Abstract:
A composite material with tailored anisotropic electrical and thermal conductivities is described. A material consists of a matrix material containing inclusions with anisotropic geometrical shapes. The inclusions are arranged in layers oriented perpendicular to the principal direction of electrical and thermal energy flow in the material. The shapes of the inclusions are such that they represent strong or weak barriers to energy flow depending on whether the major axis of the inclusions are parallel to or antiparallel to the flow direction.

Description:
BACKGROUND 
       [0001]    This invention relates to composite materials containing inclusions with anisotropic geometrical shapes in general and, more particularly, to composite materials with tailored anisotropic electrical and thermal conductivities. 
         [0002]    The ability to control the direction and magnitude of energy flow in one dimension (wire), two dimension (thin film), and three dimension (bulk) solid state components has been considered critical to device performance since the beginning of the electronic age. Diodes and other electronic valves are principal examples. Another example where directionality of thermal and electrical currents affect performance is in thermoelectric devices. The dimensionless thermoelectric figure of merit is a measure of performance and is given by the following equation: 
         [0000]    
       
         
           
             
               ZT 
               = 
               
                 
                   
                     S 
                     2 
                   
                    
                   σ 
                    
                   
                       
                   
                    
                   T 
                 
                 K 
               
             
             , 
           
         
       
     
         [0000]    where S, σ, K and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature respectively. 
         [0003]    The thermoelectric figure of merit is also related to the strength of interaction between the carriers and vibrational modes of the lattice structure (phonons) and available carrier energy states which, in turn, are a function of the materials used in the thermoelectric component. As such, the thermal conductivity, K, has an electronic component (K E ), associated with the electronic carriers and a lattice component (K L ) associated with thermal energy flow due to phonons. The thermal conductivity can then be expressed as K=K E +K L , and the figure of merit can be expressed generally as 
         [0000]    
       
         
           
             ZT 
             = 
             
               
                 
                   
                     S 
                     2 
                   
                    
                   σ 
                    
                   
                       
                   
                    
                   T 
                 
                 
                   
                     K 
                     E 
                   
                   + 
                   
                     K 
                     L 
                   
                 
               
               . 
             
           
         
       
     
         [0004]    Efforts to improve the performance of thermoelectric materials have generally focused on reducing K and maximizing σ. Unfortunately, the two quantities are closely coupled, and changing one typically results in corresponding changes in the other. 
         [0005]    Recent efforts to improve thermoelectric performance without sacrificing electrical conductivity have focused on inserting physical obstacles with nano scaled dimensions in thermoelectric structures to presumably impede phonon propagation. Venkata Subramanian et al. U.S. Pat. No. 7,342,169 teach that superlattice structures with nano scale dimensions block phonon transmission while allowing electron transmission thereby raising ZT and is included herein in its entirety for reference. Harmon et al. U.S. Pat. No. 6,605,772 disclose that quantum dot superlattices (QDSL) of thermoelectric materials exhibit enhanced ZT values at room temperature also by blocking phonon transmission and is included in its entirety for reference. Heremans et al. U.S. Pat. No. 7,365,265 and U.S. Publication No. 2004/0187905 disclose that nano scale inclusions on the order of 100 nm in size presumably block phonon transmission in lead telluride (PbTe) and other thermoelectric materials, thereby significantly improving the Seebeck coefficient. 
         [0006]    Other physical obstacles to energy flow in solids have been disclosed that affect K. Song et al., Physical Review Letters, Vol. 80, 3831 (1998), demonstrate that an asymmetric artificial scatterer in a semiconductor microjunction deflects ballistic electrons causing nonlinear transport and current voltage (IV) rectification and is included herein in its entirety for reference. 
         [0007]    Asymmetric energy flow in materials is a useful property with a multitude of applications not limited to thermoelectric materials. 
       SUMMARY 
       [0008]    In accordance with this invention, a material has a microstructure comprising of a matrix and inclusions dispersed in the matrix. The inclusions are characterized by having an anisotropic geometrical shape such that they interact with energy carriers in the matrix in an anisotropic manner, depending on how the major axes of the inclusions are oriented with respect to the principal directions of energy flow. The energy carriers are comprised of electrical energy carriers such as electrons and holes and thermal energy carriers such as phonons. By orienting the inclusions in different directions with respect to the principal directions of energy flow, anisotropic thermal and electrical conductivity can be achieved. 
         [0009]    In one aspect of this invention, the matrix material can be an electrical conductor, a semiconductor, or an insulator. The inclusions with anisotropic geometrical shapes can be electrical conductors, semiconductors, or insulators. 
         [0010]    In another aspect of this invention, the matrix is a thermoelectric material and the composite material has a high thermoelectric figure of merit ZT. 
         [0011]    In another aspect of this invention, the inclusions with anisotropic geometrical shapes are arranged in single or multiple layers oriented perpendicular to the principal direction of energy flow in the material. 
         [0012]    In another aspect of this invention, the inclusions have nano dimensions on the order of 10 nanometers to 5 microns. 
         [0013]    In another aspect of this invention, the inclusions are geometrical quantum dots. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic showing the cross-sectional shape of conical inclusions in a material. 
           [0015]      FIG. 2  is a schematic showing the cross-sectional shape of hemispherical inclusions in a material. 
           [0016]      FIGS. 3A and 3B  are schematics showing the initial steps in creating a layer of anisotropic geometric inclusions in multilayer thin films. 
           [0017]      FIGS. 4A and 4B  are schematics showing the steps to create a second layer of anisotropic geometric inclusions in multilayer thin films. 
           [0018]      FIGS. 5A and 5B  are schematics showing the final steps to create two layers of anisotropic geometric inclusions in multilayer thin films. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present invention is generally directed to a method of imparting anisotropic thermal and electrical conductivity in a composite material consisting of a matrix material and inclusions with anisotropic geometrical shapes. The anisotropic energy flow is directly related to the orientation of the principal axes of the inclusions with respect to the principal direction of energy flow. 
         [0020]    Geometrical aspects of the relation between inclusions with anisotropic geometrical shapes and energy flow are illustrated in the embodiment shown in  FIG. 1 , which is a schematic drawing showing a cross-section of material  10 . Material  10  includes matrix material  11  containing conical inclusions  12  which have an apex  13  and a base  15 . Base  15  is wider than apex  13 , thereby imparting asymmetric geometry to the inclusion. In the example, energy flow is depicted as arrow  17  in the principal direction of the flow. Arrow  19  depicts driving force for energy flow  17  and determines direction of energy flow  17 . Driving force  19  comprises, for instance, an electrical potential if energy flow  17  is electrical energy and, for instance, a temperature gradient if energy flow  17  is thermal energy. Matrix material  11  and inclusions  12  can be electrical conductors, semiconductors, or insulators. Electrical energy propagates through both matrix  11  and inclusions  12  in the form of electrons and holes, and thermal energy propagates in the form of elastic waves as phonons. Schematic wavy arrow  20  depicts deflections of energy carriers in energy flow  17  due to shaped sides of inclusions  12  as the energy carriers encounter inclusions  12 . 
         [0021]    If the elastic properties of inclusion  12  and material  10  are sufficiently different, for instance, inclusion  12  will deflect energy flow due to phonons as a result of a difference in acoustic impedance at the inclusion  12  material  10  interface. 
         [0022]    If the driving force for energy flow  19  is reversed, the energy carriers encounter obstacles (inclusions  12 ) where deflection is not possible, and barriers to energy flow are higher. This is schematically illustrated by wavy arrow  21 . Thus, the rate of energy flow in the direction of arrow  17  is higher than in the opposite direction if driving force  19  and subsequent energy flow  17  were reversed due to the anisotropic obstacle strength of the asymmetric obstacles. 
         [0023]    In the embodiment shown in  FIG. 1 , inclusions  12  are in parallel arrays stacked on top one another at spacing  16 . Inclusions  12  are spaced apart at distance  14  in each layer in an arrangement where obstacle width  18  is sufficiently less than spacing  14 , such that there is a finite aerial density of unobstructed line of sight path through composite material  10 , such that some energy carriers  20  in energy flow  17  can pass through material  10  unimpeded without encountering an obstacle. 
         [0024]    In another embodiment of the invention shown in  FIG. 2 , material  30  includes matrix material  31  containing hemispherical inclusions  32 . Driving force  39  comprises, for instance, an electrical potential for electrical energy flow  37  or, for instance, a temperature gradient, if energy flow  37  is thermal energy or both. Matrix material  31  and inclusions  32  can be electrical conductors, semiconductors, or insulators. 
         [0025]    In the embodiment shown in  FIG. 2 , schematic wavy arrow  40  depicts deflections of energy carriers of energy flow  37  due to shaped sides of inclusions  32  as they encounter inclusions  32 . Inclusions  32  are in parallel arrays stacked on top of one another at spacing  36 . Inclusions  32  are spaced apart at distance  34  in each layer in an arrangement where obstacle width  38  is sufficiently large compared to spacing  34  such that there is no aerial density where energy flow  37  can migrate through material  30  without encountering an obstacle in a line of sight path. On the other hand, energy deflection as indicated by wavy arrow  40  still has a vector component in the downward direction after impacting an obstacle allowing energy flow. 
         [0026]    If the driving force for energy flow  39  is reversed, the energy carriers encounter obstacles where deflection is not possible and the barriers to energy flow are higher. This is schematically illustrated by wavy arrow  41 . Thus, the rate of energy flow in the direction of arrow  39  is higher than in the opposite direction if driving force  39  and subsequent energy flow  37  were reversed due to the anisotropic obstacle strength of the asymmetric inclusions. 
         [0027]    In other embodiments of the invention, the inclusions with anisotropic geometrical shapes can be pyramids, oblate spheroids or other shapes wherein the base cross-sectional area is larger than the peak cross-sectional area along a principal axis of the inclusion in the direction of energy flow. 
         [0028]    To be effective as anisotropic barriers of electrical and thermal energy propagation, the barrier size and spacing need to be commensurate with the wavelength and mean free path of the carriers (i.e. electrons, holes, phonons) themselves. These quantities all have submicron dimensions. 
         [0029]    In one embodiment, for example, a superlattice structure is formed atom layer by atom layer by physical vapor deposition (PVD) methods such as molecular beam epitaxy (MBE) and others known to those in the art. The superlattice structure can be formed so that it contains nanoscale obstacles with three-dimensional anisotropic geometrical symmetry. Superlattice structures are formed by depositing alternating layers of different materials on an atom layer by atom layer basis. Under conditions where the two materials have similar crystal structures but exhibit lattice mismatch, individual three-dimensional structures or “islands” will self form in layers on the substrate to minimize strain energy during deposition. In systems where both materials are semiconductors, the particles can be quantum dots containing quantized electrons with zero dimensions. Quantum dot superlattice (QDSL) structures have enhanced thermoelectric properties over the bulk materials. 
         [0030]    In this embodiment, quantum dots with geometrical anisotropic shapes such as those shown in  FIGS. 1 and 2  are oriented such that the layers in the superlattice are perpendicular to the principal direction of electrical and thermal energy flow in the superlattice. The resulting anisotropic energy flow will raise ZT, the thermoelectric figure of merit. Thermal flow in the reverse direction will be discouraged by the shape of the obstacles. The semiconducting obstacles will allow electron flow and will impede phonon propagation due to impedance mismatch at the coherent quantum dot obstacle matrix boundaries, thereby increasing ZT by decreasing K L . 
         [0031]    In another embodiment, microstructures containing layers of particles with anisotropic geometric shapes are formed by PVD and accompanying photolithographic techniques wherein multimasking and angle etching steps form “islands” of a second material in a first material. Planarizing each layer containing the islands by depositing a layer of the first material to the top of the islands of the second material form inclusions of the second material in a layer of the first material. This process is repeated until the required dimensions are formed. 
         [0032]    This process is schematically illustrated in  FIGS. 3A ,  3 B,  4 A,  4 B,  5 A, and  5 B.  FIG. 3A  shows a structure in which layer  52  of Material B is deposited on substrate  50  of Material A. Mask pattern  54  is deposited on layer  52  and formed by liftoff and other techniques known to those in the art. The structure is then exposed to, for instance, a wet chemical etch to remove the portions of layer  52  not covered by a mask. The etched structure is shown  FIG. 3B  wherein the cross-sectional shape of the material in layer  52  under mask  54  has distinctly curved sides due to the nature of chemical activity during wet etching. 
         [0033]    The next step is shown in  FIG. 4A  in which the structure is planarized by depositing Material A to form layer  56  after removing mask  54 . As shown in  FIG. 4B , the process is then repeated by depositing Material B in layer  58  on layer  56  containing “islands”  52  of Material B. 
         [0034]    Mask pattern  60  is then deposited on layer  58  and exposed to an etch and to produce the structure shown in  FIG. 5A . In  FIG. 5A , the portion of Material B not under mask  60  has been removed to reveal islands  58  with curved sides. The structure is then planarized after removal of mask  60  by depositing a layer of Material A on layer  56  containing islands  58  to produce structure  70  shown in  FIG. 5B . The process can be repeated at will to produce composite structure  70  containing Material B inclusions  52 ,  58  etc. with anisotropic geometrical shapes in Material A matrix  50 ,  56 ,  60  etc. Inclusions  52 ,  58  etc. can have square, triangular, circular, or other cross-sections with respect to a plane parallel to layers. Cross-sectional shape of particles  52 ,  58  etc. with respect to a plane perpendicular to the layers as shown in, for instance,  FIG. 5B  can be formed by means other than wet etching such as ion beam techniques and others known to those in the art. Materials A and B can be at least one of electrical insulators, semiconductors, or conductors. Structure  70  shown schematically in  FIG. 5B  can be formed by physical vapor deposition techniques such as ion beam deposition (IBD), electron beam deposition (EBD), molecular beam epitaxy (MBE) and others known to those in the art. 
         [0035]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.