engleski

Application of the esem technique in wood research

An extensive study using the ESEM (Environmental Scanning Electron Microscopy) technique has been performed on wood objects in order that the particular advantages, possibilities and limitations of this microscopic tool are being assessed. The aim of this report is to present the valuable experience gained in optimizing operational and imaging conditions in ESEM during studies on wood and painted wood. This set of guidelines and instructions, along with the thorough and critical literature review, should help other researchers to utilize ESEM in more effective way for analysis of wood and other non-conductive polymeric materials, particularly of biological origin. In contrast to high-vacuum microscopy, ESEM is a relatively new development stage in electron microscopy, featuring a physical system in which electrons need not travel through vacuum in the chamber, but through the gaseous atmosphere which usually consists of water vapour. This enables the observation of non-conductive, polymeric, composite and porous materials (such as wood or painted wood) in their natural state, without drying, evacuating or sputtering them with a layer of carbon or metal (Au, Ag, Pt). Further advantages include observations of the object in a wide range of temperatures (up to 1000 °C), dynamic processes such as condensation, freezing and thawing of the specimen during observation (shown in this presentation), mechanical testing and other. All these options open extraordinary possibilities in microscopy that offers three-dimensional visual impression during micro-structural analysis of wood. The imaging quality of ESEM for natural samples, however, is inferior to that of classical SEM (C-SEM), especially at magnifications greater than 2000 times. Furthermore, the process of acquiring an image is much more complex in ESEM than in CSEM, demanding the optimisation of a number of interacting parameters. These include the physical conditions of the specimen, conditions of the chamber environment and electronic parameters of the formation and optimisation of the image. This poses significant demands on the knowledge and skills of the microscope operator, both about the material characteristics and microscopic routines. The analysis often requires time-consuming procedures and benefits from co-operation between the microscope operator and material scientist who supplies the object, especially in conducting dynamic, in-situ experiments within the chamber. The work on the ESEM can be performed through several operational modes, one of which is a classical (or high vacuum) mode. The environmental modes embrace a low vacuum (LV) mode, low vacuum – short distance (LVSD) mode and wet or ESEM mode. These operational systems offer various sets of environmental and imaging conditions and should be differentiated in order that optimum imaging results are obtained for a given wood object. Wood is a very difficult material for microscopic observation because of its variable chemical composition, great structural diversity, high porosity and irregular conductivity. ESEM analysis of wood requires that the essential properties of the object are known and the operational mode chosen accordingly. The report presents the analysis of interactions between influential operational and imaging parameters for the work with wood, painted wood and similar materials. The presentation also brings the observation of various painted wood specimens, which are to serve as guidelines for other ESEM operators and material scientists. A list of possibilities for further work indicates that the range of objects and possible applications of ESEM technique exceeds by far the present scale of implementation and potentials of this versatile and powerful tool in wood research.

wood; SEM; ESEM; wood structure; nanostructure; fractography

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