Electronic, Optical, Structural, and Elastic Properties of MAX Phases and (Cr2Hf)2Al3C3
Abstract
The term “MAX phase” refers to a very interesting and important class of
layered ternary transition-metal carbides and nitrides with a novel combination of both
metal- and ceramic-like properties that have made these materials highly regarded
candidates for numerous technological and engineering applications. In the present
dissertation work, the electronic structure and optical conductivities of 20 MAX phases
Ti3AC2 (A = Al, Si, Ge), Ti2AC (A = Al, Ga, In, Si, Ge, Sn, P, As, S), Ti2AlN, M2AlC
(M = V, Nb, Cr), and Tan+1AlCn (n = 1 to 4) are studied using the first-principles
orthogonalized linear combination of atomic orbitals (OLCAO) method. It is
confirmed that the N(Ef) (total density of states at the Fermi level Ef) increases as the
number of valence electrons of the composing elements increases. The local feature of
total density of states (TDOS) near Ef is used to predict structural stability. The
calculated effective charge on each atom shows that the M (transition-metal) atoms
always lose charge to the X (C or N) atoms, whereas the A-group atoms mostly gain
charge but some lose charge. Bond order values are obtained and critically analyzed
for all types of interatomic bonds in the 20 MAX phases. Also included in this work is
the exploration [using (Cr2Hf)2Al3C3 as an example] of the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead
of creating solid solution phases. The crystal structure and elastic properties of
(Cr2Hf)2Al3C3 are studied using the Vienna ab initio Simulation Package. Unlike
MAX phases with a hexagonal symmetry (P63/mmc, #194), (Cr2Hf)2Al3C3 crystallizes
in the monoclinic space group of P21/m (#11). Its structure is found to be energetically
much more favorable against the allotropic segregation and solid solution phases.
Calculations using a stress versus strain approach and the VRH approximation for
polycrystals also show that (Cr2Hf)2Al3C3 has outstanding elastic moduli
Table of Contents
Introduction of max phases -- Scope and motivation of research -- Theory and methodology -- Results and discussion on the twenty max phases -- Results and discussion on the derivative (Cr2Hf)2Al3C3 -- Summary -- Appendix A. Full basis of titanium atomic orbitals -- Appendix B. The relaxed unit cell of (Cr2Hf)2Al3C3 -- Appendix C. The relaxed 1 x 1 x 3 supercell of (Cr2Hf)2Al3C3 -- Appendix D. The relaxed segregation model -- Appendix E. The relaxed 3 x 3 x1 supercell of (Cr2Hf)2Al3C3 -- Appendix F. The relaxed solid solution model
Degree
Ph.D.