Vibrational properties of the LiNbO3 z-surfaces

The existence of localized vibrational modes both at the positive and at the negative LiNbO3 (0001) surface is demonstrated by means of first-principles calculations and Raman spectroscopy measurements. First, the phonon modes of the crystal bulk and of the (0001) surface are calculated within the density functional theory. In a second step, the Raman spectra of LiNbO(3) bulk and of the two surfaces are measured. The phonon modes localized at the two surfaces are found to be substantially different, and are also found to differ from the bulk modes. The calculated and measured frequencies are in agreement within the error of the method. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. It represents therefore an alternative method to determine the surface polarity, which does not exploit the pyroelectric or piezoelectric properties of the material.     

Fatigue properties of wood and wood composites

An overview of the state of fatigue research for wood and wood products is presented. The extreme lack of a satisfactory data base and the persistent neglect of fatigue characteristics in wood material design is emphasized. Some theoretical considerations are presented to point out the difficulties met when attempting to model the behavior of wood products exposed to cyclic loading phenomena.A major conclusion of the work is the need for the entire materials science community to consider wood as a valuable and rewarding material upon which to focus research efforts. As wood is the world's primary renewable structural resource, a concerted effort must be made to understand its fatigue performance and the degree of accuracy in predicting life expectancy.

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Résumé On présente une revue générale des recherches dans le domaine de la fatigue du bois et des produits à base de bois. On fait remarques l'extrème pauvreté de bases de données satisfaisantes et la négligence persistante avec laquelle les caractéristiques de fatigue sont prises en considération dans la conception à partir de matériaux en bois. On présente quelques considérations théoriques exprimant les difficultés rencontrées lorsqu'il s'agit de modéliser le comportement des produits ligneux exposés au phénomène de mise en charge cyclique.Une conclusion importante du travail vise à sensibiliser la communauté des sciences des matériaux à considérer le bois comme un matériau valable et prometteur sur lequel un effort de recherche doit être concentré. Comme le bois est la ressource structurelle première entièrement renouvelable de ce monde, un effort concerté doit être entrepris afin de comprendre ses performances à la fatigue et le degré d'exactitude des prévisions de vie.

     

Rheological behaviour of wood in directions perpendicular to the grain

The creep of wood in the grain direction is of major importance when deflections of structures are considered. Less studied is the creep in directions perpendicular to the grain, which is of interest when drying stresses and the cracking of wood are analysed. Experimental results obtained during 1987–1991 are reported. These included tensin, bending and some compression tests under constant and varying humidity at temperatures of 20–80°C. The results indicate that mechano-sorptive creep is still more important in directions perpendicular to the grain than in the longitudinal direction. Creep is also greatly dependent on the temperature.

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Resume Le fluage du bois dans la direction du grain est de première importance quand on examine les flèches des structures. Moin étudié, le fluage dans des directions perpendiculaires est intéressant pour l’analyse des contraintes et de la fissuration du bois. On rend compte ici des résultats expérimentaux obtenus au VTT entre 1987 et 1991: essais en traction, en flexion et en compression avec une teneur en humidité constante et variable à des teneur en humidité constante et variable à des températures de 20 à 80°C. Les résultats indiquent que le fluage mécano-sorptif est encore plus important dans des directions perpendiculaires au grain que dans la direction longitudinale. Le fluage varie aussi beaucoup avec la température.

     

Cu diffusion along Al grain boundaries

Cu grain boundary diffusion (GBD) in Al was investigated by EPMA in temperature range from 300 to 400 °C.The triple product sδD_gb (s—segregation coefficient, δ—grain boundary width, D_gb—GBD coefficient) was calculated due to Fisher-Gibbs solution using two methods—measuring of GB concentration in dependence on penetration depth and the contour angle (at the top of diffusion wedge). In the first case ; in the second .     

Vibrational properties of silicene and germanene

The structural and vibrational properties of two-dimensional hexagonal silicon (silicene) and germanium (germanene) are investigated by means of first-principles calculations. It is predict that the silicene (germanene) structure with a small buckling of 0.44 Å (0.7 Å) and bond lengths of 2.28 Å (2.44 Å) is energetically the most favorable, and it does not exhibit imaginary phonon mode. The calculated non-resonance Raman spectra of silicene is characterized by a main peak at about 575 cm^?1, namely the G-like peak. For germanene, the highest peak is at about 290 cm^?1. Extensive calculations on armchair silicene nanoribbons and armchair germanene nanoribbons are also performed, with and without hydrogenation of the edges. The studies reveal other Raman peaks mainly distributed at lower frequencies than the G-like peak which could be attributed to the defects at the edges of the ribbons, thus not present in the Raman spectra of non-defective silicene and germanene. Particularly the Raman peak corresponding to the D mode is found to be located at around 515 cm^?1 for silicene and 270 cm^?1 for germanene. The calculated G-like and the D peaks are likely the fingerprints of the Raman spectra of the low-buckled structures of silicene and germanene.      

Vibrational movement of a pair of piezoelectric oscillators along a string

The behavior of a pair of synchronously excited piezoelectric oscillators suspended on a horizontal metal string has been studied. It is established that, at certain frequencies of excitation, the oscillators move consistently at a constant velocity along the string and induce oscillations in the form of a standing transverse wave in a part of the string between the points of suspension.     

Viscoelastic properties of wood across the grain measured under water-saturated conditions up to 135 °C: evidence of thermal degradation

In this paper, the viscoelastic properties of wood under water-saturated conditions are investigated from 10 to 135 °C using the WAVE^T apparatus. Experiments were performed via harmonic tests at two frequencies (0.1 and 1 Hz) for several hours. Four species of wood were tested in the radial and tangential material directions: oak (Quercus sessiliflora), beech (Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When the treatment is applied for several hours, a reduction of the wood rigidity is significant from temperature values as low as 80–90 °C and increases rapidly with the temperature level. The storage modulus of oak wood is divided by a factor two after 3 h of exposure at 135 °C. This marked reduction in rigidity is attributed to the hydrolysis of hemicelluloses. The softening temperature of wood is also noticeably affected by hygro-thermal treatment. After three short successive treatments up to 135 °C, the softening temperature of oak shifted from 79 °C to 103 °C, at a frequency of 1 Hz. This reduction in mobility of wood polymers is consistent with the condensation of lignins observed by many authors at this temperature level. In the same conditions, fir exhibited a softening temperature decrease of about 4 °C. In any case, the internal friction clearly rises.     

Theory and practice in measuring wood grain angle using microwaves

Knowledge of wood grain inclination is critical for achieving improved lumber strength grading. This paper describes an instrumentation system and theory for using microwaves to measure wood grain angle in real time. By using an electrically modulated scattering dipole with a homodyne microwave system, it is found both theoretically and experimentally that the amplitude reading of the modulated scattered signal provides an excellent means far measuring wood grain angle. In addition, the phase output of the system can be used to identify the sign of the grain angle. Two widely used wood species, Douglas Fir and Spruce, are measured at different moisture levels to verify the theoretical prediction experimentally. Preliminary data analysis shows that reliable measurement of grain angles up to about 30° can be made for wood within a wide range of specific gravity and moisture content. Through the experimental measurements, the potential of such a system for industrial use is demonstrated     

Dielectric properties of chemically treated wood

Dielectric properties along the grain for absolutely dried untreated and seven kinds of chemically treated Sitka spruce (Picea sitchensis Carr.) woods were measured. Cole-Cole's circular arc law was applied to the results of the relaxation due to the motions of methylol groups. The following changes were caused by chemical treatments. In polyethylene glycol (PEG) impregnation, the distribution of relaxation times became very narrow, the generalized relaxation time (_m) was considerably decreased, and the relaxation magnitude (₀ – ) was slightly increased. In acetylation, the distribution of relaxation times became very broad, _m was considerably increased, and (₀ – ) was remarkably decreased. In propylene oxide treatment, the distribution of relaxation times became slightly narrow and _m was decreased. _m was slightly decreased in formalization, phenol-formaldehyde (PF) resin treatment and wood methyl methacrylate (MMA) composite. (₀ – ) was decreased in formalization and PF-resin treatment and was hardly changed in wood-MMA composite and heat treatment. The distribution of relaxation times was almost unchanged in formalization, PF-resin treatment, wood-MMA composite and heat treatment.     

Effect of various surface modifications of wood flour on the properties of PP/wood composites

Four different approaches were used for the modification of interfacial interactions in polypropylene (PP)/wood flour composites. We compare the effect of maleated polypropylene (MAPP), two surfactants (stearic acid and cellulose palmitate) and the chemical modification of wood (benzylation) on interfacial adhesion, homogeneity, processability and water absorption. Interfacial adhesion and reinforcement improves upon the addition of a maleated polymer as expected. Non-reactive surface modification leads to a moderate decrease of interaction, while benzylation decreases interfacial adhesion quite considerably. MAPP does not influence any other property of interest; homogeneity, viscosity and water absorption remain practically unchanged independently of the amount of coupling agent used. Surfactants improve homogeneity and processability, while the chemical modification of wood by benzylation decreases water absorption significantly. The results clearly prove that the proper selection of the approach and level of surface modification may lead to considerable improvement in targeted properties.     

Surface modification of wood flour and its effect on the properties of PP/wood composites

The surface of wood flour used as reinforcement in PP/wood composites was successfully modified by benzylation in NaOH solution of 20 wt% concentration at 105 °C. The time of the reaction was changed between 5 and 360 min in several steps. The progress of modification was followed by the measurement of weight increase and by diffuse reflectance infrared spectroscopy (DRIFT). The structure of the wood was characterized by X-ray diffraction (XRD) and its surface tension was determined by inverse gas chromatography (IGC). PP composites containing 20 wt% filler were prepared from a PP block copolymer and the modified wood flour. The mechanical behavior of the composites was characterized by tensile testing. The majority of the active hydroxyl groups at the surface were replaced by benzyl groups in about 2 h under the conditions used. Further increase in reaction time did not influence the properties of the filler. Both the structure of the wood flour and its surface tension changed as an effect of modification. The reduction of surface tension led to significant changes in all interactions between the wood flour and other substances resulting in a considerable decrease of water absorption, which is the major benefit of this modification. All measured mechanical properties of the composites decreased slightly with increasing degree of modification. A detailed analysis of the results proved that the dominating micromechanical deformation process of these PP/wood composites is debonding, which is further facilitated by the decrease in the surface tension of the filler. Chemical modification of wood flour slightly improved processability and the surface appearance of the composites prepared with them and considerably decreased the water absorption of these latter.     

Influence of moisture on the vibro-mechanical properties of bio-engineered wood

In this study, changes in the vibro-mechanical properties of fungi-treated wood, during sorption and desorption at different humidity levels, were investigated. Norway spruce resonance wood (with uniform narrow annual rings and high tonal quality for musical instrument craftsmanship) was incubated with Physisporinus vitreus for 36 weeks. Stiffness, internal friction, and tonal performance indices of control (untreated) and fungi-treated wood were compared after exposure to a stepwise variation of relative humidity. It was demonstrated that fungal treatment increased the internal friction and decreased the specific modulus of elasticity, during reduction of wood density. Internal friction of both control and fungi-treated wood significantly increased during dynamic sorption, especially during early stages (hours) of each humidity change step. Both specific modulus of elasticity and internal friction showed a hysteretic behavior during humidity variation cycles. Hysteresis was smaller in fungi-treated wood. Also, tonal performance indices were improved after fungal treatment and showed a reduced variation at different relative humidity conditions. Dynamic vapor sorption tests and FT-IR microscopy studies revealed changes in hygroscopicity and the supramolecular structure of wood, which may explain the observed vibrational behavior. Less dependency of wood vibrational properties to the variation of the ambient humidity is important for the acoustic performance of string instruments.     

Viscoelastic properties of the matrix substance of chemically treated wood

The temperature variations of the storage modulus and the loss tangent along the grain for four kinds of chemically modified Sitka spruce (Picea sitchensis Carr.) woods were measured at 11 Hz over a temperature range of –150 to 200 °C. By using a cell-wall model in which the amorphous matrix substance is disposed parallel to the axis of cellulose fibrils inclined at an angle to the grain direction of the wood, the storage modulus, Em, and the loss tangent, tan m of the matrix substance were estimated, and the relaxation processes detected were discussed. In formalization, the restriction of the micro-Brownian motion of the main chains due to oxymethylene bridges between the hydroxyl groups resulted in a decrease in tan m above 0 °C. In acetylation and propylene oxide treatment, a marked reduction in Em was observed over the temperature range tested, by the introduction of bulky side chains, and the tan m remarkably increased in the high-temperature range. In polyethylene glycol (PEG) impregnation, the Em increased below 20 °C due to the freezing of the micro-Brownian motion of PEG molecules in the cell lumens as well as in the cell walls, while it was reduced above this temperature by the melting of PEG molecules.     

木粉含量对木粉/酯化淀粉/聚乳酸复合材料性能影响

以甘油为相容剂,利用木粉、马来酸酐酯化淀粉和聚乳酸(PLA)进行熔融挤出制备了木粉/酯化淀粉/聚乳酸复合材料。利用XRD和SEM对复合材料的结晶度和断面形貌进行分析表征,以研究木粉含量对复合材料界面相容性的影响;并对复合材料的热稳定性、力学性能、流变性能以及吸水率进行表征。实验结果表明,随着木粉含量的增加,复合材料的界面相容性下降,拉伸强度和弯曲强度增大,断裂伸长率下降,吸水率逐渐增大;TGA测试结果表明木粉的加入使材料的热稳定性下降;流变测试表明木粉用量的增加,使复合材料的储能模量、损耗模量和复数粘度逐渐增加。     

Mechanical properties of wood examined by semi-destructive devices

This study presents a comparative evaluation of semi-destructive and destructive testing of wood. Semi-destructive methods are represented by screw-withdrawal test, microdrilling, pin penetration and a new construction of a diagnostic device for in situ evaluation of timber based on the principle of measuring the mechanical resistance to pin being pushed gradually into wood. In order to ascertain the differences between the methods, measuring of three basic softwood species was conducted. One of the aims was to capture the variability of properties brought about by the distribution of properties along the diameter and along the trunk (section) length; therefore, an entire trunk of spruce, fir and pine were used for testing. The accuracy of the prediction of mechanical properties provided by the semi-destructive devices was verified using correlations with mechanical properties established by standard pressure tests in compliance with European norms. The results show a very good correlation of the mean force necessary to push the pin in the timber with wood density and strength in compression along the grain found in standard specimens.     

Vibrational properties of single-wall carbon nanotubes: a first-principles study

We present first-principles pseudopotential-based density functional theory (DFT) calculation of structures, full phonon dispersions and thermal properties of armchair single wall armchair carbon nanotubes (SWCNTs) in the isolated and bundle forms. Comparison between the properties of isolated and bundled nanotubes is used to estimate the intertube interaction. We determine correlation between vibrational modes of a graphene sheet and of the nanotube to understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum. The radial breathing mode hardens with increasing diameter (or decreasing curvature). We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.