Granular Aluminum Research Articles

Kondo-like behavior near the metal-to-insulator transition of nanoscale granular aluminum

We show that the normal state transport properties of nanoscale granular aluminum films, near the metal to insulator transition, present striking similarities with those of Kondo systems. Those include a negative magnetoresistance, a minimum of resistance $R$ at a temperature ${T}_{m}$ in metallic films, a logarithmic rise at low temperatures, and a negative curvature of $R(T)$ at high temperatures. These normal state properties are interpreted in terms of spin-flip scattering of conduction electrons by local magnetic moments, possibly located at the metal/oxide interfaces.

High-velocity Impact Fragmentation of Brittle, Granular Aluminum Spheres

We present ballistic and fragmentation data for brittle, granular aluminum spheres following high- velocity impact (0.5-2.0km/s) on thin steel plates. These spheres, formed from isostatically pressed alu- minum powder, are representative of a wide variety of metallic reactive materials currently being studied. Simple analytic theories are introduced which provide a reasonable description of the residual velocity and hole diameter following impact. With increasing velocity there is an intriguing transition to a power-law fragment distribution, which we have interpreted as arising from extensive microbranching of fast-running cracks. The key equations of an analytic theory describing the fragment distributions are also presented.

Impact fragmentation of aluminum reactive materials

We report the fragmentation of brittle, granular aluminum spheres following high velocity impact (0.5-2.0 km/s) on thin steel plates. These spheres, machined from isostatically pressed aluminum powder, represent a prototypical metallic reactive material. The fragments generated by the impacts are collected in a soft-catch apparatus and analyzed down to a length scale of 44μm. With increasing velocity, there is a transition from an exponential Poisson-process fragment distribution with a characteristic length scale to a power-law behavior indicative of scale-invariance. A normalized power-law distribution with a finite size cutoff is introduced and used to analyze the number and mass distributions of the recovered fragments. At high impact velocities, the power-law behavior dominates the distribution and the power-law exponent is identical to the universal value for brittle fragmentation discussed in recent works. The length scale at which the power-law behavior decays is consistent with the idea that the length of side microbranches or damage zones from primary cracks is governing this cutoff. The transition in fragment distribution at high strain-rates also implies a significant increase in small fragments that can rapidly combust in an ambient atmosphere.

Ageing and relaxation times in disordered insulators

We focus on the slow relaxations observed in the conductance of disordered insulators at low temperature (especially granular aluminum films). They manifest themselves as a temporal logarithmic decrease of the conductance after a quench from high temperatures and the concomitant appearance of a field effect anomaly centered on the gate voltage maintained. We are first interested in ageing effects, i.e. the age dependence of the dynamical properties of the system. We stress that the formation of a second field effect anomaly at a different gate voltage is not a "history free" logarithmic (lnt) process, but departs from lnt in a way which encodes the system's age. The apparent relaxation time distribution extracted from the observed relaxations is thus not "constant" but evolves with time. We discuss what defines the age of the system and what external perturbation out of equilibrium does or does not rejuvenate it. We further discuss the problem of relaxation times and comment on the commonly used "two dip" experimental protocol aimed at extracting "characteristic times" for the glassy systems (granular aluminum, doped indium oxide…). We show that it is inoperable for systems like granular Al and probably highly doped InOx where it provides a trivial value only determined by the experimental protocol. But in cases where different values are obtained like in lightly doped InOx or some ultra thin metal films, potentially interesting information can be obtained, possibly about the "short time" dynamics of the different systems. Present ideas about the effect of doping on the glassiness of disordered insulators may also have to be reconsidered.

Screening and conductance relaxations in insulating granular aluminium thin films

We have recently found in insulating granular Al thin film a new experimental feature [1], namely the existence of a conductance relaxation that is not sensitive to gate voltage changes. This conductance relaxation is related to the existence of a metallic-like screening in the film and can be used to estimate its characteristic length scale. In the present paper, we give some experimental details on how this feature was measured and present our first results on the screening length temperature dependence.

Metallicity of the SrTiO3 surface induced by room temperature evaporation of alumina

It is shown that a metallic state can be induced on the surface of SrTiO3 crystals by the electron beam evaporation of oxygen deficient alumina or insulating granular aluminium. No special preparation nor heating of the SrTiO3 surface is needed. Final metallic or insulating states can be obtained depending on the oxygen pressure during the evaporation process. Photoconductivity and electrical field effect are also demonstrated.

ChemInform Abstract: Tris(trimethylsilyl)aluminum

AbstractReview: 10 refs.

Microemulsion extraction separation and determination of aluminium species by spectrofluorimetry

A simple and sensitive microemulsion extraction separation method was developed for the speciation of aluminium in tea samples by spectrofluorimetry. With 8-hydroxyquinoline (8-HQ) as the chelating agent and Triton X-100 Winsor II microemulsion as the extractant, separation of aluminium species in different pH solutions was achieved by microemulsion extraction. The formation of microemulsion, the conditions of extraction and determination of aluminium species were studied. The results showed that, the contents of aluminium species in tea leaves and infusions samples, such as total aluminium, total soluble aluminium, total granular aluminium, inorganic aluminium except Al-F, and (Al-F + Al-org), were obtained successfully under the optimal conditions. The limit of detection was 0.23 μg L −1 in pH 9.5 solution, and 0.59 μg L −1 in pH 6.0 solution respectively; the precision (RSD) for 11 replicate measurements of 10 μg L −1 aluminium was 2.1% in pH 9.5 solution, and 2.8% in pH 6.0 solution respectively; the recoveries for the spiked samples were 96.8–103.5%. The proposed method is simple and efficient, which has been applied to the speciation of aluminium in tea samples with satisfactory results.

Manifestation of ageing in the low temperature conductance of disordered insulators

We are interested in the out of equilibrium phenomena observed in the electrical conductance of disordered insulators at low temperature, which may be signatures of the electron coulomb glass state. The present work is devoted to the occurrence of ageing, a benchmark phenomenon for the glassy state. It is the fact that the dynamical properties of a glass depend on its age, i.e. on the time elapsed since it was quench-cooled. We first critically analyse previous studies on disordered insulators and question their interpretation in terms of ageing. We then present new measurements on insulating granular aluminium thin films which demonstrate that the dynamics is indeed age dependent. We also show that the results of different relaxation protocols are related by a superposition principle. The implications of our findings for the mechanism of the conductance slow relaxations are then discussed.

Ageing in granular aluminium insulating thin films

We present a new set of electrical field effect measurements on granular aluminium insulating thin films. We have explored how the conductance relaxations induced by gate voltage changes depend on the age of the system, namely the time elapsed since its quench at low temperature. A clear age dependence of the relaxations is seen, qualitatively similar to ageing effects seen in other well studied glassy systems such as spin glasses or polymers. We explain how our results differ from the previous ones obtained with different protocols in indium oxide and granular aluminium thin films. Our experimental findings bring new information on the dynamics of the system and put new constraints on the theoretical models that may explain slow conductance relaxations in disordered insulators.

Ageing in granular aluminium insulating thin films

AbstractWe present a new set of electrical field effect measurements on granular aluminium insulating thin films. We have explored how the conductance relaxations induced by gate voltage changes depend on the age of the system, namely the time elapsed since its quench at low temperature. A clear age dependence of the relaxations is seen, qualitatively similar to ageing effects seen in other well studied glassy systems such as spin glasses or polymers. We explain how our results differ from the previous ones obtained with different protocols in indium oxide and granular aluminium thin films. Our experimental findings bring new information on the dynamics of the system and put new constraints on the theoretical models that may explain slow conductance relaxations in disordered insulators.

Alternative cost-effective preparation method of polyaluminium chloride (PAC) coagulant agent: Characterization and comparative application for water/wastewater treatment

An alternative preparation procedure for polyaluminium chloride (PAC), a common inorganic coagulant, is presented in this paper. The proposed procedure is based on the use of granular aluminium metal, a common by-product of several aluminium-processing secondary industries, instead of using Al2O3 (or of using a mineral containing Al2O3) which is commonly applied by most industrial producers, and exhibits important benefits such as energy and time saving during the preparation procedure. Several efforts have been made to produce the pre-polymerized coagulant with properties, similar to the properties of a commercially available PAC solution, using appropriate, commonly found laboratory equipment and without the application of extreme conditions (in terms of temperature or pressure). The laboratory prepared PAClab was characterized mainly by means of aluminium content, basicity, density and aluminium species distribution. Furthermore, the coagulation performance of PAClab was evaluated for the treatment of contaminated tap water and of wastewater samples and it was compared with the performance of a commercially available PAC solution. Finally, the coagulation–flocculation kinetics was examined with the use of Photometric Dispersion Analyzer instrument (PDA) and the floc growth rates between the two coagulants were compared. The results suggest that PAC can be alternatively produced under mild conditions, resulting in a product with better properties than the commercial PAC used for comparison. Additionally, if the preparation takes place in industrial scale, a greater improvement is further feasible, due to the common equipment used, instead of using e.g. autoclaving.

Electrical glassy behavior in granular aluminium thin films

We present new results obtained by field effect measurements on insulating granular Al thin films. First, reproducible and stable conductance fluctuations are seen in micron size samples as a function of gate voltage. The anomalous field effect and its slow relaxation already known to exist in macroscopic samples are shown to still exist in small samples and to have no influence on the fluctuations pattern. Secondly, "true" aging is demonstrated, i.e. the anomalous field effect relaxation depends on the time elapsed since the cooling, the longer this time the longer it takes for the system to react to a gate voltage change. Interpretations and implications of these findings are discussed.

Coexistence of anomalous field effect and mesoscopic conductance fluctuations in granular aluminium

We perform electrical field effect measurements at 4 K on insulating granular aluminium thin films. When the samples size is reduced below ≃100 μm, reproducible and stable conductance fluctuations are seen as a function of the gate voltage. Our results suggest that these fluctuations reflect the incomplete self-averaging of largely distributed microscopic resistances. We also study the anomalous field effect (conductance dip) already known to exist in large samples and its slow relaxation in the presence of the conductance fluctuations. Within our measurements accuracy, the two phenomena appear to be independent of each other, like two additive contributions to the conductance. We discuss the possible physical meaning of this independence and in particular whether or not this observation is in favor of an electron glass interpretation of slow conductance anomaly relaxations.

Anomalous electric‐field effect and glassy behaviour in granular aluminium: electron glass?

AbstractWe briefly review our study of non‐equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films at low temperature. In MOS like structures, with granular aluminium channels, an anomalous field effect and its slow relaxation have been studied in some detail, the phenomenology of which is very similar to the one also observed in indium oxide. The interpretation of the experimental findings is discussed. To date, in granular systems, the present experiments can qualitatively be interpreted along two different lines. One (the “extrinsic” scenario) relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one is a purely electronic mechanism: the formation of an electron Coulomb glass in the granular met al. Some experimental facts seem difficult to reconcile with the extrinsic scenario, but more selective experiments and/or quantitative theoretical predictions are still needed to definitely distinguish between the two scenarii. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Robust Superconducting Granular Aluminium/Palladium Bilayer Microbolometers with Sub-Nanosecond Response

We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminium bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminium film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al. (Acta Phys. Pol. A 103(4):325, [2003]) and direct nanosecond-pulsed photoexcitation to obtain the microbolometer’s characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al. (J. Phys. E Sci. Instrum. 22:582, [1989]) to obtain the phonon flux in a heat pulse experiment with nanosecond resolution.

Fabrication of robust superconducting granular aluminium/palladium bilayer microbolometers with sub-nanosecond response

We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminium bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminium film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al. [1], and direct nanosecond-pulsed photoexcitation to obtain the microbolometer's characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al. [2] to obtain the phonon flux in a heat pulse experiment with nanosecond resolution.

Anomalous electric-field effect and glassy behaviour in granular aluminium thin films: electron glass?

We present a study of non-equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films. An anomalous field effect and its slow relaxation are studied in some detail. The phenomenology is very similar to the one already observed in indium oxide. The origin of the phenomena is discussed. In granular systems, the present experiments can naturally be interpreted along two different lines. One relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one relies on a purely electronic mechanism: the formation of an electron Coulomb glass in the granular metal. More selective experiments and/or quantitative predictions about the Coulomb glass properties are still needed to definitely distinguish between the two scenarios.

The influence of silica and calcium on arsenate sorption to oxide surfaces

Arsenic can be effectively removed from water by sorption to amorphous or granular iron or aluminum oxides. however, silica is known to interfere with arsenic sorption at pH >7.5. The goal of this work was to examine arsenic sorption to amorphous and granular media oxides at various levels of silica, calcium and pH, and to closely monitor the progress of sorption in these systems. At pH 8.5, calcium dramatically increased arsenic sorption to amorphous iron hydroxide in the presence of silica over relatively short reaction times, but the effects gradually disappeared over longer reaction times. Additionally, batch tests indicated that activated alumina granular media was more sensitive to silica interference than granular ferric hydroxide. However, relatively low levels of calcium could reduce silica interference to arsenic sorption onto activated alumina. While future confirmation research is needed, severe silica interference can occur in practice due to exclusion of arsenate from interstitial granular media pores.

Weak localization and magnetoconductance in percolative superconducting aluminum films

In order to investigate the crossover from a homogeneous behavior to an inhomogeneous (percolative) one, the temperature $T$ and magnetic field $H$ dependence of the sheet resistance ${R}_{◻}$ have been measured for two-dimensional granular aluminum films. Fitting the theory to data on magnetoconductance near ${T}_{C}$ with use of the diffusion constant $D(T)$ as a fitting parameter, we have obtained the anomalous $T$-dependent diffusion constant $D$. From $D(T)$, the electron diffusion index $\ensuremath{\theta}$, a certain critical exponent in the percolation theory, has been obtained. In the relation ${R}_{◻}\ensuremath{-}\ensuremath{\theta}$, the value of $\ensuremath{\theta}$ varies abruptly near $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{k}\ensuremath{\Omega}$. This behavior suggests the abovementioned crossover is similar to our previous results determined from the $T$ dependence of the upper critical magnetic field. For percolative films at $H=5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, we have found the strong ${R}_{◻}$ dependence of the prefactor ${\ensuremath{\alpha}}_{T}$ in the expression $\ensuremath{\sigma}=[{\ensuremath{\alpha}}_{T}{e}^{2}∕(2{\ensuremath{\pi}}^{2}\ensuremath{\hbar})]\mathrm{ln}\phantom{\rule{0.3em}{0ex}}T+{\ensuremath{\sigma}}_{0}$. The relation ${\ensuremath{\alpha}}_{T}\ensuremath{\propto}1∕{R}_{◻}$ can be explained qualitatively by a model of scaling law for percolation.