Chen, I-Wei

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Now showing 1 - 10 of 34
  • 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-Wei
    Self-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-Wei
    Sr3Cu2Sc2O5S2, 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
    Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3and Ni–Cu–Zn Ferrite
    (2006-02-01) Wang, X.-H.; Deng, X.-Y.; Bai, Hai-Lin; Zhou, H.; Qu, Wei-Guo; Li, L.-T.; Chen, I-Wei
    We investigated the preparation of bulk dense nanocrystalline BaTiO3 and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO3 with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y2O3, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO3 ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.
  • Publication
    Two-Step Sintering of Ceramics with Constant Grain-Size, I. Y2O3
    (2006-02-01) Wang, Xiao-Hui; Chen, Pei-Lin; Chen, I-Wei
    Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring’s scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.
  • Publication
    The effect of silica nanoparticle-modified surfaces on cell morphology, cytoskeletal organization and function
    (2008-10-01) Lipski, Anna M; Pino, Christopher J; Haselton, Frederick R; Chen, I-Wei; Shastri, V. Prasad
    Chemical and morphological characteristics of a biomaterial surface are thought to play an important role in determining cellular differentiation and apoptosis. In this report, we investigate the effect of nanoparticle (NP) assemblies arranged on a flat substrate on cytoskeletal organization, proliferation and metabolic activity on two cell types, Bovine aortic endothelial cells (BAECs) and mouse calvarial preosteoblasts (MC3T3-E1). To vary roughness without altering chemistry, glass substrates were coated with monodispersed silica nanoparticles of 50, 100 and 300 nm in diameter. The impact of surface roughness at the nanoscale on cell morphology was studied by quantifying cell spreading, shape, cytoskeletal F-actin alignment, and recruitment of focal adhesion complexes (FAC) using image analysis. Metabolic activity was followed using a thiazolyl blue tetrazolium bromide assay. In the two cell types tested, surface roughness introduced by nanoparticles had cell type specific effects on cell morphology and metabolism. While BAEC on NP-modified substrates exhibited smaller cell areas and fewer focal adhesion complexes compared to BAEC grown on glass, MC3T3-E1 cells in contrast exhibited larger cell areas on NP-modified surfaces and an increased number of FACs, in comparison to unmodified glass. However, both cell types on 50 nm NP had the highest proliferation rates (comparable to glass control) whereas cells grown on 300 nm NP exhibited inhibited proliferation. Interestingly, for both cell types surface roughness promoted the formation of long, thick F-actin fibers, which aligned with the long axis of each cell. These findings are consistent with our earlier result that osteogenic differentiation of human mesenchymal progenitor cells is enhanced on NP-modified surfaces. Our finding that nanoroughness, as imparted by nanoparticle assemblies, effects cellular processes in a cell specific manner, can have far reaching consequences on the development of "smart" biomaterials especially for directing stem cell differentiation.
  • Publication
    Demonstration and Modeling of Multi-Bit Resistance Random Access Memory
    (2013-01-29) Yang, Xiang; Chen, Albert B. K.; Choi, Byung Joon; Chen, I-Wei
    Although intermediates resistance states are common in resistance random access memory (RRAM), two-way switching among them has not been demonstrated. Using a nanometallic bipolar RRAM, we have illustrated a general scheme for writing/rewriting multi-bit memory using voltage pulses. Stability conditions for accessing intermediate states have also been determined in terms of a state distribution function and the weight of serial load resistance. A multi-bit memory is shown to realize considerable space saving at a modest decrease of switching speed.
  • Publication
    A-site substitution of SrRuO3 using La, K and Pb
    (2006-10-11) Shuba, Svetlana; Mamchik, Alexander; Chen, I-Wei
    We have investigated Sr1−xLax/2Kx/2RuO3 and Sr1−xPbxRuO3, which have a larger average size of the A-site cations. They manifest a gradual loss of ferromagnetism in a similar way as their counterparts with smaller A-site cations. There is also evidence for a magnetism-suppressing disorder effect similar to that observed in Sr1−xLax/2Nax/2RuO3. Therefore, the Stoner ferromagnetism in SrRuO3 is rather unique and cannot be easily tuned by lattice distortion to yield a higher Curie temperature.
  • Publication
    A wide-band-gap p-type thermoelectric material based on quaternary chalcogenides of Cu2ZnSnQ4 (Q=S,Se)
    (2009-05-19) Liu, Min-Ling; Huang, Fu-Qiang; Chen, Li-Dong; Chen, I-Wei
    Chalcopyritelike quaternary chalcogenides, Cu(2)ZnSnQ(4) (Q=S,Se), were investigated as an alternative class of wide-band-gap p-type thermoelectric materials. Their distorted diamondlike structure and quaternary compositions are beneficial to lowering lattice thermal conductivities. Meanwhile, partial substitution of Cu for Zn creates more charge carriers and conducting pathways via the CuQ(4) network, enhancing electrical conductivity. The power factor and the figure of merit (ZT) increase with the temperature, making these materials suitable for high temperature applications. For Cu(2.1)Zn(0.9)SnQ(4), ZT reaches about 0.4 at 700 K, rising to 0.9 at 860 K.
  • Publication
    Development of Tough Alpha-SiAlON
    (2003-01-01) Chen, I-Wei; Shuba, Roman; Zenotchkine, Misha Y
    The development of tough α-SiAlON with elongated grains in the last five years is summarized. This progress has been guided by the improved understanding of phase relations and nucleation/growth kinetics in SiAlON ceramics. Although most α-SiAlON compositions can be processed to contain some elongated grains, their microstructure, fracture toughness and R-curve behavior vary greatly. Such variability is due to the different phase stability of α-SiAlONs and the varying physical chemistry of the competing phases, including the transient/residual liquid. For this reason, microstructure control of α-SiAlON must pay close attention to the composition, starting powder and heating schedule. Seeding with single crystals of an appropriate α-SiAlON composition provides an attractive alternative that simplifies the task of microstructure control, since such seeds are thermodynamically stable and they completely dominate the nucleation statistics. Tough and hard α-SiAlON ceramics containing Ca, Y, Nd, and Yb stabilizers have been obtained using this method, some with toughness exceeding 10 MPam1/2. The ability of maintaining a uniform microstructure of highly elongated grains is the key to high toughness material.
  • Publication
    R-Curve Behavior of In Situ Toughened alpha-SiAlON Ceramics
    (2001-04-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-Sun
    R-curves of single-phase Y- and Ca-containing α-SiAlON ceramics have been measured. They range from flat ones for fine-grain ceramics to pronounced rising ones when large elongated grains are present. The highest toughness measured reached 11.5 MPa∙m1/2 over a crack extension of about 1000 μm.