Recently observed quantum corrections to the conductivity of SnO2 films suggest the existence of extended states and thus raise the question about the presence and mechanism of a metal-insulator transition. We present a comparative analysis of negative magnetoresistance, observed in fields up to 52 T on SnO2 polycrystalline films, performed in the frame of both hopping conduction model and quantum corrections to the conductivity model, with the purpose to establish the ranges of agreement between these models and the obtained data. Our results suggest that the observed negative magnetoresistance of SnO2 films is due to corrections stemming from the weak localization and electron-electron interaction.

The electrical properties of polycrystalline tin dioxide films were investigated by impedance spectroscopy in the frequency range 100 Hz-1 MHz at temperatures 4.2 K, 77 K and 300 K. Analysis of the experimental data by means of complex nonlinear least squares method made it possible to divide the contributions of grain bulk and grain boundaries to the conductivity. It was found that at room temperature charge transport processes are mainly determined by the grain volume while at the low temperatures contribution from the grain boundaries to. the impedance of the system prevails.

Physicochemical features of formation and morphology of submicron honeycomb-structure nitrocellulose films with hexagonal cell symmetry were examined.

We focus in this article on high field magnetoconductance results obtained on SnO 2 polycrystalline (cassiterite) films. The high field magnetoconductance is found to be in agreement with the magnetoconductance universal behavior due to weak localization in two dimensions as suggested by Zduniak et al. (Phys. Rev. B 56: 1996, 1997 ).

On the basis of carboxylated nitrocellulose were formed ordered micronets containing nanoparticles of nickel in their surface layer. The optimal conditions for a uniform coating with nickel of the polymer net film are as follows: activating the surface with solutions of tin(II) and palladium chlorides for 30–60 s and the action of the nickel developer for at least 3 min at 20°C.

Possibility of obtaining cellulose-iron(II, III) oxide composites with various component ratios was studied. The specific magnetization of the material was determined in relation to the mass fraction of the magnetic component in the composite.

Magnetotransport properties of the nanogranular SnO 2 films were invesigated.Non-linear current-voltage (I-V ) characteristics were observed at low temperatures.The temperature dependence of the resistance and non-ohmic I-V curves can be well approximated by fluctuation-induced tunnelling model, indicating importance of the contacts barriers between SnO2 grains.Magnetoresistance was measured within temperature range 2-15.3K and could be consistent with the variable-range hopping conduction mechanism due to existence of localized states on the surface of SnO2 grains.

A new procedure was developed for preparing carbon films with magnetic metal nanoparticles by thermolysis of a polymer precursor. The conditions for preparing carbon films with required surface concentration of the metal nanoparticles and hence with dielectric or metallic properties were determined.

2-dimensional arrays of Co- and Pd-clusters embedded in carbon films were fabricated by means of heat-treatment method of carboxylated cellulose films after the exchange of COOH-group protons by Co- and Pd-cations. The sizes of metal clusters within range 10 nm-1 mum were obtained in dependence on the heat-treatment temperature. The dependencies of the resistance on temperature and magnetic field for the samples annealed at T = 700degreesC and 900degreesC were measured. The R(T) dependencies both for carbon films with Co- and Pd-clusters can be fitted by expression R = R-0 exp(T-0/T)(1/n) inherent for variable-range hopping. In the whole range of investigated magnetic field and temperature magnetoresistance was negative and can be related to quantum interference in the variable range hopping transport along neighboring alternative paths.

Developments of intermediate-temperature solid oxide fuel cells and electrocatalytic reactors require novel electrode and catalyst materials with high performance at 800–1100 K, low-cost processing technologies, and methods for nano-scale surface modification of ion-conducting ceramics. This work presents one promising technique for the synthesis of nanocrystalline powders and components of the electrochemical cells, based on the use of structure-modified cellulose containing metal cations.

The magnetic-field dependence of the magnetoresistive effect in carbon fibers that contain cobalt nanoclusters and exhibit effects of weak localization at low temperatures (T < 45 K) is investigated on the metal side of the dielectric-metal transition. The carbon fibers are prepared by heat treatment of carboxylated cellulose after the substitution of cobalt cations for protons of COOH groups. It is found that, under conditions of two-dimensional weak localization at temperatures below 10 K, the carbon fibers possess an alternating magnetoresistance due to spin-orbit scattering of electrons by cobalt nanoclusters. The time of phase breaking of the wave function and the time of spin-orbit scattering are determined from a comparison of the experimental and theoretical magnetic-field dependences of the electrical resistance.

Magnetic characteristics of cobalt nanoclusters in carbon fibers prepared by heat-treating carboxylated cellulose after substitution of cobalt cations for the protons of the COOH groups in them have been studied. It has been established that the cobalt nanoclusters with an average diameter d congruent to 10 nm in carbon fibers prepared by vacuum annealing at a temperature T-f=700 degrees C are in a superparamagnetic state at room temperature, whereas the fibers annealed at T-f=900 degrees C are ferromagnetic and have a coercive force B-c=0.043 T. The temperature of blocking the magnetic moment of cobalt nanoclusters has been determined to be T-b=73 K, and the effective magnetic anisotropy constant, K-eff=1.7x10(5) J/m(3). It has been shown that the temperature dependence of magnetization in the superparamagnetic state is satisfactorily described by the Langevin equation with a temperature-independent average magnetic moment of the nanoclusters; the temperature dependence of the coercive force is described by a relationship B-c infinity T-0.77 characteristic of noninteracting magnetic nanoclusters.

Developments of intermediate-temperature solid oxide fuel cells (IT SOFCs) require novel anode materials with a high electrochemical activity at 800-1070 K. The polarization of cermet anodes, made of nickel, ceria and yttria-stabilized zirconia (YSZ) and applied onto a YSZ solid electrolyte, can be significantly reduced by catalytically active ceria additions, the relative role of which increases with decreasing temperature. Further improvement is observed when using Ce0.8Gd0.2O2-delta (CGO) having a high oxygen ionic conductivity instead of undoped ceria, owing to enlargement of the electrochemical reaction zone. Nanocrystalline CGO powders with grain sizes of 8-35 nm were thus synthesized via the cellulose-precursor technique and introduced into Ni-CGO-YSZ cermets, and tested in contact with a (La0.9Sr0.1)(0.98)Ga0.8Mg0.2O3-delta (LSGM) electrolyte at 873-1073 K. The results showed that the anode performance can be enhanced by additional surface activation, in particular by impregnation with a Ce-containing solution, and also by incorporation of YSZ, which probably acts as a cermet-stabilizing component. The overpotential of the surface-modified Ni-CGO (25 wt%-75 wt%) anode in a 10% H-2/90% N-2 atmosphere was approximately 110 mV at 1073 K with a current density of 200 mA/cm(2).

The electrocatalytic activity of composite anodes, consisting of micron-scale sized Pt particles and nanocrystalline Ce0.8Gd0.2O2-δ(CGO) prepared by the cellulose-precursor technique, was evaluated for the oxidation of dry methane in a solid oxide fuel cell (SOFC) with zirconia-based electrolyte at 1173 K. Increasing current density above 100 mA/cm2 and the corresponding decrease of CH4/O2 ratio down to 2–3 suppressed carbon formation, but decreased CO/CO2 molar ratio in the product mixture to 0.3–0.9. The methane conversion rate was found to increase linearly with current, suggesting an increasing role of total CH4 oxidation by oxygen electrochemically supplied onto the anode surface. The results show that, although ceria-based anode components are well known to improve SOFC performance, their presence leads to high CO2 selectivity and thus seems inappropriate for the generation of synthesis gas in SOFC-type reactors.

Full Paper: Nanosized particles of different metals in a polymer matrix have attracted considerable interest in various research fields of chemistry, because of their physical and chemical catalytic properties and their application potential in nanoelectronics. In this paper, we present an experimental method for the preparation of self-assembled honeycomb carbon network patterns with Co and Ni particles. Starting from a 2% carboxylated nitrocellulose solution in amyl acetate submerged in cooled distilled water, we already observe the above self-organized uniform-size net structure at the top of the water surface. The submergence of the carboxylated nitrocellulose network in 0.25 M water solution of cobalt acetate Co(CH3COO)(2) (or nickel acetate Ni(CH3COO)(2)) for 1 h leads to the ion-exchange introduction of inorganic cations Co2+ (or Ni2+) to a polymer matrix. In order to obtain samples with a large content of Co (or Ni) cations, we have carried out the sedimentation of the ion-connected Co (or Ni) with the oxalic acid H2C2O4 and, next, repeated the sorption of carboxylated nitrocellulose with the cations Co2+ (or Ni2+). For the fabrication of the Co (or Ni) nanoparticles, the carboxylated nitrocellulose networks, received after a first, second, and third sorption cycle, were heated under vacuum conditions (10(-5) mbar) at Tgreater than or equal to500degreesC for 2 h. The process of implementing the Co and Ni nanoparticles in the carbon network was systematically characterized with the help of measuring the specific resistivity in the temperature range from 4.2 to 295 K.

Carbon fibers with inclusions of cobalt nanoclusters are prepared by heat treatment of carboxylated cellulose containing cobalt cations. The influence of the heat treatment conditions on the structuring of the carbon matrix and cobalt clusters, the magnetization hysteresis loop, the temperature dependence of the conductivity, and the magnetoresistance is investigated. It is established that the cobalt-containing carbon fibers heat treated at T M =700 and 900°C possess superparamagnetic and ferromagnetic properties, respectively. It is shown that fibers heat treated under different conditions are characterized by different conduction mechanisms and can exhibit anisotropic and giant magnetoresistances and also the effects associated with the influence of magnetic field on the processes of weak localization and spin-orbit scattering.

The influence of the electrical field on the variable range hopping process of porous carbon networks is examined in the range of validity of the law ln σ(T)∝T−1/2, where σ and T mean electrical conductivity and temperature, respectively. We show that the field dependence of the samples investigated in the vicinity of the metal–insulator transition clearly distinguishes four characteristic regions. At low values of the applied electrical field, we have ohmic conductivity. Upon increasing the electrical field E, the electrical conductivity σ rises, first following the law ln σ(E)∝En, where n changes from 1.4 to 2.6 with increasing distance from the metal–insulator transition on the insulating side. Then, at higher electrical field, the conductivity turns to the relation ln σ(E)∝E1.0. The temperature dependence of the hopping length of the charge carriers, determined within the above field regime, develops as l(T)∝T−0.9. At temperatures where the ohmic behavior in the Coulomb gap occurs and obeys the law ln σ(T)∝T−1/2, the electrical conductivity caused by thermally nonactivated charge carriers at high fields complies with ln σ(E)∝E−1/3. The current density j changes as ln j(E)∝E−1/6. The temperature dependence of the threshold electrical field, which characterizes the transition from the low-field to the high-field range, follows Eth∝T1.5.

A successful method to fabricate large-area, microstructured sieve materials (see Figure) from a poly(p-phenylene-vinylene) (PPV) precursor and from poly(3-octylthiophene) (P3OT) is reported. Strong periodicity and a large, defect-free area make this structure extremely interesting for polymer optoelectronics. The underlying mechanism of the self-organized pattern-formation is elaborated in this work.

Dense lanthanum cobaltite ceramics with different microstructures were prepared using several processing procedures, including chemical and ceramic synthesis routes. XRD, SEM, dilatometry, total electrical conductivity and oxygen permeability measurements were used for the characterization of these materials. Submicrometer size LaCoO3−δ powders obtained via a cellulose-precursor technique or a combustion synthesis process showed much higher sinterability and poor compactability with respect to the powder prepared by the standard ceramic procedure. The influence of the processing route on crystal lattice, electronic conductivity and thermal expansion of LaCoO3−δ ceramics was negligible. At the same time, the preparation technique significantly affects the ceramic microstructure and the oxygen ionic conductivity. LaCoO3−δ membranes prepared via the standard ceramic technique, involving higher sintering temperatures, exhibit significantly higher oxygen permeation fluxes than ceramics prepared from organic precursors. This behavior was attributed to the effect of grain-boundary resistivity to ionic transport, which decreases with increasing sintering temperature and grain size, as commonly found for oxide solid electrolytes.