Ordered layered pore geopolymer based on heterogeneous mold strong gradient bidirectional freezing and preparation method of ordered layered pore geopolymer

Abstract

The invention discloses an ordered layered pore geopolymer based on heterogeneous mold strong gradient bidirectional freezing and a preparation method of the ordered layered pore geopolymer. The preparation method comprises the following steps: preparing a geopolymer slurry; the method comprises the following steps: forming an ordered layered ice crystal template by adopting graded heat conduction and strong gradient bidirectional freezing based on a heterogeneous material mold: injecting geopolymer slurry into the heterogeneous material mold, performing bidirectional freezing on the geopolymer slurry, inducing the orderly arranged layered ice crystal template, and obtaining a frozen blank containing the layered ice crystal template; performing freeze drying treatment; and maintaining to obtain the ordered layered pore geopolymer. A stable and uniform lateral temperature gradient is actively constructed in a heterogeneous mold, so that the ice crystal template with a long-range ordered layered structure is prepared, multi-stage low-temperature in-situ polymerization treatment is introduced into a frozen blank in a preferable scheme, a geopolymer matrix is reinforced in situ on the premise that the template is not damaged, and the preparation method is simple and easy to implement. And high-fidelity curing of the fine template structure is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of geopolymer preparation, in particular to an ordered layered porous geopolymer based on heterogeneous mold strong gradient bidirectional freezing and a preparation method thereof. BACKGROUND

[0002] Geopolymer, as an inorganic non-metallic material formed by polycondensation reaction of silico-alumina precursor under the action of alkaline activator, is considered as an ideal green alternative to traditional cement-based materials due to its wide raw material sources (such as industrial solid waste), low energy consumption and less carbon emission in preparation process, and excellent mechanical properties and durability. By introducing pore structure, geopolymer can be endowed with specific functional properties, such as high-efficiency adsorption, filtration, heat insulation, etc., thereby greatly expanding its application range. Ice-templating, or freeze-casting, is an advanced technology for constructing porous materials by controlling the directional solidification of solvent (usually water). In the traditional unidirectional freezing ice-templating method, by establishing a vertical temperature gradient, ice crystals can be induced to grow in a single direction, thereby preparing materials with layered or tubular pores. However, the pore structure prepared by this method is random and disordered in the dimension parallel to the freezing plane, resulting in weak anisotropy of material properties, which is difficult to meet the requirements of some applications with higher requirements for structural order. In order to achieve higher level control of pore structure, in recent years, researchers have proposed the concept of bidirectional freezing, that is, in addition to the vertical temperature gradient, a horizontal (lateral) temperature gradient is additionally introduced to precisely guide the growth direction of ice crystals in two or even three dimensions, so as to construct complex ordered structures of biomimetic natural materials (such as shells). However, the existing bidirectional freezing technology has the following interrelated and difficult to overcome fundamental technical bottlenecks when applied to geopolymer system:

[0003] First, the existing bidirectional freezing technology has limited control over the ice crystal template, and it is difficult to form a long-range ordered ideal structure. Most of the current bidirectional freezing technology relies on the geometric size difference of the homogeneous material mold to passively manufacture a lateral temperature gradient. In order to achieve higher level control of the pore structure, in recent years, researchers have proposed the concept of bidirectional freezing, that is, in addition to the vertical temperature gradient, a horizontal (lateral) temperature gradient is additionally introduced, in order to precisely guide the growth direction of ice crystals in two or three dimensions. At present, the main technical approach to achieve this lateral temperature gradient is to rely on the geometric structure design of the homogeneous material mold. However, this technical solution based on the geometric structure difference of the homogeneous material has a fundamental technical bottleneck: first, due to the extremely low thermal conductivity of the mold material itself (such as high polymer materials), the lateral temperature gradient generated by the small geometric difference is very weak and unstable, and the driving force for the directional growth of ice crystals is insufficient. Secondly, this gradient is extremely sensitive to the processing precision of the mold, the uniformity of the slurry filling and the thermal disturbance of the external environment, resulting in the shape of the isotherm being extremely easy to bend and uneven. Therefore, this weak and uneven gradient field has very limited control over the growth of ice crystals, and can only act in a local or short-range, and the ice crystal template formed on a macroscopic scale still has a large number of orientation defects and structural discontinuities, far from the ideal, long-range ordered layered pore structure. SUMMARY

[0004] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the purpose of the present application is to provide a preparation method of ordered layered porous polymer based on heterogeneous mold strong gradient bidirectional freezing, which uses a specially designed heterogeneous material mold to precisely construct the ice crystal template through strong gradient bidirectional freezing. Unlike the prior art which relies on the geometric difference of homogeneous materials to passively generate a weak lateral temperature gradient, the present application uses a combination of high thermal conductivity metal materials and low thermal conductivity polymer materials in the mold, and uses the large intrinsic physical difference in thermal conductivity between the two to actively construct a strong, stable and uniform lateral temperature gradient. The gradient has high strength and good directionality, and can precisely guide the nucleation and growth of ice crystals in a long-range manner, thereby preparing an ice crystal template with a highly ordered layered structure.

[0005] As one of the preferred solutions, the present application also introduces a multi-stage, sequential low-temperature in-situ polymerization step before freeze-drying sublimation of ice crystals. This step allows the ice crystals to partially and controllably melt by precisely controlling the temperature, and the released small amount of liquid water can initiate the polycondensation reaction of the in-situ polymer, thereby preliminarily strengthening the polymer matrix framework while maintaining the integrity of the ice crystal template structure. This "stabilization first, then strengthening" strategy ensures that the fine ice crystal template structure can be copied with high fidelity, avoiding structural collapse during subsequent processing. By combining the "precise construction" of the template with the "high-fidelity solidification" of the matrix, a porous geopolymer with long-range ordered structure and excellent mechanical properties is successfully prepared, effectively solving the technical bottleneck of balancing the strength and function of porous materials.

[0006] Another object of the present application is to provide an ordered layered porous geopolymer based on heterogeneous mold strong gradient bidirectional freezing prepared by the above method.

[0007] The object of the present application is achieved by the following technical solutions:

[0008] A preparation method of an ordered layered porous geopolymer based on heterogeneous mold strong gradient bidirectional freezing, comprising the following steps:

[0009] (1) preparing a geopolymer slurry;

[0010] (2) forming an ordered layered ice crystal template by using hierarchical heat conduction based on a heterogeneous material mold and strong gradient bidirectional freezing:

[0011] (2-1) injecting the geopolymer slurry obtained in step (1) into a heterogeneous material mold; the heterogeneous material mold comprises a bottom plate, a side wall and a wedge;

[0012] The side wall is a hollow cuboid surrounded by a first side plate, a second side plate, a third side plate and a fourth side plate; the first side plate is made of a first material; the second side plate, the third side plate and the fourth side plate are all made of a second material; the thermal conductivity of the first material is not less than 15 W / (m·K); the thermal conductivity of the first material is at least one order of magnitude higher than that of the second material;

[0013] The bottom plate is made of a material with a thermal conductivity not less than 100 W / (m·K);

[0014] When the heterogeneous material mold is used, the wedge is placed in the side wall; the bottom surface of the wedge is attached to the bottom plate; the side surface of the wedge is attached to the side wall; the top surface of the wedge is an inclined surface, the angle between the inclined surface and the bottom plate is 20°-60°; the lower end of the inclined surface is connected to the first side plate; the wedge is made of a third material with a thermal conductivity not exceeding 0.2 W / (m·K).

[0015] (2-2) by contacting the bottom plate with a single low-temperature cold source, bidirectional freezing is performed on the geopolymer slurry at a freezing temperature of -150℃ to -25℃, to induce an orderly arranged layered ice crystal template in the geopolymer slurry, and a frozen body containing the layered ice crystal template is obtained;

[0016] (3) freeze-drying treatment;

[0017] (4) curing: curing the frozen body containing the layered ice crystal template after the freeze-drying treatment under preset conditions, to obtain an ordered layered-pore geopolymer.

[0018] Preferably, after step (2) and before step (3), the following step is further performed: sequentially performing a first low-temperature in-situ polymerization stage and a second low-temperature in-situ polymerization stage;

[0019] wherein the first low-temperature in-situ polymerization stage: placing the frozen body in a temperature range of T1 and maintaining for at least 6 hours, so that part of the ice crystals in the frozen body are controlled to melt to form liquid water, and a preliminary in-situ polycondensation reaction is initiated at the temperature of T1; wherein -10℃≤T1<0℃;

[0020] wherein the second low-temperature in-situ polymerization stage: raising the body processed by the first low-temperature in-situ polymerization stage to a temperature range of T2 and maintaining for at least 6 hours, and ensuring that T2 is higher than T1, so that the liquid water formed by the further melting of the ice crystals reacts with the unreacted or partially reacted siliceous and aluminous active raw materials in the geopolymer slurry to further fully perform in-situ polycondensation reaction under the action of the original composite activator in the geopolymer slurry; wherein 0℃≤T2≤5℃.

[0021] Preferably, the first material is at least one of stainless steel, copper and aluminum; and the second material is one of polytetrafluoroethylene, polypropylene or polyether ether ketone.

[0022] Preferably, the third material is polydimethylsiloxane.

[0023] Preferably, the geopolymer slurry is prepared as follows: uniformly mixing the siliceous and aluminous active raw materials, water and the composite activator to obtain a geopolymer slurry with a viscosity controlled between 50-200 mPa·s; wherein the mass ratio of water to the siliceous and aluminous active raw materials is 0.8-1.2; and the mass of the alkali metal oxide contained in the composite activator is 4%-8% of the mass of the siliceous and aluminous active raw materials.

[0024] Preferably, the siliceous and aluminous active raw material is selected from at least one of slag, metakaolin, fly ash, coal gangue or tailings containing active silico-alumina components; the composite activator is prepared from a mixture of water glass and sodium hydroxide or potassium hydroxide, and the modulus is 1.2-1.8.

[0025] Preferably, the included angle is between 20° and 40°.

[0026] Preferably, the curing is standard curing or steam curing; the standard curing conditions are temperature 20±2℃, relative humidity no less than 95%, and curing time 3-28 days; the steam curing conditions are temperature 50-80℃, relative humidity greater than 95%, and curing time 12-48 hours.

[0027] The application also provides an ordered layered porous polymer based on heterogeneous mold strong gradient bidirectional freezing, which is prepared by the preparation method of the ordered layered porous polymer based on heterogeneous mold strong gradient bidirectional freezing.

[0028] Preferably, the ordered layered porous polymer based on heterogeneous mold strong gradient bidirectional freezing has a porosity of 45%-65%, a most probable pore diameter in the range of 10-100 μm, and a compressive strength along the vertical freezing direction of no less than 8 MPa.

[0029] Preferably, the first side plate, the second side plate, the third side plate and the fourth side plate have the same thickness.

[0030] The ordered layered porous polymer based on heterogeneous mold strong gradient bidirectional freezing prepared by the application has the following characteristics:

[0031] a) Long-range ordered structure on the macro and micro levels: the pore structure of the polymer is formed into a long-range ordered lamellar structure due to the improved bidirectional freezing in step (2) of the preparation method, and the microstructure of the polymer presents a cross-linked lamellar pore wall induced by the improved bidirectional freezing when observed under a scanning electron microscope, and the order and continuity of the structure are greatly improved compared with samples prepared by a traditional unidirectional freezing method.

[0032] b) Excellent combination of physical and mechanical properties: the polymer has an ordered layered pore structure formed by the preparation method, the porosity of the structure is 45%-65%, the most probable pore diameter is in the range of 10-100 μm, and the compressive strength along the vertical freezing direction is no less than 8 MPa.

[0033] c) Unique pore wall matrix micro-morphology and mechanical property enhancement: The pore wall matrix, after being subjected to the sequential low-temperature in-situ polymerization treatment in step (3) of the preparation method, presents a microstructure composed of a continuous and dense, fully hydrated / polymerized geopolymer gel phase under scanning electron microscope observation. The microstructure is more dense and homogeneous than that of samples without in-situ polymerization treatment. Therefore, the compressive strength along the vertical freezing direction is at least 200% higher than that of the same group of geopolymer samples prepared by using the same heterogeneous mold two-way freezing device, but without in-situ polymerization treatment, directly freeze-dried and subjected to the same curing.

[0034] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0035] 1. Realization of "precise construction" of ice crystal templates and obtaining of ideal structures with long-range order. The present application successfully constructs a strong, stable and uniform lateral temperature field by designing a heterogeneous material mold and utilizing the large intrinsic difference in thermal conductivity between high thermal conductivity metals and low thermal conductivity polymers. Compared with the weak and unstable gradient field generated by the geometric size difference in the prior art, the strong gradient field of the present application can accurately guide the nucleation and growth of ice crystals in a long-range manner, forming highly parallel isotherms with few defects, thereby preparing a long-range ordered layered ice crystal template on a macroscopic scale that cannot be obtained by traditional methods.

[0036] 2. Realization of "high-fidelity solidification" of fine structures and ensuring that the structures do not collapse. The multi-stage low-temperature in-situ polymerization process introduced by the present application before freeze-drying enables the geopolymer matrix to have sufficient mechanical strength before the ice crystals sublimate through sequential polymerization of "stabilization first and strengthening later". This ensures that the fine ice crystal template formed by the "precise construction" step can be accurately replicated, effectively resisting the shrinkage stress during the drying process and avoiding structural collapse or cracking.

[0037] 3. Realization of "synergistic leap" of the performance of the final product and solving the inherent contradiction between "strength and function". Due to the combination of the "precise construction" template and the "high-fidelity solidification" process, the final product prepared by the present application exhibits a synergistic leap effect of "1+1>2". It not only has a macroscopically ordered layered pore structure (which guarantees excellent transport and other functional properties) due to two-way freezing, but also has a microscopically dense and reinforced matrix (which guarantees excellent mechanical properties) due to in-situ polymerization. This makes the mechanical properties (such as compressive strength) and functional characteristics (such as filtration flux) of the final product far exceed those of any sample prepared by only using a single innovative method (only improving the mold or only using in-situ polymerization), successfully solving the bottleneck problem of the difficulty of balancing mechanical strength and functional characteristics in porous materials. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1FIG. 1 is a structural diagram of a heterogeneous material mold according to the present application; FIG. 1(a) is a general structural diagram of the heterogeneous material mold; and FIG. 1(b) is a cross-sectional view of the heterogeneous material mold.

[0039] Figure 2 FIG. 2 is a temperature distribution diagram of a horizontal cross-section of the mold of Comparative Example 1.

[0040] Figure 3 FIG. 3 is a temperature distribution diagram of a horizontal cross-section of the mold of Comparative Example 2.

[0041] Figure 4 FIG. 4 is a temperature distribution diagram of a horizontal cross-section of the heterogeneous material mold of Example 2.

[0042] Figure 5 FIG. 5 is a temperature distribution diagram of a horizontal cross-section of the heterogeneous material mold of Example 3.

[0043] Figure 6 FIG. 6 is a temperature distribution diagram of a horizontal cross-section of the heterogeneous material mold of Example 4.

[0044] Figure 7 FIG. 7 is a process flow diagram of the present application.

[0045] Figure 8 FIG. 8 is a scanning electron microscope (SEM) diagram of the porous polymer prepared in Comparative Example 1; FIG. 8(a) is a 50x magnification diagram; and FIG. 8(b) is a lOOx magnification diagram.

[0046] Figure 9 FIG. 9 is a SEM diagram of the porous polymer prepared in Comparative Example 2; FIG. 9(a) is a 50x magnification diagram; and FIG. 9(b) is a lOOx magnification diagram.

[0047] Figure 10 FIG. 10 is a SEM diagram of the porous polymer prepared in Example 2 of the present application; FIG. 10(a) is a 50x magnification diagram; and FIG. 10(b) is a lOOx magnification diagram.

[0048] Figure 11 FIG. 11 is a SEM diagram of the porous polymer prepared in Example 3 of the present application; FIG. 11(a) is a 50x magnification diagram; and FIG. 11(b) is a lOOx magnification diagram.

[0049] Figure 12 FIG. 12 is a SEM diagram of the porous polymer prepared in Example 4 of the present application; FIG. 12(a) is a 50x magnification diagram; and FIG. 12(b) is a lOOx magnification diagram. DETAILED DESCRIPTION

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of protection of the invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0051] Example 1: Preparation of porous geopolymer using a heterogeneous mold with bidirectional freezing but without in-situ polymerization treatment.

[0052] like Figure 1 As shown, the heterogeneous material mold of this embodiment includes a base plate 1, side walls, and a wedge 3;

[0053] The sidewall is a hollow cuboid formed by a first side plate 21, a second side plate 22, a third side plate 23, and a fourth side plate 24; the first side plate 21 is made of stainless steel with a thermal conductivity of 16.3 W / (m·K); the second side plate 22, the third side plate 23, and the fourth side plate 24 are all made of polytetrafluoroethylene with a thermal conductivity of 0.25 W / (m·K).

[0054] The base plate is made of aluminum sheet with a thermal conductivity of approximately 138 W / (m·K); the wedge is made of PDMS with a thermal conductivity of approximately 0.15 W / (m·K).

[0055] When the heterogeneous material mold is in use, the wedge 3 is placed inside the side wall; the bottom surface of the wedge 3 is in contact with the base plate 1; the side surface of the wedge is in contact with the side wall; the top surface of the wedge is an inclined plane, and the inclined plane forms an angle with the base plate, which can be 20° to 60°; the lower end of the inclined plane is connected to the first side plate.

[0056] In this embodiment, the inner cavity dimensions of the heterogeneous material mold are 30mm×30mm×100mm, the side wall thickness is 8mm, and the bottom plate thickness is 1mm.

[0057] The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold in this embodiment includes the following steps:

[0058] (1) Geopolymer slurry preparation: S95 grade slag powder 100 parts (mass parts, the same below) was weighed and water 90 parts was added. The slag powder and water were added to the ball mill tank of a planetary ball mill, and mixed at a speed of 196 rpm for 220 minutes. Then, a pre-configured composite alkali activator was added, which was prepared from industrial grade liquid water glass (with Na2O content of 8.53%, SiO2 content of 26.98%, modulus of 3.26) and analytical pure sodium hydroxide particles, so that the mass of Na2O in the final activator was 4% of the mass of the slag powder, and the modulus (molar ratio of SiO2 to Na2O) of the solution after configuration was adjusted to 1.5. After adding the composite alkali activator, mixing was continued at the above speed for 20 minutes to obtain a geopolymer slurry, the viscosity of which was controlled in the range of 50-200 mPa·s.

[0059] (2) Two-way freezing with heterogeneous material mold: The geopolymer slurry prepared in step (1) was injected into a heterogeneous material mold, and then a wedge with a 40° inclination angle was placed above the aluminum sheet, with the lowest end of the wedge tightly attached to the stainless steel side. The heterogeneous material mold containing the slurry was placed in an adiabatic container filled with liquid nitrogen (as a single low-temperature cold source) at the bottom, ensuring that the aluminum sheet at the bottom of the mold was in good contact with the liquid nitrogen through a hollow metal partition, and one-way freezing treatment was carried out for 8 hours. During this process, the temperature at the contact end of the aluminum sheet with the liquid nitrogen (i.e. the cold end temperature) was controlled at about -150±2℃, so as to form ice crystal templates aligned in the freezing direction in the slurry, and a frozen green body containing ice crystal templates was obtained.

[0060] (3) Freeze-drying: The frozen green body obtained in step (2) was directly transferred to a freeze-dryer without any in-situ polymerization treatment, and freeze-drying treatment was carried out for 48 hours under the conditions of a cold trap temperature not higher than -30℃ and a vacuum degree not higher than 10 Pa, and ice crystals and moisture were removed by sublimation to obtain a green body.

[0061] (4) Curing of the pre-cured geopolymer: The pre-cured geopolymer obtained in step (3) was standard cured under the conditions of a temperature of 20±2℃ and a relative humidity not lower than 95% for 7 days.

[0062] Example 2: Preparation of porous geopolymer wedge by two-way freezing with heterogeneous mold and in-situ polymerization treatment In this embodiment, the heterogeneous material mold is the same as in Example 1.

[0063] The preparation method of the ordered layered porous geopolymer based on heterogeneous mold strong gradient two-way freezing in this embodiment comprises the following steps:

[0064] (1) Geopolymer slurry preparation: same as step (1) in Example 1.

[0065] (2) Bidirectional ice crystal templating: The geopolymer slurry prepared in step (1) is injected into a mold with an inner cavity size of 30 mm x 30 mm x 100 mm and a side wall thickness of 8 mm, wherein the three side walls of the mold are made of polytetrafluoroethylene (thermal conductivity of 0.25 W / (m·K)), and the other side is made of stainless steel (thermal conductivity of 16.3 W / (m·K)). A piece of aluminum sheet (thermal conductivity of about 138 W / (m·K)) with a thickness of 1 mm is fixed at the bottom of the mold in advance. Then, a 40° PDMS (thermal conductivity of about 0.15 W / (m·K)) wedge is placed on top of the aluminum sheet, with the lowest end of the wedge in close contact with the stainless steel side. The mold containing the slurry is placed in an adiabatic container filled with liquid nitrogen (as a single low-temperature cold source) at the bottom, ensuring that the aluminum sheet at the bottom of the mold is in good contact with the liquid nitrogen through a hollow metal partition. Unidirectional freezing treatment is carried out for 8 hours. During this process, the temperature at the contact end of the aluminum sheet with the liquid nitrogen (i.e., the cold end temperature) is controlled at about -150±2℃, thereby forming ice crystal templates aligned in the freezing direction in the slurry, and obtaining a frozen green body containing ice crystal templates.

[0066] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment:

[0067] The frozen green body obtained in step (2) is sequentially subjected to the following sequential, at least two different and temperature-increasing low-temperature in-situ polymerization treatment stages:

[0068] First low-temperature in-situ polymerization stage: The frozen green body is placed in a temperature range of -5℃ and kept for 12 hours, so that part of the ice crystals in the frozen green body melt to form liquid water, and a preliminary in-situ polycondensation reaction is initiated at this lower temperature to stabilize the pore wall structure defined by the ice crystal templates;

[0069] Second low-temperature in-situ polymerization stage: Subsequently, the green body treated by the first low-temperature in-situ polymerization stage is warmed to a temperature range of 3℃, and the temperature of the second low-temperature in-situ polymerization stage is higher than that of the first low-temperature in-situ polymerization stage. Keep for 12 hours at this higher temperature to promote more complete in-situ polycondensation reaction by using the liquid water formed by further melting of ice crystals, thereby strengthening the geopolymer matrix skeleton;

[0070] Then, the green body treated by the multi-stage low-temperature in-situ polymerization treatment is subjected to freeze-drying treatment under the conditions of vacuum degree not higher than 10 Pa and cold trap temperature not higher than -30℃ for 12 hours, and the remaining ice crystals and part of the unreacted water are removed by sublimation, obtaining a pre-cured geopolymer that maintains the ordered layered pore structure.

[0071] (4) Curing of the pre-cured geopolymer: same as step (4) in Example 1.

[0072] Example 3: The method of the present application (using metal side walls with different thermal conductivities than in Example 2, and a wedge-shaped slope with a different angle)

[0073] In this example, the first side of the heterogeneous mold is made of 5052 aluminum alloy (thermal conductivity of 138 W / (m·K)), and the angle between the slope and the bottom of the wedge-shaped object is 20°, except that the other features of the heterogeneous mold are the same as in Example 2.

[0074] The preparation method of the ordered layered-pore geopolymer based on strong-gradient bidirectional freezing of a heterogeneous mold in this example includes the following steps:

[0075] (1) Geopolymer slurry preparation: same as step (1) in Comparative Example 1.

[0076] (2) Forming ice crystal template bidirectionally: The geopolymer slurry prepared in step (1) is injected into a mold with an inner cavity size of 30 mm x 30 mm x 100 mm and a side wall thickness of 8 mm, wherein the three sides of the mold are made of polytetrafluoroethylene (thermal conductivity of 0.25 W / (m·K)), and the other side is made of 5052 aluminum alloy (thermal conductivity of 138 W / (m·K)). A piece of aluminum sheet with a thickness of 1 mm (thermal conductivity of about 138 W / (m·K)) is pre-fixed at the bottom of the mold. Then a 20° inclined PDMS (thermal conductivity of about 0.15 W / (m·K)) wedge-shaped object is placed above the aluminum sheet, with the lowest end of the wedge-shaped object in close contact with the stainless steel side. The mold containing the slurry is placed in an adiabatic container filled with liquid nitrogen (as a single low-temperature cold source) to ensure that the aluminum sheet at the bottom of the mold is in good contact with the liquid nitrogen through the hollow metal partition, and the one-way freezing process is carried out for 8 hours. During this process, the temperature at the contact end of the aluminum sheet (i.e. the cold end temperature) is controlled at about -150±2℃, so as to form an ice crystal template oriented along the freezing direction in the slurry, and obtain a frozen blank containing the ice crystal template.

[0077] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment: same as step (3) in Example 2.

[0078] (4) Curing the pre-cured geopolymer: same as step (4) in Example 1.

[0079] Example 4: The method of the present application (using different freezing temperatures than in Example 2)

[0080] The heterogeneous material mold used in this example is the same as in Example 3.

[0081] The preparation method of the ordered layered-pore geopolymer based on strong-gradient bidirectional freezing of a heterogeneous mold in this example includes the following steps:

[0082] (1) Geopolymer slurry preparation: same as step (1) in Comparative Example 1.

[0083] (2) Bi-directional ice crystal templating: The geopolymer slurry prepared in step (1) was injected into a mold with an inner cavity size of 30 mm x 30 mm x 100 mm and a side wall thickness of 8 mm, wherein the three faces of the mold were made of polytetrafluoroethylene (thermal conductivity of 0.25 W / (m·K)), and the other face was made of stainless steel (thermal conductivity of 0.25 W / (m·K)). A piece of aluminum sheet (thermal conductivity of about 138 W / (m·K)) with a thickness of 1 mm was previously fixed at the bottom of the mold. Then a PDMS (thermal conductivity of about 0.15 W / (m·K)) wedge with a 20° inclination angle was placed above the aluminum sheet, with the lowest end of the wedge in close contact with the stainless steel side. The mold containing the slurry was placed in an adiabatic container filled with dry ice (as a single low-temperature cold source) or placed on a low-temperature freezing platform at -45°C. The aluminum sheet at the bottom of the mold was ensured to have good contact with the low-temperature medium, and the one-way freezing process was carried out for 8 hours. During this process, the temperature at the contact end of the aluminum sheet with the dry ice (i.e., the cold end temperature) was controlled at about -45±2°C, thereby forming an ice crystal template oriented along the freezing direction in the slurry, and obtaining a frozen green body containing the ice crystal template.

[0084] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment: same as step (3) in Example 2.

[0085] (4) Curing of the pre-solidified geopolymer: same as step (4) in Example 1.

[0086] Comparative Example 1: Preparation of porous geopolymer by traditional one-way freezing ice template method

[0087] This comparative example aims to illustrate the effects that can be achieved when using the multi-stage low-temperature in-situ polymerization technology of the present application without using the heterogeneous mold of the present application, but only using the traditional one-way freezing method, in order to highlight the necessity and superiority of the “strong gradient bi-directional freezing” step in the present application.

[0088] In this example, the mold side walls were all made of polytetrafluoroethylene, and no wedge was used.

[0089] (1) Geopolymer paste preparation: S95 grade slag powder 100 parts (mass parts, same below) was weighed and water 90 parts was added. The slag powder and water were added to the ball milling tank of a planetary ball mill, and mixed at a speed of 196 rpm for 220 minutes. Then, a pre-configured composite alkali activator was added, which was prepared from industrial grade liquid water glass (with Na2O content of 8.53%, SiO2 content of 26.98%, modulus of 3.26) and analytical pure sodium hydroxide particles, so that the mass of Na2O in the final activator was 4% of the mass of the slag powder, and the modulus (molar ratio of SiO2 to Na2O) of the solution after configuration was adjusted to 1.5. After adding the composite alkali activator, continue to mix at the above speed for 20 minutes to obtain a geopolymer paste, the viscosity of which is controlled in the range of 50-200 mPa·s.

[0090] (2) Unidirectional freezing to form ice crystal template: The geopolymer paste prepared in step (1) was injected into a polytetrafluoroethylene mold with an inner cavity size of 30 mm x 30 mm x 100 mm and a side wall thickness of 8 mm, and a 1 mm thick aluminum sheet (thermal conductivity of about 138 W / (m·K)) was pre-fixed at the bottom of the mold. The mold containing the paste was placed in an adiabatic container filled with liquid nitrogen (as a single low-temperature cold source) at the bottom, ensuring that the aluminum sheet at the bottom of the mold was in good contact with the liquid nitrogen through a perforated metal partition, and unidirectional freezing was carried out for 8 hours. During this process, the temperature at the contact end of the aluminum sheet with the liquid nitrogen (i.e. the cold end temperature) was controlled at about -150±2℃, so as to form an ice crystal template arranged along the freezing direction (perpendicular to the aluminum sheet direction) in the paste, and a frozen green body containing the ice crystal template was obtained.

[0091] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment: The frozen green body obtained in step (2) was sequentially subjected to the following sequential, at least two different and temperature-increasing low-temperature in-situ polymerization treatment stages:

[0092] First low-temperature in-situ polymerization stage: The frozen green body was placed in a temperature range of -5℃ and kept for 12 hours, so that part of the ice crystals in the frozen green body were controlled to melt to form liquid water, and a preliminary in-situ polycondensation reaction was initiated at the lower temperature to stabilize the pore wall structure defined by the ice crystal template;

[0093] Second low-temperature in-situ polymerization stage: Subsequently, the green body treated by the first low-temperature in-situ polymerization stage was warmed up to a temperature range of 3℃, and the temperature of the second low-temperature in-situ polymerization stage was higher than that of the first low-temperature in-situ polymerization stage, and was kept at the higher temperature for 12 hours, so as to promote more sufficient in-situ polycondensation reaction by using the liquid water formed by further melting of the ice crystals, thereby strengthening the geopolymer matrix skeleton;

[0094] Then, the green body treated by the multi-stage low-temperature in-situ polymerization process is subjected to freeze-drying treatment under the condition that the vacuum degree is not higher than 10 Pa and the cold trap temperature is not higher than -30 ℃ for 12 hours, and the remaining ice crystals and part of the unreacted water are removed by sublimation to obtain a pre-solidified polymer that maintains the ordered layered pore structure.

[0095] (4) Curing the pre-solidified polymer: the pre-solidified polymer obtained in step (3) is cured under the condition that the temperature is 20 ± 2 ℃ and the relative humidity is not lower than 95% for 7 days.

[0096] Comparative Example 2: Preparation of a porous geopolymer by a common two-way ice template method with only one wedge introduced

[0097] This comparative example aims to illustrate the influence of the preparation of a porous geopolymer by a common two-way ice template method with only one wedge introduced on its performance, as a reference for comparison with the multi-stage treatment of the present application.

[0098] In this example, the mold side wall is entirely prepared by polytetrafluoroethylene, and the same wedge as in Example 2 is used.

[0099] (1) Preparation of geopolymer slurry: same as step (1) in Comparative Example 1.

[0100] (2) Common two-way freezing to form ice crystal template: the geopolymer slurry prepared in step (1) is injected into a polytetrafluoroethylene mold with an inner cavity size of 30 mm x 30 mm x 100 mm and a side wall thickness of 8 mm, and a 1 mm thick aluminum sheet (thermal conductivity of about 138 W / (m·K)) is fixed at the bottom of the mold in advance. Then a 20° inclined PDMS wedge is placed above the aluminum sheet. The mold containing the slurry is placed in an adiabatic insulation container filled with liquid nitrogen (as a single low-temperature cold source) at the bottom, ensuring that the aluminum sheet at the bottom of the mold is in good contact with the liquid nitrogen through a perforated metal partition, so as to perform one-way freezing treatment for 8 hours. In this process, the temperature at the contact end of the aluminum sheet with the liquid nitrogen (i.e. the cold end temperature) is controlled at about -150 ± 2 ℃, so as to form an ice crystal template oriented in the freezing direction in the slurry, and a frozen green body containing the ice crystal template is obtained.

[0101] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment: same as step (3) in Comparative Example 1.

[0102] (4) Curing the pre-solidified polymer: same as step (4) in Comparative Example 1.

[0103] Performance test and result analysis:

[0104] In order to systematically evaluate the effectiveness of the method for preparing ordered porous polymer based on the improved static bidirectional frozen ice template method proposed in the present application, and to illustrate the performance advantages of the prepared polymer, the porous polymer samples prepared by the above Comparative Example 1, Comparative Example 2 and Examples 1 to 4 were subjected to detailed mechanical property tests, pore structure characterization and microscopic morphology observation. The main performance data are summarized in Table 1, and the structure of the heterogeneous mold is shown in Figure 1 The temperature distribution in the heterogeneous mold of Comparative Example 1, Comparative Example 2 and Example 1 is shown in Figures 2 to 6 The process flow is shown in Figure 7 The microscopic morphology of Comparative Example 1, Comparative Example 2 and Examples 2 to 4 is shown in Figures 8 to 12 .

[0105] Table 1 Comparison of properties of porous polymers prepared by different methods

[0106]

[0107]

[0108] (I) Significant improvement in mechanical properties and cause analysis

[0109] Table 1 systematically shows the structure parameters and mechanical properties of the porous polymer samples under different preparation conditions. Among them, the compressive strength along the z-axis direction shows great difference, which clearly reflects the profound influence of various technical strategies in the present application on the mechanical properties of the final material. The main advantages of the present application are mainly due to the two core innovations of "heterogeneous mold precise template construction" and "low-temperature in-situ polymerization high-fidelity solidification" and their synergistic effect.

[0110] (1) Heterogeneous material mold: the basis for realizing "precise template construction"

[0111] The first core innovation of the present application is to use a heterogeneous material mold, which fundamentally optimizes the temperature field quality in the freezing process.

[0112] Temperature field comparison: as shown in Figure 4 , Figure 5 and Figure 6 , using the heterogeneous material mold of the present application, the isotheral lines (color contours in the figure) in the cross section of the slurry show a highly flat, parallel and uniform spacing ideal form, no matter at what freezing temperature. This shows that by using the intrinsic physical difference of the thermal conductivity of the material, a strong and stable lateral temperature gradient has been successfully constructed. In sharp contrast, as shown in Figure 3 , using the traditional homogeneous mold of Comparative Example 2, the isotheral lines are obviously curved and uneven. This fully proves that the active construction of temperature field in the present application is much better than the passive way of relying on weak geometric size difference.

[0113] Performance improvement proof: This high-quality temperature field directly translates into the improvement of structure and performance. Compared with Comparative Example 2 (homogeneous mold, 6.31 MPa) and Example 2 (heterogeneous mold, 10.34 MPa), which both undergo in-situ polymerization, the innovation of the mold alone increases the compressive strength by about 64%.

[0114] (2) Low-temperature in-situ polymerization: the key to achieving "high-fidelity solidification" of the template

[0115] The second core innovation of the present application is the introduction of a low-temperature in-situ polymerization step before freeze-drying. This step plays a decisive role in strengthening the matrix and ensuring that the fine template structure is completely replicated.

[0116] Key data comparison: Comparative Example 3 uses a heterogeneous material mold of the present application to obtain an ordered ice crystal template, but does not undergo in-situ polymerization. Its final compressive strength is only 1.86 MPa. This value is even lower than that of Comparative Example 1 (4.52 MPa), which has a disordered structure but a partially strengthened matrix, indicating that a perfect template alone is far from enough.

[0117] Performance leap proof: When the low-temperature in-situ polymerization step is added to Example 1, Example 2 is obtained. Its compressive strength jumps from 1.86 MPa to 10.34 MPa, with an amazing increase of 456%. This data fully proves that after the construction of a fine template, in-situ strengthening of the matrix is a necessary and key step to convert from "ideal template" to "high-performance material".

[0118] (3) Synergistic effect and parameter optimization

[0119] The final superior performance of the present application is the result of the synergistic effect of the above two core innovations. Examples 2 and 3 use a heterogeneous material mold and in-situ polymerization, and their compressive strengths (10.34 MPa and 12.81 MPa) far exceed any comparative example that contains only partial improvements. In addition, by further optimizing the process parameters, the performance can be further improved:

[0120] Optimization of side wall thermal conductivity: Example 3 increases the thermal conductivity of the metal side wall from 16.3 W / (m·K) to 138 W / (m·K), constructing a more steep and uniform temperature field (see attached Figure 5 ), which further increases the compressive strength from 10.34 MPa to 12.81 MPa, reaching the optimal value in the series of experiments.

[0121] Freezing temperature effect: Comparing Example 2 (-150°C, 10.34 MPa) and Example 4 (-45°C, 9.63 MPa), it can be seen that lower freezing temperature is beneficial for forming finer pore size and denser pore wall structure, thus achieving higher mechanical strength. This highlights the importance of low-temperature rapid freezing in obtaining high-performance materials.

[0122] (4) Conclusion

[0123] In summary, by adjusting the freezing parameters and the mold thermal conductivity properties, this study successfully achieved the transition from random disorder to highly ordered pore structure, and significantly improved the macroscopic compressive strength of the material. The performance improvement can be attributed to the synergistic optimization of the following three key factors:

[0124] Improvement of sidewall thermal conductivity: Enhance radial heat flow, leading to the ordered growth of ice crystals along a specific direction.

[0125] Control of wedge angle: Adjust the directionality and range of temperature gradient distribution, optimize ice crystal arrangement.

[0126] Setting of freezing temperature: Balance the nucleation density and growth rate, regulate the final pore size and pore wall density.

[0127] The synergistic optimization of these three parameters ultimately realizes the continuous structure-performance conversion path of "strong gradient temperature field → directional ice crystal growth → ordered pore structure formation → mechanical property enhancement", providing a reliable theoretical basis and practical scheme for the design and preparation of high-performance porous inorganic materials.

[0128] (B) Analysis of the order of pore structure and pore connectivity

[0129] In addition to mechanical properties, the high order of the pore structure achieved by the present invention is also crucial for the performance of the material in functional applications such as filtration, mass transfer, and thermal conductivity. This can be fully demonstrated by SEM micro-morphology ( Figures 8 to 12 ) and functional tests (filtration flux in Table 1).

[0130] (1) Structure comparison: from "random-winding network" to "parallel-penetrating channel"

[0131] Structural characteristics of traditional methods: The samples prepared by Comparative Example 1 (unidirectional freezing) and Comparative Example 2 (homogeneous mold bidirectional freezing) have a network structure composed of high-curvature, multi-branch, and randomly oriented pore walls (see Figure 8 、 Figure 9). Although Comparative Example 2 achieved local directional induction by introducing wedges, its channels still exhibited high tortuosity and topological complexity due to the lack of long-range, stable parallel isotherms. Although this pore structure has a certain open porosity, its internal transmission path is discontinuous and highly circuitous, resulting in a serious limitation of fluid flux, showing the inherent defects of high flow resistance and low efficiency.

[0132] Structural advantages of the present application: In sharp contrast, Examples 2 to 4, which used the method of the present application, all successfully constructed ordered structures composed of a large number of mutually parallel, long-range penetrating sheet-like channels (see Figure 10 、 Figure 11 、 Figure 12 ). The superiority of this structure mainly lies in the following three points:

[0133] High continuity and low tortuosity: The parallel sheet structure forms a direct linear channel, greatly reducing the length and tortuosity of the fluid transmission path.

[0134] Strong directional selectivity: The channels induced by straight isotherms have a clear orientation, ensuring that materials can be transported along the Z-axis (perpendicular to the freezing direction) with low resistance and high efficiency.

[0135] Flow field uniformity: The regular arrangement of the pore walls makes the internal flow field uniform, effectively suppressing the turbulence and vortex that may be caused by sudden changes in flow channel cross-section and direction, thereby significantly improving the overall transmission performance.

[0136] (2) Functional verification: more than an order of magnitude improvement in filtration flux

[0137] This structural change from "random-tortuous network" to "parallel-penetrating channel" is directly reflected in the huge improvement in functional performance. As shown in Table 1, the filtration flux of the example samples (for example, Example 2 is 23203.7 L / (m 2 ·h·bar) compared with Comparative Example 1 (1952.3 L / (m 2 ·h·bar)) and Comparative Example 2 (2282.7 L / (m 2 ·h·bar)) has more than an order of magnitude improvement. This data strongly proves the huge potential of the highly ordered pore structure constructed by the present application in functional applications.

[0138] (3) Controllable adjustment of pore size

[0139] In addition, the method of the present application also gives the pore size highly adjustable. By precisely adjusting the core process parameters such as freezing temperature and mold thermal conductivity, the most probable pore size can be flexibly adjusted in the range of 11.33 μm (disordered pores of Comparative Example 1) to 24.76 μm (Example 3) or even wider. This customizable pore size provides a solid structural foundation to meet the high-precision separation needs of different particle sizes or molecular weights.

[0140] (4) Summary

[0141] In summary, the present study innovatively proposes a strong gradient two-way freezing method based on a heterogeneous material mold. By synergistically regulating the freezing temperature, wedge angle, and mold thermal conductivity, a highly ordered biomimetic layered pore structure is successfully constructed in the geopolymer matrix.

[0142] The core mechanism is that the strong gradient temperature field actively constructed by the heterogeneous material mold and determined by the intrinsic physical properties of the material induces directional nucleation and ordered growth of ice crystals, ultimately forming a continuous, low tortuosity, and highly directional pore network.

[0143] The significance lies in that this structure not only significantly improves the z-axis compressive strength of the material (with a maximum increase of 456%), but also achieves a magnitude jump in functional characteristics such as filtration flux. This effectively avoids the binary opposition problem of "strength" and "function" in traditional porous materials.

[0144] The present application provides a new and powerful solution for the structure-controllable construction of high-performance and multi-functional porous materials, and has wide engineering application potential in the fields of high-efficiency filtration, catalyst carrier, tissue engineering scaffold, etc.

[0145] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A method for preparing an ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold, characterized in that, Includes the following steps: (1) Preparation of geopolymer slurry; (2) Using hierarchical heat conduction and strong gradient bidirectional freezing based on heterogeneous material molds to form ordered layered ice crystal templates: (2-1) Inject the geopolymer slurry obtained in step (1) into a heterogeneous material mold; the heterogeneous material mold includes a base plate, side walls and wedges; The sidewall is a hollow cuboid formed by a first side plate, a second side plate, a third side plate, and a fourth side plate; the first side plate is made of a first material; the second, third, and fourth side plates are all made of a second material; the thermal conductivity of the first material is not less than 15 W / (m·K); the thermal conductivity of the first material is at least one order of magnitude higher than that of the second material; The base plate is made of a material with a thermal conductivity of not less than 100 W / (m·K); When the heterogeneous material mold is in use, the wedge is placed inside the side wall; the bottom surface of the wedge is in contact with the base plate; the side surface of the wedge is in contact with the side wall; the top surface of the wedge is an inclined plane, and the angle between the inclined plane and the base plate is 20° to 60°; the lower end of the inclined plane is connected to the first side plate; the wedge is made of a third material with a thermal conductivity not exceeding 0.2 W / (m·K); (2-2) The geopolymer slurry is subjected to bidirectional freezing at a freezing temperature of -150°C to -25°C by contacting the base plate with a single low-temperature cold source, thereby inducing an ordered layered ice crystal template in the geopolymer slurry and obtaining a frozen blank containing the layered ice crystal template. (3) Freeze-drying treatment; (4) Curing: The frozen preform containing layered ice crystal template after freeze-drying is cured under preset conditions to obtain an ordered layered porous polymer.

2. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold according to claim 1, characterized in that, After step (2) and before step (3), the following steps are performed: the first low-temperature in-situ polymerization stage and the second low-temperature in-situ polymerization stage are performed sequentially; The first low-temperature in-situ polymerization stage involves placing the frozen preform within a temperature range of T1 and maintaining it for at least 6 hours, so that some of the ice crystals in the frozen preform melt in a controlled manner to form liquid water, and initiate a preliminary in-situ polycondensation reaction at a temperature of T1; wherein -10℃≤T1<0℃; The second low-temperature in-situ polymerization stage involves raising the preform treated in the first low-temperature in-situ polymerization stage to a temperature range of T2 and maintaining it for at least 6 hours, ensuring that T2 is higher than T1. This allows the liquid water formed by the further melting of ice crystals to undergo a more complete in-situ polycondensation reaction with the unreacted or partially reacted silica-alumina active raw materials in the geopolymer slurry under the action of the original composite activator in the geopolymer slurry. The temperature range is 0℃≤T2≤5℃.

3. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold according to claim 1, characterized in that, The first material is at least one of stainless steel, copper, and aluminum; the second material is one of polytetrafluoroethylene, polypropylene, or polyetheretherketone.

4. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing of a heterogeneous mold according to claim 1 or 3, characterized in that, The third material is polydimethylsiloxane.

5. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold according to claim 1, characterized in that, The geopolymer slurry is prepared as follows: a silica-alumina active raw material, water, and a composite activator are uniformly mixed to obtain a geopolymer slurry with a viscosity controlled between 50-200 mPa·s; wherein the mass ratio of water to silica-alumina active raw material is 0.8-1.2; and the mass of the alkali metal oxide contained in the composite activator is 4%-8% of the mass of the silica-alumina active raw material.

6. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing of a heterogeneous mold according to claim 5, characterized in that, The silica-alumina active raw material is selected from at least one of slag, metakaolin, fly ash, coal gangue, or tailings containing active silica-alumina components; the composite activator is prepared from a mixture of water glass and sodium hydroxide or potassium hydroxide, with a modulus of 1.2-1.

8.

7. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold according to claim 1, characterized in that, The included angle is between 20° and 40°.

8. The method for preparing ordered layered porous geopolymer based on strong gradient bidirectional freezing of a heterogeneous mold according to claim 1, characterized in that, The curing is either standard curing or steam curing; the conditions for standard curing are a temperature of 20±2℃, a relative humidity of not less than 95%, and a curing time of 3-28 days; the conditions for steam curing are a temperature of 50-80℃, a relative humidity of greater than 95%, and a curing time of 12-48 hours.

9. An ordered layered porous geopolymer based on strong gradient bidirectional freezing using a heterogeneous mold, characterized in that, The ordered layered porous geopolymer is prepared by the method of preparing the ordered layered porous geopolymer based on strong gradient bidirectional freezing of a heterogeneous mold according to any one of claims 1 to 8; the ordered layered porous geopolymer has long-range ordered arrangement and inter-linked lamellar pore walls.

10. The ordered layered porous geopolymer based on strong gradient bidirectional freezing of a heterogeneous mold according to claim 9, characterized in that, The porosity is 45%-65%, the most probable pore size is in the range of 10-100μm, and the compressive strength along the perpendicular freezing direction is not less than 8MPa.

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