The term "thermal diode" is sometimes used for a (possibly non-electrical) device which allows heat to flow preferentially in one direction. Or, the term may be used to describe an electrical (semiconductor) diode in reference to a thermal effect or function. Or the term may be used to describe both situations, where an electrical diode is used as a heat-pump or thermoelectric cooler.
A thermal diode in this sense is a device whose thermal resistance is different for heat flow in one direction than for heat flow in the other direction. I.e., when the thermal diode's first terminal is hotter than the second, heat will flow easily from the first to the second, but when the second terminal is hotter than the first, little heat will flow from the second to the first.
Such an effect was first observed in a copper–cuprous-oxide interface by Chauncey Starr in the 1930s. Beginning in 2002, theoretical models were proposed to explain this effect. In 2006 the first microscopic solid-state thermal diodes were built. In April 2015 Italian researchers at CNR announced development of a working thermal diode, publishing results in Nature Nanotechnology.
Electrical diode thermal effect or function
A sensor device embedded on microprocessors used to monitor the temperature of the processor's die is also known as a "thermal diode".
This application of thermal diode is based on the property of electrical diodes to change voltage across it linearly according to temperature. As the temperature increases, diodes' forward voltage decreases. Microprocessors having high clock rate encounter high thermal loads. To monitor the temperature limits thermal diodes are used. They are usually placed in that part of the processor core where highest temperature is encountered. Voltage developed across it varies with the temperature of the diode. All modern Intel CPUs have on-chip thermal diodes. As they are right there in the middle it provides most relevant CPU temperature readings. The silicon diodes have temperature dependency of -2mV per degree Celsius. Thus we can determine the junction temperature by passing a current through the diode and then measuring voltage developed across it. In addition to processors, the same technology is widely used in dedicated temperature sensor IC's.
Thermoelectric heat-pump or cooler
There are two types. One uses semiconductor, or less efficient metal, i.e. thermocouples, working on the principles of the Peltier-Seebeck effect. The other relies on vacuum tubes and the principles of thermionic emission.
A team at MIT is working for construction of thermal diodes that convert heat to electricity at lower temperatures than before. This can be used in construction of engines or in electricity production. The efficiency of present thermal diodes is about 18% between the temperature range of 200-300 degree Celsius.
- Wang, Lei; Li, Baoweng (March 2008). "Phononics gets hot" (PDF). Physics World.
- https://www.cnr.it/it/comunicato-stampa/6045/ CNR - Diodo termico, dove il calore va a senso unico
- https://www.nature.com/articles/nnano.2015.11 Nature Nanotechnology - Rectification of electronic heat current by a hybrid thermal diode (Maria José Martínez-Pérez,Antonio Fornieri, Francesco Giazotto)
- MIT News - Turning heat to electricity