CN115352143A - Temperature response type electromagnetic shielding material and preparation method thereof - Google Patents
Temperature response type electromagnetic shielding material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a temperature response type electromagnetic shielding material and a preparation method thereof, and belongs to the field of composite materials. The material comprises PVDF/Fe positioned on the top layer and the bottom layer 3 O 4 The fiber film and the phase change composite material are positioned in the middle layer; the phase change composite material comprises a porous bracket and an organic phase change material adsorbed on the porous bracket; the porous scaffold is composed of a mixture of expanded graphite and multi-walled carbon nanotubes. The phase-change composite material can store and release heat energy due to the characteristics of the organic phase-change material, so that the temperature of the electronic equipment is controlled by the composite material and is stabilized within a certain temperature range. The temperature response type electromagnetic shielding material provided by the invention has excellent shielding effect on electromagnetic waves, prevents the interference of external electromagnetic waves on working electronic equipment, and simultaneously prevents the electromagnetic pollution of the electromagnetic waves generated by the high-efficiency and high-frequency work of the electronic equipment to the outside. Meanwhile, the heat-conducting material has excellent heat-conducting performance and good heat conductivity.
Description
Technical Field
The invention belongs to the field of composite materials, and particularly relates to a temperature response type electromagnetic shielding material and a preparation method thereof.
Background
With the rapid development of 5G communication technology, the integration level and the operating frequency of electronic devices are increasing, but the overall size of the electronic devices is decreasing. Electronic equipment can lead to its trouble under exposing the electromagnetic radiation of high frequency for a long time, and simultaneously, electronic equipment can lead to the ambient temperature to rise under the high frequency work, and the electromagnetic wave that also can launch simultaneously under operating condition causes electromagnetic pollution to the external world, and the electromagnetic radiation of high frequency also can harm health. Therefore, it is necessary for electronic devices to shield the electronic devices from external electromagnetic waves and prevent electromagnetic pollution to the external environment caused by electromagnetic waves generated by their operation. The most effective method for solving the electromagnetic pollution is to prepare an electromagnetic interference shielding material and a microwave absorbing material which have light weight, strong attenuation capability and good mechanical stability.
Besides ensuring that the electronic equipment shields electromagnetic wave interference, the temperature of the working environment rises under high-power operation, so that potential safety hazard occurs, and the electromagnetic wave generated by the electronic equipment also pollutes the outside, so that the temperature of the working environment of the electronic equipment is ensured to be kept in a reasonable range. The safe operation of the electronic equipment can be ensured only by using the equipment with the temperature kept within a proper working environment temperature range. Therefore, there is a strong need for thermal management of electronic devices through a reasonable and efficient temperature control method to ensure that the electronic devices operate within their proper operating temperature range. Thermal management techniques for Phase Change Materials (PCMs) have become a focus of research in comparison to conventional air and liquid based temperature control techniques. Phase change materials can store and release energy in the form of latent heat while maintaining a constant temperature. Is recognized as one of the best temperature control materials for thermal protection and electronic cooling systems. The working temperature of the electronic device can be controlled within a reasonable range by selecting a proper phase-change material within a required optimal working temperature range.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a temperature response type electromagnetic shielding material and a preparation method thereof.
In order to realize the purpose, the invention adopts the technical scheme that:
the invention discloses a sandwich structure with a temperature response type electromagnetic shielding material as three parts of a top middle sole, which comprises PVDF/Fe as a top layer and a bottom layer 3 O 4 The fiber film and the phase change composite material are positioned in the middle layer; the phase change composite material comprises a porous bracket and an organic phase change material adsorbed on the porous bracket; the porous bracket is formed by a mixture of expanded graphite and multi-wall carbon nano tubes;
the method specifically comprises the following steps:
(1) Preparing a phase-change composite material: and (3) placing the organic phase-change material in water bath heating, transferring the organic phase-change material into an ultrasonic water bath after the organic phase-change material is completely melted, and adding the multi-wall carbon nano tube. The multi-wall carbon nano-tube is uniformly dispersed by mechanical stirring and ultrasonic wave, and then the expanded graphite is added to carry out mechanical stirring in a water bath and ultrasonic environment. Stirring for 15-20 min, uniformly dispersing the mixture, transferring the mixture into a vacuum drying oven, and carrying out vacuum drying for 3 h. The mixture was then removed and placed in a cold press mold having a diameter of 30mm for cold press forming the phase change composite. Further, according to a reasonable working temperature range of the electronic equipment, the organic phase change material is selected to be n-eicosane, polyethylene glycol 2000 or polyethylene glycol 4000 and the like; the carbon nano tube is a multi-wall carbon nano tube; the mass content of the organic phase-change material in the phase-change composite material is 20-80%; the temperature of the water bath, the ultrasonic water bath and the vacuum drying is controlled to be 60-80 ℃. Furthermore, the pressure of cold pressing is 5-15MPa, and the pressing time is 5-10 minutes; the thickness of the phase change composite material is 0.5-3mm.
(2) Preparing a polyvinylidene fluoride/ferroferric oxide fiber film: adding ferroferric oxide into N-N Dimethylformamide (DMF), N-N Dimethylacetamide (DMAC) or dimethyl sulfoxide (DMSO), uniformly dispersing the ferroferric oxide by mechanical stirring, adding polyvinylidene fluoride (PVDF) into the mixture, continuously stirring the mixture, and preparing polyvinylidene fluoride/ferroferric oxide (PVDF/Fe) by using an electrostatic spinning technology after the mixture forms a stable suspension 3 O 4 ) A fibrous film. PVDF is a widely studied polymer with high dielectric constant, pyroelectric behavior, thermal stability, mechanical stability and easy processing. And the acoustic impedance of PVDF is similar to that of air and water, so that the PVDF can be well matched with the air and water in impedance, and more electromagnetic waves can enter the material. Further, PVDF and Fe 3 O 4 The mass ratio of (A) to (B) is 4:1; furthermore, in the electrostatic spinning process, the temperature is controlled to be 23-28 ℃, and the humidity is less than or equal to 60%. The electrostatic spinning needle head adopts an 18-gauge metal needle head and a 20ml syringe.
(3) Preparing the temperature response type electromagnetic shielding material: mixing PVDF/Fe 3 O 4 The fiber film is cut into a circular fiber film with the diameter of 31mm by laser, and the circular fiber film is processedAnd respectively placing the prefabricated interlayer material on the top and the bottom of the composite phase change material to obtain a prefabricated interlayer material, and placing the prefabricated interlayer material into a cold pressing die with the diameter of 30mm for cold pressing to prepare and form the temperature response type electromagnetic shielding material. Further, the cold pressing pressure is kept at 5-10MPa for 10 minutes.
The principle of the invention is as follows:
the top layer and the bottom layer of the temperature response type electromagnetic shielding material are PVDF/Fe 3 O 4 The fiber film is used, and the middle layer is made of a phase-change composite material. Wherein: PVDF/Fe as top and bottom layers 3 O 4 The fiber film has low conductivity coefficient, has insulation protection effect, and can avoid the damage of materials to electronic equipment; in addition, because the material has low electric conductivity and contains magnetic nanoparticles, the material has good impedance matching with air, so that most of electromagnetic waves enter the material, and hysteresis loss is generated to absorb the electromagnetic waves by the synergistic action of the magnetic nanoparticles and the internal structure of the fiber film. Meanwhile, the fiber film also plays a role in packaging the middle-layer phase-change composite material, and the good hydrophobicity of the fiber film also well prevents the leakage and aging of the organic phase-change material. The phase-change composite material as the middle layer is internally provided with a porous bracket consisting of expanded graphite and multi-walled carbon nano tubes for performing multiple reflection on electromagnetic waves. Meanwhile, the porous support formed by the expanded graphite and the multi-wall carbon nano tubes has strong adsorbability on the organic phase-change material, and the leakage of the organic phase-change material is well prevented. Meanwhile, the phase-change material has good conductivity, when electromagnetic waves penetrate through the top fiber film and enter the phase-change material, the difference between the conductivities of the fiber film layer and the phase-change composite material layer is large, impedance mismatch is generated, and when the electromagnetic waves contact the high-conductivity phase-change composite material layer, part of the electromagnetic waves are immediately reflected back to the fiber film layer to generate hysteresis loss again. The residual electromagnetic wave enters the porous structure in the material to be subjected to multiple reflection, which is reflected as absorption loss. The increase in interfacial contact caused by multi-walled carbon nanotubes in the network structure inside the composite phase change material layer can generate more and more heterogeneous interfaces and free charges to enhance the loss of polarization relaxation. The organic phase change material can emit heat in electronic equipment and convert the heat by shielding electromagnetic waveThe phase change occurs under the action of the quantity, the temperature rise is delayed, the time required for reaching thermal equilibrium with the outside is shortened, and the highest temperature of the surface of the electronic equipment is also reduced. The addition of the organic phase-change material also reduces impedance mismatch among the composite phase-change material in the middle layer and the top and bottom fiber films, so that more electromagnetic waves enter the material for loss. The liquefaction of the organic phase-change material in the composite material also enables the conductivity of the phase-change composite material to be increased, and the loss effect of the phase-change composite material on electromagnetic waves is further improved.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a temperature response type electromagnetic shielding material which comprises PVDF/Fe as a top layer and a bottom layer 3 O 4 The sandwich structure shields electromagnetic waves by absorption-reflection-reabsorption. Meanwhile, due to the existence of the porous structure and the multi-walled carbon nano-tube in the phase-change composite material, electromagnetic waves are subjected to multiple reflection in the structure, and are further lost. The organic phase change material in the phase change composite also provides thermal management properties. When the temperature of the phase-change composite material reaches the phase-change temperature of the organic phase-change material, the organic phase-change material is liquefied in the phase-change material to form a solid-liquid mixed state, so that the conductivity of the phase-change composite material is improved, and the electromagnetic shielding performance is further improved.
The preparation method provided by the invention has the advantages that the raw materials are easy to obtain, the operation is simple and convenient, the large-scale production can be realized, the total electromagnetic shielding of the obtained composite material reaches 98.4dB, when the temperature is increased to the phase change temperature of the organic phase change material, the total electromagnetic shielding is increased to 115.75dB, the absorption shielding ratio of electromagnetic waves reaches 85.2%, and meanwhile, the preparation method has excellent heat management performance and good heat conductivity (3.61W/mK).
Drawings
FIG. 1: scanning electron micrographs (a, d) of the composite phase change material and (e, f) of the inner surface of the composite phase change material.
FIG. 2 is a schematic diagram: PVDF/Fe 3 O 4 Scanning electron microscope (a, c) for fiber film, and EDS (d) for Fe.
FIG. 3: DSC curves (a) for the materials prepared in the different examples and comparative examples, and (b) before and after cycling for the materials prepared in example 1.
FIG. 4 is a schematic view of: the material P0 prepared in the comparative example had a change in conductivity before and after the phase transition temperature (a), and a change in electromagnetic shielding properties before and after the phase transition temperature (b), wherein: SE T For the total shielding effectiveness, SE A To absorb the shielding effect, SE R For reflective shielding effectiveness;
FIG. 5: the material P3 prepared in example 1 had a change in conductivity before and after the phase transition temperature (a) and a change in electromagnetic shielding properties before and after the phase transition temperature (b), wherein: SE T Is total shielding effectiveness, SE A To absorb shielding effectiveness, SE R For reflective shielding effectiveness;
FIG. 6: the material P1 prepared in example 2 had a change in electrical conductivity before and after the phase transition temperature (a) and a change in electromagnetic shielding properties before and after the phase transition temperature (b), wherein: SE T Is total shielding effectiveness, SE A To absorb shielding effectiveness, SE R For reflective shielding effectiveness;
FIG. 7: the material P2 prepared in example 3 had a change in conductivity before and after the phase transition temperature (a) and a change in electromagnetic shielding properties before and after the phase transition temperature (b), wherein: SE T Is total shielding effectiveness, SE A To absorb shielding effectiveness, SE R For reflective shielding effectiveness;
FIG. 8: the material P4 prepared in example 4 had a change in conductivity before and after the phase transition temperature (a) and a change in electromagnetic shielding properties before and after the phase transition temperature (b), wherein: SE T Is total shielding effectiveness, SE A To absorb shielding effectiveness, SE R For reflective shielding effectiveness;
FIG. 9: results of thermal conductivity tests of materials prepared in different examples and comparative examples.
Detailed Description
The invention is further described in the following with reference to the drawings and specific preferred embodiments of the description, without thereby limiting the scope of protection of the invention.
The materials and equipment used in the following examples are commercially available.
Example 1
(1) Preparing a phase-change composite material: accurately weighing 0.5625g of n-eicosane, 0.3g of expanded graphite and 0.075g of multi-walled carbon nano-tubes, wherein the content of the n-eicosane accounts for 60 percent. And (3) placing the n-eicosane in a water bath for heating, transferring the n-eicosane into an ultrasonic water bath after the n-eicosane is completely melted, and adding the multi-walled carbon nano tube. The multi-walled carbon nano-tubes are uniformly dispersed by mechanical stirring and ultrasonic waves, then the expanded graphite is added to carry out mechanical stirring in a water bath and ultrasonic environment, and the mixture is transferred to a vacuum drying oven to carry out vacuum drying for 3 hours after being uniformly dispersed. Then taking out the mixture and placing the mixture in a cold pressing die with the diameter of 30mm for cold pressing to form the phase change composite material; wherein: the temperature of the water bath, the ultrasonic water bath and the vacuum drying is controlled at 60 ℃. Furthermore, the pressure of cold pressing is 10MPa, and the pressing time is 10 minutes; the thickness of the phase change composite material is 1mm.
The scanning electron microscope images of the phase-change composite material are shown in fig. 1 a to d, and it can be seen from the images that the composite material has a pore structure inside, and can form multiple reflections of electromagnetic waves. The scanning electron microscope images of the inner surface of the composite phase-change material are shown as e to f in fig. 1, and it can be seen from the images that the phase-change material covers the nano graphite surface and the lamella inside the composite material, and the multi-walled carbon nano tubes are uniformly distributed on the graphite surface to form a heat-conducting and electric-conducting network.
(2) Preparing a polyvinylidene fluoride/ferroferric oxide fiber film: adding ferroferric oxide into N-N Dimethylformamide (DMF), uniformly dispersing the ferroferric oxide by mechanical stirring, adding polyvinylidene fluoride (PVDF) into the mixture, continuously stirring the mixture, and preparing the PVDF/Fe by utilizing an electrostatic spinning technology after the mixture forms a stable suspension 3 O 4 A fibrous film; wherein: PVDF and Fe 3 O 4 The mass ratio of (A) to (B) is 4:1; furthermore, in the electrostatic spinning process, the temperature is controlled to be 25 ℃ and the humidity is less than or equal to 60 percent; the electrostatic spinning needle is an 18-gauge metal needle and a 20ml syringe.
For PVDF/Fe 3 O 4 The fiber film was examined by scanning electron microscopy and EDS for Fe, the results are shown in FIG. 2. In FIG. 2, a to c are PVDF/Fe at different magnification speeds 3 O 4 The fiber film scanning electron microscope shows that the electrospun fiber has an obvious network structure and can effectively capture electromagnetic waves; in FIG. 2, d is EDS of Fe, and it can be seen that Fe is uniformly distributed in the electrospun fiber, which improves the mechanical properties and electromagnetic shielding properties of the composite material.
(3) Preparing the temperature response type electromagnetic shielding material: mixing PVDF/Fe 3 O 4 The fiber film is cut into a circular fiber film with the diameter of 31mm by laser, and the circular fiber film is placed at the top and the bottom of the composite phase change material. Placing the prefabricated sandwich material into a cold pressing die performing 30mm cold pressing, wherein: keeping the cold pressing pressure at 10MPa for 10 minutes to prepare and form the temperature response type electromagnetic shielding material, and marking as P3.
Example 2
Referring to the preparation method in example 1, the content of n-eicosane in the prepared phase change composite material is 20% by adjusting the amount of n-eicosane to 0.09375 g; and then preparing the temperature response type electromagnetic shielding material which is marked as P1 from the phase change composite material.
Example 3
Referring to the preparation method in example 1, the content of n-eicosane in the prepared phase change composite material was 40% by adjusting the amount of n-eicosane to 0.25 g; and then preparing the temperature response type electromagnetic shielding material which is marked as P2 from the phase change composite material.
Example 4
Referring to the preparation method in example 1, the content of n-eicosane in the prepared phase change composite material was 80% by adjusting the amount of n-eicosane to 1.5 g; and then preparing the temperature response type electromagnetic shielding material which is marked as P4 from the phase change composite material.
Comparative example
Accurately weighing 0.3g of expanded graphite and 0.075g of multi-walled carbon nanotubes, mixing the materials in an ultrasonic water bath environment, and mechanically stirring the materials to uniformly disperse the materials. And then taking out the mixture, putting the mixture into a cold pressing die with the diameter of not 30mm, and performing cold pressing to obtain the composite material without the n-eicosane (namely the content of the n-eicosane in the prepared phase change composite material is 0%), and marking the content as P0.
Referring to the method in step (3) of example 1, a layer of PVDF/Fe was placed on top of the composite material containing no n-eicosane, and on top of it 3 O 4 And (3) carrying out cold pressing on the fiber film to obtain the electromagnetic shielding material.
Performance testing and results analysis:
a small portion of the electromagnetic shielding material prepared in each of the above examples and comparative examples was uniformly cut and placed in an aluminum crucible for differential scanning calorimetry, and the enthalpy and specific heat capacity thereof were measured at a temperature ranging from 0 to 80 ℃ and at a temperature-rising rate of 10 ℃/min.
The results are shown in fig. 3a, and it can be seen that the samples of each example have responses to temperature, have DSC curves specific to phase change materials, and have enthalpy. Moreover, the DSC curves of the samples are similar, and the samples do not chemically change with the change in doping amount.
The electromagnetic shielding material prepared in example 1 was put into a dry type incubator and a thermal cycle experiment was performed at 20 to 70 ℃. A part of the sample after circulation is also cut out to be subjected to differential scanning calorimetry, the response to the temperature is observed, the result is shown in figure 3b, and the DSC curve of the sample in the example 1 is not obviously changed before and after circulation, which indicates that the sample has reliable circulation thermal stability.
And (3) testing the electromagnetic shielding performance:
the electromagnetic shielding materials prepared in the above embodiments and comparative examples were placed in a waveguide method X-band jig for an X-band electromagnetic shielding performance test. After the test is finished, the composite material is heated to the phase transition temperature of n-eicosane by using a heating table, the surface temperature of the composite material is observed by using an infrared camera, the composite material is placed into a clamp for electromagnetic shielding performance test after the surface temperature is raised to the phase transition temperature, the conductivity change and the electromagnetic shielding performance change of the electromagnetic shielding materials before and after the phase transition temperature are respectively tested (COLD in the figure indicates before the phase transition temperature, HOT indicates after the phase transition temperature), the results are shown in fig. 4 to 8, and the conductivity change of different materials before and after the phase transition temperature can be seen. The change of the conductivity of the composite material without the phase-change material is not obvious before and after the phase-change temperature, and the conductivity of the composite material with the phase-change material is increased to a certain extent after the temperature reaches the phase-change temperature, so that the corresponding electromagnetic shielding performance is improved.
And (3) testing thermal conductivity:
the thermal diffusivity of the electromagnetic shielding materials prepared in the above-described examples and comparative examples was measured by using a laser flash point method, and then the thermal conductivity of the materials was calculated, and the results are shown in fig. 9, and the thermal conductivity change among the samples of the examples can be seen, and the thermal conductivity of the samples prepared in the examples changes with the change in the content of the phase change material (n-eicosane).
It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Claims (10)
1. A preparation method of a temperature response type electromagnetic shielding material is characterized by comprising the following steps: the method comprises the following steps:
(1) Preparing a phase-change composite material: heating the organic phase-change material to melt the organic phase-change material into a liquid state, then adding the carbon nano tube and the expanded graphite into the organic phase-change material, uniformly mixing, and sequentially drying and cold-pressing the mixture to form the phase-change composite material;
(2) Preparing a polyvinylidene fluoride/ferroferric oxide fiber film: dispersing ferroferric oxide and polyvinylidene fluoride in an organic solvent to form a suspension, and preparing a polyvinylidene fluoride/ferroferric oxide fiber film by utilizing an electrostatic spinning technology;
(3) Preparing the temperature response type electromagnetic shielding material: respectively placing a layer of polyvinylidene fluoride/ferroferric oxide fiber film on the top and the bottom of the phase-change composite material to obtain a prefabricated interlayer material, placing the prefabricated interlayer material in a cold pressing die, and then carrying out cold pressing to prepare the temperature response type electromagnetic shielding material.
2. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (1), the organic phase change material is eicosane, polyethylene glycol 2000 or polyethylene glycol 4000.
3. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (1), the mass content of the organic phase-change material in the phase-change composite material is 20-80%.
4. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (1), the pressure of the cold pressing is 5-15MPa, and the pressing time is 5-10 minutes.
5. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (1), the thickness of the phase change composite material is 0.5-3mm.
6. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (2), the organic solvent is N-N dimethylformamide, N-N dimethylacetamide or dimethyl sulfoxide.
7. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (2), the mass ratio of polyvinylidene fluoride to ferroferric oxide is 4:1.
8. the method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (2), in the process of preparing the polyvinylidene fluoride/ferroferric oxide fiber film by using the electrostatic spinning technology, the electrostatic spinning process parameters are as follows: the temperature is controlled to be 23-28 ℃, and the humidity is less than or equal to 60%.
9. The method for preparing a temperature-responsive electromagnetic shielding material according to claim 1, wherein: in the step (3), the pressure of the cold pressing is 5-10MPa.
10. A temperature-responsive electromagnetic shielding material, characterized in that: which is prepared by the preparation method of any one of claims 1 to 9.
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