Paper on modelling the shielding characteristics of nonwoven fabrics

Paper on modelling the low frequency shielding effectiveness of nonwoven fabrics accepted for publication in IEEE Transaction on Electromagnetic Compatibility.

Nonwoven fabric materials with conductive fibres are often used as a lightweight functional layer in composite laminates to enhance the overall electromagnetic shielding performance of a structure. The structure of these types of material are random and can only be described statistically. In this paper we applied Monte Carlo simulation techniques to an equivalent circuit model of the material structure to derive semi-empirical models for the shielding performance of the material. These models can be applied in industry to the design of materials for particular applications.

The image above shows a 3D image of a nonwoven structure derived from a particular structure generated by the Monte Carlo simulator. The small spheres denote contact points between the fibres which may have a non-zero resistance and therefore contribute to the overall surface resistance of the material and hence its shielding effectiveness.

The work in the paper is part of a collaboration between the AEG and Technical Fibre Products. My particular contribution to the collaboration was initiating the percolation theory approach to the modelling and developing and implementing the Monte Simulation tool in MATLAB using elements of the AEG Mesher software package.

Full details of the work can be found in the paper (Dawson et al., 2017; Austin et al., 2015; Austin et al., 2013).

References

1. Dawson, J.F., Austin, A.N., Flintoft, I.D. and Marvin, A.C., 2017. Shielding effectiveness and sheet conductance of nonwoven carbon-fiber sheets. IEEE Transactions on Electromagnetic Compatibility, 59(1), pp.84–92.
   

   Nonwoven carbon-fiber sheets are often used to form a conductive layer in composite materials for electromagnetic shielding and other purposes. While a large amount of research has considered the properties of similar idealized materials near the percolation threshold, little has been done to provide validated analytic models suitable for materials of practical use for electromagnetic shielding. Since numerical models consume considerable computer resource and do not provide the insight which enables improved material design, an analytic model is of great utility for materials development. This paper introduces a new theoretical model for the sheet conductance of nonwoven carbon-fiber sheets built on the theory of percolation for 2-D conducting stick networks. The model accounts for the effects of sample thickness, fiber angle distribution, and contact conductance on the sheet conductance. The theory shows good agreement with Monte Carlo simulations and measurements of real materials in the supercritical percolation regime where the dimensionless areal concentration of fibers exceeds about 50. The theoretical model allows the rapid prediction of material shielding performance from a limited number of manufacturing parameters.

   @article{Dawson2017a,
     author = {Dawson, J. F. and Austin, A. N. and Flintoft, I. D. and Marvin, A. C.},
     title = {Shielding effectiveness and sheet conductance of nonwoven carbon-fiber sheets},
     journal = {IEEE Transactions on Electromagnetic Compatibility},
     year = {2017},
     volume = {59},
     number = {1},
     pages = {84-92},
     month = feb,
     issn = {0018-9375},
     note = {Date of acceptance: 04/08/2016. © 2016 ESA. Personal use of this material is permitted.},
     doi = {10.1109/TEMC.2016.2601658},
     file = {:pdfs/TEMC59-Dawson-84.pdf:PDF},
     keywords = {electromagnetic shielding effectiveness, percolation theory, sheet conductance, carbon fibre composite, nonwoven fabric},
     postprint = {https://pure.york.ac.uk/portal/files/50264656/TEMC59_Dawson_84_postprint.pdf},
     publisher = {IEEE}
   }
   

2. Austin, A.N., Dawson, J.F., Flintoft, I.D. and Marvin, A.C., 2015. Modelling the micro-structure of non-uniform conductive non-woven fabrics: Determination of sheet resistance. In: 2015 Joint IEEE International Symposium on Electromagnetic Compatibility and EMC Europe. Dresden, Germany: IEEE, pp.1–6.
   

   The plane-wave shielding effectiveness of conductive non-woven fabrics is dominated by the sheet conductance over a wide range of frequencies until the effects of skin depth, and apertures start to influence the performance (typically at 1-10 GHz depending on the areal density). This paper describes models for determining the sheet conductance from knowledge of the type, quantity, orientation and contact resistance of the fibres in the fabricated material. The stochastic nature of these materials, their complexity and local scale variability has been included in the models and correlates well with experimental results. The anisotropic sheet conductance is modelled to within 1% of the measured value in the high shielding orientation and to within 2% in the orthogonal orientation using an inter-fibre contact resistance estimated to be 10 kΩ.

   @inproceedings{Austin2015,
     issn = {2158-110X},
     booktitle = {2015 Joint IEEE International Symposium on Electromagnetic Compatibility and EMC Europe},
     month = {16-22 August},
     publisher = {IEEE},
     address = {Dresden, Germany},
     author = {Austin, A. N. and Dawson, J. F. and Flintoft, I. D. and Marvin, A. C.},
     title = {Modelling the micro-structure of non-uniform conductive non-woven fabrics: Determination of sheet resistance},
     year = {2015},
     pages = {1--6},
     note = {© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.},
     doi = {10.1109/ISEMC.2015.7256122},
     file = {:pdfs/EMCE2015-Austin-1.pdf:PDF},
     keywords = {sheet resistance, shielding effectiveness, stochastic material, contact resistance, anisotropic material, circuit model},
     owner = {idf1},
     postprint = {https://pure.york.ac.uk/portal/files/40237334/001_5072_postprint.pdf},
     timestamp = {2016.10.12}
   }
   

3. Austin, A.N., Dawson, J.F., Flintoft, I.D. and Marvin, A.C., 2013. Analysis of the shielding properties of metalised nonwoven materials. In: 2013 International Symposium on Electromagnetic Compatibility (EMC EUROPE 2013), 12th International Symposium on Electromagnetic Compatibility. Brugge, Belgium: IEEE, pp.526–531.
   

   This paper considers the shielding effectiveness of nonwoven materials in the 1-8.5 GHz range using parametric semi-empirical models and experimental techniques. A range of materials based on metalised carbon fibres are considered with weights between 10 and 75 g/m2, thicknesses between 50 and 400μm, and measured shielding effectiveness between 35 and 100dB. A numerical model is presented and compared to experimental results.

   @inproceedings{Austin2013,
     issn = {2325-0356},
     isbn = {978-1-4673-4979-6},
     booktitle = {2013 International Symposium on Electromagnetic Compatibility (EMC EUROPE 2013)},
     month = {2-6 September},
     publisher = {IEEE},
     address = {Brugge, Belgium},
     series = {12th International Symposium on Electromagnetic Compatibility},
     author = {Austin, A. N. and Dawson, J. F. and Flintoft, I. D. and Marvin, A. C.},
     title = {Analysis of the shielding properties of metalised nonwoven materials},
     year = {2013},
     pages = {526--531},
     file = {:pdfs/EMCE2013-Austin-526.pdf:PDF},
     keywords = {shielding; metallised nonwovens; high reflectivity; TLM modelling; aperture; pore; absorber box; carbon; fibre; electroplating; electrochemical; electric field; screening},
     owner = {idf1},
     timestamp = {2013.09.17}
   }