EP1438270A1

EP1438270A1 - Economical ferrite-type magnets with enhanced properties

Info

Publication number: EP1438270A1
Application number: EP02801367A
Authority: EP
Inventors: Antoine Morel; Philippe Tenaud
Original assignee: Ugimag SA
Current assignee: Ugimag SA
Priority date: 2001-10-19
Filing date: 2002-10-14
Publication date: 2004-07-21
Also published as: JP2005505944A; US20040251997A1; BR0213387A; KR100845201B1; FR2831317A1; WO2003033432A1; CN100386288C; FR2831317B1; KR20050036879A; MXPA04003449A; CN1571761A

Abstract

The invention concerns a ferrite magnet comprising a magnetoplumbite phase of formula M1-xRxFe12-yTyO19, wherein: M represents at least an element selected among the group consisting of: Sr, Ba, Ca and Pb; R represents at least an element selected among rare earths and Bi; T represents at least an element selected among Co, Mn, Ni, Zn; 0,15 < x < 0.42; 0.50 < α = y/x < 0.90, so as to provide a ferrite magnet having both a reduced level in element T and a global performance index GIP = Br + 0.5. Hk not less than 580, and preferably not less than 585, Br being the remanent induction expressed in mT, Hk corresponding to the field H expressed in kA/m, for B = 0.9.Br.

Description

ECONOMICAL AND PROPERTY FERRITE TYPE MAGNETS

IMPROVED

FIELD OF THE INVENTION

The invention relates to the field of magnets of the hexagonal ferrite type comprising the magnetoplumbite phase M.

STATE OF THE ART

Ferrite type magnets comprising the magnetoplumbite phase are already known, and of formula M Fe ₁₂ O ₁₉ with M = Sr, Ba, Ca, Pb, etc.

Magnets of this type of formula are also known (M _1-x R _x ) O • n [(Fe _12-y T _y ) ₂ O ₃ ]. Thus, European application EP 0 964 411 A1 describes magnets in which: - M is an element chosen from Sr and / or Ba,

- R is an element belonging to rare earths,

- T is an element chosen from Co, Mn, Ni and Zn, and with: - x ranging from 0.01 to 0.4,

- y going from [x / (2,6n)] to [x / (1,6n] - and n going from 5 to 6.

Similarly, European application EP 0 905 718 A1 describes magnets of this type of formula Mι _-X R _X (Fe _12-y T _y ) _z O ₁ in which:

- M is an element chosen from Sr, Ba, Ca and Pb, and essentially Sr, - R is an element belonging to rare earths or Bi, and essentially La,

- T is Co or Co and Zn, with:

- x ranging from 0.04 to 0.9,

- y ranging from 0.04 to 0.5 with x / y ranging from 0.8 to 20, and - z ranging from 0.7 to 1.2. This type of magnet is also described in European patent applications EP 0 758 786 A1, EP 0 884 740 and EP 0 940 823 Al, US 6,258,290 and EP 1 150 310 Al. The manufacture of such magnets typically includes the following steps: a) forming a mixture of raw materials either by a wet process to form a dispersion, or by a dry process to form granules, b) calcining the mixture around 1250 ° C to form a clinker, or chamotte, comprising the magnetoplumbite phase sought, said mixture, either in the form of a dispersion or of granules, being introduced into a calcination oven, c) wet grinding of the clinker until an aqueous dispersion of particles with a particle size close to 1 μm, in the form of a paste of approximately 70% dry extract, d) the paste is concentrated and compressed under a orienting magnetic field of approximately 1 Tesla and under a pressure of 30 to 50 MPa of so as to obtain a green tablet, called "green compact" in English, anisotropic, and typically 87% dry extract, e) after drying and removal of the remaining water, sintering the tablet to green, f) final machining to obtain the magnet of predetermined shape.

Manufacturing processes are also known as described in French applications No. 99 10295 and 99 15093 in the name of the applicant.

PROBLEMS POSED

The problems posed by magnets of the ferrite type of the prior art, typically magnets of the Srι- _x La _x Fe _{12 type} . _y Co _y O ₁ are of two types:

on the one hand, the iron substitution element, typically cobalt, is an expensive product,

- on the other hand, although the known magnets have high magnetic properties, typically measured by an index IP = Br + 0.5.HcJ where Br denotes the residual induction (mT) and HcJ the coercive field (kA / m ), a certain number of applications of the magnets require magnets presenting a magnetization curve Br = f (H) as square as possible, the square character (or "squareness" in English) being typically given by the ratio liκ = Hk / HcJ, Hk being the inverse field giving an induction of 0.90. Br. Hk actually corresponds to the field from which the magnetic losses are considered as irreversible.

The invention aims to simultaneously obtain magnets of the ferrite type, which have, in addition to high general magnetic properties, a reduced cost and a square character given by the ratio liκ = Hk / HcJ greater than that obtained under identical operating conditions, and typically at least equal to 0.95.

Given the preponderant importance of the factor Hk, an overall index GIP = Br + 0.5 is proposed. Hk to take into account both the final magnetic properties and the square character of the magnetization and demagnetization curves. The invention aims to obtain magnets with a global performance index GIP at least equal to 580, preferably at least equal to 585, or even at least equal to 590.

DESCRIPTION OF THE INVENTION

According to the invention, the ferrite magnet having the structure of the magnetoplumbite phase (hexaferrite of structure M) of formula M _1-X R _X Fe ₁₂ . _y T _y O ₁₉ in which:

M designates at least one element chosen from the group consisting of: Sr, Ba, Ca and Pb,

- R denotes at least one element chosen by rare earths and Bi,

- T denotes at least one element chosen from Co, Mn, Ni, Zn, - 0.15 <x <0.42

- 0.50 <α = y / x <0.90 so as to have a ferrite magnet having simultaneously a reduced rate of element T and an overall performance index GIP = Br + 0.5. Hk at least equal to 580, and preferably at least equal to 585, Br being the residual induction expressed in mT, Hk corresponding to the field H, expressed in kA / m, for B = 0.9.Br, Br being l remanent induction.

Following his research in the field of permanent magnets, more particularly ferrite magnets having the structure of magnetoplumbite or hexaferrite, permanent magnets of basic structure MFe ₁₂ O ₁₉ where M = Sr, Ba, Pb, Ca, substituted by other elements and having the chemical formula M _1-x R _X Fe _12-y T _y 0 ₁₉ , where R denotes the element Bi or a rare earth, and T denotes an element Mn, Co, Ni, Zn, the applicant continued its investigations with a view to improving on the one hand the magnetic performances represented by a performance index IP = Br + 0.5.HcJ, Br denoting the residual induction expressed in mT and HcJ being the coercive field expressed in kA / m, and with a view to improving on the other hand a second important parameter for permanent magnets, namely the character square or "squareness" in English of the demagnetization curve, generally characterized by hic ⁼ Hk / HcJ (%), Hk corresponding to the field H for B = 0.9.Br, and obtain hj_ at least equal to 0.95. Indeed, the Applicant has observed that with many types of substitution, for example with R = La and T = Co, the square character h _K was greatly degraded, which could greatly limit the applications of these magnets.

The research undertaken therefore aimed to greatly increase the square character hk, without further degrading the overall magnetic performance IP of the magnets, so as to obtain an overall performance index GIP at least equal to 580, and preferably at least equal to 585 , or even at least equal to 590.

Conventionally, to make the mixture of raw materials, the variable "x" of the formula of the ferrite magnet is taken equal to the variable "y", in order to respect the electroneutrality of the magnet whose formula is presumed to be, with R = La and T = Co: Sr _1-X ²⁺ La _x ³⁺ Fe _12-X ³⁺ Co _x ²⁺ O ₁₉

Having studied the square character of these ferrite magnets obtained as a function of the substitution rate x = y, the Applicant has observed, as illustrated in FIG. La, a deterioration of this square character as the increase in x = y - at least up to x = y = 0.3. It also observed, as illustrated in FIG. 1b, the variation of the anisotropy field Ha (kA / m) and of the coercive field HcJ (kA / m) as a function of the substitution rate x = y. It follows that, if the intrinsic magnetic properties, given by the anisotropy field Ha, increase with x = y, on the other hand the macroscopic magnetic properties of ferrite, given in particular by the coercive field HcJ, have an optimum around x = y = 0.2. Furthermore, having analyzed the above magnets obtained with x = y by X-ray diffraction, she observed in particular the presence of a spinel phase of Co (CoFe ₂ O), while in addition, lanthanum seems to replace completely strontium.

The Applicant has first hypothesized that part of the Co element probably did not participate in the formation of the actual ferrite, and that this could lead to the transformation of initial Fe ³⁺ into Fe ²⁺ in ferrite.

To verify this hypothesis, she studied the resistivity of the ferrite magnets obtained for x = y ranging from 0 to 0.4. She observed a rapid drop in resistivity (see Figure 2).

It also hypothesized that this decrease in resistivity could be related to the increasing presence of the couple of ions Fe ²⁺ - Fe ³⁺ , taking into account the possibility of having a conduction by jumps d between the Fe ²⁺ and Fe ³⁺ ions.

It also hypothesized that the presence of such a spinel phase of Co could be the cause of the degradation of the square character liκ of the ferrite magnets studied.

It is these works with the preceding hypotheses that have led the applicant to explore, with a view to solving the problems posed, the field of ferrites:

- on the one hand, weakly substituted,

- on the other hand with x different from y.

The Applicant has found that the polygonal domains shown in FIGS. 3, 4a and 4b made it possible against all odds to resolve the problems posed. As illustrated in FIG. 5e, the invention can allow, all other things being equal, both to reduce the content of element T of the ferrite magnet - element which is generally expensive, and to increase the overall performance of the ferrite magnet.

DESCRIPTION OF THE FIGURES

Figure la is a graph illustrating the variation of the square character liκ (%), on the ordinate, as a function of x and of y on the abscissa, for a ferrite of formula Sr _1-X La _x Fe ₁₂ - _y Co _y O ₁₉ in which one ax = y.

FIG. 1b is a graph illustrating the variation of the coercive field HcJ (kA / m) on the ordinate on the left - the points of the curve being squares, and the anisotropy field Ha (kA / m) on the ordinate on the right - the points of the curve being triangles, as a function of x and of y, on the abscissa, for a ferrite of formula Srι_ _x La _x Fe ₁₂ . _y Co _y 0 ₁ in which we ax = y.

FIG. 2 is a graph illustrating the variation of the resistivity, on the ordinate (log p in Ω.cm), as a function of x and of y, on the abscissa, for a ferrite of formula Sr _1-x La _x Fe _12-y Co _y O ₁₉ in which one ax = y.

FIG. 3 is a graph showing the coefficients x on the abscissa and y on the ordinate (coefficients of the ferrite formula M _1-x R _x Fe _12-y T _y O ₁₉ ) illustrating various fields of the invention, the main field being the lines:

Other sub-domains are delimited by other lines: x = 0.17 - 0.22 - 0.32, α = 0.60 - 0.65 - 0.75 - 0.80.

In FIG. 3 are shown the different tests carried out, the different series of tests having been noted: A for x = 0, B for x = 0.15, C for x = 0.20, D for x = 0.30 and E for x = 0.40.

Figures 4a and 4b are similar to Figure 3 and correspond to restricted areas:

- the polygonal domain (hatched) of FIG. 4a is limited by the lines: ai> 0.65 and ₂ <0.90

- the polygonal domain (hatched) of FIG. 4b, inscribed in the previous one, is limited by the lines:

An even more restricted domain (double hatching) is limited by the lines: X _! = 0.17 and x ₂ = 0.22

FIGS. 5a to 5e illustrate the results (on the ordinate) obtained as a function of the parameter α = y / x, for the tests Bl-1, Cl-1, C3-1, C4-1, C5-1 and Dl-1 which correspond to sintered magnets at a temperature of 1180 ° C. FIG. 5a carries on the ordinate the remanent induction Br in mT.

FIG. 5b carries on the ordinate Hk corresponding to the field H expressed in kA / m, for B = 0.9.Br, Br being the remanent induction. There figure 5c carries on the ordinate the coercive field HcJ in kA / m. Figure 5d shows on the ordinate the performance index IP = IP = Br + 0.5. HcJ Figure 5e shows on the ordinate the overall performance index GIP = Br + 0.5. Hk.

FIG. 6 illustrates an example of demagnetization curves for the tests Cl-1 in dotted lines and C3-1 in solid lines.

DETAILED DESCRIPTION OF THE INVENTION

The fields of the invention, in particular those defined by ranges of the coefficients x and α, have been defined following numerous works and tests by the applicant, a number of which appear in the exemplary embodiments.

In general, the coefficient α is taken at most equal to 0.90 so as to have simultaneously a significant reduction in the rate of the element T and an increase in the overall performance GIP, as observed in a surprising manner. On the other hand, the applicant observed a lower limit for α = 0.5 because of the degradation of the overall GIP performance.

Similarly, as regards the coefficient x, it can vary according to the invention in a range from 0.15 to 0.42. Indeed, the Applicant has observed that it was not advantageous to go beyond x = 0.2, in particular because of the very high content of element T. Because, even if good overall performance can be obtained with x high, this is not necessarily advantageous insofar as identical or better performances can be obtained for lower values of x, and consequently with a lower content of element T in the ferrite. As explained below, we prefer not to exceed a value of x = 0.32. On the other hand, there is a lower limit to the possibility of lowering the coefficients x (and therefore y) and the Applicant has observed an excessively large reduction in magnetic properties - a reduction which the improvement in square character or the reduction does not compensate for. cost, as soon as x is typically less than 0.15.

According to the invention, the magnets of formula M _1-x R _x Fe _12-y T _y O ₁₉ advantageously meet the following condition: 0.15 <x <0.32.

This sub-field of the invention is shown in Figures 3 and 4a.

Another more preferred subdomain corresponds to the following condition: 0.17 <x <

0.22. This area has been shown in Figure 4b.

In fact, the tests have shown that the best results were obtained for the tests carried out with x greater than 0.15, and typically greater than 0.17.

Furthermore, if excellent results were obtained with x = 0.4, these results were not superior to those obtained with x = 0.3. In addition, given that the magnets with x = 0.4 are significantly more expensive than those with x = 0.3 (for the same coefficient α), it is preferred that x does not exceed 0.32.

Likewise, since little difference in properties was observed between the tests with x = 0.3 and with x = 0.2, it was found advantageous to have magnets with x equal at most to 0.22, so as to have particularly economical ferrite magnets.

Other sub-domains are limited by the coefficient α = y / x, as illustrated in Figures 3 to 4b.

Tests have shown the interest of magnets for which we have the relation: 0.60 <α = y / x

<0.90, and preferably 0.65 <α = y / x <0.90, the latter domain being illustrated for example in FIG. 4a. An interesting sub-domain is also that defined by the relation 0.60 <α = y / x <0.80, and preferably that defined by the relation 0.65 <α = y / x <0.80, the latter being illustrated in Figure 4b.

Given the particular interest of the C3 test, which reconciles a very low La content and high performance, a narrow range in which α = y / x ranges from 0.67 to 0.77 is particularly advantageous. The most technically and economically interesting field is that defined by 0.17 <x <0.22 and 0.67 <α <0.77.

The invention advantageously makes it possible to have ferrites with a low content of element T in which the coefficient y is at most equal to 0.16, or even at most equal to 0.15, while at the same time retaining a very high level of performance. overall.

In addition, it is important to note that the ferrites according to the invention can be obtained under sintering conditions, in particular at a relatively low sintering temperature, and less than or equal to 1220 ° C and typically less than 1200 ° C, this which is economically advantageous.

All the ferrite tests according to the invention were carried out with M = Sr, R = La and T =

Co. However, the invention is not limited to this specific ferrite. Thus, for example, the element M can be a mixture of Sr and Ba, the atomic percentage of Sr ranging from 10% to 90% and that of Ba from 90% to 10%, and in which R =

La and T = Co.

In another embodiment of the invention, the atomic concentrations of the elements designated by T meet the condition [Co] / ([Co) + [Zn] + [Mn] + [Ni]> 30%, preferably> 50 % and even more preferably> 70%. In this embodiment, one can also choose M = Sr and R = La.

Another object of the invention is constituted by the use of a ferrite magnet according to the invention in an application requiring: - either a magnet having simultaneously a magnetic performance index IP greater than 590 mT and a high square character of the demagnetization curve, with typically a hjc≈ Hk / HcJ (%) ratio at least equal to 95%,

- or a magnet with a general performance index GIP at least equal to 580, and preferably at least equal to 585.

Another object of the invention consists of a method of manufacturing a magnet according to the invention in which: a) a mixture of precursors of the elements M, R, T and Fe is formed, corresponding to the stoichiometry of the formula M _1-x R _x Fe _12-y T _y O ₁₉ with the conditions: 0.15 <x <0.42 and 0.50 <α - y / x <0.90, b) the said mixture is calcined in temperature conditions and duration typically close to 1250 ° C. and 2 hours, so as to obtain a clinker, c) the said clinker is ground, with possible addition of additives, so as to obtain a powder with fine particles of average particle size less than 1 μm, d) said particles are subjected to a magnetic orienting field typically of 1T and sintered at a temperature typically ranging from 1150 to 1250 ° C, said temperature being chosen so as to be able to obtain a magnet having:

- either a general index of maximum GIP performance, typically at least equal to 580, and preferably at least equal to 585,

- or simultaneously a performance index IP = Br + 0.5. HcJ typically at least equal to 590 mT, and an index of the square character of the demagnetization curve hκ_ = Hk / HcJ (%), Hk corresponding to the field H for B = 0.9.Br, typically at least equal to 95% .

One can also apply to the invention the lessons brought by the manufacturing processes described in French applications No. 99 10295 and 99 15093 in the name of the applicant.

The examples which follow are given by way of illustration and are not intended to be limiting. EXAMPLES

The method described above was used for laboratory tests:

Step a):

The stoichiometric wet mixtures corresponding to the ferrite magnets of composition Srι _-x La _x Fe _{12 were produced} . _y Co _y O ₁₉ with the following values for x and y:

The following powders were used as raw materials:

- for the element Sr: SrCO ₃

- for the element La: La ₂ O ₃ in the form of powder of 1.07 m ² / g of specific surface (BET method) and an average particle diameter of 0.93 μm, diameter measured by the Fisher method,

- for the element Fe: Fe ₂ O ₃ in the form of powder of 3.65 m ² / g of specific surface and an average diameter of the particles of 0.96 μm,

- for the element Co: Co ₃ O ₄ in the form of a powder of 0.96 m ² / g of specific surface and an average diameter of the particles of 2.1 μm.

The powders were mixed in an aqueous mixer, the mixture was filtered, and then dried. The powder obtained was put into the form of pellets with a density of 2.5 kg / dm ³ using water as a binder (moisture content of 14% by weight), the pellets being dried before calcination.

Step b): the powder mixture is calcined at 1250 ° C. for 2 hours. A clinker was obtained having the following properties:

reaction.

Step c): the clinker obtained was ground in a humid environment with the addition - by weight - of:

- 0.52% SiO ₂ (in the form of a concentrated aqueous solution at 20%)

- 0.86% of CaCO ₃

- 0.95% of SrCO ₃

Granulometry of the pastes obtained: the particles have an average diameter between 0.58 μm and 0.62 μm, and a BET specific surface area between 10.3 and 11.2 m ² / g, so as to be able to make the measurements comparable of final properties of the magnets obtained.

Step d):

The particles after grinding were subjected to a magnetic orienting field typically of IT and sintered at temperatures of: 1180 ° C, 1205 ° C, 1220 ° C or 1240 ° C. Results obtained as a function of the sintering temperature T ° C with a holding time of 25 min:

Conclusions: if we compare the tests with reduced content of element T, all other things being equal, namely in particular the same value of x and of the sintering temperature (see for example the couples Cl-1 and C3-1, Cl-2 and C3-2, Cl-3 and C3-3), it is clear that the invention makes it possible to obtain simultaneously:

- less expensive ferrites, since it typically makes it possible to replace 30% of the cobalt with iron and to sinter the magnet at a relatively low temperature,

- and generally more efficient ferrites.

We can note in particular the very high performances, with GIP> 590, obtained in the case of tests C2-2, C3-3 and D2-2, the most economical ferrite being that corresponding to test C3-3.

Claims

1. Ferrite type magnet comprising a magnetoplumbite phase of formula Mι-χ R _x Fe ₁₂ _ _y T _y O ₁ in which: - M denotes at least one element chosen from the group consisting of: Sr, Ba, Ca and Pb,

- R denotes at least one element chosen by rare earths and Bi,

- T denotes at least one element chosen from Co, Mn, Ni, Zn

- 0.15 <x <0.42

- 0.50 <α = y / x <0.90 so as to have a ferrite magnet having simultaneously a reduced rate of element T and an overall performance index GIP = Br + 0.5. Hk at least equal to 580, and preferably at least equal to 585, Br being the residual induction expressed in mT, Hk corresponding to the field H expressed in kA / m, for B = 0.9.Br.

2. Magnet according to claim 1 wherein the coefficient x ranges from 0.15 to 0.32.

3. Magnet according to claim 2 wherein the coefficient x ranges from 0.17 to 0.22.

4. Magnet according to any one of claims 1 to 3 in which there is the relation: 0.60 <α - y / x <0.90, and preferably 0.65 <α = y / x <0.90.

5. Magnet according to claim 4 wherein there is the relationship 0.60 <α = y / x <0.80, and preferably 0.65 <α = y / x <0.80.

6. Magnet according to claim 5 wherein α = y / x ranges from 0.67 to 0.77.

7. Magnet according to any one of claims 1 to 6 in which the atomic concentrations of the elements designated by T meet the condition [Co] / ([Co] + [Zn] + [Mn] + [Ni])> 30%.

8. Magnet according to claim 7, in which the atomic concentrations of the elements designated by T meet the condition [Co] / ([Co] + [Zn] + [Mn] + [Ni])> 50%.

9. Magnet according to claim 7, in which the atomic concentrations of the elements designated by T meet the condition [Co] / ([Co] + [Zn] + [Mn] + [Ni]) ≥ 70%.

10. Magnet according to any one of claims 7 to 9 in which M = Sr and R = La.

11. Magnet according to any one of claims 1 to 10 wherein M is equal to a mixture of Sr and Ba, the atomic percentage of Sr ranging from 10% to 90% and that of Ba from 90% to 10%, and in which R = La. and T = Co.

12. Magnet according to any one of claims 1 to 10 in which M = Sr and R = La.

13. Magnet according to claim 12 wherein T = Co.

14. Use of a magnet according to any one of claims 1 to 13 in an application requiring a magnet which:

- either present simultaneously a magnetic performance index IP greater than 590 mT and a high square character of the demagnetization curve, with typically a hi ≈ Hk / HcJ (%) ratio at least equal to 95%. - either has a general GIP performance index at least equal to 580, and preferably at least equal to 585.

15. Method for manufacturing a ferrite type magnet comprising a magnetoplumbite phase of formula M _1-x R _x Feι _2-y T _y O ₁ in which:

M designates at least one element chosen from the group consisting of: Sr, Ba, Ca and Pb, - R denotes at least one element chosen by rare earths and Bi,

- T denotes at least one element chosen from Co, Mn, Ni, Zn said process comprising the following steps: a) a mixture of precursors of the elements M, R, T and Fe is formed, corresponding to the stoichiometry of formula M _{1 -x} R _x Feι _2-y T _y O ₁₉ with the conditions: 0.15 <x <

0.42, 0.50 <α = y / x <0.90, b) the said mixture is calcined under conditions of temperature and duration typically close to 1250 ° C. and 2 hours, so as to obtain a clinker, c ) the said clinker is ground, with possible addition of additives, so as to obtain a powder with fine particles of average particle size less than 1 μm, d) said particles are subjected to a magnetic orienting field typically of IT and sintered at a temperature typically ranging from 1150 to 1250 ° C, said temperature being chosen so as to be able to obtain a magnet having:

- or simultaneously a performance index IP = Br + 0.5. HcJ typically at least equal to 590 mT, and an index of the square character of the demagnetization curve hs = Hk / HcJ (%), Hk corresponding to the field H for B = 0.9.Br, typically at least equal to 95% .

16. Method according to claim 15, characterized in that the mixture of the precursors meets the condition 0.15 <x <0.32.

17. The method of claim 16, characterized in that the mixture of precursors meets the condition 0.17 <x <0.22.

18. Method according to one of claims 15 to 17, characterized in that the mixture of the precursors meets the condition 0.60 <α = y / x <0.90.

19. Method according to one of claims 15 to 18, characterized in that the mixture meets the condition 0.65 <α = y / x <0.90.

20. Method according to one of claims 15 to 19, characterized in that the mixture meets the condition 0.60 <α - y / x <0.80.

21. Method according to one of claims 15 to 20, characterized in that the mixture meets the condition 0.65 <α = y / x <0.80.

22. Method according to one of claims 15 to 21, characterized in that the mixture meets the condition 0.67 <α = y / x <0.77.

23. Method according to one of claims 15 to 22, wherein the sintering temperature of step d) does not exceed 1220 ° C.

24. The method of claim 23, wherein the sintering temperature of step d) is less than 1200 ° C.