Microwave dielectric properties of the (1 − x)Mg2TiO4-xCaTiO3-y wt.% ZnNb2O6 ceramics system

Composite ceramics based on (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12-0.16, y = 0-8) were prepared by a conventional mixed-oxide route. Zn²⁺ partially replaced Mg²⁺ in Mg₂TiO₄ and formed the spinel-structured (Mg_1−δZn_δ)₂TiO₄ phase. Nb²⁺, is known to be solid soluble in CaTiO₃, was found to change its shape from cubic to pliable. A bi-phase system (Mg_1−δZn_δ)₂TiO₄ and CaTiO₃ exhibited in all samples, where a small amount of second phase Mg_1−δZn_δTiO₃ was also detected. The microwave dielectric properties of specimens were strongly related to ZnNb₂O₆ and CaTiO₃ content. As y increased, ?_r and τ_f decreased, however, Q × f decreased to a minimum value and started to increase thereafter. It was also found that ?_r and τ_f increased and Q × f decreased with increasing x. The optimized microwave dielectric properties with ?_r = 18.37, Q × f = 31,027 GHz (at 6 GHz), and τ_f = 0.51 ppm/°C were achieved for (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12, y = 4) sintered at 1360 °C for 6 h.     

Solubility limits and microwave dielectric properties of Ba6−3xSm8+2xTi18O54 solid solution

Upper and lower solubility limits in Ba_6−3xSm_8+2xTi₁₈O₅₄ tungsten bronze ceramics were determined by Rietveld refinement of XRD data combined with backscattered electron images, and the variation tendency of microwave dielectric characteristics was also investigated. The upper solubility limit was confirmed as x = 2/3, while the lower solubility limit was determined as 1/4 instead of the previously reported one x = 3/10. The dielectric constant of Ba_6−3xSm_8+2xTi₁₈O₅₄ ceramics decreases monotonically with increasing x, while the small temperature coefficient of resonant frequency with complex variation tendency is observed for the compositions 1/2 ≤ x ≤ 4/5. The Qf value increases at first, reaches the maximum around x = 2/3, and turns to decrease for x > 7/10.     

Effects of CaSiO3 addition on sintering behavior and microwave dielectric properties of Al2O3 ceramics

The effects of CaSiO₃ addition on the sintering behavior and microwave dielectric properties of Al₂O₃ ceramics have been investigated. The addition of CaSiO₃ into Al₂O₃ ceramics resulted in the emergence of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which acting as liquid sintering aids can effectively lower the sintering temperature of Al₂O₃ ceramic. The Q × f value of Al₂O₃-CaSiO₃ ceramics decreased with the CaSiO₃ addition increasing because of the lower Q × f value of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Compared with the pure CaSiO₃ ceramic, the Al₂O₃-CaSiO₃ ceramic with 20 wt% CaSiO₃ addition possessed good dielectric properties of ?_r = 9.36 and Q × f = 13,678 GHz at the similar sintering temperature.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

Structural and electrochemical characteristics of Li4−xAlxTi5O12 as anode material for lithium-ion batteries

Al-doped Li₄Ti₅O₁₂ in the form of Li_4−xAl_xTi₅O₁₂ (x = 0, 0.05, 0.1 and 0.2) was synthesized via solid state reaction in an Ar-flowing atmosphere. Al-doping does not change the phase composition and particle morphology, but easily results in the lattice distortion and thus the poor crystallinity of Li₄Ti₅O₁₂. Al-doping decreases the specific capacity of Li₄Ti₅O₁₂, while improves remarkably its cycling stability at high charge/discharge rate. The substitution of Al for Li site can enhance the electronic conductivity of Li₄Ti₅O₁₂ via the generation of mixing Ti⁴⁺/Ti³⁺, whereas impede the Li-ion diffusion in the lattice. Excessive Al causes large electrode polarization due to the lower Li-ion conductivity, and thus leads to low specific capacity at high current densities. Li_3.9Al_0.1Ti₅O₁₂ exhibits a relatively high specific capacity and an excellent cycling stability.     

High piezoelectric coefficient of Pr2O3-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

Pr₂O₃-doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ-xPr) ceramics were prepared by the conventional solid-state method. A tetragonal phase is only observed in these ceramics, and the introduction of Pr₂O₃ decreases their sintering temperature without affecting negatively the piezoelectric constant. Enhanced ferroelectric properties were obtained in these BCTZ-xPr ceramics. The ceramic with x=0.06 wt% exhibits a good electrical behavior of d₃₃∼460 pC/N, k_p∼47.6%, ε_r∼4638, and tan δ∼0.015 when sintered at a low temperature of ∼1400 °C. As a result, the BCTZ-xPr ceramic is a promising candidate for lead-free piezoelectric ceramics.     

Synthesis and transport properties in La2−xAxMo2O9−δ (A = Ca, Sr, Ba, K) series

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18-20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5% H₂-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Electrical conductivity and redox stability of La2Mo2−xWxO9 materials

A series of compounds La₂Mo_2−xW_xO₉ (x = 0-2) were synthesized using a freeze-dried precursor method at relatively low temperatures (673-823 K). These materials were characterised by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetric (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and dilatometric measurements. Oxygen stoichiometry was evaluated by coulometric titration and thermogravimetric analysis at 873-1273 K. The ionic and electronic conductivities of these materials were analysed by impedance spectroscopy and a Hebb-Wagner ion-blocking method under moderately reducing conditions. The presence of W⁶⁺ leads to an increase of the stability range (about 10⁻¹⁶ Pa for La₂Mo_0.5W_1.5O₉ at 1073 K) and prevents oxygen loss and amorphisation. Within the stability range, the electronic conductivity increases gradually as the temperature increases and as the oxygen partial pressure reduces. This indicates that the electronic transport is mainly n-type as a result of the oxygen-content decreasing in the molybdate lattice. Further reduction of the oxygen partial pressure gave rise to the decomposition of La₂Mo_2−xW_xO₉, leading to the formation of new phases with molybdenum in lower oxidation states, which further enhances the electronic conductivity. The results of the coulometric titration and the thermogravimetric studies under a dry 5% H₂/Ar flow suggest that tungsten doped lanthanum molybdate materials can be used as electrolyte only at low temperature and under moderate reducing conditions.     

Electrical and microwave dielectric properties of K2Nb8O21 microwires

K₂Nb₈O₂₁ microwires with diameters of several hundred nanometers and lengths up to tens of microns were prepared by molten salt synthesis, and characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The as-synthesized microwires have semiconductor characteristics with band gap E_g of about 3.1 eV, consistent with electrical transport measurement of single microwire, which gives a resistivity of about 0.46 Ω m at room temperature. Microwave dielectric property measurement at 2–18 GHz shows that K₂Nb₈O₂₁ microwires have a relative low complex permittivity. A weak dielectric relaxation occurs in the 12–16 GHz band due to the free charges polarization on the interfaces between K₂Nb₈O₂₁ microwires and the PVB matrix.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Temperature compensating ZnAl2O4-Co2TiO4 spinel-based low-permittivity microwave dielectric ceramics

TiO₂, CaTiO₃ and SrTiO₃ were added to the 0.79ZnAl₂O₄-0.21Co₂TiO₄ (ZACT in abbreviation) system to control its temperature coefficient of resonant frequency (τ_f). The effects of these additions on sinterability, phase compositions and microwave dielectric properties of the ceramics synthesized by the solid-state reaction were investigated. The results show that TiO₂, CaTiO₃ and SrTiO₃ can all reduce the densification temperature of the ZACT ceramics within the scope from 75 to 150 °C. CoTi₂O₅ second-phase with negative τ_f value appears in the TiO₂ doped ZACT system, which inhibits TiO₂ addition's function for adjusting τ_f value of ZACT ceramics. While, CaTiO₃ and SrTiO₃ can both tune effectively τ_f value to obtain temperature-stable materials.     

Transparent polycrystalline MgAl2O4 ceramic fabricated by spark plasma sintering: Microwave dielectric and optical properties

The optical properties and microwave dielectric properties of transparent polycrystalline MgAl₂O₄ ceramics sintered by spark plasma sintering (SPS) through homemade nanosized MgAl₂O₄ powders at temperatures between 1250 °C and 1375 °C are discussed. The results indicate that, with increasing sintering temperatures, grain growth and densification occurred up to 1275 °C, and above 1350 °C, rapid grain and pore growth occurred. The in-line light transmission increases with the densification and decreases with the grain/pore growth, which can be as high as 70% at the wavelength of 550 nm and 82% at the wavelength of 2000 nm, respectively. As the sintering temperature increases, Q×f and dielectric constant ε_r values increase to maximum and then decrease respectively, while τ_f value is almost independent of the sintering temperatures and remains between −77 and −71 ppm/°C. The optimal microwave dielectric properties (ε_r=8.38, Q×f=54,000 GHz and τ_f=−74 ppm/°C) are achieved for transparent MgAl₂O₄ ceramics produced by spark plasma sintering at 1325 °C for 20 min.     

Effects of Al2O3 addition on the sintering behavior and microwave dielectric properties of CaSiO3 ceramics

The effects of Al₂O₃ addition on the densification, structure and microwave dielectric properties of CaSiO₃ ceramics have been investigated. The Al₂O₃ addition results in the presence of two distinct phases, e.g. Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which can restrict the growth of CaSiO₃ grains by surrounding their boundaries and also improve the bulk density of CaSiO₃-Al₂O₃ ceramics. However, excessive addition (≥2 wt%) of Al₂O₃ undermines the microwave dielectric properties of the title ceramics since the derived phases of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈ have poor quality factor. The optimum amount of Al₂O₃ addition is found to be 1 wt%, and the derived CaSiO₃-Al₂O₃ ceramic sintered at 1250 °C presents improved microwave dielectric properties of ?_r = 6.66 and Q × f = 24,626 GHz, which is much better than those of pure CaSiO₃ ceramic sintered at 1340 °C (Q × f = 13,109 GHz).     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Influence of A-site Ba substitution on microwave dielectric properties of (BaxMg1−x)(A0.05Ti0.95)O3 (A=Zr,Sn) ceramics

The microwave dielectric properties of (Ba_xMg_1−x)(A_0.05Ti_0.95)TiO₃ (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τ_f value increased, but the Q×f value decreased. The sintered (Ba_xMg_1−x)(Zr_0.05Ti_0.95)TiO₃ (called B_xMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (Ba_xMg_1−x)(Sn_0.05Ti_0.95)TiO₃ (called B_xMST) ceramics, the dielectric constant increased from 19 to 20.5, Q×f value increased from 120,000 to 37,000 (GHz), and the τ_f value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba_0.1Mg_0.9)(Sn_0.05Ti_0.95)TiO₃ ceramics exhibited dielectric constant of 20.5, Q×f value of 37,000 (GHz), and a near-zero τ_f value of −3.3 ppm/°C sintered at 1210 °C for 4 h.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.