MST-Department of Physics

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Thermal conductivity of EPDM rubber composites as a function of filler concentration and type.
(Kenyatta University, 1988) Oduor, Andrew Odhiambo
Thermal conductivities of EPDM (ethylene propylene diene monomer) rubber loaded with one of the following fillers: carbon black type N339, a reinforcing filler, kaolin, a non-reinforcing filler, and ground charcoal have been studied using a guarded hot plate apparatus designed at Kenyatta University. The hardness and tensile strength as well as the breaking energy of the composites have also been determined. The results show that the thermal conductivity, increases with increasing filler concentration in the composites. The thermal conductivity values also increase with increasing composite hardness. Thermal conductivity and tensile strength values were higher for the composites filled with reinforcing filler than those filled with non-reinforcing ones. The high values are explained in terms of the filler-polymer interaction energies that are higher in composite filled with reinforcing fillers than those filled with the non-reinforcing ones. Using several known thermal conductivity models, for bi component composites, the theoretical values of thermal conductivity were calculated for the various filler concentrations and compared with the experimentally determined values. None of these models, successfully predicted the experimental values of thermal conductivity of rubber composites. In almost all the cases the theoretical values of k for kaolin, charcoal and carbon black samples are at variance with the experimental values. Explanation for this discrepancy is based on the fact that the polymer filler interaction-parameter which is different in the three fillers used plays an important role in facilitating thermal conductivity in the composites. Besides other results, the conductivity-hardness and tensile strength results confirm this observation. As a major finding of this work, it is observed that the theoretical models available have been derived on the basis of conductivities of the individual components, they do not incorporate the interaction parameter which apparently plays an important role in rubber composites.
Electrical Characterization of Cu and Al Doped Amorphous Semiconductor As2Se3
(Kenyatta University, 1988-10) Okumu, John.
The electrical conductivity and thermoelectric power of an amorphous semiconductor As2Se3 in pure and doped forms were studied with temperature. The dopants used were 1 at. % Cu and 1 at. % AI. Measurements on electrical conductivity were performed in the temperature range 223-573 K and that of thermoelectric power in the temperature range 343-473 K. The effects of the impurities on the conductivity and thermoelectric power of amorphous As2Se3 are small. The samples showed conductivity activation energies. The electrical conductivity versus inverse temperature curves showed a break at low temperatures. The activation energy of conduction, E , above room temperature were found to be 0.77 ± 0.09 eV, 0.82 ± 0.09 eV, and 0.64•± 0.03 eV for pure , Cu doped, and Al doped samples respectively. The thermoelectric power varied linearly with inverse temperature and all samples were found to be p-type. The activation energy, ES' from thermoelectric power measurements were found to be 0.96 ± 0.06 eV, 0.87 ± 0.02 eV, and 0.84 ± 0.05 eV for pure, Cu doped, and Al doped samples respectively. Most of the above results have been explained on the basis of the CFO and the dangling bond models. The CFO model postulates a mobility edge and overlapping tails of localized states in the energy gap and in the dangling bond model, charged defects D+, DO, D-, exist in the energy gap, which act as traps to charge carriers.
The jahn-teller effect in the absorption band of activated crystals
(Kenyatta University, 1989) Gathua, Joseph K.; Dod, T. S.
Electrical Characterization of Iodine Doped Organic - Semiconductor Polypropylene
(Kenyatta University, 1989-10) Mulati, David Masabule
Gravity Survey in Magadi Geothermal Area
(Kenyatta University, 1999) Waithaka, J. W.
Kenya has prominent geographical features such as the Rift Valley which gives rise to geothermal reservoirs which can be exploited to produce geothermal energy for power generation and heating. Lake Bogoria, Lake Magadi as well as other areas are considered to have potential for geothermal energy because of the existing hot springs. For above mentioned regions it is known that positive gravity anomalies within the Rift Valley proper are superposed on the generally negative gravity anomaly typically for rift structures. Research has availed that these local gravity highs can be explained as result of high density intrusions in the upper crust. The aim of the project was to study the gravity highs in Magadi and determine whether they could be possible sources of geothermal energy. The area of study comprises a part of the Rift Valley, which has geothermal potential. The Magadi area is located in the southern part of the Kenyan Rift Valley. The gravity prospecting method was used in the project to investigate the gravity anomalies occurring in the area. Both qualitative and quantitative interpretations were attempted and conclusions drawn from the findings. A gravity survey was conducted to fill in the unsurveyed sectors. The data collected was subjected to the necessary corrections and a Bouguer anomaly map of the area was then drawn. The Bouguer gravity values ranges from -1800 g.u. to -1600 g.u. with the maximum peak occurring at the NNE part of the project area. The relationship between the gravity anomalies and the topographic features of the area is clearly noticeable. The quantitative interpretation involved fitting theoretical two-dimensional models to the observed data along profiles. The choice of two-dimensional models was dictated by the shape of the anomalies to be interpreted. The anomalies were interpreted in terms of intrusions of several bodies. The geometries of these bodies were also determined. Quantitative interpretation involved forward modelling along five selected profiles named PQ, MN, UV, CD and RG. The model parameters arrived at suggests that the causative bodies in this area are found to be dikes like structures except the body behind the gravity profile MN which seems to be due to a shallow magma chamber. Gravity body behind PQ is at a depth of 0.2 km to the top, 4 km to the bottom with an average width of 1 km and dips at 110° to the horizontal. It has a density contrast of about positive 280 to 290 kgm" and with an elongation of NNWSSE. The depths to the top of the bodies MN, UV, CD and HG are 0.15, 0.3, 0.5 and 0.15 km respectively while the depths to the bottom are 6.1, 4, 4 and 3.6 km respectively. The profiles mentioned above have positive density contrast ranging between 280 kgm" and 340 kgm" , with dip angles of 90°, 90°, 82° and 90° respectivelywhich are elongated in a nearlyN-S or NNW-SSE directions. It was found that the densities of the rocks causing the positive anomalies in the region is about 3 000 kgm" or slightlyless. The mode of formation of the causative bodies and shallow depths indicate that the area is potential for the generation of geothermal energy. Comparison of the results of this study with other studies in the sameregion, indicate that the anomalies are caused by bodies at shallow depth. The study of the gravity anomalies revealed that the major gravity disturbances were caused by the rifting and the subsequent tectonic activities of the Rift Valley, since the gravity anomalies were found to have north south trending nature which coincidewith the north-south running features such as dykes, fractures e.t.c. that are aligned in the direction of rifting. The agreement between the final geophysical models
Microprocessor based multifunction signal generator
(Kenyatta University, 2001-10) Mwangi, Karimi P.; Ibitola, G. A.; Stangl, R. L.; Namuye, S.
Waveform generators are excellent tools to recreate real-world signals that offer precise sirnulations for innumerable challenging test conditions. Waveforms are, for example used in airbag deployment, multi-tone amplifier testing, brake simulation, digital modulation, cardiac device calibration, power disturbance harmonic generation, laser beam control and wafer cleaning drive signals among many others. The possible waveforms that can be been generated include: Sine, Cosine, Triangle, Ramp, Exponential, Gaussian, Pulse, Sine, Hamming (Sine), Amplitude modulation (AM), Frequency modulation (FM), Frequency shift keying (FSK), Phase shift keying (PSK), Noise-Digital, Noise-Analogue, Comb, Steps, Sweep, Cardiac and Squine. In this research work, the generation of square, sine, triangular, and random waveforms using a microprocessor-based programmable waveform generator is presented. The 8088 microprocessor and other support chips, hardware and software designs are discussed. Analysis and experimental results based upon the system and its associated waveforms are evaluated using a PM 3384 autoranging combiscope (CRO). The function generator is a prototype of general-purpose, high performance instrument capable of generating standard waveform patterns at frequencies of exceptionally low distortion. The generator is capable of generating frequencies, selectable from 0 Hz to 100Hz and adjustable output amplitude from ± 4.8 V to ± 14.1 V peak to peak with high resolution. The system as a whole possesses some interesting advantages over the analogue and digital types of signal generators that are presently in use.
Dictionary Based Transform for Improving English Text Compression (Dbt)
(Kenyatta University, 2007) Owuor, Paul Otieno; Gakuru, M. K.; Merenga, A. S.
Text compression algorithms reduce the redundancy in data representation to decrease the storage and transmission time required for that data. One way of increasing text compression is to preprocess text before applying it as input to a backend compressor. There are a number of context based algorithms in this area including, Star Encoding or * -encoding, Length Index Preserving Transform (LIPT), Reverse Length Preserving Transform (RLPT), and Shortened Context Length Preserving Transform (SCLPT). However, none of these methods has been able to achieve the theoretical best-case compression ratio, suggesting that better algorithms are possible. This research focused on the dictionary-based method of preprocessing and specialized coding to increase text compression. It developed a lossless dictionary-based text-preprocessing algorithm called Dictionary Based Transform for improving English text compression (DBT). The DBT approach consisted of transforming text by replacing often-used words with special codes in a dictionary given in advance. This transformation produces the desirable effect of precompressing the original input text, maintaining some of the original context information at the word level and creating an additional stronger context in the transformed text. The transformation also ensures that frequently used characters have higher probabilities in the transformed text. The combined effect is that DBT achieve some compression at the preprocessing stage as well as retaining enough contexts and providing stronger character predictions for backend algorithms to give better results. The DBT achieved a precompression of about 17%, an average improvement of about 6.9% over LHA, 3.4% over WINZIP, 3.9% over DMC, and 0.9% over Bzip2 and PPMD, for our test corpus. Since Bzip2 and PPMD algorithms are among the best in the market, they are recommended as the preferred backend algorithms for use with DBT. Structurally, this report begins by discussing the problem of text compression, the motivation behind this work and how to achieve the stated aim of optimal textual compression. A review of relevant literature mainly on statistical and dictionary coding approaches provides the theoretical context. This is followed by the new algorithm, the methods and designs used as an attempt to solve the problem. Finally, the test results and an explanation of the results, and a conclusion, complete with suggestions for further work, is given.
Design and fabrication of an electronic system for monitoring and controlling temperature, light illuminance and humidity in a greenhouse
(2011-05-09) Nyanya, Obanda Wycliffe
Climate uncertainities pose a challenge to the production of food to feed the growing billions on the globe. Climatic conditions can change abruptly ruining farming efforts and leading to food shortages. Greenhouses attempt to solve this problem by enclosing crops in a climatically controlled environment. Crop production in greenhouse has distinct characteristics. Data on these characteristics need to be collected at regular intervals. For each type of crop, these characteristics need to be controlled within the specified limits to achieve the maximum efficiency and yields. In the past, greenhouses employed electromechanical devices such as thermostats to monitor and control the environment. Mechanical systems lack the flexibility and precision required for greenhouse control. Some modern greenhouses use computers to control the environment. Computers require programming skills, are bulky and costly. A greenhouse monitoring and control system that is compact and thus portable, adaptable, cheap and easy to assemble hs been developed. Based on the Z80 microprocessor, the hardware consists of a Z80 CPU, two 8255 PIA's, a 2716 EPROM, a 6116 RAM, an HIH-4000 humidity sensor, an LM-35 temparature sensor and two ADC 0804 analogue to digital converters, a clock and reset circuitry and 7-segment displays and LEDs. The software has been coded in the Z80 assembly language. This system monitors temperature, humidity and light illuminance on a continuous basis. It measures and displays these parameters and activates appropriate devices whenever these variables fall outside predetermined ranges. Relative humidity is controlled by a water sprinkler, temperature by a fan and heater and light illuminance by a lighting system.
Design and simulation of a finite state machine suitable for micro-code execution
(2011-05-09) Mithanga, Kithuku Elijah
Digital processors are built from very large scale integration circuits and the software to design them is only available to the manufacturers themselves. There is a need to design a central processing unit using large, medium and small scale integration circuits which can be used as a teaching aid in computer architecture and design. In this thesis a simulation model of a digital processor is designed using arithmetic logic unit, control unit and registers as building blocks with their interconnetions. The presented digital processor is a 4-bit, 80 MHz incorporating sixteen instructions. The modules for the central processing unit were built from large, medium and small scale transistor logic integrated circuits. The design and simulation is based on Electronic workbench software. The circuit is capable of modeling the functional and timing behavior of the digital processor to use instructions in the groups of data transfer, arithmetic, logical bit manipulation, shift and program control. A state machine control unit was used for sequencing of the instructions. The clock pulses were supplied by a 555 timer instead of quarts crystal in order to make it possible to vary the frequency to maximum by use of different values of resistances were also plotted; this was carried out in order to calculate the optimum simulation time for different instructions. A set of four switches was used for selecting up of the opcodes and imputing data to the the digital processor through the input port. In order to implement program control a condition jump instruction is implemented when the state of the status register is logic. 1. The mnemonic codes were also simulated and the results were found to be in agreement with theoretical values. With the sixteen opcodes, circuits for substraction, addition and multiplication were simulated. The circuits yielded good data validity at the frequency of operation. Further work can be carried out in order to improve the processor to handle large amounts of data by using 8-bit and 16-bit registers. Five or six length opcodes could also increase the number of instructions to be executed from sixteen. Verilog hardware descriptive language and very high speed hardware descriptive language can be used since they offer more tools for design.
Study of SnxSey thin films for phase change memory (PCM) applications
(2011-07-14) Muna, Karanja Joseph; Karimi, P. M.; Walter K. Njoroge
Phase Change Random Access Memory (PCM) is one of the most optimized candidates for the next generation non-volatile memory due to its fast operation speed, high scalability, low power operation and fabrication costs. The transition from the amorphous to the crystalline phase is induced by heating the material above its crystallization temperature for a long enough time, and the switching back to the amorphous phase is realized by melting and quenching the material fast enough that it solidifies in the amorphous state. In this study the electrical and optical properties of various SnXSey alloys at room temperature and during annealing from 25-250 °C was investigated and subsequent electrical characterization of fabricated SnSe4 PCM. An Edward Auto 306 RF/DC Magnetron evaporation chamber was used for thin film deposition. Transmission data in the range from 450 nm- 2500 nm was obtained using Shimadzu UV-VIS-NIR 3700/3700 DUV spectrophotometer. Optical bandgaps have been found to vary from 1.27-1.94 eV with decrease in tin concentration. IN characteristics measurements were performed on the films using Keithley 2400 source meter interfaced with four point probe using computer to determine sheet resistivity. Sheet resistivity varied from 1150-1600W cm. All the alloys have shown high sheet resistance at room temperature and low sheet resistance during annealing. This interprets that the samples were amorphous during deposition and changed to crystalline during annealing. The SnXSey alloys have different crystallization temperatures varying from 174-224 °C. Increase in tin concentration reduced the crystallization temperature. The PCM was fabricated based on SnSe4 alloy since it had high crystalline temperature, low crystallization temperature and a wide band gap hence a better choice than other alloys of SnXSey. The PCM was found to have a threshold voltage of 3.71 V and its voltage and pulse width of amorphization and crystallization processes were 3.71 V, 90 ns and 1.3 V, 26 p s, respectively
Simulation of the charge ratio of cosmic ray muons in extensive air showers using corsika
(2011-07-14) Onyango, Ochilo Livingstone; Hashim, N. O.; Okumu, J.
The interaction of primary cosmic rays in the atmosphere produces, among other particles, pions and kaons. They decay to muons, which form an important component of extensive air showers, which can be measured on the Earth's surface. The ratio of positively to negatively charged muons, called the muon charge ratio, provides important information about the cosmic ray interactions in the atmosphere. In this study, the theoretical hadronic interaction models in the cosmic ray simulation code CORSIKA are used to study the charge ratio of cosmic ray muons simulated in extensive air showers. An East - West effect on the charge ratio of simulated cosmic ray muons is observed. It is more pronounced for inclined and low-energy muons (momentum less than 100 GeV/c and zenith angle greater than 80°). Experimental data from NIINOS Near and WILLI experiments give similar results.
Design and fabrications of a simple four points probe system for electrical characterization of thin films
(2011-07-26) Agumba, O. John
The electrical characteristics of semiconductor thin films are of great practical interest in microelectronics industry hence the need to measure these parameters in a cheaper and faster manner possible. This study has embarked on design and fabrication of a simple, effective and portable computer-aided four point probe system for thin film sheet resistivity measurement. A four point probe head has been designed and fabricated from easily available materials. A relay switching device has also been designed and fabricated to perform switching of the probe tips on the sample surface as per the Van der Pauw set tip. A Keithley SourceMeter 2400 model has been interfaced to a LabVIEW running computer via the serial port (RS-232 port) for its full control by the computer. The relay switching device has also been interfaced to the computer via the printer port (LPT1 port). The fabricated probe head, the relay switching device and Keithley SourceMeter were used to probe the samples as per the Van der Pauw set up with a square symmetry adopted for sheet resistivity measurement. To test the workability and reliability of the fabricated system for thin film sheet resistivity measurement. the sheet resistivity of Cu20 semiconductor thin films prepared by DC reactive magnetron sputtering technique were measured. The sheet resistivity measured at room temperature of 23 °C was found to be 55.65 S2 cm. However, as the samples were exposed to temperature rise, the sheet resistivity was found to decrease and was at its minimum value of 29.67 Q cm at 170 °C. Cu20 thin films prepared at different sputtering pressures were also electrically characterized using the system. The sheet resistivity of the thin films were found to increase With increase in sputtering pressure. Films deposited at sputtering pressure of 1.8x10-2 mbar had sheet resistivity of 33.63 S2 cm and this increased to 62.23 SZ cm for films prepared at higher sputtering pressure of 2.4x10-2 mbar. From the measurements obtained, it was found from the study that the system offers a reliable, effective and simple technique for thin film sheet resistivity measurements.
Calculation of cross selection for elastic scattering of electrons by a magnesium atom at intermediate energies
(2011-08-01) Kariuki, Kinuthia Peter
Elastic electron-atom collision data from experimental and theoretical studies is useful in the interpretation of spectra obtained in X-ray photoelectron spectroscopy and Auger electron spectroscopy as well as in the Monte Carlo simulation of conduction electrons in solids. In principle the behavior of an electron-atom collision system is predicted by solving the Schrodinger equation. However this task is far from simple in that the equation can only be solved exactly for a two-body collision problem and consequently approximation methods are necessary for electron-atom scattering. In this study the Optical Model Potential method was used to calculate cross sections for elastic scattering of electrons by a magnesium atom at intermediate energies. Computer program PSCATT was developed to solve the Schrodinger equation numerically. Through use of a complex optical potential which accounts for possibility of inelastic processes at intermediate energies, the results obtained were found to be in fairly good agreement with recent measured results. The present results were also found to be comparable to, and in some cases better than, theoretical results obtained using sophisticated multi-channel methods. The computer program used in this study is much less demanding on computer power than the computer codes associated with multi-channel methods which has implications in terms of reducing research costs.
Design and fabrication of a microcontroller based electronic weighing machine in high mass regime
(2011-08-04) Munyao, Kitavi
Electronic weighing technology presents management with rapid, timely and accurate information that provides quick turnaround times for customers. Its popularity can be attested by its use in all the sectors of the industry. In fact, its spectrum of use spans from the traditional retail industry, to manufacturing and warehousing, to postal, health and transport industry. Ordinary microprocessor based weighing systems that utilize single point load cells, have a huge circuitry and hence render them complex to design and implement. They further have low speed and resolution. One other class of microprocessor based weighing balances utilizes faster microprocessors and has relatively high resolution. These devices often have a prohibitive price tag, as they comprise many external devices/chips. Another type of electronic weighing system utilizes a microcontroller and an 8-bit analogue-to-digital converter. It has in-built memory units to store program, data and variables. In both cases, there is compromise between range and resolution. This research work describes the design and implementation of a low-cost, high-resolution and portable digital electronic weighing system ideal for domestic, laboratory and commercial use. The system has highly reduced circuitry as it utilizes a standalone microcontroller chip. It essentially comprises of an 8bit 8051 microcontroller with a local memory module for storing data from the ADC. High resolution, without compromising range, is achieved by designing a program for interfacing a serial 10-bit ADC to the 8-bit microcontroller. This is due to the in-built Serial Programming Interface (SPI) of the microcontroller used. The developed electronic weighing balance senses, measures and displays the mass, placed on a single point load cell, on an LCD display. The system fabricated measures weights ranging from 0-19kg.
Electron impact double excitation of (2s2) 1s and (2s2p) 1p states of helium atom using a distorted wave method
(2011-08-08) Mayaka, Job Oqhoyho
Cross section studies are important as they supply fundamental level researchers with necessary information and predictions for their experiments. Once experimental measurements agree with some prediction, the theoretical model yielding the prediction is then applied to other situations and finally validated. Very few studies have been made on electron impact double excitation of (2s2) 1S and (2s2p) iP states of helium and even the results obtained in these studies do not agree among themselves. It is on this ground that the cross sections for the excitation of the above states have been calculated using a modified distorted wave method. In this method, a linear combination of the static potentials of the helium atom in the initial and final states is taken as the final channel distortion potential and the initial state static potential of the helium atom is taken as the initial channel distortion potential. Analytic wave functions for the ground and excited states are used. Numerical calculations are performed using a modified DWBAI computer program. It is noticed that the effect of the exchange is appreciable up to about 400 eV for the (2s2) 1S excitation and up to 200 eV for (2s2p) 1P excitation. It is encouraging that the results obtained agree well with some of the theoretical and experimental data.
Design and fabrication of microprocessor-based car length monitoring system
(2011-11-17) Githaiga, John Irungu; Merenga, A.S.; Mutuku, J.N.
In our local car storage facilities in some of our major towns, the drivers spend a lot of time looking out for an appropriate space for their cars. In a simple system where cars are categorized on the dimension basis, sensitive sensors may be utilized to configure the exact size with a higher degree of accuracy. This ensures maximum utilization of the storage facility simply by storing the car whose dimensions match to those of the facility. This technology is available in robotic parking. The usage of this robotic technology in our Kenyan situation is however expensive and unreliable and is thus vital to develop cheaper and cost effective means. One advantage in this cheaper technology is the standard width of modern cars and can therefore use length criteria to categorize the cars. It is possible to design a system that would continually and automatically monitor the cars length, display the length to the client (driver) and the facility currently available and suitable for such car size. To solve this problem, this research project focused on designing and fabricating a microprocessor-based car length monitoring system. The system reads the bit pattern from the length monitoring sensor stem, interprets the information and displays the length configured and later display the facility suitable for the car length. The system is implemented by constructing a microelectronic system based on Z80 processor interfaced to 2716 EPROM, 6116 RAM, two 8255 PIAs a seven-segment display, two Light Emitting Diode Displays and a photo resistor sensor circuit. The photo resistor sensor circuit consists of eight photo resistors separated at a distance of 10cm from each other. The seven segment displays and Light Emitting Diodes are both driven by 74LS48 Integrated Circuits (ICs) and are connected to port B (8255 PIA 2) and port B (8255 PIA 1) respectively. The program for driving the system is developed and coded using Z80 assembly assembler. The components used are chosen based on their cheap cost, low current and voltage ratings ( 5V ), high performance, better frequency response and easier logic family compatibility. The Z80 assembly language is preferred due to its powerful but extremely flexible instruction set (150 in number) and its ability of supporting 64K RAM memory. A car model is passed over the photo resistor sensors. As it fully by-passes the first sensor, it is stopped momentarily for 20 seconds. The length monitoring system configures the resulting binary combination due to mechanical optical beam interruption. The information is fed into port A (8255 PIA 1). The car length size and facility are displayed in port B (8255 PIA 2) and port B (8255 PIA 1) respectively. The system displays the car length.
Influence of sputter power and sputter pressure on optical properties fo WOx thin films deposited by reactive DC magnetron sputtering
(2011-11-17) Wangati, Charles Kiama
Soil fertility decline is an acute problem facing smallholder farmers in central highlands of Kenya. Availability of organic and inorganic nutrient resources, their management under spatially variable soil fertility conditions has consequences on the soil resource base, cropping patterns and crop yields. Smallholder farms in the central highlands of Kenya exhibits a high degree of heterogeneity, which are influenced by a complex set of socio-economic and biophysical factors. The farms consist of multiple plots managed differently in terms of allocation of crops, nutrient inputs and labour resources; making within-farm soil fertility gradients caused by management strategies a common feature. A monitoring study involving nutrient stocks, flows and balances was conducted in Kirege Location, central highlands of Kenya to explore soil heterogeneity with the aim of improving soil fertility management. Focus group discussions were conducted where criteria for wealth ranking and grouping farmers into different wealth status was developed. A rapid survey was conducted on a sample of 50 households randomly selected from the list of households in Kirege obtained from the local chief's office to characterize and classify the farms into three different types (rich, medium and poor). Nine case-study farms were randomly selected for detailed resource flow mapping (3 from each of the 3 farm types). The farms were visited to record movement of nutrients stock materials using a monitoring protocol covering household, crops, livestock, soil and socio-economic aspects of the farm. Soil in the plots identified in resource flow mapping at different distances from the homesteads were sampled at the end of 2006/2007 short rains cropping season (March 2007). The sampling depth was 0 - 20 cm to capture the level of nutrient stock in the topsoil. The soil samples were analyzed for pH, C, N, P, K, Mg and Ca at ICRAF laboratory using Infrared spectroscopy method. Resource flow mapping data was analyzed using IMPACT program version 2.0. Results revealed use of Results revealed use of nutrient resources varied for different field types and was strongly influenced by distance from the homestead. Home fields received more nutrient inputs compared to outfields. In wealthy farms, home fields received 109.1 kg N ha 1, 72.9 kg P ha 1, and 379 kg K ha 1, compared to 42 kg N ha-, 32.1 kg P ha-1 and 55 kg K ha -1 in the outfields. In poor farms, home fields received 56.8 kg N ha 1, 32.8 kg P ha-1 and 106 kg K ha -1 compared to 8.1 kg N ha 1, 4.6 kg P ha 1 and 16 kg K ha 1 for the outfields. Variations in resource allocation were also observed with regard to farmers' level of resource endowment. Wealthy farmers used a mean of 74.5 N kg ha 1, 50.9 kg P ha -1 and 244 kg K ha 1, compared to 27.3 kg N hat, 16.2 kg P hat and 50.7 kg K ha -1 for poor farms. Partial nutrient balances and stocks were higher in home fields and in wealthy farmers' farms as compared to outfields and poor farmers' farms. Due to nutrient heterogeneity in smallholder farms, there is a need for a more targeted approach to soil fertility intervention that differentiates between farm fields, agro-ecological zone and resource endowment status.
Design and fabrication of a microcontroller based maximum power tracker
(2011-12-02) Mutinda, Patrick Benido
A photovoltaic array exhibits non-linear I-v characteristics and its maximum power point varies with solar insolation, operating voltage and temperature. This work deals with the design and laboratory implementation of a real time microcontroller based maximum power tracker with the aim of improving conversion efficiency of the photovoltaic system. It is a digital dual tracking system that consists of a maximum point tracker (MPPT) that enables the PV array to operate at its maximum power point (MPP) and a solar path tracker, a machine for orienting the PV array towards the sun. The incremental conductance algorithm was developed using the voltage and current information and was able to track the maximum power quickly under rapid changing conditions. It was implemented using a single chip PC l 6C84 microcontroller to control the duty cycle of the boost converter with pulse width modulation. The microcontroller offered solution to weaknesses inherent in other types of control systems. Power feedback control was used to measure and maximize power at the load terminal. The MPPT consists of a DC-DC converter optimised for nominal output; a control section and a 12V 50 Ah lead acid battery as load. The solar path tracker consists of sensors, control circuitry, H-bridge, do motor and 40W polycrystalline solar panel. With reference to the set up where the panel was directly connected to the resistive load the solar path tracker (SPT) increased the performance by a factor of 1.29 while the maximum power point tracker (MPPT) increased the performance by 1.08. The maximum power tracker increased the performance by a factor of 1.49
Dynamics mechanical analysis of unirradiated and gamma ray radiated injection molded virgin and recycled high density polyethylene
(2011-12-02) King'ori, Gladys Wangechi; Merenga, Abdullah S.; Migwi, C. M.
Modification in polymeric molecular structure can be brought about by either the conventional chemical means, usually involving silanes and peroxides, or by exposure to ionizing radiation, from either radioactive sources or highly accelerated electrons. In this work ionizing radiation from a radioactive source, in this case a standard ray from a Cobalt 60 source was used to irradiate injection molded samples of both recycled high density polyethylene (RHDPE) and virgin high density polyethylene (VHDPE). The samples were then studied using a Dynamic mechanical analyzer (DMA) and the Differential Scanning Calorimetry (DSC). The DMA measurements were done in a temperature range of 298 K to 355 K in the frequency range of 0.5 Hz to 13 Hz. One relaxation process was observed which follows the Vogel Fulcher Tammann (VFT) law of the temperature dependence of the mean relaxation for both VHDPE and RHDPE. The process is assigned to chain motions between the crystals. It was noted that doses of at least 12 kRad had no effect on the frequency of the relaxation process of VHDPE or RHDPE. This suggests irradiation effect on a small (local) scale. Another observation was that irradiation resulted in an increase of the loss modulus intensities, for VHDPE and a decrease in the RHDPE. For both the gamma ray radiated and unirradiated samples the intensities of the samples cut far from the injection point were higher than for samples cut near the injection point. As regards anisotropy of the samples, a comparison of the intensities of the loss modulus for samples cut parallel and those cut perpendicular to the polymer melt flow directions indicate that irradiation has a stronger influence on the samples cut perpendicular to the melt flow direction than those cut parallel. Anisotropy between the samples cut near and those cut far from the injection point was unaffected by irradiation. DSC thermograms were obtained at a heating rate of 10 K/min from 270K to 410K. From the DSC thermogram it was observed that irradiating RHDPE to a dose of up to 6 krad decreases its melting temperature, however for a dose of up to 12 krad, the melting temperature of VHDPE was insignificantly affected.
Development and study of a microprocessor-based direct current motor controller
(2011-12-03) Mwenda, Phylis Makena; Ibitola, G. A; Merenga, Abdullah S.
Control systems for electric motors have become so vital to the proper performance and protection of modern plant equipment that they are frequently the most essential links in complex industrial applications. Electric motors play a very important part in furnishing power for all types of domestic and industrial applications. Understanding the direct current drive is particularly important because it is so widely used as a yardstick by which other drives are measured. Users who develop a good grasp of the direct current drive will find their knowhow invaluable in dealing with all other types, particularly if they can establish a firm grip on the philosophy of the control scheme. Analogue and digital motor control systems already exist. These control systems have inherent weaknesses such as lack of system's flexibility, poor responses, large power losses, several switches and relays, large complexity of wiring, and a large number of circuitry. This thesis presents a control system based on a 8088 microprocessor for starting, stopping and changing the direction of DC motors. The hardware consists of the micro-controller and the switching circuit connected to the motor. The microprocessor with the help of a clock produces pulses which are output through the port B of the Programmable Peripheral Interface (8255). This signal was fed to the switching circuit through a non inverting buffer. To run a motor both forward and reverse, an H-bridge circuit has been set that can reverse its polarity. The average voltage applied to the motor depends on the amount of time when the switch is ON with respect to the time when the switch is OFF (duty cycle). The control software was developed using assembly language and coded in 8086/8088 assembler. Experimental results were achieved using Light Emitting Diodes (LEDs) and a PM 3384 Autoranging Combiscope. The system possesses some advantages over the analog and digital motor controllers.

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