Abstract:
Disclosed is a method of consolidating a powder. The method can include obtaining a powder of semiconductor nanocrystals, obtaining a material which will form a gas when heated, and combining the powder and the material into a combined powder. The method can also include consolidating the powder by applying heat and pressure to the combined powder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of co-pending U.S. Provisional Application Ser. No. 61/618,109, filed 30 Mar. 2012, which is hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the present disclosure relate generally an increased thermoelectric performance of nanocrystals by introducing inclusion materials and gas voids into a consolidated material. Embodiments include introducing volatile compounds to powdered nanocrystals. 
       BACKGROUND OF THE INVENTION 
       [0003]    The electrical conductivity and the thermal conductivity of a material are inherently coupled. This has been the subject of study for decades. The celebrated Wiedemann-Franz law describes the relationship between the electrical and thermal conductivities. This is confounding for many interested in improving the performance of thermoelectric material. The goal for such a material is to have the lowest possible thermal conductivity with the highest possible electrical conductivity. Realistically, this is only accomplished by manipulating the lattice component to affect the thermal conductivity. The electrical component to the thermal conductivity is inherently coupled to the electrical conductivity of the material. 
         [0004]    Previous attempts to reach this goal have used dopants, impurities for scattering, rattlers like those found in skutterudites, grain boundaries, and the like for years in an effort to reduce the lattice contribution to the thermal conductivity. However, results have been very limited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    These and other features of the disclosure will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various aspects of the invention. 
           [0006]      FIG. 1  shows a cross-section view of a consolidated material that may include embodiments of the invention disclosed herein. 
           [0007]      FIG. 2  shows a cross-section view of a consolidated material that may include embodiments of the invention disclosed herein. 
       
    
    
       [0008]    It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       SUMMARY OF THE INVENTION 
       [0009]    A first aspect of the present invention includes a method of consolidating a powder comprising: obtaining a powder of semiconductor nanocrystals; obtaining a material which will form a gas when heated; combining the powder and the material into a combined powder; and consolidating the powder into a consolidated material by applying heat and pressure to the combined powder. 
         [0010]    A second aspect of the present invention includes a method of consolidating a powder comprising: obtaining a powder of semiconductor nanocrystals; obtaining an inclusion material; combining the powder and the inclusion material into a combined powder; and consolidating the powder into a consolidated material by applying heat and pressure to the combined powder. 
         [0011]    A third aspect of the present invention includes a consolidated material comprising: a plurality of semiconductor nanocrystals in a lattice structure; and a plurality of gas filled voids within the lattice structure. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    It is understood that there is a competition between the electrical conductivity and the thermal conductivity when making thermoelectric materials. The highest performing thermoelectric (TE) materials should possess low thermal conductivities and high electrical conductivities. However, these two properties are intimately connected. The thermal conductivity in fact has an electronic component that is directly proportional to the electrical conductivity. Hence a large electrical conductivity will typically result in a large electronic component to the thermal conductivity. 
         [0013]    Disclosed herein is a novel approach of creating gas-filled stable voids in a consolidated material. In addition to the thermal conductivity benefits, these void regions can inhibit grain growth of the material. 
         [0014]    Turning to  FIG. 1 , in some embodiments of the current invention, the disclosed consolidated material  100  can help to modify the lattice component of consolidated material  100  in order to affect the thermal conductivity. According to these embodiments, the consolidated material  100  made using these methods can help increase the quantum confined properties of the semiconductor nanocrystals  110  used in consolidated material  100 . Quantum confinement, a property unique to some nanomaterials, is usually lost when particles of nanomaterials are in intimate contact with particles of the same material. Accordingly, when a powder of semiconductor nanocrystals  110  is consolidated, the close proximity of semiconductor nanocrystals  110  can often reduce, or even eliminate, the extent of quantum confinement properties of consolidated material  100 . 
         [0015]    In one embodiment of the current invention, semiconductor nanocrystals  110  are utilized that have physical dimensions that are less than the Bohr radius of the material, leading to quantum confined effects. Further, semiconductor nanocrystals  110  may be colloidally grown nanocrystals. The physical dimensions of the Bohr radius vary based on the composition of the nanocrystal, but typically include at least one dimension being in the range of approximately 1 nm to 30 nm. However, when these nanocrystals are assembled into a solid such that the nanocrystals are in intimate contact with one another, the quantum confined effects can be lost as the properties of such close nanocrystals can take on the properties of a bulk material. Accordingly, in the prior methods, a consolidated material would act as a bulk material of the nanocrystals used. 
         [0016]    The inventors have found that at a high level, the electronic band structure of an individual nanocrystal may give way to the ensemble band structure of the solid material since there is no discernible distance between the nanocrystals when they are in contact with one another. The electrons are no longer “quantum confined” as they are free to pass from nanocrystal to nanocrystal without even noticing a change. If the nanocrystals were separated by a different material, especially one with a higher bandgap energy, the effects of quantum confinement can be reestablished within the nanocrystals. In many previous instances, it was undesirable to introduce a matrix material made of higher bandgap semiconductor material as a spacer between the embedded nanocrystals. In thermoelectric applications, for example, the performance of a composite like this is typically limited to the lowest performing semiconductor material in the system. It can be difficult to select two high performing semiconductor materials which can be utilized to create this matrix-nanocrystal assembly where quantum confinement is preserved. An alternative, as discussed herein, is to introduce gas filled voids  120  into consolidated material  100 . 
         [0017]    Gas filled voids  120  can be created in consolidated material  100  in a number of ways. For instance, gas filled voids  120  can be created by not fully drying a powder containing semiconductor nanocrystals  110  prior to consolidation of the powder. The powder may consist of a plurality of semiconductor nanocrystals  110 , which may consist of one or more populations of nanocrystals. The one or more populations may include the same or different compositions of material and the same or different sizes of the nanocrystals. For instance, in creating gas voids  120 , if a volatile solvent, which in some embodiments may include ether, is introduced in a controlled amount to the powder, the process of consolidation can cause the volatile solvent to vaporize. However, the pressure of consolidation may not allow the gas to escape when the volatile solvent vaporizes. This can result in a plurality of gas filled voids  120  within consolidated material  100 , which are filled with the gas released by the volatile solvent being present in the material after consolidation of the powder. 
         [0018]    In further embodiments, consolidated material  100  may be heat treated following consolidation of the powder. In these embodiments, the gas in the gas filled voids  120  may expand during heating. This expansion of the gas within gas filled void  120  can cause the lattice of the material, which can be seen as an illustration of the approximate pattern in  FIG. 1 , to be more relaxed. As shown in  FIG. 1 , a lattice has formed from the consolidation of semiconductor nanocrystals  110  and gas filled voids  120 . It is to be understood that this is just an illustration of a lattice structure, and many different lattice shapes are possible. Returning to the embodiments described, an expansion of gas filled voids  120  and the resulting relaxed lattice structure can lead to a lower density consolidated material  100  with larger gas filled voids  120 . The heat treatment can be performed as a post-consolidation treatment, or alternatively, may be performed during the consolidation step by reducing the pressure at an appropriate time as consolidated material  100  is cooling. 
         [0019]    In other embodiments, there are some materials that can be added to consolidated material  100  that will volatilize during the heating phase of the consolidation process, which may also then cool and become a solid at the operational temperatures. The materials could include, as one example, hydrocarbons. In this embodiment, the hydrogen and carbon bonds may actually break during the heating involved in consolidation, leaving a carbon residue inside gas filled voids  120 . This can be beneficial as the carbon residue can act to reduce the thermal conductivity of consolidated material  100 . Further the carbon residue can act as a grain growth inhibitor for consolidated material  100 , helping maintain the nanoscale of semiconductor nanocrystals  110 . 
         [0020]    In further embodiments, materials that undergo a phase change at the thermoelectric operational temperature may be ideally suited to reduce the thermal conductivity. For example, if the end application requires operating temperatures around 100° C., trapping water inside gas filled voids  120  could greatly reduce the thermal conductivity as the water will undergo a phase transformation to a gas. Although water is one example, there are other known volatile materials that don&#39;t have the oxidizing effect that water has. Some non-limiting examples include ether, alcohol, and hydrazine, which can all be introduced to the powder containing semiconductor nanocrystals  110  in order to create gas filled voids  120 . 
         [0021]    In ideal thermoelectric situations, it would be beneficial if each individual nanocrystal was surrounded by a gas or a void, such as gas filled voids  120 . In such a scenario, quantum confined effects would likely be present to a larger degree than usual. However, it is difficult if not impossible to create a solid material that has free-floating semiconductor nanocrystals  110 . This ‘ideal’ structure can, however, be approximated using an aerogel-type of scaffolding, or to some extent, by introducing gas filled voids  120  in the material. The electronic band structure for the semiconductor nanocrystals  110  that are close to the regions of gas filled voids  120  may exhibit more quantum confined behavior than those close to solid structures. 
         [0022]    The nanoscale size can impact the lattice thermal conductivity without affecting the Seebeck coefficient of the material. In order to create an effective thermoelectric material, it can be helpful to insulate the nanocrystals, causing the electrons to have to ‘hop’ from one isolated nanocrystal to another. Such insulation can come, in part, from gas filled voids  120 . There can be in increase in the Seebeck coefficient when the above disclosed gas filled voids  120  are present, as a portion of the nanocrystal is essentially confined. 
         [0023]    Typical mean free path (MFP) for charge carriers in average semiconductor materials are on the order of about 100 nm. This is far larger than the physical dimension of semiconductor nanocrystals  110 , which are typically only a few nanometers, sometimes between about 2 and 20 nm. In consolidated material  100  containing gas filled voids  120  surrounding each semiconductor nanocrystal  110 , or at least many of them, semiconductor nanocrystals  110  could be considered quantum confined more effectively than if the nanocrystals are in close contact with one another. In such a material, heat will not transfer so easily via lattice vibrations, as there is effectively little or no lattice structure within gas filled voids  120 . Since the charge carrier&#39;s MFP is larger than the physical dimension of semiconductor nanocrystals  110 , and larger than the typical gas filled voids  120  within consolidated material  100 , electrons and other charge carriers may be capable of travelling through a number of other semiconductor nanocrystals  110 , as well as gas filled voids  120 , with little energy lost to any thermal losses. Thus, the Seebeck coefficient is enhanced with stable inclusions and void integration in a thermoelectric material. 
         [0024]    It is understood that an efficient thermoelectric material is one with a delta-function density of states. This density of states may be approximated by a spaced super lattice of semiconductor nanocrystals  110  of the same size and stoichiometry. The disclosed consolidated material  100  approaches the density of states. 
         [0025]    In further embodiments, as illustrated in  FIG. 2 , introducing an inclusion material  130  to consolidated material  100  that will withstand the rigors of consolidation can be another way to increase the thermoelectric performance of consolidated material  100 . Once the powder of semiconductor nanocrystals  110  is ready for consolidation, nano or micro particles of other materials having a higher melting point can be introduced. The introduction of these ‘foreign’ inclusion materials  130  with a higher melting point can increase the thermoelectric performance as they will not easily bond to the lattice structure of consolidated material  100 . As such, inclusion materials  130  could act as a “rattler” to reduce the lattice thermal conductivity that can happen from vibrations of the lattice structure of consolidated material  100 . 
         [0026]    Some non-limiting examples of inclusion materials  130  can include nanoscale titania, nanoscale alumina, and other nanoscale oxides, as well as micro-sized glass beads or even elemental materials such as elemental sulfur. Other elemental materials may be used as well, such as carbon and silicon. Although illustrated in  FIG. 2  as being used in conjunction with gas filled voids  120 , it should be understood that inclusion materials  130  can be used alone with semiconductor nanocrystals  110 , or in combination with gas filled voids  120 . Further, inclusion materials  130  may comprise one or more types of inclusion material. It should be understood that inclusion material  130  can include different sizes and shapes of material, and thus the lattice structure shown and distribution of inclusion materials  130 , as well as gas filled voids  120 , are only illustrative. 
         [0027]    In addition to the “rattler” affect described above, since the disclosed inclusion materials  130  are typically all higher bandgap materials, the electron band structure of semiconductor nanocrystals  110  directly adjacent to inclusion materials  130 , which in some cases can be considered impurities, will be less like those of the bulk material and more like that of pure unattached nanocrystals  110 , i.e., quantum confined. This is aligned with the basic notion that the lower the density of states, the larger the enhancement to the thermoelectric performance. In a further embodiment, semiconductor nanocrystals  110  included in materials which can form a nanoporous matrix, such as, xerogels, aerogels, nanoporous silicon, or semiconductor nanocrystals  110  mixed with high index glass beads or nano titania, in order to form consolidated material  100 , can all demonstrate this increased thermoelectric property by creating such a lattice structure and at least partially isolating one or more semiconductor nanocrystals  110  within the lattice structure. 
         [0028]    As a further benefit of the disclosed gas filled voids  120  and/or inclusion materials  130  in a consolidated material  100 , the different gas filled voids  120  and inclusion materials  130  disclosed can also act as a grain growth inhibitor in the material. Grain growth within consolidated material  100  can decrease the effectiveness of the material, as the grains can reduce the quantum confined properties of the end material. Further, they can reduce the lattice structure&#39;s thermal conductivity in the final material. 
         [0029]    The consolidated material as described in the above embodiments can be useful for many applications. For instance, due to the unique structure, consolidated material  100  can be effective as a thermoelectric device or part of a thermoelectric device. However, this is not meant to be limiting. 
         [0030]    The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.