Electroluminescence from polyolefins subjected to a homogeneous acfield

Electroluminescence associated with ac voltage aging of polyolefins has been reported under divergent field configuration. Neither the mechanism of excitation nor the nature of luminescent centers are well understood in these polymers. Homegeneous field experiments are desirable since they are easier to run and more light can be collected, thereby allowing spectral analysis to be made. We describe a suitable experimental configuration for electroluminescence detection from polyethylene and polypropylene films submitted to a homogeneous field and report on surface roughness, field, and time dependence of the light emission. Field and time dependence of the light spectra are also investigated. It is shown that the light originates from the polymer bulk and is controlled by the surface roughness of the films. The light spectrum appears to consist of two components which exhibit different behavior as a function of field and stressing time. A tentative explanation is given in terms of competition between two different excitation processes     

Treatment of arsenic-containing solutions using chitosan derivatives: uptake mechanism and sorption performances

Modified chitosan gel beads, which had been prepared by the molybdate adsorption and coagulation (in the presence of molybdate) methods, were tested for As(III) and As(V) removal from dilute solutions (in the range 5-20 mg As L-1). The sorbent is very efficient at removing As(V) from acid solutions (optimum pH close to pH 2-3), whereas the sorption capacities are significantly lower for As(III) uptake (230 mg As(V) g-1 Mo, 70 mg As(III) g-1 Mo, respectively). Since the sorption proceeds in acidic solutions with a partial release of molybdate and with residual concentrations (ca. 500 micrograms As L-1) above the regulations for drinking water, the process appears to be directed to the treatment of industrial effluents or as a pre-concentration process. The mechanism of As(V) sorption is related to the ability of molybdate ions to complex As(V) ions in acid solutions. The uptake mechanism was confirmed by XPS analysis and desorption studies. In the case of As(III) sorption the mechanism of uptake is not identified since no complex has been cited in the literature regarding As(III) binding to Mo (VI), which was also identified by XPS analysis as the sorption site. As(V) sorption is not influenced by the presence of co-ions, with the exception of phosphate anions at low concentration, and silicate at high relative concentration. Arsenic desorption can be performed using phosphoric acid solutions.     

Ultrafast compound imaging for 2-D motion vector estimation: application to transient elastography

This paper describes a new technique for two-dimensional (2-D) imaging of the motion vector at a very high frame rate with ultrasound. Its potential is experimentally demonstrated for transient elastography. But, beyond this application, it also could be promising for color flow and reflectivity imaging. To date, only axial displacements induced in human tissues by low-frequency vibrators were measured during transient elastography. The proposed technique allows us to follow both axial and lateral displacements during the shear wave propagation and thus should improve Young's modulus image reconstruction. The process is a combination of several ideas well-known in ultrasonic imaging: ultra-fast imaging, multisynthetic aperture beamforming, 1-D speckle tracking, and compound imaging. Classical beamforming in the transmit mode is replaced here by a single plane wave insonification increasing the frame rate by at least a factor of 128. The beamforming is achieved only in the receive mode on two independent subapertures. Comparison of successive frames by a classical 1-D speckle tracking algorithm allows estimation of displacements along two different directions linked to the subapertures beams. The variance of the estimates is finally improved by tilting the emitting plane wave at each insonification, thus allowing reception of successive decorrelated speckle patterns.     

Multiple-scattering lidar retrieval method: tests on Monte Carlo simulations and comparisons with in situ measurements

A multiple-field-of-view (MFOV) lidar measurement and solution technique has been developed to exploit the retrievable particle extinction and size information contained in the multiple-scattering contributions to aerosol lidar returns. We describe the proposed solution algorithm. The primary retrieved parameters are the extinction coefficient at the lidar wavelength and the effective particle diameter from which secondary products such as the extinction at other wavelengths and the liquid-water content (LWC) of liquid-phase clouds can be derived. The solutions are compared with true values in a series of Monte Carlo simulations and with in-cloud measurements. Good agreement is obtained for the simulations. For the field experiment, the retrieved effective droplet diameter and LWC for the available seven cases studied are on average 15% and 35% (worst case) smaller than the measured data, respectively. In the latter case, the analysis shows that the differences cannot be attributed solely to lidar inversion errors. Despite the limited penetration depth (150-300 m) of the lidar pulses, the results of the studied cases indicate that the retrieved lidar solutions remain statistically representative of measurements performed over the full cloud extent. Long-term MFOV lidar monitoring could thus become a practical and economical option for cloud statistical studies but more experimentation on more varied cloud conditions, especially for LWC, is still needed.     

High-resolution full-field optical coherence tomography with a Linnik microscope

We describe an original microscope for high-resolution optical coherence tomography applications. Our system is based on a Linnik interference microscope with high-numerical-aperture objectives. Lock-in detection of the interference signal is achieved in parallel on a CCD by use of a photoelastic birefringence modulator and full-field stroboscopic illumination with an infrared LED. Transverse cross-section (en-face, or XY) images can be obtained in real time with better than 1-microm axial (Z) resolution and 0.5-microm transverse (XY) resolution. A sensitivity of approximately 80 dB is reached at a 1-image/s acquisition rate, which allows tomography in scattering media such as biological tissues.     

From LDPE to XLPE: investigating the change of electrical properties. Part I. space charge, conduction and lifetime

This paper discusses the results of space charge, conduction current and electrical lifetime measurements performed on widely-used materials for electrical insulation, that is, low density polyethylene (LDPE) and cross-linked polyethylene (XLPE). Space charge accumulation profiles were compared in LDPE, low density polyethylene plus antioxidant (LDPE+AO) and XLPE, with consideration of thermal treatment effects in LDPE and XLPE. Significant variation (decrease) of accumulated space charge and apparent mobility, as well as slight decrease of conduction current, can be seen going from LDPE to LDPE+AO and XLPE, which may be associated with formation of deeper trap levels (or an increase of their density). On the contrary, electrical life under AC voltage does not show significant differences from LDPE to XLPE. This latter result underlines that life under high AC electrical stress is mostly determined by defects (weak points) rather than material characteristics associated with charge injection and transport.     

“To be recorded” analysis in reverse-mode automatic differentiation

The automatic generation of adjoints of mathematical models that are implemented as computer programs is receiving increased attention in the scientific and engineering communities. Reverse-mode automatic differentiation is of particular interest for large-scale optimization problems. It allows the computation of gradients at a small constant multiple of the cost for evaluating the objective function itself, independent of the number of input parameters. Source-to-source transformation tools apply simple differentiation rules to generate adjoint codes based on the adjoint version of every statement. In order to guarantee correctness, certain values that are computed and overwritten in the original program must be made available in the adjoint program. For their determination we introduce a static data-flow analysis called “to be recorded” analysis. Possible overestimation of this set must be kept minimal to get efficient adjoint codes. This efficiency is essential for the applicability of source-to-source transformation tools to real-world applications.     

Possibilistic uncertainty handling for answer set programming

In this work, we introduce a new framework able to deal with a reasoning that is at the same time non monotonic and uncertain. In order to take into account a certainty level associated to each piece of knowledge, we use possibility theory to extend the non monotonic semantics of stable models for logic programs with default negation. By means of a possibility distribution we define a clear semantics of such programs by introducing what is a possibilistic stable model. We also propose a syntactic process based on a fix-point operator to compute these particular models representing the deductions of the program and their certainty. Then, we show how this introduction of a certainty level on each rule of a program can be used in order to restore its consistency in case of the program has no model at all. Furthermore, we explain how we can compute possibilistic stable models by using available softwares for Answer Set Programming and we describe the main lines of the system that we have developed to achieve this goal.     

Shifting the Design Philosophy of Spoken Natural Language Dialogue: From Invisible to Transparent Systems

Spoken natural language interfaces are characterized by a high degree of opacity. This characteristic leads many new users to converse with the machine on the basis of an inaccurate image of the system, which results in numerous dysfunctions and failures in the human-machine spoken dialogue. The application of the principle of transparency aims to correct this problem by enabling the system to make it “visible” (it can, so to speak, be seen “from the inside”). This report presents a study that compares two versions of the same spoken dialogue system that differ only by some of their dialogue strategies. In particular, one version of this system starts the dialogue with a recall of the main system's function followed by an open-ended prompt, while the other version only produces an open-ended prompt (What can I do for you?). Moreover, one version of this system makes explicit some possible actions in specific dialogue contexts, while the other version leaves these possible actions implicit. These strategies were compared by having a group of 28 users test them in real conditions of use. The results highlight that transparency strategies can help users in formulating simple and acceptable requests, satisfying their goals more easily while provoking less recognition and comprehension errors.