The sintering behavior and microwave dielectric properties of Mg4(Nb,Sb)2O9 ceramics

The sintering behavior, microstructure and microwave dielectric properties of Mg₄(Nb_2?x Sb _x )O₉ (0?≤?x?≤?2) solid solutions were investigated systematically by X-ray diffraction(XRD), scanning electron microscopy(SEM) and a network analyzer. The solid solutions of Mg₄(Nb_2?x Sb _x )O₉ was formed with x value being no more than 1.6. The dielectric constant (?) of the sintered ceramics decreased from 13.06 to 6.28 with Sb content x from 0 to 1.6. With a substitution of Sb⁵⁺ for Nb⁵⁺ (0.04?≤?x?≤?0.08), the sintering temperature of Mg₄Nb₂O₉ ceramics was decreased from 1400 to 1300 °C without degradation of the Qf values. The optimum microwave dielectric properties of ??～?12.26, Qf?～?168,450 GHz, and τ _f?～??56.4 ppm/°C were obtained in the composition of Mg₄(Nb_1.6Sb_0.4)O₉ sintered at 1300 °C.     

Microwave dielectric properties of Mg4Nb2O9 ceramics produced by hydrothermal synthesis

Mg₄Nb₂O₉ ceramics have been prepared by a hydrothermal synthesis in order to reduce the sintering temperature. The sintering and microwave dielectric properties of the hydrothermally processed Mg₄Nb₂O₉ were studied under various sintering temperatures ranging from 900 to 1300°C. The highest Q×f _o value of 26,069 GHz was obtained at the sintering temperature of 1300°C and is attributed to the increased density and appropriate grain growth. τ _f value of ?17.1 ppm/°C was improved by the addition of TiO₂ and τ _f value of 6.7 ppm/°C was obtained at 20 wt% TiO₂. Chemical compatibility of Mg₄Nb₂O₉ with Ag was tested to identity the possibility of using Mg₄Nb₂O₉ for an LTCC application. Since any secondary phase was not observed in the XRD pattern of the mixtures of Mg₄Nb₂O₉ and Ag powder heat treated at 900°C, it was considered that the Mg₄Nb₂O₉ system is applicable to the multilayer microwave devices using Ag as an electrode.     

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of the BaTi4O9 ceramics

The effect of B₂O₃ and CuO on the sintering temperature and microwave dielectric properties of BaTi₄O₉ ceramics was investigated. The BaTi₄O₉ ceramics were able to be sintered at 975^∘C when B₂O₃ was added. This decrease in the sintering temperature of the BaTi₄O₉ ceramics upon the addition of B₂O₃ is attributed to the formation of BaB₂O₄ second phase whose melting temperature is around 900^∘C. The B₂O₃ added BaTi₄O₉ ceramics alone were not sintered below 975^∘C, but were sintered at 875^∘C when CuO was added. The formation of BaCu(B₂O₅) second phase could be responsible for the decrease in the sintering temperature of the CuO and B₂O₃ added BaTi₄O₉ ceramics. The BaTi₄O₉ ceramics containing 2.0 mol% B₂O₃ and 5.0 mol% CuO sintered at 900^∘C for 2 h have good microwave dielectric properties of ε_r = 36.3, Q× f = 30,500 GHz and τ_f = 28.1 ppm/^∘C     

Liquid phase effect of La2O3 and V2O5 on microwave dielectric properties of Mg2TiO4 ceramics

Dielectric ceramics of Mg₂TiO₄ (MTO) were prepared by solid-state reaction method with 0.5–1.5 wt.% of La₂O₃ or V₂O₅ as sintering aid. The influences of La₂O₃ and V₂O₅ additives on the densification, microstructure and microwave dielectric properties of MTO ceramics were investigated. It is found that La₂O₃ and V₂O₅ additives lowered the sintering temperature of MTO ceramics to 1300 °C and 1250 °C respectively, whereas the pure MTO exhibits highest density at 1400 °C. The reduction in sintering temperature with these additives was attributed to the liquid phase effect. The average grain sizes of the MTO ceramics added with La₂O₃, and V₂O₅ found to decrease with an increase in wt%. The dielectric constant (ε_r) was not significantly changed, while unloaded Q values were affected with these additives, and the values were in the range of 92,000–157,550 GHz and 98,000–168,000 GHz with the addition of La₂O₃ and V₂O₅, respectively. The dielectric properties are strongly dependent on the densification and the microstructure of the MTO ceramics. The decrease in Q×f _o value at higher concentration of La₂O₃ and V₂O₅ addition was owing to inhomogeneous grain growth and the liquid phase which is segregated at the grain boundary. In comparison with pure MTO ceramics, La₂O₃ and V₂O₅ additives effectively improved the densification and dielectric properties with lowering of sintering temperature. The proposed loss mechanisms suggest that the oxygen vacancies and the average grain sizes are the influencing factors in the dielectric loss of MTO ceramics.     

Low-temperature sintering and microwave dielectric properties of ZnTiO3-based LTCC materials

The low-temperature sintered microwave dielectric ceramics with composition of ZnTiO₃-0.25TiO₂ were prepared by adding a small amount of low-melting compounds CuO-V₂O₅-Bi₂O₃ (CVB). The phase relationship and dielectric properties as a function of sintering temperature and the additional amount were studied. It is demonstrated that the addition of low-melting CVB can suppress the formation of Zn₂TiO₄ at low temperature, but decrease the decomposition temperature of ZnTiO₃. The sintering temperature has a significant effect on the stability of ZnTiO₃ and dielectric properties of sintered ceramics. CVB addition can promote the densification of ceramics through liquid-phase sintering. The dense 2wt% CVB-doped ZnTiO₃-0.25TiO₂ ceramics prepared at 850 °C have excellent dielectric properties of ??=?30, Q×f?=?32,000 GHz, and τ _f ?=?+12 ppm/ °C.     

Structure and dielectric properties of Cu and (K,Na) doped SrBi2Nb2O9 ferroelectric ceramics

Ferroelectric ceramics, SrBi₂Nb₂O₉ (SBN), Sr_0.8Cu_0.2Bi₂Nb₂O₉ (SCBN) and Sr_0.8K_0.1Na_0.1Bi₂Nb₂O₉ (SKNBN) were prepared by a solid state reaction process. X-ray diffraction analysis shows that the alkali and Cu almost diffuse into the SBN lattice to form a solid solution during sintering and some slight secondary phases was detected. The effect of alkali and Cu on dielectric properties of the SBN ceramics was discussed. The dielectric loss factor of (K,Na) doped SBN ceramics degraded considerably to 0.01 and their frequency and temperature stabilities were enhanced. The dielectric constant was enhanced by approximately 60% and the Curie temperature (Tc) was also improved for Cu doped SrBi₂Nb₂O₉ ceramics.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ε _r ), high quality factor (Q × f), and small temperature coefficient of resonant frequency (TCF) are required for the application of chip passive components in wireless communication low temperature co-fired ceramics (LTCC). In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₅Nb₆O₂₈ ceramics were investigated as a function of B₂O₃-CuO content. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250^∘C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ε _r of 37, TCF of −12 ppm/^∘C. The addition of B₂O₃-CuO was revealed to lower the sintering temperature of Ba₃Ti₅Nb₆O₂₈, 900^∘C and to enhance the microwave dielectric properties: Q × f of 32,500 GHz, ε _r of 40, TCF of 9 ppm/^∘C. From the X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Phase transformation and microwave dielectric properties of BiPO4 ceramics

Monazite-type compounds, BiPO₄ polymorphs were prepared by the solid-state reaction method. The phase transformation and microwave dielectric properties of sintered ceramics were investigated using the X-ray powder diffraction (XRD) and a network analyzer, respectively. The low-temperature phase of BiPO₄ has monoclinic structure, and was transformed into the high-temperature phase with a slight distortion of monoclinic when it is heated above 600^∘C. The effect of the transformation on the microwave dielectric properties was examined. It was found that the dielectric properties of each phase were significantly different. In particular, the high-temperature phase sintered at 950^∘C has good microwave dielectric properties; the relative dielectric constant (ε _r ) = 22, the quality factor (Q× f) = 32,500 GHz and the temperature coefficient of resonant frequency (τ _f ) = − 79 ppm/ ^∘C.     

Effect of microwave sintering on structural, morphological and dielectric properties of Ba(FeNb)0.5O3 ceramics

Polycrystalline Ba(FeNb)_0.5O₃/BFN ceramics were sintered conventionally and in a microwave (MW) furnace, respectively. Conventional and microwave sintering temperatures were same with different soaking times. Microwave sintering of BFN ceramics showed enhanced grain growth with improved dielectric properties. Highest dielectric constant (~29,913 at 1 kHz) at room temperature (RT) was observed in BFN ceramics sintered in MW furnace for 30 min. At RT, a non-Debye type of dielectric relaxation was observed in both conventionally and MW sintered BFN ceramics. The observed giant dielectric constant of conventionally and MW sintered BFN ceramics was attributed to intrinsic (space charge polarization) and extrinsic (Maxwell-Wagner type polarization) effects, respectively.     

Improved giant dielectric properties in microwave flash combustion derived and microwave sintered CaCu 3 Ti 4 O 12 ceramics

The nanocrystals of CaCu3Ti4O12 ceramic were prepared by microwave flash combustion technique. The microwave sintering of powders was optimized to 1025 − 1075 °C for 20 min with heating and cooling rate of 50 °C/min. Microstructural evaluation of sintered sample was carried out using SEM. The dielectric properties were measured in the frequency range 10–2 × 106 Hz and the temperature range 30–100 °C. The CCTO sample sintered at 1075 °C had giant dielectric constant 53,300 at 100 Hz. It was observed that dielectric constant was greatly increased on a slight increase in sintering temperature. Modulus and impedance analysis were performed to explore the observed unusual dielectric response. Grain and grain boundary resistance were observed as 8 Ω and 350,000 Ω, respectively. The grain boundary activation energy for electro-conduction was calculated as 0.65 eV by using the characteristic frequencies in cole-cole plots. It was noticed that the thermally activated charge carriers had long-range mobility.     

The effect of sintering atmosphere on V2O5 substituted BiNbO4 microwave ceramics

In multiplayer passive devices, low sintering temperature dielectric materials were needed to co-fire with low melting point inner electrode such as copper or silver, a major problem of base metal electrode (BME) was that the devices must be fired under low oxygen partial pressure atmosphere to protect Cu from oxidation. In this paper, dielectric properties of Bi(V_xNb_1?x)O₄ (x?=?0.001, 0.004, 0.008, 0.016, 0.048) microwave ceramics sintered under air and N₂ atmosphere have been investigated. The densification temperature sintered in different atmosphere decreased from 1010 to 830°C with the amount of V₂O₅ increasing from 0.001 to 0.048. Due to the increasing vacancy defects, the density of ceramics sintered in N₂ was smaller than that sintered in air. The ceramics sintered under N₂ have similar dielectric constant, and its Qf values are higher while x?<?0.016.     

Structural and microwave dielectric properties of Ba5-xLaxTixNb4-xO15 ceramics

The structural and microwave dielectric properties of Ba_5?x La _x Ti _x Nb_4?x O₁₅ (1?≤?x?≤?3) was investigated. The single phase with A₅B₄O₁₅-type cation-deficient hexagonal perovskite structure was obtained over the whole composition range. These ceramics have high dielectric constant up to 56, high quality factors (Q?×?f ) up to 35,000, and low temperature coefficient of resonant frequencies (τ _f ) in the range +69 to ?3 ppm °C^?1. The dielectric constants and τ _f of these ceramics gradually decrease parallel to an increase in B-site bond valence with increasing La and Ti content.     

Microwave dielectric properties of low-firing BiNbO4 ceramics with V2O5 substitution

The effect of V₂O₅ substitution on the sintering behavior and the microwave dielectric properties of BiNbO₄ ceramics were studied. The sintering temperatures of Bi(V _x Nb_1?x )O₄ ceramics decrease from 990 to 810°C with x value increasing from 0.002 to 0.064. The size of grains increased with the sintering temperature increasing and decreased with the substitution amount increasing. The dielectric properties are affected by the microstructures very much. The quality factor Q value is from 2500 to 4000 at about frequency?=?5 GHz and reach to the maximum when x?=?0.032. With the different x value, the Q _f values change between 15000 to 20000 GHz; the τ _f values changes between 0 and +20 ppm/°C between temperature range 25～85°C and decreased with the increasing of x value.     

Sintering and compositional effects on the microwave dielectric characteristics of Mg(Ta1−x Nb x )2O6 ceramics with 0.25 ≦ x ≦ 0.35

1,500 °C−sintered MgTa₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 30.5, Q × f = 56,900 GHz, and τ _f = 28.3 ppm/°C, whereas 1,400 °C-sintered MgNb₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 21.7, Q × f = 89,900 GHz, and τ _f = −68.5 ppm/°C. In order to find the dielectric resonators with τ _f value close to 0 ppm/°C, the effects of sintering condition and composition on the microwave dielectric characteristics of Mg(Ta_1−x Nb _x )₂O₆ ceramics (0.25 ≦ x ≦ 0.35) prepared under sintering temperature of 1,300–1,450 °C are investigated. The results show that as the sintering temperature increases from 1,300 to 1,450 °C, the ɛ _r, Q × f and τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics all increase and saturate at 1,450 °C. On the other hand, as the Nb₂O₅ content decreases, the τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics will shift to near 0 ppm/°C. The optimized sintering conditions and composition to obtain the Mg(Ta_1−x Nb _x )₂O₆ dielectrics with τ _f close to 0 ppm/°C are sintering temperature of 1,450 °C, sintering duration of 4 h, and composition of x = 0.25, which exhibits the microwave dielectric characteristics of ɛ _r = 27.9, Q × f = 33,100 GHz, and τ _f = −0.7 ppm/°C.     

Thermoelectric Ca3Co4O9 ceramics consolidated by Spark Plasma sintering

Hole-doped Ca₃Co₄O₉ (Co349) ceramics were prepared using solid-state reaction. Two processing strategies have been used to produce the thermoelectric oxide ceramics, Conventional and Spark Plasma (SPS) Sintering to control the grains consolidation, texturation and sample densification. Thermoelectric properties were measured and the influence of the processing conditions on the properties was evidenced. SPS favours shorter elaboration times and produces samples with larger thermoelectric properties due to better densification and alignment. The effect of the free deformation and texturation using the SPS technique is discussed. Seebeck coefficient values of 180 μV/K at 873 K are obtained.     

Effect of SnO2 on improvement on the microwave dielectric properties of Ba2Ti9O20

Effect of SnO₂ addition on the crystal structure/microstructure and the related microwave dielectric properties of the Ba₂Ti₉O₂₀ were systematically investigated. Incorporation of SnO₂ markedly stabilized the phase constituent and microstructure for the Ba₂Ti₉O₂₀ such that high quality materials can be obtained in a much wider processing window. The sintered density of the Ba₂Ti₉O₂₀ increased linearly, but the microwave dielectric constant (K) decreased monotonically, with the SnO₂ doping concentration. The quality factor (Qxf) of the materials increased firstly due to the addition of SnO₂, but decreased slightly with further increase in SnO₂ content. The best microwave dielectric properties obtained are K = 38.5 and Qxf = 31,500 GHz, which occurs for the 0.055 mol SnO₂-doped and 1350 °C/4 h sintered samples. These properties are markedly better than those for undoped materials (K = 38.8 and Qxf = 26,500 GHz).     

Microwave dielectric properties of B2O3-added Li2MgTiO4 ceramics for LTCC applications

Li₂MgTiO₄ (LMT) ceramics which are synthesized using a conventional solid-state reaction route. The LMT ceramic sintered at 1250°C for 4 h had good microwave dielectric properties. However, this sintering temperature is too high to meet the requirement of low-temperature co-fired ceramics (LTCC). In this study, the effects of B₂O₃ additives and sintering temperature on the microstructure and microwave dielectric properties of LMT ceramics were investigated. The B₂O₃ additive forms a liquid phase during sintering, which decreases the sintering temperature from 1250°C to 925°C. The LMT ceramic with 8 wt% B₂O₃ sintered at 925°C for 4 h was found to exhibit optimum microwave dielectric properties: dielectric constant 15.16, quality factor 64,164 GHz, and temperature coefficient of resonant frequency -28.07 ppm/°C. Moreover, co-firing of the LMT ceramic with 8 wt% B₂O₃ and 20 wt% Ag powder demonstrated good chemical compatibility. Therefore, the LMT ceramics with 8 wt% B₂O₃ sintered at 925°C for 4 h is suitable for LTCC applications.     

Piezoelectric and ferroelectric properties of new Pb9Ce2Ti12O36 and lead-free Ba2NdTi2Nb3O15 ceramics

In this paper, measurements of the nonlinear ferroelectric, piezoelectric and dielectric properties of Pb₉Ce₂Ti₁₂O₃₆ (Pb₉CTO) and Ba₂NdTi₂Nb₃O₁₅ (BNTN) ferroelectric ceramics are presented. Hysteresis P(E) loops were measured as a function of applied electric field, frequency and temperature. The coercive field (E _c) and remnant polarization (P _r) displayed temperature and frequency dependence. Lead-free BNTN ceramics exhibited a coercive field E _c?>?2.4 kV mm^?1 and a piezoelectric coefficient d ₃₃?=?2 pC N^?1. The hysteresis loop was pinched above 110°C and a linear response was observed at 155°C, typical of a paraelectric material. Pb₉CTO was shown to be ferroelectric with coercive field E _c?=?1.2 kV mm^?1 and a d ₃₃?=?65 pC N^?1. The frequency dependences of the impedance of the Pb₉CTO discs were analyzed.