Chen, I-Wei
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Publication Synthesis of alpha-SiAlON Seed Crystals(2001-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunSingle-phase seed crystals of Ca- and Y-α-SiAlONs have been synthesized for tailoring microstructure of α-SiAlON ceramics. The influence of composition, sintering temperature, and nitrogen pressure on the size and morphology of seeds has been explored. Guidelines for α-SiAlON seed preparation and morphology control are provided.Publication Formation of beta-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions(2003-09-01) Wang, Lingling; Tien, Tseng-Ying; Chen, I-WeiPrecipitation, growth, and coarsening of Si3N4 crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680oC has been studied. The initial nucleation of β-Si3N4 occurs mostly on α-Si3N44 because of the very high supersaturation of the liquid. After a brief period of growth, the crystals then undergo accelerated coarsening, decreasing the crystal concentration by almost 100 times with little change in the total crystal volume. Meanwhile, the crystals gradually transform from β-Si3N4, by substituting Si-N with Al-O, to β'-SiAlON of various compositions. The evolution of aspect ratio strongly depends on the Si/(Al,Mg,Y) ratio, which is rationalized by cation segregation to the interface driven by the acidity–basicity differential between the liquid and the crystal.Publication A size-dependent nanoscale metal–insulator transition in random materials(2011-01-28) Chen, Albert B.K.; Wang, Yudi; Tung, Wei-Shao; Chen, I-Wei; Kim, Soo GilInsulators and conductors with periodic structures can be readily distinguished, because they have different band structures, but the differences between insulators and conductors with random structures are more subtle. In 1958, Anderson provided a straightforward criterion for distinguishing between random insulators and conductors, based on the 'diffusion' distance ζ for electrons at 0 K (ref. 3). Insulators have a finite ζ, but conductors have an infinite ζ. Aided by a scaling argument, this concept can explain many phenomena in disordered electronic systems, such as the fact that the electrical resistivity of 'dirty' metals always increases as the temperature approaches 0 K (refs 4–6). Further verification for this model has come from experiments that measure how the properties of macroscopic samples vary with changes in temperature, pressure, impurity concentration and applied magnetic field, but, surprisingly, there have been no attempts to engineer a metal–insulator transition by making the sample size less than or more thanζ. Here, we report such an engineered transition using six different thin-film systems: two are glasses that contain dispersed platinum atoms, and four are single crystals of perovskite that contain minor conducting components. With a sample size comparable to ζ, transitions can be triggered by using an electric field or ultraviolet radiation to tune ζ through the injection and extraction of electrons. It would seem possible to take advantage of this nanometallicity in applications.Publication Nanoscale Engineering for Biomaterial Surfaces(2007-02-01) Lipski, Anna Marie; Jaquiery, Claude; Choi, Hoon; Eberli, Daniel; Stevens, Molly; Martin, Ivan; Chen, I-Wei; Shastri, V. PrasadPublication Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution(2002-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-WeiThe effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa∙m1/2 and peak R-curve toughness of ~11 MPa∙m1/2 have been obtained for seeded Y-α-SiAlON ceramics using heating rates from 1oC/min to 25oC/min.Publication Temperature-Sensitive Nanocapsules for Controlled Drug Release Caused by Magnetically Triggered Structural Disruption(2008-12-01) Liu, Ting-Yu; Liu, Kun-Ho; Liu, Dean-Mo; Chen, San-Yuan; Chen, I-WeiSelf-assembled nanocapsules containing a hydrophilic core and a crosslinked yet thermosensitive shell have been successfully prepared using poly(ethylene-oxide)-poly(propylene-oxide)-poly(ethylene-oxide) block copolymers, 4-nitrophenyl chloroformate, gelatin, and 1-ethyl-3-(3- dimethylaminopropyl) carbodiimide. The core is further rendered magnetic by incorporating iron oxide nanoparticles via internal precipitation to enable externally controlled actuation under magnetic induction. The spherical nanocapsules exhibit a hydrophilic-to-hydrophobic transition at a characteristic but tunable temperature reaching 40ºC, triggering a size contraction and shrinkage of the core. The core content experiences very little leakage at 25ºC, has a half life about 5 h at 45ºC, but bursts out within a few minutes under magnetic heating due to iron oxide coarsening and core/shell disruption. Such burst-like response may be utilized for controlled drug release as illustrated here using a model drug Vitamin B12.Publication A promising p-type transparent conducting material: Layered oxysulfide(2007-12-01) Liu, Min-Ling; Wu, Li-Bin; Chen, Qiang; Chen, Li-Dong; Chen, I-WeiSr3Cu2Sc2O5S2, a layered oxysulfide, composed of anti-PbO-like [Cu2S2] slabs alternating with perovskitelike [Sr3Sc2O5] slabs, was systematically studied as a p-type transparent conducting material. The material has a wide energy gap of 3.1 eV and a p-type electrical conductivity of 2.8 S cm−1 at room temperature. The hole mobility of +150 cm2 V−1 S−1 at room temperature, which is much higher than the typical value of ~10−1–10 cm2 V−1 S−1 found in other copper compounds. The performances of bulk undoped Sr3Cu2Sc2O5S2 show the promise of copper oxysulfides as a class of p-type transparent conductive materials that is essential for optoelectronic applications.Publication Biphosphonate-Mediated Gene Vector Delivery from the Metal Surfaces of Stents(2006-01-01) Fishbein, Ilia; Alferiev, Ivan S; Nyanguile, Origene; Vohs, John M; Gaster, Richard; Wong, Gordon Sek-Yin; Chen, I-Wei; Felderman, Howard; Choi, Hoon; Wilensky, Robert L; Levy, Robert JThe clinical use of metallic expandable intravascular stents has resulted in imporved therapeutic outcomes for coronary artery disease. However, arterial reobstruction after stenting, in-stent restenosis, remains an important problem. Gene therapy to treat in-stent restenosis by using gene vector delivery from the metallic stent surfaces has never been demonstrated. The present studies investigated the hypothesis that metal-biphosphonate binding can enable site-specific gene vector delivery from metal surfaces. Polyallylamine biphosphonate (PAA-BP) was synthesized by using Michael addition methodology. Exposure to aqueous solutions of PAA-BP resulted in the formation of a monomolecular biphosphonate later on metal alloy surfaces (steel, nitinol, and cobalt-chromium), as demonstrated by x-ray photoelectron spectroscopy. Surface-bound PAA-BP enabled adenoviral (Ad) tethering due to covalent thiol-binding of either anti-Ad antibody or a recombinant Ad-receptor protein, D1. In arterial smooth muscle cell cultures, alloy samples configured with surface-tethered Ad were demonstrated to achieve site-specific transduction with a reporter gene, (GFP). Rat carotid stent angioplasties using metal stents exposed to aqueous PAA-BP and derivatized with anti-knob antibody or D1 resulted in extensive localized Ad-GFP expression in the arterial wall. In a separate study with a model therapeutic vector, Ad-inducible nitric oxide synthase (iNOS) attached to the biphosphonate-treated metal stent surface via D1, significant inhibition of restenosis was demonstrated (neointimal/media ration 1.68 ± 0.27 and 3.4 ± 0.35; Ad-iNOS vs. control, P < 0.01). Is is concluded that effective gene vector delivery from metallic stent surfaces can be achieved using this approach.Publication Nucleation and growth mechanism of ferroelectric domain-wall motion(2007-10-07) Shin, Young-Han; Grinberg, Ilya; Chen, I-Wei; Rappe, Andrew MThe motion of domain walls is critical to many applications involving ferroelectric materials, such as fast high-density non-volatile random access memory. In memories of this sort, storing a data bit means increasing the size of one polar region at the expense of another, and hence the movement of a domain wall separating these regions. Experimental measurements of domain growth rates in the well-established ferroelectrics PbTiO3 and BaTiO3 have been performed, but the development of new materials has been hampered by a lack of microscopic understanding of how domain walls move. Despite some success in interpreting domain-wall motion in terms of classical nucleation and growth models, these models were formulated without insight from first-principles-based calculations, and they portray a picture of a large, triangular nucleus that leads to unrealistically large depolarization and nucleation energies. Here we use atomistic molecular dynamics and coarse-grained Monte Carlo simulations to analyse these processes, and demonstrate that the prevailing models are incorrect. Our multi-scale simulations reproduce experimental domain growth rates in PbTiO3 and reveal small, square critical nuclei with a diffuse interface. A simple analytic model is also proposed, relating bulk polarization and gradient energies to wall nucleation and growth, and thus rationalizing all experimental rate measurements in PbTiO3 and BaTiO3.Publication Paraffin-Based Process for Producing Layered Composites with Cellular Microstructures(2002-04-01) Dakskobler, Aleš; Kosmac, Tomaz; Chen, I-WeiA paraffin-based process that results in high-strength bimaterial ceramic layered composites is reported. The process facilitates rolling, folding, and shape retention at room temperature and allows the transition from a laminar to a cellular microstructure during deformation. The strength of sintered alumina/zirconia/alumina composites reached 700 MPa, higher than that of conventional zirconia-toughened alumina composites containing dispersed particles.