Abstract:
A method of fabricating a CoSb 3 -based thermoelectric device is disclosed. The method includes providing a high-temperature electrode, providing a buffer layer on the high-temperature electrode, forming composite n-type and p-type layers, attaching the buffer layer to the composite n-type and p-type layers, providing a low-temperature electrode on the composite n-type and p-type layers and separating the composite n-type and p-type layers from each other to define n-type and p-type legs between the high-temperature electrode and the low-temperature electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of a U.S. Provisional Application Ser. No. 60/590,632 filed Jul. 23, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to thermoelectric devices which utilize a thermal gradient to generate electrical power. More particularly, the present invention relates to an expeditious method of fabricating a CoSb 3 -based thermoelectric (TE) device by using a spark plasma sintering (SPS) process to attach a high-temperature electrode to a buffer layer and to attach the buffer layer to p-type and n-type legs of the device.  
       BACKGROUND OF THE INVENTION  
       [0003]     With the recent discovery of high-efficiency thermoelectric (TE) materials, potential applications of TE technology have attracted worldwide interest. TE devices can be used for cooling and power generation purposes in a variety of applications and have the potential for high reliability, long life and environmentally safe operation. While most of the work in thermoelectric technology has focused on the development of new materials, of equal importance is investigation of the fabrication issues regarding incorporation of the newly-developed materials into TE devices.  
         [0004]     Filled skutterudites are prospective high-efficiency materials for TE power generation by TE devices having a hot side temperature (T H ) of between 450 degrees C. and 600 degrees C. For simplicity and proof of concept, binary n-type and p-type CoSb 3  skutterudites are used to fabricate the n-type and p-type legs of thermoelectric devices. Copper is used as the electrode material at the cold side of the device.  
         [0005]     Because of the relatively high T H  at the hot side of the device, selection of the high-temperature electrode material is important. First, the high-temperature electrode material should neither react with CoSb 3  nor diffuse into the CoSb 3  at the T H . Second, the high-temperature electrode material should have high electrical and thermal conductivity values. Third, the material should have a thermal expansion coefficient which is comparable to that of CoSb 3  to prevent breakage or cracking. Finally, the material should not be oxidized easily.  
         [0006]     Due to its high electrical conductivity (18.1 10 6  Ω −1  m −1 ) and thermal conductivity (138 W/mK), molybdenum (Mo) is a good candidate for the high temperature electrode material. In addition, its room temperature thermal expansion coefficient is close to that of CoSb 3 . The room temperature thermal expansion coefficients for both n-type and p-type CoSb 3  are about 8.0×10 −6  K −1 . Furthermore, Mo does not oxidize easily. However, because it has a high melting point (2623 degrees C.), Mo is difficult to be directly joined to CoSb 3 , which has a melting point of 876 degrees C.  
         [0007]     Therefore, utilization of a titanium buffer layer between the molybdenum high-temperature electrode and the CoSb 3  n-type and p-type legs is needed in the fabrication of a thermoelectric device since titanium has relatively large electrical and thermal conductivities, a thermal expansion coefficient which is comparable to that of CoSb 3 , is oxidation-resistant and has a melting point which is much lower than that of molybdenum.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is generally directed to a novel method of fabricating a thermoelectric device having a high efficiency and durability. The method includes attaching a high-temperature electrode layer, typically molybdenum, to a buffer layer, typically titanium, using spark plasma sintering (SPS); forming adjacent composite binary skutterudite CoSb 3  n-type and p-type layers using SPS; attaching the buffer layer to the composite n-type and p-type layers using SPS; attaching a low-temperature electrode layer to the composite n-type and p-type layers; and cutting between the composite n-type and p-type layers to form separate n-type and p-type legs which connect the high-temperature electrode layer to the low-temperature electrode layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0010]      FIG. 1  is a cross-section of a high-temperature electrode subjected to an ultrasonic pretreatment process according to the method of the present invention;  
         [0011]      FIG. 2  is a cross-section of an electrode/buffer layer fabricated according to the method of the present invention;  
         [0012]      FIG. 3  is a schematic illustrating formation of composite n-type and p-type layers using a spark plasma sintering (SPS) technique according to the present invention;  
         [0013]      FIG. 4  is a schematic illustrating attachment of an electrode/buffer layer to composite n-type and p-type layers using SPS according to the present invention;  
         [0014]      FIG. 5  is a perspective view of a thermoelectric device fabricated according to the invention, prior to cutting between the composite n-type and p-type layers;  
         [0015]      FIG. 6  is a perspective view of a thermoelectric device illustrating cutting between the composite n-type and p-type layers to form the connecting n-type and p-type legs in fabrication of the device according to the present invention;  
         [0016]      FIG. 7  is a flow diagram which illustrates sequential process steps carried out according to the method of the present invention;  
         [0017]      FIG. 8  shows scanning electron micrograph (SEM) images (top panels) and elemental composition intensities (bottom panels) obtained by electron probe microanalysis (EPMA) of the CoSb 3 , Ti and Mo composite and the CoSb 3  and Ti interface of a thermoelectric device fabricated according to the method of the present invention;  
         [0018]      FIG. 9  shows SEM images (top panel) and elemental composition intensities obtained by EPMA (bottom panels) of the CoSb 3  and Ti interface after 1000 hours of thermal fatigue testing at 500 degrees C.; and  
         [0019]      FIG. 10  is a graph which illustrates voltage drop as a function of position across the CoSb 3  and the electrode interface before and after thermal fatigue testing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention contemplates a novel method of fabricating a thermoelectric device having a high efficiency and durability. According to the method, spark plasma sintering (SPS) is used to attach a typically molybdenum high-temperature electrode layer to a typically titanium buffer layer to form an electrode/buffer layer. SPS is then used to form adjacent composite binary skutterudite CoSb 3  n-type and p-type layers and to attach the electrode/buffer layer to the n-type and p-type composite layers. A low-temperature electrode layer is attached to the composite n-type and p-type layers, typically using a conventional soldering method. Finally, the composite n-type and p-type layers are cut to form separate n-type and p-type legs which connect the high-temperature electrode layer to the low-temperature electrode layer in the finished thermoelectric device. The use of SPS as a rapid sintering technique facilitates the rapid fabrication of n-type and p-type legs in the thermoelectric device.  
         [0021]     Sequential process steps carried out according to the thermoelectric device fabrication method of the present invention are shown schematically in  FIGS. 1-6  and as a flow diagram in  FIG. 7 . As a first step according to the method, as shown in  FIG. 1  and indicated in step  1  of  FIG. 7 , a high-temperature electrode foil  12 , which is preferably a molybdenum foil having a thickness of typically about 0.5˜1.5 mm, is pretreated ultrasonically for typically about 5˜10 minutes with SiC or diamond sand  13  having a particle size of typically about 0.5˜5 μm. This pre-treatment step imparts roughness to the surface  12   a  of the high-temperature electrode foil  12 .  
         [0022]     As indicated in step  2  of  FIG. 7 , buffer layer material is then placed on the pre-treated surface  12   a  of the high-temperature electrode foil  12 . The buffer layer material is preferably a titanium powder (99.9% pure, 200˜400 mesh) or a titanium foil (99.9% pure) which is laid on the pretreated surface  12   a  of the high-temperature electrode foil  12 . As indicated in step  3  and shown in  FIG. 2 , the buffer layer  14  is attached to the pretreated surface  12   a  of the high-temperature electrode foil  12  to define an electrode/buffer layer  11 . This step may be carried out using SPS under vacuum or an inert gas atmosphere for about 5˜30 minutes, with about 20˜60 MPa pressure, and at a temperature of about 950˜1000 degrees C.  
         [0023]     A SPS (spark plasma sintering) apparatus  24 , which may be conventional, is shown schematically in  FIG. 3 . The SPS apparatus  24  includes an upper punch  28  and a lower punch  30  to which are attached thermocouples  26 . A DC pulse generator  36  is electrically connected to the upper punch  28  and lower punch  30 .  
         [0024]     As shown in  FIG. 3  and indicated in step  4  of  FIG. 7 , powders of p-type and n-type CoSb 3  are loaded as alternative p-type layers  16  and n-type layers  18  between the upper punch  28  and the lower punch  30  of the SPS apparatus  24 . The desired cross-sectional thicknesses of the p-type and n-type legs in the fabricated thermoelectric device determine the quantity of p-type and n-type powders loaded in the SPS apparatus  24 . In step  5 , the p-type layers  16  and n-type layers  18  are then sintered as a composite layer at a temperature of between typically about 560 degrees C. and 590 degrees C. with a pressure  32  of typically about 20 to 80 MPa.  
         [0025]     In step  6 , the surface of the buffer layer  14  is next pre-treated ultrasonically with 0.5˜5 μm diamond sand  13 , as further shown in  FIG. 2 , for typically about 5˜10 minutes to impart surface roughness to the buffer layer  14 . As indicated in step  7  and shown in  FIG. 4 , the electrode/buffer layer  11 , which includes the high-temperature electrode foil  12  and the buffer layer  14  previously sintered together in step  3 , is next loaded with the composite p-type layers  16  and n-type layers  18  in the SPS apparatus  24 . The pretreated surface of the buffer layer  14  is placed into contact with the composite p-type layers  16  and n-type layers  18 . As indicated in step  8 , the electrode/buffer layer  11  and composite layers are then subjected to spark plasma sintering at a temperature of between typically about 560˜590 degrees C. with typically about 20˜80 MPa pressure for about 5˜60 minutes. The relatively low melting point of the titanium buffer layer  14  facilitates attachment of the high-temperature electrode foil  12  to the composite p-type layers  16  and n-type layers  18 .  
         [0026]     As indicated in step  9  and shown in  FIG. 5 , a low-temperature electrode  20  is next attached to the ends of the composite p-type layers  16  and n-type layers  18  which are opposite the electrode/buffer layer  11 . Preferably, the low-temperature electrode  20  is copper. The low-temperature electrode  20  may be formed using conventional soldering techniques known to those skilled in the art.  
         [0027]     As indicated in step  10  and shown in  FIG. 6 , fabrication of the thermoelectric device  10  may be completed by cutting a central saw line  22  through the high-temperature electrode  12  and buffer layer  14  and to the low-temperature electrode  20  to define a central p-type leg  16   a  and  n -type leg  18   a . This may be carried out using a conventional wire saw. In similar fashion, peripheral saw lines  23  may be cut through the low-temperature electrode  20  and to the buffer layer  14  to define a peripheral p-type leg  16   b  and a peripheral n-type leg  18   b . Accordingly, responsive to a thermal gradient established between the high-temperature electrode  12  and the low-temperature electrode  20 , the central p-type leg  16   a , the peripheral p-type leg  16   b , the central n-type leg  18   a  and the peripheral n-type leg  18   b  conduct the flow of electrons from the high-temperature electrode  12  to the low-temperature electrode  20  in the finished thermoelectric device  10 . The relatively large electrical conductivity of the titanium buffer layer  14  facilitates electrical conductance between the high-temperature electrode  12  and the low-temperature electrode  20 .  
         [0028]      FIG. 8  shows scanning electron microscopy (SEM) images (top panels) and elemental composition intensities obtained by electron probe microanalysis (EPMA, bottom panels) of the CoSb 3 , Ti and Mo composite and the CoSb 3  and Ti interface of a thermoelectric device fabricated according to the method of the present invention. The yield strength of the prepared sample is 65 MPa. The interfaces are crack-free and show no signs of significant inter-diffusion. As shown in  FIG. 9 , after a 1000-hour thermal fatigue test carried out at 500 degrees C., the interfaces remain unchanged and the yield strength drops slightly to 63 MPa.  
         [0029]      FIG. 10  shows the measured voltage drop as a function of position across the interfaces at room temperature using a 10 mA electrical current, before and after the thermal fatigue test. The contact resistances at the interfaces remain approximately unchanged after thermal fatigue testing.  
         [0030]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.