Layered lithium nickel manganese cobalt oxides, Li(NixMnyCoz)O2 where x + y + z = 1 (NMCs), have ... more Layered lithium nickel manganese cobalt oxides, Li(NixMnyCoz)O2 where x + y + z = 1 (NMCs), have been studied extensively due to their higher capacity, less toxicity and lower cost compared to LiCoO2. However, widespread market penetration of NMCs as cathodes for Li-ion batteries (LIBs) is impeded by their poor capacity retention and low rate capability. Coatings provide an effective solution to these problems. This article focuses on review of the recent advancements in coatings of NMCs from the mechanism viewpoint. This is the first time that coatings on NMCs are reviewed based on their functionalities and mechanisms through which the electrochemical properties and performance of NMCs have been improved. To provide a comprehensive understanding of the functions and mechanisms offered by coatings, the following functions and mechanisms are reviewed individually: (i) scavenging HF in the electrolyte, (ii) scavenging water molecules in the electrolyte and thus suppressing HF propagation during charge/discharge cycles, (iii) serving as a buffer layer to minimize HF attack on NMCs and suppress side reactions between NMCs and the electrolyte, (iv) hindering phase transitions and impeding loss of lattice oxygen, (v) preventing microcracks in NMC particles to keep participation of most NMC material in lithiation/de-lithiation, and (vi) enhancing the rate capability of NMC cathodes. Finally, the personal perspectives on outlook are offered with an aim to stimulate further discussion and ideas on the rational design of coatings for durable and high performance NMC cathodes for the next generation LIBs in the near future.
Several effective methods have been developed recently to demonstrate simultaneous high energy an... more Several effective methods have been developed recently to demonstrate simultaneous high energy and high power density in Lithium - carbon fluoride (Li-CFx) batteries. These methods can achieve as high as a 1000 Wh/kg energy density at a 60–70 kW/kg power density (40–50 C rate) in coin cells and a 750 Wh/kg energy density at a 12.5 kW/kg power density (20 C rate) in pouch cells. This performance is made possible by an ingenious nano-architecture design, controlled porosity, boron doping, and electrolyte additives. In the present study, we show that a similarly great performance, a 931 Wh/kg energy density at a 59 kW/kg power density, can be achieved by using a polyacrylonitrile binder and a LiBF4 electrolyte in Li-graphite fluoride coin cells. We also demonstrate that the observed effect is the result of the right combination of the binder and the electrolyte. We propose that the mechanistic origin of the observed phenomena is an electro-catalytic effect of the polyacrylonitrile bind...
Silicon has the potential to be a high-performance anode material, but its practical application ... more Silicon has the potential to be a high-performance anode material, but its practical application is impeded by huge volume expansion during lithiation. Many studies have revealed that the huge volume expansion problem can be mitigated by introducing engineered voids into Si/C core–shell structures. In this study, a Si/C core/shell structure with engineered voids, termed Si@void@C, is investigated for its specific capacity and cycle stability as a function of particle size and charge/discharge protocol. The study shows that finer Si@void@C particles result in higher specific capacities, but with little impact on the cycle stability. Further, lower and upper cutoff voltages in charge/discharge have a profound impact on the specific capacity and cycle stability. Importantly, cutoff voltages in formation cycles have long-lasting effects on the cycle stability, indicating the critical role of forming a robust solid electrolyte interphase (SEI) layer during formation cycles. Using a const...
Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly... more Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly from computer based solid models described by STL (Stereo Lithography) or VRML (Virtual Reality Modeling Language) files generated by Computer-Aided Design (CAD) systems. Most of the SFF processes produce parts by building them layer by layer using a row by row pattern, though it is possible to build the part using other patterns. The SFF technology represents a challenge to designers who, in addition to making decisions concerning optimum shape and functionality of the entire part, have'to take under consideration several other manufacturing factors. These factors cover a wide range of technical issues such as Computer-Aided Design model generation, part description and model slicing files, laser path files, precision of part design, rendering patterns, manufacturing tolerances, thermal expansion and residual stress phenomena. This paper investigates the effect of rendering patterns on the integrity, material characteristics and mechanical properties of the parts prepared by a desk-top SFF device using diode lasers. Fe-Bronze (Cu-Sn) premixed metal powders were used as the starting material. The particle size was about 100 /lm to 200 /lm. Density, tensile strength and microstructure of the parts prepared using different rendering patterns were characterized. The results were analyzed to seek optimal rendering patterns. It was noticed that the samples were strong along the laser scanning direction, while they were weak perpendicular to the scanning direction. These results suggest that the laser scanning patterns should be designed to minimize the warping and maximize the strength of the part in the direction depending on the part's function.
In this paper, we describe a solid freeform fabrication procedure for human dental restoration vi... more In this paper, we describe a solid freeform fabrication procedure for human dental restoration via porcelain slurry micro-extrusion. Based on submicron-sized dental porcelain powder obtained via ball milling process, a porcelain slurry formulation has been developed. The formulation developed allows the porcelain slurry to show a pseudoplastic behavior and moderate viscosity, which permits the slurry to re-shape to form a near rectangular cross section. A well-controlled cross-section geometry of the extrudate is important for micro-extrusion to obtain uniform 2-D planes and for the addition of the sequential layers to form a 3-D object. Human teeth are restored by this method directly from CAD digital models. After sintering, shrinkage of the artificial teeth is uniform in all directions. Microstructure of the sintered teeth is identical to that made via traditional dental restoration processes.
Traditional dental restorations are produced by the porcelain-fused-to-metal (PFM) process, in wh... more Traditional dental restorations are produced by the porcelain-fused-to-metal (PFM) process, in which a dental restoration is cast from a metallic alloy and then covered with dental porcelains by several firing processes, which is both labor intensive and expensive. In this paper, the feasibility of dental restorations is investigated using a multi-materials laser densification (MMLD) process. To evaluate the effectiveness of the MMLD process, nickel powders and commercial dental porcelain powders are laser densified using YAG and CO 2 lasers respectively. Effects of processing parameters, e.g. laser scanning rate and target temperature, are evaluated and the microstructure of processed nickel and porcelain materials are characterized for the optimization of laser densification. Results indicate that densities of laser processed nickel and dental porcelain are strongly dependent of processing parameters. Fully dense layers are achievable with proper processing conditions.
The authors acknowledge the support for this research by the Office of Naval Research (grant #N00... more The authors acknowledge the support for this research by the Office of Naval Research (grant #N00014-95-1-0978).
In this study, we report the first investigation of the effectiveness of the partially converted ... more In this study, we report the first investigation of the effectiveness of the partially converted carbon coating from polydopamine (PODA) to improve the cycle stability of Si anode for Li-ion batteries. It is hypothesized that by converting PODA to a partial carbonization condition, the resulting coating could have a higher electrical conductivity than PODA without carbonization, and at the same time may still contain some organic bonds and thus mechanical flexibility to accommodate the volume expansion of Si during lithiation. The results show that such a partial carbonization state can be obtained by carbonization of PODA at 400°C. Furthermore, the partially converted carbon coating can offer sufficient electrical conductivity for lithiation and delithiation of Si anode while drastically reducing the charge transfer resistance for the redox reactions. In addition, the partially-converted-carbon coated hollow Si nanospheres exhibit excellent cycle stability when the volume expansion of Si anode is not very large (~88%) even though this volume expansion is significantly larger than the engineered void space (47% of the Si volume) available inside the partially-converted-carbon coated hollow Si nanospheres, unambiguously confirming the good tolerance of the partially converted carbon coating in withstanding some tensile strain without fracture. This study offers a new direction for systematic studies in the future as a means to provide a coating on Si material with sufficient electrical conductivity along with capability to withstand some tensile strain during the volume expansion of Si, thereby improving the cycle stability of Si anodes.
Lithium-ion capacitors (LICs) have the potential to combine the high energy density of lithium io... more Lithium-ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder through mechanical activation for cathodes of LICs with non-aqueous electrolytes. Traditionally, surface functionalization is carried out via chemical activation or plasma treatment. Here, we show, for the first time, that surface functionalization can be achieved with mechanical activation using high-energy ball milling of AC powder in air at room temperature. It is found that mechanical activation is an effective way to create functional groups on the AC surface, thereby imparting pseudocapacitance and increasing the specific capacitance of the AC powder. The surface functionalization has led to an increase in the specific capacitance from 15.8 F/g for the as-received AC (AR-AC) to 33.5 F/g for the AC with 6-h high-ener...
The effects of major processing parameters of attritor mills on ball milling efficiency (i.e., mi... more The effects of major processing parameters of attritor mills on ball milling efficiency (i.e., minimum energy consumption with maximum milling progress) are investigated using discrete element modeling (DEM). The major processing parameters investigated include the size of balls, ball volume fraction inside the canister, ball milling velocity, and design of the impeller shaft of the attritor mill. Their effects are studied through examination of the output parameters including the average speed of balls, maximum speed of balls, and torque applied on the impeller shaft. The torque on the impeller shaft represents the energy consumption during ball milling, while the difference between the maximum and average speeds of balls scales with the compressive pressure during ''mini-forging'' of powder particles trapped between the colliding balls and thus scales with milling progress (particle deformation and size reduction). The simulation reveals that the ball milling velocity, ball volume fraction inside the canister, ball size, and impeller shaft design are all important parameters for energy-efficient ball milling. In particular, high ball milling velocities can lead to larger particle deformation and faster size reduction with minimum energy consumption. Further, ball sizes smaller than the gap that will not be hit by impellers directly are good for high-energy-efficient ball milling. Otherwise, energy consumption increases substantially.
Lithium ion capacitors (LICs) have the potential to combine the high energy density of lithium io... more Lithium ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder as the cathode for LICs with non-aqueous electrolytes. It is found that solution chemistry treatment is an effective way to impart pseudocapacitance and thus increase the specific capacitance of the AC powder. The surface functionalization has led to increases in the specific capacitance from 18-35 F/g to 80-140 F/g, while the areal specific capacitance per BET surface area has increased from 3.6 F/cm 2 to 74.8 μF/cm 2. The latter is 3.5 times the theorectical electrical double layer value for graphene, indicating the existence of redox reactions and their great potential in enhancing the capacitance for LICs. The mechanism of capacitance improvement has been diagnosed and attributed mainly to the pseudocapacitive redox reaction on the C=O sites. In addition, the enhancement in the specific capacitance is found to vary with the composition of electrolytes, likely due to the change in wetting behavior and the size of solvated ions. This work has opened up a new route to increase the specific capacitance of low cost and widely used AC powder for LICs.
In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell ... more In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell (HSi@C) has been investigated. The key synthesis parameters and their optimization are studied. The positively charged surface of sacrificial polystyrene (PS) nanospheres is found to be essential in forming a uniform SiO2 shell on PS through sol–gel reactions of tetraethylorthosilicate in a basic medium. Both the sol–gel reaction time and hydrolysis ratio play an important role in controlling the thickness of the SiO2 shell. The PS core is burnt via a step-wise programmed heating to prevent the formation of cracks in hollow SiO2 nanospheres which are subsequently converted to hollow Si nanospheres through magnesiothermic reduction using MgH2. Removal of the byproducts from the reduction is critical for superior electrochemical performance. The hollow Si nanospheres are coated with a carbon shell to form HSi@C which are evaluated as the active material for Li-ion battery anodes. Electrochemical results indicate significant improvements in the specific capacity and cycling stability of half-cells in comparison to other designs. The improvement is attributed to the synergistic effects of the engineered void and conductive carbon shell in [email protected] AbstractSpecific capacity and Coulombic efficiency of HSi@C half cells vs. cycle number at different current densities.
Functionalized h‐BN (FBN) is obtained by reacting h‐BN with molten LiOH in a 1:1 molar ratio (sta... more Functionalized h‐BN (FBN) is obtained by reacting h‐BN with molten LiOH in a 1:1 molar ratio (stainless steel autoclave with Cu gasket, 500 °C, 8 h) followed by an intense ball milling of the product phase.
Surface characterization of HA discs X-ray diffraction (XRD, Bruker, D5 and D8 Advanced, Karlsruh... more Surface characterization of HA discs X-ray diffraction (XRD, Bruker, D5 and D8 Advanced, Karlsruhe, Germany) was performed for both the as-synthesized HA powder and sintered HA discs. For the quantification of surface roughness of
We report an ab initio density functional theory (DFT) study that investigates the effect of tran... more We report an ab initio density functional theory (DFT) study that investigates the effect of transition metal dopants on diffusion of Mg vacancies in MgB 2. This study has implications for the diffusion-controlled hydriding kinetics of the technologically important LiBH 4 + MgH 2 hydrogen storage system. The first-principles calculation reveals that the solubility of M (M) Ni, Mn, V, Ti, Sc, and Y) in MgB 2 and the migration barriers of Mg vacancies are dictated by the dopant's atomic radius, whereas the formation energies of Mg vacancies are strongly affected by both the dopant's atomic radius and charge distribution. Mn, Sc, and Y are found to have the potential to enhance the diffusion rate of Mg, whereas V and Ti are poor dopants in enhancing the diffusion rate of Mg in MgB 2. The findings in this study are consistent with existing experimental results.
Major processing factors in forming Fe2SiO4/SiO2 and Fe2O3/SiO2 powders via sol–gel synthesis fol... more Major processing factors in forming Fe2SiO4/SiO2 and Fe2O3/SiO2 powders via sol–gel synthesis followed by solid-state reactions are investigated. The results clearly indicate that the chemical compositions of the precursors, the ratio of the precursors, the nature of the catalyst used, and the gas atmosphere during solid-state reactions can all affect the outcome of the reaction product(s). The formation of Fe2SiO4/SiO2 is enhanced by using the precursor iron(III) acetylacetonate as the Fe source with the precursor ratio of iron(III) acetylacetonate to tetraethyl orthosilicate being 1:1 and the addition of formic acid. Otherwise, crystalline Fe and Fe3C are formed in place of Fe2SiO4. By altering the gas atmosphere during solid-state reactions from argon to oxygen, the reaction products change from Fe2SiO4/SiO2 to Fe2O3/SiO2. All of the observed phenomena can be rationalized via the degree of mixing of the Fe–O and Si–O domains at the molecular level in the gel network during sol–gel reactions and the presence of a reducing or oxidizing atmosphere during the solid-state reaction.
We report here the synthesis of gadolinia-doped ceria (GDC) gels and powders via the sol–gel meth... more We report here the synthesis of gadolinia-doped ceria (GDC) gels and powders via the sol–gel method using cerium acetylacetonate and gadolinium acetylacetonate as precursors. The dependence of the morphology and nature of the sol–gel product on synthesis parameters has been investigated in detail. The sol is synthesized and gelled under various conditions to study the effects of (1) the hydrolysis ratio, (2) the reaction temperature, (3) the reaction time, (4) the addition of acetic acid, and (5) the as-synthesized condition versus the condition with subsequent calcination at 600 °C. It is found that all of these parameters exert strong influence on the morphology and nature of the sol–gel product. Crystalline GDC gels are produced when hydrolysis and gelation are carried out at 55–125 °C. Monolithic gels with low porosity and only micro cracks (shorter than 800 μm and less than 7 μm wide) can be formed with the proper drying procedure at ambient pressure. Although thermal exposure to 600 °C leads to grain growth, the size of GDC crystallites is still less than 9 nm after the thermal exposure.
Using variable temperature in situ1H NMR spectroscopy on a mixture of LiNH2+ LiH that was mechani... more Using variable temperature in situ1H NMR spectroscopy on a mixture of LiNH2+ LiH that was mechanically activated using high-energy ball milling, the dehydrogenation of the LiNH2+ LiH to Li2NH+ H2 was investigated. The analysis indicates NH3 release at a ...
... by the atom-cluster reactions, to produce the desired cluster size and distribution of the ..... more ... by the atom-cluster reactions, to produce the desired cluster size and distribution of the ... diffusion distance from the contact area to the neck zone of the particles, as predicted ... 66. A. Chatterjee and D. Chakravorty, Electrical Conductiv-ity of Sol-Gel Derived Metal Nanoparticles, J ...
Layered lithium nickel manganese cobalt oxides, Li(NixMnyCoz)O2 where x + y + z = 1 (NMCs), have ... more Layered lithium nickel manganese cobalt oxides, Li(NixMnyCoz)O2 where x + y + z = 1 (NMCs), have been studied extensively due to their higher capacity, less toxicity and lower cost compared to LiCoO2. However, widespread market penetration of NMCs as cathodes for Li-ion batteries (LIBs) is impeded by their poor capacity retention and low rate capability. Coatings provide an effective solution to these problems. This article focuses on review of the recent advancements in coatings of NMCs from the mechanism viewpoint. This is the first time that coatings on NMCs are reviewed based on their functionalities and mechanisms through which the electrochemical properties and performance of NMCs have been improved. To provide a comprehensive understanding of the functions and mechanisms offered by coatings, the following functions and mechanisms are reviewed individually: (i) scavenging HF in the electrolyte, (ii) scavenging water molecules in the electrolyte and thus suppressing HF propagation during charge/discharge cycles, (iii) serving as a buffer layer to minimize HF attack on NMCs and suppress side reactions between NMCs and the electrolyte, (iv) hindering phase transitions and impeding loss of lattice oxygen, (v) preventing microcracks in NMC particles to keep participation of most NMC material in lithiation/de-lithiation, and (vi) enhancing the rate capability of NMC cathodes. Finally, the personal perspectives on outlook are offered with an aim to stimulate further discussion and ideas on the rational design of coatings for durable and high performance NMC cathodes for the next generation LIBs in the near future.
Several effective methods have been developed recently to demonstrate simultaneous high energy an... more Several effective methods have been developed recently to demonstrate simultaneous high energy and high power density in Lithium - carbon fluoride (Li-CFx) batteries. These methods can achieve as high as a 1000 Wh/kg energy density at a 60–70 kW/kg power density (40–50 C rate) in coin cells and a 750 Wh/kg energy density at a 12.5 kW/kg power density (20 C rate) in pouch cells. This performance is made possible by an ingenious nano-architecture design, controlled porosity, boron doping, and electrolyte additives. In the present study, we show that a similarly great performance, a 931 Wh/kg energy density at a 59 kW/kg power density, can be achieved by using a polyacrylonitrile binder and a LiBF4 electrolyte in Li-graphite fluoride coin cells. We also demonstrate that the observed effect is the result of the right combination of the binder and the electrolyte. We propose that the mechanistic origin of the observed phenomena is an electro-catalytic effect of the polyacrylonitrile bind...
Silicon has the potential to be a high-performance anode material, but its practical application ... more Silicon has the potential to be a high-performance anode material, but its practical application is impeded by huge volume expansion during lithiation. Many studies have revealed that the huge volume expansion problem can be mitigated by introducing engineered voids into Si/C core–shell structures. In this study, a Si/C core/shell structure with engineered voids, termed Si@void@C, is investigated for its specific capacity and cycle stability as a function of particle size and charge/discharge protocol. The study shows that finer Si@void@C particles result in higher specific capacities, but with little impact on the cycle stability. Further, lower and upper cutoff voltages in charge/discharge have a profound impact on the specific capacity and cycle stability. Importantly, cutoff voltages in formation cycles have long-lasting effects on the cycle stability, indicating the critical role of forming a robust solid electrolyte interphase (SEI) layer during formation cycles. Using a const...
Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly... more Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly from computer based solid models described by STL (Stereo Lithography) or VRML (Virtual Reality Modeling Language) files generated by Computer-Aided Design (CAD) systems. Most of the SFF processes produce parts by building them layer by layer using a row by row pattern, though it is possible to build the part using other patterns. The SFF technology represents a challenge to designers who, in addition to making decisions concerning optimum shape and functionality of the entire part, have'to take under consideration several other manufacturing factors. These factors cover a wide range of technical issues such as Computer-Aided Design model generation, part description and model slicing files, laser path files, precision of part design, rendering patterns, manufacturing tolerances, thermal expansion and residual stress phenomena. This paper investigates the effect of rendering patterns on the integrity, material characteristics and mechanical properties of the parts prepared by a desk-top SFF device using diode lasers. Fe-Bronze (Cu-Sn) premixed metal powders were used as the starting material. The particle size was about 100 /lm to 200 /lm. Density, tensile strength and microstructure of the parts prepared using different rendering patterns were characterized. The results were analyzed to seek optimal rendering patterns. It was noticed that the samples were strong along the laser scanning direction, while they were weak perpendicular to the scanning direction. These results suggest that the laser scanning patterns should be designed to minimize the warping and maximize the strength of the part in the direction depending on the part's function.
In this paper, we describe a solid freeform fabrication procedure for human dental restoration vi... more In this paper, we describe a solid freeform fabrication procedure for human dental restoration via porcelain slurry micro-extrusion. Based on submicron-sized dental porcelain powder obtained via ball milling process, a porcelain slurry formulation has been developed. The formulation developed allows the porcelain slurry to show a pseudoplastic behavior and moderate viscosity, which permits the slurry to re-shape to form a near rectangular cross section. A well-controlled cross-section geometry of the extrudate is important for micro-extrusion to obtain uniform 2-D planes and for the addition of the sequential layers to form a 3-D object. Human teeth are restored by this method directly from CAD digital models. After sintering, shrinkage of the artificial teeth is uniform in all directions. Microstructure of the sintered teeth is identical to that made via traditional dental restoration processes.
Traditional dental restorations are produced by the porcelain-fused-to-metal (PFM) process, in wh... more Traditional dental restorations are produced by the porcelain-fused-to-metal (PFM) process, in which a dental restoration is cast from a metallic alloy and then covered with dental porcelains by several firing processes, which is both labor intensive and expensive. In this paper, the feasibility of dental restorations is investigated using a multi-materials laser densification (MMLD) process. To evaluate the effectiveness of the MMLD process, nickel powders and commercial dental porcelain powders are laser densified using YAG and CO 2 lasers respectively. Effects of processing parameters, e.g. laser scanning rate and target temperature, are evaluated and the microstructure of processed nickel and porcelain materials are characterized for the optimization of laser densification. Results indicate that densities of laser processed nickel and dental porcelain are strongly dependent of processing parameters. Fully dense layers are achievable with proper processing conditions.
The authors acknowledge the support for this research by the Office of Naval Research (grant #N00... more The authors acknowledge the support for this research by the Office of Naval Research (grant #N00014-95-1-0978).
In this study, we report the first investigation of the effectiveness of the partially converted ... more In this study, we report the first investigation of the effectiveness of the partially converted carbon coating from polydopamine (PODA) to improve the cycle stability of Si anode for Li-ion batteries. It is hypothesized that by converting PODA to a partial carbonization condition, the resulting coating could have a higher electrical conductivity than PODA without carbonization, and at the same time may still contain some organic bonds and thus mechanical flexibility to accommodate the volume expansion of Si during lithiation. The results show that such a partial carbonization state can be obtained by carbonization of PODA at 400°C. Furthermore, the partially converted carbon coating can offer sufficient electrical conductivity for lithiation and delithiation of Si anode while drastically reducing the charge transfer resistance for the redox reactions. In addition, the partially-converted-carbon coated hollow Si nanospheres exhibit excellent cycle stability when the volume expansion of Si anode is not very large (~88%) even though this volume expansion is significantly larger than the engineered void space (47% of the Si volume) available inside the partially-converted-carbon coated hollow Si nanospheres, unambiguously confirming the good tolerance of the partially converted carbon coating in withstanding some tensile strain without fracture. This study offers a new direction for systematic studies in the future as a means to provide a coating on Si material with sufficient electrical conductivity along with capability to withstand some tensile strain during the volume expansion of Si, thereby improving the cycle stability of Si anodes.
Lithium-ion capacitors (LICs) have the potential to combine the high energy density of lithium io... more Lithium-ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder through mechanical activation for cathodes of LICs with non-aqueous electrolytes. Traditionally, surface functionalization is carried out via chemical activation or plasma treatment. Here, we show, for the first time, that surface functionalization can be achieved with mechanical activation using high-energy ball milling of AC powder in air at room temperature. It is found that mechanical activation is an effective way to create functional groups on the AC surface, thereby imparting pseudocapacitance and increasing the specific capacitance of the AC powder. The surface functionalization has led to an increase in the specific capacitance from 15.8 F/g for the as-received AC (AR-AC) to 33.5 F/g for the AC with 6-h high-ener...
The effects of major processing parameters of attritor mills on ball milling efficiency (i.e., mi... more The effects of major processing parameters of attritor mills on ball milling efficiency (i.e., minimum energy consumption with maximum milling progress) are investigated using discrete element modeling (DEM). The major processing parameters investigated include the size of balls, ball volume fraction inside the canister, ball milling velocity, and design of the impeller shaft of the attritor mill. Their effects are studied through examination of the output parameters including the average speed of balls, maximum speed of balls, and torque applied on the impeller shaft. The torque on the impeller shaft represents the energy consumption during ball milling, while the difference between the maximum and average speeds of balls scales with the compressive pressure during ''mini-forging'' of powder particles trapped between the colliding balls and thus scales with milling progress (particle deformation and size reduction). The simulation reveals that the ball milling velocity, ball volume fraction inside the canister, ball size, and impeller shaft design are all important parameters for energy-efficient ball milling. In particular, high ball milling velocities can lead to larger particle deformation and faster size reduction with minimum energy consumption. Further, ball sizes smaller than the gap that will not be hit by impellers directly are good for high-energy-efficient ball milling. Otherwise, energy consumption increases substantially.
Lithium ion capacitors (LICs) have the potential to combine the high energy density of lithium io... more Lithium ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder as the cathode for LICs with non-aqueous electrolytes. It is found that solution chemistry treatment is an effective way to impart pseudocapacitance and thus increase the specific capacitance of the AC powder. The surface functionalization has led to increases in the specific capacitance from 18-35 F/g to 80-140 F/g, while the areal specific capacitance per BET surface area has increased from 3.6 F/cm 2 to 74.8 μF/cm 2. The latter is 3.5 times the theorectical electrical double layer value for graphene, indicating the existence of redox reactions and their great potential in enhancing the capacitance for LICs. The mechanism of capacitance improvement has been diagnosed and attributed mainly to the pseudocapacitive redox reaction on the C=O sites. In addition, the enhancement in the specific capacitance is found to vary with the composition of electrolytes, likely due to the change in wetting behavior and the size of solvated ions. This work has opened up a new route to increase the specific capacitance of low cost and widely used AC powder for LICs.
In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell ... more In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell (HSi@C) has been investigated. The key synthesis parameters and their optimization are studied. The positively charged surface of sacrificial polystyrene (PS) nanospheres is found to be essential in forming a uniform SiO2 shell on PS through sol–gel reactions of tetraethylorthosilicate in a basic medium. Both the sol–gel reaction time and hydrolysis ratio play an important role in controlling the thickness of the SiO2 shell. The PS core is burnt via a step-wise programmed heating to prevent the formation of cracks in hollow SiO2 nanospheres which are subsequently converted to hollow Si nanospheres through magnesiothermic reduction using MgH2. Removal of the byproducts from the reduction is critical for superior electrochemical performance. The hollow Si nanospheres are coated with a carbon shell to form HSi@C which are evaluated as the active material for Li-ion battery anodes. Electrochemical results indicate significant improvements in the specific capacity and cycling stability of half-cells in comparison to other designs. The improvement is attributed to the synergistic effects of the engineered void and conductive carbon shell in [email protected] AbstractSpecific capacity and Coulombic efficiency of HSi@C half cells vs. cycle number at different current densities.
Functionalized h‐BN (FBN) is obtained by reacting h‐BN with molten LiOH in a 1:1 molar ratio (sta... more Functionalized h‐BN (FBN) is obtained by reacting h‐BN with molten LiOH in a 1:1 molar ratio (stainless steel autoclave with Cu gasket, 500 °C, 8 h) followed by an intense ball milling of the product phase.
Surface characterization of HA discs X-ray diffraction (XRD, Bruker, D5 and D8 Advanced, Karlsruh... more Surface characterization of HA discs X-ray diffraction (XRD, Bruker, D5 and D8 Advanced, Karlsruhe, Germany) was performed for both the as-synthesized HA powder and sintered HA discs. For the quantification of surface roughness of
We report an ab initio density functional theory (DFT) study that investigates the effect of tran... more We report an ab initio density functional theory (DFT) study that investigates the effect of transition metal dopants on diffusion of Mg vacancies in MgB 2. This study has implications for the diffusion-controlled hydriding kinetics of the technologically important LiBH 4 + MgH 2 hydrogen storage system. The first-principles calculation reveals that the solubility of M (M) Ni, Mn, V, Ti, Sc, and Y) in MgB 2 and the migration barriers of Mg vacancies are dictated by the dopant's atomic radius, whereas the formation energies of Mg vacancies are strongly affected by both the dopant's atomic radius and charge distribution. Mn, Sc, and Y are found to have the potential to enhance the diffusion rate of Mg, whereas V and Ti are poor dopants in enhancing the diffusion rate of Mg in MgB 2. The findings in this study are consistent with existing experimental results.
Major processing factors in forming Fe2SiO4/SiO2 and Fe2O3/SiO2 powders via sol–gel synthesis fol... more Major processing factors in forming Fe2SiO4/SiO2 and Fe2O3/SiO2 powders via sol–gel synthesis followed by solid-state reactions are investigated. The results clearly indicate that the chemical compositions of the precursors, the ratio of the precursors, the nature of the catalyst used, and the gas atmosphere during solid-state reactions can all affect the outcome of the reaction product(s). The formation of Fe2SiO4/SiO2 is enhanced by using the precursor iron(III) acetylacetonate as the Fe source with the precursor ratio of iron(III) acetylacetonate to tetraethyl orthosilicate being 1:1 and the addition of formic acid. Otherwise, crystalline Fe and Fe3C are formed in place of Fe2SiO4. By altering the gas atmosphere during solid-state reactions from argon to oxygen, the reaction products change from Fe2SiO4/SiO2 to Fe2O3/SiO2. All of the observed phenomena can be rationalized via the degree of mixing of the Fe–O and Si–O domains at the molecular level in the gel network during sol–gel reactions and the presence of a reducing or oxidizing atmosphere during the solid-state reaction.
We report here the synthesis of gadolinia-doped ceria (GDC) gels and powders via the sol–gel meth... more We report here the synthesis of gadolinia-doped ceria (GDC) gels and powders via the sol–gel method using cerium acetylacetonate and gadolinium acetylacetonate as precursors. The dependence of the morphology and nature of the sol–gel product on synthesis parameters has been investigated in detail. The sol is synthesized and gelled under various conditions to study the effects of (1) the hydrolysis ratio, (2) the reaction temperature, (3) the reaction time, (4) the addition of acetic acid, and (5) the as-synthesized condition versus the condition with subsequent calcination at 600 °C. It is found that all of these parameters exert strong influence on the morphology and nature of the sol–gel product. Crystalline GDC gels are produced when hydrolysis and gelation are carried out at 55–125 °C. Monolithic gels with low porosity and only micro cracks (shorter than 800 μm and less than 7 μm wide) can be formed with the proper drying procedure at ambient pressure. Although thermal exposure to 600 °C leads to grain growth, the size of GDC crystallites is still less than 9 nm after the thermal exposure.
Using variable temperature in situ1H NMR spectroscopy on a mixture of LiNH2+ LiH that was mechani... more Using variable temperature in situ1H NMR spectroscopy on a mixture of LiNH2+ LiH that was mechanically activated using high-energy ball milling, the dehydrogenation of the LiNH2+ LiH to Li2NH+ H2 was investigated. The analysis indicates NH3 release at a ...
... by the atom-cluster reactions, to produce the desired cluster size and distribution of the ..... more ... by the atom-cluster reactions, to produce the desired cluster size and distribution of the ... diffusion distance from the contact area to the neck zone of the particles, as predicted ... 66. A. Chatterjee and D. Chakravorty, Electrical Conductiv-ity of Sol-Gel Derived Metal Nanoparticles, J ...
The ASM Materials Synthesis & Processing Committee and the TMS Solidification Committee fully sup... more The ASM Materials Synthesis & Processing Committee and the TMS Solidification Committee fully supported and approved this conference in October 2009. Shortly after the approval of the conference, leading scientists and researchers in the areas of processing, crystal growth, and phase equilibrium were invited to give invited presentations in this historic conference in honor of Reza Abbaschian's 40-year contribution to the theoretical and experimental aspects of materials processing, crystal growth, and phase equilibrium. The responses to the invitation and the call for papers were tremendous. As a result, a total of 42 presentations (18 invited and 24 contributed) from researchers and scientists around the world were selected by the organizing committee.
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Article by Leon Shaw
Papers by Leon Shaw