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  • American Institute of Physics (AIP)  (4)
  • Kohlhammer GmbH  (1)
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    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 408-411 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Water vapor adsorption isotherms were determined by the gravimetric method for a YBa2Cu3O7−δ powder at 298 K. Adsorption energy released per unit area, adsorption site area, adsorbate fractional coverage, minimum pore radius, surface charge, surface potential, and adsorption bond energy were calculated to characterize the surface of the powder. The adsorbed water vapor may form an oriented chemisorbed layer.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 6107-6114 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 °C for particle sizes 〈38 μm. The phase III–II transition temperature decreased sharply for particle sizes 〈240 μm, and for 38 μm particles was 44 °C lower than that reported for bulk material. Utilizing 22 μm 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 μm when physically separated by SiC particles. The phase I–III transition temperature was relatively unaffected by cooperative behavior. For particle sizes 〈61 μm, 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.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 7302-7310 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results of a thermodynamic study of the phase transitions of powdered KNO3 at two impurity concentrations (99.999% pure and 99.9% pure) are reported. Transition onset and peak temperatures have been measured between 350 and 420 K using both heat flow and photoacoustic calorimetry. The transition temperatures and characteristics observed with both techniques show excellent agreement. Specific-heat values calculated from scanning and stepped temperature heat-flow data, utilizing both positive and negative temperature increments, compare well with previously reported values. Enthalpies for the II-I, I-III, and III-II phase transitions were measured as 5.065, 2.603, and 2.084 kJ/mol, respectively, for 99.999% pure KNO3 powder using heat-flow calorimetry. The transition enthalpies and temperatures measured for 99.9% pure powder were slightly lower for all three transitions. Entropy changes for the II-I, I-III, and III-II transitions were 12.53, 6.61, and 5.30 J/mol K, respectively. Enthalpy, entropy, and Gibbs free-energy curves are presented with emphasis on the temperature range over which the ferroelectric phase transition occurs. The experimental data and calculated thermodynamic functions indicate that the ferroelectric phase III in bulk KNO3 is a metastable state at atmospheric pressure, which was not observed to exist below 350 K contrary to previous reports.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 3131-3136 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 ∼97–124 °C to that characteristic of bulk material (∼110–124 °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.
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