Claims:We Claim:
1. A manganese-zinc (Mn-Zn) ferrite having a chemical formula Mn0.75Zn0.75Fe1.5O4.
2. The Mn-Zn ferrite as claimed in claim 1, wherein the ferrite has a stressed induced distorted body centred tetragonal crystal structure.
3. The Mn-Zn ferrite as claimed in claim 1 or 2, wherein the ferrite has a coercivity of about 0.1-0.2 Oe.
4. The Mn-Zn ferrite as claimed in any one of claims 1-3, wherein the ferrite has a magnetization value of about 30-40 emu/g.
5. The Mn-Zn ferrite as claimed in any one of claims 1-4, wherein the Mn-Zn ferrite comprises about 27% to 35% Mn3+ ions at octahedral voids.
6. The Mn-Zn ferrite as claimed in any one of claims 1-5, wherein the Mn-Zn ferrite comprises about 70-75% Zn2+ ions, about 15-20% Fe2+ ions, and about 5-10% of Mn2+ ions at tetrahedral voids.
7. A method for preparing the Mn-Zn ferrite as claimed in any one of claims 1-6, comprising:
a. mixing zinc oxide (ZnO) powder, manganese oxide (MnO2) powder and ferric oxide (Fe2O3) powder to prepare a mixture;
b. heating the mixture at about 900?-1200? in an inert atmosphere to obtain a heated mixture; and
c. cooling the heated mixture under atmospheric condition to obtain the Mn-Zn ferrite.
8. The method as claimed in claim 7, wherein the mixture comprises the zinc oxide powder, the manganese oxide powder and the ferric oxide powder in 1:2:3 ratio.
9. The method as claimed in claim 7 or 8, wherein the mixture comprises about 10-35 wt% of ZnO, about 20-50 wt% of MnO2 and about 30-70 wt% of Fe2O3.
10. The method as claimed in any one of claims 7-9, wherein the mixture is heated in the inert atmosphere for about 2-4 hours.
11. The method as claimed in any one of claims 7-10, wherein the inert atmosphere is selected from an argon, helium, xenon, krypton, neon, radon, or nitrogen atmosphere or a combination thereof.
12. The method as claimed in any one of claims 7-11, wherein the mixture is heated at a rate of about 1?/min to 10?/min. , Description:TECHNICAL FIELD
The present disclosure relates to the field of manganese-zinc (Mn-Zn) ferrites. Particularly, the present disclosure relates to Mn-Zn ferrites having low coercivity and specific crystal structure and a method of preparing them.

BACKGROUND OF THE DISCLOSURE
Ferrites are very promising candidates for magnetic or electromagnetic applications. In particular, soft ferrites having very simple composition with easy manufacturing techniques and less costly starting material are useful and inexpensive for magnetic/electromagnetic applications. Ferrites are generally of inverse spinel structure having a closely related structure with normal spinel (with the same large unit cell) in which the II-site (A) ions and half of the III-site (B) ions switch places. Inverse spinels are thus formulated B(AB)O4, where the AB ions occupy octahedral sites, and the other B ions are on tetrahedral sites. There are also mixed spinels, which are intermediate between the normal and inverse spinel structure. Fe3O4 has a similar structure with Fe3+(Fe2+Fe3+)O4 as discussed.

Soft ferrites have low coercivity, so they easily change their magnetization and act as conductors of magnetic fields and efficient magnetic cores called ferrite cores. For applications below 5 MHz, Mn-Zn ferrites are used. Ferrites are low in power transfer compared to rare earth magnets but the coercivity is very low which makes it more promising for the wireless power transfer core in EVs (J.D. Widmer, R. Martin, M. Kimiabeigi, Sustainable Materials and Technologies Electric vehicle traction motors without rare earth magnets, SUSMAT. 3 (2015) 7–13. doi:10.1016/j.susmat.2015.02.001).

Many prior studies have disclosed Mn-Zn ferrites. In JP2895723B2, the powders were prepared by mixing with PVA in both dry and wet conditions for 30 mins and calcined at 700-1000? with additives: (A) <=0.3wt.% of SiO2, (B) <=0.5wt.% of CaO, (C) <=0.5wt.% of TiO2, (D) <=0.7wt.% of Nb2O5, (E) 0.1-1wt.% of a molding auxiliary and (F) 0.01-1wt.% of a wetting agent and then shaped and again baked at 1200-1350? in absence of air.

CN1447356A discloses pre-sintering at 700-1000? in a rotary kiln for 30 – 60 min to prepare the ferrite powders where the powders had undergone preheating to firing and then finally the controlled cooling in the kiln.

CN101665362A discloses solid phase chemical reaction from the carbonate salts of the Fe, Zn and Mn that were wet mixed and ground at room temperature and dried in a cake and then calcined at 350-1100? and finally a cubic structure was reported.

EP1101736B1 discloses mixing Fe2O3, ZnO, TiO2 and/or SnO2, CuO, MnO in ball mill and calcining at 800-1000? with additives: CoO, NiO or MgO to reduce the sintering temperature and granulating the mixture. Binders like PVA, polyacrylamide, methyl cellulose, polyethylene oxide, glycerin, or the like were used for the granulation and pressed at a pressure of 80 MPa or more followed by sintering at 900-1400?.

CN104446409B discloses mixing Fe2O3, Mn3O4, and ZnO slurry in sand mill, spray dried, pre-burnt, ball milled with additives like CaCO3, SiO2 and at least five of K2CO3, Y2O3, NiO, Co3O4 and Al2O3), spray dried, pressed and sintered at 750-1210?.

In CN101921102B, Fe2O3, ZnO and MnO were ball milled, spray dried and granulated and pre-burnt in air push kiln at 870-960°C for 1 to 3 hours. Additives like CaCO3, Nb2O5, TiO2, BO, BiO, MoO were added for sintering carried out at 1260-1380°C, for 2 to 4.5 hours.

JPH08138949A discloses sintering MnO, ZnO, and Fe2O3 in air and adding CaO and SiO2 to the sintered powder for intergranular insulation, which is wet-ground with binder for moulding and burned at temperatures of 1100 to 800?.

Bhalla et al. disclose production of ferrites through mixing, calcination, grinding, granules making, pressing of components, sintering in tunnel kiln or box furnace and machining on rotary table grinding machine (D. Bhalla, D. Singh, S. Singh, D. Seth, Material Processing Technology for Soft Ferrites Manufacturing, Am. J. Mater. Sci. 2 (2013) 165–170. doi:10.5923/j.materials.20120206.01). The effect of oxygen atmosphere during sintering at high temperature (1400?) were studied.

Sanyal discloses studies using nitrogen containing about 0.10 vol% O2, pure O2 and argon containing <0.001 vol % O2 in various proportions during the synthesis of ferrites (J. Sanyal, Synthesis of manganese-zinc ferrite: Influence of sintering atmosphere and types of the raw materials on the ferrite properties, Trans. Indian Ceram. Soc. 29 (1970) 19–25. doi:10.1080/0371750X.1970.10855716). The mixtures having desired compositions were homogenised in steel ball mill. In each batch, chemically pure calcium carbonate (0.02 mol% CaO in the final ferrite) was added before homogenisation. After homogenisation one half of the masses was pre-sintered in electric chamber furnace at 1100? for 4 hr in air atmosphere. The other half of the masses was directly sintered. After pre-sintering, the masses were wet ground in steel ball mill to very fine particles corresponding to specific surface 1.2 ± 0.05 sqm/gm (BET method). Toroidal cores of required sizes were prepared from the initial batches as well as from the pre-sintered batches. The ferrite specimens were sintered in protective atmospheres at 1280±5? for 4 hrs.

Masoudi et al. disclose milling and annealing to prepare the ferrite. The effect of milling time (maximum 30 hrs) was studied to produce cubic structured Mn0.5Zn0.5Fe2O4 with substitution of only Fe2+ ions from tetrahedral positions with a magnetic property varying in a range of 34 emu/g and 30 Oe to 18 emu/g and 70 Oe (M.T. Masoudi, A. Saidi, M. Hashim, A. Hajalilou, Comparison of structure and magnetic properties of Mn-Zn ferrite mechanochemically synthesized under argon and oxygen atmospheres, Can. J. Phys. 93 (2015) 1168–1173. doi:10.1139/cjp-2015-0014).

El-Badry discloses using pure MnO2, ZnO and Fe2O3 to prepare a Mn-Zn ferrite of the nominal composition Mn0.64Zn0.29Fe2.07O4 with a coercivity from 234.3 Oe to 75.81 Oe. These oxides were mixed firstly for 1 hour, and then were milled for 20 and for 40 hours with PVA, granulated, cold pressed and sintered at 1000, 1300, and 1400? (S.A. El-Badry, Influence of Processing Parameters on the Magnetic Properties of Mn-Zn Ferrites, J. Miner. Mater. Charact. Eng. 10 (2011) 397–407).

Prior studies have been done to prepare MnZn ferrites with low coercivity. For example, Hu et al. disclose an auto combustion process from nitrate precursors to synthesize MnZn ferrite with low coercivity in a range of 32 – 70 Oe with citric acid and ammonia (P. Hu, H. bo Yang, D. an Pan, H. Wang, J. jun Tian, S. gen Zhang, X. feng Wang, A.A. Volinsky, Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders, J. Magn. Magn. Mater. 322 (2010) 173–177. doi:10.1016/j.jmmm.2009.09.002).

Mirshekari et al. disclose a similar process with glycine as the additive where coercivity in the range of 40 – 60 Oe was obtained (G.R. Mirshekari, S.S. Daee, H. Mohseni, S. Torkian, M. Ghasemi, M. Ameriannejad, M. Hoseinizade, M. Pirnia, D. Pourjafar, M. Pourmahdavi, K. Gheisari, Structure and magnetic properties of Mn-Zn ferrite synthesized by glycine-nitrate auto-combustion process, Adv. Mater. Res. 409 (2012) 520–525. doi:10.4028/www.scientific.net/AMR.409.520).

Devi and Soibam disclose MnZn ferrite produced through a co-precipitation process from chloride precursors in an alkaline medium to give a range of coercivity of 12-75 Oe (E.C. Devi, I. Soibam, Effect of Zn doping on the structural, electrical and magnetic properties of MnFe2O4 nanoparticles, Indian J. Phys. 91 (2017) 861–867. doi:10.1007/s12648-017-0981-7).

US6391222 discloes reducing coercivity from 20.6 Oe to 7.5 Oe by doping Ni in the MnZn ferrites.

JP2006193343A discloses synthesizing MnZn ferrites through solid state synthesis with Li, a costly rare earth doping element, and Co and shaped to rods to achieve better magnetic properties and a coercivity in the range of less than 4376.76 Oe.

Thakur et al. disclose that coercivity of MnZn ferrites depend on the sintering temperature of the precursors and report ferrites with coercivity in the range of 20 Oe to 170 Oe (P. Thakur, S. Taneja, D. Sindhu, U. Lüders, A. Sharma, B. Ravelo, A. Thakur, Manganese Zinc Ferrites: a Short Review on Synthesis and Characterization, J. Supercond. Nov. Magn. 33 (2020) 1569–1584. doi:10.1007/s10948-020-05489-z).

Ewais et al. disclose an in-situ synthesis of MnZn ferrite powders with a coercivity from 4 Oe to 14 Oe (E.M.M. Ewais, M.M. Hessien, A.H.A. El-Geassy, In-situ synthesis of magnetic Mn-Zn ferrite ceramic object by solid state reaction, J. Aust. Ceram. Soc. 44 (2008) 57–62).

These prior studies have attempted to reduce coercivity of Mn-Zn ferrites with different synthesis techniques and different ranges of Zn and Mn concentrations. Still, there is a need in the art to develop Mn-Zn ferrites with lower coercivity values without compromising on magnetism properties. The present disclosure attempts to address said need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to a Mn-Zn ferrite having a chemical formula Mn0.75Zn0.75Fe1.5O4. The Mn-Zn ferrite of the present disclosure has a stressed induced distorted body centred tetragonal crystal structure, shows excellent low coercivity values (about 0.1-0.2 Oe) and good magnetic properties (about 30-40 emu/g).

The present disclosure also relates to a method for preparing the Mn-Zn ferrite, comprising a) mixing zinc oxide (ZnO) powder, manganese oxide (MnO2) powder and ferric oxide (Fe2O3) powder to prepare a mixture; b) heating the mixture at about 900?-1200? in an inert atmosphere to obtain a heated mixture; and c) cooling the heated mixture under atmospheric condition to obtain the Mn-Zn ferrite.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1 shows a crystal lattice (panel (a)) and the results of XRD analysis (panel (b)) of Sample 1 (Zn-rich ferrite) of Example 1.

Figure 2 shows a crystal lattice (panel (a)) and the results of XRD analysis (panel (b)) of Sample 3 (Stoichiometric ferrite) of Example 1.

Figure 3 shows a crystal lattice (panel (a)) and the results of XRD analysis (panel (b)) of Sample 2 (Mn-rich ferrite) of Example 1.

Figure 4 shows results of the magnetic study of the ferrites.

DETAILED DESCRIPTION OF THE DISCLOSURE
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Reference throughout this specification to “some embodiments”, “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The term “about” as used herein encompasses variations of +/-10% and more preferably +/-5%, as such variations are appropriate for practicing the present invention.

The present disclosure provides Mn-Zn ferrites with substantially lower coercivity values. The inventors found that a single stage sintering (i.e., heating) of Fe2O3, MnO2 and ZnO powders in solid state and in an inert atmosphere without any additives followed by atmospheric cooling of the sintered powder provides a Mn-Zn ferrite with a substantially low coercivity of about 0.1-0.2 Oe. The inventors further found that Mn2+ ions convert to Mn3+ ions which substitute the Fe3+ ions from the octahedral voids generating more stress in the cubic structure and convert the cubic structure to a distorted tetragonal structure. Further, the stressed structure is retained due to the atmospheric cooling where the stress could not get an opportunity to release and come to a low energy structure. The inventors predict that this stressed induced distorted body centred tetragonal structure provides very low coercivity values and good magnetic properties. In contrast to prior studies, the Mn-Zn ferrites of the present disclosure are prepared by a single stage solid state sintering of Fe2O3, MnO2 and ZnO powders without any additives followed by cooling the sintered powder at an atmospheric condition.

The Mn-Zn ferrites of the present disclosure have a chemical formula of Mn0.75Zn0.75Fe1.5O4 and a stressed induced distorted body centred tetragonal crystal structure.
The Mn-Zn ferrites of the present disclosure exhibit very low coercivity values. In some embodiments, the Mn-Zn ferrites show a coercivity of about 0.1-0.2 Oe, including values and range thereof. For example, in some embodiments, the Mn-Zn ferrites show a coercivity of about 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.20 Oe.

In some embodiments, the Mn-Zn ferrite has a magnetization value of about 30-40 emu/g, 32-37 emu/g, or 33-36 emu/g, including values and ranges thereof. In some embodiments, the Mn-Zn ferrites show a magnetization of about 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40 emu/g.

The inventors found that the atmospheric cooling of the sintered/heated mixture of Fe2O3, MnO2 and ZnO powders induce stress in the crystal structure leading to a Mn-Zn ferrite with a stressed induced distorted body centred tetragonal crystal structure. In the crystal structure of the Mn-Zn ferrites of the present disclosure, about 27% to 35% of Mn3+ ions, including values and ranges thereof, have replaced Fe3+ ions at octahedral voids. In some embodiments, about 28% to 33%, about 29% to 32%, or about 27, 27.1, 27.2, 27.3, 27.4, 27.5, 27.6, 27.7, 27.8, 27.9, 28, 28.1, 28.2, 28.3, 28.4, 28.5, 28.6, 28.7, 28.8, 28.9, 29, 29.1, 29.2, 29.3, 29.4, 29.5, 29.6, 29.7, 29.8, 29.9, 30, 30.1, 30.2, 30.3, 30.4, 30.5, 30.6, 30.7, 30.8, 30.9, 31, 31.1, 31.2, 31.3, 31.4, 31.5, 31.6, 31.7, 31.8, 31.9, 32, 32.1, 32.2, 32.3, 32.4, 32.5, 32.6, 32.7, 32.8, 32.9, 33, 33.1, 33.2, 33.3, 33.4, 33.5, 33.6, 33.7, 33.8, 33.9, 34, 34.1, 34.2, 34.3, 34.4, 34.5, 34.6, 34.7, 34.8, 34.9, or 35 %, of octahedral voids are occupied by Mn3+ ions.

Tetrahedral voids of the Mn-Zn ferrites of the present disclosure are mainly comprised of Zn2+ ions replacing the Fe2+ ions although very few amounts of Mn2+ ions are also present. In some embodiments, the Mn-Zn ferrite comprises about 70-75% Zn2+ ions, about 15-20% Fe2+ ions, and about 5-10% of Mn2+ ions at tetrahedral voids, including values and ranges thereof. For example, in some embodiments, the Mn-Zn ferrite comprises about 70, 70.1, 70.2, 70.3, 70.4, 70.5, 70.6, 70.7, 70.8, 70.9, 71, 71.1, 71.2, 71.3, 71.4, 71.5, 71.6, 71.7, 71.8, 71.9, 72, 72.1, 72.3, 72.4, 72.5, 72.6, 72.7, 72.8, 72.9, 73, 73.1, 73.2, 73.3, 73.4, 73.5, 73.6, 73.7, 73.8, 73.9, 74, 74.1, 74.2, 74.3, 74.4, 74.5, 74.6, 74.7, 74.8, 74.9, or 75 % Zn2+ ions; about 15, 15.1, 15.2, 15.3, 15.4, 15.5, 15.6, 15.7, 15.8, 15.9, 16, 16.1, 16.2, 16.3, 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 17, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, 17.7, 17.8, 17.9, 18, 18.1, 18.2, 18.3, 18.4, 18.5, 18.6, 18.7, 18.8, 18.9, 19, 19.1, 19.2, 19.3, 19.4, 19.5, 19.6, 19.7, 19.8, 19.9, or 20 % Fe2+ ions; and about 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, or 10 % Mn2+ ions at tetrahedral voids.

The present disclosure also provides a method for preparing the Mn-Zn ferrites of the present disclosure. The method broadly comprises mixing powders of zinc oxide, iron oxide, and manganese oxide in solid state without adding any additives to this mixture, heating or sintering the mixture at about 900?-1200? in an inert atmosphere and cooling the heated/sintered mixture at an atmospheric condition to obtain the Mn-Zn ferrite.

Iron oxides can be sourced from a variety of manufacturers. In addition to that, Fe2O3 can also be availed from iron ore beneficiation process, HSM mill scale and pickling of steel materials (acid regeneration plant) and more specifically collected from the by-product of pickling of steels to synthesize the magnets. To prepare the soft ferrites of the present disclosure with desired properties, doping is done with zinc (Zn) and manganese (Mn) where the sources are their oxides.

In one embodiment, the method for preparing a Mn-Zn ferrites comprises a) mixing zinc oxide (ZnO) powder, manganese oxide (MnO2) powder and ferric oxide (Fe2O3) powder to prepare a mixture; b) heating the mixture at about 900?-1200? in an inert atmosphere to obtain a heated mixture; and c) cooling the heated mixture under atmospheric condition to obtain the Mn-Zn ferrite.

Solid powders of ZnO, MnO2, and Fe2O3 are mixed to form a mixture. No additives are added to the mixture. In some embodiments, the powders of ZnO, MnO2, and Fe2O3 are mixed in a 1:2:3 ratio.

In some embodiments, the mixture comprises about 10-35 wt% of ZnO, about 20-50 wt% of MnO2 and about 30-70 wt% of Fe2O3, including values and ranges thereof. In some embodiments, the mixture comprises about 10-35%, 10-30%, 10-25%, 10-20%, 10-15%, 15-35%, 15-30%, 15-25%, 15-20%, 20-35%, 20-30%, 20-25%, 25-35%, 25-30%, or 30-35% by weight of ZnO, including values and ranges thereof. In some embodiments, the mixture comprises about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 % by weight of ZnO, including values and ranges thereof.

In some embodiments, the mixture comprises about 20-50%, 20-45%, 20-40%, 20-35%, 20-30%, 20-25%, 25-50%, 25-45%, 25-40%, 25-35%, 25-30%, 30-50%, 30-45%, 30-40%, 30-35%, 35-50%, 35-45%, 35-40%, 40-50%, 40-45%, or 45-50%, by weight of MnO2, including values and ranges thereof. In some embodiments, the mixture comprises about 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 % by weight of MnO2, including values and ranges thereof.

In some embodiments, the mixture comprises about 30-70%, 30-65%, 30-60%, 30-55%, 30-50%, 30-45%, 30-40%, 30-35%, 35-70%, 35-65%, 35-60%, 35-55%, 35-50%, 35-45%, 35-40%, 40-70%, 40-65%, 40-60%, 40-55%, 40-50%, 40-45%, 45-70%, 45-65%, 45-60%, 45-55%, 45-50%, 50-70%, 50-65%, 50-60%, 50-55%, 55-70%, 55-65%, 55-60%, 60-70%, 60-65%, or 65-70%, by weight of Fe2O3, including values and ranges thereof. In some embodiments, the mixture comprises about 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, or 70 % by weight of Fe2O3, including values and ranges thereof.

The mixture of ZnO, MnO2, and Fe2O3 powders in solid state is heated/sintered in a furnace at a temperature of about 900?-1200? in an inert atmosphere. In some embodiments, the mixture is heated at about 900, 925, 950, 975, 1000, 1025, 1050, 1075, 1100, 1125, 1150, 1175, or 1200 ? in an inert atmosphere. The mixture is heated to any one of the above temperatures in the inert atmosphere at a rate of about 1?/min to 10?/min, such as at a rate of about 1?/min, 1.5?/min, 2?/min, 2.5?/min, 3?/min, 3.5?/min, 4?/min, 4.5?/min, 5?/min, 5.5?/min, 6?/min, 6.5?/min, 7?/min, 7.5?/min, 8?/min, 8.5?/min, 9?/min, 9.5?/min, or 10?/min, including values and ranges thereof.

In some embodiments, the mixture is heated at any one of the above temperatures in the inert atmosphere for about 2-4 hours, such as for about 2, 2.5, 3, 3.5, or 4 hours. The inert atmosphere can be an argon, helium, xenon, krypton, neon, radon, nitrogen atmosphere or a combination thereof.

The heated/sintered mixture is cooled at atmospheric condition to obtain Mn-Zn ferrites of the present disclosure. The ferrites produced in the manner described above have a chemical formula of Mn0.75Zn0.75Fe1.5O4, a stressed induced distorted body centred tetragonal crystal structure, very low coercivity (about 0.1-0.2 Oe) and at the same time good magnetism (about 30-40 emu/g).

It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES

Example 1: Preparation of Mn-Zn ferrites
The by-product powders of Fe2O3 (20-30 nm) collected from pickling line of steel sheets and zinc oxide (ZnO) and manganese oxide (MnO2) were mixed in different weight percentages and ratios as shown in Table 1 below to obtain Mn-Zn ferrites with variations in the stoichiometry. The powders were mixed in a pulveriser for 5 minutes to obtain finer range of powders as precursors thereby enhancing the synthesis process.

Table 1 Composition Variation of Powders
Nomenclature ZnO:MnO2:Fe2O3 ZnO (wt%) MnO2 (wt%) Fe2O3 (wt%)
Sample 1: Zn-rich ferrite 2:1:3 20-50 10-35 30-70
Sample 2: Mn-rich ferrite 1:2:3 10-35 20-50 30-70
Sample 3: Stoichiometric ferrite 1:1:2 15-30 15-30 30-70

Powders were taken to the tube furnace where the heat treatment was done at 1100? with a soaking time of 3 hours in an Argon atmosphere at a heating rate of 5?/min. The heat treatment was carried out in alumina (Al2O3) crucible with least chances of impurities where the crucibles were cleaned with hydrochloric acid (HCl) thoroughly before taking to the tube furnace. After the heat treatment process through solid state reaction, the powders were directly cooled under atmospheric condition to retain the heat-treated phases.
The synthesized Mn-Zn ferrite powders were inspected under X-Ray Diffraction (XRD) and subsequent crystal structures were generated which were then corroborated with the magnetic studies with hysteresis loops generated under VSM.
Crystallographic structural study of the ferrites
The crystal structures along with the lattice parameters and crystal co-ordinates for anticipated phases were analysed through XRD.
Sample 1 being the Zn rich sample, was heat treated at 1100? and soaked for 3 hours to form a predominant phase of cubic franklinite (Fe2Mn0.1O4Zn0.9) with ICSD 98-002-8512 and space group F d -3 m having more concentration of Zn present in the cubic normal spinel as shown in Figure 1, panel (b) showing maximum content of Zn with the crystal. Higher concentration of Zn generally gives rise to normal spinel structure as Zn2+ ions prefer to occupy the one eighth of the tetrahedral voids as shown in Figure 3, panel (a). Very few amounts of unreacted cubic ZnO (space group F -4 3 m) with ICSD 98-016-2753 and tetragonal MnO2 (space group P 42/m n m) with ICSD 98-007-3716 is present in the final residue. Another iron oxide phase has got developed with Mn to form cubic ferrous manganese oxide (Fe0.497Mn0.503O1) with ICSD 98-006-0687. ZrO2 is also present and remain unreacted though has undergone a phase change from monoclinic to unstable high pressure orthorhombic with ICSD 98-017-3960 with P b c a space group. As the heat treatment was carried out in Ar atmosphere with less available oxygen, ZrO2 may get converted to oxygen deficient state with a very fine grade (2-5 nm) and a greyish black colour. This could be the reason of orthorhombic phase of ZrO2.
In Sample 3, the Mn content has further increased in the ferrite as per the doping quantity of the respective oxide with the crystal structure shown in Figure 2, panel (a). Sample 3 is predominant cubic franklinite with the chemical formula Fe1.96Mn0.36O4Zn0.68 with space group F d -3 m (ICSD 98-028-0055) where higher concentration of Mn2+ has substituted Zn2+ and Fe2+ from the tetrahedral sites of the normal spinel as observed in Figure 2, panel (b). Up to certain concentration of Mn, it prefers to be in Mn2+ state and thus favours only the tetrahedral voids available. Presence of unreacted zinc oxide is noted in this spinel also with a cubic structure (ICSD 98-016-2753). ZrO2 has converted from monoclinic to high temperature tetragonal phase (ICSD 98-009-3125) with a space group P 42/n m c. The tetragonal phase becomes stable for very fine size range of ZrO2 and presence of unreacted ZnO.
Sample 2 has the maximum content of Mn with the crystal structure shown in Figure 3, panel (a). In Sample 2, a small percentage of Mn has substituted both Zn2+ and Fe2+ from tetrahedral voids as Mn2+. The analysis indicated that the Sample 2 ferrite contained 73.5% Zn2+ ions, 17.7% Fe2+ ions and 8.8% Mn2+ at tetrahedral voids. Mn has also substituted Fe3+ from the octahedral position of the distorted body centred tetragonal spinel structure as shown in Figure 3, panel (a). As the Mn concentration increases it becomes impossible to accommodate the Mn2+ ions in the tetrahedral void and thus Mn converts to Mn3+ to substitute the Fe3+ ions from the octahedral voids. The analysis indicated that the Sample 2 ferrite comprised 33.1% Mn3+ ions at octahedral voids. This generated more stress in the cubic structure converting it to a distorted tetragonal structure. Tetragonal distortion could be due to the distortion of the Fe(Mn) octahedra (Jahn-Teller effect) as Mn3+ ions are getting incorporated into the structure. Due to more incorporation of Mn3+ ions into the cubic crystal structure of franklinite (Fe1.5Mn0.75O4Zn0.75), the structure has got stretched and converted to distorted body centred tetragonal with ICSD 98-008-2909 and space group I 41/a m d as shown in Figure 3, panel (b). This stressed structure is generated due to the special heat-treatment parameters and remained intact and the stressed phase was retained due to the atmospheric cooling where the stress could not get enough chance to release and come to a low energy structure. Cubic ZnO with ICSD 98-016-2753 and space group F -4 3 m remained unreacted throughout the reaction. During the heat treatment of ferrite B monoclinic ZrO2 has converted to high temperature tetragonal ZrO2 with ICSD 98-009-3125 and space group P 42/n m c which might get stabilised by unreacted ZnO.
The powders were characterized through magnetic studies carried out under vibrating sample magnetometer (VSM) to evaluate the ferrites as per the synthesis and doping processes. For VSM studies, each powder is kept in the crucible and the sample is first magnetized in a uniform magnetic field. It is then sinusoidally vibrated using a piezoelectric material. It gives rise to the hysteresis loop of the material and thus provides information in terms of coercivity calculated from the hysteresis loop.
The hysteresis loops for the ferrites are shown in Figure 4. For Sample 1, the saturation magnetization is lowest and as the Mn increased in Sample 3, the magnetization has further increased with a maximum at Sample 2 with a steep hysteresis curve. Having higher magnetization of Mn ions (Mn2+ and Mn3+) than Zn2+ and preferential substitution of Mn ions in both the octahedral and tetrahedral positions transforming the structure to a mixed spinel (normal and inverse) structure. For this unique structural modification, the coercivity of Sample 2 has given excellent values that has been depicted from the hysteresis data generated and reported in Table 2 and Figure 4. Mn-rich spinel has the lowest coercivity resulting in excellent novel soft magnetic property.
Table 2: Magnetic properties of the synthesized ferrites
Samples Coercivity (Oe)
Ferrite A (Sample 1)
(Zn-rich) 19.67
Ferrite B (Sample 2)
(Mn-rich) 0.13
Ferrite C (stoichiometric) (Sample 3) 19.87

As shown in Figure 4 and Table 2, for Sample 1, the saturation is achieved at a quite low magnetization value with a very low slope indicating very low permeability and high coercivity. Due to a high Zn2+ ion concentration in tetrahedral vacancies, the magnetic property of Sample 1 is not up to the mark. In Sample 3, the saturation achieved gradually with a higher slope indicating higher permeability than Sample 1 though the coercivity has not improved. The magnetic property has improved due to higher incorporation of Mn2+ in the normal spinel structure substituting more Zn2+ ions though the size of Mn2+ ions is much larger than that of Zn2+ ions and thus a lot of stress has got induced in the structure. Comparatively high saturation values achieved quite promptly with a steep slope indicating a high permeability for Sample 2 at an extremely low coercivity of 0.13 Oe. Along with the Mn2+ ions substituting Zn2+ and Fe2+ ions in Sample 2, the incorporation of Mn3+ ions substituting Fe3+ ions have improved the magnetic properties excellently in terms of coercivity giving rise to a novel characteristic. In case of Sample 2, the close packed stress induced body centred tetragonal structure has got retained with the additional stress due to the sudden atmospheric cooling after the solid-state synthesis process incorporating both Mn2+ and Mn3+ ions and thus the coercivity has got reduced excellently to 0.13 Oe generating the novel modification and observed to be the best among all the stoichiometric variations.