Patent Application
20220344574
The application claims priority to Chinese patent application No. 202110445772.6, filed on Apr. 25, 2021, the entire contents of which are incorporated herein by reference.
The present disclosure belongs to the technical field of piezoelectric ceramic materials, and in particular relates to a lead-free piezoelectric ceramic sensor material and a preparation method thereof.
The piezoelectric ceramic refers to a polycrystal formed by mixing oxides (zirconia, lead oxide, titanium oxide, etc.), sintering at high temperature, and solid-phase reaction, and the general term for ferroelectric ceramic that are made with piezoelectric effect through direct current high-voltage polarization treatment. Piezoelectric ceramic is an important part of functional ceramic. After the Curie brothers discovered the piezoelectric effect in the 1880s, the piezoelectric ceramic has been rapidly researched and developed. At present, the research on piezoelectric effect is mainly in three aspects: the piezoelectric ceramic, a piezoelectric polymer, and a piezoelectric crystal, and the piezoelectric ceramic have the most piezoelectric effect. As an important functional material with strong force, heat, electricity and light sensitivity, piezoelectric ceramic have been widely used in sensors, ultrasonic transducers, micro-displacers and other electronic devices. With the progress and optimization of processing technology, piezoelectric ceramic has high research and application value in aerospace, electronics, information and other high-tech fields due to their low cost and high-voltage power conversion advantages.
A piezoelectric ceramic sensor has important applications in many aspects of people's lives, but the piezoelectric ceramic materials currently used in large quantities in the world are still traditional lead-containing piezoelectric ceramic, whose lead element content is as high as 60% or more. Lead oxide is a volatile toxic substance, which will cause damage to humans and the ecological environment during production, preparation, use and disposal after disposal. With the increasing awareness of environmental protection, the international community is actively banning lead-containing electronic products through laws, regulations, and government directives. The volatilization of high-temperature sintered PbO not only pollutes the environment, but also causes the stoichiometric ratio of the ceramic to be higher, which affects the performance of the material. Therefore, lead-free piezoelectric ceramic has gradually become one of the hotspots in the research of piezoelectric ceramic sensor materials.
At present, the main research directions of lead-free piezoelectric ceramic are: a barium titanate based, a niobate based, a BNT-based and other lead-free piezoelectric ceramic systems. The Curie temperature of the existing barium titanate based piezoelectric ceramic is lower than other piezoelectric ceramic (Tc=120° C.), and the various electrical properties of the barium titanate based piezoelectric ceramic are poor, and its various properties are difficult to be effectively improved by doping, which limits its wide application; and niobate based lead-free piezoelectric ceramic has low density and good piezoelectric properties. therefore, the niobate based lead-free piezoelectric ceramic is good backup material for the preparation of piezoelectric materials and high-frequency transducers. However, there are no reports about its application in sensors. BNT lead-free piezoelectric ceramic has good piezoelectric properties and are considered to be a piezoelectric material that may replace PZT ceramic. However, the ceramic system has a higher coercive field, the compactness and chemical stability of the ceramic are poor, and the sintering temperature is narrow, so, compared with PZT ceramic, there is still a gap in the performance of the BNT lead-free piezoelectric ceramic and the existing BNT ceramic is difficult to apply. Therefore, the development of a lead-free piezoelectric ceramic material with excellent piezoelectricity, stability and higher Curie temperature will become the research and development direction of lead-free piezoelectric ceramic sensors.
The purpose of the present disclosure is to provide a lead-free piezoelectric ceramic sensor material, which is simple to operate, does not contain lead in the piezoelectric ceramic material, has excellent compactness and good chemical stability. And the prepared piezoelectric sensor has high sensitivity and workability, strong stability, excellent piezoelectric properties, dielectric properties, mechanical properties, and a high Curie temperature.
A lead-free piezoelectric ceramic sensor material is provided to achieve the disclosure's purpose. The lead-free piezoelectric ceramic sensor material; wherein: in terms of a mass percentage, the lead-free piezoelectric ceramic sensor material is composed of the following main raw materials: barium carbonate 32.8-35.3%, calcium carbonate 34.4-38.7%, zirconia 8.5-10.3%, titanium dioxide 8.6-11.0%, strontium carbonate 3.8-5.2%, oxidation bait 2.4-3.7% and bismuth oxide 1.5-4.2%; the lead-free piezoelectric ceramic sensor material includes lithium oxide, yttrium oxide, composite binder and dispersant agent.
Further, a mass of the lithium oxide is 0.21-0.35% of a total amount of a main raw material, and the mass of the yttrium oxide is 0.18-0.29% of the total amount of the main raw material, the mass of the composite binder is 3.5-4.5% of the total amount of the main raw material and the mass of the dispersant agent is 0.3-0.6% of the total amount of the main raw material.
Further, the composite binder is prepared by reacting polyvinyl alcohol with polysiloxane compound under an action of triethanolamine; the polysiloxane compound is a waterborne film-forming protective agent R-23 produced by a Dining Huakai Resin Co., Ltd., and a mass ratio of the polyvinyl alcohol and the polysiloxane compound is 1:(0.5-0.8).
A preparation method of the composite binder, comprising: adding 2 times a mass of the triethanolamine into the polyvinyl alcohol under a room temperature, adding organosiloxane compound, and then stirring a mixture of the polyvinyl alcohol, the triethanolamine and the organosiloxane compound under a constant temperature of 60° C. for 1.5 hours, filtering, washing with deionized water, and drying to obtain a required composite binder.
Further, the dispersant agent is one or more of sodium citrate, sodium silicate, sodium hexametaphosphate or sodium tripolyphosphate.
A preparation method of the lead-free piezoelectric ceramic sensor material, comprising the following steps:
(1) preparing ingredients: according to the mass percentage, weighing and taking a required mass of the barium carbonate, the calcium carbonate, the zirconia, the titanium dioxide, the strontium carbonate, the oxidation bait and the bismuth oxide, mixing uniformly to prepare a mixed raw material;
(2) ball milling: adding the mixed raw material into a high-energy ball mill, then adding ⅓ of a required mass of the dispersant agent, starting a wet ball milling, and a mass ratio of the mixed raw material and the deionized water is 1:1, a ball milling rate is 320-380 r/min, preforming a ball milling for 4-5 h; then continuing to adding the lithium oxide, the yttrium oxide and a remaining ⅔ mass dispersant agent into the high-energy ball mill, starting a secondary ball milling and the ball milling rate is 320-380 r/min, and preforming the ball milling for 10 h;
(3) granulating and tableting: suction filtering a slurry after ball milling, and then processing the slurry after ball milling into a 400 mesh granular ceramic powder through a centrifugal spray dryer; uniformly mixing the 400 mesh granular ceramic powder with an absolute ethyl alcohol in a mass ratio of 1:0.3, adding the composite binder, and after mixing uniformly, heating up to 70° C., stirring for 1 h, and then granulating; and placing pellets in a stainless steel mold and pressing the pellets into a green body with a required thickness under a pressure of 12-18 MPa;
(4) debinding: putting the green body in a heating furnace, heating up to 200-250° C., performing a heat preservation for 60 minutes; and then heating up to 500-600° C. at a rate of 3-5° C./min and performing the heat preservation for 2 h; then heating up to 720-780° C. and performing the heat preservation for 3 h;
(5) sintering: putting a debinding green body in a clamp pot, covering tightly, and sintering at 1050-1180° C., a heat preservation time is 3-4 h; and then naturally cooling the debinding green body to the room temperature, and finishing a preparation of the lead-free piezoelectric ceramic sensor material;
(6) applying an electrode: cleaning the lead-free piezoelectric ceramic sensor material, and then using a screen printing technology to print a silver paste on an upper surface and a lower surface of the lead-free piezoelectric ceramic sensor material, and putting a printed silver product in the heating furnace, heating up to 800° C., performing the heat preservation for 50-60 minutes, waiting the printed silver product cooling to the room temperature, and finishing a piezoelectric ceramic silver sheet;
(7) polarizing: polarizing the piezoelectric ceramic silver sheet by a direct current electric field, a polarization temperature is from 110 to 130° C., a polarization time is 30 min, and a polarization field strength is 3-6 kV/mm, and preparing the lead-free piezoelectric ceramic sensor.
The present disclosure has achieved the following beneficial effects:
1. The present disclosure is a multi-element composite ceramic material composed of barium carbonate, calcium carbonate, zirconia and titanium dioxide as main components. The ceramic material does not contain lead, and will not produce volatile and toxic lead oxide during high-temperature sintering, and in the process of disposal, it will not harm human beings and the ecological environment. The multi-element composite ceramic material is added with zirconia in the perovskite structure, which is mainly barium titanate and calcium titanate, making the ceramic material excellent in compactness, improving the high temperature strength and toughness of ceramic materials and the dielectric properties and piezoelectric properties of ceramic materials, and further increasing the Curie temperature of the piezoelectric sensor.
2. Strontium carbonate is added in the present disclosure, since the decomposition temperature of strontium carbonate is relatively low, and it is easy to decompose. Therefore, before the ceramic body is fused and sealed, the strontium carbonate has decomposed its internal CO2. Therefore, under high temperature conditions, the non-viscosity of glass phase of strontium carbonate drops too low and overburns bubbles, which improves the compactness of the lead-free ceramic material of the present disclosure, and makes the ceramic material have higher wear resistance, mechanical strength and toughness and reduces the shrinkage rate of ceramic materials and the deformation rate of the ceramic body. Strontium oxide has a good solubilizing effect. Strontium carbonate is decomposed into strontium oxide under the action of high temperature, which reduces the sintering temperature of the ceramic body, so that the disclosure can be sintered into lead-free piezoelectric ceramic material at 1050° C. Strontium carbonate can form perovskite structure strontium titanate in the sintering process, which improves the piezoelectric coefficient of lead-free piezoelectric ceramic so as to greatly improve the piezoelectric performance of piezoelectric ceramic sensors.
3. The addition of oxidation bait in the present disclosure can accelerate the electron transfer between the lead-free ceramic material and the electrode, improving the chemical stability of lead-free ceramic materials, and making the prepared lead-free piezoelectric sensor have good stability and reproducibility and further improving the piezoelectric performance of the lead-free piezoelectric sensor. By adding and synergizing the oxidation bait and the bismuth oxide, the present disclosure increases the electrical conductivity of the lead-free ceramic material, and further improves the piezoelectric coefficient and Curie temperature of the piezoelectric sensor, and the sensitivity is higher.
4. In the present disclosure, lithium oxide and yttrium oxide are added as a sintering aid for lead-free ceramic materials, reducing the sintering temperature of the ceramic body, contributing to the diffusion and migration of grain boundaries, and improving the density and mechanical properties of lead-free ceramic materials.
5. The composite binder is prepared by reacting a polyvinyl alcohol with a polysiloxane compound under an action of a triethanolamine. The composite binder can cross-link with the metal oxide in the ceramic material to form a network structure and to form a dense ceramic matrix with a well suppleness. And the composite binder is sintered and carbonized at high temperature to form silicon carbide or silicon oxide, which is conducive to the diffusion and migration of grain boundaries, thereby greatly improving the temperature resistance and mechanical properties of ceramic materials.
6. The disclosure adopts lead-free piezoelectric ceramic material based on barium carbonate, calcium carbonate, zirconia and titanium dioxide. The preparation method is simple, easy to operate, and has low dielectric constant and low dielectric loss, and since it has excellent mechanical properties, it is not easy to break. The lead-free piezoelectric ceramic sensor prepared by the disclosure has good stability, high Curie temperature, and excellent piezoelectric and mechanical properties.
The following describes the technical solutions in the embodiments of the present disclosure clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present disclosure.
The lead-free piezoelectric ceramic sensor material and the lead-free piezoelectric ceramic sensor of the present disclosure and the preparation method thereof will be described below with reference to specific embodiments.
the preparation method of the lead-free piezoelectric ceramic sensor material, comprising the following steps:
(1) preparing ingredients: weighing and taking 328 g BaCO3, 387 g CaCO3, 85 g ZrO2, 110 g TiO2, 38 g SrCO3, 37 g Er2O3 and 15 g Bi2O3 and mixing them uniformly to prepare the mixed raw material.
(2) Ball milling: adding the mixed raw material into a high-energy ball mill, and then adding 2 g sodium hexametaphosphate, starting a wet ball milling and the ball milling medium is steel ball, and adding 5 kg steel balls and 1 kg deionized water, a ball milling rate is 320-380 r/min, preforming the ball milling for 4-5 h. Continuing to adding 3.5 g Li2O, 2.9 g Y2O3 and 4 g sodium hexametaphosphate into the high-energy ball mill, starting a secondary ball milling and during the second ball milling process, 0.5 kg of deionized water needs adding into the high-energy ball mill, and the ball milling rate is 320-380 r/min, and preforming the ball milling for 10 h.
(3) Granulating and tableting: suction filtering a slurry after ball milling, and then processing the slurry after ball milling into a 400 mesh granular ceramic powder through a centrifugal spray dryer, uniformly mixing the 400 mesh granular ceramic powder with an absolute ethyl alcohol in a mass ratio of 1:0.3, adding 35 g composite binder, and after mixing uniformly, heating up to 70° C., stirring for 1 h, and then granulating; and placing pellets in a stainless steel mold and pressing the pellets into a green body with a required thickness under a pressure of 12-18 MPa.
The composite binder is prepared by reacting a polyvinyl alcohol with a polysiloxane compound under an action of a triethanolamine; the polysiloxane compound is a waterborne film-forming protective agent R-23 produced by a Dining Huakai Resin Co., Ltd. The preparation method of the composite binder, comprising: adding 200 g triethanolamine into 100 g polyvinyl alcohol under a room temperature, and then adding 50 g organosiloxane compound, and then stirring a mixture of the polyvinyl alcohol, the triethanolamine and an organosiloxane compound under a constant temperature of 60° C. for 1.5 hours, filtering, washing with a deionized water, and drying to obtain a required composite binder.
(4) Debinding: putting the green body in a heating furnace, heating up to 200-250° C., performing a heat preservation for 60 minutes; and then heating up to 500-600° C. at a rate of 5° C./min and performing the heat preservation for 2 h; then heating up to 720-780° C. and performing the heat preservation for 3 h.
(5) Sintering: putting a debinding green body in a clamp pot, covering tightly, and sintering at 1180° C., a heat preservation time is 3-4 h; and then naturally cooling the debinding green body to the room temperature, and finishing a preparation of the lead-free piezoelectric ceramic sensor material A1.
the preparation method of the lead-free piezoelectric ceramic sensor material, comprising the following steps:
(1) preparing ingredients: weighing and taking 353 g BaCO3, 344 g CaCO3, 103 g ZrO2, 86 g TiO2, 52 g SrCO3, 24 g Er2O3 and 38 g Bi2O3, and mixing them uniformly to prepare the mixed raw material.
(2) Ball milling: adding the mixed raw material into a high-energy ball mill, and then adding 1 g sodium silicate, starting a wet ball milling and the ball milling medium is steel ball, and adding 5 kg steel balls and 1 kg deionized water, a ball milling rate is 320-380 r/min, preforming the ball milling for 4-5 h. Continuing to adding 2.1 g Li2O, 1.8 g Y2O3 and 2 g sodium silicate into the high-energy ball mill, starting a secondary ball milling and during the second ball milling process, 0.5 kg of deionized water needs adding into the high-energy ball mill, and the ball milling rate is 320-380 r/min, and preforming the ball milling for 10 h.
(3) Granulating and tableting: suction filtering a slurry after ball milling, and then processing the slurry after ball milling into a 400 mesh granular ceramic powder through a centrifugal spray dryer, uniformly mixing the 400 mesh granular ceramic powder with an absolute ethyl alcohol in a mass ratio of 1:0.3, adding 45 g composite binder, and after mixing uniformly, heating up to 70° C., stirring for 1 h, and then granulating; and placing pellets in a stainless steel mold and pressing the pellets into a green body with a required thickness under a pressure of 12-18 MPa.
The composite binder is prepared by reacting a polyvinyl alcohol with a polysiloxane compound under an action of a triethanolamine. The mass ratio of the polyvinyl alcohol and the polysiloxane compound is 1:0.8.
(4) Debinding: putting the green body in a heating furnace, heating up to 200-250° C., performing a heat preservation for 60 minutes; and then heating up to 500-600° C. at a rate of 4° C./min and performing the heat preservation for 2 h; then heating up to 720-780° C. and performing the heat preservation for 3 h.
(5) Sintering: putting a debinding green body in a clamp pot, covering tightly, and sintering at 1150° C., a heat preservation time is 3-4 h; and then naturally cooling the debinding green body to the room temperature, and finishing a preparation of the lead-free piezoelectric ceramic sensor material A2.
the preparation method of the lead-free piezoelectric ceramic sensor material, comprising the following steps:
(1) preparing ingredients: weighing and taking 336 g BaCO3, 364 g CaCO3, 92 g ZrO2, 92 g TiO2, 48 g SrCO3, 26 g Er2O3 and 42 g Bi2O3 and mixing them uniformly to prepare the mixed raw material.
(2) Ball milling: adding the mixed raw material into a high-energy ball mill, and then adding 1.5 g sodium tripolyphosphate, starting a wet ball milling and the ball milling medium is steel ball, and adding 5 kg steel balls and 1 kg deionized water, a ball milling rate is 320-380 r/min, preforming the ball milling for 4-5 h. Continuing to adding 3 g Li2O, 2.5 g Y2O3 and 3 g sodium tripolyphosphate into the high-energy ball mill, starting a secondary ball milling and during the second ball milling process, 0.5 kg of deionized water needs adding into the high-energy ball mill, and the ball milling rate is 320-380 r/min, and preforming the ball milling for 10 h.
(3) Granulating and tableting: suction filtering a slurry after ball milling, and then processing the slurry after ball milling into a 400 mesh granular ceramic powder through a centrifugal spray dryer, uniformly mixing the 400 mesh granular ceramic powder with an absolute ethyl alcohol in a mass ratio of 1:0.3, adding 38 g composite binder, and after mixing uniformly, heating up to 70° C., stirring for 1 h, and then granulating; and placing pellets in a stainless steel mold and pressing the pellets into a green body with a required thickness under a pressure of 12-18 MPa.
The composite binder is prepared by reacting a polyvinyl alcohol with a polysiloxane compound under an action of a triethanolamine. The mass ratio of the polyvinyl alcohol and the polysiloxane compound is 1:0.6.
(4) Debinding: putting the green body in a heating furnace, heating up to 200-250° C., performing a heat preservation for 60 minutes; and then heating up to 500-600° C. at a rate of 3° C./min and performing the heat preservation for 2 h; then heating up to 720-780° C. and performing the heat preservation for 3 h.
(5) Sintering: putting a debinding green body in a clamp pot, covering tightly, and sintering at 1050° C., a heat preservation time is 3-4 h; and then naturally cooling the debinding green body to the room temperature, finishing a preparation of the lead-free piezoelectric ceramic sensor material A3.
the preparation method of the lead-free piezoelectric ceramic sensor material, comprising the following steps:
(1) preparing ingredients: weighing and taking 349 g BaCO3, 356 g CaCO3, 87 g ZrO2, 108 g TiO2, 41 g SrCO3, 36 g Er2O3 and 23 g Bi2O3, and mixing them uniformly to prepare the mixed raw material.
(2) Ball milling: adding the mixed raw material into a high-energy ball mill, and then adding 2 g sodium citrate, starting a wet ball milling and the ball milling medium is steel ball, and adding 5 kg steel balls and 1 kg deionized water, a ball milling rate is 320-380 r/min, preforming the ball milling for 4-5 h. Continuing to adding 2.6 g Li2O, 2.8 g Y2O3 and 4 g sodium citrate into the high-energy ball mill, starting a secondary ball milling and during the second ball milling process, 0.5 kg of deionized water needs adding into the high-energy ball mill, and the ball milling rate is 320-380 r/min, and preforming the ball milling for 10 h.
(3) Granulating and tableting: suction filtering a slurry after ball milling, and then processing the slurry after ball milling into a 400 mesh granular ceramic powder through a centrifugal spray dryer, uniformly mixing the 400 mesh granular ceramic powder with an absolute ethyl alcohol in a mass ratio of 1:0.3, adding 42 g composite binder, and after mixing uniformly, heating up to 70° C., stirring for 1 h, and then granulating; and placing pellets in a stainless steel mold and pressing the pellets into a green body with a required thickness under a pressure of 12-18 MPa.
The composite binder is prepared by reacting a polyvinyl alcohol with a polysiloxane compound under an action of a triethanolamine. The mass ratio of the polyvinyl alcohol and the polysiloxane compound is 1:0.5.
(4) Debinding: putting the green body in a heating furnace, heating up to 200-250° C., performing a heat preservation for 60 minutes; and then heating up to 500-600° C. at a rate of 4° C./min and performing the heat preservation for 2 h; then heating up to 720-780° C. and performing the heat preservation for 3 h.
(5) Sintering: putting a debinding green body in a clamp pot, covering tightly, and sintering at 1100° C., a heat preservation time is 3-4 h; and then naturally cooling the debinding green body to the room temperature, finishing a preparation of the lead-free piezoelectric ceramic sensor material A4.
Comparative example 1: a piezoelectric ceramic sensor material a1, whose preparation method is the same as that in the embodiment 3, while, the difference is that in the comparative example 1, the composition of the mixed raw materials is 349 g BaCO3, 356 g CaCO3, 87 g ZrO2 and 108 g TiO2.
Comparative example 2: a piezoelectric ceramic sensor material a2, whose material and preparation method are the same as that in the embodiment 3, while, the difference is that SrCO3 is not added into the comparative example 2.
Comparative example 3: a piezoelectric ceramic sensor material a3, whose material and preparation method are the same as that in the embodiment 3, while the difference is that Er2O3 and Bi2O3 are not added into the comparative example 3.
Comparative example 4: a piezoelectric ceramic sensor material a4, whose material and preparation method are the same as that in the embodiment 3, except that the composite binder in the comparative example 4 is polyvinyl alcohol.
Testing the performance of the piezoelectric ceramic sensor materials prepared in the foregoing embodiments 1-4 and comparative examples 1-4, and the test results are shown in Table 1 below:
It can be seen from the test results in Table 1 that the piezoelectric ceramic sensor material prepared by the present disclosure has excellent mechanical strength and toughness. The composite binder has greatly improved strength and toughness of the present disclosure, and the addition of the strontium carbonate also improve the mechanical strength and fracture toughness of the present disclosure.
Cleaning the lead-free piezoelectric ceramic sensor material made in the embodiments 1-4 and comparative examples 1-4, and then using a screen printing technology to print a silver paste on an upper surface and a lower surface of the lead-free piezoelectric ceramic sensor material, and putting a printed silver product in the heating furnace, heating up to 800° C., performing the heat preservation for 50-60 minutes, waiting the printed silver product cooling to the room temperature, and finishing a piezoelectric ceramic silver sheet. Polarizing the piezoelectric ceramic silver sheet by a direct current electric field, a polarization temperature is from 110 to 130° C., a polarization time is 30 min, and a polarization field strength is 3-6 kV/mm, and finally the lead-free piezoelectric ceramic sensor is prepared. Wherein, the lead-free piezoelectric ceramic sensors prepared by the above-mentioned electrode application and polarization methods in the embodiments 1-4 are B1, B2, B3, and B4 respectively. And the lead-free piezoelectric ceramic sensors prepared by the above-mentioned electrode application and polarization methods in the comparative examples 1-4 are b1, b2, b3 and b4 respectively.
Testing the performance of the piezoelectric ceramic sensors prepared in the foregoing embodiments 1-4 and comparative examples 1-4, and the test results are shown in Table 2 below:
It can be seen from the test results in Table 2 that the piezoelectric ceramic sensor has a higher piezoelectric coefficient, that is, higher sensitivity and lower dielectric loss, which can ensure that the piezoelectric ceramic sensor loses less internal energy during long-term operation. Further, the piezoelectric ceramic sensor will not be broken due to its heating, and has a higher Curie temperature, which can still have better piezoelectric performance at higher temperatures. The present disclosure improves the piezoelectric performance of the piezoelectric sensor and reduces the dielectric loss by adding strontium carbonate. And by adding and synergizing Er2O3 and Bi2O3, the present disclosure significantly increases the piezoelectric coefficient and Curie temperature of the piezoelectric sensor and reduces the dielectric loss, and further, the present disclosure can enjoy a higher sensitivity.
The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above-mentioned embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope of the specification.
The above-mentioned examples only express several embodiments of the present disclosure, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the present disclosure. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present disclosure, several modifications and improvements can be made, and these all fall within the protection scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021104457726 | Apr 2021 | CN | national |
