Isao Ikuta

Reversible phase‐change optical data storage in InSbTe alloy films

Some characteristics of reversible phase‐change optical data storage based on an amorphous‐crystalline transformation in InSbTe alloys are given. The reversible phase change was observed in a wide region of composition. The laser amorphized spot of a ternary compound In3SbTe2 film could be crystallized using a diode laser pulse of less than 100 ns with an incident laser power of more than 10 mW. The crystallization temperature of the amorphized spot was 280 °C and the activation energy was about 1.8 eV which shows that long‐term data retention at room temperature is possible. The repetition number of static write and erase using the pulse of 50 ns reached above 105. These data show that the ternary compound film has potential for reversible optical data storage media with high‐speed erasing and long‐term data retention.

Crystallizing mechanism and recording properties of In3SbTe2 phase-change optical disks

The crystallization mechanism and its application to improvement of the carrier-to-noise ratio (CNR) for In3SbTe2 phase-change optical disks were examined by that transmission electron microscopy (TEM). The TEM images indicated: (1) mark shapes were distorted when the interval between laser irradiations was short; (2) following dc laser irradiation onto the marks, crystallization proceeded only along the periphery of the amorphous phase; and (3) mark shapes were varied corresponding to the dc laser power. We assumed that the crystallization mechanism for In3SbTe2 was dominated by crystalline growth rather than nucleation. Then we simulated the mark shapes after dc irradiation. By controlling the thermal distribution on the marks, the CNR was improved promoted. We found that dc irradiation was a simple way of improving recording properties.

Reversible colour change in Cu-Al-Ni alloy ribbon associated with phase transformation

The colour change behaviour and its relation to phase transformation of Cu-14 wt% Al-4 wt% Ni alloy ribbon produced by the twin-roller type melt-quenching method were investigated by spectral reflectivity and X-ray diffraction, respectively. This ribbon turns copper-coloured around room temperature on quenching it from a temperature above 1020 K, and turns gold-coloured on ageing it between 670 and 970 K. By repeating these heat treatments, either of the two colours can be acquired interchangeably. The spectral reflectivity also changes with respect to the colour change. The copper-coloured alloy shows a DO3 structure which is theβ1, phase. The gold-coloured alloy shows a mixed phase ofγ2 (cubic, Cu9Al4 compound), andα (fcc, Cu-Al-Ni solid solution) and/orγ1′ which is a martensite of theβ1 phase having a (1 21) twinning structure. Therefore, the colour change between copper and gold is due to the solid-state phase transformation betweenβ1 andγ2 + (α and/orγ1′) on heat treatment.

Overwritable phase-change optical disk using an In3SbTe2 ternary compound

By employing transmission electron microscopy and thermal ana1yis, we examined overwrite repeatability of optical disks using In-Sb-Te alloys, which have high C/N ratio and excellent overwrite modulation characteristics. The overwrite repeatability of more than 105 was achieved by using an In3SbTe2 single-phase ternary compound and heat-flow balanced disk structure.

Disk structure for high performance overwritable phase-change optical disks

Disk structure for high performance overwrite using an InSbTe recording film was examined and the results were analyzed by a computer calculation. It was confirmed that dielectric films and reflective films have great influences on the sensitivities of melting and amorphization of the recording film. It was suggested that the thermal contact between the dielectric film and the reflective film is important for stable overwrite repetition.

Reversible colour change in Ag-Zn alloy ribbon

The reversibility of a colour change and its relationship to the phase transformation of Ag-50 at % Zn alloy ribbon produced by the twin-roller type melt-quenching method were investigated by spectral reflectivity measurements and X-ray diffractometry. This ribbon turns pink-coloured on quenching from a temperature above 560 K, and turns silver-coloured on ageing between 423 and 553_K. By alternating these heat treatments, either of the two colours can be acquired around room temperature interchangeably. The spectral reflectivity also changes with respect to the colour change. The pink-coloured alloy shows a CsCl structure which is the metastableβ1 phase. The silver-coloured alloy shows a hexagonal structure which is the ζ equilibrium phase. Therefore, the colour change between pink and silver is due to the solid-state phase transformation betweenβ, andζ caused by heat treatment.[/p]

40 dB OVERWRITE -MODULATION PHASE- CHANGE OPTICAL DISK USING In -Sb -Te ALLOY

A single-beam overwrite was achieved by an In-Sb-Te phase-change optical disk. The carrier to noise (C/N) ratio of more than 50 dB and the overwrite modulation of -40 dB were obtained for overwriting frequencies of 2 MHz and 3 MHz using a low power laser diode of 30 The high C/N ratio and the excellent overwrite modulation were due to the large reflectivity difference between the written and erased phases and the use of suitable write/erase power.

Design of mark shape by controlling crystallization speed of an In3SbTe2 phase-change material

In an Inj3SbTe3 phase-change optical disk, inhomogeneous crystallization determined the shape of marks on the data track. Even if a suitable laser power was irradiated to erase the amoohous marks, other amoohous portions were formed due to inhomogeneous crystallization between marks. After a second irradiation of a 6mW dc laser, the carrier-to-noise ratio (CNR) was able to be increased by 4 dB, because the latter amoohous areas were erased properly. This second irradiation procedure can be applied to realize higher density data storage by making very small clear marks and increasing the playback resolution.

Microstructures and mechanical properties of melt-quenched ferritic stainless steels

Using the twin-roller type melt-quenching technique, the effects of rapid solidification on the microstructures and mechanical properties of ferrite have been examined in 17 pct Cr and 17 pct Cr-M steels where M = Ti, Nb, Si, or Al. The melt-quenched 17 pct Cr steels have the columnar and equiaxed grains which become finer with decreasing of specimen thickness. But the yield strength of these steels, which is about 0.48 GPa, remains almost unchanged with grain size refinement, contrary to predictions based on Hall-Petch’s relationship. The columnar structure of the melt-quenched 17 pct Cr-M steels is very fine on alloying with Ti and Si, while it is slightly coarse on alloying with Nb and Al. The hardness, yield strength, and fracture strength of these 17 pct Cr-M steels increase with increased amount of alloying element, these maximum values being about 460 DPN, 1.0 GPa, and 1.35 GPa, respectively, for 17 pct Cr- 6 pct Si steel, in the ductile compositional range. These are suitable values for a fine-gauge material having high hardness and strength as well as good ductility.