Chang-Hyun Lim

Implantable flexible wireless pressure sensor module

A chip embedded flexible packaging scheme has been developed using a thinned silicon chip. Mechanical characteristics of thinned silicon chips are examined by bending test and finite element analysis. Thinned silicon chips (t<50 /spl mu/m) are fabricated by a chemical etching process to avoid possible surface damage. These technologies can be used for an implantable real-time monitoring of blood pressure. Our research targets are developing an implantable blood pressure sensor module and its telemetric measurement. By winding around the coronary arteries, we can measure the blood pressure by capacitance variation of blood vessels.

A Study of Electrodes for Thermoelectric Oxides

Oxide module studies for thermoelectric energy harvesting at a high temperature were performed in this study. Considering the degradation of the electrode properties by oxidation at normal operating temperatures in air, a silver electrode (Ag) was selected for use in this study. Two types of contact problems causing a severe loss in electrical power generation were investigated: potential barriers at the interfaces, and inter-diffusion problems. Also, two types of thermoelectric modules were fabricated using Ag electrodes: a module with n-type CaMnO3 and p-type Ca3Co4O9 oxides and another module with n-type (ZnO)7In2O3 and p-type Ca3Co4O9 oxides. The measured power density values were compared with the values calculated from the materials properties.

Free-standing Bi–Sb–Te films derived from thermal annealing of sputter-deposited Sb2Te3/Bi2Te3 multilayer films for thermoelectric applications

We introduce an easily acceptable method to produce free-standing Bi–Sb–Te films from Sb2Te3/Bi2Te3 multilayer films based on solid-state reactions. When sputter-deposited Sb2Te3/Bi2Te3 multilayer films were annealed at 400 °C under a N2 gas atmosphere, they were transformed into single layers composed of Bi–Sb–Te films. At the same time, they were spontaneously stripped on a large scale from the substrates (1 cm × 1 cm) without chemical etching. The as-fabricated free-standing Bi–Sb–Te films were very flexible, with a thickness of about 400 nm. The findings confirmed that the formation of voids and the strong flow rate of ambient gas play important roles in fabricating free-standing Bi–Sb–Te films during the thermal annealing process. The temperature-dependent crystallization behaviors of each of the Sb2Te3 and Bi2Te3 films were also investigated to understand the mechanism. Moreover, the thermoelectric properties of the free-standing Bi–Sb–Te films were examined with different thicknesses of the films. We believe that this method can contribute to the development of low-dimensional nanostructures that can be useful in various applications such as energy conversion, energy storage and wearable electronics.

Thermoelectric Properties of n-Type Half-Heusler Compounds Synthesized by the Induction Melting Method

The n -type Hf0.25Zr0.25Ti0.5NiSn0.998Sb0.002 Half-Heusler (HH) alloy composition was prepared by using the induction melting method in addition to the mechanical grinding, annealing, and spark plasma sintering processes. Analysis of X-ray diffraction (XRD) results indicated the formation of a pure phase HH structured compound. The electrical and thermal properties at temperatures ranging from room temperature to 718 K were investigated. The electrical conductivity increased with increasing temperatures and demonstrated nondegenerate semiconducting behavior, and a large reduction in the thermal conductivity to the value of 2.5 W/mK at room temperature was observed. With the power factor and thermal conductivity, the dimensionless figure of merit was increased with temperature and measured at 0.94 at 718 K for the compound synthesized by the induction melting process.

Silylation of sodium silicate-based silica aerogel using trimethylethoxysilane as alternative surface modification agent

AbstractTrimethylethoxysilane (TMES) has been recognized as a good co-precursor to increase the degree of hydrophobicity during the synthesis of a silica aerogel because of its methyl groups. Therefore, some physical properties of silica aerogels, including the contact angle and porosity, were investigated using TMES as a co-precursor at different molar ratios with the main precursor such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). In contrast to TMES, most silylating agents such as hexamethyldisilazane (HMDZ) and trimethylchlorosilane (TMCS) have been used for surface modification because of their ability to enhance the hydrophobicity of the aerogel surface. This work examines the silylation effect, which includes increasing hydrophobicity by TMES to determine the possibility of using it as an alternative silylating agent during ambient pressure drying in the synthesis of sodium silicate-based silica aerogel. In addition, the physical properties of sodium silicate-based silica aerogels with silylation under different TMES/TMCS volume ratio are investigated. The physical properties of sodium silicate-based aerogels can be changed by the TMES/TMCS volume ratio during the surface modification step. Aerogels with a high specific surface area (458 m2/g), pore volume (3.215 cm3/g), porosity (92.7%), and contact angle (131.8°) can be obtained TMES/TMCS volume ratio of 40/60. HighlightsSodium silicate-based silica aerogel was synthesized by APD using TMES/TMCS surface co-modifying agent.TMES can act as an alternative surface modification agent with TMCS.Hydrophobicity was not measured at the case of only TMES usage.TMES/TMCS surface co-modifier for silylation enhances the physical properties of silica aerogel.

Doping amount dependence of phase formation and microstructure evolution in heavily Cu-doped Bi2Te3 films for thermoelectric applications

Heavily Cu-doped Bi2Te3 films with different Cu contents (∼47.9 at%) were prepared by co-sputtering with Cu and Bi2Te3 targets at room temperature and post-annealing at ∼500 °C. When the Cu content increased, the lattice parameters of the c-axis gradually increased with the increase of Cu intercalation into van der Waals gaps between the quintets of the Bi2Te3 layers. When a Cu content of above 43.7 at% was added, formation of a Cu2Te phase was detected along with the phase transition from Bi2Te3 to Te-deficient BixTey in the films annealed at 300 °C. The porous column structure of the pure Bi2Te3 film became gradually dense with the increase of Cu content. When the Cu content was increased, the thermoelectric power factors of heavily Cu-doped Bi2Te3 films were enhanced by the increase of carrier concentration.