scholarly journals Tuning the Integration Rate of Ce(Ln)O2 Nanoclusters into Nanoparticulated ZrO2 Supports: When the Cation Size Matters

Materials ◽  
2020 ◽  
Vol 13 (12) ◽  
pp. 2818
Author(s):  
Adrián Barroso-Bogeat ◽  
Iván Daza Raposo ◽  
Ginesa Blanco ◽  
José María Pintado
Keyword(s):  
Diffusion Process ◽  
Opposite Effect ◽  
Nanostructured Catalysts ◽  
Zro2 Nanoparticles ◽  
Ceo2 Sample ◽  
Surface Phases ◽  
Fluorite Type ◽  
Doped Ceo2 ◽  
Cation Size ◽  
Successive Reduction

Three nanostructured catalysts with low total rare earth elements (REEs) content (i.e., 15 mol.%) were prepared by depositing CeO2 or Ln3+-doped CeO2 (Ln3+ = Y3+ or La3+; Ln/Ce = 0.15) on the surface of ZrO2 nanoparticles, as nanometre-thick, fluorite-type clusters. These samples were subjected to successive reduction treatments at increasing temperatures, from 500 to 900 °C. A characterisation study by XPS was performed to clarify the diffusion process of cerium into the bulk of ZrO2 crystallites upon reduction to yield CexZr1−xO2−δ surface phases, and the influence of the incorporation of non-reducible trivalent REE cations, with sizes smaller (Y3+) and larger (La3+) than Ce4+ and Ce3+. For all nanocatalysts, a reduction treatment at a minimum temperature of 900 °C was required to accomplish a significant cerium diffusion. Notwithstanding, the size of the dopant noticeably affected the extent of this diffusion process. As compared to the undoped ZrO2-CeO2 sample, Y3+ incorporation slightly hindered the cerium diffusion, while the opposite effect was found for the La3+-doped nanocatalyst. Furthermore, such differences in cerium diffusion led to changes in the surface and nanostructural features of the oxides, which were tentatively correlated with the redox response of the thermally aged samples.

scholarly journals Oxide ionic conductivity and microstructures of Pr and Sm co-doped CeO2-based systems

2018 ◽  
Vol 16 (1) ◽  
pp. 827-832 ◽  
Author(s):  
Aliye Arabacı
Keyword(s):  
Ionic Conductivity ◽  
Sem Analysis ◽  
Doped Ceria ◽  
X Ray Diffraction ◽  
X Ray ◽  
Fluorite Type ◽  
Doped Ceo2 ◽  
Oxide Ionic Conductivity ◽  
Total Ionic Conductivity ◽  
Co Doped

AbstractThe compositions Ce0.80Sm0.2-xPrxO2-δ (x=0-0.12) were prepared through the citrate-nitrate method. The synthesized Pr3+ and Sm3+ co-doped ceria powders with different compositions were calcined at 600°C for 3 h. Phase structure of the calcined powders was characterized by X-Ray diffraction (XRD) analysis.All the calcined samples were found to be ceria based solid solutions of fluorite type structures. The morphology examinations were carried out by scanning electron microscopy (SEM) analysis. Relative density of more than 91% of the theoretical can be achieved by sintering the Ce0.80Sm0.2-xPrxO2-δ pellets at 1400°C for 6 h. The two-probe a.c. impedance spectroscopy was used to study the ionic conductivity of the doped ceria samples. The Ce0.80Sm0.80Pr0.12O1.90 composition showed the highest total ionic conductivity value which is 2.39 × 10−2 S/cm at 600°C.

Titanium, Manganese, and Zirconium Dioxides

2019 ◽  
Author(s):  
Jean-Pierre Jolivet
Keyword(s):  
Electrode Materials ◽  
Alcoholic Solution ◽  
Zro2 Nanoparticles ◽  
White Pigment ◽  
Semiconducting Properties ◽  
Major Interest ◽  
Fluorite Type ◽  
Metal Organic ◽  
Particle Size And Shape ◽  
Refractory Ceramics

The dioxides of titanium (TiO2), manganese (MnO2), and zirconium (ZrO2) are important materials because of their technological uses. TiO2 is used mainly as white pigment. Because of its semiconducting properties, TiO2, in its nanomaterial form, is also used as an active component of photocells and photocatalysis for self-cleaning glasses and cements . MnO2 is used primarily in electrode materials. ZrO2 is used in refractory ceramics, abrasive materials, and stabilized zirconia as ionic conductive materials stable at high temperature. Many of these properties are, of course, dependent on particle size and shape (§ Chap. 1). Dioxides of other tetravalent elements with interesting properties have been studied elsewhere in this book, especially VO2, which exhibits a metal–isolator transition at 68°C, used, for instance, in optoelectronics (§ 4.1.5), and silica, SiO2 (§ 4.1.4), which is likely the most ubiquitous solid for many applications and uses. Aqueous chemistry is of major interest in synthesizing these oxides in the form of nanoparticles from inorganic salts and under simple, cheap, and envi­ronmental friendly conditions. However, as the tetravalent elements have re­stricted solubility in water (§ 2.2), metal–organic compounds such as titanium and zirconium alkoxides are frequently used in alcoholic solution as precursors for the synthesis of TiO2 and ZrO2 nanoparticles. An overview of the conversion of alkoxides into oxides is indicated about silica formation (§ 4.1.4), and since well-documented works have already been published, these compounds are not considered here. The crystal structures of most MO2 dioxides are of TiO2 rutile type for hexacoordinated cations (e.g., Ti, V, Cr, Mn, Mo, W, Sn, Pb) and CaF2 fluorite type for octacoordinated, larger cations (e.g., Zr, Ce), but polymorphism is common. Some dioxides of elements such as chromium and tin form only one crystal­line phase. So, hydrolysis of SnCl4 or acidification of stannate [Sn(OH)6]2− leads both to the same rutile-type phase, cassiterite, SnO2. Many other dioxides are polymorphic, especially TiO2, which exists in three main crystal phases: anatase, brookite, and rutile; and MnO2, which gives rise to a largely diversified crystal chemistry.

Facile Conversion of Layered Ruddlesden−Popper-Related Structure Y2O3-Doped Sr2CeO4into Fast Oxide Ion-Conducting Fluorite-Type Y2O3-Doped CeO2

2009 ◽  
Vol 48 (1) ◽  
pp. 257-266 ◽  
Author(s):  
Ryan Georg Gerlach ◽  
Surinderjit Singh Bhella ◽  
Venkataraman Thangadurai
Keyword(s):  
Related Structure ◽  
Fluorite Type ◽  
Doped Ceo2 ◽  
Ion Conducting ◽  
Oxide Ion

CBED analysis of impurity distributions in AlN

1990 ◽  
Vol 48 (4) ◽  
pp. 214-215
Author(s):  
Daniel Callahan ◽  
G. Thomas
Keyword(s):  
Aluminum Nitride ◽  
Bulk Material ◽  
Strong Dependence ◽  
Lattice Parameter ◽  
Promising Technique ◽  
Nitride Ceramics ◽  
Surface Phases ◽  
Convergent Beam ◽  
Oxygen Impurities

Oxygen impurities may significantly influence the properties of nitride ceramics with a strong dependence on the microstructural distribution of the impurity. For example, amorphous oxygen-rich grain boundary phases are well-known to cause high-temperature mechanical strength degradation in silicon nitride whereas solutionized oxygen is known to decrease the thermal conductivity of aluminum nitride. Microanalytical characterization of these impurities by spectral methods in the AEM is complicated by reactions which form oxygen-rich surface phases not representative of the bulk material. Furthermore, the impurity concentrations found in higher quality ceramics may be too low to measure by EDS or PEELS. Consequently an alternate method for the characterization of impurities in these ceramics has been investigated.Convergent beam electron diffraction (CBED) is a promising technique for the study of impurity distributions in aluminum nitride ceramics. Oxygen is known to enter into stoichiometric solutions with AIN with a consequent decrease in lattice parameter.

Can the Academic Self-Concept be Positive and Realistic?

2005 ◽  
Vol 19 (3) ◽  
pp. 129-132 ◽  
Author(s):  
Reimer Kornmann
Keyword(s):  
Academic Performance ◽  
Opposite Effect ◽  
Self Esteem ◽  
Point Of View ◽  
The Other ◽  
Self Concept ◽  
High Ability ◽  
Self Image ◽  
Other Hand

Summary: My comment is basically restricted to the situation in which less-able students find themselves and refers only to literature in German. From this point of view I am basically able to confirm Marsh's results. It must, however, be said that with less-able pupils the opposite effect can be found: Levels of self-esteem in these pupils are raised, at least temporarily, by separate instruction, academic performance however drops; combined instruction, on the other hand, leads to improved academic performance, while levels of self-esteem drop. Apparently, the positive self-image of less-able pupils who receive separate instruction does not bring about the potential enhancement of academic performance one might expect from high-ability pupils receiving separate instruction. To resolve the dilemma, it is proposed that individual progress in learning be accentuated, and that comparisons with others be dispensed with. This fosters a self-image that can in equal measure be realistic and optimistic.

I Like Her, Because I Like Myself: Self-Evaluation as a Source of Interpersonal Attitudes

2003 ◽  
Vol 50 (4) ◽  
pp. 239-246 ◽  
Author(s):  
Eva Walther ◽  
Claudia Trasselli
Keyword(s):  
Opposite Effect ◽  
Learning Phase ◽  
The Self ◽  
Attitude Formation ◽  
Formation Processes ◽  
False Feedback ◽  
Self Evaluation ◽  
Evaluation Group ◽  
Group A ◽  
The Difference

Abstract. Two experiments tested the hypothesis that self-evaluation can serve as a source of interpersonal attitudes. In the first study, self-evaluation was manipulated by means of false feedback. A subsequent learning phase demonstrated that the co-occurrence of the self with another individual influenced the evaluation of this previously neutral target. Whereas evaluative self-target similarity increased under conditions of negative self-evaluation, an opposite effect emerged in the positive self-evaluation group. A second study replicated these findings and showed that the difference between positive and negative self-evaluation conditions disappeared when a load manipulation was applied. The implications of self-evaluation for attitude formation processes are discussed.

Burgers fluid flow in perspective of Buongiorno’s model with improved heat and mass flux theory for stretching cylinder

2020 ◽  
Vol 92 (3) ◽  
pp. 31101
Author(s):  
Zahoor Iqbal ◽  
Masood Khan ◽  
Awais Ahmed
Keyword(s):  
Diffusion Process ◽  
Mathematical Formulation ◽  
Thermal Relaxation ◽  
Mass Diffusion ◽  
Stretching Cylinder ◽  
Discussion Section ◽  
Buongiorno’S Model ◽  
Homotopy Analysis ◽  
Burgers Fluid ◽  
Diffusion Phenomena

In this study, an effort is made to model the thermal conduction and mass diffusion phenomena in perspective of Buongiorno’s model and Cattaneo-Christov theory for 2D flow of magnetized Burgers nanofluid due to stretching cylinder. Moreover, the impacts of Joule heating and heat source are also included to investigate the heat flow mechanism. Additionally, mass diffusion process in flow of nanofluid is examined by employing the influence of chemical reaction. Mathematical modelling of momentum, heat and mass diffusion equations is carried out in mathematical formulation section of the manuscript. Homotopy analysis method (HAM) in Wolfram Mathematica is utilized to analyze the effects of physical dimensionless constants on flow, temperature and solutal distributions of Burgers nanofluid. Graphical results are depicted and physically justified in results and discussion section. At the end of the manuscript the section of closing remarks is also included to highlight the main findings of this study. It is revealed that an escalation in thermal relaxation time constant leads to ascend the temperature curves of nanofluid. Additionally, depreciation is assessed in mass diffusion process due to escalating amount of thermophoretic force constant.

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