Low work function materials for microminiature energy...

Details Low work function materials for microminiature energy... Low work function materials for microminiature energy...

1999-02-25

2003-05-13

Stafira, Michael P. (Department: 2877)

Electric lamp and discharge devices

Electrode and shield structures

Cathodes containing and/or coated with electron emissive...

C313S310000, C313S491000, C313S633000, C313S630000, C313S311000, C313S355000

Reexamination Certificate

active

06563256

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to a class of materials useful in microminiature thermionic converter (MTC) applications and methods for manufacturing those materials. More specifically, this disclosure describes heterogeneous mixed phase materials having tailored electron emission properties, and methods of manufacture and deposition of those materials in a manner consistent with fabricating thin film emitters and collectors for use in MTCs. The materials exhibit low work function, emit electrons at temperatures that are low when compared with those associated with electron emission in traditional thermionic converter applications, do not require activation to temperatures beyond operational levels of microminiature devices for energy conversion and recovery, and exhibit durability and chemical stability when exposed to the atmosphere. Additionally, the materials are suited to manufacture using integrated circuit and micro-electromechanical systems fabrication techniques, and they can be used in constructing electrode structures and coatings in a single process deposition of a multi-component film that contains necessary constituents.
2. Description of the Related Art
Thermionic conversion has been studied since the late nineteenth century, but practical devices were not demonstrated until the mid-twentieth century. Thomas Edison first studied thermionic emission in 1883 but Schicter did not propose its use for conversion of heat to electricity until 1915. Although analytical work on thermionic converters continued during the 1920's, experimental converters were not reported until 1941. The Russians, Gurtovy and Kovalenko, published data that demonstrated the use of a cesium vapor diode to convert heat into electrical energy. Practical thermionic conversion was demonstrated in 1957 by Herqvist in which efficiencies of 5-10% were reached with power densities of 3-10 W/cm
2
.
Thermionic emission depends on emission of electrons from a hot surface. Valence electrons at room temperature within a metal are free to move within the atomic lattice, but very few can escape from the metal surface. The electrons are prevented from escaping by the electrostatic image force between the electron and the metal surface. The heat from the emitting surface gives the electrons sufficient energy to overcome the electrostatic image force. The energy required to leave the metal surface is referred to as the material work function, ø. The rate at which electrons leave the metal surface is given by the Richardson-Dushman equation:
J=AT
2
exp(−
eø/kT
),
where A is a universal constant, T is the emitter temperature, k is the Boltzmann constant, and ø is the emitter work function. Large emission current densities are achieved by choosing an emitter with low work function and operating that emitter at as high a temperature as possible, with the following limitations. Very high temperature operation may cause any material to evaporate rapidly and limit emitter lifetime. Low work function materials can have relatively high evaporation rates and must be operated at lower temperatures. Materials with low evaporation rates usually have high work functions.
Choosing the correct electrode material is a key component of designing functional thermionic converters. The materials here disclosed have potential application in a wide range of thermionic conversion technologies, however they are especially suited to MTCs. An example of a class of MTCs wherein the materials of this invention find application is disclosed in a separate patent application Ser. No. 09/257,335, now U.S. Pat. No. 6,294,858 filed on the same day as the present application. That separate patent application is incorporated by reference herein in its entirety.
In all thermionic converters, once the electrons are successfully emitted, their continued travel to the collector must be ensured. Electrons that are emitted from the emitter produce a space charge in the interelectrode gap (IEG). For large currents, the buildup of charge will act to repel further emission of electrons and limit the efficiency of the converter. Two options have been considered to limit space charge effects in the IEG: thermionic converters with small interelectrode gap spacing (the close-spaced vacuum converter) and thermionic converters filled with ionized gas.
Thermionic converters with gas in the IEG are designed to operate with ionized species of the gas. Cesium vapor is the gas most commonly used. Cesium has a dual role in thermionic converters: 1) space charge neutralization and 2) electrode work function modification. In the latter case, cesium atoms adsorb onto the emitter and collector surfaces. The adsorption of the atoms onto the electrode surfaces results in a decrease of the emitter and collector work functions, allowing greater electron emission from the hot emitter. Space charge neutralization occurs via two mechanisms: 1) surface ionization and 2) volumetric ionization. Surface ionization occurs when a cesium atom comes into contact with the emitter. Volumetric ionization occurs when an emitted electron inelastically collides with a Cs atom in the IEG. The work function and space charge reduction increase the converter power output. However, at the cesium pressures necessary to substantially affect the electrode work functions, an excessive amount of collisions (more than that needed for ionizations) occurs between the emitted electrons and cesium atoms, resulting in a loss of conversion efficiency. Therefore, the cesium vapor pressure must be controlled so that the work function reduction and space charge reduction effects outweigh the electron-cesium collision effect. An example of an operational thermionic converter is that found on the Russian TOPAZ-II space reactor. These converters operate at the emitter temperatures of 1700 K and collector temperatures of 600 K with cesium pressure in the IEG of just under one torr. Typical current densities achieved are <4 amps/cm
2
at output voltages of approximately 0.5 V. These converters operate at an efficiency of approximately 6%. The control of cesium pressure in the IEG is critical to operating these thermionic converters at their optimum efficiency.
A variety of thermionic converters are disclosed in the literature, including close-spaced converters. (See: Y. V. Nikolaev, et al., “Close-Spaced Thermionic Converters for Power Systems”, Proceedings Thermionic Energy Conversion Specialists Conference (1993); G. O. Fitzpatrick, et al., “Demonstration of Close-Spaced Thermionic Converters”, 28
th
Intersociety Energy Conversion Engineering Conference (1993); Kucherov, R. Ya., et al., “Closed Space Thermionic Converter with Isothermic Electrodes”, 29
th
Intersociety Energy Conversion Engineering Conference (1994); and G. O. Fitzpatrick, et al., “Close-Spaced Thermionic Converters with Active Spacing Control and Heat-Pipe Isothermal Emitters”, 31
st
Intersociety Energy Conversion Engineering Conference (1996).) Previously demonstrated thermionic converters, however, have not been able to achieve the current densities and conversion efficiencies predicted for the present invention. Others' efforts in the field of close-space converters demonstrate that expense and difficulty arise as a result of separately manufacturing and assembling at close tolerances the converter components such as the emitter, collector and spacers. Additionally, the assembly process results in relatively large converters with spacing between the emitter and collector of up to several millimeters. A large gap spacing between the emitter and collector causes the energy conversion efficiency to drop dramatically, often necessitating Cs vapor systems even in converters otherwise designed to be “close-spaced.” Such vapor systems are usually large and cumbersome, and precise control of Cs vapor pressures needed to maximize conversion efficiency (ensuring that space-charge reduction effects outweigh electron-Cs collision effect) is difficult.
Miniature thermioni

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