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Darshil U. Shah1*, Johannes Konnerth2, Michael H. Ramage1, Claudia Gusenbauer2
1 Centre for Natural Material Innovation, Dept. of Architecture, University of Cambridge, Cambridge CB2 1PX, UK
2 Institute of Wood Technology and Renewable Materials, Department of Material Sciences and Process Engineering, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Strasse 24, 3430 Tulln an der Donau, Austria
*Corresponding author. E-mail: HYPERLINK "mailto:dus20@cam.ac.uk" dus20@cam.ac.uk. Tel: +44 (0)1223760124.
Abstract
Scanning thermal microscopy is a powerful tool for investigating biological materials and structures like bamboo and its cell walls. Alongside nanoscale topographical information, the technique reveals local variations in thermal conductivity of this elegant natural material. We observe that at the tissue scale, fibre cells in the scattered vascular tissue would offer preferential pathways for heat transport due to their higher conductivities in both anatomical directions, in comparison to parenchymatic cells in ground tissue. In addition, the transverse orientation offers more resistance to heat flow. Furthermore, we observe each fibre cell to compose of up to ten layers, with alternating thick and thin lamellae in the secondary wall. Notably, we find the thin lamellae to have relatively lower conductivity than the thick lamellae in the fibre direction. This is due to the distinct orientation of cellulose microfibrils within the cell wall layers, and that cellulose microfibrils are highly anisotropic and have higher conductivity along their lengths. Microfibrils in the thick lamellae are oriented almost parallel to the fibre cell axis, while microfibrils in the thin lamellae are oriented almost perpendicular to the cell axis. Bamboo grasses have evolved to rapidly deposit this combination of thick and thin layers, like a polymer composite laminate or cross-laminated timber, for combination of axial and transverse stiffness and strength. However, this architecture is found to have interesting implications on thermal transport in bamboo, which is relevant for the application of engineered bamboo in buildings. We further conclude that scanning thermal microscopy may be a useful technique in plant science research, including for phenotyping studies.
Keywords: Bamboo; Thermal transport properties; Scanning thermal microscopy; Structure-property relations; Cell wall ultrastructure
Introduction
Engineered bamboo is an exciting family of materials that has attracted much interest in applications for sustainable construction ADDIN EN.CITE ADDIN EN.CITE.DATA [1-3]. Today, products such as laminated bamboo are most commonly used as flooring materials due to their hardness and durability. However, their stiffness and strength is comparable to engineered wood products, making them suitable for structural uses as well ADDIN EN.CITE Sharma2014955[2, 3]95595517Sharma, B, Gatoo A, Bock M, Mulligan H, Ramage MHEngineered bamboo: state of the artProceedings of the ICE - Construction Materials57-6716822014Sharma201596196196117Sharma, B, Gatoo A, Bock M, Ramage MHEngineered bamboo for structural applicationsConstruction and Building MaterialsConstruction and Building MaterialsConstr. Build. Mater.66-73812015[2, 3]. For applications in buildings, thermal properties of materials are also relevant. Thermal conductivity, for instance, dictates the rate of temperature increase through a material, which affects fire behaviour and building energy performance. Energy use in buildings (e.g. space heating and cooling) accounts for over 30% of global energy consumption and CO2 emissions ADDIN EN.CITE ADDIN EN.CITE.DATA [4-6]. In this regard, material choices and their thermal performance have a notable role in improving building energy intensities ADDIN EN.CITE ADDIN EN.CITE.DATA [4-6].
While the ultrastructure of bamboo ADDIN EN.CITE ADDIN EN.CITE.DATA [7-9] and its relation to mechanical properties (mainly stiffness) ADDIN EN.CITE ADDIN EN.CITE.DATA [10-12] are well-known, the thermal behaviour of bamboo, particularly in relation to its structure, is only sparsely reported in literature ADDIN EN.CITE Shah20161262[13, 14]1262126217Shah, DU, Bock MCD, Mulligan H, Ramage MHThermal conductivity of engineered bamboo compositesJournal of Materials ScienceJournal of Materials ScienceJ. Mater. Sci.2991-30025162016Huang20149529529525Huang, P, Chang WS, Shea A, Ansell MP, Lawrence MAicher, S, Reinhardt HW, Garrecht HNon-homogeneous thermal properties of bambooMaterials and Joints in Timber Structures: Recent Developments of Technology2014Dordrecht, NetherlandsSpringer Netherlands[13, 14]. In previous work ADDIN EN.CITE Shah20161262[13]1262126217Shah, DU, Bock MCD, Mulligan H, Ramage MHThermal conductivity of engineered bamboo compositesJournal of Materials ScienceJournal of Materials ScienceJ. Mater. Sci.2991-30025162016[13], experiments at the macro-scale (i.e. on samples that were 10-20mm thick, >50 mm diameter) have established that thermal conductivity of bamboo is a structure-dependent property. Specifically, volumetric composition, reflected by the apparent density, has a well-predicted effect on thermal transport properties of bamboo. Based on semi-empi r i c a l c o m p o s i t e m o d e l s , t h e s t u d y w a s a l s o a b l e t o e s t i m a t e t h a t t h e t h e r m a l c o n d u c t i v i t y o f t h e b a m b o o c e l l w a l l m a t e r i a l i s k Q% = 0 . 5 5 - 0 . 5 9 W / m ( K i n t h e l o n g i t u d i n a l d i r e c t i o n ( a l o n g t h e c u l m l e n g t h ) , a n d k " = 0 . 3 9 - 0 . 4 3 W / m ( K i n t h e t r a n s v e r s e / r a d i a l d i r ection. However, these single characteristic conductivity values do not reflect the heterogeneity in bamboo cell types (e.g. in ground and vascular tissue) and bamboos complex, hierarchical, lamellar structure ( REF _Ref5545685 \h Figure 1). Information on thermal conductivity differences in bamboo cell walls would be interesting from a fundamental science perspective, as well as in indicating preferential heat pathways.
In this study, we employ scanning thermal microscopy (SThM) to image and map thermal conductivity variations across the bamboo ultrastructure, and relate this to its elegant anatomical organisation. Previous similar studies using scanning thermal microscopy on wood have been fruitful in revealing ultrastructural information (e.g. orientation of cellulose microfibrils in different cell wall layers) ADDIN EN.CITE Vay20131009[15]1009100917Vay, O, Obersriebnig M, Mller U, Konnerth J, Gindl-Altmutter WStudying thermal conductivity of wood at cell wall level by scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz155-1596722013[15], monitoring adhesive penetration at a bond line ADDIN EN.CITE Konnerth20081271[16, 17]1271127117Konnerth, J, Harper D, Lee SH, Rials TG, Gindl WAdhesive penetration of wood cell walls investigated by scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz91-98622008Xu201612721272127217Xu, D, Zhang Y, Zhou H, Meng Y, Wang SCharacterization of adhesive penetration in wood bond by means of scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz323-330702016[16, 17], and assessing the effects of carbonisation between 200 and 600 C on wood microstructure (e.g. wall thickness) and composition ADDIN EN.CITE Xu20171273[18]1273127317Xu, D, Ding T, Li Y, Zhang Y, Zhou D, Wang STransition characteristics of a carbonized wood cell wall investigated by scanning thermal microscopy (SThM)Wood Science and TechnologyWood Science and TechnologyWood Sci. Technol.831-843512017[18]. As far as we know, scanning thermal microscopy has not been used in bamboo research before. We further conclude that scanning thermal microscopy can be a very useful technique in plant science research, including for phenotyping, or even exploring the role of fire regimes and thermal resistance as evolutionary pressures in plant traits.
Figure SEQ Figure \* ARABIC 1. Schematic of bamboos hierarchical ultrastructure. a) Bamboo is a monocot grass, typically characterised by a hollow, segmented culm. b) The culm wall is functionally-graded, with an increasing density of stiff fibre-comprising vascular bundles towards the epidermis. c) These vascular bundles are embedded in ground tissue of box-shaped parenchyma cells with thin primary cell walls (PL). d-f) The fibres have thick lignified cell walls and a polylamellar structure, which includes a primary cell wall layer, and as many as eight secondary cell wall layers (SL). A pectin-rich middle lamella (ML) adjoins fibre cells together. As it can be difficult to distinguish the ML and PL from each other, the compound middle lamella (CML) refers to the combination of ML, PL and the first layer of the secondary cell wall (S0) of each fibre cell.
Experimental methods
Materials
3-5 year old raw Moso bamboo (Phyllostachys pubescens) was obtained in whole culm form from China (supplied by UK Bamboo Supplies Limited). The bamboo culms were air-dried and sun-bleached for three weeks upon harvesting, reaching an equilibrium moisture content of 10%. As a reference material, Norway spruce (Picea abies) was obtained from BSW Timber Ltd (UK). The spruce wood was cut from flat-sawn, kiln-dried timber with an equilibrium moisture content of 12%.
Specimen preparation
To measure thermal conductivity in the longitudinal (along the stem axis) and transverse directions (perpendicular to the stem axis), cross and radial sections of length 30 mm, width 10 mm and thickness 2-8 mm, were prepared using a sharp razor blade ( REF _Ref9150095 \h Figure 2a). Only inter-nodal regions of the bamboo culm were selected. These sections were impregnated with low viscosity AGAR epoxy resin (AGAR Scientific Ltd., UK) by means of alternating vacuum-pressure treatment. These sections were then glued to 15 mm metal specimen discs for observation in scanning probe microscopy. Local roughness of the sample surface can produce artefacts in the thermal image due to an increase in the tip-sample contact area ADDIN EN.CITE McConney20101274[15, 19]1274127417McConney, ME, Singamaneni S, Tsukruk VVProbing soft matter with the atomic force microscopies: imaging and force spectroscopyPolymer ReviewsPolymer Reviews235-286502010Vay201310091009100917Vay, O, Obersriebnig M, Mller U, Konnerth J, Gindl-Altmutter WStudying thermal conductivity of wood at cell wall level by scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz155-1596722013[15, 19], hence flat surfaces are preferred. To obtain smooth sample surfaces 100 nm thick slices were taken using a Leica Ultracut-R ultramicrotome equipped with a Diatome Histo diamond knife. A more detailed methodology for specimen preparation can be found in ADDIN EN.CITE Konnerth20081271[15, 16]1271127117Konnerth, J, Harper D, Lee SH, Rials TG, Gindl WAdhesive penetration of wood cell walls investigated by scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz91-98622008Vay201310091009100917Vay, O, Obersriebnig M, Mller U, Konnerth J, Gindl-Altmutter WStudying thermal conductivity of wood at cell wall level by scanning thermal microscopy (SThM)HolzforschungHolzforschungHolz155-1596722013[15, 16].
Scanning thermal microscopy (SThM)
SThM measurements were carried out using a Bru k e r D i m e n s i o n I c o n A t o m i c F o r c e M i c r o s c o p e ( B r u k e r , U S A ) e q u i p p e d w i t h a s t a n d a r d B r u k e r S T h M p r o b e ( V I T A - D M - G L A 1 , n o m i n a l t i p r a d i u s
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