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In this thesis, we will discuss that GaN template which was grown on the sapphire by metal organic vapour phase epitaxy (MOVPE). Then GaN epi-layer, intrinsic AlGaN (as spacer) and N-type AlGaN(doping Si) which offers carrier grow by molecular-beam epitaxy. We changed the content of aluminium to find out any difference on AlGaN/GaN heterostructure . For our experimental, we tried our best to keep all the parameters in steady besides the vapor of aluminum. If the vapor of aluminum every 4.65*10-9 torr is set to be one unit, then ratio of the pressure from sample A to E is 0.5, 1.0, 1.5, 2 and 3. We can get the best growth parameters by hall measurement, reflection high-energy electron diffraction, scanning electron microscope, atomic force microscopic and X-ray diffraction analysis to improve the quality of the sample. From scanning electron microscope and reflecting high energy electronic diffraction picture, the roughness of all samples is good which confirms that the samples should be in two-dimensional (2D) growth mode. We can find the same result by atom force microscope. After comparing 3D picture, we find out the surface of Sample B is the smoothest, meanwhile the roughness is 1.404 (nm) has been calculated through the functions. Due to Hall measurement in 77 K, the electron mobility of this series of samples are very high, especially Sample C is 10995(cm2/Vs) and sample D is 10697 (cm2/Vs) respectively. Quite narrow Full Width at Half Maximum(FWHM) of AlGaN which is about only 300 (arcsec) has been showed under the analysis of X-ray in rocking curve mode and these results indicate these samples have extraordinary qualities better than previous one.

A greenhouse study was conducted to determine the effects of deposition from static rocket test fires on corn and alfalfa. Seeds were germinated in a wide concentration range of depositional material, called test fire soil (TFS). Additionally, the impact of chloride and aluminum, two major components of test fire soil, on germination was also evaluated. Furthermore, plants were grown in packed columns and exposed to test fire soil, either in the root zone or on foliage. Tissue was weighed and analyzed to compare biomass production and plant composition. Corn and alfalfa exposed to test fire soil in the root zone produced less biomass than controls, but foliar treatment had no effect on biomass production. No kernels were produced by corn exposed to test fire soil in the root zone. Leaves of plants exposed to test fire soil in the root zone accumulated more metals and nutrients than controls, whereas plant tissue treated with test fire soil on the leaves contained only elevated levels of aluminum, although levels were still within reasonable concentrations for plants. Germination of seeds was not affected below 1% test fire soil in soil; however higher concentrations of test fire soil decreased percent germination. Addition of chloride to soil also inhibits germination, but addition of aluminum has no effect on germination percentage. Corn germination was restored in test fire soil leached with 200 mm artificial rainwater. The results of this research contribute information regarding the potential impact of test fire soil from static test fires on crop production. Test fire soil inhibits germination and growth if deposited in the root zone, and even foliar application alters tissue composition. However, plant composition is not altered significantly in terms of feed criteria, and germination can be restored by irrigating the TFS. The effects of test fire soil are attributed to high levels of chloride that induce salt stress. Crop damage may be avoided by conducting static test fires after crops are harvested or providing extra irrigation to soil impacted with the TFS.

This study was undertaken to develop a fast and reliable nutrient solution technique to screen grain sorghum genotypes for aluminum (Al) tolerance, and to study the gene system controlling the inheritance of Al tolerance in grain sorghum. Twenty-five sorghum genotypes representing a wide range of environmental adaptation were grown in tanks (120 seedlings/tank, approximately 303.0 ml/seedling) of nutrient solution containing 148 uM liter⁻¹ Al. Relative root lengths (RRL) as well as visual symptoms of injury on roots and leaves were the parameters used in evaluating differential Al response of sorghum genotypes. A highly significant negative correlation coefficient was found between RRL and visual symptoms on roots (r = -0.96). Eleven out of the 25 genotypes behaved as Al tolerant and the remaining behaved as Al sensitive. A half-diallel cross involving three Al-tolerant and three Al-sensitive genotypes as identified through the screening test were used to study the gene system controlling the inheritance of Al tolerance in grain sorghum. The F₁ and parents data were analyzed using both Griffing's and Jinks-Hayman methods of diallel analyses. Highly significant GCA and SCA effects were observed for the Al-tolerance trait. The GCA effects were much more important than SCA effects, with the ratio of GCA:SCA being 9:1. From the Vr, Wr graphs the Al-tolerance trait showed predominantly additive genetic effects with some degree of dominance. The six parents fell into four groups according to the relative level of dominance: (1) highly dominant (AR 3010, CI 182); (2) moderately dominant (NB 9040); (3) moderately recessive (Texas Blackhull); and (4) highly recessive (AR 3001 and AR 3006). The degree of dominance was observed to be partial. F₂ populations screened for Al tolerance showed two distinct classes of tolerance with segregating ratios of tolerant:sensitive seedlings of 3:1. This segregation indicated that the Al-tolerant trait was simply inherited, but a wide range of tolerance for Al observed among the 25 genotypes suggested a more complex gene system. Heritability of Al tolerance was very high. The narrow sense and broad sense heritabilities were 77.69% and 99.54%, respectively. These results suggested that a breeding method emphasizing additive gene effects would be favorable in developing Al-tolerant inbred lines.

This thesis reports two new methods developed to study the free ion activity of aluminum and copper. Both methods could be applied to the study of other metals. The first method measures the apparent solubility of aluminum and sulfate in a dynamic, leached system. This system is believed to give a good representation of the field situation where soils are continuously leached and never at equilibrium. This study was done under three sulfate concentrations. The part of the experiment using low sulfate concentrations showed aluminum solubility control by a gibbsite-like solid phase with a log K$ sp circ$ of 7.49. It was also shown, however, that under natural soil solution concentrations of sulfate in acidic forest soils, an interaction with sulfate controls aluminum activity rather than gibbsite solubility equilibria. This interaction is either, an aluminum-sulfate solid phase or, a stoichiometric ion-pair co-adsorption of aluminum and sulfate. / The second method is a simple determination of free Cu$ sp{2+}$ in soils using a cupric ion-selective electrode. Free copper has been demonstrated to be the toxicity controlling component in aquatic studies and this study was undertaken to measure free copper activity in soils. The possible interference due to ionic strength variations or the presence of aluminum in the soil solution was checked and found to be negligible. The free activity of copper (pCu$ sp{2+}$) measured in a variety of pristine and contaminated soils varied between 6.33 to 12.20 pCu$ sp{2+}$ units. Total soil copper content and acidity were shown to strongly increase copper solubility and free Cu$ sp{2+}$ activity in the soil solution extracts. (Abstract shortened by UMI.)

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The hydrolysis of aluminum in synthetic cation exchange resins differing in charge density was evaluated after treating aluminum and potassium saturated resins with solutions of hydrolyzed aluminum. Fixed aluminum and decrease in exchange capacity were used to calculate the OH/Al ratio of the fixed aluminum. Oowex 50 XI, the resin with lowest charge density, was found to have fixed aluminum with the highest OH/Al ratio. The highest charge density resin tested, Dowex 50 X8, was found to have a limited decrease in exchange capacity. Aluminum was exchanged from potassium saturated resins of low charge density in excess of available negative charge, yielding the conclusion that hydrolyzed forms of aluminum were exchangeable. Purified dioctahedral vermiculite from a Nason soil was expanded using successive treatments of N BaCl₂, reducing the potassium level In the sample to 0.05 percent. Tota] analysis yielded the average formula (Al_1.4Mg_.3Fe_.3)(Si_3.7Al_.3O₁₀ (0H)₂^-.70 -X^{+. 70} 8H₂0. This structural formula suggested that the lowering of the charge from the theoretical one per half cell to 0.7 was accomplished by oxidation of the iron. This was concluded to be the mechanism initiating the weathering of this type of micaceous material to dloctahedraJ vermiculite. Structural analyses of both magnesium (13.98 A) and aluminum (14.91 A) saturated dioctahedral vermiculite were carried out using structure factor calculations, one dimensional fourler synthesis, and difference synthesis. Structure factor calculation for the dioctahedral vermiculite studied indicate that the ratio of the intensities of diffraction maxima corresponding to 7 A and 14 A interplanar spacings (7A/14A ratio) should decrease as scattering matter Is added to the interlayer region through hydrolysis of alumininum. Fourier synthesis and difference synthesis indicate that scattering matter was centrally located In the Interlayer in excess of what could be accounted for by exchangeable cations present. It was concluded that a portion of the water was centrally located. A close approach between the remainder of the water and the basal oxygens was the assumption required to explain the observed intensities. The aluminum saturated dloctahedral vermiculite (14.91 A) was observed to collapse in a random interstratified fashion upon hydrolysis treatments with solutions of hydrolyzed aluminum. Upon potassium saturation, the collapse progressed, yielding a 14 A material. Orientation of interlayer water associated with triple charged aluminum ion (Al⁺) was theorized to be the cause of the stable high spacing and the hydrolysis and/or removal of the triple charge ion thought to result in the diminishing spacing. Both the experiments with synthetic cation exchange resins and the structural analysis of soil-derived vermiculite Indicate that the amount of aluminum that can be added through hydrolysis to the higher charged resins and to dioctahedral vermiculite was limited. It was theorized that hydrolyzed aluminum units could grow in association with a negatively charged surface until the diminishing charge density of the growing hydrolyzed aluminum unit approached that of the associated surface. / Ph. D.

Kinetics and mechanisms of Al adsorption on Ca-saturated kaolinite, montmorillonite, and peat were studied using miscible displacement methods. Unneutralized and partially neutralized Al solutions (OH/Al=1.0) were leached through monomineralic columns and effluent analyzed for monomeric and polymeric Al, titratable acidity, and desorbed Ca as a function of time. Observed breakthrough curves (BTC) for continuous injection of the various Al solutions through kaolinite suggested the existence of at least two mechanisms for Al adsorption. It was shown that a relatively rapid reaction, where solution Al was in equilibrium with the solid phase, involved Ca-Al exchange and a slower reaction involved Al polymerization on the adsorbent. This scenario was supported by rigorous modeling of observed BTC with nonequilibrium transport theory, analysis of desorbed Ca effluent, and potentiometric titrations of effluent solutions. The occurrence of two mechanisms for Al adsorption on montmorillonite and peat was also evident. Polymeric Al was preferentially adsorbed by kaolinite throughout the displacement experiment whereas montmorillonite preferentially adsorbed monomers at short times and polymers at longer times. Selective adsorption of either form of Al on peat was not evident. In order to accurately distinguish between monomeric and polymeric Al in solution and on solids, a method was developed which evaluates the interaction of ferron with Al. The colorimetric complexation of aqueous Al with ferron was largely described by a binary species first-order rate function. Based on the differential kinetic reactions of ferron with monomeric and polymeric Al, the fraction of the former species was well predicted using the binary species model. The method avoids arbitrary separation times for the distinction of monomeric and polymeric Al since the duration of monomer interactions with ferron depend on the initial basicity of the Al solution. / Ph. D.

Abstract available in the print copy. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.