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.
Contents |
The diode is temperature sensor. It is a pnp bipolar junction transistor whose base terminal is grounded externally and constant current is pumped through emitter which develops voltage across base emitter junction.
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.
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.[1] 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 dependancy 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.[2][3]
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 power production. The efficiency of present thermal diodes is about 18% between the temperature range of 200-300 degree celsius.The researchers have found various ways of improving the efficiency of the thermal diodes.Basically, they incorporated two changes- 1.The researchers found an efficient way of moving the electrons across the device. 2.They found a way of blocking the electrons from returning back.This efficiently increased the voltage of forward current . It made the energy conversion more efficient.[4]