Elasticity of cobalt at high pressure studied by inelastic x-ray scattering

The five independent elastic moduli of single-crystalline hcp cobalt were determined by inelastic x-ray scattering to 39 GPa and compared to ultrasonic measurements and first principles calculations. In general the agreement is good, in particular, for the evolution of the longitudinal sound velocity in the a-c plane. This confirms the calculations, suggesting that a similar evolution is valid for hcp iron, the main constituent of the Earth's inner core, up to the highest investigated pressure. Our results represent an important benchmark to further refine ab initio calculations.     

The effect of silicon impurities on the phase diagram of iron and possible implications for the Earth's core structure

The hexagonal-close-packed (hcp) structure is the accepted stable form of pure iron (Fe) under Earth's core conditions. Recently, however, a body-centred-cubic (bcc) phase of iron alloyed with lighter elements has been proposed. At relatively modest conditions, experiments have shown that small amounts of silicon (Si) can stabilise the bcc phase with respect to the hcp phase. This result has motivated our present study examining the effect of silicon on the bcc-hcp phase transition. We have performed ab initio calculations on both phases at zero Kelvin; our results are in good agreement with experiment. Extending our results to core pressures and taking into account previous studies of pure iron at core temperatures, we conclude that the bcc phase of iron alloyed with silicon is likely to be the stable crystalline phase in the inner core.     

利用激光超声技术测量介观尺度材料的声速

设计并建立了一种可用于测量介观尺度(几十微米至几毫米)材料声速的激光超声系统.该系统采用脉宽为6.3 ns的调Q激光器作为超声波激发光源,使用光纤位移干涉仪进行超声位移探测.利用新建立的激光超声系统,测量了不同厚度铜箔的纵波声速.结果表明,激光超声系统得到的声速具有较高精度,可以将其应用于介观尺度材料声速测量.     

Nanostructures of gold coated iron core-shell nanoparticles and the nanobands assembled under magnetic field

Pure metal iron nanoparticles are unstable in the air. By a coating iron on nanoparticle surface with a stable noble metal, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming self-assembling nanoarrays. Gold-coated iron core-shell structure nanoparticles (Fe/Au) synthesized using reverse micelles were characterized by transmission electron microscopy (TEM). The average nanoparticle size of the core-shell structure is about 8 nm, with about 6 nm diameter core and 1∼2 nm shell. Since the gold shell is not epitaxial growth related to the iron core, the morié pattern can be seen from the overlapping of iron core and gold shell. However, the gold shell lattice can be seen by changing the defocus of TEM. An energy dispersive X-ray spectrum (EDS) also shows the nanoparticles are air-stable. The magnetic measurement of the nanoparticles also proved successful synthesis of gold coated iron core-shell structure. The nanoparticles were then assembled under 0.5 T magnetic field and formed parallel nanobands with about 10 μm long. Assembling two dimensional ordered nanoarrays are still under going. Received 29 November 2000     

Evolution of electrical conductivity and semiconductor to metal transition of iron oxides at extreme conditions

Iron oxides are widely found as ores in Earth's crust and are also important constituents of its interiors. Their polymorphism, composition changes, and electronic structures play essential roles in controlling the structure and geodynamic properties of the solid Earth. While all-natural occurring iron oxides are semiconductors or insulators at ambient pressure, they start to metalize under pressure. Here in this work, we review the electronic conductivity and metallization of iron oxides under high-pressure conditions found in Earth's lower mantle. We summarize that the metallization of iron oxides is generally controlled by the pressure-induced bandgap closure near the Fermi level. After metallization, they possess much higher electrical and thermal conductivity, which will facilitate the thermal convection, support a more stable and thicker D^{$\prime\prime$} layer, and formulate Earth's magnetic field, all of which will constrain the large-scale dynamos of the mantle and core.     

A Strength Softening Phase Transition Observed in Shocked （Mg0.92,Fe0.08）SiO3 Perovskite at About 83 GPa

We report the experimental data of Hugoniot longitudinal sound velocity VL for natural （Mg0.92,Fe0.08）SiO3 enstatite sample at about 40-140 GPa, consisting of three new data and five previously reported data but revised by our new Hugoniot equation of state parameters. Three segments, separated by two discontinuities, appear in the VL-PH （shock pressure） plot. Analyses show that the first discontinuity at about 64 GPa, with a sharp increase of VL of about 21%, is judged to be a phase transition from enstatite to Pbnm perovskite （PV）; while the second one at about 83 GPa, with a dramatic decrease of VL of about 23%, is likely caused by a subtle structural change from Pbnm PV to tetragonal PV, accompanied by material strength softening due to melting of oxygen sublattices. This strength softening evidence is obtained first from shock wave experiments, and probably has profound implications for probing into the origin of low seismic velocity anomaly in the Earth＇s lower mantle and thus constraining the geophysical and geochemical models for the Earth＇s lower mantle.     

Forward and backward electron emission cross-sections for 23 MeVu<Superscript>-1</Superscript> C,Ni and Au projectiles traversing C,Al , Ni,Ag , Au and Bi foils

A rather complete experimental study of forward and backward electron velocity spectra from thin foils bombarded with ions at constant velocity of 30 atomic units (23MeV u^-1) was performed. Three different beams ( ¹²C³⁺ , ⁵⁸Ni¹⁴⁺ and ¹⁹⁷Au³⁶⁺ and six different targets ( ¹²C , ²⁷Al , ^natNi , ^natAg , ¹⁹⁷Au and ²⁰⁹Bi of approximately 90μg cm^-2 thickness were used. This procedure allowed to study the evolution of electron emission (velocity and angular distributions) for the [projectile; target] matrix [ C , Ni , Au ; C , Al , Ni , Ag , Au , Bi ] in a wide angular range. The projectile and target dependence of absolute cross-sections for binary encounter-, Auger- and backward-emitted electrons are analyzed.     

The velocity dependence of the transient magnetic field of O in Fe

The transient field precession has been measured for¹⁶O ions in the 3 ₁ ^– excited state recoiling into magnetized iron foils with an initial velocity of 8.0v ₀ (v ₀=c/137). The recoil velocity after passage through the Fe foils has been varied by adjusting the foil thickness. The resulting field strengths show strong deviations from the empirical linear velocity dependence of the transient field. The velocity dependence can be described well on the basis of recently measured probabilities for K-shell vacancy production for O in Fe, with a velocity independent degree of polarization =0.14 for the unpaired electrons in the K-shell.     

气液两相流中的声速研究

气液两相流在工业各领域中广泛存在,而声速是描述其声学性质的一个重要参数。本文从流体的体积弹性模量的定义出发,推导了气液两相流中的声速随含气率的变化关系式,即混合流体的Wood声速公式,将其声速的部分计算结果和其他作者的实验数据进行了比较,吻合良好。并通过COMSOL有限元模拟软件得到不同气体分布下圆管谐振腔最低阶模式的共振频率,间接数值模拟研究了含气率对声速的影响。模拟结果与理论计算结果一致,当气液两相流中含气率较低时,声速随含气率的增大急剧减小。本研究结果为确定声速与气液两相流中的含气率间的关系提供了参考依据。     

Quasihydrostatic equation of state of iron above 2 Mbar

The compression curve of iron is measured up to 205 GPa at 298 K, under quasihydrostatic conditions in a diamond anvil cell. Above 150 GPa, the compression of this metal is significantly higher than previously measured under nonhydrostatic conditions. The same compression curve is also calculated ab initio and the deviation between experiment and theory is clearly established. A formulation of the equation of state of iron over a large pressure and temperature range, based on the current data and existing shock-wave data, is also proposed. Implications for the Earth's core are discussed.     

Experimental validation of a collocated PVDF volume velocity sensor/actuator pair

The active control of sound transmission through a rectangular panel is experimentally verified. The control system is based on a collocated volume velocity sensor/actuator pair which measures and excites the first radiation mode of the panel. Suppression of the first radiation mode is an efficient strategy to control the low frequency sound radiation from the panel. This configuration leads to a simple single-input single-output control system, to which feedback control can be applied.Two implementations of the volume velocity sensor/actuator pair are tested. First, a polyvinyledene fluoride polymer (PVDF) volume velocity actuator foil with shaped electrodes is used in combination with an identical PVDF volume velocity sensor foil. Due to the mechanical coupling between the PVDF sensor and actuator foil, it is shown that a direct velocity feedback control scheme is not feasible because higher order structural modes will be destabilized. Instead integral force feedback is applied, such that the open-loop transfer function has a roll-off towards higher frequencies. Experiments show that this control strategy results in a reduction of the sound pressure in the receiving room of 10 dB at the first structural resonance without spillover to higher order modes. Due to the roll-off towards high frequencies, the control over higher order modes remains limited. Second, a discrete volume velocity sensor is constructed by summing the signals from 12 point sensors placed on the panel. The volume velocity actuator consists of two PVDF foils, glued on each side of the panel and driven in opposite phase. Direct volume velocity feedback is applied to this system, which is minimum phase. This control system is capable of reducing the sound pressure in the receiving room below 300 Hz by 10-15 dB without spillover to higher order modes.     

Structure and thermal evolution of57Fe implanted chromium

⁵⁷Fe ion implanted chromium foils have been analyzed by conversion electron Mössbauer spectroscopy. After annealing at temperatures up to 600 °C, the CEM spectra show two magnetic components which are attributed to iron atoms in the core and the outer layer of grains. By means of a simple model using the spectral data, the final average iron grain has been evaluated to be about 5 nm.     

水下对空中声源的运动参数估计

为了解决传统的交互式多模型目标跟踪算法中马尔可夫概率转移矩阵固定不变,造成的模型切换缓慢、跟踪精度不高的问题,提出了一种基于后验信息修正的时变转移概率自适应交互式多模型跟踪算法。算法定义了一种新的修正因子,利用后验信息对概率转移矩阵进行实时修正,提高匹配模型的概率,减小非匹配模型的影响,使得系统模型能够及时、准确地切换到匹配模型。蒙特卡洛仿真实验表明,该自适应交互式多模型算法能够应用于水下目标跟踪中,相比传统交互式多模型算法,模型匹配度更高,滤波效果也更好。     

铁芯托卡马克中二维和三维极向磁场计算模型的对比分析

为方便计算托卡马克磁场分布,建立了一些二维解析铁芯模型。由于前提假设的不同而给磁场分布的计算带来了不同的边界条件,因而得到的磁场分布计算结果与实际情况有所偏差。为了获得铁芯托卡马克的极向磁场三维分布,建立了带铁芯的极向磁场线圈三维数值模型,计算铁芯托卡马克的三维磁场,与不同铁芯模型的磁场计算结果进行比较,并且研究铁芯托卡马克的三维磁场的极向分量在环向上的不对称性。     

Laser heating in the diamond cell: techniques and applications

Very high temperatures (>2000 K) at megabar presures (∼100 GPa) can be achieved by classic shock compression experiments and in laser-heated diamond cells, which have evolved to a formidable tool for accurately characterizing material properties at controlled pressure and temperature conditions of the Earth's lower mantle and core. The diamond cell has great potential for future development and broad application but due to the small sample dimensions there are strict experimental requirements for obtaining reliable data, some of which are described in the present paper. Results for measuring melting temperatures of iron at very high pressure, which constrain the temperature in the Earth's centre, are reviewed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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For simplifying the calculation the magnetic distribution on tokamak, some two-dimensional analytic models including the effect of the iron core were established, such as the infinite long iron core model and the spool model. The assumptions of these two-dimensional analytic models lead to different results with the actual magnetic field due to the distinctive boundary condition. In order to accurately calculate the three-dimensional magnetic field distribution in the tokamak with iron core, a three-dimensional numerical finite element model was established based on J-TEXT tokamak. In two conditions, where the total toroidal current is nonzero or zero respectively, more comparison were carried out between the derived results of two-dimensional models and the results at different toroidal positions in three-dimensional models. Furthermore, the toroidal asymmetry of the magnetic field distribution of the three-dimensional model of tokamak with iron core was investigated. The results indicate that the three-dimensional construction of iron core causes the toroidal asymmetric poloidal magnetic field and the difference between the two- and three-dimensional models in the condition with total current of nonzero. However, in the condition with total current of zero, the intensity of toroidal asymmetric is reduced and the difference between the two- and three-dimensional models is smaller.     

Pure iron compressed and heated to extreme conditions

The results of a first-principles study supported by the temperature-quenched laser-heated diamond anvil-cell experiments on the high-pressure high-temperature structural behavior of pure iron are reported. We show that in contrast to the widely accepted picture, the face-centered cubic (fcc) phase becomes as stable as the hexagonal-close-packed (hcp) phase at pressures around 300-360 GPa and temperatures around 5000-6000 K. Our temperature-quenched experiments indicate that the fcc phase of iron can exist in the pressure-temperature region above 160 GPa and 3700 K, respectively. This, in particular, means that the actual structure of the Earth's core may be a complex phase with a large number of stacking faults.     

Techniques for measuring the elastic wave velocities of melts and partial molten systems under high pressure conditions

The Earth's deep interior is accessible only by indirect methods, first and foremost seismological studies. The interpretation of these seismic data and the corresponding numerical modelling require measurements of the elastic properties of representative Earth materials under experimental simulated in situ pressure-temperature conditions. Various experimental techniques for velocity measurements under crustal and mantle conditions and the results are described.     

线状聚焦激光与金属作用的离子速度分布

本文报道了利用法拉第筒飞行时间法进行的离子测量.通过比较高功率激光线状聚焦和点状聚焦在平面金属靶上产生的离子发射特性,总结出离子速度分布和激光辐照密度的定标规律,以及离子速度分布的空间各向异性.实验是在上海光机所钕玻璃激光装置上进行的,激光束的波长为1.06μm,脉宽为300ps,辐照密度为10~(12)～10~(14)W/cm~2.     

Measurement of noise associated with model transformer cores

The performance of a transformer core may be considered in terms of power loss and by the noise generated by the core, both of which should be minimised. This paper discusses the setting up of a suitable system for evaluation of noise in a large model transformer core (500 kV A) and issues associated with noise measurement. The equivalent continuous sound pressure level (LAeq) was used as a measure of the A-weighted sound level and measurements were made in the range 16 Hz–25 kHz for various step lap core configurations. The selection of optimum sound insulation materials between core and ground support and for enclosing the transformer is essential for minimisation of background noise. Core clamping pressure must be optimised in order to minimise noise. The use of two laminations per layer instead of one leads to an increase in noise arising from the core. Provided care is taken in building the core, good reproducibility of results can be obtained for analysis.