B. O. Aduda

Effect of morphology on electron drift mobility in porous TiO2

Porous titanium dioxide is an attractive material for solar cell application on account of its stability, electron transport properties, and the possibilities for controlling surface morphology as well as for its ease of fabrication and low cost. Nanostructured TiO2 has been intensively studied for applications to dye sensitised solar cells. The performance of the titanium dioxide based solar cells is influenced, among other factors, by the electron mobility of the porous titanium dioxide. Different fabrication processes for porous titanium films result in different film morphology, which in turn affects the electron transport. We have employed three different techniques namely, electrostatic spray assisted vapour deposition (ESAVD), D.C. reactive sputtering, and doctor blading of sol-gel dispersions to deposit thin TiO2 films onto indium tin oxide (ITO) coated glass substrates. All these films exhibited only the anatase phase as confirmed by X-ray diffraction analysis. Using the time-of-flight technique, the electron drift mobility in the porous TiO2 films was measured. The results show that in the low field region (< 55,000 V cm −1 ) the mobility, in all the films, were in the range of 10−7 to 10−6 cm2 Vs−1. The drift mobility in the films prepared by reactive sputtering was consistently higher than in the films prepared by the two other techniques. Sputter deposited films had lower porosity (∼ 10% and 36% for normal-, and oblique (60◦)-angle deposited films) compared to ∼ 50% for films deposited by the two other techniques. The relationship between the drift mobility and film morphology is discussed with the aid of scanning electron microscopy studies.

Effective thermal conductivity of loose particulate systems

The effective thermal conductivity for several loose particulate insulation systems has been measured in the temperature range from 273 K–900 K and the results compared to those predicted from three different models. The measured thermal conductivities increase with temperature. This is accounted for in terms of increased conduction by the fluid (air) and the radiative heat transfer through the media although the latter mode of heat transfer is relatively suppressed in materials containing finer particles. The model due to Zumbrunnen et al. [1] was found to predict values that closely agreed with the experimental values.

Photoactive iron pyrite films for photoelectrochemical (PEC) cells

Photoactive iron pyrite (FeS2) films have been grown by sulfurization of sprayed iron oxide films. The structural properties were studied using X-ray diffraction. The films showed good crystallinity with grain size ≈500 A. These films have been used as photoanode in photoelectrochemical cells.

Thermal conductivity of a kaolinite refractory: effect of a plant-derived organic binder

The effect of corchorus olitorius derived binder on the effective thermal conductivity of a kaolinite-based refractory was investigated. Strong dependence of (effective) thermal conductivity of fired samples on the binder concentration, temperature and porosity was noted. Comparison of experimental data with Effective Medium Approximation (EMA) and Geometric Mean Model (GMM) theories showed that predictions from EMA agreed better with the experimental data than those from GMM. This was attributed to the EMA model being more rigorous and contained more microstructural information than the simpler GMM.

Spectral Analysis of Acousto-Ultrasonic Waves for Defect Sizing

A study of the feasibility of employing acousto-ultrasonic (AU) testing to size defects has been carried out using a model system consisting of a single defect in a glass matrix. A through-transmission mode was used for the AU testing and the output signals were analysed by a counting technique (cumulative ringdown count, CRDC) and spectral analysis. The CRDC decreased with increasing defect size but the scatter in the data did not allow accurate sizing. The frequency spectra exhibited multiple peaks which occurred at constant frequency intervals. The frequency interval decreased with increasing defect size and this could be attributed to either multiple time delay or a resonance effect or to both phenomena. Most evidence supported the resonance effect and good agreement was found between defect size calculated from AU data using a resonance theory and that measured optically.

An acousto-ultrasonic study of the effect of porosity on a sintered glass system

An assessment of the applicability of an acousto-ultrasonic (AU) technique for the monitoring of porous ceramic systems has been carried out. Sintered glass was used as a model system and it was found that the AU parameters, such as normalized ringdown count, normalized pulse width, velocity and frequency interval (Δf) between adjacent peaks in the frequency spectra, decrease with increasing porosity. The porosity dependence of the normalized AU parameters has been attributed to attenuation which analysis showed depended on the pore size and content. The velocity and frequency interval changes also depended on pore content but, unlike the normalized parameters, were found to be sensitive to pore shape and size. The decreasing Δf with increasing porosity was explained in terms of the longer path lengths traversed by the waves in the higher pore-density samples.

An Investigation of Thermal Shock in Porous Clay Ceramics

The thermal shock resistance of porous ceramic materials is often characterized by the Hasselman parameters. However, in other scenarios, the room-temperature residual strengths after thermal shock are also used to quantify the damage due to thermal shock. This paper attempts to link the measured residual strengths to the dominant crack features that are introduced due to thermal shock in porous clay ceramics produced by the sintering of clay powders with well-controlled size ranges. Residual strength estimates from bend tests are compared with fracture mechanics predictions. The implications of the residual strength results are then discussed for the characterization of damage due to thermal shock.

Effect of plant-derived organic binders on fracture toughness and fatigue of kaolin-based refractories

The fracture properties of kaolin-based refractories prepared using plant-derived binders from okra and “mrenda” have been investigated and compared. It was observed that the MOR of fired samples improved from 37.5 ± 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 fracture toughness increased from 3.9 ± 0.1 MPa (for binder-free samples) to 5.6 ± 0.1 and 5.7 ± 0.1 MPa for okra and ‘mrenda’-plasticized samples, respectively. It is concluded that the use of organic binders enhances the reliability and service life of kaolin refractories used in thermally fluctuating environments.

Ultrasonic attenuation in Kenyan clay refractories

Ultrasonic attenuation in clay refractories fabricated under different processing parameters has been measured. It was established that refractory clay samples with high compaction efficiencies (CE) were less attenuating compared with those with low CE values. CE was found to be dependent on the particle size distribution of the starting powders and for a continuous particle size distribution, CE could be obtained from the slope of the compact density v. log (pressure) curves.

Emphasis on Photovoltaic (PV) Solar System Installation Training: A Case Study of a PV Solar System Installed in Makueni County, Kenya

The installation of Photovoltaic (PV) solar systems in institutions as well as homesteads in the rural areas in Kenya is increasing at a high rate; and so is the need for the stake holders to make sure the PV solar systems are professionally designed, sized, installed and maintained. In PV solar system installation, the designing, sizing and the installation are very critical steps. A wrongly designed, sized and installed system will not perform optimally and will underperform (for undersized systems) and waste energy and resources (for oversized systems). Furthermore, undersized systems do not perform to the users expectation discouraging the user and eventually a negative customer attitude creeps in which may affect the uptake of solar PV systems. On the other hand, an oversized PV system is extra expense on the side of the client, creating an exaggerated high cost of PV solar systems, again discouraging potential clients from the adoption of the technology. Both scenarios mean loss of business, jobs and the economic and social benefits associated with PV technology. We present a case study of poorly installed PV systems in Makueni County, Kenya. We observed that the solar modules specifications at the back of the modules were not clearly done, the batteries were poorly matched and the cables used in the installation were undersized. Due to these issues, even a normal television set was not able to work since the system was installed four years ago (in 2012). The above case emphasizes the need for training in PV solar system design, sizing, installation, and maintenance.