Synthesis,Thermal Properties and Electrical Conductivity of Na-Sialate Geopolymer

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.

     

Effect of Titania Additive on Structural and Mechanical Properties of Alumina–Fluorapatite Composites

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.     

A real-time distributed algorithm for an aircraft longitudinal optimal autopilot

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.     

Virtual-water content of agricultural production and food trade balance of Tunisia

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.     

Improving the transport performance in delay tolerant networks by random linear network coding and global acknowledgments

We propose and study a new set of enhancement features to improve the performance of reliable transport in Delay Tolerant Networks (DTNs) consisting of both unicast and multicast flows. The improvement in reliability is brought in by a novel Global Selective ACKnowledgment (G-SACK) scheme and random linear network coding. The motivation for using network coding and G-SACKs comes from the observation that one should take the maximum advantage of the contact opportunities which occur quite infrequently in DTNs. Network coding and G-SACKs perform “mixing” of packet and acknowledgment information, respectively, at the contact opportunities and essentially solve the randomness and finite capacity limitations of DTNs. In contrast to earlier work on network coding in DTNs, we observe and explain the gains due to network coding even under an inter-session setting. Our results from extensive simulations of appropriately chosen “minimal” topologies quantify the gains due to each enhancement feature. We show that substantial gains can be achieved by our proposed enhancements that are very simple to implement.     

Development of a molecular method for the rapid discrimination of red seaweeds used for agar production

In Tunisia, agar is produced from the species Gracilaria verrucosa. Other species, such as Gracilariopsis sp. which have a very similar morphology to Gracilaria, can be harvested in a mixture with Gracilaria with the result that the quantity and quality of agar extracted is different than initially expected. In this study, we tested the use of ITS sequences from 5.8s rDNA for species discrimination between G. verrucosa and Gracilariopsis harvested from two production sites (Bizerte and Tunis Lagoons). The amplification of genomic ITS sequences and sequencing shows that the samples collected from Tunis can be identified by PCR fragment of 1124 bp whereas the samples from Bizerte are characterized by the presence of two fragments of 1124 and 983 bp, respectively. A BLAST investigation in Genbank shows that the sequence similarities between the fragment of 1124 pb and G. verrucosa was 85% and between the fragment of 983 pb and Gracilariopsis sp. Plymoutn was 94%. Gracilariopsis sp. possesses a PCR fragment with a length of 983 bp, which discriminates it from G. verrucosa when these two species are collected in a mixture as in the lagoon of Bizerte. The RFLP method after EcoRI application provides a characteristic pattern for G. verrucosa, which is composed of two main fragments with respective sizes of 823 and 301 bp whereas amplified DNA of Gracilariopsis was uncut under these conditions.     

Sintering and mechanical properties of tricalcium phosphate–fluorapatite composites

Tricalcium phosphate and synthesized fluorapatite powder were mixed in order to elaborate biphasic ceramics composites. The effect of fluorapatite addition on the densification and the mechanical properties of tricalcium phosphate were measured with the change in composition and microstructure of the bioceramic. The Brazilian test was used to measure the mechanical resistance of the tricalcium phosphate–26.52 wt% fluorapatite composites. The densification and rupture strength increase versus sintering temperature. The composites have a good sinterability and rupture strength in temperature ranging between 1300 and 1400 °C. Thus, the densification ultimate was obtained at 1350 °C and the mechanical resistance optimum reached 9.6 MPa at 1400 °C. Above 1400 °C, the densification and the mechanical properties were hindered by the allotropic transformation of tricalcium phosphate, grain growth and the formation of both intragranular porosity and many cracks. The ³¹P magic angle spinning nuclear magnetic resonance analysis of composites reveals the presence of tetrahedral P sites.     

A shape analysis framework for neuromorphometry

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.     

Improved modelling and simulation of once-through and reverse multi-stage flash desalination configurations

An improved model for multistage flash (MSF) structures is developed and used to assess the performance of a novel MSF configuration, termed as MSF reversal (RV-MSF) and consisting of reversing the brine circulation stream. The improved model determines the temperature distribution within the stages using heat balance equations while the simplified one, which is commonly used, is based on pre-specified, constant, and equal temperature distribution throughout the stages. The performance of the RV-MSF is investigated and compared with conventional MSF once-through (OT-MSF) with and without brine mixing using simplified and full models. It is found that the simplified model overestimates the required heat transfer specific area for both MSF configurations. Moreover, it underestimates the cooling water and energy requirements for the reversal configuration. Hence, the simplified model may be good for quick analysis but leads to inaccurate design specifications and economic analysis. When brine mixing is utilized, the simplified model still provides erroneous estimates of the heat transfer area for both MSF configurations. Nevertheless, for OT-MSF structure, the simple model can provide comparable predictions with that of the improved model in terms of recovery ratio, performance ratio, and specific energy consumption. For the RV-MSF structure, a mismatch in the two model predictions of surface area, cooling water, and energy requirements is observed. Furthermore, the temperature drop in the cooling system for RV-MSF has a significant influence on the specific surface area and cooling water requirements especially at low values. The different behaviour of the simplified model between the OT-MSF and RV-MSF configurations is attributed to the fact that brine recycling does not affect the feed temperature or the temperature distribution in the system.