Charakterystyki prądowo-napięciowe wolframianów i molibdenianów srebra, kadmu, kobaltu, manganu, miedzi i metali ziem rzadkich

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 Cd1-3xGd2xxMoO4 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 Cd1-3xGd2xxMoO4 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 Cd1-3xGd2xxMoO4 (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 Cd1-3xGd2xxMoO4 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

Electrical transport properties of Yb8-xYxV2O17 (x = 0; 2; 8)

The UV-vis-NIR and electrical properties of Yb8xYxV2O17 for x = 0, 2, and 8 were investigated. The band energy gap of 2.6 eV determined for Yb6Y2V2O17 (x = 2) and comparable for the remaining compounds with x = 0 and 8 is characteristic for insulators. Low electrical conductivity with a characteristic minimum shifting to higher temperatures from 322, via 360 to 370 K in the sequence x = 0, 2 and 8, which decreases with increasing content of ytterbium was observed. Temperature dependence of thermoelectric power showed np transition at 410 and 467 K for x = 0 and 2, respectively, and n-type conductivity for x = 8, indicating mainly n-type electrical conductivity. A breakdown voltage of 26 V/mm is mainly observed for the IV characteristics at 400 K and showing a varistor-like behawior[…

Influence of Cr-substitution on the electrical properties of Fe1-xCrxSnSbO6

Fe1xCrxSnSbO6 solid solution shows semiconducting behaviour with the activation energy decreasing from EA = 0:64 eV for x = 0:0 to EA = 0:32 eV for x = 1:0 in the intrinsic conductivity temperature region as well as the n-type conduction at room temperature. The I–V characteristics and the conductance G at 300 and 400 K showed symmetrical and nonlinear behavior in the voltage range (–100, 100 V) suggesting the electron emission over the potential barrier especially for the boundary compounds FeSnSbO6 and CrSnSbO6. These effects are discussed in the context of the energy gap Eg > 1:6 eV many times greater than the thermal energy kT

Electrical properties of Sr2InV3O11

The electrical conductivity (T) and thermoelectric power S(T) measurements of Sr2InV3O11 showed the insulating state and the change of sign of thermopower from p to n at Tnp = 400 K. The I–V characteristics provided the evidence of symmetrical and non-linear behaviour typical of strong emission of charge carriers induced by temperature and voltage. Relative dielectric permittivity "r as well as loss tangent (tan ) strongly depend both on the temperature in the range of 295–400 K and the frequency in the range of 5 102 to 1 106 Hz, showing the broad maximum at 320 K. These effects are considered as a relaxation process like in the Maxwell–Wagner or Jonscher model as well as the conduction of electric current, as determined by the Joule–Lenz law[…

Electrical and optical properties of new Pr3+-doped PbWO4 ceramics

Polycrystalline samples of new scheelite-type tungstates, Pb1 3x xPr2xWO4 with 0.0098 6 x 6 0.20, where denotes cationic vacancies have been successfully prepared by a high-temperature solid-state reaction method using Pr2(WO4)3 and PbWO4 as the starting reactants. The influence of the Pr3+ substitution in the scheelite framework on the structure and optical properties of prepared new ceramic materials has been examined using powder X-ray diffraction method (XRD) and UV-Vis-NIR spectroscopy. The results of dielectric studies of Pb1 3x xPr2xWO4 samples showed both low values of dielectric constant (below 14) and loss tangent (below 0.2). The electrical conductivity and thermoelectric power measurements revealed a low conductivity ( 2 × 109 S/m) and the sign change of thermoelectric power around the temperature of 366 K suggesting the p-n transition. These results are discussed in the context of vacancy, acceptor and donor levels as well as the Maxwell-Wagner model

Correlation between the band-gap energy and the electrical conductivity in MPr2W2O10 tungstates (Where M = Cd, Co, Mn)

The values of the direct allowed energy gap determined from the UV-vis-NIR measurements and Kubelka– Munk transformation decrease from 3.38 via 2.70 to 2.42 eV for MPr2W2O10 in the sequence M = Cd, Co, Mn, while the values of the activation energy increase from 0.11 via 0.44 to 0.47 eV in the same sequence. In other words, the higher the activation energy, the smaller the energy gap. Because the energy gap is typical for insulators, so electron transport phenomena are considered under the Poole–Frenkel effect and small-polaron mechanism

Effect of tantalum substitution on dielectric constant of ZnSb2-xTaxO6 solid solution (x=0.0,0.1,0.25,0.75,1.6)

The electrical measurements of ZnSb2xTaxO6 phases with x = 0:0; 0:1; 0:25; 0:75; 1:6 have revealed insulating behavior with strongly decreasing electrical conductivity when tantalum x content is increased. As a consequence, high values of relative permittivity "r and loss tangent tan( ) were observed, that decreased with increase of temperature and frequencies, for samples with low Ta content, below x = 0:75. In turn, for samples richer in tantalum, "r reaches 14, and tan( ) becomes as low as 0.02 for x = 1:6. These effects have been interpreted either by the framework of the relaxation processes, or by the spatial charge polarization leading to the low energy loss of materials

Effect of Magnesium Substitution on Dielectric Constant of Zn2-xMgxInV3O11 (x = 0.0, 0.4, 1.6) Solid Solutions

The results of magnetic and dielectric measurements of Zn2xMgxInV3O11 phases with x = 0:0, 0.4, and 1.6 showed diamagnetic behavior above room temperature and a strong increase in the relative electrical permittivity, "r, with an increase in the magnesium content as well as the high loss tangent, tan , above 150 K, irrespective of the magnesium content in the sample. With the increase in the frequency of the electric field both "r and tan strongly decreased. At low temperatures a residual paramagnetism coming from the paramagnetic ions of vanadium was observed. These effects were interpreted within a framework of the vacancy trapping centers acting as the accumulation of electric charges on the one hand and a mixed valence of vanadium ions on the other

Dielectric and magnetic characteristics of Ca1−xMnxMoO4 (0 ≤ x ≤ 0.15) nanomaterials

Scheelite-type Ca1−xMnxMoO4 (x = 0.0,0.01, 0.05, 0.10 and 0.15) nanomaterials were successfully synthesized via a combustion route. Dielectric studies showed a weak n-type electrical conductivity characteristic for insulators and low relative permittivity (εr < 15) decreasing with increasing Mn2+ content. CaMoO4 and Mn2+-doped nanomaterials are chemically compatible with Al and Ag electrodes and promising for lowtemperature co-fired ceramic applications. Magnetic studies showed, at room-temperature diamagnetism for pure CaMoO4, the balance between diamagnetism and paramagnetism for Ca1−xMnxMoO4 (x = 0.01) and paramagnetic behaviour when 0.05 ≤ x ≤ 0.15 as well as the short-range antiferromagnetic interactions growing in strength as Mn2+ content increases. The Landé factor fitting procedure showed a spin-only contribution to the magnetic moment. CaMoO4 matrix unexpectedly revealed the residual paramagnetism at low temperatures derived probably from the molybdenum ions having unpaired 4d electrons as well as a paramagneticdiamagnetic transition at 70 K

Combustion synthesis, structural, magnetic and dielectric properties of Gd3+-doped lead molybdato–tungstates

Gd3+-doped lead molybdato–tungstates with the chemical formula of Pb1–3x□xGd2x(MoO4)1–3x(WO4)3x (where x = 0.0455, 0.0839, 0.1430, corresponding to 9.53, 18.32, 33.37 mol% of Gd3+, respectively, as well as □ denotes cationic vacancies) were successfully synthesized via combustion route. The XRD and SEM results confirmed the formation of single-phase, tetragonal scheelite-type materials (space group I41/a) with the uniform, spherical and oval grains ranging from 5 to 20 μm. Individual grains are strongly agglomerated into big clusters with the size even above 50 μm. The magnetic measurements as well as the Brillouin fitting procedure showed paramagnetic state with characteristic superparamagnetic-like behaviour and the short-range ferromagnetic interactions. The electrical and broadband dielectric spectroscopy studies revealed insulating properties with the residual electrical n-type conduction of 2×10–9 S/m and low energy loss (tanς ~ 0.01) below 300 K. Dielectric analysis showed that no dipole relaxation processes in the Gd3+-doped materials were observed. A fit of dielectric loss spectra of Gd3+-doped samples by sum of the conductivity and the Havriliak–Negami, Cole–Cole, and Cole–Davidson functions confirmed this effect