Yttria Phase Research Articles

Effect of support preparation method on water-gas shift activity of copper-based catalysts

Reducing the effect of global warming and increasing the needs for fuel sources appoint hydrogen utilization in fuel cell technology a very promising demand. A step for required pure hydrogen production for this purpose is the water-gas shift (WGS) reaction and the design of well performing but cost effective WGS catalysts, the latter being of growing research interest. In the present study catalysts containing copper (3 wt%) supported on ceria/alumina (30 wt% ceria) and Y-doped ceria/alumina (1 wt% Y2O3 regarding ceria content) were prepared by impregnation (IM) and by mechanochemical mixing (MM). The effect of support synthesis methods on the performance in the WGS reaction was discussed based on catalyst characterization by means of XRD, HRTEM, XPS, and H2-TPR methods. It was established that the effect of Y-doping was strongly dependent on the method of support preparation being positive for MM and unfavourable in the case of impregnation. The IM method caused the formation of a superficial phase with CuO, Y-dopant, and ceria in close location. Additionally, a higher amount of oxygen vacancies was related to non-reducible Y3+ ions that interrupted Cu–oxygen vacancy–Ce interaction thus leading to both lower reducibility and WGS activity. The presence of yttria phase located on ceria-free alumina in the case of MM support decreased the negative influence of Ce3+ replacement by Y3+ ions. A positive effect of separate Y2O3 could explain the best WGS performance observed. Long-term WGS tests showed good stability thus making this catalyst promising composed mainly of alumina and low amount of active phases. The study contributes to the development of an advantageous formulation of well-performing and economically profitable WGS catalysts for efficient improving of hydrogen purity for small-scale applications.

Influence of ammonium hydrogen carbonate molar ratio on structural and morphological properties of Y2O3nanoparticles

The co-precipitation synthesis is considered as one of the greatest wet chemical route to produce nanoparticles because of its yield and control over synthesis parameters. In this manuscript, Yttria (Y2O3) nanoparticles (YNPs) have been synthesized through direct titration precipitation process using ammonium hydrogen carbonate (AHC) as precipitant to metal ion molar ratio (M3+) in acidic medium condition. The molar ratio [R= (NH4HCO3)/(M3+)] varies from 0.35 M to 0.55 M with constant metal ion (M3+=0.1M) concentration. The influence of R-value on crystallographic structure, particle size, surface morphology of YNPs are comprehensively studied. Rietveld refined parameters confirm the formation of pure cubic yttria phase in calcined powders. The FTIR results confirm the presence of Y-O-Y and Y-O bands. The influence of R-value on the development of morphological aspects was observed using field emission scanning electron microscopy which shows pseudo spherical morphology with a particle size range of 24.63±5.6 nm to 31.46±5.1 nm. TGA/DSC results confirm the complete transformation of intermediate yttrium compounds to pure cubic Y2O3 single phase at a temperature of 780°C.

Enhanced Acetone-Sensing Performance of Au/Y-ZnO Composite Prepared Using a Facile Wet Chemical Method

Au/Y-ZnO composite with excellent acetone sensing was successfully prepared by a facile wet chemical method. X-ray diffraction analysis, field-emission scanning microscopy, and transmission electron microscopy investigation revealed that the obtained Au/Y-ZnO was mainly composed of ZnO wurtzite phase and Au nanoparticles with a face-centered structure in diameter of 10 nm, and Y atom replaced Zn lattice sites to form yttria phase. Compared with the pure ZnO and Y-doped ZnO, the Au/Y-ZnO-based sensor shows significantly enhanced acetone gas sensing. Its response to 100 ppm acetone is 27.6, which is about 2.3 and 5.5 times that of the Y-ZnO and the pure ZnO, respectively. The Au/Y-ZnO-based sensor has a low detection limit (1 ppm), and fast response (3 s) and recovery (9 s). Au modification can obviously decrease the operating temperature (275°C). The enhanced gas-sensing of Au/Y-ZnO towards acetone can be ascribed to YZn and VZn− defects, the Schottky barrier between Au nanoparticles and Y-doped ZnO, and the catalytic activity of the Au nanoparticles.

Synthesis of nanocrystalline yttria powder prepared by a polymer solution route

Nanometer-scaled yttria (Y2O3) powders were synthesized by a polymer solution route using polyvinyl alcohol (PVA) as an organic carrier. The PVA polymer contributed to a soft and porous powder microstructure, and homogeneous precursor gels containing PVA polymer were effective in making nanometer-sized yttria powders. In this process, the content of PVA and the calcination temperature strongly affected the microstructure and crystallization behavior of the yttria powders. The homogeneous precursors were crystallized to a stable yttria phase at 600 °C for 1 h. In this paper, a simple solution technique for the fabrication of nanometer-sized yttria powders is introduced. The effects of PVA on the powder morphology and on powder specific surface area were studied. The characterization of the synthesized powders was examined by using XRD, DTA/TG, SEM, TEM, a particle size analyzer and nitrogen gas adsorption. The yttria powder synthesized from PVA with a content ratio of 4:1 revealed a crystallite size of about 15 nm with a high surface area of 34.71 m2/g.

Study on Pyrolysis Behavior of Yttrium Oxalate and Kinetic of Yttria Grain Grown

The precursor of yttria was prepared by yttrium nitrate and ammonium oxalate in water solution. Pyrolysis behavior of the precursor and the characterization of yttria phase transformation were studied through thermoanalysis. The grain growth kinetic behavior of yttria nanopowders was investigated by the X-ray powder diffraction (XRD) result of the powders obtained through roasting the precursor. Pattern of the yttria powders and agglomeration of its grain were measured by means of SEM and laser particle size analysis methods. The results suggested that cubic crystal phase yttria could be obtained by sample calcination for 1 h under 823 K. When the precursor was calcined under conditions of temperature ranging from 823 K to 1173 K, yttria grain ranging 7 nm ~ 16 nm could be obtained ,with grain growth exponent of n< 0.2 . In addition , an significant increase in grain-growth-rate constant was observed as the temperature went up. The higher temperature, the faster growth. Grain growth activation energy was different in low-temperature and high-temperature fields. The activation energy was 77.42 kJ•mol-1 when the temperature was lower than 965 K, and became 524.36 kJ•mol-1 when the temperature was higher than 965 K. Yttria powders is highly dispersed whose agglomerates sized about 400 nm after being dispered in deionezed water.

Carbonate-precipitation synthesis of Yb 3+:Y 2O 3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb 3+:Y 2O 3) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb 3+:Y 2O 3 nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb 3+:Y 2O 3 ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the 2 F 5/2 → 2 F 7/2 transitions of Yb 3+ ion. Homogeneous Yb 3+:Y 2O 3 nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.

Effect of grain size on the sinterability of yttria nanopowders synthesized by carbonate-precipitation process

Yttria nanopowders have been synthesized by carbonate precipitation process. The thermal decomposition behavior of the carbonate precipitate, crystal phase, microstructure and sinterability of the powders were examined. The precipitate precursor was decomposed completely at 700 °C and then crystallized well into cubic yttria phase. Our results suggested that the most favorable calcination temperature was 1100 °C for obtaining good dispersion, high-reactive yttria nanopowders. The nanopowders were almost spherical and had weak agglomeration and narrow particle size distribution with a mean grain size of about 80 nm. Using these yttria nanopowders, nearly full dense yttria ceramic with ∼99.2% of theoretical density was obtained at 1600 °C for 2 h without any additives.

Molecular dynamics simulation of the structure of yttriaY2O3phases using pairwise interactions

We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.

Deposition by an aerosol assisted MOCVD process of Eu or Er doped Y2O3-P2O5 thin films

The deposition of Er or Eu doped Y 2 O 3 -P 2 O 5 thin films by an aerosol assisted MO-CVD atmospheric process is studied. 0.1 to 0.5 μm thick films were obtained on glass substrates in the temperature range 400°C-580°C. The rare earth luminescence is activated by post annealing. The properties of the films were discussed in function of the P 2 O 5 content. For high annealing post-deposition treatments the films with less than 15 Mol% P 2 O 5 tended to crystallise in the yttria phase. The YPO 4 phase appeared around 50 Mol% P 2 O 5 . Between these concentration limits the films appeared as a disordered composite Y-P-O phase and exhibited wide luminescence bands.

Processing and Creep Resistance of Nickel/Yttria Composites

In this study, pure nickel and yttria (Y2O3) were selected as a model system to investigate the feasibility of processing metal matrix composites (MMCs) through a powder metallurgy approach for the in-situ formation of a continuous three-dimensional reinforcement network or the in-situ formation of discrete reinforcements with certain degrees of interconnected clusters. Composites with a volume fraction of Y2O3 ranging from 20 to 50 % were prepared through hot pressing. The density, microstructure and creep resistance of these composites were evaluated as a function of the yttria volume fraction. It was found that a continuous Y2O3 network was formed in composites with 40 and 50 vol % Y2O3, while yttria was discrete with some degrees of interconnected clusters in composites with 20 and 30 vol % Y2O3. The creep rate was reduced by two to three orders of magnitude with the addition of 20 to 30 vol % Y2O3, and it continued to decrease with increasing the volume fraction of yttria to 50%. The analysis indicated that the load transfer to isolated yttria particles could not account for the improved creep resistance of composites with 20 and 30 vol % Y2O3, while the load transfer to a continuous yttria network in composites with 40 and 50 vol % Y2O3 could not be approximated by the model of the load transfer to continuous fibres. The discrepancies are believed to be related to the presence of interconnected yttria clusters, the low relative density of the yttria phase in the composite, and the low load-carrying capability through a three-dimensional network in comparison with the load-carrying capability through continuous fibres. It is suggested that the density of the yttria phase and hence the creep resistance of the composite can be further improved over what have been obtained in this study by densifying the composite at high temperatures and pressures.

Growth and characterization of Y 2O 3:Eu 3+ phosphor films by sol-gel process

Phosphor films play an important role in high resolution display devices, remote thermometry, gas sensors, etc. These films were grown by various methods. In the present paper, europium activated yttrium oxide (Y 2O 3:Eu) phosphor films were deposited from yttrium hydroxide gels by dip and dry and spin coating methods. XRD of the films, calcinated at 700 to 1000°C showed all the prominent lines of the yttria phase. From SEM studies, it was observed that these films were uniform and defect free. Luminescence intensity was comparable with that of Y 2O 3:Eu powder samples.

Phase stability of zirconia-based thermal barrier coatings part I. Zirconia-yttria alloys

Coatings of zirconia alloyed with 4–12 wt.% yttria were prepared by plasma spraying. A comprehensive sequence of annealing experiments comprising times from 1 to 100 h at temperatures between 1200 and 1600 °C was carried out on the alloy containing zirconia-8wt.% yttria. It was shown that the T' phase decomposes into high and low yttria phases following high temperature annealing. It was further shown in the course of thermal expansion tests that the low yttria phase will transform to monoclinic during slow heating and cooling. These findings were substantiated for coatings of alloys containing 4, 6, 10 and 12 wt.% yttria.