Effects of cobalt doping on the electrical characteristics of Al-doped ZnO varistors

The effects of cobalt additive on the electrical characteristics of the Al-doped ZnO varistors are studied in this paper. The current-voltage characteristics of the varistor samples have been investigated in a range from small to large current. With the amount of the doped cobalt increased, the leakage currents of the Al-doped ZnO varistors are inhibited, and their nonlinear coefficients increase remarkably as well. In addition, their breakdown electric fields increase in a small extent and their residual voltage ratios change slightly. Furthermore, the donor densities and the barrier heights of samples decrease with the cobalt content increased. The XRD patterns show that the Bi-rich phase and the willemite phase vary obviously in the varistor samples with various content of cobalt additive.     

Effect of ZnO doping on surface and catalytic properties of manganese oxides supported on alumina

The effect of ZnO doping (1.42–12.58 wt.%) on surface and physicochemical properties of manganese oxides supported on alumina has been investigated using nitrogen adsorption at −196 °C and catalytic decomposition of H₂O₂ reaction at 30–50 °C. The pure and doped solids were subjected to thermal treatment at 400–1000 °C. The thermal products were characterized via X-ray diffraction (XRD)-technique. The results revealed that the specific surface areas of various samples decreased monotonically as a function of both the extent of zinc oxide doping and the precalcination temperature. Moreover, the activation energy of sintering, ΔE_s, was found to decrease by increasing the concentration of ZnO to 5.44 wt.% and increased with increasing the amount of dopant to 12.58 wt.%. The pure and doped mixed solids preheated at 400 °C produced crystalline β-MnO₂ phase and its pattern intensity increased with increasing ZnO concentration up to 5.44 wt.%. Crystalline Mn₂O₃ phase was detected at 600 and 800 °C and its pattern intensity decreased with increasing the dopant concentration. Calcination of samples at 1000 °C showed the formation of crystalline γ-Al₂O₃, Mn₃O₄, ZnAl₂O₄, and ZnMn₂O₄ phases. The catalytic activities of the pure and doped mixed solids increased with increasing the reaction temperature and calcination temperature up to 600 °C. The ZnO doping led to the decrease of the catalytic activity of manganese oxides supported on alumina system. No measurable activity was observed for samples precalcined at 1000 °C because of formation of inactive compounds and/or sintering process. The changes in the catalytic activity of pure and doped solids were attributed to the change in oxidation states.     

Effects of cobalt doping on the electrical properties of MBE-grown ZnO

We investigated n-conducting Co-doped ZnO epilayers grown by MBE with a cobalt content of nominal 4, 12, and 18 at.% by means of deep level transient spectroscopy (DLTS) and resistivity measurements. We found that in materials grown at the same substrate temperature, 500 °C, the resistivity decreases with the cobalt mole fraction. The free electron concentration accounts for most of the change in the resistivity. The DLTS measurements reveal the presence of a defect center located at E_c−0.5 eV, which is (1) metastable; (2) sensitive to annealing in oxygen-free ambient, and (3) decreasing in density in the samples with higher cobalt content. We therefore assign the center to the isolated oxygen vacancy. Furthermore, our results support the hypothesis that ferromagnetism in ZnO:Co is mediated by Co-oxygen vacancy pairs.     

Ni doping effect on electrical conductivity of ZnO nanocrystalline thin films

The electrical conductivity behavior of undoped and Ni-doped ZnO nanocrystalline thin films prepared by spin-coating method was investigated as a function of temperature. The films were found to have polycrystalline structure. Grain size and the conductivity of the films were found to decrease significantly with increase in Ni concentration. This behavior was well explained by the grain boundary conduction model that takes into account electron trapping in surface states. It was observed that by increasing the Ni-doping level the surface trap density increases and implicitly the conductivity decreases.     

The influences of bismuth antimony additives and cobalt manganese dopants on the electrical properties of ZnO-based varistors

This paper investigates the influences of additive contents and the additive ratio to dopants on the electrical characteristics of ZnO-based varistors. Bismuth and antimony are chosen as the additives while cobalt and manganese are selected as the dopants in this study. Our previous works discussed the influences of the initial additive content on the electrical characteristics of ZnO-based varistors without considering the dopant content and the weight loss during processing and sintering. Therefore, in this study, we fabricated varistors with same initial formula after sintering for 1, 3 and 5 h, respectively. The sintering temperatures were 950 and 1100 °C. After sintering, the additive content and dopant content of the varistors were measured using an inductively coupled plasma-atomic emission spectrometer (ICP). The experimental results showed that when the additive-content varies from 1.44 to 3.59 at % and the dopant/additive ratio changes from 0.16 to 0.69, the nonlinear coefficient, α, reaches up to 48 and the breakdown field E_bk is to 895 V mm⁻¹. The average grain size is 2.7 μm. The α value is higher with the higher additive-content and the lower dopant/additive ratio and vice versa. The breakdown field E_bk is increased with the additive-content increasing and sintering temperature lowering at a given dopant-content. The grain size is increased with the increase in sintering temperature while the bismuth concentration decreased. The observed effects are related to the quality of grain boundaries and the conductivity of grains.     

Aqueous ferrofluids based on manganese and cobalt ferrites

Synthesis of two new aqueous ferrofluids is performed chemically according to Massart's procedure. Manganese and cobalt ferrite magnetic particles are precipitated and treated in order to obtain colloidal sols by creating a charge density on their surface. Such ionic ferrofluids can be prepared in an acidic (after a treatment by ferric nitrate) or in an alkaline medium at a concentration as high as 17moll^–1, i.e. a volumic fraction of 0.26. This result makes these new compounds of great interest.     

Effect of doping on structural and optical properties of ZnO nanoparticles: study of antibacterial properties

Sol-gel method was successfully used for synthesis of ZnO nanoparticles doped with 10 % Mg or Cu. The structure, morphology and optical properties of the prepared nanoparticles were studied as a function of doping content. The synthesized ZnO:(Mg/Cu) samples were characterized using XRD, TEM, FTIR and UV-Vis spectroscopy techniques. The samples show hexagonal wurtzite structure, and the phase segregation takes place for Cu doping. Optical studies revealed that Mg doping increases the energy band gap while Cu incorporation results in decrease of the band gap. The antibacterial activities of the nanoparticles were tested against Escherichia coli (Gram negative bacteria) cultures. It was found that both pure and doped ZnO nanosuspensions show good antibacterial activity which increases with copper doping, and slightly decreases with adding Mg.     

Effect of GaN doping on ZnO films by pulsed laser deposition

The present study reveals the codoping of Ga and N into ZnO films on sapphire substrates by pulsed laser deposition (PLD) with GaN doped ZnO targets. The glow discharge mass spectroscopy (GDMS) spectra confirm the presence of Ga and N in the doped films. The XRD measurements show that for GaN concentration up to 0.8 mol%, the full width at half maximum (FWHM) is almost unchanged thereby maintaining the crystallinity of the films, while for 1 mol% the FWHM increases. The PL spectra only show the strong near band edge (NBE) emission, whereas the deep level emissions are almost undetectable, indicating that they have been considerably suppressed. Our Hall measurements indicate that all the GaN doped ZnO films are of n-type. However, as the GaN concentration is greater than 0.6 mol%, the film shows a decrease in the carrier concentration, suggesting that N acceptors are not sufficient to compensate the native donor defects.     

Effect of Mn doping on electrical properties and accelerated ageing behaviours of ternary ZVM varistors

The electrical properties and d.c. accelerated ageing behaviour of ZVM (Zn-V-Mn) ceramics were investigated with different valences and contents of Mn. The incorporation of Mn into the ZV ceramics was found to restrict the abnormal grain growth of ZnO. The nonlinear properties and stability against the d.c. accelerated ageing stress are significantly affected by different valences and contents of Mn. The high valence of Mn (Mn⁴⁺) in the ZVM ceramics resulted in better nonlinear properties than low valence of Mn (Mn^2·66+). Furthermore, an increase in doping level of MnO² greatly improved its nonlinear properties. The ZVM ceramics doped with 2 mol% MnO² exhibited not only a high nonlinearity, in which the nonlinear coefficient is 27 and the leakage current density is 0·042 mA/cm², but also a good stability, in which %ΔE _{1 mA} = ?2·1%, %Δα = ?25·9% for the d.c. accelerated ageing stress of 0·85 E _{1 mA}/85°C/24 h.     

Effect of microwave sintering on the microstructure and electrical properties of low-voltage ZnO varistors

Coarse-grained ZnO varistors for low-voltage applications were prepared by microwave sintering technique under different soaking times of 5–150?min. For comparison, a low-voltage ZnO varistor was also prepared through a conventional sintering process. Microwave sintering remarkably enhanced the grain growth rate of ZnO varistors. Average grain size of the sample prepared by microwave sintering in 15?min was about 20?µm, which is similar to the grain size of sample prepared conventionally in 150?min time. In addition to grain growth, an increase in microwave sintering time led to precipitation of zinc titanate (Zn₂TiO₄) on the top surface of samples which sintered for long dwell times. X-ray diffraction and scanning electron microscopy results from different points of the samples declared that precipitation of Zn₂TiO₄ phase is due to the high rate of bismuth evaporation of Bi-rich liquid from top surface and the reaction between remaining titanium ions on the surface with ZnO. The results showed that increasing sintering time from 5 to 150?min increased the grain size from 14 to 33?µm, consequently, the breakdown field decreased from 90 to 27?V/mm, respectively. These changes led to a switch in the varistor application, from low to very low voltage.     

Effect of aluminum doping on electrical and dielectric aging behavior against current impulse of ZPCCY-based varistors

The electrical and dielectric aging behavior against current impulse (5–1,200 A) in the Zn-Pr-Co-Cr-Y-based varistors was investigated with aluminum doping level (0–0.01 mol%). The varistors doped with 0–0.001 mol% Al were destroyed at higher current impulse beyond 900 A and the varistors doped with 0.005–0.01 mol% Al exhibited high stability against current impulse. The clamp ratio (K) at given current impulse ranges decreased with increasing Al doping level. The varistor doped with 0.01 mol% Al exhibited the lowest K value, with 1.65 at a current impulse of 10 A and 2.38 at a current impulse of 1,200 A. The best electrical and dielectric stability against current impulse of 1,200 A was obtained at 0.01 mol% Al, where %\Updelta E_{1  \textmA/cm²} = -3.4%\%\Updelta E_{1\;\text{mA/cm}^2} = -3.4\%, %Δα = 0%, %ΔJ _L = −26.3%, %Δε′_APP = +3.4%, and %Δtanδ = −7.7%. Conclusively, Al doping level was optimized at 0.01 mol% in terms of the surge withstand capability (SWC).     

EPR study of the influence of defect structure on electrical properties of ZnO varistors

The influence of the defect structure of ZnO grains on the electrical properties of ZnO varistors, on sintering parameters and temperature of subsequent heat treatment, was studied. By analysis of the Mn²⁺ spectrum obtained by the electron paramagnetic resonance technique, the dependence of electrical properties of ZnO varistors on the defect structure of ZnO grains and impurity ions, was shown.     

HCI气相掺杂对聚苯胺电导率影响规律的探讨

采用气相掺杂这种新的掺杂方法对低电导率的导电聚苯胺进行再掺杂,气相掺杂30min的聚苯胺的电导率约为未掺杂导电聚苯胺的4倍,并对其结构进行红外表征,探讨了电导率发生变化的内在机理。该法简便易操作,不引入杂质,不污染环境,是一种很有发展前景的新方法。     

Influence of cobalt doping on the crystalline structure, optical and mechanical properties of ZnO thin films

Uniform and transparent thin films of Zn_1 − xCo_xO (0 ≤ x ≤ 0.10) were fabricated by sol-gel spin coating technique. Co addition up to x = 0.075, led to refinement in structure and improvement in film quality together with average grain size reduction from 17 nm in undoped ZnO to 15 nm with x = 0.05 and 12 nm with x = 0.10 Co additions. For x ≥ 0.035, CoO (cubic) was detected as the secondary phase. Influence of Co addition on the volume fraction of grain boundaries has been interpreted. Increase in Co content in the range 0 ≤ x ≤ 0.10 led to quenching of near-band edge and blue emissions, decrease in band gap energy (E_g) from 3.36 eV to 3.26 eV, decrease in film thickness and refractive index and an increase in extinction coefficient of Zn_1 − xCo_xO thin films. The change in nature of stress from compressive to tensile with lower to higher doping of Co is corroborative with the angular peak shift of (002) plane of ZnO lattice. An overall increase in microhardness of Zn_1 − xCo_xO thin films up to x = 0.05 is attributed to change in microstructure and evolution of secondary phase and as the secondary phase separates out the overall stress is released leading to lowering of hardness after this concentration. Hall-Petch behavior is also studied and found to obey until x = 0.05, however, considerable deviation after this dopant concentration is attributed to the increase in the volume fraction of grain boundaries, which results from the secondary phase separation from this dopant concentration.     

The influence of dislocations on the nonlinearity of ZnO:Cu varistors

Zinc Oxide doped only with Cu shows highly nonlinear I–V characteristics. Microstructural observations of these ceramics reveal the presence of extensive dislocation network. The transmission electron microscopy (TEM) indicates that the dislocations are impurity decorated which arise as a result of limited solubility of CuO in ZnO. It is envisaged that the depletion region is generated in the region containing the dislocations because of the presence of acceptor type traps.     

(Nb、Ce、Si、Ca)掺杂对TiO2压敏电阻陶瓷电性能的影响

采用正交实验方法研究了掺杂Nb2O5、CeO2、SiO2、CaCO3等对TiO2压敏电阻陶瓷电性能的影响.利用XRD及SEM实验方法分析了样品的物相和微观形貌,发现有第二相产生.这种新的第二相的存在,对压敏电压有明显的影响.优选典型配方制得样品的主要电性能指标为V1mA=10～20V,V10mA=20～30V,α=3～5,电容C=50～90nF,损耗tgδ＜0.8.     

Effect of Tm doping on the properties of electrodeposited ZnO nanorods

ZnO nanorods were fabricated by electrodeposition from solution with and without thulium precursors and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence and time-resolved photoluminescence. In spite of the low incorporation of thulium into ZnO nanorods, both the morphology and the optical properties of the nanorods were affected by the presence of thulium. The light emitting diodes with ZnO nanorods with and without Tm have been demonstrated.     

Effect of cobalt doping on the phase transformation of TiO2 nanoparticles

Co-doped TiO2 nanoparticles containing 0.0085, 0.017, 0.0255, 0.034, and 0.085 mol % Co(III) ion dopant were synthesized via sol-gel and dip-coating techniques. The effects of metal ion doping on the transformation of anatase to the rutile phase have been investigated. Several analytical tools, such as X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray analysis (EDAX) were used to investigate the nanoparticle structure, size distribution, and composition. Results obtained revealed that the rutile to anatase concentration ratio increases with increase of the cobalt dopant concentration and annealing temperature. The typical composition of Co-doped TiO2 was Ti(1-x)Co(x)O2, where x values ranged from 0.0085 to 0.085. The activation energy for the phase transformation from anatase to rutile was measured to be 229, 222, 211, and 195 kJ/mole for 0.0085, 0.017, 0.0255, and 0.034 mol % Co in TiO2, respectively.