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Metals(religion, spiritualism, and occult)
At least 4,000 years ago, various metals began to be associated with the different planets, including the Sun and the Moon, which in classical astrology were also classified as planets. By the seventh century, the following set of associations had come to be generally agreed upon: the Sun and gold, the Moon and silver, Mercury and mercury, Venus and copper, Mars and iron, Jupiter and tin, and Saturn and lead.
By extension, the signs were also associated with the metals ruled by their ruling planet (e.g., Leo, ruled by the Sun, was associated with gold, the metal ruled by the Sun; Cancer, ruled by the Moon, was associated with silver, the Moon’s metal, etc.). The analogical relationship between many metals and their rulers is fairly straightforward. It was natural, for instance, that the most important heavenly body, the “golden” Sun, should be associated with the most precious metal, gold; Saturn, the slowest of the planets known to antiquity, was naturally associated with the heaviest metal, lead; Mars, god of war, had a natural connection with iron, the metal of weapons; and so forth for the other planet—metal associations. Prior to the emergence of the modern world, these planet-metal connections were taken quite seriously as real links, not merely as symbolic analogies. Medieval alchemists, for example, paid attention to the positions of the planets when working with metals, avoiding the use of certain metals when the corresponding planets were involved in hard aspects.
When the “new” planets were discovered, astrologers experienced difficulty expanding the old system of rulerships. Certain associations seemed obvious, such as Uranus’s rulership of uranium and Pluto’s rulership of plutonium, but no astrologer has really been interested in exploring these new rulerships in any depth, largely because contemporary astrology is focused on individual human beings, with the result that almost all contemporary astrologers are primarily counselors, interested more in the psychological effects of the outer planets. If a significant number of astrologers were also metallurgists, pharmacists, and chemists, the question of the metals ruled by Uranus, Neptune, and Pluto would have been resolved long ago. The testing would be relatively easy: Assuming, as did the ancients, that there is a subtle yet tangible link between metals and planets, then some variation on the Kolisko experiments should determine precisely which metals are ruled by the planets beyond Saturn.
simple substances (chemical elements) with characteristic properties under normal conditions: high electrical and thermal conductivity, negative temperature coefficient of electrical conductivity, high reflectivity of electromagnetic waves (luster and opacity), and plasticity. In the solid state, metals have crystalline structures; in the vapor state they are monatomic.
The properties of metals listed above result from their electron structure. Metal atoms easily give up their outer (valence) electrons.
Not all the electrons in the crystal lattices of metals are bound to their respective atoms. A certain portion of them (about one per atom) is mobile. These electrons are capable of moving more or less freely within the metal. The existence of free electrons (conduction electrons) in metals is explained by the zone theory. Metals may be imagined as consisting of a skeleton of positive ions immersed in an “electron gas.” The gas compensates for the forces of electrostatic repulsion between the positive ions and thus bonds them together in the form of a solid (metallic bonding).
Of the 105 known elements (as of 1974), 83 were metals and only 22 nonmetals. If a straight line is drawn from boron to astatine in the long or “semilong” form of the Mendeleev periodic system (see Figure 1), the nonmetals may be assumed to be located on the line and to the right of it, and the metals, to the left of the line.
However, neither the properties characteristic of metals nor the distinctions between metals and nonmetals should be regarded as absolute. Metallic luster is characteristic only of the compact metals. Very thin silver and gold foils (10~4 mm thick) are translucent and have a light greenish blue color. Very fine metallic powders are frequently black or gray-black. Some metals (zinc, antimony, and bismuth) are brittle at room temperature, becoming plastic only upon heating. All these properties are characteristic of typical metals (for example, copper, gold, silver, and iron) under normal conditions (atmospheric pressure and room temperature). At very high pressures (about 105-106 atmospheres), metals may undergo substantial changes and nonmetals may acquire metallic properties.
Many elements may be classified as metals according to one set of properties and as nonmetals according to another. These “violations” are particularly common in the vicinity of the dividing line drawn in Figure 1. Thus, germanium is a metal in appearance, and it is more like a metal in its chemical behavior (it gives up electrons more readily than it accepts them), but it is a semiconductor with respect to the magnitude and type of electrical conductivity. The electrical resistance of antimony (Sb) is too high for a metal, but its temperature coefficient of resistance is large and positive, as in a metal. Antimony should also be classified as a metal according to its tendency to give up electrons. Arsenic, antimony, and bismuth are sometimes classified as semimetals. Polonium is a metal in appearance and has the chemical properties of both metals and nonmetals; in addition to positive valence (more precisely, oxidation number), it also exhibits negative valence (—2).
Metal alloys have much in common with the metals; for this reason, alloys are frequently included among the metals in the literature on physics, technology, and economics.
Historical information. The term “metal” is derived from the Greek word metallon (from metalleuo, “I excavate, extract from the ground”), which originally denoted mines or pits (the word was used in this sense by Herodotus in the fifth century B.C.). The material obtained from mines was called metalleia by Plato. In antiquity and during the Middle Ages it was believed that there were only seven metals: gold, silver, copper, tin, lead, iron, and mercury. The alchemists believed that metals were generated in the interior of the earth under the influence of planetary rays and were subsequently perfected very slowly, being transformed into silver and gold.The alchemists assumed that metals were com-pound bodies consisting of a “metallic principle” (mercury) and a “combustion principle” (sulfur). A hypothesis according to which metals consist of earth and the “combustion principle,” called phlogiston, gained wide acceptance in the 18th century.
M. V. Lomonosov enumerated six metals (gold, silver, copper, tin, iron, and lead) and defined metals as “light-colored bodies that may be forged.” In the late 18th century A. L. Lavoisier refuted the phlogiston hypothesis and demonstrated that metals are elements. In his 1789 book on chemistry, Lavoisier gave a list of simple substances that included the 17 metals known at the time (antimony, silver, arsenic, bismuth, cobalt, copper, tin, iron, manganese, mercury, molybdenum, nickel, gold, platinum, lead, tungsten, and zinc).
The development of methods of research in chemistry has led to an increase in the number of known metals. In the first half of the 19th century the metals of the platinum group were discovered, some of the alkali metals and alkaline earths were isolated by electrolysis, the division of the rare earths was begun, and unknown elements were discovered in the course of the chemical analysis of minerals. Cesium, rubidium, tellurium, and indium were discovered in 1860–63 through methods of spectral analysis. The existence of metals predicted by D. I. Mendeleev on the basis of his periodic law was brilliantly confirmed. The discovery of radioactivity in the late 19th century led to a very successful search for naturally radioactive metals. Finally, nuclear transformation methods have been used since the mid-20th century for the production of man-made radioactive metals, particularly the transuranium elements.
Metallurgy, the science of the production of metals from naturally occurring raw materials, received its physicochemical foundation at the turn of the 20th century. Studies of the properties of metals and their alloys relative to their composition and structure began at the same time.
Chemical properties. A distinction is made among metals of the major and side subgroups, according to the place occupied by the element in the periodic table (Figure 1). Metals of the major subgroups (A-subgroups) are also called the nontransition metals. They are characterized by successive filling of their atomic s- and p-orbitals. Atoms of the metals in the side subgroups (B-subgroups) are called the transitional elements, in which completion of the d- and f-orbitals takes place. Accordingly, these metals are divided into the d-group and the two f-groups, the lanthanides and the actinides, respectively. The A-subgroups contain 22 metals: lithium, sodium, potassium, rubidium, cesium, and francium (I-A); beryllium, magnesium, calcium, strontium, barium, and radium (II-A); aluminum, gallium, indium, and tellurium (III-A); germanium, tin, and lead (IV-A); antimony and bismuth (V-A); and polonium (VI-A). The B-subgroups include the following: (1) 33 transitional metals of the d-group —copper, silver, and gold (I-B); zinc, cadmium, and mercury (II-B); scandium, yttrium, lanthanum, and actinium (III-B); titanium, zirconium, hafnium, and kurchatovium (IV-B); vanadium, niobium, tantalum, and an element with Z = 105 (V-B); chromium, molybdenum, and tungsten (VI-B); manganese, technetium, and rhenium (VII-B); and iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum (VIII-B); (2) 28 elements of the f-group (14 lanthanides and 14 actinides).
A peculiarity of the electron structure of the atoms of certain d-elements is that one of the electrons passes from an external level to afld-sublevel. This occurs during the filling of the sublevel to give a population of five or ten electrons. For this reason, the electron structure of the valence sublevels in the atoms of d-elements of the same subgroup is not always identical. For example, chromium and molybdenum (subgroup VI-B) have the external electron structures 3d54sl and 4ds5sl, respectively, whereas the corresponding structure of tungsten is 5d46s2. In an atom of palladium (subgroup VIII-B), two external electrons “moved over” to the neighboring valence sublevel, and the d10 structure is observed for palladium instead of the expected dRs2.
Metals have many common chemical properties that result from the weak bonding of the valence electrons with the atomic nucleus, such as the formation of positively charged ions (cations), the appearance of positive valence (oxidation number), the formation of basic oxides and hydroxides, and the substitution of hydrogen in acids. The metallic properties of the elements may be compared in terms of their electronegativity, which is the capability of the atoms in molecules (with covalent bonding) to attract electrons, expressed in relative units; the lower the electronegativity of an element (the greater its electropositive character), the more metallic its properties.
In the Mendeleev periodic system of elements (Figure 1), the electronegativity increases from 2 to 7 within each period (beginning with the second), with increasing atomic number, starting with an alkali metal and ending with a halogen (transition from metals to nonmetals). The electronegativity usually decreases within each subgroup (A and B) with increasing atomic number, but the decrease is not always consistent. The electronegativity
|Table 1. Normal electrode potentials of nontransition metals|
|System||Normal potential at 25°C (V)|
|Li⇌Li+ + e||−3.0245|
|Cs⇌Cs+ + e||−3.020|
|Rb⇌Rb+ + e||−2.990|
|K⇌K+ + e||−2.925|
|Ra⇌Ra2+ + 2e||−2.92|
|Ba⇌Ba2+ + 2e||−2.90|
|Sr⇌Sr2+ + 2e||−2.89|
|Ca⇌Ca2+ + 2e||−2.87|
|Na⇌Na+ + e||−2.714|
|Mg⇌Mg2+ + 2e||−2.375|
|Be⇌Be2+ + 2e||−1.69|
|Al⇌AI3+ + 3e||−1.67|
|Ga⇌Ga3+ + 3e||−0.52|
|Ga⇌Ga2+ + 2e||−0.45|
|In⇌In3+ + 3e||−0.34|
|Tl⇌Tl+ + e||−0.338|
|In⇌In2+ + 2e||−0.25|
|Sn⇌Sn2+ + 2e||−0.140|
|Pb⇌Pb2+ + 2e||−0.126|
|H2⇌2H+ + 2e||0|
|Sb⇌Sb3+ + 3e||+0.20|
|Bi⇌Bi3+ + 3e||+0.23|
|Po⇌Po3+ + 3e||+0.56|
|Po⇌Po2+ + 2e||+0.65|
|Tl⇌TI3+ + 3e||+0.71|
|Pb⇌Pb4+ + 4e||+0.80|
remains at approximately the same level within the lanthanide and actinide series.
If the metals are arranged in a sequence with increasing normal potentials, a series of potentials or activities will result (Tables 1 and 2). Analysis of this series shows that, as one approaches the end of the series (from the alkali metals and alkaline earths to platinum and gold), the electropositive character of the members of the series decreases. The metals from lithium to sodium displace H2 from H2O when cold, whereas the metals from magnesium to tellurium do the same upon heating. All the metals located in the series above H2 displace it from dilute acids (either when cold or upon heating). Metals located below H2 dissolve only in oxygen-containing acids (such as concentrated H2SO4 upon heating, or HNO3), whereas platinum and gold dissolve only in aqua regia (iridium is even insoluble in the latter).
Metals from lithium to sodium react readily with O2 when cold; the subsequent members of the series combine with O2 only upon heating, whereas iridium, platinum, and gold do not react directly with it.
The oxides of metals from lithium to aluminum (Table 1) and from lanthanum to zinc (Table 2) are poorly reducible; toward the end of the series the reducibility of the oxides increases, and the oxides of the last members of the series decompose into M and O2 even upon moderate heating. The stability of metal-oxygen and metal-nonmetal compounds may also be estimated from the difference in their electronegativity (Figure 1): the greater the difference, the higher the stability.
The valences (more precisely, the oxidation numbers) of the nontransition elements are +1 for subgroup I-A; +2 for II-A; + 1 and +3 for III-A; +2 and +4 for IV-A; +2, +3, and + 5 for V-A; and -2, +2, +4, and +6 for VI-A. The transitional metals exhibit an even greater variety of oxidation numbers: +1, +2, and +3 for subgroup I-B; +2 for II-B; +3 for III-B; +2, +3, and + 4 for IV-B; +2, +3, +4, and +5 for V-B; +2, +3, + 4, +5, and +6 for VI-B; +2, +3, + 4, + 5, +6, and +7 for VII-B; and +2 to +8 for VIII-B. The oxidation numbers of +2, +3, and +4 are observed in the lanthanide family, and the oxidation numbers from +3 to +6 are observed in the actinide family. The lower oxides of metals have basic properties, and the higher oxides are anhydrides of acids. Metals with variable valence (for example, chromium, manganese, and iron) exhibit reducing properties in compounds of the lower oxidation states—Cr(+2), Mn(+2), and Fe(+2). The same metals exhibit oxidizing properties in their higher oxidation states—Cr(+6), Mn(+7), and Fe(+3).
REFERENCESNekrasov, B. V. Osnovy obshchei khimii, 2nd ed., vols. 1–3. Moscow, 1969–70.
Day, M. C., and J. Selbin. Teoreticheskaia neorganicheskaia khimiia, 2nd ed. Moscow, 1971. (Translated from English.)
Barnard, A. Teoreticheskie osnovy neorganicheskoi khimii. Moscow, 1968. (Translated from English.)
Ripan, R., and J. Ceteanu. Neorganicheskaia khimiia. Vols. 1–2: Khimiia metallov. Moscow, 1971–72. (Translated from Rumanian.)
S. A. POGODIN
Physical properties Most metals crystallize in relatively simple structures, such as cubic (body-centered and face-centered) and hexagonal close-packed, which correspond to the closest packing of atoms. Only a small number of metals have more complex types of crystal lattices. Many metals may exist in the form of two or more crystalline modifications, depending on external conditions (temperature and pressure). Polymorphic transformations are sometimes accompanied by the loss of metallic properties, such as the transition of white tin (β-Sn) to gray tin (α-Sn).
ELECTRICAL PROPERTIES. The specific conductivity or of metals at room temperature is of the order of 104–106 ohm-1cm-1 (Figure 1), whereas the specific conductivity of dielectrics—for example, sulfur—is of the order of 10-17 ohm-1cm-1. The
|Table 2. Normal electrode potentials of transition metals|
|System||Normal potential at 25°C (V)|
|Ac⇌Ac3+ + 3e||−2.60|
|La⇌La3+ + 3e||−2.52|
|Y⇌Y3+ + 3e||−2.37|
|Sc⇌Sc3+ + 3e||−2.08|
|Hf⇌Hf4+ + 4e||−1.70|
|Ti⇌Ti3+ + 3e||−1.63|
|Zr⇌Zr4+ + 4e||−1.56|
|V⇌V2+ + 2e||−1.18|
|Mn⇌Mn2+ + 2e||−1.18|
|Nb⇌Nb3+ + 3e||−1.10|
|V⇌V3+ + 3e||−0.87|
|Cr⇌Cr2+ + 2e||−0.86|
|Zn⇌Zn2+ + 2e||−0.761|
|Cr⇌Cr3+ + 3e||−0.74|
|Fe⇌Fe2+ + 2e||−0.44|
|Cd⇌Cd2+ + 2e||−0.402|
|Re⇌Re3+ + 3e||−0.3|
|Co⇌Co2+ + 2e||−0.277|
|Ni⇌Ni2+ + 2e||−0.25|
|Te⇌Te2+ + 2e||−0.24|
|Mo⇌Mo3+ + 3e||−0.20|
|H2⇌2H+ + 2e||0|
|Fe⇌Fe3+ + 3e||+0.036|
|W⇌W3+ + 3e||+0.11|
|Cu⇌Cu2+ + 2e||+0.346|
|Co⇌Co3+ + 3e||+0.40|
|Ru⇌Ru2+ + 2e||+0.45|
|Mn⇌Mn3+ + 3e||+0.47|
|Cu⇌Cu+ + e||+0.522|
|Rh⇌Rh2+ + 2e||+0.60|
|W⇌W6+ + 6e||+0.68|
|Rh⇌Rh3+ + 3e||+0.70|
|Os⇌Os2+ + 2e||+0.70|
|Ag⇌Ag+ + e||+0.779|
|Pd⇌Pd2+ + 2e||+0.83|
|Hg⇌Hg2+ + 2e||+0.854|
|Ir⇌Ir3+ + 3e||+1.0|
|Pt⇌Pt2+ + 2e||+1.2|
|Au⇌Au3+ + 3e||+1.5|
|Au⇌Au+ + e||+1.7|
semiconductors are characterized by intermediate values. A linear relationship between the current density and the intensity of the applied electric fields is a characteristic property of metals as conductors (Ohm’s law). Conduction electrons, which are highly mobile, are the current carriers in metals. According to quantum-mechanical concepts, the conduction electrons do not encounter any resistance in an ideal crystal (in the complete absence of thermal lattice vibrations). The existence of electrical resistance in real metals is due to disturbances in the periodicity of the crystal lattice. These disturbances may be associated with the thermal motion of the atoms or the presence of impurity atoms, vacancies, dislocations, and other crystal defects. Electrons are scattered by thermal vibrations and defects, and also by one another. The mean free path, which is the mean distance / between two successive collisions of electrons, is a measure of scattering. The magnitude of the specific conductivity σ is related to the length of the mean free path by the equation
(1) σ = nel/pF
where n is the concentration of conduction electrons (~ 1022~1023cm-3), e is the charge of the electron, and pF = 2πh (3n/8π)1/3 is the limiting Fermi pulse, and h is Planck’s constant. The dependence of σ or the specific electrical resistance p on the temperature T is related to the dependence between l and T. In metals at room temperature, l ~ 10-6 cm.
At temperatures appreciably exceeding the Debye temperature, the resistance p is caused mainly by the thermal lattice vibrations, and it increases linearly with temperature:
(2) α = ρrcs (1 + ρT)
The constant α is called the temperature coefficient of electrical conductivity, which has a typical value of α = 4 X 10-3deg-1 at T = 273°K (T = 0°C). At lower temperatures, where the effect of thermal oscillations of the atoms on the scattering of electrons may be neglected, the resistance virtually ceases to depend on temperature. This limiting value of the resistance is called the residual resistance. The quantity pres. characterizes the concentration of lattice defects in the metal. It is possible to prepare such pure (hyperpure) and defect-free metals that their residual resistance is 104–105 times lower than the resistance of the metals under normal conditions. The mean free path of electrons in hyperpure metals is of the order of 10-2 cm. Theoretical analysis shows that the equation for the specific resistance at low temperatures is
(3) α = αrcs + AT2 + BT5
where A and B are quantities that do not depend on T. The member BT5 is associated with the scattering of electrons by the thermal oscillations of atoms, and the member AT2 is associated with the mutual collisions of the electrons. The latter member contributes appreciably to the resistance only of some metals— for example, platinum. However, relationship (3) is obeyed only approximately.
At certain temperatures, called critical temperatures, some metals and metalloids exhibit complete disappearance of resistance—that is, a transition into the superconducting state. The critical temperatures of pure metals are in the range from several hundredths of a degree to 9°K (Figure 1).
If a sample of metal through which a current is flowing is placed in a permanent magnetic field, phenomena are generated in the metal because of the curvature of the electron trajectories in the magnetic field in the intervals between collisions in the magnetic field (galvanomagnetic phenomena). The Hall effect and the change of the electrical resistance of metals in magnetic fields (magnetoresistance) occupy an important place among such phenomena. The effect of a magnetic field increases with increasing mean free path /—that is, with decreasing temperature and decreasing quantities of impurities in the metals. A magnetic field of 107-105 oersteds changes the resistance of metals at room temperature by only fractions of a percent. At T ≦ 4°K in super-pure metals, the resistance may change manyfold. The dependence of the electrical resistance of metals on an external magnetic field is significantly influenced by the character of the energy spectrum of the electrons, particularly the shape of the Fermi surface. Many metallic single crystals (gold, copper, and silver) exhibit complex anisotropy of resistance in magnetic fields.
All metallic single crystals exhibit an oscillating dependence of the electrical resistance on the magnetic field (the Shubnikovde Haas effect) at low temperatures in magnetic fields of the order of 104–105 oersteds. This phenomenon results from quantization of the motion of electrons in the plane at right angles to the direction of the magnetic field. As a rule, the oscillating quantum dependence is superimposed in the form of a slight “ripple” on the usual dependence of the resistance on the magnetic field.
“Evaporation” of the electrons from the surface of a metal is observed upon heating of metals to high temperatures (thermal electron emission). The number of electrons emitted per unit time is determined by the relationship n ~ exp (—Φ/k7), where k is the Boltzmann constant and Φ is the electron work function for emission from the metal. The magnitude of Φ varies in different metals, and it also depends on the condition of the surface. Emission of electrons from the surface of metals also occurs under the influence of strong electric fields (of the order of 107 volts per cm) because of the tunneling of electrons through the potential barrier, which has been lowered by the field. The phenomena of photoelectronic emission, secondary electron emission, and ion-electron emission are observed in metals. The temperature gradient in metals gives rise to the appearance of an electric current or a potential difference.
THERMAL PROPERTIES. The heat capacity of metals (see Figure 1) is determined both by the ion core (lattice heat capacity Clat) and by the electron gas (electron heat capacity Ce). Although the concentration of conduction electrons in metals is very high (see above) and is independent of temperature, the electron heat capacity is low and is observable in most metals only at temperatures of the order of a few degrees Kelvin. The quantity Ce may be determined because Clat decreases proportionally to T3, and Ce ~ T. For Cu, Ce = 0.9 X 10-4RT; for Pd, Ce = 1.6 X 10-3RT (R is the gas constant). The thermal conductivity of metals is mainly due to the conduction electrons. Therefore, the specific coefficients of electrical conductivity and thermal conductivity are related in a simple way by the Wiedemann-Franz law.
INTERACTIONS OF METALS AND ELECTROMAGNETIC FIELDS. If the frequency is sufficiently high, an alternating electric current can flow along the surface of a metal without penetrating it (the skin effect). An electromagnetic field of the frequency o> penetrates the metal only to the depth of the skin layer, of thickness . For example, for copper at ω = 108 hertz (Hz), δ = 6 X 10-4 cm. An insignificant portion of the electromagnetic energy is absorbed in such a layer. Most of the energy is re-emitted by the conduction electrons and reflected. In pure metals at low temperatures, the mean free path l of the electrons frequently exceeds the depth δ. At the same time, the intensity of the field changes considerably over the length of the free path; this is manifested in the nature of reflection of electromagnetic waves from the surface of metals (anomalous skin effect).
A strong constant magnetic field exerts a significant influence on the electromagnetic properties of metals. If the frequency of the electrodynamic field is a multiple of the frequency of precession of the conduction electrons about the lines of force of the constant magnetic field, metals placed in such a field exhibit resonance phenomena. Under certain conditions, slowly decaying electromagnetic waves may propagate through a metal that has been placed in a constant magnetic field—that is, the skin effect disappears. The electrodynamic properties of metals placed in a magnetic field are similar to the properties of plasma in a magnetic field and are one of the main sources of information concerning conduction electrons.
As a rule, metals are virtually opaque to optical electromagnetic waves, and they have a characteristic luster. The internal photoelectric effect plays a certain role in the absorption of light in the visible and ultraviolet regions. Reflection of planepolarized light, incident at any angle, from the surface of a metal is accompanied by rotation of the plane of polarization and the appearance of elliptical polarization. This phenomenon is used for the determination of the optical constants of metals.
The general structures of the characteristic X-ray spectra of metals and dielectrics are identical. However, the fine structures, which correspond to the quantum transitions of electrons from the conduction zone to the deep levels, reflect the distribution of conduction electrons over the energy levels.
MAGNETIC PROPERTIES. Transition metals with incomplete f- and d-shells are paramagnetic. At certain temperatures, some of them pass into magnetically ordered states. Magnetic ordering substantially affects all properties of metals, particularly the electrical properties: scattering of electrons by the oscillation of magnetic moments contributes to the electrical resistance. Galvanomagnetic phenomena also acquire characteristic features.
The magnetic properties of the other metals are determined by the conduction electrons (which contribute to the diamagnetic and paramagnetic susceptibilities of metals), as well as by the diamagnetic susceptibility of the ion composition. The magnetic susceptibility x of most metals is relatively small (x ~ 10-6) and is weakly dependent on temperature.
All metallic single crystals exhibit a complex, oscillating dependence of the total magnetic moment on the field H at low temperatures T and high magnetic field intensities H > 104kT, which is of the same nature as the Shubnikov-de Haas effect. The study of oscillatory effects makes possible determination of the shape of the Fermi surface.
M. I. KAGANOV
MECHANICAL PROPERTIES. Many metals have a combination of mechanical properties that makes possible their use in a wide variety of industrial applications, particularly as structural materials. Such a combination is, above all, high plasticity and considerable strength and resistance to deformation (however, the ratio of these properties may be controlled over a wide range by mechanical and heat treatment of metals, as well as by the production of alloys of various compositions).
The fundamental mechanical characteristic of metals is the elastic modulus, which determines the resistance of crystal lattices to elastic deformation and directly reflects the magnitude of the bond strength within the crystal. In single crystals this quantity, like the other mechanical characteristics, is anisotropic and correlates with the melting point of the metal (for example, the mean shear modulus G varies from 0.18 X 1011 erg/cm3 for readily fusible sodium to 27 X 10ll erg/cm3 for refractory rhenium).
The resistance of an ideal crystal to failure or plastic deformation is of the order of 10-1G. However, in real crystals these characteristics, as well as all the mechanical properties, are determined by the presence of defects, mainly dislocations. Displacement of the dislocations along the close-packed planes leads to the elementary event of slippage, which is the basic mechanism of plastic deformation of metals. Other mechanisms (twinning and faulting) are important only at low temperatures.
The most important feature of metals is low resistance to slippage of dislocations in defect-free crystals. This resistance is particularly low in crystals with purely metallic bonding, which have usually close-packed structures (face-centered cubic or hexagonal). Metals with covalent components of interatomic bonds and body-centered lattices exhibit somewhat higher resistance to slippage, but the resistance is still low in comparison to purely covalent crystals. Resistance to plastic deformation, at least in metals with face-centered cubic and hexagonal lattices, is connected with the interaction between moving dislocations and other crystal defects, as well as with other dislocations, impurity atoms, and internal interfaces. The interaction of defects is determined by distortions of the lattice in their vicinity and is proportional to G. The initial resistance to deformation in annealed single crystals (the yield point) is usually of the order of 10 -3–10-4G. The number of dislocations in the crystal lattice (the dislocation density β) increases from 106–108 to 1012 cm-2 in the process of deformation. The resistance to plastic deformation, which is of the order of Gd (d is the interatomic distance), increases correspondingly. This is called strain hardening or work hardening. The existence of three stages of strain hardening is characteristic of single crystals. In the first stage, an appreciable part of the dislocations emerges at the surface, and the hardening coefficient Θ (the coefficient of proportionality between stress and deformation) is small; in the second stage, the dislocations are concentrated in the crystal and their distribution becomes significantly nonuniform: Θ ~ G/300. In the third stage β, G, and Θ decrease because of the annihilation of dislocations, which are squeezed out of their slip planes. The significance of this stage is greater for metals with body-centered lattices.
The degree of “attachment” of a dislocation to the slip plane is determined by the width of the dislocation in the plane, which in turn depends on the energy γ of the packing defect (the quantity γ/Gd in metals with face-centered lattices varies from 10-2 for aluminum, which has narrow dislocations, to 10-4 for copper alloys, with wide dislocations). The process of reduction of the dislocation density accelerates with increasing temperature and may lead to relaxation and a significant restoration of the properties of the crystal. The higher the temperature and the lower the rate of deformation, the greater the opportunity for the development of relaxation processes and the smaller the resulting strain hardening.
At T > 0.5Tm (Tm is the melting point), point defects begin to play a significant role, above all vacancies, which settle on the dislocations and cause them to move out of the slip planes. If the process is sufficiently intense, the deformation is not accompanied by hardening: the metal flows at a constant rate under a constant load (creep). The occurrence of stress relaxation processes and constant dissipation of the dislocation structure give rise to high plasticity of metals during hot working, which makes it possible to impart a variety of shapes to metal articles. Annealing of strongly deformed single crystals of metals frequently leads to the formation of polycrystals with low dislocation density in the interior of the granules (recrystallization).
The degrees of deformation of metals that may be attained are limited by the process of failure. As the dislocation density increases during cold deformation, the nonuniformity of their distribution also increases, leading to the concentration of stresses in places of concentration of dislocations and to the generation at these locations of foci of failure (cracks). In real crystals, such stress concentrations are also present in the initial undeformed state (concentration of impurities, particles of foreign phases, and other conditions). However, because of the plasticity of metals, deformation in the vicinity of dangerous locations removes the stresses and prevents failure. However, as the resistance to the movement of dislocations increases, the relaxation capacity of the material decreases, leading to the development of cracks under load (brittle fracture). This situation occurs particularly in metals with body-centered lattices, in which the mobility of dislocations decreases sharply upon a decrease in temperature because of interactions with impurities and decrease in the number of crystallographically possible slip planes. Prevention of cold brittleness is one of the most important problems in the development of metal structural materials. Another important problem is the increase of strength and resistance to deformation at high temperatures. The nuclei of failure under these conditions are micropores, which form as a result of the concentration of vacancies. An effective method for the increase of high-temperature strength is a decrease in the diffusion mobility of point defects, particularly through alloying.
The metal structural materials used in industry are polycrystalline in nature. Their mechanical properties are virtually iso-tropic and may be very different from the properties of single crystals of metals. The phase boundaries make an additional contribution to the hardening. On the other hand, they may also become locations of preferential failure (intergranular fracture) or deformation. By changing the number and structure of the phase boundaries and the form and spatial configuration of the separate structural components of multiphase systems (polycrystals and heterophase aggregates arising as a consequence of phase transformations; artificially produced compositions), and also by controlling the composition and defect structure of the separate crystals, it is possible to produce the extremely large variety of mechanical properties required for the practical application of metals.
A. L. ROITBURD
REFERENCESFrenkel’, la. I. Vvedenie v teoriiu metallov, 2nd ed. Moscow-Leningrad, 1950.
Bethe, H., and A. Sommerfeld. Elektronnaia teorüa metallov. Moscow-Leningrad, 1938. (Translated from German.)
Lifshits, I. M., M. la. Azbel’, and M. I. Kaganov. Elektronnaia teoriia metallov. Moscow, 1971.
Abrikosov, A. A. Vvedenie v teoriiu normarnykh metallov. Moscow, 1972.
Slater, J. Dielektriki, poluprovodniki, metally. Moscow, 1969. (Translated from English.)
Schulze, G. Metallofizika. Moscow, 1971. (Translated from German.)
Metals in technology Because of such properties as strength, hardness, plasticity, corrosion resistance, heat resistance, and high electrical conductivity, metals play an extremely important role in technology, and the number of metals in use is steadily increasing. It is characteristic that, before the 20th century, such very important metals as aluminum, vanadium, tungsten, molybdenum, titanium, uranium, and zirconium either were not produced at all or were manufactured in very limited quantities. Such metals as beryllium, niobium, and tantalum came into wide use just before World War II. In the 1970’s almost all naturally occurring metals are being used industrially.
All metals and alloys are divided into ferrous (including iron and its alloys—about 95 percent of world metal production) and nonferrous (all other metals and alloys). The large number of nonferrous metals and the wide range of their properties do not permit their classification according to any single criterion. An arbitrary classification convention has been adopted in industry, according to which the nonferrous metals are divided into several groups according to various criteria (physical and chemical properties; character of occurrence in the earth’s crust) that are specific for the various groups: light metals (such as aluminum and magnesium), heavy metals (copper, lead, and so on), refractory metals (tungsten and molybdenum), noble metals (gold and platinum), trace metals (gallium, indium, and tellurium), rare earths (scandium, yttrium, and the lanthanides), and radioactive metals (radium, uranium, and others). Metals that are produced in limited quantities are called rare metals. They include all the trace metals, the rare earths, the radioactive metals, most refractory metals, and some light metals.
The great capacity of metals to form numerous compounds of various types and to undergo various phase transformations creates favorable conditions for the formation of various alloys, which are characterized by desired combinations of useful properties. The number of alloys used in industry has already exceeded 10,000. The importance of alloys as structural materials, electrical materials, and materials with special physical properties is continuously increasing. The production of a number of superpure metals (purity of 99.9999 percent and higher) is increasing in connection with the development of semiconductor and nuclear technology.
The use of a particular metal or alloy is mostly determined by the practical value of its properties. However, other conditions, such as the magnitude of natural reserves, the accessibility of deposits, and the economic feasibility of mining the metal, may also be of great importance. Among the most valuable and industrially important metals, only a few are contained in large quantities in the earth’s crust: aluminum (8.8 percent), iron (4.65 percent), magnesium (2.1 percent), and titanium (0.63 percent). The natural reserves of a number of very important metals are of the order of a few hundredths of a percent (for example, copper, manganese, chromium, vanadium, and zirconium) or even a few thousandths of a percent (zinc, tin, lead, nickel, cobalt, and niobium). Some precious metals are present in the earth’s crust in even smaller quantities. The content of uranium (an important source of nuclear energy) is estimated to be 0.0003 percent, and the content of tungsten (the basis of hard alloys) is estimated at 0.0001 percent. Nature is especially poor in the noble and rare metals.
The great variety of metals predetermines the large number of methods for their production and processing. The relationship among the composition, structure, and properties of metals and alloys, as well as the principles of their transformations as a result of thermal, chemical, or mechanical treatment, is the subject of metal science. The properties, methods and volume of production, and uses of various metals are described in articles on the corresponding chemical elements and alloys based on them (see, for example, ALUMINUM, ALUMINUM ALLOYS, BERYLLIUM, and BERYLLIUM ALLOYS).
I. I. NOVIKOV