Discussion
Abstract Generated abstract
This discussion addresses experimental observations of illumination-dependent dielectric losses in phosphor or semiconductor powders at high frequencies. Participants emphasize the need for extensive controls, including variation of illumination intensity, power, electrode or grid geometry, angle, and depth of illumination, because extraneous effects from high-frequency measurement apparatus, electrodes, ionization, or powder processes could influence the measured loss tangent. Several speakers question a simple resonant explanation and suggest that temperature-dependent shifts of loss maxima may indicate a different dielectric-loss mechanism, while Agashkin argues that the time dependence of the dielectric changes follows the afterglow decay and therefore supports a luminescence-related origin. The exchange presents the reported effect as unusual and potentially important, but stresses that careful measurement procedures are required before firm conclusions can be drawn.
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Discussion
V. N. Kessenikh—I wish to draw attention to the fact that the combination of luminescent effects and high-frequency measurements requires carrying out a large number of measurements and control calculations, since extraneous effects may arise. It seems to me unlikely that there is a transition from a decrease in \(\operatorname{tg}\delta\) in the low-frequency region to an increase in the high-frequency region. In order to investigate the mechanism, it would be desirable to determine the dependence of the effect on \(J_{\text{illum.}}\) and on power, and to carry out experiments with grids of various kinds: wire grids and grids made of perforated foil, at various angles, and with illumination into the depth.
The effect of an increase in \(\operatorname{tg}\delta\) under illumination was established earlier by A. K. Krasin, but the effect discovered by Vergunas and Agashkin is so unusual and interesting that it is necessary to check especially carefully all the experimental conditions that might give rise to objections.
M. P. Tonkonogov—The mechanism of dielectric losses in the case under consideration is hardly resonant (as the speaker supposes), since the natural frequencies of oscillation of the groups responsible for dielectric losses are very low (the radio-frequency region); consequently, the groups are weakly coupled. A resonant mechanism, however, governs the motion of strongly fixed structural elements.
Since the maxima in the frequency dependence of \(\operatorname{tg}\delta\) shift toward low frequencies with increasing temperature, one may suppose that here there is a new mechanism of dielectric losses. A similar character of the process was also observed in mica by A. P. Izergin.
O. V. Agashkin—The assurance that the observed effect is caused not by ionization and other processes possible in the powder at high frequencies, but by processes connected with afterglow, is also confirmed by the fact that the decay of the dielectric changes in time coincides with the decay of the light sum both in duration and in form. In the latter case no voltage whatsoever is applied to the phosphor. Consequently, the increased voltage and the effects in the powder at high frequencies do not prevent observation of the dielectric effect in pure form.
P. F. Veselovskii. There is great distrust of \(Q\)-meters. Measurements at frequencies above \(1\) MHz must be carried out especially carefully; the rise of frequencies in the high-frequency region may be connected with an effect from the electrodes. In the journal Zavodskaya laboratoriya much attention has been devoted to this question. Before discussing the results obtained, it is necessary to work out the measurement procedure carefully.
A. A. Vorob’ev—It is of interest to elucidate dielectric losses in electronic semiconductors; therefore this work deserves attention.