La-Doped 0.3PZN-0.7PZT Ceramics with Pure Perovskite Phase Synthesized
- PDF / 1,379,452 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 6 Downloads / 205 Views
ness of the TeSW-prepared p-Si is very thin and the rms roughness is only 1.6 nm. The researchers also performed a controlled experiment of p-Si formation using NaOH etchant at various pH levels, but they observed no efficient PL emission from any of the NaOH-prepared samples. The researchers pointed out that the reproducibility appeared to be a problem for the photoetched p-Si synthesis technique. They point out that others have suggested that the preexisting state of the Si surface plays a decisive role in determining p-Si formation. TAO XU
High Frictional Anisotropy Found on Quasicrystal Surface A collaboration of scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) and the Ames Laboratory at Iowa State University have determined that friction along the surface of a quasicrystal in the direction of a periodic geometric configuration is about eight times greater than in the direction where the geometric configuration is aperiodic (i.e., without regularity). In an article published in the August 26 issue of Science (p. 1354), the research collaboration led by Miquel Salmeron, a physicist with Berkeley Lab’s Materials Sciences Division, and Patricia Thiel, a chemist in Ames Lab, reports that geometric periodicity was confirmed by rows of atoms that formed a Fibonacci sequence, a numerical pattern often observed in quasicrystals. The research combined the use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to observe the surface structures and measure the frictional forces. “That we can get such a large difference in frictional force just by scratching the surface of a material in a different direction was a major surprise,” said Salmeron. “Our results reveal a strong connection between interface atomic structure and the mechanisms by which frictional energy is dissipated.” At the atomic level, when two surfaces come in contact, the chemical bonds and clouds of electrons in their respective atoms create frictional force and cause energy to be dissipated. It has long been known that friction is greater between surfaces of identical crystallographic orientation than between surfaces of differ-
ing orientation. Some recent studies have reported frictional differences, or anisotropy, less than a factor of 3 for incommensurate crystal surfaces when there were periodicity differences. To measure the frictional effects due to periodicity alone and not to other factors such as chemical differences, the researchers worked with decagonal quasicrystals of an aluminum-nickel-cobalt (AlNi-Co) alloy. Stacked planes of Al-Ni-Co crystals exhibit both tenfold and twofold rotational symmetry. By cutting a single Al-Ni-Co quasicrystal parallel to its tenfold axis, the researchers were able to produce a two-dimensional surface with one periodic axis and one aperiodic axis, separated by 90°. In order to avoid surface damage and also wear of the tip used in the combined AFM–STM instrument, a TiN tip passivated with a molecular layer of hexadecane thiol was used. “Strong friction anisotropy was o
Data Loading...
