Topic 2 obtaining nanopowders (4:00)
Plan of the lecture
The processes that result in the formation of nano-or ultrafine structures.
Himichskie methods.
Physical methods.
Mechanical methods.
The processes that result in the formation of nano-or ultrafine structures - is crystallization, recrystallization, phase transformations, high mechanical loads, severe plastic deformation, complete or partial crystallization of amorphous structures. The choice of method is determined by the area of nanomaterials their applications, a set of desirable properties of the final product. Characteristics of the resulting product - size distribution and particle shape, impurity content, the specific surface area - can vary depending on the method of producing a very wide range.
Thus, depending on the preparation conditions, nanopowders can be spherical, hexagonal, flocculent, needle-shaped, amorphous or fine-grained structure. Methods of preparation of ultrafine materials are divided into chemical, physical, mechanical and biological.
Chemical synthesis methods include various reactions and processes, including the processes of precipitation, thermal decomposition of gas-phase chemical reactions, reduction, hydrolysis, electrodeposition. Regulation of rates of formation and growth of the new phase is carried out by changing the ratio of reactants, the degree of supersaturation, and the process temperature. As a rule, chemical methods - multi-stage, and include a set of the above-named processes and reactions.
Deposition method is the deposition of various metal compounds from fluids and salts with precipitators. Product deposition are metal hydroxides. As a precipitant use alkaline solutions of sodium, potassium and others.
Adjusting the pH and temperature of the solution, create the conditions so that the resulting high rate of crystallization and finely formed hydroxide. This method can produce powders of spherical, needle-like, scaly or irregular shape with particle size of 100 nm.
Nanopowders of complex composition is produced by co-precipitation. In this case, the reactor is a combination of two or more solutions of metal salts and alkali at a given temperature and stirring. The result is a hydroxy compound of the required composition.
Heterophase way interaction is performed by stepwise heating of mixtures of solid metal salts with an alkaline solution to form a slurry of the oxide and subsequent reduction of the metal. In this manner, metal powders with a particle size in the range 10 ... 100 nm.
Gel method is precipitated from aqueous solutions of insoluble metal compounds in the form of gels. The next stage - the restoration of the metal. This method is used to produce powders of iron and other metals.
Way to recovery and thermal decomposition - usually after the next operation in the solution of ultrafine oxides or hydroxides, followed by precipitation and drying. As a reducing agent, depending on the desired product, using gaseous reductants - usually hydrogen, carbon monoxide, or solid reducing agents.
Nanopowders Fe, W, Ni, Co, Cu and other metals produced by reduction of their oxides with hydrogen. As solid reducing agent carbon, metals or metal hydrides.
This way, metal nanopowders: Mo, Cr, Pt, Ni, and others. Typically, the particle size is in the range of 10 ... 30 nm. A strong reducing agents are metal hydrides - usually calcium hydride. So get nanopowders Hr, Ta, Nb.
In some cases, nanopowders produced by the decomposition of formates, carbonates, carbonyls, oxalates, acetates, metals due processes of thermal dissociation or pyrolysis. Thus, due to the dissociation of metal carbonyls are powders Ni, Mo, Fe, W, Cr. By thermal decomposition of a mixture of carbonyl compounds on heated substrates are base metal film. UDP metals, oxides, and mixtures of metals and oxides are produced by pyrolysis of metal formates. In this manner, powdered metals, including Mn, Fe, Ca, Zr, Ni, Co, their oxides and metal oxide compounds.
Physical methods. Methods of evaporation (condensation) or gas phase synthesis nanopowders of metals, based on the evaporation of metals, alloys and oxides with subsequent condensation in the reactor at a controlled temperature and atmosphere. Phase transitions of vapor - liquid - solid or vapor - solid place in the reactor or on the surface of the substrate or cooled walls.
The essence of the method is that the starting material is vaporized by the intense heat with gas - the carrier is fed to the reaction space, which is quenched. Heating volatile substances by means of plasma, laser, electric arc, induction method, passing an electric current through the wire. It is also possible crucible evaporation. Depending on the type of raw material and end product, evaporation and condensation in a vacuum, in an inert gas in a gas or plasma. The size and shape of the particles depends on the temperature, composition of the atmosphere and the pressure in the reaction chamber. In an atmosphere of helium particles will be smaller than in argon - a dense gas. This method to produce powders of Mo, Fe, Ti, Al. Particle size in this case - tens of nanometers.
At the time, there was, and later established a way of nanomaterials by electrical explosion of wires (conductors). In this case, the reactor is placed between the electrodes of the metal wire, which is scheduled from nanopowders, diameter 0.1 ... 1.0 mm. The electrodes is pulsed high current power (at 106 A/mm2). This results in instant heating and evaporation of the wires. Metal vapor fly, cooled and condensed. The process is under helium or argon. The nanoparticles are deposited in the reactor. In this way, metal (Ti, Co, W), oxide (TiO2, Ai2O3, ZrO2) nano-powders with particle size up to 100 nm.
Mechanical methods. Method of grinding materials mechanically in mills of various types - ball, planetary, rotary, vibrating, hygroscopic devices attritorah and simoloyerah. Attritory and simoloyery - is high-shredding machines with fixed body - with drum mixers, transmitting movement balls in the drum. Attritory have a vertical arrangement drum simoloyery - horizontal. Grinding milled material was ground balls in contrast to other types of milling equipment is not primarily due to impact, and the mechanism of wear. Drum capacity in performing these two types of up to 400 ... 600 l.
Mechanically pulverized metals, ceramics, polymers, oxides, brittle metal. Reduction ratio depends on the type of material. So, for tungsten and molybdenum oxides are particle size of about 5 nm, for iron - about 10 ... 20 nm.
A variety of mechanical grinding is mechanosynthesis or mechanical alloying, when the grinding process is an interaction of shredded materials to produce new members of the crushed material. So get nanopowders doped alloys, intermetallic compounds, silicides and dispersnouprochnennyh composites with particle size 5 ... 15 nm.
A unique advantage of the method is that, due to interdiffusion in the solid state is possible to obtain "alloys" of such elements, the mutual solubility with the use of liquid-phase methods is negligible.
The positive side of the mechanical grinding method is the relative ease of installation and technology, the ability to grind various materials and alloys to produce powders, as well as the opportunity to receive materials in large quantities.
The disadvantages of the technique include the possibility of contamination of crushed powder abrade materials and the difficulty of obtaining powders with a narrow particle size distribution, the complexity of the regulation of the product during the grinding process.
Upon receipt of any method of nanoparticles is even one of their feature - the tendency to form associations of particles. These associations are called aggregates and agglomerates. As a result, in determining the size of nanoparticles, it is necessary to distinguish between the size of individual particles (crystallites) and the size of the combined particles. The difference between aggregates and agglomerates is not strictly defined. It is believed that the crystallites in the aggregates more strongly connected and have a smaller intercrystalline porosity than in the agglomerates.
A problem associated with the aggregation of nanoparticles occurs when compaction. For example, when aggregated powder compaction by sintering, to reach a certain density of the material required temperature is higher, the larger units are available in a powder of nanoparticles.
In this regard, the development of methods for obtaining nanopowders are still seeking measures to eliminate or reduce the degree of education associations nanoparticles. Thus, in the methods of nanopowders by condensation from the vapor phase has proved useful to precise control of the temperature of formation of nanoparticles. In chemical methods is effective exclusion of water from some of the synthetic steps to reduce the degree of agglomeration. Methods are also used to reduce the contact between the particles by their cover (encapsulation), which then, before compaction is removed.
However, aggregation and agglomeration of nanoparticles complicates the compact materials. Require high mechanical stresses or temperature (sintering) to overcome the forces of agglomeration.
Recommended Reading
New materials / VN Antsiferov, FF Bezdudny, LN Belyanchikov and others, ed. YS Karabasova, Ministry of Education of the Russian Federation. - Moscow: MISA, 2002. - 736 p.
New substances, materials and products made of them as objects of inventions: Directory / VI Blinnikov etc. - Moscow, Metallurgy, 1991. - 262 p.
Rzhevskaya SV Materials science. - M. Bauman, 2000. - 280.
Kulikov V.Yu. Textbook for the course "New Materials", KSTU, 2006.
Morokhov ID, Cowards DD, Lapovok VI Physical phenomena in ultra environments. - Moscow: Nauka, 1984.
Gusev AI Nanocrystalline materials: preparation methods and properties. - Ekaterinburg, 1998.
Control tasks for independent work of the student (theme 2) [1, 2, 3, 7]
1. Mehanomintez.
2. A method of producing nanomaterials by electrical explosion of wires.
Topic 3 properties of nanopowders (4:00)
Plan of the lecture
1. Exclusive features of nanostructures.
2. Nanostructural parameters of copper.
3. Comparison of some of the fundamental properties of nanostructured metals and coarse states.
4. The unusual properties of the HCM.
Exclusive features of nanostructures. Nanostructured materials, because of the small grain size, the structure contains a large number of grain boundaries, which play a role in shaping their unusual physical and mechanical properties. As a consequence, in the ongoing experimental studies and structural models developed nanomaterials grain boundaries are central.
Even in his earliest works, made H. Glyayterom with employees, established a series of features of the structure of nanocrystalline materials obtained by condensation of the gas of atomic clusters and their subsequent compaction. This, above all, the low density of nanocrystals obtained and the presence of specific "grain boundary phase" observed the appearance of additional peaks in the Mössbauer studies. On the basis of experiments, including computer simulation was proposed structural model of nanocrystalline material composed of atoms of one kind. In this model, a nanocrystal is composed of two structural components: crystallites-grains (atoms are open circles) and the grain boundary regions (black circles). Atomic structure of the crystallites is perfect and is determined only by their crystallographic orientation. At the same time, the grain boundary area, where it joins the neighboring crystallites are characterized by reduced atomic density and changed interatomic distances.
Glyaytera model gave a powerful impetus studies of the structure of nanocrystals and the search of the unusual properties. However, subsequent studies have been identified and its important shortcomings. First, in agreement with the electron microscopy, the grain boundaries are much narrower than that predicted by the model, and their width is usually not more than 1-2 interatomic distances. Second, the atomic lattice in the nanocrystals is not perfect, and usually, as in the case of SPD nanomaterials elastically deformed. Moreover, it is now becoming clear that the method of production of nanostructured materials plays an important role in the formation of their structure and properties.
Experimental studies with different, often complementary methods, which are translucent, including high-resolution, electron microscopy, X-ray diffraction analysis, Mössbauer spectroscopy, differential scanning calorimetry, show that in nanostructured metals and alloys SDI grain boundaries are nonequilibrium due to the presence grain boundary defects with high density.
Notions of equilibrium boundaries were introduced into the scientific literature in the 1980s, based on studies of the interaction of lattice dislocations and grain boundaries. The formation of non-equilibrium state of grain boundaries is characterized by two main features - the excess energy of the grain boundaries (given crystallographic parameters of borders) and the presence of long-range elastic stresses. Assuming that the grain boundaries are crystallographically ordered structure, as sources of the elastic fields of view of violation of this discrete structure - grain boundary dislocations and their complexes.
Recent direct observations of grain boundaries made by transmission electron microscopy gave direct evidence of a specific non-equilibrium structures in HCM due to the presence of atomic steps and facets, as well as grain boundary dislocations. In turn, as a result of non-equilibrium grain boundaries, there are high voltages and lattice distortions that lead to dilations of the lattice, and results in a change in the interatomic distances, the emergence of significant static and dynamic atomic displacements experimentally detected by X-ray and Mossbauer studies. Table 1.1 lists the parameters of the nanostructure of copper, as measured by XRD.
Developed based on the concept of non-equilibrium grain boundary model representations have not only qualitatively but also quantitatively assess the changes in the fundamental parameters which are observed in many nanoscale materials.
Table 1.1 - Parameters of nanostructured copper
parameters |
The initial state |
After the high-pressure torsion |
after ECAP |
Parameter in the Debye-Waller, Å |
0,59±0,06 |
1,06±0,05 |
1,28±0,05 |
Atomic displacements <μ> 1/2, Å |
0,086±0,004 |
0,116±0,003 |
0,127±0,003 |
Debye temperature Θ, K
|
304±2 |
247±6 |
234±6 |
In the case of multiphase alloys and intermetallic compounds resulting from SDI nanostructures are very specific and are characterized not only a very small grain size of several tens of nanometers, but also strongly metastable phase structure associated with the formation of supersaturated solid solutions disorder and in some cases even with amorphization.
For example, the structure of intermetallic binary stoichiometric Ni3Al, subjected to torsion SDI consisted of very fine equiaxed grains with evidence of a high level of internal stress, as evidenced by a complex diffraction contrast in electron microscopic photographs in difficult to distinguish grain boundaries, looking diffuse and crooked. The average grain size, determined from the dark-field image, turned out to be about 20 ... 30 nm.
State after IAP Ni3Al also characterized by elevated levels of residual electrical, significant internal stresses and high microhardness. In addition, X-ray diffraction data showed a complete lack of long-range order in this state.
Table 1.2 - Some fundamental properties of metals in nanostructured (NS) and coarse (CC) states
properties |
material |
value |
|
NS |
CC |
||
The Curie temperature, K The saturation magnetization, A × m2/kg Debye temperature, K The diffusion coefficient, m2 / s Solubility limit at 293 K,% Young's modulus, GPa |
nickel
nickel iron Copper in nickel
Carbon in α-iron copper |
595
38,1 240 1·1014
1,2 115 |
631
56,2 467 1·10-20
0,06 128 |
Microstructure during annealing of the alloy undergoes a sequence of structural changes, similar to pure metals SDI. However, the characteristic of nanostructured Ni3Al was that the long-range order is recovering in a narrow temperature range near 530 K, ie at the stage of the return. This ordering is not complete, but a further increase in the parameter range order occurs only at higher temperatures close to 1300 K, whereas grains grow to relatively large sizes. Although the physical nature of the disorder in intermetallic SDI and subsequent reordering during heating requires further research, it is important to note that, following the results, it becomes clear that the re-ordering in Ni3Al is caused, first of all, not rekristallicheskimi processes, and processes the return associated with layers dislocation structure at the borders and in the body of the grains.
Using differential scanning calorimetry, was investigated heat in the heating of the material. The peak heat release is observed at temperatures well below the start of a rapid grain growth. The nature of the heat associated with the processes of return and start reordering. It should be noted the high thermal stability of the nanocrystalline state of the intermetallic compound, which allowed him to realize a unique superplastic flow.
The unusual properties of the HCM. Specific microstructure of bulk nanomaterials determine their unusual properties, many of which are unique and very attractive for practical use. These specific qualities associated with changes in some of the fundamental properties of the material with decreasing particle size or grain, as well as the changing balance of some of the bulk and surface properties.
The unique features of nanomaterials are differences between their melting points and sizes of crystal lattices of the corresponding values in the normal structure of the materials. In this regard, the question of the validity of the term "lattice constants", in relation to the size of the lattice.
With decreasing particle size increases their surface energy. As a result of changes (decreases) the melting point of the particle. The expression for the melting temperature (Tm) of the solid nanoparticle radius r is:
where Tm and Tm (r) - the melting temperature of the bulk material and nanoparticles of this material radius r, p ^, pm - the density of the liquid and solid particles, σ, σtv - surface tension of the liquid and solid particles.
It has also a decrease in the lattice parameter for metals and some compounds with decreasing particle size. For example, reducing the diameter of the aluminum particles from 20 to 6 nm grating period is reduced by about 1.5%. Size below which a decrease in the lattice parameter is different for different metals and compounds. Nanostructured metals and alloys can have a high corrosion resistance. In particular, the results demonstrate the possibility of conventional carbon steels in the nanoscopic range with higher corrosive than the special stainless steels. Recent studies indicate the possibility of a significant increase in the physical properties of the materials studied, nanostructured nitinol demonstrates exceptional superelasticity and shape memory effect, in nanokomponite Cu-Al2O3 observed combination of high temperature and conductivity; nanostructure-magnetic alloys (systems Fe-Nb-B, Co-Pt, etc .) demonstrate a record magnetic properties, and soft magnetic nanomaterials exhibit very high magnetic permeability. Discovered and studied and abnormal optical properties of nanostructured metals and semiconductors.
However, of particular interest are the mechanical properties of bulk nanostructured materials. As the theoretical estimates, in terms of the mechanical behavior of the formation of nanostructures in various metals and alloys can lead to high-strength state in accordance with the ratio of the Hall Petch, as well as to the emergence of low-temperature and / or high-speed superplasticity. The implementation of these features is directly relevant to the development of new high-strength and wear-resistant materials, advanced superplastic alloys and metals with high residual strength. All this has caused great interest among the research strength and plasticity of materials to obtain large bulk nanostructured samples for subsequent mechanical testing.
However, as noted above, there are unsolved problems in obtaining such special nanomaterials by powder metallurgy - gas condensing steam or grinding, the persistence in them during compaction some residual porosity and the presence of additional difficulties in the preparation of bulk samples. As a result, until recently, been only sporadic studies on the mechanical properties of nanostructured metals and alloys with a grain size of about 100 nm or less. Most studies related to the measurement of microhardness, and the data are contradictory. For example, some studies have found a decrease softening grains to nanometer dimensions, at the same time in a number of other studies have observed in this case hardening, although the standard curves were compared with the ratio of the Hall Petch. When stretched, these MSPs were very fragile, despite the hardness.
Intermetallic Ni3Al in the recrystallized state, the production of hot extrusion (grain size of 6 microns), show limited plasticity, including tensile at 6500 C, which is typical for this material.
Intense deformation torsion in one turn increases strength, but flexibility remains insignificant. However, further strain rate (up to 5 turns) qualitatively changes the situation where the material exhibits very high strength and ductility at the same time a record with an extension to the destruction of more than 300%.
Thus, the tests showed that exposure to severe plastic deformation, as a high-pressure torsion and compression CGS, their behavior changes qualitatively, and they not only show a very high strength, but also flexibility. This behavior of materials is fundamentally different from the behavior of metals and alloys after severe plastic deformation, such as rolling or hood, where the increase in strength is usually correlated with a decrease in ductility.
To understand the nature of this effect it is important that in the IAP is the formation of nanostructures with a very small grain size (about 100 nm). Nanostructures formed as a result of severe plastic deformation, are qualitatively different from or fragmented cellular microstructures formed after the usual large deformations. Obviously, due to the formation of nanostructures may change the mechanisms of deformation in tensile specimens, when along with the movement of lattice dislocations are beginning to take an active part in the processes of the boundaries formed by severe plastic deformation nanograins, in particular, the grain-boundary sliding.
As you know, a combination of strength and ductility is a necessary condition for the development of advanced materials. In this context, the achievement of very high strength and plasticity in metals and alloys subjected to severe plastic deformation, strategies for the creation of innovative construction materials, which are nanoscale microstructure.
These structural materials can have higher values of strength, toughness, fatigue, compared with currently used industrial materials. For example, nanostructured titanium VT1-0 after IPD shows very high values of tensile strength = 1010 ... 1040 MPa and endurance = 591 MPa, which is higher than the same parameters high-alloyed Ti alloy VT-6 (= 990 ... 1000 MPa = 567 MPa). It closed the way for a new class of structural materials with high medical fatigue properties and toughness - implants used in traumatology and orthopedics for load-bearing structures and devices trauma units.