Materials Science Forum Vols. 510-511

Abstract: The cold spray process is a relatively new process using high velocity metallic particles for surface modifications. Metallic powder particles which are injected into a converging-diverging nozzle are accelerated to supersonic velocities. In this study effects of nozzle design such as diameter of nozzle throat and length of converging and diverging part of the nozzle on gas and particle velocities developed after the nozzle exit are investigated.

Abstract: Silicon to silicon wafer bonding at low temperature of 300 °C using residual stress controlled evaporated 2 ㎛ thick Pyrex glass thin film (briefly, glass thin film) on silicon wafer was investigated. It was found that residual stresses of 2 ㎛ thick glass films on silicon wafers were strongly dependent upon moisture contents and annealing processes. Residual stresses of asdeposited glass films with compressive stress of -150 MPa could be changed to more compressive stress of -230 MPa by moisture absorption. However, after annealing process at 200 °C to 400 °C for 30 min, residual stresses were remarkably changed to tensile stresses of about 75 MPa to 130 MPa, respectively. For the reliable wafer bonding process, the evaporated glass thin films should be annealed in the range of 200 °C to 500 °C for 30 min. So, bare silicon to bare silicon and bare silicon to patterned silicon were bonded at 300 °C and 30 V ~ 60 V for 15 min using 2 ㎛ thick glass film with residual stress of 130 MPa which were generated after the annealing process of 400 °C for 30 min. These results could be used for low temperature silicon to silicon wafer bondings for applications of micro sensors, micro actuators and micro fluidics devices.

Abstract: Platinum/Palladium modified aluminide coatings prepared by aluminide pack cementation on the nickel base superalloy Inconnel 738. The platinum/palladium modified aluminide coating of cyclic oxidation behavior at 1200°C was investigated by TGA, XRD and SEM/EDS. Platinum/Palladium modified aluminide coatings showed better cyclic oxidation resistance than Platinum modified aluminide coating and palladium modified aluminide coating compared. Pt and Pd alloy played an enough role in alumina stabilization and in delaying the degradation of β-phase.

Abstract: The effects of tungsten content on the nanocrystalline structure of Ni-W alloy electrodeposits were investigated. Direct current electrodeposition was conducted with various W mole fractions in a nickel sulfate solution containing citric acid as a complexing agent. Current efficiency was an almost constant value of 52% up to W mole fraction of 0.58. However, it was sharply decreased owing to severe H2 evolution with increasing cathode overvoltage at a W mole fraction higher than 0.74. The X-ray diffraction peak was decreased and broadened with increasing W content, and an amorphous pattern appeared clearly at 49 wt.% W. It was confirmed that Ni-W alloy deposit was transformed from crystalline to amorphous structure between 41 and 49 wt.% W. This result was also verified through a TEM analysis.

Abstract: The unprecedented technology advancements in miniaturizing integrated circuits, and the resulting plethora of sophisticated, low cost electronic devices demonstrate the impact that micro/nano scale engineering can have when applied only to the area of electrical and computer engineering. Current research efforts in micro/nano fabrication technology for implementing integrated devices hope to yield similar revolutions in life science fields. The integrated life chip technology requires the integration of multiple materials, phenomena, technologies, and functions at micro/nano scales. By cross linking the individual engineering fields through micro/nano technology, various miniaturized life chips have been developed at UCI that will have future impacts in the application markets such as medicine and healthcare.

Abstract: Zn4Sb3 was successfully produced by a hot pressing technique, and its thermoelectric properties were investigated in the temperature range from 4K to 300K. The Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit showed a discontinuity in variation at 242K, indicating the α-Zn4Sb3 to β-Zn4Sb3 phase transformation. Lattice thermal conductivity was found to be dominant in the total thermal conductivity of Zn4Sb3. Therefore, it is expected that thermoelectric properties can be improved by reducing the lattice thermal conductivity inducing phonon scattering centers.

Abstract: Silicon coated graphite particles prepared by gas suspension spray method were used as a raw material for the battery anode, which showed high specific capacity and good cycle performance. The improvement of electrochemical performance seems to be due to the formation of amorphous silicon-carbon black composite layer on the surface of the graphite particles. It has a stable structure under repeated volume expansion and contraction.

Abstract: Carbon nanofibers and nano-size metal particles were incorporated into the surface of graphite particles. The carbon nanofibers prepared from the decomposition of ethylene over nickelcopper catalyst and the micro-size silicon particles were directly introduced into the graphite particles by mixing. Nano-size tin particles were also incorporated into the graphite particles by impregnation. The three different graphite composites were tested as anode base materials for lithium ion secondary battery. The incorporation of a certain amount of carbon nanofibers into the graphite electrode improved the cyclic performance as well as the initial charge/discharge capacity. With the introduction of silicon, the initial charge/discharge capacity increased but exhibited the bad cyclic characteristics. With the modification with tin, an improved electrochemical performance was observed in both initial charge/discharge capacity and cyclic characteristics.

Abstract: Lithium/sulfur battery has some problems such as low utilization of active material and poor cycle life owing to the dissolution of lithium polysulfide into electrolyte, aggregation of sulfur during charge-discharge process, and structural change of sulfur electrode. To overcome such problems, carbon nano tubes (CNTs) and graphitic nano fibers (GNFs) were added into the sulfur electrode. The addition of CNTs and GNFs having a network-like structure is expected to offer the structural stability and good electrical path of sulfur electrode. The morphology of fabricated sulfur electrode was observed by using scanning electron microscope (SEM), and the crystalline structure was characterized by using X-ray diffraction (XRD). The charge/discharge tests were conducted in the voltage range 3.2/1.5V (vs.Li) with a galvanostatic method.