水热法制备Ba6-3xNd8+2xTi18O54微波介质陶瓷

利用水热法合成了Ba6-3xNd8 2xTi18O54微波介质陶瓷,并对合成的微波介质陶瓷进行了X射线衍射分析和性能测试.结果表明:选择活性较大的前驱物[如Nd(OH)3,TiCl4],用水热法在360℃保温12h,能合成纯的Ba6-3xNd8 2xTi18O54粉体,极大地降低了粉体的合成温度.用水热法合成的Ba6-3xNd8 2xTi18O54粉体制备陶瓷,其烧结温度为1 250℃,比传统固相法要低100℃左右,陶瓷的介电常数(εr)稍大于用固相法制备的陶瓷,品质因数(Q)也有较大的提高,谐振频率温度系数(τf)也有所改善,当x=2/3时,水热法制备的Ba6-3xNd9 2xTi18O54陶瓷具有最佳微波介电性能:εr=88,Q与谐振频率(f)的乘积Q×f=8 890 GHz,τf=24x10-6/℃.     

Weak ferroelectricity and low-permittivity microwave dielectric properties of Ba2Zn(1+x)Si2O(7+x) ceramics

Ba₂Zn_(1+x)Si₂O_(7+x) ceramics were prepared using the conventional solid-state method at 1200 °C for 3 h in air. Apart from the previously reported Ba₂Zn_(1+x)Si₂O_(7+x) (x = 0) with a monoclinic structure (C 2/c), the end-member compositions at x = −1 and 1 exhibit single-phase β-BaSiO₃ with an orthorhombic structure (P2₁2₁2₁) and BaZnSiO₄ with a hexagonal structure (P6₃), and possess a coexistence of weak ferroelectricity and low-permittivity microwave dielectric properties. A reduction in Zn²⁺ content mainly decreases the intensity of the ε_r anomaly peak at lower temperature and increases the ε_r (or frequency) stability against temperature. The Zn²⁺-rich BaZnSiO₄ phase has a τ_f value of −181 ppm/°C, whereas the τ_f value of Zn²⁺-free BaSiO₃ phase decreases to −35.4 ppm/°C. The Zn²⁺ deficiency in Ba₂ZnSi₂O₇ composition could inhibit the presence of BaZnSiO₄ phase and improve the τ_f value, whereas excessive Zn²⁺ cations prompt the formation of the BaZnSiO₄ phase to deteriorate significantly the τ_f value.     

Improvement of microwave dielectric properties for Ba(Ni1/3Nb2/3)O3 ceramics by Zr-substitution

Effects of Zr-substitution on the structure, microstructure and microwave dielectric properties of Ba(Ni_1/3Nb_2/3)O₃ ceramics have been investigated. A small amount of Zr-substitution facilitates the densification of Ba(Ni_1/3Nb_2/3)O₃ ceramics. Within x≤0.05, the densification temperature decreases with increasing x in Ba[(Ni_1/3Nb_2/3)_1−xZr_x]O₃, while it turns to increase for x>0.05. With increasing x, the grains become more homogeneous and closely contacted, and significantly increase in size for x=0.15–0.20. The B-site cations 1:2 ordering is destroyed by Zr-substitution, and only stabilizes for x≤0.04. B-site cations 1:1 ordering starts to form in x=0.04, and the 1:1 ordering degree first increases and then decreases with increasing x. Qf value decreases slightly in x=0.01 and then increases monotonously with x increasing from 0.02 to 0.20. The destroyed 1:2 ordering structure is responsible for the decreased Qf value in x=0.01, while the improved grain configuration dominates the increase of Qf value for x=0.02–0.20. The dielectric constant ε_r increases monotonously with increasing x, due to the higher polarizability of Zr ion than the average value of Ni/Nb ions. The temperature coefficient of resonant frequency τ_f shifts from negative to positive through zero with increasing x, which is ascribed to the highly positive τ_f value of the end member BaZrO₃. The significant improvement of microwave dielectric properties has been achieved for x=0.10, higher ε_r, higher Qf as well as near zero τ_f value have been obtained: ε_r=31.8, Qf=36,100 GHz, τ_f=7.8 ppm/°C.     

Structure and microwave dielectric properties of novel walstromite-type MCa2Si3O9 (M = Ba,Sr) ceramics

Novel walstromite-type MCa₂Si₃O₉ (M = Ba, Sr) ceramics, with triclinic space group P-1, were prepared through a solid-state reaction method. The P–V-L theory proves that the lattice energy and bond energy of the Si–O bond play a leading role in the quality factor and the dielectric constant is mainly determined by the ionic polarization. Excellent microwave dielectric properties of BaCa₂Si₃O₉ and SrCa₂Si₃O₉ ceramics could be obtained: ε_r = 8.99 ± 0.23, Q × f = 44,542 ± 500 GHz, and τ_f = −25.9 ± 3.0 ppm/°C and ε_r = 7.39 ± 0.23, Q × f = 48,772 ± 500 GHz, and τ_f = −27.5 ± 3.0 ppm/°C, when sintered at 1240/1280 °C for 4 h. Then SrCa₂Si₃O₉ ceramic is applied to a new microstrip bandpass filter, because of its high microwave dielectric properties and low thermal expansion coefficient. With reduced dimension, the filtering performance of the circuit is also highly improved, including reduced capacitor parasitic effect and the optimized stopband insertion loss. Accordingly, the SrCa₂Si₃O₉ ceramic is a promising candidate for sub-6 GHz a filter of microstrip bandpass applications.     

Influence of Cr3+ substitution for Mg2+ on the crystal structure and microwave dielectric properties of CaMg1-xCr2x/3Si2O6 ceramics

The CaMg_1-xCr_2x/3Si₂O₆ (0?≤?x?≤?0.1) microwave dielectric ceramics were synthesized via conventional solid state reaction. In this study, the effects of Cr³⁺ substituting for Mg²⁺ on morphology, crystal structure and microwave dielectric properties of CaMg_1-xCr_2x/3Si₂O₆ ceramics were explored. XRD diffraction patterns exhibited that the CaMg_1-xCr_2x/3Si₂O₆ ceramics possessed the pure phase of CaMgSi₂O₆ when x?≤?0.06 and a small amount of secondary phase Ca₃Cr₂(SiO₄)₃ for 0.08?≤?x?≤?0.1. SEM micrographs revealed that the substitution of Mg²⁺ with Cr³⁺ could decrease the grain size. The apparent density was affected by the concentration of Mg vacancies. The correlation between crystal structure and microwave dielectric properties was investigated through the Rietveld refinement and Raman analysis. The microwave dielectric properties were mainly dependent on relative density, ionic polarizabilities, internal strain ?, disordered structure and MgO₆ octahedron distortions. Finally, CaMg_1-xCr_2x/3Si₂O₆ (x?=?0.02) ceramics sintered at 1270?°C for 3?h exhibited excellent microwave dielectric properties of ε_r?=?8.06, Q?×?f?=?89054?GHz, τ_f?=??44.92182?ppm/ºC.     

Structure,far-infrared reflectance spectra,and microwave dielectric properties of Ba2MGa11O20 (M = Bi,La) ceramics

Ba₂MGa₁₁O₂₀ (M = Bi, La; called BBG and BLG, respectively) ceramics with monoclinic space group I2/m were prepared through a solid-state reaction method. BBG ceramic sintered at 1150 °C for 6 h has the best microwave dielectric properties with low ε_r = 10.68, Q × f = 41,756 GHz, and negative τ_f = ?61.3 ppm/°C. BLG ceramic sintered at 1440 °C for 6 h exhibits ε_r = 13.94, Q × f = 45,592 GHz, and near-zero τ_f = ?16.3 ppm/°C. The large deviation between ε_r and ε_th was ascribed to the “rattling” effect of the cations and the existence of lone pair ions of Bi³⁺. The difference in Q × f of the two ceramics was discussed in terms of packing fraction, and the τ_f of BLG was closer to zero than that of BBG due to the smaller τ_ε value. Their intrinsic dielectric properties were analyzed through far-infrared reflectivity spectroscopy.     

Structure and microwave dielectric properties of 1:1 ordered Nd[(Mg1-xZnx)1/2Ti1/2]O3 complex perovskite ceramics

Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ perovskite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8) are prepared by the solid-state reaction method. The effects of Zn²⁺ substitution on the structure, microstructure, especially the B-site 1:1 cation ordering and microwave dielectric properties have been investigated. Sintered Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ ceramics all adopt dense microstructure, along with increased dimensional uniformity as Zn²⁺ substitution. All the ceramics are confirmed to have B-site 1:1 ordered monoclinic perovskite structure with P2₁/n space group. Atomic mass difference of B-site elements might be an important factor affecting the B-site 1:1 cation ordering. HRSTEM observation suggest that the doped Zn²⁺ cations have roughly entered the Mg²⁺ sites to promote 1:1 cation ordering. The degree of the 1:1 cation ordering can be negatively reflected by the full width at half maximum (FWHM) of F_2g(B) mode at 372 cm^?1 in Raman spectra. With Zn²⁺ doping, the degree of the 1:1 cation ordering first increases then decreases, and reaches its maximum at x = 0.6. Meanwhile the best combination of microwave dielectric properties is obtained, as ε_r = 31.4, Q × f = 74,000 GHz, τ_f = ?44 ppm/°C. It is found that the long-range ordering not only decreases the dielectric loss but also affects the dielectric constant, providing a theoretical foundation to understand further the correlation between ionic configuration and microwave dielectric properties.     

Upper limit of x in Ba6−3xNd8+2xTi18O54 new tungsten bronze solid solution

In the present work, the upper limit of x in Ba_6−3xNd_8+2xTi₁₈O₅₄ ceramics was determined by investigating the phase constitution as the function of composition and process conditions. For the compositions of x ≤ 0.75, the new tungsten bronze structure is stable and the single phase structure is easy to be obtained. While, the new tungsten bronze structure becomes unstable for compositions of x = 0.8, 0.9 and 1.0, where the new tungsten bronze major phase is generally detected together with some secondary phases. The excellent microwave dielectric characteristics are not only obtained at x = 0.67 but also obtained at 0.8 and 0.9.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.     

Microwave dielectric characteristics of tungsten bronze-type Ba4Nd28/3Ti18-yGa4y/3O54 ceramics with temperature stable and ultra-low loss

A series of high-k Ba₄Nd_28/3Ti_18-yGa_4y/3O₅₄ (0≤y≤2, BNTG) ceramics with temperature stable and ultra-low dielectric loss were synthesized via the conventional solid-state reaction. The main phase of all BNTG ceramics demonstrated an orthorhombic tungsten-bronze structure, but the impurity phase (gallium-rich phase) was found in BNTG (y = 2) ceramic. Partial substitution of Ga³⁺ for Ti⁴⁺ in B-site was a valid method to improve the temperature stability and dielectric loss of BNTG ceramics. The variation of ε_r values of BNTG ceramics was dominated by the ionic polarizability. The ultra-low dielectric loss (ultra-high Q × f values) was associated with grain size, suppression of Ti³⁺ and impurity phase. The decrease of TCF values was highly dependent on the tilting of Ti-O octahedra and impurity phase. Finally, outstanding combination dielectric characteristics were achieved for BNTG microwave ceramics at y = 1.5 (ε_r = 72.8, Q × f = 14,600 GHz, TCF=+4.1 ppm/°C) and at y = 2 (ε_r = 70.3, Q × f = 15,500 GHz, TCF=+3.9 ppm/°C).     

Effect of (Zn1/3Nb2/3)4+ co-substitution on the microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

Ce₂[Zr_1-x(Zn_1/3Nb_2/3)_x]₃(MoO₄)₉ (CZ_1-x(ZN)_xM) (x = 0.02–0.08) compounds were successfully prepared to scientifically examine the effect of (Zn_1/3Nb_2/3)⁴⁺ doping on phase composition, microstructures, and properties. The XRD results showed that all compounds formed a pure phase with the space group of R-3c. SEM results indicated that all compounds were compact at 675 °C, and the lattice parameters and average grain size decreased with doping. Performance analysis illustrated that ε_r was closely related to the polarizability, and Q?f was affected by the lattice energy of the Mo–O bond. The τ_f was maintained at an excellent level. Far-infrared analysis indicated that the major dielectric contribution to CZ_1-x(ZN)_xM ceramics was related to the absorption of phonon oscillation. The optimum properties (ε_r = 10.72, Q?f = 59,381 GHz, τ_f = ?11.48 ppm/°C) were obtained when x = 0.04.     

(Cu1/3Nb2/3)4+ ionic substituting effects,low-temperature sintering behaviors,and improved temperature stability of Ba3Nb4Ti4O21 microwave dielectric ceramics

In this study, (Cu_1/3Nb_2/3)⁴⁺ complex cation and BaO–ZnO–B₂O₃ glass frit were adopted to solve the high sintering temperature and poor temperature stability of Ba₃Nb₄Ti₄O₂₁ ceramics. It is shown that pure Ba₃Nb₄Ti₄O₂₁ phase was formed when Ti site was partially replaced by (Cu_1/3Nb_2/3)⁴⁺ cation. The increasing number of dopants decreases the dielectric polarizability, correspondingly, the dielectric constant and temperature coefficient of the resonance frequency values are reduced consistently. The variation of the Q × f value is determined by internal ionic packing fraction and external sintering densification. The (Cu_1/3Nb_2/3)⁴⁺ cation effectively decreases the suitable sintering temperature from 1200 to 1050 °C while greatly improving the temperature stability. BaO–ZnO–B₂O₃ glass was used to further improve the low-temperature sintering characteristics of Ba₃Nb₄Ti₄O₂₁ ceramics. It is proven that the addition of glass frits effectively decreases the temperature to 925 °C with combinational excellent microwave dielectric properties: ε_r ～55.6, Q × f ～5700 GHz, τ_f ～3 ppm/^°C, making the Ba₃Nb₄Ti₄O₂₁ ceramics promising in the applications of low-temperature cofired ceramic technology.     

Co-optimization of microwave dielectric properties in Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics through ordered domain engineering

It is an important subject to improve the temperature coefficient of resonant frequency (τ_f) and thermal conductivity (κ) of microwave dielectric ceramics without reducing the Qf value. Ordered domain engineering was applied to realize the previous objectives in Ba(Mg_1/3Ta_2/3)O₃ ceramics. With the increasing ordering degree from 0.835 to 0.897, the optimized Qf value was obtained. Meanwhile, near zero τ_f from 11.9 to 5.6 ppm °C⁻¹ was achieved, together with increased κ from 5.5 to 7.6 W m⁻¹ K⁻¹, and enhanced dielectric strength from 801 to 921 kV cm⁻¹. The noticeable ordered domain structure with large ordered domains (∼100 nm) and low-energy domain boundaries was revealed in Ba(Mg_1/3Ta_2/3)O₃. The consequent weakened phonon scattering rises the thermal conductivity. The increased bond covalency and oxygen distortion in ceramics with higher ordering degree were suggested as a cause of enlarged bandgap, which enhanced the dielectric strength. The reduced τ_f is dominated by the less “rattling” space of the cations in the ordered state by inducing more positive τ_ε. The reduced τ_f, optimized thermal conductivity, and Qf value in the present work indicate that the ordered domain engineering could open up a new direction for the optimization of microwave dielectric ceramics.     

Influence of Ti4+ substitution for Ta5+ on the crystal structure,Raman spectra,and microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 ceramics

This paper systematically investigated the influence of Ti⁴⁺ substitution for Ta⁵⁺ on the phase composition and microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1, 0.2, and 0.3) ceramics with hexagonal tungsten bronze-like structures. X-ray diffraction and Rietveld refinement results indicated that single-phase Ba₃Ta₄Ti₄O₂₁ could be obtained only with the x values of 0.1 and 0.2, and a secondary phase was detected at an x value of 0.3. The valence state of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.2) ceramics was analyzed through X-ray photoelectron spectroscopy. Increasing Ti⁴⁺/Ta⁵⁺ ratios could reduce sintering temperature and improve the microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ solid solutions. However, the dielectric properties, particularly the quality factor, of Ba₃Ta_4-4xTi_4+5xO₂₁ ceramics deteriorated severely as a result of oxygen vacancy defects caused by the transition of the valence state from Ti⁴⁺ to Ti³⁺ when x = 0.2 and the coexistence of the secondary phase when x = 0.3. Infrared reflectivity spectroscopy was performed to explore the intrinsic dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics. The measured and extrapolated microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics sintered at 1240 °C for 6 h were ε_r ~ 46.5, Q × f = 13,900 GHz, τ_f ~ +49.4 ppm/°C, and ε_r ~ 44, Q × f = 34,850 GHz.     

Effect of CuO/MnO additives on microstructure and microwave dielectric properties of 3ZrO2-3TiO2-ZnNb2O6 ceramics

The effect of CuO/MnO additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 3ZrO₂-3TiO₂-ZnNb₂O₆ (3Z-3T-ZN) ceramics prepared by conventional solid-state route were systematically investigated. CuO/MnO doped ceramics exhibited a main phase of α-PbO₂-structured ZrTi₂O₆ and a secondary phase of rutile TiO₂. SEM results showed that the grain size of MnO doped ceramics became larger with increasing amount of dopants. The presence of CuO/MnO additives effectively reduced the sintering temperature of 3Z-3T-ZN ceramics to 1220 °C. MnO doped into ceramics could enhance the Q×f values significantly. The 0.5 wt% CuO doped 3Z-3T-ZN ceramics with 0.5 wt% of MnO, sintered at 1220 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 41.02, a Q×f value of 44,230 GHz (at 5.2 GHz), and τ_f value of +2.32 ppm/°C.     

Pr取代对Ba6-3xSm8+2xTi18O54微波介质陶瓷结构和介电性能的影响

以Ba6-3xSm8-2xTi18O54(x=2／3)微波介质陶瓷为基础，通过不同量的Pr取代实验，探讨Pr取代后对陶瓷的结构和介电性能影响。结果表明：在Pr取代Sm后，陶瓷的最佳烧结温度为1390℃。在Pr取代的范围内，取代后的陶瓷基本保持了以钨青铜结构的Ba6-3x(Sm1-yPry)8 2xTi18O54固溶体为主的相。并且发现材料的介电常数和介电损耗在Pr取代量为0.2mol时获得了最佳，分别为78．08和0．0001，频率温度系数由负向正变化。