SETUP FOR MEASURING THE ELECTRICAL CONDUCTIVITY AND PHOTOCONDUCTIVITY OF SOLIDS

G. T. Shchekov, Yu. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry)

In works [1–6] the results are set forth of studies of the electrical conductivity and photoconductivity of lead and silver azides, which are initiating explosives, and mechanisms are also proposed for the thermal decomposition of these compounds. In some cases [1] discrepancies are observed in the magnitude and even in the type of the temperature dependence of the electrical conductivity of AgN$_3$, obtained on different setups, which is probably associated with different conditions under which the experiment was carried out.

This, in turn, makes it difficult to correlate the results of electrophysical measurements of azides with data on thermal decomposition, owing to the different experimental conditions. We have constructed a simple setup that makes it possible simultaneously to measure the electrical conductivity and photoconductivity of high-resistance materials.

In addition, the setup makes it possible to measure these same characteristics during the thermal decomposition of the object under investigation.

Description of the setup

The schematic of the setup is shown in Fig. 1 and consists of a measuring cell (1), a light source (6) with a monochromator, and a recording instrument V2-5. The measuring cell (1) is glass, detachable—

Fig. 1. Schematic of the setup for measuring dark conductivity and photoconductivity.

... vacuum cell, is described in detail in [8]. The use in the cell of a quartz window (2) for illuminating the specimen makes it possible to measure photocurrents over a wide spectral range \((220 \div 1100\, m\mu)\).

To eliminate errors in temperature measurements when compensation leads are used, the thermocouple (3) is brought out of the cell through vacuum-tight feedthroughs (4), made on the basis of ED-5 epoxy resin, and is connected to the PP-Sh indicating instrument (5). The light source in the setup is a DRSh-1000 super-high-pressure lamp (6), mounted in a metal housing (7). The light from the lamp is focused by a quartz lens (8) onto the prism (9) of the SF-4 spectrophotometer and then, through micrometric slits (10), falls on the specimen (11).

The compensation circuit for measuring the photocurrent is shown in Fig. 2. As the recording instrument (1) we used a domestic V2-5 tube millivoltmeter-electrometer. The voltage across the specimen (2) was supplied from a BAS-80-X-1 battery of galvanic cells (3) and regulated by a VZR potentiometer of \(1.5\, M\Omega\) (4).

Fig. 2. Compensation circuit for measuring photocurrent

The load resistance \(R_0\) is a set of standard resistances \((10^{11}, 10^{10}, 10^9, 10^8, 10^7, 10^6\, \Omega)\) with a switch from an F-57 teraohmmeter, and makes it possible to measure resistance from \(10^6\) to \(10^{15}\, \Omega\). In measuring photoconductivity, compensation of the dark-conductivity signal reaching the V2-5 indicating instrument (1) was carried out by means of the PP-Sh potentiometer (5).

The setup has the following parameters:

1. Limits of resistance measurement: \(R = 10^6 \div 10^{15}\, \Omega\);
2. Voltage applied to the specimen: \(U = 1 \div 400\, \text{V}\);
3. Operating pressure in the system: \(10^{-5}\) mm Hg;
4. Range of operating temperatures: \(t = 20 \div 250^\circ \text{C}\);
5. Measurement error: not more than \(10\%\).

On the setup described above, measurements were made of the dark and photoconductivity of lead azide and of the dark conductivity of silver azide in vacuum. A voltage of 95 V was applied to the specimen, which is certainly less than the breakdown voltage \((250\, \text{V/cm},\) see review [6]).

The results obtained are compared with literature data and are given in Table 1.

Table 1

Resistivity, photocurrent, activation energy of PbN$_6$ and AgN$_3$

where $R$ is the dark resistance of the samples, and $E_{\mathrm{act\,d}}$ and $E_{\mathrm{act\,ph}}$ are, respectively, the activation energies of dark and photoconductivity. The difference in the results for the photoconductivity of PbN$_6$ may be associated with the different field strength used by us (65 V/cm) and by Didman and Lewis [5] (600 V/cm), when injection of carriers from the electrodes [7] becomes possible, as well as with a difference in the intensities of the photoactive light.

REFERENCES

1. D. A. Young, Brit. J. Appl. Phys., 15, 499, 1964.
2. A. C. McLaren, G. T. Ragers, Proc. Roy. Soc., A240, 484, 1967.
3. B. E. Bartleff, F. S. Tompkins, F.R.S. D. A. Young. Proc. Roy. Soc., A245, 206, 1958.
4. B. L. Evans, A. D. Yobbe, A250, 346, 1959.
5. A. J. Dedman, T. J. Lewis, Trans. Farad. Soc., 62, No. 520, (part 4), 1966.
6. A. J. Dedman, G. J. Lewis, Trans. Farad. Soc., 62, No. 520, (part 4), 1966.
7. B. L. Evans, A. D. Yobbe, P. Gray. Chem. Rev. 59, 515, 1959.
8. S. M. Ryvkin. Photoelectric Phenomena in Semiconductors. Moscow, 1963.
9. Yu. A. Zakharov, G. G. Savel’ev, G. T. Shchekov. Zh. Fiz. Khim. (in press).

ERRATA NOTED