US3689782A

US3689782A - Electronic transducer for a piezoelectric line

Info

Publication number: US3689782A
Application number: US158805A
Authority: US
Inventors: Bernard Epszein
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1971-07-01
Filing date: 1971-07-01
Publication date: 1972-09-05
Anticipated expiration: 1989-09-05

Abstract

Piezoelectric transducer for creating alternating mechanical stresses in a piezoelectric material, in which said mechanical stresses are obtained by depositing electric charges on the surface of the piezoelectric material. These charges are those of electron beams impinging on the piezoelectric material, whose intensity are such and whose impact zones are so distributed at the surface of the piezoelectric material that the alternating mechanical stresses created by the electron beams are cophasal with the mechanical wave propagating within the piezoelectric material.

Description

United States Patent Epszein 1 Sept. 5, 1972 [54] ELECTRONIC TRANSDUCER FOR A 2,941,077 6/1960 Marker ..250/49.5 TE PIEZOELECTRIC LINE 2,277,007 3/ 1942 Von Ardenne ..3 l0/8 X 3 113 896 12/1963 Mann ..250/49.5 TE 72 I t Bernar 1 dEpmin France 3,446,975 5/1969 Adler et a1 ..310/s.1 x Asslgneer Th0ms9n-CSF, Pans, France 2,698,928 1/1955 Pulvari ..310/8 X 22 F1 d: ul 1 1971 1 J y Primary Examiner-J. D. Miller pp N91 158,805 Assistant Examiner-Mark 0. Budd Attorney-George B. Oujevolk [52] US. Cl. ..310/8.1, 310/8, 310/8.2,

250/495 C, 250/49.5 TE, 333/30 R, 315/30, ABSTRACT 315/31 R Piezoelectric transducer for creating alternating [51] 11 1. CI. ..H01v 7/00 mechanical stresses in a piezoelectric material, i [58] F181! of Search .310/8, 8.1, 8.3, 8.7, 9.7, which said mechanical Stresses are b i d b 310,93 315/30 3 250/495 f depositing electric charges on the surface of the 4 333/30 R piezoelectric material. These charges are those of electron beams impinging on the piezoelectric materi- [56] Referemm C'ted al, whose intensity are such and whose impact zones UNITED STATES PATENTS are so distributed at the surface of the Piezoelectric material that the alternating mechanical stresses 2,289,205 7/1942 Nagy et al ..310/8.1 X created by the electron beams are cophasal with the 2,793,288 5/1957 Pulvarl ..310/8.1 mechanical wave propagating within the piezodectric 3,614,423 10/1971 Heymck ..250/49.5 TE X material. 3,491,236 1/ 1970 Newberry ..250/49.5 C 2,547,994 4/1951 Bertein ..250/49.5 C 8 Claims, 4 Drawing figures PATENTEDSEP 51972 3.689.782

sum 1 nr 2 3044mm! fg ELECTRONIC TRANSDUCER FOR A PIEZOELECTRIC LINE The present invention relates to devices known as piezoelectric transducers, by means of which an electrical quantity can be converted into mechanical forces in a material exhibiting the piezoelectric effect. If said quantity is a time-variable one, the result is the propagation of mechanical waves within or at the surface of the material.

In the very high frequency range, the material is manufactured in one known embodiment, in the form of a parallelepiped bar and the electrical quantity which is to be transformed into mechanical waves is applied to one or more flat metal lines, of the inter-digital kind for example, between which the effective alternating electric fields are created. The mechanical stresses resulting from this are propagated through the bar in the form of mechanical waves, the wavelength being asstlaciated with the physical characteristics of the materia In order to improve the efficiency of this kind of transducer it is desirable that severalelements of the inter-digital lines should cooperate additively in terms of their electric fields, this meaning that for a given electrical frequency they should be disposed along the bar at distances which are associated with multiples of the mechanical wavelength in the material, so that the corresponding mechanical stresses are cophasal.

This geometric condition limits the passband of this kind of line transducer. This of course means drawbacks as far as certain applications are concerned, and one such drawback is that it is impossibleto easily modify the point of entry of the RF signal through the piezoelectric bloc, to accord with requirements.

Finally, if said high frequency is very high, the distance-between the elements of the inter-digital lines becomes so small that it is very difficult in practice to manufacture them.

The object of thanks present invention is to achieve the same advantages offered by the known devices without incurring their drawbacks, this by creating the effective electric fields not with the help of conventional high frequency coupling lines but with electron beams whose dimension, point of impact, impact area, shape etc., can readily be varied thank to the electrooptical techniques now available.

More precisely, the object of the present invention is a device for converting an alternating electrical quantity into a mechanical quantity propagating through a piezoelectric solid, characterized in that it comprises an electron beam source, a first group of electrodes for modulating said beam, a second group of electrodes comprising parallel slots which split said beam into several separate beams, a third group of electrodes for concentrating said beam, and a fourth group of electrodes close to the surface of said solid, said electron source being connected to the negative pole of said second group to the positive pole, of a voltage source.

The invention will be better understood from a consideration of the ensuing description by reference to the attached figures in which:

FIG. 1 illustrates a general view of the transducer device 25 in accordance with the invention FIG. 2 illustrates a variant embodiment of the device FIGS. 3 and 4 illustrate two other variant embodiments comprising a resonant cavity.

FIG. 1 illustrates a piezoelectric electronic transducer device in accordance with the invention, with an electron source 1 heated by a filament 2, a first gridtype electrode 3 for controlling the electron beam emitted by the cathode 1, a second electrode 4 containing openings 5 in the form of parallel slots, and a third electrode 6 in the form of a grid, located in proximity of the block 7, bar-shaped.

Finally, the

lateral electrodes

8 and 9 focus the electron beam and the

electrodes

13 and 14 control its direction. This is achieved by means of voltage sources (not shown) to which are connected these electrodes according to a common practice of the known art. The transducer assembly is arranged in such fashion that the direction of the beam is substantially perpendicular to the working face of the piezoelectric bloc.

The operation of this kind of transducer is as follows the electron source 1, or cathode, is connected to the negative pole of a direct voltage source 30 it emits an electron beam which is accelerated by the electrode 4 or anode, connected to the positive pole of said voltage source the grid 3 or modulating electrode enables the emission current to be regulated and in particular modulated, by the application to said grid with respect to the cathode 1, of a positive voltage, whose value undergoes variations versus time, by means of the source 31.

The electrodes, 8 and 9, enable the beam to be focused and guided, the beam, after passing through the slots in the anode, being split into several separate flat beams.

Beyond the grid 6, these beams strike the surface of the piezoelectric bar at separate impact zones. There, they release secondary electrons the grid 6 picks up these secondary electrons and, by a well-known mechanism, fixes that of the surface of the block on which the beams impinge to a value lower than that of the potential of the second group of electrodes.

The charge transfers to the impact zones on the crystal, are always at the same sign and their variations pass from the value zero to a maximum however, they can be analyzed as resulting from the sum of two separate charge transfers, namely a continuous transfer of negative charges and a purely alternating transfer, passing through positive and negative values.

The continuous negative charge transfer can be neglected in the analysis of the operation of the system, since its action is limited to the creation of a volume wave displacing perpendicularly to and terminating at the opposite surface of the crystal where, if required, it can be ansorbed the alternating transfer of charges which constitutes the effective mechanism, by contrast creates in each impact zone a mechanical stress of piezoelectric origin which leads to the same result as that obtained in the prior art electrical lines described hereinbefore.

For the same reasons enumerated in that case, it is desirable, in order to improve efficiency, to bring about addition of the action of the electric fields on each impact zone, this leading, for a given electrical modulating frequency, to the disposition of said zones at intervals which are multiples of the mechanical wavelength in the materials so that the mechanical stresses corresponding to each one are cophasal however, in contrast to the restriction of passband width which occurs in the known devices, this condition is readily satisfied in the case of the present invention by adjustment of the electrical voltages applied to the bunching and

deflection electrodes

13 and 14, it being an easy matter thus to modify the interval between the impact zones in accordance with the laws of electron optics, and thus introduce the possibility of considerably expanding the range of input frequencies at which the transducer will operate with good efiiciency.

On the other hand it is readily possible, equally, by adjustment of the voltages applied to these electrodes, to vary the location of the set of impact zones of the beam along the bar, in order, in certain applications, to produce effects of lead or lag in the establishment of the effective voltages.

FIG. 2 illustrates a variant embodiment of the invention in which the flow of static electrical charges developed on the piezoelectric bloc by the beam, is produced by a means other than the secondary emission associated with a collector electrode as occurs in the case of FIG. 1 the grid 6 of FIG. 1 is discarded and a thin conductive film 10 of high resistivity is deposited upon the block the film 10 has applied a direct positive voltage with regard to 1. In a variant embodiment, the film 10 is connected to the positive terminal of the high voltage source, as shown in FIG. 4.

Under these conditions, the static charges deposited by the electron beams on to the block 7 can flow away and propagation of mechanical waves can take place.

FIG. 3 illustrates another embodiment of the invention intended particularly for the case of very high frequencies. The modulating electrode 3 of the previous embodiments of FIGS. 1 and 2, forms part of the resonant cavity 11 of the kind used in microwave work. The emissive cathode 1 producing the electrons and the modulating electrode 3 are respectively located opposite one another in that part of the cavity where the alternating electric field is strongest.

FIG. 4 illustrates another embodiment in which the cathode 1 is separate from the cavity through which the beam passes, the beam being velocity modulated when passing through the cavity. It is well known, from the theory of electron tubes, that this kind of modulation, beyond a given drift space 20, gives rise to density modulation of the beam, producing electron bunches at high density. The piezoelectric material block is located beyond this drift space 20.

In the example shown in FIG. 4, the film 10 has applied a positive potential with respect to the electrode 4, by means of the voltage source 32 however, such a source is not essential to a satisfactory functioning of the device of FIG. 4, which may operate also with the film 10 connected to the pole of the source 30. In this case, not shown, the drift space extends up to the film 10.

What is claimed, is:

1. In a device for converting an alternating quantity into a mechanical quantity propagating through a sai bl k; a irst gi'oup of electrodes (3) for modulating the intensity of said electron by an alternating electrical quantity;

0. a second group of electrodes (8, 9, 13, 14), for

concentrating and deflecting said beam; and,

. charge means for fixing the potential of the surface on which the beam inpinges to a value depending upon the intensity of the beam; the improvement therein which comprises:

e. a direct voltage source with negative and positive poles;

f. a third group of electrodes (4) having slots which split said beam into several separate beams, said separate beams when impinging on said block serving to define said block into several impact zones, said electron source being connected to said negative pole, said third group of electrodes being connected to said positive pole; and

a second voltage source, said first group of electrodes being connected thereto, said second voltage source bringing the potential of said first group of electrodes with respect to said electron source, to a value which undergoes variations versus time.

2. A device as claimed in claim 1, in which said first group of electrodes comprises at least one electrode forming part of a high frequency resonant cavity.

3. A device as claimed in claim 2, in which, the electron source is integral part said resonant cavity and is located opposite said electrode.

4. A device as claimed in claim 2 in which said source is located apart said resonant cavity, on the side opposite the side where is located said block, and a drift space provided beyond said cavity on the path of said beam towards said block.

5. A device as claimed in claim 1, in which said parallel slots are directed perpendicularly to the direction of propagation.

6. A device as claimed in claim 1, wherein said piezoelectric solid block is made of material having secondary electron emission properties, said charge means consisting of a fourth group of electrodes located close to the surface of said block on which the electron beam impinges, said fourth group having applied a direct voltage with regard to said cathode, in order to act as a collector for secondary electrons.

7. A device as claimed in claim 1 in which said charge means consists of a thin resistive metal film applied upon the surface of said block, said metal film having applied a direct positive voltage with regard to said electron charge.

8. A device as claimed in claim 1 said third group of electrodes (4) having said slots so disposed that the defined zones are spaced at intervals which are multiples of the mechanical wavelength of the material of said block.

Claims

1. In a device for converting an alternating quantity into a mechanical quantity propagating through a piezoelectric solid block (7), having: a. an electron source (1) from which is emitted an electron beam, said electron beam impinging on said block; b. a first group of electrodes (3) for modulating the intensity of said electron by an alternating electrical quantity; c. a second group of electrodes (8, 9, 13, 14), for concentrating and deflecting said beam; and, d. charge means for fixing the potential of the surface on which the beam inpinges to a value depending upon the intensity of the beam; the improvement therein which comprises: e. a direct voltage source with negative and positive poles; f. a third group of electrodes (4) having slots which split said beam into several separate beams, said separate beams when impinging on said block serving to define said block into several impact zones, said electron source being connected to said negative pole, said third group of electrodes being conNected to said positive pole; and g. a second voltage source, said first group of electrodes being connected thereto, said second voltage source bringing the potential of said first group of electrodes with respect to said electron source, to a value which undergoes variations versus time.