Bismuth thermal conductivity

It is a white crystalline, brittle metal with a pinkish tinge. It occurs native. Bismuth is the most diamagnetic of all metals, and the thermal conductivity is lower than any metal, except mercury. It has a high electrical resistance, and has the highest Hall effect of any metal (i.e., greatest increase in electrical resistance when placed in a magnetic field). 

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, soHdus and Hquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

The physical properties of bismuth, summarized ia Table 1, are characterized by a low melting poiat, a high density, and expansion on solidification. Thermochemical and thermodynamic data are summarized ia Table 2. The soHd metal floats on the Hquid metal as ice floating on water. GaUium and antimony are the only other metals that expand on solidification. Bismuth is the most diamagnetic of the metals, and it is a poor electrical conductor. The thermal conductivity of bismuth is lower than that of any other metal except mercury. 

Steel (Quenching. Bismuth and bismuth—lead ahoys are used in the processing of some steel products. The thermal conductivity of bismuth makes it ideal for use in quenching steel. The use of a bismuth—lead ahoy in place of lead alone has the advantage of lowering the operating temperature of the bath as weh as reducing adherence of ahoy to the steel. 

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

Bismuth is more resistant to electrical current in its solid state than it is in its liquid form. Its thermal conductivity is the lowest of all metals, except mercury. Even though it is considered a metal-like element, it is a very poor conductor of heat and electricity. 

Potassium and sodium are good conductors of heat.23 If the conductivity of silver be unity, that of sodium is 0 365. J. W. Hornbeck found the temp, coeff. of the thermal conductivity of potassium or sodium falls with rise of temp. The alkali metals are also good conductors of electricity 24 for example, the conductivity of sodium for heat and electricity is exceeded only by silver, copper, and gold. According to E. F. Northrup, the metals sodium, potassium, mercury, tin, lead, and bismuth have the same value for the ratio of the coeff. of electrical resistance to the coeff. of cubical expansion at the same temp. The electrical conductivity of lithium is nearly ll-4xl04 reciprocal ohms at 20°, that is, about 20 4 per cent, of the conductivity of hard silver of sodium at 2T 70, 22 4 XlO4 reciprocal ohms, that is, about 36 5 per cent, of the value of silver. 

Equation 6.20 is a rather formidable expression. From the experimentalist s standpoint, it will be beneficial to point out some simple, yet useful, criteria when toiling with thermal conductivity. First, it is noted that thermal conductivity is not an additive property. It is generally not possible to predict the thermal conductivity of an alloy or compound from the known thermal conductivities of the substituent pure elements. For example, the thermal conductivity of polycrystaUine silver and bismuth are. 

Liquid metals are used when temperature requirement is so high that even the nitrate/nitrite salt mixture becomes unsuitable. The most commonly used liquid metal is a eutectic mixture of sodium and potassium (44%). This has a very broad temperature range (40-760°C) and very high thermal conductivity. Lead and lead-bismuth eutectic can be used up to 900° C. There are several disadvantages with the use of liquid metals. Special precautions must be taken while using alkali metals because they react violently with water and burn in air. Mercury, lead, and bismuth-based mixtures are highly toxic, hence their applications are restricted. One common use of liquid metals is in the cooling of nuclear reactors. 

Efficient thermoelectric devices require the use of high figure of merit thermoelectric materials. The thermoelectric figure of merit, ZT, can be expressed as oQ-oTIk, where a is the Seebeck coefficient, o the electrical conductivity, T, the temperature, and k, the thermal conductivity. Among the various thermoelectric materials, bismuth telluride (Bi2Te3) has been the main focus of research because of its superior ZT near room temperature [58]. 

We report on the powder metallurgical fabrication of bismuth-antimony solid solution and the thermoelectric properties of the fabricated composites. The solid solution powders were prepared by mechanical alloying (MA) aiming at large reduction of the thermal conductivity with the very fine microstructures obtained through MA process. The prepared bismuth-antimony powders (Bi-7.5at%Sb) have been sintered by hot pressing. 

Bismuth is the most diamagnetic of all metals and has low thermal conductivity. Since bismuth expands upon solidification, it is used to make castings for objects subjected to high temperatures. It is used as a replacement for lead in solders, shot for hunting, fishing sinkers, ceramic glazes, and brasses for plumbing applications. It is also used as a carrier for (an isotope of uranium) fuel in atomic reactors. Ionic compounds of bismuth are used in cosmetics and medicine. 

Thermoelectric theorists have long held that bismuth s low thermal conductivity and high carrier mobilities would make it an ideal thermoelectric material - if only the electrons would stop combining with the holes. It was... 

The best material system available at this time to meet the above criteria is the bismuth-antimony crystal system [ ]. In pure bismuth, the conduction and valence bands are slightly overlapped and, in the proper orientation, both holes and electrons have high and reasonably equal mobilities. The addition of a small amount of antimony has the effect of greatly reducing the lattice thermal conductivity. However, the addition of antimony also has the effect of decreasing the overlap of the principal conduction and valence bands in fact, for antimony concentrations greater than 5 atomic %, a gap exists and the system becomes a semiconductor, which is undesirable for these purposes [ ]. At present it appears that the optimum antimony content is about 3 atomic %. 

This progression constitutes an approach towards a symmetrical octahedral environment for each pnictide atom (as in cubic black P), and is in accord with the increasing metallic character shown as the pnictides become heavier. This metallic character is indicated by the changing appearance and increasing values of density, coefficients of expansion, electrical and thermal conductivities which are observed on moving from phosphorus through to bismuth. 

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