Electrical properties and AC degradation characteristics of low voltage ZnO varistors doped with Nd2O3

The electrical properties and degradation characteristics of low voltage ZnO varistors were investigated as a function of Nd₂O₃ content. The varistor ceramics with 0.03 mol% Nd₂O₃ sintered at 1250 °C were far more densified than those with 0.06, 0.09 and 0.12 mol% Nd₂O₃. The addition of Nd₂O₃ to the low voltage ZnO varistors greatly improved the current–voltage characteristics; the nonlinear coefficient of varistors increase from 12.2 to 34.6 with increasing Nd₂O₃ content. The samples with 0.03 mol% Nd₂O₃ showed excellent stability due to high density and relatively good V–I characteristics, with the nonlinear coefficient of 22.5 and the leakage current of 9.6 μA. Their variation rate of varistor voltage and nonlinear coefficient and leakage current were −4.7%, −5.4%, 18.3%, respectively, under AC degradation stress (1.0 V_1 mA/125 °C/24 h).     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Electrical properties of Pr6O11-doped WO3 capacitor–varistor ceramics

The microstructure and electrical properties of Pr₆O₁₁-doped WO₃ ceramics were investigated. Results showed that the breakdown voltage of doped samples was lower than that of the undoped. The dielectric constant of doped samples was higher than that of the undoped, and the high dielectric constant made Pr₆O₁₁-doped WO₃ ceramics to be applicable as a kind of capacitor–varistor materials. A small content of Pr₆O₁₁ could significantly improve nonlinear properties of the samples. The WO₃–0.03 mol% Pr₆O₁₁ obtained a large nonlinear coefficient of 3.8, a low breakdown voltage of 8.8 V/mm, and a high dielectric constant of 7.69 × 10⁴ at 1 kHz. The defects theory was introduced to explain the nonlinear electrical behavior of Pr₆O₁₁-doped WO₃ ceramics.     

Giant dielectric permittivity and non-linear electrical behavior in CaCu3Ti4O12 varistors from the molten-salt synthesized powder

CaCu₃Ti₄O₁₂ (CCTO) powder has been prepared by a molten salt method using the NaCl–KCl mixture. Crystal structure and microstructure of the powder and the resulting ceramics have been characterized by using X-ray diffraction (XRD) and scanning electron microcopy (SEM). Impedance analyzer and current–voltage meter were employed to analyze dielectric and nonlinear (I–V) properties of the CCTO ceramics with different sintering durations and subsequent cooling rates. The values of dielectric permittivity and nonlinear coefficient of the quenched sample were found to be higher than those of the slowly cooled sample. More specifically, the cooling methods (quenching and furnace-cooling) have allowed to adjust; (?) the breakdown voltage within a rather low range of 0.3–4.4 kV cm⁻¹; (??) the nonlinear coefficient between 2 and 6 and (???) the giant dielectric permittivity for the ceramics within a range from 5000 to 20000. A double Schottky barrier can be evidenced from the linear behavior between the ln J and E^1/2 in grain boundary regions. The relationship between the electrical current density and the applied electrical field indicates that the potential barrier height Φ_B is holding time dependent.     

High Nonlinearity and High Voltage Gradient ZnO Varistor Ceramics Tailored by Combining Ga2O3, Al2O3, and Y2O3 Dopants

This paper deals with the electrical characteristics of rare‐earth‐doped ZnO varistor ceramics. Multiple donor dopants (Al³⁺, Ga³⁺, and Y³⁺) were employed to improve the comprehensive performance of ZnO varistor ceramics. The leakage current of rare‐earth‐doped ZnO varistor ceramics decreased noticeably with Ga₂O₃ dopants. The Ga³⁺ dopant occupies the defect sites of grain boundaries and increases the barrier potential of ZnO varistor ceramics, so the leakage current is effectively inhibited. Y₂O₃ is primarily located around the grains, which restrains ZnO grain growth, increasing the voltage gradient. The Al³⁺ goes into the lattices of ZnO grains, decreasing the grain resistance; thus, the residual voltage ratio can be controlled at low levels under a high impulse current. With the combined incorporation of Al³⁺, Ga³⁺, and Y³, excellent electrical properties of ZnO varistor ceramics can be acquired with a nonlinearity coefficient of 87, voltage gradient of 517 V/mm, leakage current of 0.96 μA/cm², and residual voltage ratio of 1.60. These rare multiple donor dopants can aid in engineering high‐quality ZnO varistors.     

A varistor–polymer composite with nonlinear electrical-thermal switching properties

We studied the dependence of the E–J characteristics on the sintering conditions in varistor fillers, the filler content in the polymer matrix and for different polymers and suggest a possible nonlinear electrical-thermal switching mechanism. The nonlinear coefficient and breakdown field of a sample with 100 vol.% varistor fillers sintered at 950 °C for 5 h were 12.24 and 135 V/mm, separately. A decrease in the amount of filler content in polymeric matrix or using a fortified polymer improved the electrical properties by increasing the breakdown field to the 263–964 V/mm range and decreasing the leakage current density in the 1.06 × 10⁻⁵ to 1.13 × 10⁻¹⁰ A/cm² range. This varistor–polymer composite can exhibit two nonlinearities in electrical conductivity. In the conducting state above the varistor filler breakdown, the polymer matrix reduces the conductivity, if a critical temperature is reached. Thereby the over voltage is limited by the fillers (i.e. varistor effect) and the over current is limited by the polymer matrix (i.e. positive temperature coefficient effect).     

Electrical properties of new tin dioxide varistor ceramics at high currents

New dense SnO₂-based varistor ceramics with high nonlinear current–voltage characteristics (nonlinearity coefficients are of approximately 50) in a system of SnO₂–CoO–Nb₂O₅–Cr₂O₃–Y₂O₃–SrO–MgO are reported. The current–voltage behaviour at high currents is studied by using exponential voltage pulses. The obtained SnO₂ varistor ceramics exhibit low grain resistivity values of 0.23–0.64 ohm cm. To date, such values are the lowest known for SnO₂ varistors, and are closely approaching the grain resistivity of the ZnO varistor. The current–voltage characteristics of the obtained SnO₂-based varistor materials are reproducible in a wide current range from 10^?11 to approximately 10⁴ A cm^?2. The minimum current density and the minimum electric field necessary to cause the irreversible electrical breakdown are measured. It is established that a decrease in the grain resistivity leads to an increase in the minimum current density necessary for irreversible electrical breakdown to occur.     

Structure and electrical properties of Nd2O3-doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 ceramics

Nd₂O₃ doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (abbreviated to BNKT) binary lead-free piezoelectric ceramics were synthesized by the conventional mixed-oxide method. The results show that the BNKT ceramics with 0–0.15 wt.% Nd₂O₃ doping possesses a single perovskite phase with rhombohedral structure. The grain size of BNKT decreased with the addition of Nd₂O₃ dopant. The temperature dependence of the dielectric constant ?_r revealed that there were two-phase transitions from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric. A diffuse character was proved by linear fitting of the modified Curie–Weiss law. At room temperature, the specimens containing 0.0125 wt.% Nd₂O₃ with homogeneous microstructure presented excellent electrical properties: the piezoelectric constant d₃₃ = 134 pC/N, the electromechanical coupling factor K_p = 0.27, and the dielectric constant ?_r = 925 (1 kHz).     

Microstructure, electrical properties, and aging behavior of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-Er2O3 varistor ceramics

The microstructure, electrical properties, and aging behavior of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-Er₂O₃ varistor ceramics were investigated for different contents of Er₂O₃. The microstructure consisted of ZnO grain and an intergranular layer (Pr, Y, and Er-rich phases) as a secondary phase. The increase of Er₂O₃ content decreased the average grain size and increased the sintered density. As the Er₂O₃ content increased, the breakdown field increased from 4206 V/cm to 5857 V/cm and the nonlinear coefficient increased from 32.6 to 48.6. The varistor ceramics added with 1.0 mol% Er₂O₃ exhibited excellent stability by exhibiting −0.2% in the variation rate of the breakdown field and −2.7% in the variation rate of the nonlinear coefficient for aging stress of 0.95 E_1 mA/150 °C/24 h.     

Effect of Nb2O5 on ZnBiMnO varistor ceramic prepared by solid-state sintering at 850°C

Nb₂O₅ is a commonly used donor dopant for ZnO-based varistor ceramics, but its effect, especially on the low-temperature sintered ZnO varistor ceramics, is not fully understood. To provide a possible answer to this problem, ZnO–Bi₂O₃–MnCO₃ (ZnBiMnO) based varistor ceramics with 0.05%–0.3% (in mole ratio) Nb₂O₅ were fabricated by solid-state sintering at 850 °C for 3 h. Their microstructure and nonlinear electrical properties were studied by XRD, SEM and the standard current-voltage (I–V) tests to reveal the effect of Nb₂O₅. With the increase of Nb₂O₅ from 0.05 mol% to 0.3 mol%, more solid Bi₅Nb₃O₁₅ inter-granular particles form within the ceramic during sintering, thereby decreasing the Bi-rich liquid phase. As a result, the average size of ZnO grain decreases from 4.35 μm to 1.67 μm. This microstructural change leads to the increase of the breakdown voltage in the range of 821 V/mm to 1851 V/mm. The ZnBiMnO varistor ceramic with 0.1 mol% Nb₂O₅ shows the best nonlinear properties. The optimum nonlinear coefficient is 35.81, the breakdown voltage is 907.51 V/mm, and the leakage current is 7.72 μA/cm². The result of this study provides a promising candidate material for manufacturing the multilayered low-voltage varistor that may use Ag, Ni or even Cu as the inner electrodes.     

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm⁻².     

Microstructural and compositional aspects of ZnO-based varistor ceramics prepared by direct mixing of the constituent phases and high-energy milling

ZnO-based varistor samples were prepared by the direct mixing of the constituent phases (DMCP) and sintering at 1100 °C for 2 h. The influence of the starting powder mixture's composition – the amounts of the pre-reacted varistor compounds and their composition – and its preparation, either with or without mechano-chemical activation (MCA), on the microstructure, phase composition and electrical characteristics of the varistor samples was studied. It showed that MCA improved the density and microstructural homogeneity of the varistor samples. MCA strongly affected the grain growth: it enhanced the nucleation of inversion boundaries (IBs) in the ZnO grains and the IBs-induced grain-growth mechanism resulted in uniform grain growth and hence a microstructure with smaller ZnO grains and a narrower grain size distribution. The final phase composition of the samples prepared by the DMCP method mainly depended on the presence of varistor dopants that can prevent the formation of the pyrochlore phase, especially Cr₂O₃, while MCA can affect it mostly by providing a homogeneous distribution of those dopants. The DMCP varistor samples prepared with MCA had much better current–voltage characteristics than the samples of the same composition prepared from unactivated powders.     

Electrical and dielectric characteristics of erbium-added ZnO–V2O5-based varistor ceramics

This paper focuses on that the erbium-added ZnO–V₂O₅-based ceramics are attained at a sintering temperature as low as 875 °C. The effect of Er₂O₃ addition on microstructure, electrical properties, and dielectric characteristics has been investigated. Increasing the amount of Er₂O₃ slightly increased the densities of sintered pellets in the range of 5.52–5.59 g/cm³. The increase in the amount of Er₂O₃ increased the breakdown field from 6991 to 7408 V/cm up to 0.1 mol%, whereas a further addition decreased it. The sample added with 0.1 mol% Er₂O₃ exhibited the highest nonlinear coefficient (α=55) and the sample added with 0.25 mol% Er₂O₃ exhibited the lowest nonlinear coefficient (α=14). The donor concentration increased from 2.92×10¹⁷ to 8.48×10¹⁷ cm⁻³ with an increase in the amount of Er₂O₃.     

An approach to Y2O3:Eu optical nanostructured ceramics

Y₂O₃:Eu³⁺ (1 at.%) translucent nanostructured ceramics with total forward transmission achieving ∼70% of the theoretical limit has been obtained by the transformation-assisted consolidation of custom-made cubic Y₂O₃:Eu³⁺ nanopowders under high pressure (HP). Sintering under the pressure of 7.7 GPa and temperatures in the 100-500 °C range leads to the partial cubic-to-monoclinic phase transition that results in two-phase Y₂O₃:Eu³⁺ nanoceramics. The average grain size of ceramics d ≤ 50 nm for both Y₂O₃:Eu³⁺ polymorph is comparable with crystallite size of initial nanopowders (d ∼ 40 nm), indicating that the grain growth factor is near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of synthesized nanostructured ceramics have been studied.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

The effect of lithium oxide on the threshold field of ZnO varistors

Lithium oxide in form of Li₂CO₃ solution is added with contents of 0–200 ppm to two ZnO-based varistors standard formulations, once with Sb₂O₃ and the other without. According to Li₂CO₃ concentration, both threshold field and energy absorption capability evolution are studied. It is found that with the benefit of antimony, the lithium allows reaching high threshold field but concomitantly, low energy absorption capability. Without antimony, threshold fields up to 300 V/mm are attained, associated with a fair energy absorption capability. With 100 ppm of Li₂CO₃, optimum couple of values (315 V/mm; 115 J/cm³) is achieved. With 200 ppm of Li₂CO₃, threshold field exceeds 500 V/mm but energy absorption capability falls below 50 J/cm³. Correlations with SEM microstructures observations suggest that lithium increases voltage barrier height by decreasing donor density and that spinel phases (Zn₇Sb₂O₁₂) have detrimental effects on the electrical absorption capability by limiting the density of current, reducing the effective current path from one ZnO grain to another.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

The effects of doped Nd on conductivity and phase stability of BaCe0.8Y0.2O3−δ-based electrolyte for solid oxide fuel cell

This study investigates the structure, phase stability, and electrical properties of BaCe_0.8Y_0.2−xNd_xO_3−δ (x = 0-0.2) in humid air. XRD results indicate that a BaCe_0.8Y_0.2−xNd_xO_3−δ sample has an asymmetric orthorhombic structure, and this structure becomes more symmetric as the amount of Nd doping increases. The conductivity of BaCe_0.8Y_0.2−xNd_xO_3−δ depends on the amount of Nd doping and the operation temperature. AC impedance results indicate that the resistance of BaCe_0.8Y_0.2−xNd_xO_3−δ decreases as the temperature increases, with the majority of resistance coming from oxygen ion diffusion. The XRD peak intensity of BaCe_0.8Y_0.2O_3−δ apparently decreased with time, forming Ba(OH)₂ and CeO₂ second phases. The phase stability of BaCe_0.8Y_0.2−xNd_xO_3−δ (x = 0.05-0.2) samples is much better than that of BaCe_0.8Y_0.2O_3−δ, and it exhibited no second phase after tested in an 80 °C water bath for 18 h.     

Synthesis and luminescence of Y2O3:Eu inorganic–organic hybrid nanostructures with thenoyltrifluoroacetone

Cubic Y₂O₃:Eu³⁺ nanoparticles with a size about 32 nm were synthesized using a facile hydrothermal method followed by an annealing process. As expected, the Y₂O₃:Eu³⁺ nanoparticles had a broad Eu–O excitation band ranging from 200 nm to 285 nm peaking at about 260 nm. The Y₂O₃:Eu³⁺ nanoparticles were then used to fabricate the inorganic–organic hybrid nanostructures with thenoyltrifluoroacetone (TTA). The Y₂O₃:Eu³⁺–TTA hybrid nanostructures exhibited a new strong excitation band ranging from 280 nm to 390 nm peaking at about 368 nm. This new excitation band was attributed to the energy transfer mechanism of the Y₂O₃:Eu³⁺–TTA hybrid system. It is interesting to note that this energy transfer mechanism had a close interaction with the Eu–O excitation of Y₂O₃:Eu³⁺ nanoparticles. The phase structures, chemical bonding information, microstructural characteristics and luminescence properties were investigated.