Indian Institute of Science Bangalore
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An Investigation of Electronic Phases and Charge Dynamics in Low-Dimensional System
No full textIn condensed matter physics, the concepts of topology and symmetry are of paramount importance, particularly in understanding quantum phase transitions. Topology classifies objects based on their topological
properties, which are properties that are preserved under continuous deformation. This is relevant to a wide range of phenomena, such as topological insulators and the quantum Hall effect. On the other hand, symmetry is
used to understand phase transitions, where a higher symmetry group is
broken into a lower symmetry subgroup. The study of quantum phases
and phase transitions is a fundamental theme in condensed matter physics,
and topology and symmetry play a critical role in comprehending these
phenomena. This field of research is vital in understanding the behavior
of matter at the quantum level and has potential applications in quantum
computing and other technologies
This thesis mainly explored the quantum phase transitions through
resistance fluctuation spectroscopy. The first part of the thesis focused
on uncovering the electronic phases in 1T-TaS2
. The presence of a low temperature insulating phase in 1T-TaS2 has been a matter of debate
among researchers, with recent theoretical calculations suggesting that it
is a result of out-of-plane stacking rather than a Mott insulator
Our findings suggest that out-of-plane stacking might be responsible for
the observed insulating phase at low temperatures. Our study showed
that the device exhibited metallic behavior at low temperatures, but an
insulating phase was restored over a narrow range as the temperature increased. The system also exhibited signs of electrical phase separation prior to the restoration of the insulating phase, as seen through quantized jumps in conductance between two well-defined levels. These jumps were speculated to result from metallic domain walls separating insulating regions.
In the second part of the thesis, we studied the resistance fluctuations
near the Lifshitz transition in WTe2 using electrical and thermal transport
studies. The presence of two holes and two electron pockets in the band
structure at low temperatures and the disappearance of the hole pockets
above the transition temperature could be related to a topological phase
transition in the material. An electrical noise peak was observed at the
transition temperature, which was attributed to inter-band scattering at
Weyl points. The study emphasized the importance of high-quality samples for detecting the unique properties of Weyl semi-metals.
In the final section of the thesis, the behavior of charge dynamics in a
quasi-two-dimensional electron gas (q2DEG) at the LaScO3/SrTiO3
interface was studied through resistance fluctuation spectroscopy. Despite persistent efforts, the source of charge carriers at the oxide interface remains
elusive. [7, 8, 9]. Our study indicated that the role of oxygen vacancies
in transport properties at the oxide interface requires further exploration.
The q2DEG at the LaScO3/SrTiO3
interface was found to exhibit random telegraphic noise (RTN) at high temperatures, which disappeared
below 40 K, indicating a temperature-dependent behavior of the charge
dynamics. Based on these observations, we posit that the RTN is likely
the result of charge exchange between mid-gap defect states in the bulk
and the q2DEG
Molecular Approaches and Structural Insights into Correlated Triplet Pair Dynamics in Intramolecular Singlet Fission
No full textSinglet Fission (SF) is a bichromophoric process whereby a singlet exciton is converted to two triplet excitons through an overall spin-conserved process. The last decade has seen an upsurge in SF research driven by its potential application in photovoltaics to exceed the power conversion efficiency of solar cells beyond the Shockley-Quiesser (S-Q) limit. Singlet fission proceeds through a correlated triplet pair state, 1(TT), with an overall singlet character allowing the singlet state to access the triplet surface in ultrafast timescales. The formation of a coherent exciton pair makes these materials promising candidates in quantum technology and as polarized spin generators. As a result, understanding the triplet pair dynamics and their correlation with molecular structure attracts widespread attention.
It has been established that the primary governing factor that dictates the singlet fission dynamics is the electronic coupling between the two chromophores. Interestingly, the intrinsic duality of the SF process is that a strong electronic coupling leads to efficient triplet pair formation and enhanced recombination, reducing the spatial separation into free triplets. This demands the development of design approaches to optimise electronic coupling.
In my thesis, I have rationally designed molecular approaches to control triplet pair dynamics, coherences and triplet separation in molecular dimers. We have designed an array of pentacene dimers, particularly looking into the effects of molecular geometry and conformations on SF. Transient absorption measurements show that not only the static molecular geometry but dynamic molecular conformations also play a significant role in the outcome of the SF process. I have also studied the singlet fission dynamics in dimers with a resonant bridge where I have established that singlet fission is possible in systems where the initial excitation is delocalised over the two chromophores. Our observations show that the molecular structure and conformations heavily impact the dynamics within the triplet pair manifold
Engineering Nano-Electronic Devices using 2-D Materials: CMOS Logic to Biosensing
No full textTechnology scaling has driven the development of semiconductor technology that forms a
ubiquitous part of daily utilities such as smartphones, computers, and wearables. However,
efforts to continue scaling have met numerous challenges, both from engineering limitations
and the fundamental limits of silicon. Two-dimensional (2-D) materials are a potential
candidate for highly scaled nodes due to their atomically thin nature and excellent
electrostatics. The IRDS roadmap (2021) has forecasted 2-D channels as possible contenders
for 1.5 nm nodes and beyond. In this regard, there is a need to explore the potential of 2-D
materials for complementary metal oxide semiconductor (CMOS) compatible logic platforms
and address the challenges that limit their adoption. The large surface-to-volume ratio and the
sensitivity to external surroundings also make 2-D materials excellent for bio-sensing
applications. The field effect transistor (FET) technology can be leveraged for ion and
biomolecule sensing by appropriate functionalization. In this thesis, we use 2-D materials like
MoS2, WSe2, etc., along with high-k dielectric materials (HfO2, Al2O3) to develop scalable and
CMOS-compatible FETs and ion-sensitive FETs (ISFETs) for logic and biosensing
applications, respectively.
The first part of this thesis deals with process engineering and optimization for contact
engineering and short-channel devices. A modified surface treatment process using ammonium
sulfide in an alcohol medium is introduced for better Ni-MoS2 contacts with a low Schottky
barrier height of 130 m eV, resulting in lower contact resistance, low variability, and better
yield. The process is less aggressive and compatible with the back end of line processing on
pre-patterned substrates with metal interconnects. Then, we optimize electron beam
lithography for ultra-short channel device patterning for the smallest feature lengths of 80 nm
and 30 nm, using manual dose correction and algorithmic proximity error correction,
respectively. Back-gated FETs with Ni-MoS2 contacts and a short channel of 80 nm show
contact resistance as low as 1.3 kΩ μm with alcohol-based sulfur treatment.
Secondly, we explore alloying to tune the electrochemical characteristics of 2-D materials.
Ternary alloys of form MoS2(1-x)Se2x show composition-dependent bandgap, strain, and carrier
concentrations. Using these alloys, we tune the threshold voltage, subthreshold slope, mobility,
and drain currents in back-gated FETs. Integrating them with SiO2 and HfO2 dielectrics
provided exciting insights into their interfaces and highlighted the benefit of high-k dielectrics
for high-performance FETs with enhancement mode operation.
Low-power logic circuits require steep switching (<60 mV/dec) FETs, whereas ISFETs with
high sensitivity (> 59 mV/pH) are desirable for biosensing applications. Third, we engineer
new device architectures to surpass the conventional limits of these devices. A steep switching
MoS2 FET is developed using gate connected nickel ferrite (NF) threshold switching (TS)
device. By integrating the TS device to the top-gate stack of a MoS2 FET, we achieve steep
subthreshold slopes as low as 8.5 mV/dec, much lower than the Boltzmann limit of 60 mV/dec.
A super-Nernstian ISFET is developed using the vdW heterostructure of WSe2 and MoS2. The
double-gated WSe2/MoS2 ISFET achieved a sensitivity of 362 mV/pH, well above the Nernst
limit of 59 mV/pH, by exploiting the charge screening effects of the hetero-interface. Further
sensitivity enhancement can be achieved using an experiment calibrated TCAD model and
numerical solutions for the ferroelectric negative capacitance effect. The NC-WSe2/MoS2
ISFET shows a massive enhancement in sensitivity to 4.38 V/pH with a resolution of 0.002
units of pH. Biomolecule detection (total cholesterol) is also demonstrated using a
functionalized MoS2 channel. The bio-FET device uses a copper (II) oxide nanoparticle-linked
cholesterol oxidase and esterase as the primary sensing platform. The device shows a
proportional increase in current with increasing cholesterol concentrations and achieved a
normalized peak sensitivity of 1.95 μA/(μm2 mg/mL). This work opens the paths for scalable
and CMOS-compatible 2-D material platforms - for low-power computing applications; for
point-of-care diagnostic devices with high sensitivity and throughput
Improved approximation bounds on maximum edge q coloring of dense graphs
No full textThe anti-Ramsey number ar(G,H) with input graph G and pattern graph H, is the maximum
positive integer k such that there exists an edge coloring of G using k colors, in which there are
no rainbow subgraphs isomorphic to H in G. (H is
rainbow if all its edges get distinct colors). The concept of anti-Ramsey number was introduced
by Erdos, Simanovitz, and Sos in 1973. Thereafter several researchers investigated this concept
in the
combinatorial setting. The cases where pattern graph H is a complete graph K_r, a path P_r or a
star K_{1,r} for a fixed positive integer r, are well studied.Recently, Feng et al. revisited the anti-Ramsey problem for the pattern graph K_{1,t} (for t geq 3)
purely from an algorithmic point of view, due to its applications in interference modeling of
wireless networks. They posed it as an optimization problem, the maximum edge q-coloring
problem. For a graph G and an integer q geq 2, an edge q-coloring of G is an assignment of
colors to edges of G, such that edges incident on a vertex span at most q distinct colors. The
maximum edge q-coloring problem seeks to maximize the number of colors in an edge q-
coloring of the graph G. Note that the optimum value of the edge q-coloring problem of G equals
ar(G,K_{1,q+1}).
We study ar(G,K_{1,t}), the anti-Ramsey number of stars, for each fixed integer t geq 3, both from
combinatorial and algorithmic point of view. The first of our main results, presents an upper
bound for ar(G,K_{1,q+1}), in terms of number of vertices and the minimum degree of G. The
second one improves this result for the case of triangle free input graphs.
For a positive integer t, let H_t denote a subgraph of G with maximum number of possible edges
and maximum degree t. From an observation of Erdos, Simanovitz, and Sos, we get: |E(H_{q-1})|
+ 1 leq ar(G,K_{1,q+1}) leq |E(H_{q})|. For instance, when q=2, the subgraph E(H_{q-1}) refers to a
maximum matching.
It looks like |E(H_{q-1})| is the most natural parameter associated with the anti-ramsey number
ar(G,K_{1,q+1}) and the approximation algorithms for the maximum edge coloring problem
proceed usually by first computing the H_{q-1}, then
coloring all its edges with different colors and by giving one (sometimes more than one) extra
colors to the remaining edges. The approximation guarantees of these algorithms usually
depend on upper bounds for ar(G,K_{1,q+1}) in terms of |E(H_{q-1})|.
Our third main result presents an upper bound for ar(G,K_{1,q+1}) in terms of |E(H_{q-1})|.
All our results have algorithmic consequences. For some large special classes of graphs, such
as d-regular graphs, where d geq 4, our results can be used to prove a better approximation
guarantee for the sub-factor based algorithm. We also show that all our bounds are almost tight.
Results for the case q=2 were done earlier by Chandran et al. In this thesis, we extend it further
for each fixed integer q greater then
Exploring dissipation dynamics in low-dimensional superconductor
No full textTechnological advances in the past decade in fabricating two-dimensional superconducting materials with low disorder have enabled exploring many exciting phases. In recent times, two-dimensional superconductors have been the focus of research in condensed matter physics, with theories predicting such systems to be a suitable platform to observe not only exotic phases but a medium to test several statistical phenomena which are somewhat challenging to probe in other arrangements. In this thesis, my discussion has revolved around two Type II superconducting systems in which we have investigated several low-dimensional phenomena through transport studies.
Ising superconductors are predicted to host several unconventional phases through a topological pairing of electrons that arises due to spin-momentum locking. An ideal system to observe such superconductivity are the transition metal dichalcogenide (TMD) materials which show a strong Ising spin-orbit coupling (SOC) in the monolayer limit owing to the lack of in-plane inversion symmetry. In the first part of the colloquium, I will present a study of van der Waals heterostructure comprising of monolayer MoS2 – a semiconducting TMD with high Ising SOC and few-layer NbSe2 – a conventional superconducting TMD. Through systematic magnetotransport measurement, we found that the superconductivity in the heterostructure region is of 2D Ising in nature in contrast to the 3D behavior of the few-layer NbSe2. We establish that the observed phenomena can be attributed to the hybridization of the bands of an effectively thinned NbSe2 and the monolayer MoS2 on top of it. Following this, I will discuss the study of the carrier dynamics of this heterostructure around the transition temperature at zero magnetic fields through resistance fluctuation spectroscopy. We establish the universal BKT-type behavior of the superconductivity in the heterostructure region.
In the second part of the thesis, I discussed the results of our study of the dynamics of the hexatic phase in the two-dimensional vortex lattice of a weakly pinned superconducting thin film (MoGe). The hexatic phase is an intermediate phase that arises in the melting process of a two-dimensional crystalline solid. We specifically studied the effect of an external drive on the phase through fluctuation spectroscopy. We observe low-frequency oscillations in the vortex velocity that are exclusive to the thermally assisted flux flow (TAFF) regime. We also observed a long-term memory effect of the vortex motion in the Hexatic regime of the vortex lattice. Lastly, we uncovered a current-induced dynamical instability in the Hexatic phase, where the system toggles between two defined levels in the presence of a constant external drive
Structural signatures of relaxation mechanisms in glass forming and devitrifying colloidal suspensions
No full textConnecting macroscopic behavior of matter to the microscopic interactions is the central goal of condensed matter physics. Yet, achieving such a challenging goal in atomic and molecular systems would require tracking all particles in the system up to the length scale of atomic resolution (≈ nm) and timescales of picoseconds. A typical workaround through this daunting challenge is to instead model the system with colloidal suspensions, where length scales (≈ μm)
and timescales(≈ ms) are more accessible to experiments. Over the last two decades, colloidal systems have been used to mimic atomic systems as they exhibit various phases such as liquids, crystals, gases, and glasses which have enabled study of various processes such as nucleation and growth of crystals, solid-solid phase transition, liquid crystals, liquid-liquid phase transitions and many more. In this thesis, we utilize colloids to gain insights into the phenomenon of glass formation as well as relaxation of a metastable glass into a crystal. Unlike crystals which have unique structural signatures that defines their macroscopic properties, glasses possess structure that are akin to liquids. Nevertheless, they exhibit rigidity similar to solids. Physicists have tried to link the properties of glasses to the underlying aperiodic structure over the past three decades but the studies have remained inconclusive. This is attributed to the fact that standard correlation functions that could easily identify ordered structures cannot distinguish between distinct amorphous configurations. This thesis is aimed at gaining insights into the mechanisms of glass formation from experiments on colloidal glasses. It is done by designing analytical tools towards recognizing patterns in amorphous configurations which aid in understanding structure and its relation to dynamics in glassy systems. We identify distinct signatures for these disordered structures in two different ways. We designed a protocol to determine amorphous-amorphous interfaces by defining self-induced pins to critically evaluate the assumptions of Random First Order Transition (RFOT) theory, which is a prominent thermodynamic approach to glass transition. Second, we exploited machine learning algorithms to identify distinct structural features that are responsible for relaxation to predict
where crystallization occurs in glass. The method also helped unveil possible structural connections for another prominent theory for glass transition i.e, Dynamic Facilitation which is a kinetic approach. The thesis is organized into two preliminary chapters followed by work chapters which describe in detail the above findings and a concluding chapter at the end.
In Chapter 1 we introduce colloidal suspensions that are typically used as model systems to study phase behavior in condensed matter systems, which in our case was exploited to study glass transition and devitrification. Further, we
present the formulations behind relevant theories of glass formation and briefly explain the theoretical and simulation developments involving devitrification. In addition, we discuss the principle behind machine learning and describe concisely algorithms used to achieve diverse goals. In particular, we discuss supervised learning approach in detail which we utilize to develop connections between structure and dynamics of glasses.
In Chapter 2 we describe the experimental methods utilized to perform the work presented in this thesis. Experimental systems in chapters 4 and 5 utilize polystyrene particles. We describe the synthesis protocols used to develop such systems in chapter 2. Details of experimental system used to realize devitrification in chapter 3 are described in chapter 2. The chapter also describes confocal microscopy, the instrument used for performing devitrification experiments.
Chapter 3 unravels how glasses transform into crystals, a process known as devitrification. Glasses are inherently unstable to crystallization. However, the transformation remains poorly understood as it occurs whilst the dynamics in the glass stay frozen at the particle scale. In Chapter 3, through single-particle-resolved imaging experiments, we show that due to frozen-in density inhomogeneities, a soft colloidal glass crystallizes via two distinct pathways. In the poorly packed regions of the glass, crystallinity grew smoothly due to local particle shuffles, whereas in the well-packed regions, we observed abrupt jumps in crystallinity that were triggered by avalanches - cooperative rearrangements involving many tens of particles. Importantly, we show that softness - a structural order parameter determined through
machine learning methods - not only predicts where crystallization initiates in a glass but is also sensitive to the crystallization pathway. Such a causal connection between the structure and stability of a glass has so far remained elusive. Devising strategies to manipulate softness may thus prove invaluable in realizing long-lived glassy states.
Chapter 4 deals with measurement of surface tension of amorphous-amorphous interfaces which is one of the basic ingredients for Random First Order Transition (RFOT) theory - a prominent thermodynamic approach to glass transition. The relaxation dynamics in liquids on approaching their glass transition was found to become increasingly cooperative and the relaxing regions also become more compact in shape. Of the many theories of the glass transition, only RFOT anticipates the surface tension of relaxing regions to play a role in deciding both their size and morphology. However, owing to the amorphous nature of the relaxing regions, even the identification of their interfaces has not been possible in experiments hitherto.
In Chapter 4, we devise an analytical method to directly quantify the dynamics of amorphous-amorphous interfaces in bulk supercooled colloidal liquids. Our procedure also helped unveil a non-monotonic evolution in dynamical correlations with supercooling in bulk liquids. We measure the surface tension of the interfaces and show that it increases rapidly across the mode-coupling area fraction. Our experiments support a thermodynamic origin of the glass transition.
Chapter 5 explores the possible structural connections to Dynamic Facilitation theory (DF) - which is a purely kinetic approach to glass transition. Despite decades of intense research, whether the transformation of supercooled liquids into glass is a kinetic phenomenon or a thermodynamic phase transition remains unknown.
In Chapter 5, we analyzed optical microscopy experiments on 2D binary colloidal glass-forming liquids and examined a potential structural origin for localized excitations, which are building blocks of DF. To accomplish this, we utilize machine learning methods to identify a structural order parameter termed softness that has been found to be correlated with reorganization events in supercooled liquids. Both excitations and softness qualitatively capture the dynamical slowdown on approaching the glass transition and motivated us to explore spatial and temporal correlations between them. Our results show that excitations predominantly occur in regions with high softness and the appearance of these high softness regions precedes excitations, thus suggesting a causal connection between them. Thus, unifying dynamical and thermodynamical theories into a single structure-based framework may provide a route to understand the glass transition.
In Chapter6 we discuss our findings and suggest possible future directions
Climate-resilience of Dwellings in Transition and the Thermal Behaviour of their Inhabitants
No full textThermal comfort is critical for the well-being, and productivity of the inhabitants. Traditional dwellings constructed using local, low-carbon materials have for decades provided comfortable indoor thermal environment to its inhabitants through passive design. Traditional materials are increasingly being replaced by standardised, industry-manufactured, energy-intensive materials. The modern buildings are rarely designed for the climate of the region and are predominantly equipped with electro-mechanical appliances to alter the indoor thermal environment. Transitions in the building envelopes alter the thermal performance of the dwelling. Whether the changing thermal environment resulting from transitions in the buildings accompanied by the reliance on active adaptive strategies affect the thermal expectations of the inhabitants is not known. The transitioning built environment, the changing lifestyles and thermal expectation of the occupants and the looming climate change affect the thermal comfort of occupants.
Three transitioning rural settlements across three climate zones in India were selected for the study. Transitions in these settlements were studied through field investigation, habitat surveys, and historical satellite imagery. Representative traditional and modern dwellings were selected to study the effect of transitions on the embodied energy and thermal performance of the dwellings. The thermal performance of the transitioning dwellings to prevailing and changing climate was investigated through real-time measurement of environmental parameters and dynamic building simulation. The influence of long-term exposure to the thermal environment that prevailed in the traditional and modern dwellings, accompanied by the dependence on passive or active adaptive strategies on the thermal expectation of the inhabitants was investigated through general and point-in-time thermal sensation surveys of occupants.
Various drivers and consequences of transitions were identified both through literature survey and field studies. The application of secondary data, field surveys and historical satellite imagery to interpret transitions was demonstrated. A methodology to study the thermal performance of transitions dwellings was developed, and the adoption of heating and cooling load calculated based on adaptive degree-days was demonstrated. The investigation revealed that traditional dwellings performed better than the modern dwellings across climate zones. Climate change further increased the cooling load in warm climates while reducing heating load in cold climates. Both transitions and climate change had a compounding effect on the thermal performance of the dwellings in temperate and warm-humid climate zones. The study revealed varying adaptive behaviour between the two occupant groups and long-term exposure to their respective thermal environment influenced the thermal perception of occupants.
The study highlights the importance of studying the nexus between climate, habitat, and inhabitants. This is critical in understanding thermal comfort, energy implications and emissions associated with the occupation of the buildings. Studying this nexus and variations in them are imperative in designing buildings for the future that are low carbon, sustainable and resilient to climate change
Anaerobic digestion characteristics of lignocellulosic feedstocks under solid-state stratified bed mode of fermentation
No full textLignocellulosic feedstocks have become potential materials for conversion to biomethane but possess varying compositions and densities (18-50kg/m3), requiring alternative approaches for evolving sustainable anaerobic digesters. Solid-state anaerobic digesters such as solid-state stratified bed reactors (SSBR) can take such low-density biomass without extensive pre-processing. However, reactor optimization is needed due to the significant difference in physical and chemical steric properties of lignocellulose. This study attempts to identify appropriate combinations of these properties to identify ideal fermentation conditions beforehand. Temporal changes in physical, chemical, and fermentation properties were recorded as a function of SRT for ten lignocellulosic feedstocks- three dicots land weeds, three dicot tree leaves, and four agro residues in an SSBR, to represent a wide variety of feedstock in the test pool. The rates of degradation described by loss of TS, VS, and gain in bulk density indicate that for most feedstocks, there is an initial rapid weight loss ascribed to loss of extractives and some hemicellulose and an initial rapid gain in bulk density. After this period, decomposition slows down. This turning point, termed the inflection point, appears to be a good representation of economic or ideal SRT (18-25d in dicots and 46-60d in agro residues). The degradation behavior pattern of lignocellulose in SSBR- the extent of degradation, rate of degradation, and achievable average methane production rates could be predicted well by knowing the composition of feedstocks in terms of hot water extractable (HWE), oxalate extractable pectin (Ox), hemicellulose (HC), cellulose (C) and lignin (L) content expressed as ((HWE+Ox+Hc))/((C+L)), with R2=0.78, 0.81, and 0.85, respectively. It is the first time a single parameter has been able to predict multiple degradation behavior for diverse lignocellulose. At the same time, methanogenesis is addressed with a biofilm bed on the digesting feedstock. These digesting (spent) feedstocks had high specific methanogenic assay levels reaching 20L CH4/kg residual TS/d (hydrogenotrophic) and 35L CH4/kg residual TS/d (aceticlastic). Feedstocks with cellulose concentration >27% of TS recorded methane production potential between 7-10.4L/kg feed TS/d through the hydrogenotrophic route of methanogenesis. In addition, the TSMA, HSMA, and ASMA evaluation on SRT at the VS loss inflection point showed a negative correlation with “HWE+Ox+HC” (R2=0.77, 0.77, and 0.56, respectively), while a high concentration of “HWE+Ox+HC” conferred net high degradability to feedstocks. A new approach to seeding- a surrogate to substrate to inoculum ratio (S/I) is proposed for SSAD seeded with solid inoculum source after its SMA has been quantified – “(Kg TS fed/kg digestate TS)” and “SMA/S” (L TSMA required/kg TS fed). These developments show the potential to use new and untried feedstocks (and combinations) while avoiding various intermediate stages of feedstock trials and validation and end-of-life use of spent lignocellulose as high-activity biofilm support. On comparing SSBR with BMP, it was found that SSBR yields reached ≥69% of BMP yield but needed~ 53-73d to catch up. This, however, can be improved by introducing compacted feed and increasing the substrate contact with the methanogens. Methane yields estimated from BMP could be predicted with the formulated parameter- ((HWE+HC))/((C+L+Ox)). It is the first time such a close level of biomethane production potential has been predicted using the physicochemical properties of diverse feedstocks
Formulations for solving geomechanics stability problems using stress characteristics and finite element limit analysis with power type and Mohr-Coulomb yield criteria
No full textIn geotechnical engineering, the stability analysis is often carried out to determine the ultimate collapse load that a geo-structure can possibly withstand. The accurate prediction of the exact collapse load is often very critical for ensuring the safety of any structure. Various methods of performing the stability analysis are available for solving different geomechanics problems. These are the limit equilibrium method (LEM), the method of stress characteristics (MOSC), the limit analysis (LA) method with an assumption of the collapse mechanism, and the finite element limit analysis (FELA). The present research primarily focuses on predicting the numerical solutions for different geomechanics stability problems on the basis of the MOSC and the FELA. The choice of the yield criterion is very crucial in accurately modelling the behaviour of soils. The stress-dependent yield behaviour of soils cannot be modelled effectively by a linear failure envelope- like for instance the Mohr-Coulomb (MC) failure criterion. It has been observed from numerous experimental studies that soil yield parameters are generally stress-dependent, and the soil friction angle is no longer independent of normal stress. This type of soil behaviour cannot be accurately modelled using the widely employed MC yield criterion. The non-linear power type (PT) yield criterion, which simulates in a better fashion with the experimental observations, has been employed in the present thesis to arrive at much more acceptable solutions while proposing the formulations on the basis of MOSC and FELA. In addition, the MC yield criterion was also used to provide the necessary validation.
A new axisymmetric UB-FELA formulation using the second order cone programming (SOCP) has been developed for soils which obeys the MC yield criterion to provide more accurate solutions in an efficient manner. The proposed formulation is used to solve different geomechanics stability problems, including determining (i) the bearing capacity of solid circular and ring footings, and (ii) the stability numbers for unsupported vertical circular excavations for various combinations of (H/b) and the soil friction angle, ϕ; where H is the depth of the bottom of the excavation from the ground level and b is the radius of the circular excavation. The results obtained from the analysis are found to compare well with the existing studies. The proposed formulation is found to be computationally quite efficient while providing very accurate solutions.
A new FELA formulation based on the non-linear PT yield criterion in conjunction with power conic programming (PCP) has been proposed to solve both axisymmetric and planar stability problems. To validate this formulation, the solutions for (i) seismic bearing capacity of a strip footing placed on horizontal ground and (ii) the bearing capacity of a circular footing on horizontal ground were obtained. To increase the computational efficiency and to obtain more accurate solutions with a given number of finite elements in the chosen soil domain, an adaptive mesh generation technique was used. The adaptive mesh was generated using an open-source software TRIANGLE - which is based on the Delaunay triangulation technique.
While implementing the proposed formulation(s), it is intended to determine the seismic bearing capacity factors for a strip footing placed on horizontal and sloping ground surface by using the MOSC and the FELA. The objectives of this investigation include (i) computing the seismic bearing capacity factor N_σ for a rough strip footing on a sloping ground using the non-linear PT yield criterion by employing the MOSC, and (ii) finding the seismic bearing capacity factor N_γ (a component of ultimate bearing capacity due to the unit weight of the soil) for a rough strip footing using the MC yield criterion through the MOSC. The effectiveness of the PT yield criterion in determining the bearing capacity values which are in agreement with experimental values was shown. The seismic bearing capacity factor N_σ was evaluated for (i) different ground inclinations (β), (ii) varying horizontal earthquake acceleration coefficient ("α" _h), (iii) various values of shear strength parameters (c_o,m,σ_t,A) and soil surcharge (q). The resulting N_σ values were compared with previously reported results in literature, and were found to match well with the solutions from previous studies.
The employment of a rigorous failure mechanism, which overcomes the limitations of the existing MOSC formulation to determine the seismic bearing capacity factor N_γ for a rough strip footing, has also been explored. Non-dimensional charts were created to determine the factor N_γ for various combinations of soil friction angle (ϕ), ground inclination (β) and the horizontal earthquake acceleration coefficient ("α" _h). These results were also verified through modelling in the commercially available finite element software OptumG2. The failure mechanisms and N_γ magnitudes which were determined by using the MOSC were found to be in good agreement with the results using the FELA.
For all the stability problems taken up in this thesis, the solutions, including the failure mechanisms generated from the MOSC and FELA approaches, were generally found to compare well with each other. All the associated codes for performing the analysis for different problems were written in MATLAB. The optimization problem was solved by using the MOSEK toolbox.
The power type yield criterion used in the thesis for both the MOSC and the FELA approaches will be able to provide better solutions than that are available in literature at present on the basis of the Mohr-Coulomb failure criterion.MHR
Thermodynamics of Protein Adsorption on Gold and Carbonaceous Nanoparticles Probed by Second Harmonic Light Scattering
No full textOver the past two decades, interaction of small molecules, peptides, and proteins
with nanoparticles of various shapes and size have been keenly pursued for two reasons: to find
application of these nanoparticle-molecule conjugates as sensors and drug delivery vehicles
since the nanoparticle surface can be decorated with multiple agents with well-defined
functions, and to understand and model adsorption of molecules on nanosurfaces by borrowing
ideas from adsorption of gases on bulk solid substrates. In this thesis, I have investigated
protein adsorption on gold and carbonaceous nanoparticles using a variety of techniques,
including second harmonic light scattering (SHLS) from solution. The SHLS technique is fairly
new in this application. It has certain specific advantages of high sensitivity, low background
signal, etc., over the conventional analytical tools usually deployed for studying NP-protein
conjugates. Due to high surface energy of NPs, proteins stick to the surface of the nanoparticle
forming “NP-protein corona” (NPPC). The measurement of thermodynamic parameters like
binding constant, binding stoichiometry (nsat), Gibb’s free energy (ΔG), enthalpy (ΔH), and
entropy (ΔS) associated with protein adsorption will help us understand the stability of the
NPPC, which, in turn, will help design robust NPPCs for diagnostic and therapeutic
applications