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Gaskets for sealing solid oxide fuel cells
FEATURE
Gaskets for sealing solid oxide fuel cells By John R Hoyes - Technical Director Emeritus, Flexitallic Ltd, UK Achieving a reliable cost effective seal is one of the critical factors affecting the commercial development of fuel cells. This is particularly so with high temperature solid oxide fuel cells. Development of a new Themiculite material at Flexitallic has proved to be a successful solution. This feature describes the material and provides examples of the performance that can be achieved.
Introduction Fuel cells offer the potential of providing electrical power in a more efficient manner than current methods whilst also being environmentally friendly. There are a number of types of fuel cells with the solid oxide fuel cell [SOFC] showing particular promise for a wide range of applications such as remote and emergency power, auxiliary power in cars and trucks and in place of current domestic central heating units. The SOFC type of fuel cell operates at high temperature, typically 700 to 800ºC and as a result of this and their construction, sealing SOFC devices is a challenge. Flexitallic have developed a sealing material, Thermiculite 866, specifically for use as a compression seal for SOFCs. It is available in sheet form, is readily able to be cut into gaskets of complex shape and is proving to be successful around the world.
The gasket material
range of high performance, high temperature, patented, sealing materials for industrial sealing. Vermiculite is a naturally occurring mineral that is closely related to mica, a mineral noted for high temperature capability, high chemical resistance and as an electrical insulator. Both mica and vermiculite occur naturally as flakes which consist of a stack of very many crystal plates where each crystal plate is nanometres thick. Those flakes of vermiculite, unlike those of mica, are able to be exfoliated so that the crystal plates separate from each other. This exfoliation can be done by the application of heat, the result is then the well known form of vermiculite used in many everyday applications such as in gardening compost, as a thermal insulation material, as a packaging material and in fire prevention applications. The very thin crystal plates can also be more efficiently separated from each other by chemical means to produce a form which consists of just these very thin crystal plates. These separated plates, quite apart from the previously listed properties,
Figure 2. Gaskets cut from Thermiculite 866.
are also highly flexible and have the most useful property of adhering to each other to produce a thin, flexible, film. If a second material is added to the chemically exfoliated material then the second material will be bound by the chemically exfoliated material without the need for the addition of a binder. This binding action of the chemically exfoliated vermiculite means that when it is mixed with other, morphology compatible, materials it is possible to make, by methods developed by Flexitallic, a very flexible sheet material in roll form as shown in Figure 1. This sheet material is readily cut using traditional methods into gaskets of complex shapes as illustrated in Figure 2. In the case of Thermiculite 866 the second material is steatite, another silicate also with a plate-like crystal structure, which is perhaps better known as talc or soapstone.
Thermiculite 866 is a product utilizing the Thermiculite technology, based upon the use of chemically exfoliated vermiculite, that has been developed by Flexitallic to produce a
Figure 1. Thermiculite 866 as made in Roll Form.
August 2007
Figure 3. Compression curves for Thermiculite 866, as made, at various thicknesses.
Sealing Technology
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FEATURE The combination of steatite, a very soft mineral, with the very soft chemically exfoliated vermiculite results in a soft sheet material that compresses readily under modest loads and this means that on assembly of a connection it conforms easily to the surfaces thus forming a seal. The compression characteristics of this material for surface stresses of up to 20 MPa are shown in Figure 3 for three thicknesses of the material.
Seal performance Figure 4. Illustration of how the sealing capability improves with increasing stress.
Figure 5. Effect of changing gasket width on the leak rate under constant gasket stress conditions.
Figure 6. Sealing curve for Thermiculite 866, 0.5mm thick as made. Gasket dimensions 150mm OD, 130mm ID.
Figure 7. The data from Figure 6 presented as thickness achieved under load.
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Sealing Technology
Figure 4 shows how the level of seal achieved changes as the surface stress is increased and Figure 5 how the level of seal, for a given level of stress, changes as the width of the gasket, the landwidth, increases over the range 2 mm to 15 mm. In service once a connection has been assembled there will be a level of stress on the gasket that will be reduced once the system is pressurised by the opening force created by the gas pressure. An important feature of a gasket material is how it reacts to this opening force, called off-loading, caused by the pressure of the media being sealed. For a very poor sealing material the result of off-loading will be an increase in the leakage rate whereas for a good material there will be very little effect. For Helium at a pressure of 1 Bar, representative of an SOFC application, Figure 6 shows how the leakage rate for a Thermiculite 866 gasket of a landwidth of 10 mm varies during successive loading and unloading, the latter being to represent off-loading in service. The low sensitivity to off-loading indicates that good service will be obtained. In Figure 7 similar data, this time for a test conducted at 200 °C, shows how the leakage rate varies with compressed thickness. The reason for the excellent performance under both loading & off-loading conditions is that the process used to make the foil also orients the plates in such a way as to optimise sealing. This highly orientated structure ensures that not only is good sealing achieved during assembly of an SOFC stack but that it is maintained when the assembly load is reduced by external factors. As explained above, this is a unique material in a number of ways, perhaps the most remarkable being that it contains no organic binder material but is robust enough to be cut into complex shapes and be handled sufficiently to remain intact during stack assembly. The fact that it contains no organic material means that there is no creep at ambient temperature and that there is no material to burn off at elevated temperature
August 2007
FEATURE thus leaving voids which would result in increased leakage. This in turn means that sealing will continue during thermal cycling of SOFC stacks. Figure 8 shows the sealing results obtained during a test as illustrated in the schematic shown in Figure 9 where the gasket loading was kept constant and the gas pressure varied at ambient temperature. Figure 10 shows the results after a similar test at 600 °C, this temperature being the maximum safe operation temperature of the test apparatus. As can be seen the results differ very little showing the functional benefit achieved from the lack of organic material in the gasket.
Figure 8. Sealing curve for a gasket of 0.92mm as made thickness at 25ºC. Gasket size 75.1mm x 54.8mm.
Application Thermiculite 866 is available in a range of thicknesses from 0.5 mm to 1.0 mm and in the ‘as made’ form exhibits a very smooth surface on one side of the sheet and a less smooth one on the other. The material is so soft and compressible that in many applications the fact that one surface is not as smooth as the other does not inhibit its sealing performance. However, for situations where the available compressive load is low it is appreciated that a material equally smooth on both surfaces would be an advantage. Forms the material that is ‘consolidated’ rather than ‘as made’ is therefore being introduced to achieve sealing at lower stresses. For material made at 0.7 mm and then consolidated to 0.5 mm the effect on the low surface stress sealing can be seen by comparing Figure 11, this shows the sealing characteristic of 0.5 mm as made material, with Figure 12, which is consolidated to 0.5 mm from 0.7 mm as made material. The benefit of using consolidated material for SOFC systems where only modest gasket stresses can be generated, say less than 3MPa or 450lbf/ in2, can be seen readily by comparing the sealing characteristics given in Figures 11 & 12. Consolidation naturally has no advantage where stresses of above 10MPa are available but below 3MPa the benefit is very useful. As indicated previously, this gasket material is proving to be effective in SOFC stack applications and Figures 13 & 14 show actual results obtained from a stack sealed with Thermiculite 866 showing that the performance designed in to the material and shown in laboratory testing is achieved in service.
Figure 9. Test cycle for generation of the data shown in Figure 10.
Figure 10. Sealing curve for the same gasket as Figure 8 at 600ºC.
Design guidelines Beyond all of the above, no gasket material can be successful unless the basic rules of
August 2007
Figure 11. Sealing curve for a gasket of 0.5mm as made construction. Gasket size 150mm OD, 130mm ID.
Sealing Technology
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FEATURE
Figure 12. Similar size gasket to Figure 11, 0.5mm thickness consolidated material.
suc-
Figure 13. SOFC stack sealing performance achieved with Thermiculite 866 gaskets as a percentage of fuel flow rate at both ambient and operating temperature over a range of pressures showing a comparison with the target set at the start of the project.
successful gasket design are followed. Some basic considerations to be applied during the design of the gaskets of SOFC stacks are given below: • Minimize the gasket area as far as possible taking in to consideration gasket handling & cutting. • Maximize the compressive load available. • Use studs of the appropriate metal and, where the stack design allows, stress them to a high percentage of yield. • Minimize the load loss by making the studs as compliant as possible by using the minimum stud diameter suitable and by using extension collars or constant load washers such as Belleville washers. • Tighten the studs in a cross pattern manner. • Tighten the studs using either torque tensioning or hydraulic tensioners. • With torque tensioning use a reliable lubricant having a known friction factor. • Unless the gasket is compensating for connection defects, always use the minimum practical thickness. • The surfaces to be sealed should be free from transverse machining marks or scratches and be of an appropriate surface finish. Sample materials are available for evaluation. Also, to protect confidentiality of both parties in any detailed or technical discussions, please note that Flexitallic will be very willing to sign a bilateral non-disclosure agreement. The discussions that culminated in the results that are displayed in Figures 13 and 14 were covered by such an agreement and are reproduced in this document with the full permission of the other party. For more information contact: John R Hoyes, Technical Director Emeritus, Flexitallic Ltd, PO Box 21, Rooley Moor Road, Rochdale, Lancashire, OL12 7EU, UK, Tel: +44 1706 717486, Fax: +44 1274 851386, Email: [email protected].
Figure 14. Index of SOFC stack leak rate as a function of number of thermal cycles to operational temperature, showing the insensitivity of the Themiculite 866 gaskets to thermal cycling.
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Sealing Technology
Stephen Bond PhD, Vice President of Technology, Flexitallic Inc, 6915 Highway 225, Deer Park, TX 77536, USA, Tel: +1 281 604 2477, US Cell: +1 832 364 4006, UK Mobile: +44 7969 641559, Email: [email protected]. Web: www.flexitallic.com.
August 2007
Gaskets for sealing solid oxide fuel cells By John R Hoyes - Technical Director Emeritus, Flexitallic Ltd, UK Achieving a reliable cost effective seal is one of the critical factors affecting the commercial development of fuel cells. This is particularly so with high temperature solid oxide fuel cells. Development of a new Themiculite material at Flexitallic has proved to be a successful solution. This feature describes the material and provides examples of the performance that can be achieved.
Introduction Fuel cells offer the potential of providing electrical power in a more efficient manner than current methods whilst also being environmentally friendly. There are a number of types of fuel cells with the solid oxide fuel cell [SOFC] showing particular promise for a wide range of applications such as remote and emergency power, auxiliary power in cars and trucks and in place of current domestic central heating units. The SOFC type of fuel cell operates at high temperature, typically 700 to 800ºC and as a result of this and their construction, sealing SOFC devices is a challenge. Flexitallic have developed a sealing material, Thermiculite 866, specifically for use as a compression seal for SOFCs. It is available in sheet form, is readily able to be cut into gaskets of complex shape and is proving to be successful around the world.
The gasket material
range of high performance, high temperature, patented, sealing materials for industrial sealing. Vermiculite is a naturally occurring mineral that is closely related to mica, a mineral noted for high temperature capability, high chemical resistance and as an electrical insulator. Both mica and vermiculite occur naturally as flakes which consist of a stack of very many crystal plates where each crystal plate is nanometres thick. Those flakes of vermiculite, unlike those of mica, are able to be exfoliated so that the crystal plates separate from each other. This exfoliation can be done by the application of heat, the result is then the well known form of vermiculite used in many everyday applications such as in gardening compost, as a thermal insulation material, as a packaging material and in fire prevention applications. The very thin crystal plates can also be more efficiently separated from each other by chemical means to produce a form which consists of just these very thin crystal plates. These separated plates, quite apart from the previously listed properties,
Figure 2. Gaskets cut from Thermiculite 866.
are also highly flexible and have the most useful property of adhering to each other to produce a thin, flexible, film. If a second material is added to the chemically exfoliated material then the second material will be bound by the chemically exfoliated material without the need for the addition of a binder. This binding action of the chemically exfoliated vermiculite means that when it is mixed with other, morphology compatible, materials it is possible to make, by methods developed by Flexitallic, a very flexible sheet material in roll form as shown in Figure 1. This sheet material is readily cut using traditional methods into gaskets of complex shapes as illustrated in Figure 2. In the case of Thermiculite 866 the second material is steatite, another silicate also with a plate-like crystal structure, which is perhaps better known as talc or soapstone.
Thermiculite 866 is a product utilizing the Thermiculite technology, based upon the use of chemically exfoliated vermiculite, that has been developed by Flexitallic to produce a
Figure 1. Thermiculite 866 as made in Roll Form.
August 2007
Figure 3. Compression curves for Thermiculite 866, as made, at various thicknesses.
Sealing Technology
11
FEATURE The combination of steatite, a very soft mineral, with the very soft chemically exfoliated vermiculite results in a soft sheet material that compresses readily under modest loads and this means that on assembly of a connection it conforms easily to the surfaces thus forming a seal. The compression characteristics of this material for surface stresses of up to 20 MPa are shown in Figure 3 for three thicknesses of the material.
Seal performance Figure 4. Illustration of how the sealing capability improves with increasing stress.
Figure 5. Effect of changing gasket width on the leak rate under constant gasket stress conditions.
Figure 6. Sealing curve for Thermiculite 866, 0.5mm thick as made. Gasket dimensions 150mm OD, 130mm ID.
Figure 7. The data from Figure 6 presented as thickness achieved under load.
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Sealing Technology
Figure 4 shows how the level of seal achieved changes as the surface stress is increased and Figure 5 how the level of seal, for a given level of stress, changes as the width of the gasket, the landwidth, increases over the range 2 mm to 15 mm. In service once a connection has been assembled there will be a level of stress on the gasket that will be reduced once the system is pressurised by the opening force created by the gas pressure. An important feature of a gasket material is how it reacts to this opening force, called off-loading, caused by the pressure of the media being sealed. For a very poor sealing material the result of off-loading will be an increase in the leakage rate whereas for a good material there will be very little effect. For Helium at a pressure of 1 Bar, representative of an SOFC application, Figure 6 shows how the leakage rate for a Thermiculite 866 gasket of a landwidth of 10 mm varies during successive loading and unloading, the latter being to represent off-loading in service. The low sensitivity to off-loading indicates that good service will be obtained. In Figure 7 similar data, this time for a test conducted at 200 °C, shows how the leakage rate varies with compressed thickness. The reason for the excellent performance under both loading & off-loading conditions is that the process used to make the foil also orients the plates in such a way as to optimise sealing. This highly orientated structure ensures that not only is good sealing achieved during assembly of an SOFC stack but that it is maintained when the assembly load is reduced by external factors. As explained above, this is a unique material in a number of ways, perhaps the most remarkable being that it contains no organic binder material but is robust enough to be cut into complex shapes and be handled sufficiently to remain intact during stack assembly. The fact that it contains no organic material means that there is no creep at ambient temperature and that there is no material to burn off at elevated temperature
August 2007
FEATURE thus leaving voids which would result in increased leakage. This in turn means that sealing will continue during thermal cycling of SOFC stacks. Figure 8 shows the sealing results obtained during a test as illustrated in the schematic shown in Figure 9 where the gasket loading was kept constant and the gas pressure varied at ambient temperature. Figure 10 shows the results after a similar test at 600 °C, this temperature being the maximum safe operation temperature of the test apparatus. As can be seen the results differ very little showing the functional benefit achieved from the lack of organic material in the gasket.
Figure 8. Sealing curve for a gasket of 0.92mm as made thickness at 25ºC. Gasket size 75.1mm x 54.8mm.
Application Thermiculite 866 is available in a range of thicknesses from 0.5 mm to 1.0 mm and in the ‘as made’ form exhibits a very smooth surface on one side of the sheet and a less smooth one on the other. The material is so soft and compressible that in many applications the fact that one surface is not as smooth as the other does not inhibit its sealing performance. However, for situations where the available compressive load is low it is appreciated that a material equally smooth on both surfaces would be an advantage. Forms the material that is ‘consolidated’ rather than ‘as made’ is therefore being introduced to achieve sealing at lower stresses. For material made at 0.7 mm and then consolidated to 0.5 mm the effect on the low surface stress sealing can be seen by comparing Figure 11, this shows the sealing characteristic of 0.5 mm as made material, with Figure 12, which is consolidated to 0.5 mm from 0.7 mm as made material. The benefit of using consolidated material for SOFC systems where only modest gasket stresses can be generated, say less than 3MPa or 450lbf/ in2, can be seen readily by comparing the sealing characteristics given in Figures 11 & 12. Consolidation naturally has no advantage where stresses of above 10MPa are available but below 3MPa the benefit is very useful. As indicated previously, this gasket material is proving to be effective in SOFC stack applications and Figures 13 & 14 show actual results obtained from a stack sealed with Thermiculite 866 showing that the performance designed in to the material and shown in laboratory testing is achieved in service.
Figure 9. Test cycle for generation of the data shown in Figure 10.
Figure 10. Sealing curve for the same gasket as Figure 8 at 600ºC.
Design guidelines Beyond all of the above, no gasket material can be successful unless the basic rules of
August 2007
Figure 11. Sealing curve for a gasket of 0.5mm as made construction. Gasket size 150mm OD, 130mm ID.
Sealing Technology
13
FEATURE
Figure 12. Similar size gasket to Figure 11, 0.5mm thickness consolidated material.
suc-
Figure 13. SOFC stack sealing performance achieved with Thermiculite 866 gaskets as a percentage of fuel flow rate at both ambient and operating temperature over a range of pressures showing a comparison with the target set at the start of the project.
successful gasket design are followed. Some basic considerations to be applied during the design of the gaskets of SOFC stacks are given below: • Minimize the gasket area as far as possible taking in to consideration gasket handling & cutting. • Maximize the compressive load available. • Use studs of the appropriate metal and, where the stack design allows, stress them to a high percentage of yield. • Minimize the load loss by making the studs as compliant as possible by using the minimum stud diameter suitable and by using extension collars or constant load washers such as Belleville washers. • Tighten the studs in a cross pattern manner. • Tighten the studs using either torque tensioning or hydraulic tensioners. • With torque tensioning use a reliable lubricant having a known friction factor. • Unless the gasket is compensating for connection defects, always use the minimum practical thickness. • The surfaces to be sealed should be free from transverse machining marks or scratches and be of an appropriate surface finish. Sample materials are available for evaluation. Also, to protect confidentiality of both parties in any detailed or technical discussions, please note that Flexitallic will be very willing to sign a bilateral non-disclosure agreement. The discussions that culminated in the results that are displayed in Figures 13 and 14 were covered by such an agreement and are reproduced in this document with the full permission of the other party. For more information contact: John R Hoyes, Technical Director Emeritus, Flexitallic Ltd, PO Box 21, Rooley Moor Road, Rochdale, Lancashire, OL12 7EU, UK, Tel: +44 1706 717486, Fax: +44 1274 851386, Email: [email protected].
Figure 14. Index of SOFC stack leak rate as a function of number of thermal cycles to operational temperature, showing the insensitivity of the Themiculite 866 gaskets to thermal cycling.
14
Sealing Technology
Stephen Bond PhD, Vice President of Technology, Flexitallic Inc, 6915 Highway 225, Deer Park, TX 77536, USA, Tel: +1 281 604 2477, US Cell: +1 832 364 4006, UK Mobile: +44 7969 641559, Email: [email protected]. Web: www.flexitallic.com.
August 2007