A CO2-laser-based photoacoustic spectrometer was used to determine the temporal concentration profile of atmospheric ethene in Mexico City. Ethene measurements were conducted at the facilities of our institute, which is located in the north of the city and next to an avenue with heavy traffic density. Ambient air from outside our laboratory was continuously pumped into the spectrometer. This campaign was performed for 24 h a day, from November 24–30, 2001. The maximum ethene levels ranged between 26 and 81 ppbV. As expected, the lowest concentrations were monitored on weekends. These data were analyzed in combination with ozone and nitrogen oxides profiles, which were permanently monitored by an air-pollution-monitoring government network. Information on the seasonal variability of ethene was obtained by comparing the results of this campaign with data obtained in the winter of 2001. In general, the ethene concentration in November was about 30% higher than in February. On weekdays, the mean dose of human...

Atmospheric pollution profiles in Mexico City in two different seasons

Nanostructured materials: basic concepts and microstructure☆

Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling1,2,3,4. The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress ofthe material typically increase with decreasing grain size, a phenomenon known as the Hall–Petch effect5,6. Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall–Petch effect3,7) for the smallest sizes. Most of the plastic deformation is due to a large number of small ‘sliding’ events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening will ultimately impose a limit on how strong nanocrystalline metals may become.

Softening of nanocrystalline metals at very small grain sizes

The 23-volume "Encyclopedia of Applied Physics" seeks to demonstrate the synergy between physics and technological applications. Information is presented according to the following subject areas: general aspects; mathematical and information techniques; measurement sciences, general devices and/or methods; nuclear and elementary particle physics; atomic and molecular physics; electricity and magnetism; optics (classical and quantum); acoustics; thermodynamics and properties of gases; fluids and plasma physics; condensed matter - structure and mechanical properties; thermal, acoustic, and quantum properties; electronic properties; magnetic properties; dielectrical and optical properties; surfaces and interfaces; materials science; physical chemistry; energy research and environmental physics; biophysics and medical physics; geophysics, meteorology, space physics and aeronautics.

Steven J. Rothman

Papers

Encyclopedia of Applied Physics