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Recently, interest in integrated assembly sequence planning (ASP) and assembly line balancing (ALB) began to pick up because of its numerous benefits, such as the larger search space that leads to better solution quality, reduced error rate in planning, and expedited product time-to-market. However, existing research is limited to the simple assembly problem that only runs one homogenous product. This paper therefore models and optimizes the integrated mixed-model ASP and ALB using Multi-objective Discrete Particle Swarm Optimization (MODPSO) concurrently. This is a new variant of the integrated assembly problem. The integrated mixed-model ASP and ALB is modeled using task-based joint precedence graph. In order to test the performance of MODPSO to optimize the integrated mixed-model ASP and ALB, an experiment using a set of 51 test problems with different difficulty levels was conducted. Besides that, MODPSO coefficient tuning was also conducted to identify the best setting so as to optimize the problem. The results from this experiment indicated that the MODPSO algorithm presents a significant improvement in term of solution quality toward Pareto optimal and demonstrates the ability to explore the extreme solutions in the mixed-model assembly optimization search space. The originality of this research is on the new variant of integrated ASP and ALB problem. This paper is the first published research to model and optimize the integrated ASP and ALB research for mixed-model assembly problem.
Ca3Co4O9 (CCO) is a promising material for thermoelectric applications; however, this layered oxide shows a large number of physical features that complicate understanding and systematically improving its properties. A significant component of CCO’s behavior is its magnetotransport properties, particularly in the low temperature region where an incommensurate spin density wave affects its band structure. In order to improve understanding in this area, we perform low temperature magnetoresistance (MR) measurements on a bulk CCO sample. Field-less resistivity measurements confirm the conventional behavior of our sample, with a metal-to-insulator transition at approximately 70 K, and a shoulder indicating ferrimagnetism at 14 K. Resistivity vs. temperature under applied magnetic field show significant MR below around 35 K.
The electrochemical effects of embedding Cu nanoparticles in carbonized wood supercapacitor electrodes have been investigated. The nanoparticles were embedded using a solution method. Subsequent X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the Cu nanoparticles were anchored uniformly on the surface and deep within the pores of the electrode. Cyclic voltammetry measurements showed that the electrode has typical pseudocapacitive behavior, with two pairs of redox reaction peaks. The charge-discharge cycling also indicated that the redox charge transformation was a reversible process. An ultra-high specific capacitance of 888 F/g and an energy density of 123 Wh/kg were observed for the Cu loaded electrodes, as compared to the pure carbonized wood electrodes, which had a specific capacitance of 282 F/g and an energy density of 39 Wh/kg. Furthermore, both the carbonized wood and Cu loaded electrodes exhibited excellent long cycle abilities with at least 95% of the specific capacitance retained after 2000 cycles. These remarkable results demonstrate the potential for using Cu nanoparticle loaded carbonized wood as a high performance and environmentally friendly supercapacitor electrode material.
The solid state electrolyte (SSE) of Li5La3Nb2O12 (LLNO) was synthesized via a novel molten salt synthesis (MSS) method at the relatively low temperature of 900°C. The low sintering temperature prevented the loss of lithium that commonly occurs during synthesis of the SSE using conventional solid state or wet chemical reactions. Recent publications have demonstrated that preserving the Li content is critical in improving the ionic conductivity of SSEs. The LLNO in this experiment showed a high Li-ion conductivity which is comparable to other values reported for LLNO. X-ray diffraction (XRD) measurements confirmed the formation of the cubic garnet Ia-3d crystal structure. In addition, the morphology was examined by scanning electron microscopy (SEM), which showed a uniform grain size and crack-free microstructure. These results demonstrate that MSS is a powerful synthesis method to fabricate LLNO at a relatively low temperature while still achieving a high quality material.
In this study, we are reporting the time- and temperature-dependence of the electrical resistivity and temperature-dependence of the Hall voltage in neodymium nickelate thin films. The films were deposited on a lanthanum aluminate substrate [LaAlO3 (001)] by a pulsed laser deposition technique, with thicknesses ranging from 0.6 to 120 nm. Time-dependent electrical transport measurements indicated the formation of a kinetically stable metallic glassy phase rather than a stable insulating phase on cooling below the transition temperature, TM-I. Comparisons of the low-temperature behavior with that of common insulators further supported this claim. Hall effect measurements on the 1.2-nm sample showed a local maximum in the carrier concentration just below the TM-I on both the heating and cooling cycles. This again confirmed the proposed low-temperature structure, in that, for the 1.2-nm sample, there was a minimal degree of supercooling before transitioning to a kinetically stable glassy phase.
Li2FeP2O7 is a newly developed polyanionic cathode material for high performance lithium ion batteries. It is considered very attractive due to its large specific capacity, good thermal and chemical stability, and environmental benignity. However, the application of Li2FeP2O7 is limited by its low ionic and electronic conductivities. To overcome the above problem, a solution-based technique was successfully developed to synthesize Li2FeP2O7 powders with very fine and uniform particle size (< 1 μm), achieving much faster kinetics. The obtained Li2FeP2O7 powders were tested in lithium ion batteries by measurements of cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge cycling. We found that the modified Li2FeP2O7 cathode could maintain a relatively high capacity even at fast discharge rates.
A pulsed laser deposition system was employed to fabricate thin films of Li7La3Zr2O12 solid electrolyte. The deposition process was carried out at room-temperature, resulting in amorphous films. These as-deposited films had a large optical band gap of 5.13 eV, and exhibited a lithium-ion conductivity of 3.35×10-7 S/cm. The films were then annealed, and the effect of annealing on the optical and electrical properties of the films was examined. After annealing at 1000 °C, the films were found to be cubic with a narrower band gap of 3.64 eV. In addition, these annealed films showed an inferior ionic conductivity than the as-deposited ones.
Electrical transport properties in ultrathin NdNiO3 films grown on single crystal LaAlO3(001) substrate were characterized. Films with thicknesses ranging from 0.6 nm to 12 nm were grown using a pulsed laser technique. Four probe resistivity as a function of temperature measurements indicated a strong dissipation of strain effects from 0.6 nm to 6 nm as well as the presence of defects in the 12 nm sample. A proposed mechanism of kinetically stable glassy phase formation explains the time dependence of the resistivity in both cooling and heating cycles.
High quality garnet-type Li7La3Zr2O12 solid electrolyte was synthesized using a solution-based technique. The electrolyte pellets were sintered at 900 oC, resulting in tetragonal phase, which then transformed to cubic phase after annealing at 1230 oC. The ionic conductivity of both phases was studied and revealed to be 3.67x10-7 S/cm and 1.67×10-4 S/cm, respectively. A proto-type cell comprising of Li7La3Zr2O12 electrolyte, LiCoO2 cathode and lithium metal anode was assembled. The cell made with the cubic phase electrolyte exhibited superior performance than the one made with the tetragonal phase electrolyte. The former cell possessed a very promising gravimetric discharge capacity of 3.4 mAh/g, which is the highest value obtained among similar setups.
CZTS (Copper zinc tin sulfide) thin films have been synthesized on transparent conducting oxide (TCO) back contacts on a glass substrate, allowing sun light to pass through the entire solar cell. Aqueous solution based co-electrodeposition followed by elevated temperature sulfurization, was used to grow CZTS on transparent fluorinated tin oxide. Loss in conductivity of FTO is observed after sulfurization, causing reduced device efficiency. Increased resistivity of the FTO is likely due to outdiffusion process. A systematic study of resistivity of back contact at various sulfurization temperatures and times is discussed. Various remedial measures for improved conductivity of back contact were proposed and conducted.
Cu2ZnSnS4 (CZTS), an emerging p-type quaternary chalcogenide, offers many potential advantages as an absorber material. Using factorial design of experiments approach, single stage Cu-Zn-Sn co-electrodeposition from aqueous solution followed by annealing is reported in this paper. Factorial experiments facilitate to study the effects of each factor on the response variable as well as effects of interactions between individual factors on the response variable. Selected factors include concentration of individual ionic species, time of sulfurization and amount of complexing agent, whereas CZTS phase, band gap, carrier concentration, open circuit voltage, and morphological characteristics are the response variables. A model has been developed to show and predict the domain for the best possible factors for CZTS based device fabrication.
In this paper, we report the growth of ZnO films on silicon substrates using a pulsed laser deposition technique. These films were deposited on Si(111) directly as well as by using thin buffer layers of AlN and GaN. All the films were found to have c-axis-preferred orientation aligned with normal to the substrate. Films with AlN and GaN buffer layers were epitaxial with preferred in-plane orientation, while those directly grown on Si(111) were found to have random in-plane orientation. A decrease in the frequency of the Raman mode and a red shift of the band-edge photoluminescence peak due to the presence of tensile strain in the film, was observed. Various possible sources for the observed biaxial strain are discussed.
TaN has become a very promising diffusion barrier material for Cu interconnections, due to the high thermal stability requirement and thickness limitation for next generation ULSI devices. TaN has a variety of phases and Cu diffusion characteristics vary with different phases and microstructures. We have investigated the diffusivity of copper in single-crystal (NaCl-structured) and polycrystalline TaN thin films grown by pulsed laser deposition. The polycrystalline TaN films were grown directly on Si(100), while the single crystal films were grown with TiN buffer layers. Both of poly and single-crystal films with Cu overlayers were annealed at 500 °C, 600 °C, 650 °C, and 700 °C in vacuum to study the copper diffusion characteristics. The diffusion of copper into TaN was studied using STEM-Z contrast, where the contrast is proportional to Z (atomic number), and TEM. The diffusion distances are found to be about 5nm at 650°C for 30 min annealing. The diffusivity of Cu into single crystal TaN follows the relation D = (160±9.5)exp[-(3.27 ±0.1)eV/kBT]cm2s-1 in the temperature range of 600°C to 700°C. We observe that Cu diffusion in polycrystalline TaN thin films is nonuniform with enhanced diffusivities along the grain boundary.
Epitaxial ZnO films have been grown on Si(111) substrates by employing a AlN buffer layer during a pulsed laser-deposition process. The epitaxial structure of AlN on Si(111) substrate provides a template for ZnO growth. The resultant films are evaluated by transmission electron microscopy, x-ray diffraction, and electrical measurements. The results of x-ray diffraction and electron microscopy on these films clearly show the epitaxial growth of ZnO films with an orientational relationship of ZnO||Aln||Si along the growth direction and ZnO[2 11 0]||AlN[2 11 0]||Si[0 11] along the in-plane direction. High electrical conductivity (103 S/m at 300 K) and a linear I-V characteristics make these epitaxial films ideal for microelectronic, optoelectronic, and transparent conducting oxide applications.
We have developed a technique to grow self-aligned epitaxial Cu/MgO films on Si (100) using Pulsed Laser Deposition Method. In this method we deposit a uniform film of Cu/Mg (5-7%) alloy over Si (100) at room temperature using TiN as an intermediate buffer layer. As a result of HRTEM (with spatial resolution of 0.18 nm) and STEM-Z investigations we observed that when this film is annealed at 500°C (in a controlled oxygen environment), in less than 30 minutes time, all the Mg segregates at the top and at the bottom surface of Cu. This is understood to be the consequence of lower surface energy of Mg. At 500°C Mg is quite sensitive to oxygen and thin layer of MgO is immediately formed at the top surface, we also observed a thin layer of MgO at the Cu/TiN interface. Thickness of the upper MgO layer was found to be 15 nm while that of lower layer was 10 nm. MgO underneath layer acts as a diffusion barrier and inhibits the diffusion of Cu in the system. Upper MgO layer acts as a passivating layer and improves the quality of copper against oxidation. Electrical resistivity measurements (in the temperature range 12-300 K) showed MgO/Cu/MgO/TiN/Si (100) sample to be highly conducting. We also observed that the resistivity of the system is insensitive to ambient oxygen environment. Selfaligned MgO (100) layer also provides a means to grow several interesting materials over it. This technique can be used to integrate high temperature superconductors like YBa2Cu3O7 with silicon chip.
We have successfully grown epitaxial cubic (B1-NaCl structure) tantalum nitride films on Si (100) and (111) substrate using a pulsed laser deposition technique. A thin layer of titanium nitride was used as a buffer medium. We characterized these films using X-ray diffraction, high resolution transmission electron microscopy and scanning transmission electron microscopy (Zcontrast). X-ray diffraction and high-resolution transmission electron microscopy confirmed the single crystalline nature of these films with cubic-on-cubic epitaxy. The epitaxial relations follow TaN(100)//TiN(100)//Si(100) on Si(100) and TaN(111)//TiN(111)//Si(111) on Si(111). We observed sharp interfaces of TaN/TiN and TiN/Si without any indication of interfacial reaction. Rutherford backscattering experiments showed these films to be slightly nitrogen deficient (TaN0.95). High precision electrical resistivity measurements showed excellent metallic nature of these films. We also tried to deposit TaN directly on silicon, the films were found to be polycrystalline. In our method, TiN plays a key role in facilitating the epitaxial growth of TaN. This method exploits the concept of lattice matching epitaxy between TaN and TiN and domain matching epitaxy between TiN and Si. We studied the diffusion barrier properties of these films by growing a thin layer of copper on the top and subsequently annealing the films at 500°C and 600°C in vacuum. Cu diffusion layer was about 2nm after 600°C annealing for 30min. This work explores a promising way to grow high quality TaN diffusion barrier on silicon for copper interconnection.
TiN-AlN binary-components have attracted a lot of interests in coatings of high speed cutting tools, due to their higher oxidation resistance, higher hardness, lower internal stresses and better adhesion. Especially, nanometer-scale multilayer structures of AlN/TiN show superior structural and mechanical properties due to their tremendous interface area and become one of the promising candidates for superhard coatings. Here we present a novel method to grow highly aligned TiN/AlN superlattice by pulsed laser deposition. In this method TiN and AlN targets are arranged in a special configuration that they can be ablated in sequence, giving alternate layer by layer growth of TiN(1nm)/AlN(4nm). X-ray diffraction and transmission electron microscopy (TEM) analysis showed the structure to be cubic for both TiN and AlN in the nanoscale multilayers. Microstructure and uniformity for the superlattice structure were studied by TEM and Scanning transmission electron microscopy with Z-contrast (STEM). Nanoindentation results indicated a higher hardness for this new structure than pure AlN and rule-of-mixtures value. Four point probe electrical resistivity measurements showed overall insulating behavior.
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