Pathikumar Sellappan | University of California, San Diego (original) (raw)
Papers by Pathikumar Sellappan
Scientific Reports
Unique black coatings were observed in the inner wall of pottery shreds excavated from Keeladi, T... more Unique black coatings were observed in the inner wall of pottery shreds excavated from Keeladi, Tamilnadu, India. Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were used to understand the nature of the coating. The analysis revealed the presence of single, multi-walled carbon nanotubes and layered sheets in the coating. The average diameter of single-walled carbon nanotube found to be about 0.6 ± 0.05 nm. This is the lowest among the single-walled carbon nanotubes reported from artefacts so far and close to the theoretically predicted value (0.4 nm). These nanomaterials were coated in the pottery’s that date backs to sixth century BC, and still retain its stability and adhesion. The findings of nano materials in the pre-historic artifacts, its significance and impact are discussed in this article.
Journal of Materials Research
The middle initial of Matthew C. Wingert was omitted. (ii) Two in-text citations have been update... more The middle initial of Matthew C. Wingert was omitted. (ii) Two in-text citations have been updated for the following sentences due to errors in the reference list: The behavior of Ce:Al 2 O 3 is consistent with the lowtemperature optical behavior of other rare earths doped into oxides, such as Nd-[38, 39] and Er-doped [31] YAG, that exhibit optical 4f to 4f transitions that are shielded from crystal-field interactions by the outer 5d shell. The bulk ceramic Ce:Al 2 O 3 phosphors were produced using an all-solid-state, one-step reaction-densification route using CAPAD [23, 31].
APL Materials
We show tunable strain-induced perpendicular magnetic anisotropy (PMA) over a wide range of thick... more We show tunable strain-induced perpendicular magnetic anisotropy (PMA) over a wide range of thicknesses in epitaxial ferrimagnetic insulator Eu3Fe5O12 (EuIG) and Tb3Fe5O12 (TbIG) thin films grown by pulsed-laser deposition on Gd3Ga5O12 with (001) and (111) orientations, respectively. The PMA field is determined by measuring the induced anomalous Hall loops in Pt deposited on the garnet films. Due to positive magnetostriction constants, compressive in-plane strain induces a PMA field as large as 32.9 kOe for 4 nm thick EuIG and 66.7 kOe for 5 nm thick TbIG at 300 K, and relaxes extremely slowly as the garnet film thickness increases. In bilayers consisting of Pt and EuIG or Pt and TbIG, robust PMA is revealed by squared anomalous Hall hysteresis loops in Pt, the magnitude of which appears to be only related to the net magnetic moment of iron sublattices. Furthermore, the magnetostriction constant is found to be. × − for EuIG and. × − for TbIG, comparable with the values for bulk crystals. Our results demonstrate a general approach of tailoring magnetic anisotropy of rare earth iron garnets by utilizing modulated strain via epitaxial growth. †These are co-first authors.
Journal of Alloys and Compounds
Mg-Si-Sn solid solutions have interesting and useful semiconducting properties particularly for t... more Mg-Si-Sn solid solutions have interesting and useful semiconducting properties particularly for thermoelectric applications. While cubic solid solutions have been prepared by a variety of methods, solid solutions with a trigonal crystal structure have not been reported. We employed a combination of high energy ball milling and current activated pressure assisted densification (CAPAD) to induce and complete phase transformation from cubic to trigonal phase, forming dense trigonal composites. This is the first successful preparation of the trigonal phase in Mg-Si-Sn stem. In addition, we report the transport properties of both the cubic and trigonal composites measured in the 160 to 400 K range. The electrical conductivity of the trigonal composites was several times higher compared to the cubic counterparts while thermal conductivity is lower but in the range of previously reported Mg-Si-Sn materials. The Seebeck coefficient of the trigonal composites is lower compared to the cubic samples, leading to low ZT values. The ZT of the cubic samples, however is higher than previously reported for un-doped Mg-Si-Sn solid solutions which is promising for thermoelectric applications.
Physical Review B, 2016
We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically fl... more We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically flat ferrimagnetic insulator Tm 3 Fe 5 O 12 films grown with pulsed laser deposition on substituted-Gd 3 Ga 5 O 12 substrate which maximizes the tensile strain at the interface. In bilayers consisting of Pt and TIG, we observe large squared Hall hysteresis loops over a wide range of thicknesses of Pt at room temperature. When a thin Cu layer is inserted between Pt and TIG, the Hall hysteresis magnitude decays but stays finite as the thickness of Cu increases up to 5 nm. However, if the Cu layer is placed atop Pt instead, the Hall hysteresis magnitude is consistently larger than when the Cu layer with the same thickness is inserted in between for all Cu thicknesses. These results suggest that both the proximityinduced ferromagnetism and spin current contribute to the anomalous Hall effect.
Journal of Materials Research
Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-... more Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-doped Al 2 O 3 and propose it as a new phosphor converting capping layer for high-powered/high-brightness solid-state white lighting (SSWL). The bulk, dense Ce:Al 2 O 3 ceramics have a 0.5 at.% Ce:Al concentration (significantly higher than the equilibrium solubility limit) and were produced using a simultaneous solid-state reactive current activated pressureassisted densification (CAPAD) approach. Ce:Al 2 O 3 exhibits a broadband emission from 400 to 600 nm, which encompasses the entire blue and green portions of the visible spectrum when pumped with ultraviolet (UV) light that is now commercially available in UV light-emitting devices and laser diodes (LD). These broadband phosphors can be used in the commonly used scheme of mixing with other UV-converting capping layers that emit red light to produce white light. Alternatively, they can be used in a novel composite down-converter approach that ensures improved thermal-mechanical properties of the converting phosphor capping layer. In this configuration, Ce:Al 2 O 3 is used with proven phosphor conversion materials such as Ce:YAG as an active encapsulant or as a capping layer to produce SSWL with an improved bandwidth in the blue portion of the visible spectrum. To study the effect of crystallinity on the Ce photoluminescent (PL) emission, we synthesize Ce:YAG ceramics using high-pressure CAPAD at moderate temperatures to obtain varying crystallinity (amorphous through fully crystalline). We investigate the PL characteristics of Ce:Al 2 O 3 and Ce:YAG from 295 to 4 K, revealing unique crystal field effects from the matrix on the Ce dopants. The unique PL properties in conjunction with the superior thermal-mechanical properties of Ce:Al 2 O 3 can be used in high-powered/high-brightness-integrated devices based on high-efficiency UV-LD that do not suffer efficiency droop at high drive currents to pump the solid-state capping phosphors.
Materials Research Letters, 2016
Bulletin of the American Physical Society, Mar 2, 2015
Bulletin of the American Physical Society, Mar 2, 2015
Submitted for the MAR15 Meeting of The American Physical Society Combinatorial Deposition of La 2... more Submitted for the MAR15 Meeting of The American Physical Society Combinatorial Deposition of La 2−x Ba x CuO 4 Thin Films Across the x=1/8 Anomaly 1 JUSTIN LANE, ADAM WEIS, AZTON WELLS, University of Illinois at Urbana-Champaign, SO RA CHUNG, Belmont University, PATHIKUMAR SELLAPPAN, WALTRAUD KRIVEN, DALE VAN HARLINGEN, University of Illinois at Urbana-Champaign -La 2−x Ba x CuO 4 (LBCO)
Journal of the European Ceramic Society, 2015
A Brazilian disk geometry of an alumina layered composite with alternating dense and porous layer... more A Brazilian disk geometry of an alumina layered composite with alternating dense and porous layers was dynamically loaded using a Split-Hopkinson Pressure Bar (SHPB) apparatus under compression. High-speed imaging and transmitted force measurements were used to gain an insight into stress wave propagation and mitigation through such a layered system. Uniformly distributed porosities of 20 and 50 vol % were introduced into the interlayers by the addition of fine graphite particles which volatilized during heat treatment. Brazilian disk samples were cut from the cylinders which were drilled out of the sintered laminated sample. The disks were subjected to dynamic impact loading in perpendicular and parallel orientations to the layers in order to investigate the influence of the direction of impact. The dynamic failure process of the layered ceramic consisted of the initiation and propagation of the cracks mainly along the interphases of the layers.
Journal of Physics: Condensed Matter, 2014
Acoustic emission has been measured and statistical characteristics have been analyzed during the... more Acoustic emission has been measured and statistical characteristics have been analyzed during the stress-induced collapse of porous berlinite, AlPO4, containing up to 50 vol% porosity. Stress collapse occurs in a series of individual events (avalanches), and each avalanche leads to a jerk in sample compression with corresponding acoustic emission (AE) signals. The distribution of AE avalanche energies can be approximately described by a power law p(E)dE = E -ε dE (ε ~ 1.8) over a large stress interval. We observed several collapse mechanisms whereby less porous minerals show the superposition of independent jerks, which were not related to the major collapse at the failure stress. In highly porous berlinite (40% and 50%) an increase of the energy emission occurred near the failure point. In contrast, the less porous samples did not show such an increase in energy emission. Instead, in the near vicinity of the main failure point they showed a reduction in the energy exponent to 1.4, which is consistent with the value reported for compressed porous systems displaying critical behavior. This indicated that a critical avalanche regime with a lack of precursor events occurs. In this case, all preceding large events were 'false alarms' and unrelated to the main failure event. Our results identify a method to use pico-seismicity detection of foreshocks to warn of mine collapse before the main failure collapse occurs, which can be applied for highly porous materials only. PACS numbers: 62.20 mm, 61.43 Gt, 05.65.+b, 89.75.Da 2
Journal of the European Ceramic Society, 2013
The crack initiation load and fracture toughness were characterized as a function of diamond part... more The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m 1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ∼ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.
Journal of the American Ceramic Society, 2012
ABSTRACT New types of bulk SiOC glass matrix composites reinforced with diamond particles were su... more ABSTRACT New types of bulk SiOC glass matrix composites reinforced with diamond particles were successfully fabricated. Diamond particles were introduced into the SiOC matrix by the polymer-derived ceramics (PDC) route using polysiloxane and two different sized diamond particles, 2 and 30 μm as starting precursors. Dense bulk specimens were prepared by warm pressing at 150°C-160°C followed by pyrolization at 1100°C for 2 h. The composites were characterized by means of SEM, TEM, optical observation, X-ray diffraction, and Vickers indentation. Elastic properties were determined by means of ultrasonic echography, and Young's modulus was found to increase from 96 to 154 GPa when diamond content increased from 0 to 25 vol%. Reinforcement results in significant improvement, nearly 100%, in hardness compared to pristine SiOC glass sample. The size of the diamond particles has an influence on density and microstructure of the composites. The TEM investigations reveal that SiOC glass matrix and 2 μm diamond particles have excellent bonding. The present study demonstrates the possibility of fabricating bulk SiOC glass-diamond composites via the polymer-processing route, resulting in composites with promising mechanical properties.
Journal of Physics: Condensed Matter, 2013
We study the acoustic emission avalanches during the failure process of porous alumina samples (A... more We study the acoustic emission avalanches during the failure process of porous alumina samples (Al 2 O 3 ) under compression. Specimens with different porosities ranging from 30% to 59% have been synthetized from a mixture of finegrained alumina and graphite. The compressive strength as well as the characteristics of the acoustic activity have been determined. The statistical analysis of the recorded acoustic emission pulses reveals, for all porosities, a broad distribution of energies with a fat tail, compatible with the existence of an underlying critical point. In the region of 35%-55% porosity, the energy distributions of the acoustic emission signals are compatible with a power law behaviour over two decades in energy with an exponent ǫ = 1.8 ± 0.1.
Journal of Non-Crystalline Solids, 2010
Ca and Sr-based oxynitride glasses with very high nitrogen content have been synthesized using me... more Ca and Sr-based oxynitride glasses with very high nitrogen content have been synthesized using metal hydrides as primary precursors. Values of Young's modulus, shear modulus, bulk modulus and Poisson's ratio were determined by means of ultrasonic echography. Vickers micro-indentation has been used to characterize hardness and indentation fracture toughness behaviour. Elastic moduli were found to increase linearly with nitrogen content, with the highest value of Young's modulus at 135 GPa, for a Ca-glass with 58 e/o of nitrogen. The Sr-glasses exhibit lower elastic moduli than Ca glasses. Poisson's ratio, hardness, indentation fracture toughness, crack initiation load and surface damage resistance were found to increase with increasing nitrogen content for both glass series.
Comptes Rendus Mécanique, 2014
ABSTRACT The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass... more ABSTRACT The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass surface under sharp contact loading and the extent to which these cracks develop is intimately related to the way the material attempts to relax the corresponding stress field. Two processes which are known to occur upon indentation are densification and isochoric shear flow. The contributions of both mechanisms were quantitatively assessed for glasses belonging to different chemical systems in previous papers [1], [2] and [3]. In the present study, indentation cracking maps are provided, which offer guidelines to the design of glasses with better surface damage resistance based on their elastic properties and hardness.
Acta Materialia, 2013
ABSTRACT Crack initiation and deformation behaviors of oxide glasses belonging to different chemi... more ABSTRACT Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson's ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 ≤ ν ≤ 0.20; Semi-Resilient, for 0.20 ≤ ν ≤ 0.25; and Easily-Damaged for 0.25 < ν < 0.30. Radial-median cracks occur at low load (≤50 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (∼0.22 depending on the Young's modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.
Journal of the American Ceramic Society, 2007
Amorphous powders of Al 2 O 3 -37.5 mol% Y 2 O 3 (yttrium aluminum garnet (YAG)) were prepared by... more Amorphous powders of Al 2 O 3 -37.5 mol% Y 2 O 3 (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 8001C, and hot-pressed uniaxally at low temperature (6001C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasihydrostatic pressure (1 GPa) at 10001C enabled the production of fully crystallized YAG with 496% relative density and a nanocrystalline grain size of B70 nm.
Scientific Reports
Unique black coatings were observed in the inner wall of pottery shreds excavated from Keeladi, T... more Unique black coatings were observed in the inner wall of pottery shreds excavated from Keeladi, Tamilnadu, India. Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were used to understand the nature of the coating. The analysis revealed the presence of single, multi-walled carbon nanotubes and layered sheets in the coating. The average diameter of single-walled carbon nanotube found to be about 0.6 ± 0.05 nm. This is the lowest among the single-walled carbon nanotubes reported from artefacts so far and close to the theoretically predicted value (0.4 nm). These nanomaterials were coated in the pottery’s that date backs to sixth century BC, and still retain its stability and adhesion. The findings of nano materials in the pre-historic artifacts, its significance and impact are discussed in this article.
Journal of Materials Research
The middle initial of Matthew C. Wingert was omitted. (ii) Two in-text citations have been update... more The middle initial of Matthew C. Wingert was omitted. (ii) Two in-text citations have been updated for the following sentences due to errors in the reference list: The behavior of Ce:Al 2 O 3 is consistent with the lowtemperature optical behavior of other rare earths doped into oxides, such as Nd-[38, 39] and Er-doped [31] YAG, that exhibit optical 4f to 4f transitions that are shielded from crystal-field interactions by the outer 5d shell. The bulk ceramic Ce:Al 2 O 3 phosphors were produced using an all-solid-state, one-step reaction-densification route using CAPAD [23, 31].
APL Materials
We show tunable strain-induced perpendicular magnetic anisotropy (PMA) over a wide range of thick... more We show tunable strain-induced perpendicular magnetic anisotropy (PMA) over a wide range of thicknesses in epitaxial ferrimagnetic insulator Eu3Fe5O12 (EuIG) and Tb3Fe5O12 (TbIG) thin films grown by pulsed-laser deposition on Gd3Ga5O12 with (001) and (111) orientations, respectively. The PMA field is determined by measuring the induced anomalous Hall loops in Pt deposited on the garnet films. Due to positive magnetostriction constants, compressive in-plane strain induces a PMA field as large as 32.9 kOe for 4 nm thick EuIG and 66.7 kOe for 5 nm thick TbIG at 300 K, and relaxes extremely slowly as the garnet film thickness increases. In bilayers consisting of Pt and EuIG or Pt and TbIG, robust PMA is revealed by squared anomalous Hall hysteresis loops in Pt, the magnitude of which appears to be only related to the net magnetic moment of iron sublattices. Furthermore, the magnetostriction constant is found to be. × − for EuIG and. × − for TbIG, comparable with the values for bulk crystals. Our results demonstrate a general approach of tailoring magnetic anisotropy of rare earth iron garnets by utilizing modulated strain via epitaxial growth. †These are co-first authors.
Journal of Alloys and Compounds
Mg-Si-Sn solid solutions have interesting and useful semiconducting properties particularly for t... more Mg-Si-Sn solid solutions have interesting and useful semiconducting properties particularly for thermoelectric applications. While cubic solid solutions have been prepared by a variety of methods, solid solutions with a trigonal crystal structure have not been reported. We employed a combination of high energy ball milling and current activated pressure assisted densification (CAPAD) to induce and complete phase transformation from cubic to trigonal phase, forming dense trigonal composites. This is the first successful preparation of the trigonal phase in Mg-Si-Sn stem. In addition, we report the transport properties of both the cubic and trigonal composites measured in the 160 to 400 K range. The electrical conductivity of the trigonal composites was several times higher compared to the cubic counterparts while thermal conductivity is lower but in the range of previously reported Mg-Si-Sn materials. The Seebeck coefficient of the trigonal composites is lower compared to the cubic samples, leading to low ZT values. The ZT of the cubic samples, however is higher than previously reported for un-doped Mg-Si-Sn solid solutions which is promising for thermoelectric applications.
Physical Review B, 2016
We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically fl... more We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically flat ferrimagnetic insulator Tm 3 Fe 5 O 12 films grown with pulsed laser deposition on substituted-Gd 3 Ga 5 O 12 substrate which maximizes the tensile strain at the interface. In bilayers consisting of Pt and TIG, we observe large squared Hall hysteresis loops over a wide range of thicknesses of Pt at room temperature. When a thin Cu layer is inserted between Pt and TIG, the Hall hysteresis magnitude decays but stays finite as the thickness of Cu increases up to 5 nm. However, if the Cu layer is placed atop Pt instead, the Hall hysteresis magnitude is consistently larger than when the Cu layer with the same thickness is inserted in between for all Cu thicknesses. These results suggest that both the proximityinduced ferromagnetism and spin current contribute to the anomalous Hall effect.
Journal of Materials Research
Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-... more Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-doped Al 2 O 3 and propose it as a new phosphor converting capping layer for high-powered/high-brightness solid-state white lighting (SSWL). The bulk, dense Ce:Al 2 O 3 ceramics have a 0.5 at.% Ce:Al concentration (significantly higher than the equilibrium solubility limit) and were produced using a simultaneous solid-state reactive current activated pressureassisted densification (CAPAD) approach. Ce:Al 2 O 3 exhibits a broadband emission from 400 to 600 nm, which encompasses the entire blue and green portions of the visible spectrum when pumped with ultraviolet (UV) light that is now commercially available in UV light-emitting devices and laser diodes (LD). These broadband phosphors can be used in the commonly used scheme of mixing with other UV-converting capping layers that emit red light to produce white light. Alternatively, they can be used in a novel composite down-converter approach that ensures improved thermal-mechanical properties of the converting phosphor capping layer. In this configuration, Ce:Al 2 O 3 is used with proven phosphor conversion materials such as Ce:YAG as an active encapsulant or as a capping layer to produce SSWL with an improved bandwidth in the blue portion of the visible spectrum. To study the effect of crystallinity on the Ce photoluminescent (PL) emission, we synthesize Ce:YAG ceramics using high-pressure CAPAD at moderate temperatures to obtain varying crystallinity (amorphous through fully crystalline). We investigate the PL characteristics of Ce:Al 2 O 3 and Ce:YAG from 295 to 4 K, revealing unique crystal field effects from the matrix on the Ce dopants. The unique PL properties in conjunction with the superior thermal-mechanical properties of Ce:Al 2 O 3 can be used in high-powered/high-brightness-integrated devices based on high-efficiency UV-LD that do not suffer efficiency droop at high drive currents to pump the solid-state capping phosphors.
Materials Research Letters, 2016
Bulletin of the American Physical Society, Mar 2, 2015
Bulletin of the American Physical Society, Mar 2, 2015
Submitted for the MAR15 Meeting of The American Physical Society Combinatorial Deposition of La 2... more Submitted for the MAR15 Meeting of The American Physical Society Combinatorial Deposition of La 2−x Ba x CuO 4 Thin Films Across the x=1/8 Anomaly 1 JUSTIN LANE, ADAM WEIS, AZTON WELLS, University of Illinois at Urbana-Champaign, SO RA CHUNG, Belmont University, PATHIKUMAR SELLAPPAN, WALTRAUD KRIVEN, DALE VAN HARLINGEN, University of Illinois at Urbana-Champaign -La 2−x Ba x CuO 4 (LBCO)
Journal of the European Ceramic Society, 2015
A Brazilian disk geometry of an alumina layered composite with alternating dense and porous layer... more A Brazilian disk geometry of an alumina layered composite with alternating dense and porous layers was dynamically loaded using a Split-Hopkinson Pressure Bar (SHPB) apparatus under compression. High-speed imaging and transmitted force measurements were used to gain an insight into stress wave propagation and mitigation through such a layered system. Uniformly distributed porosities of 20 and 50 vol % were introduced into the interlayers by the addition of fine graphite particles which volatilized during heat treatment. Brazilian disk samples were cut from the cylinders which were drilled out of the sintered laminated sample. The disks were subjected to dynamic impact loading in perpendicular and parallel orientations to the layers in order to investigate the influence of the direction of impact. The dynamic failure process of the layered ceramic consisted of the initiation and propagation of the cracks mainly along the interphases of the layers.
Journal of Physics: Condensed Matter, 2014
Acoustic emission has been measured and statistical characteristics have been analyzed during the... more Acoustic emission has been measured and statistical characteristics have been analyzed during the stress-induced collapse of porous berlinite, AlPO4, containing up to 50 vol% porosity. Stress collapse occurs in a series of individual events (avalanches), and each avalanche leads to a jerk in sample compression with corresponding acoustic emission (AE) signals. The distribution of AE avalanche energies can be approximately described by a power law p(E)dE = E -ε dE (ε ~ 1.8) over a large stress interval. We observed several collapse mechanisms whereby less porous minerals show the superposition of independent jerks, which were not related to the major collapse at the failure stress. In highly porous berlinite (40% and 50%) an increase of the energy emission occurred near the failure point. In contrast, the less porous samples did not show such an increase in energy emission. Instead, in the near vicinity of the main failure point they showed a reduction in the energy exponent to 1.4, which is consistent with the value reported for compressed porous systems displaying critical behavior. This indicated that a critical avalanche regime with a lack of precursor events occurs. In this case, all preceding large events were 'false alarms' and unrelated to the main failure event. Our results identify a method to use pico-seismicity detection of foreshocks to warn of mine collapse before the main failure collapse occurs, which can be applied for highly porous materials only. PACS numbers: 62.20 mm, 61.43 Gt, 05.65.+b, 89.75.Da 2
Journal of the European Ceramic Society, 2013
The crack initiation load and fracture toughness were characterized as a function of diamond part... more The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m 1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ∼ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.
Journal of the American Ceramic Society, 2012
ABSTRACT New types of bulk SiOC glass matrix composites reinforced with diamond particles were su... more ABSTRACT New types of bulk SiOC glass matrix composites reinforced with diamond particles were successfully fabricated. Diamond particles were introduced into the SiOC matrix by the polymer-derived ceramics (PDC) route using polysiloxane and two different sized diamond particles, 2 and 30 μm as starting precursors. Dense bulk specimens were prepared by warm pressing at 150°C-160°C followed by pyrolization at 1100°C for 2 h. The composites were characterized by means of SEM, TEM, optical observation, X-ray diffraction, and Vickers indentation. Elastic properties were determined by means of ultrasonic echography, and Young's modulus was found to increase from 96 to 154 GPa when diamond content increased from 0 to 25 vol%. Reinforcement results in significant improvement, nearly 100%, in hardness compared to pristine SiOC glass sample. The size of the diamond particles has an influence on density and microstructure of the composites. The TEM investigations reveal that SiOC glass matrix and 2 μm diamond particles have excellent bonding. The present study demonstrates the possibility of fabricating bulk SiOC glass-diamond composites via the polymer-processing route, resulting in composites with promising mechanical properties.
Journal of Physics: Condensed Matter, 2013
We study the acoustic emission avalanches during the failure process of porous alumina samples (A... more We study the acoustic emission avalanches during the failure process of porous alumina samples (Al 2 O 3 ) under compression. Specimens with different porosities ranging from 30% to 59% have been synthetized from a mixture of finegrained alumina and graphite. The compressive strength as well as the characteristics of the acoustic activity have been determined. The statistical analysis of the recorded acoustic emission pulses reveals, for all porosities, a broad distribution of energies with a fat tail, compatible with the existence of an underlying critical point. In the region of 35%-55% porosity, the energy distributions of the acoustic emission signals are compatible with a power law behaviour over two decades in energy with an exponent ǫ = 1.8 ± 0.1.
Journal of Non-Crystalline Solids, 2010
Ca and Sr-based oxynitride glasses with very high nitrogen content have been synthesized using me... more Ca and Sr-based oxynitride glasses with very high nitrogen content have been synthesized using metal hydrides as primary precursors. Values of Young's modulus, shear modulus, bulk modulus and Poisson's ratio were determined by means of ultrasonic echography. Vickers micro-indentation has been used to characterize hardness and indentation fracture toughness behaviour. Elastic moduli were found to increase linearly with nitrogen content, with the highest value of Young's modulus at 135 GPa, for a Ca-glass with 58 e/o of nitrogen. The Sr-glasses exhibit lower elastic moduli than Ca glasses. Poisson's ratio, hardness, indentation fracture toughness, crack initiation load and surface damage resistance were found to increase with increasing nitrogen content for both glass series.
Comptes Rendus Mécanique, 2014
ABSTRACT The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass... more ABSTRACT The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass surface under sharp contact loading and the extent to which these cracks develop is intimately related to the way the material attempts to relax the corresponding stress field. Two processes which are known to occur upon indentation are densification and isochoric shear flow. The contributions of both mechanisms were quantitatively assessed for glasses belonging to different chemical systems in previous papers [1], [2] and [3]. In the present study, indentation cracking maps are provided, which offer guidelines to the design of glasses with better surface damage resistance based on their elastic properties and hardness.
Acta Materialia, 2013
ABSTRACT Crack initiation and deformation behaviors of oxide glasses belonging to different chemi... more ABSTRACT Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson's ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 ≤ ν ≤ 0.20; Semi-Resilient, for 0.20 ≤ ν ≤ 0.25; and Easily-Damaged for 0.25 < ν < 0.30. Radial-median cracks occur at low load (≤50 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (∼0.22 depending on the Young's modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.
Journal of the American Ceramic Society, 2007
Amorphous powders of Al 2 O 3 -37.5 mol% Y 2 O 3 (yttrium aluminum garnet (YAG)) were prepared by... more Amorphous powders of Al 2 O 3 -37.5 mol% Y 2 O 3 (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 8001C, and hot-pressed uniaxally at low temperature (6001C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasihydrostatic pressure (1 GPa) at 10001C enabled the production of fully crystallized YAG with 496% relative density and a nanocrystalline grain size of B70 nm.