CN115738763A - Method for dynamically adjusting aperture of super-hydrophobic membrane by regulating and controlling crystallization process - Google Patents
Method for dynamically adjusting aperture of super-hydrophobic membrane by regulating and controlling crystallization process Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 34
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- 230000008025 crystallization Effects 0.000 title claims abstract description 29
- 230000003075 superhydrophobic effect Effects 0.000 title claims abstract description 29
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- 230000001105 regulatory effect Effects 0.000 title claims abstract description 7
- 239000000758 substrate Substances 0.000 claims abstract description 151
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- 229920002981 polyvinylidene fluoride Polymers 0.000 claims abstract description 88
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims abstract description 54
- IAZDPXIOMUYVGZ-UHFFFAOYSA-N Dimethylsulphoxide Chemical compound CS(C)=O IAZDPXIOMUYVGZ-UHFFFAOYSA-N 0.000 claims abstract description 31
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- GEHJYWRUCIMESM-UHFFFAOYSA-L sodium sulfite Chemical compound [Na+].[Na+].[O-]S([O-])=O GEHJYWRUCIMESM-UHFFFAOYSA-L 0.000 claims description 14
- 238000002360 preparation method Methods 0.000 claims description 11
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A20/00—Water conservation; Efficient water supply; Efficient water use
- Y02A20/124—Water desalination
- Y02A20/131—Reverse-osmosis
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- Separation Using Semi-Permeable Membranes (AREA)
- Laminated Bodies (AREA)
Abstract
The invention discloses a method for dynamically adjusting the aperture of a super-hydrophobic membrane by regulating and controlling a crystallization process, which comprises the following steps: (1) preparing defluorinated PVDF; (2) preparing a super-hydrophobic membrane: dissolving PVDF and defluorinated PVDF in dimethyl sulfoxide, stirring to be uniform, then coating the solution on a substrate by a blade, floating above liquid nitrogen until the solution is completely crystallized, then placing the substrate in water for soaking for 12-24h, and airing to obtain the super-hydrophobic membrane. The pore size of the invention is adjustable by means of co-crystallization of solutes (PVDF and defluorinated PVDF) and solvents (DMSO) on different composite substrates. The super-hydrophobic membrane prepared by the method meets the membrane requirement for membrane distillation, has stable super-hydrophobicity and high flux property, and is extremely competitive in the field of membranes for membrane distillation.
Description
Technical Field
The invention relates to the technical field of preparation of a super-hydrophobic membrane, in particular to a method for dynamically adjusting the aperture of a super-hydrophobic membrane by regulating and controlling a crystallization process.
Background
The reserves of fresh water resources are limited and therefore the recovery of available fresh water resources from waste water or seawater is a promising strategy. Membrane distillation techniques can accomplish this. The technology uses low grade heat to generate water vapor which can permeate to the other side of the membrane and be cooled to fresh water on the other side for recovery. Membrane distillation technology is a more energy efficient way than energy consuming nanofiltration and thermal distillation.
Membrane distillation requirements for membranes include hydrophobicity and porosity to ensure rapid passage of water vapor. In addition, membranes with pore diameters of about 500nm are widely considered to be more suitable for the membrane distillation process, so that the reasonable adjustment of the pore diameter of the membrane is also very important for preparing the membrane for membrane distillation. The common methods for adjusting the pore size are to change the type and the addition amount of the pore-forming agent, which needs additional chemical addition and may increase the economic cost and even create environmental risks. Therefore, the invention provides a method for dynamically adjusting the membrane aperture by controlling the crystallization process, so that the method meets the aperture requirement of the membrane for membrane distillation, and has the advantages of simple operation and low environmental risk. The material is selected from semi-crystalline hydrophobic material PVDF, so that the membrane has excellent chemical stability and thermodynamic stability. The solvent selects a green solvent dimethyl sulfoxide (DMSO), the crystal of the DMSO is used as a membrane pore template to assist in dynamically adjusting the membrane pore size, and the membrane prepared by the method has stable super-hydrophobicity, ultrahigh gas flux and dynamically adjustable membrane pore size, and is a high-performance membrane suitable for the membrane distillation process.
Disclosure of Invention
The invention aims to provide a method for dynamically adjusting the pore diameter of a super-hydrophobic membrane by regulating and controlling a crystallization process. The crystal size of the defluorinated PVDF is more easily influenced by temperature, so that the size of crystals is controlled by considering the regulation and control of the crystallization process, thereby dynamically adjusting the pore diameter of the membrane. Blending commercial PVDF with defluorinated PVDF powder can form a multi-scale rough structure for constructing superhydrophobic membranes. Meanwhile, the crystallization process is adjusted by adjusting and controlling the combination of the substrates, so that the dynamic adjustment and control of the membrane aperture are realized. The preparation method and the aperture regulation and control means of the super-hydrophobic membrane are relatively simple, more environment-friendly and less in experimental conditions, and can be used for large-scale production.
A method for dynamically adjusting the aperture of a super-hydrophobic membrane by regulating and controlling a crystallization process specifically comprises the following steps
(1) Preparation of defluorinated polyvinylidene fluoride (PVDF): soaking commercial PVDF powder in hot alkali solution for hot alkali modification, wherein the modification conditions are as follows: the concentration of the alkali in the hot alkali solution is as follows: 8-12wt% (preferably 10 wt%), at a temperature of 50-70 deg.C (preferably 60 deg.C) for 4-6h (e.g. 4,5,6 h). And then adding the reacted mixed solution into a sodium sulfite solution for cooling, retaining a substrate, washing the substrate to be neutral by water, and drying the substrate for later use to obtain the defluorinated polyvinylidene fluoride (PVDF).
(2) Preparing a super-hydrophobic membrane: and (2) dissolving PVDF and the defluorinated polyvinylidene fluoride (PVDF) obtained in the step (1) in a dimethyl sulfoxide (DMSO) solution, and heating and stirring the solution until the solution is uniform. Then, uniformly coating the solution on a substrate plate in a blade mode, floating the substrate above liquid nitrogen (coating the upper surface of the substrate in a blade mode, and sticking the lower surface of the substrate on the liquid nitrogen) until the substrate is completely crystallized, then soaking the substrate in water for 12-24 hours, and airing to obtain a super-hydrophobic film; wherein the mass ratio of the PVDF to the defluorinated PVDF is 3-1:1 (e.g. 3.
Based on the above technical scheme, preferably, in the step (1), the concentration of the sodium sulfite solution is 1-2wt%, preferably 1.2wt%.
Based on the above technical solution, preferably, in step (1), the alkali in the hot alkali solution is KOH, and the solvent is methanol.
Based on the above technical scheme, preferably, in the step (2), the concentration of the defluorinated PVDF is 5-7wt%, and preferably 6%.
Based on the technical scheme, the time for complete crystallization in the step (2) is preferably 3-9s.
Based on the above technical solution, preferably, the substrate is at least one of a glass substrate, an Al substrate, and a Cu substrate.
Based on the above technical solution, preferably, in the step (2), the thickness of the coating is 200 to 300 μm, and preferably 250 μm.
Based on the above technical scheme, preferably, in the step (2), the substrates are one layer, two layers or three layers, the substrates in different layers are mutually attached, and the thickness of each layer of substrate is 1-3mm.
Based on the technical scheme, preferably, when the substrate is a layer of glass substrate, the upper surface of the glass substrate is used as a coating surface, and the time for the lower surface to be attached to liquid nitrogen for complete crystallization is 5-9s; when the substrate is an Al substrate or a Cu substrate, the upper surface of the Al substrate or the Cu substrate is used as a coating surface, and the time for the lower surface to be attached to liquid nitrogen for complete crystallization is 3-7s.
Based on the technical scheme, preferably, when the substrate is a two-layer Al substrate, the upper layer Al substrate is a coating surface, the other (lower layer) Al substrate is a cooling surface, and the time for the lower layer Al substrate to be attached to liquid nitrogen to be completely crystallized is 3-5s (marked as A/A); when the substrate is two-layer glass substrate, the upper layer glass substrate is a coating surface, the other (lower layer) glass substrate is a cooling surface, and the time for the lower layer glass substrate to be completely crystallized after being pasted on liquid nitrogen is 5-7s (marked as G/G) ； When the substrates are two glass substrates and one Al substrate, the uppermost glass substrate is taken as a coating surface, the other middle glass plate is taken as an intermediate layer, the other (lowermost) Al substrate is taken as a cooling surface, and the time for the lowermost Al substrate to be attached to liquid nitrogen to be completely crystallized is 5-7s (marked as AG/G); when the substrate is a three-layer glass substrate, the uppermost glass substrate is used as a coating surface, the other middle glass plate is used as an intermediate layer, the other (lowermost) glass substrate is used as a cooling surface, the time for the lowermost glass substrate to be completely crystallized after being attached to liquid nitrogen is 7-9s (marked as GG/G), and the crystallization change is observed to investigate the change of the contact angle, the surface morphology and the aperture of the film.
The invention has the beneficial effects that: the pore-size-adjustable super-hydrophobic PVDF membrane is formed by means of co-crystallization of solutes (PVDF and defluorinated PVDF) and solvents (DMSO) on different combined substrates. Crystallization of the solute results in a graded two-dimensional roughness structure, forming a superhydrophobic film. The solvent crystallization enables the membrane to obtain ultrahigh permeation flux and realizes the dynamic regulation and control of the pore diameter. Compared with the traditional pore-forming agent adding and other pore-size adjusting modes, the method is simpler, more efficient and more environment-friendly. In general, the super-hydrophobic membrane prepared by the method meets the membrane requirement for membrane distillation, has stable super-hydrophobicity and high flux property, and is very competitive in the field of membranes for membrane distillation.
Drawings
FIG. 1 is a graph of the water contact angle of a PVDF/defluorinated PVDF crystalline surface.
FIG. 2 is a graph of the effect of different substrates on the crystallization of PVDF and defluorinated PVDF.
FIG. 3 is a scale exploration of the preparation of superhydrophobic membranes.
FIG. 4 is a graph showing the effects of different substrates on the water contact angle and morphology of the membrane surface.
FIG. 5 is a graph of the effect of modulating different substrates on membrane pore size.
FIG. 6 is a schematic view of a membrane distillation apparatus.
FIG. 7 is a graph of the effect of membrane distillation application.
Detailed Description
The following non-limiting examples are presented to enable those of ordinary skill in the art to more fully understand the present invention and are not intended to limit the invention in any way.
The test methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available, unless otherwise specified.
Example 1
(1) Preparation of defluorinated polyvinylidene fluoride (PVDF): commercial PVDF powder was subjected to hot base modification under KOH:10 weight percent, the temperature is 60 ℃, and the time is 0,1,2,3,4,5h respectively. Then the mixture is put into a 1.2wt% sodium sulfite solution for cooling, the substrate is reserved, and the substrate is washed to be neutral by clear water and then is dried for standby.
(2) Preparing a nano PVDF crystal: respectively dissolving the modified materials in the step (1) at different time periods in dimethyl sulfoxide (DMSO) (6 wt%), heating and stirring to be uniform. Then, the uniform solution is coated on a glass substrate (10 cm multiplied by 1 mm) in a scraping way, the thickness of the coating is 250 mu m, then the glass plate floats above liquid nitrogen to be completely crystallized (the glass substrate is two layers, the size of each layer is the same, the upper layer glass substrate is the coating surface, the lower layer glass substrate is the cooling surface, the lower layer glass substrate is pasted on the liquid nitrogen to be completely crystallized for 5 to 7 s), and then the glass plate is placed in water to be soaked for 12h and dried for standby. The resulting surface was measured for contact angle as shown in FIG. 1. The modification time is 0-2h, and the water contact angle shows a gradient descending trend due to the increase of hydrophilic groups. However, with the decrease of crystallinity and the increase of roughness, the water contact angle tends to increase in 3-5h, and a superhydrophobic surface is formed in 4 and 5h, and the water contact angle is more than 150 degrees.
Example 2
(1) Preparation of defluorinated polyvinylidene fluoride (PVDF): commercial PVDF powder was subjected to hot base modification under KOH:10wt%, 60 deg.C and 5h. Then the mixture is put into a 1.2wt% sodium sulfite solution for cooling, the substrate is reserved, and the substrate is washed to be neutral by clear water and then is dried for standby.
(2) Preparation of crystals with adjustable size: dissolving the PVDF or the unmodified PVDF defluorinated in the step (1) in a dimethyl sulfoxide (DMSO) solution (6 wt%), and heating and stirring until the mixture is uniform. Then, the uniform solution is scraped and coated on different substrates (each substrate is 10cm multiplied by 1mm in size), the scraped and coated substrates float above liquid nitrogen until complete crystallization, and then the substrates are placed in water for soaking for 12 hours and dried for later use. When the substrates are two layers of Al substrates, one Al substrate is taken as a coating surface, the other Al substrate is taken as a cooling surface, and the substrates are stuck on liquid nitrogen until the complete crystallization time is 3-5s (marked as A/A); when the substrate is two layers of glass substrates, the upper layer of glass substrate is a coating surface, the other lower layer of glass substrate is a cooling surface, and the time for complete crystallization is 5-7s (marked as G/G) after the glass substrates are attached to liquid nitrogen; when the substrates are two glass substrates and one Al substrate, the glass substrate is a coating surface, the other glass substrate is an intermediate layer, the other Al substrate is a cooling surface, and the substrates are stuck on liquid nitrogen for 5-7s (marked as AG/G) of complete crystallization; when the substrate is a three-layer glass substrate, the upper layer glass substrate is a coating layer surface, the other glass plate is an intermediate layer, the other glass substrate is a cooling surface, and the time for complete crystallization is 7-9s (marked as GG/G) after being pasted on liquid nitrogen; the substrate after the crystallization was observed for the change of the crystallization, and the drawn-down substrate (two glass substrates) was immersed in liquid nitrogen for 3 to 5 seconds to form a film (denoted as LN). As shown in fig. 2, wherein (a) commercial PVDF (P-PVDF) and defluorinated PVDF (D-PVDF) crystallized WCA on different substrate combinations. (b) a schematic of the temperature gradient of the substrate assembly. (c) SEM morphology of commercial PVDF crystallized by immersion in liquid nitrogen. (d) SEM morphology of commercial PVDF crystallized on GG/G substrate. (e) SEM morphology of defluorinated PVDF soaked in liquid nitrogen. (f) SEM morphology of defluorinated PVDF crystals on A/A substrates. (G) SEM morphology of the crystallization of defluorinated PVDF on G/G substrates. (h) SEM morphology of crystalline defluorinated PVDF on AG/G substrates. (i) SEM morphology of defluorinated PVDF crystallized on GG/G substrate. Here, we found that with the change of the substrate, the water contact angle and the aggregate morphology of the unmodified PVDF surface did not change significantly, but the defluorinated PVDF changed significantly. The faster the temperature is reduced, the smaller the crystalline size of the defluorinated PVDF, the greater the surface roughness that is formed. The larger the crystal size of the defluorinated PVDF formed with the increased heat transfer resistance of the substrate (resistance of glass substrate > aluminum plate, larger thickness, larger temperature heat transfer resistance), the more the material loses super-hydrophobicity on GGG substrate, wherein AGG is the critical condition for forming super-hydrophobic membrane surface.
Example 3
(1) Preparation of defluorinated polyvinylidene fluoride (PVDF): commercial PVDF powder was subjected to hot base modification under KOH:10wt%, 60 deg.C and 5h. Then the mixture is put into a 1.2wt% sodium sulfite solution for cooling, the substrate is kept, and the mixture is washed by clear water to be neutral and then is dried for standby.
(2) Preparing a super-hydrophobic membrane: in order to construct a double-scale hierarchical structure, PVDF and the defluorinated polyvinylidene fluoride (PVDF) obtained in the step (1) are dissolved in a dimethyl sulfoxide (DMSO) solution (6 wt%), the mass ratio of the PVDF to the defluorinated polyvinylidene fluoride (PVDF) is 3:1,2:1 and 1:1 respectively, and the mixture is heated and stirred to be uniform. Then, coating the uniform solution on a substrate plate in a blade mode, floating the substrate above liquid nitrogen until the substrate is completely crystallized, soaking the substrate in water for 12-24 hours, and airing to obtain a super-hydrophobic film; and pure PVDF films made of PVDF alone without defluorinated polyvinylidene fluoride (PVDF) were used as a comparison. An Atomic Force Microscope (AFM) was used to observe the film surface roughness as shown in fig. 3. Wherein, (a) WCA of pure PVDF membrane and membranes of different proportions; (b) SEM morphology of pure PVDF membrane. (c) SEM morphology of membrane prepared with mass ratio of PVDF to defluorinated PVDF of 3:1. (d) SEM morphology of the membrane prepared with a mass ratio of PVDF to defluorinated PVDF of 2:1. (e) SEM morphology of the membrane prepared with a mass ratio of PVDF to defluorinated PVDF of 1:1. (f) AFM morphology of pure PVDF films. (g) The mass ratio of PVDF to defluorinated PVDF is 2:1. AGG is taken as a substrate, and found that 3:1 and 2:1 can form films, and the proportional lower surface of 1:1 collapses, which is not beneficial to film preparation. In order to ensure super-hydrophobicity, 2:1 is selected as the optimal proportion for preparing the membrane.
Example 4
The defluorinated PVDF of example 2 was replaced with PVDF and defluorinated PVDF in example 2 (mass ratio of PVDF to defluorinated PVDF was 2:1), and a film was prepared by combining the substrates of example 2 to examine the change in contact angle and the change in surface morphology of the film, as shown in fig. 4. Wherein (a) the contact angles of films formed on different substrates; (b) Immersing the substrate in liquid nitrogen to form the surface appearance of a film; (c) forming a surface topography of the film on the a/a substrate; (d) forming a surface topography of the film on the G/G substrate; (e) forming a surface topography of the film on the AG/G substrate; (f) forming a surface topography of the film on the GG/G substrate. The crystal morphology of the film on different substrates is different, and along with the increase of the heat transfer resistance of the substrate, the size of the crystal on the surface of the film is increased, and the contact angle is reduced. The membrane formed on the surface of GGG loses superhydrophobicity, so AGG substrates are also a critical condition for the membrane preparation process.
The change of the pore diameter of the prepared membrane is considered, the pore diameter of the membrane can be changed by adjusting the size of crystals of different substrates, and the pore diameter can be reduced by nearly half on the premise of ensuring super-hydrophobicity, as shown in figure 5. Wherein (a) the substrate is entirely immersed in liquid nitrogen to form a surface topography of the film; (b) forming a surface topography of the film on the a/a substrate; (c) forming a surface topography of the film on a G/G substrate; (d) forming a surface topography of the film on the AG/G substrate; (e) forming a surface topography of the film on the GG/G substrate.
Example 5
FIG. 6 is a schematic diagram of a membrane distillation apparatus in which hot saline influent flows under the drive of a peristaltic pump, water vapor permeates the other side of the membrane through the superhydrophobic membrane and is cooled to fresh water, the weight gain is recorded by an electronic balance as the cooling water is circulated into the fresh water reservoir, and the conductivity of the effluent is measured by a conductivity meter.
Direct Contact Membrane Distillation (DCMD) experiments were used to evaluate the application performance of the superhydrophobic PVDF membrane, as shown in fig. 7. The membrane distillation performance was evaluated comprehensively using different conditions including water inlet end temperature, feed concentration, pH environment and presence/absence of organic matter (humic acid, HA). Wherein the temperature of the cold water end is fixed at 20 ℃, when the influence of the temperature is studied, the temperature of the hot water end is fixed at 50, 70 and 90 ℃, the concentration of salt (NaCl) is 35g/L, the pH value is kept in a neutral state, and no organic matter exists; when the influence of the salt concentration is discussed, the temperature of the hot end is fixed at 70 ℃, the salt concentration is 35, 50 and 75g/L, the pH value is kept in a neutral state, and no organic matter exists; when the influence of pH is discussed, the temperature of the hot end is fixed at 70 ℃, the salt concentration is 35g/L, the pH is adjusted to 3,5,7,9, 11, and no organic matter exists; when the influence of the presence of organic substances on the membrane distillation process was examined, the hot-end temperature was fixed at 70 deg.C, the salt concentration was 35g/L, the pH was kept neutral, and the organic substances (HA: 10 mg/L) were compared with the case of no organic substances.
As can be seen from fig. 7a, the hot water side temperature is fixed at 50, 70, 90 deg.c, respectively, and the cold side fresh water side temperature is fixed at 20 deg.c. The initial flux of the super-hydrophobic membrane reaches 68.43, 86.83 and 109.69L/m at 50, 70 and 90 ℃ respectively 2 H. This indicates that the membrane can withstand the pressure of high temperature steam (90 ℃) and has good desalination capacity under different temperature conditions. As can be seen from fig. 7b, the membrane ensures high rejection of salts at different salt concentrations. It can be seen from fig. 7c that for different pH conditions, the flux was similar at pH 3,5,7,9, while at pH 11 the flux decreased due to slight defluorination. However, due to the excellent stability of the film, the amount of repulsion is almost constant. Meanwhile, as can be seen from fig. 7d, the presence of HA HAs little effect on the flux of the membrane, which indicates the antifouling performance of the superhydrophobic membrane. The WCA of the membranes after the distillation experiments were all above 140 ° ensuring high rejection of the salt solution (see fig. 7 e), which also illustrates the superior application properties of the membranes. In thatIn the present invention, since the membrane has stable superhydrophobicity, the rejection rate of NaCl is always maintained at 99.5% or more.
Claims (10)
1. A method for dynamically adjusting the pore size of a super-hydrophobic membrane by regulating and controlling a crystallization process is characterized by comprising the following steps:
(1) Preparation of defluorinated PVDF: performing hot alkali modification on PVDF powder, wherein the modification conditions are as follows: the concentration of alkali in the hot alkali solution is 8-12wt%, the temperature is 50-70 ℃, the time is 4-6h, then the mixed solution after reaction is added into sodium sulfite solution for cooling, the substrate is retained, and the substrate is washed to be neutral by water and then dried, so as to obtain the defluorinated PVDF;
(2) Preparing a super-hydrophobic membrane: dissolving PVDF and the defluorinated PVDF obtained in the step (1) in dimethyl sulfoxide, stirring to be uniform, then coating the solution on a substrate in a blade mode, floating above liquid nitrogen until the solution is completely crystallized, then placing the substrate in water for soaking for 12-24h, and airing to obtain a super-hydrophobic membrane; wherein the mass ratio of the PVDF to the defluorinated PVDF is 3-1:1.
2. The process according to claim 1, wherein in step (1), the concentration of the sodium sulfite solution is 1-2wt%.
3. The method according to claim 1, wherein in step (1), the base in the hot alkaline solution is KOH, and the solvent is methanol.
4. The method according to claim 1, wherein in step (2), the concentration of the defluorinated PVDF is between 5 and 7wt%.
5. The method according to claim 1, wherein in the step (2), the substrate is at least one of a glass substrate, an Al substrate, and a Cu substrate.
6. The method of claim 1, wherein in step (2), the coating has a thickness of 200-300 μm.
7. The method according to claim 1, wherein in the step (2), the time to complete crystallization is 3 to 9s.
8. The method according to claim 1, wherein in the step (2), the substrate is one layer, two layers or three layers, and the thickness of each layer of the substrate is 1-3mm.
9. The method according to claim 5 or 8, wherein when the substrate is a glass substrate, the time to complete crystallization is 5-9s; and when the substrate is an Al substrate or a Cu substrate, the time for complete crystallization is 3-7s.
10. The method according to claim 5 or 8, wherein when the substrate is a two-layer Al substrate, the upper layer Al substrate is a coating surface, the lower layer Al substrate is a cooling surface, and the time for the lower layer Al substrate to be attached to liquid nitrogen to be completely crystallized is 3-5s; when the substrate is two layers of glass substrates, the upper layer of glass substrate is a coating surface, the lower layer of glass substrate is a cooling surface, and the time for the lower layer of glass substrate to be completely crystallized after being pasted on liquid nitrogen is 5-7s ； When the substrates are two layers of glass substrates and one layer of Al substrate, the uppermost glass substrate is taken as a coating surface, the middle glass plate is taken as a middle layer, the lowermost Al substrate is taken as a cooling surface, and the time for the lowermost Al substrate to be attached to liquid nitrogen to be completely crystallized is 5-7s; when the substrate is a three-layer glass substrate, the uppermost glass substrate is taken as a coating surface, the middle glass plate is taken as a middle layer, the lowermost glass substrate is taken as a cooling surface, and the time for the lowermost glass substrate to be attached to liquid nitrogen for complete crystallization is 7-9s.
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