Studies on Determining the Feasibility of Monitoring the Percentage Content of Antimony in the Lead Sheath of a Cable by Electrical Methods

B. K. Zhukov, R. M. Zakirov

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

At present, at plants of the cable industry, determination of the percentage content of antimony (Sb) in the lead (Pb) sheath of a cable is carried out by chemical methods, which have certain drawbacks. The shortcomings of these methods are determined primarily by the relative complexity of the analyses and their duration in time (about 40 min), which does not make it possible to detect in good time deviations of the composition of the cable sheath from the standard and to introduce the appropriate corrections into the operation of the equipment, preventing the output of substandard products. From what has been said it is clear that the cable industry needs an instrument that makes it possible quickly to determine the quality of antimony in the lead sheath of a cable. The permissible antimony content is 0.3–0.8%.

At the Department of Information and Measuring Engineering, investigations were carried out with the aim of finding an electrical method of control that would make it possible quickly to determine the percentage content of Sb. In this connection, three methods most acceptable for the given case were investigated: by specific electrical resistance, by thermoe.m.f., and by the crystallization temperature of the alloy.

Experiments to establish the dependence between the percentage content of Sb in the alloy and its specific electrical resistance were carried out on an installation whose circuit is shown in Fig. 1. For the experiment, special specimens were made in the form of wire 1 mm in diameter from the Pb—Sb alloys under investigation, with an antimony content from 0.5 to 0.8%. The electrical resistance of the specimens was determined with the aid of an R-306 potentiometer from the voltage drop across them arising when a current of known magnitude was passed. The specimens were fastened in a special four-clamp device taken from the “Norma” calibration installation. The distance between the potential clamps “Z”—“Z” was chosen equal to 1 m.

As a result of the measurements carried out, it was found that the resistance of a wire 1 m long with an antimony content of 0.5% is 0.26491 ohm, while that of a wire of the same dimensions with an antimony content of 0.8% is 0.26564 ohm. Using the known relation between the resistance of a wire, its dimensions, and the specific electrical resistance of the material, the specific resistance of the alloys was calculated. It was ob-

was found to be: for the alloy with 0.5% Sb—\(2.07955 \cdot 10^{-7}\ \Omega \cdot \text{m}\), and for the alloy with 0.8% Sb—\(2.08528 \cdot 10^{-7}\ \Omega \cdot \text{m}\).

Fig. 1. Schematic diagram of the apparatus for determining the electrical resistivity of the specimen material: \(R_x\)—resistance of the specimen under investigation; \(R_y\)—standard resistance of \(0.0001\ \Omega\); PPTN—low-resistance type R306 direct-current potentiometer, accuracy class 0.015; 1—1—current clamps; 2—2—potential clamps.

If a linear dependence is assumed between the percentage content of antimony in the alloy and its electrical resistivity, then it turns out that a change in the Sb content by 0.1% will cause a change in resistivity by \(1.15 \cdot 10^{-10}\ \Omega \cdot \text{m}\), i.e., by approximately 0.06%. Recording such small changes in resistance with high accuracy by the eddy-current method or by the electrical-contact method is practically impossible. Measurement by means of a direct-current compensator is also unacceptable, since it requires producing specimens in the form of wire with a precisely maintained diameter.

Fig. 2. Schematic diagram of the apparatus for monitoring alloy components by the thermo-emf method. 1—specimen of the alloy under investigation; 2 and 3—electrodes, in combination with which the alloy under investigation gives a large thermo-emf; 4—miniature furnace serving to heat the junction of electrode 2 and alloy 1; И—recording instrument.

It is known that some alloys, when in contact with certain materials and heated, produce a thermo-emf whose magnitude depends on the composition of the alloy. This phenomenon may be used as the basis for developing an instrument for rapid monitoring of the composition of such alloys. In order to determine whether the indicated dependence exists in the Pb—Sb alloy, the corresponding experiments were carried out. The experiments were conducted on an apparatus (Fig. 2) consisting of two carbon rods with a diameter of

3.2 mm in diameter, a miniature electric furnace, an EPD-12 automatic potentiometer with a thermocouple, and an M195 galvanometer. One of the electrodes was heated by the electric furnace, while the other was at ambient air temperature. When the electrodes were placed on the specimen, the points of contact were at different temperatures, and a thermal emf appeared at the ends of the copel rods. The magnitude of this thermal emf depends on many factors, including the composition of the contacting materials. In the present case we were interested in its dependence on the composition of the material of the specimens under investigation, which had the form of plates. To ensure that the temperature of the hot contact did not change during the experiment, the temperature inside the furnace was stabilized with the aid of the EPD-12 potentiometer and a thermocouple. Copel rods of two types were used for the experiments: with a blunt end and with a pointed end. When rods with a pointed end were placed on the lead plate, the surface oxide film of the specimen was destroyed and did not participate in the formation of the thermal emf. When rods with blunt ends were placed on the plate, however, the oxide film was not destroyed and, consequently, affected the result of the experiment.

Fig. 3. Thermal emf of a copel rod in a pair with an alloy at different percentage contents:
1 — flat contact; 2 — point contact

The data obtained as a result of the experiment are presented graphically in Fig. 3. From the form of the graphs obtained it may be concluded that no unambiguous functional dependence is observed between the percentage content of antimony in Pb—Sb alloys (at low antimony contents) and the magnitude of the thermal emf.

The temperature at which crystallization of alloys begins during their cooling depends on the percentage content of the components forming the alloy. Figure 4 presents the phase diagram of the Pb—Sb alloy system. This diagram is called the solidification diagram. For any ratios between lead and antimony, when their alloy passes from the liquid state to the solid state it has two critical temperatures: the first at the transition of the liquidus line (line AOB), corresponding to the beginning of the crystallization process, and the second at the transition of the solidus line (line COD), corresponding to the end of the crystallization process. The value of the first critical temperature depends on the composition of the alloy and, consequently,

Fig. 4. Phase diagram of the Pb—Sb alloy system

can be used for monitoring purposes, if, of course, it can be recorded by available means.

To verify the detectability of this temperature in a Pb—Sb alloy with an antimony content of \(0 \div 0.9\%\), experimental studies were carried out. The metal was melted in special metal vessels in an electric furnace, in which, with the aid of an EPD-12 potentiometer, a temperature of \(400^\circ\text{C}\) was maintained. After

Fig. 5. Cooling curves of Pb—Sb alloys obtained on the chart paper of an automatic PS1-02 potentiometer

the metal had melted, it was allowed to cool. The temperature of the cooling alloy was recorded by an automatic PS1-02 potentiometer using a chromel–copel thermocouple. It was observed that, at the moment crystallization begins, the temperature of the alloy remains unchanged for a certain interval of time; this corresponds to the characteristic plateau of the cooling curve recorded on the chart paper of the self-balancing recorder. By recording cooling curves for alloys with different percentages of antimony, it was established that the position of the plateau on the cooling curve depends on the percentage content of antimony. Figure 5 presents a portion of the chart tape with a record of the cooling curves of alloys containing 0, 0.5, and 0.9% antimony. Analysis of these curves, as well as other experimental data, makes it possible to conclude that the percentage content of antimony in the Pb—Sb alloy can be monitored from the temperature at which crystallization of the alloy begins. For this purpose an automatic potentiometer may be used, the measuring circuit of which has been modified for the specified range of measurement of the thermocouple emf. One variant of such an instrument is described in [2].

REFERENCES

1. P. D. Korzh et al. Thermoelectric method for determining antimony in lead-antimony alloys. Zavodskaya laboratoriya, No. 3, p. 289, 1963.

2. V. K. Zhukov, R. M. Zakirov, V. A. Nikulin. Measurement of the percentage content of antimony in the lead sheath of a cable from the crystallization temperature of the alloy. Present collection.