ELECTRO-MAGNETIC WAVE ABSORBING MATERIAL

Abstract: An electro-magnetic wave absorbing member comprises a mixture of short fibers (2) of metal or alloy having a high elec­ troconductivity, a length of 0.1 mm to 50 mm and a length to diameter ratio larger than 10 ferrite and a high molecular weight synthetic resin. In a modification a sheet of this material is used as a low input impedance layer (13) and laminated with an electro-conductive sheet (12) and an impedance conver­ sion layer (14). The member according to this invention can in­ crease electrical loss contributing to the absorption of electro-­ magnetive waves, can stably control the absorption and can be used in many applications.

Description: The invention relates to an electro-magnet wave absorbing body comprising a metal plate and an electro-magnetic wave absorbing layer containing a mixture of a high molecular synthetic resin and a ferromagnetic material, said layer being formed onto said metal plate. Such bodies are used for preventing ghosts of iron towers, ships, etc. in radar systems, and for preventing generation of unwanted electric wave and dispersion thereof. As disclosed in JP-A-55-36987, a prior art electro-magnetic wave absorbing material was constituted by a mixture of ferrite, carbon black and a high molecular weight synthetic resin. Such material absorbs electro-magnetic wave energy by the magnetic loss caused by ferrite and electric loss caused by carbon black. Accordingly such electro-magnetic wave absorbing material is coated or deposited on the surface of a metal or electro-conductive sheet to a specific thickness so as to form an electro-magnetic wave absorber. The capability of absorbing electric waves of such an absorber, however,differs depending upon the frequency and intensity of the electric wave to be absorbed so that it has been desired to provide improved electro-magnetic wave absorbing material or member having a high absorbing efficiency. In order to increase the loss of the electro-magnetic wave imparted by the absorbing material described above, it is necessary to increase the quantities of incorporation of ferrite and carbon. However, even when the quantity of ferrite is increased there is a limit for the magnetic loss, while carbon black can increase electric loss, its function becomes unstable due to variations in electric conductivity and particle size. As is well known in the art, the electro-magnetic wave absorption capability of the electric wave absorbing member described above has the maximum absorption efficiency when the input impedance Z as measured from the surface of a layer of the electro-magnetic wave absorption material is unity, where where ε ˙₂ represents a complex dielectric constant of the absorption layer, µ ˙₂ a complex permeability, λ the wavelength and d the thickness of the absorption layer. However, the frequency bandwidth in which a condition Z = 1 bolds is very narrow. For this reason, it has been difficult to obtain satisfactory electro-magnetic wave absorption characteristic over a wide frequency bandwidth. Document EP-A-0 043 040 discloses a compound material for shielding electromagnetic radiation. This conventional compound material, in the polymer matrix of which metallized textile surface formations and/or metallized particles are contained and in which additive substances for increasing the ohmic, electric and/or magnetic losses are uniformly present in the polymer and/or on the textile surface formations and/or the particles, is intended for a high shielding effect for electromagnetic radiation in a wide frequency area. Instead of short fibers made of a metal or an alloy of a specific group, textile fibers are used which are provided with a coating of a highly conducting metal layer having a thickness of 0,05 to 20 µm. Concerning the textile fibers, monofilaments are mentioned having a length and a thickness in specific ranges, however, this document is silent as to any specific aspect ratio defined by the ratio between length and diameter of the fibers. In the document EP-A-0 043 040 textile fibers are used which are covered with a thin coating of a metallization consisting of nickel which must be deposited by a chemical process to obtain the desired metal layer of 0,05 to 20 µm in thickness. But the document is silent as to other suitable metals for such a coating, the use of specific ferromagnetic materials, the use of ferrodielectric substances and any specific weight percentages for the short fibers. Document DE-B-2 151 349 deals with a camouflage sheet comprising a single thin radar camouflage layer containing electrically conducting material. The purpose and the object described there is to provide for a flexible radar camouflage sheet which may be rolled up and by means of which it should be possible to obtain approximately the same reflection percentage as that of the camouflaged target over the whole wave length range of 1 to 15 cm. For this purpose, a thin, inhomogeneous and electrically conducting material is used, the thickness of which should be low in comparison with the shortest wave length in question. Also, specific values must be met as to the surface resistance and the reflection. The aspect of using specific fibers having specific dimensions is not mentioned in this publication. Accordingly, the object underlying this invention is to provide a novel electro-magnetic wave absorbing body that can increase electric loss contributing to the absorption of electro-magnetic waves, can stably control the loss and can widen the field of use of such material and body in order to obtain an efficient absorption of electro-magnetic waves over a wide frequency range. According to this invention the electro-magnetic wave absorbing body is characterized in that said electro-magnetic wave absorbing layer includes a low input impedance layer formed on said metal plate, said low input impedance layer being composed of said mixture of a high molecular synthetic resin and a ferromagnetic material, and comprises short fibers made of a metal or an alloy which is selected from the group consisting of Au, Ag, Cu, Al, Zn, Ni, Fe and alloys thereof and has a high electrical conductivity, in that each of said fibers has a length L of 0,1 mm to 50 mm and an aspect ratio L/D larger than 10, wherein D is the diameter of said short fibers, and in that an impedance transforming layer is formed on said low input impedance layer. A further development of the absorbing body according to the invention is characterized in that said impedance transforming layer is made of a high molecular synthetic resin. A still further development of the absorbing body according to the invention is characterized in that said impedance transforming layer comprises a mixture of a high molecular synthetic resin and either one or both of a magnetic material and high electro-conductive short fibers. A still further development of the absorbing body according to the invention is characterized in that said magnetic material comprises a ferromagnetic material selected from the group consisting of ferrite, iron, cobalt, nickel and mixtures thereof. A still further development of the absorbing body according to the invention is characterized in that said electro-conductive material comprises at least one of electro-conductive metals, alloys of metals and carbon. The inventors have found that a considerable change of the dielectric constant of the material can be expected in micro wave frequency bands when using the electro-magnetic wave absorbing material and body, respectively, according to the invention. Corresponding experiments led to excellent results when using such absorbing material and body as defined above so that the desired electro-magnetic wave absorption could be obtained over a wide frequency range. The invention will be explained in detail by means of non-limitative examples and with reference to the drawings wherein: Fig. 1 is a diagrammatic representation showing the construction of the electro-magnetic wave absorbing material of this invention; Fig. 2 is a graph showing the relationship among the quantity of short brass fibers a real part (Re(ε ˙)) and an imaginary part (lm(ε ˙)) of the complex dielectric constant; Fig. 3 is a graph showing the relationship among the quantity of short brass fibers and frequency characteristics of the complex permeability (Re(µ ˙), lm(µ ˙)); Fig. 4 is a partial sectional view showing a laminated electro-magnetic wave absorbing body embodying the invention; Fig. 5 is a Smith Chart showing an input impedance characteristic measured from above a low input impedance layer formed on a metal sheet and an input impedance characteristic measured from above an impedance transforming layer laminated on the low input impedance layer; Fig. 6 shows a reflection loss characteristic obtained by converting characteristic B shown in Fig. 5, and a reflection loss characteristic of a prior art electro-magnetic wave absorbing body; Fig. 7 is a graph comparing the frequency bandwidths of a low input impedance layer and an impedance transforming layer manifesting a magnetic loss; Fig. 8 and 9 show the input impedance characteristic and the reflection loss characteristic of only the low input impedance layer of the Example 1 to be described later; Fig. 10 and 11 show the input impedance characteristic and the reflection loss characteristic of only the low input impedance layer of Example 2; Figs. 12 and 13 show the input impedance characteristic and the reflection loss characteristic of only the low input impedance layer of Example 3; and Figs. 14 and 15 show the input impedance characteristic of the low input impedance layer alone and the reflection loss characteristic of Example 4. In a preferred embodiment shown in Fig. 1, into a mixture 1 of ferrite and a high molecular synthetic resin or a mixture of ferrite, a high molecular synthetic resin and carbon black are incorporated short fibers 2 of metal. Preferably each of the metal fibers 2 have a length (L) of 0.1-50 mm, and a length (L) to diameter (D) ratio L/D of larger than 10. The quantity of the metal fibers 2 to be incorporated should be larger than 3% by weight. The metal should have a high electric conductivity, such as Au, Ag, Cu, Al, Zn, Ni, Fe, or alloys thereof. The ferrite may be also substituted by a ferromagnetic material such as iron, cobalt and nickel. The electric wave characteristic of the electro-magnetic wave absorbing material of this invention will be described as follows. Fig. 2 shows complex dielectric constant [ε ˙=Re(ε ˙)-j lm(ε ˙)] of a composition comprising epoxide resin, iron ferrite (Fe₃O₄) and short fibers of brass measured at a frequency of 9.4 GHz. Each of short brass fibers have a length L of about 2 mm a diameter (D) of about 50 microns, and a ratio L/D of about 40. The graph shown in Fig. 2 shows that the incorporation of the short brass fibers increases Re(ε ˙) and lm(ε ˙) to greatly vary electric characteristics. This reason can be qualitatively explained as follows. When a short metal fiber is placed in a electromagnetic field, the metal fiber is polarized by the electric field component. Since the extent of polarization can be obtained by a product of the electric charge quantities separated at both ends of the short metal fiber and the length of the fiber, a thin long metal fiber is suitable for efficiently creating polarization. Where a plurality of metal short fibers is irregularly present in the electro-magnetic field, an average polarization effect of respective fibers appears. Accordingly, an electro-magnetic wave absorbing material containing a plurality of short metal fibers can be equivalently considered as a material having a large polarization, thus making large the real part Re(ε ˙) of the complex dielectric constant representing the magnitude of the polarization. Where a plurality of layers of the electric wave absorbing material each having a thickness substantially equal to the wavelength of the electric wave are laminated for use in a microwave frequency band, the overall thickness of the lamination is in a range of 0.1 mm to 50 mm. Preferably the length of the fibers is in a range of 0.1 to 50 mm. Since the fiber should have a thin and elongated configuration, the ratio L/D should be larger than 10. While in the foregoing, the real part Re(ε ˙) of the complex dielectric constant has been described, it is considered that the imaginary part lm(ε ˙) is influenced by the variation of the electric conductivity caused by the incorporation of the short metal fibers. More particularly, incorporation of the short metal fibers increases conductive current, hence electric loss, with the result that the imaginary part lm(ε ˙) of the complex dielectric constant representing the loss increases. For this reason, for the purpose of efficiently increasing the imaginary part lm(ε ˙) of the complex dielectric constant it is necessary to use short metal fibers made of metal or alloy having a large electric conductivity. The characteristics shown in Fig. 2 can be reproduced at a high fidelity as long as the same type of short metal fibers are used. Where metals of different type or fibers having different dimensions are used it is necessary to clarify the relationship among the quantity of the short metal fibers incorporated, the real part Re(ε ˙) and the imaginary part lm(ε ˙) of the complex dielectric constant. The magnetic characteristic will be discussed as follows. Since the electro-magnetic wave absorbing material of this invention contains ferrite, it can manifest a magnetic loss. Fig. 3 shows the result of measurement of the complex dielectric constant (measured at a frequency of 1 GHz-10 GHz) of the electro-magnetic wave absorbing members respectively consisting of epoxide resin, 70% by weight of iron ferrite (Fe₃O₄) and 0%, 5% and 15% by weight of short brass fibers. Even when the quantity of incorporation of short brass fibers is varied in the range of 015% by weight, both the real part Re(µ ˙) and the imaginary part lm(µ ˙) of the dielectric constant do not vary in any appreciable extent. As described above the magnetic characteristic of the material is never influenced by the incorporation of the short metal fibers and the magnetic characteristic is governed mainly by the magnetic characteristic of the ferrite. As described above, since the electric wave absorption material according to this invention is incorporated with short metal fibers not only the electric characteristic can be greatly and stably adjusted but a magnetic loss can be also increased. The type of the high molecular synthetic resin is not determined by the wave characteristic to be absorbed but rather determined by such environment conditions as weather proofness, heat resistant and shock resistant properties. In this invention, it is not necessary to use any specific resin, where the electro-magnetic wave absorbing material is applied by blasting, epoxy resin is preferred. However, when the material is applied onto a curved surface flexible rubber is advantageous. The electro-magnetic wave absorbing material can be laminated to form an electro-magnetic wave absorbing body as shown in JP-A-58010902. Moreover, since it has a large loss for the electric wave, it can be used as an electro-magnetic shield member. Some embodiments of the electric wave absorbing body using the electro-magnetic wave absorbing material characterized by the present invention will be explained as follows. The electric wave absorbing body shown in Fig. 4 comprises a low input impedance layer 13 formed on an electro-conductive sheet or plate transforming layer 14 formed on the low input impedance layer 13. The electro-conductive plate 12 constitutes a conductor portion of the basic member to be coated with the electro-magnetic wave absorbing material. The term "low input impedance layer" means a layer having a characteristic wherein the input impedance characteristic is located near the left end of a Smith Chart as shown at A in Fig. 5. Although this layer 13 manifests a large loss for electric wave, when it is merely applied onto an electro-conductive sheet nearly all electric waves are reflected by the surface ofthe layer 13 thus failing to provide a satisfactory electro-magnetic wave absorbing body. According to this embodiment, for the purpose of efficiently transmitting electromagnetic wave to the low input impedance layer 13, the impedance transforming layer 14 is provided as shown in Fig. 4 which operates in the same manner as a 1/4 wavelength impedance transformer utilized in microwave technology so that the layer 14 is designed in the same manner as the 1/4 wavelength impedance transformer. Let us denote by R the value of the real part at a frequency fo at which the imaginary part becomes zero. Then the impedance transforming layer 14 acts as an impedance transformer when the surge impedance of the layer 14 is made Zw= R and when the thickness of the layer 14 is made to be 1/4 of the wavelength at the frequency fo. Ordinary materials have dielectric constants of larger than 2 and their surge impedance is expressed by Zw= µ / ε . From the foregoing relation it can be understood that R must be less than 0.5. Where such impedance transforming layer 14 is provided the input impedance characteristic as seen from its surface becomes that shown by B in Fig. 5. When this characteristic is expressed in terms of reflection loss, a characteristic as shown by C in Fig. 6 can be obtained. Curve D shown in Fig. 6 is the characteristic of the prior art electro-magnetic wave absorbing body without including short metal fibers characterized by the present invention. From the characteristics C and D shown in Fig. 6, it will be seen that by utilizing the electro-magnetic wave absorbing material according to this invention, the frequency bandwidth in which the reflection loss becomes more than 20dB is widened. In Fig. 5 numerals 8 and 13 show the characteristics at frequencies 8 GHz-13 GHz. The low input impedance characteristic can also be improved by further adding a ferrodielectric substance for an electro-conductive substance. Typical ferrodielectric substances include barium titanate, lead titanate, strontium titanate, lead neobate, and lead zirconate. The electro-magnetic wave absorption characteristic of the electro-magnetic wave absorbing material of this invention is as follows. In order to efficiently absorb electro-magnetic wave, it is essential to form the low impedance layer with the high molecular weight resin and material having a high electric conductivity as can be clearly understood from the foregoing description. We have found that it is necessary to incorporate into the low impedance layer magnetic substance that impart magnetic loss to the low impedance layer in order to realize an excellent electro-magnetic wave absorption characteristic over a wide frequency bandwidth. Curve A shown in Fig. 5 shows the input impedance characteristic where there is a magnetic loss while curve A′ shows the input impedance characteristic where there is no magnetic loss. In both characteristics A and A′, the value R of the real part at a frequency at which the imaginary part becomes zero is about 0.3, but at a frequency of 8-13 GHz, the characteristics become different so that the electro-magnetic wave absorbing characteristics where the electro-magnetic wave absorbing body is fabricated by laminating the impedance transforming layer 14 become also different. The results of calculations of the low input impedance transforming layers 13 incorporated with magnetic loss and not are shown by curves E and F in Fig. 7 which shows the relation between a value obtained by dividing a frequency bandwidth in which a reflection loss of more than 20 dB occurs with the center frequency of the bandwidth, and the input impedance R. As can be clearly noted by comparing characteristics E and F shown in Fig. 7 the characteristic F having magnetic loss has a wider frequency band characteristic. For this reason, it is desirable for the electro-magnetic wave absorbing body to incorporate magnetic loss in the low input impedance layer 13. In Fig. 7, the relation of magnetic loss between the low impedance layer 13 and the impedance transforming layer 14 is as follows: Considering the impedance transforming layer 14, when the input impedance value of the low input impedance layer 13 is expressed by R its surge impedance must be Zw= R . Consequently, depending upon the value of R even a high molecular resin itself may be used or for the purpose of fulfilling the conditions described above magnetic substance in the form of particles, foils or short fibers is incorporated into the high molecular synthetic resin. By combining the high molecular synthetic resin and the magnetic material an electro-magnetic wave absorbing body having desired impedance conversion capability and electric loss can be provided. As a result of various calculations, we have found that impartion of magnetic loss is effective to widen the frequency band. Characteristics F and G shown in Fig. 7 show this fact, in which characteristic G has a wider frequency bandwidth. Accordingly, in the electro-magnetic wave absorbing body of this invention it is desirable to impart magnetic loss to the impedance transforming layer 14. Magnetic loss can be imparted by incorporating the same material that imparts magnetic loss to the low input impedance. Thermoplastic resins and thermosetting resins can be used as the high molecular synthetic resins utilized to form low input impedance layer 13 and the impedance transforming layer 14 and even when either one of these resins are used the same electric wave characteristic can be obtained. A flexible electro-magnetic wave absorbing body is easy to handle. To this end, natural or synthetic rubber can be used. To have better understanding of this invention the following examples are given. Example 1 Polyethylene, ferrite and short brass fibers were mixed together at a weight ratio of 30:70:24, and a sheet of this mixture having a thickness of 1.0 mm was used as a low input impedance layer 13. A sheet having a thickness of 3.6 mm and containing polyethylene and ferrite at a weight ratio of 60:40 was used as an impedance transforming layer 14 to obtain an electro-magnetic wave absorbing body. Fig. 8 shows the input impedance characteristic of the low input impedance layer 13 alone, while Fig. 9 shows the reflection loss characteristic. Example 2 A polybutadiene resin, ferrite, and iron short fibers were admixed at a weight ratio of 30:70: 15 and a sheet of this mixture having a thickness of 0.9 mm was used as the low input impedance layer 13. A sheet having a thickness of 3.8 mm and consisting of a mixture of polybutadiene resin, and ferrite at a ratio of 70:30 was used as the low impedance transforming layer 14 to form an electro-magnetic wave absorbing body. Fig. 10 shows the input impedance characteristic of the low input impedance layer 13 alone, while Fig. 11 shows the reflection loss characteristic. Example 3 A polybutadiene resin and short iron fibers were mixed together at a weight ratio of 75:25 and the mixture was shaped into a sheet having a thickness of 1.4 mm. This sheet was used as a low input impedance layer 13 and a sheet of butadiene resin having a thickness of 4.6 mm was used as an impedance transforming layer 14 to form an electro-magnetic wave absorbing body. Fig. 12 shows the input impedance characteristic of the low input impedance layer 13 alone, while Fig. 13 shows the reflection loss characteristic. Example 4 Chloroprene rubber, ferrite, carbon and short brass fibers were admixed at a weight ratio of 30:70:5:10 and the mixture was formed into a sheet having a thickness of 1.0 mm. This sheet was used as a low input impedance layer 13. A sheet having a thickness of about 3.6 mm and consisting of a mixture of chloroprene rubber, ferrite and carbon at a ratio of 70:30:5 was used as an impedance transforming layer 14 to obtain an electro-magnetic wave absorbing body. Fig. 14 shows the input impedance characteristic of the low input impedance layer 13 alone, and Fig. 15 shows a reflection loss characteristic.

Claims: An electro-magnetic wave absorbing body comprising a metal plate (12) and an electro-magnetic wave absorbing layer containing a mixture of a high molecular synthetic resin and a ferromagnetic material, said layer being formed onto said metal plate (12), characterized in that said electro-magnetic wave absorbing layer includes a low input impedance layer (13) formed on said metal plate (12), said low input impedance layer being composed of said mixture of a high molecular synthetic resin and a ferromagnetic material, and comprises short fibers (2) made of a metal or an alloy which is selected from the group consisting of Au, Ag, Cu, Al, Zn, Ni, Fe and alloys thereof and has a high electrical conductivity, in that each of said fibers (2) has a length (L) of 0,1 mm to 50 mm and an aspect ratio (L/D) larger than 10, wherein D is the diameter of said short fibers (2), and in that an impedance transforming layer (14) is formed on said low input impedance layer (13). The electro-magnetic wave absorbing body according to claim 1, characterized in that said impedance transforming layer (14) is made of a high molecular synthetic resin. The electro-magnetic wave absorbing body according to claim 1 or 2, characterized in that said impedance transforming layer (14) comprises a mixture of a high molecular synthetic resin and either one or both of a magnetic material and high electro-conductive short fibers. The electro-magnetic wave absorbing body according to claim 3, characterized in that said magnetic material comprises a ferromagnetic material selected from the group consisting of ferrite, iron, cobalt, nickel and mixtures thereof. The electro-magnetic wave absorbing body according to claim 3 or 4, characterized in that said electro-conductive material comprises at least one of electro-conductive metals, alloys of metals and carbon.

Inventors: HATAKEYAMA, KENICHI, INUI, TETSUJI
Applicants: NEC CORPORATION
Application year: 1983
Main classification: H05K9
IPC codes: C08K3, H05K9, C08K7, F41H3