Nonmetals thermal conductivity

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

Metals are distinguished from nonmetals by their strength, toughness, electrical conductivity and thermal conductivity. However, the dominant property that causes metals to be preferred over most nonmetals is their ability to deform in the presence of excessive stress rather than fracture catastrophically. This is the prime reason for the widespread use of metals for structural applications such as plant equipment, piping and pressure vessels. 

The thermal conductivities of unfilled epoxies, as with all other unfilled polymers, are quite low, typically 0.1-0.2 W/mK. When filled with metal or thermally conductive nonmetal fillers up to 80-85% by weight, the thermal conductivities increase a minimum of tenfold. Some silver-filled epoxies are reported to have thermal conductivities as high as 6.0 to approximately 8 W/m K (AI Technology ESP 8450 W and ESP 8456-00, respectively). 

Metal matrix composites (MMCs) are metals that are reinforced with fibers or particles that usually are stiff, strong, and lightweight. The fibers and particles can be metal (e.g., tungsten), nonmetal (e.g., carbon or boron), or ceramic (e.g., silicon carbide (SiC) or (alumina) AljOj). The purpose for reinforcing metals with fibers or particles is to create composites that have properties more useful than that of the individual constituents. For example, fibers and particles are used in MMCs to increase stiffness [/], strength [f ], and thermal conductivity [2], and to reduce weight [f], thermal expansion [3], fiiction [4], and wear [5]. 

Heat transfer by conduction occurs essentially by molecular vibration and movement of free electrons. As metals have more free electrons than nonmetals, they are better conductors of heat. Thermal conductivity, also known as thermal conductance, is a measure of the rate of heat transfer per unit thickness. Examples of conductivity range are presented in Table 2-16... 

Nonmetal - me-t l n (ca. 1864) An element with generally low electrical and thermal conductivities, dull luster, and a high ionization energy, electron affinity and electronegativity. 

Beryllia ceramics have these characteristics extremely high thermal conductivity, particularly in the lower temperature range excellent dielectric properties outstanding resistance to wetting and corrosion by many metals and nonmetals mechanical properties only slightly less than those of 96% alumina ceramics valuable nuclear properties, including an exceptionally low thermal neutron absorption cross section and ready availability in a wide variety of shapes and sizes. Like alumina and some other ceramics, beryllia is readily metallized by a variety of thick and thin film techniques. 

The vibrations of the atoms in the crystalline lattice are important in understanding the thermal properties of both metallic and nonmetallic solids. The energy involved in these vibrations represents thermal energy hence lattice vibrations are primarily resporrsible for the heat capacity of solids. Also, these vibrations are able to transport heat and are the dominant source of thermal conductivity in nonmetals. Therefore, in order to understand thermal properties of solids, it is necessary to start with a general understanding of the nature of lattice d5mamics. 

Metals provided additional problems for the classical theory of heat capacity. Metals are generally much better conductors of heat than nonmetals because most of the heat is carried by the free electrons. According to the classical theory, this electron gas should contribute an additional (3/2)R to the heat capacity. But the measured heat capacity of metals approached nearly the same 3R Dulong-Petit limit as the nonmetals. How can the electrons be a major contributor to the thermal conductivity and not provide significant additional heat capacity ... 

Since we know that Cv approaches 0 with a 7 dependence at low temperatures, one would expect the thermal conductivity to do likewise and this accounts for the low thermal conductivity of nonmetals at low temperature. At higher temperatures, thermal conductivity is limited by the mean free path A. Phonons are scattered inelastically from defects, impurities, and grain boimdaries. Phonons may also interact with other phonons through the anharmonic terms in the lattice potential. For example, two phonons may combine to produce a third phonon. The conservation of momentum requires... 

Since thermal conduction of nonmetals is limited at low temperatures by the falloff of the heat capacity and at high temperatures by increased Umklapp scattering, a peak in thermal conductivity would be expected near O.40d. 

Metals generally have higher thermal conductivity than nonmetals because of the presence of free electrons, but the electrons do not contribute to the heat capacity as they would be expected to form classical considerations. The low heat capacity is compensated by the high Fermi velocity, so the conductivity calculated using quantum mechanics is almost the same as the classical result. 

All nonmetals with a known high-thermal conductivity have either diamond-like, boron carbide, or graphite crystal structure. The fundamental characteristics for a crystal to exhibit... 

Figure 7.5. Qualitative temperature-dependence of thermal conductivity in metals (dashed line) and nonmetals (solid line).

Metals are located on the left side of the periodic table. Metals tend to form cations, are generally ductile and malleable, and are good electrical and thermal conductors. Nonmetals are located on the right side of the periodic table. Nonmetals tend to form anions and have a wide variety of physical properties. Metalloids look like metals but have electrical conductivity intermediate between metals and nonmetals. For this reason, metalloids are called semiconductors. 

Aluminum, the most abundant metal on earth, has metallic physical properties, such as high thermal and electrical conductivities and a lustrous appearance however, its bonds to nonmetals are significantly covalent. This covalency is responsible for the amphoteric nature of AI2O3, which dissolves in acidic or basic solution, and for the acidity of Al(H20)[Pg.879]

Metals usually differ from nonmetals by their excellent thermal and electrical conductivities, and by their great mechanical strengths and ductilities. These properties follow directly from the nonlocaUzed electronic bonds in these materials. The electrons in metals are mobile in a true metal, there are no underlying directed bonds. 

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