Production of doped ZnO powders for varistor applications using sol-gel techniques

The electrical behaviour of ZnO varistors is controlled by the characteristics of the ceramic microstructure which depends strongly on the properties of the initial powder. This paper describes a study of the effect of different chemical methods to synthesise doped ZnO powders. The ceramic powders were synthesised by the following routes: a) classical mixing of oxides (for comparison purposes), b) aqueous solution of inorganic salts and c) hydrolysis and polycondensation of metalorganic compounds in an organic solvent. The physicochemical characteristics of these powders were evaluated using a scanning electron microscope and thermoanalytical instrumentation. Standard spray drying technology was used to pelletise the powders to obtain an agglomerated powder suitable for uniaxial pressing. The discs were sintered in an electric furnace under air atmosphere using several temperature programmes. The ceramic microstructure was characterised using SEM and X-ray diffraction. The effects of powder processing route on sintering and microstructural development are discussed. Powders prepared by the metalorganic route exhibited somewhat lower sintering temperatures than conventional powders. However, the rate of sintering was slower for metalorganic and aqueous solution powders. These observations were related to powder morphology. The ZnO–Bi₂O₃–CoO system exhibited the best varistor characteristics as it was expected, whereas the binary systems supported much lower voltages at low currents than ternary systems.     

Ageing of wet-synthesized oxide powders

Wet chemical synthesis of precursor oxide ceramics is a method to obtain small particulate powders. Such powders are far more prone to ageing in air than more traditional precursors. Thermogravimetric analysis is used to highlight the species responsible for the ageing of ceramic precursors. Indeed water and carbon dioxide are observed to evolve from aged powders. Ceramics obtained from aged precursors can reach a very low final density with respect to the theoretical value. A large degree of the original sintering properties can be recovered after washing the aged powders with ethanol in a basic medium.     

Granulating titania powder by colloidal route using polyelectrolytes

A new, convenient, and inexpensive approach to process and granulate titania powders by a chemical route is proposed. It is based on the use of a formulation that includes a polyanion such as poly(sodium 4-styrenesulfonate) (PSS). Such a polyelectrolyte is most often considered to achieve dispersion of oxide powders in water. Basically, it adsorbs onto the surface of particles and induces electrical and/or steric interactions between particles in the suspension, which prevents agglomeration and rapid sedimentation. The advantages of polyelectrolytes in ceramic processing is well documented in the literature to produce low viscosity suspensions that are further used to form ceramic parts. In the case of TiO2 powders, such aqueous dispersions were obtained by adding small quantities of PSS. However, when exploring the behavior of mixtures containing lower contents of dispersant, we have discovered that, well below the optimum concentration required to get stable dispersions, the polyelectrolyte can act as a binder for titania particles. This can confer cohesion to the agglomerates, which can be processed to form large size (e.g., millimeter size) spheres. This phenomenon takes place when the oxide surface carries both positive and negative electrical charges and can be explained on a simple basis involving surface chemistry. For the optimum concentration of PSS that disperses titania, a polycation such as chitosan should be added to get spheres. This simple technique is expected to receive increasing attention due its potentialities and strong advantages with respect to other granulation techniques, such as spray-drying, which are energy consuming.     

Generation of Complex Metal Oxides by Aerosol Processes: Superconducting Ceramic Particles and Films

Complex metal oxides, including ferroelectric, ferrimagnetic and superconducting ceramics, have a variety of technologically useful properties that can be exploited for a number of applications. Fabrication of complex metal oxides ceramics with specific properties reqires high-purity powders with controlled chemical compositions, size distributions, and morphologies. Powders with these characteristics can be produced by aerosol processes in which fine particles are generated in gaseous flow systems. The particles can also be deposited from the gas phase onto surfaces to form films. This paper will discuss the use of aerosol processes for the generation of complex metal oxide powders. A review of aerosol processes will be presented first, followed by a discussion of the methods used for the generation of superconducting ceramic powders. Examples include the production of YBa₂Cu₃O₇, La_1.85Sr_0.15CuO₄, and Tl-Ca-Ba-Cu-O powders and films. Emphasis will be placed on defining the conditions required for the generation of chemically homogeneous particles with controlled morphologies.     

Effects of different compositions from magnetic and nonmagnetic dopants on structural and electrical properties of ZnO nanoparticles-based varistor ceramics

In the current study, 20 nm zinc oxide (ZnO) nanoparticles were used to manufacture high-density ZnO discs doped with Mn and Sn via the conventional ceramic processing method, and their properties were characterized. Results show that the dopants were found to have significant effects on the ZnO varistors, especially on the shape and size of grains, which are significantly different for both dopants. The strong solid-state reaction in the varistor from the 20 nm ZnO powder during the sintering process may be attributed to the high surface area of the 20 nm ZnO nanoparticles. Although Mn and Sn do not affect the well-known peaks related to the wurtzite structure of ZnO ceramics, a few of the additional peaks could be formed at high doping content (≥2.0) due to the formation of other unknown phases during the sintering process. Both additives also significantly affect the electrical properties of the varistor, with a marked changed in the breakdown voltage from 415 V to 460 V for Sn and from 400 V to 950 V for Mn. Interestingly, the electrical behaviors of the varistors, such as breakdown voltage, nonlinear coefficient, and barrier height, are higher for Mn- than Sn-doping samples, and the opposite behaviors hold for hardness, leakage currents, and electrical conductivities. Results show that the magnetic moment and valence state of the two additive dopants are responsible for all demonstrated differences in the electrical characteristics between the two dopants.     

Synthesis of Functional Oxide Nanoparticles Through RF Thermal Plasma Processing

A method of synthesizing functional nanostructured powders through reactive thermal plasma processing has been developed. Nano-sized oxide powders, including titanium dioxide and some functional oxides, were synthesized by the oxidation of liquid precursors. Oxides with the prescribed cation ratio of the liquid precursor can be synthesized with this technique, and it is possible to precisely adjust the chemical composition, which is linked to the appropriate functions of ceramic materials. Quench gases, either injected from the shoulder of the reactor or injected counter to the plasma plume from the bottom of the reactor, were used to vary the quench rate; therefore, the particle size of the resultant powders. The experimental results are well supported by numerical analysis on the effects of quench gases on the flow pattern and temperature field of thermal plasma as well as on the trajectory and temperature history of particles. Plasma-synthesized TiO₂ nanoparticles showed phase preferences different from those synthesized by conventional wet-chemical processes. Nano-sized particles of high crystallinity and nonequilibrium chemical composition were formed in one step via reactive thermal plasma processing. The plasma-synthesized nanoparticles were spherical and hardly agglomerated, and high dispersion properties were observed, i.e., the plasma-synthesized TiO₂ nanoparticles were individually dispersed in water.     

Slurry sampling fluorination assisted electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry for the direct determination of metal impurities in aluminium oxide ceramic powders

A new analytical procedure for the direct determination of metal impurities (Cr, Cu, Fe and V) in aluminium oxide ceramic powders by slurry sampling fluorination assisted electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry (ETV-ICP-AES) is reported. A polytetrafluoroethylene (PTFE) emulsion was used as a fluorinating reagent to promote the vaporization of impurity elements in aluminium oxide ceramic powders from the graphite tube. A vaporization stage with a long ramp time and a short hold time provided the possibility of temporal analyte-matrix separation. The experimental results indicated that a 10 microL 1% m/v slurry of aluminium oxide could be destroyed and vaporized completely with 600 micrograms PTFE under the selected conditions. Two aluminium oxide ceramic powder samples were used without any additional pretreatment. Analytical results obtained by using standard addition method with aqueous standard solution were checked by comparison of the results with pneumatic nebulization (PN)-ICP-AES based on the wet-chemical decomposition and analyte-matrix separation. The limits of detection (LODs) between 0.30 microgram g-1 (Fe) and 0.08 microgram g-1 (Cu) were achieved, and, the repeatability of measurements was mainly better than 10%.     

Investigation of new chelation ion chromatography procedure to determine the surface composition of powdered metal oxide samples in the solid state

Numerous tests have been conducted on the feasibility of characterizing the surfaces of metal oxide powders using HPLC. An in-line filter housing was modified to serve as a sample chamber to replace the sample loop. A gradient pump was used to gradually increase eluent acidity to find the conditions at which the surface of a metal oxide powder began to dissolve. The theoretical masses of surface monolayers of metal oxide powders were compared with the experimentally determined masses of dissolved material thought to be from the surface to test whether surface and bulk dissolution phenomena in acidic conditions are separable and quantifiable. A set of methods was tested that could first dissolve a metal oxide sample's surface, then separate and detect analyte species by chelation ion chromatography. Surface characterization by ion chromatography could be more cost-effective than existing methods, and reveal chemical properties of the sample where existing methods only give physical composition and properties.     

Plasma synthesized nano-aluminum powders

The plasma electro-condensation process was used to synthesize nano-sized aluminum powders. Adding different chemicals modified the physical and chemical properties of these powders. To characterize the nano-sized powders, X-ray diffraction, TEM, BET analyses, and simultaneous TG/DSC analyses were performed. TG/DSC analyses revealed a dramatic degradation of the aluminum oxide layer after storage of the aluminum powder in air for a period of several months. The burning rate of the model solid propellant with nano-sized aluminum was experimentally examined. The combustion behavior of nano-sized aluminum will be presented and will be compared with the combustion behavior of the micron-sized powders.     

Preparation and characterization of KTa0.9Fe0.1O3– δ perovskite electrodes

The effect of the partial substitution of tantalum by iron on the structural and electrical properties of the KTaO₃ perovskite-type oxide powders has been studied. The powders were prepared by the standard ceramic method, and the respective structural characterization performed by X-ray powder diffraction. From the synthesized oxide samples, iron-coated electrodes were fabricated and tested as the anode for the oxygen evolution reaction in alkaline medium. An estimation of the electrode's capacity has been calculated from the charging currents and the corresponding roughness factor evaluated. The electrocatalytic activity/stability of the oxide electrodes in alkaline solutions has been analysed through the kinetic parameters. Electronic Publication     

Combinatorial synthesis of oxide powders with an autopipetting system

We describe development of a relatively simple, rapid route to produce combinatorial compositional oxide powder libraries by autopipetting of liquid precursors. This partitioning approach should apply equally well to any low viscosity, liquid precursors for the synthesis of oxide powders. A commercial autopipet is modified by fitting a plastic "mask" assembly beneath the pipet array in order to partition and direct liquids into crucibles. A series of 10 mixtures from two precursor solutions can be produced quickly. After they are dispensed, the liquid mixtures are dried in the crucibles and reacted at elevated temperatures to produce oxide powders. In the present work, the viability of the pipetting process is demonstrated by using a polymerizable complex powder synthesis method to produce (1-x) LaAlO(3) - x SrTiO(3) powders. For this system, a reaction at 900 degrees C for 2 h in air was sufficient to yield solid solutions in this system. X-ray powder diffraction was used to determine the pseudocubic unit cell lattice parameter. The linear change of lattice constant with composition confirms the compositional accuracy of the partitioning.     

Slurry sampling fluorination assisted electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry for the direct determination of metal impurities in aluminium oxide ceramic powders

A new analytical procedure for the direct determination of metal impurities (Cr, Cu, Fe and V) in aluminium oxide ceramic powders by slurry sampling fluorination assisted electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry (ETV-ICP-AES) is reported. A polytetrafluoroethylene (PTFE) emulsion was used as a fluorinating reagent to promote the vaporization of impurity elements in aluminium oxide ceramic powders from the graphite tube. A vaporization stage with a long ramp time and a short hold time provided the possibility of temporal analyte-matrix separation. The experimental results indicated that a 10 μL 1% m/v slurry of aluminium oxide could be destroyed and vaporized completely with 600 μg PTFE under the selected conditions. Two aluminium oxide ceramic powder samples were used without any additional pretreatment. Analytical results obtained by using standard addition method with aqueous standard solution were checked by comparison of the results with pneumatic nebulization (PN)-ICP-AES based on the wet-chemical decomposition and analyte-matrix separation. The limits of detection (LODs) between 0.30 μg g^–1 (Fe ) and 0.08 μg g^–1 (Cu) were achieved, and, the repeatability of measurements was mainly better than 10%. Received: 28 August 2000 / Revised: 1 November 2000 / Accepted: 12 November 2000     

Homogeneity studies of powders and plasma sprayed deposits

For production of plasma sprayed protective deposits and self-standing ceramic parts many different feedstock powders are used. The powders are very often of a composite type prepared by various technologies and mostly chemically very inhomogeneous. A method has been developed for evaluation of chemical homogeneity of both, the feedstock powders and the sprayed materials. The backscattered electron images along with the element mapping of dopant have been used as the basic measurement. The homogeneity coefficients were determined by quantitative point analysis with ZAF corrections. The results show that under all spraying parameters there is always a certain redistribution of all species leading to a better homogeneity. But generally, the more homogeneous the feedstock powder, the better the homogeneity of the deposits on both micro and macro scale.     

复合金属氧化物脱硫剂脱除SO2的研究

铁系氧化物是一种有开发前途的脱硫剂，研究中发现，某些金属氧化物和铁氧化物进行匹配，在脱硫过程中具有良好的协同作用，可改善铁氧化物的脱硫性能［１～３］。进一步研究改善脱硫剂活性、稳定性、硫容、寿命、再生性能很有必要，在此基础上，本文研制出三代脱硫剂。１...     

Chromium(IV) Stabilisation in New Ceramic Matrices by Coprecipitation Method: Application as Ceramic Pigments

In this paper the stabilization of Cr⁴⁺ in new ceramic matrices as willemite Zn₂SiO₄ and galium‐gadolinium garnet GGG (Gd₃Ga₅O₁₂) is studied and compared with the chromium‐sphene ceramic pigment using the traditional solid reaction and an unconventional coprecipitation method.The chromate amount etched from the powders using Cr^III and Cr^VI precursors on ceramic and coprecipitation routes are analysed in order to evaluate the environmental impact on the waste water. Cr‐willemite and codoped Ca, Cr‐GGG produces solid solutions of Cr⁴⁺ ion occupying tetrahedral and dodecahedral sites, respectively, while Cr⁴⁺ into sphene lattice occupies octahedral sites. Cr‐willemite are stable when used as ceramic pigment in habitual tile glazes but Cr‐GGG unstabilses and produces green colour associated to Cr³⁺ solved in the glaze. Coprecipitate powders are more reactive than ceramic powders but in Cr‐GGG stabilizes the metastable Ca₃Cr₂O₈ oxide. Chromate amount in washing hot water are not dependent of the chromium precursor but depends of the allowed reactivity level and the reaction media.     

Low Temperature Sintering and Characterization of YSZ/Al Composite Coatings by Electrophoretic Deposition and Reaction Bonding Process

In practice, low temperature sintering is always the most desirable route to fabricate ceramic composites for industrial manufacturers and an economic way to produce ceramics with unique microstructure for technological applications. Normally the sintering temperature for dense YSZ coating is about 1500℃. Recently many studies have experimentally found that dense ceramic composites can be achieved at a sintering temperature lower by several hundred degrees Celsius for nano-sized powders than for coarse powders^[1-2]. Electrochemical processing is an attractive method for making ceramic films and powders because it offers the advantages of low temperature sintering, low cost and good control of the green forms^[3-4].     

Role of the interaction between forming nanocrystals and glass surface on the structure and properties of ZnO-based films

The interaction between glass surface and forming nanocrystals plays the important role in the formation of thin ZnO coatings crystal structure. The comparative study of the crystal structure of thin ZnO-based films and powders having similar chemical compositions was performed with the use of SEM, XRD analysis, optical, and luminescent spectroscopy. The influence of different coatings parameters (chemical composition, thickness) on the spectroscopic and morphological properties of thin films and powders reveals the structural features of the interaction between forming ZnO nanocrystals and glass surface. ZnO–SnO₂ coatings and powders were prepared by liquid polymer-salt technique. This method provides the close contact between the coatings’ precursors with a surface of the glass during both the nucleation and the initial growth stage of forming oxide crystals. The interaction of nanocrystals and substrate surface is responsible for the texture formation in the ZnO films and determines some features of their optical properties.     

New approaches to developing lithium polymer batteries

The electrochemical and physical-chemical properties of two families of lithium ion conducting membranes, i.e., the blends between high molecular weight poly(ethylene oxide) with a lithium salt commonly named "polymer electrolytes" and the gels of liquid solutions in a polymer matrix commonly named "gel electrolytes," are repoted and discussed. Particular attention is devoted to the newly developed approach of dispersing ceramic powders at the nanoscale particle dimension into the two types of membranes. This leads "nanocomposite" membranes having unique features, such as improved transport and interfacial properties in the case of the polymer electrolytes and enhanced liquid retention capability in the case of the gel electrolytes. Finally, the use of the gel electrolytes for the development of new-design, plastic-like, lithium-ion batteries is illustrated.     

导电二氧化钛的合成研究

综述了导电TiO2的合成方法,包括本征TiO2高温电导、磁控溅射制备纳米导电膜、表面掺杂制备TiO2压敏电阻、表面包覆制备导电粉体、晶面离子注入改性、等离子喷涂制备导电涂层等方面。并介绍了导电原理、影响因素和用途。参考文献35篇。