US 2769930 A
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' Nov. 6, 1956 R. H. STURM 2,769,930
PIEZO-ELECTRIC DEVICES Filed Oct. 15, 1952 c f l P1 1 Elecfirucle C f f Robert Heinrich Stu/ m INVENTOR ATTORNEYS 2,769,930 PIEZO-ELECTRIC DEVICES Robert Heinrich Sturm, Enfielzl, England Application October 13, 1952, Serial No. 314,563
Claims priority, application Great Britain October 15, 1951 12 Claims. (Cl. 310-83) This invention relates to piezo-electric devices of the type employing two or more layers of material which are secured together. More particularly, this invention relates to an improved method of making a multi-layer piezo-electric device .and also to the improved devic produced by such method.
A well known piezo-electric assembly, widely used in transducers such as microphones, gramophone pickups, sound reproducers and like, is a Bimorph construction in which two plates of a piezo-electric substance, having the purpose of this present specification the term Bimorph is to be taken as including not only the twois dry.
The drying and sealing of Bimorph assemblies is difficult in some cases owing to the fact that the piezoelectric material is crystalline and may contain water of crystallisation and may be hygroscopic. The problem is the piezo-elechigh piezo-clectric efliciency.
Some indication of the difiiculty of moisture-proofing Rochelle salt Bimorphs can be seen from the properties of the material itself. At 20 C. crystalline material is stable in an atmosphere of which the relative humidity lies in the range of 36 to 85%; outside this range the material will either dehydrate or dissolve. creasing temperature this safe range of humidity narrows considerably; at 57 C. Rochelle salt dissociates irreversibly.
in the past Rochelle salt Bimorphs have been made by cementing the crystal elements together, suitable electrodes being provided, and thereafter coating the Bimorph by a metallic and/ or paint-like coating of a moisture-proof nature. Despite this, the resulting Bimorphs show considerable variation, from sample to sample, of
The present invention is concerned with a method of manufacturing piezo-electric Bimorphs which have improved resistance to the ingress of water, and which have in consequence and can be used in rather more adverse circumstances than devices of known type. The invention can be applied satisfactorily to the manufacture of Bimorphs employing Rochelle salt or other materials, but since Rochelle salt presents the greatest difficulties in practice it aired States Patent Patented Nov. 6, 1956 is with that material the greatest advantage is obtained.
An improved Bimorph made in accordance with the method is shown in vertical section in the accompanying drawing.
plates of the assembly, at least one surface of plates completely with thereafter securing said The present invention has arisen from long and careful investigations into the electrical characteristics of commercially made Bimorphs. These Bimorphs consist of providing an electrode means on each of said plates, coating said a moisture-proof coating, and surfaces together.
gether with a melt of Rochelle salt. The cement is prepared by melting Rochelle salt in its water of crystallisation and supercooling the melt. The resulting high visthe atmosphere or from its surrounds. Thereafter, the composite assembly is coated with a moisture-proof compound so as to prevent the ingress of moisture.
Careful analysis of transducing elements of low resistance have shown the layer, and progressing further over the whole area of the cemented surfaces.
The Rochelle salt crystal looses water of crystallisation at a relative humidity of 36% or under at room temperature, due to the higher vapour pressure in the The dehydration is visible of .a deposit of white powder the crystal. It has been found that the deposit of White crystal absorbs water and dissolution proceeds slowhumidity falls below is irreversibly dissolved. The ly and is interrupted when the crystal also starting first on the edge of the cement layer. This deposit examined with the naked tween the electrode and the surface, the electrode is applied before the moisture-proof layer.
The elements of the Bimorph must be firmly cemented together, so as to withstand the mechanical stresses imposed upon it in use, and in accordance with another feature of the invention the cement used for this purpose is chemically affine to the coating.
In fabricating Bimorphs in accordance with the invention the crystal plates are first cut and worked to the correct size and shape, and are provided with appropriate electrodes. These steps are the same as those customarily adopted, and will be familiar to those skilled in the art. Thus the plates can be first roughly sawn from a crystal block and then milled or ground to shape; the electrodes may be of foil, or may be an evaporated or like thin metallic layer applied to the crystal.
In fabricating Binrorphs the electroded single plates are coated with the varnish to give an adherent film of at least 0.0005 inch thickness. The plates are coated and dried at a temperature of 20 to 22 C. and 40% to 50% relative humidity.
The coating varnish is preferably prepared of synthetic resins of high dielectric values and low rates of water absorption such as to name only a few: fully or partly polymerised polyester resins such as diallyl phthalate, polystyrene resins, polyvinyl resins, polyvinyl copolymer resins, polyaryl ethylene oxide resins, polyacrylic resins, chlorinated diphenyl resins alone, or a mixture of a copolymerisation product of such resins with or without the addition of modifying agents such as plasticisers like dibutyl or dioctyl phthalate, tri-cresyl phosphate or dibutyl sebacate, tackifying resins such as ester gums, polyisobutylene resins, abietic acid esters etc., fillers and pigments such as talc, silica or titanium dioxide, catalysts such as benzoyl peroxide, tertiary butyl hydroperoxide, pentaethylene diamine, etc.
The coated and dried crystal plates are cemented together to form a Bimorph or multiplate crystal element with an affine cement or by wetting the coated surfaces with a suitable solvent or with the corresponding monomer. To expedite the drying or setting time it is also possible to use instead of the cement a thin foil of the material in question as an interlaminate between the crystal plates. By this method of fabrication the foil or the coated surfaces of the crystals are wetted with a solvent or corresponding monomer and stuck together in the tacky phase.
The resulting bond has to be rigid within the temperature range of the crystal and has to exhibit a high shear strength.
Examples will now be given of suitable compositions for the moisture-proof coatings and of the chemically afiine cements for use in conjunction with them. The proportions are parts by weight.
A rnoisture-proof coating material is prepared as follows:
Example 1 Copolymer resin of 70% styrene and 30% butyl acryla'te 0 0 Chlorinated diphenyl specific gravity 1.530 to 1.630 1.60 Xylene 40.00 Butyl acetate 34.00 Ethyl lactate 2.00 5 Titanium I dioxide 3.50 Colloidal silica 2.50
The single crystal elements with the electrodes applied firmly to the adjacent surfaces are individually dip coated with the solution which on evaporation of the solvents, will provide a film round the element. The required thickness of the coating which should be at least 0.0005
inch is achieved from the dipping solution and by adjusting the viscosity and the temperature of said solution.
The coated and dried crystal plates are cemented together to form a Bimorph or multiplate crystal with an affine cement which can be formulated for instance:
Example 2 Polystyrene resin 21 Conmarone resin 4 Ethyl acetate 45 Carbon tetrachloride W 30 The cement is thinly applied to the adjacent surfaces of the elements which are to be bonded together, and the assembly left to dry under a clamping device.
A further example illustrates the use of an acrylic resin solution for coating and cementing the crystals.
Polyacrylate solutions alone or in combination with other resins such as for instance chlorinated rubber or polyvinyl resins have or impart a good wet strength to the coating with the result that an improved resistance to atmospheric conditions is obtained.
Such a coating for dipping can be formulated as follows:
Example 3 Copolymer resin of 50% butyl methacrylate and 50% methyl methacrylate 12.00 Chlorinated rubber 2.00 Chlorinated diphenyl specific gravity 1.530 1.50 Methyl ethyl ketone 65.00 Diacetone alcohol 16.00 Titanium dioxide 1.50 Talc 2.00
An afiine cement for bonding the crystal plates together consists of a polymethyl methacrylate resin dissolved in its monomer which has been catalysed, as for instance:
Example 4 Polymethyl methacrylate resin 20.00 Methyl methacrylate monomer 79.90 Tertiary butyl hydroperoxide 0.10
Alternatively to cementing, a foil of 0.0015 inch thickness of an affine resin can be used, which will reduce the drying time of the assembly.
For instance one surface of the crystal plate coated to formula Example 1 is wetted with a styrene monomer and immediately pressed against a polystyrene foil. The second crystal plate is treated in the same way and attached to the opposite side of the foil. The foil can be of the same size as the crystal plates or of larger size and cut to size of the crystal elements after the assembly has dried.
Crystal plates coated to Example 3 can be interlaminated with a olymethyl methacrylate foil after the crystal surfaces have been wetted with the corresponding monomer or a low boiling solvent such as methylene chloride.
The crystal element as described is finally coated all over with a polyblend composition of low water vapour permeability as described in British patent application No. 9,222/ 50. It will be understood that the customary connecting leads will be made to the electrodes, and that improved method is shown in the accompanying drawing. The two plates of piezo-electric material such as Rochelle salt crystal are indicated by numeral 1. Each plate has applied thereto on each face thereof an electrode layer and after the electrode layers have been applied, each plate together its electrode layers is coated on all sides with one of the moisture-proof materials which have been described, this moisture proof coating being indicated by numerals 3. After the coating 3 has dried the I claim:
1. The method of producing a moisture proof piezoelectric transducer assembly comprising a pair of plates of piezo-electnc material joined together in face to face relation which comprises the steps of applying an electrode to one face of each plate, then applying a coating of moisture-proofing material to all surfaces of said electroded plates, and then cementing the moisture proofed electroded faces of said plates together.
2. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 1, wherein the coating applied to said members is a chemical compound, and said cement is chemically afiine to said compound.
4. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 3, wherein said resin includes co-polymerised synthetic resin.
5. The method of producing a moisture-proof piezo- 6. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 3, wherein said resin includes plasticisers.
7. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 3, wherein said resin includes pigments.
8. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 3, wherein said resin includes fillers.
9. The method of material chemically afiine to said coating, said film being wetted with a solvent.
10. The method of producing a moisture-proof piezoelectric transducer assembly in accordance with claim 1, wherein said elements after being cemented together, are further coated with moisture-proof coating.
11. A moisture-proof piezo-electric transducer assembly comprising a pair of piezo-electric members of crystalline salt type, electrode means attached to a face of each of said members, a coating of moisture-proof material individually enclosing each member and its associated References Cited in the file of this patent UNITED STATES PATENTS Nicolson June 24, 1930 Ream Dec. 16, 1941