Iron crystal size

Almost all the iron oxides, hydroxides and oxide hydroxides are crystalline. The degree of structural order and the crystal size are, however, variable and depend on the conditions under which the crystals were formed. All Fe oxides display a range of crystallinities except for ferrihydrite and schwertmannite which are poorly crystalline. 

An estimate of the surface area of Fe oxides in a mixture of other fine grained compounds, e. g. in soils or sediments, may be obtained from the crystal size calculated from XRD line broadening using the Scherrer formula (see Chap. 7) or from the difference in area before and after selective removal of the iron oxides divided by the amount of oxides that were extracted (see Chap. 16). The latter method assumes that the areas of the various components are additive (Schwertmann, 1988). 

The effect of aluminium on the surface area of goethite depends on the level of Al in the system and on the source of iron. Other conditions being equal, Al reduces both the rate of growth and the crystal size its effect on surface area depends on which of these two effects predominates. The surface area (EGME) of goethite grown from ferrihydrite in 0.3 M KOH at 25 °C dropped from 52 to 26 m g as the extent of Al substitution rose from 0 to 0.16 mol mol (Schulze and Schwertmann, 1987). This effect was attributed to an increase in crystal thickness along the [001] direction... 

Iron oxides in soils have in common that they are of extremely small crystal size and/or low crystal order. This, in combination with their low concentration (only tens g kg in most soils) explains why soil iron oxides have escaped identification for a long time in spite of their obvious existence as seen from the soil colour. In the past, therefore, Fe oxides in surface environments have been considered to be amorphous to X-rays and often called limonite , which mineralogically, is an obsolete term. Furthermore, in order to identify the clay minerals in soils properly, Fe oxides are usually removed before X-ray diffraction methods are applied (Alexander et al., 1939 Mehra Jackson, 1960). 

Murad, E. Schwertmann, U. (1984) The influence of crystallinity on the Mdssbauer spectrum of lepidocrocite. Min. Mag. 48 507-511 Murad, E. Schwertmann, U. (1986) Influence of Al substitution and crystal size on the room-temperature Mdssbauer spectrum of hematite. Clays Clay Min. 34 1-6 Murad, E. Schwertmann, U. (1988) Iron oxide mineralogy of some deep-sea ferromanganese crusts. Am. Min. 73 1395-1400 Murad, E. Schwertmann, U. (1993) Temporal stability of a fine-grained magnetite. Clays Clay Min. 41 111-113... 

There appear to be two possible explanations for these central peaks in the natural hematite. One is that some of the ferric material is actually not hematite but is bound to clay that might be present. Other workers (11) obained similar results when they prepared samples of iron compounds absorbed on kaolinite or bentonite. The second reason concerns the effect of small crystal size on Mossbauer spectra and perhaps is better illustrated for goethite. 

Potentially of equal importance is the relationship between strain and catalyst stability. A calculation of the contribution to the total free energy of a catalyst crystal caused by the presence of strain-inducing microscopic precipitates50 showed that the extra free energy increases with the size of the crystal and inhibits it from sintering. This theory is an interesting one since it provides a mechanism which the catalyst scientist can exploit in his search for stable, high surface-area materials. The theory predicts the equilibrium crystallite size of the iron crystals of an ammonia synthesis catalyst with acceptable accuracy. 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

The electrical and magnetic properties of pure iron in relation to the crystal size. Phil. Mag., 31 357-366. 

For different values of n ing(i ) = i2", other kinetic expressions can be developed. Figure 8.10 [18] shows the type of powder produced on spray diydng a solution that consists of metal salts of barium and iron in the ratio 1 12 (i.e., barium ferrite). Here we see the remains of the spherical droplets with a surface that consists of the metal salt precipitates, which form a narrow size distribution of platelet crystals (see Figure 8.10(a) and (b)). This narrow crystal size distribution is predicted by the population balance model if nudeation takes place over a short period of time. When these particles are spray roasted (in a plasma gun), the particles are highly sintered into spherical particles (see Figure 8.10(c)). 

Diflusion of Fe2+ ions and electrons to the nuclei and growth of the nuclei to iron crystals of various size... 

There are two major processes for the manufacture of titanium dioxide pigments, namely (1) sulfate route and (2) chloride route. In the sulfate process, the ore limonite, Fe0Ti02, is dissolved in sulfuric acid and the resultant solution is hydrolyzed by boiling to produce a hydrated oxide, while the iron remains in solution. The precipitated titanium hydrate is washed and leached free of soluble impurities. Controlled calcinations at about 1000°C produce pigmentary titanium dioxide of the correct crystal size distribution this material is then subjected to a finishing coating treatment and milling. The process flow sheet is shown in Fig. 7.1 [4],... 

Posfai M., Cziner K., Marton E., Marton P., Buseck P. R., Frankel R. B., and Bazyhnski D. A. (2001) Crystal-size distributions and possible biogenic origin of Fe sulfides. Biogenic Iron Minerals Symposium and Workshop, Tihany, Hungary, May 20-23, 2000. Euro. J. Mineral. 13,691—703. 

The detailed description of each preparative procedure is followed by a description of the properties of the product and by conunents concerning possible variations of the method and their effects on the properties of the product. For thorough characterization numerous illustrations including color plates. X-ray diffractograms, absorption speetra (IR, Mossbauer) and electron micrographs are included. This eharacterization is necessary in view of the wide range of crystal morphologies and crystal sizes displayed by most iron oxides. It should enable the users of this book to obtain a particular product with the desired eharacteristics and also provides a check on the success of the users own efforts. 

One of the most striking features of iron oxides is their eye-catching color (see Plates V and VI). The colors include purple, red, brown, orange, yellow, black, and even greenish blue (green rust). To some extent, these colors are characteristic and, thus, diagnostic of the type of mineral, its crystal size and shape, and impurities within the crystal... 

Ferrihydrite is generally the initial precipitate that results from rapid hydrolysis of Fe solutions. Its crystallinity, i.e. crystal size and order, is usually lower than that of any of the other Fe oxides described except feroxyhyte and schwertmannite. It is usually named according to the number of its XRD peaks, with 6-8 broad peaks for well crystalline (6-line-) ferrihydrite and only two very broad ones for the most poorly crystalline form (2-line-ferrihydrite). The 2-line ferrihydrite is commonly but incorrectly called hydrous ferric oxide (HFO) or, amorphous iron oxide . In natural environments all forms of ferrihydrite are widespread usually as yoimg Fe oxides and they play an important role as an active sorbent due to their very high surface area. 

There are two types of alloys. In a substitutional alloy, some of the metal atoms in a crystal lattice are replaced by other atoms (usually of comparable size). Examples are brass, in which approximately one third of the atoms in a copper crystal are replaced by zinc atoms, and pewter, an alloy of tin that contains 7% copper, 6% bismuth, and 2% antimony. In an interstitial alloy, atoms of one or more additional elements enter the interstitial sites of the host metal lattice. An example is steel, in which carbon atoms occupy interstitial sites of an iron crystal, making the material stronger and harder than pure iron. Mild steel contains less than 0.2% C and is used for nails, whereas high-carbon steels can contain up to 1.5% C and are used in specialty applications such as tools and springs. Alloy steels are both substitutional and interstitial atoms from metals such as chromium and vanadium substitute for iron atoms, with carbon remaining in interstitial sites. Alloy steels have a variety of specialized purposes, ranging from cutlery to bicycle frames. 

This hydrolysis proceeds slowly at temperatures just below 100°C, and 1 to 2 hours is needed to complete a typical precipitation. Urea is particularly valuable for the precipitation of hydrous oxides or basic salts. For example, hydrous oxides of iron(III) and aluminum, formed by direct addition of base, are bulky and gelatinous masses that are heavily contaminated and difficult to filter. In contrast, when these same products are produced by homogeneous generation of hydroxide ion, they are dense and easily filtered and have considerably higher purity. Figure 12-5 shows hydrous oxide precipitates of aluminum formed by direct addition of base and by homogeneous precipitates with urea. Homogeneous precipitation of crystalline precipitates also results in marked increases in crystal size as well as improvements in purity. 

However, the nature, crystallinity (Kinniburg and Jackson, 1976, 1981 McKenzie, 1980), crystal size, and surface charge of metal oxides and mixed metal oxides (e.g., Fe-Al oxides Violante et al., 2003) also play an important role in the sorption selectivity of trace elements in cationic form. McBride (1982) compared the sorption behavior of different Al precipitation products of different crystallinity. The Cu sorption capacity followed tlie order noncrystalline Al-hydroxide > poorly crystalline boehmite > gibbsite. Iron and Mn oxides are... 

Owing to the large crystal size, the subsequent filtration, washing, and cake drying steps are facilitated, and the catalyst stability is enhanced. The isolated yield of product is typically in the range of 95-97% of theory, an improvement over that for the Co(II) complex. Ironically, oxidation of the Co(II) complex to the Co(III)OAc catalyst is problematic during isolation of the Co(II) species and results in non-trivial losses. Fig. 3 shows an enlarged view of representative catalyst crystals prepared via this new method. 

This result gives the dependence of crystal yield strength on particle size and work hardening due to the creation and movement of N dislocations during plastic flow. In the limit of small particle size, < 20 microns, where typically dUo/dx (N d)r Uo and the third order correction term is small, the yield stress behaves as Ty Nj if. This prediction accounts for the observed increased yield strength of small crystals and forms the basis of the specialty iron and steel industry [26.27], It is also likely to be the reason why the shock and impact sensitivity decrease with crystal size as. ... 

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