Acicular in shape

Moganite nanoaystals are pinacoid (acicular) in shape and about 1 micron in size. Quartz nanocrystals are trapezohedral in shape, about 2 or 3 micron in size, and they are present in large quantities in chalcedony crystal. 

At this point, we are most interested in the size of particles and how the other factors relate to the question of size. The next most important factor is shape. Most of the particles that we will encounter are spheroidal or oblong in shape, but if we discover that we have needle-like (acicular) particles, how do we define their average diameter Is it an avereige of the sum of length plus cross-section, or what ... 

Production. Metallic iron pigments are commercially produced by the reduction of acicular (needle-shaped) iron compounds [5.30], As in the production of magnetic iron oxide pigments, the starting materials are iron oxide hydroxides (see Section 3.1.1) or iron oxalates, which are reduced to iron in a stream of hydrogen either directly or via oxidic intermediates. 

Talcs are either platy or acicular in particle shape. Thin platelet particles have aspect ratios varying from 20 1 to 5 1. Coarse particle sizes (10 to 75 im) are commonly used in these applications at loading levels of 5 to 30 percent. Fine talcs (1 to 10 im) are more expensive and require intensive dispersion processes. Platy grades enhance barrier properties and air, water, and chemical resistance. 

This method produces 5 g of reasonably well crystalline goethite with Al substitution of ca. 30 mol%. The crystals are acicular/somatoidal in shape in contrast to the acicular form of those produced under strongly alkaline... 

However further increase of pressure may result in particle size increases and a more acicular particle shape because of increasing solubility in modified CO2. Most applications require pressures below 20 MPa. 

The term filler is very broad and encompasses a very wide range of materials. In this book, we arbitrarily define as fillers a variety of solid particulate materials (inorganic, organic) that may be irregular, acicular, fibrous, or plate-like in shape and that are used in reasonably large volume loadings in plastics. Pigments and elastomeric matrices are normally not included in this definition. 

Ashley and Innes (151) in 1952 pointed out that the low viscosity of colloidal silica at high concentrations clearly showed that the particles were spherical in shape, since any other shape, chainlike or even acicular, would cause a much higher viscosity. The conclusion is that silica in some way polymerizes to dense particles of more or less spherical shape, at least above neutrality. 

These images were obtained of the same crystal from Fig. 7. Cyan colour nanocrystals are acicular (pinacoid) in shape and nanocrystals in yellow are trapezohedral in shape. 

Excellent bonding can be achieved with zinc phosphate and mixed metal phosphates but the particle size and quantity of phosphate applied are very important. There are two main physical forms of phosphate that are used in commerce. One is an amorphous structure applied at a level typically below 4 g/m and the other an acicular (needle shaped) structure typically applied at a level of 15 g/m. The acicular form is used as an absorbent substrate for coatings and oils used to enhance the phosphate layer as protective coating. The acicular form is unsuitable for bonding as the crystal structure can fracture under the bonding agent primer. 

In the one-dimensional form, filler may be available in the thicker variety as a fiber or thinner (acicular-needle-shaped) variety as a whisker. Fillers available as fibers are glass, nylon, polyester, carbon and so on. Wollastonite stands out as a good example of an acicular... 

The chains formed by diese silica tetrahedra are cormected by calcium in octahedral coordination. Because of this chain structiu e wollastonite can occiu as acicular crystals, in some cases of macroscopic dimensions. This acicular particle shape is important in certain uses as a functional mineral filler. 

Particle Shape and Structure. Some materials exhibit particular properties owing to their particle shape or form, eg, the plate-like minerals talcum and mica or acicular woUastonite. It is often desired to maintain particle shape in such cases, an impact-type mill is usually chosen rather than a ball mill, as the latter tends to alter the original particle shape. 

Mfldelchen, n, little needle (or pin), I adel-eisenerz, n., -eisenstein, m. needle iron ore (gdthite in acicular crystals), -erz, n, needle ore (aikinite in acicular crystals), -faser, /, acicular fiber. nadelfSrmig, a. needle-shaped, acicular. Madel-holz, n. conifers (collectively), esp, pines and firs wood of conifers, soft wood, -holzer, n.pl. (Bot.) Finales, Coniferae, -holzkohle,/. soft-wood charcoal, -holzteer, m, soft-wood tar, (loosely) pine tar. -holzzellstoff, m. cellulose (or pulp) from conifers, nadelig, a. needly, acicular. 

Some of the common terms used for defining or describing particle shapes in a qualitative way are presented in Table 2.1. It is by now quite clear that particle shape cannot be very precisely defined. However, it is fortunate that mineral particles occur in a variety of generally simple shapes some are acicular, several are plate-like, most are convex, others are mildly concave, and in this manner a phraseological descriptive list is built for the different shapes that are formed or produced or generally encountered of powders of fragments of... 

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