Jean-Raymond Gavarri

Nanocrystalline vanadium dioxide: synthesis and mid-infrared properties

This work describes the first successful synthesis of nanocrystalline thermochromic VO2 powder using the low temperature irreversible structural transformation of the metastable VO2(B). At this step, the transformation is associated with a total rearrangement of VO6 octahedra, and a strong increasing of density. The reversible metal‐insulator phase transition (MIPT) of vanadium dioxide (Tta 68∞C) is associated with strong changes in electrical, magnetic and optical properties. The contrast of the optical transition in mid-infrared (MIR) region and the optical transparency are remarkably increased for these nanosized particles. Modifications in coloration are also observed. ” 2000 Elsevier Science B.V. All rights reserved.

Carbonatation and Decarbonatation Kinetics in the La 2 O 3 -La 2 O 2 CO 3 System under CO 2 Gas Flows

The carbonatation of La2O3 oxide and the decarbonatation of lanthanum carbonate phase La2O2CO3 are investigated using thermal and thermogravimetry analyses under CO2 gas flow. The initial phase La2O3 is first elaborated from pyrolysis of a LaOHCO3 precursor. Then, thermal and thermogravimetry analyses are carried out under CO2 flow, as temperature increases then decreases. The carbonatation kinetics of La2O3 is determined at three fixed temperatures. Electrical impedance spectroscopy is performed to determine the electrical responses associated with ionic mobilities and phase changes, in the temperature range 25 to 900∘C. The electrical conduction during heating under CO2 gas flow should be linked to two regimes of ionic conduction of the carbonate ions. From these electrical measurements, the ionic mobility of carbonate ions CO32− is found to be close to 0.003 ·10−4 cm2 s−1 V−1 at 750∘C for the monoclinic La2O2CO3 phase.

Elaboration and variable properties of 2223 BSCCO superconductor-polymer composites

Abstract High-Tc superconducting Bi2Sr2Ca2Cu3Oy phase (2223) was produced by a solid state reaction method. Its orthorhombic cell parameters were determined by X-ray diffraction. A series of superconductor/polymer composites was made using 2223 phase and low density polyethylene (LDPE). Complex impedance measurements at room temperature were carried out. The LDPE crystallized fraction is analyzed by X-ray diffraction: it is found to increase in size with the BSCCO volume fraction. Both sintered and composite materials were subjected to electrical and magnetic examination at low temperature. The Meissner effects associated with the 110 K superconducting transition were found to be roughly preserved in the composites. The levitation force was measured. The role of grinding is discussed.

Structural evolution, thermal expansion anomaly and the Grüneisen constants of the superconductor YBa2Cu3O7

Abstract The analyses of the continuous structural evolutions of ceramics presenting a transition can be carried out using low temperature X-ray and neutron diffraction. A specific method is recalled here for evaluating the anisotropic elastic and Gruneisen constants required for the phase study. It also provides continuous characterization of the entire evolution of the thermo-elastic properties through the superconducting transition. This method is applied to the superconductor YBa 2 Cu 3 O 7 . The compressibility coefficients of the orthorhombic phase have been determined by neutron diffraction at 300 K between 0 and 10 kbar. Using structural approximations, average elastic constants and Gruneisen parameters are then fitted to experimental thermal expansion data: C 11 + C 12 =4.83; C 13 =1.24, C 33 =2.87 in 10 11 Pa units; γ 0 =2.7 at T >200 K. These parameters are then used for interpreting thermal expansion anomalies through the Gruneisen parameter variations. Microstructure might involve local stresses thus affecting the thermal expansion anomalies in the transition range.

Ionic charge transfer in mixed compounds (Ca2P2O7)1 − x(Na4P2O7)x

To study the cationic mobilities in pyrophosphate systems, ceramics pellets were prepared by mixing, then heating powders of Ca2P2O7 and Na4P2O7 at different vol.% Na4P2O7. The transport properties of such samples were studied as a function of temperature using complex impedance spectroscopy. It is shown that for 90 and 95 vol.% Na4P2O7 the samples exhibit strongly enhanced conductivity and dielectric constant. Taking into account structural and microstructural aspects, an interpretation is proposed.

Anisotropic compressibility of the orthorhombic phase Bi2Sr2Ca1Cu2O8

Abstract The anisotropic compressibility factors of the superconducting phase Bi 2 Sr 2 Ca 1 Cu 2 O 8 (2212) are measured at 300 K by means of powder neutron diffraction under pressure up to 10 kbar. The values (in Pa −1 ) χ a =(4.9±1.0)×10 −12 , χ b =(2.0±0.4)×10 −12 and χ c =(9.8±2.0)×10 −12 have been found. The resulting volumic coefficients is χ v =17×10 −12 Pa −1 . This result is to be compared with the results concerning Y 1 Ba 2 Cu 3 O 7 : χ v =5.3×10 −12 Pa −1 . A comparison between the Gruneisen parameters of the superconductors is made.

Structural and Raman Vibrational Studies of CeO 2 -Bi 2 O 3 Oxide System

A series of ceramics samples belonging to the - phase system have been prepared via a coprecipitation route. The crystallized phases were obtained by heating the solid precursors at for 6 hours, then quenching the samples. X-ray diffraction analyses show that for a solid solution with fluorine structure is formed. For x ranging between 0.25 and 0.7, a tetragonal phase coexisting with the FCC solid solution is observed. For x ranging between 0.8 and 0.9, a new tetragonal phase appears. The phase is postulated to be a superstructure of the phase. Finally, close to , the classical monoclinic   structure is observed. Raman spectroscopy confirms the existence of the phase changes as x varies between 0 and 1.

Electrical Properties of a CeO2-Mix System Elaborated at 600∘C

The electrical conduction of a series of polycristalline [(1−x)CeO2·x/2Bi2O3] samples has been analyzed using electrical impedance spectroscopy, in the temperature range 25 to C. Samples have been prepared via a coprecipitation route followed by a pyrolysis process at C. For compositions , Ce1−xBixO2−x/2 solid solutions, with fluorite cubic structure, are obtained. In the composition range , the system is biphasic with coexistence of cubic and tetragonal structures. To interpret the Nyquist representations of electrical analyses, various impedance models including constant phase elements and Warburg impedances have been used. In the biphasic range (), the conductivity variation might be related to the increasing fraction of two tetragonal β′ and β-Bi2O3 phases. The stabilization of the tetragonal phase coexisting with substituted ceria close to composition is associated with a high conduction of the mix system CeO2-Bi2O3.

Synthesis and characterization of nanosized Ce1−xbixo2−δ solid solutions for catalytic applications

Abstract This study consists of elaborating and characterizing some nanometric materials in basic of rare-earth oxides by the soft chemistry technique. the first step of this work consists of synthesizing nanometric pure ceria by sol-gel process. In the second one, the Bismuth doped ceria by co-precipitation method was realized in order to obtain ceria-based solid solution, to improve its catalytic property by creation of oxygen vacancies. The solubility limit of Bi2O3 in CeO2 was determined to be around 20 atom %. The effect of thermal treatment temperatures on the average crystallite sizes and lattice parameters was done for pure ceria and Ce1-xBixO2-x/2 (x = 0.15 and 0.2). The different elaborated samples are subject of structural characterization (XRD). Catalytic reactivity of these materials in presence of “air- toxic gas” mixtures is studied by Fourier Transform Infra-Red Spectroscopy (FTIR).

Microstructure modifications and modulated piezoelectric responses in PLZT/Al2O3 composites

Abstract Piezoelectric ceramics for acoustic applications have been prepared by mixing the piezoelectric phase Pb1−1.5xLax□x/2(Ti1−yZry)O3 (PLZT) with variable fractions of Al2O3. The samples are in form of pellets and polarized at high temperature. After thermal treatment, X-ray diffraction and scanning electron microscopy have been used to determine the phase and morphological modifications. The morphotropic PLZT initial phase disproportionates into modified PLZT and ZrO2 phases. Using electrical impedance spectroscopy, the resonance frequencies of the composite system have been determined and analyzed. As the Al2O3 volume fractions increase, the resonance frequency and the amplitude of the electrical response both decrease. An interpretation of the role of Al2O3 additions is proposed in terms of phase and microstructure modifications. Using LRC electrical equivalent circuits, the impedance variations close to the resonance frequency are modeled: the increase of the resistance R (electrical losses) and of the capacitance C are respectively correlated to the change in microstructure and in nature of PLZT initial phase.