CERAMIC MATERIALS

Abstract

The present invention relates to glass-ceramic/silver composite precursor compositions in the form of powders, and to glass-ceramics/silver composite materials produced therefrom. Such materials find particular use as interconnect materials for high temperature electrochemical conversion devices such as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).

Claims

1. A chromium-free, glass-ceramic/silver composite precursor composition comprising: silver-based particles; and glass-ceramic precursor particles formed from a material having the general formula xAO-yAl.sub.2O.sub.3-zSiO.sub.2 in which AO represents an alkaline earth oxide or mixture of alkaline earth oxides and x, y and z represent the mol % of AO, Al.sub.2O.sub.3 and SiO.sub.2, respectively, and in which x=30-60 mol %, y=0-20 mol %, and z=35-65 mol %; wherein said composition comprises 30-70 wt % of said silver-based particles, based on the combined weight of said silver-based particles and said glass-ceramic precursor particles.

2. A composition as claimed in claim 1 in the form of a powder.

3. A composition as claimed in claim 1 or claim 2, wherein x=35-55 mol %; y=0-15 mol %; and z=40-60 mol %.

4. A composition as claimed in any one of claims 1 to 3, wherein said glass-ceramic precursor particles contain one or more of the following metal oxides: MgO, CaO, BaO, SrO, ZnO, La.sub.2O.sub.3, ZrG.sub.2 and P.sub.2O.sub.5, or mixtures thereof.

5. A composition as claimed in any one of claims 1 to 4 which comprises 40-70 wt % silver-based particles, such as 50-70 wt % silver-based particles.

6. A composition as claimed in any one of claims 1 to 5, wherein said silver-based particles are silver particles.

7. A composition as claimed in any one of claims 1 to 6, wherein said silver-based particles are silver-alloy particles, such as silver-palladium alloy particles.

8. A composition as claimed claim 7, wherein said silver-alloy particles comprise at least 50 wt % silver.

9. A composition as claimed in any one of claims 1 to 8, wherein said glass-ceramic precursor particles contain 5 mol % or less of B.sub.2O.sub.3, such as 2 mol % or less.

10. A composition as claimed in any one of claims 1 to 9, wherein said glass-ceramic precursor particles contain 5 mol % or less of P.sub.2O.sub.5, such as 2 mol % or less.

11. A chromium-free, glass-ceramic/silver composite comprising: a silver-based phase; and one or more crystalline ceramic phases formed from a material having the general formula xAO-yAl.sub.2O.sub.3-zSiO.sub.2 in which AO represents an alkaline earth oxide or mixture of alkaline earth oxides and x, y and z represent the mol % of AO, Al.sub.2O.sub.3 and SiO.sub.2, respectively, and in which x=30-60 mol %, y=0-20 mol %, and z=35-65 mol %; wherein said composite comprises 30-70 wt % of said silver-based phase, based on the combined weight of said silver-based phase, said one or more crystalline ceramic phases and any residual glass phase.

12. A composite material as claimed in claim 11, wherein said material comprises 40-70 wt % of said silver-based phase, such as 50-70 wt % of said silver-based phase.

13. A composite material as claimed in claim 11 or claim 12, wherein the one or more glass-ceramic phases in said material contain less than 10 volume % residual glass phase, preferably less than 5 volume % residual glass phase, preferably less than 2 vol % residual glass phase.

14. A composite material as claimed in any one or claims 11 to 13, wherein said silver-based phase is present in an amount of 20 vol % or more of said composite material, preferably 25 vol % or more.

15. A composite material as claimed in any one of claims 11 to 14, wherein said silver-based phase is a silver phase.

16. A composite material as claimed in any one of claims 11 to 15, wherein said silver-based phase is a silver-alloy phase, such as a silver-palladium alloy phase.

17. A method of producing a chromium-free, glass-ceramic/silver composite as defined in any one of claims 11 to 16, the method comprising the steps of: heating a chromium-free, glass-ceramic/silver composite precursor composition as claimed in any one of claims 1 to 10 to a temperature above the glass-transition temperature (T.sub.g) of the glass-ceramic precursor particles but below the melting point of the silver-based particles; and holding the temperature in said range for a duration sufficient to achieve sintering and crystallization of the glass-ceramic precursor particles.

18. A method as claimed in claim 17, wherein during crystallization the temperature is held in the range of 900-950° C., such as 925-940° C.

19. A chromium-free, glass-ceramic/silver composite material obtainable or obtained by a method as claimed in claim 17 or claim 18.

20. An interconnect for use in a high temperature electrochemical conversion device, wherein said interconnect comprises a glass-ceramic/silver composite material as claimed in any one of claims 11 to 16 and 19.

21. An electrochemical ceramic membrane reactor comprising at least two cells, the cells each having an anode and a cathode with a gas-tight interconnect between the anode of one cell and the cathode of the adjacent cell, wherein said interconnect is formed from a glass-ceramic/silver composite as claimed in any one of claims 11 to 16 and 19.

22. An electrochemical ceramic membrane reactor as claimed in claim 21 which is a solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC).

23. Use of a chromium-free, glass-ceramic/silver composite material as claimed in any one of claims 11 to 16 and 19 as a gas-tight interconnect in a high temperature electrochemical conversion device, such as a SOFC or SOEC.

Description

DESCRIPTION OF FIGURES

[0048] FIG. 1 is a schematic diagram of an exploded view of the basic components of an ‘anode supported’ SOFC stack according to an embodiment of the invention showing the arrangement of a cathode (1), an electrolyte (2), an anode (3), and an interconnect (4).

[0049] FIG. 2 is a graph showing log (viscosity) versus temperature for pre-sintered precursor glasses 538 and 595 during heating at 3° C./min.

[0050] FIG. 3 shows Scanning Electron Micrographs of polished sections of glass-ceramic/silver composite materials (A) GC588/55AgV and (B) GC538/65AgV showing the distribution of the silver (light phase) in the glass-ceramic matrix.

DETAILED DESCRIPTION OF THE INVENTION

[0051] The present invention relates to a glass-ceramic/silver composite material suitable for use as a gas-tight, electrical interconnect in a ceramic membrane reactor, for example in an ‘anode supported’ SOFC stack as depicted in FIG. 1. It further relates to a glass-ceramic/silver composite precursor composition which may be used in the production of such a composite material.

Glass-Ceramic/Silver Composite Precursor Composition

[0052] In one embodiment, the invention relates to a chromium-free, glass-ceramic/silver composite precursor composition comprising:

[0053] silver-based particles; and

[0054] glass-ceramic precursor particles;

[0055] wherein said composition comprises 30-70 wt % of said silver-based particles, based on the combined weight of said silver-based particles and said glass-ceramic precursor particles.

[0056] The glass-ceramic/silver composite precursor composition is chromium-free. Typically, the composition contains less than about 1 wt. % chromium, preferably less than about 0.1 wt. %, more preferably less than about 0.05 wt. %. Preferably the composition contains no more than trace levels of chromium and is thus considered “essentially free” from any chromium.

[0057] The glass-ceramic/silver composite precursor composition may be prepared by combining glass-ceramic precursor particles and silver-based particles as explained in greater detail below.

[0058] Generally, the glass-ceramic/silver composite precursor composition will be provided in the form of a mixture comprising an intimate blend of the silver-based particles and the glass-ceramic precursor particles. Preferably, it will be provided as a free-flowing powder.

Glass-Ceramic Precursor Particles

[0059] Glass-ceramic precursor particles for use as a component of the glass-ceramic/silver composite precursor composition can be prepared via melt processing of a glass batch. The glass batch can be a simple blend of the required metal oxide and non-metal oxides (e.g. as obtained by dry-blending). The nature of these components is not particularly limited, but these should be chosen such that they provide the desired composition of ceramic phase(s) in the final glass-ceramic. After fusing of the glass batch and homogenisation of the resulting melt, the glass melt is cooled to form an amorphous solid, preferably in the form of a frit. The amorphous solid is then comminuted (e.g. by milling) to provide a glass-ceramic precursor powder of a suitable particle size.

[0060] In some embodiments, alternative metal compounds which convert to the corresponding metal oxide during melt processing may be used in the glass batch in place of the metal oxide, e.g. metal carbonates, nitrates or phosphates. Where these alternative metal compounds are used, appropriate amounts of the compounds in the glass batch can readily be established based on the desired amount of each metal oxide in the final glass-ceramic precursor composition. In essence, the same molar amounts are required. By way of example, a glass-ceramic precursor composition comprising 25 mol % MgO, 25 mol % CaO and 50 mol % SiO.sub.2 can be prepared from a glass batch comprising 25 mol % MgO, 25 mol % CaCO.sub.3 and 50 mol % SiO.sub.2 (per 100 g of glass: 18.6 g MgO, 46.6 g CaCO.sub.3 and 55.5 g SiO.sub.2).

[0061] The use of alternative metal compounds as a precursor to the desired metal oxide can be advantageous since some metal oxides react with moisture or CO.sub.2 in the air over time and thereby change composition which can lead to inaccuracies in weighing appropriate amounts of the metal oxides. Precursor compounds which are more stable during storage than the corresponding metal oxide may be preferred. Suitable precursor compounds include those mentioned above, e.g. metal nitrates, carbonates or phosphates.

[0062] It will be appreciated that more complex compounds may also act as a source of the desired metal ions, e.g. magnesium silicate compounds. The skilled person is familiar with the formulation of the glass batch and the variety of compounds that can be used to prepare the glass batch.

[0063] The glass batch of metal/non-metal oxides or, more generally, metal/non-metal compounds may be blended as a powder and undergoes a melt processing step. In a melt processing step the glass batch (e.g. oxides and/or non-oxide compounds or perhaps a blend of compounds) is fused to form a homogenous glass melt, e.g. at a temperature above 1200° C., such as 1400-1650° C., preferably 1200 to 1800° C., especially at 1450-1600° C.

[0064] Preferably the glass batch is heated in a suitable container such as a platinum crucible to a temperature which is sufficient to produce a homogeneous melt (typically at 1400-1650° C.).

[0065] The glass melt is then cooled to room temperature. Ideally, the glass melt is cooled rapidly, preferably by quenching into water to produce a glass frit. Rapid cooling is preferred as this helps to suppress devitrification. The use of water quenching is preferred as the resulting glass frit is readily milled to form a powder. The cooled glass melt, ideally the glass frit, should therefore be amenable to milling to produce the precursor glass powder.

[0066] The amorphous solid that forms after cooling is preferably then comminuted, e.g. milled, to produce the glass-ceramic precursor in the form of a powder, preferably a free-flowing powder. The powder preferably has a mean particle size of 1-100 μm (e.g. measured by laser diffraction), such as 2 to 75 μm, e.g. 2 to 50 μm.

[0067] Suitable glass-ceramic precursor compositions for use in the invention may be based on a range of alkaline earth silicate and alkaline earth alumina-silicate systems. In one embodiment the glass-ceramic precursor composition includes one or more of the following metal oxides: MgO, CaO, BaO, ZnO, Al.sub.2O.sub.3, La.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2 and P.sub.2O.sub.5.

[0068] In certain embodiments, the glass-ceramic precursor composition comprises 35-65 mol % SiO.sub.2. For example, it may include 40-65 mol % SiO.sub.2, such as 40-60 mol % SiO.sub.2.

[0069] In certain embodiments, the glass-ceramic precursor composition comprises 5-30 mol % MgO. In other embodiments the MgO content is 10-30 mol %, such as 10-25 mol %. However, the presence of MgO is not essential.

[0070] In certain embodiments, the glass-ceramic precursor composition comprises 15-40 mol % CaO. In other embodiments the amount of CaO is 20-40 mol %, such as 20-35 mol %. However, the presence of CaO is not essential.

[0071] In certain embodiments, the glass-ceramic precursor composition comprises 5-30 mol % BaO. However, the presence of BaO is not essential.

[0072] In certain embodiments, the glass-ceramic precursor composition comprises 5-10 mol % ZnO. However, the presence of ZnO is not essential.

[0073] In certain embodiments, the glass-ceramic precursor composition comprises 5-20 mol % Al.sub.2O.sub.3. However, the presence of Al.sub.2O.sub.3 is not essential.

[0074] In certain embodiments, the glass-ceramic precursor composition comprises 1-10 mol % La.sub.2O.sub.3. However, the presence of La.sub.2O.sub.3 is not essential.

[0075] In certain embodiments, the glass-ceramic precursor composition comprises 1-10 mol % ZrO.sub.2. However, the presence of ZrO.sub.2 is not essential.

[0076] The glass-ceramic precursor composition for use in the invention may, for example, have a composition represented by the general formula xAO-yAl.sub.2O.sub.3-zSiO.sub.2 wherein “AO” denotes an alkaline earth oxide or mixture of alkaline earth oxides and x, y and z represent the mol % of AO, Al.sub.2O.sub.3 and SiO.sub.2, respectively. The sum of x, y and z may, but need not, add up to 100 mol %. As will be understood, where x+y+z=100 mol %, the glass-ceramic precursor composition consists of AO, Al.sub.2O.sub.3 and SiO.sub.2 or, where y is 0, only of AO and SiO.sub.2. However, as explained herein, additional components (e.g. B.sub.2O.sub.3, La.sub.2O.sub.3, ZrO.sub.2, etc.) may be present in the composition in which case the sum of x+y+z will be less than 100 mol %. Suitable alkaline earth oxides “AO” which may be present in the composition include MgO, CaO, BaO, SrO, and mixtures thereof. Preferably “AO” will be selected from MgO, CaO, BaO, and mixtures of these oxides.

[0077] Preferred examples of suitable glass-ceramic precursor compositions include those having the general formula xAO-yAl.sub.2G.sub.3-zSiO.sub.2, wherein AO represents an alkaline earth oxide or mixture of alkaline earth oxides, x=30-60 mol %; y=0-20 mol %; and z=35-65 mol %. Preferred compositions are those wherein x=35-55 mol %; y=0-15 mol %; and z=40-60 mol %.

[0078] In some embodiments the glass-ceramic precursor composition may include B.sub.2O.sub.3 and/or P.sub.2O.sub.5. Whilst the inclusion of low melting point oxides such as B.sub.2O.sub.3 and/or P.sub.2O.sub.5 in the glass composition may be a convenient way of reducing the sintering temperature and improving densification by delaying the onset of crystallization, the level of addition should ideally be limited as B.sub.2O.sub.3 and P.sub.2O.sub.5 can readily form volatile species under typical SOFC and SOEC process conditions which can have a detrimental effect on other components in the electrochemical cells such as electrodes. In addition, these oxides may compromise the high temperature mechanical stability of the glass-ceramic as they can promote the retention of glassy phase during heat-treatment. In an embodiment the glass-ceramic precursor composition comprises 5 mol % or less of B.sub.2O.sub.3, such as 2 mol % or less. In an embodiment the glass-ceramic precursor composition comprises 5 mol % or less of P.sub.2O.sub.5, such as 2 mol % or less.

[0079] In some embodiments the glass-ceramic precursor composition comprises 5 mol % or less of alkali metal oxides, such as 2 mol % or less. It may, for example, be substantially free from any alkali metal oxide.

Formation of Glass-Ceramic/Silver Composite Precursor Composition

[0080] The glass-ceramic/silver composite precursor compositions are prepared by mixing the glass-ceramic precursor particles herein described with the silver-based particles. Mixing will generally be carried out with suitable processing aids known in the art such as solvents, binders, dispersants, viscosity regulators, etc. to facilitate the formation of a “green body”.

[0081] In a typical procedure the glass-ceramic/silver composite precursor powder, together with any required processing aids, is shaped to form a ‘green body’ of the desired geometry. Suitable methods include those well known in the art, such as tape-casting, pressing, injection moulding, 3-D printing, gel-casting, etc. Organic processing aids such as binders and plasticizers will typically be added prior to shaping to facilitate the shaping operation and impart sufficient green-strength to the shaped part to allow handling. The shaping process may involve pressing in a suitable metal die. Any organic processing aid(s), if added, will burn-off during heating of the shaped preform to the desired heat-treatment temperature.

[0082] In some instances, it will be convenient to produce the glass-ceramic/silver composite in the form of a thin tape. In this case, tape-casting may advantageously be employed in the production of the ‘green body’. Where somewhat thicker, planar components are required, lamination of several layers of green tape may be employed to build up the thickness to the required level prior to heat-treatment.

[0083] In some embodiments the silver-based particles are silver particles, i.e. particles consisting essentially of silver, or consisting of silver (i.e. pure silver). In other embodiments, however, the silver-based particles are silver-alloy particles, such as silver-palladium alloy particles. Where a silver-alloy is used, the alloying element should not cause depression of the liquidus, and the silver-alloy should include at least 50 wt % silver.

[0084] In an embodiment the glass-ceramic/silver composite precursor composition comprises 40-70 wt % silver-based particles based on the combined amount of glass-ceramic precursor particles and silver-based particles. In some embodiments 50-70 wt % silver-based particles may be used. In some embodiments, 45-70 wt %, 45-65 wt % or 45-60 wt % silver-based particles may be used.

[0085] In an embodiment the invention provides a chromium-free, glass-ceramic/silver composite precursor composition comprising:

[0086] 40-70 wt % silver-based particles; and

[0087] 30-60 wt % glass-ceramic precursor particles;

[0088] wherein the wt % are relative to the combined amount of silver-based particles and glass-ceramic precursor particles. Such a composition may be provided in the form of a powder, or a ‘green body’ as herein described.

[0089] In an embodiment the invention provides a chromium-free, glass-ceramic/silver composite precursor composition comprising:

[0090] 40-70 wt % silver-based particles; and

[0091] 30-60 wt % glass-ceramic precursor particles containing silica (SiO.sub.2) and a combination of two or more oxides selected from the group consisting of: MgO, CaO, BaO, ZnO, Al.sub.2O.sub.3, La.sub.2O.sub.3, ZrO.sub.2 and P.sub.2O.sub.5;

[0092] wherein the wt % are relative to the combined amount of silver-based particles and glass-ceramic precursor particles. Such a composition may be in the form of a powder, or ‘green body’ as herein described.

[0093] In some embodiments the glass-ceramic precursor particles may consist of SiO.sub.2, metal oxides and non-metal oxides. In some embodiments of the invention the glass-ceramic precursor particles consist of SiO.sub.2 and metal oxides only.

Preparation of Glass-Ceramic/Silver Composite Materials

[0094] The invention provides a method of producing a glass-ceramic/silver composite material, the method comprising the steps of:

[0095] heating a glass-ceramic/silver composite precursor composition as described herein to a temperature in the range of above the glass transition temperature (T.sub.g) of the glass-ceramic precursor composition and below the melting point of the silver-based particles; and

[0096] holding the temperature in said range for a duration sufficient to achieve sintering and crystallization of the glass-ceramic precursor composition.

[0097] Initially, as the glass-ceramic/silver composite precursor composition is gradually heated to above the T.sub.g of the glass-ceramic precursor composition the material begins to sinter. The glass-ceramic/silver composite precursor composition densifies, primarily by a process of viscous sintering of the glass-ceramic precursor phase as the temperature is increased. The glass-ceramic precursor phase should remain amorphous during sintering—the development of crystalline, ceramic phase(s) occurs during crystallization which should ideally take place after sintering is complete. Sintering typically involves heating to a temperature of 100° C. or more above the T.sub.g of the glass-ceramic precursor composition, e.g. to a temperature of at least 800° C., or at least 850° C.

[0098] The process as a whole (which includes sintering, then crystallization of the glass-ceramic precursor phase) typically involves heating the glass-ceramic/silver composite precursor composition at a rate of 1 to 20° C. per minute, such as 1 to 10° C. per minute. Firstly, the sintering temperature range is reached, where densification proceeds via the viscous sintering of the glass-ceramic precursor particles. As the temperature is further increased the glass-ceramic precursor phase crystallises. Crystallization typically nucleates at the former surfaces of the glass-ceramic precursor powder particles. The temperature is typically held at a temperature within the crystallization range for a period of time which is sufficient to ensure completion of the crystallization process. A typical period is 1 to 5 hours, but longer or shorter periods may be used

[0099] The holding temperature is above T.sub.g of the glass-ceramic precursor composition and below the melting point of the silver-based particles. In some embodiments the temperature is held in the range of 900−950° C., such as 925-940° C.

[0100] A glass-ceramic/silver composite material is developed on crystallization of the glass-ceramic precursor phase. The resulting glass-ceramic comprises one or more crystalline ceramic phases, and may also contain a small volume fraction (e.g. <10 volume %) of residual glassy phase. After crystallization of the glass-ceramic precursor phase, the resulting glass-ceramic phase has a dilatometric softening temperature in excess of the crystallization temperature, preferably higher than the melting point of the silver-based phase, more preferably above 1000° C.

[0101] The invention provides a chromium-free, glass-ceramic/silver composite composition comprising:

[0102] a silver-based phase; and

[0103] one or more crystalline ceramic phases;

[0104] wherein the composition comprises 30-70 wt % of said silver-based phase, based on the combined weight of said silver-based phase, said one or more crystalline ceramic phases and any residual glass phase.

[0105] In the glass-ceramic/silver composite material herein described, the silver-based phase and crystalline ceramic phase(s) which form during heat treatment are present as interpenetrating networks within the material. Cross-sections through GC/Ag composite materials showing the distribution of the different phases (in two dimensions) are presented, by way of example only, in FIGS. 3A and 3B. As will be seen, the silver-based material is sufficiently “interconnected” to provide the composite with excellent electrical conductivity throughout the composite material, whereas the crystalline ceramic phase is sufficiently “interconnected” to impart the desired high temperature mechanical strength.

[0106] The glass-ceramic/silver composite material herein described is chromium-free. In particular, the material contains less than about 1 wt % chromium, preferably less than about 0.1 wt %, more preferably less than about 0.05 wt %. Preferably the glass-ceramic/silver composite material contains no more than trace amounts of chromium.

[0107] In an embodiment, the residual glass content of the glass-ceramic phase is less than 10 volume % of the glass-ceramic/silver composite material, preferably less than 5 volume %. In an embodiment the residual glass content of the glass-ceramic phase is less than 5 vol %, preferably less than 2 vol %.

[0108] In an embodiment, the silver-based phase is present in an amount of 20 vol % or more, preferably 25 vol % or more.

[0109] The glass-ceramic/silver composite materials may have an electrical conductivity of 1000 S/cm or more at 600° C., preferably 2000 S/cm or more at 600° C., preferably 3000 S/cm or more at 600° C. A suitable range of conductivities at 600° C. is 1000-20000 S/cm, such as 1500-20000 S/cm or 1500-15000 S/cm.

Uses of the Glass-Ceramic/Silver Composite Materials

[0110] The glass-ceramic/silver composite materials described herein find particular application as gas-tight interconnects within a high temperature electrochemical conversion device. For example, these may be used as an interconnect in a fuel cell or in an electrochemical device based on a proton-conducting ceramic membrane. Interconnects must be both mechanically and chemically stable. Unlike other contact layers (e.g. conducting coatings) in electrochemical conversion devices, an interconnect should be capable of providing mechanical support, i.e. it functions as a structural component of the device.

[0111] The invention thus provides an interconnect, for example a fuel cell interconnect, comprising the chromium-free, glass-ceramic/silver composite material as defined herein.

[0112] An interconnect may be prepared by forming the glass-ceramic/silver composite precursor material (and any required processing aids) into a ‘green body’ having the desired shape, optionally with the aid of compression. The green body is heat treated to achieve crystallization as discussed herein. In contrast to methods used to form other contact layers in electrochemical conversion devices which involve application of a paste to an underlying substrate and firing of the paste in situ to form a thin layer or track on the substrate, an interconnect will generally be formed independently from the other components of the device as a stand-alone component. The interconnect may comprise, consist essentially of, or consist only of the glass-ceramic/silver composite material. It is preferred that the interconnect will be formed only of the glass-ceramic/silver composite herein described.

[0113] The interconnect is via-free, i.e. the interconnect does not have pre-formed vias filled with a via-fill material. It will be appreciated by those skilled in the art that the silver phase is not a via-fill material because it is distributed throughout the entire glass-ceramic phase, whereas in an interconnect having vias the via-fill material is only present in selected, defined areas of the ceramic phase.

[0114] As will be understood, the size and shape of the interconnect will depend on the configuration of the stack into which it is to be incorporated and those skilled in the art will be able to conceive of suitable structures. Typical interconnects may be plate-shaped for electrochemical devices of planar geometry, and may be of cylindrical/annular shape in the case of electrochemical devices based on tubular geometries. In some cases, an interconnect may contain channels and/or ribs on one or both sides to aid in the efficient distribution of reactants over the fuel side and air side surfaces of the interconnect. This profiling of the surface may be produced via machining of a substantially planar structure, or may be achieved by pressing and sintering of the glass-ceramic/silver composite precursor material in a green body having the desired shape as described herein. An example of a suitable interconnect is illustrated by component (4) in FIG. 1.

[0115] In another aspect, the invention provides an electrochemical ceramic membrane reactor comprising at least two cells, the cells each having an anode and a cathode with a gas-tight interconnect between the anode on one cell and the cathode of the adjacent cell, wherein said interconnect is formed from a glass-ceramic/silver composite composition as described herein. The reactor may be a SOFC or SOEC, for example. An illustrative SOFC stack is shown in FIG. 1 which is an exploded view of the basic components of an ‘anode supported’ SOFC stack showing the arrangement of a cathode (1), an electrolyte (2), an anode (3), and an interconnect (4).

[0116] In one embodiment, the glass-ceramic/silver composite may provide an interconnect between a first cell and a second cell. The first and second cells may be based on yttria-stabilised zirconia (YSZ) as a solid electrolyte material.

[0117] YSZ has a CTE (20-1000° C.) of approximately 10.3×10.sup.−6 K.sup.−1. To achieve a good CTE match with YSZ, it is necessary to select a glass-ceramic precursor which will crystallize to form a glass-ceramic having a relatively low thermal expansion. The inventor has established that it is possible to tune the CTE of the glass-ceramic/silver composite material by adjusting the content of the silver-based phase. In general the CTE of the glass-ceramic phase is lower than that of the silver-based phase, so, for a given glass-ceramic composition, the greater the content of silver-based phase the higher the CTE of the glass-ceramic/silver composite. In principle, the lower the CTE of the crystallized glass phase, the more silver-based phase can be added whilst still retaining good CTE matching with YSZ.

[0118] The glass-ceramic/silver composite materials described herein also find use as an oxidation resistant, electrical feed-through material in a ceramic body, for example one made from alumina, zirconia toughened alumina (ZTA), or zirconia.

[0119] The invention will now be described with reference to the following non-limiting examples.

Examples

[0120] By way of example, a number of electrically conductive glass-ceramic/silver composite materials (“GC/Ag composites”) according to the invention were prepared as set out below. GC/Ag composites with glass-ceramic matrices from a range of different alkaline earth silicate and alkaline earth alumino-silicate systems were made. These were selected to show that the composites can be produced from a diverse range of precursor glass compositions.

Preparation and Characterisation of Glass-Ceramic Precursors

[0121] The compositions of the precursor glasses are presented in Table 1.

TABLE-US-00001 TABLE 1 Precursor Composition (mol %) Glass No. MgO CaO BaO ZnO Al.sub.2O.sub.3 La.sub.2O.sub.3 SiO.sub.2 ZrO.sub.2 P.sub.2O.sub.5 498 — 33.7 8.4 — 7.5 — 50.4 — — 516 25 25 — — — — 45 5 — 538 16 — 22 — — 5 57 — — 545 22 23 — — 9 — 46 — — 588 14 28 — 3 10 — 45 — — 590 20 22 — — 11 — 45 — 2 595 14 24 — 5 12 — 45 — —

[0122] Glass batches sufficient to yield 800 g of each glass were prepared by mixing high purity raw materials (e.g. magnesium, calcium and barium carbonates, zinc oxide, alumina, quartz, zircon and ammonium dihydrogen phosphate) in the appropriate proportions. The glass batches were melted in a zirconia grain stabilized (ZGS) platinum crucible at temperatures in the range 1450-1600° C. and, once homogenised, each melt was quenched into cold water to form a friable frit. A small bar of glass was also cast from each melt to provide a sample for dilatometric analysis. The glass bars were cast onto a pre-heated steel plate, quickly transferred to an annealing furnace at 750° C. and held at this temperature for 30 minutes. The bar samples were subsequently cooled at 3K/min or less to room temperature.

[0123] Precursor glass powders with an average particle size (d.sub.50) in the range 20-30 μm were produced by milling the frit in aluminous porcelain mill jars using alumina milling media.

[0124] The glass transition temperatures (Tg) of the precursor glasses were measured on the annealed glass bar samples cast from each glass melt. The measurements were performed on 40-50 mm long samples in a horizontal axis dilatometer equipped with an alumina pushrod and sample holder (model 801 L dilatometer, Bahr Thermoanalyse, Hüllhorst, Germany) using a heating rate of 6K/min.

[0125] Disc samples were prepared from each of the precursor glass powders for evaluation of viscosity characteristics. Disc samples 8 mm diameter×1.5-2 mm thick were prepared by uniaxial pressing of the individual glass powders in a cylindrical die. A PVA based, temporary binder (Optapix PAF 46, Zschimmer & Schwarz, Lahnstein, Germany) was added to the glass powders to aid pressing and to improve the green strength of the discs. The pressed discs were heated to a temperature which was just sufficient to achieve more or less full densification during a 15 minute dwell. The actual temperature required was determined experimentally by measuring the sintering shrinkage of disc samples which had been subjected to a 15 minute hold at various temperatures. Typically, the temperature required for full sintering in 15 minutes was 90-110° C. above the T.sub.g determined from dilatometric measurements on the glass bars.

[0126] The viscosity of the sintered glass samples was measured at a heating rate of 3K/min in a parallel plate viscometer according to ASTM C1351M-96 (2012). The viscosity was seen to decrease as the sample was heated above the glass transition temperature as would be expected from a vitreous material. However, at some point during the continued heating, the viscosity reached a minimum and was then observed to increase due to the development of crystal phase(s). For illustration, viscosity versus temperature curves for two of the sintered precursor glass powders are shown in FIG. 2. For the precursor glass materials of the present invention, it has been found that a transient minimum viscosity of below approximately 10.sup.7 Pa s is needed in order to avoid open porosity in the conductive GC/Ag composite after heat-treatment. It is also desirable for the minimum viscosity to occur at a temperature which is below the melting point of the silver or silver alloy used in the GC/Ag composite as this will allow a high degree of crystallization of the precursor glass phase to be achieved within reasonable timescales at the permissible heat-treatment temperatures.

[0127] Crystallized samples of the precursor glasses were prepared for determination of thermal expansion characteristics and density. 50 mm diameter discs, approximately 6 mm thick, were produced by uniaxially pressing the precursor glass powders to which 2 wt. % PVA binder had been added. A load of approximately 10 tons was used to press the discs. The green discs were heated to 400° C. at 2K/min in air to burn-out the temporary binder, then at 5K/min to a temperature of 940° C. where they were held for 1 hour. Samples were cooled at 5K/min to room temperature.

[0128] Density measurements were performed on the heat-treated discs by the Archimedes method, using water as the immersion medium. Bar samples, 40-45 mm long, were cut from the heat-treated discs for measurement of thermal expansion. The thermal expansion measurements were performed in air on a horizontal axis dilatometer equipped with an alumina pushrod and holder (Bähr model 801L) during heating to a temperature of 1000° C. using a heating rate of 3K/min. Corrections for pushrod/holder expansion were applied using a sapphire reference material measured under the same conditions.

[0129] The measured property data for the precursor glasses and their respective glass-ceramics after crystallization are shown in Table 2.

TABLE-US-00002 TABLE 2 Density of Dilatometric Viscosity Glass- sintered softening Precursor Glass Viscosity minimum ceramic.sup.§ glass- temperature Glass T.sub.g minimum* temperature* CTE.sub.25-900° C. ceramic.sup.§ of glass- No. (° C.) (Pa .Math. s) (° C.) (10.sup.−6 K.sup.−1) (g/cm.sup.3) ceramic 498 770 10.sup.5.6 918 10.0 2.97 >1000° C. 516 760 10.sup.5.2 890 10.1 3.03 >1000° C. 538 750 10.sup.5.2 921 12.5 3.68 >1000° C. 545 740 10.sup.5.8 898 8.9 2.77 >1000° C. 558 760 10.sup.5.4 869 9.2 3.11 >1000° C. 588 740 10.sup.5.6 906 7.7 2.87 >1000° C. 590 750 10.sup.5.7 911 7.5 2.70 >1000° C. 595 740 10.sup.6.2 884 7.3 2.83 >1000° C. *During heating of vitreous, sintered powder compact at 3K/min .sup.§Crystallization heat-treatment 940° C./1 h

Preparation of Glass-Ceramic/Silver Composites

[0130] Glass-ceramic/silver composites were produced by heat-treatment of intimate mixtures of the glass-ceramic precursor powders and silver powders. The silver powders which were used were grades AGP-V0180-4 and AGP-H3150-9 (C50) from Doduco Contacts and Refining GmbH, Pforzheim, Germany. Their mean particle sizes (d.sub.50) were in the range 15-35 μm and 4-15 μm, respectively (manufacturer's data).

[0131] Powder mixes containing 35-47.5 wt % glass-ceramic precursor and 65-52.5 wt % silver were made up from the various glass-ceramic precursor powders and the silver powders. The glass-ceramic precursor and silver powders were initially thoroughly dry mixed, then a binder (Optapix PAF 46, Zschimmer & Schwarz, Lahnstein, Germany) was blended in to produce a thick paste. After drying, the paste was crushed and sieved to yield a powder which was suitable for pressing. 50 mm diameter×5 mm thick discs were uniaxially pressed (10 tons) to provide samples for thermal expansion measurement; 20 mm diameter×2 mm thick discs were pressed (2 tons) to provide samples for determination of electrical conductivity; and 20 mm diameter×1.6 mm thick discs were pressed (4 tons) from selected materials to provide samples for measurement of mechanical strength. Heat-treatment of the discs involved heating in air to 400° C. at 1.5 K/min in a muffle furnace for binder burn-out, and thereafter at 5K/min to a holding temperature in the range 925-940° C. (holding time 1-2 hours).

[0132] Bar samples approximately 45 mm×4 mm×5 mm were cut from the larger discs for CTE measurement. The thermal expansion measurements were performed in air on a horizontal axis dilatometer equipped with an alumina pushrod and holder (Bahr model 801L). Measurements were performed in air during heating (3K/min) to 900° C. and during cooling (−3K/min or slower) to 40° C. Corrections for pushrod/holder expansion were applied using a sapphire reference material measured under the same conditions.

[0133] The flat surfaces of the conductivity samples were lightly dressed by rubbing with a 400 grit (37 μm) diamond pad prior to undertaking the measurements. Sample dimensions were approximately 18 mm diameter×1.8 mm thick. The conductivity measurements were performed at 50° C. intervals over the temperature range 300-800° C. in air using a standard 4-point probe method (Van der Pauw technique).

[0134] Mechanical strength measurements were performed at the National Physical Laboratory (NPL), Teddington, UK on selected GC/Ag composite materials. The strength was measured in biaxial flexure on a sample of 20 test-pieces of each of the selected materials. The test-pieces were discs of approximately 18.5 mm in diameter×1.5 mm thick. The faces of the discs were lightly dressed by rubbing with a 400 grit (37 μm) diamond pad to remove any surface protrusions which may have resulted from the pressing or sintering processes. A ring-on-ring test jig (16 mm/6 mm diameter) was used at a cross-head displacement rate of 0.2 mm/min.

[0135] Density measurements were performed by the Archimedes technique using water as the immersion medium. The volume fraction of silver in each of the composites was calculated from the silver weight percentage and the measured density. The porosity of the composites was calculated from the previously measured densities of the glass-ceramic matrix materials and the measured GC/Ag composite density. The calculated porosity therefore excludes any closed porosity which may have been present in the glass-ceramic matrix samples.

[0136] Composition and heat-treatment details of the GC/Ag composites are set out in Table 3. Table 4 summarises the results of the density, thermal expansion and electrical conductivity measurements on the GC/Ag composite materials. The results of mechanical strength measurements on selected materials are presented in Table 5.

TABLE-US-00003 TABLE 3 Material Precursor wt. % Heat- reference glass Silver powder silver treatment GC498/60AgV 498 AGP-V0180-4 60 930° C./2 h GC516/55AgV 516 AGP-V0180-4 55 930° C./2 h GC538/65AgV 538 AGP-V0180-4 65 940° C./1 h GC545/60AgC 545 AGP-H3150-9 60 940° C./1 h (C50) GC545/60AgV 545 AGP-V0180-4 60 940° C./1 h GC588/55AgV 588 AGP-V0180-4 55 940° C./1 h GC588/60AgV 588 AGP-V0180-4 60 940° C./1 h GC588/60AgC 588 AGP-H3150-9 60 940° C./1 h (C50) GC588/65AgV 588 AGP-V0180-4 65 930° C./2 h GC590/55AgV 590 AGP-V0180-4 55 925° C./1 h GC595/52.5AgV 595 AGP-V0180-4 52.5 930° C./2 h GC595/55AgV 595 AGP-V0180-4 55 930° C./2 h GC595/60AgV 595 AGP-V0180-4 60 930° C./2 h GC595/65AgV 595 AGP-V0180-4 65 930° C./2 h

TABLE-US-00004 TABLE 4 Material Density Vol % CTE.sub.25-900°C. Conductivity (S/cm) reference (g/cm.sup.3) silver Porosity (10.sup.−6 K.sup.−1) 400° C. 600° C. 800° C. GC498/60AgV 5.18 29.6 0.6% 11.0 6290 4650 3830 GC516/55AgV 4.65 24.3 5.2% 11.1 2810 2120 1690 GC538/65AgV 6.14 38.0 3.4% 14.8 22290 16260 13220 GC545/60AgC 4.73 27.0 4.7% 10.4 13080 9770 7720 GC545/60AgV 4.79 27.4 3.5% 10.2 6510 4910 4130 GC588/55AgV 4.56 23.9 4.6% 9.1 3780 2850 2290 GC588/60AgV 4.84 27.7 4.9% 9.8 6930 5340 4250 GC588/60AgC 4.78 27.3 6.1% 10.1 8060 6040 4800 GC588/65AgV 5.17 32.0 4.9% 10.3 16060 12070 9600 GC590/55AgV 4.49 23.5 2.5% 9.3 4470 3310 ND GC595/52.5AgV 4.43 22.2 3.5% 8.6 3010 2230 1760 GC595/55AgV 4.57 23.9 3.4% 8.8 5220 3790 3050 GC595/60AgV 4.80 27.4 4.7% 9.3 7610 5710 4580 GC595/65AgV 5.11 31.7 5.2% 9.6 15900 11990 ND ND = not determined

[0137] Table 4 shows that GC/Ag composites with high electrical conductivities can be produced with CTEs (25-900° C.) covering the range 8 to 15×10.sup.−6 K.sup.−1. This offers a major advantage in the application area of interconnect materials since it enables the CTE to be tuned to match various cell components, for example in SOFCs and SOECs.

TABLE-US-00005 TABLE 5 Biaxial flexural strength Material Average strength Std. Dev. Weibull reference (MPa) (MPa) modulus GC545/60C 164 8.5 23.0 GC588/65V 133 6.2 25.8

[0138] Table 5 shows that the selected GC/Ag composites have good mechanical strengths. More importantly, the strength is extremely uniform, characterised by an exceptionally high Weibull modulus in each case. This indicates that the materials have excellent flaw tolerance and can be expected to have high reliability in service. In the case of the GC588/65V composite, significant inelastic behaviour was observed before peak load was reached.