In this thesis, the effects of various chemical treatments on the surface modification of bulk InAs are investigated. The study focuses on the chemical processes that occur upon the exposure of the surface to sulphur-, chlorine- and bromine-containing solutions and oxygen, and the resulting changes to the electronic structure of the surface, as deduced from photoluminescence (PL) measurements, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Raman scattering and scanning electron microscopy (SEM). Three processing treatments were evaluated: i) treatment with sulphur-based solutions (Na2S:9H2O, (NH4)2S + S, [(NH4)2S / (NH4)2SO4] + S); ii) etching in halogen-based solutions (bromine-methanol and HCl: H2O); and iii) thermal oxidation. A significant overall enhancement in PL response was observed after chemical treatment or thermal oxidation, which is associated with a reduction in surface band bending. These changes correlate with the removal of the native oxide, in addition to the formation of well-ordered layers of In-S (or In-As)O as a passivating layer, indicating that electronic passivation occurs at the surface. The passivating effect on sulphide treated surfaces is unstable, however, with an increase in band bending, due to reoxidation, observed over periods of a few days. The lowest re-oxidation rate was observed for ([(NH4)2S / (NH4)2SO4] + S). Etching in HCl:H2O and Br-methanol solutions of appropriate concentrations and for moderate times (1 min) resulted in smooth and defect-free InAs surfaces. Etching completely removed the native oxides from the surface and enhanced the PL response. The adsorption of bromine and chlorine onto the InAs surface led to the formation of As-Brx , In-Brx, As-Clx and In-Clxcompounds (x = 1, 2, 3), as inferred from changes in the In 3d3/2; 5/2 and As 3d core level binding energies. The etch rate was found to decrease because of strong anisotropic effects. The improvements in surface properties were reversed, however, if the concentrations of the etchants increased or the etch time was too long. In the worst cases, pit formation and inverted pyramids with {111} side facets were observed. Surface treatments or thermal oxidisation significantly enhanced the PL intensity relative to that of the as-received samples. This was due to a reduction in the surface state density upon de-oxidation, or in some cases, to the formation of a well ordered oxide layer on the surface. The overall increase in PL intensity after treatment is ascribed to a reduction in band bending near the surface. This allows several welldefined peaks not observed or reported previously for bulk InAs (with a carrier concentration n~2x1016 cm-3), to be studied. A combination of PL and XPS measurements before and after the various treatments was used to identify the chemical nature of the impurities giving rise to bound exciton recombination in InAs (111).

Call number: LD2668 .T4 1963 W24 / Master of Science

No description available.

Environmental sustainability, strict Municipal bylaws, ever-increasing waste disposal / Thesis (M.Sc.) - University of KwaZulu-Natal, Pietermaritzburg, 2009.

The aim of this study was to develop an optimized process for the manufacturing of hexazinone and to validate laboratory results on a larger scale, in this case a miniplant. Patent protection on hexazinone has expired and the objective of this study was to improve on the known commercial process to enable the cost-effective manufacturing of hexazinone. The generic market is highly competitive and hexazinone can be sourced from several suppliers at low cost. Ownership of improved technology for the production of hexazinone can give Dow AgroSciences a competitive advantage in the generic market. Various routes for the manufacture of hexazinone are reviewed and important aspects of scale-up is considered and summarized. The known commercial five-step synthesis was optimized on a laboratory scale, using experimental design where applicable. The optimized procedures were scaled to the mini-plant. Significant improvements over patent results were realized for the process efficiencies. This is in particular true for the synthesis of N-ethoxycarbonyl-N,N,Ntrimethylguanidine in step 3. Employing experimental design to optimize the reaction was instrumental in meeting the objective of the study. Higher overall yields lead to substantial cost savings because of more efficient raw material conversion to value adding products. Die doel van hierdie studie was om die vervaardigingsproses vir heksasinoon te optimizeer. Die geldigheid van die laboratorium resultate is bevestig op ‘n groter skaal, in hierdie geval, op ‘n loodsaanleg met ‘n vyftig liter kapasiteit. Patente op die bekende vervaardigingsproses van heksasinoon het verval. Die mark vir generiese produkte is hoogs kompeterend en heksasinoon kan teen lae koste aangekoop word van verskeie vervaardigers. ‘n Betekenisvolle verbetering op die bestaande tegnologie kan aan Dow AgroSciences ‘n kompeterende voordeel besorg in die generiese heksasinoon mark. Die studie gee ‘n oorsig oor verskeie roetes vir die vervaardiging van heksasinoon en belangrike aspekte van opskalering. Die bekende vyf-stap sintese van heksasinoon is suksesvol geoptimiseer in die laboratorium met behulp van eksperimentele ontwerp. Die optimum kondisies is bevestig op ‘n loodsaanleg. Die gebruik van eksperimentele ontwerp om die vervaardigingsproses te optimiseer, het gelei tot belangrike verbetering in die tegnologie. Die totale opbrengs van die vyf-stap sintese oorskry gedokumenteerde waardes. Die geoptimiseerde proses kan lei tot betekenisvolle besparings deur die effektiewe omskakeling van uitgangstowwe na ‘n waardevolle produk. / Prof. F. Van Heerden

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A dissertation submitted to the Facultv of Engineering, University of the Witwatersrand Johannesburg, in fullilment of the requirements for the degree of Master of Science in Engineering. University of the Witwatersrand Johnnesburg, September 1996 / This research outlines a methodology for constructing chemical system models using unit operations as the building blocks. Order reduction of the nonlinear models produced is also addressed. A nonlinear analysis methodology based on bifurcation theory is presented. Bifurcation theory is introduced from an engineering perspective. Finally a nonlinear design technique is illustrated using a CSTR model. Throughout the dissertation, a simple example is used to illustrate the various metnodologies. / MT2017

Science is sometimes seen as contained in a little box on its own, separate from the rest of the world. The questions underpinning this thesis set in train thoughts about investigating the chosen topic, the teaching and learning of physical and chemical change, using a very wide range of methodologies. The perspective is of someone involved in the teaching of the physical sciences for more than 40 years. The topic still remains contentious in teaching chemistry.The thesis sets out the evidence for and against the teaching of physical and chemical change and tracks the historical origins of the concept, its changing definitions and the research relating to the concept of physical and chemical change. A number of historically based textbook case studies are presented, some of which the author considers to be pioneering. The study ends with consideration of the public understanding of science, wondering whether the concept of physical and chemical change can be helpful in this respect.The conclusions reached are not in themselves remarkable, expressing a belief, that overall the concept of physical and chemical change is of benefit to the chemical community and perhaps to the wider public.

Process control has been playing an increasingly important role in many industrial applications as an effective way to improve product quality, process costeffectiveness and safety. Simple linear dynamic models are used extensively in process control practice, but they are limited to the type of process behavior they can approximate. It is well-documented that simple nonlinear models can often provide much better approximations to process dynamics than linear models. It is evident that there is a potential of significant improvement of control quality through the implementation of the model-based control procedures. However, such control applications are still not widely implemented because mathematical process models in model-based control could be very difficult and expensive to obtain due to the complexity of those systems and poor understanding of the underlying physics. The main objective of this thesis is to develop new approaches to modeling and control of nonlinear processes. In this thesis, the multivariable nonlinear processes are approximated using a model with a static nonlinearity and a linear dynamics. In particular, the Hammerstein model structure, where the nonlinearity is on the input, is used. Cardinal spline functions are used to identify the multivariable input nonlinearity. Highlycoupled nonlinearity can also be identified due to flexibility and versatility of cardinal spline functions. An approach that can be used to identify both the nonlinearity and linear dynamics in a single step has been developed. The condition of persistent excitation has also been derived. Nonlinear control design approaches for the above models are then developed in this thesis based on: (1) a nonlinear compensator; (2) the extended internal model control (IMC); and (3) the model predictive control (MPC) framework. The concept of passivity is used to guarantee the stability of the closed-loop system of each of the approaches. In the nonlinear compensator approach, the passivity of the process is recovered using an appropriate static nonlinearity. The non-passive linear system is passified using a feedforward system, so that the passified overall system can be stabilized by a passive linear controller with the nonlinear compensator. In the extended IMC approach, dynamic inverses are used for both the input nonlinearity and linear dynamics. The concept of passive systems and the passivity-based stability conditions are used to obtain the invertible approximations of the subsystems and guarantee the stability of the nonlinear closed-loop system. In the MPC approach, a numerical inverse is implemented. The condition for which the numerical inversion is guaranteed to converge is derived. Based on these conditions, the input space in which the numerical inverse can be obtained is identified. This constitutes new constraints on the input space, in addition to the physical input constraints. The total input constraints are transformed into linear input constraints using polytopic descriptions and incorporated in the MPC design.

Understanding energy transfer processes is an essential prerequisite for the deep understanding of all chemical processes. This thesis investigates the process of vibrational relaxation (or deexcitation) of highly vibrationally and electronically excited molecular iodine (I2) induced by very low energy collisions in a supersonic free jet with six foreign gases. In an investigation of the state-to-field relaxation of I2 (B 3II+ou, v = 16) induced by collisions with He at temperatures of 2 to 12 K we find that the absolute relaxation rates are an order of magnitude smaller than those at 300 K and that the explanation of the magnitudes of these rates does not require enhancement due to low energy orbiting resonances. We find that the rates scale well with estimated collision encounter rates that account for the attractive part of the intermolecular potential. A second investigation with a much wider scope explores vibrational relaxation from v = 21 to 24 with six foreign gases: He, Ne, Ar, H2, D2 and N2. For this investigation a new type of experimental procedure has been designed and implemented that records a detailed and complete map of the fluorescence from B3II+ouI2 that is resolved with respect to both fluorescence frequency and time. These not only yield state-to-field rates, but coupled with a novel deconvolution method for growth curve fitting, yield absolute state-to-state rates for vibrational relaxation processes with Av=-1, -2, -3 and -4. The dependence of the relaxation rates on the collision partner, temperature and Av are discussed. An exponential dependence on the vibrational energy gap may be adequate to characterise the Av dependence of vibrational relaxation. The frequency resolution of the experimental data also reveals that some of the energy released by vibrational de-excitation is transferred to the rotation of the I2 molecule. We find this process is best characterised by an exponential dependence on the change of I2 angular momentum and that its extent scales with the mass of the collision partner. Measurements of the low-energy collision-induced quenching of B 3II+ouI2 are also reported for all six foreign gases. The possibility arises from the rates that the mechanism for quenching by H2 and D2 at low temperatures is different to that of the other gases and to that for H2 and D2 at high temperatures.