Elsevier

Carbon

Volume 121, September 2017, Pages 35-47
Carbon

Thermal conductivity and annealing effect on structure of lignin-based microscale carbon fibers

https://doi.org/10.1016/j.carbon.2017.05.066Get rights and content

Abstract

This work reports on systematic investigation of the structure and thermal conductivity of lignin-based carbon fibers (CF) at the microscale. The lignin-based CF is produced by melt-spinning pyrolytic lignin derived from red oak. The 0 K-limit phonon scattering mean free path uncovers a characteristic structure size of ∼1.2 nm, which agrees well with the crystallite size by X-ray scattering (0.9 and 1.3 nm) and the cluster size by Raman spectroscopy (2.31 nm). The thermal conductivity of as-prepared CFs is determined at ∼1.83 W/m·K at room temperature. The thermal reffusivity of CFs shows little change from room temperature down to 10 K, uncovering the existence of extensive defects and grain boundaries which dominate phonon scattering. The localized thermal conductivity of CFs is increased by more than ten-fold after being annealed at ∼2800 K, to a level of 24 W/m·K. Our microscale Raman scanning from less annealed to highly annealed regions shows one-fold increase of the cluster size: from 1.83 nm to 4 nm. This directly confirms structure improvement by annealing. The inverse of the thermal conductivity is found linearly proportional to the annealing temperature in the range of 1000–2800 K.

Introduction

Over the past few decades, researchers and industries are pursuing high-performance yet lightweight materials in engineering. One of the promising materials is carbon fiber (CF) [1]. The composites using CFs as the reinforced material have high strength, high modulus, high stiffness yet low density (1.75–2.0 g/cm3) [2]. The tensile modulus of CF ranges from 291 GPa for polyacrylonitrile (PAN) based CF to 940 GPa for ultrahigh modulus pitch-based CF. The typical tensile strength falls in the range of 2.47–5.69 GPa for PAN-based CF [2]. With the excellent mechanical properties, the CFs could be used in reducing the vehicles' total weight and thus to improve the fuel efficiency of vehicles [3]. Apart from this, CF also has potential applications in aerospace structures, sport equipment, turbine blades and so on [4]. Currently, the dominant precursor for producing CFs is polyacrylonitrile [2], [4], [5], [6], which is a petroleum-based polymer. Despite its unique properties, the application of CF is limited to specific fields mainly due to its high market price related to the cost of PAN, which accounts for 51% of the total production cost of CF [1]. The release of toxic gas during the processing of PAN-based CF is also an environmental concern. Thus, producing low-cost carbon fibers from alternative precursors has been of great interest to many researchers. Lignin has drawn the most attention as an alternative precursor because of its abundance, low cost and relative high carbon content [7]. Lignin is the second abundant biopolymer in nature, accounting for 10–35% of lignocellulosic biomass [8]. Isolated lignin is also available as a by-product from paper industry and bio-refineries. The estimated cost of lignin precursor could be 50–70% lower than the textile grade PAN [9]. Although economically advantageous, lignin-based CFs usually have much lower mechanical properties in comparison to PAN-based CFs mainly due to the lack of the molecular orientation in lignin polymer [10]. On the other hand, CFs produced from mixture of PAN and lignin could have lower cost than PAN-based CFs, while showing good compressive strength [4], [11].

In previous studies, research focus has been mostly on the manufacturing of lignin-based CFs and process optimization for improved mechanical properties [12], [13], [14]. However, it is also important to note that lignin-based CFs could also be applied as functional materials for other applications where material mechanical properties are not essential [15]. Past work about the relevant thermal properties was focused on carbon nanofibers (CNFs) and PAN-based CFs. Mayhew et al. reported that the k of commercially-available CNFs obtained by chemical vapor deposition is 4.6 W/m·K by using T-type probe experimental configuration [16]. The thermal conductivity for PAN-based CFs without extremely high temperature treatment is in the order of tens of W/m·K [6], [17]. High temperature (as high as ∼3000 °C) treatment is a common method used in lab and industry to improve the microstructure and thermal conductivity of CFs. The k of CNFs could be increased by around 40 times after being annealed at 2800 °C for 20 hours [16]. The k of high temperature treated PAN-based CFs could be as high as several hundreds of W/m·K [6]. However, so far, seldom work has been reported about the k of lignin-based CFs. Recently, we have produced lignin-based CFs by melt-spinning red oak derived pyrolytic lignin. Although still lower than that of PAN-based CFs, the mechanical properties of our lignin-based CFs were at the top range of previously reported lignin-based CFs [18]. In the present study, lignin-based CFs were further tested to examine their thermal properties. Our goal is to study the thermal properties of lignin-based CFs, as well as the structure effect on phonon scattering and thermal transport. Furthermore, through annealing, it is found that the k of the lignin-based CFs is expected to increase by ten folds. The k of high temperature treated lignin-based CFs could reach tens of W/m·K. One natural question is: To what extent the microstructure of the lignin-based CFs could be improved by the annealing method? We will answer this question through the structure and thermal properties study for the high temperature treated lignin-based CFs.

This work is organized as follows. In the first section, the preparation and processing of lignin-based CFs are introduced. Next, structure studies by Raman, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are presented. In the third part, we show the experimental setup for thermal properties measurement and the corresponding thermal properties study. This is followed by the study of phonon scattering in the lignin-based CFs. Finally, annealing treatment and improvement of the structure and thermal properties are discussed. We will use CF to indicate the lignin-based CF tested in this work.

Section snippets

Carbon fiber manufacturing

The carbon fiber specimens used in this study were manufactured from red oak-derived pyrolytic lignin using the melt-spinning method [18]. The production method of pyrolytic lignin and manufacturing process of the lignin-based CFs are shown in Fig. 1(a). Detailed information can be found in our previous study [18]. Briefly, red oak was pyrolyzed in a fluidized bed reactor with a staged-fractionation condenser system. Pyrolytic lignin was isolated from heavy fraction of bio-oil [19]. Because

Thermal characterization principle

The thermal diffusivity of the CFs from room temperature (RT) down to 10 K is measured by the cryogenic transient electro-thermal (TET) technique. As shown in Fig. 3(a), the to-be-measured sample is suspended between two electrodes. The contacts between the sample and the two electrodes are secured by silver paste. In this way, the thermal contact resistance could be reduced to a negligible level. Then the whole sample is placed in a vacuum chamber, whose environmental temperature from RT to

Thermal transport and properties: correlation with temperature

Fig. 3(c) shows the temperature dependence of the normalized electrical resistance for S1, S2 and S3. The normalized resistance is obtained by using the electrical resistance at RT as the base. Note that all samples exhibit similar nonmetallic-like behavior throughout the entire temperature range. When temperature is changed from RT to 10 K, the normalized electrical resistances are increased by 34%, 37% and 83% for S1, S2 and S3_r1, respectively. The resistance rise as temperature decreases is

Change of structure domain size by annealing

In this section, two samples (S6 and S7) were chosen to conduct annealing and study the thermo-physical properties over the cryogenic temperature range. We aim at gaining full understanding of the annealing effect on effective thermal conductivity and phonon mean free path. After obtaining the temperature dependence of thermal diffusivity by the TET technique, the real thermal conductivity is obtained by the following equation: k = ρcpα. Fig. 5(a) shows the temperature dependence of thermal

Conclusion

In this work, the micro-structure and thermo-physical properties of lignin-based microscale CFs were studied from various aspects. Our thermal measurement indicated that the thermal conductivity of the lignin-based CFs could be as small as 1.83 W/m·K. Our thermal reffusivity study from RT down to 10 K showed very weak variation of thermal reffusivity against temperature. This phenomenon suggested dominant boundary/defect phonon scattering-sustained heat conduction in our CFs. By utilizing a new

Acknowledgement

Support of this work by National Science Foundation (CBET1235852, CMMI1264399), Department of Energy (DENE0000671, DE-EE0007686), and Iowa Energy Center (MG-16-025, OG-17-005) is gratefully acknowledged.

References (42)

  • L. Qiu et al.

    The effect of grain size on the lattice thermal conductivity of an individual polyacrylonitrile-based carbon fiber

    Carbon

    (2013)
  • Y.J. Su et al.

    Exceptional negative thermal expansion and viscoelastic properties of graphene oxide paper

    Carbon

    (2012)
  • J. Luo

    Lignin-based Carbon Fiber

    (2010)
  • H.C. Liu et al.

    Processing, structure, and properties of lignin- and CNT-incorporated polyacrylonitrile-based carbon fibers

    ACS Sustain. Chem. Eng.

    (2015)
  • J. Heremans et al.

    Thermal conductivity and Raman spectra of carbon fibers

    Phys. Rev. B

    (1985)
  • G. Gellerstedt et al.

    The wood-based biorefinery: a source of carbon fiber?

    Open Agric. J.

    (2010)
  • D.A. Baker et al.

    Recent advances in low-cost carbon fiber manufacture from lignin

    J. Appl. Polym. Sci.

    (2013)
  • Q.N. Sun et al.

    A study of poplar organosolv lignin after melt rheology treatment as carbon fiber precursors

    Green Chem.

    (2016)
  • X.Z. Dong et al.

    Polyacrylonitrile/lignin sulfonate blend fiber for low-cost carbon fiber

    RSC Adv.

    (2015)
  • D.A. Baker et al.

    Recent advances in low-cost carbon fiber manufacture from lignin

    J. Appl. Polym. Sci.

    (2013)
  • F.S. Baker et al.

    Low Cost Carbon Fiber from Renewable Resources

    (2010)
  • Cited by (50)

    View all citing articles on Scopus
    1

    These authors contributed equally to the work.

    View full text