This article reviews recent studies and developments on ultrafine particles The author proposes the following three kinds of approaches to find out some new fields of applications based on the unique properties of the ultrafine particels. (1) particles as extremely finely divided solids (effects of particle size on the physical properties), (2) particles consisting of a large number of atoms (structure changes and property caused by increasing number of included atoms), (3) particles as assemblies of surface atoms (predominant appearance of surface activity).
Needs for ultrafine and homogeneous ceramic powders have increased and many nonconventional powder preparation techniques have been developed. In the preparation of powders, it is important to control both chemical and physical characteristics of the powders to meet the requirements in their applications. An example of requirements for the starting powders for sintering is that the powders are consisted of small, spherical, and unaggregated particles with a narrow size distribution and to have a uniform and controlled chemical composition. In the present paper, the some of the recent preparation techniques of ceramic powders from solution and vapor phases will be described. Preparation of ceramic powders from solution can be classified into precipitation and solvent-evaporation techniques. Among precipitation methods, the alkoxide process is receiving an increasing attention as a method to produce fine monosized and monodispersed spherical oxide particles. There are two methods for the powder preparation from vapor phase : evaporation-condensation (PVD) and vapor phase reaction (CVD). The application of PVD method is limited because of less volatile nature of refractory materials. CVD method as powder preparation technique can be characterized by : (1) highly discrete particles; (2) ultrafine powders with narrow particle size distribution; and (3) direct preparation of powders of metals and nonoxides in addition to oxides. In CVD method, applications of plasma and laser to the powder production is also recieving an increasing attention.
Methods for the production of fine particles which are suited for electronmicroscopic observation are briefly introduced. The usefulness of the ultra-high resolution electron microscopy for the study of fine particles is emphasized, in particular, in looking into the details of crystal structures, crystalline defects, morphologies and surface structures of small particles. As some examples, structural changes of metal clusters less than 100 Å in size which are caused by the electron beam irradiation in the microscope, structural details of spherical particles of Si and alumina, and surface coatings of small particles by carbon, are described.
The influence of prior electron beam irradiation on the growth of island structure on a silicon wafer was investigated. The substrate was irradiated by a finely focused electron beam scanning along a line. It was found that the islands tended to avoid growing (just) on the irradiated traces on the substrate. This is due to the creation of contamination pattern on the substrate surface by the electron beam irradiation. The phenomenon has been applied to provide a nanometer scale island structure in the form of a prescibed square lattice on the substrate.
Ultrafine particles less than 1000Å in diameter have unique properties such as magnetic properties, catalytic properties, sintering properties and properties of electron energy states. Among several methods of forming ultrafine particles, gas evaporation methods are chosen, because of the following advantages. Impurities in the particles are less than those in particles prepared with other methods. The diameter and the alloy compositions of the particles can be easily controlled in these methods. Several types of ultrafine particles, such as chain particles, Cu-Zn complex particles and isolated particles, can be formed. Magnetic properties of chained particles, catalytic properties of Cu-Zn complex particles and configuration of isolated particles are presented. Application fields of those particles are also presented. Finally an ionization method of the isolated particles are introduced.
Procedures for coating ultrafine particles of iron with synthetic polymers were developed. The magnetic particles were covered with synthetic polymers such as polymethyl methacrylate, polystyrene, polyacrolein or their mixtures. Those polymers are containing functional groups that can be covalently bonded to protein and biochemically active substances. At an initial step of the modification, surface hydroxyl groups of iron particles were reacted with silane coupling reagent which can be copolymerized with vinyl monomers. Polymer coated particles were conjugated with immunoglobulins, protein A, bovine serum albumin or glucose oxidase. Those coated ultrafine particles can bind proteins up to 20% of their weight. In addition, those particles are well dispersed in aqueous solutions upon sonication and still magnetically active. Those particles may be useful for immobilization of enzyme or for magnetic affinity chromatography of cell and its organella.
Recently, we often find the terms “ultrafine particles” everywhere. Ultrafine particles have been studied for the past several ten years and its particular characteritsics has been elucidated mainly on the basis of physical and/or chemical understandings. Five years ago, ERATO's ultrafine particles research project by four laboratories was started. In a research group, for biophysical applicateion of ultrafine particles, I studied the interactions between ultrafine particles and living cells and the developemnt of a new method for preparing organic ultrafine particles ranged from 10nm to sevaral 100nm in diameter. In this paper, I will introduce the biophysical application of UFP mainly using my results during my tenure of that project.
Nickel ultrafine particle (UFP) of 30nm in average diameter prepared by Gas Evaporation Technique (GET), was activated by heating in hydrogen gas in a fluidized bed. This activated Ni UFP was applied to hydrogenation catalysts; (1) selective hydrogenation of conjugated diene, 1, 3-cyclooctadiene, to cyclooctene, (2) enantioface-selective hydrogenation of methyl acetoacetate to optically active methyl 3-hydroxybutyrate. Bimetallic or rather two component UFP, copper-zinc was also prepared, and applied to methanol synthesis catalyst from carbon monoxide and hydrogen. In addition, the study of dehydorgenation of 2-propanol catalyzed by nickel UFP of 20nm in diameter has been done by M. Noda et al. From these studies it was found that GET could offer a new method to prepare a catalyst of high purity or a multicomponent catalyst.
Effects of particle size on the catalytic properties of very small metal particles embedded on a flat support have been reviewed. Among various methods for the preparation of small metal particles, a model catalyst method has been chosen because the metal particles can be well characterized. The model catalyst is prepared by vacuum evaporation of metals onto a flat support and studied by such techniques as flash desorption spectrometry, photoelectron and Auger electron spectroscopy, and so on. Adsorption-desorption behaviors of CO, oxidation of CO, hydrogenation of C2H4 or H2-D2 exchange reaction have been examined on small particles of Ni, Pd, Pt, or Ru (ranging from 0.8-12 nmφ in diameter) which were prepared on a mica sheet, and Al2O3, or carbon supports. An electrochemical method to investigate the size effect in the desorption of hydrogen and oxidation of CH3OH has also been introduced.
Control of a radial concentration of nickel metal particles in an alumina sphere was achieved by impregnating the sphere with a solution composed of nickel nitrate dissolved in ethylene glycol, ethyl silicate, and a small amount of nitric acid. The position of the nickel narrow band from the center of the sphere was well controlled merely by the impregnating time. The nickel loading in the catalyst was intentionally varied with the nickel concentration in the impregnating solution. The external surface of the catalyst prepared was covered with a thin SiO2 film resulted from gelling of ethyl silicate over the alumina sphere. The micro-pores observed in the thin SiO2 films were sharply sized around 20Å. One of the features of this technique is the formation of macro-molecules consisting of Si-O-Ni-O-Si structure during the preparation of impregnating solution. When the alumina spheres were immersed, these macro-molecules adhere to the external surface of the spheres, followed by the elimination and migration of Ni ions into spheres to form a radial distribution of the fine metal particles.
Ultrafine powders of AlN and (AlN+Al) were produced by arc melting of Al in atmospheres of N2+Ar and N2+NH3 at 0.1MPa. It was found that the rate of nitriding of Al in this process increased markedly with increasing NH3 partial pressure in nitrogen atmosphere. This paper describes the formation mechanism and some properties of ultrafine AlN particles.
New aspects of finest grinding were reviewed. Emphasis was laid on the role of chemical reactions in the region of finest grinding. Combination of chemical reaction with grinding leads to the concept of chemical comminution. Chemical reactions in situ during grinding brings about the composite formation with the chemical species in the environment. It is thus possible to produce various composite particles and microcapsules. The importance of peptization or desagglomeration was also discussed by referring the sintering phenomena.
In manufacturing ceramics by solid-state reactions, properties of powders used play an important role in the manufacturing. By the hydrolysis of metal alkoxides, fine powders with highly homogeneous composition can be produced. This enables the production of ultrafine powders with designed composition and structure. A variety of ceramics can be derived from metal alkoxides. Zirconia (ZrO2) powders with various characteristics are prepared by changing hydrolysis conditions and calcination temperatures. Optoelectronic ceramics PLZT with excellent properties are also manufactured. Because powders derived from metal alkoxides bring about the aggregation by dryng, fluidized forms such as paste, slurry and sol are required. Magnetic fluids are produced by dispersing ferromagnetic fine powders derived from metal alkoxides into the solvent. Solgel process is developed in ceramic manufacturing. Metal alkoxides easily form sols. For example, AlOOH sol is obtained and alumina thin film is fabricated. Alkoxide process is expected as a promising method in manufacturing ceramics.
Relationship between microstructure and gas sensing characteristics of ceramic gas sensors was described, and highly functional gas sensors using ultrafine particles were reviewed. Using ultrafine particles brings about activation of particle-surface (for adsorption and redox reaction) as well as increases of surface area and contact-points of particles. As a result, a high sensitivity, rapid response and detection of inactive gas species become possible.
Gold particles smaller than 10nm in diameter, immobilized with α-Fe2O3, Co3O4, or NiO, were prepared by calcining in air at 400°C. The coprecipitates were obtained from an aqueous solution of HAuCl4 and the nitrate of Fe, Co, or Ni. The ultrafine gold particles were hemispherical in shape and were strongly held by the host oxides. In most cases, hemispherical gold crystallites were deposited directing their flat (111) plane toward α-Fe2O3 (110), Co3O4 (111), and NiO (111). The gold particles with a mean diameter of 4.1nm immobilized on α-Fe2O3 were more electron deficient than evaporated gold particles of the same size, and much more than the bulk metal. The ultrafine gold particles thus immobilized with 3d transition metal oxides were extremely active for the oxidation of CO at temperatures below 0°C and appreciably selective for CO sensing.
A technique for ceramics bonding was developed by using ULTRAFINE PARTICLES (UFP) which makes the ceramic surface smooth and increases the contact area of the surface. The technique enabled the solid-state bonding of ceramics to metals or ceramics to ceramics to be achieved at lower temperature and pressures compared with those in conventional methods. Al2O3 was bonded to SUS 304 at a temperature of 1200°C with a layer of Nb UFP, prepared by the gas evaporation method. The tensile strength of the bonded specimen was 46MPa. Si3N4 ceramic was also bonded to Si3N4 ceramic at a temperature of 1500°C and a pressure of 15MPa with the Si3N4 UFP layer. The tensile strength of the bonded specimen was 70MPa, and the value was five times as large as that of the bonded specimen without the UFP layer.
The nature of electronic energy level distribution in small metallic particles and the anomalies in electronic properties associated to it are discussed. The discussion is also extended to the superconducting properties in small particles. The present states of theoretical and experimental studies are reviewed.
Fine particle magnetism involves both size and surface effects due to the large specific surface area. The recent advances in research are reviewed from an experimental point of view. The particles dealt are focused on single-domain particles and superparamagnets and their magnetic behavior and the practical applications are described. Basic magnetic properties of ferromagnetic and ferrimagnetic fine particles are discussed. Surface magnetism of spin collinearity in connection with the value of the saturation magnetization of ultrafine particles is also concerned. Recent applications of fine particle magnetism including permanent magnets, particulate magnetic-recording media and magnetic fluids are presented. Finally some future research prospects are reviewed.
Fine particles were obtained by a Calcium stearate, a linear-chain molecule, evaporating in an Ar gas of high pressure. The fine particles are germanium-decorated and observed by a transmission electron microscope. The appearance and size of the organic ultrafine particles depend on the gas pressure and the growth distance.
Structure and growth of ultrafine Cu-phthalocyanine particles produced in an inert gas of Ar or He have been discussed in comparison with the growth of fine metallic and metallic oxide particles. A new method to produce the particles a few tens of nm have been proposed. This method, called two-boat method, uses the convection flow of inert gas and temperatute distribution in the atmosphere. The particles were β-crystal of needle shape grown in b axis with a size less than a few tens of μm. The smoke ball of Cu-phthalocyanine have been produced in a He gas pressure less than 30 Torr. The coalescence growth of Cu-phthalocyanine particle has been discussed on the basis of size distribution and temperature distribution in the smoke.
Basic aspects on the synthesis of composite polymer microspheres have been described from a surface chemical point of view. In preparing composite polymer particles including inorganic fine particles as the cores, surface treatments of the seed particles with the hydrophobic molecules, and electrostatic interaction between the inorganic core particle surface and capsulating polymers play important roles. Furthermore, a method for preparing organic-inorganic composite particles based on the heterocoagulation theory has been discribed.
Research and Development Corporation of Japan inaugurated a new research system in 1981 which is called Exploratory Research for Advanced Technology (ERATO). The “Ultra Fine Particles Project” is one of its four projects launched in 1981. Each project is termed for 5 years with 15-20 researchers including few group leaders. The project leader C. Hayashi discusses in this paper, at the end of the five year project, a variety of subjects which he thinks should be pursued further hopefully by other organizations or individuals. Utilization of front end technologies for high resolution electron microscopy (HREM) and for providing well defined sample, Ultra Fine Particles (UFP), has been successfully made by R. Uyeda and S. Iijima; which revealed details of atom-layer structure and its change by electron flux of the HREM. Some of those results are exciting both for scientist and application engineers. For industrial use of UFP, gas deposition technique investigated by S. Kashu et al. should be further developed for a variety of specific applications UFP of a high moleculer organic substance made by H. Toyotama are easily dispersed in water, which may make a great contribution in pharmaceutical applications. Materials in the form of UFP may open new field both in new industries and science.
From the viewpoint of application to catalysis, ultrafine particles have attracted attention as (1) a model material for elucidating the surface characteristics of solid catalysts and (2) an industrial catalyst material suitable for practical use. As a example for the second aspect ultrafine gold particles firmly attached to the surfaces of iron oxide by a coprecipitation method provide an efficient CO oxidation catalyst. Methods to obtain ultrafine particle catalysts are the alkoxide method, the polymer-supported colloid metal method, the metal vapor synthesis method, the mist decomposition method, and the gas evaporation method. Further developments are anticipated on the ground of superiority of the ultrafine particle to the bulky one in the abundance of highly-unsaturated surface sites. As this material is intrisically deficient in thermal stability, the importance of application to endothermic reactions is pointed out.