Phase composition,Raman spectra,infrared spectra and dielectric properties of Li2MgTi1-x(Mg1/3Nb2/3)xO4 ceramics at microwave frequency

The Li₂MgTi_1-x(Mg_1/3Nb_2/3)_xO₄ (0?≤x?≤?0.5) ceramics were prepared by the conventional solid-state method. The relationship among phase composition, substitution amount and microwave dielectric properties of the ceramics was symmetrically investigated. All the samples possess the rock salt structure with the space group of Fm-3m. As the x value increases from 0 to 0.5, the dielectric constant linearly decreases from 16.75 to 15.56, which can be explained by the variation of Raman spectra and infrared spectra. The Q·f value shows an upward tendency in the range of 0?≤x?≤?0.3, but it then decreases when x?>?0.3. In addition, the temperature coefficient of resonant frequency (τ_f) is shifted toward zero with the increasing (Mg_1/3Nb_2/3)⁴⁺ addition. By comparison, the Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄ ceramics sintered at 1400?°C can achieve an excellent combination of microwave dielectric properties: ε_r=?16.19, Q·f =?160,000?GHz and τ_f =??3.14?ppm/°C.     

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.     

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).     

Effects of Li2O-B2O3-SiO2 glass on the low-temperature sintering of Zn0.15Nb0.3Ti0.55O2 ceramics

The influences of Li₂O-B₂O₃-SiO₂ glass (LBS) on the activation energy, phase composition, the stability of the structure and microwave dielectric properties of Zn_0.15Nb_0.3Ti_0.55O₂ ceramics have been systematically investigated. LBS glass acted as flux former and contributed to the reactive liquid-phase sintering mechanism, which remarkably lowed the sintering temperature from 1150?°C to 900?°C and enhanced the shrinkage and densification of ceramic at the low sintering temperatures. The ceramics with 1.5?wt% LBS glass sintered at 900?°C for 3?h show great properties: ε_r = 73.59, Q × f = 8024?GHz, τ_f = 270.54?ppm/°C.     

Microwave dielectric properties of barium substituted screen printed CaBi2Nb2O9 ceramic thick films

In the present study, Aurivillius-structured Ba²⁺ substituted CaBi₂Nb₂O₉ (CBNO) ceramic powder was synthesized by co-precipitation method. The CBNO thick films were delineated by screen printing method on alumina substrates using co-precipitated ceramic powder. The overlay method was adopted to measure the microwave dielectric properties of prepared thick films. Single phase layered perovskite structure of the prepared thick films was confirmed by X-ray Diffraction. The effects of Ba²⁺ substitution on the surface morphology, bonding, and microwave dielectric properties of thick films were systematically presented. The maximum value of microwave dielectric constant for the CBNO thick films at 11.8 GHz is 15.6 for Ba²⁺=0.8 substitution. The shift in the stretching vibration modes of the Nb-O bond of NbO₆ octahedron in the Raman spectra with a substitution of Ba²⁺ in CBNO was observed. The substitution of Ba²⁺ on A-site of CBNO improves the microwave dielectric properties of prepared thick films. This work may provide a new approach to enhance the microwave dielectric performance of Aurivillius-structured ceramic thick films.     

Crystal structure,microwave dielectric properties and low temperature sintering of (Al0.5Nb0.5)4+ co-substitution for Ti4+ of LiNb0.6Ti0.5O3 ceramics

The phase composition, microstructure, microwave dielectric properties of (Al_0.5Nb_0.5)⁴⁺ co-substitution for Ti site in LiNb_0.6Ti_0.5O₃ ceramics and the low temperature sintering behaviors of Li₂O-B₂O₃-SiO₂ (LBS) glass were systematically discussed. XRD patterns and EDS analysis result confirmed that single phase of Li_1.075Nb_0.625Ti_0.45O₃ solid solution was formed in all component. The increase of dielectric constant (ε_r) is ascribed to the improvement of bulk density. The restricted growth of grain has a negative influence on quality factor (Q×f) value. The τ_f value could be continuously shifted to near zero as the doping content increases. Great microwave dielectric properties were obtained in LiNb_0.6Ti_(0.5-x)(Al_0.5Nb_0.5)_xO₃ ceramics (x?=?0.10) when sintered at 1100?℃ for 2?h: ε_r =?70.34, Q×f =?5144?GHz, τ_f =?4.8?ppm/℃. The sintering aid, LBS glass, can effectively reduce the temperature and remain satisfied microwave performance. Excellent microwave dielectric properties for x?=?0.10 were obtained with 1.0?wt% glass: ε_r =?70.16, Q×f =?4153?GHz (at 4?GHz), τ_f =?-0.65?ppm/℃ when sintered at 925?℃ for 2?h.     

Effects of CaSiO3 addition on sintering behavior and microwave dielectric properties of Al2O3 ceramics

The effects of CaSiO₃ addition on the sintering behavior and microwave dielectric properties of Al₂O₃ ceramics have been investigated. The addition of CaSiO₃ into Al₂O₃ ceramics resulted in the emergence of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which acting as liquid sintering aids can effectively lower the sintering temperature of Al₂O₃ ceramic. The Q × f value of Al₂O₃-CaSiO₃ ceramics decreased with the CaSiO₃ addition increasing because of the lower Q × f value of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Compared with the pure CaSiO₃ ceramic, the Al₂O₃-CaSiO₃ ceramic with 20 wt% CaSiO₃ addition possessed good dielectric properties of ?_r = 9.36 and Q × f = 13,678 GHz at the similar sintering temperature.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Temperature stability,low loss and defect relaxation of MgO-TiO2 microwave dielectric ceramics modified by Ca0.8Sr0.2TiO3

Temperature-stable and low-loss microwave dielectrics based on the MgO-TiO₂ system with nominal formation Mg_n+1Ti_nO_3n+1 (n = 5, MT) were prepared via the conventional solid-state reaction method. Ca_0.8Sr_0.2TiO₃ (CST) was chosen as a τ_f compensator for matrix MT to form the composite ceramics (1-x)Mg₆Ti₅O₁₆-xCa_0.8Sr_0.2TiO₃ (0.10 ≤ x ≤ 0.26, MT-CST). The effects of CST additions on the phase composition, defect relaxation behavior, and microwave dielectric properties of MT were investigated. It revealed that undoped MT was basically consisted of MgTiO₃ as a major phase and Mg₂TiO₄ as a minor phase, and such two phases coexisted well with CST additions. Interestingly, τ_f could be tuned close to zero (?1.28 ppm/°C) for the MT-CST ceramics at x = 0.22, accompanied with a high Q×f value ~ 74,200 GHz and a proper ε_r ~ 20.25 (9.90 GHz). These materials possessed a good potential for applications in microwave components and devices. Meanwhile, significant relaxation phenomena were observed in all the MT-CST samples using dielectric spectroscopy and thermally stimulated depolarization current (TSDC) techniques. The oxygen-vacancy-related defects, shown as ($T{i}_{Ti}^{\prime }$)-($V_O^{??}$) dipoles and $V_O^{??}$, were the main types of defects in MT-CST, which was responsible for the relaxation behavior; meanwhile, the defect concentrations increased with the increase of CST content, thus resulting in the increase of dielectric loss at low and high frequencies.     

Preparation and dielectric properties of B2O3–Li2O-doped BaZr0.35Ti0.65O3 ceramics sintered at a low temperature

BaZr_0.35Ti_0.65O₃ (BZT) ceramics have been fabricated via a traditional ceramic process at a relatively low sintering temperature using liquid-phase sintering aids B₂O₃ and Li₂O. The dielectric properties of BZT ceramics have been investigated with the emphasis placed on the dielectric properties under an applied dc electric field. The temperature-dependent dielectric constant reveals that the pure BZT and B₂O₃–Li₂O-doped BZT ceramics all have a typical relaxor behavior and diffuse phase transition characteristics. The temperature-dependent dielectric constant under the applied dc electric field shows that the Curie temperature is slightly shifted to higher temperature and the peaks are suppressed and broadened. The dielectric loss is still under 0.005 and tunability is above 20% at an applied dc electric field of 30 kV/cm.     

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.     

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).     

Microwave dielectric ceramics in the system BaO–Li2O–Nd2O3–TiO2

Ceramics in the system BaO-Li₂O–Nd₂O₃–TiO₂ (BNT–LNT) were prepared by the mixed oxide route. Powders were mixed, milled, calcined and sintered at 1475°C for 4 h. Fired densities decreased steadily along the series from BNT to LNT. The microstructures of samples rich in BNT were dominated by small needle-like grains; the LNT samples comprised larger (6 μm) cubic grains. X-ray diffraction showed that there was a transition from orthorhombic BNT to cubic LNT; small amounts of LNT could be accommodated in BNT, but between 10–20% LNT there was the development of the second phase. Small additions of LNT led to a small increase in relative permittivity, but decreased the dielectric Q-value (from the maximum of 1819 at 4 GHz). As BNT and LNT exhibit negative and positive temperature dependencies of permittivity respectively, the addition of 10–20% LNT to BNT should yield samples with zero temperature dependence of _r Impedance spectroscopy showed that data could only be acquired at elevated temperatures for BNT rich samples (above 500°C), but at modest temperatures (less than 100°C) for the more conductive LNT.     

Co2O3 substitution effects on the structure and microwave dielectric properties of low-firing (Zn0.9Mg0.1)TiO3 ceramics

Low-firing (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ (x = 0.02–0.10) (ZMC_xT) microwave dielectric ceramics with high temperature stability were synthesized via conventional solid-state reaction. The influences of Co₂O₃ substitution on the phase composition, microstructure and microwave dielectric properties of ZMC_xT ceramics were discussed. Rietveld refinement results show the coexistence of ZnTiO₃ and ZnB₂O₄ phases at x = 0.02–0.10. (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ ceramic with x = 0.06 (ZMC_0.06T) obtains the best combination microwave dielectric properties of: ε_r = 21.58, Q × f = 53,948 GHz, τ_f = ? 54.38 ppm/°C. For expanding its application in LTCC field, 3 wt% ZnO-B₂O₃-SiO₂ (ZBS) and 9 wt% TiO₂ was added into ZMC_0.06T ceramic, great microwave dielectric properties were achieved at 900 °C for 4 h: ε_r = 26.03, Q × f = 34,830 GHz, τ_f = ? 4 ppm/°C, making the composite ceramic a promising candidate for LTCC industry.     

Effect of BaTiO3 addition on the microstructure and relaxor ferroelectric properties of Sr0.7Ba0.3Nb2O6 ceramics

A conventional solid-state reaction was used to synthesize (1-x) Sr_0.7Ba_0.3Nb₂O₆–xBaTiO₃ (0.00≤x≤0.10) ceramics. The phase structure, microstructure, and dielectric and relaxor ferroelectric properties of these ceramics were investigated. Tungsten bronze structure can be observed in ceramics, and addition of BaTiO₃ can make the grain size decrease and the porosity increase. The dielectric characteristics show diffuse phase transition phenomena for all samples, which were demonstrated by a linear fit of the modified Curie-Weiss law with γ varying between 1.54 and 1.88. As the BaTiO₃ content increases, the transition temperature (T_C) decreases gradually and has a minimum value of 37.53 °C at composition x=0.06, and the maximum dielectric constant (ε_max) increases gradually from 66 to 3309 and subsequently decreases to 1625 at x=0.10. In addition, the relaxor ferroelectric properties of these ceramics at x=0.8 are consistent with the Volgel-Fulcher relationship; polarization versus electric field (P-E) loops were measured at a different temperature.     

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.     

Crystal structures and microwave dielectric properties of low-firing Ca5Zn4-xMgxV6O24 ceramics

Ca₅Zn_4-xMg_xV₆O₂₄ (x?=?0–3) microwave dielectric ceramics with low sintering temperature were synthesized via the conventional solid-state reaction. Effects of the substitution of Mg²⁺ for Zn²⁺ on crystal structures and microwave dielectric properties were investigated. XRD and Rietveld refinement showed the solid solution single phase formed when 0?≤?x?≤?2, but a few ZnO was observed when x?=?3. Meanwhile, the lattice parameters were found to decrease monotonously with Mg content increasing. The vibration modes of Raman were confirmed and the relationship with microwave dielectric properties was analyzed. Appropriate substitution of Mg²⁺ improved the packing fraction, the cation ordering degree, and the Y-site bond valence, contributing to high Q×f and low | τ_f |. However, the ε_r reduced with the increasing content of Mg²⁺ due to the decrease of ion polarizability. Finally, the best microwave dielectric properties were achieved at x?=?2 with ε_r =?11.0, Q?×?f?=?66,365?GHz (at 10.0?GHz), and τ_f =??80.4?ppm/°C.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

Significantly enhanced sinterability and temperature stability of Li3Mg4NbO8-based microwave dielectric ceramics with LiF and Ba3(VO4)2 addition

Li₃Mg₄NbO₈-basic composite ceramics were elaborated via the solid-state reaction process, in which LiF and Ba₃(VO₄)₂ were utilized as a sintering aid and reinforcement phase, respectively. The sinterability, phase assemblage, microstructures, and microwave dielectric performances of Li₃Mg₄NbO₈–LiF–Ba₃(VO₄)₂ composite ceramics were thoroughly researched. The co-addition of LiF–Ba₃(VO₄)₂ can simultaneously lower the sintering temperature and improve the thermal stability of Li₃Mg₄NbO₈-basic ceramics. Solid state activated sintering is responsible for the low-temperature densification of the present ceramics. The coexistence of rock-salt structural Li₃Mg₄NbO₈/Li₄Mg₄NbO₈F and hexagonal structural Ba₃(VO₄)₂ phases was demonstrated by the combinational XRD and SEM-EDS analysis results. The 0.65(Li₃Mg₄NbO₈–LiF)-0.35Ba₃(VO₄)₂ ceramics fired at 825 °C/5 h exhibited promising microwave dielectric performances: τ_f = 0.5 ppm/°C along with ε_r = 13.8 and Qxf = 68500 GHz. The good compatibility of the developed ceramics with Ag demonstrates it potential for use in LTCC technology.     

Crystal structure,impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic

Bi(Sc_1/3Mo_2/3)O₄ ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a?=?16.9821(9)?Å, b?=?11.6097(3)?Å, c?=?5.3099(3)?Å and β?=?104.649(2)°) with a space group C12/C1, in which Bi³⁺, Sc³⁺ and Mo⁶⁺ are ?8, ?6 and ?4 coordinated, respectively. Bi(Sc_1/3Mo_2/3)O₄ ceramics were densifiedat 915?°C, giving a permittivity (ε_r) ～24.4, quality factor (Qf, Q?=?1/dielectric loss, f?=?resonant frequency) ～48, 100?GHz and temperature coefficient of resonant frequency (TCF)?～??68?ppm/°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E_a) ～0.97?eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc_1/3Mo_2/3)O₄ from 20?Hz to 30?THz. ε_r was stable from 20?Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the?ε_r.