Effect of grain size and Cu-rich phase on the electric properties of CaCu3Ti4O12 ceramics

The CaCu₃Ti₄O₁₂ ceramics were prepared by the traditional solid-state reaction method under different sintering conditions. The XRD patterns show that crystal structures of the samples are basically single-phase pseudo-cubic, except little second phases of CuO and Cu₂O in the samples sintered in air at 1050 and 1100 °C, respectively, for 12 h. The SEM results indicate that the pellet sintered at 1100 °C for 12 h possess larger grain size and more Cu-rich phases at the grain boundaries than the pellet sintered at 1050 °C for 12 h. It is interesting that the pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h shows smaller grain size (~1 μm) and no Cu-rich phases due to the higher pressure during the sintering process. The results show that the grain size has a reverse effect on the values of the permittivity and the values of breakdown electric field (E _b) and nonlinear coefficient. The pellet sintered at 1100 °C for 12 h exhibits a higher permittivity, but with a lower breakdown electric field (E _b) and a lower nonlinear coefficient due to larger grain size. The pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h exhibits a lower permittivity, but with a higher breakdown electric field (E _b) and a higher nonlinear coefficient due to smaller grain size. The Cu-rich phases at grain boundaries can raise the resistance of the grain boundary leading to the lower dielectric loss tangent, which has been supported by the results of impedance spectroscopy analysis.     

Structure and properties of Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are prepared from spray-dried powder by spark plasma sintering (SPS) and by normal sintering. By the application of SPS, ceramics with >96% relative densities could be obtained by sintering at 1,100 °C for 5 min in air atmosphere. The pellet as sintered by SPS at 1,100 °C was black and conductive. Although SPS was carried out in air atmosphere, the samples were deoxidized by heating the carbon die. By post-annealing at 1,000 °C for 12 h in air, the pellet was oxidized and became white and insulating. Grain growth was suppressed in the ceramics prepared by SPS, and the average grain size was 0.52 μm. The starting powder contained 1.90% carbon, mainly as binder, and the SPS-prepared ceramics and ordinary prepared ceramics contained 0.15 and 0.024% carbon, respectively. The BZT ceramics obtained by SPS and the subsequent annealing at 1,000 °C for 12 h exhibited a mild temperature dependence of their dielectric constant. The field-induced displacement of the BZT ceramics was less hysteretic and smaller than that of the ceramics sintered by the conventional method.     

Microwave dielectric properties and its compatibility with silver electrode of LiNb<Subscript>0.6</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript> with B<Subscript>2</Subscript>O<Subscript>3</Subscript> and CuO additions

The influences of B₂O₃ and CuO (BCu, B₂O₃: CuO = 1:1) additions on the sintering behavior and microwave dielectric properties of LiNb_0.6Ti_0.5O₃ (LNT) ceramics were investigated. LNT ceramics were prepared with conventional solid-state method and sintered at temperatures about 1,100 °C. The sintering temperature of LNT ceramics with BCu addition could be effectively reduced to 900 °C due to the liquid phase effects resulting from the additives. The addition of BCu does not induce much degradation in the microwave dielectric properties. Typically, the excellent microwave dielectric properties of ε_r = 66, Q × f = 6,210 GHz, and τ _f  = 25 ppm/^oC were obtained for the 2 wt% BCu-doped sample sintered at 900 °C. Chemical compatibility of silver electrodes and low-fired samples has also been investigated.     

Influence of BaZrO3, MnCO3 additives on dielectric properties and microstructure of Ba(Zn1/3Nb2/3)O3 ceramics and Ba(Zn1/3Nb2/3)O3-Sr(Zn1/3Nb2/3)O3 solid solutions

The effect of BaZrO₃, MnCO₃ additives on the dielectric properties, sintering temperature and microstructure of Ba(Zn_1/3Nb_2/3)O₃ (BZN) and Ba(Zn_1/3Nb_2/3)O₃-Sr(Zn_1/3Nb_2/3)O₃ (BSZN) ceramics was studied in this paper. It indicates that both BaZrO₃ and MnCO₃ can lower the sintering temperature of the ceramics and accelerate the crystallization of BZN and BSZN. The dielectric constant ɛ _r increases after MnCO₃ added, but decreases when BaZrO₃ added alone. The existence of MnCO₃ can modulate the temperature coefficient of capacitance τ _c toward positive, while BaZrO₃, can make c more negative. MnCO₃ and BaZrO₃ can restrain the appearance of the second phase; while BaZrO₃, can prevent the appearance of the superstructure. In the BSZN system, when 1 mass % MnCO₃ added, sintering temperature(t _s ) is lowered to 1240°C. In this study, the best sample that has the excellent properties is sample 5 with dielectric properties of ɛ _r = 43.6, τ _c = −8 × 10⁻⁶ °C⁻¹ and tan δ = 0.6 × 10⁻⁴ (1 MHz). The sintering temperature of BZN and BSZN system can be lowered to less than 1300°C.     

High piezoelectric d33 coefficient of lead-free (Ba0.93Ca0.07)(Ti0.95Zr0.05)O3 ceramics sintered at optimal temperature

Lead-free (Ba_0.93Ca_0.07)(Ti_0.95Zr_0.05)O₃ (BCZT) ceramics were prepared using a solid-state reaction technique. The structure and electrical properties were investigated with a special emphasis on the influence of sintering temperature. Crystalline structures and microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. The BCZT ceramics sintered at 1450 °C show the highest densification and exhibit excellent piezoelectric properties of high piezoelectric coefficient d₃₃ = 387 pC/N, planar mode electromechanical coupling coefficient k_p = 44.2%, mechanical quality factor Q_m = 140 and Curie temperature T_c = 108 °C.     

Electrical behavior of Gd-doped CeO2 electrolyte ceramics sintered from nanopowders prepared by mechanical alloying process

The densification behavior of nanocrystalline Gd-doped ceria electrolyte, synthesized via mechanical alloying process, was investigated by means of the conventional pressure less sintering and the two-step sintering methods. The effect of the heating rate and the amounts of dopant on the sinterability of Ce_1−x Gd _x O_(2−δ) x = 0.2 (2GDC) and x = 0.3 (3GDC) oxides was studied, which indicated that the gadolinium retards densification and grain growth in the final state of the conventional sintering and 2GDC samples reach 94% density at 1,550 °C. Subsequent investigation on the grain growth in the fully densified ceramics showed that lowering of the heating rate and increasing of the soaking time reduce the effect of dopant and cause samples to be densified to the higher theoretical density (97%) at lower temperatures (1,400 °C). Fully dense Gd-doped ceria ceramics with finest grain size (900–1,100 nm) can be obtained by two-step sintering method. Electrical conductivity measurement in the GDC samples was studied by impedance spectroscopy. The grain boundary conductivity in these specimens obtained by two-step sintering method was compared with normal sintered specimens. It is concluded that the reduced conductivity observed in the two-step sintering specimen is attributable to the microstructure changes obtained by increased of grain boundary resistivity.     

Influence of sintering temperature on microstructure and electric properties of CuO doped alkaline niobate-based lead-free ceramics

In this work, microstructure characteristics, dielectric, piezoelectric and ferroelectric properties of lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.87Ta_0.06Sb_0.07)O₃ (KNLNST) doped with 1?mol% copper oxide (CuO) piezoelectric ceramics prepared by a conventional solid-state reaction route are investigated with an emphasis on the influence of sintering temperature. The introduction of CuO could significantly improve the sinterability of KNLNST ceramics. It is found that the tetragonality of the ceramics increases with raising sintering temperature. A dense microstructure with increased grains is developed, probably due to liquid-phase sintering. Both the piezoelectric constant d ₃₃ and planar electromechanical coupling k _p increase with increasing relative density and grain size. The Curie temperature T _C values increase slightly when the sintering temperature is increased. In addition, the KNLNST ceramics doped with 1?mol% CuO show obvious dielectric relaxor characteristics, and the relaxor behavior of ceramics is strengthened by increasing the sintering temperature. The improved piezoelectric and dielectric properties of d ₃₃?=?241?pC/N, k _p?=?0.437, dielectric loss tanδ?=?0.0087, mechanical quality factor Q _m?=?138, dielectric constant ε _r?=?1,304 can be obtained for specimens sintered at 1,080?°C for 3?h.     

Effect of CuO on the sintering and cryogenic microwave characteristics of (Zr0.8Sn0.2)TiO4 ceramics

The effect of CuO on the sintering temperature, microstructure and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) modified with 1 wt% of ZnO has been investigated. Microwave dielectric properties of ZST ceramics are measured from cryogenic to room temperatures (15–290 K). Crystallite sizes of sintered ZST ceramics as derived from XRD are in the 30–50 nm range. The addition of CuO effectively reduced the sintering temperature to 1300 °C, possibly due to liquid-phase effects. Addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. The dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) of ZST ceramics do not significantly vary with temperature, whereas the unloaded quality factor (Q_u) changes noticeably. It is found that the Q_u factor of the sample without CuO decreased with increase in temperature, whereas the samples with addition of CuO up to 1.0 wt% showed less dependence on temperature. The Q_u factor of CuO-free ZST is 15,000 and that of ZST with 0.5 wt% of CuO is 11,800 at 15 K. The Q_u factor while measured at room temperature ranged between 2900 and 7000. Efforts were made to understand whether the increase in Q_u factor at both cryogenic and room temperatures is the result of intrinsic or extrinsic factors.     

Effects of sintering temperature on dielectric and piezoelectric properties of KNN-LS-BF-0.4mol%CuO lead-free piezoelectric ceramics

KNN-LS-BF-0.4mol%CuO piezoelectric ceramics were prepared by the traditional sintering method. The effects of sintering temperature on the dielectric and piezoelectric properties of KNN-LS-BF-0.4mol%CuO ceramics were studied. The results reveal that the sintering temperature has significant influence on the microstructure and the properties of KNN-LS-BF-0.4mol%CuO ceramics. With the increase of sintering temperature from 1,040 to 1,080 °C, the grains become more homogeneous and more tight-arrangement, resulting in the higher relative density as well as the best dielectric and piezoelectric properties. However, the properties of the samples would be deteriorated as they are sintered over 1,080 °C.     

Tantalum influence on electrical properties of lead-free (K0.4425Na0.52Li0.0375)(Nb0.93?xTaxSb0.07) O3 piezoelectric ceramics

Lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−x Sb_0.07Ta _x )O₃ (abbreviated as KNLNST _x ) piezoelectric ceramics with x = 0.045–0.075 have been prepared by an ordinary sintering technique with sintering temperature at 1,120 °C. The results of X-ray diffraction reveal that Ta⁵⁺ has diffused into the perovskite lattice to form a solid solution. The grain growth of the ceramics was inhibited by substituting Ta⁵⁺ for Nb⁵⁺. It has been shown that the substitution of Ta decreases Curie temperature T _C and orthorhombic-tetragonal phase transition temperature T _O-T. Besides, the dependence of the ceramics with different Ta content on the dielectric, piezoelectric and ferroelectric properties has been investigated. The results indicate that Ta substitution provides “soft” piezoelectric characteristics, owing to improvements in d ₃₃, k _p and ε _r and a decease in Q _m. For the ceramics with x = 0.06, the piezoelectric coefficient d ₃₃ becomes maximum at a value of 270 pC/N, while the other electrical properties remain reasonably high: dielectric constant ε _r = 1,577, planar mode electromechanical coupling factors k _p = 0.4, Curie temperature T _C = 252 °C and the remanent polarization P _r = 16.03 μC/cm². These results show that (K, Na, Li) (Nb, Sb)O₃ ceramics with small amount of Ta (x <8 mol%) are good lead-free piezoelectric ceramic.     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

Influence of Ca0.8Sr0.2TiO3 on the microstructures and microwave dielectric properties of Nd(Mg0.4Zn0.1Sn0.5)O3 ceramics

The influence of Ca_0.8Sr_0.2TiO₃ on the microstructures and microwave dielectric properties of Nd(Mg_0.4Zn_0.1Sn_0.5)O₃ ceramics were investigated by the conventional solid-state method. The X-ray diffraction peaks of (1 − x)Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–xCa_0.8Sr_0.2TiO₃ ceramic system shifted to higher angles as x increased. The dielectric constant increased from 31.8 to 47.7, the quality factor (Q × f) decreased from 54,200 to 42,800 GHz, and the temperature coefficient of resonant frequency (τ _f ) increased from −43 to +41 ppm/°C as x increased from 0.5 to 0.7 when (1 − x)Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–xCa_0.8Sr_0.2TiO₃ ceramic system sintered at 1,600 °C for 4 h.     

Crystallite structure,microstructure, dielectric,and piezoelectric properties of (Pb1.06−xBax)(Nb0.94Ti0.06)2O6 piezoelectric ceramics prepared using calcined powders with different phases

In an attempt to obtain dense lead metaniobate-based ceramics with improved dielectric and piezoelectric properties, the (Pb_1.06−xBa_x)(Nb_0.94Ti_0.06)₂O₆ (x = 0, 0.04, 0.08, 0.12) piezoelectric ceramics were prepared separately from the two kinds of calcined powders, i.e., the powders with the rhombohedral phase and orthorhombic phase. For obtaining the calcined powders with the different phases, two different calcination temperatures of 900 °C and 1250 °C were chosen. The calcined powders were characterized using X-ray diffraction, scanning electron microscope, laser particle size analyzer and differential scanning calorimetry. Effects of the phase structures of the calcined powders on crystallite structure, microstructure, dielectric and piezoelectric properties of the ceramics were studied in detail. The lattice parameters and grain size of the ceramics are related to the phase structures of the calcined powders. The doping of Ba²⁺ has an influence on the dielectric and piezoelectric properties of the ceramics. The ceramics with x = 0.08 fabricated from the calcined powders with the orthorhombic phase demonstrate the optimum dielectric and piezoelectric properties.     

Microwave and photoluminesent characterizations of (Ca<Subscript>2</Subscript>Mg<Subscript>3</Subscript>)(X<Subscript>1.75</Subscript>Sb<Subscript>0.25</Subscript>)TiO<Subscript>12</Subscript> [X = Nb and Ta] ceramics

(Ca₂Mg₃)(X_1.75Sb_0.25)TiO₁₂ [X = Nb and Ta] ceramics are prepared through the conventional solid-state route. The samples are calcined at 1,100 and 1,180 °C, and are sintered at 1,250 and 1,375 °C. The substitution of Sb decreases the calcination and sintering temperatures of pure (Ca₂Mg₃)(Nb/Ta)₂TiO₁₂. The structure of the samples is analyzed using X-ray diffraction method. The microstructure of the sintered pellet is studied using scanning electron microscopy. The dielectric properties such as dielectric constant (ε_r), quality factor (Q_uxf) and temperature coefficient of resonant frequency (τ_f) are measured in the microwave frequency region. By Sb substitution, thermal stability is achieved, with the increase in dielectric constant, without much change in the quality factor. The materials have intense emission lines in the wavelength region 500–700 nm. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Ferroelectric, dielectric and piezoelectric properties of Pb1?xCex(Zr0.60Ti0.40)O3, 0?≤?x?≤?0.08

The ferroelectric, dielectric and piezoelectric properties of compositions Pb_1−x Ce _x (Zr_0.60Ti_0.40)O₃, (x = 0.0, 0.01, 0.02, 0.04, 0.06 and 0.08) are studied. The above compositions are prepared from their constituent oxides, calcined at 900 °C for 4 h and various phases present are characterized by X-ray diffraction (XRD) technique. The above powders are uniaxially pressed into circular compacts, sintered at 1,250 °C for 2 h, electroded, poled at 2 kV/mm D.C. voltage and their electrical properties are measured. The XRD analysis shows the presence of rhombohedral phase up to 2 mol% ceria while tetragonal phase found at higher concentrations. It is observed that the ferroelectric, dielectric and piezoelectric properties increase with the addition of ceria with a maximum at 2 mol% and then decreases. The higher piezo properties associated with low ceria concentration are attributed to rhombohedral phase.     

Microwave dielectric properties of BaNb(2?x)TaxP2O11 (x?=?0, 0.5, 1, 1.5 and 2) ceramics

BaNb_(2−x)Ta_xP₂O₁₁ (x = 0, 0.5, 1, 1.5 and 2) ceramics were prepared by the conventional solid state ceramic route. The relative permittivity (ε_r) of the ceramics decreased from 25.3 for x = 0 to 12.9 for x = 2. BaTa₂P₂O₁₁ (x = 2) sintered at 1250°C showed good microwave dielectric properties with Q_u × f = 28,900 GHz and τ_f = − 29 ppm/°C. The addition of 5 wt% TiO₂ lowered the sintering temperature to 1225°C and improved τ_f to −6 ppm/°C with ε_r = 13.4 and Q_u × f = 17,200 GHz.     

Low-temperature sintering of (Zn0.8 Mg0.2)2SiO4-TiO2 ceramics

Effect of Li₂O-B₂O₃-SiO₂ (LBS) glass on the sintering behavior and the microwave dielectric properties of (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ (ZMST) ceramics were investigated. The Li₂O-B₂O₃-SiO₂ glass lowered the sintering temperature of ZMST ceramics effectively from 1250 to 870 °C. The unknown second phase, which was formed in the ZMST ceramics increased with the addition of LBS glass. With increasing the LBS glass content, the bulk density, dielectric constant (ε_r) and the maximum Q × f value decreased, and the temperature coefficient of resonant frequency (τ_f) shifted to a negative value. (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ ceramics with 3 wt.% Li₂O-B₂O₃-SiO₂ glass sintered at 870 °C for 2 h shows excellent dielectric properties: ε_r = 8.48, Q × f = 11500 GHz, and τ_f = 0 ppm/°C.     

Comparison of sintering behavior and piezoelectric properties of (K,Na)NbO3-based ceramics sintered in conventional and microwave furnace

In this study (1 − x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ − xSrTiO₃ (0.0 ≤ x ≤ 0.10) ceramics were fabricated by sintering in microwave furnace for first time as well as in conventional furnace (either via single step or two-step procedures). Sintering behavior and piezoelectric properties of sintered samples were studied and compared. It was found that two-step sintering decreases sintering temperature effectively and enhances densification compared to single step sintering. Microstructure analysis revealed that, two-step sintering suppresses grain growth and promotes densification. On the other hand, microwave sintering enhanced densification more effectively and reduced sintering time and temperature. The maximum piezoelectric constants of ceramics were measured for those sintered in microwave furnace. Piezoelectric constant of the sample containing 1 mol% SrTiO₃ which was sintered in microwave furnace was measured 310 pC N⁻¹ while by sintering in conventional furnace via single and two-step sintering it was obtained 208 and 278 pC N⁻¹, respectively.     

Annealing temperature dependence of effective piezoelectric coefficients for Bi3.15Eu0.85Ti3O12 thin films

The effects of annealing temperatures 600, 650, 700, and 750 °C on microstructure, chemical composition, leakage current, ferroelectric, dielectric, and piezoelectric properties of Bi_3.15Eu_0.85Ti₃O₁₂ (BET) thin films prepared by metal–organic decomposition were studied in detail. The largest spontaneous polarization 2P _s (98.7 μC/cm² under 300 kV/cm), remnant polarization 2P _r (81.7 μC/cm² under 300 kV/cm), dielectric constant ε_r (889.4 at 100 kHz), effective piezoelectric coefficient d ₃₃ (46.7 pm/V under 260 kV/cm), and lowest leakage current (1.3 × 10⁻⁶ A/cm² under 125 kV/cm) of BET thin film were obtained with annealing at 700 °C. The mechanisms concerning the dependence of the enhancement d ₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make BET thin film a promising candidate for piezoelectric thin film devices.