Technical problems

Frequency- and angular-dependent longitudinal Zeeman rdaxation in liquid crystals 

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic 

To test the quality of both the established and the disputed relaxation models in the light of the new angular-dependent results, we performed simulations and tried model fits of the pertinent theories for proton spin relaxation by reorientations of dipolar coupled inter- and intra-molecular proton pairs. Assuming fast magnetization transfer between unlike spin pairs and a superposition of the three independent reorientations (M) known to be important mechanisms for common nematic liquid crystals, namely nematic 

In FC experiments, where the external Zeeman field, 5o (with selectable 

For simplicity, in equation (3c) S oc is assumed to be mainly along the director axis n, and n is aligned parallel tol. For Btoc Bo, the total 7, is equal to 7,2, whereas in the opposite case B,oc Bq one has 7, = 7,o- Evidently, the finite local field contribution complicates the control of the angle adjustment without an exact knowledge of, oc, and because of equation (3c) the inclinations of 90° become impossible by external field switches. Furthermore, the spectral densities for 7,0 are not discussed in the literature to the same extent as for 7,2, nor does there exist a critical experimental examination of the validity of the basic expression equation (3a). Approximate predictions about with the Redfield formalism give, for the completely isolated, i.e., uncoupled proton spin-pair (/ = 1) and high spin-temperature approach  

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Prices of spandex fibers are highly dependent on thread size selling price generally increases as fiber tex decreases. Factors that contribute to the relatively high cost of spandex fibers include (/) the relatively high cost of raw materials, (2) the small size of the spandex market compared to that of hard fibers which limits scale and thus efficiency of production units, and (J) the technical problems associated with stretch fibers that limit productivity rates and conversion efficiencies. 

There are many technical problems to be considered when developing a new commercial and viable filter. However, the filtration hardware in itself is not enough, as the control of a continuous pressure filter is much more difficult than that of its equivalents in vacuum filtration the necessary development may also include an automatic, computerized control system. This moves pressure filtration from low to medium or even high technology. Disk Filters. 

Early in the twentieth century, the first attempts to manufacture formamide directiy from ammonia and carbon monoxide under high temperature and pressure encountered difficult technical problems and low yields (23). Only the introduction of alkaU alkoxides in alcohoHc solution, ie, the presence of alcoholate as a catalyst, led to the development of satisfactory large-scale formamide processes (24). 

Wood Pulping. The system Mg(OH)2 S02 H20 is also used in acid bisulfite pulping. Compared to a calcium-based system which is not as amenable to regeneration of the pulping bisulfite (87), fewer technical problems are encountered in the digesters, evaporators, or recovery boiler of the Mg-based process. In the presence of excess SO2, bisulfite forms in a 43% MgSO solution, at 25°C and 101.3 kPa (1 atm) SO2 pressure, to increase MgSO solubihty. 

Operational Constraints and Problems. Synthetic ammonia manufacture is a mature technology and all fundamental technical problems have been solved. However, extensive know-how in the constmction and operation of the faciUties is required. Although apparendy simple in concept, these facihties are complex in practice. Some of the myriad operational parameters, such as feedstock source or quaUty, change frequendy and the plant operator has to adjust accordingly. Most modem facihties rely on computers to monitor and optimize performance on a continual basis. This situation can produce problems where industrial expertise is lacking. 

The highest power of a reactor of the HTGR type was 330 MWe in Fort St. Vrain, Colorado. The reactor, started in 1979, had many technical problems, including helium leaks, and did not perform up to expectations. It was shut down in 1989. 

Neither of these processes has been commercialized, although some aspects of the methodology were incorporated into a plant operated for a short time by the Elcor Company (31). This company, which operated briefly in western Texas in 1968 using natural gypsum, is the only one known to have commercially attempted to recover elemental sulfur from this material by a two-step thermal process. The Elcor plant was shut down shortly after it began operation. Although most technical problems were said to have been solved, production costs were prohibitive. 

For operations producing 30,000 tons or less of copper annuaHy, hydrometaHurgy offers an alternative to smelting that avoids problems associated with sulfur dioxide recovery and environmental controls. Techniques include the Anaconda oxygen—ammonia leaching process, the Lake Shore roast-leach-electrowin process, and ferric chloride leaching processes for the treatment of copper sulfides. AH the facHities that use these techniques encountered serious technical problems and were shut down within a few years of start-up. 

Many of the technical problems of fabrication that formerly inhibited the use of titanium alloys in dental castings (164—166) have been effectively solved, and titanium castings may now be obtained for virtually any type of dental appHance at prices that are increasingly competitive. Special induction or electric-arc furnaces are necessary for casting titanium alloys, and this specialized equipment has, until now, been available in only a limited number of commercial dental laboratories. However, the relatively high price of this equipment, attributed to development costs, is expected to decline significantly this should help to improve the general availabihty of cast titanium appHances. 

The revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

The technical problem in die high teiiiperamre application of Si3N4 is that unlike the pure material, which can be prepared in small quantities by CVD for example, die commercial material is made by sintering the nitride with additives, such as MgO. The presence of the additive increases the rate of oxidation, when compared with the pure material, by an order of magnitude, probably due to the formation of liquid magnesia-silica solutions, which provide short-circuits for oxygen diffusion. These solutions are also known to reduce the mechanical strength at these temperatures. 

Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

Process investigation manager responsible for investigating technical problems and for transferring laboratory results to plant-scale operations. 

Attention must be paid to the specific technical problems posed by measuring flow in industrial ventilating systems, such as high turbulence level and long time-variation of mean velocity. The LDA measurement conditions (statistically sufficient number of LDA data, suitably long duration of LDA measurements for recognition of long-term phenomena) must be carefully selected for an appropriate treatment of these problems. 

Perhaps the most visible technical problems studied and the most data available on shock-compressed solids are focused on the loading portion of wave profiles. Often, the portion of the wave profile corresponding to the release of pressure to atmospheric, but elevated temperature, values is the more descriptive of solids in the high pressure state. 

Studies of the electrical and mechanical responses of ferroelectric solids under shock compression show this technical problem to be the most complex of any investigated. The combination of rate-dependent mechanical and electrical processes along with strong electromechanical coupling has clouded physical interpretation of the numerous investigations. 

Technical problems. How does one actually mn the program and what does one look for in the output ... 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

SOME TECHNICAL PROBLEMS IN KKR CALCULATIONS FOR ANY BRA VAIS LATTICE... 

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