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Atomic-level Investigation and Proposals to Address Technological Roadblocks and Reliability Challenges in 2D Material Based Nanoelectronic Devices
No full textThe transistor scaling is witness to many extraordinary inventions during its consecutive miniaturization. The journey began from Dennard’s classical constant field scaling, crossing through the milestones like strain engineering, high ‘k’ gate dielectric, ultrathin body transistor (UTB), silicon on insulator (SOI), and multi-gate 3D architectures, and continues in the form of advanced FinFET technology. However, further downscaling is sensing a dead-end because of the various challenges due to fundamental limitations of silicon, the building material of the transistor. Among these, two significant challenges are mobility degradation due to boundary scattering by surface dangling bonds and loss of gate control due to quantum confinement. To keep downscaling alive, the research community is looking for an alternate material that can mitigate these issues and consist of better fundamental properties from silicon like intrinsic mobility, thermal conductivity, optical response, and mechanical strength.
Two-dimension material (2D material) shows great potential for next-generation electronic material and provides multiple avenues for further exploration. The material is one or a few atomic-layer 2D thin sheets of covalently bonded atoms stacked using weak van der Walls (vdW) forces in the third dimension. The lack of surface dangling bond and atomic-scale thickness mitigates the significant challenges of low mobility and inadequate gate control of the silicon material, respectively. Presently, more than 150 materials exist in the 2D material family. Graphene, Transition Metal Dichalcogenides (TMDs), and Phosphorene are well ahead of other family members due to their extraordinary properties, thereby plenty of investigations. Despite these properties, the materials have several roadblocks to their technological application. Opening bandgap and minimizing contact resistance are significant challenges in graphene, and reducing contact resistance and mature growth and reliability are big concerns for the TMDs. Phosphorene, which has hybrid properties of graphene and TMDs, is relatively less explored due to its spontaneous degradation in the ambient environment. Understanding and mitigating its spontaneous ambient degradation is still an open challenge for the electronics and material research communities across the globe. Keeping in mind these limitations, we explore the problems one by one and find their reasonable solutions.
Based on DFT investigations, the discussion begins with a proposal for a reliable direct bandgap opening technique in graphene. Graphene possesses zero bandgap due to its highly symmetric hexagonal structures, which touch its π and π* orbitals’ energy states near the Fermi level, known as the Dirac point. Breaking this symmetry by carbon vacancy or Stone-Wales (SW) defects opens the bandgap at the Dirac point. However, the carbon vacancy creates unwanted mid-gap (trap) states, attributed to unbound orbitals of the nearest unsaturated carbon atoms at the vacant site. Moreover, the unsaturated carbon atoms react with ambient gases like oxygen, making graphene unstable. Interestingly, hydrogenation or fluorination of the unsaturated carbon atoms near the vacant site helps prevent the trap states while contributing to promising direct band gaps in graphene. The opened bandgap is tunable in the infrared regime and persists for different sizes and densities of hydrogenated or fluorinated patterns. The proposed approach is thermodynamically favorable as well as stable.
The next work demonstrates the contact resistance reduction of graphene with palladium (Pd) by carbon vacancy engineering. The discussion begins with fundamental insights into the Pd-graphene interface and carbon vacancy-assisted contact resistance reduction using Density Functional Theory (DFT), followed by its experimental validation by various processes. Our study reveals significant interaction of Pd with graphene. Their orbitals overlap leads to potential barrier lowering at the interface, which can be reduced further by bringing graphene closer to the bulk Pd using carbon vacancy engineering at the contacts. Thus, the carbon vacancy-assisted barrier modulation reduces contact resistance by increasing carrier transmission probabilities at the interface. The theoretical findings have been emulated experimentally by carbon vacancy engineering at the graphene Field Effect Transistors (FETs). Different contact engineered graphene devices with Pd contacts shows significant contact resistance reduction, measuring as low as ~78 Ω-µm at room temperature. The contact resistance shows a ‘V’ shape curve as a function of defect density. The optimum contact resistance achieved is significantly lower than their pristine counterpart, as predicted by the theoretical estimates.
Subsequently, the journey turns towards an atomic level investigation of phosphorene ambient degradation using the first-principles Molecular Dynamics (MD) simulations in the following work. The study reveals that the oxygen molecule dissociates spontaneously over pristine phosphorene in the ambient environment resulting in an exothermic reaction, which is boosted further by increasing partial pressure, temperature, and the presence of oxygen free radicals. The surface reaction is mainly due to lone pair electrons of phosphorous atoms, making the degradation directional and spontaneous under oxygen atoms. Furthermore, water molecules play a vital role in the degradation process by changing the reaction dynamics path of phosphorene-oxygen interaction and reducing activation energy and reaction energy due to its catalyzing action. In addition, phosphorous vacancy acts as an epicenter for oxidation. The oxygen attacks directly over the vacant site and reacts faster than its pristine counterpart. As a result, phosphorene edges resembling extended vacancy are prominent reaction sites that oxidize anisotropically due to different bond angle strains. The edge initiated spontaneous degradation, and rapid oxidation under the free radicals are validated using consistent probing under an optical microscope and Transmission Electron microscope (TEM), respectively.
After material exploration, the next work reveals a unique reliability issue in the Phosphorene FETs. Here, we investigate the role of channel excess holes (due to inversion) in phosphorene degradation using the first-principles MD computations and electrical and Raman characterization. The results show that phosphorene degrades faster under negative gate bias (excess hole) than in pristine conditions (unbiased). The rapid degradation is mainly due to the enhanced chemical interaction of oxygen with the available hole in the channel. The computational findings are experimentally verified over phosphorene FETs. Compared to the unbiased condition, the devices show a faster change in drain current and fast decay of all primary Raman peaks in the ambient environment under negative gate bias.
At the risk of ambient degradation, phosphorene thin flakes are to be identified quickly using a non-destructive technique like Raman to make their FETs for further exploration. The next work shows that the Raman signature of a low-frequency interlayer out-of-plane phonon mode, known as breathing mode, helps in identifying the thin flake quickly. Further, the work talks about thermal evolution and estimates the first-order temperature coefficient of different breathing modes. All the captured modes show a negative temperature coefficient around -0.002-0.003 cm-1/K across different flake thicknesses. Moreover, a closer look at the thermal evolution reflects that the modes follow three-phonon and four-phonon process dominant scattering phenomena at low and high-temperature ranges. The three-phonon process scattering is dominant below ~100 K, shifting to four-phonon process dominant scattering beyond ~150 K. Besides, the work discusses pristine instrumental error in the Raman shift characterization and suggests a mitigation method using Stokes and Anti-stokes scattering lines.
Finally, the last work discusses the interactions of different metals (Au, Cr, Ni, and Pd) with TMDs (MoS2, MoSe2, WS2, and WSe2). The work reflects that Au has a weak interaction with all the TMDs. Thus, it stays more than 2 Å away from the TMDs surfaces. However, other metals show strong chemistry with TMDs. Due to weak interaction, Au offers very few metal-induced gap states (MIGS) in all the TMDs. On the other hand, metals like Cr, Ni, and Pd flood many MIGS in the bandgap region of the TMDs. During interactions, all the metals offer n-type doping to TMDs. Chalcogen vacancy enhances the interaction of the metals with all the TMDs. The vacancy leaves the unbounded orbitals, which bond strongly with the approaching metals. The bonding enhancement reduces the metal-TMDs distances that can be used in contact resistance engineering in their bulk counterparts. Chalcogen interstitial impurity also enhances the bond strength of some metal-TMDs interfaces.
Our journey helps in overall technological advancement in the leading 2D materials. The work digs into the leading roadblocks like contact resistance reduction and method of bandgap opening in graphene, understanding the degradation issue of phosphorene at the material and device level, and exploring metal-TMDs interactions for their contact resistance engineering
Investigation of Structural relaxation in IV - V -VI Chalcogenide glasses for Infrared applications
No full textChalcogenide glasses are well known for their application in infrared transmission and sensing. These
glasses are usually prepared by quenching the melt. The arrangement of atoms in the liquid state
gets arrested into the solid state possessing high enthalpy in the structure and reaching a metastable
state. Upon heating, these glasses undergo an endothermic reaction and exhibit a glass transition
temperature (Tg) which is about 2/3 of the melting temperature. The viscosity of the glass decreases
at Tg and the glass softens. This property is being exploited by glass technologists to mold the glass
into a required shape according to the need of the application. Since glasses are kinetically arrested
systems, they tend to relax with time to their most stable state in a thermodynamically non equilibrium state. Structural relaxation depends on the viscosity variation with temperature. Angell
classified the glass-forming liquids into strong and fragile based on the viscosity variation with
temperature. If the variation of viscosity with temperature is Arrhenius then the melt is strong and if
it is non-Arrhenius it is a fragile melt. For device applications, the glass needs to be stable. To mold
into the required shape, the temperature of the glass needs to be taken a little above Tg and
pressures of the order of 30MPa applied. So, the knowledge of viscosity variation with temperature
and structural relaxation is very much essential in designing the parameters for molding the glass.
The other interesting aspect is that, unlike their crystalline counterparts, glasses can be prepared
over a wide composition range offering flexibility in fine-tuning the properties according to the
requirement of the desired application. Thus understanding structural relaxation and the structure property relations and their composition dependence forms an important aspect of glass science.
Current work is focused on preparing chalcogenide glasses containing Te as one of the major
components to extend the IR transmission range. Compared to S and Se, Te is known to increase the
glass forming difficulty. Compositions of Te containing IV(Ge)-V(As)-VI(Se/Te) glasses are carefully
designed to have a high glasse forming ability and a wide IR transmission range. Melt quenched
GexTeySe(100-x-y) (10 ≤ x ≤ 40, 20 ≤ y ≤ 45) glasses covering the average coordination number (Zav)
between 2.20 and 2.80 are found to have high thermal stability, high glaas forming ability and high
activation energy for Tg. The fragility index varies between 20 and 35 indicating strong nature of the
melts. The viscosity as a function of reduced temperature follows almost an Arrhenius behavior
confirming the melts are strong in nature. The non-reversing heat flow (ΔHnr) calculated fron MDSC
measurements shows that these glasses are non-ageing.
Glasses in the pseudo-binary joint GeTe4-As2Se3. and GeSe4-As2Se3 (0 ≤ x ≤ 100) having a fixed
connectivity of 2.40 were also prepared to study the composition effects keeping the network
connectivity constant. In covalent network glasses, the properties are mainly determined by the
network connectivity and Tg is expected to undergo minimal changes. On the other hand, a large
variation in Tg has been observed indicating the effect of compositions in modulating the properties.
The change in bond energy and the structural motifs are responsible for the observed variations in
thermal, optical and structla properties. The fragility index varies between 15 and 32 indicating that
the melts of these glasses are super strong in nature. The strong nature of the melts of these glasses
indicates that the structural relaxation is minimal and are non-ageing.
All the three systems studied in this work were found to transmit IR light up to 18 microns. There
were absorption peaks due to oxide impurities. A distillation process using Al as an oxygen getter has
been developed to purify these glasses. The glasses subjected to distillation are found to be free
from absorption due to impurities. The strong nature of the melts and the non-ageing exhibited by
these glasses indicate that these glasses are suitable for device applications
Metal Ion Mediated Riboswitch Folding and Cognate Ligand Sensing
No full textRiboswitches are noncoding RNA molecules that can control gene expression upon cognate ligand binding. Riboswitches are primarily present in bacteria and are crucial for bacteria's survival, which makes riboswitches attractive targets for discovering new antimicrobials. Designing drugs that target a riboswitch function can be accelerated by understanding the effect of physicochemical factors (like ions, temperature, pressure, cosolvents, and pH) on the riboswitch folding and cognate ligand binding. The magnesium (Mg2+) ions possess the unique capability to exhibit site-specific binding along the RNA chain and modulate the population of functionally relevant RNA tertiary structures. In this thesis, I have discussed the effect of Mg2+ on the folding thermodynamics and kinetics of the riboswitches and how cations assist riboswitches in attaining specific tertiary structures. I have provided insight into how the anionic cognate ligands bind to the polyanionic RNA backbone. Further, I have discussed the properties that contribute to cations' binding at specific riboswitch sites. I then highlighted, how riboswitch responds to the cognate ligand binding and transmits the ligand binding information to the gene expression machinery
Design of Electroactive Materials and Mechanistic Investigations of Metal (Li, Na, Mg)-Sulfur Batteries
No full textThe thesis entitled “Design of Electroactive Materials and Mechanistic Investigations of Metal (Li, Na, Mg)-Sulfur Batteries’’ deliberates on some of the important issues impeding the progress of metal sulfur battery (Li/S, Na/S, Mg/S) and discusses possible materials design as well as alternative cell configuration strategies to alleviate them.
The major factor hindering the practical applications of metal sulfur battery is the dissolution of intermediate polysulfides into the ether-based electrolyte during the battery cycling. The present thesis discusses in detail the usage of conductive additive in sulfur cathode as one of the important strategies for the confinement of intermediate polysulfides at the S-cathode. This chemical design strategy is highly effective for both Na/S and Li/S batteries. Apart from the polysulfides dissolution, volume expansion and safety concerns are the other challenges in practical applications of metal sulfur battery. To alleviate such issues, an alternative cell configuration has been proposed. Instead of the sulfur element cathode, fully expanded state of polysulfides viz. lithium sulfide (final discharge product of S8 Li2S) is used as the cathode and lithium metal is replaced by lithiated anatase TiO2. The various stages of redox reaction occurring in the metal-sulfur battery have been extensively investigated using various operando and ex-situ spectroscopic techniques. Apart from the design strategy of the S-cathode, the present thesis also discusses the major challenges associated with electrolyte in bivalent metal sulfur battery system viz. the Mg/S system. Majority of the literature reports the Mg/S battery performance with TEGDME and THF solvent-based electrolyte. However, the persistent concern regarding the lower current density and poor cyclability of TEGDME and higher volatility of THF put Mg/S on the backfoot for practical applications. The present thesis discusses a new class of electrolyte using 1,3-Dioxalane (DOL)/1,2-Dimethoxyethane (DME) binary solvent in Mg/S battery. Like Li/S and Na/S battery system, various intermediate polysulfides formation take place in the Mg/S system as well. The present thesis discusses in detail the polysulfide confinement mechanism in the Mg/S system using operando and ex-situ spectroscopic techniques.DST Nano Missio
Raman Studies of Topological Crystalline Insulator, Natural Heterostructures, Excitonic Insulator and Layered Oxide Under Pressure
No full textThe theme of this thesis is Raman spectroscopic study of a variety of exotic states of matter under extreme conditions, such as, hydrostatic pressure as high as 25 GPa and a wide temperature range from 77 K to 390 K. Raman spectroscopy and in some cases, X-ray diffraction studies were performed on systems such as; Topological Crystalline Insulator (TCI) SnTe, normal semiconductor SnSe, natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n (with m = 1 and n = 1, 2) homologous family, which are also predicted to be Topological Insulators (TI), then Excitonic Insulator (EI) Ta2NiSe5, and its S-counterpart, a normal semiconductor Ta2NiS5. In addition, a Ru2O6-layer honeycomb lattice compound, the silver ruthenium oxide AgRuO3 was also studied. The work presented in this thesis is divided into three parts. (1) Pressure dependent Raman studies were performed to look for signatures of topological phase transitions in SnTe, and a comparative study with a normal semiconductor SnSe. High-pressure Raman studies were also performed on SnBi2Te4 and SnBi4Te7 to look for electronic topological transitions as well as structural phase transitions. For structural characterization as a function of pressure, X-ray diffraction measurements using synchrotron source have been pursued. (2) Pressure and temperature dependent Raman studies were performed to look for signatures of stability of the excitonic insulating phase in Ta2NiSe5 and a comparative temperature dependent study on the S-counterpart, Ta2NiS5, to look for signatures of any phase transition. (3) Lastly, our temperature dependent Raman studies on AgRuO3 reveal a signature of subtle phase transition in addition to an antiferromagnetic transition. For pressure-dependent structural characterization, Raman and X-ray diffraction measurements have been pursued.Department of Science and Technology (DST) India, Fellowshi
Geoelectrical Characterization for Relating Electrical Resistivity and Geotechnical Properties of Soils
No full textThis thesis proposes electrical resistivity measurement as a potential method to characterize different types of geomaterials under different environmental conditions. Electrical resistivity method is a promising non-destructive method for subsurface investigations compared to conventional geotechnical methods which pose difficulties in obtaining good quality undisturbed soil samples. This method provides a simple way to identify the variation of geotechnical properties as well as the presence of heterogeneity in the subsurface. Integration of electrical resistivity with geotechnical properties and understanding their interactions with each other allow characterizations of soils properties. The focus of the thesis is to relate the electrical behaviour of various types of soils to changes in their physical and mechanical properties. Controlled laboratory investigations are carried out on different types of soils such as fine sand, medium sand, coarse sand, bentonite, sand-bentonite mixes, mine tailing, red soil, kaolinite, black cotton soil and gravels. To study resistivity variation with physical properties, resistivity boxes are fabricated using acrylic sheet to facilitate four-electrode method of measurement. For investigating resistivity changes with engineering properties, the conventional triaxial setup is modified using two-electrode method, and the rigid wall falling head permeameter and oedometer are modulated using Wenner four array method. Prior to testing soil samples in fabricated apparatus, calibration studies are carried out using standard salt solution. The experimental investigations are compensated with model studies in an acrylic tank to detect the anisotropies and numerical tomography studies.
The work in this thesis can be divided into four major parts. The first part of the thesis focuses on the assessment of sand-bentonite mix characteristics using the electrical resistivity method. Effects of particle size, surface conductivity, volumetric water content, dry density, void ratio, bentonite content, and temperature on electrical resistivity is investigated. In the second part of the thesis, an attempt is made to characterize mine tailings using electrical resistivity method through a series of laboratory experiments. Results from triaxial compression tests, permeability tests and electrical resistivity studies are coupled to establish correlations between electrical resistivity and engineering properties of tailings including permeability and shear strength. The third part of the thesis deals with the effectiveness of the electrical resistivity technique for geo-electrical characterization of compacted clays. A comprehensive study is carried out to investigate the variation in the electrical resistivity with change in mechanical characteristics of black cotton soil, bentonite, kaolinite clay, and red soil. Observations show that soils belonging to the same classification group can behave electrically different due to the differences in their mineralogical compositions. During consolidation, the changes in soil structure and expulsion of water voids alter the electrical pathways, which result in increased resistivity in case of red soil and reduced resistivity in case of black cotton soil. In the fourth part of the thesis, subsurface anisotropy is studied for sand, red soil, black cotton, and gravels using the square array method that allows measurement of resistivity along different azimuths. The electrical anisotropy of the clayey samples is studied with induced anisotropy in the form of cracks. Finally, azimuthal cross-square array resistivity soundings are carried out in field to investigate the features of anisotropy of the subsurface. Further, numerical investigations are carried out on simulated models of soil deposits to detect the inhomogeneities or anomalies in the subsurface using electrical resistivity distributions.
Based on the studies carried out in this thesis, electrical resistivity technique is found to be very efficient for basic characterizations of all types of geomaterials, quick evaluation of their engineering properties with reasonable accuracy and anisotropy and fault detection in soil or rock layers. Results from this study have great significance for many applications like quick subsurface surveys of large areas, geotechnical characterizations of different materials including hazardous wastes, detection of leakage in landfills, detection of faults and fractures in rocks, identifying failure zones in slopes for early detection of landslides and detection of anisotropy in soil deposits
INTERPIN: identifying INtrinsic transcription TERminators, hairPINs in bacteria
No full textThe conversion of DNA to RNA through transcription is an important step in the life cycle of every organism. It ensures that the genetic information in DNA is converted through RNA into instructions/blueprints for the formation of functional molecules, such as proteins. The property of RNA to fold onto itself, creating secondary and tertiary structures supports biochemical activities such as catalysis, ligand or protein binding, control of gene expression, protein transport, translation, and other regulatory functions in the cell. For intrinsic termination of transcription, a basic RNA secondary structure needed is known as the hairpin. The hairpin consists of a stem and a loop structure whose location of formation is central to tight and accurate regulation of the transcription process that has an essential role in the cell life cycle. There have been many in silico and experimental studies on the identification and analyses of RNA hairpins. Despite this, termination sites are known only in 30-40% of operons, and the analyses are limited only to a few bacteria including E. coli, M. tuberculosis, B. subtilis, and Streptomyces. All studies propose a single hairpin structure (canonical hairpins) as capable of stalling the RNA polymerase (RNAP) molecule to trigger the termination of transcription. This event constantly competes with the process of RNA elongation and kinetic rates of hairpin folding. Through this work, we have shown that a group of hairpins in a cluster can also work in tandem to cause transcription termination. The size of the hairpin groups could vary and all hairpins are present at ≤15 bases from each other. The two-member cluster hairpins have the highest occurrence and the size distribution decrease exponentially. The overall occurrence of cluster hairpins was found to be higher than single hairpins in prokaryotes (58:42), which can be explained by kinetic and thermodynamic considerations. Our prediction of intrinsic terminators in 13 bacterial genomes across 6 bacterial phyla has matched in 72% of cases when cross-checked against terminator locations inferred from high throughput RNA-seq data. Our method can predict hairpins in both AT and GC-rich genomes. The energy scores of predicted hairpins majorly fall within [-5,5] kcal/mol and lie within 50 base pairs from the stop codon, suggesting efficient termination. We did not find the occurrence of poly U/A pattern as a necessary feature of hairpins to drive the intrinsic transcription termination. Even though the terminator hairpin sequences themselves are not conserved, the process of intrinsic termination is highly conserved which can be explained by subtle differences in the features of hairpins across bacterial phyla.
To disseminate our work, we have archived the results in a public database named INTERPIN, which is the largest collection of intrinsic transcription termination units in bacteria.The database covers 12,745 bacterial genomes, from 10 different bacterial phyla, namely, Firmicutes, Chlamydia, Actinobacteria, Spirochaetes, Planctomycetes, Fusobacteria, Cyanobacteria, Thermodesulfobacteria, Acidobacteria, and Proteobacteria (which is divided into α-, β-, γ-, δ-, ε-, and other proteobacteria), and covering approximately 2.5*107 operons. The database provides a one-stop solution to obtain bacterial intrinsic termination predictions as well as visualize them. We also predicted hairpins in 27,938 bacterial plasmids, and in both cases, hairpins were predicted in >90% of interoperonic regions (IR), out of which cluster hairpins formed ∼58% of the total pool of predicted hairpins. We analyzed these hairpins across bacterial chromosomes and plasmids and found the relationship between hairpin energy and stem/ loop lengths, distance from stop codon, GC content, correlation with the occurrence of cluster/single hairpins, etc., and noted differences and similarities of hairpins across bacterial phyla. We have also delineated alternate transcription termination sites, which add another layer of regulation to prevent read-through of RNA transcripts in case the first default terminator is inadequate in terminating the transcription process. We found these sites in >80% IRs, ranging from 2-10 additional termination sites being present downstream of the first detected intrinsic terminator. The alternate termination sites were found in two categories: first where intrinsic hairpin terminator was found downstream of the first intrinsic terminator and second where intrinsic terminators were found downstream of rho-termination sites. Further, we have exhaustively compared our results with those available from other software like the WebGeSTer which makes similar predictions but uses a distinct hairpin detection approach. Through this analysis, we were able to identify common determinants of intrinsic transcription termination in bacteria, distinguishing features that are conserved across the hairpins predicted by the two software. These include localization of hairpins close to the stop codon (within 50 bp), energy scores between [-20,5] kcal/mol, and non-essentiality of poly U pattern at the hairpin base for effecting transcription termination. The studies were supplemented by validations made against the corpus of experimentally derived intrinsic terminators depicted in the literature. In summary, a new group of terminators, cluster hairpins have been identified and characterized, which is expected to fill the knowledge gap on intrinsic termination sites in bacteria. The prediction provided can help in advancing microbial genome annotations, guide experimental design strategies, and technology development, and provide potential targets for drug discovery studies
Hydrodynamic and hydromagnetic instabilities leading to turbulence in astrophysical accretion disks and their connection to laboratory shear flows
No full textAccretion disks are astrophysical objects formed around a denser object, mainly in the form of a disk. In an accretion disk, matter spirals in and falls onto the central object. To match the observations, the underlying flow has to be turbulent. Nevertheless, the
accretion flow model is stable against the infinitesimal perturbations according to Rayleigh criterion. A similar kind of discrepancy
between theory and experiments prevails there in the case of some laboratory flows, i.e., plane Couette flow and plane Poiseuille
flow. However, the presence of a weak magnetic field in the hot enough accretion flow could plausibly explain the onset of turbulence
through magnetorotational instability (MRI). Nonetheless, among its many caveats, MRI operates when the ionization is significant.
We, therefore, look for hydrodynamic instability as it is a generic case in all kinds of flows. We consider and extra force of stochastic
nature with a nonzero mean to be present in the local flow. The plausible origins of the force could be the small thermal fluctuation
present in the systems, the disturbances of arbitrary origins, etc. However, in the context of an accretion disk, the extra force could
originate from the interaction between the dust grains and fluid parcel in protoplanetary disks; back reactions of outflow/jet to accretion
disks; external forcing of the disk, i.e. tidal forcing, shock wave debris, outburst, or internal forcing by nonlinear terms.
We start by undertaking the problem with the introduction of an extra force in Orr-Sommerfeld and Squire equations along with the
Coriolis force mimicking the local region of the accretion disk. For plane Couette flow, the Coriolis term drops. Subsequently, we solve
the equations by the WKB approximation method. We investigate the dispersion relation for the Keplerian flow and plane Couette flow
for all possible combinations of wave vectors. Due to the very presence of extra force, we show that both the flows are unstable for a
certain range of wave vectors. However, the nature of instability between the flows is different. We also study the Argand diagrams
of the perturbation eigenmodes. It helps us compare the different time scales corresponding to the perturbations and accretion. We
ultimately conclude with this formalism that fluid gets enough time to be unstable and hence plausibly turbulent, particularly in the local
regime of the Keplerian accretion disks.
We, then, provide one of the plausible models of the extra stochastic force rigorously in the context of the accretion flow. In the
presence of the extra force and Coriolis force, we eventually establish the evolution of nonlinear perturbation by numerical solutions.
We show that even in the linear regime, under suitable forcing and Reynolds number, the otherwise least stable perturbation evolves to a
very large saturated amplitude, leading to nonlinearity and plausible turbulence. Hence, forcing essentially leads a linear stable mode to
unstable. We further show that nonlinear perturbation diverges at a shorter timescale in the presence of force, leading to a fast transition
to turbulence. Interestingly, the emergence of nonlinearity depends only on the force but not on the initial amplitude of perturbation.
Next, we explore the effect of forcing on the linear shear flow or plane Couette flow, which is also the background flow in the
very small region of the Keplerian accretion disk. We show that depending on the strength of forcing and boundary conditions suitable
for the systems under consideration, the background plane shear flow and, hence, accretion disk velocity profile modifies to parabolic
flow, which is plane Poiseuille flow or Couette-Poiseuille flow, depending on the frame of reference. In the presence of rotation, plane
Poiseuille flow becomes unstable at a smaller Reynolds number under pure vertical as well as three-dimensional perturbations compared
to their non-rotating two-dimensional counterpart. Hence, while rotation stabilizes plane Couette flow, the same destabilizes plane
Poiseuille flow faster and forced-local accretion disk. Depending on the various factors, when local linear shear flow becomes Poiseuille
flow in the shearing box due to the presence of extra force, the flow becomes unstable even for the Keplerian rotation, and hence
turbulence will pop in there.
In the end, we venture for the comparison between growth rates for MRI and hydrodynamics instability in the presence of an
extra force in the local Keplerian accretion flow. The underlying model is described by the Orr-Sommerfeld and Squire equations in the
presence of rotation, magnetic field, and an extra force, plausibly noise with a nonzero mean. We obtain MRI using WKB approximation
without extra force for purely vertical magnetic field and vertical wavevector of the perturbations. Expectedly, MRI is active within a
range of the magnetic field, which changes depending on the perturbation wavevector magnitude. Next, to check the effect of noise
on the growth rates, a quartic dispersion relation has been obtained. Among the four solutions for growth rate, the one that reduces to
MRI growth rate at the limit of vanishing mean of noise in the MRI active region of the magnetic field is mostly dominated by MRI.
However, in MRI inactive region, in the presence of noise, the solution turns out to be unstable, which is almost independent of the
magnetic field. Another growth rate, which is almost complementary to the previous one, leads to stability at the limit of vanishing
noise. The remaining two growth rates, which correspond to the hydrodynamical growth rates at the limit of the vanishing magnetic
field, are completely different from the MRI growth rate. More interestingly, the latter growth rates are larger than that of the MRI. If
we consider viscosity, the growth rates decrease depending on the Reynolds number.
Hence, we have established that the presence of an extra stochastic force with a nonzero mean makes the linearly stable flow
effectively unstable. Once the instability and, therefore, turbulence kicks in inside the shearing box, we consider the shearing box
repeatedly throughout the radial extension of the accretion disk. Hence, the angular momentum transport can be interpreted in the
Keplerian accretion disk
Sub-Newtonian Coalescence in Polymeric Fluids
No full textDroplet coalescence is a thermodynamic equilibration process driven by surface energy
minimization. The physics of this phenomenon is characterized by the temporal evolution
of a liquid bridge formed upon the proximate approach of two droplets. This phenomenon
is ubiquitous, manifesting in processes linked to life like those in raindrop formation,
growth of tumor cells, and industrial processes like those in combustion, spray paintings,
and coatings. Despite these universal occurrences, studies on the coalescence of complex
fluid droplets remain scarce in the literature. Unlike Newtonian fluids, complex fluids
have signature micro-structures that can result in a wide range of responses depending
on external perturbations making a unified model elusive. Such diversity in micro structures and flow behaviors have resulted in classifying these in sub-classes ranging
from polymers to suspensions. But there has been a recent surge in studies investigating
coalescence dynamics in macromolecular-based micro-structure fluids, i.e., polymeric
fluids. However, a detailed work on developing a theoretical framework for coalescence
in complex fluids remain unknown. Here in this thesis we propose such frameworks for
polymers, suspensions and dispersions. Further based on our observations for polymeric
fluids, we propose the existence of new coalescence regime, namely the sub-Newtonian
regime with arrested coalescence as its limiting case.MHR
Multi-layered Composite Structures for Electromagnetic Interference Shielding Applications
No full textWith the recent surge in the usage of electronic devices, electromagnetic interference (EMI) poses a serious threat. Unwanted electromagnetic (EM) waves not only interfere with the normal functioning of electronic components, but certain studies suggest that it is a serious threat to human health. Polymer nanocomposites serve as a promising solution for EMI shielding as they can be tuned to meet the commercial shielding requirements by incorporating suitable fillers. Moreover, polymers are lightweight, corrosion resistive, easy to process, and can be molded into complex geometries. In this dissertation, multi-layered composite structures have been fabricated and studied for EMI shielding applications. Multi-walled carbon nanotubes (CNTs) were chosen as one of the fillers owing to their electrically conducting nature. To improve the thermal conductivity and/or EM wave absorption properties, a hybrid functional filler composed of Fe3O4 decorated reduced graphene oxide (rGO) was chemically synthesized and incorporated into selected composites. Alternatively, polyurethane (PU) foam-based multi-layered structures were also fabricated to enhance the absorption-based shielding performance.
As a background study, we started with polycarbonate (PC)-based composites fabricated using the conventional melt mixing approach. PC was chosen as it has a low electrical percolation threshold for CNTs (<0.5 wt%). Total shielding effectiveness (SET) of -23 dB (1 mm thick) was obtained for PC composites with 3 wt% CNT and 10 wt% rGO-Fe3O4. A high filler loading in PC may result in either processing difficulties or poor structural properties. Therefore, PC was blended with polyvinylidene difluoride (PVDF) to improve the structural stability and also to take advantage of the double percolation effect. The selective localization of CNTs in the PC component of the PC/PVDF blend resulted in double percolation, i.e., improved bulk electrical conductivity in blend-based composites compared to single polymer composites. Despite the double percolation, the maximum SET value of -24 dB (1 mm thick) was observed with 3 wt% loading of CNTs. We further added a mutually soluble homopolymer, polymethylmethacrylate (PMMA), as a compatibilizer for the PC/PVDF immiscible blend to reduce the interfacial tension and refine the blend morphology. Despite the morphology refinement, the shielding performance declined due to the diffusion of PMMA in the individual components (PC or PVDF) and the redistribution of fillers.
In the subsequent chapters, multi-layered composite structures were opted over the conventional melt mixed composites to improve upon the shielding performance. Thin films of PVDF and PC nanocomposites were interfacially locked using a mutually miscible polymer (PMMA) to obtain a shield with enhanced structural properties and EMI shielding performance. By stacking multi-layered films one above the other, reaching an assembly thickness of ca. 0.5 mm, the maximum SET was found to be -26 dB, which is a significant improvement compared to melt mixed composites. In the next chapter, porous structures (synthesized foams and 3D printed mesh structure) were sandwiched between composite sheets of PC and PVDF with an aim to dissipate the EM signals through multiple internal reflections. PU neat and composite foams were synthesized through a polymerization reaction between 4,4’-Methylenebis(phenyl isocyanate) and polyethylene glycol. Using PU-CNT foam as an inner layer between composite sheets of PC and PVDF, a maximum SET of -39 dB (approx. 5.3 mm thick) with absorption-dominated shielding. In order to further enhance the shielding effectiveness, Ag was sputtered on the PU-foam, which resulted in the highest SET value of -50 dB in the X-band but with a significant reflection component. The results presented here begin to suggest that in-situ synthesized foam with non-uniform and dead pores enhances the shielding performance compared to non-porous structures.
Towards the end of the dissertation, PU foam was fabricated using a simpler technique of salt-leaching. By stacking freestanding CNT papers (approx. 200 µm in thickness) on both sides of lightweight PU foam, we could limit the filler content yet maximizing the absorption-based EMI shielding performance. The multi-layered structure exhibited a high SET value of -49 dB (92% absorption @ 26.5 GHz; approx. 4.6 mm thick), whereas CNT paper by itself showed a maximum SET value of -35 dB (73% absorption @ 26.5 GHz). The porous uniform PU structure enhances the absorption component of shielding due to the trapped air, adequate impedance match, and multiple internal reflections. Further, we arrived at a remarkably interesting conclusion, i.e., if the incoming wave encounters PU foam before the CNT paper (in multi-layered structure with foam and CNT paper on one side), the absorption percentage of shielding can be further enhanced (98% absorption @both 8.2 and 26.5 GHz, SET ~ -37 dB, approx. 4.4 mm thick). In such asymmetric structures, reversing the direction of the incoming EM wave can change the absorption percentage.
The results presented in this dissertation suggest the various methodology for composite fabrication and the approaches to maximize the absorption-based EMI shielding performance