KR20170065308A - Bi-Te-Se based thermoelectric powder and materials with improved thermostability and manufacturing methods thereof - Google Patents
Bi-Te-Se based thermoelectric powder and materials with improved thermostability and manufacturing methods thereof Download PDFInfo
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Abstract
The present invention provides a thermoelectric material with improved thermal stability and a method of manufacturing the same. In particular, the present invention proposes a thermoelectric powder capable of producing thermoelectric materials with improved thermal stability. This thermoelectric powder includes a Bi-Te-Se thermoelectric material core part; And a graphene oxide shell portion coated on the surface of the core portion.
Description
BACKGROUND OF THE
Due to the temperature difference at both ends of the solid state material, the electrons (or holes) having a thermal dependence cause a difference in the concentration at both ends, and this is caused by an electric phenomenon, that is, a thermoelectric phenomenon. Such a thermoelectric phenomenon can be classified into a thermoelectric power generating electric energy and a thermoelectric cooling / heating which causes a temperature difference at both ends by electric power supply.
Thermoelectric materials show such a thermoelectric phenomenon, and many researches have been made because of their environmental and sustainable advantages in power generation and cooling. Particularly, there is a high interest as a technology to improve fuel efficiency and reduce CO 2 by producing electric power from industrial waste heat and automobile waste heat.
Generally, a thermoelectric element is composed of a p-type thermoelectric element made of a p-type thermoelectric material that moves a hole by moving a hole, and a p-type thermoelectric element made of an n-type thermoelectric element made of an n- One pair of elements may be a basic unit, and may be configured as a module type comprising a plurality of pairs of pn thermoelectric elements, electrodes above and below the pn thermoelectric element, and an insulating substrate.
The energy conversion efficiency of a thermoelectric device depends on ZT (= S 2 σTk -1 ) which is the dimensionless performance index value of the thermoelectric material. Where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and k is the total thermal conductivity. Many thermoelectric materials have been proposed and developed so far. The thermoelectric materials are largely classified into a metal system and an oxide system, and the metal system includes a chalcogenide system, a silicide system, a clathrate system, a Half Heusler system, a skutterudite system, And so on.
Generally, the higher the temperature difference between the two materials is, the higher the efficiency and the electromotive force are formed. However, a high temperature for forming a large temperature difference causes thermoelectric material composition change through atomic physical chemical phenomena such as melting, diffusion, and sublimation, leading to degradation of the thermoelectric material , A thermoelectric element, a module as an aggregate thereof, and a thermoelectric element including the same.
This problem is called degradation due to thermal stability. In order to improve the thermal stability of a thermoelectric element, a metal thin film coating technique utilizing various vapor deposition is known. However, due to various problems such as secondary diffusion of the metal thin film, interfacial adhesion, reduction of electromotive force due to contact resistance, and the like, it is necessary to develop a new technology.
It is an object of the present invention to provide a thermoelectric material having improved thermal stability and a method of manufacturing the same.
In order to achieve the above object, the present invention proposes a thermoelectric powder that can produce a thermoelectric material having improved thermal stability.
The thermoelectric powder according to the present invention is a composite of a Bi-Te-Se based thermoelectric material and graphene oxide. Particularly, the thermoelectric powder according to the present invention comprises a Bi-Te-Se thermoelectric material core part; And a graphene oxide shell portion coated on the surface of the core portion.
The core portion may have an average particle size of 50 nm to 500 mu m. The shell portion may include a single layer of graphene oxide.
The thermoelectric material according to the present invention includes a sintered body of such a thermoelectric powder. Therefore, the method of manufacturing a thermoelectric material according to the present invention includes the step of sintering this thermoelectric powder.
According to another aspect of the present invention, there is provided a thermoelectric material comprising: a Bi-Te-Se thermoelectric material grain; And graphene oxide located at grain boundaries between the thermoelectric material grains are consolidated by sintering.
The thermoelectric material having such a structure can be produced by a simple mixing and sintering method of a general Bi-Te-Se thermoelectric material powder and graphene oxide, but it is preferable to sinter the thermoelectric powder according to the present invention. In the case of sintering by simple mixing, it may be difficult to obtain a fine structure in which graphene oxide surrounds the Bi-Te-Se thermoelectric material grain depending on the degree of mixing. However, in the case of Bi- -Se-based thermoelectric material The sintered powder of the structure wrapped with the graphene oxide shell part can easily and reproducibly obtain the microstructure surrounding the Bi-Te-Se based thermoelectric material grains by grapin oxide.
The thermoelectric material according to the present invention may further include amorphous carbon in the grain boundaries. The amorphous carbon may be due to thermal decomposition of the graphene oxide.
In the present invention, a bulk thermoelectric material including such a thermoelectric material, a thermoelectric element dicing the same, and a thermoelectric module such as a thermoelectric module integrated therewith are also proposed.
According to the present invention, a thermoelectric material having thermal stability can be produced by coating graphene oxide on the surface of a Bi-Te-Se based thermoelectric material. Graphene oxide wrapped around the Bi-Te-Se thermoelectric material suppresses the volatilization of the Te element from the Bi-Te-Se thermoelectric material and prevents or suppresses the diffusion of the material in the thermoelectric material, The heat resistance and thermal stability of the material are improved. A thermoelectric element such as a thermoelectric element or thermoelectric module including such a thermoelectric material is excellent in thermal stability, and the degree of deterioration is relatively small even after long-term use at a high temperature.
In the present invention, graphene oxide can be coated on the surface of a Bi-Te-Se thermoelectric material by utilizing graphene oxide having excellent adhesive strength without using other organic materials, and phonon scattering is increased through graphene oxide, And the thermal stability can be ensured while improving the figure of merit.
As described above, the thermoelectric material according to the present invention is remarkably effective in improving the performance index and securing the thermal stability.
In addition, the thermoelectric powder and the thermoelectric material manufacturing method according to the present invention are economical because they are performed by a simple solution-based method without expensive deposition processes and the like.
BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the description of the invention given below, serve to further augment the technical spirit of the invention. Should not be construed as limiting.
FIG. 1 is a diagram schematically illustrating a configuration of a thermoelectric powder according to an embodiment of the present invention. Referring to FIG.
2 is a flowchart schematically showing a method of manufacturing a thermoelectric powder according to an embodiment of the present invention.
FIG. 3 is a diagram schematically showing a configuration of a thermoelectric material manufactured using a thermoelectric powder according to an embodiment of the present invention. Referring to FIG.
4 shows XRD (X-Ray Diffractometer) analysis results of Examples and Comparative Examples of the present invention.
Figure 5 shows the results of Raman analysis for Examples and Comparative Examples.
6 is an SEM photograph of the thermoelectric powder of Comparative Example.
7 is an SEM photograph of the thermoelectric powder of Example.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as 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. It is provided to let you know. It should be noted that the embodiments described in the present specification and the configurations shown in the drawings are only the most preferred embodiments of the present invention and are not intended to represent all of the technical ideas of the present invention so that various equivalents And variations are possible.
FIG. 1 is a diagram schematically illustrating a configuration of a thermoelectric powder according to an embodiment of the present invention. Referring to FIG.
Referring to FIG. 1, a
The
Bi-Te-Se thermoelectric materials are n-type thermoelectric materials by mixing Se with a base material, Bi-Te thermoelectric material. Representative materials are represented by the formula Bi 2 Te x Se a -x ( 2? X <3, 3? A <3.2). In order to adjust the temperature range in which the maximum figure of merit (ZT) appears, the composition of any one of Bi, Se and Te can be changed.
The Bi-Te-Se based thermoelectric material may further include at least one of metal oxides and metal antimonides other than Bi, Te and Se which are the main elements. For example, a material represented by the formula Bi 2 Te x Se ax M b (2? X <3, 3? A <3.2, 0 <b). In particular, M may be any one or more of oxides or antimonides of any one of Cu, Bi, Sn, In, Co and Fe. M is preferably contained in a range of 0 to 1 wt% based on the total weight. More preferably, M is contained in an amount of 0.01 wt% to 0.2 wt% based on the total weight. However, the Bi-Te-Se-based thermoelectric material used in the present invention is not limited thereto, and can be applied to all the composition ranges corresponding to known Bi-Te-Se-based thermoelectric materials.
On the other hand, the shape of the
In one example, the
As described above, the
On the other hand, the
The
As described above, the
In the
The
Graphite is a structure in which planes of carbon are arranged like honeycomb hexagonal nets, and one layer of this graphite is called graphene. Graphene has a very high physical and chemical stability with a thickness of 0.35 nm. As graphene emerges as a new material with excellent properties, a composite with graphene applied to a thermoelectric material has been studied for improving thermoelectric properties. However, the results of the studies so far have not been able to realize or realize the thermoelectric property improving effect, and various surfactants have been utilized for improving the interfacial adhesion, which is also an cause of increasing the process cost. There are other known methods for utilizing graphene as an electrode of a thermoelectric element, as a thin film layer for preventing heat conduction, or for making a carbon material composite for reinforcing flexibility. However, a thermoelectric material column There is no known method of applying graphene directly to thermoelectric materials for stability.
At present, graphene is produced by a physical separation method, a chemical vapor deposition (CVD) method, an epitaxial synthesis method, a chemical stripping method, a graphite interlayer compound method, etc. Generally, a CVD method and a chemical stripping method .
The chemical stripping method is a method utilizing the oxidation-reduction characteristics of graphite. First, graphite oxide is produced by oxidizing graphite with strong acid and oxidizing agent. Since graphite oxide is hydrophilic, it is easy for water molecules to be inserted between the surface and the surface, and when it comes into contact with water, water molecules permeate between the surface and the surface due to the strong hydrophilic nature of the graphite oxide. As a result, the interplanar spacing is increased, and it can be easily peeled off by stirring for a long time or using an ultrasonic mill. The exfoliated graphite oxide is graphen oxide which is contained in the shell portion of the thermoelectric powder of the present invention.
The Bi-Te-Se thermoelectric material is a thermoelectric material suitable for use in a low-temperature region of 200 ° C or less. In the case of a conventional Bi-Te-Se thermoelectric material, Te or Se vacancies and anti-site defects (for example, Bi Te , Bi Se ) generated during a bulk manufacturing process, The reproducibility was not good because it was very difficult to control the film, and this has been a big obstacle to commercialization. Te has a very high vapor pressure at high temperatures and is highly volatile. Te volatilization during material synthesis or sintering is a major requirement for lowering the thermoelectric performance of a thermoelectric device. In addition, in the case of controlling the ratio of Te and Se, which is generally used for controlling the physical properties, composition control is very difficult in controlling the carrier density and tuning properties. In addition, There have been many cases where the change in physical properties is large and the power factor is rapidly decreased.
In the present invention, graphene oxide is coated on the surface of a Bi-Te-Se thermoelectric material core portion to enhance safety, thereby suppressing non-uniform Te or Se baking and anti-site defect generation. Therefore, a Bi-Te-Se based thermoelectric material having good physical property reproducibility can be produced.
As described above, the thermoelectric material powder according to the present invention is a high-efficiency n-type thermoelectric material having a thermoelectric performance improved by providing a graphene oxide coating layer as compared with a conventional Bi-Te-Se thermoelectric material. Specifically, graphen oxide suppresses the volatilization of Te element, prevents or suppresses diffusion in the material, improves thermal stability, increases thermal conductivity by increasing phonon scattering, and increases thermoelectric performance of the material itself.
According to the present invention, the graphene oxide coated on the surface of the Bi-Te-Se based thermoelectric material core can prevent or suppress the volatilization of the main elements constituting the Bi-Te-Se based thermoelectric material, Thermal stability of the thermoelectric powder and the thermoelectric material sintered therewith can be ensured. Although graphene oxide itself is insulating, according to the thermoelectric powder manufacturing method of the present invention, since it can be thinly coated with a single layer, the thermoelectric powder itself including the Bi-Te-Se thermoelectric material core portion and the whole thermoelectric material sintered therewith The fraction occupied by graphene oxide is extremely small and there is therefore little problem of lowering the electric conductivity. Especially, increase of phonon scattering decreases the thermal conductivity and increases the figure of merit. Therefore, the thermal stability can be enhanced by the graphene oxide coating, and the performance of the thermoelectric material itself can be improved.
Particularly, in the present invention, graphene oxide is used instead of graphene. When the graphene is complexed, the dispersion characteristics are deteriorated and the mechanical strength is lowered. In addition, graphene has a low surface charge and tends to self-aggregate due to Van der Waals forces, and exhibits very low dispersion in any solvent, which has been a significant obstacle to practical application and academic research. Just as it is a problem with compounding with other materials, applying graphene to thermoelectric materials will require that the surface of the graphene be surface-charged with a surfactant or block copolymer and modified for polarity control.
On the other hand, graphene oxide is well dispersed in water and does not aggregate. In the present invention, the use of such graphene oxide does not require the use of a surfactant or the like.
2 is a flowchart schematically showing a method of manufacturing a thermoelectric powder according to an embodiment of the present invention.
As shown in FIG. 2, the thermoelectric powder manufacturing method according to the present invention includes a material synthesis step (S10), a powder formation step (S20), and a graphene oxide coating step (S30).
The material synthesis step (S10) is a step of synthesizing a Bi-Te-Se based material, and a conventional Bi-Te-Se based material synthesis method may be employed. For example, the material synthesis step (S10) includes a step of mixing raw materials for forming a Bi-Te-Se-based material and a step of synthesizing a Bi-Te-Se-based compound by heat-treating the thus- .
In this material synthesis step S10, the mixing of the raw materials can be performed by hand milling using a mortar, ball milling, planetary ball mill or the like, The invention is not limited by these specific modes of mixing.
In the material synthesis step S10, the heat treatment may be performed using an ampoule method, an arc melting method, a solid state reaction (SSR), a metal flux method, a Bridgeman method ) Method, an optical floating zone method, a vapor transport method, and a mechanical alloying method.
In the method using the ampoule, the raw material element is put into an ampoule made of a quartz tube or metal at a predetermined ratio, and is vacuum-sealed to perform heat treatment. In the arc melting method, a raw material element is put into a chamber at a predetermined ratio, and an arc is discharged in an inert gas atmosphere to dissolve the raw material element, thereby forming a sample. The SSR is a method including a step of mixing and hardening a predetermined proportion of the raw material powder, followed by heat treatment, or heat treatment of the mixed powder, followed by processing and sintering. In the metal flux method, an element that provides an atmosphere so that a predetermined proportion of a raw material element and a raw material element can grow well at a high temperature is placed in a crucible, and then a crystal is grown by heat treatment at a high temperature. In the Bridgman method, a predetermined amount of raw material is placed in a crucible, heated at a high temperature until the raw material element is dissolved at the end of the crucible, and then slowly moved in a high temperature region to dissolve the sample locally so that the entire sample passes through the high temperature region It is a way to grow crystals. In the optical flow region method, a seed rod and a feed rod are formed in a bar shape at a predetermined ratio, and then the feed rod is fused to the focal point of the lamp to dissolve the sample at a locally high temperature. Slowly pulling up the part upward to grow crystals. The steam transfer method is a method in which a raw material element is introduced into a quartz tube at a predetermined ratio, the raw material element is heated, and the quartz tube is set at a low temperature to vaporize the raw material and cause a solid phase reaction at a low temperature. The mechanical alloying method is a method in which a raw material powder and a steel ball are added to a container of a cemented carbide material and rotated, and the steel ball mechanically impacts the raw material powder to form an alloy type thermoelectric material.
Particularly, in the present invention, such a heat treatment step may be performed by an SSR method in which the mixture is put into an electric furnace and heated at a predetermined temperature for a predetermined time. Even thermoelectric materials of the same composition may have different thermoelectric performances depending on the reaction method between the raw materials. In the case of Bi-Te-Se system, the raw materials are reacted by other methods such as SSR method rather than melting method The thermoelectric performance of the manufactured material can be further improved.
For example, in the material synthesis step (S10), a green body is prepared by hand-pressing the raw material mixture powder, and the powder is charged into a quartz tube to set a high vacuum state to 10 -5 torr, Followed by heating.
The powder forming step (S20) is a step of forming a Bi-Te-Se based composite formed in step S10 in powder form. As described above, when the Bi-Te-Se based compound is formed into a powder form, it has a high surface area, so that coating of graphene oxide on the Bi-Te-Se based material can be more successfully performed in step S30. In addition, when the Bi-Te-Se based composite is formed into a powder form, the sintered density can be further increased. Preferably, in the step S20, the particle size may be 50 nm to 500 mu m. More specifically, in step S20, a material having a particle size of 26 mu m or less can be produced to a size of 10 mu m.
The steps S10 and S20 correspond to the step of preparing the thermoelectric material core portion of the thermoelectric powder and if necessary, a commercially available Bi-Te-Se thermoelectric material powder is purchased and directly supplied to the Bi-Te-Se The thermoelectric material core portion may be used.
The coating step S30 is a step of coating a Bi-Te-Se-based material formed in powder form with graphene oxide. For example, in the coating step S30, the Bi-Te-Se based powder is mixed with a solution in which graphene oxide is dispersed, and at least one of ultrasonic treatment, heating, stirring, and shaking Processing may be carried out in a manner.
In the present invention, graphene oxide, which is low in graphene value and easy to utilize / apply, is used. As mentioned above, graphene oxide can be used after concentration adjustment by purchasing a product obtained by chemical peeling of graphite oxide and dispersing it in deionized water.
The Hummers method is well known as a method of obtaining graphite oxide from graphite. In this method, to form oxidized graphite, NaNO 3 , H 2 SO 4 , and KMnO 4 are used to break graphite intergranular bonds and to attach functional groups such as -OH and -COOH. This process will be described in more detail as follows.
To form graphite from graphite, graphite can be pretreated with a strong acid such as H 2 SO 4 and then oxidized with an oxidizing agent such as KMnO 4 . When H 2 SO 4 with further addition of the NaNO 3 HNO 3 is generated and HNO 3 as well as serve to act as an oxidizer to help a graphite oxide, and also helps the oxidation of the impurities contained in graphite. The mixture of graphite and NaNO 3 , H 2 SO 4 , and KMnO 4 is added with deionized water to form an aqueous solution, and H 2 O 2 is added to the graphite.
This aqueous graphite oxide solution is subjected to ultrasonic treatment to peel off the graphite oxide layer to obtain a solution in which graphene oxide is dispersed. This solution is commercially available and is diluted to a desired concentration, and a thermoelectric material powder such as the Bi-Te-Se thermoelectric material powder prepared through steps S10 and S20 is mixed and ultrasonicated to form the graphene An oxide coating step (S30) may be performed. After the ultrasonic treatment, a subsequent treatment for generally obtaining the powder can be performed in the order of washing and drying the precipitate.
Graphene oxide is easily adsorbed on the surface of a metallic thermoelectric material such as a Bi-Te-Se thermoelectric material powder due to an oxide system on the surface of graphene. Therefore, there is no need to use a surfactant in comparison with the conventional methods using graphene directly. Even if a solution in which graphene oxide is dispersed for use in the production of the thermoelectric powder of the present invention is made of only graphene oxide and deionized water, Easy to operate, economical and other variables to control are reduced, and the process is simple and reproducible.
When graphene oxide is physically dispersed in deionized water, adsorption between graphene oxide and graphene oxide is not well induced. Therefore, graphene oxide can be coated as a single layer on the surface of the Bi-Te-Se based thermoelectric material powder.
The surface of the material can be coated 100% so that the remaining dispersion can be used to coat the surface of other materials. It is advantageous for mass synthesis if the concentration can be maintained to enable continuous coating.
The thermoelectric material according to the present invention can be manufactured using the thermoelectric powder according to the present invention as described above. In particular, the thermoelectric material according to the present invention can be obtained by sintering the thermoelectric powder according to the present invention. For example, the method of manufacturing a thermoelectric material according to the present invention may further include a step of sintering the powders coated in steps S 10 to
Therefore, the
This sintering step is a step of sintering the Bi-Te-Se-based powder coated with graphene oxide in step S30. Here, the sintering step may be performed by a hot press (HP) method or a spark plasma sintering (SPS) method.
The SPS or the hot press is a pressure sintering method. When the sintering is performed by this pressure sintering method, the thermoelectric material can easily obtain high sintering density and thermoelectric performance improving effect. The SPS method is a process in which pulsed electric energy is directly applied to the particle gap of the powder compact, and the high energy of the discharge plasma generated instantaneously by the spark discharge is effectively applied by the action of heat diffusion and electric field. As the energizing and pressing method using a DC pulse, the firing starts and a discharge phenomenon occurs in the green compact, and joule heating occurs between the particles, and the firing proceeds due to thermal diffusion and electric field diffusion. It is possible to control the growth of particles, to obtain a dense sintered body in a short period of time, and to sinter an egg sintered material easily. Such short-time sintering can be used to limit the growth of the particles, and thus the thermal diffusivity can be controlled by having more grain boundaries in the sintered matrix.
A hot press is a method of using a high pressure of 10 to 200 MPa and a high temperature, filling a capsule with a predetermined amount of a powder or a molded body, deaerating and sealing, and simultaneously heating and sintering the mixture while pressurizing.
In the case of the thermoelectric material according to the present invention, when sintered by this pressure sintering method, a high sintering density and thermoelectric performance improving effect can be easily obtained. However, the present invention is not necessarily limited to this sintering method, and the sintering step may be performed in various other ways such as HPHT (High Pressure High Temperature) and HPT (High Pressure Torsion).
In addition, the sintering step may be performed in a vacuum state or while flowing a gas containing a part of hydrogen or containing no hydrogen such as Ar, He, N 2 , or in an inert gas atmosphere.
P-type and n-type thermoelectric elements can be obtained if the bulk thermoelectric material obtained by the pressure sintering is formed by a cutting method or the like and made into a desired size sintered body from the beginning. A module can be manufactured by integrating such thermoelectric elements together with electrodes on a substrate. As the substrate, sapphire, silicon, pyrex, a quartz substrate, or the like can be used. The material of the electrode may be selected from a variety of materials such as aluminum, nickel, gold, titanium, and the like. The electrode may be patterned by any known patterning method without limitation. For example, a lift-off semiconductor process, a deposition method, a photolithography process, or the like may be used.
The thermoelectric module thus manufactured may be, for example, a thermoelectric cooling system or a thermoelectric power generation system. The thermoelectric cooling system may be a universal cooling device such as a non-refrigerant refrigerator, an air conditioner, a CPU cooler, a laser diode cooling device, a CCD A micro cooling system such as a cooling element, a high output transistor cooling element, and an IR sensor cooling element, an air conditioner, a waste heat generation system, and the like, but is not limited thereto. The construction and the manufacturing method of the thermoelectric cooling system are well known in the art, and a detailed description thereof will be omitted herein.
During the production of the thermoelectric material through the sintering of the thermoelectric powder, graphene oxide prevents the material from moving, thereby ensuring thermal stability during manufacture. When thermoelectric elements, thermoelectric modules or thermoelectric elements using sintered thermoelectric materials are used, graphene oxide prevents oxidation, prevents volatilization, and changes the composition of the thermoelectric element, thereby ensuring thermal stability during use.
FIG. 3 is a diagram schematically showing a configuration of a thermoelectric material produced by sintering a thermoelectric powder according to an embodiment of the present invention, which is a schematic representation of a microstructure in a cross section of a sintered body.
3, the
Here, the Bi-Te-Se based thermoelectric material grain (A) is a grain containing a Bi-Te-Se based thermoelectric material and may be a matrix in which a plurality of adjacent grains are gathered. The graphene oxide (B) can be located at the grain boundary of the Bi-Te-Se based thermoelectric material grain (A).
The Bi-Te-Se thermoelectric material grain (A) can be formed in various sizes and shapes. For example, the size of the Bi-Te-Se thermoelectric material grain (A) may be several tens of nanometers to several hundreds of micrometers. Furthermore, the size of the Bi-Te-Se thermoelectric material grain (A) may be, for example, 0.5 to 500 mu m. The Bi-Te-Se thermoelectric material grain (A) may be formed into various shapes such as a spherical shape, an acicular shape, a plate shape, and the like depending on synthesis conditions and the like.
Particularly, in the
The thermoelectric material grain (A) is a material represented by the formula Bi 2 Te x Se a - x M b where 2? X <3, 3? A <3.2 and 0? B, M is Cu, Bi, Sn , An oxide or an antimonide of any one of In, Co and Fe. At this time, it is preferable that M is included in a range of 0 to 1 wt% based on the total weight of the grain (A). More preferably, M is contained in an amount of 0.01 wt% to 0.2 wt% based on the total weight of the grain (A).
The graphene oxide (B) in the thermoelectric material (110) according to one aspect of the present invention may be interposed in a continuous or discontinuous film form in the grain boundary of the Bi-Te-Se thermoelectric material grain (A). That is, graphene oxide (B) can be formed along the crystal interface of the thermoelectric material matrix, as shown in FIG. The grain boundaries including graphene oxide (B) may be formed to have a uniform thickness as a whole, or may be formed to have a partially different thickness. Further, graphene oxide (B) may be entirely filled in the grain boundary, or partially filled.
Meanwhile, the graphene oxide included in the grain boundaries in the thermoelectric material according to an embodiment of the present invention may exist in the form of a film or may exist in the form of agglomerated particles.
As described above, the thermoelectric material according to the present invention is a composite of a Bi-Te-Se thermoelectric material grain and graphene oxide located at the grain boundary between the Bi-Te-Se thermoelectric material grains.
The thermoelectric material having such a structure can be produced by sintering the thermoelectric powder according to the present invention as described above, or by mixing and sintering Bi-Te-Se thermoelectric material powder and graphene oxide. In the case of using the thermoelectric powder according to the present invention, it is possible to uniformly coat graphene oxide with a single layer on the large surface area of the Bi-Te-Se based thermoelectric material core portion, Te-Se-based thermoelectric material in the sintering process, compared with the case of mixing and sintering Bi-Te-Se based thermoelectric material grains after sintering. It is easy to distribute.
As mentioned above, the figure of merit is related to the Seebeck coefficient, electrical conductivity, and thermal conductivity. The high performance index means that the energy conversion efficiency of the thermoelectric material is high. In order to improve the performance index, it is necessary to increase the electric conductivity or decrease the thermal conductivity. In particular, among the functions that influence the performance index of the thermoelectric material, the Seebeck coefficient and the electrical conductivity depend mainly on the charge scattering, and the thermal conductivity depends mainly on the scattering of the phonons. .
In the present invention, the scattering of the charge in the thermoelectric material is reduced as much as possible, and the scattering of the phonon constituting the thermoelectric material is increased, thereby reducing the thermal conductivity and improving the figure of merit.
The thermoelectric material according to the present invention can constitute a thermoelectric element. Such a thermoelectric element can secure thermal stability and can maintain the efficiency without deterioration even when it is used for a long time at a high temperature.
Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. It should be understood, however, that the embodiments of the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.
Production Example 1: Preparation and pulverization of n-BTS
Bi, Te, and Se, which are raw materials in powder form, were weighed in amounts of 2, 2.5 and 0.5 at a total amount of 30 g, and then mixed through a vortex machine. Pelletized at room temperature pressing, placed in a quartz tube, and vacuum sealed. The airtight tube was heated in a box furnace in a box furnace for 450 hours, and then the obtained gray product was hand-milled to obtain an n-type BTS material (hereinafter also referred to as BTS). The obtained product itself is Comparative Example 1P.
Production Example 2: Control of GO dispersion concentration
The concentration of commercially available graphene oxide (GO) dispersion (~ 80% monolayer / deionized water) is diluted 1/20.
Example: Surface adsorption rate 100% GO @ n-BTS Synthesis and sintering
13 g of the product obtained in Preparation Example 1 is added to 60 ml of the diluted solution of Preparation Example 2 and subjected to ultrasonic treatment for 5 minutes (using a general cleaner sonicator). Thereafter, the precipitate obtained through centrifugation (3000 rpm, 5 min) is washed once with deionized water and then dried in an oven at 70 캜 for one day. The thus prepared powder is Example 1P (hereinafter also referred to as GO @ BTS).
Example 1 P powder was placed in a graphite mold and sintered at SPS (50 MPa, 390 DEG C, 5 minutes) to form a 12.7Φ sintered body. This sintered body is the example 1S.
Comparative Example: n-BTS sintering of Preparation Example 1
COMPARATIVE EXAMPLE 1S in which the powder of Comparative Example 1P obtained in Production Example 1 was sintered under the same conditions as in the above Example.
Evaluation Example 1: XRD comparison between Examples and Comparative Examples
4 shows the XRD analysis results of Examples and Comparative Examples of the present invention.
4 shows the XRD analysis results of Comparative Example 1P and Example 1P together. As shown in Eh 4, the XRD patterns of Comparative Example 1P and Example 1P are substantially the same, confirming that there is no denaturation of BTS even in the GO treatment of graphene oxide coating in the manufacturing process of Example 1P.
Evaluation Example 2: Sublimation test (300 DEG C)
EXAMPLES Sintered body 1S and comparative example Sintered body 1S was molded into a hexagonal column having a surface area of about 1 to 2 cm < 2 & gt ;. After the initial mass was measured, it was placed in a quartz tube and vacuum-tight. The mass change (m) was determined by measuring the mass of the sintered body after maintaining the temperature at 300 ° C for 96 hours, and the sublimation rates with and without graphene oxide were compared. Table 1 summarizes the results.
Thus, as a result of the sublimation test, the mass change of the sintered product 1S GO @ BTS of the sintered body 1S GO @ BTS subjected to the GO treatment as in the present invention was 0.0019g in the case of the comparative sintered body 1S BTS, The mass change is reduced. Accordingly, it was confirmed that the sublimation speed of the present invention was slowed down to about 15%.
Evaluation Example 3: Evaluation of thermoelectric property
Example The sintered body 1S and the comparative sintered body 1S were processed to a proper size and then the thermal conductivity was measured by laser flash analysis. The electrical conductivity of the sample and the defects The power factor (PF) and the figure of merit (ZT) were measured and the results were summarized in Table 2.
Referring to Table 2, it can be seen that the electrical conductivity and the whitening coefficient of the sintered body 1S according to the present invention are decreased over the entire temperature range, and the whitening coefficient is slightly increased. This is an effect of graphene oxide having a low electric conductivity, Within the scope, it can be regarded as almost similar level. Particularly, it is revealed that scattering of phonons leads to scattering, which impedes free movement, thereby lowering thermal conductivity. Accordingly, it is possible to form a thermo-blocking-electron transmission structure in a bulk-phase thermoelectric material having a thickness of several micrometers to several centimeters. For this reason, the electrical conductivity and the Seebeck coefficient, ie, the power factor, can be improved by lowering the thermal conductivity while maintaining a similar level.
Evaluation example 4: Raman spectroscopy confirmed the presence of GO
5 shows the results of Raman analysis of Examples and Comparative Examples.
Fig. 5 shows Raman analysis results of the sintered body 1S of the embodiment and the sintered body 1S of the comparative example. Referring to FIG. 5, in Comparative Example 1S, D and G peaks do not appear, and in Example 1S, D and G peaks are formed due to GO. The G peak near 1580 cm -1 is due to graphene because it is the peak due to the sp 2 bond of carbon, and the D peak near 1350 cm -1 appears when the sp 2 bond of carbon is defective. That is, in the case of the present invention, it is shown that the GO exists without decomposition.
Table 3 shows the comparison values before and after the evaluation of the thermoelectric properties in the examples. Referring to Table 3, it can be seen that the D / G peak ratio is maintained at 1.016 before thermoelectric property evaluation, 1.018 after thermoelectric property evaluation, and about 1.0 at known graphene oxide. This means that graphene oxide does not cause a change in composition such as oxidation / reduction at a temperature close to the processing sintering temperature of the Bi-Te-Se type thermoelectric material of 400 and a relatively low temperature of room temperature to 250, do.
As a result of the test, the thermoelectric material of the present invention has a tendency that the D / G peak ratio is 0.5 to 2 and the G peak is observed at 1500 to 1620 cm -1 in Raman measurement, and graphen oxide is maintained in the thermoelectric material .
Evaluation Example 5: SEM measurement of Examples and Comparative Examples
6 is an SEM photograph of Comparative Example 1P, and Fig. 7 is an SEM photograph of Example 1P.
6 and FIG. 7, graphene oxide is coated in the case of FIG. 7, and a thin thin film that is transparent on the SEM is observed (the arrow mark portion in FIG. 7).
That is, in the case of the thermoelectric powder according to the present invention, that is, the sample of Example 1P, unlike the sample of the comparative example, the coating layer is formed on the surface of the particle. That is, in the configuration of Fig. 7, a bright, thin film surrounds the surface of the particle. In this case, the particle itself may be referred to as a core portion of the thermoelectric powder, and the layer surrounding the surface of the particle may be referred to as a shell portion of the thermoelectric powder.
On the other hand, referring to FIG. 6, it is possible to observe the conventional BTS particles of the conventional Bi-Te-Se thermoelectric powder shape, and no separate coating layer is observed on the surfaces of these particles. That is, in the case of the comparative 1P sample shown in FIG. 6, it can be seen that no coating layer is formed on the particle surface as in the sample of Example 1P of FIG.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but many variations and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. It will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the appended claims.
10: thermoelectric powder
20: core part
30: Shell part
Claims (20)
And a graphene oxide shell portion coated on the surface of the core portion.
Here, 2? X <3, 3? A <3.2, 0? B,
Wherein M is at least one of an oxide or an antimony of any one of Cu, Bi, Sn, In, Co and Fe.
Wherein graphene oxide located at grain boundaries between the thermoelectric material grains is consolidated by sintering.
Here, 2? X <3, 3? A <3.2, 0? B,
And M is at least one of an oxide or an antimony of any one of Cu, Bi, Sn, In, Co and Fe.
And forming a graphene oxide shell part on the surface of the core part.
Preparing a graphene oxide dispersion; And
And mixing the thermoelectric material core portion with the graphene oxide dispersion.
Bi-Te-Se based material and preparing it in powder form,
Wherein forming the graphene oxide shell portion comprises:
Preparing a graphene oxide dispersion;
Mixing the thermoelectric material core portion with the graphene oxide dispersion;
Ultrasonic treatment and / or stirring; And
And washing and then drying the thermally conductive powder.
And sintering the thermoelectric powder.
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