A method for preparing porous ceramics via polymer-derived ceramic route

Abstract

This invention discloses a method for preparing porous ceramics via a polymer-derived ceramic route, belonging to the field of ceramic preparation technology. The method includes: crosslinking of the polymer ceramic precursor: the polymer ceramic precursor undergoes multi-stage heat treatment for crosslinking; preparation of casting slurry: the above polymer ceramic precursor is prepared into a casting slurry; phase inversion molding: the casting slurry is dried to obtain a hollow fiber ceramic membrane precursor; and high-temperature sintering: high-temperature sintering yields a porous ceramic. The porous ceramic preparation method of this application combines polymer-derived ceramic technology with phase inversion molding. It only requires the preparation of the casting solution and the adjustment of spinning process parameters, without the need for other additional conditions, to easily achieve the formation of the fiber membrane. The casting solution prepared with crosslinked polymethyl silsesquioxane or MK solidifies quickly during the phase inversion process, making the preparation process of the hollow fiber ceramic ultrafiltration membrane simpler and more efficient.

Description

Technical Field

[0001] This invention relates to the field of ceramic preparation technology, and in particular to a method for preparing porous ceramics via a polymer-derived ceramic route. Prior Technology

[0002] Ceramic membranes, due to their excellent separation performance and reliable chemical, mechanical, and thermal stability, have been widely used in water treatment, food industry, energy engineering, and biopharmaceutical engineering. Ceramic ultrafiltration membranes with pore sizes ranging from 2 nm to 100 nm are widely used in oily wastewater, heavy metal ion wastewater, and papermaking wastewater. By selectively separating impurities and pollutants in wastewater through the pore size of the ultrafiltration membrane, wastewater purification and resource recovery are achieved.

[0003] Common ceramic membranes typically consist of a support layer with a certain mechanical strength, an intermediate transition layer, and a functional separation layer, exhibiting an asymmetric structure. The preparation of ceramic membranes requires the separate fabrication of the support layer, intermediate layer, and separation layer, each requiring high-temperature firing. Commercially available ceramic microfiltration membranes usually have at least three layers, while ceramic ultrafiltration membranes often have four to five layers, resulting in a thicker overall membrane, lower porosity (<35%), and higher permeation resistance. This leads to a complex fabrication process, longer preparation cycle, and poorer permeation performance. Problems at any step during ceramic membrane fabrication can introduce defects into the prepared ceramic ultrafiltration membrane, causing a decrease in membrane performance and yield.

[0004] Compared to the traditional multi-stage preparation and sintering process of ceramic membranes, the one-step phase transformation molding combined with a single high-temperature sintering technique offers significant advantages. Preparing hollow fiber ceramic membranes with self-supporting asymmetric structures through one-step phase transformation molding and a single high-temperature sintering effectively shortens the cost and cycle time of ceramic membrane production. Hollow fiber ceramic membranes possess advantages such as thin membrane layers, excellent permeate flux, high packing density, and flexible application, with strong controllability in microstructure and performance. Currently, phase transformation is mainly used for the preparation of hollow fiber ceramic microfiltration membranes. Directly preparing hollow fiber ceramic ultrafiltration membranes via phase transformation requires ceramic powders with even smaller particle sizes. However, reducing the particle size of ceramic powder leads to a dramatic increase in specific surface area, resulting in excessively high viscosity of the casting solution, ceramic particle agglomeration, and poor stability of the casting solution, posing difficulties for dry / wet spinning. Another method for preparing hollow fiber ceramic ultrafiltration membranes is to deposit another membrane layer on the hollow fiber ceramic microfiltration membrane support. Common methods include the sol-gel method. However, the overall preparation cost of the membrane is high, the preparation process is complicated, and the membrane layer is prone to cracking after drying, which cannot guarantee the yield and performance of the membrane product.

[0005] In recent years, the polymer-derived ceramics (PDCs) route for preparing porous ceramics has attracted widespread attention due to its advantages such as simple process, controllable micro / nano-scale fine structure, and low-temperature preparation. The PDCs method can prepare various ceramic materials, such as oxides, silicates, nitrides, and carbides. If hollow fiber ceramic ultrafiltration membranes can be prepared via the PDCs route and applied to oily wastewater and heavy metal ion wastewater, it is expected to solve the problems of low porosity, poor permeability, and complex preparation processes of existing ceramic ultrafiltration membranes. Currently, no literature reports the preparation of hollow fiber ceramic ultrafiltration membranes via the PDCs route. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing porous ceramics via a polymer-derived ceramic route, so as to solve the problems mentioned in the background art, and to prepare hollow fiber ceramic ultrafiltration membranes using a polymer-derived ceramic route.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0008] A method for preparing porous ceramics via a polymer-derived ceramic route, the method comprising the following steps:

[0009] Crosslinking of polymer ceramic precursors:

[0010] The polymer ceramic precursor undergoes multi-stage heat treatment crosslinking. The temperature is raised from 20℃ to 80℃ at a rate of 1℃ / min and held for 2 hours, then raised to 140℃ at a rate of 1℃ / min and held for 2 hours, and finally raised to 200℃ at a rate of 1℃ / min and held for 2 hours before natural cooling.

[0011] Preparation of casting slurry:

[0012] When the polymer ceramic precursor used is polymethylsilsesquioxane, polyether ether (PES) is added to the solvent N-methylpyrrolidone (NMP), and the mixture is stirred for 3 hours using a stirrer equipped with a polytetrafluoroethylene (PTFE) impeller to obtain a polymer solution. Pre-crosslinked polymethylsilsesquioxane and inert filler α-Al₂O₃ are added to the polymer solution in small batches and stirred for 2 hours. Then, polyvinylpyrrolidone (PVP) is added and stirred for 1 hour. The resulting casting solution is then degassed under vacuum. Note that when the synthesized ceramic ultrafiltration membrane is mullite, α-Al₂O₃ needs to be added to the casting solution; otherwise, it is not added.

[0013] When the polymer ceramic precursor used is MK, MK is dissolved in a mixed solvent of DMF and anhydrous ethanol and stirred thoroughly. Then, the polymer binder PMMA is added and stirred until complete to obtain the casting solution. Vacuum degassing treatment is performed. When the chemical composition of the synthesized ceramic ultrafiltration membrane is mullite, inert filler α-Al2O3 needs to be added to the casting solution. Otherwise, no filler is added.

[0014] Phase transformation molding:

[0015] When the polymer ceramic precursor used is polymethylsilsesquioxane, the core liquid is tap water, the external coagulation bath is tap water, the air distance is 2 cm, the core liquid flow rate is 48 ml / min, and the nitrogen pressure is 1 bar. The casting solution is extruded from one end of the spinning needle of the spinning device, passes through an air distance, and then enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24 hours, the solvent and non-solvent are fully exchanged. After drying, the hollow fiber ceramic membrane precursor is obtained. When the polymer ceramic precursor used is MK, the external coagulation bath is anhydrous ethanol, the air distance is 6 cm, the core liquid flow rate is 48 ml / min, and the nitrogen pressure is 1 bar. The casting solution is extruded from one end of the spinning needle of the spinning device, passes through an air distance, and then enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24 hours, the solvent and non-solvent are fully exchanged. After drying, the hollow fiber ceramic membrane precursor is obtained.

[0016] High-temperature sintering:

[0017] When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula mullite, a certain length of hollow fiber ceramic membrane precursor is cut and loaded into an alumina tubular membrane tube. Under air atmosphere, the temperature is first raised from room temperature to 950 ℃ at a rate of 2℃ / min and held for 2 h, and then raised to 1400℃~1600℃ at a rate of 5℃ / min and held for 2 h.

[0018] When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula SiO2, the temperature is first raised from room temperature to 800℃ at a rate of 2℃ / min and held for 2 hours in an air atmosphere, and then raised to 1000℃~1200℃ at a rate of 5℃ / min and held for 2 hours.

[0019] When preparing a hollow fiber ceramic ultrafiltration membrane with the chemical formula SiOC, the temperature is raised to 800 ℃ at a rate of 2℃ / min and held for 2h under a nitrogen atmosphere, then raised to 1000 ℃ at a rate of 4℃ / min and held for 5h by introducing oxygen.

[0020] Preferably, when the polymer ceramic precursor used is polymethylsilsesquioxane and the chemical formula of the hollow fiber ceramic ultrafiltration membrane is mullite, wherein 1:4≤PES:NMP≤1:6, 14wt%≤polymethylsilsesquioxane≤25wt%, 26wt%≤α-Al2O3≤35wt%, and 40wt%≤NMP≤50wt%.

[0021] Preferably, when the polymer ceramic precursor used is polymethylsilsesquioxane and the hollow fiber ceramic ultrafiltration membrane has the chemical formula of mullite, the weight percentage of the casting solution is: 40.68 wt% NMP, 9.18 wt% PES, 29.08 wt% α-Al2O3, and 15.81 wt% polymethylsilsesquioxane, wherein the particle size of polymethylsilsesquioxane is ≤5 μm and D50 = 2 μm.

[0022] Preferably, when using the polymer ceramic precursor MK, the solvent is DMF and anhydrous ethanol, the mass ratio of DMF to anhydrous ethanol is 3:2, and 10wt%≤MK≤25wt%.

[0023] Preferably, when using the polymer ceramic precursor MK, the casting solution comprises, by weight percentage: 16 wt% anhydrous ethanol, 24 wt% DMF, 8 wt% PMMA, 34 wt% α-Al2O3 and 16 wt% MK.

[0024] The above technical solution has the following beneficial effects:

[0025] 1. The preparation of hollow fiber ceramic ultrafiltration membranes combines polymer-derived ceramic technology with phase inversion. Only the casting solution needs to be prepared and the spinning process parameters need to be adjusted. No other additional conditions are required to easily achieve the formation of fiber membranes. The casting solution prepared with cross-linked polymethyl silsesquioxane or MK solidifies and forms quickly during the phase inversion process, making the preparation process of hollow fiber ceramic ultrafiltration membranes simpler and more efficient.

[0026] 2. Polymer-derived ceramics technology can obtain the desired ceramic composition by synthesizing precursor polymers with specific chemical structures. Since the ceramics are derived from polymers, this method can achieve high purity and controllable composition of ceramic materials. Oxide ceramic films and non-oxide ceramic films can be prepared by controlling the composition of precursors. However, the preparation of these oxide and non-oxide ceramic films using traditional powder molding requires extremely high firing temperatures and complex processing techniques.

[0027] 3. Ceramic ultrafiltration membranes prepared from nano-ceramic powders have low porosity and the sintered pore structure is uneven and difficult to control. Polymer-derived ceramic technology can utilize gases or other substances generated during polymer decomposition to form a uniform nanoscale pore structure. This gives hollow fiber ceramic membranes uniformly distributed nanoscale pores, high porosity, and high permeation flux. Simple Explanation of the Diagram

[0028] Figure 1 shows the XRD pattern of porous ceramics prepared by a polymer-derived ceramic route according to the present invention. Implementation

[0029] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions of embodiments are for illustrative purposes and do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] The purpose of this invention is to provide two high-performance, low-cost polymer-ceramic precursor materials to overcome the technical problem of difficulty in matching the PDCs route with phase inversion technology, simplify the ceramic ultrafiltration membrane preparation process, and improve the performance of ceramic ultrafiltration membranes. The pore size and porosity of the porous materials prepared by PDCs can be controlled by adjusting the composition of the precursor and process parameters. As an alternative to the powder route for ceramic preparation commonly used in the field of electrostatic chucks, the combination of PDCs and phase inversion molding technology is expected to directly prepare hollow fiber ceramic ultrafiltration membranes with high porosity and uniformly distributed nanoscale pores. This method effectively simplifies the membrane preparation process and reduces the preparation cost.

[0031] Currently available organosilicone polymer precursors include polysiloxanes, polysilanes, polycarbosilanes, polysilazanes, and polyboron silazanes, providing a wide range of choices for porous ceramic applications. Depending on the type of polymer precursor and heat treatment conditions, amorphous or crystalline ceramic membranes can be prepared, with pore sizes ranging from nanometers to micrometers. The challenge in combining phase inversion technology with the PDCs route to prepare hollow fiber ceramic membranes lies in determining a suitable solvent / non-solvent system for highly hydrophobic ceramic precursors. Many organosilicone precursors, due to their inherent chemical stability, cannot be effectively matched with phase inversion technology. Traditional solvents used for phase inversion in the preparation of hollow fiber ceramic membranes, such as NMP, DMF, DMSO, and DMAC, also cannot effectively dissolve polymer ceramic precursors.

[0032] To address the aforementioned technical challenges, this application employs novel polymer precursors of polymethylsilsesquioxane and polymethylhydrosiloxane (MK), and utilizes a dry / wet spinning-phase inversion technique. Using the polymer ceramic precursor and molding method provided by this invention, high-flux, high-porosity (>40%), narrow pore size distribution, uniform pore structure, and high mechanical strength and toughness hollow fiber ceramic ultrafiltration membranes can be effectively prepared. Furthermore, the membrane's microstructure is highly tunable, allowing for the preparation of high-performance ceramic ultrafiltration membranes for specific applications. This effectively solves the technical problems of low porosity and permeate flux, and complex preparation processes, inherent in hollow fiber ceramic ultrafiltration membranes prepared from ceramic powder.

[0033] The present invention provides two high-performance polymer ceramic precursors, polymethylsilsesquioxane or polymethylhydrosiloxane (the molecular formulas of polymethylsilsesquioxane and polymethylhydrosiloxane are [Si(CH3)2O]n and [CH3)(H)SiO]n, respectively, and the ceramic yields of the two precursors are 72% and 83%, respectively;

[0034] A method for preparing porous ceramics via a polymer-derived ceramic route includes the following steps:

[0035] Crosslinking of polymer ceramic precursors:

[0036] The polymer ceramic precursor undergoes multi-stage heat treatment crosslinking. This multi-stage heat treatment crosslinking is performed as follows: the temperature is raised from 20°C to 80°C at a rate of 1°C / min and held for 2 hours; then raised to 140°C at a rate of 1°C / min and held for 2 hours; finally, the temperature is raised to 200°C at a rate of 1°C / min and held for 2 hours, followed by natural cooling. This gradual increase in temperature ensures the full progress of the crosslinking reaction and the stability of the material structure.

[0037] Preparation of casting slurry:

[0038] When the polymer ceramic precursor used is polymethylsilsesquioxane, polyether ether (PES) is added to the solvent N-methylpyrrolidone (NMP), and stirred for 3 hours using a stirrer equipped with a polytetrafluoroethylene (PTFE) impeller to obtain a polymer solution. The pre-crosslinked polymethylsilsesquioxane and inert filler α-Al₂O₃, prepared in the previous step, are added to the polymer solution in small batches and stirred for 2 hours. Then, polyvinylpyrrolidone (PVP) is added and stirred for 1 hour. The resulting casting solution is then degassed under vacuum. Note that when the synthesized ceramic ultrafiltration membrane is mullite, α-Al₂O₃ needs to be added to the casting solution; when the synthesized ceramic ultrafiltration membrane is SiO₂ or SiOC, α-Al₂O₃ is not added.

[0039] When the polymer ceramic precursor used is polymethylsilsesquioxane, and the final hollow fiber ceramic ultrafiltration membrane to be prepared has the chemical formula of mullite, α-Al2O3 needs to be added during the preparation of the casting solution. At this time, the quality ratio of PES to NMP satisfies: 1:4≤PES:NMP≤1:6, preferably PES:NMP=1:5; the weight percentage of polymethylsilsesquioxane satisfies: 14wt%≤polymethylsilsesquioxane≤25wt%; the weight percentage of added α-Al2O3 satisfies: 26wt%≤α-Al2O3≤35wt%; and the weight percentage of added NMP satisfies: 40wt%≤NMP≤50wt%.

[0040] Specifically, when the polymer ceramic precursor used is polymethylsilsesquioxane and the hollow fiber ceramic ultrafiltration membrane has the chemical formula mullite, the preferred weight percentage of the casting solution is: 41 wt% NMP, 8.2 wt% PES, 29 wt% α-Al₂O₃, and 16 wt% polymethylsilsesquioxane, wherein the size of the polymethylsilsesquioxane satisfies: particle size ≤ 5 μm, D50 = 2 μm;

[0041] In other embodiments of this example, when the polymer ceramic precursor used is polymethylsilsesquioxane and the final hollow fiber ceramic ultrafiltration membrane to be prepared has the chemical formula of mullite, α-Al2O3 needs to be added during the preparation of the casting solution. At this time, the mass ratio of PES to NMP satisfies: PES:NMP=1:4, the weight percentage of polymethylsilsesquioxane is 14wt%, the weight percentage of added α-Al2O3 is 26wt%, and the weight percentage of added NMP is 40wt%.

[0042] In other embodiments of this example, when the polymer ceramic precursor used is polymethylsilsesquioxane and the final hollow fiber ceramic ultrafiltration membrane to be prepared has the chemical formula of mullite, α-Al2O3 needs to be added during the preparation of the casting solution. At this time, the mass ratio of PES to NMP is 1:6, the weight percentage of polymethylsilsesquioxane is 25wt%, the weight percentage of added α-Al2O3 is 35wt%, and the weight percentage of added NMP is 50wt%.

[0043] When the polymer ceramic precursor used is polymethylsiloxane (MK), MK is dissolved in a mixed solvent of DMF (dimethylformamide) and anhydrous ethanol and stirred thoroughly. Then, the polymer binder PMMA (polymethyl methacrylate) is added and stirred until complete to obtain the casting solution. Vacuum degassing treatment is then performed. When the chemical composition of the synthesized ceramic ultrafiltration membrane is mullite, inert filler α-Al2O3 needs to be added to the casting solution. When the synthesized ceramic ultrafiltration membrane is SiO2 or SiOC, α-Al2O3 is not added.

[0044] When using MK as the polymer ceramic precursor, the solvents are DMF and anhydrous ethanol, wherein the mass ratio of DMF to anhydrous ethanol is 3:2, and the weight percentage of MK satisfies: 10wt%≤MK≤25wt%.

[0045] When using the polymer ceramic precursor MK, the specific weight percentages of the casting solution are: 16 wt% anhydrous ethanol, 24 wt% DMF, 8 wt% PMMA, 34 wt% α-Al2O3, and 16 wt% MK;

[0046] In other embodiments of this example, when the polymer ceramic precursor is MK, the solvent is DMF and anhydrous ethanol, wherein the mass ratio of DMF to anhydrous ethanol is 3:2, and the weight percentage of MK is 10wt% or 25wt%.

[0047] Phase transformation molding:

[0048] When the polymer ceramic precursor used is polymethylsilsesquioxane, the core liquid is tap water, the external coagulation bath is tap water, the air distance is 2 cm, the core liquid flow rate is 48 ml / min, and the nitrogen pressure is 1 bar. The casting solution is extruded from one end of the spinning needle of the spinning device, passes through an air distance, and then enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24 hours, the solvent and non-solvent are fully exchanged. After drying, the hollow fiber ceramic membrane precursor is obtained.

[0049] When the polymer ceramic precursor used is MK, the external coagulation bath is anhydrous ethanol, the air distance is 6 cm, the core liquid flow rate is 48 ml / min, and the nitrogen pressure is 1 bar. The casting solution is extruded from one end of the spinning needle of the spinning device, passes through an air distance, and then enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24 h, the solvent and non-solvent are fully exchanged. After drying, the hollow fiber ceramic membrane precursor is obtained.

[0050] High-temperature sintering:

[0051] When preparing a hollow fiber ceramic ultrafiltration membrane with the chemical formula mullite, a certain length of the hollow fiber ceramic membrane precursor is cut and loaded into an alumina tubular membrane tube. Under air atmosphere, the temperature is first raised from room temperature to 950 °C at a rate of 2 °C / min and held for 2 hours. Then, the temperature is raised from 950 °C to 1400 °C at a rate of 5 °C / min and held for 2 hours to complete the high-temperature sintering of the hollow fiber ceramic ultrafiltration membrane. In other embodiments of this example, after raising the temperature from room temperature to 950 °C, the temperature is then raised from 950 °C to 1600 °C at a rate of 5 °C / min and held for 2 hours to complete the high-temperature sintering of the hollow fiber ceramic ultrafiltration membrane.

[0052] When preparing a hollow fiber ceramic ultrafiltration membrane with the chemical formula SiO2, the temperature is first raised from room temperature to 800°C at a rate of 2°C / min and held for 2 hours in an air atmosphere. Then, the temperature is raised from 800°C to 1000°C at a rate of 5°C / min and held for 2 hours to complete the high-temperature sintering of the hollow fiber ceramic ultrafiltration membrane. In other embodiments of this example, after raising the temperature to 800°C at a rate of 2°C / min, the temperature is then raised from 800°C to 1200°C at a rate of 5°C / min and held for 2 hours to complete the high-temperature sintering of the hollow fiber ceramic ultrafiltration membrane.

[0053] When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula SiOC, the temperature is raised from room temperature to 800℃ at a rate of 2℃ / min under a nitrogen atmosphere and held for 2 hours. Then, the temperature is raised from 800℃ to 1000℃ at a rate of 4℃ / min, and oxygen is introduced and held for 5 hours to complete the high-temperature sintering of the hollow fiber ceramic ultrafiltration membrane.

[0054] Referring to Figure 1, the XRD pattern of porous ceramics prepared by the polymer-derived ceramic route of this application is shown. The XRD pattern of porous ceramics usually contains multiple peaks, which correspond to different grains in the ceramic material. XRD (X-ray diffraction) is a technique used to analyze the crystal structure of materials. It determines the crystal structure and grain size of the material by measuring the diffraction pattern after X-rays pass through the material.

[0055] Compared with existing methods, the method for preparing porous ceramics using the above-described technical solution has the following advantages:

[0056] 1. The preparation of hollow fiber ceramic ultrafiltration membranes combines polymer-derived ceramic technology with phase inversion. Only the casting solution needs to be prepared and the spinning process parameters need to be adjusted. No other additional conditions are required to easily achieve the formation of fiber membranes. The casting solution prepared with cross-linked polymethyl silsesquioxane or MK solidifies and forms quickly during the phase inversion process, making the preparation process of hollow fiber ceramic ultrafiltration membranes simpler and more efficient.

[0057] 2. Polymer-derived ceramics technology can obtain the desired ceramic composition by synthesizing precursor polymers with specific chemical structures. Since the ceramics are derived from polymers, this method can achieve high purity and controllable composition of ceramic materials. Oxide ceramic films and non-oxide ceramic films can be prepared by controlling the composition of precursors. However, the preparation of these oxide and non-oxide ceramic films using traditional powder molding requires extremely high firing temperatures and complex processing techniques.

[0058] 3. Ceramic ultrafiltration membranes prepared from nano-ceramic powders have low porosity and the sintered pore structure is uneven and difficult to control. Polymer-derived ceramic technology can utilize gases or other substances generated during polymer decomposition to form a uniform nanoscale pore structure. This gives hollow fiber ceramic membranes uniformly distributed nanoscale pores, high porosity, and high permeation flux.

[0059] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing porous ceramics via a polymer-derived ceramic route, characterized in that the method comprises the following steps: Crosslinking of the polymer ceramic precursor: The polymer ceramic precursor is polymethylsilsesquioxane or polymethylhydrosiloxane (MK), which undergoes multi-stage heat treatment crosslinking. The temperature is raised from 20°C to 80°C at a rate of 1°C / min and held for 2 hours, then raised to 140°C at a rate of 1°C / min and held for 2 hours, and finally raised to 200°C at a rate of 1°C / min and held for 2 hours, followed by natural cooling; Preparation of the casting slurry: When the polymer ceramic precursor used is polymethyl silsesquioxane, polyether ether (PES) is added to the solvent N-methylpyrrolidone (NMP), and stirred for 3 hours using a mixer equipped with a polytetrafluoroethylene (PTFE) impeller to obtain a polymer solution. Pre-crosslinked polymethyl silsesquioxane and inert filler α-Al₂O₃ are added to the polymer solution in small batches, stirring for 2 hours each time. Then, polyvinylpyrrolidone (PVP) is added and stirred for 1 hour. The resulting casting solution is then degassed under vacuum. When the synthesized ceramic ultrafiltration membrane is mullite, α-Al₂O₃ needs to be added to the casting solution; otherwise, it is not added. Specifically, when the hollow fiber ceramic ultrafiltration membrane has the chemical formula mullite, the following ratios apply: 1:4 ≤ PES:NMP ≤ 1:6, 14wt% ≤ polymethylsilsesquioxane ≤ 25wt%, 26wt% ≤ α-Al₂O₃ ≤ 35wt%, 40wt% ≤ NMP ≤ 50wt%. When the polymer ceramic precursor used is polymethylhydrosiloxane (MK), MK is dissolved in a mixed solvent of dimethylformamide (DMF) and anhydrous ethanol and stirred thoroughly. Then, polymethyl methacrylate (PMMA) is added and stirred completely to obtain the casting solution. Vacuum degassing is then performed. When the synthesized ceramic ultrafiltration membrane has the chemical composition mullite, inert filler α-Al₂O₃ needs to be added to the casting solution; otherwise, it is not added. Specifically, dimethylformamide (DMF) The mass ratio of DMF and anhydrous ethanol is 3:2, 10wt%≤MK≤25wt%; Phase inversion molding: When the polymer ceramic precursor used is polymethylsilsesquioxane, the core liquid is tap water, the external coagulation bath is tap water, the air distance is 2cm, the core liquid flow rate is 48ml / min, and the nitrogen pressure is 1bar. The casting solution is extruded from one end of the spinning needle of the spinning device, passes through an air distance, and then enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24h, the solvent N-methylpyrrolidone (NMP) and the non-solvent tap water are fully exchanged. After drying, the hollow fiber ceramic membrane precursor is obtained. When the polymer ceramic precursor used is MK, the external coagulation bath is anhydrous ethanol, the air distance is 6 cm, the core liquid flow rate is 48 ml / min, and the nitrogen pressure is 1 bar. The casting solution is squeezed out from one end of the spinning needle of the spinning device, and after passing through a certain air distance, it enters the external coagulation bath for solidification. After soaking in the external coagulation bath for 24 h, the solvent dimethylformamide (DMF) and anhydrous ethanol are fully exchanged with the anhydrous ethanol in the external coagulation bath, which is a non-solvent. After drying, the hollow fiber ceramic membrane precursor is obtained. High-temperature sintering: When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula mullite, a certain length of hollow fiber ceramic membrane precursor is cut and loaded into an alumina tubular membrane tube. Under air atmosphere, the temperature is first raised from room temperature to 950 ℃ at a rate of 2℃ / min and held for 2 h, then raised to 1400℃~1600℃ at a rate of 5℃ / min and held for 2 h. When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula SiO2, under air atmosphere, the temperature is first raised from room temperature to 800℃ at a rate of 2℃ / min and held for 2 h, then raised to 1000℃~1200℃ at a rate of 5℃ / min and held for 2 h. When preparing hollow fiber ceramic ultrafiltration membranes with the chemical formula SiOC, under nitrogen atmosphere, the temperature is raised to 800℃ at a rate of 2℃ / min and held for 2 h, then raised to 1000℃ at a rate of 4℃ / min, and oxygen is introduced and held for 5 h.

2. A method for preparing porous ceramics using a polymer-derived ceramic route according to claim 1, wherein, When the polymer ceramic precursor used is polymethylsilsesquioxane and the hollow fiber ceramic ultrafiltration membrane has the chemical formula of mullite, the weight percentage of the casting solution is: 40.68 wt% NMP, 9.18 wt% PES, 29.08 wt% α-Al2O3, and 15.81 wt% polymethylsilsesquioxane, wherein the particle size of polymethylsilsesquioxane is ≤5 μm and D50 = 2 μm.

3. A method for preparing porous ceramics using a polymer-derived ceramic route according to claim 1, wherein, When using the polymer ceramic precursor MK, the casting solution comprises the following weight percentages: 16 wt% anhydrous ethanol, 24 wt% DMF, 8 wt% PMMA, 34 wt% α-Al2O3, and 16 wt% MK.

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