Physicists in the Netherlands have built a heat engine that might be the tiniest ever created. Based on “piezoresistive” silicon, and smaller than a typical biological cell, the engine could find applications in watch mechanisms or as a mechanical sensor.
Engines come in a variety of sizes. The smallest include biological engines such as the flagella that bacteria use for locomotion, which are driven by chemical reactions, or manmade electrostatic engines, which drive ions with electric fields.
But heat engines, which usually rely on the expansion and contraction of liquids or gas, are trickier to downscale. As the devices get smaller, engineers find it harder to design structures that can handle the high pressures and fluid velocities required for a reasonable power output. The efficiency also tends to decrease, because it requires large temperature differences as given by the famous Carnot heat-engine equations. For these reasons, liquid- or gas-driven heat engines rarely get smaller than around 107 µm3.
In a paper published today in Nature Physics, however, Peter Steeneken and colleagues at NXP Semiconductors in Eindhoven easily overcome this threshold with a heat engine driven by the movement of a solid – in particular, a piezoresistive mass of crystalline silicon. Piezoresistive materials are unique in that their electrical resistance changes with applied stress: when a piezoresistive material is compressed, its resistance increases, and when it expands, its resistance decreases.
The tiny engine consists of a flat resonator of crystalline silicon, 1125 µm3 in size, with two small parallel beams, 0.34 µm3 in size, at one end – rather like a tuning fork with a heavy base. Both beams are anchored such that the compression or extension of one beam, the “engine” beam, heated by a DC current of just over one milliamp, bends the entire device up or down.