Note: Descriptions are shown in the official language in which they were submitted.
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ANNEALING OF TH~RMALLY INSULATED CORE
BACKGROUND OF THE INVENTION
This invention relates to an improved process for
annealing amorphous metal alloy cores and, more
particularly to a process for rapidly heat-treating and
magnetically annealing amorphous metal alloy cores that
tailors the magnetic properties thereof to specific
product applications.
DESCRIPTION OF THE PRIOR ART
Amorphous metal alloys have been developed that
evidence magnetic properties superior to conventional
crystalline all~oys. These amorphous alloys can be wound
to form magnetic cores, approximating a toroid, and are
well adapted for use as cores of magnetic devices such
as transformers, inductors, electrodeless fluorescent
lamps or the like. The adaptation of an amorphous metal
core for use in an electrodeless fluorescent lamp is
disclosed by U.S. Patent No. 4,227,120 to Luborsky.
Amorphous metal cores have been annealed to enhance the
magnetic properties thereof. Typically, the annealing
process includes the steps of heat heating the core to a
temperature sufficient to achieve stress relief without
initiating crystallization and cooling in the presence
oi- a magnetic field. Annealing processes of the type
described are disclosed by U.S. Patent Nos. 4fll6,728,
4,~49,969, 4,262,233, and 4,298,409.
One of the major problems with conventional
annealing processes is the extended time period required
to effect the heating step. The problem is particularly
troublesome with larger cores. Rapid heating of the
core to the annealing temperature produces hot spots at
exterior portions thereof, which so degrade the core's
magnetis properties that it is rendered unsuitable for
use in the aforementioned product applications. To
alleviate this problem, the temperature of the core must
be elevated to the annealing temperature by a laborious
process involving a plurality of graduated heating
steps, which is both time consuming and expensive.
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SUMMARY OF THE INVENTION
The present invention provides an improved process
for annealing a magnetic core that substantially reduces
the time period required for the anneal and markedly
improves the magnetic properties thereof. Generally
stated, a magnetic core is wound from amorphous metal
ribbon to form an outside surface, an inside surface,
and top and bottom surfaces. The core is heated to and
held at a first temperature for a preselected period of
time and then cooled to a second temperature. The
improvement comprises the step of insulating the inside
and outside surfaces of said core prior to the heating
step.
It has been found that by insulating the inside and
outside surfaces of the core prior to the heating step
substantially larger cores can be annealed in an
economical, reliable manner. Heat is transferred
rapidly through the top and bottom surfaces along metal
paths to interior portions of the core, while the rate
of heat transfer to the inside and outside core surfaces
is substantially reduced. Inasmuch as the heating step
is the rate determining step the overall process time
and production costs are minimized. Conventional
graduated heating steps are eliminated, with the result
that the number of process steps is reduced and the
reliability of the annealing process is increased. The
core is heated in a highly uniform manner without
substantial temperature variations which, if present,
would produce mechanical distortions, thermal stresses,
and hot spots. Advantageously, magnetic cores produced
in accordance ~ith the method of this invention exhibit
enhanced magnel:ic properties (ie. AC core loss ranging
from about 0.16 to 0.25 W/Kg, exciting power ranging
from about 0.25 to 0.45 VA/Kg, and coercive force
ranging from about 1.1 to 1.6 A/m at an induction of
1.40 Tesla and a frequency of 60 Hz). Accordingly,
magnetic cores annealed in accordance with the present
invention are especially well suited for use in
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inductors, transformers and elec~rodeless flourescent
lamps.
BRIEF D~SCRIPTION OF THE DRAWINGS
The invention will be more fully understood and
further advantages will become apparent when reference
is made to the following detailed description and the
accompanying drawings in which:
Fig. 1 is a plot showing a time-temperature profile
of a magnetic core annealed in accordance with a
conventional annealing process; and
Fig. 2 is a plot showing a time-temperature profile
of a magnetic core annealed in accordance with the
annealing process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Amorphous metal alloys are produced by rapidly
quenching molten metals, at a rate of approximately
106C/sec., to develop glassy substances directly in the
form of thin ribbons or wires. Typically, the ribbon-
thickness ranges from about 20 to 30 ~m and the ribbon
width ranges from about 25 to 100 mm. A magnetic core
is wound from the amorphous ribbon forming an outside
surface, an inside surface, and top and bottom
surfaces. The inside surface defines a center aperture
extending substantially coaxially with a centroid of the
core. In addition, the top and bottom surfaces lie,
respectively, in planes substantially perpendicular to
cylindrical surfaces formed from the inside and outside
surfaces thereof.
The present invention provides an improved process
for annealing a magnetic core that substantially reduces
the time period required for the anneal and markedly
improves the cores magnetic properties, wherein the
improvement comprises insulating the inside and outside
surfaces of the core before initiating the heating
portion of the annealing procedure. The insulation
procedure further comprises selecting a thermal
insulative substrate having, in combination, a thermal
conductivity ranging from about 0.03 to 0.14 W/mC, and
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linear shrinkage ranging from about 1 to 3 percent up to
500C. It is within the scope of the present invention
to employ a preEabricated insulation or one that is
manually prepared from component parts and which
satisfies the above mentioned criteria. Once selected,
the insulation is applied to the inside and outside
surfaces by a method selected from the group consisting
of wrapping, pain~ing, casting and dipping.
Wrapping of the insulation is readily accomplished
using conventional equipment and procedures, and at low
cost. According, wrapping is the preferred method for
applying the insulation to the core. Typically, the
insulation is applied such that the thickness dimension
thereof extends radially outward from the outside
surface and radially inward from the inside surface of
the core and ranges from about 25 to 75mm.
In accordance with the present invention, the
magnetic properties of the core can be enhanced by
annealing. The process of annealing generally comprises
rapidly heating the core to and holding it at a first
temperature for a preselected period of time, which is
sufficient to relieve the material of all stresses but
which is less than that required to initiate
crystallization. Preferably, the oven housing the core
is initially rapidly heated to a peak oven temperature
ranging from 100 to 160C higher than the first
temperature of the particular amorphous alloy chosen.
As the core approaches its annealing temperature the
oven temperature is lowered to correspond to that o~ the
core (see figure # 2). With this procedure, the time
required for heat treatment of the core is substantially
reduced. The core is then cooled at a cooling rate
ranging from about 0.1 to 100C per minute to a second
temperature ranging from about 200 to 25C. The first
temperature typically ranges from about 325 to 400C and
is below the Curie temperature of the particular
amorphous alloy chosen, whereas the second temperature
is usually ambient temperature. Preferably, at least
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the heating step and, most preferably, each of the
heating and cooling steps is carried out in the presence
of a magnetic field, the direction of which may be
either parallel or perpendicular to the core's
longitudinal dXiS depending upon its specific product
application. Certain alloys, such as those having a
composition consisting essentially of a member selected
from the group consisting of Co66Fe4Ni1B14Sil5 and
Fe76.85Cr2B16.1Si4.gC0,2s can be annealed in the absence
of a magnetic field to attain a substantial improvement
in the magnetic properties thereof, as hereinbefore
described.
The method of construction of a magnetic core is
such that a percentage of the gross area is air, which
is trapped between the layers of the spirally wound
ribbon. Thus,
LGROSS AREA-NET CROSS SECTIONAL AREA] x 100 = PERCENT
AIR AREA.
GROSS AREA
It is also well established that metals are excellent
conductors of heat while gases are poor conductors of
heat. The thermal conductivity values (k) for metal and
air, respectively, are 50.2 J5~l m~l (C) 1 and 0.024 J5-
1 m~l (C)-l, Consequently, the air trapped between the
layers of ribbon of an uninsulated core serves to impede
the heat current (H) to the center along the ribbon
thickness, wherein
H = kA (T~ -dTl)
k is the thermal condivity of the material, A is the
material's cross-sectional area, T2-Tl is the
temperature difference between two points, and d is
distance between the two points. As a result of this
poor conduction, hot spots develop at exterior portions
of the core which so degrade its magnetic properties
that it is rendered unsuitable for use in transformers,
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inductors, electrodeless flourescent lamps, or the
like. Furthermore, thermal conductivity tests performed
on uninsulated sample cores at ~00C reveal that the
thermal conductivity through the ribbon width is over 20
times greater than through the ribbon thickness for both
the heating and cooling stepsO We have found that by
thermally insulating the inside and outside surfaces
prior to the heating step while, at the same time,
leaving the top and bottom surfaces exposed,
substantially larger cores can be annealed in an
economical reliable manner. This is because heat is
transferred rapidly through the top and bottom surfaces
along metal paths to interior portions of the core,
while the rate of heat transfer to inside and outside
core surfaces is substantially reduced. Inasmuch as the
heating step is the rate determining step, the overall
process time, and production costs are minimized.
Conventional, graduated heating steps are eliminated,
with the result that the number of process steps are
reduced and the reliability of the annealing process is
increased. The core is heated in a highly uniform
manner without substantial temperature variations which,
if present, would produce mechanical distortions,
thermal stresses, and hot spots. Advantageously,
magnetic cores produced in accordance with the method of
this invention exhibit enhanced magnetic properties
(i.e~ AC core loss ranging from about 0.16 to 0.25 W/Kg,
exciting power ranging from about 0.25 to 0.45 VA/Kg,
and coercive force ranging from about 1.1 to 1.6 A/m at
an induction of 1.40 Tesla and a frequency of ~0 Hz~.
Accordingly, magnetic cores annealed in accordance with
the present invention are especially well suited for use
in inductors, transformers, and electrodeless
flourescent lamps.
The following examples are presented to provide a
more complete understanding of the invention. The
speci~ic techniques, conditions, materials,
~preparations, and reported data set forth to illustrate
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the principles and practice of the invention are
exemplary an~ should not be construed as limiting the
scope of the invention.
EXA
Toroidal test samples were prepared by spirally
winding 100 mm wide alloy ribbon of Metglas~ 2605 S-2,
having a nominal composition of Fe78B13Sig, The average
inside and outside diameters of the various test samples
were 172 mm and 377 mm, respectively, with an average
weight of 55 kg. Six turns of high temperature magnetic
wire were wound parallel to the longitudinal axis of the
core to provide a magnetic field of 800 A/m for
annealing purposes. Several samples were annealed
without insulation by a conventional process while
others were annealed with insulation by the method of
the present invention. The samples were placed in an
inert gas atmosphere and heated to their respective
annealing temperatures with the 800 A/m field applied
during the heating and cooling steps. The samples were
cooled to 200C at an average rate of 1.5C/min.
The time-temperature profiles for samples annealed
by the conventional annealing process and samples
annealed by the process of the present invention are
plotted in figures 1 and 2, respectively. Thermocouples
were strategically placed the center 2 of each core and
5 mm from the outside surface 1 of the core in order to
monitor variations in temperature throughoùt the
annealing process. AS shown in Figure 1, the
temperature at the center 2 of each core annealed by the
conven~ional process differs substantially from the
temperature at the outside surface 1 thereof. When
cores were annealed using the process of the present
invention, as shown in Figure 2, the temperature
differential between the center 2 and at the outside
surface 1 is minimized. As a result, cores exhibiting
the time temperature profile shown in Figure 2 can be
annealed within a substantially reduced time period to
produce markedly improved magnetic properties.
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The magnetic properties i.e. coercive force
(Amperes/meter), A.C. core losses (Watts/kilogra~), and
exciting power (Volt-ampers/kilogram) of the samples
were measured at an induction o~ 1.40 Tesla and a
frequency of 60 Hz. The magnetic values for samples
subjected to a conventional anneal and for samples
annealed by the process of the present invention are
shown in tables I and II, respectively.
TABLE I
CONVENTION ANNEAL PROCESS
Example Alloy Core Core
# Dimension mm Wt. kg
1 METGLAS~Inside Diameter 17257
2605 S-2Outside Diameter 383
2 METGLAS~Inside Diameter 17256
2605 S-2Outside Diameter 380
Anneal Temp. Total Anneal Coercive 60Hz, 1.4T
C Cycle Time* Force Magnetic Losses
_ _ hr_ A/m W/kg VA/kg
355 10 1.8 0.24 0.39
370 10 1.8 0.25 0.3
* Cool to 200C.
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TABLE II
WITHIN THE SCOPE OF THE INVENTION
CO~ES ANNEALED WITH INSULATION
Example Alloy Core Core
# Dimension mm Wt. kg
1 METGLAS~ Indise Diameter 17253
2605 S-2 Outside Diameter 375
2 METGLAS~ Inside Diameter 17254
2605 S-2 Outside Diameter 375
3 METGLAS~ Inside Diameter 17256
2605 S-2 Outside Diameter 37~
4 METGLAS~ Inside Diameter 17255
2605 S-2 Outside Diameter 378
Anneal Temp. Total Anneal Coercive 60Hz, 1.4T
C Cycle Time* Force Magnetic Losses
hr A/m _ W/kgVA/k~
340 6.5 1.4 0.24 0.44
350 6.5 1.1 0.16 0.29
360 6.5 1.3 0.19 0.36
370 6.5 1.6 0.22 0.41
* Cool to 200C.
Three more sample cores were then annealed without
insulation, but with rapid heat treatment, and their
magnetic values were measured at an induction of 1.40
Tesla and a frequency of 60 Hz. Magnetic properties of
these rapidly annealed, uninsulated cores are set forth
in Table III.
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TAELE III
OUTSIDE THE SCOPE OF THE INVENTION
CORE9 ANNEALED WITHOtJT INSULATION
Example Alloy Core Core
~ Dimension mmWt. kg
1 METGLAS3 Inside Diameter 172 54
2605 S-2 Outside Diameter 373
2 METGLAS~ Inside Diameter 54
2605 S-2 Outside Diameter 373
3 METGLAS~ Inside Diameter 172 56
2605 S-2 Outisde Diameter 382
Anneal Temp. ~otal Anneal Coercive 60Hz, 1.4T
C Cycle Time*Force Magnetic Losses
hr _ A/m W/kg VA/k~
350 6.5 1.9 0.36 0.49
360 6.5 2.1 0.37 0.55
360 6.5 2.2 0.32 0.77
* Cool to 200C.
In contrast to the cores of Table II, the cores of Table
III evidenced high coercive forces and high magnetic
losses.
Having thus described the invention in rather full
detail it will be understood that such detail need not
be strictly adhered to but that further changes and
modifications may suggest themselves to one skilled in
the art, all falling within the scope of the invention
as defined by the subjoined claims.
