Physics

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Formation of trimetallic complexes containing redox-active nitrosyl molybdenum tris(3,5-dimethylpyrazolyl)-borato groups. Schiff base compounds containing two Mo and one Cu centre
(Elsevier, 1994) Kagwanja, Stanley M.; Jones, Christopher J.; Maher, John P.; McCleverty, Jon A.
The reaction of [Mo(NO)Tp*Cl2] [Tp* = tris(3,5-dimethylpyrazolyl)borate] with copper(II) Schiff base complexes derived by condensation of two mole equivalents of 2,5-dihydroxybenzaldehyde with α,ω-diamines [1,2-C6H4(NH2)2, NH2(CH2)nNH2, n = 2–5] affords trimetallic species containing three potential redox centres. The IR, UV-vis and EPR spectroscopic properties of these compounds are described and it is shown that, with increasing chain length in the diimino bridges of the Schiff base complexes, there are small distortions from planarity, probably towards tetrahedral structures. From cyclic and differential pulse voltammetry it is shown that the trimetallic species primarily undergo two accessible sequential one-electron reductions associated with the {Mo(NO)Tp*Cl} centres, and by determination of conproportionation constants for the mono-reduced species, it is established that there is weak interaction between these two centres. Attempts to obtain reduced trimetallic species are described.
Reaction of molybdenum mononitrosyl halides with Schiff bases: Hydrolysis of azomethine links,formation of monometallic tris
(Elsevier, 1996-06) Kagwanja, Stanley M.; Jeffery, John C.; Johns, Christopher J.; McCleverty, John A.
The reaction of 2 mol equiv. of [Mo(NO){HB(dmpz)3}X2] ((HBdmpz)3 = tris(3,5-dimethylpyrazolyl)borate; X = Cl or I) with Schiff base ligands derived by condensation of 2 mol equiv. of para- and meta-hydroxybenzaldehyde with 1,4-C6H4(NH2)2 and 1,6-NH2(CH2)6NH2 afforded monometallic complexes [Mo(NO) {H(dmpz3)} X(OC6H4 CHO)]. Sodium borohydride reduction of the Schiff bases obtained from the hydroxybenzaldehydes and 1,4-C6H4(NH2)2 followed by reaction with [Mo(NO){HB(dmpz)3 Cl2}] afforded the bimetallic species [{Mo(NO)HB(dmpz)3}Cl{(OC6H4CH2NH)2C6H4}]. These compounds were characterized spectroscopically and the structure of a closely related compound, [Mo(NO){HB(dmpz)3}Cl{OC6H3(3-CHO)(4-OH)}], has been determined crystallographically.
Trimetallic complexes containing Nickel or Palladium and two Molybdenum nitrosyl groups
(1997) Kagwanja, Stanley M.; Johns, Christopher J.; McCleverty, Jon A.
The reaction of [Mo(NO){HB(dmpz)3}Cl2] [HB(dmpz)3 = tris(3,5-dimethylpyrazolyborate] with nickel(II) and palladium(II) Schiff-base complexes derived by condensation of two mole equivalents of 2,5-dihydroxybenzaldehyde with α,ω-diamines [1,2-C6H4(NH)2),(CH2)nNH2, n = 2–4 for Ni, n = 2 for Pd) affords trimetallic species. From cyclic, square wave and differential pulse voltammetric data it is shown that the species may be reduced to a dianion and that the sites of reduction are probably the two molybdenum nitrosyl fragments. From the electrochemical and limited EPR spectral data obtained from reduced species, it is clear that there is very weak electronic and minimal magnetic interaction between the redox centres.
Synthesis and electrochemical studies of heterobinuclear complexes containing copper and molybdenum nitrosyl groups linked by Schiff base ligands
(Springer, 2000-08) Lutta, Samwel T.; Kagwanja, Stanley M.
Abstract The reaction of [MoCl(NO)Tp * = tris(3,5-dimethylpyrazolylborate] with copper Schiff base complexes derived by condensation of one mole each of 2,5-dihydroxybenzaldehyde and salicylaldehyde with α,ω diamines [NH2(CH2) n NH2, n = 2–4] yields heterobinuclear complexes with two potential redox centres. I.r., electronic and e.s.r. spectroscopic properties of these complexes are described. Cyclic voltammetric data of the base complexes in DMSO reveal that the copper redox centres undergo irreversible one electron reduction at potentials which vary slightly with the polymethylene carbon chain backbone of the Schiff base ligands. Incorporation of [MoCl(NO)Tp *]+ groups in the copper Schiff base complexes, results to a slight anodic shift (100 mV) in the reduction potential of the copper centre which remains invariant as the polymethylene carbon chain lengthens. Electrochemical data of the heterobinuclear complexes using CH2Cl2 and DMSO as solvents indicate the solvent dependence of the reduction potentials of these complexes. In CH2Cl2, the reduction potential of the copper centre shifts cathodically by 100 mV, while that of the molybdenum centre shifts anodically by 200 mV. However, accumulated electrochemical data of the heterobinuclear complexes indicate minimal electronic interactions between the copper and molybdenum redox centres.
The effect of organic binders on the fracture properties of kaolin refractories, Journal of Material Science
(Springer, 2008-06) Njogu, M. S.; Nyongesa, W.; Aduda, B. O.
The fracture properties of kaolin-based refrac-tories prepared using plant-derived binders from okra and‘‘mrenda’’ have been investigated and compared. It wasobserved that the MOR of fired samples improved from37.5±0.1 MPa (for binder-free samples) to 69.6±0.1 MPa, and to 120.0±0.1 MPa for okra- and ‘mrenda’-plasticized samples, respectively, while the fracturetoughness increased from 3.9±0.1 MPa (for binder-freesamples) to 5.6±0.1 and 5.7±0.1 MPa for okra and‘mrenda’-plasticized samples, respectively. It is concludedthat the use of organic binders enhances the reliability andservice life of kaolin refractories used in thermally fluctu-ating environments
Synthesis, characterization and electrochemistry of heterobimetallic complexes containing molybdenum(II) nitrosyl and manganese(II)-schiff base centers
(Chemical Society of Ethiopia, 2010) Odhiambo, Ruth; Muthakia, Gerald K.; Kagwanja, S. M.
New complexes derived from the reaction of Mo (NO)Tp *Cl2; Tp * = tris(3,5-dimethylpyrazolyl) borate with a series of manganese(II) Schiff bases have been synthesized. Characterization by UV, IR, MS and elemental analysis support their formulations. Cyclic and differential pulse voltammograms of manganese (II) Schiff base monometallic complexes exhibit one reduction peak which may be associated with the reduction of the manganese (II) center. In CH3CN solutions, the molybdenum (II)-manganese (II) bimetallic complexes showed two reversible reduction peaks which are associated with the reduction of the molybdenum and manganese centers. In CH2Cl2, only the reduction of molybdenum (II) center exhibited reversibility while the reduction of manganese (II) center was irreversible. These observations demonstrate that reduction potentials for bimetallic complexes are solvent dependent.
Synthesis, characterization and electrochemistry of heterobimetallic complexes containing molybdenum(II) nitrosyl and manganese(II)-Schiff base Bull.
(Chemical Society of Ethiopia, 2010) Odhiambo, R.; Muthakia, G. K.; Kagwanja, S. M.
ABSTRACT. ABSTRACT New complexes derived from the reaction of Mo(NO)Tp *Cl2; Tp * = tris(3,5-dimethylpyrazolyl) borate with a series of manganese(II) Schiff bases have been synthesized. Characterization by UV, IR, MS and elemental analysis support their formulations. Cyclic and differential pulse voltammograms of manganese(II) Schiff base monometallic complexes exhibit one reduction peak which may be associated with the reduction of the manganese(II) center. In CH3CN solutions, the molybdenum(II)-manganese(II) bimetallic complexes showed two reversible reduction peaks which are associated with the reduction of the molybdenum and manganese centers. In CH2Cl2, only the reduction of molybdenum(II) center exhibited reversibility while the reduction of manganese(II) center was irreversible. These observations demonstrate that reduction potentials for bimetallic complexes are solvent dependent.
Geophysical structure of Homa Hills Geothermal field based on Gravity and magnetic survey
(2011) Adero, B. Otieno; Odek, A.; Ambusso, W.; Gitonga, G. J.
Gravity and ground magnetic surveys were conducted over an area of 76 Km 2 in order to investigate thermal structures responsible for geothermal manifestations such as hot springs in the Homa Hills geothermal prospect. The two surveys show that the southern part of the prospect has a potential for geothermal resource and should be investigated further to establish presence of an exploitable hydrothermal system. The magnetic survey indicates higher demagnetization in this part of the field compared to other parts of the field. The gravity survey is relatively sharp with a classic dome shaped profile associated with a large shallow dike
“Effect of AC Helmholt magnetic field on the mass of Rosecoco deans”
(Taylor and Francis Online, 2011-06-09) Kamweru, P. K.; Ndiritu, F. G.; Kinyanjui, T. K.; Muthui, Z. W.; Ngumbu, R. G.; Odhiambo, P. M.
Plastic bags mostly made of polyethylene (PE) cause pollution as solid waste due to their non-degradability nature. Initiation of a degradative process by enhanced photo-oxidation is a possible method for an accelerated degradation. This paper presents temperature treatment effects on PE films where photodegradation was initiated using ultraviolet (UV) irradiation in the ranges of 200–300 nm and 300–400 nm for 2 hr. Effects of temperature of 40°C and 55°C on non-UV-irradiated and UV-irradiated PE films processed by conventional methods were investigated and evaluated after 50 hr, 150 hr, and 350 hr of temperature exposure. The effects of UV wavelength range irradiation on the degradation were deduced. Measuring the dynamic moduli using a dynamic mechanical analyzer monitored the degradation. The decrease in average storage modulus was 62% with treatment at 55°C, higher than the 16% drop at 40°C for unirradiated samples after 350-hr exposure. Cross-linking in UV-exposed samples, characterized by an increase in dynamic modulus (stiffening), was observed followed by a reduction of storage modulus. Temperature treatment at 55°C together with 300–400-nm UV range irradiation resulted in the largest increase, i.e., 22% after 150 hr, followed by the largest reduction of storage modulus, i.e., 74.6% for a cumulative 350-hr exposure.
Study of Temperature and UV wavelength Range Effects on degradation of Photo Irradiated polyethylene films using DMA
(2011-06-09) Kamweru, P. K.; Ndiritu, F. G.; Kinyanjui, T. K.; Muthui, Z. W.; Ngumbu, R. G.; Odhiambo, P.M.
Plastic bags mostly made of polyethylene (PE) cause pollution as solid waste due to their non-degradability nature. Initiation of a degradative process by enhanced photo-oxidation is a possible method for an accelerated degradation. This paper presents temperature treatment effects on PE films where photodegradation was initiated using ultraviolet (UV) irradiation in the ranges of 200–300 nm and 300–400 nm for 2 hr. Effects of temperature of 40°C and 55°C on non-UV-irradiated and UV-irradiated PE films processed by conventional methods were investigated and evaluated after 50 hr, 150 hr, and 350 hr of temperature exposure. The effects of UV wavelength range irradiation on the degradation were deduced. Measuring the dynamic moduli using a dynamic mechanical analyzer monitored the degradation. The decrease in average storage modulus was 62% with treatment at 55°C, higher than the 16% drop at 40°C for unirradiated samples after 350-hr exposure. Cross-linking in UV-exposed samples, characterized by an increase in dynamic modulus (stiffening), was observed followed by a reduction of storage modulus. Temperature treatment at 55°C together with 300–400-nm UV range irradiation resulted in the largest increase, i.e., 22% after 150 hr, followed by the largest reduction of storage modulus, i.e., 74.6% for a cumulative 350-hr exposure.
Study of Humidity and UV Wavelength Effects on Degradation of Photo-Irradiated Polyethylene Films Using DMA
(2011-06-09) Kamweru, P. K.; Ndiritu, F. G.; Kinyanjui, T. K.; Muthui, Z. W.; Ngumbu, R. G.; Odhiambo, P. M.
Plastic bags, mostly made of polyethylene, cause pollution as solid waste due to their nondegradable nature. Accelerated degradation, as a solution to mitigate the menace, can be achieved through moisture enhanced photolysis. This study evaluated the effect of three relative humidity environments, i.e., 25%, 40%, and 60% RH, at a constant temperature of 55°C. The effect was studied for ultraviolet (UV) irradiated and nonirradiated samples of polyethylene (PE) films processed under conventional ways. Photodegradation was initiated using ultraviolet irradiation in the ranges (200–300) nm and (300–400) nm for two hours and the effects of subsequent humidity treatment analyzed. Dynamic mechanical analysis was used to measure the dynamic storage modulus to monitor degradation. For nonirradiated samples, there was essentially no change in storage modulus at the three relative humidity environments after 550 hrs. Irradiation in the (300–400) nm range showed faster degradation than for the (200–300)nm range with the highest drop in storage modulus being 67% after 550 hrs. Raising the humidity from 25% to 40% and 60% RH resulted in 41%, 62%, and 67% drop of storage modulus, respectively, at the 550 hrs.
Determination of Hall Effect Parameters of Gallium Arsenide and Gallium Manganese Arsenide by Van Der Pauw Geometry
(2012) Kemei, S.K*; Kirui, M.S.K.,; Ndiritu, F.G.,; Odhiambo, P.M.,; Ngumbu, R.G.,; Amollo, T.A .
Gallium Arsenide (GaAs) has been used widely in electronic industry to make diodes and transistors. As a semiconductor, it can be doped up with impurities with magnetic properties such as manganese to increase its electron conductivity. The storage capacity of the electronic devices made of gallium manganese arsenide (Ga1-xMnxAs) and the proportion of manganese atoms is worth studying. Here, GaAs was doped at different manganese levels, x, and the charge carrier concentrations at varied applied magnetic fields was investigated using Van der Pauw configuration. The tests were conducted at room temperature of 230C with magnetic field, 0.9 ≤ B ≤ 3.6mT and direct current of 1.19A. All the samples were studied for their hall voltage VH, carrier mobility µ, hall resistivity ρH and charge carrier concentration for different values of x. It was determined for Ga1-xMnxAs, 10% ≤ x ≤ 20% range, has maximum hall resistivity at B≈ 1.9 mT. For x=10%, 44.0 . ρ H ≈ Ω m ; x=20%, 79.0 . ρ H ≈ Ω m and for x=1%, the applied magnetic field has no effect on hall resistivity at initial states until B≈1.7mT. Beyond this point, magnetic field increases linearly with the hall resistivity to a maximum of 72.0 . ρ H ≈ Ω m . Maximum hall resistivity for x=50% was 2596.0 . ρ H ≈ Ω m at B mT ≈ 0.9 . For 0 20% ≤ ≤ x , carrier mobility µ , was of order of 7 2 1 1 10 m V s − − − while for x = 50% , µ was of order 9 2 1 1 10 m V s − − − . It was found out that the most probable doping percentage of GaAs with Mn dopants is approximately 20% and 10% as they show a hysteric response to an applied magnetic field. It suggests a good doping level of GaAs for making of volatile memory chips.
Determination of optical band gap and heat dissipation of Ga1-xMnxAs with Light in UV-VIS-IR Region using OSA SPECTRO 320”,
(International Knowledge Sharing Platform, 2012) Kemei, S. K* .,; Kirui, M.S.K.,; Ndiritu, F.G.,; Ngumbu, R.G.,; Odhiambo, P.M.,; Amollo, T.A.
Heat generated by silicon-based transistors due to high energy photo irradiation interferes with the electronic conductivity of transistors in the ICs. There is need to search for an alternative semiconductor material for making diodes and transistors with little or no heat dissipation. Dilute magnetic semiconductor such as Gallium Arsenide (GaAs) has demonstrated to be a better candidate to substitute silicon in electronic technology. UV-VIS-IR light was illuminated on the Ga1-xMnxAs samples of thickness 500nm−1000nm of varied doping levels during the study. Reflectance and transmittance spectra were determined using OSA SPECTRO 320 with light obtained from sodium lamp (240V, 100 W) with irradiance of 33.4807 W/m2 . The maximum absorbance within the UV-VIS-IR range was, A≤ 83.82% at λ≈200nm and minimum absorbance was, A≥0.96% at λ≈300nm with Ga1-xMnxAs, x=20% having the highest absorbance value and Ga1- xMnxAs, x=1%, the least absorbance. For Ga1-xMnxAs; x=0, x=10%, x=1% and x=20%; maximum absorbance occurred at UV region while for x=50%, maximum absorbance was observed at λ≈707nm. The results show that GaAs generate most heat due to its wide optical energy gap of 1.43eV while for x=1% dissipates little heat because of its small optical energy gap of 0.36eV.
A Comparative study of the transmission parameters of the sound of mosquito predators and EMR”
(2012) Mang’are, P. A. 1; Maweu, O. M. 2; Ndiritu, F. G. 3; Vulule, J. M. 3
The wide bandwidth EMR ultrasound, exploited in mosquito repellency, yielded only 20 % effectiveness. Due to this wide bandwidth, ultrasound from Amolops tormotus and Coleura afra, mosquito predators, was investigated. The sound was recorded, filtered and anaysed by Avisoft software; transmission parameters determined and compared with those of EMR. Spectrogram analysis showed harmonics, FM and CF components. The fundamental frequency of A. tormotus and C. afra sound was 5.371 kHz and 6.836 kHz respectively. The bandwidth of unfiltered sound of A. tormotus and C. afra was 10.98 kHz and 17.71 kHz respectively. The maximum peak amplitude of unfiltered sound of A. tormotus and C. afra was 135.19 dB SPL and 134.97 dB SPL; maximum acoustic energy was 19.57 Pa2s and 35.80 Pa2s respectively. Maximum acoustic energy of A. tormotus and C. afra sound was 10.843 Pa2s and 14.857 Pa2s recorded in 35-60 kHz and 10-34 kHz ranges respectively. Optimum acoustic parameters included 58.5 kHz peak frequency, 12.32-10.84 Pa2s acoustic energy, 19.40-19.85 kHz bandwidth, 55.13-55.48 kHz and 34.66-44.26 kHz as mean maximum and minimum frequency respectively, 134.08-134.28 dB SPL peak amplitude and 132.06-133.27 dB SPL minimum peak amplitude; bandwidth significantly narrowed from EMR. These parameters are critical in realizing effective EMR.
“The startling effect of the sound of C. Afra And A. Tormotus on the female A. gambiae”
(2012) Mang’are, P. A.1,*; Maweu, O. M.2,; Ndiritu, F. G.3,; Vulule, J. M.4
The female Anopheles gambiae, a malaria vector, detects ultrasound by its antenna, initiating an attractive or repulsive response. Modern electronic mosquito repellents exploit this concept in attempt to control malaria, but have shown only 20 % effectiveness. This work determines the startle response of the female A. gambiae to recorded sound of C. afra and A. tormotus and optimum acoustic transmission parameters needed for the design of an effective electronic mosquito repellent. A bioassay involving 3-4 day old fe ma le A. gambiae bred and reared under standard conditions was conducted in a standard glass cage yielding evasive behavioural responses on exposure to varied frequencies. The 35-60 kHz sound of A. tormotus and C. afra, the optimum frequency range, evoked evasive responses in an average of 46 % and 23 % of the mosquitoes, higher than the reported 20 % effective repulsion of EM R sound. The evasive response was characterized by 58.5o antenna erection, physical injury, unusual rest and movement, fatigue and falls; attributed to neural stress and fear for predation. The steady increase in signal intensity, maximum and mean acoustic energy in the sound of A. tormotus over all frequency ranges yielded greatest startle response in the female A . gambiae.
Volatility Modelling of the Nairobi Securities Exchang
(2012-03) Wagala, Adolphus; Nassiuma, Dankit K.; Islam, Ali S.; Mwangi, Jesse W.
In this paper we identify the most efficient ARCH-type model that can be applied to the Nairobi stock exchange data for forecasting and prediction of volatility which in turn is important in pricing financial derivatives, selecting portfolios, measuring and managing risks more accurately. The establishment of an efficient stock market is indispensable for an economy that is keen on utilizing scarce capital resources to achieve its economic growth. The purpose of this study was to determine the most efficient model from the symmetric and the asymmetric GARCH models. The models were evaluated by use of the Shwartz Bayesian Criteria (SBC), Akaike Information Criteria (AIC) and the Mean Squared Error (MSE). The results show that the AR-Integrated GARCH (IGARCH) models with student’s t-distribution are the best models for modelling volatility in the Nairobi Stock Market data.
Study of Humidity and UV wavelength effects on degradation of photo irradiated polyethylene films using DMA
(2012-03-12) Kamweru, P. K.; Ndiritu, F. G.; Kinyanjui, T. K.; Muthui, Z. W.; Ngumbu, R. G.; Odhiambo, P. M.
Plastic bags, mostly made of polyethylene, cause pollution as solid waste due to their nondegradable nature. Accelerated degradation, as a solution to mitigate the menace, can be achieved through moisture enhanced photolysis. This study evaluated the effect of three relative humidity environments, i.e., 25%, 40%, and 60% RH, at a constant temperature of 55°C. The effect was studied for ultraviolet (UV) irradiated and nonirradiated samples of polyethylene (PE) films processed under conventional ways. Photodegradation was initiated using ultraviolet irradiation in the ranges (200–300) nm and (300–400) nm for two hours and the effects of subsequent humidity treatment analyzed. Dynamic mechanical analysis was used to measure the dynamic storage modulus to monitor degradation. For nonirradiated samples, there was essentially no change in storage modulus at the three relative humidity environments after 550 hrs. Irradiation in the (300–400) nm range showed faster degradation than for the (200–300)nm range with the highest drop in storage modulus being 67% after 550 hrs. Raising the humidity from 25% to 40% and 60% RH resulted in 41%, 62%, and 67% drop of storage modulus, respectively, at the 550 hrs.
Efficiency Evaluation When Modelling Nairobi Security Exchange Data Using Bilinear and Bilinear-Garch (Bl-Garch) Models
(2012-06) Wagala, Adolphus; Islam, Ali S.; Nassiuma, Dankit K.
In this paper, the weekly returns of the Nairobi Securities Market (NSE) are modelled using bilinear models and the bilinear-GARCH models so as to determine the most efficient and adequate model for forecasting of the Nairobi Equity market. The data used was obtained from the Nairobi Stock Exchange (NSE) for the period between 3rd June 1996 to 31st 30th October 2011for the company share prices while for the NSE 20-share index was for period between 2nd March 1998 to 30th October 2011.The share prices for three companies; Bamburi Cement, National Bank of Kenya and Kenya Airways which were selected at random from each of the three main sectors as categorized in the Nairobi Stock Exchange were used. The results indicate that the combination of bilinear-GARCH model is more adequate and efficient in modelling the weekly returns of the Nairobi Securities Exchange.
Mathematical Analysis of a Comprehensive HIV AIDS Model: Treatment versus Vaccination
(HIKARI Ltd,, 2013) Okongo, M. O.; Kirimi, J.; Murwayi, A. L. Murwayi; Muriithi, D. K.
A comprehensive deterministic HIV/AIDS transmission model incorporating social behaviour, treatment, vaccination, stages of infection, age structures, discrete time delay and vertical transmission is presented and rigorously analyzed. Two age structures are considered with group one consisting of children aged (0 - a) years and group two consisting of adults aged (a) years and above. In this study we investigate wether a trade-off exists between vaccination and treatment. Numerical simulations shows that treatment that does not reduce infectiousness is worse than when the treatment is not applied at all, however when coupled with effective counseling, then it is very effective in combating the spread of the disease and finally eliminating it. A trade off seems to exists between vaccination and treatment and therefore careful considerations should be made when vaccination and treatment is to be applied together because a combination of the two could be counterproductive or helpful depending on how it is implemented.
2D-Euler Deconvolution and Forward Modeling of Gravity Data of Homa-Hills Geothermal Prospect, Kenya
(2013) Odek, O. 1; Otieno, A.B. 1; Ambusso, W.J. 1; Githiri, J. G 2
In order to fully assess the potential of Homa Hills Geothermal prospect, the heat source which is one of the main features of a geothermal system had to be located based on its perturbation on the gravity field. Ground gravity survey was conducted in an area covering about 76 km2 and the data processed to remove all other effects which are not of geological interest. Qualitative interpretation was attempted and cross sections drawn across the anomalous areas on the complete Bougu er anomaly map. Quantitative interpretation attempted involved both Euler Deconvolution and 2 ‐D Forward modelling. The parameters obtained from Euler Deconvolution were used as the start up parameters for 2 ‐D Forward modeling. Well clustered Euler solutions were obtained at a shallow depth of approximately 200‐750 m which is consistent with the modeled shallow dike like intrusive probably of carbonatite origin

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