Physicists design material that conducts one way and insulates the other.
Stiffness and springiness could make heat stop and go.
European physicists have sketched out a blueprint for a valve that lets heat pass only one way. The proposed material conducts heat flowing in one direction, but also behaves as an insulator, stopping it going the other way1.
In theory, a heat valve could keep parts of microelectronic circuitry cool or channel heat to chip-sized chemical reactors, which are currently being developed for high-efficiency chemical synthesis or ultra-sensitive analysis.
Heat corresponds to the movement of atoms. When atoms are joined together in molecules, they vibrate back and forth. The larger the vibrations, the hotter the material.
Heat is conducted along a chain of particles because vibrations travel from one particle to the next. If one end is attached to a hot material and the other to something cooler, the hot end jiggles more. This jiggling goes down the chain to the cool end.
A chain of particles of identical weights linked by ideal, so-called harmonic springs vibrates at the same frequency irrespective of the amplitude of the vibrations. In real chains, like DNA, the links are not ideal, but anharmonic: their vibration frequency depends on amplitude.
One-way heat transfer would make use of anharmonicity. A chain divided into three sections, say Terraneos team, can insulate heat if the middle section acts like a strongly anharmonic spring while the outer two are softer and more harmonic.
If one of the end sections is stiffer than the other, more heat can flow in one direction than the other. The anharmonic middle of the chain can jiggle in sympathy with the stiff end when it is cold and the soft end when it is hot - but not vice versa. So the chain transmits heat from a hot, soft end to a cool stiff end. If the stiff end gets hot and the soft cold, the middle section blocks vibrations, so the chain as a whole acts as an insulator.
PHILIP BALL | © Nature News Service
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