The reaction sequence and dielectric properties of BaSm2Ti4O12 ceramics

To establish the correct reaction sequence of BaO–Sm₂O₃–4TiO₂, phases present in different calcining temperatures are identified by X-ray diffraction patterns. When different calcining temperatures are used, the source phase BaO (BaCO₃) consumes below 850°C, the source phases TiO₂ and Sm₂O₃ consume at 1000 and 1150°C; the intermediate phases BaTiO₃, BaTi₄O₉, and Sm₂Ti₂O₇ consume at 1050, 1200, and 1250°C, respectively. The BaSm₂Ti₄O₁₂ phase starts to reveal at the 1100°C-calcined powder. The integrating intensity of BaSm₂Ti₄O₁₂ phase increases with the raising of calcining temperatures, accompanying with the decrease of integrating intensities of the source and intermediate phases. As the sintering temperature increases, the densities, quality values, and dielectric constants of BaSm₂Ti₄O₁₂ ceramics increase and saturate at 1325^oC. The BaSm₂Ti₄O₁₂ ceramics sintered at 1325°C have the properties of Q^*f=5180,_r=81.8, and τ_f=−19.2 ppm/°C.     

Preparation, microstructure and microwave dielectric properties of ZrxTi1−xO4 (x=0.40–0.60) ceramics

Zr_xTi_1−xO₄ (x=0.40–0.60) ceramics sintered without additives were prepared from powders made by the coprecipitation of metal salts from aqueous solutions in order to investigate the existence range of a homogeneous phase and the relationships between composition, microstructure and the dielectric properties. XRD, TEM, SEM, EDS, and the dielectric measurements were used to characterize the products. A homogeneous solid solution was obtained. Its crystal structure was isomorphous with ZrTiO₄. The variation of the lattice parameters with TiO₂ content was discussed. The optimum sintering temperature of samples was dependent of composition. TiO₂ suppressed the densification and acted as a grain growth enhancer during the sintering process. With the increase in TiO₂ content the relative densities of the sintered bodies decrease, while the grain sizes increase. The dielectric properties at microwave frequency (1.8 GHz) in this system, especially Q value, were poor, due to low densification, impurities and lattice defects. The dielectric constant _r and Q value exhibited a significant dependence on the relative density and composition. Both _r and Q increased with the increase in relative density, but they were primarily influenced by the composition and the effect of the relative density could be ignored when the relative density was greater than 90% theoretical. _r increased slightly with increasing TiO₂ content, while Q value decreased.     

Direct and hybrid microwave solid state synthesis of CaCu3Ti4O12 ceramic: Microstructures and dielectric properties

CaCu₃Ti₄O₁₂ (CCTO) electroceramic possesses unusual giant dielectric permittivity up to ε?=?10⁴ at low frequency range and room temperature. CCTO dielectric properties strongly depend on its microstructure therefore it is essential to pay attention to the processing techniques which impact grain size and microstructure. In this work, direct and hybrid microwave solid state synthesis was specifically designed and used for the synthesis of CCTO. The microwave process was also compared to the conventional process which involves usual infrared heating. The structural (XRD) and microstructural (SEM) characterizations indicate that microwave synthesis is particularly efficient to get rapidly pure CCTO powder. The fully automated 915?MHz single-mode microwave cavity used for hybrid synthesis allows a perfect control of the temperature distribution and heating rate. Therefore hybrid microwave synthesis leads to a fine, mono-disperse and practically pure CCTO powder in the range of 300 – 500?nm. The advantages of the hybrid microwave heating method are discussed and compared to the conventional and direct microwave heating processes. From the powders synthesized by the different routes, dense compacts were sintered in air at 1050?°C in a conventional furnace. Microstructural characterizations reveal abnormal grain growth during sintering which levels dielectric properties. All exhibit a giant dielectric constant ε?>?10³ at room temperature which decreases drastically to ε?=?90 at 10?K. Those properties are discussed according to the well-established Internal Barrier Layer Capacitor (IBLC) model.     

Effects of Mg/Fe substitution on perovskite stabilization and dielectric properties of Pb(Zn1/2W1/2)O3

The effects of compositional modification by substituting Mg or Fe for Zn on the perovskite formation, crystallographic aspects, and dielectric properties of Pb(Zn_1/2W_1/2)O₃ were investigated. Microstructural evolutions in the sintered ceramics were also examined. Stabilization of the perovskite structure was accomplished by a much smaller concentration of Mg substitution. Lattice parameters of the perovskite decreased with increasing substituent fractions in general. By contrast, the permittivity values increased significantly with increasing Mg/Fe substitution.     

Sintering and dielectric properties of Cu2Ta4O12 ceramics

In this work, Cu₂Ta₄O₁₂ ceramic was investigated as a promising, lead-free, nonferroelectric material with high dielectric permittivity. The results of impedance spectroscopy studies carried out at frequencies 10 Hz to 2 MHz over a wide temperature range from −55 to 700 °C were analyzed in the impedance, dielectric permittivity and electric modulus formalisms. In complex impedance plots two distinct arcs were distinguished, ascribed to the semiconducting grains and to the insulating grain boundaries. Cu₂Ta₄O₁₂ ceramic was found to exhibit a high dielectric permittivity exceeding 10,000 at low frequencies in the temperature range 150–740 °C. High permittivity of this material was attributed to the formation of internal (grain boundary) barrier layer capacitors. The influence of sintering conditions on microstructure, composition and dielectric properties of Cu₂Ta₄O₁₂ ceramics was also studied.     

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.     

Microstructure and dielectric properties of low temperature sintered ZnNb2O6 microwave ceramics

Low sintering temperature ZnNb₂O₆ microwave ceramics were prepared by doping with mixed oxides of V₂O₅–Bi₂O₃ and V₂O₅–Bi₂O₃–CuO. The effects of additives on the microstructure and dielectric properties of the ceramics were investigated. The results show that doping with V₂O₅–Bi₂O₃ can reduce the sintering temperature of ZnNb₂O₆ from 1150 °C to 1000 °C due to the formation of V₂O₅ and Bi₂O₃ based eutectic phases. The combined influence of V₂O₅ and Bi₂O₃ resulted in rod-like grains. Co-doping CuO with 1 wt.% V₂O₅–1 wt.% Bi₂O₃ further lowered the sintering temperature to 880 °C, because eutectic phases could be formed between the CuO, V₂O₅ and Bi₂O₃. A second phase of (Cu₂Zn)Nb₂O₈ also forms when the content of CuO is greater than 2.5 wt.%. A pure ZnNb₂O₆ phase can be obtained when the amount of CuO was 1.0–2.5 wt.%. The Q × f values of ZnNb₂O₆ ceramics doped with V₂O₅–Bi₂O₃–CuO were all higher than 25,000 GHz. The dielectric constants were 22.8–23.8 at microwave frequencies. In addition, theτ_f values decreased towards negative as the content of CuO increased. The ceramic with composition of ZnNb₂O₆ + 1 wt.%V₂O₅ + 1 wt.% Bi₂O₃ + 2.5 wt.% CuO sintered at 880 °C exhibited the optimum microwave dielectric properties, is 23.4, Q × f is 46,975 GHz, and τ_f is −44.89 ppm/°C, which makes it a promising material for low-temperature co-fired ceramics (LTCCs).     

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.     

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.     

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.     

Microwave dielectric properties of Ba3Ti4−x(Zn1/3Nb2/3)xNb4O21 for low temperature co-fired ceramics

The effects of substitution of (Zn_1/3Nb_2/3) for Ti on the sintering behavior and microwave dielectric properties of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (0 ≤ x ≤ 4) ceramics have been investigated. The dielectric constant (?_r) and the temperature coefficient of the resonant frequency (τ_f) of Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ ceramics decreased with increasing x. However, the Q × f values enhanced with the substitution of (Zn_1/3Nb_2/3) for Ti. It was found that a small amount of MnCO₃-CuO (MC) and ZnO-B₂O₃-SiO₂ (ZBS) glass additives to Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics lowered the sintering temperature from 1250 to 900 °C. And Ba₃Ti_4−x(Zn_1/3Nb_2/3)_xNb₄O₂₁ (x = 2) ceramics with 1 wt% MC and 1 wt% ZBS sintered at 900 °C for 2 h showed excellent dielectric properties: ?_r = 53, Q × f = 14,600 GHz, τ_f = 6 ppm/°C. Moreover, it has a chemical compatibility with silver, which made it as a promising material for low temperature co-fired ceramics technology application.     

Cation distribution of high-performance Mn-substituted ZnGa2O4 microwave dielectric ceramics

In current study, only 5?mol% Mn²⁺ was applied to fabricate high performance microwave dielectric ZnGa₂O₄ ceramics, via a traditional solid state method. The crystal structure, cation distribution and microwave dielectric properties of as-fabricated Mn-substituted ZnGa₂O₄ ceramics were systematically investigated. Mn²⁺-substitution led to a continuous lattice expansion. Raman, EPR and crystal structure refinement analysis suggest that Mn²⁺ preferentially occupies the tetrahedral site and the compounds stay normal-spinel structure. The experimental and theoretical dielectric constant of Zn_1-xMn_xGa₂O₄ ceramics fit well. In all, this magnetic ion, Mn²⁺, could effectively adjust the τ_f value to near zero and double the quality factor from 85,824?GHz to 181,000?GHz of Zn_1-xMn_xGa₂O₄ ceramics at the meantime. Zn_1-xMn_xGa₂O₄ (x?=?0.05) ceramics sintered at 1400?°C for 2?h exhibited excellent microwave dielectric properties, with ε_r =?9.7(@9.85?GHz), Q?×?f?=?181,000?GHz, tanδ?=?5.44?×?10^?5,and τ_f =???12?ppm/°C.     

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.     

Enhanced dielectric and piezoelectric properties in Na0.5Bi4.5Ti4O15 ceramics with Pr-doping

The bismuth layer-structured Na_0.5Bi_4.5-xPr_xTi₄O₁₅ (x?=?0, 0.1, 0.2, 0.3, 0.4, and 0.5) (NBT-xPr³⁺) ceramics were fabricated using the traditional solid reaction process. The effect of different Pr³⁺ contents on dielectric, ferroelectric and piezoelectric properties of Na_0.5Bi_4.5Ti₄O₁₅ ceramics were investigated. The grain size of Pr³⁺-doping ceramics was found to be smaller than that of pure one, the maximum dielectric constant and Curie temperature T_c gradually decreased with increasing Pr³⁺ contents, and the dielectric loss decreased at high temperature by Pr³⁺-doping. Moreover, the activation energy (E_a), resistivity (Z’), remanent polarization (2P_r) and piezoelectric constant (d₃₃) increased by Pr³⁺-doping. The NBT-xPr³⁺ ceramics with x?=?0.3 achieved the optimal properties with the maximum dielectric constant of 1109.18, minimum loss of 0.00822 (250?kHz), E_a of 1.122?eV, Z’ of 7.9?kΩ?cm (725 ºC), d₃₃ of 18 pC/N, 2P_r of 12.04 μC/cm². The enhancement was due to the addition of Pr³⁺ which suppressed the decreasing of resistivity at high temperature and made it possible for NBT-xPr³⁺ ceramics to be poled in perpendicular direction, implying that it is a great improvement for Na_0.5Bi_4.5Ti₄O₁₅ ceramics in electrical properties.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

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.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

Effects of Bi2Mo2O9 addition on the sintering characteristics and microwave dielectric properties of BiSbO4 ceramics

In this study, densification, microstructural evolution and microwave dielectric properties of (1 − x)BiSbO₄–xBi₂Mo₂O₉ ceramics (x = 0–0.25) were investigated. Bi₂Mo₂O₉ was selected as the sintering aid as well as the modifier of the dielectric properties for BiSbO₄ ceramic. In comparison with pure BiSbO₄ densified at 1100 °C, the 0.75BiSbO₄–0.25Bi₂Mo₂O₉ composites emerged to reach maximum sintering density at 775 °C. No other second phase was detected while the microstructure exhibited a bimodal grain size distribution as both 1–2 μm large grains of Bi₂Mo₂O₉ and 0.2–0.5 μm fine grains of BiSbO₄ were observed. The ceramic with the best performance in terms of microwave dielectric properties in this system is found to be the 0.82BiSbO₄–0.18Bi₂Mo₂O₉ composite, which reports a ?_r of 24.3, a Q × f of 24,019 GHz, and a τ_f of −4 ppm/°C when sintered at 825 °C.