Preparation and characterization of nanostructured La2Zr2O7 feedstock used for plasma spraying

The nanostructured La₂Zr₂O₇ (LZ) feedstock with high density, suitable size distribution and nearly spherical morphology which can be used for plasma spraying was prepared by spray drying in this study. The spray drying process was discussed. In addition, the formation mechanism of feedstock with hollow shell structure was discussed by finite element method in this paper. The double ceramic layer (DCL) LZ/YSZ (yttria stabilized zirconia) thermal barrier coatings were prepared using the as prepared LZ feedstock. The average grain size computed by Scherrer formulation, the observation of powder size by Transmission Electron Microscope (TEM) and “single splat” deposition experiment indicate that the as prepared LZ feedstock is nanostructured feedstock.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Influence of VO2 nanostructured ceramics on hydrogen desorption properties from magnesium hydride

The hydrogen desorption properties and kinetics of MgH₂–VO₂ composite prepared by mechanical milling of MgH₂ and VO₂ have been investigated. Structural characterization of produced nanocomposite was done by X-ray powder diffraction (XRD), particle size analysis and scanning electron microscopy (SEM). The structure and morphology of the composite have been correlated with hydrogen desorption properties investigated by differential thermal analysis (DTA). It has been shown that short mechanical milling of nanostructured VO₂ and MgH₂ leads to decrease of hydrogen desorption temperature of MgH₂ by 80 K. The mechanism of desorption has been changed from phase boundary reaction, spherical symmetry for untreated MgH₂ to phase boundary reaction, cylindrical symmetry for the composite material. The activation energy for desorption has been reduced by adding VO₂ ceramics as a catalyst.     

Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

High‐temperature time‐of‐flight neutron diffraction experiments were performed on cubic yttria‐stabilized zirconia (YSZ, 10 mol% YO_1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO₂ laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10^?6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10^?6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10^?6/K at 2350°C–2550°C with oxygen displacement parameters (U_iso) reaching 0.1 Å², similar to behavior observed in UO₂. Acquisition of powder‐like high‐temperature neutron diffraction data from solid‐levitated samples is feasible and possible improvements are outlined. This methodology should be applicable to a wide range of materials for high‐temperature applications.     

Self-propagating high-temperature synthesis of intermetallide/oxide nanocomposite powders using mechanocomposite precursors

It was found that mechanical activation of a mixture containing iron, aluminum, and oxide iron resulted in mechanochemical reduction of iron oxide and formation of a Fe/Al/Al₂O₃ nanostructural mechanocomposite. High-temperature self-propagating synthesis (SHS) using this composite as a precursor yielded a FeAl/Al₂O₃ composite, which retained the morphology and dimensional characteristics of the precursor. During mechanical activation of a mixture of iron, aluminum, and chromium oxide, complete reduction of the latter did not occur but SHS also led to the formation of an intermetallide/oxide nanocomposite, in which chromium atoms were incorporated in the structure of iron aluminide.     

Synthesis of ZrB2 Powders from ZrO2, BN,and C

Submicrometer (200–800 nm) ZrB₂ powders have been successfully prepared via a new ZrO₂–C–BN precursor powder system at a relatively low temperature (1550°C) for 1.5 h. As a moderate cost boron source, BN has a well‐defined stoichiometry and low impurity. Both thermodynamic and experimental results indicated that ZrC was formed below 1300°C, a temperature required for ZrB₂ formation. Moreover, the reaction of ZrC–BN mixture at 1300°C indicated that the ZrC acted as an effective direct reducing reagent for BN to form ZrB₂, indicating that the pathway involving the formation of intermediate phase ZrC determined the formation mechanism.     

Effect of geometric parameter on thermal stress generation in fabrication process of double-ceramic-layers thermal barrier coating system

In this work, quenching stress generated during the deposition process and the Coefficient of Thermal Expansion (CTE) thermal mismatch stress produced during the cooling down process of Double-Ceramic-Layers Thermal Barrier Coating System (DCL-TBCs) have been intensively examined. The thickness ratio of Lanthanum Zirconate (LZ, La₂Zr₂O₇) coating to stabilized Zirconia (YSZ, ZrO₂-8%Y₂O₃) coating, have been theoretically analyzed. In addition, DCL-TBCs specimens with different thickness ratio of LZ to YSZ coatings were fabricated, to study the effect of this thickness ratio by specimen curvature and crack density analysis. Meanwhile, Finite Element Method (FEM) has been carried out to validate results obtained theoretically. The results reveal that by comparison to CTE thermal mismatch stress, quenching stress has remarkable effect on total thermal stress. By increasing thickness ratio of YSZ to LZ coatings, average thermal stress and crack densities in YSZ and LZ coatings increased. Nevertheless, the curvature ratio of DCL-TBCs specimen decreases.     

Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.     

Catalytic oxidation of methane on CuO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/FeAlO/FeAl cermet catalysts

The activity of plates of CuO/Al₂O₃/FeAlO/FeAl structured cermet catalysts is compared by varying their alumina content. The catalysts were prepared by impregnation of cermet supports obtained by mechanochemical activation of powder mixtures of an alumina precursor [20–50% (wt.)], iron, and aluminum, followed by hydrothermal treatment and calcination. It is shown that increasing the content of the alumina precursor (product of thermal activation of gibbsite) increases the specific surface area of the support and the mesopore and macropore volumes and reduces its mechanical strength. The content of the active component (CuO) also increases, resulting in an increase in the specific activity of catalyst despite a reduction in the effectiveness of using the active component. The activity of catalysts with a moderate concentration of alumina is sufficient to initiate methane oxidation.     

Preparation of lanthanum zirconate films with a widely controllable La/Zr ratio by LCVD

Lanthanum zirconate (LZ) films with a controllable La/Zr composition were prepared by laser enhanced chemical vapor deposition (LCVD). The effects of different precursors ratio, i.e. the La(dpm)₃/Zr(dpm)₄ molar ratio, on composition, crystal structure, morphology and electrical conductivity of films were investigated. The formation region of columnar and purely cubic pyrochlore structured LZ films with a controllable La/Zr molar ratio in a wide range of 0.51–2.53 was mapped. Crystal structure changed with the different precursor's ratio, which was caused by atomic substitution between lanthanum and zirconium, being proved by combining with experimental and theoretical XRD patterns. It is found that electrical conductivity of non-stoichiometric LZ films is up to 4.3?×?10^?3 S?cm^?1 at 1073?K. The columnar pyrochlore structured LZ films with a wide region of non-stoichiometric compositions are expected to be candidates for many potential applications, such as dielectric, thermal barrier coatings and nuclear waste treatment materials.     

Influence of the precursors on the formation and the properties of ZnFe2O4

The phase ZnFe₂O₄, franklinite, with spinel structure, has been synthesised using solid-state reactions from the mechanically mixed powders of several precursor salts. However, this method requires a high-temperature treatment for some hours to homogenise the whole composition to obtain a single phase. In this paper ZnFe₂O₄ has been prepared by the ceramic method using different precursors of Fe and Zn. The spinel has also been prepared by a citrate route that allows to prepare a homogeneous single phase at lower temperature and shorter annealing time. The citrate method has let to obtain single phase of ZnFe₂O₄ at 600°C/2 h. In addition, this processing increases the inversion grade with respect to the ceramic ones obtained with the same precursor salts. The samples have been studied and characterised by means of differential thermal analysis (DTA), thermal gravimetric analysis (TGA), powder X-ray diffraction (XRD), Rietveld refinement method, Mössbauer spectroscopy, scanning electron microscopy (SEM), energy dispersion of X-ray spectrometry (EDX) and magnetic measurements. The inversion grade has been detected by means of Rietveld analyses and corroborated by magnetic measurements. ©     

The Identification of Different Active Sites on Mo/Al2O3 Metathesis Catalysts

Mo/Al₂O₃ catalysts prepared via fixation of Mo(³-C₃H₅)₄ on Al₂O₃ or by conventional impregnation (2.2 or 2.9 wt% Mo) have been compared with regard to their catalytic behavior in the metathesis of propene in different temperature ranges (293-323 K, 473 K). Different active sites have been distinguished. A site derived from a Mo(VI) precursor by thermal activation in inert gas exhibits stable activity, with a propene reaction order near 1. Other sites that are derived from a reduced Mo precursor, probably Mo(IV), are of higher activity but unstable with time-on-stream and also at elevated temperatures (>323 K). These sites support the metathesis at a propene reaction order of 0.5 and with activation energies between 10 and 25 kJ/mol depending on unknown structural details. Due to their instability, they cannot contribute to the high-temperature (T > 373 K) metathesis activity observed with Mo/Al₂O₃ catalysts. The latter is supported by Mo(VI)-derived sites or, at after reduction of catalysts with higher Mo contents, by Mo(IV)-derived sites that are different from those identified in the present study.     

Influence of the deposition energy on the composition and thermal cycling behavior of La2(Zr0.7Ce0.3)2O7 coatings

Lanthanum–zirconium–cerium composite oxide (La₂(Zr_0.7Ce_0.3)₂O₇, LZ7C3) coatings were prepared under different conditions by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphologies, cyclic oxidation behavior of these coatings were studied. Elemental analysis indicates that the coating composition has partially deviated from the stoichiometry of the ingot, and the existence of excess La₂O₃ is also observed. The optimized composition of LZ7C3 coatings could be effectively achieved by the addition of excess CeO₂ into the ingot or by properly controlling the deposition energy. Meanwhile, when the deposition energy is 1.15 × 10⁴–1.30 × 10⁴ J/cm², the coating has a similar X-ray diffraction (XRD) pattern to the ingot, and the thermal cycling life of the coating is also superior to other coatings. The spallation of the coatings occurs either within the ceramic layer approximately 6–10.5 μm above its thermally grown oxide (TGO) layer or at the interface between ceramic layer and bond coat.     

Hydrothermal synthesis of crystalline α-/β-MnO2 nanorods via γ-MnOOH nanorod precursors

The crystalline α-MnO₂ and β-MnO₂ nanorods have been successfully prepared via a facile hydrothermal method from γ-MnOOH nanorods precursor, respectively. The samples were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Fourier transformed infrared spectra (FTIR). The morphology and structure of γ-MnOOH nanorods precursors have a great influence on the crystal structure of the obtained products. The α-MnO₂ nanorods are prepared from the 100°C γ-MnOOH precursor, while the β-MnO₂ nanorods are obtained from the 150°C γ-MnOOH precursor, respectively. Besides, the catalytic activity of the prepared α-MnO₂ and β-MnO₂ nanorods for the H₂O₂ decomposition has been investigated comparatively, and the latter shows better catalytic activity.     

Design of a ductile carbon nanofiber/ZrB2 nanohybrid film with entanglement structure fabricated by electrostatic spinning

Polyacrylonitrile (PAN), a kind of multi-purpose man-made polymer material, has been widely used in various products, including carbon fiber precursor fiber manufacturing. Organic/inorganic nanocomposites can provide precursor material with unique properties due to optimal structural design. Herein, PAN based carbon nanofiber (CNF) coated zirconium borate (ZrB₂) particles fiber film was prepared via electrostatic spinning strategy. Crosslinking network between carbon atoms formed at 280 °C due to long chain PAN molecules, which underwent pyrolysis at 800–1200 °C. Scanning electron microscope analysis showed that ductile CNF/ZrB₂ hybrid material with entanglement structure was successfully fabricated. Phase composition of the materials was analyzed by X-ray diffractometer, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, which confirmed the presence of carbon atoms in the materials. Entanglement structure between CNFs and ZrB₂ enhanced tensile performance of nanohybrid film, in which CNF film with 25% ZrB₂ content exhibited optimal mechanical properties. The design of nanohybrid structure provides facile and universal approach for exploration of organic/inorganic nanocomposites with controlled structures and excellent mechanical properties.     

Effect of the excess of bismuth on the morphology and properties of the BaBi2Nb2O9 thin films

In this study, the effect of bismuth content on the crystal structure, morphology and electric properties of barium bismuth niobate (BaBi₂Nb₂O₉) thin films was explored with the aid of X-ray diffraction (XRD), scanning electron microcopy (SEM), atomic force microscopy (AFM) and dielectric properties. BaBi₂Nb₂O₉ (BBN) thin films have been successfully prepared by the polymeric precursor methods and deposited by spin coating on Pt/Ti/SiO₂/Si (1 0 0) substrates. The phase formation, the grain size and morphology of the thin films were influenced by the addition of bismuth in excess. It was observed that the formation of single phase BBN for films was prepared with excess of bismuth up to 2 wt%. The films prepared with excess of the bismuth showed higher grain size and better dielectric properties. The 2 wt% bismuth excess BBN thin film exhibited dielectric constant of about 335 with a loss of 0.049 at a frequency of 100 kHz at room temperature.     

Preparation of K2Ti6O13 fibers by electrospinning for near-infrared reflectivity

Ceramic coatings have been widely used in industrial fields. K₂Ti₆O₁₃ fibers could be a potential coating material for low thermal conductivity, high mechanical strength and infrared reflectivity. The precursor fibers, using potassium acetate (CH₃COOK) and tetrabutyl titanate (TBOT) as potassium and titanium sources, were prepared by the electrospinning technique with the sol-gel method. To prepare K₂Ti₆O₁₃ fibers, the mole ratio of K/Ti was adjusted from 1:2 to 1:3. The transformation of other phases of potassium titanates was characterized by XRD patterns and Raman spectra. The effects of mole ratio of K/Ti on morphology of samples have been investigated. Well-crystallized fibers were prepared with single-crystal K₂Ti₆O₁₃ structure. The corresponding morphology and microstructure evolution were studied by SEM and TEM. The as-obtained K₂Ti₆O₁₃ fibers possessed the high near-infrared (NIR) reflectivity of 98%.     

CaZrTi2O7 zirconolite synthesis: From ceramic to glass-ceramic

The formation of CaZrTi₂O₇ zirconolite and its crystallization in sodium alumino-borosilicate glass has been investigated via sintering in air. The ceramic precursor is prepared using a soft chemistry route to ensure ultimate mixing of the reactants at the molecular level. A nearly phase pure CaZrTi₂O₇ zirconolite is formed after sintering the ceramic precursor at 1400°C for 12 hours. In order to form zirconolite glass-ceramics, various processing conditions are investigated including sintering temperatures, different glass composition precursors, and ceramic to glass weight ratios. Zirconolite crystallization on the surface and inside of glass-ceramic-pelletized samples is investigated. Higher ceramic to glass weight ratio leads to the formation of zirconolite as crystalline phase, which is quite dominant in glass.     

Phase formation and dielectric properties of (Pb,Ba)[(Zn1/2W1/2),Ti]O3 ceramics

PbTiO₃ and/or BaTiO₃ were systematically introduced into Pb(Zn_1/2W_1/2)O₃ and resultant phase developments in terms of perovskite formation were investigated. Ceramic powders were prepared via a B-site precursor route to further assist the perovskite formation. Weak-field dielectric properties of the sintered samples were examined. For Pb(Zn_1/2W_1/2)O₃-rich compositions, multiphase ceramics resulted and formation of monophasic perovskite turned out to be not successful even by the B-site precursor method. Values of the perovskite formation yield and the maximum dielectric constant increased with increasing fractions of the substituent species.