Vibrations of amorphous, nanometric structures: When does continuum theory apply?

Structures involving solid particles of nanometric dimensions play an increasingly important role in material sciences. These structures are often characterized through the vibrational properties of their constituent particles, which can be probed by spectroscopic methods. Interpretation of such experimental data requires an extension of continuum elasticity theory down to increasingly small scales. Using numerical simulation and exact diagonalization for simple models, we show that continuum elasticity, applied to disordered system, actually breaks down below a length scale of typically 30 to 50 molecular sizes. This length scale is likely related to the one which is generally invoked to explain the peculiar vibrational properties of glassy systems.Comment: 4 pages, 5 figures, LATEX, Europhysics Letters accepte

"m=1" coatings for neutron guides

A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section

"m=1" coatings for neutron guides

A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section

Low temperature phase transformation of nanocrystalline tetragonal ZrO₂ by neutron and Raman scattering studies

International audienceThe structural properties of three, undoped, nanocrystalline, tetragonal zirconia powders, characterized by 6.9, 13 and 18 nm average crystallite size, respectively, have been studied at low temperature by neutron diffraction and Raman spectrometry. Raman spectrometry has shown that the 6.9 nm sample undergoes a continuous and reversible phase transition, towards the monoclinic structure below 175 K. This transformation could be at the origin of the appearance of components near 22.5 and 35 meV in the inelastic neutron spectrum when the temperature is lowered to 100 K. Such a transformation is not observed for the 13 and 18 nm samples. This transformation seems to depend on the crystallite size and on the kinetics of thermal treatments. This could explain why the transformation of the 6.9 nm zirconia could not be induced by the slow cooling and heating rates of the cryostat used in neutron diffraction. This transition has some similarities with infrared, low temperature measurements on cerium-doped zirconia reported in the literature. The neutron diffraction data have allowed the determination of thermal expansion coefficients: βa varies from 0.76 to 0.78 and βb from 0.80 to 0.91×10−5 K−1 for the 6.9 and the 18 nm zirconia, respectively