Nanocrystalline Materials: Synthesis and Properties Part 1 (Nanotechnology)

INTRODUCTION

The synthesis of nanocrystalline bulk and powder materials is one of the problems facing the modern materials scientist. In recent decades, the interest paid to this problem has grown remarkably because it was found that the properties of nanocrystalline substances change considerably when the size of crystallites decreases below a threshold value.[1-8] Such changes arise when the average size of crystal grains does not exceed 100 nm and are most pronounced when grains are less than 10 nm in size. Ultrafine-grain substances should be studied considering not only their composition and structure, but also particle size distribution. Ultrafine-grain substances with grains 300 to 40 nm in size on the average are usually referred to as submicrocrystalline, while those with grains less than 40 nm in size on the average are called nanocrystalline. The classification of substances by the size D of their particles (grains) is shown in Fig. 1.

Nanosubstances and nanomaterials may be classified by geometrical shape and the dimensionality of their structural elements. The main types of nanomaterials with respect to the dimensionality include cluster materials, fibrous materials, films and multilayered materials, and also polycrystalline materials whose grains have dimensions comparable in all the three directions (Fig. 2).

The main objective of this paper is to give a general idea about diverse nanocrystalline substances and materials. The chemical composition, the microstructure, the grain size distribution, and, consequently, the properties of nanosubstances largely depend on their production method. It is for this reason that the paper first describes main methods for production of powders and bulk samples in the nanocrystalline state and then considers specific features of the microstructure of nanocrystalline substances. The influence of the nanocrystalline state on properties of various substances and determination of causes of this influence present the objective of the final part of this paper.


BACKGROUND OF FINE MATERIALS

In December 1959, at his talk at the California Institute of Technology, Feynman[9] stressed the problem of control over the substance in the interval of extremely small dimensions as an insufficiently explored, but very promising field of science. He noted in particular that ”.. .when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do. .. .The problems of chemistry.. .can be greatly helped if our ability to see what we are doing, and to do things on an atomic level, is ultimately developed.”

In 1982-1985, Gleiter[1] proposed a concept of the nanostructure of solids and was the first to realize a method for production of bulk materials with nanometer-sized grains (crystallites). From that time on, bulk and powder substances, which contained nanometer-sized particles, have been called nanocrystalline. Gleiter’s works spurred studies into the synthesis, the structure, and the properties of nanocrystalline substances.

Differences in the properties of fine particles from those of bulk materials have been known and used for a relatively long time. Examples are aerosols, dyeing pigments, and glasses colored with colloidal particles of metals. A very significant field of successful application of fine particles is catalysis of chemical reactions. Mul-tilayered nanostructures are used in electronics. These structures represent a crystal, which has, in addition to the usual lattice of periodically arranged atoms, a superlattice comprising alternating layers of different compositions.

Semiconductor nanoheterostructures, which realize quantum-size effects, are of special interest to electronics. Nanoheterostructures, especially double ones, including quantum wells, wire, and dots, allow controlling basic parameters of semiconductors (the energy spectrum, the forbidden gap width, the effective mass, and the mobility of carriers).

DISTINCTIVE PROPERTIES

The density of states N(E) is a continuous function in a three-dimensional (3-D) semiconductor. When the electron gas dimensionality decreases, the energy spectrum becomes split and discrete (Fig. 3). A quantum well is a two-dimensional structure, in which charge carriers are limited to the direction perpendicular to the layers and can move freely in the layer plane. Charge carriers are limited to two directions in a quantum wire and only move along the wire axis. A quantum dot is a quasi-zero-dimensional (”0” D) structure, in which charge carriers are limited in three directions. The electron energy spectrum of an ideal quantum dot is fully discrete (Fig. 3) and corresponds to the spectrum of a single atom, although a real quantum dot (a ”superatom”) can include hundreds of thousands of atoms.

Classification of substances and materials by their particle (grain) size D.

Fig. 1 Classification of substances and materials by their particle (grain) size D.

A small size of grains determines a large length of grain boundaries. Also, grains may have various atomic defects (vacancies or their complexes, disclinations, and dislocations), whose number and distribution differ from those in coarse grains 5 to 30 mm in size. If dimensions of a solid in one, two, or three directions are comparable with characteristic physical parameters having the length dimensionality (the size of magnetic domains, the electron free path, the size of excitons, the de Broglie wavelength, etc.), dimensional effects will be observed for the corresponding properties. Thus dimensional effects imply a set of phenomena connected with changes in properties of substances, which are caused by 1) a change in the particle size, 2) the contribution of interfaces to properties of the system, and 3) comparability of the particle size with physical parameters having the length dimensionality. Specific features of the structure of nanocrystalline substances make their properties differ considerably from those of usual polycrystals. Therefore the decrease in the grain size is viewed as an efficient method for adjustment of properties of solids.

Types of nanocrystalline materials: 0-D (zero-dimensional) clusters; 1-D (one-dimensional) nanotubes, filaments, and rods; 2-D (two-dimensional) films and layers; 3-D (three-dimensional) polycrystals.

Fig. 2 Types of nanocrystalline materials: 0-D (zero-dimensional) clusters; 1-D (one-dimensional) nanotubes, filaments, and rods; 2-D (two-dimensional) films and layers; 3-D (three-dimensional) polycrystals.

Density of states of charge carriers of states N(E) as a function of the semiconductor dimensionality: (3-D) three-dimensional semiconductor; (2-D) quantum well; (1-D) quantum wire; (0-D) quantum dot.

Fig. 3 Density of states of charge carriers of states N(E) as a function of the semiconductor dimensionality: (3-D) three-dimensional semiconductor; (2-D) quantum well; (1-D) quantum wire; (0-D) quantum dot.

Nanocrystalline substances represent a special state ofcondensed matter, namely, macroscopic ensembles of superfine particles up to several nanometers in size. Properties of nanosubstances are determined by both specific features of separate particles and their collective behavior, which depends on the interaction between nanoparticles.

SYNTHESIS OF NANOCRYSTALLINE POWDERS

Gas Phase Synthesis

Isolated nanoparticles are prepared by the evaporation of a metal, an alloy, or a semiconductor at a controlled temperature in the atmosphere of a low-pressure inert gas and the subsequent condensation of the vapor near or on a cold surface. The gas phase synthesis provides particles between 2 and several hundreds of nanometers in size. Nanoparticles < 20 nm in size have a spherical shape, while coarser particles are faceted.

The metal may be evaporated from a crucible or fed to the evaporation zone as a wire or a powder. A beam of argon ions serves for the metal evaporation. The energy may be injected via direct heating, passage of an electric current, an electric-arc plasma discharge, inductive heating with currents of high and superhigh frequencies, laser radiation, or electron beam heating. A vacuum, a motionless inert gas, a gas flow, or a plasma jet may serve as the working medium. The composition and the size of nanoparticles may be controlled by changing the atmosphere pressure and composition (an inert gas or a reagent gas) and the temperature gradient between the evaporated substance and the surface, on which the vapor condensates.

Properties of isolated nanoparticles largely depend on the contribution of the surface layer. In the case of a spherical particle with the diameter D and the surface ayer thickness d, the fraction of the surface layer in the total volume of the particle is ~ 6d/D. When the surface layer thickness equals 3-4 atomic monolayers (0.5-1.5 nm) and the size of nanoparticles is 10-20 nm, the surface layer accounts for up to 50% of the whole substance.

Structure of most significant fullerenes C60 and C70. The C60 molecule is shaped like a soccer-ball and its cage is about 0.7 nm in diameter. All fullerenes exhibit hexagonal and pentagonal rings of carbon atoms.

Fig. 4 Structure of most significant fullerenes C60 and C70. The C60 molecule is shaped like a soccer-ball and its cage is about 0.7 nm in diameter. All fullerenes exhibit hexagonal and pentagonal rings of carbon atoms.

Plasmachemical Technique

Low-temperature (4000-8000 K) nitrogen, ammonium, hydrocarbon, or argon plasma of the arc, glow, high-frequency, or superhigh-frequency discharge is used in the plasmachemical synthesis. Elements, their halogenides, and other compounds serve as the starting material. Particles of plasmachemical powders represent single crystals 10 to 100-200 nm in size. Laser heating provides nano-powders with a narrow particle size distribution. The gas phase synthesis with laser radiation for generation and maintenance of the plasma, in which the chemical reaction takes place, proved to be an efficient method for production of molecular clusters.

Molecular clusters occupy a special place among nano-structured substances. The best known of these structures are the fullerenes,[10] representing a new allotropic modification of carbon in addition to graphite and diamond.

The C60 and C70 fullerenes are produced by electric arc sputtering of graphite in the helium atmosphere at a pressure of ~ 104 Pa. However, electron beam evaporation and laser heating are also used.

A C60 molecule has the structure of a truncated regular icosahedron (Fig. 4), where carbon atoms form a closed hollow spherical surface comprising 5- and 6-member rings. Each atom has its coordination number equal to 3 and is located at vertices of two hexagons and one pentagon. Crystallization of C60 from a solution or a gas phase leads to appearance of fullerites, which are molecular crystals with a cubic lattice having the constant of 1.417 nm. The C70 fullerene is shaped like a closed spheroid (Fig. 4).

In 1992, a stable Ti8Ci2+ cluster was discovered1-11-1 corresponding to a Ti8Ci2 molecule in the form of a distorted pentagon dodecahedron (Fig. 5). The Ti18Ci2 cluster has the linear dimension of about 0.5 nm. Ti8C12 clusters were produced by the plasmachemical gas phase synthesis in a helium atmosphere using hydrocarbons (methane, ethylene, acetylene, propylene, and benzene) and titanium vapors as reagents. Titanium was evaporated under irradiation from a Nd-laser with a 532-nm wavelength. The Ti18Ci2 cluster is the first member in the new class of molecular clusters, that is, M8C12 metallo-carbohedrenes, where M=Zr, Hf, V, Cr, Mo, or Fe.

Precipitation from Colloid Solutions

A standard method for producing nanoparticles from colloid solutions consists of a chemical reaction between the solution components and interruption of this reaction at a certain moment of time. After this, the dispersed system changes from the liquid colloid state to the dispersed solid state. For example, nanocrystalline cadmium sul-fide CdS is produced by precipitation from a mixture of Cd(ClO4)2 and Na2S solutions. The solution pH is increased abruptly to stop the growth of CdS particles.

Dodecahedral structure of the molecular cluster Ti8C12 with the symmetries Th and Td taking into account different length of Ti—C and C—C bonds.

Fig. 5 Dodecahedral structure of the molecular cluster Ti8C12 with the symmetries Th and Td taking into account different length of Ti—C and C—C bonds.

Mixed-composition nanoparticles, i.e., nanocrystalline heterostructures, are synthesized by precipitation from colloid solutions. The core and the shell of a mixed nano-particle are made of semiconductor substances having different structures of electronic levels. Heterostructures, such as CdSe/ZnS or ZnS/CdSe, HgS/CdS, ZnS/ZnO, and TiO2/SnO2, are formed through a controlled precipitation of one type of semiconductor molecules on presynthesized nanoparticles of a semiconductor of another type.

Precipitation from colloid solutions is highly selective and allows producing stabilized nanoclusters with a very narrow particle size distribution.

Thermal Decomposition and Reduction

Subject to thermal decomposition are elemento- and me-tallo-organic compounds, carbonyls, formates, oxalates, amides, and imides of metals, which decompose at some temperature and form the synthesized substance. For example, metal powders with particles 100 to 300 nm in size on the average are prepared by pyrolysis of iron, cobalt, nickel, and copper formates in a vacuum or an inert gas at 470-530 K.

Superfine metal powders are also produced by hydrogen reduction of hydroxides, chlorides, nitrates, and carbonates of metals at <500 K. Advantages of this method include a low concentration of impurities and a narrow particle size distribution of powders (Fig. 6).

Mechanical Synthesis

Mechanical synthesis as a method for producing nano-powders may be divided into two categories: mechanical milling and mechanical alloying. Mechanical milling is used both for grinding and amorphization of the starting material. Mechanical alloying requires grinding, mixing, mass transfer, and chemical interaction of powders of several pure elements, compounds, or alloys. Substances in the crystalline and amorphous states may be prepared by mechanical alloying.

Mechanical synthesis is the most efficient method for large-quantity production of nanopowders.

Grinding and mechanical synthesis are performed in high-energy planetary, ball, or vibrating mills. The average size of powder particles is 200 to 5-10 nm. For example, Fe-Ni and Fe-Al nanocrystalline alloys with grains 5 to 15 nm in size were synthesized by grinding of metal powders in a ball vibrating mill during 300 hr.[8]

Typical distributions of metal particles by their size D. The particles were synthesized via reduction of metals from compounds in a hydrogen flow.

Fig. 6 Typical distributions of metal particles by their size D. The particles were synthesized via reduction of metals from compounds in a hydrogen flow.

Synthesis by Detonation and Electric Explosion

One more type of the mechanical treatment, which provides conditions for synthesis and dispersion of the final product, is a shock wave. Nanocrystalline diamond powders are prepared from mixtures of graphite and metals under the shock wave pressure of a few dozens of gigapascals. Diamond powders may be produced more conveniently by explosion of organic substances with a high concentration of carbon and a low percentage of oxygen.

Detonation of condensed explosives, which decompose with liberation of free carbon, is used for commercial production of diamond nanopowders. The volume of explosion chambers is not less than 2-3 m3. Synthesized diamond powders consist of cubic particles about 4 nm in size on the average.

Fine powders with particles up to 50 nm in size are produced by an electric explosion of wire when it passes a strong current pulse, 10- 5-10-7 sec long, having a density of 104-106 A mm-2.[12] A wire of diameter 0.1 to 1.0mm is used. A current pulse quickly heats the metal up to a temperature T>104 K (above the melting point) and the overheated metal is dispersed as in explosion. The average size of particles diminishes as the current density grows and the pulse length shortens. Electric explosion in an inert atmosphere provides powders of metals and alloys. Fine powders of oxides, nitrides, carbides, or their mixtures may be synthesized when reagents (O2+He, N2, H2O, and Ci0H22) are added into the reactor.

Synthesis of Superfine Oxides in Liquid Metals

In this method, molten gallium, lead, or Pb-Bi alloy serve as the working medium. A metal M, whose chemical affinity for oxygen is larger than the oxygen affinity of the molten metal, is dissolved in the melt. Then the dissolved metal M is oxidized by bubbling water vapor or an oxidizing gas mixture (H2O+Ar) through the melt. Superfine amorphous oxides of metals are formed as a result of selective oxidation. For example, oxidation of aluminum in molten gallium leads to formation of flocks of amorphous Al2O3- H2O. It consists of fibers 5 to 100 nm in diameter, which are spaced 5 to 400 nm. The synthesized material has a porosity of 97-99 vol.% and a specific surface of 30 to 800 m2 g-1

Nanostructured oxides SbO2, TeO, NiO, GeO2, SnO2, In2O3, K2O, ZnO, Ga2O3, Na2O, MnO, Li2O, Al2O3, BaO, SrO, MgO, and CaO may be produced via selective oxidation. The method is also applicable to synthesis of superfine nitrides, sulfides, and halogenides. In this case, a mixture of an inert gas and nitrogen N2, hydrogen sulfide H2S, or gaseous gallium or lead chlorides is passed through the melt with a dissolved metal.

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