Amorphous surface layer

The most obvious future data needs concern the missing, uncertain, and conflicting data identified above. Additional experimental investigations are needed in the case of Fe(III) and Zr(IV) carbonate complexation, and in the case of the Sn(IV)/Sn(II) and the Se(0)/Se(-II) redox couples. The molecular structure of metal silicate complexes needs clarification in order to remove ambiguities in the speciation scheme of these complexes. A rather challenging topic concerns the supposed transformation of crystalline tetra-valent actinide oxides, AnOz(cr), to solids with an amorphous surface layer as soon as the An4+ ion hydrolyses. The consequences of such... 

Grinding or abrasion during sample preparation produces a disturbed, disordered or amorphous surface layer which is hydrous. Acid leaching removes this material. 

Fig. 7. (a) A HRTEM image of a 40-nm freestanding bismuth nanowire, showing lattice fringes. The amorphous surface layer is bismuth oxide formed upon air exposure of bismuth nanowire, (b) SAED pattern of a single Bi nanowire (Zhang et at, 1999). 

The actual mechanism by which the amorphous surface layer is produced is probably a combination of many processes very large shearing... 

The disturbance of the normal structure of the solid gradually decreases, from the completely amorphous surface layer, down through layers of varying degrees of crystal size and distortion, to the undistorted structure several thousand A. below the surface. 

To check on the dependence of AHt on crystallinity, a cristobalite sample was prepared at temperatures high enough to cause a phase transformation (22), and it was assumed that at this temperature any amorphous surface layer would also crystallize. As predicted, the Afi/s increased (approximately 200 ergs per sq. cm.) above the original quartz values. Difficulties of interpretation were not completely obviated, since the surface areas differed by a factor of 2. Also, the Aff/s were for two different crystalline modifications and thus would not be expected to display exactly the same interaction energies with water. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Furthermore, these catalysts were found to be active from the start of operation, whereas catalysts derived from V0HP04 7iH20 take several hours to achieve full activity. These observations were considered to support the proposal that the active catalyst comprises an amorphous surface layer—and the crystalline vanadyl pyrophosphate that has been so well studied may be nothing more than an elaborate support (Figure 28). 

Hutchings and coworkers [72-74] have prepared vanadium phosphate catalysts using supercritical precipitation methods. These materials were found to be amorphous by XRD and electron diffraction, but showed activity comparable to standard vanadium phosphate catalysts. This demonstrates that an amorphous surface layer can be the active phase in these catalysts and that the crystalline VPP that has been so well studied may be nothing more than an elaborate support. 

The minimum size of a colloidal particle has been arbitrarily chosen to be 10 mn to ensure that a sufficient number of constituent atoms/molecules may be considered to have the bulk material properties. The surface layer is typically amorphous, but may recrystallize (slowly) to the final bulk material. The surface chemistry outlined above thus relates to the amorphous surface layer, but not to the actual (recrystallized) surface of the bulk solid phase. 

Moreover, the amorphous and crystalline components are not independent of each other. The amorphous surface layers between adjacent lamellae are formed by free chain ends, chain folds, and tie molecules. The free chain ends are fixed at the other end in the crystal lattice. Chain folds and tie molecules are fixed at both ends, the former on the same lamellae, the latter on different ones. These constraints modify deeply the properties of the amorphous layer. Even above Tgi the mobility of the amorphous chains is substantially less than in a rubber with the same average number of mers in the chain segments between subsequent crosslinks. 

Transmission electron microscopy of the Sn-Cr-0 catalyst corroborates these observations. For example, the TEM micrograph of the catalyst with a Sn Cr ratio of 0.015 at 600° shows only the presence of small crystallites of tin(IV) oxide and no cluomium-containing phase although cliromium is detected by EDXa analysis. We deduce therefore that the cliromium is present in an amorphous surface layer of an as yet unknown composition on the crystallites of the tin(IV) oxide. 

An important observation discussed in [2001NEC/K1M] is that experimental solubility data in neutral and alkaline solutions are approximately the same for amorphous hydroxides An(OH)4(am, hyd) or hydrous oxides An02(am, hyd) and crystalline oxides An02(cr), although their solubility constants differ by 6-7 orders of magnitude. Solubility data determined with An02(cr) at pH > 5 do not reflect an equilibrium with the bulk solid but with amorphous fractions or an amorphous surface layer. 

Figure 10-13. The melting temperature dependence of the reciprocal lamellar thickness Ijd of lamellae for poly(trifluorochloroethylene). The lamellar thickness was measured as the inter-lamellar distance by small-angle X-ray analysis, and thus contains both the crystalline component and the amorphous surface layer (after J. D. Hoffman from data of P. H. Geil and J. J. Weeks).

Fig. 4. Molecular model of a stack of parallel lamellae of the spherulitic structure A, interlamellar tie molecule B, boundary layer between two mosaic blocks C, chain end in the amorphous surface layer (c ilium) D, thickness of the crystalline core of the lamella E, linear vacancy caused by the chain end in the crystal lattice L. long period I, thickness of the amorphous layer (Peterlir ).

The two-component system—crystal lamellae or blocks alternating with amorphous layers which are reinforced by tie molecules— results in a mechanism of mechanical properties which is drastically different from that of low molecular weight solids. In the latter case it is based on crystal defects and grain boundaries. In the former case it depends primarily on the properties and defects of the supercrystalline lattice of lamellae alternating with amorphous surface layers (in spherulitic, transcrystalline or cylindritic structure) or of microfibrils in fibrous structure, and on the presence, number, conformation and spatial distribution of tie molecules. It matters how taut they are, how well they are fixed in the crystal core of the lamellae or in the crystalline blocks of the microfibrils and how easily they can be pulled out of them. In oriented material the orientation of the amorphous component (/,) is a good indicator of the amount of taut tie molecules present and hence an excellent parameter for the description of mechanical properties. In fibrous structure it directly measures the fraction and strength of microfibrils present and therefore turns out to be almost proportional to elastic modulus and strength in the fibre direction. 

Mirror and polished surfaces do not always reduce fouling effects. In manufacturing processes, these surfaces receive an amorphous surface layer which is... 

This conclusion seems inconsistent with mineralogical observations indicating that uraninite is in fact present in the Tono Uranium Deposit (Section 2). A possible explanation for this apparent dichotomy is the experimental observation of Neck and Kim [28] that uraninite surfaces in contact with an aqueous phase at pH >3 may be coated with a thin layer of U02(am). Under such conditions, uraninite dissolution is effectively irreversible, and solubility is controlled by the amorphous surface layer. Additional experimental studies and observations of relevant natural systems are needed to test this hypothesis. 

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