Growth rate needles

The injector needle and seat will experience impact loading and cyclic loading, so hydrogen embrittlement will cause chips and fractures of these components. Clark and Walter and Chandler showed that the fatigue crack growth rate of steel is increased in... 

Hematites with a particular crystal shape (plates, needles, spindles, pseudocubes, peanuts) and a narrow size distribution (monodisperse) can be obtained by adding various chemicals (shape controllers) to the system. The mechanism behind the control of shape is most likely to be the adsorption of impurities on certain crystal faces thereby reducing their growth rate in favor of that of the other faces. Internally these crystals may be either mono- or polydomainic, depending on the type and concentration of the additive. It must be kept in mind that higher additive concentrations may lead to product contamination. Some examples are summarized in the following section. 

Figure 11. Growth rate of the basal plane (diffusion-controlled) and the prismatic planes (interface-controlled) as a function of oversaturation in the oxinitride liquid. The oversaturation after the a/P-transformation depends on the surface energy (a), the grain thickness (2D) and the length of the needle like grains (2L).

Another interesting growth form exhibited by several substances is the development of regular isometrical shapes towards the end of the precipitation process. The phenomenon was described for the case of barium sulphate by Melikhov and Kelebeev (1979) who proposed the mechanism that, in a strongly supersaturated solution, homogeneous nucleation followed by growth yields isotropic crystals of about 2 nm in size. Their surfaces then develop zones with enhanced growth rates which lead to the formation of anisotropic needles. 

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