Human beings are very visual animals in that a lot of their understanding of the world around them comes from what they see.
The invention of the microscope in the early 17th Century opened up a new world, the observation of which paved the way for significant advances in biology, geology and medical science. The resolution possible with light microscopy is limited to around 200 nano metres (2×10-7 metres) because of the wavelength of light. This resolution is sufficient to understand the anatomy of wood cells, but is not sufficient to see the complex detail of the ultra-structure of the cell walls – that is, the layers of fibrils that make up the cell walls.
Higher-resolution images are possible using electron microscopes that use electrons rather than light photons to generate an image. The most common electron microscopes are the scanning type (SEM). Modern SEMs have a resolution of around 3nm, so almost 100 times better than light microscopes, and they have a much greater depth of field, giving pin-sharp, 3D-like images. However, conventional SEM techniques require specimens to be electrically conductive, which wood is not, so specimens must be coated in a thin layer of gold. In addition, the images are obtained in a vacuum chamber and this causes wood, and other organic specimens, to dry out and shrink during ‘exposure’. Therefore, one can never be sure that the image seen is a correct representation of material in its natural ‘wet’ state.
An environmental SEM (ESEM) is a relatively recent evolution of the standard SEM. It can capture images in low gas pressure environments, or partial vacuums, which reduces the drying problem, especially if combined with a stage which cools the specimen during imaging. It can also photograph non-conductive specimens. Therefore a wood sample can be inspected visually by an ESEM without any pre-treatment.
A major secondary advantage to this is that the effects of a process, eg paint application, glue mixing, chemical modification, etc can be followed. First an electron micrograph is taken of the sample before treatment and then a second after treatment. The main limitation to this is that the samples are relatively small, being only a few tens of millimetres. However, careful and ingenious experiment design can minimise these limitations.
I am delighted to say that the Ecole Supérieure du Bois (ESB) is about to buy an ESEM thanks to a local government grant of nearly 50% of the purchase price (around €350,000). This ESEM also has an EDAX attachment which will allow identification and quantification of all elements down to, and including, boron. In addition, the ESEM will have a micro-testing machine built into the chamber. Researchers at ESB will, therefore, be able to visualise, mechanically test and chemically analyse wood samples, including individual fibres, in almost a single step.
The ESEM will help our research into understanding the complex structure of wood based composites, how adhesives, paints and other additives interact with wood, and to estimate the mechanical properties of small parts, fibres and particles of composites.
