Effect of pyrolyzation temperature on wood-derived carbon and silicon carbide

doi:10.1016/j.jeurceramsoc.2009.04.040

Journal of the European Ceramic Society

Volume 29, Issue 14, November 2009, Pages 3069-3077

https://doi.org/10.1016/j.jeurceramsoc.2009.04.040 Get rights and content

Abstract

The structure of carbon and silicon carbide produced through the pyrolyzation of wood and the subsequent melt-infiltration with silicon was studied as a function of initial carbon pyrolyzation temperature. Scanning electron microscopy, transmission electron microscopy, mercury intrusion porosimetry, X-ray diffraction and Raman spectroscopy were used to characterize material derived from initial carbon pyrolyzation temperatures in the range of 300–2400 $°$ C. It was determined that, although structural differences abound in carbon pyrolyzed at different temperatures, the resulting silicon carbide is independent of the initial temperature of carbon pyrolyzation.

Introduction

Biomorphic silicon carbide (bioSiC) utilizes a unique method of processing porous ceramics from wood. This technique involves the pyrolysis of natural wood precursors, followed by the infiltration of silicon to form silicon carbide (SiC), retaining the initial wood structure.[1], [2], [3], [4] Utilizing the natural wood structure takes advantage of pre-existing porosity, thus eliminating high-energy pore-forming routes from the processing steps. There are many potential high-temperature applications for these porous ceramics, including heat exchangers, molten metal filters, catalyst supports, and heating elements.⁴

Hardwood microstructures include three types of pores: vessels, which are large diameter cells used for transportation of nutrients, and fibers and rays, which are smaller diameter cells used for strength and storage.⁵ The vessels and fibers are elongated and run in the axial (longitudinal) direction of the tree. The rays are aligned perpendicularly to the vessels and fibers, in the transverse direction.⁵ Depending on the wood species, the relative size and size distribution of the various pores can vary considerably. The mechanical and thermal properties of the resulting porous silicon carbide have been well characterized as a function of porosity and orientation.[3], [4], [6], [7], [8] The effects of the pyrolyzation temperature on the resulting silicon carbide, however, have not yet been determined.

Traditionally, to produce bioSiC, the carbon is pyrolyzed to 1000 $°$ C, but previous research has shown that the majority of weight loss as a result of pyrolyzation occurs before the temperature has reached 500 $°$ C.[7], [8], [9] However, X-ray diffraction of wood samples pyrolyzed from 400–2500 $°$ C showed a gradual narrowing of the (0 0 2) reflection (near $2 θ = 26 °$ ) as the pyrolyzation temperature increased, indicating that although the weight loss is complete, some structural changes are occurring.¹⁰ Although increased order is observed, wood-based carbon will not arrange into graphite, and thus is classified as non-graphitizable.¹¹ Strong cross-linked bonds are formed at low temperatures, such that even with high-temperature heat treatments (3000 $°$ C), crystalline graphite never results.[11], [12] Instead, turbostratic carbon is formed, characterized by misaligned graphene sheets with an average lattice spacing larger than that of graphite.

The goal of the current work is to investigate further the progression towards structural order in wood-derived carbon, and then determine how the pyrolyzation temperature affects the resulting silicon carbide material. While previous work has utilized X-ray diffraction to monitor structural changes of carbon pyrolyzed at different temperatures,¹⁰ this work further explores these structures through first studying the atomic level crystal structure with transmission electron microscopy, the bonding characteristics using Raman spectroscopy, and finally investigating the samples at a microscopic level by analyzing the pore size distribution using mercury porosimetry. In addition, the resultant SiC is similarly characterized, in order to determine the effect of the pyrolyzation temperature on the final ceramic product and optimize the processing conditions.

Section snippets

Materials processing

Carbon and biomorphic silicon carbide were processed from five hardwood precursors: beech (Fagus sylvatica), mahogany (Swietenia macrophylla), poplar (Liriodendron tulipifera), red oak (Quercus rubra) and sapele (Entandrophragma cylindricum).¹³ Wood samples were cut and pyrolyzed at temperatures of 300, 500, 700, 1000 and 1200 $°$ C in argon for one hour to produce carbon. These carbon scaffolds were subsequently melt-infiltrated at 1500 $°$ C in vacuum for one hour with excess silicon powder to form

Materials

Fig. 1(a)–(e) show examples of the microstructures of each wood precursor used in this study after pyrolyzation to 1000 $°$ C, with their corresponding porosity values listed in each figure. It is obvious from these images that the microstructures vary greatly from wood to wood. Beech and poplar-based carbon, Fig. 1(a) and (b), respectively, have similar microstructures; the vessels are evenly distributed throughout the microstructure and are similar in size. However, they have vastly different

Conclusions and implications

Carbon was pyrolyzed at temperatures ranging from 300–2400 $°$ C, and silicon carbide was processed from carbon pyrolyzed at temperatures from 300–1200 $°$ C. Transmission electron microscopy demonstrated that there is no visible difference in the structure of either carbon (amorphous) or silicon carbide (cubic crystalline) as a result of these processing schemes. X-ray diffraction indicated an increase in order in the carbon samples with an increased carbon pyrolyzation temperature, which was further

Acknowledgements

This work was funded by the National Science Foundation (DMR-0710630).

The SEM, TEM, and Raman Spectroscopy work was performed in the EPIC and Keck-II facilities of NUANCE Center at Northwestern University. NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University.

This work made use of the J.B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-0520513) at the Materials Research

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¹: Present address: Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.

²: Present address: Ethicon, Inc., Somerville, NJ 08876, USA.

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Effect of pyrolyzation temperature on wood-derived carbon and silicon carbide

Abstract

Introduction

Section snippets

Materials processing

Materials

Conclusions and implications

Acknowledgements

Journal of the European Ceramic Society

Journal of the European Ceramic Society

Journal of the European Ceramic Society

Materials Science and Engineering A

Carbon

Carbon

Carbon

Materials Science and Engineering C

Biomimetic process for producing SiC “Wood”

Journal of the American Ceramic Society