Chemistry: electrical current producing apparatus – product – and – Deferred action type – Responsive to heat
Reexamination Certificate
2002-04-12
2004-11-16
Dove, Tracy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Deferred action type
Responsive to heat
C429S115000, C429S057000, C429S103000, C429S163000
Reexamination Certificate
active
06818344
ABSTRACT:
BACKGROUND OF THE INVENTION
Thermal batteries are designed for immediate and short-duration activation under extreme operating conditions. In an inert state suitable for storage, a thermal battery is dormant, and can remain inactive for long periods of time. Upon initiation, a thermal battery instantly activates to serve as an accurate voltage source that is stable for a predetermined time duration.
Contemporary thermal batteries include an anode and cathode separated by a solid electrolyte. In a solid state, the electrolyte is dormant, and serves as an electrical buffer between the anode and cathode. When converted to a molten state, for example by means of heat produced by an activated pyrotechnic charge, the electrolyte becomes a conductor, serving as a conduit between the anode and cathode. The thermal battery remains active for a predetermined period of time until the charge is exhausted.
Examples of thermal batteries are disclosed in U.S. Pat. Nos. 5,895,730 and 6,198,249, the contents of which are incorporated herein by reference. Such thermal batteries are limited in their operation in that they suffer from relatively low energy density, short-duration activation period, limited shelf life in storage, poor reliability under exposure to extreme acceleration, large size and weight, limited altitude operation range, and narrow temperature operation range.
SUMMARY OF THE INVENTION
The present invention is directed to a thermal battery and process for forming a thermal battery that overcome the limitations of conventional embodiments. In particular, the battery and method of the present invention are well suited for battery applications that require highly integrated thermal batteries that are relatively small in physical size, yet are capable of reliable performance over a wide range of operating conditions.
In one aspect, the thermal battery of the present invention is housed in a chamber that utilizes micro-electromechanical systems (MEMS)-based technology to offer superior chemical stability and advantageous mechanical and thermal properties. In another aspect, the thermal battery of the present invention is activated by heat, for example heat generated by a pyrotechnic charge, for immediate and thorough activation of the electrolyte. In another aspect, the anode, cathode and electrolyte may be formed of pellets having a curved interface for increased current density.
In another aspect of this invention, the pyrotechnic charge consists of a heating pellet including suitable chemical ingredients, which is utilized to provide rapid, controlled, high-temperature heating of the electrolyte to achieve rapid melting. In a preferred embodiment, the heating pellet consists essentially of thermite, a blended mixture of two solid components, iron (III) oxide and aluminum powder, that may be pressed and shaped as described hereinafter. Upon ignition, thermite produces a large quantity of heat (relative to the mass of the components) and two distinct solid-based byproducts (iron and aluminum oxide) with zero moles of gas. By “solid-based” it is meant that the byproduct is a solid at ambient conditions. (The thermite reaction may initially produce molten iron.)
Because all of the thermite reaction byproducts are solid-based, i.e., no gases are evolved, all of the evolved energy (847.6 kJ/mole of energy) is available for heating the solid eutectic carbonate electrolyte of this invention. When gases are evolved as byproducts of a chemical reaction, there is a variability in the reaction kinetics, i.e. turbulence, which creates oscillations in pressure and heat output. This variability, in the case of a thermal battery, leads to uncontrolled or erratic melting of the carbonate electrolyte and to possible inefficiencies and interruptions in the generation of electrical power. Such limitations are avoided in this aspect of the present invention by using a suitable material, e.g. thermite, as the primary heat source.
In another aspect of this invention, the thermite pyrotechnic charge is activated by means of an ignition strip that burns at a high enough temperature to ignite the thermite. In a preferred embodiment, the ignition strip includes a fuse roll or foil strip consisting essentially of about 54 wt. % magnesium powder, about 30 wt. % Teflon™, and about 16 wt. % Viton™ (hereinafter “MTV”). Teflon™ and Viton™ are materials available from E. I. DuPont de Nemours and Company, Wilmington, Del. An MTV ignition strip is preferred to a simple magnesium strip for purposes of this invention because it has been found that the heat output from combustion of the MTV strip is much higher and more controlled. Also, an MTV ignition strip can be easily processed into the sizes and shapes required for use with the thermal batteries of the present invention.
Alternatively, an ignition strip in accordance with the present invention may consist essentially of bisnitro cobalt-3-perchlorate (BNCP), which is synthesized according to known techniques.
In another aspect of this invention, the electrolyte is in the form of a thin, solid tablet or pellet at ambient conditions, and is positioned between the anode and cathode elements of a cell unit. A preferred electrolyte in accordance with the present invention includes a three-component blended eutectic salt mixture selected to have a melting temperature in the range of about 490° C.-520° C. In a particularly preferred embodiment, the electrolyte consists essentially of one of the following two ternary eutectic mixtures of alkali carbonate salts.
A first preferred eutectic carbonate salt mixture consists essentially of lithium carbonate (Li
2
CO
3
), sodium carbonate (Na
2
CO
3
), and potassium carbonate (K
2
CO
3
), hereinafter abbreviated as “(LNk)
2
CO
3
”. In general, this mixture may include about 38-49 wt. % lithium carbonate, 26-37 wt. % sodium carbonate, and 20-30 wt. % potassium carbonate. For example, a preferred mixture of about 43.5 wt. % lithium carbonate, 31.5 wt. % sodium carbonate, and 25 wt. % potassium carbonate has been determined to have a eutectic melting point of 518° C., within the preferred electrolyte melting temperature range.
A second preferred eutectic carbonate salt mixture consists essentially of lithium carbonate (Li
2
CO
3
), sodium carbonate (Na
2
CO
3
), and rubidium carbonate (Rb
2
CO
3
), hereinafter abbreviated as “(LNR)
2
CO
3
”. In general, this mixture may include about 34-44 wt. % lithium carbonate, 33-44 wt. % sodium carbonate, and 17-28 wt. % rubidium carbonate. For example, a preferred mixture of about 39 wt. % lithium carbonate, 38.5 wt. % sodium carbonate, and 22.5 wt. % rubidium carbonate has been determined to have a eutectic melting point of 499° C., also within the preferred electrolyte melting temperature range.
In accordance with the present invention, it has been found that a ternary eutectic salt mix achieves superior performance in thermal battery applications as compared with single or two-component mixtures. In particular, it has been found that the heat capacity of the ternary eutectic mix is much higher than that for a two component (carbonate or non-carbonate based) eutectic molten salt. This ensures that the molten electrolyte salt in the ternary composition remains as a liquid melt for a much longer time, thus leading to the longer operation life and markedly improved electronic transfer.
In another preferred embodiment of this invention, the electrolyte consists essentially of a ternary inorganic alkali carbonate eutectic salt composition blended with a minor proportion, e.g., about 0.005%-10% by weight, most preferably about 1% by weight, of a surfactant to enhance electron mobility during electrolyte activation and to improve wetting of the molten electrolyte to the internal walls of a zeolite molecular sieve as hereinafter described. A particularly preferred surfactant for such purposes is sodium lauryl sulfate.
In still another aspect of this invention, an anode element of a thermal battery according to the present invention includes an alkali/alkaline earth metal alloy shaped as a lozenge
Dove Tracy
Mills & Onello LLP
Textron Systems
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