Page 34 - Spring 2025 Electron

The tube is pumped on the unit and a (ii) There is a considerable reduction in size over pump the large quantities of water vapour or plasticiser
filament loaded with aluminium is in- the normal dielectric and foil construction. which are evolved, special pumping systems are re-
serted into the neck. The gun end of The difficulties involved in the continuous coating of quired, using booster type diffusion pumps with a high
the tube is shielded and the aluminium rolls of dielectric material in vacuum have only been critical backing pressure, and air-ballasted rotary
is evaporated onto the upper parts of overcome in the last few years. The special problems pumps to prevent oil contamination. In such outgassing
the tube and on the lacquer film. After arise in three ways:- plant, the rolls of paper may be heated in vacuum for
baking out the lacquer the aluminium periods of 24 hours or the material may be wound over
remains as a reflecting layer. The (i) Papers and plastics may contain large quantities radiant heaters in order to reduce the time required for
principal advantages are:- of volatiles, for example, papers may contain up the outgassing operation.
(i) The aluminium forms the anode to 8% water vapour by weight. Plant for the continuous evaporation of aluminium
of the tube. (ii) The construction of a continuously evaporating is also available, but a much lower working pressure is
source of metal vapour is complicated by the
required for aluminium than for the successful coating
(ii) Light normally lost behind the interaction of the charge with its container. of zinc or cadmium. Aluminium coatings are more
screen is reflected forward to brilliant and more resistant to atmospheric deteriora-
give a higher intensity output. (iii) Condensation of very volatile metals on to the tion. There are no problems of nucleation but a source
dielectric at a fast enough rate is sometimes
(iii) The screen is protected from difficult. temperature of about 1,400°C is required, and since
positive ion bombardment. Figure 4. aluminium is extremely reactive at that temperature,
The last remarks chiefly concern the continuous
The process is carried out on a Schematic diagram of a new plant for the deposition of electrodes on quartz evaporation of zinc and cadmium, metals which require special sources have been developed. These have now
been used for the continuous coating of different kinds
whole range of special tubes, and in crystals by evaporation. special conditions for condensation at room tempera- of plastic foils and papers, but again the outgassing
certain applications a silver film is de- tures. Either a high beam intensity must be provided, problem can be severe, and special pumping systems
liberately aggregated by heating to make it non-con- The chamber is pumped by a pumping system which is or a nucleating layer of impurity atoms must be de- are required to deal with this.
ducting, the metal is left on the surface as a mosaic. capable of reaching coating pressure of 0.5 microns in posited first.
The method is used in some image converter tubes, less than 45 seconds. A very fast time cycle is possible, One of the most important advances from the point
and to make emitron camera screens. giving a production output potentially greater than a Plant for the continuous coating of capacitor tissue of view of metallising plastics has been the introduction
multiple head assembly housed in a larger vacuum with zinc or cadmium, using silver as the nucleating of polyester foils such as “Mylar” by Du Pont and
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The Coating of Quartz Crystals chamber. The plant has no high vacuum valve, SO medium has been constructed . Two steel crucibles “ Melinex ” by I.C.I. These materials have very good
are used for the evaporation of zinc, and two silver
that while the chamber is open to atmosphere, the hot
The requirement here was for a thin metallic film in pump oil is in contact with oxygen. Silicone oils are sources are provided for coating paper or plastic up to dielectric properties and have already been used for the
close contact with the face of the crystal which would used in the diffusion pump since they are particularly 5 inches wide at a speed of 500 ft./min. Some control manufacture of high grade capacitors by a continuous
not interfere seriously with the fundamental mode of resistant to oxidation. In order to avoid losing the on the evaporation rate is provided by measuring the aluminising process. Their properties are also good
vibration of the crystal. This was an obvious applica- diffusion pump fluid to the backing pump, a special resistance of the coated paper passing over the rolling from the vacuum point of view and they have been
tion of vacuum deposition and it remains one of the baffle known as a fluid economiser is fitted to con- jig. The rolling jigs and sources in a typical plant, coated readily by a continuous process. The coated
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few applications of a sputtered metal film . Gold is dense the oil vapour. A magnetic valve is also fitted which is housed in a 24 inch chamber, are illustrated in material is finding many useful applications, apart from
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used, principally because the sputtering can be carried in the backing line, in order to prevent large quantities Fig. 5. the manufacture of capacitors.
out using air in the discharge. Gold oxide is usually of air from being drawn through the diffusion pump
unstable at the temperature of deposition so that air whilst the chamber is open to atmosphere. The mag- Electrostatic Loudspeaker
can safely be used for sputtering. The thickness of the netic valve is bypassed by a small leak so that a small The possibility of providing a conducting surface on
film can also be readily controlled on a time basis, and quantity of air is drawn through the system, thereby a thin dielectric membrane has led to the development
is more suitable for operation by unskilled workers. preventing oil vapour from rising into the work of the electrostatic loudspeaker 30, 31 . This has long been
It is also possible to obtain less wastage of metal in chamber. A baffle is also provided over the diffusion suggested as a better alternative to the moving coil
the sputtering process compared with the evaporation pump to reduce backstreaming, and the chamber itself speaker, but has awaited the development of a plastic
method. has a heated jacket to reduce the possibility of oil con- foil for the vibrating membrane which had good
The presence of the gold film influences the frequency densation on the crystal with subsequent contamination mechanical and electrical properties and was unaffected
of vibration of the crystal, so that for accurate work of the deposited film. by small temperature changes. The new polyester foils
it is also necessary to perform a second calibration, or The crystal holder, masks and filament are mounted are ideal for this purpose, and have already been used
alternatively, bring the crystal to a desired frequency on the top plate of the chamber. The complete head to great advantage in the construction of prototype
of oscillation by measurement during a second deposi- comprises two lids so that during the pumping of one speakers with several membranes covering the greater
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tion of gold, sometimes carried out by electro-plating. crystal, the next may be mounted in position and the part of the audio frequency range. Different mem-
Special evaporation plant has been devised for cali- filament recharged in preparation for the next sequence. branes are used in conjunction with a crossover net-
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bration to frequency and jigs have been made on which The filament is preheated during the pumping cycle, work to give good linear response at treble and inter-
12 crystals may be mounted, each one being brought and is raised to evaporation temperature by push button mediate frequencies. Fig. 6 shows a diagram of an
into position for frequency control by evaporation of control. Insulated leads are brought through the lid electrostatic loudspeaker in its most useful form,
the small amount of gold required. for calibration during coating. namely, the balanced push-pull output constant charge
In the most up-to-date method of coating quartz electrostatic loudspeaker. The membrane has a con-
crystals with gold , an evaporation technique is used The Metallising of a Capacitor Tissue Figure 5. ducting coating, and is held close to the fixed perfor-
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and both base coating and frequency calibration can be View inside the chamber of a plant for the continuous ated outer electrodes. The foil is extremely thin and
performed in the same apparatus during the same The advantages of a metallised paper or plastic for coating of capacitor dielectric, showing the rolling jigs light, yet has sufficient mechanical strength to withstand
vacuum cycle. A schematic diagram of the system is capacitor construction are:- end sources. the forces applied to it. The foil is constrained by a
shown in Fig. 4. (i) In the event of dielectric breakdown the metal polarising voltage which may be of the order 1 kV, and
The degassing of the rolls of paper prior to coating
A very small coating chamber is employed for simul- coating is burnt away thus giving the material is an important part of the process and this can most a signal of the order 700V r.m.s. is applied through a
high resistance R. This high resistance by providing a
taneous evaporation on to both sides of one crystal. valuable self-healing properties. conveniently be done in a separate plant. In order to long time constant with the speaker, gives it constant
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