M P Manoharan

The interfacial strength of carbon nanofiber epoxy composite using single fiber pullout experiments.

Carbon nanotubes and nanofibers are extensively researched as reinforcing agents in nanocomposites for their multifunctionality, light weight and high strength. However, it is the interface between the nanofiber and the matrix that dictates the overall properties of the nanocomposite. The current trend is to measure elastic properties of the bulk nanocomposite and then compare them with theoretical models to extract the information on the interfacial strength. The ideal experiment is single fiber pullout from the matrix because it directly measures the interfacial strength. However, the technique is difficult to apply to nanocomposites because of the small size of the fibers and the requirement for high resolution force and displacement sensing. We present an experimental technique for measuring the interfacial strength of nanofiber-reinforced composites using the single fiber pullout technique and demonstrate the technique for a carbon nanofiber-reinforced epoxy composite. The experiment is performed in situ in a scanning electron microscope and the interfacial strength for the epoxy composite was measured to be 170 MPa.

Synthesis and elastic characterization of zinc oxide nanowires

Zinc oxide nanowires, nanobelts, and nanoneedles were synthesized using the vapor-liquid-solid technique. Youngs modulus of the nanowires was measured by performing cantilever bending experiments on individual nanowires in situ inside a scanning electron microscope. The nanowires tested had diameters in the range of 200-750 nm. The average Youngs modulus, measured to be 40 GPa, is about 30% of that reported at the bulk scale. The experimental results are discussed in light of the pronounced electromechanical coupling due to the piezoelectric nature of the material.

Influence of strain on thermal conductivity of silicon nitride thin films

We present a micro-electro-mechanical system-based experimental technique to measure thermal conductivity of freestanding ultra-thin films of amorphous silicon nitride (Si3N4) as a function of mechanical strain. Using a combination of infrared thermal micrography and multi-physics simulation, we measured thermal conductivity of 50 nm thick silicon nitride films to observe it decrease from 2.7 W (m K)?1?at zero strain to 0.34 W (m K)?1?at about 2.4% tensile strain. We propose that such strong strain?thermal conductivity coupling is due to strain effects on fraction?phonon interaction that decreases the dominant hopping mode conduction in the amorphous silicon nitride specimens.

In-plane thermal conductivity of nanoscale polyaniline thin films

Thermal characterization of conducting polymers is important in understanding transport phenomena in energy conversion and flexible electronics devices. We present an experimental technique to determine the in-plane thermal conductivity and the thermal contact resistance of thin films on substrates simultaneously. For 20 nm thick polyaniline films on SiO2 substrate, the respective values were measured to be 0.0406 W/m K and 0.806 K m2/W. We also observed thickness dependence of in-plane thermal conductivity, which suggests that heat transfer is governed by phonon-boundary scattering when the film thickness is close to the mean free path.

Role of adhesion in shear strength of nanowire–substrate interfaces

The shear strength of nanowire–substrate contact critically influences the electrical and mechanical contact characteristics of nanowire-based sensors, actuators and nanoelectronic devices. Yet, very few studies are available in the literature because of the experimental challenges associated with these one-dimensional nanostructures and none of the existing contact mechanics models account for their ultra-high bending compliance. Using a novel experimental setup that effectively decouples adhesion and friction forces, we show that the friction coefficient for the zinc oxide nanowires and silicon system can be about two orders of magnitude higher than the bulk values, even under zero externally applied normal loads. We model nanowire bending compliance and capillary line tension as competing mechanisms to explain the observed anomalous adhesion–friction coupling and establish a criterion for contact area dependence in friction in one-dimensional interfaces.

Elastic Properties of 4–6 nm-thick Glassy Carbon Thin Films

Glassy carbon is a disordered, nanoporous form of carbon with superior thermal and chemical stability in extreme environments. Freestanding glassy carbon specimens with 4–6 nm thickness and 0.5 nm average pore size were synthesized and fabricated from polyfurfuryl alcohol precursors. Elastic properties of the specimens were measured in situ inside a scanning electron microscope using a custom-built micro-electro-mechanical system. The Young’s modulus, fracture stress and strain values were measured to be about 62 GPa, 870 MPa and 1.3%, respectively; showing strong size effects compared to a modulus value of 30 GPa at the bulk scale. This size effect is explained on the basis of the increased significance of surface elastic properties at the nanometer length-scale.

Fracture toughness characterization of advanced coatings

We present an experimental technique for the measurement of the fracture toughness of advanced surface coatings in situ in a scanning electron microscope. The technique is demonstrated on titanium–titanium nitride multi-layer thin films. Titanium–titanium nitride multi-layers are part of a new class of advanced erosion-resistant coatings with optimized toughness and hardness for performance and life respectively, and the fracture properties of these surface coatings determine their effectiveness. Thin film specimens were prepared using a lift-out technique in focused ion beam-scanning electron microscope. The fracture toughness of a 300 nm thick specimen perpendicular to the multi-layer thickness was measured to be 2.90 ± 0.3 MPa m+1/2. The fracture characterization technique can be applied for a wide variety of surface coatings, and thin films in general.

Room temperature amorphous to nanocrystalline transformation in ultra-thin films under tensile stress: an in situ TEM study

The amorphous to crystalline phase transformation process is typically known to take place at very high temperatures and facilitated by very high compressive stresses. In this study, we demonstrate crystallization of amorphous ultra-thin platinum films at room temperature under tensile stresses. Using a micro-electro-mechanical device, we applied up to 3% uniaxial tensile strain in 3-5 nm thick focused ion beam deposited platinum films supported by another 3-5 nm thick amorphous carbon film. The experiments were performed in situ inside a transmission electron microscope to acquire the bright field and selected area diffraction patterns. The platinum films were observed to crystallize irreversibly from an amorphous phase to face-centered cubic nanocrystals with average grain size of about 10 nm. Measurement of crystal spacing from electron diffraction patterns confirms large tensile residual stress in the platinum specimens. We propose that addition of the externally applied stress provides the activation energy needed to nucleate crystallization, while subsequent grain growth takes place through enhanced atomic and vacancy diffusion as an energetically favorable route towards stress relaxation at the nanoscale.

Enhancement of dielectric breakdown strength in glass using polymer coatings

Traditional polymer film capacitors used in pulsed power and power electronics applications have severe limitations in terms of high temperature reliability. Alkali-free glass is a promising material for high temperature and high power capacitors because of its low dielectric loss and high breakdown strength (12 MV/cm), resulting in energy density values exceeding 35 J/cm3. Several alkali-free glass compositions are commercially available with a permittivity range of 5 to 7 and low dielectric loss (tan δ <; 0.5%) up to 200°C. Polymer coatings, such as parylene-C and fluorinated polyester (FPE) significantly increase the breakdown strength of alkali-free glass sheets; in some cases, by over 50% at temperatures up to 125°C, compared to uncoated glass.

High temperature - High energy density polymer-coated glass capacitors

Traditional polymer capacitors used for pulsed power and power electronics applications have severe limitations in terms of portability and high temperature reliability. Alkali-free glass is a promising material for high temperature and high power capacitors because of its low dielectric loss and high breakdown strength (10 MV/cm), resulting in energy density values higher than 35 J/cm3. Thin glass sheet production has grown substantially because of the strong demand from the flat panel display industry and a large investment in the development of new glass fabrication methods. Several alkali-free glass compositions, such as Schotts AF-45 and NEGs OA-10G barium-boro-alumino-silicate glass, are commercially offered with a permittivity range of 5 to 7 and low dielectric loss (tan δ < 0.5%) values up to temperatures of 200°C. Glass ribbons and sheets are available with thicknesses between 5 and 50 µm and length of up to a few meters.