Enrico Franceschi

M5X3 intermediate phases formed by Ca, Sr and Ba

Abstract The crystal structures of some M 5 X 3 compounds (M = Ca, Sr, Ba; f,X = element of Group II B, III B, IV B or V B) were determined by single crystal and powder techniques. The phases Ca 5 Zn 3 , Sr 5 Cd 3 , Ca 5 Ga 3 , Sr 5 In 3 , Sr 5 Tl 3 , Sr 5 Sn 3 , Ba 5 Sn 3 and Sr 5 Pb 3 crystallize with the tetragonal Cr 5 B 5 structure type; Ca 5 As 3 , Sr 5 As 3 , Ca 5 Sb 3 and Ba 5 Bi 3 belong to the hexagonal Mn 5 Si 3 type; Sr 5 Bi 3 is isostructural with orthorhombic β-Yb 5 Sb 3 . A survey of the known M 5 X 3 phases formed by the alkaline earth elements is given and the influence of the size factor on the occurrence of the various structural types is discussed.

Phase equilibria in the Nd-Cu system

Abstract The Nd-Cu phase diagram was established by differential thermal analysis, metallography and X-ray analysis. A microprobe analysis was performed, and the microhardness and resistivity of the intermediate phases were determined. The following phases were observed: NdCu (670°C), NdCu 2 (840°C), Nd 2 Cu 7 (825°C), NdCu ≈4 (745°C), NdCu 5 (870°C) and NdCu 6 (920°C). Only NdCu 2 and NdCu 6 melt congruently. The compound Nd 2 Cu 7 has a limited stability range. Eutectics occur at 30 at.% Cu (520°C), 73 at.% Cu (770°C) and about 90 at.% Cu (865°C). The presence of a thermal anomaly around the 1:1 composition caused by a first-order transition is pointed out.

On the occurrence of M2X intermetallic compounds (M = Ca, Sr, Ba; X = Si, Ge, Sn, Pb)

Abstract The M 2 X phases formed by the alkaline earth metals with the elements of Group IVB of the periodic system have been prepared and studied. Their crystal structure has been determined by the X-ray powder and single crystal methods. They crystallize in the orthorhombic anti-PbCl 2 type (space group D 16 2 h ). The relation between this structure and the Co 2 Si or anti-CaF 2 structures, which occur for other homologous compounds formed by transition elements or magnesium, is discussed. The stability of these three types of structure as a function of atomic sizes, electronegativities and the electronic configuration of the valence shell of the component atoms is examined.

Dimorphism of La5Sn3, Ce5Sn3 and Pr5Sn3 compounds

Abstract A low temperature modification, having a tetragonal structure of the W5Si3 type, has been found for the compounds La5Sn3, Ce5Sn3 and Pr5Sn3, which at high temperature crystallize with the hexagonal Mn5Si3 type of structure. The low temperature form appears to be denser than the high temperature form. For all the known W5Si3-type M5X3 compounds, the shortest M-M distance in the cell shows a linear variation with the radius of the M atom. A comparison with the analogous trend shown by the compounds of the Mn5Si3 type is also discussed.

On the Sr-Pb system

Abstract The Sr-Pb system was studied by thermal analysis and X-ray analysis. Seven intermediate phases were found to exist. Two of these phases melt congruently: Sr2Pb (1155 °C); SrPb3 (675 °C). The remaining five phases form peritectically: Sr5Pb3 (1054 °C); Sr5Pb4 (943 °C); SrPb (785 °C); Sr2Pb3 (717 °C); Sr3Pb5 (645 °C). Two eutectics are formed, at 87.5 at.% Sr (725 °C) and at 30.5 at.% Sr (627 °C). The crystal structures of the compounds Sr2Pb (anti-PbCl2 type), Sr5Pb3 (Cr5B3 type), SrPb (CrB type) and SrPb3 (Ti3Cu type) are confirmed; Sr5Pb4 crystallizes with the Gd5Si4-type structure.

On the conservation of architectural artistic handwork of the ‘Pietra di Finale’

Abstract This paper focuses on the characteristics of alteration of the Pietra di Finale, a stone with long tradition in Ligurian regional architecture. The Pietra di Finale is quarried near Finale Ligure in western Liguria, and widely used in the ancient buildings (e.g. by the architect Galeazzo Alessi). Three main types are exploited: white, pale pink and dark pink, and all were the object of the present study. The stone was exposed to atmospheric agents, to investigate weathering processes and eventual suitable protection against the Ligurian climate. In particular we studied a laboratory procedure of ageing by exposing the stone to an artificial acid rain whose composition is the same as the most polluted rains fallen on Ligurian region. Consequently we observed the alterations that occurred on stone after exposure to the same amount of artificial and natural rains. The same procedure was applied to samples protected by a fluorinated copolymer such as Akeogard LTX (Syremont, Italy).

Topics on the preparation of HTSC via solid state reaction and melting process

The problems of the preparation of High Temperature Superconductors by sintering and melting are examined. Results of the thermal properties of the total substitution of Yttrium by a rare earth element are reported. The process of fabrication of HTSC after high temperature decomposition is indicated. The results of first characterisation are also reported.ZusammenfassungProbleme der Herstellung von Hochtemperatur-Supraleitern durch Sinter- und SchmelzvorgÄnge wurden untersucht.Es werden Resultate der thermischen Auswirkungen der vollstÄndigen Substitution von Yttrium durch Seltenerdelemente wiedergegeben. Der Prozess einer Herstellung von HT-Supraleitern nach thermischer Zersetzung im Bereich hoher Temperatur wird beschrieben, und es werden Resultate erster Charakterisierungen mitgeteilt.

The influence of Ag and Ag(In) alloy on Y-Ba-Cu-O equilibrium diagram around the Y-123 superconducting phase

We studied the chemical compatibility of Ag, Ag(In)alloy, Y2BaCuO5 (the so-called Y-211 green phase) and YBa2Cu3O7–δ (Y-123) phase in order to check if better grain growth and alignment with minimum contamination were possible during the Y-123melt texturing. We demonstrate that the addition of silver to Y-123 always led to an enhanced texture. The typical microstructure of the composite Y-123 plus Y-211 (that is believed essential for high critical currents in these High Tc superconductors) was not disturbed by addition of silver up to 7.5 wt%, while higher concentrations of metal led to a degradation of the texture. DTA–TG analysis was used to investigate the influence of Ag and Ag(In) alloy on the Y–Ba–Cu–O equilibria around the Y-123 phase. We found a very unexpected thermal behaviour, similar to an eutectic equilibrium, when the silver concentration was increased to35 wt%. We believe that this effect was essentially due to the increasingly higher concentration of silver that could react with the secondary phases present in the melt.

The influence of silver and Ag(In) alloy on Y-Ba-Cu-O equilibria around the YBa2Cu3O7-δ superconducting phase

Abstract We investigated the effect of silver, Ag(In) alloy and Y2BaCuO5 (the so-called Y-211 green phase) on melt-textured samples of YBa2Cu3O7-δ (Y-123) phase in order to check whether better grain growth and alignment were possible. We demonstrate that the addition of silver to Y-123 always led to an enhanced texture. The typical microstructure of the composite Y-123 plus Y-211 (that is believed essential for high critical currents in these high-T c superconductors) was not disturbed by addition of silver up to 7.5 wt% while higher concentrations of metal led to a degradation of the texture. Differential thermal analysis and thermogravimetric analysis were used to investigate the influence of silver and Ag(In) alloy on the Y-Ba-Cu-O equilibria around the Y-123 phase. We found a very unexpected thermal behaviour, similar to eutectic equilibrium, when the silver concentration was increased to 35wt%. We believe that this effect was essentially due to the increasingly higher concentration of silver that could react with the impurities present in the melt.

Interactions between paper and adsorbed molecules : A preliminary study and some experimental results

We have studied the geometry and energy involved in the interactions between adsorbed molecules and paper substrate, over a wide range of partial pressures. A two-dimensional virial equation of state was employed to evaluate the extent of interactions by the compressibility factor (Z). Data of H-bonded molecules (water and butan-1-ol) and of more weakly interacting ones (n-decane and dioxane) were selected from the literature. At low surface coverage, the compressibility factor Z exceeds 1, showing a deviation from ideality due to interactions, and reaches a maximum at a value close to the BET monolayer.As preliminary results we have measured by means of a differential microcalorimeter (Calvet) at 25‡C the energy of interactions of water andn-heptane vapor on chromatographic grade Whatman paper.At very low coverage, the differential heats of adsorption of water on paper are higher than those ofn-heptane adsorption on paper: 114 kJ mol−1 > 77 kJ mol−1. During the whole process of adsorption, in the first case, the specific interaction predominates, while in the second non-specific interactions predominate i.e. the dispersion or Van der Waals forces.