Intercrystalline component

The shell of living rhynchonellate brachiopods typically consists of an organic perio-stracum succeeded by a thin calcitic primary layer and a secondary layer composed of alternately stacked calcitic fibres sheathed in membranes (Williams, 1966). The only significant variations in this sequence are the virtual suppression of secondary shell secretion in thecideidines and the deposition of a prismatic tertiary layer in some terebratulides (Williams, 1968). In 1965 Jope reported the extraction of amino acids, lipids and carbohydrates from living (and fossil) rhynchonellates. These organic residues were mainly intercrystalline components of the shell. However, organic traces. 

Model of a nanostructured material filled circles represent crystalline component (CC) atoms whereas the open circles represent intercrystalline component (IC) atoms... 

If a NS monocrystal takes up a single component from a fluid phase and Intercrystalline transport does not influence the uptake rate, one should be aware of the possibility that, besides intracrystalline diffusion, the following processes may either contribute or even govern the uptake rate ... 

The ability of PPG NMR, to monitor simultaneously the mobihty of different components [159,164] makes it a very effective tool for studying the mobility of the reactant and product molecules during chemical reaction. Figure 19 shows the results of in-situ PPG NMR measurement during the conversion of cyclopropane to propene in zeohte Na-X [165]. hi addition to the diffusivity of the reactant molecule (cyclopropane) and the product molecule (propene), also the time dependence of the relative amounts of the involved molecular species is presented. Since the conversion times are much larger than the intercrystalline exchange times as following from the diffusivities, the considered reaction may clearly be assumed to be reaction controlled. 

Of interest in this connection is the fact that atmospheric corrosion mainly occurs in the form of a more or less uniformly distributed scar-shaped erosion. However, other forms of corrosions are also encountered. Thus, for instance, the impact of a humid atmosphere on zinc alloys may produce intercrystalline corrosion, and the impact of a humid atmosphere containing NH3 may lead to stress cracks in brass components. 

Failure of copper alloys may occur by cracking due to the combined influence of tensile stress and exposure to a corrosive environment. When the stresses are produced in components during manufacture the trouble is usually known as season cracking and failures of brass components due to this form of stress corrosion have been known for many years Only certain specific environments appear to produce stress corrosion of copper alloys, notably ammonia or ammonium compounds or related compounds such as amines. Mercury or solutions of mercury salts (which cause deposition of mercury) or other molten metals will also cause cracking, but the mechanism is undoubtedly different. Cracks produced by mercury are always intercrystalline, but ammonia may produce cracks that are transcrystalline or intercrystalline, or a mixture of both, according to circumstances. As an illustration of this, Edmunds" found that mercury would not produce cracking in a stressed single crystal of brass, but ammonia did. 

Cr-8Ni-2Mo, available as covered electrodes, sohd wire, and sohd wire associated with a flux. It was used for Phenix components, but its reputation has been tarnished since it would not fully fulfill intercrystalline corrosion tests. 

The structure of high modulus polyethylene fibres obtained by optimized drawing of linear polyethylene is viewed as crystalline lamellae linked by intercrystalline bridges.Accordingly, the component B is then viewed as crystalline, and its content (1 — >1) corresponds to the volume fraction of the material incorporated in the crystalline bridges. A more complex model consisting of four components has been proposed for these fibres by Grubb. 

---