CN111402965B - Evaluation method for high-frequency electromagnetic shielding effectiveness of carbon fiber/mullite composite material - Google Patents
Evaluation method for high-frequency electromagnetic shielding effectiveness of carbon fiber/mullite composite material Download PDFInfo
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- KZHJGOXRZJKJNY-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Si]=O.O=[Al]O[Al]=O.O=[Al]O[Al]=O.O=[Al]O[Al]=O KZHJGOXRZJKJNY-UHFFFAOYSA-N 0.000 title claims abstract description 85
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Abstract
The invention provides an evaluation method of electromagnetic shielding effectiveness of a carbon fiber/mullite composite material under high frequency. The traditional predictive model cannot predict the equivalent electromagnetic shielding effectiveness of a composite material at gigahertz frequencies. The evaluation method provided by the invention mainly considers the influence of the microstructure under high frequency and the frequency-related interface effect between the carbon fiber and mullite ceramic, and establishes a high-frequency equivalent electromagnetic shielding effectiveness theoretical evaluation model of the carbon fiber/mullite composite material based on mesomechanics. The invention solves the problems of high research cost, long design and experiment time of the electromagnetic shielding material under the current gigahertz frequency.
Description
Technical Field
The invention belongs to the field of research on a homogenization method of electromagnetic performance of a functional composite material at high frequency, and more particularly relates to an evaluation method of electromagnetic shielding effectiveness of a carbon fiber/mullite composite material at gigahertz frequency.
Background
In recent years, with the rapid development of the communication industry, electromagnetic interference has become a very important issue for electronic devices. Due to the high electrical conductivity of carbon materials, carbon-containing composites have been widely used in electromagnetic shielding devices in the gigahertz (8.2-12.4 GHz) frequency range. Typical carbonaceous composites include polymer-based, metal-based, ceramic-based composites. However, electronic devices are inevitably subjected to complex electromagnetic loads and high temperature environments during manufacture and use, such as spacecraft in the aerospace field. Metal-based or polymer-based composites are no longer suitable in extreme environments. Carbon or silicon carbide material filled reinforced ceramic matrix composites are therefore of great interest in electromagnetic shielding applications in extreme environments due to their high temperature resistance, low mass density characteristics. Recently, mullite ceramics have been widely used in high temperature structural materials due to their excellent thermal stability, high strength and low density characteristics. Conductivity and dielectric properties are key factors in determining the electromagnetic shielding effectiveness of the composite material. Carbon material filled reinforced mullite composites are considered as the material of choice for high temperature electromagnetic shielding devices at gigahertz frequencies. The carbon fiber/mullite composite material prepared by the spark plasma sintering method can obtain higher electromagnetic shielding efficiency under a small thickness. Research shows that microstructure and interfacial effect are important to electromagnetic shielding effectiveness of the carbon fiber/mullite composite material.
The search finds that: at present, no physical mechanism capable of explaining the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material based on the theory of complex interface effect and microstructure among components exists. The invention is to establish an evaluation method of equivalent electromagnetic shielding effectiveness of a carbon fiber/mullite composite material under high frequency.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a method for evaluating high-frequency electromagnetic shielding effectiveness of a carbon fiber/mullite composite material, the method comprising the steps of:
step one: measuring geometric parameters and electromagnetic properties of carbon fiber and mullite ceramic component materials
1.1 measuring the slenderness ratio alpha (the ratio of the major axis to the minor axis of the material) and the diameter D of a carbon fiber material;
1.2 measurement of in-plane electromagnetic Properties of carbon fiber Material including in-plane dielectric epsilon 1 Conductivity sigma 1 Magnetic permeability mu' 1 And magnetic loss mu' 1 The method comprises the steps of carrying out a first treatment on the surface of the Measuring out-of-plane electromagnetic properties of carbon fiber materials, the properties including out-of-plane dielectric ε 3 Conductivity sigma 3 Magnetic permeability mu' 3 And magnetic loss mu' 3 ；
1.3 measurement of the electromagnetic Properties of mullite ceramics, including its dielectric ε 0 Conductivity sigma 0 Magnetic permeability mu' 0 Magnetic loss mu' 0 ；
1.4 inquiring the practical physical constant table to obtain the vacuum intermediate constant epsilon vac And magnetic permeability mu vac And air dielectric properties epsilon air Conductivity sigma air Magnetic permeability mu' air And magnetic loss mu' air ；
Step two: preparing a carbon fiber/mullite composite material sample, and measuring part of microscopic parameters and electromagnetic properties under high frequency
2.1 with short carbon fibers, al 2 O 3 and SiO2 Taking the powder as a raw material, taking yttrium oxide powder as a sintered additive, adopting a spark plasma rapid sintering (SPS) method to prepare a plurality of carbon fiber/mullite composite material samples with different carbon fiber contents (the range is 0vol% to 5 vol%) and the thickness of 2-3mm, and measuring the thickness d of the samples;
2.2, adopting a Scanning Electron Microscope (SEM) to characterize the morphology and microstructure of the carbon fiber/mullite composite material sample, and measuring the interface layer thickness h between the carbon fiber and mullite ceramic;
2.3, measuring complex dielectric property and complex magnetic conductivity of carbon fiber/mullite composite material samples with different carbon fiber contents at gigahertz frequency by using a vector network analyzer, and measuring a distance r between a wave source and the carbon fiber/mullite composite material;
step three: establishing an evaluation model of electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency
According to formula (13), the electromagnetic Shielding Effectiveness (SE) of the carbon fiber/mullite composite material with a given carbon fiber content at gigahertz frequency is calculated:
se=r+a+m formula (13)
Wherein R, A and M represent reflection loss, absorption loss and multiple reflection loss in electromagnetic shielding, respectively, and the expressions are:
wherein d is the thickness of the composite sample, which has been obtained by measurement in step two, 2.1;
and
The complex permittivity and complex permeability of air are obtained by looking up a table in step one, 1.4; ω=2pi f is the angular frequency of the electromagnetic wave, f represents the corresponding ac frequency, i is a virtual constant;
and
Representing the effective complex dielectric properties and complex permeability of the composite material, will be given by solving the following equation set (1):
wherein ,
wherein ,
and
Conductivity and dielectric properties of the interfacial layer, respectively; mu' (int) and μ”(int) Magnetic permeability and magnetic loss of the interface layer, respectively, +.>
and
The dimension parameters of electron tunneling effect and nano capacitor formation under direct current load are respectively +.>
For the dimensional parameters of the formation of nano-capacitors at infinite frequency, t σ and tε Characteristic times of interface electronic transition effect and dielectric relaxation effect, respectivelyThe method comprises the steps of carrying out a first treatment on the surface of the The parameters are obtained through experimental data fitting in the fourth step; thus, the evaluation model of the invention is built;
step four: method for evaluating electromagnetic shielding effectiveness of carbon fiber/mullite composite material under high frequency and calculating and extracting material parameters
4.1 bringing the equivalent complex dielectric properties measured in step 2.3 into the evaluation model established in step three, determining complex dielectric related material parameters in the evaluation model in step three by data fitting, wherein the material parameters comprise
t σ and tε ；
4.2 bringing the equivalent complex permeability measured in step 2.3 into the evaluation model established in step three, determining the complex permeability-related material parameters in the evaluation model in step three by data fitting, wherein the material parameters comprise mu '(int) and mu'; (int) ；
step five: calculation and verification of prediction curve of electromagnetic shielding effectiveness evaluation method of carbon fiber/mullite composite material under high frequency
5.1 measuring scattering parameters of carbon fiber/mullite composite samples at gigahertz frequencies by waveguide method using vector network analyzer (S 11 、S 12 、S 22 、S 21 ) The method comprises the steps of carrying out a first treatment on the surface of the The electromagnetic Shielding Effectiveness (SE) of the composite material is calculated from the measured scattering parameters according to the following equation (16):
wherein r= |s 11 | 2 and T＝|S21 | 2 Respectively representing a reflection coefficient and a transmission coefficient;
and 5.2, bringing different carbon fiber contents and gigahertz alternating current frequencies into the evaluation model established in the step three of the invention to obtain a prediction curve of equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material relative to the alternating current frequency, and verifying the prediction curve with experimental data in 5.1.
The effective effects are as follows:
the traditional homogenization method can not predict the equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency. According to the evaluation method based on the mesomechanics, complex interface effects and microstructures among component materials are considered, so that the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at the gigahertz frequency can be accurately evaluated, and the design efficiency of an electromagnetic shielding device is improved. The principle of the electromagnetic shielding effectiveness evaluation method of the carbon fiber/mullite composite material of the present invention at high frequencies will be explained from the following two parts.
1 determination of equivalent electromagnetic Properties at high frequencies
The invention introduces complex dielectric constant epsilon and complex magnetic permeability mu as homogenization variables of an evaluation method, and the expression is as follows:
wherein ω=2pi f is the angular frequency of the electromagnetic wave, f represents the corresponding ac frequency, i is a virtual constant; epsilon, sigma, mu' and mu "represent dielectric, conductive, magnetic permeability and magnetic loss, respectively.
Secondly, three types of interface effects need to be considered at the interface between the carbon fiber and the mullite ceramic: 1. dielectric aspect: carbon fiber content dependent nanocapacitors form and frequency dependent dielectric relaxation effects; 2. conductivity aspect: electron tunneling effect and frequency-dependent electron transition effect associated with carbon fiber content; 3. interface connection aspect: non-ideal interfacing.
In terms of dielectric properties, electrons cannot freely pass through the interface between the carbon fiber and the mullite ceramic due to the large difference in conductivity between the carbon fiber and the mullite, and a large number of nano capacitors are formed at the interface, which will significantly improve interface dielectric properties. In addition, as the ac frequency increases, more electrons will cross the interface between the carbon fiber and mullite ceramic, creating a dielectric relaxation effect. Taking into account the above-described nanocapacitor formation and dielectric relaxation effects, the interfacial dielectric is expressed as:
wherein
t ε Representing the characteristic time of formation of the nanocapacitors,
and
Interface dielectric properties in DC and infinite frequency states, respectively, < >>
and
Characteristic parameters representing the corresponding polarization effects, which parameters are to be given by data fitting; c 1 Representing the volume fraction of carbon fiber phase in the composite material, which has been given during sample preparation;
The percolation threshold of the carbon fiber/mullite composite material is represented by the following expression:
wherein ,S33 Is a component of the eshellby tensor and will be given in equation (12). Function of
The growth trend of interfacial dielectric after the carbon fiber content reaches the percolation threshold is shown, and the expression is as follows:
threshold value of seepage
For composite materials, a strict geometrical parameter is used, the value of which depends on the slenderness ratio of the carbon fibers.
In terms of conductivity, when the carbon fiber volume fraction reaches the percolation threshold, carbon conductive networks begin to build up, which will significantly promote interfacial conductivity. In addition, as the ac frequency increases, more electrons transfer from one carbon fiber to another, creating an electron transfer effect. Taking the electron tunneling effect and the electron transition effect into consideration, the interfacial conductivity has the following expression:
wherein p (ω) is a transition function,
interface conductivity +.representing the electron tunneling under DC loading of the interface layer>
wherein ,
is a characteristic parameter of electron tunneling, +.>
Is the initial conductivity of the interfacial layer,t σ the characteristic time representing the electron transition phenomenon, the above parameters will be given by data fitting.
In terms of interface connection, because the carbon fiber/mullite composite material inevitably has defects in the preparation and use processes, an undesirable interface is usually formed between the carbon fiber and the mullite ceramic. After the three types of interface effects are considered, the effective double dielectric property of the carbon fiber considering the interface effect can be calculated by a Mori-Tanaka method
And complex permeability->
wherein ,
μ' (int) and μ”(int) Respectively representing interfacial dielectric properties, conductivity, magnetic permeability and magnetic loss taking the above interface effects into consideration; epsilon i ,σ i ,μ' i and μ”i Is the electromagnetic property of the corresponding carbon fiber. c int The expression is the volume fraction of the interface layer in the carbon fiber containing the interface, and the expression is:
c int ＝1-LD 2 /(L+h)(D+2h) 2 ,L＝Dα/2 (9)
wherein D and α represent the diameter and slenderness ratio of the carbon fiber, respectively, and h represents the thickness of the interface layer, which will be given in experimental measurements.
The carbon fiber complex dielectric property and complex permeability taking the interface effect between the carbon fiber and mullite ceramic into consideration are obtained by the formula (8). The effective dielectric method is adopted to calculate the equivalent complex dielectric property of the carbon fiber/mullite composite material
Complex permeability
The expression is as follows:
wherein ,
and
The complex permittivity and permeability tensor of the carbon fiber are considered for the interfacial effect, respectively, and are given in formula (8); c 1 Representing the volume fraction of carbon fiber phase in the composite material, which has been given during sample preparation; s is S 11 and S33 Is a component of the Eshelby tensor of the carbon fiber phase, and has the expression:
by solving the formulas (10) - (11), the effective complex permittivity and permeability of the carbon fiber/mullite composite material at gigahertz frequency are obtained, and the method can be used for calculating the electromagnetic shielding effectiveness of the whole composite material.
2 determining electromagnetic shielding effectiveness at high frequencies
The equivalent electromagnetic shielding effectiveness of the composite material at the gigahertz frequency is calculated by the equivalent complex dielectric property and complex magnetic permeability related to the carbon fiber/mullite composite material frequency based on the plane wave shielding theory. The electromagnetic shielding schematic diagram of the carbon fiber/mullite composite material is shown in fig. 1, and can be divided into the following three parts: reflection loss (R), absorption loss (a), and multiple reflection loss (M). Based on the plane wave shielding theory, the electromagnetic Shielding Effectiveness (SE) of the carbon fiber/mullite composite material can be given by the ratio of the incident wave to the output wave, and the expression is as follows:
SE＝R+A+M (13)
η and γ represent the intrinsic impedance and propagation constant, respectively; the subscripts "e" and "air" denote the properties of the composite material and air, respectively, and are expressed as:
wherein 
and
Representing the effective complex dielectric properties and permeability of the overall composite, has been derived in equations (10) - (11); z is Z w ≈1/ωε vac r is the wave impedance of the near field, and describes the influence on near field shielding; d is the thickness of the composite sample, r represents the distance between the wave source and the composite barrier, and has been obtained by experimental measurements;
And
the complex dielectric properties and complex permeability of air have been obtained by looking up a table.
The equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency is completely obtained by the formula (13), and the establishment of the evaluation model is completed.
A physical mechanism for explaining the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material adopts the evaluation method of the high-frequency electromagnetic shielding effectiveness of the carbon fiber/mullite composite material.
The physical mechanism is based on complex interface effects between components and microstructure.
Drawings
Fig. 1 is a schematic diagram of the principle of electromagnetic shielding of a carbon fiber/mullite composite material. In the figure, numeral 1 indicates an incident electromagnetic wave, numeral 2 indicates a reflected electromagnetic wave, and numeral 3 indicates an outgoing electromagnetic wave.
FIG. 2 is a graph of the microstructure of the carbon fiber/mullite composite material.
Fig. 3 is a graph of theoretical prediction curves and experimental data of equivalent dielectric properties and conductivity at gigahertz frequencies of carbon fiber/mullite composites. In the figure, numeral 4 represents an equivalent dielectric theoretical prediction curve, numeral 5 represents an equivalent conductivity theoretical prediction curve, numeral 6 represents equivalent dielectric experimental data, and numeral 7 represents equivalent conductivity experimental data.
Fig. 4 is a theoretical prediction curve and experimental data graph of equivalent permeability and magnetic loss at gigahertz frequency of the carbon fiber/mullite composite material. In the figure, numeral 8 represents an equivalent magnetic permeability theoretical prediction curve, numeral 9 represents an equivalent magnetic loss theoretical prediction curve, numeral 10 represents equivalent magnetic permeability experimental data, and numeral 11 represents equivalent magnetic loss experimental data.
Fig. 5 is a graph of experimental data of electromagnetic shielding effectiveness at gigahertz frequencies of carbon fiber/mullite composites.
FIG. 6 is a graph comparing the prediction curve of the evaluation model of the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at gigahertz frequency with experimental data. In the figure, numeral 12 represents a theoretical prediction curve of equivalent electromagnetic shielding effectiveness of a sample S0, numeral 13 represents a theoretical prediction curve of equivalent electromagnetic shielding effectiveness of a sample S1, numeral 14 represents a theoretical prediction curve of equivalent electromagnetic shielding effectiveness of a sample S2, numeral 15 represents experimental data of equivalent electromagnetic shielding effectiveness of a sample S0, numeral 16 represents experimental data of equivalent electromagnetic shielding effectiveness of a sample S1, and numeral 17 represents experimental data of equivalent electromagnetic shielding effectiveness of a sample S2.
FIG. 7 is a graph comparing the predicted curve of the evaluation model of the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at the gigahertz frequency with the predicted curve based on the theoretical model in document [1 ]. Numeral 18 represents a theoretical prediction curve of the electromagnetic shielding effectiveness of the sample S1 based on the document [1], numeral 19 represents a prediction curve of the electromagnetic shielding effectiveness of the sample S1 based on the evaluation method of the present invention, and numeral 20 represents experimental data of the electromagnetic shielding effectiveness of the sample S1.
Detailed Description
In order to facilitate understanding of the present invention, the following specific embodiments of the present invention are described in conjunction with examples and comparative examples, respectively, but the present invention is not limited to the following examples and comparative examples.
Example 1
The present invention will be described more fully with reference to the following examples. The method comprises the following specific steps:
1. measuring the slenderness ratio alpha=254 and the diameter D=10μm of the T700 12k short carbon fiber; the in-plane electromagnetic properties of the carbon fiber material were measured, and as a result: in-plane dielectric epsilon 1 /ε vac =10, conductivity σ 1 =0.83S/m, permeability μ' 1 /μ vac =1.01 and magnetic loss μ' 1 /μ vac =0.001; the out-of-plane dielectric characteristics of the carbon fiber were measured to obtain the out-of-plane dielectric ε 3 /ε vac =15, conductivity σ 3 ＝8.32×10 2 S/m, permeability μ' 3 /μ vac =1.05 and magnetic loss μ' 3 /μ vac =0.001; the electromagnetic properties of mullite ceramics were measured, as a result of which: conductivity sigma 0 ＝3.5×10 -9 S/m, dielectric ε 0 /ε vac =7.0, permeability μ' 0 /μ vac =0.982, magnetic loss μ″ " 0 /μ vac =0.005; inquiring the practical physical constant table to obtain the vacuum medium constant epsilon vac ＝8.85×10 -12 F/m, permeability in vacuum. Mu. vac ＝4π×10 -7 N/A 2 And air dielectric properties epsilon air /ε vac =1, conductivity σ air Approximately 0, magnetic permeability mu' air /μ vac =1 and magnetic loss μ' air /μ vac ≈0。
2. With T700 12k short carbon fiber (length 2-3 mm), commercially available Al 2 O 3 and SiO2 The powder is used as a raw material, 6.5vol% of yttrium oxide powder is used as a sintering additive, a spark plasma rapid sintering method is adopted to prepare 3 carbon fiber/mullite composite material samples with different carbon fiber contents, the volume fractions of the carbon fibers are respectively 0 percent (sample S0), 1.32 percent (sample S1) and 1.65 percent (sample S2), and the sizes of the samples are 22.86mm multiplied by 10.16mm multiplied by 2mm.
(1) The thickness of the composite sample was measured to be d=2mm;
(2) Characterization of the microstructure of the composite material sample by a scanning electron microscope (SEM, nova Nano SEM 230), wherein the morphology diagram is shown in figure 2; measurement of interfacial layer thickness h=3×10- 7 m；
(3) Measuring complex dielectric properties and complex magnetic permeability of carbon fiber/mullite composite material samples with different carbon fiber contents at gigahertz frequency by using a vector network analyzer, wherein experimental data are shown in figures 3 to 4; the distance r=0.0015 m between the measuring wave source and the carbon fiber/mullite composite.
3. The experimental data of the complex dielectric property and complex magnetic conductivity of the carbon fiber/mullite composite material and the parameters of the sample materials, which are measured by the experiment, are brought into an evaluation model established by the invention, and the residual parameters of the material of the evaluation model are determined by data fitting, and are shown in table 1. Thus, a complete evaluation model of electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at gigahertz frequency is obtained.
Table 1 material parameters for calculation
4. Measuring scattering parameters of a sample by a waveguide method using a vector network analyzer (Agilent N5230A) (S 11 、S 12 、S 22 、S 21 . Electromagnetic Shielding Effectiveness (SE) is calculated from the measured scattering parameters according to the following formula:
wherein r= |s 11 | 2 and T＝|S21 | 2 Respectively representing the reflection coefficient and the transmission coefficient. The experimental results of electromagnetic shielding effectiveness at gigahertz frequencies of the carbon fiber/mullite composite are shown in fig. 5.
5. Substituting the carbon fiber content and gigahertz alternating frequency into the evaluation model established by the invention to obtain a prediction curve of equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at gigahertz frequency, as shown in figure 6. And the electromagnetic shielding effectiveness evaluation model prediction curve of the carbon fiber/mullite composite material is consistent with experimental data of the prepared sample. Therefore, the method for evaluating the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency has feasibility. According to the actual design requirement of the electromagnetic shielding device, the content, alternating frequency and composite material thickness of the carbon fiber can be selected from the graph, so that the design process of the carbon fiber/mullite functional composite material electromagnetic shielding device is guided, the experiment times are reduced, and the design efficiency is improved.
Comparative example 1
The present invention will be explained below with reference to comparative examples. By comparing the evaluation method of the invention with the theoretical prediction result in the document [1], the invention proves that the defect that the traditional evaluation model can not accurately predict the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at the gigahertz frequency can be effectively improved. The method comprises the following specific steps:
1. the predicted curve of equivalent electromagnetic shielding effectiveness at gigahertz for carbon fiber/mullite composites according to the evaluation method of the present invention with respect to ac frequency has been given in example 1 as shown in fig. 7.
2. The material parameters of table 1 in example 1 were taken into the prediction model of document [1], and a prediction curve of the equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material with respect to the alternating current frequency was drawn, as shown in fig. 7.
3. The prediction curves obtained in steps 1 and 2 of comparative example 1 above were compared with the experimental data obtained in example 1, as shown in fig. 7. It can be seen that the theoretical model in the document [1] is only suitable for calculating the electromagnetic shielding effectiveness of the graphene/polymer composite material at low frequency, and cannot accurately predict the variation trend of the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at gigahertz frequency with respect to alternating current frequency. The evaluation method provided by the invention can overcome the defects of the existing model, effectively predict the electromagnetic shielding efficiency of the carbon fiber/mullite composite material at the gigahertz frequency and improve the design efficiency of the electromagnetic shielding device.
Among these, in this comparative example, document [1] is: xia XD, wang Y, zhong Z, weng GJ. Journal of Applied Physics,120:085102 (2016).
Claims (4)
1. The evaluation method of the high-frequency electromagnetic shielding effectiveness of the carbon fiber/mullite composite material is characterized by at least comprising the following steps:
step one: measuring geometric parameters and electromagnetic properties of carbon fiber and mullite ceramic component materials
1.1 Measuring slenderness ratio of carbon fiber material
And diameter->
；
1.2 Measuring in-plane electromagnetic properties of carbon fiber materials, including in-plane dielectric properties
Conductivity->
Magnetic permeability->
And magnetic loss->
The method comprises the steps of carrying out a first treatment on the surface of the Measuring out-of-plane electromagnetic properties of carbon fiber materials, including out-of-plane dielectric +.>
Conductivity->
Magnetic permeability->
And magnetic loss->
；
1.3 Measuring the electromagnetic properties of mullite ceramics, including their dielectric properties
Conductivity->
Magnetic permeability->
Magnetic loss->
；
1.4 Inquiring the practical physical constant table to obtain the vacuum medium constant
And permeability->
And air dielectric properties
Conductivity->
Magnetic permeability->
And magnetic loss->
；
Step two: preparing a carbon fiber/mullite composite material sample, and measuring part of microscopic parameters and electromagnetic properties under high frequency
2.1 With short carbon fibres, al 2 O 3 and SiO2 The powder is used as raw material, yttrium oxide powder is used as sintering additive, a spark plasma rapid sintering method is adopted to prepare a plurality of carbon fiber/mullite composite material samples with different carbon fiber contents and thickness of 2-3mm, and the thickness of the samples is measured
；
2.2 The morphology and microstructure of the carbon fiber/mullite composite material sample are characterized by adopting a scanning electron microscope, and the interface layer thickness between the carbon fiber and mullite ceramic is measured
；
2.3 Measuring complex dielectric property and complex magnetic conductivity of carbon fiber/mullite composite material samples with different carbon fiber contents at gigahertz frequency by using vector network analyzer, and measuring distance between a wave source and the carbon fiber/mullite composite material
；
Step three: establishing an evaluation model of electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency
According to the formula, the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material with given carbon fiber content at gigahertz frequency is calculated:
wherein ,
for the thickness of the composite sample, 2.1 has been obtained by measurement in said step two;
and
The complex permittivity and complex permeability of air are obtained by looking up a table in step one, 1.4;
Is the angular frequency of electromagnetic waves, < >>
Representing corresponding alternating frequency, i being a virtual constant;
And
representing the effective complex dielectric properties and complex permeability of the composite material, will be given by solving the following equation set (1): />
wherein ,
wherein ,
and
Conductivity and dielectric properties of the interfacial layer, respectively;
and
Magnetic permeability and magnetic loss of the interface layer, respectively, +.>
and
The dimension parameters of electron tunneling effect and nano capacitor formation under direct current load are respectively +.>
Size parameters for the formation of nano-capacitors at infinite frequency, < >>
and
Characteristic time of interface electronic transition effect and dielectric relaxation effect respectively;
step four: calculating and extracting material parameters of an evaluation method of electromagnetic shielding effectiveness of the carbon fiber/mullite composite material at high frequency;
step five: the method for evaluating the electromagnetic shielding effectiveness of the carbon fiber/mullite composite material under high frequency predicts the calculation and verification of the curve.
2. The method for evaluating high-frequency electromagnetic shielding effectiveness of a carbon fiber/mullite composite material according to claim 1, wherein the carbon fiber content is 0-5 vol%.
3. The method for evaluating the high-frequency electromagnetic shielding effectiveness of the carbon fiber/mullite composite material of claim 1, wherein the specific operation of the fourth step is as follows:
4.1 Bringing the equivalent complex dielectric property measured in the step 2.3 into the evaluation model established in the step three, and determining the complex dielectric property related material parameters in the evaluation model in the step three through data fitting, wherein the material parameters comprise
，
，
，
，
，
，
and
；
4.2 bringing the equivalent complex permeability measured in step 2.3 into the evaluation model established in step three, determining complex permeability-related material parameters in the evaluation model in step three by data fitting, wherein the material parameters comprise
and
。
4. The method for evaluating the high-frequency electromagnetic shielding effectiveness of the carbon fiber/mullite composite material of claim 1, wherein the specific operation of the fifth step is as follows:
5.1 Measuring scattering parameters of carbon fiber/mullite composite samples at gigahertz frequencies by a waveguide method using a vector network analyzer: s is S 11 、S 12 、S 22 、S 21 The method comprises the steps of carrying out a first treatment on the surface of the The electromagnetic shielding effectiveness of the composite is calculated from the measured scattering parameters according to the following equation (16):
5.2 And (3) bringing different carbon fiber contents and gigahertz alternating current frequencies into the evaluation model established in the third step, obtaining a prediction curve of equivalent electromagnetic shielding effectiveness of the carbon fiber/mullite composite material with respect to the alternating current frequency, and verifying the prediction curve with experimental data in the step 5.1.
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