Takeyuki Uchida

Thermoelastic properties of MgSiO3 perovskite determined by in situ X ray observations up to 30 GPa and 2000 K

In situ X ray experiments on MgSiO3 perovskite at pressures of 21–29 GPa and temperatures of 300–2000 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P-V-T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as αT,25 = a25 + b25T − c25T2 with the best-fit parameters of a25 = 2.11 × 10−5 K−1, b25 = 1.80 × 10−9 K−2, and c25 = 1.93 K. The equation of state of MgSiO3 perovskite has been determined using our new data combined with the lower-pressure data of Wang et al. [1994] and Utsumi et al. [1995]. The optimal set of parameters of the third-order Birch-Murnaghan equation of state, which is expressed as P = (3/2)KT,0[(VT,0/V)7/3 − (VT,0/V)5/3]{1 − (3/4)(4 − KT,0′)[(VT,0/V)2/3 − 1]}, where KT,0 = K300,0 + (∂KT,0/∂T)P(T − 300), KT,0′ = K300,0′, VT,0 = V0exp ∫300TαT,0dT, and αT,0 = a0 + b0T − c0T−2, is K300,0 = 261 GPa, K300,0′ = 4, a0 = 1.982 × 10−5 K−1, b0 = 0.818 × 10−8 K−2, c0 = 0.474 K, and (∂KT,0/∂T)P = −0.0280 GPa/K. The reliability of the result is discussed in detail.

Stability field and thermal equation of state of ε‐iron determined by synchrotron X‐ray diffraction in a multianvil apparatus

In situ synchrotron X-ray diffraction measurements have been carried out on Fe using a “T cup” multianvil high-pressure apparatus up to 20 GPa and 1500 K. The stability field of the hexagonal phase (e-Fe) is characterized by the triple point of the body-centered cubic (bcc) (α), e, and face-centered cubic (fcc) (γ) phases, located at 8.0(±0.3) GPa and 680(±50) K with the slope of the phase boundary between the e and γ phases being 36±3 K GPa−1. Pressure-volume-temperature (P-V-T) data for the e-Fe enable us to extract thermal equation of state (EOS) parameters accurately. Least squares fit of a combination of our room temperature data with previous results using the diamond anvil cell (DAC) to the third-order Birch-Murnaghan EOS yields KT,0 = 135±19 GPa, K′T,0 = 6.0±0.4, and V0 = 22.7±0.3 A3, where KT,0, K′T,0 and V0 are zero-pressure isothermal bulk modulus, its pressure derivative, and zero-pressure volume, respectively. Volume data at high temperatures are fit with various high-temperature EOSs. A fit using the high-temperature Birch-Murnaghan EOS yields the temperature derivative of the bulk modulus (∂KT,0/∂T)P = −4.48 ±0.56 × 10−2 GPa K−1, with the zero-pressure thermal expansivity in the form αT,0 = a + bT − cT−2, where α = 3.98 ± 0.24 × 10−5 K−1, b = 5.07 ± 0.88 × 10−8 K−2, and c is nonresolvable from 0. The thermal pressure approach based on the Mie-Gruneisen-Debye theory gives (αT,0KT,0) and (∂2P/∂T2)v to be 6.88 ± 0.30 × 10−3 GPa K−1 and 4.63 ± 0.53 × 10−6 GPa K−2, respectively. The thermoelastic parameters obtained from various EOSs are mutually consistent. The edge lengths (a and c) for the e-Fe are also fit with the Mie-Gruneisen-Debye EOS based on fictitious volumes (a3 and c3, respectively) to obtain pressure and temperature dependence of c/a. Linear thermal expansivity for the c axis is slightly larger than that of the a axis while incompressibilities are similar. Thus pressure dependence of c/a at each temperature is quite similar, although absolute values of c/a become higher with increasing temperature. Below 20 GPa, no new phase between the e- and γ-Fe stability fields was observed, and no anomaly in the c/a ratio was detected. Under the assumption that e-Fe is stable at the corresponding P and T conditions of the Earths inner core, the density of e-Fe is significantly higher than that of the Preliminary Reference Earth Model, indicating light element(s) must be present not only in the outer core but also in the inner core.

High-pressure and high-temperature in situ x-ray Diffraction study of iron to above 30 Gpa using MA8-type apparatus

In situ x-ray diffraction experiments on iron at pressures of 22–32 GPa and temperatures of 300–1500 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. No phases other than e(hcp) and γ(fcc) were observed in this pressure and temperature range. It became clear that non-hydrostaticity or chemical reaction with hydrogen causes serious problems in high-pressure and high-temperature x-ray diffraction study on iron. The average thermal expansivity of e phase between 300 K and 1000 K and the zero-pressure bulk modulus of γ phase at 1400 K were determined by assuming that no chemical reaction occurred in the present study. The thermal expansivities of e iron at 22 GPa and 32 GPa are determined to be 3.88 × 10−5 K−1 and 3.16 × 10−5 K−1, respectively. In this case, the volume at 22–32 GPa and room temperature is about 1% larger than the literature value. The bulk modulus of γ iron is determined to be 120 GPa, when its pressure derivative is fixed at 5.

In situ measurement of viscosity of liquids in the Fe-FeS system at high pressures and temperatures

Abstract The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 × 10-2 to 4 × 10-3 Pa-s in FeS at 1450 to 1700 °C and 2 × 10-2 to 8 × 10-3 Pa-s in Fe-Seut at 1150 to 1380 °C. These results are consistent with recent viscosity measurements in Fe-Seut at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vočadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 × 10-2 Pa-s for an outer core composition with ~10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity

LATTICE STRAINS IN CRYSTALS UNDER UNIAXIAL STRESS FIELD

An expression for the lattice strains in a polycrystalline specimen under uniaxial stress field has been extended for all crystal systems. Apparent Miller indices (HKL) are introduced from Miller indices (hkl) and lattice parameters. The lattice strain e(l1l2l3) of the direction l1l2l3, normal to the plane HKL, can be uniquely expressed for all crystal systems as follows: e(l1l2l3)={αβ(l1l2l3)+(1−α)[1/(3KV)]}σ p+α(−(t/3)(1−3 cos2 ψ){(1/2)[3/E(l1l2l3) −β(l1l2l3)]})+(1−α){−(t/3)(1−3 cos 2 ψ)[1/(2GV) ]}, where β(l1l2l3) and E(l1l2l3) denote the linear compressibility and the Young modulus, respectively. Bulk modulus KV and shear modulus GV are values for isostrain model. Variable ψ is the angle between loading axis and the normal of the plane HKL. The first term is the strain caused by the hydrostatic stress component σp. The second and third term, strains caused by the differential stress t, correspond to the isostress and the isostrain model, respectively. The parameter α takes a value between 0 (isostrain...

Deviatoric stress measurement under uniaxial compression by a powder x‐ray diffraction method

The complete stress field in a polycrystalline sample compressed in a modified Drickamer‐type apparatus was determined from x‐ray diffraction data. The incident x rays, from a synchrotron source, were perpendicular to the compression axis, and the diffracted energy‐dispersive signals were simultaneously determined for two directions relative to the compression axis. The two sets of d values measured by this system were analyzed by making use of a new equation derived by Singh, and the uniaxial stress component σ1−σ3 and the parameter α, which describes the stress and strain conditions across the grain boundaries of the sample, were obtained. This method was applied to NaCl and the results give the important information on the stress state and the pressure determination method under direct compression of a solid sample.

High-pressure viscometry of polymerized silicate melts and limitations of the Eyring equation

Abstract In situ falling-sphere measurements of viscosity have been performed to determine the viscosity of dacite melt (68 wt% SiO2) from 1.5 to 7.1 GPa at temperatures between 1730 and 1950 K, using the T-25 MA8 multianvil apparatus at the GSECARS 13-ID-D beamline at the Advanced Photon Source, Argonne National Lab. The viscosity of dacite melt decreases between 1.5 and 7.1 GPa. At 1.5 GPa and 1825 K the viscosity is 86.6 ± 17.3 Pa⋅s, whereas at 6.6 GPa and 1900 K it is 2.8 ± 0.6 Pa·s. The negative pressure dependence of viscosity results in an activation volume of .12.4 ± 1.4 cm3/mol at 1800 K and .5.1 ± 0.9 cm3/mol at 1900 K. These new data are compared with viscosities estimated from the Eyring equation using oxygen self-diffusion data for the same bulk composition at high pressures. The Eyring equation generally predicts viscosities that are greater than measured viscosities. In addition, the Eyring equation predicts a minimum viscosity at 5 GPa, but no minimum was seen in our falling sphere data set. These discrepancies suggest that the mechanisms for viscous flow and self-diffusion of oxygen in polymerized melts may differ at high pressures, thus limiting the utility of the Eyring equation for high-pressure extrapolations. Further development of the Adam-Gibbs theory may provide an alternative for relating self-diffusion and viscosity at high pressures.

High-pressure x-ray tomography microscope: Synchrotron computed microtomography at high pressure and temperature

A new apparatus has been developed for microtomography studies under high pressure. The pressure generation mechanism is based on the concept of the widely used Drickamer anvil apparatus, with two opposed anvils compressed inside a containment ring. Modifications are made with thin aluminum alloy containment rings to allow transmission of x rays. Pressures up to 8GPa have been generated with a hydraulic load of 25T. The modified Drickamer cell is supported by thrust bearings so that the entire pressure cell can be rotated under load. Spatial resolution of the high pressure tomography apparatus has been evaluated using a sample containing vitreous carbon spheres embedded in FeS matrix, with diameters ranging from 0.01to0.2mm. Spheres with diameters as small as 0.02mm were well resolved, with measured surface-to-volume ratios approaching theoretical values. The sample was then subject to a large shear strain field by twisting the top and bottom Drickamer anvils. Imaging analysis showed that detailed microst...

Amorphization of Serpentine at High Pressure and High Temperature

Pressure-induced amorphization of serpentine was observed at temperatures of 200° to 300°C and pressures of 14 to 27 gigapascals with a combination of a multianvil apparatus and synchrotron radiation. High-pressure phases then crystallized rapidly when the temperature was increased to 400°C. These results suggest that amorphization of serpentine is an unlikely mechanism for generating deep-focus earthquakes, as the temperatures of subducting slabs are significantly higher than those of the rapid crystallization regime.

Investigation of pressure-induced amorphization in hydrated zeolite Li-A and Na-A using synchrotron X-ray diffraction

High pressure synchrotron X-ray diffraction measurements of hydrated zeolite Li-A and Na-A were carried out at pressures up to 4.1 GPa and at room temperature in a large volume press. Energy dispersive X-ray diffraction measurements showed progressive pressure-induced amorphization of both Li-A and Na-A samples. The most rapid loss in long-range ordering occurred at pressures up to 2.2 GPa followed by a gradual, continued decrease in ordering up to the maximum pressure. At 4.1 GPa the samples appeared to be X-ray amorphous. After decompression, diffraction patterns at 1 atm indicated that the sample reverted back to their initial crystal structure. q 2001 Elsevier Science Ltd. All rights reserved.