Chemical stability of zirconia-stabilized alumina fibers during pressure infiltration by aluminum

Metal matrix composites composed of high-purity aluminum and Du Pont PRD-166 continuous zirconia-stabilized polycrystalline alumina fibers are fabricated by liquid metal infiltration using three different casting procedures. The microstructure of the composites is analyzed using optical and electron microscopy, including analytical electron microscopy. It is found that discrete faceted particles of ZrAl₃ form at the interface and grow into the matrix of samples processed above the melting point of the matrix for 13 minutes or more. The formation of this compound is in agreement with thermodynamic stability calculations. It is also found that there is a reaction between solid aluminum and the fibers at 913 K, yielding a reaction product which has the same morphology as that observed with molten aluminum. When the fibers are infiltrated with an initial preform temperature below the metal melting point and a solidifination time below 1 minute, no reaction products were visible in the composite using the scanning electron microscope (SEM). This leads to the conclusion that aluminum matrix composites can be cast with no apparent interfacial reaction product using these fibers provided that adequate processing parameters are chosen.     

A Versatile Flat-Deposit Impactor-Type Aerosol Collector Part 1: Design and Qualitative Study

Cascade impactors provide an efficient method for sampling aerosols according to their size, grossly between 0.1 mu m and a few tens of microns in diameter. We have designed such an apparatus, with rotatable substrate plates and radially aligned rectangular orifices. The nominal cutpoint diameters are, respec tively, for stages 1-7 of the collector: 10, 5, 2, 1, 0.5, 0.25, and 0.1 mu m aed for an airflow of 10/L min. The sampler has an auxiliary set of orifice plates for operation at 20/L min giving the same cutpoint characteristics. Annular deposits are obtained following complete rotation of the collecting substrates. The airflow is controlled with a critical orifice at the last stage. A calculation program was written for a rectangular orifice impactor in order to determine the cutpoints in different conditions. The constant exit pressure mode of operation appears to be the best way to minimize fluctuations of the effective cutpoints against the varying conditions of ambient temperature and pressure. The SPAL sampler has proven satisfactory with respect to manipulations required, maintenance, and results delivered.     

Infiltration of fibrous preforms by a pure metal: Part V. Influence of preform compressibility

Infiltration by a pure matrix in the presence of preform deformation and partial matrix solidification is analyzed using a bounding approach for the preform rheology where solid metal is present. It is found, using parameters for the infiltration of short alumina fiber preforms by aluminum, that the two bounds are close in comparison to other factors of uncertainty in the prediction of infiltration rate. Using this approach, preform compression is shown to exert a significant influence on the infiltration rate for the system explored; in particular, the analysis shows the existence of an optimal value of applied pressure. Simplifications in the analysis are also presented, which yield fairly accurate results while easing their computation significantly.     

Resin transfer moulding of anionically polymerised polyamide 12

Liquid composite moulding of Lactam 12 monomer and activating system (APLC12) into satin weave carbon fabrics is investigated, with emphasis on minimising the void content in the final part. The main sources for void formation are identified. The solidification shrinkage is quantified to account for at most 9% in the matrix. Optimal flow conditions are determined to minimize void content during liquid moulding. Finally, as the monomer is kept under Nitrogen prior to processing, diffusion and solubility of Nitrogen in the monomer are characterized, to indicate that Nitrogen coalescence during injection is a major cause of voids in the final part. The average void content is reduced from initially 15% to below 1% in polyamide 12 based composite plates with optimised process conditions.     

Low‐profile mechanisms in a PVAc/polyester blend

The different steps associated with the curing of a PVAc/polyester blend are identified and correlated to the mechanism of shrinkage control in the presence of a low‐profile additive (LPA). Poly(vinyl acetate) (PVAc) is used as a LPA and is shown to induce a phase separation upon curing that leads to an interconnected globule morphology. This morphology strongly modifies the rheokinetics of the blend compared to that of the neat polyester resin. In particular, the presence of PVAc delays the cure kinetics and the gel time. A comparison between these delays, called shift times, demonstrates an increase in the gel conversion of polyester in the presence of PVAc. This, coupled to the thermal expansion of PVAc at the early stages of curing, contributes to the low‐profile effect. Microvoids in the LPA‐rich phase, which are believed to play a key role in the mechanism of shrinkage control, are efficient at the later stages of curing and during cooling and complete the low‐profile effect. However, it is also shown that the formation of microvoids may indirectly induce macroscopic voids that could be at the origin of pinholes at the surface of the parts molded with this material. POLYM. ENG. SCI. 46:303–313, 2006. © 2006 Society of Plastics Engineers     

Autoimmune hepatitis treated with cyclosporin revealed by acute hepatocellular failure

The kidney bears the brunt of the demands of a tropical climate for water and electrolyte homeostasis. We hypothesised that a tropical climate may cause adaptive changes in the entire organism leading to altered renal function in our subjects. Hence renal function data for residents of a temperate climate may not be applicable to tropical residents. We therefore sought to elucidate renal function in subjects residing in a tropical climate. We used lithium clearance, CLi, a non-invasive tool for assessing proximal tubular function in humans, and endogenous creatinine clearance, CCr, to estimate proximal tubular function and glomerular function, respectively, in our subjects. We did this in order to establish whether or not nephron function in our subjects differs from that for residents of a temperate climate. Nineteen male and 12 female Ghanaian subjects aged between 15 and 48 years were studied. The estimated GCr was 117.3 +/- 6.6 ml/min for male subjects and 97 +/- 6.4 ml/min for female subjects. CLi was 20.3 +/- 1.6 ml/min for male and 19.1 +/- 0.4 ml/min for female subjects, respectively. The estimated absolute reabsorption rate of fluid of proximal tubules was 97.0 +/- 6.0 ml/min for males and 78.1 +/- 6.0 ml/min for females. The percentage proximal fluid reabsorption for male and female subjects was 81.2 +/- 1.4 and 79.5 +/- 1.6, respectively. The differences between male and female values (mean +/- SEM) were not statistically significant. The data suggest that the proximal tubule in residents of a tropical climate may reabsorb more fluid compared to that in residents of a temperate climate. Our values for proximal tubular reabsorption are higher than those reported for residents of a temperature climate. Our estimate of glomerular filtration, however, is similar to published data for Caucasians. The difference in proximal tubular function may reflect possible renal adaptation to a hot, humid climate. We conclude that renal function of tropical residents differs from that of residents of a temperate climate. This difference may be due to renal adaptation to the hot, tropical climate.     

Reversal by naloxone of the effects of morphine on the unanesthetized dog]

In unasthetized dogs naloxone induced effects opposed to those of morphine (tachycardia, agitation, hyperthermia, tachypnea) and mydriasis. These effects were moderate and transient; some of them were elicited with low doses being border-line after 0.03 mg.kg-1 s.c., statistically significant after 0.1 mg.kg-1 s.c.; they increased slightly with the dose. After repeated administrations, acute tolerance developed and some moderate morphine-like effects (miosis, sedation) were observed. The stimulatory effects described here may result from antagonism of a morphinomimetic natural ligand,and represent thus indirect arguments in favour of normal functions of this ligand; these functions would be to temper not only algesic but also other stimulant reactions. The limitation of the effects might result from the limited release of this ligand in normal dogs and (or) from interfering morphinomimetic properties of naloxone, which are apparently unmasked when administrations are repeated. Both, stimulatory and inhibitory effects of naloxone are not liable to represent noticeable side-effects of this drug, but they both might play some role in the mechanisms of precipitated abstinence.     

Infiltration of fiber preforms by an

Preforms of 20 vol pct SAFFIL alumina fibers are infiltrated with Al-4.4 wt pct Cu-0.3 wt pct Mg using a horizontal die casting machine. Fiber preform temperature is varied from 973 to 673 K. Solute distribution, fiber volume fraction, and matrix microstructure are characterized using optical metallography and electron microprobe analysis. Increases in fiber volume fraction are observed in the composites downstream of the infiltration path. We propose that these result from locking of the compressed fibers by solid metal present during infiltration. With this as- sumption, we find good agreement between theory presented in Parts I [1] and II [2] for solute concentration, fiber volume fraction distributions, as well as matrix microstructure and exper- iments. With an initial preform temperature of 673 K, freckles are found in the composite, which are interpreted to result from the combined effects of pressure and significant enrichment in solute at the infiltration front.     

Infiltration of fiber preforms by a binary

In a previous article, [1] a theoretical analysis was developed to describe the infiltration of fiber preforms by a binary alloy, and its solution was given for unidirectional adiabatic infiltration under constant applied pressure. This article further develops the analysis by proposing a model to predict the permeability of fibrous preforms containing solidified primary metal, by deducing the final composite microstructure from processing parameters, and by addressing the influence of external cooling on macrosegregation within the composite. Experimental procedures estab- lished for the infiltration of fiber preforms by pure aluminum are modified to produce samples infiltrated under nearly adiabatic conditions. Samples of SAFFIL alumina fiber preforms infil- trated adiabatically under constant applied pressure with Al-4.5 wt pct Cu show longitudinal variations in copper concentration, which are well predicted by theory presented in Part I. [1] The microstructures in the infiltrated composite samples also agree with analysis: the grain size is small where solid and liquid matrix coexisted during infiltration, whereas it is large where remelting occurred, indicating that SAFFIL fibers do not promote nucleation of Al-Cu. The model proposed here for permeability of the preform in the presence of solidified metal yields infiltration rates in agreement with experimental data. Finally, samples produced under non- adiabatic conditions exhibit transverse macrosegregation; this is explained for simple limiting cases of heat transfer at the die wall.