Patent Description:
The present invention belongs to the sector of magnetic materials and their industrial applications.

Permanent magnets are crucial materials since they allow storing, supplying and converting electrical energy into mechanical energy and vice versa, such that better magnets lead to greater energy efficiency.

The most competitive magnets at an industrial level are based on rare earths, however, the global crisis associated with these elements has highlighted the economic, geostrategic and environmental relevance of producing alternative magnets of competitive performance for certain applications in order to reduce dependency on rare earths.

The combination of a hard magnetic material based on rare earths and a hard magnetic material based on ferrites of an oxidic nature is a way to reduce the rare earths content in permanent magnets, without worsening or maintaining the magnetic properties thereof, as claimed by <CIT>.

<CIT> discloses a method for preparing composite ferrite powder with exchange coupling effect and single-phase magnetic behaviour through grinding between a soft magnetic phase of FeB and a hard magnetic phase of ferrite according to a mass ratio of <NUM>:<NUM>. With the method adopted, the exchange coupling effect can be carried out without the need for high-temperature sintering. However, the resulting product does not exhibit a permanent magnet behaviour since it exhibits soft magnetic behaviour with remanent magnetisation values less than <NUM> T.

<CIT> relates to a method for preparing magnetic nanocrystalline particles, characterised in that an aluminium-doped barium ferrite nanocrystalline powder presenting hard magnetic behaviour and a nickel-copper-zinc ferrite nanocrystalline powder with soft magnetic behaviour are mixed, dried and thermally treated, obtaining a complex nanocrystalline phase having a saturation magnetisation of <NUM> emu·g-<NUM>, and a coercive field of <NUM> kOe.

<CIT> claims hybrid permanent magnet microcomposites comprising an oxide-based ferrite hard magnetic material, a metal-based soft magnetic material and an organic coupling agent. The microcomposite has improvements of <NUM> % in the remanence magnetisation with respect to the ferrite phase, of isotropic powders without magnetic orientation. In the method of obtainment, the presence of a coupling agent is required to protect the soft magnetic material, which has particles with sizes greater than <NUM> from oxidation. The material of <CIT> is an isotropic powder having an improvement in remanence with respect to an isotropic ferrite powder; however, the invention of <CIT> does not cover the case of anisotropic materials (i.e. magnetically oriented). In this manner, although the isotropic composite has a higher remanence, it does not necessarily imply that said improvement can be transferred to a magnetically oriented or anisotropic composite.

Permanent magnets are usually sintered at high temperatures in order to obtain dense bodies and maximise the volume-dependent energy product. A dense magnetic material also has the advantage of the mechanical integrity thereof and greater resistance to processes of environmental degradation. The densification or sintering processes take place at high temperatures which, in the case of ferrite magnets, require temperatures ><NUM> and times greater than <NUM> to <NUM> hours at maximum temperature. During heat treatment, grain growth occurs as a result of mass transport phenomena. The consequence of the increase in grain size is a reduction in the magnetic response, in particular a strong reduction of the coercive field, Hc.

<CIT> claims the sintering of ceramic and composite materials with at least one inorganic element by means of the cold sintering method called "cold sintering process". This methodology allows sintering at least one inorganic compound in the form of particles by using an aqueous-based solvent with at least <NUM> % by weight of H<NUM>O. The use of this aqueous solvent in a proportion of no more than <NUM> % by weight with respect to the inorganic component partially solubilises the inorganic compound to give rise to a densified material at temperatures below <NUM> and applying a uniaxial pressure of less than <NUM> MPa. As a result of the cold sintering process, densified parts with at least a relative density of <NUM> % can be obtained. However, this process limits the controlled grain growth of the particles.

Therefore, it is necessary to develop new methods for producing competitive permanent magnets comprising minimal amounts of rare earths.

Next, the definitions of certain terms that will be used throughout the description are presented:
The term "hard magnetic particles" is understood, in the present invention, as that particle that has a coercive field between <NUM> kOe and <NUM> kOe and a saturation magnetisation between <NUM> emu/g and <NUM> emu/g. Said values were determined by means of a vibrating sample magnetometer for a maximum magnetic field of <NUM> T. These measurement conditions were maintained in all the examples of the present invention.

The term "soft magnetic particles" is understood in the present invention as that particle that presents a saturation magnetisation between <NUM> emu/g and <NUM> emu/g.

In the present invention, the term "organic solvent" encompasses both acidic and basic organic compounds. The most common organic acids are carboxylic acids, whose acidity is associated with its carboxyl group -COOH. Sulphonic acids, containing the group - SO<NUM>OH, are relatively stronger acids. Alcohols, with -OH, can act as acids but are usually very weak. The relative stability of the conjugate base of the acid determines the acidity thereof. Other groups can also confer a generally weak acidity: the thiol group -SH, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally called organic acids.

An organic base is an organic compound that acts as a base. Organic bases are generally, but not always, proton acceptors. In general, they contain nitrogen atoms which can be easily protonated. Amines and nitrogen-containing heterocyclic compounds are organic bases. Examples include: pyridine, alkanamines, such as methylamine, imidazole, benzimidazole, histidine, guanidine, phosphazene bases, quaternary ammonium cation hydroxides or some other organic cations.

The term "magnetic anisotropy" is understood as the non-homogeneity of the magnetic properties when measured in different directions in space. In other words, the magnetic response is different depending on the direction in which it is examined.

The term "magnetocrystalline anisotropy" is understood as the non-homogeneity of the magnetic properties along the examined axes of the crystalline structure. A material will be magnetically harder the greater the total magnetocrystalline anisotropy thereof.

The term "shape anisotropy" is understood as the magnetic response as a consequence of the geometric shape of the material or of the particles that make it up.

The equivalences between magnetic units are <NUM> T = <NUM> = <NUM> Oe = <NUM>,<NUM> kA·m-<NUM> and <NUM> emu·g-<NUM> = <NUM>·π·<NUM>-<NUM> A·m<NUM>·kg-<NUM>.

The density or absolute density is the magnitude that expresses the relationship between the mass and the volume of a substance or a solid object. Its unit in the International System is kilogram per cubic meter (kg/m<NUM>), although it is also frequently expressed in g/cm<NUM>.

The relative density of a substance is the relationship between its density and that of another reference substance; therefore, it is a dimensionless quantity (without units).

In the present invention, the term "relative or apparent density" is understood as that relationship between the measured density and the theoretical density of a magnet.

The present invention relates to a method for obtaining a permanent ceramic magnet that does not contain rare earths, that is magnetically anisotropic and exhibits an apparent density greater than <NUM> %. Said method is characterised in that it comprises a step of mixing hard magnetic microparticles with a basic or acidic organic solvent, a heating step under pressure, a cooling step, and a sintering step.

The permanent ceramic magnet that is produced following the methods of the present invention has a coercive field greater than <NUM> Oe, for some preferred embodiments, permanent magnets with a coercive field of up to <NUM> Oe have been produced.

The main applications of said permanent magnets are framed within the renewable energy generation and storage sector (wind turbine generators, generators in wave parks, flywheels) and the automotive industry (a car contains an average of <NUM> magnets), for example, magnets for power steering and power windows, and in particular alternators and engines for electric or hybrid vehicles. In this sense, the new electric motors require the use of permanent magnets that withstand high temperatures with high energy products that require high coercive field values such as those claimed in the present invention, aspect that will expand the use of these materials.

Next, the detailed description of the invention is presented:
In a first aspect, the present invention relates to a method for obtaining a permanent ceramic magnet comprising.

characterised in that it comprises the following steps:.

and e) sintering the product from step (d) at a temperature between <NUM> and <NUM> in the presence of an air atmosphere.

The step (a) of the method of the present invention relates to homogeneously mixing.

Said mixing process of step (a) is simple and low cost.

The mixture is preferably made in an agate mortar, until the powder is homogeneously moistened by the acidic or basic organic solvent. In a scaled-up process, the mixture is carried out in an Eirich-type intensive mixer until the required degree of homogeneity is achieved.

In a preferred embodiment of the method of the present invention, the mixture of step (a) further comprises.

obtaining a permanent magnet with a coercive field of up to <NUM> Oe.

In another preferred embodiment of the method of the present invention, the mixture of step (a) further comprises.

In a preferred embodiment of the method of the present invention, step (a) is carried out in the presence of an atmosphere of the same acidic or basic organic solvent used in the mixture.

Preferably, the hard magnetic ferrite nanoparticles/microparticles and the soft magnetic nanoparticles/microparticles are premixed, for example, by means of a dry grinding process for <NUM> minutes in a Mixermill <NUM> equipment using a nylon container and yttria-stabilised zirconia balls of <NUM> in diameter, to produce a homogeneous mixture of powders.

In a preferred embodiment of the method of the present invention, the organic solvent of step (a) is an acidic organic solvent selected from glacial acetic acid (CH<NUM>COOH), oleic acid (C<NUM>H<NUM>O<NUM>), lactic acid (C<NUM>H<NUM>O<NUM>), formic acid (CH<NUM>O<NUM>), citric acid (C<NUM>H<NUM>O<NUM>), oxalic acid (C<NUM>H<NUM>O<NUM>), uric acid (C<NUM>H<NUM>N<NUM>O) or malic acid (C<NUM>H<NUM>O<NUM>); or a combination thereof.

In a preferred embodiment of the method of the present invention, the organic solvent of step (a) is a basic organic solvent selected from anisole (C<NUM>H<NUM>O), aniline (C<NUM>H<NUM>N), purine (C<NUM>H<NUM>N<NUM>), triethylamine (C<NUM>H<NUM>N), oleamine (C<NUM>H<NUM>N); or a combination thereof.

The step (b) of the method relates to compacting the homogeneous mixture obtained in step (a) with a uniaxial pressure of between <NUM> MPa and <NUM> MPa, for a period of time between <NUM> and <NUM>, at room temperature between <NUM> and <NUM>.

Said compaction step (b) is industrially feasible.

The compaction step (b) is carried out in a period of time between <NUM> and <NUM>. This period of time ensures the alignment of the magnetic particles in the pressing direction to thus produce an anisotropic magnetic material.

In another preferred embodiment of the method of the present invention, the compaction step (b) is carried out in the presence of a magnetic field of at least <NUM> T oriented in the direction perpendicular to the direction of the pressure exerted. The applied magnetic field is between <NUM> T and <NUM> T.

On the one hand, the application of an external magnetic field in the direction of pressure application during the compaction process allows the hard magnetic particles with platelet morphology to be magnetically aligned along the magnetisation axis thereof which, due to its magnetocrystalline anisotropy, is perpendicular to the plane of the platelets. On the other hand, the presence of an acid or basic organic solvent during the application of a magnetic field reduces the friction between the particles and favours the alignment processes of the easy magnetisation axis of the hard magnetic particles and the applied external magnetic field.

The step c) of the method of the present invention relates to heating the product from step b) under uniaxial pressure between <NUM> MPa and <NUM> MPa up to the boiling temperature of the organic solvent used in step (a), preferably at a temperature of between <NUM> and <NUM>, for a time between <NUM> and <NUM>.

The pressure and temperature provide an increase in the density of the product from step b) and exert localised pressure between the particles that is concentrated in the contact areas between them. The solubility of the strontium ferrite with the organic solvent used gives rise to a partial solubilisation of the crystalline structure that takes place on the surface of the inorganic particles, preferably in the areas of greatest chemical potential and/or in the areas where mechanical stresses accumulate. Thus, the process of partial solubilisation of the magnetic structure allows for mass transport that favours the creation of sintering necks between the particles. This process is favoured by a supersaturation of the solution that is accentuated with the removal by evaporation of the organic solvent with temperature.

By way of comparison, it must be noted that pressure and temperature can cause a degradation of the hard magnetic condition which is generally partial after the step (c) of the method of the present invention and results in amorphous phases or phases that do not have hard magnetic order at room temperature, such as α-Fe<NUM>O<NUM>. Said degradation is non-reversible by means of a subsequent heat treatment when an aqueous-based acid solution is used, i.e., a solution in which the percentage by weight of water is greater than <NUM> %.

The product obtained in step (c) is allowed to cool until it reaches a temperature of between <NUM> and <NUM> and, with the help of a press applying uniaxial pressure, it is extracted from the mould. The extraction of the compacted part is favoured at said temperatures since the differences in the coefficient values of thermal expansion between the compacted part and the mould are smaller than those that take place when the assembly is cooled to room temperature. At room temperature, the metal mould exerts a maximum compression force on the compact and extraction is made difficult. Subsequently, the product extracted from the mould is allowed to cool to room temperature, between <NUM> and <NUM>. After this step (d) of the method of the present invention, parts are produced with a relative density greater than <NUM>% with respect to the theoretical density.

In the present invention, the term "relative or apparent density" is understood as that relationship between the measured density of the permanent ceramic magnet according to the Archimedes' method or by the relationship between the mass determined by weighing and its volume determined by dimensional measurements the theoretical density of hard magnetic particles of ferrite or hexaferrite composition of formula MFe<NUM>O<NUM>, wherein M is a divalent alkaline earth metal selected from Sr+<NUM>, Ba+<NUM> and any of the combinations thereof.

Note that the theoretical density of SFO is <NUM>/cm<NUM>, the theoretical density of α-Fe<NUM>O<NUM> is <NUM>/cm<NUM> and the theoretical density of Fe<NUM>O<NUM> is <NUM>/cm<NUM>. As used herein, the term relative density relates to % of density relative to the theoretical density of SFO. Said term is used both for SFO materials and for materials that initially comprise a proportion of iron in the oxidised or non-oxidised form thereof.

The step (e) of the method of the present invention relates to the sintering of the product from step (d) by means of a heat treatment at a temperature between <NUM> and <NUM> in the presence of an air atmosphere. The purpose of said step is to improve the densification and, where appropriate, recover the hard magnetic condition. The sintered parts obtained after the step (e) result in dense parts with a relative density greater than <NUM> % compared to the theoretical density.

In a preferred embodiment of the method of the present invention, the maximum temperature reached in the step (e) of sintering, between <NUM> and <NUM>, is maintained constant for a period of time between <NUM> and <NUM>.

In another preferred embodiment of the method of the present invention, the sintering step (e) is carried out by means of a thermal cycle with a heating rate of between <NUM>/min and <NUM>/min. These speed conditions are preferred because they favour the removal of organic solvent residues present in the product and the solid state chemistry reactions to recover the hard magnetic phase.

Throughout the description and the claims, the word "comprises" and its variants do not intend to exclude other technical features, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention may be partially deduced from both the description and the embodiment of the invention. The following examples and figures are provided by way of illustration and are not intended to limit the present invention.

Next, the invention will be illustrated by means of assays carried out by the inventors that demonstrate the effectiveness of the product of the invention.

In order to show the advantages of the materials obtained in the present invention, two comparative examples related to a conventional sintering process and a cold sintering process are presented. Likewise, the conventional sintering process has been carried out using an organic solvent in the compaction step since it is favorable for the mechanical orientation of the particles in the production of compacts due to the morphology thereof.

The term "sintering by the conventional method" is understood as the sintering of parts obtained from powders previously compacted by heat treatment to achieve a final density greater than <NUM> % of the theoretical density of the material. In a conventional sintering process, the mass transport responsible for the densification is thermally activated, requiring high temperatures.

The term "cold sintering" is understood as the sintering of parts obtained from powders and an aqueous solution through a combined process of temperature up to <NUM> and pressure. In a cold sintering process, the mass transport responsible for the densification occurs by means of the supersaturation of the aqueous solution by removing the solvent with temperature and in the presence of high pressures to achieve a final density greater than <NUM> % of the theoretical density of the material.

In the comparative examples and in the examples of the present invention, the same starting powder of strontium hexaferrite or strontium ferrite, SFO, is used, in order to show the advantages of the method followed in the present invention.

To prepare a permanent ceramic magnet, platelet-shaped particles of strontium hexaferrite (SFO) were used, which presented a bimodal distribution of particle sizes, whose two contributions correspond to platelet thicknesses between <NUM> and <NUM> and to platelet diameters between <NUM> and <NUM>. <NUM> of the particles in powder form were manually homogenised in an agate mortar for <NUM> minutes. <FIG> shows the morphology of the starting particles by means of scanning electron microscopy.

In a second step, the homogeneous powder was compacted in a stainless steel die with an internal diameter of <NUM> and a pressure of <NUM> MPa was applied for <NUM> minutes.

Subsequently, the compacted parts were extracted and the conventional thermal sintering of the samples was carried out at a maximum temperature of: <NUM> (M1), <NUM> (M2), <NUM> (M3), <NUM> (M4) and <NUM> (M5). The heat treatment was carried out in an air atmosphere and with a heating rate of <NUM>/min until reaching the maximum temperature, where it was maintained for <NUM> and the subsequent cooling to room temperature according to the oven.

<FIG> show the scanning electron microscopy images of a magnet sintered by means of conventional sintering at a sintering temperature of <NUM> (M2) and <NUM> (M5) respectively, compared with the starting powder of strontium ferrite (<FIG>). In both sintered parts, an increase in grain size is produced with respect to the particle size of the starting material. During the thermal process, sintering necks are produced between the particles and, if applicable, an increase in the density of the part by increasing the maximum temperature of the process, as shown in Table <NUM>. Specifically, relative densities of <NUM> % were obtained with respect to the theoretical density for sample M2 and <NUM> % with respect to the theoretical density of the SFO for sample M5. The grain size of the ferrite magnets presented a growth with respect to the average size of the starting particle, becoming on average <NUM> for magnet M2 and <NUM> for magnet M5. In the latter case, grains with sizes greater than <NUM> were observed. The data relating to magnets M1-M5 sintered by means of the conventional method, and the compact before heat treatment (M0) are summarised in Table <NUM>.

<FIG> shows the diffraction patterns obtained by means of X-ray diffraction, using a D8 Bruker equipment with Cu Kα radiation (λ = <NUM>), of the samples sintered by means of conventional thermal sintering at temperatures of <NUM> (M2) and <NUM> (M5). Both parts show diffraction patterns in which the Bragg peaks corresponding to strontium ferrite are identified. By means of a treatment of the data obtained by X-ray diffraction from a Rietveld refinement, it was identified that the crystalline phase in both samples is made up of <NUM> % strontium ferrite.

<FIG> represents the results obtained by means of confocal Raman spectroscopy, using a Witec ALPHA 300RA with a Nd:YAG laser (<NUM>). The Raman spectra corroborate the data obtained by means of X-ray diffraction presented in <FIG>, showing a Raman spectrum corresponding to strontium ferrite, identifying the main modes of vibration and without the presence of other phases.

The magnetic response of the magnets M1, M2, M3, M4 and M5 is represented in <FIG>. M1 has a coercive field of <NUM> kOe, a ratio of remanence magnetisation versus saturation magnetisation of <NUM>. Note that a randomly oriented material would have a ratio of <NUM> and a fully oriented material would achieve a ratio of <NUM> and a saturation magnetisation of <NUM> emu/g. It is therefore found at present that a slight orientation of the SFO particles occurs during the pressing step due to mechanical effects related to the morphology of the platelet-shaped particles.

The observed magnetic response is associated with a magnetic hysteresis loop that is characteristic of a hard material, with improved magnetic characteristics with respect to the unsintered compacted starting SFO material (M0). As we increase the sintering temperature, the magnetic properties are affected by lowering the coercive field to <NUM> kOe for M2 and <NUM> kOe for M3. In both M1, M2 and M3, the low density thereof did not provide sufficient mechanical integrity due to the low densification to handle samples properly. For M4, although the relative density compared to the theoretical density improves, reaching a value of <NUM>%, a drastic decrease in the magnetic response was obtained with a decrease in the coercive field at <NUM> kOe. Magnetic response that decreases as the sintering temperature increases to <NUM> (M5) having a coercive field of <NUM> kOe, with a reduction of the ratio between remanence magnetisation versus saturation magnetisation to <NUM>. This worsening of the magnetic response is related to the increase in grain size (<FIG>) associated with the densification of the magnet.

Therefore, from strontium ferrite particles, obtaining a permanent densified magnet by means of conventional heat treatment is associated with a growth in grain size that reduces the magnetic response thereof, in particles the coercive field that decreases by an amount greater than <NUM> % with respect to the value of the initial particles in powder form.

A mixture was prepared based on the particles obtained in Comparative Example <NUM> of strontium hexaferrite, corresponding to <NUM> of SFO particles and <NUM>% by weight of <NUM> or <NUM> acetic acid as aqueous-based solvent. The mixture was manually homogenised in an agate mortar for about <NUM> minutes until it reached a moistened powder.

In a second step, the moistened mixture was compacted in a hardened steel die with an internal diameter of <NUM> at a pressure of <NUM> MPa for <NUM> minutes.

Subsequently, the set of the die with the compacted moistened powder was subjected to a heating process by means of a heating press in an air atmosphere, maintaining the sample under pressure. This process of heating under pressure is referred to as "cold sintering" in the literature. The conditions of the process were a temperature of <NUM> for <NUM> under a uniaxial pressure of <NUM> MPa. After the process, the part was extracted from the die at a temperature of <NUM> and subsequently cooled under ambient conditions to room temperature. The magnets thus obtained are called M6 and M8 using <NUM> or <NUM> acetic acid, respectively.

In an additional process, the magnets were subjected to a heat treatment at a temperature of <NUM> for <NUM> in an air atmosphere with a heating rate of <NUM>/min. The resulting magnets after the heat treatment are named magnet M7 and M9 using <NUM> or <NUM> acetic acid, respectively.

The M6 magnet reached a relative density <<NUM> % with respect to the theoretical density and presented a percentage of crystalline phases of <NUM> % SFO and <NUM> % Fe<NUM>O<NUM> phase iron oxide. After heat treatment at <NUM> for <NUM> hours, the M7 magnet presented a density of <NUM> % with respect to the theoretical density with a decrease in the percentage of Fe<NUM>O<NUM> iron oxide of up to <NUM>%. Increasing the molarity of the aqueous solution from <NUM> to <NUM>, the obtained magnet M8 presented a density of <NUM>% with respect to the theoretical density with a percentage of crystalline phases of <NUM> % of SFO and <NUM> % of α-Fe<NUM>O<NUM> iron oxide, while after the heat treatment at <NUM> for <NUM> hours, the M9 magnet began to increase relative density to <NUM> % with a decrease in the percentage of α-Fe<NUM>O<NUM> to <NUM> %. In both cases, the process gave rise to a partial recrystallisation of the ferrite and a densification of the part after the heat treatment at high temperature.

The magnetic response of M6, M7, M8 and M9 is represented in <FIG>. M6 and M8 presented a coercive field of <NUM> kOe and a ratio of remanence magnetisation versus saturation magnetisation of <NUM> and <NUM>, respectively, indicating that the mechanical orientation of the platelet morphology particles did not improve with respect to the samples obtained in comparative example <NUM>. The observed magnetic response is associated with a magnetic hysteresis loop characteristic of the combination of SFO and the corresponding iron oxide. In both cases, for magnet M6 and M8, the low density thereof did not provide sufficient mechanical integrity due to the low densification to handle the sample properly.

For magnets M7 and M9, after the heat treatment process at <NUM>, an increase in the magnetic response was obtained by increasing both the coercive field to <NUM> kOe and <NUM> kOe, respectively, and the ratio between remanence magnetisation versus saturation magnetisation to <NUM> for both magnets. This improvement of the magnetic signal is related to the partial recovery of the SFO phase and the retention of the particle size after the heat treatment process. However, the density of the permanent magnet is less than <NUM> % of the theoretical density of the SFO and it presents water absorption and brittleness that prevents the use thereof as a permanent ceramic magnet.

Therefore, by means of the cold sintering process or heating under pressure using an aqueous solvent, a densification level greater than <NUM> % was not reached. Moreover, a notable loss of magnetic properties is observed that correlates with the non-reversible chemical decomposition of the hard magnetic phase SFO to give rise to iron oxide phases. The subsequent heat treatment allows the recrystallisation of the SFO phase that produces an improvement in the magnetic properties, however, the density of the ceramic material does not exceed <NUM> % of the theoretical density of the SFO.

A mixture of <NUM> of the particles obtained in Comparative Example <NUM> of strontium hexaferrite, SFO, and a certain percentage by weight of an organic solvent (glacial acetic acid, oleic acid or oleamine). The mixture was manually homogenised in an agate mortar for about <NUM> minutes until it reached a moistened powder.

In a second step the homogeneously moistened powder was compacted in a hardened steel die with an internal diameter of <NUM> and a pressure of <NUM> MPa was applied for <NUM> minutes.

Subsequently, the compacted parts were extracted and a conventional thermal sintering process of the samples was carried out at the maximum temperature of <NUM> for <NUM>. The heat treatment was carried out in an air atmosphere using a heating rate of <NUM>/min until reaching the maximum temperature, where it was maintained for <NUM> and subsequent cooling to room temperature according to the oven.

The data relating to magnets using glacial acetic acid (M10), oleic acid (M11) and oleamine (M12) sintered by conventional thermal methodology are summarised in Table <NUM>. The sintered parts presented as the only crystalline phase, <NUM> % SFO and reached a relative density against the theoretical density of SFO ≤ <NUM> %, regardless of the organic solvent used for sintering.

The magnetic response of M10, M11 and M12 is represented in <FIG>. In all cases, the magnets presented a coercive field ≥ <NUM> kOe, a ratio of remanence magnetisation to saturation magnetisation of about <NUM> and a saturation magnetisation of <NUM> emu/g. The use of an organic solvent favored the mechanical alignment of the particles with platelet morphology of SFO compared to the processes that use an aqueous-based solvent. The observed magnetic response is associated with a magnetic hysteresis loop that is characteristic of a hard material, with improved magnetic characteristics with respect to the compacted SFO material without sintering (M0). However, in all cases, the density is less than <NUM> % of the theoretical density and the mechanical integrity is insufficient for use as a permanent sintered magnet.

Therefore, a conventional sintering process in which the compaction step is improved by the use of an organic solvent does not solve the problem of obtaining densified compacts with a coercive field equal to or greater than the coercive field of the starting SFO particles.

To prepare a permanent ceramic magnet, platelet-shaped particles of strontium hexaferrite (SFO, the same powder described in Comparative Example <NUM> was used) which presented a bimodal distribution of particle sizes corresponding to platelet thicknesses between <NUM> and <NUM> and platelet diameters between <NUM> and <NUM> were used.

Mixtures were prepared comprising <NUM> of SFO and the organic solvents indicated in Table <NUM>, which were glacial acetic acid, oleic acid or oleamine. The mixtures were manually homogenised in an agate mortar for about <NUM> minutes until reaching a moistened powder.

In a second step, the moistened mixture resulting from the first step was compacted in a hardened steel die with an internal diameter of <NUM> at a pressure of <NUM> MPa for <NUM> minutes.

Subsequently, the set of the die with the compacted moistened powder was subjected to a heating process under pressure by means of a heating press in an air atmosphere. The conditions of the heating process under pressure with parameters related to pressure, temperature and time are specified in table <NUM>. The temperature of the heating process under pressure was selected above the decomposition temperature of the organic solvent. After the process, the part was extracted from the die at a temperature of <NUM> and subsequently cooled under ambient conditions. The resulting magnets are called M13, M15 and M17 magnets (see Table <NUM>).

In a subsequent process the M13, M15 and M17 magnets were subjected to a heat treatment at a maximum temperature of <NUM> for <NUM> in an air atmosphere. The heat treatment consisted of: a heating ramp of <NUM>/min until reaching the maximum temperature where it was maintained for <NUM> and the subsequent cooling to room temperature according to the oven. The magnets thus obtained are called M14, M16 and M18 (See Table <NUM>).

<FIG> shows the scanning electron microscopy micrograph of a sintered magnet using glacial acetic acid as organic solvent at <NUM>% by weight with respect to the SFO material, at a temperature of <NUM> for <NUM> under a uniaxial pressure of <NUM> MPa (M13 magnet). The grain size of the M13 ferrite-based magnet presented a bimodal microstructure with grain sizes of approx. <NUM> and <NUM> respectively, which is a reflection of the size distribution of the starting particles. During the process sintering necks are produced between the particles, obtaining a relative density with respect to the theoretical density of <NUM>%. After the heat treatment at <NUM> for <NUM> hours (<FIG>), the M14 magnet presented a density of <NUM> % with respect to the theoretical density, identifying a growth in the particle size that maintains the bimodal microstructure and becomes approx. <NUM> and <NUM>.

An X-ray diffraction analysis from a Rietveld refinement determined a partial decomposition of the SFO for sample M13 using glacial acetic acid with a percentage of crystalline phases of <NUM>% SFO and <NUM>% α-Fe<NUM>O<NUM> iron oxide. After the heat treatment process at <NUM>, sample M14 presented a total recrystallisation of the SFO, not identifying any crystalline or amorphous impurity as shown in <FIG> (analysis carried out by means of X-ray and Raman spectroscopy). Therefore, it is observed that the heat treatment allows reaching an adequate density and recrystallising the SFO phase to obtain a sintered ferrite permanent magnet material.

The magnets prepared in the present invention using oleic acid (M15) and oleamine (M17) maintained the crystalline phase of SFO after the compaction process followed, and a low densification of <NUM> % and <NUM> % of the theoretical density of the SFO, respectively. After the heat treatment process at <NUM> for <NUM>, the M16 and M18 magnets using oleic acid and oleamine, respectively, reached adequate densification with relative density values ≥ <NUM> %.

The magnetic response of M13, M15 and M17 show a coercive field of around <NUM> kOe and a ratio of remanence magnetisation to saturation magnetisation of <NUM>, <NUM> and <NUM>, respectively. The low density thereof did not provide sufficient mechanical integrity to handle the sample properly.

After the heat treatment of the parts M13, M15 and M17 at <NUM> for <NUM>, the M14, M16 and M18 magnets presented a density of <NUM> % with respect to the theoretical one, a coercive field of at least <NUM> kOe and an improvement in the ratio of remanence magnetisation to saturation magnetisation greater than <NUM>. The improvement in particle orientation combines mechanical orientation during the compaction process and preferential recrystallisation during treatment at <NUM>. The magnetic response of the M13, M14, M16 and M18 magnets is presented in <FIG>.

The permanent magnets of the present invention therefore have the advantages of a density greater than <NUM> %, which gives them adequate mechanical integrity for use as permanent ceramic ferrite magnets. The method of the invention allows to control the growth of the grain size and the crystalline phases of hard ferrite that provide a high coercive field. The properties of the permanent magnets of the present invention represent an improvement with respect to conventional sintering processes (Comparative Example <NUM>) and with respect to cold sintering processes that use aqueous-based solvents (Comparative Example <NUM>) given that in the comparative examples, a similar degree of densification was not reached while maintaining controlled grain size. Likewise, in comparative examples <NUM> and <NUM>, the magnetic properties for dense materials reached by means of the method of the present invention are not achieved. It is shown that the method of the present invention solves the problem of densifying permanent magnets by limiting grain growth to provide an improved permanent magnet.

It starts from the precursor mixture used in sample M17 manually mixed in an agate mortar for about <NUM> minutes until reaching a homogeneous moistened powder.

In a second step, the moistened mixture resulting from the first step was compacted in a hardened steel die with an internal diameter of <NUM> at a pressure of <NUM> MPa, simultaneously applying a magnetic field of <NUM> T. Process prior to sintering.

Subsequently, the set of the die with the compact was subjected to heating under pressure by means of a heating press in an air atmosphere. The conditions of the process were a temperature of <NUM>, above the boiling temperature of the organic solvent, for a time of <NUM> under a uniaxial pressure of <NUM> MPa. After the process, the part was extracted from the die at a temperature of <NUM> and subsequently cooled to room temperature.

In a subsequent process, the magnets obtained were subjected to a heat treatment at a maximum temperature of <NUM> for <NUM> in an air atmosphere. The heat treatment consisted of heating at a rate of <NUM>/min until reaching the maximum temperature, where it was maintained for <NUM>, and subsequent cooling to room temperature. In this manner, the M19 magnet was obtained, the properties of which are presented in Table <NUM>.

The M19 magnet presented a <NUM> % relative density with respect to the theoretical density. This relative density is similar to that obtained without applying a magnetic field during the previous compaction of the mixture (M18 magnet).

The magnetic response corresponding to the M19 magnet is shown in <FIG>. A slight reduction in the coercive field was observed with respect to the M18 magnet, going from <NUM> kOe to <NUM> kOe. In addition, an improvement of the remanence magnetisation compared to the saturation magnetisation is obtained, going from <NUM> for the M18 magnet to <NUM> for the M19 magnet. The increase in said ratio indicates a greater degree of orientation of the crystalline structure in the pressing direction as a consequence of the orientation of the particles in the presence of the external magnetic field favored by the organic solvent, the mechanical action of the pressing process itself and the crystalline orientation during the heat treatment. The M19 magnet of the present invention therefore has an anisotropic behaviour.

A mixture with <NUM> % by weight of SFO, and <NUM> % by weight of strontium hexaferrite nanoparticles was prepared, ("nSFO" hereinafter). The mixture was homogenised by means of a dry grinding process for <NUM> in a Mixermill <NUM> equipment using a nylon container and yttria-stabilised zirconia balls with <NUM> diameter.

From said mixture, the method described in example <NUM> was followed, which comprised:.

and e) sintering the product obtained in step (d) at a temperature between <NUM> and <NUM> in the presence of an air atmosphere.

In this example, a magnetic field was not used in order to compare the samples obtained with the magnets of example <NUM>. The application of a magnetic field in step b) would provide an increase in anisotropy according to example <NUM> and is applicable to the different materials of this example <NUM>.

<FIG> shows a scanning electron microscopy micrograph of a magnet based on <NUM>% by weight of SFO and <NUM>% by weight of starting nSFO and processed using glacial acetic acid as organic solvent at <NUM>% by weight with respect to the starting material which, after compaction, was subjected to heating under pressure at a temperature of <NUM> for <NUM> and at a uniaxial pressure of <NUM> MPa (M20).

Compared to the M13 magnet obtained under the same conditions using <NUM>% SFO as starting powder, a lower degradation of the SFO crystalline phase was identified. After a heat treatment at <NUM> for <NUM> (M21), a greater effectiveness in grain growth control was observed, which is limited to an average value of ~<NUM> as shown in <FIG>. During the heat treatment at <NUM> for <NUM>, the densification of the compact increases until reaching <NUM> % relative density with respect to the theoretical density. An advantage of the incorporation of nanoparticles in the initial mixture consists in the refinement of the grain sizes that show a single mode. In this manner, larger particles in the starting powder are refined during the processing of the present invention to generate a monomodal microstructure, i.e., with a homogeneous size distribution.

In the case of using oleic acid together with the starting powder after the compaction process at a temperature of <NUM> for <NUM> under a uniaxial pressure of <NUM> MPa (M22), it gives rise to a lower relative density after the heating process under pressure and the crystalline phase of SFO remains without degradation. After a heat treatment at <NUM> for <NUM> (M23), a relative density of <NUM>% was reached with respect to the theoretical density, as in the case of using glacial acetic acid (M21).

An X-ray diffraction analysis from a Rietveld refinement determined a partial decomposition of the SFO for the M20 magnet using glacial acetic acid with a percentage of crystalline phases of <NUM> % SFO and <NUM> % α-Fe<NUM>O<NUM> iron oxide, while using oleic acid as organic solvent, there is no decomposition of the SFO phase (M22, <NUM> % SFO). After the heat treatment process at <NUM> for <NUM>, regardless of the organic solvent used in the process, <NUM> % SFO was identified for M21 and M23. The magnetic signal after the presented process is associated with a magnetic hysteresis cycle characteristic of a hard material such as SFO, <FIG>.

Therefore, the incorporation of SFO nanoparticles in the mixture with SFO improves the crystalline stability of the compacts after heating under pressure, which translates into greater control of the grain size of the magnets once thermally treated. This greater homogeneity in the grain size of the sintered ceramic magnet of the present invention has a monomodal microstructure that results in an advantage related to the functional response of the material.

A mixture of <NUM>-<NUM> % by weight of strontium hexaferrite microparticles, SFO and <NUM>-<NUM> % by weight of a soft magnetic phase, FMB selected from.

The mixture was homogenised by means of a dry grinding process in an air atmosphere for <NUM> in a Mixermill <NUM> equipment using a nylon container and yttria-stabilised zirconia balls with <NUM> diameter.

The percentage by weight of SFO and FMB, as well as the conditions during sintering are indicated in table <NUM>, where the main parameters of the method of the invention followed to obtain the magnets M24 to M29 are summarised.

<FIG> show scanning electron microscopy micrographs of SFO-based magnets incorporating <NUM>% mFe (M24), <NUM>% nFe (M26) and <NUM> % nFe<NUM>O<NUM> (M28), respectively, and compacted using glacial acetic acid as organic solvent at <NUM> % by weight of the magnetic material (SFO+FMB) and heated under uniaxial pressure of <NUM> MPa at a heating temperature of <NUM> for <NUM>. In all cases, a bimodal particle size was distinguished, consisting of microparticles with a size of <NUM>-<NUM> and nanostructures with particle size <<NUM>. During the process of heating under pressure an improvement in apparent density is obtained, reaching densities greater than <NUM> % with respect to the theoretical density of SFO.

After heat treatment of the compounds at <NUM> for <NUM>, the M25 (<FIG>), M27 (<FIG>) and M29 magnets (<FIG>), obtained from the mixture of SFO + <NUM> % mFe, + <NUM> % nFe and + <NUM> % nFe<NUM>O<NUM>, respectively, an increase in particle size was observed, reaching an average of <NUM> with a more homogeneous particle distribution and a relative density compared to the theoretical density of the resulting magnets greater than <NUM> %; said apparent density is advantageous for the functionality of the magnet.

The M24, M26 and M28 magnets (samples obtained from a method without sintering heat treatment) showed a partial decomposition of the starting SFO. In the case of sample M24 the decomposition was almost complete. After the heat treatment process at <NUM> for <NUM> hours, the M25, M27 and M29 magnets presented a partial recrystallisation of the SFO, identifying a percentage of secondary phase corresponding to iron oxide. α-Fe<NUM>O<NUM> less than <NUM> %, <FIG>.

The magnetic response of composite M24 to M29 magnets is presented in <FIG>. The M24, M26 and M28 parts presented magnetic hysteresis cycles that can be explained by the composition of the phases comprising them, due to the transformation of phases induced during the process. For example, the M24 magnet presented a cycle associated with a soft magnetic phase that is explained by the <NUM> % of the crystalline phase as Fe<NUM>O<NUM> identified after sintering. Specifically for the M24 and M26 magnets, the low density thereof did not provide sufficient mechanical integrity.

After the sintering heat treatment at <NUM> for <NUM>, recrystallisation of the SFO results in an improvement in the magnetic properties. M25, M27 and M29 magnets presented a coercive field greater than <NUM> kOe, even <NUM> kOe for the M29 magnet. The ratio of remanence magnetisation versus saturation magnetisation became <NUM>, <NUM> and <NUM>, and the saturation magnetisation became <NUM> emu/g, <NUM> emu/g and <NUM> emu/g for the M25, M27 and M29, respectively.

Claim 1:
A method for obtaining a permanent ceramic magnet comprising
• hard magnetic particles in the form of platelets of ferrite or hexaferrite composition of formula MFe<NUM>O<NUM>, wherein M is a divalent alkaline earth metal selected from Sr+<NUM>, Ba+<NUM> and any of the combinations thereof, and wherein said magnetic particles have a bimodal particle size distribution with a first particle size between <NUM> and <NUM> and a second particle size between <NUM> and <NUM>,
characterised in that it comprises the following steps:
a) mixing homogeneously
• hard magnetic particles of ferrite or hexaferrite composition of formula MFe<NUM>O<NUM>, wherein M is a divalent alkaline earth metal selected from Sr+<NUM>, Ba+<NUM> and any of the combinations thereof, with particle size between <NUM> and <NUM>,
• and an organic solvent;
wherein the percentage of organic solvent ranges from <NUM> % to <NUM> % by weight with respect to the total weight of the mixture,
b) compacting the homogeneous mixture obtained in step (a) at a uniaxial pressure of between <NUM> MPa and <NUM> MPa for a period of time between <NUM> and <NUM>;
c) heating the product obtained in step b) under uniaxial pressure between <NUM> MPa and <NUM> MPa at a temperature between <NUM> and <NUM>, and for a period of time between <NUM> and <NUM>;
d) cooling the product from step (c) until reaching a temperature between <NUM> and <NUM> to facilitate the extraction of the compacts, and subsequent cooling to room temperature between <NUM> and <NUM>;
and e) sintering the product from step (d) at a temperature between <NUM> and <NUM> in the presence of an air atmosphere.