Gen Satoh

Annealing Effect on the Shape Memory Properties of Amorphous NiTi Thin Films

Thin film shape memory alloys have recently become a promising material for the actuation of devices on the microscale such as micropumps and microvalves. Their utilization, however, has been limited due to the difficulty in tailoring their properties for different applications. Control over the transformation temperatures as well as mechanical and shape memory properties is required to enable their widespread use. This study examines the effects of heat treatment time and temperature on the properties of amorphous, Ti-rich NiTi thin films on silicon substrates. The effects on the transformation temperatures are investigated through the use of temperature dependent optical microscopy and temperature dependent X-ray diffraction. The indentation modulus and hardness, as well as dissipated energy and depth recovery, are obtained through nanoindentation and atomic force microscopy. The role of microstructure and composition in altering both the mechanical and shape memory properties of the films is discussed.

Femtosecond Laser-Induced Simultaneous Surface Texturing and Crystallization of a-Si:H Thin Film: Absorption and Crystallinity

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost. Their overall efficiency and stability, however, are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. In this study, both surface texturing and crystallization on a-Si:H thin film are achieved through one-step femtosecond laser processing. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by femtosecond laser irradiation in both air and water are presented and enhanced light absorption is observed due to light trapping based on surface geometry changes, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially be increased. The relationship among crystallinity, fluence, and scan speed is also discussed. Furthermore, a comparison of absorptance spectra for various surface morphologies is developed. Finally, the absorptance measurement across the solar spectrum shows that the combination of surface texturing and crystallization induced by femtosecond laser processing is very promising for a-Si:H thin film solar cell applications. [DOI: 10.1115/1.4006548]

Transmission Welding of Glass by Femtosecond Laser: Mechanism and Fracture Strength

Femtosecond laser pulses were focused on the interface of two glass specimens. Proper use of optical and laser processing parameters enables transmission welding. The morphology of the weld cross section was studied using differential interference contrast optical microscopy. In addition, a numerical model was developed to predict the absorption volumes of femtosecond laser pulses inside a transparent material. The model takes into account the temporal and spatial characteristics and propagation properties of the laser beam, and the transmission welding widths were subsequently compared with the absorption widths predicted by the model. The model can lead to the achievement of a desirable weld shape through understanding the effects of laser pulse energy and numerical aperture on the shape of the absorption volume. The changes in mechanical properties of the weld seams were studied through spatially resolved nanoindentation, and indentation fracture analysis was used to investigate the strength of the weld seams.

The Laser Interlaminar Reinforcement of Continuous Glass Fiber Composites

The Laser Inter-Laminar Reinforcement of Continuous Glass Fiber Composites Huade Tan (Corresponding Author) Graduate Research Assistant Email: ht2288@columbia.edu Phone: 646-691-0720 Fax: 212-666-2393 Gen Satoh Email: gs2358@columbia.edu Y. Lawrence Yao Email: yly1@columbia.edu Manufacturing Research Laboratory Department of Mechanical Engineering Columbia University 500 West 120 st Mudd bldg, rm 220 New York, New York, 10027

CHARACTERIZATION AND PREDICTION OF TEXTURE IN LASER ANNEALED NITI SHAPE MEMORY THIN FILMS

Thin film shape memory alloys are a promising material for use in microscale devices for actuation and sensing due to their strong actuating force, substantial displacements, and large surface to volume ratios. NiTi, in particular, has been of great interest due to its biocompatibility and corrosion resistance. Effort has been directed toward adjusting the microstructure of as-deposited films in order to modify their shape memory properties for specific applications. The anisotropy of the shape memory and superelastic effects suggests that inducing preferred orientations could allow for optimization of shape memory properties. Limited work, however, has been performed on adjusting the crystallographic texture of these films. In this study, thin film NiTi samples are processed using excimer laser crystallization and the effect on the overall preferred orientation is analyzed through the use of electron backscatter diffraction and X-ray diffraction. A threedimensional Monte Carlo grain growth model is developed to characterize textures formed though surface energy induced abnormal grain growth during solidification. Furthermore, a scaling factor between Monte Carlo steps and real time is determined to aid in the prediction of texture changes during laser crystallization in the partial melting regime. [DOI: 10.1115/1.4007459]

Beneficial Interface Geometry for Laser Joining of NiTi to Stainless Steel Wires

Joining the dissimilar metal pair of NiTi to stainless steel is of great interest for implantable medical applications. Formation of brittle intermetallic phases requires that the joining processes used for this dissimilar pair limits the amount of over-melting and mixing along the interface. Thus, because of its ability to precisely control heat input, laser joining is a preferred method. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear stress component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing parameters.

FEMTOSECOND LASER-INDUCED SURFACE TEXTURING AND CRYSTALLIZATION OF A-SI:H THIN FILM

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost, however, the overall efficiency and stability are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. In this study, both surface texturing and crystallization on a-Si:H thin film are achieved through one-step femtosecond laser processing in water. Light absorption is enhanced by light trapping based on surface geometry changes, and the formation of a mixture of hydrogenated microcrystalline silicon (µc-Si:H) and a-Si:H after crystallization suggests that the overall stability may be increased. Furthermore, the formation mechanism for the surface spikes is discussed. A comparison of absorptance spectra for various surface morphologies and crystallinities shows that the combination of surface texturing and crystallization induced by femtosecond laser processing is very promising for a-Si:H thin film solar cell applications.

Laser Autogenous Brazing of Biocompatible, Dissimilar Metals in Tubular Geometries

The successful joining of dissimilar metal tubes would enable the selective use of the unique properties exhibited by biocompatible materials such as stainless steel and shape memory materials such as NiTi, to locally tailor the properties of implantable medical devices. The lack of robust joining processes for the dissimilar metal pairs found within these devices, however, is an obstacle to their development and manufacture. Traditional joining methods suffer from weak joints due to the formation of brittle intermetallics or use filler materials that are unsuitable for use within the human body. This study investigates a new process, Laser Autogenous Brazing, that utilizes a thermal accumulation mechanism to form joints between dissimilar metals without filler materials. This process has been shown to produce robust joints between wire specimens but requires additional considerations when applied to tubular parts. The strength, composition, and microstructure of the resultant joints between NiTi and Stainless Steel are investigated and the effects of laser parameters on the thermal profile and joining mechanism are studied through experiments and numerical simulations.

Effects of Interfacial Geometry on Laser Joining of Dissimilar NiTi to Stainless Steel Wires

Joining of the dissimilar metal pair NiTi to stainless steel is of great interest for implantable biomedical applications. Formation of brittle intermetallic phases requires that the joining processes limit the amount of over-melting and mixing along the interface. Thus, laser joining is a preferred method due to its ability to precisely control heat input. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing.

Applications of surface structuring with lasers

Surface properties are significant in determining the applications of a material, and the modifications of surface properties have been drawing wide attention. The properties attracting the most attention include hydrophilicity, microhardness, optical properties, and tribological properties. To modify these surface characteristics, numerous methods have been developed and investigated over the years; however, they have limitations such as the involvement of toxic chemicals or the lack of spatial resolution. Free from these drawbacks, laser surface structuring is a promising method for surface property modifications. It provides high temporal and spatial resolution, noncontact processing and hence can maintain a high degree of purity. It also delivers a high quantum of energy to induce a very high heating and cooling rate, thermal gradient and resolidification rate. In this paper, laser surface structuring is reviewed in two fronts: the topography change, in which a desired surface topography is created, and the microstructure change, which alters the surface crystallinity and crystal structure. Simultaneous modification of topography and microstructure is also considered. Recent studies on the modifications of surface properties and applications caused by laser structuring are reviewed. Future perspectives are also presented.Surface properties are significant in determining the applications of a material, and the modifications of surface properties have been drawing wide attention. The properties attracting the most attention include hydrophilicity, microhardness, optical properties, and tribological properties. To modify these surface characteristics, numerous methods have been developed and investigated over the years; however, they have limitations such as the involvement of toxic chemicals or the lack of spatial resolution. Free from these drawbacks, laser surface structuring is a promising method for surface property modifications. It provides high temporal and spatial resolution, noncontact processing and hence can maintain a high degree of purity. It also delivers a high quantum of energy to induce a very high heating and cooling rate, thermal gradient and resolidification rate. In this paper, laser surface structuring is reviewed in two fronts: the topography change, in which a desired surface topography is created, a...