Grain-growth studies

2 Metastable eutectic condensation and magnesium-rich minerals in astrophysics 

As the peaks observed near 33 and 69 p.m are due to phonon transitions in the crystalline silicate lattice, the ISO observations are an unambiguous indication of the presence of crystalline silicates in cometary dust. More recently, crystalline olivine and orthopyroxene have been identified in Comet C/2001 Q4 (NEAT) via 10 pm observations (Wooden et al. 2004). There is considerable controversy concerning the origin of these crystalline grains. 

As much as 2 to 5% of the interstellar silicate grain population could be crystalline, yet still lie below our current limits of detection (Li Draine 2001 Kemper et al. 2005). As said before, it could equally well be true that any crystalline silicates injected into the interstellar medium become amorphous after long exposure to galactic cosmic rays. Finally, studies on the spectral evolution of silicate grains in protoplanetary disks indicate that the degree of crystallinity is not simply correlated with age (see Chapter 8 for more details). These observations, argue that processes within the protostellar nebula itself are responsible for the production of crystalline silicates. 

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Grain-growth studies have also been carried out under 1 atm of nitrogen pressure [25]. The results were found to be different from those studies performed at 10 atm. The time exponent n for both directions was equal to 3. This will result in an equation in which the exponent of the time term is equal to one, and the aspect ratio is time independent during sintering. 

M. Kramer, M. J. Hoffmann and G. Petzow, Grain Growth Studies of Silicon nitride Dispersed in an Oxynitride glass , J. Am. Ceram. Soc., 76, 1993, 2778-84. 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

Plant and Animal Nutrient. Copper is one of seven micronutrients that has been identified as essential to the proper growth of plants (87). Cereal crops are by far the most affected by copper deficiency (see Wheat and other cereal grains). Greenhouse studies have shown yield increases from 38% to over 500% for wheat, barley, and oats (88) using copper supplementation. A tenfold increase in the yield of oats was reported in France (89). Symptoms of copper deficiency vary depending on species, but often it is accompanied by withering or chlorosis in the leaves that is not ammenable to iron supplementation. In high concentrations, particularly in low pH sods, copper can be toxic to plants. 

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

Recrystallisation and grain growth after deformation has been studied by in situ SXRT experiments by Gastaldi, Jourdan and co-workers for over a decade. 

Lodder. J., C.P. Kurtzman, and J.W. Fell The Yeasts, A Taxonomic Study, 4th Edition, Elsevier Science, New York. NY, 1998 Milner, M. and W.F. Geddes Grain Storage Studies. III. The Relation between Moisture Content, Mold Growth, and Respiration of Soybeans." Cereal Chem., 23, 225 (1946)... 

Painter, G.S., Becher, P.F., Shelton, W.A., Satet, R.L. and Hoffmann, M.J., (2004), First-principles study of rare-earth effects on grain growth and microstructure in P-Si3N4 ceramics , Phys. Rev. B, 70, article 144108. 

The motivation of this study was to establish a relationship between the structure of the films and their electrical and optical characteristics, which strongly depend on the indium concentration [11, 20], The conclusion of this study is clearly beyond the scope of this review. But the polarized EXAFS study has clearly evidenced that the indium doping affects the grain growth process by increasing the surface anisotropy of films. Finally this example shows that we have powerful experimental but also theoretical tools to study surfaces or oriented systems. 

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