High-Q (Na1-xAgx)2WO4 (x = 0.1, 0.2) ceramics with ultra-low sintering temperature

The solid solution (Na_1-xAg_x)₂WO₄ (x = 0.1, 0.2) ceramics with ultra-low sintering temperatures were prepared by a modified solid-state reaction method. Through introducing Ag⁺ substitution at the Na⁺-site, the sintering temperature of the (Na_1-xAg_x)₂WO₄ ceramics have been lowered from 565℃ to 510–520℃, while their dielectric lose is still keeping low. The (Na_0.9Ag_0.1)₂WO₄ ceramic can be sintered well at 520℃ with a permittivity of 5.8, a Q × f value of 97 600 GHz and a temperature coefficient of ?70 ppm/℃ at 12.6 GHz. After being sintered at 510℃, the (Na_0.8Ag_0.2)₂WO₄ ceramic possesses the best properties with a permittivity of 6.1, a Q × f value of 70 600 GHz and a temperature coefficient of ?72 ppm/℃ at a frequency of 12.5 GHz. Due to the excellent dielectric properties, the (Na_1-xAg_x)₂WO₄ (x = 0.1, 0.2) ceramics are potential candidate for Low-Temperature Co-fired Ceramics (LTCC) applications.     

Temperature stable 0.35Ag2MoO4-0.65Ag0.5Bi0.5MoO4 microwave dielectric ceramics with ultra-low sintering temperatures

In this study, the novel temperature-stable (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ microwave dielectric ceramics were prepared by a modified solid-state reaction method. The phase composition, microstructures and microwave dielectric properties of the (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ ceramics were investigated. All the compounds can be sintered well at ultra-low temperatures (<540 °C). The XRD and SEM analysis indicate that the Ag₂MoO₄ and the Ag_0.5Bi_0.5MoO₄ can coexist with each other. When x = 0.65, the ceramics exhibit the best microwave dielectric properties with a relative permittivity of 23.9, a Q × f value of 16,200 GHz (at 7.3 GHz) and a near-zero TCF value of -2.4 ppm/°C at 520 °C. The results indicate that temperature-stable (1-x)Ag₂MoO₄-xAg_0.5Bi_0.5MoO₄ ceramics are promising candidates for low temperature co-fired ceramics (LTCC) applications.     

Distribution of Tm3+ and Ni2+ in chalcogenide glass ceramics containing Ga2S3 nanocrystals: Influence on photoluminescence properties

The distribution of Tm³⁺ and Ni²⁺ ions is unambiguously exhibited in 80GeS₂-20Ga₂S₃ chalcogenide glass ceramics (GCs) containing Ga₂S₃ nanocrystals (NCs) by using advanced analytical transmission electron microscopy. Distinctively different distribution patterns of Tm³⁺ and Ni²⁺ ions are observed in the GCs obtained by controlled crystallization. The distribution of the dopants imposes strong influence on their optical properties which are revealed by absorption and photoluminescence (PL) spectra. Detailed discussions are given of the mechanisms of the crystallization-induced PL enhancement and quenching of the Tm³⁺ mid-infrared and Ni²⁺ near-infrared emissions, respectively.     

Mechanical and dielectric properties of 3D printed highly porous ceramics fabricated via stable and durable gel ink

A novel, efficient, versatile strategy was carried out to fabricate highly porous ceramic parts based on the combination of strong colloidal gel ink fabricated with high boiling point organic solvents and DIW technique. The preparation and optimization of inks and the effect of heating temperature on the phase composition, microstructure, mechanical properties and dielectric properties of ceramic parts were systematically investigated. The strong colloidal ink exhibits excellent ambient stability and printability. The sintering temperatures bring about the evolution of phases, structural mechanical properties and dielectric properties of ceramic parts. Ultimately, Si₂N₂O single wall ceramic parts with a frame density of 1.07?1.14 g/cm³ and an apparent porosity of 53.13 ± 1.29% were successful fabricated. The dielectric constant and dielectric loss of Si₂N₂O sample (1650℃) are only 4.24 and 0.0049, respectively. This strategy provides a reference for in-situ synthesis of high-performance porous ceramic components based on the DIW.     

Improvement of Al3+ ion conductivity by F doping of (Al0.2Zr0.8)4/3.8NbP3O12 solid electrolyte for mixed potential NH3 sensors

New Al³⁺ ion conducting solid electrolytes (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x(0?≤x?≤?0.4) with Nasicon-structure are successfully prepared by solid state reaction method. The influences of the doped F^- content on the properties of the (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x samples are investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The results show that F^- doping can effectively improve the sinterability and the total conductivity of the (Al_0.2Zr_0.8)_4/3.8NbP₃O_12-xF_2x samples. Among the solids series, (Al_0.2Zr_0.8)_4/3.8NbP₃O_11.7F_0.6 shows the highest conductivity of 1.53?×?10^?3 S?cm^?1at 500?°C, which is approximately 7.9 times higher than that of the undoped (Al_0.2Zr_0.8)_4/3.8NbP₃O₁₂. The ion transference number of the samples is higher than 0.99 at 300–700?°C. On the basis of the promising properties, a mixed-potential type NH₃ sensor based on (Al_0.2Zr_0.8)_4/3.8NbP₃O_11.7F_0.6 electrolyte and In₂O₃ sensing electrode has been developed. The sensing performance of the sensor is evaluated. The mixed-potential type sensor can work at relatively low temperatures of 200–350?°C and an excellent sensitivity of 99.71?mV/decade at 250?°C is obtained. The sensor also displays excellent stability and reproducibility, accompanied by low cross-sensitivities to CO₂, CH₄ and H_2.     

Improved H2-generation performance of Pt/CdS photocatalyst by a dual-function TiO2 mediator for effective electron transfer and hole blocking

Surface modification with noble metal cocatalysts was proved to be a useful route for boosting photocatalytic efficiency of various photocatalysts. Nevertheless, considering the random dispersion of metallic cocatalysts on the photocatalyst surface, the noble metal-loaded photocatalyst generally shows a limited enhancement of its activity because the noble metals can also work as the recombination sites of photoinduced charges. In this paper, TiO₂ as a dual-function mediator (for effective electron transport and hole block) is successfully introduced into the interface of Pt and CdS to form PtTiO₂/CdS photocatalyst, with an aim of suppressing the high recombination rate of electron-hole pairs on the Pt active sites. Under visible light, all the prepared PtTiO₂/CdS displayed distinctly enhanced photocatalytic hydrogen-generation performance and the PtTiO₂/CdS(8%) attains the highest photocatalytic H₂-production rate (294.2?μmol/h), a value significantly higher than that of Pt/CdS about 3.2 time. A dual-function TiO₂-mediated mechanism was put forward to account for the superior hydrogen production of PtTiO₂/CdS photocatalyst, namely, the TiO₂ layer in the PtTiO₂/CdS not only works as electron-transport layers to effectively transfer photogenerated electrons to promote the H₂-production reaction on Pt cocatalysts, but also acts as hole-block layer to prevent the possible recombination of photogenerated charges on the Pt active sites, resulting in a distinct improvement of final H₂-generation activity.     

A novel ultra-high-temperature oxidation protective MoSi2-TaSi2 ceramic coating for tantalum substrate

To protect refractory metal against oxidation at ultra-high temperatures, a MoSi₂-TaSi₂ ceramic coating was prepared on a pure tantalum (Ta) substrate using a novel three-step process, which included dip-coating with a molybdenum slurry, vacuum sintering, and halide-activated pack cementation (HAPC). The original coating had a MoSi₂-TaSi₂ double-layer structure from the surface to the substrate. After oxidation at 1700°C for 8 h in air, the coating exhibited a complex multi-layer structure composed of SiO₂-Mo₅Si₃-MoSi₂-(Mo,Ta)₅Si₃-TaSi₂-Ta₅Si₃ from the outer layer to the inner layer, due to the high-temperature phase transition and diffusion of Si and O. The coating effectively protected the Ta substrate at 1700°C for 12 h without failure, thereby demonstrating great improvement to its service life in an ultra-high-temperature aerobic environment. The protective effect was attributed to the integrity of the ceramic coating and the formation of a dense, uniform SiO₂ film that effectively lowered the inward oxygen diffusion rate.     

Reactive wetting and interfacial characterization of ZrO2 by SnAgCu-Ti alloy

The reactive wetting behavior of zirconia with SnAgCu-x%Ti (SAC-x%Ti, wt%, x?=?1, 4) alloy was investigated via the sessile drop method in isothermal experiments. As temperatures elevated, the final contact angle decreased and the minimum contact angle of 21° and 7° were obtained at 1000?°C for SAC-1%Ti and SAC-4%Ti droplets, respectively. Kinetic calculations indicated that the spreading of SAC-Ti droplets on zirconia was controlled by interfacial reaction and the wetting activation energy was 108.8?kJ/mol. The reaction products distribution and morphology in droplets were influenced vastly by the addition of Ti. Along with the increase of Ti content from 1% to 4%, a great deal of Ti-Sn intermetallic compounds (IMCs) were generated in droplets, thereby the outline of droplets were transformed from hemispherical into similar trapezoidal due to the limited spreading and fluidity of droplets. Owing to the interfacial reaction between active elements Ti and zirconia and the subsequent formation of the Ti-O layer, the wettability of SAC-Ti/zirconia was greatly promoted. According to transmission electron microscopy (TEM) analysis, the thin Ti-O reaction layer consisted of the Ti₂O, Ti₄O₇, Ti₇O₁₃ and TiO₂ phase.     

Effects of sintering method and BiFeO3 dopant on the dielectric and ferroelectric properties of BaTiO3–BiYbO3 based solid solution ceramics

(0.95–x) BaTiO₃–0.05 BiYbO₃–x BiFeO₃ (x?=?0, 0.01, 0.02, and 0.04) (abbreviated as (0.95–x) BT–0.05 BY–x BFO) ceramics were fabricated by conventional sintering (CS) and microwave sintering (WS) methods. Effects of sintering method and BFO dopant on the microstructure and electric properties of (0.95–x) BT–0.05 BY–x BFO ceramics were comparatively investigated. X-ray diffraction showed that all CS and WS samples presented a single perovskite phase. It was also found that WS ceramics possessed denser microstructure and finer grains compared to CS samples as indicated by the surface morphology characterization. Dielectric measurements revealed that all samples exhibited the weak relaxation behavior; however, the degree of relaxation behavior of BT–BY based ceramic could be strengthened by addition of BFO and by WS method. Moreover, the temperature and frequency stability could be improved with doped BFO. The density of 0.93BT–0.05BY–0.02BFO ceramic was found to be the largest while that of 0.94BT–0.05BY–0.01BFO ceramic was the smallest, thus, the dielectric constant of 0.93BT–0.05BY–0.02BFO was significantly larger than that of 0.94BT–0.05BY–0.01BFO and 0.94BT–0.05BY–0.04 BFO ceramics. minimum dielectric constant of (0.95–x) BT–0.05 BY–x BFO ceramic was obtained at x?=?0.01. Ferroelectric measurements indicated that all samples showed the slim hysteresis loop. The remnant polarization (P_r) and coercive field (E_C) of (0.95–x) BT–0.05 BY–x BFO ceramics first decreased and then increased with increasing x,the minimum values were obtained at x?=?0.01. Moreover, P_r and E_C of WS ceramics were slightly larger than those of CS ceramics, indicating that higher density and larger grain sizes contributed to enhancing the ferroelectric characteristic. These findings indicate that addition of moderate amount of BFO and use of WS technique can strengthen the degree of relaxation behavior and improve the ferroelectric properties of BT–BY based ceramics.     

Hot corrosion behavior of TiO2 doped,Yb2O3 stabilized zirconia exposed to V2O5 + Na2SO4 molten salt at 700–1000 °C

3.5 mol% Yb₂O₃ stabilized zirconia (YbSZ) doped with 10 mol% TiO₂ (Ti-YbSZ) was produced, and its hot corrosion behavior exposed to Na₂SO₄ + V₂O₅ molten salt was investigated. The as-fabricated ceramic mainly consists of metastable tetragonal (t′) phase. When exposed to the molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 2 h and 10 h, YbVO₄ and m-ZrO₂ formed as corrosion products due to chemical reactions between the ceramics and the salt. Ti⁴⁺ in Ti-YbSZ solid solution keeps stable during the hot corrosion tests, which acts as a stabilizer for ZrO₂, preventing total decomposition of the t′ phase. After the hot corrosion tests, Ti-YbSZ has an apparently lower m phase content than Y₂O₃ doped Zirconia and YbSZ, indicative of better corrosion resistance. The hot corrosion mechanism of Ti-YbSZ is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Engineering an ultrathin amorphous TiO2 layer for boosting the weatherability of TiO2 pigment with high lightening power

TiO₂ pigments are typically coated with inert layers to suppress the photocatalytic activity and improve the weatherability. However, the traditional inert layers have a lower refractive index compared to TiO₂, and therefore reduce the lightening power of TiO₂. In the present work, a uniform, amorphous, 2.9-nm-thick TiO₂ protective layer was deposited onto the surface of anatase TiO₂ pigments according to pulsed chemical vapor deposition at room temperature, with TiCl₄ as titanium precursor. Amorphous TiO₂ coating layers exhibited poor photocatalytic activity, leading to a boosted weatherability. Similarly, this coating method is also effective for TiO₂ coating with amorphous SiO₂ and SnO₂ layers. However, the lightening power of amorphous TiO₂ layer is higher than those of amorphous SiO₂ and SnO₂ layers. According to the measurements of photoluminescence lifetime, surface photocurrent density, charge-transfer resistance, and electron spin resonance spectroscopy, it is revealed that the amorphous layer can prevent the migration of photogenerated electrons and holes onto the surface, decreasing the densities of surface electron and hole, and thereby suppress the photocatalytic activity.     

Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y₂O₃-5Al₂O₃ (mole ratio of 3:5) and 3Y₂O₃-5Al₂O₃-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y₂O₃-5Al₂O₃-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y₂O₃-5Al₂O₃ is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.     

Excellent optical transparency of potassium-sodium niobate-based lead-free relaxor ceramics induced by fine grains

Here we report a lead-free multifunctional material (1-x)(K_0.5Na_0.5)NbO₃-xBa(Mg_1/3Nb_2/3)O₃ prepared by pressure-less sintering procedure. X-ray diffraction indicates a gradual crystal structure transformation with increasing x. Microstructural observation demonstrates that the addition of Ba(Mg_1/3Nb_2/3)O₃ additive reduces the size of grains with clear grain boundary, which is favorable for a high optical transmittance. The relaxor characteristics of the ceramics could lead to further enhancement of the transparency, owing to the low defects and weak light scattering. Notably, the ceramics with 0.04 ≤ x ≤ 0.06 all show good transparency over 70% in visible region and 80% in infrared region.     

Ultra-precision grinding of AlON ceramics: Surface finish and mechanisms

Aluminum oxynitride (AlON) can be effectively finished by ultra-precision grinding. In this work, the ultra-precision grinding experiment was conducted on AlON to investigate surface characteristics and material removal mechanism. The ground surface has an unusual non-uniform morphology resulted from the different material removal modes. Grazing incidence X-ray diffraction (GIXRD), nanoindentation and Electron Back-Scattered Diffraction (EBSD) were carried out to study the micro-properties of AlON. The results revealed that the micro mechanical properties vary with the grain orientation on the surface. The morphologies of ground surface are consistent in the twinned grains and change with the grain orientation. By comparing the relationship of machining size and grain size, the material removal modes of individual grains should be taken into consideration during ultra-precision grinding. Based on this, a simple theoretical model was proposed to explain the material removal mechanism of AlON under ultra-precision grinding.     

Optimized strain with small hysteresis and high energy-storage density in Mn-doped NBT-ST system

In this paper, optimized strain obtained at low electric field and accompanied with small hysteresis was achieved simultaneously by inducing defect dipoles into 0.7NBT-0.3ST (NBST) system. Narrow S-E curves with none S_rem and large strains of 0.29% and 0.32% with small hysteresis of 24% and 28% were realized at 60 kV/cm in 1 and 5 mol.‰ Mn-doped NBST caramics, respectively. High electrostriction coef?cient of 0.0215 and 0.021 m⁴C² with pure electrostrictive characteristics and large strain can also be obtained in Mn-doped NBST ceramics. Meanwhile, the electric breakdown field (BDS) of NBST ceramics has been significantly improved after doping with MnO, and large recoverable energy density of 0.93 and 0.97 J/cm³ with a relatively high energy-storage efficiency were obtained accordingly. The findings may pave a way for further optimizing strains, electrostrictive effects and energy-storage properties through the proper selection of base composition and effective chemical modifier.     

Simple route to fabricate nanowire-assembled 3D AlGaN nanourchins by chemical vapour deposition

Large-scale nanowire-assembled three-dimensional (3D) AlGaN nanourchins are synthesized under various growth conditions via a facile chemical vapour deposition (CVD) method. Characterizations of the morphology, microstructure, and crystalline phase of the as-prepared products reveal that the urchin-like AlGaN nanostructures with a hexagonal wurtzite structure are pure, and have a homogenous composition distribution, and also that they are composed of an assembly of nanowires with sharp tips and lengths of 1–2.5?µm. Effects of growth temperature, growth time, and introduction of NH₃ at different substrate temperatures (T_sub) on the growth of the AlGaN nanostructures are systematically investigated. The investigation results show that T_sub is a crucial factor influencing the formation of the 3D AlGaN nanourchins, and an increase in growth temperature results in a variation of their crystal quality and size. The growth mechanism of the urchin-like AlGaN nanostructures is discussed on the basis of the morphological evolution of the samples obtained under different growth conditions. The unique optical properties of the AlGaN nanostructures as evidenced by the photoluminescence spectra suggest the potential of their applicability in nanoscale optoelectronics.     

Microwave dielectric properties of low firing temperature stable scheelite structured (Ca,Bi)(Mo,V)O4 solid solution ceramics for LTCC applications

In the present work, a systematic study on microwave properties of Ca_1-xBi_xMo_1-xV_xO₄ (0.2 ≤ x ≤ 0.5) solid solution ceramics synthesized by using the traditional solid-state reaction method was conducted. A scheelite structured solid solution was formed in the composition range 0.2 ≤ x ≤ 0.5. We successfully prepared a microwave dielectric ceramic Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ with a temperature coefficient of resonant frequency (TCF) near to zero and a low sintering temperature by using (Bi, V) substituted (Ca, Mo) in CaMoO₄ to form a solid solution. The Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ ceramic can be well sintered at only 870 °C and exhibits good microwave dielectric properties with a permittivity (ε_r) ?21.9, a Qf ?18,150 GHz (at 7.2 GHz) (Q = quality factor = 1/dielectric loss; f = resonant frequency), a TCF ? + 0.1 ppm/°C. The chemical compatibility with silver indicated that the Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ ceramic might be a good candidate for the LTCC applications.     

Effects of deformation rate on properties of Nd,Y-codoped CaF2 transparent ceramics

Different deformation rates of Nd,Y-codoped CaF₂ transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF₂ ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF₂ ceramic (△a?=?62%) sample exhibited a high transparency (T_a?>?91%, 3-mm thick,250?～?1200?nm), low light scattering, superior fracture toughness (K_1c?～?0.71?MPa·m^1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material.