Aaberg push-pull

For tests on plain or notched specimens, a simple approach can be to use a conventional Wohler rotating cantilever beam modified so as to permit the specimen to be brought into contact with the corrodent. This may be achieved by surrounding the specimen with a cell through which the corrosive solution is circulated or by applying it by a pad wick or drip feed . Four-point loading or push-pull machines can be used in a similar way, and have the advantage over the Wohler machine when testing plain specimens that the length of the test-piece between the two points of loading is subjected to an approximately uniform stress.  [c.1052]

In Chapter VIII, Haas and Zilberg propose to follow the phase of the total electronic wave function as a function of the nuclear coordinates with the aim of locating conical intersections. For this purpose, they present the theoretical basis for this approach and apply it for conical intersections connecting the two lowest singlet states (Si and So). The analysis starts with the Pauli principle and is assisted by the permutational symmetry of the electronic wave function. In particular, this approach allows the selection of two coordinates along which the conical intersections are to be found.  [c.770]

When you are ready to start, cover the first page with a card and pull it down to reveal the first frame. Read and act on that frame, then reveal frame 2 and so on. If you are unfamiliar with the disconnection approach, I suggest you read the introduction Wby bother with disconnections so that you can see what I m driving at. Otherwise the first sections of the programme may seem rather pointless.  [c.2]

Classification of the many different encapsulation processes is usehil. Previous schemes employing the categories chemical or physical are unsatisfactory because many so-called chemical processes involve exclusively physical phenomena, whereas so-called physical processes can utilize chemical phenomena. An alternative approach is to classify all encapsulation processes as either Type A or Type B processes. Type A processes are defined as those in which capsule formation occurs entirely in a Hquid-filled stirred tank or tubular reactor. Emulsion and dispersion stabiUty play a key role in determining the success of such processes. Type B processes are processes in which capsule formation occurs because a coating is sprayed or deposited in some manner onto the surface of a Hquid or soHd core material dispersed in a gas phase or vacuum. This category also includes processes in which Hquid droplets containing core material are sprayed into a gas phase and subsequentiy solidified to produce microcapsules. Emulsion and dispersion stabilization can play a key role in the success of Type B processes also.  [c.318]

Centrifugal Discharge. In centrifugal discharge elevators, the buckets are spaced apart on the chain or belt. Material is scooped from the boot and then discharged by centrifugal force as the buckets approach and pass over the head pulley or sprocket. Speed of the chain or belt is critical to proper discharge of the material. The critical speed has been defined as the speed at the point where the centrifugal force at the center of mass of the material in the bucket is equal to the gravitational force (30). The best operating point for an elevator is where the centrifugal force is two-thirds the gravitational force. The effects of bucket geometry and speed on material discharge have also been reported (31,32). Changes in exit trajectory of the material leaving the bucket as a result of material friction on the bucket wall and the change in center of gravity of the mass as it sHdes along the wall have been investigated.  [c.159]

Another approach to the design phase of the structure-based design cycle utilizes the computet program DOCK, which iacreases the chances that compounds chosen for testing will be inhibitors. Novel lead compounds have been developed for l ctobacillus casei thymidylate synthase by usiag DOCK, x-ray crystallography, and additional computational analysis for the selection of potential inhibitors from a pool of commercially available compounds (61). Initial, weak-binding inhibitors were found by using DOCK and the x-ray crystal stmcture of TS to screen the small molecule database, FCD. X-ray crystal stmctures of resulting enzyme-inhibitor complexes were further analyzed, generating a pool of potential inhibitors. DOCK was then used to assess the complementarity of these molecules to the enzyme active site, and they were rank-ordered accordingly. As can be seen in Table 2, many of the top-scoring compounds showed inhibition in the ]lM range. The stmctures of the top tanked compounds with IC qS at or below 100 ]lM are shown in Figure 9. These phenolphthalein derivatives are quite different in stmcture from the natural substrates of TS, which is useful for novel lead discovery.  [c.327]

However, for specific load demands and accurate minimum and maximum torque requirements, the number of steps can be calculated on the basis of actual requirement as discussed in Section 5.2.2. Sometimes, for an economical design of the resistance unit, the number of steps can be reduced without jeopardizing the basic requirement of torque except for slightly higher or lower limits in its values as shown in Figure 5.6(a). Also, as we approach the rated speed, the closer become the steps and the greater the tendency to become infinite. In such a. situation it becomes essential to liberalize the torque limits to achieve a quicker objective. Moreover, by now the motor will have almost run to its full speed and will pose no problem of a current or torque kick. Sometimes, however, when the motor possesses a substantially high pull-out torque than is required by the eonnected load, a quick removal of external resistanee may cause an abrupt jump in the torque and may exert a jerk on the load, whieh may not be desirable. To overcome such a situation and to economize on the resistance design, a small resistance is sometimes left permanently connected in  [c.90]

The vibrationally excited products in the ground electronic state are created when the end O atom is abstracted from the O3 molecule as a result of the approach of the NO molecule at an ONO angle of 110°. The barrier height equals 3.57 kcal/mol. The activation energy 2.13 kcal/mol is close to the measured one. In the transition state the saddle point is shifted towards the reactants valley and the distances Rqo and Rnq are equal, respectively, to 1.277 A and 1.957 A, while the equilibrium bond lengths in O3 and NO are 1.272 A and 1.150 A. Comparison of the configurations of the van der Waals complex (1) and of the transition state shows that the arrangement of the reactants provides the same attack angle as in the gas-phase reaction. The 0-0 bond stretches by less than 0.2 A. The O-NO length becomes reduced by 0.45 A. Seemingly, intermolecular vibrations as well as the hindered rotation play a promoting role. Because of the low frequencies of these modes the low-temperature limit is not reached even at 10 K.  [c.129]

The morphology of urethane adhesives, i.e., hard segments amid low Tg rubbery soft segments, provides certain advantages and disadvantages. If a high flexural modulus is desired, urethanes may not be the best choice. Equally disadvantageous would be other adhesives with similar morphologies, i.e., plastic domains within a rubbery matrix, e.g., a high rubber SBS triblock, flexible epoxies, etc. Urethanes can be designed to approach the structural modulus of some of the rigid epoxies, cyanate esters, etc., and this will be discussed later. In order to raise the modulus of the adhesive, it is necessary to raise the glass transition temperature of the soft segment, thereby moving away from the standard urethane morphology of hard segments in a rubber matrix. Increased modulus within the standard urethane morphology can be accomplished by raising the level of hard segment (chain extender) and adding certain fillers [38]. Higher levels of chain extenders would also be expected to increase shrinkage in the bond line, if the reaction takes place within the confines of the bond itself. If too much chain extender is added to the urethane, a phase inversion may occur, in which the plastic portion of the urethane, derived from the chain extender/isocyanate reaction, becomes the continuous phase. The rubber portion of the urethane, from the polyol/isocyanate reaction, becomes the discontinuous phase. At these high chain extender levels, increased modulus is achieved, but this creates a network that may easily undergo brittle fracture.  [c.775]

The application of environmentally benign principles to laboratory-scale synthesis can be illustrated by revisiting the oxidation of alcohols. As noted in Section 15.10, the most widely used methods involve Cr(VI)-based oxidizing agents. Cr(VI) compounds are carcinogenic, however, and appear on the EPA list of compounds requiring special disposal methods. The best way to replace Cr(VI)-based oxidants would be to develop catalytic methods analogous to those used in industry. Another approach would be to use oxidizing agents that are less hazardous, such as sodium hypochlorite. Aqueous solutions of sodium hypochlorite are available as "swimming-pool chlorine," and procedures for their use in oxidizing secondary alcohols to ketones have been developed. One is described on page 71 of the January 1991 edition of the Journal of Chemical Education.  [c.644]

Ring opening reactions of aziridines which are initiated by nucleophilic reagents have been shown to proceed with extensive, if not complete, inversion of configuration at the point of attack (S7JA734). When unsymmetrical aziridines are involved, ring opening can occur in either of two different directions. Frequently, the nucleophile attacks the less hindered carbon atom preferably with the result that one direction of ring opening is predominant (S9CRV737). However, such reactions are generally difficult to predict because the product ratio can easily be affected by changes in the solvent and in the proportion of the reagents. The diverse and often seemingly contradictory facts pertaining to the opening of this strained ring can be correlated in terms of a push-pull mechanism (64HC(19-1)524). According to this concept the major factors involved in such processes are approach of the nucleophilic reagent, the rupture of the C—X bond, and the effect of the electrophilic reagent. As a result, steric factors are less influential than usual, while sensitivity to factors such as solvent, resonance and the presence of electron releasing substituents is substantially increased.  [c.70]

The Stem and double-layer systems play an important role in stabilizing colloidal suspensions, and conversely, the study of suspensions has provided much information about the electrical nature of the interface. In particular, lyopho-bic sols are often stabilized by the presence of a surface charge. Two particles approaching closely will flocculate because of the dispersion interaction discussed in Chapter VI. The electrostatic repulsion discussed in Section V-6 deters this approach hence, variations in ionic strength and or f will have a marked effect on the stability of such sols. Some classic data [89] shown in Table V-4, illustrate the generd relation between stability and f potential for a gold sol in a solution containing added Al. A review of these stability issues in ceramic materials is given by Pugh [90].  [c.189]

The application of environmentally benign principles to laboratory scale synthesis can be illus trated by revisiting the oxidation of alcohols As noted m Section 15 10 the most widely used meth ods involve Cr(VI) based oxidizing agents Cr(VI) com pounds are carcinogenic however and appear on the EPA list of compounds requiring special disposal methods The best way to replace Cr(VI) based 0x1 dants would be to develop catalytic methods analo gous to those used m industry Another approach would be to use oxidizing agents that are less hazardous such as sodium hypochlorite Aqueous solutions of sodium hypochlorite are available as swimming pool chlorine and procedures for their use in oxidizing secondary alcohols to ketones have been developed One is described on page 71 of the January 1991 edition of the Journal of Chemical Education  [c.644]

Baddeleyite, a naturally occurring zirconium oxide, has been found in the Poco de Caldas region of the states of Sao Paulo and Minas Geraes in Brazil, the Kola Peninsula of the former USSR, and the northeastern Transvaal of the Repubflc of South Africa. BraziUan baddeleyite occurs frequently with zircon, and ore shipments are reported to contain 65—85% zirconium oxide, 12—18% siUca, and 0.5% uranium oxide. Veryhttle of this ore is exported now because all radioactive minerals are under close control of the BraziUan government.  [c.426]

SFA measurements on monolayers deposited on mica. Chen and coworkers [69] studied the molecular mechanism associated with contact adhesion and contact angle hysteresis of monolayers adsorbed on mica surfaces. The mono-layer was adsorbed on mica either by self-assembly or by the Langmir-Blodgett technique. The phase states of these monolayers were varied systematically. The monolayers studied in this work include crystalline layers of phospholipids, amorphous solid layers of surfactants and liquid-like layers of surfactants. In these experiments, Chen et al. found that no contact adhesion hysteresis occurred when the contact was between crystalline-crystalline or crystalline-amorphous layers. However, significant adhesion hysteresis was observed for amorphous-amorphous and liquid-liquid contacts. In the case of the amorphous solid, and liquid-like monolayers, the pull-off force increased with time in contact. The extent of increase in pull-off force was about 35% in about 10 min for amorphous-amorphous contacts. The rate of increase of pull-off force with time, dP /dtc, decreased with increase in temperature. The rate of increase in pull-off was rather low for liquid-liquid like interfaces, and crystalline-crystalline contacts. This behavior was attributed to ease of relaxation and interdigitation of the monolayers. The hysteresis disappeared when the monolayers were exposed to vapors of dode-cane. This behavior was due to capillary condensation of the hydrocarbon vapors. Based on these studies, Chen et al. concluded that the hysteresis in the above monolayers was associated with chemical non-equilibrium effects associated with approach and separation. The authors argued that chemical non-equilibrium occurred when the surface groups on separation were differently positioned or oriented from those on approach.  [c.110]

An enediol may be generated from molecules such as benzoin or benzil which can then be treated with a diol-ditosylate to afford the crown. Merz demonstrated the utility of this approach in forming both the 9-crown-3 and 18-crown-6 derivatives (i.e., 1 + 1 and 2 + 2 products) of diphenylstilbenediol. The synthesis is remarkable since it utilizes phase transfer conditions to generate the crown ether. A benzene solution of benzoin is treated with 50% aqueous sodium hydroxide, diethylene glycol ditosyl-ate and tetra- -butylammonium bromide. The latter is intended to be the phase transfer catalyst for this reaction. The reaction is clearly successful both 9- and 18-membered rings are isolated as shown in Eq. (3.41). One wonders, however, what roles the various reagents play in the synthesis. Presumably, the enediol of benzoin is generated at the phase boundary in accordance with the understanding of such phase transfer ether syntheses. The enediolate must then ion-pair with the quaternary ammonium salt and be transported into the benzene layer where reaction with diethylene glycol ditosylate occurs. Since hydroxide anion is virtually insoluble in benzene solution even when paired with tetrabutylammonium cation, and since extraction of NaOH from such a concentrated salt solution must be extremely difficult, it is not clear whether there is a templating ion present during the synthesis at all. One might argue that the incipient crown could complex sodium and draw it into benzene, but the incipient crown would be an alkoxide and the latter should be paired with tetrabutylammonium, not sodium. In any event, the success of the reaction is documented and shown schematically below.  [c.42]

Phase transitions in two-dimensional layers often have very interesting and surprising features. The phase diagram of the multicomponent Widom-Rowhnson model with purely repulsive interactions contains a nontrivial phase where only one of the sublattices is preferentially occupied. Fluids and molecules adsorbed on substrate surfaces often have phase transitions at low temperatures where quantum effects have to be considered. Examples are molecular layers of H2, D2, N2 and CO molecules on graphite substrates. We review the path integral Monte Carlo (PIMC) approach to such phenomena, clarify certain experimentally observed anomalies in H2 and D2 layers, and give predictions for the order of the N2 herringbone transition. Dynamical quantum phenomena in fluids are analyzed via PIMC as well. Comparisons with the results of approximate analytical theories demonstrate the importance of the PIMC approach to phase transitions where quantum effects play a role.  [c.78]

Phase transitions in two-dimensional (adsorbed) layers have been reviewed. For the multicomponent Widom-Rowlinson model the minimum number of components was found that is necessary to stabilize the non-trivial crystal phase. The effect of elastic interaction on the structures of an alloy during the process of spinodal decomposition is analyzed and results in configurations similar to those found in experiments. Fluids and molecules adsorbed on substrate surfaces often have phase transitions at low temperatures where quantum effects have to be considered. Examples are layers of H2, D2, N2, and CO molecules on graphite substrates. We review the PIMC approach, to such phenomena, clarify certain experimentally observed anomahes in H2 and D2 layers and give predictions for the order of the N2 herringbone transition. Dynamical quantum phenomena in fluids are also analyzed via PIMC. Comparisons with the results of approximate analytical theories demonstrate the importance of the PIMC approach to phase transitions, where quantum effects play a role.  [c.123]

There are several techniques, however, that modify the system parameters to manipulate the power flow in localized regions. The one that has been in use the longest is modifying the parameters of generator operation, the quantity of power being produced, and the voltage at the terminals of the machine. This sonictinics requires operation of less efficient and more costly generating stations, instead of optimizing efficiency and cost, and other limitations frequently come into play as well. The second approach is to install a vei y costly ty pe of specialized transformer, a phase angle regulating transformer, at crucial points in the system. This device provides limited control, but can of modify power flows by iiiiposiiig a change in phase angle in the voltage and current going through it. Switched series capacitor banks are sometimes used to reduce the effective inductance of a transmission line, permitting more power to flow through it, but these are not highly controllable devices either.  [c.436]

See pages that mention the term Aaberg push-pull : [c.328]    [c.70]    [c.68]    [c.136]    [c.568]    [c.430]   
Industrial ventilation design guidebook (2001) -- [ c.1484 ]