Stress-dependent electrical transport and its universal scaling in granular materials

We experimentally and numerically examine stress-dependent electrical transport in granular materials to elucidate the origins of their universal dielectric response. The ac responses of granular systems under varied compressive loadings consistently exhibit a transition from a resistive plateau at low frequencies to a state of nearly constant loss at high frequencies. By using characteristic frequencies corresponding to the onset of conductance dispersion and measured direct-current resistance as scaling parameters to normalize the measured impedance, results of the spectra under different stress states collapse onto a single master curve, revealing well-defined stress-independent universality. In order to model this electrical transport, a contact network is constructed on the basis of prescribed packing structures, which is then used to establish a resistor-capacitor network by considering interactions between individual particles. In this model the frequency-dependent network response meaningfully reproduces the experimentally observed master curve exhibited by granular materials under various normal stress levels indicating this universal scaling behaviour is found to be governed by i) interfacial properties between grains and ii) the network configuration. The findings suggest the necessity of considering contact morphologies and packing structures in modelling electrical responses using network-based approaches.Comment: 12 pages, 4 figure

Effects of heat treatment and strain rate on the microstructure and mechanical properties of 6061 Al alloy

In the present work, the effects of heat treatment and strain rate on mechanical behaviour and microstructure evolution of aluminium alloy (AA) 6061 have been investigated. The micro-crack initiation and crystallographic texture evolution are obtained from scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) experiments. Quasi-static and high strain rate compression tests are conducted on AA6061 specimens that underwent two different heat treatments: the as-received material with the original T6 heat treatment and the heat treated and artificially aged (HT) specimens. For the high strain rate compression (~2000 s-1 and ~4000 s-1) tests, the split Hopkinson pressure bar apparatus is used. It is observed that the additional heat treatment has significantly reduced the yield strength of the material. Furthermore, EBSD results show that the higher the applied strain rate is, the less significant change will happen to the texture. SEM images show that, for both T6 and HT specimens, the number and size of micro-cracks in the dynamic compressed specimens are smaller than in the quasi-static deformed specimen. Therefore, the strain rate is considered to be the dominant factor in forming micro-cracks.Australian Research Council through Centre of Excellence for Design in Light Metals (CE0561574). Discovery Projects (DP130101291 and DP140100945). LIEF Project (LE100100045). The National Natural Science Foundation of China through Grant No. 11232003

Effects of heat treatment and strain rate on the microstructure and mechanical properties of 6061 Al alloy

In the present work, the effects of heat treatment and strain rate on mechanical behaviour and microstructure evolution of aluminium alloy (AA) 6061 have been investigated. The micro-crack initiation and crystallographic texture evolution are obtained from scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) experiments. Quasi-static and high strain rate compression tests are conducted on AA6061 specimens that underwent two different heat treatments: the as-received material with the original T6 heat treatment and the heat treated and artificially aged (HT) specimens. For the high strain rate compression (~2000 s-1 and ~4000 s-1) tests, the split Hopkinson pressure bar apparatus is used. It is observed that the additional heat treatment has significantly reduced the yield strength of the material. Furthermore, EBSD results show that the higher the applied strain rate is, the less significant change will happen to the texture. SEM images show that, for both T6 and HT specimens, the number and size of micro-cracks in the dynamic compressed specimens are smaller than in the quasi-static deformed specimen. Therefore, the strain rate is considered to be the dominant factor in forming micro-cracks.Australian Research Council through Centre of Excellence for Design in Light Metals (CE0561574). Discovery Projects (DP130101291 and DP140100945). LIEF Project (LE100100045). The National Natural Science Foundation of China through Grant No. 11232003

Interfacial electro-mechanical behaviour at rough surfaces

International audienceIn a range of energy systems, interfacial characteristics at the finest length scales strongly impact overall system performance, including cycle life, electrical power loss, and storage capacity. In this letter, we experimentally investigate the influence of surface topology on interfacial electro-mechanical properties, including contact stiffness and electrical conductance at rough surfaces under varying compressive stresses. We consider different rough surfaces modified through polishing and/or sand blasting. The measured normal contact stiffness, obtained through nanoindentation employing a partial unloading method, is shown to exhibit power law scaling with normal pressure, with the exponent of this relationship closely correlated to the fractal dimension of the surfaces. The electrical contact resistance at interfaces, measured using a controlled current method, revealed that the measured resistance is affected by testing current, mechanical loading, and surface topology. At a constant applied current, the electrical resistance as a function of applied normal stress is found to follow a power law within a certain range, the exponent of which is closely linked to surface topology. The correlation between stress-dependent electrical contact and normal contact stiffness is discussed based on simple scaling arguments. This study provides a first-order investigation connecting interfacial mechanical and electrical behaviour, applicable to studies of multiple components in energy systems

High-Performance Metal/Carbide Composites with Far-From-Equilibrium Compositions and Controlled Microstructures

The prospect of extending existing metal-ceramic composites to those with the compositions that are far from thermodynamic equilibrium is examined. A current and pressure-assisted, rapid infiltration is proposed to fabricate composites, consisting of reactive metallic and ceramic phases with controlled microstructure and tunable properties. An aluminum (Al) alloy/Ti(2)AlC composite is selected as an example of the far-from-equilibrium systems to fabricate, because Ti(2)AlC exists only in a narrow region of the Ti-Al-C phase diagram and readily reacts with Al. This kind of reactive systems challenges conventional methods for successfully processing corresponding metal-ceramic composites. Al alloy/Ti(2)AlC composites with controlled microstructures, various volume ratios of constituents (40/60 and 27/73) and metallic phase sizes (42–83 μm, 77–276 μm, and 167–545 μm), are obtained using the Ti(2)AlC foams with different pore structures as preforms for molten metal (Al alloy) infiltration. The resulting composites are lightweight and display exceptional mechanical properties at both ambient and elevated temperatures. These structures achieve a compressive strength that is 10 times higher than the yield strength of the corresponding peak-aged Al alloy at ambient temperature and 14 times higher at 400 °C. Possible strengthening mechanisms are described, and further strategies for improving properties of those composites are proposed

Additive Manufacturing of Carbon Fiber and Graphene – Polymer Composites using the technique of Fused Deposition Modelling

Adding micro or nano-carbon reinforcements to polymers enhances their mechanical and electrical properties. In this paper, the effects of the addition of short carbon fibres (SCF) and graphene into acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) polymer to create composite filaments for fused deposition modelling (FDM) are investigated. After creating carbon polymer composite filaments, using a commercial 3D printer, samples were printed and tested for mechanical and electrical properties. The measured values for these composites were compared to those obtained for pure ABS and pure PLA. It was found that by using only 2% SCF it was possible to achieve a 22% increase in tensile strength with no significant impact on printability. With addition of graphene, PLA was made to be conductive. These results show the feasibility of developing new materials for 3D printing that will create structurally sound and conductive designs.Mechanical Engineerin

Electrical transport in granular metals

In this paper, we studied the frequency-dependent AC conductance of randomly packed stainless steel spheres by means of impedance spectroscopy. Two types of power-law behaviour have been observed: (a) at low frequencies, the dependence of the measured impedance on the applied load; (b) at high frequencies, the dependence of the impedance modulus on frequency. Under different loading conditions, the imaginary parts of the measured conductances exhibit respective peaks at critical frequencies, corresponding to the onset of conductance dispersion. Using these critical points as scaling parameters to normalize the measured conductance, results in the spectra from different loading levels collapsing onto a single master curve. Both the electron tunnelling and capacitive paths among particles contribute to the conduction in granular metallic media, resulting in well-characterized universal behaviour

Electrical transport in granular metals

In this paper, we studied the frequency-dependent AC conductance of randomly packed stainless steel spheres by means of impedance spectroscopy. Two types of power-law behaviour have been observed: (a) at low frequencies, the dependence of the measured impedance on the applied load; (b) at high frequencies, the dependence of the impedance modulus on frequency. Under different loading conditions, the imaginary parts of the measured conductances exhibit respective peaks at critical frequencies, corresponding to the onset of conductance dispersion. Using these critical points as scaling parameters to normalize the measured conductance, results in the spectra from different loading levels collapsing onto a single master curve. Both the electron tunnelling and capacitive paths among particles contribute to the conduction in granular metallic media, resulting in well-characterized universal behaviour