Abstract:
Artificial dielectric material characterized by an organic matrix composed of woven and/or non-woven fibers coated with a metallic material wherein the article exhibits a measurable conductivity at microwave frequencies when the article is overall non-conductive at DC.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a Continuation-In-Part of application Ser. No. 10/863,849, filed Jun. 7, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to the partial metallization of a non-conductive cloth or fabric employed in the manufacture of an artificial dielectric, which includes the dielectric constant varying with frequency.  
         [0004]     2. Description of Related Art  
         [0005]     Various procedures exist for metallizing cloth or fabric. These include vapor deposition and electroless plating. Prior art in these fields relates to the production of a metal or metallic coating that will yield the properties of heat resistance, electromagnetic insulation, or reflection or bulk conductivity.  
         [0006]     While useful for the various applications for which they are intended, the dielectric constant of a metallic or conductive material is very high (e.g., above 10,000). These materials are not applicable to electromagnetic applications where low or medium dielectric constants are desired. High dielectric materials effectively exclude electromagnetic energy and can function as insulators simply by blocking the transmittance of such energy.  
         [0007]     For purposes herein, a low dielectric constant is less than 10, a medium dielectric constant is 10-100 and a high dielectric constant is above 100.  
         [0008]     Alternatively, a low or medium dielectric constant material will allow the penetration of the energy into the material where it may be attenuated by resonant cancellation or simply absorbed.  
         [0009]     Furthermore, the dielectric constant of the prior art material being metallic, would tend to remain constant over any frequency range. This limits applicability since a certain dielectric constant is useful for only a small frequency in many systems. By contrast, the material produced by the techniques presented herein have a dielectric constant that varies with frequency. This allows the insulating or attenuation effects to function over a broader range of frequencies.  
         [0010]     Percolating composite materials typically use powders, fibers, microspheres or microcylinders in conjunction with a polymer matrix. Often, these composites require advanced technology, i.e. in terms of shape and size of particles, in order to produce a significant effect.  
         [0011]     U.S. Pat. No. 5,607,743 discloses a metallized and electrically conducting gauze, deformed by deep drawing, based on a flat-shaped resin-coated textile material which has a metallized surface. The surface metal coating is up to 300 microns thick, although it is 20-100 microns thick in the preferred embodiment. The gauze product is made by impregnating a gauze fabric with a suitable resin suitable for mechanical stabilization and then pre-treating the resin-coated gauze by activating it with a solution containing noble metal ions or noble metal colloid followed by acceleration treatment in an aqueous acid followed by the step of depositing a metal such as copper, nickel or gold. The metal is deposited by treating the prepared gauze with an aqueous solution containing the relevant metal ions and a reducing agent. Another layer of same or different metal can then be deposited electrolytically on the chemically deposited metal layer.  
         [0012]     The Browning et al article in Journal of Applied Physics, in Vol. 84, No. 11, on pp. 6109-6113, entitled “Fabrication and radio frequency characterization of high dielectric loss tubule-based composites near percolation” discloses microscopic lipid tubules with an average aspect ratio of about 12 that were metallized elecrolessly with copper or nickel-on-copper and mixed with vinyl to make composite dielectric panels. As loadings increased, the metal tubule composites displayed an onset of electrical percolation with accompanying sharp increases in real and imaginary permitivities. Gravity-induced settling of the tubules, while the vinyl was drying, increased true loading density at percolation threshold for nickel/copper tubules to about 12 volume percent. This threshold was at a significantly lower loading density than that previously measured for percolation by composites containing spherical conducting particles. Qualitatively, the shape of the composite permitivity versus loading density curves followed predictions by the effective-mean field theory for conducting stick composites. Changes in permitivity of the vinyl panels were observed for several days after fabrication and were apparently associated with solvent evaporation from the matrix.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     An object of this invention is a metallized artificial dielectric material with a dielectric constant of low, medium or high magnitude that is especially useful for electromagnetic applications.  
         [0014]     Another object of this invention is a metallized fiber, woven or non-woven, wherein the metallized surface is provided by electroless plating wherein the motive force is imparted by a reducing agent.  
         [0015]     Another object of this invention is a metallized fabric with non-continuous or semi-continuous electrically conducting path that can be used in the general electromagnetic insulation, isolation and/or absorbance fields.  
         [0016]     Another object of this invention is a metallized fabric with a dielectric constant that varies with frequency.  
         [0017]     Another object of this invention is metallized fabric with a negative dielectric constant.  
         [0018]     These and other objects are achieved by an artificial dielectric article comprising a non-conductive matrix coated, for a selected period of time, with a metallic material wherein the article yields a measurable conductivity at microwave frequencies despite the article being non-conductive at DC, and wherein the article includes properties of electromagnetic percolation and the article includes an additional property of a dielectric constant that varies over frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0020]      FIG. 1  illustrates a metal deposition process onto a cellulose surface.  
         [0021]      FIG. 2  illustrates a comparison of resistance to plating time of the article as metal deposition is increasing.  
         [0022]      FIG. 3  illustrates limited variation in real and imaginary dielectric constants over the frequency range of up to 19 GHz at a loading of 5 minutes of electroless metal deposition.  
         [0023]      FIG. 4  illustrates a pronounced variation in real and imaginary dielectric constants over the frequency range of up to 19 GHz at a loading of 10 minutes of electroless metal deposition.  
         [0024]      FIG. 5  illustrates a more pronounced variation in real and imaginary dielectric constants over the frequency range of up to 19 GHz at a loading of 15 minutes of electroless metal deposition.  
         [0025]      FIG. 6  illustrates a very pronounced variation in real and imaginary dielectric constants over the frequency range of up to 19 GHz at a loading of 20 minutes of electroless metal deposition.  
         [0026]      FIG. 7  illustrates a dramatic variation in real and imaginary dielectric constants over the frequency range of up to 19 GHz at a loading of 25 minutes of electroless metal deposition.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]      FIG. 1  is illustrative of an electroless metallic deposition process used to generate a conductive fiber to be used in a composite of conductive fibers within a non-conductive matrix. Accordingly, cellulose surface  10  of a cloth or fabric, for example, is treated with metal substance  15  to act as the “catalyst”, activating the surface for further treatment,  20 . The treatment is continued, as shown by items  30   a - d . Based on existing commercial technology for electroless metallization, Shipley Cataposit 44, a Palladium compound, is used for the catalyst, for example. However, it is important to note, an embodiment of the present invention is not limited to this particular catalyst. Other catalysts have been described in the prior art, including Platinum. The Palladium compound of Cataposit 44 has been found to bind strongly to cellulose and is particularly suitable. In addition, gold or silver fulminate can also be employed as the catalyst. Silver is typically used because it is less expensive. Fulminates, when mixed with a reducing compound, precipitate conductive metal non-selectively on any available surface. Thus, they can be used in place of the Palladium compound to activate the surface.  
         [0028]     Following catalysis, the fabric  10  is treated with copper electroless plating bath, not shown. This bath is composed of two parts. The first part is a concentrated solution of copper salt, a reducing agent (such as formaldehyde) and stabilizers. The second part is a pH adjuster, usually Sodium Hydroxide. When properly diluted and mixed, the resulting bath will specifically deposit Copper onto the Palladium catalyst.  
         [0029]     The electroless copper used for this study includes Shipley 328, for example. Other metals are also capable of being deposited in this manner.  
         [0030]     If the catalyst is uniformly and densely attached to the surface, with each particle directly adjacent to other particles, the fabric will become conductive upon metallization with Copper. However, if, as with an embodiment of the present invention, the catalyst is applied less fully then significant metallization is achieved without the fabric becoming DC conductive, as shown in  30   a - c . In other words, materials that are significantly polarizable under the influence of an electric field, can result in materials that have a measurable conductivity at microwave frequencies despite being non-conductive at DC. Over time the individual domains of Copper deposition are connected to form an increasingly connected network. Only after extended plating is a large-scale conductive fabric  30   d  obtained.  
         [0031]     The invention disclosed herein has properties of a percolating system of materials. In these systems, an insulating matrix is combined with metal or metallic inclusions to form an artificial dielectric material. As the amount of metal or metallic inclusions increases, such materials approach the percolation threshold where they begin to take on the bulk properties of an electrical conductor, as described above and seen in  FIG. 1 .  
         [0032]     For ordinary non-conductors, the dielectric constant (ε) is usually presented as a real number related to the ability of a material to store electric field energy. However, more generally considered, the dielectric constant is a complex number with a real part and an imaginary part which is proportional to the conductivity (σ) of the material, as shown by the following equations. 
 
ε=ε′+ iε″ 
 
ε″=σ/ωε 0  
 
 Where ω is the radian frequency and δ 0  is the permittivity of free space, or 8.85×10 −12  F/m. So for a non-conductor σ is close to zero and the ε″ term can be neglected. The dielectric constant is usually expressed as a term relative to ε 0  so the value for free space is given as 1+0i and is dimensionless. Some typical values of a real dielectric constant for nonconductors are 2.5 (polystyrene), 2.1 (Teflon), 3.8 (quartz), and 90 (titanium dioxide). 
 
         [0033]     For metal conductors (such as gold, silver or copper) the conductivity is in the range of 4-7×10 7  (Ωm) −1 . The ε″ or imaginary dielectric constant for these metals is very high, but dependant on frequency. A typical value will be on the order of ˜1×10 8 .  
         [0034]     The corresponding real dielectric constant (ε′) is indeterminate or at least very hard to measure. This property is a measure of the interaction of an electric field with the material. However, materials with high conductivity also have a very high attenuation. Skin depth is a measure of the penetration of electromagnetic energy into a material, and for copper is given as 
 
δ S =6.6 f   −1/2  cm 
 
 or about 0.6 microns at 10 GHz. Thus a highly conductive material has insufficient penetration of energy for ordinary measurements to be valid. For theoretical purposes the complex dielectric constant of conductors is often taken to be ε=1+1×10 8 i. 
 
         [0035]     The materials described in this disclosure are intermediate between the two extremes (i.e., non-conductors and those materials that are highly conductive) mentioned above. The imaginary dielectric constant is significantly greater than zero, and significantly less than 1×10 6 , at microwave frequencies. At DC, or low AC, the conductivity (and thus ε″) of these materials is essentially that of a non-conductor, about zero.  
         [0036]     The difference between DC and microwave dielectric constants is determined by the structure of the material. A unique aspect to an embodiment of the present invention includes materials that are significantly polarizable under the influence of an electric field thus yielding a measurable conductivity at microwave frequencies despite the fact that they are fundamentally non-conductors at DC. For example, composites made of a polymer mixed with conductive inclusions (powders, fibers, etc) may be non-conductive at DC or low frequencies. However, at higher frequencies, the induced polarization, switching orientation at the microwave frequencies, can lead to electromagnetic effects (such as loss or reflection) that are realized in terms of conductivity or imaginary dielectric constant.  
         [0037]     This effect is sometimes referred to as electromagnetic percolation. It is known to those skilled in the art that when the loading of conductive particles is increased from a low value to a high value the dielectric constant varies in a complicated manner. Initially, a slightly loaded composite has a low value of ε′ and ε″ near zero, as typical for a non-conducting polymer. Increasing the loading a small amount tends to increase ε′ while leaving ε″ unchanged, or increasing very slightly. At higher loadings still, both values continue to increase, but ε″ begins to increase faster, as the increasing polarizability of the composite begins to generate significant RF conductivity. The point at which the magnitude of the real and imaginary dielectric constants are equal is often termed the “critical loading” or the “percolation threshold”. The percolation threshold is the situation where a non-conductive matrix (in this case the fabric) has enough metallic inclusions (in this case the plated metal) that it begins to take on the large scale properties of a conductor. Conventionally, the percolation threshold is defined as the point when the real and imaginary components of the dielectric constant are approximately equal within about 10%.  
         [0038]     When loadings are higher still, the imaginary dielectric constant will take on very large values. This is taken to indicate the formation of conductive interactions between the metal particles. At very high loadings these particles can form a somewhat continuous network and yield values of ε″&gt;1000. Conductivity at DC can be achieved at the very highest metallic loadings when the metallic particle density is so high as to be continuous in all directions. At this point the microwave ε″ is much greater than 1000. At and near the percolation threshold, these materials have unique dielectric properties that are useful in electromagnetic applications.  
         [0039]     This concept is best illustrated by  FIG. 2  which illustrates the resistivity in Ohms/Sq. as plating time increases, in the process mentioned above. It is important to note that the experimentation conducted to achieve this result is different from the experiments conducted to achieve the results illustrated in  FIGS. 3-7 . Specifically, an embodiment of the present invention includes materials whose resistive properties decrease as plating time increases. The materials that are of interest with respect to the present invention are those that lie within the dotted lines of the graph as depicted in  FIG. 2 . Thus, the graph illustrates the concept that resistivity decreases as plating time increases. By contrast the resistivity of copper is on the order of 10 −8  Ohm.M.  
         [0040]     In a process used to achieve an embodiment of the present invention, an artificial dielectric material was fabricated conventionally from an organic matrix that can be a cloth or a fabric composed of common textile materials selected from natural materials such as cotton, wool, hemp, jute and synthetic material such as polybutadiene polyester, acrylics, and the like. Hereinafter, fabric will be used to denote the organic matrix, be it a cloth or a fabric, woven or non-woven, and can be composed of any of the common textile materials. In the example given herein, the cotton fabric includes a white Workhorse brand Manufactured Rags (Kimberly Clark). This cotton fabric is described as a high pulp content non-woven composite fabric.  
         [0041]     The fabric was first rinsed in water for about a quarter of an hour in order to hydrate the fibers and remove any loose or soluble matter that was present.  
         [0042]     A commercial tin-palladium catalyst was then used to sensitize the fabric to the metal plating bath. In this case, the catalyst was Shipley Cataposit 44 and Cataprep 404. The amounts used followed the manufacturer&#39;s recommendation of 270 g/l for the solid Cataprep 404 and for liquid Cataposit 44, the final concentration of 0.01% by volume was used. Cataprep 404 can be used at concentrations of 50-300 g/l whereas Cataposit 44 can be used at concentrations of 0.001-2.0%.  
         [0043]     The fabric was agitated in the catalyst aqueous solution for a quarter of an hour during which time, the fabric changed in color from white to brown, i.e. the color of the palladium catalyst, indicating that the palladium catalyst was bound to the fibers of the fabric. The fabric was then rinsed with water to remove excess catalyst solution.  
         [0044]     The fabric may be metallized with any plating bath, according to manufacturer&#39;s instructions. In this example, the plating bath was Shipley Cuposit 328 which was a multi-part aqueous solution for copper plating. The plating bath may be heated, according to the manufacturer&#39;s instructions, but it was found that plating at room temperature resulted in slower plating and allowed greater control over the level of plating.  
         [0045]     Continuing with the procedure, the fabric was immersed in the plating bath and allowed to react for an amount of time appropriate for the level of metallization required. In this example, different samples were plated for 5, 10, 15, 20 and 25 minutes, yielding fabrics with low to high dielectric properties.  
         [0046]     To terminate the plating reaction, the fabric was immersed in a large volume of water and then rinsed to remove residual plating bath. It was then air dried, with or without heating.  
         [0047]     If desired, the fabric may be formed into a composite by the addition of an epoxy coating or other polymer treatment to yield rigidity or other mechanical properties.  
         [0048]     The materials employed in an embodiment of the present invention have the electromagnetic properties of a percolation system but without the use of a non-conductive polymer and a conductive filler. Instead a non-conductive textile fabric is partially metallized with a conductive metal to yield the appropriate dielectric constants. In contrast to a traditional percolating system, which is composed of conductive particles formed into a network within a non-conducting polymer; this system uses a pre-formed non-conducting network based on a textile fabric and adds metal to it in a partial, incomplete fashion.  
         [0049]     Between the onset of metallization and the onset of DC conductivity the materials properties (i.e. dielectric constant) of the product will be similar to those of a percolating system. Data presented with the disclosure show a range of samples, plated between 5 minutes and 25 minutes under the conditions described. The data show how the complex dielectric constant varies with increasing plating time, and also demonstrates such phenomena as frequency dispersion (change in dielectric with frequency).  
         [0050]     The benefit of this disclosure lies in the properties of the resultant fabric product. The table below, i.e., Table 1, summarizes the dielectric properties of the samples in the example above at the frequency range of 2 Mhz-20 GHZ.  
                                                                       TABLE 1                           Plating   Dielectric Constant   Percolation   Frequency            Time   Real   Imaginary   Threshold   Dispersion                    5   ˜2-5   ˜0   below   low       10   ˜4-6   ˜0.5-1.0   below   minor       15    ˜7-15   ˜5-7   near   strong       20    ˜50-˜25    ˜75-300   above   strong       25   &lt;˜50     ˜250-˜&gt;1000   above   strong                  
 
         [0051]     Measurements of dielectric constant as a function of frequency over the range of 2 MHz-20 GHz is shown in  FIGS. 3-7  for plating times of 5-25 minutes. It should be noted that  FIG. 7  shows dielectric constant variation with frequency for a material with a negative real dielectric constant. This is an example of a so-called “left-handed” material. By classical theory, negative dielectric constants are impossible. However, recently several approaches to this class of material have been presented, and are of interest for various applications including employing left-handed materials to manufacture perfect lens and other products.  
         [0052]     The examples summarized in Table 1, above, show the expected result for a percolating system that as the amount of metal increases, both the real and imaginary dielectric constants increase until the threshold at which the imaginary value rises dramatically while the real value decreases.  
         [0053]     For purposes herein, useful dielectric constants are estimated to be in the range of 1-1000, typically 1-50, for the real dielectric constants and 0-1000, typically 0-50, for the imaginary dielectric constants over the microwave frequency range of 2 MHz-100 GHz. Thickness of the metallic coating is expected to be in the range of 0.05-50 microns.  
         [0054]     Frequency dispersion is a measure of the change in dielectric constant with frequency. Most materials retain a dielectric constant that does not change across a frequency range. The materials described here demonstrate a variable dielectric constant over the range tested. The importance of this is that for an insulator/absorber of microwave energy to function at different frequencies (i.e., to be broadband), the optimal dielectric constant is different at each frequency. Hence, with this material, it is possible to design higher performance electromagnetic composites. Optimal dielectric constant for a particular frequency can be determined by trial and error.  
         [0055]     The fact that dielectric constant varies with frequency allows insulating or attenuating effects to function over broader range of frequencies. This should be understood in the context of using multiple coatings each imparting a different dielectric constant that is effective for energy absorbance at a different frequency.  
         [0056]     It is known from electromagnetic theory that optimal absorbance over a broad range of frequency is achieved with appropriate materials having a dielectric constant as a function of frequency. For best performance, the real component of the dielectric constant should vary as an inverse proportion to the square of the frequency, while the imaginary dielectric constant should vary as a simple inverse proportion to the frequency.  
         [0057]     Shielding from radar or antennae isolation are principal concerns for the artificial dielectrics of this invention which involve wave reflection or attenuation. One way to provide for antireflection is to provide a coating on a structure, for example an aircraft, which would produce at least two reflections of which, one reflection would be off the structure and the second reflection would be off the coating. Cancellation of the two waves causing the reflections is possible only if the waves are 180° out of phase. Thus the waves cancel each other out and in theory, the result can be a zero reflection. However, in order to observe this 180° out of phase reflection, the spacing between the reflecting structure and the coating is typically odd multiples of ¼ wavelength of the impinging energy.  
         [0058]     Although a typical microwave wavelength is about 3 centimeter, ¼ thereof is about 0.8 cm which is considerable and impractical cancellation spacing. However, in a dielectric material, a wavelength is able to shrink allowing for wave cancellation and essentially zero reflection. For instance, if wavelength of 10 GHz radiation is 3 cm, its wavelength in a dielectric medium with a dielectric constant of 2 would be 2.1 cm; in a dielectric medium with a dielectric constant of 5, the wavelength would be 1.3 cm: for a medium with a dielectric constant of 10, the wavelength would be 9.5 mm; and for a medium with a dielectric constant of 25, the wavelength would be 6 mm. Thus, in a situation where maximum cancellation is desired, matrix dielectric constant is adjusted, as by matrix material selection, and thickness, and other adjustments are made in order to achieve the desired result.  
         [0059]     Rather than plating uniformly across the fabric, it is possible to plate non-uniformly. Plating also can be limited to only one side of the fabric. Plating can be carried out in such a manner as to create a gradient of dielectric properties across the length or breadth of the fabric. It is also possible to pattern the fabric by various techniques and a complex geometric pattern of dielectric properties can thus be created.  
         [0060]     Artificial dielectrics as represented by an embodiment of the present invention can be used in wearable antenna applications, for example. In this application radar shielding is a major concern and this intention has shown promise in gain enhancement and radiation hazard reduction and particularly in antenna isolation or shielding.  
         [0061]     While the above embodiments have been shown of the novel artificial dielecrics, and of the several modifications thereof, persons skilled in this art will readily appreciate that various additional changes and modifications can be made without departing from the spirit of the invention, as defined and differentiated by the following claims.