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 A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

) 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 A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...

or thermoelectric cooler.

One-way heat-flow

A thermal diode in this sense is a device whose

thermal resistance Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. * (Absolute) thermal resistance ''R'' in kelvi ...

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 Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

– 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 ''Nature Nanotechnology'' is a monthly peer-reviewed scientific journal published by Nature Publishing Group. It was established in October 2006. The editor-in-chief is Fabio Pulizzi. It covers all aspects of nanoscience and nanotechnology. Abst ...

. Thermal siphons can act as a one-way heat flow.

Heat pipe A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor b ...

s operating i
gravity
may also have this effect.

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 AMD and Intel CPUs, as well as AMD and Nvidia GPUs have on-chip thermal diodes. As the sensor is located directly on the processor die, it provides most local and relevant CPU and GPU temperature readings. The silicon diodes have temperature dependency of -2mV per degree Celsius. Thus the junction temperature can be determined by passing a set 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

, or less efficient metal, i.e.

thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...

s, working on the principles of the

Peltier-Seebeck effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...

. The other relies on vacuum tubes and the principles of

thermionic emission Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...

Peltier devices

* a heat engine working backwards as a refrigerator, such as a

Peltier device Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side o ...

(diode)

Advancements

As of 2009 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.

References

External links

A news article on the increases in semiconductor thermal diode efficiency Thermoelectric Power Generation: Converting Low-Grade Heat into Electricity (Equations for semiconductor thermal diodes)
{{DEFAULTSORT:Thermal Diode Diodes Thermodynamics