Wood Microstructure and Identification

Leah C. Frenette

University of Rochester, Department of Chemistry

Introduction

Wood is classified into hardwood and softwood based different classifications of trees on a biological level. Angiosperm trees are usually deciduous and have large pores, called vessels, which run with the grain of the wood and are used to transfer water and nutrients to different parts of the tree. This complex cellular structure creates a dense wood and has lead to angiosperms being known as hardwoods. Gymnosperm trees are conifers and mostly evergreen. Their cellular structure is less complex, resulting in softer wood.


Scanning Electron Microscopy (SEM) is able to provide high resolution, high magnification images, allowing for the imaging of features on the sub-micron scale. The scanning electron microscope exploits the interaction of an electron beam focused on a sample, providing elastic and inelastic scattering events, to create an image. Secondary electrons result from inelastic interactions of the beam with the sample. These interactions provide valuable information about the surface features of a sample. SEM images can be used to compare surface features of different samples, making it a powerful tool for comparing the microstructure of hard- and softwoods. SEM also has the ability of detecting X-rays excited from the sample through an EDAX attachment, which will be used to determine the elemental composition of the sample. In this study it will be used to determine if the samples have been treated with chemical coatings.


In this project images of hardwood and softwood at a high magnification were examined for differences between the two types of wood on the microscale. The findings were compared to known characteristics of each type of wood. Secondary electron imaging was used to image the surface features of four wood samples, two hardwood and two softwood. The samples were prepared by microtomy and sputter coated with gold to reduce charging. After imaging, some images were colorized for added effect and 3D anaglyphs were created to show depth of features. Post collection processing was done in Adobe Photoshop CS6.


Samples and Preparation




Photographs (upper) and micrographs (lower) of hardwood samples A and B and softwood samples C and D


Four unknown wood samples, two hardwood and two softwood, were collected. The samples were split into appropriately small pieces using a screwdriver and trimmed and given flat edges using The Chopper. The samples were then shaved using a Microtome to provide a clean flat surface to image. Samples were then mounted onto a SEM sample stub with a metal ring mounted on it using conductive adhesive tape and secured using conductive glue. To make the samples themselves conductive and reduce charging, the samples were sputter coated with gold for 90 seconds using a gold sputter coater.



"The Chopper" at the Institute of Optics at the University of Rochester


RMC MT6000 Microtome at the Institute of Optics at the University of Rochester


Denton Vacuum sputter coater at the Institute of Optics at the University of Rochester

Imaging and Results

Secondary Electron Imaging

Secondary Electron (SE) images were collected using the Zeiss Auriga Crossbeam SEM at the Institute of Optics at the University of Rochester. SE electron images were collected of all four samples at varying magnification, working distance, and accelleration voltage.

Hardwood Samples

Exposed canal showing a pit. (A)

Delicate microstructure. (A)


Interesting microstructure next to a resin canal. (B)

Resin canal opening. (B)



Softwood Samples

Bordered pits characteristic of pines. (C)

Slow and fast growth (left to right) (C).

rays from a transverse view (C).

X-ray Microanalysis

X-ray microanalysis was conducted using the Zeiss Auriga Crossbeam SEM at the Institute of Optics at the University of Rochester. X-ray measures the amount of x-ray signal generated from the electron beam interacting with the sample. The Zeiss Auriga Corssbeam SEM is equipped with an energy dispersive x-ray spectrometer consisting of a solid-state detector mounted inside the vacuum chamber at as close to perpendicular to the sample plane as possible. X-ray microanalysis was conducted on each of the four different wood samples. It was hypothesized that the samples may have been treated with a chemical coating that would be detectable by X-ray, as can be seen from the resulting micrographs this was not the case. It did confirm, however, that the samples are indeed organic materials.



EDAX Analysis for samples A,B,C, and D.

Anaglyphic Image Creation

To create a 3D anaglyph, two secondary electron micrographs were collected of the sample, differing only by a tilt of approximately two degrees. Superposition of the images and colorization using Adobe Photoshop CS 6 produced the final anaglyphic image. Secondary Electron images were collected using the Zeiss Auriga Crossbeam SEM at the Institute of Optics at the University of Rochester.

An interesting surface structure.

Resin canal opening.

Image Colorization


Adobe Photoshop CS6 was used for image colorization.

 

 

 

Electron Flight Simulation

Electon flight simulation was usperfomeding EDS Software owned by the Institute of Optics at the University of Rochester. The electron trajectories were simulated for bulk organic material (wood) at 5kV and 20kV accelleration voltage. The higher accelleration voltage electrons penetrate deeper within the surface of the material, meaning that the there are less secondary electrons generated from the surface.

Conclusions

Scanning Electron Microscopy is a powerful tool for imaging and understanding the world on the microscale. As a scientific tool it can be used to obtain qualitative as well as quantitative information about structures that are unable to be seen by eye or with light microscopes. As an art form it provides high magnification and high-resolution images of surface structures that are exotic to the general public because it is such a specialized tool. In this study it was able to create beautiful images of surface structures unique to hardwood and softwood tree species.

Acknowledgements

I would like to thank Brian McIntyre for his instruction, dedication, help and support throughout the duration of this semester, and Diana Olvera for her time and energy serving as a TA for this course. I would also like to thank Amanda Amori for encouraging me to take this course and for being an amazing person to share a cubicle with, and the best graduate school support system a girl could ask for. Big thanks to Steven Kean for the wood samples; I literally could not have done this project without them! I would also like to thank my Dad, Donald Frenette, for sharing his love of trees and tree puns with me throughout this investigation, and for supporting me through all of my endeavors.

References and Further Reading:

Egerton, R.F. Physical Principles of Electron Microscopy. Springer Science+Business Media, Inc., New York, 2005.

Core, Cote and Day. Wood Structure and Identification. Syracuse Wood Science Series 6. 1979.

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