A silicon heat engine, about the size of a bacterium and thought to be the smallest ever created, has been developed by scientists from NXP Semiconductors in Eindhoven. It is powered by an electrical current that heats an expanding and contracting silicon beam, and offers evidence of the potential for applying heat engines on the micro- and nano-scale.
The study, published in Nature Physics [Steeneken et al Nature Physics (2011) doi: 10.1038/nphys1871], revealed that a tiny silicon crystal can be made to mechanically oscillate when a D.C. current is applied to it. When there is enough current, a 280 nm silicon beam begins to operate as a heat engine similar to that in cars or steam engines.
The researchers have been working on mechanical micro-electro-mechanical system (MEMS) oscillators whose motion is detected by the piezoresistive effect, as they are better than quartz oscillators due to both the mechanical resonator and the transistor amplifier being made in/on silicon crystals. Silicon crystals can also be made much smaller than quartz crystals, as they can be structured using the same lithographic tools used to structure transistors, bringing down costs and allowing for higher resonance frequencies.
The team discovered the new method for generating motion using thermodynamic/thermal expansion forces from the piezoresistive effect; the change of electrical resistance of a material when it is mechanically compressed, for power supply and amplification. With no transistors necessary for amplification, a 1 mW current can be run through a piece of silicon crystal, making it start to vibrate at a very stable frequency, simplifying the production of mechanical oscillators significantly.
They also showed that the heat engines can modify the Brownian motion of a resonator and amplify these Brownian vibrations. And it can also be reversed, reducing the Brownian motion, making it operate as a tiny refrigerator.
When the current is applied to the crystal, it spontaneously oscillates mechanically at a frequency of 1.3 MHz, more than a million times each second. They measured the high-frequency motion by using slow-motion video under a microscope with stroboscopic illumination.
The silicon beam, called the engine beam, was used to drive a larger silicon structure in the shape of a pendulum. When electricity flows through a resistor, it generates heat that makes the resistor’s temperature increase. With the power focused in a silicon resistor beam of very small volume, a huge heating power per volume is generated in the beam.
The oscillation from the heat engine could have applications in smaller, simpler and cheaper watches or other electronic devices, or for use in microscopic clocks or as a sensor. The researchers now hope to demonstrate applications for the oscillator, and improve its performance and operation.


Laurie Donaldson