Alumina ciystal phases

The reflection of y-alumina and PdO ciystals can be distinguished on the samples calcined at 500 °C. When the calcination temperature is increased fi om 500 "C to 1000 "C, alumina undergoes changes of crystal phase as reported in previous studies [17], This modification of the crystal structure is usually accompanied by a decrease of the surface area. In the case of Ba- and La-modified material, this modification of the crystal structure is noticeably diminished. On the samples calcined at 1000 C (right part of the figure), the increase in crystalline character affects the palladium more than the alumina support. The presence of La and Ba within the alumina structure effectively avoids these transformations. In contrast, the samples supported on pure alumina suffer from the formation of various phases, mostly 0-AI2O3. However, it is possible to observe traces of a-ALOs phases. 

The method using the picrate [Packer et al. J Am Chem Soc 80 905 1958] is as follows quinoline is added to picric acid dissolved in the minimum volume of 95% EtOH, giving yellow ciystals which are washed with EtOH, air-dried and crystallised from acetonitrile. These are dissolved in dimethyl sulfoxide (previously dried over 4A molecular sieves) and passed through a basic alumina column, onto which the picric acid is adsorbed. The free base in the effluent is extracted with n-pentane and distilled under vacuum. Traces of solvent can be removed by vapour-phase chromatography. [Moonaw Anton J Phys Chem 80 2243 1976 ] The ZnCl2 and dichromate complexes have also been used [Cumper et al. J Chem Soc 1176 1962], [Beilstein 20 H 339, 20 I 134, 20 II222, 20 III/IV 3334, 20/7 V 276.]... 

For elements such as silicon, alumina and sodium, there is a fairly good agreement between atomic absorption (AA) determination and EDS data for the parent Mg-rectorite. However, there are differences with respect to other elements, as shown in Table 6-1. This is not unexpected, since AA determination reports bulk analysis, whereas EDS reports the composition of the individual clay ciystal surface. The iron content measured by EDS is much higher than reported by AA analysis indicating that Fe is preferentially concentrated on the clay platelets surface, and that FejOj-containing phase impurities are negligible. The magnesium distribution is similar to that observed for Fe. 

The majority of ceramics are multiphased materials that comprise both ciystallized and vitreous phases. Porcelain thus consists of silicate glass reinforced by acicular crystals of crystallized muUite, but we can also observe millimetric ciystal agglomerates with a very porous microstracture (iron and steel refiactoiy materials), or fine grained polycrystals (< 10 pm) without vitreous phases and with very low porosity (hip prosthesis in alumina or zirconia). It should be reiterated that, in addition to the chemical nature of the compound(s) in question, it is the microstracture of the material (size and shape of the grains, rate and type of porosity, distribution of the phases) that controls the properties. 

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