CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to United States provisional patent application entitled, "Nanostructured Copper Zinc Tin Sulfide-based Thermoelectric Energy Conversion," having U.S. Serial No. 61/442,947 and filed on February 15, 2011, the disclosure of which is hereby incorporated by reference.

INTRODUCTION TO THE INVENTION

[0002] The present disclosure is directed to thermoelectric materials and, more specifically, to nanostructured thermoelectric materials and methods of utilizing nanostructured materials including, without limitation, copper zinc tin sulfide-based materials.

[0003] Thermoelectric materials directly convert temperature difference into electric voltage and vice versa. Thermoelectric materials can be used to generate electricity from waste heat or used as a heater or cooler when electrically powered. The performance of a thermoelectric material is evaluated by a quantity called the dimensionless figure of merit, ZT. The dimensionless figure of merit can be expressed as an equation:

ZT= S ² -a-T/K

where:

S is the Seebeck coefficient,

σ is the electrical conductivity of the thermoelectric material,

T is the temperature, and

K is the thermal conductivity.

Greater values of ZT indicate greater thermodynamic efficiency and better device performance. [0004] A novel technique has been developed to use nanostructured copper zinc tin sulfide (CZTS) as a high efficient thermoelectric material. Using this novel nanostructured material is advantageous because it comprises relatively inexpensive, abundant, and non-toxic elements. This novel nanostructure improves electrical conductivity (σ = 1/ ) and the Seebeck coefficient (S) due to the enhanced electron density of states (DOS), while the phonon scattering from the nanostructure surface and lattice confinement reduce the thermal conductivity (κ). As a result, ZT is enhanced.

[0005] It is a first aspect of the present invention to provide a nanocrystalline form of copper zinc tin sulfide having a mean particle size ranging between five to thirty nanometers.

[0006] In a more detailed embodiment of the first aspect, a Seebeck coefficient is greater than 65 μν/Κ and less than 301 μΎ Κ within a temperature range of 300 K to 700 K. In yet another more detailed embodiment, an electrical conductivity is greater than 409 S/m and less than 1388 S/m within a temperature range of 300 K to 700 K. In a further detailed embodiment, a thermal conductivity is greater than 0.645 W/mK and less than 0.695 W/mK within a temperature range of 300 K to 700 K. In still a further detailed embodiment, a dimensionless figure of merit is greater than 0.0008 and less than 0.14 within the temperature range of 300 K to 700 K.

[0007] It is a second aspect of the present invention to provide a method of fabricating crystalline copper zinc tin sulfide nanoparticles, the method comprising: (a) mixing a copper precursor, a zinc precursor, and a tin precursor to create a reactant mixture; (b) degassing the reactant mixture under vacuum for a predetermined time; (c) triggering a reaction by injection of a sulfur precursor into the degassed reactant mixture at a predetermined temperature to create a product mixture; (d) precipitating a portion of the product mixture to form an intermediate including copper zinc tin sulfide nanocrystals; and, (e) refining the intermediate to increase a concentration of copper zinc tin sulfide nanocrystals in a refined product.

[0008] In a more detailed embodiment of the second aspect, the method further includes drying the refined product to create dried copper zinc tin sulfide nanocrystals, and grinding the dried copper zinc tin sulfide nanocrystals into a copper zinc tin sulfide nanocrystal powder. In yet another more detailed embodiment, the method further includes hot pressing the dried copper zinc tin sulfide nanocrystals. In a further detailed embodiment, the copper precursor comprises copper

acetylacetonate. In still a further detailed embodiment, the zinc precursor comprises zinc acetate dihydrate. In a more detailed embodiment, the tin precursor comprises tin acetate dihydrate. In a more detailed embodiment, after the degassing step, the reactant mixture is purged with an inert gas. In another more detailed embodiment, after the purging step, the reactant mixture is heated to 300 °C upon the injection of a sulfur precursor and then reacted for one hour at 300 °C. In yet another more detailed embodiment, the precipitating step includes mixing the product mixture with ethanol, followed by centrifugation. In still another more detailed embodiment, the refining step includes dispersing the intermediate in chloroform, followed by centrifugation, to create a supernatant comprising copper zinc tin sulfide nanocrystals.

[0009] It is a third aspect of the present invention to provide a method of fabricating crystalline copper zinc tin sulfide nanoparticles, the method comprising: (a) mixing a copper precursor, a zinc precursor, and a tin precursor to create a reactant mixture; (b) triggering a reaction by mixing a sulfur precursor with the reactant mixture to create a product mixture; and, (c) precipitating a portion of the product mixture to form an intermediate including copper zinc tin sulfide nanocrystals.

[0010] In a more detailed embodiment of the third aspect, the method further includes dispersing the intermediate in a dispersing agent to create a suspension, and processing the suspension to retain a supernatant that includes copper zinc tin sulfide nanocrystals. In yet another more detailed embodiment, the method further includes precipitating a portion of the supernatant to form a refined product comprising a higher percentage of copper zinc tin sulfide nanocrystals than in the intermediate, and at least one of centrifuging, drying, and pressing the refined product.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is an X-ray diffraction on the synthesized exemplary material of the instant disclosure, with the diffraction pattern being indexed to tetragonal CZTS. [0012] FIG. 2 is a transmission electron microscopy analysis on the synthesized exemplary material of the instant disclosure.

[0013] FIG. 3 is a UV-Vis spectrum of the synthesized exemplary material of the instant disclosure.

[0014] FIG. 4 is a magnification image from a scanning electron microscope of a bulk sample of synthesized exemplary material of the instant disclosure after hot press.

[0015] FIG. 5 is a plot of thermoelectric properties of the synthesized exemplary material of the instant disclosure as a function of temperature.

DETAILED DESCRIPTION

[0016] The exemplary embodiments of the present disclosure are described and illustrated below to encompass thermoelectric materials and, more specifically, to nanostructured thermoelectric materials and methods of utilizing and creating nanostructured materials including, without limitation, copper zinc tin sulfide-based materials. Of course, it will be apparent to those of ordinary skill in the art that the preferred embodiments discussed below are exemplary in nature and may be reconfigured without departing from the scope and spirit of the present invention. However, for clarity and precision, the exemplary embodiments as discussed below may include optional steps, methods, and features that one of ordinary skill should recognize as not being a requisite to fall within the scope of the present invention.

[0017] A novel nanostructure thermoelectric energy conversion material fabricated in accordance with the instant disclosure is believed to have the following chemical formula: Cu ₂ ZnSnS (abbreviated hereafter as CZTS). An exemplary synthesis of CZTS in the form of nanocrystals includes adding 0.98 g of copper acetylacetonate (97%), 0.295 g of zinc acetate dihydrate (reagent grade), and 0.355 g of tin chloride (98%) to 55 mL of oleylamine in a 100 mL three-neck flask on a Schlenk line.

Meanwhile, 0.195 g of sulfur (99.5%) is dissolved in 5 mL of oleylamine in a separate vial at 100 °C.

[0018] The resulting reaction mixture is thereafter degassed under vacuum at 80 °C for one hour, then purged with nitrogen for thirty minutes at 1 10 °C, and then heated 2012/024973

to 300 °C when the sulfur in oleylamine solution is injected. After injection the temperature is held at 300 °C for one hour, followed by cooling until the contents reach room temperature.

[0019] It should be noted that greater quantities of CZTS nanocrystals may be created simply by multiplying the amount of each ingredient by a predetermined factor.

Accordingly, the instant disclosure is not limited by the foregoing amounts listed for each ingredient.

[0020] The contents are then combined with ethanol, followed by centrifugation, in order to precipitate the CZTS nanocrystals. The precipitated CZTS nanocrystals are further refined to remove solid reaction by products and aggregates of poorly capped nanocrystals by dispersing the nanocrystals in chloroform, followed by centrifugation conducted for two minutes at 8000 rpm. The resulting precipitation is discarded, while the CZTS nanocrystals comprise a supernatant. The supernatant with the CZTS nanocrystals is washed with diluted hydrazine hydrate/ethanol solution (v:v = 1 : 10) and then with ethanol/chloroform for three times to remove the capping ligands. The CZTS nanocrystals are thereafter collected by centrifugation, dried in vacuum, and ground into powder.

[0021] A hot press (HP) process is thereafter carried out on the CZTS nanocrystals to form a bulk material while preserving the nanostructure. In this exemplary HP process, the CZTS nanocrystals are consolidated at 523 K for 15 minutes under an axial pressure of 120 MPa.

[0022] Referring to FIG. 1, X-ray diffraction was performed on the as-synthesized CZTS nanocrystals and the hot pressed CZTS bulk, where the diffraction pattern was indexed to tetragonal CZTS.

[0023] Referencing FIG. 2, a transmission electron microscopy analysis confirmed the formation of CZTS nanocrystals. The diameters of the majority of CZTS

nanocrystals were in the range of 5 to 30 nanometers. [0024] Referring to FIG. 3, a UV-Vis spectrum indicated that the band gap of the CZTS nanocrystals was 1.5 eV. The relative density of the final CZTS nanocrystals is 89%, which indicates a porous structure.

[0025] Referencing FIG. 4, scanning electron microscopy analysis was performed on the CZTS nanocrystals in order to demonstrate that CZTS nanocrystals hot pressed are truly nanostructured. As is visible in FIG. 4, the individual crystalline domains have a diameter of approximately 5 to 30 nanometers, which is consistent with the diameter of as-synthesized CZTS nanocrystals.

[0026] Referring to FIGS. 5a-5d, the temperature dependence of the thermoelectric properties for the CZTS nanocrystals were investigated from 300 to 700 K.

[0027] Referencing FIG. 5a, Seebeck coefficients for the CZTS nanocrystals were measured using MMR Technologies' MMR SB 100 Seebeck Measurement System (www.mmr.com). Comparing the measured Seebeck coefficients for the CZTS nanocrystals with bulk CZTS, it was observed that the Seebeck coefficient was enhanced with the CZTS nanostructure at high temperature. The Seebeck coefficient for CZTS nanocrystals increases from 65 μΥ/Κ at room temperature to 301 μν/Κ at 700 K, which is 43% higher than the Seebeck coefficient of the CZTS bulk crystals. A positive Seebeck coefficient indicates that the conduction in CZTS nanocrystals is p-type.

[0028] Referring to FIG. 5b, electrical conductivity of the CZTS nanocrystals were measured with a home-made electrical measurement system. The electrical conductivity of the CZTS nanocrystals increases from 409 S/m at 300 K to 1388 S/m at 700 K.

[0029] Referencing FIG. 5c, thermal conductivity measurements were taken of the CZTS nanocrystals by Thermophysical Properties Research Laboratory, Inc., 3080 Kent Ave., West Lafayette, Indiana. Thermal diffusivity (a) of the CZTS

nanocrystals was measured using the laser flash technique. Bulk density (p) was calculated from the CZTS nanocrystals' geometry and mass. Specific heat (Cp) was measured using differential scanning calorimeters. Thermal conductivity (κ) was calculated as a product of these quantities, i.e. κ = a · Cp · p. The results of these measurements and calculations are plotted separately as a function of temperature. As can bee seen from these plots, the thermal conductivity of the Cu-doped CZTS nanocrystals remains low between 300 and 700 K and reaches a minimum of 0.645 W/(m-K) at 700 K, which corresponds to a 28.3% decrease compared to the Cu-doped CZTS bulk crystals (0.9 W/m-K at 700 K).

[0030] Referring to FIG. 5d, the power factor and figure of merit (ZT) is calculated with the aforementioned thermoelectric properties. The power factor and overall ZT for the CZTS nanocrystal samples reach to their peak values of 125 μΨ/m-K ² and 0.14 at 700 K, respectively.

[0031] Following from the above description and disclosure summaries, it should be apparent to those of ordinary skill in the art that, while the methods and apparatuses herein described constitute exemplary embodiments of the present disclosure, the invention contained herein is not limited to this precise embodiment and that changes may be made to such embodiments without departing from the scope of the invention as defined by the claims. Additionally, it is to be understood that the invention is defined by the claims and it is not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the interpretation of any claim element unless such limitation or element is explicitly stated. Likewise, it is to be understood that it is not necessary to meet any or all of the identified advantages or objects of the disclosure in order to fall within the scope of any claims, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.

[0032] What is claimed is: