Crystals, fracturing

Trapped liquor may cause crystal fracture due to vapour pressure build-up on... 

Note that these hardness values are approximate because these crystals fracture very easily. The fracture surface energy of PETN is only 0.11 J/m2 (110erg/cm2) (Hagan and Chaudhri, 1977). 

Fracture often determines the reliability of a material in its practical applications. Brittle fracture of a material is the reason for a sudden catastrophe. The mechanical property ductile or brittle determines, in essence, whether or not a tool can be made from a given material. Let us identify the imperfections of a crystal and the chemical processes which cause ductility and brittleness. We distinguish two limiting cases of failure 1) A crystal, under external stress, deforms by forming a narrowing neck until eventually ductile rupture occurs. Dislocations are the only imperfections involved in this process of failure. 2) Crystals fracture suddenly. A sharp crack propagates and causes the failure. 

Fig. 12. Dependence of zinc single-crystal fracture strain on temperature and environment (a) air,(b) mercury (after Rozhanskii et al. - ref. 15)...

This idea was confirmed experimentally by Berschtein [29]. The solid surfaces contain potentially reactive interatomic bonds, therefore, generation and propagation of crystal fracture and under the action of stresses (applied and residual) in the presence of water proceed via mechanically induced hydrolysis. The experimental data obtained for loaded bonds in quartz in the presence or absence of water were treated according to equation... 

Some of these minerals possess high cation exchange capacities greatly in excess of that attributable to surface area, crystal fracture and edge effects. The reason for this difference in behaviour arises out of a third and major cause of exchange capacity, namely isomorphous... 

Optimization of mixing is also critical because of the need to achieve rapid blending while avoiding crystal fracture. 

Test high and low agitator speeds to determine the effect on the number of final crystals, either by increased nucleation or crystal fracture. 

Diffusion processes may also take place in the bulk of the solid, e.g., migration of ions along lattice vacancies or along crystal fractures and migration of sorbates in solid materials with a loose structure. These processes are also activated and may obey equations similar to Eq. [6] or [8]. 

By the use of a generalized population balance the MSMPR modelf is extended to account for unsteady-state operation, classified product removal, crystals in the feed, crystal fracture, variation in magma volume, and time-dependent growth rate. These variations are not included in the following derivations. 

Grinding produces crystal fracture surfaces and increases the surface activity of extenders. Since the surface also bears hydroxyl groups, the ground extenders are generally hydrophilic. 

Under polarization microscopa, liquid crystal exhibit Maltese-crosses optical textures, while crystals produced more angular (needle-like, rhombus, or dot-shapa) diffraction patterns The two states also reacted differently to pressure experiments, with the liquid-crystals dividing into smaller Maltese-cross droplets, while crystals fractured imer duress [Xu XH et al 2009, 2011a]. 

Fig. 9.19 The Hy of an nt-cBN bulk sample as a function of applied load (E). The Hy of the nt-cBN bulk decreases from 196 GPa at 0.2 N to its asymptotic value, 108 GPa, beyond 3 N. Error bars indicate s.d. (n = 5). Left inset, an optical micrograph of the Vickers indentation with cracks produced at a load of 19.6 N. Right inset, the Hy-E curve of a 0.3-mm cBN single crystal. Hy does not reach its asymptotic value, and the crystal fractures when E exceeds 4.9 N [33]. With kind permission of the Nature Publishing Group...

The most important imperfections, so far as the mechanical properties of crystals are concerned, are the various imperfections called dislocations. The ease with which dislocations move through a crystal determine to a large extent its ranges of elastic and plastic deformation under an applied stress and its ultimate yield point —that is, the stress under which the crystal fractures. One kind of dislocation, called an edge dislocation, is shown in Figure 17-11. An edge dislocation can be described as involving removal of one-half of a plane of atoms from the crystal. 

This theory of metallic bonding explains the physical properties of metals. If a stress is applied to the metal, the structure can change shape without the crystal fracturing. This is in contrast to the effect of stress on an ionic crystal, which will shatter. 

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