CN113307624B - Method for sintering ceramic at room temperature - Google Patents

Method for sintering ceramic at room temperature Download PDF

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CN113307624B
CN113307624B CN202110522622.0A CN202110522622A CN113307624B CN 113307624 B CN113307624 B CN 113307624B CN 202110522622 A CN202110522622 A CN 202110522622A CN 113307624 B CN113307624 B CN 113307624B
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ceramic
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green body
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CN113307624A (en
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王希林
黄荣厦
祝志祥
吴昂轩
贾志东
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Foshan Huajun Special Porcelain Technology Co ltd
Shenzhen International Graduate School of Tsinghua University
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Shenzhen International Graduate School of Tsinghua University
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Abstract

The invention discloses a method for sintering ceramics at room temperature, which is characterized in that the local dielectric breakdown of the ceramics flash firing at room temperature is controlled by calculating air pressure variable. Mixing the raw materials, and pressing the mixed materials into a formed green body; heating the formed green body to remove the binder, drying and continuously heating to obtain a ceramic green body; coating silver paste on two ends of the ceramic green body to reduce the contact resistance with a circuit, and baking and curing the silver paste to obtain a cured ceramic green body; placing the solidified ceramic blank in a vacuum chamber, and connecting a high-voltage alternating-current power supply by a copper wire to prepare flash firing; in the flash firing process, a corundum plate is placed between the solidified ceramic blank and the bottom of the vacuum chamber for insulation, and a high-voltage resistor is connected with a sample and a power supply in series; the local dielectric breakdown of the room-temperature flash firing of the ceramic material is adjusted by adjusting the firing pressure value, voltage and current. And, have following 2 advantages: (1) increasing the relative density of the ceramic sample; (2) the effect of reducing the average grain size of the ceramic samples.

Description

Method for sintering ceramic at room temperature
Technical Field
The disclosure belongs to the field of ceramic material preparation, and particularly relates to a room temperature sintering method for ceramics.
Background
The prior common ceramic materials are all prepared by using a traditional powder metallurgy method, relate to pretreatment forming, sintering and post-sintering treatment of raw material powder, and are widely applied to the industrial field. In the flash sintering process, a strong electric field and a large current flow through a ceramic sample, compared with other sintering technologies, the densification time of the ceramic is shorter, and the furnace temperature of the flash sintering is lower. To date, flash sintering has been used successfully to produce a variety of materials, including ionic conductors, semiconductors, and insulators
However, the above methods cannot avoid too long heat preservation time, which results in too large ceramic grains, making the preparation of the nano ceramic material very difficult.
Disclosure of Invention
The present disclosure provides a method for sintering a ceramic at room temperature, in which Yttrium Stabilized Zirconia (YSZ) is a widely used material in a flash sintering research, and local dielectric breakdown of the flash sintering at room temperature of the ceramic is controlled by calculating a gas pressure variable, so as to achieve the effects of increasing the relative density of a ceramic sample and reducing the average grain size of the ceramic sample.
In order to achieve the above object, according to an aspect of the present disclosure, there is provided a method of sintering a ceramic at room temperature, the method including the steps of:
step 1, mixing raw materials, and pressing the mixed materials into a formed green body;
step 2, heating the formed green body to remove the binder, drying and continuously heating to obtain a ceramic green body;
step 3, coating silver paste on two ends of the ceramic green body to reduce the contact resistance with a circuit, and baking and curing the silver paste to obtain a cured ceramic green body;
step 4, placing the solidified ceramic blank in a vacuum chamber, and connecting a high-voltage alternating-current power supply by a copper wire to prepare flash firing;
step 5, in the flash-firing process, placing a corundum plate between the solidified ceramic blank and the bottom of the vacuum chamber for insulation, and connecting a high-voltage resistor, a sample and a power supply in series;
and 6, adjusting local dielectric breakdown of the ceramic material subjected to room-temperature flash firing by adjusting the firing air pressure value, voltage and current.
Further, in step 1, the method of mixing the raw materials and pressing the mixed material into a shaped green body is to mix the average particles>Mixing the 99% diameter high-purity 3YSZ yttrium-stabilized zirconia powder with adhesive by a spraying machine, granulating, and averaging the particle diameter>99% high purity 8YSZ yttrium stabilized zirconia powder is mixed with 5 wt% (5 wt%) PVA binder added to 10 wt% binder in deionized water and mixedThe mixture was uniaxially pressed at a pressure of 540MPA and a length of 14.40mm and a cross-sectional area of 5.68mm2To obtain a shaped green body.
Further, in the step 2, the method for heating the formed green body to remove the binder therein, drying and continuously heating to obtain the ceramic green body includes the steps of putting the formed green body generated in the step 1 into a muffle furnace, raising the temperature to 400 ℃ at the heating rate of 2 ℃/min, keeping the temperature for 2 hours, stopping heating for pre-sintering and glue discharging, drying, and continuing to raise the temperature to 900 ℃ at the heating rate of 10 ℃/min for 4 hours to enhance the strength of the green body to obtain the ceramic green body.
Further, in step 3, coating silver paste on both ends of the ceramic green body to reduce contact resistance with the circuit, and baking the cured silver paste to obtain a cured ceramic green body by the following method: in order to reduce the contact resistance between the ceramic green body and a circuit, high-temperature silver paste is coated on two ends of the ceramic green body, and the ceramic green body is baked at 650 ℃ for 10 minutes to be solidified by the silver paste, so that a solidified ceramic green body is obtained.
Further, in step 4, the method for placing the cured ceramic green body in a vacuum chamber and connecting a high-voltage alternating-current power supply with a copper wire to prepare flash firing comprises the following steps: and (4) placing the cured ceramic blank obtained in the step (3) in a vacuum chamber, connecting the cured ceramic blank to a high-voltage alternating-current test power supply by using a copper wire with the radius of 0.5mm, and preparing to carry out a flash-firing experiment by using two new copper wires.
Further, in step 5, in the flash firing process, a corundum plate is placed between the solidified ceramic blank and the bottom of the vacuum chamber for insulation, and the method for connecting the high-voltage resistor in series with the sample and the power supply comprises the following steps: after step 4, a sapphire plate is placed between the cured ceramic green body and the bottom of the vacuum chamber for insulation, and a high voltage resistor with a total resistance of 6k Ω or 10k Ω or 16k Ω is connected in series with the sample and the power supply to limit the flash sintering current.
Further, in step 6, the method for adjusting the local dielectric breakdown of the room temperature flash firing of the ceramic material by adjusting the firing pressure, voltage and current is as follows: and 5, connecting a circuit, adjusting the air pressure in the vacuum chamber to a preset value of 20.265kPa by using an air pump, then turning on a power supply to start a flash experiment, controlling the initial output voltage of the alternating current power supply to be about 0.9kV, raising the output voltage until the current exceeds 50mA, starting the local dielectric breakdown of flash, controlling the average voltage raising rate to be below 0.4kV/s, keeping the flash current for 30s, then reducing the output of the power supply to be off, and recording the output voltage and the current.
Further, in step 7, the functional relationship between the firing pressure value variable and the flash firing voltage amount is calculated, and the pressure during flash firing in step 6 is adjusted, so as to adjust the local dielectric breakdown of the ceramic material in the room-temperature flash firing process, and thus, the room-temperature flash firing of the zirconia ceramic is realized.
Further, in step 8, the crystal structure of the zirconia ceramic is detected by the specific method: the crystal structure of the zirconia ceramic sample produced in step 7 was examined by an X-ray diffractometer (XRD, D8-Advance, Bruker) and a scanning electron microscope (SU8010, Hitachi), high-resolution microstructural images of the surface and the fracture of the sample were obtained, and the luminescence of the zirconia ceramic sample during flash sintering was recorded by a high-speed camera (Phantom V2012).
The beneficial effect of this disclosure does: the invention provides a ceramic room temperature sintering method, which is characterized in that local dielectric breakdown of ceramic room temperature flash firing is controlled by calculating air pressure variables, and the method has the following 2 advantages: (1) increasing the relative density of the ceramic sample; (2) the effect of reducing the average grain size of the ceramic samples.
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The foregoing and other features of the present disclosure will become more apparent from the detailed description of the embodiments shown in conjunction with the drawings in which like reference characters designate the same or similar elements throughout the several views, and it is apparent that the drawings in the following description are merely some examples of the present disclosure and that other drawings may be derived therefrom by those skilled in the art without the benefit of any inventive faculty, and in which:
FIG. 1 is a flow chart of a method for sintering ceramics at room temperature;
FIG. 2 is a diagram illustrating a snapshot capture of a flash process;
FIG. 3 is a crystal structure detection chart of a zirconia ceramic sample.
Detailed Description
The conception, specific structure and technical effects of the present disclosure will be clearly and completely described below in conjunction with the embodiments and the accompanying drawings to fully understand the objects, aspects and effects of the present disclosure. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.
Referring to fig. 1, a flow chart of a method for sintering a ceramic at room temperature according to the present disclosure is shown, and a method for sintering a ceramic at room temperature according to an embodiment of the present disclosure is described below with reference to fig. 1.
The invention provides a room-temperature sintering method of ceramic, which specifically comprises the following steps:
step 1, mixing raw materials, and pressing the mixed materials into a formed green body;
step 2, heating the formed green body to remove the binder, drying and continuously heating to obtain a ceramic green body;
step 3, coating silver paste on two ends of the ceramic green body to reduce the contact resistance with a circuit, and baking and curing the silver paste to obtain a cured ceramic green body;
step 4, placing the solidified ceramic blank in a vacuum chamber, and connecting a high-voltage alternating-current power supply by a copper wire to prepare flash firing;
step 5, in the flash firing process, placing a corundum plate between the solidified ceramic blank and the bottom of the vacuum chamber for insulation, and connecting a high-voltage resistor with the sample and a power supply in series;
and 6, adjusting local dielectric breakdown of the ceramic material subjected to room-temperature flash firing by adjusting the firing air pressure value, voltage and current.
Further, in step 1, the method of mixing the raw materials and pressing the mixed material into a shaped green body is to mix the average particles>Mixing the 99% diameter high-purity 3YSZ yttrium-stabilized zirconia powder with adhesive by a spraying machine, granulating, and averaging the particle diameter>99% high purity 8YSZ yttrium stabilized zirconia powder bonded at 5 wt% PVAAdding 10 wt% binder generated in deionized water into the binder, mixing, and pressing the mixture under 540MPA pressure to obtain a cross-sectional area of 5.68mm with a length of 14.40mm2To obtain a shaped green body.
Further, in the step 2, the method for heating the formed green body to remove the binder therein, drying and continuously heating to obtain the ceramic green body includes the steps of putting the formed green body generated in the step 1 into a muffle furnace, raising the temperature to 400 ℃ at the heating rate of 2 ℃/min, keeping the temperature for 2 hours, stopping heating for pre-sintering and glue discharging, drying, and continuing to raise the temperature to 900 ℃ at the heating rate of 10 ℃/min for 4 hours to enhance the strength of the green body to obtain the ceramic green body.
Further, in step 3, coating silver paste on both ends of the ceramic green body to reduce contact resistance with the circuit, and baking the cured silver paste to obtain a cured ceramic green body by the following method: in order to reduce the contact resistance between the ceramic green body and a circuit, high-temperature silver paste is coated on two ends of the ceramic green body, and the ceramic green body is baked at 650 ℃ for 10 minutes to be solidified by the silver paste, so that a solidified ceramic green body is obtained.
Further, in step 4, the method for placing the cured ceramic green body in a vacuum chamber and connecting a high-voltage alternating-current power supply with a copper wire to prepare flash firing comprises the following steps: and (4) placing the cured ceramic blank obtained in the step (3) in a vacuum chamber, connecting the cured ceramic blank to a high-voltage alternating-current test power supply by using a copper wire with the radius of 0.5mm, and preparing to carry out a flash-firing experiment by using two new copper wires.
Further, in step 5, the method of placing a corundum plate between the solidified ceramic blank and the bottom of the vacuum chamber for insulation and connecting the high voltage resistor in series with the sample and the power supply during the flash firing process comprises: after step 4, a sapphire plate is placed between the cured ceramic green body and the bottom of the vacuum chamber for insulation, and a high voltage resistor with a total resistance of 6k Ω or 10k Ω or 16k Ω is connected in series with the sample and the power supply to limit the flash sintering current.
Further, in step 6, the method for adjusting the local dielectric breakdown of the room-temperature flash-firing of the ceramic material by adjusting the firing pressure, voltage and current comprises: and 5, connecting a circuit, adjusting the air pressure in the vacuum chamber to a preset value of 20.265kPa by using an air pump, then turning on a power supply to start a flash experiment, controlling the initial output voltage of the alternating current power supply to be about 0.9kV, raising the output voltage until the current exceeds 50mA, starting the local dielectric breakdown of flash, controlling the average voltage raising rate to be below 0.4kV/s, keeping the flash current for 30s, then reducing the output of the power supply to be off, and recording the output voltage and the current.
Further, in step 7, the pressure during the flash firing in step 6 is adjusted by calculating a functional relationship between a firing pressure value variable and a flash firing voltage amount, so as to adjust the local dielectric breakdown of the ceramic material in the flash firing at room temperature and achieve the flash firing at room temperature of the zirconia ceramic, and the specific method is as follows:
step 7.1, setting the cumulative time second number of the solidified ceramic blank entering the vacuum chamber as a variable i (i is a positive integer from 0 to n, n belongs to (0, infinity), and each second is taken as one time), setting the indoor air pressure variable as t and the voltage variable as v, and acquiring the air pressure value t corresponding to the time i in the chamber by using an air pressure gaugeiObtaining a voltage value v corresponding to the moment i in the flash furnacei
Step 7.2, recording t acquired in each acquisitioniAnd each time v that should be collectediSeparately find all tiAverage value of (2)
Figure BDA0003064640260000051
And all viAverage value of (2)
Figure BDA0003064640260000052
Step 7.3, calculate each i moment tiReal-time deviation t from tiWhen the natural number e is used as the base number, the air pressure deviation degree at each time i is t1`=ln(t1-t`),t2`=ln(t2-t`),…,tn-1`=ln(tn-1-t`),tn`=ln(tn-t'); and calculates the time v of each iiAnd the real-time deviation v of v' from viWhen the natural number e is used as the base number, the voltage deviation degree at each time i is v1`=ln(v1-v`),v2`=ln(v2-v`),…,vn-1`=ln(vn-1-v`),vn`=ln(vn-v'); from ti' and viCalculating the product of the air pressure deviation and the voltage deviation at each time ii' indicating that the common deviation degree of the air pressure and the voltage at each time i is a1`=ln(t1-t`)*ln(v1-v`),a2`=ln(t2-t`)*ln(v2-v`),…,an-1`=ln(tn-1-t`)*ln(vn-1-v`),an`=ln(tn-t`)*ln(vn-v'); to measure the variance deviation of the voltage population, the total deviation degree of the voltage value is calculated
Figure BDA0003064640260000053
7.4, calculating the common deviation between the air pressure and the voltage at all the moments i and dividing the common deviation by the total deviation lambda of the air pressure value to obtain the correlation between the air pressure and the voltage in the flash furnace
Figure BDA0003064640260000054
The average of the pressure distribution deviates from the weighted mathematical expected value of the pressure variable by
Figure BDA0003064640260000055
Setting the firing function to T (v)i) Setting the voltage value at the moment of flash-over as the optimal voltage vexpectedOr taking the optimum voltage vexpectedIn the range of 40-80kV, according to the optimum voltage v required for real-time firingexpectedCalculating to obtain the expected air pressure value t which is to be regulated and controlled in the flash furnace at the momentexpectedThen can obtain
Figure BDA0003064640260000061
Figure BDA0003064640260000062
texpected=T(vexpected) The air pressure variable and the voltage variable are functionally related, i.e. by applying a function T (v)i) Input required voltage value can be calculatedCalculating the corresponding expected regulation air pressure value texpected
Step 7.5, monitoring real-time air pressure t through the barometeriWhen t isi>texpectedWhen the pressure is reduced to the desired pressure value, ti≤texpectedAnd closing the vacuum air pump to control the firing process to realize the room-temperature flash firing of the zirconia ceramic.
Further, in step 8, the crystal structure of the zirconia ceramic is detected by the specific method: the crystal structure of the zirconia ceramic sample generated in the step 7 was detected by an X-ray diffractometer (XRD, D8-Advance, Bruker) and a scanning electron microscope (SU8010, Hitachi), a high-resolution microstructure image of the surface and fracture of the sample was obtained, and the luminescence of the zirconia ceramic sample during flash sintering was recorded by a high-speed camera (Phantom V2012). As shown in fig. 2: wherein, (a) is a snapshot of 0.000s after flash burn occurs, (b) is a snapshot of 0.312s after flash burn occurs, (c) is a snapshot of 0.503s after flash burn occurs, and (d) is a snapshot captured at 0.724s after flash burn occurs. As shown in fig. 3: wherein, on the flash sintered zirconia ceramic sample, no porosity was observed in (d) because the 8YSZ sample was not fully densified, whereas no porosity was observed in (b) due to a relative density of 98.58%; according to figures (a) and (b), the average grain size of 3YSZ increased from about 200nm to 3um after pre-sintering and flash-sintering; graphs (c) and (d) show that flash-sintered 8YSZ has an average grain size of about 1um and 8YSZ grains have an average grain size of about 50nm, i.e., both 3YSZ and 8YSZ grains grow moderately around 20 times larger, which means that flash sintering at room temperature can limit the grain growth of YSZ ceramics.
While the present disclosure has been described in considerable detail and with particular reference to a few illustrative embodiments thereof, it is not intended to be limited to any such details or embodiments or any particular embodiments, but it is to be construed as effectively covering the intended scope of the disclosure by providing a broad, potential interpretation of such claims in view of the prior art with reference to the appended claims. Furthermore, the foregoing describes the disclosure in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the disclosure, not presently foreseen, may nonetheless represent equivalent modifications thereto.

Claims (5)

1. A method for sintering ceramics at room temperature, which is characterized by comprising the following steps:
step 1, mixing raw materials, and pressing the mixed materials into a formed green body;
step 2, heating the formed green body to remove the adhesive, drying and continuously heating to obtain a ceramic green body;
step 3, coating silver paste on two ends of the ceramic green body to reduce the contact resistance with a circuit, and baking and curing the silver paste to obtain a cured ceramic green body;
step 4, placing the solidified ceramic blank in a vacuum chamber, and connecting a high-voltage alternating-current power supply by a copper wire to prepare flash firing;
step 5, in the flash firing process, placing a corundum plate between the solidified ceramic blank and the bottom of the vacuum chamber for insulation, and connecting a high-voltage resistor with the sample and a power supply in series;
step 6, adjusting local dielectric breakdown of the ceramic material subjected to room-temperature flash firing by adjusting the firing air pressure value, voltage and current;
step 7, adjusting the air pressure during the flash firing in the step 6 by calculating the functional relationship between the firing air pressure value variable and the flash firing voltage quantity so as to realize the adjustment of the local dielectric breakdown of the ceramic material in the flash firing at the room temperature and realize the flash firing at the room temperature of the zirconia ceramic;
in step 2, the method for heating the formed green body to remove the binder, drying and continuously heating to obtain the ceramic green body comprises the following steps: putting the molded green body generated in the step 1 into a muffle furnace, raising the temperature to 400 ℃ at a heating rate of 2 ℃/min, preserving the heat for 2 hours, stopping heating, pre-sintering and removing glue, and after drying, raising the temperature to 900 ℃ at a heating rate of 10 ℃/min for 4 hours to enhance the strength of the green body to obtain a ceramic green body;
in step 7, the air pressure during the flash firing in step 6 is adjusted by calculating a functional relationship between a firing air pressure value variable and a flash firing voltage value, so as to adjust the local dielectric breakdown of the ceramic material in the room-temperature flash firing process and realize the room-temperature flash firing of the zirconia ceramic, and the method specifically comprises the following steps:
step 7.1, setting the cumulative time seconds of the solidified ceramic blank entering the vacuum chamber as a variable i, setting the indoor air pressure variable as t and the voltage variable as v, and acquiring the indoor air pressure value t corresponding to the moment i by using an air pressure gaugeiObtaining a voltage value v corresponding to the moment i in the flash furnaceiI is a positive integer from 0 to n, n ∈ (0, ∞), as one time per second;
step 7.2, recording t acquired in each acquisitioniAnd each time v that should be collectediSeparately find all tiAverage value of (2)
Figure 579346DEST_PATH_IMAGE001
And all viAverage value of (2)
Figure 531122DEST_PATH_IMAGE002
Step 7.3, calculate each i moment tiReal-time deviation t from tiWhen the natural number e is used as the base number, the air pressure deviation degree at each time i is
Figure 534850DEST_PATH_IMAGE003
And calculates the time v of each iiAnd the real-time deviation v of v' from vi`With the natural number e as the base number, the voltage deviation at each time i is
Figure 382720DEST_PATH_IMAGE004
From ti' and viCalculating the product of the air pressure deviation and the voltage deviation at each time ii"indicating that the common deviation degree of the air pressure and the voltage at each time i is
Figure 635847DEST_PATH_IMAGE005
Figure 594576DEST_PATH_IMAGE006
To measure the variance deviation of the voltage population, the total deviation degree of the voltage value is calculated
Figure 482504DEST_PATH_IMAGE007
7.4, calculating the common deviation between the air pressure and the voltage at all the moments i and dividing the common deviation by the total deviation lambda of the air pressure value to obtain the correlation between the air pressure and the voltage in the flash furnace
Figure 298013DEST_PATH_IMAGE008
The average of the pressure distribution deviates from the weighted mathematical expected value of the pressure variable by
Figure 241699DEST_PATH_IMAGE009
Setting the firing function to T (v)i) Setting the voltage value at the moment of flash-over as the optimal voltage vexpectedOr taking the optimum voltage vexpectedIn the range of 40-80kV, according to the optimum voltage v required for real-time firingexpectedCalculating to obtain the expected pressure value t to be regulated and controlled in the flash furnace at the momentexpected
Then obtain
Figure 4118DEST_PATH_IMAGE010
,
Figure 716859DEST_PATH_IMAGE011
The air pressure variable and the voltage variable are in functional relation, i.e. by applying a function T (v)i) Inputting the required voltage value to calculate the corresponding regulation expected air pressure value texpected
Step 7.5, byBarometer monitoring real-time barometric pressure tiWhen t isi>texpectedWhen the pressure is reduced to the desired pressure value, ti≤texpectedAnd closing the vacuum air pump to control the firing process to realize the room-temperature flash firing of the zirconia ceramic.
2. The method for sintering ceramics at room temperature according to claim 1, wherein in step 4, the method for preparing flash firing by connecting a high voltage ac power source with a copper wire by placing the cured ceramic green body in a vacuum chamber comprises: and (4) placing the cured ceramic blank obtained in the step (3) in a vacuum chamber, connecting the cured ceramic blank to a high-voltage alternating-current test power supply by using a copper wire with the radius of 0.5mm, and preparing to carry out a flash-firing experiment by using two new copper wires.
3. A method for sintering ceramics at room temperature according to claim 1, wherein in step 5, a corundum plate is placed between the solidified ceramic green body and the bottom of the vacuum chamber for insulation during flash firing, and the method for connecting the high voltage resistor in series with the sample and the power supply comprises: after step 4, a corundum plate is placed between the cured ceramic blank and the bottom of the vacuum chamber for insulation, and a high-voltage resistor with total resistance of 6k omega, 10k omega or 16k omega is connected in series with the sample and the power supply to limit the flash sintering current.
4. The method for sintering ceramics at room temperature according to claim 1, wherein in step 6, the method for adjusting the local dielectric breakdown of the room-temperature flash-firing of the ceramic material by adjusting the firing pressure, voltage and current comprises: and 5, connecting a circuit, adjusting the air pressure in the vacuum chamber to a preset value of 20.265kPa by using an air pump, then turning on a power supply to start a flash experiment, controlling the initial output voltage of the alternating current power supply to be 0.9kV, increasing the output voltage until the current exceeds 50mA, starting the local dielectric breakdown of flash, controlling the average voltage increase rate to be below 0.4kV/s, keeping the flash current for 30s, then reducing the output of the power supply to be off, and recording the output voltage and the current.
5. The method for sintering the ceramic at room temperature according to claim 1, further comprising, in step 8, detecting the crystal structure of the zirconia ceramic by: and (3) detecting the crystal structure of the zirconia ceramic sample generated in the step (7) by using an X-ray diffractometer and a scanning electron microscope to obtain high-resolution microstructure images of the surface and the fracture of the sample, and recording the luminescence of the zirconia ceramic sample in the flash sintering process by using a high-speed camera.
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