Primary Accelerators

Accelerating primary fragments s under coulomb interaction > approximately 10 20 sec for fragments to reach 90% of their final K.E. 

Accelerated primary fragments separated by a relatively large distance... 

Use Basic rubber accelerator, primary standard for acids. 

A series of lenses and apertures in a primary beam column allow the accelerated primary beam to be focused on the sample surface. Generally, the ion source and primary column are tuned by the analyst to get the maximum amount of primary beam current into the smallest spot size. After focusing, the beam may be moved on the sample surface in a raster pattern. The current density of the primary beam is often expressed as mA/cm. Eor example, a 100 nA primary beam rastered over a 250 pm square has a current density of ... 

The limitations in sensitivity and in depth resolution of the electron-probe microanalyzer prompted the development of ion-probe microanalysis. This technique is based on mass spectro-graphic analysis of the secondary ions emitted from a sample under the impact of a focused and accelerated primary ion beam. This type of analysis also offers, in comparison with the electron probe, the possibilities of isotopic analysis and the investigation of elements of low atomic number, including hydrogen, at trace concentrations. 

Primary accelerators Primary accelerators are mercapto based accelerators, generally efficient and confer good processing safety to the rubber compounds, exhibiting a broad vulcanisation plateau with relatively low crosslink density. Examples are sulfenamides and thiazoles. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. 

The combination of accelerated primary nucleation and suppressed secondary nucleation described here has the general effect of increasing the size of crystals on a surface and hence to increasing grain size in a thin film. 

The primary source energy can be switched between 6, 9 and 11 MeV. So it is possible to penetrate and examine up to 0.6 m solid steel objects. In front of the accelerator a collimator... 

The source of radiation is a linear accelerator with selectable primary energies of 6, 9 or 11 MeV ( VARIAN Linatron 3000 A). The output of the LINAC at 9 MV is 3000 rad ( 30 Gy) per minute. The pulse length is 3.8 microseconds with repetition frequencies between 50 and 250 Hertz. 

Figure Bl.17.8. Iron oxide particles coated with 4 nm of Pt in an m-planar magnetron sputter coater (Hennann and Mtiller 1991). Micrographs were taken in a Hitachi S-900 in-lens field emission SEM at 30,000 primary magnification and an acceleration voltage of 30 kV. Image width is 2163 nm.

Nucleophilic substitution by azide ion on an alkyl halide (Sections 8 1 8 13) Azide ion IS a very good nucleophile and reacts with primary and secondary alkyl halides to give alkyl azides Phase transfer cata lysts accelerate the rate of reaction... 

The reaction of an aryl diazonium salt with potassium iodide is the standard method for the preparation of aryl iodides The diazonium salt is prepared from a primary aro matic amine m the usual way a solution of potassium iodide is then added and the reac tion mixture is brought to room temperature or heated to accelerate the reaction... 

A typical single microchannel electron multiplier. Note how the primary ion beam causes a shower of electrons to form, The shower is accelerated toward the other end of the microchannel, causing the formation of more and more secondary electrons. 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

Primary alkyl groups are more reactive than secondary and tertiary. PivaUc acid accelerates the rate of protonolysis of trialkylboranes with water and alcohols (207,208). The reaction can be controlled to give excellent yields of dialkylbotinic acids and esters. 

The reactions of trialkylboranes with bromine and iodine are gready accelerated by bases. The use of sodium methoxide in methanol gives good yields of the corresponding alkyl bromides or iodides. AH three primary alkyl groups are utilized in the bromination reaction and only two in the iodination reaction. Secondary groups are less reactive and the yields are lower. Both Br and I reactions proceed with predominant inversion of configuration thus, for example, tri( X(9-2-norbomyl)borane yields >75% endo product (237,238). In contrast, the dark reaction of bromine with tri( X(9-2-norbomyl)borane yields cleanly X(9-2-norbomyl bromide (239). Consequentiy, the dark bromination complements the base-induced bromination. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Guanidines. Guanidines (10) were one of the first aniline derivatives used as accelerators. They are formed by reaction of two moles of an aromatic amine with one mole of cyanogen chloride. Diphenylguanidine (DPG) has enjoyed a resurgence ia demand as an activator for sulfenamides and a co-accelerator ia tire tread compounds which employ siUca fillers for low rolling resistance. Guanidines alone show too Htde activity to be extensively used as primary accelerators. There were no U.S. producers as of mid-1996. 

It is common practice in the mbber industry for a compounder to use combinations of several accelerators in developing a cure system. Typically these cure systems are comprised of a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary types (kickers) are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (20). 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

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