US2786955A - Transducer tube - Google Patents

Description

J- K. TROLAN TRANSDUCER TUBE March 26, 1957 Filed Feb. 2, 1954 E t han i Fig. 4

Fig. 3.

' INVEN TOR. J, KENNEL TROLAN By K fiffarnry United S e o The invention relatesfto devices which convert ame- 'cha'nical motion into an electrical. responseand particularly to' transducers employing an electronic vacuum tube with a field emission cathode.

The purposes of the invention are to provide a means of altering the surface electric field at "the cathodeof a 'field emission tube, 'by "the motion of one or more electrodes, thuslproducing simultaneously a change .inthe 'field dependent emission current; to ip'rovidea transducer utilizing the characteristics ofthefield emission cathode to obtain high sensitivity ofcurrent response to electrode "motion, miniaturization (and high frequency response,

negligible power required by the cathode q and ..dissipated elsewhere, and wide range of current an'd' voltage over which the device is useful. I

Numerous types of transducers are known .in .theart.

Most transducers are ,of either the -magnetic or piezoelectric type' although thermionic tubes with movable .con-

n01 grids are used in certain specialized applications.

The thermionic transducers with'movable control grids are difiicult to manufacture and are also quite fragile and susceptible to damageby mechanical shock. They lack the sensitivity of thedevices of pres'ent invention andrequire appreciable heat energy, and its dissipation, with correspondingly increased weight'and size. The latter also limits the frequency response of the device. .Piezoelectric and magnetic transducers are essentially low power and lowvoltage devicesa'nd cannotb'e used in high voltage'orhigh power applicationswithout bulky and expensive amplifying systems. "Magnetic transducers employing an electrical conductor moving in a magnetic "field will convert mechanical motion into'electrical re-.

- cathode electric field F,which 'dependence is given approximately by the equation:

J: Ae--L% 1 where J is in amperes/cm. ,'F.isthe-electridfield at the cathode surface in volts/cm. and A and b are constants which depend'qon. the type of 'cathodefmaterial used. It

' emitter is dependent on both the electrode geometries and their :mutualspacings and applied potentials (The Field --Emitter;. Fabrication, Electron' -Microscopy, andzElectric Field: Calculations, Journal of Applied, Physics; vol.: 24, o No; 5, 570.- 576,'May"1953), and that: a small'change in :the spatial relationship of the;electrodes,:;at"constant'volt- 1 age; can: cause a large. change in the emitted? field current i density from" the cathode- V -Field Emission: Large Current Densities, Space Charge, and the Va'cuum Arc,

' 2 .Theprimary purpose of this invention, is to utilize the characteristics of the field emitter ina vacuum tube in which the electric fieldatthe cathode. surface and hence itsfield emission current density can be variedbythe movement of oneor more of the electrodes jin the tube. Because of the exponential dependence of J. on jF (Eq. '1), there results fromthe present invention a,,highl-y sensitive transducer of mechanical motion to ,e1eotrica1, 1:esPQnSe.

The invention will be more ,fully described with reference, to the accompanying drawing in WhlCh- Fig. 1 is a selection elevation of a transducer mbe embodying the principles ofthe invention;

Fig. 2 is a fragmentary enlarged view of the electrode elements of the tube;

Fig. 3 is a'fragmentary enlarged .view of the electrode arrangement of another embodiment of the invention; and

.Fig. 4 is afragmentary enlarged view of theelectrq'de arrangement of a further modification of the invention.

Equation 1 requires an electric field of 10" to 10 volts/cm. at the cathode surface for appreciablefield .p,arent electron suppressor. 5 and being collectedat the plate electrode 6 Motion of the field electrode 4 relative to the cathodeZ causes a changein the cathodefield in spite of a constant potential difierence betweenthose electrodes and that change causes an appreciablygreater change in the amplitude in thefcurrent at theplatej, in view of Eq. 1. The device shown in Fig. IQthere'tore provides a highly sensitive transducer of mechanical mo tion between'electrodesl and 4 into electrical response at electrode '6. Relative, motion of the cathodet 2.with respect to the tube electrode 4 may be effected'by either or both of two means, (a) motion of the cathode 2'which is permitted by compression of the expansionbellows'!) through externally applied forces and (b) motion of the electrode 4 through a force applied on its lever arm 7 which is permitted by the flexible inseal 8.

"Figure 1 shows the details of .tube construction. An

electric current may be passed through the wire filament -3 to heat the filament by the resistive generation of heat.

- movable and its position relative to the cathode 2 controls the electric field at the cathode surface, fora given value of the applied potential. In this tube both the cathode andelectrode 4 are movable although satisfactory operafha'ving-only onemovable electrode.

"is'shown as a wire loop or ring, but may take other forms.

Position4a represents the probable'mean position of the -=controlelectrode 4 when it is desired tohold it'at high potential relative" to: the cathode. The electric. field :at r the: cahtode surfacei will then increase: as the control-electrode 4' is moved towards thewcathode'causing a large c 'change in theemission current from thecathode. The

=2 emissioncurren'cwill :be: contained ina cone Wilma-vertex angle of about 100, the axis of the cone being parallel to the emitter axis so the electrons will pass through the center of the control electrode 4 and be collected on the anode 6. In this case the operation of the transducer is relatively independent of the potential of anode 6 which can be adjusted to meet other circuitry requirements as long as it is at least a few volts positive with respect to the cathode. The secondary electron suppressor grid 5 should be at or near cathode potential.

Position 4b of the control electrode 4 is suitable if it is desired to maintain the control electrode 4 at a potential near that of the cathode (positive or negative). In this case fora suitable choice of potential on electrode 4, the emission current will decrease as the control electrode 4 is moved toward the emitter tip and will increase as it is moved away from the emitter tip. In this case the electric field at the cathode tip is influenced by the chargeon both of the electrodes 5 and 6; this necessitates that they be at a sufficiently high potential with respect' to the cathode to obtain the desired field current. Electrodes 5 and 6 may be tied together electrically or electrode 5 may be made slightly less positive than electrode 6 to minimize electron collection on electrode 5.

There are several methods for transmitting mechanical. motion through vacuum seals. The flexible bellows 9 (Fig. l) is designed to form a vacuum tight junction. It can be made of glass but such a bellows is very fragile so that some material such as Kovar that can be sealed to the glass envelope 10 is preferable. The flexible diaphragm 8 may also be made of Kov-ar or other material that can be sealed into the glass envelope. For most applications the tube envelope 1% may be made of metal or some ceramic material. This provides a more rugged design and simplifies the sealing on of flexible diaphragms or adjustable bellows.

Fig. 3 illustrates a multi-electrode transducer in which the controlling electrode 13 is separate from the field emission cathode 12, and the electron collecting anode 11. In this case the electric field at the cathode surface is varied by altering the spacing between 12 and 13. If the control electrode 13 is near cathode potential, electron emission from the cathode will decrease as the electrode 13 is moved towards the cathode 2; if, however, 13

is considerably more positive than'12, electron emission will increase as the spacing between 12 and 13 is dccreased. The potentials of the three electrodes can be altered independently to obtain desired operating conditions. A secondary electron suppressor grid similar to electrode 5 in Fig. 1 may be included in a practical tube design in order to minimize secondary electron effects in cases where the electrode 13 is more positive than electrode 11. In Fig. 3, the control electrode 13 is shown as a hemispherically capped cylinder, but there are many geometries that would serve the purpose, such as a ring, helix or cylinder around the emitter, or a disc with a hole through which the emitter could protrude.

Fig. 4 shows a two electrode transducer with an anode 14 which is maintained at a positive potential relative to the field cathode 15. The potential necessary to cause usable field emission from the cathode is dependent on both the cathode-anode spacing and on the physical dimensions of both electrodes (primarily of the cathode); however, by a suitable choice of electrode geometries, the tube can be designed for operation at a desired voltage level. For a given potential, the magnitude of the electric field at the cathode surface is dependent on the cathode-anode spacing so that any variation in this spacing causes a large change in the emission from the cathode in view of Eq. 1. Fig. 4 indicates motion of the cathode 15 from position 15a to 15b; however, that movement is mentioned only as an illustration since the same desired effect couldbe obtained by moving the anode or by moving, the cathode and anode, simultaneously.

Single needle-shaped emitters whose tips have radii of curvature of 10- to 10- cm. have proven' satisfactory in operation as have numerous needlespring is directly proportional to the displacement.

shaped emitters operating in parallel. Other geometries with small radii of curvature such as razor edges, have given substantial field emission. it is well known that the effective work function of metals can be altered by coating the surfaces with suitable materials. For example, coatings of barium, cesium or thorium lower the effective work function of tungsten while oxygen or hydrogen increase its work function. In some cases it may be desirable to coat the cathode surface with suitable materials to alter the work function or to increase the stability of electron emission or both.

The transducer of theinvention has several advantages due to the characteristics of the field emission cathode. The primary advantage is the extreme sensitivity of the emission current to changes in electric field. For example, the emission current from a cathode whose work function is 4.5 electron volts, will increase by a factor of 100 if the electric field at the cathode surface is increased from 3.0)(10' volts/cm. to 3.8x l0 volts/cm. The transducer of the invention is also extremely versatile in that it can'be used over a wide range of voltage and current. This is a distinct advantage over piezoelectric or magnetic transducers as these devices are limited to lower power and low voltage application. A further advantage is the micro-geometry of the cathode which facilitates miniaturization of the tube and makes possible tube designs in which a very small amount of mass needs to be moved. This increases both the sensitivity of the device and frequency at which it may be operated.

The motion of the transducer electrodes can be controlled by means other than the direct transmittal of mechanical motion from sources external to the device.

Simplified construction can be obtained for many appli cations by using a magnetic field to move the electrodes. Ferromagnetic material can be included in an electrode which could then be moved by forces exerted by a magnetic field. Theelectrode can be mounted so that a displacement of the electrode is necessary to balance the force caused by the magnetic field, for example by spring mounting an electrode so that the force exerted by the If the force exerted by the spring is equal and opposite to the force caused by the magnetic field, the displacement of the control electrode will be directly proportional to the magnetic field strength. Such a device can be used to measure magnetic fields or as a magnetic amplifier.

The movement of the electrodes can also be controlled by the thermal expansion of a metal support or a bimetallic strip. For example, the movable electrode can be mounted on or connected to a bimetallic strip of wire that can be heated directly by the passage of electric current or indirectly by having a small heating element adjacent to it. The movement caused by this thermal expansion controls the field emission current so that low voltage used for heating can be made to control a high voltage circuit. Infra-red radiation can beused to control the temperature of the tube elements and the device can thus be used as an infra-red detector.

Obviously, there are many similar uses for tubes embodying the principles of the invention.

I claim:

l. A transducer tube for converting mechanical movement into electrical variations comprising'an evacuated envelope, an electrode assembly in said envelope includmg a field emission cathode having an electron emitting portion of radius of curvature not exceeding about 10* centimeters, a collector electrode spaced therefrom and at least one auxiliary electrode positioned adjacent the electron emitting portion of said cathode and transparent to theiflow of electrons between said cathode and collector electrode, andmeans for effecting relative motion of an electrode of said assembly to'alter the field strength at said electron emitting portion of said'cathode.

2. A transducer tube as defined in claim 1 including envelope to the interior thereof supported by said flexible means for moving said cathode with respect to said colmember and interiorly connected to an electrode of said lector electrode. electrode assembly.

3. A transducer tube as defined in claim 1 including means for moving said auxiliary electrode with respect to 5 Ref n Cited in the file of this patent said cathode.

4. A transducer tube as defined in claim 1 wherein UNITED STATES PATENTS said envelope includes a flexible member in the wall 2,407,979 Findlay Sept. 24, 1946 thereof and a member passing from the exterior of the

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