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Journal ArticleDOI

Electrical Properties of Nanocomposites

01 Jan 2000-Transactions of The Indian Ceramic Society (Taylor & Francis)-Vol. 59, Iss: 1, pp 1-11

AbstractThe present state-of-the-art for synthesizing composites of nanometer-sized phases (metal, semiconductor or ceramic) dispersed in a matrix has been reviewed. Both dc and ac electrical properties of different nanocomposites synthesized have been described. The theoretical models used to explain the experimental results are discussed. It appears that there is a need to have newer theoretical models developed to understand the ac electrical properties of metal-ceramic nanocomposites.

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Citations
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Abstract: The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superiattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.

36 citations

Journal ArticleDOI
TL;DR: This paper reviews and enhances numerical models for determining thermal, elastic and electrical properties of carbon nanotube-reinforced polymer composites and validated by comparison with various experimental datasets reported in the recent literature.
Abstract: This paper reviews and enhances numerical models for determining thermal, elastic and electrical properties of carbon nanotube-reinforced polymer composites. For the determination of the effective ...

25 citations

Journal ArticleDOI
Abstract: Ceramic based composites with dispersion of nano sized metal/metal carbide particles have generated wide technological interest for their improved mechanical properties — hardness, fracture strength as well as fracture toughness, superior electrical properties and magnetic properties. In the present investigation alumina–silica gels have been prepared along with nickel chloride and dextrose distributed in the nanometric pores of the gel. The gels are prepared with different molar proportions of alumina and silica containing 5 wt% of nickel chloride and 50 wt% excess dextrose. During heat treatment at a temperature of 900°C for half an hour in nitrogen atmosphere, nickel chloride is reduced to metallic nickel by in situ generated hydrogen in the silica–alumina matrix. X-ray analyses indicate that no nickel chloride reduction is possible upto 50 mol% silica in alumina–silica matrix. Beyond this range, higher the silica content, higher is the reduction of nickel chloride. The presence of metallic nickel has been substantiated further by SAD analysis. Particle size analysis based on X-ray diffraction as well as transmission electron micrograph shows the presence of nickel particles of size ∼20 nm distributed in the alumina–silica nanocomposite.

14 citations

Journal ArticleDOI
Abstract: Molybdenum carbide has been formed in the silica gel matrix by the in situ reduction of ammonium molybdate. The average particle size of molybdenum carbide is in the range of 5-15 nm. The detailed XRD analyses coupled with thermodynamic arguments show that reduction of ammonium molybdate is predominantly by in situ generated hydrogen.

2 citations

Journal ArticleDOI
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.

1 citations


References
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Journal ArticleDOI
24 Nov 1995-Science
Abstract: The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superlattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.

1,926 citations

Journal ArticleDOI
Abstract: In this paper we present a novel and versatile t e c h n i q u e f o r t h e p r o d u c t i o n o f u l t r a f i n e m e t a l p a r t i c l e s by evaporation from a temperature‐regulated oven containing a reduced atmosphere of an inert gas. An extensive investigation of particles of oxidized Al, with diameters of 3 to 6 nm, has been performed. We have also studied ultrafine particles of Mg,Zn, and Sn produced in the same manner. A supplementing investigation has been carried out for particles of Cr, Fe, Co, Ni, Cu, and Ga, as well as larger Al particles, produced by ’’conventional’’ inert‐gas evaporation from a resistive filament. Diameter as a function of evaporation rate, inert‐gas pressure, and the kind of inert gas are reported. Crystalline particles smaller than 20 nm look almost spherical in the electron microscope, while larger ones often display pronounced crystal habit. S i z e d i s t r i b u t i o n s have been investigated in detail, and consistently the logarithm of the particle diameter has a Gaussian distribution to a high precision for the smallest sizes, whereas larger particles deviate from such a simple behavior. A statistical growth model, based on the Central Limit Theorem, has been formulated for liquidlike coalescence of particles; this theory accounts satisfactorily for all our data, as well as for most size distributions published in the literature. Applications of the model to colloids, discontinuous films, and supported catalysts are discussed. By comparing size distributions for particles produced by a variety of techniques we found a number of empirical rules for the width of the distributions, as defined by a (geometric) standard deviation σ. For crystalline inert‐gas‐ evaporated particles we obtained consistently 1.36?σ?1.60; for coalescing islands in discontinuous films we found 1.22?σ?1.34; and similar rules are applicable to colloids, supported catalysts, and to ultrafine droplets.

1,448 citations

Journal ArticleDOI
20 Sep 1996-Science
Abstract: Close-packed planar arrays of nanometer-diameter metal clusters that are covalently linked to each other by rigid, double-ended organic molecules have been self-assembled. Gold nanocrystals, each encapsulated by a monolayer of alkyl thiol molecules, were cast froma colloidal solution onto a flat substrate to form a close-packed cluster monolayer. Organic interconnects (aryl dithiols or aryl di-isonitriles) displaced the alkyl thiol molecules and covalently linked adjacent clusters in the monolayer to form a two-dimensional superlattice of metal quantum dots coupled by uniform tunnel junctions. Electrical conductance through such a superlattice of 3.7-nanometer-diameter gold clusters, deposited on a SiO2 substrate in the gap between two gold contacts and linked by an aryl di-isonitrile [1,4-di(4-isocyanophenylethynyl)-2-ethylbenzene], exhibited nonlinear Coulomb charging behavior.

1,072 citations

Journal ArticleDOI
Abstract: Granular metal films (50–200,000 A thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO2, Al2O3), where the volume fraction of metal, x, was varied from x = 1 to x = 0.05. The materials were characterized by electron micrography, electron and X-ray diffraction, and measurements of composition, density and electrical resistivity at electric fields e up to 106 V/cm and temperatures T in the range of 1.3 to 291 K. In the metallic regime (isolated insulator particles in a metal continuum) and in the transition regime (metal and insulator particles in a metal continuum) and in the transition regime (metal and insulator labyrinth structure) the conduction is due to percolation with a percolation threshold at x⋍0.5. Tunnelling measurements on superconductor-insulator-granular metal junctions reveals that the transition from the metallic regime to the dielectric regime (10–50 A size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal ...

1,065 citations

Journal ArticleDOI
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.

747 citations