Experimental evidence of cooperative behavior during the ferroelectric phase transitions in granular and powder KNO3 at atmospheric pressure is presented. Three different experimental studies were performed in which p...
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Experimental evidence of cooperative behavior during the ferroelectric phase transitions in granular and powder KNO3 at atmospheric pressure is presented. Three different experimental studies were performed in which phase transitions were detected and characterized by heat flow calorimetry: (1) the distribution of SiC powder in granular KNO3 was varied;(2) the volume fraction of SiC in powdered KNO3 was varied;and (3) pure KNO3 powder was thermally cycled. All three studies provided evidence of cooperative behavior between the KNO3 particles during the III-II phase transition. The cooperative behavior reduced the temperature range of phase III stability from approximately 97-124-degrees-C to that characteristic of bulk material (approximately 110-124-degrees-C). Separate KNO3 particles behaved as individual ferroelectric domains, with each particle making the phase transition independently near the expected Curie temperature. Particles of KNO3 in intimate physical contact tended to behave cooperatively as a single large ferroelectric domain leading to sharper phase transitions more characteristic of single crystals. The degree of cooperative behavior was dependent upon the extent to which the individual particles were in physical contact. The absence of the III-II phase transition in KNO3 powder that has been reported in the literature can be understood from the results obtained using SiC powder to separate KNO3 particles during heat flow calorimetry measurements.
The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increase...
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The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increases with decreasing crack length. But the opposite is true if the dislocation is originated from elsewhere. At the same distance from the crack tip, the image force for the former is always larger than that for the latter. They become equal only if the crack is semi-infinitely long. If the dislocation is emitted from one of the crack tips, it generally will shield this crack tip but antishield the other tip. However the dislocation will shield or antishield both crack tips if it is originated from elsewhere.
Geological carbon sequestration (GCS) offers a promising approach to mitigating greenhouse gas emissions and supporting global carbon neutrality goals. Offshore GCS, which leverages subsea geological formations, provi...
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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...
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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.
Particle size effects on the phase transition temperatures of KNO3 at atmospheric pressure were characterized using heat-flow calorimetry. The phase II-I transition temperature was relatively unaffected by particle si...
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Particle size effects on the phase transition temperatures of KNO3 at atmospheric pressure were characterized using heat-flow calorimetry. The phase II-I transition temperature was relatively unaffected by particle size, whereas the phase I-III transition temperature was reduced up to 4-degrees-C for particle sizes < 38 mum. The phase III-II transition temperature decreased sharply for particle sizes < 240 mum, and for 38 mum particles was 44-degrees-C lower than that reported for bulk material. Utilizing 22 mum SiC powder to separate the same range of KNO3 particle sizes, it was shown that cooperative behavior played a significant, but lesser role than the KNO3 particle size in determining the phase transition temperatures. The phase II-I transition temperature increased with decreasing particle size for particle sizes < 240 mum when physically separated by SiC particles. The phase I-III transition temperature was relatively unaffected by cooperative behavior. For particle sizes < 61 mum, cooperative behavior was unimportant and particle size assumed the dominant role in controlling the phase III-II transition temperature. The general trend of transition temperature as a function of particle size was similar to reported results for KNO3 thin films, but exhibited significant differences when compared to reported results for KNO3 powder. The dependence of transition enthalpy on particle size for the three phase transitions is discussed and the specific heat of KNO3 powder in phase III at atmospheric pressure is reported.
Investigations on the junctions made between Pb and polycrystalline Bi2Sr2CaCu2O8+x (BSCCO) are reported. The BSCCO surfaces were treated with number of methods including mechanically scraping, plasma sputtering, plas...
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Investigations on the junctions made between Pb and polycrystalline Bi2Sr2CaCu2O8+x (BSCCO) are reported. The BSCCO surfaces were treated with number of methods including mechanically scraping, plasma sputtering, plasma oxidation, and Br etch before junctions were fabricated. The current-voltage characteristics and dynamic resistance (dV/dI) vs voltage have been measured at both 77K and 4.2K. We observed, among other features, a Pb gap structure similar to that seen in Pb/YBa2Cu3O7-delta junctions, a magnetic dependent supercurrent, an asymmetric I-V curve, and narrow resistance peaks. Properties of junctions prepared in different manners are compared. Both XPS and electrical studies indicated that Br etch is not suitable for the BiSrCaCuO system in spite of its successful application in YBa2Cu3O7-delta.
A large number and variety of metal-matrix composites have been developed, many of them tailored to specific applications and the properties of interest often include strength, stiffness, wear resistance or density. L...
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A large number and variety of metal-matrix composites have been developed, many of them tailored to specific applications and the properties of interest often include strength, stiffness, wear resistance or density. Low density is important for uses in the transport industry and in electronics applications where low inertia is required for rapidly moving parts. Conventional magnesium alloys are already attractive candidate materials in these fields but could be made even more attractive by a further reduction in density; indeed, a reduction in density is often desirable in many other alloy systems.
A model for indentation creep is proposed using the stress field produced by the indenter and the material flow from a high pressure region to a low pressure region by diffusion. The model was found to be incomplete i...
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A model for indentation creep is proposed using the stress field produced by the indenter and the material flow from a high pressure region to a low pressure region by diffusion. The model was found to be incomplete if not incorrect because the effect calculated may not be the indentation creep produced by diffusion. For the vacancy mechanism of diffusion in a single crystal in which the partial molar volumes of atoms and vacancies are equal, pressure gradient is not a driving force for diffusion.
Conventional powder processing methods have been used to fabricate superconducting cermets, using non-noble metals. Two distinct classes of superconducting cermets have been made. The first expels magnetic fields at l...
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Conventional powder processing methods have been used to fabricate superconducting cermets, using non-noble metals. Two distinct classes of superconducting cermets have been made. The first expels magnetic fields at liquid N 2 temperatures. The second exhibits zero resistivity on a macroscopic scale. We report preliminary success in the fabrication of these cermets.
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