Ideal powders

A crystal smaller than about 10 6 cm in diameter gives a broadened diffracted beam. For quantitative treatment it is first necessary to define breadth . A line on a powder photograph (Fig. 233) shows maximum intensity at the centre, and fades away on either side a simple definition of breadth is the angular width between the points at which the intensity falls to half its maximum value. In an ideal powder photograph of an extremely narrow specimen taken with a very narrow beam, the relation between this breadth at half height j3 and... 

The characteristics of ideal powders and real powders produced by hydrothermal processing are shown in Table 1.6 and Table 1.7. Hydrothermal powders are close to ideal powders. 

Ideally powders should be taken while standing or sitting upright... 

In the case of spray flux. Fig. 21-103 illustrates an idealized powder bed of width B moving past a flat spray of spray rate dV/dt at a solids velocity of w. For a given spray rate, the number of drops is determined by drop volume, which in turn defines the drop area a per unit time that will be covered by the spray, giving a spray flux of... 

Changing the orientation of the crystal reorients the reciprocal lattice bringing different reciprocal lattice points on to the surface of the Ewald sphere. An ideal powder contains individual crystallites in all possible orientations with equal probability. In the Ewald construction, every reciprocal lattice point is smeared out onto the surface of a sphere centered on the origin of reciprocal space. This is illustrated in Figure 1.9. The orientation of the vector is lost and the three-dimensional vector space is reduced to one dimension of the modulus of the vector A ti-... 

Fig. 2a-e. Idealized powder ESR patterns, together with their corresponding absorption profile a isotropic b axial and c rhombic... 

Mechanical milling cannot produce fine powders even after long-term milling, the limiting particle size is 0.2 pm. Extended milling has several shortcomings the spheres of the mill become worn and introduce significant quantities of undesired impurities into the powder the distribution in particle size becomes extremely wide. Such characteristics are different from those expected for an ideal powder. Table 3.1. 

Each method yields powders suitable for sintering but with different morphologies, crystallinity, specific surface area, oxygen, carbon and impurity contents, all of which can significantly influence the rate of densification. The ideal powder should have the following characteristics ... 

Figure 8.30 Stress intensification factor versus relative density for various idealized powder geometries and for the data of Shima and Oyane (49), (From Ref. 50.)...

For structure solution and refinement, diffraction peak intensities need to be highly reproducible to represent X-ray scattering from the crystal. An ideal powder sample is composed of grains <5 pm in size. If the grains are shaped as needles or plates, steps must be taken to prepare the sample in a fashion that minimizes a preferred orientation of the grains. In this respect, measurements of their Debye-Scherrer ring by means of CCD area detectors or e-scanning with point detectors may be necessary to screen powder samples. 

One of the advantages that thoria presents is that it is very stable with respect to high temperatures. Once a thoria produced powder is high fired to 1600°C, it is virtually physically stable. The surface chemistry is also stable with no change in stoichiometry. It is therefore an ideal powder with which to perform basic research. 

The need for ceramics with improved mechanical properties has led to tremendous interest in methods for preparation of superior powders. In particular, many studies have examined methods for making monodisperse particles from solution. This is based on the argument that the ideal powder should be pure, stoichiometric, dense, equiaxed (i.e., spheroidal), and nearly monodisperse. Uniform particle size is said to facilitate preparation of stable dispersions, as well as dense, uniform powder compacts (76]. However, if the particles are well dispersed, a denser body can be made with a range of particle sizes, as the smaller ones fit into the spaces between the larger ones. It is sometimes argued that the best sintering... 

P.L. Bransby and P.M. Blair-Fish. Initial deformations during mass flow from a bunker Observation and idealizations. Powder Technology, ll(3) 273-288, 1975. 

It should be noted that this set of ideal powder characteristics has been found to be generally useful for many ceramic systems. One must, however, be careful in determining the exact set of characteristics that are appropriate for the particular application at hand, i.e., the exact characteristics for a SigN powder for structural applications will be different from those required for a 1-2-3 high T. superconducting powder. 

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