Low melting point alloys

Because bismuth expands on solidification and because it alloys with certain other metals to give low melting point alloys, bismuth is particularly weU suited for a number of uses. Alloys of bismuth can be made that expand, shrink, or remain dimensionally stable on solidification. AH other metals except gallium and antimony contract on solidification. Bismuth aHoys and uses are summarized in Table 5. 

Some alloys are softer than the component metals. The presence of big bismuth atoms helps to soften a metal and lower its melting point, much as melons would destabilize a stack of oranges because they just do not fit together well. A low-melting-point alloy of lead, tin, and bismuth is employed to control water sprinklers used in certain fire-extinguishing systems. The heat of the fire melts the alloy, which activates the sprinklers before the fire can spread. 

Calculate the relative number of atoms of each element contained in each of the following alloys (a) Wood s metal, which is a low-melting-point alloy used to trigger automatic sprinkler systems and is 12.5% tin, 12.5% cadmium, and 24% lead by mass in bismuth (b) a steel that is 1.75% by mass carbon in iron. 

A die is a reusable mold, usually made of steel, for the mass production of small parts in low-melting-point alloys—usually zinc or aluminum alloys. For the mass production of small parts that... 

A heated mixture of low melting point alloys or of chemical salts used to provide the heat for curing extrudates in one method of continuous vulcanisation. 

Ultrasonic Gas Atomization <30 Low melting point alloys, Fc, stainless steels, Ni, Co alloys, Carbon steels I04-,06 Annular 2.4-15 Linear much higher Steel strip -2.4 Al strip -0.45 Fine smooth droplets, High gas efficiency High cost ... 

Nicolas Lemery (1645—1715), a French chemist, studied and analyTed antimony and its compounds in detail. He published his findings in 1707. Antimony was used primarily as a treatment for diseases, but as a chemical element it was neglected it until modern days, when it was used to produce low-melting-point alloys. 

TABLE 1. SOME REPRESENTATIVE LOW-MELTING-POINT ALLOYS CONTAINING BISMUTH... 

Die casting Reusable metal mold Copper or low-melting-point alloys Good dimensional tolerances, thinnest cast sections possible, quick cycle time High die cost makes it impractical for low volume, limited alloys, need draft/parting lines, size/weight limit Small intricate parts, housings, valves, heat sinks, toys... 

Consequently, their use is best confined to short run or prototype use. In normal production, the improved heat transfer capability of a metal mold will more than repay the greater cost. Aluminum is most commonly used for thermoforming molds other options include cast or sprayed low melting point alloys, porous sintered metals, and copper alloys (Chapter 17). 

The technique of impregnation with low melting point alloy results in a freezing of the state of penetration in 3-dimensions amongst the pore spaces. An impregnated sample of powder can then be sectioned and polished and, if viewed on an SEM, affords a view of a 2-D random plane through the 3-D pore spaces. The alloy used melts at 47 C. Impregnation has been performed at around 60 C (in a hot water bath) so that samples are solid at room temperature. 

Figure 8 shows a part of a section of an impregnated powder sample with a field size of approximately 500 pm x 500 pm. This field contains sections through about 250 powder particles, and it is clear that the extent of penetration amongst individual particles shows a very great variance. Indeed, about 50 particles (around l/5th of the total) show negligible penetration of low melting point alloy For the purposes of the present analysis, attention will be focused on the typical particle shown in Figure 9. This particle is the one located just lower than, and left of the centre of Figure 8. It has a typical penetrated porosity of 0.33 and an apparent diameter of about 60 pm. The section in Figure 9 probably passes close to the particle centre, since particles are of this order of diameter.

Figure 8. Low melting point alloy (LMPA) impregnation at 50 atmos (Reproduced with permission. Copyright 1993 Institution of Chemical Engineers.)...

In conclusion it would appear that the combination of 3-D stochastic pore networks with mercury porosimetry and low melting point alloy impregnation offers a new framework for the description and calculation of the role of pore spaces in typical porous catalyst particles. 

In this section, two examples are presented for the application of a technique of low-melting-point alloy (LMPA) impregnation that provides for a visualization of the invasion of a nonwetting fluid into the pore spaces in a typical porous article. The visualization can be linked to the modeling of mercury porosimeter curves using 3-D stochastic pore networks. This makes the quantification of pore structure more direct. Quantified structures can be visually examined against sample particle sections. The visual comparison can be made more precise by image analysis of the accessible porosity made visible by metal penetration over a series of pressures. 

Figure 17 Low-melting-point alloy impregnation of FCC catalyst panicles.

An interesting property of cadmium is its effect in alloys. In combination with certain metals, it lowers the melting point. Some common low-melting-point alloys are Lichtenberg s metal, Abel s metal, Lipowitz metal, Newton s metal, and Wood s metal. The Wood s metal alloy melts at 158°F (70°C), and is used in fire sprinkler systems as a plug. 

Currently, thallium is used in some electronic devices, in low melting point glass, and in the creation of low melting point alloys, see also Halogens Potassium. 

J. J. Berzelius in 1814. Bismuth occurs less rarely than mercury, but shows a more frequency of appearance as silver. It is found in its native form, and also in minerals such as bismuthite (bismuth sulfide) and bismite (bismuth oxide). The main use of bismuth is in pharmaceuticals and in low-melting point alloys which are used as fuses ( 4000 tons annually). Occupational intoxication by these alloys are rare, and in most instances the adverse effect is caused by other metals present in the alloys such as lead and cadmium. Bismuth as a metal is classed as nontoxic. 

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