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Dopant roles

Since the work of Coble in 1961, the MgO-doped AI2O3 system has represented one of the most celebrated examples of the effectiveness of the solid solution approach to microstructure control. Whereas the dramatic effect of MgO on the sintered microstructure of AI2O3 was easily reproduced (see Fig. 11.43), understanding the mechanism by which MgO acts proved to be more difficult despite numerous investigations. One of the main obstacles in the earlier work has been the use of powders containing impurities that have tended to mask the true effect of MgO. However, as described in a thorough review by Bennison and Harmer (81), the dopant role in this system is now fairly well understood, at least at a phenomenological level. [Pg.751]

Repeatedly in this book, the important functions of dopants , intentional additives made in small amounts to materials, have been highlighted the use of minor additives to the tungsten used to make lamp filaments is one major example. The role of impurities, both intentional and unintentional, in matters such as phase transformations, mechanical properties and diffusion, was critically reviewed in one of the early seminar volumes published by the American Society for Metals (Marzke 1955). But extreme purity was not considered that came a little later. [Pg.357]

The theoretical hmit of 5.4% (NaAlH4+2 mol% TiN) for the two subsequent decomposition reactions is in both cases only observed in the first cycle. The reason for the decrease in capacity is stiU unknown and litde is known about the mechanism of alanate activation via titanium dopants in the sohd state. Certainly, the ease of titanium hydride formation and decomposition plays a key role in this process, but whether titanium substitution in the alanate or the formation of a titanium aluminum alloys, i.e., finely dispersed titanium species in the decomposition products is crucial, is stiU under debate [41]. [Pg.288]

As mentioned in the introduction, the first empirical correlation between the absolute configuration of dopants and the handedness of induced cholesterics was proposed in 1975.20 The first attempt to find a general correlation was a few years later Krabbe et al.58 related the sense of the cholesteric to a stereochemical descriptor of the dopant based on the effective volume of the substituents and listed many compounds following this rule. However, exceptions were described at that time,59 and, furthermore, this approach neglects the role of the structure of the nematic solvent in determining the sense of the cholesteric. It is well known that chiral compounds may induce cholesterics of opposite handedness in different nematics.60,61... [Pg.442]

The role of dopants such as Sr2+ and Pb2+ is to alter the Curie temperature of the barium titanate so that devices that switch at a desired temperature can be fabricated. [Pg.128]

The hugely complicated chemistry of the catalytic converter is described in some depth in the Open University s excellent book Physical Chemistry Principles of Chemical Change, Topic study 2 . Its part 1 is entitled the three-way catalytic converter , Open University, Milton Keynes, 1996. It covers the composition of the catalytic surface and the role of each dopant, the actual chemical reactions occurring, and details of the current legal situation regarding atmospheric pollution. [Pg.556]

The physical and chemical properties of the entrapped dopants are generally retained. Yet, the efficient isolation of one molecule from another and the active role played by the sol gel cage, for instance in dictating accessibility, gives place to a vast new chemistry and physics of sol gel entrapped molecules which largely encompasses and goes beyond traditional solution chemistry. [Pg.16]

What role does the dopant play in APPI (Ionization of molecules having a low photoionization cross section (probability) has been shown to be enhanced by the use of a dopant that is introduced into the vaporized plume of analyte molecules the dopant is selected on the basis of its high UV absorptivity and serves as a charge transfer reagent). [Pg.400]

Choi, W., Termin, A., Hoffmann, M. 1994. The role of metal ion dopants in quantum-sized TiOj. J Phys Chem 98 13669-13679. [Pg.154]

Figure 6.1 shows the mass spectmm of an impurity run in the positive APPI mode and shows both at miz 352 and MH miz 353. This is an example where the mechanisms of proton transfer and electron transfer are both taking place. This can be confusing when dealing with complete unknowns and demonstrates why it is unsuitable for routine use in an open access multi-user environment or by inexperienced users. A better understanding of the processes involved and the role of mobile phase and dopant is required before this can be put to routine use. There are a number of papers published on this topic [20, 21]. [Pg.160]

Lattice defects also play a prominent role in the reactivity of pyrotechnic materials, where the displacement or absence of atoms or ions from their normal positions within a lattice can have a pronounced effect on properties, as can the introduction of foreign species or dopants within a crystal. [Pg.20]

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). [Pg.147]

Much less studied has been the role of V species in n-pentane oxidation to MA and PA. Papers published in this field are aimed mainly at the determination of the reaction mechanism for the formation of PA (2-4,11). Moreover, it has been established that one key factor to obtain high selectivity to PA is the degree of crystallinity of the VPP amorphous catalysts are not selective to PA, and the progressive increase of crystallinity during catalyst equilibration increases the formation of this compound at the expense of MA and carbon oxides (12). Also, the acid properties of the VPP, controlled by the addition of suitable dopants, were found to play an important role in the formation of PA (2). [Pg.110]


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See also in sourсe #XX -- [ Pg.633 ]




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