US3452224A

US3452224A - Method of operating a thermionic converter

Info

Publication number: US3452224A
Application number: US480664A
Authority: US
Inventors: Karl G Hernqvist; Jules D Levine
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1965-08-18
Filing date: 1965-08-18
Publication date: 1969-06-24
Anticipated expiration: 1986-06-24
Also published as: DE1539339A1; BE685670A; GB1146713A

Description

United States Patent O D. Levine, New

U.s. c1. 9 Claims ABSTRACT F THE DISCLOSURE A method of operating a gas tube containing cesium involves heating the tube including a sole cesium reservoir to a temperature above the condensation temperature of cesium, and storing within the reservoir by surface absorption only, at least ten times as many atoms of cesium vapor as are present elsewhere within the tube.

Our invention relates to gas tubes and particularly to a method of operating a gas tube such as a thermionic energy converter containing a source of gas or metal vapor.

A thermionic energy converter is an electron tube having a cathode and an anode, for converting heat energy to the cathode to electrical energy in the form of a voltage produced by the tube itself between its cathode and anode terminals. The materials of the cathode and anode are usually chosen to provide a cathode having an electron work function substantially higher than that of the anode, to produce an internal electric field for accelerating electrons from the cathode to the anode.

In order to facilitate travel of electrons from the cathode to the anode, the space charge of the electrons between the cathode and anode may -be neutralized by the use of positive ions. One source of such ions may be an alkali metal vapor having a low ionization potential, such as cesium, introduced into the interelectrode space.

One way of introducing an alkali metal vapor such as cesium into the interelectrode space has involved the provision within the tube envelope of a liquid pool of alkali metal. Such liquid pool is usually accommodated in an appendage communicating with the interelectrode space of the device. v

A serious disadvantage characterizing the use of such a liquid reservoir or pool resides in the fact that the device is rendered undesirably temperature sensitive. It u is well known that the vapor pressure of a liquid pool of alkali metal utilized as a source of alkali metal vapor to produce desired positive ions, is determined mainly by the pool temperature. The relation between the vapor pressure and the pool temperature hereafter referred to as the normal condensation temperature, is known for most elements. See for intsance K. K. Kelley, Bureau 0f Mines Bulletin, 383, 1935. I

Accordingly, it is an object of our invention to provide an improved method of operating a gas tube characterized by reduced sensitivity of gas or vapor pressure within the t-ube to te-mperature.

It is common practice to operate thermionic energy converters under such conditions that most, if not all, metal surfaces within the envelope of the device are covered with a monolayer or less of cesium. For example, the cathode may be covered with one-half monolayer of cesium. This coverage of the internal metal surfaces of the device is an equilibrium state between an arrival and evaporation rate of cesium at such surfaces.

We propose to take advantage of this tendency of cesium to cover metal surfaces, by providing a relatively large surface area means within the device for storage thereon of cesium in its vapor phase. The storage means may comprise a metal sponge having pores therein and formed for example, by sintering together tungsten metal particles or other material, such as aluminum oxide. The sponge may also be in the form of an unsintered powder. For a given volume of such structure, the surface area of the pores therein is relatively large, thus providing storage for a relatively large amount of cesium in its adsorbed state.

The greater afinity of cesium for surfaces other than liquid cesium renders it feasible to store cesium in an adsorbed state within the device at a relatively wide temperature range. This temperature range extends from a temperature slightly higher than the normal condensation temperature of cesium to any substantially higher temperature at which the converter may be operated. In other words, a temperature at which the cesium pressure is optimum for best converter efficiency may be used even though it is substantially higher or lower than one or more elements of the converter.

This greatly increased independence of reservoir temperature from the pressure of cesium within the converter is due to the fact that the binding energy of cesium atoms to a pool is relatively small, being 0.75 electron volt at a temperature of 550 K. whereas the binding energy 0f cesium atoms to the sponge pore wall is relatively large such as, for example, 2.00 electron volts at the temperature indicated. Therefore, with a sponge type of cesium reservoir, it is possible to operate the reservoir at an appreciably higher temperature than normal. A cesium vapor reservoir is, therefore, seen to be less dependent on critical temperatures than liquid pool type of reservoirs.

A cesium reservoir as herein `described stores cesium in the form of vapor adsorbed on metal surfaces. The volume of this type of storage reservoir is appreciably larger than in the liquid pool type of reservoir, for the same cesium mass. However, the volume of cesium stored as an adsorbed vapor Should be adequate throughout a normal operating lifetime of the device. It should be noted in this connection that unavoidable losses of cesium occur during operation of a converter. Causes for such losses may be grain boundaries, cracks, or fissures in the inner wall of the device envelope into which cesium may migrate. We have found that such losses are fully compensated for during the normal operating life of the device when the surface area of pore walls of a sponge type of cesium vapor reservoir is at least ten times as large as the normal inner surface area of the envelope of the converter. However, we prefer an appreciably larger ratio of pore wall area to normal internal envelope area.

For a more detailed consideration of an exemplary embodiment of the invention, reference is now made to the accompanying drawing in which:

FIG. 1 shows a section of a thermionic energy converter in which the invention is used; and

FIG. 2 is a family of functional pressure versus temperature curves illustrating the appreciable difference between the vapor temperature relations of liquid type cesium reservoir, and the sponge type cesium vapor reservoir.

The invention is illustrated in the drawing as ernbodied in a thermionic energy converter tube 10 having a vacuum tight envelope 12 containing cesium and made up of two similar circular metallic plates 14, 16 made of tungsten, for example. The plates 14, 16 are suitably spaced from each other by a ring 18 of insulating material such as aluminum oxide.

On the inner surface of plate 14 is fixed .as by sintering a matrix 20 which may be made of tungsten particles sintered together. The sintered matrix 20 constitutes a reservoir for cesium vapor. On the surface of the Imatrix 20 is a relatively thin layer 22 of highly smooth metal such as tungsten xed to the matrix 20 as by sintering. The matrix 20 and layer 22 may constitute a cathode, and a plate 16 may serve as .an anode of the device. An exhaust tubulation 24, shown tipped o, serves as a means for evacuating the envelope 12 and introducing cesium thereinto. The anode 16, which is maintained at a lower temperature than the cathode is suitably cooled by radiators 26. The cathode may be heated by suitable means such as a flame directed to the outer surface of cathode supporting plate 14, as shown by arrows 28. The power output from the converter may be taken from

leads

30, 32.

In order to serve effectively as a reservoir for adsorbed cesium throughout the length of a practical lifetime of the converter, it is desirable that the metal matrix or sponge reservoir 20 have pore walls of large surface area to accommodate a quantity of Iadsorbed cesium atoms that is large in relation to the number of cesium atoms within a normally operating converter i. We have found that a practical converter should have at least ten times as many cesium atoms adsorbed on pore walls of the sponge reservoir, as are present elsewhere in the envelope of the converter during normal operation. However, we prefer a larger ratio than 10 to 1 of the number of cesium atoms adsorbed in the reservoir to those present elsewhere in the converter envelope. Thus, where space accommodations are available, the matrix reservoir may be large enough to hold one thousand times as many cesium atoms as are normal elsewhere within the converter envelope during operation. In this way normal losses of cesium from the envelope 12 during converter operation will be replenished by the reservoir over a period of time that may extend to the limit of and beyond the normal lifetime of a practical converter.

It is feasible to tailor the volume size of the matrix cesium reservoir to the space within a converter. We have found that the size of the metal particles of which the reservoir matrix is composed affects the volume of the matrix. Thus, if the particle size is reduced, the volume of the matrix body 20 is also reduced for a given storage capacity.

A metal sponge or matrix reservoir of the invention can be shown to have the ability to store on the surfaces of its pores X times as many cesium atoms as are on the inner surfaces of and in the space `volume defined by the converter, at a vapor pressure of 1 torr by the following expression:

X(Nvol+Nsurf) 'I 14 d X 5X 10 where 5 1014 is a gas constant, d is diameter of each metal particle of the reservoir, it being assumed that each particle is spherical, and NW1 and Nsurf are the number of cesium atoms in the volume and on inner surfaces of the converter, respectively.

It will be noted from the foregoing that the volume of the sponge 20 is related to the diameter of the metal particles of the sponge, as indicated by the expression (l). Therefore, by selecting a particular particle size for the sponge 20, the volume of the sponge may be tailored to the space requirements of any type of converter.

For example, where the combined areas of 4anode 16 and the metal structure comprising plate 14 and cathode 22 is 100 cm.2 and the spacing between the anode 16 and cathode 28 is 0.025 cm. the converter may accommodate conveniently .a sponge 20 having a volume of 1 cm. This volume of sponge 20 can be shown to have a storage capacity for cesium atoms that is 100 times that of the inner volume and inner surface area of the converter, provided the diameter of the particles making 4 up the sponge is l04 cm., and the converter is operated at a vapor pressure of 1 torr.

The storage of cesium by surface adsorptionon pore walls of the sponge or matrix reservoir 20 results in the feasibility of preserving a desired cesium vapor pressure of, say, 1 torr within the converter, with appreciably less temperature dependence than in the case of the cesium pool type of converter. Curve 34 of FIG. 2 indicates the vapor pressure response to temperature, Tp, of a pool type of cesium reservoir. It will be noted that this curve is rather steep, that is to say, a change in temperature control undesirably causes a relatively large change in pressure. This is because the force with which the cesium atoms are bound to the pool is relatively small, corresponding to only 0.75 electron volt binding energy. However, curve 36, which indicates the vapor pressure response of the cesium when stored on the pore walls of a lmetal sponge type of reservoir 20 is much less steep and thus desirably the cesium vapor pressure varies less for the before mentioned change in temperature control. This is because the force with which the cesium atoms in the reservoir are bound to the pore walls thereof.is substantially higher than in the case of a pool type of reservoir, i.e. corresponding to a binding energy of 2 electron volts.

vIf the cathode temperature is Tk, it will be noted that appreciable departures from this temperaure either to the left (FIG. 2) to a lower temperature T1, or to the right to a higher temperature Th, are permissible when using the sponge reservoir of the invention without appreciably changing the vapor pressure of cesium within the converter from a value such as 1 torr. This permits unavoidable iuctuations in the cathode temperature of the -converter without reducing converter efficiency.

If the temperature Ta of FIG. 2 is the temperature of the anode 16 of FIG. 1, it will be seen that the reservoir can be positioned in heat transfer relation with respect to the anode 16 instead of the cathode 22, for preserving a desired cesium vapor pressure of 1 torr. The pressure versus temperature relation may then behave as shown by curve 38 of FIG. 2. This renders it feasible to employ the sponge cesium reservoir of the invention adjacent to either the cathode or anode. It is also possible to locate the matrix cesium converter adjacent to the converter envelope and in a region spaced from both the anode and cathode.

If two or more separate matrix reservoirs are used, more complicated pressure-temperature relations are obtained than shown in FIG. 2. In fact, a larger degree of freedom in choosing a desired relation is possible.

The binding energy of the cesium atoms to the pore walls of the sponge reservoir 20, as has been indicated before herein, is about 2 electron volts at a temperature of 553 K. However, the binding energy of the cesium atoms varies with cesium coverage. Thus, at higher coverage the binding energy is reduced, while at lower coverage the binding energy is increased. In any case, the pressure is determined by temperature as shown by the following expression:

Vr p=K1 exp.

wherein V is the binding energy of cesium atoms, Tc is the temperature of the element to which the sponge reservoir is attached, p is the vapor pressure, and K is a constant having small variations with temperature.

In order to assure that a predetermined operating temperature does not extend beyond the temperature range at which a desired cesium vapor pressure is to be preserved, it is preferable that such predetermined operating temperature constitute a mean Tk, between the extremes of the range T1, Th, as shown in FIG. 2. In this way, temperature departures above and below the mean or predetermined operating temperature, are tolerable without affecting the cesium vapor pressure.

In makin-g a thermionic energy converter 10 (FIG. 1) having a tungsten sponge reservoir of cesium, a sponge mass 20 having a volume for adsorbing on the pore walls thereof at least ten times as many cesium `atoms as are adsorbed on inner surfaces of metal elements of the converter envelope 12 and present within the space defined by the envelope, is fixed as by sintering to the end wall 14 to form part of the cathode of the device. The cathode should have a highly smooth surface facing anode 16, in view of the relatively small spacing between opposed cathde and anode surfaces. This smooth surface may be provided by a layer 22 of tungsten or other suitable metal having a thickness of about mils, applied over the surface of the sponge facing the anode 16. 'Ihe exposed surface of the thin layer 22 is rendered smooth by suitably polishing the same and it is therefore adapted to constitute the effective cathode surface of the device. If the layer 22 is not used, the surface of the sponge 22 facing the anode 16 should be suitably polished.

The insulating ring 18, having a length to provide a desired spacing between the facing surfaces of the cathode 22 and the anode 16, which may be 0.025 cm. in one type of converter, is suitably metallized at its end surfaces and fixed as by ybrazing to the end walls 14, 16 of the converter.

Thereafter the envelope 12 is exhausted through tubulation 24 to a desired relatively low pressure. The conyverter envelope 12 and

electrodes

16, 22 are then raised to their operating temperatures. The envelope temperature may be 553 K. Cesium is admitted to the evelope 12 through a suitable appendage which is also heated to this temperature. Such appendage is well known and therefore not shown. The appendage contains a pool of cesium. It is important in processing the thermionic energy converter 10 that all of the converter be operated at a temperature above that of the cesium pool. After equilibrium is established, for example, at a cesium vapor pressure of 1 torr within the envelope, the appendage is hermetic-ally closed as by pinching. In such an equilibrium state, cesium 'will form a partial monolayer on the cathode surface 22 and on the pore walls of the tungsten sponge 20. A monolayer of cesium will be formed on the surface of the cooler anode 16. Cesium atoms will also permeate the inner space of the envelope 12. It is important in operating the converter 10 that all of the converter be operated at temperatures above the normal condensation temperature corresponding to the operating pressure. This will assure that no liquid cesium pools are formed anywhere in the converter and that the pressure at all times is controlled by the temperature of the sponge reservoir.

It is thus apparent that at any selected temperature of operation, the sponge cesium reservoir will serve as a continuing source of cesium so that normal losses of cesium within the container will not affect adversely the life of the device. Furthermore, the sponge reservoir described is less dependent on temperature stability for preserving a desired vapor pressure, than liquid pool type reservoirs, and therefore assures a high efficiency of operation in the presence of varying reservoir temperatures within a relatively wide tolerable range.

What is claimed is:

1. Method of operating a thermionic energy converter having an envelope containing cesium and a porous sponge reservoir only for cesium, comprising:

(a) heating said converter to a predetermined temperature above the normal condensation temperature of cesium at a given pressure, and

(b) heating said sponge reservoir to a temperature above the normal condensing temperature of cesium and differing appreciably from said predetermined temperature without substantially disturbing said given pressure.

2. Method of operating a thermionic energy converter containing cesium and only a surface reservoir for cesium, comprising:

(a) heating said converter to a predetermined temperature above the condensation temperature of said cesium at a given pressure,

(b) absorbing on the surfaces of said reservoir at said temperature at least ten times as many cesium atoms as are present in said converter apart from said reservoir, and

(c) heating said reservoir to a temperature higher than said predetermined temperature for releasing said cesium from said reservoir,

(1) said higher temperature having a different pressure-temperature relation than said predetermined temperature and given pressure.

3. Method of operating a thermionic energy converter according to claim 5 and wherein the pressure in said different pressure-temperature relation is substantially the same as said given pressure.

4. Method of operating a thermionic energy converter containing cesium comprising:

(a) heating said cesium to a temperature above the condensation temperature of cesium to produce a predetermined operating vapor pressure of the cesium, and

(b) keeping said vapor pressure substantially constant with variations in said temperature above Said condensation temperature.

5. Method of operating a thermionic energy converter containing cesium and having a cathode including a sponge reservoir for cesium vapor and wherein said reservoir is the only cesium reservoir of said converter, comprising:

(a) heating said converter to a first temperature above the normal condensation temperature of cesium at a predetermined pressure to produce cesium vapor, and

(b) heating said cathode to a temperature above said condensation temperature and different from said first temperature without substantially changing said predetermined pressure,

(c) whereby said cathode may be operated at a relatively wide range of temperatures with freedom from substantial pressure change of said cesium vapor in said converter for increased eiiiciency of said converter.

6. Method of operating a thermionic energy converter containing cesium, comprising:

(a) heating said converter to a first temperature above the normal condensation temperature of cesium to obtain a given vapor pressure, and

(b) heating a cesium vapor reservoir in said converter to a temperature higher than said tirst temperature and releasing cesium vapor into said converter from said reservoir,

(c) said released cesium vapor replacing cesium lost from said converter for preserving said given vapor pressure at said rst temperature.

7. Method of operating a thermionic energy converter containing cesium and a sole reservoir of cesium, comprising:

(a) heating said converter including said reservoir to a ttirst temperature above the normal condensation temperature of cesium at a given pressure,

(b) adsorbing cesium vapor on surfaces of said reservoir with a binding force capable of release only at a temperature substantially higher than said first temperature,

(c) whereby said reservoir may be heated to a temperature substantially different from said temperature while substantially preserving said given pressure.

8. Method of operating a thermionic energy converter having an envelope containing a cathode, cesium vapor, and a sole sponge reservoir for cesium in heat transfer relation to said cathode, said method comprising:

(a) heating said converter including said reservoir to an operating temperature above the normal condensation temperature of cesium to provide a predetermined vapor pressure, and

(b) storing on surfaces only, in said sponge reservoir at least ten times as many cesium atoms as are present in said envelope apart from said sponge, with a binding force substantially greater than that of a pool of cesium, l

(c) whereby said cathode is adapted to tolerate substantial variations from said operating temperature Without appreciably affecting said predetermined vapor pressure, and said reservoir is adapted to replenish cesium lost from said envelope throughout a relatively long operating life of said converter.

9. Method of operating a gas tube having therein a Huid condensable at a predetermined temperature and a sole reservoir for said uid comprising:

(a) heating all portions of said tube including said reservoir to a temperature higher than said predetermined condensing temperature, whereby all of said iluid within said tube and in said reservoir is transformed to the gas state, and

(b) storing on surfaces only within said reservoir at said temperature at least ten times as many atoms of said uid in the gas state as are present elsewhere in said tube.

References Cited MILTON O. HIRSHFIELD, Primary Examiner.

15 D. F. DUGGAN, Assistant Examiner..

U.S. Cl. X.R.