High-temperature creep behavior in Ni2GeO4 spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4
microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to
1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following
flow laws:
where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion
(Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels
although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are
normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation
creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size
reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis
of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less
than 2 μm.
Received: 18 June 2000 / Accepted: 3 April 2001 相似文献
In this paper, numerical methods for flow simulation in the well vicinity are discussed. Flux truncation errors are analyzed on gridblocks near the well. Due to the singularity of the well, standard numerical schemes are not efficient for near well flow simulations. To improve simulation accuracy, a new methodology, using a change of coordinates to make near well pressure linear, is presented for the singularity modelling. Based on this new approach, two numerical schemes, a two-point flux approximation scheme and a multi-point one, are proposed. These schemes are accurate for the near well modelling and are suitable for any kinds of gridblocks. 相似文献
The structures of Ca2CoSi2O7, Ca2MgSi2O7, and Ca2(Mg0.55Fe0.45)Si2O7 have been determined in the temperature range between 297 and 773 K with arbitrary intervals. The structures of the incommensurate
phase of the three compounds are characterized by the presence of the six-, seven-, and eight-coordinated Ca–O polyhedra and
of the bundles along the c-axes consisting of four arrays of the six-coordinated Ca–O polyhedra and an array of T1O4 (T1: Co, Mg, or Mg–Fe) tetrahedra in the structures. The number of bundles in each material decreases at elevated temperatures.
The incommensurate phase undergoes a phase transition into the normal phase at 493 K in Ca2CoSi2O7, at 360 K in Ca2MgSi2O7, and at 510 K in Ca2(Mg0.55Fe0.45)Si2O7. The features of the structures of the normal phase are almost the same as those found in the basic structures (the averaged
structures of the incommensurate structures), and this fact implies that the characteristics of the structures, such as the
six-coordinated Ca–O polyhedra or fragments of the bundles, should be partially preserved at higher temperatures both in the
incommensurate structures and also in the structures of the normal phase. Analyses of anisotropic displacement parameters
clarified that disorder of the modulation waves is developed in the structures at higher temperatures. The evolution of a
disorder in the structures was ascertained by observation of the circular diffuse streaks in the vicinity of the transition
temperature between the incommensurate and normal phases.
Received: 3 July 2000 / Accepted: 26 October 2000 相似文献
The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and
temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater
5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content
was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three
relatively intense bands at 3575, 3516, and 3459 cm−1 (ν1 to ν3, respectively) and two very weak bands at 3296 and 3210 cm−1 (ν4 and ν5, respectively). The band at 3516 cm−1 is strongly asymmetric and can be resolved into two bands, 3528 (ν2a) and 3508 (ν2b) cm−1, with nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1
0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative
analysis of OH in coesite (ɛi,tot=190 000 ± 30 000 l mol−1H2O cm−2). The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa
were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions
of the ν1, ν2, and ν3 bands decrease linearly with pressure. The mode Grüneisen parameters for ν1, ν2, and ν3 are −0.074, −0.144 and −0.398, respectively. There is a linear increase of the pressure derivatives with band position which
follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν1, ν2, and ν3 bands increase from 35, 21, and 28 cm−1 in the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results,
a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the
substitution Si4+(Si2)+4O2−= [4]□(Si2) + 4OH−, which gives rise to four vibrations, ν1, ν2a, ν2b, and ν3. Because the OH(D)-bearing samples do contain traces of Al and B, the bands ν4 and ν5 may be coupled to Al and/or B substitution.
Received: 19 December 2000 / Accepted: 23 April 2001 相似文献
An 18-yr chlorophyll time series for Narragansett Bay based on weekly samples collected without regard to tidal phase revealed a long-term decrease in mean annual levels. The potential influence of neglecting tidal phase in the sampling strategy on measured chlorophyll and its apparent long-term decrease is evaluated. A two year data set (1995–1996) is used as a proxy for the 1973–1990 time series together with an observed relationship between continuous measurements of in situ chlorophyll fluorescence and accompanying tidal phase. The deviations in chlorophyll from long-term means relative to deviations from mean low water at the time of sample collection are also analyzed, as is the potential influence of tidally-induced advective increases or dilution on measured chlorophyll levels. The analyses, which compare the magnitude and trends in tidally adjusted and directly measured chlorophyll, indicate that semi-diurnal intratidal variations in chlorophyll had little apparent effect on the long-term and seasonal patterns and trends deduced from the chlorophyll measurements. Neither tidal advection of the chlorophyll gradient, nor bloom magnitude appear to compromise application of the model. The 18-yr decline in annual mean chlorophyll observed between 1973–1990 in narragansett Bay is considered to be a bonafide portrayal of actual events, and not an artifact of failure to consider tidal phase in the weekly sampling strategy. The results also suggest that intratidal variability in chlorophyll does not seriously confound its meaningful measurement and usefulness as a representative index of phytoplankton abundance at the permanent monitoring station established for Narragansett Bay. Nonetheless, there is need to refine and to incorporate temporal sampling strategies more closely attuned to the tempo of growth, grazing, and nutrient recycling which accompany estuarine phytoplankton dynamics. 相似文献
Agricultural field boundary information is important in the study of agricultural crops by remote sensing. In this paper, the dyadic wavelet transform is adopted to extract field boundaries directly from TM imagery. A field boundary is a set of locally connected pixels characterized by their abrupt spectral intensity variation (singularity) on the image. Preliminary extraction is made on the near-infrared Band 4 (0.76–0.90 μm) image. It is found that most of the boundaries are well characterized by the wavelet modulus maxima curves, particularly for fully vegetated fields, while better results are obtained from the NDVI image for cropped field boundaries. To extract additional boundaries while suppressing unwanted noisy edges, the wavelet transform is then applied to the Wetness image calculated by the Tasselled Cap Transform. A technique is implemented to integrate the edges extracted from each source. The combined results show that the majority of the field boundaries are delineated. The use of a single date image fails to delineate some of the fallow field boundaries and boundaries between two cropped fields with similar spectral properties. Although techniques have been applied to reduce noise, still great difficulties are encountered when trying to delineate edges that are field boundaries from all the detected singularities. Manual editing is still required to trace the remaining boundaries to obtain a complete boundary map. It is recommended that a multi-temporal data set be used to obtain a complete delineation with a fully automated algorithm. 相似文献
We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ18O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19.
Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed.
The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of 40Ar/39Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28).
The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface.
Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions. 相似文献
The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m−2. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales. 相似文献