Microwave dielectric properties and its compatibility with silver electrode of LiNb<Subscript>0.6</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript> with B<Subscript>2</Subscript>O<Subscript>3</Subscript> and CuO additions

The influences of B₂O₃ and CuO (BCu, B₂O₃: CuO = 1:1) additions on the sintering behavior and microwave dielectric properties of LiNb_0.6Ti_0.5O₃ (LNT) ceramics were investigated. LNT ceramics were prepared with conventional solid-state method and sintered at temperatures about 1,100 °C. The sintering temperature of LNT ceramics with BCu addition could be effectively reduced to 900 °C due to the liquid phase effects resulting from the additives. The addition of BCu does not induce much degradation in the microwave dielectric properties. Typically, the excellent microwave dielectric properties of ε_r = 66, Q × f = 6,210 GHz, and τ _f  = 25 ppm/^oC were obtained for the 2 wt% BCu-doped sample sintered at 900 °C. Chemical compatibility of silver electrodes and low-fired samples has also been investigated.     

Low-firing Li<Subscript>2</Subscript>ZnTi<Subscript>3</Subscript>O<Subscript>8</Subscript> microwave dielectric ceramics with BaCu(B<Subscript>2</Subscript>O<Subscript>5</Subscript>) additive

Phase purity, microstructure, sinterability and microwave dielectric properties of BaCu(B₂O₅)-added Li₂ZnTi₃O₈ ceramics and their cofireability with Ag electrode were investigated. A small amount of BaCu (B₂O₅) can effectively reduce the sintering temperature from 1075°C to 925°C, and it does not induce much degradation of the microwave dielectric properties. Microwave dielectric properties of ε _r = 23·1, Q × f = 22,732 GHz and τ _f = − 17·6 ppm/°C were obtained for Li₂ZnTi₃O₈ ceramic with 1·5 wt% BaCu(B₂O₅) sintered at 925°C for 4 h. The Li₂ZnTi₃O₈ +BCB ceramics can be compatible with Ag electrode, which makes it a promising microwave dielectric material for low-temperature co-fired ceramic technology application.     

Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

The effects of CuO-doping on dielectric and piezoelectric properties of Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–Ba(Zr,Ti)O<Subscript>3</Subscript> lead-free ceramics

CuO-doped lead-free ceramics based on bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) and barium zirconate titanate (Ba(Zr_0.07Ti_0.93)O₃, BZT) were prepared via a multi-step solid-state reaction process. The BNT–BZT with CuO dopant ceramics sintered at 1150–1180 °C for 2 h in air showed a pure perovskite structure. SEM images reveal that a small amount of CuO (<2 mol%) play a significant role on the microstructure to improve its sintering attributes, while it will degrade when the dopant is added beyond 2 mol%. The dielectric and piezoelectric properties of CuO-doped BNT–BZT ceramics were evaluated. At room temperature, the sample doped with 2 mol% CuO shows quite good properties such as a high piezoelectric constant (d ₃₃ ∼156.5 pC/N) and a high electromechanical coupling factor (k _t ∼52%). The depolarization temperature increased dramatically and the maximum permittivity temperature decreased slightly.     

Optimized sintering properties and temperature stability of MgZrTa<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics with CuO addition for microwave application

Microwave dielectric ceramics CuO–modified MgZrTa₂O₈ were synthesized by the conventional solid-state reaction method. The effects of CuO additives on the sintering characteristics and microwave dielectric properties have been investigated. With CuO addition, the sintering temperature of MgZrTa₂O₈ ceramics can be effectively lowered from 1475 to 1375 °C without decreasing its dielectric properties obviously and the temperature coefficient of the resonant frequency of MgZrTa₂O₈ ceramics have been optimized to near-zero. The crystalline phase exhibited a wolframite crystal structure and no second phase was detected at low addition levels. The grain growth of CuO–modified MgZrTa₂O₈ ceramics was accelerated due to liquid phase effect. The relative dielectric constants (ε_r) were correlated with apparent density and were not significantly different for all levels of CuO concentration. The quality factors (Q?×??) and temperature coefficient of resonant frequency (τ_?), which were strongly dependent on the CuO concentration, were analyzed by the grain size and the dielectric constant respectively. A best Q?×?? value of 116400 GHz and τ_? value of ?6.19 ppm/℃ were obtained for specimen with 0.05 wt% CuO addition at 1375 °C.     

Low-temperature sintering and microwave dielectric properties of Mg<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript>-LMZBS composites

The Mg₃B₂O₆ ceramics with lithium magnesium zinc borosilicate (LMZBS) glass were prepared at a lower sintering temperature. The effects of the glass addition on the densification, phase development, microstructure and microwave dielectric properties of the Mg₃B₂O₆ ceramics were investigated. The addition of LMZBS glass improved the densification and lowered the sintering temperature of Mg₃B₂O₆ ceramics from 1,300 to 950 °C. X-ray diffraction patterns showed that Mg₃B₂O₆ transformed into Mg₂B₂O₅ and a new phase, Li₂ZnSiO₄, crystallized from the glass phase. Because of the high dielectric performance of these phases, Mg₃B₂O₆ mixed with 55 wt% LMZBS sintered at 950 °C for 3 h had ε_r = 6.8, Q × f = 50,000 GHz, and τ_f = ?64 ppm/°C at 7.28 GHz. The chemical compatibility of ceramic-glass composites with Ag was also investigated for LTCC.     

Low temperature preparation of the Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript> ceramics with the addition of BaO and B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Zn₂SiO₄ ceramics with the addition of BaO and B₂O₃ are fabricated by traditional solid-state preparation process at a sintering temperature of 900 °C. The introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is achieved by adding 10 and 20 wt. % flux BB to the mixed ZnO-SiO₂ ceramic powders pre-sintered at 1,100 °C, respectively. The chemical composition of the flux BB (50 wt.%BaO-50 wt.% B₂O₃) is located at a liquid phase zone with a temperature range of about 869–900 °C in the binary diagram BaO-B₂O₃. In addition, the introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is also achieved by the means of a chemical combination of H₂SiO₃, H₃BO₃, ZnO and Ba(OH)₂·8H₂O, which can result in the formation of the hydrated barium borates with low melting characteristics. In turn, by the liquid sintering aid of the barium borate melts, the preparation process of the Zn₂SiO₄ ceramics can be further simplified. In the two preparation methods, the Zn₂SiO₄ ceramics with the 1.5–2.0 ZnO/SiO₂ molar ratios and the addition of a 10 wt. % flux BB can show good dielectric properties whereas the bending strength mainly depends on the microstructure of the Zn₂SiO₄ ceramics and SiO₂ content in the composition of the specimen.     

Influence of Primary Milling on Structural and Electrical Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Ceramics Obtained by Sintering Process

In this article, the influence of primary mechanical milling of precursors on the microstructure and dielectric properties of Bi₄Ti₃O₁₂ ceramics was studied. Precursor material (mixture of Bi₂O₃ and TiO₂ powders) was ground by a high-energy attritorial mill for (1, 3, 5, and 10) h. Bi₄Ti₃O₁₂ ceramics were obtained by a solid-state reaction process, synthesized at an intermediate temperature (800 °C) and finally sintered at high temperature (1140 °C). Structure studies were performed by X-ray diffraction (XRD) and scanning electron microscopy techniques. XRD patterns were analyzed by the Rietveld method using the DBWS 9807a program. The thermal properties of the studied materials were measured using differential thermal analysis and thermal gravimetric techniques. These studies indicate that one-, three-, and five-hour primary high-energy ball milling followed by sintering is a promising technique for pure Bi₄Ti₃O₁₂ ferroic ceramics preparation. The investigation of Bi₄Ti₃O₁₂ shows that ceramics obtained from a precursor and milled for 5 h have the best dielectric properties.     

Sol–gel synthesis, densification, and electrical properties of CuO–B2O3 doped Ba6?3xR8+2xTi18O54 (R?=?Nd) microwave dielectric ceramics

The low-temperature confirmable Ba_6−3x R_8+2x Ti₁₈O₅₄ (BNT, R = Nd, x = 2/3) ceramics were prepared by means of a citrate sol–gel soft-chemical method and the addition of sintering aid. Nano-sized BNT crystallite powders (~80 nm) were successfully synthesized as indicated by transmission electron microscopy and X-ray diffractometry. The powder compacts exhibit enhanced sintering activity and can be well densified at 925 °C with the aid of a small amount of CuO and B₂O₃. Compared to pure BNT ceramics prepared by a solid-state reaction method, not only was the sintering temperature significantly decreased, but also the good microwave dielectric properties of dielectric constant ε_r = 63 and quality factor Q × f = 5200 GHz were maintained. Moreover, the relationship between the microstructure, densification, and electrical properties was discussed.     

Varistor properties and aging behavior of ZnO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> ceramics with sintering process

The microstructure, electrical properties, and DC-accelerated aging behavior of the ZnO-V₂O₅-Mn₃O₄ ceramics were investigated at different sintering temperatures of 850–925°C. The microstructure of the ZnO-V₂O₅-Mn₃O₄ ceramics consisted of ZnO grain as a primary phase, and Zn₃(VO₄)₂ which acts as a liquid-phase sintering aid, in addition to Mn-rich phase as secondary phases. The maximum value (3,172 V/cm) and minimum value (977 V/cm) of breakdown field were obtained at sintering temperature of 850 and 900°C, respectively. The nonlinear coefficient exhibited the highest value, reaching 30 at 925°C and the lowest value, reaching 4 at 850°C. The optimum sintering temperature was 900°C, which exhibited not only high nonlinearity with 24 in nonlinear coefficient, but also the high stability, with %ΔE_1mA = −0.9% and %∆α = −12.5% for DC-accelerated aging stress of 0.85 E_1mA/85°C/24 h.     

Microstructure and electrical properties of Pr<Subscript>6</Subscript>O<Subscript>11</Subscript> doped ZnO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-based varistors

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped by Pr₆O₁₁ in the content range of 0–5.49 wt% were investigated at different sintering temperatures (1,100, 1,150, 1,175, 1,200 °C). The increase of sintering temperature leads to more dense ceramics, which increases the nonlinear property, whereas it decreases the voltage-gradient and leakage current. With increasing Pr₆O₁₁ content, the breakdown voltage increases because of the decreases of ZnO grain size. The improvement of non linear coefficient together with the decrease of leakage current are related to the uniformly distribution of secondary phases along the grain boundaries of the ZnO. The varistors sintered at 1,175 °C with the 3.37 wt% Pr₆O₁₁ doping possess the best electrical properties: the varistor voltage, nonlinear coefficient, and leakage current are 340 V/mm, 46 and 0.63 μA, respectively.     

Structure and microwave dielectric properties of ZnTa<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics with TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> additions

Ceramic samples based on ZnTa2O₆ and ZnTa₂O₆–MO₂ (M = Ti, Zr) systems have been obtained by the solid state ceramic route. The phase composition and microstructure of samples were investigated. The effect of the aliovalent substitution of ions Zn²⁺ and Ta⁵⁺ by M⁴⁺ (M = Ti, Zr) in the structure of ZnTa₂O₆ on microwave dielectric properties of ceramics was studied. The way of the compensation of the positive temperature coefficient of resonant frequency of dielectric resonators based on ZnTa₂O₆ ceramics with using the aliovalent substitution of cations was proposed. Dielectric resonators with the high temperature stability of the resonant frequency and high dielectric properties in the microwave range based on the ZnTa₂O₆–ZrO₂ system were obtained for application in electronics.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on sintering behavior and microwave dielectric properties of ZnTa<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics

ZnTa₂O₆ ceramics with various amount of Al₂O₃ additive were synthesized by a conventional mixed-oxide route. The grain growth of ZnTa₂O₆ ceramics was accelerated with Al₂O₃ additive. However, excessive addition (>1.0 wt%) of Al₂O₃ leaded to abnormal grain growth. With Al₂O₃ addition, the Al₂O₃ additive did not solubilized into ZnTa₂O₆ structure but resulted in forming the second phase. The Al₂O₃ addition resulted in the lower sintering temperature of ZnTa₂O₆ ceramics and improved microwave dielectric properties. The dielectric constant (ε_r) of the samples did not change much and ranged from 32.41 to 34.33 with different amount of Al₂O₃ addition. The optimized quality factor (Q × f) was higher than 70,000 GHz as a result of the denser ceramics. The temperature coefficient of resonant frequency (τ _f ) of the doped ZnTa₂O₆ ceramics could be optimized to near-zero.     

Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

Relationship among the phase equilibria,microstructures, and dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics via different sintering time

The structure–property relationship of the CaCu₃Ti₄O₁₂ ceramics processed via conventional solid-state method was studied in terms of the different processing conditions. X-ray diffraction patterns of the tenorite CuO and cuprite Cu₂O secondary phases found on the unpolished and polished surfaces of CaCu₃Ti₄O₁₂ were explained by the reduction/reoxidation reaction as a function of sintering time. Based on the microstructures, grain growth of CaCu₃Ti₄O₁₂ continued from 0.5 to 4 h sintering while the further growth was limited to the small-sized grains after 8 h sintering. Also, WDS data indicated the Cu-deficient and Ti-excessive stoichiometry of CaCu₃Ti₄O₁₂ on both outer and inner regions regardless of sintering time. The change of dielectric constant and tan δ were shortly discussed with regard to the secondary phases and the microstructures of the different sintering hours.     

Investigation of Li<Subscript>2</Subscript>Zn<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>-based temperature stable dielectric ceramics for LTCC applications

A low temperature co-fired ceramic (LTCC) was fabricated at 910 °C /2 h from the powder mixture of Li₂Zn₃Ti₄O₁₂, TiO₂ and a B₂O₃–La₂O₃–MgO–TiO₂ glass (BLMT), and the influence of TiO₂ on microstructure and dielectric properties of the composite was investigated in the composition range (wt%) of 20BLMT–(80???x)Li₂Zn₃Ti₄O₁₂–xTiO₂ (x?=?0, 2.5, 5, 7.5, 9 and 10). The results showed that all samples consisted of Li₂Zn₃Ti₄O₁₂, TiO₂, LaBO₃ and LaMgB₅O₁₀ phase. And LaBO₃, LaMgB₅O₁₀ and a small amounts of TiO₂ were crystallized from BLMT glass during sintering process. As x increases, dielectric constant and temperature coefficient of resonance frequency of the composites demonstrated gradually increase, whereas the quality factor of the sample of x?=?0 wt% was about 41,500 GHz and the ones maintained stable at a high level of 49,000–51,000 GHz for other samples. The composite with x?=?9 wt% had an optimal microwave dielectric properties with the dielectric constant of 20.2, quality factor of 50,000 GHz and temperature coefficient of resonant frequency of ??0.33 ppm/°C.     

Effect of B2O3 and CuO additions on the sintering temperature and microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics

The effects of B₂O₃–CuO (BCu, the weight ratio of B₂O₃ to CuO is 1:1) addition on the sintering behavior, microstructure, and the microwave dielectric properties of 3Li₂O–Nb₂O₅–3TiO₂ (LNT) ceramics have been investigated. The low-amount addition of BCu can effectively lower the sintering temperature of LNT ceramics from 1125 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 2 wt% BCu-added ceramic sintered at 900 °C has better microwave dielectric properties of ε _r  = 50.1, Q × f = 8300 GHz, τ _f  = 35 ppm/°C. Silver powders were cofired with the dielectric under air atmosphere at 900 °C. The SEM and EDS analysis showed no reaction between the dielectric ceramic and silver powders. This result shows that the LNT dielectric materials are good candidates for LTCC applications with silver electrode.     

Low-temperature sintering behavior and properties of <Emphasis Type="Italic">monoclinic</Emphasis>-SrAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics prepared via an aqueous suspension milling process

In this work, by an aqueous suspension milling process, boric acid (H₃BO₃), calcium hydroxide [Ca(OH)₂], strontium carbonate (SrCO₃) and barium hydroxide octahydrate [Ba (OH)₂·8H₂O] are mixed with strontium carbonate (SrCO₃) and kaolin (Al₂O₃·2SiO₂·2H₂O) to prepare SrAl₂Si₂O₈ ceramics with a sintering temperature of 950 °C. According to chemical compositions of flux agents B₂O₃, CaO·2B₂O₃, SrO·2B₂O₃ and BaO·2B₂O₃, raw materials boric acid, calcium hydroxide, strontium carbonate and barium hydroxide octahydrate were introduced to the suspension slurries of strontium carbonate and kaolin to decrease the densification sintering temperature of SrAl₂Si₂O₈ ceramics. In addition, the Sr element in SrAl₂Si₂O₈ ceramics are partly substituted with Ba and Ca elements, respectively, to investigate the low-temperature sintering behavior of partly substituted SrAl₂Si₂O₈ ceramics. The results indicated that the addition of flux agents to SrAl₂Si₂O₈ ceramics can availably achieve the densification sintering of SrAl₂Si₂O₈ ceramics at 950 °C, whereas the substitution of Sr with Ca or Ba have a great effect on sintering behaviors and dielectric properties of SrAl₂Si₂O₈ ceramics. Additionally, main crystal phases of the SrAl₂Si₂O₈ ceramics are monoclinic- SrAl₂Si₂O₈ and small quartz, but the evolution of crystal phases also depend on flux agents.     

Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Influence of different dopant sites by lanthanum on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The effects of La³⁺ doped in calcium copper titanate (CCTO) at Ca²⁺ site and Cu²⁺ site were examined. The doped compositions, La_0.1Ca_0.85Cu₃Ti₄O₁₂ (LCCTO) ceramics and CaLa_0.1Cu_2.85Ti₄O₁₂ (CLCTO) ceramics were prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristics were studied. And it was found that La³⁺ doped at Ca²⁺ site achieved lower sintering temperatures than that doped at Cu²⁺ site in CCTO ceramics. The dielectric loss (tan δ) of LCCTO ceramics was about 0.05 at 40 kHz when the sample was sintered at 1080 °C. Dielectric constant (ε′) of LCCTO ceramics was about 3.2 × 10⁴ when the sample was sintered at 1100 °C, which was larger than CLCTO ceramics examined under the same process condition with sintering temperatures vary. The impedance analysis revealed that LCCTO ceramics had an influence of resistance of grain boundaries, which was stronger than that of CLCTO ceramics. Meanwhile, both LCCTO ceramics and CLCTO ceramics had a nonlinear-Ohmic property.