CN115521765B - Preparation method of phase-change energy storage material - Google Patents
Preparation method of phase-change energy storage material Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
- C09K5/08—Materials not undergoing a change of physical state when used
- C09K5/14—Solid materials, e.g. powdery or granular
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
- C09K5/02—Materials undergoing a change of physical state when used
- C09K5/06—Materials undergoing a change of physical state when used the change of state being from liquid to solid or vice versa
- C09K5/063—Materials absorbing or liberating heat during crystallisation; Heat storage materials
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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
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/14—Thermal energy storage
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Abstract
The invention provides a preparation method of a phase-change energy storage material, which comprises the following steps: chopping hollow asphalt-based pre-oxidized fibers, dispersing the chopped hollow asphalt-based pre-oxidized fibers by a liquid medium, enabling the liquid medium to be close to a cold source, supercooling the liquid medium, and then freeze-drying to obtain a fiber network; applying pressure to the fiber network, and performing preliminary carbonization in a protective atmosphere to obtain a first carbon fiber network; the method of the pressure is perpendicular to the arrangement direction of the fibers in the fiber network; secondary carbonizing the first carbon fiber network to obtain a second carbon fiber network; or graphitizing the first carbon fiber network to obtain a second carbon fiber network; and placing the second carbon fiber network in a molten organic phase change material, vacuumizing and cooling to obtain the carbon fiber composite material.
Description
Technical Field
The invention relates to a preparation method of a phase-change energy storage composite material with high directional heat conduction and high heat storage, and belongs to the field of novel composite materials.
Background
The phase change energy storage material utilizes the absorption and release of material latent heat in the phase change process, realizes the controllable storage and utilization of heat energy, solves the contradiction between the time and space mismatching of energy supply and demand, is one of important technologies for improving the energy utilization efficiency and protecting the environment, and has wide application prospect in the fields of solar energy utilization, building energy conservation, electronic device heat dissipation, aerospace and the like.
The pure phase change material mainly has two problems of poor heat conduction performance and solid-liquid phase flow leakage. The poor thermal conductivity directly leads to poor heat transfer performance, long heat storage reaction time, low heat storage utilization rate and easy overheating phenomenon of the phase change temperature control system, thereby reducing the efficiency of the phase change temperature control system. In order to improve the heat conductivity, various high-heat-conductivity materials are generally added to prepare the composite phase-change energy storage material. The carbon material has high heat conductivity, corrosion resistance, low density, good compatibility with phase change material and other features. The mesophase pitch is a high-quality precursor of a high-heat-conductivity carbon material, and the carbon fiber prepared by the method has low resistivity and extremely high heat conductivity due to the high preferential orientation of the graphite crystal structure along the fiber axis, and is particularly suitable for preparing a composite material with high heat conductivity, stable size or matched heat expansion coefficient due to the addition of negative heat expansion coefficient, high modulus and low density.
However, in the application of the phase change energy storage material at present, carbon fibers are filled in the phase change energy storage material in a dispersed form, and unstable phenomena such as deposition and the like are easily caused in the phase change circulation. In order to fully utilize the high heat-conducting property of the carbon fiber and improve the heat stability of the phase-change composite material, a three-dimensional network of the carbon fiber is required to be constructed to provide a channel for rapid heat transfer, and meanwhile, a continuous network structure can provide a supporting framework with stable shape for the phase-change energy storage material, so that the problem of heat stability in the phase-change process is well solved.
Because the energy storage density (latent heat) of the phase change energy storage composite material is directly related to the duty ratio of the phase change material, the mass and the volume of the heat conduction reinforcing body need to be reduced as much as possible to ensure the energy storage density, and therefore, the heat conductivity of the phase change composite material and the heat storage coefficient (Thermal effusivity) of the latent heat are considered at the same time to become important parameters for describing the potential of the material for dynamically storing and exchanging environmental heat energy.
The thermal storage coefficient is the square root of the product of thermal conductivity (k), bulk density (ρ), and latent heat (h), as shown in equation 1:
Therefore, in order to obtain the phase-change energy storage composite material with high heat storage coefficient, the material needs to obtain high heat conductivity and high latent heat at the same time, so that the low-density high-heat-conductivity porous carbon network body becomes an ideal phase-change energy storage heat-conduction reinforcing body material.
Invention patent CN106044742B: the preparation process of self-adhering asphalt-base carbon fiber network material includes the steps of pre-oxidizing asphalt fiber, short cutting, mechanical dispersing, dispersing in gas medium or water solution, molding, carbonizing or graphitizing to obtain the self-adhering carbon fiber network material. The network material obtained by the method has the advantages that the high-heat-conductivity carbon fibers are randomly arranged on the XY surface, the high heat-conductivity performance of the one-dimensional carbon fibers in the axial direction cannot be fully exerted, the directional heat conduction design is carried out, and meanwhile, the random fiber arrangement cannot realize the sufficient and effective infiltration and filling of the phase-change material.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a preparation method of a phase change energy storage composite material with high directional heat conduction and high heat storage, which builds a high directional carbon fiber network by taking high heat conduction mesophase pitch-based hollow fibers as raw material fibers, can realize continuous penetration of the heat conduction network, can reduce the mass and the volume ratio of the carbon fibers by utilizing a hollow structure, and is beneficial to forming capillary force in micron-sized hollow slender space in the carbon fibers, thereby effectively solving the problem of solid-liquid phase change leakage.
In order to solve the technical problems, the invention adopts the following technical scheme:
A preparation method of a phase-change energy storage material comprises the following steps:
Chopping hollow asphalt-based pre-oxidized fibers, dispersing the chopped hollow asphalt-based pre-oxidized fibers by a liquid medium, enabling the liquid medium to be close to a cold source, supercooling the liquid medium, and then freeze-drying to obtain a fiber network;
Applying pressure to the fiber network, and performing preliminary carbonization in a protective atmosphere to obtain a first carbon fiber network; the method of the pressure is perpendicular to the arrangement direction of the fibers in the fiber network;
secondary carbonizing the first carbon fiber network to obtain a second carbon fiber network; or alternatively
Graphitizing the first carbon fiber network to obtain a second carbon fiber network;
and placing the second carbon fiber network in a molten organic phase change material, vacuumizing and cooling to obtain the carbon fiber composite material.
The hollow asphalt-based pre-oxidized fiber is prepared from mesophase asphalt fiber;
The oxidation weight gain of the hollow asphalt-based pre-oxidized fiber is 50-90% of the total pre-oxidation weight gain;
The temperature of the cold source is between 10 ℃ below zero and 196 ℃ below zero;
The cold source comprises liquid nitrogen.
The preliminary carbonization temperature is 500-1000 ℃;
the temperature rising rate of the preliminary carbonization is 0.1-10 ℃/min;
The preliminary carbonization time is 1-600 min.
The outer diameter of the hollow asphalt-based pre-oxidized fiber is 10-60 mu m, and the inner diameter is 2-58 mu m;
the chopping is to cut the hollow asphalt-based pre-oxidized fiber into chopped fibers with the length of 0.5-10 mm.
The liquid medium comprises water.
The pressure is less than or equal to 20MPa;
the protective atmosphere comprises at least one of nitrogen, argon and helium.
The temperature of the secondary carbonization is 1000-1800 ℃;
the temperature rising rate of the secondary carbonization is 0.1-10 ℃/min.
Graphitization temperature is 2000-3200 deg.c and heating rate is 0.1-100 deg.c/min.
The temperature of the freeze drying is between 10 ℃ below zero and 40 ℃ below zero.
The temperature during vacuumizing is 40-200 ℃.
Compared with the prior art, the invention has the following advantages:
1. According to the method provided by the invention, the high orientation of the carbon fiber along the ice crystal direction can be realized by regulating and controlling the freeze-drying temperature field, the advantage of high thermal conductivity in the one-dimensional carbon fiber surface is fully utilized, and the heat conduction orientation design is carried out, so that the phase-change energy-storage composite material has ultrahigh thermal conductivity in a specific direction, the heat emitted in the specific direction can be quickly introduced into the phase-change energy-storage composite material, and the heat storage and release characteristics of the phase-change material are exerted;
2. The method provided by the invention can obtain the carbon fiber network material structure with adjustable porosity, density and heat conductivity by regulating and controlling the pressure vertical to the fiber orientation; by regulating and controlling the carbonization temperature and time, the phase change energy storage material reinforcement with high adjustable heat conductivity can be obtained.
3. The method provided by the invention can realize the adjustable porosity and the height of the hollow capillary structure by using the hollow carbon fibers with different hollowness and outer diameters, and is particularly suitable for effectively packaging liquid phase change materials with different viscosities in the phase change process. Compared with the carbon fiber with a solid structure, the porosity of the carbon fiber with a hollow structure can be increased by 10-30%, and under the condition of ensuring high heat conductivity, the accommodated phase change material is increased, so that the heat storage coefficient of the phase change material is increased by more than 20%, and the application of the phase change energy storage composite material in various fields is facilitated.
4. The technical scheme provided by the invention has low requirements on equipment, is simple to operate and is easy for industrial production.
Drawings
FIG. 1 shows a flow chart of a method for preparing a phase change energy storage material provided by the invention;
FIG. 2 shows SEM pictures of the oriented arrangement of fibers in a hollow carbon fiber network body of example 1;
Fig. 3 shows SEM photographs of the hollow structure of the carbon fibers in the hollow carbon fiber network body of example 1.
FIG. 4 shows SEM pictures of carbon fiber bonds in a hollow carbon fiber network sample of example 1
Fig. 5 shows a schematic of oxygen bridging of bonds between mesophase pitch-based pre-oxidized fibers.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
The invention provides a preparation method of a phase change energy storage material, which comprises the steps of firstly chopping hollow asphalt-based pre-oxidized fibers, dispersing the chopped fibers by a liquid medium, enabling the liquid medium to be close to a cold source, supercooling the liquid medium, and then freeze-drying to obtain a fiber network; applying pressure to the fiber network, and performing preliminary carbonization in a protective atmosphere to obtain a first carbon fiber network; the method of the pressure is perpendicular to the arrangement direction of the fibers in the fiber network; secondary carbonizing the first carbon fiber network to obtain a second carbon fiber network; the first carbon fiber network can be graphitized to obtain a second carbon fiber network; placing the second carbon fiber network in a molten organic phase change material, vacuumizing, filling the molten organic phase change material in the second carbon fiber network, filling the molten organic phase change material in a hollow structure of carbon fibers forming the second carbon fiber network, and cooling to obtain the carbon fiber composite material. The hollow fiber is selected to obtain the phase change energy storage composite material with high storage coefficient, and the smaller the proportion of the high heat conduction reinforcing material of the non-phase change part is, the better the proportion is, and the high heat conduction carbon fiber with the hollow structure is beneficial to increasing the proportion of the phase change material without obviously reducing the heat conduction efficiency.
The invention provides that the liquid medium is brought close to the cold source to supercool it in order to orient the fibres. The fibers were oriented for the following reasons:
1. Because the hollow size of the fiber is in the micron order, the fiber is favorable for the molten phase transition to flow into the pores for complete filling after orientation. The flow resistance must be increased if the layers are randomly arranged.
2. The fiber arrangement is relatively regular after orientation, which is beneficial to self-bonding to form developed crystal structure to promote heat conduction, and meanwhile, after orientation, the heat conduction along the fiber orientation direction can be greatly enhanced.
The oxidation weight of the hollow asphalt-based pre-oxidized fiber is increased by 50-90% of the total pre-oxidation weight; mesophase pitch is a very complex mixture of aromatic hydrocarbons with nematic liquid crystal properties. In the production of carbon fibers, the stabilization of the highly oriented arrangement of the molecular structure must be ensured by preoxidation (introduction of oxygen bridges). The oxygen bridge structure is usually formed by thermal oxidation and dehydrogenation, first occurring on the aliphatic side chains, then aromatization and crosslinking, and the pre-oxidation process is one of the key factors controlling the structure and properties of the carbon fiber. While the incompletely oxidized fibers still have an appropriate amount of phenolic hydroxyl and carboxyl functionality, which facilitates bonding between the fibers. Chemical reactions between phenolic hydroxyl and carboxyl functionalities in adjacent fibers occur as shown in fig. 5. The formation of ethers, ketones, aromatic lactones, anhydrides and other "oxygen bridge" structures provides a strong bonding capability between fibers. During the further high temperature sintering process, phenolic ether, lactone, anhydride and other 'oxygen bridge' bonds in the fibers are gradually broken, releasing H 2O、CO、CO2 and other small molecules, so that the fibers are self-bonded together to form a carbon network structure. The heat conduction of carbon materials is mainly lattice vibration, and in order to establish an effective heat conduction network, the carbon fibers are first required to have large-size graphite crystallites with high orientation degree, and the fewer lattice defects, the higher the heat conductivity coefficient. Second, the carbon crystallites are discontinuous at the interface, requiring that the carbon fiber portions be meltable to bond with adjacent fibers in order to reduce interfacial thermal resistance and increase thermal conductivity. Therefore, it is desirable to reasonably select the degree of pre-oxidation (controlled in the range of 50 to 90% according to the present invention) to produce pre-oxidized fibers with a sufficiently developed crystal structure while still having good interactions at the fiber interface region during carbonization, too low to meet the requirements. The pre-oxidation degree is 50-90% of the weight of the asphalt fiber calculated by taking the weight of the complete pre-oxidation as 100%, namely the weight of the asphalt fiber is 50-90% of the weight of the asphalt fiber when the asphalt fiber is completely oxidized.
Specifically, the hollow asphalt-based pre-oxidized fiber is prepared from mesophase asphalt fiber; it should be noted that isotropic asphalt and mesophase asphalt are distinguished. The mesophase pitch is a nematic liquid crystal substance generated by a heavy aromatic hydrocarbon mixture, and the obtained carbon fiber internal structure carbon layer has obvious orientation and graphitization easiness, so that the carbon fiber with high thermal conductivity can be obtained. The isotropic pitch achieves disordered layer arrangement of the carbon layers of the internal structure of the carbon fiber, and graphitization is difficult, so that the thermal conductivity of the carbon fiber is relatively low.
Specifically, the temperature of the cold source is between 10 ℃ below zero and 196 ℃ below zero; the liquid in the sample is gradually and completely frozen so as to orient the fiber, the fiber is not easy to be oriented due to overhigh temperature, and a cold source with lower temperature is not easy to be obtained;
specifically, the cold source comprises liquid nitrogen. It will be appreciated by those skilled in the art that other gases having boiling points below-10 ℃ may also be used as a heat sink.
Specifically, the preliminary carbonization temperature is 500-1000 ℃; the bonding points among the fibers are not firm at too low temperature, and the requirement of the hot pressing equipment is higher due to too high temperature, so that the production cost is increased;
specifically, the temperature rising rate of the preliminary carbonization is 0.1-10 ℃/min; the low heating rate leads to the reduction of production efficiency and the rise of energy consumption cost; the temperature rising speed is too fast, so that the temperature field in the heating equipment is uneven, and the uniformity of the samples is easy to be different;
specifically, the preliminary carbonization time is 1-600 min. Too short a time is unfavorable for the process operation and the formation of the bonding process, and too long a time leads to the reduction of production efficiency and the rise of energy consumption and cost
Specifically, the outer diameter of the hollow asphalt-based pre-oxidized fiber is 10-60 mu m, and the inner diameter is 2-58 mu m; when the fiber outer diameter is larger, the homogenization and stabilization control of the pre-oxidation degree of the fiber is not easy to carry out; the smaller fiber outer diameter leads to smaller inner diameter of the fiber, and the energy storage advantage is not obvious when the fiber is applied to the phase change energy storage composite material;
Specifically, the chopping is to cut the hollow pitch-based carbon fiber into chopped carbon fiber with the length of 0.5-10 mm. Too short fibers can significantly reduce the thermal conductivity of the carbon fiber network, and the fiber process is detrimental to the dispersion of the fibers and to the alignment of the fibers in the vertical direction.
In particular, the liquid medium comprises water. Those skilled in the art will appreciate that other liquid media having freezing points above 35 c may also be used to practice the present invention.
Specifically, the pressure is less than or equal to 20MPa; greater pressure tends to cause breakage of the fibers and thereby reduce the thermal conductivity of the carbon fiber backbone.
Specifically, the protective atmosphere comprises nitrogen. Those skilled in the art will appreciate that other inert gases are also possible.
Specifically, the temperature of the secondary carbonization is 1000-1800 ℃; too low carbonization temperature cannot remove as much non-carbon elements as possible, so that the heat conductivity coefficient of the carbon fiber network is too low to be applied, the high temperature of common carbonization equipment is 1800 ℃, the equipment is more easily damaged, and if the heat treatment is carried out at a higher temperature, graphitization equipment is needed.
Specifically, the temperature rising rate of the secondary carbonization is 0.1-10 ℃/min. The temperature rate is too low, so that the production efficiency is reduced, and the energy consumption cost is increased; the temperature rising speed is too fast, so that the temperature in the heating equipment is too uneven, and the uniformity of the samples is easy to be different;
Specifically, the graphitization temperature is 2000-3200 ℃, and the temperature rising rate is 0.1-100 ℃/min. The graphitization stage is mainly affected by the highest temperature, so that the production efficiency is reduced and the energy consumption cost is increased due to the excessively low temperature rate; the heating rate is too fast, so that the heating equipment is easy to fail, and the production cost is indirectly increased;
Specifically, the freeze-drying temperature is from-10 ℃ to-40 ℃. The water is not easy to sublimate at the excessively low temperature, so that the production time is prolonged, the production efficiency is reduced, and the production cost is indirectly improved; too high a temperature tends to reduce the degree of orientation of the sample during drying.
Specifically, the temperature during vacuumizing is 40-200 ℃. The melting point of the phase change energy storage material used in the present invention is within this range.
The invention is further illustrated below with reference to examples.
Example 1
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. Pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 4.5% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 1mm length, mixing with deionized water, and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 10 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network oriented along the Z direction.
3. Placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 2KPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 550 ℃ at 1 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) carrying out secondary carbonization treatment on the first carbon fiber network obtained in the step (3) in a nitrogen atmosphere to obtain a second carbon fiber network. The heating rate during carbonization is 1 ℃/min, the carbonization temperature is 1400 ℃, and the heat preservation is carried out for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten paraffin, heating to 70 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten paraffin is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
SEM images of carbon fibers in the hollow carbon fiber network samples obtained in step 4 of example 1 after being peeled off, it can be seen from the images that the carbon fibers are bonded by self-bonding, and the peeling of the carbon fibers causes a failure interface to appear on the fiber surface, which indicates that the bonding between the fibers is tight.
Example 2
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 5.0% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 5mm length, mixing with deionized water, and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 20 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network oriented along the Z direction.
3. Placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 10KPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 650 ℃ at 1 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) carrying out secondary carbonization treatment on the first carbon fiber network obtained in the step (3) in a nitrogen atmosphere to obtain a second carbon fiber network. The heating rate during carbonization is 10 ℃/min, the carbonization temperature is 1800 ℃, and the heat preservation is carried out for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten paraffin, heating to 70 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten paraffin is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
Example 3
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. and pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 5.0% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 7mm length, and mixing with deionized water and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 30 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network oriented along the Z direction.
3. Placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 100KPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 700 ℃ at 10 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) graphitizing the first carbon fiber network obtained in the step (3) in nitrogen atmosphere to obtain a second carbon fiber network. The temperature rising rate during graphitization is 10 ℃/min, the graphitization temperature is 2600 ℃, and the temperature is kept for 2 hours.
5. Immersing the second carbon fiber network obtained in the step 4 into molten paraffin, heating to 70 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten paraffin is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
Example 4
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. and pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 5.5% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 10mm length, and mixing with deionized water and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 10 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network oriented along the Z direction.
3. Placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 20MPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 800 ℃ at 10 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) graphitizing the first carbon fiber network obtained in the step (3) in nitrogen atmosphere to obtain a second carbon fiber network. The temperature rising rate during graphitization is 1 ℃/min, the graphitization temperature is 3200 ℃, and the temperature is kept for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten paraffin, heating to 70 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten paraffin is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
Example 5
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. and pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 5.5% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 10mm length, and mixing with deionized water and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 35 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network oriented along the Z direction.
3. Placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 20MPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 1000 ℃ at 10 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) graphitizing the first carbon fiber network obtained in the step (3) in nitrogen atmosphere to obtain a second carbon fiber network. The temperature rising rate during graphitization is 1 ℃/min, the graphitization temperature is 3200 ℃, and the temperature is kept for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten erythritol, heating to 140 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten erythritol is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
Example 6
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. Pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 4.5% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 1mm length, mixing with deionized water, and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. And (3) injecting the aqueous solution obtained in the step (1) into a square mould (the mould is made of heat-insulating nylon materials around and is made of copper metal materials at the bottom), placing the mould on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top, driving hollow pre-oxidized fibers to orient after the growth of the ice crystals, and finally freezing into ice cubes. And (3) demolding, taking out, putting into a freeze dryer at the temperature of minus 10 ℃ under vacuum condition, and taking out after 3 days to obtain the hollow pre-oxidized fiber network body oriented along the Z direction.
3. And (3) placing the high-orientation hollow pre-oxidized fiber network body into a die with adjustable pressure on the side, adjusting the pressure to 2KPa, and placing into a carbonization furnace under the protection of nitrogen, and heating to 500 ℃ at 1 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) graphitizing the first carbon fiber network obtained in the step (3) in nitrogen atmosphere to obtain a second carbon fiber network. The temperature rising rate during graphitization is 100 ℃/min, the graphitization temperature is 3000 ℃, and the temperature is kept for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten erythritol, heating to 140 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten erythritol is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
Example 7
This example differs from example 5 in that the oxidized fiber used in step 1 had an oxidized weight gain of 2.8% and the remaining steps were the same as in example 4.
Example 8
This example differs from example 5 in that the oxidized fiber used in step 1 had an oxidized weight gain of 5.9% and the remaining steps were the same as in example 4.
Comparative example 1
The embodiment is a specific implementation mode of the invention, and specifically comprises the following steps:
1. and pre-oxidizing 100% mesophase pitch fiber in an oxygen atmosphere until the oxidation weight gain is 5.5% (the complete pre-oxidation weight gain is 6.20%), cutting 2g into 10mm length, and mixing with deionized water and stirring for 2h to obtain a uniformly dispersed aqueous solution.
2. Pouring the aqueous solution obtained in the step 1 into a die for suction filtration, and drying the obtained filter cake in a vacuum drying oven at 80 ℃ for 5 hours.
3. Placing the hollow pre-oxidized fiber network body into a die with adjustable pressure in the gravity direction, adjusting the pressure to 20MPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 550 ℃ at 10 ℃/min for preliminary carbonization. Naturally cooling to room temperature, and demoulding to obtain the high-orientation first carbon fiber network.
4. And (3) graphitizing the hollow carbon fiber network body obtained in the step (3) in a nitrogen atmosphere to obtain a second carbon fiber network. The temperature rising rate during graphitization is 1 ℃/min, the graphitization temperature is 3200 ℃, and the temperature is kept for 30min.
5. Immersing the second carbon fiber network obtained in the step 4 into molten paraffin, heating to 70 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten paraffin is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
The framework materials and phase change energy storage composite materials provided in examples 1 to 8 and comparative example 1 were tested, and the tests included:
1. calculating the density of the material according to the ratio of the mass to the volume of the material, wherein the mass is measured by an analytical balance, and the volume is calculated after the size is measured;
2. Testing the thermal diffusion coefficients of the sample in the horizontal direction and the vertical direction by a laser heat conduction instrument, and then calculating according to a thermal conductivity formula to obtain the thermal conductivity of the corresponding composite material;
3. Measuring the phase change enthalpy of the composite material by a differential scanning calorimeter;
4. calculating the heat storage coefficient of the composite material according to the formula 1;
The results of the above test are shown in Table 1
TABLE 1
As can be seen from the data results shown in examples 1 to 6, the increase in the heat treatment temperature, the increase in the fiber length and the increase in the molding pressure are all favorable for the improvement of the heat conductivity coefficient and the improvement of the heat storage coefficient; in addition, the phase change material with high self density and high heat accumulation enthalpy value is favorable for preparing the composite material with high heat accumulation coefficient. And it can be seen from fig. 4 that the carbon fibers are bonded by self-bonding, the peeling of the carbon fibers causes a failure interface to appear on the surface of the fibers, and a distinct carbon layer is visible at the interface, which indicates that the bonding between the fibers is tight.
As is clear from the data in examples 5 and 7, when the pre-oxidation degree of the mesophase pitch hollow fiber is too low, the bonding effect is increased, resulting in an increase in the skeleton density, but the crystal development inside the fiber is not complete enough, so that the thermal conductivity of the phase change energy storage composite material in the Z direction is significantly reduced, and the filling amount of the phase change material is reduced due to the too high skeleton density, thereby reducing the thermal storage coefficient of the phase change energy storage composite material.
As can be seen from the data of examples 5 and 8, when the pre-oxidation degree of the mesophase pitch hollow fiber is too high, the bonding effect is poor, the skeleton density is reduced, the interface thermal resistance is large, and the thermal conductivity of the phase-change energy storage composite material in the Z direction is remarkably reduced, so that the thermal storage coefficient of the phase-change energy storage composite material is reduced.
As can be seen from the data of example 5 and comparative example 1, in the comparative example, the high thermal conductivity fibers tend to be randomly arranged on the XY plane and less arranged in the Z direction by adopting the conventional suction filtration molding method, whereas the phase change energy storage composite material obtained by orienting the arrangement of the hollow fibers in the Z direction and preparing the hollow carbon fiber skeleton in example 5 has higher thermal conductivity and thermal storage coefficient and is more remarkable in the Z direction.
Claims (1)
1. The preparation method of the phase-change energy storage material is characterized by comprising the following steps of:
S1, pre-oxidizing 100% mesophase pitch fiber to an oxidation weight gain of 5.5% in an oxygen atmosphere, chopping 2g to a length of 10mm, and mixing with deionized water and stirring for 2 hours to obtain a uniformly dispersed aqueous solution; the weight gain of the 100% mesophase pitch fiber fully pre-oxidized was 6.20%;
S2, injecting the aqueous solution obtained in the step S1 into a square mold, wherein the mold is made of a heat-insulating nylon material at the periphery and is made of a copper metal material at the bottom; placing a die on a copper column with the height of 10cm, immersing the lower end of the copper column into liquid nitrogen, growing ice crystals from bottom to top by using an aqueous solution in the die, driving the hollow pre-oxidized fibers to orient by the growth of the ice crystals, and finally freezing into ice cubes; demoulding, taking out, putting into a freeze dryer at-35 ℃ under vacuum condition, taking out after 3 days, and obtaining a hollow pre-oxidized fiber network oriented along the Z direction;
S3, placing the high-orientation hollow pre-oxidized fiber network into a die with adjustable pressure on the side, adjusting the pressure to 20MPa, placing into a carbonization furnace under the protection of nitrogen, and heating to 1000 ℃ at 10 ℃/min for preliminary carbonization; naturally cooling to room temperature, and demolding to obtain a high-orientation first carbon fiber network;
S4, graphitizing the first carbon fiber network obtained in the step S3 in nitrogen atmosphere to obtain a second carbon fiber network; the temperature rising rate during graphitization is 1 ℃/min, the graphitization temperature is 3200 ℃, and the temperature is kept for 30min;
and S5, immersing the second carbon fiber network obtained in the step S4 into molten erythritol, heating to 140 ℃ in a vacuum drying oven, vacuumizing for 2 hours, removing vacuum, repeating for a plurality of times until the molten erythritol is completely filled into the second carbon fiber network, cooling to room temperature, solidifying, and taking out the polished surface to be smooth.
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| CN110128792A (en) * | 2019-06-04 | 2019-08-16 | 中国科学院深圳先进技术研究院 | A kind of hot interface composites and its preparation method and application |
| CN113881228A (en) * | 2021-09-10 | 2022-01-04 | 中国科学院金属研究所 | High-thermal-conductivity carbon fiber composite material and preparation method thereof |
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| CN110128792A (en) * | 2019-06-04 | 2019-08-16 | 中国科学院深圳先进技术研究院 | A kind of hot interface composites and its preparation method and application |
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