Tensile behavior of AI2O3/FeAI + B and AI2O3/FeCrAIY composites

The feasibility of Al₂O₃/FeAl + B and Al₂O₃/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the single-crystal A12O3 fibers. The interfacial shear strengths were low for Al₂O₃/FeAl + B and moderate to high for Al₂O₃/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites. The strength of the A1₂O₃ fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al₂O₃/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in coefficient of thermal expansion (CTE). Development of the Al₂O₃/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing and elevated temperature service.     

A technique to reduce the contact resistance to 4H-silicon carbide using germanium implantation

The effects of implanted Ge on the resistance of nickel-metal contacts to n-type and p-type 4H-SiC are reported. The Ge was implanted with an energy of 346 keV and a dose of 1.7×10¹⁶ cm⁻², and the wafer was annealed up to 1700°C for 30 min. Contact resistance measurements using the transfer length method (TLM) were performed on etched mesas of n-type and p-type 4H-SiC, with and without the Ge. For the annealed-Ni metal contacts, the Ge lowered the specific contact resistivity from 5.3×10⁻⁴ Ωcm² to 6.0×10⁻⁵ Ωcm² for n-type SiC and from 1.2×10⁻³ Ωcm² to 8.3×10⁻⁵ Ωcm² for p-type SiC. For the as-deposited (unannealed) Ni, the Ge produced ohmic contacts, whereas the contacts without Ge were rectifying. These results suggest that the addition of Ge can be an important process step to reduce the contact resistance for SiC-device applications.     

The Fowler breathholding study revisited: continuous rating of respiratory sensation

The respiratory distress of breathholding has been shown to be relieved by breathing, even without correction of worsening blood gases (Fowler, 1954). We repeated the study by having untrained normal subjects perform maximal breathholds which were followed by the rebreathing of a gas mixture containing 7.5% CO2 and 8.2% O2, and then by second breathholds. In addition, we had the subjects continuously rate their respiratory distress using a visual analog scale (VAS). The ratings were easy to perform and were highly reproducible on repeated trials in a given subject. Subjects experienced increasing distress during the breathhold, rapid and substantial relief upon rebreathing, and then were capable of performing second breathholds, all consistent with Fowler's results. The findings are consistent with animal studies in which a neural mechanism associated with stimulation of pulmonary stretch receptors inhibits the firing of midbrain neurons which may be involved in transmission to the cortex of sensory information about breathing.     

Growth and characterization of large diameter undoped semi-insulating GaAs for direct ion implanted FET technology

The growth of large diameter, semi-insulating GaAs crystals of improved purity by Liquid Encapsulated Czochralski (LEC) pulling from pyrolytic boron nitride (PBN) crucibles and characterization of this material for direct ion implantation technology, is described. Three-inch diameter, 〈100〉-oriented GaAs crystals have been grown in a high pressure Melbourn crystal puller using 3 kg starting charges synthesized in-situ from 6/9s purity elemental gallium and arsenic. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 10⁷ and 10⁸ Ω cm range, respectively. High sensitivity secondary ion mass spectrometry (SIMS) demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the mid 10¹⁴ cm^?3 range, compared to concentrations up to the 10¹⁶ cm^?3 range for growths in fused silica containers. The residual chromium content in undoped LEC grown GaAs crystals is below the SIMS detection limit for Cr (4 × 10¹⁴ cm^?3).The achievement of direct ion implanted channel layers of near-theoretical mobilities is further evidence of the improved purity of undoped, semi-insulating GaAs prepared by LEC/PBN crucible techniques. Direct implant FET channels with (1–1.5) × 10¹⁷ cm^?3 peak donor concentrations exhibit channel mobilities of 4,800–5,000 cm²/V sec in undoped, semi-insulating GaAs substrates, compared with mobilities ranging from 3,700 to 4,500 cm²/V sec for various Cr-doped GaAs substrates. The concentration of compensating acceptor impurities in semi-insulating GaAs/PBN substrates is estimated to be 1 × 10¹⁶ cm^?3 or less, and permits the implantation of 2 × 10¹⁶ cm^?3 channels which exhibit mobilities of 5,700 and 12,000 cm²/V sec at 298K and 77K, respectively.Discrete power FET's which exhibit 0.7 watts/mm output and 8 dB associated gain at 8 GHz have been fabricated using these directly implanted semi-insulating GaAs substrates.     

A comparison of passive cutaneous anaphylaxis guinea pig responses using an intravenous or an intradermal route for antigen challenge

The passive cutaneous anaphylaxis method has been used to evaluate residual antigenicity in castor bean pomace. This paper compares the results obtained using two routes of antigen challenge, intradermal (ID) and intravenous (IV). Passively sensitized guinea pigs were challenged with untreated and steam treated castor meal extract. An analysis of variance of the data shows there was greater variability between pigs within dose for the IV assay method compared to the ID technique. The ID assay also showed greater precision of the potency estimates. Biometrical Services Staff, ARS. USDA. W. Utiliz. Res. Dev. Div., ARS, USDA.     

Colour and turbidity removal with reusable magnetic particles—I. Use of magnetic cation exchange resins to introduce aluminium ions

Clarification of hard, turbid surface water by Al³⁺ ions introduced on a sulphonic acid ion exchanger is described. If the resin beads are sufficiently fine, they act as nuclei for floc formation and accelerate the subsequent settling of the flocs. Incorporation of magnetic iron oxide in the resin beads enables them to be conveniently separated from the clarified water. Acidification of the resin-containing sludge then results in partial reloading of the resin with Al³⁺ ions. However, the regenerated resin proved ineffective as a coagulant, a result attributed chiefly to hydrolysis of Al³⁺ ions within the resin. The findings of this work are significant for the theory of coagulation and also suggested the possibility of enhancing clarification by adding fine particles with an absorbing surface.     

Chromosomal rearrangements in rock wallabies, Petrogale (Marsupialia: Macropodidae). VIII. An investigation of the nonrandom nature of karyotypic change

Although the nonrandom nature of many chromosome breaks is well known, few studies have investigated the potential significance of this breakage specificity in chromosome evolution. The macropod genus Petrogale is an ideal group in which to investigate this phenomenon, since it comprises a large number of chromosomal forms, many of which appear to have differentiated relatively recently. By exposing Petrogale cells to mutagenic agents it should be possible to compare the distribution and abundance of induced breaks with those that are known to have occurred in vivo during the chromosomal differentiation of the genus. In this study, breaks were induced in mitotic chromosomes from P. assimilis and P. mareeba by exposing synchronized cultured fibroblasts to low doses of gamma radiation. The results were remarkably similar for both species and the distribution of breaks among the chromosomes appeared to be nonrandom. It was found that chromosomes 5, 6, and (possibly) 10 had a substantially higher rate of breakage than expected. These are also the chromosomes that occur disproportionately among the rearrangements identified in Petrogale. While the distribution of breaks along the chromosome appeared uniform or normal for most chromosomes, a putative "hot spot" was identified near the centromere in chromosome 5 of P. mareeba and in a homologous position near the telomere of chromosome 5 in P. assimilis. In a further experiment, a 1- to 2-h pulse of mitomycin C was used to induce centric fusions in cultured fibroblasts of P. penicillata (2n = 22); 2408 cells were examined and 112 fusions were identified. While it was found that all chromosomes participated in forming fusions, chromosome 10 was found to be most frequently involved, being present in 28.6% of the identified fusions. This frequency is far greater than would be expected if fusions were to occur at random (10%). It is significant then that chromosome 10 has been involved in five of the eight centric fusions that have been identified in Petrogale and that it is also the chromosome that has been most frequently rearranged in Petrogale. These results suggest that features of the karyotype may influence the distribution and frequency of chromosome breaks and therefore the rate and nature of chromosome evolution.     

Transverse Strength of SCS-6 Silicon Carbide Fibers

A diametral compression test was used to measure the transverse strength of SCS-6 SiC fibers before and after heat treatment. Subjecting fibers to diametral compression successfully produced transverse tensile failure in the form of fiber cracking along the same diametral plane in which the compressive load was applied. An analysis of the hoop stress along the diametral plane, in which the effects of the C core were included, showed that there is a large tensile hoop stress concentration in the SiC sheath at the interface between the C core and the SiC sheath, where the stress is 6.3 times greater than the stress present in a solid SiC fiber under identical loading. This high tensile hoop stress concentration promotes crack initiation near the core and significantly limits the capability of these fibers to withstand transverse compressive loading. The maximum tensile hoop stresses, located at the interface between the C core and SiC sheath, at the measured failure loads were 850 MPa for the as-received SCS-6 fiber and 1210 MPa for fibers exposed to a 1-h heat treatment at 1850°C in 138 MPa of Ar.     

A fluid handling system with finger-tightened connectors for biological studies at kiloatmosphere pressures

We present a high-pressure fluid handling system based around a simple-to-construct seal for applications in the biologically relevant kiloatmosphere range. Connectors are compact and finger tightened, as compared to the wrench tightening required of cone-type seals commonly used. The seal relies on an O-ring compression, and the system has been tested up to 2000 atm. While the system was designed for biological studies, it should be versatile enough for a wide range of applications, thus contributing finger-tightened convenience to the kiloatmosphere range.