This article is devoted to the assessment of Tunisian agricultural production and food trade balance water-equivalent. A linear regression model relating annual rainfall to crop yields is developed to estimate the agricultural production water-equivalent. Its implementation is based on national data for crop and animal production, leading to food demand water-equivalent quantification. Results highlight the relationship between agricultural and water policies and provide a picture of food security in the country in relation to local agricultural production, and to virtual water fluxes related to foodstuffs trade balance. 相似文献
A frequency agile extended cavity diode laser using an integrated Bragg reflector in a Ti : Fe : LiNbO/sub 3/ waveguide is developed and characterized. The laser emits up to 7 mW in the 1.5-/spl mu/m telecommunication window. The emission spectrum exhibits a 18-kHz linewidth, >40-dB sidemode suppression ratio, and a wavelength stability of /spl plusmn/1 pm over hours. Very fast mode hop-free frequency tuning is achieved through the electrooptic effect, with a tuning slope of 55.5 MHz/V. 相似文献
Mechanical properties of alumina-fluorapatite composites with different titania additive amounts (0, 0.5, 1, 1.4, 2, 3, 4 and 5 wt%) have been investigated between 1200 and 1600℃. The optimum values of densification and mechanical properties of composites have been reached with 1.4 wt% of titania after the sintering process at 1500℃ for 1 h. Thus, the rupture strength of alumina-26.52 wt% Fap-1.4 wt% TiO2 reaches 75 MPa. At higher temperature and beyond 1.4 wt% TiO2 ,the densification and mechanical properties were hindered by the formation of both intergranular porosity and secondary phase. X-ray diffraction (XRD) analysis of alumina-Fap-TiO2 composites shows the formation of aluminium titanate (Al2O3-TiO2:Al2TiO5 ). The 27Al magic angle scanning nuclear magnetic resonance analysis of Al2O3-Fap-TiO2 composites reveals the presence of octahedral and pentahedral aluminium and novel environment relative to tetrahedral aluminium sites. 相似文献
This work aims to study the thermal behavior of basic-geopolymers derived from metakaolin (clay). The geopolymers were characterized by different techniques: thermal analysis (DTA, TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy. Some physicochemical properties of the products were also determined: the phases obtained after geopolymer heat treatment and their electrical properties. The results obtained after drying and heat treatment showed that the products kept their initial shapes, but revealed variable colors depending on the temperatures at which they were treated. The products obtained are amorphous between 300 up to 600 °C with peaks relating to the presence of nanocrystallites of muscovites and zeolite, thus at 900 °C it is quite amorphous but only contains nanocrystallites of muscovites. From the temperature of 950 °C, we notice that the geopolymer has been transformed into a crystalline compound predominated by the Nepheline (NaAlSiO4) with the presence of a crystalline phase by minor peaks of Muscovite, this crystalline character has been increased at 1100 °C to obtain a whole phase crystalline of a Nepheline. The treatment of this geopolymer for one hour at 1200 °C shows an amorphous phase again corresponding to corundum (α-Al2O3). This indicates that the dissolution of the grains by the liquid phase induces the conversion of the material structure from sialate [–Si–O–Al–O] to sialate siloxo [–Si–O–Al–O–Si–O–] and the formation of a new crystalline phase (α-Al2O3). This development of sialate to sialate-siloxo was confirmed by IR spectroscopy. As mentioned above, from 300 to 900 °C, Na-sialate geopolymer exhibits the same disorder structure of nepheline. The crystal structure of nepheline is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built by Regularly alternating tetrahedral AlO4 and SiO4. Stacking the layer’s parallel to the c axis gives a three-dimensional network containing channels occupied by Na cations. This topology favors easy movement of Na+ ions throughout the structure. For this reason, ionic migration in nepheline is widely reported. The refinement of Na-Sialate geopolymer at room temperature gives bulk high ionic conductivity of about 5 × 10?5 S cm?1 and this is due to the probable joint contribution of H+ and Na+ ions. Above 200 °C, Na+ seems to remain the only charge carrier with a low activation energy of about Ea?=?0.26 eV. At higher temperatures, the characteristic frequencies become so close that it is impossible to distinguish the contributions. A total resistance comprising both grain and grain boundaries contribution is then determined.
This paper is concerned with fast and efficient computations for the solution of the two-point boundary-value problem that arises in optimal control of an aircraft longitudinal mode. The solution proposed is implemented on a T800 transputer network using parallel C and is shown to be adequate for real-time control purposes. 相似文献
Since industry is rapidly developing, either locally or globally, manufacturers witness harder challenges due to the growing competitivity. This urges them to better consider the four factors linked to production and output: quality, quantity, cost and price, quality being of course the most important factor which constitutes their main concern. Efforts will be concentrated??in this research??on improving the quality and securing more accuracy for a machined surface in ball-end milling. Quality and precision are two essential criteria in industrial milling. However, milling errors and imperfections, due mainly to the cutting tool deflection, hinder the full achieving of these targets. Our task, all along this paper, consists in studying and realizing the simulation of the deflected cutting tool trajectory, by using the methods which are available. In a future stage, and in the frame of a deeper research, the simulation process will help to carry out the correction and the compensation of the errors resulting from the tool deflection. The corrected trajectory which is obtained by the method mirror will be sent to the machine. To achieve this goal, the next process consists??as a first step??in selecting a model of cutting forces for a ball-end mill. This allows to define??later on??the behavior of this tool, and the emergence of three methods namely the analytical model, the finite elements method, and the experimental method. It is possible to tackle the cutting forces simulation, all along the tool trajectory, while this latter is carrying out the sweeping of the part to be machined in milling and taking into consideration the cutting conditions, as well as the geography of the workpiece. A simulation of the deflected cutting tool trajectory dependent on the cutting forces has been realized. 相似文献
This paper addresses in an integrated and systematic fashion the relatively overlooked but increasingly important issue of measuring and characterizing the geometrical properties of nerve cells and structures, an area often called neuromorphology. After discussing the main motivation for such an endeavour, a comprehensive mathematical framework for characterizing neural shapes, capable of expressing variations over time, is presented and used to underline the main issues in neuromorphology. Three particularly powerful and versatile families of neuromorphological approaches, including differential measures, symmetry axes/skeletons, and complexity, are presented and their respective potentials for applications in neuroscience are identified. Examples of applications of such measures are provided based on experimental investigations related to automated dendrogram extraction, mental retardation characterization, and axon growth analysis. 相似文献