Search Results (5)

Marble waste is produced on a large scale in many countries, resulting in serious pollution problems. This investigation aimed to study the valorization potential of marble waste from the ornamental rock industry used in the synthesis of a novel calcium niobate–magnesium niobate composite powder prepared by a solid-state reaction between 1000 °C and 1200 °C. The chemical and mineralogical characteristics of the marble waste were determined. Structural and morphological characterizations of the synthesized calcium niobate–magnesium niobate composite powders were conducted by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The thermophysical properties were measured using open photoacoustic cell and photothermal techniques. Structurally, at all synthesis temperatures, the calcium niobate–magnesium niobate powders were found to be composed of a complex mixture of CaNb₂O₆/Ca₂Nb₂O₇/MgNb₂O₆/CaMg_0.33Nb_0.67O₃. In addition, the calcium niobate–magnesium niobate composite powders exhibited low values of thermal diffusivity (1.88–2.15 × 10⁻⁷ m²/s) and thermal conductivity (0.12–0.16 W/mK). The findings of this investigation highlight the potential of marble waste as a promising sustainable source of carbonate for obtaining calcium niobate–magnesium niobate composite powder, which has thermophysical properties that should be explored in low-thermal-conductivity applications. Full article

Uniaxial ferroelectrics with tetragonal tungsten bronze structure are important functional materials with photorefractive, electrooptic, piezoelectric, and pyroelectric properties. Sr_xBa_1−xNb₂O₆ (SBN100x) with x > 50 is known as a typical uniaxial relaxor ferroelectric, while Ca_xBa_1−xNb₂O₆ (CBN100x) undergoes nearly normal ferroelectric phase transitions. Single crystals of CSBN100x = [x(CBN28) + (1 − x) (SBN61)] = xCa_0.28Ba_0.72Nb₂O₆ + (1 − x) Sr_0.61Ba_0.39Nb₂O₆ with nominal x = 0.00, 0.25, 0.50, 0.75, and 1.00 were studied to clarify the dynamical properties at the crossover from relaxor (x = 0) to normal (x = 1) ferroelectric behavior. The longitudinal acoustic (LA) and transverse acoustic (TA) modes and a central peak (CP) related to the relaxation process of polarization fluctuations along the polar c-axis were studied in uniaxial ferroelectric CSBN single crystals as a function of temperature via Brillouin scattering spectroscopy. A CBN28 (x = 1.00) crystal shows the sharp elastic anomaly of the LA mode in the gigahertz range toward Curie temperature, T_c. However, those of CSBN25 (x = 0.25) and SBN61 (x = 0.00) crystals show diffusive anomalies due to stronger random fields. The relaxation time determined from the width of a CP shows a critical slowing down in the vicinity of T_c. The elastic anomaly and slowing down of relaxation time of CSBN100x crystals become diffusive in the vicinity of T_c as the CBN28 content decreases. The origin of the crossover from relaxor to normal ferroelectric phase transitions is discussed in terms of the difference in the A1 and A2 sites’ occupancies. Full article

Modifying lithium niobate cation composition improves not only the functional properties of the acousto- and optoelectronic materials as well as ferroelectrics but elevates the protonic transfer in LiNbO₃-based electrolytes of the solid oxide electrochemical devices. Molten chlorides and other thermally stable salts are not considered practically as the precursors to synthesize and modify oxide compounds. This article presents and discusses the results of an experimental study of the full or partial heterovalent substitution of lithium ion in nanosized LiNbO₃ powders and in the surface layer of LiNbO₃ single crystal using molten salt mixtures containing calcium, lead, and rare-earth metals (REM) chlorides as the precursors. The special features of heterovalent ion exchange in chloride melts are revealed such as hetero-epitaxial cation exchange at the interface PbCl₂-containing melt/lithium niobate single crystal; the formation of ${\mathrm{Li}}_{\left(1-\mathrm{x}\right)}{\mathrm{Ca}}_{\left(\mathrm{x}/2\right)}{\mathrm{V}}_{\left(\mathrm{x}/2\right)}^{{\mathrm{Li}}^{+}}{\mathrm{NbO}}_3$ solid solutions with cation vacancies as an intermediate product of the reaction of heterovalent substitution of lithium ion by calcium in LiNbO₃ powders; the formation of lanthanide orthoniobates with a tetragonal crystal structure such as scheelite as the result of lithium niobate interaction with trichlorides of rare-earth elements. It is shown that the fundamental properties of ion-modifiers (ion radius, nominal charge), temperature, and duration of isothermal treatment determine the products’ chemical composition and the rate of heterovalent substitution of Li⁺-ion in lithium niobate. Full article

Sodium-calcium-phosphate based oxynitride glasses and glass-ceramics doped with Mg, Si, and Nb were studied in vitro in simulated body fluid (SBF) under static conditions. The release of ions and pH changes up to 7 days of immersion were investigated. The nitrogen incorporation into phosphate glass matrix was found to notably influence in vitro dissolution only of homogenous glasses. Increasing the nitrogen content in the samples decreased the mean mass loss, while the niobate incorporation increased it. The correlation between the nitrogen content and increase in pH of SBF was also observed. The presence of phosphates crystallites was found to support the dissolution process at the beginning step (up to 3 days). Full article

Phase transitions of Mg₂TiO₄ and Fe₂TiO₄ were examined up to 28 GPa and 1600 °C using a multianvil apparatus. The quenched samples were examined by powder X-ray diffraction. With increasing pressure at high temperature, spinel-type Mg₂TiO₄ decomposes into MgO and ilmenite-type MgTiO₃ which further transforms to perovskite-type MgTiO₃. At ~21 GPa, the assemblage of MgTiO₃ perovskite + MgO changes to 2MgO + TiO₂ with baddeleyite (or orthorhombic I)-type structure. Fe₂TiO₄ undergoes transitions similar to Mg₂TiO₄ with pressure: spinel-type Fe₂TiO₄ dissociates into FeO and ilmenite-type FeTiO₃ which transforms to perovskite-type FeTiO₃. Both of MgTiO₃ and FeTiO₃ perovskites change to LiNbO₃-type phases on release of pressure. In Fe₂TiO₄, however, perovskite-type FeTiO₃ and FeO combine into calcium titanate-type Fe₂TiO₄ at ~15 GPa. The formation of calcium titanate-type Fe₂TiO₄ at high pressure may be explained by effects of crystal field stabilization and high spin–low spin transition in Fe²⁺ in the octahedral sites of calcium titanate-type Fe₂TiO₄. It is inferred from the determined phase relations that some of Fe₂TiO₄-rich titanomagnetite inclusions in diamonds recently found in São Luiz, Juina, Brazil, may be originally calcium titanate-type Fe₂TiO₄ at pressure above ~15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions. Full article