P-type thermoelectric material GeSi target and preparation method thereof
阅读说明：本技术 一种P型热电材料GeSi靶材及其制备方法 (P-type thermoelectric material GeSi target and preparation method thereof ) 是由 沈文兴 白平平 童培云 于 2021-09-15 设计创作，主要内容包括：本发明公开了一种P型热电材料GeSi靶材及其制备方法,涉及合金靶材技术领域,所述制备方法包括如下步骤：(1)将锗颗粒、硅颗粒放入磁悬浮感应熔炼炉中加热至熔化,保温,水冷,得到锗硅合金锭；(2)破碎锗硅合金锭,球磨,得到锗硅合金粉,加入硼粉,所述硼粉占锗硅合金粉的质量分数为0.3～1.5％,均质,得到掺硼锗硅合金粉；(3)将掺硼锗硅合金粉放入模具中,在真空热压炉中预压以排除模具中的空气,得到掺硼锗硅合金坯体；关闭炉门,抽真空,升温至1300～1400℃后保温,保温30～60min后加压,压力为30～35MPa,保温保压60～90min,降压,冷却,得到所述GeSi靶材。由本发明所述方法制备的GeSi靶材的相对密度均可达到98％以上,氧含量低于500ppm,电阻率小于0.001Ω·cm,组分均匀。(The invention discloses a P-type thermoelectric material GeSi target material and a preparation method thereof, relating to the technical field of alloy target materials, wherein the preparation method comprises the following steps: (1) putting germanium particles and silicon particles into a magnetic suspension induction smelting furnace, heating to be molten, preserving heat, and cooling with water to obtain a germanium-silicon alloy ingot; (2) crushing a germanium-silicon alloy ingot, performing ball milling to obtain germanium-silicon alloy powder, adding boron powder, homogenizing to obtain boron-doped germanium-silicon alloy powder, wherein the boron powder accounts for 0.3-1.5% of the mass of the germanium-silicon alloy powder; (3) putting the boron-doped germanium-silicon alloy powder into a mold, and prepressing in a vacuum hot pressing furnace to remove air in the mold to obtain a boron-doped germanium-silicon alloy blank; closing the furnace door, vacuumizing, heating to 1300-1400 ℃, preserving heat for 30-60 min, pressurizing, preserving heat and pressure for 30-35 MPa, preserving heat and pressure for 60-90 min, reducing pressure, and cooling to obtain the GeSi target material. The GeSi target material prepared by the method has the relative density of more than 98 percent, the oxygen content of less than 500ppm, the resistivity of less than 0.001 omega cm and uniform components.)
1. A preparation method of a P-type thermoelectric material GeSi target is characterized by comprising the following steps:
(1) putting germanium particles and silicon particles into a magnetic suspension induction smelting furnace, heating to be molten, preserving heat, and cooling with water to obtain a germanium-silicon alloy ingot;
(2) crushing a germanium-silicon alloy ingot, performing ball milling to obtain germanium-silicon alloy powder, adding boron powder, homogenizing to obtain boron-doped germanium-silicon alloy powder, wherein the boron powder accounts for 0.3-1.5% of the mass of the germanium-silicon alloy powder;
(3) putting the boron-doped germanium-silicon alloy powder into a mold, and prepressing in a vacuum hot pressing furnace to remove air in the mold to obtain a boron-doped germanium-silicon alloy blank; closing the furnace door, vacuumizing, heating to 1300-1400 ℃, preserving heat for 30-60 min, pressurizing, preserving heat and pressure for 30-35 MPa, preserving heat and pressure for 60-90 min, reducing pressure, and cooling to obtain the GeSi target material.
2. The method for preparing the P-type thermoelectric material GeSi target material according to claim 1, wherein in the step (1), the mass ratio of the germanium particles to the silicon particles is 20-40: 60-80, and the purity of the germanium particles to the silicon particles is more than 4N.
3. The preparation method of the P-type thermoelectric material GeSi target material as claimed in claim 1, wherein in the step (1), the magnetic suspension induction melting furnace is vacuumized before heating, then inert gas is introduced to ensure that the pressure in the furnace is 0.05-0.08 MPa, and after the material is melted, the temperature is kept for 10-20 min.
4. The method for preparing the P-type thermoelectric material GeSi target material according to claim 3, wherein in the step (1), the heating temperature is 1400-1500 ℃.
5. The preparation method of the P-type thermoelectric material GeSi target material as claimed in claim 1, wherein in the step (2), the SiGe alloy ingot is firstly crushed to a particle size of less than 5mm, then put into a ball milling pot, ball milled for 4-6 h, kept still for 1-2 h, and sieved to obtain SiGe alloy powder with a particle size of less than 45 μm.
6. The method for preparing the P-type thermoelectric material GeSi target material according to claim 1, wherein in the step (3), the pre-pressing condition is as follows: 2-3T/min, 5-10 MPa, 3-5 min.
7. The method for preparing the P-type thermoelectric material GeSi target material according to claim 1, wherein in the step (1), a water-cooled copper crucible is used for water cooling.
8. The method for preparing the P-type thermoelectric material GeSi target material according to claim 1, wherein in the step (2), a zirconium oxide grinding tank and a hammer are adopted to break the germanium-silicon alloy ingot, the broken germanium-silicon alloy particles are ball-milled in a polyurethane ball-milling tank by using zirconium balls, the volume of the zirconium balls accounts for 30-50% of the total volume of the ball-milling tank, the ball-milling speed is 80-90 rpm, a double-motion mixer is used for homogenizing, and the ball-milling and homogenizing processes are carried out under inert gas.
9. The method for preparing the P-type thermoelectric material GeSi target material according to claim 1, wherein in the step (3), the hot pressing is carried out in a graphite mold, and graphite paper with the thickness of 0.35-0.4 mm is filled in the graphite mold in advance; the heating rate is 5-10 ℃/min; and after the heat preservation and pressure maintaining are finished, reducing the pressure to 10MPa at the speed of 5-10T/min, cooling along with the furnace, and processing the product after cooling to obtain the P-type thermoelectric material GeSi target.
10. The P-type thermoelectric material GeSi target material prepared by the preparation method of the P-type thermoelectric material GeSi target material as claimed in any one of claims 1 to 9.
Technical Field
The invention relates to the technical field of alloy targets, in particular to a P-type thermoelectric material GeSi target and a preparation method thereof.
Background
Thermoelectric materials are also called thermoelectric materials, have cross-coupled thermoelectric transmission properties, are novel functional materials with mutual conversion of thermal effect and electric effect, and can directly convert heat energy and electric energy into semiconductor functional materials by utilizing the properties of the thermoelectric materials. The thermoelectric device made of thermoelectric material has the advantages of small volume, light weight, no pollution, no noise, safety, reliability and the like, and has very wide application prospect.
The thermoelectric material which is well researched at present and is suitable for a high-temperature system is GeSi alloy, and the GeSi alloy is already practically applied to manufacturing a thermoelectric generator for supplying heat by radioactive isotopes. However, the silicon germanium alloy can be a completely miscible substitutional solid solution in any proportion, a crystallization process is carried out in a wider temperature range, the process is very easy to cause component segregation, phase separation can be formed in serious cases, the formed component segregation alloy is difficult to eliminate through subsequent high-temperature heat treatment and other modes, and the performance of the thermoelectric material is seriously influenced.
In patent CN 101550495B, germanium, silicon and their dopant powders are used as raw materials, and the silicon-germanium alloy is prepared through the processes of homogeneous mixing, sheet-pressing smelting, powder-making, homogeneous mixing, solid-phase reaction, etc. The method has the problems of complicated preparation procedures, insufficient target material purity and the like due to easy introduction of impurities in the processes of powder preparation and homogenization; the operation difficulty is high, although the operation difficulty is improved compared with the preparation of the germanium-silicon alloy by a smelting method, the segregation phenomenon of components still exists, and a Ge-rich area appears.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide the P-type thermoelectric material GeSi target material with uniform components, high density, low oxygen content and low resistivity and the preparation method thereof.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a preparation method of a P-type thermoelectric material GeSi target comprises the following steps:
(1) putting germanium particles and silicon particles into a magnetic suspension induction smelting furnace, heating to be molten, preserving heat, and cooling with water to obtain a germanium-silicon alloy ingot;
(2) crushing a germanium-silicon alloy ingot, then carrying out ball milling to obtain germanium-silicon alloy powder, adding boron powder, and homogenizing to obtain boron-doped germanium-silicon alloy powder, wherein the boron powder accounts for 0.3-1.5% of the mass of the germanium-silicon alloy powder;
(3) putting the boron-doped germanium-silicon alloy powder into a mold, and prepressing in a vacuum hot pressing furnace to remove air in the mold to obtain a boron-doped germanium-silicon alloy blank; closing the furnace door, vacuumizing, heating to 1300-1400 ℃, preserving heat for 30-60 min, pressurizing, preserving heat and pressure for 30-35 MPa, preserving heat and pressure for 60-90 min, reducing pressure, and cooling to obtain the GeSi target material.
The invention adopts a magnetic suspension induction melting furnace to melt, and rapidly cools the germanium-silicon alloy solution to obtain a germanium-silicon alloy ingot without segregation, and the components of the prepared target are very uniform. The boron content is controlled to be 0.3-1.5%, and the prepared germanium-silicon alloy target material can be ensured to have good thermoelectric property. The vacuum hot pressing condition is limited, so that the target material has good stability and density, and when the parameter condition does not meet the limit, the target material is easy to crack and has poor mechanical property.
Preferably, in the step (1), the mass ratio of the germanium particles to the silicon particles is 20-40: 60-80, and the purity of the germanium particles to the silicon particles is more than 4N. Ge. The mass ratio of Si can influence the mobility of a current carrier, and when the mass ratio of Ge to Si is 20-40: 60-80, the prepared product can be ensured to have good thermoelectric performance.
Preferably, in the step (1), before heating, the magnetic suspension induction melting furnace is vacuumized, then inert gas is introduced, the pressure in the furnace is 0.05-0.08 MPa, the heating temperature is 1400-1500 ℃, and after the materials are melted, the temperature is kept for 10-20 min. The conditions are adopted for smelting, so that the distribution of Ge and Si in the alloy ingot is more uniform.
Preferably, in the step (2), the germanium-silicon alloy ingot is firstly crushed to a particle size of less than 5mm, then the crushed germanium-silicon alloy ingot is put into a ball milling tank, ball milling is carried out for 4-6 h, standing is carried out for 1-2 h, and sieving is carried out to obtain germanium-silicon alloy powder with a particle size of less than 45 μm. Firstly, crushing the germanium-silicon alloy ingot to be less than 5mm, then carrying out ball milling to improve the ball milling efficiency, controlling the grain diameter of the germanium-silicon alloy powder to be less than 45 mu m to enable the germanium-silicon alloy powder and the boron powder to be mixed more uniformly, and preparing the GeSi target material with uniform components.
Preferably, in the step (3), the pre-pressing condition is as follows: 2-3T/min, 5-10 MPa, 3-5 min. The prepressing aims at exhausting air in the die, the powder can be firmer by adopting the process, and the finished product rate of the GeSi target material is improved.
Preferably, in the step (1), water cooling is performed by using a water-cooled copper crucible; in the step (2), a zirconium oxide grinding tank and a hammer are adopted to break the germanium-silicon alloy ingot, zirconium balls are used for ball-milling the broken germanium-silicon alloy particles in a polyurethane ball-milling tank, the volume of the zirconium balls accounts for 30-50% of the total volume of the ball-milling tank, the ball-milling rotating speed is 80-90 rpm, a double-motion mixer is used for homogenizing, and the ball-milling and homogenizing processes are carried out under inert gas.
Preferably, in the step (3), hot pressing is carried out in a graphite mold, and graphite paper with the thickness of 0.35-0.4 mm is padded in the graphite mold in advance; the heating rate is 5-10 ℃/min; and after the heat preservation and pressure maintenance are finished, slowly reducing the pressure to 10MPa, cooling along with the furnace, processing the product after cooling, and cutting the product into a required shape according to a drawing, polishing and the like to obtain the P-type thermoelectric material GeSi target.
In addition, the invention also discloses the P-type thermoelectric material GeSi target prepared by the method.
Compared with the prior art, the invention has the beneficial effects that: the method adopts a 3-step method to prepare the germanium-silicon alloy target material with no segregation and uniform components, and has low manufacturing cost. Firstly, melting germanium particles and silicon particles in a magnetic suspension induction heating furnace according to a ratio to completely form a GeSi alloy solid solution, and then quenching to form a GeSi alloy ingot; secondly, preparing germanium-silicon alloy powder, and then uniformly mixing the germanium-silicon alloy powder and boron powder; and the third step adopts the vacuum hot-pressing sintering technology to prepare the P-type thermoelectric material GeSi target material, thereby fundamentally solving the series problems of segregation of alloy target material components, high preparation price and the like. In addition, the P-type thermoelectric material GeSi target prepared by the invention also has the advantages of high density, low oxygen content, low resistivity, uniform distribution, no component segregation and the like.
Detailed Description
To better illustrate the objects, aspects and advantages of the present invention, the present invention will be further described with reference to specific examples.
Example 1
In an embodiment of the preparation method of the P-type thermoelectric material GeSi target of the present invention, the preparation method includes the following steps:
(1) selecting 5N germanium particles and 5N silicon particles as raw materials, accurately weighing the raw materials by using an electronic scale according to the mass ratio of Ge to Si of 40:60, and then putting the raw materials into a graphite crucible of a magnetic suspension smelting furnace to obtain a mixed material. Vacuum-pumping to 1 × 10-1And Pa, filling argon for replacing again, vacuumizing again, and filling new argon as protective gas to control the pressure in the smelting furnace to be 0.06 MPa. Under the protection of inert gas, the oxidation of raw materials can be effectively avoided, the material is further prevented from splashing under high vacuum to cause loss, and the component content of the alloy is ensured. Heating the mixture to 1400 ℃ in the vacuum environment, melting the mixed materials to form solid solution alloy, preserving the temperature for 10min, and quenching in a water-cooled copper crucible to obtain the germanium-silicon alloy ingot.
(2) And knocking the cooled SiGe alloy ingot into particles smaller than 5mm by using a zirconia grinding tank and a hammer, putting the knocked particles into a polyurethane ball-milling tank filled with inert gas, putting zirconium balls accounting for 30% of the volume fraction of the ball-milling tank, then, filling argon again to further discharge air in the ball-milling tank, sealing, carrying out ball-milling for powder preparation, carrying out ball-milling for 4 hours at a ball-milling rotating speed of 90rpm, standing for 1.5 hours, taking out, and sieving the SiGe alloy powder by using a stainless steel sieve to obtain the SiGe alloy powder with the particle size smaller than 45 microns. And then placing the germanium-silicon alloy powder and boron powder accounting for 0.3 wt% of the mass fraction of the germanium-silicon alloy powder into a double-motion homogenizer filled with inert gas, and homogenizing for 1h to obtain the boron-doped germanium-silicon alloy powder.
(3) The boron-doped germanium-silicon alloy powder is placed into a graphite mould padded with graphite paper with the thickness of 0.38mm, and then the graphite mould is placed into a vacuum hot-pressing furnace smoothly and stably by using a lifter, so that the center of a pressing column is aligned with the center of the mould. Firstly, pre-pressing at 2.5T/min, wherein the pre-pressing pressure is 5MPa, so as to remove air in the graphite mould, and releasing pressure after keeping for 3min, thereby obtaining the boron-doped germanium-silicon alloy blank. Closing the furnace door, vacuumizing until the vacuum degree in the hearth reaches 10Pa, starting heating, heating to 1300 ℃ at a speed of 10 ℃/min, and preserving heat, wherein the heat preservation comprises the following specific steps: and (3) keeping the temperature for 60min, then starting pressurizing, then keeping the temperature and the pressure, wherein the pressure is 30MPa, the time is 90min, slowly reducing the pressure to 10MPa after the temperature and the pressure are kept, cooling along with the furnace, opening a furnace door, demolding to obtain a germanium-doped silicon alloy blank, and processing to obtain the P-type thermoelectric material GeSi target.
Example 2
In an embodiment of the preparation method of the P-type thermoelectric material GeSi target of the present invention, the preparation method includes the following steps:
(1) selecting 5N germanium particles and 5N silicon particles as raw materials, accurately weighing the raw materials by using an electronic scale according to the mass ratio of Ge to Si of 20:80, and then putting the raw materials into a graphite crucible of a magnetic suspension smelting furnace to obtain a mixed material. Vacuum-pumping to 1 × 10-1And Pa, filling argon for replacing again, vacuumizing again, and filling new argon as protective gas to control the pressure in the smelting furnace to be 0.06 MPa. Heating the mixture to 1500 ℃ in the vacuum environment, melting the mixture to form solid solution alloy, preserving the temperature for 10min, and quenching in a water-cooled copper crucible to obtain the germanium-silicon alloy ingot.
(2) And knocking the cooled SiGe alloy ingot into particles smaller than 5mm by using a zirconia grinding tank and a hammer, putting the knocked particles into a polyurethane ball-milling tank filled with inert gas, putting zirconium balls accounting for 40% of the volume fraction of the ball-milling tank, then, filling argon again to further discharge air in the ball-milling tank, sealing, carrying out ball-milling for powder preparation, carrying out ball-milling for 4 hours at a ball-milling rotating speed of 80rpm, standing for 1.5 hours, taking out, and sieving the SiGe alloy powder by using a stainless steel sieve to obtain the SiGe alloy powder with the particle size smaller than 45 mu m. And then placing the germanium-silicon alloy powder and boron powder accounting for 0.5 wt% of the mass fraction of the germanium-silicon alloy powder into a double-motion homogenizer filled with inert gas, and homogenizing for 1.5h to obtain the boron-doped germanium-silicon alloy powder.
(3) The boron-doped germanium-silicon alloy powder is placed into a graphite mould padded with graphite paper with the thickness of 0.38mm, and then the graphite mould is placed into a vacuum hot-pressing furnace smoothly and stably by using a lifter, so that the center of a pressing column is aligned with the center of the mould. Firstly, pre-pressing at 2T/min, wherein the pre-pressing pressure is 5MPa, so that air in a graphite mold is removed, and the pressure is relieved after keeping for 3min, thereby obtaining a boron-doped germanium-silicon alloy blank. Closing the furnace door, vacuumizing until the vacuum degree in the hearth reaches 10Pa, starting heating, heating to 1350 ℃ at the speed of 10 ℃/min, and preserving heat, wherein the heat preservation specific steps are as follows: and (3) keeping the temperature for 60min, then starting pressurizing, then keeping the temperature and the pressure, wherein the pressure is 35MPa, the time is 90min, slowly reducing the pressure to 10MPa after the temperature and the pressure are kept, cooling along with the furnace, opening a furnace door, demolding to obtain a germanium-doped silicon alloy blank, and processing to obtain the P-type thermoelectric material GeSi target.
Example 3
In an embodiment of the preparation method of the P-type thermoelectric material GeSi target of the present invention, the preparation method includes the following steps:
(1) selecting 4N germanium particles and 4N silicon particles as raw materials, accurately weighing the raw materials by using an electronic scale according to the mass ratio of Ge to Si of 30:70, and then putting the raw materials into a graphite crucible of a magnetic suspension smelting furnace to obtain a mixed material. Vacuum-pumping to 1 × 10-1And Pa, filling argon for replacing again, vacuumizing again, and filling new argon as protective gas to control the pressure in the smelting furnace to be 0.06 MPa. Heating the mixture to 1450 deg.C in vacuum environment, melting the mixture to form solid solution alloy, holding the temperature for 10min, and quenching in water-cooled copper crucible to obtain SiGe alloy ingot.
(2) And knocking the cooled SiGe alloy ingot into particles smaller than 5mm by using a zirconia grinding tank and a hammer, putting the knocked particles into a polyurethane ball-milling tank filled with inert gas, putting zirconium balls accounting for 50% of the volume fraction of the ball-milling tank, then, filling argon again to further discharge air in the ball-milling tank, sealing, carrying out ball-milling for powder preparation, carrying out ball-milling for 4 hours at a ball-milling rotating speed of 80rpm, standing for 1.5 hours, taking out, and sieving the SiGe alloy powder by using a stainless steel sieve to obtain the SiGe alloy powder with the particle size smaller than 45 mu m. And then placing the germanium-silicon alloy powder and boron powder accounting for 1 wt% of the mass fraction of the germanium-silicon alloy powder into a double-motion homogenizer filled with inert gas, and homogenizing for 2 hours to obtain the boron-doped germanium-silicon alloy powder.
(3) The boron-doped germanium-silicon alloy powder is placed into a graphite mould padded with graphite paper with the thickness of 0.38mm, and then the graphite mould is placed into a vacuum hot-pressing furnace smoothly and stably by using a lifter, so that the center of a pressing column is aligned with the center of the mould. Firstly, prepressing at 3T/min, wherein the prepressing pressure is 5MPa, so as to remove air in the graphite mould, and releasing pressure after keeping for 3min, thereby obtaining the boron-doped germanium-silicon alloy blank. Closing the furnace door, vacuumizing until the vacuum degree in the hearth reaches 10Pa, starting heating, heating to 1320 ℃ at a speed of 10 ℃/min, and preserving heat, wherein the heat preservation comprises the following specific steps: and (3) keeping the temperature for 60min, then starting pressurizing, then keeping the temperature and the pressure, wherein the pressure is 35MPa, the time is 90min, slowly reducing the pressure to 10MPa after the temperature and the pressure are kept, cooling along with the furnace, opening a furnace door, demolding to obtain a germanium-doped silicon alloy blank, and processing to obtain the P-type thermoelectric material GeSi target.
Comparative example 1
A method for preparing a P-type thermoelectric material GeSi target, which is different from the method in example 1 only in that, in the step (3), the vacuum hot-pressing temperature is 1200 ℃.
The performance of the P-type thermoelectric material GeSi target materials prepared in the examples 1-3 and the comparative example 1 is tested, the test method is as follows, and the test results are shown in tables 1-3:
component detection: the measurement was carried out by using an ICP-OES apparatus (hereinafter referred to as Inductively Coupled Plasma Optical Emission Spectrometry). And detecting the components of the center and the edge of the target and the middle part of the center and the edge of the target, and taking an average value.
And (3) density detection: detecting by adopting an Archimedes principle;
and (3) resistivity detection: the method comprises the following steps of (1) adopting a four-probe method for testing, specifically connecting a current source to two ends of a sample, placing leads of a voltmeter according to a set distance, and calculating resistivity according to the cross-sectional area of the sample and the distance between the leads of the voltmeter;
N/P type test: a method for rapidly detecting the Seeback coefficient (RIGOL equipment of Beijing Puyuan smart electrical technology Co., Ltd.) is adopted;
the purity measurement was carried out by an ICP-MS instrument (hereinafter referred to as Inductively coupled plasma mass spectrometer).
TABLE 1
Item
Ge(wt.％)
Relative density%
Resistivity (omega cm)
O(ppm)
N/P type
Example 1
39.92％
98.1％
0.0005
495
P
Example 2
19.90％
98.4％
0.0006
462
P
Example 3
29.93％
98.2％
0.0005
421
P
Comparative example 1
39.89％
93.3％
0.0015
486
P
Table 2 (wt.%)
Item
Ge (center)
Ge (edge)
Ge (center and edge middle)
Example 1
39.94
39.92
39.89
Example 2
19.91
19.89
19.90
Example 3
29.94
29.91
29.93
Table 3 (wt.%)
Item
Al
Ca
Fe
Mg
Na
Zn
Cu
Mn
Cr
Example 1A
＜1
1.4
＜1
＜1
＜1
＜1
2.1
＜1
＜1
Example 1B
＜1
1.5
＜1
＜1
＜1
＜1
2.0
＜1
＜1
Example 2A
＜1
1.4
＜1
＜1
＜1
＜1
1.9
＜1
＜1
Example 2B
＜1
1.4
＜1
＜1
＜1
＜1
1.9
＜1
＜1
Example 3A
＜1
1.5
＜1
＜1
＜1
＜1
1.8
＜1
＜1
Example 3B
＜1
1.4
＜1
＜1
＜1
＜1
1.8
＜1
＜1
Comparative example 1A
＜1
1.3
＜1
＜1
＜1
＜1
1.7
＜1
＜1
Comparative example 1B
＜1
1.4
＜1
＜1
＜1
＜1
1.8
＜1
＜1
A is the boron-doped germanium-silicon alloy powder prepared in the step (2), and B is a GeSi target material.
As can be seen from tables 1 to 3, the target materials prepared in examples 1 to 3 have higher density, which can reach 98%, and the resistivity is less than 0.001 Ω · cm, and the target materials have uniform components and no segregation. The density of the target material prepared in comparative example 1 is only 93.3%, and the result shows that the hot pressing condition has an extremely important influence on the performance of the target material.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the protection scope of the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.