Natural rubber based composites containing different carbon nanofillers (fullerenes, carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs)) at different concentrations have been prepared. Their dielectric properties (dielectric permittivity, dielectric loss) have been studied in the 1 - 12 GHz frequency range. Some factors (electromagnetic field frequency, fillers concentration, fillers intrinsic structure) influencing the dielectric behavior of the composites have been investigated. The dielectric properties of the developed natural rubber composites containing conductive fillers (fullerenes, CNTs, GNPs) indicate that these composites can be used as broadband microwave absorbing materials.
A growing number of demanding applications in electronics and telecommunications rely on the unique properties of carbon allotropes. The need for microwave absorbers and radar-absorbing materials is steadily increasing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference (EMI) among components and circuits, reduction of the back-radiation of microstrip radiators [
In recent times researchers have tried three types of carbonaceous fillers, i.e., carbon black (CB), carbon fiber (CF) and carbon nanotubes including multiwalled (CNTs/MWNTs) [
A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (carbon black, carbon nanotube, carbon fiber, graphene) and the embedding matrix (rubber, thermoplastic) [
Polymer composites containing conductive fillers have been developed in recent times as alternative EMI shielding materials since they possess the advantages like light weight, low cost, resistant to corrosion and processing advantages. However, in such materials, the EMI shielding effectiveness depends on many factors, including filler’s intrinsic conductivity, filler loading, dielectric constant, aspect ratio and filler-polymer matrix interactions [
Dielectric properties of materials are those electrical characteristics of poorly conducting materials that determine their interaction with electric fields. Those properties determine how well energy from the high-frequ- ency alternating electric fields can be absorbed and thus how rapidly the materials will be heated. The dielectric properties of the load materials are also important in the design of the radio frequency or microwave power equipment [
The complex permittivity relative to free space may be represented as
In principle, the dielectric properties of most materials vary with several influencing factors. The dielectric properties depend on the frequency of the applied alternating electric field, the temperature and the water content of the material, its density, etc. The dielectric properties are dependent also on the presence of mobile ions and the permanent dipole moments. The distribution of the phases, including voids and cracks, has also a major influence on the dielectric properties of the composite materials [
Among the fillers used in the last years, carbon-based nanostructures, such as fullerenes (Fs), carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) have been studied extensively. The reason for the interest in the above mentioned carbon nanostructures and their application in rubber composites can be explained with their unique properties (
These materials have different aspect ratios and specific surface areas, and structures: CNTs are known to have high aspect ratio (several hundred to thousand), outstanding electrical and mechanical properties [
It is often difficult to explain most of the complex dielectric properties in the disordered composites [
Material | Density g/cm3 | Specific surface area m2/g | Length µm | Diameter or thickness nm | Aspect ratio | Electrical conductivity S/cm | Thermal conductivity W/mK |
---|---|---|---|---|---|---|---|
MWCNTs | 2.6 | 50 - 1315 | 1 - 10 | 1 - 50 | 300 - 1000 | 102 - 106 | 2000 - 6000 |
GNPs | 0.4 | up to 2670 | <50 | <100 | 500 | 106 | 800 - 5300 |
Fs | 1.7 | 50 - 500 | >1 | 40 - 50 | 3000 - 5000 | 10−5 | 0.4 |
The aim of this work is to prepare natural rubber based composites containing different amounts (2 - 10 phr) (phr = parts of filler by weight per hundred parts of rubber) of fullerenes, CNTs and GNPs; to analyze the role of some factors and specific structure features of these fillers on the microwave dielectric behavior of the composites with a view to propose lightweight, flexible materials suitable for microwave absorbers in wide frequency range (1 - 12 GHz).
Natural rubber SMR 10 (Moony viscosity, ML1+4 at 100˚C = 60; dielectric constant ε = 2.1) was purchased from North Special Rubber Corporation of Hengshui, Hebei Province, China.
Other ingredients such as zinc oxide (ZnO), stearic acid (SA), N-tert-Butyl-2-benzothiazolesulfenamide (TBBS) and sulphur (S) were commercial grade and used without further purification.
The investigations reported in this paper were on neat fullerene powder comprising 99.5% of С60 fullerene produced by Alfa Aesar (Johnson Matthey Company). Fullerene density was 1.65 g/cm3. The micrograph in
Multiwalled carbon nanotubes produced by Grafen Chemical Industries Co., Ankara, Turkey, were used in our investigation. The material’s purity was more than 95%, density −2.6 g/сm3. Carbon nanotubes were with average diameter about 15 nm and length 1 - 10 microns.
Graphene used in the investigation was also produced by Grafen Chemical Industries Co., Ankara, Turkey. Graphene nanoplatelets (GNP) have a “platelet” morphology, meaning they have a very thin but wide aspect. Aspect ratios for this material can range into the thousands. Each particle consisted of several sheets of graphene with an overall thickness of 50 nm and average plate diameter 40 micron (
The rubber compounds were prepared according to a specific recipe. The pre-characterized filler powder was incorporated into the natural rubber matrix at various loadings on an open two-roll laboratory mill (L/D 320 × 360 and friction 1.27). The speed of the slow roll was 25 min−1. The formulations of the compounds prepared are shown in
NR1 | NR2 | NR3 | |
---|---|---|---|
Natural rubber | 100 | 100 | 100 |
Stearic acid | 1 | 1 | 1 |
Zinc oxide | 4 | 4 | 4 |
Processing oil | 10 | 10 | 10 |
Filler (Fs, CNTs, GNPs) | 2 | 6 | 10 |
MBTSa | 2 | 2 | 2 |
TMTDb | 1 | 1 | 1 |
Vulkanox 4020c | 1 | 1 | 1 |
Sulphur | 2 | 2 | 2 |
aMercaptobenzothiazole sulphenamide; bTetramethylthiuram disulphide; cDimethyl butyl- phenyl-p-phenylendiamine.
ZnO and stearic acid were added after 5 min. After 3 min of homogenization the filler was added. Following another homogenization for 7 min the accelerator and sulphur were added and the compound was rehomogenized for 4 min. The temperature of the rolls did not exceed 70˚C. The experiments were repeated for verifying the statistical significance. The ready compounds in the form of sheets stayed 24 hours prior to their vulcanization. The optimal vulcanization time was determined by the vulcanization isotherms taken on an oscillating disc vulcameter MDR 2000 (Alpha Technologies) at 150˚C, according to ISO 3417:2002. The vulcanization was performed on an electric hydraulic press which plate was 400 × 400 mm, at 10 МРа.
The properties of the composites obtained―dielectric constant and dielectric loss―were determined in the 1 - 12 GHz frequency range.
The determination of complex permittivity was carried out by the resonance method, based on the cavity perturbation technique [
The sample was in the form of a disc with a diameter of 11 mm and about 1.5 mm thick. Its location in the cavity was at the maximum electric field. Because the thickness of the sample was not equal to the height of the resonator, a dielectric occurred with an equivalent permittivity εe at the place of its inclusion. The parameter was determined by Equation (1) and instead
where k = l/Δ and l is the distance from the disk to the top of the resonator.
The loss factor tan δ was calculated from the quality factor of the cavity [
where Qε―quality factor of the cavity with a sample and Qr―quality factor of the cavity without a sample.
The measurement setup used several generators for the whole range: HP686A and G4-79 to 82, frequency meters: H 532A; FS-54, a cavity resonator. The scheme of the equipment used is shown on
The dielectric properties were measured in the frequency range from 1 GHz to 12 GHz.
The frequency dependence of the dielectric permittivity of NR based composites, containing a different amount of fullerenes, CNTs and GNPs is shown in Figures 3-5. It is evident that the dielectric permittivity values increase with the increasing frequency in the case of any of the fillers used. In the 1 - 7 GHz range the increase is slightly pronounced while in the 7 - 12 GHz range it is more drastic. The specifics of the fillers used are expressed in the slope of plotted curves.
Figures 6-8 present the dielectric loss in the 1 - 12 GHz frequency range of the composites investigated. The changes in the imaginary part of the relative complex permittivity of the material also known as dielectric loss angle tangent―tan δε―depend on the frequency. As expected, with the increasing frequency the dielectric loss decreases, more rapidly in the region 5 - 12 GHz. Evidently, when GNPs are used as filler, the frequency increase has a stronger effect on the dielectric losses of the samples at lower frequencies (1 - 4 GHz). Noteworthy is the fact that in the 11 - 12 GHz frequency range the values of dielectric losses of the samples examined become closer.
The results in Figures 3-8 allow the conclusion that the dielectric properties of most materials vary considerably with the frequency of the applied electromagnetic field. An important phenomenon contributing to the frequency dependence of the dielectric properties is the polarization, arising from the orientation of molecules which have permanent dipole moments with the imposed electric field. The mathematical formulation developed by Debye to describe the permittivity for polar materials [
where ω = 2πƒ is the angular frequency, ε∞ represents the dielectric constant at frequencies so high that molecular orientation does not have time to contribute to the polarization, εs represents the static dielectric constant, i.e., the value at zero frequency (dc value), and τ is the relaxation time in seconds, the period associated with the time for the dipoles to revert to random orientation when the electric field is removed. Separation of Equation (4) into its real and imaginary parts yields
Obviously, ε∞, εs and in particular, τ are specific parameters for each of the fillers used, which depend on the fillers chemical and crystallographic nature. Therefore, the dielectric properties of the composites studied depend differently on the frequency, provided all other conditions are identical (the same polymer matrix and filling degree). The dependence of ε∞, εs and τ on the fillers chemical nature and structure explains why in some frequency range (usually at lower frequencies) the dielectric properties change monotonously, while in some higher frequency range, when relaxation is hindered, the increase is drastic. According to the Debye theory of dielectric properties [
The Debye relaxation is one of the important dielectric loss mechanisms, which can be characterized by
fullerenes has an S-shaped pattern with two regions wherein the dielectric loss values decrease almost linearly with the increasing dielectric permittivity, in the third region of curve that decrease is intermittent. As
On the other hand, since the elastomer matrix is non-polar, apparently, the relaxation and polarization processes are greatly dependent on the fillers used and on their specific features. With regard to those specifics, the polarization may proceed according to three different mechanisms: electronic, ionic and orientational. All non-conducting materials are capable of electronic polarization. Therefore, we consider the polarization of the elastomer matrix used for the studied composites to proceed according to that mechanism. The ionic and orientational polarizations occur only in materials possessing ions and permanent dipoles, respectively. It might be assumed that the matrix and the fillers introduced into it polarize according to a different mechanism. The more polarization mechanisms of a composite are, the higher its dielectric constant will be. For example, a natural rubber based composite containing fillers with permanent dipoles have a dielectric permittivity higher than the one of non polar natural rubber. On the other hand, the more easily the various polarization mechanisms can act, the higher the dielectric permittivity will be. For example, among elastomers, the more mobile the chains are (i.e. the lower the degree of crystallinity), the higher the dielectric permittivity will be. It is important for the composites investigated, because the natural rubber crystallizes and the chemical nature of the fillers used and their amounts can change the degree of crystallinity and the dielectric permittivity. For polar structures the magnitude of the dipole also affects the magnitude of the occurring polarization. The fillers with non-centrosymmetric structure have especially strong spontaneous polarization, hence a high dielectric permittivity, respectively.
It is obvious from Figures 3-8 that, in the investigated concentration interval, the values of the dielectric permittivity are too close to each other with a tendency to a slight increase with the increasing filler concentration. A similar tendency has been described by the authors [
When CNTs are used as fillers (
Of particular interest is the 9 - 12 GHz range wherein there is a relatively fast increase in the dielectric permittivity and its dependence on the amount of GNPs is the most prominent.
From the very beginning of that range the values of tanδ are clearly distinguished as dependent on the different degree of filling. As expected, the increase in filler amount leads to an increase in tanδε values (Figures 6-8).
The imaginary part of the complex relative permittivity is more sensitive to changes in the filler amount than the real part is.
Carbon nanofillers used have different geometries and exhibit different surface area/volume relations [
The investigations carried out and the obtained results (Figures 3-9) show irrevocably the impact that the crystallographic structure and the chemical nature of the used fillers have upon the dielectric permittivity values and dielectric loss. The effect could be explained first of all by the polarization and by the mechanism according to which the polarization proceeds, as well as by the proceeding of relaxation and the time needed for the process. The aforementioned two processes are crucial for the dielectric properties of the materials when applying an electromagnetic field. The difference in the crystallographic structure and chemical nature of the fillers predetermine the differences in the dielectric permittivity values and dielectric losses of composites based on the same matrix. The effects of the two factors (crystallographic structure and chemical nature) sometimes intermingle and are hardly distinguishable. That could be the scope of future investigations.
Natural rubber (NR) based composites containing different carbon nanofillers (fullerenes, carbon nanotubes and graphene nanoplatelets) at concentrations from 2 to 10 phr have been prepared. Their dielectric properties (dielectric permittivity, dielectric loss) have been studied in the 1 - 12 GHz frequency range. It was found that the dielectric constant and dielectric loss of filled composites depend on amount and type of filler loading. It has been established that with increasing the frequency of the electromagnetic field the dielectric permittivity values increase, while those of the dielectric losses get lower. The higher the concentration of the fillers is (keeping identical all other conditions), the higher the values of dielectric permittivity and dielectric losses are. The observed effects are related first of all to the impact that the chemical nature and crystallographic structure of the fillers and elastomer matrix studied have upon the polarization time and mechanism as well as upon the relaxation time and mechanism. The latter processes determine the dielectric properties of the materials. The dielectric properties of the developed natural rubber composites containing conductive fillers (fullerenes, CNTs, GNPs) indicate that these composites can be used as broadband microwave absorbing materials.
The work is a part of a project funded by King Abdulaziz University, Saudi Arabia under grant number MB/11/12/436. The authors acknowledge the technical and financial support.
Ahmed A.Al-Ghamdi,Omar A.Al-Hartomy,Falleh R.Al-Solamy,NikolayDishovsky,DianaZaimova,RossitsaShtarkova,VladimirIliev, (2016) Some Factors Influencing the Dielectric Properties of Natural Rubber Composites Containing Different Carbon Nanostructures. Materials Sciences and Applications,07,108-118. doi: 10.4236/msa.2016.72011