Patent Number: 055524559
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the field of radar absorbing coatings and, in particular, to an improved coating incorporating iron particles. 2. Description of Related Art Typical radar absorbing material (RAM) coatings incorporate iron particles in a resin that is either spray painted on the surface of the vehicle or applied thereon in the form of decals. The iron particles can also be incorporated into a ceramic matrix material. For example, U.S. Pat. Nos. 5,164,242 "Electromagnetic Wave Attenuating And Deicing Structure" by S. D. Webster, et. al, and 5,338,617 "Radio Frequency Absorbing Shield And Method" by D. M. Workinger, et. al. discloses the use of Carbonyl iron in a resin matrix, while U.S. Pat. No. 5,085,931 "Microwave Absorber Employing Acicular Magnetic Metallic Filaments" by C. E. Boyer, et al. discloses the use of filaments having an average length of 10 microns and diameters of about 0.1 micron. for use in an absorber. U.S. Pat. No. 4,003,840 "Microwave Absorber" by K. Iishino, et. al. suggests 1.65 mm ferrite powder in an organic high molecular compound; for example 0.2 to 0.9 part by volume ferrite powder and 0.8 to 0.1 organic high molecular compound. U.S. Pat. No. 3,568,195 "Electromagnetic Wave Attenuating Device" by L. Wesch, et. al. discloses an absorber comprising an outer radar wave attenuating layer that can incorporate iron powders and a non-metallic backing sheet. In a good light weight specular RAM coating high attenuation level and broad frequency range are important. However, with such coatings peak attenuation band width decreases with decreasing frequency and causes attenuation at frequencies other than the peak attenuation frequencies to be less than 5 dB. One common technique to improve the broad band response of a specular RAM is to use multiple coatings separated by some kind of a band pass filter. For example in U.S. Pat. Nos. 5,169,713 "High Frequency Electromagnetic Radiation Absorbent Coating Comprising A Binder And Chips From A Laminate Of Alternating Amorphous Magnetic Films And Electrically Insulating" by P. Kmurdjian. Kmurdjian discloses the use of multiple layers having a thickness in the 2-5 nanometer range, with each layer including an amorphous magnetic film and an insulating film of 1-5 electrically insulating material. In U.S. Pat. No. 4,581,284 "Fiber Compound Material" by D. Ggumbh a structure is disclosed made of fiber plies impregnated with a radar absorbing compounds in a concentration varying from the exterior to the interior side. U.S. Pat. No. 5,147,718 "Radar Absorber" by S. A Papoulias, et. al. discloses the use of a multi-layer absorber having a first layer with 4 to 5 micron carbonyl iron powder and a second layer with 0.5 to 1.5 micron powder. The inventor claims that such an absorber provides a relatively high radar attenuation magnitude over a selected broad band frequency range. U.S. Pat. No. 4,024,318 "Metal-Filled Plastic Material" by E. O. Forster, et. al. discloses the use of a multi-layer material wherein the first layer is filed with metal particles in a resin matrix and a second contains metal oxides in a resin matrix. However, such multiple layer absorbers have weak shear planes between layers, are expensive and, additionally, create field maintenance problems. A problem of both single and multiple coating is their high unit weight. The performance of these coatings, particularly those using spherical particles, is dependent upon how closely the spheres are packed together. Thus the most efficient coating would be one approaching the density of solid iron with a minimum amount of resin included to electrically insulate the particles from one another. That is, the attenuation efficiency increases faster than the weight, so that a thinner coating with the same attenuation, can be used, providing an overall weight savings. Unfortunately, the particles, when produced, are of non-uniform diameter and not necessarily uniformly round. Even with filtering for size or centrifugal particle separation methods, a Gaussian distribution about the selected diameter occurs. Thus the best packing densities are around 4.5 grams per cubic centimeter for 5 micron diameter particles, when 5.7 grams per cubic centimeter could be obtained if all the particles were of exactly one diameter. Thus it is a primary object of the subject invention to provide an improved radar absorbing material. It is another primary object of the subject invention to provide an improved radar absorbing material that is lighter in weight than conventional absorbers having equal performance. It is a further object of the subject invention to provide an improved single layer radar absorbing material that is lighter in weight than conventional absorbers having equal performance. It is a still further object of the subject invention to provide an improved radar absorbing material that has a greater packing density when the spheres of magnetic material are distributed about a mean diameter. SUMMARY OF THE INVENTION The invention is a RAM coating and a process for making the coating. In detail, the coating includes a binder material that can be a resin or ceramic material containing a mixture of two groups of spheres made of a magnetic material. The spheres of the first group have a specific average diameter and the spheres of the second group have an average diameter generally 0.73 times the specific average diameter of the spheres of the first group. The first and second groups contain generally equal numbers of spheres and the amount of the binder material is just sufficient to bind the mixture together while maintaining the individual spheres separated from each other. In most applications, the average diameter of the first group of spheres should be about 5 microns. In detail, the process for the manufacture of a radar absorbing material comprising the steps of: 1. providing a first group of spheres made of a magnetic material; PA1 2. providing a second group of spheres made of a magnetic material containing a number of spheres equal to the number of spheres of the first group with an average diameter of generally 0.73 times the average diameter of the spheres of the first group; PA1 3. mixing the first and second groups of spheres together forming a mixture; PA1 4. mixing an amount of binder material to the mixture sufficient to bind the mixture together while maintaining the individual spheres separated from each other; and PA1 5. solidifying the ceramic or resin binder material. However, precise particle sizes are unavailable from suppliers; they are more in the form of a Gaussian distribution. Thus, upon receipt of various quantities and sizes of spherical iron particles from suppliers, they are sorted by separators into specific size cuts. Particle size distribution is measured on the sized iron and calculations are made to control the number of large and small particles using a weight basis and the measured particle size distribution. Appropriate amounts of sizes of iron particles are mixed together and measurements are made of their tap density and true density. The measured tap and true densities of the iron particles and the true density of the binder are used to calculate how much matrix binder is required to attain a given theoretical percolation factor. The percolation factor is defined as the volume of all particles when optimally packed divided by the volume of particles and binder after the RAM coating cures and optimal packing occurs when all particles touch and therefore occupy a minimum volume. Ideally, the procedure to determine the weights of particles that must be mixed to get optimum packing assumes two groups of perfect uni-size particles with the smaller diameter group having a diameter that is 0.73 times the larger diameter group particle size. Mixing an equal number of particles is accomplished by calculating the weights of large and small particles. If one assumes that the material for the small and large particles are the same, and therefore have the same density, the weight ratio is a function of only the cube of the radius, or 2.5707. This means that 2.5706 pounds of large diameter sorted material must be mixed with one pound of small diameter sorted material to get equal numbers of particles with a size ratio of 1 to 0.73 in the resultant mix. However, iron particles available from suppliers have a distribution that typically varies from less than one micron to over ten microns in size. Even after the iron particles are separated by size, a Gaussian distribution exists for each size. Mixing these Gaussian distribution size separated materials using the 2.5707 weight ratio may not provide optimum or repeatable results. This requires that the small and large particle size distributions be measured so that a "best" fit can be used to determine the optimum weight ratios. Therefore, after separation, size distributions of the small diameter and large diameter size cuts are made by use of a particle size analyzer. The particle size analyzer output separates the range of particle sizes in the sample into mulitple segments and provides a minimum and maximu diameter and a volume percent per segment. The number of particles in a measured segment is calculated using an average particle radius and equating it to the segment radius. Calculations are made by assuming a unit volume of one cc and dividing it into fractions equal to the measured volume fractions. The number of particles in a given fraction is then calculated by dividing the fractional cc volume by the volume of one particle calculated by using the average measured diameter within the volume fraction. This process is repeated for all the fractions of each particle size, which are thereafter plotted. A visual technique is used to compare plots of the number of particles in the smaller diameter size cut to the number of particles in the larger diameter size cut. Before visual comparisons are performed the distribution of the number of particles in the size cuts must be normalized. The normalization is accomplished by multiplying the large particle sizes by 0.73 and displacing the original large diameter sort particle number distribution to lower diameters. The normalized particle number distribution curve of the larger diameter sort is visually compared to the non-normalized particle distribution curve of the smaller diameter sort. The normalized distribuition curve is multiplied by multiplicaton factors until a best "visual fit" between the two curves is obtained. Once the best fit is obtained, that multiplication factor is used to determine mixture ratio on a pound basis for mixing the large particles to the small particles in a similar manner the smaller diameter particle number distribution can be normalized by dividing its diameters by 0.73 and comparing the resultant curve to the non normalized particle number distribution curve of the larger diameter sort. Thereafter the binder, in the form of a resin (thermosetting or thermoplastic) or ceramic material, is added in the proper amount to the mixture of particles and solidified by curing or the like. In this step, the mixture of binder and particles maybe cast in a mold or formed into sheets. It may even be sprayed on to a surface as a coating. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.