Literatura científica selecionada sobre o tema "973.8/092 b"

Room-temperature single-crystal X-ray structure determinations carried out on ‘maximally’ hydrated rare earth(III) trifluoroacetates, Ln(tfa)3.x H2O, crystallized at room temperature, show the Ln = La, Ce adducts to be isomorphous and monoclinic, P 21/c, a ≈ 11·9, b ≈ 12·8, c ≈ 9·8 8 Å, β ≈ 103·7°, Z = 4; they are trihydrates. The Ln = Pr, Lu (and, implicitly, intermediate Ln) adducts are also monoclinic, P 21/c, Z = 4, and trihydrates, but of a different polymorph, with a ≈ 9·2, b 18·8, c ≈ 9·8 Å, β ≈ 114°. For the four determinations, conventional R values on |F| were 0·038, 0·032, 0·036, and 0·034 for No 2952, 4821, 4544, and 4092 independent ‘observed’ (I > 3σ(I)) diffractometer reflections respectively. The Ln = La, Ce adducts are two-dimensional polymers, the sheets parallel to the bc plane; the other systems are binuclear, the two metal atoms being linked by four bridging carboxylate O-tfa-O′ ligands. In both structural types, the metal atoms are eight-coordinate, but differ in the number of water molecules (2 cf. 3) in the O8 array. Extension of previous studies by single-crystal X-ray methods on the structural characterization of trivalent rare earth trichloroacetates, ‘maximally’ hydrated at local ambience, Ln(tca)3.x H2O, suggests the following arrays to be prevalent. The Ln = La adduct is a pentahydrate, monoclinic, P21/c, a 5·636(7), b 22·454(4), c 16·58(1) Å, β 90·52(8)°, Z = 4 f.u., R 0·035 for No 4154. The compound is a linear polymer along a, successive nine-coordinate La (separated by a) being linked by three O-tca-O′ bridging ligands at the opposite faces of a tricapped trigonal prismatic array, the equatorial sites being filled by water molecules. The Ln = Ce adduct is a trihydrate, monoclinic, P 21/c, a 10·071(2), b 22·973(2), c 20·222(5) Å, b 119·48(2)°, Z= 8 f.u., R 0·050 for No 5019. The array is also linear polymeric, but with successive Ce being linked alternately now by sets of two and then four O-tca-O′ bridging carboxylates along b, the Ln = Ce coordination number being diminished (relative to La) to eight with the coordination of two water molecules to each metal. Ln = Pr, Lu (and, presumptively, intermediate Ln) are dihydrates, triclinic, P 1, a ≈ 11·70, b ≈ 12·8, c ≈ 15·3 Å, α ≈ 71, β ≈ 77·85, γ ≈ 65·5°, Z = 4 f.u., R 0·056, 0·059 for No 5650, 5398. The array is a linear polymer, similar to that of the Ln = Ce adduct but alongside the bridging acetate pair one of the water molecules now bridges, resulting in a stepped Ln 1 array (along c) rather than a quasi-straight one as is found for the Ln = Ce (and La) adduct. Structure determinations are also recorded for rare earth(III) trichloroacetate ethanol trisolvates, Ln(tca)3.3EtOH. Adducts of Ln = La, Yb (and, implicitly, intermediate Ln) are isomorphous, triclinic, P 1, a ≈ 12, b ≈ 11·8, c ≈ 11·4 Å, α ≈ 114, β ≈ 100, γ ≈ 104°, Z = 2 f.u., R 0·056, 0·050 for No 3843, 4171. The complexes are centrosymmetric dimers [(EtOH)3(tca-O)Ln(O-tca-O′)4Ln(O-tca)(HOEt)3], the two metal atoms being linked by four O-tca-O′ bridging carboxylate groups; the metal atoms are eight-coordinate, the other four sites being occupied by four oxygen atoms from unidentate ethanol and carboxylate moieties. Bis(bis(2-pyridyl)aminium) bis(diaquatetrakis(trichloroacetato)lanthanate(III)), 2(dpaH+) [(H2O)2-(tca-O)(tca-O,O′)2La(O-tca-O′)2La(O,O′-tca)2(O-tca)(OH2)2]2-, is triclinic, P 1, a, 13·901(2), b 13·764(3), c 10·073(2) Å, α 104·04(2), β 108·93(2), γ 101·50(2)°, Z = 1 binuclear f.u., R 0·045 for No 4999. The anion is binuclear, the two nine-coordinate lanthanum atoms being linked by a pair of bridging O-carboxylate-O′ groups. The other seven sites of the LaO9 array are occupied by a pair of O,O′ -chelating and one O-unidentate carboxylate groups and a pair of water molecules.

The effect of strain rate and temperature on the mechanical behavior of the Ni -18 Si -3.0 Nb -1.0 Cr -0.2 B alloy was investigated by atmosphere-controlled tensile testing in vacuum and air atmosphere at different strain rates and different temperatures. The results reveal that the Ni -18 Si -3.0 Nb -1.0 Cr -0.2 B alloy exhibits ductile mechanical behavior (UTS > 1200 MPa, ε > 8%) at room temperature under different atmosphere conditions. In addition, both of the ultimate tensile strength and elongation exhibit quite insensitive response with respect to the loading strain rate when tests are held at temperatures below 973K. The elongation of the samples tested in vacuum and air for the Ni -18 Si -3 Nb -1 Cr -0.2 B alloy exhibits a significantly increase with temperature from 973K to 1073K. In addition, all fracture surfaces tested at 1073K in vacuum and air atmosphere presents a typical ductile fracture surface, a fully dimpled fracture pattern. The fact of increasing in elongation at high temperature (1073K) is suggested to be attributed by the dynamic recrystallization that occurs preferentially around the dispersion phase or grain boundaries and so as to enhance the ductility by reducing the stress concentration at or near grain boundaries.