Patent Publication Number: US-6713774-B2

Title: Structure and method for controlling the thermal emissivity of a radiating object

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract DE-AC0676RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a structure and method for changing or controlling the thermal emissivity of the surface of a radiating object in situ, and thus, for changing or controlling the radiative heat transfer between the object and its environment in situ. More particularly, changing or controlling the degree of blackbody behavior of the object is accomplished by changing or controlling certain physical characteristics of a structure defining a plurality of cavities on the surface of the object. As described herein, this cavity structure may be integral to the radiating object or added to the surface of the object to form a new radiating surface. 
     BACKGROUND OF THE INVENTION 
     Heat transfer between an object and its environment is achieved by up to three main processes: conduction, convection, and radiation. While conduction occurs at solid/solid and solid/fluid interfaces, the principal means of transferring heat into or out of many systems is by a combination of convective media and radiation. Terrestrial system designs typically exploit both convective and radiative heat transfer, however, heat management in many space (i.e., extraterrestrial) systems relies essentially on radiation because of the lack of a convective medium. 
     Convective heat transfer is provided by the natural or forced flow of a fluid over the surface of an object and can be controlled by changing parameters such as the fluid medium and/or its physical properties, flow rate, and surface roughness. In contrast, radiative heat transfer depends on the degree of blackbody behavior exhibited by the surface and the fourth power of surface temperature. Thermal energy radiated by a surface is expressed by the Stefan-Boltzmann equation: 
     
       
           Q   rad   =A σε( T   b   4   −T   a   4 )  (1) 
       
     
     where 
     Q rad =thermal power radiated (W) 
     A=area of radiating surface (m 2 ) 
     σ=the Stefan-Boltzmann Constant (5.67×10 −8 W/m 2 /K 4 ) 
     ε=thermal emissivity factor of radiating surface 
     T b =temperature of the radiating surface (K) 
     T a =ambient temperature (K) 
     The thermal emissivity factor (ε) is the ratio of an object&#39;s radiative emission efficiency to that of a perfect radiator, also called a blackbody. The thermal emissivity factor of most materials ranges between 0.05 and 0.95 and is relatively constant over a significant temperature range. Therefore, the radiative heat transfer capability of an object is typically a predetermined, monotonic function of its temperature raised to the fourth power. 
     The following example illustrates the expected impact of changing the thermal emissivity, or degree of blackbody behavior, of an object that is transferring heat by free convection and radiation. In this example, the reference object is a horizontal cylinder 1 m long with a 10 cm outer diameter, rejecting heat to a 300K environment through free convection and radiation. A simplified equation for the laminar flow convective heat transfer coefficient, h, for the object is: 
     
       
           h =1.32(Δ T/D   c ) 0.25   (2) 
       
     
     (Holman, J. P., Heat Transfer, Sixth Edition, McGraw-Hill) where 
     ΔT=temperature difference between surface and ambient (K) 
     D c =diameter of cylinder (m) 
     Heat transferred by convection (Q conv ) is expressed by: 
     
       
           Q   conv   =hA ( T   b   −T   a )  (3) 
       
     
     where A, T b , and T a  are the same variables as in Equation 1. 
     FIG. 1 shows the amount of heat rejected from the reference object by convection and radiation using Equations 1 and 3, respectively, over a ΔT range of 1-1000 K, which covers a principal range of engineering interest. This figure shows the convection term (Q conv ) to be approximately an order of magnitude larger than radiation (Q rad ) from a surface with ε=0.1 for ΔT up to about 100 K. Beyond this temperature, the T 4  dependence of radiation increases more rapidly, making the two modes of heat transfer approximately equal when ΔT approaches 1000 K. In contrast, radiation from a surface exhibiting ideal blackbody behavior (i.e., ε=1.0) is always greater than convection and is at least an order of magnitude larger when ΔT is near or above 1000 K. More importantly, FIG. 1 illustrates the potential impact on the heat transfer capability of the reference object as the thermal emissivity of its surface changes, by changing the thermal emissivity factor from ε=1.0 to ε=0.1, and vice versa. 
     Thus, the ability to change or control the degree of blackbody behavior of a radiating object, while it is in service (i.e., in situ), analogous to changing or controlling the convective term in a fluid system during operation by altering the flow rate of the fluid, would enable a remarkable improvement in the thermal design and control of many systems where radiative heat transfer is important. For example, the surface of an object or system with controllable thermal emissivity could be activated at some limiting temperature as a thermal safety valve. In this mode of operation, the surface would be triggered to switch to a higher thermal emissivity that, in turn, radiates more heat to prevent the temperature of the object or system increasing above safe limits. Similarly, switching thermal emissivity to a lower value could protect against a system operating at less than a desirable temperature limit. 
     In addition, changing the thermal emissivity of an object will effectively change its thermal, or infrared (IR), signature. This is especially important in detection, recognition, and camouflage applications. For example, the ability to change or control the thermal emissivity of an object provides an opportunity for an object to match its thermal emission characteristics with those of other objects or structures in its vicinity, thereby enabling an IR camouflage effect. 
     In current systems where radiative heat transfer is important, the surface material and/or surface preparation of a radiating object is carefully selected to obtain the desired fixed thermal emissivity and resulting radiative heat transfer characteristic. Typical surface preparations include a variety of coating, etching, and polishing techniques. Etching techniques are also being used to create fixed surface textures for spectroscopic applications. For example, Ion Optics Inc. (Waltham, Mass.) has developed tuned infrared sources using ion beam etching processes that create a random fixed surface texture consisting of sub-micron rods and cones (http://www.ion-optics.com). Such a surface texture has a high emissivity over a narrow band of wavelengths and low emissivity in other bands and is an attractive alternative to IR light-emitting diodes. 
     Applying the emerging field of solid state microelectromechanical technology, tunable IR filters for IR spectral analysis are also being developed. An example of such a device is reported by Ohnstein, T. R., et al (“Tunable IR Filters With Integral Electromagnetic Actuators,” Solid State Sensor and Actuator Workshop Proceedings, 1996, pp 196-199, Hilton Head, S.C.). Such tunable IR filters comprise arrays of waveguides whose transmittance can be varied by changing the spacing between them using linear actuators. The wavelength cutoff range from 8 μm to 32 μm achieved by Ohnstein et al with this technology is typical of its narrowband selectivity. Such IR spectral analysis devices, like the devices developed by Ion Optics, Inc., are purposely designed with surface microstructures having dimensions comparable to specific wavelengths in the electromagnetic spectrum to be effective at wavelengths that are discrete or in narrow bandwidths. Consequently, these devices are ineffective for applications which require the changing or controlling of broader ranges of wavelengths important in radiative heat transfer. 
     Accordingly, there is a need for a capability to change or control broadband radiative heat transfer between an object and its environment while the object is in service. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a structure and method for changing or controlling the thermal emissivity of the surface of an object in situ, and thus, changing or controlling the radiative heat transfer between the object and its environment in situ. Changing or controlling the degree of blackbody behavior of the object is accomplished by changing or controlling certain physical characteristics of a cavity structure on the surface of the object. The cavity structure, defining a plurality of cavities, may be formed by selectively removing material(s) from the surface, selectively adding a material(s) to the surface, or adding an engineered article(s) to the surface to form a new radiative surface. 
     The physical characteristics of the cavity structure that are changed or controlled in accordance with the present invention include cavity area aspect ratio, cavity longitudinal axis orientation, and combinations thereof. Controlling the cavity area aspect ratio may be performed by controlling the size of the cavity surface area, the size of the cavity aperture area, or a combination thereof. As described herein, the cavity structure may contain a gas, liquid, or solid that further enhances radiative heat transfer control and/or improves other properties of the object, for example surface finish, while in service. 
     The subject matter of the present invention is particularly disclosed and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description and examples taken in connection with accompanying drawings wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the temperature dependence of convective and radiative heat transfer at two extreme values of the thermal emissivity factor at temperatures between 1 and 1000 K; 
     FIG. 2 illustrates a simplified representation of a cavity structure; 
     FIG. 3 shows the top view of one example of a cavity structure defining a plurality of cavities and a geometric array of cavity apertures with circular shapes; 
     FIG. 4 a  illustrates an example of a cavity structure defining a plurality of cylindrically-shaped cavity surfaces having cavity longitudinal axes oriented relative to the radiative surface; 
     FIG. 4 b  illustrates an alternative mode of obtaining a cavity structure, similar to that of FIG. 4 a , by the addition of a cavity article on the surface of the object; 
     FIG. 5 a  illustrates a test cavity structure used to determine the effect of cavity area aspect ratio on radiative heat transfer from the structure; 
     FIG. 5 b  graphically depicts the radiance as a function of hole number (i.e., cavity area aspect ratio) in the test cavity structure of FIG. 5 a  at two operating temperatures; 
     FIGS. 6 a - 6   c  illustrate an embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 6 a ), to an intermediate emissivity state (FIG. 6 b ), and then to a low emissivity state (FIG. 6 c ), by changing the cavity area aspect ratio (i.e., by changing A a ) by translational movement of a cavity article relative to the object; 
     FIGS. 7 a - 7   b  illustrate an embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 7 a ) to a low emissivity state (FIG. 7 b ) by changing the cavity area aspect ratio (i.e., by changing A a ) by the application of tensile or compressive forces resulting from pressure differences across the object; 
     FIG. 8 a  illustrates another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed by changing the cavity area aspect ratio (i.e., by changing A a ) by the movement of caps over the cavity apertures; 
     FIGS. 8 b - 8   c  illustrate another embodiment of the present invention, similar to that of FIG. 8 a , whereby the caps and cavity structure are designed so as to produce thermal emissivity changes with less cap movement than that of the embodiment shown in FIG. 8 a;    
     FIGS. 9 a - 9   b  illustrate another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from high emissivity state (FIG. 9 a ) to a low emissivity state (FIG. 9 b ) by changing the cavity area aspect ratio (i.e., by changing A a ) by the rotation of a shutter; 
     FIGS. 10 a - 10   b  illustrate another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 10 a ) to a low emissivity state (FIG. 10 b ) by changing the cavity area aspect ratio (i.e., by changing A a ) by the application of a shear force on the object; 
     FIGS. 11 a - 11   b  illustrate another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 11 a ) to a low emissivity state (FIG. 11 b ) by changing the cavity area aspect ratio (i.e., by changing A a ) by the application of a shear force on a cavity article rigidly attached to the surface of the object; 
     FIGS. 12 a - 12   b  illustrate another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 12 a ) to a low emissivity state (FIG. 12 b ) by changing the cavity area aspect ratio (i.e., by changing A c ) by the movement of a block within the cavity structure; 
     FIGS. 13 a - 13   b  illustrate another embodiment of the present invention whereby the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 13 a ) to a low emissivity state (FIG. 13 b ) by changing the cavity area aspect ratio (i.e., by changing A c ) by the raising of the level of a selector within the cavity structure; 
     FIGS. 14 a - 14   b  illustrate another embodiment of the present invention whereby the cavity structure is a fiber mat and the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 14 a ) to a low emissivity state (FIG. 14 b ) by changing the orientation of the cavity longitudinal axes; 
     FIG. 15 illustrates another mode of changing the orientation of the cavity longitudinal axes of a cavity structure similar to that of FIGS. 14 a - 14   b ; and 
     FIGS. 16 a - 16   b  illustrate another embodiment of the present invention whereby the cavity structure comprises bladders and the degree of blackbody behavior (i.e., thermal emissivity) of the cavity structure is changed from a high emissivity state (FIG. 16 a ) to a low emissivity state (FIG. 16 b ) by changing the orientation of the cavity longitudinal axes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An aspect of the present invention is a structure on the surface of an object wherein the structure defines a geometric or random array of cavities open to the environment, for example pits, thru-holes, closed-end holes, and the like. The structure may be formed by selectively removing material(s) from the surface, selectively adding a material(s) to the surface, or adding an engineered article(s) to the surface to form a new radiative surface. The structure may be similar to closed-cell or open-cell foams in the regard that the cavities may be physically separated from other cavities or may be interconnected with one or more other cavities. The structure may be formed by a suitable mechanical, chemical, electrical, or biological process including but not limited to drilling, pressing, coining, etching, lithography, irradiation, laser ablation, vapor deposition, explosive forming, spallation, bacterial, enzyme and viral action, and combinations thereof. It is to be understood that this list of processes, and others disclosed herein, are exemplary only and that those skilled in this art will appreciate that the present invention is not limited to a particular method of forming a structure defining a plurality of cavities. 
     A purpose of the following FIGS. 2-5 is to provide a basis for terminology used herein to describe the structure defining a plurality of cavities. FIGS. 6-16 illustrate various embodiments of the present invention whereby the degree of blackbody behavior of an object (i.e., thermal emissivity) can be changed or controlled using such a structure. 
     FIG. 2 illustrates a simplified representation of a cavity structure  100  comprising a radiative surface  110  of an object  120  wherein the cavity structure  100  defines a plurality of cavities open to the environment with which the object  120  is transferring radiant energy  130 . In particular, the cavity structure  100  defines a plurality of cavity apertures  140  at the radiative surface  110 , each with a cross-sectional area A a  and an effective diameter D equal to 2×(A a /π) ½ , and a plurality of cavity surfaces  150 , each with a cavity surface area A c . The cavity area aspect ratio R is defined as A c /A a . The cavity apertures  140  (and cavity surfaces  150 ) are not necessarily the same size and shape for a given cavity structure  100 . As is known to those skilled in the art, the cavity apertures  140  and cavity surfaces  150  may be any size and shape, including those that define slots, consistent with (1) the theory behind blackbody behavior of cavities (e.g., Chapter 3 of Wolfe, W. L., 1965, Handbook of Military Infrared Technology, Office of Naval Research, Department of the Navy, Washington, D.C.), (2) the desired range of thermal emissivity control of the cavity structure  100 , and (3) other desired surface properties of the cavity structure  100 , for example surface finish or roughness. 
     The number and density of cavity apertures  140  and cavity surfaces  150  are variable and depends on the desired degree of blackbody behavior of the cavity structure  100  and desired degree of radiative control. Measurable thermal emissivity changes were obtained with the present invention when the cumulative sum of the cross-section areas of the cavity apertures  140  (i.e., ΣA a ) was as low as 20% of the object&#39;s surface (i.e., the ratio of total cavity aperture area, ΣA a , to the area of the radiative surface  110  was 1:4). In most engineered systems, however, it is typically desirable to have a higher percentage of the object&#39;s surface occupied by cavity apertures  140  and cavity surfaces  150  so that a larger range of radiative control is obtained. 
     FIG. 3 shows the top view of an example of a cavity structure  100 , comprising the radiative surface  110  of an object  120 , and defining a dense array of cavity apertures  140  with circular shapes (the cavity surfaces  150  below the cavity apertures  140  are not shown for clarity). In this example, the cavity apertures  140  have an effective diameter D (equal to the diameter of the cavity apertures  140 ) centered on a square pitch array with the thinnest section of wall of the cavity structure  100  between adjacent cavity apertures  140  designated by the minimum wall thickness d. Choosing a minimum wall thickness d equal to about 0.08D provides the following ratio of total cavity aperture area, ΣA a , to the area of the radiative surface  110  (i.e., surface area unoccupied by cavity apertures  140  and excluding ΣA c ): 
     
       
         0.25 πD   2 :[(1.08 D ) 2 −0.257 πD   2 ]≅2:1 
       
     
     The cavity structure  100  in FIG. 3 will increase the radiative heat transfer capability of the object  120  (relative to an unaltered object surface) proportional to the sum of the respective Aε terms in Equation 1. In this example, if ε=0.1 for the radiative surface  110  (i.e., the object surface unoccupied by cavity apertures) and the cavities represent blackbodies (i.e., ε=1.0), the cavity structure  100  enhances the radiative power of the object  120  by a factor of: 
     
       
         (2×1+1×0.1)/((2+1)×0.1)=7 
       
     
     The ultimate potential enhancement of radiative power by this means can closely approach that of the whole surface of the object  120  acting as a single blackbody. Both larger and smaller radiation enhancement factors will be achieved with different minimum wall thicknesses d and different emissivities of the radiative surface  110 . For example, if the cavities represent blackbodies, d=0.08, and ε=0.05 for the radiative surface  110 , there is the potential of a nearly 15-fold enhancement, whereas having ε=0.2 allows only a 3-fold enhancement. As will be described later, such a cavity structure  100  can be physically altered in situ to reduce the degree of blackbody behavior (and then physically altered again in situ to increase the degree of blackbody behavior) so that a range of radiative control is obtained. 
     The shape of the cavity aperture  140  may be any regular shape (e.g., circular, elliptical, rectangular, quadrilateral, and other polygonal shapes) or any irregular shape, although the shape will typically be limited by manufacturing and economic constraints. Effective diameters of the cavity apertures  140  in the range from about 1 μm to several 1000 μm are practical and provide the principal benefits of the present invention for most engineered systems. Larger effective diameters may be optimal for very large systems. Smaller effective diameters may be optimal for systems operating at very high temperatures. As is evident to those skilled in the art, the range of radiative heat transfer control depends upon the radiation bandwidth emitted by the object  120 . Consequently, it is preferred that the size of the cavity apertures  140  be chosen to achieve an acceptable amount of radiative heat transfer control by virtue of the temperature of the object  120 . In most applications, it is preferred that the average effective diameter of the plurality of cavity apertures  140  is at least 10 μm. 
     FIG. 4 a  illustrates an example of a cavity structure  100  defining a plurality of cylindrical cavity surfaces  150  (and circular cavity apertures  140 ) with an effective diameter of D (equal to the diameter of the cylinder) and a depth/length of L. In this example, the cavity area aspect ratio R equals A c /A a =(πDL+πD 2 /4)/πD 2 /4=4L/D+1. When the cavity structure  110  defines closed-end holes made by drilling or boring, the bottom of the cavity surfaces  150  will typically have a shape that conforms to the machine tool used to form the cavities (e.g., a taper produced by a standard drill bit). In cases whereby the shape of the cavity surface  150  has a longitudinal axis (as conventionally defined) the orientation of such a cavity longitudinal axis  160 , relative to the radiative surface  110 , is measured by the angle α as shown in FIG. 4 a.    
     FIG. 4 b  illustrates another example of a cavity structure  100 , similar to that of FIG. 4 a , except that the cavity structure  100  in FIG. 4 b  is formed by the addition, or deposition, of a cavity article  170  to the surface  180  of the object  120  to form a new radiative surface  110 . As is evident to those skilled in the art, the cavity article  170  would typically be in contact with the object  120  in a manner to assure a good thermal bond. The cavity article  170  may be formed by a variety of material deposition techniques, including those used in semiconductor manufacture, or may comprise a separately manufactured component (e.g., a perforated plate, screen, mesh, fiber mat) that is in contact with the object  120 . Furthermore, the cavity structure  100  may be formed by combining the cavity structure  100  shown in FIG. 4 a  with the cavity article  170  shown in FIG. 4 b  such that the cavity surface  150  resides in both the object  120  and the cavity article  170 . This may be advantageous from a manufacturing perspective since boring cavities deep enough to act as blackbodies may be difficult in some sizes, materials and/or configurations. Furthermore, such a shared arrangement also provides another mode of thermal emissivity control as explained in more detail below. 
     As is evident from the previous discussion, the amount of radiative control of an object  120  depends upon the amount by which its thermal emissivity can be changed. For maximum radiative control, the thermal emissivity factor should be capable of being changed from a value of near 0 to near 1 and vice versa. In accordance with the present invention, a cavity structure  100  having an average cavity area aspect ratio of approximately 8 or greater provides a means by which the thermal emissivity of an object&#39;s surface can be significantly increased. This is illustrated in FIGS. 5 a - 5   b  whereby a test cavity structure  100 ′, defining  190  cavity apertures  140 ′ and cavity surfaces  150 ′, has variable cavity area aspect ratios ranging between 5 and 41 (1 st /19 th  holes and 10 th  hole, respectively). FIG. 5 b  shows the measured normalized radiance of each cavity (i.e., hole) which is proportional to its thermal emissivity. The 1 st  and 19 th  holes, each with a depth to diameter ratio of 1:1 (i.e., a cavity area aspect ratio of 5) have a radiance close to that of the unmodified surface of the test object  120 ′. The 2 nd  and 18 th  holes, each with a depth to diameter ratio of 2:1 (i.e., a cavity area aspect ratio of 9) show an onset of increased radiance, and hence increased thermal emissivity. Holes with higher depth to diameter ratios show the expected higher radiance. As is evident to those skilled in the art, the aforementioned cavity area aspect ratio range of approximately 8 or greater would also be expected to be suitable for a cavity structure  100  defining a plurality of cavities whereby one or more of the cavities are backfilled with a material that is substantially transparent to incident and emitted radiation. Such backfilling would be advantageous, for example, in applications whereby the object  120  requires a smooth surface finish. 
     Embodiments Whereby ε is Changed by Changing R 
     An embodiment of the present invention is a cavity structure  100  for an object  120  whereby the radiative heat transfer between the object  120  and its environment is controlled in situ by controlling the cavity area aspect ratio R of the cavity structure  100  in situ (in some circumstances, changing the cavity area aspect ratio R also changes the ratio of total cavity aperture area to surface area unoccupied by cavity apertures  140 ). Controlling the cavity area aspect ratio R may be implemented by controlling the effective size of the cavity aperture area A a , the effective size of the cavity surface area A c , or a combination thereof. The controlling may be by passive means, active means, or a combination thereof (e.g., spontaneous or externally applied stimuli such as temperature, chemistry, biology, humidity, pressure, electrical current or field, voltage, magnetic field, electromagnetic radiation, particle radiation, mechanical force, and combinations thereof). 
     FIGS. 6-13 illustrate specific embodiments of the present invention whereby thermal emissivity control is obtained by varying the cavity area aspect ratio R. In these figures, the control systems and actuators are not shown because it is evident to those skilled in the art that a variety of control systems and actuators could be utilized and interfaced with the cavity structure  100  without undue experimentation. 
     FIGS. 6 a - 6   c  illustrate an embodiment of the present invention comprising a cavity article(s)  170  that is a perforated plate/sheet, sleeve, screen, or mesh. The cavity surface  150  defining the combination of thru-hole surfaces  190  in the cavity article  170  and the hole surfaces  195  in the object  120 . The array of perforations in the cavity article  170  dimensionally match the array of holes in object  120  so that when the thru-hole surfaces  190  are aligned in full coincidence with the hole surfaces  195 , the thermal emissivity of the resulting cavity is at a maximum (FIG. 6 a ). As the cavity article(s)  170  is translated relative to the object  120 , the effective size of the aperture area A a  is changed such that the thermal emissivity of the cavity structure  100  is controlled over a range of different values (FIG. 6 b ). A minimum thermal emissivity is obtained when there is no coincidence between the thru-hole surfaces  190  and hole surfaces  195  (FIG. 6 c ). Relative movement between the cavity article(s)  170  and the object  120  may be by passive means, active means, or a combination thereof including mechanical, thermal, electrical, magnetic, and chemical means (e.g., the cavity article(s)  170  may be moved by a solenoid, motor, bimetallic actuator, piezoelectric element, etc. (not shown) that is connected to a control system (not shown)). 
     FIGS. 7 a - 7   b  illustrate a further embodiment of the present invention whereby the cavity area aspect ratio is changed (i.e., by changing the area of the cavity aperture  140 ) by applying tensile or compressive forces to the radiative surface  110  of the object  120  (e.g., by applying a differential pressure across the object  120  as shown FIGS. 7 a - 7   b ). Similar cavity aspect ratio changes can be induced by exposing a cavity structure  100  that is made of a hydrophilic, or hydrophobic, material to water. The present invention anticipates all means of swelling, shrinking, deforming, exposing, or obscuring single or multiple cavity apertures  140  to change the effective aperture area A a , and thus cavity area aspect ratio R. 
     FIG. 8 a  illustrates a further embodiment of the present invention whereby the size of the cavity apertures  140  is controlled by controlling the movement of caps  800  positioned proximate the cavity apertures  140 . The caps  800  are attached (e.g., by fasteners, hinges, welding, soldering, adhesives, slide rails) to the radiative surface  110  to allow angular movement of the caps  800  through an angle β (up to 90° in this embodiment) relative to the radiative surface  110 . In another embodiment, the movement of the caps  800  may be in the same plane as the radiative surface  110  (e.g., a cap  800  that slides across the radiative surface  110  along rails, not shown). The caps  800  may be moved relative to the radiative surface  110  individually, or in sets, by incorporating into the caps  800  active elements such as bimetallic, shape memory, piezoelectric, magnetic, magnetostrictive, and combinations thereof and then activating the caps  800  by the application of heat/cold, voltage, magnetic field, etc. Such activation causes the caps  800  to bend or otherwise move to change the size of the cavity apertures  140 . The caps  800  may also be moved individually or in sets by mechanical means (e.g., a stepping motor). Furthermore, it is anticipated that an individual cap  800  may be sized such that it changes the size of more than one aperture  140  at one time upon its movement. In the embodiment shown in FIG. 8 a , the cavity longitudinal axes  160  are approximately perpendicular to the plane of the cavity apertures  140 , but the present invention is not limited to such an orientation. 
     For example, FIGS. 8 b - 8   c  illustrate a further embodiment of the present invention, similar to that shown in FIG. 8 a , except that the cavity longitudinal axis  160  is not perpendicular to the plane of the cavity apertures  140 , and slots  810  are incorporated in the cavity structure  100 . Such a cavity longitudinal axis  160  orientation reduces the amount of cap  800  angular movement (as indicated by the angle β) required for a given change in the size of the cavity aperture  140  (and thus, thermal emissivity). The slots  810  facilitate the attachment of the caps  800  to the cavity structure  100  (e.g., by using the slots  810  as a sliding track for the caps  800 , or using the slots  810  as a means to press-fit or weld the caps  800  to the cavity structure  100 ). 
     FIGS. 9 a - 9   b  show a further embodiment of the present invention whereby the size of the cavity aperture  140  is changed by the rotation of a shutter  900  located proximate to the cavity aperture  140  in a cavity article  170 . The shutter  900  is penetrated by a hole such that by rotating the shutter  900  (e.g., by magnetic, electrical, pressure, or mechanical means), the effective size of the cavity aperture  140  is increased (FIG. 9 a ) or decreased (FIG. 9 b ). The hole in the shutter  900  can be any size or shape, depending upon the degree of radiative control desired. 
     FIGS. 10 a - 10   b  show a further embodiment of the present invention whereby the size of the cavity aperture  140  is changed by mechanical deformation of the radiative surface  110  by shear force. FIGS. 11 a - 11   b  show a further embodiment of the present invention, similar to the embodiment shown in FIGS. 10 a - 10   b , whereby the cavity surfaces  150  are shared between the cavity article  170  and the object  120 . In this embodiment, the cavity article  170  would typically be rigidly fixed to the object surface  180 . The cavity article  170  may be manufactured from a material different from that of the object  120  and which has desirable shear deformation properties. 
     FIGS. 12 a - 12   b  illustrate a further embodiment of the present invention whereby the effective cavity surface area A c  is changed by the movement of a block  1200  relative to the cavity surface  150 . In this embodiment, if the shape of the cavity surface  150  is cylindrical, the block  1200  may be cylindrical or spherical in shape (similar configuration to that of a ball valve). The block  1200  may be moved by pressure differences across the block  1200  (e.g., by pneumatic or hydraulic means via channel  1210 ) or by magnetic, electrical, or mechanical means. 
     FIGS. 13 a - 13   b  illustrate a further embodiment of the present invention whereby the cavity structure  100  defines a plurality of cavities whereby one or more of the cavities are backfilled with a selector  1300 . The selector  1300  may be a liquid, solid, condensable gas, or a combination thereof. In applications whereby the selector  1300  is fluid, the cavity surface area A c  may be changed by the raising or lowering of the level of the selector  1300  using a feed/drain channel  1310 . Furthermore, the cavity surface area A c  may be changed by eliminating the feed/drain channel and relying on the evaporation, sublimation, or condensation of the selector  1300  to effect a change in the level of the selector  1300 . 
     The thermal emissivity of a cavity structure  100  can further be changed or controlled by backfilling the cavities in the cavity structure  100  with a selector  1300  and then changing the radiative characteristics (e.g., reflectivity, transmissivity, and absorptivity) of the selector  1300  by the application of physical and/or environmental stimuli to the selector  1300 . For example, the selector  1300  may be a luminescent material, liquid crystal, photochrome, electrochrome, or a combination thereof. Backfilling cavities with a selector  1300  that is reflective and/or opaque to incident radiation while transparent to emitted radiation or vice versa will have the character of a thermal diode. This is a further modification of the present invention that increases the ability to thermally engineer and independently control the radiative heat transfer properties of an object  120 . This control approach can be designed to modify both emitted and absorbed radiation to effect thermal control of the object  120 . 
     Activation of the selector  1300  by physical means and/or environmental stimuli provides a broad range of IR detection, recognition and tagging possibilities. For example, the cavity structure  100  of the present invention provides reservoirs for a variety of selectors  1300  to aid detection, inspection, tracking and tracing activities commonly practiced in law-enforcement, customs and excise, brand and fraud verification, etc. Selectors contained in cavity structures  100  having a high average cavity area aspect ratio will be superior to surface-applied taggants in resisting wear, erasure or alteration. Thermal or other means of activating the cavity structure  100  could dispense new selector  1300  to the radiative surface  110  to restore the desired IR signature of the surface and replace surface-active material that may have been removed or obscured. 
     Embodiments Whereby ε is Changed by Changing α 
     Another embodiment of the present invention is a cavity structure  100  for an object  120  whereby the radiative heat transfer between the object  120  and its environment is controlled in situ by controlling the orientation of the cavity longitudinal axes  160  in the cavity structure  100  relative to the radiative surface  110 . The controlling may be by passive means, active means, or combinations thereof (e.g., spontaneous or externally applied stimuli such as temperature, chemistry, biology, humidity, pressure, electrical current or field, voltage, magnetic field, electromagnetic radiation, particle radiation, mechanical force, and combinations thereof). 
     FIGS. 14-16 illustrate specific embodiments of the present invention whereby thermal emissivity control is obtained by varying the orientation of the cavity longitudinal axis  160  (measured by the angle α). In these figures, the control systems and actuators are again not shown because it is evident to those skilled in the art that a variety of control systems and actuators could be utilized and interfaced with the cavity structure  100  without undue experimentation. 
     FIGS. 14 a - 14   b  illustrate a further embodiment of the present invention whereby the cavity structure  100  comprises a plurality of filaments  1400  whereby each end of a filament  1400  is attached to the object surface  180  to form a mat of filaments  1400  approximately parallel with one another. The mat may comprise, for example, 5-10 μm diameter filaments  1400  that are synthetic, ceramic, metal, or combinations thereof and typically would have a high surface reflectivity. The filaments  1400  may be solid or hollow and may be coated with a metal. For example, hollow filaments could be incorporated as micro heat pipes containing a working fluid. Such an arrangement would deploy in the high emissivity condition when the working fluid pressure reaches some design value as a consequence of heating. Such a cavity structure  100  has an array of interconnected cavity surfaces  150  and interconnected cavity apertures  140 . The filaments  1400 , oriented approximately perpendicular to the object surface  180  (i.e., α≅90°), presents a maximum thermal emissivity configuration (FIG. 14 a ) while the filaments  1400  oriented approximately parallel to the object surface  180  (i.e., α≅0°) presents a minimum thermal emissivity configuration (FIG. 14 b ). Maximum and minimum thermal emissivity configurations of the filaments  1400  may be obtained, for example, by applying a electrostatic charge to the filaments and grounding the filaments, respectively. An alternative embodiment of the present invention includes selective treating or coating of the filaments  1400  to obtain selective electrostatic behavior. Another means by which the orientation of the filaments  1400  may be changed is by attaching a substantial number of the free ends of the filaments  1400  to a perforated sheet  1500  as shown in FIG. 15 to form the cavity structure  100 . As shown in FIG. 15, low and high thermal emissivity configurations may be obtained by translating the perforated sheet  1500  relative, and parallel, to the object surface  180 . Relative movement between the perforated sheet  1500  and object  120  may be accomplished by a variety of means including, mechanical, thermal, electrical, magnetic, and chemical. 
     FIGS. 16 a - 16   b  illustrate a further embodiment of the present invention whereby the cavity structure  100  comprises a perforated membrane  1600  defining a plurality of cavity apertures  140 . The perforated membrane  1600  may be a perforated plate/sheet, mesh, or screen. The perforated membrane  1600  is typically thermally bonded to the object  120  by one or more connecting members  1610 . The cavity structure  100  further comprises a plurality of inflatable and/or deflatable bladders  1620  that have an open end  1630  attached to the perforated membrane  1600  at the cavity aperture  140 . As shown in FIGS. 16 a - 16   b , high and low thermal emissivity configurations may be obtained by inflating or deflating the bladder  1620 , effectively changing the orientation of the cavity longitudinal axis  160  relative to the radiative surface  110 . Such inflation/deflation may be accomplished by such means as mechanical suction, pressurization through heating, and mechanical expansion. 
     As is evident from the foregoing, another embodiment of the present invention is a cavity structure  100  for an object  120  whereby the radiative heat transfer between the object  120  and its environment is controlled in situ by controlling the cavity area aspect ratio R and orientation of the cavity longitudinal axis  160  in combination. 
     The present invention has extremely broad and diverse applications with the potential of providing preset or dynamic control of temperature and heat transfer in objects as diverse as automobiles, machinery, buildings, power generation equipment, and military and space systems. Cavity structures  100  used in the manner disclosed herein enable a variety of components (e.g., engines, exhaust components, transmission lines, reactors) to run cooler, thereby reducing the size and power requirements of the conventional cooling system, increasing safety, and extending component life. Furthermore, the cavities in the cavity structures  100  can be backfilled with a material (e.g., glass, polymer), that is substantially transparent to the incident and emitted radiation, to restore the original surface smoothness and prevent the cavities from being filled with dirt or other undesirable materials. 
     The present invention also offers the potential of controlling the sizes of individual, or groups, of cavity apertures  140 , cavity surfaces  150 , or combinations thereof, to effect local control of thermal zones on an object  120 . Among many possible uses, the ability to program individual or groups of cavity apertures  140 , cavity surfaces  150 , or combinations thereof, with a different thermal emissivity could be used to disguise or randomize the IR signature of objects  120  so that they escape detection by IR cameras or sensors. Such an arrangement could provide thermal camouflage of objects  120  having, for example, military or law-enforcement significance. Furthermore, actively controlled cavity structures  100  could enable a programmable identification friend or foe (IFF) capability necessary in warfare and law-enforcement. IFF signatures expressed by patterns of open and closed cavity apertures  140  would be detectable by IR cameras or sensors. These patterns could be reprogrammed frequently to avoid recognition or use by an enemy. Another application is maintaining optical precision in relatively large structures such as telescopes. With computer control of a cavity structure  100  in the base of a large mirror system, for example, thermal emissivity could be manipulated to provide a means of ultrafine-tuning the mirror&#39;s shape by local thermally-induced contractions and expansions of the structure. 
     CLOSURE 
     While numerous embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.