Browsing by Author "Manyali, George S."

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DFT calculations of elastic, electronic and thermal properties of TiB2Mo
(Oxford Open Materials Science, 2022-02-02) Wafula, Job W.; Manyali, George S.; Makokha, John Wanjala
Ceramics are materials with good mechanical properties; however, low fracture toughness, intrinsic brittleness and poor resistance against oxidation at high temperatures are challenges limiting their applications. TiB2Mo is a ceramic material whose all elastic properties have not been calculated. In this study, we investigated the elastic, electronic and thermal properties of TiB2Mo structure using first principles calculations. All first principles calculations were based on the density functional theory as implemented in Quantum ESPRESSO code with the help of Thermopw as a post-processing code. Obtained lattice parameters of TiB2Mo structure were in good agreement with other previous theoretical studies. TiB2Mo structure was found to be mechanically and dynamically stable at ground state conditions. The results also show that TiB2Mo is brittle, anisotropic and metallic in nature. Based on the calculated Vicker’s hardness Hv, we noted that TiB2Mo is classified as a hard material with fracture toughness of above 7MPam0:5; therefore, it is a promising ultra-high temperature ceramic.
DFT study of structural, mechanical, electronic, and thermoelectric properties of YNbNi2X2 (X=Si or Ge) double half-Heusler alloy
(Results in Materials, 2025-12-28) Wafula, Job W.; Manyali, George S.; Makokha, John Wanjala
This work examines the structural, mechanical, electronic, and thermoelectric characteristics of YNbNi2X2 (X = Si, Ge) double half-Heusler alloys using density functional theory (DFT) computations. Thermodynamic stability was demonstrated by formation energy estimates, and mechanical parameters show ductile behavior with good mechanical stability. YNbNi2Si2 and YNbNi2Ge2 exhibit semiconducting characteristics, with indirect band gaps of 0.51 eV and 0.56 eV, respectively. The Boltzmann transport equation is used to investigate thermoelectric properties, which yield promising Seebeck coefficients and electrical conductivities. The dimensionless figure of merit (ZT) for YNbNi2Ge2 and YNbNi2Si2 peaks at 0.85 and 0.68 at 300 K respectively. However, these values should be regarded as upper limits, as only the electronic contribution to thermal conductivity was considered, and the lattice contribution was not explicitly calculated. The figure of merit (𝑍𝑇 ) increases with temperature suggesting that these are potential materials for energy harvesting applications at high temperature.
DFT+U investigation of electronic, optical, and thermoelectric properties of YAuX (X=Si or Ge or Sn) half-Heusler alloys
(Results in Physics, 2024-05-06) Wafula, Job W.; Manyali, George S.; Makokha, John Wanjala; Madallah, Yusuf; Bouhmaidi, Soukaina; Setti, Larbi
Half-heusler alloys are fascinating thermoelectric materials because they have superior mechanical and transport properties. In this study, we used dft+u calculations and the Boltztrap equation to examine the electronic, optical, and thermoelectric properties of YAuSi, YAuGe, and YAuSn half-heusler alloys. The DFT+U approach predicted band gaps of 1.2767 eV, 0.611 eV, and 1.5741 eV for YAuGe, YAUSn, and YAuSi, respectively. We also observed that all materials under consideration have a broad absorption spectrum ranging from 1 eV to 12 eV, with notable peaks in the visible and UV ranges. The obtained opto-electronic properties position the three alloys as promising candidates for photovoltaic applications. Finally, thermoelectric property calculations revealed that the figure of merit values for YAuSi, YAuGe, and YAuSn HH alloys were 0.730, 0.726, and 0.736 at 800 K, respectively suggesting the materials are also suitable candidates for application in the field of thermoelectricity
First-principles calculations to investigate structural, elastic, electronic and thermodynamic properties of NbCoSn and VRhSn Half-Heusler compounds
(Results in Physics, 2022-11-30) Wafula, Job W.; Makokha, John Wanjala; Manyali, George S.
In this study, we investigated the structural, elastic, electronic, and thermodynamic properties of NbCoSn and VRhSn HH compounds using the first-principles calculations as implemented in the density functional theory (DFT). The computed lattice constants of NbCoSn and VRhSn compounds were found to be consistent with the available theoretical as well as the experimental data. The compounds are mechanically stable since their elastic constants satisfy the Born-Huang criteria for cubic system stability. Due to the absence of imaginary phonons, NbCoSn is dynamically stable, whereas VRhSn is unstable. NbCoSn is harder than VRhSn HH because it has a higher Vicker’s hardness and shear modulus. Both compounds feature band gaps, indicating that they are semiconductors. When compared to NbCoSn HH compound, VRhSn has a narrow band gap. Furthermore, thermodynamic properties are computed and thoroughly explored. As a result of the findings, NbCoSn and VRhSn HH compounds are viable thermoelectric materials; however, doping and alloying could be employed to enhance the stability of VRhSn HH compound.
Pressure Dependency of Elasticity in α-TiZr Shape Memory Alloys
(Kabarak University, 2019-09-18) Wafula, Job W.; Manyali, George S.; Makokha, John Wanjala; Sifuna, James
Shape memory alloys are a group of materials with two noteworthy properties; shape memory effects and super elasticity thus they have attracted a number of industrial applications. Elasticity is the ability of a material to resume its normal shape after being stretched or compressed when the elastic limit is not exceeded. Titanium Nickel, copper-based and iron-based shape memory alloys are mostly applied in constructions sector but they face challenges of pressure dependency. To provide a solution, we investigated the pressure dependency of elasticity in α-TiZr shape memory alloy. Elastic constants, bulk modulus, Young modulus, shear modulus and Poisson’s ratio of α-TiZr shape memory alloy were calculated at different pressure (0-10GPa) using Quantum ESPRESSO code with post-processing of the data done using Thermo_pw code. Projector augmented wave pseudo-potential with Generalized Gradient Approximations (GGA) within Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional was applied in this study. Elastic constants; C11, C12, C13 C33, and C66 of α-TiZr shape memory alloy increase with pressure monotonically except C44 which slightly decrease linearly. Furthermore, Bulk modulus, Young’s modulus shear modulus and Poison’s ratio were also observed to increase with external pressure. This indicates that elasticity of α-TiZr shape memory alloy is improved when external force is exerted on crystal. The study of pressure dependency of elasticity in α-TiZr shape memory alloy provides information which may lead to adoption of this alloy in construction of intelligent reinforced concrete (IRC).
Temperature Dependent Elastic Constants (TDECs) and Thermodynamic Properties of B19 TiZr; Shape Memory Alloy (SMAs)
(International Journal of Research and Innovation in Applied Science, 2020-08-05) Wafula, Job W.; Manyali, George S.; Makokha, John Wanjala
We report temperature dependent elastic constants (TDECs) and thermodynamic properties of B19 TiZr Shape Memory Alloy (SMAs) computed by Quasi-static approximations (QSA) as implemented in thermo_pw code. B19 TiZr is mechanically and dynamically stable at zero pressure with lattice parameter of 3.110 Å and 1.578 for a and the ratio a/c respectively. The first-principles calculations were performed within quantum ESPRESSO code.