Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

Enhancement in dielectric and ferroelectric properties of lead free Bi0.5(Na0.5K0.5)0.5TiO3 ceramics by Sb-doping

Lead free piezoelectric Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ (pure and 1 wt.%, 2 wt.%, 4 wt.% Sb-doped) ceramics were synthesized away from its MPB. The crystalline nature of the BNKT ceramic was studied by XRD and SEM. Depolarization temperature (T_d) and transition temperature (T_c) were observed through phase transitions in dielectric studies which were found to increase after Sb-doping, thus increasing its usable temperature range. In the study of relaxation behavior, the activation energy for relaxation was found to be 0.33, 0.43, 0.57 and 0.56 eV for pure and Sb-doped samples, respectively. All samples were found to exhibit normal Curie-Weiss law above their T_c. Doping of Sb was found to restrain the diffused character of the pure sample. In P-E loop, Sb-doping was found to increase the ferroelectric properties.Pure and Sb-doped BNKT ceramics exhibited high values of piezoelectric charge coefficient (d₃₃) as 115, 121, 129 and 100 pC/N, respectively.     

ZnLi2/3Ti4/3O4: A new low loss spinel microwave dielectric ceramic

A new low loss spinel microwave dielectric ceramic with composition of ZnLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state ceramic route. The ceramic can be well densified after sintering above 1075 °C for 2 h in air. X-ray diffraction data show that ZnLi_2/3Ti_4/3O₄ ceramic has a cubic structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.40172 Å, V = 593.07 Å³, Z = 8 and ρ = 4.43 g/cm³. The best microwave dielectric properties can be obtained in ceramic with relative permittivity of 20.6, Q × f value of 106,700 GHz and τ_f value of −48 ppm/°C. The addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1075 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added ZnLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

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.     

Fabrication and characterization of (Pb(Mg1/3Nb2/3)O3, Pb(Yb1/2Nb1/2)O3, PbTiO3) ternary system ceramics

New ternary compositions in the Pb(Mg_1/3Nb_2/3)O₃-Pb(Yb_1/2Nb_1/2)O₃–PbTiO₃ (PMN-PYbN-PT) system were prepared using 0.5Pb(Yb_1/2Nb_1/2)O₃-0.5PbTiO₃ (PYbNT) and (1-x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (x = 0.26; PMNT26 or x = 0.325; PMNT32.5) powders synthesized via the columbite method. Dense (≥ 96% of theoretical density) ceramics with PMN/PYbN mole ratios of 25/75 (R-25), 50/50 (R-50) and 75/25 (R-75T and R-75R) were fabricated by reactive sintering at 1000 °C for 4 h. Therefore, incorporation of PYbNT to PMNT successfully decreased sintering temperature of PMNT from 1200 °C-1250 °C to 1000 °C. Samples with higher density and perovskite ratio together with lower weight loss possessed higher dielectric and piezoelectric values in each composition. The R-75 samples had remanent polarization (P_r) values of 34-36 μC/cm² and piezoelectric charge coefficient (d₃₃) of 560 pC/N. The sharp phase transition PMNT as a function of temperature became broader or more diffuse with increasing PYbNT content. However, PYbNT addition to PMNT increased Curie temperature (T_c) from 183 °C (for PMNT32.5) to 220-242 °C (for R-75T and R-75R) to 336 °C (for R-25). Therefore, these ternary compositions can be tailored for various high temperature applications due to the relatively higher T_c with enhanced piezoelectric and dielectric properties as compared to PMNT.     

Microwave dielectric characteristics of ceramics in Mg2SiO4–Zn2SiO4 system

(Mg_1−xZn_x)₂SiO₄ ceramics were prepared and characterized. The densification temperatures of the present ceramics are much lower than those for Mg₂SiO₄ and Zn₂SiO₄ end-members. Small solid solution limits of Zn in Mg₂SiO₄ and Mg in Zn₂SiO₄ are observed, and the bi-phase structure is confirmed in (Mg_1−xZn_x)₂SiO₄ ceramics with x = 0.1–0.9. Even though, it is clear that the Qf value of Zn₂SiO₄ ceramics can be significantly improved together with a suppressed temperature coefficient of resonant frequency τ_f by substituting Mg for Zn. (Mg_0.4Zn_0.6)₂SiO₄ ceramics indicate a good combination of microwave dielectric characteristics: _r = 6.6 Qf = 95,650 GHz, and τ_f = −60 ppm/°C.     

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.     

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.     

Structural anomaly and electrical relaxation in (Na2/3Pb1/3)(Mn1/2Nb1/2)O3 ceramics

The X-ray diffraction patterns of (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics were measured within 15–850 K temperature range. The anomaly in the thermal expansion temperature dependence occurred in 250–365 K range. The generalised Cole–Cole model was proposed to describe the measured effective electric permittivity influenced by high electric conduction and the coexistence of two contributions ?*(T,f) = ?*_lattice + ?*_carriers was considered. The analysis of the electric permittivity and conduction exhibited two relaxation processes. The electric conduction relaxation characteristic time values indicated the small polaron mechanism with τ₀ ≈ 10⁻¹³ s occurring in 240–345 K range and the ionic mechanism with τ₀ ≈ 10⁻¹¹ s involved in the other relaxation occurring in the 320–510 K range. The ionic relaxation process was ascribed to a subsystem of defects, which was weakly interrelated to the anomaly in thermal expansion of the (Na_2/3Pb_1/3)(Mn_1/2Nb_1/2)O₃ ceramics. The Gate model was proposed to describe the ionic relaxation mechanism.     

Low-temperature preparation and microwave dielectric properties of cold sintered Li2Mg3TiO6 nanocrystalline ceramics

This study aims to fabricate Li₂Mg₃TiO₆ ceramics with ultrafine grains using a novel cold sintering process combined with a post-annealing treatment at a temperature <?950?°C. In this study, phase composition, sintering behavior, microstructure evolution, and microwave dielectric properties of the resultant nanocrystalline ceramics were investigated for the first time. The as-compacted green pellets at 180?°C yielded a high relative density of ~ 90% and the ceramics that were post-sintered over a broad temperature range (800–950?°C) possessed highly dense microstructure with a relative density of ~ 96%. The average grain size varied from 100 to 1200?nm for the samples sintered at 800–950?°C. Furthermore, the quality (Q × f) values of the obtained specimens exhibited a strong positive dependency on the grain size, which increased from 17,790 to 47,960?GHz for grain sizes ranging between 100 and 1200?nm, while the dielectric permittivity (ε_r) and temperature coefficient of the resonant frequency (τ_f) values did not undergo any significant changes over this range of grain size.     

Phase formation and dielectric properties of (Pb,Ba)[(Zn1/2W1/2),Ti]O3 ceramics

PbTiO₃ and/or BaTiO₃ were systematically introduced into Pb(Zn_1/2W_1/2)O₃ and resultant phase developments in terms of perovskite formation were investigated. Ceramic powders were prepared via a B-site precursor route to further assist the perovskite formation. Weak-field dielectric properties of the sintered samples were examined. For Pb(Zn_1/2W_1/2)O₃-rich compositions, multiphase ceramics resulted and formation of monophasic perovskite turned out to be not successful even by the B-site precursor method. Values of the perovskite formation yield and the maximum dielectric constant increased with increasing fractions of the substituent species.     

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.     

Structure and dielectric characteristics of Ca(Fe1/2Ta1/2)O3 complex perovskite ceramics

Ca(Fe_1/2Ta_1/2)O₃ complex perovskite ceramics have been prepared and characterized in the crystal structure and microstructures, and the dielectric characteristics have been evaluated over a broad temperature and frequency range. There are two dielectric relaxations in low and high temperature ranges, respectively. Differing from the situation for Ba(Fe_1/2Ta_1/2)O₃, Sr(Fe_1/2Ta_1/2)O₃ and Ba(Fe_1/2Nb_1/2)O₃, O₂-annealing has little effect on the dielectric properties of Ca(Fe_1/2Ta_1/2)O₃, and the much lower dielectric constant and low loss are ascribed to the low concentration of Fe²⁺ according to the XPS measurements. The microwave dielectric properties (at 5.38 GHz) for Ca(Fe_1/2Ta_1/2)O₃ ceramics are obtained as: ?_r = 30.7, Qf = 3070 GHz.     

Structure and electrical properties of Ca0.28Ba0.72Nb2O6 ceramics with addition of rare earth oxides (CeO2, La2O3)

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with addition of CeO₂ and La₂O₃, were prepared by the conventional ceramic fabrication technique. XRD results showed that the single tungsten bronze structure of CBN28 was not changed by adding CeO₂ or La₂O₃. SEM results indicated that both CeO₂ and La₂O₃ dopants were effective in inhibiting the grain growth and suppressing the anisotropic growth behavior in tungsten bronze structure. It was also found that both two kinds of dopants had remarkable effects on the dielectric and ferroelectric properties of CBN28 ceramics. Compared with CBN28 ceramics, the dielectric constant around room temperature ε_r, dielectric loss tan δ, the degree of diffuseness γ and coercive field E_c were all ameliorated when doping proper amount of CeO₂ or La₂O₃. The comprehensive electric performance was obtained in CBN28–0.3 wt% CeO₂ and CBN28–0.4 wt% La₂O₃ ceramics. Besides, the underlying mechanism for variations of the electrical properties due to different dopants was explained in this work.     

Structure and electrical properties of Ca2+-doped (Na0.47Bi0.47Ba0.06)TiO3 lead-free piezoelectric ceramics

The lead-free piezoelectric ceramics (Na_.47Bi_.47Ba_.06)_1-xCa_xTiO₃ (x?=?0, 0.01, 0.02, 0.03, 0.05, and 0.08, abbreviated as BNBTC/0, BNBTC/1, BNBTC/2, BNBTC/3, BNBTC/5, and BNBTC/8, respectively) were obtained using the solid-state reaction method. The structure, electric conductivity, and dielectric, ferroelectric, and piezoelectric properties of the Ca²⁺-doped (Na_.47Bi_.47Ba_.06)TiO₃ ceramics were thoroughly investigated. The ceramics sintered at 1200?°C exhibit dense microstructures, having relative densities higher than 96%. The X-ray diffraction results demonstrate that all ceramics have a pure perovskite structure. The mean grain sizes of the ceramics are related to the Ca²⁺ quantity. A small quantity of Ca²⁺ ions (x?≤?0.03) improves the piezoelectric and ferroelectric properties of the samples. The dielectric behavior of the samples is sensitive to the Ca²⁺ content and electric poling. The results demonstrate that the electrical properties of the (Na_.47Bi_.47Ba_.06)TiO₃ lead-free ceramics can be well tuned by varying the Ca²⁺ quantity.     

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.     

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.     

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.     

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.     

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li_2(1+x)Mg₃ZrO₆ microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000 GHz. Particularly, desirable microwave dielectric properties of ε_r=12.8, Q×f=307,319 GHz (@9.88 GHz), and τ_f=−35 ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6 h.