Effects of sizes of additive particles on suspensions,microstructures, and electrical properties of ZnO varistors

The effects of the sizes of the additives on the dispersion of the mixed suspensions, the microstructures, and the comprehensive electrical properties of the ZnO varistors were investigated. The particle size distribution, the viscosity, and the zeta potential of the suspension were characterized to evaluate the effect of the sizes of the additives on the dispersion of the metal oxide particles. The potential gradient, the leakage current, the nonlinear coefficient, the voltage ratio, and the aging coefficient were considered to estimate the effect of the sizes of the additives on the performance of the ZnO varistors. The electrical testing results showed that a proper amount of milling of the additive particles could improve the comprehensive electrical properties, while an excessive milling produced the opposite effect. When the milling time of the additive particles reached 30 minutes, the dispersion of ZnO-additive mixed suspension was the best. The as-prepared varistors showed the optimal electrical performance with potential gradient of 310 V/mm, leakage current of 1 μA, nonlinear coefficient of 32.7, voltage ratio of 1.69, and aging coefficient of 0.59.     

Preparation,sintering and electrical properties of nano-grained multidoped ceria

Multiply doped ceria nanopowders were synthesized by applying MGNP (modified glycine/nitrate procedure). The overall concentration of dopants was kept constant (x = 0.2) whereby Gd ion as the main dopant was gradually substituted by Sm and by Sm + Y. The compositions of solid solutions were calculated by applying defect model introducing anion vacancy radius. Characterization of powders involved BET, TEM, XRD and chemical analyses. Densification was performed at 1500 °C, in an oxygen atmosphere for 1 h. The results showed that with increasing number of dopants, specific surface area of powders increased, followed by decrease of crystallite and grain sizes. Densification degree was also found to rise with increasing number of dopants. According to impedance measurements it was found that ionic conductivity was the highest 1.14 × 10^?3 S cm^?1 at 450 °C in sample doped with Gd, Sm and Y simultaneously.     

Synthesis,sintering and electrical properties of yttria–calcia-doped ceria

Crystalline yttria and calcia doped ceria powder, with a composition of Ce_0.8Y_0.18Ca_0.02O_2?δ has been prepared by a coprecipitation procedure from the corresponding nitrates of component cations. Nanopowder was obtained after thermal treatment at 700 °C 2 h of the coprecipitated mixtures. Specific surface area was 45 m²/g. Isostatically and uniaxially pressed pellets were prepared from the powder. Sintering behaviour was followed by CHR dilatometer. Isothermal sintering was carried out between 1100 and 1300 °C. Apparent density as high as 98% D_th was attained by firing isostatically pressed pellets at 1150 °C 4 h. Uniaxially pressed pellets attained the same apparent density at 1275 °C 2 h, being in both cases very low the densification temperatures. Microstructure was observed by scanning electron microscopy (SEM). Ionic conductivity was determined by complex impedance spectroscopy. Bulk and grain boundary conductivities have similar values, and the total conductivity attains good value compatible with the use as electrolyte in solid oxide fuel cell (SOFC).     

Rheological,electrical, and microwave properties of polymers with nanosized carbon particles

The rheology, dc‐conductivity, and microwave properties of acrylic, polyurethane, and epoxy composites containing 0–15 vol % of nanosized carbon particles have been investigated. Carbon nanoparticles (1–3 nm) are produced by a shock wave technology. Steady‐state shear and oscillatory flow tests are applied to investigate the rheological properties of dispersions; dc‐conductivity and MW absorption/reflection are investigated for solid composite films. Rheological characteristics are used for the evaluation of agglomeration processes of nanoparticles in dispersions, as controlled by volume fraction and processing technology. The percolation threshold is interpreted as a structural transition from a dispersed to an agglomerated state and it is found to depend significantly on the type of the matrix polymer. Above the percolation threshold, the presence of carbon nanoparticles produces a strong increase in the viscosity of dispersions as well as of the electrical conductivity and microwave properties of solid composites. A good correlation between the three characteristics is found for the systems in a wide range of carbon volume fractions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2220–2227, 2004     

Grain size effect on the electrical properties of nanocrystalline ceria

The total and partial electronic conductivities of gadolinium doped ceria (Ce_0.95Gd_0.1O_1.95−δ: GDC) with nanometer grain size have been evaluated in an attempt to identify the nanosize effect in heavily doped ceria. Nanocrystalline GDC bulk specimens with relatively high densities (≥96% of theoretical density) and various grain sizes (70, 100, 170 nm) were successfully fabricated by a conventional solid-state sintering method. According to the measurements of total and partial electronic conductivity via AC-impedance and DC polarization methods, respectively, no significant grain size dependence appeared for either type of conductivity. Furthermore, both total and partial electronic conductivity were not significantly different from those of microcrystalline GDC, which indicated that, upon nanostructuring within the examined grain size range, nanostructured bulk GDC was not affected by any nanosize effect: either space charge layer effect or grain boundary blocking effect.     

Mechanical,electrical properties and microstructures of hot-pressed B4C-WB2 composites

In this paper, dense B₄C-WB₂ composites were fabricated at 1950 °C using B₄C and WB₂ as raw materials via a hot press method. The phase composition, microstructures and mechanical properties of the B₄C-WB₂ composites with different B₄C volume fraction were studied. The obtained 68.7 vol%B₄C-WB₂ composites demonstrated good comprehensive properties with high flexural strength of 696 MPa, superior hardness of 34.8 GPa, and acceptable fracture toughness of 3.3 MPa m^1/2. The high flexural strength mainly resulted from the pinning effect of preferentially oriented strip-shape WB₂ grains and clean grain interfaces between B₄C and WB₂ phases. The toughening mechanism of the B₄C-WB₂ composites was associated with the interfacial residual stress induced by the mismatch of thermal expansion coefficient. In addition, the B₄C-WB₂ composites demonstrated good electrical conductivity (3.3 × 10⁵ S/m) with a low density of 5.589 g/cm³, making them of potential interest for cutting tools and armor protection applications.     

Sintering,microstructural development,and electrical properties of gadolinia-doped ceria electrolyte with bismuth oxide as a sintering aid

A considerable reduction (≥250 °C) in the sintering temperature, enhancement of the sintering density, and a slight improvement of the electrical properties, can be achieved by using bismuth oxide in the range of 0.2 to 2 wt.%, as a sintering aid for gadolinia-doped ceria (GDC) ceramic electrolytes. Dilatometric experiment (CHR) and SEM observations indicate that a liquid phase-assisting sintering mechanism contributes to the improvement in sintering density for bismuth oxide concentrations exceeding 0.5 wt.%. The addition of small amount of Bi₂O₃ ≤0.5 wt.% also results in the achievement of highly dense ceramic bodies (≥99% of theoretical density) after sintering at 1200 °C for 4 h, which indicates that the addition of Bi₂O₃ to gadolinia-doped ceria promoted the sintering process by a cooperating volume diffusion-liquid phase-assisting mechanism. Based on the lattice constant data, the solid solubility limit of Bi₂O₃ in gadolinia-ceria is, probably, lower than 1.0 wt.%. Grain size also increased with increasing Bi₂O₃ content up to 0.5 wt.% and then it decreased with further addition of Bi₂O₃. The addition of the smaller amounts of bismuth oxide, i.e., ≤1.0 wt.% Bi₂O₃ slightly enhanced the total ionic electrical conductivity of the gadolinia-doped ceria electrolyte. The sintering temperature strongly influenced the electrical conductivity of the doped-GDC ceramics. The best sample was that containing 1.0 wt.% Bi₂O₃ sintered at 1400 °C for 2 h which had an ionic electrical conductivity of 4 S m⁻¹ at 700 °C, and an activation energy of 0.58 eV for the oxide-ion conduction process in air.     

Preparation,characterization, and electrical properties of ZSM‐5/PEG composite particles

ZSM‐5/PEG composites were synthesized by a simple solution method with polyethylene glycol (PEG) and H‐ZSM‐5 zeolite (Si/Al = 11.4). The obtained composites were characterized using X‐ray powder diffraction and Fourier transform infrared spectroscopy. The obtained results indicated that the ZSM‐5 was physically combined with PEG. The thermal properties and thermal stability were investigated by thermogravimetric and differential thermal analyses. In situ electrical conductivity was used to follow‐up the changes in the electrical conductance during the heating of the ZSM‐5/PEG composite. It was found that ZSM‐5 is able to effectively enhance the electrical conductivity of PEG. The results showed that the obtained weight loss during the composite decomposition to charcoal is accompanied by a decrease in the electrical conductivity. Moreover, the removal of the formed charcoal is associated with an electrical conductivity increase. Calcining the ZSM‐5/PEG composite having a content of 30% results in many effects on the structural, textural, and electrical properties of the obtained products. POLYM. COMPOS., 35:1160–1168, 2014. © 2013 Society of Plastics Engineers     

Sm-doped CaTiO3 phosphor: Synthesis, structure, and photoluminescent properties

Photoluminescent properties of samarium-doped calcium titanate for near ultra-violet excitation were studied. CaTiO₃:Sm³⁺ phosphor was synthesized by using the solid-state reaction method. The structure and properties of the phosphor were characterized by using X-ray diffractometer, scanning electron microscope, UV-visible spectrophotometer, high-resolution secondary ion mass spectrometer, and X-ray photoelectron spectrometer. The photoluminescent properties were studied by taking excitation and emission spectra. A strong red-orange luminescence corresponding to ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺ for near ultra-violet excitation was observed. It was found that CaTiO₃:Sm³⁺ was a red-orange emitting phosphor and had higher efficiency for the operation with near ultra-violet excitation.     

Influence of a Ni interlayer on the optical and electrical properties of trilayer GZO/Ni/GZO films

Trilayer GZO/Ni/GZO films were deposited onto polycarbonate (PC) substrates with RF and DC magnetron sputtering, and then the influence of a Ni interlayer on the optical and electrical properties of the films was investigated. A 2-nm-thick Ni interlayer decreased the resistivity to 6.4×10⁻⁴ Ω cm and influenced the optical transmittance.Although optical transmittance deteriorated with Ni insertion, the films showed a relatively high optical transmittance of 74.5% in the visible wavelength region. The figure of merit (FOM) of a GZO single layer film was 1.2×10⁻⁴ Ω⁻¹, while that of the GZO/Ni/GZO films reached a maximum of 8.2×10⁻⁴ Ω⁻¹.Since a higher FOM results in higher quality transparent-conductive oxide (TCO) films, it is concluded that GZO films with a 2 nm Ni interlayer have better optoelectrical performance than single-layer GZO films.     

Bipolar charging and neutralization of nanometer-sized aerosol particles

An experimental study of charging and neutralization of 2.5–10 nm aerosol particles by bipolar air ions has been carried out. The time required to attain the charge equilibrium state, both for charging of initially uncharged particles and for neutralization of charged particles, has been previously determined using different-sized diffusive type chargers. To avoid sizing errors caused by space charge field and Brownian diffusion effects, a tandem DMA (differential mobility analyzer) system has been used. The first DMA classifies a certain fraction of particles, and the second one allows precise determination of their size. The experimental charging probabilities at equilibrium are in relatively good agreement with calculations based on Fuchs' theory, except in the particle size range below 3 nm where large deviations have been observed.     

复合二氧化硅纳米颗粒的制备及在生物分析方面的应用

描述了二氧化硅纳米颗粒的3种常见制备方法,并对其在生物分析方面的应用如生物传感器、DNA/RNA检测和收集、基因送递等进行了讨论。     

稀土Sm掺杂棒花状ZnO的制备及其光催化性能研究

由硝酸锌、硝酸钐、十二烷基硫酸钠制备Sm掺杂棒花状纳米氧化锌,考察制备温度、分散剂浓度、Sm掺杂等因素对氧化锌形貌和光催化性能的影响,采用X射线衍射仪(XRD)、场发射扫描电子显微镜(FESEM)、能量色散X射线谱(EDS)、X射线光电子能谱(XPS)进行表征。结果表明,制备温度、分散剂浓度、Sm掺杂量等对棒花状纳米氧化锌的形貌和光催化性能有显著的影响。在60℃下,分散剂浓度为28 mmol/L、Sm掺杂量为5%时,所制备的棒花状纳米氧化锌的形貌规整且光催化性能最好。     

Tri-reforming of surrogate biogas over Ni/Mg/ceria–zirconia/alumina pellet catalysts

The performance of catalytic tri-reforming under industrially relevant situations (e.g., pellet catalysts, pressurized reactor) was investigated using surrogate biogas as the feedstock. Tri-reforming using Ni/Mg/Ce_0.6Zr_0.4O₂/Al₂O₃ pellet catalysts was studied in a bench scale fixed-bed reactor. The feed molar ratio for CH₄:CO₂:air was fixed as 1.0:0.70:0.95. The effects of temperature (800–860°C), pressure (1–6?bar), and H₂O/CH₄ molar feed ratio (0.23–0.65) were examined. Pressure has substantial impact on the reaction and transport rates and equilibrium conversions, making it a key variable. At 860°C, CO₂ conversion increased from 4 to 61% and H₂/CO molar ratio decreased from 2.0 to 1.1 as the pressure changed from 1 to 6?bar. CO₂ conversion and H₂/CO molar ratio were also influenced by the temperature and H₂O/CH₄ molar ratio. At 3?bar, CO₂ conversion varied between 4 and 43% and the H₂/CO molar ratio varied between 1.2 and 1.9 as the temperature changed from 800 to 860°C. At 3?bar and 860°C, CO₂ conversion decreased from 35 to 8% and H₂/CO molar ratio increased from 1.7 to 2.4 when the H₂O/CH₄ molar ratio was increased from 0.23 to 0.65. This work demonstrates that the tri-reforming technology is feasible for converting biogas under scaled-up conditions in a fixed-bed reactor.     

Durability and thermal cycling of Ni/YSZ cermet anodes for solid oxide fuel cells

The long-term properties of Ni/yttria stabilized zirconia (YSZ) cermet anodes for solid oxide fuel cells were evaluated experimentally. A total of 13 anodes of three types based on two commercial NiO powders were examined. The durability was evaluated at temperatures of 850 C, 1000 C and 1050 C over 1300 to 2000h at an anodic d.c. load of 300mA cm^–2 in hydrogen with 1 to 3% water. The anode-related polarization resistance, R _P, was measured by impedance spectroscopy and found to be in the range of 0.05 to 0.7 cm². After an initial stabilization period of up to 300h, R _P varied linearly with time within the experimental uncertainty. At 1050 C no degradation was observed. At 1000 C a degradation rate of 10 m cm² per 1000 h was found. The degradation rate was possibly higher at 850 C. A single anode was exposed to nine thermal cycles from 1000 to below 100 C at 100 C h^–1. An increase in R _P of about 30m cm² was observed over the first two cycles. For the following thermal cycles R _P was stable within the experimental uncertainty.     

Effect of Ni substitution on electrical and thermoelectric properties of LaCoO3 ceramics

LaCo_1−xNi_xO₃ (0 ≤ x ≤ 0.2) ceramics were prepared by solid state reaction and their thermoelectric properties were investigated from room temperature (RT) to 400 °C. In the range from RT to 180 °C, LaCoO₃ showed a large negative Seebeck coefficient, but it changed to a positive value above 180 °C. However, the Seebeck coefficient became positive in the whole investigated temperature span due to Ni substitution for Co even for a tiny amount, but its absolute value decreased significantly with increasing Ni content. The LaCo_0.9Ni_0.1O₃ composition showed an enhanced power factor with a maximum value of 1.41 × 10⁻⁴ W m⁻¹ K⁻² at room temperature, which is about 3.5 times higher than that of un-doped LaCoO₃. Because the power factor decreased and the thermal conductivity increased apparently with temperature, the ZT values were not increased at elevated temperatures, in spite of a relatively large ZT value of 0.031 at a low temperature (50 °C) obtained in the composition LaCo_0.9Ni_0.1O₃.     

Chloride-induced modifications of the properties of rhodia/ceria catalysts

A RhO_x/CeO₂ catalyst, prepared using RhCl₃, is studied by EPR and IR spectroscopies, comparing the results with those obtained for Cl-free RhO_x/CeO₂. The substitution of basic hydroxyls by chlorides produces an extensive modification of the ceria surface, describable as formation of a cerium oxychloride surface layer, which improves the dispersion of rhodia. The more covalent character of Ce⁴⁺–O ₂ ⁻ bonds formed on the surface, the absence of ceria–rhodia electron transfer and the stability of the surface against reoxidation are interpreted in terms of a Cl-induced lowering of the cerium electron energy levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanical,tribological and electrical properties of ZrB2 reinforced Cu processed via milling and high-pressure hot pressing

The present work investigates the effect of (0–10 wt%) ZrB₂ reinforcement on densification, mechanical, tribological and electrical properties of Cu. The consolidation of Cu–ZrB₂ samples was carried out using a hot press (temperature: 500 °C, pressure: 500 MPa, time: 30 min, vacuum pressure: 1.3 × 10^-2 mbar). The bulk density of the hot-pressed Cu composites decreased from 8.84 g/cc to 8.16 g/cc and the relative density of samples lowered from 98.6% to 92.1% with the addition of ZrB₂. The incorporation of hard ZrB₂ (up to 10 wt%) improved the hardness of Cu (1.32–2.55 GPa). However, the yield strength and compressive strength of Cu composites increased up to 5 wt% ZrB₂, and further addition of ZrB₂ lowered its strength. The yield strength of Cu samples varied from 602 to 672 MPa and the compressive strength between ~834 and 971 MPa. On the other hand, the coefficient of friction (COF) (from 0.49 to 0.18) and wear rate (from 49.3 × 10^-3 mm³/Nm to 9.1 × 10^-3 mm³/Nm) of Cu–ZrB₂ samples considerably decreased with the addition of ZrB₂. Significantly low wear was observed with Cu-10 wt% ZrB₂ (Cu-10Z) samples, which is 5.41 times less than pure Cu. As far as the wear mechanisms are concerned, in pure Cu, continuous chips (wear debris) were formed during sliding wear by plowing. Whereas the major amount of material loss was occurred due to the plowing mechanism with discontinuous and short chip formation for Cu–ZrB₂ composites. The electrical conductivity of Cu–ZrB₂ samples decreased from 75.7% IACS to 44.1% IACS. In particular, Cu with ZrB₂ (up to 3 wt%) could retain the conductivity of 66.8% IACS. This study reveals that the addition of ZrB₂ (up to 3 wt%) is advantageous to have a good combination of properties for Cu.     

Composite polypyrrole-containing particles and electrical properties of thin films prepared therefrom

Preparation of core-shell particles consisting of polystyrene-poly(ethylene glycol) monomethacrylate (PS-PEGMA) core covered with polypyrrole (PPy) shell is described. The thickness of PPy shell, which strongly influences electrical properties of the films prepared from the particles, can be varied by changing pyrrole load, controlling the overall template surface area in the system and by influencing the pyrrole polymerization kinetics in the presence of different oxidants. The type of anions and PPy loading strongly influence the electrical conductivity. Typical value of the resistivity of thin film consisting of core-shell particles was 34 Ωm (PPy oxidized by FeCl₃, shell thickness 3 nm). Current-voltage dependences of low conductivity samples (thin PPy shell layer) are characteristic of contact-limited currents. The conductivity of the particles changes with humidity, which can be utilized in humidity sensors.