Fe 3 O 4 -CNT Water-based Magnetic Fluid and Preparation Method Thereof, and Medium-temperature Magnetic-controlled Heat Collection System

Abstract

The present invention discloses an Fe3O4-CNT water-based magnetic fluid, a preparation method thereof, and a magnetically controlled medium-temperature heat collection system. The preparation method includes: S1, acidifying carbon nanotubes; S2, modifying the acidified carbon nanotubes with iron tetroxide to obtain magnetic carbon nanotubes; S3, performing micro-aggregation of the magnetic carbon nanotubes based on a porous alumina template to form and collect a magnetic carbon nanotube array; S4, preparing a water-based magnetic fluid based on the magnetic carbon nanotube array. The iron tetroxide composite carbon nanotubes prepared by the present invention based on a porous alumina template are used as nanofluids in a heat collection medium, and an ideal nanodot array with artificially adjustable length is prepared, which can not only achieve a high light absorption rate, but also perform micro-regulated aggregation under the action of a magnetic field, further enhancing the photothermal conversion efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanofluids, and particularly relates to an Fe 3 O 4 -CNT water-based magnetic fluid, a preparation method thereof, and a magnetically controlled medium-temperature heat collection system. Background Art

[0002] Energy is an essential material basis for the survival and development of human society. The sun is the source of many energies, such as wind energy, water energy, ocean thermal energy conversion, wave energy, biomass energy, and part of tidal energy. At present, the existing solar energy utilization in China is mainly photovoltaic power generation and solar thermal power generation. Solar collectors can be divided into low-temperature heat collection, medium-temperature heat collection, and high-temperature heat collection according to different working temperature ranges. The medium-temperature heat collection part is mainly used for industrial and agricultural heat use, which is related to and affects the development of industry and agriculture. The key to improving the thermal utilization of the medium-temperature part of solar energy is to improve the heat collection efficiency of the collector. The efficiency of the collector can be improved from two aspects. One is to optimize the structure of the collector, and the other is to develop a new type of heat collection working fluid.

[0003] In a traditional indirect absorption type collector, a coating is applied on the surface of the heat collection tube. The cost of the heat collection tube is high, and the highest temperature point is on the coating, and then it is transferred to the heat collection medium by the coating, which will cause the problem of uneven heat transfer temperature and easily cause the rupture of the heat collection tube. Therefore, it is considered to use a nanofluid direct absorption type collector.

[0004] The key to the high heat collection efficiency of a nanofluid direct absorption type heat collection tube is that the heat collection working fluid must have high absorptivity and high heat conduction efficiency. Traditional heat collection working fluids tend to select single types of nanomaterials such as carbon nanotubes or magnetite and add them to pure liquids. Although magnetite nanofluid can achieve micro-regulation under the action of a magnetic field, due to the low light absorptivity of magnetite particles, the final photothermal conversion efficiency of the nanofluid is not high; while carbon nanotubes have a relatively high light absorptivity, but they cannot be micro-moved and aggregated under the action of a magnetic field.

[0005] Therefore, in view of the above technical problems, it is necessary to provide an Fe 3 O 4 -CNT water-based magnetic fluid, a preparation method thereof, and a magnetically controlled medium-temperature heat collection system. Summary of the Invention

[0006] In order to solve the problem that traditional nanofluids cannot have both high absorptivity and magnetic control, the purpose of the present invention is to provide an Fe 3 O 4-CNT aqueous magnetic fluid, preparation method thereof, and magnetic-controlled medium-temperature heat collection system. In order to improve the heat absorption of a direct absorption type collector, the present invention combines two materials, carbon nanotubes with a high absorption rate and magnetic iron tetroxide that can be controlled by a magnetic field, by using a porous alumina template. The carbon nanotubes convert light energy into heat energy, with a high absorption rate and high heat collection efficiency; under the action of a magnetic field, iron tetroxide aggregates to form a heat channel, further enhancing heat transfer.

[0007] To achieve the above object, the technical solution provided by an embodiment of the present invention is as follows:

[0008] A preparation method of Fe 3 O 4 -CNT aqueous magnetic fluid, the preparation method comprising:

[0009] S1. Acidify the carbon nanotubes;

[0010] S2. Modify the acidified carbon nanotubes with iron tetroxide to obtain magnetic carbon nanotubes;

[0011] S3. Perform micro-aggregation of the magnetic carbon nanotubes based on a porous alumina template to form a magnetic carbon nanotube array and collect it;

[0012] S4. Prepare an aqueous magnetic fluid based on the magnetic carbon nanotube array.

[0013] In one embodiment, the step S1 is specifically:

[0014] Put the carbon nanotubes into a flask, sequentially add concentrated nitric acid and concentrated sulfuric acid, install a reflux condenser and an absorption device, and perform heating reflux;

[0015] After cooling to room temperature, filter the excess acid to obtain pure carbon nanotubes;

[0016] Wash the carbon nanotubes with deionized water until the pH value is 7, and dry.

[0017] In one embodiment, in the step S1:

[0018] The volume ratio of concentrated nitric acid to concentrated sulfuric acid is 1:2 to 4; and / or,

[0019] The heating reflux time is 0.1 to 1 h; and / or,

[0020] The drying time is 0.5 to 5 h.

[0021] In one embodiment, the step S2 is specifically:

[0022] Add the acidified carbon nanotubes to a triethylene glycol solution and perform ultrasonic treatment;

[0023] During the magnetic stirring process, add iron oxide nanoparticles, and then place the mixed solution in a thermostatic heating magnetic stirrer for heating and maintaining;

[0024] Naturally cool to room temperature;

[0025] Dilute with ethanol and separate the product from the aqueous solution by magnetic adsorption using a magnet;

[0026] Wash the product with ethanol and dry it to obtain magnetic carbon nanotubes modified with iron oxide.

[0027] In one embodiment, in step S2:

[0028] The ultrasonic treatment time is 1 - 30 min; and / or,

[0029] The heating temperature of the thermostatic heating magnetic stirrer is 220 - 300 °C, and the holding time is 10 - 60 min; and / or,

[0030] The mass ratio of the carbon nanotubes to the iron oxide is 1 - 3:4 - 6; and / or,

[0031] The particle size of the iron oxide nanoparticles is 10 - 30 nm; and / or,

[0032] The outer diameter of the carbon nanotubes is 20 - 30 nm, and the inner diameter is 5 - 10 nm.

[0033] In one embodiment, step S3 forms a magnetic carbon nanotube array through a micro - aggregation device. The micro - aggregation device includes a flask and a magnetic sieve pool. The flask and the magnetic sieve pool are connected by a pipeline, and a valve is provided on the pipeline. The flask includes a first injection port, a second injection port, a third injection port, and an air pump inlet. A magnet is installed at the bottom of the magnetic sieve pool. The magnetic sieve pool includes a first collection port and a second collection port, and the first collection port is located above the second collection port;

[0034] The specific steps of step S3 are as follows:

[0035] Close the valve between the flask and the magnetic sieve pool;

[0036] Ball - mill the porous alumina template with a ball mill, dissolve the ball - milled porous alumina template in anhydrous ethanol, and use a first peristaltic pump to connect to the first injection port of the flask;

[0037] Dissolve the magnetic carbon nanotubes in anhydrous ethanol, pour them into a sprayer, and connect the sprayer to the second injection port of the flask;

[0038] Connect the penetrant dimethyl sulfoxide to the third injection port of the flask using a second peristaltic pump;

[0039] After pre-stirring the nebulizer, switch to the ultrasonic spray stirring mode, and spray the magnetic carbon nanotubes into the flask from the second injection port. At the same time, the first peristaltic pump and the second peristaltic pump pump the porous alumina template solution and the penetrant dimethyl sulfoxide into the flask from the first injection port and the third injection port respectively;

[0040] Blow air into and suck air out of the flask through an air pump, so that the magnetic carbon nanotubes and the porous alumina template are repeatedly fused under the action of buoyancy and gravity, enabling the magnetic carbon nanotubes to fully adhere to the inside of the porous alumina template to form a magnetic carbon nanotube array;

[0041] Open the valve between the flask and the magnetic sieve tank, and let the substances in the flask flow into the magnetic sieve tank;

[0042] Based on the magnetic property of iron oxide, suspend the magnetic carbon nanotube array at the upper part of the magnetic sieve tank through a magnet, and deposit other substances at the lower part of the magnetic sieve tank;

[0043] Collect the magnetic carbon nanotube array through the first collection port, and collect other substances through the second collection port.

[0044] In one embodiment, in step S3:

[0045] The ball milling time of the porous alumina template is 10 - 60 min; and / or,

[0046] The stirring speed of the pre-stirring of the nebulizer is 500 - 2000 r / min, and the pre-stirring time is 5 - 60 min; and / or,

[0047] The pumping speed of the first peristaltic pump and / or the second peristaltic pump is 1 - 10 mL / min; and / or,

[0048] The pore diameter of the porous alumina template is 100 - 300 nm, and the thickness is 30 - 100 μm.

[0049] In one embodiment, step S4 is specifically:

[0050] Add gum arabic to water, and perform ultrasonic oscillation for 5 - 60 min under the water bath condition of 30 - 70 °C to obtain an aqueous solution of gum arabic;

[0051] Add the magnetic carbon nanotube array to the aqueous solution of gum arabic, and perform ultrasonic oscillation for 10 - 100 min under the water bath condition of 30 - 70 °C to prepare Fe 3 O 4 -CNT water-based magnetic fluid.

[0052] The technical solution provided by another embodiment of the present invention is as follows:

[0053] A kind of Fe 3 O4 -CNT aqueous magnetic fluid, which is prepared by the above preparation method.

[0054] The technical solution provided by another embodiment of the present invention is as follows:

[0055] A magnetically controlled medium-temperature heat collection system, which includes a heat collection tube, a magnet and an aqueous magnetic fluid located inside the heat collection tube, and the aqueous magnetic fluid is prepared by the above preparation method.

[0056] The present invention has the following beneficial effects:

[0057] Based on the Fe3O4 composite carbon nanotubes prepared by the porous alumina template as the nanofluid in the heat collection medium, the present invention prepares an ideal nanodot array with artificially adjustable length, which can not only achieve a high light absorption rate, but also can be micro-controlled and aggregated under the action of a magnetic field to further enhance the photothermal conversion efficiency. Description of the Drawings

[0058] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0059] Figure 1 For the Fe 3 O 4 -CNT aqueous magnetic fluid preparation method in Embodiment 1 of the present invention;

[0060] Figure 2 For the structural schematic diagram of the magnetic carbon nanotubes in Embodiment 1 of the present invention;

[0061] Figure 3 For the structural schematic diagram of the micro-aggregation device in Embodiment 1 of the present invention;

[0062] Figure 4 For the structural schematic diagram of the magnetic carbon nanotube array in Embodiment 1 of the present invention;

[0063] Figure 5 For the structural schematic diagram of the magnetically controlled medium-temperature heat collection system in Embodiment 2 of the present invention.

[0064] Wherein: 1-Ferroferric oxide (Fe 3 O 4), 2 - Carbon nanotube (CNT), 10 - Flask, 20 - Magnetic sieve cell, 11 - First injection port, 12 - Second injection port, 13 - Third injection port, 14 - Air pump inlet, 21 - First collection port, 22 - Second collection port, 23 - Magnet, 31 - Anodic aluminum oxide template (AAO), 32 - Magnetic carbon nanotube, 41 - Heat collection tube, 42 - Magnet, 43 - Fe 3 O 4 -CNT water-based magnetic fluid. Detailed implementation mode

[0065] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0066] In order to solve the problem that traditional nanofluids cannot have both high absorption rate and magnetic control, the present invention proposes to use magnetite composite carbon nanotubes (Fe 3 O 4 -CNT) as nanoparticles in the heat collection medium to prepare an ideal nanodot array with artificially adjustable length. It can not only achieve high light absorption rate, but also be micro-regulated and aggregated under the action of a magnetic field to further enhance the photothermal conversion efficiency.

[0067] The present invention discloses a preparation method of Fe 3 O 4 -CNT water-based magnetic fluid, including:

[0068] S1. Acidify the carbon nanotubes;

[0069] S2. Modify the acidified carbon nanotubes with magnetite to obtain magnetic carbon nanotubes;

[0070] S3. Micro-aggregate the magnetic carbon nanotubes based on the anodic aluminum oxide template to form a magnetic carbon nanotube array and collect it;

[0071] S4. Prepare a water-based magnetic fluid based on the magnetic carbon nanotube array.

[0072] The present invention also discloses an Fe 3 O 4 -CNT water-based magnetic fluid, which is prepared by the above preparation method.

[0073] The present invention also discloses a magnetically controlled medium-temperature heat collection system, which includes a heat collection tube, a magnet and a water-based magnetic fluid located in the heat collection tube. The water-based magnetic fluid is prepared by the above-mentioned preparation method.

[0074] The following further describes the present invention with specific embodiments.

[0075] Embodiment 1:

[0076] As shown Figure 1 In this embodiment, the preparation method of the Fe 3 O 4 -CNT water-based magnetic fluid includes:

[0077] S1. Acidify carbon nanotubes (CNT).

[0078] Put 1 g of carbon nanotubes into a flask, sequentially add 15 mL of concentrated nitric acid and 45 mL of concentrated sulfuric acid, install a reflux condenser and an absorption device, and carry out heating reflux for 0.5 h;

[0079] After cooling to room temperature, filter the excess acid, and the remaining powdery substance is pure carbon nanotubes;

[0080] Wash the obtained carbon nanotubes with deionized water until the pH value is 7, and place them in an oven to dry for 2 h.

[0081] In this embodiment, the volume ratio of concentrated nitric acid to concentrated sulfuric acid is 1:3. Adding concentrated acid is to remove impurities in the carbon nanotubes and improve the purity of the carbon nanotubes.

[0082] S2. Use iron tetroxide (Fe 3 O 4 )1 to modify the acidified carbon nanotubes (CNT)2 to obtain magnetic carbon nanotubes. The structure of the magnetic carbon nanotubes is as Figure 2 shown.

[0083] Weigh 80 mg of carbon nanotubes and add them to 25 mL of triethylene glycol solution, and ultrasonically treat for 10 min;

[0084] During magnetic stirring, add 200 mg of iron tetroxide nanoparticles, and then place the mixed solution in a constant temperature heating magnetic stirrer and heat it to 280 °C and keep it for 30 min;

[0085] Then naturally cool to room temperature;

[0086] Dilute with ethanol and separate the product from the aqueous solution by magnetic adsorption using a magnet;

[0087] Finally, wash the product repeatedly with ethanol and dry it in a vacuum oven. The obtained composite particles are magnetic carbon nanotubes modified with iron tetroxide.

[0088] Among them, the mass ratio of carbon nanotubes to iron tetroxide is 1-3:4-6, preferably 2:5; the particle size of the iron tetroxide nanoparticles is 10-30 nm, preferably about 20 nm; the carbon nanotubes are obtained by acidification, drying, and grinding, with an outer diameter of 20-30 nm and an inner diameter of 5-10 nm.

[0089] Triethylene glycol solution is used as a solvent, and the carbon nanotubes are dispersed in the solution, which is convenient for being dispersed by a stirrer.

[0090] Furthermore, when the iron tetroxide nanoparticles are heated to 220-300 °C, the magnetism decreases extremely rapidly, the magnetism is weak, and the response of the iron tetroxide to the magnetic field is weak, enabling the iron tetroxide to be uniformly compounded with the carbon nanotubes under the action of a stirrer.

[0091] A magnet is used to attract the iron tetroxide compounded with the carbon nanotubes to separate the impurities and the desired product. It cannot be guaranteed that all the carbon nanotubes in the composite particles are compounded with iron tetroxide. Therefore, through the cooperation of the magnet, the carbon nanotubes without the compounded iron tetroxide particles are removed by washing with ethanol.

[0092] Ethanol is used as a cleaning solution to wash away the unnecessary impurities.

[0093] S3. Based on the porous anodic alumina template (AAO), micro-aggregation of magnetic carbon nanotubes is carried out to form a magnetic carbon nanotube array and collect it.

[0094] Step S3 forms a magnetic carbon nanotube array through a micro-aggregation device. As shown in Figure 3 , the micro-aggregation device includes a flask 10 and a magnetic sieve pool 20. The flask 10 and the magnetic sieve pool 20 are connected by a pipeline, and a valve (not shown) is provided on the pipeline. The flask 10 includes a first injection port 11, a second injection port 12, a third injection port 13, and an air pump inlet 14. A magnet 23 is installed at the bottom of the magnetic sieve pool 20. The magnetic sieve pool includes a first collection port 21 and a second collection port 22, and the first collection port 21 is located above the second collection port 22. Figure 3 As shown in Figure 3 , the micro-aggregation device includes a flask 10 and a magnetic sieve pool 20. The flask 10 and the magnetic sieve pool 20 are connected by a pipeline, and a valve (not shown) is provided on the pipeline. The flask 10 includes a first injection port 11, a second injection port 12, a third injection port 13, and an air pump inlet 14. A magnet 23 is installed at the bottom of the magnetic sieve pool 20. The magnetic sieve pool includes a first collection port 21 and a second collection port 22, and the first collection port 21 is located above the second collection port 22.

[0095] Step S3 in this embodiment is specifically as follows:

[0096] Close the valve between the flask and the magnetic sieve pool;

[0097] Use a ball mill to ball mill the porous anodic alumina template for 30 min, and dissolve 1 g of the ball-milled porous anodic alumina template in 50 mL of absolute ethanol, and use the first peristaltic pump to connect to the first injection port 11 of the flask 10;

[0098] Dissolve 5 g of magnetic carbon nanotubes in 50 mL of absolute ethanol, pour them into a sprayer, and connect the sprayer to the second injection port 12 of the flask 10;

[0099] The penetrant dimethyl sulfoxide is connected to the third injection port 13 of the flask 10 using a second peristaltic pump;

[0100] After the sprayer pre-stirs for 15 min at a stirring speed of 1000 r / min, it switches to the ultrasonic spray stirring mode. The magnetic carbon nanotubes are sprayed into the flask 10 from the second injection port 12. At the same time, the first peristaltic pump and the second peristaltic pump respectively pump the porous alumina template solution and the penetrant dimethyl sulfoxide into the flask at a speed of 3 mL / min from the first injection port 11 and the third injection port 13;

[0101] Turn on the air pump, blow air and suck air into the flask 10 through the air pump, so that the magnetic carbon nanotubes and the porous alumina template are repeatedly fused under the action of buoyancy and gravity, and finally the magnetic carbon nanotubes are fully attached to the inside of the porous alumina template to form a magnetic carbon nanotube array;

[0102] After sufficient penetration, open the valve between the flask 10 and the magnetic sieve pool 20, and let the substances in the flask flow into the magnetic sieve pool;

[0103] Based on the magnetic property of iron tetroxide, the magnetic carbon nanotube array is suspended at the upper part of the magnetic sieve pool 20 by a magnet, and other substances are deposited at the lower part of the magnetic sieve pool 20;

[0104] Collect the magnetic carbon nanotube array through the first collection port 21, and collect other substances through the second collection port 22. And wash the magnetic carbon nanotube array collected at the first collection port 21 with absolute ethanol.

[0105] Among them, the pore diameter of the porous alumina template is 100 - 300 nm, preferably about 200 nm, and the thickness is 30 - 100 μm, preferably 60 μm.

[0106] Dimethyl sulfoxide as a penetrant can help the magnetic carbon nanotubes penetrate into the porous alumina template.

[0107] In this embodiment, the structure of the magnetic carbon nanotube array is as Figure 4 shown. The pores of the porous alumina template 31 are compounded with magnetic carbon nanotubes 32, and the magnetic carbon nanotubes 32 form a dot matrix inside the porous alumina template 31.

[0108] S4. Prepare an aqueous magnetic fluid based on the magnetic carbon nanotube array.

[0109] Weigh a certain amount of gum arabic and add it to water, and ultrasonically oscillate it for 20 min under the condition of a 50 °C water bath to prepare an aqueous solution of gum arabic with a mass concentration of 2 g / L;

[0110] Add the magnetic carbon nanotube array to the aqueous gum arabic solution and ultrasonically oscillate it for 30 min under the condition of a 50 °C water bath to prepare stable ink-like Fe 3 O 4 -CNT water-based magnetic fluid.

[0111] Example 2:

[0112] As shown in the reference Figure 5 The magnetically controlled medium-temperature heat collection system in this example includes a heat collection tube 41, a magnet 42 located inside the heat collection tube, and Fe 3 O 4 -CNT water-based magnetic fluid 43. The Fe 3 O 4 -CNT water-based magnetic fluid is prepared by the preparation method in Example 1.

[0113] Put the prepared Fe 3 O 4 -CNT water-based magnetic fluid into the heat collection tube of the magnetically controlled medium-temperature heat collection system. Under the action of the magnet, the Fe 3 O 4 -CNT water-based magnetic fluid is subjected to a force and shows an orderly arrangement in the magnetic field.

[0114] To intuitively illustrate the effect of this device, a comparative experiment was conducted on Fe 3 O 4 water-based magnetic fluid, CNT water-based magnetic fluid, and Fe 3 O 4 -CNT water-based magnetic fluid prepared based on the AAO template in the same environment (no magnetic field and with magnetic field). The experimental data are shown in Table 1.

[0115] Table 1: Temperature test table of different nanofluids

[0116]

[0117] It is found through experiments that in the same environment, when the three groups of water-based magnetic fluids are placed in sunlight, within the same time, compared with the nanofluids of single nanoparticles of magnetite and carbon nanotubes, the reading of the temperature measuring device of the nanofluid of magnetite composite carbon nanotubes prepared based on the AAO template increases faster and higher.

[0118] From the above technical solutions, it can be seen that the present invention has the following advantages:

[0119] The present invention combines carbon nanotubes and magnetite, combines a material with high light absorption rate and a material with magnetism, which not only realizes high-efficiency light absorption, but also forms a heat conduction channel under the action of a magnetic field to further enhance heat absorption;

[0120] Considering that the magnetic field cannot completely control the movement of materials in the solution and can only achieve micro-regulation, a porous alumina template is adopted. A lattice is formed by using the porous alumina template, and the magnetic carbon nanotubes are orderly arranged in the solution to form the expected ideal thermal conduction chain elements. At the same time, under the action of the magnetic field, the magnetic carbon nanotubes form a thermal conduction network, which further enhances the absorption of sunlight.

[0121] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

[0122] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing Fe 3 O 4 -CNT aqueous magnetic fluid It is characterized in that the preparation method includes: S1. Acidify the carbon nanotubes; S2. Modify the acidified carbon nanotubes with iron trioxide to obtain magnetic carbon nanotubes; S3. Micro-aggregate the magnetic carbon nanotubes based on a porous alumina template to form a magnetic carbon nanotube array and collect it; S4. Prepare an aqueous magnetic fluid based on the magnetic carbon nanotube array; In step S3, a magnetic carbon nanotube array is formed by a micro-aggregation device, and the micro-aggregation device includes a flask and a magnetic sieve pool. The flask and the magnetic sieve pool are connected by a pipeline, and a valve is provided on the pipeline. The flask includes a first injection port, a second injection port, a third injection port, and an air pump inlet. A magnet is installed at the bottom of the magnetic sieve pool. The magnetic sieve pool includes a first collection port and a second collection port, and the first collection port is located above the second collection port; Specifically, step S3 is as follows: Close the valve between the flask and the magnetic sieve pool; Ball-mill the porous alumina template with a ball mill, dissolve the ball-milled porous alumina template in absolute ethanol, and use a first peristaltic pump to connect to the first injection port of the flask; Dissolve the magnetic carbon nanotubes in absolute ethanol, pour them into a sprayer, and connect the sprayer to the second injection port of the flask; Connect penetrant dimethyl sulfoxide to the third injection port of the flask using a second peristaltic pump; After pre-stirring the sprayer, switch to the ultrasonic spray stirring mode, spray the magnetic carbon nanotubes into the flask from the second injection port, and at the same time, the first peristaltic pump and the second peristaltic pump pump the porous alumina template solution and penetrant dimethyl sulfoxide into the flask from the first injection port and the third injection port respectively; Blow air and draw air into the flask through an air pump, so that the magnetic carbon nanotubes and the porous alumina template are repeatedly fused under the action of buoyancy and gravity, so that the magnetic carbon nanotubes are fully attached to the inside of the porous alumina template to form a magnetic carbon nanotube array; Open the valve between the flask and the magnetic sieve pool, and let the substances in the flask flow into the magnetic sieve pool; Based on the magnetic property of iron trioxide, suspend the magnetic carbon nanotube array in the upper part of the magnetic sieve pool through a magnet, and deposit other substances in the lower part of the magnetic sieve pool; Collect the magnetic carbon nanotube array through the first collection port and collect other substances through the second collection port.

2. The preparation method according to claim 1, It is characterized in that Specifically, step S1 is as follows: Put the carbon nanotubes into a flask, add concentrated nitric acid and concentrated sulfuric acid in sequence, install a reflux condenser and an absorption device, and carry out heating reflux; After cooling to room temperature, filter the excess acid to obtain pure carbon nanotubes; Wash the carbon nanotubes with deionized water until the pH value is 7, and dry.

3. The preparation method according to claim 2, It is characterized in that In step S1: The volume ratio of concentrated nitric acid to concentrated sulfuric acid is 1:2-4; and / or, The heating reflux time is 0.1-1 h; and / or, The drying time is 0.5-5 h.

4. The preparation method according to claim 1, It is characterized in that Specifically, step S2 is as follows: Add the acidified carbon nanotubes to a triethylene glycol solution and perform ultrasonic treatment; Add Fe₃O₄ nanoparticles during magnetic stirring, and then place the mixed solution in a constant-temperature heating magnetic stirrer for heating and maintaining; Naturally cool to room temperature; Dilute with ethanol and separate the product from the aqueous solution by magnetic adsorption using a magnet; Wash the product with ethanol and dry it to obtain magnetic carbon nanotubes modified with Fe₃O₄.

5. According to the preparation method described in claim 4, It is characterized in that, In the step S2: The ultrasonic treatment time is 1 - 30 min; and / or, The heating temperature of the constant-temperature heating magnetic stirrer is 220 - 300 °C, and the holding time is 10 - 60 min; and / or, The mass ratio of the carbon nanotubes to Fe₃O₄ is 1 - 3:4 - 6; and / or, The particle size of the Fe₃O₄ nanoparticles is 10 - 30 nm; and / or, The outer diameter of the carbon nanotubes is 20 - 30 nm, and the inner diameter is 5 - 10 nm.

6. According to the preparation method described in claim 1, It is characterized in that, In the step S3: The ball milling time of the porous alumina template is 10 - 60 min; and / or, The stirring speed of the pre-stirring of the sprayer is 500 - 2000 r / min, and the pre-stirring time is 5 - 60 min; and / or, The pumping speed of the first peristaltic pump and / or the second peristaltic pump is 1 - 10 mL / min; and / or, The pore diameter of the porous alumina template is 100 - 300 nm, and the thickness is 30 - 100 μm.

7. According to the preparation method described in claim 1, It is characterized in that, The step S4 is specifically: Add gum arabic to water, and ultrasonically oscillate for 5 - 60 min under a water bath condition of 30 - 70 °C to obtain an aqueous solution of gum arabic; Add the magnetic carbon nanotube array to the aqueous solution of gum arabic and ultrasonically oscillate it for 10 - 100 min under the condition of a water bath at 30 - 70 °C to prepare Fe 3 O 4 -CNT water-based magnetic fluid.

8. A kind of Fe 3 O 4 -CNT aqueous magnetic fluid, It is characterized in that, The water-based magnetic fluid is prepared by the preparation method described in any one of claims 1 - 7.

9. A magnetically controlled medium-temperature heat collection system, It is characterized in that, The magnetically controlled medium-temperature heat collection system includes a heat collection tube, a magnet and a water-based magnetic fluid located in the heat collection tube, and the water-based magnetic fluid is prepared by the preparation method described in any one of claims 1 - 7.

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