Mikio Higuchi

Thermal conductivity/diffusivity of Nd3+ doped GdVO4, YVO4, LuVO4, and Y3Al5O12 by temperature wave analysis

Thermal diffusivity and thermal conductivity of single crystals of Nd3+ doped GdVO4, YVO4, LuVO4, and Y3Al5O12 are precisely measured over a wide range of doping concentration from 0.5to15at.% by temperature wave analysis. Thermal diffusivity serves as the most sensitive parameter to detect the effect of doping on thermal properties, where Nd3+ doped GdVO4 exhibits a decrease in thermal diffusivity (it has changed about 20% in their values in the c axis) but an increase in heat capacity (only 1.7%). It has long been understood that the thermal conductivity of YVO4 is inferior to that of Y3Al5O12; however, the thermal conductivity of YVO4 in the c axis shows the highest value in all four crystals compared at 1at.% of Nd3+ doping concentration. Thermal conductivity exhibits a decrease (∝e−1∕2, e: mass variance) with an increase of doping concentration, that is characteristic of Klemens’ point defect model for the phonon scattering. In the numerical fitting, the anisotropic decrease of thermal conductivity i...

Floating zone growth and scintillation characteristics of cerium-doped gadolinium pyrosilicate single crystals

Growth of cerium-doped gadolinium pyrosilicate (Gd₂Si₂O₇:Ce) single crystals, which show 2.5 times greater light output for gamma-rays and five times greater light output for alpha particles than GSO single crystals, is accomplished using floating zone growth method (FZ method). Although growth of Gd₂Si₂O₇ (GPS) single crystal is considered to be difficult because it melts incongruently according to the phase diagram in a Gd₂O₃-SiO₂ system, we attempted crystal growth of Ce:GPS because the possibility exists that heavy Ce doping changes the phase diagram. Transparent single crystals were obtained, but cracks were observed in the crystals. The crystal structure was triclinic with space group P_/1 and density of 5.5 g/cm³. Two peaks were observed by photoluminescence spectrum measurement at 374 nm and 394 nm caused by 5d-4f transition in Ce³⁺ ion. Decay times of Ce:GPS were 46 ns for gamma-ray and 39 ns for alpha particles; its density was 5.5 g/cm³. We consider that the energy resolution of 23% will be improved by fabrication of large crystals and improvement of crystal perfectability.

Efficient laser performance of N:dGdVO 4 crystals grown by the floating zone method

Efficient laser performance is demonstrated with Nd:GdVO4 crystals grown by the floating zone method. With a 2-at. % Nd-doped crystal a slope efficiency of 67% is achieved with pumping at 808 nm. We also find that pumping at 879 nm with a bandwidth of 1.8 nm is practical for laser diode pumping. With this pumping level the slope efficiency reaches 78%. High-quality Nd:GdVO4 crystals are successfully grown with as much as 15-at.% Nd concentration by the floating zone method without inclusions or macroscopic defects. Homogeneity and high reproducibility of crystal growth are confirmed.

Preparation of thin Nd-doped YVO4 single crystal rods by the floating zone method

Thin neodymium-doped yttrium orthovanadate (Nd:YVO4) single crystal rods were successfully prepared by the floating zone method with an infrared convergence type heater. The minimum diameter obtained was about 0.8 mm. The double pass technique was used to keep a stable molten zone throughout the growth of thin single crystals. The as-grown crystals were violet and transparent, and did not have macroscopic defects such as cracks and inclusions. The growth in the pure oxygen flow suppressed the evaporation of the vanadium oxides effectively. Dislocation density was decreased with decreasing crystal diameters. No low-angle grain boundaries and no strains were observed for any crystals up to 3 mm in diameter.

Growth of apatite-type neodymium silicate single crystals by the floating-zone method

Apatite-type neodymium silicate single crystals, which show high oxide ionic conductivity, were successfully grown by the floating-zone method. At a growth rate of 5 mm/h, numerous tiny bubbles were easily incorporated into the crystal, whereas those grown at 2 mm/h contained no bubbles. The crystals were found to have sufficient quality for electrical measurements, that is, no low-angle grain boundaries nor twin structures were observed by polarizing microscopy.

Float zone growth and characterization of Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 single crystals with an apatite structure

Single crystals of apatite-type praseodymium silicate (Pr9.33(SiO4)6O2) and samarium silicate (Sm9.33(SiO4)6O2) with high oxide ionic conductivity have been grown by the floating zone method. The as-grown crystals of Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 are green and orange, respectively, and both crystals are transparent. The crystals do not contain inclusions, low-angle grain boundaries or twin structures but the samarium silicate crystals contain a few cracks perpendicular to the c-axis whereas the praseodymium silicate crystals are crack-free. Microcracks are also introduced in the samarium silicate crystals during the cutting and polishing processes. The oxide ionic conductivities of these crystals are comparable to that of neodymium silicate with the same structure.

Floating zone growth and characterization of aluminum-doped rutile single crystals

Abstract Transparent and grain-boundary-free rutile (TiO 2 ) single crystals, to which a small amount of Al 2 O 3 was added, were successfully grown by the floating zone method. The most effective Al addition was 0.4 at%. On the other hand Al-free, pure rutile crystals were dark-blue and comprised many low-angle grain boundaries. Al 3+ ions have two roles in rutile crystals. One is to pin down the migration of dislocations during cooling because of the difference in the ionic radii between Ti 4+ and Al 3+ , so that polygonization, namely formation of low-angle grain boundaries, does not occur. The other role is to form oxygen vacancies, via which oxygen ions can easily migrate in the rutile crystal during cooling to room temperature after the crystal growth. The conductivity of the Al-doped crystal was larger by one order of magnitude than that of the pure crystal at a temperature range of 600–900°C, which indicates that the diffusion rate of oxygen ions is much higher in the Al-doped crystal than in the pure crystal.

Electron-beam induced recrystallization in amorphous apatite

Electron-beam induced recrystallization of irradiation-induced amorphous Sr2Nd8(SiO4)6O2 is investigated in situ using transmission electron microscopy with 200keV electrons at room temperature. Epitaxial recrystallization is observed from both the amorphous/crystalline interface and the surface, and the recrystallization is more pronounced with increasing electron-beam flux. Since the temperature increase induced by electron-beam irradiation is estimated to be less than 7K and maximum energies transferred to target atoms are below the displacement energies, ionization-induced processes are considered to be the primary mechanisms for the solid-phase epitaxial recrystallization observed in the present study.

Growth of rutile single crystals by floating zone method

Rutile single crystals of optical grade were grown by the floating zone (FZ) method. The formation of low angle grain boundaries was depressed by controlling oxygen partial pressure of growth atmosphere. The upper limit of the oxygen partial pressure was 1 × 103 Pa to grow rutile single crystals without low angle grain boundaries. Fluctuation of refractive index was estimated to be up to 3 × 10−6 and etch pit density on (110) plane was 5 X 104 cm-2 for the as-grown crystal. The extinction ratio was 55 dB for Glan-Thomson prisms made of the FZ-grown rutile. The FZ method was an excellent technique to grow rutile single crystals of optical grade.

Reinvestigation of phase relations around the oxyapatite phase in the Nd2O3–SiO2 system

Phase relations around the oxyapatite phase in the Nd2O3–SiO2 system have been investigated by means of a slow cooling floating zone (SCFZ) method and solid-state reactions. The results on SCFZ proved that the oxyapatite phase melts congruently at the composition of Nd2O3:SiO2=7:9, corresponding to Nd9.33(SiO4)6O2. The 2:3 phase (Nd8(SiO4)6), which had been believed to have an apatite structure and to melt congruently, does not exist and consists of a mixture of Nd9.33(SiO4)6O2 and Nd2Si2O7 phases. The solid-state reaction revealed that a solid solution region of the oxyapatite phase is extremely narrow or does not exist below 1650°C.