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Amorphous materials: their properties, application in modern technology, methods of production

Completed:

student of group 206 HFMM

Dorozhkin A.P.

Checked:

Head of the department

physical chemistry

Tomilin O.B.

Introduction

For a long time it seemed that the most interesting thing in Physics was the study of the microcosm and microcosm. It was there that they tried to find answers to the most important, fundamental questions explaining the structure of the surrounding world. And now a third front of research has emerged - the study of solids.

Why is it so important to study solids?

Of course, practical human activity plays a huge role here. Solids are metals and dielectrics, without which electrical engineering is unthinkable; they are semiconductors that form the basis of modern electronics, magnets, superconductors, and structural materials. In short, it can be argued that scientific and technological progress is largely based on the use of solids.

But not only the practical side of the matter is important when studying them. The very internal logic of the development of science - solid state physics - led to an understanding of the importance of the collective properties of large systems.

A solid body consists of a billion particles that interact with each other. This causes the appearance of a certain order in the system and special properties of the entire number of microparticles. Thus, the collective properties of electrons determine the electrical conductivity of solids, and the ability of a body to absorb heat - heat capacity - depends on the nature of the collective vibrations of atoms during thermal motion. Collective properties explain all the basic patterns of behavior of solids.

The structure of solids is diverse. However, they can be divided into two large classes: crystals and amorphous solids.

1. General characteristics of amorphous bodies

Not all solids are crystals. There are many amorphous bodies.

Amorphous bodies do not have a strict order in the arrangement of atoms. Only the nearest neighbor atoms are located in some order. But there is no strict directionality in all directions of the same structural element, which is characteristic of crystals in amorphous bodies.

Often the same substance can be found in both crystalline and amorphous states. For example, quartz SiO2 can be in either crystalline or amorphous form (silica). The crystalline form of quartz can be schematically represented as a lattice of regular hexagons. The amorphous structure of quartz also has the appearance of a lattice, but of irregular shape. Along with hexagons, it contains pentagons and heptagons.

In 1959, the English physicist D. Bernal conducted interesting experiments: he took many small plasticine balls of the same size, rolled them in chalk powder and pressed them into a large ball. As a result, the balls were deformed into polyhedra. It turned out that in this case predominantly pentagonal faces were formed, and the polyhedra had an average of 13.3 faces. So there is definitely some order in amorphous substances.

Amorphous bodies include glass, resin, rosin, sugar candy, etc. Unlike crystalline substances, amorphous substances are isotropic, that is, their mechanical, optical, electrical and other properties do not depend on direction. Amorphous bodies do not have a fixed melting point: melting occurs in a certain temperature range. The transition of an amorphous substance from a solid to a liquid state is not accompanied by an abrupt change in properties. A physical model of the amorphous state has not yet been created.

Amorphous solids occupy an intermediate position between crystalline solids and liquids. Their atoms or molecules are arranged in relative order. Understanding the structure of solids (crystalline and amorphous) allows you to create materials with desired properties.

Under external influences, amorphous bodies exhibit both elastic properties, like solids, and fluidity, like liquids. Thus, under short-term impacts (impacts), they behave like solid bodies and, under a strong impact, break into pieces. But with very long exposure, amorphous bodies flow. Let's follow a piece of resin that lies on a smooth surface. Gradually the resin spreads over it, and the higher the temperature of the resin, the faster this happens.

Amorphous bodies at low temperatures resemble solids in their properties. They have almost no fluidity, but as the temperature rises they gradually soften and their properties become closer and closer to the properties of liquids. This happens because as the temperature increases, the jumps of atoms from one position to another gradually become more frequent. Amorphous bodies, unlike crystalline ones, do not have a specific body temperature.

When a liquid substance is cooled, it does not always crystallize. under certain conditions, a nonequilibrium solid amorphous (glassy) state can form. In the glassy state there can be simple substances (carbon, phosphorus, arsenic, sulfur, selenium), oxides (for example, boron, silicon, phosphorus), halides, chalcogenides, many organic polymers. In this state, the substance can be stable for a long period of time, for example, some volcanic glasses are millions of years old. The physical and chemical properties of a substance in a glassy amorphous state can differ significantly from the properties of a crystalline substance. For example, glassy germanium dioxide is chemically more active than crystalline one. Differences in the properties of the liquid and solid amorphous state are determined by the nature of the thermal movement of particles: in the amorphous state, particles are capable of only oscillatory and rotational movements, but cannot move within the substance.

Under the influence of mechanical loads or temperature changes, amorphous bodies can crystallize. The reactivity of substances in the amorphous state is much higher than in the crystalline state. The main feature of the amorphous (from the Greek “amorphos” - shapeless) state of matter is the absence of an atomic or molecular lattice, that is, the three-dimensional periodicity of the structure characteristic of the crystalline state.

There are substances that can only exist in solid form in an amorphous state. This refers to polymers with an irregular sequence of units.

2. Amorphous metal alloys

Amorphous metal alloys (metallic glasses) are metallic solids in which there is no long-range order in the arrangement of atoms. This gives them a number of significant differences from ordinary crystalline metals.

Amorphous alloys were first obtained in 1960 by P. Duvez, but their extensive research and industrial use began a decade later - after the spinning method was invented in 1968. Currently, several hundred amorphizing alloy systems are known, the structure and properties of metallic glasses have been studied in sufficient detail, and the scope of their application in industry is expanding.

2.1 Methods for producing amorphous alloys

Ultra-high cooling rates of liquid metal to obtain an amorphous structure can be realized in various ways. What they have in common is the need to ensure a cooling rate of at least 106 degrees/s. There are known methods for catapulting a drop onto a cold plate, spraying a jet with a gas or liquid, centrifuging a drop or jet, melting a thin film of a metal surface with a laser with rapid heat removal by the mass of the base metal, ultra-fast cooling from a gaseous medium, etc. The use of these methods makes it possible to obtain a tape of various widths and thickness, wire and powders.

The most effective methods for the industrial production of amorphous tape are cooling a jet of liquid metal on the external (disc quenching) or internal (centrifugal quenching) surfaces of rotating drums or rolling the melt between cold rollers made of materials with high thermal conductivity.

Fig.1. Methods for producing thin strip by hardening from a melt: a) centrifugal hardening; b) hardening on a disk; c) melt rolling; d) centrifugal hardening; e) planetary hardening

Figure 1 shows schematic diagrams of these methods. The melt obtained in an induction furnace is squeezed out of the nozzle by a neutral gas and solidifies upon contact with the surface of a rotating cooled body (refrigerator). The difference is that in centrifugal quenching and disk quenching methods, the melt is cooled on only one side.

The main problem is getting a sufficient degree of cleanliness of the external surface, which does not come into contact with the refrigerator. The melt rolling method produces good quality on both surfaces of the tape, which is especially important for amorphous tapes used for magnetic recording heads. Each method has its own limitations on the size of the tapes, since there are differences both in the course of the solidification process and in the hardware design of the methods. If during centrifugal hardening the strip width is up to 5 mm, then rolling produces strips with a width of 10 mm or more.

The disk hardening method, which requires simpler equipment, allows the strip width to be varied within a wide range depending on the size of the melting crucibles. This method makes it possible to produce both narrow tapes with a width of 0.1-0.2 mm, and wide ones - up to 100 mm, and the accuracy of maintaining the width can be ± 3 microns. Installations with a maximum crucible capacity of up to 50 kg are being developed. In all installations for hardening from a liquid state, the metal quickly solidifies, spreading in a thin layer over the surface of a rotating refrigerator. If the composition of the alloy is constant, the cooling rate depends on the thickness of the melt and the characteristics of the refrigerator. The thickness of the melt on the refrigerator is determined by the speed of its rotation and the flow rate of the melt, i.e., it depends on the diameter of the nozzle and the gas pressure on the melt. Of great importance is the correct choice of the angle of supply of the melt to the disk, which allows you to increase the duration of contact of the metal with the refrigerator. The cooling rate also depends on the properties of the melt itself: thermal conductivity, heat capacity, viscosity, density.

To obtain thin amorphous wire, various methods of drawing fibers from the melt are used.

Fig.2 Methods for producing thin wire hardened from a melt: a) drawing the melt through a cooling liquid (melt extrusion); b) pulling the thread from the rotating drum; c) drawing out the melt in a glass capillary; 1 - melt; 2 -- coolant; 3 -- glass; 4 -- nozzle; 5 -- winding wire

In the first method (Fig. 2, a) molten metal is drawn in a round tube through an aqueous solution of salts.

In the second (Fig. 2, b) a stream of molten metal falls into a liquid held by centrifugal force on the inner surface of a rotating drum: the solidified thread is then unwound from the rotating liquid. A known method consists of obtaining an amorphous wire by drawing the melt as quickly as possible in a glass capillary (Fig. 2, c).

This method is also called the Taylor method. The fiber is obtained by drawing the melt simultaneously with a glass tube, and the fiber diameter is 2-5 microns. The main difficulty here is to separate the fiber from the glass covering it, which naturally limits the composition of the alloys amorphized by this method.

2.2 Mechanical properties

The first feature of the mechanical properties of amorphous alloys that should be noted is their very high strength. As is known, the theoretical strength, that is, the stress required to break all interatomic bonds in the fracture plane, is 1~10E? (E is Young's modulus). The strength of real metals is two to three orders of magnitude lower - only the strength of whiskers (whiskers) approaches the theoretical one.

For amorphous alloys, values ​​of 0.040.05Ey?… close to the theoretical strength are also typical. This is due, firstly, to lower elastic moduli compared to crystals, and secondly, to the specific mechanisms of deformation and fracture. The Poisson's ratio of amorphous alloys is usually close to 0.4 - this is an intermediate value between crystalline metals (0.3) and liquids (0.5). A rather unexpected property of amorphous alloys is their ability to undergo plastic flow. In crystals, as is known, plastic behavior is ensured by the movement of dislocations. But in a body without translational symmetry, dislocations in the classical sense are impossible, and one would expect that amorphous substances would be absolutely brittle. Inorganic glasses behave this way, but in amorphous metals plastic deformation still occurs.

The ability to deform is associated, as for crystals, with the itinerant, non-directional nature of the metallic bond. In this case, it is possible to realize the high strength that is inherent in amorphous bodies, provided that brittle fracture is suppressed at stresses less than the yield point. Plastic deformation of amorphous alloys can be homogeneous, when each element of the volume is deformed and the sample experiences uniform deformation, and inhomogeneous, when plastic flow is localized in thin shear bands.

Homogeneous deformation occurs at high temperatures (close to the crystallization temperature) and low stresses (0.01Gf<, где G -- модуль сдвига). При этом скорость деформации пропорциональна приложенному напряжению. Вязкость з по мере развития деформации непрерывно растёт, и с повышением температуры этот рост ускоряется по аррениусовскому закону. Степень пластической деформации при гомогенном течении практически неограничена, и при правильно подобранных условиях можно добиться эффекта сверхпластичности с деформацией в сотни процентов. По-видимому, гомогенная деформация происходит за счёт непрерывной релаксации структуры, хотя она может протекать и после предварительного отжига при более высокой температуре.

As a result, after homogeneous deformation, alloys usually become sharply embrittled. Inhomogeneous plastic flow occurs at low temperatures and high stresses (cr0.8TT<0,02Gф>). It is little sensitive to loading rate and is practically not accompanied by strain hardening. In contrast to homogeneous deformation, inhomogeneous deformation causes a decrease in the degree of order in the amorphous structure. During inhomogeneous deformation, the flow is concentrated in shear bands, the number of which determines the plasticity of the alloy. Plasticity varies greatly depending on the loading pattern. When stretching, it is usually small - destruction occurs after a deformation of 1...2%, while when rolling, deformations of 50...60% can be achieved, and when bending, the radius can be comparable to the thickness of the tape (30...40 microns).

The fracture of amorphous alloys, like conventional crystalline alloys, can be brittle and tough. Brittle fracture occurs by cleavage without external traces of macroscopic flow and along planes perpendicular to the tensile axis. Ductile fracture occurs after or simultaneously with plastic deformation. It develops along the planes where the maximum tangential stresses act. A characteristic feature of the ductile fracture of amorphous alloys is the presence of two zones on the fracture surface: almost smooth cleavage areas and areas in which a system of intertwining “veins” is observed - traces of the emergence of areas of highly localized plastic flow with a thickness of ~0.1 μm.

2.3 Physical properties

First of all, we should dwell on the magnetic properties of amorphous alloys. In the amorphous state, despite the disordered arrangement of atoms, an ordered arrangement of magnetic moments can arise. Therefore, many amorphous alloys based on iron, cobalt, nickel, and also some rare earth metals are ferromagnetic. Their behavior is qualitatively similar to the behavior of crystalline ferromagnets: magnetic domains appear in them, during magnetization reversal there is a hysteresis loop, there is a Curie point above which spontaneous magnetization disappears, etc. In amorphous alloys, there are no barriers to the movement of domain walls during magnetization reversal, such as dislocations or grain boundaries; however, local inhomogeneities, magnetostriction from internal stresses, etc. can act as barriers. Annealing below the crystallization temperature, leading to relaxation of the amorphous structure and a decrease in internal stresses, usually reduces the coercive force. However, in some cases, on the contrary, it can lead to an expansion of the hysteresis loop due to the stabilization of domain boundaries.

The electrical resistance of amorphous alloys is significantly higher than that of crystalline alloys due to the lack of long-range order. In addition, their electrical resistance varies slightly with temperature. There are also amorphous superconductors.

2.4 Application of amorphous alloys

1. About 80% of industrial amorphous alloys are produced for their magnetic properties. They are used as soft magnetic materials that combine isotropic properties, high magnetic permeability, high saturation induction, and low coercive force. They are used for the manufacture of magnetic screens, magnetic filters and separators, sensors, recording heads, etc. Transformer cores made of amorphous alloys are characterized by very low magnetization reversal losses due to a narrow hysteresis loop, as well as high electrical resistance and small thickness, which reduces losses associated with eddy currents.

Although amorphous materials are chemically more active than crystalline ones, if they contain chromium and other elements that contribute to the formation of a passivating film, they can have exceptionally high corrosion resistance and be used in aggressive environments; for example, the alloy Fe45Cr25Mo10P13C7 is even superior to tantalum in durability. Amorphous alloys are also used as high-strength alloys (for example, as a component of composite materials and even car tire cord). Some amorphous alloys exhibit invar and elinvar properties (that is, they have a coefficient of thermal expansion close to zero or elastic moduli that are weakly temperature dependent) and can be used in precision devices. Finally, amorphous alloys are used to produce nanocrystalline materials. The use of amorphous alloys is hampered by both technological limitations (small thickness of the resulting semi-finished products, complete inability to weld) and low stability of properties - their structure and properties change significantly not only when heated, but also during operation at room temperature.

In the Chelyabinsk region there is an enterprise that produces amorphous metal alloys on an industrial scale - this is Ashinsky Metallurgical Plant OJSC. The first work on the production of amorphous alloys began there in 1984, and the workshop for the production of amorphous tape (ESPTs-1) was built in 1989.

Amorphous tape is produced on Ural-100 units by casting a flat jet of liquid metal onto the surface of a rotating cooled drum with a diameter of about 1000 mm and a width of 200 mm (see Fig. 1, a). The resulting tape has a width from 3 to 80 mm and a thickness of 20...30 microns. Soft magnetic amorphous alloys based on iron 2NSR, 9KSR, 30KSR and cobalt 71KNSR, 86KGSR, 82K3KHSR, 84KKHSR, as well as a nanocrystalline alloy of the “finmet” type 5BDSR are produced. (The designations of elements in alloy grades are the same as for alloyed steels.) The alloys are supplied to consumers both in the form of tape wound into rolls, and in the form of finished products - magnetic circuits. In addition to twisted magnetic circuits, magnetic screens, cores of magnetic sensors and transformers, resistive elements, etc. can be made from amorphous tape.

The tape is supplied without heat treatment, however, finished products from most alloys require mandatory thermomagnetic treatment (less often, heat treatment without a magnetic field) at 400...460 °C for 10...60 minutes. Thermomagnetic treatment of the 5BDSR alloy, accompanied by nanocrystallization, is carried out at 520...550 °C. Without heat treatment, only 71KNSR alloy is used for magnetic shields. For each batch of tape, not only the chemical composition is controlled, but also a whole set of magnetic characteristics after thermal (thermomagnetic) treatment.

Amorphous elinvars are used for the manufacture of seismic sensors, pressure gauge membranes, speed, acceleration and torque sensors; springs of clock mechanisms, scales, dial indicators and other precision spring devices. In Germany, an alloy of the Vitrovac-0080 brand was developed, containing 78% nickel, boron and silicon. The alloy has tensile strength = 2000 MPa, Young's modulus 1.5*105 MPa, density 8 g/cm3, electrical resistance 0.9 Ohm*mm2/m, bending endurance limit about 800 MPa based on 107 cycles. The alloy is recommended for the manufacture of springs, membranes and contacts.

Amorphous materials are used for reinforcing high-pressure tubes, making tire steel cords, etc. In the future, it is possible to use amorphous alloys for making flywheels. Such flywheels can be used to store energy and cover peak loads in power plants, improve vehicle performance, etc.

Iron-based AMCs are used as materials for the cores of high-frequency transformers for various purposes, chokes, and magnetic amplifiers. This is due to low total losses, which in the best AMS of this class are an order of magnitude lower than in silicon electrical steels.

Fe-Si-B alloys with high magnetic saturation have been proposed to replace conventional crystalline Fe-Si alloy in transformer cores, as well as high-permeability Ni-Fe alloys. The absence of magnetocrystalline anisotropy, combined with a fairly high electrical resistance, reduces eddy current losses, especially at high frequencies. Losses in cores made of the amorphous alloy Fe81B13Si4C2 developed in Japan are 0.06 W/kg, i.e., approximately twenty times lower than losses in grain-oriented transformer steel sheets. Savings due to the reduction of hysteresis energy losses when using the Fe83B15Si2 alloy instead of transformer steels will amount to $300 million/year in the USA alone. This area of ​​application of metallic glasses has a wide perspective.

In addition to extremely high initial magnetic permeability, especially at high frequencies (10 kHz), as well as zero magnetostriction, cobalt-based metal glasses have high hardness and good corrosion characteristics, so they are used as materials for magnetic recording heads. The Fe5Co70Si10B15 alloy developed in Japan has found high performance and wide application. The roll hardening method produces a strip 50 µm thick and 15 mm wide with excellent quality on both surfaces (roughness ± 3 µm). Due to the high magnetic flux density and high wear resistance, recording heads made from this tape have better overall performance than ferrite heads and permalloy heads. These materials are used in audio, video, computer and other recording equipment.

Tapes made of amorphous cobalt alloys are used in the cores of small-sized high-frequency transformers for various purposes, in particular for secondary power supplies and magnetic amplifiers. They are used in current leakage detectors, telecommunications systems and as sensors (including fluxgate types), for magnetic screens and temperature-sensitive sensors, as well as highly sensitive magnetic converters. High strength combined with corrosion resistance allows the use of amorphous alloys for the manufacture of cables operating in contact with sea water, as well as products whose operating conditions are associated with exposure to aggressive environments.

The combination of high strength, corrosion and wear resistance, as well as soft magnetic properties makes it possible for other applications. For example, it is possible to use such glasses as inductors in magnetic separation devices. Products woven from tape were used as magnetic screens. The advantage of these materials is that they can be cut and bent into desired shapes without compromising their magnetic properties.

Since glasses are highly supercooled liquids, their crystallization when heated usually occurs with strong nucleation, resulting in a homogeneous, extremely fine-grained metal. Such a crystalline phase cannot be obtained by conventional processing methods. This opens up the possibility of obtaining special solders in the form of a thin strip. This tape bends easily and can be cut and stamped to obtain the optimal configuration. It is very important for soldering that the tape is homogeneous in composition and provides reliable contact at all points of the products being soldered. Solders have high corrosion resistance. They are used in aviation and space technology.

In the future, it is possible to obtain superconducting cables by crystallization of the initial amorphous phase.

It is also known to use amorphous alloys as catalysts for chemical reactions. For example, the amorphous Pd - Rh alloy turned out to be a catalyst for the decomposition of NaCl into NaOH and C12, and iron-based alloys provide a higher yield (about 80%) compared to iron powder (about 15%) in the synthesis reaction

4H2 + 2CO = C2H4 + 2H2O - (12.1)

Amorphous metals are often called materials of the future, due to the uniqueness of their properties, which are not found in ordinary crystalline metals. Information on the main areas of application of amorphous metallic materials is contained in Table 12.4.

The widespread use of amorphous metals is hampered by high cost, relatively low thermal stability, as well as the small size of the resulting tapes, wires, and granules. In addition, the use of amorphous alloys in structures is limited due to their low weldability.

3. Amorphous and glassy semiconductor materials

Amorphous and glassy substances exhibiting semiconductor properties. They are characterized by the presence of short-range order and the absence of long-range order. A glassy semiconductor material, which can be considered as a special type of amorphous substance, is characterized by the presence of a spatial lattice, in which, in addition to covalently bonded atoms, there are polar groups of ions. In such materials, the bond between groups of atoms and ions is carried out due to short-range covalent van der Waals forces. Inorganic glassy semiconductors exhibit electronic conductivity.

Unlike crystalline semiconductors, glassy semiconductors do not have impurity conductivity. Impurities in glassy semiconductors affect the deviation from stoichiometry, and thereby change their electrical properties. These semiconductors are colored and opaque in thick layers. Glassy semiconductor materials are characterized by misoriented structure and unsaturated chemical bonds.

Amorphous and glassy semiconductors according to composition and structure are divided into oxide, chalcogenide, organic, and tetrahedral.

Oxide oxygen-containing glasses are produced by fusing metal oxides with variable valence, for example, V2O5-P2O5-ZnO. The metal oxides that form these glasses simultaneously have at least two differently valent states of the same element, which determines their electronic conductivity. Oxygen-free chalcogenide glasses are produced by fusing chalcogens (S, Se, Te) with elements of groups III, IV, V of the periodic system. Chalcogenide glassy semiconductors are produced mainly either by cooling the melt or by evaporation in a vacuum. Typical representatives are arsenic sulfide and selenide. These also include two- and multicomponent glassy alloys of chalcogenides (sulfides, selenides and tellurides) of various metals (for example, Ge-S, Ge-Se, As-S, As-Se, Ge-SP, Ge-As-Se, As -S-Se, As-Ge-Se-Te, As-Sb-S-Se, Ge-S-Se, Ge-Pb-S). Chalcogenide glasses have high transparency in the IR region of the spectrum from 1 to 18 microns. Amorphous films of complex chalcogenide compounds have great potential for varying their physicochemical properties.

Amorphous films of Si, Ge, GaAs and other semiconductor substances are not of practical interest due to their properties. The absence of long-range order in these semiconductors and the presence of a large number of defects such as micropores leads to the presence of unsaturated dangling bonds in many atoms. The consequence of this is a high density of localized states (1020 cm-3) in the band gap. Due to the specific nature of the process of electrical conductivity in amorphous semiconductors, it is almost impossible to control the electrical properties of such materials.

The introduction of hydrogen into amorphous silicon films significantly changes its electrical properties. Dissolving in amorphous silicon, hydrogen closes the dangling bonds (saturates them), as a result, in such a “hydrogenated” material, called Si:H, the density of states in the band gap sharply decreases (to 1016-1017 cm-3). Such a material can be doped with traditional donor (P, As) and acceptor (B) impurities, giving it an electronic or hole type of conductivity, and creating p-n junctions in it. A series of hydrogenated amorphous semiconductors with interesting electrical and optical properties Si1-xCx:H, Si1-xGex:H, Si1-xNx:H, Si1-xSnx:H have been synthesized based on silicon.

The practical applications of amorphous and glassy semiconductors are varied. Amorphous silicon has emerged as a cheaper alternative to monocrystalline silicon, for example, in the manufacture of solar cells based on it. The optical absorption of amorphous silicon is 20 times higher than that of crystalline silicon. Therefore, for significant absorption of visible light, a -Si:H film with a thickness of 0.5-1.0 μm is sufficient instead of expensive 300-μm silicon substrates. Compared to polycrystalline silicon cells, products based on -Si:H are produced at lower temperatures (300 °C).

Hydrogenated silicon is an excellent material for creating photosensitive elements in xerography, primary image sensors (sensors), videocon targets for transmitting television tubes. Optical sensors made of hydrogenated amorphous silicon are used for recording video information in memory, for flaw detection purposes in the textile and metallurgical industries, in automatic exposure and brightness control devices.

Glassy semiconductors are photoconducting semi-insulators and are used in electrophotography, information recording systems and a number of other fields. Due to their transparency in the long-wavelength region of the spectrum, chalcogenide glassy semiconductors are used in optical instrumentation, etc.

4. General methods for obtaining amorphous materials

General methods for obtaining amorphous materials can be depicted in the form of a picture.

amorphous metallic crystalline physical

Conclusion

The dual nature of amorphous materials is highly valued from an industrial point of view. Experimental and theoretical work on amorphous solids has led to a better understanding of the paradoxical nature of the solid structure of these materials. Also, why did there arise interest in amorphous metal alloys? First of all, because metal alloys with short-range order in the arrangement of atoms are to this day very interesting objects in the physics of condensed matter.

In recent years, important results have been obtained in the study of the mechanical, electrical and magnetic properties of amorphous metallic materials. However, the complete completion of research on amorphous structures is still ahead. The question of the structure of short-range order in accordance with reality requires an unambiguous solution. But next in line are amorphous structures in which there is no even short-range order. So the study of the beneficial properties of amorphous materials continues to this day.

List of used literature

1. A. West Solid State Chemistry, Part 2, M.: Mir, 1988

2. Zolotukhin I.V. Physical properties of amorphous metallic materials. M.: Metallurgy, 1986. 176 p.

3. B.V. Nekrasov, Fundamentals of general chemistry, M.: Chemistry, 1973.

4. Felts A. Amorphous and glassy inorganic solids / A. Felts. - M.: Mir, 1986. - 556 p.

5. Henney N. Solid State Chemistry / N. Henney. - M.: Mir, 1971. -223 p.

6. Amorphous metal alloys / V.V. Nemoshkalenko and others / resp. ed. V.V. Nemoshkalenko. - Kyiv: Naukova Dumka, 1987. - 248 p.

7. Suzuki, K. Amorphous metals / K. Suzuki, H. Fujimori, K. Hashimoto; edited by Ts. Masumoto. - M.: Metallurgy, 1987. - 328 p.

8. Ryabov, A.V. Modern methods of steel smelting in arc furnaces: textbook / A.V. Ryabov, I.V. Chumanov, M.V. Shishimirov. - Chelyabinsk: SUSU Publishing House, 2007. - 188 p.

9. Website of OJSC "Asha Metallurgical Plant": http://www.amet.ru.

10. Website "Wikipedia": http://ru.wikipedia.org

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Amorphous metal alloys (metallic glasses) are metallic solids in which there is no long-range order in the arrangement of atoms. This gives them a number of significant differences from ordinary crystalline metals.

Amorphous alloys were first obtained in 1960 by P. Duvez, but their extensive research and industrial use began a decade later - after the spinning method was invented in 1968. Currently, several hundred amorphizing alloy systems are known, the structure and properties of metallic glasses have been studied in sufficient detail, and the scope of their application in industry is expanding.

Methods for producing amorphous alloys

Ultra-high cooling rates of liquid metal to obtain an amorphous structure can be realized in various ways. What they have in common is the need to ensure a cooling rate of at least 106 degrees/s. There are known methods for catapulting a drop onto a cold plate, spraying a jet with a gas or liquid, centrifuging a drop or jet, melting a thin film of a metal surface with a laser with rapid heat removal by the mass of the base metal, ultra-fast cooling from a gaseous medium, etc. The use of these methods makes it possible to obtain a tape of various widths and thickness, wire and powders.

The most effective methods for the industrial production of amorphous tape are cooling a jet of liquid metal on the external (disc quenching) or internal (centrifugal quenching) surfaces of rotating drums or rolling the melt between cold rollers made of materials with high thermal conductivity.

Fig.1. Methods for producing thin strip by hardening from a melt: a) centrifugal hardening; b) hardening on a disk; c) melt rolling; d) centrifugal hardening; e) planetary hardening

Figure 1 shows schematic diagrams of these methods. The melt obtained in an induction furnace is squeezed out of the nozzle by a neutral gas and solidifies upon contact with the surface of a rotating cooled body (refrigerator). The difference is that in centrifugal quenching and disk quenching methods, the melt is cooled on only one side.

The main problem is getting a sufficient degree of cleanliness of the external surface, which does not come into contact with the refrigerator. The melt rolling method produces good quality on both surfaces of the tape, which is especially important for amorphous tapes used for magnetic recording heads. Each method has its own limitations on the size of the tapes, since there are differences both in the course of the solidification process and in the hardware design of the methods. If during centrifugal hardening the strip width is up to 5 mm, then rolling produces strips with a width of 10 mm or more.

The disk hardening method, which requires simpler equipment, allows the strip width to be varied within a wide range depending on the size of the melting crucibles. This method makes it possible to produce both narrow tapes with a width of 0.1-0.2 mm, and wide ones - up to 100 mm, and the accuracy of maintaining the width can be ± 3 microns. Installations with a maximum crucible capacity of up to 50 kg are being developed. In all installations for hardening from a liquid state, the metal quickly solidifies, spreading in a thin layer over the surface of a rotating refrigerator. If the composition of the alloy is constant, the cooling rate depends on the thickness of the melt and the characteristics of the refrigerator. The thickness of the melt on the refrigerator is determined by the speed of its rotation and the flow rate of the melt, i.e., it depends on the diameter of the nozzle and the gas pressure on the melt. Of great importance is the correct choice of the angle of supply of the melt to the disk, which allows you to increase the duration of contact of the metal with the refrigerator. The cooling rate also depends on the properties of the melt itself: thermal conductivity, heat capacity, viscosity, density.

To obtain thin amorphous wire, various methods of drawing fibers from the melt are used.

Fig.2 Methods for producing thin wire hardened from a melt: a) drawing the melt through a cooling liquid (melt extrusion); b) pulling the thread from the rotating drum; c) drawing out the melt in a glass capillary; 1 - melt; 2 -- coolant; 3 -- glass; 4 -- nozzle; 5 -- winding wire

In the first method (Fig. 2, a) molten metal is drawn in a round tube through an aqueous solution of salts.

In the second (Fig. 2, b) a stream of molten metal falls into a liquid held by centrifugal force on the inner surface of a rotating drum: the solidified thread is then unwound from the rotating liquid. A known method consists of obtaining an amorphous wire by drawing the melt as quickly as possible in a glass capillary (Fig. 2, c).

This method is also called the Taylor method. The fiber is obtained by drawing the melt simultaneously with a glass tube, and the fiber diameter is 2-5 microns. The main difficulty here is to separate the fiber from the glass covering it, which naturally limits the composition of the alloys amorphized by this method.

PRESENTATION

discipline: Processes for obtaining nanoparticles and nanomaterials

on the topic: “Preparation of nanomaterials using solid-phase transformations”

Completed:

Student gr. 4301-11

Mukhamitova A.A.

Kazan, 2014

INTRODUCTION

Recently, a number of methods have been developed for the production of nanomaterials in which dispersion is carried out in a solid without changing the state of aggregation.

Controlled crystallization from amorphous state is one of the methods for producing bulk nanomaterials. The method consists of obtaining an amorphous material, for example, by quenching from a liquid state, and then crystallizing it under controlled heating conditions.

Amorphous are metals that are in a solid state, in which the arrangement of atoms does not have long-range order, characteristic of metals in the usual state, i.e. crystalline state. To characterize metals in this state, the terms “metallic glass” and, less commonly, “non-crystalline metals” are also used. The amorphous state is the limiting case of thermodynamic instability of solid metal systems, opposite to the thermodynamic state of a defect-free crystal.

For thousands of years, humanity has used solid metals exclusively in the crystalline state. Only in the late 30s of the 20th century did attempts to obtain non-crystalline metal coatings in the form of thin films using vacuum deposition appeared. In 1950, an amorphous film of the Ni–P alloy was obtained by electrodeposition from solutions. Such films were used as hard, wear-resistant and corrosion-resistant coatings.

The situation changed significantly when in 1960 a method was discovered for producing amorphous metal alloys by hardening the liquid state, and in 1968 a method was discovered for hardening the melt on the surface of a rotating disk to produce an amorphous ribbon of large length (hundreds of meters). This opened up the possibility of large-scale production of amorphous metals at relatively low cost and led to an explosive growth in research in the field of amorphous alloys.

Today, about 80% of industrial amorphous alloys are produced for their unique magnetic properties. They are used as soft magnetic materials that combine isotropic properties, high magnetic permeability, high saturation induction, and low coercive force. They are used for the manufacture of magnetic screens, magnetic filters and separators, sensors, recording heads, etc. Transformer cores made of amorphous alloys are characterized by very low magnetization reversal losses due to a narrow hysteresis loop, as well as high electrical resistance and small thickness of the amorphous tape, which reduces losses associated with eddy currents.

Recently, approximately since the mid-90s of the twentieth century, interest in the structural elements of various materials, including metals, having a nanoscale scale (1...100 nm) has increased significantly. With such sizes of structural formations, in particular crystals, the proportion of surface particles that have an interaction different from those located inside the particle volumes increases significantly. As a result, the properties of materials formed by such particles may differ significantly from the properties of materials of the same composition, but with larger sizes of structural units. To characterize such materials and methods of their production, special terms nanomaterials, nanotechnology, and nanoindustry have appeared and are widely used.

In the modern understanding, nanomaterials are a type of product in the form of materials containing structural elements of nanometer dimensions, the presence of which provides a significant improvement or the emergence of qualitatively new mechanical, chemical, physical, biological and other properties determined by the manifestation of nanoscale factors. And nanotechnology is a set of methods and techniques used in the study, design, production and use of structures, devices and systems, including targeted control and modification of the shape, size, integration and interaction of their constituent nanoscale (1...100 nm) elements to obtain objects with new chemical, physical, biological properties. Accordingly, the nanoindustry is the production of nanomaterials that implements nanotechnologies. When applied to metals, the term “nanocrystalline” usually refers to metals whose crystal sizes fall within the above nanometer range.

The development of nanomaterials, nanotechnologies and the use of objects with controlled nano-sized structures have become possible largely due to the advent of research instruments and direct methods for studying objects at the atomic level. For example, modern transmission electron microscopes with a magnification of about 1.5x10 6 allow visual observation of atomic structure.

There are different ways to obtain nanostructured materials, including metals. For example, a nanostructure can be obtained in a bulk metal workpiece by grinding ordinary crystals to nanosized ones. This can be achieved, in particular, by intense plastic deformation. However, methods of structure refinement by deformation do not allow the production of nanocrystalline metals on an industrial scale and do not belong to traditional metallurgical technologies.

At the same time, a nanocrystalline, as well as an amorphous, metal structure can be obtained by traditional metallurgical methods, in particular by rapid cooling of the melt. Depending on the quenching conditions of the liquid state, three options for the formation of the structure are possible:

· nanocrystallization directly during the melt quenching process (the limiting case of conventional accelerated crystallization, leading to the formation of not just a fine-grained, but a nanostructure);

· in the process of melt quenching, partial crystallization occurs, so that a composite amorphous-crystalline structure is formed;

· during quenching, an amorphous structure is formed, and a nanocrystalline structure is formed during subsequent annealing.

Nanocrystalline, as well as amorphous, metals obtained by liquid hardening are also used primarily as magnetic and electrical materials with unique properties. They are used as soft and hard magnetic materials, conductors, semiconductors, dielectrics, etc.

In particular, soft magnetic alloys of the Finemet type have found widespread use. These are nanocrystalline alloys of the Fe–Si–B system with additions of Cu and Nb or other refractory metals. Alloys are obtained by partial crystallization of the amorphous state. Their structure consists of ferromagnetic crystallites with a size of 10...30 nm, distributed in an amorphous matrix, which makes up from 20 to 40% of the volume. Finemet type alloys have a very low coercive force, high magnetic permeability and magnetization, and low magnetization reversal losses, surpassing in their characteristics other soft magnetic alloys, including amorphous ones.

Magnetically hard nanocrystalline alloys of the Fe–Nd–B and Fe–Sm–N systems are also widely used. Since many magnetic materials (Fe–Si, Fe–Nd–B) are brittle, reducing the grain size not only improves their magnetic characteristics, but also increases ductility.

METHODS FOR PRODUCING AMORPHOUS METALS

The production of amorphous metals is possible by crushing the initial crystalline body to obtain an amorphous structure (the “top-down” path). The path involves disruption of the regular arrangement of atoms in a crystalline body as a result of external influences on the crystal and the transformation of a solid crystalline body into an amorphous solid.

To date, several technical methods for implementing these paths are known (Fig. 1). Since an amorphous metal, from a thermodynamic point of view, is an extremely nonequilibrium system with large excess energy, its production, in contrast to the production of a crystalline metal, requires nonequilibrium processes. In this figure, the equilibrium processes of phase transformations of the metal are represented by solid arrows, and the nonequilibrium processes of obtaining an amorphous metal are represented by dashed arrows.

Fig.1. Methods for achieving equilibrium and nonequilibrium states of metals

As follows from the above diagram, a thermodynamically nonequilibrium amorphous (and nanocrystalline) metal can be obtained from any equilibrium phase:

· condensation from the gas phase. With some reservations, methods of electrolytic deposition of amorphous films from electrolyte solutions can also be included in this group;

· amorphization of the crystalline state by introducing a large number of defects into the crystals;

· hardening of the liquid state from a metal melt.

The first two methods for producing amorphous metals - from the gas phase and crystalline metals - appeared in the first half of the last century and have been used for a relatively long time, but they do not relate to metallurgical technologies.

V S Suchkov, A N Imatov

Kama State Polytechnic Institute

Naberezhnye Chelny

Prospects for the use of amorphous materials

The development of modern technology creates a need for the search and development of new metallic materials that have not only higher physical and chemical properties, but also a combination of properties that are different in nature, which cannot be achieved on the basis of traditional materials. Amorphous metal alloys are such a new class of materials.

Amorphous alloys are materials with high strength and corrosion resistance; these are soft magnetic materials that have hysteresis magnetic properties, the level of which is characteristic of the best crystalline soft magnetic materials (permalloy, sendust); these are materials with invar properties; these are materials with special elastic (elinvar) and magnetomechanical properties (materials with a high magnetomechanical coupling coefficient and piezomagnetic coefficient); These are also materials with special electrical properties.

It is possible to use amorphous soft magnetic alloys on a large scale:

Improving the quality of products by using amorphous alloys that have higher performance characteristics than traditional crystalline materials;

Replacement of crystalline materials based on scarce metals with amorphous alloys consisting either of more accessible elements or containing scarce elements in smaller quantities;

The transition from traditional multi-stage, labor-intensive and energy-intensive technology for obtaining the final product to a new material - and energy-saving technology for obtaining products by melt hardening, which in many ways bears the features of a waste-free and environmentally friendly technology.

The main methods for producing amorphous powders are based on rapid quenching from melts classified according to the location of the heat sink:

A method of forming a liquid material in contact with a heat sink. This approach has the advantage that the product is formed sequentially drop by drop (atomization methods). Spray solidification can occur in several stages, and the thermal history of specific areas of the samples can be quite complex.

A method in which the melt is delivered to the heat sink continuously, uniformly, without crushing (pouring onto a cooling surface).

A method (which includes all welding processes) associated with rapid localized melting and subsequent rapid solidification while maintaining constant contact with a heat sink (usually an unmelted part of the same material). A cold heat sink is usually a solid metal with high thermal conductivity (for example, copper). During spraying, when cooling and solidification of droplets occurs in the process of free flight through a gaseous medium, and when extruding a melt filament into a liquid cooling medium, a gas or liquid serves as a heat sink.

For each material, a so-called C-shaped diagram of the onset of crystallization can be constructed. It is based on calculations of the time dependence t, which is required for crystallization of a given fraction of the melt volume X, on the amount of hypothermia Δ T = (Tm- T) This diagram is called TTT – diagram (the initial letters of the English words: temperature-time-transformation). Shows the critical cooling rate Rc. The specific shape of the TTT curve is determined by the superposition of two factors acting in opposite directions, namely, an increase in the driving force of the crystallization process with increasing supercooling and a decrease in the diffusion mobility of atoms. First, with increasing supercooling, the time for the onset of crystallization t decreases at a certain temperature TN it reaches its minimum value tN. With further supercooling of the melt, a progressive increase in the time for the onset of crystallization is determined mainly by an increase in the viscosity of the melt.

Rc = (Tm- TN)/ tN

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Scheme of the temperature dependence of the free volume in a liquid without its transition to the amorphous state (1) and in the case of its transition to the amorphous state at Tg(2). Only the change in free volume is shown, without taking into account thermal expansion caused by the anharmonicity of atomic vibrations: Vo– specific volume of supercooled liquid at absolute zero

temperature; Δ V- excess (“frozen”) free volume in the amorphous phase

Free volume. By free volume, firstly, we can mean the difference between the volume of the melt V at selected temperature T and its volume Vo at absolute zero. Secondly, the definition of free volume can be formulated as follows: free volume is the difference between the volume of the melt V at the selected temperature and the total volume of its constituent atoms. Usually the first definition is followed. In accordance with the “hole” theory of liquid, the physical foundations of which were formulated by J. Frenkel, liquid is represented as an inhomogeneous, discontinuous system in which there are equilibrium micropores (“holes”) with an average volume ν h and the number of which Nh changes depending on temperature. The total volume of these pores Nh" ν h and determines the free volume Δ Vf. . If the melt loses the ability to form micropores in sufficient quantities (free volume Δ Vf reaches low values), then the melt viscosity η accordingly, it increases sharply and its amorphization occurs.

Based on the study of these methods, a method has been developed for producing an amorphous powder in an electric discharge plasma.

When slowly cooled below the crystallization point, the liquid finds itself in a supercooled state. This state of the liquid is metastable, that is, after some time it must go into a crystalline state, which is energetically favorable below the crystallization point. If crystallization of the liquid has taken place, then glass transition will no longer be observed. However, if the crystallization of a liquid is difficult for some reason, that is, the lifetime of the metastable state is sufficiently long, then when the supercooled liquid is cooled sufficiently quickly, its viscosity quickly increases and it turns into a solid amorphous state.

The transition from a glassy to a crystalline state, although possible, is associated with long waiting times, and in many cases is practically unobservable.

The possibility of obtaining a glassy state of a substance is determined by how easily its crystallization occurs. Based on this criterion, substances can be divided into three groups. The first group includes many organic polymer liquids. Crystallization of such liquids is difficult due to the low mobility of its long polymer molecules, which are in a complex intertwined state. Even with very slow cooling of such a liquid, it reaches the temperatures at which glass transition occurs without crystallizing. Such liquids are sometimes called naturally amorphous. Naturally amorphous are many

native resins. The second group is formed by substances that lend themselves well to both crystallization (at a slow cooling rate) and glass transition. A classic example is glycerin. For such substances, it is possible to measure the characteristics of both the crystal and the supercooled liquid at the same temperatures, which turns out to be important for understanding the nature of the glass transition. Liquids of the first and second groups are called glass-forming. The third group includes easily crystallizing substances for which the existence of a glassy state was long considered impossible. A classic example of such substances is pure metals and various alloys. However, recently methods have emerged for obtaining ultra-fast cooling up to 108 K/s. With such rapid cooling, it was possible to obtain the amorphous state of many metals and alloys.

4.2 Methods for obtaining amorphous metallic materials

Methods for producing amorphous materials can be divided into three groups:

Cooling at ultra-high speeds (10 5 -10 7 K/s) of molten metal (quenching from a liquid state). This includes shooting a drop of melt onto a heat-conducting substrate (refrigerator), flattening a drop between copper plates, casting a jet of metal melt onto a rotating refrigerator (disk or drum), rolling a jet of melt between rolls, freezing a thin layer of melt on the edge of a highly heat-conducting disk rapidly rotating in a vertical plane. material. Using these methods, tape, powders, and fibers from metal alloys are produced.

Deposition of metals from the gas (vapor) phase onto a cooled substrate. These include thermal evaporation, ion sputtering, plasma spraying, etc. These methods are characterized by a high quenching rate, which allows the formation of an amorphous state also for alloys that do not amorphize during quenching from the melt. The disadvantages of these methods are low productivity, complexity and high cost of equipment.

Destruction of the crystalline structure of a solid due to external influences. Here, ion implantation is of greatest interest, with the help of which it is possible to obtain amorphous layers on finished products made of certain metals.

A common feature of the first methods is the creation of conditions for rapid cooling of the melt that would prevent the crystallization process. Practice shows that it is possible to prevent crystallization and fix the glassy state by contacting the liquid melt with a cold metal substrate, which should be made of a material with good thermal conductivity. Typically, copper, beryllium bronze, and brass are used for this purpose. The melt is heated with an induction heating device or a resistance furnace.

There are several main conditions, the fulfillment of which makes it possible to obtain an amorphous alloy by quenching from a liquid state at room temperature and normal atmospheric pressure:

The volumetric flow rate of the melt through the nozzle opening onto the surface of the rotating disk must be constant throughout the entire time of formation of the amorphous alloy.

The flow of the molten jet must be stable and protected from the influence of small dust particles and uncontrolled air flows created by rotating parts of the equipment.

The forming surface of the disk must be well polished and have good mechanical and thermal contact with the molten jet.

In recent years, the method of high-speed ion-plasma sputtering of material onto a substrate has been used to obtain amorphous structures. The sputtering rate depends on both the voltage and the density of the ion current entering the target. The sputtered atoms leave the target. Some of the atoms fall on the substrate and are deposited on it, while some are lost on special screens. Spraying is carried out in 2 stages:

Preliminary. Its goals are: 1 - the top contaminated layer of the target is removed; 2- a film of the sputtered substance is deposited on the screens, which can serve as a getter, etc. in the area of ​​the substrate, an area with a reduced content of impurities is created; 3- the sputtering process becomes more stationary in nature and the composition of the deposited layer will correspond to the composition of the target only after some time has elapsed, during which the composition of the sputtered atoms equalizes. After pre-sputtering is completed, the substrate is ionically cleaned for several minutes by applying a negative potential of 100V to it. Then spraying begins in operating mode. This method makes it possible to create amorphous structures of complex composition up to 1 cm thick.

Also, to produce amorphous metals, laser radiation is currently used, which allows the metal to be quickly heated and provides cooling of the melt at a rate of at least 10 5 -10 6 K/s. With rapid melting, a homogeneous liquid appears, which, after solidification, turns into the so-called. glass with unusual physical and mechanical properties. The process of formation of a similar structure on the surface of metallic materials is called “laser glass transition”.