KR102046142B1 - Thermoelectric powder and materials with improved thermostability and manufacturing methods thereof - Google Patents
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 having improved thermal stability and a method of manufacturing the same. In particular, the present invention proposes a thermoelectric powder capable of producing a thermoelectric material having improved thermostability. Such thermoelectric powders include thermoelectric material cores; And a graphene oxide shell portion coated on the surface of the core portion.
Description
BACKGROUND OF THE
Due to the temperature difference between the materials in the solid state, electrons (or holes) having thermal dependences generate concentration differences at both ends, which are represented by an electrical phenomenon called thermoelectric power, that is, a thermoelectric phenomenon. These thermoelectric phenomena can be classified into thermoelectric power generation, which produces electrical energy, and thermoelectric cooling / heating, which causes a temperature difference between both ends by supplying electricity.
Thermoelectric materials exhibit such thermoelectric phenomena, and many studies have been conducted due to environmentally friendly and sustainable advantages in power generation and cooling. In particular, it is of great interest as a technology that can improve fuel efficiency and reduce CO 2 by producing electric power from industrial waste heat and automotive waste heat.
In general, a thermoelectric device has a p-type thermoelectric element made of a p-type thermoelectric material in which holes move and transfers thermal energy, and a pn thermoelectric made of n-type thermoelectric element made of an n-type thermoelectric material in which electrons move and transfer thermal energy. One pair of elements may be a basic unit, and may include a plurality of pairs of such pn thermoelectric elements, and may be configured as a module type including electrodes on and under the pn thermoelectric element, and an insulating substrate.
The energy conversion efficiency of a thermoelectric device depends on ZT (= S 2 σ Tk -1 ), which is a dimensionless figure of merit value of the thermoelectric material. Where S is the Seebeck coefficient, sigma 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. Thermoelectric materials are largely divided into metal and oxide, and the metal is based on chalcogenide, silicide, clathrate, half heusler, and skutterudite. And so on.
Although the temperature range indicating high electromotive force / efficiency is different for each material of the thermoelectric material, in general, the greater the temperature difference across the material, the higher the efficiency and electromotive force. However, the high temperature for the formation of a large temperature difference leads to a change in the thermoelectric material composition through physicochemical phenomena such as melting, diffusion, sublimation, etc., leading to degradation of the thermoelectric material. , Thermoelectric elements, modules that are a collection of these, and a reduction in the lifetime of thermoelectric devices including the same.
This problem is called a deterioration problem due to thermal stability. To increase the thermal stability of thermoelectric devices, metal thin film coating techniques utilizing various vapor depositions are known. However, the development of new technologies is required due to various problems that are formed for high cost, such as secondary diffusion of metal thin films, interfacial adhesion, and reduction of electromotive force due to contact resistance.
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 capable of manufacturing a thermoelectric material having improved thermostability.
The thermoelectric powder according to the present invention is a composite of thermoelectric material and graphene oxide. In particular, the thermoelectric powder according to the present invention includes a thermoelectric material core portion; And a graphene oxide shell portion coated on the surface of the core portion.
In an embodiment of the thermoelectric powder according to the present invention, the core part may include skutterudite particles. For example, the core part may include In-Co-Sb-based field skutterudite particles. The core portion may be one having an average particle size of 50 nm to 500 ㎛. The shell part may include a single layer of graphene oxide.
The thermoelectric material according to the present invention includes a sintered body of such thermoelectric powder. The thermoelectric material manufacturing method according to the invention therefore comprises the step of sintering such thermoelectric powder.
In addition, another thermoelectric material according to the present invention for achieving the above object, 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 may be prepared by a method of simply mixing and sintering the thermoelectric material powder and graphene oxide, but it is preferable to prepare the sintered thermoelectric powder according to the present invention. In the case of sintering by simple mixing, it may be difficult to obtain a microstructure in which graphene oxide surrounds the thermoelectric material grains depending on the degree of mixing. However, the thermoelectric core part may be preliminarily formed into the graphene oxide shell part like the thermoelectric powder according to the present invention. This is because sintering the powder of the wrapped structure allows the graphene oxide to easily and reproducibly obtain the microstructure surrounding the thermoelectric material grain.
The thermoelectric material according to the present invention may further include amorphous carbon at the grain boundaries. The amorphous carbon may be due to thermal decomposition of the graphene oxide.
In addition, the grains may comprise scattererite. In an embodiment of the thermoelectric material of the present invention, the grains may further include a compound including In—Co—Sb-based field scattererite and including at least one of In, Sb, and Co at the grain boundaries. The grain is In-Co-Sb-based field scattererite, and may further include at least one of InSb, In 2 O 3 , CoSb-based material, and amorphous carbon at the grain boundary.
The present invention also proposes a thermoelectric device such as a bulk thermoelectric material including such a thermoelectric material, a thermoelectric element dicing it, a thermoelectric module incorporating the same, and the like.
According to the present invention, it is possible to prepare a thermoelectric material having thermal stability by coating graphene oxide on the surface of the thermoelectric material. Graphene oxide wrapped around the surface of the thermoelectric material prevents or inhibits diffusion of material from the thermoelectric material, thereby improving the heat resistance and thermal stability of the thermoelectric material. Thermoelectric devices such as thermoelectric elements and thermoelectric modules including such thermoelectric materials are excellent in thermal stability and relatively low in deterioration even in long-term use at high temperatures.
In the present invention, it is possible to coat graphene oxide on the surface of the thermoelectric material by using graphene oxide having excellent adhesion even without using other organic materials, and graphene oxide itself is insulating but when a thin layer is coated with a single layer, the performance index ZT is a conventional characteristic. It can maintain or improve and at the same time ensure thermal stability.
As described above, the thermoelectric material according to the present invention has a remarkable effect of securing thermal stability without deteriorating ZT.
In addition, the manufacturing method of the thermoelectric powder and the thermoelectric material according to the present invention is economical because it is performed by a simple solution-based method without expensive deposition processes and the like.
The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to.
1 is a view schematically showing the configuration of a thermoelectric powder according to an embodiment of the present invention.
2 is a flow chart schematically showing a method of manufacturing a thermoelectric powder according to an embodiment of the present invention.
3 is a diagram schematically illustrating a configuration of a thermoelectric material manufactured using a thermoelectric powder according to an embodiment of the present invention.
4 is a form of a specimen for the sublimation test of the sintered body of the present invention and the comparative example.
5 shows the results of XRD (X-Ray Diffractometer) analysis of the inventive examples and the comparative example sintered bodies.
6 (a) is a SEM photograph of a comparative thermoelectric powder, and (b) is a SEM photograph of an example thermoelectric powder.
(A) is a SEM image of the fracture surface of an Example sintered compact, (b) is an enlarged image of (a). (c) is a SEM image of the fracture surface of the sintered compact of the comparative example, and (d) is an enlarged image of (c).
8 shows Raman analysis results of the comparative example sintered body, the Example sintered body, and the Example sintered body after thermoelectric property evaluation.
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and only the present embodiments are intended to complete the disclosure of the present invention and to those skilled in the art to fully understand the scope of the invention. It is provided to inform you. Configurations shown in the embodiments and drawings described herein are only one of the most preferred embodiments of the present invention and do not represent all of the technical spirit of the present invention, various equivalents that may be substituted for them at the time of the present application It should be understood that there may be variations and examples.
1 is a view schematically showing the configuration of a thermoelectric powder according to an embodiment of the present invention.
Referring to FIG. 1, the
The
As mentioned above, the efficiency of the thermoelectric material is determined by the value of ZT (= S 2 σ Tk -1 ). Therefore, good thermoelectric materials should have higher power factor (PF = S 2 σ) values and lower thermal conductivity values. In other words, a good thermoelectric material must have a special transport performance, for which the phonon mean free path should be as short as possible, and the electron mean free path should be as long as possible. In this respect, the sputter ludite based on CoSb 3 has received great attention in engineering. In addition, since CoSb 3 has an excellent output factor value, scutterite is being evaluated as an effective method for reducing thermal conductivity.
For example, field scutterudite can be prepared by filling heterogeneous ions such as rare earth elements, alkali metals, and alkaline earth metals in the Sb-icosahedron cavities of CoSb 3 scutterrudite, which is a lattice thermal conductivity ( It is very effective in reducing k L ) and increasing ZT value. As described above, heterogeneous ions (hereinafter referred to as filler atoms) filled in the cavity have an independent vibration mode and thus have low mutual bonding force, and thus, have been reported to strongly inhibit lattice thermal conductivity by interacting with the general vibration mode in the lattice structure.
It has been reported that each filler atom becomes a phonon resonance scattering center close to a certain frequency, and the normal phonon mode with a frequency close to the local resonance frequency strongly interacts with the vibration mode of the filler atom. . Typically, phonon scattering should occur at a wide range of frequencies to further reduce lattice thermal conductivity. Hence, field squatterrudites, filled with a plurality of atoms with locally different vibration frequencies, have been reported to be very effective in lowering the lattice thermal conductivity even further. Therefore, many research reports have been drawn on field scutterudites which are filled by many kinds of filler atoms, and these filler atoms are filled with a single or a plurality of filler atoms. In the present invention, such field scattered particles can be used as the
Representatively, the
For example, it may further include other elements based on Co-Sb. For example, the
In addition, the
As another example, the
In addition, other scattering materials, such as Fe-Sb-based, Co-Fe-Sb-based, Co-Ni-Sb-based, and Co-As-based materials, may be used as the scattererite material capable of forming the
The
In addition, the shape of the
In one example, the
As such, the
Meanwhile, the
This
As such, the
In the
According to the embodiment as described above, since the
Graphite is a structure in which carbons are stacked in layers with planes arranged like honeycomb hexagonal nets. One layer of 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 applied with graphene in thermoelectric materials has been studied to improve thermoelectric properties. However, the results of the studies to date have not been realized / realized the thermoelectric properties improvement effect, and various surfactants are used to improve the interfacial adhesion, which is also the cause of increasing the process cost. Graphene is used as an electrode of thermoelectric elements, as a thin film layer for preventing thermal conduction, or a method for making a carbon material composite for enhanced flexibility, but there is a known method of thermoelectric material heat, which is an essential item for practical use. It is not known how to apply graphene directly to thermoelectric materials to ensure stability.
Currently, graphene is manufactured by physical exfoliation, chemical vapor deposition (CVD), epitaxial synthesis, chemical exfoliation, graphite intercalation, and the like. It is used.
Chemical exfoliation method utilizes the oxidation-reduction properties of graphite. First, graphite is oxidized with a strong acid and an oxidizing agent to produce graphite oxide. Since graphite oxide is hydrophilic and water molecules are easily inserted between the surfaces, the contact with water causes the water molecules to penetrate between the surfaces due to the strong hydrophilicity of the graphite oxide. As a result, the interval between planes increases, so that it can be easily peeled off using a long time stirring or an ultrasonic grinder. The exfoliated graphite oxide is graphene oxide contained in the shell portion of the thermoelectric powder of the present invention.
According to the present invention, by the graphene oxide coated on the surface of the thermoelectric material core portion, it is possible to prevent or suppress the volatilization of the main elements constituting the thermoelectric material, through which the thermal stability of the thermoelectric powder, the thermoelectric material sintered it It can be secured. Graphene oxide itself is insulating, but according to the method of manufacturing a thermoelectric powder of the present invention, since it can be thinly coated with a single layer, the graphene oxide occupies the whole of the thermoelectric powder itself including the core of the thermoelectric material and the sintered thermoelectric material. Is extremely small and thus there is little problem of lowering the conductivity. Accordingly, the graphene oxide coating may increase the thermal stability but hardly change the performance of the thermoelectric material itself.
In particular, the present invention uses a graphene oxide rather than graphene, graphene, when combined with the dispersion properties are lowered, leading to a decrease in mechanical strength. In addition, graphene has been a significant obstacle to practical applications and academic research because of its low surface charge, which tends to self-aggregate under van der Waals forces and exhibits very low solubility in any solvent. As is problematic for complexation with other materials, the application of graphene to thermoelectric materials will require modification of the graphene surface with surfactants or block copolymers.
In contrast, graphene oxide is well dispersed in water and does not aggregate. In the present invention, use of such a 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 forming step (S20), and a graphene oxide coating step (S30).
The material synthesizing step (S10) is, for example, a step of synthesizing the field scatterrudite material, a conventional scatterrudite material synthesis method may be employed. For example, the material synthesis step S10 may include mixing a raw material for forming a scatterudite material and synthesizing a scatterudite compound by heat treating the mixed raw materials. . Depending on the material type, n-type and p-type may be determined, but if necessary, a dopant may be further added to the raw material in order to obtain n-type and p-type semiconductor properties.
In the material synthesis step (S10), the mixing of the raw materials may be performed by a method such as hand milling using ball mortar, ball milling, planetary ball mill, etc. The invention is not limited by this specific mixing mode.
In addition, the heat treatment step in the material synthesis step (S10) is a method using an ampoule, arc melting method, solid state reaction (SSR), metal flux method (metal flux) method, Bridgeman (Bridgeman) ) Method, optical floating zone method, vapor transport method (vapor transport) method, or mechanical alloying method.
The method using an ampoule is to heat-process the raw material by sealing it in a vacuum tube or a metal ampoule in a predetermined ratio. The arc melting method includes a step of preparing a sample by dissolving a raw material element by discharging an arc in an inert gas atmosphere by inserting a raw material element into a chamber at a predetermined ratio. SSR is a method comprising a step of mixing the raw material powder in a predetermined ratio and processing it hardly and then heat treatment, or heat-treating the mixed powder and then processing and sintering. The metal flux method is a method comprising a step of growing a crystal by placing a predetermined ratio of the raw material element and an element providing an atmosphere so that the raw material element can grow well into crystals at a high temperature in a crucible and heat treatment at a high temperature. The Bridgeman method puts a certain proportion of raw material into the crucible and heats it at a high temperature until the raw material is dissolved at the end of the crucible, and then slowly moves the high temperature zone to locally dissolve the sample so that the entire sample passes through the high temperature zone. It is a way to grow crystals. The optical flow domain method creates a rod-shaped seed rod and feed rod in a proportion of the raw elements, and then dissolves the feed rod by melting the sample locally at a high temperature by focusing the light of the lamp. This is how you grow the crystal by slowly pulling the part upwards. The vapor transfer method is a method of growing a crystal at a low temperature as a raw material element is vaporized by putting a predetermined ratio of a raw material element under the quartz tube, heating the raw material part and keeping the upper portion of the quartz tube at a low temperature. The mechanical alloying method is a method in which an alloy-type thermoelectric material is formed by applying a raw powder and a steel ball to a container made of a cemented carbide and rotating the steel ball to mechanically impact the raw powder.
In particular, the heat treatment step in the present invention is preferably carried out by the SSR method of heating the mixture at a predetermined temperature by putting the mixture in a furnace (furnace). Even thermoelectric materials of the same composition, there may be a difference in the thermoelectric performance according to the reaction method between the raw materials, in the case of the sputter rudite, when each raw material is reacted by other methods, such as SSR method than the melting method, The thermoelectric performance of the manufactured material can be further improved.
For example, the heat treatment step in the material synthesis step (S10), by pressing the raw material mixture powder to produce a green body (green body), it is charged into a chamber to maintain a vacuum to 10 -2 torr with a rotary pump and then heated Can be performed. At this time, it can heat in Ar atmosphere.
The powder forming step (S20) is a step of forming a powder of the sputter ludite-based composite formed in step S10. As such, when the scauterite compound is formed in a powder form, it has a high surface area, and thus, coating of graphene oxide on the scaterite material may be better performed in step S30. In addition, the sintered density may be further increased by forming the scatterudite composite in powder form. Preferably, in the step S20, the particle size may be 50 nm to 500 ㎛. More specifically, in the step S20, the particle size may be 26 ㎛ or less to prepare a 10 ㎛ grade material.
Such steps S10 and S20 correspond to preparing the thermoelectric material core part of the thermoelectric powder. If necessary, a commercial thermoelectric material powder may be purchased and used as the thermoelectric material core part.
The coating step (S30) is a step of coating the grapher oxide material of the sputter rudite formed in the form of a powder. For example, the coating step (S30), the scattered powder is mixed with a solution in which graphene oxide is dispersed and subjected to at least one treatment of ultrasonication, heating, stirring and shaking. It can be done in a way.
In the present invention, graphene oxide, which is cheaper than graphene and is easy to use / apply, is used. As mentioned above, graphene oxide may be used after concentration adjustment by purchasing a product dispersed in deionized water obtained through chemical exfoliation of graphite oxide.
Conventionally, the Hummers method is well known as a method of obtaining graphite oxide from graphite. In this method, in order to form graphite oxide, NaNO 3 , H 2 SO 4 , and KMnO 4 are used to break the graphite interlayer bond and attach a functional group such as -OH or -COOH. This process is described in more detail as follows.
To form graphite oxide from graphite, the graphite may 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. Deionized water is added to the mixture of NaNO 3 , H 2 SO 4 , and KMnO 4 in graphite to form an aqueous solution, and H 2 O 2 is added to form graphite oxide.
When the graphite oxide aqueous solution is sonicated and separated between the graphite oxide layers, a solution in which graphene oxide is dispersed is obtained. This solution is commercially available, and after diluting it to a desired concentration, a thermoelectric material powder such as a sputter ludite powder prepared through steps S10 and S20 is mixed and sonicated, thereby obtaining the graphene oxide coating step of the present invention ( S30) may be performed. After sonication, the subsequent treatment of obtaining the powder may be further performed in order to wash and dry the precipitate.
Graphene oxide is easily adsorbed on the surface of metal-based thermoelectric material such as thermoelectric material powder due to the oxide-based surface of the graphene. Therefore, there is no need to use a surfactant as compared to conventional methods of directly using graphene, and a solution containing graphene oxide dispersed in the thermoelectric powder of the present invention may be coated with graphene oxide and deionized water. It's easy, economical and reduces the number of other variables to control, making the process simple and highly reproducible.
When graphene oxide is physically dispersed in deionized water, adsorption between graphene oxide and graphene oxide is not well induced. Thus, graphene oxide may be coated in a single layer on the surface of the thermoelectric material powder.
This 100% coating of the material surface and the remaining dispersion can be used to coat the surface of other materials. Maintaining the concentration to allow continuous coating is advantageous for mass synthesis.
The thermoelectric material according to the present invention may 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 coated powder after the steps S10 to S30 and the step S30 in FIG. 2.
Therefore, the
This sintering step is a step of sintering the sputtered rudite 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 hot press is a pressure sintering method, and when sintered by this pressure sintering method, the thermoelectric material may easily obtain a high sintered density and a thermoelectric performance improvement effect. The SPS method is a step of applying pulsed electrical energy directly to a particle gap of a powder compact to effectively apply high energy of a discharge plasma generated by a spark discharge by thermal diffusion, an action of an electric field, or the like. As a pressurized pressurization using DC pulse, firing starts and discharge occurs in the green compact, and Joule heating occurs between the particles, which causes the firing to proceed due to thermal diffusion and electric field diffusion. Since rapid temperature rising is possible, the growth of particles can be controlled, a compact sintered body can be obtained in a short time, and an sintered material can be easily sintered. Such a short time sintering method can be used to limit the growth of the particles, so that the thermal diffusivity can be controlled by allowing more grain boundaries to exist in the sintered body matrix.
The hot press uses a high pressure and a high temperature of 100 to 200 Mpa, and is a method of filling a capsule with a predetermined amount of powder or a molded product, degassing sealing, pressurizing and simultaneously heating and sintering.
In the case of the thermoelectric material according to the present invention, when sintered by the pressure sintering method, it is easy to obtain a high sintered density and an effect of improving thermoelectric performance. However, the present invention is not necessarily limited to this sintering method, and the sintering step may be performed by various other methods such as HPHT (High Pressure High Temperature) and HPT (High Pressure Torsion).
In addition, the sintering step may be performed in a vacuum state, while flowing a gas such as Ar, He, N 2 or the like containing some hydrogen or no hydrogen, or in an inert gas atmosphere.
The p-type and n-type thermoelectric elements can be obtained if the bulk thermoelectric material obtained by pressure sintering is molded by a cutting process or the like and manufactured from the beginning into a sintered body of a desired size. Integrating such thermoelectric elements together with electrodes onto a substrate allows fabrication of modules. As the substrate, sapphire, silicon, pyrex, quartz substrate and the like can be used. The material of the electrode may be variously selected, such as aluminum, nickel, gold, titanium, etc., and the size thereof may also be variously selected. As the method for patterning the electrode, a conventionally known patterning method can be used without limitation, and for example, a lift-off semiconductor process, a deposition method, a photolithography method, or the like can be used.
The module-type thermoelectric device manufactured as described above may be, for example, a thermoelectric cooling system or a thermoelectric power generation system. The thermoelectric cooling system may be a general-purpose cooling device such as a refrigerantless refrigerator, an air conditioner, a CPU cooler, a laser diode cooling device, a CCD. Micro cooling system, such as a cooling device, a high output transistor cooling device, an IR sensor cooling device, an air conditioner, a waste heat generation system and the like, but is not limited thereto. The construction and manufacturing method of the thermoelectric cooling system are well known in the art, and thus detailed description thereof is omitted.
In the production of thermoelectric materials through such thermoelectric powder sintering, graphene oxide prevents mass transfer, thereby ensuring thermal stability during manufacturing. When using a thermoelectric element, a thermoelectric module, or a thermoelectric device using a sintered thermoelectric material, graphene oxide prevents oxidation, volatilization, and compositional change, thereby ensuring thermal stability in use.
3 is a diagram schematically illustrating the configuration of a thermoelectric material manufactured by sintering a thermoelectric powder according to an embodiment of the present invention, and illustrates a microstructure in a sintered body cross section.
Referring to FIG. 3, the
Here, the scattered grain (A) is a grain containing a scattered raw material, it can form a matrix in the form of a plurality of adjacently gathered. And, the graphene oxide (B) may be located at the grain boundary of the scattererite grains (A).
The scattererite grains A may be formed in various sizes or shapes. For example, the size of the scattererite grains A may be several tens of nm to several hundred micrometers. Moreover, the size of the scattererite grains A may be, for example, 1 μm to 500 μm. In addition, the scattererite grains (A) may be formed in various forms, such as spherical, needle-like, plate-like, depending on the synthetic conditions.
In particular, in the
In the
In addition, the
For example, when grain (A) includes In—Co—Sb-based field scattererite, the grain boundary may further include a compound including at least one of In, Sb, and Co. For example, the grain boundary may further include at least one of InSb, In 2 O 3 , CoSb-based material including CoSb 2 , and amorphous carbon. These materials may be formed during the sintering process by being derived from some components of the thermoelectric material constituting the core portion of the thermoelectric material powder, or may be due to pyrolysis upon sintering. These may appear when the synthetic conditions are unstable and have a wide range. Or small volatilization of Sb or Sb oxide.
On the other hand, the graphene oxide included in the grain boundary in the thermoelectric material according to an embodiment of the present invention, may exist in the form of a film or in the form of agglomerated particles.
As such, the thermoelectric material according to the present invention is one in which thermoelectric material grains and graphene oxide located at grain boundaries between the thermoelectric material grains are consolidated.
The thermoelectric material having such a structure may be prepared by sintering the thermoelectric powder according to the present invention as described above, or may be prepared by mixing and sintering the thermoelectric material powder and graphene oxide. In the case of using the thermoelectric powder according to the present invention, since the graphene oxide can be uniformly coated with a single layer over a large surface area of the thermoelectric material core part, the thermoelectric powder in the sintering process compared with the case of mixing and sintering the thermoelectric material powder and graphene oxide The effect of preventing the material diffusion of the thermoelectric material core portion is excellent, and it is easy to distribute the graphene oxide evenly at the grain boundaries of the thermoelectric material after sintering.
The thermoelectric material according to the present invention may constitute a thermoelectric device. Such a thermoelectric device is ensured thermal stability can maintain the efficiency without deterioration even for long time use at high temperature.
Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples. However, the embodiment according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.
Preparation Example 1 ICS Synthesis and 10㎛ Class Grinding Classification
The raw materials of powder form In 0.776g, Co 2.671g and Sb 16.554g are mixed and pelletized by pressing at room temperature, and then put into a quartz tube and vacuum sealed. After the airtight tube was heated in a box furnace at 650 ° C. for 36 hours, the gray product obtained was hand milled, and a 500-mesh sieve was used for In-Co-Sb material of 26 μm or less. Hereinafter, ICS06 material) is obtained.
Preparation Example 2: GO dispersion concentration control
Dilute the commercially available graphene oxide dispersion (~ 80% monolayer / deionized water) to 1/50.
Example: Synthesis and Sintering of Surface Adsorption Rate 100% GO @ ICS06 10㎛
6 g of the obtained product of Preparation Example 1 was added to 30 ml of the dilution solution of Preparation Example 2 and sonicated for 5 minutes (using a sonicator for general cleaning). The precipitate obtained through centrifugation (5000 rpm, 5 min) is then washed once with deionized water and then dried in a 70 ° C. oven for one day. Thus prepared powder is Example 1.
The powder of Example 1 was put into a graphite mold and sintered with SPS (50 MPa, 650 ° C. for 10 minutes) to form a 12.7 sintered body. This sintered compact is Example 2.
Comparative Example: Sintering of ICS06 10㎛ Classifier of Preparation Example 1
Preparation Example 1 The obtained ICS-based powder itself is Comparative Example 1.
The powder of Comparative Example 1 was placed in a graphite mold and sintered with SPS (50 MPa, 650 ° C. for 10 minutes) to form a 12.7 sintered compact, which is Comparative Example 2.
Evaluation example 1: sublimation test of two sintered bodies (500 degreeC, 650 degreeC)
The sintered bodies of Example 2 and Comparative Example 2 were processed as shown in FIG. 4 to make a specimen, measure dimensions, measure initial mass, and place in a quartz tube to vacuum-tighten. One set was placed at 500 ° C. and the other 1 set was placed at 650 ° C. and after 96 hours of measurement, the dimensions and mass of the sintered body were measured to compare the sublimation rate according to the presence or absence of graphene oxide. Each set was first removed organics by heating at 300 ° C. for 6 hours before testing.
Table 1 summarizes the results.
In Table 1, Ratio is based on the sublimation ratio of ICS without graphene oxide, that is, when Comparative Example 2 is sublimed under 500 ° C. As shown in the ratio, when the ratio of subliming the comparative example 2 at 650 ° C. was very high at 31.181, the ratio at 500 ° C. was 0.242 and the ratio at 650 ° C. was 7.304 in the present invention.
As such, as a result of the sublimation test of ICS, in the case of ICS 06 powder of 100 nm to several um, Sb volatilization and sintering due to the relatively large specific surface area during the sintering process (SPS, 650 ° C, 15min, Ar / Vac atmosphere) for thermoelectricization Although oxidation appears strongly, it was confirmed that the ratio becomes very low after GO treatment as in the present invention.
5 shows the results of XRD analysis of the inventive examples and the comparative examples.
XRD analysis was performed on the powder of Comparative Example 1, the sintered compact of Comparative Example 2 and the sintered compact of Example 2. In the case of Comparative Example 2, an Sb 2 O 5 peak is detected as compared with Example 2. That is, in Comparative Example 2, it is confirmed that there is volatilization and oxidation of Sb, but not in the case of the present invention.
Evaluation Example 2: Thermoelectric Property Evaluation
After processing the sintered body of Example 2 and Comparative Example 2 to a suitable size, the thermal conductivity is measured by laser flash analysis, electrical conductivity and backing of the sample at a predetermined temperature interval through the ZEM-3 (Ulvac-Riko, Inc) equipment By evaluating the coefficients, the power factor (PF) and the performance index (zT) were measured, and the results are summarized in Table 2.
Contribution in Table 2 is shown based on the value of the comparative example. Referring to Table 2, in the embodiment according to the present invention over the entire temperature range it can be seen that the electrical conductivity is reduced and the Seebeck coefficient is increased. That is, the graphene oxide may improve the thermal stability in the embodiment of the present invention, the electrical conductivity is slightly reduced, but the ZT is improved in the low temperature region of less than 300 ℃ due to the reduced Seebeck coefficient and reduced heat transfer rate. In conclusion, the experimental results show that the thermoelectric properties are largely maintained because there is only a small variation that can be observed at the experimental error to the measurement error level.
Evaluation Example 3 SEM / EDX Measurement of Examples and Comparative Examples
6 (a) is a SEM photograph of Comparative Example 1, and (b) is a SEM photograph of Example 1. FIG.
When comparing (a) and (b) of FIG. 6, in the case of (b), graphene oxide is coated and a transparent thin film is observed on the SEM.
That is, in the case of the thermoelectric powder according to the present invention, that is, Example 1 sample, it can be seen that the coating layer is formed on the surface of the particles, unlike the sample of Comparative Example 1. That is, in the configuration of FIG. 6B, a bright and thin film surrounds the surface of the particle. In this case, the particles themselves may be referred to as the core portion of the thermoelectric powder, and the layer surrounding the surface of the particles may be referred to as the shell portion of the thermoelectric powder.
On the other hand, referring to Figure 6 (a), there is shown a particle of a conventional general sputter rudder thermoelectric powder, and no separate coating layer is observed on the surface of such particles. That is, in the case of the sample of Comparative Example 1 shown in (a) of FIG. 6, it can be seen that the coating layer is not formed on the particle surface as in the sample of Example 1 of FIG.
(A) is a SEM image of the fracture surface of the sintered compact of Example 2, (b) is an enlarged image of (a). As a result of EDX measurement at three grain boundaries, elemental ratio results as shown in Table 3 below were obtained, and GO was observed.
As a result of EDX observation of the corresponding position in the image (b) of FIG. 7, the positions P1 and P3 where the GO shape of the grain boundary was observed showed a relatively higher ratio of carbon, oxygen, and indium compared to P2 determined by the ICS06 surface. Is determined by GO and In 2 O 3 .
(C) is a SEM image of the fracture surface of the sintered compact of the comparative example 2, (d) is an enlarged image of (c). The shape of the grain boundary seen in FIG. 7D is different from that of (B). That is, in Example 2 in which the thermoelectric powder according to the present invention, that is, Example 1 sample is sintered, a matrix is formed including a plurality of scattererite grains, but the grain boundary of the matrix is different from that of the scattererite grains. It can be seen that other phases are distributed. That is, as can be seen from Figure 7 (b), when the thermoelectric material is manufactured by sintering the thermoelectric powder according to the present invention, it may be configured in the form of filling a predetermined material as a whole grain boundary. At this time, the material filled in the grain boundary, GO and In 2 O 3 was confirmed through the EDS analysis.
On the other hand, referring to Figure 7 (d), in Comparative Example 2 prepared by sintering the sample of Comparative Example 1, the matrix is formed, including a plurality of scattererite grains is similar to that in Example 2 sample As shown in (b) of FIG. 7, it can be seen that a form in which a predetermined material is filled at the grain boundaries of the matrix is not observed. Therefore, in the case of the sample of Comparative Example 2, even if the thermoelectric material is manufactured through sintering, it can be said that there is no configuration including graphene oxide at the grain boundary like the thermoelectric material according to the present invention.
Evaluation Example 4: Confirmation of GO by Raman Spectroscopy
8 is a Raman analysis result of the sintered compact of Comparative Example 2, the sintered compact of Example 2 and the sintered compact of Example 2 after thermoelectric characteristic evaluation.
Referring to FIG. 8, D and G peaks do not appear in Comparative Example 2 ICS, and in Example 2, D and G peaks are formed due to GO. The G peak near 1580 cm −1 is due to graphene because it is a peak due to sp 2 bond of carbon, and the D peak near 1 350 cm −1 appears when a defect exists in sp 2 bond of carbon. That is, in the case of the present invention indicates that GO is present without decomposition.
Table 4 is a comparative value before and after thermoelectric characteristic evaluation in Example 2. FIG. Referring to Table 4, it can be seen that the tendency of the D / G peak ratio to form about 1.2 in graphene using the conventional graphite redox method is maintained. In particular, in the case of the present invention, it can be seen that after the thermoelectric characteristic evaluation, the D / G peak ratio is increased and the G peak is shifted toward the shorter wavelength, and this shows that some oxides of graphene oxide are present after the thermoelectric characteristic evaluation is removed. Point. That is, a portion of the graphene oxide refers to reduced graphene oxide or reduced graphene oxide. Analogous graphene is known to have properties between graphene and graphene oxide. Therefore, the reduction of graphene oxide shows a better effect on the thermoelectric properties. So by controlling the reducing degree of the pin-oxide D / G peak ratio may be increased to about two further increased to more than 1.2, peak G may be reduced by moving towards a short wavelength than 1580cm -1 to 1500 cm -1. It was confirmed that the D / G peak ratio of the thermoelectric material according to the present invention was changed to 0.5 or more by changing the conditions in the manufacture of the sintered body, and the G peak was obtained at 1620cm -1 . Therefore, the thermoelectric material according to the present invention is a composite of thermoelectric material-graphene oxide having a D / G peak ratio of 0.5 to 2 and a G peak of 1500 to 1620 cm -1 .
As mentioned above, although the preferred embodiment of the present invention has been illustrated and described, the present invention is not limited to the specific preferred embodiment described above, and the present invention belongs to the present invention without departing from the gist of the present invention as claimed in the claims. Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.
10: thermoelectric powder
20 core part
30 shell part
Claims (20)
A core-shell structured thermoelectric powder comprising a graphene oxide shell portion coated on the surface of the core portion to surround the outside of the core portion, wherein the shell portion prevents or inhibits volatilization of the core portion material.
Graphene oxide located at the grain boundaries between the thermoelectric material grains is consolidated by sintering,
The graphene oxide is a thermoelectric material, characterized in that the presence of the particles in the form of a continuous or discontinuous film in the form of a continuous or discontinuous film in the grain boundary of the thermoelectric material grain.
And forming a graphene oxide shell on the surface of the core in a form surrounding the outside of the core to produce a thermoelectric powder having a core-shell structure.
Preparing a graphene oxide dispersion consisting of graphene oxide and deionized water only; And
And mixing the thermoelectric material core part with the graphene oxide dispersion.
Preparing a powder form by synthesizing a field scatterudite material,
Forming the graphene oxide shell portion,
Preparing a graphene oxide dispersion consisting of graphene oxide and deionized water only;
Mixing the thermoelectric material core portion with the graphene oxide dispersion;
Sonicating and / or stirring; And
Thermoelectric powder manufacturing method comprising the step of drying after washing.
Sintering the thermoelectric powder,
And wherein said graphene oxide prevents material diffusion of said thermoelectric material core portion during said sintering step.
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