Investigation of the effect of thermal emf in carbon fibers

. The article presents the result of a study of the occurrence of the thermoelectric effect in contact "modified carbon fiber - the original carbon fiber". The description of the method of modification of carbon fiber by pulsed current is given. The supposed physical process of this phenomenon is shown. The possibility of implementing this effect in rocket and space technology is being considered.


Introduction
Currently, the idea of using carbon fiber (CF) as a reinforcing element in the elements of rocket and space technology is being implemented.It is known that the first samples of such products have already been created (Fig. 1).
Carbon fiber has a number of positive properties, such as high strength, low specific gravity, low coefficient of thermal expansion and chemical inertia.But, in addition, when studying the properties of carbon fiber, the presence of a thermoelectric effect was recorded [1][2].
The problems that are traditional for thermo-EMF are especially acute for organic nanocomposites, since they have high resistance and a low coefficient of thermo-EMF.But because of their outstanding physical and mechanical properties, research is being conducted in the field of increasing the power factor of these thermoelectric materials [3][4][5][6][7][8][9][10][11][12][13][14][15].
There are many examples of studies of the thermoelectric effect in polymers with carbon nanofillers [6] and composite films [7].For this direction, the obtained flexible and lightweight thermoelectric materials are used.These are n-and p-type semiconductors [10,14].To increase the power factor of such materials, alloying and other changes in their chemical composition or structure are used [5,[11][12][13].The method discussed below for obtaining thermoelectric inhomogeneity differs from the methods mentioned.The relevance of the work lies in the use of structural elements of launch vehicles and spacecraft as an additional source of energy that feeds on-board systems.This will make it possible to exclude the placement on launch vehicles and spacecraft of other equipment designed to generate energy.This will lead to a decrease in the mass and size characteristics of the rocket, an increase in its efficiency and the power-to-weight ratio.

Production of modified carbon fiber
During the experiments, TenaxR HTS 5331 carbon fiber was used (Fig. 2.).The presence of thermal EMF was noticed after experiments on passing a pulsed current through a carbon fiber using an installation (Fig. 3).A 500 microfarad pulse capacitor bank was charged to 600 V -5 kV, then switched to carbon fiber.As a result, a current passed through the carbon fiber for microseconds, the values of which reached about 1 kA [2].Carbon fiber was instantly heated to high temperatures.An electrical explosion was visually recorded.Plasma formation was assumed on the basis of a sharp decrease in the electrical resistance of the sample [3].
The experiment was conducted in an air atmosphere.The main result of the experiment was that a contact potential difference is created at the point of transition "modified CFinitial CF".

Fixing the thermal emf in a carbon fiber circuit
The manifestation of this effect is observed when the free ends of the original HC and the modified CF are connected.A circuit is formed, which is a closed loop in which there are two contacts "modified CF -original CF" (Fig. 4).When one of the contacts heats up, an EMF appears in the circuit (Fig. 5), which indicates the presence of a potential difference at the contacts and a change in the properties of the modified CF.

Results and discussion
The appearance of this effect can be explained by a change in the type of carbon conductivity from n-type to p-type [4].But the question arises: what led to the appearance of new levels of acceptors in carbon fiber or the disappearance of previously existing levels of donors?
The chemical composition of the CF was not changed by the addition of new impurities during the experiment.It can be assumed that the reason for the change in the type of conductivity was the interaction of atmospheric oxygen with heated CF.But as a result of the experiment in a technical vacuum, there was also a change in the CF with further fixation of the thermal EMF.
It was also found that the effect of thermal EMF on the "modified CF -initial CF" contact persists over time.Repeated fixation of the effect was carried out after a few months.That is, any changes in the type of oxide film burnout or CF ionization cannot explain the presence of this effect, since after a short period of time, the CF would return to its original state.
It was verified that during prolonged heating of the CF current, the effect of thermal EMF on the contact "modified CF -original CF" was not observed.The high speed of the process certainly plays a crucial role.
Based on this, a new assumption was put forward about the nature of the processes occurring during the modification of CF.
Carbon fiber, depending on the processing method, has a carbon content of 80% to 90.5%, as well as hydrogen and heteroatoms N, O, Si.Since carbon (like silicon) belongs to the fourth group of the periodic table of chemical elements of D.I. Mendeleev, it acts as its own semiconductor (has an equal number of positive and negative charge carriers).
As is known, the contact potential difference is provided by the difference in the work of the electron output of dissimilar materials, calculated by the formula: where Fp is the work of the electron output from the semiconductor into vacuum, e is the electron charge.In direct contact, an internal contact potential difference is observed: where Фп is the work of the electron exit from the semiconductor into vacuum, e is the electron charge.
With direct contact, an internal contact potential difference is observed: where EF is the Fermi energy of the contacting material.Thus, it turns out that the observed contact potential difference is a consequence of a change in the Fermi level CF.In addition, there is no explicit temperature in this formula, but thermal EMF directly depends on it.Since the electron charge is constant, the Fermi energy must depend on temperature.Since the assumption of impurity conductivity as the cause of the contact potential difference does not fully reveal the picture of the processes taking place, we can consider CF as its own semiconductor.
In the case of intrinsic semiconductivity with increasing temperature, the Fermi energy can linearly increase, decrease or remain unchanged (Fig. 6.), which follows from the formula: where EF is the Fermi energy,   is the lower bound of the valence band,   is the band gap, k is the Boltzmann constant, T is the temperature,   is the effective mass of the hole,   is the effective mass of the electron.
Usually these changes are neglected because they are insignificant.But there are also substances (InSb), when working with which it is necessary to take into account such changes.If our materials have different Fermi dependence on temperature, as a result of different effective mass of charge carriers, then with increasing temperature the contact potential difference will increase and, as a consequence, EMF will be observed in the circuit during heating.At T = 0 K energy the Fermi level coincides with the middle of the forbidden zone.If the effective mass of holes is greater than the effective mass of electrons, then the natural logarithm takes positive values, and the Fermi energy increases with increasing temperature.If the effective mass of the electron is higher, then the Fermi energy decreases with increasing temperature.If the effective masses are equal, the Fermi energy will remain unchanged.
Thus, the presence of the thermo-EMF effect in carbon fiber can tell us that the ratio of the effective mass of the positive charge carrier to the effective mass of the electron in the modified CF has changed.
The effective mass of the charge carrier is calculated by the formula: where k is the wave vector of the electron, ћ is the cyclic Planck constant, E is the energy of the charge carrier.
Since the cyclic Planck constant is a constant, let's turn to the functional dependence of the electron energy on the wave vector: () =   +  + 2() , (5) where EA is the energy of the atomic level from which the zone was formed; the shift of the c-level under the action of the field of neighboring atoms; A is the exchange integral, which takes into account the possibility of moving electrons from atom to atom as a result of the overlap of their wave functions; a is the crystal parameter (the distance between atoms).It follows from the obtained equality that the second derivative of the energy in the wave vector clearly depends on the crystal parameter and the exchange integral.The larger the exchange integral, the greater the overlap of wave functions, and the overlap of wave functions also depends on the crystal parameter.That is, the reason for the appearance of a contact potential difference could be a change in the distance between CF atoms.
Thus, the appearance of the thermoelectric effect at the contact of "modified CForiginal CF" is a consequence of the restructuring of the carbon fiber structure.This allows us to draw an analogy between the processes occurring in carbon fiber and the hardening of steel, which occurs as a result of lagging the restructuring of the iron-carbon crystal lattice from the heating-cooling processes, which leads to a change in the properties of the material.

Conclusion
For the first time, a mechanism for the occurrence of thermal EMF at the "modified FCoriginal FC" contact was proposed due to changes in the effective masses of the charge carriers of the FC.This effect is observed only in the case of very rapid heating of CF, so an analogy was drawn with the process of hardening steel.The considered method of obtaining the thermoelectric effect allows to obtain electricity from elements of rocket and space technology made of carbon materials.It is known that the temperature difference on the illuminated and shadow side of the spacecraft reaches hundreds of degrees, and the temperature differences inside and outside the LRE are measured in thousands of degrees.So it will be possible to receive energy from the body of the spacecraft or any units, during the operation of which a temperature drop will be formed.In addition, this phenomenon is promising for the development of alternative methods and techniques for obtaining thermoelements for the implementation of thermocouples, Peltier elements and other devices operating on contact phenomena.

Fig. 1 .
Fig.1.An example of the use of carbon composites for the manufacture of a launch vehicle body[1]

Fig. 3 .
Fig. 3. Schematic diagram of the installation used to replace carbon fiber: E -power supply, Aswitch, C -capacitor bank, L0 -inductance of the simulated circuit, R0 -resistance of the simulated circuit, V -voltmeters

Fig. 4 .
Fig. 4. Diagram of the contour formed by the connection of the original CF (1) and the modified FC (2)

Fig. 5 .
Fig. 5.The occurrence of thermal EMF at the time of heating of one of the contacts of thermoelectrically heterogeneous sections of CF