Jadran Maček

Preparation of Ni–YSZ composite materials for solid oxide fuel cell anodes by the gel-precipitation method

Abstract Nickel oxide–yttria stabilized zirconia powder mixtures (Ni–YSZ) were prepared by the gel-precipitation method from aqueous or methanol solutions of the corresponding chlorides. The sinterability of the powder mixtures and hence their appropriateness as anodic material in solid oxide fuel cell technology (SOFC) is affected both by the history of the powder preparation and its composition. Relative densities of greater than 97% of theoretical of the mixed NiO–YSZ powder were achieved at sintering temperatures as low as 1300°C. In sintered samples, both NiO and YSZ, composites could be prepared as continuous phases if the chemical composition of the prepared powder mixture were carefully controlled. The continuity of the YSZ and Ni phases was preserved after reduction as well. If the ceramic YSZ phase was prepared as a continuous rigid framework there was practically no shrinkage of the sintered body during reduction, but the reduced matrices became porous and the relative densities were lowered. The microstructural and electrical properties of Ni–YSZ cermets, where the Ni content in the YSZ matrix is close to 35 vol.%, showed that the materials are appropriate for SOFC anode preparation.

Preparation of submicrometer nickel powders by the reduction from nonaqueous media

Abstract Nickel powders in the submicrometer size range were prepared by a chemical reduction-precipitation method from nonaqueous solutions of nickel salts. Hydrazine was used as the reducing agent and paraffin oil, ethylene glycol, di- and tri-ethanolamine as the reaction media in order to carry out the reactions at higher temperatures, and, for the latter two media, to provide the appropriate basic environment for the reduction. The characteristics of the resulting end products, especially the particle size and size distribution, as well as the micromorphological characteristics, the yield of the reaction and the purity of the nickel powders, depend on the reaction conditions. The mean particle size of the nickel powders can be lowered also by changing the homogenous nucleation into a heterogeneous one by adding traces of palladium. Nickel powders with mean particle sizes ranging from 0.1 to several μm and with up to 99.8% purity were obtained in this way.

Citrate-nitrate gel transformation behavior during the synthesis of combustion-derived NiO-yttria-stabilized zirconia composite

NiO-yttria-stabilized zirconia powder mixtures were prepared from reactive citrate-nitrate gels using the combustion technique. The influence of the fuel/oxidant molar ratio in the precursor on the combustion rate and itsthermal characteristics was studied by thermal analysis and evolved gas analysis. It was found that the precursor thermal decomposition properties depended strongly on the citrate/nitrate ratio prior to the combustion. Intermediate precursors and final powder ashes were also analyzed by x-ray diffraction.

Formation of nickel and zirconia nanocomposites by the coprecipitation method

Abstract The precursors for the Ni-YSZ cermets preparation were either a solution of nickel, zirconium and yttrium chlorides or separately prepared NiO and YSZ. When preparing Ni-YSZ cermets by utilising the gel-precipitation technique the average particle size of the Ni and YSZ grains after the thermal treatment up to 1000 °C is no larger than 50–100 nm, but is in the micrometer range after the prolonged thermal treatment at 1300 °C. The mechanical mixing of the cermet components results in a single phase region larger than several micrometer range.

Combustion synthesis and the influence of precursor packing on the sintering properties of LCC nanopowders

The sintering characteristics of calcium-substituted lanthanum chromite powders (LCC) prepared via the citrate-nitrate combustion route were examined. The effects of sample packing prior to combustion synthesis and (La+Ca)/Cr cation ratios were studied in terms of the degree of reaction conversion and agglomeration of the nanoparticles obtained. The difference in sintering behaviour of samples milled for 5 and 60 min indicates that the sintering properties of the samples are not related only to the presence of liquid phase but also to the agglomerate size reduction during milling.

Growth of zinc oxide particles in the presence of silicon

In this study we report on the growth behavior of zinc oxide in the presence of different concentrations of silicon. We performed reactions in a continuous tubular reactor in aqueous and ethanolic-aqueous media at different reaction temperatures and for different residence times. It was found that the zinc oxide particles grew from the aqueous medium through the crystallization of amorphous zinc oxide units via different growth stages, i.e., plate-like particles, double-plate-like particles and ellipsoidal particles. The Zn/OH atomic ratio and, consequently, the pH control the particle size and the morphology (i.e., the inclusions on the particle surfaces). The presence of silicon in the reaction solution influences the particle morphology (the formation of planar defects), the particle growth stages (the double-plate-like particles) as well as the particle growth rate. By shifting the UV-VIS absorption maxima of the reaction suspensions we showed that an increased silicon concentration and a decreased reaction temperature significantly retarded the growth rate of the zinc oxide.

Starved Water Hydrolysis of Different Precursors and its Influence on the Properties of Precipitated Zirconia

The chemical composition of zirconia gels precipitated from methanol solutions with excess, stoichiometric or deficient amounts of water as well, as the phase composition of fine ZrO2 powders obtained by thermal treatment of gels prepared by this method, were investigated. It was observed that both the stoichiometry and crystalline phase formation during thermal treatment of zirconia gels are strongly influenced by the amount of water added to the initial reaction mixture. Heating the hydrated zirconia gels in an inert oxygen-free atmosphere produced a black nonstoichiometric oxide. The degree of nonstoichiometry of zirconia and its microstructure are influenced by the initial conditions in the reaction mixture. The X-ray patterns of thermally treated samples prepared with a substoichiometric amount of water show power lines of monoclinic and tetragonal zirconia, while after the same thermal treatment to 700°C, those prepared with excess water in the initial methanol solution, show mainly tetragonal diffraction lines.

Study of the drying zirconia gel-precipitates using thermal analysis

Thermoanalytical techniques (TG, DTG, DTA and EGA) can be helpful in studying gel compositions and relating the chemical history and the role of the precipitant to the chemical properties of gels. The objective of our work was to determine the influence of the initial conditions in the reaction mixture on the product characteristics. Zirconia gels were prepared by the gelprecipitation method by the hydrolysis of zirconium tetrachloride dissolved in methanol. The substitution of water by methanol was chosen in order to obtain a reaction medium in which better control of the hydrolysis and condensation reactions of the zirconium precursor were achieved and thus also better control of the final properties of the dried zirconia gel-precipitates could be obtained. The nature of the hydrated zirconia gels obtained during the gelation process is strongly influenced by the conditions of the reaction mixture, e.g. concentration of the zirconium precursor, the mixing rate and particularly by the amount of water added. To distinguish between various types of water in the gel-precipitates, the furnace atmosphere and the partial water pressure were altered appropriately. It was shown that initial zirconium to water molar ratio has a significant effect on the properties of the final products. It was assumed that, according to the preparation conditions, three different types of water are present in the formed zirconia gel-precipitates. Modification of the dehydration processes of zirconia gel-precipitates enables control of the final microstructural and surface properties of the dried gels which are suitable for further catalysts support as well as mixed oxide preparation.

Thermal properties and reactivity determination of micro- and submicrometer nickel powders

Monodispersed fine metal nickel powders of uniform shape and high purity are increasingly required for specific uses in many technological areas, especially in the preparation of electronic materials such as the manufacture of conductive inks and pastes and the formation of catalysts. Metallic nickel powders were prepared in ethylene glycol by the reduction of a nickel solution. Hydrazine was used as a reducing agent. Metal powders were characterized by chemical analysis, scanning electron microscopy (SEM), thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). Particle size distributions were determined using laser light scattering. The reactivity and purity of these fine nickel powders were tested by repeated oxidation and reduction of nickel powders in oxidative and reductive atmospheres.

Crystallization of gypsum from batch chemical neutralizations

Abstract The sulphuric acid-calcium hydroxide-water system similar to those which occur in batch neutralization processes in industry, has been studied. The rate of precipitation of calcium sulphate dihydrate during batch neutralization of sulphuric acid solutions with lime, in the temperature range of 10 to 60°C and acid concentrations from 0.03M to 0.125M, was studied. On the basis of the theoretical correlation between the individual parameters and the experimental data, a mathematical-statistical model has been developed. With the aid of this model it is possible to observe and predict changes in the supersaturation/concentration of calcium sulphate with time.