Method for improving thermoelectric performance of tin selenide block material

Abstract

The invention relates to the technical field of thermoelectric materials, and discloses a method for improving thermoelectric performance of a tin selenide block material, which comprises the following steps of: 1, mixing Sn powder and Se powder according to a stoichiometric ratio, and adding Na2S and Li2S as codopants; 2, the mixed raw material is subjected to vacuum sealing treatment and then subjected to solid-phase melting reaction treatment, and a tin selenide cast ingot is obtained; step 3, carrying out ball-milling refining on the molten product to obtain tin selenide powder; and 4, carrying out spark plasma sintering on the tin selenide powder, and applying pulse current in the sintering process to obtain the tin selenide block material. By introducing alkali metal sulfides Na2S and Li2S, the solid solubility of alkali metal in a tin selenide matrix can be remarkably improved, so that the electrical transport performance and thermoelectric performance of the tin selenide material are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of thermoelectric materials, and in particular to a method for improving the thermoelectric performance of a tin selenide bulk material. Background Art

[0002] Thermoelectric conversion technology enables direct conversion between heat and electricity, and has important applications in waste heat recovery, solid-state refrigeration, and deep space energy supply. As a new thermoelectric material, tin selenide (SnSe) has attracted widespread attention in recent years due to its intrinsically low thermal conductivity and potentially high thermoelectric figure of merit (ZT).

[0003] Currently commercially available thermoelectric materials include PbTe and GeTe for the medium-temperature range, and Bi2Te3 for the low-temperature range. However, tellurium (Te) is expensive and toxic, prompting researchers to search for alternative thermoelectric materials. In 2014, Professor Zhao Lidong of the Beijing University of Aeronautics and Astronautics reported in the journal Nature that single-crystalline tin selenide achieved an ultra-high thermoelectric figure of merit of 2.6 along the b-axis at 923K. However, single-crystalline tin selenide is complex to prepare, requiring long cycles and high costs. It also easily dissociates along the dissociation plane, resulting in poor mechanical properties, making device assembly difficult and shortening the service life of thermoelectric devices. Therefore, there is an urgent need to develop polycrystalline tin selenide and optimize its thermoelectric properties.

[0004] The polycrystalline SnSe bulk materials prepared by existing technologies still have the following key problems: 1) Insufficient carrier transport performance: low intrinsic carrier concentration: the carrier concentration of pure SnSe (~10 16 -10 17 cm -3 ) is far below the range required for optimal thermoelectric performance (10 19 -10 20 cm -3 ), resulting in low conductivity (σ). 2) Limitations of traditional doping: Although single element doping (such as Na, K, Ag) can increase the carrier concentration, it will introduce strong ionized impurity scattering, resulting in a significant decrease in carrier mobility (μ) (usually <40cm 2 / V·s); the limited solid solubility of dopant atoms in the SnSe lattice (e.g., the solid solubility of Na in SnSe is <1 at.%) makes it difficult to precisely control the carrier concentration. Therefore, it is urgent to find suitable dopants to synergistically optimize carrier concentration and carrier mobility to optimize the electrical transport properties of tin selenide materials, thereby improving the thermoelectric figure of merit across the entire temperature range. Summary of the Invention

[0005] The present invention aims to provide a method for improving the thermoelectric properties of tin selenide bulk materials, so as to solve the problem that the existing alkali metal solid solubility in the tin selenide matrix is ​​low, resulting in poor electrical transport performance and thermoelectric performance.

[0006] To achieve the above object, the present invention adopts the following technical solution: a method for improving the thermoelectric performance of tin selenide bulk material, comprising the following steps:

[0007] Step 1: Sn and Se powders are mixed in a stoichiometric ratio, and Na2S and Li2S are added as co-dopants;

[0008] Step 2: vacuum-sealing the mixed raw materials and then subjecting them to a solid-phase melting reaction to obtain a tin selenide ingot;

[0009] Step 3: ball-milling the molten product to obtain tin selenide powder;

[0010] Step 4: Spark plasma sintering the tin selenide powder, applying a pulse current during the sintering process, to obtain a tin selenide bulk material.

[0011] Preferably, as an improvement, in step 1, the amount of Na2S added is 0.25-2at%, and the amount of Li2S added is 0.025-0.5at%.

[0012] Preferably, as an improvement, in step 2, the vacuum degree during the vacuum sealing process is 1×10 -4 Pa.

[0013] Preferably, as an improvement, in step 2, the conditions for the solid-phase melting reaction are: heating from room temperature to 950° C. within 10 hours, keeping the temperature for 24 hours, and then cooling to room temperature within 10 hours.

[0014] Preferably, as an improvement, in step three, the ball milling speed is 600-800 rpm / min, and the ball milling time is 5-10 min.

[0015] Preferably, as an improvement, in step three, the ball milling process is carried out in an argon atmosphere.

[0016] Preferably, as an improvement, in step 4, the sintering temperature is 450-550° C., the sintering pressure is 45-55 MPa, the holding time is 3-10 min; and the vacuum degree is ≤4 Pa.

[0017] Preferably, as an improvement, in step 4, the heating rate is 50-150°C / min.

[0018] Preferably, as an improvement, in step 4, the pulse mode is a DC pulse with a duty cycle of 12:2 and a current density of 500-1500 A / cm 2 .

[0019] Preferably, as an improvement, in step 4, the density of the obtained tin selenide bulk is greater than 95%, and the grain size is 0.5-10 μm.

[0020] The principle and advantages of this solution are: in actual application, in this technical solution, the problem of low solid solubility of alkali metals in the tin selenide matrix in the prior art (<1at.%), resulting in poor electrical transport performance and thermoelectric performance, is addressed. For traditional p-type SnSe, the prior art usually adds alkali metal elements (Li, Na, K) or Ag, Cu as dopants. This technical solution breaks through the existing technical barriers and creatively introduces alkali metal sulfides Na2S and Li2S as co-dopants to significantly improve the solid solubility of alkali metals in the tin selenide matrix, thereby improving the electrical transport performance and thermoelectric performance of the tin selenide material. Na2S and Li2S decompose into Na + 、Li + 、S 2- into the SnSe lattice, where Na + (ionic radius 0.102nm) limited occupation of Sn 2+ Position (ion radius 0.118nm), providing hole carriers and optimizing carrier concentration; part of Li+ (ion radius 0.076nm) enters the interstitial position in the SnSe lattice, and the other part fills the intrinsic Sn vacancies in the SnSe lattice, compensating for lattice distortion, reducing carrier scattering, and optimizing carrier mobility. 2- (ionic radius 0.184nm) occupies Se 2- Position (ionic radius 0.198nm), through the difference in electronegativity, the carrier concentration transition is suppressed, thereby optimizing the Seebeck coefficient and obtaining a better electrical transport performance. + 、Li + Together they provide holes, increasing the carrier concentration to 2-5×10 19 cm -3 (Undoped material~10 18 cm -3 ), which can regulate the carrier concentration; Li+ enters the lattice interstitial position of SnSe and fills the intrinsic Sn vacancies, which can reduce the lattice distortion and maintain the carrier mobility at 40-70cm 2 / V·s(single Na doping~25cm 2 / V·s). In addition, this technical solution uses alkali metal sulfides Na2S and Li2S as precursors, which can effectively inhibit the volatilization of alkali metals (composition deviation <5%) and has good process repeatability. Therefore, this technical solution combines S 2- The introduction of has achieved the coordinated optimization of carrier concentration, mobility and Seebeck coefficient.

[0021] In terms of sintering process, this technical solution comprehensively upgrades the sintering temperature, heating rate and holding time based on SPS technology. It adopts the synergistic effect of low-temperature sintering, high heating rate and short holding time to improve the density of the bulk material and avoid the grain coarsening problem caused by high temperature (>700℃) in traditional hot pressing process.

[0022] During the technology research and development stage, the inventors tried to dope elemental alkali metals (Na, Li single doping), and the results showed that the solid solubility was low, and the alkali metals volatilized severely at high temperatures, and composition segregation would be formed. The inventors also tried to dope with a single sulfide Na2S, and the results showed that the carrier mobility was low and the lattice thermal conductivity was high. In addition, the amount of alkali metal doping also has a key influence on the effect. When Li2S is added excessively, it will lead to excessive carrier compensation, too much Li+ entering the interstitial position, resulting in a sudden drop in carrier concentration, aggravated lattice distortion, and a significant decrease in carrier mobility. In terms of sintering process, the inventors tried traditional hot pressing sintering (sintering temperature>700℃, holding temperature for 1h), which will lead to grain coarsening (grain size growth, reduced density, poor mechanical properties), and will also cause alkali metal volatilization, making the actual composition of the sample uncontrollable and reproducible. In addition, in the optimization of dopants, the inventors also tried the combination of Li2S and K2S and Na2S and K2S, but failed to obtain ideal results.

[0023] In summary, the beneficial effects of this technical solution are:

[0024] 1. In this technical solution, the synergistic addition of Na2S and Li2S can increase the total solid solubility of alkali metals in tin selenide to 1.5-2.5at.%, thereby improving the electrical conductivity, which is significantly higher than that of alkali metal elemental doping (<1at.%). The solid solubility of alkali metals in the SnSe matrix can be obtained through XRD pattern shift and Na2Se precipitation phase.

[0025] 2. In this technical solution, the synergistic addition of Na2S and Li2S can synergistically optimize the carrier concentration and carrier mobility of the tin selenide material, thereby obtaining better electrical transport performance in the near-room temperature region.

[0026] 3. In this technical solution, the thermoelectric figure of merit is significantly improved by the synergistic addition of Na2S and Li2S. At 773K, the thermoelectric figure of merit reaches 2.1 (undoped sample ~0.6, single Na-doped sample ~1.2), and the average thermoelectric figure of merit between 323-773K reaches 1.2.

[0027] 4. In this technical solution, by optimizing the sintering process, the density of the bulk material is >95%, avoiding the grain coarsening problem caused by high temperature (>700°C) in the traditional hot pressing process, making the grain size 0.5-10μm, and the bulk material presents a (h00) crystal plane preferential orientation, which helps to improve the thermoelectric performance.

[0028] 5. In this technical solution, Na2S and Li2S are used as precursors, which can effectively inhibit the volatilization of alkali metals (composition deviation <5%); the process has good repeatability and is suitable for large-scale production; and it can avoid the chemical and physical properties of alkali metal elements and reduce production hazards. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the EPMA-mapping spectrum of Example 1 of the present invention.

[0030] Figure 2 Graph showing the change of conductivity with temperature in an embodiment of the present invention.

[0031] Figure 3 Graph showing the change of ZT value with temperature in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The following is further described in detail through specific embodiments, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following embodiments are conventional means well known to those skilled in the art; the experimental methods used are all conventional methods; and the materials, reagents, etc. used are all commercially available.

[0033] Program Overview:

[0034] A method for improving the thermoelectric performance of a tin selenide bulk material comprises the following steps:

[0035] Step 1: Sn and Se powders are mixed in a stoichiometric ratio, and sodium sulfide (Na2S) and lithium sulfide (Li2S) are added as co-dopants; the amount of Na2S added is 0.25-2at%, and the amount of Li2S added is 0.025-0.5at%;

[0036] Step 2: Place the mixed raw materials in a high-purity quartz tube and perform vacuum sealing through a vacuum sealing system. The vacuum degree is drawn to 1×10 -4 Pa, vacuum sealing and solid phase melting reaction treatment; the solid phase melting reaction process is as follows: heating from room temperature to 950 ° C within 10 hours, keeping at this temperature for 24 hours, and then cooling to room temperature within 10 hours to obtain a tin selenide ingot;

[0037] Step 3: Using a high-energy ball mill to refine the molten product in an argon atmosphere, the ball milling speed is 600-800 rpm / min, and the ball milling time is 5-10 min to obtain tin selenide powder;

[0038] Step 4: Place the tin selenide powder obtained by ball milling into a graphite mold with a diameter of 15mm, and place it in a spark plasma sintering furnace. The sintering temperature is 450-550℃, the heating rate is 50-150℃ / min, the sintering pressure is 45-55MPa, and the holding time is 3-10min; the vacuum degree is ≤4Pa; and the tin selenide block is obtained through the above sintering process. A pulse current is applied during the sintering process. The pulse mode is DC pulse with a duty cycle of 12:2; the current density is 500-1500A / cm 2 .

[0039] The tin selenide bulk obtained by the above preparation process has the following microstructural characteristics: 1) the elements in the tin selenide matrix are evenly distributed without obvious second phase; 2) the grain size is 0.5-10μm and exhibits a (h00) preferred orientation.

[0040] Example 1

[0041] A method for improving the thermoelectric performance of a tin selenide bulk material comprises the following steps:

[0042] Step 1: Mix Sn and Se powders in a stoichiometric ratio, and add sodium sulfide (Na2S) and lithium sulfide (Li2S) as co-dopants. The specific formula composition is: SnSe matrix: Sn 50at.%, Se 50at.%; dopant: Na2S 1.2at.% + Li2S 0.3at.%;

[0043] Step 2: Place the mixed raw materials in a high-purity quartz tube and perform vacuum sealing through a vacuum sealing system. The vacuum degree is drawn to 1×10 -4 Pa, vacuum sealing and solid phase melting reaction treatment; the solid phase melting reaction process is as follows: heating from room temperature to 950 ° C within 10 hours, keeping at this temperature for 24 hours, and then cooling to room temperature within 10 hours to obtain a tin selenide ingot;

[0044] Step 3: Using a high-energy ball mill to refine the molten product in an argon atmosphere, the ball milling speed is 600-800 rpm / min, and the ball milling time is 5-10 min to obtain tin selenide powder;

[0045] Step 4: Place the tin selenide powder obtained by ball milling into a graphite mold with a diameter of 15 mm and place it in a spark plasma sintering furnace with a sintering temperature of 500°C, a heating rate of 100°C / min, a sintering pressure of 50 MPa, and a holding time of 5 min; the vacuum degree is ≤ 4 Pa; and a tin selenide block is obtained through the above sintering process. A pulse current is applied during the sintering process, the pulse mode is DC pulse, the duty cycle is 12:2, and the current density is 1000 A / cm 2 .

[0046] Example 2

[0047] The difference between this embodiment and embodiment 1 is that: in this embodiment, the dopant in step 1 is Na2S 2.0at% + Li2S 0.2at.%.

[0048] Example 3

[0049] The difference between this embodiment and embodiment 1 is that: in this embodiment, the dopant in step 1 is Na2S 0.8at%+Li2S 0.5at.%.

[0050] Comparative Example 1

[0051] The difference between this comparative example and Example 1 is that in this comparative example, the dopant in step 1 is Na2S 1.5at%.

[0052] Comparative Example 2

[0053] The difference between this comparative example and Example 1 is that in this comparative example, the dopant in step 1 is Na 1.5 at %.

[0054] Comparative Example 3

[0055] The difference between this comparative example and Example 1 is that in this comparative example, the dopant in step 1 is Li 0.2 at %.

[0056] Comparative Example 4

[0057] The difference between this comparative example and Example 1 is that in this comparative example, in step 1, the dopant is Na2S 2.5at%+Li2S 0.3at.%.

[0058] Comparative Example 5

[0059] The difference between this comparative example and Example 1 is that in this comparative example, in step 1, the dopant is Na2S 1.2 at% + Li2S 0.7 at.%.

[0060] Comparative Example 6

[0061] The difference between this comparative example and Example 1 is that in this comparative example, the dopant is Na2S 2.5at% + Li2S 0.7at.%.

[0062] Comparative Example 7

[0063] The difference between this comparative example and Example 1 is that in this comparative example, in step 4, the sintering temperature of spark plasma sintering is 400°C.

[0064] Comparative Example 8

[0065] The difference between this comparative example and Example 1 is that in this comparative example, in step 4, the sintering temperature of spark plasma sintering is 600°C.

[0066] Comparative Example 9

[0067] The difference between this comparative example and Example 1 is that in this comparative example, in step 4, the heating rate of spark plasma sintering is 30° C. / min.

[0068] Comparative Example 10

[0069] The difference between this comparative example and Example 1 is that in this comparative example, in step 4, the heating rate of spark plasma sintering is 200° C. / min.

[0070] Experimental Example 1 Performance Test

[0071] The performance tests of the tin selenide blocks prepared in the above embodiments and comparative examples were carried out, and the test indicators and test methods are as follows:

[0072] 1. Carrier concentration / cm -3 :The test method adopts Hall Effect Measurement

[0073] Test standard reference: ASTM F76 "Standard Test Methods for Measuring Resistivity and Hall Coefficient and Determining Hall Mobility in Single-Crystal Semiconductors"

[0074] Equipment example: Van der Pauw Hall test system (such as Quantum Design PPMS)

[0075] Test conditions: room temperature to 773K, magnetic field strength 0.5-1.5T, current 1-10mA.

[0076] Principle: The carrier type (n / p) and concentration (n=1eRHn=eRH1, RHRH is the Hall coefficient) are calculated by the voltage difference under a perpendicular magnetic field.

[0077] 2. Carrier mobility / cm 2 / V·s: The test method uses Hall effect synchronous calculation

[0078] Formula: μ H =RHρ (ρ is the resistivity, measured by the four-probe method).

[0079] Four-probe resistivity test (ASTM F84)

[0080] Equipment: Linear four-probe station (such as Keithley 2400 source meter + probe station).

[0081] 3. Power factor / μWm -1 K -2 :The test method uses the combined test of conductivity (σ) and Seebeck coefficient (S) Standard reference: ASTM E1225《Standard Test Method for Thermal Conductivity of Solids》

[0082] Equipment: ZEM-3 (ULVAC-RIKO) or Linseis LSR-3

[0083] Calculation formula: PF = σS 2

[0084] Conductivity (σ): Four-probe method (ASTM F390).

[0085] Seebeck coefficient (S): temperature difference method (ΔT = 5-10K, measuring thermoelectric potential ΔV).

[0086] 4. Lattice thermal conductivity / Wm -1 K -1 :The test method uses laser flash method (LFA) + specific heat capacity test

[0087] Standard reference: ASTM E1461《Standard Test Method for Thermal Diffusivity by the Flash Method》

[0088] Equipment: Netzsch LFA 467 or TA Instruments Flashline 5000

[0089] Calculation formula: κ lat =D·ρ·Cp-κelec

[0090] Thermal diffusivity (D): directly measured by LFA.

[0091] Density (ρ): Archimedean drainage method (ASTM B962).

[0092] Specific heat capacity (Cp): DSC (differential scanning calorimetry, ASTM E1269) or comparison method (with standard sapphire samples).

[0093] Electronic thermal conductivity (κ elec =LσT, Lorentz number L = 2.44×10 -8 WΩK -2 ).

[0094] 5. Thermoelectric figure of merit: test method

[0095] Comprehensive calculation: ZT=S 2 σT / κ

[0096] Data source: Test results of power factor (PF) and total thermal conductivity (κ).

[0097] Verification equipment: Synchronous test system (such as Netzsch SBA 458Nemesis, which can measure σ, S, and κ simultaneously).

[0098] 6. Relative density: The test method adopts Archimedes drainage method

[0099] Standard reference: ASTM B962 "Standard Test Methods for Density of CompactedPowder Metallurgy Materials"

[0100] Calculation formula: κ=DρCp

[0101] Theoretical density: calculated from XRD refined unit cell volume.

[0102] Measured density: A precision balance (±0.1 mg) measures the mass difference between the sample in air and in the immersion liquid (such as ethanol).

[0103] Each group was subjected to three repeated tests, and the test results are shown in Table 1. The results show that the tin selenide bulk prepared in Examples 1-3 of the present invention significantly improves the carrier concentration and carrier mobility of the tin selenide material by the synergistic addition of Na2S and Li2S, thereby obtaining better electrical transport performance in the near-room temperature region. Moreover, the density of the bulk material is improved by optimizing the sintering process. The density of the bulk material in Examples 1-3 of the present invention is all greater than 95%. In Comparative Example 1, Li2S was not added, resulting in low mobility of the bulk material; in Comparative Example 2, the dopant was replaced by Na, resulting in volatilization of elemental Na and precipitation of Na2Se phase; in Comparative Example 3, the dopant was replaced by Li, resulting in an extremely low carrier concentration due to insufficient solid solubility; the results of Comparative Examples 4-6 show that the addition ratio of Na2S and Li2S has a certain effect on the performance of the tin selenide bulk; the results of Comparative Examples 7-10 show that the sintering temperature and the heating rate have a key influence on the density of the product and the uniformity of the grains, thereby affecting the thermoelectric properties of the bulk material.

[0104] Table 1

[0105]

[0106]

[0107] Experimental Example 2 Electron Probe Microanalysis

[0108] Electron probe microanalysis was performed on the tin selenide block prepared in Example 1. The elemental composition and distribution were determined by bombarding the sample surface with a focused electron beam and detecting characteristic X-rays generated from the sample. The results are shown in FIG. Figure 1 As shown, Figure 1 (a) BSE diagram, (b) Sn, (c) Se, (d) Na, (e) S, and Li elements cannot be detected because they are too light.

[0109] Experimental Example 3 Effect of Temperature on Conductivity and ZT Value

[0110] Test method:

[0111] Conductivity: Seebeck coefficient is tested by ZEM-3 equipment;

[0112] The thermal conductivity was calculated using the formula κ = DρCp, where the thermal diffusivity D was measured using a laser thermal conductivity meter (LFA-467), ρ was measured using the Archimedes drainage method, and Cp was measured using a synchronous thermal analyzer (STA-449).

[0113] The test results are as follows Figure 2-3The results show that conductivity increases with increasing temperature, and further increases with the gradual addition of Na2S and Li2S. This is due in part to the contribution of Na2S to hole carriers, and in part to the stabilization of Li2S to carrier mobility, further improving conductivity and ultimately electrical transport performance. This optimized electrical transport performance significantly enhances the thermoelectric figure of merit, reaching 2.4 at 773K.

[0114] The above is only an embodiment of the present invention, and the common knowledge such as the specific technical solutions and / or characteristics in the solution are not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the description can be used to interpret the content of the claims.

Claims

1. A method for improving the thermoelectric performance of tin selenide bulk material, characterized in that: The steps include: Step 1: Sn and Se powders are mixed in a stoichiometric ratio, and Na2S and Li2S are added as co-dopants; Step 2: vacuum-sealing the mixed raw materials and then subjecting them to a solid-phase melting reaction to obtain a tin selenide ingot; Step 3: ball-milling the molten product to obtain tin selenide powder; Step 4: Spark plasma sintering the tin selenide powder, applying a pulse current during the sintering process, to obtain a tin selenide bulk material.

2. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 1, characterized in that: In step 1, the amount of Na2S added is 0.25-2at%, and the amount of Li2S added is 0.025-0.5at%.

3. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 2, characterized in that: In step 2, the vacuum degree during vacuum sealing is 1×10 -4 Pa.

4. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 3, characterized in that: In step 2, the conditions for the solid phase melting reaction are: heating from room temperature to 950° C. within 10 h, keeping the temperature for 24 h, and then cooling to room temperature within 10 h.

5. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 4, characterized in that: In step 3, the ball milling speed is 600-800 rpm / min, and the ball milling time is 5-10 min.

6. The method for improving the thermoelectric performance of a tin selenide bulk material according to claim 5, characterized in that: In step 3, the ball milling process is carried out in an argon atmosphere.

7. The method for improving the thermoelectric performance of a tin selenide bulk material according to claim 6, characterized in that: In step 4, the sintering temperature is 450-550° C., the sintering pressure is 45-55 MPa, the holding time is 3-10 min, and the vacuum degree is ≤4 Pa.

8. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 7, characterized in that: In step 4, the heating rate is 50-150°C / min.

9. The method for improving the thermoelectric performance of a tin selenide bulk material according to claim 8, characterized in that: In step 4, the pulse mode is DC pulse, the duty cycle is 12:2, and the current density is 500-1500A / cm 2 .

10. The method for improving the thermoelectric performance of tin selenide bulk material according to claim 9, characterized in that: In step 4, the density of the obtained tin selenide block is greater than 95%, and the grain size is 0.5-10 μm.

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