Sol–gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

The sol–gel-hydrothermal processing of K_0.5Bi_0.5TiO₃ (KBT) nanowires as well as their sintering behavior at 1000–1100 °C were investigated. The morphological analyses indicated that sol–gel-hydrothermal route led to the formation of KBT nanowires with diameters of 4 nm and lengths of 100 nm at low processing temperature of 200 °C with KOH concentration of 6 M. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires. The KBT ceramics with a relative density of more than 95% can be successfully fabricated from the high quality KBT nanowires even by a conventional sintering process. The KBT ceramics sintered at 1050 °C showed typical characteristics of relaxor ferroelectrics, and the dielectric properties were better than that prepared by all other methods reported previously.     

Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics

Microwave dielectric ceramics ZnTa₂O₆ were prepared by conventional mixed oxide route. The effects of CaF₂ addition on the microstructures and microwave dielectric properties of ZnTa₂O₆ ceramics were investigated. Formation of second phase can be detected at the high addition of CaF₂ (0.5–1.0 wt.%). Variation of grain shapes were observed with CaF₂ content increasing. The sintering temperature of CaF₂-doped ZnTa₂O₆ ceramics can be effectively lowered from 1400 °C to 1225 °C due to liquid phase effect. The microwave dielectric properties were affected by the amount of CaF₂ addition. At 1225 °C for 4 h, ZnTa₂O₆ ceramics with 0.25 wt.% CaF₂ possesses excellent microwave dielectric properties: ε_r = 31.32, Q × ? = 73600 GHz(6.8 GHz) and τ_? = ? 6.97 ppm/°C.     

Low-firing of BiSbO4 microwave dielectric ceramic with V2O5–CuO addition

Effects of 1.0 wt.% V₂O₅–CuO mixture addition on the sintering behavior, phase composition and microwave dielectric properties of BiSbO₄ ceramics have been investigated. BiSbO₄ ceramics can be well densified below temperature about 930 °C with 1.0 wt.% V₂O₅–CuO mixtures addition with different ratios of CuO to V₂O₅. The formation of BiVO₄ phase and substitution of Cu²⁺ can explain the decrease of sintering temperature. Dense BiSbO₄ ceramics sintered at 930 °C for 2 h exhibited good microwave dielectric properties with permittivity between 19 and 20.5, Qf values between 19,000 and 40,000 GHz and temperature coefficient of resonant frequency shifting between ?71.5 ppm °C^?1 and ?77.8 ppm °C^?1. BiSbO₄ ceramics could be a candidate for microwave application and low temperature co-fired ceramics technology.     

Temperature stable microwave dielectric ceramic 0.3Li2TiO3–0.7Li(Zn0.5Ti1.5)O4 with ultra-low dielectric loss

In the present work, the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic was prepared via the conventional solid state reaction route, and the phase composition, microstructure, and sintering behavior were investigated. The ceramic sample sintered at 1100 °C for 2 h demonstrated high microwave dielectric performance with a relative permittivity of 23.5, a high quality factor (Qf) ~ 88,360 GHz (at 7.4 GHz), and near zero temperature coefficient of resonant frequency about ? 0.8 ppm/°C. These results indicate that the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic might be a good candidate for dielectric resonators, filters and other microwave electronic device applications.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.     

Investigation on the calcining condition and sintering temperature of perovskite-type La0.7Ca0.3CrO3 complex oxides by a combustion method

Perovskite-type La_0.7Ca_0.3CrO₃ composite oxides were synthesized by a combustion method. The calcining condition of the synthesized powders and sintering temperature of ceramic specimens were studied. It was found that a pure perovskite structure was direct acquired in the combustion ash and the fine morphology (～100 nm) was observed in the sample calcined at 600 °C. In view of the relative density, microstructure and electrical conducting properties of the sintering ceramics at different temperatures, the preferred sintering temperature was ascertained to be 1300 °C, at which the sample attains a high relative density of 96.8%, showing an electrical conductivity of 53.6 S cm^?1 at 800 °C and much lower activation energy of 0.122 ev. Compared with La_0.7Ca_0.3CrO₃ synthesized by the conventional solid state reaction method, that synthesized by the combustion method exhibits fine morphology and superior sintering activity, effectively lowering the sintering temperature and enhancing electrical conducting properties of the material.     

Influence of CuO addition to BaSm2Ti4O12 microwave ceramics on sintering behavior and dielectric properties

Microwave dielectric ceramics of tungsten–bronze-type BaSm₂Ti₄O₁₂ were prepared by doping CuO (up to 2 wt.%) as the liquid-phase sintering aid. The effects of CuO additive on the densification, micro structure and dielectric properties were investigated. Due to the liquid-phase effect, the sintering temperature of BaSm₂Ti₄O₁₂ ceramics with 1 wt.% CuO addition can be effectively reduced to 1160 °C, about 200 °C lower than that of pure BaSm₂Ti₄O₁₂ ceramics, while good microwave dielectric properties of ɛ_r = 75.8, Q*f = 4914.6 GHz and τ_f = −7.65 ppm/°C were still achieved.     

Sintering temperature dependence of microwave dielectric properties in Mg4(TaNb1−xVx)O9 compounds

The relationship between the sintering temperature and the microwave dielectric properties of Mg₄(TaNb_1−xV_x)O₉ (MTNV) compounds were investigated in this study in order to reduce the sintering temperature of the compound. A small amount of V₂O₅ doping lowered the sintering temperature of the MTNV compounds. The variations in the dielectric constant and the quality factor of the MTNV compounds depended on the amount of V₂O₅ doping and the sintering temperature; a small amount of V₂O₅ doping was effective in allowing low sintering temperatures without a detrimental effect on these dielectric properties. As a result, a dielectric constant of approximately 12 and a quality factor of approximately 200,000 GHz were obtained when the MTNV compounds with x = 0.2 was sintered at 1200 °C. The temperature coefficient of resonant frequency of the MTNV compound with x = 0.025 slightly changed from −63 to −73 ppm/ °C with an increased sintering temperature because of the presence of a secondary phase.     

Structural and microstructural characterizations of nanocrystalline hydroxyapatite synthesized by mechanical alloying

Single phase nanocrystalline hydroxyapatite (HAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO₃ and CaHPO₄ powders in open air at room temperature, for the first time, within 2 h of milling. Nanocrystalline hexagonal single crystals are obtained by sintering of 2 h milled sample at 500 °C. Structural and microstructural properties of as-milled and sintered powders are revealed from both the X-ray line profile analysis and transmission electron microscopy. Shape and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Particle size of HAp powder remains almost invariant up to 10 h of milling and there is no significant growth of nanocrystalline HAp particles after sintering at 500 °C for 3 h. Changes in lattice volume and some primary bond lengths of as-milled and sintered are critically measured, which indicate that lattice imperfections introduced into the HAp lattice during ball milling have been reduced partially after sintering the powder at elevated temperatures. We could achieve ~ 96.7% of theoretical density of HAp within 3 h by sintering the pellet of nanocrystalline powder at a lower temperature of 1000 °C. Vickers microhardness (VHN) of the uni-axially pressed (6.86 MPa) pellet of nanocrystalline HAp is 4.5 GPa at 100 gm load which is close to the VHN of bulk HAp sintered at higher temperature. The strain-hardening index (n) of the sintered pellet is found to be > 2, indicating a further increase in microhardness value at higher load.     

Synthesis and microwave dielectric properties of Ba4Ti3P2O15 ceramics

Present work introduces a new kind of microwave dielectric ceramic, Ba₄Ti₃P₂O₁₅. Ba₄Ti₃P₂O₁₅ ceramic can be prepared by solid state reaction method and be well densified after being sintered at above 1175 °C for 4 h in air. All the XRD patterns can be fully indexed as single-phase structure. The best microwave dielectric properties can be obtained in ceramic sintered at 1200 °C for 4 h with permittivity about 20.7, Q × f about 42,210 GHz and TCF about 37 ppm °C^?1. Measurements of the microwave dielectric properties of Ba₄Ti₃P₂O₁₅ ceramic revealed the existence of a maximum in the temperature dependence of the dielectric loss because of the defect dipoles relaxation.     

Powder synthesis and properties of LiTaO3 ceramics

The LiTaO₃ powders with sub micrometer grade grain size have been synthesized successfully using a molten salt method. Lithium tantalate began to form at 400 °C reaction temperature and transformed to pure phase without residual reactants when it was processed at 500 °C for 4 h in static air. The undoped LiTaO₃ ceramics with a Curie temperature about 663 °C were obtained by pressureless sintering at 1300 °C for 3 h. The relative dielectric constant (ɛ_r) increases from 50 to 375 at temperature ranging from 30 to 663 °C and then decreases quickly as the temperature increases above 663 °C. The ceramics shows a relative dielectric constant of 49.4, a dielectric loss factor (tan δ) of 0.007, a coercive field (Ec) of 28.66 kV/cm and a remnant polarization (Pr) of 32.48 μC/cm² at room temperature.     

Synthesis of nanocrystalline MMoO4 (M = Ni,Zn) phosphors via a citrate complex route assisted by microwave irradiation and their photoluminescence

Nanocrystalline MMoO₄ (M = Ni, Zn) phosphors, which have wolframite-type structure, were successfully synthesized at low temperatures via a modified citrate complex route assisted by microwave irradiation. The citrate complex precursors were heat-treated from 300 to 600 °C for 3 h. Crystallization of the MMoO₄ (M = Ni, Zn) nanocrystallites were detected at 500 °C, and entirely completed at a temperature of 600 °C. The nanoparticles presented primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The nanocrystalline MMoO₄ (M = Ni, Zn) phosphors prepared at 600 °C exhibited broad luminescence in green and blue wavelength region, respectively.     

Low temperature sintering and dielectric properties of BaTiO3 with glass addition

To develop low-temperature-fired BaTiO₃-based ceramics, the effects of various glasses (BaO–B₂O₃–SiO₂ (BBS), PbO–B₂O₃–SiO₂ (PBS), and ZnO–B₂O₃–SiO₂ (ZBS1: 57 mol%ZnO–33 mol%B₂O₃–10 mol%SiO₂ and ZBS2: 60.7 mol%ZnO–24.9 mol%B₂O₃–14.4 mol%SiO₂)) addition on both the sintering behavior and dielectric properties of BaTiO₃ were investigated. X-ray diffractometer (XRD), scanning electron microscopy (SEM), dilatometer were used to examine the effect of various glasses on the densification of BaTiO₃ and the chemical reaction between the glass and BaTiO₃. The results indicate that ZBS2 glass can be used as a sintering aid to reduce the sintering temperature of BaTiO₃ from 1300 °C to 900 °C without the formation of secondary phase. The dielectric properties of BaTiO₃ with ZBS2 glass sintered at 900 °C show a relative density of 95%, a high dielectric constant of 994, and a dielectric loss of 1.6%.     

Microwave dielectric properties of BaO–TeO2 binary compounds

A series of BaO–TeO₂ binary ceramic compounds were explored for microwave dielectric applications with ultra-low processing temperatures. During the calcination of mixed BaCO₃ and TeO₂ raw powders, BaTe₄O₉, BaTe₂O₆, BaTeO₃, and Ba₂TeO₅ phases were obtained through the sequential phase formations from Te-rich to Ba-rich phases at temperatures ranging from 500 to 850 °C. Sintering temperatures were as low as only 550 °C for the Te-rich phases. Barium tellurate ceramics exhibited excellent microwave dielectric properties with intermediate dielectric permittivities and high quality factors (Q). The dielectric properties at microwave frequencies were ε_r = 10–21, Q × f = 34,000–55,000 GHz, and TCf = − 51 to − 124 ppm/°C, depending on compositions.     

Influence of copper substitution on magnetic and electrical properties of MgCuZn ferrite prepared by microwave sintering method

Polycrystalline MgCuZn ferrites with chemical formula Mg_0.50-xCu_xZn_0.50Fe₂O₄ (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared by microwave sintering method. These powders were calcined, compacted and sintered at 950 °C for 30 min. Structural, microstructural and elemental analyses were carried out using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS), respectively. The lattice parameter is found to increase with increasing copper content. A remarkable densification is observed with the addition of Cu ions in the ferrites. The sintered ferrite was characterized for initial permeability, dielectric constant and dielectric loss tangent and ac conductivity measurements. The temperature variation of the initial permeability of these samples was carried out from 30 °C to 200 °C. The dielectric constant, dielectric loss tangent and ac conductivity have been measured in the frequency range of 100 Hz to 1 MHz. Initial permeability and dielectric constant were found to increase and dielectric loss decreased with Cu substitution for Mg, up to x = 0.20. The ferrite powder prepared is suitable for the application in multilayer chip inductor due to its low-temperature sinterability, good magnetic properties and low loss at high frequency.     

Dielectric properties of Sb2O3-doped Ba0.672Sr0.32Y0.008TiO3

Sb₂O₃-doped Ba_0.672Sr_0.32Y_0.008TiO₃ (BSYT) dielectric ceramics were prepared by conventional solid state method, and their dielectric properties were investigated with variation of Sb₂O₃ doping content and sintering temperature. The X-ray diffraction patterns indicated that all the BSYT specimens possessed the perovskite polycrystalline structure. The experimental results reveal that the introduction of Sb₂O₃ into Ba_0.672Sr_0.32Y_0.008TiO₃ can control the grain growth, reduce the relative dielectric constant and dielectric loss, shift the Curie temperature to lower temperature and significantly improve the thermal stability of the BSYT ceramics. The samples doped with 1.6 wt.% Sb₂O₃ sintered at 1320 °C for 2 h exhibited attractive properties, including high relative dielectric constant (> 1500), low dielectric loss (< 40 × 10^− 4), low temperature coefficient of capacitor(< ± 35%) over a wide temperature range from − 25 °C to + 85 °C.     

Effect of BaCu(B2O5) on the sintering temperature and microwave dielectric properties of BaO–Ln2O3–TiO2 (Ln = Sm,Nd) ceramics

BaCu(B₂O₅) (BCB) ceramic powder was used to decrease the sintering temperatures of BaSm₂Ti₄O₁₂ (BST) and BaNd₂Ti₅O₁₄ (BNT) ceramics. The sintering temperature of the BST and BNT ceramics was reduced from approximately 1350 °C to 850 °C by the addition of BCB. The bulk density of the specimens increased and reached the saturated value with increasing BCB content. The variation of the dielectric constant (ɛ_r) was similar to that of the bulk density and, thus, the relative density plays an important role in determining the ɛ_r value of the specimens. The Q-value initially increased with the addition of BCB but decreased considerably when a large amount of BCB was added because of the presence of the liquid phase. Good microwave dielectric properties of Qxf = 4500 GHz, ɛ_r = 60 and τ_f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h.