Cleavage plane splitting

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

Spaltiings-energie, /, energy of splitting or fission. -flache,/. cleavage plane, cleavage face. 

Cleavage Plane— Preferred direction along smooth plane surfaces in which a mineral tends to split or cleave. Planes of cleavage are governed by atomic structure and represent direction in which atomic bonds are relatively weak. Amphiboles exhibit prismatic cleavage with an angle of about 55° between cleavage planes. 

However, if a force causes one layer to move slightly, ions with the same charge will be positioned next to each other. The cations in one layer are now lined up with other cations in a nearby layer. In the same way, anions from one layer are lined up with other anions in a nearby layer. Because the anions are next to each other, the like charges will repel each other and the layers will split apart. This is why all salts shatter along a line extending through the crystal known as a cleavage plane. 

Drawn wires and rolled sheet exhibit elongated fibrous grains, which can alleviate the embrittlement caused by interstitials. However, delamination can occur in heavily woriced materials during bending along the fiber boundaries. The splitting mode can be related to contaminated 100 cleavage planes [1.41,1.43]. 

In case of brittle solid materials, especially single crystals with clearly defined layer structure (e.g. mica), it is possible to use the cleavage method developed by Obreimow [44], In this method the crystal is split along the cleavage plane (Fig. 1-23), and the force that has to be applied to cause further development of the crack, Fc, is measured. 

Copper(II) azide crystallizes in an orthorhombic Pnma space group with four molecules per unit cell. The crystals are stable in air and mostly grow with a few well-developed faces of holohedral symmetry. There are also perfect cleavage planes along the needle axis which cause the crystals to split at the slightest pressure into a bundle of thin fibers. 

At T = 2 K, in the jt-polarised reflection spectrum of the (OOl)-cleavage planes of anthracene crystals, one observes spectrally-resolved surface excitons (Fig. 6.17). The Davydov splittings of their 0,0 transitions are only 10-20% smaller than those of the volume excitons (Sect. 6.7 and Ref [39]). Discuss this experimental result qualitatively (cf Sect. 6.5.2). 

Since most sample holders have some type of alkali metal-halide window, a brief discussion of salt polishing is in order. It is not difficult for the spectroscopist to prepare and polish salt plates. If the NaCl is bought as a crystalline block, it can be cleaved into suitable windows with a single edged razor which is hit with a small hammer or screw driver shaft. When a cleavage is started, it is followed until complete. Blocks may be split into halves or thirds safely, but if much smaller sections are cleaved from the block, the smaller pieces may break. The halves of thirds can then be split. The cleavage planes are usually (but not always) parallel to the planes of the uncleaved block. NaCl and KBr will cleave but CsBr does not cleave and plates are bought preshaped. 

If we split this structure between the (TeO) chains and rotate every second chip by ISO"", we end up with the H2Te206 structure [300] disregarding the H atoms. In H2Te206, the Te octahedra, still linked through corners, now form chains which are connected by the same —Te —O—Te—O— chains to infinite sheets. These sheets are held together by H bonds and van der Waals forces only, resulting in perfect cleavage planes. 

They are brittle. Ionic crystals may split apart when hit in the same direction asthe layers of ions. The layers of ions may be displaced by the force of the blow so that ions with the same charge come together. The repulsions between thousands of ions in the layers, all with the same charge, cause the crystal to split along these cleavage planes. 

Fig. 2. Surface of E. coli B after freeze etching. At a few narrow areas, the outer membrane (OM) is split in an intermediate plane (arrows). The granular layer represents the cleavage plane within the plasma (= inner) membrane (IM). (80,000x)...

Metals that fracture with a relatively small or negligible amount of plastic strain exhibit brittle fracture. Cracks propagate rapidly. Brittle failure results from cleavage (splitting along definite planes). Ductile fracture is better than brittle fracture, because ductile fracture occurs over a period of time, where as brittle fracture is fast, and can occur (with flaws) at lower stress levels than a ductile fracture. Figure 1 shows the basic types of fracture. 

Cleavage n. (1) Breaking of a laminate due to the separation of the strata. (2) Portion of the material adhering to the side of a container after the major portion has been drained. (3) The property of a crystalline substance of splitting along definite crystal planes. 

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