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.     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.     

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.     

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.     

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.     

Large-size-mismatch co-dopants for colossal permittivity rutile TiO2 ceramics with temperature stability

Colossal permittivity (CP) in donor-accepter co-doped rutile TiO₂ has attracted significant interest. Here, the CP behavior of (Ta?+?La) co-doped rutile TiO₂ ceramics were studied, where the ionic radii of Ta⁵⁺ and La³⁺ are much larger than that of Ti⁴⁺. The ceramics with an extremely low doping exhibit colossal dielectric permittivity (～2.6?×?10⁴) with an acceptable low dielectric loss (<0.07) in the frequency range from 40 to 10⁶?Hz. The CP properties obtained in (Ta?+?La) co-doped TiO₂ ceramics show excellent temperature stability over a wide temperature range of 20–400?°C. The X-ray diffraction analysis and the density functional theory calculation illustrates that the La₂³⁺${\text{V}}_{\text{o}}^{??}$Ti₂³⁺ and Ta₂⁵⁺Ti³⁺Ti⁴⁺ defect complexes with the lowest energy are responsible for the enhanced dielectric properties. Moreover, the defect complex formed by large-size trivalent substitutions and oxygen vacancy is very stable, and assists in improving temperature stability of the dielectric properties of co-doped rutile TiO₂ ceramics.     

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.     

Effects of Sr2+ substitution on the crystal structure,Raman spectra,bond valence and microwave dielectric properties of Ba3-xSrx(VO4)2 solid solutions

The effects of Sr²⁺ substitution for Ba²⁺ on microwave dielectric properties and crystal structure of Ba_3-xSr_x(VO₄)₂ (0 ≤ x ≤ 3, BSVO) solid solution were investigated. Such Sr²⁺ substitution contributes to significant reduction in sintering temperature from 1400 °C to 1150 °C. Both permittivity (∑_r) and quality factor (Q × f) values decreased with increasing x value, which was determined to be related with the descending values of average polarizability and packing fraction, whereas the increase in τ_f value was explained by the decreased average VO bond length, A-site bond valence. BSVO ceramics possessed encouraging dielectric performances with ∑_r = 12.2–15.6 ± 0.1, Q × f = 44,340 - 62,000 ± 800 GHz, and τ_f = 24.5–64.5 ± 0.2 ppm/°C. Low-temperature sintering was manipulated by adding B₂O₃ as sintering additive for the representative Sr₃V₂O₈ (SVO) ceramic and only 1 wt.% B₂O₃ addition successfully contributed to a 21.7% decrease in sintering temperature to 900 °C, showing good chemical compatibility with silver electrodes, which render BSVO series and SVO ceramics potential candidates in multilayer electronic devices fabrication.     

Significant improvement on quality factor in Bi3+/Ta5+ codoped Ce2Zr3-3xBi1.5xTa1.5xMo9O36 microwave dielectric ceramics with ultra-low sintering temperatures

Molybdenum oxide-based ceramics have attracted intense interest due to ultra-low sintering temperatures. However, low quality factors (Q × f) hinder their practical applications. Although Q × f can be improved by ions doping, the sintering temperature is greatly increased. Accordingly, it is still a challenge to obtain high Q × f ceramics sintered at ultra-low temperatures (<660 °C). Herein, (Bi_0.5Ta_0.5)⁴⁺ ions are utilized to tackle this issue in the Ce₂Zr₃(MoO₄)₉ ceramic as a prototype. Density and scanning electron microscope (SEM) results uncover good sintering states, and X-ray diffraction (XRD) results reveal the formation of solid solutions. Interestingly, the Ce-O bonds exhibit a dominant contribution to the bond ionicity (f_i), while Mo-O bonds play an important role in the lattice energy (U), the bond energy (E) and the thermal expansion coefficient (α). The remarkable increase of Q × f can be interpreted by the enhancement of the packing fraction and the mean U of Mo-O bonds. Moreover, the variations of the dielectric constant (ε_r) and the temperature coefficient of the resonance frequency (τ_f) can be explained by the variations of the intrinsic parameters. More interestingly, a negative correlation between Q × f and τ_f is first found. Typically, the CZ_0.98B_0.02 ceramic sintered at 650 °C exhibits optimum microwave dielectric properties: ε_r = 9.92, Q × f = 110,670 GHz, and τ_f = −19.20 ppm °C⁻¹. Notably, Q × f of the Ce₂Zr_2.94Bi_0.03Ta_0.03Mo₉O₃₆ (CZ_0.98B_0.02) ceramic is about 6 times larger than that of the matrix while retaining a low sintering temperature of 650 °C and a low ε_r of 9.92, making it a promising candidate for ultra-low temperature cofired ceramics (ULTCC) applications.     

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.     

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.     

Anisotropic magnetic property and exchange bias effect in a homogeneous Sillen-Aurivillius layered oxide

A new Sillen-Aurivillius oxide Bi₇Fe₂Ti₂O₁₇Cl (BFTOCl), in which four perovskite layers are sandwiched by Sillen slabs, is designed and prepared by the solid-state reaction. Compared with pure Aurivilius structure, the Sillen slab requires one less positive charge for charge compensation due to the intercalation of Cl anions between two (Bi₂O₂)²⁺, which increases the concentration of magnetic Fe³⁺ ions in perovskite slab, and may depress the interaction of Fe³⁺ ions existing in neighboring perovskite slabs. This unique natural superlattice structure and the highly orientated texture of the ceramic result in a complex spin structure and an interesting magnetic anisotropy. The magnetism in in-plane direction (0.00487 emu/g at 100 K) is about 2.5 times larger than that in out-of-plane direction. Remarkably, exchange bias anisotropy has been observed in the BFTOCl ceramic, of which the exchange bias fields in in-plane and out-of-plane directions are 345 and 174 Oe at 100 K, respectively.     

Structural,infrared reflectivity spectra and microwave dielectric properties of the Li7Ti3O9F ceramic

A cubic rock salt structured ceramic, Li₇Ti₃O₉F, was fabricated via the conventional solid-state reaction route. The synthesis conditions, sintering characteristics, and microwave dielectric properties of Li₇Ti₃O₉F ceramics were investigated by X-ray diffraction (XRD), thermal dilatometer, Scanning Electron Microscopy (SEM) accompanied with EDS mapping, and microwave resonant measurements. Rietveld refinement, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that Li₇Ti₃O₉F adopts a cubic rock-salt structure. The ceramic sintered at 950?°C presented the optimal microwave properties of ε_r?=?22.5, Q×f?=?88,200?GHz, and τ_f?=??24.2?ppm/^oC. Moreover, good chemical compatibility with Ag was verified through cofiring at 950?°C for 2?h. These results confirm a large potential for Li₇Ti₃O₉F ceramic to be utilized as substrates in the low temperature cofired ceramic (LTCC) technology. This work provides the possibility to exploit low-temperature-firing ceramics through solid solution between oxides and fluorides.     

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.     

n-Hexadecane and pyrene biodegradation and metabolization by Rhodococcus sp. T1 isolated from oil contaminated soil

The high-molecular weight polycyclic aromatic hydrocarbons(PAHs) pyrene and typical long chain alkane nhexadecane are both difficult to degrade. In this study, n-hexadecane and pyrene degrading strain Rhodococcus sp. T1 was isolated from oil contaminated soil. Strain T1 could remove 90.81% n-hexadecane(2 vol%) and 42.79% pyrene(200 mg·L~(-1)) as a single carbon within 5 days, respectively. Comparatively, the degradation of pyrene increased to 60.63%, but the degradation of n-hexadecane decreased to 87.55% when these compounds were mixed. Additionally, identification and analysis of degradation metabolites of Rhodococcus sp. T1 in the above experiments showed that there were significant changes in alanine, methylamine, citric acid and heptadecanoic acid between sole and dual substrate degradation. The optimal conditions for degradation were then determined based on analysis of the pH, salinity, additional nutrient sources and liquid surface activity.Under the optimal conditions of pH 7.0, 35 °C, 0.5% NaCl, 5 mg·L~(-1) of yeast extract and 90 mg·L~(-1) of surfactant,the degradation increased in single or dual carbon sources. To our knowledge, this is the first study to discuss metabolite changes in Rhodococcus sp. T1 using sole substrate and dual substrate to enhance the long-chain alkanes and PAHs degradation potential.     

Wind field reconstruction for the dispersion modeling of accidental chemical spills on complex geometry

Chemical spills on complex geometry are difficult to model due to the uneven concentration distribution caused by air flow over ground obstacles. Computational fluid dynamics (CFD) is one of the powerful tools to estimate the building-resolving wind flow as well as pollutant dispersion. However, it takes too much time and requires enormous computational power in emergency situations. As a time demanding task, the estimation of the chemical spill consequence for emergency response requires abundant wind field information. In this paper, a comprehensive wind field reconstruction framework is proposed, providing the ability of parameter tuning for best reconstruction accuracy. The core of the framework is a data regression model built on principal component analysis (PCA) and extreme learning machine (ELM). To improve the accuracy, the wind field estimation from the regression model is further revised from local wind observations. The optimal placement of anemometers is provided based on the maximum projection on minimum eigenspace (MPME) algorithm. The fire dynamic simulator (FDS) generates high-resolution data of wind flow over complex geometries for the framework to be implemented. The reconstructed wind field is evaluated against simulation data and an overall reconstruction error of 9% is achieved. When used in real case, the error increases to around 12% since no convergence check is available. With parameter tuning abilities, the proposed framework provides an efficient way of reconstructing the wind flow in congested areas.     

Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics

NiCuZn ferrite with superior magnetic performance is vital ceramic material in multilayer chip inductors (MLCI) applications. In this study, low-temperature-sintered Ni_0.22Cu_0.2Zn_0.58Fe₂O₄ ferrite ceramic doped with 1.0?wt% Bi₂O₃-x?wt% Nb₂O₅ (where x?=?0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized via solid-state reaction method. Effects of Bi₂O₃-Nb₂O₅ additives on microstructures and magnetic properties of NiCuZn ferrite ceramics sintered at 900?°C were systematically investigated. Results indicate that an appropriate amount of Bi₂O₃-Nb₂O₅ composite additives can significantly promote grain growth and densification of NiCuZn ferrite ceramics when sintered at low temperatures. Specifically, samples doped with 1.0?wt% Bi₂O₃ and 0.4?wt% Nb₂O₅ additives exhibited excellent initial permeability (~ 410 @ 1?MHz), high cutoff frequency (~ 10?MHz), high saturation magnetization (~ 54.92?emu/g), and low coercive force (~ 20.32?Oe). These observations indicate that NiCuZn ferrite ceramics doped with appropriate amounts of Bi₂O₃-Nb₂O₅ additives are great candidate materials for MLCI applications.     

Combustion synthesis of MoSi2 based composite and selective laser sintering thereof

Additive manufacturing is gaining increasing attention as it provides cost-effective and waste-less production of materials with multi-axis geometries. Selective laser sintering of ceramics is very challenging in terms of poor sinterability caused by low thermal shock resistance and insufficient electron conductivity blocking absorption of laser beam energy.Here, we present a novel strategy for manufacturing dense, hierarchically structured ceramics, particularly, MoSi₂-based composites by selective laser sintering. MoSi₂-Si composite powders were prepared by combustion synthesis technique, where the ceramic grains were covered with different amount of Si. MoSi₂-Si powder was consolidated by selective laser sintering reaching 92% of density. The hardness of the manufactured samples varied with the amount of Si and applied laser current from 7.7–11.4?GPa. The maximum value of the compressive strength was determined to be 636?MPa. The manufactured MoSi₂-Si was subjected to nitridation, which resulted in the growth of Si₃N₄ fibres on the surface and pores of the samples.