열전도도 향상을 위한 직물섬유 복합재의 최적구조 설계

김명수 ( Myungsoo Kim ),성대한 ( Dae Han Sung ),박영빈 ( Young Bin Park ),박기원 ( Kiwon Park ) 한국복합재료학회 2017 Composites research Vol.30 No.1

본 연구에서는 직물섬유 복합재의 열전도를 구하는데 있어 기존의 연구보다 개선된 방법을 제시하고, 직물섬유의 기하학적 구조가 복합재의 열전도도 향상에 미치는 영향, 그리고 유전 알고리즘(Genetic algorithm)을 이용하여 복합재의 열전도도 향상을 위한 최적구조 설계에 관한 연구를 하였다. 직물섬유의 구조를 토우의 물결무늬와 너비 및 두께를 이용하여 구현하였고, 열전도도는 열전기유사법(Thermal-electrical analogy)을 이용하여 구하였다. 유전 알고리즘에서 염색체 문자열은 fill과 warp tow의 두께와 너비로 하였고 복합재의 열전도도를 향상 시키는 방향으로 목적함수를 정하였다. 연구결과 직물섬유 복합재의 열전도도를 예측을 위한 향상된 방법이 제시되었고, 섬유토우 사이의 간격(inter-tow gap)이 넓어 질수록 복합재의 열전도도가 감소하는 것으로 나타났다. 직물섬유 복합재의 구조 최적화에서는 이론적 수치해석 결과가 제시되었는데, 전체적으로 섬유토우(tow)의 축의 수직 방향보다는 축 방향의 열전도도 성분이 복합재의 전체 열전도도 향상에 크게 기여를 하는 것으로 나타났다. This research presents studies on an improved method to predict the thermal conductivity of woven fabric composites, the effects of geometric structures of woven fabric composites on thermal conductivity, and structural optimization to improve the thermal conductivity using a genetic algorithm. The geometric structures of woven fabric composites were constructed numerically using the information generated on waviness, thickness, and width of fill and warp tows. Thermal conductivities of the composites were obtained using a thermal-electrical analogy. In the genetic algorithm, the chromosome string consisted of thickness and width of the fill and warp tows, and the objective function was the maximum thermal conductivity of woven fabric composites. The results confirmed that an improved method to predict the thermal conductivity was built successfully, and the inter-tow gap effect on the composite`s thermal conductivity was analyzed suggesting that thermal conductivity of woven fabric composites was reduced as the gap between tows increased. For structural design, optimized structures for improving the thermal conductivity were analyzed and proposed. Generally, axial thermal conductivity of the fiber tow contributed more to thermal conductivity of woven fabric composites than transverse thermal conductivity of the tows.

Temperature dependence and cation effects in the thermal conductivity of glassy and molten alkali borates

Kim, Youngjae,Morita, Kazuki Elsevier 2017 Journal of non-crystalline solids Vol.471 No.-

<P><B>Abstract</B></P> <P>The thermal conductivity of Li<SUB>2</SUB>O-B<SUB>2</SUB>O<SUB>3</SUB>, Na<SUB>2</SUB>O-B<SUB>2</SUB>O<SUB>3</SUB> and K<SUB>2</SUB>O-B<SUB>2</SUB>O<SUB>3</SUB> glass systems was measured as a function of the temperature. As the temperature increases, the thermal conductivity of the glass phase initially increases and then reaches a plateau. Afterwards, in the liquid phases, a further increase in the temperature leads to a decrease in the thermal conductivity. The thermal conduction phenomenon can be better described by considering the glass and molten oxide systems as a one-dimensional continuum. It was found that the temperature corresponding to the highest thermal conductivity lies close to the one-dimensional Debye temperature (<I>Θ</I> <SUB> <I>D</I>1</SUB>). According to phonon gas model, the variables affecting the thermal conductivity were evaluated. Below <I>Θ</I> <SUB> <I>D</I>1</SUB>, the increase in heat capacity with the temperature leads to a corresponding increase in the thermal conductivity. The heat capacity then becomes constant above <I>Θ</I> <SUB> <I>D</I>1</SUB> leading to the observed plateau in the thermal conductivity of the glass phase. After melting the glass, the decrease in thermal conductivity with increasing temperature is due to changes in sound velocity and mean free path. The relative content of 3- and 4-coordinated boron was analyzed by <SUP>11</SUP>B MAS-NMR. The cation effect on the thermal conductivity of alkali borate glasses was evaluated through their ionization potentials.</P> <P><B>Highlights</B></P> <P> <UL> <LI> By phonon gas model, variables determining thermal conductivity in the glassy and molten oxide system were evaluated. </LI> <LI> The glassy and molten oxide system was treated as a conducting medium of one-dimensional continuum. </LI> <LI> In the non-crystalline oxide system, maximum thermal conductivity could be found near one-dimensional Debye temperature. </LI> <LI> Effect of cation on thermal conductivity in the alkali borate system was evaluated through the ionization potential. </LI> </UL> </P>

반도체 재료의 격자열전도도 분석

임종찬,양희선,김현식,Lim, Jong-Chan,Yang, Heesun,Kim, Hyun-Sik 한국마이크로전자및패키징학회 2020 마이크로전자 및 패키징학회지 Vol.27 No.4

열전소재의 격자열전도도 저감은 열전성능 증대를 위해 가장 빈번하게 사용되는 방법이다. 하지만 전체 열전도도에서 다른 열전도도 기여분을 제외하는 방법으로만 격자열전도도를 구할 수 있기 때문에 격자열전도도를 정확하게 분석하는 것을 간단한 작업이 아니다. 본 연구에서는 먼저 전자/홀에 의한 열전도도 기여분 (모든 소재 적용)과 쌍극 전도에 의한 기여분 (작은 밴드 갭 소재 적용)을 정확하게 계산해야만 격자열전도도를 정확하게 분석할 수 있음을 설명한다. 전자/홀에 의한 기여분을 계산하기 위해 필수적인 로렌츠 숫자 계산법 (싱글 파라볼릭 모델링 및 간단한 식 이용)과 쌍극 전도에 의한 기여분 계산법 (투 밴드 모델링) 또한 소개한다. 격자열전도도의 정확한 분석은 격자열전도도 저감을 위한 여러 결함 제어 전략의 효과를 객관적으로 평가할 수 있는 강력한 분석 도구로 사용될 수 있다. Suppressing lattice thermal conductivity of thermoelectric materials is one of the most popular approach to improve their thermoelectric performance. However, accurate characterization of suppressed lattice thermal conductivity is challenging as it can only be acquired by subtracting other contributions to thermal conductivity from the total thermal conductivity. Here we explain that electronic thermal conductivity (for all materials) and bipolar thermal conductivity (for narrow band gap materials) need to be determined accurately first to characterize the lattice thermal conductivity accurately. Methods to calculate Lorenz number for electronic thermal conductivity (via single parabolic model and using a simple equation) and bipolar thermal conductivity (via two-band model) are introduced. Accurate characterization of the lattice thermal conductivity provides a powerful tool to accurately evaluate effect of different defect engineering strategies.

Dispersion behavior and thermal conductivity characteristics of Al2O3–H2O nanofluids

Dongsheng Zhu,Xinfang Li,Nan Wang,Xianju Wang,Jinwei Gao,Hua Li 한국물리학회 2009 Current Applied Physics Vol.9 No.1

Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. In this study, Al2O3–H2O nanofluids were synthesized, their dispersion behaviors and thermal conductivity in water were investigated under different pH values and different sodium dodecylbenzenesulfonate (SDBS) concentration. The sedimentation kinetics was determined by examining the absorbency of particle in solution. The zeta potential and particle size of the particles were measured and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate attractive and repulsive potentials. The thermal conductivity was measured by a hot disk thermal constants analyser. The results showed that the stability and thermal conductivity enhancements of Al2O3–H2O nanofluids are highly dependent on pH values and different SDBS dispersant concentration of nano-suspensions, with an optimal pH value and SDBS concentration for the best dispersion behavior and the highest thermal conductivity. The absolute value of zeta potential and the absorbency of nano-Al2O3 suspensions with SDBS dispersant are higher at pH 8.0. The calculated DLVO interparticle interaction potentials verified the experimental results of the pH effect on the stability behavior. The Al2O3–H2O nanofluids with an ounce of Al2O3 have noticeably higher thermal conductivity than the base fluid without nanoparticles, for Al2O3 nanoparticles at a weight fraction of 0.0015 (0.15 wt%), thermal conductivity was enhanced by up to 10.1%. Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. In this study, Al2O3–H2O nanofluids were synthesized, their dispersion behaviors and thermal conductivity in water were investigated under different pH values and different sodium dodecylbenzenesulfonate (SDBS) concentration. The sedimentation kinetics was determined by examining the absorbency of particle in solution. The zeta potential and particle size of the particles were measured and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate attractive and repulsive potentials. The thermal conductivity was measured by a hot disk thermal constants analyser. The results showed that the stability and thermal conductivity enhancements of Al2O3–H2O nanofluids are highly dependent on pH values and different SDBS dispersant concentration of nano-suspensions, with an optimal pH value and SDBS concentration for the best dispersion behavior and the highest thermal conductivity. The absolute value of zeta potential and the absorbency of nano-Al2O3 suspensions with SDBS dispersant are higher at pH 8.0. The calculated DLVO interparticle interaction potentials verified the experimental results of the pH effect on the stability behavior. The Al2O3–H2O nanofluids with an ounce of Al2O3 have noticeably higher thermal conductivity than the base fluid without nanoparticles, for Al2O3 nanoparticles at a weight fraction of 0.0015 (0.15 wt%), thermal conductivity was enhanced by up to 10.1%.

Opto-thermal technique for measuring thermal conductivity of polyacrylonitrile based carbon fibers

Jang, Dawon,Lee, Dong Su,Lee, Aram,Joh, Han-Ik,Lee, Sungho Elsevier 2019 Journal of industrial and engineering chemistry Vol.78 No.-

<P><B>Abstract</B></P> <P>Thermal conductivity of carbon fibers (CFs) is an important property because CFs are used as heat dissipation fillers in composites for aerospace and electronics applications. However, evaluating thermal conductivity of a single filament of CFs is an arduous task due to dimensional issue of specimens and limitations of conventional measurement system. Therefore, we suggest an opto-thermal technique using Raman spectroscopy to measure thermal conductivity of commercial polyacrylonitrile based CFs (T300, T700SC and T800H). The opto-thermal technique used that G band from Raman spectroscopy of carbon materials is shifted depending on temperature. For verifying an accuracy of the technique, the laser absorbance of CFs were estimated, and the thermal conductivity was measured depending on the length of CF. The measured data were reflected in the thermal conductivity calculation formula. It was demonstrated that the method provides more reasonable thermal conductivity values compare to a conventional Angstrom method. In addition, this simple technique confirmed that graphitic structure of CFs played a critical role in their thermal conductivity.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An opto-thermal technique was demonstrated to measure thermal conductivity of commercial polyacrylonitrile based CFs. </LI> <LI> The G band shift depending on temperature in Raman spectra was used for obtaining thermal conductivity of CFs. </LI> <LI> For an accurate measurement, a laser absorbance of CFs was estimated by UV–vis-NIR spectrometer and simulation. </LI> <LI> The thermal conductivity of T300, T700SC, and T800H were found to 13.8, 12.7, and 37.5W/mK, respectively. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Suppression of bipolar conduction via bandgap engineering for enhanced thermoelectric performance of p-type Bi_0.4Sb_1.6Te₃ alloys

Kim, Hyun-Sik,Lee, Kyu Hyoung,Yoo, Joonyeon,Shin, Weon Ho,Roh, Jong Wook,Hwang, Jae-Yeol,Kim, Sung Wng,Kim, Sang-il Elsevier 2018 Journal of alloys and compounds Vol.741 No.-

<P><B>Abstract</B></P> <P>Substitutional doping is known to be effective when used to enhance the thermoelectric figure of merit <I>zT</I>, and this is generally explained as resulting from a reduction in the thermal conductivity caused by an additional atomic-scale defect structure. However, a comprehensive analysis of the substitutional doping effect on the electrical and thermal properties together has not been undertaken, especially when the bipolar thermal conductivity becomes serious. A previous study by the authors also showed that the <I>zT</I> of Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> thermoelectric alloys was enhanced by indium (In) doping due to the reduction of the total thermal conductivity. Here, we more closely analyze the electrical and thermal transport properties of a series of indium (In)-doped p-type Bi<SUB>0.4</SUB>Sb<SUB>1.6-x</SUB>In<SUB>x</SUB>Te<SUB>3</SUB> (x = 0, 0.003, 0.005, 0.01) using both the single-parabolic-band model and the Debye-Callaway model in an effort to investigate the origin of the observed thermal conductivity reduction more closely. The bipolar contribution to the total thermal conductivity was estimated exclusively based on a two-band model based on a single-parabolic-band model. Furthermore, the lattice thermal conductivity was calculated using the Debye-Callaway model while taking additional In substitutional defects into consideration. The calculations indicated that the significant suppression of bipolar thermal conductivity was achieved as a result of the increased bandgap in Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> caused by In doping. Additional point defects from In doping also reduced the lattice thermal conductivity, but not as much as the bipolar thermal conductivity did. The study suggests that the suppression of bipolar conduction by means of a bandgap modification can be an effective approach for enhancing <I>zT</I> further via a simple In-doping process in Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Thermal conductivity reduction in In-doped Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> is analyzed. </LI> <LI> Bandgap increase by In doping suppresses bipolar conduction significantly. </LI> <LI> Extra point defects by In doping reduces lattice thermal conductivity. </LI> <LI> The <I>zT</I> enhancement in In-doped Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> is mainly due to bipolar suppression. </LI> </UL> </P>

Determination of Thermo-Electrical Properties in Sn Based Alloys

Sezen Aksöz,Yavuz Ocak,Kâzlm Keslioslu,Necmettin Mara ll 대한금속·재료학회 2010 METALS AND MATERIALS International Vol.16 No.3

The variation of thermal conductivity of solid phase versus temperature for Sn-21 wt.% Bi, Sn-25 wt.% In and Sn-35 wt.% In-26 wt.% Bi alloys were measured with a radial heat flow apparatus. From the graphs of thermal conductivity versus temperature, the thermal conductivity of the solid phases at their melting temperatures and the thermal temperature coefficients for the same alloys were obtained. The ratios of thermal conductivity of liquid phase to solid phase for the same materials were measured with a Bridgman type directional solidification apparatus. The variations of electrical conductivity of solid phases versus temperature for the same alloys were determined from the Wiedemann-Franz law by using the measured values of thermal conductivity. From the graphs of electrical conductivity versus temperature, the electrical temperature coefficients for the same alloys were also determined. According to present experimental results it can be concluded that the thermal and electrical conductivity of Sn based alloys depend on the thermal and electrical conductivity of the alloying elements. If the thermal and electrical conductivity of the alloying elements are lower than the thermal conductivity of Sn, the thermal conductivity of Sn based alloys decreases, whereas, otherwise,it increases.

Effects of Y2O3 Addition on Densification and Thermal Conductivity of AlN Ceramics During Spark Plasma Sintering

채재홍,박주석,안종필,김경훈,이병하 한국세라믹학회 2008 한국세라믹학회지 Vol.45 No.12

Spark plasma sintering (SPS) of AlN ceramics were carried out with Y2O3 as sintering additive at a sintering temperature 1,550~ 1,700 o C. The effect of Y2O3 addition on sintering behavior and thermal conductivity of AlN ceramics was studied. Y2O3 added AlN showed higher densification rate than pure AlN noticeably, but the formation of yttrium aluminates phases by the solid-state reaction of Y2O3 and Al2O3 existed on AlN surface could delay the densification during the sintering process. The thermal conductivity of AlN specimens was promoted by the addition of Y2O3 up to 3 wt% in spite of the formation of YAG secondary phase in AlN grain boundaries because Y2O3 addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. However, the thermal conductivity rather decreased over 3 wt% addition because an immoderate formation of YAG phases in grain boundary could decrease thermal conductivity by a phonon scattering surpassing the contribution of Y2O3 addition. Spark plasma sintering (SPS) of AlN ceramics were carried out with Y2O3 as sintering additive at a sintering temperature 1,550~ 1,700 o C. The effect of Y2O3 addition on sintering behavior and thermal conductivity of AlN ceramics was studied. Y2O3 added AlN showed higher densification rate than pure AlN noticeably, but the formation of yttrium aluminates phases by the solid-state reaction of Y2O3 and Al2O3 existed on AlN surface could delay the densification during the sintering process. The thermal conductivity of AlN specimens was promoted by the addition of Y2O3 up to 3 wt% in spite of the formation of YAG secondary phase in AlN grain boundaries because Y2O3 addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. However, the thermal conductivity rather decreased over 3 wt% addition because an immoderate formation of YAG phases in grain boundary could decrease thermal conductivity by a phonon scattering surpassing the contribution of Y2O3 addition.

Steady- and transient-state analyses of fully ceramic microencapsulated fuel loaded reactor core via two-temperature homogenized thermal-conductivity model

Lee, Yoonhee,Cho, Nam Zin Elsevier 2015 Annals of nuclear energy Vol.76 No.-

<P><B>Abstract</B></P> <P>Fully ceramic microencapsulated (FCM) fuel, a type of accident-tolerant fuel (ATF), consists of TRISO particles randomly dispersed in a SiC matrix. In this study, for a thermal analysis of the FCM fuel with such a high heterogeneity, a two-temperature homogenized thermal-conductivity model was applied by the authors. This model provides separate temperatures for the fuel-kernels and the SiC matrix. It also provides more realistic temperature profiles than those of harmonic- and volumetric-average thermal conductivity models, which are used for thermal analysis of a fuel element in VHTRs having a composition similar to the FCM fuel, because such models are unable to provide the fuel-kernel and graphite matrix temperatures separately.</P> <P>In this study, coupled with a neutron diffusion model, a FCM fuel-loaded reactor core is analyzed via a two-temperature homogenized thermal-conductivity model at steady- and transient-states. The results are compared to those from harmonic- and volumetric-average thermal conductivity models, i.e., we compare <I>k<SUB>eff</SUB> </I> eigenvalues, power distributions, and temperature profiles in the hottest single-channel at steady-state. At transient-state, we compare total powers, reactivity, and maximum temperatures in the hottest single-channel obtained by the different thermal analysis models. The different thermal analysis models and the availability of fuel-kernel temperatures in the two-temperature homogenized thermal-conductivity model for Doppler temperature feedback cause significant differences as revealed by comparisons.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Fully ceramic microencapsulated fuel-loaded core is analyzed via a two-temperature homogenized thermal-conductivity model. </LI> <LI> The model is compared to harmonic- and volumetric-average thermal conductivity models. </LI> <LI> The three thermal analysis models show ∼100pcm differences in the <I>k<SUB>eff</SUB> </I> eigenvalue. </LI> <LI> The three thermal analysis models show more than 70K differences in the maximum temperature. </LI> <LI> There occur more than 3 times differences in the maximum power for a control rod ejection accident. </LI> </UL> </P>

Opto-thermal technique for measuring thermal conductivity of polyacrylonitrile based carbon fibers

장다원,이동수,이아람,조한익,이성호 한국공업화학회 2019 Journal of Industrial and Engineering Chemistry Vol.78 No.-

Thermal conductivity of carbonfibers (CFs) is an important property because CFs are used as heatdissipationfillers in composites for aerospace and electronics applications. However, evaluating thermalconductivity of a singlefilament of CFs is an arduous task due to dimensional issue of specimens andlimitations of conventional measurement system. Therefore, we suggest an opto-thermal techniqueusing Raman spectroscopy to measure thermal conductivity of commercial polyacrylonitrile based CFs(T300, T700SC and T800 H). The opto-thermal technique used that G band from Raman spectroscopy ofcarbon materials is shifted depending on temperature. For verifying an accuracy of the technique, thelaser absorbance of CFs were estimated, and the thermal conductivity was measured depending on thelength of CF. The measured data were reflected in the thermal conductivity calculation formula. It wasdemonstrated that the method provides more reasonable thermal conductivity values compare to aconventional Angstrom method. In addition, this simple technique confirmed that graphitic structure ofCFs played a critical role in their thermal conductivity.