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http://hdl.handle.net/123456789/469
Acoustic characterization of helmholtz resonators based metamaterial (MS)
Dogra, Sourabh
Noise control is one of the major challenges with rapid urbanization due to its adverse effects on living beings. To keep the noise level up to an adequate limit, various noise insulation materials such as sound insulation panels, and noise barriers are available. But transmission loss of these materials is based on the mass law, which intends to increase the mass and volume of the materials to achieve higher transmission loss, and thus these materials cannot be used in lightweight applications. Therefore, controlling the sound of lower frequencies is always a major concern in acoustics. The sound attenuating properties of these materials should be known when choosing a suitable material for a particular case. The sound attenuation characteristics of these materials are determined
by calculating the different coefficients such as absorption coefficient,reflection coefficient, and transmission coefficient. A design of a four- microphone impedance tube of brass is proposed to estimate these coefficients of noise absorbing materials. Transfer matrix formulation and two-load boundary condition method are used to calculate the absorption coefficient, reflection coefficient, and transmission coefficient. The accuracy of the impedance tube is confirmed by comparing the experimental data with the data provided by an accredited laboratory.
The acoustic metamaterial is an artificially engineered structure that provides an extraordinary sound absorption property that is not achievable with conventional materials. A design of an acoustic metamaterial plate with inbuilt Helmholtz resonators is proposed. Helmholtz resonator is a well-proven design for the attenuation of the desired frequency. This is achieved by altering the geometric parameters of the Helmholtz resonator. The plate was made of Polylactic acid (PLA) and fabricated using an additive manufacturing technique. It consists of Helmholtz resonator-shaped cavities of different sizes. We have analyzed the acoustic properties of the plate experimentally in impedance tube as well as numerically in COMSOL Multiphysics. The plate behaves as a reflective surface at lower frequencies, while at higher frequencies, the resonators start absorbing the sound. There is an additional advantage of being lightweight because of the Helmholtz resonator-shaped cavities built inside the plate. Thus, these types of metamaterial plates can find their application in the design sector, requiring lighter materials with high sound
transmission loss. Finally, to control the sound of the lower frequencies in the duct, a
Helmholtz resonator arrangement in series and the parallel combination is proposed.
The arrangement is studied numerically as well as theoretically. This arrangement can
broaden the attenuation band over the different frequencies band. The proposed arrangement can control the noise coming from the ducts due to air movement in the
ventilation system.
2022-05-19T00:00:00ZFirst- principles study of semiconducting heusler alloys for high temperature thermoelectric applications (PhD)
http://hdl.handle.net/123456789/468
First- principles study of semiconducting heusler alloys for high temperature thermoelectric applications (PhD)
Shastri, Shivprasad Shivaram
The aim of the thesis is to study, understand and prediction of thermoelectric properties of
semiconducting Heusler compounds by using first-principles methods for high temperature thermoelectric applications. In this direction the electronic structure, phonon spectrum, the derived transport properties and heat and charge carrier lifetimes are studied. The figure of merit zT and efficiency are predicted for the few Heusler thermoelectric materials. The density functional theory (DFT), semiclassical transport theory, DFT based harmonic and anharmonic phonon calculations are mainly applied.
The present thesis is divided into seven chapters. In chapter one, we give an introduction
to the thermoelectric energy conversion, the efficiency of thermoelectric generator (TEG),
and Heusler family of materials. The formula for efficiency of TEG and its relation to material’s properties connected through zT are described. Next, the cause of limitation
in conversion efficiency, need for high zT materials, desired material’s features for good
thermoelectric material, conflicting relation among transport coefficients and criteria for high zT are discussed. We provide some of the general properties of Heusler family of compounds, crystal structure, possible disorders and the features of Heusler materials that attract them to explore for thermoelectric applications. After this, we discuss a brief theoretical background of DFT, phonon calculations, semiclassical transport theory and a short overview of carrier lifetime calculations approaches in chapter two.
Chapter three explains the effect of exchange-correlation (XC) functionals on the electronic and phonon properties of Fe2VAl and Fe2TiSn. Using different functionals electronic dispersion and density of states (DOS) are calculated to see the effect on electronic structure.Mainly, the effect on Eg and band features and effective mass are checked. Further, the effect on the phonon properties is studied by comparing the phonon dispersion and thermal properties. The mBJ functional is found to be suitable to obtain proper Eg values. However, the band features described by mBJ shows reasonable differences compared to other functionals which is also observed through effective mass. Comparison of phonon energy in dispersion and DOS from various functionals shows the lowest phonon energies from PBE while LDA and SCAN are giving higher energy values. The change in XC functionals has negligible effect on thermal properties.
The dynamical stability and thermoelectric properties of Fe2ScX (X=P, As, Sb) Heuser
compounds are predicted in chapter four. We use the two functionals approach i.e., mBJ for the Eg and SCAN functional to describe band features in reliable prediction of properties of these new compounds. The total energy calculations suggested the ground state structure of these compounds is ordered L21 phase compared to other possible Heusler structure. The phonon calculations showed the dynamical stability in the L21 phase. The zT predicted for the n- and p-type compounds suggested Fe2ScX compounds are worth considering for high temperature thermoelectric applications on successful synthesis.
In chapter five, we try to explain the experimental Seebeck coefficient S of a ZrNiSn
sample using combined DFT and semiclassical transport calculations. The ZrNiSn is a
promising thermoelectric material with high S and moderately high electrical conductivity.
The thermal expansion behaviour is calculated which can be helpful in TEG design. The
analysis of S suggested the Eg in sample could be ∼0.18 eV with the possibility of disorder or defect. Based on this result, we further predict the zT and efficiency obtainable by doping the pure (stoichiometric and ordered, Eg of ∼0.54 eV) ZrNiSn. The highest zT predicted for the n-type and p-type compounds are ∼0.5 and ∼0.6 at 1200 K, respectively.
The study of electronic structure, phonon properties, thermal expansion, prediction of
zT and efficiency, as well as calculation of carrier lifetimes of FeVSb are carried out in
chapter six. We try to understand the experimental S of two samples using combined DFT
and transport calculations. The best possible explanation to experimental S is found for Eg of 0.7 eV which is also in agreement to Eg from mBJ calculation. From ab-initio anharmonic lattice dynamics calculations the lattice thermal conductivity of FeVSb is calculated by considering phonon-phonon interaction under single mode relaxation time approximation. Further, we extend our study on FeVSb by calculating the charge and heat carrier lifetimes from first-principles considering the three intrinsic scattering mechanisms viz. electron-electron interaction, electron-phonon interaction and phonon-phonon interaction. Using the calculated lifetime values thermoelectric properties are predicted.
At last in chapter seven, we summarize the thesis, with a brief overview of the significant
conclusions drawn and give direction for future work.
2022-01-25T00:00:00ZOn the information flow in undirected unicast network (PhD)
http://hdl.handle.net/123456789/467
On the information flow in undirected unicast network (PhD)
Qureshi, Mohammad Ishtiyaq
One of the important unsolved problems in information theory is the conjecture that network coding has no rate benefit over routing in undirected unicast networks. If the conjecture is true then the undirected unicast net- work information capacity is the same as the routing capacity. However, the conjecture is unsolved and the undirected unicast network information capacity is not characterized yet. Even upper bounding the symmetric information rate is a challenging problem. Only two explicit upper bounds on symmetric information rate are known for general undirected networks: (1) sparsest cut bound on symmetric rate is a trivial bound on both commodity and information flow and (2) the linear programming bound using Shannon-type inequalities is generally not used for evaluation due to prohibitively large problem size.
In this work, we characterize an upper bound, called the partition bound, on the symmetric rate for information flow in general undirected unicast networks and present a partitioning technique to obtain converse results for undirected network information flow. We give two proof methods for the partition bound. This bound is further generalized for non-symmetric
rates. We show that the partition bound is not tight in general and also demonstrate an approach to tighten the bound. As a result, we present an alternative proof of the undirected unicast network information capacity of the well known Hu’s 3-pairs network. We give explicit routing solutions achieving the partition bound for (1) two classes of complete n-partite networks called Type-I and Type-II n-partite networks, and (2) a class of
3-layer networks called Type-I 3-layer networks. These results prove that the undirected unicast network coding conjecture holds for these classes of networks. A parameter is defined as an optimal partition which delivers the partition bound. We present a procedure to compute a lower bound for this parameter. This lower bound renders a computable upper bound for the partition bound. We also show that the decision version problem
of computing the partition bound is an N P-complete problem. Thus, both the upper bounds, the sparsest cut bound, and partition bound are not polynomial-time computable unless P=N P. Recently, the undirected unicast network coding conjecture was proved for a new class of networks and it was shown that all the network instances for which the conjecture is proved previously, and the cut based bound is not achievable by commodity flow, are elements of this class. The conjecture was also proved for all undirected unicast networks (1) with six or less number of nodes and (2) with up to three sessions and seven nodes except one particular network. We show the existence of a Type-I n-partite network for which the partition bound is tight and achievable by routing and is not an element of this class of networks. This result establishes that there exist networks outside of the class of networks with unverified conjecture such that the partition bound is tight and attainable by routing.
2022-02-01T00:00:00ZInvestigating the effect of electronic correlation on the physical properties of late 3d transition-metal based system using ab-initio methods (PhD)
http://hdl.handle.net/123456789/466
Investigating the effect of electronic correlation on the physical properties of late 3d transition-metal based system using ab-initio methods (PhD)
Dutta, Paromita
The aim of the thesis is to understand how the electronic correlations are affecting various physical properties of late 3d transition-metal based systems like LaCoO3, FeSi and CoSi. The thesis presents a consistent study of these systems by using different ab-initio methods such as DFT, DFT+U, G0W0 and DFT+DMFT with the self-consistently calculated values of U & J. Each of these ab-initio methods deal differently with the different levels of electronic correlation existing in a material. The thesis shows an interesting study of LaCoO3 and CoSi where they have different strength of correlations
while having same Co 3d orbital. At the same time, FeSi and CoSi exhibits same structure and just differ by one electron, but still their properties are differently affected by electronic correlation.
The present thesis is divided into seven chapters. In chapter one, we discuss the history and development of the ab-initio methods for studying the properties of a material in condensed matter physics. This historical aspect is followed with the introduction of on-site Coulomb interaction U & Hund’s like exchange interaction J. For performing the electronic structure calculations these two parameters play an important role. After this, we reasoned out the need of these parameters and then followed by the description of first-principles based methods developed for calculating these parameters. After this chapter, we provide an overview of the theoretical methods such as DFT,DFT+U, GW, DFT+DMFT and semiclassical transport theory used for the investigations carried out
in chapter two.
Chapter 3 provides a systematic study of the electronic structure of LaCoO3 by using DFT,DFT+U and DFT+DMFT methodologies. The objective of this chapter is to see what level of electronic correlation is existing in the electronic structure of the compound. In this chapter we compared the density of states (DOS) as calculated from DFT, DFT+U and DFT+DMFT separately with photoemission (PES) and inverse photoemission spectra (IPES), respectively. The chapter brings out the difference between theoretical methods with their individual limitations in describing the experimental spectral attributes of the occupied and unoccupied states of LaCoO3. For the investigation, U is calculated using cDFT method ∼ 6.9 eV for Co 3d and using this U value J is calculated as ∼ 1.18 eV. The study shows that DFT has failed in even creating the hard gap while the DOS distribution seems to be similar to the experimental attributes. However, DFT+U created the gap ∼ 1.8 eV but eventually failed in explaining the DOS distribution with PES mainly. However, DFT+DMFT created the gap ∼ 1.1 eV and successfully explained the DOS distribution for both PES and IPES. This indicates that the level of correlations as treated within DFT+DMFT seems appropriate to understand the electronic states of LaCoO3.
In chapter 4, we studied the effect of correlations of Fe (and Co) 3d electrons in FeSi (and CoSi) using DFT and DFT+DMFT methods. This is because FeSi has been reported to possess unusual temperature-dependent properties which are the consequences of its electron-electron interactions. At the same time, CoSi exhibits the same structure as FeSi does and just differs by one electron. So, there is an expectation of seeing the similar unusual temperature-dependent behavior in CoSi. We compared the calculated spectral functions from both DFT and DFT+DMFT methods with the available experimental x-ray PES data of both compounds. The calculated spectral functions from DFT+DMFT are found to provide fairly good representation for the experimentally observed PES for both compounds. This motivated us to carry forward the investigation by using DFT+DMFT
method. From the study of the temperature-dependent (100-800 K) DOS and momentum resolved spectral functions the influence of electronic correlations on the electronic states of FeSi are found be largely affected than CoSi. From momentum resolved spectral functions, the excitations have shown enhanced broadening with temperature rise in FeSi whereas an opposite behavior is observed in CoSi.
Chapter 5 studies the effect of pressure and temperature on the role of electronic correlation Fe 3d electrons in FeSi. In this chapter, the we tried to understand the pressure-dependent experimental observations of resistivity and bandgap by using ab-initio methods. At first, we studied the DFT obtained partial DOS and then DFT+DMFT obtained total DOS for T = 100 K and T = 300 K. Both the calculations have revealed the widening of the bandgap with increase in pressure which is similar to other experimental observations for stoichiometric FeSi. However, they could not explain the experimental observation of insulator-to-metal transition when T < 50 K with the condition P ≥ 14.4 GPa for nearly stoichiometric FeSi samples. As it is generally seen that the samples prepared
experimentally are often found to be off-stoichiometric. Considering this fact, we then studied the electronic structure of Fe1.02Si0.98 using DFT based KKR-CPA method. KKR-CPA calculations revealed that the impurity states have been generated in the gapped region around the Fermi level and they have half-metallic behavior. With pressure increment the closure of the energy gap (in one channel) appears to be responsible for the experimental observation for the Fe excess sample. Further, KKR-CPA calculations have suggested the presence of strong Hund’s coupling responsible for creation of net magnetic moment at Si site when Fe is in excess. CoSi having the crystal structure which belongs to the B20 type with P213 spacegroup lacks the inversion center. Such a symmetry causes the emergence of unconventional fermionic quasiparticles (QPs) in CoSi which has been shown experimentally and theoretically (studies based on DFT). Since interacting picture is quite important because this picture takes account of the electronic correlations differently. Therefore, it is very important to see its effect onto these newly found fermions. In chapter 6, we studied the spectral functions of bulk and (001) surface of CoSi using DFT, DFT + DMFT (T = 100 K) and G0W0 methodologies with and without inclusion of spin–orbit coupling (SOC), respectively. SOC and electronic correlations appear to modify the nature of bands involved at Γ point. The existence of both coherent and incoherent features indicates the presence of QP–QP interactions which is eventually affecting the lifetime (τ) of exotic fermionic QPs. As CoSi has been reported to have exotic fermions at three nodal points in the band structure such as two at Γ (G1 & G2) and one at R (R1). Thus, if the chemical potential is kept close to the energies of these nodal points then there is a possibility of capturing the unconventional charge carriers’ contribution to electronic transport of CoSi. Based on this, the chapter 6 further studies the transport coefficients at these three nodal points for the temperature 40-300 K by using first principle based DFT method.
At last in chapter 7, we summarize the thesis, with a brief overview of the significant conclusions drawn and give directions for future work.
2022-01-13T00:00:00Z