Alkali halide substrates

The two-step process of epitaxial polymerization has been applied to symmetrically substituted diacetylenes First, the monomers have been crystallized epitaxially on alkali halides substrates from solution and the vapor phase. The oriented monomer crystals are then polymerized under the substrate s influence by gamma-irradiation. The diacetylenes in this study are 2,4-hexadiyn-l,6-diol (HD) and the bis-phenylurethane of 5,7-dodecadiyn-l,12-diol (TCDU). The polydiacetylene crystal structures and morphologies have been examined with the electron microscope. Reactivity and polymorphism are found to be controlled by the substrate. 

Two diacetylenes have been epitaxially polymerized as thin films in contact with alkali halide substrates. These films in contact with alkali halide substrates. These films consisted of highly oriented single crystals aligned along both <110> directions of the substrate. The structures of both poly(TCDU) and poly(DMDA) were modified by this technique and, in all cases, highly crystalline near-perfect films were achieved. 

Polyphosphazene. Hexachlorocyclotriphosphazene (N P C1.) was obtained as purified crystals from the Army Research Laboratories. This trimer was deposited as thin films on alkali-halide substrates from the vapor, melt, and solution. Vapor phase deposition was accomplished by subliming N P Cl at 80 C in 10 torr vacuum. It was found that deposition times of around 20 minutes yielded incomplete films suitable for electron microscopy. 

Alkali Halide Substrates. (100) freshly cleaved surfaces of KCl, KI, KBr, NaCl, KF, and Rbl were used as epitaxial substrates for these materials. 

Ichihashi T, Tanigaki K, Ebbesen T W, Kuroshima S and lijima S 1992 Structures of Cgg thin films fabricated on alkali halide substrates by organic MBE Chem. Phys. Lett. 190 179-83... 

Synthesis.—In efforts to increase crystal perfection and hence electrical characteristics, SjNj vapour has been epitaxially crystallized onto a variety of alkali halide substrates, and the resulting crystals polymerized in the solid state to (SN) . While the normal a form of (SN) was obtained on KCl, a novel y form was discovered... 

Kintzel J. Edward, Smilgies Detlef Matthias, Skofronick G. James, Safron A. Safron, and Winkle Van H. David. Ultrathin film growth of p-phenylene oligomers on alkali halide substrates. J. Cryst. Growth. 289 no. 1 (2006) 345-350. 

The monoclinic phase of PE has also been obtained under different growth conditions, in the absence of mechanical stress. Teare and Holmes [15] suggested that it can be produced by crystallization at low temperatures. Keller and coworkers [17] found traces of this phase in germ-free PE droplets crystallized at very high supercooling, which is indeed known to induce unstable crystal forms in many systems. In [18-20] it was reported that epitaxial crystallization of PE from dilute solutions on alkali halide substrates may give rise to the monoclinic phase that transforms to the orthorhombic phase. Transformation takes place within a few nanometers from the substrate surface, and growth proceeds thereafter in the orthorhombic phase. 

Figure 6.11. Lattice matching conditions of fee C( and C70 crystals on layered materials and alkali halide substrates. The lattice matching conditions are expressed as amoi/Osub, where 0 01 denotes the intermolecular distance of C o/Cvo and as b the lattice parameter defined in the Figure. (Reproduced by permission from ref 42.)...

One method we might use is to cool the melt to incipient nuclei-formatlon. toss in the seed-crystal, and allow the melt to freeze into a single crystal. This is the KYROPOULOS method which we will discuss in detail later. Alas, this method only works for a few systems, notably alkali halides (cubic) and the like. We find that we can use a seed-crystal to grow single crystals, but only if we use it under carefully defined conditions. A modified K5rropoulos method has been used for many years to form single-crystal sapphire up to 13.0 inches in dieimeter. Plates cut from such crystals are used as windows and substrates for all sorts of integrated circuits, as well as watch "crystals". 

Epitaxial crystallization was accomplished by the immersion of a preheated substrate (the substrates for epitaxial crystallization were single crystals of alkali halides from Harshaw Chemical Co.) into the monomer solution followed by its in-situ cleavage to expose two fresh (100) surfaces. 

The synthesis of aliphatic nitro compounds from the reaction of alkyl halides with alkali metal nitrites was discovered by Kornblum and co-workers and is known as the modified Victor Meyer reaction or the Kornblum modification. The choice of solvent in these reactions is crucial when sodium nitrite is used as the nitrite soiuce. Both alkyl halide and nitrite anion must be in solution to react, and the higher the concentration of nitrite anion, the faster the reaction. For this reason, both DMF and DMSO are widely used as solvents, with both able to dissolve appreciable amounts of sodium nitrite. Although sodium nitrite is more soluble in DMSO than DMF the former can react with some halide substrates.Urea is occasionally added to DMF solutions of sodium nitrite to increase the solubility of this salt and hence increase reaction rates. Other alkali metal nitrites can be used in these reactions, like lithium nitrite,which is more soluble in DMF than sodium nitrite but is also less widely available. 

Epitaxial crystallization of polymers has been investigated for a wide variety of substrates minerals (alkali halides, talc, mica, and so on), low molecular weight organic materials (condensed and linear aromatics, benzoic acid and many of its substituted variants and their salts or hemiacids, other organic molecules of different types), and other crystalline polymers. 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

Salt metathesis reactions between a metal halide substrate and an alkali metal guanidinate (with the latter normally being generated by one of the routes (a) or (b)). 

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