Eiichi Ozawa

Low-temperature sintering and gas desorption of gold ultrafine powders

Abstract Sintering and gas desorption phenomena were examined in ultrafine gold powders. Sintering characteristics were observed by means of Brunauer-Emmett-Teller specific surface area measurements, dilatometric measurements and SEM observations. The decrease in the surface area and shrinkage simultaneously occurred at approximately 360 K. H 2 O, CO 2 , H 2 and CO gases were mainly detected while heating the gold ultrafine powders at temperatures up to 873 K in a high vacuum. These gases were considered to be produced by surface reactions.

Sintering characteristics of Fe and FeCo alloy ultrafine powders

Sintering characteristics of three kinds of iron and FeCo alloy ultrafine powders (UFPs) in vacuum and a hydrogen atmosphere were examined by continuously measuring their dimensions, and observing their structural changes. The UFPs exposed to air contain mixtures of oxide and hydroxide phases. The oxide phase increased during heating in vacuum. The compacts of the UFPs shrunk slightly at temperatures between 450 and 700 K, where the surface oxides sintered, and then densified rapidly above 700 K. The shrinkage in a stream of hydrogen occurred at much lower temperatures than that in vacuum. The reduction reaction is ratecontrolled by an interface reaction and the reduction rate of the UFPs depends on the oxidation level, where the more highly oxidized UFPs tend to the lower reduction rate. Activation energies of the reduction rate constants of the iron UFPs and FeCo UFPs lie in the range 48–59 kJ mol−1.

Coherent array of tungsten ultrafine particles by laser irradiation

We found a coherent array of tungsten ultrafine particles of about 700 nm around a laser-irradiated mark on a single-crystalline surface of tungsten. The tungsten specimens were irradiated by a short-pulse Nd:YAG laser under low pressure in an inert gas atmosphere. Two types of coherent arrays were observed that included the two-dimensional cubic and hexagonal systems. Such a coherent array of ultrafine particles may have potential applications for emission devices of plasma display and microelectronic devices.

Nonlithographic coherent array of ultrafine particles on an irradiated material using Nd:YAG laser: influence of the laser fluence on the microstructure

A non-lithographic coherent array of ultrafine tungsten particles (150-500 nm diameter) is self-assembly arranged around a laser-irradiated mark on a tungsten substrate. Single and poly-crystal tungsten substrates were irradiated by a Q-switched Nd:YAG laser under low pressure in an inert gas atmosphere. We studied the effect of the laser fluence on the morphology of the coherent arrays. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for these observations. The spacing of the coherent array increases with increasing laser fluence. For a 10.5 J/cm2 irradiation, many droplets and peeling of the surfaces induced by rapid thermal expansion of the tungsten surface were observed. The recrystallizatoin texture on the surface was also confirmed by TEM observations. This suggests that the irradiated surface layer was partially recrystallized during laser irradiation. We concluded that coherent arrays are related to the laser fluence, the crystal surface structure, and the crystal growth. The recrystallization of the slightly melted tungsten on the top surface induced by the repeated laser irradiation plays a vital role in the formation of the array. If we can produce a coherent array of ultrafine tungsten particles with a higher aspect ratio, it may have potential applications for the emission cathodes of field emission displays (FED) and microelectronic devices.

Characteristics of electric devices made by direct nanoparticle spraying

Metal nanoparticles films were prepared by a gas deposition technique couple with the nanosecond pulsed Nd:YAG laser ablation of two kinds of metal targets. Two generated nanoparticles were insitu mixed in the near space of the targets in the generation chamber and transported by a helium carrier gas to the deposition chamber and deposited on a substrate to form electric devices such as a resistor, a capacitor, wiring, etc., composed of nanoparticles composites. The electrical resistivity and capacity of the devices were measured. The relationship between the experimental conditions such as ambient pressure and laser fluences and the electrical properties of each device were analyzed. A heater was produced as an example of the application of a resistor.

Progressive growth of tungsten microcones using nanosecond pulsed-Nd:YAG laser irradiation in various atmospheres

We have discovered coherent arrays of ultrafine particles around a laser-irradiated mark. We tried to apply these arrays for electron emission guns. However, it was not successful due to their low aspect ratio. We observed microcones, which were made in the laser-irradiated mark on the initial surface of the target material. They seem to have a higher aspect ratio than the coherent arrays because heavy irradiation was needed to obtain cones with a high aspect ratio. Therefore, we examined the tungsten microcone growth in various gas atmospheres. The experimental results indicated that tungsten microcone growth did not depend on the impurities in the gases. In this paper we experimentally show the morphologies of the microcone in order to understand the microcone formation mechanism in various ambient atmospheres such as helium, SF6, and air. The difference in the microcone formation mechanism among the tungsten and Si substrate is discussed as related to the VLS (vapor-liquid-solid) mechanism.

Tungsten microcone arrays grown by nanosecond pulsed Nd: YAG laser irradiation

Tungsten microcone arrays with a high aspect ratio, which protrude from the initial surface of a target material, have been formed by Nd:YAG laser irradiation of tungsten in a low pressure inert gas atmosphere. The laser fluences were 1.5- 9.6 J/cm2 at SHG. The tungsten substrate was irradiated with 1-3600 pulses. The microcone growth was strongly affected by the number of laser pulses. The microcones were up to 20 micrometers tall and had about a 1.5 micrometers diameter at the tip. Several ten or several hundred pulses caused only a rough surface while subsequent pulses created the microcone arrays. Silicon, polymer, and oxides were used as the target materials in the former studies. The growth on these substrates could be attributed to the presence of various impurities and chemical gases. However, we used no impurities or chemical gases in this experiment. We believe that the growth mechanism of the microcones might be different from this study. It was concluded that the melted/solidification tungsten tip induced by the repetition of the pulsed laser irradiation on the top surface plays a significant role in the formation of the microcones. These tall tungsten microcone arrays might be very attractive for various industrial applications.

Sintering of Copper Ultrafine Powders

Sintering characteristics were examined for the Oxidized (subjected to slow oxidation treatment) Cu and the Fresh (not subjected to slow oxidation treatment) Cu ultrafine powders(UFPs). Gas desorption, reduction and densification of two kinds of Cu UFPs during sintering in a vacuum or in a hydrogen atmosphere were discussed.