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Title: Charakterystyki prądowo-napięciowe wolframianów i molibdenianów srebra, kadmu, kobaltu, manganu, miedzi i metali ziem rzadkich
Authors: Sawicki, Bogdan
Advisor: Groń, Tadeusz
Górny, Krzysztof
Keywords: wolframiany; molibdeniany; metale ziem rzadkich
Issue Date: 2015
Publisher: Katowice : Uniwersytet Śląski
Abstract: The PhD thesis presents the study on magnetic, paramagnetic resonance spectra, broadband dielectric spectroscopy, optical in the UV-vis-NIR combined with the determination of an energy gap as well as measurements of electrical conductivity and thermoelectric power versus a function of temperature and current-voltage characteristics I-V including conductance as a function of voltage curried out for following tungstates RE2W2O9 (RE = Pr, Sm-Gd), MRE2W2O10 (M = Cd, Co, Mn; RE = Y, Pr, Nd, Sm - Ho), AgY1-xGdx(WO4)2 (0,005  x  1), AgY1-xNdx(WO4)2 (0,005  x  1) CuSm2W2O10, Cu3Sm2W4O18, CoEu4W3O16 and for Cd1-3xGd2xxMoO4 molybdates (0,0005  x  0,2222). Detailed measurements of magnetic susceptibility, magnetic isotherms and paramagnetic resonance spectra for AgY1-xGdx(WO4)2 (0.005  x  1) revealed the nature of the paramagnetic spin visible in the Landé factor (g  2), estimated both from the Curie constant and the Brillouin procedure. Based on UV-vis-NIR measurements, the values of the energy gap ranging from 2.4 eV to 3.73 for MPr2W2O10 (M = Cd, Co i Mn) and RE2W2O9 (RE = Pr, Sm-Gd) as well as for Cd1-3xGd2xxMoO4 molybdates (x = 0,0, 0,0238 i 0,1667) were determined. These values are typical for insulators, for which electrons cannot be activated only by means of thermal energy kT. Relative dielectric constant and loss tangent measurements for CuSm2W2O10, Cu3Sm2W4O18, CoEu4W3O16 and RE2W2O9 (RE = Pr, Sm-Gd) suggest that only those ions that have a large number of unpaired electrons on unscreened orbitals are responsible for the colossal dielectric effect as it takes place for CoEu4W3O16. Measurements of electrical conductivity and thermoelectric power for RE2W2O9 (RE = Pr, Sm-Gd), MRE2W2O10 (M = Cd, Co, Mn; RE = Y, Pr, Nd, Sm - Ho), AgY1-xGdx(WO4)2 (0,005  x  1) i AgY1-xNdx(WO4)2 (0,005  x  1) as well as for Cd1-3xGd2xxMoO4 (0,0005  x  0,2222) revealed mostly poor electrical conductivity of n-type defects associated with the dominance of anionic acting as double donors. For some tungstates and molybdates, such as MPr2W2O10 (M = Co, Mn) of unfilled and unscreened 3d orbital of transition metals, a significant increase of the electron emission and the conductance of about two orders of magnitude of 300 K and about three orders of magnitude of 400 K were observed. Similar behavior was found for Cd1-3xGd2xxMoO4 solution in the concentration range of 0.0238  x  0.0839 and a strong emission of electrons at 400 K on the current-voltage characteristics I-V was observed. Measurements of current-voltage characteristics I-V are an excellent tool for studying electron emission materials weakly conductive. They also found that a strong electron emission does not depend on the value of the energy gap. Small electrical conductivity and low emission of electrons were explained with the aid of the deep vacancy trapping centers associated with high energy of creation of silver vacancy of 1.10 eV/ vac. in AgY1 x(Gd,Nd)x(WO4)2. High electrical conductivity and strong emission of electrons for MPr2W2O10 (M = Co, Mn) containing ions that have unfilled and unscreened orbitals explained using the Poole-Frenkel model by the emission of electrons over the potential barrier. High electrical conductivity and strong emission of electrons for Cd1-3xGd2x xMoO4 (0.0238 ≤ x ≤ 0.0839) containing ions, which are paired electrons, completed and screened orbitals explained through the mechanism of small polarons. Optical studies for tungstates and molybdates which have a strong emission of white and red light, and suitable for the production of ceramic matrix laser also exhibit strong electron emission visible in the current-voltage characteristics I-V. These results have been published in the international journals as Materials Science and Engineering: B, Ceramics International i Journal of the European Ceramic Society and presented at many national Polish and international conferences.
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