Carbon Fiber Reinforced Polymer

Abstract

A carbon fiber reinforced polymer (CFRP) laminate, with the top layer consisting of shredded fibers, is proposed and manufactured. The shredded fibers are aligned randomly on the surface to achieve a more isotropic conductivity, as is desired in antenna applications. Moreover, fiber shreds can be recycled from carbon fiber composites. Conductivity, permittivity, and permeability are obtained with the Nicolson-Ross-Weir method from material samples measured inside rectangular waveguides in the frequency range of 4 to 6 GHz. The decrease in material anisotropy results in negligible influence on antennas. This is shown by measuring the proposed CFRP as ground plane material for both a narrowband wire monopole antenna for 5.9 GHz and an ultrawideband conical monopole antenna for 1–10 GHz. For comparison, all measurements are repeated with a twill-weave CFRP.

1. Introduction

Composites are materials consisting of a mixture of components present as separate phases. They are created for engineering applications to combine the desired qualities of its individual components. Carbon fiber reinforced polymers (CFRP) are carbon fiber composites (CFC) consisting of carbon fibers embedded in a polymer matrix, typically a resin. The most common production technique is to build CFRP as laminates. These laminates are stacked from unidirectional or woven carbon fiber plies preimpregnated with resin, which are commonly referred to as prepreg. The laminate is stacked in a mold to form the desired part geometry, vacuum-bagged, and cured in an autoclave.

The motivation to use CFRP is found in their mechanical properties. While Young’s modulus

of CFRP is lower than that of many metals, the density of CFRP is much lower. Young’s modulus per material density of CFRP is much higher than that of metals; for example, the specific tensile modulus of steel is about , while for unidirectional CFRP (UD-CFRP) values

can be easily achieved in fiber direction. This makes CFRP a suitable material for lightweight construction.

Carbon fibers are electric conductors, while the matrix is almost exclusively nonconductive. The electromagnetic properties of CFRP laminates are in general anisotropic and depend on fiber and matrix materials, ply weave, orientation of the layers in the laminate, and frequency. In the past, due to their application in avionics and aeronautics, the research focus was on the electromagnetic shielding properties of carbon fiber composites [1, 2]. For antenna design the electrical conductivity, permittivity, and magnetic permeability of the materials in the vicinity of the antenna are of interest.

Measurements of radio-frequency electromagnetic properties are performed by inserting a material sample (material under test, MUT) inside a well understood system, such as a waveguide [3, 4] and coaxial cable, or between horn antennas [5]. The electrical conductivity and permittivity transverse and parallel to UD-CFRP and with different fiber volume fractions were measured with waveguides in [6]; values are given in a large frequency range up to

. The conductivity of CFRP is much higher than that of carbon-black or graphite particulate composites [7]; conductivity of UD-CFRP in and perpendicular to fiber direction differs by a factor of . Horn antenna measurements have shown that the conductivity of CFC with unidirectional fibers increases with frequency if the electrical field is perpendicular to the fibers but is flat with the field parallel to the fiber direction [8]. Waveguide measurements of electrical conductivity and permittivity with the Nicolson-Ross-Weir (NRW) method and different ply orientation in the range of

to have been conducted in [9], where they also provide measurements of CFRP slot antennas. CFRP are diamagnetic; the magnetic permeability of unidirectional carbon fiber polymer laminates was measured with horn antennas and waveguides in [10]. In most investigations only CFRP made with unidirectional fiber direction, and plies which are all oriented in the same direction, are considered. UD-CFRP are more interesting from a theoretical viewpoint as the material anisotropy is more pronounced and the material is easier to model. In applications however, woven fabrics are often used to compensate the small E-module and/or low electric conductivity perpendicular to fiber direction.

Modeling approaches for the electrical properties of CFC have been proposed in [11, 12] in addition to simple law of mixture models used in [1, 13].

CFRP are used in a variety of antenna applications. The whole antenna can be built from CFRP. Measurements of braided CFRP patch antennas are compared to law of mixture and geometry based simulations in [14]. The performance of bow-tie antennas with unidirectional and braided CFRP is measured in [15]. Various monopole antennas made from CFRP and carbon nanotubes are investigated in [16, 17]. Slotted waveguide antennas manufactured from CFRP are investigated in [18]. This type of antenna is especially relevant in aeronautical applications. Load bearing structures are formed from CFRP and the geometry obtained for mechanical stability is close to the geometry of rectangular waveguides. CFRP bearers can be used as rectangular waveguides and slots can be cut into them to build rectangular waveguide antennas [19]. Several elements can be combined into an antenna array, the slotted waveguide antenna stiffened structure [20].

CFRP are used for lightweight construction of reflectors, mostly in large parabolic dishes [21] or space applications [22]. Measurements in [23] show that CFRP is applicable as reflector material for millimeter-wave antennas at GHz. Antenna gain with a reflector made from woven CFRP is close to the gain with a chrome plated reflector. It should be noted that CFRP cannot be used in some antenna applications as they are a source of intermodulation products [24].

CFRP are used as antenna ground plane material. In specialized applications the high anisotropy of UD-CFRP can be utilized. A mechanically reconfigurable antenna with an anisotropic CFRP ground plane is presented in [25]. Surface currents on the ground plane can flow in fiber direction, while they are blocked perpendicular to fiber direction due to the low conductivity of CFRP. This mechanism acts as a mode filter for a patch antenna which is rotated against the CFRP ground plane. In general, however, the conductivity of the antenna ground plane should be isotropic.

A typical use case as ground plane is when antennas are mounted on large CFRP structures such as an aircraft fuselage or a car chassis. The influence of the ground plane material on the antennas should be small to allow antenna design independent of composite design. To achieve this, CFRP with near anisotropic conductivity are preferred, such that electric currents are not obstructed. This is especially the case in automotive antenna design, where antenna modules are used on different types of vehicle and are required to function on the CFRP roof of electric cars as well as on steel roofs. An automotive roof mounted antenna module (shark-fin) is measured on a CFRP car roof in [26]. An antenna cavity for integration into CFRP sheets, such as aircraft skin panels or car chassis, is proposed and prototyped in [27]. In vehicular applications the antenna cavity can be manufactured as part of a carbon fiber reinforced car roof as is described in [28].

A CFRP material with carbon fiber shreds in random alignment as its top layer is proposed for antenna applications and manufactured. Fiber shreds can be obtained from recycled CFRP, resulting in a sustainable material. Due to the skin effect, it is sufficient to design the top layer of the laminate for antenna applications in the gigahertz range; the other plies can be chosen independently to meet the mechanical requirements of the composite. The material is described in Section 2. Its electromagnetic properties are measured with the NRW method in a rectangular waveguide and compared to a CFRP with a 2/2 twill weave in Section 3.1. Material measurements are performed in the frequency band from

to , which includes the frequency band from to , that is reserved for dedicated short range communication (DSRC) in intelligent transportation systems (ITS), IEEE 802.11p. The influence of the proposed CFRP as ground plane material is measured with several monopole antennas in Section 4. The proposed CFRP is measured as ground plane for a wire monopole antenna for the DSRC band and with broadband conical monopole antennas in a frequency range from 1 to 10 Ghz.

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