CaO-CeO

Figure 4-10. Band structure and DOS profiles of Cao Ceo 90 9 along high-symmetty directions of the irreducible Brillouin zone. The valence-band maximum is taken as the energy zero.

The intial densification kinetics in the sintering of powder compacts have been examined for five oxides ZrOz-CaO, MgO, CaO, CeO, and SiOz. Study of the initial sintering, the densification which occurs as the temperature of the specimen is increased, permits the observation of densification kinetics which may not be diffusion controlled. The densification kinetics demonstrate several features which are not compatible with a diffusion-controlled process. For example, there is a disproportionately large increase in the densification rate when the temperature is increased compared to that expected from a volume diffusion process. Other tests indicate that isostatic pressing of partially sintered powder compacts causes densification on subsequent heat treatment. An explanation of these effects by a diffusion sintering process, either volume or grain boundary diffusion, is not evident however, a qualitative explanation based on dislocation transport of material can be proposed. This explanation rests on the premise that as the temperature is increased, the stress required for motion and generation of dislocations decreases. 

Thirteen kinds of metal oxides eontaining calcium, barium, magnesium and lanthanium were tested for transesterification of triglycerides. Among the tested catalysts, CaZrOj and CaO-CeO heterogeneous base eatalysts were effective catalyst for the transesterification of rapeseed oil at a temperature of 60° C and reaction time of 10 h to produce biodiesel in 90% yield [38]. 

Fig. 10. Unpolarized Raman spectra (T = 300 K) for solid Ceo, KaCeo, RbsCeo, NaeCeo, KaCco, RbeCeo and CseCeo [92, 93], The tangential and radial modes of Ag symmetry are identified, as are the features associated with the Si substrates. From the insensitivity of these spectra to crystal structure and specific alkali metal dopant, it is concluded that the interactions between the Cao molecules are weak, as are also the interactions between the Cao anions and the alkali metal cations.

Diffraction studies see Diffraction Methods in Inorganic Chemistry) show pure, sublimed Ceo to be crystalline with the near spherical molecules arranged in a face-centered cubic (fee) array, with a center-to-center inter-Cao distance of 10.02 A. The presence of traces of solvent impurities appears to stabilize an hexagonal close-packed (hep) arrangement (see Close Packing). In the fee phase at room temperature, each Ceo molecule rotates essentially... 

Figure 4 FTIR spectra for MgO, CeO Figure 5 FTIR spectra for pure ceria, pure CaO, and and solids with composition Mg Ce O, 12Ca96Ce oxide precipitated by co-precipitation (cp)...

The structure of Cso has been proposed to be a truncated icosahedron with twenty six-membered rings and twelve five-membered rings. In this allotrope, the atoms are equivalent, giving a closed-shell electronic structure and a molecule that is unique in nature. While the existence of Ceo has been known for several years, moderately large-scale production and phase separation of Cao and other fullerenes were not possible before the work of Kratschmer et al. That breakthrough virtually assured rapid development in understanding the properties of these novel forms of matter. 

Ceo.8Cdo.202-8. perovskite-like Sro 7Ceo.3lVlno 9Alo/ 03 5 [63], Cao.ySro Feo.sAlosOt-s [67],... 

Figure 2.56 collects some exemplary reactions of Ceo with different bromoma-lonates and other applicable keto compounds. This collection shows that there is virtually no Umit to the choice of side chains, which renders the Bingel-Hirsch reaction an attractive and flexible starting point for the synthesis of fuUerene-containing materials. For example, the base-mediated conversion of a malonate and Cao into the respective methanofullerene can also be achieved. With the semiester of malonic acid instead of a malonate, the monosubstituted methanofullerene is obtained because the primary product is instantaneously decarboxylated (Figure 2.56). P-Ketoesters may also be reacted under Bingel-Hirsch conditions to give methanofuUerenes.

Therefore, what we plan to plot will be Ca(T>/Cao, Cb(r)/Cao, Cd(T)/Cdo, and Ce(r)/Ceo against the dimensionless time r. We can use Mathematica to do this, the beauty of which is that we can let the product kt = r vary as natural numbers without actually assigning a specific value to k, and for the same reason the concentrations will vary as natural numbers between zero and unity. To emphasize the nondimensional nature of the concentrations, we can introduce a new variable, namely, the Greek letter for dimensionless concentrations. When the initial concentration of B is divided by that of A, we will call this bo, and likewise for the other two species. The new expressions in dimensionless form will be ... 

Since the discovery by Kroto et al. [232] of the third molecular form of carbon, Ceoj named buckminsterfiillerene, and especially after the development of effective ways of production of these type of molecules, there has been great interest in the thermochemistry of these molecules. The enthalpy of formation of Cao is a key value in establishing its thermodynamic stability. Several micro- and macro-combustion calorimeters have been used for the experimental determination of the enthalpies of formation of Ceo in the crystalline state at 298.15 K [233]. A graphical representation of the available results collected in... 

The photocurrent quantum efficiencies of undoped PDS(Th)4 and PDS(Th)4 doped with 1.5 wt % Cao versus wavelength are shown in Fig. 5. The photocurrent quantum efficiency of the doped sample is greatly enhanced, compared with that of the undoped one. In fact, the photocurrent quantum efficiency of undoped PDS(Th)4 was only 0.5 % E= 3x10 V cm ) upon irradiation at 480 nm. The photocurrent quantum efficiency of the Ceo-doped PDS(Th)4, however, increased to 11.5 % E = 1.5x10 V cm" ) upon irradiation at 470 nm. Furthermore, Fig. 6 shows the field... 

Pure and mixed metal oxides have been widely investigated as Lewis acid and/or Lewis base for the reaction epoxides with CO to form the corresponding cyclic carbonates. Bhanage et al. [31] investigated several kinds of basic metal oxides (MgO, CaO, ZnO, ZrO, La Oj, CeO, Al Oj and K CO ) for the reactions of EO and PO with CO to the corresponding cyclic carbonates and for the transesterification reactions of the cyclic carbonates with methanol to DMC and the corresponding glycols. The catalytic performance of the various metal oxides for the synthesis of PC (express in Yp ) was (54.1%) >MgO (32.1%) > ZrO (10.9%) > K CO (8.7%) =... 

DCM and chloroform are solvents in which fullerenes have an acceptably high solubility. Accordingly theses solvents have been used frequently in chromatographic systems that separate, analyze, and/or purify the fullerenes. For the lower molecular weight fullerenes, (e.g., Ceo and C70) DCM has been used in hexane at the cAo/. fr, r>htain seoaration [652,759]. However, a significantly higher level... 

Thermodynamic stability of the interface with silicon is also important for correct selection of sensor material. According to Hubbard and Schlom (1996), for MgO and ZrO, thermodynamic stability of their interfaces with silicon was predicted. For HfO, AI2O3, CaO, and Y2O3, experimental evidence showed that these oxides also form stable interfaces with silicon. The CeO /Si interface is thermodynamically unstable, in accordance with experimental observations of formation of a Ce203 interlayer at the CeO /Si interface. Present analysis shows that, for the majority of metal oxides used, the interface with Si is stable. However, ZrO or YSZ thin films are permeable for oxygen at elevated temperatures, so that oxygen diffuses to the silicon substrate and a SiO layer is formed at the interface (Jia et al. 1995). It should be noted that this problem exists for all metal oxides used for gas sensor design. 

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