U.S. patent application number 15/737429 was filed with the patent office on 2018-12-20 for method of manufacturing soft magnetic dust core and soft magnetic dust core.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE PRECISION CORPORATION, JFE STEEL CORPORATION, National Institute of Advanced Industrial Science and Technology, TOKIN CORPORATION. Invention is credited to Yu KANAMORI, Mineo MURAKI, Naomichi NAKAMURA, Makoto NAKASEKO, Koichi OKAMOTO, Kimihiro OZAKI, Shoichi SATO, Takuya TAKASHITA, Hoshiaki TERAO, Toshinori TSUDA, Akiri URATA, Raita WADA, Makoto YAMAKI.
Application Number | 20180361474 15/737429 |
Document ID | / |
Family ID | 57942706 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180361474 |
Kind Code |
A9 |
NAKAMURA; Naomichi ; et
al. |
December 20, 2018 |
METHOD OF MANUFACTURING SOFT MAGNETIC DUST CORE AND SOFT MAGNETIC
DUST CORE
Abstract
Provided is a soft magnetic dust core having high density and
favorable properties. A method of manufacturing a soft magnetic
dust core includes: preparing coated powder including amorphous
powder made of an Fe--B--Si--P--C--Cu-based alloy, an
Fe--B--P--C--Cu-based alloy, an Fe--B--Si--P--Cu-based alloy, or an
Fe--B--P--Cu-based alloy, with a first initial crystallization
temperature T.sub.x1 and a second initial crystallization
temperature T.sub.x2; and a coating formed on a surface of
particles of the amorphous powder; applying a compacting pressure
to the coated powder or a mixture of the coated powder and the
amorphous powder at a temperature equal to or lower than
T.sub.x1-100 K; and heating to a maximum end-point temperature
equal to or higher than T.sub.x1-50 K and lower than T.sub.x2 with
the compacting pressure being applied.
Inventors: |
NAKAMURA; Naomichi;
(Chiyoda-ku, Tokyo, JP) ; NAKASEKO; Makoto;
(Chiyoda-ku, Tokyo, JP) ; TAKASHITA; Takuya;
(Chiyoda-ku, Tokyo, JP) ; MURAKI; Mineo;
(Chiyoda-ku, Tokyo, JP) ; TERAO; Hoshiaki;
(Niigata-shi, Niigata, JP) ; WADA; Raita;
(Niigata-shi, Niigata, JP) ; URATA; Akiri;
(Sendai-shi, Miyagi, JP) ; KANAMORI; Yu;
(Sendai-shi, Miyagi, JP) ; YAMAKI; Makoto;
(Sendai-shi, Miyagi, JP) ; OKAMOTO; Koichi;
(Sendai-shi, Miyagi, JP) ; TSUDA; Toshinori;
(Sendai-shi, Miyagi, JP) ; SATO; Shoichi;
(Sendai-shi, Miyagi, JP) ; OZAKI; Kimihiro;
(Nagoya-shi, Aichi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION
JFE PRECISION CORPORATION
TOKIN CORPORATION
National Institute of Advanced Industrial Science and
Technology |
Chiyoda-ku, Tokyo
Niigata-shi, Niigata
Sendai-shi, Miyagi
Chiyoda-ku, Tokyo |
|
JP
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
JFE PRECISION CORPORATION
Niigata-shi, Niigata
JP
TOKIN CORPORATION
Sendai-shi, Miyagi
JP
National Institute of Advanced Industrial Science and
Technology
Chiyoda-ku, Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180169759 A1 |
June 21, 2018 |
|
|
Family ID: |
57942706 |
Appl. No.: |
15/737429 |
Filed: |
July 28, 2016 |
PCT Filed: |
July 28, 2016 |
PCT NO: |
PCT/JP2016/003512 PCKC 00 |
371 Date: |
December 18, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01F 1/15308 20130101;
H01F 1/15375 20130101; C22C 38/00 20130101; B22F 3/03 20130101;
B22F 1/02 20130101; B22F 3/00 20130101; C22C 38/16 20130101; H01F
1/15325 20130101; B22F 3/14 20130101; C22C 38/02 20130101; C22C
2200/04 20130101; H01F 27/255 20130101; B22F 2301/35 20130101; H01F
3/08 20130101; B22F 3/02 20130101; B22F 2304/10 20130101; H01F
1/15333 20130101; C22C 38/002 20130101; H01F 41/0246 20130101; C22C
33/02 20130101; C22C 2202/02 20130101; C22C 2200/02 20130101; C22C
45/02 20130101; B22F 1/00 20130101 |
International
Class: |
B22F 3/14 20060101
B22F003/14; C22C 45/02 20060101 C22C045/02; C22C 38/16 20060101
C22C038/16; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; B22F 1/02 20060101 B22F001/02; H01F 1/153 20060101
H01F001/153; H01F 41/02 20060101 H01F041/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2015 |
JP |
2015-152804 |
Claims
1-13. (canceled)
14. A method of manufacturing a soft magnetic dust core comprising:
preparing coated powder including amorphous powder made of an
Fe--B--Si--P--C--Cu-based alloy, an Fe--B--P--C--Cu-based alloy, an
Fe--B--Si--P--Cu-based alloy, or an Fe--B--P--Cu-based alloy, with
a first initial crystallization temperature T.sub.x1 and a second
initial crystallization temperature T.sub.x2; and a coating formed
on a surface of particles of the amorphous powder; applying a
compacting pressure to the coated powder or a mixture of the coated
powder and the amorphous powder at a temperature equal to or lower
than T.sub.x1-100 K; and heating to a maximum end-point temperature
equal to or higher than T.sub.x1-50 K and lower than T.sub.x2 with
the compacting pressure being applied.
15. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein the amorphous powder has a
composition containing, in atomic percent: Fe: 79% or more and 86%
or less; B: 4% or more and 13% or less; Si: 0% or more and 8% or
less; P: 1% or more and 14% or less; C: 0% or more and 5% or less;
Cu: 0.4% or more and 1.4% or less; and incidental impurities.
16. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein the composition contains total 3 at.
% or less of at least one selected from the group consisting of Co,
Ni, Ca, Mg, Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Al, Mn, Ag, Zn, Sn, As,
Sb, Bi, Y, N, O, S, and rare earth elements, instead of part of
Fe.
17. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein a mean particle diameter D.sub.50 of
the amorphous powder is 1 .mu.m to 100 .mu.m.
18. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein a mean particle diameter D.sub.50 of
the amorphous powder is 1 .mu.m to 100 .mu.m.
19. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein an apparent density AD (Mg/m.sup.3)
of the amorphous powder and the mean particle diameter D.sub.50
(.mu.m) satisfy AD.gtoreq.2.8+0.005.times.D.sub.50.
20. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein an apparent density AD (Mg/m.sup.3)
of the amorphous powder and the mean particle diameter D.sub.50
(.mu.m) satisfy AD.gtoreq.2.8+0.005.times.D.sub.50.
21. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein a crystallization degree of the
amorphous powder is 20% or less.
22. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein a crystallization degree of the
amorphous powder is 20% or less.
23. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein crystalline soft magnetic powder is
mixed with the amorphous powder or the coated powder.
24. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein crystalline soft magnetic powder is
mixed with the amorphous powder or the coated powder.
25. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein the compacting pressure is 100 MPa
to 2000 MPa, and a holding time is 120 minutes or less, the holding
time being defined as a time after the heating to the maximum
end-point temperature, during which the maximum end-point
temperature is kept while the compacting pressure is applied.
26. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein the compacting pressure is 100 MPa
to 2000 MPa, and a holding time is 120 minutes or less, the holding
time being defined as a time after the heating to the maximum
end-point temperature, during which the maximum end-point
temperature is kept while the compacting pressure is applied.
27. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein the heating is performed by
electrical heating.
28. The method of manufacturing a soft magnetic dust core,
according to claim 15, wherein the heating is performed by
electrical heating.
29. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein the heating is performed using at
least one heating source placed inside, outside, or both inside and
outside a mold used for the application of the compacting
pressure.
30. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein the heating is performed by both
electrical heating, and heating using at least one heating source
placed inside, outside, or both inside and outside a mold used for
the application of the compacting pressure.
31. The method of manufacturing a soft magnetic dust core,
according to claim 14, wherein prior to the application of the
compacting pressure, the amorphous powder is preformed at a filling
rate of 70% or less.
32. A soft magnetic dust core manufactured by the method according
to claim 14, the soft magnetic dust core having a green density of
78% or more, a crystallization degree of 40% or more, and
.alpha.-Fe crystallites with a size of 50 nm or less.
33. A soft magnetic dust core manufactured by the method according
to claim 15, the soft magnetic dust core having a green density of
78% or more, a crystallization degree of 40% or more, and
.alpha.-Fe crystallites with a size of 50 nm or less.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a method of manufacturing a soft
magnetic dust core, and in particular, relates to a method of
manufacturing an iron-based soft magnetic dust core having a
nanocrystalline structure. Further, this disclosure relates to a
soft magnetic dust core manufactured by the above-mentioned
manufacturing method.
BACKGROUND
[0002] A dust core is a magnetic core manufactured by green
compacting magnetic powder. An insulation coating is typically
applied to the surface of the particles of the material magnetic
powder, and a binder is added to the powder as necessary to improve
mechanical strength. Because of their structure, dust cores have
features including reduced eddy current losses and isotropic
magnetic properties, compared with laminated magnetic cores
obtained by stacking, for example, electrical steel sheets.
Accordingly, dust cores are being developed in the field of
high-frequency technology.
[0003] Of dust cores, dust cores made using crystalline powder as a
material have already been in practical use in a variety of
applications such as choke coils. Further, in parallel with the
dust cores using a crystalline material, nanocrystalline dust cores
using a nanocrystalline soft magnetic material are also being
developed.
[0004] A nanocrystalline soft magnetic material is a soft magnetic
material containing fine crystals. For example, an iron-based
nanocrystalline material, which is a typical nanocrystalline soft
magnetic material, can be obtained by subjecting an alloy to heat
treatment, the alloy including, as the main phase, an amorphous
structure having a structure that can exhibit a nanocrystalline
structure. The heat treatment is performed at a temperature equal
to or higher than the crystallization temperature determined in
accordance with the composition of the alloy. Performing the heat
treatment at an excessively high temperature would cause for
example coarsening of crystal grains and precipitation of a
non-magnetic phase. Accordingly, studies have been made to
manufacture iron-based nanocrystalline dust cores having favorable
properties.
[0005] For example, JP 2004-349585 A (PTL 1) and JP 2014-103265 A
(PTL 2) disclose techniques of manufacturing a nanocrystalline dust
core by mixing powder made of for example an
Fe--Si--B--Nb--Cu--Cr-based amorphous alloy with a binder and
pressing the mixed powder; and then performing heat treatment to
harden the binder, thereby precipitating the nanocrystalline phase
during the heat treatment.
[0006] Further, JP 5537534 B2 (PTL 3) discloses a method of
manufacturing a soft magnetic dust core by performing heat
treatment on Fe--B--Si--P--C--Cu-based amorphous powder and
nanocrystallization is performed on the powder followed by
pressing.
[0007] However, amorphous particles and the nanocrystallized
particles having been subjected to heat treatment are extremely
hard; in particular, the Vickers hardness of
Fe--B--Si--P--C--Cu-based powder described above in an amorphous
state at room temperature is approximately 800, and the Vickers
hardness of the powder having been nanocrystallized exceeds 1000.
Even when the powder made of such hard particles is green
compacted, the resulting dust core has low density, and the
magnetic properties cannot be improved sufficiently. To address
this problem, studies have been made to provide a method of
increasing the density of a nanocrystalline dust core made using
amorphous powder as a material.
[0008] For example, JP H07-145442 A (PTL 4) discloses a method of
manufacturing a high-density dust core by extruding Fe--B-based
amorphous powder having been heated to a temperature near its
softening point. The extrusion temperature in the above method is
set to be 300.degree. C. to 600.degree. C.
[0009] Further, JP H08-337839 A (PTL 5) discloses a method of
pressing and heating Fe--B-based amorphous powder similar to one in
PTL 4, in which the density of the green compact is increased by
setting the heating temperature to T.sub.x-100.degree. C. or higher
and T.sub.x+100.degree. C. or lower where T.sub.x is the initial
crystallization temperature of the amorphous powder. In the above
method, the density of the green compact is described as being
increased because the amorphous powder is softened in the above
temperature range.
[0010] In addition, JP 4752641 B2 (PTL 6) discloses a method in
which when metallic glass powder is sintered by pulsed electric
current sintering, the pattern of pressing and heating is
controlled, thereby preventing insulating layers applied to the
surface of powder particles from breaking and increasing the
density of the powder.
CITATION LIST
Patent Literature
[0011] PTL 1: JP 2004-349585 A
[0012] PTL 2: JP 2014-103265 A
[0013] PTL 3: JP 5537534 B2
[0014] PTL 4: JP H07-145442 A
[0015] PTL 5: JP H08-337839 A
[0016] PTL 6: JP 4752641 B2
SUMMARY
Technical Problem
[0017] However, even if the methods disclosed in PTLs 4 to 6 are
used, it has been difficult to form Fe--B--Si--P--C--Cu-based
amorphous powder having significantly high hardness as described
above into a dense green compact without damaging the insulation
coating applied to the surface of the powder particles and to
prevent secondary phases of borides or the like which would affect
the magnetic properties from being crystallized.
[0018] It could thus be helpful to provide a soft magnetic dust
core having high density and favorable properties.
Solution to Problem
[0019] Specifically, primary features of the present disclosure are
as follows.
[0020] 1. A method of manufacturing a soft magnetic dust core
comprising:
[0021] preparing coated powder including amorphous powder made of
an Fe--B--Si--P--C--Cu-based alloy, an Fe--B--P--C--Cu-based alloy,
an Fe--B--Si--P--Cu-based alloy, or an Fe--B--P--Cu-based alloy,
with a first initial crystallization temperature T.sub.x1 and a
second initial crystallization temperature T.sub.x2; and a coating
formed on a surface of particles of the amorphous powder;
[0022] applying a compacting pressure to the coated powder or a
mixture of the coated powder and the amorphous powder at a
temperature equal to or lower than T.sub.x1-100 K; and
[0023] heating to a maximum end-point temperature equal to or
higher than T.sub.x1-50 K and lower than T.sub.x2 with the
compacting pressure being applied.
[0024] 2. The method of manufacturing a soft magnetic dust core,
according to 1. above, wherein the amorphous powder has a
composition containing, in atomic percent:
[0025] Fe: 79% or more and 86% or less;
[0026] B: 4% or more and 13% or less;
[0027] Si: 0% or more and 8% or less;
[0028] P: 1% or more and 14% or less;
[0029] C: 0% or more and 5% or less;
[0030] Cu: 0.4% or more and 1.4% or less; and
[0031] incidental impurities.
[0032] 3. The method of manufacturing a soft magnetic dust core,
according to 2. above, wherein the composition contains total 3 at.
% or less of at least one selected from the group consisting of Co,
Ni, Ca, Mg, Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Al, Mn, Ag, Zn, Sn, As,
Sb, Bi, Y, N, O, S, and rare earth elements, instead of part of
Fe.
[0033] 4. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 3. above, wherein a mean particle
diameter D.sub.50 of the amorphous powder is 1 .mu.m to 100
.mu.m.
[0034] 5. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 4. above, wherein an apparent density
AD (Mg/m.sup.3) of the amorphous powder and the mean particle
diameter D.sub.50 (.mu.m) satisfy
AD.gtoreq.2.8+0.005.times.D.sub.50.
[0035] 6. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 5. above, wherein a crystallization
degree of the amorphous powder is 20% or less.
[0036] 7. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 6. above, wherein crystalline soft
magnetic powder is mixed with the amorphous powder or the coated
powder.
[0037] 8. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 7. above, wherein the compacting
pressure is 100 MPa to 2000 MPa, and
[0038] a holding time is 120 minutes or less, the holding time
being defined as a time after the heating to the maximum end-point
temperature, during which the maximum end-point temperature is kept
while the compacting pressure is applied.
[0039] 9. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 8. above, wherein the heating is
performed by electrical heating.
[0040] 10. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 8. above, wherein the heating is
performed using at least one heating source placed inside, outside,
or both inside and outside a mold used for the application of the
compacting pressure.
[0041] 11. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 8. above, wherein the heating is
performed by both
[0042] electrical heating, and
[0043] heating using at least one heating source placed inside,
outside, or both inside and outside a mold used for the application
of the compacting pressure.
[0044] 12. The method of manufacturing a soft magnetic dust core,
according to any one of 1. to 11. above, wherein prior to the
application of the compacting pressure, the amorphous powder is
preformed at a filling rate of 70% or less.
[0045] 13. A soft magnetic dust core manufactured by the method
according to any one of 1. to 12. above, the soft magnetic dust
core having a green density of 78% or more, a crystallization
degree of 40% or more, and a-Fe crystallites with a size of 50 nm
or less.
Advantageous Effect
[0046] According to this disclosure, a soft magnetic dust core
having high density and favorable properties can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a flow diagram illustrating a method of
manufacturing a soft magnetic dust core according to one embodiment
of this disclosure.
DETAILED DESCRIPTION
[0048] FIG. 1 is a flow diagram illustrating a method of
manufacturing a soft magnetic dust core according to one embodiment
of this disclosure. In the embodiment illustrated by the flow
diagram, first, the surface of particles of amorphous powder is
coated to prepare coated powder to be a material. Next, the coated
powder is subjected to pressing and heating processes, thereby
obtaining a dust core as a formed body. In the pressing and heating
processes, a compacting pressure is applied to the material under
predetermined temperature conditions, and the heating is then
performed to a predetermined maximum end-point temperature with the
compacting pressure being applied. As illustrated in FIG. 1,
crystalline magnetic powder having a smaller mean particle diameter
than the amorphous powder can be added to the amorphous powder
before being coated and the coated powder. Alternatively, the
uncoated amorphous powder may be added to the coated powder and the
powders can be subjected to the pressing and heating processes in
the form of a mixture of the coated powder and the amorphous
powder. The coated powder may be preformed before pressing and
heating processes. Further, heat treatment can be performed on the
dust core obtained through the pressing and heating processes.
Materials and the steps that can be used in this disclosure will
now be described in detail. In the description below, the symbol %
used to express the composition denotes at. % unless otherwise
specified.
[0049] <Coated Powder>
[0050] In the disclosed method of manufacturing a soft magnetic
dust core, coated powder having amorphous powder and a coating
formed on the surface of the particles of the amorphous powder is
used as a material.
[0051] <Amorphous Powder>
[0052] The above amorphous powder used may be any given amorphous
powder made of an Fe--B--Si--P--C--Cu-based alloy, an
Fe--B--P--C--Cu-based alloy, an Fe--B--Si--P--Cu-based alloy, or an
Fe--B--P--Cu-based alloy.
[0053] The amorphous powder used may be for example, the
Fe--B--Si--P--C--Cu-based amorphous powder disclosed in PTL 3. The
preferred content range of each component of the composition will
be further described below.
[0054] A higher Fe content improves the saturation magnetic flux
density. Accordingly, in terms of sufficiently improving the
saturation magnetic flux density, the Fe content is preferably 79%
or more. In particular, when a saturation magnetic flux density of
1.6 T or more is required, the Fe content is preferably 81% or
more. On the other hand, when the Fe content is excessively high, a
higher cooling rate is required in producing amorphous powder,
which would make it difficult to produce amorphous powder having
uniform particles. Therefore, the Fe content is preferably 86% or
less. When more uniformity is sought, the Fe content is more
preferably 85% or less. Further, in particular when the amorphous
powder is produced by a method using a low cooling rate, such as
gas atomization, the Fe content is yet more preferably 84% or
less.
[0055] Si is an element which serves to form an amorphous phase.
The lower limit of the Si content is not limited and may be 0%;
however, adding Si can improve the stabilization of nanocrystals.
When Si is added, the Si content is preferably 0.1% or more, more
preferably 0.5% or more, and still more preferably 1% or more. On
the other hand, an excessively high Si content reduces the glass
forming ability and degrades soft magnetic properties. Accordingly,
the Si content is preferably 8% or less, more preferably 6% or
less, and still more preferably 5% or less.
[0056] B is an essential element which serves to form an amorphous
phase. When the B content is too low, it would be difficult to form
an amorphous phase under rapid liquid cooling conditions for
example in water atomization. Accordingly, the B content is
preferably 4% or more, more preferably 5% or more. On the other
hand, an excessively high B content reduces the difference between
T.sub.x1 and T.sub.x2, which makes it difficult to obtain a uniform
nanocrystalline structure, in which case, the soft magnetic
properties of the dust core would be degraded. Therefore, the B
content is preferably 13% or less. In particular, when the alloy
powder is required to have a low melting point for mass production,
the B content is more preferably 10% or less.
[0057] P is an essential element which serves to form an amorphous
phase. When the P content is too low, it would be difficult to form
an amorphous phase under rapid liquid cooling conditions for
example in water atomization. Accordingly, the P content is
preferably 1% or more, more preferably 3% or more, and still more
preferably 4% or more. On the other hand, an excessively high P
content would reduce the saturated magnetic flux density and
degrade the soft magnetic properties. Therefore, the P content is
preferably 14% or less, more preferably 9% or less.
[0058] C is an element that serves to form an amorphous phase. The
lower limit of the C content is not limited in particular and may
be 0%. However, when C is used in combination with B, Si, P, and
the like, the glass forming ability and the stabilization of the
nanocrystals can be further increased compared with the case of
using only one of those elements. When C is added, the C content is
preferably 0.1% or more, more preferably 0.5% or more. On the other
hand, an excessively high C content would make the alloy
composition brittle and would degrade the soft magnetic properties.
Therefore, the C content is preferably 5% or less. In particular, a
C content of 2% or less can prevent variation of the composition
due to the evaporation of C in melting.
[0059] Cu is an essential element that contributes to
nanocrystalization. When the Cu content is excessively low,
nanocrystallization would hardly occur. Therefore, the Cu content
is preferably 0.4% or more, more preferably 0.5% or more. On the
other hand, when the Cu content is excessively high, the amorphous
phase becomes nonuniform, so that a nonuniform nanocrystal
structure cannot be obtained through heat treatment and the soft
magnetic properties would be degraded. Accordingly, the Cu content
is preferably 1.4% or less, more preferably 1.2% or less, and still
more preferably 0.8% or less. Considering the oxidation of the
alloy powder and the grain growth of the alloy powder into
nanocrystals in particular, the Cu content is preferably 0.5% or
more and 0.8% or less.
[0060] Amorphous powder used in one embodiment of this disclosure
is substantially composed of the above-described elements and
incidental impurities. The incidental impurities may contain
elements such as Mn, Al, and O, in which case, the total content of
Mn, Al, and O is preferably 1.5% or less.
[0061] More preferably, the above amorphous powder used has a
composition containing 79%.ltoreq.Fe.ltoreq.86%,
0%.ltoreq.Si.ltoreq.8%, 4%.ltoreq.B.ltoreq.13%,
1%.ltoreq.P.ltoreq.14%, 0%.ltoreq.C.ltoreq.5%,
0.4%.ltoreq.Cu.ltoreq.1.4%, and incidental impurities. Still more
preferably, the amorphous powder has a composition containing
81%.ltoreq.Fe.ltoreq.85%, 0%.ltoreq.Si.ltoreq.6%,
4%.ltoreq.B.ltoreq.10%, 3%.ltoreq.P.ltoreq.9%,
0%.ltoreq.C.ltoreq.2%, 0.5%.ltoreq.Cu.ltoreq.0.8%, and incidental
impurities. Most preferably, the amorphous powder has a composition
containing 81%.ltoreq.Fe.ltoreq.84%, 0%.ltoreq.Si.ltoreq.5%,
4%.ltoreq.B.ltoreq.10%, 4%.ltoreq.P.ltoreq.9%,
0%.ltoreq.C.ltoreq.2%, 0.5%.ltoreq.Cu.ltoreq.0.8%, and incidental
impurities.
[0062] Note that the above composition may contain other trace
elements unless the operation and effect of this disclosure are
adversely affected. In order to improve corrosion resistance and
control electric resistance, provided that the saturated magnetic
flux density does not excessively decrease, the composition of the
amorphous powder may contain total 3 at. % or less of at least one
selected from the group consisting of Co, Ni, Ca, Mg, Ti, Zr, Hf,
Nb, Ta, Mo, W, Cr, Al, Mn, Ag, Zn, Sn, As, Sb, Bi, Y, N, O, S, and
rare earth elements, instead of part of Fe.
[0063] In other words, the amorphous powder used may have a
composition containing, in at. %:
[0064] Fe: 79% or more and 86% or less;
[0065] B: 4% or more and 13% or less;
[0066] Si: 0% or more and 8% or less;
[0067] P: 1% or more and 14% or less;
[0068] C: 0% or more and 5% or less;
[0069] Cu: 0.4% or more and 1.4% or less;
[0070] optionally at least one selected from the group consisting
of Co, Ni, Ca, Mg, Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Al, Mn, Ag, Zn,
Sn, As, Sb, Bi, Y, N, O, S, and rare earth elements: 3 at. % or
less in total; and
[0071] incidental impurities.
[0072] Since Co, Ni, Ca, Mg, Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Al, Mn,
Ag, Zn, Sn, As, Sb, Bi, Y, N, O, S, and rare earth elements above
are optional additional elements, the lower limit of the total
content of those elements may be 0%.
[0073] <Initial Crystallization Temperature>
[0074] The amorphous powder used in this disclosure has a first
initial crystallization temperature T.sub.x1 and a second initial
crystallization temperature T.sub.x2. In other words, the amorphous
powder has at least two exothermic peaks indicating
crystallizations in the heating stage in a differential scanning
calorimetry (DSC) curve obtained by differential scanning
calorimetry. Of the exothermic peaks, the exothermic peak on the
lowest temperature side indicates a first crystallization in which
an a-Fe phase is crystallized, and the next exothermic peak
indicates a second crystallization in which a boride or the like is
crystallized.
[0075] Here, the first initial crystallization temperature T.sub.x1
is defined as the temperature of the intersection point of the base
line of the DSC curve and a first rising tangent line that is a
tangent line passing through a point having the largest positive
slope in a first rising portion from the base line to the first
peak that is the exothermic peak on the lowest temperature side.
Further, the second initial crystallization temperature T.sub.x2 is
defined as the temperature of the intersection point of the base
line and a second rising tangent line that is a tangent line
passing through a point having the largest positive slope in a
second rising portion from the base line to the second peak that is
the exothermic peak following the first peak. A first final
crystallization temperature T.sub.z1 is defined as the temperature
of the intersection point of the base line and a first falling
tangent line that is a tangent line passing through a point having
the largest negative slope in a first descending portion from the
first peak to the base line.
[0076] The method of producing the amorphous powder used is not
limited in particular. For example, the method can include melting
materials of an alloy, having a predetermined composition, followed
by powdering the melt by atomization. For a specific technique of
the atomization, various methods can be used, for example, water
atomization or gas atomization. Preferred examples of the technique
used include water atomization as disclosed in EXAMPLES of PTL 3,
atomization using the centrifugal force of a rotating disc as
disclosed in JP 2013-55182 A, a combination of gas atomization and
water cooling as disclosed in JP 4061783 B2 and JP 4181234 B2, and
a method including water cooling after water atomization as
disclosed in JP 2007-291454 A.
[0077] <Mean Particle Diameter D.sub.50>
[0078] The mean particle diameter D.sub.50 of the amorphous powder
used herein is preferably in a range of 1 .mu.m to 100 .mu.m.
Particles having D.sub.50 of less than 1 .mu.m are not readily
industrially produced at low cost. Therefore, D.sub.50 is
preferably 1 .mu.m or more, more preferably 3 .mu.m or more, and
still more preferably 5 .mu.m or more. On the other hand, D.sub.50
exceeding 100 .mu.m can have a detrimental effect, for example,
particle segregation. Accordingly, D.sub.50 is preferably 100 .mu.m
or less, more preferably 90 .mu.m or less, and still more
preferably 80 .mu.m or less. The mean particle diameter D.sub.50
used herein refers to a particle diameter obtained when the
volume-based integrated particle size distribution measured by
laser diffraction or laser diffusion is 50%.
[0079] <Apparent Density AD>
[0080] The particle shape of the amorphous powder used herein is
preferably as spherical as possible. When the particles are less
spherical, projections would be formed on the surface of the
particles, and the coating would be damaged by concentrated stress
on the projections from the surrounding particles when a compacting
pressure is applied, leading to insufficient insulation. As a
result, the magnetic properties of the dust core to be obtained
would be degraded (in particular, the iron loss would be
increased). Accordingly, the apparent density AD which is an
indication of the sphericity of the particles preferably satisfies
AD.gtoreq.2.8+0.005.times.D.sub.50. Here, the unit of the AD is
Mg/m.sup.3, and the unit of D.sub.50 is .mu.m. Further, the AD can
be measured by a method defined in HS Z 2504. Since a higher
apparent density AD is preferred, the upper limit of the AD is not
limited in particular; for example, the AD may be 5.00 Mg/m.sup.3
or less and may be 4.50 Mg/m.sup.3 or less.
[0081] The sphericity of the particles can be controlled to a
preferable range by adjusting the conditions for producing the
amorphous powder, for example, the amount of water, water pressure
of a high pressure water jet used for atomization, the temperature
of materials to be melted, and the feed rate of the materials in
water atomization. Specific production conditions vary depending on
the composition of the amorphous powder to be produced and the
desired productivity.
[0082] The particle size distribution of the amorphous powder in
this disclosure is not limited in particular; however, an
excessively wide particle size distribution may have an adverse
effect, for example, particle segregation. Therefore, the maximum
particle diameter of the amorphous powder is preferably 2000 .mu.m
or less. Further, as described in A. B. Yu and N. Standish,
"Characterisation of non-spherical particles from their packing
behavior", Powder Technol. 74 (1993) 205-213, when amorphous powder
having two peaks in the particle size distribution is used, the
filling rate is improved, resulting in an improved density of a
dust core. A particle size distribution having two peaks can be
obtained for example by mixing powders having two particle
diameters obtained by classification based on the particle
diameters of the desired peaks. Given methods and apparatus can be
used, for example, sieve classification or air classification can
be employed for the classification; and hand mixing or machine
blending using a V blender, a double cone blender, or the like can
be employed for mixing. The probability of particle segregation can
be reduced by attaching the powder particles with the smaller
particle diameter to the surface of the powder particles with the
larger particle diameter. In order to attach the powders, any given
method can be used. For example, the adhesion force of the coating
material itself may be used, or a binder may be added.
[0083] Further, crystal soft magnetic powder may be mixed with the
amorphous powder or the coated powder. The magnetic powder that can
be mixed is not limited in particular; for example, iron powder
(pure iron powder), carbonyl iron powder, Sendust powder, Permendur
powder, or Fe--Si--Cr-based soft magnetic powder can be used. The
crystalline soft magnetic powder may be selected depending on the
use of the nanocrystalline dust core to be manufactured.
Particularly preferably, crystalline soft magnetic powder having
smaller mean particle diameter than that of the amorphous powder is
used. This makes voids between the amorphous powder particles being
filled with the magnetic particles, thereby improving the density
of the dust core, so that advantageous effects such as improvement
of the saturation magnetic flux density can be achieved. The amount
of the crystalline soft magnetic powder mixed is preferably 5 mass
% or less of the total amount of the crystalline soft magnetic
powder and one of the amorphous powder and the coated powder. Since
the disclosed amorphous powder densification effect is not exerted
on crystalline soft magnetic powder, the mixed amount exceeding 5
mass % rather reduces the density of the dust core.
[0084] <Crystallization Degree>
[0085] As the crystallization degree of the amorphous powder used
herein is lower, a dust core to be manufactured is uniformly
nanocrystallized, and exhibits favorable soft magnetic properties.
Accordingly, the crystallization degree of the amorphous powder is
preferably 20% or less, more preferably 10% or less, and still more
preferably 3% or less. Here, the crystallization degree is a value
calculated by the whole-powder-pattern decomposition (WPPD) method
using an X-ray diffraction pattern. On the other hand, since the
crystallization degree of the amorphous powder is preferably as low
as possible, the lower limit of the crystallization degree is not
limited. For example, the crystallization degree may be 0%.
[0086] <Coating>
[0087] A coating is applied to the above-described amorphous powder
for example in order to improve insulation and mechanical strength.
The material of the coating is not limited in particular, and any
given material, an insulating material in particular can be used. A
given material can be used as the material, for example, resins
(silicone resin, epoxy resin, phenol resin, polyamide resin,
polyimide resin, and the like), phosphates, borates, chromates,
metal oxides (silica, alumina, magnesia, and the like), and
inorganic polymers (polysilane, polygermane, polystannane,
polysiloxane, polysilsesquioxane, polysilazane, polyborazylene,
polyphosphazene, and the like) can be used depending on the desired
insulation performance. Further, a plurality of materials may be
used in parallel; for example, the coating may be formed to have a
multi-layer structure with two or more layers using different
materials. When amorphous powder having two peaks in the particle
size distribution as described above is used, the above-described
powders having two particle diameters may be mixed and then formed
with only one of the powders having been subjected to insulation
and without the other having been subjected to insulation
coating.
[0088] The method of coating can be selected from various methods
including powder mixing, dip coating, spray coating, fluidized bed
coating, the sol-gel process, CVD, and PVD in view of the kind of
the material used for coating and cost efficiency.
[0089] When the coating weight (coating coverage) of the coating is
excessively high, the saturated magnetic flux density would be
reduced. Therefore, the coating weight is preferably 15 parts by
volume or less, more preferably 10 parts by volume or less, per 100
parts by volume of the amorphous powder. On the other hand, the
lower limit of the coating weight is not limited in particular, yet
if the coating weight is excessively low, the effects of the
coating in improving insulation and strength might not be
sufficiently achieved. Therefore, the coating weight is preferably
0.5 parts by volume or more, more preferably 1 part by volume or
more, per 100 parts by volume of the amorphous powder.
[0090] <Preformation>
[0091] In this disclosure, before applying a compacting pressure to
be described to the above coated powder, preformation can be
performed. However, when the filling rate of the preformed body
obtained by the preformation exceeds 70%, the coating would be
partially damaged, so that sufficient insulating effects would not
be obtained. Accordingly, when preformation is performed, the
filling rate of the formed body after the preformation is
preferably 70% or less. On the other hand, the lower limit of the
filling rate is not limited in particular; however, when the
filling rate is less than 30%, the strength of the preformed body
would be reduced, and the preformed body would be broken while
being handled in the subsequent steps. Therefore, the filling rate
is preferably 30% or more. Note that the filling rate here is a
ratio of the actual density with respect to the theoretical density
determined in accordance with the composition. For the
preformation, any given method used for example for the powder
metallurgical technique, such as uniaxial pressing, isostatic
pressing, or slip casting can be selected and used depending on the
desired shape and cost efficiency. The preformation is preferably
performed at a temperature lower than T.sub.x1.
[0092] <Application of Compacting Pressure (Pressing)>
[0093] Next, a compacting pressure is applied to the coated powder
obtained as described above under predetermined temperature
conditions. The application of the compacting pressure can be
performed by filling a mold with the coated powder and pressing in
accordance with the conventional method. On that occasion, a higher
compacting pressure has a larger densification effect. Accordingly,
the compacting pressure is preferably 200 MPa or more, more
preferably 300 MPa or more, still more preferably 500 MPa or more.
On the other hand, an excessively high compacting pressure
saturates the densification effect and increases the risk of mold
damage. Accordingly, the compacting pressure is preferably 2000 MPa
or less, more preferably 1500 MPa or less, and still more
preferably 1300 MPa or less.
[0094] In this disclosure, it is important to apply the compacting
pressure to the coated powder at a temperature of T.sub.x1-100 K or
less. Here, "applying a compacting pressure at a temperature of
T.sub.x1-100 K or less" means that the temperature of the coated
powder at a time when the compacting pressure is applied is
T.sub.x1-100 K or less. In respect of this, the temperature of the
coated powder before the application of the compacting pressure can
be set to be T.sub.x1-100 K or less. When the temperature exceeds
T.sub.x1-100 K, the density after the formation is not sufficiently
improved. It is inferred that this is caused because if the
temperature exceeds T.sub.x1-100 K, partial crystallization starts,
and particles start to be hardened due to the high crystallization
rate. Meanwhile, the density of the Fe--B-based amorphous material
of PTL 4 is improved by a method in which the amorphous material is
heated to a temperature near the crystallization temperature and
then pressed. Accordingly, a phenomenon in which a high-density
dust core cannot be obtained unless the temperature of the material
before being pressed is kept at T.sub.x1-100 K or less is unique to
the alloys used in this disclosure, and the phenomenon has been
first revealed by the studies involving this disclosure. The
phenomenon is attributed to a feature of the alloys used herein,
that is, the alloys require a shorter time for crystallization than
other alloys.
[0095] Further, since the temperature of the amorphous powder is
T.sub.x1-100 K or less at a time of applying a compacting pressure,
the hardness of the amorphous powder is high at the start of the
pressing. However, as stated above, when amorphous powder having a
particle shape satisfying AD.gtoreq.2.8+0.005.times.D.sub.50 is
used, the insulation coating on the particle surface can be
prevented from being damaged even if pressing is performed in a
state where the particles have high hardness. Thus, high resistance
can be kept. Therefore, when amorphous powder satisfying
AD.gtoreq.2.8+0.005.times.D.sub.50 is used, a formed body can be
obtained which has higher density and extremely high resistance.
The thus obtained formed body is more preferred as a dust core.
[0096] <Heating>
[0097] Next, the coated powder is heated to a maximum end-point
temperature of T.sub.x1-50 K or more and less than T.sub.x2, with
the compacting pressure being applied. Various methods can be used
for the heating. Examples include but not limited to for example
electrical heating (direct electrical heating, pulsed electrical
heating, and the like), a method using a heat source such as an
electrical heater, provided inside the mold, and a method of
externally heating a mold placed in a heating chamber. When the
temperature reaches T.sub.x1-50 K, structure relaxation of the
amorphous structure starts and the amorphous powder is softened, so
that the density of the formed body is improved. When the
temperature exceeds T.sub.x1, a first crystallization starts and
the particles are softened further, so that the density of the
formed body is improved further. On the other hand, when the
temperature is T.sub.x2 or more, secondary phases of borides or the
like are crystallized, resulting in degraded soft magnetic
properties. Accordingly, the maximum end-point temperature here is
set to be less than T.sub.x2. The maximum end-point temperature is
preferably T.sub.x2-0.4.DELTA.T K or less, where
.DELTA.T=T.sub.x2-T.sub.x1, more preferably T.sub.x2 0.6.DELTA.T K
or less, still more preferably T.sub.x2-0.8.DELTA.T K or less.
[0098] In this disclosure, after heating to the maximum end-point
temperature, the maximum end-point temperature can be held for a
given period with the compacting pressure being applied. However,
when the holding time is excessively long, for example, a-Fe
crystal grains would be coarsened and secondary phases of borides
or the like would be partly crystallized. Accordingly, the holding
time is preferably 120 min or less, more preferably 100 min or
less. On the other hand, the lower limit of the holding time is
preferably, but not limited to, 1 min or more and more preferably 5
min or more.
[0099] <Heat Treatment>
[0100] In this disclosure, the dust core obtained by green
compacting in the above-mentioned process may further be heat
treated at a temperature in a range of T.sub.x1 or more and
T.sub.x2 or less. The heat treatment further promotes
nanocrystallization and allows the soft magnetic properties to be
further improved.
[0101] <Soft Magnetic Dust Core>
[0102] In this disclosure, pressing and heating are performed under
predetermined conditions as described above, thereby obtaining a
soft magnetic dust core having a green density of 78% or more, a
crystallization degree of 40% or more, and an a-Fe crystallite size
of 50 nm or less. The green density is preferably 80% or more, more
preferably 85% or more, and still more preferably 90% or more. On
the other hand, the upper limit of the green density may be, but
not limited to, 100% or 99% or less. The upper limit of the
crystallization degree may typically be, but not limited to, 60% or
less, 55% or less, or 50% or less. The a-Fe crystallite size is
preferably 40 nm or less, more preferably 30 nm or less, and still
more preferably 25 nm or less. On the other hand, the lower limit
of the a-Fe crystallite size is not limited in particular. The size
is preferably as small as possible, and may typically be 10 nm or
more, or 15 nm or more.
[0103] The green density here is expressed as a percentage obtained
by dividing the density calculated from the size and the weight of
a dust core (formed body) by the true density of the coated powder
determined based on the composition and the coating weight.
Further, the a-Fe crystallite size is the crystallite diameter D
(nm) calculated from a half width .beta. of an X-ray diffraction
peak corresponding to the a-Fe (110) plane using the Scherrer
equation D=0.9.lamda./.beta.cos .theta.. Here, .lamda. is the
wavelength (nm) of the X-ray, .theta. is the diffraction angle of
the a-Fe (110) plane, and 2.theta.=52.505.degree.. The
crystallization degree of a soft magnetic dust core can be measured
by the same method as the crystallization degree of the
above-described amorphous powder.
EXAMPLES
[0104] Next, a more detailed description is given below based on
Examples. The following examples merely present preferred examples,
and this disclosure is not limited to those examples.
[0105] (Production of Amorphous Powder)
[0106] As feedstocks, electrolytic iron, ferrosilicon,
ferrophosphorus, ferroboron, and electrolytic copper were weighed
to achieve a predetermined ratio. Molten steel obtained by vacuum
melting of the feedstocks was water atomized in an argon
atmosphere, thereby producing an amorphous powder having a
composition presented in Table 1. Amorphous powders Nos. 3-1 to 3-4
and Nos. 6-1 to 6-3 were produced using molten steel having the
same composition; however, their mean particle diameter D.sub.50
and apparent density AD is varied by adjusting the water
atomization conditions and the conditions of classification after
the atomization. The amorphous powder No. 3-4 was obtained by
mixing two kinds of powders prepared by water atomization to
achieve a weight ratio of 50:50. One of the powders had been
separated by passing through a sieve of aperture size 53 .mu.m, and
the other had been classified by passing through a sieve of
aperture size 106 .mu.m and by remaining on a sieve of aperture
size 75 .mu.m. Accordingly, the amorphous powder No. 3-4 has a
bimodal particle size distribution with two peaks. In a water
atomizer system and a classifier system used in this example, the
yield was extremely reduced when the mean particle diameter was
intended to be adjusted to 1 .mu.m or less, so that it was
difficult to produce a sufficient amount of powder being green
compacted to be evaluated.
Example 1
[0107] In order to determine the influence of the pressing and
heating conditions, the same kind of coated powders were subjected
to pressing and heating under various conditions, thereby
evaluating the density and the crystal state of the resultant soft
magnetic dust cores. The specific steps were as follows.
[0108] The amorphous powder No. 1 having a first initial
crystallization temperature T.sub.x1 of 454.degree. C. and a second
initial crystallization temperature T.sub.x2 of 567.degree. C. was
used as an amorphous powder, and an insulation coating was formed
on the surface of the amorphous powder particles. The insulation
coating was prepared by immersing the amorphous powder in a
solution in which a silicone resin (SR 2400 produced by Dow Corning
Toray Co., Ltd.) was diluted with xylene and then volatilizing the
xylene. The coating weight of the silicone resin was set to be 1
part by weight as a solid content of the silicone resin per 100
parts by weight of the amorphous powder. When converted to a volume
fraction, the resin coating weight corresponds to approximately 6
parts by weight per 100 parts of weight of the amorphous
powder.
[0109] The application of a compacting pressure and heating was
performed on the coated powder obtained as described above
according to the following steps. First, a cylindrical mold having
an internal diameter of 15 mm was filled with the coated powder
with a punch being inserted into the mold from the bottom; another
punch was inserted into the mold from the top; and a pressing force
of 1 GPa was applied thereto. Next, with the pressing force being
applied, a direct current was flown using the upper and lower
punches as electrodes, thereby raising the temperature at a rate of
10.degree. C./min to a predetermined maximum end-point temperature.
After the maximum end-point temperature was achieved, the
temperature was held for a predetermined period, and a green
compact was removed from the mold after cooling to the first
initial crystallization temperature or lower. The temperature at a
time of the compacting pressure application, the maximum end-point
temperature, and the holding time of the maximum end-point
temperature are presented in Table 2.
[0110] The green density, crystallization degree, and crystallite
size of the resultant soft magnetic dust core were measured. The
measurement results are presented in Table 2. Table 2 also presents
whether secondary phases other than a-Fe were formed or not, which
was determined by X-ray diffraction. Here, the green density was
determined by dividing the density calculated from the size and
weight of the soft magnetic dust core by the true density of the
coated powder determined based on the composition and the coating
weight.
[0111] Under each of the forming conditions Nos. 2 to 7, 9, 11, and
14 meeting the conditions of this disclosure, a green density of
78% or more and a crystallization degree of 40% or more was
obtained. Further, in those examples, the crystallite size was 50
nm or less, and secondary phases were not formed or even when
formed, the amount was very small. In contrast, under the forming
condition No. 1 in which the end-point temperature was low,
sufficient green density was not achieved and the crystallization
degree was low. Under the forming condition No. 8, in which the
maximum end-point temperature was high, secondary phases were
significant. Under the forming condition No. 10 in which the
temperature at a time of compacting pressure application was high,
sufficient green density was not achieved. Under the forming
condition No. 12 in which the holding time of the maximum end-point
temperature was as long as 140 min, the crystallite size was large
compared with the case where the holding time was 10 min, and
secondary phases were slightly formed. Further, under the forming
condition No. 13 in which the compacting pressure was as low as 80
MPa, the green density was low compared with the case where the
compacting pressure was 1100 MPa.
TABLE-US-00001 TABLE 1 First initial First final Second initial
Amorphous Crystallization crystallization crystallization
crystallization Apparent powder Composition (at. %) degree
temperature temperature temperature D.sub.50 density No. Fe B Si P
Cu C (%) (.degree. C.) (.degree. C.) (.degree. C.) (.mu.m)
(Mg/m.sup.3) 1 80.3 10 5 4 0.7 0 1 454 471 567 45.3 3.25 2 81.3 9 5
4 0.7 0 2 440 466 565 46.8 3.41 3-1 81.4 10 0 8 0.6 0 3 436 454 509
50.2 3.37 3-2 81.4 10 0 8 0.6 0 3 436 454 509 19.8 2.91 3-3 81.4 10
0 8 0.6 0 3 436 454 509 48.2 2.85 3-4 * 81.4 10 0 8 0.6 0 3 436 454
509 62.1 3.81 4 81.4 10 3 5 0.6 0 7 449 466 551 48.3 3.08 5 81.4 6
5 7 0.6 0 5 434 457 542 47.6 3.05 6-1 82.4 11 1 5 0.6 0 2 426 444
537 51.2 3.27 6-2 82.4 11 1 5 0.6 0 2 426 444 537 86.3 3.18 6-3
82.4 11 1 5 0.6 0 2 426 444 537 104.2 3.45 7 82.4 11 0 5 0.6 1 2
430 451 541 53.8 3.37 8 83.3 8 4 4 0.7 0 20 415 434 555 48.6 3.29 9
83.4 10 0 6 0.6 0 8 422 439 523 42.3 3.31 10 84.8 10 2 2 1.2 0 25
396 426 523 45.5 3.28 11 84.8 10 0 4.5 0.7 0 11 425 446 536 49.3
3.39 12 85.6 9.5 0 4.5 0.4 0 15 408 429 519 50.1 3.22 13 86.5 11 0
2 0.6 0 28 388 418 521 49.5 3.41 * The particle size distrbution
includes two peaks.
TABLE-US-00002 TABLE 2 Forming conditions Maximum Temperature end-
of Maximum point Dust core compacting end- temperature Crystal-
Forming pressure Compacting point holding Green lization .alpha.-Fe
condition application pressure temperature time density degree
crystallite Secondary No.* (.degree. C.) (MPa) (.degree. C.) (min)
(%) (%) size (nm) phase Note 1 25 1100 380 10 73 28 20 Absent
Comparative Example 2 25 1100 410 10 81 40 21 Absent Example 3 25
1100 450 10 95 42 21 Absent Example 4 25 1100 460 10 95 43 22
Absent Example 5 25 1100 470 10 95 42 22 Absent Example 6 25 1100
480 10 95 42 22 Present Example (Slight) 7 25 1100 500 10 95 43 26
Present Example (Slight) 8 25 1100 570 10 95 45 41 Present
Comparative (Significant) Example 8-1 25 1100 650 10 95 46 53
Present Comparative (Significant) Example 9 250 1100 460 10 92 42
21 Absent Example 10 410 1100 460 10 72 42 22 Absent Comparative
Example 11 25 1100 460 100 98 44 40 Absent Example 12 25 1100 460
140 98 44 38 Present Example (Slight) 13 25 80 450 10 78 41 21
Absent Example 14 25 150 450 10 83 42 22 Absent Example *The
amorphous powder No. 1 in Table 1 is used in each example.
Example 2
[0112] Next, in order to determine the influence of the amorphous
powder to be used, the amorphous powders Nos. 1 to 13 presented in
Table 1 was subjected to pressing and heating under the same
conditions, thereby evaluating the density and the like of the
resultant soft magnetic dust cores. The specific steps were as
follows.
[0113] An insulation coating made of a silicone resin is formed on
each of the amorphous powders Nos. 1 to 13 presented in Table 1
under the same conditions as Example 1, thereby obtaining coated
powders. Next, the resulting coated powders were molded in the same
manner as Example 1 except that the forming conditions were fixed
to the condition No. 3 in Table 2, thereby manufacturing soft
magnetic dust cores. The green density, the crystallite size, and
the specific resistance of the soft magnetic dust cores were
measured. The measurement results are presented in Table 3. Here,
the green density was determined by the above-described method.
Further, the specific resistance was measured by four-terminal
sensing.
[0114] As seen from the results presented in Table 3, when pressing
and heating is performed by a method meeting the conditions of this
disclosure, a green density of 78% or more, a crystallization
degree of 40% or more, and a crystallite size of 50 nm or less were
achieved when any one of the amorphous powders was used.
[0115] For Samples Nos. 1 to 4 and 6 to 18 using amorphous powders
in which the apparent density AD (Mg/m.sup.3) and the mean particle
diameter D.sub.50 (.mu.m) satisfy
AD.gtoreq.2.8+0.005.times.D.sub.50, a sufficiently high specific
resistance of 1000.mu..OMEGA.m or more was achieved. Presumably,
this is because high sphericity of the amorphous powders might have
prevented the insulating coating from being damaged by the
projections formed on the surface of the particles. Further, for
Sample No. 6 using the amorphous powder No. 3-4, a higher green
density was achieved compared with the other cases. Presumably,
this is because the amorphous powder No. 3-4 had a bimodal particle
size distribution leading to increased filling rate. For Sample No.
11 using the amorphous powder No. 6-3, the green density varied
significantly. Presumably, this is because the mean particle
diameter D.sub.50 of the amorphous powder No. 6-3 exceeding 100
.mu.m might have caused particle segregation. Further, for Samples
Nos. 15 and 18 using the amorphous powders No. 10 and No. 13, the
green density was lower than in the other cases. Presumably, this
is because the crystallization degree of the amorphous powders
prior to molding exceeding 20% could not have sufficiently caused
the softening phenomenon induced by structure relaxation of the
amorphous structure or crystallization.
[0116] For Samples No. 6 and No. 6-1, the amorphous powder No. 3-4
having a bimodal particle size distribution was used. Note that for
Sample No. 6, an insulation coating was applied to all the
amorphous powder particles in the same manner as Example 1, whereas
in Sample No. 6-1, an insulation coating was applied to the powder
classified between the sieves having apertures of 106 .mu.m and 75
.mu.m in the same manner as Example 1, and no insulation coating
was applied to the powder separated using a sieve having an
aperture of 53 .mu.m. The same conditions were used for Samples No.
6 and No. 6-1 other than the above respects. As a result, the
specific resistance of the dust core of Sample No. 6-1 was close to
1000 .mu..OMEGA.m although it was slightly lower than the specific
resistance of Sample No. 6.
[0117] For Samples No. 1-1 to No. 1-3 in Table 3, dust cores were
manufactured under the same conditions as Sample No. 1 except that
carbonyl iron powder having a mean particle diameter of
approximately 1 .mu.m was mixed with the amorphous powder No. 1.
Carbonyl iron powder is an iron powder (pure iron powder) obtained
by the thermal decomposition of pentacarbonyliron (iron
pentacarbonyl). The amount of the carbonyl iron powder added was 2
mass % (No. 1-1), 4 mass % (No. 1-2), and 6 mass % (No. 1-3) of the
total mass of the amorphous powder No. 1 and the carbonyl iron
powder. The green density of Samples Nos. 1-1 and 1-2 were higher
than that of Sample No. 1, whereas the green density of Sample No.
1-3 was lower than that of Sample No. 1.
TABLE-US-00003 TABLE 3 Dust core Amor- Crystal- .alpha.-Fe phous
Green lization crystallite Specific powder density degree size
resistance No. *.sup.1 No. (%) (%) (nm) (.mu..OMEGA.m) Note 1 1 95
42 21 .gtoreq.1000 1-1 1 98 42 21 .gtoreq.1000 Carbonyl iron powder
mixed: 2 mass % 1-2 1 96 42 21 .gtoreq.1000 Carbonyl iron powder
mixed: 4 mass % 1-3 1 92 42 21 .gtoreq.1000 Carbonyl iron powder
mixed: 6 mass % 2 2 96 43 19 .gtoreq.1000 3 3-1 96 44 22
.gtoreq.1000 4 3-2 91 44 21 .gtoreq.1000 5 3-3 89 45 23 807 6 3-4
98 44 22 .gtoreq.1000 6-1 3-4 98 44 22 975 7 4 91 42 26
.gtoreq.1000 8 5 92 45 23 .gtoreq.1000 9 6-1 96 47 22 .gtoreq.1000
10 6-2 97 46 21 .gtoreq.1000 11 6-3 95 *.sup.2 47 24 .gtoreq.1000
12 7 96 45 21 .gtoreq.1000 13 8 82 46 37 .gtoreq.1000 14 9 93 44 26
.gtoreq.1000 15 10 80 43 38 .gtoreq.1000 16 11 87 44 31
.gtoreq.1000 17 12 85 43 35 .gtoreq.1000 18 13 80 40 40
.gtoreq.1000 *.sup.1 The forming conditions of each example are the
same as those of No. 3 in Table 2. *.sup.2 The value varies between
92% to 98%.
* * * * *