SOME TECHNICAL APPLICATIONS OF ULTRASOUND

L. A. Levshuk, D. A. Noskov

(Presented by the scientific-technical conference of the Radio Engineering Faculty)

At the present time, in the Soviet Union and abroad, much attention is being devoted to the application of ultrasonic oscillations in the electrovacuum industry. Ultrasonic methods are used for the purpose of intensifying certain technological processes, for degreasing and cleaning internal lamp parts, for grinding oxide masses, etc. The use of ultrasonic oscillations gives a particularly good effect in cleaning parts of oils, mastics, and other contaminants.

At the Department of “Electronic Devices” of TPI, work was carried out to determine the possibility of using ultrasonic oscillations for cleaning the bases of incandescent lamps before zinc coating and for intensifying the zinc-coating process. To carry out this work, an ultrasonic-oscillation generator with a power of 2 kW at a frequency of 20–30 kHz, with a magnetostrictive vibrator, was developed. The choice of the vibrator type and of the generator frequency was determined by the reliability and safety in operation of magnetostrictive vibrators, used mainly at frequencies up to 100 kHz. As the generator of ultrasonic power for exciting the vibrator, a self-excited tube generator with a tuned circuit on a GDO-10 tube is used. The generator circuit is shown in Fig. 1. From the power-supply unit, assembled on gas-filled rectifiers $L_1$, $L_2$, $L_3$ according to a three-phase half-wave rectification circuit, the direct voltage is supplied through the protective choke $DR_2$ to the anode of the generator tube $L_5$, in whose anode circuit an oscillatory circuit tuned to the ultrasonic frequency is connected. From the anode circuit $C_3$, through the coupling coil $L_2$ and the blocking capacitor $C_4$, the ultrasonic oscillations are fed to the winding of the magnetostrictive vibrator $L_3$ (excitation current). At the same time, a direct current (polarizing current) is passed through the vibrator winding from a polarizing-current rectifier assembled on selenium valves in a bridge circuit. The magnetostrictive vibrator is built up from nickel plates 0.1 mm thick, previously annealed at a temperature of 750°C for 25–30 minutes. The winding is wound on the rods of the vibrator in such a way that the magnetic fluxes in the rods add. The external appearance of the vibrator is shown in Fig. 2. After adjustment of the generator and additional processing of the vibrator, it was possible to obtain a specific radiated power of up to 2.5 W/cm².

At this radiation power, experiments were carried out on degreasing and cleaning metal surfaces of various contaminants and on intensifying the galvanizing process by means of ultrasonic vibrations.

Fig. 1.

Experiments on degreasing glass and metal surfaces were carried out using tap water as the washing liquid. The use of such a washing medium is based on the property of ultrasonic vibrations to create emulsions of oil in water. Experiments show that \(0.5\ \mathrm{cm}^3\) of transformer oil at a temperature of \(35^\circ\mathrm{C}\) is completely emulsified in \(300\ \mathrm{cm}^3\) of water during 5 minutes of irradiation, and the emulsion, without noticeable sediment, is preserved for several weeks.

The part to be degreased was placed in the ultrasonic field and irradiated for from 30 sec to 3 minutes. The oil was intensively separated from the surface of the part with the formation of a stable emulsion. The most intense separation of oil was observed from areas of the surface located normally to the propagation of the vibrations. Experiments show that water can be used as a washing liquid only with preliminary cleaning, when the main contaminants are removed. It is not possible to remove contaminants completely by irradiation with ultrasonic vibrations in water. On the po—

Fig. 2.

traces of emulsion remain on the surface of the part after it is removed from the washing medium, and, when the water evaporates, the oil remains on the surface in the form of the finest droplets. For complete degreasing it is necessary to use some solvent as the washing liquid. In our opinion, it is advisable to carry out degreasing in two stages. Preliminary degreasing is performed by irradiation with ultrasound in ordinary water (in this case the main contaminants are removed, leaving only the finest drops of oil on the surface), and final degreasing—with the use of a solvent. Such a cleaning method will reduce solvent consumption, since the bulk of the fatty contaminants dissolves in water.

Experiments on cleaning parts of contaminants not containing oils were carried out under irradiation by ultrasonic oscillations with a specific power of \(2\text{--}2.5\ \text{W}/\text{cm}^2\). Ordinary water was used as the washing medium, as in the degreasing experiments. Parts contaminated with oxide pastes or watercolor paints were placed in the washing medium at a distance of \(3\text{--}5\ \text{cm}\) from the vibrator. After irradiation for \(1\text{--}1.5\) minutes, the surface of the part was completely cleaned of contaminants. At the same time, fragmentation of the contaminant particles was observed; for a long time these particles remained suspended in the washing liquid. The experiments show that even a small specific power of ultrasonic oscillations (\(1.5\text{--}2\ \text{W}/\text{cm}^2\)) can be used both for cleaning parts and for pulverizing powders and obtaining suspensions. Contaminants that do not contain oils are successfully removed in ordinary water and may be removed during preliminary degreasing.

At the same power, experiments were conducted on the intensification of the process of zinc plating iron bases of incandescent lamps. The experiments were carried out with the normal zinc electrolyte used at the Tomsk Electric Lamp Plant. The only departure from the factory formulation was that we did not use the substance that imparts gloss to the coating.

The vibrator was placed directly in the zinc-plating bath. The bath was cooled by passing tap water through a coil placed in the electrolyte. The temperature of the electrolyte was maintained at \(25\text{--}30^\circ\text{C}\). Before coating, the base was treated in acid and washed in water. The base was placed in the zinc-plating bath in such a way that it could be rotated in the ultrasonic field for uniform irradiation. Zinc plating was carried out at a current density from \(5\ \text{A}/\text{dm}^2\) to \(20\ \text{A}/\text{dm}^2\). The zinc-plating time ranged from 1 to 5 minutes. After a series of experiments it was established that the best results are obtained at a specific radiation power of \(1.18\ \text{W}/\text{cm}^2\), a current density of \(16\ \text{A}/\text{dm}^2\), and a zinc-plating time of 3 minutes. To obtain the same coating thickness at the plant, the zinc-plating process lasts 1.5 hours, and attempts to accelerate zinc plating by increasing the current density lead to loose coatings. The coating time under ultrasonic irradiation can be reduced still further, but in this case the coating becomes nonuniform because of nonuniform rotation of the base in the ultrasonic field.

Examination of bases zinc-plated by the ordinary method according to the factory technology and zinc-plated with the use of ultrasound showed that, when the base is irradiated, the coating obtained is of better quality. The structure of the coating is more fine-grained than in the case of coating by the ordinary method. The strength and uniformity of the coating fully satisfy factory requirements.

The effect of accelerating galvanic deposition at high current densities can be explained by two factors. First, by compaction of the coating under the action of cavitation; second, by intensive stirring of the electrolyte by ultrasonic vibrations.

As a result of the work carried out, the practical possibility has been demonstrated of using ultrasonic vibrations of comparatively low specific power for degreasing and cleaning parts. Especially good results are obtained by using ultrasound to intensify the processes of galvanic deposition. We obtained an acceleration of the zinc-plating process by several times in comparison with the technology adopted at the plant.