Woong-Ryeol Yu

SnO2 nanotubes fabricated using electrospinning and atomic layer deposition and their gas sensing performance

A novel method is developed to fabricate a SnO(2) nanotube network by utilizing electrospinning and atomic layer deposition (ALD), and the network sensor is proven to exhibit excellent sensitivity to ethanol owing to its hollow, nanostructured character. The electrospun polyacrylonitrile (PAN) nanofibers of 100-200 nm diameter are used as a template after stabilization at 250 degrees C. An uniform and conformal SnO(2) coating on the nanofiber template is achieved by ALD using dibutyltindiacetate (DBTDA) as the Sn source at 100 degrees C and the wall thickness is precisely controlled by adjusting the number of ALD cycles. The calcination at 700 degrees C transforms the amorphous nanofibers into SnO(2) nanotubes composed of several nanometer-sized crystallites. The SnO(2) nanotube network sensor responds to ethanol, H(2), CO, NH(3) and NO(2) gases, but it exhibited an extremely high gas response to ethanol with a short response time (<5 s). The results demonstrate that the combination of electrospinning and ALD is a very effective and promising technique to fabricate long and uniform metal oxide nanotubes with the precise control of wall thickness, which can be applied to various applications such as gas sensors and lithium ion batteries.

Effect of pores in hollow carbon nanofibers on their negative electrode properties for a lithium rechargeable battery.

The effect of pores in hollow carbon nanofibers (HCNFs) on their electrochemical performance is investigated because the carbon shell itself acts as a reservoir for accommodating Li-ions through intercalation and simultaneously becomes a transport medium through which Li-ions migrate into the core materials in HCNFs. Porous HCNFs (pHCNFs) are prepared by the coaxial electrospinning of a sacrificial core solution and an emulsified shell solution containing sacrificial islands for pore generation. After a thermal treatment, a systematic study is carried out to relate the resulting pore size in pHCNFs to the sacrificial islands in the emulsified shell. As the pores are introduced in pHCNFs, their initial capacity and reversible capacity rate are proved to increase significantly to 1003 mAhg(-1) and 61.8%, respectively, compared to those (653 mAhg(-1) and 53.9%) of nonporous HCNFs. The increased pore size and expanded graphene layers are believed to facilitate lithium insertion/extraction behavior.

Two-way shape memory behavior of shape memory polyurethanes with a bias load

Thermo-responsive shape memory polyurethane (SMPU) is a smart material that can respond to external heat by changing its macroscopic shape from a temporary configuration to a memorized permanent one. The temporary shape can be processed using mechanical forces above a certain temperature (the transition temperature) and can be maintained until the material acquires a certain thermal energy. Thereafter, the material will recover its memorized permanent shape. However, it is unclear what will occur if the thermal energy is then dissipated, i.e., the material temperature decreases. There are two possibilities: the material will respond to the dissipated energy, resulting in another macroscopic shape change; or nothing will happen beyond the thermal contraction. The former is called two-way shape memory (TWSM) behavior and the latter is called one-way shape memory behavior. This paper reports novel findings showing that TWSM behavior can be imparted to SMPUs using a thermo-mechanical treatment, i.e., imposing a constant stress on them after their temporary shaping. A series of experiments were carried out to characterize the TWSM behavior of SMPUs and to explain its mechanism.

Mechanical modeling of self-expandable stent fabricated using braiding technology.

The mechanical behavior of a stent is one of the important factors involved in ensuring its opening within arterial conduits. This study aimed to develop a mechanical model for designing self-expandable stents fabricated using braiding technology. For this purpose, a finite element model was constructed by developing a preprocessing program for the three-dimensional geometrical modeling of the braiding structure inside stents, and validated for various stents with different braiding structures. The constituent wires (Nitinol) in the braided stents were assumed to be superelastic material and their mechanical behavior was incorporated into the finite element software through a user material subroutine (VUMAT in ABAQUS) employing a one-dimensional superelastic model. For the verification of the model, several braided stents were manufactured using an automated braiding machine and characterized focusing on their compressive behavior. It was observed that the braided stents showed a hysteresis between their loading and unloading behavior when a compressive load was applied to the braided tube. Through the finite element analysis, it was concluded that the current mechanical model can appropriately predict the mechanical behavior of braided stents including such hysteretic behavior, and that the hysteresis was caused by the slippage between the constituent wires and their superelastic property.

Degradation and Healing Mechanisms of Carbon Fibers during the Catalytic Growth of Carbon Nanotubes on Their Surfaces

This study reports on the main cause of the reduced tensile strength of carbon fibers (CFs) by investigating the microstructural changes in the CFs that are undergoing mainly two processes: catalyst nanoparticle formation and chemical vapor deposition (CVD). Interestingly, the two processes oppositely influenced the tensile strength of the CFs: the former negatively and the latter positively. The catalysts coating and nanoparticle formation degraded the CF surface by inducing amorphous carbons and severing graphitic layers, while those defects were healed by both the injected carbons and interfaced CNTs during the CVD process. The revealed degradation and healing mechanisms can serve as a fundamental engineering basis for exploring optimized processes in the manufacturing of hierarchical reinforcements without sacrificing the tensile strength of the substrate CFs.

Simulation of mechanical behavior of temperature-responsive braided stents made of shape memory polyurethanes.

Polymeric stents can be considered as an alternative to metallic stents thanks to their lessened incidence of restenosis and controlled deployment. The purpose of this study was to investigate the feasibility of developing a temperature-responsive braided stent using shape memory polyurethane (SMPU) through finite element analysis. It was assumed that braided stents were manufactured using SMPU fibers. The mechanical behavior of SMPU fibers was modeled using a constitutive equation describing their one-dimensional thermal-induced shape memory behavior. Then, the braided stents were analyzed to investigate their mechanical behavior using finite element analysis software, in which the constitutive equation was implemented through a user material subroutine. The diameter of the SMPU fibers and braiding angle were chosen as the design parameters and their values were adjusted to ensure that the mechanical properties of the braided polymer stents match those of metallic stents. Finally, the deployment process of the braided stents inside narrowed vessels was simulated, showing that the SMPU stents can be comfortably implanted while minimizing the overpressure onto the vessel walls, due to their thermo-responsive shape memory behavior.

Two-way actuation behavior of shape memory polymer/elastomer core/shell composites

Semi-crystalline shape memory polymers (SMPs) show net two-way shape memory (2W-SM) behavior under constant stresses by the recoverable creep strain upon heating and stress-induced crystallization under the application of creep stress upon cooling. The applied constant stress is the key factor in this 2W-SM behavior. A core/shell structure is manufactured for the purpose of imparting a constant stress upon SMPs. An SMP in film or fiber form is dipped into a solution of an elastomer, photoinitiator, and curing agent and then dried out. After this dip coating process is repeatedly carried out, the SMP/elastomer core/shell composite is deformed into a temporary shape after being heated up above the transition temperature of the SMP. Under constant strain conditions, the composite is cooled down, after which the shell elastomer is cured using ultraviolet light. Then, the SMP/elastomer core/shell composite extends and contracts upon cooling and heating, respectively, without any external load. This cyclic deformation behavior is characterized, demonstrating that the current method offers a simple macroscopic processing technique to manufacture 2W-SM polymer composites.

Drape Simulation of Woven Fabrics by Using Explicit Dynamic Analysis

For the three-dimensional drape simulation of large samples of woven fabric, an explicit-dynamic analysis code is developed to predict the shape of woven fabric as it deforms into a natural shape under gravity. A contact algorithm is incorporated into the code, which is based on explicit algorithms for the non-linear dynamics of shells with a simple damping effect. For validation purposes, various simulations are performed, and the results are compared with those obtained in experiments.

An effective method for manufacturing hollow carbon nanofibers and microstructural analysis

Hollow carbon nanofibers (HCNFs) were successfully manufactured by co-axial (core/shell) electrospinning of poly(styrene-co-acrylonitrile) (SAN) and poly(acrylonitrile) (PAN) solutions. The shell component (PAN) was converted into a turbostratic carbon structure by thermal treatment, whereas the sacrificial core component (SAN) was eliminated. SAN was found to be a very suitable material for the sacrificial core. SAN exhibited excellent co-axial electrospinnability to produce a uniform core/shell nanofiber precursor because of its immiscibility with PAN. Also, SAN had a good thermal sustainability that prevented the PAN shell from shrinking during the stabilization and carbonization processes, thus maintaining the shell structure. These two predominant properties of SAN enabled the manufacturing of uniform HCNFs with controlled inner diameters and wall thickness that ranged from 120–510 nm and 52–145 nm, respectively. The core solution properties, such as solution concentration and flow rate, were mostly effective in controlling both the outer diameters and the wall thicknesses of HCNFs. The microstructure of these HCNFs was investigated using high resolution transmission electron microscopy. The crystallite size and crystallinity of HCNFs were dependent on their wall thicknesses. As the wall thicknesses of HCNFs decreased, they developed smaller crystallites and higher crystallinities.

Silicon/Carbon Nanotube/BaTiO3 Nanocomposite Anode: Evidence for Enhanced Lithium-Ion Mobility Induced by the Local Piezoelectric Potential

We report on the synergetic effects of silicon (Si) and BaTiO3 (BTO) for applications as the anode of Li-ion batteries. The large expansion of Si during lithiation was exploited as an energy source via piezoelectric BTO nanoparticles. Si and BTO nanoparticles were dispersed in a matrix consisting of multiwalled carbon nanotubes (CNTs) using a high-energy ball-milling process. The mechanical stress resulting from the expansion of Si was transferred via the CNT matrix to the BTO, which can be poled, so that a piezoelectric potential is generated. We found that this local piezoelectric potential can improve the electrochemical performance of the Si/CNT/BTO nanocomposite anodes. Experimental measurements and simulation results support the increased mobility of Li-ions due to the local piezoelectric potential.