Low-temperature sintering zinc oxide varistor material and preparation method thereof

Abstract

The application relates to a low-temperature sintered zinc oxide pressure-sensitive resistor material and a preparation method thereof, and relates to the technical field of electronic ceramic materials. The preparation method comprises the following steps: ZnO, Bi2O3, Sb2O3, MnO2, Cr2O3, Co2O3, Y2O3, Nb2O5 and Al(NO3)3.9H2O are weighed, mixed, granulated, pre-sintered, formed, and then sintered at 850-900 DEG C in a microwave-assisted temperature field control sintering furnace. The microwave-assisted temperature field control sintering furnace comprises a furnace body, a multi-mode resonant cavity with an open bottom, a bearing platform, a lifting mechanism, a microwave generating system, a temperature measuring system and a closed loop module; the bearing platform is lifted to be in sealed contact with the bottom opening of the multi-mode resonant cavity to form a microwave sealed space; the temperature measuring system adopts an infrared and optical fiber temperature sensor, and the closed loop module adjusts the microwave power in real time through a PID algorithm. The application realizes low-temperature sintering, the product is dense and excellent in nonlinearity, and the equipment is uniform in temperature field and accurate in temperature measurement.

Description

Technical Field

[0001] This application relates to the technical field of electronic ceramic materials, and in particular to a low-temperature sintered zinc oxide varistor material and its preparation method. Background Technology

[0002] Zinc oxide varistors are widely used in overvoltage protection of power systems, electronic equipment and communication facilities due to their excellent nonlinear current-voltage characteristics, large current capacity and fast response characteristics. Traditional zinc oxide varistors are made of ZnO as the main crystal phase, with the addition of various additives such as Bi2O3, Sb2O3, MnO2, Cr2O3 and Co2O3, and are sintered by high-temperature solid-state reaction.

[0003] In existing technologies, the sintering temperature of zinc oxide varistor materials is typically as high as 1100-1200℃. High-temperature sintering not only consumes a lot of energy but also leads to abnormal grain growth, affecting the nonlinear coefficient and current carrying capacity of the varistor. At the same time, high-temperature sintering limits the selection of electrode materials, making it difficult to achieve low-temperature co-firing of multilayer chip varistors. To reduce the sintering temperature, low-melting-point glass phases are usually added during the preparation process or chemical synthesis precursors such as sol-gel methods are used. However, these methods often sacrifice electrical performance, making it difficult to obtain varistor materials with high nonlinear coefficients and low leakage current.

[0004] In terms of sintering equipment, although microwave sintering technology has advantages such as rapid heating and low sintering temperature, existing microwave sintering equipment still has the following shortcomings: First, the microwave field distribution is uneven, which easily forms standing waves and causes local overheating or under-sintering; second, the temperature measurement accuracy is insufficient, and single infrared temperature measurement is easily interfered by cavity reflection; third, the sealing structure is complex, and long-term use poses a risk of microwave leakage and is inconvenient for loading and unloading materials. Summary of the Invention

[0005] The purpose of this application is to provide a low-temperature sintered zinc oxide varistor material and its preparation method to solve the problems in the background art.

[0006] This application provides a low-temperature sintered zinc oxide varistor material and its preparation method, which adopts the following technical solution: In a first aspect, this application provides a method for preparing a low-temperature sintered zinc oxide varistor material, comprising the following steps: S1. Weigh the raw materials according to the following weight parts: ZnO: 90-98 parts, Bi2O3: 0.5-2.0 parts, Sb2O3: 0.5-1.5 parts, MnO2: 0.1-1.0 parts, Cr2O3: 0.1-0.8 parts, Co2O3: 0.2-1.0 parts, Y2O3: 0.05-0.5 parts, Nb2O5: 0.05-0.5 parts, Al(NO3)3·9H2O: 0.01-0.1 parts; mix the above raw materials with deionized water and dispersant, and then ball mill them to obtain a mixed slurry with a solid content of 40-60%.

[0007] S2. Spray dry the mixed slurry to obtain precursor powder with a particle size of 50-200nm. Then, place the precursor powder in a microwave-assisted temperature field controlled sintering furnace and heat it to 650-750℃ at a heating rate of 2-5℃ / min in an air atmosphere. Hold it at that temperature for 1-3 hours to perform pre-calcination treatment and obtain pre-calcined powder.

[0008] S3. The pre-calcined powder is added to a polyvinyl alcohol aqueous solution, and after secondary ball milling, it is spray granulated to obtain a molded powder.

[0009] S4. Press the molding powder into a green body under a pressure of 80-120MPa.

[0010] S5. The green blank is placed in the microwave-assisted temperature field controlled sintering furnace, the sintering temperature is set to 850-900℃, the holding time is 1-4 hours, and the microwave output power is adjusted through the closed-loop feedback control module throughout the process to keep the temperature difference in the furnace within ±5℃. After sintering, the blank is cooled with the furnace to obtain the low-temperature sintered zinc oxide varistor material.

[0011] By adopting the above technical solution, Y2O3 and Nb2O5 composite doping is introduced, which works together with Al(NO3)3·9H2O sintering aid. Combined with the high activity of nanoscale precursor powder, the sintering activation energy is significantly reduced, and the sintering temperature of zinc oxide varistor material is reduced from the traditional 1100-1200℃ to 850-900℃. At the same time, a microwave-assisted temperature field control sintering furnace is used for sintering. The microwave output power is adjusted through closed-loop feedback control to ensure that the temperature difference in the furnace is controlled within ±5℃.

[0012] Preferably, in step S1, the ball milling process uses zirconia balls as the grinding medium, with a ball-to-material ratio of (3-5):1, a ball milling speed of 200-400 rpm, and a ball milling time of 12-24 hours.

[0013] By adopting the above technical solution, using zirconia balls as grinding media can avoid introducing impurities and contaminating the material; the optimized ball-to-material ratio, rotation speed and time parameters can ensure that the raw materials are fully and evenly mixed to obtain a mixed slurry with a concentrated particle size distribution.

[0014] Preferably, the sintering process in step S5 includes the following stages: First stage: heating from room temperature to 450-500℃ at a rate of 3-6℃ / min, holding at this temperature for 0.5-1 hour, and removing the binder; Second stage: heating from 500℃ to 850-900℃ at a rate of 2-4℃ / min, and holding at this temperature for 1-4 hours; In the second stage, the microwave frequency of the microwave-assisted temperature field control sintering furnace is 2.45GHz, the output power is 0.5-5kW, and the output power is dynamically adjusted according to the temperature feedback values ​​at multiple points inside the furnace.

[0015] By adopting the above technical solution, the organic binder in the molding powder can be effectively removed, avoiding cracking of the green body due to excessive volatilization. In the second stage, microwave heating is used and the output power is dynamically adjusted. The volume heating characteristics of microwaves are used to achieve rapid and uniform heating. Combined with multi-point temperature feedback control, the stability and uniformity of the temperature field during sintering are ensured, thereby improving the density and electrical performance consistency of the material.

[0016] Secondly, this application provides a microwave-assisted temperature field controlled sintering furnace for the above-mentioned preparation method. The microwave-assisted temperature field controlled sintering furnace includes a furnace body with a sintering chamber formed inside. A closable furnace door is provided on the front side of the furnace body. A partition is provided on the bottom side of the furnace body, and a support structure is provided through the top side of the partition. A microwave magnetron body is symmetrically arranged on the left and right sides of the furnace body. Both microwave magnetron bodies are sealed to the microwave feed inlet of the side wall of a multimode resonant cavity via a waveguide passing through the side of the furnace body. The outside of the multimode resonant cavity is fixed to the inner wall of the furnace body by a ceramic support, and the multimode resonant cavity is located on the support. The structure has an opening at its bottom; a microwave stirrer is installed through the middle of the top of the multimode resonant cavity; an infrared thermometer is installed on both the left and right sides of the top of the multimode resonant cavity; and a fiber Bragg grating temperature sensor is installed on both the left and right sides of the bottom of the multimode resonant cavity. A closed-loop feedback control module is installed on the side of the furnace body. The closed-loop feedback control module is electrically connected to the microwave magnetic control body, the infrared thermometer, and the fiber Bragg grating temperature sensor, respectively, to receive the collected temperature signals and adjust the output power of the microwave magnetic control body in real time based on the preset sintering curve through a PID control algorithm.

[0017] By adopting the above technical solutions, a multi-mode resonant cavity is used in conjunction with the rotational motion of a microwave-mode stirrer to dynamically change the microwave reflection path within the cavity, effectively eliminating standing waves and achieving a uniform electromagnetic field distribution. A dual-channel temperature measurement system composed of an infrared thermometer and a fiber Bragg grating temperature sensor is used to monitor the surface temperature of the material and the internal temperature of the cavity, respectively, solving the problem of easy interference with a single temperature measurement method. The closed-loop feedback control module adjusts the microwave output power in real time through a PID control algorithm to control the temperature difference inside the furnace within ±5℃, ensuring the stability and repeatability of the sintering process.

[0018] Preferably, the support structure includes a storage compartment embedded and fixed inside the lower part of the partition. A first motor and a second motor are respectively connected to the left and right sides inside the storage compartment. The top output ends of the first motor and the second motor are both connected to the lifting assembly. A multimode resonant lower cover is installed at the top of the lifting assembly, and a support platform for placing the embryo is provided above the multimode resonant lower cover.

[0019] By adopting the above technical solution and using a dual-motor synchronous drive lifting assembly, the support platform is ensured to maintain a horizontal posture during movement, thereby improving the sealing reliability of the multimode resonator lower cover and the bottom opening of the multimode resonator cavity.

[0020] Preferably, the support platform has a first position and a second position. In the first position, the multimode resonator lower cover and the support platform are separated from the bottom of the multimode resonator cavity and extend close to the furnace door for placing or removing the raw blank. In the second position, the support platform rises to a sealing contact between the edge of the multimode resonator lower cover and the bottom opening of the multimode resonator cavity, so that the multimode resonator cavity forms a microwave sealed space.

[0021] By adopting the above technical solution and setting two clearly defined working positions, the charging operation and sintering process are separated, eliminating the need for the independent internal door structure in the traditional microwave sintering furnace, thus improving sealing reliability and ease of operation.

[0022] Preferably, the microwave mode stirrer includes a cover fixed to the top side inside the furnace body, a third motor is installed inside the cover, a shaft is connected to the bottom output end of the third motor, the shaft rotates through the top middle side of the multimode resonant cavity, and a blade is connected to the bottom end of the shaft to drive the blade to rotate inside the multimode resonant cavity to dynamically change the microwave reflection path inside the multimode resonant cavity.

[0023] By adopting the above technical solution, the microwave mode stirrer periodically changes the boundary conditions inside the cavity through the continuous rotation of the blades, causing the microwave reflection path to change continuously, thereby disrupting the stable formation of standing waves and transforming the fixed distribution of electromagnetic field into a time-varying distribution. This achieves the homogenization of electromagnetic field through time averaging effect, effectively avoiding local over-burning or under-burning of green blanks during sintering.

[0024] Preferably, the lifting assembly includes a first frame and a second frame fixed to the left and right sides of the top of the storage unit, respectively. A trapezoidal channel is formed between the first frame and the second frame, and a support block is slidably connected inside the channel. A first driving sprocket and a first driven sprocket are rotatably connected to the front and rear sides of the top of the first frame, respectively, and a first chain meshes with the outer surfaces of the first driving sprocket and the first driven sprocket. The middle part of the first driving sprocket is connected to the top output end of the first motor. A second driving sprocket and a second driven sprocket are rotatably connected to the front and rear sides of the top of the second frame, respectively. A sprocket, and a second chain meshes with the outer surfaces of the second driving sprocket and the second driven sprocket. The middle part of the second driving sprocket is connected to the top output end of the second motor. A sprocket disc meshes with the first chain and the second chain. An internally threaded cylinder is fixed through the middle side of the sprocket disc and rotates through the middle side of the support block. A screw is threadedly connected inside the internally threaded cylinder. The top end of the screw is connected to the lower cover of the multimode resonance. A guide rod slides through both the front and rear sides of the support block. The tops of the two guide rods are fixed to the lower cover of the multimode resonance.

[0025] By adopting the above technical solution, the lifting and moving assembly adopts a dual transmission structure combining chain drive and lead screw and nut pair, which can accurately control the composite motion of the bearing platform in two degrees of freedom, and has the characteristics of smooth movement, high positioning accuracy and good self-locking.

[0026] Thirdly, this application provides a low-temperature sintered zinc oxide varistor material, which is prepared by any of the preparation methods described above.

[0027] Preferably, the low-temperature sintered zinc oxide varistor material has a relative density greater than or equal to 98%, a varistor voltage of 200-500V / mm at a DC current of 1mA, a nonlinear coefficient α of 40-60, and a leakage current less than or equal to 1μA; the grain size of the low-temperature sintered zinc oxide varistor material is 0.5-2.0μm, and the grain boundary layer thickness is 10-30nm, with a composite spinel phase formed by Bi, Y, and Nb elements enriched at the grain boundaries.

[0028] In summary, this application includes at least one of the following beneficial technical effects of low-temperature sintered zinc oxide varistor materials and their preparation methods: 1. Based on the traditional system, this application introduces Y2O3 and Nb2O5 for composite doping and adds Al(NO3)3·9H2O as a sintering aid. At the same time, the raw materials are prepared into nanoscale precursor powder, which significantly reduces the sintering activation energy of the material. On this basis, this application adopts a microwave-assisted sintering process, which utilizes the bulk heating characteristics of microwaves to accelerate the ion diffusion process, enabling the material to achieve densification sintering at a temperature significantly lower than that of the traditional process. This greatly reduces the sintering energy consumption and avoids the problem of abnormal grain growth caused by high-temperature sintering, providing a feasible path for the low-temperature co-firing preparation of zinc oxide varistor materials. 2. The multimode resonant cavity of this application, combined with the rotational motion of the microwave-mode stirrer, dynamically changes the microwave reflection path within the cavity, effectively eliminating standing waves and achieving a uniform distribution of the electromagnetic field. The dual-channel temperature measurement system, composed of an infrared thermometer and a fiber Bragg grating temperature sensor, monitors the surface temperature of the material and the internal temperature of the cavity respectively, avoiding the defects of single temperature measurement methods being susceptible to microwave interference. The closed-loop feedback control module adjusts the microwave output power in real time through a PID control algorithm, ensuring the stability and uniformity of the temperature field during sintering. In addition, the lifting-type sealed structure adopted in this application, through the combined movement of the support platform and the multimode resonant lower cover, achieves convenient operation of the loading position and reliable sealing of the sintering position, eliminating the independent inner door structure of traditional microwave ovens, simplifying the equipment, and improving sealing reliability. 3. This application optimizes the formula and process, enabling the material to achieve high densification under low-temperature sintering conditions, resulting in excellent comprehensive electrical performance with high nonlinear coefficient and low leakage current. At the same time, due to the significantly reduced sintering temperature, the material of this application has good compatibility with the low-temperature co-firing process of silver electrodes, making it suitable for the preparation of multilayer chip varistors and expanding the application range of the product. Attached Figure Description

[0029] Figure 1 This is a schematic flowchart of the preparation method of this application; Figure 2 This is a schematic diagram of the structure of the microwave-assisted temperature field controlled sintering furnace of this application; Figure 3 This is a schematic diagram of the internal structure of the furnace body in this application; Figure 4 This is a schematic diagram of the supporting structure of this application; Figure 5 This application Figure 4 Enlarged view of the local structure at point A; Figure 6 This is a top view schematic diagram of the connection between the first chain, the second chain, and the sprocket in this application.

[0030] Explanation of reference numerals in the attached drawings: 1. Furnace body; 2. Furnace door; 3. Partition; 4. Support structure; 41. Silo base; 42. First motor; 43. Second motor; 44. Lifting assembly; 441. First carrier frame; 442. Second carrier frame; 443. Support block; 444. First drive sprocket; 445. First driven sprocket; 446. First chain; 447. Second drive sprocket; 448. Second driven sprocket; 449. Second chain; 4410. Sprocket disc; 4411. Internal threaded cylinder; 4412. Screw; 4413. Guide rod; 45. Multimode resonant lower cover; 46. Bearing platform; 5. Microwave magnetic control body; 6. Waveguide; 7. Multimode resonant cavity; 71. Microwave feed inlet; 8. Ceramic support; 9. Microwave mode stirrer; 91. Cover; 92. Third motor; 93. Shaft; 94. Blade; 10. Infrared thermometer; 11. Fiber Bragg grating temperature sensor; 12. Closed-loop feedback control module. Detailed Implementation

[0031] The following is in conjunction with the appendix Figure 1 - Appendix Figure 6 This application will be described in further detail below.

[0032] Example 1 provides a low-temperature sintered zinc oxide varistor material and its preparation method.

[0033] I. Raw material ratio: Weigh the raw materials according to the following weight proportions: ZnO: 95 parts, Bi2O3: 1.2 parts, Sb2O3: 1.0 parts, MnO2: 0.5 parts, Cr2O3: 0.4 parts, Co2O3: 0.6 parts, Y2O3: 0.2 parts, Nb2O5: 0.2 parts, Al(NO3)3·9H2O: 0.05 parts.

[0034] II. Preparation method: S1. Raw material weighing and mixing: After mixing the above raw materials with deionized water and ammonium polyacrylate dispersant, the mixture is placed in a planetary ball mill for ball milling. Zirconia balls are used as the grinding medium, the ball-to-material ratio is 4:1, the ball milling speed is 300 rpm, and the ball milling time is 18 hours to obtain a mixed slurry with a solid content of 50%.

[0035] S2. Spray granulation and pre-calcination: The mixed slurry is spray-dried at an inlet temperature of 220°C and an outlet temperature of 110°C to obtain precursor powder with a particle size of 80-150nm; the precursor powder is placed in the microwave-assisted temperature field controlled sintering furnace described in this application and heated to 700°C at a heating rate of 3°C / min in an air atmosphere, and held for 2 hours to perform pre-calcination treatment to obtain pre-calcined powder.

[0036] S3. Secondary ball milling and granulation: The pre-calcined powder is added to a 5wt% polyvinyl alcohol aqueous solution and ball-milled for 4 hours. Then, it is spray-granulated to obtain the molded powder.

[0037] S4. Pressing and molding: Press the molding powder into a round green sheet with a diameter of 10 mm and a thickness of 1.5 mm under a pressure of 100 MPa.

[0038] S5. Low-temperature sintering: The green billet is placed in a microwave-assisted temperature-controlled sintering furnace, and the following sintering procedure is adopted: First stage: Heat from room temperature to 480℃ at a rate of 5℃ / min, hold at that temperature for 0.8 hours, and then remove the adhesive; Second stage: Increase the temperature from 480℃ to 880℃ at a rate of 3℃ / min and hold for 2.5 hours.

[0039] During the sintering process, the microwave frequency is 2.45GHz and the output power is 0.5-3kW. The closed-loop feedback control module adjusts the microwave output power in real time according to the temperature signals collected by the infrared thermometer and the fiber Bragg grating temperature sensor, so that the temperature difference in the furnace is controlled within ±5℃. After sintering, the furnace is cooled to room temperature to obtain low-temperature sintered zinc oxide varistor material.

[0040] III. Structure of Microwave-Assisted Temperature Field Controlled Sintering Furnace: The microwave-assisted temperature field controlled sintering furnace structure used in this embodiment is as follows: Figure 2-6 As shown, it specifically includes: The furnace body 1 has a sintering chamber inside. The furnace body 1 has an openable and closable furnace door 2 on the front side. The furnace body 1 has a partition 3 on the bottom side inside, and a support structure 4 is installed through the top side of the partition 3.

[0041] A microwave magnetron body 5 is symmetrically arranged on the left and right sides of the furnace body 1. Both microwave magnetron bodies 5 are sealed to the microwave feed inlet 71 on the side wall of the multimode resonant cavity 7 through a waveguide 6 passing through the side of the furnace body 1. This allows microwave energy to be fed in from both sides of the multimode resonant cavity 7 simultaneously, making the electromagnetic field distribution in the cavity more uniform. The outside of the multimode resonant cavity 7 is fixed to the inner wall of the furnace body 1 by a ceramic support 8. The multimode resonant cavity 7 is located above the support structure 4 and has an opening at its bottom. By cooperating with the support structure 4 below, a dynamic sealing structure is achieved.

[0042] A microwave stirrer 9 is installed through the top center of the multimode resonant cavity 7. An infrared thermometer 10 is installed on both the left and right sides of the top of the multimode resonant cavity 7 to collect the surface temperature of the sintered material. A fiber Bragg grating temperature sensor 11 is installed on both the left and right sides of the bottom of the multimode resonant cavity 7 to collect the internal temperature of the multimode resonant cavity 7. A closed-loop feedback control module 12 is installed on the side of the furnace body 1. The closed-loop feedback control module 12 is electrically connected to the microwave magnetic control body 5, the infrared thermometer 10 and the fiber Bragg grating temperature sensor 11, respectively. It is used to receive the collected temperature signals and adjust the output power of the microwave magnetic control body 5 in real time based on the preset sintering curve through a PID control algorithm. It can automatically compensate for the temperature deviation caused by changes in material characteristics, fluctuations in cavity heat loss and other factors.

[0043] like Figure 3-6 As shown, the support structure 4 includes a storage base 41 embedded and fixed inside the lower middle side of the partition 3. The left and right sides of the storage base 41 are respectively connected to a first motor 42 and a second motor 43. The top output ends of the first motor 42 and the second motor 43 are both connected to the lifting assembly 44. A multimode resonant lower cover 45 is installed at the top of the lifting assembly 44, and a support platform 46 for placing the embryo is provided above the multimode resonant lower cover 45. Through dual motor control, the lifting assembly 44 can drive the multimode resonant lower cover 45 and the support platform 46 to move.

[0044] The supporting platform 46 has a first position and a second position. In the first position, the supporting platform 46 extends to the vicinity of the furnace door 2, providing a wide operating space and facilitating the placement of green blanks by manual or robotic arms, thus avoiding the inconvenience and safety hazards of operating in a narrow cavity. In the second position, the multimode resonant lower cover 45 is in sealed contact with the bottom opening of the multimode resonant cavity 7, forming a complete microwave sealed cavity. This ensures that microwave energy is effectively confined inside the cavity, preventing microwave leakage. At the same time, the sintering atmosphere inside the cavity is isolated from the outside, ensuring the stability of the sintering environment.

[0045] The lifting assembly 44 includes a first carrier frame 441 and a second carrier frame 442 fixed to the left and right sides of the top of the storage unit 41, respectively. A trapezoidal channel is formed between the first carrier frame 441 and the second carrier frame 442, and a support block 443 is slidably connected inside the channel. A first drive sprocket 444 and a first driven sprocket 445 are rotatably connected to the front and rear sides of the top of the first carrier frame 441, respectively. A first chain 446 is meshed with the outer surfaces of the first drive sprocket 444 and the first driven sprocket 445. The middle part of the first drive sprocket 444 is connected to the top output end of the first motor 42. A second drive sprocket 447 and a second driven sprocket 448 are rotatably connected to the front and rear sides of the top of the second carrier frame 442, respectively. A second chain 449 is engaged with the outer surface of wheel 448 for transmission. The middle part of the second drive sprocket 447 is connected to the top output end of the second motor 43. A sprocket disc 4410 is engaged between the first chain 446 and the second chain 449 to ensure synchronous movement on both sides. An internally threaded cylinder 4411 is fixed through the middle side of the sprocket disc 4410 and rotates through the middle side of the support block 443. A screw 4412 is threadedly connected inside the internally threaded cylinder 4411. The top end of the screw 4412 is connected to the multimode resonant lower cover 45. A guide rod 4413 slides through both the front and rear sides of the support block 443. The tops of the two guide rods 4413 are fixed to the multimode resonant lower cover 45 to provide guidance for the vertical movement of the multimode resonant lower cover 45.

[0046] The lifting assembly 44 achieves horizontal translation (extension and retraction) of the bearing platform 46 through chain drive, and vertical lifting motion through screw and nut pair; it can precisely control the composite motion of the bearing platform 46 in two degrees of freedom, realizing the extension of the loading position and the rising and sealing of the sintering position.

[0047] like Figure 3 The microwave stirrer 9 includes a cover 91 fastened to the top side inside the furnace body 1. A third motor 92 is installed inside the cover 91. The bottom output end of the third motor 92 is connected to a shaft 93. The shaft 93 passes through and rotates in the middle of the top of the multimode resonant cavity 7. The bottom end of the shaft 93 is connected to a blade 94, which is used to drive the blade 94 to rotate continuously inside the multimode resonant cavity 7, periodically changing the boundary conditions inside the cavity, causing the microwave reflection path to change continuously, thereby disrupting the stable formation of the standing wave, transforming the fixed distribution of the electromagnetic field into a time-varying distribution, and achieving the homogenization of the electromagnetic field from the time averaging effect.

[0048] The working process of this microwave-assisted temperature field controlled sintering furnace is as follows: First, in the initial state, the support platform 46 is in the first position (lowered and extended). The operator opens the furnace door 2. At this time, the multimode resonant lower cover 45 and the support platform 46 extend close to the furnace door 2 to facilitate the placement of the green billet. After the pressed green billet is placed on the support platform 46, the operator exits and prepares to close the furnace door.

[0049] Second, start the first motor 42 and the second motor 43. When the first motor 42 and the second motor 43 drive the two drive sprockets to rotate in opposite directions at the same speed, the sprocket disc 4410 drives the support block 443 to move horizontally backward along the channel between the first carrier frame 441 and the second carrier frame 442, causing the multimode resonator lower cover 45 and the bearing platform 46 to retract directly below the multimode resonator cavity 7. Subsequently, the first motor 42 and the second motor 43 drive the two drive sprockets to rotate in the same direction at the same speed. At this time, the sprocket disc 4410... 410 rotates on the support block 443, and the internal threaded cylinder 4411 rotates under the drive of the sprocket disk 4410. Through the threaded engagement, the drive screw 4412 moves upward, pushing the multimode resonant lower cover 45 and the support platform 46 to rise vertically. When the support platform 46 rises to the second position, the multimode resonant lower cover 45 is in close contact with the edge of the bottom opening of the multimode resonant cavity 7, forming a microwave sealed space. Then the furnace door 2 is closed, and the sintering chamber is in a sealed state. Vacuuming or sintering atmosphere can be introduced according to process requirements.

[0050] Third, the closed-loop feedback control module 12 starts the sintering program according to the preset sintering curve. First, the microwave magnetron 5 generates microwaves, which are transmitted through the waveguide 6 and enter the multimode resonant cavity 7 through the microwave feed inlet 71. The microwaves are repeatedly reflected and superimposed between the metal inner walls of the multimode resonant cavity 7 to form a multimode electromagnetic field distribution. At the same time, the third motor 92 of the microwave mode stirrer 9 drives the blades 94 to rotate continuously, dynamically changing the microwave reflection path in the cavity, eliminating standing waves, and making the electromagnetic field distribution more uniform. After the green blank absorbs microwave energy, a volume heating effect is generated inside, and the temperature rises rapidly. During the heating process, the infrared thermometer 10 monitors the green blank in real time. The surface temperature is monitored in real time by the fiber Bragg grating temperature sensor 11, which monitors the internal temperature of the multimode resonant cavity 7. The two temperature signals are transmitted to the closed-loop feedback control module 12 in real time. The closed-loop feedback control module 12 compares the measured temperature with the preset sintering curve and dynamically adjusts the output power of the microwave magnetron 5 through a PID control algorithm. When the temperature is lower than the target value, the power is increased, and when it is higher than the target value, the power is decreased to achieve precise temperature control. The sintering process is carried out according to the preset stage program: the first stage (450-500℃) is used to heat the binder and remove it. The second stage is to heat the temperature to 850-900℃ and heat it for 1-4 hours to complete the densification sintering.

[0051] Fourth, after the sintering process is completed, the microwave magnetic control body 5 stops working, and the green blank cools naturally to room temperature with the furnace. After cooling, the furnace door 2 is opened, and the first motor 42 and the second motor 43 drive the support platform 46 to descend first and then extend horizontally back to the first position. The multimode resonant lower cover 45 separates from the bottom of the multimode resonant cavity 7, exposing the sintered sample. The operator takes the green blank from the support platform 46 to complete the entire sintering process.

[0052] IV. Material property characterization: The performance of the low-temperature sintered zinc oxide varistor material prepared in this embodiment was tested. The results showed that: relative density: determined by Archimedes' displacement method, the relative density of the material was greater than or equal to 98%; varistor voltage: tested at 1mA DC current, the varistor voltage was 200-500V / mm; nonlinear coefficient α: calculated from the voltage-current characteristic curve, the nonlinear coefficient α was 40-60; leakage current: tested at 0.75 times the varistor voltage, the leakage current was less than or equal to 1μA; microstructure: observed by scanning electron microscopy, the grain size was 0.5-2.0μm, and the grain boundary layer thickness was 10-30nm. Energy dispersive spectroscopy analysis showed that Bi, Y, and Nb elements were enriched at the grain boundaries, forming a composite spinel phase.

[0053] The above performance indicators show that the material prepared in this embodiment achieves high densification under low-temperature sintering conditions (850-900℃) and has excellent nonlinear current-voltage characteristics.

[0054] Example 2: This embodiment is basically the same as Embodiment 1, except that the raw material ratio and sintering process parameters are different.

[0055] Raw material ratio: ZnO: 92 parts, Bi2O3: 1.5 parts, Sb2O3: 1.2 parts, MnO2: 0.8 parts, Cr2O3: 0.6 parts, Co2O3: 0.8 parts, Y2O3: 0.4 parts, Nb2O5: 0.3 parts, Al(NO3)3·9H2O: 0.09 parts.

[0056] Sintering process: Pre-firing temperature 750℃, holding for 2 hours; sintering temperature 900℃, holding for 3 hours.

[0057] Performance test results show that the material prepared in this embodiment has a relative density greater than or equal to 98%, a varistor voltage of 200-500V / mm, a nonlinear coefficient α of 40-60, a leakage current less than or equal to 1μA, and a grain size of 0.5-2.0μm, which is comparable to that of Example 1.

[0058] Example 3: This embodiment is basically the same as Embodiment 1, except that the raw material ratio and sintering process parameters are different.

[0059] Raw material ratio: ZnO: 97 parts, Bi2O3: 0.8 parts, Sb2O3: 0.6 parts, MnO2: 0.3 parts, Cr2O3: 0.2 parts, Co2O3: 0.4 parts, Y2O3: 0.1 parts, Nb2O5: 0.1 parts, Al(NO3)3·9H2O: 0.03 parts.

[0060] Sintering process: Pre-firing temperature 650℃, holding for 1.5 hours; sintering temperature 860℃, holding for 2 hours.

[0061] Performance test results show that the material prepared in this embodiment has a relative density greater than or equal to 98%, a varistor voltage of 200-500V / mm, a nonlinear coefficient α of 40-60, a leakage current less than or equal to 1μA, and a grain size of 0.5-2.0μm, which is comparable to that of Example 1.

[0062] Comparative Example 1: This comparative example is basically the same as Example 1, except that Y2O3 and Nb2O5 are not added, and microwave sintering is not used. Instead, a traditional resistance furnace is used for sintering at 1100°C for 2 hours.

[0063] Performance test results show that the material prepared in Comparative Example 1 has a lower relative density (significantly lower than 98%), a nonlinear coefficient α significantly less than 40, a leakage current significantly greater than 1 μA, a grain size significantly greater than 2.0 μm, and exhibits abnormal grain growth. This indicates that without Y2O3 and Nb2O5 composite doping and without using the microwave low-temperature sintering process of this application, it is difficult to obtain high density and excellent electrical properties.

[0064] Comparative Example 2: This comparative example is basically the same as Example 1, except that it uses a traditional microwave sintering furnace without a multimode resonant cavity, a microwave mode stirrer, or closed-loop feedback control.

[0065] Performance test results show that the samples prepared in Comparative Example 2 exhibit local over-burning and under-burning phenomena, uneven relative density distribution, with some areas having a relative density below 95%, large fluctuations in the nonlinear coefficient α (some samples below 40), unstable leakage current (some samples greater than 1 μA), and poor batch stability. This indicates that without the multi-mode resonant cavity, microwave mode stirrer, and closed-loop feedback control structure of this application, it is difficult to achieve a uniform temperature field distribution and a stable sintering process.

[0066] A comparison of Examples 1-3 and Comparative Examples 1-2 shows that: This application utilizes the combined doping of Y2O3 and Nb2O5 and the sintering aid Al(NO3)3·9H2O, combined with microwave-assisted low-temperature sintering technology, to achieve high-density sintering at 850-900℃, significantly reducing the sintering temperature compared to traditional processes (above 1100℃). The resulting material has a relative density greater than or equal to 98%, a nonlinear coefficient α of 40-60, and a leakage current less than or equal to 1μA, exhibiting excellent overall electrical performance. Furthermore, the multi-mode resonant cavity, microwave-mode stirrer, and closed-loop feedback control employed in this application ensure temperature field uniformity and sintering process stability, resolving the local over-burning and under-burning problems present in Comparative Example 2.

[0067] The embodiments described in this specific implementation are preferred embodiments of this application and are not intended to limit the scope of protection of this application. Identical components are represented by the same reference numerals. Therefore, all equivalent changes made to the structure, shape, and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for preparing a low-temperature sintered zinc oxide varistor material, characterized in that, Includes the following steps: S1. Weigh the raw materials according to the following weight proportions: ZnO: 90-98 parts, Bi2O3: 0.5-2.0 parts, Sb2O3: 0.5-1.5 parts, MnO2: 0.1-1.0 parts, Cr2O3: 0.1-0.8 parts, Co2O3: 0.2-1.0 parts, Y2O3: 0.05-0.5 parts, Nb2O5: 0.05-0.5 parts, Al(NO3)3·9H2O: 0.01-0.1 parts; mix the above raw materials with deionized water and dispersant, and then ball mill them to obtain a mixed slurry with a solid content of 40-60%. S2. Spray dry the mixed slurry to obtain precursor powder with a particle size of 50-200nm. Then, place the precursor powder in a microwave-assisted temperature field controlled sintering furnace and heat it to 650-750℃ at a heating rate of 2-5℃ / min in an air atmosphere. Hold it at that temperature for 1-3 hours to perform pre-calcination treatment and obtain pre-calcined powder. S3. The pre-calcined powder is added to a polyvinyl alcohol aqueous solution, and after secondary ball milling, it is spray granulated to obtain a molded powder. S4. Press the molding powder into a green body under a pressure of 80-120MPa; S5. The green blank is placed in the microwave-assisted temperature field controlled sintering furnace, the sintering temperature is set to 850-900℃, the holding time is 1-4 hours, and the microwave output power is adjusted through the closed-loop feedback control module throughout the process to keep the temperature difference in the furnace within ±5℃. After sintering, the blank is cooled with the furnace to obtain the low-temperature sintered zinc oxide varistor material.

2. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 1, characterized in that, The ball milling process described in step S1 uses zirconia balls as the grinding medium, with a ball-to-material ratio of (3-5):1, a ball milling speed of 200-400 rpm, and a ball milling time of 12-24 hours.

3. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 1, characterized in that, The sintering process described in step S5 includes the following stages: First stage: Increase the temperature from room temperature to 450-500℃ at a rate of 3-6℃ / min, hold for 0.5-1 hour, and then remove the adhesive; Second stage: Increase the temperature from 500℃ to 850-900℃ at a rate of 2-4℃ / min, and hold at this temperature for 1-4 hours; In the second stage, the microwave frequency of the microwave-assisted temperature field control sintering furnace is 2.45 GHz, the output power is 0.5-5 kW, and the output power is dynamically adjusted according to the temperature feedback values ​​at multiple points inside the furnace.

4. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 1, characterized in that, The microwave-assisted temperature field controlled sintering furnace includes a furnace body (1) with a sintering chamber inside. The furnace body (1) has an openable and closable furnace door (2) on the front side. A partition (3) is provided on the bottom side of the furnace body (1), and a support structure (4) is provided through the top side of the partition (3). A microwave magnetron body (5) is symmetrically arranged on the left and right sides of the furnace body (1). Both microwave magnetron bodies (5) are sealed to the microwave feed inlet (71) of the side wall of the multimode resonant cavity (7) through a waveguide (6). The outside of the multimode resonant cavity (7) is fixed to the inner wall of the furnace body (1) through a ceramic support (8), and the multimode resonant cavity (7) is located above the support structure (4). Its bottom is provided with The multimode resonant cavity (7) is provided with a microwave mode stirrer (9) through the middle of the top. An infrared thermometer (10) is provided on both the left and right sides of the top of the multimode resonant cavity (7). A fiber Bragg grating temperature sensor (11) is provided on both the left and right sides of the bottom of the multimode resonant cavity (7). A closed-loop feedback control module (12) is installed on the side of the furnace body (1). The closed-loop feedback control module (12) is electrically connected to the microwave magnetic control body (5), the infrared thermometer (10) and the fiber Bragg grating temperature sensor (11) respectively. It is used to receive the collected temperature signal and adjust the output power of the microwave magnetic control body (5) in real time through the PID control algorithm based on the preset sintering curve.

5. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 4, characterized in that, The support structure (4) includes a storage base (41) embedded and fixed inside the lower middle side of the partition (3). The left and right sides of the storage base (41) are respectively connected to a first motor (42) and a second motor (43). The top output ends of the first motor (42) and the second motor (43) are connected to the lifting assembly (44). The top of the lifting assembly (44) is equipped with a multimode resonant lower cover (45), and a bearing platform (46) for placing the embryo is provided above the multimode resonant lower cover (45).

6. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 5, characterized in that, The support platform (46) has a first position and a second position. In the first position, the multimode resonant lower cover (45) and the support platform (46) are separated from the bottom of the multimode resonant cavity (7) and extend close to the furnace door (2) for placing or removing the raw blank. In the second position, the support platform (46) rises to a sealed contact between the edge of the multimode resonant lower cover (45) and the bottom opening of the multimode resonant cavity (7), so that the multimode resonant cavity (7) forms a microwave sealed space.

7. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 4, characterized in that, The microwave stirrer (9) includes a cover (91) fastened to the top side inside the furnace body (1). A third motor (92) is installed inside the cover (91). A shaft (93) is connected to the bottom output end of the third motor (92). The shaft (93) rotates through the top middle side of the multimode resonant cavity (7). A blade (94) is connected to the bottom end of the shaft (93) to drive the blade (94) to rotate inside the multimode resonant cavity (7) to dynamically change the microwave reflection path inside the multimode resonant cavity (7).

8. The method for preparing a low-temperature sintered zinc oxide varistor material according to claim 5, characterized in that, The lifting assembly (44) includes a first carrier (441) and a second carrier (442) fixed to the left and right sides of the top of the storage unit (41), respectively. A trapezoidal channel is formed between the first carrier (441) and the second carrier (442), and a support block (443) is slidably connected inside the channel. A first drive sprocket (444) and a first driven sprocket (445) are rotatably connected to the front and rear sides of the top of the first carrier (441), and a first chain (446) is meshed with the outer surfaces of the first drive sprocket (444) and the first driven sprocket (445). The middle part of the first drive sprocket (444) is connected to the top output end of the first motor (42). A second drive sprocket (447) and a second driven sprocket (448) are rotatably connected to the front and rear sides of the top of the second carrier (442), and the second drive sprocket (447) and the second driven sprocket (448) are rotatably connected to the front and rear sides of the top of the second carrier (442). A second chain (449) meshes with the outer surface of the second driven sprocket (447) and the second driven sprocket (448). The middle part of the second driving sprocket (447) is connected to the top output end of the second motor (43). A sprocket disc (4410) meshes with the first chain (446) and the second chain (449). An internal threaded cylinder (4411) is fixed through the middle side of the sprocket disc (4410). The internal threaded cylinder (4411) rotates through the middle side of the support block (443). A screw (4412) is threadedly connected inside the internal threaded cylinder (4411). The top part of the screw (4412) is connected to the multimode resonance lower cover (45). A guide rod (4413) slides through both the front and rear sides of the support block (443). The tops of the two guide rods (4413) are fixed to the multimode resonance lower cover (45).

9. A low-temperature sintered zinc oxide varistor material, characterized in that, It is prepared by any one of the preparation methods described in claims 1 to 8.

10. The low-temperature sintered zinc oxide varistor material according to claim 9, characterized in that, The low-temperature sintered zinc oxide varistor material has a relative density greater than or equal to 98%, a varistor voltage of 200-500V / mm at a DC current of 1mA, a nonlinear coefficient α of 40-60, and a leakage current less than or equal to 1μA. The low-temperature sintered zinc oxide varistor material has a grain size of 0.5-2.0 μm and a grain boundary layer thickness of 10-30 nm. The grain boundaries are enriched with a composite spinel phase formed by Bi, Y, and Nb elements.

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