Comparaison entre les solutions solides PLZT et les materiaux PZT bisubstitues

A systematic study of PLZT materials with composition close to the rhombohedral-tetragonal phase transition boundary at room temperature showed that piezoelectric properties of such materials are comparable with those exhibited by PZT with similar composition but containing two substituents. The best results were obtained with 6% lanthanum and 58% zirconium samples.     

The morphology of isomorphous substituted hectorites

Small-angle scattering (SAS) is used to investigate geometric structure parameters of isomorphically substituted hectorites. The aluminum content, the layer charge, and the purity of the substituted hectorites are related to the Al supply. The structural information contained in the scattering curves I(h) about the morphology of the samples in four different range orders from 1 nm to 300 nm is analyzed using the theory of the chord length distribution. The geometric structure parameters obtained are compared with results from XRD (L ₀-parameter), N₂-adsorption (L ₁ and mesopore size) and photon-correlation spectroscopy (largest dimension L ₃ and characteristic dimension of the secondary particles). Accepted after revision: 13 November 2001 Electronic Publication     

Gosteli-Claisen rearrangement of propargyl vinyl ethers: cascading molecular rearrangements

The ambivalent reactivity of 2-alkoxycarbonyl-substituted propargyl vinyl ethers has been explored. Depending on the conditions, the catalyzed and uncatalyzed Gosteli-Claisen rearrangement triggers downstream transformations that cascade from initially formed γ-allenyl α-keto esters to highly substituted furanes and cyclopentenes. In support of a mechanistic hypothesis, the results of a DFT study using the B1B95 and B3LYP functionals are revealed.     

Pseudomorphy, surface alloys and the role of elementary clusters on the domain orientations in the Cu/Al13Co4(100) system

We have used the pseudo-tenfold surface of the orthorhombic Al(13)Co(4) crystal as a template for the adsorption of Cu thin films of various thicknesses deposited at different temperatures. This study has been carried out by means of low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). From 300 to 573 K, Cu adatoms grow pseudomorphically up to one monolayer. At 300 K, the β-Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the β-Al(Cu, Co) phase further transforms into the γ-Al(4)Cu(9) phase. Both β and γ phases grow as two (110) domains rotated by 72° ± 1° from each other. Instead of following the substrate symmetry, it is the orientations of the bipentagonal motifs present on the clean Al(13)Co(4)(100) surface that dictate the growth orientation of these domains. The initial bulk composition and structural complexity of the substrate have a minor role in the formation of the γ-Al(4)Cu(9) phase as long as the amount of Al and the Cu film thickness reach a critical stoichiometry.     

Novel tocopherol derivatives. Part 32: On the bromination of pyrano[3,2-f]chromenes related to γ-tocopherol

While bromination of γ-tocopherol (2) with elemental bromine affords 5-bromo-γ-tocopherol quantitatively (3), the analogous reaction of its truncated model compound, 2,2,7,8-tetramethylchromanol (2a) is known to be accompanied by side reactions and to produce hitherto unknown byproducts. These compounds originate from pyrano[3,2-f]chromene (6), a byproduct in the synthesis of model compound 2a, which affords bromochromene 7 as the major product. The reaction mechanism was shown to proceed via chromene 8 and its 1,2-dibromo addition compound 9, which eliminates HBr in an E1 process to finally afford 7. Analytical data including crystal structures of both 6 and 7 are reported.     

Isolated Pd Sites on the Intermetallic PdGa(111) and PdGa($\bar 1$$\bar 1$$\bar 1$) Model Catalyst Surfaces

Scratching the surface: The high selectivity of PdGa catalysts towards the partial hydrogenation of acetylene was previously attributed to the separation of Pd atomic sites at the surfaces. The atomic structures of PdGa surfaces are determined by means of a combined experimental and computational approach, allowing investigation of the catalytic dissociation of hydrogen. The two opposite (111) and ${{\left( {\bar 1\bar 1\bar 1} \right)}}$ surfaces show a very different arrangement of catalytic centers.     

Gasanalyse

Ohne Zusammenfassung     

Equivariant pretheories and invariants of torsors

In the present paper we introduce and study the notion of an equivariant pretheory (basic examples are equivariant Chow groups of Edidin and Graham, Thomason??s equivariant K-theory and equivariant algebraic cobordism). Using the language of equivariant pretheories we generalize the theorem of Karpenko and Merkurjev on G-torsors and rational cycles. As an application, to every G-torsor E and a G-equivariant pretheory we associate a ring which serves as an invariant of E. In the case of Chow groups this ring encodes the information about the motivic J-invariant of E, in the case of Grothendieck??s K ₀ indexes of the respective Tits algebras and in the case of algebraic cobordism ?? it gives a quotient of the cobordism ring of G.