Denise Morin

Role of lead substitution in the production of 110-K superconducting single-phase Bi-Sr-Ca-Cu-O ceramics

We have systematically investigated the influence of the three parameters (temperature of sintering, lead content, and duration of the sintering) on the growth of the (2:2:2:3) phase in ceramics of nominal composition Bi2−xPbxSr2Ca2Cu3O10−x/2, and have determined the compositions and thermal conditions leading to ceramics with Tc≊110 K and containing more than 95% of this phase. We show that this result can be obtained by usual calcination near 880°C in air, without recurring to the introduction of excess in the constituants, providing that intermediate grinding are effected, in order to reactivate the formation of the (2:2:2:3) phase which otherwise saturates for long durations of sintering. The suitable nominal compositions (0.3≤x≤0.4) correspond to those that yield inside the grains a lead content in the vicinity of the limit of solubility for lead. From the results of the preceding investigations and of experiments effected on lead‐substituted single crystals of the (2:2:1:2) phase, we have clarified ...

Etude dilatometrique de la transition ferroelastique de l'orthophosphate de plomb monocristallin

Abstract Single crystals of lead phosphate Pb3 (PO4)2 have been grown by Czochralski pulling. Thermal expansion has been investigated in this material by standard dilatometry and by X-ray powder diffraction, between room-temperature and 250° C, across the 180° C ferroelastic phase transition. Behaviour of the spontaneous strain, a most important quantity for ferroelastics, is deduced from thermal expansion measurements along the b and c monoclinic axes. The large room-temperature value of the strain component corresponding to a shear in the (100) plane of the crystal, 22 × 10−3, agrees with the value derived from lattice parameters. A jump of the spontaneous strain is observed at the Curie point which discloses a first order transition. This result, confirmed by differential thermal analysis measurements, is also consistent with recent birefringence data. The estimated transition heat is 0.17 cal/g.

Equilibre de phases et caracterisation dielectrique des niobates mixtes de strontium et de sodium

Abstract Phase equilibrium and dielectric characterisation of the binary system SrNb 2 O 6 -NaNbO 3 were investigated through DTA, crystallographic, Curie point, and hysteresis loop measurements. Similarly to previous results on alkaline-alkaline earth niobates, ferroelectric solid solutions with the tungsten bronze structure are stable at room temperature between 69 and 79 mol.% SrNb 2 O 6 . In the 65–69 % range a tungsten bronze phase occurs at high temperature. On the other hand, a new compound with cubic symmetry at room temperature is determined for 33 mol.% SrNb 2 O 6 , which undergoes a phase transition at − 100°C into a ferroelectric state. The solubility range of this compound extends to 14 mol.% SrNb 2 O 6 . Between 14 and 33 % ferroelectric solid solutions have a slightly distorted cubic symmetry below their Curie temperature, which is found-to vary from − 100°C to + 200°C. Finally it has been shown that small amounts of SrNb 2 O 6 incorporated in NaNbO 3 (2,5 mol.% SrNb 2 O 6 ) drive ferroelectric the room temperature antiferroelectric phase of NaNbO 3 .

Influence of lead substitution on the properties of the superconducting materials Bi2 − xPbxSr2Can − 1CunO2n + 4

Abstract It is shown that the structural modifications induced by the partial substitution of bismuth by lead in Bi-Sr-Ca-Cu-O is to establish bonding between the building blocks of the structure of these material, these blocks being almost independent in the lead-free compounds. On this basis one can understand two important aspects concerning the lead-subtituted materials: 1. i) the possibility of obtaining 110K superconducting single phase ceramics, 2. ii) the increase of the percolation threshold for the 110K zero-resistivity transition in thin films, with respect to the situation which is observed in lead-free thin films.

Phase characterization and superconductivity transitions in fluorinated YBaCuO

We present the results of an investigation of ceramics of nominal composition YBa2Cu3OxFy. X-ray diffraction and electron microscopy have determined the presence of BaF2, CuO, Cu Y2O6 and YBaCu0 phases. The occurence of a resistivity anomaly at 150K in y = 1.5 ceramics is atiributed to an important presence of insulating layers of BaF2 which strongly pertubs the electrical measurements.

Etude du diagramme de phases et cristallogenese des phosphovanadates de plomb

Abstract The phase diagram of lead phosphovanadate Pb 3 (P x V 1−x O 4 ) 2 for 0.5 ≤ x ≤ 1 has been investigated. Liquidus and solidus have been determined by means of differential thermal analysis and X-ray techniques. These results are confirmed through a molecular absorption spectrophotometry analysis of phosphor and vanadium, by comparing the compositions of lead phosphovanadate single crystals, grown by Czochralski pulling, with that of the starting melts. Good optical and crystallographic quality crystals of large size are obtained for x = 0 and x = 0.5, while for other compositions, they are still suitable for physical measurements. A definite, congruently melting compound with trigonal symmetry at room temperature is found for x = 0.5.