Kazuhiro Mihama

Formation of Ultrafine Metal Particles by Gas-Evaporation VI. Bcc Metals, Fe, V, Nb, Ta, Cr, Mo and W

The crystal structures and habits of bcc metal particles have been investigated systematically by electron microscopy. The habits for the bcc structure are rhombic dodecahedra truncated by six {100} faces with various degrees of truncation from 0 to 100%. The truncation degree for Fe and V particles grown in the intermediate zone of a metal smoke is in good agreement with that for the Wulff polyhedron expected from the surface energies calculated for {110} and {100} faces. Particles of Cr, Mo and W have the A-15 type structure besides the ordinary bcc structure. The present results support the hypothesis that the A-15 type structure is stable when the particle size is small. The habits for the A-15 type structure are rhombic dodecahedra (Cr), {211} icositetrahedra (Cr and Mo) and rounded cubes (Mo and W).

Preparation of extremely fine particles by vacuum evaporation onto a running oil substrate

Abstract A large amount of extremely fine particles of Ag were prepared by vacuum evaporation onto a running oil substrate. The size distribution of Ag particles thus prepared was studied by electron microscopy. It was found that the mean diameter of the particles strongly depends upon the deposition rate, and kind and viscosity of the oil. Particles of mean dimater from 30 to 80 A can be produced. The sticking coefficient on the oil surface for arriving atoms is, in general, smaller than that on a glass surface. The coefficient increases with increasing viscosity for the same kind of oil. Coalescence growth of particles is also detected by heating the oil containing the particles. The heat treatment also enables us to control the practicle size between 30 to 80 A.

High resolution electron microscopy of composite films of gold and magnesium oxide

Abstract Composite films of gold crystallites embedded in magnesium oxide (001) and (110) single crystalline films are prepared by the simultaneous vacuum deposition technique for the study of metal-magnesium oxide composite films. The epitaxially grown gold crystallites of a few nm in size are analyzed from the contrast of the lattice images by using computer-simulated images. The lattice fringes at the crystallites are observed with two kinds of spacing of gold or magnesium oxide depending on the defocus condition, which suggests that the thickness is a few atomic layers. The result is also inferred from the growth condition in the simultaneous deposition. The fact that no strain contrast is apparent shows that gold crystallites grow almost coherently inside the magnesium oxide. The imaging characteristics of the composite films are studied in detail on the thickness and the depth of the gold crystallites.

A New Technique for the Formation of Ultrafine Particles by Sputtering

A gas evaporation technique using sputtering is newly developed. The characteristic of this new technique is the use of sputtering in an atmosphere of higher gas pressure such as 0.5 to 100 Torr than is used for ordinary sputtering. W, Ag and Cu are tested and their ultrafine particles are formed. WC1-x particles are also prepared by reactive sputtering.

An electrostatic field effect in the epitaxial growth of gold particles evaporated onto sodium chloride

Abstract A tendency for gold particles to take a parallel orientation at the initial stage of the epitaxial growth on sodium chloride is enhanced by putting a negative potential (

Attempt to form hydride and amorphous particles, and introduction of a new evaporation method

Al and TiH2 particles of fcc structure can be produced in an atmosphere of gaseous H2 at reduced pressure. Al particles with definite habit are obtained, which has been never observed in the ordinary gas evaporation technique using a HV system. The habit of TiH2 particles grown in the intermediate zone of the smoke is determined to be a dodecahedron. The growth is considered as the result of the martensite transformation from the bcc structure initially formed to the fcc structure accompanying a slight modification of the characteristic habit as observed for Ti particles. For the preparation of amorphous particles, first, the quenching rate of a particle, dTdt was estimated to be more than 104°Cs. Ultrafine particles of Pd80Si20 chosen as a test sample did not show the amorphous structure, but the crystalline. Application of the sputtering method as a new evaporation source in the gas evaporation technique is attempted. With the sputtering method, W particles with definite habits are produced.

Formation of Ultrafine Particles by Gas-Evaporation Technique. : V.Silicon and Germanium in Argon

Fine particles of silicon and germanium were prepared by evaporation in argon gas at low pressures, and they were studied by electron microscopy. Relatively large particles showed distinct crystal habits for both materials. A typical habit of silicon was an polyhedron bounded by twenty four {311} and eight {111} faces, and that of germanium, an icositetrahedron bounded by {311} faces. Crystallites with these habits have the diamond structure as usual. However, relatively small germanium particles, which were formed in the inner part of the smoke, showed no distinct habit and had an unknown structure with a tetragonal unit cell, a=5.37 A and c=9.04 A.

Formation and Ionization Potentials of Lead Clusters

Clusters of lead atoms have been produced by an adiabatic expansion of the vapour in a high temperature crucible through a small hole into a vacuum. These clusters have been ionized by electron bombardment and analysed with a time-of-flight mass spectrometer. Clusters of up to ten atoms have been observed, and Pb7, with the highest intensity. The relative abundance of the clusters is higher at higher crucible temperatures and/or with larger hole diameters. Ionization potentials of the clusters of up to seven atoms have also been measured. The change in the ionization potentials shows the same trend as that in an effective work function of a continuum metal drop.

Attempt to form ultrafine particles with hydride and amorphous structure

TiH2 particles with fcc structure can be produced in an atmosphere of reduced pressure of H2, instead of an ordinary inactive gas, by the gas evaporation technique. The habit of the particles grown in the intermediate zone of a smoke is determined by means of electron microscope to be dodecahedral and consists of 8 {111} and 4 {100}. As in the case of Ti particles, the growth mechanism can be considered as follows: The bcc TiH2 particles initially formed, the high temperature phase, are transformed into fcc structure, the low temperature phase, through the martensite transformation with a slight change of the habit, from the rhombic dodecahedral to simple dodecahedral. For the preparation of amorphous particles, first the quenching rate of a particle, dT/dt was estimated to be more than 104°C/s. The quenching rate was estimated from measurements of the temperature gradient around the evaporation source, dT/dx and the rising velocity of the particles along the convection flow of residual gas, dx/dt. The preparation of ultrafine particles of Pd80Si20 chosen as a test material was attempted. However, the particles showed crystalline rather than amorphous structure.

Crystal structure and habit of fine metal particles formed by gas-evaporation technique; bcc metals (V, Fe, Cr, Mo and W)

Abstract Fine particles of several kinds of bcc metals were prepared by evaporation in an argon atmosphere. They were collected at various growth zones in the smokes and their structures and habits were studied by electron microscopy. Vanadium and iron particles grown in all zones have the ordinary bcc structure. However, chromium and molybdenum particles grown in the inner part of the smokes have the bcc structure, and those in the outer part have the A-15 type structure. Habits of particles with the bcc structure can be generally described in terms of rhombic dodecahedra truncated by six (100) faces with various degrees of truncation from 0 to 100%. The truncated rhombic dodecahedron is a common habit to the bcc metals, though the degree of truncation is dependent on the metal. For vanadium it varies from zone to zone systematically. Molybdenum particles of rounded cube and {211} icositetrahedron are found to have the A-15 type structure in addition to the well known {211} icositetrahedron and rhombic dodecahedron found in chromium.