Process for producing a high-temperature ceramic/metal seal - Architecture- microstructure of the composition employed

The invention relates to the technological field of ceramic/metal joints. At the present time, many known solutions are applied in the production of

SOFCs (solid-oxide fuel cells). These solutions involve the use either of simple glasses or crystallizable glasses or glass/ceramic mixtures.

When simple glasses are used in solid form at the use temperature, their physical properties, such as their thermal expansion coefficient (TEC), must be matched to the other elements of the joint, especially the metal and ceramic parts.

Thus, R. Zheng et al. have developed formulations of the SiO ₂ -CaO-

B ₂ O ₃ AI ₂ O ₃ type (R. Zheng, S.R. Wang, H.W. Nie and T.-L. Wen, J. of Power

Sources, Vol. 128, pages 165-172); K. Eicher et al. and R. Loehman et al. have developed formulations of the BaO-AI ₂ O ₃ -SiO ₂ type (K. Eichler, G. Solow, P. Otschick and W. Schaffrath; 1999, J. of Euro. Ceram. Soc, VoI 19, pages 1101-1104; R. Loehman, M. Brochu, B. Gauntt, R. Shah, D. Zschiesche and H-P Dumm, SECA Core technology programme review, 11-14 May 2004); and R. Cassidy has developed formulations of the LiO ₂ -AI ₂ O ₃ -SiO ₂ type (R. Cassidy, United States Patent 4 921 738). However, the glasses that have the highest softening point generally have a thermal expansion coefficient different from those of metal alloys, metals and ceramics. In addition, I. D. Bloom et al. have shown that glasses with a high thermal expansion coefficient often contain water-soluble compounds, for example boron oxide B ₂ O ₃ or phosphorus oxides P ₂ O ₅ (I. D. Bloom and K.L. Ley, 1995, United States Patent 5 453 331 ) thereby accelerating their rate of evaporation in atmospheres containing water vapour. In the same patent, I. D. Bloom et al. propose the use of a glass that is fluid at the operating temperature in order to make a perfect seal, ensuring perfect contact at the interfaces and having no defects, such as cracks or pores. However, this technique is limited in terms of mechanical strength, since the sealing material can flow under the effect of pressure. The composition must therefore be adjusted so that its softening temperature lies within the use range and so that the thermal expansion coefficient of the solid glass is close to those of

the constituent materials of the joint, at low temperature, so as to avoid any cracking. Now, over time, crystallization of the glass reduces its fluidity and modifies its properties. Under these conditions, the characteristics of the crystal formed are difficult to control as they depend essentially on the initial composition and on the thermal cycle undergone by the material. Crystallization-limiting agents may be needed to keep the glass in the fluid state. Although this approach has a certain advantage in terms of gas-tightness, it has however proved to be a handicap in terms of mechanical strength of the material under a large pressure difference. The two approaches, using a single glass, whether solid or fluid at the use temperature, are therefore unsatisfactory.

H. McCollister et al. have shown that by controlling the crystallization of the glass it is possible to improve the mechanical strength and the chemical resistance of the joint material and that the two parameters to be controlled are, in this case, the formulation of the glass and the thermal cycle to be applied to it in order to form a crystalline phase possessing the desired properties. (H. McCollister and ST. Reed, 1983, United States Patent 4 414 282). This approach has led to the development of compositions of the following type:

- LAS (Li ₂ O-AI ₂ O ₃ -SiO ₂ ; United States Patent 4 921 738);

- BAS (BaO-AI ₂ O ₃ -SiO ₂ ; K. Eichler, G. Solow, P. Otschick and W. Schaffrath, 1999, J. of Euro. Ceram. Soc, VoI 19, page 1101-1104;

K.D. Meinhardt et al., United States Patent US 6 430 966 B1 ); and

- BCAS (barium calcium aluminosilicate; Z. Yang et al., 2003, Solid State Ionics, Vol. 160, 213-225).

These compositions are more stable than uncrystallized glasses. Nucleating agents and/or a particular heat treatment are used to promote the growth of a phase whose thermal expansion coefficient is matched to the other components of the joint (N. Lahl, K. Singh, L. Singheiser, K. Hilpert and D. Bahadur, 2000, J. Mat.

Sci., VoI 35, pages 3089-3096; and R. Loehman, M. Brochu, B. Gauntt, R. Shah,

D. Zschiesche and H-P Dumm, SECA Core technology programme review, 11-14 May 2004). However, refining the formulation and the heat treatment is tricky as it is necessary to provide for phases liable to be formed and to evolve over time. In addition, modifying the characteristics of the sealing material during creation of the interfaces between the various components is a source of defects

that may result in leaks. This approach is therefore potentially useful for controlling the characteristics of the sealing material, but it does require complex and lengthy development of the formulation and of the heat treatment, which at the present time precludes it from being implemented on an industrial scale. A simple means of modifying the characteristics of a glass is to add a crystallized phase to it (U. Balachandran, JT. Dusek, J. E. Emerson, P. S. Maiya and J.J. Picciolo, 1998, United States Patent 5 725 218). This type of sealing is commonly used in electronics in order to match the thermal expansion coefficients of glasses to the metal components used for electronic conducting joints. Chou et al. (Y-S. Chou and J.W. Stevenson, 2002, J. of Power Sources,

VoI 112, pages 376-383; Y-S. Chou and J.W. Stevenson, 2002, Advances in joining of ceramics, pages 175-184; Y-S. Chou, J. W. Stevenson and L.A. Chick, 2002, J. of Power Sources, Vol. 112, pages 130-136; and S. P. Simner and J.W. Stevenson, 2001 , Journal of Power Sources, Vol. 102, pages 310-316) have produced, and studied its behaviour, a stack of mica single crystals or sheets as sealant for an SOFC of planar geometry. Localized leaks at the interfaces between the mica and the components of the joint have led the authors to add a glass layer, which improves the sealing. Application of a compressive force also reduces the leaks. R. Loehman et al. have studied the spreading and the variation in the wetting angle, viscosity and thermal expansion coefficient at high temperature of mixtures of glasses and ceramic particles of various shapes and sizes. The wetting angle depends on the quantity and the shape of the particles introduced. The viscosity increases with the reduction in size of the ceramic particles and their concentration. Since the intended application is for fuel cells of the SOFC type, a metal is added to ensure electrical conduction of the joint. The TEC of the mixture increases with the addition of ceramic particles and may vary because of an onset of crystallization of the glass R. Loehman, M. Brochu, B. Gauntt, R. Shah, D. Zschiesche and H-P Dumm, SECA Core technology programme review, 11-14 May 2004).

A similar approach is presented by G. L. Larsen et a/., the objective being to control the viscosity, the flow temperature and the thermal expansion coefficient of the sealing material by adding ceramic particles. This approach is similar to the

use of a glass that is fluid at the use temperature. In this case, control of the glass properties, such as the glass transition temperature and the thermal expansion coefficient, is facilitated by the incorporation of a ceramic phase rather than by adjusting its chemical composition. However, a large quantity glass phase may pose a corrosion or evaporation problem at high temperature and a mechanical strength problem if the pressure different is very large (G. L. Larsen, P. H. Larsen and C. Bagger, 2004, United States Patent 6 828 263 B1).

Shutz et al. have studied glass/ceramic-based mixtures. For applications involving gas separation, the ceramic is identical to that used for producing a membrane. Compositions containing 50 vol% to 80 vol% ceramic seem suitable for their application and provide unexpected sealing properties (J. B. Schutz and, T.F. Barton, 2002, United States Patent US 6,402,156 B1).

The inventors have sought, during their research, both on trying to improve the stability of a joint between a ceramic and a metal alloy and on trying to improve the gas-impermeability of the seal in the temperature range from about 20 ⁰ C to about 900 ⁰ C, so as to separate two atmospheres, one an oxidizing atmosphere and the other a reducing atmosphere, and with a pressure difference extending from 0 up to 3 x 10 ⁶ Pa (30 bar) on either side of said joint, to develop an architecture/microstructure of this joint that improves the thermal and chemical stability and the mechanical strength of the seal under the abovementioned use conditions.

One subject of the invention is a process for producing a ceramic/metal joint between a ceramic part and a part made of metal or a metal alloy, by means of a material, called sealing material, characterized in that said sealing material consists, for 100% of its weight, of:

- 10 wt% to 90 wt% of a glass or a glass mixture (G); and

- 10 wt% to 90 wt% of at least one ceramic (C) of chemical nature identical to or different from that of the ceramic part to be bonded; and in that said process comprises the following successive steps: either

- a step (a) of preparing a powder blend (B) consisting, for 100% of its mass, of:

- 10 wt% to 90 wt% of a glass or a glass mixture (G), and

- 10 wt% to 90 wt% of at least one ceramic (C), from a glass powder (G) and a ceramic powder (C);

- a step (b) of pressing the preform of the sealing material, obtained from the powder blend (B) prepared in step (a);

- a step (c) of densifying the preform of the sealing material prepared in step

(b);

- a step (d) of positioning the elements of the ceramic joint that are complementary to the preform of said sealing material, the preform being densified in step (c); and

- a step (e) of positioning heat treatment of the seal prepared positioned in step (d), or else

- a step (b) of pressing a preform of a powder of at least one ceramic (C); - a step (c ¹ ) of partially densifying the preform prepared in step (b);

- a step (d) of positioning the elements of the ceramic/metal joint that are complementary to the preform of said ceramic material (C), said preform being densitifed in step (c); and

- a step (e ¹ ) of heat treating the preform positioned in step (d) so as to cause the glass or glass mixture (G) to infiltrate into the porosity of said preform, in order to form said constituent sealing material of the ceramic metal joint.

The term "ceramic part" is understood mainly to mean a part made of an ionic conductor, preferably an ionic and electronic conductor, said ceramic more preferably comprising at least one crystal lattice having at least one oxygen vacancy, said ceramic being, even more preferably, chosen from ceramics having a perovskite crystal structure and cerium oxides.

The expression "metal or metal alloy part" is understood to mean mainly a part made of a metal or a metal alloy exhibiting an oxidation resistance in an oxidizing atmosphere up to 900 ⁰ C, a reduction resistance in a reducing atmosphere up to 900 ⁰ C, a creep strength up to 900°C, a melting point of 900 ⁰ C or higher and a TEC within the [20°C to 900°C] temperature range of between 8 x 10 ^{" 6} /°C and 25 x 10 ^"6 /°C, preferably between 10 ^"5 /°C and 18 x 10 ^"6 /°C. The TEC is determined by means of a dilatometer. This apparatus is a furnace combined with

a displacement sensor. A parallelepipedal specimen is introduced and its length variation recorded when the temperature is raised at a controlled rate. The TEC is defined in a given temperature range and corresponds to the length change divided by the initial length of the specimen. Step(a) of preparing the powder blend (B) in proportions of glass (G) and ceramic (C) may be carried out starting from a glass powder and a ceramic powder, each being already finely divided and dispersed, that is to say prepared independently of each other, in terms of particle size, specific surface area and particle size distribution. Step (a _g ) of pretreating the glass powder (G) and step (a _c ) of pretreating the ceramic powder (C) include, independently of each other, one or more milling steps in order to obtain the desired particle size distribution, and optionally a heat treatment, in order to remove any organic components present in one or other of these powders. Where appropriate, said heat treatment is carried out either before the powder is milled, or after the powder has been milled, this is the glass powder, and before the powder is milled when it is the ceramic powder C. This method of preparation and the relative proportions of ceramic and glass make it possible to obtain a microstructure consisting of a ceramic backbone, the pores of which are filled with the glass. Once they have been prepared, the two powders are then blended together, the blend being homogenized by rotation, for example using a Turbula device, which is a device for stirring the powders, or in a jar, possibly in a liquid medium, such as water, in order to obtain a homogeneous suspension of the two components. If necessary or if desired, the liquid medium is then removed by heating, before the preform is prepared.

It is also possible to carry out step (a) of the process as defined above without the prior treatment of the powders. In this case, it is then necessary to make the blend (B) obtained undergo a possible heat treatment, in order to remove the organic components possibly present, and one or more milling and/or attrition steps in order to end up with the desired particle size distribution before preparing the preform. Step (b) of pressing the preform of the sealing material based on the powder blend (B) prepared in step (a) or by pressing a preform of a powder of at least one ceramic (C), is generally carried out by uniaxial pressing or by isostatic

pressing. The second method allows more homogeneous compaction, but the first is easier to implement.

The powder blend (B) or, where appropriate, the ceramic powder (C), is optionally dried in order to facilitate the pressing operation. An organic binder, for example PEG 6000, may optionally be added to the powder in order to favour the green strength.

Step (c) of densifying the preform obtained in step (b) generally comprises a densification heat treatment in order to minimize the volume shrinkage during the subsequent production of the seal. This precaution improves the adhesion between the sealing material and the parts to be joined and to ensure that the entire volume is filled. The densification thermal cycle comprises a solvent- removal hold, allowing the organic substances used for preparing the powder and for pressing it to be removed, and a densification hold in air, with the rates of temperature rise and fall being controlled. In the case preparing a ceramic preform with no glass, a partial densification step (c ¹ ) is carried out by adjusting the temperature and the duration of the temperature hold in order to prevent complete densification. Step (c ¹ ) may also consist in adding a pore former to the preform obtained in step b). Step (c ¹ ) also consists in adding a pore former (an organic compound) which is removed during a thermal cycle identical to that of step c as defined above, namely a solvent-removal hold and a densification hold.

Step (d) of positioning the elements of the ceramic/metal joint that are complementary to the preform of said sealing material densified in step (c), or where appropriate (c ¹ ), comprises the assembling of said sealing material or its precursor, the ceramic part and the metal or metal alloy part and the ceramic or ceramic precursor part, said assembling operation generally including a setting step whereby the sealing material is kept in place in the zone for joining to the metal or metal alloy part. When the ceramic part is a tube, the metal part is for example a socket, and the preform of said sealing material is an annular part. Step (d) optionally includes a step (di) of at least partial preoxidizing one surface of the joint zone of the metal or metal alloy part, especially a socket, so as at least partially to form at least one layer of at least one metal oxide on said surface.

Step (e) of positional heat treatment of the seal prepared in step (d) allows the preform to be deformed so as to fill the volume between the parts to be joined,

to ensure formation of the interfaces between the sealing material and the parts to be joined, and to produce bridges between the ceramic particles in order to confer mechanical properties on the sealing material. The softening temperature of the sealing material is controlled by the prior steps of the process, namely the powder and preform preparation steps. If necessary, pressure may be applied to the sealing material so as to encourage it to spread and/or to reduce the temperature for producing the joint. The thermal cycle for this treatment generally comprises two holds. The first hold allows the preform to flow in order to fill the space between the parts to be bonded and to form the interfaces. The temperature and duration of this hold depend on the characteristics of the blend. The second hold, for annealing the glass, makes it possible to limit the thermal stresses that may be created as the glass solidifies. These heat treatments are carried out either in air or in a controlled atmosphere. The spread of the preform may be modified according to the atmosphere used. Where appropriate, a heat treatment step (e ¹ ) for infiltrating the glass or glass mixture (G) into the pores of the preform positioned in step (d), in order to form said constituent sealing material of said ceramic/metal joint, consists in supplying the glass either using a preform or powder placed above the porous ceramic, or by pouring molten glass. The first technique seems more favourable for avoiding the thermal shocks to the ceramic tube. Infiltration of the glass sometimes requires a particular atmosphere in order to improve its wettability, and therefore its flow into the pores. This atmosphere may be a gas or a vacuum. Two temperature holds are generally necessary, the first for ensuring that the glass flows and the second for relieving the stresses that may appear as the glass solidifies.

The expression "glass or glass mixture" is understood to mean, within the process as defined above, any glass or glass mixture which is compatible with the oxides formed on the surface of the metal or of the metal alloy, which is resistant to reduction in a reducing atmosphere at temperatures up to 900 ⁰ C and resistant to oxidation in an oxidizing atmosphere at temperatures up to 900 ⁰ C, and which, when mixed with the ceramic C in the claimed proportions, results in a mixture that preferably possesses a TEC that is higher than the TEC of the metal or the metal alloy and lower than the TEC of the ceramic.

According to one particular aspect of the process as defined above, glass or glass mixture is used that has a thermal expansion coefficient of between 3 x 10 ^~6 and 15 x 10 ^"6 /°C between 20 ⁰ C and 500 ⁰ C.

The expression "ceramic of chemical nature identical to or different from that of the ceramic part to be bonded" is understood to mean, within the process, in particular a material (A) comprising, for 100% of its volume:

(i) - at least 75% by volume and at most 100% by volume of a compound (Ci) chosen from doped ceramic oxides which, at the use temperature, are in the form of a crystal lattice with oxide ion vacancies of perovskite phase, of formula (I): Med.™ Mα'x Mα" _u Mβi _-y-v Mβ' _y Mβ" _v O _3- w (I) in which:

- Ma represents an atom chosen from scandium, yttrium or from the families of lanthanides, actinides or alkaline-earth metals;

- Ma' which differs from Ma, represents an atom chosen from scandium, yttrium or from the families of lanthanides, actinides or alkaline-earth metals;

- Ma" which differs from Ma and Ma', represents an atom chosen from aluminium (Al), gallium (Ga), indium (In), thallium (Tl) or from the family of alkaline-earth metals; - Mβ represents an atom chosen from transition metals;

- Mβ' which is different from Mβ, represents an atom chosen from transition metals, aluminium (Al), indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb) or titanium (Ti);

- Mβ" which differs from Mβ and Mβ', represents an atom chosen from transition metals, metals of the alkaline-earth family, aluminium (Al), indium

(In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb) or titanium (Ti);

- 0 < x < 0.5;

- 0 < u < 0.5; - (x + u) < 0.5;

- 0 ≤ y ≤ 0.9;

- 0 ≤ v ≤ 0.9;

- 0 < (y + v) < 0.9

and w is such that the structure in question is electrically neutral; (ii) - optionally up to 25% by volume of a compound (C2), which differs from compound (Ci), chosen either from oxide-type materials such as boron oxide, aluminium oxide, gallium oxide, cerium oxide, silicon oxide, titanium oxide, zirconium oxide, zinc oxide, magnesium oxide or calcium oxide, preferably from magnesium oxide (MgO), calcium oxide (CaO), aluminium oxide (AI2O3), zirconium oxide (Zrθ2), titanium oxide (TϊO2) or ceria (Ceθ2); strontium-aluminium mixed oxides SrAI ₂ O ₄ or Sr ₃ AI ₂ Oe; barium-titanium mixed oxide (BaTiOs); calcium- titanium mixed oxide (CaTiOs); aluminium and/or magnesium silicates, such as mullite (2SiO ₂ .3AI ₂ O ₃ ), cordierite (Mg ₂ AI ₄ Si ₅ Oi ₈ ) or the spinel phase MgAI ₂ O ₄ ; calcium-titanium mixed oxide (CaTiOs); calcium phosphates and their derivatives, such as hydroxylapatite Caio(PO ₄ )6(OH) ₂ or tricalcium phosphate Cas(PO ₄ ) ₂ ; or else materials of the perovskite type, such as LaosSro sFeo θTio iOs-δ, Lao εSro ₄ Feo θGao 103-δ, Lao sSro sFeo θGao 103-δ or Lao εSro ₄ Feo θTio 103 _; δ . or else from materials of the non-oxide type, preferably chosen from carbides or nitrides such as silicon carbide (SiC), boron nitride (BN), aluminium nitride (AIN) or silicon nitride (Si ₃ N ₄ ), "sialons" (SiAION), or from nickel (Ni), platinum (Pt), palladium (Pd) or rhodium (Rh); metal alloys or mixtures of these various types of material; and,

(iii) - optionally up to 2.5% by volume of a compound (Ci _-2 ) produced from at least one chemical reaction represented by the equation: xFci + yF _C 2 > zFci _-2 , in which equation F _C i, F _C2 and F _C i _-2 represent the respective raw formulae of compounds (Ci), (C ₂ ) and (Ci _-2 ) and x, y and z represent rational numbers greater than or equal to O. As examples of material (A) used in the process as defined above, there are those in which the volume proportion of compound (Ci _-2 ), optionally present, tends toward O and/or those in which the volume proportion of optionally present compound (C ₂ ) is greater than or equal to 0.1 % and less than or equal to 10 %.

Among constituent compounds (Ci) of material (A), there are for example compounds (Ci) chosen: either from compounds of formula (Ia):

Lai _-x-u Mα'χMα" _u Mβi _-y- vMβ' _y Mβ"vθ3-w (Ia), corresponding to formula (I), in which Ma represents a lanthanum atom;

or from compounds of formula (Ib):

Mαi _-x-u Sr _x Mα" _u Mβi _-y- vMβ' _y Mβ"vO ₃ -w (Ib), corresponding to formula (I), in which Ma' represents a strontium atom; or from compounds of formula (Ic): Mαi-χ _-u Mα'χMα" _u Fei-y-vMβ' _y Mβ"vO ₃ -w (Ic), corresponding to formula (I), in which Mβ represents an iron atom; and among them there are for example compounds (Ci) chosen: either from compounds of formula (Id):

Lai-χ _-u Sr _x Mα" _u Fei-y-vMβ' _y Mβ"vO ₃ -w (Id), corresponding to formula (Ia) in which Ma' represents a strontium atom and Mβ represents an iron atom; or from compounds of formula (Ie):

Lai _-x-u Mα'χAl _u Fei _-y- vMβ' _y Mβ" _v O ₃ -w (Ie), corresponding to formula (Ia) in which Ma" represents an aluminium atom and Mβ represents an iron atom; or from compounds of formula (If):

Lai _-x Sr _x Fei _-y Mβ' _y O _3-w (If), corresponding to formula (Ia) in which Ma' represents a strontium atom, Mβ represents an iron atom and u and v are equal to 0; or from compounds of formula (Ig):

Lai _-u Ca _u Fei _-y Mβ' _y O _3-w (Ig), corresponding to formula (Ia) in which Ma' represents a calcium atom, Mβ represents an iron atom and x and v are equal to 0; or from compounds of formula (Ih): Lai _-u Ba _u Fei _-y Mβ' _y O _3-w (Ih), corresponding to formula (Ia) in which Ma' represents a barium atom, Mβ represents an iron atom and x and v are equal to 0; or from compounds of formula (Ii):

Lai _-x-u Sr _x Ca _u Fei _-y-v Mβ' _y Mβ" _v O _3-w (N), corresponding to formula (Id) in which Ma" represents a calcium atom; or from compounds of formula (Ij):

Lai _-x-u Sr _x Ba _u Fei _-y-v Mβ' _y Mβ" _v O _3-w (Ij), corresponding to formula (Id) in which Ma" represents a barium atom.

As such compounds, mention may be made of compounds (Ci) chosen from compounds of formulae: Lai-xSrxFei-yGayOs-w, Lai-xSrxFei-yTϊyOs-w, Lai _-x Sr _x Feθ3-w, Lai _-u Ca _u Fei _-y Gavθ3-w,

Lai _-u Ca _u Fei _-y Tiyθ3-w, Lai _-u Ca _u Feθ3-w, Lai _-u Ba _u Fei _-y Gavθ3-w, Lai _-u Ba _u Fei _-y Tiyθ3-w, Lai _-u Ba _u Feθ3-w, Lai-x-uSrxAluFei-yTiyOs-w, Lai-x-uSrxCauFei-yTiyOs-w,

Lai-x-uSrxBauFei-yTiyOs-w, Lai-x-uSrxAluFei-yGavOs-w, Lai-x-uSrxCauFei-yGavOs-w, Lai-x-uSrxBauFei-yGavOs-w, Lai-xSrxFei-yTiyOs-w, Lai _-u Ca _u Fei _-y Tiyθ3-w, Lai _-u Ba _u Fei _-y Tiyθ3-w, Lai-xSrxFei-yGayOs-w, Lai _-u Ca _u Fei _-y Gavθ3-w, Lai _-u Ba _u Fei _-y Gavθ3-w, Lai _-u Ba _u Feθ3-w, Lai _-u Ca _u Feθ3-w or Lai-χSrχFeθ3 _-w , and more particularly those of formulae: Lao 6Sr ₀ 4Feo 9Gao iθ3 _-w , Lao θSγO i Feo θGao i 03 _-w , Lao sSro sFeo θTio 103 _-w , Lao θSro i Feo θTio 103-W, Lao εSro ₄ Feo 2C008O3-W or Lao θSro 1 Feo 2C008O3-W

Among constituent compounds (C2) of material (A), there are for example compounds (C ₂ ) chosen from magnesium oxide (MgO), calcium oxide (CaO), aluminium oxide (AI2O3), zirconium oxide (Zrθ2), titanium oxide (TϊO2), mixed strontium aluminium oxides SrAI ₂ O ₄ or Sr ₃ AI ₂ Oe' mixed barium titanium oxide (BaTiOs), mixed calcium titanium oxide (CaTiOs), Lao 5Sr ₀ sFeo 9Ti ₀ 103.5 or Lao εSro ₄ Feo θGao lOβ-δ-

The subject of the invention is also a composition, characterized in that it consists, for 100% of its mass, of:

- 10 wt% to 90 wt% of a glass or a glass mixture (G); and - 10 wt% to 90 wt% of at least one material (A) comprising, for 100% of its volume:

(i) - at least 75% by volume and at most 100% by volume of a compound (Ci) chosen from doped ceramic oxides which, at the use temperature, are in the form of a crystal lattice with oxide ion vacancies of perovskite phase, of formula (I) as defined above:

(ii) - optionally up to 25% by volume of a compound (C ₂ ) as defined above, which is different from compound (Ci), and (iii) - optionally up to 2.5 % by volume of a compound (Ci _-2 ) as defined above.

As examples of the composition defined above, there are those for which the volume proportion of compound (Ci _-2 ), optionally present in material (A), tends

towards 0, those for which the volume proportion of compound (C2) optionally present in material (A) is greater than or equal to 0.1 % and less than or equal to 10%, and those for which compound (Ci) present in material (A) is chosen: either from compounds of formula (Ia): Lai _-x-u Mα'χMα" _u Mβi _-y-v Mβ' _y Mβ" _v θ3-w (Ia), corresponding to formula (I), in which Ma represents a lanthanum atom; or from compounds of formula (Ib):

Mαi-χ _-u Sr _x Mα" _u Mβi _-y- vMβ' _y Mβ"vO ₃ -w (Ib), corresponding to formula (I), in which Ma' represents a strontium atom; or from compounds of formula (Ic):

Mαi _-x-u Mα'χMα" _u Fei _-y- vMβ' _y Mβ"vθ3-w (Ic), corresponding to formula (I), in which Mβ represents an iron atom; more particularly those for which compound (Ci) present in material (A) is chosen: either from compounds of formula (Id): Lai _-x-u Sr _x Mα" _u Fei _-y- vMβ' _y Mβ"vO ₃ -w (Id), corresponding to formula (Ia) in which Ma' represents a strontium atom and Mβ represents an iron atom; or from compounds of formula (Ie):

Lai _-x-u Mα' _x Al _u Fei _-y-v Mβ' _y Mβ" _v O ₃ -w (Ie), corresponding to formula (Ia) in which Ma" represents an aluminium atom and Mβ represents an iron atom; or from compounds of formula (If):

Lai _-x Sr _x Fei _-y Mβ' _y O _3- w (If), corresponding to formula (Ia) in which Ma' represents a strontium atom, Mβ represents an iron atom and u and v are equal to 0; or from compounds of formula (Ig):

Lai _-u Ca _u Fei _-y Mβ' _y O _3- w (Ig), corresponding to formula (Ia) in which Ma' represents a calcium atom, Mβ represents an iron atom and x and v are equal to 0; or from compounds of formula (Ih):

Lai _-u Ba _u Fei _-y Mβ' _y O _3-w (Ih), corresponding to formula (Ia) in which Ma' represents a barium atom, Mβ represents an iron atom and x and v are equal to 0;

or from compounds of formula (Ii):

Lai _-x-u Sr _x Ca _u Fei _-y- vMβ' _y Mβ" _v O ₃ -w (Ii), corresponding to formula (Id) in which Ma" represents a calcium atom; or from compounds of formula (Ij): Lai _-x-u Sr _x Ba _u Fei _-y- vMβ' _y Mβ" _v O ₃ -w (Ij), corresponding to formula (Id) in which Ma" represents a barium atom; those for which compound (Ci) present in material (A) is chosen from compounds of formulae:

Lai _-x Sr _x Fei _-y Ga _v θ3-w, Lai _-x Sr _x Fei _-y Ti _y θ3- _w , Lai _-x Sr _x FeO _3- w, Lai _-u Ca _u Fei _-y Ga _v θ3-w, Lai _-u Ca _u Fei _-y Ti _y θ3-w, Lai _-u Ca _u Feθ3-w, Lai _-u Ba _u Fei _-y Ga _v θ3-w, Lai _-u Ba _u Fei _-y Ti _y θ3-w, Lai _-u Ba _u Feθ3-w, Lai _-x-u Sr _x Al _u Fei _-y Ti _y θ3-w, Lai _-x-u Sr _x Ca _u Fei _-y Ti _y θ3-w, Lai _-x-u Sr _x Ba _u Fei _-y Ti _y θ3-w, Lai _-x-u Sr _x Al _u Fei _-y Ga _v θ3-w, Lai _-x-u Sr _x Ca _u Fei _-y Ga _v θ3-w, Lai _-x-u Sr _x Ba _u Fei _-y Ga _v θ3-w, Lai _-x Sr _x Fei _-y Ti _y θ3- _w , Lai _-u Ca _u Fei _-y Ti _y θ3-w, Lai _-u Ba _u Fei _-y Ti _y θ3-w, Lai _-x Sr _x Fei _-y Ga _v θ3-w, Lai _-u Ca _u Fei _-y Ga _v θ3-w, Lai _-u Ba _u Fei _-y Ga _v θ3-w, Lai _-u Ba _u Feθ3-w, Lai _-u Ca _u Feθ3-w or Lai _-x Sr _x FeO _3- w, and more particularly those of formulae:

Lao 6Sr _{0 4} Fe ₀ 9Ga ₀ 103 _-w , Lao 9Sr ₀ I Feo 9Ga ₀ 103 _-w , Lao 5Sr ₀ 5Fe ₀ 9Ti ₀ 103 _-w , Lao 9Sr ₀ i Feo 9Tϊ0 103-W, Lao 6Sr _{0 4} Feo 2C008O3-W or Lao 9Sr ₀ 1 Feo 2C008θ3 _-w ; and/or those for which compound (C ₂ ) present in the material (A) is chosen from magnesium oxide (MgO), calcium oxide (CaO), aluminium oxide (AI ₂ O3), zirconium oxide (Zrθ ₂ ), titanium oxide (Tiθ ₂ ), mixed strontium aluminium oxides SrAI ₂ O ₄ or Sr ₃ AI ₂ Oe, mixed barium titanium oxide (BaTiO ₃ ), mixed calcium titanium oxide (CaTiO ₃ ), La ₀ SSr _{0 5} FeO gTi ₀ IO _3-5 or La _{0 6} Sr ₀₄ Fe _{0 9} Ga ₀ IO _3-5 .

According to another aspect, the subject of the invention is the use of the composition as defined above as sealing material, for a ceramic/metal joint, between a ceramic part and a part of metal or a metal alloy.

According to a final aspect, the subject of the invention is a ceramic/metal joint assembly (5), said assembly (5) comprising: 1 ) at least one ceramic part (3) in the form of an approximately cylindrical hollow tube of axis (X'X), closed at one of its ends (3b) and open at the other end (3a), defining an inner zone called the ceramic zone (CZ) and an outer zone called the metal zone (MZ), said ceramic (CZ) and metal (MZ) zones not communicating with

each other and being at least partially separated by the ceramic part (3), which part is at least partially sheathed by:

2) at least one socket (2) made of metal or metal alloy, comprising an approximately cylindrical hollow portion (2b) of axis (X ¹ X), hereafter called "joint zone", which at least partially surrounds said tube (3), an approximately annular space (4) of axis (X'X) being provided between said tube (3) and said joint zone (2b), characterized in that the sealing between said tube (3) and said socket (2) is provided by at least one ceramic/metal joint element (1 ) which is in contact with the tube (3) and with said joint zone (2b) of the socket (2), the joint element (1 ) being at least partially present in said annular space (4) by occupying an subspace

(4a) thereof and preferably being in the form of an approximately annular part (1 ); in that the joint element (1 ) comprises at least one sealing material consisting of a composition as defined in one of claims 11 to 17; and in that said joint assembly (5) is such that said joint zone (2b) has a small dimension (I) along any axis (Y'Y) passing through said joint zone (2b) and perpendicular to the axis (X'X).

In the assembly as defined above, the ratio of the dimension along the axis

(X'X) of the joint zone (2b) to the dimension of the subspace (4a) is generally at least equal to 2/1 and preferably is in the 2/1 to 100/1 range.

In the assembly as defined above, said small dimension (I) is generally between about 20 μm and 500 μm, preferably about 50 μm to 400 μm and even more preferably about 200 μm to 300 μm.

In the assembly as defined above, the socket (2) may include at least one shoulder (2c) for supporting the tube (3) and/or at least one shoulder (2a) for setting the tube (3).

Such an assembly is illustrated in Figure 1.

The invention finds use in applications involving, at least in part, ceramics bonded to metal alloys and operating at high temperature. Mention may in particular be made of catalytic membrane reactors (CMRs) for the production and/or separation of gases and more particularly for the production of syngas, solid oxide fuel cells (SOFCs), or oxygen generators operating by electrochemistry through a ceramic membrane.

The following description illustrates the invention without however limiting it.

(I) - Demonstration of the softening temperature and thermal expansion coefficient of glass/ceramic mixtures.

Schott 8350™ glass was chosen as its physicochemical characteristics (chemical stability, softening temperature, etc.) are compatible with the ceramic tube that has to be bonded and with the metal alloy constituting the ring. The ceramic introduced into the sealing material is the ceramic constituting the tube. However, any other ceramic possessing good chemical stability and no chemical reactivity with respect to the glass, and also having a TEC close to that of the tube, may be used. The morphology (size, shape, specific surface area) of the particles (ceramic and/or glass particles) is adjusted so as to make it easier to process the material or to improve its flow or mechanical properties. In the following table, the composition of the glass employed (Schott 8350™) and the formulation of material C used are given.

Figure 2 shows the variation in the dilatometric behaviour of the ceramic/glass mixtures as a function of the densification temperature of the preform for different ceramic contents in the glass.

The softening temperature of the glass/ceramic mixtures is substantially above that of the pure glass, which allows the use temperature to be increased. It should be pointed out that, depending on the temperature at which the preform was produced, the behaviour of the mixtures is different. At 1000 ⁰ C, it seems that partial crystallization of the glass has taken place (Figure 3), whereas at 700 ⁰ C, for a ceramic content of greater than 30%, the behaviour is essentially due to interactions between the ceramic particles. These observations are important with regard to the "ageing" of the sealing material and its positioning by viscous flow. A

crystallized glass requires a higher processing temperature, but is resistant to higher temperature. The rate of evaporation of a partially crystallized glass could also be lower. A compromise has to be made between properties and processability. It is therefore preferable, in our case, to densify the preforms at low temperature in order to make it easier for the glass to flow, and to subject the glass to a crystallization treatment after or during the cycle for producing the seal, in order to improve its high-temperature properties.

The thermal expansion coefficient of the glass/ceramic mixtures is also of paramount importance in the development of thermal stresses between the materials of the bond. The pure glass has a TEC of 9 x10 ^"6 /°C between 20 ⁰ C and 500 ⁰ C. This value is sufficiently close to the TECs of the other materials in order to produce an uncracked seal. However, increasing the TEC of the sealing material would allow the thermal stresses to be reduced further. The glass/ceramic mixtures systematically have a TEC above that of the glass alone, this being favourable to the mechanical stability of the seal (Figure 2).

The atmosphere during production of the seal also plays a role in terms of wettability and spreading of the glass/ceramic mixtures. A constraint in terms of chemical stability may also require the use of a controlled atmosphere at high temperature.

(II) Production of the ceramic/metal joint

The objective is to produce a joint using a ceramic/glass mixture by forming a ceramic backbone the pores of which are filled with glass.

(11-1) Starting with a powder blend

(11-1-1) Treatment of the powders

The powders used contained organic additives used during the process to synthesize them. These compounds were removed by a heat treatment at 350 ⁰ C in air. These organic residues are a source of bubbles in the molten glass. Their presence may significantly modify the properties of the sealing material, may induce stresses, limit the mechanical strength or the impermeability.

(11-1-2) Milling of the powders

The particle size distributions of the powders had to be adjusted in order to obtain a compact and homogeneous sealing material, and also to control the melting point of the glass. The powders were then milled separately in order to achieve the desired particle size distribution. Figure 4 shows the initial particle size distributions (after synthesis) of the material C and Schott 8350™ glass powders. Diameters of agglomerates and large grains of greater than 100 μm were observed. Figure 5 shows the powder size distribution of the same powders after attrition milling for 2 hours in the case of the ceramic and jar milling for 1 hour in the case of the glass. The milling parameters used are given in the following table:

The powders had a d ₅ o (50% of the particles had a diameter below this value) of 1 μm in the case of the ceramic and 5 μm in the case of the glass. These values were chosen so as to obtain a final particle stacking as compact as possible. Another technique for further improving the compactness would be to separate the particle size classes and blend them in proportions known to those skilled in the art. This would give a "customized particle size distribution". After milling, the powders were intimately blended, using a turbular device or in a jar, in an aqueous medium, in order to obtain a homogeneous suspension of the two components. Figure 6 shows the microstructures obtained for different steps in the preparation of the powders. The homogeneity of the preform microstructure was controlled by the particle size and size distribution of the powders, and also the ceramic/glass ratio. In the case of unmilled powders, the microstructure was composed of coarse glass and ceramic particles. The final microstructure was completely heterogeneous and the user has to put up with this. The bridging between the

ceramic particles was random and difficult to establish. Glass-depleted areas could appear and a complete lack of glass could result in leaks.

The powder preparation step, especially the powder milling operation, makes the microstructure homogeneous, improves the stacking of the ceramic particles and promotes their bridging (Figure 6). The shape of the ceramic particles has a major influence on the subsequent flow during the heat treatment of the mixture. The more equiaxed and closer to spherical the shape of the particles, the easier the spreading. Compromise must therefore be found between the ease of forming the seal and its properties. Access of the glass at the interfaces is also of paramount importance for the impermeability and adhesion of the seal. A lack of glass could result in leaks and poor adhesion. Trapped gas bubbles could appear in the preforms or during production. These bubbles are deleterious to the mechanical strength and to sealing. The application of a load and pre-treatment of the powders allows the phenomenon to be limited.

(11-1-3) Heat treatment for producing the seal

The objective of this thermal cycle is to fill the volume between the parts to be joined, to ensure formation of the interfaces between the sealing material and the parts to be joined, and to form the bridges between the ceramic particles in order to give the sealing material mechanical properties. The softening temperature of the sealing material is controlled firstly by the behaviour of the glass, and it is then set when the bridges between ceramic particles have formed. If necessary, a load may be applied to the sealing material so as to encourage it to spread and to reduce the temperature at which the joint is formed. The thermal cycle comprises two temperature holds (Figure 7). The first, at a temperature between 900 ⁰ C and 1200 ⁰ C, allows the powder blend to flow, in order to fill the space between the parts to be bonded, and forms the interfaces. The temperature and duration of this hold depend on the characteristics of the blend. A second hold, for annealing the glass, limits the thermal stresses that may be created as the glass solidifies. The annealing temperature depends on the glass used. It lies within a temperature range in which the glass is highly viscous, thereby allowing the stresses to be slowly relieved by plastic deformation. This information is

obtained from the viscosity/temperature curve that characterizes a glass. In the case of the Schott 853™ glass, it is about 550 ⁰ C.

(11-2) Starting from a preform A second technique for producing the joint consists in producing a preform of the sealing material.

(11-2-1) Preparation of the powders

The powders were treated as given in the above paragraphs, so as to give them the properties (particle size distribution, BET surface area, morphology) necessary for the pressing operation. The particle size distribution and the specific surface area were checked before the pressing operation, in order to obtain the most compact possible stacking of the particles.

(11-2-2) Pressing of the preforms

The preform may be produced by uniaxial pressing or isostatic pressing. The second method produces more homogeneous compaction but the first method is easier to implement. The powder blend is dried in order to facilitate the preform pressing. The incorporation, by granulation, of an organic binder (PEG 6000) increases the green strength. The mixture obtained thus contains: x% material C

100-x% Schott 8350™ glass

1 wt% of dispersant (relative to the mass of glass/ceramic mixture); and 1 wt% of PEG 6000 (relative to the mass of the glass/ceramic mixture). The preforms are pressed uniaxially at 15 klSI for 10 seconds.

(II-2-3) Preform densification heat treatments

The preforms undergo a densification heat treatment before the seal is formed, so as to minimize their volume shrinkage during formation. This precaution makes it possible to improve the adhesion between the sealing material and the parts to be joined and to ensure that the entire volume will be filled. After this step, the ceramic particles will preferably be still free to move in the glass, so as to allow the preform to spread during formation of the seal. The densification

thermal cycle comprises a binder-removal hold at 400 ⁰ C, allowing the organic substances used for preparing and pressing the powder to be removed, and a densification hold at 600 ⁰ C for 10 minutes in air, the rates of temperature rise and fall being controlled. The densification is carried out above the softening temperature of the glass (Figure 8). In the case of the Schott 8350™ glass, the temperature is lowered slowly between 550 ⁰ C and 450 ⁰ C so as to allow the stresses to relax as the glass solidifies, without causing the preform to crack. The preforms are then characterized by SEM (scanning electron microscope) micrographs of the microstructure so as to check: - the distribution homogeneity of the components the presence of porosity; the presence of point defects (cracks, etc.)

Moreover, to determine the heat treatment temperature to form the seal, the preforms are characterized by their spreadability according to the treatment temperature and treatment atmosphere. This purely qualitative analysis consists in placing pellets of the glass/ceramic mixture in a furnace and looking to see if they form a ball after heat treatment. The spreadability may also be measured quantitatively, by measuring the wetting angle, but this measurement is very difficult to carry out in the case of such materials.

(11-2-4) Heat treatment to form the seal

The objective of this thermal cycle is to deform the preform so as to fill the volume between the parts to be joined, to ensure formation of the interfaces between the sealing material and the parts to be joined, and to produce bridges between the ceramic particles in order to give the sealing material mechanical properties. The softening temperature of the sealing material is controlled by the above steps, namely the step of preparing the powders (morphology, particle size distribution, glass/ceramic ratio) and the preforming step (geometry and dimensions of the preform, associated heat pre-treatment). If necessary, pressure may be applied to the sealing material so as to promote its spreading or to reduce the temperature at which the joint is formed. The thermal cycle comprises two holds (Figure 9). The first at a temperature between 900°C and 1200C so as to

allow the preform to flow, in order to fill the space between the parts to be bonded, and forms the interfaces. The temperature and the duration of this hold depend on the characteristics of the mixture. A second hold, for annealing the glass at 550 ⁰ C, limits the thermal stresses that may be created as the glass solidifies. These heat treatments may be carried out in air or in a controlled atmosphere. The spreading of the preform may be modified according to the atmosphere used.

(11-3) Liquid infiltration

The formation of the seal, by infiltration of the liquid glass into a porous ceramic, may be carried out in one or two steps. The one-step process consists in manufacturing a porous ceramic preform, possibly integral with the ceramic tube to be bonded. In this case, the ceramic particles are already bonded together by a heat treatment, which may be that of sintering the tube if the preform is integral therewith. Once the parts - tube, porous preform, refractory alloy ring - are in place, the glass may be added, either by means of a preform or a powder placed on top of the porous ceramic, or by pouring molten glass thereonto. The first technique seems more favourable for avoiding thermal shocks on the ceramic tube. The two-step process consists in moulding the preform made of porous ceramic in the interstice between the tube and the refractory alloy ring, so as to minimize the gap between these parts. In this case, the heat treatment for consolidating the porous ceramic may not be identical to that for sintering the tube. The glass is then added using the same techniques as described above. Infiltration of the glass may require a particular atmosphere in order to improve its wettability and therefore its flow into the pores. This atmosphere may be a gas or a vacuum. The microstructure of the porous ceramic preform must also be perfectly controlled in order to ensure that the glass infiltrates throughout the thickness of the preform. The preparation of the powders and the choice of particle size distribution are therefore very important. Figure 10 shows one possible cycle for forming such a seal. Two temperature holds are required, the first for ensuring that the glass flows and the second for relieving the stresses that may appear as the glass solidifies.

The latter technique has several advantages over the previous ones: • the microstructure of the seal is perfectly controlled;

• the interfaces are formed with a less viscous fluid than with the ceramic/glass mixtures, hence better contact.

However, it does require precise control of the distribution of pores to be infiltrated and the formation of a porous preform having dimensions very close to the space to be filled. Figure 11 shows a cross section through a porous preform infiltrated by glass. The pores are completely filled by the flow of glass.

(Ill) - Exemplary embodiments

(111-1) Figures 12 and 13 show a seal produced with preforms of mixtures containing 30 vol% and 50 vol% of ceramic C, respectively. The joints were produced at 900 ⁰ C and 1050 ⁰ C, in both cases with a hold for 1 hour at 550°C in order to relieve the stresses as the glass solidifies. These photos show the perfect matching of the chemical and mechanical properties of the materials constituting the bond. The refractoriness of the more highly filled material is greater. This allows it to be used at a higher temperature, but it does require a higher forming temperature.

(111-2) Figure 14 shows the formation of a seal using a load placed on the preform. A ceramic ring is placed between the glass/ceramic preform and the load in order to avoid any bonding between these two parts. Applying a load allows the formation temperature to be reduced slightly and encourages bridging between the ceramic particles.

(IH-3) Figures 15 and 16 show SEM micrographs of the interfaces between the refractory alloy and the sealing material, and also the microstructure of the latter. It is apparent that the phases are distributed homogeneously and that there is bridging between the ceramic particles. The pores thus formed are completely filled with glass, which also ensures adhesion at the interfaces to the ceramic tube and to the refractory alloy.