VOLTAGES AND CURRENTS DURING DEPOLARIZATION OF A FERROELECTRIC MATERIAL

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VOLTAGES AND CURRENTS DURING DEPOLARIZATION OF A FERROELECTRIC

MATERIAL

M. Bernard, Y. Fetiveau, G. Grange, J. Perrigot

To cite this version:

M. Bernard, Y. Fetiveau, G. Grange, J. Perrigot. VOLTAGES AND CURRENTS DURING DE-

POLARIZATION OF A FERROELECTRIC MATERIAL. Journal de Physique Colloques, 1972, 33

(C2), pp.C2-260-C2-262. �10.1051/jphyscol:1972291�. �jpa-00215027�

C2-260 M. BERNARD, Y. FETIVEAU, G. GRANGE ET J. PERRIGOT

2) Le courant de court-circuit est proportionnel à la vitesse de montée en température :

Au cours d'autres expériences une céramique de titanate de baryum a été déchargée dans différentes résistances. On a ainsi montré que, pour de faibles vitesses de montée en température, la céramique peut être considérée, à chaque température comprise entre 20 OC et 120 OC, comme une source linéaire de tension à vide V, et de résistance interne

Ri

III. Applications [7]. - Si nous supposons que la céramique débite, à chaque température, sur une charge adaptée :

l'énergie électrique fournie est :

Dans cette formule le facteur de vitesse dT/dt n'ap- paraît plus et l'énergie électrique ne dépend pas de la vitesse de montée en température mais seulement du 2 0 120 2 2 0 matériau et de l'intervalle de température Tl, T,.

T O C Les énergies obtenues, quelques mJ/cm3 pour la

FIO. 2. - Enregistrement simultané de Vv et Zcc pendant la dépo- céramique de BaTiO3 de la figure 2, sont faibles devant larisation d'une céramique de BaTi03. celles que l'on obtient en dépolarisation adiabatique.

VOLTAGES AND CURRENTS DURING DEPOLARIZATION OF A FERROELECTRIC MATERIAL

Abstract. - We describe a new method to calculate the electric energy that a polarized material is liable to supply, to any sort of load, during a phase transition. This method can also be used to improve the way of polarization and to compare different energy storage materials.

When an electric field is applied on a ferroelectric c) At high temperatures ( T > 120 OC for BaTiO,) ceramic material, the polarization may result from the grain boundaries trap the conduction electrons.

three different mechanisms [l]. This is space charge polarization, which is also rema- a) The external applied field displaces ions from nent Pl, 131, FI.

their stable equilibrium position and the electro- I t is interesting t0 be able t0 set in motion one of nic paths. This is called an induced polarization. It these mechanisms and to measure the corresponding disappears together with the field. energy.

b) When a molecule possesses a permanent electric 1. Operating procedure. - We can get rid of the dipolar moment it may be oriented by an applied space charge polarization (whis is less energetic and electric field. This polarization of orientation remains uninteresting for pyroelectric applications) by follow- after the field has been removed. ing the process illustrated in figure 1. 151, [6].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972291

VOLTAGES AND CURRENTS DURMG DEPOLARIZATION OF A FERROELECTRIC MATERIAL C2-261

vi .c cd,

a 3 2 0

U C) a,

<-I a, Fd u

Pi Cu

.

Tc ,120

al

V)

Fd 2 0

,G a

U U a,

<-I al O , +l +l al

FIG. 1. - Way of polarizing a material in the ferroelectric phase only.

Step 1) An alternative electric field is applied on the sample while we raise, then lower its temperature down to Curie temperature.

Step 2) While keeping the Curie temperature we slowly reduce to zero the alternative field.

Step 3) A continuous field is applied while the temperature is lowered to room-temperature.

Step 4) The sample is short-circuited at room- temperature.

Then to study the depolarization the sample under- goes a linear raising of temperature (from 20 OC to 220 OC during 20 mn) and we plot the open circuit voltage Voc the short-circuit current Isc.

II. Experimental results. - Figure 2 shows a recording of Vo, and Isc for a barium titanate ceramic.

The values of Voc and Isc depend on the raising speed of temperature :

1) For a given thickness of material, the open circuit voltage varies slightly with the raising speed of tempera- ture. For instance Voc varies from 1 to 1.5 when the speed varies from 1 to 15. Therefore Voc is approxima- tely a function of temperature only : Voc = f (T).

2) The short-circuit current is proportional to the raising speed temperature

Other experiments, in which a BaTiO, ceramic was discharged in different resistances, showed that for a slow raising in temperature a BaTi0, ceramic may be considered, at any temperature between 200C and

FIG. 2. - Simultaneous recording of Voe and Zsc during depola- rization of a BaTiO3 ceramic.

120 OC, as a fairly linear source of open circuit vol- tage Voc and of interna1 resistance

R i

III. Applications [7]. - If we suppose that the ceramic is loaded by a matched load,

at each temperature, the supplied electric energy is

t 2

W = ₄

1

$1

In this formula the speed factor dT/dt is no more present and the electric energy does not depend on the raising speed of temperature but only on the material and on the temperature interval Tl, T,.

The energies (a few mJ/cm3) which we get, with the BaTi03 ceramic of figure 2, are weak compared to those obtained by adiabatic depolarization.

M. BERNARD, Y. FETIVEAU, G. GRANGE AND J. PERRIGOT

References

Il] EYRAUD (L.), Diélectriques solides anisotropes et ferro- [5] GRANGE (G.), Etude expérimentale de la conversion électricité. Gauthiers-Villars. 1967. thermodiélectriaue. Thèse Lyon, 1967.

[2] TREDWLD (R. H.), Space charge conduction in solids, Elsevier, 1966.

[3] CHANUSSOT (G.), GODEFROY (G.), Etude des thermo- courants dans le titanate de baryum monocristal- lin. C. R. Acad. Sei. Paris, 1968, 266, 793 à 795.

[4] CHYNOWETH (A. G.), Surface space charge layers in barium titanate. Phys. Rev., 1956, 102,705 à 714.

- .

[6] PERRIGOT (J.), La pyroélectricité des céramiques pola- risées. Application à la conversion de l'énergie.

Thèse Lyon, 1969.

[7] EYRAUD (L.), La ferroélectricité et la conversion de i'énergie. Revue Générale de I'Electricité, 1968.

pp. 955 à 959.