Maria Atanasova | University of Pretoria (original) (raw)

Papers by Maria Atanasova

An entry from the Inorganic Crystal Structure Database, the world's repository for inorganic ... more An entry from the Inorganic Crystal Structure Database, the world's repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Southern African Journal of Epidemiology and Infection, 2010

Research Square (Research Square), Apr 23, 2024

Journal of Thermal Analysis and Calorimetry

This manuscript reports the investigation of the polymorphic behaviour of fipronil using a system... more This manuscript reports the investigation of the polymorphic behaviour of fipronil using a systematic comparison of the thermochemical and structural properties of different crystal forms obtained in this study as well as those previously reported in literature. The analytical techniques employed include DSC, TGA, PXRD, SCXRD and hot stage microscopy. DSC proved particularly useful because it made it possible to differentiate between the two different crystal forms found in the as-received neat fipronil. The DSC scans revealed the presence of two polymorphs which had melting endotherms with peak maxima at ca. 196 °C and 205 °C, respectively. These polymorphs were successfully separated via sublimation and resulted in a metastable, lower melting polymorph in the sublimate and a thermodynamically stable, higher melting form in the sublimation residue. Clear evidence for the instability of the lower melting polymorph was found when the endotherms were examined under a range of heating ...

Applied Clay Science, 2015

Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminium or i... more Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminium or iron(III) ions does not affect the exfoliation onset temperature (456  11 C). However, co-intercalation of minor amounts of urea lowers the exfoliation onset temperature to 209  35 C. This significant decrease is attributed to the low thermal stability of urea which releases ammonia and water as blowing agents. Thermo-mechanical analysis indicates that the expanded vermiculite exhibits spring-like properties. Part of the original expansion is recovered when the applied force is removed. The observed hysteresis is attributed to the progressive flattening of warped/buckled sheets that make up the expanded worm-like structure. Provided the expanded flake has not been heated beyond 450 C, it can be reconstituted by applying a sufficiently high compressive force followed by equilibration with distilled water. Such flakes show the conventional exfoliation behaviour when heated again.

The South African electricity producer, Eskom, generated 36.01 million tons of coalcombustion fly... more The South African electricity producer, Eskom, generated 36.01 million tons of coalcombustion fly ash (FA) in 2010. About 5.6% were reused for the production of cement. The remaining 33.89 million tons were safely disposed of and managed on Eskom ash dumps and dams which are located adjacent to their corresponding power stations. South Africa has a long history regarding the development of new applications for this material. For instance, Ash Resources, South Africa’s leading manufacturer and supplier of fly ash products, has pioneered the re-use of FA and has positioned itself as one of the world leaders in ash technology over the last 30 years. South Africa remains very active in the development of ash technologies. Concurrently, the power industry is also a major carbon dioxide (CO2) emitter, with Eskom’s emissions approximating 225 million tons for 2010.

Organo-bentonites containing quaternary ammonium surfactants were 5 prepared using purified bento... more Organo-bentonites containing quaternary ammonium surfactants were 5 prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation 6 was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash 7 activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 8 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt 9 angles of 27 o to 33 o; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain 10 surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the 11 single chain surfactant. However, FTIR analysis revealed disordered alkyl chain 12 conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic 13 basis) featured high viscosities. They showed strong shear thinning behaviour confirming the 14 ability of the montmorillonite particles to exfoliate. 15

Since 2002, acid mine drainage (AMD) has discharged from the West Rand Gold3eld of the Witwatersr... more Since 2002, acid mine drainage (AMD) has discharged from the West Rand Gold3eld of the Witwatersrand into nearby streams and through subsurface 4ow into the adjacent dolomitic aquifer. Surface discharge has also been shown to recharge the dolomitic aquifer. This has led to fears of increased dissolution of the dolomite which, it has been proposed, could lead to land subsidence in the a2ected area. Laboratory investigations show that precipitation of resistant mineral coatings on reactive carbonate surfaces limit the potential for dissolution of dolomite and the potential for natural attenuation.

Metal stearate blends are potential lubricants and filler dispersants in polymer compounding. The... more Metal stearate blends are potential lubricants and filler dispersants in polymer compounding. Therefore, the thermal phase behaviour of binary blends of calcium stearate and magnesium stearate with zinc stearate was investigated. Cooling curve studies indicated lower melting temperatures in both systems. X-ray diffraction studies revealed that this was due to the formation of solid solutions of variable composition rather than to eutectic formation. Rheology measurements showed that true fluidification only happened well above the apparent melting points established from cooling curves. Calcium carbonate filled polyethylene containing metal stearate mixtures showed lower melt viscosities than were achievable using only calcium- or magnesium stearate as the lubricant. These results suggest that metal stearate blends, in combination with a judiciously selected wax component, may offer advantages in lowering compound melt viscosity and facilitating better particle dispersion.

The aim of this study was to conduct a comparative analysis using remote sensing and conventional... more The aim of this study was to conduct a comparative analysis using remote sensing and conventional sample analysis to assess asbestos pollution in rehabilitated former asbestos mining areas. The study focused on the Mafefe and Mathabatha areas of Limpopo Province, South Africa. Field-based remote sensing techniques were used to spectrally differentiate various types of asbestos minerals in order to determine their efficacy in assessing asbestos pollution. X-ray diffraction and scanning electron microscopy were employed for the identification and characterisation of traces of asbestos minerals in soil and water samples collected from the rehabilitated areas. The samples were also examined using in situ remote sensing. An Analytical Spectral Devices field spectrometer was used for spectral analysis of asbestos minerals and that of soil and water samples to compare and validate reflectance spectroscopy findings with laboratory results. Results show that in situ remote sensing techniques are able to reveal traces of asbestos minerals on rehabilitated dry surface areas, suggesting that they can play a significant role in monitoring the distribution of the asbestos minerals for epidemiological risk assessment. However, the spectral characteristics of asbestos minerals in the water medium were not as distinct as compared to laboratory methods. Overall, the results show potential for using remote sensing techniques in spatial epidemiology studies.

Propellants, Explosives, Pyrotechnics, 2016

The binary Mn + Sb 2 O 3 pyrotechnic composition was investigated for mining detonator time delay... more The binary Mn + Sb 2 O 3 pyrotechnic composition was investigated for mining detonator time delay applications. EKVI thermodynamic modelling predicted two maxima in the adiabatic reaction temperature. The local maximum, at a manganese fuel content of ca. 36 wt-%, corresponds to a pure thermite-type redox reaction: 3Mn + Sb 2 O 3  3MnO + 2Sb. The overall maximum in the adiabatic reaction temperature (ca. 1640 K), at the fuel-rich composition of 49 wt-% Mn, is consistent with the reaction 5Mn + Sb 2 O 3  3MnO + 2MnSb, i.e. a combination of the standard thermite with an additional exothermic intermetallic reaction. XRD analysis of combustion residues confirmed the formation of MnSb and Mn 2 Sb for fuel-rich compositions. Burn rates were measured using delay elements assembled into commercial detonators. The d 50 particle sizes were 23.4 and 0.92 m for the Mn fuel and Sb 2 O 3 oxidant powders respectively. The delay elements comprised rolled lead tubes with a length of 44 mm and an outer diameter of 6.4 mm. The rolling action compacted the pyrotechnic compositions to 74  2 % theoretical maximum density. The burning rate increased linearly from 4.2 to 9.4 mm s 1 over the composition range 25 -50 wt-% Mn.

Applied Clay Science, 2016

Three rectorite samples from the Beatrix Gold Mine, South Africa were characterized. Scanning ele... more Three rectorite samples from the Beatrix Gold Mine, South Africa were characterized. Scanning electron microscopy revealed a layered morphology. High resolution transmission microscopy showed well distinguished light and dark layers of about 2.20 nm consistent with the 1:1 interstratified mica-smectite nature. X-ray diffraction measurements confirmed the basal spacing d 001 of 2.20 nm consistent with a one-water-layer structure. Unit cell parameters, for a monoclinic unit cell with primitive lattice, refined to a = 5.177 Å; b = 8.980 Å; c = 22.489 Å and β = 97.335° with mean crystallite size around 14 nm and calculated cell volume of 1045 Å 3 . The Greene-Kelly test suggested that the expandable smectite layers have montmorillonite-beidellite composition. Nuclear magnetic resonance spectroscopy indicated a high degree of Al substitution and the presence of two different Al sites corresponding to six-and four-fold octahedral and tetrahedral aluminum respectively. The chemical composition and diffraction data suggest that the mica is Na-Ca-rich, i.e. of paragonitemargarite series. The fixed interlayer regions (mica interlayers) contains proportionally dominant Na + and Ca 2+ and minor amounts of K + . The exchangeable smectitic interlayers contain almost equal amounts of Na + and Ca 2+ ions. The distribution of the interlayer Na + ions was quantified by 23 Na solidstate NMR spectroscopy. It points to a three component mixed-layer structure with considerable variation in the composition of the mica layer of the different samples.

Earth and Planetary Science Letters, 2013

Applied Clay Science, 2010

Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentoni... more Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt angles of 27 º to 33 º; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the single chain surfactant. However, FTIR analysis revealed disordered alkyl chain conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic basis) featured high viscosities. They showed strong shear thinning behaviour confirming the ability of the montmorillonite particles to exfoliate. similar to the corresponding Boane (SC14 + DC16) modified bentonite. This suggests that the abrasive properties derived primarily from the bentonite itself and that the cristobalite impurity probably did not contribute significantly to increased abrasiveness of the Boane bentonite. This was expected in view of the extreme fineness of the particles constituting this impurity as revealed by the FESEM pictures.

Applied Clay Science, 2010

Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. ... more Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. The sodium-exchanged form was prepared by immersion in saturated brine at ambient conditions. The sodium content reached the equilibrium level within the first month of exposure. However, XRD spectra continued to evolve and sharpen over a six month period. The thermal-activated exfoliation was studied using thermo-mechanical analysis. The expansion of the neat "vermiculite" commenced at temperatures above 420 C. In some applications a lower exfoliation temperature is desirable, i.e. in the range 200 to 350°C. This study revealed that simple sodium exchange is effective towards this goal as it lowered the onset temperature by about 120 C. The linear expansion reached a maximum of about eight-fold at temperatures around 700 C and decreased again at higher temperatures.

Applied Clay Science, 2010

Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. ... more Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. The sodium-exchanged form was prepared by immersion in saturated brine at ambient conditions. The sodium content reached the equilibrium level within the first month of exposure. However, XRD spectra continued to evolve and sharpen over a six month period. The thermal-activated exfoliation was studied using thermo-mechanical analysis. The expansion of the neat "vermiculite" commenced at temperatures above 420 C. In some applications a lower exfoliation temperature is desirable, i.e. in the range 200 to 350°C. This study revealed that simple sodium exchange is effective towards this goal as it lowered the onset temperature by about 120 C. The linear expansion reached a maximum of about eight-fold at temperatures around 700 C and decreased again at higher temperatures.

Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminum or ir... more Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminum or iron (III) ions does not affect the exfoliation onset temperature (456 ± 11 °C). However, co-intercalation of minor amounts of urea lowers the exfoliation onset temperature to 209 ± 35 °C. This significant decrease is attributed to the low thermal stability of urea which releases ammonia and water as blowing agents. Thermo-mechanical analysis indicates that the expanded vermiculite exhibits spring-like properties. Part of the original expansion is recovered when the applied force is removed. The observed hysteresis is attributed to the progressive flattening of warped/buckled sheets that make up the expanded worm-like structure. Provided the expanded flake has not been heated beyond 450 °C, it can be reconstituted by applying a sufficiently high compressive force followed by equilibration with distilled water. Such flakes show the conventional exfoliation behavior when heated again.

An entry from the Inorganic Crystal Structure Database, the world's repository for inorganic ... more An entry from the Inorganic Crystal Structure Database, the world's repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Southern African Journal of Epidemiology and Infection, 2010

Research Square (Research Square), Apr 23, 2024

Journal of Thermal Analysis and Calorimetry

This manuscript reports the investigation of the polymorphic behaviour of fipronil using a system... more This manuscript reports the investigation of the polymorphic behaviour of fipronil using a systematic comparison of the thermochemical and structural properties of different crystal forms obtained in this study as well as those previously reported in literature. The analytical techniques employed include DSC, TGA, PXRD, SCXRD and hot stage microscopy. DSC proved particularly useful because it made it possible to differentiate between the two different crystal forms found in the as-received neat fipronil. The DSC scans revealed the presence of two polymorphs which had melting endotherms with peak maxima at ca. 196 °C and 205 °C, respectively. These polymorphs were successfully separated via sublimation and resulted in a metastable, lower melting polymorph in the sublimate and a thermodynamically stable, higher melting form in the sublimation residue. Clear evidence for the instability of the lower melting polymorph was found when the endotherms were examined under a range of heating ...

Applied Clay Science, 2015

Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminium or i... more Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminium or iron(III) ions does not affect the exfoliation onset temperature (456  11 C). However, co-intercalation of minor amounts of urea lowers the exfoliation onset temperature to 209  35 C. This significant decrease is attributed to the low thermal stability of urea which releases ammonia and water as blowing agents. Thermo-mechanical analysis indicates that the expanded vermiculite exhibits spring-like properties. Part of the original expansion is recovered when the applied force is removed. The observed hysteresis is attributed to the progressive flattening of warped/buckled sheets that make up the expanded worm-like structure. Provided the expanded flake has not been heated beyond 450 C, it can be reconstituted by applying a sufficiently high compressive force followed by equilibration with distilled water. Such flakes show the conventional exfoliation behaviour when heated again.

The South African electricity producer, Eskom, generated 36.01 million tons of coalcombustion fly... more The South African electricity producer, Eskom, generated 36.01 million tons of coalcombustion fly ash (FA) in 2010. About 5.6% were reused for the production of cement. The remaining 33.89 million tons were safely disposed of and managed on Eskom ash dumps and dams which are located adjacent to their corresponding power stations. South Africa has a long history regarding the development of new applications for this material. For instance, Ash Resources, South Africa’s leading manufacturer and supplier of fly ash products, has pioneered the re-use of FA and has positioned itself as one of the world leaders in ash technology over the last 30 years. South Africa remains very active in the development of ash technologies. Concurrently, the power industry is also a major carbon dioxide (CO2) emitter, with Eskom’s emissions approximating 225 million tons for 2010.

Organo-bentonites containing quaternary ammonium surfactants were 5 prepared using purified bento... more Organo-bentonites containing quaternary ammonium surfactants were 5 prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation 6 was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash 7 activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 8 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt 9 angles of 27 o to 33 o; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain 10 surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the 11 single chain surfactant. However, FTIR analysis revealed disordered alkyl chain 12 conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic 13 basis) featured high viscosities. They showed strong shear thinning behaviour confirming the 14 ability of the montmorillonite particles to exfoliate. 15

Since 2002, acid mine drainage (AMD) has discharged from the West Rand Gold3eld of the Witwatersr... more Since 2002, acid mine drainage (AMD) has discharged from the West Rand Gold3eld of the Witwatersrand into nearby streams and through subsurface 4ow into the adjacent dolomitic aquifer. Surface discharge has also been shown to recharge the dolomitic aquifer. This has led to fears of increased dissolution of the dolomite which, it has been proposed, could lead to land subsidence in the a2ected area. Laboratory investigations show that precipitation of resistant mineral coatings on reactive carbonate surfaces limit the potential for dissolution of dolomite and the potential for natural attenuation.

Metal stearate blends are potential lubricants and filler dispersants in polymer compounding. The... more Metal stearate blends are potential lubricants and filler dispersants in polymer compounding. Therefore, the thermal phase behaviour of binary blends of calcium stearate and magnesium stearate with zinc stearate was investigated. Cooling curve studies indicated lower melting temperatures in both systems. X-ray diffraction studies revealed that this was due to the formation of solid solutions of variable composition rather than to eutectic formation. Rheology measurements showed that true fluidification only happened well above the apparent melting points established from cooling curves. Calcium carbonate filled polyethylene containing metal stearate mixtures showed lower melt viscosities than were achievable using only calcium- or magnesium stearate as the lubricant. These results suggest that metal stearate blends, in combination with a judiciously selected wax component, may offer advantages in lowering compound melt viscosity and facilitating better particle dispersion.

The aim of this study was to conduct a comparative analysis using remote sensing and conventional... more The aim of this study was to conduct a comparative analysis using remote sensing and conventional sample analysis to assess asbestos pollution in rehabilitated former asbestos mining areas. The study focused on the Mafefe and Mathabatha areas of Limpopo Province, South Africa. Field-based remote sensing techniques were used to spectrally differentiate various types of asbestos minerals in order to determine their efficacy in assessing asbestos pollution. X-ray diffraction and scanning electron microscopy were employed for the identification and characterisation of traces of asbestos minerals in soil and water samples collected from the rehabilitated areas. The samples were also examined using in situ remote sensing. An Analytical Spectral Devices field spectrometer was used for spectral analysis of asbestos minerals and that of soil and water samples to compare and validate reflectance spectroscopy findings with laboratory results. Results show that in situ remote sensing techniques are able to reveal traces of asbestos minerals on rehabilitated dry surface areas, suggesting that they can play a significant role in monitoring the distribution of the asbestos minerals for epidemiological risk assessment. However, the spectral characteristics of asbestos minerals in the water medium were not as distinct as compared to laboratory methods. Overall, the results show potential for using remote sensing techniques in spatial epidemiology studies.

Propellants, Explosives, Pyrotechnics, 2016

The binary Mn + Sb 2 O 3 pyrotechnic composition was investigated for mining detonator time delay... more The binary Mn + Sb 2 O 3 pyrotechnic composition was investigated for mining detonator time delay applications. EKVI thermodynamic modelling predicted two maxima in the adiabatic reaction temperature. The local maximum, at a manganese fuel content of ca. 36 wt-%, corresponds to a pure thermite-type redox reaction: 3Mn + Sb 2 O 3  3MnO + 2Sb. The overall maximum in the adiabatic reaction temperature (ca. 1640 K), at the fuel-rich composition of 49 wt-% Mn, is consistent with the reaction 5Mn + Sb 2 O 3  3MnO + 2MnSb, i.e. a combination of the standard thermite with an additional exothermic intermetallic reaction. XRD analysis of combustion residues confirmed the formation of MnSb and Mn 2 Sb for fuel-rich compositions. Burn rates were measured using delay elements assembled into commercial detonators. The d 50 particle sizes were 23.4 and 0.92 m for the Mn fuel and Sb 2 O 3 oxidant powders respectively. The delay elements comprised rolled lead tubes with a length of 44 mm and an outer diameter of 6.4 mm. The rolling action compacted the pyrotechnic compositions to 74  2 % theoretical maximum density. The burning rate increased linearly from 4.2 to 9.4 mm s 1 over the composition range 25 -50 wt-% Mn.

Applied Clay Science, 2016

Three rectorite samples from the Beatrix Gold Mine, South Africa were characterized. Scanning ele... more Three rectorite samples from the Beatrix Gold Mine, South Africa were characterized. Scanning electron microscopy revealed a layered morphology. High resolution transmission microscopy showed well distinguished light and dark layers of about 2.20 nm consistent with the 1:1 interstratified mica-smectite nature. X-ray diffraction measurements confirmed the basal spacing d 001 of 2.20 nm consistent with a one-water-layer structure. Unit cell parameters, for a monoclinic unit cell with primitive lattice, refined to a = 5.177 Å; b = 8.980 Å; c = 22.489 Å and β = 97.335° with mean crystallite size around 14 nm and calculated cell volume of 1045 Å 3 . The Greene-Kelly test suggested that the expandable smectite layers have montmorillonite-beidellite composition. Nuclear magnetic resonance spectroscopy indicated a high degree of Al substitution and the presence of two different Al sites corresponding to six-and four-fold octahedral and tetrahedral aluminum respectively. The chemical composition and diffraction data suggest that the mica is Na-Ca-rich, i.e. of paragonitemargarite series. The fixed interlayer regions (mica interlayers) contains proportionally dominant Na + and Ca 2+ and minor amounts of K + . The exchangeable smectitic interlayers contain almost equal amounts of Na + and Ca 2+ ions. The distribution of the interlayer Na + ions was quantified by 23 Na solidstate NMR spectroscopy. It points to a three component mixed-layer structure with considerable variation in the composition of the mica layer of the different samples.

Earth and Planetary Science Letters, 2013

Applied Clay Science, 2010

Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentoni... more Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt angles of 27 º to 33 º; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the single chain surfactant. However, FTIR analysis revealed disordered alkyl chain conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic basis) featured high viscosities. They showed strong shear thinning behaviour confirming the ability of the montmorillonite particles to exfoliate. similar to the corresponding Boane (SC14 + DC16) modified bentonite. This suggests that the abrasive properties derived primarily from the bentonite itself and that the cristobalite impurity probably did not contribute significantly to increased abrasiveness of the Boane bentonite. This was expected in view of the extreme fineness of the particles constituting this impurity as revealed by the FESEM pictures.

Applied Clay Science, 2010

Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. ... more Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. The sodium-exchanged form was prepared by immersion in saturated brine at ambient conditions. The sodium content reached the equilibrium level within the first month of exposure. However, XRD spectra continued to evolve and sharpen over a six month period. The thermal-activated exfoliation was studied using thermo-mechanical analysis. The expansion of the neat "vermiculite" commenced at temperatures above 420 C. In some applications a lower exfoliation temperature is desirable, i.e. in the range 200 to 350°C. This study revealed that simple sodium exchange is effective towards this goal as it lowered the onset temperature by about 120 C. The linear expansion reached a maximum of about eight-fold at temperatures around 700 C and decreased again at higher temperatures.

Applied Clay Science, 2010

Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. ... more Palabora represents an interstratified vermiculite-biotite containing less than 50% vermiculite. The sodium-exchanged form was prepared by immersion in saturated brine at ambient conditions. The sodium content reached the equilibrium level within the first month of exposure. However, XRD spectra continued to evolve and sharpen over a six month period. The thermal-activated exfoliation was studied using thermo-mechanical analysis. The expansion of the neat "vermiculite" commenced at temperatures above 420 C. In some applications a lower exfoliation temperature is desirable, i.e. in the range 200 to 350°C. This study revealed that simple sodium exchange is effective towards this goal as it lowered the onset temperature by about 120 C. The linear expansion reached a maximum of about eight-fold at temperatures around 700 C and decreased again at higher temperatures.

Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminum or ir... more Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminum or iron (III) ions does not affect the exfoliation onset temperature (456 ± 11 °C). However, co-intercalation of minor amounts of urea lowers the exfoliation onset temperature to 209 ± 35 °C. This significant decrease is attributed to the low thermal stability of urea which releases ammonia and water as blowing agents. Thermo-mechanical analysis indicates that the expanded vermiculite exhibits spring-like properties. Part of the original expansion is recovered when the applied force is removed. The observed hysteresis is attributed to the progressive flattening of warped/buckled sheets that make up the expanded worm-like structure. Provided the expanded flake has not been heated beyond 450 °C, it can be reconstituted by applying a sufficiently high compressive force followed by equilibration with distilled water. Such flakes show the conventional exfoliation behavior when heated again.