SIMPLE RECORDING SETUPS FOR STUDYING THERMAL DECOMPOSITION

G. G. Savel’ev, V. V. Bordachev

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

1. Mac-Bain Automatic Balance

For automatic recording on an ÉPP-09 potentiometer of thermal-decomposition curves, we used an ordinary Mac-Bain balance. The vertical displacement of the lower end of the quartz spiral, caused by a change in the weight of the substance, was converted, by means of a projection lamp, a long-focus objective, and a system of mirrors, into the horizontal motion of the shadow image of the end of the spiral and was projected onto the chart tape of the recorder. When the weight changes, the shadow moves along the scale of the recorder, to the pen of which a sensitive element “following” the shadow is attached; thus the displacement of the shadow is recorded.

Fig. 1. Bridge circuit. D—photodiodes of type FD-1; \(R_1=R_2=360\ \text{k}\Omega\); \(E\)—battery of \(1.5\div 6.3\ \text{V}\); \(K\)—switch.

The “following” circuit used by us for recording differs from that described in [1]. As the sensitive element we used two FD-1 photodiodes, mounted on the carriage of the recorder at a distance of 1 cm from one another. Both photodiodes were connected into the arms of a bridge (Fig. 1). Variable resistances \(R_1\) and \(R_2\) were connected into the other two arms. As the voltage source \(E\), a dry cell of \(1.5\)—\(15\ \text{V}\) may be used.

To record the displacement of the shadow, an ÉPP-09 M3 recorder with a sensitivity of 10 or 46 mV over the full scale is connected into the diagonal of the bridge. With the aid of resistances \(R_1\) and \(R_2\), the circuit was adjusted so that, at the same illumination level of photodiodes \(D_1\) and \(D_2\), there was no signal at the input of the recorder. If the shadow moved over one of the photodiodes, then, owing to the imbalance of the bridge, a signal appeared at the ÉPP input, deflecting the pen of the recorder. But as soon as the second photodiode also entered the shadow region, the signal again decreased, since the arrangement of the photodiodes was such that darkening of the right photodiode shifted the pen to the left, and darkening of the left one—to the right. In this case the pen of the recorder always stood so that the shadow was between the photodiodes and the pen “followed the shadow.” Otherwise the pen will “follow the light—

…with a spot of light and be located to the side of the shadow. In the case of a McBain balance it is more convenient to work in the first mode.

The advantages of the circuit with two photodiodes over one with a single photodiode are obvious. First, in the circuit with two photodiodes the signal is doubled and, second, the circuit becomes insensitive to changes in the illumination of the room. Thus, if one works with a circuit with a single photodiode, then when meteorological conditions change or the lighting is switched on, adjustment of the circuit is necessary, which is inconvenient.

On the setup described, by using appropriate spirals, changing the distance to the recorder, or changing the objective, one can obtain practically any sensitivity down to \(10^{-4}\) g (and possibly less) over the entire scale of the EPP-09. With such a high sensitivity, smooth curves are naturally obtained only when thermal decomposition is carried out in vacuum. When decomposition is carried out in a gas flow or in air, however, the recorder registers oscillations of the spiral caused by convection of the gas in the vessel. In order to obtain smooth curves at normal pressure as well, it is necessary to damp the recorder.

It should be noted that, when an electronic amplifier with a voltage gain of about 20 is used in this circuit, or when a recorder with a sensitivity of \(\sim 3\) mV is used, the need for the dc voltage source \(E\) disappears, since the photodiodes themselves possess a considerable valve photo-e.m.f., which under these conditions proved sufficient for operation of the circuit. Curves recorded on one such setup, which has been operating reliably for about 1.5 years, are given in [2].

2. Differentiating Automatic Setup

For studying the rate of thermal-decomposition reactions it is convenient to use setups whose operation is based on measuring the pressure of the gases evolved, since in this case heat removal can easily be arranged and the productivity of the setup increased. In our work we use a differentiating setup, since in this case automatic recording of decomposition curves can be carried out especially easily. The operating principle of such a setup has been described in several papers (see, for example, [3]) and consists in carrying out the decomposition with continuous pumping through a capillary; the pressure in the vessel does not rise above \(10^{-2}\) mm Hg, so that the condition for molecular beams, \(\lambda \gg d/3\), is maintained, where \(\lambda\) is the mean free path of the molecules in vacuum and \(d\) is the diameter of the capillary. In this case, at equilibrium in the vacuum system, the decomposition rate is

\[ \frac{dv_p}{dt}=\frac{P_v-P_0}{P_0 W}, \]

where \(P_v\) is the pressure in the reaction vessel, \(P_0\) is the pressure beyond the capillary (in the high-vacuum part), and \(W\) is the resistance, constant for the given geometry of the vacuum system [4]. Passing from the change in volume \(\frac{dv_p}{dt}\) at constant pressure to the change in pressure at constant volume, we obtain

\[ \frac{d\cdot P_v}{dt}\cdot v_c=\frac{P_v-P_0}{W}, \]

where \(v_c\) is the volume of the vessel up to the capillary. Thus, for \(P_0 \ll P_v\) the rate of thermal decomposition is proportional to the pressure in the re-

reaction vessel, and, by recording the pressure during decomposition in the vessel, we record, on a certain scale, the change in the rate of thermal decomposition with time. This scale can be determined if the rate of occurrence of some process proceeding at a constant rate is measured. In particular, such calibration can be carried out in each experiment on any section with a constant rate, for example, in the stage of decay of decomposition; in this case, naturally, it is necessary to know \(v_c\).

To record the thermal-decomposition curves we used a VT-2 (or VIT-1) thermocouple vacuum gauge with an LT-2 lamp and an EPP-09M3 recorder. From the vacuum gauge a stabilized heating voltage was supplied to the LT lamp, but the thermocouple emf was measured not by the vacuum-gauge instrument itself, but was recorded on the EPP-09-M3. The ends of the thermocouple were connected to the recorder by a shielded wire.

Fig. 2. Diagram of the reaction vessel of the differentiating setup. 1—branch for samples; 2—ground joint for loading samples; 3—iron cylinder for moving samples; 4—rotating metal stand for holding the sample during decomposition; 5—thermocouple lamp; 6—capillary; 7—valve; 8—device for collecting the sample; 9—metal block; 10—furnace; 11—ground joint for unloading samples; 12—LM-2 lamp.

The curves recorded in this way in the range \(10^{-4}\)—\(10^{-2}\) mm Hg do not require recalculation by the calibration curve of the LT-2 lamp, since precisely in this range the calibration of the lamp in the coordinates pressure—thermocouple emf is linear. This can be verified by representing the calibration supplied with the lamp in the coordinates \(\lg P—f(E)\) in the form \(P—f(E)\) (here \(P\) is pressure in mm Hg, \(E\) is emf in mV).

Figure 2 shows a diagram of the reaction vessel of our setup; the diameter of the constriction (capillary) is about \(0.5\) mm. The design of the vessel made it possible to carry out a large number of experiments without breaking the vacuum. Loading and changing of samples were performed as follows. Several samples (in the form of closed foil weighing bottles with a weighed portion of substance) were placed into branch (1) through ground joint (2). A vacuum better than \(10^{-4}\) mm Hg was created in the vessel, which was monitored by the LT-2 lamp on the vessel side and by the LM-2 lamp on the pump side. By means of an electro-

the furnace heated the reaction space to the required temperature, measured by a thermocouple and regulated by means of a contact thermometer and a relay. After this, by moving the iron cylinder 3 with a permanent magnet, one sample was dropped onto support 4. The rate of decomposition was recorded from thermocouple lamp 5, while the pressure beyond capillary 6 was measured by lamp 12 in order to monitor fulfillment of the condition \(P_0 \ll P_c\). Calibration of the decomposition curve was usually carried out during the period when decomposition was dying out, by closing stopcock 7. After the decomposition had ended, the sample was dropped by turning the flat support 4 with a permanent magnet by means of gate 8. The setup was ready for the next experiment. Figure 3 in [2] presents curves of the rate of thermal decomposition of lead azide recorded on this setup. The dependence of the rate on time has two distinct maxima, whereas in decomposition on a balance setup only one maximum is observed. Thus, the setup described above made it possible to study new regularities in the thermal decomposition of lead azide [2].

The authors express their gratitude to Yu. A. Zakharov for discussion of the work.

REFERENCES

1. V. I. Eroshkin, V. T. Pismenko. Kinetics and Catalysis, 6, No. 6, 1965.
2. R. A. Vasyutkova, G. G. Savel’ev, Yu. A. Zakharov. Present collection, p. 131.
3. V. V. Boldyrev. Methods for Studying the Kinetics of Thermal Decomposition of Solids, Tomsk, Tomsk State University Press, p. 275, 1958.
4. G. Mönch. High-Vacuum Technology, Moscow—Leningrad, Energiya, pp. 394–412, 1965.