Real-time Atomic-resolution Probing of Lithium Ion Intercalation in TiO2-Related Anodes using Transmission Electron Microscopy.

Nanostructured TiO2 polymorphs such as rutile and bronze have been considered for lithium ion battery (LIB) application due to their chemical stability and accessibility. Despite their promising performance as anodes and coating materials, understanding of lithium ion behaviors in TiO2-related systems is still controversial, since most studies have relied on bulk characterization techniques which do not present local changes in morphology. Here, we employed in-situ transmission electron microscopy (TEM) and high-resolution scanning TEM (HRSTEM) to perform nano-scale structural studies of TiO2-related anodes upon Li intercalation. The electrochemical study of a single-crystalline rutile TiO2 nanowire (NW) was conducted under high-resolution TEM aided by selected area electron diffraction. The result demonstrates the two-step lithiation accompanied by the highly anisotropic volumetric expansion and phase transformation. An in-depth study of lithiation in bronze TiO2 (TiO2-B) had been possible by the growth of a high-quality TiO2-B thin film templated by Ca-modified bronze phase, CaTi5O11 (Ca:TiO2-B). Various interfaces and defects in TiO2-B and Ca:TiO2-B thin films deposited on (100) and (110) SrTiO3 substrates were first studied under HRSTEM. High crystallinity of (001) TiO2-B on a Ca:TiO2-B template motivated us to perform lithiation on the structure under HRTEM. Revealed by high-resolution observation of electrochemical lithiation into TiO2-B, many defects were induced by strain relaxation upon Li-induced TiO2-B lattice expansion. In fact, depending on Li intercalation direction into the structure, either high-symmetry structural transformation or plain shears was generated. The observations were corroborated by post-mortem HRSTEM characterization and theoretical calculation. The capability of a TiO2 nanostructure as a coating material for a LIB anode was also investigated via potentiostatic lithiation of a rutile TiO2-coated Si NW. It was found that lithium intercalation into this NW occurred locally by having each segment achieving full lithiation throughout the Li migration pathway. The TiO2 coating also influenced final lithiated morphology of a coated Si (Li15Si4) to be different from that of an uncoated one. The results discussed in this thesis provide the in-depth knowledge on the Li ion dynamics in two TiO2 polymorphs. The application of in-situ high-resolution TEM technique can be expanded towards other polymorphs of various different structural forms.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113420/1/sjkkorea_1.pd

Experimental Evidence of Ferroelectric Negative Capacitance in Nanoscale Heterostructures

We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric, Pb(Zr0.2Ti0.8)O3 and dielectric, SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau's mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in MOSFETs, the sub threshold slope can be lowered below the classical limit (60 mV/decade)

Stacked Polymeric Multimode Waveguide Arrays for Two-Dimensional Optical Interconnects

Two-dimensional (2-D) polymeric multimode waveguide arrays with two reflection-mirrors have been fabricated for optical interconnects between 2-D arrayed vertical-cavity surface-emitting lasers and detectors. Contact printing lithography was adopted for simple and low-cost process using ultraviolet-curable epoxy-based polymers. Fabricated waveguides were diced of the same size and stacked one by one with lateral positional errors less than ±20 μm. Two kinds of mirrors were fabricated: single-reflection mirror and double-reflection mirror. Double-reflected mirrors resulted in lower losses with 1.2 dB than single reflected mirrors with 2.1 dB. The average insertion losses of 16-channel arrayed waveguides with two single-reflection mirrors and with two double-reflection-mirrors were measured to be 6.1 and 4.4 dB for 6-cm-long waveguides at a wavelength of 830 nm, respectively. The crosstalk between the waveguides was less than -25 dB. The characteristics of the waveguide arrays are good enough for applications to optical interconnects.This work was supported by the Ministry of Commerce, Industry and Energy and by the BK 21 program

Developing the Korean Educational Needs Assessment Tool (Korean ENAT) in rheumatoid arthritis: Cross-cultural validation using Rasch analysis

Background/aims: This study was performed to undertake cross-cultural adaptation and validation of the Educational Needs Assessment Tool (ENAT) in rheumatoid arthritis (RA) for use in Korea.Methods: The study involved two main phases: cross-cultural adaptation of the ENAT from English into Korean, and validation of the Korean ENAT. The first phase followed the established process of cross-cultural adaptation of self-report measures, and in the second phase, the Korean ENAT data were analyzed using the Rasch measurement model. Fit to the model was determined using the observed data infit and outfit statistics. Additional tests of validity included unidimensionality and internal consistency.Results: Adequate conceptual equivalence was achieved following the adaptation process. A total of 123 patients completed the Korean ENAT. The mean age was 46.7 ± 12.3 years and the majority of patients (81.3%) were female. Thirty-five of the 39 items gave good fit to the model. The four items deviating from the model had infit and outfit > 1.50. The item separation index (5.26) and item reliability index (0.97) provided evidence for good reliability of items. All seven domains of the Korean ENAT fit the Rasch model. The internal consistency of the Korean ENAT was high, and unidimensionality was confirmed (person separation index, 3.41; reliability index, 0.92; item separation index, 16.82; reliability index, 1.00).Conclusions: Using the standard procedure for cross-cultural adaptation, the ENAT has been adapted into Korean, and Rasch analysis has confirmed the construct validity, reliability, and unidimensionality of the Korean ENAT

Influence of the waist diameters on transmission characteristics and strain sensitivity of microtapered long-period fiber gratings

Transmission characteristics of microtapered long-period fiber gratings (MTLPGs) and their strain and temperature sensitivities with variations in the waist diameters are investigated theoretically and experimentally. Transmission characteristics of MTLPGs strongly depend on the waist diameter of the tapered optical fiber (TOF) because of the modification of the effective index difference between the core and the cladding modes. Based on the photoelastic effect, the resonant wavelengths of MTLPGs with variations in strain shift to shorter wavelengths. The strain sensitivity of the MTLPG with a waist diameter of 25 μm is improved by a factor of 20 compared with that of a 125 μm long-period fiber grating. The temperature sensitivities of MTLPGs are also enhanced by reducing the waist diameter of the TOF