Hematite shape

Iron Oxide Reds. From a chemical point of view, red iron oxides are based on the stmcture of hematite, a-Fe202, and can be prepared in various shades, from orange through pure red to violet. Different shades are controlled primarily by the oxide s particle si2e, shape, and surface properties. Production. Four methods are commercially used in the preparation of iron oxide reds two-stage calcination of FeS047H2 O precipitation from an aqueous solution thermal dehydration of yellow goethite, a-FeO(OH) and oxidation of synthetic black oxide, Fe O. ... 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

Chen, M., Jiang, J., Zhou, X. and Diao, G. (2008) Preparation of akaganeite nanorods and their transformation tosphere shape hematite. Journal for Nanoscience and Nanotechnology, 8, 3942-3948. 

The most common ore of iron is hematite that appears as black sand on beaches or black seams when exposed in the ground. Iron ores (ferric oxides) also vary in color from brownish-red to brick red to cherry red with a metallic shine. Small amounts of iron and iron alloys with nickel and cobalt were found in meteorites (siderite) by early humans. This limited supply was used to shape tools and crude weapons. 

TEM and differential X-ray line broadening (expressed by the ratio of the width at half height of the 104 relative to that of the 110 reflection) indicate that the thickness of the platy Al-hematite crystals decreases as Al/(Fe-t Al) increases (Schwertmann et al., 1977 Barron et al., 1984). It is this change in morphology, rather than the structural Al, which governs the IR spectra, in particular the shape factor and the absorp-... 

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Hematites grown hy forced hydrolysis of acidic Fe " solutions at elevated temperatures also show a range of crystal morphologies. The type of anion, the acidity and the presence of additives appear to be main factors that influence the shape (Matijevic Scheiner, 1978 Kandori et al., 1991 Bailey et al., 1993). Additives appear to act by adsorbing on specific planes of the growing crystal. At close to 100 °C, rhombohedral crystals, 50-100 nm in size, sometimes showing intergrowths (stepped appearance)... 

Fig. 4.22 I Uniform ( monodispersed ) ellipsoidal and peanut-shaped hematites produced by the so-called gel-sol method ofT. Sugimoto (Sugimoto et al.,1993, 1998, with permission).

Tab. 4.4 Experimental conditions for the production of monodispersed hematites with various crystal shapes (Schwertmann and Cornell, 2000 with permission)...

Hematite formed by dehydroxylation of oxide hydroxides at temperatures below 500-600 °C is porous. That formed by heating goethite in vacuo at 300 °C contains slit shaped meso pores which coalesce to circular macropores at temperatures >400°C (Naono and Fujiwara, 1980). At even higher temperatures, these pores are... 

Fig. 6.12 Diffuse reflectance spectra of hematites of different crystal size (left) and different crystal shape (right) (Hund, 1981, with permission).

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

For all three types of anisotropy the coercivity can be calculated (Tab. 7.7). For SD magnetite and maghemite, shape anisotropy, dominates over strain and crystal anisotropy, whereas for hematite and goethite, morphology has little influence on coer-civity. 

Hematite derived from dehydroxylation of FeOOH at temperatures below 600 °C shows marked, non-uniform (differential) broadening of the XRD lines. Some authors have attributed this effect to the anisotropic shape of the coherently diffracting domains of hematite (Duvigneaud Derie, 1980), and others to the development... 

The sample used by Naono et al. (1982) was a non-porous one (based on a t-plot) (Fig. 14.8) with a BET surface area of 22 m g . It developed a maximum surface area of 178 m g at 200 °C due to the formation of a system of slit-shaped pores ca. one nm wide (see Fig. 14.2 c). During this process, a contraction of ca. 30% occurred along [100] and [010], but not along [001], i.e. not along the tunnels. With increasing temperature, the pores widened to mesopores and irregular macropores. The surface area of the hematite that finally formed at 500 °C was only 23 m g . ... 

Fig. 14.13 Extent of transformation at 70 °C of akaganeite to goethite and hematite versus time, a) rod-shaped akaganeite in M KOH b-d) spindle-shaped akaganeite b) M KOH c) 0.1 M KOH d) 0.1 M KOH -h Mn -" (Mn/(Fe-hMn) = 0.1) (Cornell Giovanoli, 1990,1991 with permission).

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