Microstructure and electrical properties of rare earth doped ZnO-based varistor ceramics

ZnO-based varistor ceramics doped with Nd₂O₃ and Y₂O₃ have been prepared by the conventional ceramics method. The phase composition, microstructure and electrical properties of the ceramics have been investigated by XRD, SEM and a V–I source/measure unit. The XRD and EDS analyses show the presence of ZnO, Bi₂O₃, Zn₇Sb₂O₁₂, Y₂O₃, Nd-rich phase and Y-containing Bi-rich phase. The electrical properties analyzed show that the nonlinear coefficient of the varistor ceramics is in the range of 4.4–70.2, the threshold voltage is in the range of 247.1–1288.8 V/mm, and the leakage current is in the range of 1.51–214.6 μA/cm². The 0.25 mol% Nd₂O₃ added varistor ceramics with 0.10 mol%Y₂O₃ sintered at 1050 °C exhibits excellent electrical properties with the high threshold voltage of 556.4 V/mm, the nonlinear coefficient of 61 and the leakage current of 1.55 μA/cm². The results illustrate that doping Nd₂O₃ and Y₂O₃ in ZnO-based varistor ceramics may be a very promising route for the production of the higher threshold voltage and the nonlinear coefficient of ZnO-based varistor ceramics.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Effect of ZnBi2O4 and Bi2O3 addition on the densification,microstructure, and varistor properties of ZnO varistors

In this study, 1 wt% Bi₂O₃ (1B), 1 wt% ZnBi₂O₄ (1BZ), and a composite (a mixture of 1 wt% Bi₂O₃ and various amounts (1-4 wt%) of ZnBi₂O₄ ,1B1BZ-1B4BZ) were added to ZnO varistors to investigate the effects of additives on the densification, microstructure, and varistor performance. The results showed that the addition of ZnBi₂O₄ can lower the densification temperature to about 850^oC. When the additive was changed from 1 wt% Bi₂O₃ to 1 wt% ZnBi₂O₄, the α value increased from 42 to 54, the breakdown voltage increased from 775 V/mm to 1011 V/mm, and the leakage current decreased to 0.11 μA. Additions of ZnBi₂O₄ and transition metal cations as donor dopants for the ZnO varistors promote oxygen chemisorption at grain boundaries, resulting in greater α value and lower leakage currents. This suggests the addition of ZnBi₂O₄ can effectively promote densification and improve the varistor properties of ZnO varistors.     

Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk

Ga₂O₃ (5 wt.%) doped zinc oxide (ZnO, 95 wt.%) bulk was fabricated by underwater shock compaction technique. The microstructural, crystal structure and electrical properties of shock-consolidated samples were investigated and compared to a commercially available sintered Ga₂O₃ (5 wt.%) doped ZnO (95 wt.%). The relative density of shock-consolidated sample was about 97% of the theoretical density, and no grain growth and lattice defects were confirmed. The grain boundary resistance was remarkably higher than that of commercial sintered Ga₂O₃ doped ZnO and nonlinear current-voltage (I-V) characteristics of shock-consolidated ZnO and Ga₂O₃ doped ZnO were very lower than that of commercial ZnO varistor.     

Characteristics of ZnO-based varistor ceramics doped with Al2O3

The influence of Al₂O₃ doping in the range 0.00–0.83 mol% on the microstructure and current–voltage characteristics of ZnO-based varistor ceramics sintered at 1200 °C for 2 h was studied. The threshold voltage V_T (V/mm) increased up to a dopant level of about 0.08 mol% Al₂O₃; the nonlinear coefficient α was significantly increased by additions of up to 0.04 mol% Al₂O₃, although larger additions of Al₂O₃ caused it to decrease; and the leakage current increased sharply with increasing amounts of Al₂O₃. Doping with Al₂O₃ up to about 0.12 mol% Al₂O₃ resulted in a significantly decreased ZnO grain size, which is mainly responsible for the significantly increased threshold voltage, V_T. No ZnAl₂O₄ spinel phase was detected in any of the samples, and EDXS and WDXS analyses showed that most of the added Al₂O₃ distributed between the Zn₇Sb₂O₁₂ spinel phase and the ZnO phase, while only trace amounts were detected in the Bi₂O₃-rich phase. The spinel phase incorporates an appropriate amount of Al₂O₃; however, with an increasing amount of added Al₂O₃, more of it remains outside the spinel phase in the Bi₂O₃-rich liquid, where it can incorporate into the growing ZnO grains at the sintering temperature. The amount of Al in the ZnO grains was determined. A mechanism for the grain growth inhibition resulting from the small amounts of Al₂O₃ in the Bi₂O₃-rich liquid phase is also proposed.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

Band gap tuning of Ga2O3–Al2O3 ceramics

Translucent ceramics comprising monoclinic Ga₂O₃ and non-cubic Ga₂O₃–Al₂O₃ solid solution with a composition of Ga_1.5Al_0.5O₃, GaAlO₃ and Ga_0.4Al_1.6O₃ were fabricated through two-step spark plasma sintering by using calcinated commercial mixture oxide ceramic powders. All sintered ceramics were nearly fully densified, with a relative density exceeding 99.2%. The average grain sizes decreased with increasing Al content. Band gap was tuned from 4.08 to 6.37 eV. The highest total transmittance (73.6% at 1100 nm) was measured in Ga_0.4Al_1.6O₃ ceramics with both hexagonal (rhombohedral) and monoclinic crystal structures and a minimum average grain size of 0.288 ± 0.067 μm. Defect related photoluminescence, measured under excitation by a 254 nm UV lamp in the GaAlO₃ and Ga_0.4Al_1.6O₃ ceramics, lasted more than 15 s after removal of the UV source. These translucent ceramics from Ga₂O₃–Al₂O₃ solid solution provide an alternative form of a potential transparent conducting material.     

Grain‐Boundary and Thermally Stimulated Current Characteristics of Y2O3‐Doped ZnO Varistor

The doping of rare‐earth oxides can greatly improve the electrical characteristics of ZnO varistors. Thermally stimulated current (TSC) characteristic test, capacitance voltage (C–V) characteristic test, scanning electron microscope (SEM) test, and voltage current (V–I) test were carried out to study the influence of Y₂O₃ content on the electrical properties of ZnO varistors in this study. The results show that the grain size decreases while the voltage gradient increases as the Y₂O₃ content is increased. The reaction of Y₂O₃ with other additives leads to the decrease in grain‐boundary defects, which accounts for the decrement of barrier height, donor density, and surface state density. The trap level and trapped charge of ZnO varistors decrease as the Y₂O₃ content is increased from 0.3 to 0.9 mol%, which means the shallow traps inside ZnO varistors reduce, and the Y₂O₃ additive can greatly improve the TSC characteristic of ZnO varistors.     

Improvement of surge current performances of ZnO varistor ceramics via C3N4-doping

ZnO-Bi₂O₃ based varistor ceramics doped with C₃N₄ were fabricated via solid state method. Experimental results show that C₃N₄ additive acts as an inhibitor in ZnO grain growth, and the average grain size decreases from 10.2–7.1 μm. The varistor breakdown voltage gradient increases from 222.3–282.3 V/mm, and the nonlinear coefficient increases from 51.9–58.2 with the increase of C₃N₄ content from 0 to 3.0 wt%. The C₃N₄-added samples exhibited smaller residual voltage ratio and better surge current withstanding capability. It is proposed that the C₃N₄-doping leads to substitution of nitrogen for oxygen in the grain boundary region, forming acceptor type defects. The acceptor type defects act as electron traps, increasing the barrier height from 1.31 to 1.50 eV and the depletion layer width from 54.9–61.8 nm, which increases the nonlinearity, and the surge current performances of the C₃N₄ doped ZnO varistor ceramics are improved.     

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.     

Triple-doping of (Ga1/2Nb1/2)xTi1-xO2 ceramics with Al3+ for enhanced giant dielectric response with simultaneous decrease in dielectric loss

An acceptor-donor co-doped (Ga_1/2Nb_1/2)_0.1Ti_0.9O₂ ceramic is triple-doped with Al³⁺, followed by sintering at 1450 °C for 5 h to obtain (Al_xGa_1/2-xNb_1/2)_0.1Ti_0.9O₂ ceramics with improved giant dielectric properties. Homogeneous dispersion of all dopants inside the grains, along with the partially segregated dispersion of the Ga³⁺ dopant along the grain boundaries, is observed. The (Al_xGa_1/2-xNb_1/2)_0.1Ti_0.9O₂ ceramics exhibit high dielectric permittivities (ε′～4.2–5.1 × 10⁴) and low loss tangents (tanδ～0.007–0.010), as well as a low-temperature coefficients (<±15%) between ? 60 and 200 °C. At 1 kHz, tanδ is significantly reduced by ～4.4 times, while ε′ is increased by ～3.5 times, which is attributed to the higher Al³⁺/Ga³⁺ ratio. The value of tanδ at 200 °C is as low as 0.04. The significantly improved dielectric properties are explained based on internal and surface barrier-layer capacitor effects, which are primarily produced by the Ga³⁺ and Al³⁺ dopants, respectively, whereas the semiconducting grains are attributed to Nb⁵⁺ doping ions.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Effect of Y2O3 and ZnO co-doping on the densification and properties of magnesium aluminum spinel

The effects of Y₂O₃ and ZnO co-doping on the densification and properties of magnesium aluminum spinel were investigated. The physical phase composition, microstructure, elemental distribution, densification, apparent porosity, particle size distribution and average corrosion depth of the specimens were investigated by XRD, SEM-EDS, Archimedes drainage method, Nano Measurer 1.2 software, and molten salt corrosion tests. The results showed that after co-doping with Y₂O₃ and ZnO, the cations in the sintering aids could dissolve into the spinel structure, forming solid solution ZnAl₂O₄, second phase Y₃Al₅O₁₂ and Al₂Y₄O₉, which inhibited the abnormal grain growth and made the grain distribution more uniform, thus promoting the densification of the samples. The best co-doping amount of Y₂O₃ and ZnO was 1 wt% Y₂O₃-3 wt.% ZnO, the relative density of the sample was 99.3%, the apparent porosity was 0.021%, and the grain size was the most uniform (6.52 μm). After the sample was placed into the aluminum electrolytic molten salt electrolyte for 1 h, and it was found that the sample doped with 1 wt% Y₂O₃-3 wt.% ZnO had the minimum average corrosion depth (131.9 μm) and the best corrosion resistance.     

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 ZnO and TiO2 doping on the sintering behavior of Y2O3 ceramics

Full densification of Y₂O₃ is challenging and requires a very high sintering temperature (above 1700 °C). In this study, the effect of ZnO and TiO₂ dopants on its densification was investigated, showing that both dopants lowered the sintering temperature and improved the process. Moreover, ZnO promoted the grain growth, while TiO₂ inhibited it; hence, the ZnOTiO₂ co-doping and the change in the ZnO/TiO₂ ratio allowed the control of the sintered body microstructure while maintaining high densification. Since Y₂O₃ has a higher plasma erosion resistance than conventional Si-based materials, the plasma dry etching resistance of the sintered Y₂O₃ was also evaluated and found superior due to the improved densification and controlled grain size of the doped samples.     

Influence of Ca-doping on the nonlinear properties of novel ZnO-Cr2O3-based varistor ceramics

The influence of Ca-doping on the microstructure development and electrical characteristics was studied in the novel, ZnO-Cr₂O₃-based, varistor ceramics with the nominal composition (99.47-x) mol% ZnO + 0.1 mol% Cr₂O₃ + 0.33 mol% Co₃O₄ + 0.1 mol% La₂O₃ + x mol% CaCO₃, with x ranging from 0 to 6, sintered at 1200 °C. The results showed that, within the limits of the solid solubility of CaO in ZnO, the addition of CaO greatly enhanced the diffusion processes and hence the grain growth, thus broaden the range of breakdown voltages. Moreover, the Ca-doping simultaneously increases the height of the electrostatic Schottky barrier, resulting in a higher nonlinear coefficient of 40 and a lower leakage current of 2.8 μA/cm². The results are important for the potential application of these novel ZnO-Cr₂O₃-based varistors, having different nominal voltages and yet suitable dimensions, in overvoltage protection across a broad range of voltages.     

Modeling and Monte Carlo simulation on photothermal effect in Gd3Al3Ga2O12:Ce3+/Y3Al5O12:Cr3+ layered composite ceramic

Layered composite ceramics have wide application in solid-state lasers. However, the photothermal effect in the layered ceramics has not been clarified, due to the interface effect between layers. In this work, the model of photon propagation and thermal distribution in the Gd₃Al₃Ga₂O₁₂:Ce³⁺/Y₃Al₅O₁₂:Cr³⁺ layered ceramics are established. The property of photon absorption, reflection, transmission, and thermal distribution are studied by combining the Monte Carlo method and the convolution method. It is found that the photon absorption distribution and thermal distribution of this layered ceramics show the gradient change. Furthermore, this change is strongly dependent on the type, beam width, and power of laser. The temperature of layered ceramics induced by Gaussian beam is higher than that induced by flat circular beam. This work provides a useful research method for the design of layered ceramic materials with excellent scintillation performance.     

The Band Structure of Polycrystalline Al2O3 and Its Influence on Transport Phenomena

The electronic (band) structure of polycrystalline Al₂O₃, in particular the density of near‐band edge grain‐boundary localized states, plays a significant role in a host of high‐temperature phenomena, including sintering, high‐temperature creep, oxygen permeability in dense “dry” Al₂O₃ ceramics, and Al₂O₃ scale formation on Al₂O₃ scale‐forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al₂O₃ assume great importance, and has been calculated using DFT for both nominally undoped and Y‐doped Σ7 bi‐crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y₂O₃ segregation to Al₂O₃ grain boundaries is so effective in enhancing high‐temperature creep resistance of polycrystalline Al₂O₃, and in understanding the reactive element effect in Al₂O₃ scale‐forming alloys. Finally, a case will be made that grain‐boundary diffusion is mediated by the migration of a class of grain‐boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.