Kiyoto Fukuoka

Shock-induced phase transition of M2O3 (M = Sc, Y, Sm, Gd, and In)-type compounds

Abstract Sintered specimens of cubic Sc2O3-type structure (C-type, Ia 3 ), Sc2O3, Y2O3, Sm2O3, and In2O3, and monoclinic Sm2O3-type structure (B-type, C2 m ) Sm2O3, and powdered specimens of C-type Gd2O3 were shock-loaded to 2–50 GPa using flyer plates accelerated by a propellant gun. Recovered specimens were studied by X-ray powder diffraction analysis at room and high temperature. Y2O3 began to transform to the B-type structure above 12 GPa, and the transformation was completed at 20 GPa. Gd2O3 also transformed to the B-type structure, but residual temperature effects were observed in the yield and crystallinity of the high pressure phase. In2O3 transformed to the corundum structure ( R 3 c ) in the pressure range of 15–25 GPa, although the yield was very small. Shock-induced phase transition from the C-type to the B-type structure was inferred to proceed via hexagonal La2O3-type structure (A-type, P 3 m1 ), in comparison with static high pressure experiments.

Shock behavior of zircon: phase transition to scheelite structure and decomposition

Abstract Both single-crystal and powdered specimens of zircon (ZrSiO4) were shocked to peak pressures between 30 and 94 GPa using the gun method, and specimens recovered were studied by means of X-ray diffraction analysis, transmission electron microscopy and infrared spectroscopy. Transformation to the scheelite structure started above 30 GPa, and was completed above 53 GPa in the case of single crystal specimens. Tetragonal unit cell parameters of the scheelite type ZrSiO4 at room condition are measured to bea = 4.7341(1)A, c = 10.51(1)A, c/a = 2.219(2) andV = 235.5(2)A3, which is smaller than that of the zircon type by 9.9%. The recovered scheelite-type ZrSiO4 reverts to the zircon type after rapid heating to 1200°C at room pressure. This transformation from the zircon type to the scheelite type is unique in that it is fast, displacive-like, but does not reverse. Tetragonal ZrO2 was detected as decomposition product in the single-crystal specimen shocked to 94 GPa, and further confirmed in a powdered specimen shocked to 53 GPa where enhancement of temperature is expected because of high porosity. Decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results.

The development of high performance Nd–Fe–Co–Ga–B die upset magnets

The magnetic properties and the microstructure of Nd–Fe–B and Nd–Fe–Co–Ga–B die upset magnets produced from amorphous materials were studied. The Nd–Fe–B die upset magnets had fine polygonal Nd2Fe14B grains and showed magnetic anisotropy. The compositional modification and optimization of the die upset condition led to the increase in the remanence and coercivity values of the Nd–Fe–B die upset magnets. The optimally deformed Nd–Fe–Co–Ga–B die upset magnets showed the maximum energy product of 54.4 MGOe.

Anisotropic phase transition of rutile under shock compression

Shock recovery experiments for single crystal and powdered specimens of TiO2 with the rutile structure were performed in the pressure range up to 72 GPa. Single crystal specimens were shocked parallel to [100], [110] and [001] directions. X-ray powder diffraction analysis showed that the amount of α-PbO2 type TiO2 produced by shock-loading depended strongly on the shock propagation direction. The maximum yield (about 70%) was observed for shock loading to 36 GPa parallel to the [100] direction. In the [001] shock direction, the yield is much smaller than that of the [100] direction. This anisotropic yield was consistent with the observed anisotropy of the phase transition pressure in shock compression measurements. However, transformation to the α-PbO2 type cannot explain the large volume change observed above about 20 GPa. On the basis of the high pressure behavior of MnF2, we assumed that the high pressure phase was either fluorite or distorted fluorite type and that the phase conversion to the α-PbO2 type was induced spontaneously in the pressure reduction process.We present a displacive mechanism of phase transition under shock compression from the rutile structure to the fluorite structure, in which the rutile [100] is shown to correspond to the fluorite [001] or [110] and the rutile [001] to the fluorite [110]. Direct evidence is obtained by examining the [100] shocked specimen by high resolution electron microscopy.

Yield properties, phase transition, and equation of state of aluminum nitride (AlN) under shock compression up to 150 GPa

Inclined-mirror Hugoniot measurements were performed on pure AlN polycrystals in the pressure range up to 150 GPa to study the yield properties, phase transition, and equation of state. The Hugoniot-elastic limit (HEL) stress was approximately 9.4 GPa. Above the HEL, the Hugoniot data converged to a static compression curve despite the high thermal conductivity, which indicated that the thermal property is not an important factor in determining the shock yield property. The phase transformation from wurtzite-type (B4) to rock salt-type (B1) structure took place at approximately 19.4 GPa, and was completed by about 75 GPa. The corrected transition pressure at 298 K was 19.2 GPa. Shock velocity (Us) versus particle velocity (Up) relation of the final phase was given by Us=3.27+1.81Up km/s. The Birch–Murnaghan fitting curve of the calculated isothermal compression curve of the B1-type phase roughly coincided with the recent static x-ray diffraction data up to over 100 GPa. The Gruneisen parameter, bulk modul...

Shock compression behaviors of boron carbide (B4C)

Hugoniot measurements on the highly dense, pure B4C polycrystal were performed by the inclined-mirror method to study the elastoplastic transition and to search phase transition. In inclined-mirror streak photographs, the smoothly jagged structure was observed at the free-surface shape in the plastic region. The Hugoniot-elastic limit (HEL) has been determined to be approximately 19.5GPa. In the plastic region, a kink was observed at a particle velocity of around 1.26km∕s. The shock velocity (US)–particle velocity (UP) Hugoniot relations in the plastic region were given by US=3.7+5.4UPkm∕s in the Up range of 0.54–1.26km∕s and US=9.61+0.73UPkm∕s in the Up range of 1.26–4.3km∕s. The S value (0.73) in US=C0+SUP above UP=1.26km∕s is significantly small compared with the result of Vogler et al. [J. Appl. Phys. 95, 4173 (2004)], and was much smaller than those of many oxides and nitrides. This material behaved as an elastoisotropic solid above the HEL and showed a large and linear change in the pressure-density...

Yielding and phase transition under shock compression of yttria-doped cubic zirconia single crystal and polycrystal

Inclined‐mirror Hugoniot measurements of yttria (Y2O3) ‐doped (9.6 and 8.0 mol %) cubic zirconia single crystal and polycrystal were performed in the pressure range up to 120 GPa to study yielding and phase transition. The Hugoniot‐elastic limit (HEL) stresses parallel to the 〈100〉 and 〈110〉 axes were approximately 14 and 25 GPa, respectively, while that of the polycrystal was approximately 13 GPa. Above the HELs the Hugoniot data parallel toward the 〈100〉 and 〈110〉 axes converged on each other, and showed large relief to an isotropic compression state, while those of the polycrystal preserved a considerably larger shear strength. A phase transformation took place at approximately 53 GPa (both 〈100〉 and 〈110〉 axis directions), and was completed by about 70 GPa. The phase transition pressure was much higher than those of the monoclinic‐ or tetragonal‐orthorhombic II phase transitions under static compression. The shock velocity Us versus particle velocity Up relation of the final phase of the single crysta...

High-pressure phase transformation of corundum (α-Al2O3) observed under shock compression

A discontinuous volume change was observed on the Hugoniot curve of corundum (α-Al2O3) along the axis by the inclined-mirror streak photographic technique, probably due to a high-pressure phase transformation. The transition pressure was measured to be 79 GPa, where shock temparature was estimated to be 860°C, in good agreement with a fully optimized quantum mechanical calculation. The volume change at the transition point was considered to be larger than 3%, which was larger than the theoretically predicted value of the corundum-Rh2O3 (II) transition, and was consistent with the recent static compression result by using diamond-anvil cell. The occurence of the phase transformation in alumina under shock compression presents significant implication in earth interior science or ceramic science, and also seriously affects the interpretation of static and shock compresion experiments at ultra-high pressures.

Formation of rutile-type Ta(IV)O2 by shock reduction and cation-deficient Ta0.8O2 by subsequent oxidation

Ta2O5 is reduced to Ta(IV)O2 with the rutile structure by shock-loading to 50–60 GPa. Tetragonal unit cell parameters at room conditions are measured to be a = 4.7518(5)A, c = 3.0878(4) A, ca = 0.6498(1), and V = 69.72(1) A3. The chemical composition is thermogravimetrically determined to be Ta0.97±0.04O2 by heating shock-reduced products in an oxygen gas flow to 1200°C. In the oxidation process a cation-deficient rutile-type compound Ta0.8O2 is found to be metastably formed.

Shock‐induced phase transition of MnO around 90GPa

Shock compression experiments on single crystal MnO grown by the Verneuil method have been carried out for the pressure range of 41–114GPa using both propellant and two-stage light-gas guns. The shock states, recorded with the inclined mirror method, were analyzed by the free-surface approximation and impedance-match solution. The observed shock compression curve of MnO with the rock salt structure(B1) is found to be slightly more compressible than that measured statically to 60GPa using a diamond-anvil cell. We observed a phase transition around 90GPa with a volume decrease of approximately 8%. The present phase transition pressure falls on a trend of B1-B2(CsCl-type structure) transformation among alkaline earth metal monoxides, rather than transformation to the B8(NiAs-type structure) that wustite (FeO) demonstrates.