U.S. patent application number 12/471479 was filed with the patent office on 2009-10-08 for fe-based amorphous magnetic powder, magnetic powder core with excellent high frequency properties and method of making them.
Invention is credited to Deren Li, Caowei Lu, Zhichao Lu, Ko Sun, Shaoxlong Zhou.
Application Number | 20090249920 12/471479 |
Document ID | / |
Family ID | 36784540 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090249920 |
Kind Code |
A1 |
Lu; Zhichao ; et
al. |
October 8, 2009 |
Fe-based amorphous magnetic powder, magnetic powder core with
excellent high frequency properties and method of making them
Abstract
The present invention provides an amorphous alloy powder and
magnetic powder cores exhibiting excellent high frequency
properties and a method for making themof. The composition of said
alloy powder by atomic percentage satisfies the following formula:
(Fe.sub.1-xM.sub.x).sub.100-a-b-cP.sub.aT.sub.bD.sub.c, wherein M
represents at least one element of Co and Ni; T is over three
elements selected from Al, C, B and Si, D is at least one element
of Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Pt, Pd and Au; the
subscripts x, a, b, and c satisfy the relationships
0.01.ltoreq.x.ltoreq.0.16, 8.ltoreq.a.ltoreq.15,
10.ltoreq.b.ltoreq.25 and 0.5.ltoreq.c.ltoreq.6. The said amorphous
alloy powder is made by atomization method and a magnetic powder
core comprises a molded article of mixture of the said alloy powder
and an insulating material. A method of making the amorphous alloy
powder core includes the steps of screening, insulating,
compacting, annealing and spray painting.
Inventors: |
Lu; Zhichao; (Beijing,
CN) ; Lu; Caowei; (Beijing, CN) ; Li;
Deren; (Beijing, CN) ; Sun; Ko; (Beijing,
CN) ; Zhou; Shaoxlong; (Beijing, CN) |
Correspondence
Address: |
Nixon Peabody LLP
200 Page Mill Road, Suite 200
Palo Alto
CA
94306
US
|
Family ID: |
36784540 |
Appl. No.: |
12/471479 |
Filed: |
May 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11558478 |
Nov 10, 2006 |
|
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12471479 |
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Current U.S.
Class: |
75/352 |
Current CPC
Class: |
C22C 33/0257 20130101;
B22F 2003/248 20130101; C22C 2202/02 20130101; H01F 1/15333
20130101; H01F 1/15366 20130101; H01F 1/15308 20130101; C22C
2200/02 20130101; B22F 2998/10 20130101; H01F 41/0246 20130101;
B22F 2998/10 20130101; B22F 9/082 20130101; B22F 1/0059 20130101;
B22F 3/02 20130101; B22F 3/24 20130101 |
Class at
Publication: |
75/352 |
International
Class: |
B22F 9/02 20060101
B22F009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2005 |
CN |
200510114933.4 |
Claims
1. A method for making amorphous alloy powder exhibiting excellent
soft magnetic properties in high frequency characterized in that
the alloy powder with composition in atomic percent of the
following formula:
(Fe.sub.1-xM.sub.x).sub.100-a-b-cP.sub.aT.sub.bD.sub.c, wherein M
represents at least one element of Co and Ni; T is over three
elements selected from Al, C, B and Si, D is at least one element
selected from Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Pt, Pd and
Au, the subscripts x, a, b, and c satisfy the relationships of
0.0.ltoreq.x.ltoreq.0.16, 8.ltoreq.a.ltoreq.15,
10.ltoreq.b.ltoreq.25 and 0.5.ltoreq.c.ltoreq.6 is made by
atomization method, wherein said atomization maybe water
atomization and/or gas atomization, while said gas atomization may
be vacuum gas atomization, non-vacuum gas atomization, adjustable
gas atomization or their combination.
2. The method for making amorphous alloy powder according to claim
1, characterized in that the loose packed density of the powder
.rho. satisfies the relation: .rho..gtoreq.2.4 g/cm.sup.3.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/558,478 filed Nov. 10, 2006, which claims priority to
Chinese Patent Application No. 200510114933.4 filed Nov. 16, 2005,
both of which are incorporated by reference herein.
RELATED TECHNICAL FIELD
[0002] The present invention relates to a Fe-based amorphous alloy
powder, and more particularly, relates to an amorphous soft
magnetic alloy powder core manufactured by said amorphous alloy
powder and a method for making themof.
ART OF BACKGROUND
[0003] Fe-based amorphous and nanocrystalline soft magnetic alloy,
for instance, amorphous Fe--Si--B series alloy disclosed by U.S.
Pat. No. 4,217,135 and Fe--Cu-M-Si--B series (wherein M is one of
Nb, Mo, Hf, Ta, etc.) nanocrystalline soft magnetic alloy disclosed
by U.S. Pat. No. 4,881,989 have been widely used in various
electronic parts and components because of their excellent soft
magnetic properties. To obtain the said Fe-based amorphous alloy, a
high cooling rate of approximately 10.sup.-5K/s is necessary.
Though amorphous alloy ribbon can be produced in large scale by
single roller rapidly quenched technology, it is still difficult to
obtain amorphous alloy powder directly from rapid quenching.
[0004] Fe-based amorphous and nanocrystalline alloy powder can be
obtained by pulverizing ribbons, wherein the magnetic powder core
can be obtained by several procedures such as sticking, pressing
annealing and so forth. The problem of said magnetic powder core is
that the powder obtained by pulverizing ribbons contains much
deformed powder and the insulation of the powder is difficult, thus
the core generally has low quality factor, high core losses.
[0005] Bulk amorphous Fe--Al--Ga--P--C--B--Si systems disclosed by
U.S. Pat. No. 5,876,519 have large glass formation ability, wherein
the supercooled liquid region is over 50K, bulk amorphous alloy of
1.5 mm in thickness can be obtained by mold casting and the said
alloy has excellent soft magnetic properties. By using the large
glass formation ability of the said alloy system, amorphous powder
can be prepared by atomization method, wherein a magnetic powder
core can be made thereof. The problem of said soft magnetic alloy
powder is that, firstly, it contains expensive element of Ga, which
is difficult to popularize due to its high price; secondly, it does
not contain antioxidation elements such as Ni, Cr, therefore,
antioxidation properties are poor and the powder easily oxidates
and its properties deteriorate while preparing powder by
atomization method. Moreover, a Fe-based amorphous alloy system
disclosed by Chinese Patent Publication No. CN1487536A at least
contains elements of P, C, B and a small amount of elements such as
Cr, Mo, W, V, Nb, etc. Said alloy system contains only a small
amount of antioxidation element such as Cr and Mo etc, resulting in
poor antioxidation ability during the process of atomization.
SUMMARY OF THE INVENTION
[0006] Accordingly, a primary object of the present invention is to
provide an amorphous alloy powder with excellent high frequency
properties, large glass formation ability, low-cost and low oxygen
content, and a method for making it.
[0007] Another object of the present invention is to provide a
magnetic powder core with excellent high frequency properties and
method for making them thereof.
[0008] In order to achieve the objects mentioned above, the present
invention involves the following aspects:
[0009] In one aspect, the present invention provides an amorphous
alloy powder with excellent soft magnetic properties in high
frequency. The compositions of said alloy powder by atomic percent
satisfies the following formula:
(Fe.sub.1-xM.sub.x).sub.100-a-b-cP.sub.aT.sub.bD.sub.c, wherein M
represents at least one element of Co and Ni; T is over three
elements selected from Al, C, B and Si; D is at least one element
selected from Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Pt, Pd and
Au. The subscripts x, a, b, and c satisfy the relationships
0.01.ltoreq.x.ltoreq.0.16, 8.ltoreq.a.ltoreq.15,
10.ltoreq.b.ltoreq.25 and 0.5.ltoreq.c.ltoreq.6. Preferably,
0.01.ltoreq.x.ltoreq.0.12, 9.ltoreq.a.ltoreq.12,
10.ltoreq.b.ltoreq.23 and 1.ltoreq.c.ltoreq.5,
22.ltoreq.(a+b+c).ltoreq.38.
[0010] The reduced glass transition temperature T.sub.rg of said
amorphous alloy powder satisfies T.sub.rg.gtoreq.0.53, wherein
T.sub.rg=T.sub.g/T.sub.m, T.sub.g represents glass transition
temperature, T.sub.m represents melting point of alloy. The
supercooled liquid region .DELTA.T.sub.x.gtoreq.20K, wherein
.DELTA.T.sub.x=T.sub.x-T.sub.g, T.sub.x represents crystallization
temperature. The oxygen content of the powder is below 4000
ppm.
[0011] In another aspect, the present invention provides a method
for making an amorphous alloy powder with excellent soft magnetic
properties in high frequency. The alloy components mentioned above
are used to prepare the alloy powder by atomization method, wherein
said atomization is water atomization and/or gas atomization, while
said gas atomization is one of vacuum gas atomization, non-vacuum
gas atomization, adjustable gas atomization or their
combination.
[0012] The loose packed density of the powder P obtained by the
process should satisfy the relation: .rho..gtoreq.2.4
g/cm.sup.3.
[0013] In the third aspect, the present invention provides an
amorphous magnetic powder core with excellent soft magnetic
properties in high frequency, comprising the components by weight
percentage as follows: 0.2%-7% of insulating agent, 0.01%-5% of
adhesives, 0.01%-2% of lubricants, the rest is said amorphous alloy
powder.
[0014] Wherein, said insulating agent is at least one selected from
following groups of substances: [0015] Oxide powder selected from
SiO.sub.2, CaO, Al.sub.2O.sub.3 and TiO.sub.2, [0016] Salts
selected from silicates and phosphates, [0017] Mineral powder
selected from mica powder and kaolinite, and [0018] surface film
produced by chemical deposition or self-oxidation.
[0019] Said adhesives are organic adhesives and/or inorganic
adhesives, wherein, the organic adhesives are at least one selected
from epoxy resins, the inorganic adhesives are at least one
selected from phosphates.
[0020] Said lubricants are one or a combination selected from
stearates and talc powder.
[0021] The magnetic properties of said magnetic powder core shall
satisfy the requirements of at least one, several or their
combination of the followings: [0022] Magnetic permeability is no
less than 35; [0023] Quality factor Q is not less than 30 at 1 MHz;
[0024] Per unit initial permeability is not less than 98% at 100 k
Hz, not less than 90% at 1M Hz; [0025] Coercive force H.sub.c
corresponding with static magnetic hysteresis loop in maximum
magnetic field of 2000 A/m is below 70 A/m.
[0026] In the fourth aspect, the present invention provides a
method for making the amorphous magnetic powder core with excellent
soft magnetic properties in high frequency. Said method comprises
the following steps:
(a) Using said amorphous alloy powder and a required content of
insulating agent, adhesives and lubricants, mixing them and then
drying them to obtain dry powder; (b) Compacting said dried powder
under a pressure of 500 MPa-3000 MPa to make magnetic powder core;
(c) Annealing said molded magnetic powder core below the
crystallization temperature of said amorphous powder.
[0027] After step (c), the process further including: (d) spray
painting magnetic powder core and (e) testing the properties of
magnetic powder core.
[0028] In step (c), the temperature for annealing said magnetic
powder core is between (T.sub.x-100.degree. C.) and T.sub.x,
wherein T.sub.x represents the crystallization temperature of said
alloy powder; annealing time is from 5 minutes to 300 minutes; the
atmosphere is one of vacuum, nitrogen and argon.
[0029] In step (c), the annealing temperature is between
(T.sub.x-70.degree. C.) and (T.sub.x-20.degree. C.), wherein
T.sub.x represents the crystallization temperature of said alloy
powder.
[0030] The magnetic properties of magnetic powder core obtained by
said process shall satisfy the requirements of at least one, or
their combination of the followings: [0031] Magnetic permeability
is over 35; [0032] Quality factor Q is not less than 30 at 1 MHz;
[0033] Per unit of initial permeability is not less than 98% at 100
kHz, not less than 90% at 1 MHz; [0034] Coercive force Hc
corresponding with static magnetic hysteresis loop in maximum
magnetic field of 2000 A/m is below 70 A/m.
[0035] To sum up, the present invention provides a technical
solution for making amorphous alloy powder and amorphous magnetic
powder core, wherein the antioxidation properties and glass
formation ability of the alloy are enhanced by the improvement of
the components of the alloy, therefore, the amorphous alloy powder
can be made by atomization method. Said powder is prepared into
dried powder after insulating treatment and mixing with a small
amount of adhesives, then molded into core. After proper heat
treating, magnetic powder core can be obtained. The detailed
procedures will be described hereinafter.
Improvement of Alloy Composition
[0036] The iron element is the main component of the amorphous
alloy powder of the present invention and a small amount of Co and
Ni is included thereof, so soft magnetic properties are improved
and antioxidation properties of powder are enhanced in the presence
of Co and Ni; meanwhile the present invention contains more glass
formation elements such as P, Al, C, B and Si, thus amorphous alloy
powder is formed. Moreover, the present invention contains more
than one kind of elements such as Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti,
Zr, Hf, Pt, Pd, Au, etc., which will further improve the glass
formation ability of alloy and antioxidation properties. For
instance, the glass formation ability is improved in presence of
the elements such as Sn, Zr, etc., and the addition of Cr and Mo
and some other elements can not only improve glass formation
ability, but also enhance the antioxidation properties of powder as
well.
[0037] In said amorphous alloy powder of the present invention, the
compositions of atomic percent of the alloy should satisfy the
following formula:
(Fe.sub.1-xM.sub.x).sub.100-a-b-cP.sub.aT.sub.bD.sub.c
[0038] Wherein M represents at least one element of Co and Ni; T is
over three elements selected from Al, C, B and Si; D is at least
one element selected from Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti, Zr, Hf,
Pt, Pd and Au; and all by atomic percentage
0.01.ltoreq.x.ltoreq.0.16; 8.ltoreq.a.ltoreq.15;
10.ltoreq.b.ltoreq.25; 0.5.ltoreq.c.ltoreq.6.
[0039] As to the amorphous alloy powder of the present invention,
the content of Co and Ni is preferably from 1 at. % to 12 at. %;
the content of P is preferably from 9 at. % to 12 at. %; the
content of Al, C, B and Si is preferably from 10 at. % to 23 at. %;
the content of Sn, Cr, Mn, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Pt, Pd and
Au is preferably from 1 at. % to 5 at. %.
[0040] As to the amorphous alloy powder of the present invention,
the sum of elements of Fe, Co and Ni is preferably from 62 at. % to
78 at. %; the sum of elements of P, Al, C, B, Si, Sn, Cr, Mn, Mo,
W, V, Nb, Ta, Ti, Zr, Hf, Pt, Pd and Au is preferably from 22 at. %
to 38 at. %.
[0041] The alloy of the present invention will inevitably contain a
small amount of 0 and other impurities, wherein the total quantity
of these impurities is no more than 0.5 wt. %.
Preparation of Amorphous Alloy Powder by Atomization Method
[0042] The amorphous alloy powder of the present invention has
large glass formation ability, so amorphous alloy powder can be
prepared by atomization method; the supercooled liquid region
.DELTA.T.sub.x.gtoreq.20K (.DELTA.T.sub.x=T.sub.x-T.sub.g, wherein
T.sub.x represents crystallization temperature and, T.sub.g
represents glass transition temperature, the reduced glass
transition temperature T.sub.rg.gtoreq.0.53
(T.sub.rg=T.sub.g/T.sub.m, wherein T.sub.m represents melting point
of alloy).
[0043] The amorphous alloy powder of the present invention can be
prepared by water atomization method under non-vacuum condition,
wherein the maximum particle size of said amorphous alloy powder is
greater than 75 .mu.m and the oxygen content of the powder is below
4000 ppm. The loose packed density of said water atomized amorphous
alloy powder is characterized in that, when the particle size is
between -200 mess (about 74 .mu.m) and +300 mess (about 49 .mu.m),
the loose packed density of powder is more than 2.7 g/cm.sup.3;
when the particle size is between -300 mess (about 49 .mu.m) and
+400 mess (about 38 .mu.m), the loose packed density of powder is
more than 2.6 g/cm.sup.3; when the particle size is below -400 mess
(about 38 .mu.m) and over 5 .mu.m, the loose packed density of
powder is over 2.5 g/cm.sup.3.
[0044] In the present invention, the water atomized amorphous alloy
powder is used to make magnetic powder core, wherein oxygen content
is below 4000 ppm. If the oxygen content is too high, magnetic
properties of powder will deteriorate. If properties of magnetic
powder core prepared by using said powder are not fine which
results in the decrease of permeability and increase of coercive
force of magnetic powder core.
[0045] In the present invention, the water atomized amorphous alloy
powder is used to prepare a magnetic powder core, wherein the loose
packed density of the amorphous alloy powder is over 2.5
g/cm.sup.3. If the loose packed density is too small, properties of
magnetic powder core will not be fine. Too small loose packed
density is generally resulted from small particle size of powder or
complicated shapes of powder or much more pores contained in
powder. The magnetic powder core prepared by using said powder has
the drawbacks of low density, large air gaps distributed in
magnetic powder core, low magnetic permeability of powder core,
larger coercive force and high core losses. The loose packed
density of amorphous alloy powder of the present invention is over
2.5 g/cm.sup.3, preferably over 2.8 g/cm.sup.3.
[0046] Compared with water atomization, gas atomization has lower
cooling rate. In the alloy system of the present invention, the
nearly ball-shaped amorphous alloy powder is made by vacuum gas
atomization, non-vacuum gas atomization and adjustable gas
atomization (a method that alloy liquid is atomized to prepare
powder by gas and then the powder particles are cooled by water)
and so on, wherein the particle size of said amorphous alloy powder
is greater than 50 .mu.m and the oxygen content of the powder is
below 1500 ppm. The loose packed density of gas atomized amorphous
alloy powder is over 3.5 g/cm.sup.3.
Magnetic Powder Core Prepared by Amorphous Alloy Powder
[0047] In the present invention, the gas atomized amorphous alloy
powder is used to prepare magnetic powder core, wherein the oxygen
content is below 1500 ppm. The magnetic powder core prepared by
using said amorphous alloy powder has good magnetic properties,
high magnetic permeability and low coercive force. Compared with
water atomized powder, magnetic powder core prepared by using gas
atomized powder has a higher cost and advanced properties, which
will satisfy the requirements of some high-level products.
[0048] The process for preparing magnetic powder core by using
amorphous alloy powder of the present invention comprises the
following steps:
1. Mixing the powder with insulating agent, adhesives and lubricant
and then drying them to made dried powder; 2. Pressing the powder
to form magnetic powder core; 3. Annealing magnetic powder core; 4.
Spray painting magnetic powder core; 5. Testing properties of
magnetic powder core.
[0049] The amorphous alloy powder prepared is screened by test
sieve, standard spanking vibration sieve, other types of vibration
sieves and pneumatic powder classifier equipments. Magnetic powder
core is prepared according to the following steps:
Step 1
[0050] In order to increase the resistivity of the magnetic powder
core, reduce eddy current losses and enhance magnetic permeability
in high frequency, the present invention shall preferably select
the following types of insulating agent and amorphous alloy powder
to mix and insulate: 1. Oxide powder, such as SiO.sub.2, CaO,
Al.sub.2O.sub.3, TiO.sub.2, etc., generally, oxide powder has the
advantages of stable properties, excellent properties of insulation
and heat-resistance and low cost. 2. Silicates, phosphates, etc. 3.
Other mineral powder such as mica powder, kaolinite, etc. 4.
Surface film chemically formed or surface oxide occurred.
[0051] If said insulating agent is used to insulate the amorphous
alloy powder, the weight percentage of insulating agent should be
between 0.2 wt. % and 7 wt. % of the total mixture weight. If the
insulating agent is less, the amorphous alloy powder will be
difficult to be fully separated, thus resulting in more contact
surface; if the insulating layer is too thin, the insulating layer
between powder will easily breakdown, losing insulating effect
under the action of electromagnetic induction, causing larger
losses of magnetic powder core and low magnetic permeability in
high frequency. If too much insulating agent is provided, the gap
between powders will be too large, resulting in a decrease of
magnetic permeability of the magnetic powder core. The weight
percentage of insulating agent more preferably is from 0.5 wt. % to
5 wt. %.
[0052] Molding properties of amorphous alloy powder are not good.
The molding of gas atomized powder is especially difficult. So the
following types of adhesive substances are preferable to serve as
the adhesives for the present invention: 1. Organic adhesives, such
as epoxy resin, which has been commonly used in industrial product
and the mixing with curing agent shall have better effect on
sticking. 2. Inorganic adhesives, such as phosphates, etc.,
inorganic adhesives have the advantages of good heat-resistant
properties and excellent insulation properties in itself and dual
functions of insulation and sticking, and an additional proper
amount will make the powder fully adhesive.
[0053] The content of adhesive shall not more than 5% of the total
weight while using said adhesive materials. If too much adhesive is
added, properties of magnetic powder core will deteriorate and so
will magnetic permeability.
[0054] The mixture of lubricant functions as: 1. The powder is easy
to flow while pressing the powder, thus increase the density of
magnetic powder core; 2. Magnetic core is not prone to stick with
press mold, thus demolding becomes easier. Stearates and talc
powder is selected preferably as lubricant substances for the
present invention, wherein the weight is no more than 2 wt. % of
total weight of mixture. If too much lubricant is added, the
density of amorphous alloy powder in magnetic powder core will
decrease, resulting in the deterioration of magnetic properties and
reduction of magnetic permeability.
[0055] In order to obtain fully insulated and mixed amorphous
magnetic powder and excellent magnetic properties, the insulating
substances, adhesives and lubricants preferably occupy from 0.5 wt.
% to 10 wt. % of total weight of mixture for the present invention,
more preferably from 1 wt. % to 7 wt. %.
Step 2
[0056] The molding pressure of the amorphous alloy powder of the
present invention should be preferably from 500 MPa to 3000 MPa. If
the pressure is less than 500 MPa, the powder will be difficult to
mold or cracks will still exist after molding, the magnetic
permeability will be low and the properties of magnetic powder core
will not be good. If the pressure is over 3000 MPa, the withstand
pressure of mold will be large, thus the mold will be easily
destroyed, and the powder will be difficult to insulate, losses of
powder core will be high and the quality factor will not be fine.
The molding pressure of magnetic powder core is more preferably
from 800 MPa to 2500 MPa.
Step 3
[0057] The cooling rate of amorphous alloy powder is comparatively
large during the preparation process, so stress will inevitably
exist inside said powder; magnetic powder core shall have stress
inside it while preparing under the action of stirring and
compression and these stress shall influence the properties of
magnetic powder core. The internal stress of powder and magnetic
powder core can be eliminated and the magnetic properties can be
improved by annealing the amorphous magnetic powder core. The
temperature of annealing amorphous magnetic powder core shall
satisfy the requirements of: 1. Annealing temperature should be
below the crystallization temperature of said alloy powder. The
crystallization of amorphous alloy powder in said powder core
results in magnetic permeability and the increase in losses. 2.
Annealing temperature shall not be below (T.sub.x-100.degree. C.),
wherein T.sub.x represents the crystallization temperature of said
alloy powder. 3. The annealing temperature of powder shall
preferably be between (T.sub.x-70.degree. C.) and
(T.sub.x-20.degree. C.). Since if the annealing temperature of the
powder core is too low, heat disturbance will be un-uniform, the
internal stress in amorphous powder core will not be eliminated and
the magnetic properties will not be fully enhanced.
[0058] The annealing time of amorphous powder core shall satisfy
the requirements of: 1. The annealing time of powder core is less
than 3 hours since the manufacture cost will increase with too long
annealing time and low effectiveness. 2. The annealing time of the
powder core is more than 5 minutes since uniform treatment cannot
be achieved while batch processing and the properties of powder
core cannot be uniform if the annealing time is too short. 3. The
annealing time of the powder core shall preferably be between 30
minutes and 90 minutes. For the present invention, the annealing
process mentioned above should preferably be carried out in a
protective atmosphere, which can be vacuum, hydrogen, nitrogen and
argon atmosphere.
Step 4
[0059] In order to protect magnetic powder core from powder
dropping and being eroded by air and from the deterioration of
magnetic properties, the magnetic powder core is protected by spray
painting. The spray painting materials shall preferably be of epoxy
resin or a mixture of epoxy resin and estrodur compounds with
relatively small curing stress. The thickness of the spray painting
is preferably from 50 .mu.m to 300 .mu.m.
Step 5
[0060] Various parameters of properties of amorphous magnetic
powder core for the present invention is tested respectively by the
following methods: 1. Test method for inductance and quality factor
Q, using enameled copper wires with diameter of 0.2 mm to wind 10
turns uniformly and to measure by Agilent 4294A precision impedance
analyzer; magnetic permeability of magnetic powder core is
calculated by the formula
.mu. e = l _ L 0.4 .pi. N 2 A 10 8 , ##EQU00001##
wherein l (unit is cm) represents average length of magnetic path,
N represents the turns of winding, A represents across section of
magnetic path (unit is cm.sup.2). 2. Measuring static magnetic
hysteresis loop of magnetic powder core by galvanometer, wherein
the maximum magnetic field is 2000 A/m.
[0061] The magnetic powder core prepared by alloy powder of the
present invention via said method has excellent soft magnetic
properties in high frequency. In detail, the properties of
amorphous magnetic powder core satisfy the following requirements:
the quality factor is larger than 50 and the magnetic permeability
is larger than 40 at 500 kHz; the quality factor is larger than 30
and the magnetic permeability is larger than 40 at 1 MHz; the
quality factor of magnetic powder core is larger than 20 and the
magnetic permeability is larger than 40 at 3 MHz; while the
decrease of magnetic permeability should be less than 10% in the
frequency range from 100K to 1M.
[0062] The amorphous alloy prepared according to the technical
solution of the present invention has large glass formation ability
and good antioxidation properties. Said alloy is characterized in
large glass formation ability, good antioxidation properties and
excellent soft magnetic properties. By preparing amorphous alloy
powder with low oxygen content via atomization method, the
amorphous alloy powder with excellent high-frequency magnetic
properties, low oxygen content and low cost and corresponding
magnetic powder core can be obtained.
BRIEF DESCRIPTION OF DRAWINGS
[0063] FIG. 1 is a photo of morphology of amorphous alloy powder
having composition of
Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4 prepared
according to embodiment 1 of the present invention.
[0064] FIG. 2 is a graph illustrating an X-ray diffraction pattern
of amorphous alloy powder having composition of
Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4 prepared
according to embodiment 1 of the present invention.
[0065] FIG. 3 is DSC thermogram of amorphous alloy powder having a
composition of prepared according to embodiment 1 of the present
invention.
[0066] FIG. 4 is a graph illustrating the static magnetic
hysteresis loop of magnetic powder core prepared according to
embodiment 1 of the present invention with maximum magnetic field
of 2000 A/m.
[0067] FIG. 5 is a graph illustrating the dependence of
permeability on frequency of an amorphous magnetic powder core
prepared according to embodiment 1 of the present invention in
different frequency.
[0068] FIG. 6 is a graph illustrating the dependence of quality
factors on frequency of an amorphous magnetic powder core prepared
according to embodiment 1 of the present invention and of MPP
magnetic powder core as comparison in different frequency.
[0069] FIG. 7 is a graph illustrating the dependence of magnetic
permeability on annealing temperature of an amorphous magnetic
powder core prepared according to embodiment 5 of the present
invention under different annealing temperature at 100 kHz.
[0070] FIG. 8 is a graph illustrating the dependence of quality
factor on annealing temperature of an amorphous magnetic powder
core prepared according to embodiment 5 of the present invention
under different annealing temperatures at 100 kHz and 1 MHz.
[0071] FIG. 9 is a graph illustrating the dependence of coercive
force H, on annealing temperature corresponding with the static
magnetic hysteresis loop of magnetic powder core prepared according
to embodiment 5 of the present invention under different annealing
temperatures in a maximum magnetic field of 2000 A/m.
EMBODIMENTS OF THE INVENTION
Embodiment 1
[0072] In said embodiment, amorphous
Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4
(composition serial number is 1) alloy powder is prepared by water
atomization method. The raw materials shall be Fe, Ni, Al, Sn,
P--Fe alloy, B--Fe alloy, graphite and Si.
[0073] The morphology of the powder having composition of
Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4 is shown
in FIG. 1. As shown in this figure, the small particle powder is
basically spherical, the big particle powder is elliptical and a
small amount of powder is irregular.
[0074] The X-ray diffraction pattern of powder having composition
of Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4 is
shown in FIG. 2. As shown in this figure, distinct wide atlas
exists in X-ray diffraction pattern and there is no notable
crystallization peak in it, which indicates that the alloy powder
is amorphous. Thereby,
Fe.sub.72Ni.sub.2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4
alloy can prepare amorphous alloy powder by water atomization
method.
[0075] The DSC thermogram of
Fe.sub.72Ni2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4 powder
is shown in FIG. 3. The speed is 10K/minute while performing DSC
testing. As shown in this figure, alloy glass transition
temperature T.sub.g is 646K and crystallization temperature T.sub.x
is 695K. The melting point T.sub.m tested by high temperature DSC
is 1209K, so we can calculate that the supercooled liquid region of
said alloy .DELTA.T.sub.x is 49K and the reduced glass transition
temperature T.sub.rg is 0.54.
[0076] The analysis of oxygen content of the powder indicates that
the oxygen content of the powder is 3500 ppm, which implies that
alloy bears stronger antioxidation properties and lower oxygen
content.
[0077] The amorphous alloy powder of -300 mess obtained by
screening is isothermal annealed for 30 minutes at a temperature of
440.degree. C., wherein the annealing process is in nitrogen
atmosphere. Said amorphous alloy powder uniformly mixes with 1.5
wt. % of SiO.sub.2 powder, 1 wt. % of epoxy resin and 0.3 wt. % of
zinc stearate and then the mixture is dried in the help of alcohol
as a co-solvent. The molding of the magnetic powder core is pressed
under the pressure of 2 GPa. The magnetic powder core is annealed
under vacuum condition. The annealing temperature is 400.degree. C.
and the annealing time is 90 minutes. The epoxy resin and estrodur
compounds are used to spray and paint the surface of magnetic
powder core. The thickness of the spray painting layer is 100
.mu.m.
[0078] Static magnetic hysteresis loop of said magnetic powder core
with the maximum magnetic field of 2000 A/m is shown in FIG. 4. As
shown in this figure, magnetic powder core basically keeps constant
permeability properties within the range of testing magnetic field
and coercive force H.sub.c is 45 A/m.
[0079] The measuring result of magnetic permeability of said
magnetic powder core is given in FIG. 5. As shown in this figure,
magnetic permeability of said magnetic powder core is 46.9. With
the increase of frequency, magnetic powder core keeps excellent
constant permeability properties. Within the range of 6.3 kHz to 5
MHz, magnetic permeability drops from 46.9 to 46.4. The percentage
of dropping is less than 2%.
[0080] FIG. 6 shows the dependence of quality factor of said
magnetic powder core on frequency and the comparison example of
FeNiMo magnetic powder core is also shown. As shown in this figure,
amorphous alloy magnetic powder core has higher quality factor in
high frequency.
Embodiment 2
[0081] In said embodiment, amorphous Fe--Ni--Sn--Al--P--C--B--Si
series alloy powder is prepared by water atomization method. The
raw materials shall select Fe, Ni, Sn, Al, P--Fe alloy, graphite,
B--Fe alloy and Si.
[0082] The nominal compositions of said embodiment are listed in
Table 1. Glass transition temperature of corresponding alloy,
crystallization temperature, melting point, reduced glass
transition temperature and width of supercooled liquid region are
listed in Table 1 respectively. As seen from this table, except
that composition 10 is in a crystallization process, alloy with
other composition of said series alloy has high reduced glass
transition temperature. The minimum is 0.54 and the maximum is
0.58. The width of supercooled liquid region is over 20K.
TABLE-US-00001 TABLE 1 Alloy composition in atomic No. percentage
T.sub.g/K T.sub.x/K T.sub.m/K T.sub.g/T.sub.m (T.sub.x - T.sub.g)/K
1 Fe.sub.72Ni.sub.2Al.sub.4Sn.sub.2P.sub.10C.sub.2B.sub.4Si.sub.4
646 695 1209 0.54 49 2
Fe.sub.68.5Ni.sub.3Al.sub.7P.sub.11.5C.sub.3B.sub.5 650 700 1209
0.54 50 3
Fe.sub.67Ni.sub.7Sn.sub.2Al.sub.4P.sub.10B.sub.4C.sub.2Si.sub.4 654
694 1208 0.54 40 4
Fe.sub.70.5Ni.sub.10Sn.sub.1Al.sub.4P.sub.13C.sub.2B.sub.3Si.sub.6
665 690 1204 0.55 25 5
Fe.sub.71Ni.sub.4Sn.sub.4Al.sub.1P.sub.10C.sub.4B.sub.2Si.sub.4 666
698 1206 0.55 32 6
Fe.sub.72Ni.sub.2Sn.sub.1Al.sub.3P.sub.9C.sub.2B.sub.8Si.sub.3 675
703 1211 0.56 28 7
Fe.sub.73Ni.sub.1Sn.sub.2Al.sub.4P.sub.10C.sub.2B.sub.4Si.sub.4 668
715 1202 0.56 47 8
Fe.sub.73.5Ni.sub.2Sn.sub.5P.sub.9.5C.sub.2B.sub.5Si.sub.3 680 702
1200 0.57 22 9
Fe.sub.74.5Ni.sub.1Sn.sub.2Al.sub.2P.sub.11.5B.sub.4Si.sub.5 698
729 1210 0.58 31 10
Fe.sub.77Ni.sub.3Sn.sub.1Al.sub.3P.sub.8C.sub.2B.sub.2Si.sub.4 --
-- 1208 -- -- Comparative 1
Fe.sub.68.5Ni.sub.3Al.sub.7P.sub.11.5C.sub.3B.sub.3Si.sub.2 652 700
1209 0.54 48
[0083] In the amorphous alloy powder with the composition mentioned
above, the oxygen content and the loose packed density of amorphous
alloy powder of -300 mess are listed in Table 2. As can be seen in
this table, except for the Comparison 1 of said series alloy, the
loose packed density of said series alloy is over 2.5 g/cm.sup.3
and the average oxygen content is below 3900 ppm, which implies
that the alloy has stronger antioxidation properties.
[0084] The amorphous alloy powder of -300 mess obtained by
screening the amorphous alloy powder with the compositions
mentioned above is isothermal annealed for 30 minutes at
440.degree. C., wherein annealing process is in vacuum protection.
Said amorphous alloy powder uniformly mixes with 1 wt. % of
SiO.sub.2 powder, 1.5 wt. % of epoxy resin and 0.3 wt. % of zinc
stearate and then the mixture is dried in the help of alcohol as
co-solvent. The mold magnetic powder core is pressed under the
pressure of 2 GPa. The magnetic powder core is annealed under
vacuum situation. The annealing temperature is 440.degree. C. and
annealing time is 60 minutes. The epoxy resin and estrodur
compounds are used to spray and paint the surface of magnetic
powder core. The thickness of spray painting layer is 100
.mu.m.
[0085] The properties of amorphous alloy powder core prepared by
said method mentioned above are also listed in Table 2, wherein the
atomization method is not suitable for Comparison 1, which will get
the amorphous alloy powder with high oxygen content and low loose
packed density. It can be concluded from Table 2 that the magnetic
powder core made of amorphous alloy powder with lower coercive
force and higher magnetic permeability compared with Comparison 1.
Magnetic properties of composition 10 deteriorate because of
crystallization; coercive force of alloy with other compositions
(except 10 and Comparison 1) is below 60 A/m, the average magnetic
permeability is over 35 which is 98% or more than the magnetic
permeability at 100 kHz and 90% or more at 1 MHz, wherein the
quality factor is over 40 at 1 MHz.
TABLE-US-00002 TABLE 2 Powder characteristic Loose Oxygen packed
content density H.sub.c Properties of powder core (ppm)
(g/cm.sup.3) structure (A/m) .mu..sub.10 kHz .mu..sub.100 kHz
.mu..sub.1 MHz Q.sub.10 kHz Q.sub.100 kHz Q.sub.1 MHz 1 3500 2.7
amorphous 45 47 47 46 7 38 67 2 3500 2.6 amorphous 53 43 43 41 6 32
72 3 2600 2.7 amorphous 40 60 60 59 8 35 77 4 3800 2.5 amorphous 40
53 53 53 5 29 66 5 3000 2.7 amorphous 26 42 42 42 6 33 75 6 3300
2.6 amorphous 34 39 39 39 4.5 30 69 7 2800 2.6 amorphous 35 62 62
60 5.5 32 75 8 3900 2.5 amorphous 50 38 38 38 4 27 48 9 3500 2.6
amorphous 47 46 46 44 4.5 28 63 10 3200 2.8 crystallized 120 8 8 8
5 7 33 Comparative 1 5200 2.3 amorphous 65 35 35 35 4.5 27 60
Embodiment 3
[0086] In said embodiment, amorphous
Fe--Co--Cr--Sn--Al--P--C--B--Si series alloy powder is prepared by
water atomization method. The raw materials are Fe, Co, Cr, Sn, Al,
P--Fe alloy, graphite, B--Fe alloy and Si.
[0087] The nominal composition of said embodiment is listed in
Table 3. The lists of glass transition temperature of corresponding
alloy, crystallization temperature, melting point, approximate
glass transition temperature and width of super cooled liquid
region are listed in Table 3 respectively. As seen from this table,
except that composition 20 cannot be fully formed into amorphous
materials, alloy with other compositions all has high reduced glass
transition temperature. The minimum is 0.58 and the maximum is
0.60. The width of super cooled liquid region is over 20K.
TABLE-US-00003 TABLE 3 3 No. Alloy composition in atomic percentage
T.sub.g/K T.sub.x/K T.sub.m/K T.sub.g/T.sub.m (T.sub.x - T.sub.g)/K
11
Fe.sub.70Co.sub.2Cr.sub.2Sn.sub.2Al.sub.4P.sub.10C.sub.2B.sub.4Si.sub.4
705 744 1208 0.58 39 12
Fe.sub.68.5Co.sub.2Cr.sub.1Sn.sub.2Al.sub.7P.sub.11.5C.sub.3B.sub.3Si.s-
ub.2 715 740 1197 0.60 25 13
Fe.sub.68.5Co.sub.1Cr.sub.5Sn.sub.3Al.sub.2P.sub.11.5C.sub.5B.sub.2Si.s-
ub.2 715 753 1223 0.58 38 14
Fe.sub.69.5Co.sub.1Cr.sub.0.5Sn.sub.1Al.sub.4P.sub.13C.sub.2B.sub.3Si.s-
ub.6 701 738 1183 0.59 37 15
Fe.sub.69.5Co.sub.2Cr.sub.4Sn.sub.4Al.sub.1P.sub.10C.sub.4B.sub.2Si.sub-
.4 723 758 1215 0.60 35 16
Fe.sub.70Co.sub.2Cr.sub.2Sn.sub.1Al.sub.3P.sub.9C.sub.2B.sub.8Si.sub.3
704 739 1216 0.58 35 17
Fe.sub.71Co.sub.2Cr.sub.1Sn.sub.2Al.sub.4P.sub.10C.sub.2B.sub.4Si.sub.4
671 724 1204 0.56 53 18
Fe.sub.71.5Co.sub.5Cr.sub.1Al.sub.2P.sub.10.5C.sub.2B.sub.5Si.sub.3
698 731 1201 0.58 33 19
Fe.sub.72.5Co.sub.2Cr.sub.2Al.sub.4P.sub.9.5C.sub.1B.sub.4Si.sub.5
701 732 1209 0.58 31 20
Fe.sub.75Co.sub.3Cr.sub.3Al.sub.3P.sub.8C.sub.2B.sub.2Si.sub.4 --
-- 1250 -- --
[0088] In the amorphous alloy powder with the compositions
mentioned above, the oxygen content and the loose packed density of
amorphous alloy powder of -300 mess are listed in Table 4. As shown
in this table, the average loose packed density of said series
alloy is not less than 2.5 g/cm.sup.3 and the average oxygen
content is below 3300 ppm, which implies that the alloy has
stronger antioxidation properties.
[0089] The amorphous alloy powder of -300 mess obtained by
screening the amorphous alloy powder with the compositions
mentioned above is isothermal annealed for 30 minutes at a
temperature of 440.degree. C., wherein annealing process is in
vacuum protection. Said amorphous alloy powder uniformly mixes with
1 wt. % of SiO.sub.2 powder, 1.5 wt. % of epoxy resin and 0.3 wt. %
of zinc stearate and then the mixture is dried in the help of
alcohol as co-solvent. The molding of magnetic powder core is
pressed under the pressure of 2 GPa. The magnetic powder core is
annealed under vacuum situation. The annealing temperature is
440.degree. C. and annealing time is 60 minutes. The epoxy resin
and estrodur compounds are used to spray and paint the surface of
magnetic powder core. The thickness of spray painting layer is 100
.mu.m.
[0090] The properties of amorphous alloy magnetic powder core
prepared by said method mentioned above are also listed in Table 4.
It can be seen from Table 4 that, except that composition 20 is in
a crystallization process and properties of magnetic powder core
deteriorate, other compositions all have more excellent magnetic
properties and magnetic permeability is not less than 45.
[0091] The coercive force of alloy with other compositions (except
10 and Comparison 1) is below 60 A/m, the average magnetic
permeability is over 35 which is 98% or more than the magnetic
permeability at 100 kHz and 90% or more at 1 MHz, wherein the
quality factor is over 60 at 1 MHz.
TABLE-US-00004 TABLE 4 Powder characteristic Oxygen Loose packed
Properties of powder core content density H.sub.c No. (ppm)
(g/cm.sup.3) structure (A/m) .mu..sub.10 kHz .mu..sub.100 kHz
.mu..sub.1 MHz Q.sub.10 kHz Q.sub.100 kHz Q.sub.1 MHz 11 2600 2.8.0
Amorphous 46 48 48 47 7 34 72 12 2900 2.9 Amorphous 37 60 60 58 8
38 85 13 2500 2.8 Amorphous 40 65 65 64 7 35 80 14 3000 2.8
Amorphous 41 65 65 64 6 33 75 15 2700 2.6 Amorphous 49 60 60 59 6
30 72 16 2800 2.6 Amorphous 50 53 53 52 6 32 75 17 3300 2.5
Amorphous 47 49 49 48 5 27 64 18 3000 2.7 Amorphous 54 45 45 44 4.5
28 68 19 3200 2.7 Amorphous 52 46 46 45 4.5 28 67 20 2800 2.7
crystallized 118 12 12 12 5 7 34
Embodiment 4
[0092] In said embodiment, amorphous
Fe--Co--Ni--Sn--Al--P--C--B--Si series alloy powder is prepared by
non-vacuum gas atomization method. The raw materials are Fe, Co,
Ni, Cr, Sn, Al, P--Fe alloy, graphite, B--Fe alloy and Si.
[0093] The nominal compositions of said embodiment are listed in
Table 5. The lists of glass transition temperature of corresponding
alloy, crystallization temperature, melting point, approximate
glass transition temperature and width of super cooled liquid
region are also listed in Table 5 respectively. As shown in this
table, except that composition 25 cannot be fully formed into
amorphous materials, alloy with other compositions of said series
alloy all has high reduced glass transition temperature. The
minimum is 0.58 and the maximum is 0.60. The width of super cooled
liquid region is over 20K.
TABLE-US-00005 TABLE 5 Series Compositions of alloy by atomic No.
percentage T.sub.g/K T.sub.x/K T.sub.m/K T.sub.g/T.sub.m (T.sub.x -
T.sub.g)/K 21
Fe.sub.71Co.sub.2Sn.sub.2Al.sub.4P.sub.10C.sub.2B.sub.5Si.sub.4 703
740 1208 0.58 37 22
Fe.sub.71Ni.sub.2Cr.sub.1Al.sub.4P.sub.11C.sub.2B.sub.6Si.sub.2 715
745 1197 0.60 30 23
Fe.sub.72.5Co.sub.1Ni.sub.1Sn.sub.3Al.sub.2P.sub.11.5C.sub.5B.sub.2Si.s-
ub.2 708 746 1223 0.58 38 24
Fe.sub.63Ni.sub.3Co.sub.3Cr.sub.0.5Al.sub.4P.sub.13C.sub.2B.sub.3Si.sub-
.6 720 743 1183 0.61 23 25
Fe.sub.63Ni.sub.5Co.sub.3Cr.sub.2Sn.sub.4Al.sub.1P.sub.10C.sub.4B.sub.2-
Si.sub.4 -- -- 1215 -- --
[0094] In the amorphous alloy powder with the components mentioned
above, the oxygen content and the loose packed density of amorphous
alloy powder of -350 mess are given in Table 6. As can be seen in
this table, the average loose packed density of said series alloy
is not less than 3.6 g/cm.sup.3 and the average oxygen content is
below 2000 ppm.
[0095] The amorphous alloy powder of -350 mess obtained by
screening the amorphous alloy powder with the composition mentioned
above is isothermal annealed for 30 minutes at a temperature of
400.degree. C., wherein annealing process is in vacuum protection.
Said amorphous alloy powder uniformly mixes with 1 wt. % of
SiO.sub.2 powder, 2 wt. % of epoxy resin and 0.3 wt. % of zinc
stearate and then the mixture is dried in the help of alcohol as
co-solvent. The molding of magnetic powder core is pressed under
the pressure of 2.4 GPa. The magnetic powder core is annealed under
vacuum situation. The annealing temperature is 440.degree. C. and
annealing time is 60 minutes. The epoxy resin and estrodur
compounds are used to spray and paint the surface of magnetic
powder core. The thickness of spray painting layer is 100
.mu.m.
[0096] The properties of amorphous alloy magnetic powder core
prepared by said method mentioned above are given in Table 6. It
can be concluded from Table 6 that, except that composition 25 is
in a crystallization process and properties of magnetic powder core
deteriorate, the coercive force of alloy with other components is
below 40 A/m, the average magnetic permeability is over 60 which is
98% or more than the magnetic permeability at 100 kHz and 90% or
more at 1 MHz, wherein the quality factor is over 60 at 1 MHz.
TABLE-US-00006 TABLE 6 Loose Oxygen packed Series content density
H.sub.c No. (ppm) (g/cm.sup.3) Structure (A/m) .mu..sub.10 kHz
.mu..sub.100 kHz .mu..sub.1 MHz Q.sub.10 kHz Q.sub.100 kHz Q.sub.1
MHz 21 1500 3.6 amorphous 32 72 72 70 9 45 98 22 1200 3.7 amorphous
28 70 70 68 9 40 88 23 1250 3.7 amorphous 26 68 68 66 9 40 92 24
1100 3.7 amorphous 25 75 75 73 9 41 96 25 800 3.7 crystallization
68 13 13 13 6 9 42
Embodiment 5
[0097] Amorphous alloy powder is prepared by the same method as
embodiment 1. The procedures for preparing magnetic powder core are
also the same. The only difference is the change of temperature of
heat treatment.
[0098] FIG. 7 shows the dependence of magnetic permeability on
annealing temperature at 100 kHz. As shown in the figure, the
magnetic permeability of said magnetic powder core increases with
the increasing of the temperature of heat treatment and the maximum
value is obtained at 440.degree. C., then the magnetic permeability
decreases with the increasing of annealing temperature. The maximum
magnetic permeability is about 50.
[0099] FIG. 8 shows the dependence of quality factor on annealing
temperature at 100 kHz and 1 MHz.
[0100] The dependence of the quality factor on annealing
temperature is similar to that of magnetic permeability. The
difference is that the maximum quality factor of magnetic powder
core appears under the annealing temperature of 380.degree. C. The
maximum quality factor is about 75 at 100 kHz; the maximum quality
factor is about 35 at 1 MHz.
[0101] FIG. 9 shows the dependence of coercive force H.sub.c on
annealing temperature corresponding to the static magnetic
hysteresis loop of magnetic powder core under maximum magnetic
field of 2000 A/m. As shown in the figure, the change trend of
coercive force is completely opposite to that of magnetic
permeability and the minimum coercive force is about 44 A/m.
* * * * *