Improved dielectric properties of Ba0.5Sr0.5TiO3–ZnAl2O4 composite ceramics using double sintering

Ba_0.5Sr_0.5TiO₃–ZnAl₂O₄ composite ceramics were prepared by double sintering and conventional sintering. The results show that the double sintering can effectively reduce the ion diffusion between Ba_0.5Sr_0.5TiO₃ and ZnAl₂O₄ phases. The double sintered samples exhibit higher density and more uniform grain size distribution than the conventional sintered samples. The dielectric permittivity of double sintered samples is lower than that of conventional sintered samples. Impedance spectrum analysis shows that the oxygen vacancy content and grain boundary resistance of the double sintered samples is lower than that of the conventional sintered samples, which indicates that the Q value of the double sintered samples is higher than that of the conventional sintered samples. The optimum dielectric tunability and Q value of double sintered 60 wt%Ba_0.5Sr_0.5TiO₃-40 wt%ZnAl₂O₄ sample are 23.4% at 30 kV/cm and 276 at 2.257 GHz, respectively. Therefore, double sintering is a strategy that can effectively adjust the dielectric tunability and Q value of BST-ZA composite ceramics.     

Microwave dielectric properties of high dielectric tunable - low permittivity Ba0.5Sr0.5TiO3–Mg2(Ti0.95Sn0.05)O4 composite ceramics

Ba_0.5Sr_0.5TiO₃–Mg₂(Ti_0.95Sn_0.05)O₄ composite ceramics have been synthesized by the solid-state reaction. Phase structure, microstructure and microwave dielectric properties have been systematically characterized. The permittivity is tailored to a certain extent with the addition of Mg₂(Ti_0.95Sn_0.05)O₄. Both X-ray diffraction (XRD) and back electric image (BEI) analysis show the co-existence of two-phase structures of ABO₃ perovskite and A₂BO₄ spinel structure. A high dielectric tunablity can be obtained and a high Q value can be achieved at microwave frequency. The composition 30 wt.%Ba_0.5Sr_0.5TiO₃–70 wt.%Mg₂(Ti_0.95Sn_0.05)O₄ exhibits good dielectric properties with ? of 79, Q of 152 (at 2.997 GHz) and T of 15.8% (30 kV/cm & 10 kHz) at room temperature, which make it a promising candidate for tunable microwave device applications in the wireless communication system.     

Property optimization of Ba0.4Sr0.6TiO3–BaMoO4 composite ceramics for tunable microwave applications

(1 ? x)Ba_0.4Sr_0.6TiO₃–xBaMoO₄ ceramics with x = 5, 10, 20, 30, 40 and 60 wt% were prepared by traditional solid-state reaction method. Two crystalline phases, a cubic perovskite structure Ba_0.4Sr_0.6TiO₃ (BST) and a tetragonal scheelite structure BaMoO₄ (BM) were obtained by XRD analysis. The microwave dielectric properties of Ba_0.4Sr_0.6TiO₃–BaMoO₄ composite ceramics were investigated systematically. The results show that the composite ceramics exhibited promising microwave properties. The dielectric constant can be adjusted in the range from 900 to 78, while maintaining relatively high tunability from 27.3% to 12.8% under a direct current electric field of 60 kV/cm and Q values from 619 to 67 in the gigahertz frequency region.     

In situ synthesis of Ba0.5Sr0.5TiO3–Mg3B2O6 composites and their dielectric response

Ba_0.5Sr_0.5TiO₃–Mg₃B₂O₆ (BST–MB) composites have been prepared in situ by a citrate gel process, and their structure and effective dielectric response have been investigated. The precursor with pH≥7 is suitable for in situ formation of the diphase structure consisting of BST and MB. Accordingly, MB particles homogeneously disperse in BST particles, accompanied by the formation of boron-rich grain boundary resulting from liquid phases sintering of B₂O₃. Related with the existence of boron-rich grain boundary and the incorporation of Mg²⁺ into BST lattice, permittivity decreases rapidly with increasing volume fraction of MB from 0.0 to 0.2 and then decreases slowly with further increasing, which coincides with theoretical prediction of the layered model.     

Phase analysis and microwave dielectric properties of BaO–Nd2O3–5TiO2 composite ceramics using variable size TiO2 reagents

There are significant inconsistencies in published literature surrounding the phase analysis and physical properties of ceramics with the nominal composition BaO–Nd₂O₃–5TiO₂ (BNT125). A careful phase analysis investigation of BNT125 ceramics using variable size TiO₂ reagents was therefore undertaken using XRD, FESEM and EPMA with corresponding dielectric properties characterised over 2–3 GHz. Three distinct phases were consistently formed: Ba_6?3xNd_8+2xTi₁₈O₅₄ (x ～ 0.67), Ba₂Ti₉O₂₀ and TiO₂. The use of nano-scale TiO₂ reagents significantly reduced porosity and improved the dielectric properties of the composite ceramics, while markedly reducing processing times. Structural and crystal chemical indications as to the origin of this system's physical properties are discussed, with these results providing new insights into optimisation paths for microwave dielectric materials of this type.     

Fabrication and thermoelectric properties of SrTiO3–TiO2 composite ceramics

The paper presents the results of preparing biphase SrTiO₃–TiO₂ ceramics as a promising system for n-type thermoelectrics using the features of a two-dimensional electron gas. Ceramics was obtained by reactive spark plasma sintering of SrCO₃ and TiO₂. The dynamics of phase transformations are shown; it is clarified that phase transformations are not the driving force of sintering. The mutual stabilization of the SrTiO₃ and TiO₂ phases is shown. Unique data on the assessment of the temperature gradient in the system have been obtained. A comparison of the thermoelectric characteristics of biphasic ceramics and its constituent phases allows concluding that the role of the two-dimensional electron gas is reduced to modulating the properties of bulk phases. Clear signs of size quantization were detected by the X-ray luminescence method, which is expressed in the blueshift of the luminescence spectrum by 22.3 ± 0.8 meV.     

Microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics

The densification, microstructural evolution and microwave dielectric properties of (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba_1−ySr_y)WO₄ phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics is increased by raising the Sr²⁺ content, which significantly lowers the sintering temperature. The microstructure of the (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic sintered at 1400 °C results in the best microwave dielectric properties, including ε_r of 20.6, Q×f of 152,600 GHz and τ_f of +24.0 ppm/°C.     

Microwave dielectric properties of BaO–4.3TiO2–0.5ZnO ceramics with BaCu(B2O5)

Effect of BaCu(B₂O₅) (BCB) addition on microwave dielectric properties and sintering behaviors of BaO–4.3TiO₂–0.5ZnO system (BTZ) ceramics were investigated to develop middle-k dielectric composition with low sintering temperatures. When a small amount of BCB was added to BTZ system, the sintering temperature can be lowered from 1100 °C to 900 °C due to the formation of BCB liquid phase. The system added with 7 wt% BCB was sintered at 900 °C for 2 h and ?_r of 31, Q × f of 18,200 GHz and τ_f of 3.8 ppm/°C were obtained. The suitability of BTZ ceramics for tape casting and cofiring with Ag electrodes was investigated, and no evidence of chemical reaction between Ag and ceramics was observed. The dielectric properties of the stacked multilayer plate without any electrodes were also measured. The result shows that the as-prepared BTZ ceramics are suitable for low-temperature co-fired ceramics applications.     

CaMn0.05Zr0.95O3–NiMn2O4 composite ceramics with tunable electrical properties for high temperature NTC thermistors

A series of novel high temperature negative temperature coefficient (NTC) composite ceramics based on (1-x)CaMn_0.05Zr_0.95O₃-xNiMn₂O₄ (x = 0, 0.1, 0.2, 0.3) were fabricated by using the solid-state method. The Rietveld refinement method and backscattered electron (BSE) image confirmed the absence of any other phase. The conduction mechanism of the composite ceramics was determined by analyzing the complex impedance and resistance-temperature characteristics, which could be related to the formation of a continuous Mn³⁺-O-Mn⁴⁺ network. The effect of spinel content on the high temperature stability was analyzed using aging tests. The ρ_400°C, ρ_700°C and B_400/700 values of the NTC thermistors were determined to be 4.9 × 10⁶–1.2 × 10⁴, 1.4 × 10⁵–0.8 × 10³ Ω cm and 12739-5712 K, respectively. This indicated that the electrical properties could be tuned by adjusting the weight ratio of CaMn_0.05Zr_0.95O₃ and NiMn₂O₄. The findings obtained in this study reveal that the composite NTC thermistor exhibits a good application potential at high temperatures and under harsh environments.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Structure,dielectric and ferroelectric properties of 0.92Na0.5Bi0.5TiO3–0.06BaTiO3–0.02K0.5Na0.5NbO3 lead-free ceramics: Effect of Co2O3 additive

0.92Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–0.02K_0.5Na_0.5NbO₃+x wt% Co₂O₃ (NBKT–xCo, x=0, 0.2, 0.4, 0.6, 0.8) lead-free ferroelectric ceramics were prepared via a conventional solid state reaction method. Effects of Co₂O₃ additive on crystallite structure, microstructure, dielectric and ferroelectric properties of the NBKT–xCo ceramics were studied. X-ray diffraction results showed that the rhombohedral–tetragonal morphotropic phase boundary existed in all the ceramics, with relative amount of tetragonal phase varying with the content of Co₂O₃. Average grain size, maximum value of dielectric constant, Curie temperature and ferroelectric properties of the ceramics were close related to the content of Co₂O₃. The dielectric anomaly caused by the phase transition between the ferroelectric phase and the so-called “intermediate phase” was observed in the ceramics with x≤0.2, while it disappeared with further increasing x. All the ceramics showed a diffuse phase transition between the “intermediate phase” and the paraelectric phase. The change in the ferroelectric properties with changing the content of Co₂O₃ was discussed by considering the competitive effects among grain size, relative amount of the tetragonal phase and oxygen vacancies.     

Preparation of bioactive TiO2–MgO composite ceramics with bone-promoting properties

In the field of hard tissue repair, titanium-based materials have excellent mechanical properties and magnesium-based materials have good bioactivity, but their shortcomings are that titanium-based materials do not have good bioactivity, while magnesium-based materials are limited in application due to their rapid degradation rate. In order to give full play to the advantages of these two materials, the TiO₂–MgO composite ceramic materials were prepared by combining the two elements and sintering at high temperature. By changing sintering temperature and MgO content, the structure composition and bioactivity of composite ceramic materials can be controlled. The surface morphology, mineralization ability in vitro, cytotoxicity and bone-promoting properties of composite ceramic materials were studied. The experimental results show that high MgO content composite ceramic materials will bring too strong alkalinity to the environment, which will accelerate the mineralization ability of materials, but is not conducive to the survival of cells. Composite ceramic materials with suitable sintering temperature and MgO content have good bioactivity and bone-promoting performance, while the porous structure produced by MgO degradation is beneficial to cell spreading and can form a good combination between the material and bone tissue at an early stage. Porous structure and Mg²⁺ can adjust the bone-promoting properties of materials together. Through the above experimental research, it is found that TiO₂–MgO composite ceramic material is a new type of material which is used in the field of hard tissue repair due to its good bioactivity.     

Microwave dielectric properties of low-fired Li2MnO3 ceramics co-doped with LiF–TiO2

Li₂MnO₃ ceramics co-doped with 2 wt% LiF and x wt% TiO₂ (x=0, 3, 5, 7, 10) were prepared by solid-state reaction for low-temperature co-fired ceramics (LTCC) applications. The sintering temperatures of Li₂MnO₃ ceramics were successfully lowered to 925°C due to the formation of a LiF liquid phase. Their temperature stability was improved by doping with TiO₂. A typical Li₂MnO₃-2 wt% LiF-5 wt% TiO₂ sample with well-densified microstructures displayed optimum dielectric properties (ε_r=13.8, Q×f= 23,270 GHz, τ_f=1.2 ppm/°C). Such sample was compatible with Ag electrodes, which suggests suitability of the developed material for LTCC applications in wireless communication systems.     

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl₂O₄–(1 − x)YCr_0.5Mn_0.5O₃ (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase isomorphic to YCrO₃. The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr³⁺, Cr⁴⁺, Mn³⁺, and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The obtained ρ₂₅, B_25–150, and B_700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 10⁶–9.92 × 10⁹ Ω cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl₂O₄ concentration.     

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd_2?xCe_xTi₂O₇ has been investigated. The solubility limit of Ce in Nd_2?xCe_xTi₂O₇ was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd_2?xCe_xTi₂O₇ (x?=?0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point T_c of Nd_2?xCe_xTi₂O₇ compounds. The results suggest that the T_c of Ce₂Ti₂O₇ is <1445°C.     

Broad sintering temperature range and stable microwave dielectric properties of Mg2TiO4–CeO2 composite ceramics

There has been extensive research on microwave dielectric materials considering their application in 5G and 6G communication technologies. In this study, the sintering temperature range of Mg₂TiO₄–CeO₂ (MT-C) ceramics was broadened using a composite of CeO₂ and Mg₂TiO₄ ceramics, and their microwave dielectric performance was stabilized. Low-loss MT-C composite ceramics were prepared using the solid-phase reaction method, and their microwave dielectric properties, microscopic morphologies, and phase structures were investigated. The proposed MT-C ceramics contained Mg₂TiO₄ and CeO₂ phases; their average grain size was maintained at 2–4 μm in the sintering temperature range of 1275–1425 °C, and the samples were uniformly dense without porosity. The cross-distribution of Mg₂TiO₄ and CeO₂ grains in the samples inhibited the growth of ceramic grains, providing uniform and dense surfaces. The dielectric loss of MT-C ceramics remained constant in the temperature range of 1300–1425 °C at 9 × 10^?4 (8.45 ≤ f ≤ 8.75 GHz). As opposed to the base material, MT-C ceramics are advantageous owing to their wide sintering temperature range and the stable microwave dielectric properties, and there are suitable substrate materials for further industrial applications.     

Ba0.7Sr0.3TiO3–glass–silver percolative composite

Ba_0.7Sr_0.3TiO₃–glass–silver (BST–glass–Ag) composites were prepared by the solid state ceramic route. Their percolation behavior and dielectric properties were examined. The pure BST had a percolation limit of 24 vol% of silver whereas an addition of 8 wt% of 50PbO–30B₂O₃–20SiO₂ (PBS) glass lowered the percolation limit to 14 vol% of Ag. Glass addition lowered the sintering temperature of BST from 1300 to 975 °C and addition of Ag further lowered the sintering temperature to 925 °C, minimizing the Ag loss during sintering. The relative permittivity increased from 2700 for pure BST to about five orders of magnitude in the BST–glass–Ag composites near the percolation threshold.     

Microwave-assisted sintering and improved dielectric,ferroelectric properties of 0.5[(Ba0.7Ca0.3)TiO3]–0.5[Ba(Zr0.2Ti0.8)O3] lead-free ceramics

0.5[(Ba_0.7Ca_0.3)TiO₃]–0.5[Ba(Zr_0.2Ti_0.8)O₃] lead-free ceramics were synthesised by coprecipitation method and sintered by fast microwave sintering (MWS) and by conventional sintering (CS) at 1200°C. After being sintered with the two different methods, the materials were characterised for structural, microstructural, frequency and temperature-dependent dielectric properties, Raman spectroscopy, and ferroelectric measurements. Results are compared and discussed in the present paper. X-ray diffraction confirms the presence of the tetragonal and rhombohedral phases in the composites sintered by both methods. The ferroelectric to paraelectric transition temperature (Tc) is increased in microwave-sintered composite. Diffuse constant (γ) values show BCT–BZT ceramics to be neither normal ferroelectrics nor relaxor ferroelectrics. Raman spectra confirm phase transition in the ceramic samples. Saturation polarisation (Ps) values are 7.62 and 4.28?µC?cm^?2 and nearly equal remanant polarisation (Pr) values were observed for BCT–BZT composite sintered with MWS and CS, respectively.     

Dielectric properties and relaxor behavior of high Curie temperature (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3–Bi(Mg0.5Ti0.5)O3 Lead-free ceramics

The (1?x)(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃?xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BCZT–xBMT, x=0.1–0.7] lead-free solid solution ceramics were prepared by the conventional mixed oxide method. The phase structure was investigated by X-ray diffraction and results show that a single perovskite phase was obtained in all of these samples, suggesting that the added BMT diffused into BCZT to form a solid solution. Dense ceramics with relative densities >95% were obtained, and a small amount of BMT (≤50 mol%) acted as grain growth promoter, had an evident effect on grain size growth. Further increase of the BMT content inhibited the grain growth of BCZT samples. Temperature dependence of the dielectric properties showed that all the BCZT–BMT solid solution ceramics exhibited relaxor-like characteristics. With increasing BMT content, the Curie temperature was first increased and then decreased, giving a maximum value of 218 °C for the 0.4BCZT–0.6BMT composition. Furthermore, stable dielectric constants and low losses were obtained with 0.5≤x≤0.7 in the wide temperature range, indicating that the system possess potential for high-temperature application.