Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique
01 Feb 2006-Czechoslovak Journal of Physics (Kluwer Academic Publishers-Consultants Bureau)-Vol. 56, Iss: 2, pp 201-210
TL;DR: The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behavior of ceramic nanocomposite (Ni-SiO2) at low temperature down to 77 K are reported in this article.
Abstract: The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO2) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω
s
. The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.
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TL;DR: A novel catalyst has been produced that has been shown to be effective in the production of hydrogen from the steam reforming of ethanol by a simple sol-gel method and compared to catalysts prepared by an impregnation method.
Abstract: Catalytic steam reforming of ethanol has been regarded as a promising way to produce hydrogen. However, catalytic deactivation is a key problem in the process. In this paper, a novel nano-Ni/SiO2 catalyst was prepared by a simple sol−gel method and compared to catalysts prepared by an impregnation method in relation to the steam reforming ethanol process. Good Ni dispersion and high BET surface areas (>700 m2 g−1) were obtained for sol−gel catalysts, whereas only 1 m2 g−1 surface area was obtained for the Ni/SiO2 impregnation catalyst. The results of catalytic steam reforming of ethanol showed that about twice of the hydrogen production was produced with the Ni/SiO2 catalyst prepared by sol−gel (around 0.2 g h−1) compared with that prepared by impregnation (around 0.1 g h−1). The analysis of the used catalysts showed that 10Ni/SiO2-B and 20Ni/SiO2-B presented the highest stability, while other catalysts were fragmented into small pieces after the reforming process, especially the catalysts prepared by imp...
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References
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01 Jan 1984
TL;DR: In the last fifteen years, there has been a noticeable shift towards impure semiconductors -a shift which came about because it is precisely the impurities that are essential to a number of major semiconductor devices.
Abstract: First-generation semiconductors could not be properly termed "doped- they were simply very impure. Uncontrolled impurities hindered the discovery of physical laws, baffling researchers and evoking pessimism and derision in advocates of the burgeoning "pure" physical disciplines. The eventual banish ment of the "dirt" heralded a new era in semiconductor physics, an era that had "purity" as its motto. It was this era that yielded the successes of the 1950s and brought about a new technology of "semiconductor electronics." Experiments with pure crystals provided a powerful stimulus to the develop ment of semiconductor theory. New methods and theories were developed and tested: the effective-mass method for complex bands, the theory of impurity states, and the theory of kinetic phenomena. These developments constitute what is now known as semiconductor phys ics. In the last fifteen years, however, there has been a noticeable shift towards impure semiconductors - a shift which came about because it is precisely the impurities that are essential to a number of major semiconductor devices. Technology needs impure semiconductors, which unlike the first-generation items, are termed "doped" rather than "impure" to indicate that the impurity levels can now be controlled to a certain extent."
1,904 citations
TL;DR: In this article, a comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials, and it is concluded that the behavior at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.
Abstract: The various origins of a frequency-dependent conductivity in amorphous semiconductors are reviewed, stressing particularly recent advances and the influences that factors such as correlation and non-random spatial distributions of electrically active centres can have on the a.c. conductivity. A comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials. It is concluded that the a.c. behaviour at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.
1,814 citations
TL;DR: The current state of polaron theory as applicable to transition metal oxides is reviewed in this article, including problems such as impurity conduction where disorder plays a role, and an estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy.
Abstract: The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO 2 and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.
1,063 citations
TL;DR: In this article, a unified theoretical treatment of the complex a.c. conductivity is given, within the pair approximation, for single electron tunnelling and hopping in both uncorrelated and strongly correlated cases, and the discussion is extended to pair processes and to atomic relaxation.
Abstract: Recent developments in the theoretical analysis and experimental study of frequency-dependent loss by relaxation in amorphous semiconductors are reviewed. A unified theoretical treatment of the complex a.c. conductivity is given, within the pair approximation, for single electron tunnelling and hopping in both uncorrelated and strongly correlated cases, and the discussion is then extended to pair processes and to atomic relaxation. The problems associated with measuring the frequency dependent conductivity of amorphous samples are considered, and relevant measurements reported for the different classes of amorphous semiconductors, tetrahedral and group V materials and chalcogenides are reviewed in the light of the available theoretical models. The similarity in the magnitudes and frequency, temperature and electric field dependences of the losses observed in many different systems at liquid helium temperatures is noted, and the possible physical reasons for this are examined.
1,025 citations
TL;DR: In this paper, it was shown that the observed temperature dependence of low-field conductivity in granular metals can be attributed to a relationship between the separation of neighboring metal grains and the electrostatic energy required to create a positive-negative charged pair of grains.
Abstract: We present evidence that in granular metals the observed temperature dependence of the low-field conductivity, $\mathrm{exp}(\ensuremath{-}\frac{b}{{T}^{\ensuremath{\alpha}}})$ with $\ensuremath{\alpha}=\frac{1}{2}$, can be ascribed to a relationship $s{E}_{c}=\mathrm{const}$ between $s$, the separation of neighboring metal grains, and ${E}_{c}$, the electrostatic energy required to create a positive-negative charged pair of grains. This relationship results from simple considerations of the structure of granular metals. The predictions of the theory, for both the high- and the low-field electrical conductivity, are in excellent accord with experimental results in granular Ni-Si${\mathrm{O}}_{2}$ films.
765 citations