RUBBER BASED COMPOSITES WITH ACTIVE BEHAVIOUR TO MICROWAVES (REVIEW)

Size: px

Start display at page:

Download "RUBBER BASED COMPOSITES WITH ACTIVE BEHAVIOUR TO MICROWAVES (REVIEW)"

Eugenia Harmon
6 years ago
Views:

Journal of the University of Chemical N. Technology Dishovsky and Metallurgy, 44, 2, 2009, 115-122 RUBBER BASED COMPOSITES WITH ACTIVE BEHAVIOUR TO MICROWAVES (REVIEW) N.

1 Journal of the University of Chemical N. Technology Dishovsky and Metallurgy, 44, 2, 2009, RUBBER BASED COMPOSITES WITH ACTIVE BEHAVIOUR TO MICROWAVES (REVIEW) N. Dishovsky University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria dishov@uctm.edu Received 05 March 2009 Accepted 30 April 2009 ABSTRACT Electromagnetic wave is a self-propagating wave in space with electric and magnetic components. Electromagnetic radiation may be classified into types according to the frequency of the wave: these types include, in order of increasing frequency, radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. In the paper are discussed: Characteristics, influencing on the materials behaviour to microwaves: complex permittivity, complex permeability, dielectric loss, magnetic loss, dielectric loss tangent, magnetic loss tangent, wave impedance; Basic requirements to microwave absorbing materials: high attenuation, small external surface reflection, low weight, keeping the stability of the structure and the properties of the rubber matrix and the absorption active fillers during the exploitation; Microwave absorbing materials application: in antenna techniques and production, for protection of humans and other biological objects from the harmful action of the electromagnetic waves, in military applications for anti-radar camouflage, for improving the electromagnetic compatibility between different electronic devices; Basic principles of microwave absorbing materials development: suitable rubber matrix and suitable conductive filler or system of fillers, interaction between composite and microwaves in whole composite volume, rubber matrix to insulate completely the filler particles one from another, fillers with high values of their dielectric and magnetic loss, multilayered gradient composite structure in respect to the fillers concentration; Different types of rubber based composites with active behaviour to microwaves, developed and investigated in the last 15 years and possibilities for their real application - liquid (with application like paint), flexible multilayered coatings, rubber-textile combinations, etc. Keywords: microwave absorbers, rubber composites, fillers. INTRODUCTION Electromagnetic wave is a wave self-propagating in space having electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in phase with each other. According to the wave frequency electromagnetic radiation is classified into types that include, in order of increasing frequency: radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Microwaves are electromagnetic waves with wavelengths ranging from 1 mm to 1 m, or frequencies between 300 MHz and 300 GHz. The microwave range includes ultra-high frequency (UHF) (0.3 3 GHz), super high frequency (SHF) (3 30 GHz), and extremely high fre- 115

2 Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009 quency (EHF) ( GHz) signals. The most important characteristics influencing materials behaviour to microwaves are the following: complex permittivity ( ε = ε jε ), complex permeability ( µ = µ jµ ), dielectric loss (ε ), magnetic loss ( µ ), dielectric loss tangent ( tgδ ε = ε / ε ), magnetic loss tangent µµ r ( tg δ µ = µ / µ ), wave impedance ( Z = ). εε r The behaviour of a composite to microwaves may be considered as inactive, when µ << µ ( µ 0) ε << ε ( ε 0) In the other case, when the values for µ, µ, ε, ε are comparable, the composites exhibit active behaviour to microwaves. The most important microwave characteristics of materials are: Pt Coefficient of attenuation α = 10 lg, db Ð. Z Coefficient of reflection 2 Z1 r = Z + Z 2 1 (Z 1 and Z 2 wave impedances of medium 1 and medium 2) 1+ r Standing wave ratio SWR = 1 r Balance of the absorbed, reflected and transited powers Pr+ Pa + Pt = 1 Basic requirements to microwave absorbing materials may be summarized as follows: high attenuation; small external surface reflection, especially in case of plane absorbers; low weight; preserving the structural stability and the properties of the rubber matrix and those of the absorption active fillers during exploitation. The main principles for developing microwave absorbing composites cover: Finding suitable rubber matrix and suitable conductive filler or system of fillers; i Ensuring interaction between composite and microwaves in the entire volume of the composite; Availability of a rubber matrix to insulate the filler particles completely from each other; Using fillers with high values of their dielectric and magnetic loss; Affording a multilayer gradient structure of the composite with respect to the filler concentration. Literature data show that most of the microwave absorbers consist of a dielectric polymer matrix and specific functional fillers. The latter must have high values of the imaginary part of the complex dielectric permittivity and magnetic permeability that absorb high frequency energy [1-6]. The aim of the paper is to present the results of our investigations on rubber-based composites developed for production of microwave absorbers for various applications. Different rubbers and versatile absorbing fillers have been used for the purpose. EXPERIMENTAL Different kinds of rubbers natural rubber (NR), styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), nitrile butadiene rubber (NBR) were tested as a polymer matrix. Graphite, furnace carbon black (N 330 and N 550), acetylene carbon black (ACB), active carbon (AC) were experimented as fillers with high dielectric losses. They are accessible, lightweight and convenient for different applications. Usually they enhance to a great extent the conductivity and dielectric losses of the composite. These fillers are very effective in a broad frequency range. The carbonyl iron and some ferrites having undergone special treatment for increase their Fe 2+ content are the most friendly used fillers with high magnetic losses. We supposed that it would be worth using the natural magnetite as a filler in composite structures. High magnetic losses and high conductivity are the characteristic features of natural magnetite because of high Fe 2+ content and electronic exchange between Fe 2+ and Fe 3+. RESULTS AND DISCUSSION Influence of the chemical nature of the polymer matrix on the microwave properties of the absorbers 116

3 N. Dishovsky Our investigations indicated that the chemical nature of the elastomeric matrix had significant influence on the interaction between the electromagnetic waves and the composite [7]. The elastomers with highly polar functional groups or bonds (NBR, CR) produce better microwave properties of the absorbers and the polarity is a more important factor than the ability of the elastomer to crystallize. The improved absorbing properties of the absorbers based on polar crystallizing elastomers is due mainly to the decrease of the passed through absorber power (the interaction between the electromagnetic waves and absorbing composite is more active). Crystallization ability of the elastomer exerts a noticeable positive influence on the absorbing properties, if the elastomer is non-polar (comparison between SBR, NR and BR). The elastomers of the same nature have very similar microwave properties. The analyses of the obtained experimental results allowed us to conclude that the nitrile butadiene rubber would perform best as a polymer matrix owing to the highly polar nitrile group -C N in its macromolecules. The availability of this group is the reason for NBR semiconductor properties and low volume resistivity of the microwave absorbers based on NBR. DEVELOPMENT OF DIFFERENT SYSTEMS OF FILLERS FOR RUBBER BASED COMPOSITES WITH ACTIVE BEHAVIOUR TO MICROWAVES We will describe four different cases in our investigations: I CASE: ε 0, µ = 0 (conductive carbon black, active carbon, graphite) The effectiveness of different fillers with high dielectric losses and their combinations was studied at frequencies of 9, 4 GHz and 22 GHz. With respect to the absorption ability the graphite and furnace carbon black (N 330) have a priority at 9, 4 GHz. In the case of higher frequencies we have to prefer combinations of furnace and acetylene carbon black (ACB) at a 50:50 mass ratio and such of ACB: graphite at a mass ratio 75:25. The final aim of our investigations is to develop elastomer composites with maximum attenuation while at the same time their surface reflections are minimum. That is why the most important criterion for evaluation is the so-called Power balance that gives a correct idea about the percentage of absorbed, transitioned and reflected energy : Pi = Pr + Pa + Pt, where: Pi, Pr, Pt and Pa incident, reflected, transitioned and absorbed power. Obviously the effectiveness of the absorption composite structure will be acceptable when it possesses low reflection, and minimum transition and maximum absorption at the same time. In accordance with this criterion the acetylene carbon black (ACB) and the active carbon (AC) have priority as individual fillers as well as the combinations ACB AC and ACB-N330 (at a 50:50 mass ratio)[7-9]. II CASE: ε = 0, µ 0 (carbonyl iron, magnetite, Mn-Zn ferrites) Carbonyl iron, Mn-Zn ferrites and magnetite were used in our investigation as high magnetic loss fillers. We suppose that the natural magnetite is a more attractive filler than the carbonyl iron and Mn-Zn ferrites. Besides having high magnetic loss, high conductivity (due to the high Fe 2+ content) and electron exchange between Fe 2+ and Fe 3+, it is cheaper. Our investigations show also that the characteristics of the microwave absorbers containing natural magnetite are improved in comparison with those of the compositions containing carbonyl iron and Mn-Zn ferrites. Meanwhile the composition density is lower[10-11]. III CASE: double systems of fillers (magnetite and nanosized carbon black) The development of elastomeric composites that contain fillers with both high magnetic and high dielectric losses is demanded by the necessity to expand the working frequency band of the elastomer absorbers already designed and to improve their effectiveness. Theoretically this approach is based on the equation: '' '' ε µ tgδ = tgδε + tgδµ = + ' ' ε µ where: tg δ ε - tangent of dielectric losses; tg δ µ tangent of magnetic losses; 117

4 Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009 ε ',ε '' real and imaginary part of the complex dielectric permittivity; µ ', µ '' - real and imaginary part of the complex magnetic permeability. The theoretical necessity of having fillers with high values both of eand m in the respective frequency range is obvious. A lot of preliminary experiments were carried out to establish the optimal combinations of fillers. The natural magnetite as a high magnetic losses filler as well as the following high dielectric losses fillers were selected: - active carbon (AC); - acetylene carbon black (ACB); - furnace carbon black (N550); - graphite. The results indicate that the samples with the highest attenuation have also high VSWR, i.e. a higher percent of reflected power compared to the incident power. Therefore we have to reach a compromise between these indices. The influence that the nature of a filler with high dielectric losses has is evident. This effect is manifested greatly in reflection. Active carbon (AC) produces the lowest reflection probably because of its porous structure and large specific surface. The attenuation is also improved when even a micro quantity of metal oxides is present in the filler. With respect to the attenuation it is the highest in the case of graphite acting as filler with high dielectric losses. The rest three fillers ensure very close results. To minimise the composites reflected power a four-layer structure based on filler concentration gradient principle was developed. A nitrile butadiene rubber (NBR) and a chloroprene rubber (CR), as well as their combinations, were used as rubber matrices. The structure of the multilayer absorber, namely the composition, the thickness and the ordering of the layers was experimentally optimised, so that reduced reflection from a metal surface placed below the absorber could be achieved. The best results were obtained for the following acetylene carbon black and magnetite contents (quantities in mass parts per hundred of rubber): layer 1 - thickness 1.5 mm; carbon black P m. p., magnetite 50 m. p. layer 2 - thickness 1.5 mm ; carbon black P m. p.,magnetite m. p.. layer 3 - thickness 1 mm; carbon black P m. p., magnetite m. p. layer 4 - thickness 3-6 mm; carbon black P m. p., magnetite 50 m. p.. The total thickness of the absorber is 7-10 mm depending on layer N4 thickness. Two minima of the reflection coefficient of the absorber attached to a metal surface were observed in X- and Ku- bands, respectively. It is shown that the position of the minima of the reflection coefficient of the absorber attached to a metal surface may be shifted by changing the thickness of the fourth layer only. As a result of the investigations described above a dual-band multilayer rubber absorber having two minima of the reflection coefficient located in X- and Ku- bands, respectively, was developed. It is shown that the position of the minima of the reflection coefficient of the absorber attached to a metal surface may be shifted by changing the thickness of the fourth layer only. The absorber can be readily cut and bonded to metal surfaces, making it a good camouflage material for vehicles and structures [8,12]. IV CASE: triple system of fillers (magnetitesubstituted barium hexaferrites-carbon black) The microwave absorbing properties of rubber composites containing substituted barium hexaferrites as fillers were investigated. It is shown that both the frequency of natural ferromagnetic resonance and the internal magnetic anisotropy field intensity may be changed altering the extent of Fe 3+ ions substitution with Co 2+ and Ti 4+ ions in fillers crystal structure. The purposeful changes in the structure of the used specific fillers having a high field of internal magnetic anisotropy are implemented by substitution of Fe 3+ ions in ferrites crystal structure with Co 2+ and Ti 4+ ions at different extent during the synthesis of the fillers. The substitution of the Fe-ions is based theoretically on their three types of positions in the barium hexaferrites crystal structure (hexagonal M-type structure isomorphous to a magnetoplumbite one, consisting of alternating S and R blocks ), wherein Co 2+ and Ti 4+ ions may be introduced. It has been also found that there is a strong magnetic anisotropy in the hexaferrite structure used. Purposeful substitution of Fe 3+ in the hexaferrite crystal structure 118

N. Dishovsky Fig. 1. Light weight rubber based composites for improving the antenna parameters (the arrow indicates the place of the absorbing material) [23].

The process gives an opportunity for adjustment and control over these basic hexaferrite characteristics as fillers influence strongly the elastomer composites properties [13].

We explain the above phenomena with the existence of an optimum internal anisotropy field value (at level of Fe 3+ substitution 0,85) that leads to NFMR within the investigated frequency range.

5 N. Dishovsky Fig. 1. Light weight rubber based composites for improving the antenna parameters (the arrow indicates the place of the absorbing material) [23]. leads to a change both in the frequency of natural ferromagnetic resonance (NFMR) and in the internal magnetic anisotropy field (IMAF) intensity. The process gives an opportunity for adjustment and control over these basic hexaferrite characteristics as fillers influence strongly the elastomer composites properties [13]. It has been revealed that at equal level of filling the microwave absorption increases with the extent of substitution. The effect observed is better pronounced at the higher frequencies. We explain the above phenomena with the existence of an optimum internal anisotropy field value (at level of Fe 3+ substitution 0,85) that leads to NFMR within the investigated frequency range. Our further investigations on the hexaferrite BaCo0,85Ti0,85Mn0,1Fe10,2O19 (NFMR frequency - 12 GHz, IMAF intensity - 4,4 koe) allowed optimization of the procedure. Thus we developed a composite containing a ternary system of fillers (hexaferrite-magnetite-carbon black) with improved absorbing properties and lower density [14-15]. The analysis of the results demonstrates the strong positive influence of the hexaferrite NFMR on the microwave absorption of a rubber composite. Introduction of highly anisotropic magnetic filler into the compos ite affects the magnetic properties of the absorber. The magnetic studies on a composite comprising a magnetite filler and those on a sample of neat hexaferrite showed a coercive force He higher by a factor of two in the hexaferrite case. In the composite containing their combination we observed an He rise corresponding to the hexaferrite concentration, as well as a reproducible increase in the substance induction. These facts give us reason to assume that addition of highly anisotropic magnetic filler improves the long-range magnetic order in the composite matrix [16-17]. 119

Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009 Fig. 2. Rubber based microwave absorbing materials for humans protection [16-17].

electromagnetic radiation. At the same time they depend strongly on the type and quantities of the fillers used. We tried to investigate how they are affected by the presence of hexaferrite.

6 Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009 Fig. 2. Rubber based microwave absorbing materials for humans protection [16-17]. It is well known that the complex magnetic permeability µ* and complex dielectric permittivity å* are the most important absorber characteristics determining its level of interaction with electromagnetic radiation. At the same time they depend strongly on the type and quantities of the fillers used. We tried to investigate how they are affected by the presence of hexaferrite. The comparative analysis of the magnetic permeability frequency spectra of the samples with and without hexaferrite shows a well pronounced natural ferromagnetic resonance behaviour; larger magnetic losses were observed within hexaferrite NFMR frequencies, when the best absorbing properties of the rubber composite were measured. Moreover, the presence of a highly anisotropic filler in the composite leads to substantial changes in its dielectric properties its dielectric losses decrease considerably. We assume that these effects are due to the changes in the supermolecular structure of the rubber matrix, i.e. to changed location and the orientation of NBR macromolecules on the surface of the hard magnetic filler determined by their high internal anisotropy magnetic field [18]. APPLICATIONS The most important applications of microwave absorbing materials are the next: antenna techniques and production improving the antenna parameters; protection of humans and other biological objects from the harmful action of the electromagnetic waves; military application for anti-radar camouflage; improving the electromagnetic compatibility between different electronic devices. In the last 15 years we have investigated all described above cases of filled rubber based composites. The results from the measurements of their microwave characteristics in waveguide lines or by free space methods in the frequency region 1-18 GHz show real opportunities for application of these rubber based composites as microwave absorbers for different purposes. Different types of microwave absorbing materials were developed on the basis of this research: liquid (with application like paint), flexible multilayered coatings, rubber-textile combinations, etc. (Figs. 1-3). FUTURE INVESTIGATIONS The influence of used as a filler nanosized carbon particles, obtained by detonation synthesis [24], on the microwave absorption and exploitation properties of rubber composites is investigated in the last years. The measurements carried out in free space, giving us information about the behavior of the composites in real conditions, and the results obtained, are very prospective[25]. Some possibilities to use as absorption active fillers some waste metallurgical phases, rich in magnetite, will be also investigated [26, 27]. 120

N. Dishovsky a) b) c) d) Fig. 3. Rubber based microwave absorbing materials for anti-radar camouflage [19-23] a) camouflage net; b, c, d) camouflage coatings. REFERENCES 1.

Emerson and Cuming, Eccosorb Microwave Products-Electromagnetic Principles and Applications, Technical Bulletin, February, 2006. 3.

February, 2006. 4. Eccosorb-Dual band flexible rubber sheet absorbers, Technical Bulletin, December, 2002. 5.

Ferrosorb, Microwave Absorbing Materials, Mi crowave Filter Company, Inc, Ferro Catalog, Vol. 1, 1, 2005. 7. R. Shtarkova, N.

7 N. Dishovsky a) b) c) d) Fig. 3. Rubber based microwave absorbing materials for anti-radar camouflage [19-23] a) camouflage net; b, c, d) camouflage coatings. REFERENCES 1. Microwave Materials, Ed. V. Murthy, S. Sundaram, B. Viswanathan, Springer-Verlag, Narosa Publishing House, Emerson and Cuming, Eccosorb Microwave Products-Electromagnetic Principles and Applications, Technical Bulletin, February, Emerson and Cuming, Eccosorb Microwave Products-Energy propagation in dielectric and magnetic materials: Theoretical notes, definitions and calculations, Technical Bulletin, February, Eccosorb-Dual band flexible rubber sheet absorbers, Technical Bulletin, December, Emerson and Cuming, Eccosorb Microwave Products, Radar Cross Section Reduction, Technical Bulletin, March, Ferrosorb, Microwave Absorbing Materials, Mi crowave Filter Company, Inc, Ferro Catalog, Vol. 1, 1, R. Shtarkova, N. Dishovsky, Journal of Elastomers and Plastics, 41, 2, 2009, N. Dishovsky, K. Kostov, B. Vichev, Microwave Physics and Technique, 33, 1997, N. Dishovsky, M. Grigorova, Mat. Res. Bull., 35, , N. Dishovsky, K. Ruskova, Macromolecular Symposia, 169, 2001, N. Dishovsky, Il. Iliev, Macromolecular Symposia, 169, 2001, F. El-Tantawy, N. Dishovsky, J. Applied Polymer Science, 91, 2004, N. Dishovsky, Eurofillers 97, Manchester, UK, 1997, Proceedings,

8 Journal of the University of Chemical Technology and Metallurgy, 44, 2, N. Dishovsky, Iv. Nedkov, Eurofillers 95, Mulhouse, France, 1995, Proceedings, N. Dishovsky, Journal of Applied Electromagnetism, June, 2000, N. Dishovsky, Iv. Nedkov, IEEE Trans. on Magnetics, 30, 2, 1994, I. Nedkov, L. Milenova, N. Dishovsky, IEEE Trans. on Magnetics, 30, 6, 1994, N. Dishovsky, Iv. Nedkov, Eurofillers 95, Mulhouse, France, 1995, Proceedings, N. Dishovsky, Bulgarian Patent No N. Dishovsky, Bulgarian Patent No N. Dishovsky, Bulgarian Patent No N. Dishovsky, Bulgarian Patent No R. Starkova, Ph. D. Thesis, Technical University, Sofia, S. Stavrev, US Patent, 5, 353, D. Pishinkov, N. Dishovsky, S. Stavrev, V. Iliev, J. Univ. Chem. Technol. Met. (Sofia), 41, 2, 2006, V. Iliev, D. Grigorova, N. Dishovsky, S. Borros, M. Marinov, J. Univ. Chem. Technol. Met. (Sofia), 42, 2, 2007, N. Dishovsky, D. Grigorova, Vl. Iliev, S. Borros, Journal of Elastomer and Plastics, 2009, (submitted). 122

CHARACTERIZATION OF MAGNETICALLY LOADED MICROWAVE ABSORBERS

Progress In Electromagnetics Research B, Vol. 33, 277 289, 2011 CHARACTERIZATION OF MAGNETICALLY LOADED MICROWAVE ABSORBERS I. Zivkovic * and A. Murk Institute of Applied Physics, University of Bern Siedlerstrasse