Copolyesters Containing Terephthalic and Bio‐Based Furanic Units by Melt‐Polycondensation

5,5′‐Isopropylidene‐bis(ethyl 2‐furoate), a monomer prepared from bio‐based ethyl 2‐furoate, was reacted with dimethyl terephthalate and ethan‐1,2‐diol (ED) by melt polycondensation in order to obtain copolyesters containing both terephthalate and furoate units. The conventional two‐step method involving (i) the formation of a hydroxyethyl‐terminated oligomer by reaction of starting diester mixture with excess ED and (ii) a polycondensation step with elimination of ED was used to obtain high molar mass copolyesters. Copolymers of various compositions were synthesized and characterized by ¹H NMR, DSC, and TGA. For all compositions, the degree of randomness, determined by ¹H NMR, was close to 1, reflecting a random distribution of terephthalic and furanic ester units in polymer chains. The resulting materials are amorphous polymers (T_g = 70–80 °C) with good thermal stability.

     

Modelling skin pelvic landmark coordinates into corresponding internal bone for wheelchair users

The purpose of this study was to investigate the relationships, by linear regression, between internal and external pelvic landmarks identified by two techniques: manual digitization or skin markers. It was hypothesized that the body mass index or the skinfold thickness are significant variables in these relationships. The internal pelvic landmarks were obtained with a stereoradiographic method. Results showed that the external coordinates are generally statistically different from the internal ones; manual digitization of the landmark reduces the soft tissue artifacts compared to the use of skin markers. Different regression models were obtained according to the external acquisition method. Body mass index or skinfold thickness was generally included as a significant variable in models along the direction of the soft tissue thickness: postero-anterior direction for the anterior-superior iliac spine, medio-lateral direction for the apex of the iliac crests. With the use of skin markers, models obtained for a specific internal landmark coordinate include generally many variables, such as the other two coordinates of the landmark, body mass index, or skinfold measurements. This study presented preliminary results on the relationships between internal and external pelvic landmark coordinates. More research is needed before the full relationships are understood and adequate models are developed.     

Résistance à la traction et module d’élasticité des bétons calcaires: application à des mélanges tunisiens

This article presents the results of the tensile strength and the elastic modulus obtained between one day and a year on a vast set of mortars, micro-concretes and concretes carried out with limestone aggregates out coming from five Tunisian careers. For the tensile strength, the results were compared with the compression strength measured on the same mixtures. The obtained evolutions do not coincide with the power-laws generally adopted for the predicted tensile strength. An approach, where the law parameters (coefficient, exponent) can be adjusted, gives better results. The resort to another more precise model, described by a hyperbolic law, makes it possible to provide a physical interpretation to the tensile strength behavior of the hydraulic mixtures. Its parameters are linked to the aggregate strength and cohesion in the paste-aggregate transition zone. For the elastic modulus, the use of a model taking into account the properties of the granular phase and the binding phase made it possible to reach the elastic modulus of the aggregates.

Real-time sliding-mode observer and control of an induction motor

This paper deals with the control and observation of an induction motor using a sliding-mode technique. The authors' aim is to regulate the speed and the square of the rotor flux magnitude to specified references. Assuming that all the states are measured, sliding surfaces are proposed within a sliding-mode control framework. Then, the stator voltages are derived such that the sliding surfaces are asymptotically attractive since, in practice, the rotor fluxes are not usually measurable, a sliding-mode observer is derived to estimate the rotor fluxes. Furthermore, it is shown that their observer is robust against modeling uncertainties and measurement noise. To illustrate their purpose, they present experimental results for a 0.37-kW induction motor obtained on a digital-signal-processor-based system (TMS 320C31/40 MHz). The experimental results show that the proposed control system is robust against rotor resistance variations     

Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 2. Mine Waste Geochemical Behavior

Weathering and humidity cell tests were used to predict the potential for acid mine drainage (AMD) and to estimate the mineral reaction rates and depletion of fine and coarse tailings from the abandoned Kettara mine, Morocco. The geochemistry of the fine and coarse mine wastes was similar and, as expected by static tests, the wastes produced significant amounts of AMD. The sulfate production rate of both fine and coarse tailings was very high (2,000–8,000 and 2,400–560 mg SO₄/kg/week, respectively) during the first weeks of kinetics tests. After 9 weeks, sulfate release became low, ranging between 600 and 78 mg SO₄/kg/week for fine tailings and 500–120 mg SO₄/kg/week for coarse tailings. Effluent water samples had low pH (2.9–4.2) and elevated concentrations of acidity, sulfate, iron, copper, and zinc. Most or all of the dissolved K, Na, Al, Mg, and Si in the AMD result from the acidic dissolution of silicates (chlorite, talc, muscovite, and albite). Fine tailings produce much higher concentrations of acidity and sulfate than coarse tailings. However, due to greater transport of oxygen and water within the coarse waste, coarse tailings could be of greater environmental significance than fine tailings. The coarse waste continued to release acid after 378 days of leaching, whereas the fine tailings naturally passivates. These laboratory results agree with field observations; the upper profile of the coarse waste rock dam is highly oxidized (75 cm) whereas oxidation in the fine tailings does not extend more than 5–15 cm beneath the surface. A comparison between weathering and humidity cell tests indicated that the general trend of dissolution of metals was essentially similar for both methods. However, sulfate depletion rates were higher for the weathering cell tests. These tests indicate that the Kettara tailings piles and dam will continue to release acid for a long time unless remedial action is taken.     

Large signal design method of distributed power amplifiers applied to a 2–18-GHz GaAs chip exhibiting high power density performances

A suitable large signal design method of distributed power amplifiers, based on the optimum FET load requirement for high power operation, is proposed in this article. The gate and drain line characteristic admittances are determined, providing both the initial values and right directions for an optimum design. To validate the proposed design method, a FET amplifier demonstrator with a gate periphery of 1.2 mm has been manufactured at the Texas Instruments foundry. The MMIC distributed amplifier demonstrated an improved power density performance of 340 mW/mm over the 2–18-GHz frequency band associated with a minimum of 13% power-added efficiency and 24% drain efficiency at 1-dB compression in CW operation. © 1996 John Wiley & Sons, Inc.     

Ab Initio Calculations of the Atomic and Electronic Structure of β-Silicon Nitride

The atomic and electronic structure of the β-silicon nitride (β-Si₃N₄) crystal have been determined using the ab initio pseudopotential method based on the density functional theory. We have obtained the stable lattice parameters and the stable positions of 14 atoms in the unit cell for the structure P 6₃/ m for the first time. The electronic structure and the charge distribution indicate that the Si–N bond has both ionic and covalent characters. The band structure is in good agreement with the other first-principles results and consistent with the experiments.     

Electrosynthéses de composés carbonylés catalysées par des complexes du nickel: Activation du monoxyde de carbone ou de composés acylés

The complex Ni⁰bpy generated from Nibpy²⁺ is an efficient catalyst for the electrochemical conversion of organic halides into symmetrical or unsymmetrical ketones. Carbonylation reactions are achieved from either carbon monoxide or carbonyl metal complexes or carbon dioxide. Acylation reactions are obtained by coupling halocarbons with acid chlorides or anhydides. All syntheses are conducted at normal pressure and room temperature at a constant current intensity in an individed cell fitted with a sacrificial anode.     

H–Si Functionalized Silicon Surfaces and Powders as Photoinitiators of Polymerization

Nowadays, silicon represents the most important material used for microelectronic applications. In this paper, both H–Si (111) surfaces and H–Si powders are used to initiate a multifunctional acrylate photopolymerization. The polymers formed are characterized by IR spectroscopy. This should be the way to create either an acrylate polymer coating on a Si wafer or a polymer film containing covalently linked silicon particles.

     

The effect of resultant force at the pushrim on shoulder kinetics during manual wheelchair propulsion: a simulation study

The aim of this study was to determine, by simulation on real data, the effect of modifying the direction or effectiveness of a given force amplitude on the load sustained by the shoulder estimated by joint forces and moments. Kinematics and kinetics data were recorded on 14 manual wheelchair users 68.2+/-5.2 years for 10 s at sub-maximal speed (0.96-1.01 m/s). The simulation consisted in modifying force effectiveness at the pushrim while maintaining the same initial force amplitude. Shoulder kinetics were computed for simulated resultant forces from radial to tangent directions and also for initial force effectiveness. The results show that as the force was simulated tangent to the wheel, there was a significant increase in the average proximal and anterior shoulder joint forces. Also, significant increases in average internal rotation, flexion in the sagittal and horizontal plane moments were reported. Higher shoulder kinetics could accelerate the onset of fatigue and increase the risk of injury. A single-case analysis revealed an improvement window for force effectiveness ( approximately 10%) in which shoulder kinetics were not substantially increased. Our results provide useful information on what would happen to shoulder kinetics if we were able to teach manual wheelchair users to modify their force pattern at the pushrim. The results suggest that for an elderly population, it is not wise to aim at producing a mechanically optimal resultant force at the pushrim (i.e., tangent). Smaller increases of the initial force effectiveness would be preferable.