Emmanuel Osei Boakye - Academia.edu (original) (raw)
Papers by Emmanuel Osei Boakye
Mechanical Properties and Performance of Engineering Ceramics and Composites VI
... 10 Chawla, KK, Liu, H., Janczak-Rusch, J. & Sambasivan, S. Microstructure and Properties ... more ... 10 Chawla, KK, Liu, H., Janczak-Rusch, J. & Sambasivan, S. Microstructure and Properties of Monazite ... 12 Curtin, WA, Eldredge, JI & Srinivasan, GV Push-Out Test on a New Silicon ... 27 Popa,K., Sedmidubsky, D., Benes, O., Thiriet, C. & Konings, RJM The High Temperature ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Ceramic Transactions Series, 2010
... 19 Chawla, KK, Liu, H., Janczak-Rusch, J., and Sambasivan, S., Microstructure and Properties ... more ... 19 Chawla, KK, Liu, H., Janczak-Rusch, J., and Sambasivan, S., Microstructure and Properties of Monazite (LaPQi ... 1 Curtin.WA,Eldredge, JI, and Srinivasan, GV, Push ... 40 Popa, K., Sedmidubsky, D., Benes, O., Thiriet, C, and Konings, RJM, The High Temperature Heat Capacity of ...
Journal of the American Ceramic Society, 2018
It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for... more It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y 2 Si 2 O 7 coatings on SiC. The proposed method is validated using an experimental procedure in which the in-situ formation of Y 2 Si 2 O 7 on a commercial SiC-based fiber is demonstrated. The method involves the deposition of YPO 4 on pre-oxidized fine diameter SiC-based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of pre-oxidation, and coating thickness. The results are promising for the development of environmentally-resistant interfacial coatings for SiC-fiber reinforced SiC-based matrix composites. The proposed and validated approach allows a low cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.
International Journal of Climate Change Strategies and Management, 2019
Purpose This paper aims to assess the rate and land category contributing to the changes in seven... more Purpose This paper aims to assess the rate and land category contributing to the changes in seven land-uses in the Kintampo North Municipality of Ghana and the effect of the decisions of land users on future landscapes. Design/methodology/approach LANDSAT images were classified to generate land use/cover maps to detect changes that had occurred between 1986 and 2014. In total, 120 farmers were also interviewed to determine their perceptions on land use changes. Interval, category and transition levels of changes were determined. Savanna woodland, settlement and forest were mostly converted to farmland in both intervals (1986-2001 and 2001-2014). Findings Results showed that rock outcrop, plantation, cropland and savanna woodland increased at an annual rate of 13.86, 1.57, 0.82 and 0.33 per cent, respectively, whilst forest, settlement and water body decreased at 4.90, 1.84 and 1.17 per cent annual rate of change, respectively. Approximately, 74 per cent of farmers will not change la...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
SummaryA zircon-carbon precursor was continuously coated on Nextel 720™ fibers using the sol-gel ... more SummaryA zircon-carbon precursor was continuously coated on Nextel 720™ fibers using the sol-gel method. Zircon was synthesized by the hydrolysis of zirconyl nitrate hydrate and tetraethoxysilane (TEOS) in ethanol. Polyacrylic acid was added as a carbon precursor. Ammonium vanadate was added to decrease the zircon formation temperature. The effects of prehydrolysis of TEOS and preheating of zircon and zircon-carbon precursor powders were investigated. Zircon formation was retarded by the presence of carbon.
Ceramic Engineering and Science Proceedings
ABSTRACT
24th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: B: Ceramic Engineering and Science Proceedings, Volume 21, Issue 4
... 3. KM Prewo, JJ Brennan, and S. Starrett, “Silicon Carbide Fiber-reinforced Glass Ceramic Com... more ... 3. KM Prewo, JJ Brennan, and S. Starrett, “Silicon Carbide Fiber-reinforced Glass Ceramic Composite Tensile Behavior at Elevated Temperature,” J ... 7. DB Marshall, PED Morgan, RM Houley, and JT Cheung, “High-Temperature Stability of the Al203-LaPO4 System,” J. Ceram. ...
ABSTRACT Spherical 10 nm rhabdophane (LaPO4·H2O) particles were made by controlled precipitation ... more ABSTRACT Spherical 10 nm rhabdophane (LaPO4·H2O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit+/H3PO4) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO3)3/H3PO4). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed.
Ceramic Engineering and Science Proceedings, 2011
Page 1. Composites: Fibers, Matrices, Interfaces, and Applications Mechanical Properties and Perf... more Page 1. Composites: Fibers, Matrices, Interfaces, and Applications Mechanical Properties and Performance of Engineering Ceramics and Composites VI Edited by Dileep Singh and Jonathan Salem Copyright © 2011 The American Ceramic Society Page 2. ...
Ceramic Transactions Series, 2011
Page 1. MONAZITE FIBER COATINGS FOR OXIDE COMPOSITES Emmanuel E. Boakye , Pavel Mogilevsky* Randa... more Page 1. MONAZITE FIBER COATINGS FOR OXIDE COMPOSITES Emmanuel E. Boakye , Pavel Mogilevsky* Randall S. Hay, and Michael K. Cinibulk Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7817 ...
Journal of the European Ceramic Society, 2014
Phase transformations and deformation mechanisms were characterized in polycrystalline rare-earth... more Phase transformations and deformation mechanisms were characterized in polycrystalline rare-earth orthophosphates by SEM and TEM after indentation. Xenotime-phase pellets with TbPO 4 and (Gd X ,Dy 1−X)PO 4 compositions close to the monazite stability field were softer than DyPO 4 compositions far from the monazite stability field, or monazite-phase GdPO 4 compositions. Transformation to monazite was observed by TEM beneath indents in TbPO 4. Grains with alternating (0 1 0) anhydrite and {0 1 0} xenotime lamellae were adjacent to areas that transformed to monazite. Pressure and/or shear stress in xenotime may cause three phase transformations: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. These transformations were not observed in indented DyPO 4 xenotime. Large displacement rebounds in some indentation load-displacement curves suggest that at least one of these transformations is ferroelastic. The phase transformations and material softening are consistent with previous observations made for sheared (Gd 0.4 ,Dy 0.6)PO 4 xenotime fiber coatings.
Journal of the American Ceramic Society, 2003
The effectiveness of monazite (LaPO 4) in providing an oxidation-resistant weak fiber/matrix inte... more The effectiveness of monazite (LaPO 4) in providing an oxidation-resistant weak fiber/matrix interface was evaluated in a fiber roving/thin coating/ceramic-matrix composite with >20% fiber volume fraction. Nextel 610/monazite/alumina composites were fabricated and tensile tested after isothermal exposures of up to 1000 h. Some strength loss was seen after short-term exposures (1100°-1200°C/5-250 h); however, no further loss was observed after 1000 h at 1200°C. Conversely, control samples containing uncoated fiber displayed >70% strength losses after only 5 h at 1200°C. Fiber pullout was seen in monazite-containing samples even after 1000 h at 1200°C. Debonding was predominantly in the coating or at either the fiber/coating or coating/matrix interface. Push-out testing confirmed the weakness of the monazite coating interface.
Journal of the American Ceramic Society, 2013
ABSTRACT The formation of the Y4.69(SiO4)3O apatite phase was observed in compositions targeted a... more ABSTRACT The formation of the Y4.69(SiO4)3O apatite phase was observed in compositions targeted at the yttrium monosilicate and disilicate phases. Through the use of high-temperature in situ X-ray diffraction, the lattice parameters of the phase were characterized as a function of temperature, and thermal expansion coefficients were calculated. Phase fractions were assessed as a function of sintering temperature, and the apatite phase fraction increased with temperature.
Journal of the American Ceramic Society, 2013
HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protecti... more HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protection systems because of favourable temperature stability, good mechanical properties and oxidation resistance. In this work HfB 2 powder has been synthesised via a sol-gel based route using phenolic resin, hafnium chloride and boric acid as the source of carbon, hafnium and boron respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB 2 formation investigated using FTIR, TGA, DSC and XRD. The results showed that whilst temperatures as low as 1300 o C could be used to produce HfB 2 particles, the calcination needed to last for about 25 h and the long duration resulted in particle growth along the c-axis of the HfB 2 crystals yielding tube like structures of about 10 µm long. Equiaxed particles 1-2 µm in size were obtained when the precursor was calcined at 1600 o C for 2 h. The reaction mechanism involved carbo/borothermal reduction and the indications were that the formation of HfB 2 at 1300 o C is through
Journal of the American Ceramic Society, 2004
Page 1. Fiber Strength Retention of Lanthanum-and Cerium Monazite-Coated NextelTM 720 Emmanuel E.... more Page 1. Fiber Strength Retention of Lanthanum-and Cerium Monazite-Coated NextelTM 720 Emmanuel E. BoakyeV and Pavel Mogilevsky UES Inc., Dayton-Xenia Rd., Dayton, Ohio 45432 NextelTM 720 fibers were coated with LaPO4 and CePO4 monazite. ...
Journal of the American Ceramic Society, 2007
The composition and lattice parameters of co-precipitated (La 0.3 Y 0.7) orthophosphate were stud... more The composition and lattice parameters of co-precipitated (La 0.3 Y 0.7) orthophosphate were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate that the as-precipitated powder consists of single-phase (La 0.3 Y 0.7-PO 4. H 2 O) rhabdophane nanoparticles. Heat treatment at 9501C caused the decomposition of rhabdophane into a (La 1Àx Y x)PO 4 monazite solid solution and YPO 4 xenotime. The solid solubility of Y in LaPO 4 monazite from 10001 to 16001C was studied using XRD, TEM, and EDX. The implications of the findings for controlling the coefficient of thermal expansion of the prospective two-phase monazite-xenotime fiber coatings for ceramic composites applications are discussed.
Journal of the American Ceramic Society, 2003
Microstructure and texture development of fiber coatings of rhabdophane-sol-derived monazite was ... more Microstructure and texture development of fiber coatings of rhabdophane-sol-derived monazite was studied. As-deposited textures and orientation relationships during phase transformations were determined by TEM. Monazite coatings had a crystallographic texture relict from that of as-deposited rhabdophane, with layers of rod-shaped particles that changed orientation by 90°across layers. Heat treatment at 1200°C of minicomposites with these coated fibers caused considerable monazite grain coarsening, and disappearance of the texture.
Journal of the American Ceramic Society, 2013
The reduction in fiber push-out stress by transformation plasticity in xenotime rare-earth orthop... more The reduction in fiber push-out stress by transformation plasticity in xenotime rare-earth orthophosphate fiber-matrix interphases was demonstrated. Processing methods for transformable xenotime coatings were explored. For conversion to xenotime during processing, (Gd x Dy 1−x)PO 4 solid solutions had to be more Dy-rich than those for pellets. Single-crystal alumina fibers were coated with 10-20 lm of (Gd 0.4 Dy 0.6)PO 4 xenotime and incorporated into polycrystalline alumina matrices. Coated fiber push-out stresses were between 10 and 80 MPa, significantly lower than those for fibers with other rare-earth orthophosphates coatings. Phase transformations and deformation mechanisms were characterized by SEM and TEM in fiber coatings after push out. Bands of deformed coating several micrometers in width formed during fiber push out. Cataclastic flow with fracture, granulation, translation, rotation, and intense plastic deformation of coating grains was observed. Three phase transformations may occur in heavily deformed particles in the deformation band: xenotime ? monazite, xenotime ? anhydrite, and anhydrite ? monazite. Anhydrite was abundant as a fine lamellar phase on (100) planes in xenotime. Selected area electron diffraction and highresolution TEM confirmed formation of monazite in a variety of heavily deformed particles. Issues for use of rare-earth orthophosphate transformation plasticity to lower fiber pull-out stress in ceramic matrix composites are discussed. D. Marshall-contributing editor Manuscript No. 32464.
Mechanical Properties and Performance of Engineering Ceramics and Composites VI
... 10 Chawla, KK, Liu, H., Janczak-Rusch, J. & Sambasivan, S. Microstructure and Properties ... more ... 10 Chawla, KK, Liu, H., Janczak-Rusch, J. & Sambasivan, S. Microstructure and Properties of Monazite ... 12 Curtin, WA, Eldredge, JI & Srinivasan, GV Push-Out Test on a New Silicon ... 27 Popa,K., Sedmidubsky, D., Benes, O., Thiriet, C. & Konings, RJM The High Temperature ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Ceramic Transactions Series, 2010
... 19 Chawla, KK, Liu, H., Janczak-Rusch, J., and Sambasivan, S., Microstructure and Properties ... more ... 19 Chawla, KK, Liu, H., Janczak-Rusch, J., and Sambasivan, S., Microstructure and Properties of Monazite (LaPQi ... 1 Curtin.WA,Eldredge, JI, and Srinivasan, GV, Push ... 40 Popa, K., Sedmidubsky, D., Benes, O., Thiriet, C, and Konings, RJM, The High Temperature Heat Capacity of ...
Journal of the American Ceramic Society, 2018
It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for... more It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y 2 Si 2 O 7 coatings on SiC. The proposed method is validated using an experimental procedure in which the in-situ formation of Y 2 Si 2 O 7 on a commercial SiC-based fiber is demonstrated. The method involves the deposition of YPO 4 on pre-oxidized fine diameter SiC-based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of pre-oxidation, and coating thickness. The results are promising for the development of environmentally-resistant interfacial coatings for SiC-fiber reinforced SiC-based matrix composites. The proposed and validated approach allows a low cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.
International Journal of Climate Change Strategies and Management, 2019
Purpose This paper aims to assess the rate and land category contributing to the changes in seven... more Purpose This paper aims to assess the rate and land category contributing to the changes in seven land-uses in the Kintampo North Municipality of Ghana and the effect of the decisions of land users on future landscapes. Design/methodology/approach LANDSAT images were classified to generate land use/cover maps to detect changes that had occurred between 1986 and 2014. In total, 120 farmers were also interviewed to determine their perceptions on land use changes. Interval, category and transition levels of changes were determined. Savanna woodland, settlement and forest were mostly converted to farmland in both intervals (1986-2001 and 2001-2014). Findings Results showed that rock outcrop, plantation, cropland and savanna woodland increased at an annual rate of 13.86, 1.57, 0.82 and 0.33 per cent, respectively, whilst forest, settlement and water body decreased at 4.90, 1.84 and 1.17 per cent annual rate of change, respectively. Approximately, 74 per cent of farmers will not change la...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
SummaryA zircon-carbon precursor was continuously coated on Nextel 720™ fibers using the sol-gel ... more SummaryA zircon-carbon precursor was continuously coated on Nextel 720™ fibers using the sol-gel method. Zircon was synthesized by the hydrolysis of zirconyl nitrate hydrate and tetraethoxysilane (TEOS) in ethanol. Polyacrylic acid was added as a carbon precursor. Ammonium vanadate was added to decrease the zircon formation temperature. The effects of prehydrolysis of TEOS and preheating of zircon and zircon-carbon precursor powders were investigated. Zircon formation was retarded by the presence of carbon.
Ceramic Engineering and Science Proceedings
ABSTRACT
24th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: B: Ceramic Engineering and Science Proceedings, Volume 21, Issue 4
... 3. KM Prewo, JJ Brennan, and S. Starrett, “Silicon Carbide Fiber-reinforced Glass Ceramic Com... more ... 3. KM Prewo, JJ Brennan, and S. Starrett, “Silicon Carbide Fiber-reinforced Glass Ceramic Composite Tensile Behavior at Elevated Temperature,” J ... 7. DB Marshall, PED Morgan, RM Houley, and JT Cheung, “High-Temperature Stability of the Al203-LaPO4 System,” J. Ceram. ...
ABSTRACT Spherical 10 nm rhabdophane (LaPO4·H2O) particles were made by controlled precipitation ... more ABSTRACT Spherical 10 nm rhabdophane (LaPO4·H2O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit+/H3PO4) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO3)3/H3PO4). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed.
Ceramic Engineering and Science Proceedings, 2011
Page 1. Composites: Fibers, Matrices, Interfaces, and Applications Mechanical Properties and Perf... more Page 1. Composites: Fibers, Matrices, Interfaces, and Applications Mechanical Properties and Performance of Engineering Ceramics and Composites VI Edited by Dileep Singh and Jonathan Salem Copyright © 2011 The American Ceramic Society Page 2. ...
Ceramic Transactions Series, 2011
Page 1. MONAZITE FIBER COATINGS FOR OXIDE COMPOSITES Emmanuel E. Boakye , Pavel Mogilevsky* Randa... more Page 1. MONAZITE FIBER COATINGS FOR OXIDE COMPOSITES Emmanuel E. Boakye , Pavel Mogilevsky* Randall S. Hay, and Michael K. Cinibulk Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7817 ...
Journal of the European Ceramic Society, 2014
Phase transformations and deformation mechanisms were characterized in polycrystalline rare-earth... more Phase transformations and deformation mechanisms were characterized in polycrystalline rare-earth orthophosphates by SEM and TEM after indentation. Xenotime-phase pellets with TbPO 4 and (Gd X ,Dy 1−X)PO 4 compositions close to the monazite stability field were softer than DyPO 4 compositions far from the monazite stability field, or monazite-phase GdPO 4 compositions. Transformation to monazite was observed by TEM beneath indents in TbPO 4. Grains with alternating (0 1 0) anhydrite and {0 1 0} xenotime lamellae were adjacent to areas that transformed to monazite. Pressure and/or shear stress in xenotime may cause three phase transformations: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. These transformations were not observed in indented DyPO 4 xenotime. Large displacement rebounds in some indentation load-displacement curves suggest that at least one of these transformations is ferroelastic. The phase transformations and material softening are consistent with previous observations made for sheared (Gd 0.4 ,Dy 0.6)PO 4 xenotime fiber coatings.
Journal of the American Ceramic Society, 2003
The effectiveness of monazite (LaPO 4) in providing an oxidation-resistant weak fiber/matrix inte... more The effectiveness of monazite (LaPO 4) in providing an oxidation-resistant weak fiber/matrix interface was evaluated in a fiber roving/thin coating/ceramic-matrix composite with >20% fiber volume fraction. Nextel 610/monazite/alumina composites were fabricated and tensile tested after isothermal exposures of up to 1000 h. Some strength loss was seen after short-term exposures (1100°-1200°C/5-250 h); however, no further loss was observed after 1000 h at 1200°C. Conversely, control samples containing uncoated fiber displayed >70% strength losses after only 5 h at 1200°C. Fiber pullout was seen in monazite-containing samples even after 1000 h at 1200°C. Debonding was predominantly in the coating or at either the fiber/coating or coating/matrix interface. Push-out testing confirmed the weakness of the monazite coating interface.
Journal of the American Ceramic Society, 2013
ABSTRACT The formation of the Y4.69(SiO4)3O apatite phase was observed in compositions targeted a... more ABSTRACT The formation of the Y4.69(SiO4)3O apatite phase was observed in compositions targeted at the yttrium monosilicate and disilicate phases. Through the use of high-temperature in situ X-ray diffraction, the lattice parameters of the phase were characterized as a function of temperature, and thermal expansion coefficients were calculated. Phase fractions were assessed as a function of sintering temperature, and the apatite phase fraction increased with temperature.
Journal of the American Ceramic Society, 2013
HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protecti... more HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protection systems because of favourable temperature stability, good mechanical properties and oxidation resistance. In this work HfB 2 powder has been synthesised via a sol-gel based route using phenolic resin, hafnium chloride and boric acid as the source of carbon, hafnium and boron respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB 2 formation investigated using FTIR, TGA, DSC and XRD. The results showed that whilst temperatures as low as 1300 o C could be used to produce HfB 2 particles, the calcination needed to last for about 25 h and the long duration resulted in particle growth along the c-axis of the HfB 2 crystals yielding tube like structures of about 10 µm long. Equiaxed particles 1-2 µm in size were obtained when the precursor was calcined at 1600 o C for 2 h. The reaction mechanism involved carbo/borothermal reduction and the indications were that the formation of HfB 2 at 1300 o C is through
Journal of the American Ceramic Society, 2004
Page 1. Fiber Strength Retention of Lanthanum-and Cerium Monazite-Coated NextelTM 720 Emmanuel E.... more Page 1. Fiber Strength Retention of Lanthanum-and Cerium Monazite-Coated NextelTM 720 Emmanuel E. BoakyeV and Pavel Mogilevsky UES Inc., Dayton-Xenia Rd., Dayton, Ohio 45432 NextelTM 720 fibers were coated with LaPO4 and CePO4 monazite. ...
Journal of the American Ceramic Society, 2007
The composition and lattice parameters of co-precipitated (La 0.3 Y 0.7) orthophosphate were stud... more The composition and lattice parameters of co-precipitated (La 0.3 Y 0.7) orthophosphate were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate that the as-precipitated powder consists of single-phase (La 0.3 Y 0.7-PO 4. H 2 O) rhabdophane nanoparticles. Heat treatment at 9501C caused the decomposition of rhabdophane into a (La 1Àx Y x)PO 4 monazite solid solution and YPO 4 xenotime. The solid solubility of Y in LaPO 4 monazite from 10001 to 16001C was studied using XRD, TEM, and EDX. The implications of the findings for controlling the coefficient of thermal expansion of the prospective two-phase monazite-xenotime fiber coatings for ceramic composites applications are discussed.
Journal of the American Ceramic Society, 2003
Microstructure and texture development of fiber coatings of rhabdophane-sol-derived monazite was ... more Microstructure and texture development of fiber coatings of rhabdophane-sol-derived monazite was studied. As-deposited textures and orientation relationships during phase transformations were determined by TEM. Monazite coatings had a crystallographic texture relict from that of as-deposited rhabdophane, with layers of rod-shaped particles that changed orientation by 90°across layers. Heat treatment at 1200°C of minicomposites with these coated fibers caused considerable monazite grain coarsening, and disappearance of the texture.
Journal of the American Ceramic Society, 2013
The reduction in fiber push-out stress by transformation plasticity in xenotime rare-earth orthop... more The reduction in fiber push-out stress by transformation plasticity in xenotime rare-earth orthophosphate fiber-matrix interphases was demonstrated. Processing methods for transformable xenotime coatings were explored. For conversion to xenotime during processing, (Gd x Dy 1−x)PO 4 solid solutions had to be more Dy-rich than those for pellets. Single-crystal alumina fibers were coated with 10-20 lm of (Gd 0.4 Dy 0.6)PO 4 xenotime and incorporated into polycrystalline alumina matrices. Coated fiber push-out stresses were between 10 and 80 MPa, significantly lower than those for fibers with other rare-earth orthophosphates coatings. Phase transformations and deformation mechanisms were characterized by SEM and TEM in fiber coatings after push out. Bands of deformed coating several micrometers in width formed during fiber push out. Cataclastic flow with fracture, granulation, translation, rotation, and intense plastic deformation of coating grains was observed. Three phase transformations may occur in heavily deformed particles in the deformation band: xenotime ? monazite, xenotime ? anhydrite, and anhydrite ? monazite. Anhydrite was abundant as a fine lamellar phase on (100) planes in xenotime. Selected area electron diffraction and highresolution TEM confirmed formation of monazite in a variety of heavily deformed particles. Issues for use of rare-earth orthophosphate transformation plasticity to lower fiber pull-out stress in ceramic matrix composites are discussed. D. Marshall-contributing editor Manuscript No. 32464.