US2123227A

US2123227A - Piezoelectric device

Info

Publication number: US2123227A
Application number: US28414A
Authority: US
Inventors: Carl A Bieling
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1935-06-26
Filing date: 1935-06-26
Publication date: 1938-07-12
Anticipated expiration: 1955-07-12

Description

July 12, 1938 PIEZOELECTRIC DEVICE Filled June 26, 1935 A TTORNEY c. A. BIELENG 2,123,227

Patented in, 12, 1938 UNITED STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application June 26, 1935, Serial No. 28,414

Claims. (Cl. 171-427) This invention relates to piezoelectric elements and particularly to metallic coatings for such elements.

An object of the invention is to improve the per- 5 formance of piezoelectric crystals.

Another object of the invention is to facilitate the conductive association of piezoelectric crystals with electrical circuits.

A more specific object of the invention is to provide a metallic coating on the surface of a piezoelectric crystal.

Piezoelectric crystals are, as is well known, commonlyused as elements in electrical circuits where a constant frequency is essential and are of 5 increasing importance inconnection withwave filters and similar electrical networks. when the piezoelectric elements are used in such electrical circuits, it is necessary, of course, that some means be provided whereby the piezoelectric ele- 2g ment may be operatively associated with the circuit. It is common practice to provide for this Purpose a metallic coating on one or more of the surfaces of the piezoelectric crystal element, this coating being electrically connected to the crystal 25 surface to which it is applied. The desired conductors of the circuit may then be attached, for

example by soldering, to the coatings. These coatings are commonly referred to as the electrodes of the crystal.

30 Certain somewhat stringent requirements exist with.respect to coatings of the nature referred to above. For example, for some purposes in order that the coating may perform its role of an electrode satisfactorily, it must extend evenly over 35 the surface of the crystal and must be in intimate contact therewith.- Moreover, the process of applying the coating must be such that the structure of the crystal is not damaged. The coating, in addition to its use as an electrode may be 40 utilized to regulate certain characteristics of performance of the crystal, as outlined for example in Hulbert United States Patent 1,848,630 and the process of applying the coating should therefore be of such a nature that the thickness 45 of the coating be uniform and easily regulated within narrow limits.

According to a feature of the invention the metalliccoating is applied to the surface of a piezoelectric crystal element by the spraying 50 process commonly referred to as the Schoop process. It has been found that a coating applied by this process meets the requirements referred to above in an economical and otherwise satisfactory manner particularly with respect to Rochelle salt 55 crystals although the process may also be used to the coating and the crystal.

advantage in coating other types ofpiezoelect'ric crystal elements. Methods previously followed in i the application of electrode coatings to piezoelectric crystal elements have proven generally unsatisfactory when used in connection with 5,

Rochelle salt crystals. In some instances where the known method involved considerable heat, dehydration of the crystal resulted while, in instances of methods involving less heat, it was found diflicult to obtain a firm adherence between 10 However with respect to the Schoop process,'the Rochelle salt body is not damaged in any way thereby, the resulting coating adheres satisfactorily and the electrical characteristics of the piezoelectric device produced thereby are such that the device is particularly adaptable for use in electrical filters and similar circuits. The value of r, for-example, is comparatively ;low while the value of "Q is substantially higher than. that obtained with Rochelle salt piezoelectric elements, the electrodes of which have been applied by other known methods. (The terms 1'" and Q are derived from consideration of the so-called equivalent electrical circuit of the piezoelectric device which circuit is considered to comprise an inductance in series with a capacity C1 and shunted by a capacity Co, 1' being the ratio of Co to C1 and Q being the ratio of the reactance to the resonance resistance of the equivalent circuit referred to. The theory and derivation of the equivalent electrical circuit of piezoelectric devices is explained on page 45 of Quartz Resonators and Oscillators by P. Vigoureux published by His Majestys Stationery Oflice, London.)

In accordance with another feature of the inventionrefinemen'ts are introduced in the design of the pistol used whereby it is better adapted to the application of the coating in the accurate and easily regulated manner required. 40 In accordance with an additional featureof the invention a novel conveying arrangement is V utilized for feeding the crystal plates to the spraying device whereby the thickness of the coating may be automatically controlled within certain narrow limits.

A thorough understanding of the invention and of the various advantageous features thereof may be gained from consideration of the following detailed description and the annexed drawing in which:

Fig. 1 is 'a perspective view of a piezoelectric crystal plate, the surfaceof which has been coated by the process contemplated by the present invention; I

Fig. 2 is an end view'of the crystal shown in Fig.

Fig. 3 is a side elevation of .a Schoops pistol of the. type adapted 'for the coating process contem- -plated;'

-rangement adapted to be used in the coating process contemplated by the invention.

Referring now to the drawing, a piezoelectric plate is shown in Fig. 1 which may be, for example, of the Rochelle salt type. A metallic electrode coating i2 is shown applied to the upper major surface of the plate, a portion H of the plate being left uncoated, thereby effectively separating the coating I2 into two distinct'sections.

If desired a coating of similar or difierent pattern maybe applied to the lower major surface of the plate as Well as to the ends and sides thereof.

As shown most clearly in Fig.2, certain of the edges'of the plate have been rounded out, it having been found that this procedure is effective in preventing fracturing of the coating and chipping of the crystal itself during the preparation and use of the plate.

In accordance with the invention the coating I2 is preferably applied to the plate by'means of a Schoops pistol of the general type shown in Fig. 3. This pistol comprises a casing l5 which encloses a feeding mechanism (not shown) to which wire I6 is supplied. A pistol-grip handle ll, control trigger I8 and supply pipes 2i and 22 are also provided.

In accordance with the usual method pf operation of the pistol, the wire I6 is fed through an opening in nozzle 23 (Fig. 4) and is reduced to a liquid state by an oxy-hydrogen flame formed by the combustion of these gases in chamber 24. The metal particles are then projected through orifice 28 of cap 25 at a high velocity, air, supplied under pressure through chamber 26, forcing the particles out through the orifice.

According to a feature of the invention orifice 28 provided in cap 25 is not round but is, as shown -most clearly in Fig. 5, elongated or flattened. It

has been found that the provision of an orifice of this particular shape greatly enhances the value of the pistol for fine work such as that'involved in crystal coating as it permits the application of the particles in a thin well defined patternmuch as if a small paint brush were used. Coatings may therefore be applied in separate distinct sections as shown in Fig. 1, whereas the use of circular orifices of the type previously provided on Schoops pistols does not permit this fine definition of pattern.

previously used, it has been observed that the tips became damaged in use by the flame and soon presented a distorted orifice. While this did not prove particularly troublesome in coarse work, it

work such as crystal coating as it is paramount,

nozzle and of the cap remain the same.

It has been observed further that, with respect to the type ofv pistols previously used, the "uni formity of the coating was destroyed duetothe nozzle partially clogging particularly at low velocities. investigation that this trouble can be overcome by providing a particularly smooth surface, such as obtained, for example, by chromium plating, on the inside surface of cap 25.

Fig. '6, which shows a piezoelectric plate in the process of being coated, well illustrates the adaptability of a pistol provided with the refinements referred to above to fine 'work such as crystal coating. As shown the particles are projected in a definite well defined pattern thereby permitting the production of separate section coatings of the natureshown in Fig. l. The thickness of the coating produced, moreover, is uniform and easily regulated.

Referring now to Fig. 7 there is shown, schematically, a conveying arrangement adaptable for use in the crystal spraying process referred to above. The arrangement contemplates theprovision of a conveyor belt 38 which is supported on Applicant has found, after considerable if uniform particle size and definite coating thickness are to be attained, that the orificesofthe.

39, polarized relay 52, meter d5, batteries 36 and ll and a power source 58 are also included.

- It will be noticed that one or more of the three fixed resistances 35, 36'or 31 is included in the operating circuit of driving motor 38, the particular resistance or resistances included depending upon the position of armature 5! of relay 32.

When armature 5| is in the neutral position of normal speed. When the higher resistance 31 is included, however, the speed of the motor will decrease and when resistances 36 and 36 are included in parallel the resulting lower efiective resistance in the motor circuit will result in an increase-in the operating speed of the ,motor.

The left-hand winding of relay 42 is energized by current from battery 41, the strength'of this energizing current being varied as desired by operation of rheostat 38 while the right-hand winding of the relay is, energized by current from battery 46, the strength of this current being variedv by operation of rheostat 39. The last mentioned energizing circuit includes contact members 54 and 55, each of which is provided ,with a roller which is adapted to be placed in engagement with a coated piezoelectric plate.

The coating of the plate may in this way be associated with the energizing circuit for the righthand winding of relay 42. The arrangement of the relay circuit is such that, if the resistance of a coating be within certain predetermined limits thereby indicating thecorrectcoating thickness, the current flowing in the energizing circuit will be such that armature 5| willnot be moved from its normal 'p osition shown in Fig.- 1. Should, however, the resistance of the coating be less than the predetermined minimum, thereby indicating a coating of too great a thickness, the increase of current flowing through the right-hand winding of relay 42 will cause armature 5| to swing to the "up position in which position, as pointed out above, resistances 35 and 36 in parallel are connected in the motor circuit and the speed of motor 34 is increased. Conversely, should the resistance of the coating exceed the predetermined maximum limit thereby indicating a coating of insufllcient thickness, armature 5| is moved to the down position and the higherresistance 31 is brought into the motor circuit.

In order to illustrate the operation of the arrangement of Fig. v'I, let us assume that piezoelectric plate 56 has been coated and that plate 51 is in position under the spray emanating from nozzle of the Schoops pistol; In order to simplify the description, it is assumed that the nozzle is so adjusted in this instance that the path of the particles sent forth is sufliciently extended to cover the entire plate. Parts of the plate to be left free of the coating may in such an instance be protected by a mat.

The energizing circuits for the right-hand and left-hand windings of relay 42 havingbeen properly adjusted by manipulation of

respective rheostats

33 and 33, members 34 and 53 are placed in contact with the coating of plate 53 and driving motor34 is set into operation thereby causing" rotation of drum 33 and movement of belt 3| and plate 51 under the spray of metal particles projected by the gun. Assuming, first, that the thickness of the coating on plate 53 is correct within the predetermined limits, the resistance of the energizing circuit of right-hand winding of relay 42 remains such that armature Si is not moved from its normal position, the resistance of the motor circuit is not changed and the motor continues at the predetermined normal" speed.

Assuming, however, that the thickness of the coating on plate 56 is too great, obviously the speed at which the belt is moving should beincreased so thatplate' 51 and the plates following it will be moved through the spray at an increased speed in order that the thickness of the coating applied to these plates may be "out do This desired increase-in speed is automatically attained as the decreased resistance in the energizing circuit of the right-hand winding of relay 42 due tothe undue thickness of the coating results in moving armature 5| to the "up position, in which position, as previously pointed out, resistances 35 and 36 in parallel are connected into the motor circuit. The resulting drop in the resistance of the motor circuit results in an increase in speed of motor 34 and 8, corresponding increase in the speed of the conveying belt.

Had the coating thickness been found insufliclent the proper remedy would, of course, have been to decrease the speed of the belt in order that the subsequent plates might receive a coating of proper thickness. In such an instance the increase of resistance in the energizing circuit of the right-hand winding of relay 42 would be effective to swing armature 5! to the down position, thereby connecting resistance 31 in the motor circuit. The magnitude of resistance 31 is higher than that of resistance 36 and the increase of resistance in'the motor circuit would, of course, cause a decrease in the operating speed of motor 34.

Insulating strips 8| are placed on the belt between each two adjacent piezoelectric plates.

Contact members 54 and 55 engage these strips after leaving one plate and before engaging the subsequent plate. While a momentary interruption of the energizing circuit of the right-hand winding of relay 42 results from engagement of the contact members with these insulating strips, relay 42 is slow-to-operate and the armature does not change its position during the comparatively short time the circuit is interrupted.

While certain specific embodiments of the invention have been selected for detailed description, the invention is not, of course, limited in its application to the specific embodiments described. These embodiments should be considered illustrative of the invention rather than restrictive thereof to prevent fracturing of said coating, the

coating on at least one of said surfaces being separated into a plurality of sections to form a plurality of electrodes.

4. A piezoelectric crystal plate having metallic coating formed integral therewith, an edge of said plate between a major and another surface thereof being rounded to prevent fracturing of said coating.

5. A piezoelectric crystal plate having metallic coating formed integral therewith, an edge of said plate between a major and another surface thereof being rounded to prevent fracturing of said coating, the coating on said major surface being separated into a plurality of sections to form a plurality of electrodes.

CARL A. BIELING.