General summary
Until recently, refractory metals - vanadium, chromium, niobium, tantalum, molybdenum and tungsten were used mainly for alloying alloys based on metals such as iron, nickel, cobalt, aluminum, copper, and in very limited quantities in other industries, for example in the electric lamp and chemical industry.
For alloying, it was quite enough to have metals with 1–2% impurities. Refractory metals with such an impurity content are extremely fragile and unsuitable for use as structural materials. However, the plasticity of refractory metals increases with an increase in their purity, and the problem of their use as structural materials became quite real after the development of methods for obtaining these metals with a very low content of impurities.
Refractory metals are usually obtained by reduction of their salts or oxides with active metals or hydrogen, as well as by electrolysis.
Vanadium is obtained by reduction of its pentoxide with calcium or vanadium trichloride with magnesium or calcium. The purest vanadium is obtained by the iodide method, as well as by electrolytic refining in molten salts.
A simple way to obtain sufficiently pure chromium is its electrolytic deposition from aqueous solutions. Electrolytic chromium contains, however, quite significant amounts of oxygen and hydrogen. Very pure chromium is obtained by the iodide method, as well as vacuum distillation and hydrogen refining of commercially pure chromium.
Niobium is usually found in nature together with tantalum. Therefore, when obtaining these metals in their pure form, careful separation is necessary. After separation, pure tantalum is obtained by reduction of its fluorotantalate with sodium or other active metals. Niobium is extracted from niobium carbide or oxide, which is formed by the separation of tantalum and niobium. Niobium can also be obtained by electrolysis of potassium fluoroniobate and reduction of niobium pentachloride with hydrogen. For final purification, tantalum and niobium are remelted in a high vacuum.
Molybdenum and tungsten are obtained by the reduction of their purified oxides, chlorides or ammonium salts with hydrogen.
It should be noted that after extraction from ores, most refractory metals are in the form of a powder or a sponge. Therefore, to obtain them in a compact form, the methods of powder metallurgy, arc melting, and, more recently, very effective electron beam melting are used.
Physical and chemical properties of pure refractory metals
Refractory metals discussed here are in the VA (vanadium, niobium and tantalum) and VIA (chromium, molybdenum and tungsten) subgroups.
Some physical properties of pure refractory metals are given in table. 25.
Of the other physical properties of pure refractory metals, a relatively small cross-section for the capture of thermal neutrons should be noted: 1.1 for niobium, 2.4 for molybdenum, 2.9 for chromium, and 4.7 bar for tungsten. The purest tungsten and molybdenum at temperatures near absolute zero are superconductors.
This also applies to vanadium, niobium, and tantalum, whose superconducting transition temperatures are 5.9 and 4.5 ° K, respectively.
The chemical properties of pure refractory metals are very different. Chromium at room temperature is resistant to air and water. As the temperature rises, the activity of chromium increases and it directly combines with halogens, nitrogen, carbon, silicon, boron and a number of other elements, and burns out in oxygen.
Vanadium is chemically active. It begins to interact with oxygen, hydrogen and nitrogen already at temperatures above 300 ° C. With halogens, vanadium reacts directly when heated to 150-200 ° C.
At room temperature, molybdenum is stable in air and in oxygen, but when heated above 400 ° C, it begins to oxidize intensively. It does not chemically react with hydrogen, but weakly absorbs it. Molybdenum actively interacts with fluorine at ordinary temperatures, begins to interact with chlorine at 180 ° C, and it hardly reacts with iodine vapor.
Tungsten is also stable in air and oxygen at room temperature, but strongly oxidizes when heated above 500 ° C. Tungsten does not react with hydrogen up to the melting point. It reacts with fluorine at room temperature, with chlorine at temperatures above 300 ° C and interacts very difficult with iodine vapor.
Of the metals under consideration, pure tantalum and niobium are characterized by the highest corrosion resistance. They are stable in hydrochloric, sulfuric, nitric and other acids and somewhat less in alkalis. In many media, pure tantalum is close to platinum in its chemical resistance. A characteristic feature of tantalum and niobium is their ability to absorb large amounts of hydrogen, nitrogen and oxygen. When heated above 500 ° C, these metals are intensively oxidized in air.
For the possibility of using refractory metals at elevated temperatures, their tendency to oxidation is of particular importance. Of the metals under consideration, only pure chromium has a high resistance to oxidation. All other refractory metals are intensively oxidized at temperatures above 500-600 ° C. The high resistance of chromium to oxidation is due to the formation of a dense refractory oxide film on its surface, which protects the metal from further oxidation. On the surface of the remaining refractory metals, no protective oxide films are formed.
Oxides of molybdenum and vanadium are very fusible (their melting points are 795 and 660 ° C, respectively) and volatile. The oxides of niobium, tantalum, and tungsten have relatively high melting points (1460, 1900, and 1470 ° C, respectively), but their specific volumes significantly exceed the specific volumes of the corresponding metals. For this reason, oxide films, even with their very small thickness, crack and peel off from the metal, opening access to oxygen to its clean surface.
Mechanical properties of pure refractory metals and the effect of impurities on these properties
Since all the described refractory metals have a body-centered lattice, their mechanical properties have a number of features characteristic of metals with such a structure. The mechanical properties of refractory metals (tensile strength, plasticity, hardness) strongly depend on the presence of impurities in them. The negative influence of even minute amounts of impurities on their plastic properties is extremely great.
The decisive role in changing the mechanical characteristics of body-centered metals is played by such interstitial impurities as carbon, nitrogen, oxygen, hydrogen entering the interstitial spaces.
Thus, in molybdenum melted in an arc furnace, the carbon content can be reduced to 0.01%, and the gas content can be brought to very low values, for example oxygen up to 1 ppm. Such a bar can be bent without destruction to a temperature of the order of -50 ° C, and upon impact testing it breaks.
By zone melting, the carbon content in molybdenum can be reduced from 0.01 to 0.002% and below. During impact testing, zone-refined rods retain their ductility down to -140 ° C. Hence, it clearly follows that the ductility of molybdenum (as well as other refractory metals) is a function of their purity with respect to interstitial impurities. Freed from these impurities, molybdenum and other refractory metals can easily withstand cold working (rolling, stamping and other similar operations).
The degree of purification of molybdenum from oxygen greatly affects the transition temperature to the brittle state: at 0.01% O2 it is equal to plus 300 ° C, at 0.002% O2 - plus 25 ° C, and at 0.0001%) O2 - minus 196 ° WITH.
At present (by the method of zone melting with electron beam heating) large single crystals of molybdenum with a length of about 500 mm and a cross section of 25x75 mm are grown. These single crystals achieve high material purity with a total interstitial impurity content of less than 40 ppm. Such single crystals of the purest molybdenum are characterized by very high plasticity up to the temperature of liquid helium.
A single crystal of molybdenum can be bent without destruction by 180 degrees, from a single crystal of molybdenum with a diameter of 12 mm, cold deformation can be used to obtain a wire with a diameter of 30 microns and a length of 700-800 m or a foil with a thickness of 50 microns, which can be cold pressed with drawing, which is very important to obtain a number of critical parts of electrovacuum devices.
A similar method is used to obtain single crystals of other refractory metals - tungsten, vanadium, niobium, tantalum. Tungsten is currently produced by the method of electron beam zone melting in the form of single crystals with a diameter of about 5 mm and a length of about 250 mm of high density and purity (99.9975% W). Such tungsten is ductile even at temperatures of - 170 ° C.
Tungsten single crystals obtained by electron beam melting withstand twofold bending at room temperature, which indicates a very low transition temperature of this metal from a ductile to a brittle state. For ordinary tungsten, the beginning of the transition to the brittle state is at temperatures above 700 ° C.
Tungsten single crystals easily withstand cold processing and are currently used for the manufacture of wire, bar material, sheets and other semi-finished products. Monocrystalline niobium can deform at room temperature up to 90% reduction and retains a sufficiently high plasticity at liquid nitrogen temperature (-194 ° C). A single crystal of tantalum, compressed by 80%, in the manufacture of wire also has sufficient ductility.
Excellent plasticity, minimal work hardening, high corrosion resistance and good stability are characteristic of high-purity refractory metals obtained in the form of single crystals by the method of electron beam zone melting. Vanadium, niobium and tantalum in the form of polycrystalline ingots of electron beam melting or single crystals refined by zone melting, even with very deep cooling, do not pass into a brittle state.
Application of pure refractory metals
The use of pure refractory metals (and in the future they will obviously be used only in this form) is developing in two main directions: 1) for supersonic aviation, guided missiles, rockets and spaceships; 2) for electronic equipment. In both cases, the purest metals with very high ductility are required, which, as we saw above, is achieved by deep cleaning of refractory metals from interstitial impurities.
Heat-resistant steels and alloys based on nickel and cobalt, which can operate at temperatures of 650-870 ° C, no longer meet the requirements of supersonic aviation and rocketry. Materials are required that have a sufficiently long-term strength at temperatures above 1100 ° C. Such materials are pure refractory metals (or alloys based on them) capable of plastic deformation.
For the manufacture of the skin of supersonic aircraft and missiles, sheets of pure molybdenum and niobium are required, which have a higher specific strength than tantalum and tungsten, up to 1300 ° C.
Parts of air-jet, rocket and turbojet turbines operate in more severe conditions. For the manufacture of these parts operating at temperatures up to 1370 ° C, it is advisable to use pure molybdenum and niobium, but at higher temperatures, only tantalum and tungsten are suitable. For operation at temperatures above 1370 ° C, pure tantalum and its alloys, which have a relatively high plasticity at such temperatures, and are not inferior to tungsten in terms of heat resistance, are of greatest interest.
Parts of gas turbines work in the most severe conditions. Pure niobium and niobium-based alloys with acceptable oxidation resistance are most suitable for such parts.
The purest refractory metals find various applications in electronic and vacuum technology. Tantalum is a good getter and is widely used in the manufacture of vacuum tubes. Niobium is used in vacuum technology for the manufacture of anodes, grids, tubes and other parts. Molybdenum and tungsten are used in electric vacuum devices and radio tubes for the manufacture of filaments, electrodes, hooks, pendants, anodes and meshes.
High-purity and non-porous tungsten single crystals are used as cathode heaters in electric vacuum devices, for electrical contacts, in vacuum switches, in bushings in vacuum installations - where the absence of gases is an important factor.
Pure refractory metals produced using electron beam melting will find direct application in the manufacture of miniature electronic devices. Of interest are coatings of pure refractory metals obtained by sputtering or thermal decomposition of refractory metal compounds.
Pure vanadium and niobium, due to their small thermal neutron capture cross section, are also successfully used in nuclear power engineering. Vanadium is used to make thin-walled pipes for nuclear reactors, shells of fuel elements, since it does not fuse with uranium and has good thermal conductivity and sufficient corrosion resistance.
Pure niobium does not interact with molten sodium and bismuth, which are often used as heat transfer fluids, and does not form brittle compounds with uranium.
Due to its high corrosion resistance, pure tantalum is used for the manufacture of parts of chemical equipment operating in acidic corrosive environments, for example, in the production of artificial fibers. Recently, tantalum is often replaced here with pure niobium, which is cheaper and more widespread in nature. Pure chromium has similar applications. All the expanding fields of application of the purest refractory metals are far from being exhausted by these examples.
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For a very long time, some other metals were also considered brittle - chromium, molybdenum, tungsten, tantalum, bismuth, zirconium, etc. However, this was the case until they learned how to obtain them in a sufficiently pure form. As soon as this was successful, it turned out that these metals are very ductile even at low temperatures. In addition, they do not rust and have a number of valuable properties. Now these metals are widely used in various industries.
But what is pure metal? It turns out that this also cannot be answered unambiguously. Conventionally, according to their purity, metals are divided into three groups - technically pure, chemically pure and extra pure. If the alloy contains at least 99.9 percent of the base metal, this is technical purity. 99.9 to 99.99 percent chemical purity. If 99,999 or more, this is a particularly pure metal. In everyday life, scientists use another definition of purity - by the number of nines after the decimal point. They say: "purity three nines", "purity five nines", etc.
At first, the industry was quite satisfied with chemically, and often even technically pure metals. But the scientific and technological revolution has made much more stringent demands. The first orders for ultrapure metals came from the nuclear industry. Ten-thousandth, and sometimes even millionths of a percent of some impurities made uranium, thorium, beryllium, graphite unusable. Obtaining ultrapure uranium was perhaps the main difficulty in creating the atomic bomb.
Then the jet technology presented its demands. Ultrapure metals were required to obtain particularly high-temperature and heat-resistant alloys, which were to work in the combustion chambers of jet aircraft and rockets. The metallurgists did not have time to cope with this task, when a new "application" was received - for semiconductors. This task was more difficult - in many semiconductor materials, the amount of impurities should not exceed a millionth of a percent! Don't let this scanty amount confuse you. Even with such a purity, where there is one impurity atom per 100,000,000,000 atoms of the basic substance, there are still more than 100,000,000,000 "foreign" atoms in each gram of it. So it's far from perfect cleanliness. However, there is no such thing as absolute purity. This is an ideal to strive for, but which is impossible to achieve at this level of technological development. Even if by a miracle it is possible to obtain an absolutely pure metal, then atoms of other substances contained in the air will immediately penetrate into it.
A curious incident with the famous German physicist Werner Heisenberg is indicative in this respect. He worked with a mass spectrograph in his laboratory. And suddenly the device showed the presence of gold atoms in the test substance. The scientist was amazed, because this could not be. But the device stubbornly stood its ground. The misunderstanding was clarified only when the scientist took off and hid his glasses in gold frames. Individual gold atoms, "escaping" from the crystal lattice of the mount, got into the substance under study and "confused" the extremely sensitive device.
But this happened in a laboratory where the air is clean. What can we say about modern industrial areas, the air of which is more and more polluted by industrial waste?
We started this chapter by talking about how in one case the presence of impurities in the metal is good, but in the other it is bad. Moreover, at first we said that alloys have better strength and heat resistance than pure metals, but now it turns out that pure metals have the highest properties. There is no contradiction. In many cases, the alloy is stronger, more heat-resistant, etc., than any of the metals included in its composition. But these qualities are enhanced many times over when all the components of the alloy perform a specific task necessary for a person. When there is nothing "superfluous" in it. This means that the components themselves should be as pure as possible, contain a minimum amount of "extraneous" atoms. Therefore, now the question of the purity of the obtained metallurgical products is becoming more and more acute. How is this problem solved?
In metallurgical plants, where a large amount of metal is produced for conventional products, vacuum is increasingly used. In a vacuum, the metal is melted and poured, and this makes it possible to protect it from the ingress of harmful gases and molecules of other substances from the surrounding air. And in some cases, melting is carried out in an atmosphere of inert gas, which further protects the metal from unwanted "penetration".
Pure metals metals with a low content of impurities. Depending on the degree of purity, metals of high purity (99.90-99.99%), metals of high purity, or chemically pure (99.99-99.999%), metals of high purity, or spectrally pure, Ultrapure metals (over 99.999 %).
Great Soviet Encyclopedia. - M .: Soviet encyclopedia. 1969-1978 .
See what "Pure Metals" is in other dictionaries:
pure metals- Metals with a low content of impurities (< 5 мас. %). Выделяют м. повыш. чистоты (от 99,90 до 99,99 %) и особой чистоты (от 9,999 до 99,9999 %). Тематики металлургия в целом EN pure metals … Technical translator's guide
Metals or alloys with a low content of impurities. Depending on the degree of purity, metals are distinguished cf. purity, or technically pure (99.0 99.90%). increase. purity (99.90 99.99%), high purity, or chemically pure (99.99 99.999%). special ... ... Big Encyclopedic Polytechnic Dictionary
pure metals- metals with a low content of impurities (< 5 мас. %). Выделяют металлы повышенной чистоты (от 99,90 до 99,99 %) и особой чистоты (от 9,999 до 99,9999%); Смотри также: Металлы щелочные металлы ультрачистые металлы тяжелые металлы …
PURE METALS- see Degree of purity of metal or alloy ... Metallurgical Dictionary
Simple substances with characteristic properties under normal conditions: high electrical conductivity and thermal conductivity, negative temperature coefficient of electrical conductivity, the ability to reflect electromagnetic waves well ... ...
- (from the Greek metallon, originally mine, ore, mines), simple in va, possessing, under normal conditions, characteristic connections: high electrical conductivity and thermal conductivity, negative temperature coefficient. electrical conductivity, ability well ... ... Physical encyclopedia
ultrapure metals- high-purity, extra-pure metals, in which the mass fraction of impurities does not exceed 1 10 3%. The main stages of the technology for the production of ultrapure metals: obtaining pure chemical compounds, their reduction to ... ... Encyclopedic Dictionary of Metallurgy
High-purity metals, especially pure metals, metals, the total content of impurities in which does not exceed 1 - 103% (by weight). The main stages of the production technology of U.M .: obtaining pure chemical compounds, their restoration to ... ... Great Soviet Encyclopedia
radioactive metals- metals that occupy places in the Periodic Table of Elements with an atomic number greater than 83 (Bi), emitting radioactive particles: neutrons, protons, alpha, beta particles or gamma quanta. Found in nature: At, Ac, Np, Pa, Po ... Encyclopedic Dictionary of Metallurgy
transition metals- elements Ib and VIIIb of the subgroup of the Periodic system. The inner shells of transition metal atoms are only partially filled. Distinguish between d metals, which are gradually filled with 3d (from Se to Ni), 4d (from Y to ... ... Encyclopedic Dictionary of Metallurgy
It allows you to save energy resources (coke, coal), get a greater yield of finished products from raw materials, shorten the production cycle while improving quality and improving the ecological state of the atmosphere. This is metallurgy, namely, the reduction of metals with the help of hydrogen.
Prehistory, or Forward to the past for pure metals
Metallurgy has accompanied humanity since the Bronze and Iron Ages. As early as 14 centuries BC. e. ancient people smelted iron using a critical method. The principle was to reduce iron ore with coal at a relatively low temperature of 1000 ° C. As a result, they received a kritsa - an iron sponge, then forged it until a blank was obtained, from which household items and weapons were made.
Already in the XIV century, primitive forges and blast furnaces began to appear, which laid the foundation for modern metallurgical processes: blast furnace, open-hearth and converter. The abundance of coal and iron ore has long established these methods as the main ones. However, the increasing requirements for product quality, resource saving and environmental safety led to the fact that already in the middle of the 19th century they began to return to their origins: to use the direct recovery of pure metals. The first modern such installation appeared in 1911 in Sweden, producing small batches of metals obtained with the help of hydrogen with a purity of 99.99%. At that time, consumers were only research laboratories. In 1969, a factory was opened in Portland (USA), producing up to 400 thousand tons of pure metals. And already in 1975, 29 million tons of steel were produced in the world by this method.
Now such products are awaited not only by the aviation, instrument-making industry, enterprises for the manufacture of medical instruments and electronics, but also many others. This technology received a special advantage in non-ferrous metallurgy, but in the near future, and "hydrogen ferrous metallurgy".
Pure metals and alloys used in electronics
Lecture 8. Conducting materials and wires
Appointment of conductive materials;
Appointment and types of wires.
Lecture objectives:
Study of conductive materials;
Study of wires.
8.1 nvalue n conductor materials
Most metallic conductive materials have high electrical conductivity ( ρ = 0.015 ÷ 0.028 μOhm m). These are predominantly pure metals that are used for the manufacture of winding and radio installation wires and cables.
Along with this, conductors with high electrical resistance are used in radio electronics - alloys of various metals. For metal (resistive) ρ = 0.4 ÷ 2.0 μOhm m. These alloys constitute a group of metallic materials with a low temperature coefficient of resistivity (TC ρ ) and are used for the manufacture of wire-wound resistors and other radio components.
Copper- the main material with high ductility, sufficient mechanical strength and high electrical conductivity. The melting point of copper is 1083 ° С, the coefficient of thermal expansion KTP = 17 · 10 -6 1 / ° С. For the manufacture of products (winding, radio-mounting wires and cables) use pure copper grades M00k; The IOC; Mock; M1k and M00b; Mob; M1b. Copper content 99.99 - 99.90%. Products made of soft copper (at 20 ° C) have a density of 8900 kg / m 3; σ p = 200 ÷ 280 MPa; e = 6 ÷ 35%; ρ = 0.072 ÷ 0.01724 μOhm m. Temperature coefficient of resistivity for all TK copper grades ρ = 0.0041 / ° C.
Bronze represents alloys of copper with tin (tin bronze), aluminum (aluminum), beryllium (beryllium) and other alloying elements. In terms of electrical conductivity, bronze is inferior to copper, but surpasses it in mechanical strength, elasticity, abrasion resistance and corrosion resistance. Bronze is used to make spring contacts, contact parts of connectors and other parts.
Brass- an alloy of copper with zinc, in which the highest zinc content can be 45% (by weight). Various parts are made from sheet brass: clamps, contacts, fasteners. The main characteristics of bronze, brass and copper are shown in table 8.1.
Kovar- an alloy of nickel (about 29% by weight), cobalt (about 18%), iron (the rest). A characteristic feature of kovar is the closeness of the values of its CTE = (4.3 ÷ 5.4) · 10 -6 1 / ° С to the values of the CTE of glass and ceramics in the temperature range 20-200 ° С. This makes it possible to produce consistent, hermetically sealed Kovar joints with glass and ceramics. It is used for the manufacture of IC packages and semiconductor devices.
Aluminum is the second conductive material after copper due to its relatively high electrical conductivity and resistance to atmospheric corrosion.
The density of aluminum is 2700 kg / m 3, ᴛ.ᴇ. it is 3.3 times lighter than copper, with a melting point of 658 ° C. Aluminum is characterized by low hardness and low tensile strength (σ p = 80 ÷ 180 MPa) and higher than copper CTE = 24 · 10 -6 1 / ° С. This is a disadvantage of aluminum.
High-purity aluminum grades are used to make plates for electrolytic capacitors, as well as foil. Aluminum wire is produced from Ø0.08 - 8mm in three varieties: soft (AM), semi-hard (APT), hard (AT).
Table 8.1
Silver belongs to the group of noble metals that do not oxidize in air at room temperature. Oxidation begins at 200 ° C. Silver is distinguished by its high ductility, allowing to obtain foil and wire Ø up to 0.01mm, and the highest electrical conductivity.
The main characteristics of silver: density 1050 kg / m 3; melting point 960.5 ° C; σ p = 150 ÷ 180 MPa (soft silver); σ p = 200 ÷ 300 MPa (solid silver); ρ = 0.0158 μOhm m; TC ρ = 0.003691 / ° C; CTE = 24 · 10 -6 1 / ° С.
Protective layers are made of silver on copper conductors of radio installation wires used at temperatures up to 250 ° С. Silver is applied to the inner surface of the waveguides to obtain a layer with high electrical conductivity, and is also introduced into solders (PSr10, PSr50), which are often used for soldering conductive ones in CEA.
Gold- unlike silver, it does not oxidize in air even at high temperatures. It has a very high ductility, it is used to produce foil up to 0.005 mm thick and wire Ø up to 0.01 mm.
The main characteristics of gold: density 1930 kg / m 3; melting point 1063 ° C; σ p = 150 ÷ 180 MPa, ρ = 0.0224 μOhm m; TC ρ = 0.003691 / ° C;
CTE = 14.2 10 -6 1 / ° C.
Gold is used for thin-film contact coatings when switching low currents in microcircuits, as well as for coating walls
waveguides and microwave resonators.
Pure metals and alloys used in electronics - concept and types. Classification and features of the category "Pure metals and alloys used in radio electronics" 2017, 2018.
