Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms

This work investigates emulsion templating to synthesize hexadecane oil/geopolymer composites. In a system with hexadecane as the internal (dispersed) phase and an alkali activated continuous phase without added surfactant, adding aluminosilicate clay particles does not increase resistance against creaming or coalescence, while adding a surfactant (L35 or CTAB) stabilizes the solid-liquid interface. Infrared studies and rheological studies of the associated geopolymerization determined that the presence of the organic phase or surfactant has no significant effect on the geopolymerization kinetics, as determined by the change in time of the Si-O-T IR stretching frequency and the rheological moduli involved during the process. The stabilization of the organic template is reminiscent of Pickering emulsion even though we employ a much greater amount of inorganic material for geopolymer formation. Although the addition of surfactant has a significant effect on the behavior of the paste, the percolation of the network remains unmodified, highlighting the fact that the phenomenon is not dependent on viscosity. Finally, rheological measurements were used to obtain the mass fractal dimension of the as-made gel network, which is able to differentiate the interfacial effect between surfactant molecules with a slightly denser interphase when a cationic surfactant is used.     

Temperature stability of a pure metakaolin based K-geopolymer: Part 2. Variations in the mesoporous network and its rehydration stability

The thermal behavior of a model MK-based K-geopolymer was investigated between room temperature and 1400°C in order to evaluate its potentiality for high-temperature applications. The purpose of our study was to monitor the behavior of a geopolymer during a temperature rise in order to better understand its variations with respect to temperature. The works from the present paper focus only changes in the porous network; it follows a first part devoted to variations in the mineral matrix. The results obtained here show that the geopolymer material preserves its porous integrity up to 800°C, while maintaining the reversibility of water exchanges corresponding to about 25 weight percent. Together with the results of part 1, the findings of this study allow us to affirm that geopolymer materials are only very little affected by temperatures up to 800°C, or even 900°C (keeping its mesoporous amorphous structure).     

Natural roller bearing fault detection by angular measurement of true instantaneous angular speed

The challenge in many production activities involving large mechanical devices like power transmissions consists in reducing the machine downtime, in managing repairs and in improving operating time. Most online monitoring systems are based on conventional vibration measurement devices for gear transmissions or bearings in mechanical components. In this paper, we propose an alternative way of bearing condition monitoring based on the instantaneous angular speed measurement. By the help of a large experimental investigation on two different applications, we prove that localized faults like pitting in bearing generate small angular speed fluctuations which are measurable with optical or magnetic encoders. We also emphasize the benefits of measuring instantaneous angular speed with the pulse timing method through an implicit angular sampling which ensures insensitivity to speed fluctuation. A wide range of operating conditions have been tested for the two applications with varying speed, load, external excitations, gear ratio, etc. The tests performed on an automotive gearbox or on actual operating vehicle wheels also establish the robustness of the proposed methodology. By the means of a conventional Fourier transform, angular frequency channels kinematically related to the fault periodicity show significant magnitude differences related to the damage severity. Sideband effects are evidently seen when the fault is located on rotating parts of the bearing due to load modulation. Additionally, slip effects are also suspected to be at the origin of enlargement of spectrum peaks in the case of double row bearings loaded in a pure radial direction.     

Factorial design of experiment (DOE) for parametric exergetic investigation of a steam methane reforming process for hydrogen production

Hydrogen is expected to play a significant role in future energy systems. The efficient production of hydrogen at a minimum cost and in an environmentally acceptable manner is crucial for the development of a hydrogen-including economy. The exergy analysis is a powerful tool to quantify sustainable development potential. An important aspect of sustainable development is minimizing irreversibility. The purpose of this study is to perform the exergy analysis of a steam methane reforming (SMR) process for hydrogen production. As a first step, an exergy analysis of an existing process is shown to be an efficient tool to critically examine the process energy use and to test for possible savings in primary energy consumption. The results of this investigation prove that the exergetic efficiency of the SMR process is 65.47%, and the majority of destroyed exergy is localized in the reformer with a 65.81% contribution to the whole process destroyed exergy. Next, an exergetic parametric study of the SMR has been carried out with a factorial design of experiment (DOE) method. The influence of the reformer operating temperature and pressure and of the steam to carbon ratio (S/C) on the process exergetic efficiency has been studied. A second-order polynomial mathematical model has been obtained through correlating the exergetic efficiencies with the reformer operating parameters. The results of this study show that the rational choice of these parameters can improve the process exergetic performance.     

RTD studies in a falling film graphite evaporator with internal spiral fins: Influence of the geometry

A hydrodynamic study of a new graphite evaporator has been performed. In this exchanger the fluid flows as a thin film on a spirally wound fin which forms a spiral staircase inside a tube. The slope θ_m of the spirally wound fin is modifiable. The residence time distribution (RTD) of the film was measured for different fin slopes θ_m and several film flowrates m_f. The flow pattern was then modelled thanks to the DTS software as a cascade of perfect mixed cells and the mean residence time τ_m was calculated. The evolution of τ_m as a function of θ_m and m_f is accurately represented by a power law relation.     

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

The effects of milling and doping NaAlH₄ with TiCl₃, TiF₃ and Ti(OBuⁿ)₄, and of cycling doped NaAlH₄ have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (900 Å for milled and 700 Å for doped, as compared to 1600 Å for unmilled and undoped NaAlH₄) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by 15 cm⁻¹ to higher frequencies for the Al–H asymmetric stretching mode ν₃, and by 20 cm⁻¹ to lower frequencies for one part of the H–Al–H asymmetric bending mode ν₄, thus suggesting structural changes in the local environment of the [AlH₄]⁻ units. The broad ν₃ bands become sharpened which suggests a more homogeneous local environment of the [AlH₄]⁻ units, and there appears a new vibration at 710 cm⁻¹. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF₃: downward shift by 10 cm⁻¹) and a general broadening of the ν₃ mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation.     

Evaluation architecturale VLSI de systèmes standard de codage ďimages

Image coding systems currently undergoing standardisation within ISO and CCITT are the final outcome of a process of incremental improvements to classical hybrid (transform-predictive) algorithms. The task of VLSI architecture synthesis for these complete systems is made somewhat awkward due to the unstructured, irregular and non-modular nature of these algorithms. An ad hoc methodology for pruning the architectural search space, directed by the goal of minimizing the overall internal memory, leads to a strongly control-flow solution, using a pipeline scheme more efficient than with the original signal-flow graph. A generic image coding processor using a parallel programmable architecture is another solution. It may be inferred that second generation image coding techniques should be designed with massive fine-grain parallelism in view, if they are to take advantage of the full potential of dedicated VLSI implementations.     

Cloning and nucleotide sequences of the topoisomerase IV parC and parE genes of Mycoplasma hominis

The topoisomerase IV parC and parE genes from the wall-less organism Mycoplasma hominis PG21 were cloned and sequenced. The coupled genes are located far from the DNA gyrase genes gyrA and gyrB. They encode proteins of 639 and 866 amino acids, respectively. As expected, the encoded ParE and ParC proteins exhibit higher homologies with the topoisomerase IV subunits of the gram-positive bacteria Staphylococcus aureus and Streptococcus pneumoniae than with their Escherichia coli counterparts. The conserved regions include the Tyr residue of the active site and the region involved in quinolone resistance (quinolone resistance-determining region [QRDR]) in ParC and the ATP-binding site and the QRDR in ParE.     

A comparative structural study of wet and dried ettringite

Two different techniques were used to compare structural characteristics of “wet” ettringite (stored in the synthesis mother liquid) and “dried” ettringite (dried to 35% relative humidity over saturated CaCl₂ solution). Lattice parameters and the water content in the channel region of the structure (site occupancy factor of the water molecule not bonded to cations) as well as microstructure parameters (size and strain) were determined from a Rietveld refinement on synchrotron powder diffraction data. Local environment of sulphate anions and of the hydrogen bonding network was characterized by Raman spectroscopy. Both techniques led to the same conclusion: the “wet” ettringite sample immersed in the mother solution from the synthesis presents similar structural features as ettringite dried to 35% relative humidity. An increase of the a lattice parameter combined with a decrease of the c lattice parameter occurs on drying. The amount of structural water, the point symmetry of sulphate and the hydrogen bond network are unchanged when passing from the wet to the dried ettringite powder. Ettringite does not form a high-hydrate polymorph in equilibrium with alkaline solution, in contrast to the AFm phases that lose water molecules on drying. According to these results we conclude that ettringite precipitated in aqueous solution at the early hydration stages is of the same chemical composition as ettringite present in the hardening concrete.