Particles containing iron carbide

Abstract: The present invention 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 α-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 are controllable in coercive force and usable for magnetic record media of various standards.

Description: 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 to 400°C. It is known that the acicular iron carbide particles contain Fe₅C₂ as the main iron carbide component, Fe₃O₄ (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 Fe₃O₄ 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. Still 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. The present invention 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 α-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 ferric hydroxide to be used for producing acicular particles 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 as 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 α-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-, >C=O, -SO₃H, -PO₃H₂, -NH₂, =N-OH, N, -SH, -S-, >C=S, -CS₂H, -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, nitrilotriacetic acid and like polycarboxylic acids, especially di- and tri-carboxylic acids; citric acid, tartaric acid, glycollic acid, malic acid, α-methylmalic acid, α-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 β-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 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 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 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 acid 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 α-ferric oxide crystals in the hydrothermal reaction. The amount is usually 1×10⁻⁵ to 3 moles, preferably 1×10⁻⁴ to 1×10⁻¹ 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 α-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 α-Fe₂O₃ which is substantially free from voids can be obtained. If the hydrothermal reaction is conducted in the presence of seed crystals of α-Fe₂O₃ in addition to the water-soluble compound, acicular α-Fe₂O₃ can be obtained with a greatly diminished particle size distribution. The seed crystals of α-Fe₂O₃ 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 0.4µm, preferably up to 0.2µm, on the average. If the smallest width of the iron oxide seed crystals is over 0.4µm on the average, the acicular α-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 α-iron oxide seed crystals, it is usually about 100Å. α-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 α-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 α-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 crystals 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°C in the preparation of the particles. At lower reaction temperatures, branched particles such as cross-shaped or T-shaped particles or α-FeOOH particles are formed, presenting difficulty in obtaining α-Fe₂O₃ in the desired acicular form. When conducted at a temperature of at least 100°C, the reaction produces no branched particles or α-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 to 250°C, preferably about 130 to about 200°C. Although not limited specifically, the reaction time is usually several tens of minutes to several hours. In this way, acicular α-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 α-Fe₂O₃ 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 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 to about 700°C. As the reducing-and-carburizing agent, at least one of the following compounds can be used. 1 ○ CO 2 ○ aliphatic, linear or cyclic, saturated or unsaturated hydrocarbons such as methane, propane, butane, cyclohexane, methylcyclohexane, acetylene, ethylene, propylene, butadiene, isoprene, town gas, etc. 3 ○ aromatic hydrocarbons such as benzene, toluene, xylene, alkylated or alkenylated derivatives thereof having a boiling point up to 150°C. 4 ○ aliphatic alcohols such as methanol, ethanol, propanol, cyclohexanol, etc. 5 ○ esters such as methyl formate, ethyl acetate and like ester having a boiling point up to 150°C 6 ○ ethers such as lower alkyl ether, vinyl ether and like ether having a boiling point up to 150°C 7 ○ aldehydes such as formaldehyde, acetaldehyde and like aldehyde having a boiling point up to 150°C 8 ○ ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and like ketone having a boiling point up to 150°C Particularly preferable reducing-and-carburizing agents are CO, CH₃OH, HCOOCH₃, saturated or unsaturated aliphatic hydrocarbons having 1 to 5 carbon atoms. The reducing-and-carburizing agent can be used as it is or as diluted. Examples of diluents are N₂, 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 above acicular α-ferric oxide. The preferred contacting temperature is about 250 to about 400°C, more preferably about 300 to about 400°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 1000ml S.T.P./min, more preferably about 5 to about 500ml 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 atm. although not limited particularly. The particles of the present invention are in the form of generally uniform acicular particles when observed under an electron microscope. They are present as primary particles and have the same acicular particulate form as the starting acicular particles. The acicular particles 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 Å, 2.20 Å, 2.08 Å, 2.05 Å and 1.92 Å. Such pattern corresponds to Fe₅C₂. The iron carbide component of the acicular particles chiefly comprises Fe₅C₂. In most cases, the acicular particles further contain an iron oxide component which is chiefly Fe₃O₄. While the acicular particles 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 slightly smaller than but almost the same as the starting acicular particles in average axial ratio and average particle size. Accordingly they 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 µm, preferably 0.1 to 1.0 µm. The particles of the present invention containing an 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 H₂. The iron carbide-containing acicular particles of the present invention are characterized by being relatively low in coercive force and great in magnetization. More specifically, they 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 magnetic characteristics were determined by the following method. 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. Example A 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 (13g) 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°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 α-Fe₂O₃ by X-ray diffraction. Observation under an electron microscope revealed that the powder was 0.3µm in average particle size (long axis) and 4 in average axial ratio. The α-Fe₂O₃ powder (100g) was dispersed in 2 liters of water. A silane coupling agent (6g) was added to the dispersion for adsorption, the dispersion was thereafter filtered, and the particles were dried. The powder obtained (2g) was placed as contained in a porcelain boat into a tubular furnace and treated at 340°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. Comparative Example Alpha-Fe₂O₃ obtained by heating α-iron oxyhydroxide powder for dehydration in the usual manner was brought into contact with CO in the same manner as in the above example to obtain a powder, which was 950 Oe in coercive force and 75 emu/g in saturation magnetization.

Claims: 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 α-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. Acicular particles as defined in claim 1 which are at least 3 in average axial ratio. Acicular particles as defined in claim 1 or 2 which have a coercive force of about 600 to about 750 Oe and a saturation magnetization of about 85 to about 95 emu/g. Acicular particles as defined in any one of claims 1 to 3 wherein the water-soluble compound, capable of coordinating to iron, has within the molecule at least one coordination group selected from -OH, -COOH, -O-, >C=O, -SO₃H, -PO₃H₂, -NH₂, =N-OH, N, -SH, -S-, >C=S, -CS₂H, -COSH and -OCN. Acicular particles as defined in any one of claims 1 to 4 wherein the water-soluble compound is used in an amount of 1x10⁻⁵ to 3 moles per gram atom of the iron of the ferric hydroxide. Acicular particles as defined in any one of claims 1 to 5 wherein the reducing-and-carburizing agent is CO, CH₃OH, HCOOCH₃ or a saturated or unsaturated aliphatic hydrocarbon having 1 to 5 carbon atoms. A process for preparing the acicular particles of claim 1 which comprises 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 α-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. A process as defined in claim 7 wherein the heating of the aqueous dispersion is conducted further in the presence of seed crystals of α-Fe₂O₃. A process as defined in claim 8 wherein the seed crystals are used in an amount of from 0.1 to 25 mole-%, calculated as iron based on the starting ferric hydroxide.

Inventors: ARASE TAKUYA, DAIMON SHIGEO, SHIBUYA YOSHIYUKI, ARASE, TAKUYA, DAIMON, SHIGEO, SHIBUYA, YOSHIYUKI
Applicants: DAIKIN IND LTD, DAIKIN INDUSTRIES, LIMITED
Application year: 1986
Main classification: G11B5
IPC codes: C01B31, G11B5, B01J27