Dielektrika

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Thermodielectric effect                                                                     Cz     En


    This interestig effect was discovered by Joaquim da Costa Ribeiro in 1944.  This Brazilian physicist observed that solidification and melting of many dielectrics are accompanied by charge separation. Thermodielectric effect was demonstrated with carnauba wax, naphtalene and paraffin. Charge separation in ice was also expected. This effect was observed during water freezing period, electrical storm effects can be caused by this strange phenomenon. Effect was measured by many researches - Bernardo Gross, Armando Dias Tavares, Sergio Mascarenhas etc. César Lattes (co-discoverer of the pion) supposed that this was the only effect ever to be discovered entirely in Brazil.
 
Fig. I - Joaquim da Costa Ribeiro                    Fig. II - Sergio Mascarenhas

Costa Ribeiro concluded that the effect was a general one: Production of currents and charge separation during phase transition. The basic mechanism is explained as follows. A contact potential difference PD (whether for ions or electrons makes no material difference) exist between the solid and the liquid. This contact PD results in the formation of a double layer at the interface under static conditions. If the interface moves, as in solidification or melting (Fig. III), the double layer would have to move with it. However, owing to the low conductivity of the solid site, the solid component of the charge can not dissipate fast enough to keep up with the interface; hence as the interface advances it leaves behind a trail of space charge in the solid which builds up a potential difference between the region deep in the solid and the liquid (electrolyte) which can be many orders of magnitude larger than the contact PD. Thus the interface behaves like a selectively permeable membrane allowing one kind of charge carriers to pass through more easily than the other, and "pushing along" with it a surface charge consisting of the excess charge left in the liquid. The combined fields of space and surface charges results in a counter electromagnetic field at the interface which gradually neutralizes the contact PD, so that the trail of space charge decreases as the interface advances and a limiting interface field is reached. The fraction of the double layer charge which is actually left in behind as space charge depends upon the velocity of advance of the interface and the rate at which charge is dissipated across the interface by back-diffusion. [1]

   thermodielectric effect
Fig. III - Thermodielectric effect   

Thermodielectric effect measurement
    For my measurement I used condenser with two electrodes positioned in a glass tray. One electrode was a circuit metal plate, the second electrode was a small metal sieve (Fig. IV). As a dielectric material I used paraffin.
Condenser
Fig. IV - Condenser in the glass tray

    I melted paraffin in an acceptable pot and then I fill the glass tray with it (Fig. V).
Liquid paraffin
Fig. V - Liquid paraffin in the glass tray

    Then I laid the glass tray to the block of ice. The bottom of glass tray was colder than other parts and the interface was formed at plate electrode. Then I linked the condenser with accurate voltmeter and surveyed the potential difference between electrodes (Fig. VI).
Glass tray before measurement
Fig. VI - The condenser linked to voltmeter

    I measured the potential difference between electrodes in convenient segments of time (1 minute). Results are published in following table (Tab. I).
Time dependency of potential diffenence U during thermodielectric effect
t [min] U [mV]
1 0
2 0
3 200
4 580
5 960
6 880
7 640
8 380
9 420
10 360
11 240
12 160
13 100
14 80
15 30
16 0
Tab. I - Time dependency of potential difference during thermodielectric effect
    Graphic representation:
graph
    The measurement was repeated for several times. The time dependency of potential difference was similar in these experiments.
The tray after experiment
Fig. VII - The glass tray after paraffin solidification


REFERENCES:
[1] Gross, B.: Theory of Thermodielectric effect. Physical Review 94, pp. 1545-1551, 1954.