US2439466A - Piezoelectric crystal element and method of fabricating same - Google Patents

Description

April 13, 1948. c. K. GRAVLEY 2,439,456 PIEZOELECTRIC CRYSTAL ELEMENT AND METHOD OF FABRICATING SAME Filed Oct. 20, 1944 JNVENTOR. CHARLES l1. GR/WLEY ATTORNEY Patented Apr. 13, 1948 PIEZOELECTRIC CRYSTAL ELEMENT AND METHOD OF mnmcarmc SAME Charles K. Gravley, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application October 20, 1944, Serial No. 559,524

My invention pertains to a method of fabricating a moisture-proofed piezoelectric crystal element, and to a new and novel piezoelectric crystal element.

An object of my invention is to provide a piezoelectric crystal element which is more moisture resistant than hitherto known piezoelectric crystal elements of comparable type.

Another object of my invention is to provide a method of constructing a piezoelectric crystal element so that it is more moisture resistant.

A further object of my invention is to provide a method of evenly distributing a liquid coating material on a piezolectric crystal element.

It is also an object of my invention to provide a method of fabricating a piezoelectric crystal element having a moisture resistant coat of substantially even thickness throughout its flat surface areas and at its edges and corners.

Another object of my invention is toprovide a piezoelectric crystal element whose' corner and edge areas are substantially as moisture resistant as its face areas.

Another object of my invention is to so shape a piezoelectric crystal element that the maximum effectiveness in waterproofing may be obtained from any given waterproofing material.

Other objects and a fuller understanding of my invention may be had by referring to the following description and claims read in conjunction with the drawings in which:

Figure 1 is a cross-sectional view of a mom-- electric plate which has not been moistureproofed according to my invention.

Figure 2 is a cross-sectional view of a piezoelectric crystal element which has been moisture-proofed according to my invention.

Figure 3 is a cross-sectional view of a multiplate flexing type piezoelectric element fabricated according to my invention.

Figure 4 is a diagrammatic illustration of a 15 Claims. (Cl. 171-327) causing it to pull away from sharp edges and corners. In waterproofing piezolectric crystals, which are usually rectangular, square, or trapezoidal, this "pulling away" of the coating material from the corners results in weak spots in the coating through which moisture passes to the detriment of the crystal element.

In my invention I provide a practical method for rounding the edges and corners of a piezoelectric crystal plate sufliciently to cause the liquid coating material to be evenly distributed on the crystal element so that when the coating dries the resulting moisture-resistant coat is of substantially even thickness throughout the area of the crystal element. Thus there is no weak spot in the moisture-resistant coat at the edges and corners of the element.

Figure 1 is a cross-sectional view of a piezoelectric crystal element 10 whose corners i2 have not been rounded. A coat of water-proofing material l3 has been applied and the figure shows how the coating material l3, due to surface tension, tends to pull away from the corners leaving the comers l2 bare or only very slightly covered by the moisture-proofing material [3. My Patent No. 2,359,476 shows a device and method for reducing the extent to which the coating pulls away from the corners and edges.

When the crystal element I0 is subjected to an atmosphere having high humidity the moisture will first break through at the comers and edges and is then apt to travel under the moisture-proof coating thus spreading over a relativelylarge area. of the crystal element and leading to failure in or serious reduction of its piezoelectric action.

Piezoelectric crystal elements almost always have electrodes associated with them, such as the electrode [4, and it has been found that if moisture once gets under the electrode it is apt to spread rapidly. Also if moisture spreads on 'the face of the crystal element between the two electrodes H-H', a shunt circuit is established which materially adversely affects the operation of the crsytal element.

Figure 2 illustrates a piezoelectric crystal element I5 whose comers it have been rounded by my process. Subsequent to the rounding operation the crystal element I! has been coated with a liquid coating material such as shellac, chlorinated rubber. or the like, and it shows how the liquid coating material l3 dries to form a moisture-resistant film of substantially uniform thickness throughout the face. edge and corner areas of the crystal element. Thus the element shown in Figure 2 is as moisture resistant at its corners' and edges as it is at its face areas.

Figure 3 illustrates a multiplate flexing element of the type disclosed in Sawyers Patents Re. 20,213 and Re. 20,680, moisture-proofed in accordance with my invention. The corners and edges 38 on the upper or outer face of the plate 31 are rounded, but the corners and edges on the lower or inner face are not rounded. The corners and edges 40 on the lower or outer face of the plate 39 are rounded, but the corners and edges on the upper or inner face are not. I

A preferred method of fabricating a multiplate flexing element comprises the following steps: select two plates of crystalline material of proper size and orientation, as has been described in the Sawyer patents; machine one face of each plate until it is smooth, as has been described in Wil- 'iiams Patent No. 2,414,574, or in Sawyer et al.

2,112,636; connect an electrode 36, 36' to each of said machined faces using the teachings of Williams 2,106,143, or Sawyer 1.995257 and 1,994,- 487; cement the two plates together face-to-face with the electrodes inbetween, and if desired, with a lead extension in contact with said inner electrodes and extending out away from said element; one of the-outer faces is machined to provide a smooth surface and to reduce one of the connected plates to its proper thickness, and the edges and corners of the plate are rounded; the other outer face of the multiplate assembly is then machined and its edges and corners are rounded; outer electrodes 35, 35' are then connected to the outer faces, Leads may be connected to the electrodes 35, 35 if desired and the assembly is then dipped in or sprayed with liquid material which, upon drying, forms a moisture-resistant coating 13 of uniform thickness around the element.

Figure 4 illustrates the preferred method of rounding the corners and edges it of a piezoelectric crystal element. A circular work-supporting table I is keyed to a vertical spindle 3 which receives relatively slow rotary motion from a prime mover which is not illustrated. Suitable bearings are provided for the spindle and the table, if desired, may be supported at its periphery upon a fixed circular track of conventional type. The table is provided with a large number of small channels 5 that terminate in openings in the upper and lower faces thereof. A rotary milling cutter l is mounted to extend inwardly over the upper face of the table from the periphery toward the spindle, the axis of rotation of the cutter being parallel to the plane of rotation of the table. A plurality of wire brushes 20 and 2| are mounted to extend over the table I and are driven by means such as a motor 2'2. Means (not shown) are provided for raising and lowering-the cutter and the brushes, either together or independently, with respect to the surface of the table and the means may be set to hold the cutter and the brushes a spaced distance above the table.

The upper surface of the table I carries a. masking disk 9 having a plurality of spaced crystal receiving windows H therein. The spindle 3 has a threaded extension which extends through a central aperture in the masking disk 9 and a nut 25 having a wide laterally extending flange 26 is threaded on the spindle and so holds the disk 9 rigidly yet removably in fixed position that it revolves with the table. The windows ii in the disk 9 expose only the openings of the channels extending through the table directly beneath them; the remaining channels are Ill blocked ofl by the imperforate portions of the disk. The windows ii are only slightly larger than the crystal section which is to be put into it thus facilitating loading and unloading of the crystal. Should it be desired to operate upon crystal sections of a different size or shape, a different masking disk QImay be placed on the table l. i

In order that atmospheric pressure /may (be utilized automatically for holding a crystal section in place upon the table during the milling operation, the open end of an intake pipe 25 leading to an exhaust pump (not shown) is positioned beneath the table directly below the milling cutter and the gap between the lower surface of the moving table and the edge of the open end is made air-tight through utilization of packing means such as a resilient gasket disposed around the pipe and maintained in engagement with the underneath side of the table I. A more detailed description of this gasketmay be had by referring to Alfred L. W. Williams Patent No. 2,414,574.

Similar exhaust pipes 26 and 27 and resilient gaskets are provided for exhausting the air under those crystal elements which are directly beneath the brushes 2!] and 2!. Obviously means may be provided whereby a singl pipe exhausts the air at all three work stations.

It is preferable that the milling cutter i rotate in a direction to oppose the movement of a crystal section moving past it. One of the wire brushes, for example, brush 20 should also oppose the movement of the crystal section past it but the other brush 2! should rotate in a direction to aid the movement of a crystal section past it. The work table 5 rotates in a direction indicated by the arrow 28 such that each crystal element is milled prior to being brushed. After passing the milling cutter 1 each crystal element is brushed for a period of'time of about to 1 second in order to round two of its edges and it then passes to the brush 2| where it is brushed in the reverse direction for about to 1 second in order to round two more of its edges. After being brushed the second time the crystal elements are removed from the work table at a point where atmospheric pressure does not tend to hold them down and if they are not to form part of a multiplate flexing element, may go to a second similar work table where the opposite face is milled and the other 4 edges are brushed.

By suitable choice of the brushes 20, 2| and by suitably correlating the rate of rotation of the brush and the time period during which the crystal is operated on by the brush, the edges and corners may be rounded without detrimentally affecting the face area of the elements. Factors which must be taken into consideration when choosing a suitable brush include the material from which the bristles are made, the diameter of the brush, the compactness of the bristles of the brush, and the diameters of the individual bristles in the brush. Thus it is virtually impossible to here define all brushes which will work especially when the best type of brush will also depend somewhat upon the size and type of crystal element whose corners are to be rounded. As a practical example, I have found that on Osborn brush model No. 604-0 is satisfactory for rounding the edges and corners of Rochelle salt crystal elements. Two of these brushes may be positioned with respect to the crystal elements as shown in Figure 3. Their elevation with respect to the crystal should be such that about a: to %4 of an inch of the brush hits the crystal. This brush has steel wire bristles, 4 mils in diameter and about 1.75 inches long. The brush itself is 6 inches in diameter and it was rotated at 1,800 revolutions per minute. [have also found that if the bristles are of brass wire then thew may be 4 or 6 mils in diameter and still give satisfactory performance. Care must be exercised that the brush does not rotate too rapidly as it has been found that the centrifugal force stiffens the Wires to an extent that they will scratch the face surface of the. crystal elements. The brush 20 rotating in the direction shown by the arrow 29 opposes the motion of the crystal elements and rounds the two leading edges 3t, 32 of the crystal. The brush 2| rotating in the direction of the arrow 30 to aid the motion of the crystal elements rounds the trailing edges 33 and 34 of the crystal. Thus after each crystal element passes the second brush 2| its four top edges and corners are rounded. By a subsequent operation the four bottom edges and corners may be rounded. I have found that about a m or a 3 inch radius is sufficient rounding to give good results when water-proofing material such as shellac or chlorinated rubber is used for covering the crystal elements. Generally speaking the higher the surface tension of the material to be used for covering the crystal elements the more the corners and edges should be rounded in order to assure good coverage.

This disclosure has been made by way of example, and many changes may be made without departing from the spirit and scope of the invention as defined by the claims.

I claim as my invention:

1. In the process of fabricating a piezoelectric crystal element, the steps of rounding the edges and corners of said element, subsequently covering said element with a coating of hardenable liquid material, and causing said liquid coating to become hard to form a moisture-proof coating.

2. In the process of fabricating a platelike piezoelectric crystal element, the steps of rounding all of the edges and corners of said element adjacent one of its major faces. substantially covering said element with a coating of hardenable liquid material, and causing said liquid coating to become hard to form a moisture-proof coating, said hard moisture-proof coat being substantially as thick at said edges and corners as it is on the said major face of said element.

3. In the process of fabricating a piezoelectric crystal element, the steps of causing said element to come into contact with a rotating brush to cause said brush to remove crystalline material at the edges and corners of said element, and subsequently coating said element with a thin layer of liquid hardenable waterproofing material.

4. In the process of fabricating a plate-like piezoelectric crystal element, the steps of causing said element to come into contact with a rotating brush to cause said brush to remove crystalline material to round all of the edges and corners of said element adjacent one of its major faces, and subsequently coating said element with a thin layer of. liquid hardenable waterproofing material, said moistureproof layer upon becoming hard being substantially as thick at said edges and corners as it is on the faces of said element.

5. The process at set forth in claim 4, further characterized in this: that said brush is a wire brush.

6. The process as set forth inclaim 4, further characterized in this: that said brush is a wire brush comprised of brass wires 4-6 mils in diameter.

7. The process as set forth in claim 4, further characterized in this: that said brush is a wire brush comprised of steel wires 4 mils in diameter.

8. The process as set forth in claim 4, further characterized in this: that said brush is about 6 inches in diameter and rotates at about 1,800 r volutions per minute.

9. I'he process as set forth in claim 4, further characterized in this: that only about 3% to %4 of an inch of the ends of the brush bristles come in contact with said crystal element.

10. The process as set forth in claim 4, further characterized in this: that the direction of rotation of said brush is at an angle with respect to the directions of the edges of said crystal element.

11. The process as set forth in claim 4, further characterized in this: that said brush is in contact with said crystal element for about one second.

12. The process as set forth in claim 4, further characterized in this: that said brush is about six inches in diameter and is comprised of wire bristles about 5 mils in diameter, said brush rotating at about 1,800 revolutions per minute and havirg about as of an inch of the ends of the bristles in contact with the crystal element for about one second.

13. As an article of manufacture, a piezoelec tric crystal element covered with a thin layer of substantially waterproof material, said crystal element having all of its edges and corners rounded.

14. As an artcle of manufacture, a piezoelectric crystal element covered with a thin layer of substantially waterproof material, said crystal element having all of its edges and corners rounded. and said layer of substantially waterproof material being of substantially uniform thickness at all points on said element.

15. An article of manufacture as set forth in claim 14, further characterized in this: that the said edges of said crystal element are rounded on a radius of about 41 of an inch.

CHARLES K. GRAVLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 300,711 Ingersoll June 17, 1884 1,950,372 Cross Mar. 6, 1934 1,964,174 Raiche June 26, 1934 2,123,227 Bieling July 12, 1938 2,405,425 Higgins Aug. 6, 1946 2,359,476 Gravley Oct. 3, 1944

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