Ferrite material with garnet structure with low sintering temperature for co-sintering with a silver or gold metal finish, and method for producing the ferrite material

Description

[0001] The field of the invention is that of microwave circulators and isolators. These non-reciprocal devices are used in the case of simultaneous transmission / reception or in isolator mode, in order to protect the source from impedance mismatches.

[0002] More specifically, the present invention relates to new materials and their production for integration into microwave circulators / insulators.

[0003] There figure 1 illustrates the diagram of a three-plate circulator consisting of 3 main elements: the ferrite 10, the cause of the circulator's non-reciprocity under the influence of a magnetic field. A garnet can advantageously be used for applications between 1 and 12 GHz; the conductive lines 11, which can be made of gold or silver; ground plans 12 the permanent magnet 13.

[0004] A challenge in the development of such devices, particularly those used in active scanning electronic antennas, is to reduce their cost and size without compromising their performance.

[0005] For this reason, these devices can be advantageously manufactured by co-sintering. This technique allows: to reduce the number of steps involved in assembly: (indeed, the various components of a circulator are mechanically assembled) and thus simplify the construction of a circulator using triple-plate or microstrip line technology ( microstrip) ; to eliminate the use of glue which impairs the performance of devices as well as their lifespan.

[0006] THE figures 2a et 2b to illustrate this point, diagrams respectively illustrate the triple-plate and microstrip line technologies (microstrip), or respectively a structure featuring a magnet 13, a central ferrite10 a Y Junction 11 and a site plan 12. There figure 2b highlights a site plan 12, a ferrite 10 and a Y junction 11.

[0007] The manufacturing process for these devices is lengthy and expensive, and the mechanical assembly of the various components is complex. To simplify this process, adapting the different materials to a co-sintering process could be a solution.

[0008] In order to cofritter with gold or silver, it is essential to adapt the central material, the ferrite, which is responsible for the non-reciprocal effect of the circulator when it is subjected to a magnetic field.

[0009] A ferrite material particularly well suited to the applications referred to in the present invention and in particular for applications in the frequency range [1GHz, 12GHz] is a ferrimagnetic yttrium iron garnet (YIG).

[0010] Since gold and silver are generally the metals considered for co-sintered metal tracks, and their melting points are 1064 and 962 °C respectively, it is crucial to adapt the sintering temperature of the ferrite. Yttrium iron garnet (YIG) has a high sintering temperature of 1450-1500 °C. Therefore, it cannot be co-sintered with gold or silver.

[0011] Studies published in patent applications EP1829061A1 and EP2622613A1, the thesis of L. Pinier (New ferrimagnetic garnets with low sintering temperatures for microwave applications. Doctoral thesis, École Polytechnique, 2006) and the thesis of L. Qassym (Study and development of garnet-structured ferrites with low sintering temperatures for integration into microwave circulators. Doctoral thesis, Université Paris-Saclay, 2017) have shown that it is possible to sinter garnet ferrites derived from the YIG below their initial sintering temperatures. More precisely: The Cu and Bi substituted YIG has a sintering temperature lowered to 950°C; the Cu, Bi, V substituted YIG has a sintering temperature lowered to 900°C.

[0012] It has thus been demonstrated that these new Bi-Cu-V substituted garnets can be co-frittered with gold, but, with a view to reducing costs, it becomes desirable to be able to co-fritter these new garnets with silver.

[0013] Indeed, the combined use of copper and bismuth allows for a lower sintering temperature. However, this reduction proves insufficient for the production of devices co-sintered with silver (the melting point of silver – 962°C – being too close to the sintering temperature – 950°C). Vanadium was therefore added as a further substitute. This effect on the sintering temperature is significant, and the densification of this material occurs below 900°C. In theory, this material would be compatible for co-sintering with silver. Unfortunately, this is not the result obtained. In fact, due to an affinity between vanadium oxides and silver at low temperatures, a diffusion phenomenon (and alteration of the metallic pattern) was observed by the Applicant, as shown in the images illustrated in figures 3a et 3b representative of test results with a silver deposit on a raw substrate corresponding to a substrate obtained after calcination and before sintering ( figure 3a ) and after sintering at 850°C ( figure 3b ).

[0014] These substitutions for vanadium, although allowing the sintering temperature to be sufficiently reduced, therefore do not allow for a ferrite that can be co-sintered with silver.

[0015] In this context, the Applicant proposes an alternative solution involving a novel ferrite material containing lithium. This new ferrite material can be advantageously obtained by adding lithium salt as a flux to calcined (or chamotted) ferrite powder, known as "raw" ferrite, prior to sintering. The flux phase, having a low melting point, facilitates the sintering process.

[0016] The lithium salt used can advantageously be, for example, lithium carbonate, its melting point being 723 °C.

[0017] More specifically, the present invention relates to a ferrite material characterized in that it corresponds to the following chemical formula: (1-x) (Y a TR b Bi b ,< Fe c Al d In e Ca f Cu g Zr h Co i Si j O 12±γ ) + x (Li 2 CO 3 or Li 2 O or LiF or LiCl) with x% by mass between 0.1 and 2, where TR is a rare earth element or a combination of rare earth elements not including element Y. − 1 ≤ γ ≤ 1 ; 3 a + b + b ′ + c + d + e + 2 f + g + i + 4 h + j = 24 ± 2 γ 0 ≤ a ≤ 2 ; 0 ≤ b ≤ 1,5 ; 0,1 < b ′ < 3 ; 4 ≤ c ≤ 5 ; 0 ≤ d ≤ 1,5 ; 0 ≤ e ≤ 0,8 ; 0 ≤ f ≤ 1 ; 0 < g < 0,05 0 ≤ h ≤ 1 ; 0 ≤ i ≤ 0 ,5 ; 0 ≤ j ≤ 0 ,5 . Advantageously the Bi rate can be relatively high and at least greater than or equal to 0.67.

[0018] The invention also relates to a component comprising the ferrite material according to the invention and metallizations advantageously in silver. The component may be a circulator or an insulator.

[0019] The invention also relates to a method for manufacturing a ferrite material according to the invention, said method comprising: the weighing of raw materials of the oxide or carbonate type; a first grinding of said raw materials; a calcination step (also called chamotte) of said ground raw materials at a temperature between 750°C and 850°C allowing to obtain a powder of raw ferrite material; a second grinding of said powder of raw ferrite material; the addition of a flux of lithium salt being Li 2 CO 3 , Li 2 O, LiF or LiCl , to said powder of raw ferrite material.

[0020] The ferrite material resulting from the raw ferrite material powder and lithium salt flux can advantageously be sintered at a temperature between 850°C and 950°C.

[0021] The invention also relates to a method for manufacturing a component comprising several elements, including a ferrite material according to the invention, characterized in that: said ferrite material is prepared according to the process of the invention; a sintering operation is carried out during which said ferrite is bonded to at least one of said other elements of said component or to a precursor (gold or silver metal, low sintering temperature dielectric, low sintering temperature ferrite ...) of said element by a co-sintering process.

[0022] In the component, at least one of the other elements is a layer of metal and advantageously silver.

[0023] The invention will be better understood and other advantages will become apparent upon reading the following description, given by way of example only, and with reference to the accompanying figures, among which: [ Fig 1 ] illustrates the diagram of a three-plate circulator according to the known art; [ Fig 2a ] illustrates a three-plate circulator according to the known art that can be made by co-sintering; [ Fig 2b ] illustrates a microstrip line circulator (microstrip) according to the known art that can be achieved by co-fritting; [ Fig 3a ] illustrates annealing test results with a silver deposit on a raw substrate, made from a prior art ferrite comprising a vanadium substituent; [ Fig 3b ] illustrates test results of a prior art ferrite casing with the addition of a vanadium substitute with a silver deposit, after sintering at 850°C; Fig 4 ] provides the evolution of shrinkage in % as a function of sintering temperature for various ferrites of the known art and for ferrites of the invention; [ Fig 5 ] illustrates the permittivity values ​​of ferrite materials of the invention as a function of frequency expressed in Hertz; [ Fig 6a ] illustrates the results of annealing tests with a silver deposit on a raw substrate and of formwork tests carried out using an example of ferrite B of the invention; [ Fig 6b ] illustrates the results of annealing tests with a silver deposit on a raw substrate and of formwork tests carried out from an example of ferrite E of the invention.

[0024] In general, the ferrite material of the invention corresponds to the following chemical formula: (1-x) (Y a TR b Bi b ,< Fe c Al d In e Ca f Cu g Zr h Co i Si j O 12±γ ) + x (Li 2 CO 3 or Li 2 O or LiF or LiCl) with x% by mass between 0.1 and 2, where TR is a rare earth element or a combination of rare earth elements not including element Y. − 1 ≤ γ ≤ 1 ; 3 a + b + b ′ + c + d + e + 2 f + g + i + 4 h + j = 24 ± 2 γ 0 ≤ a ≤ 2 ; 0 ≤ b ≤ 1 ,5 ; 0 ,1 < b ′ < 3 ; 4 ≤ c ≤ 5 ; 0 ≤ d ≤ 1 ,5 ; 0 ≤ e ≤ 0,8 ; 0 ≤ f ≤ 1 ; 0 < g < 0,05 0 ≤ h ≤ 1 ; 0 ≤ i ≤ 0,5 ; 0 ≤ j ≤ 0,5 .

[0025] It should be noted that the ferrite material conforming to the chemical formula of the present invention can be either a raw ferrite material or a sintered material. In the case of a raw material, the material can be in the form of a cast strip, 50 to 200 µm thick, preferably approximately 100 µm thick. In this case, the material consists mainly of ferrite and a flux, which can be Li₂CO₃, LiF, or LiCl.

[0026] In the case of a sintered material, the material will consist mainly of ferrite and Li2O.

[0027] Rare earth elements can be used to modify saturation magnetization or improve power handling.

[0028] The use of copper and bismuth makes it possible to lower the sintering temperature.

[0029] The introduction of zirconium allows the saturation magnetization to be adapted (increased or decreased).

[0030] Bismuth can also be used to increase permittivity.

[0031] Calcium can be used to counteract excess positive charges resulting from the introduction of zirconium. It can also help stabilize the lattice when using a high bismuth content, for example.

[0032] Substituting aluminum for iron can reduce linewidths and decrease saturation magnetization.

[0033] The ferrite materials of the present invention are produced using a conventional ceramic synthesis process. The different stages are: The process involves weighing the raw materials; grinding the previously obtained mixture; a calcination step, also called chamotte grinding, aimed at synthesizing the garnet phase into a powder and obtaining a raw ferrite powder; a second grinding step; and the addition of a lithium-based flux: the flux is added to the calcined phase (the garnet phase formed by heat treatment). It is added to the calcined phase using a mixer (paddle mixer, attitor, jar, etc.).

[0034] After the addition of flux, standard shaping and sintering steps can be carried out. Sintering the reground chamotte powder at a higher temperature aims to densify the ceramic while giving it the desired shape.

[0035] The first and second grinding are advantageously carried out in a humid environment.

[0036] The sintering of the reground powder is advantageously carried out under air or oxygen.

[0037] Grinding operations are advantageously carried out with a ball mill and / or by attrition.

[0038] The performance obtained is described below in the context of an example, not in accordance with the invention, involving the use of lithium carbonate as a flux and in comparison with ferrite materials not including the addition of lithium-based flux.

[0039] To highlight the effect of using fluxing lithium carbonate, four materials were compared. The reference material chosen was bismuth- and copper-substituted yttrium iron garnet (sintering temperature of 950 °C): Ferrite A : Y 2.28 Cu 0.05 Bi 0.67 Fe 5 O 12 .

[0040] The Applicant also used a raw Y 2.28 Cu 0.05 Bi 0.67 Fe 5 O 12 ferrite during the lithium-based flux addition operation to produce the following ferrites: Ferrite B obtained from Y 2.28 Cu 0.05 Bi 0.67 Fe 5 O 12 + 0.16% by mass of Li 2 CO 3; Ferrite C obtained from Y 2.28 Cu 0.05 Bi 0.67 Fe 5 O 12 + 0.32% by mass of Li 2 CO 3; Ferrite D obtained from Y 2.28 Cu 0.05 Bi 0.67 Fe 5 O 12 + 0.65% mass of Li 2 CO 3 .

[0041] The Applicant also made comparisons with Ferrite E : Y 2.23 Cu 0.05 Ca 0.05 Bi 0.67 Fe 4.95 V 0.05 O 12 .

[0042] The Applicant carried out thermomechanical analysis measurements on ferrite cylinders to monitor the material shrinkage during the heat treatment process. The technique consists of measuring the change in cylinder height during the sintering cycle using an alumina probe.

[0043] Curves C4A, C4B, C4C, C4D, C4E illustrated in figure 4 show the evolution of shrinkage (following the cylinder thickness) in % as a function of sintering temperature for the different ferrite materials respectively. A, B, C, D And E. During these tests, we monitor the evolution of shrinkage during sintering, as a function of the sintering temperature.

[0044] The dilatometry curves make it possible to highlight the impact on densification of the addition of melting lithium carbonate.

[0045] In comparison with the ferrite material A,the sintering operation for materials B, C, D And E It starts at a low temperature (between 750 and 800°C compared to over 850°C for the ferrite material). A ).

[0046] We also note that when the lithium content increases from 0.16% to 0.32%, sintering begins at a lower temperature, whereas it appears to stabilize at a level of 0.65%, in the example concerned.

[0047] These initial results have highlighted the effectiveness of the molten lithium carbonate in reducing the sintering temperature.

[0048] Another aspect to consider concerns the impact of melting lithium carbonate on the electromagnetic properties of the material.

[0049] Curves C 5B, C 5C and C 5D of the figure 5 provide the permittivity values ​​of ferrite materials B, C And D,illustrating the measurement results of the saturation magnetization of different materials as well as their dielectric properties at low frequency (between 1MHz and 1GHz).

[0050] The permittivity values ​​do not appear to be affected by the addition of lithium flux with a coefficient ε' of about 21, a high value due to the presence of bismuth in the lattice (as described in a previous patent application filed by the applicant and published under number FR 3044309).

[0051] The dielectric losses of these materials are on the order of a few 10⁻³, values ​​that are quite acceptable for the type of application targeted and of the same order of magnitude as those of ferrite materials. A And E.

[0052] The Applicant also carried out co-sintering tests. The parts were sintered at 850°C, the recommended sintering temperature for metallic inks.

[0053] In order to make a comparison, tests are carried out on the materials B And E (exhibiting sufficient density when sintered at low temperature).

[0054] THE figures 6a et 6b are respectively related to the ferrite material B and to the ferrite material E.

[0055] More specifically, the figures on the left relate to annealing (calcining) tests with silver on the materials B And E Raw, the figures on the right relate to materials tests B And E sintered at 850°C.

[0056] The co-sintering of the material E and silver highlights diffusion. This diffusion is only observed for the material E. In the case of ferrite co-sintering B With silver, the screen-printed design remains intact.

[0057] The effectiveness of adding Li2CO3 was also observed for garnets substituted with copper, bismuth, calcium, aluminum and gadolinium.

[0058] The direct application of the present invention is the production of circulators and insulators using LTCC (Low Temperature Cofired Ceramics) technology or other additive manufacturing technologies. The present invention simplifies complex assemblies through co-sintering, reducing the number of manufacturing steps. Furthermore, it eliminates the need for adhesives, which degrade the properties and make the technology less durable.

[0059] It is also recalled that until now it was not possible to co-frit a garnet ferrite with silver (problem of sintering temperature or compatibility with silver), the present invention makes it possible to overcome the diffusion problem and makes it possible to co-frit the aforementioned garnet and silver at less than 900 °C (saving time and cost).

Claims

1. A ferrite material, characterised in that it has the following chemical formula:         (1-x) (YaTRbBib,FecAldIneCafCugZrhCoiSijO12±y) + x (Li2CO3 or Li2O or LiF or LiCI) with x mass percent ranging between 0.1 and 2, with TR being a rare earth or a combination of rare earths which does not comprise the element Y, − 1 ≤ γ ≤ 1 ; 3 a + b + b' + c + d + e + 2 f + g + i + 4 h + j = 24 ± 2 γ; 0 ≤ a ≤ 2 ; 0 ≤ b ≤ 1.5 ; 0.1 < b' < 3 ; 4 ≤ c ≤ 5 ; 0 ≤ d ≤ 1.5 ; 0 ≤ e ≤ 0.8 ; 0 ≤ f ≤ 1 ; 0 < g < 0.05 ; 0 ≤ h ≤ 1 ; 0 ≤ i ≤ 0.5 ; 0 ≤ j ≤ 0.5 .

2. The ferrite material according to claim 1, wherein 0.67 ≤ b' < 3.

3. A component comprising the ferrite material according to any of claims 1 or 2 and metallisations that can be made of silver.

4. A circulator comprising the ferrite material according to any of claims 1 or 2 and metallisations that can be made of silver.

5. An insulator comprising the ferrite material according to any of claims 1 or 2 and metallisations that can be made of silver.

6. A method for manufacturing a ferrite material according to any of claims 1 or 2, said method comprising: - weighing raw materials such as oxides or carbonates; - a first step of milling said raw materials; - a step of calcinating (also called firing) said milled raw materials at a temperature ranging between 750 °C and 800 °C in order to obtain the raw ferrite material; - a second step of milling said raw ferrite material powder; - adding a lithium salt flux, which can be Li2CO3, Li2O, LiF or LiCl, to said raw ferrite material powder.

7. A method for manufacturing a component containing several elements including a ferrite material according to any of claims 1 or 2, characterised in that: - said ferrite material is prepared using the method according to claim 6; - a sintering operation is carried out, during which operation said ferrite material is consolidated with at least one of said other elements of said component or with a precursor of said element using a co-sintering method.

8. The method for manufacturing a component according to claim 7, characterised in that at least one of the other elements is a metal layer.

9. The method for manufacturing a component according to claim 8, characterised in that the metal is silver.