Sodium Excess Ta‐Modified (K0.5Na0.5)NbO3 Ceramics Prepared by Reactive Template Grain Growth Method

Lead‐free sodium excess Ta‐modified (K_0.470Na_0.545)(Nb_0.55Ta_0.45)O₃ (KNNT) ceramics were synthesized by a conventional and reactive templated grain growth methods, and their degree of grain orientation, microstructure, dielectric, ferroelectric, and field‐induced strain properties were systematically investigated. A high degree of grain orientation (Lotgering factor F = 80%) was obtained in textured KNNT ceramics. Results showed that textured KNNT ceramics exhibit high grain orientation, dielectric constant, and field‐induced strain as compared to nontextured samples of the same composition. Room temperature unipolar field‐induced strain of K_0.5Na_0.5NbO₃ (KNN) ceramics was enhanced from 0.080% for nontextured sample to 0.115% for textured sample, and their corresponding dynamic piezoelectric coefficients () were improved from 320 pm/V to 460 pm/V, respectively.     

Temperature‐insensitive strain behavior in 0.99[(1−x)Bi0.5(Na0.80K0.20)0.5TiO3−xBiFeO3]–0.01Ta lead‐free piezoelectric ceramics

Lead‐free 0.99[(1?x)Bi_0.5(Na_0.80K_0.20)_0.5TiO₃?xBiFeO₃]–0.01Ta (BNKT20–100xBF–1Ta) lead‐free piezoelectric ceramics were fabricated through conventional solid state sintering method. Results showed that change of BF content in the BNKT20–100xBF–1Ta induced a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order. A large electric‐field‐induced strain of 0.36% (at 80 kV/cm driving field, corresponding to a large signal of ~450 pm/V) which is derived from a reversible field‐induced ergodic relaxor to ferroelectric phase transformation, was obtained in the composition with x=0.01 near the ferroelectric‐ergodic relaxor phase boundary. Moreover, an attractive property for application in nonlinear actuators demanding enhanced thermal stability was obtained in this material, which showed a temperature‐insensitive strain characteristic in the temperature range from room temperature to 100°C.     

Fe doping effect on EuTiO3: The magnetic properties and giant magnetocaloric effect

The magnetic properties and magnetocaloric effect for EuTi_1-xFe_xO₃ (x = 0.05, 0.1) compounds are investigated. When a part of Ti⁴⁺ions were substituted by Fe ions, the AFM ordering can be significantly changed to be FM. The EuTi_1-xFe_xO₃ (x = 0.05, 0.1) compounds exhibit a PM to FM transition with decreasing temperature and the Curie temperature is 6 K. Under the field changes of 1 T, and RC are valued to be 10.1 J/kg K and 50.2 J/kg for EuTi_0.95Fe_0.05O₃; 9.6 J/kg K and 47.7 J/kg for EuTi_0.9Fe_0.1O₃, without magnetic and thermal hysteresis. RC is almost twice as much as EuTiO₃ (27 J/kg) as substitution of Fe³⁺ ions for Ti⁴⁺ions, which may be attributed to the magnetic transition (AFM to FM). Therefore, the giant and large RC suggest the EuTi_1-xFe_xO₃ compounds are good materials for magnetic refrigerant.     

Visible light active heterostructured photocatalyst system based on CuO plate-like particles and SnO2 nanofibers

In this study, CuO–SnO₂ p-n type heterostructures were produced and tested for the degradation of methylene blue and 4-nitrophenol under visible light irradiation. CuO particles were produced in plate-like morphology using hydrothermal synthesis. SnO₂ nanofibers were obtained by electrospinning. Structural, morphological, optical and semiconducting property characterization of heterostructured CuO–SnO₂ and individual phases were performed. The photocatalytic activity was found to change depending on the amount of CuO particles in heterostructured samples. Among others, the sample with 0.35 wt.% CuO–SnO₂ showed the highest photocatalytic efficiency with a degradation rate constant ~2 h⁻¹. Active specie scavenger tests revealed that the decomposition reaction occurs through direct oxidation mechanism by the holes in the valence band of SnO₂ in pure samples whereas in CuO–SnO₂ samples and radicals also form and involve in the reactions. Further, the photocatalytic degradation mechanism was revealed using relative band potentials and p-n junctions of the heterostructured photocatalyst.     

Effect of interfacial energy on microstructure of a directionally solidified Al2O3/YAG eutectic ceramic

An Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic was prepared with a c‐axis sapphire seed by an optical floating zone furnace. The crystallographic relationships in the initial and the steady growth sections of the as‐grown eutectic ceramic were investigated by electron backscattering diffraction and transmission electron microscopy. The corresponding results were: <111 > {101}Y₃Al₅O₁₂ || <0001 > {}Al₂O₃ and <110 > {}Y₃Al₅O₁₂ || < > {0001}Al₂O₃, respectively. The steady orientation of the latter one shows smaller planar disregistry. Interfacial strain played the decisive role in affecting the solidification behavior of the Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic. The stability of interfaces with minimum interfacial strain and better ionic charge balance in irregular microstructure prevail upon the constraint of the seed. These results might cast light for the interfacial design of the Al₂O₃/Y₃Al₅O₁₂ binary eutectic ceramic.     

Dissolution of BaZrO3 refractory in titanium melt

Although numerous investigations have studied BaZrO₃ as a crucible refractory for melting titanium alloys, the interaction mechanism between them has not been clarified. In this study, a set of three designed alloys with different Ti composition (TiNi, Ti_1.5Ni, and Ti₂Ni) were melted in the BaZrO₃ crucibles. By using the X‐ray diffraction, optical, and scanning electron microscopy analysis, the interactions between the BaZrO₃ crucibles and the titanium melts were investigated. It was found that dissolution of the BaZrO₃ refractory into the titanium melts resulted in the crucible erosion and the melt contamination, the degree of which were both increased with the increasing of Ti content in the melts. The dissolution reaction could be determined as follows: . The oxygen content dissolved in TiNi, Ti_1.5Ni, and Ti₂Ni melts was thermodynamically calculated as 0.0055, 0.1922, and 0.2263 wt%, respectively, which were in agreement with the experimental results.     

Development of Ce‐PSZ‐/Y‐PSZ‐Toughened Alumina Inserts for High‐Speed Machining Steel

The nanocomposite CeO₂/Y₂O₃ partially stabilized zirconia (Ce‐PSZ/Y‐PSZ)‐toughened alumina was prepared by wet chemical simultaneous coprecipitation process. The thermal stability of phases and morphology of powders were characterized by TG‐DTA, FTIR, and FESEM. The microstructure, stabilization of phases and compositional analysis with different mol% CeO₂/Y₂O₃‐doped zirconia in alumina are characterized by FESEM, XRD, and EDAX spectra. Significant improvement in fracture toughness and flexural strength has been observed in 10 vol% of partially stabilized zirconia (2.5 mol% Y₂O₃ in ZrO₂/9 mol% CeO₂ in ZrO₂)‐toughened alumina, which is suitable for high‐speed machining applications.     

A Novel Scheme to Obtain Y2O2S:Er3+ Upconversion Luminescent Hollow Nanofibers via Precursor Templating

Y₂O₃:Er³⁺ hollow nanofibers were prepared by calcination of the monoaxial electrospinning‐derived PVP/[Y(NO₃)₃+Er(NO₃)₃] composite nanofibers, and then Y₂O₂S:Er³⁺ hollow nanofibers were synthesized by sulfurization of the as‐obtained Y₂O₃:Er³⁺ hollow nanofibers via a double‐crucible method using sulfur powders as sulfur source. X‐ray diffraction (XRD) analysis shows that the Y₂O₂S:Er³⁺ hollow nanofibers are pure hexagonal phase with the space group of . Scanning electron microscope (SEM) observation indicates that the Y₂O₂S:Er³⁺ hollow nanofibers are obvious hollow‐centered with the outer diameter of 176 ± 25 nm. Upconversion emission spectrum analysis manifests that Y₂O₂S:Er³⁺ hollow nanofibers emit strong green and weak red upconversion emissions centering at 526, 546, and 667 nm, respectively. The green emissions and the red emission are, respectively, originated from ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_l5/2 energy levels transitions of the Er³⁺ ions. The emitting colors of Y₂O₂S:Er³⁺ hollow nanofibers are located in the green region in CIE chromaticity coordinates diagram. The formation mechanism of the Y₂O₂S:Er³⁺ hollow nanofibers is also advanced. This preparation technique can be applied to prepare other rare‐earth oxysulfides hollow nanofibers.     

Characteristics,Thermodynamics, and Preparation of Nanocaged 12CaO·7Al2O3 and Its Derivatives

Crystal s tructures, characteristics, and preparations of 12CaO·7Al₂O₃ family and CaO–Al₂O₃ (C–A) system have been reviewed in detail with relevant thermodynamic parameters being assessed or recalculated. 12CaO·7Al₂O₃ (shortened as C12A7) can form several derivatives of type C12A7:M^n? or C12A7–M^n? through replacing so‐called “free oxygen ion” by many anions including OH^?, H^?, O^?, , F^?, Cl^?, and e^? in their cages, or being adopted by rare earth metals or alkaline earth metal oxides at cation sites (Ca²⁺ or Al³⁺). These doped C12A7 derivatives show unique material properties of transparent conduction, catalysis, and antibacterial with potential applications in fast ion conductors, optoelectronics, oxidants, and catalysts etc.     

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

The reactivity between rare‐earth (RE‐) oxide stabilized ZrO₂ or HfO₂ thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1?x) ZrO₂‐xY₂O₃ coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO₂ system. In general, samples with nominal compositions ZrO₂‐9Dy₂O₃, HfO₂‐7Dy₂O₃, ZrO₂‐8Y₂O₃, HfO₂‐6Er₂O₃, ZrO₂‐9.5Y₂O₃‐2.25Gd₂O₃‐2.25Yb₂O₃, and ZrO₂‐30Y₂O₃ exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.     

Competitive growth of Al2O3/YAG/ZrO2 eutectic ceramics during directional solidification: Effect of interfacial energy

The microstructure evolution and growth behavior of the Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ ternary eutectic ceramics during directional solidification were well investigated. During directional solidification of the Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics, {} Al₂O₃ paralleled with {001}ZrO₂ while they did not parallel with {001}YAG at the same time in the competitive growth stage. All of the interfaces parallel to each other finally. The area percentage of the Al₂O₃/ZrO₂ and YAG/ZrO₂ interfaces are 40.4 ± 0.2% and 30.8 ± 0.1%, respectively, higher than that of the Al₂O₃/YAG (28.8 ± 0.2%). The content of Al₂O₃ and YAG phases are 39.9% and 41.1%, respectively, almost double of that of ZrO₂. The interfaces of Al₂O₃/ZrO₂ and YAG/ZrO₂ are shorter and more dispersed than that of the Al₂O₃/YAG. It was found that the interfacial energy of Al₂O₃/ZrO₂ and YAG/ZrO₂ interfaces are lower than that of Al₂O₃/YAG. It can be concluded that interfacial energy plays a decisive role in affecting the crystallographic orientation and interfaces distribution in the Al₂O₃/YAG/ZrO₂ eutectic since the interfaces of Al₂O₃/ZrO₂ and YAG/ZrO₂ with lower interfacial energy can be formed more easily during directional solidification. Therefore, the contents of Al₂O₃/ZrO₂ and YAG/ZrO₂ interfaces are higher. This study can provide theoretical guidance for interface design of multi-phase materials.     

Low Thermal Conductivity of SnO2‐Doped Y2O3‐Stabilized ZrO2: Effect of the Lattice Tetragonal Distortion

Considering the phonon scattering effect and the stability of t′ zirconia, Sn⁴⁺ ion is recognized as an appropriate dopant to achieve the best combination of thermal insulating capability and durability of yttria‐stabilized zirconia thermal barrier coatings (TBCs). In this research, unusual lattice expansion and strong structural disordering were observed in a series of SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds, which are caused by the tetragonal distortion of oxygen coordination. Phonon scattering due to the structural disordering rather than point defects of Sn⁴⁺ substitutions predominates in reducing the thermal conductivity. However, deterioration of the thermal properties was observed at high doping content, which may be attributed to the t‐m phase transformation during the measurements. Considering the structure stability and thermal properties, SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds can be promising candidates for TBCs.     

Nickel element doping impacts on structure features and Faraday effects of magneto-optical transparent holmium oxide ceramics

The nickel element doped holmium oxide (Ho₂O₃:Ni) transparent magneto-optical ceramics were fabricated by vacuum sintering and the dopant impacts on structure features and Faraday effects were investigated. The starting oxide powders were synthesized by pyrolyzing the resulting layered holmium-based hydroxide nanosheets prepared from a chemical precipitation route using the sodium hydroxide as precipitant at the freezing temperature. Upon high-temperature sintering, the ${\mathrm{Ni}}_i^{••}$ defect is introduced by Ni²⁺ substitution for Ho³⁺ to form the interstitial solid solution. The 1 at.% Ni²⁺ doped Ho₂O₃ ceramic sample exhibits an in-line transmittance of ∼70.04% at 1 550 nm with a relative density of ∼99.88%, while more Ni²⁺ incorporation (e.g., 2‒5 at.%) even leads to a completely opaque state. The magneto-optical transparent Ho₂O₃:1%Ni ceramic developed in this work has Verdet constants of ∼−195, −65, and −29 rad/(T·m) at 635, 1 064, and 1 550 nm, respectively, which are ∼1.8-fold higher than the commercial terbium gallium garnet crystal or ∼1.4-fold higher than the pure Ho₂O₃ ceramic. This material also possesses relatively large figure of merit of ∼14.6°/T at 1 064 nm and relatively high thermal conductivity of ∼7.5 W/(m·K) at room temperature.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

Novel mullite-cordierite ceramic refractory fabricated from halloysite and talc

This research work aims to fabricate industrial refractory bricks using cheap and abundant kaolin DD₃ constituted of Halloysite. The fabrication process consists on mixing the crushed raw materials (Kaolin/Clay, Fireclay, Talc) with the respective proportions and an optimized granulometry, shaped by a uniaxial hydrostatic pressure of 250 bars, followed by drying at 100°C and sintering at 1300°C. The addition of Talc - at first with water, Fireclay and AP/DD₃- as a source of MgO promotes the increase of Cordierite formation in order to obtain high heat resistance shock and reduced thermal expansion. Three assays are studied with AP Clay and DD₃ Kaolin weight ratios (100/0, 50/50 and 0/100) % corresponding to (ER₀, ER₁ and ER₂) respectively. Mineralogical analysis results of the new refractory bricks, indicate the presence of Mullite and Cordierite as major phases besides Corundum and Quartz as minor phases. All products (ER₀, ER₁ and ER₂) are found to be argillaceous refractory with Alumina concentration (40 < Al₂O₃ < 45%, FC 40 group) and a melting point higher than 1600°C. The mechanical resistance of the samples is of the same order of magnitude; reaching 51 MPa.     

High-surface-area mesoporous silica-yttria-zirconia ceramic materials prepared by coprecipitation method ― the role of silicon

A high surface area is one of desired properties for yttria-zirconia (Y₂O₃–ZrO₂) ceramic materials given their catalytic applications. The objective of this study is to develop high-surface-area Y₂O₃–ZrO₂ materials by silicon (Si) modification and investigate the role of Si. Si-modified yttrium-zirconium hydroxides were prepared via a one-step precipitation process and calcined at 800 or 950 °C to form Si-modified Y₂O₃–ZrO₂ (denoted as SiO₂–Y₂O₃–ZrO₂) materials containing 0-20 wt% Si as SiO₂. These hydroxides or materials were characterized by ²⁹Si NMR, XPS, TG-DSC, XRD, UV Raman, TEM, and N₂ physisorption measurements. Si species uniformly distributed in the hydroxides tended to be enriched on the material surface at high temperatures. These Si species dominated by the silicates blocked the migration of Y and Zr atoms, which resisted the crystallite growth of Y₂O₃–ZrO₂ components and reduced their crystallite size. Therefore, the SiO₂–Y₂O₃–ZrO₂ possessed a surface area of 59-112 m²/g after calcination at 950 °C for 9 h, which was significantly higher than that of the Y₂O₃–ZrO₂ (23 m²/g). This study may stimulate ideas for developing high-surface-area crystalline ceramic materials calcined at high temperatures.     

Effect of Oxygen Vacancy on Electrical Property of Acceptor Doped BaTiO3–Na0.5Bi0.5TiO3–Nb2O5 X8R Systems

In this study, we reported a new BaTiO₃–Na_0.5Bi_0.5TiO₃–Nb₂O₅–Mn₂O₃/Fe₂O₃/Co₃O₄/In₂O₃ X8R system with high dielectric constant (>2100) at room temperature. The impacts of oxygen vacancy ( ) on dielectric, electrical conductivity, and ferroelectric properties were systematically studied. The Curie point is largely depended on the concentration, which can be confirmed by the dielectric behavior and A_1g octahedral breathing modes in Raman spectrum. In addition, the activation energy of diffusion is greatly reduced with the increase in concentration. It was found that the remnant polarization and coercive field were both decreased with increasing concentration, due to the facilitated defect dipoles reorientation and domain switching.     

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x = 0.10, Y_2/3?0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.