Charge buildup

Charge buildup is dehimental in all types of SIMS instruments as this imposes an additional potential field around the sample over which the incoming primary ions and emitting secondary ions have to deal with. As an example, positive charge buildup will further accelerate positive secondary ions/decelerate negative 

If uncompensated, charge buildup will increase over time with the recorded secondary ion signals steadily decreasing. In extreme cases, complete electrical break down will occur. The break down may result from the decomposition of an insulating film on the sample analyzed or may result from arcing to conductive elements in close proximity to the area experiencing charge buildup. 

Charge buildup also affects primary ion trajectories, irrespective of the instrument type. This is particularly evident when low-energy primary ions are used. That being said, primary ions are less susceptible to charge buildup owing to their greater energies with respect to secondary ions. 

Charge compensation in SIMS is most commonly carried out by coirradiating the affected sample surface region with electrons. High-energy electrons (typically 

Other methods that can be used in conjunction with electron beam irradiation include  

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Thus, HyperChem occasionally uses a three-point interpolation of the density matrix to accelerate the convergence of quantum mechanics calculations when the number of iterations is exactly divisible by three and certain criteria are met by the density matrices. The interpolated density matrix is then used to form the Fock matrix used by the next iteration. This method usually accelerates convergent calculations. However, interpolation with the MINDO/3, MNDO, AMI, and PM3 methods can fail on systems that have a significant charge buildup. 

Reduce linear flow velocities to eliminate static charge buildup during feed... 

As Cu2+ ions are reduced, the solution at the cathode becomes negatively charged and the solution at the anode begins to develop a positive charge as the additional Zn2+ ions enter the solution. To prevent this charge buildup, which would quickly stop the flow of electrons, the two solutions are in contact through a porous wall ions provided by the electrolyte solutions move between the two compartments and complete the electrical circuit. 

A DFT study found a corresponding TS to be the lowest energy.167 This study also points to the importance of the solvent, DMSO, in stabilizing the charge buildup that occurs. A further computational study analyzed the stereoselectivity of the proline-catalyzed aldol addition reactions of cyclohexanone with acetaldehyde, isobu-tyraldehyde, and benzaldehyde on the basis of a similar TS.168 Another study, which explored the role of proline in intramolecular aldol reactions, is discussed in the next section.169... 

Figure 6.8 Deformation densities for the F2 molecule, (a) Standard deformation density. Note that there is no charge buildup-in the bonding region between the nuclei, (b) Modified deformation density molecular density minus the density of atoms in the (ls)2(2s)2(2px)2(2py)2( 2pe)1 reference state showing a buildup of charge in the bonding region. (Reproduced with permission from P. Coppers [1997].)...

A common ignition source within chemical plants is sparks resulting from static charge buildup and sudden discharge. Static electricity is perhaps the most elusive of ignition sources. Despite considerable efforts, serious explosions and tires caused by static ignition continue to plague the chemical process industry. 

Static charge buildup is a result of physically separating a poor conductor from a good conductor or another poor conductor. When different materials touch each other, the electrons move across the interface from one surface to the othpr. Upon separation, more of the electrons remain on one surface than on the other one material becomes positively charged and the other negatively charged. 

If both the materials are good conductors, the charge buildup as a result of separation is small because the electrons are able to scurry between the surfaces. If, however, one or both of the materials are insulators or poor conductors, electrons are not as mobile and are trapped on one of the surfaces, and the magnitude of the charge is much greater. 

Estimate the charge buildup, and accumulated energy, as a result of a person (insulated from the floor) charging 30 lb of a dry powder, using a scoop, into a 20-gal insulated drum. Assume that the person capacitance is 300 X 10 12 farad. 

This operation is a sliding-contact type operation. From Table 7-3 this operation gives a charge of 10 5 coulomb/kg. Therefore the charge buildup is... 

Figure 7-17 Charge buildup with complex vessel system.

This is the charge buildup just before reaching the overflow line. 

Charge buildup, resulting sparks, and the ignition of flammable materials is an inevitable event if control methods are not appropriately used. In practice, however, design engineers recognize... 

In actual practice,8 it was found that a hold time equal to or greater than one-half the calculated relaxation time is sufficient to eliminate charge buildup. The twice the relaxation time rule, therefore, provides a safety factor of 4. The American Petroleum Institute9 recommends a ud, from Equation 7-21, of less than 0.5 m2/s for road tanker filling and 0.8 m2/s for rail car filling. 

The conductivity of nonconducting organic materials can sometimes be increased using additives called antistatic additives. Examples of antistatic additives include water and polar solvents, such as alcohols. Water is effective only when it is soluble in the offending liquid, because an insoluble phase gives an additional source of separation and charge buildup. 

Estimate the charge buildup and accumulated energy as a result of pneumatically conveying a dry powder through a Teflon duct. The powder is collected in an insulated vessel. 

Highly stereoselective intermolecular C-H activation is possible at C-H bonds adjacent to nitrogen [130]. The ability of the nitrogen lone pair to stabilize charge buildup... 

Disadvantages stiff fibers (aromatic rings), poor drape except with cotton blends, hydrophobic, pilling, static charge buildup, absorbs oils and greases easily (stains)... 

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