Preparation and Characterization of Continuous Fe-containing Silicon Carbide Radar Absorbing Fibers from Precursor-based Processing

A new polymer polyferrocarbosilane (PFCS) was synthesized using polydimethylsilane and ferrocene as the raw materials. The polymer was then melt-spun into continuous PFCS fibers, cured in air and heat-treated in a nitrogen atmosphere up to 1300, a kind of Fe-containing SiC fibers were finally obtained. The SiC(Fe) ceramic fibers were combined with epoxy resin and a structural material was prepared showing excellent radar absorbing properties. HRTEM, XRD and XPS were used to characterize the SiC(Fe) fibers. The results show that the fibers are composed of-SiC, amorphous SiC x O 4-x , free carbon and small amount of Fe 3 Si-like microcrystals. A carbon-enriched layer of about 120nm was formed on the surface of the fibers.


INTRODUCTION
It is well known that silicon carbide fibers prepared by precursor conversion method are one of the most promising fibers for many high-technology composites due to their high tensile strength, high Young's modulus and good oxidation resistance at high temperatures.It is interesting to study the electromagnetic properties of the fibers when they are used as functional materials.Since SiC fibers derived from polycarbosilane (PCS) generally have rather high specific resistance and no magnetic properties, modification of the electromagnetic properties of SiC fibers with hetero-atoms are necessary and attracts much attention [1].
Polytitanocarbosilane (PTCS) with 1.5-4% of Ti was synthesized from polydimethylsilane and titanium tetraalkoxide By Yamamura [2], and low specific resistance (10 -1 cm) Si-Ti-C-O fibers {Tyranno Lox [3] (trade name) by Ube industries, Japan} were produced from PTCS through melt-spinning, curing and pyrolysis.With a similar procedure, Si-Zr-C-O fibers [4,5](with about 2% of Zr) were prepared with polyzirconocarbosilane (PZCS) and Si-Al-C-O fibers [5][6][7] (with about 1% of Al) from polyaluminacarbosilane (PACS) were manufactured by Ube with trade names Tyranno-ZE and Tyranno-SA, respectively.The heteroatoms in both Si-Zr-C-O and Si-Al-C-O fibers mentioned above play an important role in inhibition of crystalline grain of -SiC at very high temperatures and thus improve the high temperature resistance.
It was reported that SiC fibers containing Ti or Zr may be applied as electromagnetic wave absorbing materials [3].For *Address correspondence to this author at the School of Materials Science & Engineering, Central South University of Forestry & Technology, Changsha 410004, P.R. China; Tel: +86-731-85623221; Fax: +86-731-85623303; E-mail:spchen@163.com example, SiC fibers with small amount of Fe, Co and Ni were prepared by physical mixing of the nano-meter metal particles with PCS solution by ultra-sonic processing followed by melt-spinning, curing and pyrolysis.These fibers show electromagnetic wave absorbing properties because of the existence of the magnetic metal particles [8].
In this paper, we tried to prepared a new polymer using polydimethylsilane and ferrocene as the raw materials, and studied the electromagnetic wave absorbing properties of the final SiC(Fe) fibers.

Experimental Procedure
Polydimethylsilane (PDMS) was synthesized by dechlorination of dimethaldichlorosilane with sodium in xylene [9,10].The synthesized PDMS was rearranged at 420 o C under N 2 to produce viscous liquid with some Si-C backbone and some Si-Si groups, which was then mixed with ferrocene and reacted at 400 o C for 10h.PFCS was obtained through dissolving in xylene, filtration and distillation.Continuous SiC(Fe) fibers were prepared by melt-spinning PFCS at about 300 o C using a multiple-hole nozzle, cured in air at 200 o C, and heat-treated continuously at 1300 o C in a stream of nitrogen gas.The radar-wave absorbing material was obtained with this fibers and epoxy resin by hot-pressing technique.

Characterization
The FT-IR spectroscopy was performed using a Vector 22 spectrophotometer (Bruker, Germany) in KBr wafers.The XRD patterns were recorded on a D8 advance X-ray diffractometer (CuK radiation, =0.15418nm), during which fibers were grounded into powders.XPS semiquantitative analysis of the surface of SiC(Fe) fibers were performed with an X-ray photoelectron spectroscopy apparatus (PHI-5702, American physics electron, AlK ) under ultrahigh vacuum (3.4 10 -9 torr).The HRTEM micrographs were taken with a 200KeV high-resolution transmission electron microscope (JEOL JEM-2010FEF, Japan).The conductivity was measured in bulk with dc frequency.The microwave reflection was measured using network analyser 8720ET by the arc method (180mm 180mm wide and 3mm thick) [11].

Synthesis and Characterization of PFCS
The FT-IR spectra of the synthesized PFCS with different percentages of ferrocene in feed are shown in Fig. (1), where PFCS-n and PCS stand for the polymers synthesized with n wt% and 0 wt% ferrocene in feed, respectively.All the samples show a similar structure with absorption bands of C-H stretching (2950 and 2900cm -1 ), Si-H stretching (2100cm -1 ), CH 3 deformation of Si-CH 3 (1400cm -1 ), CH 2 deformation in Si-CH 2 -Si (1350cm -1 ), CH 2 wagging in Si-CH 2 -Si (1020cm - 1 ), and Si-CH 3 rocking/Si-C stretching (820cm -1 ) [9].However, the Si-H bond becomes weakened with the increasing of ferrocene.In the case of high ferrocene sample, such as PFCS-10, some small characteristic absorbing bands of 5 -Cp in ferrocene emerge at 3088, 1106 and 490 cm -1 [12].Thus, PFCS can be assumed to take the following general structure, a much smaller than b.
The results of the chemical analysis of PFCS are shown in Table 1, where the content of Fe gradually increases with more amount of ferrocene added, which is quantitatively in accordance with the above structure.It is interesting to find that the ratio of C/Si of PFCS is close to 2.0, while in the case of PCS, the ratio is significantly less than 2.0 [9,13,14].This can be explained that in the rearrangement reaction from PDMS (C/Si = 2.0) to PCS without ferrocene, degradation of methyl group and hydrogen free radicals occurs simultaneously to form volatile small molecules and the evaporation of large amount of CH 4 leads a lower C/Si ratio.In the presence of ferrocene, however, the degradation reaction seems less important and the resulted PFCS have a higher C/Si ratio.In practice, we find that ferrocene also acts as a catalyst of the rearrangement reaction because it makes the reaction time much shorter even at lower temperature.

Melt Spinning of PFCS Precursor
The spinning parameters include spinning temperature, spinning rate and pressure.The spinning temperature depends on the melting point range of PFCS.In order to gain better spinnability, it was found that the spinning temperature was generally 50-120 higher than the melting point of the polymer.If the spinning temperature is too low, the melt viscosity will be too large to raise the rate of spinning, due to the cohesive rupture; and if spinning temperature is too high, the melt viscosity will be too low to remain the stability of  spinning, due to the fluctuations caused by capillary rupture [15].Fig. (2) is the relationship between fiber diameter and spinning temperature at various spinning rate, with a spinning pressure of 0.4MPa.It can be seen the fiber diameter increases with increasing spinning temperature and lowering spinning rate.

Fig. (2).
The effect of spinning temperature on average diameter of PFCS green fibers.

Exploration of Curing
The reaction degree of the Si-H bonds was used to evaluate the reaction progress of precrusor fibers, calculated as P Si  3), when the reaction degree Si-H bonds is about 70% (by weight gain about 11%), the continuous SiC(Fe) fibers with higher tensile strength will be achieved.The reason is that if the curing degree is not enough, the fibers might merge together, and more volatile will evaporate, which will definitely result in more defects lowering fibers strength.On the contrarily, if the curing degree is too high, after pyrolysis, more amorphous SiC x O 4-x and SiC x O 4-x structures will generate which will decompose completely and yield gaseous CO and SiO, result in more defects in the fiber, thus lowering fiber strength [16].

XPS Depth Analysis
In Fig. (4), XPS depth elementary profiles of continuous SiC(Fe) fibers show that a carbon-enriched layer of about 120nm was formed on the surface, C/Si mole ratio is about 1.3 in the fiber core, and O and Fe atomic concentrations are about 15% and 2%, respectively, and there are some Fe 3 Silike microcrystals in the fibers.The fluctuation of the elementary composition may be due to the heterogenious distribution of small amount of Fe atoms.

Fig. (4).
The distribution of the elements in the diametric direction of continuous SiC(Fe) fibers by XPS depth analysis (Etching rate 15nm/min, etching degree 45°).

XRD Analysis
From the XRD patterns of SiC(Fe) fibers shown in Fig. (5), it is obvious that higher amount of ferrocene in PFCS or the content of Fe in the fibers leads to sharper diffraction peaks.The crystalline grain sizes were estimated, using Scherrer equation from the 111 peak of -SiC at 2 = 35.6 [5], to be 1.8, 3.1, 3.3, 4.2 and 93.4nm for SiC, SiC(Fe)-1, SiC(Fe)-2, SiC(Fe)-3 and SiC(Fe)-10 (powder) respectively.Therefore, Fe contained in the SiC(Fe) fibers seems to accelerate the growth of -SiC crystals in the pyrolysis process.In addition, for the high Fe content sample (Fig. 5a), some new diffractions at 2 = 45.34 , 66.05 and 83.75 emerge, which may possibly be assigned to Fe 3 Si as 220, 400 and 422 respectively.

Radar Wave Absorbing Properties
Fig. (7) shows the reflection attenuation curve of radarwave absorbing material having a size of 180mm 180mm 2mm, which was prepared with continuous SiC(Fe) fibers and epoxy resin by the thermal-pressure technique.The composite exhibits a reflection attenuation peak of -15.4dB at 12GHz.The reflection band below -10dB is in the frequency region of 10 to 14GHz, up to 4GHz, indicating an excellent radar-wave absorbing property.

CONCLUSIONS
A new pre-ceramic polymer, polyferrocarbosilane (PFCS), was synthesized from polydimethylsilane and ferrocene.The precursor was melt-spun into fibers, cured in air, pyrolyzed above 1200 in nitrogen and continuous SiC(Fe) fibers were obtained.The SiC(Fe) fibers showed good radar wave absorbing properties, namely, 4GHz band below -10GHz at the thickness of 2mm.The continuous SiC(Fe) fibers were composed of amorphous SiC x O 4-x phase, -SiC crystalline phase, small amount of Fe 3 Si crystalline phase and free carbon phase, which were confirmed in XPS, XRD and HRTEM analyses.
High-resolution electron micrographs of continuous SiC(Fe)fibers are shown in Fig. (6).Fig. (6a) is considered to be in the amorphous state consisting of Si, C, O and a trace amount of Fe.In Fig. (6b), the lattice image corresponding to a 0.25nm interlayer spacing of -SiC(111) planes is observed in some areas of continuous SiC(Fe) fibers.In Fig. (6c), the lattice image corresponding to a 0.335nm interlayer spacing is assigned to ordered free carbon.So continuous SiC(Fe)fibers are primarily composed of amorphous SiC x O 4- x , small amount crystallization of -SiC and free carbon.A small number of -SiC and free carbon are distributed in amorphous SiC x O 4-x .