Single crystal sphere

Except for support effects, structure sensitivity has usually appeared in one of two aspects, variation of rate with svirface crystal face or with particle size. In ICC 1 Gwathmey reported in one of the first experiments with single crystal faces that different faces machined fi om Ni single crystal spheres catalyzed the hydrogenation of ethylene at different rates (ICC 1 paper 5). 

Fig. 7. 164. Fast- and slow-growing faces as applied to a single crystal sphere. The facets that develop and are observed are those crystal faces with the slowest growth rates.

Otter (18) also obtained striking interference color patterns showing variation of rate of oxidation with face, when single crystal spheres of nickel were heated in oxygen at a pressure of 1 atmosphere and a temperature of 800°C for periods up to 15 minutes. The relative order of the rates of oxidation on the different faces and the smoothness of the starting surface were not given or discussed. 

Cathcart (21) oxidized single crystal spheres of niobium and found large differences of rate with crystal face. The Hi and 110 faces oxidized most rapidly and at about the same rate, and the 100 face oxidized the most slowly. The time required for crack formation in the oxide varied with face. 

The adherence of the oxide to the metal substrate may be related to the epitaxial relationships of the oxide-metal system. Bardolle (53) showed that die adherence of the oxide diminished with the multiplication of the epitaxial possibilities. Thus on iron the oxide was less adherent on the Oil and 113 faces than on the 001 faces. The authors observed that when a copper single crystal sphere was heated in oxygen at a high temperature until a thick oxide scale formed on the high rate faces, the oxide cracked and flaked off easily on these faces but adhered tightly on the low rate faces. In this case the high rate faces show the greatest number of possible orientations, whereas the low rate faces show only one orientation. It should be noted that Bardolle s observations indicated that the faces with multiple orientations on iron were slow rate faces. 

R. Heimann, Dissolution forms of single crystal spheres of quartz in acid alkali fluoride melts, J. Cryst. Growth 18, 61, 1973. 

N2, NH3, NO, CO, and O2.—Considerable success has resulted from the application of sophisticated techniques to the study of these small molecules. Changes of morphology on Pt single crystal spheres have been observed during oxidation of NH3, CaHs, and CO. A weakly bound state appears in N2 adsorption on Fe(lll), but not on Fe(lOO), and there are similarities observed between N on Fe(lOO) at 6 0.5, and Fe4N. Reconstruction of Fe(lll) occurs on covering the surface with N and this can be related to work by Boudart et al, on small particles. N2 weakly bound states have been observed for polycrystalline Fe and Kishi and Roberts have related this to the mechanism of adsorption and dissociation. No evidence for weakly bound N2 on Pt(lll) at r > 165 K has been found. 

When single-crystal spheres of Mg were allowed to react with 0.05 M ethyl bromide in THF at 0 C. clearly defined crystal planes and geometric shapes did not develop, l his shows that there is no strong preference lor reaction at any particular crystal plane 11 I9. ... 

FIGURE 24.22 Using a single-crystal sphere to model exaggerated grain growth in AI2O3 doped with 1 wt% MgO at 1800°C for 60 minutes. 

What is the pressure for shrinkage of a spherical pore entrapped within a single crystal sphere What are your assumptions for the answer ... 

Kumagai et al. (1975) bear on the question of the coexistence of superconductivity and ferromagnetism and are referred to again in section 4.2.2. in connection with ESR studies of these compounds. The observation of the NMR in a wall-free single crystal sphere of GdAl2 has been achieved by Fekete et al. (1975). In this case, the quadrupole splitting of the Al resonance was observed as well as the dependence of the resonance frequencies on the orientation of the crystal with respect to an applied field. We refer to this work again in connection with the analysis of transferred hyperfine interactions in section 2.2.3.3. 

Magnetization studies on single crystal spheres with fields in the [110], [111], and [100] directions indicate that the NNS state at 1.4 K can be easily modified into a canted spin state, Fukuma et al. [21]. 

Fig. 105. Experimental and calculated spontaneous magnetostriction (Ij-lo)/ along [100] vs. temperature for an EuSe single crystal sphere.

Zinc oxide and nickel oxide powders were pressed isostatically. Zinc oxide pellets were sintered in air at 900°C for 100 hr, resulting in compacts 85.5% theoretical density. Nickel oxide specimens were sintered in air at 1000°C for 120 hr, yielding compacts of only 78% theoretical density. The spheres of these oxides were obtained by crushing the pellets and then grinding them in a modified Jordan air-grinder ( ). The spheres thus obtained had diameters varying from 0.5 to 0.8 mm. In addition MgO single-crystal spheres of uniform diameter of 1 mm were employed. 

Magnesium Oxide-Ferric Oxide System. In these experiments either magnesium oxide single-crystal spheres of 1 mm diameter were sintered to polycrystalline ferric oxide plates or ferric oxide polycrystalline spheres were sintered to single-crystal blocks of magnesium, oxide. In general the results were similar to those obtained in the nickel oxide-ferric oxide system. 

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