Note: Descriptions are shown in the official language in which they were submitted.
CA 02024226 1999-11-18
MAGNESIUM OXIDE COATING FOR ELECTRICAL STEELS
AND THE METHOD OF COATING
BACKGROUND OF THE INVENTION
The present invention relates to a coating composition which provides
good insulative properties and acts an annealing separator during the final
high
temperature anneal for grain oriented electrical steels. Magnesia is used
extensively as a separator for high temperature annealing of electrical steels
1 0 after cold rolling. The coating is normally applied after decarburizing
and forms a
glass film during the final high temperature anneal.
DESCRIPTION OF THE PRIOR ART
1 5 Magnesia coatings which are composed mainly of magnesium oxide and
magnesium hydroxide, are used extensively as a separator coating on electrical
steel during the high temperature anneal to prevent the coil laps from
sticlang. A
glass film forms from a reaction between the steel surface and the magnesia. A
magnesia coating must possess certain physical qualities and also improve the
2 0 overall magnetic qualities of the electrical steel. To provide all of
these
properties, the prior workers have done extensive modification to the basic
magnesia composition.
Magnesium oxides when present with water can hydrate to magnesium
hydroxide in a short period of time. The degree of hydration has a strong
2 5 influence on the viscosity of the magnesia slurry and the control of the
process
for applying the coating. The amount of water in the coating can have an
adverse
effect on the glass film formation. To control hydration, prior workers have
changed the size and distribution of the magnesia particles. They have
adjusted
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CA 02024226 1999-11-18
the calcining temperatures for magnesia production. U.S. Patent No. 4,168, 189
(Haselkorn) is representative of this work.
Far more work has been done with additives to the magnesia to improve
the glass film formation. A thin insulative glass film requires a reaction at
S elevated temperatures between the magnesia and the oxide film on the surface
of
the silicon steel after decarburization. Various silica and silicate compounds
have been added to improve the reactions. Oxides of titanium, chromium and
manganese have been added to improve the adherence and glass film.
Phosphate additions have been taught to act as binders in the coating and
1 0 improve hygroscopicity. Various coating additives are used to improve the
appearance, thickness, oxidation resistance and other properties of the glass
film
produced during the high temperature anneal.
Considerable work has also been done to improve the magnetic properties
of the silicon steel by adjusting the composition of the magnesia. The
magnesia
1 5 has a strong influence on surface reactions relating to atmosphere
interactions
and grain size control. U.S. Patent No. 3,627,594 adds titanium dioxide and
manganese oxides. U.S. Patent No. 3,676,227 adds boron compounds to the
magnesia.
Chlorides have been added to magnesias in the past but in combination
2 0 with other compounds. U.S. Patent No. 4,543,134 adds a chloride of Sb, Sr,
Ti
or Zr with an antimony compound, such as Sb2(S04)3, to seal the strip surface
and prevent the atmosphere from reacting with the base metal. The chlorides
are used to increase the silica fom~ed on the surface and reduce the Fe0
content.
The sealing function of the magnesia coating is attributed to the antimony
2
compound which prevents the removal and absorption of the inhibitor elements.
The level of chlorine in th~ coating may range from 0.0025 to 0.4%.
U.S. Patent No. 3,841,925 adds a chlorine contributor and sodium
metasilicate to magnesia to resist hydration and form a nonporous insulative
coating. The critical balance between these additions results in a magnesia
with
sodium chloride and magnesium silicate which delay hydration and provide a
longer residence time for coating. The magnesia has a high level of chlorine,
typically about 0.22 to 3.4% based on the weight of magnesia.
U.S. Patent No. 4,287,006 in Fig. 1 clearly shows the importance of
1 0 eliminating chlorine by requiring the temperature of ca~ining to be above
1300°C
for the control of hydration of magnesias. Column 7, line 27 of this patent
states
annealing separator should contain no magnesium chloride or magnesium
sulfate because they hinder the formation of the glass film.
U.S. Patent No. 3,956,029 states chlorine should be below 0.04% in
1 5 magnesia coatings because it forms a corrosive gas which attacks the base
metal
and causes a rough surface. The irregular coating thickness which results
causes a poor glass ~Im with subsequent peeling problems.
U.S. Patent No. 3,941,623 teaches the control of the moisture which
remains from the hydration of magnesium oxide during the final high
temperature
2 0 anneal. The patent uses metal nitrides which are subsequently converted
into
oxides during the anneal to consume water and lower the dew point. This
reduces th~ steel oxides and provides an improved glass ~Im and grain growth
control in the (110)[001j direction.
Magnesium oxides used for annealing separators in processing electrical
2 5 steels during the final anneal at temperatures between 1100°C and
1300°C have
3
2a 24~ 22~
been modified in many ways. The problems of hydration control, glass film-
metal
surface reactions, impurity removal at the surface and excellent magnetic
properties in the glass film and base metal have been so complex that the
solutions have been only partially successful. The additions to magnesia in
the
past have also been very complex in nature due to the interactions with other
additives.
The present invention has provided magnesia additives used for
annealing separators which do not cause an unsafe environmental working
condition and which are less expensive to use. The interactions with the
1 0 magnesia components are less complex but still provide the desired
benefits of a
high quality glass film and excellent improvements to magnetic quality. The
additives are carefully controlled within critical limits to provide the
desired
combination of properties.
SUMMARY OF THE INVENTION
The present invention has discovered the addition of a metal chloride
(selected from the group of Mg, Na, K and Cad to magnesia will provide
improved
orientation and magnetic quality without the combined addition of sodium
metasilicate or antimony sulfate. The level of chlorine from the chloride
addition
2 0 within the range of 0.01 to 0.2% was found to produce excellent glass film
quality
and magnetic improvements equivalent to prior art magnesias but without the
environmental concerns of antimony. The chloride addition of the present
invention lowers the glass film formation temperature to seal the surface at a
lower temperature. The control of coating porosity using chlorides without the
2 S need for another additive which reacts with the chlorine is unexpected
based on
prior work with chlorides. The chloride addition provides an improved control
of
4
2A 2~ 22~
final grain orientation and grain size by limiting the diffusion and surface
interactions.
The use of Mg, Ca, Na and/or K to provide the surface of chlorine is also
critical to the
quality of the glass film and the magnetic properties of the electrical steel
strip. It is
important to note that the total level of chlorine in the magnesia must be
considered to
optimize the level of metal chloride being added. The production of the
magnesia may
inherently have some level of chlorine which may be adjusted in combination
with the
metal chloride addition.
The magnesia of the present invention may also contain additions of titanium
dioxide to stabilize the aqueous suspension and improve the glass film quality
and the
magnetic properties of the steel strip. Boron, chromium, silica and calcium
phosphate
additions are also optional with the present coating composition. The magnesia
of the
present invention may also be modified to optimize the benefits for regular
grain oriented
or high permeability grain oriented.
The present invention also provides a process for coating silicon containing
electrical steel strip with an adherent electrically insulative coating prior
to the final high
temperature anneal. The aqueous slurry of magnesia is conventionally applied
to the
decarburized strip, heated to remove water and dry the coating and annealed
above about
1,000°C to form a glass film and develop the desired magnetic
properties.
Accordingly, in one aspect, the invention provides a magnesia slurry for
coating
cold rolled oriented silicon steels prior to a final high temperature anneal,
said slurry
being maintained below 25°C and consisting essentially of:
Sa
~~'
CA 02024226 1999-07-20
(a) magnesia with a majority of the particles having a citric acid activity
(CAA) less
than 200 seconds;
(b) a total chlorine level in said magnesia of 0.01 to 0.20 weight % based on
the
weight of said magnesia with at least 0.01 weight % chlorine being from a
metal chloride
selected from the group of Mg, Ca, Na and K;
(c) 0 to 15 weight % Ti02;
(d) 0 to 10 weight % Si02;
(e) 0 to 15 weight % Cr;
(f) 0 to 0.3 weight % B; and
(g) 0 to 20 weight % phosphate.
In another aspect, the invention provides a method for producing a glass film
on
oriented silicon steel strip, said method comprising the steps of:
(a) providing a magnesia bath consisting essentially of magnesia particles
with a
majority of said magnesia particles having a citric acid activity less than
200 seconds, 0 to 15
weight % Ti02 based on the weight of said magnesia, 0 to 10 weight % Si02
based on the
weight of said magnesia, 0 to 15 weight % Cr based on the weight of said
magnesia, 0 to 0.3
weight % B based on the weight of said magnesia, 0 to 20 weight % phosphate
based on the
weight of said magnesia, and a metal chloride selected from the group
consisting of magnesium
chloride, calcium chloride, sodium chloride and/or potassium chloride;
(b) maintaining said magnesia bath at temperature from above freezing to
25°C;
(c) applying an aqueous magnesia slurry from said bath to said strip;
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CA 02024226 1999-07-20
(d) drying said magnesia slurry to remove excess water and provide a dried
magnesia coating weighing about 6.4 grams per square meter, said dried coating
having 0.01 to
0.20 weight % total chlorine based on the weight of said dried magnesia with
at least 0.01
weight % based on the weight of said dried magnesia being from said metal
chloride; and
(e) providing a final high temperature anneal to form said glass film wherein
said
metal chloride seals said coating surface during said anneal to control grain
growth inhibitors in
said steel to stabilize secondary grain growth and develop improved magnetic
properties.
In a further aspect, the invention provides a method for providing a glass
film on
oriented silicon steel strip, said method comprising the steps of:
(a) providing a magnesia bath consisting essentially of magnesia particles
with a
majority of said magnesia particles having a citric acid activity less than
200 seconds, 0 to 15
weight % Ti02 based on the weight of said magnesia, 0 to 10 weight % Si02
based on the
weight of said magnesia, 0 to 15 weight % Cr based on the weight of said
magnesia, 0 to 0.3
weight % B based on the weight of said magnesia, 0 to 20 weight % phosphate
based on the
weight of said magnesia, and a metal chloride selected from the group
consisting of magnesium
chloride, calcium chloride, sodium chloride and/or potassium chloride;
(b) maintaining said bath at a temperature from above freezing to 25°C;
(c) applying a magnesia slurry from said bath to said strip;
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CA 02024226 1999-07-20
(d) drying said magnesia slurry to remove excess water and provide a dried
magnesia coating having from 1.8 x 10-9 to 3.6 x 10-g moles chlorine per
square centimetre per
side of said step with at least 1.8 x 10-9 moles chlorine per square
centimetre per side of said
steel being provided by said metal chloride; and
(e) providing a final high temperature anneal to form said glass film wherein
said
metal chloride seals said coating surface during said anneal to control grain
growth inhibitors in
said steel to stabilize secondary grain growth and develop improved magnetic
properties.
DETAILED DESCRIPTION OF THE INVENTION
The annealing separator of the present invention is a magnesia with a
controlled level of
hydration to allow the aqueous slurry to be applied by conventional
processing. Magnesia
slurries will have some degree of hydration which require the water of
hydration to be driven
off during the high temperature anneal. The water remaining after drying will
cause porosity in
the final glass. To provide a magnesia slurry with controlled hydration, the
majority of the
particles should have a citric acid activity (CAA) below 200 seconds and
preferably below 100
seconds. CAA is a measure of the activity of the magnesia determined by the
time required for
a predetermined amount of hydroxyl ions to neutralize a given weight of citric
acid. The test is
disclosed fully in U.S. Patent No. 3,841,925 at lines 22-46 of column 4 as
being conducted by:
(1.) 100 ml. of 0.400 normal aqueous citric acid containing 2 ml. of 1 percent
phenophthalein
indicator is brought to 30°C in an 8 ounce wide mouth jar (the jar is
fitted with a screw cap and
a magnetic stirrer bar); (2.) magnesia weighing 2.00 g is admitted to the jar
and a stopwatch is
started at the same instant; (3.) as soon as the magnesia sample is added the
lid is screwed on
the jar. At the 5 second point the jar and contents are vigorously shaken and
shaking is
terminated at the 10-second point; (4.) at the 10-second point the sample is
placed on a
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CA 02024226 1999-07-20
magnetic stirrer assembly and mechanical stirring produces a vortex about 2
centimetres deep
at the centre when the inside diameter of the jar is 6 cm; and (5.) the
stopwatch is stopped the
instant the suspension turns pink and the time is noted. This time in seconds
is the citric acid
activity. The magnesias of the present invention may also include up to about
45% inactive
magnesia which has a CAA above 200 seconds and typically about 500-5,000. The
inactive
magnesia tends to control hydration since it hydrates more slowly and also is
less expensive.
The amount of inactive magnesia which can be effectively used is related to
the quality of the
glass film and the control of porosity in the film.
The magnesia of the present invention requires a chlorine addition within the
critical
range of 0.01 to 0.20 weight % to provide good glass film formation and
improved magnetic
quality in the grain oriented electrical steel. The chlorine level required
may be partially
provided by the magnesia production in combination with at least 0.01 % metal
chloride. It is
the total level of chlorine present which must be controlled within the ranges
of 0.01 to 0.20
weight %.
The chlorine in a metal compound selected from the group of Mg, Na, K and Ca
may be
added to the magnesia in an amount of 0.01 to 0.20 weight % based on the
weight of Mg0
depending on the level of chlorine present in the magnesia initially. The
metals used with the
chlorides are selected to provide improved magnetic quality without any
adverse effects on
safety, cost and glass film and may be used alone or in combination. The
magnesia of the
present invention will have a citric acid activity of less than 200 seconds
for the majority
of the
6
CA 02024226 1999-11-18
particles. The magnesia may also contain up to about 45% inactive magnesia
particles having a citric acid activity above 200 and typically from above 500
to
5,000.
The magnesia coating of the present invention is applied to cold rolled
strip of electrical steel prior to the final high temperature anneal. The
electrical
steel strip is typically grain oriented silicon steel containing up to 4 %
silicon, up
to 0.08 % carbon, and any of the well known grain growth inhibitors, such as
AIN,
MnS, MnSe, BN, and others. High permeability silicon steel is generally
considered to possess a permeability above 1880 at 796 A!m and has an
1 0 aluminum nitride inhibitor system as a result of adding about 0.01 to
0.065%
aluminum. U.S. Patent No. 3,676,227 is typical of this technology.
Decarburization of the strip produces a carbon level below about 0.003% and a
surface oxide which reacts with the magnesia during the final high temperature
anneal to form the glass film of forsterite. The oxide film formed during
1 5 decarburizing is basically fayalite and Si02 with some iron oxide present.
The chloride addition of the present invention modifies the surface
reactions and the level must be carefully controlled. Total chlorine levels
above
0.20% produce a glass film with too high a level of iron to be acceptable
quality.
Excessive chlorine levels also result in poor oxidation resistance and poor
2 0 surface resistivity due to the iron content on the surface. The interface
between
the glass film and the base metal also becomes too rough with high levels of
chlorine. Chlorine is preferably added at levels below about 0.15% and more
preferably below about 0.12%. To obtain the magnetic improvement to the
electrical steel, a minimum level of 0.01 % chlorine must be present . A
preferred
2 5 minimum chlorine added as a metal chloride of the invention is about
0.015%
7
CA 02024226 1999-11-18
(and more preferably 0.02%) which provides an optimum balance between
improving the glass film and the magnetic properties of the base metal.
The chlorides of the present invention act to seal the surface during
annealing to control the grain growth inhibitors. This plays a major role in
the
stability of secondary grain growth. In prior chloride additions to magnesia,
such
as U.S.Patent No. 3,841,925, the formation of a nonporous coating was provided
by the reaction with a compound such as sodium metasilicate which was
balanced stoichiometrically to the chlorides. The reaction produced magnesium
silicate and sodium chloride which formed the nonporous coating and controlled
1 0 hydration. Examples with magnesium chlorides within the ranges of the
present
invention were shown to be unsuitable for coating (viscosity so low that the
slurry
was too thin and resulted in excessive porosity). The work done in this patent
clearly shows a quantity of sodium silicate less than the stoichiometric
equivalent
to react with magnesium chloride will produce a coating which is deficient in
1 5 insulative properties and is porous. The present invention has found that
a
critical lower level of chlorine does not require the sodium silicate
addition.
The other prior work of interest was the addition of Sb, Sr, Ti or Zr chloride
with antimony sulfate in U.S.Patent No. 4,543,134. The chlorine was selected
in
an amount from 0.0025 to 0.4%. With less than 0.05% antimony sulfate, the
2 0 patent taught magnetic properties would not be improved. The present
invention
provides the same improvements without the antimony sulfate addition which
this
patent taught was required. The present invention also uses different metals
to
provide the addition of the chlorine.
In order to apply the magnesia of the present invention by conventional
2 5 means, the hydration of the magnesia must be controlled to provide a
slurry
8
CA 02024226 1999-11-18
having a viscosity within a workable range and a stability sufficient to
provide a
reasonable range of operation. This is accomplished by controlling the
temperature of the magnesia slurry, the particle size of the magnesia and the
use
of various additives.
The temperature of the magnesia is controlled to be above freezing and
below about 75°F (25°C) and preferably between about 32-
45°F (0-7°C). This
eliminates the need for additional additives for hydration control which may
have
an adverse influence on glass film quality or the magnetic properties of the
silicon
steel. To maintain the temperature range of the slurry, the magnesia is
1 0 maintained in an insulated vessel with refrigeration coils. The magnesia
is mixed
with cold water and never stored for prolonged periods of time. By maintaining
this cold condition of the magnesia, the slurry does not hydrate to a
significant
degree which would interfere with the coating thickness or uniformity of the
glass
film. The temperature of the magnesia has a general relationship to the
storage
1 5 life before hydration adversely affects the application of the coating.
The higher
the temperature, the more quickly it must be used.
The particle size and citric acid activity of the magnesia for high
permeability silicon steel plays an important role in the glass film quality.
The
majority of the particles will have a CAA below 200 and preferably below 100.
2 0 The magnesia may contain up to about 45 % inactive magnesia which has a
CAA
above 200 and typically from 500 to 5,000. Regular grain oriented silicon
steel
may use a magnesia with larger particle size and have more inactive magnesia.
The bulk density or packing factor of the dried magnesia coating is dependent
on
the particle size distribution and CAA to control the interactions with the
2 5 atmosphere and surface reactions on the steel. The degree of hydration
will also
9
CA 02024226 1999-11-18
influence the magnesia particles during drying. The amount of water of
hydration
will be reduced with larger particle sizes. Too coarse of particles will tend
to
settle out of the slurry and not undergo reaction with the silica during the
final
anneal. To avoid having a porous coating due to excess hydration and have a
magnesia which may be applied as an aqueous slurry by dipping, spraying or
metering rolls, all of these variables must be controlled. The magnesia
coatings
of the invention will produce a good glass film under these conditions and
will
eliminate the need for the sulfate and silicate additions required in prior
coatings.
The chloride addition has another important consideration which has not
1 0 been addressed in the prior art. Laser scribing for domain refinement has
become a required practice for high permeability grain oriented silicon steel.
The
nature of the surface film has a considerable influence on the amount of
energy
from the laser which passes through the glass film and the amount of damage
caused to the glass film during domain refinement. The present glass film
1 5 developed by the chloride additions of the present invention is controlled
to
provide a glass film which may be laser treated without surface damage. A
laser
process such as taught in U.S. Patent No. 4,456,812 has been found to be very
beneficial in providing domain refinement without damage to the glass film.
To evaluate the properties of glass film produced from a magnesia having
2 0 a chlorine addition using a compound of the invention, a series of
experiments
were conducted. Cold rolled strip of high permeability silicon steel having an
AIN
inhibitor system was coated with a magnesia having MgCl2 of various amounts.
The results were compared to a magnesia having an addition of antimony sulfate
as described in U.S. Patent No. 4,543,134 and are shown in TABLE 1. The
2 5 material was 0.23 mm in thickness, 76 mm in width and 305 mm In length.
The
CA 02024226 1999-11-18
results are the average of 10 samples coated with Tateho A1120 magnesia with
5% Ti02. The samples were heated up to 1200°C in a 25% nitrogen-75%
hydrogen atmosphere with a 4°C dew point. The samples were soaked at
1200°C for 15 hours in 100% H2. After the final anneal was completed,
the
samples were scrubbed and stress relief annealed. The magnesia used had a
chlorine level of 0.02 weight %.
TABLE 1
TOTAL 15 KG 17 KG
%CI WATTSJLB WATTS/LB H-10
ADDIT~ WEIGHT % CORE LOSS CORE LOSS p~$~
1 ------------ 0.02 .397 .566 1925
5
Sb2(S04)3
+SbCi3 0.035 .392 .549 1938
2 0.065 .389 .531 1939
0
0.095 .396 .561 1931
--------------0.02 .397 .566 1925
25
MgCl2 0.035 .392 .548 1927
" 0.065 .398 .547 1940
3 0.095 .393 .533 1941
0
A comparison of the magnetic properties between the 2 additives to the
magnesias indicate the magnesium chloride addition which adds about 0.015 to
0.075 % chlorine produces magnetic properties equal to or better than the same
3 5 level of chloride addition with the antimony sulfate within the range of
about 0.03
11
CA 02024226 1999-11-18
to 0.10% total chlorine. It is clearly suggested that levels of chlorine up to
0.20%
should produce better magnetic properties with the magnesium chloride addition
of the present invention based on the trend up to the 0.10% level. The higher
the
chlorine level, the more the magnesium chloride is preferred over the Sb
chloride additives.
As part of the investigation, other samples were also evaluated for glass
film quality using comparative evaluations on secondary coating adherence,
oxidation resistance and Franklin resistivity measurements on the glass film.
These results are shown in TABLE 2.
TABLE 2
PIiYSICAL GLASS FILM QUALITY
Franklin
1 5 Total Oxidation % Secondary Amps
l~1 FLB;?lS,Zr~108 Coatin~p Flaking (ass Film
0.011 Good Fair ,
.506
0.031 Very Good Very Good .587
0.061 Fair Very Good .665
2 0 0.111 Poor Best .755
All chloride additions are MgCL2 , Mg0 has 0.011 % CI
All coating weights are about 6.4 gm/m2/side
0.23mm High Permeability Silicon Steel
A preferred level of total chlorine has been determined to be about 0.015
to about 0.15%. A more preferred total level of chlorine is about 0.015 to
about
0.12%, which provides a good balance of magnetic improvements with a glass
film having good physical properties. The optimum total chlorine level appears
to
3 0 be from 0.02 to 0.10%.
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The resulting glass film must also permit laser scribing without coating
damage. The laser scribing process of U.S Patent No. 4,456,812 provides
improved domain refinement with the present glass films and avoids coating
damage. The magnesia composition of the present invention provides improved
optical characteristics for laser treatments. While any of the metals selected
from
the group of Mg, Ca, K or Na may be used alone or in combination, the use of
Mg
and Na are preferred. The magnesia may include up to 15 weight % Ti02 and
preferably about 5 to 10% when added. Colloidal silica may also be added in
amounts up to 10 weight %. For high permeability grain oriented silicon steel,
the
1 0 level of silica is preferably about 3 to 7% and boron is preferably about
0.05 to
0.15%. Chromium is also an optional addition up to 15 weight %. Preferably the
level is restricted to about 2.5 to about 5% when added.
The magnesias of the present invention may also be used for insulative
coatings for regular grain oriented electrical steels. These magnesias may be
1 5 varied slightly to include up to about 20% phosphate additions with
calcium
phosphate additions preferred within the range of 4-15%, up to 15% chromium
additions with 2-10% preferred, up to 10% silica with about 3-7% preferred and
up to 0.15% boron with a preferred maximum of 0.10%.
The glass film formed from the magnesia may have an insulative coating
2 0 applied over the surface and the secondary coating will have good
adherence.
The addition of the metal chloride in the present invention does not require
a precipitation reaction with a solution of a silicate salt as claimed in U.S.
Patent
No. 3,941,622. A magnesia-silica complex is not applied in the process of the
present invention. TABLE 3 below shows the influence of the metal chloride
2 5 addition within the ranges of the invention for magnesium, calcium and
sodium.
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The permeability and core losses are dramatically improved by adding these
levels of chlorides. The results also indicate that increasing calcium to the
more
preferred level for magnesium and sodium does not provide any additional
benefit and, in fact, may cause a slight degradation in properties. While no
data
is provided for potassium, it is expected to behave similar to sodium in the
levels
required to obtain similar benefits. Both sodium and potassium tend to smooth
the metal interface. Magnesium tends to be more neutral in this regard.
Calcium
appears to improve adherence of the glass film. All of the metal chloride
additives of the invention provide a level of chlorine which roughen the strip
1 0 surface. As stated previously, the chlorine also lowers the temperature of
glass
film formation. It is important to note that magnesias may have an inherent
level
of chlorine, such as 0.011% in the first example of TABLE 3. This chlorine
level
must be considered as contributing to the total level of chlorine reacting in
the
system. The minimum metal chloride addition to provide 0.01 % chlorine must be
1 5 adhered to regardless of the chlorine content of the magnesia. Part of the
preferred higher chlorine contents may include chlorine from the magnesia in
combination with the metal chlorides.
14
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TABLE 3
CHLORIDE ADDITIVES
Total Stress Relief ealed Laser Scribed
Ann
Additiye 1~r1 ~Q ~$ ~Y1?L~ W17/60
cw~~~ cwn~~~ m cw~g~ cw~g~
------ 0.011 .901 1.258 1.914 .835 1.141
MgCl2 0.041 .881 1.231 1.925 .802 1.081
MgCl2 0.071 .872 1.218 1.927 .793 1.070
NaCt 0.041 .874 1.225 1.923 .800 1.084
NaCI 0.071 .874 1.203 1.932 .786 1.048
CaCl2 0.041 .881 1.231 1.923 .802 1.084
CaCl2 0.071 .894 1.242 1.927 .804 1.079
Stress relief anneal at 1525°F, 2 hours, in 95% N2 -5%H2
The levels of the metal chloride required to improve glass film and
magnetic properties appear to vary slightly depending on the metal selected.
The
preferred maximum level of chlorine with calcium chloride appears to be lower
2 0 than with magnesium, sodium or potassium chloride additions. While the
reason
for this difference is not completely understood, the improved properties
occur
with a preferred calcium addition of about 0.015-0.07%. The preferred levels
with
the other metal chloride additions of the invention is about 0.015-0.10%.
These
addition levels may be adjusted to compensate for the level of chlorine
present in
2 5 the source of magnesia.
The present invention has been described with reference to particular
embodiments but is to be understood that numerous modifications may be made
without departing from the scope of the invention. The appended claims are
intended to cover all such equivalent variations as come within the true
spirit and
3 0 ape of the invention.
