Silicide alloy material and element using same

Abstract

To provide a silicide-based alloy material capable of reducing environmental load and obtaining high thermoelectric conversion performance at room temperature, and an element using the same.SOLUTION: A silicide-based alloy material is mainly composed of silver, barium, and silicon, in which when the contents of silver, barium, and silicon are Ag, Ba, and Si, respectively, the atomic ratios of the elements constituting the alloy material are: 9 atom%≤Ag / (Ag+ Ba+Si)≤27 atom%, 20 atom%≤Ba / (Ag+Ba+Si)≤53 atom%, and 37 atom%≤Si / (Ag+Ba+Si)≤65 atom%, and the silicide-based alloy material has an average crystal grain size of 20 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a silicide-based alloy material and a device using the same. [Background technology]

[0002] Thermoelectric power generation using waste heat has long been known as a renewable energy candidate. Currently, Bi2Te3 is in practical use for waste heat below 200°C, but Bi-Te materials have problems in that both Bi and Te are expensive, and Te is highly toxic. For these reasons, there is a demand for thermoelectric conversion elements that can reduce power generation costs and environmental impact.

[0003] In addition, in recent years, applications such as wearable devices that come into direct contact with human skin have begun to be considered, creating a demand for materials that are resistant to harm to the human body.

[0004] Silicide materials are excellent materials in terms of low environmental impact, low toxicity, and low cost, and have attracted much attention. Mg2Si is particularly well known (see, for example, Patent Document 1), and a mixture of Mg2Si and CaMgSi has been proposed as a p-type thermoelectric material using homologous elements (see, for example, Patent Document 2). However, the Seebeck coefficient at 400°C is as low as 70 μV / K or less, and thermoelectric properties sufficient for practical use have not been obtained.

[0005] Therefore, there is a demand for thermoelectric conversion materials that have a low environmental impact, low toxicity, and low cost, and that can provide high thermoelectric conversion efficiency in the temperature range from room temperature to about 100°C.

[0006] Silicides consisting of silver, barium, and silicon are known to form clathrate compounds, known for their superconductivity and rattling phenomena, at specific composition ratios. These compounds also exhibit properties as thermoelectric conversion materials, but their performance remains low, with a thermoelectric conversion performance ZT of approximately 0.02 at room temperature (see, for example, Non-Patent Document 1).

[0007] Furthermore, knowledge is not available for all composition ranges of silicides made of silver, barium, and silicon, and only the physical properties in the above-mentioned specific composition range have been investigated.

[0008] Here, T is the absolute temperature, and the figure of merit Z is defined by the following formula:

[0009]

number

[0010] S is the Seebeck coefficient (V / K), σ is the electrical conductivity, which is the reciprocal of the electrical resistance (Ω·m), and κ is the thermal conductivity (W / K·m). The numerator of Z (the product of the square of S and σ) is the power factor (W / K 2 It is called m).

[0011] Additionally, thermoelectric conversion performance is defined as (power factor / thermal conductivity) x temperature (K: Kelvin).

[0012] The inventors have discovered that by controlling the crystal grain size at a specific composition ratio and creating a silicide-based alloy material containing multiple crystalline phases and primarily composed of silver (Ag), barium (Ba), and silicon (Si), it is possible to suppress thermal conductivity and improve thermoelectric conversion performance. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-368291 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-147261 [Non-patent literature]

[0014] [Non-Patent Document 1] I.Zeiringer et al.Japanese Journal of Applied Physics 50(2011)05FA01-1 Summary of the Invention [Problem to be solved by the invention]

[0015] An object of the present invention is to develop a silicide-based alloy material that can reduce the environmental load and achieve high thermoelectric conversion performance at room temperature, and an element using the same. [Means for solving the problem]

[0016] The silicide-based alloy material according to the present invention has the following features. (1) A silicide-based alloy material whose main components are silver, barium, and silicon, and the atomic ratio of the elements that make up the alloy material is Ag, Ba, and Si, respectively. 9at%≦Ag / (Ag+Ba+Si)≦27at% 20at%≦Ba / (Ag+Ba+Si)≦53at% 37at%≦Si / (Ag+Ba+Si)≦65at% and an average crystal grain size of 20 μm or less. (2) The silicide-based alloy material according to (1), which has a relative density of 95% or more. (3) A thermoelectric conversion element using the silicide-based alloy material according to (1) or (2). (4) (3) A thermoelectric conversion module using the thermoelectric conversion element described above.

[0017] The present invention will be described in detail below.

[0018] The silicide-based alloy material of the present invention is a silicide-based alloy material mainly composed of silver (Ag), barium (Ba), and silicon (Si), and the atomic ratio of the elements constituting the alloy material is as follows, when the contents of silver (Ag), barium (Ba), and silicon (Si) are Ag, Ba, and Si, respectively: 9at%≦Ag / (Ag+Ba+Si)≦27at% 20at%≦Ba / (Ag+Ba+Si)≦53at% 37at%≦Si / (Ag+Ba+Si)≦65at% and preferably 9at%≦Ag / (Ag+Ba+Si)≦27at% 20at%≦Ba / (Ag+Ba+Si)≦45at% 37at%≦Si / (Ag+Ba+Si)≦65at% and particularly preferably 15at%≦Ag / (Ag+Ba+Si)≦27at% 30at%≦Ba / (Ag+Ba+Si)≦39at% 40at%≦Si / (Ag+Ba+Si)≦55at% is.

[0019] This is because, within the above-mentioned range, a silicide-based alloy material of silver (Ag), barium (Ba), and silicon (Si) exhibits a semiconductor crystalline phase with excellent thermoelectric conversion performance in the low temperature range. If the composition is outside this range, the physical properties of the silicide-based alloy material will exhibit a metallic crystalline phase or a crystalline phase with low conductivity, resulting in a significant deterioration in thermoelectric conversion performance.

[0020] The average crystal grain size of the silicide-based alloy material of the present invention is 20 μm or less, preferably 1 nm to 20 μm, more preferably 100 nm to 20 μm, or even more preferably 100 nm to 5 μm, and most preferably 100 nm to 1 μm, and most preferably 100 nm to 700 nm. Regarding the average crystal grain size, the reason why the performance of thermoelectric conversion elements remains low is largely due to high thermal conductivity. Therefore, by reducing the average crystal grain size, it is possible to reduce thermal conductivity. However, the average crystal grain size suitable for efficiently suppressing thermal conduction varies depending on the type of material. This is because the phonons that transmit heat have different mean free paths depending on the type of material. In addition, an average crystal grain size below 1 nm is likely to result in low electrical conductivity, which can lead to problems such as poor performance of the thermoelectric conversion element.

[0021] The average crystal grain size referred to here means a value calculated by counting the number of crystal grains having a specific size relative to the number of crystal grains observed in a specific region and calculating the average value.

[0022] The crystalline structure of a silicide-based alloy material for low-temperature thermoelectric materials preferably contains both a crystalline phase of space group 229 and a crystalline phase of space group 70. Furthermore, in order to exhibit high thermoelectric conversion performance in the low-temperature range around 100°C, the ratio of phase 1, whose crystalline structure is space group 229, to phase 2, whose crystalline structure is space group 70 (phase 1 / (phase 1+phase 2)) should be 0.00001≦(Phase 1 / (Phase 1+Phase 2))≦0.1 It is preferred that More preferably 0.00001≦(Phase 1 / (Phase 1+Phase 2))≦0.08 and most preferably 0.0001≦(Phase 1 / (Phase 1+Phase 2))≦0.05 is.

[0023] The silicide-based alloy material of the present invention may contain unavoidable trace amounts of impurities, such as metal elements other than Si, Ag, and Ba, and compounds such as oxides of these metal elements.

[0024] Furthermore, in order to achieve both a high Seebeck coefficient and low electrical resistivity, the relative density of the silicide-based alloy material of the present invention is preferably high. However, from the viewpoint of suppressing thermal conductivity, it is also preferable that the silicide-based alloy material has minute pores inside. Therefore, the relative density is preferably 90% or more. It is more preferably 90 to 99.5%, even more preferably 95 to 99.5%, and most preferably 97 to 99.5%.

[0025] The silicide-based alloy material of the present invention has a high Seebeck coefficient and low electrical resistivity, and is characterized by a high Seebeck coefficient, the absolute value of which is preferably 100 to 1000 μV / K, and particularly preferably 200 to 500 μV / K.

[0026] The silicide-based alloy material of the present invention is characterized by low electrical resistivity, which is preferably 1.00e-3 to 1.00e-1 Ω·cm, and particularly preferably 1.00e-3 to 1.00e-2 Ω·cm.

[0027] The silicide-based alloy material of the present invention is a material with suppressed thermal conductivity, and its thermal conductivity is preferably 0.1 to 20 W / mK, and particularly preferably 0.5 to 5 W / mK.

[0028] The silicide-based alloy material of the present invention is characterized by high thermoelectric conversion performance, and the thermoelectric conversion performance preferably has a figure of merit of 0.01 to 5, and particularly preferably 0.1 to 5.

[0029] Next, a method for producing the silicide-based alloy material of the present invention will be described.

[0030] The method for producing the silicide-based alloy material of the present invention preferably comprises the steps of: synthesizing an alloy from silver, barium, and silicon; optionally pulverizing or quenching the alloy to obtain a powder; and sintering the alloy powder by hot pressing at a sintering temperature of 650°C to 950°C.

[0031] First, in the process of synthesizing an alloy from silver, barium, and silicon, silver, barium, and silicon are prepared in a predetermined ratio and pre-melted in an arc melting furnace to synthesize silver-barium silicide. This is done to remove impurities from the powder and to refine the alloy structure, as described below. Furthermore, as for melting conditions, it is preferable to process the material at high power for a short time rather than at low discharge power for a long time. The current amount is preferably 30 A / g or more per unit sample amount. However, it is desirable for the current amount to be 100 A / g or less. This is because if the discharge power is too high, some of the metals will evaporate, causing the composition ratio to fluctuate.

[0032] The alloy obtained under the above preferred conditions is a silver-barium silicide alloy.

[0033] The subsequent step of crushing the alloy to a powder or quenching, as the case may be, reduces the particle size of the resulting powder.

[0034] When pulverizing, it is preferable to perform the pulverization in an inert gas atmosphere to prevent the oxygen content from increasing after the synthesis of the alloy. This is because oxidation of the powder surface can be prevented and the oxygen content can be kept low. Furthermore, the pulverization method can control the microstructure when the silicide-based alloy material is obtained. Methods that can be used for pulverization and granulation include mortar grinding, ball milling, jet milling, bead milling, spray drying, and gas atomization. The primary particle size of the resulting powder is preferably as small as possible. Furthermore, the average particle size of the granulated powder is not particularly limited, but is preferably approximately 10 to 100 μm in terms of handleability, etc.

[0035] Furthermore, depending on the composition ratio of the raw materials, it is possible to create silicide-based alloy materials suitable for high-temperature thermoelectric materials and silicide-based alloy materials suitable for low-temperature thermoelectric materials.Silicide-based alloy materials suitable for high-temperature thermoelectric materials and silicide-based alloy materials suitable for low-temperature thermoelectric materials can be obtained by mixing raw material powders so that the composition ratio corresponds to the above-mentioned Ag, Ba, and Si ratio.

[0036] When quenching, the molten silicide alloy material is sprayed onto a water-cooled copper roller rotating at 3000 rpm, which allows it to instantly solidify from the molten state and form a thin ribbon (quenched ribbon). In this case, the cooling rate is approximately 8 × 10 5 It may also be K / s.

[0037] Finally, in the sintering process in which the alloy powder is hot-pressed at a sintering temperature of 650°C to 950°C, sintering methods that can be used include atmosphere-controlled furnaces, hot pressing, which is a type of pressure sintering, and discharge plasma sintering.

[0038] An example of the spark plasma sintering method (hereafter abbreviated as SPS method), which is a type of pressure sintering, is explained below. Spark plasma sintering is a device that applies pressure to powder while passing a large current directly through the powder to sinter it. By applying uniaxial pressure during heating, it is possible to heat the powder to high temperatures in a short time, and it is a sintering method that can obtain a dense silicide alloy material while maintaining a fine grain size. Sintering using the SPS method improves density compared to conventional methods, and the theoretical density of AgBa2Si3 has been increased to 4.78 g / cm 3 In this case, it is possible to obtain a silicide-based alloy material with a relative density of 80% or more. The firing temperature in the SPS method is 650°C to 950°C, and preferably 700°C to 900°C. At temperatures lower than 650°C, firing does not proceed well and the relative density increases by only about 60%. Furthermore, firing at temperatures higher than 950°C may cause the alloy to melt and adhere to the hot press mold, resulting in a decrease in yield.

[0039] In the case of a silicide-based alloy material suitable for use as a thermoelectric conversion material, the firing temperature is preferably 750° C. or higher, as this makes it easier to obtain high thermoelectric conversion performance.

[0040] The pressure during firing is preferably 10 MPa to 100 MPa.

[0041] There are no particular restrictions on the holding time at the firing temperature for the SPS method, but it is preferable that it be within 10 minutes. If the holding time is extremely short, the inside will not be heated uniformly and it will be difficult to maintain the shape as a polycrystalline body. On the other hand, if the holding time is more than 10 minutes, it will induce an increase in grain size, which may result in an increase in thermal conductivity.

[0042] The silicide-based alloy material of the present invention may be processed to a predetermined size. The processing method is not particularly limited, and a surface grinding method, a rotary grinding method, a cylindrical grinding method, or the like can be used. By using these methods, it is possible to process it into a shape suitable for use in a thermoelectric conversion element.

[0043] The silicide-based alloy material of the present invention is preferably used as a thermoelectric conversion element.

[0044] Thermoelectric conversion elements are fabricated using p-type and n-type semiconductors. Therefore, it is preferable that the semiconductor material used can be controlled to be p-type or n-type. In the present invention, it is possible to control the p-type or n-type by adding a specific element to an alloy of silver, barium, and ruthenium.

[0045] An example of a method for manufacturing a thermoelectric conversion element using the above-mentioned silicide-based alloy material will be described below. The p-type and n-type silicide alloy materials are placed in parallel so that they do not touch each other, and an electrode bridges the upper part. When these structures are used to form a Pi-shaped element, the upper part of the element is in contact with high temperatures, and when heated, a temperature gradient occurs between the upper and lower parts of the element, and a current is generated from the potential difference caused by the Seebeck effect according to the temperature difference ΔT (=TH-TL). Therefore, by attaching electrodes to the p-type and n-type silicide alloy materials at the bottom of the element and forming a circuit via an appropriate resistor, it can be used as a battery.

[0046] By integrating the above-mentioned thermoelectric conversion elements, a thermoelectric conversion module capable of extracting large amounts of electric power can be produced. Examples of thermoelectric conversion modules include single-stage thermoelectric conversion modules, cascade modules, and segment modules, among which single-stage thermoelectric conversion modules are preferred. Below, a single-stage thermoelectric conversion module, which is preferred among thermoelectric conversion modules, will be described.

[0047] A single-stage thermoelectric module has a structure in which the above-mentioned thermoelectric conversion elements are integrated. For example, by integrating multiple thermoelectric conversion elements in series in a circuit continuing from the low-temperature side of the thermoelectric conversion element, it is possible to improve the output voltage, and by integrating them in parallel, it is possible to increase the output current. By creating an integration structure required depending on the application, it is possible to control the output voltage and output current. [Effects of the Invention]

[0048] By using the silicide-based alloy material of the present invention, it is possible to fabricate a highly efficient thermoelectric conversion element over a wide temperature range. [Example]

[0049] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0050] (Method for measuring average crystal grain size) Measurements were made using a JEOL JSM-7100F field emission scanning electron microscope with EBSD attachment. Crystal boundaries with a misorientation of 5° or more were determined to be grain boundaries.

[0051] (Method for measuring crystalline phase) The crystalline phase was identified from the diffraction peaks obtained by X-ray diffraction measurement.

[0052] (Method of measuring composition) Quantitation was performed by ICP-MS mass spectrometry.

[0053] (Method of measuring electrical characteristics) The measurements were carried out using a Hall effect measuring device (ResiTest8400 manufactured by Toyo Corporation).

[0054] (Method for measuring Seebeck coefficient) A Seebeck coefficient measurement system (ResiTest8400 option manufactured by Toyo Corporation) was attached to the Hall effect measurement device, and measurements were carried out.

[0055] (Method for measuring thermal conductivity) Measurements were carried out using a laser flash thermal conductivity measuring device (TC-1200RH manufactured by Advance Riko Co., Ltd.).

[0056] Example 1 Silicon pieces (4N purity, 1cm average size, manufactured by Kojundo Chemical Co., Ltd.), silver granules (99.9% purity, 1mm average particle size, manufactured by Kojundo Chemical Co., Ltd.), and barium (99% purity, 2cm x 1mm average size, manufactured by Kojundo Chemical Co., Ltd.) were mixed at Ag / (Ag + Ba + Si) = 17 at%, Ba / (Ag + Ba + Si) = 34 at%, and Si / (Ag + Ba + Si) = 49 at%, then filled into a water-cooled mold and arc-melted. The resulting raw material was ground in an agate mortar to produce powder. The resulting powder was filled into a 10mm diameter circular carbon mold and subjected to spark plasma firing. The firing conditions were a heating rate of 100°C / min above 600°C, a 10-minute hold at 900°C, and a pressure of 75 MPa. The vacuum was 5.0e-3 Pa. The temperature was measured using a radiation thermometer (IR-AHS manufactured by Chino Corporation).

[0057] As a result of EBSD measurement, the AgBa2Si3 crystalline phase (space group 70) containing approximately 0.001% Ba phase in area ratio was observed in the obtained alloy material.

[0058] Assuming that the alloy material is pure AgBa2Si3, the relative density of the obtained alloy material is 4.78 g / cm3, which is the theoretical density of AgBa2Si3. 3 Using the Archimedes method, the result was 95.2%.

[0059] The alloy material was then processed into a size of 10 mm diameter x 1 mm thick to prepare samples for measuring electrical properties and thermal conductivity, which were then measured. The Seebeck coefficient and electrical resistance were measured under vacuum conditions at 50°C, while thermal conductivity was measured under a He atmosphere at 50°C. The measurement results are shown in Table 1.

[0060] Examples 2 to 9 A silicide-based alloy material was prepared using the composition shown in Table 1 in the same manner as in Example 1 except for the composition.

[0061] (Examples 10 to 16) A silicide-based alloy material was prepared using the composition shown in Table 2 in the same manner as in Example 1 except for the composition.

[0062] (Comparative Examples 1 to 4) A silicide-based alloy material was prepared using the composition shown in Table 3 in the same manner as in Example 1 except for the composition.

[0063] [Table 1]

[0064] [Table 2]

[0065] [Table 3]

[0066] The alloy materials of Examples 1 to 16, which are within the composition range of the present invention, exhibited high thermoelectric conversion performance, whereas the alloy materials of Comparative Examples 1 to 4, which are outside the composition range of the present invention, exhibited only low thermoelectric conversion performance. [Industrial Applicability]

[0067] By using the present invention, it becomes possible to fabricate a thermoelectric conversion element with high performance, and to efficiently utilize waste heat over a wide temperature range.

Claims

1. A silicide-based alloy material mainly composed of silver, barium, and silicon, wherein the atomic ratio of the elements constituting the alloy material is: 9at%≦Ag / (Ag+Ba+Si)≦27at% 20at%≦Ba / (Ag+Ba+Si)≦53at% 37at%≦Si / (Ag+Ba+Si)≦65at% % of Ag+Ba+Si, and an average crystal grain size of 20 μm or less.

2. 2. The silicide-based alloy material according to claim 1, which has a relative density of 80% or more.

3. A thermoelectric conversion element using the silicide-based alloy material according to claim 1 or 2.

4. A thermoelectric conversion module using the thermoelectric conversion element according to claim 3.

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