Tae-Jung Ha

Variations in mechanical and thermal properties of mesoporous alumina thin films due to porosity and ordered pore structure.

Ordered mesoporous aluminum oxide films with porosity ranging between 5% and 37% were synthesized by evaporation-induced self-assembly (EISA) using surfactant templating. To investigate the effects of mesoporous structure on thermal properties, changes in pore structure including pore size, pore distribution, and porosity were monitored as a function of surfactant concentration (Pluronic P-123, poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)(20)). The ordered mesoporous alumina films were then examined to determine how their morphology influences their thermal properties. These alumina films had a body-centered cubic pore structure or a random-oriented pore structure, depending on the surfactant concentration used, and superior thermal properties were obtained by controlling porosity and pore structure. Therefore, the ordered mesoporous alumina films synthesized in this study can be used as high-temperature thermo-isolating materials.

Investigation of the properties of organically modified ordered mesoporous silica films.

Organically modified, ordered mesoporous silica films, which can provide hydrophobicity and low polarizability to the framework, were prepared using Brij-76 block copolymer as a template. Due to a fast condensation reaction of the silica precursor, mesostructured silica films were not properly synthesized. To circumvent this problem, a synthesis procedure was modified to provide an enhancement of pore periodicity through the incorporation of methyl ligands on the framework. The micropore volume was reduced, and the pore size was enlarged, as the concentration of the methyl ligands on the framework was increased. A mesophase transition from a two-dimensional hexagonal structure to a body-centered cubic (BCC) structure was observed according to the concentration of incorporated methyl ligands. The mechanical properties of the fabricated films were investigated according to the pore ordering and film density. The mechanical properties of the films with random pore geometry show a positive correlation between film density and elastic modulus. Meanwhile, the mechanical behavior of organically modified mesoporous silica films with periodic pore distribution represents a negative correlation within a certain density range, which is advantageous to the low-k materials. Especially, film with a low micropore volume fraction and BCC pore ordering is more applicable to a low-k material due to low dielectric constant and high mechanical strength.

Roughness and pore structure control of ordered mesoporous silica films for the enhancement of electrical properties

Mesostructured silica films with a well-ordered Im3m structure and enhanced surface properties can be synthesized by means of cosolvent addition and the optimization of sol aging time. Mesostructured silica films fabricated using ethanol, a commercial solvent, result in wavy surface properties induced by the Marangoni effect irrespective of rotation speed during spin coating. In order to improve the surface roughness, we used an ethanol-acetone mixture as a solvent with a certain molar ratio. Wavy and rough films could be avoided when the acetone molar ratio was optimized. The simultaneous increase of unit cell size and thickness of the film was observed when the amount of cosolvent was increased. The unit cell was also enlarged with sol aging time, and well-ordered mesoporous silica films could be synthesized by controlling the silica sol aging. The dielectric property could be improved by means of cosolvent addition only after the development of a highly ordered mesoporous nature of the film by calcinat...

Thermal conductivity of BCC-ordered mesoporous silica films

Experimental investigations on thermal conductivity of ordered mesoporous silica films (MSFs) are conducted. Ordered MSFs are prepared using Brij-76 block copolymer. Porosities of ordered MSFs are 30%, 40% and 50% with thicknesses of 162 nm, 192 nm and 177 nm, respectively. Pores with size of a few nanometres are distributed along the entire film in body-centered cubic (BCC) phase. Thermal conductivities of BCC-ordered MSFs are measured using the 3ω method at temperature ranging from room temperature (300 K) to 600 K. Thermal conductivities are measured to be up to several factors higher compared with random porous silicates, which is desirable for effective thermal management in integrated circuits.

Investigation of the effect of calcination temperature on HMDS-treated ordered mesoporous silica film.

To reduce signal delay in ultra-large-scale integrated circuits, an intermetal dielectric with low dielectric constant is required. Ordered mesoporous silica film is appropriate for use as an intermetal dielectric due to its low dielectric constant and superior mechanical properties. To reduce the dielectric constant, an ordered mesoporous silica film prepared by a tetraethoxysilane/methyltriethoxysilane silica precursor and Brij-76 block copolymer was surface-modified by hexamethyldisilazane (HMDS) treatment. HMDS treatment substituted OH with Si(CH(3))(3) groups on the silica surface. After treatment, ordered mesoporous silica films were calcined at various calcination temperatures, and the calcination temperature to obtain optimal structural, electrical, and mechanical properties was determined to be approximately 300 degrees C.

Use of ordered mesoporous SiO2 as protection against thermal disturbance in phase-change memory

To commercialize phase change memory (PCM), a drastic change of resistivity at specific temperatures and a low power consumption to minimize heat transfer to neighboring cells are needed. Therefore, in this work, an ordered mesoporous SiO2 thin film of 45% porosity was introduced as an intercell dielectric in Ge1Sb4Te7 PCM because it has a low thermal conductivity (0.177 W/m K). By using a hybrid layer structure of mesoporous and dense SiO2 films, the temperature of neighboring cells could be decreased from 393.3 K to 353.2 K, corresponding to a 100-fold change in resistivity.

Pore structure control of ordered mesoporous silica film using mixed surfactants

Materials with nanosized and well-arranged pores have been researched actively in order to be applied to new technology fields. Especially, mesoporous material containing various pore structures is expected to have different pore structure. To form a mixed pore structure, ordered mesoporous silica films were prepared with a mixture of surfactant; Brij-76 and P-123 block copolymer. In mixed surfactant system, mixed pore structure was observed in the region of P-123/(Brij-76 + P-123) with about 50.0wt.% while a single pore structure was observed in regions which have large difference in ratio between Brij-76 and P-123 through the X-ray diffraction analysis. Regardless of surfactant ratio, porosity was retained almost the same. It is expected that ordered mesoporous silica film with mixed pore structure can be one of the new materials which has distinctive properties.

Study on the Electrical and Thermal Conductivity of Ordered Mesoporous TiO2 Thin Film Incorporated with Pt Nanoparticles

Ordered mesoporous TiO2 films with incorporated Pt nanoparticles were prepared using titanium tetraisopropoxide, hexachloroplatinic acid hexahydrate, and Pluronic P-123 as a titania precursor, a Pt precursor, and a structure-directing agent, respectively. Pt nanoparticles were introduced to enhance the electrical properties of the mesoporous films, which have excellent thermal insulation properties. We confirmed that the synthesized composite films with Pt nanoparticles had an ordered pore structure with anatase phase TiO2 by both small-angle and wide-angle X-ray diffraction analyses. Although the porosity of the composite film decreased from 37.6 to 29.3% when Pt nanoparticles were included, the conductivity ratio (σ/κ) of the film increased greatly, up to approximately 1370 K V-2, due to the greater increase in electrical conductivity than thermal conductivity. Based on our results, we conclude that by incorporating Pt nanoparticles into ordered mesoporous TiO2 film, the thermoelectric properties of the mesoporous films can be improved.

Preparation and Characterization of Mesoporous Ceramic Materials

Ordered mesoporous oxide films have been focused because of their low density, high interior specific surface area, and high thermal insulation. Specially, the ordered mesoporous oxide films prepared by self-assembly has many advantages due to easy process and high reproducibility. In this work, ordered mesoporous , , and films were synthesized by control of composition and processing parameter. Also, their structural, thermal, and mechanical properties were characterized variously. In conclusion, ordered mesoporous oxides will be one of core materials in new technology due to their excellent and unique properties.

HMDS Treatment of Ordered Mesoporous Silica Film for Low Dielectric Application

In order to reduce signal delay in ULSI, an intermetal material of low dielectric constant is required. Ordered mesoporous silica film is proper to intermetal dielectric due to its low dielectric constant and superior mechanical properties. The ordered mesoporous silica film prepared by TEOS (tetraethoxysilane) / MTES (methyltriethoxysilane) mixed silica precursor and Brij-76 surfactant was surfacemodified by HMDS (hexamethyldisilazane) treatment to reduce its dielectric constant. HMDS can substitute -Si(CH₃)₃ groups for - OH groups on the surface of silica wall. In order to modify interior silica wall, HMDS was treated by two different processes except the conventional spin coating. One process is that film is dipped and stirred in HMDS/n-hexane solution, and the other process is that film is exposed to evaporated HMDS. Through the investigation with different HMDS treatment, it was concluded that surface modification in evaporated HMDS was more effective to modify interior silica wall of nano-sized pores.