis of thermal computation.
In principle, a thermal computer could be used for the same tasks as a normal computer, such as word processing or surfing the Internet, says Jiping Huang, a physicist at Fudan University in Shanghai, China. But a thermal computer would benefit from being energy saving, because it could run off waste heat in the environment—even heat produced by a human body.
Huang stresses that there are practical hurdles, however. Unlike electricity, heat can transfer by three processes: conduction, convection, and radiation. This means physicists will have to learn how to limit the influence of unwanted heat. "The dream is pretty brilliant," says Huang, "but the reality is quite cruel."