W. O. Soboyejo

Electrical response of amorphous silicon thin-film transistors under mechanical strain

We evaluated amorphous silicon thin-film transistors (TFTs) fabricated on polyimide foil under uniaxial compressive or tensile strain. The strain was induced by bending or stretching. The on- current and hence the electron linear mobility μ depend on strain e as μ=μ0(1+26×e), where tensile strain has a positive sign and the strain is parallel to the TFT source-drain current path. Upon the application of compressive or tensile strain the mobility changes “instantly” and under compression then remains constant for up to 40 h. In tension, the TFTs fail mechanically at a strain of about +0.003 but recover if the strain is released “immediately.”

Mechanical properties of engineered materials

Overview of Crystal/Defect Structure and Mechanical Properties and Behavior. Defect Structure and Mechanical Properties. Basic Definitions of Stress and Strain. Introduction to Elastic Behavior. Introduction to Plasticity. Introduction to Dislocation Mechanics. Dislocations and Plastic Deformation. Dislocation Strengthening Mechanisms. Introduction to Composites. Further Topics in Composites. Fundamentals of Fracture Mechanics. Mechanisms of Fracture. Toughening Mechanisms. Fatigue of Materials. Introduction to Viscoelasticity, Creep, and Creep Crack Growth. Index.

On the measurement of the plasticity length scale parameter in LIGA nickel foils

This paper presents measurements of the plasticity length scale for polycrystalline LIGA nickel foils produced by electroplating in a sulfamate bath. The micro-bend test method developed by Stolken and Evans [Acta Mater. 46 (1998) 5109] is used to obtain a composite length scale parameter, lc that is primarily associated with rotational gradients. The micro-bend test utilizes the measurement of the curvature change associated with elastic spring-back of bent micro-beams in the extraction of a composite length scale parameter, lc. The length scale is measured to be ∼5.6 μm for foils between 25 and 175 μm thick. This is in the range (3–5 μm) reported from prior micro-bend experiments on nickel foils (loc. cit.). The measured length scale is also similar to values reported previously for polycrystalline copper wires using torsion tests in which rotation gradients dominate.

Biofunctionalization, cytotoxicity, and cell uptake of lanthanide doped hydrophobically ligated NaYF4 upconversion nanophosphors

Surface biofunctionalization of the hydrophobic lanthanide ion doped hexagonal phase NaYF4:Yb,Er upconversion nanophosphors (UCNPs) was achieved by introducing amino and carboxyl groups, respectively. Amino groups were added by using the 3-aminopropyltrimethoxysilane reaction after a thin layer of SiO2 coating. The carboxyl groups on surface were added directly by coating modified amphiphilic polyacrylic acid polymer. Experimental studies of cytotoxicity and cell uptake of UCNPs were conducted. The cytotoxicity analysis of the functionalized UCNPs was conducted by methylthiazol tetrazolium assays. Cell uptake was accomplished by incubating the UCNPs with human osteosarcoma cells and proved by transmission electron microscopy. The results showed that the functionalized UCNPs had very low toxicity compared with the control group, while UCNPs taken into the cells indicated that they had very high biocompatibility. The imaging of UCNPs, which were incubated with AB12 mouse mesothelioma cells and excited by 1 ...

Mechanical properties of functionally graded hierarchical bamboo structures

This paper presents the results of a series of multi-scale experiments and numerical models concerning the mechanical properties of moso culm functionally graded bamboo structures. On the nano- and microscales, nanoindentation techniques are used to study the local variations in the Youngs moduli of moso culm bamboo cross-sections. These are then incorporated into finite element models in which the actual variations in Youngs moduli are used to model the deformation and fracture of bamboo during fracture toughness experiments. Similarly, the measured gradations in moduli are incorporated into crack bridging models that predict the toughening observed during resistance curve tests. The implications of the results are discussed for the bio-inspired design of structures that mimic the layered, functionally graded structure of bamboo.

Surface topography evolution and fatigue fracture in polysilicon MEMS structures

This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture.

Nanoindentation study of plasticity length scale effects in LIGA Ni microelectromechanical systems structures

This paper presents the results of a nanoindentation study of the effects of strain gradient plasticity on the elastic-plastic deformation of lithographie, galvanoformung, abformung (LIGA) Ni microelectromechanical systems (MEMS) structures plated from sulfamate baths. Both Berkovich and North Star/cube corner indenter tips were used in the study to investigate possible effects of residual indentation depth on the hardness of LIGA Ni MEMS structures between the micro- and nanoscales. A microstructural length scale parameter, l = 2.2 μm, was determined for LIGA nickel films. This is shown to be consistent with a stretch gradient length-scale parameter, l s , of approximately 0.9 μm.

Nanoindentation of Ni–Ti Thin Films

Ni–Ti thin films of various compositions were sputtered-deposited on silicon substrates. Their mechanical properties (hardness and Youngs modulus) were then determined using a nanoindenter equipped with a Berkovich tip. This paper examines the effects of composition on the mechanical properties (hardness and Youngs modulus) of the sputter deposited Ni–Ti thin films. This is of particular interest since the actuation properties of these shape memory alloy films are compositionally sensitive. The surface-induced deformation is revealed via Atomic Force Microscopy (AFM) images of the indented surfaces. Which show evidence of material pile-up that increases with increasing load. The measured Youngs moduli are also shown to provide qualitative measures of the extent of stress-induced phase transformation in small volumes of Ni–Ti films.

On the evolution of surface morphology of polysilicon MEMS structures during fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures.

An Investigation of Fatigue Crack Nucleation and Growth in a Ti–6Al–4V/TiB in Situ Composite

Abstract This paper presents the results of an experimental study of fatigue crack nucleation and growth in a Ti–6Al–4V/TiB in situ whisker-reinforced composite. The onset of crack nucleation is shown to correspond to ∼20% of the total life at a stress range of 480 MPa. This is associated with transverse cracking across TiB whiskers, and interfacial decohesion between the TiB whiskers and the Ti–6Al–4V matrix. Subsequent cracking occurs by the formation of multiple cracks across the elongated α grains. These cracks are retarded initially by the β phase. However, subsequent fatigue damage results in transgranular crack growth across α and β phases prior to the onset of catastrophic failure. The long fatigue crack growth rates in the Paris regime in the Ti–6Al–4V/TiB composite are comparable to those of Ti–6Al–4V processed under nominally identical conditions. However, the fatigue crack growth rates in the composite are faster than those in the matrix alloy at lower Δ K values. Cyclic deformation of the composite is associated with strain softening, presumably as a result of progressive interfacial decohesion around the TiB whiskers early in the fatigue deformation process. The implications of the results are assessed for potential structural applications of the Ti–6Al–4V/TiB composite.