A two-step co-firing method for preparing a silicon carbide ceramic support and a film layer

Abstract

The application discloses a two-step co-firing method for preparing a silicon carbide ceramic support and a film layer. Firstly, a small-particle-size silicon carbide film layer is coated on the surface of a green silicon carbide support body, and the film thickness is controlled and the film layer defects are reduced by adjusting the spraying time and spraying distance. Then, the film layer and the support body are co-fired in one step by adopting a two-step sintering process, and a silicon carbide ceramic microfiltration membrane with excellent performance is prepared. Compared with a step-by-step sintering method, the heat preservation time is shortened by half, the energy consumption for preparing the silicon carbide film is greatly reduced, and a new way is provided for the low-cost preparation of the ceramic membrane.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of porous ceramic membrane material preparation, and relates to a two-step co-firing preparation method of a silicon carbide ceramic support body and a membrane layer. BACKGROUND

[0002] SiC membrane is a new type of inorganic membrane material, which has excellent mechanical strength, chemical stability, thermal shock resistance and hydrophilicity. In recent years, it has received more and more attention from the academic and industrial circles. Compared with polymers (such as PVDF) and inorganic materials (such as Al2O3 and ZrO2), SiC membrane has higher efficiency in oil-water emulsion separation. However, most of the commercial SiC ceramic membranes are prepared by high-temperature (>2000 °C) recrystallization process in an inert atmosphere, and its wide application is hindered by high manufacturing cost. Therefore, reducing the sintering energy consumption has become a key problem in the field of ceramic membranes.

[0003] In order to reduce the preparation cost of inorganic ceramic membranes, in addition to the method of adding sintering aids to reduce the temperature, the co-firing process is the most promising technology to reduce the cost, which not only can reduce the preparation cost, but also can shorten the preparation period. However, there are still many difficulties in the co-firing process of the membrane layer and the support layer with large difference in particle size, and the differences in thermal expansion rate, sintering shrinkage rate and sintering process of the membrane layer and the support body need to be considered. In the co-firing process, the sintering temperature suitable for the membrane layer is difficult to reach the sintering temperature required by the support body, which leads to too low bending strength of the support body; the sintering temperature suitable for the support body will lead to excessive sintering of the membrane layer, and the two are not matched, which will cause cracking and other defect problems.

[0004] For example, Chinese invention patent CN117820007A reports a method for preparing high-flux silicon carbide ceramic membranes at ultra-low temperature. By adding alkaline liquid-phase sintering additives, both the sintering temperature and the rheological properties of the membrane-forming solution can be adjusted, and high-purity water-permeable asymmetric silicon carbide ceramic membranes can be prepared at a low temperature of 600 °C. This method avoids the thermal stress difference between the membrane layer and the support layer and the temperature mismatch problem. Chinese invention patent CN108911706A reports a method for co-sintering preparation of fly ash ceramic microfiltration membranes. By doping rigid fibers into the fly ash support, the matching problem between the support and the membrane layer during sintering is alleviated, and microfiltration membranes with a pore size of about 100 nm and a membrane thickness of 40-50 µm are prepared at 1050 °C for 2 h. Chinese invention patent CN115572178A reports a method for co-sintering preparation of silicon carbide support and membrane layer. By adjusting the particle size of the support and the membrane layer, the sintering temperature, and the sintering atmosphere, a defect-free membrane layer is prepared, and the support and the membrane layer are sintered in one step, saving a lot of energy. It can be seen that in order to realize the co-sintering preparation of the support and the membrane layer, the existing solutions are to adjust the formula of the support to reduce the sintering temperature of the support or slow down the shrinkage of the support. SUMMARY

[0005] The purpose of the present application is to reduce the energy consumption of the preparation of asymmetric ceramic membranes and to provide a new way for the low-cost preparation of ceramic membranes. The present application proposes to use the optimal sintering temperature of the support and the membrane layer respectively, to optimize the sintering process by segmenting the holding process, to reduce the shrinkage of the membrane layer on the basis of meeting the strength application of the support, and to realize the optimized preparation of asymmetric pure silicon carbide membranes.

[0006] To achieve the above-mentioned purpose, the technical solutions of the present application are as follows:

[0007] (1) uniformly mix silicon carbide powder and NaA molecular sieve sintering aid powder according to a mass ratio, and add a certain proportion of polyvinyl alcohol (PVA) and water to mix uniformly, press into silicon carbide sheet-shaped (Ф30 mm×3 mm) and strip-shaped (50 mm×6 mm×6 mm) green bodies under 8 MPa, and dry to shape;

[0008] (2) disperse the silicon carbide powder in a solution containing polyammonium methacrylate and methyl cellulose, stir thoroughly, adjust the pH of the coating solution to 9-10 by adding ammonia water, and then add a defoaming agent TL-56NQ and stir at low speed for 20 min. Before coating, the solution is vacuum degassed to prevent too many bubbles in the solution causing membrane defects;

[0009] (3) coat the coating solution obtained in step (2) on the surface of the silicon carbide support green body, and dry the prepared pure silicon carbide asymmetric green membrane in an oven;

[0010] (4) The asymmetric green film is placed in a muffle furnace for sintering to perform two-step sintering, and finally cooled to room temperature with the furnace to obtain the prepared silicon carbide ceramic film.

[0011] In step (1), the proportion of the sintering aid in the total mass after mixing with the silicon carbide powder is 5-20 wt%, and the addition amount of PVA is about 5-10 wt% of the total mass.

[0012] In step (2), the solid content of the film-forming solution is 2-10 wt%, and the methyl cellulose content is 0.5-2 wt% of the solid content.

[0013] In step (3), the prepared silicon carbide green film is dried in an oven at 40-80 °C for 8-24 h.

[0014] In step (4), the sintering procedure is a two-step holding procedure of high temperature first and then low temperature, the high temperature uses the best sintering temperature (1000-1100 °C) of the support body, the low temperature uses the best sintering temperature (900-1000 °C) of the film layer, the co-sintering procedure is to control the high-temperature holding time to be (0.5-1 h), the low-temperature holding time to be (1-4 h), the heating rate is controlled at 1-2 °C / min, and the sintering atmosphere is air.

[0015] The asymmetric silicon carbide ceramic film prepared by the present application has a separation layer film thickness of about 10-30 µm and an average pore size of 0.1-0.3 µm.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] 1. Based on the sintering temperature of the film layer and the support layer, the present application realizes one-time sintering of the silicon carbide support body and the film layer by using a two-step co-sintering method, which greatly reduces the preparation process and sintering energy consumption, and provides a new way for low-cost preparation of inorganic ceramic membranes such as silicon carbide.

[0018] 2. By using the two-step co-sintering method, high-temperature calcination for a short time makes the strength of the support body meet the requirements and avoids excessive sintering of the film layer. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 SEM image of the surface of the asymmetric silicon carbide ceramic film prepared in Example 1.

[0020] Figure 2 Pore size distribution graph of the asymmetric silicon carbide ceramic film prepared in Example 1.

[0021] Figure 3 Oil-in-water emulsion separation performance of the asymmetric silicon carbide ceramic film prepared in Example 1. DETAILED DESCRIPTION

[0022] The application will be described in detail below with reference to the accompanying drawings and specific embodiments. The following examples are only used to illustrate the application, but not to limit the scope of the application. Example 1

[0023] A two-step co-firing method for preparing a silicon carbide ceramic support and a membrane layer, the specific steps are as follows:

[0024] (1) First, silicon carbide powder with an average particle size of 5 µm and NaA molecular sieve powder with an average particle size of 3.9 µm are mixed in a ratio of 84:16, and a binder PVA is added, with the amount of PVA being 5% of the total mass.

[0025] (2) The powders are thoroughly ground, 2.5 g of uniformly mixed powders are weighed, and the mixed powders are pressed into sheet and strip-shaped green bodies by a tablet press at 8 MPa, and then placed in a constant temperature environment for drying for 12 h.

[0026] (3) A coating solution of silicon carbide particles with a solid content of 10 wt% is prepared. First, a dispersant poly-methacrylic acid ammonium (1 wt% of the solid content of silicon carbide) is weighed and dispersed in deionized water, then 10 wt% of silicon carbide powder is added, and 2 wt% of MC solution is added, and stirred at 400 r / min for 6 h. Before spraying, add a defoaming agent TL-56NQ and stir at low speed for 30 min.

[0027] (4) A membrane layer is coated on the green sheet-shaped support by spraying, with a spraying time of 5 s each time, and then placed in a 60 °C oven for drying before spraying again, with a total spraying time of 25 s. After drying in the oven, sintering is performed. In an air atmosphere, the temperature is raised from 200 °C to 500 °C at a rate of 2 °C / min, then raised to 1050 °C at a rate of 1 °C / min and held for 1 h, the cooling rate is set to 1 °C / min according to the natural cooling rate of the muffle furnace, and the temperature is lowered to 900 °C and held for 1 h, and finally cooled to room temperature with the furnace.

[0028] The surface microstructure is observed by scanning electron microscopy, and it can be seen from Figure 1 that the surface of the silicon carbide is defect-free and has a porous structure, and there are obvious sintering necks between the particles. The bending strength of the porous silicon carbide ceramic support is tested by a fine ceramic three-point bending method, and the result shows that the strength is ~21.7 MPa. The average pore size of the asymmetric silicon carbide ceramic membrane is about 0.22 µm (as shown in Figure 2 ), the membrane thickness is about 21 µm, and the pure water permeability is about 1920 L·m -1 ·h -1 ·bar -1The rejection rate of 100 ppm oil-in-water emulsion reached 94.3% under the condition of 0.5 bar and 0.5 m / s, and the oil content in the permeate after filtration was only 5.7 mg / L -1 , which met the national emission standard. The permeate liquid after filtration was clear and transparent, and almost no oil droplets were observed, which proved that the silicon carbide membrane had excellent oil-water separation performance. Figure 3 Example 2

[0029] A two-step co-firing method for preparing a silicon carbide ceramic support and a membrane layer, the specific steps are as follows:

[0030] (1) First, the silicon carbide powder with an average particle size of 5 µm and the NaA molecular sieve powder with an average particle size of 3.9 µm are mixed in a ratio of 84:16, and a binder PVA is added, the amount of PVA is 5% of the total mass.

[0031] (2) Grind the powder thoroughly, weigh 2.5 g of uniformly mixed powder, press the mixed powder into a sheet and strip-shaped green body by a tablet press at 8 MPa, and place it in a constant temperature dryer for 12 h.

[0032] (3) Prepare a 10 wt% solid content silicon carbide particle coating solution. First, weigh the dispersant poly-methacrylic acid ammonium (1 wt% of the solid content of silicon carbide) and disperse it in deionized water, then add 10 wt% silicon carbide powder, and then add 2 wt% MC solution, stir thoroughly at 400 r / min for 6 h. Before spraying, add a small amount of defoamer TL-56NQ and stir at low speed for 30 min.

[0033] (4) Coat the membrane layer on the green sheet-shaped support by spraying, spray for 5 s at a time, place it in a 60 °C oven for drying before spraying again, the total spraying time is 25 s, and then sinter after drying in the oven. In an air atmosphere, increase the temperature from 200 °C to 500 °C at a rate of 2 °C / min, then increase the temperature to 1050 °C at a rate of 1 °C / min and keep it for 1 h, set the cooling rate to 1 °C / min according to the natural cooling rate of the muffle furnace, cool to 900 °C and keep it for 0.5 h, and finally cool to room temperature with the furnace.

[0034] The fine ceramic three-point bending method was used to test the bending strength of the porous silicon carbide ceramic support, and the results showed that the strength was ~17.2 MPa. The thickness of the asymmetric silicon carbide ceramic membrane was about 21 µm, and the pure water permeability was about 1650 L·m -1 ·h -1 ·bar -1 . Example 3

[0035] ​A two-step co-firing method for preparing a silicon carbide ceramic support and a film layer, the specific steps are as follows:

[0036] (1) First, silicon carbide powder with an average particle size of 5 µm and NaA molecular sieve powder with an average particle size of 3.9 µm are mixed in a ratio of 84:16, and a binder PVA is added, with the amount of PVA being 5% of the total mass.

[0037] (2) The powders are thoroughly ground, 2.5 g of uniformly mixed powders are weighed, the mixed powders are pressed into sheet and strip-shaped green bodies by a tablet press at 8 MPa, and are placed in a drying oven at room temperature for 12 h.

[0038] (3) A coating solution of silicon carbide particles with a solid content of 10 wt% is prepared. First, a dispersant poly-methacrylic acid ammonium (1 wt% of the solid content of silicon carbide) is weighed and dispersed in deionized water, then 10 wt% of silicon carbide powder is added, followed by the addition of 2 wt% of MC solution, and the mixture is stirred at 400 r / min for 6 h. Before spraying, a defoaming agent TL-56NQ is added and stirred at low speed for 30 min.

[0039] (4) The film layer is coated on the green sheet-shaped support by spraying, with a spraying time of 5 s each time, and the total spraying time is 25 s. After drying in an oven at 60 °C, the sample is sintered. In an air atmosphere, the temperature is raised from 200 °C to 500 °C at a rate of 2 °C / min, then raised to 1050 °C at a rate of 1 °C / min and held for 0.5 h. The cooling rate is set to 1 °C / min according to the natural cooling rate of the muffle furnace, and the temperature is lowered to 900 °C and held for 2 h, and finally cooled to room temperature with the furnace.

[0040] The bending strength of the porous silicon carbide ceramic support is tested by a fine ceramic three-point bending method, and the result shows that the strength is ~15.0 MPa. The thickness of the asymmetric silicon carbide ceramic membrane is about 21 µm, and the pure water permeability is about 2200 L·m -1 ·h -1 ·bar -1 .

Claims

1. A two-step co-firing method for preparing a silicon carbide ceramic support and a film layer, characterized in that, The specific steps are as follows: (1) uniformly mix the silicon carbide powder and the NaA molecular sieve sintering aid powder according to the proportion, add a certain proportion of polyvinyl alcohol and water, mix uniformly, press into a silicon carbide sheet or strip green body under 8 MPa, and dry and shape; (2) disperse the silicon carbide powder in a solution containing polyamine methacrylate and methyl cellulose, stir well, adjust the pH of the coating solution to 9-10 by adding ammonia water dropwise, add defoaming agent TL-56NQ at low speed and stir for 20 min, and vacuum degassing the solution before coating to prevent too many bubbles in the solution causing film defects; (3) coat the coating solution obtained in step (2) on the surface of the silicon carbide support green body prepared in step (1), and dry the prepared silicon carbide asymmetric green film in an oven; (4) place the asymmetric green film in a muffle furnace for sintering, and prepare it by two-step co-sintering process. The sintering program is as follows: in air atmosphere, from room temperature to 200 ℃, then from 200 ℃ to 500 ℃ at a rate of 1-2 °C / min, then from 500 ℃ to 1050 ℃ at a rate of 1 ℃ / min, and keep for 0.5-1 h, according to the natural cooling rate of the muffle furnace, set the cooling rate to 1 ℃ / min, cool to 900 ℃ and keep for 0.5-2 h, and finally cool to room temperature with the furnace, to obtain the prepared asymmetric pure silicon carbide ceramic membrane.

2. The two-step co-firing method of a silicon carbide ceramic support and a film layer according to claim 1, characterized in that, In step (1), the proportion of sintering aid in the total mass after mixing with silicon carbide powder is 5-20 wt%, and the addition amount of PVA is 5-10 wt% of the total mass.

3. The two-step co-firing method of a silicon carbide ceramic support and a film layer according to claim 1, characterized in that, In step (2), the solid content of the coating solution is 2-10 wt%, and the content of methyl cellulose is 0.5-2 wt% of the solid content.

4. The two-step co-firing method of a silicon carbide ceramic support and a film layer according to claim 1, characterized in that, In step (3), the prepared silicon carbide green film is dried in an oven at 40-80 ℃ for 8-24 h.

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