Note: Descriptions are shown in the official language in which they were submitted.
1~3~69
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to a method of
making magnetic powders which have high coercive force values and
improved rectangular ratios (Br/B ) and improved print-through
characteristics when used in a magnetic recording medium and is
directed more particularly to a method of making novel acicular
iron oxide powders.
Description of the Prior Art
Recently, magnetic iron oxide which includes cobalt
CO and is suitable for high density recording has been investi-
gated. As well known, when Co2+ ion is contained in acicular
magnetic iron oxide, its magnetic characteristics are further
improved due to the crystal anisotropy in combination with the
shape anisotropy. On this point reference is invited to "Cobalt
substitutedr-Fe2O3 as high Density Recording Tape" by IEEE Trans
Action On Electronics Computers Vol. EC-15 No. 5 1966 and so on.
In general, there are three methods for producing
y-Fe2O3 containing Co.
1. Ferric hydroxide containing cobalt hydroxide is subjected to
water-heat treatment or processing in aqueous alkali solution,
thus produced powders are reduced and then oxided into r-Fe2O3
containing Co.
2. In case of making acicular goethite, goethite, containing CO
is produced by adding an aqueous cobalt salt solution to adjust
the pH, then reduced and oxidized to produce y-Fe2O3 containing
Co .
3. Goethite containing no CO is used as a nucleus, a reaction
the same as the method 2 is carried out on the nucleus to grow
goethite containing CO~ and then this goethite is reduced and
oxidized to produce y-Fe2O3 containing CO.
-1-
11~3~69
According to the above prior art methods, it is pos-
sible to control the coercive force of the powders over a wide
range by adjusting the amount of C contained therein, but there
are defects such as the manufacturing Processe~ thereo-~ are rather
complicated, the powders thus produced are not stable, being
reduced much in magnetic characteristics by pressure and heat and
being poor in print-through characteristics. These defects may
be explained by the phenomena that the crystal magnetic aniso-
tropy of 3 axes directions of the cubic lattice becomes dominant
due to the diffusion of Co2+ ion entered into 16d site of the
spinel crystal structure.
The assignee of this application filed a patent appli-
cation on a novel method free from the prior art defects which
was laid open as Japanese patent application publication No.
10994/73, which is now considered effective. The method of this
publication is generally referred to as a cobalt hydroxide
adsorbing method, in which cobalt hydroxide is adsorbed on the
surface of acicular goethite, y-Fe2o3 or Fe3O4 and when the
nucleus is y-Fe2O3 an acicular magnetic material of high coer-
cive force can be obtained only by subjecting the same to asuitable thermal heating process. According to various experi-
ments, however, when r-Fe2O3 adsorbed with cobalt hydroxide is
subjected to a thermal treatment or processing at, for example,
high temperature (400C) to diffuse in the particles thereof
Co2+ ion, the coercive force increases remarkably but the insta-
bility of its magnetic characteristics becomes great and its
print-through property becomes worse than that of the original
material. Further, even when Co2+ ion is not diffused in the
particles, when CO-ferrite appears on the surface of the particle,
the print-through property is also deteriorated.
It is also known that iron oxide powders adsorbed with
3Gi~9
- cobalt hydroxide as such are used in the magnetic medium. Iron
oxide magnetic powders of this type are disclosed in the Japanese
Publication Nos. 74399/74 (Toda Kogyo Ltd., Co.), 74400/74 (Toda
Kogyo Ltd., Co.), 113159/74 (Tokyo Denki Kagaku Kogyo Ltd., Co.)
and so on. According to the methods of making iron oxide magnet-
ic powers disclosed therein, alkali is added to an aqueous solu-
tion of cobalt salt, into which iron oxide powders are dispersed,
to deposit a cobalt compound on the surface of iron oxide pow-
ders. In this case, the concentration of alkali is lower than
3 mol/~e. The thus obtained iron magnetic powders have the depos-
ited cobalt hydroxide on their surface, so that they have a large
hydrophilic property due to OH radicals on their surface. Thus,
their dispersion property in organic solvents is low which is
necessary during magnetic paint manufacturing process in magnetic
recording medium making process, and hence the rectangular ratio
Br/Bm of the magnetic recording medium is low. Further, even
if the iron oxide magnetic powder which is made by the above
method is subjected to thermal treatment to decompose the cobalt
hydroxide on the surface of powder and to decrease the hydro-
philic property on the surface and hence to increase the disper-
sion property in the paint, the increase in dispersion property
is not sufficient when the alkali concentration is lower than
3 mol/~, because the deposition of cobalt compound on the surface
of iron oxide powder is not homogeneous. Further, the iron
oxide magnetic powder, on which a cobalt compound is deposited
from a solution whose alkali concentration is lower than 3 mol/~,
is lacking in improvement of its coercive force Hc with respect
to the depositing amount of cobalt compound. If high coercive
force is desired to be obtained, it is necessary to deposit a
large amount of cobalt compound, which results in the iron oxide
magnetic powder being lowered in magnetization degree and hence
' ' . . ., - ~
3~
a magnetic recording medium made by using such a powder is low
in output.
OBJECTS AND SUMMA:RY OF THE INVENTION
The present invention has as an object to provide a
method of making a magnetic iron oxide powder which is high in
coercive force for use with a novel magnetic recording medium
which is improved in print-through characteristic and high in
rectangular ratio Br/Bm.
It is another object of the invention to provide a
method of making a magnetic iron oxide powder in which iron
oxide is dispersedin an aqueous solution of cobalt salt, alkali
is added to this solution to such an extent that the alkali con-
centration of the liquid phase the reacted dispersion is
higher than 3 mol/Ae but lower than 12 mol/A~ and the dispersion
is heated to deposit~a cobalt compound on the surface of iron
oxide particles to modify the properties thereof~
A further object of the invention is to provide a
method of making magnetic iron oxide particles in which the
magnetic iron oxide powder having deposited thereon the above
modified cobalt compound is subjected to thermal treatment in
a non-reduction atmosphere at a temperature between 100C and
200C.
According to an aspect of the present invention there
is provided a method of making magnetic iron oxide powder which
comprises the steps of dispersing iron oxide particles into an
aqueous solution of a cobalt salt, adding alkali to said aqueous
solution to such an extent that the alkali concentration of the
liquid phase of the reacted dispersion becomes higher than 3
mol/~ but lower than 12 mol/~ , and heating the resultant dis-
persion so as to modify said iron oxide particles with cobalt.
The other objects, features and advantages of the
~ 3~6i9
present invention will become apparent from the following de-
scription taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figs. 1 to 5 are graphs showing the relations between
the coercive force of magnetic powders and their thermal treat-
ment or processing time used for explaining the method of the
present invention;
Fig. 6 is a graph plotting the coercive force against
the atomic ratio of cobalt to iron of magnetic powders which are
used for the explanation of the invention;
Fig. 7 is a graph showing the relation between the
coercive force of magnetic powders and their thermal processing
temperature;
Fig. 8 is a graph showing the relation between the
rectangular ratio of a magnetic tape and the thermal processing
temperature of magnetic powders; and
Fig. 9 is a graph showing the relation between the
print-through value of a magnetic tape and the thermal processing
` temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the present invention will be described
hereinbelow. According to the present invention, acicular iron
oxide particles such as r-Fe2O3, Fe3O4 and a substance whose
oxidized state is therebetween, i.e., O~Fe2+ /Fe3+ ~0.5 are
dispersed into an aqueous solution of cobalt salt; alkali is
added to this aqueous solution to such an amount that the alkali
concentration of the liquid phase of the resultant reaction dis-
persion becomes more than 3 mol/~but less than 12 mol/A?; and
the resultant reaction dispersion is heated to modify the acicular
iron oxide particle with cobalt. That is, the surface of the
acicular iron oxide particle is coated with a cobalt compound
--5--
~3q~
consisting of cobalt hydroxide, cobalt oxide or an intermediate
product. Then, the reaction compound is washed by water, dried
at a predetermined temperature, rinsed with water again, and
dried again. Thereafter, the product is subjected to a heating
process at a predetermined temperature in non-reducing atmos-
phere to produce desired magnetic powders.
In case of making the magnetic powders, it is desired
that the mixing ratio of acicular iron oxide particles with
cobalt salt be such that the atomic ratio of cobalt with iron,
Co/Fe, of the resultant particle is within the range between
0.1 and 10 at. %. As the aqueous solution of cobalt salt, an
aqueous solution of cobalt chloride, cobalt bromide,
cobalt sulfate, cobalt nitrate, cobalt acetate, or mixtures of
two or more of the foregoing or the like can be employed. Also,
the alkali which is used in the present invention is a strong
alkali or hydroxide of alkali metal such as lithium hydroxide,
sodium hydroxide, potassium hydroxide, or mixtures of two or
more of the foregoing or the like. The alkali concentration of
the liquid phase of the resultant reaction dispersion, which
consists of the aqueous solution of cobalt salt dispersed with
acicular iron oxide particles and treated with alkali, is select-
ed between 3 mol/~ and 12 mol/~, as set forth previously, and
more desirably between 3.5 mol/~ and 10 mol/~. When the said
alkali concentration becomes lower than 3 mol/~, the coercive force
Hc becomes low, the dispersion property is deteriorated, and the
rectangular ratio of a magnetic recording medium using the par-
ticles is lowered. While, when the said alkali concentration
exceeds 12 mol/~, the surface of the iron oxide particle is
partially dissolved and its shape is deformed, which results in
that the increase of coercive force Hc is ended and the rectangu-
lar ratio becomes low. In this case, the alkali concentration
is defined as the concentration of the ion pair of alkali metal
' -6-
~ .,
~3Ç~
atom cation and hydroxy ion in the liquid phase of the resultant
reaction dispersion.
When the acicular iron oxide particles are dispersed
into the aqueous solution of cobalt salt, it is desired that the
acicular iron oxide particles be dispersed into an aqueous
solution of cobalt salt whose weight is same as or greater than
that of the acicular iron oxide particles and that the aqueous
solution of cobalt salt have a concentration lower than 1.5
mol/~, more desirably lower than 1.0 mol/~ so as to deposit a
cobalt compound homogeneously on the surface of the acicular
iron oxide particles. The heating temperature of the dispersion
is generally desired to be between 70C and the boiling point
thereof, and as the atmosphere outside the dispersion, a non-
oxidizing atmosphere such as nitrogen, argon, mixture of the
foregoing or the like can be used, but an oxidizing atmosphere
such as oxygen, air, or a mixture of the foregoing with nitrogen
or argon or the like therewith are more desirable.
The acicular iron oxide particles having a cobalt compoun~
deposited thereon are washed with water, dried at a predetermined temperature,
or after being washed with water and dried, they are subjected
to a heating process at a predetermined temperature in non-
reducing atmosphere to achieve desired magnetic powders or parti-
cles. In this case, the range of temperatures at which the
particles, which have been dried and are subjected to the heating
process in non-reduction atmosphere, is desirably between 100C
and 200C, more desirably between 120C and 200C. When the
temperature exceeds 200C, the coercive force Hc of magnetic
particle is lowered, while when the temperature becomes lower
than 120C and further lower than 100C, the dispersion property
thereof becomes poor. The print-through characteristic is good
at the temperature range between 100C and 200C, more desirably
~3~
between 120C and 200C, and becomes poor when outside the above
temperature ranges. The heating or thermal process time in non-
reducing atmosphere is required more than 0.5 hours, but if the
heating process time is carried out in more than 5 hours at the
high temperature side (~200C), the coercive force Hc becomes
low, which is not desired. As the atmosphere for the thermal
process, oxidizing, inert and reducing atmospheres may be used,
but if the coercive force Hc, dispersion property, print-through
characteristic and so on are taken into account, oxidizing and
inert atmospheres are desired. Especially, in order to change
Co(OH)2 on the surface of the magnetic powder (master powder) to
oxide easily and stably without causing any lowering of the
magnetic characteristic, an oxidizing atmosphere is most desired.
If a reducing atmosphere such as hydrogen gas is employed,
cobalt ferrite appears partially and hence the print-through
characteristic is deteriorated. In the case of inert atmosphere
i.e. nitrogen gas, when compared with air, the dispersion proper-
ty i.e. rectangular ratio Br/Bm and print-through value are
lowered somewhat but are better than those of the prior art.
In the case that alkali is mixed into the aqueous
solution of cobalt salt, into which iron oxide magnetic particles
are dispersed, when the mixture is heated to deposit cobalt com-
pound on the surface of the magnetic powders and magnetic powders
of high coercive force are made, it is generally said that as
the amount of added cobalt is great, the coercive force of mag-
netic particles is high. However, the inventors of the present
invention have found that the rectangular ratio Br/Bm of a
magnetic tape using the magnetic powder, which is prepared under
the condition of an alkali concentration between 3 mol/~ and 12
mol/~e regardless of the amount of added cobalt, is improved.
As set forth above, according to the invention, the
magnetic record~ng med~um which is high in coercive force,
superior in dispersion property and good in print-through
effect is made. In the invention, even if Co2+ ion is not
diffused into iron oxide particles, the coercive force Hc
becomes high. The reason for this may be that some magnetic
interaction appears on the boundary between the surface of
the iron oxide particle and the adsorbed substance i.e. cobalt
oxide (surface magnetic anisotropy).
Hereinbelow, the present invention will be described
further with reference to Examples.
Example 1
Acicular y-Fe2O3 particles (whose coercive force
Hc is 380 e~ whose longer axis is 0.5 ~m (micrometer) and
whose axis ratio is about 8) in an amount of 3 Kg had been
dispersed into 20Q of water, 2Q of cobalt salt aqueous solution
in which 894g of CoCQ2 6H2O had been dissolved, was added
to the former, and the resultant mixture was stirred sufficiently,
which means that Co was added at 10 at. ~ (Co/Fe ratio).
Then, 8Q of alkali aqueous solution in which 3.8 Kg of NaOH
had been dissolved was added to the above dispersion. The
resultant dispersion was in an aqueous medium which had a
strong alkali concentration of about 3 mol/Q. The final
dispersion was heated at 100C for 4 hours while being stirred
sufficiently. After the heating, the dispersion was rinsed
with water to have a neutral pH, and filtered with a suction
filter. Thus, ~-Fe2O3 particles, each having Co on the surface
thereof, were produced. These particles were dried and then
subjected to a thermal processing at 100C in air for 5
hours. The magnetic characteristics of magnetic particles
thus produced were as follows;
Saturated magnetization as = 71.8 emu/g
Coercive force Hc = 675 e
g _
3~
Ratio ~r (residual magnetism~/sS = 0.48
The thus produced iron oxide powders, each con-
taining Co, were mixed with the following composition for
about 48 hours in a ba~l mill to produce magnetic paint.
Magnetic iron oxide powders 100 wt. parts
containing Co
Vinyl chloride-vinyl acetate 17.5 wt. parts
copolymer
(Binding agent)(VAGH: Trade name of UCC Ltd, Co.)
Polyurethane Resin 7.5 wt. parts
(Binding agent)(Estane 5702: BF Goodrich Chemical Co.)
Lecithin (Dispersion agent) 2.0 wt. parts
Methyl-ethyl ketone (Solvent) 100 wt. parts
Cyclohexanone ~Solvent) 100 wt. parts
The above magnetic paint was coated on a film,
which is made of polyethylene terephthalate with 12 ~m in
thickness such that the thickness of the paint after being dried
is 6 ~m. Thus, a magnetic tape was produced. In this case,
the coercive force Hc thereof was 660 e and the rectangular
ratio Br/Bm thereof was 0.79.
Figs. 1 to 5 are graphs, respeativel~, showing the
relations between the coercive force Hc of the magnetic
powder and vary;ng Co adsorbing conditions.
Fig. 1 is a graph showing the relation between the
coercive force Hc of the magnetic powder and the heating
time (hr) of the reaction dispersion under the condition that
the added amount of Co was 1 at. % and NaOH concentrations of
the reaction dispersion (1 mol/Q, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol/Q) were varied.
Figs. 2 to 5 are graphs showing the characteristics
3Q similar to that of Fig. 1 when the added amount of Co was
3 at. %, 5 at. %, 10 at. %, and 15 at. %, respectively. In
the respective graphs, the marks X,~;O ,~ and ~ show the cases
-- 10 _
~ ~3~
of NaOH concentration of 1 mol~, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol~Q, respectively.
The following Tables I, II, III and IV are
respectively made from the characteristic graphs of Figs. 1
to 5 and show the changes of the coercive forces and rectangular
ratios of magnetic tapes which are prepared by using the
magnetic powders- made by varying one of Co adsorbing con-
ditions.
3~
Table I
_
Co absorbing condition
Concent- Adding Heating Heating Coercive Rectan-
ration of amount temper- time force of gular
NaOH of Co ature tape ratio
(at.~, of tape
(mol/") Co/Fe) (C) (hour) (e) (Br/Bm)
Comparison 1 10 100 4 528 0.73
_ __ _
Example 3 10 100 4 660 0.79
Example 5 10 100 4 710 0.80
. _
3 10 10 100 4 750 0.81
.
~ Dn ; 15 ~ ~ ~ o l o ~l
~3~
,:
~able II
. . Co absorbing condition
Concent- Adding Heating Heating Coei-cive Rectan-
ration amount temper- time force of gular
of NaOH of Co ature tape ratio
(at. %, of tape
(mol/Q) Co/Fe) (~C) (hour) (e) (Br/Bm)
._.. _
. Example 3 1 100 1 470 0.79
_ _ _ .~ _. _. _ .
Example 3 5 100 1 575 0.79
... __ .....
6 3 10 100 1 608 0.79
. __ ..
Comparison 1 1 100 1 447 0.72
.
Comparison
. 4, 1 5 100 1 488 0.74 . .
_ _ _ __
Comparison 1 10 100 1 491 0.73 .
~ 13 -
1~3069
Table III
.~
Co absorbing condition
Concent- Ac-ding Heating Heating Coercive Rectan-
ration amount temper- time force of gular
of NaOH of Co ature tape ratio
(at.%, of tape
(mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm)
_ _
Example 35 100 1 575 0.79
_ _ __ _ _ .
Exa8mPle 3 5 100 4 658 0.80
.. __ _ _ _
Example 3 5 100 24 690 0.82
.. .; _ . . . . _ ._ ._
Comparison 1 5 100 1 488 0.74
_ _ I .
Comparison
7' 1 5 100 4 513 0.75
.. .. _ . . ._ . .
Comparison
1 5 100 ~ 549 0.76
- 14 -
~3~69
Table IV
Co absorbing condition
. Concent- Adding Heating Heating Coercive Rectangu-
ration amount temper- time force of lar rati
of NaOH o(atC,~o, ature tape of tape
(mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm)
Comparison 1 10 100 24 580 0.74
_
Example 3 3 100 4 576 0.80 ~
_ ...
Example 5 3 100 1 580 0.81
Example 10 3 100 0.5 586 0.81
_
Comparison15 3 100 0.5 570 0 76
- 15 -
3~
Table I shows the characteristics of a tape which
is made by varying the amount of sodium hydroxide in aqueous
medium, with all other conditions being held constant.
In Table I, Examples 1, 2 and 3 are the cases that the
NaOH concentration is at 3 mol/Q, 5 mol/Q and 10 mol/Q,
respectively, and Comparisons 1' and 2' are the cases that
the NaOH concentration is at 1 mol/Q and 15 m~l/Q, respectively.
Further, in Table I the reason why the rectangular ratio of
Comparison 1' in which NaOH concentration 1 mol/Q, is low may
be explained that under this condition Co is not adsorbed on
the surfaces of magnetic powders homogeneously, and the reason
why the rectangular ratio of Comparison 2', in which NaOH concen-
tration is selected 15 mol~Q, is low may be considered that
under this condition the magnetic powders are partially
dissolved due to the high alkali content and hence their
physical forms are deformed or poor. In this case, it is
ascertained that even ~f the concentration of NaOH in the
reaction solution was higher than 12 mol/Q, no magnetic
powders of higher coercive force could be obtained.
The above Table II shows the characteristics of
respective tapes in which the amounts of Co and NaOH were
varied but the other conditions were kept constant. In
Table II, the Examples 4, 5 and 6 show the cases that the
NaOH concentration is held constant at 3 mol/Q but the
adding amount of Co is varied 1 at. %, 5 at. % and 10 at. %,
respectively, while Comparisons 3', 4' and 5' show the
cases that the NaOH concentration is held constant at
1 mol/Q but the adding amount of Co is selected 1 at. ~,
5 at. % and 10 at. % respectively. From Table II it is
noted that, regardless of the added amount of Co, in case
of making magnetic powders with a NaOH concentration higher
than 3 mol/Q, the rectangular ratio of a magnetic tape increases.
- 16 -
3~
The above Ta~1e III shows the characteristics of
a magnetic tape in which the heating time of the reaction
dispersion and NaOH concentration therein are varied but
the other conditions are kept the same. In Table III,
Examples 7 (~which is the same as Example 5), 8 and 9 show
the cases that NaOH concentration is held constant at
3 mol/Q and the heating time is 1, 4 and 24 hours, respectively,
while. Comparisons 6' (which is the' same as Comparison 4'),
7~ and 8' show the cases that NaOH concentration is held
constant at 1 mol~Q and the heating time is 1, 4 and 24 hours, ' '.
respectively. From Table III it will be noted that, in the
case where magnetic powders are made wi.th an NaOH concentration
higher tI.an 3 mol~Q, the rectangular ratio of the magnetic tape
increases regardless of the heating time.
When such a relation between NaOH concentration
and the coercive force of produced magnetic powders is con-
sidered, as NaOH concentration becomes lower than 3 mol/Q,
the coercive force of the produced magnetic powders becomes
low, and also even if the added amount of Co is increased
and the heating time is increased under the same condition,
the coercive force is not increased. Accordingly, in order
to obtain a coercive force higher than a coercive force
HC-600 e' it is necessary to use a NaOH concentration
higher than 3 mol/Q.
Further, even if the magnetic powder of the required
coercive force can be produced with a NaOH concentration
of lower than 3 mol~Q, it is hetter, in view of the rectangular
ratio of magnetic tape, to reproduce under the condition of
NaOH concentration higher than 3 mol/Q, selecting the condition
of the adding amount of cobalt and the heating time.
The above Table IV shows the rectangular ratios of
magnetic tapes using magnetic powders with the coercive
- 17 -
~ 3~
force of about 580 e which are made by varying NaOH con-
centrations. In Table IV Comparison 9', Examples 10, 11~
12, and Comparison 10' are the cases that NaOH concentration
was 1 mol/Q, 3 mol~Q, 5 mol~Q, 10 mol~Q and 15 mol/Q,
respectively. From Table IV, it will be noted that the
magnetic powders, wh~ch are produced with a NaOH concentration
3 to 10 mol/Q, have good rectangular ratios when they are
used to form magnetic tapes and such magnetic powders can be
produced with smaller amounts of cobalt and shorter heating
times. As described above, in order to improve the rectangular
ratio of a magnetic tape, it is preferred that NaOH concen-
tration during coba;lt adsorption is within a range of 3 mol/Q
to 12 mol~Q.
Fig. 6 is a graph showing the coercive force of
magnetic powders at respective Co adding amounts with varying
NaOH concentrations. The graph of Fig. 6 is prepared from
those of Figs. 1 to 5 in which only their maximum values
are extracted and in which the values inscribed in the
vicinit~ of the respective marks represent the heating time
in hours. From the graph of Fig. 6 it will be noted that
if NaOH concentration is maintained within the range of
the present invention the coercive force increases and if
the added amount of Co is lower than 10 at. % in the atomic
ratio of Co~Fe, the improved results are achieved.
Example 13
3 Kg of acicular ~-Fe2O3 powders or particles
(having a coercive force HC=380 e' long axis of 0.5 ~m and
axis ratio of about 8) had been dispersed into 20Q ot
water, aqueous solution of 2Q into which 447 g of CoCQ2-6H2O
(on market) had been dissolved was added to the former and
then the mixture dispersion was stirred sufficiently, which
resulted in a dispersion which contained Co at 5 at. % (in
- 18 -
3q~
Co/Fe atomic .ratio~. Then, 8Q of aqueous solution into which6.0 Kg of NaOH has l~een dissolved was added to the above
resultant dispersion, so that the finally resultant suspension
had a solution of strong alkali of about 5 mol/~. This final
dispersion was heated at 100C for about 1 hour while being
stirred sufficiently. After heating, the solution was rinsed
with water to be neutral in pH, y-Fe2O3 particles on which cobalt
hydroxide was deposited were extracted by means of a suction
funnel, and then the y-Fe2O3 particles were dried. The
10 magnetic characteristics of thus obtained magnetic particles
were such that saturation magnetization ~s was 73.3 emu/g,
coercive force Hc was 606 e and ~r (residual magnetization)/aS
was 0.48, respectively. These magnetic particles will be
referred as a master powder A.
The master powder ~ was subjected to thermal
processing in air with the temperature being changed from 70C
to 400C. Then, when the surface condition of the magnetic
particle or Co adsorption condition was analyzed by the
electron ray d~ffraction and X-ray photoelectron spectrometry
20 (ESCA), the following Table V was obtained.
Table V
State of Co adsorbed on magnetic
particle
Thermal processing (8y Electron ray difraction and
condit'ion ' ' X-'ray photoelectron spectrometry~
70C - 20 hours Co(OH)2
100C - 1 hour Co(OH~2
130C - 1 hour CoOOH, Co3O4
150C - 1 hour CoOOH, Co3O4
150 C - 5 hours 3O4
200C - 1 hour Co3O4
370C - 1 hour C3O4, CoFe2O4
400C - 1 hour CoFe2O4
-- 19 --
~3~
According to this Table V, when the temperature is
lower than 100C, Co~OH~2 on the surface of the master
powder A is still as it is or not changed. Butf when the
temperature goes up to 130C, it is observed that CotOH)2
is changed into Co3O4. It is ascertained that as the
temperature becomes higher, Co is gradually diffused into
the master powder A.
The following Tables VI and VII respectively show
the results when the master powder A was subjected to the
thermal processing in nitrogen and hydrogen atmospheres
and a similar analysis was carried out on the resultant
product. According to the thermal processing in nitrogen,
it is noted that the adsorbed cobalt hydroxide is changed
over about 130C, and as the temperature of the thermal
processing is raised further, Co starts its diffusion into
the magnetic powder. While, according to the thermal processing
in the hydrogen atmosphere, it is noted that Co starts its
diffusion into the magnetic powders at the temperature of
about 200C and at the same time the master powder starts to
be reduced.
Fig. 7 is a graph showing the measured results
of a coercive force Hc f the magnetic powder after the
similar thermal processing has been carried out for about
1 hour. In the graph of Fig. 7, a curve I connecting the
marks O , a curve II connecting the marks X and a curve
III connecting the marks ~ show the thermal processes in
the atmospheres of air, nitrogen and hydrogen, respectively.
It is noted in the thermal processes of air and nitrogen
atmospheres that the coercive force Hc is lowered considerably
within the temperature range of 200C to 350C, which may
be due to the fact that as the temperature of the thermal
process becomes high, Co2~ ion is caused to be moved and the
- 20 -
~3~
structure of the interface between the magnetic particle and
the substance adsor~ed thereon, which interfacial structure
causes the increase of t~e coercive force Hc, will disappear.
As the temperature of the thermal process becomes higher, the
coerci~e force Hc again increases, which is caused by the fact
that Cois diffused into the magnetic powders. Since the
similar diffusion of Co occurs at relatively low temperature
in the thermal process in the hydrogen atmosphere, it may
be considered that no temperature range within which the
coercive force Hc is lowered has been observed.
- 21 -
~ .
'~
3~
Table VI
State of Co adsorbed on magnetic
Thermal processing (By electron-ray diffraction
condition and X-ray photoelectron spec-
trometry)
... ..
130C - 1 hour CoOOH, Co(OH)2
150C - 1 hour CoO, CoOOH
150C - 5 hours CoO
200C - 1 hour CoO
300C - 1 hour CoO, CoFe2O4
400C - 1 hour _ .
Table VII
. .
State of Co adsorbed on magnetic
Thermal processing (By electron-ray diffraction
conditlon and X-ray photoelectron spec-
trometry)
. . _ __.
150C - 1 hour Co(OH)2
200C - 1 hour CoO, CoFe2O4
300C - 1 hour 2 4
,~ .
~ ~,
~i3~
Fig. 8 is a graph showing the results obtained
when the magnetic powders, which are subjected to the thermal
processes in the above respective atmospheres, are used to
provide magnetic tapes and the rectangular ratios (Br/Bm) of the
respective magnetic tapes are measured. In the graph of
Fig. 8, a curve rv connecting the marks ~, a curve V connect-
ing the marks ~ and a curve VI connecting the marks ~ represent
the cases of the thermal processing in air, nitrogen and hydro-
gen atmospheres, respectively. In this case the magnetic tape
is made by the same manner as that of Example 1.
From the graph of Fig. 8 it is noted that the
cobalt oxides derived from cobalt hydroxide which is coated on
the surface of the master powder is superior in the dis-
persion property.
Fig. 9 is a graph showing themeasured print-through
values of the magnetic tape having the characteristics
of Fig. 8 according to JISC-5542. In the graph of Fig. 9,
a curve VII connecting the marks O , a curve VIII connect-
ing the marks ~ and a curve IX connecting the marks ~ re-
present the thermal processes in air, nitrogen and hydrogen
atmospheres, respectively. It is noted that in air and
nitrogen atmospheres, the print-through value begins to be
improved with the thermal processing at temperature higher
than 100C and further about 120C. While it is noted
that when the temperature of the thermal processing becomes
high, the print-through value is deteriorated due to the
diffusion of Co into the master powder. In the graph of
Fig. 9, the print-through value of the tape lower than -50
dB can not be used as a practical tape.
From the above resul~s, it will be understood that
magnetic particles, which will represent superior characteristics
when coated on a tape base, are obtained in the case that
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they are subjected to the thermal processing in air or inert
gas at the temperaturé range between 100C and 200C, and
preferably 120C and 200C.
It is also possible that even if magnetic particles
of Fe3O4 or a substance whose oxidizing condition is be-
tween Fe3O4 and Fe2O3 (intermediate substance), are used,
such magnetic particles whose surface is covered by cobalt
oxide can be made and hence a magnetic tape having improved
characteristics is obtained.
Example 14
2 Kg of magnetic particles, whose divalent to triva-
lent iron ratio Fe2~Fe3~ is 0.20 (which have the coercive
force Hc of 260 e~ long axis of 0.5 ,um and axis ratio of
about 8) had been dispersed into 20Q of water, 2Q of aqueous
solution into which 300g of CoCQ2 6H2O had been dissolved was
added to the dispersion, and the resultant suspension was
stirred sufficiently. Then, 8Q of aqueous solution into
which 6.0 Kg of NaOH had been dissolved was added to the
dispersion, and the resulting dispersion was heated at 100C
for 1 hour while being stirred sufficiently. The magnetic
characteristics of the particles are as follows.
~s = 80.2 emu/g
Hc = 576 e
ar~aS = 0.46
This magnetic powder will be referred as a master
powder B.
The following Table VIII shows the characteristics
of the master powder B after being subjected to thermal
processing at 150C for 1 hour in air and those of a magnetic
tape which is made by using thus prepared magnetic powders
in the manner as recited in Example 1. From Table VIII
it is noted that the characteristics of the tape are improved
- 24 -
by the thermal processing.
Example 15
3 Kg of ~-Fe2O3 particles (having a coercive force
Hc = 405 e~ long axis = 0.4 ,um and axis ratio of about
8) had been dispersed into 20Q of water, aqueous solution
into which 268g of CoCQ2-6H2O (on market) had been dissolved
was added to the dispersion, and the resulting dispersion
was stirred sufficiently, which results in that about
3 at. ~ (Co/Fe atomic ratio) of Co was added. Then 8Q of
aqueous solution into which 4.2 Kg of NaOH had been dissolved
was added to the above dispersion. Thus, the finally resultant
liquid phase had a strong alkali content of about 3.5 mol/Q.
This resulting dispersion was heated at 100C for 1 hour while
being stirred sufficiently. Thus prepared magnetic powders
have the magnetic characteristics that their ~s = 74.6 emu/g,
Hc = 587 e and ~r~s = 0.48. These magnetic powders will
be referred to as a master powder C.
The magnetic characteristics of a magnetic tape
which uses magnetic powders prepared by subjecting the master
powder C to the thermal processing in the manner similar
,
.
l~C3~
-- 26 --
to that of Example 1 are shown in the following Table IX.
Table IX
Thermal process~ng Coercive Rectangular Print-through
condition force of ratio of value of
magnetic tape tape
powd~Or~ Br/B (dB)
None 569 0.74 -51.1
Air
130C - 1 hour 563 0.83 -56.3
Nitrogen
atmosphere
200C - 1 hour 558 0.82 -54.7
From the above Table IX is is ascertained that the
characteristics of the magnetic tape are improved by the
thermal processing.
Also, according to the method of the present
invention the master powders can be made by using substances
different from the cobalt salt and alkali used in Example 13.
Example 16
3 Kg of acicular y-Fe2o3 particles (having the coer-
cive force = 380 e' long axis = 0.5 ,um and axis ratio = 8)
were dispersed into 20~ of water, 2Q of aqueous solution
into which 528 g of CoSO4 7H2O (on market) had been dissolved
was added to the dispersion, and the resultant dispersion
was stirred sufficiently, which meant that Co was added
at about 5 at. % (Co/Fe atomic ratio). Next, 8~ of aqueous
solution into which 6.3 Kg of LiOH H2O had been dissolved
was added to the above aqueous dispersion. Thus, the final
dispersion had a strong alkali solution of about 5 mol/Q.
The magnetic characteristics of magnetic powders, which were
obtained by processing the above solution in the manner similar
to that of Example 13, are as follows. These are ~s = 74.2 emu/g,
Hc = 628 e and ~r/~s = 0.48. This magnetic powder will be
referred to as master powder D.
- 27 -
, 3~
The characteristics of a magnetic tape, which uses
magnetic powders provided by subjecting the master powder
D to the thermal processing similar to Example 13, are
shown in the following Table X.
Table X
Thermal Processing Coercive Rectangular Print-through
condition force of ratio of value of
magnetic tape tape
H Wd(or) Br/Bm (dB)
None 628 0.75 -50.8
Air
160C - 1 hour 619 0.83 -55.1
Nitrogen
atmosphere
200C - 0.5 hour 651 0.81 -49.0
From the above Table X it is noted that the
dispersion property and print-through characteristics are
especially improved by the thermal processing in air.
As described above, according to the method of
making magnetic powders of the invention, a magnetic recording
medium, which has high coercive force and superior disper-
sion property and Is superior in print-through characteristic,
can be made. This magnetic medium is preferred for use
as a high density recording tape and so on.
It will be apparent that many modifications and
variations could be effected by one skilled inthe art without
departing from the spirits or scope of the novel concepts of
the present invention so that the scope of the invention should
be determined by the appended claims only.
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