Papers by Keyword: Pyrochlore

Abstract: Yttria stabilized zirconia (YSZ) based composite topcoats were prepared with three advanced ceramic materials as second phase component; CaZrO₃, (La_0.75Nd_0.25)₂Zr₂O₇ and Nd₂Ce₂O₇. The solid particles erosion (SPE) testing of the air plasma sprayed composite topcoats was carried out at room temperature and 900 °C to study the effects of second phase on erosion behavior. The erodent was angular fused alumina of 40-45 mm size. The erosion mechanism was followed in all topcoats with cracking and fracturing of the plasma sprayed splats due to erodent particles impact and impingement, whereas micro-ploughing was observed after 900 °C SPE testing as an additional feature. The hardness and intrinsic properties of second phase component in the composite topcoats played a crucial role in improving the erosion rate (ER) at ambient as well as 900 °C. The Nd₂Ce₂O₇/YSZ topcoat show lower ER due to combatively higher hardness of Nd₂Ce₂O₇ and good interfacial bonding with YSZ. The overall lowering of ER at 900 °C as compared to that of at ambient conditions was ascribed to the anelastic mechanical response of the ceramic topcoats due to thermal cycling involved in the SPE testing at high temperature.

Abstract: Thermal barrier coatings (TBCs) serve as thermal insulator in the hot region of an aircraft engine. Besides this, it also protects the underlying metal surface from the harsh corrosive and eroding environment. The associated lower thermal conductivity of TBC ceramic materials plays an important role in the improvement of thermal efficiency of the engine in term of increased combustion temperature and power. The thermal conductivity of the conventional yttria stabilized zirconia (YSZ) and three advanced ceramic materials with perovskite (CaZrO₃) and pyrochlore structure (La_0.75Nd_0.25)₂Zr₂O₇ & Nd₂Ce₂O₇) have been determined using differential scanning calorimetry (DSC). With thin metallic disk on the ceramic samples of different heights were heated / scanned using a standard DSC apparatus. The results were evaluated for the thermal conductivity measurement using well established procedure /calculations. The analyzed results were compared with that of other techniques given by other researchers and found to be in good agreement with an error of 10-15%. The result of coefficient of thermal expansion (CTE) that was measured using a dilatometer up to 1273°K has also given.

Abstract: In this paper, pyrochlore-like Na₂Ta₂O₆ nanocrystals have been successfully synthesized in mixed solvents (water-ethanol) by a single-step and low-temperature solvothermal method. During the synthesis process, tantalum chloride and sodium hydroxide are used as starting materials; diethanolamine is acted as a stabilizer. The influence of alkaline concentration, reaction temperature and time on the crystallinity and morphology of Na₂Ta₂O₆ nanocrystals is investigated in detail. The samples are characterized by X-ray diffraction and transmission electron microscope. The results show that the pure-phase pyrochlore-like Na₂Ta₂O₆ nanocrystals can be formed with 10.5 mmol sodium hydroxide at 353K for 12 h.

Abstract: Chemical co-precipitation method was used to prepare Gd₂Zr₂O₇ powders. The powders were then heated in air at 1500°C, 1510°C, 1525°C, 1530°C, 1550°C, 1575°C, 1600°C for 5 h, and 1575°C, 1600°C for 10 h, respectively. The samples after heat treatment were characterized by X-ray diffraction and Raman spectroscopy. X-ray diffraction patterns reveal that order-disorder transition of Gd₂Zr₂O₇ occurs between 1550°C and 1575°C. Prior to the transition, the relative intensity of peaks corresponding to the super-lattice of pyrochlores increases with the increasing temperature. On the other hand, Raman spectra give an inconsistent result from the X-ray diffraction data. No appreciable difference can be observed for all the samples though with peaks broadening as temperature increases. The spectra of the samples indicated as fluorite structure in X-ray diffraction patterns appear with six resolvable peaks which is quite different from the spectrum of ideal fluorite structure.

Abstract: Ceramic materials of (La_0.5Sm_0.5)₂(Zr_0.7Ce_0.3)₂O₇ composition powder was synthesized by chemical coprecipitation method using Sm₂O₃, La₂O₃, Ce (NO₃)₃·6H₂O and ZrOCl₂·8H₂O powders as raw materials. Microstructure and phases of (La_0.5Sm_0.5)₂(Zr_0.7Ce_0.3)₂O₇ (LSZCO) powder was examined by Transmission Electron Microscope (TEM) and X-ray diffraction (XRD). The specific surface area of powders were determined by BET specific surface area test. The structure of LSZCO powder remained in pyrochlore with the increase of temperature more than 1200 °C. According to the impact on the specific surface area of LSZCO powder, the factors of the process is ordered in descending sequence as follows: pH value, dispersant content, concentration of reactants, the reaction temperature.

Abstract: This work is aimed at synthesizing lead zinc niobate powders which consist of pyrochlore phases by modified solid-state reaction via a rapid vibro-milling in difference three synthesis routes: mixed oxide precursor (PZN-M) , columbite precuresor (PZN-C) and lead niobate precuresor (PZN-P). It involves the fabrication of PZN powder and study of phase formation and local structural environment of Zn ions by using X-ray Diffraction (XRD) and X-Ray Absorption Spectroscopy (XAS) techniques that compared the amount of pyrochlore phases with perovskite phase.

Abstract: Technetium-99 is a key radioisotope from a nuclear waste perspective because of its long half-life (t_1/2= 2.13x10⁵ years) and its abundance in used nuclear fuel. As such, it is targeted in separation strategies such as UREX+, for isolation and encapsulation in solid waste forms for final disposal in a nuclear repository. We report here results regarding the incorporation of Tc-99 into ternary oxides of different structure types: pyrochlore (Nd₂Tc₂O₇), perovskite (SrTcO₃), and layered perovskite (Sr₂TcO₄). The objective was to determine synthesis conditions of these ceramic waste forms to immobilize Tc-99 as Tc (IV) and to harvest crystallographic, thermophysical and hydrodynamic data. The fabricated ceramic technetates exhibit good crystallinity and lattice parameters and atomic coordinates could be refined with high accuracies. Thermophysical properties of the oxides, such as the critical temperature (T_c) for superconductivity, were analyzed using AC magnetic susceptibility measurements. In our efforts to compare hydrodynamic properties of the ceramic waste forms with those to Tc-bearing borosilicate glass, we applied the standard test method ASTM C1220-10. Hereby matrix corrosion and Tc-leaching of Tc-99 containing LAWE4 type borosilicate glass and ceramic specimens were compared by static leaching experiments.

Abstract: Neodymium zirconate nanocrystals doped aluminium ion Nd_xAl_2-xZr₂O₇ with pyrochlore structure were prepared by a salt-assistant glycine combustion method. Zirconium nitrate, aluminium nitrate and neodymium nitrate were used as chemicals, and glycine was used as burning agent. The as-prepared Nd_xAl_2-xZr₂O₇ nano-crystals were characterized by XRD, FT-IR, TEM and HRTEM. The results showed the Nd ions can be partially replaced by Al ions. For crystalline Nd_1.9Al_0.1Zr₂O₇, there are four strong diffraction peaks at 2θ = 29.02°, 33.70°, 48.37° and 57.37° corresponding crystal faces are 0.308nm, 0.266nm, 0.188nm and 0.161 nm, respectively, which indicates that the interstices of corresponding crystal faces are smaller than that of Nd₂Zr₂O₇. Furthermore, the fluorescent properties of Nd_1.9Al_0.1Zr₂O₇ nano-crystals evaluated by the fluorescence spectra indicate that the prepared nano-crystals are a kind of potential fluorescent-emitted material.

Abstract: MCC-1 static short-term corrosion tests were performed to pyrochlore simulated waste forms in deionized water, at 90°C. Experimental results show that pyrochlore simulated waste forms has a good anti-leaching property. Both the normalized mass loss rate and the normalized mass loss of element Ce decrease with soaking time increasing in the short test. The corrosion process is co-dominated by the leaching agent and the corrosion temperature.

Abstract: Nanocrystalline Fe³⁺-doped La₂Zr₂O₇ series solid solutions were prepared by a convenient salt-assisted combustion process using glycine as fuel. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results showed the La ion can be partially replaced by Fe ion. The partial substituted products were still single-phase solid solutions and the crystal form remained unchanged. TEM images reveal that the products are composed of well-dispersed square-shaped nanocrystals. The method provides a convenient and low-cost route for the synthesis of nanostructures of oxide materials. The fluorescence of La_1.8Fe_0.2Zr₂O₇ nanocrystals was evaluated by the UV-visible absorption spectra and the fluorescence spectra. The results indicate that (La_xFe_1-x)₂Zr₂O₇ nanocrystals prepared by this method are a kind of potential fluorescent-emitted material.