US2402269A - Deposition of metals - Google Patents

Description

June 13, 1946. P. ALEX NDER ETAL 2,402,269

DEPOSITION OF METALS Filed Aug. 18, 1942 y//L g 2/3 \OOOOO/ @W kW Patented June 18, 1946 DEPOSITION OF METALS Paul 'Alexander, Berkhamsted, and Edwin Laurence Cranstone, Mill Hill, London N. W., England, assig-nors to Vapco Limited, London, England, a company of Great Britain Application August 18, 1942, Serial No. 455,224 In Great Britain September 3, 1941 'Claims.

This invention relates to the deposition of metals by thermal evaporation in a vacuum, on a support of paper or Cellophane or other moisture-absorbent material. I

Most metals can be deposited on paper or Celloration of the metal in a vacuum without dim-- culty, but it has been found impracticable to maintain the necessary vacuum in the evaporation chamber when the metal to be evaporated is aluminium or magnesium. A certain minimum degree of vacuum must be maintained, in order to obtain a bright surface of metal on the paper or Cellophane. In the usual process of evaporation, in which evaporation takes place from a bath or layer of liquid metal, the vacuum must be of the order of 0.0001 mm. of Hg but, in the process described in the United States specification No. 2,353,786, a vacuum with pressure as high as 0.01 mm. of Hg can be used.-

Deposition of, say, silver can be made on a roll of paper or Cellophane, the necessary vacuum being maintained by keeping the pumping equipment in operation during the operation of deposition, any water vapour which i evolved from the paper or Cellophane being thereby re-' moved. When, however, aluminium'is evaporated, the evolution of gas is many times as much. and is so great that the vacuum cannot be maintained by any practicable pumping equipment. It has been found that the gas evolved is hydrogen and not water vapour, and it is believed that the hydrogen is released from an adsorbed layer on the surface of the paper or Cellophane containing H0 or H2O molecules, by the action of the aluminium atoms.

According to the present invention a process of depositing a film of aluminium or magnesium with a bright surface on a support of paper,

Cellophane or other moisture-absorbent material is characterised by exposing one surface only of the support, being the surface on which the metal is to be deposited, to radiant heat in a vacuum for a period less than that required to deprive the body of the material of its moisture content. and then, without prolonged exposure of the material to atmospheric air, effecting the deposition of the metal on the said surface by thermal evaporation in a vacuum, while preventing appreclable deposition on the other surface of the support.

Apparatus for depositing a bright film of metal on a support of moisture absorbent material by thermal evaporation in a vacuum chamber according to the present invention comprises a preheating chamber including 'means for maintaining a vacuum in said preheating chamber, and a heater within the said chamber mounted in re 'lation to the support so that one surface of the .support faces the heater whereby a radiant heat treatment in a vacuum to eliminate moisture from the said surface is performed prior to depositing the metallic fllm on said preheated surface, and shielding means comprised in the depositing chamber whereby appreciable deposition on the other surface of the support is prevented.

The means of shielding the unheated surface of the support against receiving any appreciable deposit of the metal preferably consists of a tubular shield which directs the vapours of the metal on to the surface tobe coated.

In one arrangement according to the invention the material is on a storage roll and is unwound from the roll on to a receiving roll and, in the passage from one roll to the other, the material passes close to a heater, which may be a plurality of wires or wire spirals in a plane parallel to and close to the plane in which the material passes, the wires being maintained I electrically at red heat. The apparatus, including means for winding the material from one roll to the other, is contained in a chamber which heating is superficial only, that is to say, the

heating does not penetrate the material so as to effect any appreciable drying. In the case of Cellophane particularly, drying of the material is to be avoided, because Cellophane when dried becomes too brittle for convenient handling. It has been found that the effect of the heating, which is confined to the surface presented to the heater, is wholly ineffective in preventing the evolution of gas if deposition be effected on the other surface. Consequently, during the operation of thermal deposition, thi other surface should be so placed or so shielded in the vacuum ohamberthatno appreciable amount of metal is deposited on it.

After the above superficial heating has been completed the roll carrying the treated material is removed from the heating chamber and placed in the other vacuum chamber in which the thermal deposition is carried out.

In an alternative arrangement according to the invention the superficial heating and depositaaoaaeo ing of the metal is effected in a single chest divided into two compartments by a partition. one compartment being allocated to deposition and the other to the superficial heating. The two roll are then placed in the heating chamber;

and the material passes from one roll to-the other through slits in the partition and over two idle rollers in the deposition part.

In this latter case, the superficial heating and the deposition may be effected simultaneously on different parts of the support, the material passing from one roll close to the heaterand then through a slit in the partition into the deposition chamber where the deposition is effected, and then through another slit in the partition to the other roll. Alternatively, this latter roll may be in the deposition chamber.

It may be noted that, if the slits are narrow, a vacuum of, say, 0.001 mm. Hg may be maintained in the deposition chamber while a vacuum of, say, 0.05 mm. Hg is maintained in the heating chamher, the leakage through the slits being very small.

The roll of material after heat treatment must not be left exposed to the atmosphere for more than a few minutes before the deposition is effected, butit may be kept in a vacuum for many hours without losing'the effect of the treatment. In order that the invention may be mor clearly understood, reference will now be made to the accompanying diagrammatic drawing whichshows by way of example preferred embodiments thereof.

In the accompanying drawing:

Figure l is a verticalsection of a vacuum chamber with apparatus for heating superficially'the support as it is unwound from a roll of material;

Figure 2 is a vertical section of a vacuum chamher with apparatus for thermally depositing a film of metal on the surface of the material superficiallyheated inthe chamberof Figure 1;

Figure 3 is a vertical section of a vacuum chest divided by a partition into two chambers of which motor l0. During the transfer the roll of matefor a prolonged period, otherwise moisture may be re-absorbed" on to theheated surface. In

I damp air, the exposure should not exceed a few minutes.

The apparatus for thermal deposition is of known type. It consists of a heater II, which may be a rod of tungsten heated by the passage of electric. current, and a wire l2. of the metal to be deposited. which is fed on to the heater H from a spool of wire l3 by a clockwork motor 14 by which the feed rolls 15 are driven. The source of the metal vapour is the point of contact of the wire 12 with the heater II. and thisis so disposed as to face the surface of the material 4 which had been superficially heated. Means are provided to prevent any appreciable quantity of the metal vapour from striking. the unwound surface of the material 4 which has not beensuperfioially heated. In the arrangement of Figure 2 the said means is the tubular screen l8, which shields the surfaces of the outer. convolution of one contains apparatus similar to the apparatus shown in Figure 1 for first heating a roll of material superficially and the other contains apparatus similar to the apparatus shown in Figure 2 for depositing a film of metal on said roll, and V Figure 4 is a vertical section of apparatusof alternative form to the apparatus of Figure 3.

In the drawing like reference numerals desig-' nate the same'or similar parts.

Referring to Figure l, the vacuum chamber I is evacuated through the hole 2. An electric heater 3 is disposed so as to heat superficially one surface only of the material 4 which is wound from a'storage roll 5 mounted on an axis 5a on to a receiving spool 6 mounted on-an axis. do by means of the clockwork motor 1. A moderate vacuum is suiiicient such as can be produced; by a mechanical vacuum pump, preferably between 0.05 and 0.5 mm. of Hg. The amount of superficial heating which should be done depends on the nature of the support material and is best found by trial. It may be adjusted by varying the electric energy supplied to the heater 3 or by varying the rate of passage of the material. past the heater. The amount of heating, however, should be less than is required to deprive the material of its moisture content.

After the superficial heating process has been effected, the roll of material 5 having the one surface superficially freed of moisture is transferred to the vacuum chamber 8, where it is wound on to the roll 9 mounted on axis 9a by the clockwork the rolls 6" and 9 from the metal vapour. Preferably, a device of the kind described in the speciflcation of United States application Serial No. 258,416, filedFebruary 25', 1939, is employed to facilitate maintenance of the vacuum, this device consisting in a plate. of aluminium l1 insulated from the chamber by insulators l8 and connected by 'a wire Him a source of. electrical high tension. Prior to starting the termal deposition, an alternating electrical discharge-is maintained for a short time between the plate I! and an. aluminium casing surrounding the heater. which may be the screen l6.

The thickness of the film depositedv on the material 4 is determined by the rate of passage of the material from, the roll 6 to the roll 9. The chamber 8 is evacuated by means of the hole 2 to a vacuum which, for the thermal deposition device shown, maybe from 0.001 to 0.01 mm. Hg. Once the roll 6, after th superficial heating, has been transferred to the chamber 8., and a moderate vacuum produced therein, the roll may be kept for many hours before the thermal deposition, without the material losing the effect of the superficial heating.

Figure 3 shows the parts of the apparatus shown in Figures 1 and 2 combined in a single vacuum chest 20, divided into two chambers by a partition'Z'l. The material 4 passes from the roll 5 to the roll 9 over idle rollers 22, and through a narrow slit 23' in the partition ll. It may be noted that a vacuum of, say, 0.1 mm. Hg may be maintained in the heating chamber to the left of the partition 2 I, while a vacuumof, say, 0.001 mm. Hg is maintainedin thedeposition chamber to the right of the partition,.since at these pressures, the leakage through the. slit 23 is small.

In this apparatus it is seen that the processes of superflcialheating and of thermal deposition are carried on simultaneously.

Figure 4.shows an alternative form of the apparatus 'of Figure 3, in which the two rolls are face of the support opposite to the heated surface is exposed to impact by the metal atoms and consequently shielding means I8 is not required. The walls of the chamber are, however, provided asoaaco with an aluminium lining 24 which constitutes the device of the aforementioned United States application Serial No. 258,416, for facilitating maintenance of the vacuum.

We claim:

1. The process of depositing a film of one of the metals of a group consisting of aluminium and magnesium with a bright surface upon a support of moisture absorbent material in sheet form. characterized by continuously passing a sheet of moisture containing absorbent material past a heating unit in a vacuum chamber at such a rate and in such close proximity thereto that an exposed suriace only of the sheet, which is the surface to be coated, is superficially heated to remove moisture from said surface and without heating the body of the material sufiiclently to effect appreciable drying thereof, and thereafter,

without appreciable prolonged exposure to the atmospheric air, continuously moving said sheet in a vacuum chamber and eifecting the deposition of the metal by thermal evaporation on said moisture free surface, while preventing appreciable deposition of the metal on the opposite surface of the sheet.

2. Apparatus for depositing a bright film of metal on a support of moisture absorbent material by thermal evaporation in a vacuum, comprising a deposting chamber, means in the chamber for vaporizing metal to bev deposited on the support, means for maintaining asuitable vacuum in the depositing chamber, a preheating chamber comprising means for maintaining a vacuum in said preheating chamber and a heater within the chamber mounted in relation to the support so that one surface of the support faces the heater whereby a radiant heat treatment in a vacuum to eliminate moisture from the said surface is performed prior to depositing the metallic film on said preheated surface.

3. Apparatus according to claim 2 comprising in the depositing chamber a receiving roll on to which the support material is rolled after heat ing in the preheating chamber means for rotating the receiving roll and a tubular shield about the metal vapourising means mounted to direct the vapour to the front face of the support to be coated and to prevent the vapour from coating the back face of the support material which is exposed on the receiving roll.

4. Apparatus according to claim 2 comprising a chest divided into two chambers by a partition. one of said chambers being the reheating chamber and the other the depositing chamber the said preheating chamber containing means for rotatably mounting a storage roll for the support material and mechanism for guiding support material on the roll past the heater into the depositing chamber, the partition having an opening in the form of a slit which closely fits about the support in its passage through the partition, a receiving roll in the depositing chamber, means for rotating said receiving roll and means for guiding the surface of the support which has been heated past the evaporating means to the receiving roll, the vaporizing means being surrounded by a shield mounted to direct the vapour of the metal to the surface to be coated and to prevent the vapour from being deposited on the back face of the support which is exposed on the receiving roll.

5. Apparatus according to claim 2, comprising a chest divided into two chambers by a partition, one of said chambers being the preheating chamber containing the heater, and the other the depositing chamber said preheating chamber con-.

taining means for rotatably mounting a storage roll for the support material, means for rotatably mounting a receiving roll for said material and means for rotating said receiving roll, the depositing chamber containing parallel guide rollers for the support, the guide rollers being disposed to cause the heated surface of the support to be exposed to the evaporating means, and the partition between the chambers having openings in the form of slits which closely fit about the support in its passage from the storage roll into and out of the said second depositing chamber back to the receiving roll.

PAUL ALEXANDER. EDWIN LAURENCE CRANB'I'ONE.

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