Method of producing Ni-Cu-Zn ferrite material

A method of producing a Ni--Cu--Zn ferrite material comprises the steps of preparing a mixture of an iron compound powder having a specific surface area of about 8.5 m.sup.2 /g or more, a nickel compound powder, copper compound powder and a zinc compound powder, the mixture having a specific surface area of about 8.0 m.sup.2 /g or more; pre-calcining the mixture such that the pre-calcined mixture has a surface area of about 5.0 m.sup.2 /g or more and a spinel crystal synthesizability within a range of about 80.5% to 98%; and milling the pre-calcined mixture to obtain a powder of a Ni--Cu--Zn ferrite material having a specific surface area of about 6.0 m.sup.2 /g or more.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method of producing a ferrite material,
 in particular a Ni--Cu--Zn ferrite material which is suitable for use as a
 chip inductor.
 2. Description of the Related Art
 An inductor element has been used as a noise filter in an electric circuit
 of an electronic device. In order to achieve a compact size and effect a
 high density attachment, there has been suggested and put into practical
 use a chip inductor involving only a small amount of a magnetic flux
 leakage, occupying only a small area, and having a structure in which the
 inner conductor is disposed within a ferrite ceramic (core).
 Such chip inductors, for example, may be obtained by simultaneously
 calcining a plurality of ferrite material layers and electrically
 conductive material layers formed between the ferrite layers. In general,
 there has been used a Ni--Cu--Zn ferrite material as the ferrite material
 for forming the chip inductors, and, as an electrically conductive
 material, there has been used a Ag material which has a high electric
 conductivity.
 When Ag is used as an electrically conductive material and the above chip
 inductor is obtained by means of a simultaneous calcining treatment, the
 melting point of Ag is 950.degree. C. under the oxygen equilibrium
 conditions in the atmosphere. If it is heated to a temperature of
 900.degree. C. or higher, a plastic deformation of Ag will begin as the
 time of heating progresses, thereby causing it to penetrate and disperse
 throughout the ferrite. Because of this, there will occur some problems
 which can include the cross sectional area of the internal conductor will
 decrease, the direct current resistance value will increase, and the
 consumption electric power will thus increase. Moreover, if it is heated
 to a high temperature which is higher than 950.degree. C., a part of the
 internal conductor will be disconnected, losing the predetermined function
 as an inductor. For this reason, it is required that the calcining
 treatment should be conducted at a temperature of 950.degree. C. or lower,
 preferably 900.degree. C. or lower, in order to obtain a chip inductor
 using Ag as an internal conductor.
 However, it is necessary to calcine a Ni--Cu--Zn ferrite material which was
 used as a core material (ferrite ceramic) of a chip inductor at a
 temperature of 1000.degree. C. or higher so as to obtain a dense calcined
 body. If the calcining treatment is conducted at a temperature lower than
 such a value, it is impossible to obtain a sufficient calcining density,
 hence causing a problem in that the initial magnetic permeability becomes
 low and pores are adversely created in the calcined body.
 Further, in order to effectively remove a noise component having a low
 frequency of 30 MHz or lower, it is required that, as a characteristic of
 a noise filter for use in an electric circuit, the cross point frequency
 at the intersection of an R component frequency curve and the X component
 frequency curve should be controlled to a value of 10 MHz or lower. For
 this reason, it is required that the initial magnetic permeability of the
 Ni--Cu--Zn ferrite material, which is used as a core material of a chip
 inductor, be maintained at a value of 800 or higher for this purpose.
 Furthermore, in order to inhibit a wave form distortion in a frequency
 component, it is required that, as a characteristic of a noise filter for
 use in an electric circuit, the cross point frequency at an intersection
 of an R component frequency curve and an X component frequency curve
 should be controlled at a value of 5 MHz or lower. For this reason, it is
 required that the initial magnetic permeability of the Ni--Cu--Zn ferrite
 material, which is for use as a core material of a chip inductor, be
 maintained at a value of 1200 or higher.
 SUMMARY OF THE INVENTION
 The present invention can solve the aforementioned problems associated with
 the conventional art and provides a method of producing a Ni--Cu--Zn
 ferrite material can be calcined at a temperature of 900 degrees or lower
 so as to obtain a high density, and has an initial magnetic permeability
 of 800 or higher or an initial magnetic permeability of 1200 or higher.
 The method of producing a Ni--Cu--Zn ferrite material comprises the steps
 of: preparing a mixture of an iron compound powder having a specific
 surface area of about 8.5 m.sup.2 /g or more, a nickel compound powder,
 copper compound powder and a zinc compound powder, the mixture having a
 specific surface area of about 8.0 m.sup.2 /g or more; pre-calcining the
 mixture such that the pre-calcined mixture has a surface area of about 5.0
 m.sup.2 /g or more and a spinel crystal synthesizability within a range of
 about 80.5% to 98%; and milling the pre-calcined mixture to obtain a
 powder of a Ni--Cu--Zn ferrite material having a specific surface area of
 about 6.0 m.sup.2 /g or more.
 To achieve larger initial magnetic permeability, the nickel compound powder
 and the zinc compound powder have a specific surface area of about 8.0
 m.sup.2 /g or more, respectively, the mixture has the specific surface
 area of about 10.0 m.sup.2 /g or more, the pre-calcining is performed such
 that the pre-calcined mixture has a surface area of about 6.0 m.sup.2 /g
 or more and the spinel crystal synthesizability within a range of about
 90% to 95% and the powder of a Ni--Cu--Zn ferrite material has the
 specific surface area of about 8.0 m.sup.2 /g or more.
 According to the present invention, it is possible to conduct a dense
 calcining treatment at a temperature of 900.degree. C. or lower, and thus
 one can obtain Ni--Cu--Zn ferrite material having an initial magnetic
 permeability of 800 or higher or an initial magnetic permeability of 1200
 or higher.
 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 According to the method of producing a Ni--Cu--Zn ferrite material of the
 present invention, a mixture of an iron compound, a nickel compound
 powder, copper compound powder and a zinc compound powder is first
 prepared. The mixture has a specific surface area of about 8.0 m.sup.2 /g
 or more, and the iron compound has a specific surface area of about 8.5
 m.sup.2 /g or more.
 The mixture is then pre-calcined. The pre-calcining process is performed
 such the obtained pre-calcined mixture has a surface area of about 5.0
 m.sup.2 /g or more and a spinel crystal synthesizability of the obtained
 pre-calcined mixture within a range of about 80.5% to 98%.
 Thereafter, the pre-calcined mixture is milled to obtain a powder of a
 Ni--Cu--Zn ferrite material having a specific surface area of about 6.0
 m.sup.2 /g or more. Note that the spinel crystal synthesizability referred
 in this specification is defined as a value calculated by the following
 equation using a peak intensity (I.sub.Fe101) of (104) surface of Fe.sub.2
 O.sub.3 and a peak intensity (I.sub.sp311) of (311) surface of the spinel
 crystal, both of which are measured during X-ray diffraction on the powder
 material:
 Spinel crystal synthesizability (%)=I.sub.sp311 /(I.sub.Fe104
 +I.sub.sp311).times.100.
 The iron compound used present invention is preferably .alpha. Fe.sub.2
 O.sub.3, and more preferably .alpha. Fe.sub.2 O.sub.3 synthesized in a wet
 method. In addition, the thus obtained Ni--Cu--Zn ferrite material
 preferably consists of about 48.0 mol % to 49.8 mol % of Fe.sub.2 O.sub.3,
 about 20.0 mol % to 34.0 mol % of ZnO, about 6.0 mol % to 20.0 mol % of
 CuO, with the remainder being NiO.
 When the specific surface area of the powder of the Ni--Cu--Zn ferrite
 material after milling is smaller than about 6.0 m.sup.2 /g, a reactivity
 of the powder will be low, thus making it impossible to effect a
 sufficient calcining treatment at a temperature of 900.degree. C. or lower
 and the calcining density will be low, making it impossible to obtain an
 initial magnetic permeability of 800 or higher. For this reason, it is
 preferred that the specific surface area of the powder material after
 disintegration is about 6.0 m.sup.2 /g or larger.
 If the specific surface area of the pre-calcined mixture is smaller than
 about 5.0 m.sup.2 /g, the particle of the pre-calcined mixture is
 undergrown. It may be necessary to extend the milling time or to employ a
 milling machine of a medium stirring type so to obtain a specific surface
 area of about 6.0 m.sup.2 /g or larger after milling. If so, the amount of
 impurity coming from a medium such as cobble stones will increase, thus
 causing a deterioration in the characteristic of the Ni--Cu--Zn ferrite
 after the calcining treatment. For this reason, it is preferred that the
 specific surface area of the powder material after pre-calcining is about
 5.0 m.sup.2 /g or larger.
 If spinel crystal synthesizability after pre-calcining treatment is less
 than 85%, there will be a lot of unreacted Fe.sub.2 O.sub.3 remaining and
 thus calcining capability will decrease. Hence, it is impossible to obtain
 a uniform Ni--Cu--Zn ferrite during the calcining treatment, and thus an
 initial magnetic permeability of 800 or higher can not be obtained. On the
 other hand, if the temperature of the pre-calcining is elevated until the
 spinel crystal synthesizability exceeds 98%, particle growth of the spinel
 crystal will occur, the specific surface area of the powder material will
 decrease and hence its reactivity will become low, resulting in a problem
 that it can not be calcined sufficiently at a temperature of 900.degree.
 C. or lower. For this reason, it is preferred that the spinel crystal
 synthesizability after pre-calcining be controlled within a range from
 about 85% to 98%.
 Further, if the specific surface area of the mixture before pre-calcining
 is smaller than about 8.0 m.sup.2 /g, the reactivity of the mixture will
 be low. As a result, if it is desired to obtain a spinel crystal
 synthesizability of 85% to 98% during the pre-calcining treatment, the
 mixture will have to be pre-calcined at a temperature higher than that for
 obtaining a specific surface area of about 8.0 m.sup.2 /g or larger.
 Consequently, the powder particle growth of the powder material will
 proceed rapidly so that the specific surface area after pre-calcining will
 become smaller than about 5.0 m.sup.2 /g. For this reason, it is preferred
 that the specific surface area of the powder material after compound
 mixing is at least about 8.0 m.sup.2 /g.
 Even when specific surface area of the powder material of the iron compound
 is smaller than about 8.5 m.sup.2 /g, by increasing the specific surface
 area of powder of nickel compound, zinc compound and copper compound, it
 is still possible to obtain a specific surface area of about 8.0 m.sup.2
 /g or larger for the powder material after compound mixing. During the
 pre-calcining process for treating the Ni--Cu--Zn ferrite material, when
 there is an increase in temperature, Zn ferrite will at first occur at a
 low temperature range, and then the Cu and Ni will be solid solved so as
 to form the Ni--Cu--Zn ferrite. Therefore, when the specific surface area
 of the iron compound is smaller than about 8.5 m.sup.2 /g, the temperature
 for the formation of an initial Zn ferrite will become high. If in the end
 a spinel crystal synthesizability from about 85% to 98% is to be obtained,
 the pre-calcining treatment is required to be conducted at a higher
 temperature than that necessary for treating an iron compound whose
 specific surface area is about 8.5 m.sup.2 /g or larger. As a result, the
 particle growth of the powder material will become too rapid, resulting in
 a problem that the specific surface area of the powder material after the
 pre-calcining is smaller than about 5.0 m.sup.2 /g. Accordingly, it is
 preferred that the specific surface area of the iron compound powder
 material is about 8.5 m.sup.2 /g or larger.
 In addition, as to the composition of the Ni--Cu--Zn ferrite material, if
 the amount of Fe.sub.2 O.sub.3 is less than about 48.0 mol %, saturated
 magnetization of the ferrite material will become small and its initial
 magnetic permeability will be lower than 800. On the other hand, if the
 amount of Fe.sub.2 O.sub.3 is more than about 49.8 mol %, its friability
 will become extremely low, and hence it can not be calcined at a
 temperature of 900.degree. C. or lower. Further, if the amount of CuO is
 less than about 6.0 mol %, its calcining density can not become high at a
 temperature of 900.degree. C. or lower. On the other hand, if the amount
 of CuO is more than about 20.0 mol %, its Curie temperature will become
 80.degree. C. or lower. Further, if the amount of ZnO is less than 20.0
 mol %, the saturated magnetization produced by a ferri-magnetism will not
 be sufficient, causing the initial magnetic permeability to become lower
 than 800. In contrast, when the amount of ZnO is over about 34.0 mol %,
 its Curie temperature will become 80.degree. C. or lower. For this reason,
 it is preferred that Ni--Cu--Zn ferrite material has a Fe.sub.2 O.sub.3
 amount from about 48.0 mol % to 49.8 mol %, a ZnO amount from about 20.0
 mol % to 34.0 mol %, a CuO amount from about 6.0 mol % to 20.0 mol %, with
 the balance being NiO.
 In addition to the aforementioned conditions, in the case where the
 Ni--Cu--Zu ferrite material which has an initial magnetic permeability of
 1200 or higher is to be produced, it is necessary to employ conditions to
 obtain more fine powder of the Ni--Cu--Zn ferrite material.
 Specifically, it is preferably that the nickel compound powder and the zinc
 compound powder have a specific surface area of about 8.0 m.sup.2 /g or
 more, respectively, in addition to use of iron compound powder having a
 specific surface area of about 8.0 m.sup.2 /g or more, and the mixture of
 these compounds has a specific surface area of about 10 m.sup.2 /g or
 more. At least one of the iron compound powder, the nickel compound powder
 and the zinc compound powder may be synthesized by a wet method.
 The pre-calcining process is preferably performed such the obtained
 pre-calcined mixture has a surface area of about 6.0 m.sup.2 /g or more
 and a spinel crystal synthesizability of the obtained pre-calcined mixture
 is within a range of about 90% to 95%.
 Further, the pre-calcined mixture is preferably milled to obtain a powder
 of a Ni--Cu--Zn ferrite material having a specific surface area of about
 8.0 m.sup.2 /g or more.

EXAMPLE 1
 At first, .alpha.-Fe.sub.2 O.sub.3 powder materials obtained by a wet
 synthesizing method were prepared at the various specific surface areas
 shown in Table 1. Further, there were prepared NiO powder material serving
 as a nickel compound, CuO powder material serving as a copper compound and
 ZnO powder material serving as a zinc compound. Subsequently, these
 powders were combined so that the weights were such that the Fe.sub.2
 O.sub.3 is 48.7 mol %, ZnO is 26.9 mol %, CuO is 10.5 mol % with the
 balance being NiO. The compounds were then wet mixed in a ball mill,
 followed by drying.
 Next, the powder materials obtained after mixing were pre-calcined at
 various temperatures shown in Table 1. Subsequently, the powder materials
 were wet milled in the ball mill. Then, a binder was added in the powder
 materials thus obtained, and dried, granulated and further treated by
 pressing, thereby obtaining toroidal members each having a diameter of 20
 mm, an inner diameter of 10 mm and a height of 2 mm. After calcining at a
 temperature of 870.degree. C. for 2 hours, ferrite ceramics were thus
 obtained.
 During the above process, the specific surface area of the powder material
 after mixing, the powder material after pre-calcining and the powder
 material after disintegrating were measured by the BET method. Further, an
 X-ray diffraction analysis was conducted on the powder material after
 pre-calcining, and a spinel crystal synthesizability was calculated. The
 above results are shown in Table 1.
 For the ferrite ceramic thus obtained, the density was calculated by using
 Archimedes method, thereby obtaining a relative density (indicated by %)
 with respect to a theoretical density. Further, initial magnetic
 permeability was measured with the use of an impedance analyzer. The
 results are indicated in Table 1. In Table 1, samples labeled with marks *
 are used to represent those outside of the ranges of the present
 invention, while the other samples are within the ranges of the present
 invention.
 TABLE 1
 Specific
 Surface Specific Surface Area
 Relative
 Area of Pre-calcining (m.sup.2 /g) Spinel
 crystal Calcining
 .alpha.-Fe.sub.2 O.sub.3 Temperature After After Pre-
 synthesizability Density Initial Magnetic
 Sample No. (m.sup.2 /g) (.degree. C.) Mixing calcining After Milling (%)
 (%) Permeability
 *1 5.50 700 5.2 4.2 4.6 83
 82 350
 *2 7.30 700 7.1 4.5 4.9 85
 93 750
 3 9.21 700 9.0 5.3 6.3 88
 97 833
 4 10.97 700 10.1 5.5 6.9 89
 96 815
 5 13.55 700 12.8 5.7 7.5 92
 98 920
 6 16.21 700 15.3 6.0 7.8 93
 98 935
 7 18.20 700 16.2 6.3 8.2 95
 97 940
 8 22.50 700 20.5 7.2 9.2 98
 98 960
 *9 6.0 500 5.5 5.1 5.4 70
 67 90
 *10 6.0 700 5.5 3.9 4.1 88
 88 450
 *11 6.0 750 5.5 3.7 3.8 95
 78 370
 *12 6.0 800 5.5 3.4 3.5 97
 72 240
 *13 6.0 850 5.5 3.0 3.1 98
 68 130
 14 13.55 600 12.5 6.9 8.7 85
 96 850
 15 13.55 700 12.5 5.1 6.9 97
 95 830
 16 13.55 750 12.5 5.7 7.5 92
 98 920
 *17 13.55 550 12.5 8.8 10.6 74
 82 620
 *18 13.55 800 12.5 4.2 5.3 100 87
 670
 *19 13.55 850 12.5 3.9 4.5 100 77
 320
 It is understood from sample numbers 3 to 8 and 14 to 16 shown in Table 1,
 that if the method of the present invention uses .alpha.-Fe.sub.2 O.sub.3
 as an iron compound having a specific surface area of about 8.5 m.sup.2 /g
 or larger, the specific surface area of the powder material after mixing
 is set to be about 8.0 m.sup.2 /g or larger, the specific surface area of
 the powder material after pre-calcining is set to be about 5.0 m.sup.2 /g
 or larger, the specific surface area of the powder material after
 disintegration is set to be about 6.0 m.sup.2 /g or larger, and spinel
 crystal synthesizability after pre-calcining is set to be about 85% to
 98%, a Ni--Cu--Zn ferrite material made by using such manufacturing method
 will exhibit a relative calcining density of 95% or higher when calcined
 at a temperature of 870.degree. C. Further, the initial magnetic
 permeability of the ferrite ceramic is 800 or higher, which is necessary
 for inhibiting a cross point frequency of the chip inductor at 10 MHz or
 lower.
 In contrast to this, as shown in sample numbers 1 and 2, when
 .alpha.-Fe.sub.2 O.sub.3 serving as an iron compound has a specific
 surface area which is smaller than 8.5 m.sup.2 /g, initial magnetic
 permeability will be lower than 800 and is this undesirable.
 When .alpha.-Fe.sub.2 O.sub.3 serving as an iron compound has a specific
 surface area which is smaller than 8.5 m.sup.2 /g as shown by sample
 number 9, even if its pre-calcining temperature is as low as 500.degree.
 C. and the specific surface area of the powder material after
 pre-calcining is set to be 5.0 m.sup.2 /g or larger, the spinel crystal
 synthesizability after pre-calcining will still be as low as 70%,
 resulting in an insufficient pre-calcining treatment. As a result, its
 relative calcining density is low and its initial magnetic permeability is
 also low, therefore it is not desirable. On the other hand, as shown by
 sample numbers 10 to 13, if the pre-calcining temperature is set to be
 700.degree. C. or higher, although the spinel crystal synthesizability
 after pre-calcining will become as high as 85% to 98%, the specific
 surface area of the powder material after pre-calcining will be smaller
 than 5.0 m.sup.2 /g, and the specific surface area of the powder material
 after disintegration will be smaller than 6.0 m.sup.3 /g, resulting in a
 low relative calcining density and a low initial magnetic permeability and
 which is not desirable.
 Further, as shown by sample number 17, even when the specific surface area
 of the powder material after pre-calcining is 5.0 m.sup.2 /g or larger and
 the specific surface area of the powder material after disintegration is
 6.0 m.sup.2 /g or larger, if the spinel crystal synthesizability after
 pre-calcining is low and out of a range of 85% to 98%, its relative
 calcining density will become low and its initial magnetic permeability
 will also become low which is not desirable. On the other hand, as shown
 by sample numbers 18 and 19, even when the spinel crystal synthesizability
 after pre-calcining is high, if the specific surface area of the powder
 material after pre-calcining is smaller than 5.0 m.sup.2 /g and the
 specific surface area of the powder material after disintegration is
 smaller than 6.0 m.sup.2 /g, its relative calcining density will become
 low and its initial magnetic permeability will also become low and this is
 not desirable.
 Although in the above embodiment .alpha.-Fe.sub.2 O.sub.3 obtained in a wet
 synthesizing method was used as an iron compound, the present invention is
 not be limited by this. It is also possible to use as an iron compound
 .alpha.-Fe.sub.2 O.sub.3 obtained in a method other than the wet
 synthesizing method, and also to use an iron compound such as Fe.sub.3
 O.sub.4, FeOOH or the like.
 EXAMPLE 2
 At first, an .alpha.-Fe.sub.2 O.sub.3 powder material having a specific
 surface area of 12.0 m.sup.2 /g or 6.2 m.sup.2 /g was prepared as an iron
 compound, a ZnO powder material having a specific surface area of 45.3
 m.sup.2 /g or 4.1 m.sup.2 /g was prepared as a zinc compound, and a NiO
 powder material having a specific surface area of 9.0 m.sup.2 /g or 4.1
 m.sup.2 /g was prepared as a nickel compound. Further, a CuO powder
 material was prepared as a copper compound. Thereafter, the specific
 surface areas of the powder materials were selected as shown in Table 2
 and these compounds were combined in weights so that Fe.sub.2 O.sub.3 is
 48.7 mol %, ZnO is 26.9 mol %, CuO is 10.5 mol %, with the balance being
 NiO. The compounds were then wet mixed in a ball mill, followed by drying.
 Next, powder materials obtained after mixing were pre-calcined at various
 temperatures shown in Table 2. Subsequently, the powder materials were wet
 milled in the ball mill. Then, a binder was added in the powder materials
 thus obtained, and dried, granulated and further treated by pressing,
 thereby obtaining toroidal members each having a diameter of 20 mm, an
 inner diameter of 10 mm and a height of 2 mm. After calcining at a
 temperature of 870.degree. C. for 2 hours, ferrite ceramics were thus
 obtained.
 During the above process, the specific surface areas of the powder material
 after mixing, powder material after pre-calcining and powder material
 after disintegrating were measured by the use of BET method. Further, an
 X-ray diffraction analysis was conducted on the powder material after
 pre-calcining, and a spinel crystal synthesizability was calculated. The
 above results are shown in Table 2.
 The density of the ferrite ceramic thus obtained was calculated by using
 Archimedes method, thereby obtaining a relative density (indicated by %)
 with respect to a theoretical density. Further, the initial magnetic
 permeability was measured with the use of an impedance analyzer. The above
 results are indicated in Table 2. In Table 2, marks * are used to
 represent those samples outside of the ranges of the present invention,
 while the other samples are all within the ranges of the present
 invention.
 TABLE 2
 Specific Surface Areas
 of Various Specific Surface Area Spinel
 Relative
 Compounds Pre-calcining (m.sup.2 /g) Synthesiz-
 Calcining
 Sample (m.sup.2 /g) Temperature After After Pre- After ability
 Density Initial Magnetic
 No. Fe.sub.2 O.sub.3 ZnO NiO (.degree. C.) Mixing calcining
 Milling (%) (%) Permeability
 1 12.0 45.3 9.0 600 20.5 7.5 9.8 92 98
 1250
 2 12.0 45.3 9.0 700 20.5 6.7 8.5 95 98
 1280
 *3 12.0 4.1 4.1 500 12.5 8.8 10.6 74
 82 620
 *4 12.0 4.1 4.1 700 12.5 6.9 8.7 85
 96 850
 *5 12.0 4.1 4.1 750 12.5 5.7 7.5 92
 98 970
 *6 12.0 4.1 4.1 800 12.5 5.1 6.9 97
 95 830
 *7 12.0 4.1 4.1 850 12.5 4.2 5.3 100 87
 670
 *8 6.2 4.1 4.1 500 5.5 5.1 5.4 70
 67 90
 *9 6.2 4.1 4.1 700 5.5 3.9 4.1 88
 88 450
 *10 6.2 4.1 4.1 750 5.5 3.7 3.8 95
 78 370
 *11 6.2 4.1 4.1 800 5.5 3.4 3.5 97
 72 240
 *12 6.2 4.1 4.1 850 5.5 3.0 3.1 98
 68 130
 *13 12.0 45.3 9.0 550 20.5 10.5 12.7 88
 95 860
 *14 12.0 45.3 9.0 750 20.5 5.5 7.2 100 92
 1120
 *15 12.0 45.3 9.0 800 20.5 4.5 6.0 100 88
 970
 As is understood from sample numbers 1, 2 shown in Table 2, when the method
 of the present invention uses, as an iron compound, an .alpha.-Fe.sub.2
 O.sub.3 powder material having a specific surface area of 8.5 m.sup.2 /g
 or larger, and uses, as said nickel compound and said zinc compound,
 powder materials each having a specific surface area of 8.0 m.sup.2 /g or
 larger, and when the specific surface area of the powder material after
 mixing is set to be 10.0 m.sup.2 /g or larger, the specific surface area
 of the powder material after pre-calcining is set to be 6.0 m.sup.2 /g or
 larger, the specific surface area of the powder material after
 disintegration is set to be 8.0 m.sup.2 /g or larger, the spinel crystal
 synthesizability after pre-calcining is set to be 90% to 95%, a Ni--Cu--Zn
 ferrite material made by using such manufacturing method will exhibit a
 relative calcining density of 98% or higher when calcined at a temperature
 of 870.degree. C. Further, the initial magnetic permeability of the
 ferrite ceramic 1200 or higher which is necessary for inhibiting a cross
 point frequency of the chip inductor at 5 MHz or lower.
 In contrast to this, as shown by sample numbers 3, 4, when an
 .alpha.-Fe.sub.2 O.sub.3 powder material for use as an iron compound has a
 specific surface area of 8.5 m.sup.2 /g or larger, and when each of a ZnO
 powder material for use as a zinc compound and a NiO powder material for
 use as a nickel compound have a specific surface area smaller than 8.0
 m.sup.2 /g, if the pre-calcining temperature is set to be 700 C or lower,
 it is possible that the specific surface area of the powder material after
 pre-calcining may be made to be 6.0 m.sup.2 /g or larger and further that
 the specific surface area of the powder material after disintegration may
 be made to be 8.0 m.sup.2 /g or larger, but the spinel crystal
 synthesizability will be lower than 90% and it is impossible to obtain an
 initial magnetic permeability of 1200 or higher, which is not desirable.
 When the specific surface area of ZnO powder material and the specific
 surface area of NiO powder material are smaller than 8.0 m.sup.2 /g, as
 shown by sample numbers 5 to 7, if the pre-calcining temperature is made
 to be 750.degree. C. or higher, it is possible to increase the spinel
 crystal synthesizability, but it is impossible to obtain an initial
 magnetic permeability of 1200 or higher, which is not desirable.
 As shown by sample numbers from 8 to 12, when the specific surface area of
 .alpha.-Fe.sub.2 O.sub.3 powder is less than 8.5 m.sup.2 /g and when the
 specific surface area of ZnO powder material and the specific surface area
 of NiO powder material are all smaller than 8.0 m.sup.2 /g, even if the
 pre-calcining temperature is elevated or lowered, it is still impossible
 to obtain a pre-calcined powder material having a specific surface area of
 6.0 m.sup.2 /g or larger. Moreover, if the pre-calcining temperature is
 elevated, although it is possible to increase the spinel crystal
 synthesizability, the reactivity of the powder material will become low.
 For this reason, it is impossible to increase the relative calcining
 density and thus it is impossible to obtain an initial magnetic
 permeability of 1200 or higher, which is not desirable.
 In addition, as shown by sample number 13, even when the specific surface
 area of the powder material after pre-calcining is 6.0 m.sup.2 /g or
 larger and the specific surface area of the powder material after
 disintegration is 8.0 m.sup.2 /g or larger, if the spinel crystal
 synthesizability after pre-calcining is out of a range extending from 90%
 to 95% and is low, it is impossible to obtain an initial magnetic
 permeability of 1200 or higher, which is not desirable. Moreover, as shown
 by sample numbers 14 and 15, even when the specific surface area of the
 powder material after pre-calcining is 6.0 m.sup.2 /g or larger and the
 specific surface area of the powder material after disintegration is 8.0
 m.sup.2 /g or larger, if the spinel crystal synthesizability after
 pre-calcining is undesirably high, such as 95%, it is impossible to
 increase the relative calcining density and it is also impossible to
 obtain an initial magnetic permeability of 1200 or higher.
 It is preferred that the .alpha.-Fe.sub.2 O.sub.3, ZnO and NiO powder
 materials are those synthesized in a wet method involving the use of a
 simplified disintegrating process for producing fine powder particles.
 Such powder materials are effective in preventing characteristic
 deterioration due to an invasion of an impurity.
 The iron compound is not limited to .alpha.-Fe.sub.2 O.sub.3 powder
 material. It is possible to use other iron compounds such as Fe.sub.3
 O.sub.4 and FeOOH. The nickel compound is not limited to NiO as it is
 permitted to use other nickel compounds such as Ni.sub.2 O.sub.3.
 Furthermore, the copper oxide raw material for use as a copper compound is
 preferred to have a specific surface area of about 4.0 m.sup.2 /g or
 larger, since it is useful for obtaining a uniform reactivity and thus
 more preferable.
 While preferred embodiments of the invention have been disclosed, various
 modes of carrying out the principles disclosed herein are contemplated as
 being within the scope of the following claims. Therefore, it is
 understood that the scope of the invention is not to be limited except as
 otherwise set forth in the claims.