GB2271832A - Thermal control layer - Google Patents

Abstract

Device for controlling the emission of radiated heat, particularly for space vehicles, consisting of cells the degree of heat emission from which can be adjusted independently of one another by electrical activation. The cells consist preferably of a controllable layer (4) and an ion conductive layer (3) disposed between a front electrode (1) and a rear electrode (2) and which reversibly vary their degree of heat emission by the application of an electrical voltage.

Description

Thermal control layer

The invention relates to a device for controlling the emission of radiant heat according to the preamble to Claim 1.

The heat management of a space vehicle is determined essentially by the interaction of three effects: external irradiation, mainly from the sun and the earth, internal heat sources, e.g.

electrical appliances, and the radiation of heat into space.

According to the mission, so these heat currents are subject to more or less intense fluctuations so that special regulating measures are needed for the temperatures of the space vehicle to be maintained within certain acceptable limits. The possibilities for varying the energy absorption and the development of internal heat are very limited, so that regulating the radiant output constitutes the most important and most used measure of controlling temperature. In cases involving solar or albedo irradiation onto the radiant surface, the presence of a low absorption coefficient for visible rays is advantageous or even essential.

At present, mainly moving elements such as shutters and blinds, heat pipes of variable conductivity or compensating heaters are used for this purpose. These solutions are expensive as well as being unsatisfactory with regard to weight, energy and space requirements and are to a certain extent susceptible to breakdown.

Also in other fields of technoiogy, surfaces are required from which the heat emission can be controlled or regulated, e.g.

surfaces of radiant panels, radiators, heat exchangers or heat engines general'y wherever radiated heat should be emitted under control, whether it emanates from heat processes, electrical processes or friction losses.

The invention is based on the problem of providing a device for controlling the emission of radiant heat, which makes it possible electrically to control the degree of heat emission from radiant surfaces without mechanically moving parts, and which is characterised by simple lightweight construction and minimal power requirements. For space travel applications, it is intended at the same time to achieve a low degree of absorption of solar radiation.

According to the invention, this problem is resolved by a device having the features indicated in the Claims.

The basic idea is variation of the degree of heat emission. In particular, this variation can be made by an electrical control system. To this end, the emitting body is provided with a flat arrangement of cells. Each cell consists of a system of thin layers on a suitable backing, a synthetic plastics film or a plate and an electrical connection. By means of electrical voltage signals, the & values of the cells can be adjusted individually so that more or less heat can be given off although the cells are on a uniform or an arbitrary temperature level.

The solution according to the invention is particularly suitable for automatically regulating the heat management of space vehicles.

Quite generally, it is possible to control or regulate the heat given off via radiation of heat wherever it is important quickly to vary the thermal properties. Radiant panels, heat exchangers, heat engines, all machine parts on which friction heat, ohmic or eddy current losses have to be dissipated via radiation, can, so long as they have not yet reached their working temperature, emit less and be switched over to a higher level of emission when tey are (too) hot.

The preferred mechanism of ; -control according to the invention is based on the fact that in the vicinityof the surface of a layer system the high frequency conductivity of a zone is varied by an electrochemical mechanism so occasioning a change in the degree of heat emission. Control of the infra-red optical effect is achieved by a time-related or space-related variation of the density of free electrons, in fact in a layer zone which is responsible for the heat emission from the system.

A distinction is drawn between two forms of # - control: 1. Reversible variation of the active zone in the region of high electrical conductivity between a condition with a predominant absorption character and a condition with a predominant reflection character ("reflector control").

2. Reversible variation of the active zone with a range of moderate conductivity (e.g. ( 10 # -1cm-1) 10#-1cm-1 ) between a condi- tion of predominantly absorption character and predominantly transmission character ("absorber control").

These two principles are shown in Figs. 1 and 2. The basic configuration contains, to provide for electrical activation, an IR-transparent front electrode 1 and a thin metallic rear electrode 2. In between there is an ion conductive layer 3, 5 and a controllable layer 4, 6. The two switch conditions are designated H (high emission, X 1) and N (low emission, ; B 0).

Where the reflector control in Fig. 1 is concerned, the controllable layer 4 in condition H consists of an IR-absorbing or transparent material. Regardless of the cell temperature Tz, 4 irradiation from the surface has a density of L FC. # T (e.g. 9 = 0.8, r = Stefan-Boltzmann constant).

After switching to condition N, the controllable layer 4 becomes highly conductive and thus reflective. The inherent radiation of the cell recedes accordingly (e.g. ç = 0.2).

The absorber control arrangement in Fig. 2 works similarly.

There, the controllable layer 6 is infra-red transparent in condition N. On account of the transparency of the ion conductive layer 5 and the higher reflection at the rear electrode 2, the cell has an -value greater than 0. The switching process renders layer 6 infra-red absorbent (indicated by the dots) and & rises.

To adjust the desired radiant output from the entire arrangement, there are two options. On the one hand, the radiation can be adjusted directly (analogue) by the extent of the selected degree of emission of one or more units of surface area. The control voltage is supplied by a control circuit, e.g. for thermostatic control of a component. The second option resides in subdividing the radiant area into a number of smaller units or cells, switching each cell to two extreme & values and regulating the effective entire radiant output digitally, in other words via the number of cells which are connected in one specific condition.

With regard to the need for a low solar absorption level in the system, the following possibilities are available: The controllable radiation panel is not mounted directly on the outer surface of the space vehicle but behind a thin screen, e.g.

a metal film which is coated white on the outside and is provided on the inside, the face which is towards the panel, with a thermally highly absorbent surface. The control function (g travel) is limited somewhat by this second plane). The low solar absorber effect can also be achieved by a direct coating of the radiant surface with a selective coating which has a high infra-red transmissive capacity and at the same time a high reflection or remission degree in the solar spectral range.

Examples of the selective layer are very thin metal films (50 to 200 ) thick consisting of titanium, vanadium, zirconium, manganese, lead or molybdenum, interference filters of infrared transparent dielectric materials and adaption to high reflection at .0 = 500 nm, as well as multiple layers or pigmented layers consisting of two (or more) transparent components which in the thermal infra-red range have identical but otherwise different refraction indices.

A further description of the invention, particularly the material realisation of the electro-optical switching process, is given with reference to Figs. 1 and 3 to 5.

Fig. 1 shows a cell according to the invention. On a suitable carrier, which is not shown here, for example on a synthetic plastics plate or film, firstly a conductive thin layer of lead is applied which acts as a rear electrode 2. The controllable layer 4 and the ion conductive layer 3 consist in this case of lead fluoride PbF2 totalling a few micrometers thickness, and in this case the layer 3 is of pigments of lead, lead oxide or other substances in order to enhance infra-red absorption. The front electrode 1 consists of an inert thin semi-conductor film of, for example, silicon, indium oxide, tin oxide, lead oxide, lead sulphide, with adequate infra-red transparency. If the front contact is negatively pretensioned, then a metallic layer of lead will be deposited in the layer 4. A corresponding quantity of lead passes in turn to the rear electrode 2 and changes to PbF2.For controlling the t-value, it is sufficent to generate a layer of metal 100 to 300 A thick. The magnitude of the control voltage of typically 1 to 10 volts determines the velocity of the effect. The process takes place in a matter of seconds. It is also possible for the layers 3 and 4 to be of identical construction. At the front, the cell can be protected by an infra-red transparent covering layer or a film of, for example, polyethylene.

Fig. 3 shows an embodiment which is optimum in terms of effect and cycle strength, in which the formation and disintegration of the reflector layer occur homogeneously, in other words regularly over the surface area. This is achieved in that the zone of reaction Pb/PbF2 is spatially bounded. The active zones consist here of two thin layers 7 (PbF2) and 9 Pb) of a few 100 A layer thickness, separated by a selectively ion conductive layer (solid electrolyte) or an ion conductive polymer membrane 9. If an anion conductor such as PbF2 is used for the layers 7 and 8, then the polymer membrane 9 must be permeable to the movable anion, in this case fluroion. For instance, other (lead-free) fluroidic ionic conductors such as KBiF4, BaF2, SrF2, Bal xLaxF2+x and similar compounds are suitable for this.

Switching causes the PbF2 layer 7 to be converted quantitatively to Pb 7' while pcle reversal causes the reaction to take place in the reverse direction. The function of the rear electrode 2 is taken over by a metal layer which is inert to the fluorine ion, such as molybdenum or a semi-conductor layer such as is used at the front contact. What is not shown is that the PbF2/Pb cell can also be used in the absorber mode if the PbF2 layer 7 is at the time of manufacture made substantially thicker than the Pb layer 8. The effect of this asymmetry is that with a negatively poled front contact, the lead in the PbF2 layer is precipitated dispersively in the form of grains or whiskers (condition H), whereas if the front contact is positive, the rear thin PbF2 layer is completely converted to Pb, so producing 2 reflective closed metal layer (condition N).In this case, the layer 9 must be infra-red transparent.

The system PbF2/Pb is here representative of the group of the solid ionic conductors emanating from a combination with a stable metal. Examples are anionic conductors such as halogen compounds of tin, bismuth, manganese, zirconium, and cationic conductors of the AgJ type and the anionic conductive copper halides.

Figs. 4 and 5 show other embodiments of the invention which are based on electrically conductive polymer layers, the conductivity of which can be controlled by electrochemical doping during operation. Examples are polyporrole, polyaniline and polythiophene.

Fig. 4 shows the construction for a reflector control arrangement. Expediently, two identical reactable polymer layers 10 and 11 are used adjacent to the two electrodes. In between there is an ionic conductor 12 capable of transmitting the ion used for doping. This may consist of an aqueous electrolyte, a solid electrolyte or a semi-solid gel-like substance. Good results have been obtained with a combination of polyaniline layers which can be doped with protons, with electrolytes consisting of polymeric sulphonic acids and polypyrrole layers in combination with Nay104 or LiC1-4-electrolytes and doping using Na± or Li±ions.

Under the influence of the electrical field, the doping and thus the conductivity of the two active polymer layers 10 and 11 vary alternately. In condition N in Fig. 4 on the left, the front polymer layer is highly conductive (resembling metal), in condition H it is less conductive or insulating so that there is a more or less pronounced absorbent effect in respect of infra-red radiation.

The reflector mode is suitable only for polymers which can actually assume a metal-like conductivity (e.g. better than 10 cm 1cm1), in order to achieve a sufficiently high reflectivity or (low emission level). Modifications which offer only average conductivity and which in the alternative switched condition move to the infra-red transparent range are better used in the absorber mode.

Fig. 5 shows such an embodiment of a cell which is operated in the absorber mode. Here, the reflector effect in condition N is produced by an additional metal layer 13 behind the front active polymer layer 10. This reflector layer must be microporous in order not to inhibit diffusion of the doping ions, but on the other hand it must have a sufficient layer thickness (preferably less than 10 nm) and a sufficiently closed structure in order to maintain the low-emission effect. Good results can be achieved here with thin layers of rare metal of gold and platinum in layer thicknesses of 100 to 300 A. For systems with proton doping, palladium layers are particularly suitable by virtue of their high hydrogen permeability.

Instead of the above-described rear controllable polymer layer 11, it is also possible to provide another organic or inorganic ion storage layer which as required stores or gives out the ions needed for electrochemical doping of the controllable layers 6 or 10.

The layer systems described can basically be produced by known coating processes. In order to show (sick) test cells according to Figs. 1 to 5, the following processes have so far been successfully used: The inorganic ionic conductors (PbF2, AgJ et al) and metal layers (Pb, Mo, Au, Pt, Pd, Ag) were obtained by high vacuum evaporation. For separation of transparent semi-conductor layers on an indium-tin-oxide (ITO) basis, the cathode atomising (sputtering) process was used as usual. The polymer layers were produced by known chemical and electrochemical processes.

For example, homogeneous layers of polyaniline can be produced by anodic oxidation of aniline from an acid, aqueous solution consisting of H2 SO4N aSO4, HCl-NH4Cl or HC10-NaC104. Typical oxidation potentials are 0.8 to 1.2 volts.

Polypyrrole layers are preferably polymerised from anhydrous electrolytes, e.g. from Et4NBF4 (tetraethyl ammonium fluoroborate) in acetonitrile with pyrrole as a monomer. Electrochemical polymersiation can take place potentiostatically (1.3 to 2 volts), galvanostatically (approx. 1 mA/sq.m) or with a triangular alternating current voltage (0.2 to 2 volts).

Preferred layer thicknesses and materials are the objects of sub-claims.

Two sheets of drawings

Claims (13)

1. Patent claims: 1. Device for controlling the emission of radiated heat, parti cularly for radiators, heat exchangers or space vehicles, characterised by cells, of which the degree of heat emission is adjustable independently of one another, by means of electrical activation.
2. 2. Device according to Claim 1, characterised in that the cells consist of a layer system and in that the high-frequency con ductivity of at least one layer zone can be varied by a reversible electrochemical reaction in the cell.
3. 3. Device according to Claims 1 and 2, characterised by the following construction of the cells: a) a continuous or intermittent infra-red transmissive front electrode (1); b) a rear electrode (2), c) a controllable layer (4) which preferably directly behind the front electrode (1) can in at least one switch condi tion assume a high electrical metal-like conductivity (reflector control) and d) behind it an infra-red absorbing ion-conductive layer (3).
4. 4. Device according to Claims 1 and 2, characterised by the following construction of the cells: a) an infra-red transmissive front electrode (1), b) a rear electrode (2), c) a controllable layer (6) which preferably immediately behind the front electrode (1) is in at least one switch condition infra-red absorbing and in the other switch con dition can assume a high infra-red transmissive capacity (absorber control) and d) behind it an infra-red transmissive ion conductive layer (5).
5. 5. Device according to one of Claims 3 or 4, characterised in that the controllable layer (4) consists of an inorganic ionic conductor which by electrochemical reduction and oxida tion, with the joint action of an inorganic solid electrolyte or an ion-conductive polymer membrane (ion conductive layers 3 and 5) shows separation and disintegration of metallic phases.
6. 6. Device according to Claim 5, characterised in that a second controllable layer (8) of very small layer thickness is dis posed in front of the rear electrode (2) and in that the con trollable layer (7) is on the front substantially thicker in construction than the rear thin controllable layer (8).
7. 7. Device according to one of Claims 3 or 4, characterised in that the controllable layer is an electrically conductive polymer layer (10) of which the conductivity and infra-red absorption level can be sharply varied by electrochemical doping, with the joint action of a solid, semi-solid or liquid electrolyte (12).
8. 8. Device according to Claims 4 and 7, characterised by an additional thin reflective metal layer (13) behind the con trollable polymer layer (10), the metal layer (13) being ion permeable and the ionic conductor layer (12) behind it not necessarily having to be infra-red transmissive.
9. 9. Device according to one of Cairns 7 or 8, characterised by an additional inorganic or organic ionic storage layer (11) in front of the rear electrode (2), the ionic storage layer (11) as required storing or giving out ions necessary for electrochemical doping of the controllable layer (10).
10. 10. Device according to one of the preceding Claims, charateri sed by the following layer thicknesses: - front electrode (1): 0.1 - 0.5 jim - controllable layer (4, 6, 10) 2 - 5 tim - controllable thin layer (8): 10 - 20 nm - ion permeable reflective layer (13); < 10 nm - ion conductive layer (3, 5, 9, 12): 5 - 10 /lem - ion storage layer (11): 2 - 5 jim - rear electrode (2): 0.1 - 1 11m
11. 11.Device according to one of the preceding Claims, characteri sed in that the layers consist of one of the following materials: - front electrode (1): In203: SnO2 (=IT0), SnO2, Si, Mo, ZnSe and other semi-conductors - controllable layer PbF2, AgJ, RbAg4J5, polyaniline (salt (4, 6, 7, 8, 10): of sulphuric acid), polypyrrole, polythiophene and other conductive polymers - ion permeable Pd, Pt, Rh reflective layer (13): - ion conductive layer PbF2, AgJ, RbAg4J5, polymeric sul (3, 5, 9, 12): phonic acids, polymeric carboxylic acids, H2S04 buffered HC1 buffered, H2C104 buffered - ion storage layer (11): polyaniline, Pd, Pt, Rh, ITO - rear electrode (2): Ti, Mo, Pt, Pd, Pb, Ag, ITO.
12. 12. Device according to one of the preceding Claims, characteri sed in that the radiant surface to be controlled is sub divided into smaller units or cells, each cell being switch able into two g-conditions, the magnitude of the totally radiated output being controlled by the number of cells which are in one condition.
13. 13. Device according to one of the preceding Claims, characteri sed in that the radiant surface to be controlled is protected from irradiation in the shortwave (solar) spectral range by the use of a screen or a selectively infra-red transmissive coating.