A thermally assisted cold emission gun is similar to the cold field emission gun except, as its name implies, the tip is heated to help in the extraction of electrons. The thermal field emission gun is very different from the other FEGs. The work function is lowered by coating the emitter with zirconium oxide and the tip is heated to about 1800K. This change in the mode of operation is summarized where a comparison of the performance of the various Fegs is given. It can be seen that the choice between the various FEGs must be made with the particular applications in mind. There is no clear-cut advantage of one FEG over another although the cold field emission source is probably capable of the best performance in the probe mode (if mechanical stability and vacuum specifications are appropiate) and the thermal FEG has some advantages if it has to operate both as a small and large probe. Hence in a dedicated STEM the cold field source is preferred and in a STEM/TEM the thermal FEG is preferred.
(M. H. Loretto, Electron Beam Analysis of Materials, p.8)
Synthetic Adhesives
Multi-Part Adhesives (Two-Part Adhesives)
Synthetic Adhesives
A simple classification is depicted in Figure 4.1. Adhesives are either produced from a natural source such as starch glue or as is the case with the majority of consumptions, they are synthesized from basic hydrocarbons. The synthetic group consists of thermoplastic and thermosetting adhesives, both of wich follow the definitions used in plastics for thermoplastic and thermosetting polymers.
Synthetic adhesives term is usually used to apply to all adhesives other than natural adhesives (i.e., elastomeric, thermoplasitc, thermosetting, and alloys). All structural adhesives are synthetic.
Multi-Part Adhesives (Two-Part Adhesives)
The most important adhesives of this group are polyester, cold-setting epoxy resins, polurethanes and acrylic adhesives. Common two-part adhesives consists of several components that are mixed in a specific ratio before application. For acrylic adhesives of the ‘second generation’, it is possible to apply each of the components seperately to one of the adherent surfaces, and then to press the adherents together, thereby initiating the curing process.
In addition to the specific mixing ratio, the so-called ‘pot life’ must also be respected; this is the period of time during which the adhesive can be used after mixing the components, and is dependent on the type of adhesive used and the volume prepared. As the crosslinking process gradually starts during the pot life, and the viscosity of the adhesive gradually increases during this time, exceeding the pot life will cause a lack of wetting of the adherent surfaces by the adhesive and result in poor adhesion properties within the bond-line. The cross linking of two- part or multi- part adhesives, respectively, almost always takes place as an exothermic reaction, so that the adhesive will be heated up during this process.
(Walter Brockmann, Paul Ludwig Geiss, Adhesive Bonding, Materials, Applications and Technology,p.36)
One-Part Adhesives
One-part adhesives are preferred over two part adhesives for surface mounting because it is a nuisance to have to mix two-part adhesives in the right proportions for the right amount of time. One-part adhesives, eliminating one process variable in manufacturing, are easier to apply, and one does not have to worry about the short working life(pot life) of teh mixture. The single- part adhesives have a shorter shelf life, however. The terms “shelf life” and “pot life” can be confusing. “Shelf life” refers to the usable life of the adhesive as it sits in the container, whereas “pot life,” as indicated above, refers to the usable life of the adhesive after the two main components( catalyst and resin) have been mixed and catalysis has begun.
(Ray P. Prasad, Surface Mount Technology: Principles and Practice, p. 340)
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