JP4752934B2 - Radio wave absorber and manufacturing method thereof - Google Patents

Description

  The present invention relates to a radio wave absorber made of a MnZn-based ferrite material that does not contain Ni as a main component, and relates to a radio wave absorber used for an anechoic chamber, a radio wave absorption wall, and the like, and a method for manufacturing the same.

  In recent years, with the development of information communication technology and the widespread use of various electrical devices, the effect of unnecessary electromagnetic noise on precision device-related devices has become a problem.

  Measurement of such electromagnetic noise requires an anechoic chamber (electromagnetic anechoic chamber) that does not reflect electromagnetic waves, and an electromagnetic wave absorber is used on the inner wall of the anechoic chamber.

  Moreover, in order to prevent a reception failure caused by reflection of television radio waves by a high-rise building or the like, a radio wave absorber is used on the outer wall of the building or the like.

  Since such a radio wave absorber is used in a large amount inside or outside the anechoic chamber, the product cost is required to be low.

  As a conventional wave absorber, for example, as a wave absorber having a characteristic of a return loss of 20 dB or more in a frequency band of 40 MHz to 450 MHz, a wave absorber obtained by sintering magnesium-zinc ferrite, nickel-zinc ferrite is used. There are sintered radio wave absorbers, manganese-nickel-copper-zinc ferrites, and the like.

  Among these, although magnesium-zinc ferrite material is relatively inexpensive, the radio wave absorber has a large matching thickness of about 8 mm, and is used for the inner walls of anechoic chambers and outer walls of buildings, etc. There is a limit to reducing the total body weight.

  On the other hand, ferrite containing nickel as a main component can be said to be an advantageous material for obtaining desired radio wave absorber characteristics, but the cost becomes high and does not conform to the gist of the present invention for the purpose of cost reduction. . In an anechoic chamber for measuring electromagnetic noise of precision equipment-related devices, the frequency band for evaluating electromagnetic noise is standardized, and the return loss in the range of 30 to 1000 MHz is required to be 20 dB or more.

  Under such circumstances, the present applicant has already filed Japanese Patent Application No. 2007-272587 (application date: October 19, 2007) and Japanese Patent Application No. 2008-59056 (application date: March 10, 2008). ) As an invention of an Mn—Zn-based electromagnetic wave absorber. According to these proposals, it is possible to reduce the manufacturing cost while maintaining the required characteristic level at a high level.

Such a proposal by the applicant of the present application particularly increases the volume ratio of the nonmagnetic part due to the grain boundary component by adding an appropriate amount of CaCO ₃ or the like, and in particular, μ ′ (complex permeability) at a high frequency around 30 MHz. It can be said that this includes a technique of reducing the real part) and improving the return loss.

  However, since this method also reduces the value of μ ″, which is the imaginary part of the complex permeability at a frequency of 30 MHz, the matching thickness that is one of the important elements as a radio wave absorber is increased. There is a tendency.

That is, in the effect of adding the amount of CaCO ₃ , the effect of improving the return loss and the effect of reducing the matching thickness are in a trade-off relationship.

JP 2001-53483 A

  Under such circumstances, the present invention has been invented, and the purpose thereof can be said to be in the above trade-off relationship. In the relationship between the return loss and the matching thickness, the effect of improving the return loss is obtained. An object of the present invention is to provide a radio wave absorber having an extremely excellent characteristic that the matching thickness can be reduced while maintaining a high level, and a method for manufacturing the same.

In order to solve such a problem, the radio wave absorber of the present invention is
An electromagnetic wave absorber made of a MnZn ferrite sintered body,
The radio wave absorber is
45.0 to 49.0 mol% iron oxide in terms of Fe ₂ O _3,
Zinc oxide has a main component composed of 19.0 to 23.0 mol% in terms of ZnO, and manganese oxide has 28.0 to 36.0 mol% in terms of MnO,
As a subcomponent with respect to 100 parts by weight of this main component,
Cobalt oxide is 1000-7000 ppm by weight in terms of CoO,
10 to 200 ppm by weight of silicon oxide in terms of SiO _2,
0 to 500 ppm by weight of vanadium oxide in terms of V ₂ O ₅ ,
And 200 to 2500 ppm by weight of calcium oxide in terms of CaO,
The ratio of Mn ³⁺ / Mn ²⁺ is 0.04 to 0.3,
The grain boundary thickness is configured to be 1.0 to 2.0 nm.

Moreover, as a preferred embodiment of the radio wave absorber of the present invention,
The cobalt oxide is 3500-6500 ppm by weight in terms of CoO,
30-150 ppm by weight in the silicon oxide in terms of SiO _2,
The vanadium oxide is 50 to 500 ppm by weight,
The said calcium oxide is comprised so that 500-1500 weight ppm may be contained in conversion of CaO.

  Moreover, as a preferable aspect of the radio wave absorber of the present invention, it is configured to have a characteristic that the matching thickness is 6 mm or less and a characteristic that the return loss at 30 MHz and 25 ° C. is 20 dB or more.

  Moreover, as a preferable aspect of the radio wave absorber of the present invention, it is configured to have a characteristic that the matching thickness is 6 mm or less and a characteristic that the Curie temperature is 80 ° C. or more.

  Moreover, as a preferable aspect of the electromagnetic wave absorber of this invention, it is comprised so that plate-shaped tile shape may be made.

The method for producing a radio wave absorber according to the present invention is a method for producing a radio wave absorber as described above, comprising a firing step for firing a molded body to form ferrite,
The firing step has a temperature raising operation unit, a high temperature holding operation unit, and a temperature lowering operation unit in this order, and the temperature raising operation unit gradually increases the firing temperature from room temperature until reaching the maximum temperature. The high temperature holding operation unit is an operation region in which the reached maximum temperature is maintained for a predetermined time, and the temperature lowering operation unit gradually decreases the reached maximum temperature to near room temperature. The temperature raising operation part and the high temperature holding operation part are each operated in air, and the temperature lowering operation part is a reducing atmosphere in which nitrogen is introduced into the air atmosphere, Regarding the operation of oxygen partial pressure and temperature in the temperature lowering operation section, the following equilibrium relational expression showing the equilibrium relation between oxygen partial pressure (PO ₂ (unit:%)) and temperature (T (unit: absolute temperature K)) (1)
Log (PO ₂ ) = B + A / T (1)
Is used to determine the A value and the B value in the range of B = 2.57 to 20.4 and the range of A = −30000 to −2000,
A method of manufacturing a radio wave absorber in which a temperature lowering operation is performed at an equilibrium oxygen partial pressure (PO ₂ ) based on the above formula (1) at least from a temperature decrease start temperature to a temperature drop of 900 ° C.

  As a preferred embodiment of the method for manufacturing a radio wave absorber according to the present invention, the temperature lowering operation section is configured such that when the temperature is lowered to 900 ° C., the atmosphere is completely replaced with nitrogen.

The radio wave absorber made of the MnZn-based ferrite sintered body of the present invention is a radio wave absorber made of an MnZn-based ferrite sintered body, and the radio wave absorber has an iron oxide content of 45.0 to _{4 in} terms of Fe ₂ O _3. The main component is 49.0 mol%, zinc oxide is 19.0 to 23.0 mol% in terms of ZnO, and manganese oxide is 28.0 to 36.0 mol% in terms of MnO. As auxiliary components with respect to parts by weight, cobalt oxide is 1000 to 7000 ppm by weight in terms of CoO, silicon oxide is 10 to 200 ppm by weight in terms of SiO ₂ , vanadium oxide is 0 to 500 ppm by weight in terms of V ₂ O ₅ , and Calcium oxide is contained in an amount of 200 to 2500 ppm by weight in terms of CaO, the ratio of Mn ³⁺ / Mn ²⁺ is 0.04 to 0.3, and the grain boundary thickness is 1.0 to 2.0 nm. Composed Runode, while maintaining the improvement of return loss in high level, that it is possible to reduce the matching thickness, expressed extremely excellent effect.

Embodiments of the present invention will be described below.
( Description of main components constituting the radio wave absorber of the present invention )
The radio wave absorber of the present invention includes a main component composed of iron oxide, zinc oxide, and manganese oxide. Iron oxide, 45.0 to 49.0 mol% in terms of Fe ₂ O ₃ (preferably, 46.0 to 48.0 mol%), zinc oxide from 19.0 to 23.0 mol% in terms of ZnO ( Preferably, 20.5 and ˜22.5 mol%) and manganese oxide are contained in an amount of 28.0 to 34.5 mol% in terms of MnO.

  In a composition region outside the above range, the frequency characteristics of the complex permeability and the frequency characteristics of the complex dielectric constant required for the radio wave absorption characteristics are not satisfied, or the matching thickness of the radio wave absorber exceeds 6.0 mm. , There is a tendency that inconvenience that an appropriate Curie point cannot be obtained as a radio wave absorber.

  Here, the radio wave absorption characteristic is expressed by the following formula (1). When the frequency causing the decrease of the real part μ ′ of the complex permeability is increased, the good radio wave absorption characteristic from the low frequency band cannot be obtained. End up. Further, when the imaginary part μ ″ of the complex magnetic permeability is low, the matching thickness is increased.

  Moreover, if the real part ε ′ of the complex dielectric constant is not an appropriate value, the return loss is reduced.

  In addition, when the Curie point is extremely low, the temperature of the radio wave absorber itself easily exceeds the Curie point due to the converted heat, resulting in inconvenience that the magnetism is lost and the radio wave absorber does not function.

  When the content of the iron oxide is less than 45.0 mol%, there is a disadvantage that the matching thickness becomes 6 mm or more and the Curie temperature becomes 80 ° C. or less due to the decrease of the imaginary part μ ″ of the complex magnetic permeability. Tend to occur. Further, when the iron oxide content exceeds 49.0 mol%, the frequency causing the reduction of the real part μ ′ of the complex permeability increases, and at room temperature (25 ° C.) in the low frequency region of 30 MHz. There is a tendency for the inconvenience that the reflection loss amount becomes 20 dB or less.

  In addition, when the content of zinc oxide is less than 19.0 mol%, the frequency causing a decrease in the real part μ ′ of the complex permeability is increased, and at room temperature (25 ° C.) in a low frequency region of 30 MHz. There is a tendency for the inconvenience that the reflection loss amount becomes 20 dB or less. On the other hand, if the zinc oxide content exceeds 23.0 mol%, there is a tendency that the matching thickness becomes 6 mm or more and the Curie temperature becomes 80 ° C. or less.

( Description of subcomponents added to the main component )
(1) Addition of Cobalt Oxide as Subcomponent With respect to 100 parts by weight of the above-mentioned MnZn-based ferrite main component, cobalt oxide is contained as a subcomponent in an amount of 1000 to 7000 ppm by weight, preferably 3500 to 6500 ppm by weight in terms of CoO.

  Appropriate addition of cobalt has the effect of shifting the attenuation of the real part μ ′ of the complex permeability to the low frequency side. When the content of cobalt oxide is less than 1000 ppm by weight, the real part μ of the complex permeability There is a tendency that the frequency causing the decrease of 'becomes high. As a result, in the low frequency region of 30 MHz, the return loss at room temperature (25 ° C.) is 20 dB or less, and the return loss at −20 ° C. (−20 ° C.) is 15 dB or less. Tend.

  On the other hand, when the content of cobalt oxide exceeds 7000 ppm by weight, the frequency that causes the real part μ ′ of the complex permeability to decrease is increased, and at a low frequency region of 30 MHz at room temperature (25 ° C.). There is a tendency that the return loss becomes 20 dB or less and the return loss at minus 20 ° C. (−20 ° C.) becomes 15 dB or less.

(2) with respect to the addition above MnZn ferrite main component 100 parts by weight of silicon oxide as a secondary component (SiO _2), 10 to 200 ppm by weight silicon oxide in terms of SiO ₂ as an auxiliary component, preferably 30 to 150 wt Contains ppm. When the content of silicon oxide is less than 10 ppm by weight, there is a tendency for the disadvantage that the sintered density is remarkably lowered. Moreover, when content of silicon oxide exceeds 200 weight ppm, there exists a tendency for the problem that abnormal grain growth will appear.

  Further, the addition of 10 to 200 ppm by weight of silicon oxide tends to contribute to the control of the grain boundary thickness in combination with the addition of vanadium oxide and calcium oxide.

(3) Addition of vanadium oxide (V ₂ O ₅ ) as an auxiliary component With respect to 100 parts by weight of the above MnZn-based ferrite main component, vanadium oxide as an auxiliary component is 0 to 500 ppm by weight in terms of V ₂ O ₅ , preferably Is contained in an amount of 50 to 500 ppm by weight. The vanadium oxide content is preferably 50 ppm by weight or more, and if it is less than 50 ppm by weight, the strength of the sintered body tends to decrease.

Further, when the content of vanadium oxide exceeds 500 ppm by weight, the frequency causing the decrease of the real part μ ′ of the complex relative permeability increases, and the reflection attenuation at room temperature (25 ° C.) in the low frequency region of 30 MHz. The amount tends to be 20 dB or less. The matching thickness also tends to increase.
Further, the addition of 50 to 500 ppm by weight of vanadium oxide tends to contribute to the control of the grain boundary thickness in combination with the addition of calcium oxide and silicon oxide.

(4) Addition of calcium oxide (CaO) as a subcomponent With respect to 100 parts by weight of the MnZn-based ferrite main component, calcium oxide is contained as a subcomponent in an amount of 200 to 2500 ppm by weight, preferably 500 to 1500 ppm by weight in terms of CaO. Is done. When the content of calcium oxide is less than 200 ppm by weight, the real part μ ′ of the complex relative permeability increases at a frequency of 30 MHz (the attenuation of the real part μ ′ of the complex relative permeability shifts to the high frequency side. Therefore, there is a tendency that a return loss of 20 dB or more cannot be obtained.

  On the other hand, if the content of calcium oxide exceeds 2500 ppm by weight, the imaginary part μ ″ of the complex relative permeability at a frequency of 30 MHz tends to be small and the matching thickness tends to increase.

  Further, the addition of 200 to 2500 ppm by weight of calcium oxide tends to contribute to the control of the grain boundary thickness in combination with the addition of vanadium oxide and silicon oxide.

(5) as other subcomponents other _{subcomponents, Nb 2 O 5, SnO 2} , TiO 2, NiO, Ta 2 O 5, ZrO 2, HfO 2, GeO 2, MoO 3, WO 3, Bi 2 O 3 Various subcomponents such as In ₂ O ₃ , Cr ₂ O ₃ , and Al ₂ O ₃ may be contained within a range not departing from the operational effects of the present invention.

The electromagnetic wave absorber of the present invention as described above is manufactured by sintering a MnZn-based ferrite material blended so that the composition after sintering is in the above range. The ratio of Mn ³⁺ / Mn ^{2+ in} the MnZn-based ferrite sintered body is 0.04 to 0.3, preferably 0.1 to 0.25, and more preferably 0.15 to 0.22. Is done.

  When this value is less than 0.04, there is a disadvantage that the return loss is reduced, and when this value exceeds 0.3, there is a disadvantage that the matching thickness is increased.

The ratio of Mn ³⁺ / Mn ²⁺ can be obtained from calculation by composition analysis and titration analysis of Mn ³⁺ .

Furthermore, the grain boundary thickness δ of the crystal in the MnZn-based ferrite sintered body of the present invention is 1.0 to 2.0 nm, preferably 1.2 to 1.8 nm. When the grain boundary thickness δ is less than 1.0 nm, there arises a disadvantage that the return loss is reduced. Further, when the grain boundary thickness δ exceeds 2.0 nm, there arises a disadvantage that the matching thickness is increased.

  The grain boundary thickness δ can be obtained by observing the grain boundary layer with a TEM and measuring the length in a region where the sample is inclined to show the narrowest width. Ten locations per sample and the number of measurement samples N were three.

In order to achieve the effect of the present invention, in addition to the above composition range, it is necessary to satisfy both requirements of the ratio value of Mn ³⁺ / Mn ²⁺ and the grain boundary thickness δ.

In order to satisfy the requirements of both the value of the ratio of Mn ³⁺ / Mn ²⁺ and the grain boundary thickness δ, it is necessary to carry out the following MnZn-based ferrite manufacturing method.

[ Description of Manufacturing Method of MnZn Ferrite (Radio Wave Absorber) of the Present Invention ]
In the method for producing MnZn-based ferrite of the present invention, the other steps are performed in accordance with the conventional method for producing MnZn-based ferrite except that the firing step is different from the conventional method. That is, the steps up to the firing step, that is, the steps up to forming the raw material powder compact are performed according to the conventional method for producing MnZn-based ferrite.

  Examples of the steps up to the firing step include the following steps (1) to (4).

(1) A step of weighing so that the ratio of metal ions is a predetermined component so as to obtain a target ferrite. As a main component material, an oxide or a compound that becomes an oxide by heating, for example, carbonate, hydroxide Powders such as oxalate and nitrate are used. What is necessary is just to select suitably the average particle diameter of each raw material powder in the range of about 0.1-3.0 micrometers. In addition, not only the raw material powder mentioned above but it is good also considering the powder of the complex oxide containing 2 or more types of metals as raw material powder. Each raw material powder is weighed so as to have a predetermined composition.

(2) Temporary baking step after mixing the weighed material by wet or drying After the raw material powder is wet-mixed by, for example, a ball mill, dried, pulverized and sieved, it is held within a temperature range of 700 to 1000 ° C. for a predetermined time. Calcination is performed. The atmosphere temperature of calcination is set to nitrogen or an air atmosphere. What is necessary is just to select suitably the holding time of calcination within the range of 1 to 5 hours.

(3) Crushing step of calcined powder After calcining, the calcined body is pulverized, for example, to an average particle size of about 0.5 to 5.0 μm.
In addition, the timing which adds raw material powder is not limited to what was mentioned above. For example, first, only a part of the powder of components is weighed, mixed, calcined and pulverized. Then, a predetermined amount of raw material powders of other components may be added to and mixed with the main component powder obtained after calcining and pulverization.

(4) Granulation / molding process The pulverized powder is granulated into granules to facilitate the subsequent molding process. At this time, it is desirable to add a small amount of an appropriate binder such as polyvinyl alcohol (PVA) to the pulverized powder. The particle size of the obtained granules is preferably about 80 to 200 μm. For example, a toroidal shaped body is formed by pressing the granulated powder.

(5) Firing Step Next, the firing step for firing the formed body, which is the main part of the present invention, to form ferrite will be described in detail.

  The firing step includes a temperature raising operation unit, a high temperature holding operation unit, and a temperature lowering operation unit in this order.

  The temperature raising operation unit is an operation region in which the firing temperature is gradually increased from room temperature until reaching the maximum temperature. The high temperature holding operation unit is an operation region that keeps the reached maximum temperature stable for a predetermined time. The temperature lowering operation unit is an operation region in which the reached maximum temperature is gradually lowered to near room temperature. In the present invention, the vicinity of room temperature refers to a temperature range of 0 to 50 ° C.

  Hereinafter, each operation unit will be described in more detail.

In the temperature raising operation part, it is desirable to perform the temperature raising operation in the atmosphere (oxygen: about 21 Vol%). The rate of temperature rise is 50 to 300 ° C./hr, more preferably 50 to 150 ° C./hr.

The high temperature holding temperature in the high temperature holding operation unit is appropriately set within a range of 1200 to 1350 ° C.

  The firing atmosphere of the high temperature holding operation unit is desirably an operation of an air atmosphere (oxygen: about 21 Vol%) as in the case of the above temperature rising operation unit.

The operation of the oxygen partial pressure and temperature in the temperature lowering operation unit is a reducing atmosphere in which nitrogen is introduced into the air atmosphere, and the operation of the oxygen partial pressure and temperature in the temperature lowering operation unit is the oxygen partial pressure ( The following equilibrium relational expression (1) showing the equilibrium relation between PO ₂ (unit:%) and temperature (T (unit: absolute temperature K))

Log (PO ₂ ) = B + A / T Equilibrium relational expression (1)
Is operated at an equilibrium oxygen partial pressure determined by an equilibrium relationship with temperature.

In the above formula (1), the B value is determined in the range of B = 2.57 to 20.4 (preferably in the range of 5 to 15),
The A value is determined in the range of A = −30000 to −2000 (preferably in the range of −15000 to −10000).

  The operation at the equilibrium oxygen partial pressure based on the above formula (1) is performed along the temperature gradually lowered from the terminal temperature (temperature decrease start temperature) of the high temperature holding operation section, and is balanced until at least the temperature reaches 900 ° C. Operation with oxygen partial pressure is performed.

  In the temperature lowering operation section, it is preferable that the atmosphere is operated so as to be completely replaced with nitrogen when the temperature is lowered to 900 ° C.

  It is desirable that the atmosphere be operated so that it is completely replaced with nitrogen when a temperature is reached that no longer requires operation at equilibrium oxygen partial pressure.

By incorporating such a special operation of the temperature lowering operation part, in the production of the ferrite of the present composition, the sintered body satisfying both requirements of the value of the ratio of Mn ³⁺ / Mn ²⁺ and the grain boundary thickness δ. Formation is possible.

  For more specific production methods, refer to experimental examples in the examples described later. In addition, as a tile-shaped magnitude | size, the plate-shaped object whose vertical dimension is about 50-200 mm, horizontal dimension is about 50-200 mm, and thickness dimension is about 3-10 mm can be illustrated.

  Hereinafter, the present invention will be described in more detail with reference to specific examples.

[Experimental Example I]
Each raw material component was weighed so that the composition after firing would be the composition shown in Table 1 below, and wet mixed in a steel ball mill for 16 hours.

  Next, this mixed powder was calcined in the atmosphere at 900 ° C. for 2 hours.

  Subcomponents were added to the obtained calcined product so that the composition after firing was as shown in Table 1 below, and wet pulverized with a steel ball mill for 16 hours.

The MnZn-based ferrite powder thus obtained is granulated by adding 10% by weight of a polyvinyl alcohol aqueous solution, and shaped into a predetermined shape capable of performing the following wave absorber characteristic test at a pressure of 1 ton / cm ^2. Molded.

  The molded product thus formed was fired.

The firing conditions were as follows.
Heating operation section
In the temperature raising operation part, the temperature was raised in an air atmosphere (oxygen: about 21 Vol%). The heating rate was 100 ° C./hr.

High temperature holding operation part The high temperature holding temperature in the high temperature holding operation part was set to 1280 ° C. The holding time was 3 hours.

  The firing atmosphere of the high temperature holding operation section was an operation of an air atmosphere (oxygen: about 21 Vol%) as in the case of the above temperature increase operation section.

About the operation of the oxygen partial pressure and temperature in the temperature- decreasing operation part temperature-decreasing operation part, it was set as the reducing atmosphere which introduced nitrogen in the air atmosphere.

Regarding the operation of oxygen partial pressure and temperature in the temperature lowering operation section, the following equilibrium relational expression showing the equilibrium relation between oxygen partial pressure (PO ₂ (unit:%)) and temperature (T (unit: absolute temperature K)) (1)

Log (PO ₂ ) = B + A / T Equilibrium relational expression (1)
Was operated at an equilibrium oxygen partial pressure determined by an equilibrium relationship with temperature.

  In the above formula (1), B = 9.6 and A = -12500.

Based on the above formula (1), the oxygen partial pressure was controlled from the temperature lowering start temperature to a temperature drop of 900 ° C., and firing was performed in a nitrogen (N ₂ ) atmosphere at a temperature lower than 900 ° C.

About the sample of the radio wave absorber obtained in this way,
(1) Mn ³⁺ / Mn ²⁺ value,
(2) Grain boundary thickness δ (nm),
(3) Matching thickness d (mm) of the wave absorber,
(4) Return loss at 25 ° C. at a frequency of 30 MHz (RD ₂₅ ), and (5) Curie point (Tc),
Each was measured.

(1) Method of measuring Mn ³⁺ / Mn ²⁺ value As described above, the Mn ³⁺ / Mn ²⁺ value was determined by calculation by composition analysis and Mn ³⁺ titration analysis.

(2) Measuring Method of Grain Boundary Thickness δ (nm) Grain boundary thickness δ was obtained by observing the grain boundary layer with TEM, and measuring the length in the region showing the narrowest width by tilting the sample. Ten samples per sample and the number N of measurement samples were three, and the average value of a total of 30 locations was taken.

(3) Matching thickness d (mm) of radio wave absorber
Regarding the radio wave absorption characteristics of the radio wave absorber, a reflection coefficient was measured with a network analyzer in a state of being inserted into a coaxial tube using a ring-shaped sample processed to have an outer diameter of 19.8 mm and an inner diameter of 8.6 mm. From the obtained measurement results, the return loss and the normalized impedance of the front surface of the radio wave absorber were calculated.

  The relationship between the normalized impedance (Z) and the reflection coefficient (S) is as follows.

Z = (1 + S) / (1-S)
S = (Z-1) / (Z + 1)
S = (S _sample / S _metal )
−20 log | S | = dB

  The normalized impedance of each thickness was plotted on a Smith chart, the thickness passing through the center of the Smith chart was obtained by calculation, and the thickness was taken as the matching thickness (d).

  The target value for the alignment thickness (d) is 6 mm or less.

(4) Return loss at 25 ° C. at a frequency of 30 MHz (RD ₂₅ )
A ring having the calculated matching thickness was actually produced, and the return loss (RD ₂₅ ) at 25 ° C. at a frequency of 30 MHz was measured by the coaxial tube method. The target value of RD ₂₅ is 20 dB or more.

(5) Measuring method of Curie point (Tc) The sample was placed in a high temperature layer and held at each temperature until it was sufficiently stable, and then the temperature characteristics of the initial permeability μi were measured using an LCR meter. In the descending portion exceeding the maximum value of the initial magnetic permeability, a point where an extension line connecting the point of 80% and the point of 20% of the maximum value intersects with the line of μi = 1 was obtained and set as the Curie temperature Tc. The measurement frequency was 1 kHz.

  The target value of the Curie temperature Tc is 80 ° C. or higher.

  The measurement results for each of these items are shown in Table 1 below.

  Although the data is not clearly shown in the table, the sample No. 1 and Sample No. 1 of the present invention. When the strength of the sintered body of No. 2 was compared, 1 is Sample No. As compared with 2, the sintered body strength was about 80%.

[Experimental Example II]
Sample No. in Experimental Example I above. 4 as a composition base, the B value and the A value in the above formula (1) were variously changed as shown in Table 2 below. 4-1 to 4-11 were produced. That is, the experiment was performed by changing the oxygen partial pressure and temperature operating conditions in the temperature lowering operation section with the base composition basically the same (except for sample Nos. 4-8 and 4-9).

  About the obtained sample, the same physical property and characteristic evaluation as the said Experimental example I were performed.

  The results are shown in Table 2 below.

From the above experimental results, the effect of the present invention is clear. That is, in the radio wave absorber composed of the MnZn ferrite sintered body of the present invention, iron oxide is 45.0 to 49.0 mol% in terms of Fe ₂ O ₃ , and zinc oxide is 19.0 to 23.0 in terms of ZnO. Mol% and manganese oxide has a main component of 28.0 to 36.0% in terms of MnO, and as a subcomponent with respect to 100 parts by weight of this main component, cobalt oxide is 1000 to 7000 ppm by weight in terms of CoO. And 10 to 200 ppm by weight of silicon oxide in terms of SiO ₂ , 0 to 500 ppm by weight of vanadium oxide in terms of V ₂ O ₅ , and 200 to 2500 ppm by weight of calcium oxide in terms of CaO, Mn ³⁺ / Mn ^Since the ratio of ²⁺ is 0.04 to 0.3 and the grain boundary thickness is 1.0 to 2.0 nm, the effect of improving the return loss is maintained at a high level. Alignment thickness That can be reduced, expressing extremely excellent effect.

  The method for producing MnZn-based ferrite of the present invention can be widely used in various electric component industries.

Claims (7)

An electromagnetic wave absorber made of a MnZn ferrite sintered body,
The radio wave absorber is
45.0 to 49.0 mol% iron oxide in terms of Fe ₂ O _3,
Zinc oxide has a main component composed of 19.0 to 23.0 mol% in terms of ZnO, and manganese oxide has 28.0 to 36.0 mol% in terms of MnO,
As a subcomponent with respect to 100 parts by weight of this main component,
Cobalt oxide is 1000-7000 ppm by weight in terms of CoO,
10 to 200 ppm by weight of silicon oxide in terms of SiO _2,
0 to 500 ppm by weight of vanadium oxide in terms of V ₂ O ₅ ,
And 200 to 2500 ppm by weight of calcium oxide in terms of CaO,
The ratio of Mn ³⁺ / Mn ²⁺ is 0.04 to 0.3,
An electromagnetic wave absorber having a grain boundary thickness of 1.0 to 2.0 nm. The cobalt oxide is 3500-6500 ppm by weight in terms of CoO,
30-150 ppm by weight in the silicon oxide in terms of SiO _2,
The vanadium oxide is 50 to 500 ppm by weight,
The radio wave absorber according to claim 1, wherein the calcium oxide is contained in an amount of 500 to 1500 ppm by weight in terms of CaO. The matching thickness is 6 mm or less, and the return loss at 30 MHz and 25 ° C. is 20 dB or more,
The radio wave absorber according to claim 1 or claim 2, wherein The matching thickness is 6 mm or less, and the Curie temperature is 80 ° C. or more,
The radio wave absorber according to claim 1 or claim 2, wherein   The radio wave absorber according to any one of claims 1 to 4, which has a plate-like tile shape. The method for producing a radio wave absorber according to any one of claims 1 to 5, further comprising a firing step for firing the formed body to form ferrite.
The firing step has a temperature raising operation part, a high temperature holding operation part, and a temperature lowering operation part in this order,
The temperature raising operation part is an operation region until the firing temperature is gradually increased from room temperature to reach the maximum temperature,
The high-temperature holding operation unit is an operation region that maintains the reached maximum temperature for a predetermined time,
The temperature lowering operation part is an operation region in which the maximum temperature reached is gradually lowered to near room temperature,
Each of the temperature raising operation unit and the high temperature holding operation unit is an operation in air,
In the temperature lowering operation section, a reducing atmosphere is introduced in which nitrogen is introduced into the air atmosphere. Regarding the operation of the oxygen partial pressure and temperature in the temperature lowering operation section, oxygen partial pressure (PO ₂ (unit:%)) and temperature The following equilibrium relational expression (1) showing the equilibrium relation with (T (unit: absolute temperature K))
Log (PO ₂ ) = B + A / T (1)
Is used to determine the A value and the B value in the range of B = 2.57 to 20.4 and the range of A = −30000 to −2000,
A method of manufacturing a radio wave absorber in which a temperature lowering operation is performed at an equilibrium oxygen partial pressure (PO ₂ ) based on the above formula (1) at least from a temperature decrease start temperature to a temperature drop of 900 ° C.   The method for manufacturing a radio wave absorber according to claim 6, wherein when the temperature is lowered to 900 ° C. in the temperature lowering operation unit, the atmosphere is completely replaced with nitrogen.