Title of Invention

"A NEW METHOD OF PREPARING W-TYPE HEXAGONAL FERRITES"

Abstract

A process for preparing W-type hexagonal ferrites of the formula Ae2+Me2+2Fe3+16O27 where Ae is an alkaline earth metal such as Ba, Sr, or Pb or a partial replacement with trivalent and monovalent metals thereof and Me is a divalent metallic ion such as Fe Zn, Cu, Ni, Mn, Mg, or a combination of these divalent metals or a partial replacement with trivalent and monovalent metals thereof, comprising: taking commercially available pure chemical ingredients of the said formula in the ratio of Ae2+ to Me2+ and Fe3+ as 1:18, mixing the said chemical ingredients by ball-milling process and forming pellets by pressing, calcining the said pellets between 950°C - 1000°C and quenching from calcination temperature to liquid nitrogen temperature to avoid the formation of S and M phases separately, ball-milling the quenched material and forming pellets by using an organic binder and a sintering aid, sintering the said pellets between 1250° C - 1300° C and again quenching from the sintering temperature to liquid nitrogen temperature to get W-type ferrite.

Full Text

This invention relates to a new method of preparing W-type hexagonal ferrites of the formula Ae2+ Me2+2 Fe3+ 16 O27 where Ae is alkaline earth metal such as Ba, Sr or Pb or a partial replacement with a combination of a trivalent and monovalent metals thereof and Me is a divalent metallic ion such as Fe, Zn, Cu, Ni, Mn, Mg, Co or combination of these divalent metals or a partial replacement of a combination of a trivalent and a monovalent metals thereof. BACKGROUND One of the most important member of the hexagonal ferrite family is : (Formula Removed) where Ae is an alkaline earth metal such as Ba, Sr, or Pb and the atomic ratio of Ae to iron is 1:12. These ferrites, are known as M-type hexagonal ferrites or M-type ferrites and are available in the market under a trade name "Ferroxdure". The crystal structure of an M-type ferrite is : (Formula Removed) where R is Ba Fe 6 O11 and S is Me2 Fe4 O8 and R* and S* stand for mirror images of R and S respectively. These ferrites are used as permanent magnets or in magnetic recording media. Some of the important parameters which determine the quality of these ferrites are Saturation magnetisation, Coercive force and Energy product (BH) max. The highest values obtained for an M-type ferrite are 72 emu/gr, 1.6 Koe, 4.0 X 106 gauss-Oe respectively. Although, these parameters appear to be excellent for a general purpose magnet, there is a need to increase some of the parameters such as the saturation magnetisation, energy product in order to keep them on par with the commercially available permanent magnets such as the alloy magnets -Alnico, metal magnets - SmCo, NdFeB. These M-type ferrites have been described in US Patent Nos. 4.622,159, 4,671,885, 4,786,430, 4,810,402, 5,649.362 and the Indian Patent Nos. 129140 and 134384. Since it is not possible to increase the above mentioned technical parameters in the M-type ferrite, W-type hexagonal ferrite was developed. This new ferrite (W-type) is represented by a stoichiometric composition given by the formula : (Formula Removed) where Ae is an alkaline earth metal such as Ba, Sr, or Pb and Me is divalent metallic ion such as Fe Zn, Cu, Ni, Mn, Mg or a combination of these divalent metals. Unlike in the case of M-type ferrites, in the case of W-type ferrites, the ratio of Ae to Me and Fe is 1:18 and has crystal structure given below, which is closely related to that of an M-type ferrite : (Formula Removed) where R is Ba Fe 6 O11 and S is Me2 Fe4 O8 and R* and S* stands for the images of R and S. The additional advantage of this W-type ferrite is that their magnetisation values are about 10% higher with almost same value of anisotropy constant. This offers 20% higher value of (BH)max and as such they are more promising than M-type materials. These W-type ferrites have been. described in US Patent Nos, 4,397,796 and 5,593,612 The additional S block shown in the crystal structure of W-type ferrites, plays a very important role and unless it is properly incorporated into the structure, it might create unnecessary complications. In US Patent No. 4,397,796 a W-type ferrite was prepared from a composition defined by the formula MeFe22 + Fe163 + O27, where Me is one or more of the metals barium or strontium, optionally partly replaced by calcium and/or lead. A pre-fired product having a ferrous iron content which corresponds to the stoichiometric ferrous content of the product composition is sintered at a temperature between 1160° and 1250° C, in an atmosphere having such an oxygen concentration that substantially no oxygen exchange takes place between the product and the atmosphere. US Patent No,5,593,612 described U,W,X,Y, or Z type hexagonal ferrite and method tor making such a ferrite are disclosed. The hexagonal ferrite has a formula as follows : (Formula Removed) According to the method, the required weights of the oxides and carbonates are measured and then pulverized and mixed to form a material, which is sintered in air. This material is then cooled to produce U, W, X, Y or Z-type hexagonal ferrites. It is known that, if the material is sintered at normal temperatures, where M and other type of ferrites are generally sintered, both M and S phases are likely to be formed separately, maintaining their separate identity. However, if S phase is to be incorporated into the material so that a unified RSS R structure is obtained, one has to sinter at much higher temperatures. If the material is sintered at higher temperatures, Hc falls rapidly due to the abnormal growth of grain size. Falling of Hc results in the production of inferior quality permanent magnets. Accordingly, there is a need to control the grain growth to get at least a moderately good value of Hc. The achieve this object, this invention provides a process for preparing a W-type hexagonal ferrites of the formula (Formula Removed) where Ae is an alkaline earth metal such as Ba, Sr or Pb or a partial replacement with the trivalent or monovalent metals thereof and Me is a divalent metallic ion such as Fe, Zn, Cu, Ni, Mn, Mg, Co or a combination of these divalent metals or a partial replacement with trivalent and monovalent as herein described metals thereof, comprising: taking commercially available chemical ingredients of the said formula in the ratio of Ae2+ to Me2+ and Fe3+ as 1:18, mixing the said chemical ingredients by ball- milling process and forming pellets by pressing, calcining the said pellets between 950°C - 1000°C and quenching from calcinations temperature to liquid nitrogen temperature to avoid the formation of S and M phases separately, ball - milling the quenched material and forming pellets by using conventional organic binder and a sintering aid of the kind as herein described, sintering the said pellets between 1250°C -1300°C and again quenching from the sintering temperature to liquid nitrogen temperature to get W-type ferrite. The sintering aid used in the process if Bi2 O3 weighing about 3% of the material and the organic binder is Poly Vinyl Alcohol (PVA) of 2% weight of the material. The calcinations temperature and the sintering temperatures are preferably 1000°C and BOOT respectively and the liquid nitrogen temperature is -196°C. The invention will now be described with reference to the following examples: Examples 1: A material with following composition, (Formula Removed) was prepared taking Ba Co3 Zn O, Co O and Fe2 O3 in a stoichiometric ratio. The ingredients were then ground in a ball-mill using ethanol for about 50 hours. The ground product in the form of pellets was calcined at a temperature of l000o C in air for about 4 hours and was then quenched quickly into liquid nitrogen. The pellets were powered and the process of grinding was undertaken again for about 45-50 hours. To this powder, a 2% (by weight of the material) organic binder - Poly Vinyl Alcohol (PVA) was added. Later, a 3% (by weight of the material) sintering aid viz; - The powder then is palletized by applying a pressure of 6 tons per sq.inch. The pallets then slowly heated up to 400° C so that the organic binder gets evaporated. Later, the samples were sintered at 1300°C in air for 4 hours followed by a quick quenching to liquid nitrogen temperature. The properties of the pellets were then measured as follows Bulk density = 4.66 gms/cc Magnetic Transition Temperature (Tc) = 404° C Saturation Magnetisation (as) = 77.1 emu/gr Coercivity(Hc) = 1.61KOe Example 2 : The compositional formula of the material prepared under this example is (Formula Removed) In order to prepare the above material, Sr Co3, Li Co3, Zn O and Fe2O3 were taken in the stoichiometric ratio. The entire process of preparing this material is also the same as mentioned under example 1, except the starting chemicals. The properties of the material so prepared were : Bulk density = 4.60 gms/cc Magnetic Transition Temperature (Tc) = 414° C Saturation Magnetisation (as) = 81.90 emu/gr Coercivity (Hc) = 1.60 KOe Example 3 : In this material Barium is partially replaced with a combination of trivalent lanthnam and monovalent sodium to get a sample with the compositional formula : (Formula Removed) For this purpose, the carbonates of barium and sodium along with the oxides of lanthnam, zinc and iron were taken in the stoichiometric ratio. The entire process of preparing this material is also the same as mentioned under example 1. The properties of the material so prepared were : Bulk density = 4.95 gms/cc Magnetic Transition Temperature (Tc) = 432° C Saturation Magnetisation (as) = 92.0 emu/gr Coercivity(Hc) = 1.62KOe We claim: 1. A process of preparing W-type hexagonal ferrites of the formula (Formula Removed) where Ae is an alkaline earth metal such as Ba, Sr or Pb or a partial replacement with the trivalent or monovalent metals thereof and Me is a divalent metallic ion such as Fe, Zn, Cu, Ni, Mn, Mg, Co or a combination of these divalent metals or a partial replacement with trivalent and monovalent as herein described metals thereof, comprising: taking commercially available chemical ingredients of the said formula in the ratio of Ae2+ to Me2+ and Fe3+ as 1:18, mixing the said chemical ingredients by ball- milling process and forming pellets by pressing, calcining the said pellets between 950°C - 1000°C and quenching from calcinations temperature to liquid nitrogen temperature to avoid the formation of S and M phases separately, ball - milling the quenched material and forming pellets by using conventional organic binder and a sintering aid of the kind as herein described, sintering the said pellets between 1250°C -1300°C and again quenching from the sintering temperature to liquid nitrogen temperature to get W-type ferrite. 2. A process as claimed in claim 1, wherein the sintering aid is Bi2O3 weighing about 3% of the material and the organic binder is Poly Vinyl Alcohol(PVA) of 2% weight of the material. 3. A process as claimed in claim 1, wherein the calcinations temperature and the sintering temperature are preferably 1000°C and 1300°C respectively and the liquid nitrogen temperature is -196°C. 4. A process as claimed in claim 1, wherein the chemical ingredients used were BaCo3, ZnO, CoO and Fe2O3 in stoichiometric ratio and the composition was Ba Zn 5. A process as claimed in claim 1, wherein the chemical ingredients were Sr Co3, Li CO3, Zn O and Fe3 O2 in stoichiometric ratio to form the composmon (Formula Removed) 6. Process as claimed in claim 1, wherein barium is partially replaced with a combination of trivalent lanthnam. and monovalent sodium to get a sample with the compositional formula : (Formula Removed) preparing W-type hexagonal ferrites of the formula : (Formula Removed) where Ae is an alkaline earth metal such as Ba, Sr, or Pb or a partial replacement with trivalent and monovalent metals thereof and Me is a divalent metallic ion such as Fe Zn, Cu, Ni, Mn, Mg, or a combination of these divalent metals or a partial replacement with trivalent and monovalent metals thereof, substantially as herein described with reference to forgoing examples.

Documents:

770-del-1998-abstract.pdf

770-del-1998-claims.pdf

770-del-1998-correspondence-others.pdf

770-del-1998-correspondence-po.pdf

770-del-1998-description (complete).pdf

770-del-1998-form-1.pdf

770-del-1998-form-19.pdf

770-del-1998-form-2.pdf

770-del-1998-form-3.pdf

770-del-1998-gpa.pdf

770-del-1998-pa.pdf