Centrifugal casting of an Al-12Si-2Mg/Al2O3-particulate MMC. Part II: Evaluation of soundness and structure

A technique for the preparation of an MMC using centrifugal casting has been developed and tested for its feasibility in preparing Al-12Si-2Mg/Al₂O₃- particulate composites. The process is evaluated by observing the structure, measuring the homogeneity in the distribution of the ceramic particles, the porosity type and distribution, and by analysing the metal/ceramic interface for possible reactions.

The different processing conditions applied are: rotational frequency 16, 22.7 and 33.3 Hz (960, 1360 and 2000 rpm), Al₂O₃ particle size 30, 47, 60 and 89 μn, melt superheat 20, 100 and 150°C, specimen radius of rotation from 145 to 180 mm.

Because the ceramic particles are close packed, a uniform particle distribution with no agglomeration is obtained, and the interparticle distance depends only on the alumina particle size. The metal/ceramic interface was sharp with no reaction. Microporosity is observed in some locations due to incomplete infiltration between the alumina particles. Increasing rotational speed, particle size, superheat, and radius of rotation help to decrease the microporosity. The macrostructure along the composite length showed columnar grains followed by equiaxed grains. The type and size of the structure depend mainly on the composition of the matrix and not on the presence of the alumina particles.     

The influence of bearing misalignment on the performance of helicopter gear boxes

The misalignments in the bearings of a gear box affect the life of the bearings and the deflection of the tooth contact points of the gears and consequently the performance of the gear box as a whole. Owing to the interaction of the deflections of all component parts of a gear box such as gears, shafts, bearings, spacers etc. it is necessary to consider these in combination rather than individually. A computer program was developed for analysing typical helicopter gear boxes with cantilevered housings in which the gear shafts are supported by taper roller bearings. In this program such factors as the misalignment of the bearings, the torque transmitted, the bearing preload, the rigidity of the casing and shaft complete with the bearing spacer, the spacing between the bearings, the location of the external load and the elements of elastic deformations in the bearings are considered. The particular assembly used in this paper is based on a Wessex tail rotor gear box, although the program developed is of a general nature.The influence of the misalignments between the bearing races that may be present before the application of an external load is investigated in this paper. The effect of crowning of the rollers or race tracks, to reduce the edge loading, on the deflection components of the gear tooth contact points is also included in the investigation.     

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.

     

Kinetic and rheokinetic study of dicarboxylic fatty acid chain extension using a dioxazoline coupling agent

A kinetic and rheokinetic study of the condensation reaction of a dicarboxylic fatty acid, Pripol®1009 (C36), and a dioxazoline coupling agent (1,3‐Phenylene)‐bis(2‐Oxazoline) (OO) was made. The kinetic study showed a similar reactivity of the two acid groups of C36 and also a similar reactivity of the two oxazoline groups of OO. The reaction kinetics can be described using a second‐order kinetic model. A kinetic constant k = 16.1 × 10⁻⁴ mol⁻¹ s⁻¹ at 156°C with an activation energy E_a = 80.6 kJ mol⁻¹ was calculated. A rheological evaluation of the reactants and the obtained polymers showed that the reactive system had Newtonian behavior during all the reaction times for shear rates lower than 100 s⁻¹. Using this kinetic modeling and measured viscosity evolution of the reactive system at different temperatures, rheokinetic models were proposed for viscosity evolution with the molar mass evolution of the synthesized polymer and the reaction time and conversion. Viscosity evolution of the reactive system during the first 10 min, corresponding to a typical mean residence time in reactive extrusion, were calculated using the proposed rheokinetic model. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1017–1024, 1999     

Geological Engineering,Civil Engineering,Traffic and Transportation Engineering Electrical capacitance volume tomography for measurement soil water infiltration in vessel experiments简

Electrical capacitance volume tomography （ECVT） is a recently-developed technique for real-time, non-invasive 3D monitoring of processes involving materials with strong contrasts in dielectric permittivity. This work is first application of the method to visualization of water flow in soil. We describe the principles behind the method, and then demonstrate its use with a simple laboratory infiltration experiment. 32 ECVT sensors were installed on the sides of an empty PVC column. Water was poured into the column at a constant rate, and ECVT data were collected every second. The column was then packed with dry sand and again supplied with water at a constant rate with data collected every second. Data were analyzed to give bulk average water contents, which proved consistent with the water supply rates. Data were also analyzed to give 3D images （216 voxels） allowing visualization of the water distribution during the experiments. Result of this work shows that water infiltration into the soil, wall flow, progress of the unstable wetting front and the final water distribution are clearly visible.     

Accelerated sintering of PtRh/Al2O3 catalysts in an oxygen periodic pulse flow

PtRh catalysts were prepared by coimpregnation, with chlorine-free precursors of a -alumina. They were sintered at 973 K under a continuous flow of O₂ (1% in Ar or air) or of a flow of Ar into which pulses of O₂ were injected periodically. Under the latter conditions the sintering was significantly accelerated, particularly for a certain value of the period of O₂ pulses.     

Rheology of poly(n‐butyl methacrylate) and its composites with calcium carbonate

Poly(n‐butyl methacrylate) (PBMA) composites with calcium carbonate (CaCO₃) were prepared by in situ radical copolymerization of butyl methacrylate (BMA) and methacrylic acid (MA) with precipitated calcium carbonate. To compare the different rheological behaviors of the monomer mixtures with CaCO₃ and the composites, the steady and dynamic viscosities of BMA/MA/CaCO₃ and poly(BMA/MA/CaCO₃) were measured by means of steady and oscillatory shear flows. The viscosity of the mixture BMA/MA/CaCO₃ was found to increase evidently with the increasing of CaCO₃%. The influence of MA% on viscosity of BMA/MA/CaCO₃ was slight. During the in situ polymerization, the viscosity of the reacting system was measured to be enhanced by a factor of about 10⁴ from the monomer/CaCO₃ mixture to composites. The dependency of zero‐shear viscosity on molar mass of PBMA was also investigated. The relation between the zero‐shear viscosity and molar mass is η₀ = 10^?15 M_w^3.5. The evolution of the viscosity with the temperature for both PBMA and its composites was obtained and time–temperature superposition was used to build master curves for the dynamic moduli. The flow activation energies were found to be 115.0, 148.6, and 178.7 kJ/mol for PBMA, composite PBMA/CaCO₃ (90/10), and PBMA/MA/CaCO₃ (89/1/10), respectively. The viscosity of the composites containing less than 10% CaCO₃ was lower than that of pure PBMA with the same molar mass. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1376–1383, 2003     

Reversibly crosslinked self‐healing PCL‐based networks

Poly(ε‐caprolactone) (PCL)‐based thermoreversible networks with self‐healing properties were prepared through Diels–Alder (DA) and retro‐DA reactions. Bis‐ or Tris‐maleimide compounds and a series of copolymer(caprolactone‐diene) PCL_XF_Y (X: degree of polymerization and Y: furan‐average functionality) with Y between 2.4 and 4.9 were used. The successive sequences of formation and dissociation of polycaprolactone networks via DA and retro‐DA reactions were observed repeatedly by dynamic mechanical analyses (DMA) and their gel‐temperatures determined. The cross‐linking densities, thermal properties, and thermal reversibility of the PCL_XF_Y/multimaleimide polymers have been modulated by the structure and functionalities of the used diene and dienophile moieties. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Radical grafting of polar monomers onto polypropylene by reactive extrusion

Polypropylene (PP) was functionalized in the melt by grafting polar monomers using an internal mixer and a corotating twin‐screw extruder. 2,5‐Bis(tertbutylperoxy)‐2,5‐dimethylhexane (Luperox 101) and dicumyl peroxide (DP) were the used radical initiators. The polar monomers were itaconic acid (IAc), 2‐octen‐1‐ylsuccinic anhydride (OY), 2‐hydroxyethyl methacrylate (HEMA), and 3‐allyloxy‐1,2‐propanediol (AP). Grafting was quantified by FTIR combined to Elemental Analysis. Grafting degree depends mainly on monomer and initiator natures and concentrations. Grafting degree maxima were 3.9, 2, 9.5, and 3.9 wt %, respectively, for IAc, OY, HEMA, and AP. Some properties of the modified PP were evaluated. Thermal analysis indicated that the polarity of PP increased by grafting reaction and size‐exclusion chromatography showed that the grafting was not accompanied by a significant M_w and viscosity decrease. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013