Marjan Marinšek

Ni–YSZ cermet anodes prepared by citrate/nitrate combustion synthesis

Abstract The synthesis of Ni–YSZ cermets with tailored particle package, shape and microstructural characteristics is essential when preparing anodes for solid oxide fuel cells (SOFC). These materials are generally prepared by sintering and subsequent reduction of the mixture of metal oxides. In order to obtain cermets with an adequate contact area between electrocatalyst (Ni) and ionic conductor (YSZ), an alternative route was used based on mixed gel combustion with the material synthesis, calcination and partial sintering achieved in one step. The precursor for the combustion synthesis was a mixed citrate/nitrate gel prepared from nickel, zirconium and yttrium nitrates and citric acid by vacuum evaporation of the solution. The combustion reaction of this gel produces submicrometer crystalline NiO–YSZ composite. The influence of the fuel/oxidant molar ratio of the precursor on the combustion rate and end product characteristics was investigated. The reaction period, phase composition, morphology and agglomerate formation were studied in detail. It was shown that the initial fuel/oxidant ratio strongly influences the characteristics of the powder mixtures thus obtained. The morphological properties of the prepared mixed oxides after the combustion synthesis reveal that the particle size distribution and the agglomerate formation in the voluminous intermediate mixed oxide product (green body) differ with the initial fuel/oxidant molar ratio. Narrower agglomerate and pore size distribution has a great influence on the subsequent sintering and reduction of the mixed material. If the particle and pore size distribution in the green body are narrow, the coarsening of the YSZ and NiO grains, and subsequently, YSZ and Ni grains are less pronounced.

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.

Influence of citrate–nitrate reaction mixture packing on ceramic powder properties

Abstract Lanthanum chromite-based materials have a good prospect for use in various high temperature applications, as well as an SOFC separator. A citrate–nitrate gel combustion reaction was used for the preparation of submicron crystalline strontium-substituted lanthanum chromite (LSC). The effect of the fuel-oxidant molar ratio and sample form prior to combustion was investigated in terms of reaction period, phase formation, particle size, morphology and agglomerate formation. Several characterization methods including scanning electron microscopy, mercury porosimetry, BET measurement, X-ray powder diffraction and thermal analysis were used to evaluate the influence of reaction mixture packing on powder characteristics for different citrate–nitrate (c/n) ratios. It was shown that the reaction period depends on the fuel/oxidant ratio and reaction mixture packing. The LSC powders prepared via the combustion route exhibited surface areas of about 12 m 2 /g for the loose packed layer prepared samples and 7 to 11 m 2 /g for samples prepared from a pellet. The nature of the agglomerates was studied from the pore size distribution in the green compacts pressed at different pressures. The sintering behaviour of powders and some of the electrical properties of sintered samples are reported. Sintering tests on LSC powders prepared via the combustion route showed that the sintering process started at about 900°C and proceeded in two steps in the presence of a liquid phase.

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.

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.

The role of Cr/Co substitution on dielectric properties of gadolinium orthochromite

The dielectric relaxation of GdCr1−x Co x O3 (x = 0, 0.33, 0.5 and 1) was studied using dielectric spectroscopy. The study was performed in a frequency range from 10 Hz to 10 MHz and a temperature interval of 297 K–337 K. Changes in dielectric constant and electric modulus with increasing Cr/Co substitution in the structures were observed. The obtained results of dielectric loss were analyzed introducing the model of a hopping of charge carrier over a potential barrier between charged defects. The conduction mechanism for pure and mixed structures was discussed in detail.

SYNTHESIS AND CHARACTERIZATION OF EPOXY-SINGLE-WALL CARBON NANOTUBE COMPOSITES

We present a study on the composite of 1 wt% single-wall carbon nanotubes (SWNTs) in epoxy resin. Composites were prepared by solution mixing of epoxy resin XY 444 (Vantico), polyamine hardener XV 279 (Vantico) and single-wall carbon nanotubes (SWNTs), as a thin film. We observed that addition of surfactant together with sonification were necessary in order to obtain more uniform composite. We used several techniques to investigate the nanocomposite material, such as scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The observed changes of the glass transition temperature (Tg) and electron micrographs indicate that the compatibility and interfacial adhesion between the nanotubes and epoxy resin are poor.