Speranta Tanasescu

Thermal and structural characterization of synthetic and natural nanocrystalline hydroxyapatite

The aim of this work was to study the thermal stability on heating and to obtain the processing parameters of synthetic and bone-derived hydroxyapatite over temperatures between room temperature and 1400°C by thermal analysis (thermogravimetry (TG)/differential scanning calorimetry (DSC) and thermo-mechanical analysis-TMA). Structural and surface modifications related to samples origin and calcination temperature were investigated by Fourier transformed infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD) and BET method. FTIR spectra indicated that the organic constituents and carbonate are no longer present in the natural sample calcined at 800°C. Raman spectra highlighted the decomposition products of the hydroxyapatite. The calcination treatment modifies the processes kinetics of the synthetic samples, being able to isolate lattice water desorption processes of decarbonization and the dehydroxylation processes. Shrinkage of calcined synthetic sample increases by 10% compared to uncalcined synthetic powder. From the TMA correlated with TG analysis and heat capacity data it can be concluded that sintering temperature of the synthetic samples should be chosen in the temperature range of the onset of dehydroxylation and the temperature at which oxyapatite decomposition begins.

The influence of compositional variables on the thermodynamic properties of lanthanum strontium ferrite manganites and lanthanum strontium manganites

Abstract In order to obtain a better understanding of the stability limits of perovskite phases used in new SOFC (solid oxide fuel cell) cathodes materials, a study of the thermodynamic properties of some perovskite-type oxides based on lanthanum strontium ferrite manganite and lanthanum strontium manganite was initiated. For the determination of thermodynamic quantities, a solid state electrochemical technique has been employed. The relative partial molar free energies, enthalpies, and entropies of oxygen dissolution in the perovskite phase, as well as the partial pressures of oxygen, have been obtained in the temperature range of 1073–1273 K. The results evidence the influence of different compositional variables on the thermodynamic properties.

Energetics of lanthanide cobalt perovskites: LnCoO3−δ (Ln = La, Nd, Sm, Gd)

Lanthanide cobalt perovskites LnCoO3−δ (Ln = La, Nd, Sm, and Gd) are important materials for electroceramics, catalysts, and electrodes in solid oxide fuel cells. Formation enthalpies of LnCoO3−δ compounds were measured using high temperature oxide melt solution calorimetry. The formation enthalpies of LaCoO2.992, NdCoO2.985, SmCoO2.982 and GdCoO2.968 from constituent binary oxides (Ln2O3, CoO) and O2 gas are −111.87 ± 1.36, −98.49 ± 1.33, −91.56 ± 1.46 and −88.16 ± 1.45 kJ mol−1, respectively. Thus these perovskites become energetically less stable with decrease in ionic radius of the lanthanide (from La to Gd), which corresponds to a decreasing tolerance factor and increasing oxygen deficiency. The thermodynamic stability of LaCoO2.992, NdCoO2.985, SmCoO2.982 and GdCoO2.968 was also assessed considering their oxygen partial pressures for decomposition, with good agreement between thermochemical and equilibrium data.

Effects of dopants and oxygen non-stoichiometry on thermodynamic and magnetic properties of micro- and nanostructured perovskite type manganites

Abstract The aim of the paper is to evidence the effect of the composition and oxygen stoichiometry on the thermodynamic, spin dynamics and magnetic properties of some micro- and nanostructured substituted manganites La1–xAxMn1–yByO3–δ (A=Ca; B=Co, Fe; x=0·33; y=0, 0·02 and 0·04). The partial molar thermodynamic properties of oxygen dissolution in the perovskite phase as well the equilibrium partial pressures of oxygen have been obtained using solid electrolyte electrochemical cells [electromotive force measurement (EMF)] method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. Using electron paramagnetic resonance spectroscopy, the spin dynamics has been investigated. Particular attention was devoted to the discussion concerning the influence of doping on polaron activation energy E a and exchange coupling integral J between Mn spins. The results were correlated with the variation of both saturation magnetisation M s and Curie temperature T c. The data were discussed in relation to significant changes in the overall concentration of defects. New features related to the modifications in properties connected with the nanocrystalline state were evidenced.

Thermodynamic Properties of Nano- and Micro-Structured Perovskite-Type Compounds

The thermodynamic properties of the ABO3 perovskite-type compounds are strongly affected by the A- and B- site substitution and the oxygen nonstoichiometry. The paper presents the thermodynamic properties of some doped nano- and microstructured lanthanum manganites of general formula La1-xMxMn1-yMeyO3 (where M=Sr, Ca; Me=Al; x=0.3;0.33, y=0;0.05) which have been studied within the temperature range of 873-1273 K in a reducing atmosphere (10-6 Pa). The relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase and the equilibrium partial pressures of oxygen have been obtained by using a solid electrolyte galvanic cell method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with measurements of the electromotive force (EMF). The focus of the research was to emphasise modifications of the thermodynamic properties connected with the nanocrystalline state. The results obtained are discussed in relation to significant changes in the overall concentration of defects.

Low-Temperature synthesis and thermodynamic and electrical properties of barium titanate nanorods

Studies regarding the morphology dependence of the perovskite-type oxides functional materials properties are of recent interest. With this aim, nanorods (NRs) and nanocubes (NCs) of barium titanate (BaTiO3) have been successfully synthesized via a hydrothermal route at temperature as low as 408 K, employing barium acetate, titanium isopropoxide, and sodium hydroxide as reagents without any surfactant or template. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD), used for the morphology and structure analyses, showed that the NRs were formed by an oriented attachment of the NCs building-blocks with 20 nm average crystallites size. The thermodynamic properties represented by the relative partial molar free energies, enthalpies, and entropies of the oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressure of oxygen, indicated that NRs powders have lower oxygen vacancies concentration than the NCs. This NRs characteristic, together with higher tetragonallity of the structure, leads to the enhancement of the dielectric properties of BaTiO3 ceramics. The results presented in this work show indubitably the importance of the nanopowders morphology on the material properties.