Nickel-containing thin-film coatings on fibrous materials..
Nickel-containing thin-film coatings on fibrous materials
© L.M. Lynkov, V.A. Bogush, E.A. Senkovets, S.M. Zavadsky
Belarusian State University of Informatics and Radioelectronics, Minsk E-mail: aleks@gw.bsuir.unibel.by
A promising method for creating electromagnetic screens is vacuum deposition of thin films on fibrous materials. To study the electromagnetic properties, machine-knitted fabrics were obtained, onto which a nickel coating was applied by magnetron sputtering. The shielding characteristics of knitted fabrics with nickel deposited on one side were studied. When the electromagnetic radiation source was located on the sprayed side of the fabric, a decrease in the reflection coefficient by 1.4 times was found compared to the case of its location on the unsputtered side. At the same time, the absorption coefficient remained unchanged. A structural analysis of the obtained material was also carried out.
Introduction.Electromagnetic radiation (EMR) is widely used in many areas of human activity. However, elevated levels of electromagnetic fields have a negative impact on biological objects, the human body, complicate the operation of low-power high-precision measuring equipment, and exacerbate the problems of electromagnetic compatibility of electronic equipment and information security.
In this regard, the task of developing highly effective and technologically advanced shielding and absorbing materials and coatings is becoming relevant. Of particular interest are methods for creating elastic radio-absorbing materials based on the use of widespread knitting equipment and characterized by low cost and material intensity [1].
One of the promising methods for producing electromagnetic screens is vacuum deposition of thin films on fibrous materials.
Methodology for producing samples.The use of ion-beam and ion-stimulated magnetron sputtering processes makes it possible to obtain coatings from various materials that have high adhesion to the substrate at relatively low temperature loads [2].
To study the shielding properties, samples were made on which a metallic coating of nickel was applied by magnetron sputtering.
The coating was applied on a VU-2MP vacuum unit equipped with two dual-beam ion sources. Before sputtering, the working chamber was evacuated to a pressure of 2×10-3 Pa, then the surface of the substrate (knitted fabric) was ion-cleaned.
The coating was formed by sputtering a nickel target with an ion-beam source with discharge parameters Up = 5 kV, Ip = 200 mA and working pressure in the chamber P = 2×10-2 Pa.
The knitted base — the substrate on which the nickel was applied, was produced on a knitted jersey equipment according to a technological scheme that ensures the formation of knitted fabrics with a fleecy volumetric structure.
The coating thickness was controlled over the process time using a calibration curve representing the dependence of the nickel coating thickness on the glass on the sputtering time. The formed nickel film has a surface resistance of 3 W/D. A single-sided nickel coating with a thickness of 0.1/mm was applied to the knitted base (polyacrylonitrile).
The resulting coating has good adhesion to the substrate, is characterized by stable electrophysical properties, and does not degrade over time.
Experimental part. The structure of the synthesized material was studied by X-ray diffractometry methods on a DRON-30 setup using Cuk-a radiation (wavelength l = 1.5417737 A) for general phase analysis and evaluation of the average size of crystalline particles. The diffraction pattern was recorded on a chart tape at a rate of 2000 pulses/s in the angle range from 10 to 80°. The X-ray patterns were recorded in the coordinates intensity (I)-angle (2q).The resulting X-ray diffraction pattern was processed by assessing the relative intensity of the diffraction lines on a 100-point scale by measuring the peak area and taking the area of the largest peak as 100.
The noise detected in the diffraction patterns, as well as the increase in the intensity of scattered X-ray radiation in the region of small diffraction angles, is explained by the influence of the organic component of the fibers [3].
Identification of the deposited metal was carried out by analyzing the diffractograms according to the ASTM card index. The study of the nickel-coated samples was carried out from the front (sputtered) and back (non-sputtered) sides.
The shielding properties of the materials were studied using a set of vector network analyzers measuring the S-parameters of four-terminal networks in the microwave range (1.5…18-18… 37 GHz) [4].
Before measurements were taken, the equipment underwent a calibration procedure, as a result of which the amplitude-frequency characteristic of the paths was taken as a zero level and all measurements of the samples were made relative to this level.
The test sample, a square piece of knitted fabric measuring 15×15 cm, was fixed between the receiver and the emitter, which were horn systems or flanges of the waveguide system. The reflection and transmission coefficients (S11 and S21, respectively) were chosen as the measured parameters. The fabric sample was stretched uniformly on all sides to prevent distortion of the knitted fabric structure and was placed in a plane perpendicular to the direction of wave propagation.
The study considered the case of the location of the electromagnetic radiation source both on the front and back sides.
Results and discussion. When studying the shielding properties of knitted fabrics with a coating containing thin layers of nickel on the front side, the values of the transmission coefficients (S21) and reflection coefficients (S11) were obtained from the sprayed and unsprayed sides in the frequency range of 1.5-37.7GHz. It was noted that at lower frequencies the reflection coefficient is higher (-8-10 dB), while at higher frequencies it decreases to -4-6 dB. An inverse relationship is observed with the transmission coefficient — with increasing frequency, the transmission coefficient also increases from -2 to -10 dB.
Fig. 1. Diffraction pattern of nickel-sprayed fibers
polyacrylonitrile on the non-sprayed side.
A decrease in the reflection coefficient was found when electromagnetic radiation fell from the uncoated side of the fabric. It was found that when using such a design, the screen reflects on average 1.4 times more electromagnetic energy than in the case of installing the knitted fabric with the uncoated side to the source. At the same time, the transmission coefficient of the screen from the uncoated side is no greater than from the coated side. This indicates that when an electromagnetic wave falls on the uncoated side of the screen, its attenuation due to absorption is greater than when falling on the coated side.
The shielding characteristics of pseudo-pyramidal structures obtained from sprayed flat material with a corrugation height of 5.7.12 mm at a fixed frequency of 9.5 GHz were also studied. An increase in the transmission and reflection coefficients by 2-3 dB was noted, and with an increase in the corrugation height, the shielding characteristics also increase.
It was found that the diffraction pattern of the synthesized material obtained from the non-deposited side of the sample, in addition to the peak caused by the influence of the substrate — modified polyacrylonitrile (interplanar distance 5.069 A), contains peaks corresponding to the compounds NiS2, Ni2S2, Ni3S2 (Fig. 1). In the structural analysis of the deposited side, in addition to these peaks, there is a reflex indicating the presence of metallic nickel of the cubic modification in the coating (Fig. 2).
Fig. 2. Diffraction pattern of nickel-sputtered polyacrylonitrile fibers from the sprayed side.
According to the authors, it is the presence of crystalline nickel on the surface of the sprayed side that causes an increase in the reflection coefficient when electromagnetic radiation falls on it. The polyacrylonitrile-nickel interface has a geometrically more heterogeneous structure, which leads to increased absorption of the fibers.
Conclusion. In As a result of the research it was found that knitted fabrics with applied nickel coatings have different properties of absorption of electromagnetic radiation depending on the operating conditions. The developed fabrics were used as an outer layer for the construction of multilayer shielding materials, which made it possible to obtain multilayer elastic EMI structures with a Ksv — 1.5 with an attenuation coefficient of 20-23 dB in the frequency range of 1.5-115 GHz. The discovered features of the interaction of elastic structures with electromagnetic radiation make it possible to use them in the development of highly effective broadband radio-absorbing coatings.
List of references
[1] Lynkov L.M., Bogush V.A., Glybin V.P. et al. Flexible structures of electromagnetic radiation screens/Ed. L.M. Lynkova. Minsk: BSUIR, 2000.
[2] Bogush V.A., Zavadsky S.M., Lynkov L.M. etc. //Izv. Belarusian Engineering Academy. 1999. No. 1(7)/2. S. 171-174.
[3] Golerik S.S., Rastorguev L.N., Skakov Yu.A. X-ray and electron-optical analysis. M.: Metallurgy, 1970. 108 pp.
[4] Elizarov A.S., Kostrikin A.M., Gusinsky A.V. etc. //Radio engineering and electronics. 1996. T. 41. No. 5. P. 602-610.