Method for preparing sheet-shaped heating elements

Abstract

This invention belongs to the field of heating element technology, specifically relating to a method for preparing sheet-like heating elements. The preparation steps include: dissolving lanthanum nitrate, strontium nitrate, manganese nitrate hexahydrate, and a solvent by stirring to obtain a metal salt solution; preparing a precursor from the metal salt solution, which may be a precursor solution or a precursor sol; casting the precursor into a sheet, drying it to obtain a green sheet; removing the binder from the green sheet; and sintering it to obtain the sheet-like heating element. This invention optimizes the precursor system, improves the dispersion stability of the precursor, and, combined with the casting, drying, and sintering processes, solves the problem of powder agglomeration, reduces defects in the green sheet, decreases the drying shrinkage rate and surface roughness of the green sheet during the preparation process, and improves the stability, heat output uniformity, and density of the heating element.

Description

Technical Field

[0001] This invention belongs to the field of heating element technology, specifically relating to a method for preparing sheet-shaped heating elements. Background Technology

[0002] Traditional slurries are usually formed into heating elements by scraping or pressing combined with drying or sintering processes. However, heating elements formed in this way are prone to problems such as thickness fluctuations, local stress concentration, drying cracks or incomplete film formation. In severe cases, it can also affect the stability of the heating element and the uniformity of heat output.

[0003] Subsequently, tape casting technology emerged, which is highly suitable for the fabrication of large, thin ceramic components. However, tape casting has disadvantages, including the tendency for powder agglomeration, green defects (cracking, uneven thickness, etc.), and low product density during the fabrication process. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a sheet-like heating element.

[0005] The above-mentioned objective of the present invention is achieved by the following technical solution:

[0006] The method for preparing a sheet-like heating element includes the following steps:

[0007] Lanthanum nitrate, strontium nitrate, manganese nitrate hexahydrate, and solvent were stirred and dissolved to obtain a metal salt solution;

[0008] A metal salt solution is used to prepare a precursor, which is either a precursor solution or a precursor sol.

[0009] The precursor is cast and dried to obtain a blank sheet. The blank sheet is then debonded and sintered to obtain a sheet-shaped heating element.

[0010] According to one aspect of the present invention, a method for preparing a precursor solution includes the following steps:

[0011] Polyvinyl butyral solution was added to the metal salt solution, followed by dibutyl phthalate. The mixture was stirred, and the solid content and viscosity were adjusted. Impurities were removed to obtain the precursor solution.

[0012] It should be noted that, using the above technical solution, the traditional method utilizes pre-synthesized powder as a raw material. This invention uses a metal salt solution as a starting material, thus solving the problem of powder agglomeration.

[0013] According to one aspect of the present invention, a method for preparing a precursor sol includes the following steps:

[0014] The metal salt solution is heated to 80-90℃, citric acid is added and stirred, then ethylene glycol is added and the reaction continues. After the reaction is completed, the solution is cooled to obtain a sol.

[0015] The solvent and hexagonal boron nitride were ultrasonically mixed, stirred, and reacted to obtain a suspension.

[0016] The suspension was added to the sol while stirring and dispersed to obtain a mixed sol;

[0017] Low-melting-point glass powder was added to the mixed sol and stirred. Polyvinyl butyral solution was added and dibutyl phthalate was added and stirred. The solid content and viscosity were adjusted, impurities were removed, and the mixture was degassed under vacuum to obtain the precursor sol.

[0018] It should be noted that, using the above technical solution, metal salt chelation yields a metal-citric acid complex. Under heating conditions, the carboxyl group on citric acid undergoes an esterification reaction with the hydroxyl group on ethylene glycol to form a long polymer chain of "citric acid-ethylene glycol-citric acid". The previously formed metal complex serves as a crosslinking point, ultimately forming a three-dimensional, elastic polymer gel network that fixes the metal ions in their respective positions.

[0019] In the suspension, hexagonal boron nitride exists in a monodisperse or few-layer state, avoiding later agglomeration. The addition of hexagonal boron nitride to the sol can improve the thermal conductivity of the finished product and prevent local overheating of the heating element. It can also inhibit excessive shrinkage and deformation during the sintering of the green sheet.

[0020] Adding low-melting-point glass micropowder before adding the polyvinyl butyral solution, i.e. after the gel network is formed and before the organic reagent is added, allows the low-melting-point glass micropowder to be partially encapsulated or adsorbed by the gel network, achieving a preliminary uniform distribution and avoiding competitive adsorption or entanglement with the polymer chains of polyvinyl butyral, which would affect the subsequent casting performance.

[0021] In some embodiments, the solid content is 35-45 wt%, and the viscosity at room temperature is 500-1500 mPa·s.

[0022] In some embodiments, the solvent is obtained by mixing ethanol, ethylene glycol methyl ether, glacial acetic acid and deionized water.

[0023] In some embodiments, casting the precursor includes the following steps:

[0024] Set the doctor blade height of the casting machine to 250-300μm, pour the precursor into the hopper of the casting machine, start the casting machine, and move the base belt at a constant speed of 2-3cm / min. At the same time, control the flow rate of the hopper to make the precursor evenly coated on the base belt to form a continuous wet film. During the casting process, maintain the ambient temperature at 23-27℃ and the relative humidity at 40%-50%.

[0025] In some embodiments, drying includes the following steps:

[0026] The wet film after casting is dried together with the base tape at 35-45℃ for 2-3 hours, then the temperature is raised to 55-65℃ and dried for 4-6 hours; finally, the temperature is raised to 75-85℃ and dried for 1-2 hours. After drying, the dried wet film is peeled off from the base tape to obtain the raw sheet.

[0027] It should be noted that, using the above technical solution, the drying process adopts a gradient temperature increase method: 35-45℃ to initially dry the surface of the wet film; 55-65℃ to further remove the solvent; and 75-85℃ to completely dry the wet film.

[0028] In some embodiments, the debinding process for raw sheet metal includes the following steps:

[0029] Place the raw sheet into the furnace and heat it from room temperature to 550-650℃ at a rate of 1-2℃ / min. Hold it at this temperature for 2-3 hours to complete the glue removal process.

[0030] In some embodiments, sintering includes the following steps:

[0031] After the glue removal is completed, the temperature is continued to rise at a rate of 3-5℃ / min in an atmosphere, and the reaction is maintained at this temperature for 4-6 hours. After the reaction, the temperature is cooled to obtain a sheet-like heating element.

[0032] It should be noted that, using the above technical solution, during sintering and heat preservation, the green blank undergoes a solid-state reaction, forming a dense crystal structure.

[0033] According to one aspect of the present invention, when the precursor is a precursor solution, the sintering atmosphere is air, and the temperature is raised to 1200-1250°C.

[0034] According to one aspect of the present invention, when the precursor is a precursor sol, the sintering atmosphere is nitrogen, and the temperature is raised to 1075-1125°C.

[0035] It should be noted that when the precursor is a precursor sol, the addition of low-melting-point glass powder can synergistically improve the densification degree of sintering with hexagonal boron nitride. The chemical inertness of hexagonal boron nitride results in a high interfacial energy with the LSMO matrix, which theoretically is unfavorable for sintering. However, this invention incorporates low-melting-point glass powder. The molten glass phase preferentially wets the surface of hexagonal boron nitride and fills the interface between hexagonal boron nitride and LSMO, solving the interfacial bonding problem on the one hand, and promoting mass transport through the liquid-phase sintering mechanism on the other, achieving high densification of the material at a lower sintering temperature. Furthermore, using a low-temperature nitrogen atmosphere for sintering also creates more oxygen vacancies, which is beneficial for improving electrical and thermal conductivity.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] 1. Unlike existing technologies that use La 0.7 Sr0.3 Using MnO3 pre-synthesized powder as a raw material, this invention directly dissolves the metal salt in a solvent to prepare a metal salt solution, and uses it as a starting material to solve the problem of powder agglomeration.

[0038] With fewer precursor defects and improved stability, the drying shrinkage rate of the green body is reduced after tape casting, and the surface roughness is decreased, thereby reducing the internal stress of the green body and making it less prone to cracking and warping. Also, because the precursor has good uniformity and high sintering activity, the final heating element is more dense and has fewer defects, forming a more efficient electron conduction pathway and reducing surface resistance.

[0039] 2. When the precursor is a precursor solution, the metal salt solution is mixed with the organic carrier to form a molecular-level and homogeneous precursor. Then, through tape casting and sintering, a dense exothermic body is synthesized in situ. This method is more uniform than directly using pre-synthesized powder (such as Comparative Example 1), proving that tape casting does not necessarily require the use of powder slurry; using a molecular-level precursor solution is equally feasible and yields better results.

[0040] 3. When the precursor is a precursor sol, the two-dimensional plate-like hexagonal boron nitride (BN) tends to align parallel to the casting substrate direction under the shear force and interfacial energy during the casting process, resulting in its in-plane orientation. The BN is confined at the grain boundaries of the LSMO grains. However, the BN does not directly contact the LSMO because the molten low-melting-point glass phase wets the surface of the BN and then encapsulates it. Simultaneously, this glass phase also bonds with the surrounding LSMO grains. This forms a three-phase interface structure of LSMO grains, the glass phase, and the BN. The glass phase acts as a "bridge" between the BN and the LSMO matrix, solving the problem of poor adhesion caused by chemical incompatibility.

[0041] Furthermore, because hexagonal boron nitride is distributed at grain boundaries, it pins these boundaries, preventing grain boundary migration and suppressing abnormal LSMO grain growth at high temperatures. This results in a finer, more uniform LSMO grain structure. This fine-grained structure reduces the material's resistivity. Detailed Implementation

[0042] To facilitate understanding of the present invention, a more comprehensive description is provided below, along with preferred embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0044] Unless otherwise specified, the equipment and materials used in the embodiments can be readily obtained from commercial companies.

[0045] The raw materials and equipment involved in this invention are as follows:

[0046] (a) Raw materials

[0047] Lanthanum nitrate: Purity ≥ 99.0%

[0048] Strontium nitrate: Purity ≥ 99.0%

[0049] Manganese nitrate hexahydrate: purity ≥ 98.0%

[0050] Ethylene glycol methyl ether: purity ≥ 99.5%

[0051] Polyvinyl butyral (PVB): degree of polymerization approximately 1500

[0052] Dibutyl phthalate (DBP), purity ≥99.0%

[0053] Ethanol: Purity ≥ 99.7%

[0054] Glacial acetic acid: purity ≥ 99.5%

[0055] Hexagonal boron nitride: D50 < 1 μm, purity ≥ 99.0%

[0056] Low-melting-point glass powder: low-melting-point glass powder, melting temperature 500℃, D50 < 2μm

[0057] La 0.7 Sr 0.3 MnO3 pre-synthesized powder: purity ≥ 99.5%, D50 < 1 μm, specific surface area (BET) 5 m² 2 / g.

[0058] (II) Equipment Preparation

[0059] Casting machine: Clean the casting machine components, including the base tape and doctor blade, sequentially with ethanol and deionized water, then wipe them dry with a clean, lint-free cloth to ensure no impurities remain on the surface. Install the cleaned base tape onto the casting machine and adjust its tension to ensure it is flat and at a suitable level. Adjust the distance between the doctor blade and the base tape according to the required thickness of the sheet-like heating element to be prepared.

[0060] (III) Reagent Preparation

[0061] The solvent was prepared by mixing ethanol, ethylene glycol methyl ether, glacial acetic acid and deionized water in a volume ratio of 7:3:1:0.2.

[0062] Example 1

[0063] The specific steps for preparing a metal salt solution are as follows:

[0064] According to the chemical formula La 0.7 Sr 0.3 To determine the stoichiometric ratio of MnO3, the metal nitrates were weighed using an electronic balance to ensure a molar ratio of La, Sr, and Mn of 0.7:0.3:1. Specifically, 3.031 g of lanthanum nitrate, 0.635 g of strontium nitrate, and 2.870 g of manganese nitrate hexahydrate were weighed and poured into a beaker, along with 13 mL of solvent. The mixture was stirred with a magnetic stirrer at 300-400 rpm until the metal nitrates were completely dissolved, forming a homogeneous metal salt solution.

[0065] Example 2

[0066] The specific steps for preparing the precursor solution are as follows:

[0067] 0.419 g of polyvinyl butyral was first dissolved in a mixture of 5 mL of ethanol and glacial acetic acid to obtain a polyvinyl butyral solution. The volume ratio of ethanol to glacial acetic acid was 7:1.

[0068] Add polyvinyl butyral solution to the metal salt solution of Example 1, add 0.209 g of dibutyl phthalate, stir for 10-15 min, add solvent to adjust the solid content to 40 wt%, the target viscosity at room temperature is 500-1500 mPa·s, centrifuge at 4000-5000 r / min for 10-15 min, take the supernatant, sonicate for 30-45 min to obtain the purified precursor solution.

[0069] Example 3

[0070] The specific steps for preparing the precursor sol are as follows:

[0071] The metal salt solution from Example 1 was heated to 80-90°C, 17.29g of citric acid was added, and the mixture was stirred for 30-60 minutes. Then, 5.59g of ethylene glycol was added at 80-90°C, and the reaction was continued for 30-60 minutes. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain a sol.

[0072] Add 4 mL of solvent to another beaker, add 0.034 g of hexagonal boron nitride, sonicate for 10-15 min, stir at room temperature for 30 min, then heat to 40-50 °C and react at a constant temperature for 60-90 min to obtain a suspension.

[0073] Add the suspension to the sol while stirring, sonicate for 15-30 minutes to disperse evenly, and obtain a mixed sol.

[0074] 0.419 g of polyvinyl butyral was first dissolved in a mixture of 5 mL of ethanol and glacial acetic acid to obtain a polyvinyl butyral solution. The volume ratio of ethanol to glacial acetic acid was 7:1.

[0075] Add 0.016g of low-melting-point glass powder to the mixed sol and stir for 10 min. Add polyvinyl butyral solution and 0.209g of dibutyl phthalate and stir for 10-15 min. Add solvent to adjust the solid content to 40wt%. The target viscosity at room temperature is 500-1500 mPa·s. Centrifuge at 4000-5000 r / min for 10-15 min, filter, and degas under vacuum for 10-20 min to obtain the purified precursor sol.

[0076] Example 4

[0077] The specific steps for preparing the green preform are as follows:

[0078] The doctor blade height of the casting machine was set to 200 μm. The precursor solution after impurity removal in Example 2 was slowly poured into the hopper of the casting machine. The casting machine was started, and the substrate moved at a constant speed of 2-3 cm / min. At the same time, the flow rate of the hopper was controlled to ensure that the precursor solution after impurity removal was evenly coated on the substrate to form a continuous wet film. During the casting process, the ambient temperature was maintained at 23-27℃ and the relative humidity at 40%-50%.

[0079] The cast wet film, along with the substrate, is placed in a forced-air drying oven for drying. The drying process employs a gradient temperature increase: first, drying at 40℃ for 2-3 hours to initially dry the surface of the wet film; then, increasing the temperature to 60℃ and drying for 4-6 hours to further remove the solvent; finally, increasing the temperature to 80℃ and drying for 1-2 hours to completely dry the green sheet. After drying, the green sheet is carefully peeled off from the substrate to obtain the green sheet.

[0080] Example 5

[0081] The specific steps for preparing the green preform are as follows:

[0082] The doctor blade height of the casting machine was set to 200 μm. The precursor sol, after impurity removal in Example 3, was slowly poured into the hopper of the casting machine. The casting machine was started, and the substrate moved at a constant speed of 2-3 cm / min. Simultaneously, the flow rate of the hopper was controlled to ensure that the impurity-removed precursor sol was evenly coated onto the substrate, forming a continuous wet film. During the casting process, the ambient temperature was maintained at 23-27℃, and the relative humidity at 40%-50%.

[0083] The cast wet film, along with the substrate, is placed in a forced-air drying oven for drying. The drying process employs a gradient temperature increase: first, drying at 40℃ for 2-3 hours to initially dry the surface of the wet film; then, increasing the temperature to 60℃ and drying for 4-6 hours to further remove the solvent; finally, increasing the temperature to 80℃ and drying for 1-2 hours to completely dry the green sheet. After drying, the green sheet is carefully peeled off from the substrate to obtain the green sheet.

[0084] Example 6

[0085] The specific steps for preparing a sheet-shaped heating element are as follows:

[0086] Debinding process: The raw sheet from Example 4 is placed in a high-temperature muffle furnace and heated from room temperature to 600°C at a rate of 1-2°C / min. It is then kept at 600°C for 2-3 hours to allow the organic matter in the raw sheet to fully decompose and volatilize, thus completing the debinding process.

[0087] High-temperature sintering: After the glue removal is completed, the temperature is raised to 1200℃ in air at a rate of 3-5℃ / min. The temperature is held at 1200℃ for 4-6 hours and then naturally cooled to room temperature to obtain a sheet-like heating element.

[0088] Example 7

[0089] The specific steps for preparing a sheet-shaped heating element are as follows:

[0090] Debinding process: The raw sheet from Example 5 is placed in a high-temperature muffle furnace and heated from room temperature to 600°C at a rate of 1-2°C / min. It is then kept at 600°C for 2-3 hours to allow the organic matter in the raw sheet to fully decompose and volatilize, thus completing the debinding process.

[0091] High-temperature sintering: After the binder is removed, the temperature is raised to 1125℃ in a nitrogen atmosphere at a rate of 3-5℃ / min. The temperature is held at 1125℃ for 3-5 hours and then naturally cooled to room temperature to obtain a sheet-like heating element.

[0092] Comparative Example 1

[0093] Add 13 mL of solvent to a beaker, then add 0.419 g of polyvinyl butyral and 0.209 g of dibutyl phthalate. Stir with a magnetic stirrer at 300-400 r / min until the mixture is completely dissolved to form an organic carrier solution.

[0094] Add 4.5g of La to the organic carrier solution. 0.7 Sr 0.3 MnO3 pre-synthesized powder was mechanically stirred for 2-4 hours to obtain a slurry. The solid content was adjusted to 40 wt%, and the target viscosity at room temperature was 500-1500 mPa·s. The slurry was centrifuged at 4000-5000 r / min for 10-15 min to remove impurities and obtain the purified precursor slurry.

[0095] Set the doctor blade height of the casting machine to 200μm, slowly pour the purified precursor slurry into the hopper of the casting machine, start the casting machine, and move the base belt at a constant speed of 2-3cm / min. Simultaneously control the flow rate of the hopper to ensure the purified precursor slurry is evenly coated onto the base belt, forming a continuous wet film. During the casting process, maintain the ambient temperature at 23-27℃ and the relative humidity at 40%-50%.

[0096] The wet film, along with the substrate, is placed in a forced-air drying oven for drying. The drying process uses a gradient temperature increase: first, drying at 40℃ for 2-3 hours; then increasing the temperature to 60℃ and drying for 4-6 hours; finally, increasing the temperature to 80℃ and drying for 1-2 hours. After drying, the green film is carefully peeled off the substrate to obtain the green film.

[0097] Debinding process: Place the raw sheet into a high-temperature muffle furnace and heat it from room temperature to 600℃ at a rate of 1-2℃ / min. Then, keep it at 600℃ for 2-3 hours to complete the debinding process.

[0098] High-temperature sintering: After the glue removal is completed, the temperature is raised to 1200℃ in air at a rate of 3-5℃ / min. The temperature is held at 1200℃ for 4-6 hours and then naturally cooled to room temperature to obtain a sheet-like heating element.

[0099] In Example 1 of this invention, a metal salt solution was prepared.

[0100] Metal salt solutions were used to prepare precursors, which were available in two forms: precursor solutions (Example 2) and precursor sols (Example 3).

[0101] The precursor was then cast, dried (Examples 4 and 5), and sintered (Examples 6 and 7) to obtain a sheet-like heating element.

[0102] Comparative Example 1: Directly using commercially available La 0.7 Sr0.3 MnO3 powder was used as a raw material to prepare sheet-like heating elements.

[0103] The precursors prepared in Examples 2 and 3 and Comparative Example 1 were tested, and the results are as follows:

[0104] Example 2 has a room temperature viscosity of 980 mPa·s, a pH of 5.8, and a Zeta potential of -27.4 mV;

[0105] Example 3 has a room temperature viscosity of 1130 mPa·s, a pH of 6.1, and a Zeta potential of -33.2 mV.

[0106] The precursor slurry in Comparative Example 1 had a room temperature viscosity of 1400 mPa·s, a pH of 6.3, and a Zeta potential of -10 mV.

[0107] Samples were prepared from the raw blanks used in Examples 4 and 5 and Comparative Example 1, with a thickness of 100 μm and a side length of 20 mm. The samples were then tested, and the results are as follows:

[0108] The drying shrinkage rate of Example 4 was 8.6%, and the surface roughness Ra was 1.8 μm;

[0109] The drying shrinkage rate of Example 5 was 7.2%, and the surface roughness Ra was 1.3 μm;

[0110] The drying shrinkage rate of the raw blank in Comparative Example 1 was 12.2%, and the surface roughness Ra was 3.5 μm.

[0111] The sheet-shaped heating elements prepared in Examples 6 and 7, as well as Comparative Example 1, were made from raw blanks with dimensions matching those of the aforementioned raw blank samples, i.e., a thickness of 100 μm and a side length of 20 mm. The sheet-shaped heating elements were tested, and the results are as follows:

[0112] The surface resistivity of Example 6 is 72.4 Ω·sq. -1 Its thermal conductivity is 2.4 W / m·K;

[0113] The surface resistivity of Example 7 is 61.7 Ω·sq. -1 Its thermal conductivity is 3.1 W / m·K;

[0114] The surface resistance of the sheet-like heating element in Comparative Example 1 is 108.5 Ω·sq. -1 Its thermal conductivity is 1.6 W / m·K.

[0115] analyze:

[0116] Regarding the precursors: (1) Zeta potential: The absolute values ​​of the Zeta potentials of Examples 2 and 3 are higher than those of Comparative Example 1, indicating that the precursors of Examples 2 and 3 are less prone to agglomeration and have better dispersion stability. Compared with Examples 2, the colloid of Example 3 exhibits better stability. (2) Room temperature viscosity: The viscosity of Comparative Example 1 is higher than that of Examples 2 and 3. The precursor slurry of Comparative Example 1 may have uneven particle dispersion and agglomerates, which leads to an increase in viscosity and subsequent deterioration of casting uniformity.

[0117] Regarding the preforms: Compared with Comparative Example 1, Examples 4 and 5 showed lower shrinkage and smoother surfaces, indicating that the precursors in Examples 4 and 5 had better dispersibility, which could reduce particle agglomeration and stress concentration during the preparation of preforms.

[0118] Regarding the sheet-like heating element: Compared to Comparative Example 1, Examples 6 and 7 exhibited lower resistance, indicating that the precursors synthesized in Examples 6 and 7 were more uniform, had better crystallinity, and resulted in a more compact sheet-like heating element structure. In Comparative Example 1, the powder agglomerated, and the sintering was not dense enough, leading to high resistance and low thermal conductivity in the finished product.

[0119] Combining Examples 2, 4, and 6, we see that metal salts are directly dissolved in a solvent and mixed with an organic carrier to form a molecular-level mixed solution. During subsequent drying and sintering, the metal salts decompose and react to generate the target product. This method is more uniform than directly using pre-synthesized powder (Comparative Example 1), demonstrating that tape casting does not necessarily require powder slurry; using molecular-level precursor solutions is equally feasible and yields better results.

[0120] Looking at the combination of Examples 3, 5, and 7, the fine and uniform LSMO grains are firmly bonded together by an extremely thin glassy grain boundary layer. Simultaneously, hexagonal boron nitride is embedded at the grain boundary junctions, its surface wetted and encapsulated by a glassy phase layer, thus forming a highly efficient heat conduction network in conjunction with the LSMO grains.

[0121] By comparing the embodiments, we can further understand that:

[0122] Compared with Examples 2, 4, and 6, Examples 3, 5, and 7 show further improved performance. The precursor sol makes the material more uniform, shortens the solid-phase reaction path, and reduces grain boundary defects, thereby achieving a higher degree of densification at a lower temperature.

[0123] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a sheet-like heating element, characterized in that, Includes the following steps: Lanthanum nitrate, strontium nitrate, manganese nitrate hexahydrate, and solvent were stirred and dissolved to obtain a metal salt solution; A metal salt solution is used to prepare a precursor, which is either a precursor solution or a precursor sol. The precursor is cast and dried to obtain a blank sheet. The blank sheet is debonded and sintered to obtain a sheet-shaped heating element. The preparation method of the precursor solution includes the following steps: Add polyvinyl butyral solution to metal salt solution, add dibutyl phthalate, stir, adjust solid content and viscosity, remove impurities, and obtain precursor solution; The preparation method of precursor sol includes the following steps: The metal salt solution is heated to 80-90℃, citric acid is added and stirred, then ethylene glycol is added and the reaction continues. After the reaction is complete, the solution is cooled to obtain a sol. The solvent and hexagonal boron nitride were ultrasonically mixed, stirred, and reacted to obtain a suspension. The suspension was added to the sol while stirring and dispersed to obtain a mixed sol; Low-melting-point glass powder was added to the mixed sol and stirred. Polyvinyl butyral solution was added and dibutyl phthalate was added. The mixture was stirred, and the solid content and viscosity were adjusted. Impurities were removed and the mixture was degassed under vacuum to obtain the precursor sol. The viscosity is 500-1500 mPa·s.

2. The method for preparing a sheet-like heating element according to claim 1, characterized in that, The solvent is prepared by mixing ethanol, ethylene glycol methyl ether, glacial acetic acid and deionized water.

3. The method for preparing a sheet-like heating element according to claim 1, characterized in that, The precursor casting process includes the following steps: Set the doctor blade height of the casting machine to 250-300μm, pour the precursor into the hopper of the casting machine, start the casting machine, and move the base belt at a constant speed of 2-3cm / min. At the same time, control the flow rate of the hopper to make the precursor evenly coated on the base belt to form a continuous wet film. During the casting process, maintain the ambient temperature at 23-27℃ and the relative humidity at 40%-50%.

4. The method for preparing a sheet-like heating element according to claim 3, characterized in that, Drying includes the following steps: The wet film after casting is dried together with the base tape at 35-45℃ for 2-3 hours, then the temperature is raised to 55-65℃ and dried for 4-6 hours; finally, the temperature is raised to 75-85℃ and dried for 1-2 hours. After drying, the dried wet film is peeled off from the base tape to obtain the raw sheet.

5. The method for preparing a sheet-like heating element according to claim 1, characterized in that, The process of removing glue from raw blanks includes the following steps: Place the raw sheet into the furnace and heat it from room temperature to 550-650℃ at a rate of 1-2℃ / min. Hold it at this temperature for 2-3 hours to complete the glue removal process.

6. The method for preparing a sheet-like heating element according to claim 1, characterized in that, Sintering includes the following steps: After the glue removal is completed, the temperature is continued to rise at a rate of 3-5℃ / min in an atmosphere, and the reaction is maintained at this temperature for 4-6 hours. After the reaction, the temperature is cooled to obtain a sheet-like heating element.

7. The method for preparing a sheet-like heating element according to claim 6, characterized in that, When the precursor is a precursor solution, the sintering atmosphere is air, and the temperature is raised to 1200-1250℃.

8. The method for preparing a sheet-like heating element according to claim 6, characterized in that, When the precursor is a precursor sol, the sintering atmosphere is nitrogen, and the temperature is raised to 1075-1125℃.