Melt laboratory techniques

Samples investigated were 0.3-3.0 o,m amorphous Sb Sei (0 < x < 0.05) films. These were preferentially prepared by conventional vacuum evaporation onto room-temperature sihca-glass substrates (plates). The a-Sb Sei source material was made by the usual melt-quenching technique. Coohng rate was estimated to be 100-200 K/s. Prior to measurements, the films prepared were aged at laboratory conditions (natural aging) for several weeks to allow their structure to equilibrate. 

If you have been highlighting a polymer a week, the first four experiments in Section A— Free Radical Polymerization, Synthesis of Nylon, Synthesis of Polyesters in the Melt, and Synthesis of a Polyurethane Foam —are excellent demonstrations to intersperse with the content as it is presented. If you want your students to actually perform the experiments, it might be best to wait until the end of a first-year chemistry course when the students have developed their laboratory techniques to the greatest extent. Another use for the four experiments would be to introduce a different one each quarter and discuss the polymer produced in the experiment. This is a good way to use the information on polymer chemistry if time does not permit the presentation of a Polymer of the Week. 

Filtration is employed to remove insoluble impurities from solutions and melts. The techniques of filtration of the laboratory and small manufacturing scales need not be repeated here. It is appropriate to remember the wide range of filtration media and aids that are now available and that filtration may be performed hot or cold and under temperature control. For the efficient removal of relatively small quantities of fine suspended matter, centrifugation provides a suitable approach. 

This section presents simple laboratory techniques that can be used to screen the potential of melt crystallization as a purification or concentration technique for chemicals, pharmaceuticals, and foods. These substances will be in most of the cases of organic nature, but the described techniques are suitable for water or other inorganic substances and metals as well. 

Traditional methods of additive analysis and the required instruments are often expensive and require the efforts of a skilled technician or chemist. In some cases a single instmment can not provide analyses for the wide variety of additives a particular organisation utilises. Additionally, laboratory techniques rarely provide results in a timely fashion. Determination of physical properties is not the least important if one thinks of pigments, talc and other fillers. Application of spectroscopic techniques to polymer production processes permits real-time measurement of those qualitative variables that form the polymer manufacturing specification, i.e. both chemical properties (composition, additive concentration) and physical properties (such as melt index, density). On-line analysis may intercept plant problems such as computer error, mechanical problems and human error with respect to additive incorporation in the resin production. Characterisation and quantitative determination of additives in technical polymers is an important but difficult issue in process and quality control. 

These apparent restrictions in size and length of simulation time of the fully quantum-mechanical methods or molecular-dynamics methods with continuous degrees of freedom in real space are the basic reason why the direct simulation of lattice models of the Ising type or of solid-on-solid type is still the most popular technique to simulate crystal growth processes. Consequently, a substantial part of this article will deal with scientific problems on those time and length scales which are simultaneously accessible by the experimental STM methods on one hand and by Monte Carlo lattice simulations on the other hand. Even these methods, however, are too microscopic to incorporate the boundary conditions from the laboratory set-up into the models in a reahstic way. Therefore one uses phenomenological models of the phase-field or sharp-interface type, and finally even finite-element methods, to treat the diffusion transport and hydrodynamic convections which control a reahstic crystal growth process from the melt on an industrial scale. 

Arc furnaces (cold crucible technique). Small, commercial, laboratory arc-melting equipment generally includes a high-vacuum/aigon atmosphere pumping... 

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

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