Materials Research Centre (MRC)
https://etd.iisc.ac.in/handle/2005/12
2021-09-20T01:15:26ZAb-initio Modeling and Designing of Materials for Thermoelectrics, Optoelectronics and High Temperature Applications
https://etd.iisc.ac.in/handle/2005/4896
Ab-initio Modeling and Designing of Materials for Thermoelectrics, Optoelectronics and High Temperature Applications
Samanta, Atanu
Designing a material for a particular application requires an atomistic understanding of its
properties. Recent development in first principles methods and supercomputing speeds has
enabled researchers to compute materials properties accurately. This has opened up a window
for computational designing of materials for various applications such as optoelectronics,
thermoelectrics, magnetic shape memory alloys etc. In this thesis, first principles methods
have been utilized to understand the properties of various materials such as TiS2, TiS3, GeO2,
Co3(MoTaAl) alloys, Ni2MnGa and graphene. This thesis has been organized as follows:
• Chapter 1 introduces various functional materials and their application in the thermoelectric,
optoelectronic, high temperature and magnetic shape memory. The microscopic
understanding of materials properties such as structure, energetics, electronic structure,
electronic transport, and lattice dynamics etc. can lead to novel ways of designing materials
properties for various applications.
• Chapter 2 describes the theoretical methodology adopted in this work. It gives a brief
understanding of first principles based density functional theory (DFT) and various approximations
to obtain accurate electronic properties. Methods employed for calculation
of electronic and thermal transport are also discussed briefly.
• In Chapter 3 we explore the tuning of the electronic structure of the transition metal
dichalogenide TiS2. We show that by engineering its electronic structure, it transforms
from a semimetal to a semiconductor under biaxial strain. The thermoelectrics study
shows that a 3 fold enhancement in thermopower can be achieved by application of 5%
biaxial strain. This enhancement is driven by a small bandgap opening of ∼0.15 eV, which
increases its thermopower at the same time decreasing its lattice thermal conductivity
indicating improvement in ZT.
• In Chapter 4 we study the possibility of inherent stacking fault in bulk TiS3 and its effect
on the electronic properties. We find that TiS3 can exist in AB′ and AB′′ geometries. The
energy difference between two structures is about 0.011 eV/f.u. The electronic structure
is independent of the stacking fault due to the weak vdW interaction between the layers.
The calculated thermopower is 200 μV/K in the carrier concentration range of 1×1020
cm−3 - 5×1020 cm−3, which is comparable with other state of the art thermoelectric
materials. The high thermopower and electrical conductivity in the carrier concentration
range of 1×1020 cm−3 - 5×1020 cm−3 leads to a high power factor for both p- and n-type.
Moreover, the power factor for p-type is three times higher than that of n-type carriers
indicating that the thermoelectric performance for p-type will be much better than that
of n-type.
• Chapter 5 reveals the origin behind the large variation in the band gap (∼ 2 eV) of
GeO2 calculated by standard DFT within LDA/GGA, which had remained unresolved.
Using the many-body perturbation theory (GW approximation), we find that this large
variation observed in literature is independent of the method used and depends strongly
on the lattice parameter (volume strain). This strong dependence originates from a change
in hybridization among O-p and Ge-(s and p) orbitals.
• Chapter 6 deals with the structural stability of order intermetallic Co-based superalloys.
We have shown that W free Co3Al order structure can be stabilized in L12 structure by
addition of Mo and Ta atoms. The enthalpy of formation of L12 structure significantly
becomes more negative compared to the DO19 structure by the addition of ≥ 4% of Ta
atoms. This implies that the L12 structure of Co3(Al,Mo,Ta) structure is more stable
compared to DO19. The lowering in the enthalpy of formation is found due to the formation
of the pseudo gap and the decrease in the states at the pseudo gap with increasing Ta
concentration. The stability of the L12 structure can be further improved by the addition
of Ni and Ti atoms.
• In Chapter 7, the lattice dynamics and electronic structure of X2YZ [where X = Ni,
Fe, Co; Y = Mn; Z = Al, Ga, Ge, In, Sn, Sb] stoichiometry compounds are investigated.
The lattice instability of X2MnZ depends on the position of the Fermi energy (EF ) with
respect to the pseudo gap. The phonon mode softening along the Γ-K symmetry direction
is observed for Ni2MnZ in the austenite phase since EF is located above the pseudo gap.
This mode softening is mainly responsible for the MSM effect. On the other hand, Fe2MnZ
and Co2MnZ [Z = Al, Ga, Ge, In, Sn, Sb] in the cubic phase do not show any phonon
mode softening because EF lies in the vicinity of the pseudo gap or at the pseudo gap.
Thus, alloying Fe or Co at the Ni site in Ni2Mn (Z = groups-IV and V) can tune the lattice
modulation. In addition, the magnetic moments of Fe2Mn (Z = groups-IV and V) and
Co2Mn (Z = groups-IV and V) are much higher than those of Ni2Mn (Z = groups-IV and
V), indicating that the magnetic moments of Ni2MnZ can be enhanced. The calculated
phonon dispersion with magnetic moment indicates that the phonon mode softening is
sensitive to the change in the local magnetic moment of the atoms, thereby enabling
tunability in the MSM effect.
• In chapter 8, we show that the mono vacancy defects in graphene can be used as precursors
to form novel clipped structures without explicit use of functional groups. These
clipped structures can be transformed into one-dimensional (1D) double wall nanotubes
(DWCNT) or multi-layered three dimensional (3D) structures. The clipped structures
show good mechanical strength due to covalent bonding between multi-layers. Clipping
also provides a unique way to simultaneously harness the conductivity of both walls of
a double wall nanotube through covalently bonded scattering junctions. With additional
conducting channels and improved mechanical stability, these clipped structures can lead
to a myriad of applications in novel devices.
• Chapter 9 summarizes and concludes the work presented in this thesis.
Accelerated search for thermoelectric and topological materials using first-principles and machine learning
https://etd.iisc.ac.in/handle/2005/4699
Accelerated search for thermoelectric and topological materials using first-principles and machine learning
Juneja, Rinkle
In summary, we have performed first-principles calculations to study the topological phase transitions
in chalcopyrite compounds as a function of hydrostatic pressure. These compounds are
topological insulators in the native phase with an inverted band order around BZ center. Upon
hydrostatic compression, there is a transition from nontrivial TI phase to a Dirac semimetallic
state at a critical pressure. Further increase in pressure drives the materials into a trivial semiconductors
along with normal ordering of bands. Different quantum phases are characterized
by topological invariants as well as surface states. These quantum phase transitions are further
validated by model calculations based on L¨uttinger Hamiltonian, which unravels the critical
role played by pressure-induced anisotropy of frontier bands in driving the phase transitions.
Such a manoeuvre between various topological phases by hydrostatic pressure can stimulate
the search for TQPTs in future experiments
Addressing the Challenges in Designing of Efficient Thermoelectric Materials
https://etd.iisc.ac.in/handle/2005/4806
Addressing the Challenges in Designing of Efficient Thermoelectric Materials
Pandey, Tribhuwan
Thermoelectric materials offer applications in conversion of waste heat to useful electrical energy
and are promising sources of renewable energy. To use a material efficiently in thermoelectric
application it is necessary to have high figure of merit (ZT). The task of increasing ZT is
challenging because of the competing relation between electrical conductivity, thermopower,
and thermal conductivity. For enhancing ZT of a thermoelectric material, it is necessary to
have a good understanding of its electronic structure as well as transport properties. The goal
of the present work is to develop an understanding of the thermoelectric properties of selected
materials and address some of the fundamental challenges for achieving enhanced performance
in these materials. In this thesis, I have employed density functional theory based calculations
combined with Boltzmann transport theory, to study electronic structures, and electronic and
thermal transport properties of several promising class of thermoelectric materials, including
the transition metal silicides (FeSi2, CrSi2), Zintl compounds, sulphide (Bi2S3), intermetallics
and transition metal dichacogenides (MX2 (M = Zr, Hf and X = S, Se), and MoS2). Based on
the comprehensive study of electronic and thermoelectric properties we conclude that different
strategies are required to improve the ZT of different class of materials. Our findings provide a
better understanding of materials properties and can be generalized to other materials as well
Alluaudite Class of High-Voltage Cathodes for Sodium-Ion Battery Applications: A Synthesis, Structure, Property Correlation Study
https://etd.iisc.ac.in/handle/2005/5295
Alluaudite Class of High-Voltage Cathodes for Sodium-Ion Battery Applications: A Synthesis, Structure, Property Correlation Study
Dwibedi, Debasmita
The work presented in this thesis demonstrates
alluaudites as a niche class of compounds forming the
rich treasure house of sodium insertion hosts. The
potential of this open frame-worked alluaudites is
immense and further research will open up newer
domains of applications. The current thesis work has also elucidated in-depth crystal/ magnetic structure analysis
and Rietveld refinement of these novel compounds by combining X-ray, neutron and
synchrotron diffraction techniques. It has also explored the synthesis of various metastable
phases and polymorphism at different synthetic conditions to gauge their structural and
electrochemical properties