Crystallization manipulation

Figure 2. Schematic view of the spectrometer. The components illustrated include M, crystal manipulator Q.M.S., quadrupole mass spectrometer I.G., primary ion source E.S., energy spectrometer G, Bayard-Alpert gauge T, crystal target and G.I., gas inlet. Auxiliary components are omitted for graphical clarity. The SIMS experimental geometry and coordinate systems are defined in the inset. Reproduced with permission from Ref. 4. Copyright 1981, American Institute of Physics.

Figure 2 Schematic view of the apparatus used in studies of the steric effects in gas-surface scattering. A detail of the crystal mount with die orientation rod at 1 cm in front of the surface is shown in die right hand corner. A detailed drawing of the hexapole state selector is given below the main figure. The voltage is applied to die six small rods indicated by an arrow. Key Q quadrupole mass spectrometer R Rempi detector M, crystal manipulator SI, beam source for state selected molecules H electric hexapole state selector C mechanical beam chopper V pulsed gas source S2, continuous molecular beam source. From Tenner et al. [34].

Fig. 3. Cross-section of the combined UPS-XPS-LEED system described by Bradshaw and Menzel (25). A - Hemispherical electron energy analyser B - Slit-change mechanism C - Qua-drupole mass spectrometer D - Crystal manipulator E - LEED optics F - Assembly for electron bombardment of the sample replaceable by an argon ion gun G - Rare gas resonance lamp with axis perpendicular to the plane of the cross-section H - X-ray source. [Reproduced with permission from Ber. Bunsenges. 78, 1140 (1974)]...

Essential diffraction apparatus consists of electron gun (98-101), crystal manipulator, and beam detection components. These are housed in a bakeable chamber, in which vacuums of 1 X 10 Torr or better are easily developed. The usual material is stainless steel, rather than glass as in earlier years. Sputter-ion pumps are more commonly used than diffusion pumps, because their pumping speeds are greater and back-streaming problems associated with hydrocarbon (or mercury) vapors are reduced, and of course they are much more convenient. Gases are introduced through bakeable leak valves, either directly from flasks or from a header where they can be mixed to a desired composition. 

The most desirable characteristics of a solvent for recrystalhsation are (a) a high solvent power for the substance to be purified at elevated temperatures and a comparatively low solvent power at the laboratory temperature or below (6) it should dissolve the impurities readily or to only a very small extent (c) it should yield well-formed crystals of the purified compound and (d) it must be capable of easy removal from the crystals of the purified compound, i.e., possess a relatively low boiling point. It is assumed, of course, that the solvent does not react chemically with the substance to be purified. If two or more solvents appear to be equally suitable for the recrystallisation, the final selection will depend upon such factors as ease of manipulation, inflammability and cost. 

The isothermal curves of mechanical properties in Chap. 3 are actually master curves constructed on the basis of the principles described here. Note that the manipulations are formally similar to the superpositioning of isotherms for crystallization in Fig. 4.8b, except that the objective here is to connect rather than superimpose the segments. Figure 4.17 shows a set of stress relaxation moduli measured on polystyrene of molecular weight 1.83 X 10 . These moduli were measured over a relatively narrow range of readily accessible times and over the range of temperatures shown in Fig. 4.17. We shall leave as an assignment the construction of a master curve from these data (Problem 10). 

For sodium palmitate, 5-phase is the thermodynamically preferred, or equiUbrium state, at room temperature and up to - 60° C P-phase contains a higher level of hydration and forms at higher temperatures and CO-phase is an anhydrous crystal that forms at temperatures comparable to P-phase. Most soap in the soHd state exists in one or a combination of these three phases. The phase diagram refers to equiUbrium states. In practice, the drying routes and other mechanical manipulation utilized in the formation of soHd soap can result in the formation of nonequilibrium phase stmcture. This point is important when dealing with the manufacturing of soap bars and their performance. 

The piopeities of a ceramic material that make it suitable for a given electronic appHcation are intimately related to such physical properties as crystal stmcture, crystallographic defects, grain boundaries, domain stmcture, microstmcture, and macrostmcture. The development of ceramics that possess desirable electronic properties requires an understanding of the relationship between material stmctural characteristics and electronic properties and how processing conditions maybe manipulated to control stmctural features. 

However, in rare cases, crystallisation is not a satisfactory method of purification, especially if the impurity forms crystals that are isomorphous with the material being purified. In fact, the impurity content may even be greater in such recrystallised material. For this reason, it still remains necessary to test for impurities and to remove or adequately lower their concentrations by suitable chemical manipulation prior to recrystallisation. 

SIMS, and SNMS in rare cases, such as for HgCdJTei samples or some polymers, the sample structure can be modified by the incident ion beam. These effects can often be eliminated or minimized by limitii the total number of particles incident on the sample, increasing the analytical area, or by cooling the sample. Also, if channeling of the ion beam occurs in a crystal sample, this must be included in the data analysis or serious inaccuracies can result. To avoid unwanted channelii, samples are often manipulated during the analysis to present an average or random crystal orientation. 

The most arresting development is the use of an STM tip, manipulated to move both laterally and vertically, to shepherd individual atoms across a crystal surface to generate features of predeterminate shapes an atom can be contacted, lifted, transported and redeposited under visual control. This was first demonstrated at... 

IBM in California by Eigler and Schweizer (1990), who manipulated individual xenon atoms aeross a niekel (110) crystal surfaee. In the immediate aftermath of this achievement, many other variants of atom manipulation by STM have been published, and DiNardo surveys these. 

After the seed crystals had arrived it was found that crystallization practically always occurred when amounts of 100 g of any laboratory sample were slowly warmed over a period of a day, after cooling to liquid-air temperatures. This occurred even when great precautions were taken to exclude the presence of seeds. However, it was found readily possible, by temperature manipulation alone, to produce crystalline or supercooled glycerol at will. 

Cyclodimer 3 proved to be somewhat difficult to manipulate, thus contributing to the complexity of its characterization. The bowed diacetylenic linkages revealed in the X-ray data impart surprising physical characteristics to the molecule. The energy-rich hydrocarbon was sufficiently strained that it decomposed explosively upon grinding (i. e. preparing a Nujol mull) or when heated above 80°C. At room temperature, crystals blackened within a few days and apparently auto-polymerized, even when stored under vacuum in the dark. Only dilute solutions of 3 in benzene or pyridine were fairly stable over time, especially when stored cold under an inert atmosphere. 

Figure 7.6 shows a schematic set up for TPD. The crystal, mounted on a manipulator in an ultrahigh vacuum chamber, is heated such that the temperature increases linearly in time. The concentration of desorbing species is monitored with a mass spectrometer or with a simple pressure gauge. 

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