Photocatalytic self-assembly synthesized semiconductor ceramic as well as preparation method and application thereof

Abstract

The invention relates to a photocatalytic self-assembly synthesized semiconductor ceramic as well as a preparation method and application thereof, and belongs to the technical field of semiconductor ceramic materials. The method comprises the following steps: providing a solution containing Zn < 2 + > and doped metal ions; introducing anatase type nano titanium dioxide and a slow-release alkali source into the solution to form a mixed reaction system; carrying out photocatalytic self-assembly reaction under ultraviolet irradiation and heating conditions to form a ceramic precursor; and forming and sintering to obtain the semiconductor ceramic. According to the invention, ZnO is driven to grow directionally on the surface of TiO2 by utilizing a photo-generated electric field, a uniform ZnO crystal grain network taking TiO2 as heterogeneous nodes is constructed, and meanwhile, controllable distribution of doped elements in a crystal boundary is realized. The obtained ceramic has a high nonlinear coefficient and low leakage current, and is suitable for a transient overvoltage protection element of a high-voltage system of a new energy automobile.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor ceramic materials technology, and relates to a photocatalytic self-assembly synthesis of semiconductor ceramics, its preparation method and application. Background Technology

[0002] Semiconductor ceramics, such as zinc oxide (ZnO)-based ceramics, are widely used in overvoltage protection, varistors, and sensors due to their unique nonlinear current-voltage characteristics and excellent varistor and dielectric properties. Especially in the high-voltage systems of new energy vehicles, stable and fast-responding transient overvoltage protection components are needed to ensure the safety of power electronic equipment.

[0003] Traditional ZnO semiconductor ceramics are typically prepared using solid-state sintering, which involves mixing ZnO powder with various metal oxide additives, ball milling, shaping, and then sintering at high temperatures. This method is essentially a macroscopic physical mixing process, making it difficult to achieve uniform molecular or nanoscale distribution of the additives within the ZnO matrix. Consequently, the microstructure of the sintered ceramic is poorly controllable, with random and unstable heterojunctions and grain boundaries. This directly leads to poor product consistency and repeatability, and limits further improvements in its electrical properties.

[0004] To improve microstructure and properties, researchers have developed various wet chemical synthesis methods, such as co-precipitation and sol-gel methods, to prepare more uniform ceramic precursor powders. However, these methods still face challenges in constructing uniform and stable heterojunction structures between ZnO and other metal oxides (such as TiO2). The formation of heterojunctions is crucial to the electrical properties of ceramics, but traditional methods struggle to achieve precise and uniform distribution of the second phase (such as TiO2) at the ZnO grain boundaries, resulting in less than ideal electrical properties (such as nonlinear coefficients and leakage current). Therefore, developing a new method capable of precisely controlling the microstructure at the molecular level to achieve heterojunction self-assembly is of great significance for the preparation of high-performance semiconductor ceramics. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-performance semiconductor ceramics with uniformly distributed structures through a photocatalytic self-assembly process.

[0006] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a method for preparing semiconductor ceramics by photocatalytic self-assembly, comprising the following steps: (1) Provide Zn 2+ Solutions containing ions and doped metal ions; (2) Introduce nano-titanium dioxide and slow-release alkali source into the solution to form a mixed reaction system; (3) Under light and heating conditions, the mixed reaction system hydrolyzes the alkali source and drives the growth of ZnO crystal nuclei to form a ceramic precursor; (4) The ceramic precursor is formed and sintered to obtain the semiconductor ceramic.

[0007] As used in this text, "photocatalytic self-assembly" refers to the process by which nanomaterials spontaneously organize and form an ordered structure under illumination, initiated and driven by a photocatalytic reaction. In this invention, it specifically refers to the process by which ultraviolet light excites nano-titanium dioxide to generate photogenerated electron-hole pairs, thereby simultaneously driving the directional growth of ZnO crystal nuclei and constructing a heterojunction with TiO2.

[0008] As used in this text, "ceramic precursor" refers to an intermediate material prepared by chemical methods that possesses the basic microstructure of the target ceramic product. In this invention, it specifically refers to a transitional structure formed by photocatalytic self-assembly, containing a three-dimensional ZnO-TiO2 heterojunction network and doping elements, which will be transformed into the final semiconductor ceramic after sintering.

[0009] Preferably, the nano-titanium dioxide is anatase type with a particle size of 5-20 nm; the slow-release alkali source is urea or hexamethylenetetramine.

[0010] As used in this text, "slow-release alkali source" refers to a substance that can slowly and continuously release OH- under heating conditions. - Ionic compounds (such as urea and hexamethylenetetramine) provide a stable alkaline environment for crystal growth by controlling the hydrolysis rate, thus avoiding explosive nucleation.

[0011] Preferably, the illumination is ultraviolet light with a wavelength of 200-400 nm; and the reaction temperature is 60-90℃.

[0012] The photocatalytic self-assembly reaction is carried out under heating conditions, with a reaction temperature range of 60°C to 90°C. This temperature range ensures that the slow-release alkaline source (urea or hexamethylenetetramine) hydrolyzes at a suitable rate, providing kinetic conditions for the orderly formation of ZnO crystal nuclei and the simultaneous construction of a three-dimensional heterojunction network.

[0013] Preferably, the molar amount of Ti in the nano-titanium dioxide is related to that of Zn. 2+ The ratio of ion molar amounts is from 0.5:100 to 3:100.

[0014] Preferably, the slow-release alkali source is related to Zn. 2+The molar ratio of ions is 1.5:1 to 5:1. Within this range, a slow-release alkaline source (such as urea or hexamethylenetetramine) can establish and maintain a stable alkaline environment, ensuring the orderly growth of crystals. When the ratio is below 1.5:1, the alkalinity provided is insufficient, and the reaction is incomplete; when the ratio is above 5:1, the rapid release of alkalinity may lead to explosive nucleation, disrupting the orderly self-assembly process.

[0015] Preferably, the doped metal ions include one or more of Bi, Co, Mn, and Sb; and Zn 2+ The total molar ratio of the ions to all doped metal ions is 100:1-8.

[0016] Preferably, the sintering is carried out in an air atmosphere, the sintering temperature is 800-1000℃, and the holding time is 1-4 hours.

[0017] The core of this invention lies in combining chemical self-assembly with photocatalytic reactions to achieve the construction of semiconductor ceramic microstructures. First, a slow-release alkaline source establishes a stable and controllable alkaline environment under heating conditions, providing a fundamental guarantee for the orderly growth of crystals. On this basis, pre-dispersed nano-TiO2 becomes a photocatalytic active center under ultraviolet light, and photogenerated electron-hole pairs separate on its surface, generating a local built-in electric field and active species.

[0018] Driven by the photogenerated electric field, newly generated ZnO nuclei and precursor ions are effectively guided and enriched near TiO2 particles. Driven by the interface energy minimization mechanism of heterogeneous nucleation, specific high-energy crystal planes tend to preferentially orient and tightly adhere to the TiO2 surface. This selective adhesion effectively reduces the surface energy of the crystal plane, thereby guiding the ZnO crystal to continuously grow along this direction. Ultimately, the TiO2 particles are captured by the growing ZnO grains and precisely fixed at the grain boundary junctions, forming a three-dimensional heterojunction network of nodes throughout the entire microstructure.

[0019] At the same time, Bi in the solution 3+ Co 2+ Under the influence of the electric-chemical dual composite field composed of a photogenerated electric field and a slow-release alkali source, the doped ions migrate to the growing ZnO-TiO2 interface region and are transformed into its hydroxide or oxide nanoclusters.

[0020] Through this collaborative self-assembly process, TiO2 is precisely fixed as a heterojunction node, while dopants such as Bi and Co are embedded.

[0021] Secondly, this invention provides a photocatalytic self-assembly synthesis of semiconductor ceramics, employing the following technical solution: A photocatalytic self-assembly synthesis of semiconductor ceramics, obtained using the preparation method described in the first aspect.

[0022] The beneficial effects of this invention are: (1) This invention combines photocatalytic reaction with chemical self-assembly mechanism, and uses ultraviolet light to excite nano-titanium dioxide to generate a photogenerated electric field, which can guide the preferential growth of ZnO crystal nuclei on TiO2 surface. At the same time, the composite field formed by the photogenerated electric field and the slow-release alkali source can also effectively drive doped ions to migrate to and accumulate in the grain boundary region.

[0023] (2) The product obtained by this invention exhibits a higher nonlinear coefficient and lower leakage current due to its uniform microstructure distribution and optimized grain boundary state. When used as an overvoltage protection element, it has a more sensitive and faster voltage response and lower self-energy consumption and heat generation, which can effectively improve the reliability and service life of the element. Detailed Implementation

[0024] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0025] Main raw materials used: Unless otherwise specified, all reagents used in the following examples are of analytical grade. The nano-titanium dioxide is anatase type with an average particle size of 10 nm.

[0026] Example 1 A method for preparing semiconductor ceramics through photocatalytic self-assembly includes the following steps: (1) Weigh 50.0 mmol of zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 1.0 mmol of bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), and 1.0 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), dissolve them in 500 mL of deionized water, and stir until completely dissolved.

[0027] (2) Add nano-titanium dioxide (corresponding to 0.75 mmol of Ti) to the solution and ultrasonically disperse the mixture (using a 300W probe ultrasonic instrument for 30 minutes) to ensure that the nano-titanium dioxide is fully deagglomerated and uniformly distributed. Then, add urea (150 mmol) and stir at room temperature for 0.5 hours to form a uniform suspension.

[0028] (3) The mixture was placed in a constant temperature water bath at 85℃ and vertically irradiated with a 300 W, 365 nm wavelength ultraviolet lamp, and reacted for 4 hours under magnetic stirring. During the reaction, the system gradually changed from a suspension to a white precipitate. After the reaction was completed, the mixture was cooled, filtered, washed with deionized water and ethanol, and dried at 80℃ for 12 hours to obtain ZnO-TiO2-Bi-Co composite ceramic precursor powder.

[0029] (4) Add 5 wt% polyvinyl alcohol (PVA) aqueous solution to the precursor powder as a binder, granulate, and press into 20 mm diameter round green blanks under 100 MPa pressure. Place the green blanks in a box furnace, heat to 550℃ at 5℃ / min and hold for 2 hours to remove the binder, then heat to 900℃ at 3℃ / min and sinter for 2 hours, and cool with the furnace to obtain semiconductor ceramic samples.

[0030] Example 2 A method for preparing semiconductor ceramics through photocatalytic self-assembly includes the following steps: The difference from Example 1 is as follows: The doping elements were changed to manganese nitrate (Mn(NO3)2) and antimony nitrate hydrate (Sb(NO3)3·5H2O), and Zn 2+ With Mn 2+ Sb 3+ The molar ratio is 100:2:2.

[0031] The amount of nano-titanium dioxide was adjusted to a Ti / Zn molar ratio of 2:100.

[0032] The sintering process was changed to 950℃ and held for 1 hour.

[0033] The remaining steps are exactly the same as in Example 1.

[0034] Example 3 A method for preparing semiconductor ceramics through photocatalytic self-assembly includes the following steps: The difference from Example 1 is as follows: The amount of nano-titanium dioxide used is reduced, resulting in a Ti / Zn molar ratio of 0.5:100.

[0035] The remaining steps are exactly the same as in Example 1.

[0036] Example 4 A method for preparing semiconductor ceramics through photocatalytic self-assembly includes the following steps: The difference between this embodiment and Embodiment 1 is that: The increased amount of nano-titanium dioxide resulted in a Ti / Zn molar ratio of 3:100.

[0037] The remaining steps are exactly the same as in Example 1.

[0038] Comparative Example 1 Weigh commercially available nano TiO2, micron-sized ZnO, Bi2O3, and Co3O4 powders according to the same proportions as the final stoichiometry in Example 1.

[0039] The mixed powder was ball-milled with zirconia balls and deionized water for 12 hours.

[0040] The ball-milled and dried powder was shaped and sintered in the same manner as in Example 1 (900°C, 2 hours).

[0041] Comparative Example 2 The steps are exactly the same as in Example 1, except that the ultraviolet light is not turned on during the reaction, and the mixture is heated and stirred at 85°C for 4 hours.

[0042] Performance testing: The semiconductor ceramic samples prepared in the examples and comparative examples were subjected to the following performance tests and characterizations. All tests were performed at room temperature (25°C).

[0043] 1. Varistor voltage test Referring to the GB / T 10193 standard, silver paste electrodes are first prepared on the upper and lower surfaces of the ceramic sheet and sintered at 550℃ to form ohmic contacts. Then, an incremental DC voltage is applied using a varistor testing system to accurately measure the voltage value corresponding to when the current flowing through the sample reaches 1 mA. This parameter directly reflects the overvoltage protection threshold of the component.

[0044] 2. Nonlinear coefficient test Referring to the IEC 61051-1 international standard, first measure the sample at 1 mA / cm². 2 and 10 mA / cm 2 Voltage values ​​V1 and V under current density 10 Then substitute it into the formula α = 1 / log(V 10 The coefficient / V1) is used for calculation; this coefficient is the core indicator characterizing the sensitivity of the material's resistance to changes in voltage. The higher the value, the better the nonlinear characteristics.

[0045] 3. Leakage current test Referring to GB / T 10194 standard, at 0.75 times V 1mA Under DC voltage conditions: The value read after the ammeter reading stabilizes is the leakage current. This parameter directly reflects the energy consumption level and operational stability of the component during normal operation and is a key indicator for evaluating product reliability.

[0046] 4. Microstructure Characterization Microstructure analysis was performed using field emission scanning electron microscopy (FE-SEM): after grinding, polishing and hot etching, the grain morphology, size distribution and second phase distribution of the sample cross-section were observed.

[0047] Performance test results: The comparison between the above embodiments and comparative examples shows that the photocatalytic self-assembly method provided by the present invention can prepare high-performance semiconductor ceramics with uniform heterojunction structure, high nonlinear coefficient and low leakage current. Its technical effect is significant, and this excellent effect comes from its unique core process that combines "photocatalysis" and "self-assembly".

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing semiconductor ceramics through photocatalytic self-assembly, characterized in that, Includes the following steps: (1) Provide Zn 2+ Solutions containing ions and doped metal ions; (2) Introduce nano-titanium dioxide and slow-release alkali source into the solution to form a mixed reaction system; (3) Under light and heating conditions, the mixed reaction system hydrolyzes and slowly releases the alkali source and drives the growth of ZnO crystal nuclei to form a ceramic precursor; (4) The ceramic precursor is formed and sintered to obtain the semiconductor ceramic.

2. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The nano-titanium dioxide is anatase type with a particle size of 5-20 nm; the slow-release alkali source is urea or hexamethylenetetramine.

3. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The illumination is ultraviolet light with a wavelength of 200-400 nm; the reaction temperature is 60-90℃.

4. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The molar amount of Ti in the nano-titanium dioxide and Zn 2+ The ratio of ion molar amounts is from 0.5:100 to 3:

100.

5. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The slow-release alkali source and Zn 2+ The molar ratio of ions is 1.5:1 to 5:

1.

6. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The doped metal ions include one or more of Bi, Co, Mn, and Sb; and Zn 2+ The total molar ratio of the ions to all doped metal ions is 100:1-8.

7. The method for preparing semiconductor ceramics by photocatalytic self-assembly according to claim 1, characterized in that, The sintering is carried out in an air atmosphere at a temperature of 800-1000℃ and a holding time of 1-4 hours.

8. A semiconductor ceramic precursor prepared by the method according to any one of claims 1-7, characterized in that, Its microstructure consists of a composite structure composed of ZnO and introduced TiO2.

9. A photocatalytic self-assembly synthesis of semiconductor ceramics, formed by sintering the precursor described in claim 8, characterized in that, Its microstructure is a ZnO grain network with TiO2 as the heterojunction node.

10. The application of the photocatalytic self-assembly synthesized semiconductor ceramic as described in claim 9 in the preparation of transient overvoltage protection elements for high-voltage systems of new energy vehicles.

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