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• 1. [Article] Long-Term Stability of 14YT-4Sc Alloy at High Temperature

  14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) ...

  Citation × Citation Citation Abstract Copy Title: Long-Term Stability of 14YT-4Sc Alloy at High Temperature Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) for 1 hour. Here we report that the microstructure and mechanical behavior of 14YT-4Sc alloy after long-term annealing for up to 60 hours at 1000 °C. FIB analysis shows abnormal grain growth with annealing time, while a large fraction of the matrix still consists of nanoscale grains. TEM images reveal a slight growth of nano grains, with estimated grain growth exponent, n, to be 0.29. Sc-Ti-Y-O enriched nano oxide particles (nm) were observed in EDS mapping of the 14YT-4Sc-60h sample. The nano-structure retention at the high temperature of 1000 °C for 60 hours is attributed to these complex nano oxides. Title: Long-Term Stability of 14YT-4Sc Alloy at High Temperature Author: Li, Lulu, Xu, Weizong, Saber, Mostafa, Zhu, Yuntian, Koch, Carl C., Scattergood, Ronald O. Year: 2015 14YT alloy (Fe-14Cr-0.25wt.%Y2O3-0.4wt.%Ti) with 4 at.% Sc addition was previously reported to exhibit a nanoscale microstructure and high strength when annealed at temperatures up to 1000 °C (0.65Tm) for 1 hour. Here we report that the microstructure and mechanical behavior of 14YT-4Sc alloy after long-term annealing for up to 60 hours at 1000 °C. FIB analysis shows abnormal grain growth with annealing time, while a large fraction of the matrix still consists of nanoscale grains. TEM images reveal a slight growth of nano grains, with estimated grain growth exponent, n, to be 0.29. Sc-Ti-Y-O enriched nano oxide particles (nm) were observed in EDS mapping of the 14YT-4Sc-60h sample. The nano-structure retention at the high temperature of 1000 °C for 60 hours is attributed to these complex nano oxides. Close

• 2. [Article] Analysis of Cellulose Nanocrystal Aerogels

  Aerogels are interesting material systems with outstanding properties such as low density, high surface area, and low thermal conductivity. They can be used for a variety of applications, such as solar ...

  Citation × Citation Citation Abstract Copy Title: Analysis of Cellulose Nanocrystal Aerogels Author: Buesch, Christian Aerogels are interesting material systems with outstanding properties such as low density, high surface area, and low thermal conductivity. They can be used for a variety of applications, such as solar cells, sensors, or catalysts. Additionally they provide a unique model system to study the properties of small particles. Cellulose Nanocrystals (CNCs) are the nanoscale extractions of large cellulose fibers in a variety of plants and animals such as wood, paper, cotton, or tunicates. Their nanoscale size coupled with high mechanical strength, biodegradability, and renewability make them an interesting alternative to oil based materials for composites with applications in various areas such as electronics, packaging, or tissue engineering. In this work aerogels are used to study CNCs by first creating CNC aerogels followed by structure investigations of aerogels, and finally the study of their mechanical properties to find the strength of CNC bonds. With the understanding of the bond strength, CNC nanocomposite material system development can advance to the next level. CNC aerogels are produced from carboxylated nanocrystalline cellulose organogels by acetone exchange followed by supercritical CO₂ drying. Atomic layer deposition is used to coat CNC aerogels with a thin conformal layer of Al₂O₃. The resulting three dimensional nanoporous structure is analyzed via transmission electron microscope tomography to find the number of CNC bonds within the aerogel. Uniaxial compression tests are performed to determine the mechanical properties of the aerogels. The resulting aerogels are highly transparent, extremely light weight and have a large surface area. The deposited layer of alumina increases the resistance of the aerogels to moisture and elevated temperatures, and provides protection for CNCs against the electron beam for imaging. Tomographic data shows the nanoscale structure of the CNCs in the aerogel and enables the qualitative and quantitative characterization of the bonds. The mechanical properties are found to depend mostly on the density of the aerogel including the number of CNCs and the number of bonds. The required work to mechanically break CNC bonds is measured and the resulting CNC bond strength suggests that the CNCs are bonded together by hydrogen bonds located at the surface of the CNCs. Title: Analysis of Cellulose Nanocrystal Aerogels Author: Buesch, Christian Aerogels are interesting material systems with outstanding properties such as low density, high surface area, and low thermal conductivity. They can be used for a variety of applications, such as solar cells, sensors, or catalysts. Additionally they provide a unique model system to study the properties of small particles. Cellulose Nanocrystals (CNCs) are the nanoscale extractions of large cellulose fibers in a variety of plants and animals such as wood, paper, cotton, or tunicates. Their nanoscale size coupled with high mechanical strength, biodegradability, and renewability make them an interesting alternative to oil based materials for composites with applications in various areas such as electronics, packaging, or tissue engineering. In this work aerogels are used to study CNCs by first creating CNC aerogels followed by structure investigations of aerogels, and finally the study of their mechanical properties to find the strength of CNC bonds. With the understanding of the bond strength, CNC nanocomposite material system development can advance to the next level. CNC aerogels are produced from carboxylated nanocrystalline cellulose organogels by acetone exchange followed by supercritical CO₂ drying. Atomic layer deposition is used to coat CNC aerogels with a thin conformal layer of Al₂O₃. The resulting three dimensional nanoporous structure is analyzed via transmission electron microscope tomography to find the number of CNC bonds within the aerogel. Uniaxial compression tests are performed to determine the mechanical properties of the aerogels. The resulting aerogels are highly transparent, extremely light weight and have a large surface area. The deposited layer of alumina increases the resistance of the aerogels to moisture and elevated temperatures, and provides protection for CNCs against the electron beam for imaging. Tomographic data shows the nanoscale structure of the CNCs in the aerogel and enables the qualitative and quantitative characterization of the bonds. The mechanical properties are found to depend mostly on the density of the aerogel including the number of CNCs and the number of bonds. The required work to mechanically break CNC bonds is measured and the resulting CNC bond strength suggests that the CNCs are bonded together by hydrogen bonds located at the surface of the CNCs. Close

• 3. [Article] Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors

  Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks ...

  Citation × Citation Citation Abstract Copy Title: Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors Author: Jin, Hyung Dae Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks are used to lower the fabrication cost of the absorber layers of the solar cells. In addition, some quantum confined nanocrystals display electron-hole pair generation phenomena with greater than 100% quantum yield, called multiple exciton generation (MEG). These quantum dots could potentially be used to fabricate solar cells that exceed the Schockley-Queisser limit. At present, continuous syntheses of nanoparticles using microreactors have been reported by several groups. Microreactors have several advantages over conventional batch synthesis. One advantage is their efficient heat transfer and mass transport. Another advantage is the drastic reduction in the reaction time, in many cases, down to minutes from hours. Shorter reaction time not only provides higher throughput but also provide better particle size control by avoiding aggregation and by reducing probability of oxidizing precursors. In this work, room temperature synthesis of Au₁₁ nanoclusters and high temperature synthesis of chalcogenide nanocrystals were demonstrated using continuous flow microreactors with high throughputs. A high rate production of phosphine-stabilized Au₁₁ nanoclusters was achieved using a layer-up strategy which involves the use of microlamination architectures; the patterning and bonding of thin layers of material (laminae) to create a multilayered micromixer in the range of 25-250 µm thick was used to step up the production of phosphine-stabilized Au₁₁ nanoclusters. Continuous production of highly monodispersed phosphine-stabilized Au₁₁ nanoclusters at a rate of about 11.8 [mg/s] was achieved using a microreactor with a size of 1.687cm³. This result is about 30,000 times over conventional batch synthesis according to production rate/per reactor volume. We have elucidated the formation mechanism of CuInSe₂ nanocrystals for the development of a continuous flow process for their synthesis. It was found that copper-rich CuInSe₂ with a sphalerite structure was formed initially followed by the formation of more ordered CuInSe₂ at longer reaction times, along with the formation of Cu₂Se and In₂Se₃. It was found that Cu₂Se was formed at a much faster rate than In₂Se₃ under the same reaction conditions. By adjusting the Cu/In precursor ratio, we were able to develop a very rapid and simple synthesis of CuInSe₂ nanocrystals using a continuous flow microreactor with a high throughput per reactor volume. The microreactor has a simple design which uses readily available low cost components. It comprised an inner microtube to precisely control the injection of TOPSe into a larger diameter tube that preheated CuCl and InCl₃ hot mixture was pumped through. Rapid injection plays an important role in dividing the nucleation and growth process which is crucial in getting narrow size distribution. The design of this microreactor also has the advantages of alleviating sticking of QDs on the growth channel wall since QDs were formed from the center of the reactor. Furthermore, size-controlled synthesis of CuInSe₂ nanocrystals was achieved using this reactor simply by adjusting ratio between coordinating solvents. Semiconductors with a direct bandgap between 1 and 2eV including Cu(In,Ga)Se₂ (1.04 – 1.6eV) and CuIn(Se,S)₂ (1.04 - 1.53eV) are ideal for single junction cells utilize the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic QD-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, Cd[subscript X]Hg[subscript 1-X]Te), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters. First continuous synthesis of shape-controlled SnTe nanocrystals were also accomplished in this work. SnCl₂, and TOPTe were used as reactants successfully in coordinating OA and TOP solvents. Both rod shape and dot shape SnTe nanocrystals with uniform size distributions could be obtained. A blue shift was observed from these SnTe nanocrystals. Production rate at about 5mg/min (300mg/hr) was achieved using a microreactor at a size of 1.78cm³. Title: Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors Author: Jin, Hyung Dae Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks are used to lower the fabrication cost of the absorber layers of the solar cells. In addition, some quantum confined nanocrystals display electron-hole pair generation phenomena with greater than 100% quantum yield, called multiple exciton generation (MEG). These quantum dots could potentially be used to fabricate solar cells that exceed the Schockley-Queisser limit. At present, continuous syntheses of nanoparticles using microreactors have been reported by several groups. Microreactors have several advantages over conventional batch synthesis. One advantage is their efficient heat transfer and mass transport. Another advantage is the drastic reduction in the reaction time, in many cases, down to minutes from hours. Shorter reaction time not only provides higher throughput but also provide better particle size control by avoiding aggregation and by reducing probability of oxidizing precursors. In this work, room temperature synthesis of Au₁₁ nanoclusters and high temperature synthesis of chalcogenide nanocrystals were demonstrated using continuous flow microreactors with high throughputs. A high rate production of phosphine-stabilized Au₁₁ nanoclusters was achieved using a layer-up strategy which involves the use of microlamination architectures; the patterning and bonding of thin layers of material (laminae) to create a multilayered micromixer in the range of 25-250 µm thick was used to step up the production of phosphine-stabilized Au₁₁ nanoclusters. Continuous production of highly monodispersed phosphine-stabilized Au₁₁ nanoclusters at a rate of about 11.8 [mg/s] was achieved using a microreactor with a size of 1.687cm³. This result is about 30,000 times over conventional batch synthesis according to production rate/per reactor volume. We have elucidated the formation mechanism of CuInSe₂ nanocrystals for the development of a continuous flow process for their synthesis. It was found that copper-rich CuInSe₂ with a sphalerite structure was formed initially followed by the formation of more ordered CuInSe₂ at longer reaction times, along with the formation of Cu₂Se and In₂Se₃. It was found that Cu₂Se was formed at a much faster rate than In₂Se₃ under the same reaction conditions. By adjusting the Cu/In precursor ratio, we were able to develop a very rapid and simple synthesis of CuInSe₂ nanocrystals using a continuous flow microreactor with a high throughput per reactor volume. The microreactor has a simple design which uses readily available low cost components. It comprised an inner microtube to precisely control the injection of TOPSe into a larger diameter tube that preheated CuCl and InCl₃ hot mixture was pumped through. Rapid injection plays an important role in dividing the nucleation and growth process which is crucial in getting narrow size distribution. The design of this microreactor also has the advantages of alleviating sticking of QDs on the growth channel wall since QDs were formed from the center of the reactor. Furthermore, size-controlled synthesis of CuInSe₂ nanocrystals was achieved using this reactor simply by adjusting ratio between coordinating solvents. Semiconductors with a direct bandgap between 1 and 2eV including Cu(In,Ga)Se₂ (1.04 – 1.6eV) and CuIn(Se,S)₂ (1.04 - 1.53eV) are ideal for single junction cells utilize the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic QD-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, Cd[subscript X]Hg[subscript 1-X]Te), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters. First continuous synthesis of shape-controlled SnTe nanocrystals were also accomplished in this work. SnCl₂, and TOPTe were used as reactants successfully in coordinating OA and TOP solvents. Both rod shape and dot shape SnTe nanocrystals with uniform size distributions could be obtained. A blue shift was observed from these SnTe nanocrystals. Production rate at about 5mg/min (300mg/hr) was achieved using a microreactor at a size of 1.78cm³. Close

• 4. [Article] Poly(vinyl alcohol) / cellulose nanocomposite barrier films

  There is a continual interest in developing robust, flexible, durable, lightweight, waterborne polymer barrier coatings which are increasingly resistant towards both chemical warfare agents as well as ...

  Citation × Citation Citation Abstract Copy Title: Poly(vinyl alcohol) / cellulose nanocomposite barrier films Author: Paralikar, Shweta There is a continual interest in developing robust, flexible, durable, lightweight, waterborne polymer barrier coatings which are increasingly resistant towards both chemical warfare agents as well as an ever growing number of toxic industrial chemicals. In this study, barrier films were prepared with poly(vinyl alcohol) (PVOH) and varying amounts of cellulose nanocrystals (CNXLs) as filler. Poly(acrylic acid) (PAA) was used as a crosslinking agent to provide water resistance to PVOH. The films were heat treated at various temperatures (125, 150, 170, 185 °C) in order to determine the optimum crosslinking density. Heat treatment at 170 °C for 45 minutes resulted in films with improved water resistance without polymer degradation. Infrared spectroscopy (FTIR) indicated ester bond formation with heat treatment. Mechanical tests showed that films with 10%CNXLs/ 10%PAA/ 80%PVOH had the highest tensile strength, tensile modulus and toughness of all the films studied. Polarized optical microscopy and atomic force microscopy showed agglomeration of CNXLs at filler loadings of 15% CNXLs. A thermogravimetric analysis (DTGA) showed highly synergistic effects with 10%CNXLs/ 10%PAA/ 80%PVOH and supported the tensile test results. The purpose of these barrier films is to prevent the diffusion of chemical warfare agents while allowing moisture to pass through to allow breathability. Water vapor transmission indicated that all the films allowed moisture to pass. However, moisture diffusion was reduced by the presence of both CNXLs and PAA compared to pure PVOH. The crystalline nature of CNXLs causes the diffusing molecules to undergo a tortuous path, while the crosslinking forms a network structure which reduces diffusion. A standard time lag diffusion test utilizing permeation cups was used to study the chemical barrier properties. The film containing 10%CNXL/ 10%PAA/ 80%PVOH showed an improvement of 90% compared to 100% PVOH film. Surface modification of CNXLs was successful and well dispersed carboxylated CNXLs were obtained. Carboxylated cellulose nanocrystals (C.CNXLs) showed less agglomeration, improved interaction, slightly reduced flux and slightly increased time lags compared to CNXLs. Title: Poly(vinyl alcohol) / cellulose nanocomposite barrier films Author: Paralikar, Shweta There is a continual interest in developing robust, flexible, durable, lightweight, waterborne polymer barrier coatings which are increasingly resistant towards both chemical warfare agents as well as an ever growing number of toxic industrial chemicals. In this study, barrier films were prepared with poly(vinyl alcohol) (PVOH) and varying amounts of cellulose nanocrystals (CNXLs) as filler. Poly(acrylic acid) (PAA) was used as a crosslinking agent to provide water resistance to PVOH. The films were heat treated at various temperatures (125, 150, 170, 185 °C) in order to determine the optimum crosslinking density. Heat treatment at 170 °C for 45 minutes resulted in films with improved water resistance without polymer degradation. Infrared spectroscopy (FTIR) indicated ester bond formation with heat treatment. Mechanical tests showed that films with 10%CNXLs/ 10%PAA/ 80%PVOH had the highest tensile strength, tensile modulus and toughness of all the films studied. Polarized optical microscopy and atomic force microscopy showed agglomeration of CNXLs at filler loadings of 15% CNXLs. A thermogravimetric analysis (DTGA) showed highly synergistic effects with 10%CNXLs/ 10%PAA/ 80%PVOH and supported the tensile test results. The purpose of these barrier films is to prevent the diffusion of chemical warfare agents while allowing moisture to pass through to allow breathability. Water vapor transmission indicated that all the films allowed moisture to pass. However, moisture diffusion was reduced by the presence of both CNXLs and PAA compared to pure PVOH. The crystalline nature of CNXLs causes the diffusing molecules to undergo a tortuous path, while the crosslinking forms a network structure which reduces diffusion. A standard time lag diffusion test utilizing permeation cups was used to study the chemical barrier properties. The film containing 10%CNXL/ 10%PAA/ 80%PVOH showed an improvement of 90% compared to 100% PVOH film. Surface modification of CNXLs was successful and well dispersed carboxylated CNXLs were obtained. Carboxylated cellulose nanocrystals (C.CNXLs) showed less agglomeration, improved interaction, slightly reduced flux and slightly increased time lags compared to CNXLs. Close