A sintering aid-free silicon carbide ceramic room temperature ultrafast field-induced sintering method and device

Abstract

The application discloses a sintering aid-free silicon carbide ceramic room-temperature ultrafast field-induced sintering method and device. The method comprises the following steps: (1) placing a ceramic green body in a sealed vacuum box, and arranging electrodes at both ends of the ceramic green body and connecting the electrodes to an alternating current power supply; in the sealed vacuum box, the bottom of the ceramic green body is padded with an alumina plate, and an alumina plate is arranged above the ceramic green body; the alumina plates above and below are parallel and are supported by alumina blocks arranged at both sides of the ceramic green body; (2) adjusting the atmosphere in the sealed vacuum box to an inert atmosphere with a pressure of less than 1 atm; (3) turning on the power supply, increasing the voltage to a value at which surface discharge of the ceramic green body occurs, so that the electrical conductivity of the ceramic green body changes and an electric current channel is generated, continuously increasing the voltage to a predetermined value of the current density, then turning off the power supply, and obtaining the silicon carbide ceramic.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ceramic material preparation, and particularly relates to a sintering aid-free silicon carbide ceramic room-temperature ultrafast field-induced sintering method and device. BACKGROUND

[0002] As an important functional material, ceramic materials are widely used in various industries. Among them, silicon carbide ceramics not only have excellent room-temperature mechanical properties, such as high bending strength, excellent oxidation resistance, good corrosion resistance, high wear resistance and low friction coefficient, but also have the best high-temperature mechanical properties (strength, creep resistance, etc.) among known ceramic materials. Therefore, silicon carbide ceramics have been widely used in the fields of petroleum, chemical industry, microelectronics, automobile, aerospace, aviation, papermaking, laser, mining and atomic energy.

[0003] Silicon carbide ceramics are prepared by sintering ceramic green bodies prepared by granulation, tabletting, degassing and pre-sintering of silicon carbide powder. Sintering is a key step in the preparation of ceramic materials and is also the main energy-consuming step in the production process of ceramic materials, which has a significant impact on the performance of ceramics. Silicon carbide ceramics are covalently bonded and belong to non-oxide materials, which are much more difficult to sinter than oxide ceramics.

[0004] At present, the sintering methods of silicon carbide ceramics include pressureless sintering, hot-pressing sintering, reactive sintering, hot isostatic pressing sintering, microwave sintering and discharge plasma sintering. However, in the above sintering methods, pressureless sintering requires high temperature and sintering aids; hot-pressing and hot isostatic pressing sintering require high temperature and high pressure conditions and sintering aids; the performance of the products of reactive sintering is poor; and microwave sintering and discharge plasma sintering have high requirements for equipment and are difficult to mass-produce. Therefore, it is necessary to propose a new SiC ceramic sintering method that requires lower equipment and environment and does not require sintering aids.

[0005] Flash sintering is a new type of electric field assisted rapid sintering technology and an effective method for rapidly preparing electrical ceramics at a lower temperature. Specifically, by applying a suitable electric field to the two ends of the ceramic green body, an internal conductive channel is formed after the ceramic green body flashes along the surface, and rapid sintering of the ceramic is realized by utilizing the Joule heating effect.

[0006] Chinese patent CN110606751B discloses a method for graphene-assisted room-temperature flash sintering of ceramic materials, in a specific embodiment, silicon carbide powder is added to a reduced graphene oxide solution to obtain a composite powder of silicon carbide and reduced graphene oxide, and an electric field is applied to the ends of a green body pressed from the composite powder to achieve sintering. The above method uses flash sintering technology to achieve sintering of silicon carbide ceramics, but requires the addition of reduced graphene oxide as a sintering aid, which may change the structure of the ceramic and affect the density of the ceramic. If the sintering aid cannot be completely removed, it will also affect the purity of the ceramic.

[0007] At present, there are still relatively few related researches on flash sintering of non-oxides such as silicon carbide, especially on sintering aid-free flash sintering of silicon carbide. SUMMARY

[0008] The first object of the present application is to provide a sintering aid-free silicon carbide ceramic room-temperature ultra-fast field-induced sintering method.

[0009] The second object of the present application is to provide a sintering aid-free silicon carbide ceramic room-temperature ultra-fast field-induced sintering device.

[0010] The object of the present application is achieved by the following scheme.

[0011] A sintering aid-free silicon carbide ceramic room-temperature ultra-fast field-induced sintering method, comprising sintering a ceramic green body prepared by granulation, tabletting, degassing and pre-sintering of silicon carbide powder, characterized in that the sintering comprises the following steps:

[0012] (1) placing the ceramic green body in a sealed vacuum box, and setting electrodes at both ends of the ceramic green body and connecting them to an alternating current power supply;

[0013] In the sealed vacuum box, the bottom of the ceramic green body is padded with an alumina plate, and an alumina plate is also provided above, and the upper and lower alumina plates are parallel and supported by alumina blocks provided on both sides of the ceramic green body;

[0014] (2) adjusting the atmosphere in the sealed vacuum box to an inert atmosphere of <1 atm;

[0015] (3) turning on the power supply, raising the voltage to cause surface discharge of the ceramic green body, so that the electrical conductivity of the ceramic green body changes and an electric current channel is generated, continuously raising the voltage to a predetermined value of current density, then turning off the power supply, and obtaining a silicon carbide ceramic.

[0016] The present application reduces the initial voltage of arc on the sample surface and the current of initial arc by using low pressure inert atmosphere of <1 atm, inhibits the heat dissipation of sample surface to the surrounding space, and avoids the oxidation of silicon carbide. Meanwhile, the alumina blocks are arranged on the upper and lower sides of the ceramic green body to constrain the arc height and accelerate its settlement, so that the silicon carbide ceramic green body is superfast sintered at room temperature when the alternating high voltage and current are applied to the silicon carbide ceramic green body.

[0017] The ceramic green body is prepared by the processes of granulation, tabletting, glue removal and pre-burning in the art, and the present application is not particularly limited, and the shape can be one of round tablet, long strip and dog bone shape.

[0018] Preferably, the distance between the alumina plate above the ceramic green body and the upper surface of the ceramic green body in step (1) is 3-5 mm, which is beneficial to constrain the arc height to ensure the success rate of sintering.

[0019] Preferably, the electrode is a molybdenum electrode.

[0020] Preferably, the alternating current source is a high-voltage alternating current source without local discharge.

[0021] Preferably, in step (2), the air in the sealed vacuum box is extracted, and argon is filled into the box to adjust the atmosphere in the box.

[0022] Preferably, the atmosphere in the sealed vacuum box is adjusted to an inert atmosphere of 0.4-0.6 atm; as a specific embodiment of the present application, the atmosphere in the sealed vacuum box is composed of 0.01 atm of air and 0.59 atm of argon.

[0023] Preferably, the specific operation of step (3) is: after the power is turned on, the voltage is increased in steps, during which surface flashover occurs on the surface of the silicon carbide ceramic green body, and dazzling white light is observed, at this time the current flowing through the silicon carbide ceramic green body suddenly increases, the voltage across the silicon carbide ceramic green body drops sharply, the power supply is switched from voltage control mode to current control mode, the voltage is continuously increased to a predetermined value of current density, and after maintaining for a period of time, the power is turned off to obtain the silicon carbide ceramic.

[0024] Preferably, in step (3), the voltage increasing method adopts manual stepwise voltage increasing; the predetermined value of current density is controlled to be 100-102 mA / mm 2 , and the maintenance time is 110-130 s. Regardless of the mode of stepwise voltage increasing, the purpose of the present application can be achieved, so the present application is not limited in this regard.

[0025] As a specific embodiment of the present application, the binder used in the granulation process for preparing the ceramic green body is polyvinyl alcohol.

[0026] The present application also provides a sintering aid-free silicon carbide ceramic room temperature ultrafast field sintering device, comprising: a sealed vacuum box, an alternating current power supply, an argon gas cylinder and a vacuum pump.

[0027] The sealed vacuum box is provided with two upper and lower parallel placed alumina plates supported by alumina blocks arranged on both sides to form a sintering chamber for the silicon carbide ceramic green body.

[0028] The sealed vacuum box is communicated with the argon gas cylinder and the vacuum pump through gas inlet and outlet ports.

[0029] The alternating current power supply is electrically connected with the conductive column of the sealed vacuum box through wires, and the conductive column is electrically connected with both ends of the silicon carbide ceramic green body through wires with electrodes in the sealed vacuum box.

[0030] The electrode is made of molybdenum.

[0031] The alternating current power supply is a high-voltage alternating current power supply without partial discharge.

[0032] A gas pressure gauge and a gas valve are arranged on the pipeline for communicating the sealed vacuum box with the argon gas cylinder and the vacuum pump.

[0033] Compared with the prior art, the present application has the following beneficial effects.

[0034] The method of the present application can realize the flash sintering of silicon carbide ceramic at room temperature without sintering aid, which eliminates the influence of impurities on the performance of silicon carbide ceramic, greatly reduces the environmental temperature required for sintering of silicon carbide ceramic, eliminates the need for heating furnace, further improves the energy utilization rate of power supply, simplifies the sintering equipment and reduces the sintering cost, and realizes the sintering aid-free room temperature ultrafast sintering of silicon carbide ceramic, providing a feasible way for room temperature flash sintering of non-oxide materials. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 The present application provides a sintering aid-free silicon carbide ceramic room temperature ultrafast field sintering device;

[0036] Figure 2 The present application provides a sintering aid-free silicon carbide ceramic room temperature ultrafast field sintering device; DETAILED DESCRIPTION

[0037] In order to better understand the technical solutions of the present application, the present application will be described in detail below through specific examples and in conjunction with the drawings.

[0038] The present application provides a sintering aid-free silicon carbide ceramic room temperature ultrafast field sintering device, comprising a sealed vacuum box 1, a high-voltage alternating current power supply 2 without partial discharge, an alumina plate 3, an alumina block 4, an argon gas cylinder 5, a vacuum pump 6, etc. Figure 1The specific devices used are all commercially available products, wherein the non-arcing high-voltage alternating current power supply is a YDTW 100 / 50 non-arcing alternating current power supply produced by Xinyuan Electrical Equipment Co., Ltd. in Yangzhou.

[0039] The sealed vacuum box 1 is provided with two upper and lower parallel placed alumina plates 3, which are supported by alumina blocks 4 arranged on both sides. Specifically, stacked alumina blocks 4 can be arranged on both sides between the upper and lower parallel placed alumina plates 3 as supports to form a sintering chamber of the silicon carbide ceramic green body 7, and the silicon carbide ceramic green body 7 is placed in the sintering chamber.

[0040] The sealed vacuum box 1 is in communication with the argon gas cylinder 5 and the vacuum pump 6 through the gas inlet and outlet provided with the air pressure gauge 8 and the gas valve 9.

[0041] The non-arcing high-voltage alternating current power supply 2 is electrically connected to the conductive column 10 of the sealed vacuum box 1 through a wire; in the sealed vacuum box, the conductive column 10 clamps the molybdenum electrode through a wire with a clamp to electrically connect both ends of the silicon carbide ceramic green body 7.

[0042] The thickness of the alumina plate is not limited in the present application, and the length and width are adapted to the size of the silicon carbide ceramic green body, that is, to ensure that the silicon carbide ceramic green body can be placed in the sintering chamber formed thereby.

[0043] Example 1

[0044] (1) Select 6H-SiC powder for testing, and prepare ceramic green body according to the conventional granulation, tabletting, degassing, and pre-burning processes.

[0045] The specific operation used in this example is as follows: 6H-SiC powder with an average particle size of 0.47 μm is selected, 6wt% of polyvinyl alcohol (PVA) is mixed with SiC powder at a mass ratio of 1:10, and granulation is completed by hand grinding for 15 min and passing through an 80-mesh screen (pore size 180 μm); considering that SiC ceramic has a low theoretical density of 3.20 g / cm 3 , 0.5 g of granulated powder is weighed each time, and a dog bone-shaped green body is obtained by uniaxial pressing at 135 MPa for 45 s, the size of the strip-shaped area in the middle of the sample is 13*3.3*2 mm, and the relative density is 65.8%; the SiC green body obtained by pressing is loose and fragile, the SiC green body is heated at 400°C for 2 h for degassing to remove the organic binder polyvinyl alcohol, and then pre-burning at 1000°C for 1 h to enhance the mechanical strength of the green body.

[0046] (2) As shown in Figure 1 , the silicon carbide ceramic green body in (1) is placed in the sintering chamber composed of alumina plates in the sealed vacuum box, and the upper surface of the silicon carbide ceramic green body is about 5 mm away from the upper alumina plate;

[0047] The molybdenum electrode is clamped by a wire clamp, and the electrode end is tightly attached to the two ends of the dog bone-shaped ceramic green body as an electrode and is connected to both ends of the non-partial discharge high-voltage alternating current power supply, and the power supply is kept in the off state.

[0048] (3) Open the gas valve, vacuum pump and argon cylinder valve, and perform two cycles of removing 90% of the gas and introducing argon to leave only 0.01 atm. of air in the sealed vacuum box. The atmosphere in the box is adjusted to 0.01 atm. of air + 0.59 atm. of argon.

[0049] (4) Turn on the non-partial discharge high-voltage alternating current power supply, and manually increase the voltage in steps, with a specific increase of 1000V each time in this embodiment. During the voltage increase, surface flashover occurs on the surface of the silicon carbide ceramic green body, and a dazzling white light is observed. At this time, the power supply is switched from voltage control mode to current control mode, the voltage across the ceramic green body drops suddenly, the current increases instantaneously, and a stable conductive channel is generated inside the ceramic green body. The voltage is continuously increased to a current density of about 110 mA / mm 2 , and the power supply is turned off after maintaining for 120s. After waiting for 10 min, the sample is taken out after the temperature naturally decreases to room temperature, and a silicon carbide ceramic is obtained.

[0050] (5) Replace the new silicon carbide ceramic green body, and repeat steps (2) to (4) above to obtain multiple groups of silicon carbide ceramics, and analyze the experimental data of each group.

[0051] The microstructure of the sample is observed by SEM, as shown in Figure 2 , the ceramic green body is sintered into a dense silicon carbide ceramic.

[0052] The density of the sintered sample is measured by the Archimedes drainage method, and the relative density is calculated, and the dog bone-shaped silicon carbide sample is successfully sintered to a density of 96.8%.

[0053] Example 2

[0054] The difference between this embodiment and Example 1 is that the pressure in the sealed vacuum box is adjusted to 0.4 atm, and the atmosphere is adjusted to 0.01 atm. of air + 0.39 atm. of argon; the other steps are the same as in Example 1.

[0055] The density of the sintered sample is measured by the Archimedes drainage method, and the relative density is calculated, and the dog bone-shaped silicon carbide sample is successfully sintered to a density of 95.1%.

[0056] Example 3

[0057] The difference between this embodiment and Example 1 is that the current density reaches about 110 mA / mm 2 , and the power supply is turned off after maintaining for 100s; the other steps are the same as in Example 1.

[0058] Density of sintered samples was measured using Archimedes displacement method and relative density was calculated, successfully sintering dog-bone shaped silicon carbide samples to 92.6% of theoretical density.

[0059] The above-described embodiments of the present application are merely intended to further explain the purpose, technical solutions and advantages of the present application, and should not be used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for room temperature ultra-fast field-induced sintering of silicon carbide ceramics without sintering aids, comprising granulating, pressing, debinding, and pre-firing silicon carbide powder to form a ceramic green body, followed by sintering, characterized in that... The sintering process includes the following steps: (1) Place the ceramic green body in a sealed vacuum chamber and set electrodes at both ends of the ceramic green body and connect them to an AC power source; In a sealed vacuum chamber, an alumina plate is placed at the bottom of the ceramic green body, and an alumina plate is also placed on top of it. The upper and lower alumina plates are parallel to each other and are supported by alumina blocks on both sides of the ceramic green body, forming a sintering chamber for the ceramic green body. The distance between the alumina plate above the ceramic green body and the upper surface of the ceramic green body is 3~5mm. (2) Adjust the atmosphere inside the sealed vacuum chamber to an inert atmosphere of 0.4~0.6 atm; (3) Turn on the power supply and increase the voltage until the ceramic green body undergoes surface discharge, which changes the conductivity of the ceramic green body and generates a current channel. Continue to increase the voltage to the predetermined current density, and then turn off the power supply to obtain silicon carbide ceramic.

2. The method for room temperature ultrafast field-induced sintering of silicon carbide ceramics without sintering aids according to claim 1, characterized in that, The electrode is a molybdenum electrode; the AC power supply is a high-voltage AC power supply without partial discharge.

3. The method for room temperature ultrafast field-induced sintering of silicon carbide ceramics without sintering aids according to claim 2, characterized in that, In step (2), the air inside the sealed vacuum chamber is extracted, and argon gas is introduced into it to adjust the atmosphere inside the chamber to an inert atmosphere of 0.4~0.6 atm.

4. The method for room temperature ultrafast field-induced sintering of silicon carbide ceramics without sintering aids according to claim 3, characterized in that, The specific operation of step (3) is as follows: after turning on the power, the voltage is increased stepwise, so that the current flowing through the silicon carbide ceramic green body suddenly increases and the voltage across the silicon carbide ceramic green body drops sharply. The power supply is switched from voltage control mode to current control mode, and the voltage is continuously increased until the current density reaches the predetermined value. After maintaining this for a period of time, the power supply is turned off to obtain silicon carbide ceramic. The predetermined value of the current density is controlled at 100~102 mA / mm. 2 The duration is 110~130s.

5. The method for room temperature ultrafast field-induced sintering of silicon carbide ceramics without sintering aids according to claim 4, characterized in that, The binder used in the granulation process for preparing ceramic green bodies is polyvinyl alcohol.

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