Fumiyoshi Hakoe

Synthesis of a metal oxide with a room-temperature photoreversible phase transition

Photoinduced phase-transition materials, such as chalcogenides, spin-crossover complexes, photochromic organic compounds and charge-transfer materials, are of interest because of their application to optical data storage. Here we report a photoreversible metal-semiconductor phase transition at room temperature with a unique phase of Ti(3)O(5), lambda-Ti(3)O(5). lambda-Ti(3)O(5) nanocrystals are made by the combination of reverse-micelle and sol-gel techniques. Thermodynamic analysis suggests that the photoinduced phase transition originates from a particular state of lambda-Ti(3)O(5) trapped at a thermodynamic local energy minimum. Light irradiation causes reversible switching between this trapped state (lambda-Ti(3)O(5)) and the other energy-minimum state (beta-Ti(3)O(5)), both of which are persistent phases. This is the first demonstration of a photorewritable phenomenon at room temperature in a metal oxide. lambda-Ti(3)O(5) satisfies the operation conditions required for a practical optical storage system (operational temperature, writing data by short wavelength light and the appropriate threshold laser power).

External stimulation-controllable heat-storage ceramics

Commonly available heat-storage materials cannot usually store the energy for a prolonged period. If a solid material could conserve the accumulated thermal energy, then its heat-storage application potential is considerably widened. Here we report a phase transition material that can conserve the latent heat energy in a wide temperature range, T<530 K and release the heat energy on the application of pressure. This material is stripe-type lambda-trititanium pentoxide, λ-Ti3O5, which exhibits a solid–solid phase transition to beta-trititanium pentoxide, β-Ti3O5. The pressure for conversion is extremely small, only 600 bar (60 MPa) at ambient temperature, and the accumulated heat energy is surprisingly large (230 kJ L−1). Conversely, the pressure-produced beta-trititanium pentoxide transforms to lambda-trititanium pentoxide by heat, light or electric current. That is, the present system exhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phase transitions. The material may be useful for heat storage, as well as in sensor and switching memory device applications.

Sol–gel synthesis of nanosized λ-Ti3O5 crystals

In this study, we show a synthesis of λ-Ti3O5 nanocrystals dispersed in silica by sol–gel method. The X-ray diffraction measurements, Rietveld analyses, and transmission electron microscope images of the obtained samples showed that tuning the sintering temperature in the synthesis process can control the size of the λ-Ti3O5 nanocrystals, i.e., 8±2 nm (1123°C; sample 1), 9±3 nm (1133°C; 2), 9±2 nm (1143°C; 3), 10±3 nm (1153°C; 4), 11±4 nm (1163°C; 5), 13±4 nm (1173°C; 6), 25±12 nm (1200°C; 7), and 36±15 nm (1250°C; 8), whereas adjusting the hydrogen flow rate can tune the oxidation-reduction state of the sample without apparent change in the crystal size. At the lowest sintering temperature of 1123°C, the smallest X-Ti3O5 nanocrystals of 8 nm in size were produced.