Particle shape acicular

Table 2.4 Qualitative terms for particle shape Acicular needle-shaped...

Particle shapes are classified as acicular. A, monoclinic and blocky, M, or monoclinic and needle, N. 

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. 

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... 

Depending on operation conditions and metal properties, the shapes of the atomized particles may be spheroidal, flaky, acicular, or irregular, but spherical shape is predominant. The spheroidal particles are coarse. For example, roller-atomized Sn particles exhibited a mass median diameter of 220 to 680 pm. The large particle sizes and highly irregular particle shapes suggested that the disintegration process may be arrested either by the premature solidification or by the formation of a thick, viscous oxide layer on the liquid surface. The particle size distributions were found to closely follow a log-normal pattern even for non-uniform particle shapes. 

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. 

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. 

At this point, we are most interested in the size of particles and how the other factors relate to the problem of size. However, particle shape will determine how we define size. Most of the particles that we will encounter are spheroidal hi shape. But. if we discover that we have needle-like (acicular) particles, how do we define their average diameter Is it an average of the sum of length plus cross-section, or what ... 

We have already discussed measurement of particle size by the use of particle dimensions in Section II. where particle shape as relating to particle size was a major concern. Note that even though the shape of the particle may be quite elongated, the stated size wiU be predicated upon a spheroidal shape. The stated (measured) size of a PSD based upon weight will depend upon how well the particles were separated into fractions. For example, if we use a set of sieves to separate particle fractions, the fractions of particles measured may, or may not, be a function of shape. Acicular particles will not pass through a given screen the same way as... 

FIGURE 9.3 Particle Shapes (a) Spherical - Calcite (b) Acicular - Calcite with Stibnite (c) Fibrous - ActinoUte (d) Dendritic - Gold (e) Hakes - Abhurite (f) Polyhedron - Apophyllite (g) CyUndrical Particles (For color version of this figure, please refer the color plate section at the end of the book.)... 

Particles may be nearly spherical, cubic, nodular (a rounded irregular shape), acicular (needle- or rod-like) or lamellar (plate-like). Since particle shape affects pigment packing, it therefore affects hiding power. Rod-shaped particles can reinforce paint films, like iron bars in concrete, or they may tend to poke through the surface reducing gloss. 

Using hydrotriorganoborates in the presence of compressed hydrogen (20 - 100 bar), we were able for the first time to reduce iron oxides to yield well shaped acicular iron particles at only 80 °C. 

Particle shape spherical j acicular spherical irregular... 

Particle shape spherical spherical/ prismatic acicular spherical... 

The active element, or material, within the active element gives the fired film its electrical properties. If the active material is a metal, the fired film will be a conductor if it is a conductive metal oxide, a resistor and, if it is an insulator, a dielectric. The active metal is in powder form ranging from 1 to 10 m, with a mean diameter of about 5 m. Particle morphology can be varied greatly depending on the method used to produce the metallic particles. Spherical, flaked, or acicular shapes (both amorphous and crystalline) are available from powder manufacturing processes. Structural shape and particle morphology is critical to the development of the desired electrical performance and, therefore, the control on the particle shape, size, and distribution must be maintained to ensure uniformity of the properties of the fired film [23,24]. 

Shape of particles may be assessed descriptively by terms such as spherical, elongated, acicular, angular, or a host of other terms. Although these are descriptive terms, if accurately used, they can convey a general idea of the particle shape. However, they reveal little about the degree to which the particles take upon a particular shape. Without a comparable quantitative measure, it may be difficult to assess the effects of particle shape on a process or product. 

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. 

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