X-ray amorphous structure

All industrially important glass fibers arc manufactured from silicate melts. They retain their glassy X-ray amorphous structure in the solid state. 

The films containing more than 14 at. % of boron have X-ray amorphous structure. The surface also consists of hemispherical formations, and appears to be more smoothed. The grain boundaries are less pronounced. 

By rapid solidification, it is possible to produce alloys that show an X-ray amorphous structure. Typically, these alloys contain a large fraction of non-metallic elements, such as boron or phosphorus, which slow down crystallization. Atoms in amorphous alloys are less densely packed than in crystalline alloys. Furthermore, amorphous structures are thermodynamically unstable. Both factors should induce a... 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

Figure 2. These high-resolution micrographs show how a so-called x-ray amorphous, nonstoichiometric molybdenum sulfide catalyst exhibits structural (as well as compositional) heterogeneity. Amorphous, quasi-crystalline, and crystalline regions coexist at the ultramicro level (18,).

X-ray crystal structure analysis showed no crystallinity. Fructan is amorphous. X-ray analysis was performed on a General Electric X-ray diffraction refractometer. 

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

The most serious limitation of XRD as a catalyst characterization method is often related to the fact that many of the phases present in a catalyst may not give rise to any well-defined diffraction line at all. Absence of a diffraction pattern is a consequence of the requirement that a structure must contain a periodicity extending more than about 2-3 nm to yield a diffraction pattern measurable in a sense of the Bragg equation [Eq. (1)]. Thus, particles or domains with sizes smaller than 2-3 nm will appear to be X-ray amorphous in XRD experiments i.e., they do not exhibit sharp diffraction lines. 

Fig. 1. Schematic representation of different size distributions of structures present in typical catalysts. The X-ray amorphous region is indicated by the shaded area.

At the present time much effort is being devoted to tailor-making of new nanomaterials with specific catalytic properties. In this quest for constantly decreasing the dimensions of the catalytically active components, one will unavoidably encounter materials that will be partly or completely X-ray amorphous. The present review has shown that the combined EXAFS/ XRD techniques are uniquely well suited for providing the necessary structural understanding. Thus, in view of the trend in catalyst technologies and advances in technique developments, the application of the combined techniques will no doubt play an increasing role in future catalyst characterization efforts. We now briefly discuss some likely applications and technique developments which involve the X-ray techniques discussed presently. 

The above discussion exemplifies how a study of the different Mbssbauer parameters and their temperature dependences can give detailed information about the location of a non-Mossbauer isotope, lead, in its surrounding structure. It should perhaps for comparison be mentioned that the conventional technique of structure analysis, X-ray diffraction, did not enable the above information to be obtained, again showing the advantage of Mossbauer spectroscopy in the study of catalyst systems, which often may show X-ray amorphous features. 

Due to structural disordering caused by activation, the substance gets excess energy. A maximum amount of energy (up to 33 kJ/mol A1(0H)3) is accumulated at a moment when the thickness of plates reaches its minimum ( 2 nm). The product as prepared is X-ray amorphous and highly active. It is dissolved several times faster than non-activated one. 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

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