Primary crystallites

Line-width of XRD peaks of these acidic Cs salts (x = 2.1. 2.2, and 2.5) show that the size of the primary crystallites was about 120 A and the primary crystallites themselves are nonporous [29]. Therefore, the pores observed in the present study correspond to the interparticle voids (not intraparticle). The pore structure and the mechanism of the formation of pores will be discussed in our forthcoming paper. 

Re-organization of amorphous chains in the interstitial regions between pre-existing crystallites leads to the formation of secondary crystallites. The new crystallites have smaller a , b , and "c dimensions than the primary crystallites. 

Electron microscopy 0.07 Detecting primary crystallite size... 

Fig. 12. The ratio between the activity of propylene polymerization catalysts and their specific surface. The specific surface of the samples was calculated from the average sizes of primary crystallites, determined by using X-ray techniques, on the assumption that the crystallites have the form of hexagonal prisms (001 axis). Polymerization at 70°C, cocatalyst AlEtjCl, monomer concentration 1 M. From Yermakov et at. (13).

In the Fries rearrangement of PA, high yield and selectivity for o-HAP can be obtained with large pore zeolites such as HFAU, HBEA and average pore zeolites such as HMFI. HBEA zeolite modified by ion exchange with cerium[77] and especially a commercial HMFI made of primary crystallites and agglomerates joined by finely dispersed alumina[78] were claimed to be particularly stable and selective for the formation of o-HAP. 

With continuing exposure to oxygen compounds, irreversible damage to catalyst activity may occur due to the growth of iron primary crystallites. This damage depends on the water vapor partial pressure and is especially serious at high temperatures. 

During milling, the fragmentation of polycrystalhne material and primary crystallites proceeds until the fragmentation limit is reached. The particles undergo mainly plastic deformations and accumulate defects, which leads to an amorphization of the solid. 

The primary effect of milling a solid is to increase its specific surface area. Most reactions between a solid and another phase are proportional to the extent of specific surface area being in contact. This includes outer surfaces as well as the inner surfaces of pore systems and primary crystallites. Intensive milling decreases the size of the primary crystallites which tend to lower the energy by recrystallization. Therefore, a limiting crystallite size can be reached for a given experimental set-up. Most of the crystallite surface is not accessible for fluid reactants, but changes in the pore system and the outer surface allow for an increased reaction rate. 

As outlined above, nucleation rates are extremely sensitive to temperature changes. Therefore, precipitation temperature is a decisive factor in controlling precipitate properties such as primary crystallite sizes, surface areas, and even the phases formed. However, it is very difficult to state how the precipitation temperature has to be adjusted to achieve a product with specific properties. The optimum precipitation temperature usually is a parameter which has to be determined experimentally. 

Chang PL, Yen FS, Cheng KC, Wen HL (2001) Examirratiorrs on the critical and primary crystallite sizes during 0-to a-phase transformation of rrltrafine alumina powders. Nano Letters 1 253-261 Charlet L, Manceau A (1992) X-ray absorption spectroscopic study of the sorption of Cr(IIl) at the oxide/water interface. 11 Adsorptiorr, coprecipitation and surface precipitation on ferric hydrous oxides. J Colloid Interface Sci 148 425-442... 

Properties Primary crystallites 50-150 nm, BET specific surface area 23 m /g [618,2524]. 

The nature of this acceleration period was a subject of debate for a long time. Early research by Natta and Pasquon [12] using a-TiCls catalysts established that breakdown of the a-TiCls matrix took place due to the mechanical pressure of the growing polymer chains in the early stages of the polymerization, thus exposing fresh Ti atoms and leading to the formation of new active centers with consequent increase in the polymerization rate. After breakdown of the a-TiCls matrix to primary crystallites has taken place, a steady rate is observed for a significant period of time. Such attainment of a steady-rate behavior permits accurate measurement of polymerization kinetics. 

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