RESEARCH
Electro-Mechanical Coupling in Cellular CNT (or CNT Foam/CNT Turf)
Carbon nanotube (CNT) foam is a multifunction material revealing high energy absorption capacity along with electric field induced actuation, photo-actuation, etc. Through a series of experimental and analytical works, we have discovered that the mechanical behaviour, including stress-strain response under quasi-static as well as dynamic loading condition and viscoelastic stress relaxation, can be greatly engineered through application by static or fluctuating electric field. Due to the non-linear electro-mechanical coupling in CNT strands, the strength as well as energy absorption capacity of ~1 mm thick CNT foams increases several folds (> 7 times with a potential difference of only 1V, which is easily available in electronic devices!) under quasi-static loading conditions. However, this quantum of the increase reduces under dynamic loading as well as due to magnified effect of electric field on viscoelastic relaxation. Piyush Jagtap is leading this effort.
In this context, we have also developed a new equipment for conducting ball drop impact test. This work has been covered by a national daily (Click here for the article in The Hindu.)
Left figure shows the custom fabricated impact tester used in this study. Right figure shows the variation of the energy absorbed as function of the electric field applied across the CNT foam.
Relevant References
Creep of Sn-based Lead-free Solders
In past, we have studied effect of microstructure on the creep behavior. In this context, we have also studied microstructural evolution during usual service of a microelectronic device and incorporated the effect of microstructural evolution due to the thermo-mechanical excursion on the creep behavior. In this way, we finally developed a microstructurally adaptive creep model for solders.
Creep behaviour of lead-free Sn-3.8%Ag-0.7%Cu (SAC387) solder was studied. The ball array grid (BGA) shaped samples were isothermally aged under various temperature-time conditions, and were creep tested at different stresses and temperatures. The effects of aging on the microstructural evolution of the solder was studied and analytically correlated with the creep behaviour of solder joints. A microstructurally adaptive creep model for Sn-based lead-free solder joints was developed which can accommodate the thermo-mechanical history of the solder and predict the primary-cum-secondary creep behaviour of a solder joint.
Relevant References
High Strain Rate Fracture of Solder Joints
We are interested in studying fracture behavior of solder joints under mixed mode loading conditions and at high strain rates - mimicking the fall condition. We are currently also performing fracture tests on highly miniaturized solder joints. We are collaborating with Professor Indranath Dutta (Washington State University, Pullman, USA) in this area.
(a) Fractographs and (b) crack profile of partially propagated crack showing different fracture modes. Profiles of partially propagated cracks also reveal the damage zone ahead of the crack tip.
A test fixture was designed to study the mixed-mode fracture behaviour of SAC387 (Sn-3.8%Ag-0.7%Cu)/Cu joints at high strain rates (up to 200 per second ). The effects of strain rate, solder microstructure and the morphology of intermetallic compounds (IMCs) layer at Cu/solder interface were studied, and a fracture mechanism map was developed.
Effects of (c) strain rate and (d) mode mixity on the fracture toughness of the solder joints. The effects of dwell time and cooling rates are also shown in above figures.
A solder microstructure facilitating low yield strength, and a smooth, thin IMC layer yielded high fracture toughness for all testing conditions, although the relative differences in the fracture behaviour (toughness and crack propagation mechanism) diminished at high strain rates as well as at high mode-mixites.
Fracture mechanism map for solder joints (e) Mode I and (f) mixed mode with 45o loading angle.
Relevant References
Strength-Ductility Paradox: Achieving High Strength-High Ductility through Severe Plastic Deformation
We are interested in exploring the possibility of transition from high strength-low ductility regime to high strength-high ductility regime in metallic alloys. We specifically like to employ one of the severe plastic deformation (SPD) processes to achieve such transition. Tarang Mungole conducted experiments on Al-7% Si alloys processed through high pressure torsion or HPT. We observed that imposing very high strains in this material may produce microstructre suitable for obtaining both high strength and high ductility
Left figure shows variation of the normalised ultimate tensile strength as function of the normalised elongation to failure. This is a new way of showing the strength-ductility paradox, as compared to the traditional way of depicting the same, as shown in right figure.
Earlier, experiments were conducted using Al-3%Mg, to develop a processing route for simultaneously achieving both high ductility and high strength. Attaining UFGs in this alloy through a SPD process, such as equal channel angular pressing (ECAP), was shown to be a route for the same, especially if the samples are deformed or tested at the high strain rates.
Stress-strain curves at various strain rates showing effect of ECAP processing on simultaneously increasing strength and ductility.
Relevant References
Critcal Examination of Harper-Dorn Creep: Does It Really Exist in Classical Sense?
We conducted creep tests on single crystalline, high purity Al in the Harper-Dorn regime
(i.e. very low stresses and very high temperatures) to critically examine the validity of classical Harper-Dorn creep. Interestingly, we did
not observe a stress exponent of 1 in the so called Harper-Dorn regime; as a matter of fact, we observed a stress exponent of 1. Furthermore,
we also observed that the dislocation density in the Harper-Dorn regime varied as the square of the applied stress - the same dependence
which we often observe in the 5-power law regime. Based on the experimental results, we proposed a dislocation climb (in Frank net) based creep mechanism predicting
a stress exponent of ~3 for the Harper-Dorn regime.
(a) Comparison of our results (present investigation) with the reported data, including orginal work of Harper and Dorn. (b) Our measurements revealed that the stress dependence of the dislocation density in the Hraper-Dorn regime is identical to that in the power law creep.
Relevant References
Grain Boundary Sliding in Ultra-Fine Grained Materials
Severe plastic deformation (SPD) via equal channel angular pressing (ECAP) was performed on Zn-22%Al alloy to produce ultra-fine grains (UFG). The samples machined from this material were tested in tension under superplastic conditions, and grain boundary sliding (GBS) was measured using marker method. It was shown that, similar to their large grain sized counterparts, GBS is the most dominant deformation mechanism for superplasticity in these ultra-fine grained materials also.
(a) Micrograph showing GBS (top) and table showing the contribution of GBS in the total strain (bottom) and (b) relative GBS at different types of grain boundaries
Relevant ReferencesEffects of thermal cycling on integrity of through silicon vias (TSVs)
TSVs are used in advanced, new generation 3-D microelectronic packages to connect electronic circuitries at various levels/layers. Thermal cycles with various ΔT ranges, and heating & cooling rates were imposed on Cu filled TSVs in both as-fabricated and annealed conditions. Due to thermal cycling, Cu pillars either intruded or extruded relative to Si jeopardizing integrity of the TSV containing microelectronic package. The extent of intrusion or extrusion of Cu depended on the annealing conditions, heating & cooling rates, and the ΔT range of thermal cycles. An analytical model explaining results was also developed.
Relevant References
Development of Thermal Interface Materials (TIMs) for Next Generation Microelectronic Packaging
Thermal interface materials should have high thermal conductivity and high shear compliance. Such materials may be produced via liquid phase sintering (LPS), which enables uniform distribution of the high melting phase (HMP) constituents in the low melting phase (LMP) matrix. The thermal conductivity of optimized Cu-In based solders was shown to be at least 2 times higher than that of pure In with only ~50% increase in the yield strength of the composite relative to the pure indium.
Lately, we improved the thermal and mechanical properties of Cu-In composites by adopting a two steps procedure consisting of liquid phase sintering and severe plastic deformation. We have observed that ARB can significantly homogenize the distribution of Cu in In, dramatically improve the thermal conductivity without compromising the compliance of the sample, and also allows inclusion of a foreign species (such as CNT, rGO, etc.) in a certain architecture in the material.
Relevant References
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