Varistor behavior of Mn doped ZnO ceramics prepared from nanosized precursors

Polycrystalline ZnO doped with MnO, from 2 to 15?mol%, was prepared from nanosized precursors. The effect of Mn doping and sintering temperature on phase evolution, microstructure and V-I characteristics were investigated. SEM images showed that the great merit of using nanoparticles is that the samples with high microstructural uniformity and lower grain size can be achieved. Varistor behavior was observed in all specimens, even in the undoped ceramics due to the oxidation process of zinc interstitial defects at grain boundaries. The electric field versus current density (E-J) curves indicated that the breakdown field E_b increased and the nonlinear coefficient ?? decreased with the increase in doping level. 2?mol% Mn doped ceramic sintered at 1100?°C exhibited the highest nonlinear coefficient, ???=?40. The stability test under DC stress was performed for the undoped ZnO ceramics. ZnO varistor sintered at 1300?°C showed not only high nonlinearity, but also high stability under DC stress.     

Microstructure and densification mechanism of low temperature sintering Bi-Substituted yttrium iron garnet

The low temperature sintering and densification mechanism of Bi-substituted yttrium iron garnet (Bi:YIG) polycrystalline samples were studied in this paper. The Bi:YIG polycrystalline samples, with the composition of Y_3?x Bi _x Fe₅O₁₂ (x?=?0–1.2), were prepared by the solid-state reaction method. The X-ray diffraction (XRD) patterns show that Bi-substitution can lower the formation temperature of garnet phase from about 1200 to 900 °C and the thermo-mechanical analysis (TMA) indicates that the sintering temperature of ceramics can be decreased from over 1350 °C to below 1000 °C. The microstructure of grains and grain boundaries was observed by high resolution electron microscopy (HREM). The bismuth distribution in grains and grain boundaries was performed by X-ray energy dispersive spectroscopy (EDS). The occurrence of liquid phase with Bi-contained oxide in the sintering process caused the decrease in sintering temperature.     

Analysis on the temperature dependent structural properties of Li2CO3 doped (Ba,Sr)TiO3 ceramics

We have fabricated various amount of Li₂CO₃ doped (Ba,Sr)TiO₃ (BST) ceramics for LTCCs (Low Temperature Co-fired Ceramics) applications through the conventional sintering method. By introducing Li₂CO₃ into BST ceramics, the sintering temperature was decreased from 1350°C to 900°C. In this study, we discussed the crystalline and structural properties of Li₂CO₃ doped BST ceramics. By scanning X-ray diffraction analysis, we found that 1, 3, and 5 wt% Li₂CO₃ doped (Ba,Sr)TiO₃ ceramics have perovskite structure without any pyrochlore phases. Frequency dependent dielectric properties were analyzed and discussed. Scanning Electron Microscopy (SEM) images depending on the sintering temperature and dopants were prepared and discussed. The crystalline and dielectric properties of Li₂CO₃ doped (Ba,Sr)TiO₃ were discussed.     

Effects of doping Y2O3 on the microstructure and electrical properties of ZnO-Bi2O3−based varistor ceramics

ZnO-based varistor ceramics doped with different amount of Y₂O₃ have been made by two-step solid-state reaction route including the pre-calcination and subsequent sintering procedures, using nanosized ZnO powder and corresponding additives as the raw material. The phase composition, microstructure and electrical properties were studied by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and direct current electrical measurement. It was found that the electrical properties of the varistor ceramics sintered at 950 °C from the powder pre-calcined at 800 °C were enhanced by doped appropriate amount of Y₂O_3. Particularly, ZnO varistors doped with 1.2 mol% Y₂O₃ possessed the best comprehensive electrical properties with the breakdown field of 2113 V/mm, the nonlinear coefficient of 184.6 and the leakage current of 0.4 μA. Y₂O₃ phase, Y-rich phase and the other secondary phase particles were confirmed to distribute along the grain boundaries of predominant ZnO grains from XRD and SEM analyses. The results illustrated that doping Y₂O₃ should be a promising route to obtain varistor ceramics with excellent electrical properties.

     

Magnetization and Electric Properties of Pr-doped ZnO

Magnetic susceptibility of ZnO-varistors doped with Pr-ion was measured to elucidate the valence state of the Pr-ion and the effect of thermal treatment. Magnetic susceptibility _χ(T) of the Pr-ion obeyed Curie-Weiss law, from which its valence state was estimated. The valence state of the Pr-ion varied with annealing condition of specimens, and it was indicated that the Pr-ion in ZnO ceramics was more reducible in comparison with pure \hboxPrO_x ceramics. Nonlinear current-voltage characteristics was diminished with the reduction of the valence state of Pr ions. According to Auger electron spectra, the Pr-ions segregated at grain boundaries in ZnO ceramics.     

Dielectric and sintering properties of NaNbO3 ceramic prepared by Pechini method

NaNbO₃ powders and ceramics were prepared by Pechini method. The pure phase NaNbO₃ was obtained at temperature as low as 350 °C, and then they obtained fine powders were used to prepared ceramics through conventional sintering process. The effect of sintering temperature on microstructure evolution and dielectric properties of NaNbO₃ ceramics has been determined. Results shown that the microstructure of NaNbO₃ ceramics consisted of stacked plates, which was related the liquid phase. It was important to note that the stacked plate configuration forming the grain has not been emphasized before, and these grains support adequate wetting characteristics for the liquid phase in order to achieve fully dense microstructure. Above 1195 °C, a number of angular grains with flat interfaces started to appear, and joined to each other with the sintering temperature increase. The effect of grains morphology on dielectric properties of the NaNbO₃ ceramics was also detected.     

Effects of ZnO–xV2O5 substitution on the microstructure and microwave dielectric properties of ZnNb2O6 ceramics

The ZnO–xV₂O₅ substituted ZnNb₂O₆ ceramics with chemical formula Zn(Nb_0.9V _x )₂O_5.5+5x (0?<?x?≤?0.10) were prepared by solid-state reaction route. The densities, microstructures and microwave dielectric properties were investigated according to the different substitution amount of V₂O₅ and sintering temperature. A small amount of substitution of ZnO–xV₂O₅ was effective to lower sintering temperature of ZnNb₂O₆ ceramics from 1,150 °C to 900 °C. The V₂O₅ substitution led to growth of rod-like grains with the help of liquid phase formed from ZnO and V₂O₅. The dielectric properties depended largely on the amount of V₂O₅ substitution and sintering temperature. The dense ceramics with x?=?0.05 were obtained at 950 °C, which had excellent dielectric properties: ?_r?=?24, Q?×?f?=?72,800 GHz and τ_f?=??63.5 ppm/°C. The interface analysis for cofired multilayer composites composed of the present LTCC and metal Ag demonstrated good co-firing chemical compatibility at co-sintering temperature.     

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.     

Growth characteristics and film properties of gallium doped zinc oxide prepared by atomic layer deposition

We carried out comprehensive studies on structural, optical, and electrical properties of gallium-doped zinc oxide (Ga:ZnO) films deposited by atomic layer deposition (ALD). The gallium(III) isopropoxide (GTIP) was used as a Ga precursor, which showed pure Ga₂O₃ thin film with high growth rate. Using this precursor, conductive Ga doped ZnO thin film can be successfully deposited. The electrical, structural and optical properties were systematically investigated as functions of the Ga doping contents and deposition temperature. The best carrier concentration and transmittance (7.2?×?10²⁰ cm^?3 and 83.5 %) with low resistivity (≈3.5?×?10^?3?Ωcm) were observed at 5 at.% Ga doping concentration deposited at 250 °C. Also, low correlation of deposition temperature with the carrier concentration and film structure was observed. This can be explained by the almost same atomic radius of Ga and Zn atom.     

Effect of heat treatment on electrical properties of zinc oxides

The electrical properties of ZnO ceramics were studied from their voltage-current characteristics, thermally stimulated current (TSC), and heat-treatment effects. ZnO ceramic samples with a few mol% of insulating oxide components were prepared. After sintering at 1200°C in air, some of the sample were heat treated at various temperatures between 450 and 900°C in air. On the voltage-current curves, the current varies as a function of applied dc voltage according to the relation I = CV^α, where α is the nonlinear exponent. The nonlinear exponent α decreased with the heat-treatment temperature in the low-temperature region, became the lowest around 600–750°C, and almost recovered at 900°C. Three TSC peaks (designated by P₁, P₂ and P₃ from the low-temperature side) were observed in the nonheat-treated samples. The samples heat treated at 600 and 750°C showed only the P₁ and P₂ peaks; the P₃ peak increased in magnitude with the increase of biasing temperature t_b, and it shifted toward the high-temperature side. These results suggest that the electrical properties of ZnO ceramics will be associated with the phase transition of the Bi₂O₃- rich intergranular layers by heat treatment.     

The Effect of Sintering Processes on the Properties of Mn-F Doped PZT Ceramics

Fluoridated PZT ceramics were produced by solid-state and liquid-phase sintering methods, according to the formula Pb(Zr_0.52Ti_0.48)_1-xMn_xO_3-yF_y, where 0 < x < 0.015 and 0 < y < 0.1. The effects of sintering processes on the phase development and microstructure of Mn-F doped PZT ceramics have been investigated using XRD and FEGSEM. In solid-state sintering, the fluoride additive enhanced the densification of PZT ceramics, enabling densification to >95% relative density at a temperature as low as 1000°C. However, fluoride loss at high temperatures was found to be a significant problem. Alternatively, ceramics with a density >92% were prepared by sintering at a temperature of 850°C by incorporating a eutectic mixture of PbO and V₂O₅ as sintering aid. Problems associated with volatilization of fluoride compounds during sintering could be alleviated using this approach. EPMA was employed to analyze the distribution of the additives in the calcined powders and sintered ceramics. The nonlinear dielectric properties were determined by measuring P-E loops, using an AC electric field in the range 0.1 to 2.0 kV mm^?1.     

Effect of sintering temperature on piezoelectric and dielectric properties of 0.98(Na0.5K0.5)NbO3-0.02Li(Sb0.17Ta0.83)O3+0.01wt%ZnO ceramics

We studied sintering temperature to enhance the piezoelectric and dielectric properties of 0.98(Na_0.5?K_0.5)NbO₃-0.02Li(Sb_0.17Ta_0.83)O₃?+?0.01wt%ZnO (hereafter NKN-LST+ZnO) lead free piezoelectric ceramics. The synthesis and sintering method were the conventional ceramic technique and sintering was executed at 1080?～?1120°C. We found that optimal sintering temperature and NKN-LST+ZnO ceramics showed the highest piezoelectric properties and dielectric properties at the optimal sintering temperature. The NKN-LST+ZnO ceramics sintered at 1090°C show a superior performance with piezoelectric constant d ₃₃?=?185 pC/N, k _p?=?0.36, ε ₃₃ ^T /ε₀?=?491 respectively. These results reveal that NKN-LST+ZnO ceramics are promising candidate materials for lead-free piezoelectric application.     

Microwave dielectric properties of Na<Superscript>+</Superscript> and M<Superscript>3+</Superscript>-doped Ba(Mg<Subscript>0.5</Subscript>W<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics

In this study, phase evolution, microstructure, and microwave dielectric properties of (Ba_0.98Na_0.02)(Mg_0.48M³⁺_0.02W_0.5)O₃ (M³⁺?=?Al, Ga, Sc, In, Yb, Y, Dy, Gd, and Sm) ceramics sintered at 1700 °C for 1 h were investigated. All the compounds exhibited an ordered cubic perovskite structure. Regardless of the ionic radius of the doped M³⁺ ions, BaWO₄ was detected as the secondary phase in all the compounds. The field emission scanning electron microscopy (FE-SEM) images revealed a dense microstructure in all the compounds, except in the Al-doped compound, which exhibited an insufficient grain growth. The large and irregularly shaped grains indicated that the liquid phase sintering occurred. Splitting of the A_1g(O) mode was observed in the Raman spectra of large M³⁺ ion-doped compounds. Splitting of the F_2g modes did not occur and the bands were sharp, indicating that the cubic symmetry was retained. As the ionic radius of the doped M³⁺ ions increased, the dielectric constant (ε_r) increased slightly. The compounds doped with M³⁺?=?Sc, In, Yb, and Y exhibited a very high quality factor (Q?×?f₀) in the range of 250,000 ~ 280,000 GHz. In the case of the compounds doped with M³⁺?=?Al, Ga, Sc, In, Yb, Y, and Dy, the value of the temperature coefficient of resonant frequency (τ_f) was in the range of ?24 ~ ?19 ppm/°C, while the Gd and Sm-doped compounds exhibited positive values of 2.8 and 31.2 ppm/°C, respectively. The dielectric constant, quality factor, and temperature coefficient of resonant frequency of the In-doped compound, i.e., (Ba_0.98Na_0.02)(Mg_0.48In_0.02W_0.5)O₃, were 18.7, 286,557 GHz, and???24.4 ppm/°C, respectively.     

Effect of the firing temperatures on the phase formation,microstructure and electrical properties of BaTi0.91 Sn0.09O3 ceramics synthesized via the solid state combustion method

Abstract

BaTi_0.91Sn_0.09O₃ (BTS) ceramics were synthesized by the solid state combustion method with glycine as the reducting agent and metal nitrates as the oxidants. The calcination and sintering temperatures were in the range of 1200?°C–1300?°C and 1300?°C–1450?°C, respectively, for 2?h. Pure perovskite powders were obtained from the samples calcined at ≥1275?°C for 2?h. The crystal size calculated by Scherrer equation was in the range of 22–30?nm. XRD results of the sintered ceramics showed the coexistence of the tetragonal (T) and orthorhombic (O) phases in all samples, and were confirmed by Rietveld refinement. The grain sizes increased from 9.04 to 29.83 µm when the sintering temperatures increased from 1300?°C–1450?°C for 2?h. The highest density (5.95?g?cm^?3), large piezoelectric strain (d₃₃* = 830?pm/V) and best dielectric constant (ε_r = 14841) were obtained from the sample sintered at 1350?°C for 2?h. This study clearly demonstrates the potential of the solid state combustion method for producing high density and good dielectric properties in BTS ceramics.     

Piezoelectric properties in textured ceramics of bismuth layer-structured ferroelectrics

Piezoelectric properties and their temperature stability were studied for the <001> textured ceramics of bismuth layer structured ferroelectrics SrBi₂Nb₂O₉ (SBN) type compound. The <001> textured ceramics of 10% Nd substituted SBN were obtained by the templated grain growth (TGG) method, and the orientation degree reached 96%. Electro-mechanical coupling coefficient of thickness shear vibration for the 96%-textured specimen was three times larger than that for the randomly oriented specimen. In addition, the resonance frequency increased with increasing temperature for the 96%-textured specimen, while it decreased for the randomly oriented specimen. Therefore, textured ceramics with various orientation degrees were prepared. The gradient of the temperature variation in the resonance frequency changed from negative to positive with increasing grain orientation, and an excellent temperature coefficient of resonance frequency (TCF) of ?0.4 ppm/°C was obtained for the specimen with 76% orientation. Moreover, the specimens with the major faces formed at various angles to <001> oriented direction were fabricated from a 96%-textured ceramics. The gradient of temperature variation in resonance frequency changed from negative to positive with decreasing angle. the minimum value of TCF was ?2.6 ppm/°C for the specimen with a 30° angle. Optimization of the orientation degree or the major face angle was effective to improve TCF for thickness shear vibration of SBN type ceramics.     

Electrical and mechanical properties of MgO added 0.5Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript>–0.5(Ba<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>)TiO<Subscript>3</Subscript> (BZT–0.5BCT) composite ceramics

MgO (0–2.0 vol%) added Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT) ceramics have been prepared by the conventional solid-state reaction method. The effects of MgO powder on the phase formation, densification, dielectric, piezoelectric and mechanical properties (flexural strength, hardness) of the BZT-0.5BCT ceramics have been studied systematically. The synthesized powder could be densified to 97 % of true density at a temperature of 1350 °C. The MgO addition also provided materials with better mechanical properties. The most interesting aspect of MgO added samples is their relative permittivity vs. temperature response. MgO additions effectively suppress the relative permittivity around phase transition temperature. The aging rate of d₃₃ observed for BZT-0.5BCT is 14 %/decade. MgO addition reduces the ageing rate and for 1 vol% MgO added, BZT-0.5BCT shows aging rate of 3 %/decade. BZT-0.5BCT/MgO ceramics possesses good mechanical properties viz., flexural strength 93 MPa, which is almost 25 % higher than that of monolithic BZT-0.5BCT (73 MPa).     

The structure and dielectric properties of low temperature sintering barium titanate based x7r ceramics

In this paper we try to do systematic investigation on the structure and dielectric properties of low temperature sintered barium titanate based X7R ceramics, which are doped with rare earth oxide, Nb₂O₅, Co₃O₄ using ZnO–B₂O₃ as the sintering aids. The dielectric ceramic powder can be sintered at the temperature below 950°C to satisfy X7R requirement and also with good permittivity and low dielectric loss. The research strongly suggest that this doped BaTiO₃ based ceramic, which can be sintered at a rather low temperature, may be used in the production of MLCC with low content of Pd in the Ag–Pd electrode, and for sure, the production cost would be greatly reduced.     

Effect of reoxidation on positive temperature coefficient of resistance behavior for BaTiO3-K0.5Bi0.5TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, (1-x)BaTiO₃-xK_0.5Bi_0.5TiO₃ (BT-KBT, 0.05≦ x ≦0.15) ceramics without any donor doping were prepared by a conventional oxide mixing method. All samples were sintered in an Ar atmosphere at 1280?~?1350°C, subsequently, reoxidized at 800?~?1100°C in a gas mixture (99 %Ar–1 %O₂). The PTCR behavior of BT-KBT ceramics were investigated in terms of KBT content, reoxidation temperature and time. The results showed that the BT-KBT ceramics exhibited an abrupt increase in their resistivity near the Curie temperature (Tc) after annealing in gas mixture, Tc of 0.9BT-0.1KBT ceramic was shifted to a higher temperature (~150°C). Furthermore, the room-temperature resistivity (ρ_RT) of ceramic samples sintered in Ar and reoxidized in a gas mixture decreased to 10² Ω·cm. The jump in resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) was enhanced by three orders of magnitude through a suitable reduction–reoxidation method without sacrificing the ρ_RT.     

Effect of CuO addition on sintering temperature and piezoelectric properties of 0.05Pb(Al0.5Nb0.5)O3−0.95Pb(Zr0.52Ti0.48)O3+0.7 wt.% Nb2O5 + 0.5 wt.% MnO2 ceramics

Effect of CuO addition on piezoelectric properties of 0.05Pb(Al_0.5Nb_0.5)O₃?0.95Pb(Zr_0.52Ti_0.48)O₃+0.7 wt.% Nb₂O₅ + 0.5 wt.% MnO₂ (PAN-PZT) ceramics was studied to decrease the sintering temperature below 900°C for LTCC. The PAN-PZT ceramics sintered at 1200°C had piezoelectric properties of d ₃₃ = 340 pC/N, k _p = 61.6%, Q _m = 1,725, and density of 7.5 g/cm³. The addition of CuO significantly decreased the sintering temperature due to the formation of liquid phase containing a binary combination of PbO and CuO in grain boundary. Piezoelectric properties of d ₃₃ = 361 pC/N, k _p = 57%, Q _m = 145, and density of 7.8 g/cm³ were achieved at sintering temperature of 900°C. The CuO doped PAN-PZT ceramics show high density and d ₃₃ at low sintering temperature though its electromechanical quality factor abruptly decreases due to the CuO additive effect.     

Low temperature sintering of Ba0.91Ca0.09Ti0.916Sn0.084O3 lead-free piezoelectric ceramics with the additives of ZnO and MnO2

The objective of this work is to lower the sintering temperature of Ba_0.91Ca_0.09Ti_0.916Sn_0.084O₃ (BCTS) ceramics without sacrificing the piezoelectric performance. The low-temperature sintering technique has been conducted to prepare the BCTS ceramics by adding two additives of ZnO and MnO₂. The ceramics endure a phase transition from a ferroelectric tetragonal phase to a pseudo-cubic relaxor ferroelectric with increasing MnO₂ content. The addition of ZnO and MnO₂ decreases the sintering temperature greatly, positively affecting their dielectric and piezoelectric properties. An enhanced electrical behavior of d ₃₃?～?495 pC/N, k _p?～?43.0 %, ε _r?～?5429, and tan δ?～?1.54 % has been observed in the ceramic with x?=?0.1 wt% when sintered at ~1315 °C. As a result, the method to dope two additives of ZnO and MnO₂ can effectively improve the piezoelectric properties of BaTiO₃-based ceramics sintered at a low temperature.