This study focuses on (1) the co-dispersion behaviors of dissimilar phases exhibiting different surface chemistry in ceramic suspensions and (2) the effect of local whisker distribution on matrix densification. Si3N4 ...
This study focuses on (1) the co-dispersion behaviors of dissimilar phases exhibiting different surface chemistry in ceramic suspensions and (2) the effect of local whisker distribution on matrix densification. Si3N4 whiskers and Al2O3 powders are mixed and slipcast in aqueous solution. Local whisker clusters are observed and remain stable even after prolonged sintering. High shear-rate agitation of the composite suspension does not eliminate the whisker agglomeration. Surface pre-treatment of whiskers, such as acid/base-washing, effectively reduces the clustering and sintered flaw size. From SEM observation it is found that the matrix powder densifies highly inhomogeneously because of the constraint imposed by the reinforcement. Matrix powder is inhibited from diffusing into “inter-whisker” voids.
For Fe78B13Si9 (Allied 2605 S2) amorphous ribbon the extent of annealing embrittlement was measured by a bending test. The stress relief by Joule heating at high temperatures and short times obeys a kinetic relation a...
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For Fe78B13Si9 (Allied 2605 S2) amorphous ribbon the extent of annealing embrittlement was measured by a bending test. The stress relief by Joule heating at high temperatures and short times obeys a kinetic relation and has an activation energy of 25 kJ/mol. Comparing the stress relief at lower temperatures shows the possibility of two consecutive processes. Some atoms at critical positions must diffuse away (due to high activation energy) before the surrounding atoms can rearrange themselves (low activation energy) to relieve the stresses. The stress relief and improvement of magnetic properties could be achieved by Joule heating without much annealing embrittlement.
Contacts between the individual ceramic particles dictate the resistive behavior of bulk, polycrystalline, preformed YBa 2 Cu 3 O 7−δ compacts in the low temperature sintering regime. Ag/Ag 2 O additions to performed...
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Contacts between the individual ceramic particles dictate the resistive behavior of bulk, polycrystalline, preformed YBa 2 Cu 3 O 7−δ compacts in the low temperature sintering regime. Ag/Ag 2 O additions to performed 123 are investigated, with an aim to lower the sintering temperature required to achieve macroscopic zero resistivity in particulate cermet superconductors. The addition of Ag/Ag 2 O results in enhanced resistive properties due to improved interparticle contacts. The thermal decomposition of Ag 2 O in the presence of 123 is unaltered, as shown by DTA and TGA analysis. Hence, the presence of Ag 2 O at temperatures higher than its normal decomposition temperature appears unlikely. For sintering temperatures below 600°C, internal oxidation due to dissociation of Ag 2 O may serve as an efficient mechanism for oxygenating the 123 phase. However, for sintering temperatures close to 800°C, Ag additions are as effective as Ag 2 O additions and the benefit of oxygen donation by Ag 2 O appears to be small. The addition of Ag/Ag 2 O and the use of high compaction pressures, reduce the minimum sintering temperature to attain macroscopic zero resistivity from 900–950°C to ∼ 800°C.
The ferromagnetic amorphous alloy Fe 78 B 13 Si 9 (Allied 2605 S2) can be magnetized below the critical magnetic field of about 0.025 Oe by superposing a suitable a.c. magnetic field such as 0.05 Oe at 100 Hz. With th...
The ferromagnetic amorphous alloy Fe 78 B 13 Si 9 (Allied 2605 S2) can be magnetized below the critical magnetic field of about 0.025 Oe by superposing a suitable a.c. magnetic field such as 0.05 Oe at 100 Hz. With this additional a.c. field, the d.c. magnetic induction is 5.6 kG in a d.c. field of 0.02 Oe as opposed to 60 G without the a.c. field.
The critical magnetic field Hcr for the Barkhausen effect of ferromagnetic amorphous alloys can be modified and reduced by applying ac high‐frequency magnetic fields. Both straight and toroidal specimens of amorphous...
The critical magnetic field Hcr for the Barkhausen effect of ferromagnetic amorphous alloys can be modified and reduced by applying ac high‐frequency magnetic fields. Both straight and toroidal specimens of amorphous ribbons Fe78B13Si9 (Allied 2605S2) and Fe40Ni38Mo4B18 (Allied 2826MB) were used in our experiment. The waveforms of ac magnetic fields were sine, triangular, or square waves, and the directions of the applied ac magnetic field were either parallel or perpendicular to the length direction of the specimen. For as‐cast Fe78B13Si9, applying ac magnetic fields of 100 Hz–1 kHz and magnitudes 0.1–0.3 Oe, the critical magnetic field of the specimen could be reduced from 0.075 to 0.02 Oe under the same exciting field and magnetic induction. For as‐cast Fe40Ni38Mo4B18, applying ac magnetic field of 100 Hz–1 kHz and magnitudes 0.1–0.3 Oe, could reduce the Hcr from 0.06 to ≤0.01 Oe under the same exciting field and magnetic induction. If the specimens were heat treated first by ac Joule heating and then applying the same ac magnetic fields when measuring the B‐H curves, the critical magnetic fields could be reduced to lower than 0.001 Oe under the same exciting magnetic field and magnetic induction. By using a suitable frequency and magnitude of the ac magnetic field, we observed a magnetic behavior similar to superparamagnetism (i.e., coercive force Hc=0).
Anomalously low laser damage resistance thresholds have been observed in many thin film systems. Thermal conductivity values well below bulk have been suggested as a possible cause. A series of reactively sputtered A1...
Anomalously low laser damage resistance thresholds have been observed in many thin film systems. Thermal conductivity values well below bulk have been suggested as a possible cause. A series of reactively sputtered A1N films were deposited on fused quartz and sapphire substrates. Their thermal conductivities were measured using the thermal comparator technique. Important factors contributing to the apparently large thermal resistance in the A1N films include film surfaces, small grain size, and high oxygen concentration. Preferred orientation and columnar morphology also affect film conductivity. Film microstructures were observed by transmission electron microscopy, and correlated with measured values of thermal conductivity.
Characteristics of both thermoplastic and thermoset composite materials as they pertain to marine vehicle applications are discussed. Comparison of various material selection factors such as strength, damage and moist...
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Characteristics of both thermoplastic and thermoset composite materials as they pertain to marine vehicle applications are discussed. Comparison of various material selection factors such as strength, damage and moisture resistance, and flammability and toxicity as well as cost and availability of thermoset and thermoplastic composite materials are presented. Methods for testing and reducing the flammability and toxicity are discussed. Many commercially available composite systems are reported to provide favorable characteristics for marine applications. Although there seems to be a need for improved production technology for thermoplastics, they present potential advantages in physical properties over thermoset composites.
A second order phase transformation in a dielectric ceramic superconductor is shown to relate jumps in physical properties to the ratio of state variables, i.e., to the phase boundary, near the critical point. The maj...
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A second order phase transformation in a dielectric ceramic superconductor is shown to relate jumps in physical properties to the ratio of state variables, i.e., to the phase boundary, near the critical point. The major jumps in specific heat, compressibility, permittivity, permeability, and spinodal composition curvature, between the normal and superconducting phases, are found from the continuity of the entropy, volume, electromagnetic fields (with zero fields), and chemical equilibrium. The thermal expansion, pyroelectric effects, and the piezoelectromagnetic effects are important differences between ceramic and metallic superconductors. The most important conclusions from experimental measurements are that the lattice plays a minor role in the superconductor transformation while the magnetoelectric jump may be related to the jump in permeability, i.e., the Meissner effect, and the jump in permittivity. The oxygen miscibility gap which controls order/disorder transformations thermodynamically mandate that only metastable compositions are obtained in the metal oxide, so absolute stability of the system may never be achieved. An explicit criterion to suggest other superconductor systems is given from magnetoelectric materials, i.e., some of the pervoskites.
The discontinuity of the isothermal elastic compliances across the superconducting transition are calculated for a single crystal. The discontinuity is based on measurements of jumps in the thermal expansion coefficie...
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The discontinuity of the isothermal elastic compliances across the superconducting transition are calculated for a single crystal. The discontinuity is based on measurements of jumps in the thermal expansion coefficient matrix, the adiabatic elastic compliances and the specific heat for the second order superconducting phase transformation. It is shown that the jump in the adiabatic compliance is significantly smaller than the jump in the isothermal compliance. Wide differences exist between the reported values of the thermal expansion coefficients and the measured jumps in the leastic compliances. These physical differences might result from the spinoidal decomposition of Y 1 Ba 2 Cu 3 O 7−δ into a series of homologous ordered phases at low temperatures, with each phase having significantly different physical properties with relatively no change in the microscopic transition temperature.
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