Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 1 66~0~
This invention relates to an aqueous composition
for forming an insulative glass coating directly on silicon
steel strip and sheet stock, and more particularly to an
aqueous magnesia slurry of high concentration wherein a
substantial portion o the magnesia (magnesium oxide) is an
inactive magnesia (having a citric acid activity, as
hereinafter de~ined, of greater than 200 seconds), and
wherein a thermally decomposable phosphate containing
compound is present as a stabilizing agent which keeps the
inactive magnesia in suspension and increases the viscosity
of the slurry. The invention further relates ~o a method
o suspending an inactive magnesia in water to produce a
stable slurry which can be readily applied to silicon steel
surfaces to form a dried layer which will react with the
steel to form a glass film during a subsequent hlgh
temperature anneal.
In the production of oriented silicon ste~l
decarburized strip and sheet stock is conventionally coated
with an aq~eous slurry of magnesia which is dried at 1GW
heat. During decarburization a layer of fayalite (an iron
silicate) if formed on the silicon steel stock surfaces.
The stocX which is coated with a dried layer of magnesia is
then subjected to a finalt high temperature anneal at about
1095 to about 1260C, during which the magnesia in the
coating reacts with the fayalite layer to form a glass
film, and the cube-on-edge orientation is deveLoped by
secondary recrystallization, as is well known in the art.
Heretofore, it has been considered nece3sary to
use an active maynesia having a citric acid activity of
less than ~00 seconds, in order to provide an aqueous
1 3 ~)6~ 0 ~
slurry from which the magnesia would not settle out rapidly
and in order to obtain reaction between the magnesia and
the fayalite surface layex. An active magnesia of the type
conventionally used hydrates, with consequent increase in
viscosity of the magnesia - water slurry, and thi~ creates
a problem since the viscosity of the slurry must be kept
within a range which will permit application of a coating
of uniform thickness by dipping, spraying, metering rolls,
or the like.
Inactive magnesias have never previously been
considered suitable for forming an insulative glass ilm on
oriented silicon steel strip and sheet stock ~ecause the
dense particles could not be kept in suspension resulting
in the formation of a slurry with very low viscosity. The
dense inactive particles also would not react with the
fayalite surface layer, at least within the time limits
imposed by commercial production rates.
However, since inactive magnesias are available at
much lower cost than the active magnesias, substitution of
inactive magnesia in an annealing separator composition
offers the prospect of very ~ubstantial economies in the
processing of cube-on-edge oriented silicon steel.
Although additives to magnesia slurries have been
proposed Xor the purpose of improving glass film properties
and/or to facilitate the magnesia-fayalite reaction, to the
best of applicant's knowledge no additives have been
developed which are successful in stabilizing an aqueous
slurry o an inactive magnesia and increasing the viscosity
thereof.
The addition of phosphates to an active magnesia
I ~ 66~0~
slurry is known, in order to i~prove the glass ilm
properties and magnetic properties o the silicon steel
base stock. Reference may be made to United Sta~es Patent
3,615,918, issued to J. D~ Evans and D. W. Taylor, wherein
a magnesia composition containing a decomposable phosphate
compound is disclosed. According to this patent, the
phosphate, which ranges between 1~ and 25% by weight
calculated as P2O5 is reduced to elemental phosphorus
during the final high temperature anneal which diffuses
inwardly from ~he coating into the silicon steel.
Pho~phate base coatings which may be applied
directly to metallic surfaces, or as secondary coatings
over a mill glass magnesia-base coatiny, are known in the
art. Pho~phate and magnesia-containing coatings suitable
for application to oriented silicon steel surfaces, either
directly or as a secondary coatings, are disclosed in
United States Patents 3,840,378 and 3,948,876, issued to
J. D. Evans.
As indicated above, to the best o~ applicant's
knowledge, the prior art has never suggested the use of
inactive magnesia for annealing separator composition on
oriented silicon steel strip and sheet ~tock, nor have
additives been proposed which would remedy the problems
inherent in using an inactive magnesia for this purpose.
It is a principal object of the present invention
to provide an aqueous magnesia slurry of high concentration
containing a substantial portion of an inactive magnesia
having a citric acid activity greater than 200 seconds
which is stable against settling, which has a viscosity
suitable for application by conventional means and which
o ~
will react with the Eayalite layer on the stock
surfaces to form a uniform insulating glass filrn o~
satisfactory quality.
It is a further object of the invention to
provide a method of suspending an inactive magnas.ia in
wa-ter to produce a stable slurry which can be readily
applied to silicon steel surfaces and which will react
to form an insulating glass film during a subsequent
high temperature anneal.
According to the invention there is provided a
composition for forming an insulative glass film on
oriented silicon steel strip and sheet surfaces, com-
prising an aqueous slurry of magnesia, at least 25% by
weight oE said magnesia having a citric acid activity
greater than 200 seconds, a phosphate-containing com~
pound chosen from the group consisting of calcium phos-
phates, water soluble ammonium polyphosphate, aluminum
phosphate, magnesium phosphates, phosphoric acid, and
mixtures thereof, said compound being present within
the range Oe 2% to 25~ by weight calculated as P205,
based on the weight of magnesia, and balance essenti-
ally water, the magnesia concentration being up to 006
gram per cubic centimeter of slurry, said slurry being
stable against settling for periods of time up to 10
hours.
The present invention further provides a method
of suspending in water a magnesia at least 25% of which
consists of magnesia having a citric acid activity
greater than 200 seconds to produce a slurry having
increased viscosity and stability against settling,
which comprises the step of providing in said slurry a
- t ~ ~6~
phosphate-containing compound within the range o~ 2~ to
25% by weight calculated as P20s, based on the weight
of magnesia, said phosphate-containing compound being
chosen from the group consisting of calcium phosphates,
water soluble ammonium polyphosphate, aluminum phos-
phate, magnesium phosphates, phosphoric acid and
mixtures thereof.
It has been found that the invention can be
practiced successfully with inactive magnesia from a
commercial source having a citric acid activity greater
than 200 seconds, a Cl- level of 0.02~ maximum and a
S04-- level of 0.02% maximum. Tests have been
conducted on such a magnesia having a citric acid
activity ranging from about 1500 to about 3000 secondsr
wherein the inactive magnesia comprised 100% of the
total magnesia content of the slurry. Blends of 50% by
weight inactive magnesia with 50~ by weight active
magnesia have also been tested and have been found -to
provide optimum adherence of -the dried coating to the
stock and improved glass film quality. It is evident
that lower proportions of inactive magnesia with an
active magnesia can also be used, down to 10% inactive
magnesia and 90% active magnesia for improved glass
film quality, although the economic advantage o the
inactive magnesia is thereby lost. At the other
extreme, satisfactory results have been obtained with
slurries in which 100% of the magnesia had a citric
acid value greater than 200 seconds. The principal
disadvantage in use of a composition in which all the
magnesia is inactive arises from the fact that the
~ ~ ~6~30~
adherence of the dried coating is only fair, ~o that care
must be exercised in handling the coated coils in order to
prevent removal of hte magnesia coating. Where the
inactive magnesia constitutes 25~ to 75% of the ~otal
magnesia content, this problem is minimized. Thi~ range is
thus preferred. The active magnesia preferably wiLl have a
citric acid activity of about 20 to 40 seconds.
The phosphate addition not only stabilizes the
inactive magnesia slurry against settling but also
increases the viscosity to a range desirable for
application by conventional means such as dipping,
spraying, metering rolls and the like. Moreover, the
magnesia concentration can be highsr t~an that o the prior
art slurries containing only active magnesia. Such high
concentrations are advantageous in that drying can be
effected with less heat and in a shorter period of time,
and in that ignition 105s can be maintained below 1~. With
such low ignition losses the glass film quality has been
~ound to remain very uni~orm along both the length and the
width of a coil.
Pre~erred phosphate compounds are monocalcium
phosphate monohydrate, dicalcium phosphate dihydrate and
water soluble ammonium polyphosphate. Preferably rom ~.5~
to 10~ by weight calculat~d as P205, based on the weight of
magnesia, is added. When adding monocalcium phosphate
monohydrate or dicalcium phosphate dihydrate, which are
particulate, the material can be dry blended with the
magnesia prior to forming an aqueous slurry, or can be
added to the water before, during or after mixing the
magnesia therewith. Accordingly the manner in which and
stage at which the pho~phate compound is introduced
into the slurry do not constitute a limitation on
6 ~3 V ~
the practi~e of the invention. In the case of water soluble
a~monium polyphosphate, which is available as an aqueou~
solution only, ~he addition is made to the water of the
aqueous slurry either before or after mixing the magnesia
therewith.
~ The high calcining tem~eratures used in the
production of inactive maynesia result in sintering and/or
agglomeration of the magnesia producing a dense particle
which requires grinding to obtain the de~ired particle size
distribution, This grinding produces magnesia particles with
an active surface. Although not intending to be bound by
theory, it is belieYed that the phosphate compound added in
accordance with the present invention reacts with the magnesia
particles forming a thin~phosphate layer on the surface of
the particles. This thin phosphate layer ~P205 ) giv~s the
particles a net negative char~e preventing agglomeration
and settling from occurring. ?his is believed to be the
theoretical basis for the markedly improved stability
against settling out which ha~ been observed. Additionally,
the stable suspension of su¢h magnesia particles also increases
the viscosity of the slurry to a desixable range.
It has been foun~ that concentratio~s up
to 0.6 grams per cubic centimeter (5 pounds per gallon) can
be used without settling and without increasing the
viscosity to an objectionable extent when the magnesia
component is lO0~ inactive magnesia. When using a mixture
of 50~ inactive and 50% active magnesia, co~cen~rations
ranging from about 0.24 grams per cubic centimeter to
about 0.36 grams per centimeter (3 pounds per gallon)
can be used. Dried coating weights of about 7.5 to about
1 11 6680~
15 grams per square meter were obtained within the above
concentrations ranges when applied in conventional manner
with metering rolls.
The amount of phosphate containing compound
which may be added depends u~on the decarburizing and final
annealing conditions, with a relatively low phosphate level
for highly oxidizing decarburizing conditions, and a
relatively high phosphate level for less oxidi~ing conditions.
Comparision of a 100% inactive magnesia slurry
conta1nin~ a phosphate compound in accordance with~ the
present invention with a control slurry containing 100~ of
the same inactive magnesia slurry but with no additive
indicated that there was no appreciable settling of the
slurry of the invention after lO hours, whereas the slurry
lS with no additive settled out completely within one hour
leaving a substantially clear supernatant liquid layer.i
By way of background, it is pointed out that a
cold reduced decarburi2ed silicon steel strip and sheet
stoc~ may be prepared by conventional procedures wherein a
suitable melt is cast in the form of ingots or continuously
cast into slab form. If cast into ingots the steel is bloomed
and slabbed in conventional manner and the slabs are hot rolled
to intermediate thickness from a temperature of about 1200 to
about 1400C with annealing a~ter hot rolling. The hot mill scale
is removed, and the material is then cold rolled to final guage
in one or more stages, followed by decarburixation preferably
in a wet hydrogen-containing atmosphere. If the steel
is continuously cast into slab form, the method disclosed
in United 5tates Patent 3,764,406, to ~O F Littman is
~ 1 ~6~3(3~
preferably ~ollowed.
A magnesia coating is then applied to the
surfaces of the decarburized stock by dipping, spraying,
metering rolls, or other conventional means. Th~ coati~g
is then dried by heating to a temperature sufficient to
~evaporate the water. The dried coating weigh~ should
preferably range between about 6 and 20 grams per square metsr.
The coated stock is then subjected to a final high temper-
ature anneaL at about 1095 to about l260C, which may
be a box annaal or an open coil anneal, in a reducing
atmosphere. During this anneal the magnesia reacts to
form an insulative glass film, and the cube-on-edge
orientation is obtained by secondary recrystallization.
The composition and method of processing the
silcion s~eel to final strip and sheet form do not constitute
a limitation on the present invention. Any of the regular or
high permeability grades of oriented silicon steel on which
a magnesia coating is utilized may be treated in the practice
of the present invention. By way of non-limiting example,
compositions and methods in which the present invention
finds utility include thosedisclosed in United States Patents
3,287,183; 3f873,381; 3,90S,842; 3,905,B43; 3,932,~234;
3,957,546,and 4,000,015. Except for No. 3,932,234, these
~atents relate to the production of high permeability
_ material (relative permeability greater than 1850 at 796 A/m)
by means of boron and nitrogen additions to the steel.
Manganese and sulfur (and/or selenium) may also be presen~.
For a reguIar g~ain oriented silicon steel
wherein manganese and sulfur (and/or selenium) are present
as grain qrowth inhibitors, a typical but non~limiting
1 ~ ~)6~0~
composition fox the cold rolled and decarburized stock
may comprise, in weight percent, about 2% to about 4%
silcion, about 0.01~ to about 0~4% manganese, about 0.01%
to about 0.03% sulfur, about 0.002~ ~o about 0.005% carbon,
up to about 0.065% (total) aluminum, and balance iron plus
incidental impurities.
Two series of laboratory tests were conducted
which demonstrated the effectiv~ess of phosphate addition
to an aqueous slurry of an inactive magnesia. In both
series of tests the mag,nesia was from a commercial souxce
and had a citric acid activity of about 2000 seconds. ,The
phosphate compound used was monocalc~um phosphate monohydrate
(sold by S-tauffer Chemlcal Company under the designation
"12 XX" and sold by Monsanto Corporation under the designation
"MC.P~"). Both source~ were in particulate form.
- In the first serias aqueous slurries wPre
,prepared each containing 0.6 grams per cubic centimeter of
magnesia, the first slurry containin~ no additive, and the
remaining slurries containing 2.5~, 5~, 7.5% and lOg respectively
- of the calcium phosphate calculated as P205, ba~ed on the
weight of magnesia. D~carburized silicon steel stoc~ of
14 mil thickness was coated with each of the slurries~
The slurry-containing no additive and the slurry containing -
2.5% phosphate did not wet the stock well and without
25_ agitation the magnesia tended to settle out rapidly. No
problems were encountered wi~h coating with the remaining slurries.
After a final high temperature anneal at 1200C,
examination of the glass film developed on each sample indicated.
that they ranged from a thin rainbow glass for the composition
containing no additive to a light gray continuous film for
.
11
I ~ 66~0~
the slurries containing more than 5% phosphate. A progressive .
improvement in the physical appearance of the glass f~m was
noted for each higher amount of phosphate additive.
The second series involved preparation of slurries
containing 100~ i~active magnesia with 0~, nominal 5~, 7.5%
and 10% additions o calciunl phosphate calculated as P205i
50~ inactive magnesia by we~ight with 50~ active magnesia by
weight having a citric acid activity of about 30 seconds
with 0%, nominal 2.5% and 5% calcium phosphate calculated as
P2~5, and 50~ by weight inactive magnesia with 50~ active
magnesia from another source having citric acid activity of
30 seconds with 0%, nominal 2.5~ and 5% calcium phosphate,
calculated as P205. A control composition was also prepared
contàining 50~ each by weight of the active magnesias.from
the two sources with 2.5% by weight addition of calcium phos-
phate calculated as P205.
The active magnesias which were used had the
following s~ecifications:
. F-: less than 500 ppm
Cl-: less than 200 ppm
Na l: less than 200 ppm
Ignition loss at 100C: 1.5-13.0%
Citric acid activity: 20 to 40 seconds
Particle size: 99.9% through 200 mesh
25_ 99.5% throu~h 325 mesh
These Specifications should be observed in the practice of
the invention, although the citric acid activity may range
up to less than 200 seconds, as indicated previously.
- The slurries in which all the magnesia was
inactive weFe prepared with a concentration of 0.60 grams
l?
1 J ~i6~04
per cc, whereas the slurries containing 50% inactive and
50% active magnesias were prepared with concentrati~n ~or
0.36 grams per cc. The control slurry was prepared with a
concentration of 0.144 grams per cc.
Decarburized silicon steel sheets of 11 mil
thic~ness were-coa~ed with each of the above slurries and
were subjected to a box anneal in a reducing atmosphere
at about 120QC. No coating problems were encountered with
any of the.slurries except the one containing only inactive
}o magnesia with no addition of calci~n phosphate. All the
remaining slurries remained in suspension and wet the
stock well.
Evalua~on of the glass:films formed by the
various s}urries indicated the following:
The inactive magnesia with no calcium phosphate
addition formed a thin, discon~inuous glass filmO
The slurries of 100% inactive magnesia with
additions of 5% and 7.5% calcium phosphate calculated as
P205, formed a thin, rough, light gray glass film~ The
slurry con~aining lOn% inactive magnesia and 10~ by weight
calcium phosphate resulted in a rough, re~dish gray poor
quality glass film.
. The mixtures of 50% inactive magnasia and 50%
.
active magnesia formed ~lass films with improved ph~sical
appearance. The slurries containing no calcium phosphate
additions to these mixtures exhibited a continuous gray
glass film with a slightly rough texture. ThP calcium
, . . .
1 3 t~6~0'1
phosphate additions of 2.5~ and 5% calculated as P2O5 re-
sulted in the forrnation of a smoother, lighter gray glass
film.
All -the slurries containing 100~ inactive
magnesia had poor oxidation resistance regardless of
the calcium phosphate additions.
~ the slurries containing 50~ inactive magnesia
and 50% active magnesia with no calcium phosphate addition
also exhibited poor oxidation resistance. Slurries
containing 2.5% calcium phosphate calculated ag P205 exhibited
somewhat improved oxidation resistance. The best oxidation
resistance was exhibited by slurries containing 50% inaçtive
and 50% active magnesias with 5~ calcium phosphate calculated
as PzO5. By way of comparison the control sample containing
equal parts of the two active magnesias with 2.5% calcium
phosphate addition, had poor oxidatio~ resistance.
Secondary coatings of the type disclosed in the
above-mentioned United States Patents 3,840,378 and 3,948,876
(containing magnesia, phosphate and alurninum) were applied to
all samples, and tested for adherence. The adherence of the
secondary coatings followed the same trend as oxidation
resistance. Glass films which demonstrated good oxidation
resistance had good adherence of the secondary coating
while those which had poor oxidation resistance exhibited poor
adherence of the secondary coatings. The best adherence
was exhibited by ~ixtures of 50% active and 50% inactive
magnesias with 5% calcium phosphate calculated as P205.
Various magnetic properties of the samples of the
second series were determined at 60Hz and are summarized in
Ta~le I. It will be noted that of the compositions containing
14
3 0 4
100% inactive magnesia the be5t magnetic quality was eY.hibited
by the sample containing a nominal 5% calcium phosphate additio~
calculated as P205. Additions of nomlnal 7.5% and 10% ~;
calcium phosphate resulted in a decrease in magnetic quality.
The magnetic quality,of samples containing
50% inactive and 50% active magnesia~ exhibiting a sux-
pr-ising improvement, particularly with calcium phosphate
additions. With no calcium phosphate addition the magnetic
quality was co~parable to that of the control sample. With
calcium phosphate additions of nominal 2.5 and 5% calculated
as P205, all samples exhi,bited a significant improvement
in magnetic quality as compared to the control sample.
Plant trials wexe next conducted using
decarburized silicon steel coils of 11 mil thickness within
the conventional steel composition ranges set forth above,
and coils were coated with the following'composition:' '
5~% by weight inactive magnesia CAA 2000 seconds
50% active magnesia CAA 30 seconds
5% mo,nocalcium phosphate monohydrate calculated
as P2s
balance water
Slurry concen-trations and coating weights,of the
dried coatings are set forth in Table II. No coating
problems were encountered. The slurry wet the strip very well
producing smooth, uniform coatings; the as dried adherence
_- was air. No appreciable settling was observed in the coating
pan.
7 ~
It was found tha~ the coatings dried much more
rapidly than conventlonal coatings containing only ac ~ve
magnesia. Consequently coatings containing inactive magnesia
can be dried at lower temperature, thus reducing the amount
of energy required.
The coated coils were box annealed in a reducing
atmosphere at about 1150C in conventional manner for 24
hours. The coils formed an excellent ylass film which was
light gray:, smooth, continuous and devoid of any oxide o~
; rainbow. Moreover the coil front end to coil back end
variation in physical appearance o~ the ylass film whlch
, normally occurs with conventional active ma~nesias, was not
! present in any of these coils.
Magnetic properties of the above coils are reported
in Table III together with magnetic properties o~ control
coils from the same heat coated witp a mixture of 50%
active magnesia CAA 30 seconds fxom a first source and 5Q~
acti~e magnesia CAA 30 seconds from a second source. It is
evident that the magnetic quality of coils coated with the
composition of the invention was good and e~ual to or better
than-those of ~he control coils~
Further tests have been conducted showing that
the composition and me~hod of the present invention are
applicable to the production of very high pe~meability
silicon steel conkaining boron and nitrogen in accordance
with the teachings of United States Patent 3,873,381, and
wherein boron and titanium dioxide are added to the magnesia
composition. Prior art magnesia compositions to which boron
and titanium dicxide addit~ons are made have always
contained only an active magnesia having a citric acid
16
1 1 ~6~0~
activity ranging from, e.g., about 20 to about 80 seconds.
The boron addition may be in the form of a boron compound
such as boric acid, sodium tetraborate, and the liXe.
In these tests a mixture of 50% by weiyht of an
inactive magnesia from a domestic source having a CAA of
about 2000 seconds, and 50% by weight of an active
magnesia from a domestic source ha~ing a CAA of about 30
seconds, was used as the base composition. An active
magnesia from a Japanese source having a CAA of about 30.
seconds was used as a control since this had produced
optimum glass film properties and magnetic quality in th0
production of very high permeability material by prior
art methods. This contained 0.08% boron as supplied.
. The base composition was mixed with water to
form a slurry having a concent~ation of 0.36 gm per cc.
Various combinatlons of additives were mixed with slurry
samples, based on the total dry magnesia weight, as follows:
% monocalcium
phosphate mono- % .
hydrate as P205. Boron _2_
S O O
0 O
, 0 0
1 0 0
.12 0
.12 0
-. .12 0
.12 0
S' O ' S
. ln o
~ 15 0 5
0 5
S o 10
0 10
0 10
0 10
17
8 0 ~
.12 5
.12 5.
.12 5
.L2 ~ 5
.12 10 ~,
12 10
lS 12 . 10
.12 lQ
The control composition was mixed with water
to form a slurry of 0.147 gm per cc concentxation, and 5%
titanium dioxide (based on the weight of magnesia) was
a~ded.
Coatings were applied to decarburized blanks of
very high permeability silicon steel of 11 mil thi.ckness r
and coating weights as-dried ranged from 10 to 12 gm per m2.
The coated blanks were then subjected to a final anneal,in
a reducing atmosphere which included holding at 1190C
in pure hydrogen for 24 hours..
Evaluation of the various coated and annealed
blanks showed the following results:
The samples with 5%, 10%, 15% and 20% calcium
phosphate with and without boron additions plus titanium dioxide,
developed smooth, light gray glass films.
Titanium dioxide additions made the glass
film rougher and ~arker gray. At 5% calcium phosphate and
5% and 10% t.itanium dioxide the physical appearance was
similar to that of the control sample (containing 5~ titanium
dioxide). At 10%, 15~ and 20% calcium phosphate levels
the glass film was progressively rougher and darker in color.
Secondary coatings of the type disclosed in
the above mentioned United States Patent 3,840,378 were
. 18
/ o ~
applied. Samples containing 5~ and 10~ calcium phosphate
plus 5~ and 10% titanium dioxide had the best appe ~ance.
In samples containing no titanium dioxid~ the
secondary coating adherence was poor. The titanium dioxide-
containing samples had improved adherence. The samplecontaining 5% calcium phosphate and S% titanium dioxide
had adherance similar to that of the control sample,
while the adherence of the sample containing 10% calcium
phosphate and 5% titanium dioxide was superior to that of
the control sample.
With respect to magnetic quality, samples
containing 5%, 10% and 15~ calcium phosphate (with no
boron and titanium dioxide additions) had simiLar properties,
i.e., core loss at 1.7T and 60 Hz of 1.675watts/kg, and relative
15 permeability of 1875 at 796 A/m.
In the samples containing 0.12~ boron and
no titanium dioxide, the magnetic quality was inferior to
that of samples above containing no boron and no titanium
dioxide.
Samples containing titanium dioxide and no
- boron exhibited inferior magnetic quality.
Samples containing boron and titanium dioxide
exhibited improved magnetic quality, the best being obtained
with 5% calcium phosphate, 0.12~ boron and 5% titanium dioxide.
25 The core loss for the best sample was 1.679 wat~.s/kg at 1.7T
and 60~z, and relative permeability was1880 at 796 ~/m, as com
pared to 0.763 and 1905 for the control sample.
. '
19 ..
~ '
t3V~
. It was further observed that boron additions
increased the final grain size significantly, and tha~
calcium phosphate additions greater than 5% (wi~h boron
and titanium dioxide) were detrimental to magnetic quality.
It is thereforeevident that in the production
~of very high permeability material a best combination of
properties is obtained with 5% to 10X phosphate compound
calculated as ~205,about 0.10% to about 0~15% bor~n, and
about 5~ to about 10% titanium dioxide, based on the
weight of magnesia~ These ranges are applicable to a
magnesia composition wherein about 25% to about 75~ is
an inactive magnesia having a CAA of about 1500 to about
3000 seconds and the remainder is an active magnesia
having a CAA of about 20 to 40 seconds.
The above data show that calcium phosphate
additions significantly change the reactions which occur
and the composition of the glass ilm during the f.inal high
temperature anneal. The phosphate oxidizes the silicon
and iron on the stock surface increasing the amount of
fayalite at the surface. The phosphorus is thus reduced,
- . and most of lt diffuses harmlessly into the steel, as
described in the above-mentioned United States Patent
3,615,918. On the other hand, calcium, in the form of
calcium oxide, acts as a ~uxing agent causing liquid
formation at a lower temperature. The calcium oxide
reacts with the fayalite on the silicon steel suxfaces
to form wollastonite, which is a magnesium and calcium
oxide~silicon dioxide complex. Wollastonite melts about
200C lower than fayalite, and this incxeased amount of
. 30 liquid phase formation is thus able ~o take th~ magnesia
2n
I ~ 6680~
into solu~ion more readily to complete the glass ilm
formation.
The citric acid activity is a measure of the
hydration rate of magnesium oxide and is determined by
measuring the time required for a given weight of a
magnesia to provide hydroxyl ions sufficient to neutralize
a given weight of citric acid. The test is the same as that
reported in United States Patent 3,841,925, viz.;
1. 100 ml of 0.400 normal aqueous citric acid
containing 2 ml of 1~ phenolphthalein indicator is brought to
30C in an 8 oun¢e wide mouth jar. The jar is fitted with a
screw cap and a magnetic stirrer bar.
2. Magnesia weighins 2.00g is admitted to the
jar, and a stop watch is started at the same instant.
3. As soon as the magnesia sample is added the
lid is screwed on the ja~. At the 5 second point the jar and
contents are vigorously shaken. Shaking is terminated at ~he
10 second point.
4. At the 10 second point the sample is placed
on a magnetic stirrer assembly. Mechanical stirring should
produce a vortex abou~ 2 cm deep at the center when the
inside diameter of the jar is 6 cm.
5~ The stop watch is stopped at the lnstant
the suspension turns pink, and ~he time i5 noted~ This time
- 25 in seconds is the citric acid activity.
21
~ 3 ~6~30~
T~BL~ I
Magnetic Properties
Ca (H2P4) ~2n Core Loss Relative perm
nominal wt. percent Watts/kg at 6011z eability at
Magnesia 2 5 1.5 T 1~7 T- ?96_A/m
100% inactive
(CA~ 2000 secs.) O 1.065 1.583 1840
100% i~active 5 1O054 1.526 1840
100% inactiye 7.5. 1.116 }.662 1823
100% inactive 10 1.111 1.638 1824
50% inactive.+
50% acti~e CAA 3Q sec. 0 1.056 1.563 1834
,
50% inactive +
50~ active CAA 30 sec. 2.5 1.038 1.519 1835
50~ inactive +
50~ active CAA 30 sec. 5 1.038 1.493 1844
50% inactive +
50~ active CA~ 30 sec. 0 1.052 1.55Z 1836
50% inactive +
50~ active CAA 30 sec. 2.5 1.038 1.515 1837
50% inactive +
50~ active CAA 3n sec. 5 1.042 1.506 1842
50% active CA~ 30lsec.
50% active C~A 30 ~ec. 2.5 1.060 1.559 1837
T~B1E II
,
Coating Conditions
Sample . Concentration Coating Weight
/cc =,~
(Coil 1) 0.1~5 11.34
(Coil 2) . 0.248 15.12
(Coil 3) 0.225 . 11~34
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23
