Mechanical properties of pyrolysed wood: a nanoindentation study

TLDR: In this article, changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400°C were investigated. But the authors focused on changes in the indentation modulus and elasto-plastic/brittle behavior of the carbonaceous residues.

Abstract: The present work focuses on changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400°C. Nanoindentation tests are used for the determination of mechanical properties at the scale of single wood cell walls. Hardness, indentation modulus and elasto-plastic/brittle behaviour of the carbonaceous residues are derived as function of pyrolysis temperature. Hardness values increase continuously by more than one order of magnitude to 4.5 GPa at 700°C. The indentation modulus shows complex changes with a minimum of 5 GPa around 400°C and a maximum of 40 GPa around 1000°C. The deformation induced by the indenter is largely visco-plastic in native wood, but it is almost purely elastic in the carbonaceous residue, with particular low values of the indentation ductility index around 700°C. A low density and a strongly cross-linked carbon structure may explain the mechanical behaviour at these intermediate temperatures. A final decrease of the modulus and a slight decrease of duc...

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Mechanical properties of pyrolysed wood – a
nanoindentation study
Gerald Zickler, Thomas Schöberl, Oskar Paris
To cite this version:
Gerald Zickler, Thomas Schöberl, Oskar Paris. Mechanical properties of pyrolysed wood – a
nanoindentation study. Philosophical Magazine, Taylor & Francis, 2006, 86 (10), pp.1373-1386.
�10.1080/14786430500431390�. �hal-00513635�

For Peer Review Only
Mechanical properties of pyrolysed wood Â

 a
nanoindentation study
Journal:
Philosophical Magazine & Philosophical Magazine Letters
Manuscript ID:
TPHM-05-Apr-0091.R1
Journal Selection:
Philosophical Magazine
Date Submitted by the
Author:
22-Sep-2005
Complete List of Authors:
Zickler, Gerald; Max Planck Institute KGF, Biomaterials; Max Planck
Institute of Colloids and Interfaces, Biomaterials
Schöberl, Thomas; Austrian Academy of Sciences, Erich Schmid
Institute of Materials Science
Paris, Oskar; Max Planck Institute of Colloids and Interfaces,
Biomaterials; Max Planck Institute KGF, Biomaterials
Keywords:
nanoindentation, mechanical properties, carbon
Keywords (user supplied):
wood, pyrolysis
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Philosophical Magazine & Philosophical Magazine Letters

For Peer Review Only
1
Mechanical properties of pyrolysed wood – a nanoindentation study
G. A. ZICKLER†, T. SCHÖBERL‡ and O. PARIS†*
†Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, Am Mühlenberg 1,
D-14476 Potsdam-Golm, Germany
‡Erich Schmid Institute of Materials Science, Austrian Academy of Sciences and Institute of Metal
Physics, University of Leoben, Jahnstr. 12, A-8700 Leoben, Austria
*Corresponding author: e-Mail:
oskar.paris@mpikg.mpg.de (O. Paris)
The present work is focused on changes of mechanical properties in pyrolysed
spruce wood as a function of temperature up to 2400°C. Nanoindentation tests are
used for the determination of mechanical properties at the scale of single wood cell
walls. Hardness, indentation modulus and elasto-plastic/brittle behaviour of the
carbonaceous residues are derived as function of pyrolysis temperature. Hardness
values increase continuously by more than one order of magnitude to 4.5 GPa at
700°C. The indentation modulus shows complex changes with a minimum of 5 GPa
around 400°C and a maximum of 40 GPa around 1000°C. The deformation induced
by the indenter is largely visco-plastic in native wood, but it is almost purely elastic
in the carbonaceous residue with particular low values of the indentation ductility
index around 700°C. A low density and a strongly cross-linked carbon structure
may explain the mechanical behaviour at these intermediate temperatures. A final
decrease of the modulus and a slight decrease of ductility for temperatures above
2000°C can be attributed to a continuous structural transition of the material
towards graphite-like stacking of carbon sheets and to preferred carbon orientation
along the wood cell axis.
Keywords: Wood; Carbon; Pyrolysis; Nanoindentation; Mechanical properties
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1. Introduction
Natural plant resources present a large potential for transforming hierarchically ordered and
mechanically optimised structures into carbonaceous materials by simple pyrolysis processes. They
have been used for millennia to generate charcoal [1], and also for quite some years to produce
activated carbons with high micro- and mesoporosity for filters or catalyst supports [2-4]. These
applications do not usually require preservation of the hierarchical structure, and the mechanical
integrity of the precursor is not retained. However, there is a growing interest of a one-to-one
transformation of the unique anatomical characteristics of plants into structural materials with
entirely different composition, overcoming eventually some physical limitations of the biological
precursor.
Carbonised wood monoliths can be used as templates for near net shape manufacturing of a
diversity of materials, including structural activated carbons, carbon/polymer- and carbon/carbon
composites as well as carbide- or oxide ceramics [5-12]. In particular the cellular morphology of
wood tissues and the large diversity of their pore sizes and size distributions promise a broad range
of potential applications such as filters, catalyst carriers, biocatalysts, sensors or even cancellous
bone replacement [9]. Consequently, a growing number of current research activities is
concentrating on structural aspects of wood pyrolysis [13-17], in particular concerning fundamental
questions of the carbon nanostructure development and preferred carbon orientation as a possible
consequence of the cellulose microfibril orientation in wood [13; 17].
Concerning its mechanical properties, wood is a material of excellent optimisation strategies
at several levels of hierarchy. Apart from macroscopic optimisation [18] a cellular honeycomb-like
structure at the micrometre level provides high directional stiffness and strength at low weight [19].
Another optimisation is performed at the nanometre
level where the so-called cellulose microfibril
angle controls the needs either for high stiffness or for high extensibility of the cell walls, and this
provides an adaptive tool for the tree to react upon external stresses [20-22]. Moreover, subtle
deformation mechanisms at the molecular scale allow large plastic deformation of the cell wall
material without damage [23]. Upon pyrolytic conversion some of these optimisation features might
be transferred into the inorganic carbonaceous material by retaining preferred orientation as well as
the nanocomposite character. Thus, some of the architectural optimisations in wood may be
combined with the superior properties of composite carbon materials such as high tensile modulus
and tensile strength at very high temperatures. However, there are only a few investigations of the
mechanical properties of pyrolysed wood [7; 8; 14; 24; 25], and to the best of our knowledge, there
are no studies of the mechanical properties on the level of single cell walls.
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Nanoindentation provides the ability to measure both elastic and plastic deformation at a
very small scale [26; 27]. Thus, it can be used for the characterisation of hardness and elastic
modulus, as well as other elastic and plastic parameters of the carbonaceous residue after wood
pyrolysis at the level of single cell walls. The method was applied to study native wood by Wimmer
et al. [28; 29], and it was shown that it can provide new insights into local mechanical properties of
the wood cell wall. Gindl et al. extended this matter, investigating the influence of lignin content
[30], microfibril angle [31] and chemical wood modifications [32; 33] on local mechanical
properties. A critical discussion about the general significance of mechanical parameters from
nanoindentation tests on wood samples was provided by Gindl and Schöberl [34], regarding the
effect of high elastic anisotropy on the indentation modulus. The present work is focusing on
changes of mechanical properties of cell walls from spruce wood by pyrolysis up to temperatures of
2400°C with narrow temperature intervals. Additionally to the mechanical parameters, mass loss
and dimensional changes of the material are determined as a function of heat treatment temperature,
from which the bulk density change of the material is obtained. The aim of this work is to
continuously follow the development of the mechanical response of the material during
decomposition of wood and the subsequent formation of carbon. Together with the structural
changes published elsewhere [17], we derive a detailed survey of the relation between local
microstructures and mechanical properties of biomorphous carbon from wood.
2. Experimental Methods
A board of air dried spruce wood (Picea abies Karst.) of error-free quality with year rings of regular
intervals containing normal wood with small microfibril angle was selected for the experiment. The
samples were stored in an environmental chamber, kept at 20°C and a relative ambient moisture
content of 65% before machining. Cubic shaped specimens of 15×15×15 mm
3
size were cut using a
band saw in a way that the faces of the cubes were parallel to the axial, radial and tangential growth
directions of the tree. Prior and after pyrolysis the specimens were weighed and axial, radial and
tangential dimensions were measured using a micrometre screw. Macroscopic bulk density was
calculated by taking the sample mass and dividing it by the volume determined from macroscopic
dimensions. Five samples were heat treated at every temperature value between 220°C and 600°C
with intervals of 20°C and between 600°C and 1000°C with intervals of 100°C. This was performed
in a tube furnace (Heraeus Thermicon) equipped with a quartz glass tube under continuous flow of
nitrogen gas (approximately 0.02 m
3
h
-1
) to ensure a non-oxidising atmosphere. For all specimens
the heating rate was 2°C min
-1
. Further specimens were pyrolysed at 1300°C, 1600°C, 2200°C and
2400°C after pre-pyrolysis at 1000°C in a vacuum-seal inert gas furnace (HTM Reetz) under
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Frequently asked questions

Q1. What have the authors contributed in "Mechanical properties of pyrolysed wood – a nanoindentation study" ?

Mechanical properties of pyrolysed wood – a nanoindentation study Gerald Zickler, Thomas Schöberl, Oskar Paris 

Q2. What have the authors stated for future works in "Mechanical properties of pyrolysed wood – a nanoindentation study" ?

The peculiar mechanical behaviour at intermediate temperatures around 1000°C is attributed qualitatively to a particular nanostructure of the carbonaceous material, consisting of a strongly cross-linked 3D carbon network, quite different from the graphite-like, turbostratic stacking of extended carbon sheets present at higher temperatures. 

Q3. How was the force programmed to decrease in the unloading segment?

In the unloading segment the force was programmed to decrease to 20% of the maximum, where a second hold segment of 15 s was used to estimate viscoelastic recovery, before the sample was completely unloaded. 

Q4. What are the advantages of carbonised wood monoliths?

Carbonised wood monoliths can be used as templates for near net shape manufacturing of adiversity of materials, including structural activated carbons, carbon/polymer- and carbon/carbon composites as well as carbide- or oxide ceramics [5-12]. 

Q5. What were the key parameters obtained during indentation experiments?

The key parameters obtained during indentation experiments were peak load Pmax, thedisplacement at peak load h and the unloading stiffness S. According to the method developed by Oliver and Pharr [27], based on considerations by Doerner and Nix [26], the elastic modulus of a sample, which exhibits plastic deformation during loading, was determined from the initial unloading curve, which was supposed to be purely elastic. 

Q6. What is the potential of wood pyrolysis?

Natural plant resources present a large potential for transforming hierarchically ordered and mechanically optimised structures into carbonaceous materials by simple pyrolysis processes. 

Q7. How many indents were performed on different wood year rings?

An average number of 25 indents on radial and tangential latewood cell walls of different wood year rings were performed on every specimen. 

Q8. What is the way to explain the vacancy-like defects in carbonaceous materials?

Since the carbonaceous residue from pyrolysed wood contains presumably a high concentrations of vacancy-like defects from the evaporation of volatile molecular fragments, such energetically metastable cross-links could contribute significantly to the carbon structure at low temperatures. 

Q9. What is the definition of the reduced modulus of the indenter?

The reduced modulus Er accounts for the effect of elastic deformation of the indenter based on the assumption that compliance occurs in the indenter as well as in the sample. 

Q10. how much pyrolysis temperature affects the cell wall material?

The effects of pyrolysis temperature on mechanical properties of the cell wall material from wooden precursor can roughly be separated into three temperature regions: i) T < 400°C, ii) T = 400°C to 1000°C and iii) T > 1000°C. 

Q11. How much hydrogen can be expected in the carbonaceous residue above 600°C?

Taking the elemental analysis on cellulose by Tang and Bacon [48; 49] as representative also for wood, less than 5 at% of oxygen, but around 20 at% of hydrogen can be expected in the carbonaceous residue above 600°C. 

Q12. What is the reason for the unexpected mechanical properties of pyrolysed wood?

also porosity cannot be made responsible for the effect, since the material density increases above 2000°C (i.e. decrease of porosity), which should result in an increase of E. Based on these considerations it is proposed, that the unexpected mechanical properties of pyrolysed wood around 1000°C are a consequence of a particular structure, based on a low-density 3D carbon network with a high amount of crosslinking. 

Q13. What is the definition of the indentation ductility index?

According to this definition, the indentation ductility index D is equal to one for a fully plastic material without any elastic recovery during unloading, and it is equal to zero for a fully elastic material exhibiting a complete unloading recovery in an elastic manner along its previous loading path. 

Q14. How much of the original wood density is lost at 340°C?

Above 600°C, the density increases and displays a relative maximum at about 900°C and a minimum at about 1800°C, reaching eventually a value of 80% of the original wood density at 2400°C. 

Q15. What is the role of the cellulose microfibril angle in wood pyro?

Another optimisation is performed at the nanometre level where the so-called cellulose microfibril angle controls the needs either for high stiffness or for high extensibility of the cell walls, and this provides an adaptive tool for the tree to react upon external stresses [20-22]. 

Q16. How does the pyrolysis curve change with increasing temperature?

Figure 4 shows the effect of pyrolysis temperature on Er. Up to a temperature of about 280°C only a slight decrease of Er is observed, whereas the curve drops very fast to about half of the original value between 280°C and 320°C and displays a characteristic broad minimum around 400°C. 

Q17. What is the slope of the load-displacement curves?

In figure 3b the slope of the load-displacement curves decreases with increasing heat treatment temperature, and the unloading path does not completely retrace the loading path but fairly well returns to the origin of the curve with a small hysteretic loop. 

Q18. What is the description of the sp3-type bonding?

Such a high H-content could enable a significant amount of sp3-type bonding, similarly to a class of materials known as a-C:H, or diamond-like carbons [50]. 

Q19. How does the indentation ductility index D change with temperature?

Figure 6 illustrates the effect of the heat treatment temperature on the indentation ductility index D, which stays almost constant at D ≈ 0.8 up to temperatures of 300°C and then decreases rapidly to a value below D ≈ 0.1.