Powder metallurgical molding and method of producing same

Abstract

A powder metallurgical molding forms an interconnector or an end plate for an electrochemical cell. The molding has a chromium content of at least 80% by weight, a basic shape of a plate and one or more flow fields with structuring formed on one or both of the main faces of the molding. A ratio of a maximum diameter D.sub.max of the molding, measured along the main face, to a minimum thickness d.sub.min of a core region of the molding which extends along the flow field or fields and is not affected by the structuring lies in a range of 140D.sub.max/d.sub.min350.

Claims

1. A powder metallurgical molding for an electro-chemical cell, the molding comprising: a plate having a basic plate shape with two main faces; said plate being formed with a chromium content of at least 80% by weight and being configured as an interconnector for a solid oxide fuel cell (SOFC); each of said main faces having: a flow field formed on each of said main faces, said main face having, in a region of said flow field, a structuring with a plurality of knob-shaped or ridge-like elevations and intermediate depressions; an edge region laterally of the respective said flow field and surrounding the respective said flow field completely or partially; wherein a ratio of a maximum diameter D.sub.max of the molding, measured along the main face, to a minimum thickness d.sub.min of a core region of the molding that extends along the flow field(s) and is not affected by the structuring lies in a range of 140D.sub.max/d.sub.min350; and wherein a maximum thickness d.sub.max of the molding in the region of said flow field is 2.3 mm (millimeters).

2. The molding according to claim 1, wherein the minimum thickness d.sub.min of the core region of the molding is 1.1 mm (millimeters).

3. The molding according to claim 1, wherein the maximum diameter D.sub.max of the molding, measured along the main face, is 200 mm.

4. The molding according to claim 1, wherein precisely one said flow field is formed on each of said two main faces of the molding.

5. The molding according to claim 1, wherein a ratio of a size of the main face in mm.sup.2 (square millimeters) to the minimum thickness d.sub.min of said core region in mm (millimeters) is 1.310.sup.4 mm.

6. The molding according to claim 1, wherein a ratio of a total weight of the molding in g (grams) to a size of the main face in cm.sup.2 (square centimeters) is 1.1 g/cm.sup.2.

7. The molding according to claim 1, wherein one or more flow fields are formed on each of said two main faces of the molding, and said structuring of mutually opposite flow fields have main extension directions along the respective said main faces running substantially parallel to one another.

8. The molding according to claim 1, wherein said plate is formed with four circumferential side edges, said four circumferential side edges including two mutually opposite side edges running parallel to one another, and wherein a ratio of a side length L.sub.max of a longest side edge to the minimum thickness of the core region d.sub.min is greater than or equal to 110.

9. The molding according to claim 1, wherein, in the core region, a proportion by surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions which have a surface area of 100 m.sup.2 (square micrometers), relative to a total surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions, is 60%, evaluated by quantitative image analysis on a scanning electron microscope image of a measurement area which is located in a cut surface, running along a thickness direction, through the molding into the core region.

10. The molding according to claim 1, wherein, in the core region, a proportion by surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions which have a surface area of 70 m.sup.2 (square micrometers), relative to a total surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions, is 70%, evaluated by quantitative image analysis on a scanning electron microscope image of a measurement area which is located in a cut surface, running along a thickness direction, through the molding into the core region.

11. The molding according to claim 1, wherein, in the core region, a proportion by surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions which have a surface area of 50 m.sup.2 (square micrometers), relative to a total surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions, is 80%, evaluated by quantitative image analysis on a scanning electron microscope image of a measurement area which is located in a cut surface, running along a thickness direction, through the molding into the core region.

12. The molding according to claim 1, wherein, in the core region, a proportion by surface area P of pores that are empty or partly filled with metal oxide and of oxide inclusions, provided they lie wholly within a measurement area under consideration, relative to a total surface area of the measurement area, lies in a range of 80%(1P)95%, evaluated by means of quantitative image analysis on a scanning electron microscope image of the measurement area which is located in a cut surface, running along a thickness direction, through the molding into the core region.

13. The molding according to claim 1, wherein the core region is formed with a uniform distribution of pores that are empty or partly filled with metal oxide and of oxide inclusions, and wherein, in the core region, a mean spacing of the pores that are empty or partly filled with metal oxide and of the oxide inclusions is 9 m, wherein the mean spacing is calculated according to the formula $={\left(\frac{\frac{4}{3}{a}^3}{P}\right)}^{\frac{1}{3}},$ wherein: 2a corresponds to a mean equivalent pore or oxide inclusion diameter; P is a proportion by surface area of pores that are empty or partly filled with metal oxide and of oxide inclusions, provided they lie wholly within a measurement area; and values for 2a and P are evaluated by quantitative image analysis on a scanning electron microscope image of a measurement area that is located in a cut surface, running along the thickness direction, through the molding into the core region.

14. A method for producing a powder metallurgical molding according to claim 1, the method comprising: a) providing a powder batch which, based on a total metal content, has a chromium content of 80% by weight and in which at least a portion of the powder batch has a BET surface area of 0.05 m.sup.2/g; b) pressing the powder batch to form a compact; and c) sintering the compact at a temperature from 1100 to 1500 C.; to form the molding according to claim 1.

15. The method according to claim 14, wherein the pressing step (b) is the only pressing operation carried out during the production of the molding.

16. The method according to claim 14, which comprises presintering the compact at a temperature from 500 to 1000 C. between the pressing step (b) and the sintering step (c).

17. The method according to claim 14, which comprises oxidizing the molding obtained after the sintering step (c) in the presence of an oxygen source to form an oxide layer, and subsequently removing the oxide layer from at least part of a surface of the molding.

18. The method according to claim 14, wherein the providing step (a) comprises producing a chromium-containing powder having a BET surface area of 0.05 m.sup.2/g and a chromium content of 90% by weight by reduction of at least one compound selected from the group consisting of chromium oxide and chromium hydroxide, optionally with an added solid carbon source, under an at least temporary exposure of hydrogen and hydrocarbon and adding the chromium-containing powder at least proportionately to the powder batch.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 shows a top view of an interconnector of co-flow design;

(2) FIG. 2 is a cross-sectional view of a section of the interconnector of FIG. 1;

(3) FIG. 3 shows a cross-sectional view of a section of an end plate;

(4) FIG. 4 shows a scanning electron microscope image of an interconnector with an advantageous microstructure; and

(5) FIG. 5 shows a scanning electron microscope image of an interconnector with a conventional microstructure.

DETAILED DESCRIPTION OF THE INVENTION

(6) Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a top view of a main face 4 of a (rectangular) interconnector 2 of co-flow design. The main face 4 is delimited by four circumferential side edges 8, 10, 16, 18. On the main face 4 shown, the interconnector 2 has precisely one flow field 6 with a structuring 5 formed in the interconnector 2. Adjacent to the flow field 6 on each of two mutually opposite sides there is an edge region 12, which extends to the side edges 8 or 10. Slot-like through-openings 14 for supplying and conveying away the process gases are provided in each of the edge regions 12. On the remaining two sides, the flow field 6 extends to the respective side edges 16, 18. The flow field 6 has a plurality of ridge-shaped elevations 20 with intermediate depressions 22 which extend continuously, substantially parallel to one another, along a main extension direction 24 from one side edge 16 to the opposite side edge 18 of the interconnector 2 (shown only schematically in FIG. 1). The flanks of the ridge-like elevations 20 each fall away obliquely towards the depressions 22 (see FIG. 2). FIG. 2 shows a cross-sectional view (along the cut surface A-A shown in FIG. 1) of a section of the interconnector 2 shown in FIG. 1 in the region of the flow field 6. On the opposite main face 26 there is likewise formed in a corresponding manner precisely one flow field 28 with a structuring 30 which has a plurality of ridge-like elevations 32 with intermediate depressions 34. Edge regions (not shown) are again provided adjacent to the side edges 8, 10. The main extension directions 24 of the structuring 5, 30 of both main faces 4, 26 run parallel to one another (co-flow design), the depressions 22 of one main face 4 each being arranged opposite the elevations 32 of the other main face 26 and vice versa (arrangement at gaps).

(7) As is apparent with reference to FIG. 2, the structural depth S.sub.A of the opposite main face 26, which during use faces, for example, an anode of an adjacent fuel cell, is lower in the present case than the structural depth S.sub.K of the main face 4 shown in FIG. 1, which during use faces, for example, a cathode of an adjacent fuel cell. The maximum diameter D.sub.max measured along the main face 4 or 26 corresponds in the case of the shape of the interconnector 2 shown to the spacing between two opposite corners, as is shown in FIG. 1. Here, the term diameter is used in its broadest meaning as straight line passing from side to side through the center of a body or a figure. In a rectangle, the maximum diameter is the diagonal, in a round body it is the longest straight line from one side to an opposite side. The side length of the longest side edge L.sub.max is likewise shown in FIG. 1. The thickness of the core region, which is not affected by the structuring 5, 30, in the region of the flow fields 6, 28 is substantially constant over the main faces 4, 26 in the interconnector 2 shown and thus corresponds to the minimum thickness d.sub.min (see FIG. 2). In the embodiment shown, the maximum thickness d.sub.max of the interconnector 2 corresponds to the spacing of the elevations 20, 32 in the region of the flow fields 6, 28, projected onto a thickness direction running perpendicular to the plane of the main faces 4, 26, and is constant over the region of the flow fields 6, 28 (see FIG. 2).

(8) FIG. 3 shows a cross-sectional view of a section of an end plate 36. The end plate 36 has a flow field 6 with a structuring 5 in a corresponding manner to that explained in relation to FIGS. 1 and 2 on only one main face 4, which (in the embodiment shown) faces the cathode of an adjacent fuel cell during use. Accordingly, the same reference numerals as in FIGS. 1 and 2 have been used in FIG. 3 for corresponding component sections, and reference is made to the explanations relating to FIGS. 1 and 2. In contrast to FIGS. 1 and 2, the core region is followed by a structuring 5 in the region of the flow field 6 on only one side, so that the minimum thickness d.sub.min of the core region and the maximum thickness d.sub.max of the end plate 36 are each measured along the thickness direction from the depressions 22 or elevations 20 to the opposite main face 26 (see FIG. 3).

(9) Exemplary embodiments for the production of interconnectors according to the present invention are explained herein below. Interconnectors having different geometries were thereby produced, all of which had a ratio D.sub.max/d.sub.min in the range 140D.sub.max/d.sub.min350 and at the same time exhibited sufficient stability. In particular, interconnectors having the dimensions indicated below were produced.

(10) TABLE-US-00001 Width No.: Length [mm] [mm] d.sub.max [mm] d.sub.min [mm] D.sub.max/d.sub.min 1 100 100 1.8 0.89 158.9 2 110 110 1.8 0.9 172.8 3 180 180 2.5 1.0 254.6 4 200 160 2.5 1.0 256.1 5 Main face round: 2.5 1.0 200.0 diameter: 200 mm

(11) Obtaining Chromium Powder Having a Large BET Surface Area:

(12) 500 g of Cr.sub.2O.sub.3 of pigment grade (Lanxess Bayoxide CGN-R) having a mean particle size d.sub.50, measured by means of laser diffraction, of 0.9 m (for powder morphology see FIG. 3) were heated in the course of 80 min. to 800 C. in H.sub.2 (75% by volume)-CH.sub.4(25% by volume) (flow rate 150 l/h, pressure approximately 1 bar). Thereafter, the reaction mixture was heated slowly to 1200 C., the reaction mixture being in the temperature range from 800 to 1200 C. for 325 min. The reaction mixture was then heated in the course of 20 min. to T.sub.R, where T.sub.R=1400 C. The holding time at 1400 C. was 180 min. Heating from 1200 C. to T.sub.R and holding at T.sub.R were carried out with the supply of dry hydrogen with a dew point <40 C., the pressure being approximately 1 bar. Furnace cooling was likewise carried out under H.sub.2 with a dew point <40 C. There was obtained a metallic sponge which could very easily be deagglomerated to a powder.

(13) Powder Batch:

(14) A powder batch consisting of 95% by weight fine Cr powder (having a BET surface area of 0.05 m.sup.2/g, granulated to form a more readily pourable powder having a particle size fraction of 45-250 m, e.g. produced by the process for obtaining chromium powder explained above) and 5% by weight of an FeY master alloy (alloy with 0.8% by weight Y, particle size <100 m) is then prepared. 1% by weight of pressing aid (wax) is added to the powder batch. This mixture is then mixed for 15 min in a tumbling mixer.

(15) Exemplary Embodiment Single Pressing Operation:

(16) This mixture is introduced into a mold and pressed at a specific pressing pressure of from 500 to 1000 MPa (in the present case e.g. at 800 MPa), so that a compact is formed. The compact is then presintered at from 500 to 1000 C. (in the present case e.g. at 900 C.) for 20 min (time at maximum temperature) under a hydrogen atmosphere in a conveyor furnace for the purpose of dewaxing the compact. After presintering, high-temperature sintering of the component is carried out at from 1100 C. to 1450 C. (in the present case e.g. at 1450 C.) for from 1 to 7 h (time at maximum temperature; in the present case e.g. for 7 h) under a hydrogen atmosphere for the purpose of further densification and alloy formation. Oxidation of the component is then carried out at 950 C. for a period of from 10 to 30 h (in the present case e.g. for 20 h; h: hour) in order to close up any residual porosity to such an extent that the permeability is sufficiently low. The surface of the oxidized component is freed of the oxide layer by a sand-blasting process on all sides.

(17) Exemplary Embodiment Two Pressing Operations:

(18) The advantages of and further information regarding the two-stage pressing operation are described, for example, in U.S. Pat. No. 8,173,063 B2. Obtaining the chromium powder and production of the powder batch are carried out as explained above. Production of the compact, including the pressing step and the presintering step, are carried out as in the exemplary embodiment single pressing operation above. After presintering, calibration pressing of the presintered component is carried out at a specific pressing pressure of from 500 to 1000 MPa (in the present case e.g. at 800 MPa). High-temperature sintering, oxidation and processing by a sand-blasting process are then carried out in a corresponding manner to the exemplary embodiment single pressing operation.

(19) In relation to the pressing pressure in the pressing step, it should be added that, when a powder having a large BET surface area (in particular 0.05 m.sup.2/g) is used, and in particular when a sponge-like powder is used, as can be produced, for example, by the process according to the invention (see in particular the process for obtaining chromium powder explained above), significantly better pressability is obtained and lower pressing pressures are accordingly sufficient. This is advantageous in view of the production costs and also in view of minimal wear of the pressing tools. While a typical pressing pressure of >900 MPa (MPa: megapascal) is used in the case of a maximum thickness in the region of the flow field/fields of 2.5 mm and in the case of conventional chromium powder produced by aluminothermic methods, a pressing pressure of 600 MPa is sufficient when using the advantageous powder for a maximum thickness of both 2.5 mm and 2.2 mm. In the case of a maximum thickness in the region of the flow field/fields of 2.0 mm and of 1.8 mm, a pressing pressure of only 500 MPa was sufficient when using the advantageous powder.

(20) Furthermore, when a powder having a large BET surface area (in particular 0.05 m.sup.2/g) is used, and in particular when a sponge-like powder is used, as can be produced, for example, by the process according to the invention (see in particular the process for obtaining chromium powder explained above), the molding has a particularly advantageous microstructure, as is explained below with reference to FIGS. 4 and 5. FIG. 4 shows a scanning electron microscope image of a cross-sectional ground section of an interconnector which has such an advantageous microstructure. By comparison, FIG. 5 shows a corresponding image of an interconnector produced by conventional means (using chromium powder produced by aluminothermic means) on the same scale. The pores that are empty or partly filled with metal oxide(s) and also the oxide inclusions are significantly darker in these images than the surrounding metallic matrix. As is apparent with reference to FIGS. 4 and 5, the advantageous microstructure is distinguished by a finely divided pore pattern. The proportion of large pores (pores that are empty or partly filled with metal oxide(s)) and oxide inclusions is especially very small. In particularbased on a specific minimum size of the pores that are empty or partly filled with metal oxide(s) and of oxide inclusionsthe proportion by surface area thereof, relative to the total surface area of pores that are empty or partly filled with metal oxide(s) and of oxide inclusions, is significantly smaller in the case of the advantageous microstructure than in the case of interconnectors produced by conventional means, as is shown by the following table (evaluated by means of the quantitative image analysis described below):

(21) TABLE-US-00002 Minimum size 100 m.sup.2 70 m.sup.2 50 m.sup.2 Conventional interconnector 88% 92% 94% Interconnector produced with 5% 14% 24% novel powder

(22) Description of the Quantitative Image Analysis for Determining the Microstructure

(23) For the quantitative image analysis, the moldings were cut perpendicularly to their main face by means of a diamond wire saw into segments having an edge length of approximately 20 mm. The cut surface was located through the flow field(s). The blanks were cleaned with water and then dried. The dried blanks were embedded in epoxy resin. After a curing time of at least 8 hours, the cut edges of the samples were prepared metallographically, that is to say an examination over the thickness of the component can later be carried out. The preparation comprises the steps: grinding at from 150 to 240 N with firmly bonded SiC paper of grit sizes 240, 320, 400, 800, 1000, 1200 and 2400 grit; fine grinding with 9 m Al.sub.2O.sub.3 lapping paper; polishing with diamond suspensions, first with 3 m grain size and then with 1 m grain size; final polishing with a diamond suspension of grain size 0.04 m; cleaning of the specimens in an ultrasonic bath; drying of the specimens.

(24) Five images of different, representative areas of the ground surface were then prepared for each specimen, the areas each being chosen within the core region. This was carried out by means of scanning electron microscopy (Ultra Plus 55 from Zeiss) using a 4-quadrant annular detector to detect back-scattered electrons (BSE). The excitation voltage was 20 kV, the tilt angle was 0. The images were focussed; the resolution should be at least 1024768 pixels for correct image analysis. The contrast was so chosen that both the pores and any partial metal oxide fillings of the pores as well as oxide inclusions clearly stand out from the metallic matrix andas explained abovecan be evaluated together. The magnification for the images was so chosen that each image contains at least 100 pores/oxide inclusions. In the present case, this gave image areas of from 0.04 to 0.25 mm.sup.2.

(25) The quantitative image analysis was carried out using QWin software from Leica. The QXCount module was used. Each image analysis followed the steps: setting a grey level threshold so that pore volume in the pores that is open and possibly partly filled with metal oxide(s), and also oxide inclusions (i.e. without open pore volume) were detected together as pore/oxide inclusion; fixing the measure frame, in this case the entire image area; measurement options: classification by equivalent diameter; detection adjustment: dark objects, fill holes, remove edge particles, open reconstruct.

(26) Filter functions should not be used either in the image or in the analysis of the images. Because the pores/oxide inclusions (as defined above) appear darker in a back scattered electron image than the metallic matrix, the dark objects must be defined as pores/oxide inclusions (as defined above) in the detection adjustment. It can occur, for example owing to partial filling of the pore volume with metal oxide(s), that this combination of pore volume and metal oxide filling is not detected as an object and is thus detected as an area (for the evaluation of the area sizes of the pores/oxide inclusions explained above). The option fill holes is to be used in order to detect this combination, and thus its area, as an associated object. By means of the option remove edge particles, incomplete pores/oxide inclusions (as defined above) in the edge region of the image area are not included in the evaluation.

(27) After the 5 images had been analyzed individually in each case, a statistical evaluation of the data of all 5 images was carried out. The following parameters were used for this evaluation: proportion by surface area of the pores (%) density (1/mm.sup.2) of the pores/oxide inclusions (as defined above) equivalent diameter (m) of the individual pores/oxide inclusions area (m.sup.2) of the individual pores/oxide inclusions.