Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Investigation on the origin of the giant dielectric constant in CaCu3Ti4O12 ceramics through analyzing CaCu3Ti4O12-HfO2 composites

A full range of CaCu₃Ti₄O₁₂-HfO₂ (CCTO-HfO₂) composites were prepared by sintering mixtures of the two components at 1000 °C for 10 h. X-ray diffraction studies confirmed the two-phase nature of the composites. The evolution of the microstructure in the composites, in particular, the size distribution of CCTO grains, was examined by scanning electron microscopy. The studies showed that, as more HfO₂ was added, the abnormal grain growth of CCTO and coarsening of the microstructure were gradually suppressed. As a result, the average CCTO grain size was reduced from 50 to 1 μm. The measured dielectric constants agree well with the values calculated from Lichtenecker's logarithmic law, using only the dielectric constants of pure CCTO and HfO₂ as two end points. The agreement suggests to us that the dielectric constant of CCTO is dominated by domain boundaries within the grains rather than by grain boundaries between the grains.     

Microwave assisted sintering of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ electroceramic was prepared by a microwave assisted solid-state reaction technique from CaCO₃, CuO and TiO₂ powders. Processing involved the preparation of raw material, mixing and milling, calcination, pellet forming and sintering processes. Conventional furnace and microwave assisted sintering processes were employed in order to improve phase structures, morphology and dielectric properties of CaCu₃Ti₄O₁₂ ceramics. Surface and fracture FESEM analysis showed that the microwave assisted sintered CaCu₃Ti₄O₁₂ produced better densification and more uniform grain size compared to the conventional sintered sample.     

Effect of various dopants on the tunable and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

Acceptor (Mn-, Co-) doped and donor Y-doped Ba_0.6Sr_0.4TiO₃ have been prepared by traditional ceramic processing and their structural, surface morphological, dielectric and tunable properties were investigated. The results show all dopants to have a strong effect on the average grain size. The curie temperature of all doped specimens decreases and their temperature spectrum broadens. Loss tangent at 15 °C and low frequency (10 kHz) increases for all doped specimens whereas the loss tangent at high frequency (100 MHz) is not affected. The tunability of all the doped specimens is lower than the undoped one.     

Effects of Al2O3 addition on the microstructure and microwave dielectric properties of Ba4Nd9.33Ti18O54 ceramics

Ba₄Nd_9.33Ti₁₈O₅₄·x wt%Al₂O₃ (BNT-A) ceramics (x=0, 0.5, 1.0, 1.5, 2.0, 2.5) were prepared by the conventional solid state reaction. The effects of Al₂O₃ on the microstructure and microwave dielectric properties of Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) ceramics were investigated. X-ray diffraction and backscatter electronic images showed that the Al₂O₃ additive gave rise to a second phase BaAl₂Ti₅O₁₄ (BAT). The formation mechanism and grain growth of the BAT phase were first discussed. Dielectric property test revealed that the Al₂O₃ additive had improved the dielectric properties of the BNT ceramics: increased the Q×f value from 8270 to 12,180 GHz and decreased the τ_f value from 53.4 to 11.2 ppm/°C. A BNT-A ceramic with excellent dielectric properties: ε_r=70.2, Q×f=12,180 GHz, τ_f=20 ppm/°C was obtained with 2.0 wt% Al₂O₃ added after sintering at 1320 °C for 4 h.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

Structural,dielectric properties and electrical conduction behaviour of Dy substituted CaCu3Ti4O12 ceramics

Dy substituted CCTO ceramics were synthesized using solid state reaction method. Effect of Dy on structural, microstructural, dielectric and electrical properties has been studied over a wide temperature (300–500 K) and frequency range (100 Hz–1 MHz). Rietveld refinement, carried out on the samples, confirmed single phase formation and indicated overall decrease in lattice constant. Microstructure showed bimodal distribution of grains in CCTO with bigger grains surrounded by smaller grains. Dy substitution reduced grain size. Dy substitution in CCTO reduces the dielectric constant which may be attributed to increase of the Schottky potential barrier. The dielectric constant remains nearly constant in temperature range 300–400 K. The AC conductivity obeys a power law, σ_ac=A fⁿ, where n is the temperature dependent frequency exponent. The AC conductivity behaviour can be divided into three regions, over entire temperature range, depending on conduction processes. The relevant charge transport mechanisms have been discussed.     

Microstructure and dielectric properties of Ba0.6Sr0.4TiO3-MgAl2O4 composite ceramics

(1 − x)Ba_0.6Sr_0.4TiO₃-xMgAl₂O₄(x = 25, 30, 35 and 40 wt%) composite ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and tunability of the composites have been investigated. The XRD patterns analysis reveals two crystalline phases, a cubic perovskite structure Ba_0.6Sr_0.4TiO₃ (BST) and a spinel structure MgAl₂O₄ (MA). SEM observations show that the BST grains slightly dwindle and agglomerate with increasing amounts of MA. A dielectric peak with very strong frequency dispersion is observed at higher MA content, and the Curie temperature shifts to a higher temperature with increasing MA content. The ceramic sample with 30 wt% MA has the optimized properties: the dielectric constant is 1503, the dielectric loss is 0.003 at 10 kHz and 25 °C, the tunability is 23.63% under a dc electric field of 1.0 kV/mm, which is suitable for ferroelectric phase shifter.     

Low-temperature sintering of 0.85CaWO4–0.15LaNbO4 ceramics

Microwave dielectric properties of 0.85CaWO₄–0.15LaNbO₄ (CWLN) ceramics were investigated as a function of H₃BO₃, Li₂CO₃ content and sintering temperature. With the co-addition of 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃, the sintering temperature could be effectively reduced from 1150 °C for pure CWLN ceramics to 900 °C without any degradation of dielectric properties. These results are due to the enhancement of the sinterability of CWLN by liquid phase sintering. For the specimens with H₃BO₃–Li₂CO₃ sintered at 900 °C for 3 h, the dielectric constant (K) did not changed remarkably. However, the quality factor (Qf) and the temperature coefficient of resonant frequency (TCF) increased up to y = 1.0 of 3.0 wt.% H₃BO₃–y wt.% Li₂CO₃, and then decreased due to the formation of the secondary phases. Typically, K of 11.8, Qf of 45,200 GHz and TCF of −23.1 ppm/°C were obtained for the specimens of CWLN with 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃ sintered at 900 °C for 3 h.     

Effect of Bi2O3 and B2O3 additives on the sintering temperature, microstructure, and microwave dielectric properties for Sm(Mg0.5Ti0.5)O3 ceramics

The microwave dielectric properties of Sm(Mg_0.5Ti_0.5)O₃ incorporated with various amount of Bi₂O₃ and B₂O₃ additives have been investigated systematically. In this study, both Bi₂O₃ and B₂O₃ additives acting as a sintering aid can effectively lower the sintering temperature from 1550 °C to 1300 °C. The ionic radius of Bi³⁺ for a coordination number of 6 is 0.103 nm, whereas the ionic radius of B³⁺ is 0.027 nm. Clearly, the ionic radius of Bi³⁺ is greatly larger than one of B³⁺, which resulted in the specimens incorporated with Bi₂O₃ having larger lattice parameters and cell volume than those incorporated with B₂O₃. The experimental results show that no second phase was observed throughout the entire experiments. Depending on the interfacial tension, the liquid phase may penetrate the grain boundaries completely, in which case the grains will be separated from one another by a thin layer as shown in Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with Bi₂O₃. Whereas, in Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with B₂O₃, the volume fraction of liquid is high, the grains may dissolve into the liquid phase, and rapidly rearrange, in which case contact points between agglomerates will be dissolved due to their higher solubility in the liquid, leading plate-like shape microstructure.A dielectric constant (?_r) of 29.3, a high Q × f value of 26,335 GHz (at 8.84 GHz), and a τ_f of −32.5 ppm/°C can be obtained for Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with 10 mol% Bi₂O₃ sintered at 1300 °C. While Sm(Mg_0.5Ti_0.5)O₃ ceramics incorporated with 5 mol% B₂O₃ can effectively lower temperature coefficient of resonant frequency, which value is −21.6 ppm/°C. The Sm(Mg_0.5Ti_0.5)O₃ ceramic incorporated with heavily Bi₂O₃ and B₂O₃ additives exhibits a substantial reduction in temperature (∼250 °C) and compatible dielectric properties in comparison with that of an un-doped one. This implied that this ceramic is suitable for miniaturization in the application of dielectric resonators and filters by being appropriately incorporated with a sintering aid.     

Temperature dependence of piezoelectric properties on Nd and V co-substituted Bi4Ti3O12 ceramics for ceramic resonator applications

The temperature dependences of the piezoelectric properties of (Bi_4−yNd_y)_1−(x/12)(Ti_3−xV_x)O₁₂ [BNTV-x, y (x = 0.01, y = 0.00–1.00)] were investigated for environmentally friendly lead-free piezoelectric ceramic resonators with low-temperature coefficients of resonance frequency, TC-f. The |TC-f| in the (33) mode improved with increasing concentration of modified Nd ions, y, and exhibited the smallest |TC-f| value of 77.4 ppm/°C at y = 0.75 (BNTV-0.75). The |TC-f| in the other vibration mode (t), was also investigated for the BNTV-0.75 ceramic, and a smaller value of 42 ppm/°C was obtained. The (t) mode of the BNTV-0.75 ceramic showed excellent piezoelectric properties: Q_m = 4200, Q_e max = 31 and TC-f = −49.8 ppm/°C. These properties are very similar to those of commercialized hard PZT ceramics for resonator applications. The BNTV-0.75 ceramic seems to be a superior candidate material for lead-free piezoelectric applications of ceramic resonators.     

Dielectric properties of Bi-doped Ba0.8Sr0.2TiO3 ceramic solid solutions

Bismuth-doped barium–strontium–titanate ceramics of the formula (Ba_0.8Sr_0.2)_(1−1.5y)Bi_yTiO₃ were prepared using a conventional solid-state reaction method. The structure, dielectric properties, and ferroelectric relaxor behaviour of all compositions were thoroughly investigated. The findings revealed a broad dielectric anomaly and a shift in dielectric maxima towards higher temperatures with increasing frequency. The diffuseness degree indicator γ was about 1.68, and dielectric relaxation was noted to follow the Vogel–Fulcher relationship, with T_f=185 K, f₀=1.18×10¹⁰ Hz, and Ea=0.35 eV, which further supported the spin-glass-like properties of BBSTs. The latter were also noted to display significant ferroelectric relaxor behaviour that could be attributed to the presence of Bi³⁺ doping ions. The degree of relaxation behaviour was noted to increase with the increase in bismuth concentration. Raman spectra were investigated as a function of temperature, and the findings confirmed the results from X-ray and dielectric measurements. Among the compositions assayed in this solid solution, 10% Bi-doped Ba_0.8Sr_0.2TiO₃ yielded promising relaxor properties that make it a strong candidate for future industrial application in the production of efficient and eco-friendly relaxor ferroelectric materials.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

A study on the interfacial reaction and dielectric properties of Ba0.88(Nd1.40Bi0.42La0.30)Ti4O12/alkali-borosilicate glass composites

This study investigated the effects of sintering parameters and the addition of alkali-borosilicate glass into the Ba_0.88(Nd_1.40Bi_0.42La_0.30)Ti₄O₁₂ B(NBL)T ceramic. The microstructure evolution, ionic exchange phenomenon at phase interfaces and the dielectric properties variation of composites were examined by XRD, EPMA, TEM, RF impedance analyzer and network analyzer, respectively. XRD patterns revealed that interactions between B(NBL)T ceramic and glass during sintering could have caused the change in the preferred orientation as well as the shifting of the crystals’ diffraction angles. EPMA mapping showed that the concentrations of Ba, and Bi decreased along the edge of the B(NBL)T ceramic that is closest to the glass phase, while the opposite trend was seen for Na and Ca. TEM and EDS analyses confirm that an ionic exchange took place during sintering with the glass phase wetting the B(NBL)T ceramic and was responsible for the change in the crystal plane and the variation in lattice parameters. The ionic exchange that occurred between the B(NBL)T ceramic and the glass phase resulted in a decrease in the electrical resistivity of the glass phase, which in turn reduced the dielectric loss.     

Synthesis,structural analysis and dielectric properties of Ba8(Mg1−xZnx)Nb6O24 hexagonal perovskites

Ba₈(Mg_1−xZn_x)Nb₆O₂₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered at 1375–1425 °C. The structure of the system was analyzed using X-ray diffraction and vibrational spectroscopic studies. The microstructure of the sintered pellet was analyzed using scanning electron microscopy. The dielectric constant (ε_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) were measured in the microwave frequency region. The τ_f values of the compositions were reduced by varying the value of x from 0 to 1. The dielectric responses to frequency were also studied in the radio frequency region. The compositions have good microwave dielectric properties and hence are suitable for dielectric resonator applications.     

B-site modification of (Ba0.6Sr0.4)TiO3 ceramics with enhanced diffuse phase transition behavior

(Ba_0.6Sr_0.4)TiO₃ (BST) ceramics modified with both Zr and Mn were synthesized via a traditional solid state reaction route. Pure perovskite structure and dense morphology were obtained for all specimens. An enhanced diffuse phase transition (DPT) behavior induced by Mn substitution was detected. We suggest that such enhanced DPT behavior was beneficial for BST practical applications due to the improved dielectric thermal stability while maintaining good dielectric performance. The impact of Mn concentration on the microstructure, dielectric properties, and DPT diffuseness was investigated.     

Enhancement of the dielectric,piezoelectric, and ferroelectric properties in BiYbO3-modified (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free ceramics

Lead-free (1−x)(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃−xBiYbO₃ [(1−x)BCTZ−xBYO] piezoelectric ceramics in the range of BYO concentrations were prepared by the conventional oxide-mixed method, and the effect of BYO content on their microstructure, crystalline structure, density and electrical properties was investigated. A dense microstructure with large grain was obtained for the ceramics with the addition of BYO. The ceramics with x=0.1% exhibit an optimum electrical behavior of d₃₃~580 pC/N, r~10.9 Ω, k_p~56.4%, and tan δ~1.12% when sintered at a low temperature of ~1350 °C. When the measuring electric field is 40 kV/cm, the well-saturated and square-like P–E loops for the ceramics were observed with P_r~12.2 μC/cm² and E_c~1.83 kV/cm.     

Microstructure and mechanical properties of dental 3Y-TZP ceramics by using CaO-P2O5 glass as additive

This study investigated the effects of glass frits on the sintering and mechanical properties of dental 3Y-TZP ceramics. The glass frits, which consisted of MgO, CaO, Al₂O₃, SiO₂, and P₂O₅, were selected to lower the sintering temperature of zirconia via liquid phase sintering.The results of the experiment showed that these glass frit additives neither destroy the stability of the high temperature t-phase nor induce grain growth. All the mechanical properties and the relative densities were strongly correlated with the addition of glass frits. At lower sintering temperatures, the presence of glass additives resulted in an increase in mechanical properties. At higher sintering temperatures, the presence of glass additives decreased the mechanical properties.     

Fabrication and electrical properties of Pb(Co1/3Nb2/3)O3 ceramics

Pb(Co_1/3Nb_2/3)O₃ (PCN) ceramics have been produced by sintering PCN powders synthesized from lead oxide (PbO) and cobalt niobate (CoNb₂O₆) with an effective method developed for minimizing the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development, dielectric and ferroelectric properties of the sintered ceramics. From X-ray diffraction analysis, the optimum sintering temperature for the high purity PCN phase was found at approximately 1050 and 1100 °C. The densities of sintered PCN ceramics increased with increasing sintering temperature. However, it is also observed that at very high temperature the density began to decrease. PCN ceramic sintered at 1050 °C has small grain size with variation in grain shape. There is insignificant change of dielectric properties with sintering temperature. The P–E hysteresis loops observed at −70 °C are of slim-loop type with small remanent polarization values, which confirmed relaxor ferroelectric behavior of PCN ceramics.