Crystals of urea have been grown by physical vapour transport (PVT) in semi-open (effusive) cells. The reason why effusive cells are used is in relation with the particular vaporisation scheme of ...urea, which is briefly discussed. X-ray topography (XPT) and diffraction experiments point to a very high structural quality, as demonstrated by the absence of extended defects and narrow Bragg peak. However, dark and bright bands parallel to the c-axis, whose nature is not yet understood, have been revealed. Optical characterisation has also been performed by adsorption spectra in the near-IR-visible-near-UV. The results put in evidence intrinsic absorption in the near IR region and possible defect (impurity)-related transparency losses whose distribution is not homogeneous through the sample.
A new growth method, based on a vapour‐liquid‐solid (VLS) mechanism, is reported for preparing single crystals of N‐methylurea (NMU), a material which appears to be a good alternative to urea for ...non‐linear optical applications in the “near UV‐visible” region of the spectrum. Details of the growth procedure are given and it is shown that large single crystals, with volumes up to 7 ÷ 10 cm3, can be obtained with satisfactory reproducibility and very fast growth rate. Structural and optical characterisation, still preliminary, are reported, which evidence a crystalline quality comparable to that of urea and N‐methylurea as previously grown with other growth techniques. Cryst. Res. Technol., Vol. 32, No. 1.
In this paper, we consider a geometric formalism for optimal control of underactuated mechanical systems. Our techniques are an adaptation of the classical Skinner and Rusk approach for the case of ...Lagrangian dynamics with higher-order constraints. We study a regular case where it is possible to establish a symplectic framework and, as a consequence, to obtain a unique vector field determining the dynamics of the optimal control problem. These developments will allow us to develop a new class of geometric integrators based on discrete variational calculus.
Référence bibliographique : Robert de Cotte, 130
Référence bibliographique : Fossier, 358
Appartient à l’ensemble documentaire : Des18Cotte
Plan au sol
Bibliographical Reference: Robert de Cotte, 130
...Bibliographical Reference: Fossier, 358
Part of the documentary ensemble: Des18Cotte
Ground plan
Référence bibliographique : Robert de Cotte, 130
Référence bibliographique : Fossier, 358
Appartient à l’ensemble documentaire : Des18Cotte
Plan au sol
Bibliographical Reference: Robert de Cotte, 108
Bibliographical Reference: Fossier, 358
Part of the documentary ensemble: Des18Cotte
Mass plane
Référence bibliographique : Robert de Cotte, 108
...Référence bibliographique : Fossier, 358
Appartient à l’ensemble documentaire : Des18Cotte
Plan de masse
Référence bibliographique : Robert de Cotte, 108
Référence bibliographique : Fossier, 358
Appartient à l’ensemble documentaire : Des18Cotte
Plan de masse
