Surface checking of wood is increased by photodegradation caused by ultraviolet and visible light
The aim of this research was to test the hypothesis that exposure to solar radiation increases the checking of wood exposed to the weather, and to examine the causes and spectral dependency of such an effect. Lodgepole pine decking samples were exposed outdoors under filters, which blocked selected regions of the solar spectrum while allowing other weathering factors to act on samples. Surface checking in samples was quantified after 12, 24 and 36 weeks of exposure, and the chemical and micro-structural changes occurring at weathered wood surfaces were examined. Check numbers and dimensions were greater in samples exposed under a filter to the full solar spectrum than in samples exposed under filters that blocked the transmission of UV, visible or infrared radiation. Samples that were shielded from more energetic wavelengths developed less checking and also showed less delignification at the exposed wood surfaces. Checks developed at the margins of rays and propagated at the interface between adjacent tracheids, close to the middle lamella. We conclude that exposure to UV and visible light increases the tendency of wood to check during exterior exposure. Our findings point to a link between changes in cell micro-structure as a result of photodegradation of lignin and the development of visible checks in wood exposed outdoors.
Spectral sensitivity in the photodegradation of fir wood (Abies alba Mill.) surfaces: colour changes in natural weathering
Activation spectra of wood under natural irradiation were investigated in detail in this work. The main purpose was to study colour changes on the wood surface over time and into the depth during natural light exposure and thus to further contribute to the optimization of surface-protecting treatments. In a natural weathering test, three 80-μm-thick strips of fir wood forming the surface layer of a wood composite were exposed to light under a series of glass cut-off filters. Samples were withdrawn at intervals and tested for colour changes. Identification of the most detrimental wavebands of light causing photodegradation was performed based on recorded colour changes. With chronological development of exposure, the colour changes shifted ever deeper into the surface and further into the visible region of the spectrum. A relatively narrow waveband from 360 to 435 nm was identified in the activation spectra to be the most active band, causing the greatest proportion of recorded colour changes. However, also visible light of wavelengths up to 515 nm significantly contributed to colour changes of the surface layers.
Refinement of crystal structure parameters using convergent‐beam electron diffraction: the low‐temperature phase of SrTiO3
The method to refine crystal structure parameters (atom positions and Debye–Waller factors) using convergent-beam electron diffraction (CBED), which is applicable to crystal structure analysis of a small specimen area down to a few nm in diameter, is studied. The line profiles of higher-order Laue-zone reflections are recorded using imaging plates. Theoretical intensities are calculated based on the dynamical theory of electron diffraction with the aid of the generalized Bethe approximation to shorten calculation time. The structural parameters are determined by fitting the experimental profiles with the theoretical ones using a nonlinear least-squares method. The present method has been applied to the low-temperature phase of SrTiO3. The structural parameter or the rotation angle of the oxygen octahedron has been determined to be \varphi = 1.12 (4)°, which shows good agreement with that obtained from electron spin resonance experiments.