On the growth and characterisation of AIGaN alloys for optoelectronic applications
- Authors: James, Grant Robert
- Date: 2005
- Subjects: Gallium nitride -- Electric properties , Photoluminescence
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/8824 , vital:26433
- Description: In this study the growth and characterisation of undoped and Si-doped AlxGa1-xN has been performed. The layers were grown using low-pressure metalorganic vapour phase deposition (MOCVD) on sapphire substrates. The optical and electrical properties of the AlxGa1-xN layers were studied using variable temperature Hall effect and photoluminescence measurements. AlxGa1-xN layers were grown over the entire composition range. Room temperature ultraviolet (UV) transmission measurements showed that the material quality was very good for layers with an Al content, x, of 0 _ x _ 0.5. However, the quality of layers of higher composition was seen to rapidly decrease with increasing x. The electrical and optical properties of AlxGa1-xN with x < 0.5 were also good, comparable to those reported on in literature. The study of the Si-doping of AlxGa1-xN was performed in two parts; firstly a series of Al0.23Ga0.77N samples was grown in which the doping level was increased from zero to n _ 3 × 1018 cm-3. A similar, albeit a less rigorous, study was performed for Al0.41Ga0.59N and Al0.5Ga0.5N. A second series of samples was then grown in which the doping level was kept constant, while the Al content was incrementally increased. Room temperature Hall effect measurements performed on Si-doped Al0.23Ga0.77N showed that the electron concentration did not scale linearly with the silane flow, as was the case in GaN. It was also seen that the electron mobility of the layers increased with slight Si-doping, possibly due to an improvement in the crystalline quality and/or a change in the conduction mechanism. It was also found that at higher compositions (x = 0.41 and 0.50) an increase in the doping level resulted in an increase in the mobility. Variable temperature Hall effect and photoluminescence measurements, performed on the Al0.23Ga0.77N samples, revealed a good correlation between the first PL activation energy E1 and the donor activation energy ED, prompting the conclusion that the first PL recombination channel in AlxGa1-xN is due to the delocalisation of excitons bound at neutral Si donors. Furthermore, E1 and ED were seen to decrease with n1/3, as is the case for GaN and other semiconductor materials. It was also observed that strong exciton localisation occurs in slightly Si-doped material, with the amount of localization becoming less at higher doping levels. Possible mechanisms responsible for the second PL recombination channel of activation energy E2 were also proposed. The electrical and optical properties of the second set of AlxGa1-xN samples was then studied. The PL properties of undoped AlxGa1-xN were typical of a homogeneous alloy system, with the increase in the PL FWHM and exciton localisation energies with x following the trend predicted by alloy disorder theory. The variation of the band gap energy with the Al content could not, however, be fitted over the entire composition range using a single bowing parameter. It was proposed that this was due either to an effect of the 9 7 valence band crossover, or due to exciton localisation at alloy disorder and/or impurities. As was the case for GaN and Al0.23Ga0.77N, all undoped material was highly resistive. As was mentioned earlier, the exciton localisation energies increased according to alloy disorder theory in undoped AlxGa1-xN. In the doped samples, however, a large increase in the donor localisation energy was measured for x > 0.3. The possibility that Si could become a DX-centre in AlxGa1-xN was then investigated. However, Hall effect measurements showed that the Si activation energy increased in good agreement with the model of a shallow effective mass state donor, with no sudden increase in ED being observed up to x = 0.4. It was then suggested that the increase in the E1 and E2 activation energies, as well as the exciton localisation energies, could be due to the 9 7 valence band crossover, which occurs at roughly the same composition. However, due to the scarcity of reports on the valence band structure in AlxGa1-xN no conclusions could be made at this stage as to the effect of the 9 7 valence band crossover on the PL properties of AlxGa1-xN.
- Full Text:
- Date Issued: 2005
- Authors: James, Grant Robert
- Date: 2005
- Subjects: Gallium nitride -- Electric properties , Photoluminescence
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/8824 , vital:26433
- Description: In this study the growth and characterisation of undoped and Si-doped AlxGa1-xN has been performed. The layers were grown using low-pressure metalorganic vapour phase deposition (MOCVD) on sapphire substrates. The optical and electrical properties of the AlxGa1-xN layers were studied using variable temperature Hall effect and photoluminescence measurements. AlxGa1-xN layers were grown over the entire composition range. Room temperature ultraviolet (UV) transmission measurements showed that the material quality was very good for layers with an Al content, x, of 0 _ x _ 0.5. However, the quality of layers of higher composition was seen to rapidly decrease with increasing x. The electrical and optical properties of AlxGa1-xN with x < 0.5 were also good, comparable to those reported on in literature. The study of the Si-doping of AlxGa1-xN was performed in two parts; firstly a series of Al0.23Ga0.77N samples was grown in which the doping level was increased from zero to n _ 3 × 1018 cm-3. A similar, albeit a less rigorous, study was performed for Al0.41Ga0.59N and Al0.5Ga0.5N. A second series of samples was then grown in which the doping level was kept constant, while the Al content was incrementally increased. Room temperature Hall effect measurements performed on Si-doped Al0.23Ga0.77N showed that the electron concentration did not scale linearly with the silane flow, as was the case in GaN. It was also seen that the electron mobility of the layers increased with slight Si-doping, possibly due to an improvement in the crystalline quality and/or a change in the conduction mechanism. It was also found that at higher compositions (x = 0.41 and 0.50) an increase in the doping level resulted in an increase in the mobility. Variable temperature Hall effect and photoluminescence measurements, performed on the Al0.23Ga0.77N samples, revealed a good correlation between the first PL activation energy E1 and the donor activation energy ED, prompting the conclusion that the first PL recombination channel in AlxGa1-xN is due to the delocalisation of excitons bound at neutral Si donors. Furthermore, E1 and ED were seen to decrease with n1/3, as is the case for GaN and other semiconductor materials. It was also observed that strong exciton localisation occurs in slightly Si-doped material, with the amount of localization becoming less at higher doping levels. Possible mechanisms responsible for the second PL recombination channel of activation energy E2 were also proposed. The electrical and optical properties of the second set of AlxGa1-xN samples was then studied. The PL properties of undoped AlxGa1-xN were typical of a homogeneous alloy system, with the increase in the PL FWHM and exciton localisation energies with x following the trend predicted by alloy disorder theory. The variation of the band gap energy with the Al content could not, however, be fitted over the entire composition range using a single bowing parameter. It was proposed that this was due either to an effect of the 9 7 valence band crossover, or due to exciton localisation at alloy disorder and/or impurities. As was the case for GaN and Al0.23Ga0.77N, all undoped material was highly resistive. As was mentioned earlier, the exciton localisation energies increased according to alloy disorder theory in undoped AlxGa1-xN. In the doped samples, however, a large increase in the donor localisation energy was measured for x > 0.3. The possibility that Si could become a DX-centre in AlxGa1-xN was then investigated. However, Hall effect measurements showed that the Si activation energy increased in good agreement with the model of a shallow effective mass state donor, with no sudden increase in ED being observed up to x = 0.4. It was then suggested that the increase in the E1 and E2 activation energies, as well as the exciton localisation energies, could be due to the 9 7 valence band crossover, which occurs at roughly the same composition. However, due to the scarcity of reports on the valence band structure in AlxGa1-xN no conclusions could be made at this stage as to the effect of the 9 7 valence band crossover on the PL properties of AlxGa1-xN.
- Full Text:
- Date Issued: 2005
Metalorganic vapour phase epitaxial growth and characterisation of Sb-based semiconductors
- Authors: Vankova, Viera
- Date: 2005
- Subjects: Compound semiconductors , Epitaxy , Organometallic compounds , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10548 , http://hdl.handle.net/10948/d1019678
- Description: This study focuses on the growth and characterization of epitaxial InAs and InAs1-xSbx. Layers are grown on InAs, GaAs and GaSb substrates by metalorganic vapour phase epitaxy, using trimethylindium, trimethylantimony and arsine as precursors. The growth parameters (V/III ratio, Sb vapour phase compositions) are varied in the temperature range from 500 ºC to 700 ºC, in order to study the influence of these parameters on the structural, optical and electrical properties of the materials. The layers were assessed by X-ray diffraction, electron and optical microscopy, photoluminescence and Hall measurements. Furthermore, the influence of hydrogenation and annealing on the electrical and optical properties of GaSb was investigated. It is shown that the growth temperature and the V/III ratio play a vital role in the resulting surface morphology of homoepitaxial and heteroepitaxial InAs layers. Growth at low temperatures is found to promote three-dimensional growth in both cases, with improvements in the surface morphologies observed for higher growth temperatures. All the investigated epilayers are n-type. It is shown that the electrical properties of heteroepitaxial InAs epilayers are complicated by a competition between bulk conduction and conduction due to a surface accumulation and an interface layer. The low temperature photoluminescence spectra of homoepitaxial InAs are dominated by two transitions. These are identified as band-to-band/excitonic and donor-acceptor recombination. The incorporation efficiency of antimony (Sb) into InAs1-xSbx is dependent on the growth temperature and the V/III ratio. Under the growth conditions used in this study, the incorporation efficiency of Sb is controlled by the thermal stability of the two constituent binaries (i.e. InAs and InSb). Changes in the low temperature photoluminescence spectra are detected with increasing x. From temperature and laser power dependent measurements, the highest energy line is attributed to band-to-band/excitonic recombination, while the peak appearing approximately 15 meV below this line is assigned to donor-acceptor recombination. The origin of an additional “moving” peak observed for higher Sb mole fraction x is tentatively attributed to quasi-donor-acceptor-recombination, arising from increased impurity/defect concentrations and a higher compensation ratio in the material. However, the unusual behaviour of this peak may also be ascribed to the presence of some degree of ordering in InAsSb. The exposure of a semiconductor to a hydrogen plasma usually leads to the passivation of shallow and deep centres, thereby removing their electrical and optical activity. In this study, the passivation and thermal stability of the native acceptor in p-type GaSb is also investigated. It is shown that this acceptor can be passivated, where after improvements in the electrical and optical properties of GaSb are observed. Upon annealing the passivated samples above 300 °C, the acceptor is reactivated.
- Full Text:
- Date Issued: 2005
- Authors: Vankova, Viera
- Date: 2005
- Subjects: Compound semiconductors , Epitaxy , Organometallic compounds , Metal organic chemical vapor deposition
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10548 , http://hdl.handle.net/10948/d1019678
- Description: This study focuses on the growth and characterization of epitaxial InAs and InAs1-xSbx. Layers are grown on InAs, GaAs and GaSb substrates by metalorganic vapour phase epitaxy, using trimethylindium, trimethylantimony and arsine as precursors. The growth parameters (V/III ratio, Sb vapour phase compositions) are varied in the temperature range from 500 ºC to 700 ºC, in order to study the influence of these parameters on the structural, optical and electrical properties of the materials. The layers were assessed by X-ray diffraction, electron and optical microscopy, photoluminescence and Hall measurements. Furthermore, the influence of hydrogenation and annealing on the electrical and optical properties of GaSb was investigated. It is shown that the growth temperature and the V/III ratio play a vital role in the resulting surface morphology of homoepitaxial and heteroepitaxial InAs layers. Growth at low temperatures is found to promote three-dimensional growth in both cases, with improvements in the surface morphologies observed for higher growth temperatures. All the investigated epilayers are n-type. It is shown that the electrical properties of heteroepitaxial InAs epilayers are complicated by a competition between bulk conduction and conduction due to a surface accumulation and an interface layer. The low temperature photoluminescence spectra of homoepitaxial InAs are dominated by two transitions. These are identified as band-to-band/excitonic and donor-acceptor recombination. The incorporation efficiency of antimony (Sb) into InAs1-xSbx is dependent on the growth temperature and the V/III ratio. Under the growth conditions used in this study, the incorporation efficiency of Sb is controlled by the thermal stability of the two constituent binaries (i.e. InAs and InSb). Changes in the low temperature photoluminescence spectra are detected with increasing x. From temperature and laser power dependent measurements, the highest energy line is attributed to band-to-band/excitonic recombination, while the peak appearing approximately 15 meV below this line is assigned to donor-acceptor recombination. The origin of an additional “moving” peak observed for higher Sb mole fraction x is tentatively attributed to quasi-donor-acceptor-recombination, arising from increased impurity/defect concentrations and a higher compensation ratio in the material. However, the unusual behaviour of this peak may also be ascribed to the presence of some degree of ordering in InAsSb. The exposure of a semiconductor to a hydrogen plasma usually leads to the passivation of shallow and deep centres, thereby removing their electrical and optical activity. In this study, the passivation and thermal stability of the native acceptor in p-type GaSb is also investigated. It is shown that this acceptor can be passivated, where after improvements in the electrical and optical properties of GaSb are observed. Upon annealing the passivated samples above 300 °C, the acceptor is reactivated.
- Full Text:
- Date Issued: 2005
A commercial process development for plant food formulation using polyprotic acids from natural extracts as chelating agents
- Authors: Ndibewu, Peter Papoh
- Date: 2005
- Subjects: Chelates , Lemon juice , Liquid fertilizers
- Language: English
- Type: Thesis , Doctoral , DTech (Chemistry)
- Identifier: vital:10368 , http://hdl.handle.net/10948/153 , Chelates , Lemon juice , Liquid fertilizers
- Description: The citrus industry is one of South Africa's largest agricultural sectors in terms of export earnings with lemon fruits and juice as a trendsetter due to their high grade quality. According to growers, the Eastern Cape Province of South Africa alone produces an excess of about 10-14,000 tons of lemon juice which is presently of no economic value due to the sour taste and “bitterness”. As a result of this excess and in order to make use of the polyprotic acids naturally occurring in the lemon juice, four fertilizer nutrient mixtures are formulated, using lemon juice as base. From a conceptual scientific approach, characterization (physico-chemical and functional properties determinations) of Eureka Lemon fruit juices were undertaken, followed by smaller scale batch formulation experiments. On the basis that these lemon juice-based fertilizer mixtures are prepared following standard liquid fertilizer formulation guidelines, a field test was conducted to evaluate their potential effectiveness to influence plant growth. A growth chamber testing on tomato plants revealed high growth response (> 99.9 % certainty) potential in two of the semi-organic mixtures formulated while the organic mixture showed a relatively good growth rate as compared to the control (pure tap water). According to statistical analysis (ANOVA) comparison, two of the semi-organic mixtures performed considerably better than the two commercial samples evaluated. Potential benefits profoundly associated with these nutrient mixtures as compared to similar liquid fertilizer products on the market is that most nutrients are chelated and dissolved in solution. Also, the mixtures contain all necessary nutrients including plant growth substances required for healthier plant growth. The most important socioeconomic impact is the value addition to the technology chain in the citrus industry. The use of fluid fertilizers in significant quantities is less than twenty years old. Nevertheless, growth has been so rapid that in South Africa demand for mixed liquid fertilizer has greatly increased from 90 000 tons NPK & blended micronutrients in 1955 to more than 600 000 per annum tons today (Report 41/2003, Department of Minerals and Energy). The liquid fertilizers market is sparsely specialized with major competitors like Omnia, Kynoch and Foskor supplying more than 50 % of the market demand. Amongst the nutrient mixtures formulated, mixture one is an NPK (1-1-2) based nutrient mixture containing both secondary nutrients (0.5 % Mg & 1.0 % Ca) and seven micronutrients (0.1 % Fe, 0.05 % Cu, 0.05 % Zn, 0.05 % Mn, 0.02 % B, 0.0005 % Mo and 0.0005 % Co). The composition of this mixture offers the formula a potential to be used as a general purpose (all stages of plant growth) fertilization mixture in view of its balanced composition (containing all essential plant nutrients). Mixture two contains essentially the micronutrients and in higher concentrations (0.3 % Fe, 0.3 % Cu, 0.1 % Zn, 0.2 % Mn, 0.02 % B, 0.0005 % Mo and 0.0005 % Co) as compared to mixture one except for boron, molybdenum and cobalt. The concentration of the micronutrients contained in this mixture is adequately high which offers a potential for it to be used in supplementing nutrition in plants with critical micronutrient-deficient symptoms. Mixture three is very similar to mixture two (1.0 % Fe, 0.05 % Cu, 0.05 % Zn, 0.05 Mn, 0.05 % B, 0.0005 % Mo and 0.0005 % Co) except that the concentrations of all seven micronutrients are considerably less than those of contained in mixture two. However, the concentration of iron in this mixture is as high as 1.0 %. The mixture has a potential to be used in high iron-deficient situations. Mixture four is an organic formula with relatively low nutrient concentrations (NPK-0.02-0.02-1, 0.27 % Mg, 0.02 % Ca, 0.008 % Fe, 0.26 % Cu, 0.012 % Zn, 0.009 % Mn). Nevertheless, this mixture is appealing for organically grown crops where the use of chemicals is prohibited by standards. These lemon juice-based nutrient mixtures were further characterized and tested for stability and storability over a period of eight weeks. This study revealed no major change in the physical quality (colour, pH and “salt out” effect). The basic formulation methodology is a two-step procedure that involves filtration of the lemon juice to remove membranous materials, mixing at ambient temperature and stabilization of the nutrient mixtures. However, for the organic nutrient formula mix, filtration follows after extraction of nutrients from plant materials using the lemon juice.
- Full Text:
- Date Issued: 2005
- Authors: Ndibewu, Peter Papoh
- Date: 2005
- Subjects: Chelates , Lemon juice , Liquid fertilizers
- Language: English
- Type: Thesis , Doctoral , DTech (Chemistry)
- Identifier: vital:10368 , http://hdl.handle.net/10948/153 , Chelates , Lemon juice , Liquid fertilizers
- Description: The citrus industry is one of South Africa's largest agricultural sectors in terms of export earnings with lemon fruits and juice as a trendsetter due to their high grade quality. According to growers, the Eastern Cape Province of South Africa alone produces an excess of about 10-14,000 tons of lemon juice which is presently of no economic value due to the sour taste and “bitterness”. As a result of this excess and in order to make use of the polyprotic acids naturally occurring in the lemon juice, four fertilizer nutrient mixtures are formulated, using lemon juice as base. From a conceptual scientific approach, characterization (physico-chemical and functional properties determinations) of Eureka Lemon fruit juices were undertaken, followed by smaller scale batch formulation experiments. On the basis that these lemon juice-based fertilizer mixtures are prepared following standard liquid fertilizer formulation guidelines, a field test was conducted to evaluate their potential effectiveness to influence plant growth. A growth chamber testing on tomato plants revealed high growth response (> 99.9 % certainty) potential in two of the semi-organic mixtures formulated while the organic mixture showed a relatively good growth rate as compared to the control (pure tap water). According to statistical analysis (ANOVA) comparison, two of the semi-organic mixtures performed considerably better than the two commercial samples evaluated. Potential benefits profoundly associated with these nutrient mixtures as compared to similar liquid fertilizer products on the market is that most nutrients are chelated and dissolved in solution. Also, the mixtures contain all necessary nutrients including plant growth substances required for healthier plant growth. The most important socioeconomic impact is the value addition to the technology chain in the citrus industry. The use of fluid fertilizers in significant quantities is less than twenty years old. Nevertheless, growth has been so rapid that in South Africa demand for mixed liquid fertilizer has greatly increased from 90 000 tons NPK & blended micronutrients in 1955 to more than 600 000 per annum tons today (Report 41/2003, Department of Minerals and Energy). The liquid fertilizers market is sparsely specialized with major competitors like Omnia, Kynoch and Foskor supplying more than 50 % of the market demand. Amongst the nutrient mixtures formulated, mixture one is an NPK (1-1-2) based nutrient mixture containing both secondary nutrients (0.5 % Mg & 1.0 % Ca) and seven micronutrients (0.1 % Fe, 0.05 % Cu, 0.05 % Zn, 0.05 % Mn, 0.02 % B, 0.0005 % Mo and 0.0005 % Co). The composition of this mixture offers the formula a potential to be used as a general purpose (all stages of plant growth) fertilization mixture in view of its balanced composition (containing all essential plant nutrients). Mixture two contains essentially the micronutrients and in higher concentrations (0.3 % Fe, 0.3 % Cu, 0.1 % Zn, 0.2 % Mn, 0.02 % B, 0.0005 % Mo and 0.0005 % Co) as compared to mixture one except for boron, molybdenum and cobalt. The concentration of the micronutrients contained in this mixture is adequately high which offers a potential for it to be used in supplementing nutrition in plants with critical micronutrient-deficient symptoms. Mixture three is very similar to mixture two (1.0 % Fe, 0.05 % Cu, 0.05 % Zn, 0.05 Mn, 0.05 % B, 0.0005 % Mo and 0.0005 % Co) except that the concentrations of all seven micronutrients are considerably less than those of contained in mixture two. However, the concentration of iron in this mixture is as high as 1.0 %. The mixture has a potential to be used in high iron-deficient situations. Mixture four is an organic formula with relatively low nutrient concentrations (NPK-0.02-0.02-1, 0.27 % Mg, 0.02 % Ca, 0.008 % Fe, 0.26 % Cu, 0.012 % Zn, 0.009 % Mn). Nevertheless, this mixture is appealing for organically grown crops where the use of chemicals is prohibited by standards. These lemon juice-based nutrient mixtures were further characterized and tested for stability and storability over a period of eight weeks. This study revealed no major change in the physical quality (colour, pH and “salt out” effect). The basic formulation methodology is a two-step procedure that involves filtration of the lemon juice to remove membranous materials, mixing at ambient temperature and stabilization of the nutrient mixtures. However, for the organic nutrient formula mix, filtration follows after extraction of nutrients from plant materials using the lemon juice.
- Full Text:
- Date Issued: 2005
The development of an electrochemical process for the production of para-substituted di-hydroxy benzenes
- Authors: Rautenbach, Daniel
- Date: 2005
- Subjects: Phenol , Benzene
- Language: English
- Type: Thesis , Doctoral , DTech (Science)
- Identifier: vital:10367 , http://hdl.handle.net/10948/159 , Phenol , Benzene
- Description: The project was concerned with the investigation of the electrochemical oxidation of various phenols, and to develop a viable reaction system for the production of the respective hydroquinones. Current production routes utilizing phenol as starting material have the limitations of using large amounts of acids, having to be stopped at low conversions and producing a mixture of the hydroquinones and catechols. Of the possible routes to the respective hydroquinones from the various phenols, the electrochemical oxidation of these phenols offers commercial and environmental advantages and hence formed the theme of the investigation. The synthetic possibilities proved to be more prevalent in a system when the electrochemical oxidation of these phenols was performed in an aqueous medium utilizing an organic co-solvent. Results obtained during this investigation made it possible to make certain predications about the mechanism taking place. This was found to depend on the anode material used for the oxidation. The results showed that the process developed for the electrochemical oxidation of these phenols, yields mainly the para-isomers of the respective hydroquinones and benzoquinones in good yields and selectivities, with fair current efficiencies and good mass balances at high conversions. For example: * Phenol (batch) 8 F: 345% current efficiency, 70% hydroquinone, 6% catechol, 9% benzoquinone, 9% phenol and 94 mass balance. * 2-Tert-butylphenol (flow) 10 F: 37% current efficiency, 65% hydroquinone, 33% benzoquinone, 2% phenol and 100% mass balance. * 2,6-Di-tert-butylphenol (flow) 11 F: 23% current efficiency, 92 % hydroquinone, 6% benzoquinone, 1% phenol and 99% mass balance. The developed electrochemical oxidation system offers the following advantages over previous and current methods: simplified isolation and extraction procedures, smaller amounts of acid usage, reasonably selective synthesis of the para-isomer and a less corrosive system all at high conversions.
- Full Text:
- Date Issued: 2005
- Authors: Rautenbach, Daniel
- Date: 2005
- Subjects: Phenol , Benzene
- Language: English
- Type: Thesis , Doctoral , DTech (Science)
- Identifier: vital:10367 , http://hdl.handle.net/10948/159 , Phenol , Benzene
- Description: The project was concerned with the investigation of the electrochemical oxidation of various phenols, and to develop a viable reaction system for the production of the respective hydroquinones. Current production routes utilizing phenol as starting material have the limitations of using large amounts of acids, having to be stopped at low conversions and producing a mixture of the hydroquinones and catechols. Of the possible routes to the respective hydroquinones from the various phenols, the electrochemical oxidation of these phenols offers commercial and environmental advantages and hence formed the theme of the investigation. The synthetic possibilities proved to be more prevalent in a system when the electrochemical oxidation of these phenols was performed in an aqueous medium utilizing an organic co-solvent. Results obtained during this investigation made it possible to make certain predications about the mechanism taking place. This was found to depend on the anode material used for the oxidation. The results showed that the process developed for the electrochemical oxidation of these phenols, yields mainly the para-isomers of the respective hydroquinones and benzoquinones in good yields and selectivities, with fair current efficiencies and good mass balances at high conversions. For example: * Phenol (batch) 8 F: 345% current efficiency, 70% hydroquinone, 6% catechol, 9% benzoquinone, 9% phenol and 94 mass balance. * 2-Tert-butylphenol (flow) 10 F: 37% current efficiency, 65% hydroquinone, 33% benzoquinone, 2% phenol and 100% mass balance. * 2,6-Di-tert-butylphenol (flow) 11 F: 23% current efficiency, 92 % hydroquinone, 6% benzoquinone, 1% phenol and 99% mass balance. The developed electrochemical oxidation system offers the following advantages over previous and current methods: simplified isolation and extraction procedures, smaller amounts of acid usage, reasonably selective synthesis of the para-isomer and a less corrosive system all at high conversions.
- Full Text:
- Date Issued: 2005
- «
- ‹
- 1
- ›
- »