Frequency-dependent electrical properties in the system SnO2-TiO2

The a.c. impedance response of polycrystalline ceramics Sn_xTi_-x1O₂ was studied in the air in the temperature range 298–1073 K. The impedance spectra were analyzed in terms of two proposed equivalent circuits involving both resistor and non-Debye constant phase element (CPE). The presence of CPE elements may be interpreted by diffusion phenomena. The results indicate a negligible contribution of grain boundary resistance to the total resistivity of the studied materials. The determined activation energy of the bulk conductivity of TiO₂ was compared with literature data. © 2001 Kluwer Academic Publishers     

Dependence of electrical properties on thermal temperature in nanocrystalline SnO2 thin films

Nanocrystalline SnO2 thin films were prepared by pulsed laser deposition techniques on clean glass substrates, and the films were then annealed for 30 min from 50 to 550 degrees C with a step of 50 degrees C, respectively. The investigation of X-ray diffraction confirmed that the various SnO2 thin films were consisted of nanoparticles with average grain size in the range of 23.7-28.9 nm. Root-mean-square surface roughness of the as-prepared SnO2 thin film was measured to be 25.6 nm which decreases to 16.2 nm with thermal annealing. Electrical resistivity and refractive index were measured as a function of annealing temperature, and found to lie between 1.24 to 1.45 momega-cm, and 1.502 to 1.349, respectively. The results indicate that nearly opposite actions to root-mean-square surface roughness and electrical resistivity make a unique performance with thermal annealing temperature. The post annealing shows greater tendency to affect the structural and electrical properties of SnO2 thin films which composed of nanoparticles.     

Seebeck coefficient and electrical conductivity of mesoscopic nanocrystalline SrTiO3

In the present study, we investigate the effect of the grain boundaries on both the electrical transport and the thermoelectric properties. For this purpose, the Seebeck coefficient and the electrical conductivity of a model material, such as nominally pure SrTiO₃ (single crystal, microcrystalline, and nanocrystalline), is measured under oxidizing conditions. The impedance spectroscopy measurements reveal a strong change of the conduction properties of the nanocrystalline sample compared with the unperturbed bulk properties, namely a reduction of the p-type conductivity by two orders of magnitude at high oxygen partial pressure. Similarly, the Seebeck coefficient values of the nanocrystalline sample exhibit remarkable deviations from the single crystal ones: Under oxidizing conditions, values up to 2160 μV K^?1 (at 575 °C) are detected. More importantly, in the nanocrystalline sample, the dependence of the Seebeck coefficient on the concentration of the charge carriers is found to be four times larger than in the single crystal.     

A novel method to prepare nanocrystalline SnO2

5 nm crystallites of SnO₂ were prepared by hydrothermal technique in nitric acid solution starting from pure metallic Sn. The phase and size were determined by XRD and TEM. The specific surface area measured by BET method was as high as 150.6 m²/g. IR spectrum revealed that the content of water was rather low. The contents of other metallic impurities were determined by XPS and AES, respectively.     

Ni doping effect on electrical conductivity of ZnO nanocrystalline thin films

The electrical conductivity behavior of undoped and Ni-doped ZnO nanocrystalline thin films prepared by spin-coating method was investigated as a function of temperature. The films were found to have polycrystalline structure. Grain size and the conductivity of the films were found to decrease significantly with increase in Ni concentration. This behavior was well explained by the grain boundary conduction model that takes into account electron trapping in surface states. It was observed that by increasing the Ni-doping level the surface trap density increases and implicitly the conductivity decreases.     

Structural and magnetic properties of nanocrystalline Fe-doped SnO2

Single phase Sn_1?xFe_xO₂ (x = 0.1, 0.3 and 0.5) nanoparticles having size in the range 9–17 nm were prepared by a chemical route. XRD analysis of the samples confirmed the formation of cassiterite phase without any impurity. The nanocrystalline nature of the samples and their crystallinity were confirmed by TEM measurements. Raman and Mössbauer spectroscopy studies indicate structural disorder and vacancies in the nanostructures. M–H and M–T data recorded by a SQUID magnetometer show that the samples are essentially paramagnetic with a weak ferromagnetic component. Ferromagnetism is argued to be associated with the vacancies and defects present in the grain boundaries and interfaces of the nanoparticles.     

Synthesis of nanocrystalline SnO2 powder by amorphous citrate route

Nanocrystalline SnO₂ has been synthesized by liquid mix technique using citric acid as the complexing agent. The tin oxide powder obtained at different calcination temperatures (773–1223 K) is characterized using powder X-ray diffraction (XRD), SEM, TEM, TG-DTG and UV spectroscopic techniques. The material obtained is nanocrystalline, having particle size in the range of 10–14 nm. The technique is cost-effective and yields the desired product at temperatures as low as 773 K.     

119Sn magic angle spinning NMR of nanocrystalline SnO2

Nanocrystalline SnO2 samples of different grain sizes, prepared by inert gas condensation technique (IGCT) and chemical precipitation method and conforming to the tetragonal phase, have been studied by variable speed (3-10 kHz) 119Sn MAS NMR at 11.74 Tesla field. 119Sn solid-state NMR results show that the IGCT prepared samples have good crystallinity and phase purity compared to the samples prepared by the chemical method. The determination of 119Sn chemical shielding parameters (delta iso, delta delta and eta) from slow MAS spectra shows that the 119Sn isotropic chemical shift (delta iso) is strongly influenced at smaller grain sizes, attributable to the change in the O2- local symmetry for the surface 119Sn ions at smaller grain sizes. The observed line widths in MAS spectra are significantly larger than the life-time broadening due to spin-lattice (T1) and spin-spin (T2) relaxation. The 119Sn MAS NMR spectra are thus inhomogeneously broadened by a distribution of isotropic chemical shifts, the line broadening increasing with decrease in grain size.     

Structural and electrical conductivity studies on undoped and copper-doped nanocrystalline zinc sulphide

Zinc sulphide (ZnS) and copper-doped zinc sulphide nanocrystallites (ZnS:Cu) of average size 4 and 3 nm, respectively, have been synthesized by chemical precipitation method. Structural and morphological studies using X-ray and high resolution transmission electron microscopy (HRTEM) measurements have confirmed hexagonal structure for the samples. Using impedance spectroscopy, the effect of grain interior and grain boundary regions on the electrical conductivity have been studied at various temperatures. In the high temperature region, the grain boundary contribution to conduction is found to be larger than that of the grain interior region. Further, the activation energies of charge carriers in both the grain interior and grain boundary regions have been determined. The conduction mechanism of copper-doped zinc sulphide nanocrystallites have been studied at various temperatures and the results are reported.     

Effect of nanocrystalline phase on the electrical conductivity of amorphous tantalum-containing silicon-carbon films

We have studied the effect of metal concentration on the electrical conductivity of tantalum-containing silicon-carbon nanocomposite films. The results demonstrate that carrier transport in the films follows different mechanisms at low and high metal concentrations. When the concentration of the conducting nanocrystalline tantalum carbide phase is under 28 at %, conduction is due to hopping. Conducting-phase concentrations from 28 to 34 at % correspond to a percolation threshold. At higher tantalum carbide concentrations, conduction is through the tantalum carbide nanocrystals. The effect of heat treatments on the electrical conductivity of the material is analyzed.     

Structural and electrical conductivity studies on nanocrystalline undoped and silver doped zinc sulphide

Zinc sulphide (ZnS) and silver doped zinc sulphide nanocrystallites (ZnS:Ag) of average size 4 and 8 nm, respectively, have been synthesized by chemical precipitation technique. The structural and morphological studies using X-ray and high resolution transmission electron microscopy (HRTEM) measurements have confirmed hexagonal structure for the samples. Using the impedance spectroscopy method, the effect of grain interior and electrode–sample interface effect on their conductivity have been studied at various temperatures. In high temperature region, the electrode–sample interface effect is found to be larger than that of the grain interior region. Further, the results of the activation energies of the charge carriers in both the regions are determined and analyzed. The conduction mechanism of silver doped zinc sulphide nanocrystallites has been studied at various temperatures and the results are reported.     

Microstructure correlated electrical conductivity of Manganese alloyed nanocrystalline cubic zirconia synthesized by mechanical alloying

Fully stabilized cubic (c) ZrO₂ phase has been synthesized by mechanical alloying (MA) the stoichiometric powder mixture of elemental Mn (5–20 mol%) and monoclinic (m) ZrO₂ at room temperature. XPS study reveals that major part of metallic Mn is ionized to Mn²⁺ oxidation state during MA. Mn-alloyed c-ZrO₂ nanoparticles with ～18 nm particle size have been synthesized within 10 h of MA. Microstructures of the compounds have been precisely evaluated by analyzing the X-ray powder diffraction patterns employing Rietveld refinement and transmission electron microscopy images. A decrease in lattice parameter from 5.11 Å to 5.09 Å is correlated with an increase in oxygen vacancy from 14% to 26% with increasing Mn concentrations. Elemental compositions in the compounds are obtained from electron probe microanalysis. The role of Mn alloying in the polymorphic phase transformation (m → c) has been established with changes in structure and microstructure parameters. Electrical conductivities of all c-ZrO₂ compounds are measured in the temperature range 350–550 °C. Grain and grain boundary contributions to total conductivity are calculated from frequency dependent real and imaginary impedance. Conductivity of Mn alloyed c-ZrO₂ increases with increasing temperature and Mn concentrations. Electrical transport mechanism in the compound is studied by impedance and modulus spectroscopy. The relaxation frequency is found to be temperature, microstructure and composition dependent.     

Microwave-assisted hydrothermal synthesis of nanocrystalline SnO powders

Tin oxide (SnO) powders were obtained by the microwave-assisted hydrothermal synthesis technique using SnCl₂·2H₂O as a precursor. By changing the hydrothermal processing time, temperature, the type of mineralizing agent (NaOH, KOH or NH₄OH) and its concentration, SnO crystals having different sizes and morphologies could be achieved. The powders were characterized by X-ray diffraction (X-ray), Field Emission Scanning Electron Microscopy (FE-SEM), High Resolution Transmission Electron Microscopy (HR-TEM) and Selected Area Electron Diffraction (SAED). The results showed that plate-like form is the characteristic morphology of growth and the TEM analyses indicate the growth direction as (200).     

Microstructure and electrical properties of MgO-doped SnO2 varistor ceramics

The effect of MgO addition on the properties of (Co, Nb, Cr)-doped SnO₂ varistors was investigated. The samples with different MgO concentrations were fabricated by the conventional ceramic method and sintered at 1,250, 1,300, 1,350 and 1,400 °C for 2 h. It was found that the nonlinear coefficient presented a peak value of 28 and lowest leakage current density of 7 μA/cm² when 0.5 mol% MgO was added. The breakdown electrical field increased from 174 to 531 V/mm with increasing MgO from 0.0 to 2.0 mol%. The relative dielectric constant decreased with increasing MgO from 0.0 to 0.5 mol%, but increased with more MgO added. The dielectric loss decreased obviously in the case of low frequency with MgO added, and it had the lowest value when 0.5 mol% MgO added. The optimal samples were obtained by doping MgO with 0.5 mol% and sintering at 1,350 °C.     

Sensitivity to oxygen and response characteristics of nanocrystalline SnO2 at room temperature

The electrical properties of compressed nanocrystalline tin (IV) oxide were studied. It was observed that the room temperature electrical resistance of nanocrystalline SnO₂ samples increased with increasing partial pressure of oxygen. In addition, when there is a sudden change in the oxygen partial pressure, a short pulse of spontaneous electromotive force of the millivolt order can be detected. This new finding has the potential application of leakage detection in vacuum systems or pressure vessels.     

Comparison between two combustion routes for the synthesis of nanocrystalline SnO2 powders

Two nitrate–citrate gel combustion routes for the synthesis of nanocrystalline SnO₂ powders were compared. Metallic tin and tin dichloride were used as raw materials. The combustion process was more intense for the metallic tin route, resulting in smaller crystallite size (10.5 nm) and higher specific surface area (49.4 m²/g). The presence of chloride ions in powders obtained by the tin dichloride route was also studied.     

Preparation, characterization and electrical conductivity studies of nanocrystalline La doped BaMoO4

Different compositions of scheelite type nanocrystalline La doped BaMoO₄ [Ba_1−xLa_xMoO_4+x/2, where x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5] samples were prepared by acrylamide assisted sol-gel combustion process. Dried gels prepared at 60 °C were heated at different temperatures and characterized using TG/DTA, XRD, FTIR and SEM-EDX techniques. From XRD patterns, crystalline phases for La doped BaMoO₄ samples were confirmed and their average crystallite sizes were calculated using Scherrer's formula and it was found to be less than 80 nm. Structure and thermal behavior of scheelite type nanocrystalline La doped BaMoO₄ samples were identified respectively using FTIR and TG/DTA measurements. Microstructure and existence of O, La, Ba and Mo elements in the La doped BaMoO₄ samples were obtained from SEM-EDX and HRTEMtechniques. The ‘d’ spacing values were obtained for different (h k l) planes and were well matched with the standard BaMoO₄. (h k l) values for different directions of planes were assigned for the observed HRTEM images and were matched with standard BaMoO₄. Grain and grain boundary conductivities were evaluated by analyzing the impedance data, using the Winfit software, measured at different temperatures.