Hematite seeds

PZC/IEP of Hematite Obtained by Aging of Hematite Seeds in Acidified FeClj Solution... 

Figure 6 presents the particle characteristic face length as a function of the number concentration of seed material to compare the effectiveness of alumina seeds against hematite seeds. The particle size control is relatively linear against the log of concentration. The number concentration of hematite seeds required to achieve equivalent results to the alumina seeds is greater by approximately two orders of magnitude. Lattice mismatch in the hematite seeds is not a probable cause for the difference, as hematite seeds have been shown to act as epitaxial sites for seed growth (22). 

Wet preparation of red iron oxides can involve either a hydrothermal process (see Hydrothermal processing) or a direct precipitation and growth of iron oxide particles on specially prepared nucleating seeds of Fe202- In the hydrothermal process, iron(II) salt is chemically oxidized to iron(III) salt, which is further treated by alkahes to precipitate a hydrated iron(III) oxide gel. The gel can be dehydrated to anhydrous hematite under pressure at a temperature around 150°C. 

Transparent red iron oxide is composed mainly of hematite, a-Ee202, having primary particles about 10 nm. It is prepared by a precipitation reaction from a dilute solution of an iron salt at a temperature around 30°C, foUowed by a complete oxidation in the presence of some seeding additives,... 

Hirse, /. millet, -erz, n. oolitic hematite, -fieber, n. miliaria, -kom, Hirsenkom, n. millet seed-... 

At temperatures of up to 70 °C, akaganeite grown by hydrolysis of FeCl3 is stable for months in the acidic mother liquor (Cornell, 1992). If, however, the system is seeded with goethite or hematite, the akaganeite gradually transforms into these com-... 

The mechanism by which hematite is formed from ferrihydrite in an aqueous system, appears more complicated than that by which goethite forms. If hematite crystals are added to the system they do not function as seeds for hematite formation but induce epitaxial growth of goethite instead (Atkinson et al. 1968 Cornell Giovanoli, 1985). 

Particularly, they proved by a seeding analysis that hematite particles in such... 

Size control of the hematite particles must be performed during this nucleation period by controlling the temperature or pH, or by adding very fine seeds of ot-Fe203. 

The seeding technique can also be used for controlling the size of the monodis-persed hematite particles. If we combine seeding technique with a shape controller such as sulfate or phosphate ions, the size and shape with different aspect ratios of... 

Fig. 1.3.12 TEM images of ellipsoidal hematite particles of different sizes prepared by the seeding technique at 100°C with different contents of seed suspension (S.S.) in the presence of a fixed concentration of sulfate ions at 3.0 X 10-2 mol dm-3. The aging times were fixed at 2 days for (a), (b), and (c) and 3 days for (d), (e), and (f). (From Ref. 15.)...

Transparent red iron oxides containing iron oxide hydrate can also be produced directly by precipitation. A hematite content of > 85 % can be obtained when iron(II) hydroxide or iron(II) carbonate is precipitated from iron(II) salt solutions at ca. 30 °C and when oxidation is carried out to completion with aeration and seeding additives (e.g., chlorides of magnesium, calcium, or aluminum) [5.271], Transparent iron oxides can also be synthesized by heating finely atomized liquid pentacarbonyl iron in the presence of excess air at 580-800 °C [5.272], [5.273]. The products have a primary particle size of ca. 10 nm, are X-ray amorphous, and have an isometric particle form. Hues ranging from red to orange can be obtained with this procedure, however, it is not suitable for yellow hues. 

Hematite forms competitively with akaganeite at reaction temperatures above 90 °C and goethite forms competitively if seed crystals of goethite are added (Atkinson et al., 1977). 

For the larger particles. Hematite I is used as a seed sol, providing the nuclei for further growth on their surface. Seed sol and ferric chloride are added in this case to boiling perchloric acid. 

Examples of the uses of ceramic particles in a matrix (rather than as a powder) include the use of glass beads for hardening rubber for tires and adding kaolin to paper to make it smoother and easier to print on. The properties depend not only on the particle but also on the PB that encloses it. Particles are used to seed crystallization and other phase transformations. Hematite particles can be used to seed the growth of a-Al203 at temperatures lower than the usual phase transformation so that the grain size can be kept small. 

Seed preparation, a-hematite (a-Fe203) sol particles were synthesized as seed materials by the MacCallum method (21). 22.7 grams of Fe(N03)3 9 H O was... 

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