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 resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
)
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A 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 h ...
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 kelvin ...
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.
Abstr ...
.
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 ...
s operating i
gravitymay 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
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
, 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 the ...
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 mate ...
.
Peltier devices
* a heat engine working backwards as a
refrigerator
A refrigerator, colloquially fridge, is a commercial and home appliance consisting of a thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to its external environment so th ...
, 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.
See also
*
Drinking bird
Drinking birds, also known as insatiable birdies, dunking birds, drinky birds, water birds, dipping birds, and “Sippy Chickens” are toy heat engines that mimic the motions of a bird drinking from a water source. They are sometimes incorrectly ...
*
Loop heat pipe A loop heat pipe (LHP) is a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being able to provide rel ...
*
Thermosiphon
Thermosiphon (or thermosyphon) is a method of passive heat exchange, based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heat ...
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