Particles containing iron carbide

The present invention provides acicular particles (A) containing an iron carbide, which have (a) a free carbon content of at most 20 wt. %, and (b) an Fe.sub.5 C.sub.2 ratio(x) of at least 70% and a coercive force of 3x+300 to 7x+270 Oe, the ratio(x) being expressed by ##EQU1## wherein S1 is the X-ray diffraction strength of Fe.sub.5 C.sub.2, and S2 is the X-ray diffraction strength of Fe.sub.3 O.sub.4. The invention further provides particles (B) containing nickel and an iron carbide. The invention further provides acicular particles containing an iron carbide which are obtained by heating an aqueous dispersion of ferric hydroxide in an alkaline system in the presence of a water-soluble compound capable of coordinating to iron to obtain acicular .alpha.-ferric oxide and contacting the product with a carbon-containing reducing-and-carburizing agent or with a mixture of the agent and a carbon-free reducing agent, with or without contacting the product with the carbon-free reducing agent. These particles (A) to (C) are controllable in coercive force and usable for magnetic record media of various standards.

The present invention relates to acicular particles containing iron 
carbide. 
To give an increased recording density to recording media constructed of a 
magnetic material, e.g. magnetic tape, it is generally desirable that the 
magnetic material have a high coercive force. However, the coercive force 
of the magnetic material is required to accord with the ability of the 
head of recording and play back system. 
Acicular particles containing iron carbide have a high coercive force, are 
electrically conductive, possess high hardness and are therefore excellent 
magnetic particles for preparing magnetic recording media. Such iron 
carbide-containing acicular particles are prepared, for example, by 
contacting acicular iron oxyhydroxide particles or acicular iron oxide 
particles with a reducing-and-carburizing agent at 250.degree. to 
400.degree. C. It is known that the acicular iron carbide particles 
contain Fe.sub.5 C.sub.2 as the main iron carbide component, Fe.sub.3 
O.sub.4 (magnetite) and free carbon. However, such acicular particles 
containing iron carbide are not always usable for heads of any ability 
because of their exceedingly high coercive force. Accordingly, it has been 
desired to develop particles containing an iron carbide having a 
relatively low coercive force or an optionally designed coercive force. 
To give an increased output, it is further required that magnetic particles 
for magnetic recording media be greatly magnetized. 
The coercive force of iron carbide-containing particles is controllable 
either (1) by interrupting the carburization reaction, or (2) by elevating 
the temperature of the carburization reaction. Nevertheless, the product 
obtained by the method (1) contains a large amount of Fe.sub.3 O.sub.4 and 
has a poor distribution of coercive forces and is small in magnetization, 
giving a magnetic recording medium which permits changes with time-lapse, 
in the record and magnetic transfer of the record. Further the method (2) 
involves decomposition of the reducing-and-carburizing agent as a side 
reaction which deposits a large quantity of carbon, with the result that 
acicular particles which are generally low in magnetization are obtained. 
An object of the present invention is to provide acicular particles 
containing an iron carbide which is relatively low in coercive force and 
great in magnetization. Another object of the present invention is to 
provide acicular particles containing an iron carbide and outstanding in 
coercive force distribution, acicular shape retentivity, etc. 
Another object of the present invention is to provide acicular particles 
containing an iron carbide and having a coercive force suited to the 
ability of the head to be used. 
Still another object of the present invention is to provide particles 
containing an iron carbide, low in the contents of Fe.sub.3 O.sub.4 and 
elemental carbon and controllable in coercive force. 
The present invention provides acicular particles (A) containing an iron 
carbide, which have (a) a free carbon content of at most 20 wt. %, and (b) 
an Fe.sub.5 C.sub.2 ratio(x) of at least 70% and a coercive force of 
3x+300 to 7x+270 Oe, the ratio(x) being expressed by 
##EQU2## 
wherein S1 is the X-ray diffraction strength of Fe.sub.5 C.sub.2, and S 2 
is the X-ray diffraction strength of Fe.sub.3 O.sub.4. 
The invention further provides particles (B) containing nickel and an iron 
carbide. 
The invention further provides acicular particles (C) containing an iron 
carbide and obtained by heating an aqueous dispersion of ferric hydroxide 
in an alkaline system in the presence of a water-soluble compound capable 
of coordinating to iron to obtain acicular .alpha.-ferric oxide and 
contacting the product with a carbon-containing reducing-and-carburizing 
agent or with a mixture of the agent and a carbon-free reducing agent, 
with or without contacting the product with the carbon-free reducing 
agent. 
The acicular particles (A) are prepared, for example, by heating acicular 
FeOOH particles at 700 to 1000.degree. C. to obtain acicular 
.alpha.-Fe.sub.2 O.sub.3 particles, optionally contacting the Fe.sub.2 
O.sub.3 particles with a reducing agent not having carburizing ability at 
200.degree. to 700.degree. C., and contacting the resulting particles with 
a reducing-and-carburizing agent at 250.degree. to 400.degree. C. 
Iron carbide-containing acicular particles heretofore known (e.g. those 
disclosed in Unexamined Japanese Patent Publication No. 71509/1985) are 
above 7x+270 to not greater than 7x+470 Oe in coercive force y when 
containing up to 20 wt. % of free carbon and having an Fe.sub.5 C.sub.2 
ratio(x) of at least 70. None of iron carbide-containing acicular 
particles are known which have a low coercive force y of up to 7x+270 Oe 
as contemplated by the present invention. 
With reference to FIG. 1 showing the relation between the Fe.sub.5 C.sub.2 
ratio(x) and the coercive force(y), the area ABDC represents conventional 
iron carbide-containing acicular particles, while the area CDFE represents 
the particles of the invention. 
Although the reason why iron carbide-containing acicular particles having a 
relatively low coercive force can be obtained according to the invention 
is not yet clarified, the reason will presumably be as follows. The 
conventional .alpha.-Fe.sub.2 O.sub.3 particles are obtained by 
dehydrating FeOOH at a relatively low temperature of about 350.degree. C. 
and have relatively small crystallites, whereas FeOOH is dehydrated at a 
high temperature of 700.degree. to 1000.degree. C. according to the 
invention so that relatively large .alpha.-Fe.sub.2 O.sub.3 particles can 
be obtained. 
The acicular iron oxyhydroxide (FeOOH) to be used n the present invention 
may be .alpha.-, .beta.- or .gamma.-FeOOH, which gives acicular 
.alpha.-Fe.sub.2 O.sub.3 when heated at about 700.degree. to about 
1000.degree. C., preferably 750.degree. to 900.degree. C. 
With the present invention, the acicular .alpha.-Fe.sub.2 O.sub.3 thus 
obtained is subsequently brought into contact with a 
reducing-and-carburizing agent at 250.degree. to 400.degree. C., whereby 
the desired iron carbide-containing acicular particles are prepared. This 
step can be performed after contacting the Fe.sub.2 O.sub.3 with a 
reducing agent having no carburizing ability, e.g. hydrogen, at 
200.degree. to 700.degree. C. 
The starting acicular iron oxyhydroxides can be those usually at least 3, 
preferably 3 to 20, in average axial ratio and having an average particle 
size (long axis) of usually up to 2 .mu.m, preferably 0.1 to 2 .mu.m, most 
preferably 0.1 to 1.0 .mu.m. As will be described later, the acicular 
particles (A) produced are slightly smaller than, but almost unchanged 
from, the starting material in average axial ratio and in average particle 
size, so that the acicular particles (A) in general preferably have such 
sizes as already stated. 
The acicular iron oxyhydroxides to be used for the process for producing 
acicular particles (A) may have added thereto a small amount or small 
amounts of a compound, such as oxide or carbonate of copper, magnesium, 
manganese or nickel; silicon oxide; potassium salt, sodium salt, etc., 
insofar as the starting material chiefly comprises an iron oxyhydroxide. 
As the reducing-and-carburizing agent, at least one of the following 
compounds can be used. 
CO 
aliphatic, linear or cyclic, saturated or unsaturated hydrocarbons such as 
methane, propane, butane, cyclohexane, methylcyclohexane, acetylene, 
ethylene, propylene, butadiene, isoprene, town gas, etc. 
aromatic hydrocarbons such as benzene, toluene, xylene, alkylated or 
alkenylated derivatives thereof having a boiling point up to 150.degree. 
C. 
aliphatic alcohols such as methanol, ethanol, propanol, cyclohexanol, etc. 
esters such as methyl formate, ethyl acetate and like ester having a 
boiling point up to 150.degree. C. 
ethers such as lower alkyl ether, vinyl ether and like ether having a 
boiling point up to 150.degree. C. 
aldehydes such as formaldehyde, acetaldehyde and like aldehyde having a 
boiling point up to 150.degree. C. 
ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and 
like ketone having a boiling point up to 150.degree. C. 
Particularly preferable reducing-and-carburizing agent are CO, CH.sub.3 OH, 
HCOOCH.sub.3, saturated or unsaturated aliphatic hydrocarbons having 1 to 
5 carbon atoms. 
In the process of preparing the acicular particles (A), the 
reducing-and-carburizing agent can be used as it is or as diluted. 
Examples of diluents are N.sub.2, argon, helium, etc. The dilution ratio 
is suitably selected but is preferably about 1 to about 10 times (by 
volume). The contacting temperature, contacting time, gas flow rate and 
other conditions depend, for example, on the production history, average 
axial ratio, average particle size and specific surface area of the 
acicular iron oxide. The preferred contacting temperature is about 
250.degree. to about 400.degree. C., more preferably about 300.degree. to 
about 400.degree. C. The preferred contacting time is about 0.5 to about 6 
hours. The preferred gas flow rate (excluding diluent) is about 1 to about 
1000 ml S.T.P./min, more preferably about 5 to about 500 ml S.T.P./min, 
per gram of the acicular iron oxide. The contacting pressure inclusive of 
that of the diluent is usually 1 to 2 atom. although not limited 
particularly. 
The particles (A) are in the form of generally uniform acicular particles 
when observed under an electron microscope. The particles (A) are present 
as primary particles and have the same acicular particulate form as the 
starting acicular particles. The acicular particles (A) are found to 
contain carbon by elementary analysis and to contain an iron carbide by 
its X-ray diffraction pattern, which exhibits plane spacings at 2.28.ANG., 
2.20.ANG., 2.08.ANG., 2.05.ANG. and 1.92.ANG.. Such pattern corresponds to 
Fe.sub.5 C.sub.2. The iron carbide component of the acicular particles (A) 
chiefly comprises Fe.sub.5 C.sub.2. 
In most cases, the acicular particles (A) further contain an iron oxide 
component which is chiefly Fe.sub.3 O.sub.4. 
While the acicular particles (A) are likely to contain free carbon, the 
content thereof must not exceed 20 wt. %. If it is over 20 wt. %, a 
reduced amount of magnetization will result undesirably. 
The acicular particles of the present invention are at least 70%, 
preferably at least 80%, in Fe.sub.5 C.sub.2 ratio(x) expressed by the 
equation 
##EQU3## 
wherein S1 is the X-ray diffraction strength of Fe.sub.5 C.sub.2, and S2 
is the X-ray diffraction strength of Fe.sub.3 O.sub.4. The present 
acicular particles are further in the range of 3x+300 to 7x+270 Oe, 
preferably 3x+300 to 6x+300 Oe, in coercive force. 
The acicular particles (A) are slightly smaller than but almost the same as 
the starting acicular particles in average axial ratio and average 
particle size. Accordingly the acicular particles (A) have an average 
axial ratio usually of at least 3, preferably of 3 to 20, and an average 
particle size (long axis) usually of up to 2 .mu.m, preferably of 0.1 to 2 
.mu.m, most preferably of 0.1 to 1.0 .mu.m. 
The particles (B) of the present invention which contain nickel and an iron 
carbide are prepared, for example, by mixing together an aqueous solution 
of nickel salt, an aqueous solution of ferrous salt and an aqueous 
solution of alkali to form a dispersion, introducing oxygen or an 
oxygen-containing gas into the dispersion to obtain particles of 
.alpha.-iron oxyhydroxide containing nickel, and filtering up, drying and 
carburizing the particles. Alternatively, the particles (B) can be 
prepared by mixing together iron oxyhydroxide or iron oxide particles, a 
water-soluble nickel salt and water to form a slurry, admixing with the 
slurry a reagent for converting the nickel salt to an insoluble or 
sparingly soluble nickel compound to cover the surfaces of the particles 
with the nickel compound, and filtering up, drying and carburizing the 
particles. 
Examples of nickel salts useful for the first process are nickel chloride, 
nickel sulfate, nickel nitrate, nickel acetate and the like. Examples of 
useful ferrous salts are ferrous chloride, ferrous sulfate and the like. 
Examples of useful alkalis are hydroxides or carbonates of Na, K, Ca, Mg, 
etc., ammonia and the like. The nickel salt and the ferrous salt are used 
in such a ratio that usually 0.2 to 30 atomic %, preferably 0.2 to 10 
atomic %, of nickel is present based on the iron. The aqueous alkali 
solution is used in at least two times, preferably at least three times, 
the amount equivalent to the iron. 
In the above first process, the aqueous solutions of nickel salt, ferrous 
salt and alkali are mixed together to form a dispersion, and oxygen or 
oxygen-containing gas, such as air, is introduced into the dispersion to 
form nickel-containing .alpha.-iron oxyhydroxide. Oxygen or 
oxygen-containing gas is passed through the dispersion for oxidation, 
preferably at a rate of 10 to 1000 c.c./min reduced to oxygen basis per 
mole of the Fe(OH).sub.2 (ferrous hydroxide) formed in the dispersion. 
The precipitate thus produced is then filtered off, dried and carburized to 
obtain particles containing nickel and iron carbide. The precipitate is 
dried preferably by being allowed to stand at room temperature or being 
heated at a temperature of up to 250.degree. C. The carburizing agents are 
the same compounds as those mentioned in the reducing-and-carburizing 
agent in the preparation of the acicular particles (A). The carburization 
can be conducted in the same manner as in the preparation of the particles 
(A) with the exception of using the above precipitate. 
For the second process of the preparation of the particles (B), examples of 
iron oxyhydroxides are .alpha.-FeOOH, .beta.-FeOOH, .gamma.-FeOOH and the 
like, examples of iron oxides are .alpha.-Fe.sub.2 O.sub.3, 
.gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and the like, and examples of 
nickel salts are those exemplified for the first process. According to the 
second process, to a slurry prepared from iron oxyhydroxide or iron oxide 
particles, water-soluble nickel salt and water is added a reagent for 
converting the nickel salt to an insoluble or sparingly soluble nickel 
compound to cover the surfaces of the particles with the nickel compound, 
and the covered particles are filtered up, dried and carburized, whereby 
particles containing nickel and iron carbide are obtained. Whether the 
nickel is in the form of nickel carbide or nickel oxide or in some other 
form is not yet clarified. The nickel salt and the particulate iron are 
used in such a ratio that usually 0.2 to 30 atomic %, preferably 0.2 to 20 
atomic %, of nickel is present based on the iron. Examples of reagents 
useful for converting the nickel salt to an insoluble or sparingly soluble 
compound are alkali hydroxides such as KOH and NaOH; alkali carbonates 
such as Na.sub.2 CO.sub.3 and NaHCO.sub.3 ; ammonia; alkali cyanates such 
as KCN and NaCN; phosphoric acid or phosphates such as Na.sub.2 HPO.sub.4 
and NaH.sub.2 PO.sub.4 ; hydrogen sulfide; oxalic acid or oxalates such as 
potassium oxalate; chelating agents such as acetylacetone and 
dimethylglyoxime; etc. The reagent is used in an amount sufficient to 
produce an insoluble or sparingly soluble nickel compound. The particles 
covered with the nickel compound can be filtered up, dried and carburized 
in the same manner as in the first process. 
The particles (B) obtained by the above process are in the form of 
generally uniform particles when observed under an electron microscope. 
The particles (B) contain nickel in the interior or surfaces thereof. The 
particles (B) obtained are found to contain carbon by elementary analysis 
and to contain an iron carbide by its X-ray diffraction pattern, which 
exhibits plane spacings at 2.28.ANG., 2.20.ANG., 2.08.ANG., 2.05.ANG. and 
1.92.ANG.. Such pattern corresponds to Fe.sub.5 C.sub.2. The iron carbide 
component of the particles (B) chiefly comprises Fe.sub.5 C.sub.2. 
The particles (B) may further contain an iron oxide component which is 
chiefly Fe.sub.3 O.sub.4. 
While the particles (B) are likely to contain free carbon, the content 
thereof must not exceed 20 wt. %. If it is over 20 wt. %, a reduced amount 
of magnetization will result undesirably. 
The particles (B) are slightly smaller than but almost the same as the 
starting particles in average axial ratio and average particle size. 
Accordingly the particles (B) have an average axial ratio usually of 1 to 
20, preferably 3 to 20, and an average particle size (long axis) usually 
of up to 2 .mu.m, preferably 0.1 to 2 .mu.m, most preferably 0.1 to 1.0 
.mu.m. 
FIG. 2 shows the relationship between the Ni content and the coercive force 
for particles (B) containing Ni in the interior and for those covered with 
Ni on their surfaces. More specifically, Curve A represents the 
relationship between the Ni content and the coercive force determined for 
magnetic particles which were prepared by adding NiCl.sub.2 to 0.5 liter 
of 0.8 mole/liter solution of FeCl.sub.2 in an Ni/Fe ratio of 1 to 10 
atomic %, adding the solution to 1.5 liters of 2.67 moles/liter solution 
of NaOH, passing air through the mixture at a rate of 0.5 liter/min to 
produce .alpha.-FeOOH containing Ni, heating the product in air at 
350.degree. C. and thereafter heating the product in a CO stream at 
340.degree. C. Further Curve B represents the relationship corresponding 
to Curve A determined for magnetic particles which were prepared by adding 
NaOH solution to 2 liters of an aqueous slurry containing 20 g of 
.alpha.-FeOOH particles and NiCl.sub.2 solute in the Ni/Fe ratio of 0 to 
25 atomic % to cause Ni(OH).sub.2 to deposit on the surfaces of the 
.alpha.-FeOOH particles, heating the particles in air at 600.degree. C. 
and thereafter heating the particles in a CO stream at 300.degree. C. 
The drawing indicates that the coercive force of the particulate magnetic 
material can be controlled as desired by varying the Ni content. The 
particles were low in the contents of magnetite and elemental carbon and 
were almost free of sintering. 
The ferric hydroxide to be used for producing acicular particles (C) of the 
present invention can be prepared by any method. Usually, however, an 
alkali such as sodium hydroxide, potassium hydroxide or ammonia is added 
to an aqueous solution of a ferric salt such as ferric chloride, sulfate 
or nitrate, whereby ferric hydroxide is obtained in the form of an 
amorphous precipitate. This method is already well known. Depending on the 
reaction condition, it is likely that the precipitate obtained still has a 
constituent atom thereof the anion constituting the ferric salt. Such a 
precipitate is also usable in the invention as ferric hydroxide. 
The aqueous dispersion to be used may have such a ferric hydroxide 
concentration that the mixture of the dispersion and a water-soluble 
compound capable of coordinating to iron can be stirred as will be 
described below without difficulty, preferably in the presence of seed 
crystals. Usually, the concentration is up to 1.5 moles/liter, preferably 
0.1 to 1 mole/liter, calculated as iron. 
The water-soluble compound having coordinating ability for iron and to be 
used in the present invention is selected from among the water-soluble 
organic or inorganic compounds which act as crystallization control agents 
to control the direction and speed of growth of the .alpha.-ferric oxide 
crystals formed when the aqueous dispersion of ferric hydroxide is heated 
so as to produce acicular crystals. Such compounds have within the 
molecule at least one coordination group containing atoms having 
coordinating ability for iron, such as oxygen, nitrogen and/or sulfur 
atom(s). Examples of such coordination groups are --OH, --COOH, --O--, 
&gt;C.dbd.O, --SO.sub.3 H, --PO.sub.3 H.sub.2, --NH.sub.2, .dbd.N--OH, 
.fwdarw.N, --SH, --S--, &gt;C.dbd.S, --CS.sub.2 H, --COSH, --OCN, etc. 
Preferably, the water-soluble compound capable of coordinating to iron and 
to be used for the invention has within the molecule at least two such 
coordination groups which may be the same or different. 
Examples of preferred water-soluble compounds capable of coordinating to 
iron for use in the present invention are succinic acid, maleic acid, 
nitrotriacetic acid and like polycarboxylic acids, especially di- and 
tri-carboxylic acids; citric acid, tartaric acid, glycollic acid, malic 
acid, .alpha.-methylmalic acid, .alpha.-hydroxyglutaric acid, 
dihydroxyglutaric acid, salicylic acid and like hydroxycarboxylic acids; 
lysine, glycine and like aminocarboxylic acids; ethylenediamine and like 
polyamines; hydroxylamine; aminotri(methylenephosphonic acid), 
ethylenediaminotetra(methylenephosphonic acid), ethylene-1,1'-diphosphonic 
acid, 1-hydroxyethylene-1,1'-diethylenephosphonic acid and like 
organophosphonic acids; cysteine, mercaptoacetic acid and like 
thiocarboxylic acids; mannitol, pentaerythritol and like polyhydric 
alcohols; acetylacetone, ethyl acetoacetate and like .beta.-dicarbonyl 
compounds, sulfophenyliminodiacetic acid and like aromatic sulfonic acids; 
etc. Water-soluble salts or esters of these compounds are also usable in 
the present invention insofar as they have coordinating ability for iron. 
Examples of such salts or esters are sodium citrate, sodium tartrate, 
sodium 1-hydroxypropyl-1,1'-diphosphonate, triethyl citrate, dimethyl 
hydroxysuccinate, ethyl mercaptoacetate, etc. Further according to the 
invention, phosphoric acid salts are also usable which include, for 
example, sodium phosphate, potassium phosphate, ammonium phosphate, etc. 
While various compounds are usable in the present invention as mentioned 
above, especially preferable are the above-mentioned aliphatic 
hydroxycarboxylic acids, organophosphonic acids to be mentioned below, and 
salts and esters thereof. 
Organophosphonic acids represented by the formula 
##STR1## 
wherein n is an integer of 2 to 6, and m is 0 or an integer of 1 to 5, and 
salts or esters of the acids. 
Organophosphonic acids represented by the formula 
##STR2## 
wherein X and Y are each hydrogen, hydroxyl, amino, alkyl or aryl, and q 
is an integer of 1 to 6, and salts or ester of the acids. 
Organophosphonic acids represented by the formula 
##STR3## 
wherein R is hydrogen or alkyl, and salts or esters of the acids. 
More specific examples of organophosphonic acids represented by the formula 
(I) are aminotri(methylenephosphonic acid), 
ethylenediaminotetra(methylenephosphonic acid), 
diethylenetriaminopenta(methylenephosphonic acid), 
triethylenetetraaminohexa(methylenephosphonic acid), 
tetraethylenepentaaminohepta(methylenephosphonic acid), 
pentaethylenehexaaminoocta(methylenephosphosphonic acid) and the like. In 
the formula (II), the alkyl group preferably has 1 to 6 carbon atoms, and 
the aryl group preferably has 6 to 14 carbon atoms. Examples of such 
organophosphonic acids are methylenediphosphonic acid, 
ethylene-1,1'-diphosphonic acid, ethylene-1,2-diphosphonic acid, 
propylene-1,1'-diphosphonic acid, propylene-1,3--diphosphonic acid, 
hexamethylene-1,6-diphosphonic acid, 
2,6-dihydroxypentamethylene-2,4-diphosphonic acid, 
2,5-dihydroxyhexamethylene-2,5-diphosphonic acid, 
2,3-dihydroxybutylene-2,3-diphosphonic acid, 
1-hydroxybenzyl-1,1'-diphosphonic acid, 1-aminoethylene-1,1'-diphosphonic 
acid and the like. 
In the formula (III), the alkyl group preferably has 1 to 5 carbon atoms. 
Examples of such organophosphonic acids are hydroxymethylenediphosphonic 
acid, 1-hydroxyethylene-1,1'-diphosphonic acid, 
1-hydroxypropylene-1,1'-diphosphonic acid, 
1-hydroxybutylene-1,1'-diphosphonic acid, 
1-hydroxyhexamethylene-1,1'-diphosphonic acid and the like. 
The water-soluble compound capable of coordinating to iron is used in an 
amount which is not limited specifically provided that it is sufficient to 
control the direction and speed of growth of .alpha.-ferric oxide crystals 
in the hydrothermal reaction. The amount is usually 1.times.10.sup.-5 to 3 
moles, preferably 1.times.10.sup.-4 to 1.times.10.sup.-1 mole, per gram 
atom of the iron of the ferric hydroxide. Generally, if the water-soluble 
compound is used in too small an amount, it is difficult to obtain 
.alpha.-ferric oxide in the desired acicular form, whereas use of an 
excess of the compound requires a longer period of time for the reaction, 
hence undesirable. 
When the aqueous dispersion of ferric hydroxide is heated in an alkaline 
system in the presence of the water-soluble compound capable of 
coordinating to iron, acicular .alpha.-Fe.sub.2 O.sub.3 which is 
substantially free from voids can be obtained. If the hydrothermal 
reaction is conducted in the presence of seed crystals of .alpha.-Fe.sub.2 
O.sub.3 in addition to the water-soluble compound, acicular 
.alpha.-Fe.sub.2 O.sub.3 can be obtained with a greatly diminished 
particle size distribution. 
The seed crystals of .alpha.-Fe.sub.2 O.sub.3 are not limited specifically 
in shape but can be acicular, spherical, cubic or of any desired shape, 
provided that the smallest width thereof is up to 0.4 .mu.m, preferably up 
to 0.2 .mu.m, on the average. If the smallest width of the iron oxide seed 
crystals is over 0.4 .mu.m on the average, the acicular .alpha.-iron oxide 
obtained is smaller in axial ratio and/or greater in average particle size 
than is desired. Although there is no particular lower limit for the 
average smallest width of the .alpha.-iron oxide seed crystals, it is 
usually about 100.ANG.. .alpha.-Iron oxide which is in such a range in the 
average smallest width is commercially available or can be prepared by a 
known method. 
The .alpha.-iron oxide serving as seed crystals is used in an amount of 0.1 
mole % to 25 mole %, preferably 0.5 to 15 mole %, calculated as iron and 
based on the starting material, i.e. ferric hydroxide. If the amount is 
less than 0.1 mole %, the acicular .alpha.-iron oxide particles obtained 
are excessively large, whereas if it is more than 25 mole %, the product 
is smaller than is desired in axial ratio and/or particle size. 
According to the present invention, satisfactory results can be achieved 
insofar as the water-soluble compound capable of coordinating to iron (and 
also seed crystals) is present when the aqueous dispersion of ferric 
hydroxide is heat-treated. The compound and the seed crystals can be added 
in a desired order. For example, the water-soluble compound and/or seed 
crystals may be admixed with the aqueous solution of ferric salt before 
ferric hydroxide is to be precipitated from the solution. In this case, 
the water-soluble compound capable of coordinating to iron is attached by 
coordination to the iron atom of the ferric hydroxide and consequently 
contained in the precipitate, so that there is no need to additionally 
admix the compound with the dispersion of ferric hydroxide to be 
heat-treated. Usually, however, it is desirable to add the seed crystal 
and the water-soluble compound to the aqueous dispersion of ferric 
hydroxide. 
The thermal reaction is conducted at a pH of at least 7, preferably 8 to 
12.5. Although the alkali to be used is not limited specifically, sodium 
hydroxide, potassium hydroxide, ammonia or the like is usually used. The 
alkali is added to the aqueous dispersion of ferric hydroxide before or 
after the addition of the water-soluble compound or seed crystals. 
Generally, the reaction temperature is preferably at least 100.degree. C. 
in the preparation of the particles (C). At lower reaction temperatures, 
branched particles such as cross-shaped or T-shaped particles or 
.alpha.-FeOOH particles are formed, presenting difficulty in obtaining 
.alpha.-Fe.sub.2 O.sub.3 in the desired acicular form. When conducted at a 
temperature of at least 100.degree. C., the reaction produces no branched 
particles or .alpha.-FeOOH. The upper limit of the reaction temperature is 
below a temperature level at which the water-soluble compound capable of 
coordinating to iron starts thermal decomposition. While the reaction may 
be conducted at an increased pressure, it is usually unnecessary to 
intentionally apply pressure to the reaction system, such that the mixture 
to be reacted is heated and stirred merely in a closed reactor. In this 
case, the reaction temperature is usually 100.degree. to 250.degree. C., 
preferably about 130.degree. to about 200.degree. C. Although not limited 
specifically, the reaction time is usually several tens of minutes to 
several hours. 
In this way, acicular .alpha.-iron oxide can be obtained. The shape and 
dimensions of the oxide particles are controllable as desired by varying 
the heating temperature for the above reaction and selecting the kind and 
amount of the water-soluble compound capable of coordinating to iron, the 
amount and dimensions of the seed crystals, etc. 
According to the present invention, the acicular .alpha.-Fe.sub.2 O.sub.3 
is then brought into contact with a carbon-containing reducing-and 
-carburizing agent or with a mixture of the agent and a carbon-free 
reducing agent, with or without contacting the oxide with the carbon-free 
reducing agent, whereby the desired acicular particles (C) containing iron 
carbide can be obtained. 
The carbon-free reducing agent is, for example, hydrogen or the like. When 
the oxide is to be contacted with this agent, the contact is effected 
usually at about 200.degree. to about 700.degree. C. 
As the reducing-and -carburizing agents are used the same compounds as 
those mentioned in the preparation of the acicular particles (A). The 
carburization can also be carried out in the same manner as in the 
preparation of the particles (A) with the exception of using the above 
acicular .alpha.-Fe.sub.2 O.sub.3. 
The particles (C) are in the form of generally uniform acicular particles 
when observed under an electron microscope. The particles (C) are present 
as primary particles and have the same acicular particulate form as the 
starting acicular particles. The acicular particles (C) are found to 
contain carbon by elementary analysis and to contain an iron carbide by 
its X-ray diffraction pattern, which exhibits plane spacings at 2.28.ANG., 
2.20.ANG., 2.08.ANG., 2.05.ANG. and 1.92.ANG.. Such pattern corresponds to 
Fe.sub.5 C.sub.2. The iron carbide component of the acicular particles (C) 
chiefly comprises Fe.sub.5 C.sub.2. 
In most cases, the acicular particles (C) further contain an iron oxide 
component which is chiefly Fe.sub.3 O.sub.4. 
While the acicular particles (C) are likely to contain free carbon, the 
content thereof must not exceed 20 wt. %. If it is over 20 wt. %, a 
reduced amount of magnetization will result undesirably. 
The acicular particles (C) are slightly smaller than but almost the same as 
the starting acicular particles in average axial ratio and average 
particle size. Accordingly the acicular particles (C) have an average 
axial ratio usually of at least 3, preferably 3 to 20, and an average 
particle size (long axis) usually of up to 2 .mu.m, preferably 0.1 to 2 
.mu.m, most preferably 0.1 to 1.0 .mu.m. 
The particles (A), (B) and (C) of the present invention containing iron 
carbide are useful as a magnetic material for magnetic recording as is 
apparent from the foregoing characteristics, etc., while the use thereof 
is not limited thereto. For example, the particles are usable as a 
catalyst for preparing lower aliphatic hydrocarbons from CO and H.sub.2. 
The iron carbide-containing acicular particles (A) of the present invention 
are excellent in coercive force distribution and acicular shape 
retentivity. 
The acicular particles (A) of the invention can be adapted to have a 
coercive force suited to the ability of the head to be used and is 
outstanding in electrical conductivity and hardness. 
The acicular particles (A) of the invention are about 10 to about 30% 
greater n magnetization than cobalt-modified magnetic particles which are 
comparable to the particles (A) in coercive force. 
The particles (B) containing nickel and iron carbide are low in the 
contents of Fe.sub.3 O.sub.4 and elemental carbon, controllable in 
coercive force and usable for magnetic record media of various standards. 
The iron carbide-containing acicular particles (C) are characterized by 
being relatively low in coercive force and great in magnetization. More 
specifically, the acicular particles (C) have a relatively low coercive 
force of about 600 to about 750 Oe and a great saturation magnetization of 
about 85 to about 95 emu/g. 
The present invention will be described in greater detail with reference to 
the following examples, in which the characteristics were determined by 
the following methods. 
(1) Magnetic characteristics 
Unless otherwise stated, the magnetic characteristics were determined by an 
automatic recorder of direct current magnetization characteristics, Model 
BHH-50, product of Riken Denshi Co., Ltd., having a maximum magnetic field 
of 2500 Oe. 
(2) Analysis of total carbon content 
According to the usual method of C elementary analysis using MT2 CHN CORDER 
Yanaco, product of Yanagimoto Seisakusho Co., Ltd. and oxygen (helium 
carrier) which was passed through the tube containing the particles to be 
analysed at 900.degree. C. 
(3) Free carbon content analysis 
The free carbon content was calculated from the total carbon content 
determined by CHN elementary analysis and the theoretical carbon content 
and ratio of Fe.sub.5 C.sub.2. The ratio (x) determined from X-ray 
diffraction strength is used for this purpose since this ratio 
approximately matches the ratio (z) by weight which is expressed by the 
equation

EXAMPLE 1 
Five gram of acicular .alpha.-FeOOH particles were placed into a muffle 
furnace and heated at 800.degree. C. for 1 hour to obtain acicular 
.alpha.-Fe.sub.2 O.sub.3 particles which were 0.7 .mu.m in average 
particle size (long axis) and 10 in average axial ratio. A 2 g quantity of 
the product was placed into a porcelain boat, then inserted into a tubular 
furnace, heated to 340.degree. C. and contacted with CO which was fed at a 
rate of 200 ml/min for 3 hours. Table 1 shows the characteristics of the 
powder obtained. 
EXAMPLES 2 AND 3 
Powders were prepared in the same manner as in Example 1 except the 
.alpha.-FeOOH heating temperature and .alpha.-Fe.sub.2 O.sub.3 -CO contact 
conditions listed in Table 1. Table 1 also shows the results. 
REFERENCE EXAMPLES 1 TO 3 
Powders were prepared in the same manner as in Example 1 except the 
.alpha.-FeOOH heating temperature and .alpha.-Fe.sub.2 O.sub.3 -CO contact 
conditions listed in Table 1. The results are also given in Table 1. 
TABLE 1 
__________________________________________________________________________ 
heating contact 
contact 
Fe.sub.5 C.sub.2 
free 
temp temp 
time 
ratio(x) 
carbon 
Hc *(b) 
(.degree.C.) 
(.degree.C.) 
(hr) 
(%) (%) (Oe) 
*(a) 
(emu/g) 
__________________________________________________________________________ 
Ex. 1 800 340 3 93 8 740 
1.4 
89.1 
Ex. 2 900 340 4 92 9 710 
1.4 
89.3 
Ex. 3 1000 
340 5 92 9 680 
1.3 
89.5 
Ref. Ex. 1 
350 340 3 90 6 990 
1.1 
78.7 
Ref. Ex. 2 
350 340 1 61 4 730 
1.7 
85.3 
Ref. Ex. 3 
350 400 3 93 28 600 
2.2 
68.3 
__________________________________________________________________________ 
Note: 
*(a) coercive force distribution 
*(b) magnetization 
EXAMPLE 4 
A 0.5 liter quantity of 0.8 mole/liter solution of FeCl.sub.2 containing 
0.02 mole of NiCl.sub.2 so that the Ni/Fe ratio was 5 atomic % was added 
to 1.5 liters of 2.67 moles/liter solution of NaOH. Air was passed through 
the mixture at a rate of 0.5 liter/min to obtain .alpha.-FeOOH particles 
containing Ni. The particles obtained were filtered off (with up to 1 ppm 
of Ni contained in the filtrate), washed with water, dried, pulverized, 
heated in air at 350.degree. C. for 1 hour and subjected to reduction 
carburization at 340.degree. C. for 2 hours while contacting the 
heat-treated product with CO which was fed at a rate of 200 ml/min. The 
powder obtained was 0.7 .mu.m in average particle size (long, axis), 10 in 
average axial ratio, 695 Oe in coercive force and 79.7 emu/g in saturation 
magnetization. 
COMATIVE EXAMPLE 1 
A powder was prepared in the same manner as in Example 4 with the exception 
of using no Ni. The powder was 930 Oe in coercive force and 88.1 emu/g in 
saturation magnetization. 
EXAMPLE 5 
NiCl.sub.2 .multidot.6H.sub.2 O (6.16 g) was dissolved in 2 liters of 
water, and the solution was adjusted to a pH of 3 with HCl. Goethite (20 
g) was admixed with the solution to obtain a uniform slurry having an 
Ni/Fe ratio of 10 atomic %. NaOH was added dropwise to the slurry with 
vigorous stirring to adjust the slurry to a pH of 10. Water glass No. 3 
(0.35 g) was further added to the slurry to give an SiO.sub.2 /FeOOH ratio 
of 0.5 wt. %. The slurry was thereafter filtered, and the particles were 
washed with water, dried and pulverized. The Ni content of the filtrate 
was up to 1 ppm. The resulting powder was heated in air at 600.degree. C. 
for 1 hour and subsequently treated at 310.degree. C. for 6 hours in a CO 
stream which was fed at a rate of 200 ml/min. The powder obtained was 0.7 
.mu.m in average particle size (long axis), 10 in average axial ratio, 710 
Oe in coercive force and 77.5 emu/g in saturation magnetization. 
EXAMPLE 6 
NiSO.sub.4 .multidot.7H.sub.2 O (6.32 g) was dissolved in 2 liters of 
water, and the solution was adjusted to a pH of 3 with H.sub.2 SO.sub.4. 
Lepidocrocite (20 g) was admixed with the solution to prepare a uniform 
slurry having an Ni/Fe ratio of 10 atomic %. Na.sub.2 CO.sub.3 was added 
dropwise t the slurry with vigorous stirring to adjust the slurry to a pH 
of 9. The slurry was then filtered, and the particles were washed with 
water, dried and pulverized. The Ni content of the filtrate was up to 1 
ppm. The resulting powder was heated in air at 350.degree. C. for 1 hour 
and thereafter treated at 300.degree. C. for 6 hours in a CO stream which 
was fed at a rate of 200 ml/min. The powder obtained was 0.7 .mu.m in 
average particle size (long axis), 10 in average axial ratio, 643 Oe in 
coercive force and 72.9 emu/g in saturation magnetization. 
EXAMPLE 7 
Goethite prepared by the conventional method was heated at 350.degree. C. 
for 1 hour for dehydration to obtain hematite. Ni(CH.sub.3 COO).sub.2 
.multidot.4H.sub.2 O (5.60 g) was dissolved in 2 liters of water, and the 
solution was adjusted to a pH of 4.7 with CH.sub.3 COOH. The hematite was 
admixed with the solution to obtain a uniform slurry having an Ni/Fe ratio 
of 10 atomic %. Concentrated aqueous solution of ammonia was added to the 
slurry with vigorous stirring to adjust the slurry to a pH of 10, and the 
slurry was filtered. The particles were washed with water, dried and 
pulverized. The powder obtained was treated at 300.degree. C. for 6 hours 
in a CO stream which was fed at a rate of 200 ml/min. The powder thus 
prepared was 0.7 .mu.m in average particle size (long axis), 10 in average 
axial ratio, 833 Oe in coercive force and 77.2 emu/g in saturation 
magnetization. 
EXAMPLE 8 
Concentrated aqueous solution of ammonia was added dropwise to 10 liters of 
aqueous solution of ferric chloride (0.2 mole/liter in concentration) with 
vigorous stirring until the solution was adjusted to a pH of 8. The 
resulting precipitate was filtered off and washed with water to obtain 
ferric hydroxide particles, which were then dispersed in water to prepare 
2 liters of slurry. Sodium citrate (13 g) was added to the slurry, and 
sodium hydroxide was further added thereto until the slurry was adjusted 
to a pH of 12. The mixture thus obtained was stirred in a closed reactor 
at 150.degree. C. for 3 hours. The resulting precipitate was filtered off, 
washed with water and dried, giving a reddish orange powder. The powder 
was found to be .alpha.-Fe.sub.2 O.sub.3 by X-ray diffraction. Observation 
under an electron microscope revealed that the powder was 0.3 .mu.m in 
average particle size (long axis) and 4 in average axial ratio. 
The .alpha.-Fe.sub.2 O.sub.3 powder (100 g) was dispersed in 2 liters of 
water. A silane coupling agent (6 g) was added to the dispersion for 
adsorption, the dispersion was thereafter filtered, and the particles were 
dried. 
The powder obtained (2 g) was placed as contained in a porcelain boat into 
a tubular furnace and treated at 340.degree. C. for 5 hours in a CO stream 
which was fed at a rate of 200 ml/min. The powder thus prepared was 725 Oe 
in coercive force and 92 emu/g in saturation magnetization. 
COMATIVE EXAMPLE 2 
Alpha-Fe.sub.2 O.sub.3 obtained by heating .alpha.-iron oxyhydroxide powder 
for dehydration in the usual manner was brought into contact with CO in 
the same manner as in Example 8 to obtain a powder, which was 950 Oe in 
coercive force and 75 emu/g in saturation magnetization.