Hidenori Terasaki

In situ determination of the phase boundary between Wadsleyite and Ringwoodite in Mg2SiO4

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 composition was determined by in situ observation using synchrotron X-ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X-ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=10.32(28)+0.00691(9)×T, where P is pressure in gigapascals and T is temperature in degrees Celsius.

Effect of structural transitions on properties of high-pressure silicate melts: 27Al NMR, glass densities, and melt viscosities

Abstract The densities and viscosities of silicate melts depend strongly on pressure, in part because of potentially measurable structural rearrangements. In an attempt to further understand these changes and how they affect macroscopic properties, we have used 27Al MAS NMR to determine the coordination of the Al cations in a series of aluminosilicate glasses quenched from melts at pressures of 2 to 8 GPa, have measured the glass densities, and have applied an in-situ falling sphere method to measure melt viscosities at high pressure. Spectra from these four- and five-component glasses show increasing Al coordination with increasing pressure and with increasing average field strength of the modifier cation, as was previously reported for simpler compositions. These data also indicate that when multiple modifier cations are present (e.g., Ca and K), the Al coordination is lower than what would be expected from linear combinations of the appropriate aluminosilicate end-members. The viscosity of Ca3Al2Si6O18 melts, measured using a falling sphere method that combines multianvil techniques with synchrotron X-ray radiography, may reach a minimum at a pressure below 6 GPa. A quasi-thermodynamic approach using equilibrium constants for the reactions that generate high-coordinated Al suggests that this pressure may be related to a maximum in the concentration of five-coordinated Al. These results further support the concept that pressure-induced network structural transitions have direct implications for the macroscopic properties of high-pressure melts.

Numerical study of a free-burning argon arc with anode melting

Numerical modeling of free burning arcs and their electrodes is useful for clarifying the heat transfer phenomena in the welding process and to elucidate those effects which determine the weld penetration. This paper presents predictions for a stationary welding process by the free-burning argon arc. The whole region of the welding process, namely, tungsten cathode, arc plasma and stainless steel anode is treated in a unified numerical model to take into account the close interaction between the arc plasma and the molten anode. The time dependent development of two-dimensional distributions of temperature and velocity, in the whole region of the welding process, are predicted at a current of 150 A. The weld penetration geometry as a function of time is thus predicted. It is shown also that different surface tension properties can change the direction of re-circulatory flow in the molten anode and dramatically vary the weld penetration geometry.

The effect of temperature, pressure, and sulfur content on viscosity of the Fe–FeS melt

Abstract The Fe–FeS melt is thought to be the major candidate of the outer core material. Its viscosity is one of the most important physical properties to study the dynamics of the convection in the outer core. We performed the in situ viscosity measurement of the Fe–FeS melt under high pressure using X-ray radiography falling sphere method with a novel sample assembly. Viscosity was measured in the temperature, pressure, and compositional conditions of 1233–1923 K, 1.5–6.9 GPa, and Fe–Fe 72 S 28 (wt%), respectively. The viscosity coefficients obtained by 17 measurements change systematically in the range of 0.008–0.036 Pa s. An activation energy of the viscous flow, Q =30.0±8.6 kJ/mol, and the activation volume, Δ V =1.5±0.7×10 −6 m 3 /mol, are determined as the temperature and pressure dependence, and the viscosity of the Fe 72 S 28 melt is found to be smaller than that of the Fe melt by 15±10%. These tendencies can be well correlated with the structural variation of the Fe–FeS melt.

Fe-Mg partitioning between perovskite and ferropericlase in the lower mantle

Abstract Fe-Mg partitioning between perovskite and ferropericlase in the MgO-FeO-SiO2 system has been studied up to about 100 GPa at around 2000 K using a laser-heated diamond anvil cell (LHDAC). The compositions of both phases were determined by using analytical transmission electron microscopy (ATEM) on the recovered samples. Present results reveal that the Fe-Mg apparent partition coefficient between perovskite and ferropericlase [KDPv/Fp = (XFePv XMgFp)/(XMgPv XFeFp)] decreases with increasing pressure for a constant FeO of the system, and it decreases with increasing FeO content of ferropericlase. The gradual decrease of KDPv/Fp with increasing pressure is consistent with the spin transition in ferropericlase occurring in the broad pressure range from 50 to 100 GPa at around 2000 K.

Direct Observation that Bainite can Grow Below M-S

In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

Density measurement of liquid FeS at high pressures using synchrotron X-ray absorption

Abstract The density of liquid iron sulfide (FeS) was measured up to 3.8 GPa and 1800 K using an X-ray absorption method. The compression curve of liquid FeS was fitted using the Vinet equation of state. The isothermal bulk modulus and its temperature and pressure derivatives were determined using a nonlinear least-squares fit. The parameter sets determined were: K0T = 2.5 ± 0.3 GPa at T = 1500 K, (dK0/dT)P = -0.0036 ± 0.0003 GPa/K, and (dK0/dP)T = 24 ± 2. These results suggest that liquid FeS is more compressible than Fe-rich liquid Fe-S.

Radiographic study on the viscosity of the Fe-FeS melts at the pressure of 5 to 7 GPa

Abstract Stokes’ viscometry combined with in situ X-ray radiographic observation, using the 6-8 type multi-anvil press and synchrotron radiation, has been applied to the viscosity measurement of the Fe- FeS melt up to pressures of 7 GPa. The viscosity is found to be about 2 × 10-2 Pa-s at 5 to 7 GPa and temperatures about 1350 K, in marked contrast to previous viscosity measurements, which showed high viscosity, 0.5 to 14 Pa-s, at 2 to 5 GPa (LeBlanc and Secco 1996). Our viscosity data, however, is consistent with all other evidence, which include 1 atm viscosity data, X-ray structure analysis, and ab initio simulations. Recent viscosity measurements (Dobson et al. 2000) also showed the viscosity of Fe-FeS melt to be about 10-2 Pa-s at 2.5 GPa. Thus, we are confident that the viscosity of the Fe- FeS melt is close to a typical value (10-2 Pa-s) of viscosity for liquid metal even at high pressures.

Density of carbonated peridotite magma at high pressure using an X-ray absorption method

Abstract The density of carbonated peridotite magma was measured up to 3.8 GPa and 2100 K using an X-ray absorption method. A fit of the pressure-density-temperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus, KT0 = 22.9 ± 1.4 GPa, its pressure derivative, K0′ = 7.4 ± 1.4, and the temperature derivative of the bulk modulus (∂KT/∂T)P = -0.006 ± 0.002 GPa/K at 1800 K. The bulk modulus of carbonated peridotite magma is larger than that of hydrous peridotite magma. The partial molar volume of CO2 in magma under high pressure and temperature conditions was calculated and fit using the Vinet equation of state. The isothermal bulk modulus was KT0 = 8.1 ± 1.7GPa, and its pressure derivative was K0′ = 7.2 ± 2.0 at 2000 K. Our results show that the partial molar volume of CO2 is less compressible than that of H2O, suggesting that, on an equal molar basis, CO2 is more effective than H2O in reducing peridotite melt density at high pressure.

In situ viscosity measurements of albite melt under high pressure

The viscosities of albite (NaAlSi3O8) melt under high pressures have been measured using an x-ray radiography falling sphere method with synchrotron radiation. This method has enabled us to determine the precise sinking velocity directly. Recent experiments of albite melt showed the presence of a viscosity minimum around 5 GPa (Poe et al1997 Science 276 1245, Mori et al2000 Earth Planet. Sci. Lett. 175 87). We present the results for albite melt up to 5.2 GPa at 1600 and 1700°C. The viscosity minimum is clearly observed to be around 4.5 GPa, and it might be explained not by the change of the compression mechanism in albite melt but by change of the phase itself.