Niloy Mukherjee

The use of poly-L-lysine to form novel silica morphologies and the role of polypeptides in biosilicification

Silicification at neutral pH and under ambient conditions in vitro is of great interest due to its relationship with silicification in vivo as well as for the benign conditions of the process. As it is important to know the exact group(s) or a particular site in the macromolecules that are responsible for the silicification under these conditions in vivo, poly-L-lysine (PLL) was chosen for this investigation in vitro. Here we report the use of tetramethoxysilane (TMOS) as a silica precursor and the utilization of poly-L-lysine (PLL) for silicification at neutral pH and under ambient conditions. We describe (1) the use of PLL to precipitate silica, (2) the effect of mixing of macromolecules PLL and poly(allylamine hydrochloride) (PAH) to control morphologies of the product, and (3) the formation of novel silica morphologies.

Effect of process parameters on the polymer mediated synthesis of silica at neutral pH

We report herein the synthesis of well-defined silica structures atneutral pH and ambient conditions using poly(allylamine hydrochloride)(PAH), a cationically charged synthetic polymer, as a catalyst/template.Tetramethoxysilane (TMOS) was used as the precursor and the synthesisprocess parameters varied include TMOS pre-hydrolysis time(tP), reaction time (tR), buffer, molecular weightof the polymer, TMOS concentration, polymer concentration andperturbation of the reaction mixture. It was found that the TMOSpre-hydrolysis time was an important parameter governing the resultingsilica morphology along with the reaction time and the TMOSconcentration. Characterization of the silica was performed using SEM,FTIR, EDS and XRD. The poly(allylamine hydrochloride), which was thecatalyst/template, was found to be incorporated into the silicaparticles. These findings are of importance for understanding the roleof polypeptides, in nature, and macromolecules, in general, that arecapable of forming similar silica structures.

Formation of fiber-like amorphous silica structures by externally applied shear

The structure of amorphous silica determines its properties and governs its applications. Here we report the synthesis of elongated silica chains/rods on the nanometer size scale formed by the orientation of a growing silica sol. We have utilized a cationically charged synthetic organic polymer as a catalyst/template and perturbed the system by externally applied shear. It is proposed that the polymer orientation plays an important role in the formation of such morphologies.

PTCR Effect in BaTiO3: Structural Aspects and Grain Boundary Potentials

Extensive microstructural and structure-property studies on donor doped barium titanate have revealed that the PTCR phenomenon is strongly controlled by the density, number of grain boundaries available to conduction, domain orientation and grain boundary domain coherence. Structural heterogeneities lead to a wide range of grain boundary structures, potential barriers and, therefore, depletion widths. Conduction thus occurs primarily by percolation of electrons through favorably aligned domain pathways and low potential barrier grain boundaries. At the Curie point, the increase in the potential barriers along these pathways is likely to dominate the PTCR effect. To improve theoretical understanding a model needs to take heed of local values of parameters and also incorporate the fact that the bulk of the current flow is only through a certain percentage of grain boundaries. The specific structural factors that have led to an improved qualitative understanding of overall PTCR phenomenon are discussed.

The piezoelectric cochlear implant: Concept, feasibility, challenges, and issues

A better understanding of the fundamental phenomena occurring in both the healthy and the artificially stimulated cochlea will greatly aid in the engineering of more effective cochlear implant devices and will, in general, enhance mankinds knowledge of inner ear function. This study was initiated to probe the feasibility of use of artificial piezoelectric transducer devices, both for the understanding of cochlear phenomena and as a possible cochlear implant. Aspects of feasibility of such an implant, the issues involved, the materials science challenges that need to be overcome to fabricate such a device, and results from initial in vivo experiments are discussed.

Synthesis of C-60 fullerene-silica hybrid nano structures

We have recently demonstrated a procedure for the synthesis of silica nanometer and micrometer particles under modest conditions. Here we report the synthesis of C60 fullerene-silica hybrid nanometer sized materials via sol-gel processing at neutral pH and under ambient conditions. The C60 fullerene, when functionalized, was water-soluble and also able to facilitate the formation of silica structures from an aqueous silica precursor. This C60 fullerene had similar functionality to the cationically charged polymers, which have been reported earlier to act as catalysts/templates for silicification. The resulting organic-inorganic hybrid was studied using SEM, EDS and UV/Vis spectroscopy. These hybrid materials may have applications in areas such as optical devices, semiconductors, chemical sensors, catalysis and in the medical field. The results presented in this study may be useful in developing a process for the synthesis of novel organic-inorganic nanometer sized materials and for the biomimetic synthesis of silica.

Microstructural dependence of the voltage sensitivity of the PTCR effect in donor doped barium titanate

Barium titanate positive temperature coefficient of resistivity (PTCR) ceramics are widely used in the electrical device industry. It is known that the PTCR effect shows a voltage sensitivity similar to voltage-dependent resistors. However, the microstructural aspects controlling this phenomenon have not been studied in detail. In this study, the voltage sensitivity of PTCR materials displaying a wide range of microstructures and properties was measured. Results show that though all materials tested showed considerable voltage sensitivity, the extent of the effect is strongly controlled by microstructural features. The presence of a grain boundary insulating second phase has the most significant effect. In these materials the room temperature resistivity is relatively insensitive to the voltage but the high temperature resistivity is significantly decreased. In the absence of such a second phase, resistivities are lowered by similar amounts at all temperatures, including a large decrease in the room temperature value. The difference in behavior between these two types of microstructures has been attributed to the difference in the number of available conducting pathways, and the resultant differences in local electric fields generated in the near-grain boundary regions. Thus, microstructural aspects play a pivotal role in determining the voltage sensitivity of these materials. An understanding of these relationships will enable engineering of better devices.

Vibrational and acoustic studies of bending mode piezoelectricity in millimeter-size polyvinylidene fluoride cantilevers

The dynamic bending piezoelectric properties of polyvinylidene fluoride cantilevers in the millimeter size range is reported. These devices are being investigated with the intention of developing a piezoelectric device based inner ear cochlear implant. The size restrictions and fluid environment of the inner ear place special requirements on a piezoelectric device, and it is essential to perform basic studies on sensor materials, deformation modes and device configurations to develop a successful implant. Results from both basic vibration tests and underwater acoustic measurements are presented. Experimental modal analysis reveals that millimeter length cantilevers exhibit three bending resonances under 1 kHz. The modal frequencies are sensitive functions of the length and thickness of the film, and are also affected substantially by the width of the cantilever and the nature of the electrode material. Further, all bending piezoelectric modes display high piezoelectric coupling coefficients in the range 0.2 - 0.35, and damping of < 2%. Experimental results are compared with a theoretical model of unimorph piezoelectric cantilever beams. Underwater acoustic measurements also reveal that single-cantilever devices in the millimeter length display acoustic sensitivities in the -195 to -210 dB range, in the 2 - 10 kHz regime. These sensitivities are comparable to commercial devices of larger size and more complex design. The viability of use of the conducting polymer polypyrrole as the electrode material in polymer piezoelectric sensors is also investigated. Results show that devices with polypyrrole electrodes are at least as sensitive as devices with metal electrodes, and these type all-polymer devices thus have great promise. The results presented in this paper can be used to design an appropriate sensory implant, as well as in other audio frequency applications.