NMR Dynamics with Large Dipolar Interactions in Liquid Helium-3 Films

In a growing number of experimental situations NMR dynamics is found to be deeply affected by strong dipolar couplings. We have developed models based on approximations of dipolar interactions and applied them to a thin film situation. Weakly damped, robust eigenmodes are found as solutions of the simplified Bloch equations in 2D systems. 3D numerical simulations also reveal the existence of spectral clustering (collapse of NMR spectra into sets of narrow lines) at low tip angles. Systematic pulsed NMR experiments previously performed in thin films of laser-polarised helium-3 have been re-analysed. The resulting spectra consist of several well-resolved lines with the expected frequencies. The effect of applied field gradients on the characteristics of the modes has been studied. The results demonstrate the robustness of spectral clustering in highly polarised thin films.     

Model-based decoupling control method for dual-drive gantry stages: A case study with experimental validations

Industrial motion control of dual-drive gantry stages is usually performed by position controllers acting independently on each actuator. This approach neglects the unbalance and the mechanical coupling between actuators, leading to poor positioning performances. To overcome this drawback, a model-based decoupling control is proposed. Initially, a dynamic model of the gantry stage is proposed. Once identified and validated, such model is written in terms of a decoupling basis. Then, by model inversion, a feedback?feedforward decoupling control is presented. Experimental results show that, in comparison to the independent axis control approach, the proposed solution leads to improved motion control.     

Analyse de signaux à composantes quasi sinusoïdales

Estimation of spectral lines of a finite duration observation can be viewed as an application for linear prediction models. The authors discuss the feasibility in describing a quasisinusoidal signal with noise, either as a sum of spectral lines in additive noise or as anarma noise with few poles near the unit circle. To both of these models, correspond two problems of estimation which aims are two different characterizations of the observation. In these two cases, is given the form of the maximum likelihood estimator, and the consequences of the use of one estimator instead of the other are examined.     

Application of Computational Mechanics to the Analysis of Seismic Time-series via Numerical Optimisation

We apply the Computational Mechanics approach to the analysis of time-series representative of geophysical measurements. The algorithm employed is the Causal-State Splitting Reconstruction (CSSR) algorithm. We address a number of data pre-processing steps which are necessary when analysing complex time-series and specific to symbolised time-series analysis tools such as CSSR. We cast the choice of input parameters for the CSSR algorithm and time-series symbolisation, into an optimisation problem, with the aim of maximising the predictability of events of specific interest. Our approach is problem independent and can be easily extended to other applications. This research highlights the challenges to be overcome when analysing complex time-series using the Computational Mechanics approach. We also discuss further developments necessary to extend the approach to real data applications.

NMR Instabilities in Spin-Polarised Liquids: 3He, 3He-4He Mixtures and 129Xe

We report on pulsed NMR experiments performed in low magnetic field (1–2 mT) on various spin-polarised liquid samples obtained from optically pumped gas. Thin films of pure ³ He and bulk samples of dilute ³ He- ⁴ He mixtures are prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) within a narrow U-shaped tube with polarisations of order 6%. In all these highly magnetised anisotropic samples, dipolar magnetic couplings have a strong influence on spin dynamics. Spectral clustering is obtained for small tipping angles and usually results in precession lifetimes of several seconds. Much shorter precession lifetimes (down to 10 ms) are obtained for larger tipping angles in highly polarised samples. In all systems, 90° tipping pulses are followed by precession signals which abruptly decay after a variable delay, as long as 1 s for moderate polarisations in xenon. Use of different liquids permits us to vary the effects of dipolar couplings, quantum exchange and spin diffusion, and to probe their influence on the observed phenomena.     

The use of nanocarbons as chemical filters for the selective detection of nitrogen dioxide and ozone

The gas filtering abilities of different nanocarbon materials such as nanocones/nanodiscs, and nanofibres, either as-prepared or modified by physical (annealing, grinding) or chemical (fluorination) treatment are reported. The aptitude to filter nitrogen dioxide and ozone, two of the most significant gaseous pollutants of the atmosphere, have been correlated to both the BET specific surface area studied by N2 adsorption at 77 K, and the presence of chemical functional groups at the surface. Valuable information regarding the mechanisms of gas-nanocarbon interaction has been obtained, in terms of chemisorption and physisorption. A prototype microsystem is proposed for the selective measurement of nitrogen dioxide and ozone concentration by means of organic semiconductor gas sensors.     

Impacts of temperature and surface finish upon steam oxidation of austenitic steel TP347HFG

Abstract

The effects of the steam oxidation process on an austenitic steel (TP347HFG) exposed under isothermal conditions between 600 and 800 °C for up to 2500 h have been investigated. Samples with both as-received and ground surfaces have been exposed and the impact of surface finish on the oxidation process analysed using scanning electron microscopy with energy dispersive X-ray analysis. Exfoliated oxide flakes have also been examined to characterise their microstructures on fractured sections as well as external and spalled surfaces. Microscopic analyses demonstrated that ground surfaces possess better steam oxidation resistance than as-received surfaces due to their ability to form a more protective chromium-rich layer. The formation of regions of thicker multi-layered oxides was noted on both types of surface finish, covering large areas on as-received surfaces and only nodules on ground surfaces (spreading with increasing exposure temperature and time).     

碳纤维缠绕叠合梁冲击破坏效果模型参数的监测

借助振动试验,对两种类型缠绕叠合材料(重、轻)的冲击韧性进行研究。由于尚缺少有效的缠绕叠合材料冲击试验相关文献,本文采用简单的对称冲击。对两种破坏水平(BVID和表面不明显破坏)模型参数的变化进行分析,振动试验结果表明:与重缠绕试件相比,轻缠绕试件具有较好的阻尼,对冲击破坏十分敏感。而且,与刚度变化相比,阻尼与对破坏更敏感。因此,可推断出,阻尼可用以替代自然频率作为结构健康监测破坏的指标。     

Visualizing gas evolution on graphite and oxygen-evolving anodes

Recent progress in material science might soon allow the replacement of the consumable carbon anode by an inert material. This is likely to induce changes in the overall process, and particularly in the gas evolution. Video recordings of oxygen-evolving anodes (SnO₂, Cu, Cu-Ni) and carbon anodes were performed in laboratory electrolysis cells, using direct observation from above, a see-through cell, and radiography techniques. The gas behavior was very different between the two kinds of anodes, and probably linked to the wettability of the material by the electrolyte.