Thermal diode

The term thermal diode is sometimes used for a (possibly non-electrical) device which causes 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.

One-way heat-flow

A thermal diode can be:

Due to the second law of thermodynamics a passive system cannot move heat from a cold source to a hot destination. But the law allows to avoid or minimize the flow in this direction. If the source is hotter than the destination heat flows with a low thermal resistance towards the destination.

Electrical diode thermal effect or function

This application of thermal diode is based on its property 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.

Peltier devices

Advancements

A team at MIT is working for construction of thermal diodes that convert heat to electricity at lower temperatures than before.[1] 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.[2]

In April 2015 Italian researchers at CNR announced development of a working thermal diode,[3] publishing results on Nature Nanotechnology [4]

See also

References

  1. MIT News - Turning heat to electricity
  2. http://www.cnr.it/cnr/news/CnrNews.html?IDn=3225| CNR - Diodo termico, dove il calore va a senso unico
  3. http://www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2015.11.html| Nature Nanotechnology - Rectification of electronic heat current by a hybrid thermal diode (Maria José Martínez-Pérez,Antonio Fornieri, Francesco Giazotto)

External links

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