As there is the transformation of heat energy to electric
Direct transformation of heat energy in elektricheyosky can be carried out, using the phenomena in contact of two metalyol or semiconductors where third-party forces which caused diffusion of the charged particles work.
The size of contact potential difference depends not only on properties of the contacting materials, but also on the contact temperature as energy of free electrons and their concentration are connected with temperature.
Considering the closed circuit from two different metals (fig. 1a), it is possible to be convinced that at the identical temperayotura of contacts 1 and 2 electric current in the chain will not turn out as the contact potential differences determined by the formula
Uk = (A1 – A2): e0
in both contacts are identical, but are sent to opposite stoyorona on the chain:
Uk1 - Uk2 = (A1 – A2) + (A2 - A1): e0 = 0
If one of contacts, for example 1 to heat (t1 > t2), then ravnoveyosy it will be broken — in contact 1 there will be the additional jump of potential connected with heating. In this case Uk1> UK2. In the chain thermo-electromotive force is formed (thermo-e. of page) which absolute value is proportional to the difference of temperatures of contacts:
Et = UKl - UK2 = E0 (t 1 t2),
where E0 — the size depending on properties of the metals forming contact.
Thus, thermo-e. of page arises in the chain consisting of razyony metals at the different temperature of junctions.
Termo-e. of page in the considered chain is supported thanks to heating of the seal 1, i.e. at the constant expense of heat energy. In turn, thermo-e. of page is the reason of electric current.
However concentration of free electrons in metals is big and upon transition from one metal to another changes very little. In this regard the contact potential difference is neznachitelyyony and depends on temperature a little. For this reason metal thermoelements have very small э. of page (in the seal of platinum and iron — 1,9 mV at the difference of temperatures of hot and cold seals of 100 °C), and efficiency does not exceed them 0,5%. Such thermoelements apply to measurement of temperatures (thermocouple).
For this purpose the chain of the thermocouple turns on the measuring instrument thermo-e. of page — the millivoltmeter (fig. 1, 6). The thermocouple in this case is the source of electrical energy, and the measuring device — the receiver.
Except contact of 1 main metals of the thermocouple their contacts with bonding wires are among themselves formed (Fig. 1 - 2, 3). In these contacts there are contact potential differences too, but they do not change thermo-e. of page if their temperature is maintained by identical.
In the presence of any pin count of different metals the sum of contact potential differences in the closed circuit remains equal to zero if all contacts have identical temperature. It is possible to be convinced of it, having worked out the equation similar to the aforesaid. Irrespective of the pin count, thermo-e. to of page it is proportional to the difference of temperatures of more heated contact and all other contacts which are at the identical temperature.
Unlike metals, in semiconductors at increase in temperature concentration of free electrons and holes strongly increase. This property of semiconductors allows to receive higher thermo-e. of page (up to 1 mV on 1 °C of the difference of temperatures) and efficiency of thermoelements up to 7%.
The semiconductor thermoelement consists of two semiconductors (п and р in fig. 2). One of them has electronic, and another hole conduction. When heating semiconductors in the junction their metal plate concentration of free charge carriers strongly increases. Therefore in semiconductors there is their diffusion from the hot end to cold. In the semiconductor with electron conduction by the cold end electrons therefore this end is loaded negatively move. In other semiconductor by the cold end holes, forming the positive charge move. The arisen potential difference counteracts diffusion, and at its some value the balance of forces of electric field and third-party forces under the influence of which there is the process of carrier diffusion is established. This potential difference also is thermo-e. of page of the semiconductor thermoelement.
If to connect the conducting element to the cold ends of semiconductors, for example, the resistor, then the closed circuit and electric current in it is formed.Top