Abstract:
Microwave absorber comprising a mixture of ferrite powder having particle size of less than 1.65 mm and an organic high molecular weight compound.

Description:
BACKGROUND OF THE INVENTION 
     It is generally known to those skilled in the art that ferrites absorb microwave such as those of 500 MHz to 12 GHz to change the microwave energy to a thermal energy. The ferrite is a sintered body having the spinel structure and it is a compound having the following general formula: 
     
         MFe.sub.2 O.sub.4 
    
     (wherein M is a divalent metal such as Mn, Ni, Cu, Zn, Mg, Co, etc.). 
     The use of a sintered body of the ferrite for preventing the leakage of microwave in &#34;microwave heating oven&#34; has been disclosed in U.S. Pat. No. 2,830,162. The microwave heating oven is a device for heating a material with microwaves generated by a microwave generating device such as magnetron. The microwave, however, can be absorbed more effectively by the powder of ferrite (ferrite powder) than the sintered body of ferrite. A mixture mainly composed of ferrite powder for use in absorbing the microwave has been disclosed in U.S. Pat. No. 3,742,176. In this patent, a mixture of ferrite powder and an insulating material such as rubber has been disclosed. The invention of this (U.S. Pat. No. 3,742,176) has been accomplished by the same inventor as one of the inventors of the present invention. The inventors of the present invention found that the effect of absorption of the microwave depends on both the frequency of microwave and the particle size of ferrite powder. Namely, the ferrite powder having a certain limited particle size can absorb more effectively microwave having a certain frequency. The present invention is based on the findings as set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a microwave absorber comprising a mixture of the ferrite powder having particle size of less than 1.65 mm and an organic high molecular compound. 
     Said microwave is a wave having a frequency of from 500 MHz to 12 GHz. Said ferrite is a ferrimagnetic material having the general formula: 
     
         MFe.sub.2 O.sub.4 
    
     wherein M is a divalent metal such as Mn, Ni, Cu, Zn, Mg, Co, etc. Said mixture comprises the ferrite powder and an insulating organic high molecular compound which are mixed in a ratio as shown below: 
     
         ______________________________________Ferrite powder  0.2-0.9 parts by volumeOrganic high molecular           0.8-0.1 part by volumecompound______________________________________ 
    
     As the insulating organic high molecular compound, the following resins and rubbers may be used: 
     Thermosetting resin such as phenol resin, polyester resin, epoxy resin and silicone resin; thermoplastic resin such as polyethylene, polypropylene and polyvinyl chloride; natural rubber and synthetic rubber such as polychloroprene, acrylonitrile-butadiene-styrene and fluorine-contained rubber. 
     The ferrite powder serves to absorb microwave. However, in order to form a shaped body of ferrite powder, the ferrite powder is mixed with an insulating organic high molecular weight compound. Namely, the organic high molecular compound is used as a binder of the ferrite powder. 
     Relations between particle sizes of ferrite powder and frequencies of microwave absorbed effectively by the ferrite powder are as follows: 
     1. In the event that the powders of ferrites having the initial permeability of more than 300 (the initial permeability is measured at the frequency of 10 kHz) are used; 
     
         ______________________________________Particle sizes of the              Frequencies of the micro-ferrite powder     wave absorbed effectively______________________________________1.65  mm    -     701   μ   500 MHz - 1.5 GHz701   μ  -     351   μ   1.0 GHz - 2.0 GHz351   μ  -     104   μ   1.8 GHz - 3.0 GHz104   μ  -     43    μ   2.5 GHz - 7.5 GHz&lt;43   μ                     6.0 GHz - 12.0 GHz______________________________________ 
    
     2. In the event that the powders of ferrites having the initial permeability of less than 300 (the initial permeability is measured at the frequency of 10 kHz) are used; 
     
         ______________________________________Particle sizes of the              Frequencies of the micro-ferrite powder     wave absorbed effectively______________________________________1.65  mm    -     701   μ   1.0 GHz - 3.0 GHz701   μ  -     351   μ   2.0 GHz - 4.5 GHz351   μ  -     104   μ   4.0 GHz - 6.0 GHz104   μ  -     43    μ   5.0 GHz - 7.5 GHz&lt;43   μ                     6.0 GHz - 12.0 GHz______________________________________ 
    
     As shown above, by using the powder of ferrites having the different initial permeability (i.e. more than or less than 300), the frequency range of microwave absorbed effectively shifts in some degree. 
     Microwave having a frequency of 2.45 GHz used in a microwave heating oven, so-called a microwave oven or an electronic oven, is absorbed effectively by the ferrite powders having particle size of from 351 μ to 104 μ (the initial permeability: &gt;300) or from 701 μ to 351 μ (the initial permeability: &lt;300). 
    
    
     In order to explain the present invention, reference is made to the accompanying drawings, in which: 
     Fig. 1(a) and FIG. 1(b) are drawings showing the arrangement of the microwave absorber 1 in the coaxial tube 2 (WX-14D; inner conductor 6.2 mmφ; outer conductor 14.2 mmφ), 
     FIG. 1(a) being a longitudinal sectional view and 
     FIG. 1(b) being a cross sectional view along the line I--I of FIG. 1(a). 
     In the manner as shown in FIG. 1(a) and FIG. 1(b), the attenuation (dB/cm) of the microwave absorber against microwave was measured. 
     FIG. 2 and FIG. 3 are a diagram showing the relation of attenuation (dB/cm) and frequency of the microwave (GHz) for the particle sizes of the ferrite powder having the initial permeability of more than 300 and less than 300 at 10 kHz, respectively. 
     In FIG. 2 and FIG. 3, Curves I, II, III, IV and V are those obtained by the particle size of the ferrite powder as shown below: 
     
         ______________________________________Curve       Particle size______________________________________I           1.65    mm    -   701 μII          701     μ  -   351 μIII         351     μ  -   104 μIV          104     μ  -   43  μV           &lt;43     μ______________________________________ 
    
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples are given to illustrate the present invention. 
     EXAMPLE 1 
     The microwave absorbers of the present invention were prepared as follows: 
     724 g of Fe 2  O 3 , 175 g of MnO and 101 g of ZnO were each weighed out to provide a Mn-Zn-ferrite including 55 mol% of Fe 2  O 3 , 30 mol% of MnO and 15 mol% of ZnO. Fe 2  O 3 , MnO and ZnO were mixed in a ball mill for 20 hours. The mixture was compression molded at about 1 ton/cm 2  to form a shaped body 110 mm × 18 mm × 5 mm. The shaped body was heated at a temperature of 1,350° C for 2 hours. The resulting sintered body, i.e. Mn-Zn-ferrite, had the initial permeability of 2,500 at 10 kHz. This Mn-Zn-ferrite was pulverized for 2 hours using a stamp mill to give Mn-Zn-ferrite powder. The resulting ferrite powder was sifted through eight sieves of different mesh such as 10, 24, 42, 150 and 325 mesh to obtain ferrite powder having different particle sizes as follows: 
     
         ______________________________________Pass through           Not pass through                        Particle size ofNo.  the sieve of           the sieve of ferrite powder______________________________________1      10 mesh     24 mesh   1.65 mm -   701μ2      24 &#34;        42 &#34;      701μ -   351μ3      .increment.&#34;             150 &#34;      351μ -   104μ4     150 &#34;       325 &#34;      104μ -    43μ5     325 &#34;                  &lt;43μ______________________________________ 
    
     Each shifted ferrite powder Nos. 1-5 obtained above was mixed with silicone resin in the ratio of 9 (ferrite powder) to 1 (silicone resin) by volume. Each mixture was compression molded at about 100 kg/cm 2  to form a shaped body having an inside diameter of 6.2 mm, an outside diameter of 14.2 mm and a thickness of 3 mm. Each shaped body was heated at a temperature of 100° C for 2 hours to provide Microwave absorbers Nos. 1-5 of the present invention. 
     In the manner as shown in FIG. 1(a) and FIG. 1(b), the attenuations (dB/cm) of Microwave absorbers No. 1 - 5 against microwaves of different frequencies such as 500 MHz, 1 GHz, 2.45 GHz, 4 GHz, 6 GHz and 12 GHz were measured, and the following results were obtained: 
     
                                           TABLE 1__________________________________________________________________________Microwave Attenuation (dB/cm)absorber FrequencyNo.   500 MHz       1 GHz            2.45 GHz                  4 GHz                       6 GHz                            12 GHz__________________________________________________________________________1     5.3   8.8  6.5   5.0  3.8  3.32     3.0   7.8  11.0  9.5  8.2  7.23     3.5   7.6  14.4  15.4 13.5 12.04     1.4   4.6  13.3  19.0 24.5 23.05     0.5   2.8  11.0  18.0 23.0 26.0__________________________________________________________________________ 
    
     The resulting attenuations of Microwave absorbers Nos. 1, 2, 3, 4 and 5 are rspectively shown by Curves I, II, III, IV and V in FIG. 2. 
     EXAMPLE 2 
     Microwave absorbers Nos. 6-10 were prepared by repeating the same procedure as that shown in Example 1 except that: 
     1. 739 g of Fe 2  O 3 , 119 g of NiO, 136 g of ZnO and 6 g of CoO were used to provide a Ni-Zn-Co-ferrite including 58 mol% of Fe 2  O 3 , 20 mol% of NiO, 21 mol% of ZnO and 1 mol% of CoO instead of the Mn-Zn-ferrite of Example 1, 
     2. the shaped body was heated at a temperature of 1,250° C to a ferrite sintered body, 
     3. polychloroprene (chloroprene rubber) was used instead of silicone resin as a binder, and 
     4. the mixture of ferrite powder and binder was heated at a temperature of 175° C for 2 minutes. 
     The resulting Ni-Zn-Co-ferrite had the initial permeability of 150 at 10kHz. 
     In the same manner as that described in Example 1, the attenuations (dB/cm) of Microwave absorbers No. 6 - 10 were measured, and the following results were obtained: 
     
                                           TABLE 2__________________________________________________________________________Microwave Attenuation (dB/cm)absorber Frequency -No.   1 GHz      2 GHz           2.45 GHz                 4 GHz 6 GHz                           8 GHz                                12 GHz__________________________________________________________________________6     3.5  6.2  8.0   8.8   6.07     3.0  5.6  6.8   11.0  11.0                           9.58     2.8  5.4  6.5   10.0  12.3                           11.09               5.0   8.4   12.6                           14.0 13.010              4.0   7.3   11.2                           14.4 14.0__________________________________________________________________________ 
    
     The resulting attenuations of Microwave absorbers Nos. 6, 7, 8, 9 and 10 are respectively shown by Curves I, II, III, IV and V in FIG. 3. 
     Microwave absorbers Nos. 11 and 12 for use in comparative tests were provided as follows: 
     Microwave absorber No. 11 was prepared by repeating the same procedure as that of Example 1 except that Mn-Zn-ferrite powder having particle size of from 3μ to 2 mm was used. Such ferrite powder was obtained by sifting the pulverized ferrite through a sieve of 6 mesh. 
     Microwave absorber No. 12 was prepared by repeating the same procedure as that of Example 2 except that Ni-Zn-Co-ferrite powder having particle size of from 3μ to 2 mm was used in the same manner as that shown in providing Microwave absorber No. 11. 
     In the same manner as that described in Example 1, the attenuations (dB/cm) of Microwave absorbers No. 11 and No. 12 were measured, and the following results were obtained: 
     
                       TABLe 3______________________________________Microwave   Attenuation (dB/cm)absorber   FrequencyNo.    500 MHz  1 GHz    2.45 GHz                            4 GHz  6 GHz______________________________________11     5.5      4.8      3.5     3.3    3.012     3.8      4.2      3.3     2.8    2.5______________________________________ 
    
     As can be seen from the results as obtained above, microwave of a certain frequency is absorbed more effectively by ferrite powder having a certain limited particle size as hereinbefore described. 
     In the above Examples 1 and 2, the effects of the present invention was illustrated by using the powders of Mn-Zn-ferrite and Ni-Zn-Co-ferrite. 
     However, ferrites having high or low initial permeability can be obtained by using divalent metals such as Cu and Mg instead of Mn and Ni, and similar results to those described in Examples 1 and 2 can be obtained by using powders of Cu-Zn-ferrite and Mg-Zn-ferrite. 
     Many kinds of ferrites can be obtained by changing composition and process of production of ferrite. The powders of the ferrites thus obtained can be used in the present invention in the same manner as that described in Examples 1 and 2.