Page 309 - Battery Reference Book

27/6 High-temperature thermally activated primary batteries

parts and dried. Once dry, they must be handled very Dehydrated asbestos, ceramic fibres, and Kapton (poly-
carefully since they are very susceptible to ignition imide) have been used. Insulative layers are tightly
by static discharge. Heat paper has a burning rate of bound around the periphery of the battery stack, and
about 10-15 cds and a heat content of about 1675 J/g. several asbestos or ceramic fibre discs are placed on
Heat paper combusts to an inorganic ash with high each end to ensure a tight pack in the metal can.
electrical resistivity. Consequently, its use necessitates Special end reservoir pellets are often-life (> 10 min)
addition of nickel or iron electrode collectors and inter- thermal batteries often low thermally conductive prod-
cell connectors to conduct current around each cell’s uct called John-Manville Co.), which is manufactitania
heat paper pad. and silica.
Fine iron powder (1 - 10 pm) and potassium perchlor-
ate are blended dry and pressed to form heat pellets. 27.1.6 Production batteries
The iron content ranges from 80 to 88% by weight
and is considerably in excess of stoichiometry. Excess Thermal batteries produced by various suppliers (Cat-
iron provides the combustible pellet with sufficient alyst Research Corporation, Eagle Picher, Mine Safety
electronic conductivity, eliminating the need for inter- Appliances) have an electrochemical system that con-
cell connectors. The heat content of iron-potassium sists of an active calcium anode, a fused salt electrolyte
perchlorate pellets ranges from 920 J/g for 88% iron to (lithium chloride-potassium chloride eutectic mixture)
1420 J/g for 80% iron. Burning rates of pellets are gen- and a cathode consisting of a relatively inert metal
erally slower than those of heat paper, and the energy current iron or nickel collector containing calcium
required to ignite them is greater. The heat pellet has chromate (or tungstic oxide, ferric oxide, vanadium
higher activation energy and is therefore less suscepti- pentoxide cathode active materials). In typical thermal
ble to inadvertent ignition during battery manufacture. battery designs the nickel or iron cathode cup, pos-
However, the battery must be designed so that there sibly with an added grid, contains a cathode active
is good contact of the heat pellet with the first fire or (on a depolarized) layer, one or two electrolyte lay-
ignition source. ers, a central calcium anode layer followed by further
After combustion, the heat pellet is an electronic electrolyte and depolarizer layers, and a cathode cell
conductor, simplifying intercell connection and battery cover. The edge of the closed cell is crimped over and
design. The heat pellet bums at a lower rate than heat sealed with a gasket. This cell is thus a double cell
paper, and so heat pellet batteries generally start about having a common anode. The cathode active mater-
0.2 s slower than heat paper batteries under load. Upon ial (depolarized) is fabricated as a chemical layer on
combustion, however, the heat pellet ash retains its inorganic fibre paper, and the electrolyte is dip coated
original shape and, since it has a higher enthalpy than on woven glass tape. The open-cell system is a simpler
heat paper ash, it serves as a heat reservoir, retaining construction in which the electrolyte and depolarizer
considerable heat, reaching lower peak temperatures, chemicals are fabricated into a two-layer pellet disc
and releasing its heat to the cell as the electrolyte using an inorganic binder. Simple disc cathodes and
starts to cool. The heat paper combusts to shapeless anodes are used, connected together as a dumb-bell to
refractory oxides causing slumping of the battery stack link adjacent cells in a series configuration.
during ignition and less resistance to environmental Batteries are made up from both the closed and
effects. open systems in any series or parallel configuration
of stacked cells interleaved with the thermite layers
required to activate the battery. Complete batteries are
27.1.4 Methods of activation thus usually cylindrical, in the form of hermetically
Thermal batteries are initiated by either mechanical sealed metal canisters. Terminals providing connec-
action using a percussion-type primer or an electrical tions to cells are located in glass or ceramic insulating
pulse to an integral electric match (squib). For most seals.
military applications, safety considerations require the Before activation, the electrolyte is an inert solid.
squib to be non-ignitable under a load of 1 W, 1 A. In Since the case is hermetically sealed there is no
deterioration of the electrochemical system. Battery
heat pellet batteries, an intermediate heat paper firing
train is often added to carry the ignition from the storage and operation over a wide range of environ-
primer or squib to each pellet. mental conditions can be achieved. Because of heat
losses and ultimate resolidification of the electrolyte,
the active life of thermally activated reserve batteries
27.1.5 Insulation materials is necessarily short, generally less than 5min. Typi-
cal battery designs are shown in Figure 27.8. In this
Thermal batteries are designed to maintain hermeticity type of cell the cathode and electrolyte are combined
throughout service life, even though internal temper- in a single homogeneous ‘DEB’ pellet, consisting of
atures approach 600°C. Thermal insulation used to Depolarizer, Electrolyte and Binder. An electrically
minimize peak surface temperatures must be anhy- conductive heat pellet functions as the cell heat source
drous and, if organic, must have high thermal stability. and intercell connector.