S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
A study on the high performance nanocomposite materials for power generation : 나노복합재료의 고성능 기전 특성에 관한 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Piezoelectricity (PE) ; Thermoelectricity (TE) ; Power generator ; Poly (vinylidene fluoride) (PVDF) ; Multiwalled Carbon nanotubes (MWCNTs) ; Bismuth antimony telluride (BST) ; poly(3 ; 4-ethylenedioxythiophene) (PEDOT) : poly(styrenesulfonate) (PSS) (PEDOT:PSS)
- 학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 서용석.
- Piezoelectric (PE) and thermoelectric (TE) materials have been extensively studied by several researchers to enhance their performance as applications for power generation. But still there are many restrictions like power efficiency, environmental problems, financial constraint, etc. Here, we studied on some piezoelectric and thermoelectric materials, which showed high potential to overcome such restrictions.
As a piezoelectric material, PVDF membranes of high β-phase content could be obtained by using an electrospinning process, adding small amounts of MWCNTs (0.2 wt%). The electrospinning process is similar in some ways to uniaxial mechanical stretching, which helps the α-phase converse into the β-phase. MWCNTs promoted the conversion of the PVDF molecules α-phase into the β-phase, by acting as nuclei in the crystallization process and inducing charge accumulation at the interface. The change of the physical properties could be attributed to the alterations in the polymer microstructure produced by drawing and poling processes, which showed synergetic effect with the MWCNTs. The cooperative effect of the drawing process and MWCNTs led to a high degree of conversion from the nonpolar α-phase to the polar β-phase in the electrospun PVDF/MWCNT composite nanofibers. The significant effects of the polymer chain orientation originated mainly from their determining role in confining the dipole through chain rotation under drawing and poling. Different from the solution casting membranes, depoling was not observed with the excessive MWCNT addition that led to the β-phase content decrease. For electrospun samples, it appeared that depoling did not happen because of better dispersion of MWCNTs which leads to the charge accumulation at the interface, not exceeding the coercive field. For the poled samples, the amount of the β-phase increased with a content of MWCNTs due to the efficient charge accumulation. The high conversion of the α-phase into the β-phase was improved remarkably by further drawing of the membranes, which resulted in the rapid enhancement of the ferroelectric and piezoelectric properties of the PVDF/MWCNT membranes.
Nonisothermal crystallization analysis of a PVDF and its MWCNT nanocomposites provides some important information about the crystalline structure development of these materials. In this study, nonisothermal crystallization kinetics analysis method proposed by Seo was successfully applied to the DSC data for a PVDF and its nanocomposites at various low cooling rates. The Avrami exponent of the pristine PVDF was close to 3 indicating that the crystallites had a spherulite structure whereas that of the nanocomposites was around 4, indicating the addition of MWCNT into PVDF caused heterogeneous nucleation and created the growth mechanism of 3- dimensional shape. By observing the morphologies of these materials as well as crystallographic analysis, we confirmed these findings. The morphology of the pristine PVDF was spherulites whereas that of the PVDF/MWCNT nanocomposites was consisted of smaller three dimensional structures. The observed behavior could be attributed to a large number of nuclei (MWCNT). Though the addition of MWCNT could affect the nonisothermal crystallization kinetics, it did not change the crystalline polymorphs of PVDF at all. Consequently, all composites showed α-phase polymorph only. This means that the interaction between the functionalized MWCNT and -CF2 group of PVDF was not strong enough to induce the alignment of the -CF2 moiety into the same direction. Therefore, strong external stimulus such as drawing and/or poling should be provided to convert the α-form PVDF crystalline phase into the β-form.
Strong correlation between the contents of β-phase in the film and the cellular activity/growth was observed. All cells showed very high proliferation rate on the PVDF nanocomposite films. Scaffolds containing 0.5 wt% MWCNT reached the highest cell activity values (more than two folds of the drawn and poled neat PVDF film), while the ferroelectricity reached its maximum at 0.2 wt% MWCNT concentration. This was ascribable to the additional effects of scaffolds stiffness and the ferroelectricity. The stiffness of the scaffold increased with the amount of MWCNT. Since the cellular activity increased with the scaffold stiffness, the MWCNT composition of the maximum of proliferation rate moved to higher concentration. The nanocomposite films containing excessive amount of MWCNT (1 wt% in this study) which had more reduced ferroelectricity due to depoling showed reduced cell growth in spite of increased film stiffness. The ferroelectricity seemed dominant over the modulus increase at this composition. The cell proliferation rate was the outcome of the cell response to the stiffness of the scaffolds and to the polarization surface of them. This study demonstrates the importance of the surface polarization and high modulus, which implicates that highly ferroelectric films with high stiffness can be applied to many polymeric scaffolds and contribute to the development of new highly functional scaffolds for the cell culture.
BST-PEDOT:PSS composite film showed enhanced TE performance compared to the organic-inorganic composites studied in previous researches. High electrical conductivity of BST at room temperature was successfully combined with the highly conductive organic conductor, PEDOT:PSS, due to the structure control in synthetic processes and chemical post-treatments. The reduction process of alloy precursors near 90°C did not make the PEDOT:PSS decomposed, and we could get the homogeneous BST-PEDOT:PSS composite suspension after the synthesis. With the addition of DBSA, BST-PEDOT:PSS suspension could be used as a conductive ink that is easily cast on substrates controlling its thickness and pattern.
From SEM analysis, we recognized that the BST fillers had grown to fibers which can act like low resistive pathways in themselves. And when all the components were homogeneously dispersed in the composite, PEDOT:PSS could play a role as a bridge among the fibrous pathways. A large increase in electrical conductivity was attributed to the structural development. Treated by DMSO at the dipping method, cast BST-PEDOT:PSS film reached the electrical conductivity of 1371 S/cm and the Seebeck coefficient of 48μV. As a result, optimized power factor of 316 μV/mK^2 was achieved.