EFFECT OF γ-IRRADIATION ON THE DIELECTRIC PROPERTIES OF CERAMICS BASED ON BARIUM TITANATE

G. A. URALOV

(Presented by the scientific seminar of the Research Institute of Nuclear Physics)

A large number of works have been devoted to the study of the properties of single crystals and ceramics of barium titanate [1–4]. However, despite the great interest in barium titanate, the effect of ionizing radiation on BaTiO\(_3\) and ceramics based on it has at present been almost unstudied. The existing works describe the behavior of barium titanate under neutron irradiation [5]. The effect of γ-quanta on ceramics based on barium titanate, however, has not yet been studied.

Ceramics based on barium titanate may find broad application in microwave electronics [6] and accelerator technology [7]. In the latter case, the ceramics will operate in a γ-radiation field. It is therefore of interest to investigate the effect of γ-radiation on the electrical properties of ceramics in the microwave range.

The electrical properties of materials can be characterized by two quantities: the permittivity \(\varepsilon'\) and the loss tangent \(\operatorname{tg}\delta\). Since these quantities are related to conductivity, they must change under the action of ionizing radiation.

In the present work, experimental results are described for a study of the properties of ceramics based on barium titanate under the action of γ-radiation from a Co\(^ {60}\) source in the microwave range \((\lambda = 10 \text{ cm})\).

To measure \(\varepsilon'\) and \(\operatorname{tg}\delta\) of ceramics in a γ-radiation field, an apparatus was constructed that satisfied the following requirements:

1) the apparatus must allow remote measurement of dielectrics with large values of dielectric permittivity. This requirement follows from considerations of radiation safety;

2) the apparatus must allow simultaneous measurement of permittivity and the loss tangent. This requirement follows from the fact that the samples have a certain spread in \(\varepsilon'\) and \(\operatorname{tg}\delta\), and also from the fact that \(\operatorname{tg}\delta = f(\varepsilon')\);

3) to increase the accuracy of measurement, it is necessary to measure quantities that depend on \(\varepsilon'\) and \(\operatorname{tg}\delta\) in as short a time interval as possible.

For measuring the permittivity and losses of dielectrics with large \(\varepsilon'\), resonant methods are the most acceptable [8]. A block diagram of the apparatus is shown in Fig. 1.

The requirement of remote measurement leads to the fact that a considerable fraction of the energy of the microwave generator will be lost in the transmission lines of the apparatus. Therefore generator 1 was chosen with a high output power level (GS-22). The signal from the microwave generator, through attenuator 2, is fed to resonator 3, which is a rectangular resonator on the \(H_{10}\) wave, partially filled with the dielectric under study. The ceramics of grades T-80 and T-150 under investigation were

Fig. 1. Block diagram of the setup:
1—microwave pulse generator (GS-22), 2—attenuator, 3—resonator, 4—detector, 5—oscilloscope, 6—rectangular-pulse generator, 7—delay line

made in the form of rectangular plates with a cross section equal to the cross section of the resonator and with a thickness close to a quarter-wave layer. The plate is placed at a distance of 1 mm from the end face of the resonator. This is due to the fact that, as calculations showed, in this case the highest accuracy is obtained in measuring \(\varepsilon'\) and \(\tg \delta\) of dielectrics at large values of the permittivity. The signal from the resonator, through detector 4, is fed to the input of oscilloscope 5.

In order to satisfy the requirement that the measurement of the resonant frequency and quality factor be short in time, the apparatus used a modification of the decay-decrement method [9], which makes it possible to measure rapidly the parameters of low-\(Q\) resonators. For this purpose rectangular-pulse generator 6 and delay line 7 are used. A negative pulse from generator 6, through delay line 7, is fed to the modulating electrode of oscilloscope 5, while a positive pulse is fed to the microwave modulation of generator 1. Standard instruments 26I and MGI-1 were used as the rectangular-pulse generator and the delay line, respectively.

As calculations showed, using the described circuit it is possible to measure the permittivity and the tangent of the loss angle with maximum errors of 5 and 30%, respectively.

The apparatus described above made it possible to carry out studies of changes in the electrical properties of barium-titanate-based ceramics in the microwave range before, during, and after irradiation. The source of \(\gamma\)-quanta was \(^{60}\mathrm{Co}\) from the gamma installation of the Institute of Nuclear Physics of the Academy of Sciences of the Uzbek SSR. The experiments were carried out at a temperature of \(28^\circ\mathrm{C}\). The experimental results are presented in Figs. 2–4.

Figure 2 shows the change in the ratio of the loss tangent in the field of \(\gamma\)-radiation, \(\tg \delta\), to the loss tangent before irradiation, \(\tg \delta_0\), as a function of the logarithm of the integral dose, \(\lg D\), at a dose rate of 11 r/sec. As can be seen from Fig. 2, in the dose interval \(10^3\)–\(10^7\) r the ratio \(\tg \delta/\tg \delta_0\) remains constant. It is quite possible that the initial point 1 is shifted to the left because the irradiation time up to an integral dose of \(10^3\) r is short and it is difficult to obtain experimental points

in this region proved impossible. Similarly, the annealing time (here and below, temporary annealing is meant) after irradiation (points 2–3) is also small and, for the given dose rate in the dose range \(10^3—10^7\) r, does not exceed two minutes. At the same time \(\operatorname{tg}\delta\) falls to the value corresponding to the unirradiated specimens.

Fig. 2. Dependence of the ratio of the loss-angle tangents on the irradiation dose (dose rate 11 r/sec)

With increasing integral dose the annealing time increases, as shown in Fig. 3, but in the investigated dose interval the values of the loss-angle tangent, within experimental error, fall to the initial value.

It is interesting to note that the loss-angle tangent depends not only on the integral dose, but also on the dose rate. In Fig. 4,a the dependence of the loss-angle tangent on temporary annealing at different dose rates is shown for an integral dose of \(2 \cdot 10^7\) r, and in Fig. 4,b for an integral dose of \(2.2 \cdot 10^8\) r.

Let us note that the initial value of the loss-angle tangent \(\operatorname{tg}\delta_0\) for specimens obtained in a field with a dose rate of 270 r/sec is somewhat

Fig. 3. Effect of annealing on the loss-angle tangent as a function of the integral irradiation dose (dose rate 425 r/sec)

larger than the initial values of \(\operatorname{tg}\delta_0\) for specimens irradiated in a field with a dose rate of 425 r/sec. Taking this into account, one can see that the “annealing” time in these cases is approximately the same. From Figs. 4,a and 4,b it can be concluded that the number of defects increases with increasing dose rate of the ionizing radiation. Evidently, this is connected with temporary annealing during irradiation. As for the permittivity, no changes in it were detected at doses up to \(2 \cdot 10^8\) r.

The most important effect of the action of $\gamma$-radiation on dielectrics, as of X-rays, is the process of ionization. In addition, part of the energy brought by the $\gamma$-quanta may be expended on the displacement of atoms and on the effects proposed by Seitz and Varley [10]. The fact that the loss tangent after irradiation in a $\gamma$-radiation field with a dose of up to $2 \cdot 10^8$ r is restored to its initial value indicates that atomic displacements and the Seitz and Varley effects are very small in this dose interval. The fact that, at these doses, the permeability of the specimens does not change also indicates the same.

Fig. 4. Presence of annealing in the loss tangent as a function of dose-rate power during irradiation:
a—integral dose $2 \times 10^7$ r; b—integral dose $2.2 \times 10^8$ r.

Thus, as a result of $\gamma$-irradiation of barium titanate–based ceramics, an increase in the loss tangent is observed, which is proportional not only to the integral dose but also to the dose rate. After irradiation these effects disappear with time.

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