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Acicular .alpha.-iron particles, their preparation and recording media employing same
| Details |
Inventors: Berry, Kenneth L.;
Assignee: E. I. Du Pont de Nemours and Company (Wilmington, DE)
Primary Examiner: Pianalto; Bernard D.
Assistant Examiner:
Attorney, Agent or Firm:
Particulate product containing at least 75 weight % iron (total iron content) and at least 25 weight % acicular .alpha.-iron metal, process for making said product and magnetic recording medium having said product incorporated therein, said product having a length less than 1 micron (.mu.), a length to diameter ratio of at least about 10:1, a coercivity (iH.sub.c), of greater than about 800 oersteds, a saturation magnetization (.sigma..sub.s) of at least 110 electromagnetic units/gram (emu/g), a residual (remanent) magnetization (.sigma..sub.r) of at least 50 emu/g, and a ratio (.sigma..sub.r /.sigma..sub.s) of residual (remanent) magnetization (.sigma..sub.r) to saturation magnetization of at least about 0.45. |
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DETAILED DESCRIPTION OF THE INVENTION The crux of the present invention lies in the hydrogen reduction of acicular particles of .
alpha.
-FeOOH which are intimately codispersed with calcium oxide, to form acicular .
alpha.
-iron particles having a coercivity greater than about 800 oersteds, preferably greater than 900 oersteds.
The process of the invention comprises (1) dispersing acicular particles of .
alpha.
-FeOOH, preferably submicron in size with the largest dimension less than 1 micron, intimately with calcium oxide, the weight ratio of CaO/.
alpha.
-FeOOH being at least 1:1, preferably in the range of about 1.
5-5:1, more preferably about 3-4:1, (2) contacting and reacting the dispersion with hydrogen at a temperature in the range of about 150.
degree.
-350.
degree.
C.
, preferably about 150.
degree.
-250.
degree.
C.
, and at a pressure in the range of about 1-1000 atmospheres, preferably over 400 atmospheres, more preferably in the range of about 450-550 atmospheres, and (3) recovering from the reaction mixture particulate product containing at least 75 weight % iron (total iron content) and at least 25 weight % acicular .
alpha.
-iron metal, said product having a length less than 1 micron (.
mu.
), a length to diameter ratio of at least 10:1, a coercivity (iH.
sub.
c) of greater than about 800 oersteds, a saturation magnetization (.
sigma.
.
sub.
s) of at least 110 electromagnetic units/gram (emu/g), a residual (remanent) magnetization (.
sigma.
.
sub.
r) of at least 50 emu/g, and a ratio (.
sigma.
.
sub.
r /.
sigma.
.
sub.
s) of residual (remanent) magnetization (.
sigma.
.
sub.
r) to saturation magnetization of at least about 0.
45.
This process provides microscopic, acicular .
alpha.
-iron particles which possess superior magnetic properties, particularly saturation and residual magnetizations and .
sigma.
.
sub.
r /.
sigma.
.
sub.
s ratios and high coercivities (iH.
sub.
c), making them ideal for use in coatings on magnetic recording tapes.
Because these particles are acicular or needle-like in configuration, they possess greatly enhanced magnetic properties due to the phenomenon known as shape anistropy
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Manufacturing Materials 
