PARTICLES CONTAINING IRON CARBIDE

Abstract: 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 Fe₅C₂ ratio(x) of at least 70% and a coercive force of 3x+ 300 to 7x+270 Oe, the ratio(x) being expressed by wherein S1 is the X-ray diffraction strength of Fe₅C₂, and S2 is the X-ray diffraction strength of Fe₃O₄. 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 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 (A) to (C) are controllable in coercive force and usable for magnetic record media of various standards.

Description: The present invention relates to particles containing nickel and an 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. EP-A-179490 discloses particles containing an iron oxide which particles are prepared starting from an iron oxyhydroxide or iron oxide which may contain a small amount of a compound selected from oxides or carbonates of copper, magnesium, manganese or nickel, silica, potassium salts and sodium salts. GB-A-2 156 836 discloses a wear resistant magnetic recording medium which comprises a substrate coated with a magnetic paint composition comprising a mixture of magnetic metal particles and magnetic metal carbide particles or metal carbide-coated metal particles dispersed in a binder. Suitable metal particles are e.g. iron particles which may be prepared from iron oxyhydroxides or iron oxides optionally doped with at least one element selected from Co, Mn, Ni, Ti, Bi, Ag, B, etc. and having an aluminum or silicon compound adsorbed and adhered to the surface. An object of the present invention is to provide particles containing an iron carbide and having a coercive force suited to the ability of the head to be used. Another object of the present invention is to provide particles containing an iron carbide, low in the contents of Fe₃O₄ and elemental carbon and controllable in coercive force. The present invention provides iron carbide particles having a nickel content of 0.2 to 30 atomic %, based on the iron. The particles 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 α-iron oxyhydroxide containing nickel, and filtering up, drying and carburizing the particles. Alternatively, the particles 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 α-iron oxyhydroxide. Oxygen or oxygen-containing gas is passed through the dispersion for oxidation, preferably at a rate of 10 to 1000 ml./min reduced to oxygen basis per mole of the Fe(OH)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°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 agent 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 precipitate. 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 precipitate. The contacting pressure inclusive of that of the diluent is usually 1.013 to 2.026 bar (1 to 2 atm) although not limited particularly. For the second process of the preparation of the particles, examples of iron oxyhydroxides are α-FeOOH, β-FeOOH, γ-FeOOH and the like, examples of iron oxides are α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄ 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 Na₂CO₃ and NaHCO₃; ammonia; alkali cyanates such as KCN and NaCN; phosphoric acid or phosphates such as Na₂HPO₄ and NaH₂PO₄; 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 obtained by the above process are in the form of generally uniform particles when observed under an electron microscope. The particles contain nickel in the interior or surfaces thereof. The particles 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 0.228 nm, 0.220 nm, 0.208 nm, 0.205 nm and 0.192 nm (2.28Å, 2.20Å>, 2.08Å, 2.05Å and 1.92Å). Such pattern corresponds to Fe₅C₂. The iron carbide component of the particles chiefly comprises Fe₅C₂. The particles may further contain an iron oxide component which is chiefly Fe₃O₄. While the 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 particles are slightly smaller than but almost the same as the starting particles in average axial ratio and average particle size. Accordingly the particles 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 µm, preferably 0.1 to 2 µm, most preferably 0.1 to 1.0 µm. Fig. 1 shows the relationship between the Ni content and the coercive force for particles 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 NiCl₂ to 0.5 liter of 0.8 mole/liter solution of FeCl₂ 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 α-FeOOH containing Ni, heating the product in air at 350°C and thereafter heating the product in a CO stream at 340°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 20g of α-FeOOH particles and NiCl₂ solute in the Ni/Fe ratio of 0 to 25 atomic % to cause Ni(OH)₂ to deposit on the surfaces of the α-FeOOH particles, heating the particles in air at 600°C and thereafter heating the particles in a CO stream at 300°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 particles of the present invention containing nickel and 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 particles containing nickel and iron carbide are low in the contents of Fe₃O₄ and elemental carbon, controllable in coercive force and usable for magnetic record media of various standards. 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°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 Fe₅C₂. 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 Fig. 1 is a graph showing the relation between the Ni content and the coercive force as determined for particles incorporating Ni in the interior (Curve A) and for those covered with Ni over their surfaces (Curve B). Example 1 A 0.5 liter quantity of 0.8 mole/liter solution of FeCl₂ containing 0.02 mole of NiCl₂ 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 α-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°C for 1 hour and subjected to reduction carburization at 340°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 µm in average particle size (long, axis), 10 in average axial ratio, 695 Oe in coercive force and 79.7 emu/g in saturation magnetization. Comparative 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 2 NiCl₂·6H₂O (6.16g) was dissolved in 2 liters of water, and the solution was adjusted to a pH of 3 with HCl. Goethite (20g) 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 (0.35g) was further added to the slurry to give an SiO₂/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°C for 1 hour and subsequently treated at 310°C for 6 hours in a CO stream which was fed at a rate of 200 ml/min. The powder obtained was 0.7 µm 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 3 NiSO₄·7H₂O (6.32g) was dissolved in 2 liters of water, and the solution was adjusted to a pH of 3 with H₂SO₄. Lepidocrocite (20g) was admixed with the solution to prepare a uniform slurry having an Ni/Fe ratio of 10 atomic %. Na₂CO₃ was added dropwise to 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°C for 1 hour and thereafter treated at 300°C for 6 hours in a CO stream which was fed at a rate of 200 ml/min. The powder obtained was 0.7 µm 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 4 Goethite prepared by the conventional method was heated at 350°C for 1 hour for dehydration to obtain hematite. Ni(CH₃COO)₂·4H₂O (5.60g) was dissolved in 2 liters of water, and the solution was adjusted to a pH of 4.7 with CH₃COOH. 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°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 µm in average particle size (long axis), 10 in average axial ratio, 833 Oe in coercive force and 77.2 emu/g in saturation magnetization.

Claims: Iron carbide particles having a nickel content of 0.2 to 30 atomic percent, based on the iron. Particles as defined in claim 1 which are 1 to 20 in average axial ratio. A process for preparing the particles of claim 1 which comprises 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 α-iron oxyhydroxide containing nickel, and filtering up, drying and carburizing the particles. A process for preparing the particles of claim 1 which comprises 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. A process as defined in claim 3 or 4 wherein the carburizing is conducted with use of a carburizing agent selected from CO, methanol and methyl formate.

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