GAS GENERATOR

This invention relates to gas generators, more especially oxygen generators, and in particular to a generator for providing breathable oxygen.

Oxygen is generated on an industrial scale by numerous different methods. These include isolation of molecular oxygen from air and decomposition of water by, for example, electrolytic means. On an industrial scale, however, these methods require substantial capital investment, and the resulting plant is large, heavy, and immobile. On a commercial scale, for providing breathable oxygen by portable means, oxygen is generally supplied from an oxygen cylinder, i.e., the oxygen is stored in molecular form, or generated on demand by a chemical reaction, a term used herein to exclude electrolytic decomposition and other methods in which generation requires an external source of energy. Breathable oxygen generation by a chemical reaction has been known for a very long time, and portable oxygen candles are articles of commerce. A typical candle comprises an oxygen-containing chemical, for example an alkali metal chlorate or perchlorate, in admixture with a catalyst that facilitates lower temperature decomposition of the chemical to oxygen and residual solids, and optionally a fuel, for example iron. A typical catalyst is manganese dioxide or cobalt oxide, which reduces the temperature at which alkali metal chlorates decompose.

However, the overall reaction is exothermic, and the exterior of the reaction mass generally reaches very high temperatures, of the order of 600-1200°C. Considerable efforts have been made to provide portable devices that can safely be held by the user during operation despite the high temperatures reached during the reaction.

A number of thermally insulated devices are described in US 5 725 834. This patent describes a prior art device in which silica is used as thermal insulation, which has the dual function of maintaining the exterior of the housing at an acceptably low temperature and maintaining the interior of the candle at a high enough temperature to ensure decomposition of all contained materials, to maximize oxygen output. In one embodiment of the Nishii invention itself, a cylindrical candle is surrounded by porous solid insulation within a copper jacket which is separated by an annular air space from the housing. The oxygen generated passes through the insulation to an outlet of the device at the head of the cylinder. In other embodiments there are more coaxial airspaces, and the i

generated oxygen traces a convoluted passage through them, ensuring that it is cooled before reaching the user and that the outer surface of the housing does not get too hot. All these measures, while being effective, inevitably add bulk and weight to the device.

WO 2004/024239 describes a chemical gas generator, for example an oxygen candle, in which insulation is provided using a vacuum jacket. This is an effective means of insulation for certain portable gas generators, but because of the very high temperatures, the technology is difficult to apply successfully to very small oxygen candles, for example those intended to supply oxygen for a short period of, say, up to 15 minutes.

There accordingly remains a need for a gas generator which provides satisfactory performance despite being lighter and/or less bulky than existing apparatus.

Accordingly, the present invention provides a gas generator for producing gas by chemical reaction, the generator being of generally cylindrical shape having first and second ends and comprising (i) a generally cylindrical core having first and second ends, said core being capable on ignition of producing gas by chemical reaction; (ii) ignition means for igniting said core; and (iii) means for collecting gas from said core; and also comprising:

(a) a heat-conducting inner jacket in thermal contact with said core, located circumferentially about said core and extending longitudinally to or beyond each end of said core; (b) respective closure members located beyond each end of said core, each providing a seal to the respective end of the inner jacket;

(c) an insulating layer located circumferentially about said inner jacket, said insulating layer comprising an aerogel;

(d) an outer jacket located circumferentially about and in contact with said insulating layer; and

(e) respective closures at each end of said outer jacket, each defining an insulating space at the respective end of the gas generator.

Advantageously, the gas generated is oxygen, and for simplicity will be referred to as such in much of the discussion below. However, unless the context requires otherwise, the following discussion should be deemed to apply to chemical gas generators in general. Such generators are often referred to, and are referred to herein, as candles.

In general, when constructing an oxygen candle, insulation must be provided between the outer jacket of the candle and the core or any inner jacket. One disadvantage of previous proposals is that, whatever the insulation means that has been proposed (e.g. a

gas, a vacuum, or a solid), it has been necessary to provide support or fixing means between the outer jacket and the core or any inner jacket. These support or fixing means inevitably provide a thermal path to the outer jacket which circumvents the insulation provided, and results in the undesirable transfer of heat. The present invention prevents this problem by using an insulating layer comprising aerogel between an inner jacket and an outer jacket. This layer acts as support for the outer jacket, cushioned separation being provided by the aerogel between the inner and outer jackets. Because of the unique physical properties of aerogels, no separate support or fixing means between the inner jacket and the outer jacket is required, and there is therefore no thermal path radially from the inner jacket to the outer jacket. The provision of an insulating space at each end of the candle results in a coherent design in which all heat flows to the outer jacket of the candle are adequately controlled.

The invention is sufficiently effective to enable the safe operation of oxygen generators that may come into contact with skin. For example, the invention may be used in the construction of small oxygen candles capable of being carried by, for example, paramedics, for use in a medical emergency.

The core of the candle is of generally cylindrical shape having first and second ends. The cylinder may be of any desired cross-section, for example rectangular or square, but is preferably of generally circular cross-section. Located circumferentially about the core and in thermal contact therewith is a heat- conducting inner jacket. This jacket is made of a material of high thermal conductivity, for example a metal, for example aluminium, stainless steel, copper, tungsten or brass. The inner jacket transmits heat from the burn front (i.e. the interface region between burnt and unburnt regions of the core) and the burnt region of the core to the unburnt region or regions, thereby pre-heating the unburnt region and helping to establish an even burn. The jacket may comprise two or more separate layers of different materials, e.g. different metals; in one preferred embodiment, an inner layer of the inner jacket is constructed of stainless steel, while an outer layer of the inner jacket is constructed of aluminium. Where two or more separate layers are present, at least one of these must be gas-impermeable, but one or more others may if desired be perforated or in the form of a mesh or gauze. The inner jacket is advantageously of complementary shape to the core. Its ends extend at least up to the respective ends of the core. In a preferred embodiment, one or both ends of the inner jacket extend beyond the respective end of the core.

A closure member is located beyond each end of the core, each closure member providing a seal to the respective end of the inner jacket. The closure member provides a seal to the respective end of the inner jacket, preferably by being shaped to be a close- fitting plug inserted into an end of the inner jacket extending beyond the respective end of the core. In one preferred embodiment, a closure member is an insulated closure member. This may for example take the form of a bowl-shaped structure containing an insulating material. Any insulating material may be used, but preferably the insulating material is an aerogel as described below. The bowl-shaped structure is preferably made of metal, for example aluminium, copper, brass or, especially, stainless steel. Alternatively, the closure member may comprise a hollow sealed structure in which insulation is provided by air or a vacuum. In such an arrangement, the point at which an insulated closure member makes contact with the inner jacket does potentially provide a thermal path from the inner jacket to the outer jacket. This thermal path is circumvented by the provision of a closure defining an annular insulating space. In a further embodiment, a closure member is simply a solid disc close-fitted, e.g. welded, into an end of the inner jacket.

An insulating layer is located circumferentially about the inner jacket. This layer comprises an aerogel. An aerogel is a solid-state material that has been produced from a gel in which the liquid of the gel has been replaced by a gas. In other words, it has the physical structure of a gel, but contains gas instead of a liquid. Aerogels are of low density, and this confers a benefit to the gas generator by keeping the weight of the device to a minimum. An aerogel typically comprises generally spherical particles of about 2 nm to about 1 mm, for example about 2 to 5 nm in diameter, agglomerated into clusters. These clusters are in turn coalesced together in an extremely porous structure, with the pores normally being less than lOOnm across. Thus, the structure of an aerogel can be thought of as being like a rigid foam, but on a nanoscale. The manufacturing processes for aerogels enable the size and density of these pores to be controlled. Aerogels can be produced from a number of base materials and one favoured material for the purposes of the present invention is silica. Silica aerogels can be manufactured by drying a hydrogel made from colloidal silica. Aerogels have a number of interesting and unusual properties. One of these properties, particularly pertinent to the present invention, is their extremely high effectiveness as thermal insulators. Silica aerogel, which features in a preferred embodiment of the present invention, has an extremely low thermal conductivity, with

values typically in the range of from 0.03 to 0.004 W.mW.K. Silica aerogels typically also have a high melting point.

Aerogels are commercially available in a number of different forms, including particles, beads and blocks, and also including aerogels in composite form with other materials. One particular composite form, which finds application in a preferred embodiment of the present invention, is in the form of a blanket that consists of a mass of aerogel particles carried in a fibrous web. This fibrous web can be made of fibres of various types, including nylon, carbon, or more refractory materials such as glass or mineral, or any combination of such materials. All of these forms are embraced within the scope of the present invention. Preferably, the aerogel used in the present invention is in the form of a blanket comprising a mass of aerogel, especially silica aerogel, particles, carried in a fibrous web.

Further information on aerogels may be found in J. Fricke, A. Emmerling (1992), "Aerogels — Preparation, properties, applications", Structure & Bonding 77: 37-87. The insulating layer may be composed entirely of aerogel, or it may also contain other materials. In one preferred embodiment, two or more sub-layers of aerogel blanket are separated by an intermediate rigid solid layer. This intermediate layer may comprise any suitable material which is tolerant of the temperatures to which it is likely to be subjected when the candle is used. Suitable heat-conducting layers include metal layers, for example a steel or aluminium layer. Suitable less-conducting or insulating layers include rigid plastics materials, for example rigid glass-filled nylon layers; glass-filled rigid polymer structures are generally manufactured by extrusion or by injection moulding of polymer powder in admixture with glass powder. The intermediate layer is generally air- impermeable. Surprisingly, the use of an intermediate layer has been shown to provide superior heat-insulation, even when the intermediate layer is of a material which has less good heat insulation properties than the aerogel.

Preferably, the insulating layer (including all sub-layers and intermediate layers) has a thickness of from 2 to 25, preferably from 2 to 12mm. Of course, the optimal thickness depends upon the form of the aerogel used, and on the precise structure of candle. It has been found that from two to four sub-layers of such blanket, each sub-layer being around

3mm thick, and each preferably separated by an intermediate layer of a different temperature-tolerant material, provides very satisfactory support and insulation for the outer jacket of the generator of the present invention.

Located circumferentially about, and in contact with, the insulating layer, is an outer jacket. This jacket is supported by and spaced away from the inner jacket by the insulating layer. No additional support means or fixing means are required between the inner and outer jackets; thus, the only direct thermal path between the inner and outer jackets is through the insulating layer. It is a key feature of the present invention that an aerogel can provide not only the necessary thermal insulation, but also sufficient physical support to dispense with additional support means or fixing means between the inner and outer jackets of the generator according to the invention.

The external jacket may be made of any suitable material, for example a metal, a plastics material, or a robust glass. If desired, the external surface of the external jacket may be textured, as this can improve the comfort of a person handling the candle in use.

The candle is provided with closures at each end of the outer jacket. Each closure may be a feature integral with the outer jacket which enables closure of the candle; alternatively, a separate closure, for example an end cap, may be provided to effect closure. Each closure is structured to define an insulating space at the respective end of the candle, preferably an annular space. This space may contain air, or an insulating material, or it may be a vacuum. If the respective closure member is itself insulating, preferably this space contains air. If the respective closure member is not itself insulating, preferably at least some of this space contains an insulating material, preferably aerogel as described above. In any event, the structure is such that no thermal path is available between the core and the exterior of the candle. When the closures are in the form of end caps, they may be made of any suitable material, but are preferably made of plastics material.

In one preferred embodiment of the invention, the ignition means for the candle is provided at one end of the candle, and the closure either includes an aperture through which the ignition means projects in a snug fit thus giving an air-tight seal, or carries the ignition means. In this way, an annular insulating space is created. In a further preferred embodiment of the invention, the gas collection means for the candle is provided at the opposite end of the candle from the ignition means, and the closure at this end of the candle includes an aperture through which the gas collection means projects. Preferably, this closure is an end cap which includes an external wall securely fixed to the respective end of the outer jacket, and an internal wall defining, together with the external wall, an annular insulating space. The gas collection means projects through the cylindrical space enclosed by the internal wall of the closure. Alternatively, the gas collection means may simply project through an aperture in the closure giving an air-tight seal.

The candle is provided with ignition means. In use, a burn front travels through the core from the point of ignition towards the unburnt region of the core. The core may be a cast body or may contain a loose-filled or compacted powder or pelletized or granular material. For oxygen generation, the core advantageously comprises a metal chlorate or perchlorate, preferably an alkali metal, especially sodium or lithium, chlorate or perchlorate, in admixture with a catalyst, for example manganese dioxide or cobalt oxide, and a fuel, for example a metal, especially iron or magnesium.

If desired, the core may comprise at least one ignition element positioned and arranged for ignition by the igniting means, and a further element positioned and arranged for ignition by the first element. There may be a succession of such elements throughout the generator, each arranged for ignition by the immediately preceding element.

Advantageously, the composition of the element of the core in which, in use, ignition takes place, is extremely reactive. It may contain, for example, in addition to the main candle material, for example, the chlorate or perchlorate and fuel, a strong oxidizing agent, for example, manganese dioxide, cobalt oxide or, especially, a permanganate, e.g., potassium permanganate. Advantageous ranges of proportions in the ignition region of the candle may be, for example, sodium chlorate up to 95%, and fuel, e.g., iron, up to 50%. A catalyst, such as cobalt oxide up to 10%, is also advantageous. These percentages are by weight of the total composition. Elements of the core adjacent to and closer to the ignition means are advantageously more reactive, i.e., contain a greater proportion of fuel, e.g., iron, than elements more remote from it, to improve stability of the candle and render it less susceptible to accidental ignition. The most remote element may, if desired, contain no fuel, it having been found that this provides a lower maximum external temperature after reaction is complete. Step- wise gradation of the elements facilitates manufacture, as each element has a uniform composition, and the candle is then assembled from discrete components. The interfaces between elements may be of conical or frusto-conical form, assisting in control of burn front propagation between adjacent elements.

Preferably the ignition means is located at one end of the core, and preferably one end of the inner j acket extends beyond the respective end of the core in order to accommodate part or all of the ignition means. In one preferred embodiment, the ignition means passes into the body of the candle via an aperture in the respective closure member, the ignition means fitting sufficiently closely within said aperture to prevent any leakage of heat or gas.

The ignition means may be, for example, electrically powered or mechanically driven, or operated by percussion cap or chemical reaction. Such means include an electrically resistive heating device, a percussion cap detonator or a cartridge ignition device, or an exothermic chemical reaction, initiated, for example, by causing or allowing contact between previously separated reagents. In one preferred embodiment, the ignition device is a friction-generated spark ignition means; in another preferred embodiment, the ignition device uses a phosphorus-coated impact or friction device.

Electrically resistive heating devices have the advantage of being intrinsically safe until energized. When the ignition means comprises a resistive heating device, the generator advantageously further comprises electronic control means for controlling electrical power applied to the heating device to initiate combustion. Use of electronic control means enables an improved degree of ignition control to be achieved. The control means preferably includes coupling means for inductively coupling electrical power from the control means to the heating device; such inductive coupling is of benefit in that exposed electrical contacts may be avoided in the generator, such contacts being susceptible to oxidation and being potentially unreliable.

To conserve battery power and yet ensure reliable ignition of the generating means, the control means may include timing means for automatically controlling the period during which electrical power is applied to the heating device for initiating combustion. When electrical or electronic ignition is employed, the control means advantageously includes at least one, preferably more than one, battery providing power for the ignition means to initiate oxygen generation, the control means further comprising battery monitoring means for monitoring remaining power deliverable from the at least one battery. The monitoring means indicates to a user the status of the battery needs. The monitoring means advantageously includes a light emitting diode indicator and/or a liquid crystal display indicator for indicating remaining power deliverable from the battery or batteries. Light emitting diode indicators are more visible in subdued lighting, for example at dusk or night-time, whereas liquid crystal display indicators are more visible in strong illumination, for example in bright sunlight. More details of a gas generator having these features are given in International

Patent Application No. PCT/GB02/02603.

Other electrically powered ignition means include means for electrically generating a spark, e.g., by piezo-electric generation.

In a preferred embodiment, however, the ignition means is mechanically operable and more especially comprises a means for generating a spark as, for example, a friction- generated spark. For friction-induced generation of sparks there may be mentioned, for example, contact of a friction wheel or other friction member with a flint, e.g. a rare-earth containing flint, for example a cerium-iron alloy.

Advantageously, there are provided when spark-induced generation is used means for abrading the part of the surface of the element of the generating device to be ignited, preferably immediately before ignition. Such abrasion ensures that any surface changes that might take place during storage do not result in inhibition of ignition and, more importantly, that loosened material, for example in the form of small particles, e.g. dust, is available for ignition by the spark. The abrasion means may be, for example, in the case where a friction member is used to generate the spark, the friction member itself or, advantageously, an associated member that is actuated when the friction member is actuated. Advantageously, in its rest position the means for abrading the element surface to be ignited is not in contact with the surface and is moved into contact only when the friction member is actuated. When the ignition region is firmly compacted, as in preferred embodiments of the invention, it tends to be resistant to ignition by sparking. This, especially when in combination with the abrasion means being, when in the storage position, out of contact with the candle, makes for additional safety in storage and transport. Advantageously, the means for actuating the ignition means comprises a rotatable shaft on which the friction member is carried, and, if present, its associated abrasion member is also mounted. The shaft also carries, advantageously remote from the ignition means, means for rotating the shaft, for example a gear means driven directly or indirectly by handle means. Operation of the handle means may actuate the ignition means directly but advantageously, to ensure uniformity of actuation. Actuation may also be indirect, for example by the handle deforming spring means, disengagement of the handle causing or allowing the spring means to actuate the ignition means. Actuation may also take place, for example, by gear means associated with the spring means rotating the shaft by way of the gear means carried on the shaft. In a novel and preferred embodiment of the invention, there is provided a gas generator for producing gas by chemical reaction, comprising a core capable on ignition of producing gas by chemical reaction, and ignition means adapted to ignite said core using a friction-generated spark, said ignition means comprising a rotatable shaft, a priming spring, a priming wheel, a flint wheel and a flint arranged such that, in use, a first rotation of said

shaft in a first direction produces tension in said priming spring and also causes said priming wheel to scrape against a surface of said core, and subsequent release of tension in said spring causes a second rotation of said shaft in the opposite direction, said second rotation causing said flint wheel to strike said flint, generating a spark for ignition of said core; characterised in that the ignition means is also provided with an actuation gear mounted on an axis arranged at an angle of from 20 to 70° relative to said rotatable shaft, said activating gear being constructed and arranged such that in use its operation causes the first and second rotations of said rotatable shaft.

In an alternative preferred embodiment of the invention, the activation means generates a spark by chemical means using a chemically-coated impact or friction device. Here, a spring-loaded rotatable shaft is provided with a head coated with a layer of friction- ignitable material, for example phosphorus, or a potassium permanganate/iron powder mix, or other suitable powdered oxidiser/fuel mix, and under storage conditions is held in a position where the coated head is a distance away from the surface of the core of a gas generator. The spring loading works such that in use the shaft is urged forward and also rotates such that the coated head comes into frictional contact with said surface, generating a spark which ignites the core. This may be achieved by embedding the ignition means in the respective end cap, and providing means permitting said end cap to be rotated and urged forward by the user. The generator according to the invention also comprises gas collection means. This may for example be via a gas spigot and a valve to an exit tube. An oxygen candle for personal use may be provided with a face mask at the end of the oxygen collection means remote from the candle. The candle may if desired be provided with means to blend the oxygen produced with atmospheric air, to deliver oxygen-enriched air, rather than pure oxygen, to a user. The generator may also be provided with one or more filters or absorbents to remove any traces of undesirable substances such as particulates, impurities, and unwanted or toxic gases, for example carbon monoxide, carbon dioxide and chlorine. In a preferred embodiment, one or more filters and/or absorbents are provided adjacent to the end of the core from which the generated oxygen will be removed. In a preferred embodiment of the invention, one end of the inner jacket extends beyond the respective end of the core to accommodate at least part of such filters and/or absorbents.

The invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal cross-sectional view of an oxygen generator according to the invention;

Figure 2 is an axial cross section across line x-x of Figure 1;

Figures 3, 4 and 5 are perspective views of one embodiment of an ignition mechanism shown schematically as 40 of Figure 1 ;

Figure 6 is a longitudinal cross-sectional view of a further embodiment of an oxygen generator according to the invention.

Figures 1 and 2 illustrate an oxygen generator generally indicated at 1. A core 2 comprising materials capable, on ignition, of generating oxygen, is constructed in the shape of an elongate cylinder having a circular cross-section and having ends 2' and 2". A fibre mat 3 surrounds core 2 to ensure an air-tight seal with inner jacket 4, which comprises a first layer 5 of stainless steel and a second layer 6 of aluminium. In an alternative embodiment (not shown) the second layer 6 of aluminium may be omitted. The ends 4' and 4" of jacket 4 extend in an axial direction beyond the ends T and 2" of core 2. Inner jacket 4 is surrounded by an insulating layer 7 of a fibrous blanket saturated with silica aerogel particles, which is in turn surrounded by outer jacket 8.

Core 2 is provided at its end 2' with an ignition zone 11 which comprises materials which are particularly easy to ignite when subject to the action of an ignition mechanism, and at its other end 2" with a series of filters and absorbents shown generally at 12. In a preferred embodiment, not shown, the materials comprising the bulk of core 2 are arranged into zones such that the ease of thermal degradation of the core is greater at end 2' than it is at end 2".

Abutting end 2' of core 2 and cooperating with inner jacket 4 is a steel cap 13 having an aperture 14. Further remote from end 2' of core 2 there is provided a first closure member 15 comprising a stainless steel bowl 16 filled with insulating aerogel blanket 17. Closure member 15 acts as a plug for end 4' of inner jacket 4 to produce a tight seal. An aperture 18 is provided in closure member 15 through which an ignition mechanism shown generally at 40 projects forming an air-tight seal.

Abutting the series of filters and absorbents 12 located at end 2" of core 2, there is provided a second closure member 21 comprising a stainless steel bowl 22 filled with insulating aerogel 23. Closure member 21 cooperates with end 4" of jacket 4 to produce a tight seal. An aperture 24 having a raised rim 25 is provided in closure member 21. Oxygen collection means shown generally at 60 is seated around rim 25 providing an airtight seal.

Generator 1 is provided with a closure in the form of a plastic end cap 31 which is securely fixed to end 8' of outer jacket 8. A cylindrical wall 32 supports a concave end portion 33 having an aperture 34 through which an ignition mechanism 40 projects forming an air-tight seal. The shape and configuration of end cap 31 is such that an air-tight annular space 35 is provided between end portion 33 of end cap 31 and the cooperating end sections of jacket 4 and sealing cap 15. In the embodiment shown, annular space 35 contains air.

Ignition mechanism 40 comprises a base portion 41 of generally complementary shape to end portion 33 of end cap 31, a stem portion 42 which passes through apertures 34 and 18 giving an air-tight seal, and an ignition portion 43 which, in use, is adapted to ignite ignition zone 11 of core 2 via aperture 14.

Generator 1 is also provided with a closure in the form of a plastic end cap 51 which is securely fixed to end 8" of external jacket 8. A cylindrical wall 52 supports a convex end portion 53 having an aperture 54 through which oxygen collection means 60 projects. Support 55 spaces end portion 53 from sealing cap 21 to define an air-tight annular space 56 between end portion 53 of end cap 51 and the cooperating end section 4' of inner jacket 4 and closure member 21. In the embodiment shown, annular space 56 contains air.

In use, ignition mechanism 40 is operated by means not shown in order to ignite ignition zone 11 of core 2 which in turn ignites further materials present in core 2, generating oxygen gas. The gas is forced under its own pressure through the series of filters and absorbents 12 and exits the generator 1 via oxygen collection means 60. Heat generated by combustion in the core 2, initially near end 2' and progressively towards end 2", is spread along the length of the core by inner jacket 4. Insulating layer 7 keeps the external surface of outer jacket 8 sufficiently cool to be handled and because the aerogel of layer 7 provides physical support as well as thermal insulation, no fixing points which could act as thermal leakage points are required between jackets 4 and 8. Insulated closure members 15 and 21 prevent excessive heat transmission to the user from the core 2 except for the points at which steel bowls 16 and 22 respectively seal ends 4' and 4" of jacket 4. These points are located in annular air-tight spaces 35 and 56, and these spaces prevent excessive heat transmission to the user from these points. When the oxygen generator of Figure 1 is constructed as described such that generator 1 is approximately 220mm in length and approximately 75mm in diameter, the core composition includes sodium chlorate and iron powder, and layer 7 consists of 3 layers of fibre-reinforced aerogel blanket each 2mm thick, preferably interspersed with a rigid solid layer, satisfactory heat insulation is provided even though the temperature reached in

the core 2 is estimated to reach about 1200 ⁰ C and the temperature of the stainless steel cap 13 is estimated to reach about 800 ⁰ C.

Referring now to Figures 3, 4 and 5, an ignition mechanism 40 comprises an actuation gear 60 mounted on a base plate 61 via an axle 62. Actuation gear 60 co-operates with a priming gear 63 such that in use, rotation of the actuation gear 60 initiates the rotation of the priming gear 63, through engagement of the gear teeth. Priming gear 63 is mounted on a priming shaft 64 arranged at an angle of approximately 60° to axle 62, and this rotation of the priming gear 63 in turn produces rotation of a priming wheel 65 and a flint wheel 66 that are also mounted on the priming shaft 64. The purpose of the priming wheel 65 is to scrape the surface of the ignition zone 11 of Figure 1 to create an easily ignitable local dust cloud of particles. As the priming shaft 64 rotates, a priming spring 61 is wound up to create a tension. The actuation gear 62 is designed with teeth missing at two diametrically opposite points, with the result that continued rotation ultimately leads to release of the priming gear 63, priming shaft 64 and priming spring 67. This in turn causes the priming shaft 64 to rotate very rapidly in the opposite direction, resulting in a frictional strike between the flint wheel 66 and flint 68 located in flint holder 69, thus generating the required spark.

Referring now to Figure 6, an alternative embodiment of an oxygen generator is shown. An oxygen generator is generally indicated at 1. A core 2 comprising materials capable, on ignition, of generating oxygen, is constructed in the shape of an elongate cylinder having a circular cross-section and having ends 2' and 2". A fibre mat, consisting of an inner layer 3' of kaowool and an outer layer 3" of glass fibre surrounds core 2 to ensure a close fit with inner stainless steel jacket 4. The ends 4' and 4" of jacket 4 extend in an axial direction beyond the ends 2' and 2" of core 2. Inner jacket 4 is surrounded by an insulating layer 81 of a fibrous blanket saturated with silica aerogel particles, which is in turn surrounded by a second stainless steel jacket 82 and a second insulating layer 83 of a fibrous blanket saturated with silica aerogel particles. Layers 81, 82 and 83 together form an insulating layer. Finally, layer 83 is surrounded by outer jacket 8. Core 2 is provided at its end 2' with an ignition zone 11 which comprises materials which are particularly easy to ignite when subject to the action of an ignition mechanism, and at its other end 2" with a series of filters and absorbents shown generally at 12.

Spaced slightly from end 2' of core 2 and securely welded to inner jacket 4 is a steel closure member 84 which acts as a plug for end 4' of inner jacket 4 to produce a tight seal.

An aperture 18 is provided in closure member 84 through which an ignition mechanism shown generally at 70 projects.

Abutting the series of filters and absorbents 12 located at end 2" of core 2, there is provided a second closure member 21 comprising a stainless steel bowl 22. Closure member 21 cooperates with end 4" of jacket 4 to produce a tight seal. An aperture 24 having a raised rim 25 is provided in closure member 21. Oxygen collection means shown generally at 60 is seated around rim 25 giving an air-tight seal.

Generator 1 is provided with a closure in the form of a plastic end cap 85 which is fixed to end 8' of outer jacket 8. Means (not shown) are provided to permit end cap 85 to be rotated relative to jacket 8, and to be pushed inwards towards core 2. A cylindrical wall 86 supports an end portion 87 in which an ignition mechanism 70 is firmly embedded. The shape and configuration of end cap 85 is such that a space 88 is provided between end portion 87 of end cap 85, jacket 4 and closure member 84. Space 88 contains two layers 89 and 90 of a fibrous blanket saturated with silica aerogel particles. Generator 1 is also provided with a closure in the form of a plastic end cap 91 which is securely fixed to end 8" of external jacket 8. A cylindrical wall 92 supports an end portion 93 having an aperture 54 through which oxygen collection means 60 projects. End portion 93 is spaced from sealing cap 21 to define an annular space 94 between end portion 93 of end cap 91 and the cooperating end section 4' of inner jacket 4 and closure member 21. In the embodiment shown, a layer of a fibrous blanket saturated with silica aerogel particles is provided in part of annular space 94, while the remaining part 96 contains air.

Ignition mechanism 70 comprises a striking head 71 carried on a shaft 72 surrounded by a spring 73 retained by a nut 74 and washer 75, normally acting under compression and hence retaining striking head 71 at a distance from ignition zone 11. The surface of striking head 71 is coated with a layer of phosphorus or a potassium permanganate/iron powder mix, or other suitable powdered oxidiser/fuel mix. In use, actuation is carried out by the user grasping end cap 85 and pushing and rotating it. This causes rotation of shaft 72 and hence striking head 71. Spring 73 is further compressed, and striking head 71 moves towards and comes into contact with ignition zone 11. The coated surface of striking head 71 is subjected to frictional force against ignition zone 11, and ignition occurs.