US 2540187 A
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Description (OCR text may contain errors)
Feb. 6, 1951 w L. CHERRY, JR 0,187
PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME Original Filed Dec. 26, 1947 2 Sheets-Sheet l 5 F762 55 F/6f3 WALTER L. CHERRY JR.
HIS AGENT Feb. 6, 1951 CHERRY JR 2,540,187
W. L. PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME Original Filed Dec. 26, 1947 2 Sheets-Sheet 2 WALTER L. CHERRY JR. INVENTOR.
HIS AGENT Patented Feb. 6, 1951 PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME Walter L. Cherry, Jr., Northbrook, Ill., assigno'r to Zenith Radio Corporation, a corporation of minois Original application December 26, 194'! Serial No. 793,892. Divided and this application May 2, 1950, Serial No. 159,813
3 Claims. 1
This application is a division of the copending application of Robert Adler, Serial No. 793,892, filed December 26, 1947, assigned to the present assignee, and relates to piezo-electric transducers and to methods for producing such transducers. It is a primary object of the invention to provide an improved artificial transducer having permanent piezo-electric properties and to provide an improved method of producing such a transducer.
The use of polycrystalline aggregate such as barium titanate or barium strontium titanate, bonded with a ceramic binder, in the production of artificial piezo-electric transducers is specifically disclosed and claimed in the copending application of Walter L. Cherry, Jr., Serial Number 770,163, filed August 22, 1947, for Piezo-.
Electric Transducers, and which application is assigned to the same assignee as the present application.
Such artificial piezo-electric transducers as are specifically disclosed in the aforementioned copending Cherry application are particularly useful for high frequency applications. However, for optimum coupling, the direction of mechanical stress must be identical with that of the piezo-electric axis and that of the alternating field; consequently transducers of this type are not readily applicable to audio frequency devices. The minimum frequency at which such transducers may be operated is determined by the maximum capacitance, or electrical compliance, commensurate with practical values of mechanical compliance. If the mechanical compliance is made large enough to suit practical requirements by making the cross-sectional area small, the associated capacitance is too small for audio frequency applications. If, on the other hand, a practical value of capacitance is obtained by making the cross-sectional area large, the mechanical compliance becomes inconveniently small.
It is a particular object of this invention to provide an improved artificial piezo-electric transducer which/is suitable for operation at audio frequencies;
In accordance with the above-identified Adler application, it has been found that the capacitance or electrical compliance of the artificial transducers may be increased while maintaining a practical value of mechanical compliance by varying the direction of the piezo-electric axis within a single-unit transducer. It is a further object of the invention to provide a novel ceramic transducer, sensitive to bending stress, which utilizes this principle.
Frequent applications of piezo-electric transducers in the audio frequency range are made in phonograph pickups, microphones, and the like. In such applications, it is necessary to provide a transducer which has permanent piezoelectric properties with respect to bending stress. It is an important object of the present invention to provide a novel piezo-electric polycrystalline aggregate transducer, sensitive to bending stress, which is suitable for use as a phonograph pickup or the like, and which is rugged, inexpensive, and durable.
In the specification and claims, the term direction, as applied to the piezo-electric axis or to the polarizing field, includes the concept of sense; therefore the phrase "piezo-electric axes having different directions includes a piezo-electric axes of opposite sense.
The term unidirectional, as applied to polarizing fields, is employed with reference to time, and not with reference to space. Hence a "unidirectional polarizing field is one which is produced by a unidirectional potential difference.
The terminology polycrystalline aggregate is employed to connote a unitary structure comprising a large number of minute crystals. The term ceramic necessarly implies such a structure.
The features of the invention which are believed to be novel are set forth particularly in the appended claims. The invention may more readily be understood, however, by reference to the folowing description taken in connection with the accompanying drawings, in which like reference numerals indicate like elements, and in which:
Figure 1 is a schematic representation of a theoretical means for attaining the objects set forth above.
Figure 2 is a schematic representation of a practical method for approximating the theoretical optimum condition shown in Figure 1.
Figure 3 is an enlarged view of a section of Figure 2 showing the electrostatic fiux distribution.
Figure 4 is a schematic representation, partly in section, of a ceramic element sensitive to bending stress.
Figures 5 and 6 are schematic representations of an embodiment constructed in accordance with the invention.
Figures 7-9 are schematic representations of another type of ceramic transducer sensitive to bending stress.
Figure is a perspective view of a physical embodiment of the invention.
Referring now to Figure 1, there is shown schematically a slab of piezo-electric material, the direction of the piezo-electric axis being assumed to be longitudinal. It is seen that the mechanical compliance in the longitudinal direction may well be high enough to suit practical requirements, since the cross-sectional area is small relative to the length, but the capacitance between electrodes located on the small'surfaces 2i and 22 separated by the full length of the piece 20 is inconveniently small. In order to increase the capacitance or electrical compliance, it is assumed that slab 28 is theoretically divided into n slices 23-42 of equal length; the dotted lines indicate any desired number of intermediate slices. The capacitance between the imaginary transverse surfaces of each slice is therefore n times that between the two end surfaces 2! and 22 of slab 28, while the mechanical compliance between surfaces 2i and 22 remains unchanged. Furthermore, by interconnecting alternate transverse faces of the imaginary slices 23-32, as shown, and by connecting each set of alternate transverse faces to one of a pair of terminals 88 and 24, the total capacitance between surfaces 2| and 22 is made equivalent to n times the capacitance between the end faces of an individual imaginary slice 23; consequently the effective capacitance between surfaces 2! and 22 is increased by a factor of n.
Ideally such a-condition may be accomplished by polarizing adjacent slices 28-32 along the same axis but in opposite directions. Such a condition is shown schematically by the arrows which indicate the direction of polarization, and hence the direction of the piezo-electrlc axis, in each of the imaginary slices.
It has been found that permanent pleas-electric properties may be induced in certain polycrystalline aggregates, and furthermore, that the direction of the piezo-electric axis induced in such aggregates is identical to that of the electrostatic flux set up by the polarizing fields. Consequently, such a condition as shown schematically in Figure 1 might be accomplished if suitable unidirectional polarizing fields could be set up between the transverse faces of each of the imaginary slices 23-32. Obviously, such a situation is impractical, since no electrodes exist on the imaginary transverse surfaces between the slices.
As a practical approach to the optimum condition shown in Figure 1, there is shown in Figure 2 a slab of suitable polycrystalline aggregate, such as a ceramic comprisi barium titanate or barium strontium titanate mixed with a small amount of a glass forming oxide and fired to vitrification in accordance with the aforementioned copending Cherry application, on opposite faces 4i and 42 of which have been disposed a plurality of electrical terminals or electrodes 43-44. For purpose of illustration, twelve terminals have been shown: however, other numbers of terminals may be employed. Alternate pairs of opposite terminals 43, ll, l1, 48, 5|, 52 and 45, 48, 49, 50, 53, 54 are interconnected and brought out to a pair of input terminals 55 and 56 respectively. when a unidirectional potential diflerence is applied between input terminals 55 and 58, adjacent portions of slab III are polarized in substantially opposite directions.
In order more fully to show and explain such polarization, there is shown in Figure 3 a detail view of a portion of slab 40 of Figure 2. The electrostatic flux distribution, and hence the direction of polarization, is assumed, for the purposes of explanation, to have a sense from to If then, it is assumed that input terminal 55 is the positive terminal and input terminal 58 is the negative terminal, the electrostatic field, and hence the direction of the piezo-electric axis, differs abruptly from portion to portion as schematically shown in Figure 3.
It is seen that the embodiment shown and described in conjunction with Figures 2 and 3 is only an approximation of the ideal shown and described in conjunction with Figure 1. Since those portions of the slab 40 of ceramic material which lie between opposite pairs of terminals, l1 and 48, for example, carry little or no electrostatic flux, these portions may be regarded as being waste portions." In order to effect a compromise between the desired effective increase in electrical compliance and a minimum of "waste material, it has been found that the width of the individual terminals 43-54 should be of the order of the thickness of the slab l8, and the distance between successive terminals, 41 and 49, for example, should be of the order of twice the thickness of the slab 40. These proportions are the result of practical experiment and are intended in no sense to be construed as limitations, as other proportions may be used with varying degrees of efficiency.
In certain audio frequency applications, it is 'desirable to employ a piezo-electric element which is sensitive to bending. Two elements, each formed in accordance with the method shown and described in connection with Figure 2, may be fastened together, by cement or in some other suitable manner, to provide such a dimorphic element. Such a configuration is shown in Figure 4, wherein a pair of elements 60 and 6| are fastened together to form a composite body 82. In operation one element 60 is instantaneously subjected to tensile stress while the other element 8! is instantaneously subjected to compressive stress, or vice versa. Therefore, care must be taken that elements 60 and iii are assembled and electrically connected in such a manner that the useful outputs of the individual elements are additive in the composite transducer 62.
While a transducer having permanent piezoelectric properties with respect to bending stress may be produced in the manner shown and described in conjunction with Figure 4, a simplified transducer of this type may be provided. In
accordance with the invention, an element sensitive to bending stress may be provided by properly applying polarizing fields to a single unitary polycrystalline aggregate body. Such an element is shown in Figure 5, wherein a unitary slab 18 of barium titanate or other suitable aggregate is provided with a plurality of electrical terminals or electrodes ll-82 similarly disposed along opposite faces 83 and 84. Alternate terminals H, 15, and 19 on face 83 and alternate terminals 14, I8, and 82 on face 84 are interconnected and brought out to an input terminal 85, and the remainder of the terminals 12, 13, I8, l1,
10 80, and 8| are interconnected and brought out to a second input terminal 86.
By applying a unidirectional potential differ-i ence between input terminals 85 and 86, unidirectional polarizing fields are applied between 1 successive terminals, II and 13, for example, on
s each face, 04 of slab I0. Furthermore, each terminal (H, for example) is oppositely polarized from the terminal (12, for example) which is directly opposite therefrom, and the polarizing fields between opposite pairs'of successive terminals ill, I and II, I4 for example) are opposite in sense. By these means, the direction or sense of the piezo-electric axis is made to differ abruptly from portion to portion throughout the single piece II in a manner similar to the piezoelectric axis distribution in a composite dimorphlc'element such as that shown in Figure 4.
After maintaining the polarizing fields for a suiiicient period of time at least to approach saturation of the piezo-electric eifect, such fields are removed, and the connections of terminals I I-fl are changed to insure additive outputs in response to bending. Slab I0 with terminals II-02 reconnected for proper output at terminals 00 and 08 is shown in Figure 6, in which alternate pairs of opposite terminals II, I2, I0, I0, 10, and 00, and I3, I4, TI, I0, 8|, and 02 are interconnected. Such an interconnection is desirable in order to minimize the shunt capacitance between output terminals 85 and 00 which effectively reduces the useful output of such a reducing any shunt capacitance across the output terminals. In this embodiment, a pair of polycrystalline aggregate bodies 90 and 0i are fastened together by cementing means 92 of low dielectric constant thereby to form a composite body. Cementing means 02 may comprise a layer of cementing material of low dielectric constant, although the desired results may be achieved by glazing slabs 00 and 0| and firing with the glazed sides in contact, by firing a "sandwich of three ceramic slabs, or by other suitable means. Opposite faces of the composite body are provided with a plurality of similarly disposed electrical terminals 93-98 and 99-404, alternate pairs of opposite terminals 83, 99, 95, MI, 91, I03, and 94, I00, 96, I02, 08, I04 being interconnected and brought out to a pair of input terminals I05 and I06 respectively. After polarizing in the usual manner, the terminal connections on one face of the composite body are reversed to insure additive outputs in response to bending, as shown in Figure 8, and terminals I05 and I06 become output terminals. Although opposite terminals (93 and 90, for example) have opposite polarities associated therewith, the thin layer of cementing material 92 so reduces the effective capacitance shunting the output that satisfactory operation is insured. The embodiment of Figures 7 and 8 is specifically disclosed and claimed in the copending application of Alexander Ellett, Serial No. 159,620, filed concurrently herewith, for Pie'zo-Eiectric Transducer and Method for Producing Same, and assigned to the present assignee.
While the embodiment shown in Figures '7 and 8 is formed by cementing bodies 90 and SI together before polarization, it is to be understood that the individual bodies 00 and SI may be polarized separately and then firmly united in such manner as to insure additive outputs in response to bending. Such a method is shown schematically in Figure 9, in which body .90 is separately polarized. Alternate terminals 93, 95, 91, and 94, 06, 90 are connected to respective input terminals I00 and I06. After polarization, two such bodies are cemented or otherwise fastened together to produce a composite body such as that shown in Figure 8.
It is also to be understood that, although pairs of terminals 03-404 have been shown connected in parallel, pairs of terminals may be connected In series in certain applications in which such connection is desirable.
As a variant of this embodiment, two slabs of polycrystalline aggregate such as slab 00 in Figure 9 may be provided each with electrical terminals differently disposed in such a way that, after polarization, such slabs may be fastened together in a manner similar to that shown and described in conjunction with Figure 4. In this way, objectionable shunt capacitance between the output terminals may be avoided without the use of low dielectric constant cementing means.
As a further variant, the composite body shown in Figures 7 and 8 maybe polarized with the terminals 93 I04 connected as in Figure 8, in which case the terminals 83-I04 are reconnected as shown in Figure 7 to insure additive outputs. In this manner, the shunt capacitance across the output is minimized by interconnecting opposite terminals.
There is shownin Figure 10 a phonograph pickup IIO constructed in accordance with the invention. The terminals III and H2 are shown in the form of a pairof intermeshing combs for efilcient polarization, and may be of silver paint or other suitable material applied by silk screen, vacuum evaporation, or other suitable process. In a practical application, the terminals applied on the back side (not shown) of element I I0 may be made in theform of a mirror image of those applied to the front side in order to minimize the undesired capacitance shunting the output. One end H3 of transducer 0 is firmly clamped in a bracket I I4, and the other end I I5 is provided with a rigid extension H0 so constructed and arranged that the edges I I1 and I I0 of transducer H0 and extension H0 all converge to a point H0. Lateralmotion at point I I9, which may be translated from the undulations of a record disk by means of a stylus II9' secured to extension H6 at point H9, then results in corresponding electrical output between terminals III and. H2; by making transducer IIO trapezoidal in shape, the conditions of a uniform-stress beam are approached.
While there .has been shown and described a certain preferred embodiment of the invention, it will be understood that numerous variations and modifications may be made, and it is contemplated, in the appended claims, to cover all such variations and modifications as fall within the true spirit and scope of the invention.
1. The process of producing a permanently piezo-electric ceramic body sensitive to bending stress, said process comprising similarly disposing along each of twoopposite faces of a solid polycrystalline aggregate a plurality of parallel electrodes, applying unidirectional polarizing fields having substantially-opposite directions between successive pairs of electrodes on each of said faces, the direction of the polarizing fields between successive pairs of electrodes on one of 2. A ceramic element having permanent piezoelectric properties, said element including a solid polycrystalline aggregate and a plurality of electrodes similarly disposed along each of two opposite faces of said aggregate, the direction of the piezo-electric axis being substantially opposite between successive pairs of electrodes on each of said faces, and the direction of the piezo-electric axis between successive pairs of electrodes on one of said faces being opposite to the direction of the piezo-electric axis between corresponding pairs of electrodes on the other of said faces.
3. A ceramic element having permanent piezoelectric properties, said element including a solid polycrystalline aggregate, said aggregate comprising individual crystals of barium titanate bonded together with a ceramic binder, and a plurality of electrodes disposed along each of two opposite faces of said aggregate, the direction of the piezoelectric axis being substantially opposite between successive pairs of electrodes on each of said faces, and the direction of the piezo-electric axis between successive pairs of electrodes on one of said faces being opposite to the direction of the piezo-electric axis between corresponding pairs of electrodes on the other of said faces.
- WALTER L. CHERRY, JR.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS It! Number Name Date 2,281,778 Mason May 5, 1942 2,486,560 Gray Nov. 1, 1949