Surface tension compresses a water droplet and equivalently the effect of surface stress is to decrease the lattice parameter in metal particles. The effect of surface stress and elastic properties like Poisson’s ratio can be accentuated in lower dimensional systems like metal nanoparticles. The I.I.T. Kanpur team, using a two scale computational method and transmission electron microscopy, not only show a lattice expansion due to purely surface stress effects in a metallic system, but also discover anomalous lattice expansion in the case of very thin walled in metal nanoshells. This effect where “compression is causing expansion” has been hitherto unanticipated. The Kanpur team not only establish that this is a Poisson effect driven phenomenon and but generalize it all metal nanoshells.
Surface stress can have profound effects on nanoscale materials and can lead to a contraction of the lattice in nanoparticles to compensate for the under-coordination of the surface atoms. The effect of elastic properties like Poisson’s ratio can be accentuated in lower dimensional systems. The current study focuses on hollow metal nanoshells (MNSs), wherein there is interplay between the surface stresses existing in the inner and outer surfaces. Using a two scale computational method and transmission electron microscopy, we not only show a lattice expansion (in the radial direction) due to purely surface stress effects in a metallic system but also discover anomalous lattice expansion in the case of very thin walled MNSs. We argue that this effect, wherein the stress in the outer surface causes expansion in the radial lattice parameter (instead of compression), is a Poisson effect driven phenomenon. Although Ni nanoshells are used as an illustrative system for the studies, we generalize this effect for all metal nanoshells.
THERMAL EXPANSION AND CONTRACTION
Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled. When free to deform, concrete will expand or contract due to fluctuations in temperature. The size of the concrete structure whether it is a bridge, a highway, or a building does not make it immune to the effects of temperature. The expansion and contraction with changes in temperature occur regardless of the structure’s cross-sectional area.
Concrete expands slightly as temperature rises and contracts as temperature falls. Temperature changes may be caused by environmental conditions or by cement hydration (the exothermic chemical process in which the cement reacts with the water in a mixture of concrete to create the calcium silicate hydrate binder and other compounds). An average value for the coefficient of thermal expansion of concrete is about 10 millionths per degree Celsius (10×10-6/C), although values ranging from 7 to 12 millionths per degree Celsius have been observed. This amounts to a length change of 1.7 centimeters for every 30.5 meters of concrete subjected to a rise or fall of 38 degrees Celsius.
Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel, granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. Of these factors, aggregate type has the greatest influence on the expansion and contraction of concrete.
Severe problems develop in massive structures where heat cannot be dissipated. Thermal contraction on the concrete’s surface without a corresponding change in its interior temperature will cause a thermal differential and potentially lead to cracking. Temperature changes that result in shortening will crack concrete members that are held in place or restrained by another part of the structure, internal reinforcement or by the ground. For example, a long restrained concrete section is allowed to drop in temperature. As the temperature drops, the concrete tends to shorten, but cannot as it is restrained along its base length. This causes the concrete to be stressed, and eventually crack.
Joints are the most effective way to control cracking. If a sizable section of concrete is not provided with properly spaced joints to accommodate temperature movement, the concrete will crack in a regular pattern related to the temperature and restraint directory. Control joints are grooved, formed, or sawed into sidewalks, driveways, pavements, floors, and walls so that cracking will occur in these joints rather than in a random manner. Contraction joints provide for movement in the plane of a slab or wall, and induce cracking caused by thermal shrinkage at preselected locations. One of the most economical methods for making a contraction joint is by simply sawing a continuous cut in the top of the slab with a masonry saw.
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