CN115230252B - Aerogel multilayer spacing material and preparation method thereof - Google Patents
Aerogel multilayer spacing material and preparation method thereof Download PDFInfo
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- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
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- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C67/00—Shaping techniques not covered by groups B29C39/00 - B29C65/00, B29C70/00 or B29C73/00
- B29C67/24—Shaping techniques not covered by groups B29C39/00 - B29C65/00, B29C70/00 or B29C73/00 characterised by the choice of material
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- B32B27/281—Layered products comprising a layer of synthetic resin comprising synthetic resins not wholly covered by any one of the sub-groups B32B27/30 - B32B27/42 comprising polyimides
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- B32B9/00—Layered products comprising a layer of a particular substance not covered by groups B32B11/00 - B32B29/00
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- C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
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- C08J2205/02—Foams characterised by their properties the finished foam itself being a gel or a gel being temporarily formed when processing the foamable composition
- C08J2205/026—Aerogel, i.e. a supercritically dried gel
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- C08J2379/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen, or carbon only, not provided for in groups C08J2361/00 - C08J2377/00
- C08J2379/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors
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Abstract
The invention relates to the technical field of heat insulation materials, in particular to an aerogel multilayer spacing material and a preparation method thereof. The aerogel multilayer spacing material comprises aerogel spacing layers and reflecting screens, wherein the aerogel spacing layers and the reflecting screens are arranged in a staggered mode, and one layer of reflecting screen is arranged on each of two sides of any one aerogel spacing layer; the number of layers of the aerogel spacing layer is n, the number of layers of the reflecting screen is n +1, and the value of n is more than or equal to 2; the aerogel spacer layer gradually decreases in thickness from one side of the aerogel multilayer spacer material to the other. The aerogel spacing layer is prepared by adopting a directional freezing technology, so that the aerogel spacing layer has an anisotropic porous structure and has different heat conductivity coefficients along different directions. The aerogel multilayer spacing material has wide application prospect in aerospace and various fields.
Description
Technical Field
The invention relates to the technical field of heat insulation materials, in particular to an aerogel multilayer spacing material and a preparation method thereof.
Background
The increasing depletion of fossil fuels has led to a growing awareness of the importance of thermal insulation materials for industrial production and life. The heat insulation materials (including cellular glass, rigid foam or spray foam) generally adopted by the industry at present can be roughly divided into two types, one is an organic heat insulation material, such as phenolic foam, polyurethane foam and the like, and is suitable for short-term low-temperature heat insulation occasions; and another inorganic heat-insulating material, such as ceramic fiber and the like, is more suitable for heat-insulating occasions with higher temperature, but the heat-insulating property of the inorganic heat-insulating material is reduced along with the increase of the temperature. In addition to the above two materials, a Vacuum Insulation Panel (VIP) composed of a core insulation material, a gas adsorption material and a gas barrier film is widely used in the fields of cold chain transportation, aerospace and the like, but the operation environment of the VIP needs to maintain a high vacuum degree. Due to the aforementioned shortcomings of conventional insulation materials, such as the outer surface of spacecraft, space launch facilities, and other types of equipment that operate under severe environmental conditions such as high and low temperature alternating cycles, temperature shock, and intense radiation, there is a need for more effective thermal insulation cladding materials.
The multilayer thermal insulation (MLI) material is the most commonly used thermal insulation material under the vacuum strong radiation situation known at present, and has low density and good thermal insulation performance: the thermal conductivity under the ideal condition of vacuum can reach 10 at the lowest -5 On the order of magnitude, but the layer density is difficult to control, and the heat insulation performance is greatly reduced by heat leakage caused by interlayer contact due to environmental pressure or external load.
In addition, under certain temperature and surface radiation characteristics, the conventional purely passive thermal control method and device have limited capability of dissipating waste heat and cannot dynamically respond to solar heat gain when the temperature changes, and the conventional mechanical shutter, variable heat conduction pipe and other methods can increase the weight cost and implementation difficulty of a thermal control system and increase the complexity of system design.
Disclosure of Invention
The invention aims to solve the technical problem of providing an aerogel multilayer spacing material and a preparation method thereof.
The technical scheme for solving the technical problems is as follows:
the invention provides an aerogel multilayer spacing material which comprises aerogel spacing layers and reflecting screens, wherein the aerogel spacing layers and the reflecting screens are alternately arranged, and one layer of reflecting screen is arranged on each of two sides of any one aerogel spacing layer; the number of layers of the aerogel spacing layer is n, the number of layers of the reflecting screen is n +1, and the value of n is more than or equal to 2; the aerogel spacer layer tapers in thickness from one side of the aerogel multilayer spacer material to the other.
The invention can be implemented by the following further technical scheme:
further, the thickness of each aerogel spacing layer is D, dmin is the minimum value of D, dmax is the maximum value of D, and the thickness ratio of Dmax to Dmin is 3-4: 1.
further, the aerogel of the aerogel spacing layer has an anisotropic porous structure, the pore size of the porous structure is 1-100nm, and the porosity is greater than or equal to 90%.
Further, the aerogel of the aerogel spacing layer has different thermal conductivities along different directions, and the thermal conductivity of each direction at room temperature is less than or equal to 0.035W/(m.K).
Further, the aerogel spacing layer is made of one of silicon dioxide aerogel, metal oxide aerogel, organic aerogel and carbon aerogel.
Further, the metal oxide aerogel is one of zirconia aerogel, titania aerogel and alumina aerogel; the organic aerogel is one of polyimide aerogel, resorcinol-formaldehyde aerogel, polydicyclopentadiene aerogel, polyisocyanate aerogel, polyurethane-based aerogel and cellulose-based aerogel; the carbon aerogel is graphene aerogel.
Furthermore, intelligent thermal control coatings are respectively arranged outside the reflecting screens at the two sides; the intelligent thermal control coating comprises a high-reflectivity bottom layer coated on the outer surface of the reflecting screen and a thermochromic coating coated on the high-reflectivity bottom layer; the thermochromic coating comprises alternating layers of vanadium dioxide and silicon.
The invention provides a preparation method of the aerogel multilayer spacing material, which comprises the steps of preparing a plurality of aerogel spacing layers, and then alternately arranging and fixing the aerogel spacing layers and a plurality of reflecting screens.
Further, the aerogel spacing layer is prepared from polyamic acid and comprises the following steps:
s1), sequentially carrying out electrospinning and thermal imidization on polyamic acid to prepare a polyimide nanofiber membrane;
s2) cutting the polyimide fiber membrane into small pieces, and putting the small pieces into an N-methylpyrrolidone solution to obtain a suspension; the mass ratio of the polyimide fiber membrane to the N-methyl pyrrolidone is 1;
s3), homogenizing the suspension obtained in the step S2) for the first time to obtain a polyimide nanofiber solution;
s4), carrying out suction filtration and drying on the polyimide nanofiber solution to obtain polyimide fibers;
s5), mixing the polyimide nano fibers with benzoxazine powder to obtain a mixture; the mass ratio of the polyimide nano fiber to the benzoxazine powder is 4:1;
s6), dissolving the mixture obtained in the step S5) in a 1, 4-dioxane solution to obtain a mixed solution; the mass ratio of the total mass of the mixture to the 1, 4-dioxane solution is 5:1000, parts by weight;
s7) homogenizing the mixed solution obtained in the step S6) for the second time to obtain polyimide nanofiber dispersion liquid, and then carrying out ultrasonic oscillation on the polyimide nanofiber dispersion liquid;
s8), directionally freezing the polyimide nanofiber dispersion liquid until the polyimide nanofiber dispersion liquid is completely frozen; the temperature of the directional freezing is 0 ℃, 10 ℃ below zero or 50 ℃ below zero;
s9), putting the completely frozen sample into a freeze dryer for freezing and vacuum drying treatment to obtain a cross-linked polyimide aerogel framework;
s10), putting the crosslinked polyimide aerogel framework into a high-temperature furnace for heating treatment, and completing thermal crosslinking to obtain the polyimide aerogel.
Further, in the step S10, the temperature rise treatment is performed at 200 ℃ for 30min at a temperature rise rate of 10 ℃/min.
The invention has the beneficial effects that:
1) The aerogel multilayer spacing material of the invention introduces aerogel into the multilayer spacing material, and the thickness of the aerogel spacing layer is set to be gradually reduced; in use, the side with the greater thickness is adjacent the cold boundary and the side with the lesser thickness is adjacent the hot boundary to reduce heat leak for a given system.
2) According to the aerogel multilayer spacing material, the aerogel spacing layer has ultrahigh porosity, the pore size of the nanometer skeleton is 1-100nm, and the nanometer porosity can reach more than 90%; the material greatly reduces the heat leakage caused by solid heat conduction and gas heat conduction, has excellent heat insulation property, and has very low density almost close to air; 3) The solar absorptivity and emissivity characteristics of the intelligent thermal control coating can be correspondingly changed along with the temperature change, the outermost surface of the aerogel multilayer thermal insulation material can be automatically controlled by temperature, when the working temperature is too high, the heat dissipation with high emissivity is realized, and when the temperature is too low, the heat preservation characteristic with low emissivity is realized. By controlling the critical temperature, continuous adjustment of different thermal emissivity and solar absorptivity characteristics of the material can be realized at different temperatures.
4) According to the preparation method disclosed by the invention, the internal microstructure of the polyimide aerogel can be adjusted in a directional freezing manner, and the equivalent thermal conductivity of the aerogel spacing layer is further reduced in a set direction.
5) The aerogel multilayer spacing material has a wide applicable temperature range, and can provide excellent heat preservation and insulation effects for various extremely complex environments such as liquid hydrogen, liquid nitrogen, liquid oxygen or liquid methane and the like under low-temperature liquid and low-temperature scenes.
6) The aerogel multilayer spacing material can resist the combined action of various severe conditions such as all-weather air pressure exposure environment, vibration and structural load impact conditions, and the like, and the light heat-insulating material with heat-insulating property stability can be widely concerned in aerospace and various fields, and has wide application prospect.
Drawings
FIG. 1 is an electron micrograph of an ordered array microstructure of polyimide aerogel in example 1, with a dimension of 500um, of an aerogel multilayer spacer material according to the present invention;
FIG. 2 is an electron micrograph of an aerogel multilayer spacer material according to the present invention, in example 1, a polyimide aerogel ordered arrangement microstructure, with a dimension of 200um;
FIG. 3 is a flow chart of an iterative method for optimizing aerogel spacer layers of different thicknesses in example 2, which is an aerogel multilayer spacer material of the present invention.
Detailed Description
The principles and features of this invention are described below in conjunction with the following drawings, which are set forth by way of illustration only and are not intended to limit the scope of the invention.
The aerogel multilayer spacing material comprises aerogel spacing layers and reflecting screens which are arranged in a stacked mode, wherein the aerogel spacing layers and the reflecting screens are arranged in a staggered mode, and a layer of reflecting screen is arranged on each of two sides of any one aerogel spacing layer; the number of layers of the aerogel spacing layer is n, the number of layers of the reflecting screen is n +1, and the value of n is more than or equal to 2; the thickness of the aerogel spacer layer gradually decreases from one side of the aerogel multilayer spacer material to the other.
The aerogel multilayer spacing material of the invention introduces aerogel into the multilayer spacing material, and the thickness of the aerogel spacing layer is set to be gradually reduced; in use, the side with the greater thickness is adjacent the cold boundary and the side with the lesser thickness is adjacent the hot boundary to reduce heat leak for a given system.
The reason for arranging the aerogel spacing layers with different thicknesses is that in practical use, under the conditions of different temperatures, different gas pressure atmospheres, different gas compositions, different material compositions and the like of the use environment, the heat insulation property of the multi-layer heat insulation material is also influenced. For example, the radiation at the cold and hot boundaries is significantly different from the thermal conductivity. Under the condition of low temperature, the solid heat conduction and radiation heat transfer through the multi-layer heat insulation material are larger than those under the condition of high temperature, and the heat conduction and heat leakage heat flow cannot be effectively inhibited by increasing the number of the metal plating foils of the reflecting screen. Therefore, in order to further reduce the heat leakage between the internal environment and the external environment, the multilayer heat insulation material adopts the arrangement mode of the spacing layers with different thicknesses, and the thickness of the aerogel spacing layer is non-single so as to ensure that the aerogel spacing layer is matched with the heat transfer ratio of the aerogel spacing layer in different modes.
Near the cold boundary, the amount of thermal leakage of solid conductive heat through the aerogel material is inversely proportional to the material thickness. Since the temperature difference between the reflecting screens increases with decreasing temperature, solid conduction is much more important near the cold boundary than near the hot boundary. The effect of increasing the thickness of the spacer material near the cold boundary is therefore most pronounced.
Preferably, the thickness of each aerogel spacing layer is D, dmin is the minimum value of D, dmax is the maximum value of D, and the thickness ratio of Dmax to Dmin is selected from the range of 3-4: 1. More preferably, the D value is in the range of 0.1mm to 2 mm.
Preferably, the thickness of the aerogel spacing layer can be varied, and the thickness of the polyenergetic adjacent aerogel spacing layers can be the same. Taking three groups of thicknesses as an example, the thickness of D1-Di is D1, the thickness of Di +1-Dj is D2, the thickness of Dj-Dj is D3, the thickness relation of the spacer layer is D1> D2> D3, namely the maximum thickness of the inner layer D1 is Dmax, and Dn is Dmin.
The aerogel of the aerogel multi-layer spacing material has an oriented porous structure, the pore size of a nanoscale framework of the aerogel multi-layer spacing material is 1-100nm, the nano porosity is greater than or equal to 90%, and the thermal conductivity (thermal conductivity) at room temperature is less than or equal to 0.035W/(m.K).
The porous structure of the orientation of the aerogel enables the aerogel multilayer spacer material to have anisotropy and different thermal conductivity coefficients in different directions, and further enhances the thermal insulation performance of the aerogel.
According to the invention, the aerogel with ultralow heat conductivity is introduced into the traditional multilayer heat insulation (MLI) material to be used as the aerogel spacing layer, so that the problem of unstable layer density of the traditional MLI material is effectively solved. Meanwhile, the aerogel has light and excellent heat insulation characteristics due to the nanometer skeleton size and the nanometer porosity, the density is very low and almost close to air, and the stability of the heat insulation performance of the aerogel spacing layer is ensured.
The aerogel multilayer spacing material greatly reduces the heat leakage caused by solid heat conduction and gas heat conduction, thereby being capable of well isolating the external high temperature. Meanwhile, uncertain interlayer contact between the multilayer reflecting screens can be effectively prevented, so that the mechanical compression resistance of the aerogel multilayer spacing material is enhanced.
The structural form of the aerogel spacing layer material and the heat transfer principle thereof are that the heat transfer in the aerogel in a non-vacuum state is formed by three mechanisms: thermal conduction through the solid framework, gas phase heat transfer and radiative heat transfer present in the open pore aerogel structure. For solid framework heat conduction, according to a phonon diffusion model of Debye, the mean free path of phonons in an aerogel framework is lower than that of most dielectric particles, so that extremely low framework heat conductivity is shown, and meanwhile, the structural characteristics of a nano network and high porosity are realized, so that the volume density is greatly reduced, the heat transfer path is prolonged, and the solid heat conductivity coefficient is reduced; for gas phase heat conduction, the characteristic aperture in the aerogel reaches the nanometer level, and compared with the heat transfer in free gas, the heat transfer of gas phase in the material has good inhibition effect on gas convection heat transfer.
In addition, the aerogel of the invention can be doped with an opacifier, so that the optical property of the aerogel is improved, and the effect of inhibiting radiation heat transfer is achieved.
In the above embodiment, preferably, the specific connection manner of the aerogel multi-layer thermal insulation material of the present invention is that a label glue needle is used for connection and fixation, and compared with the conventional connection manner in which sewing threads are used for puncturing or materials of each layer are bonded together by using a high temperature resistant adhesive, such connection manner can reduce puncture holes, reduce heat leakage, and enhance mechanical properties.
In the above embodiment, preferably, the reflective screens at both sides are respectively provided with an intelligent thermal control coating; the intelligent thermal control coating comprises a high-reflectivity bottom layer coated on the outer surface of the reflecting screen and a thermochromic coating coated on the high-reflectivity bottom layer; the thermochromic coating comprises alternating layers of vanadium dioxide and silicon.
To more effectively adapt to a dynamic temperature changing environment, a smart thermal control coating is introduced into the outermost layer of the multi-layer aerogel insulation material: the smart thermal control coating is represented by a combination of a high reflectivity underlayer and a thermochromic coating. Depending on the desired properties for a particular application, the combination of a base layer and a coating layer with different spectral properties can be selected to control the amount of radiation that can be introduced and removed to provide a surface of the outermost layer of the multi-layer aerogel insulation with properties that can be tailored to provide a particular thermal emissivity and/or solar absorption at a particular temperature.
In the above embodiments, it is preferable to use alternating vanadium dioxide layers and silicon layers as the thermochromic coating, which shows different spectral characteristics in terms of heat amounts below the transition critical temperature and above the critical temperature. VO when the temperature is lower than the critical temperature Tc 2 The nanoparticles are in an insulating state, in which case the material is transparent to solar radiation. VO when the temperature is higher than Tc 2 The nanoparticles are converted into a metal phase, the visible light band can penetrate through the material, and the near-infrared band is partially blocked, so that the overall emissivity is increased at a temperature higher than the critical temperature, the heat dissipation capacity is increased, and the emissivity is reduced at a temperature lower than the critical temperature, so that the heat preservation effect is realized. In addition, VO can be quantitatively changed by adjusting the amount ratio of the vanadium dioxide layer and the silicon layer or doping other elements such as magnesium or tungsten 2 The phase transition temperature of the phase transition device achieves the purpose of dynamic thermal control.
In the above embodiments, it is preferred that the high reflectivity bottom layer comprise, but not limited to, a combination of solar barrier multilayer coatings of alternating magnesium fluoride and zinc sulfide layers, with some alternative materials including BiF 3 、CaF 2 、CeO 2 、CeF 3 、ZrO 2 And ZnS and other high-reflectivity materials.
Preferably, the material of the aerogel spacing layer is one of silicon dioxide aerogel, metal oxide aerogel, organic aerogel and carbon aerogel. The specific material selection can be determined according to the actual application scene, and the specific characteristics of the various aerogels are as follows:
the silicon dioxide aerogel is an inorganic non-metallic material which is prepared by carrying out sol-gel reaction on silicon sources such as silicon ester and the like and is constructed by silicon dioxide particles and has a three-dimensional nano porous network structure, the lowest thermal conductivity under atmospheric pressure is not higher than 0.03W/(m.K), and the thermal conductivity coefficient under medium vacuum degree can reach 0.004W/(m.K). Meanwhile, the density can reach 3mg/cm at the lowest 3 The maximum specific surface area can reach 1200m 2 The maximum thermal resistance value can reach 10 times of that of a typical wall insulation layer with the same thickness. In addition, the material has the characteristics of ultralow dielectric constant, low sound wave propagation rate, high optical transparency (the highest light transmittance can reach 90 percent) and the like. Therefore, the silicon dioxide aerogel has wide application prospects in the fields of heat insulation, aerospace, sound insulation and noise reduction, catalytic carriers, adsorption cleaning, biomedicine and the like.
The metal oxide aerogel has a wide application temperature range and is ZrO 2 And TiO 2 2 For example, ambient temperatures up to 800 ℃ may be used. After the special modification and enhancement process, the environmental temperature can be raised to over one thousand ℃. In addition, the metal oxide aerogel has stable high temperature thermal conductivity, and in the case of alumina aerogel, the porous structure of the metal oxide aerogel is formed by alumina nanoparticles. The alumina aerogel not only has various properties of common aerogels, but also has other characteristics, and is mainly characterized in that the microstructure of the alumina aerogel is jointly composed of amorphous state and polycrystalline state, has better high temperature resistance and thermal stability than the silica aerogel, the thermal conductivity of the alumina aerogel is only 0.029 mW/(m.K) at 30 ℃, the thermal conductivity is only 0.098W/(m.K) at 800 ℃, the thermal conductivity is only 0.11W/(m.K) when approaching the upper limit of the service temperature, the maximum service temperature can reach more than 1000 ℃, and the alumina aerogel can be used as an aviation aircraft and can be used in the fields of aviation and aerogelsAnd (4) an insulation layer under an extreme environment.
Common organic aerogels include resorcinol-formaldehyde (RF) aerogel, polydicyclopentadiene (PDCPD) aerogel, polyisocyanate aerogel, polyurethane-based aerogel (PU), cellulose-based aerogel, polyimide (PI) aerogel, and the like. The solid framework of organic aerogels has inherently lower thermal conductivity than inorganic aerogels; in terms of mechanical properties, organic aerogels exhibit better compressibility and resilience, and better durability. In addition, phenolic aerogel is another common organic aerogel, and is an aerogel material which is obtained by utilizing a chemical reaction between a phenol compound and an aldehyde compound to form a large number of nanoclusters in a solution, crosslinking the nanoclusters with each other through surface functional groups of the nanoclusters to form gel, and performing supercritical drying, wherein the density of the aerogel material is generally not more than 0.06g/cm 3 The specific surface area reaches 350m at least 2 The pore diameter is not more than 50nm, the size of the network colloid particle is generally in the range of 3 to 20nm, the minimum room temperature thermal conductivity is 0.012W/(m.K), and the low-temperature freezing target material can be used for preparing low-temperature freezing target materials for adsorbing nuclear fusion fuels.
Carbon aerogels absorb radiation and may reflect only 0.3% of the radiation at a minimum in the 250nm to 14.3 μm wavelength range. The maximum pyrolysis temperature of the aerogel treated by a special process can reach 2500 ℃, and the aerogel can be used under the condition of extremely high temperature. The graphene aerogel is a three-dimensional ultra-light porous macroscopic material assembled by graphene sheets with a single atomic layer in a specific structure. The unique two-dimensional structure of graphene gives the aerogel material excellent properties such as ultra-low density (not more than 10 mg/cm) 3 ) The composite material has the advantages of ultrahigh porosity (the porosity can generally reach more than 99%), compressive superelasticity, good environmental stability, excellent photo-thermal conversion and electrothermal conversion capabilities, excellent wave-absorbing and sound-absorbing properties and good electrical conductivity, so that the composite material has great application potential in the fields of energy storage and conversion, environment, sensors and the like. It is worth mentioning that the super-elastic behavior of the graphene aerogel prepared by the special method is almost unchanged in the range from the deep low temperature of-269.15 ℃ to the high temperature of 1000 ℃, and the graphene aerogel has the same elasticity as the graphene aerogel prepared by the special method under the ultralow temperature condition of-269.15 DEG CMechanical properties at the same temperature: almost completely reversible super-elastic behavior (strain up to 90 percent), unchanged Young modulus, nearly zero Poisson ratio and good cycle stability, and is expected to be used for heat preservation and heat insulation at deep low temperature such as outer space and the like.
Preferably, the reflective screen in the aerogel multilayer spacer material of the present invention can be selected from, but is not limited to, polyimide film, polyester film, polytetrafluoroethylene, aluminum foil, molybdenum foil, titanium-plated, tantalum foil, aluminum oxide-plated foil, stainless steel foil, nickel foil, tungsten foil.
The durable temperature ranges of the above-mentioned reflective screens are shown in table 1:
TABLE 1 durable temperature of each reflecting screen
| Kind of reflecting screen | Durable temperature range (degree centigrade) |
| KAPTON (polyimide film) | >-269,<+400 |
| MYLAR (polyester film) | >-60,<+150 |
| TEFLON (Polytetrafluoroethylene) | <327 |
| PEEK (polyether-ether-ketone) | <300 |
| Aluminum foil | <660 |
| Molybdenum foil/titanium plating | <2930 |
| Tantalum foil | <2000 |
| Aluminum oxide coated foil | <2054 |
| Stainless steel foil | <800 |
| Nickel foil | <900 |
| Tungsten foil | <3000 |
The preparation method of the aerogel multilayer spacing material comprises the steps of firstly preparing the aerogel spacing layer and then fixing the aerogel spacing layer and the reflecting screen, wherein the filling, fixing or combining modes of the aerogel spacing layer and the reflecting screen can be carried out by adopting a conventional method in the field, and the specific fixing mode can also be selected from the label glue needle binding mode.
The preparation method of the aerogel spacing layer comprises the following steps: preparing wet gel, directionally freezing the wet gel, freeze-drying the directionally frozen wet gel to obtain an aerogel framework, and finally carrying out temperature programming treatment on the aerogel framework to obtain the aerogel.
The directional freezing is a freezing technology, belonging to the ice template method. The invention adopts a directional freezing mode to prepare the aerogel, can prepare the aerogel with a directional porous structure, realizes anisotropy, and leads the aerogel to have different heat conductivity coefficients along different directions. Further enhancing the thermal insulation properties of the aerogel.
Preferably, the specific step of directional freezing is to add the wet gel to a directional freezing device containing a freezing medium, and the wet gel is positioned on one side of the freezing medium.
The temperature of the directional freezing is lower than the freezing point of the 1, 4-dioxane solution. The temperature is set by the experiment, and the selectable temperature values are 0 ℃, 10 ℃, 50 ℃ and the like. This temperature affects the directional freezing results, e.g., the lower the temperature, the smaller the material pores.
In the preparation method, the wet gel is generally prepared by a sol-gel method, and after the sol generates the gel with a certain space structure, the prepared gel is further dried to obtain the aerogel. The evaporation of the liquid within the gel will normally collapse the fragile matrix of the gel due to surface tension. But the liquid in the gel can be converted into solid by drying and then separated from the gel framework, so that the problem of framework collapse caused by liquid volatilization is effectively avoided, and the aerogel structure finally presenting an open type nano pore structure network configuration is obtained.
The technical solution of the present invention is illustrated by examples below.
Example 1 preparation of polyimide aerogel
This example illustrates a specific process for preparing an imide aerogel to illustrate the preparation method of the aerogel multi-layer thermal insulation material of the present invention.
The specific preparation steps of this example are:
s1), carrying out electric spinning and thermal imidization on polyamic acid in sequence to prepare the polyimide nanofiber membrane.
Preferably, in the polyimide nanofiber membrane, the diameter of the fiber is 200nm.
S2), cutting the polyimide fiber membrane into small pieces, and putting the small pieces into an N-methylpyrrolidone solution to obtain a suspension, wherein the mass ratio of the polyimide fiber membrane to the N-methylpyrrolidone is 1.
Preferably, the mass of the polyimide fiber chips is 5g, and the mass of the N-methylpyrrolidone is 500g.
S3), shearing and carrying out primary homogenization on the suspension in the step S2) by using a homogenizer to obtain a polyimide nanofiber solution; the homogenization time is 20min.
And S4), carrying out suction filtration and drying on the polyimide nanofiber solution to obtain the polyimide fibers.
S5), mixing the polyimide nano fiber with benzoxazine powder to obtain a mixture, wherein the mass ratio of the polyimide nano fiber to the benzoxazine powder is 4:1.
s6), dissolving the mixture obtained in the step S5) in a 1, 4-dioxane solution to obtain a mixed solution, wherein the mass ratio of the total mass of the mixture obtained in the step S5) to the 1, 4-dioxane solution is (5): 1000.
s7), homogenizing the mixed solution obtained in the step S6) for the second time, wherein the homogenizing time is 10min, so as to obtain polyimide nanofiber dispersion liquid, and then carrying out ultrasonic oscillation on the polyimide nanofiber dispersion liquid for 30min, so as to obtain the polyimide nanofiber.
S8), transferring the polyimide nanofiber dispersion liquid into an oriented freezing mould, and putting the oriented freezing mould into oriented freezing equipment for oriented freezing. In this example, the temperature of the directional freezing was-10 ℃.
In this embodiment, the cold source liquid in the directional freezing apparatus is located at the lower side of the freezing mold. The cold source carries out directional freezing on the polyimide nano-fiber dispersion liquid at the lower side of the polyimide nano-fiber dispersion liquid, so that the polyimide nano-fiber dispersion liquid forms an oriented pore diameter from bottom to top, thereby realizing different thermal conductivity coefficients in different directions, and the lowest thermal conductivity coefficient can be 0.014 mW/(m.K).
S9), putting the completely frozen sample into a freeze dryer for freeze vacuum drying, and treating for at least 24h to obtain the cross-linked polyimide aerogel framework.
S10), placing the aerogel framework into a high-temperature furnace, treating for 30min at 200 ℃, and completing thermal crosslinking at a heating rate of 10 ℃/min to obtain the polyimide aerogel.
S11), placing the aerogel framework into a high-temperature furnace for programmed heating treatment, specifically, treating at 200 ℃ for 30min, wherein the heating rate is 10 ℃/min, and completing thermal crosslinking to obtain the polyimide aerogel.
The polyimide aerogel of the embodiment is prepared by condensation reaction of polyanhydride and polyamine monomer, has good flexible mechanical property, low thermal conductivity, light weight, high elasticity, good (alternating) temperature resistance, a service temperature range of plus and minus three hundred degrees and radiation resistance.
As shown in fig. 1 and 2, the polyimide aerogel of this example has an ordered arrangement microstructure.
As a high-quality heat insulation material, the high-elasticity characteristic of the polyimide aerogel can be applied to coating the surface of a load structure with any geometric topology, but the polyimide aerogel as an organic aerogel is easy to generate structural damage under an aerobic environment to cause heat insulation performance failure, and the highest tolerance temperature is not more than 400 ℃.
Through tests, the polyimide aerogel of the embodiment realizes a thermal conductivity coefficient of 22.4 mW/(m.K) in the directional freezing direction, and the thermal conductivity in the direction perpendicular to the directional freezing direction is 25.2 mW/(m.K).
Example 2
In this embodiment, the polyimide aerogel prepared in embodiment 1 is used as the material of the aerogel spacer layer to obtain a specific number of layers and a thickness of the aerogel multilayer spacer material. And in the embodiment, the aerogel multilayer spacing material optimizes the thickness and the heat insulation effect of the container containing the cryogenic liquid under the condition of room temperature (298K).
Specifically, the number of layers of the aerogel spacer layer in this embodiment is 15, and the number of layers of the reflective screen is 16. The total number of layers of the aerogel multilayer insulation material was set to 15. The thickness of each aerogel spacing layer is sequentially represented as D1, D2, D3 \8230 \ 8230and D15 from a cold boundary close to the low-temperature storage container to a hot boundary far away from the low-temperature storage container, three different layer thickness spacing layers are available, the thickness of D1-Di is D1, the thickness of Di +1-Dj is D2, the thickness of Dj-D15 is D3, the three groups are counted, the thickness relationship of the spacing layers is D1> D2> D3, namely the maximum thickness of the inner layer D1 is Dmax, and D15 is Dmin.
In the case of insulating materials, the distance from the cryogenic container or installation has a significant influence on their insulating effect. In this embodiment, the cryogenic liquid in the cryogenic storage vessel is liquid nitrogen at a temperature of 4K or liquid hydrogen at a temperature of 20K.
For the 1 st to 3 rd groups of units consisting of the reflecting screen and the aerogel with different interval thicknesses, corresponding energy balance equations can be listed:
q total representing the total heat flow, it can be seen that the heat flow value through each cell is composed of a heat radiating portion together with a heat conducting portion, and according to the fourier heat transfer law:
deltax represents the material thickness, and it can be seen that for the value of the heat conduction heat flow, the thickness D of each aerogel spacer layer will affect the magnitude of the heat flow in the heat conduction portion, and thus the overall thermal conductivity.
As shown in fig. 3, the heat leak for a given system is reduced by iteration through different aerogel spacer layer thicknesses during the optimization process until the leak heat limit for a given environment is met.
It should be noted that although the thickness of each interlayer can be set to be variable, and the most excellent thermal performance can be obtained through a certain number of iterations, the flow is simplified in consideration of the preparation of the aerogel interlayer in practical situations, and in this embodiment, three different thicknesses of interlayer materials are used in consideration of the practical applicability as described above.
Through the optimization, the values of D1, D2 and D3 in the embodiment are 1.25mm (D1-D5), 0.83mm (D6-D10) and 0.625mm (D11-D15), respectively.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (7)
1. An aerogel multilayer spacing material comprises aerogel spacing layers and reflecting screens, and is characterized in that the aerogel spacing layers and the reflecting screens are alternately arranged, and one layer of reflecting screen is arranged on each of two sides of any one aerogel spacing layer;
the number of layers of the aerogel spacing layer is n, the number of layers of the reflecting screen is n +1, and the value of n is more than or equal to 2;
the thickness of the aerogel spacer layer gradually decreases from one side of the aerogel multilayer spacer material to the other side; the thickness of each aerogel spacing layer is D, dmin is the minimum value of D, dmax is the maximum value of D, and the thickness ratio of Dmax to Dmin is 3-4: 1;
the aerogel of the aerogel spacer layer has an anisotropic porous structure;
the pore size range of the porous structure is 1-100nm, and the porosity is greater than or equal to 90%;
the aerogel of the aerogel spacing layer has different heat conductivity coefficients along different directions, and the heat conductivity coefficient of each direction at room temperature is less than or equal to 0.035W/(m.K);
the aerogel multilayer spacer material is applied to the aerospace field.
2. The aerogel multilayer spacer material of claim 1, wherein the aerogel spacer layer is made of one of silica aerogel, metal oxide aerogel, organic aerogel and carbon aerogel.
3. An aerogel multilayer spacer material as claimed in claim 2, wherein said metal oxide aerogel is one of zirconia aerogel, titania aerogel, alumina aerogel;
the organic aerogel is one of polyimide aerogel, resorcinol-formaldehyde aerogel, polydicyclopentadiene aerogel, polyisocyanate aerogel, polyurethane-based aerogel and cellulose-based aerogel;
the carbon aerogel is graphene aerogel.
4. The aerogel multilayer spacing material as claimed in any one of claims 1 to 3, wherein intelligent thermal control coatings are respectively arranged outside the reflection screens at two sides;
the intelligent thermal control coating comprises a high-reflectivity bottom layer coated on the outer surface of the reflecting screen and a thermochromic coating coated on the high-reflectivity bottom layer;
the thermochromic coating comprises alternating layers of vanadium dioxide and silicon.
5. The preparation method of the aerogel multilayer spacing material as claimed in any one of claims 1 to 4, characterized by preparing a plurality of aerogel spacing layers, and then alternately arranging and fixing the aerogel spacing layers and the plurality of reflection screens to obtain the aerogel multilayer spacing material.
6. The method for preparing the aerogel multilayer spacing material according to claim 5, wherein the aerogel spacing layer is prepared from polyamic acid, and comprises the following steps:
s1), sequentially carrying out electrospinning and thermal imidization on polyamic acid to prepare a polyimide nanofiber membrane;
s2) shearing the polyimide nanofiber membrane into small pieces, and putting the small pieces into an N-methylpyrrolidone solution to obtain a suspension; the mass ratio of the polyimide nanofiber membrane to the N-methyl pyrrolidone is 1;
s3) homogenizing the suspension obtained in the step S2) for the first time to obtain a polyimide nanofiber solution;
s4), carrying out suction filtration and drying on the polyimide nanofiber solution to obtain polyimide nanofibers;
s5) mixing the polyimide nano fibers with benzoxazine powder to obtain a mixture; the mass ratio of the polyimide nano fibers to the benzoxazine powder is 4:1;
s6) dissolving the mixture obtained in the step S5) in a 1, 4-dioxane solution to obtain a mixed solution; the mass ratio of the total mass of the mixture to the 1, 4-dioxane solution is (5): 1000;
s7), homogenizing the mixed solution obtained in the step S6) for the second time to obtain polyimide nanofiber dispersion liquid, and then carrying out ultrasonic oscillation on the polyimide nanofiber dispersion liquid;
s8), directionally freezing the polyimide nanofiber dispersion liquid until the polyimide nanofiber dispersion liquid is completely frozen; the temperature of the directional freezing is 0 ℃, 10 ℃ below zero or 50 ℃ below zero;
s9), putting the completely frozen sample into a freeze dryer for freezing and vacuum drying treatment to obtain a cross-linked polyimide aerogel framework;
s10), putting the crosslinked polyimide aerogel framework into a high-temperature furnace for heating treatment, and completing thermal crosslinking to obtain the polyimide aerogel.
7. The method for preparing an aerogel multilayer spacer material according to claim 6, wherein the temperature rise process is performed at 200 ℃ for 30min and at a temperature rise rate of 10 ℃/min in step S10.
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