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.     

Transport properties of the organic conductor (TMTSF)2BrO4: Evidence of variable-range hopping

A study of d.c. and microwave conductivity and thermoelectric power of the organic conductor (TMTSF)₂BrO₄ is presented. The transport properties are in qualitative agreement with charge transport via variable-range hopping among localized states. The localization is attributed to the anions, which are ordered only over short ranges, probably due to impurities.     

Validation of a new transit time ultrasound flowmeter for measuring blood flow in colonic mesenteric arteries

OBJECTIVE: To validate a transit time ultrasound flowmeter (CardioMed CM 4000) for measuring blood flow in isolated colonic mesenteric arteries. DESIGN: Experimental and clinical study. SETTING: Teaching hospital, Denmark. ANIMALS AND SUBJECTS: One female pig, and 6 patients being operated on for carcinoma of the sigmoid colon and rectum. INTERVENTIONS: Volume blood flow measured by Cardiomed CM 4000 and pump withdrawal flow recordings. MAIN OUTCOME MEASURES: Correlation between the two methods. RESULTS: There was good agreement between transit time flow recordings and pump withdrawal flow recordings (correlation coefficient of 1.0). Of the differences between the two methods, 95% were between -0.16 ml min(-1) and 1.29 ml min(-1), mean 0.57 ml min(-1), or (in percentages) 105, 95% lying between 97-115. There was also good reproducibility in transit time flow recordings, the mean difference between repeated measurements being 0.06 ml min(-1), 95% lying between -0.66 ml min(-1) and 0.78 ml min(-1). CONCLUSION: Ultrasound transit time flow recordings gave precise measurements of blood flow in isolated colonic mesenteric arteries.     

Heart failure and echocardiographic changes during long-term follow-up of patients with sick sinus syndrome randomized to single-chamber atrial or ventricular pacing

OBJECTIVE: In the present study, physical and sexual abuse among incarcerated youth were examined with respect to sexual activity, contraceptive use, and pregnancy. METHOD: The self-report survey data were collected from 62 females and 334 males who were incarcerated in Nevada youth correctional facilities in the summer of 1994. Of the males, 46.8% (n = 156) reported a history of physical abuse and 9.9% (n = 33) reported sexual abuse. A surprisingly high 73% (n = 46) of the females reported a history of physical abuse and 68.3% (n = 43) reported sexual abuse. RESULTS: The analyses indicated that females who reported a history of sexual abuse had an earlier mean age of first intercourse than those who reported no sexual abuse. In addition, male and female respondents with a history of physical and/or sexual abuse reported using no method of contraception as compared to youth with no history of abuse. CONCLUSIONS: Implications of these results for detention-based counseling and prevention programming are discussed.     

Columnar dendritic solidification in a metal- matrix composite

Results are presented of a study on columnar dendritic solidification of the matrix of a fibrous metal-matrix composite, the fibers of which are aligned SiC fibers 140 m in diameter and the matrix of which is Al-4.5 wt pct Cu. Samples were produced by pressure infiltration of the metal-matrix into a preform of the fibers. The matrix was subsequently remelted and resolidified under controlled thermal gradient and growth rate. Dendrite growth begins in the center of the interstices left between the fibers. The dendrite tip temperature is not significantly influenced by the fibers, but the usual linear dependency of dendrite arm spacing ont ^1/3 (wheret is time during solidification) is altered significantly in the narrower interstices at long solidification times. The underlying mechanism is dendrite arm coalescence which takes place at a sufficiently rapid rate in the composite that the microstructure gradually becomes nondendritic. The solid/liquid interface then is parallel to the matrix/fiber interface. A model is presented for the kinetics of dendrite arm coalescence and compared with experimental results. The amount of microsegregation that was found in the matrix within interstices is significantly less than that found in the usual cast alloy, especially at long solidification times (low cooling rates). The mechanism responsible for the observed reduction in microsegregation is solid-state diffusion which is enhanced in the composite by the fact that the fibers place an upper limit on the dendrite arm spacing, and hence on the required diffusion distance.     

Synthesis of bulk and reinforced

Preforms of nickel powder or nickel/alumina powder blends were infiltrated with molten aluminum to produce nickel aluminides. Application of a pressure of 3.6 and 6.9 MPa on the melt allowed infiltration of preforms with nickel powder particles between 5 and 15 μm in diameter, which could not be infiltrated under the sole action of capillary forces. By varying the initial preform temperature from 705 °C to 280 °C, the diameter of nickel powder particles from 15 to 150 μm, and the volume fraction of alumina from 0 to 34 vol pct, pressure-infiltrated samples with large variations in the extent of reaction between nickel and aluminum were pro- duced. The range of microstructure extended from fully reacted nickel aluminide to essentially unreacted nickel/aluminum samples containing low levels of final porosity. In particular, it was found that reducing the initial preform temperature below the melting point of aluminum results in a reduction of the rate and extent of aluminide formation, allowing, in turn, reduction of macrosegregation within the resulting infiltrated material. Formerly Research Assistant, Department of Materials Science and Formerly Research Assistant, Department of Materials Science and     

Molecular characterization of Vibrio cholerae O1 strains isolated during cholera outbreaks in Guinea-Bissau

In the present study, 19 strains of Vibrio cholerae O1 biotype El Tor isolated during outbreaks of cholera in Guinea-Bissau in 1987, 1994, and 1995 were characterized to investigate a possible epidemiological relationship among the isolates. On the basis of ribotyping with the restriction enzyme BglI, 5 strains isolated in 1987 showed two closely related ribotypes, while 14 strains isolated in 1994 and 1995 showed the same ribotype that was distinct from the ribotypes of strains isolated in 1987. Southern blot hybridization of BglI-digested genomic DNA with a cholera toxin probe demonstrated that the strains isolated in 1987 showed an identical cholera toxin genotype, whereas O1 strains isolated in 1994 and 1995 showed the same genotype that was distinct from the genotype of strains isolated in 1987. These results were supported by the results of antibiotic susceptibility testing, in which strains isolated in 1987 showed resistance to polymyxin B only, while each of the strains from 1994 and 1995 showed resistance to polymyxin B, trimethoprim-sulfamethoxazole, and the vibriostatic agent O/129. Although our results are based on a limited number of V. cholerae O1 strains, they suggest that the epidemic in Guinea-Bissau in 1994 and 1995 was due to the introduction of a new strain to the country.