Patent Application
20140342488
The present invention relates to a method of manufacturing a thermoelectric nanowire, and more particularly, to a method of manufacturing a thermoelectric nanowire having a core/shell structure including a bismuth (Bi) nanowire as a core and a thermoelectric material as a shell.
In general, semimetals, such as bismuth (Bi), antimony (Sb), arsenic (As), silicon (Si), and germanium (Ge), have properties common to both metals and nonmetals, and are commonly used in electric device components as single compound or an alloy. Particularly, semimetals alloyed with semiconductors have received a great deal of attention as thermoelectric materials.
A thermoelectric material is a material having a low degree of thermal conductivity and a high degree of electrical conductivity, and in-depth research into thermoelectric materials is currently being conducted. For example, since an alloy of Bi and tellurium (Te), BixTe1-x has a relatively large mass and has a small spring constant, due to Van der Waals bonding between Bi and Te and covalent bonding between Te atoms, the thermal conductivity thereof may be decreased. Thus, the figure of merit (ZT), exhibiting the thermoelectric efficiency of the thermoelectric material may be increased, and so, BixTe1-x is currently in common use as a thermoelectric material.
In addition, by manufacturing a thermoelectric nanowire from such a BixTe1 x alloy, the electrical density of states may be controlled, and the Seebeck coefficient affecting the thermoelectric effect may be controlled by matching the shape and the peak position of the function of the electrical density of states with a Fermi level. In addition, the movement of electrons may be increased by the quantum confinement effect, and thus, electrical conductivity may be maintained at a high level. Thus, the limit of the thermoelectric material in a bulk phase may be overcome, and a relatively high ZT value may be obtained.
To obtain a high degree of thermoelectric efficiency, the manufacturing of a single crystal thermoelectric nanowire is necessary. However, when considering the inherent properties of the general thermoelectric materials, it may be somewhat difficult to grow a single crystal structure therein. Therefore, the growth of thermoelectric nanowires may be somewhat restricted, and growth methods for the single crystal thermoelectric nanowires have not been commonly known to date.
Generally, it is necessary to grow thermoelectric nanowires from an alloy rather than a single material, and so, a method of growth employing a solvent in which respective materials are dissolved is mainly used. This method includes a template-assisted method, a solution-phase method, a pressure injection method, and the like.
However, according to the template-assisted method, the preparation of a template is not easy. According to other methods, a relatively complicated process including the preparation of a starting material, etc. is necessary. In addition, the removal of an appropriate template and the removal of remaining chemical materials from the surface of a nanowire are necessary for conducting a single nanowire device process. Further, the formation of various patterns during the manufacturing of a device is difficult due to a low aspect ratio. Particularly, since a thermoelectric nanowire grown using a common method has polycrystallinity, thermoelectric efficiency is low, and the observation of the inherent properties of the single crystal thermoelectric nanowire is limited.
Along with the development of nano technology in the late 1990s, research into thermoelectric applications has been actively undertaken. The theoretical background of such research has shown that a thermoelectric figure of merit (ZT) that has reached the limit thereof may be increased in the case that Bi2Te3, known as the most appropriate material among bulk state materials for thermoelectric applications, is manufactured on the nanoscale level. However, the thermoelectric figure of merit obtained by using a single thin film or a nanowire is still a considerable distance from being commercially viable. Rather, a high thermoelectric figure of merit was measured in a thermoelectric application using a hetero structure such as a 2D ultra lattice thin film. Venkatasubramanian's group has reported the manufacturing of a 2D ultra lattice thin film in which a high thermoelectric figure of merit of 2.4 was obtained.
However, the group that measured a thermoelectric figure of merit by manufacturing a heterostructure nanowire, that is, a core/shell nanowire structure and by using a single nanowire was not present until now. Since the synthesis of a core/shell structure by using a general method of synthesizing a nanowire is relatively difficult, groups corresponding to masters in thermoelectric field also merely analyzed mechanisms using computer simulations.
An aspect of the present invention provides a method of manufacturing a thermoelectric nanowire having a core/shell structure of Bi/thermoelectric material through manufacturing a single nanowire and sputtering a thermoelectric material.
Particularly, another aspect of the present invention provides a method of manufacturing a thermoelectric nanowire having a core/shell structure having a desired degree of thermal conductivity by controlling interface roughness between a core and a shell of the thermoelectric nanowire having the core/shell structure.
According to an aspect of the present invention, there is provided a method of manufacturing a thermoelectric nanowire having a core/shell structure including preparing a substrate provided with an oxide layer formed thereon, and forming a Bi thin film on the oxide layer; heat treating a structure produced during forming the Bi thin film to induce compressive stress due to differences in coefficients of thermal expansion between the substrate, the oxide layer and the Bi thin film, to grow a Bi single crystal nanowire on the Bi thin film; and cooling the substrate of a structure on which the nanowire is grown to a low temperature, and sputtering a thermoelectric material on the Bi single crystal nanowire in a cooled state to manufacture a thermoelectric nanowire having a core/shell structure of Bi/thermoelectric material.
In an embodiment of the present invention, the manufacturing of the thermoelectric nanowire may include controlling roughness of an interface between the Bi single crystal nanowire and the thermoelectric material by controlling the temperature for cooling the substrate.
In an embodiment of the present invention, the cooling to a low temperature may be performed by using liquid nitrogen.
In an embodiment of the present invention, the forming of the Bi thin film may include forming the Bi thin film on the oxide layer in the cooled state using a sputtering method.
In an embodiment of the present invention, the thermoelectric material may be one selected from Te, Bi2Te3, PbTe, Sb and S.
In an embodiment of the present invention, the thickness of the shell, a thermoelectric material layer composing the thermoelectric nanowire may be may be equal to half the diameter of the Bi single crystal nanowire.
In an embodiment of the present invention, the single crystal Bi nanowire may have a diameter of 50 to 1,000 nm.
In an embodiment of the present invention, the oxide layer may be at least one selected from the group consisting of SiO2, BeO, and Mg2Al4Si5O18.
In an embodiment of the present invention, the heat treating temperature may be 200 to 270° C.
In an embodiment of the present invention, the method may further include final heat treating of the core/shell thermoelectric nanowire thus manufactured in the manufacturing operation of the thermoelectric nanowire. In an embodiment of the present invention, a temperature of the final heat treating may be selected from a temperature less than or equal to a melting point of Bi, or a temperature greater than or equal to a melting point of Bi to a temperature less than or equal to a melting point of the thermoelectric material.
As described above, a single crystal core/shell nanowire may be synthesized without difficulty. In addition, the nanowire may be synthesized without a separate template or a catalyst, and a core/shell nanowire based on a Bi nanowire may be synthesized by using various thermoelectric materials.
Further, the thermal conductivity of a thermoelectric nanowire manufactured by the method of the present invention may be determined by controlling the roughness of the interface of a core and a shell. Thus, a nanowire satisfying requirements required in various fields may be manufactured.
Further, a tube structure of various thermoelectric materials may be synthesized, and the observation of the physical properties of various novel materials such as thermoelectric properties and magnetic Kondo effect, may be helpful in the case when using the tube structure of the nanowire.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In addition, the terminology used herein is for the purpose of describing particular example embodiments only and is not intended to limit the present inventive concept. It will be further understood that terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
First, as shown in
In an embodiment of the present invention, the Bi thin film 50 may be formed on the oxide layer 30. The Bi thin film may be effectively manufactured by a commonly known sputtering method. For example, the Bi thin film 50 may be formed on the oxide layer 30 by a radio frequency (RF) magnetron sputtering method under a pressure of 4×10−8 torr at a rate of 32.7 Å/s. Particularly, the substrate 10 may be cooled using liquid nitrogen during forming the Bi thin film 50. The cooling process is performed to form small particle tissues to decrease the diameter of a Bi nanowire to be formed in a following process.
In an embodiment of the present invention, the Bi thin film 50 is preferably formed as a single crystal thin film. Generally, when the Bi thin film is a single crystal layer, the orientation of (003), (006), and (009) in an X-ray diffraction pattern may be obtained.
In addition, the preferable thickness of the Bi thin film 50 may be from 50 nm to 4 μm in an embodiment of the present invention.
After that, in an embodiment of the present invention, as shown in
As shown in
For example, the Bi thin film has a coefficient of thermal expansion of 13.4×10−6/° C., the oxide layer (SiO2) has a coefficient of thermal expansion of 0.5×10−6/° C., and the substrate (in the case of a Si substrate) has a coefficient of thermal expansion of 2.4×10−6/° C. Due to the large difference of the coefficients of thermal expansion, compressive stress may be applied to a structure including the substrate 10, the oxide layer 30 and the Bi thin film 50, and a nanowire may be grown on the Bi thin film 50 to relieve the compressive stress. That is, the compressive stress induced by the heat treatment may provide a driving force during the growth of the nanowire.
Meanwhile, the heat treating temperature of the Bi thin film 50 may be preferably 200 to 270° C. in an embodiment of the present invention. In addition, the heat treating time may be to 15 hours. In the case that the heat treating time increases, the Bi thin film expands much more, and more compressive stress may be induced.
Then, as shown in
Generally, a thermoelectric material is a material having thermoelectric properties of Seebeck effect by which a voltage is generated due to temperature difference at both ends and the Peltier effect by which one side generates heat and the other side absorbs heat when current flows between both ends. Particular examples of the thermoelectric material according to an embodiment of the present invention may include one selected from Te, Bi2Te3, PbTe, Sb and S.
In an embodiment of the present invention, a sputtering process and a cooling process of the substrate 10 to a low temperature may be performed at the same time. That is, in an embodiment of the present invention, the substrate 10 may be exposed to a cooling medium such as liquid nitrogen during performing the sputtering process so that the substrate 10 may maintain a cool state of a low temperature while sputtering the thermoelectric material 70 on the grown surface of a nanowire. Through the cooling process to the low temperature of the substrate 10, the kinetic energy of the thermoelectric material may be minimized, and the interface roughness between the Bi nanowire and the thermoelectric material of the core/shell structure may be smooth. From the above description, it may be confirmed that the interface roughness may be controlled to a desired level by appropriately controlling the cooling temperature to the low temperature by the cooling process.
As shown in
Meanwhile, a final heat treating process of a thermoelectric nanowire having a core/shell structure and produced through the above described processes may be further conducted in an embodiment of the present invention. In an embodiment of the present invention, the final heat treating temperature may be smaller than the melting point of Bi, or may be in the temperature range of the melting point of Bi to the melting point of the thermoelectric material. In the case that the final heat treating temperature is less than the melting point of Bi, the diffusion phenomenon of a material may be generated, and the diffusion of a thermoelectric material such as Te may occur in a Bi core area to produce a BiTe compound. Through this phenomenon, a thermoelectric nanowire having a core/shell structure in which the thermoelectric material is diffused in a core area may be manufactured.
Meanwhile, when the final heat treating temperature is greater than and equal to the melting point of Bi and less than or equal to the melting point of the thermoelectric material, a Bi component may be evaporated, and a nanowire of a thermoelectric material having a tube structure may be synthesized. That is, the tube structure of the thermoelectric material may be synthesized by applying the above-described heat treating temperature, and physical properties of various novel materials such as magnetic Kondo effect may be observed by using the nanowire having the tube structure as well as the thermoelectric properties.
As described above, a thermoelectric nanowire having a core/shell structure including a Bi nanowire manufactured by using compressive stress as a core and a thermoelectric material layer as a shell may be effectively obtained in the present invention.
Hereinafter, the present invention will be described through various embodiments.
A Si substrate provided with a SiO2 oxide layer formed thereon was prepared, and a Bi thin film was formed on the oxide layer by a RF magnetron sputtering method. In this case, the basic pressure of the sputtering was 4×10−8 torr, and the forming rate of the Bi thin film was 32.7 Å/s. In addition, during performing the sputtering of the Bi thin film, the substrate was cooled by using liquid nitrogen.
The substrate provided with the Bi thin film formed thereon was mounted on an alumina boat in a reaction furnace as shown in
Then, a thermoelectric material Te was deposited on the Bi nanowire at room temperature by using a RF magnetron sputtering method. During performing of the sputtering process, the substrate on which the liquid Bi nanowire was grown was cooled to a low temperature. The power of the RF magnetron sputtering was 12 W (rf).
Bi nanowire was formed as in Example 1, and a thermoelectric material, Te was deposited on the Bi nanowire at room temperature by using a RF magnetron sputtering method. During sputtering, a cooling process with respect to a substrate provided with a liquid Bi nanowire grown thereon to a low temperature was not performed differently from Example 1. The power of the RF magnetron sputtering was 30 W (rf).
Meanwhile,
a) illustrates a TEM photographic image of the cross-section of a thermoelectric nanowire having a core/shell structure manufactured in Example 2. A Bi single crystal nanowire core is found to be surrounded with a shell of a thermoelectric layer. The outer part corresponds to a deposited Pt part.
A Bi/Te thermoelectric nanowire having a BiTe compound in a core area and having a core/shell structure was manufactured by finally heat treating a core/shell thermoelectric nanowire manufactured by the same conditions as those in Example 2 to a temperature less than or equal to the melting point of Bi, that is at 250° C. for 2 hours.
A Bi/Te thermoelectric nanowire having a core/shell structure was manufactured by the same conditions as those in Example 2. Then, heat treating was performed at the temperature greater than or equal to the melting point of Bi (above 271 degrees) to the temperature less than or equal to the melting point of Te (449 degrees), particularly at 330° C. for 10 hours.
As described above, the manufacturing technique of a core/shell thermoelectric nanowire and an applying technique of a nano device may be confirmed according to the present invention. The improvement of the properties of common devices may be obtained, and the appearance of new devices may be possible.
In addition, since a thermoelectric device using a core/shell thermoelectric nanowire has super-high-efficient thermoelectric effect, a method of developing a new system may be served.
Further, developments to a higher level in various fields such as a Space Generator, a heater, an aeronautical heat regulator, a military infrared detector, and a circuit cooler for missile guidance, may be realized by using the technique of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2011-0123806 | Nov 2011 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR2012/002566 | 4/5/2012 | WO | 00 | 5/22/2014 |
