Physics
http://hdl.handle.net/2263/1185
2024-03-29T09:27:43ZEffects of sintering temperature on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite
http://hdl.handle.net/2263/95413
Effects of sintering temperature on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite
Ogunbiyi, Olugbenga; Jamiru, Tamba; Sadiku, Rotimi; Salifu, Smith; Maepa, Charity E.
While IN738 Ni-based superalloy is a high strength alloy, it is feasible to improve its
properties at the bulk level by reinforcing with graphene nanoplatelets (GNPs), taking
advantage of the superior mechanical, tribological and thermal properties using the spark
plasma sintering technique. In the present study, the influence of spark plasma sintering
temperature range between 900 and 1100 C on the microstructure, mechanical, tribological
and thermophysical properties of GNPs/IN738 composite is assessed. The dispersion of
GNPs reinforcement and alloying metals to form composite powder is conducted using a
turbular mixer and low-frequency planetary ball milling, followed by spark plasma sintering.
The relative density of the sintered samples assessed following Archimedes'
method indicates increasing densification with the increasing sintering temperature from
94.7% (900 C) to 98.5% (1100 CÞ. The microstructure assessed via SEM, XRD and Raman
spectroscopy indicates the formation of precipitate gamma, intermetallic gamma prime,
solid solution and GNPs strengthening phases. Thus, the mechanical (micro/nano hardness
and Young's modulus), tribological (wear rate and coefficient of friction), and thermophysical
(thermal diffusivity, thermal conductivity, and specific heat capacity) properties
increased with the increasing sintering temperature. The microhardness increased from
354HV (900 C) to 469HV (1100 C), nanohardness from 8 GPa (900 C) to 17 GPa (900 C), and
Young's modulus from 190 GPa (900 C) to 291 GPa (1100 C). The wear rate reduced with an
increase in sintering temperature for the three loads of 5, 10 and 20N. The thermophysical
properties assessed from 25 to 600 C show the formation of few inflection points as the temperature increases, which is attributed to the dissolution and rearrangement of precipitate
gamma prime and Cr in solid solution phases. Similarly, the small increment in the
thermal diffusivity is equally associated with the smoother phonon transition at the GNPs/
matrix interface.
2023-03-01T00:00:00ZWasserstein distance between noncommutative dynamical systems
http://hdl.handle.net/2263/95367
Wasserstein distance between noncommutative dynamical systems
Duvenhage, Rocco
We introduce and study a class of quadratic Wasserstein distances on spaces consisting of generalized dynamical systems on a von Neumann algebra. We emphasize how symmetry of such a Wasserstein distance arises, but also study the asymmetric case. This setup is illustrated in the context of reduced dynamics, and a number of simple examples are also presented.
2023-11-01T00:00:00ZStructural and optical characterization of beta‑gallium oxide
http://hdl.handle.net/2263/95365
Structural and optical characterization of beta‑gallium oxide
Muramba, Valentine W.; Ali, Abdulraoof I.; Nel, Jacqueline Margot
Demands for cheaper solar cells have led researchers to less complex, low-temperature,
vacuum-free thin-film deposition processes, like spray pyrolysis and sol–
gel spin coating. Previous studies of β-Ga2O3 thin-film deposition have used aqueous
solutions of gallium nitrate which have strong tendencies to form hydroxide
precipitates. This leads to the blockage of spray gun nozzles. To avoid precipitation,
tetrahydroxogallate (III) ammonium was used as a novelty precursor in this
study for the synthesis of β-Ga2O3. In the spray pyrolysis technique, the precursor
was deposited on sapphire substrates at 200 ℃, with a carrier gas pressure of
200 kPa. The same precursor, with added monoethanolamine to enhance viscosity,
was used in the spin coating method. A polycrystalline β-Ga2O3 structure was
obtained by post-annealing films at 750 ℃ in ambient air. The spin-coated films
with thicknesses ranging from 165 to 354 nm exhibited an average crystallite size
of 17.78 nm and an optical band gap range between 4.80 eV and 4.95 eV. Films
produced by spray pyrolysis had thicknesses ranging between 158 and 255 nm,
an average crystallite size of 17.55 nm, and a band gap ranging between 4.69 eV
and 4.93 eV. From Raman spectroscopy, the molecular vibrational modes Ag
and
Bg
were detected, featuring three blue shifts and two red shifts. Films showed a
UV-blue region originating from oxygen and gallium vacancies in the lattice, an
important characteristic for good photodetectors and vital for solar cell passivation.
When utilizing β-Ga2O3 as dielectric coating, the refractive index between air
and solar cells is reduced, enhancing solar energy absorption. Similar results were
obtained for both synthesis techniques confirming the reliability of the methods.
DATA AVILABILITY : The author declares that the data supporting the findings
of this study are available within the paper. Any
secondary data the author has reused, repository DOI
is cited in the reference. Any raw data generated during
this study are available from the corresponding
author on reasonable request.
2023-12-01T00:00:00ZInvestigation of the structural and temperature-dependent electrical properties of MZnO (M = Ce and Sm) Schottky diode devices fabricated using the sol-gel spin-coating technique
http://hdl.handle.net/2263/95364
Investigation of the structural and temperature-dependent electrical properties of MZnO (M = Ce and Sm) Schottky diode devices fabricated using the sol-gel spin-coating technique
Ahmed, Mustafa Abaas Mohamedelkhair; Meyer, W.E. (Walter Ernst); Nel, Jacqueline Margot
In the present study, the Schottky diode devices based on Ce and Sm co-doped
ZnO thin films were fabricated using the sol–gel spin-coating technique with Pd
and Pt as Schottky contacts. The structural and electrical properties of the fabricated
Schottky diode devices were investigated at room temperature and in
the temperature range of 320–160 K. The crystalline structure of the prepared
films was studied using X-ray diffraction spectroscopy. The I–V characteristics
of the fabricated Schottky diode devices based on Pd and Pt Schottky contacts
manifest good diode behavior with the rectification of nine and ten orders of
magnitudes, respectively. The ideality factor for Pd-based Schottky diode was
found to decrease with temperature, and the lowest value obtained at 160 K was
1.3, while for the Pt-based Schottky diode, the ideality factor was found to
increase with decreasing temperature. Moreover, by using two Gaussian models,
the barrier height for Pt Schottky diodes was found to decrease with
decreasing temperature indicating the inhomogeneity in the barrier height.
Furthermore, alongside the thermionic emission theory, the fabricated Schottky
diode parameters were also analyzed with Cheung–Cheung and the modified
Nord methods. Finally, the current transport mechanism in both fabricated Pd
and Pt was found to be controlled by an Ohmic, trap-filled voltage and spacecharge-
limited current mechanisms in the low, moderate and higher voltage
bias, respectively.
DATA AVAILABILITY : Data will be made available on request.
2023-06-09T00:00:00Z