Abstract:
An imaging technology for fuel cells is based on x-ray microscopy. A metrology system images the electro-chemical interaction areas of solid-oxide fuel cells (SOFC) in-situ. This system takes advantage of both the penetrating power and elemental absorption contrast of hard x-ray radiation to image the internal interaction areas in a SOFC. The technology can further take advantage of the strong dependence of the x-ray absorption on material type and energy to distinguish the four major material types: cathode, electrolyte, air, and low-Z contaminants such as sulfur.

Description:
BACKGROUND OF THE INVENTION 
   Fuel cell devices convert chemical energy directly into electrical energy without combustion. The immediate benefit is much higher energy conversion efficiency and the drastic reduction of pollutants. Solid-oxide fuel cells (SOFC), in particular, are able to utilize existing widely used hydrocarbon fuel to simultaneously generate thermal and electrical power with combined heat and power (CHP) efficiency of over 80%. In comparison, internal-combustion engines (ICE) provide 25-30% efficiency with most of the energy loss in the form of exhaust heat and pollution. SOFC technology therefore can significantly reduce the fuel consumption as well as pollutant emission of vehicles, and without the need of exotic fuels. 
   A critical step in making these devices practical is developing electrode and electrolyte components with high efficiency and high resistance to aging and contamination. The electrochemical reactions occur in a narrow zone along the three-phase boundary (TPB), where the three reaction elements: cathode, solid electrolyte, and gas are in contact. Better electrochemical performances are expected for components with larger TPB length per unit area. Much of SOFC research and development efforts are therefore focused on producing nano-porous structures with maximum TPB length as well as optimizing operating conditions that lead to high performance and corrosion and contamination resistance, e.g. from sulfur. 
   SUMMARY OF THE INVENTION 
   A critical tool in the development of SOFC is a three-dimensional imaging technique to visualize and measure the size, distribution, and connectivity of the pores and channels in these nano-porous structures. Furthermore, it is particularly desirable to study the dynamic changes of these structures in-situ and while the device is in operation. 
   The imaging techniques used today are primarily based on electron microscopy because of their high spatial resolution. But because of the shallow penetration depth of electrons, the imaging processes are generally destructive to the device, as the region of interest (ROI) must be mechanically cross-sectioned to reveal buried structures. This is a tedious and difficult process that is prone to introducing artifacts, particularly with hard and porous ceramic materials used in fuel cells. Furthermore, electron microscopy only provides a surface picture and does not reveal the pore connectivity. 
   The present invention concerns the use of an imaging technology for fuel cells based on x-ray microscopy. The large penetration depth of x-ray radiation enables imaging through the ROI without sectioning, and when used in combination with the tomography technique used in medical CT scans, the exact three dimensional (3D) structure of a sample can be obtained non-destructively, even while the device is in operation. This technology can further take advantage of the strong dependence of the x-ray absorption on material type and energy to distinguish the four major material types: cathode, electrolyte, air, and low-Z contaminants such as sulfur. This means that that the geometry of porous structures in the TPB can be directly measured from the 3D images to understand a SOFC&#39;s electrochemical state and predict its performance. This represents a metrology capability that is important for the development of fuel cell technology. By providing the most direct and relevant information on the exact operating conditions of the device, it has the potential to significantly reduce the development time and improve the reliability. 
   In one example, a metrology system images the electro-chemical interaction areas of solid-oxide fuel cells (SOFC) in-situ. This system takes advantage of both the penetrating power and elemental absorption contrast of hard x-ray radiation to image the internal interaction areas in a SOFC. Combined with computer tomography (CT) technique, the 3D morphology of the interaction volume can be obtained in-situ at 50-nm 3D resolution, and different material compositions can also be determined. Consequently, the exact aging mechanism can be tracked and analyzed dynamically while the device is in operation. This system provides the most direct and relevant information on the exact operating conditions of the device, having the potential to significantly accelerate the development of the SOFC technology and reduce the time-to-market. 
   The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
       FIG. 1  is a schematic diagram illustrating an x-ray microscope (TXM) according to an embodiment of the present invention; 
       FIG. 2  is a schematic diagram illustrating a microscope sample chamber to image a SOFC device in operation according to an embodiment of the present invention; 
       FIG. 3  shows the 3D volume data obtained using the inventive system from a typical SOFC sample shown in virtual cross-sectional views: (a-d) four slices in the different depth direction (perpendicular to imaging beam), (e-h) four slices along the rotation axis (parallel to beam), and (i-l) perpendicular to the previous axes; and 
       FIG. 4  is a plot of l/e attenuation length of bulk LSM, YSZ, sulfur, and air as a function of x-ray energy. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The disclosed tool is designed to nondestructively image the three-dimensional structure of the interface area of LSM/YSZ at 1-100 nm resolution while being able to distinguish four different materials: LSM, YSZ, sulfur, and air (empty gap space). There are two important elements to this approach: (1) how to resolve the structures in 3D and (2) how to distinguish different materials. 
     FIG. 1  shows an x-ray microscope (TXM) 100. In a setup similar to a conventional visible light microscope, x-ray microscope  100  comprises an x-ray source  110 , condenser lens  112 , objective lens  114 , and an area detector system  116 . This microscope  100  is used to image a SOFC anode substrate  10 . In one example, the anode substrate  10  comprises a cathode layer  12  (thickness approximately 50 micrometers (μm)), an electrolyte layer  14  (thickness approximately 10 μm), functional anode layer  16  (thickness approximately 5-10 μm), and anode substrate  18  (thickness approximately 1500 μm). 
   Preferably, a sample stage  150  is used to position the substrate  10  in an x-ray beam  105  from the source propagating along beam axis  106 . The rotation stage facilitates tomographic data acquisition by rotating the sample of fuel cell material in a controlled manner. Specifically, the stage  150  rotates the fuel cell material  10  to acquire images at different angles of inclination relative to the axis  106  (see arrow  62 ) and around axis  106  (see arrow  60 ) to obtain the multiple projection images needed for tomography data reconstruction. 
   In the preferred embodiment, the x-ray source  110  is a rotating anode type with molybdenum (Mo) target to generate 17.5 kilo-electron-Volt (keV) x-ray radiation. In other embodiments, the target is copper, silver, or rhodium. 
   Preferably, the condenser  112  is a capillary reflective lens, and a Fresnel zone plate lens is used as the objective lens  114 . A central stop  152  is also preferably provided along the beam axis to block x-ray that will not be or have not been focused by the zone plate lens  114 . This improves signal to noise ratio of the system. In one example, the stop is installed on or made part of the condenser  112 . 
   A lens-coupled scintillated CCD camera will be used as the detector  116 . In one embodiment, a scintillated CCD camera detector is used similar to that described in U.S. Pat. No. 7,057,187, entitled Scintillator Optical System and Method of Manufacture, which is incorporated herein in its entirety by this reference. 
   The 17.5 keV x-ray radiation allows samples with 100-200 μm thickness to be imaged. In one example, the anode substrate  18  of the SOFC  10 , and also possibly the cathode layer  12 , is thinned locally near the region of interest (ROI)  5 . A preferred sample preparation procedure is a mechanical process called dimpling that has been used routinely in semiconductor industries to remove the silicon (Si) substrate while keeping the circuitry intact. Skilled operators can routinely use this technique to remove nearly all the substrate while leaving the active circuitry without damage to the delicate interconnects. The materials are removed around the ROI  5 , but leaving the active area intact. 
   As shown in  FIG. 2 , the preferred embodiment further includes a chamber  210  to facilitate observation of dynamic changes of the TPB in situ. The chamber  210  functions to enable the SOFC device  10  to operate normally and also allows the imaging x-ray beam  105  to pass through. It is essentially a standard test chamber for planar SOFC devices modified with one or two x-ray windows. 
   The test chamber  210  has seals  212  to isolate the two sides of the device. It also has gas feed-through tubes to inject and remove fuel air and exhaust gases, heaters to start the electrochemical reaction, electrodes to draw electric power, and measuring devices  214 . Upper and lower cone-shaped windows devices  216 ,  218  are provided, each made from a thin metallic or ceramic membrane. Using hard x-ray radiation with 17.5 keV energy, there is a wide choice of window materials with acceptable absorption for the upper and lower window elements  220 ,  222  at the peaks of the fustoconical window devices  216 ,  218 . For example, 200 μm thick sapphire (Al2O3) provides about 80% transmission and are commercially available. 
   The objective lens  114  of this imaging system  100  is preferably a zone plate lens that focuses electromagnetic radiation by diffraction, as opposed to the refraction effect used by glass lens for focusing visible light. It is essentially a circular diffraction grating with the grating period decreasing towards the periphery. With this arrangement, the diffraction angle increases with the increasing radius, and all radiation passing through the zone plate converges to a focal point. The diffraction-limited resolution of a zone plate measured by the Rayleigh criterion is simply 1.22 times the width of the outer-most grating line, or the outermost zone width. The focal length of a zone plate is expressed as  f=DDr/l , where is  D  the diameter,  Dr  is the outer-most zone width, and  l  is the wavelength. The numerical aperture is  NA=l/2Dr . For example, in order to achieve the 60 nanometer (nm) resolution, a zone plate lens with an outer most zone width of 50 nm is preferred. 
   The depth of field (DOF) is generally defined as twice the resolution divided by the numerical aperture. Here, DOF is about 200 μm. Therefore a region of interest with 100-200 μm is contained completely within the DOF. In other words, all structures in the region of interest are imaged with 60 nm resolution and there is no defocus effect. 
   The volume of the ROI  5  is a region having a thickness of a few micrometers and extends over the area of the SOFC. One typically needs to image a 20 μm×20 μm sample area per measurement. A computed tomography (CT) technique is used to study the 3D structures of the interface area between the cathode  12  and the electrolyte  14 , or the anode  18 . The TPB is then identified from the 3D image and the effected interaction length can be measured. 
   A computed tomography (CT) algorithm originally developed in the medical imaging community can resolve these features in depth. In the imaging system shown in  FIG. 1 , the x-ray beam passes through the sample to record an image containing all features that overlap in the depth direction. With tomography imaging process, the sample is imaged at different view angles to acquire a series of tomographic projections, and these projections are then mathematically backprojected into a volume data to form a 3D representation of the object. The features in the sample 10 can then be analyzed by studying the data volume. Typical techniques used in the analysis include studying the cross-sectional views of the volume, or virtual sectioning (see  FIGS. 3   a - l ), as is most often used in medical imaging applications, and studying the volume rendering. This is done without physically modifying the object, while in contrast, to obtain equivalent cross-section views with SEM, one must physically cut and section the sample through the region of interest—a completely destructive process. In the medical imaging analogy, the TXM is compared with non-invasive imaging methods such as CT or MRI while SEM imaging can be compared to the invasive procedures such as biopsy or exploratory surgery. And their applications should comparable as well: the non-invasive imaging method should be used as the first screening or diagnosis procedure, while the invasive methods applied when the non-invasive methods cannot provide a definite answer. 
   A typical result from a planar SOFC sample is shown in  FIGS. 3   a - l . These results were acquired using 8.05 keV x rays generated from a rotating anode x-ray source with a copper (Cu) target rather than the 17.5 keV preferred design. Its objective lens was a zone plate lens with 50 nm outer most zone width with theoretical maximum resolution of 60 nm. A scintillator coupled CCD camera with 1024×1024 pixels was used as the detector. The substrate of the SOFC sample was thinned to a wedge shape. 
   The reconstructed 3D structure is shown in  FIGS. 3   a - l  with 4 slices along each of the 3 axes: (a-d) in the depth direction, where the slices are perpendicular to the imaging beam direction  z ; (e-h) in the rotation axis direction  y  (images parallel to the beam); and (i-l) along  x  x axis perpendicular  y-z  plane. The TPB can be easily identified from the image, and furthermore, the interaction length of the TPB can be measured from these images or from the 3D volume, for example automatically with specialized software. Furthermore, this measurement is made non-destructively and at about 2 centimeter (cm) stand-off working distance, the 3D images can also be made with a device in operation. 
   An important advantage of x-ray microscopy is that the absorption depends strongly on the elemental composition and x-ray energy. Thus, different materials can be distinguished as summarized in  FIG. 4  where the l/e attenuation lengths of LSM, YSZ, Sulfur, and air are plotted as a function of x-ray energy. Air is practically transparent with this combination of x-ray energy range and length scale. Note that the attenuation length for all three solid materials increases with the x-ray energy and the use of higher energy x rays allows thicker samples to be imaged. For example, the images from the preliminary experiments were acquired using 8.05 keV x rays. This requires the sample to be thinned to about 20 μm at the region of interest. However, a system using 17.5 keV x rays from Mo emission will allow a region of interest to remain in 100-200 μm thickness, thus greatly simplifying the sample preparation process and furthermore allowing observation at native operating conditions. 
   At 17.5 keV, the attenuation of LSM (La 1-x Sr x MnO 3-δ ), YSZ (La 0.65 Sr 0.3 MnO 3-δ ), and sulfur differ considerably. For example, with nearly half the attenuation length, LSM is more than twice more absorbing than YSZ, thus producing twice the image contrast. The two materials can then be identified in the 3D image from their very different absorptive properties, for example with a simple threshold. Air is practically transparent at this energy. It can be distinguished from both LSM and YSZ as empty space, while the solid materials can be distinguished from each other by their different pixel density. With this method, the three-dimensional structures of all three materials in the region where the electro-chemical reaction take place can be identified from a 3D image. This gives the ability to study the TPB from the 3D data without physically altering the sample through destructive processes. 
   Because of strong absorption edges of Y and Zr at 17.1 keV and 18 keV, there is actually a contrast reversal at x-ray energies above 18 keV. For example, at silver emission line at 22 keV, YSZ actually becomes about three times more absorbing than LSM. This means that when two images are taken, one using a Mo source with 17.5 keV emissions and one with Ag source with 22 keV emissions, the YSZ material will appear more absorbing than LSM in the first image, but their contrast will reverse in the second image. 
   In one embodiment, the x-ray source  110  of  FIG. 1 , comprises two separate sources, or a single source with two different target materials  110   a ,  110   b . The data obtained with emissions at the two energies are then combined to increase the accuracy of the material identification. This ability to map different materials in 3D not only provides new and unique capability for studying the TPB of SOFC sample, it also provides a powerful tool for materials science research in general. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.