Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

High‐Q microwave ceramics of Li2TiO3 co‐doped with magnesium and niobium

In order to solve the problems of acceptor/donor individual doping in Li₂TiO₃ system and clarify the superiority mechanism of co‐doping for improving the Q value, Mg + Nb co‐doped Li₂TiO₃ have been designed and sintered at a medium temperature of 1260°C. The effects of each Mg/Nb ion on structure, morphology, grain‐boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg_1/3Nb_2/3)⁴⁺ inhibits not only the diffusion of Li⁺ and reduction in Ti⁴⁺, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q × f value of Li₂TiO₃ ceramics co‐doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li₂TiO₃ ceramics. And the co‐doped ceramics have an appropriate dielectric constant of 19.01 and a near‐zero resonance frequency temperature coefficient of 13.38 ppm/°C. These results offer a scientific basis for co‐doping in Li₂TiO₃ system, and the outstanding performance of (Mg + Nb) co‐doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.     

Colossal permittivity in (In1/2Nb1/2)xTi1−xO2 ceramics prepared by a glycine nitrate process

(In + Nb) co-doped TiO₂ nanoparticles with very low dopant concentrations were prepared using a glycine nitrate process. A pure rutile—TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved. By doping TiO₂ with 1.5% (In + Nb) ions, a very high dielectric permittivity of ε′ = 42,376 and low loss tangents of tanδ = 0.06 (at room temperature) were achieved. The large conduction activation energy at the grain boundary decreased with decreasing dopant concentration. The colossal permittivity was primarily attributed to the internal barrier layer capacitor (IBLC) effect. The dominant effect of interfacial polarization at the non–Ohmic sample–electrode contact was observed when the dopant concentration was ≤1.0 mol%. Interestingly, the sample–electrode contact and resistive–outer surface layer effects, i.e., surface barrier layer capacitor (SBLC) effect, has also an effect on the colossal dielectric response in (In + Nb) co-doped TiO₂ ceramics.     

Colossal permittivity in TiO2 co‐doped by donor Nb and isovalent Zr

Effect of isovalent Zr dopant on the colossal permittivity (CP) properties was investigated in (Zr + Nb) co‐doped rutile TiO₂ ceramics, i.e., Nb_0.5%Zr_xTi_1?xO₂. Compared with those of single Nb‐doped TiO₂, the CP properties of co‐doped samples showed better frequency‐stability with lower dielectric losses. Especially, a CP up to 6.4 × 10⁴ and a relatively low dielectric loss (0.029) of x = 2% sample were obtained at 1 kHz and room temperature. Moreover, both dielectric permittivity and loss were nearly independent of direct current bias, and measuring temperature from room temperature to around 100°C. Based on X‐ray photoelectron spectroscopy, the formation of oxygen vacancies was suppressed due to the incorporation of Zrions. Furthermore, it induced the enhancement of the conduction activation energy according to the impedance spectroscopy. The results will provide a new routine to achieve a low dielectric loss in the CP materials.     

Enhancing giant dielectric properties of Ta5+-doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

Various strategies to improve the dielectric properties of ACu₃Ti₄O₁₂ (A = Sr, Ca, Ba, Cd, and Na_1/2Bi_1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta⁵⁺–doped Na_1/2Y_1/2Cu₃Ti₄O₁₂ (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid-state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta⁵⁺ doping ions owing to the increased Cu⁺/Cu²⁺, Cu³⁺/Cu²⁺, and Ti³⁺/Ti⁴⁺ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta⁵⁺ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.     

Colossal dielectric permittivity in (Al+Nb) co-doped Ba0.4Sr0.6TiO3 ceramics

Stimulated by the outstanding colossal permittivity behavior achieved in trivalent and pentavalent cations co-doped rutile TiO₂ ceramics, the co-doping effects on the dielectric behavior of Ba_0.4Sr_0.6TiO₃ ceramics were further explored. In this work, (Al + Nb) co-doped Ba_0.4Sr_0.6TiO₃ ceramics were synthesized via a standard solid state ceramic route. The structural evolution was analyzed using X-ray diffraction patterns and Raman spectra. Dense microstructures with no apparent change of grain morphology were observed from the scanning electron microscopy. A huge enhancement of dielectric permittivity was obtained with 1 mol% (Al + Nb) doping and excellent dielectric performances (ε_r ∼ 20,000, tanδ ∼ 0.06 at 1 kHz) were achieved after further heat treatment. The formation of electron pinned defect dipoles localized in grains may account for the optimization of dielectric behaviors and the corresponding chemical valence states were confirmed from the XPS results.     

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of SnxTa0.025Ti0.975-xO2 ceramics

The Sn_xTa_0.025Ti_0.975-xO₂ (x%Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn⁴⁺ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO₂ phase were observed as x% increased. The mean grain size was slightly reduced (～17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x%, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x%Sn(TTO) ceramics remained unchanged owing to the same Ta⁵⁺ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x% increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ～0.02 and high ε′～10⁴ with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn⁴⁺–isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO₂ without the creation of complex defect clusters inside the grains.     

Improved Pyroelectric Properties of CaBi4Ti4O15 Ferroelectrics Ceramics by Nb/Mn Co‐Doping for Pyrosensors

The pyroelectric properties of Nb(Mn)‐doped and Nb/Mn co‐doped CaBi₄Ti₄O₁₅ (CBT) bismuth layer‐structured ferroelectric ceramics were investigated. It was found that Nb/Mn co‐doping resulted in stronger enhancement of pyroelectric properties than that of single Nb or Mn doping. The mechanism of doping effect was explained by the distortion of the [BO₆] octahedra induced by the doped Nb and Mn cations occupying the B‐site of the pseudoperovskite structure. A large pyroelectric coefficient of 84.4 μC/m²K was obtained at room temperature for Nb/Mn co‐doped CBT (CBTN‐Mn) ceramics, higher than that of pure, Nb or Mn‐doped counterparts, being on the order of 35.9, 58.2, 44.0 μC/m²K, respectively. The enhanced pyroelectric coefficient together with reduced dielectric constant (99) and dielectric loss (0.002) led to greater improvement of figures of merit (FOMs), including FOMs for voltage responsivity (F_v ~ 3.95 × 10^?2 m²/C) and detectivity (F_d ~ 2.44 × 10^?5 Pa^?1/2), in CBTN‐Mn ceramics. Furthermore, the temperature variations of F_v and F_d were found to be 24% and 68%, respectively, over a broad temperature range from room temperature to 350°C, making CBTN‐Mn ceramics potential candidate for high‐temperature pyroelectric devices.     

Srn+1TinO3n+1 (n=1, 2) microwave dielectric ceramics with medium dielectric constant and ultra‐low dielectric loss

Sr_n+1Ti_nO_3n+1 (n=1, 2) ceramics with tetragonal Ruddlesden–Popper structure were prepared via a standard solid‐state reaction process, and their microstructures and microwave dielectric properties were investigated systematically. The phase composition, grain morphology, and densification behavior were explored using X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Outstanding microwave dielectric properties were achieved in the present ceramics: ε_r=42, Q × f=145 200 GHz, τ_f=130 ppm/°C for Sr₂TiO₄, and ε_r=63, Q × f=84 000 GHz, τ_f=293 ppm/°C for Sr₃Ti₂O₇, respectively. The present ceramics might be expected as excellent candidates for next‐generation medium‐permittivity microwave dielectric ceramics after the further optimization of τ_f value.     

Electromechanical Properties of Acceptor‐Doped Lead‐Free Piezoelectric Ceramics

We have studied the processing and electromechanical properties of Mn and Fe‐doped 0.88[Bi_0.5Na_0.5TiO₃]–0.08[Bi_0.5K_0.5TiO₃]–0.04[Bi_0.5Li_0.5TiO₃] piezoelectric ceramics prepared by the mixed oxide route. Different amounts of Mn (0.01, 0.014, 0.015, 0.016, 0.017, 0.02, 0.022) or Fe (0.0125, 0.015, 0.0175) were doped to this lead‐free piezoelectric composition. Ceramics were sintered at different temperatures (1075°C–1150°C) to achieve the highest density and mechanical quality factor. Mn or Fe doping resulted in a considerable enhancement of Q_m in both planar and thickness resonance modes. In 1.5 mol% Mn‐doped ceramics sintered at 1100°C, a planar Q_m of about 970 and tanδ of 0.88% were obtained. In Fe‐doped ceramics, a planar Q_m as high as 900 was achieved. Acceptor dopants also resulted in decreasing the coupling coefficients, the piezoelectric charge coefficient, and the dielectric constant.     

Giant dielectric behavior of monovalent cation/anion (Li+, F−) co-doped CaCu3Ti4O12 ceramics

Giant dielectric behavior and electrical properties of monovalent cation/anion (Li⁺, F⁻) co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction route were systematically investigated. Substitution of Li⁺ and F⁻ led to a significantly enlarged mean grain size. A reduced loss tangent (tanδ ~0.06) with the retainment of an ultra-high dielectric permittivity (ε′ ~7.7-8.8 × 10⁴) was achieved in the co-doped ceramics, while the breakdown electric field and nonlinear coefficient of CaCu₃Ti₄O₁₂ ceramics were increased by co-doping with (Li⁺, F⁻). The variations in nonlinear electrical properties and giant dielectric response, as well as the dielectric relaxation, were well explained by the Maxwell-Wagner polarization model for an electrically heterogeneous microstructure, in which a Schottky barrier height at the grain boundaries (GBs) was formed. ε′ was closely correlated to the GB capacitance. Significantly decreased tanδ value and enhanced nonlinear properties were related to a significant increase in the GB resistance, which was attributed to the significantly increased potential barrier height and conduction activation energy at the GBs. The semiconducting nature of the grains was also studied using X-ray photoelectron spectroscopy and found to originate from the presence of Cu⁺ and Ti³⁺ ions.     

Dielectric and mechanical properties of CaCu3Ti3.925(Nb0.5Al0.5)0.075O12 & Al reinforced epoxy-composites (0–3) for embedded capacitor applications

CaCu₃Ti_3.925(Nb_0.5Al_0.5)_0.075O₁₂ [CCTNAO] ceramics were synthesized by microwave assisted solid state reaction technique. CCTNAO ceramics possessed room temperature (RT) dielectric constant (ε_r) ～ 24,173 with tanδ ～0.149 at 1 kHz frequency. Commercially available epoxy-resin, hardener, Al-powder along with CCTNAO powder were used to prepare epoxy based 0–3 composites. Maximum ε_r ～33.37 with tanδ ～0.107 at RT were obtained for 40 vol% CCTNAO loading in epoxy. For x = 0.2 in (1-x)[0.8 Epoxy-0.2 CCTNAO]-x Al Epoxy composites, highest ε_r ～77.6 with tanδ ～ 0.15 at 1 kHz frequency were observed. Increase in ε_r with the increase of Al filler content in composites is attributed to interfacial polarization and cluster formations. Different theoretical models were discussed to explain the dielectric properties of synthesized composites. Experimentally measured values of ε_eff were in close agreement with EMT model (n = 0.13) and Yamada Model (η = 7). An empirical proposed power law ε_eff = ε_m(1+x)ⁿ, with n ～ 10 had a considerable agreement with the experimental results. Vickers hardness test study was carried out to ascertain the mechanical properties of the synthesized composites.     

Aluminum‐vacancy‐related dielectric relaxations in AlN ceramics

The dielectric properties of AlN ceramics were investigated comprehensively in the temperature range from room‐temperature to 950 K and frequency range of 10² to 5 × 10⁶ Hz. The sample exhibits intrinsic dielectric behavior when T < 500 K, showing a flat dielectric permittivity about 10 and an extremely low dielectric loss factor (tanδ < 2 × 10^?3). In the temperature above 500 K, two thermally activated dielectric relaxations related to bulk and interfacial effects were observed. Both relaxations strongly depend on the concentration of oxygen atoms. Our results indicate that the bulk relaxation, occurring in lower temperature range, is caused by aluminum vacancy hopping motion inside grains. The interfacial relaxation, occurring in higher temperature range, is caused by surface‐layer effect due to aluminum vacancies being blocked by sample‐electrode contact.     

High Power Performance of Manganese‐Doped BNT‐Based Pb‐Free Piezoelectric Ceramics

Electromechanical properties and high power characteristics of Pb‐free hard piezoelectric ceramics in the (BiNa_0.88K_0.08Li_0.04)_0.5 (Ti_1?xMn_x)O₃ (x = 0, 0.014, 0.015, and 0.016) system were studied. Mn doping resulted in a considerable enhancement of mechanical quality factor Q_m and vibration velocity. The lowest mechanical and dielectric losses were achieved in 1.5 mol% Mn‐doped ceramics with a planar Q_m of about 970 and tanδ of 0.89%. The heat dissipation and resonance frequency shift under high drive condition were remarkably suppressed upon Mn doping. The maximum vibration velocity was increased from 0.28 m/s in undoped ceramic to 0.6 m/s in 1.5 mol% Mn‐doped composition. The results of this study revealed that Mn‐doped BNT‐based piezoelectrics exhibited a superior high power performance compared to their lead‐based counterparts such as PZT4 and PZT8 ceramics.     

Frequency and voltage dependence of dielectric properties,complex electric modulus,and electrical conductivity in Au/7% graphene doped‐PVA/n‐Si (MPS) structures

In order to increase the capacitance of Au/n‐Si (MS) structure, 7% graphene doped PVA was coated on n‐Si as an interfacial layer. The measured data of capacitance (C) and conductance (G/ω) of Au/7% graphene doped‐PVA/n‐Si (MPS) structure was utilized for the calculation of real and imaginary parts of complex permittivity (ε^* = ε′ − jε″), loss tangent (tanδ), complex electric modulus (M^* = M′ + jM″), and electrical conductivity (σ). The admittance measurements (C and G/ω) were carried out in the frequency range of 0.5 kHz to 1 MHz at room temperature. Frequency dependence of the dielectric constant (ε′), dielectric loss (ε″) and tanδ shows a dispersive behavior at low frequencies. This behavior was explained by Maxwell–Wagner relaxation. Due to the dipolar and the interfacial polarizations, as well as the surface states (N_ss) and the interfacial PVA layer, the parameters exhibited a strong dependence on frequency and applied bias voltage. The σ versus log(f) plot exhibited both low and high frequency dispersion phenomena such that at low frequencies σ value corresponding to the dc conductivity (σ_dc), but at high frequencies it corresponds to the ac conductivity (σ_ac). M′ and M″, both, have low values in the low frequency region. However, an increase is observed with the increasing frequency due to the short‐range mobility of charge carriers. As a result, the change in dielectric parameters and electric modulus with frequency is the result of relaxation phenomena and surface states. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43827.     

Structure and Dielectric Properties of Re0.02Sr0.97TiO3 (Re = La,Sm, Gd,Er) Ceramics for High‐Voltage Capacitor Applications

Rare–earth‐doped strontium titanate ceramics yielding the formula Re_0.02Sr_0.97TiO₃ (Re–ST, Re = La, Sm, Gd, Er) were prepared by solid‐state reaction route. All Re–ST ceramics had single cubic perovskite structure similar to pure SrTiO₃ (ST). The grain size of Re–ST ceramics dramatically decreased to 1–10 μm, depending on different rare‐earth elements, as compared to ~30 μm of pure ST. The relative dielectric constant of Re–ST ceramics (ε_r = 2750–4530 at 1 kHz) showed about 10–15 times higher than that of pure ST (ε_r = 300 at 1 kHz), whereas the dielectric loss of Re–ST ceramics still remained lower than 0.03 (at 1 kHz) at room temperature. Under 0–1.63 × 10⁶ V/m bias electric field testing conditions, the ε_r of Re–ST ceramics at room temperature changed within 14%. The P–E results indicated that the Re–ST ceramics were linear dielectrics. Together with their relatively high breakdown strength (E_b > 1.4 × 10⁷ V/m), the Re–ST ceramics could be very promising for high‐voltage capacitor applications. Meanwhile, the temperature stability of the ε_r of Re–ST ceramics was evaluated in a temperature range of ?60°C–200°C.     

Microstructure,colossal permittivity,and humidity sensitivity of (Na,Nb) co-doped rutile TiO2 ceramics

(Na_0.25Nb_0.75)_xTi_1−xO₂ (NNTO) ceramics (x = 0, 0.005, 0.01, 0.02, and 0.05) were prepared by the conventional solid-state reaction. The microstructure, dielectric, and humidity sensitivity of the ceramics were systematically investigated. Results showed that all ceramics exhibit pure rutile TiO₂ phase with dense microstructures. Co-doping of (Na, Nb) can effectively improve the microstructure homogeneity of the ceramics. When the doping level x ≥ 0.01, the co-doped samples show colossal permittivity higher than 10⁴ and dielectric loss tangent lower than 0.38. This dielectric behavior features the merit of both frequency and temperature stability in the range of 10²-10⁶ Hz and 100-300 K, respectively. The co-doped ceramics were found to be sensitive to the environment moisture. The humidity sensitivity incurs a Maxwell-Wagner relaxation near room temperature, which further enhances the dielectric permittivity. Excellent humidity sensitive properties of sensitivity to be 102.6 pF/%RH, response/recovery time to be 115/20 seconds, as well as good repeatability, were achieved in the sample with the doping level x = 0.05. This work underscores that the room temperature dielectric properties of doubly doped TiO₂ system depends strongly on the environmental condition and suggests that the (Na + Nb) co-doped TiO₂ ceramics might be promising humidity sensing materials.     

The dielectric properties for (Nb,In,B) co-doped rutile TiO2 ceramics

Recently, colossal permittivities (~10⁵) and low loss factors (<0.1) were reported in (Nb+In) co-doped rutile TiO₂ ceramics, which have attracted considerable attention. In this work, (Nb,In,B) co-doped rutile TiO₂ ceramics were investigated for achieving temperature- and frequency- stable dielectric properties in TiO₂ based colossal dielectric ceramics. The (Nb,In,B) co-doped rutile TiO₂ ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and complex impedance of 1 mol.% (Nb+In) co-doped rutile TiO₂ (TINO) and xwt% B₂O₃ (x=0.5, 1, 2 and 4) doped TINO were systematically investigated and compared. It was found that by doping B₂O₃ the sintering temperature of TINO ceramics can be reduced by 100 °C. Meanwhile, the dielectric loss of TINO ceramics was decreased by doping B₂O₃. In the ₂wt% B₂O₃ doped TINO ceramics, the dielectric permittivity kept a high value of >2.0×10⁵ and the dielectric loss was lower than 0.1 in a frequency range of 10²−10⁵ Hz and a temperature range of 25–200 °C.     

Large electrocaloric responses in [Bi1/2(Na,K)1/2]TiO3‐based ceramics with giant electro‐strains

The electrocaloric effect (ECE) is investigated through indirect measurement in two lead‐free [Bi_1/2(Na,K)_1/2]TiO₃‐based ceramics that were previously reported to display giant electro‐strains. In the Nb‐doped ceramic, denoted as BNKT‐2.5Nb, a decent temperature change of ΔT=1.85 K and an electrocaloric responsivity of ΔT/ΔE=0.37 (10^?6Km V^?1) are found around room temperature (32°C). While in the Ta‐doped ceramic, BNKT‐1.5Ta, a wide operation temperature range (T_span ~55 K) is observed near room temperature. Additional electrical measurements, as well as transmission electron microscopy experiments, are performed to identify the mechanisms of the ECE in both ceramics.     

Effects of ZnO and B2O3 on the Microstructure and Microwave Dielectric Properties of 5Li2O‐1Nb2O5‐5TiO2 Ceramics

The effects of ZnO and B₂O₃ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O‐1Nb₂O₅‐5TiO₂ (LNT) ceramics have been investigated. With addition of low‐level doping of ZnO and B₂O₃, the sintering temperature of the LNT ceramics can be lowered down to near 920°C due to the liquid phase effect. The Li₂TiO₃ss and the “M‐phase” are the two main phases, whereas other phase could be observed when co‐doping with ZnO and B₂O₃ in the ceramics. And the amount of the other phase increases with the ZnO content increasing. The addition of ZnO does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the good microwave dielectric properties of ε_r = 36.4, Q × f = 8835 GHz, τ_f = 4.4 ppm/°C could be obtained for the 1 wt% B₂O₃ and 4 wt% ZnO co‐doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as internal electrode.