Abstract:
Electromagnetic-energy absorbing materials are combined with thermally conductive materials, such as those used for thermal management in association with electronic equipment, thereby suppressing the transmission of electromagnetic interference (EMI) therethrough. Disclosed are materials and processes for combining EMI-absorbing materials with thermally conductive materials thereby improving EMI shielding effectiveness in an economically efficient manner. In one embodiment, a thermally conductive EMI absorber is prepared by combining an EMI-absorbing material (for example, ferrite particles) with a thermally conducting material (for example, ceramic particles), each suspended within an elastomeric matrix (for example, silicone). In application, a layer of thermally conductive EMI-absorbing material is applied between an electronic device or component, and a heat sink

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to thermal management in electronic applications and, more specifically, to thermal conductors incorporating electromagnetic-energy-attenuating properties.  
         [0003]     2. Description of the Prior Art  
         [0004]     As used herein, the term EMI should be considered to refer generally to both electromagnetic interference and radio-frequency-interference (RFI) emissions, and the term “electromagnetic” should be considered to refer generally to electromagnetic and radio frequency.  
         [0005]     Electronic devices typically generate thermal emissions as an unavoidable byproduct. The amount of thermal emissions generated can correlate to the switching speed and complexity of the source electronic component or device. As newer electronic devices tend to operate at greater and greater switching speeds, they will also result in greater thermal emissions. These increased thermal emissions, at some level, pose a risk of interfering with the function of the source electronic component, and with the functions of other nearby devices and components.  
         [0006]     Accordingly, the unwanted thermal emissions should be dissipated benignly to preclude or minimize any undesirable effects. Prior-art solutions addressing the removal of unwanted thermal emissions include providing a thermal pad over the electronic component and attaching a heat sink to the thermal pad. Heat sinks generally include material with high thermal conductivity. When placed in intimate contact with a heat-generating electronic component, the heat sink conducts thermal energy away from the component. Heat sinks also include attributes that facilitate heat transfer from the heat sink to the ambient environment, for example, through convection. For example, heat sinks often include “fins” that result in a relatively large surface area for a given volume.  
         [0007]     Furthermore, under normal operation, electronic equipment typically generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can exist over a wide range of wavelengths and frequencies. To minimize problems associated with EMI, sources of undesirable electromagnetic energy can be shielded and electrically grounded to reduce emissions into the surrounding environment. Alternatively, or additionally, susceptors of EMI can be similarly shielded and electrically grounded to protect them from EMI within the surrounding environment. Accordingly, shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a barrier, a housing, or other enclosure in which the electronic equipment is disposed.  
         [0008]     Sound EMI design principles recommend that EMI be treated as near as possible to the source to preclude entry of unwanted EMI into the local environment, thereby minimizing the risk of interference. Unfortunately, components and devices requiring the use of heat sinks are not well suited for protective treatment for EMI at the source, because such treatment would interfere with the operation of the heat sink. The heat sink should be in intimate contact with the electronic component to provide a thermal conduction path and also be open to the surrounding environment to allow for the heat sink to function through convective heat transfer.  
       SUMMARY OF THE INVENTION  
       [0009]     In general, the present invention relates to an electromagnetic-interference-absorbing thermally-conductive gap filler, such as an elastomeric (for example, silicone) pad treated with an electromagnetic-interference-absorbing material. The EMI-absorbing material absorbs a portion of the EMI incident upon the treated thermal pad, thereby reducing transmission of EMI therethrough over a range of operational frequencies. The absorbing material may remove a portion of the EMI from the environment through power dissipation resulting from loss mechanisms. These loss mechanisms include polarization losses in a dielectric material and conductive, or ohmic, losses in a conductive material having a finite conductivity.  
         [0010]     Accordingly, in a first aspect, the invention relates to a composite material for reducing electromagnetic emissions generated by an electronic device, the composite material including, in combination, a thermally conductive material and an electromagnetic-energy-absorptive material. The thermally conductive material facilitates transfer of thermal energy from the device and the electromagnetic-energy-absorptive material reduces electromagnetic emissions generated by the device.  
         [0011]     In one embodiment, at least one of the thermally conductive material and the electromagnetic-energy-absorptive material are granules. The granules may be generally spherical, such as microspheres, or other shapes, such as powder, fibers, flakes, and combinations thereof. The composite further includes a matrix material in which the thermally conductive material and the electromagnetic-energy-absorptive material are suspended.  
         [0012]     In general, the matrix material is substantially transparent to electromagnetic energy, for example, being defined by a relative dielectric constant of less than approximately 4 and a loss tangent of less than approximately 0.1. In one embodiment, the matrix is prepared as a liquid. In another embodiment, the matrix is prepared as a solid. In another embodiment, the-matrix is prepared as a phase-change material existing in a solid phase at ambient room temperature and transitioning to a liquid phase at equipment-operating temperatures. In another embodiment, the matrix is prepared as a thermosetting material.  
         [0013]     In some embodiments, the thermally conductive EMI absorber is formed in a sheet having a thickness greater than approximately 0.010 inch and less than approximately 0.18 inch. In other embodiments, the sheet includes a thermoconductive adhesive layer.  
         [0014]     In another aspect, the invention relates to a method for reducing electromagnetic emissions produced by a device, the method including the steps of providing a thermally conductive material, providing an electromagnetic-absorbing material, and combining the thermally conductive material with the electromagnetic-absorbing material.  
         [0015]     In one embodiment, the process includes the additional step of suspending the combined thermally conductive material and electromagnetic-absorbing material in a matrix material.  
         [0016]     In another embodiment, the process includes the additional step of placing the combined thermally conductive material and electromagnetic-absorbing material between the device and proximate structure, such as between an integrated circuit and a heat sink. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The advantages of the invention may be better understood by referring to the following description, taken in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a schematic diagram depicting a perspective view of an embodiment of a thermally conductive EMI absorber identifying exemplary constituent components;  
         [0019]      FIG. 2  is a schematic diagram depicting a perspective view of an exemplary application of a thermally conductive EMI absorber, such as the embodiment illustrated in  FIG. 1 ;  
         [0020]      FIGS. 3A and 3B  are schematic diagrams depicting perspective views of alternative embodiments of a thermally conductive EMI absorber formed as a sheet and as a rollable tape, respectively;  
         [0021]      FIG. 4  is a schematic diagram depicting an alternative embodiment of the thermally conductive EMI absorber depicted in  FIG. 1 , in which desired shapes are cut, for example, from the sheet of  FIG. 3A ;  
         [0022]      FIG. 5  is a schematic diagram depicting a perspective view of an alternative embodiment of the thermally conductive EMI absorber depicted in  FIG. 1 , in which the shield is pre-formed according to a predetermined shape;  
         [0023]      FIG. 6  is a schematic diagram of an alternative embodiment of a thermally conductive EMI absorber in a flowable form, such as a liquid;  
         [0024]      FIG. 7  is a schematic diagram depicting a perspective view of an exemplary application of a flowable, thermally conductive EMI absorber, such as the embodiment illustrated in  FIG. 6 ;  
         [0025]      FIG. 8  is a flow diagram depicting an embodiment of a process for preparing a thermally conductive EMI absorber, such as the embodiment illustrated in  FIG. 1 ; and  
         [0026]      FIG. 9  is a schematic plan view of a test fixture used to measure the thermal conductivity of the thermally conductive EMI shield of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Materials having electromagnetic-energy absorbing properties can be used to suppress the transmission of EMI over a broad range of frequencies. Such EMI-absorbing materials can provide substantial electromagnetic-shielding effectiveness, for example, up to about 5 dB or more at EMI frequencies occurring from about 2 GHz up to about 100 GHz.  
         [0028]     According to the present invention, a thermally-conductive EMI absorber can be formed by combining EMI-absorbing fillers and thermally conducting fillers in a base matrix (for example, an elastomer) capable of being applied as a thermal gap filler, or pad. Generally, the resulting thermally-conductive EMI absorber can be applied as any thermal conductive material, for example, as between an electronic component (e.g., a “chip”) and a heat sink.  
         [0029]     Referring to  FIG. 1 , a thermally-conductive EMI absorber (thermal EMI shield)  100  is illustrated as a rectangular volume. The front face of the thermal EMI shield  100  represents a cross-sectional view of the interior composition of the shield  100 . Namely, the thermal EMI shield  100  includes a number of EMI absorbers  110  and a number of thermal conductors  120 , both being suspended within a matrix material  130 . Although none of the EMI absorber particles  110  and the thermal conductor particles  120  are illustrated as being in contact with any neighboring particles  110 , 120 , configurations in which such contact occurs are anticipated. For example, thermal conductivity of the thermal EMI shield  100  would generally be enhanced for configurations in which thermal conductor particles  120  are in close proximity and contact with each other.  
         [0030]     The relative sizes of the individual EMI absorbers  110 , thermal conductors  120 , and the thickness of the matrix  130  as shown in  FIG. 1  are for illustration purposes only. In general, the suspended fillers  110 , 120  are extremely small (that is, microscopic). Small filler particles  110 , 120  allow for embodiments in which the overall thickness of the thermal EMI shield  100  is thin, for example, the thickness of the thermal EMI shield  100  is substantially less than the thickness of either the electronic component/device or the heat sink.  
         [0031]     Similarly, the relative shapes of the suspended particles  110 , 120  can be any arbitrary shape. The elliptical shapes of the suspended particles  110 , 120  shown in  FIG. 1  are for illustration purposes only. In general, the shape of the suspended particles  110 , 120  can be granules, such as spheroids, ellipsoids, or irregular spheroids. Alternatively, the shape of the suspended particles  110 , 120  can be strands, flakes, a powder, or combinations of any or all of these shapes.  
         [0032]     The EMI absorbers  110  function to absorb electromagnetic energy (that is, EMI). Specifically, the EMI absorbers  110  convert electromagnetic energy into another form of energy through a process commonly referred to as a loss. Electrical loss mechanisms include conductivity losses, dielectric losses, and magnetization losses. Conductivity losses refer to a reduction in EMI resulting from the conversion of electromagnetic energy into thermal energy. The electromagnetic energy induces currents that flow within an EMI absorber  110  having a finite conductivity. The finite conductivity results in a portion of the induced current generating heat through a resistance. Dielectric losses refer to a reduction in EMI resulting from the conversion of electromagnetic energy into mechanical displacement of molecules within an absorber  110  having a non-unitary relative dielectric constant. Magnetic losses refer to a reduction in EMI resulting from the conversion of electromagnetic energy into a realignment of magnetic moments within an EMI absorber  110 .  
         [0033]     In some embodiments, the EMI absorber  110  exhibits better thermal conductivity than air. For example, spherical iron particles selected as an EMI absorber  110  because of their EMI-absorbing properties also offer some level of thermal conductivity. Generally, however, the thermal conductivity of the EMI absorbers  110  of comparable thicknesses is substantially less than the value of thermal conductivity offered by substantially non-EMI-absorbing thermal conductors  120 , such as ceramic particles.  
         [0034]     In general, the EMI absorber  110  is selected from the group consisting of electrically conductive material, metallic silver, carbonyl iron powder, SENDUST (an alloy containing 85% iron, 9.5% silicon and 5.5% aluminum), ferrites, iron silicide, magnetic alloys, magnetic flakes, and combinations thereof. In some embodiments, the EMI absorber  110  is a magnetic material. In one particular embodiment, the EMI absorber  110  has a relative magnetic permeability greater than about 3.0 at approximately 1.0 GHz, and greater than about 1.5 at 10 GHz.  
         [0035]     The thermal conductor  120  includes a thermal impedance value substantially less than that of air. A low value of thermal impedance allows the thermal conductor  120  to efficiently conduct thermal energy. In general, the thermal conductor  120  is selected from the group consisting of aluminum nitride (AlN), boron nitride, iron (Fe), metallic oxides and combinations thereof. In some embodiments, the thermal conductor includes a ceramic material. In one particular embodiment, the thermal conductor  120  includes an Fe—AlN (40% and 20% by volume, respectively) having a thermal conductivity value greater than about 1.5 Watts/m-° C. An exemplary test report including a test procedure for measuring the thermal conductivity of a test sample, as well as measured thermal conductivity test results, is provided herein as Appendix A and incorporated herein in its entirety.  
         [0036]     In general, the matrix material  130  is selected to have properties allowing it to conform to surface imperfections encountered in many heat-sink applications (for example, surface imperfections of the mating surfaces of either the electronic component or device and the heat sink). Other desirable properties of the matrix material  130  include an ability for the material  130  to accept and suspend a substantial volume of particles  110 , 120 , (for example, up to about 60% by volume) without compromising the other advantageous properties of the matrix material  130 , such as conformability, compliance, and resilience. Generally, the matrix material  130  is also substantially transparent to electromagnetic energy so that the matrix material  130  does not impede the absorptive action of the EMI absorbers  110 . For example, a matrix material  130  exhibiting a relative dielectric constant of less than approximately 4 and a loss tangent of less than approximately 0.1 is sufficiently transparent to EMI. Values outside this range, however, are also contemplated.  
         [0037]     Generally, the matrix material  130  can be selected as a solid, a liquid, or a phase-change material. Embodiments in which the matrix material  130  is a solid further include thermoplastic materials and thermoset materials. Thermoplastic materials can be heated and formed, then reheated and re-formed repeatedly. The shape of thermoplastic polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point. Thermoset materials can also be heated and formed; however, they cannot be reprocessed (that is, made to flow under pressure when reheated). Thermoset materials undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network.  
         [0038]     In some solid embodiments, the matrix material  130  is selected from the group consisting of elastomers, natural rubbers, synthetic rubbers, PDP, ethylene-propylene diene monomer (EPDM) rubber, and combinations thereof. In other embodiments the matrix material  130  includes a polymer. The matrix material  130  can also be selected from the group consisting of silicone, fluorosilicone, isoprene, nitrile, chlorosulfonated polyethylene (for example, HYPALON®), neoprene, fluoroelastomer, urethane, thermoplastics, such as thermoplastic elastomer (TPE), polyamide TPE and thermoplastic polyurethane (TPU), and combinations thereof.  
         [0039]     Referring to  FIG. 2 , an exemplary application is illustrated in which an electronic component  200 , shown mounted on a circuit board  210 , is fitted with a heat sink  220 . The electronic component  200  can be an electronic circuit (for example, a microcircuit, or “chip”). Alternatively, the electronic component  200  can be an electronic device, such as a packaged module including one or more electronic components (for example, mounted within a metallic housing, or “can”). In either instance, the electronic component  200  creates, as a byproduct of its electronic function, thermal energy that should be dissipated to ensure that the electronic component  200  continues to operate within its design parameters and is protected from physical damage due to overheating.  
         [0040]     In general, a heat sink  220  is a device for dissipating heat from a host component  200 . The heat sink  220  first absorbs heat from the host component  200  through conduction. The heat sink  220  then dissipates the absorbed heat through convection to the surrounding air. The particular type or form of heat sink  220  selected is not critical. Rather, the heat sink  220  can be any one of a numerous variety of commercially available heat sinks, or even a custom designed heat sink.  
         [0041]     The thermal EMI shield  230  facilitates thermal conduction from the component  200  to the heat sink  220 . Generally, the thickness of the thermal EMI shield  230  (the dimension between the protected component  200  and the heat sink) is less than a predetermined maximum value. For example, in one embodiment, the thermal EMI shield  230  has a maximum thickness less than approximately 0.18 inch. Furthermore, the thickness of the thermal EMI shield  230  is generally greater than a predetermined minimum value. If the thermal EMI shield is too thin, an insufficient volume of EMI absorbing material will be provided to sufficiently absorb EMI from the component  200 . For example, in one embodiment, the thermal EMI shield  230  has a minimum thickness greater than approximately 0.01 inch.  
         [0042]     In one exemplary configuration, a thermal EMI shield  230  having a thickness of 0.125 inch, exhibits an attenuation of at least about 5 dB in a frequency range from about 5 GHz up to at least about 18 GHz. In another exemplary configuration, a thermal EMI shield  230  having a thickness of 0.02 inch, exhibits an attenuation of at least about 3 dB for a frequency range extending upward from about 10 GHz. In another exemplary configuration, a thermal EMI shield  230  having a thickness of 0.04 inch, exhibits an attenuation of at least about 10 dB in a frequency range from about 9 GHz up to at least about 15 GHz and an attenuation of at least about 6 dB in a frequency range extending upward from about 15 GHz. In yet another exemplary configuration, a thermal EMI shield  230  having a thickness of 0.060 inch, ±0.005 inch, exhibits an attenuation of at least about 5 dB in a frequency range extending upward from about 4 GHz, having a greater attenuation of at least about 10 dB in a frequency range from about 6 GHz up to at least about 10 GHz. Exemplary values of the complex (real and imaginary) relative permittivity (ε r ) and complex (real and imaginary) relative magnetic permeability (μ r ) for a nitrile rubber compound are tabulated and provided herein as Appendix B, incorporated herein in its entirety.  
         [0043]     Referring to  FIG. 3A , a thermal EMI shield is illustrated in a sheet configuration. Generally, the thermal EMI shield can be formed as a sheet  300 . The sheet  300  includes a length (L′) a width (W′) and a thickness (T′). In one embodiment, the length and width may be selected according to the dimensions of a particular application, such as the length and width of an electronic component  200  to which a heat sink  220  will be applied. In another embodiment, the sheet  300  can be fabricated in a predetermined size, such as a length of 26 inches, a width of 6 inches, and a thickness of either 0.030 inch or 0.060 inch. Any size, however, is contemplated.  
         [0044]     Yet other embodiments of a thermal EMI shield  100  may include a sheet  300  as just described, further including an adhesive layer  310 . The adhesive layer  310  a may be a thermoconductive adhesive to preserve the overall thermal conductivity. The adhesive layer  310  can be used to affix the heat sink  220  to the electronic component  200 . In some embodiments, the sheet  300  includes a second adhesive layer, the two layers facilitating the adherence of the heat sink  220  to the electronic component  200 . In some embodiments, the adhesive layer  310  is formulated using a pressure-sensitive, thermally-conducting adhesive. The pressure-sensitive adhesive (PSA) may be generally based on compounds including acrylic, silicone, rubber, and combinations thereof. The thermal conductivity is enhanced, for example, by the inclusion of ceramic powder.  
         [0045]     In an alternative embodiment, referring now to  FIG. 3B , the thermal EMI shield may be formed as a tape  320 . The tape  320 , for example, can be stored on a roll  330 , similar in form to a conventional roll of adhesive-backed tape. The tape  320  generally exhibits construction and composition features similar to those already described in relation to the sheet  300  of  FIG. 3A . Similar to the sheet  300 , the tape  320  includes a second width (W″) and a second thickness (T″). In general, the length for a tape roll embodiment is arbitrary, because the length of the tape  320  is substantially longer than any individual application. Accordingly, lengths of tape  320  suitable for intended applications can be separated (for example, “cut”) from the roll  330 . Again, similar to the previously described sheet  300 , the tape  320  can include a first adhesive layer  340 . The tape  320  can also include a second adhesive layer, similar to two-sided fastening tape.  
         [0046]     Referring now to  FIG. 4 , an alternative embodiment of a thermal EMI shield  100  configured as a sheet  400  is illustrated. In this embodiment, desired application shapes, such as a rectangle  410 ′ and an ellipse  410 ″ (generally  410 ) can be die-cut from the sheet  400 , thereby yielding thermal EMI absorbers  100  of any desired two-dimensional shape. Accordingly, the sheet  400  can be die-cut to produce the desired outlines of the application shapes  410 . Alternatively, the desired outlines of the application shapes  410  can be custom cut from the blank sheet  300  shown in  FIG. 3A .  
         [0047]     In yet another embodiment, the thermal EMI shield material may be preformed in any desired shape. Referring now to  FIG. 5 , a preformed shield  500  in a non-planar application is illustrated. The thermal EMI shield may be molded or extruded in any desired shape, such as the rectangular trough shown, a cylindrical trough, and semi-circular trough. Such non-planar thermal EMI shields  500  can be used in connection with non-planar electrical components  200 , such as cylindrical devices or components (for example, “cans”).  
         [0048]     Referring to  FIG. 6 , an embodiment of a liquid thermal EMI shield  600  is illustrated. In general, a vessel  610  is shown holding a liquid thermal EMI shield solution  620 . A portion of the solution  620  “A” is illustrated in greater detail in an insert labeled “Detail View A.” The detail view illustrates that the solution  620  includes EMI absorber particles  630  and thermal conductor particles  640 , each suspended within a liquid matrix  650 . Generally, the attributes of the particles  630 , 640  are similar to the attributes of the corresponding particles  110 , 120  described in relation to  FIG. 1 . Similar to the matrix described in relation to  FIG. 1 , the liquid matrix  650  is substantially transparent to electromagnetic radiation. The liquid matrix  650  can be formed as a liquid that may be painted onto an applicable surface to be treated. Alternatively, the liquid matrix  650  can be formed as a gel, such as grease, or as a paste or pour-in-place compound. In some embodiments, the liquid thermal EMI shield  600  can be applied to the intended surface by painting, spraying, or other suitable method. The matrix material may also be a liquid selected from the group consisting of silicones, epoxies, polyester resins and combinations thereof.  
         [0049]     In one embodiment, the matrix  130  illustrated in  FIG. 1  is a suitably selected phase-change material having properties of both a solid and a liquid. At ambient room temperatures, the phase-change material behaves as a solid offering ease of handling and storage. The phase-change material, however, exhibits a reflow temperature at or below the equipment operating temperature thereby enabling a “wetting action.” The matrix  130  reflows allowing the EMI-absorbing particles  110  and the thermally conductive particles  120  of the composite material  100  to flow into any gaps, such as those caused by surface imperfections.  
         [0050]     Referring to  FIG. 7 , a close-up detail of a cross-sectional view of an electronic component  700 , a heat sink  710 , and a thermally conducting EMI shield  720  is illustrated. Also shown are the surface imperfections  730  of each or both of the component  700  and heat sink  710 . The surface imperfections  730  are portrayed in an exaggerated manner for the purpose of illustration. With an ability to flow into surface imperfections  730 , a matrix  650  formulated as a liquid, or phase-change material removes air gaps, thereby minimizing the thermal impedance between the device  700  and an associated heat sink  710 . The overall effect of removing air gaps reduces the thermal impedance between the electrical component  700  and the heat sink  710 , leading to improved heat transfer efficiency. The matrix material may be a mixture of a paraffin wax having a melting point of approximately 51° C. and a 28% ethylene-vinyl acetate copolymer having a melting point of approximately 74° C. For example, a mixture of ninety-five parts by weight of the paraffin wax and five parts by weight of the ethylene-vinyl acetate copolymer may be used. Alternatively, a mixture of twenty-five parts by weight of the paraffin wax and six parts by weight of the ethylene-vinyl acetate copolymer may be used. Alternatively still, the matrix material may be a synthetic wax having a melting point of approximately 100° C. and a molecular weight of approximately 1000. Such a wax is of a type known as a Fischer-Tropsch wax.  
         [0051]     Referring to  FIG. 8 , a flow diagram is illustrated depicting a process of preparing a thermally-conductive EMI absorber  100 , such as the embodiments illustrated in either  FIG. 1  or  FIG. 6 . EMI absorber particles  110 , 630  are provided at step  800 . Thermally conductive particles  120 , 640  are also provided at step  810 . The EMI absorber particles  110 , 630  and thermally conducting particles  120 , 640  are combined and suspended within either a solid matrix material  130 , or a liquid matrix material  650 . Once prepared, the composite thermal EMI shield  100 , 600  is applied between an electronic component  200 , 700  and a heat sink  220 , 710  at step  830 .  
         [0052]     Having shown exemplary and preferred embodiments, one skilled in the art will realize that many variations are possible within the scope and spirit of the claimed invention. It is therefore the intention to limit the invention only by the scope of the claims, including all variants and equivalents.  
       APPENDIX A  
       [0000]     Test Report  
         [0000]     Scope:  
         [0053]     This report summarizes the thermal conductivity testing of multiple electromagnetic-energy-absorbing materials including a thermally conductive filler to also provide good thermal conductivity.  
         [0000]     Part Description:  
         [0054]     Three test samples were prepared and tested for thermal performance. Each of the samples consisted of iron (Fe)-filled elastomeric materials formulated to absorb electromagnetic surface waves. Some specific details for the test samples are listed below in Table 1.  
                         TABLE 1                           Test Samples            Sample No.   Test Sample Description               1   50% Fe by volume in isoprene, test slab           thickness of 30, 60, 90 and 125 mils.       2   41.5% Fe by volume in silicone, test slab           thickness of 20, 30, 60 and 100 mils.       3   40% Fe plus 20% aluminum nitride (AlN) by           volume in silicone, test slab thickness of 30,           60, 90 and 120 mils.                  
 
 Test Procedure: 
 
         [0055]     Thermal resistance testing was conducted in accordance with in internal test procedure and in accordance with ASTM specification D5470. The test samples were first die-cut into 1-inch-diameter circles to match the size of the thermal impedance probes. All of the thermal resistance measurements were made at 50° C., and 100 psi.  
         [0056]     The test fixture  900  is shown in  FIG. 9 . The test sample  910  is placed between two polished metal plates  920 , 930  that are stacked within the test assembly  900  as shown in  FIG. 9 . The heat is input from the heater plate  940 , which is protected from heat loss in all directions other than the testing direction by applying the same temperature to a guard heater  950  that is located above and around the heater plate  940 . An upper meter block  920  is located directly below the heater  940  and is followed by the test sample  910  find then a lower meter block  930 . Heat is drawn out from the bottom of the test stack with a water-cooled chiller plate  960 . Thermocouples  970  embedded in the meter blocks  920 , 930  are used to extrapolate the surface temperature on each side of the test sample  910 . This is done using a SRM 1462 reference material that has a thermal conductivity much greater than that of the test sample.  
         [0057]     During the test the sample is compressed at a constant pressure using a pneumatic cylinder. The stack is then permitted to reach a steady state at which point the thermal resistance of the sample is calculated. Once the thermal resistance of several thickness of material (nominally five) is measured and plotted the thermal conductivity is calculated as the inverse of the slope of the least squares best fit line through this data.  
         [0000]     Test Results:  
         [0058]     The thermal conductivity of the three absorbing test samples is shown in Table 2. The two standard absorbing materials, Sample No. 1 and Sample No. 2, have very similar thermal conductivities (approximately 1.0 Watts/m-° C.), whereas the third absorber material, Sample No. 3, has a substantially higher thermal conductivity (approximately 1.5 Watts/m-° C.).  
                                 TABLE 2                           Thermal Conductivity                    Thermal Conductivity   Standard           Test Sample   (Watts/m-° C.)   Deviation                       Sample No. 1   0.986   0.0632           Sample No. 2   1.022   0.0959           Sample No. 3   1.511   0.0637                      
 
         [0059]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 APPENDIX B 
               
             
             
               
                   
               
               
                   
               
               
                 NITRILE RUBBER (40%) 
               
             
          
           
               
                 Frequency 
                   
                   
                   
                   
               
               
                 (GHz) 
                 μ r   
                 μ i   
                 ∈ r   
                 ∈ i   
               
               
                   
               
             
          
           
               
                 0.915 
                 4 
                 −1.77 
                 12.277 
                 −0.251 
               
               
                 1.15 
                 4 
                 −1.77 
                 12.277 
                 −0.251 
               
               
                 2 
                 3.4 
                 −1.74 
                 12.277 
                 −0.251 
               
               
                 2.245 
                 3.29 
                 −1.735 
                 12.277 
                 −0.251 
               
               
                 3 
                 2.95 
                 −1.72 
                 12.277 
                 −0.251 
               
               
                 4 
                 2.58 
                 −1.67 
                 12.277 
                 −0.251 
               
               
                 5 
                 2.219 
                 −1.624 
                 12.277 
                 −0.251 
               
               
                 6 
                 2.05 
                 −1.58 
                 12.277 
                 −0.251 
               
               
                 7 
                 1.88 
                 −1.55 
                 12.277 
                 −0.251 
               
               
                 8 
                 1.65 
                 −1.52 
                 12.277 
                 −0.251 
               
               
                 9 
                 1.5 
                 −1.48 
                 12.277 
                 −0.251 
               
               
                 9.5 
                 1.45 
                 −1.43 
                 12.277 
                 −0.251 
               
               
                 10 
                 1.39 
                 −1.4 
                 12.277 
                 −0.251 
               
               
                 11 
                 1.34 
                 −1.36 
                 12.277 
                 −0.251 
               
               
                 12 
                 1.27 
                 −1.32 
                 12.277 
                 −0.251 
               
               
                 13 
                 1.201 
                 −1.273 
                 12.277 
                 −0.251 
               
               
                 14 
                 1.18 
                 −1.24 
                 12.277 
                 −0.251 
               
               
                 15 
                 1.14 
                 −1.21 
                 12.277 
                 −0.251 
               
               
                 15.5 
                 1.1 
                 −1.18 
                 12.277 
                 −0.251 
               
               
                 16 
                 1.057 
                 −1.147 
                 12.277 
                 −0.251 
               
               
                 17 
                 1.04 
                 −1.125 
                 12.277 
                 −0.251 
               
               
                 18 
                 1.03 
                 −1.1 
                 12.277 
                 −0.251 
               
               
                 20 
                 0.854 
                 −0.955 
                 12.277 
                 −0.251 
               
               
                 25 
                 0.68 
                 −0.74 
                 12.277 
                 −0.251 
               
               
                 30 
                 0.6 
                 −0.54 
                 12.277 
                 −0.251 
               
               
                 35 
                 0.533 
                 −0.34 
                 12.277 
                 −0.251 
               
               
                 40 
                 0.461 
                 −0.165 
                 12.277 
                 −0.251