Theses and Dissertations
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- ItemOpen AccessEffects of Extrinsic Parameters on Friction and Wera of Alloy Steels.(Obafemi Awolowo University, 1985) Agu, Joseph Ifeanyi; Adewoye, O. O.Parameters governing the characteristics and microstructural damages in single sass war runs of as-received and carburized austenitic stainless steel type 304 are considered. The significance of the variation of Meyer index with load regimes is interpreted in terms of the deformation of the underlying microstructure. Stress-strain curves are examined as a function of carburization case-depth. There is a marked load dependence of micro hardness and wear resistance at low loads traceable to the presence of chrome films. Carburization, though improves wear resistance, depletes the chromium content of 304 and makes the steel more susceptible to chemical attack, and hence corrosion.
- ItemOpen AccessPhofluorescence Lifetime Measurement by Single Photon Counting(Obafemi Awolowo University, 1985) Aweda, Moses Adebayo; Aladekomo, J. B.A fluorescence lifetime measuring instrument in the nanosecond time range has been assembled using components from the ORTEC, EMI, TEKTRONIX, BRANDENBURG AND CAMBERRA. The fluorescence emmission spectra of two aromatic hydro-carbons (naphthalene and pyrene) were measured for identification and proper classification. The lifetime measuring instrument was calibrated and tested with 1µg/ml solution of quinine sulphate in. 0.05M sulphuric acid. A value of 21.50 ns was obtained for the fluorescence lifetime of quinine sulphate. The fluorescence lifetimes of naphthalene and pyrene were then determined with this system using four different concentration of each sample. A vacuum system was also constructed for removing the dissolved oxygen in these aromatic hydrocarbons. After degassing the measurement was repeated for each sample. The monomer fluorescence lifetime of 437.50 ns was obtained for naphthalene while 577.02 ns was obtained for pyrene.
- ItemOpen AccessMossbauer Spectroscopy of Tin Compounds.(Obafemi Awolowo University, 1985) Arua, Awa Eke; Osadebe, F. A. N.The Mossbauer spectrometer has been used in the transmission geometry to measure the isomer shifts and quadrupole splitting of tin and some of its inorganic compounds. The spectrometer has a Moire interferometer for absolute velocity calibration and the velocity data are fitted to a polynomial by the method of least squares. As a check on the velocity calibration, a natural iron foil absorber has been used. The ratio of the g-factors extracted from the iron data is in agreement with published values. The Mossbauer spectral data has been fitted with a sum of Lorentzians and the isomer shift and quadrupole splitting values obtained from these fits are in good agreement with published ones. Using s-electron densities Calculated from relativistic Hartree-Fock method for the various oxidation states of tin the fractional change in nuclear charge radius ΔR/R following a gamma transition has been calculated as (+2.05 ± 0.17) x 10-4. This is in good agreement with published values. From molecular orbital calculations, a configuration of 5s1..25p 2..8 has been suggested for α-tin. No equivalent data exists for β-tin. However, since the isomer shift is directly proportional to the s-electron density at the nuclear site, the measured isomer shift value has been used to suggest a configuration of 5s1.42 5p2.8 for β-tin. This is consistent with the saner shift values for the two allotropes of tin.
- ItemOpen AccessWork-Hardening and Recovery Studies of Pure Aluminium.(Obafemi Awolowo University, 1985) Ojediran, Surajudeen Oluwafemi; Ajaja., O.The effects of work-hardening and recovery on the strength of pure aluminium have been investigated. The modes of interaction of these two processes during creep deformation were closely examined. The Bailey-Orowan equation was derived without recourse to a mechanical equation of state. This new method of derivation clearly showed that the Bailey-Orowan equation is applicable only to steady-state but not to transient creep deformation. The results of recovery tests performed on prestrained samples showed that the flow stress decreased with recovery time, eventually approaching an asymptotic value which was appreciably higher than the flow stress of the annealed sample. It was demonstrated that the recovery rate was not a unique function of the instantaneous dislocation density. Stress increment and stress removal (state recovery) tests performed on the creeping samples showed the generation of appreciable instantaneous plastic strains during the stress hike, and high strain rates following recovery. Both of these observations are consistent with the promise that creep is recovery-controlled in this material.
- ItemOpen AccessCreep Studies in Aluminium-2.25WT.% Magnesium.(Obafemi Awolowo University, 1985) Fagbulu, J. Babaniyi; Ajala, OlawumiIn this project, a study of creep mechanism in an Al-2.25wt% Mg alloy is conducted at temperatures between 573K and 623K and stresses (α ) ranging from 7.70 to 48.10 Mpa. Using the dislocation network model, the characteristics features of recovery-controlled and glide-controlled creep mechanisms are delineated. One of the manifestations of the fundamental differences between these two mechanisms, it is demonstrated, is the disparity which should be observed between the experimentally observed obstacle strength parameter α exp in the equation σ =α exp Gb/p where G, b and p are the shear modulus, the Burgers vector of dislocations and the dislocation density respectively and the theoretically calculated value, αo. Specifically, the theoretical model predicts that α exp for glide creep should be greater than α exp for recovery creep. The conditions under which a transition from one creep mechanism to the other could occur in a given material are investigated (theoretically). It is demonstrated that for creep to be glide-controlled, the dislocation glide velocity Vg must be related to the annihilation rate p a according to the expression Vg << β pa / (α1p3/2), where α1 and β are constants. Otherwise, creep should be recovery-controlled. The implications of the theoretical model on the shape of creep curves for both mechanisms are discussed and are shown to be generally consistent with literature data. The creep tests performed on the Al-Mg alloy show that inverse transient creep curves are observed at low stresses and normal transients at high stresses. Also a change in transient creep curve shapes are observes when the annealed samples are restrained prior to creep. Stress increase and stress removal tests performed (in the high stress range) during creep reveal certain creep characteristics which are trademarks of recovery-controlled creep at high stresses. Recovery tests performed on restrained samples show that the flow stress decreases with recovery time, even though it never quite approaches the value characteristics of annealed samples.
- ItemOpen AccessA Critical View of Various Radiation Dosimetric Techniques.(Obafemi Awolowo University, 1985) Okunade, Akintunde Akanbi; Olomo, J. B.There has been tremendous increase in the application of ionising radiation in medicine, agriculture, basic and applied science and other industrial applications which involve hazard to man. Hence, there is an increasing need to optimize the benefits of the uses of nuclear energy for these activities through accurate and reliable dosimetric systems. This work reports a critical view of various dosimetric techniques with the intent and purpose of evaluating the extent to which quantities such as energy of radiation, dose rate, environmental parameters, fading, linearity, direction of incidence, electrical requirements, methods of calibration, etc, contribute to uncertainties in dose values. The limits within which certain accuracy holds for these techniques were used to pin-point proper areas of safe and reliable applications. Areas for further studies in concepts and methods of these techniques are also discussed. A national survey of existing dosimetric systems at several places of radiological and nuclear facilities in Nigeria was conducted and result of analysis which reflects lack of adequate dosimetric systems sufficient to meet the required optimisation of benefits of these of ionising radiation is presented. Lastly, steps in organising an efficient regulatory body, backed up by appropriate law, to deal with licensing, inspection and regulation of the use of ionising radiation within the country are presented. This is utterly important and necessary if the current abuses of the uses of ionising radiation are to be stopped.
- ItemOpen AccessGeneration of the Basis States for the Calculation of the Effects of Core Excitations on the Low-lying Negative Parity States of 90 Zr.(Obafemi Awolowo University, 1985) Jegede, Oluwagbemiga Olawale; Amusa, A.From the various theoretical studies reported on the low-lying levels in 90 Zr, it has been established that the spectrum is rather anomalous in that it has attributes of a simple two-proton coupling and some collectivity, notably with the 21+, 22+ and 31- low-lying states. The basis states necessary for the inclusion of core excitations in the low-lying negative parity states (J<7-) in 90 Zr are generated in this work. It is hoped that the eventual inclusion of core excited states in manner specified by Lawson in the study of the low-lying positive parity states for 18 O would lead to reproducing the experimental energy levels for these low-lying states better than was obtained with the previous models.
- ItemOpen AccessAbsolute Normalization of the Three-Nucleon Transfer Reactions Cross Section in the Distorted-Wave Born Approximation Formalism.(Obafemi Awolowo University, 1987) Nubi, Olatunbosun Owolabi; Amusa, A.The three-nucleon transfer reaction cross section form factor for direct pick-up reactions (p,α) and (n,α), using DWBA formalism and shell model basis states, is obtained following an approach analogous to that by Towner and Hardy. Explicit calculations, with gaussian form for the interaction, are performed on the portion of the form factor which depends on the a-particle wave function and on the interaction potential The main emphasis is directed towards carrying out exact calculations that avoids drastic approximations in the two-body interaction potential, and such approximation as the point-alpha approximation and other simplifications of the a-particle wave function as it has been successfully carried out by Amusa for two-nucleon pick-up reactions induced by light ions. Two-body interaction parameters which reproduce low-energy scattering data very well are employed in computing values of the normalization and spectroscopic factors that are extracted from the form factor.
- ItemOpen AccessRe-association Kinetics of the DNA of the Reptile, Agama Agama Agama (L).(Obafemi Awolowo University, 1987) Ighavini, Bulex Oroghekowho; Adegoke, J.A.The re-association kinetics of the Agama agama agama DNA and some of the characteristics of the highly repetitive DNA sequence were investigated on hydroxyapatite column with the specific objectives of determining the re-association profile at 60°C for this organism and also determining the amount and nature of the highly repetitive DNA sequences. The re-association curve showed that the genome consists of the three components characteristic of eukaryotic genomes. These components are highly repetitive, intermediate repetitive and single or unique copy sequences which form 18%, 8% and 74% of the total genome respectively. A Cot 1/2 of 370 was obtained for the genome, and this is a reflection of a high proportion of unique sequences in this animal and also indicates a fairly complex genome. Thermal elution studies on the total DNA, highly repetitive DNA and on mouse DNA (for comparison) were carried out on HA, and melting profiles deduced from the results. Two peaks (a minor and a major) were obtained for both the native DNA and highly repetitive. The minor peak formed about 3.6% of the native genome and about 9.6% of the highly repetitive DNA sequences. It elutes at 65°C to 75°C in both cases, and this indicates that it is AT-rich. The major peak of the highly repetitive DNA has a melting temperature of 89. 5°C, giving a base composition of 49. 5% G + C. The results suggest that repetitive sequence is localised on the chromosomes, while the bulk may be dispersed throughout the genome. The native DNA shows a Tm of 85.60C which gives a base composition of 40. 2% G + C. This value does not vary from a general recently evolved eukayotic pattern, and the totality of the results obtained for the DNA of Agama shows that the animal is evolutionarily advanced.
- ItemOpen AccessDevelopment of a Meteorological Database Algorithm for Radio Channel Evaluation(2015-04-02) Ajileye, Oluwaseun OlasunmboThis study developed an instrumentally and computationally efficient linkage between a meteorological database' and the performance of a radio communication channel by determining the effect of meteorological conditions on the range of frequencies suitable for radio waves under varying meteorological conditions. It also developed an algorithm for evaluating radio channels from meteorological data with a view to predicting the emerging radio communication characteristics. Radio meteorological data, including net radiation, were extracted from the meteorological database of an experiment conducted at the campus dam site of the Obafemi Awolowo University, Ile-Ife, by the micrometeorology group of the Physics Department in October/November, 1998. The values of motivation of net radiation were estimated and used in calculating the equivalent earth radius factor (K) using the market paradigm. The hourly minimum, maximum, median and average values of K were subsequently evaluated. The results showed that the variation of the equivalent earth radius factor (K) with time had a direct application in describing the propagation properties of radio waves in terms of "diffraction", "reflection", "refraction", "extinction", "attenuation': and "ducting" modes. The median values of K evaluated were applicable in the description of climate types and weather conditions. The hourly-average values of K were found suitable in describing and predicting the real-time channel evaluation of the range of frequencies suitable for radio wave propagation. The statistical distributions of the values of K were found to be a random function of time. The study concluded that the meteorological database algorithm (MEDAL) developed was suitable for carrying out the real time channel evaluation of radio communication frequencies and in the estimation of the equivalent earth radius factor (K).
- ItemOpen AccessBroadband Solar Irradiance and Photometric Illuminance at the Tropical Station in Ileife Nigeria(2015-04-30) Okogbue, Emmanuel ChilekwuThe main objectives of this research were to investigate the diurnal variations of the solar radiation fluxes and the associated solar radiation ratios with a view to characterising the sky conditions over Ile-lfe, Nigeria and establishing models for estimating the radiation fluxes at the station. Hourly solar radiation flux densities (global and diffuse) and ambient temperature measured at the rooftop of the Physics Department Building, Obafemi Awolowo University (OAU), Ile-Ife, Nigeria, between 1992 and 2002 and photometric illuminance measured between 1992 and 1996 were utilized for the study. Additional solar radiation data measured during the Nigerian Micrometeorological Experiments (NIMEX-1, 2 and 3) at OAU during the months of February and March of 2004, 2005 and 2006 were also utilized. The solar radiation ratios (the clearness index, the cloudiness index, and the luminous efficacy) computed from the measured solar fluxes and relevant astronomical relationships were investigated for diurnal and seasonal variations. These ratios (which are sky conditions indicators) were also used to characterize the local sky conditions over Ile-lfe and to develop statistical models for estimating the solar irradiance (global and diffuse) and photometric illuminance at the station. Very strong diurnal variations in the solar input and solar radiation ratios were observed resulting from the influence of the intervening atmosphere such that on particular days in August and December 1993 only about 45% and 530/0 respectively, of the total solar radiation arriving at the edge of the atmosphere at local noon actually got to the surface. The local sky conditions at the station were almost devoid of clear skies (clear skies occurred for only about 3.5% of the time). Overcast skies were also very scarce (overcast skies occurred for only about 4.8% of the time). The sky conditions were rather predominantly cloudy (cloudy skies occurred for above 72% of the time) all the year round. Daylight availability at Ile-Ife in terms of luminous efficacy was as low as 35 lm/W and as high as 115 lm/W and varied seasonally. The probability of having daylight at Ile-Ife with luminous efficacies more than 100 Im/W was about 24%.of the time on hourly and 33% of the time on daily bases. Empirical models for estimating diffuse solar radiation and photometric illuminance from the more readily measured global solar radiation were established for Ile-Ife. Models for estimating global solar radiation from the more readily measured ambient temperature were also established for the station. Inter comparison of the models using mean bias error (MBE), root mean square error (RMSE) and t-statistic showed that for any of the models, estimated values were not significantly different from their measured counterparts. The t-values ranged between t = 0.22, P > 0.05 and t = 0.88, P > 0.05. The models therefore predicted the measured values accurately. It was concluded that the local sky conditions over Ile-Ife were predominantly cloudy all the year round and almost devoid of clear skies and overcast skies and that the models predicted the measured values accurately.