Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO₂ doping on the microstructure and varistor properties of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics, which were prepared via the traditional ball milling–molding–sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO₂ changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta₂O₅, CaCO₃, Ge, and GeO₂ exhibited the highest nonlinear coefficient (α=14.6), a lower breakdown voltage (E_B=18.7 V mm⁻¹), the least leakage current (J_L=10.5 μA cm⁻²), and the highest grain boundary barrier (Φ_B=1.05 eV). In addition, Ge and GeO₂ function as sintering aids, which reduce the sintering temperature because of their low melting points.     

High-performance and low-voltage SnO2-based varistors

This paper presents the results of a thorough study conducted on the action mechanism of one-dimensional single-crystalline SnO₂ nanobelts in decreasing the breakdown electric field (E_b) in SnO₂-based varistors. The proposed method has general validity in that our investigation was focused on the traditional varistor composition SnO₂-CoO-Cr₂O₃-Nb₂O₅. To accomplish our study objective, two methods of decreasing E_b value were compared; one involving the increase in average grain size of the varistor through the sintering time and the other one related to the addition of nanobelts. The morphological results show that the method involving the increase in average grain size is limited by the formation of intragranular pores. Furthermore, despite contributing successfully towards decreasing the E_b value (which underwent a decline from 3990 V cm⁻¹ to 1133 V cm⁻¹ with an increase in sintering time from 1 h to 2 h), the reduction obtained by this method is found to be much lower compared to that obtained via the nanobelts insertion method (E_b = 270 V cm⁻¹). Impedance spectroscopy results showed that the insertion of nanobelts caused a decline in the grain boundary resistance while surface potential measurements proved that this decline in resistance is attributed to the absence of potential barriers along the belts which leads to the formation of a lower resistance percolation path in the varistor.     

Varistor and dielectric properties of SiO2 doped SnO2-Zn2SnO4 ceramic composites

In this work, SiO₂ doped SnO₂-Zn₂SnO₄ ceramic composites with excellent varistor and dielectric properties were prepared through traditional ceramic processing. The obtained nonlinear coefficient α was as high as 9.6, and the breakdown electrical field EB and leakage current density JL was as low as 5.9 V/mm and 62 μA/cm², respectively. At a low frequency of 40 Hz, the relative permittivity εr measured at room temperature was higher than 2.5×10⁴. The nonlinear decrease of the semicircle diameter in the complex impedance spectra with increasing DC bias voltage indicates that the grain boundary effect is an important origin of the varistor and giant permittivity properties. With an increase of temperature, the relaxor peak of the imaginary part M″ of the complex electric modulus shifted to high frequency and the activation energy E_a obtained from the M″ spectrum was about 0.31 eV, much lower than the grain boundary barrier height ϕ_b. The results suggest that other mechanisms may also be responsible for the giant permittivity property besides grain boundary barriers.     

Controlling the breakdown electric field in SnO2 based varistors by the insertion of SnO2 nanobelts

Semiconducting metal oxides have many practical applications, including varistors. Varistors based on SnO₂ exhibit both high nonlinear coefficients and high breakdown electric fields. In this work we present a facile means for controlling the breakdown electric field in the SnO₂-CoO-Cr₂O₃-Nb₂O₅ varistor system by the introduction of 1D SnO₂ nanobelts. The materials were prepared by a solid state reaction method with Nb₂O₅ doping levels at 0.10 and 0.20, mole percent. The materials were studied in detail by dual beam microscopy, direct current and impedance measurements. Exaggerated three-dimensional growth of the tin dioxide belts was observed, which was attributed to Ostwald ripening. The breakdown electric field was observed to decrease from 2510 V/cm to 2280 V/cm and from 1700 V/cm to 804 V/cm after nanobelt insertion, into the systems with 0.10 and 0.20, mole percent Nb₂O₅ respectively. A model for the observed results was proposed based on the percolation of electrons through the belts, decreasing the number of effective potential barriers at the grain boundaries. The simulations of the impedance data showed one order of magnitude decrease in the grain boundary resistance due to the nanobelt insertion for both studied systems.     

Microstructure and electrical characteristics of ZnO–B2O3–PbO–V2O5–MnO2 ceramics prepared from ZnO nanopowders

A peculiar kind of ZnO–B₂O₃–PbO–V₂O₅–MnO₂ ceramics was produced from the ZnO nanopowders directly co-doped with the oxides instead of lead zinc borate frit in this investigation. The 8 wt.% (PbO+B₂O₃) co-doped ceramics sintered at 950 °C for 2 h displayed the optimum electrical properties, that is, leakage current density J_L=6.2×10⁻⁶ A/cm², nonlinear coefficient α=22.8 and breakdown voltage V_BK=331 V/mm. The co-doping of 8 wt.% (PbO+B₂O₃) resulted in an increase in nonlinear coefficient and a decrease in leakage current density of the ZnO–V₂O₅ varistors while the sintering temperature showed no evident influence on nonlinear coefficient and leakage current density at the range of 800–950 °C.     

The effect of Sm2O3 on the microstructure and electrical properties of SiO2-doped SnO2-Zn2SnO4 ceramic varistors

In this work, Sm₂O₃- and SiO₂-codoped SnO₂-Zn₂SnO₄ ceramic varistors were prepared through traditional ceramic processing, and the effect of Sm₂O₃ on the resulting microstructure and electrical properties was investigated. The results demonstrated that the ceramics were composed mainly of SnO₂ and Zn₂SnO₄, and Sm was distributed homogeneously in the grains and along the grain boundaries. With 0.2 mol% Sm₂O₃ doping, the grain growth was obviously promoted. Further increases in Sm₂O₃ to 0.4 mol% resulted in trace amount of SiO₂ and segregations containing elemental Sm via X-ray diffraction patterns and microstructure photos, respectively. In the sample doped with 0.3 mol% Sm₂O₃, optimal electrical characteristics of α=9.4, E_B=10 V/mm, J_L=46 μA/cm² and ε′=1.2×10⁴ were obtained. Simultaneously, the sample doped with 0.3 mol% Sm₂O₃ had the lowest conductance activation energy of 0.16 eV at temperatures lower than 110 °C. This good performance indicates that Sm₂O₃- and SiO₂-codoped SnO₂-Zn₂SnO₄ composite ceramics are viable candidate for the manufacture of capacitor-varistor functional devices.     

Highly conducting and transparent antimony doped tin oxide thin films: the role of sputtering power density

Sb doped SnO₂ thin films were deposited on quartz substrates by magnetron sputtering at 600 °C and the effects of sputtering power density on the preferential orientation, structural, surface morphological, optical and electrical properties had been studied. The XRD analyses confirm the formation of cassiterite tetragonal structure and the presence of preferential orientation in (2 1 1) direction for tin oxygen thin films. The dislocation density analyses reveal that the generated defects can be suppressed by the appropriate sputtering power density in the SnO₂ lattice. The studies of surface morphologies show that grain sizes and surface roughness are remarkably affected by the sputtering power density. The resistivity of Sb doped SnO₂ thin films gradually decreases as increasing the sputtering power density, reaches a minimum value of 8.23×10⁻⁴ Ω cm at 7.65/cm² and starts increasing thereafter. The possible mechanisms for the change in resistivity are proposed. The average transmittances are more than 83% in the visible region (380–780 nm) for all the thin films, the optical band gaps are above 4.1 eV. And the mechanisms of the variation of optical properties at different sputtering power densities are addressed.     

Ge-added TiO2–Ta2O5–CaCO3 varistor ceramics

The influence of doping with Ge on the nonlinear coefficient α and the breakdown electric field E_B of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics was investigated. In this study, TiO₂–Ta₂O₅–CaCO₃ varistor ceramics added with Ge was successfully prepared using the traditional method of ball milling–molding–sintering. The electrical performance, including the nonlinear coefficient α, the breakdown electric field E_B, and the leakage current J_L, are tested using a varistor direct current parameter instrument. The average barrier height Φ_B of each sample is calculated using the relevant formula. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electronic microscopy analyses demonstrated that Ge doping notably changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing α and decreasing E_B. When the doping contents of Ta₂O₅ and CaCO₃ were 0.2 and 0.4 mol%, respectively, the optimum doping content of 0.9 mol% Ge exhibited high α (10.2), low E_B (14.1 V mm⁻¹), and high Φ_B (0.95 eV). These results are superior to previous findings. In addition, Ge as sintering aid reduced the sintering temperature caused by the low melting point. The optimal sintering temperature was 1300 °C for the TiO₂–Ta₂O₅–CaCO₃ ceramics doped with Ge.     

Electrical resistivity of silicon carbide ceramics sintered with 1 wt% aluminum nitride and rare earth oxide

The influence of additive composition on the electrical resistivity of hot-pressed liquid-phase sintered (LPS)-SiC was investigated using AlN–RE₂O₃ (RE = Sc, Nd, Eu, Gd, Ho, Er, Lu) mixtures at a molar ratio of 60:40. It was found that all specimens could be sintered to densities >95% of the theoretical density by adding 5 wt% in situ-synthesized nano-sized SiC and 1 wt% AlN–RE₂O₃ additives. Six out of seven SiC ceramics showed very low electrical resistivity on the order of 10^?4 Ω m. This low electrical resistivity was attributed to the growth of nitrogen-doped SiC grains and the confinement of non-conducting RE-containing phases in the junction areas. The SiC ceramics sintered with AlN–Lu₂O₃ showed a relatively high electrical resistivity (～10^?2 Ω m) due to its lower carrier density (～10¹⁷ cm^?3), which was caused by the growth of faceted grains and the resulting weak interface between SiC grains.     

Electrical impedance spectroscopy and structural characterization of liquid-phase sintered ZnO–V2O5–Nb2O5 varistor ceramics doped with MnO

The influence of the MnO content on the microstructure and the electrical response of liquid-phase sintered ZnO–V₂O₅ varistor ceramics wereanalysed using A.C. impedance spectroscopy on samples prepared via the conventional solid state route.The impedance spectra were analysed with the help of one model equivalent circuit at high frequencies and another at low frequencies, involving both resistor and non-Debye constant phase elements (CPEs). The results indicate a significant contribution of grain boundary resistance to the total resistance and non-ohmic characteristic of the studied materials. The Arrhenius plots show two slopes with a turnover at 150 °C/200 °C for both the higher- and lower-frequency time constants. These behaviours can be related to the decrease of the minor charge carrier density. These activation energies were associated with the adsorption and reaction of O₂ (as well as V) species at the grain boundary interface. Consequently, better varistor performance is achieved for 96.9 mol% ZnO+0.5 mol% V₂O₅+0.10 mol% Nb₂O₅+2.5 mol% MnCO₃ with nonlinear coefficient α=21.6, breakdown field E_1mA=191.47 V/mm, leakage current density J_L=36.46 µA/cm² and activation energies of 0.639 eV and 0.644 eV. X-ray diffraction shows that in addition to the major ZnO phases, Zn₃(VO₄)₂ and ZnV₂O₄, were detected as minor secondary phases. SEM analysis of the morphology shows that the grain growth increases with increases in the MnO doping level.     

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

Electrical resistivity of α-SiC ceramics sintered with Al2O3 or AlN additives

Highly resistive SiC ceramics were prepared by hot pressing α-SiC powders with Al₂O₃-Y₂O₃ additives with a 4:1 molar ratio. X-ray diffraction patterns, Raman spectra, electron probe microanalysis (EMPA), and scanning electron microscopy (SEM) images revealed that the bulk SiC ceramics consisted mostly of micron-sized 6H-SiC grains along with Y₂O₃ and Si clusters. As the additive content increased from 1 to 10 vol%, the electrical resistivity of the ceramics increased from 3.0 × 10⁶ to 1.3 × 10⁸ Ω cm at room temperature. Such high resistivity is ascribed to Al₂O₃ in which Al impurities substituting Si site act as deep acceptors for trapping carriers. More resistive α-SiC ceramics were produced by adding AlN instead of Al₂O₃. The highest resistivity (1.3 × 10¹⁰ Ω cm) was achieved by employing 3 vol% AlN-Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG) as an additive.     

Characterisation of SnO2–CoO–MnO–Nb2O5 ceramics

Varistors based on SnO₂ have attracted increasing interest in recent years. However, the combined effect of CoO–MnO on SnO₂ ceramics is still unclear. In this study, the non-Ohmic behaviour of the 98.95 mol%SnO₂–0.5 mol%CoO–0.5 mol%MnO–0.05 mol%Nb₂O₅ system, the microstructures and the influence of sintering temperature were investigated. The samples were prepared by the mixed oxide route, and were sintered at temperatures in the range 1250–1450 °C. SEM observation and EDS analysis revealed that the ceramics have a two-phase microstructure comprising SnO₂ primary grains and a Mn, Co rich secondary phase of small particles. The sintered density of the samples increased with the increase in sintering temperature. The maximum non-linear coefficient (α = 10) was obtained at a sintering temperature of 1350 °C.     

Mechanical and optical properties of spark plasma sintered transparent Y2O3 ceramics

Commercial Y₂O₃ nanopowder was used to fabricate transparent Y₂O₃ ceramics by spark plasma sintering under the pressure of 100 MPa for 20 min with the heating rate of 100 °C/min. The microstructures, mechanical and optical properties of the Y₂O₃ ceramics sintered at different temperatures were investigated in detail. Densification occurred up to a sintering temperature of 1500 °C, and above 1500 °C, rapid grain growth and pore growth occurred. The highest relative density of 99.58% and the minimum average grain size of 0.58±0.11 µm were obtained at 1500 °C. The flexural strength, hardness and fracture toughness of the optimal spark plasma sintered Y₂O₃ ceramic were 122 MPa, 7.60 GPa and 2.06 MPa.m^1/2, respectively. The Y₂O₃ ceramic sintered at 1500 °C had the in-line transmission of about 11–54% and 80% in the wavelength range of 400–800 nm and 3–5 µm, respectively.     

Electric field-assisted flash sintering of tin dioxide

SnO₂ green pellets were submitted to ac electric fields at temperatures below 1350 °C. Electric current pulses occurred and a substantial modification was found in the microstructure of the pellets after application of 80 V cm⁻¹ at 900, 1100 and 1300 °C. Similar experiments were carried out in SnO₂ mixed to 2 wt.% MnO₂. The linear shrinkage of the pellets was monitored with a dilatometer during the application of the electric field. Scanning electron microscopy micrographs of the pellets show the grain structure evolution after the electric current pulses. The larger is the electric current flow through the SnO₂ pellet, the larger are the shrinkage and the average grain size. Even though sintering occurs without significant densification in SnO₂, the welding of the grains is evident. The apparent density of green pellets of SnO₂ with MnO₂ addition sintered at 1100 °C increased 110% with the application of 80 V cm⁻¹, 5 A.     

Fabrication and positive temperature coefficient of resistivity properties of semiconducting ceramics based on the BaTiO3–(Bi1/2K1/2)TiO3 system

BaTiO₃–(Bi_1/2K_1/2)TiO₃ (BT–BKT) ceramics have a low ρ_RT of 10¹–10² Ω cm like that of semiconducting materials prepared by sintering in a N₂ flow with low O₂ concentration. By annealing in air, the BT–BKT ceramics show an abrupt increase in their resistivity near the T_c, namely, a positive temperature coefficient of resistivity (PTC) characteristic. With 5 mol% and 10 mol% BKT added to BT, the ceramics display the PTC characteristic at 155 °C and 165 °C, respectively. Furthermore, the ratio, ρ_max/ρ_RT, of the highest resistivity (ρ_max) and the resistivity at room temperature (ρ_RT) of the ceramics increased on adding a small amount of Mn and a sintering aid.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

One-pot synthesis of core–shell-structured tin oxide–carbon composite powders by spray pyrolysis for use as anode materials in Li-ion batteries

Core–shell-structured tin oxide–carbon composite powders with mixed SnO₂ and SnO tetragonal crystals are prepared by one-pot spray pyrolysis from a spray solution with tin oxalate and polyvinylpyrrolidone (PVP). The aggregate, made up of SnO_x nanocrystals (several tens of nanometers), is uniformly coated with an amorphous carbon layer. The initial discharge capacities of the bare SnO₂ and SnO_x–carbon composite powders at a current density of 1 A g⁻¹ are 1473 and 1667 mA h g⁻¹, respectively; their discharge capacities after 500 cycles are 78 and 1033 mA h g⁻¹, respectively. The SnO_x–carbon composite powders maintain their spherical morphology even after 500 cycles. On the other hand, the bare SnO₂ powder breaks into several pieces after cycling. The structural stability of the SnO_x–carbon composite powders results in a low charge transfer resistance and high lithium ion diffusion rate even after 500 cycles at a high current density of 2 A g⁻¹. Therefore, the SnO_x–carbon composite powders have superior electrochemical properties compared with those of the bare SnO₂ powders with a fine size.     

Low-temperature microwave and THz dielectric response in novel microwave ceramics

Low-temperature dielectric properties of BaZn_1/3Nb_2/3O₃-based ceramics, CeO₂-based ceramics and Ruddlesden–Popper Sr_n+1Ti_nO_3n+1 (n = 1–4) ceramics has been studied in microwave, THz and infrared frequency range down to 10 K. Extrinsic dielectric losses originating probably from diffusion of charged defects are observed in two families of compounds by a minimum in the temperature dependence of microwave quality Q. The rise of microwave permittivity and dielectric losses at low temperatures in Sr_n+1Ti_nO_3n+1 (n = 2–4) ceramics was explained by softening of an optical polar mode in SrTiO₃, which is in the Sr_n+1Ti_nO_3n+1 (n = 3, 4) ceramics contained as a second phase.     

Synthesis,characterization, and electrical conduction of 10 mol% Dy2O3-doped CeO2 ceramics

Well-densified 10 mol% Dy₂O₃-doped CeO₂ (20DDC) ceramics with average grain sizes of ∼0.12–1.5 μm were fabricated by pressureless sintering at 950–1550 °C using a reactive powder thermally decomposed from a carbonate precursor, which was synthesized via a carbonate coprecipitation method employing nitrates as the starting salts and ammonium carbonate as the precipitant. Electrical conductivity of the ceramics, measured by the dc three-point impedance method, shows a V-shape curve against the average grain size. The sample with the smallest grain size of 0.12 μm exhibits a high conductivity of ∼10^−1.74 S/cm at the measurement temperature of 700 °C, which is about the same conduction level of the micro-grained 10 mol% Sm₂O₃- or Gd₂O₃-doped CeO₂, two leading electrolyte materials.