Single and hybrid electromagnetic absorbing coatings on polyacrylonitrile precursor to motivate the microwave pre-oxidation

Highlights

• Single and hybrid coating were applied on PAN fibers.

• Coated fibers were used as precursors for the microwave pre-oxidation process.

• The pre-oxidation led to polymer decrystallization, cyclization and cross linking reaction.

• The polymer crosslinking induced the rearrangement of layer plane cause change of the microstructure parameters.

• More developed conjugated structure was obtained in pre-oxidized hybrid fibers.

Abstract

Nano silicon carbide was used separately and as an auxiliary absorber for the modification of the PAN precursor fibers to upgrade their microwave absorbability in order to undergo the subsequent pre-oxidization by using microwave. Surface coatings were successfully applied to the PAN precursor and the coated fibers were pre-oxidized over the microwave furnace for 60 min. To study the effect of the coatings on the pre-oxidized fibers, a series of characterizations were carried out. FTIR analysis results showed clear changes in the basic chemical structure of pre-oxidized fibers, which approved the occurrence of the cyclization and crosslinking reactions. Furthermore, the cyclization was confirmed by the thermal analysis results. According to the listed oxygen values, it can be suggested that the pre-oxidized fibers can resist the high temperatures in the following stage (carbonization). In addition, the crystalline analysis revealed a reduction in crystallinity, while the mechanical analysis demonstrated a reduction in tensile strength and elongation at break of pre-oxidized fibers.

Introduction

Generally, the pre-oxidation as one of the main manufacturing stages for polyacrylonitrile (PAN-) based carbon fibers has a great effect on the final properties of carbon fibers (CFs), and the PAN precursor fibers which were pre-oxidized at favorable conditions can produce high modulus CFs [1]. During the pre-oxidation process, several reactions are involved such as oxygen uptake reactions, dehydrogenation, and the cyclization which is generated by the polymerization of nitrile groups to form ladder polymer structure [2]. CH₂ and CN groups disappeared completely due to elimination, cyclization and aromatization reactions and formed C=C, C=N, and = C–H groups. These chemical changes result in physical changes in the PAN structure. The pre-oxidation is carried out by the conventional heating way (conduction and convection) at the temperature range of 180–300 °C [3], but it is considered as an energy consuming process. The relatively higher production cost of CFs has limited its usage in industrial sectors [4]. In addition, the properties of CFs (in particular tensile strength) have remained only a fraction of the theoretical predictions. Various techniques, such as the application of magnetic field [5], and gamma irradiation [6] have been used to improve the properties of carbon fibers [7]. S. Y. Kim et al. [8] heated the PAN precursor fibers under an atmospheric pressure plasma followed by the carbonization process, thus slightly increased the strength. A. K. Naskar et al. [9] exposed the PAN fibers to ultraviolet (UV) radiation for a few seconds prior to pre-oxidizing it at 320 °C and resulted in better crosslinking and cyclization.

Nowadays microwave heating has become applicable in different industrial sectors instead of conventional heating because it is an effective way to accelerate heating and enhance material properties [10]. In the area of carbon fiber manufacturing Liu et al. [11] and C. Zhang et al. [12] modified a microwave pre-oxidation system equipped with ceramic rod for PAN fibers treatment. Although the obtained results demonstrate an enhancement of skin core structure of the pre-oxidized fibers, there was no any information regard the effect of microwave treatment on the properties enhancement or deterioration of the structural defects such as small crystallite size. Moreover, one of the most important values that gives predict on the success of the next stage is the Aromatization Index (AI). The obtained AI values indicating the incomplete pre-oxidation, consequently the resulted fibers cannot withstand the high carbonization temperature.

The materials which interact with microwaves to produce heat are called microwave absorbers. The microwave absorbers can be classified as dielectric, magnetic or hybrid. Classifications are made based on the behavior of the wave-material interactive loss process, which varies in result due to the type of absorber used. An ideal microwave absorber should exhibit low-reflective properties, strong-reflective loss in broad bandwidth, low density, and minimal thickness in order to suit a wide range of applications [13]. Nano-carbon black (CB) is widely used as microwave absorber due to its excellent electrical conductivity, lightweight, and low cost, but it has small absorption peak and narrow absorption bandwidth [14,15]. CB coated PAN fibers has been reported in our another article [16]. On the other hand, silicon carbide (SiC) is one of the dielectric absorbers by virtue of its intrinsic electric dipolar polarization [17]. It has good thermal conductivity and stability, chemical stability, and adjustable electrical conductivity, which make it a good candidate for microwave absorption [18,19]. SiC is usually applied in the fabrication of multi-band radar absorbing materials and utilized as an auxiliary absorber in other materials or after surface treatments to enhance the absorption capacity [[20], [21], [22]]. Combination of SiC with other dielectric or magnetic absorber such as nano-carbon black (CB) can enhance microwave absorption by achieving an equilibrium between permeability and permittivity [23].

The aim of this contribution is that, silicon carbide was utilized separately and as an auxiliary absorber on the PAN precursor surface to assay its role on the microwave pre-oxidation. The practical value of this modification lies in the fact that, they aid to improve the interaction between microwave field and the modified surface to polarize charges in the coating, and the inability of this polarization to follow rapid reflections of this field.