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@PHDTHESIS{Rahmani:848213,
      author       = {Rahmani, Meimanat},
      title        = {{O}ptimization of powder and ceramic processing, electrical
                      characterization and defect chemistry in the system
                      {Y}b$_{x}${C}a$_{1-x}${M}n{O}$_{3}$},
      volume       = {54},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2018-03475},
      isbn         = {978-3-95806-323-5},
      series       = {Schriften des Forschungszentrums Jülich. Reihe Information
                      / Information},
      pages        = {XIV, 164 S.},
      year         = {2018},
      note         = {RWTH Aachen, Diss., 2018},
      abstract     = {Mixed valence alkaline earth manganites
                      RE$_{x}$A$_{1-x}$MnO$_{3}$ with very rich and complex
                      crystal physics and chemistry have attracted a great deal of
                      attention recently. Two innovative potential applications
                      for rare earth manganites are non-volatile memories based on
                      resistive switching and waste heat recovery techniques by
                      thermoelectric generators. The resistive switching mechanism
                      and thermoelectric properties strongly depend on the
                      concentration and nature of the charge carriers. Defect
                      chemistry mechanism involved in these process have not been
                      understood clearly yet. The lack of information in the
                      literature is certainly also due to complexity of this
                      material system, since manganese cations possess several
                      valence states in the quaternary oxide. The other reason
                      possibly is due to the formation of cracks or deformation of
                      ceramic compounds in to the powder during electrical
                      measurements in the reduction regions. Therefore, in the
                      present thesis comprehensive studies were performed for the
                      first time in order to find the way for reducing or
                      preventing crack formation that occurs during sintering. For
                      this purpose the effect of different calcination and
                      sintering conditions on the physical properties of
                      stoichiometric, nonstoichiometric (Ca-excess and Mn-excess)
                      and 0.1 to 10 at. \% of Yb-doped CaMnO$_{3}$ ceramics were
                      investigated in dependence of temperature T and partial
                      pressure of oxygen p(O$_{2}$). After finding the required
                      parameters to produce desired crack free-ceramics such as
                      calcination temperature, sintering gas composition types and
                      heating/cooling rates, etc., electrical characterizations
                      were performed including DC conductivity and impedance
                      spectroscopic measurements. Bulk conductivity measurements
                      are performed by impedance spectroscopy accompanied by
                      modeling the compounds in terms of an electrical equivalent
                      circuit in a wide range of temperatures and frequencies.
                      These experiments reveal the main role of grain boundaries
                      and electronic and ionic contributions in conductivity. A
                      novel schematic illustration based on the
                      double-Schottky-barrier model for polycrystalline ceramics
                      is proposed which clearly describes the contribution of
                      different resistance components in electric transport
                      properties. Migration of oxygen vacancies and their
                      participation in conductivity are discussed in the present
                      study and the results are confirmed by observing oxygen
                      released using a ZrO$_{2}$ oxygen sensor during dilatometry
                      measurements in a wide range of temperatures. The defect
                      chemistry model is proposed to clarify the details of the
                      chemistry of point defects. This theoretical model
                      accompanied by experimental DC-conductivity measurements,
                      dilatometry analysis, SEM, EDX, XRD-measurements, Raman
                      spectroscopy, iodometric titration and thermogravimetric
                      (TGA) experiments reveal ionic as well as electronic charge
                      transport conductivity contribution in dependence ofthe
                      oxygen partial pressure p(O$_{2}$) for donor-doped
                      Yb$_{x}$Ca$_{1-x}$MnO$_{3}$ systems. A comparison of the
                      experimental observations and the theoretical defect
                      chemical models clearly show the way for controlling charge
                      carriers in dependence of p(O$_{2}$) and dopant
                      concentrations. A negative slope of electrical conductivity
                      versus p(O$_{2}$)indicates that n-type conductivity occurs.
                      A p(O$_{2}$) independent conductivity (plateau region) is
                      observed for both undoped and donor-doped CaMnO$_{3}$.
                      Electrical measurements accompanied by thermal analysis and
                      phase purity assessment reveal that the origin of the
                      plateau region in conductivity is due to intrinsic or
                      extrinsic electronic compensation. In this region probably
                      due to formation of secondary phases oxygen ions do not
                      considerably influence the electron and hole concentrations.
                      In addition, the origin of the drastic decrease in
                      conductivity at reduction region is elucidated. Overall, the
                      present experiments clearly demonstrate the effect of donor
                      dopant concentration, formation of oxygen vacancies and
                      corresponding change in density of Mn$^{3+}$-Mn$^{4+}$ pair
                      sites on phase transition, charge migration and conductivity
                      mechanism of the complex systems
                      Yb$_{x}$Ca$_{1-x}$MnO$_{3}$. The obtained information
                      probably will be useful for better understanding the
                      conduction mechanism in future technological applications.},
      cin          = {PGI-7},
      cid          = {I:(DE-Juel1)PGI-7-20110106},
      pnm          = {899 - ohne Topic (POF3-899)},
      pid          = {G:(DE-HGF)POF3-899},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/848213},
}