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Thermally decomposition of high quality flower-like ZnO nanorods from zinc acetate dihydrate
Madavali, B.,Kim, H.S.,Hong, S.J. North-Holland 2014 Materials letters Vol.132 No.-
In the present work, high purity flower-like ZnO nano-rods have been synthesized by thermal decomposition method maintained at 300<SUP>o</SUP>C in air without using any organic solvent additive surfactant or catalyst. The phase and structural analysis were characterized by X-ray diffraction (XRD) which confirmed the high crystal quality of ZnO with hexagonal (Wurtzite phase) crystal structure. The morphology was carried out by scanning electron microscopy (SEM) which indicates that the formation of well-formed ZnO nanorods as a flower-like ZnO structure. The optical bandgap of synthesized ZnO nanorods found as 3.37eV for all samples by using UV-vis absorption spectra and further verified by diffuse reflectance spectroscopy analysis. The room-temperature photoluminescence (PL) spectrum of all samples shows strong UV emission peak at ~384nm and a weak broad green-yellow due to the defects in the samples. The Raman spectroscopy was performed for the studies of vibrational bands and the quality of the crystalline structure of flower-like ZnO nanorods.
Madavali, B.,Kim, H. S.,Hong, S. J. Springer Science + Business Media 2014 Journal of electronic materials Vol.43 No.6
In this research, n-type (Bi2Te3)(1-x) (Bi2Se3) (x) -based thermoelectric (TE) materials were produced through a gas atomization process, and subsequently hot extruded with an extrusion ratio of 10:1 at 400 A degrees C. The effect of chemical composition on TE properties was investigated. The microstructure of all extruded bars showed a homogeneous and fine distribution of grains due to the dynamic recrystallization during the hot extrusion process. With increasing Bi2Te3 content, from 0.85 to 0.90, both electrical resistivity and Seebeck coefficient values were increased. The maximum figure of merit (ZT) 0.673 was obtained at room temperature for (Bi2Te3)(0).(90)(Bi2Se3)(0.10) alloys due to them exhibiting higher seebeck coefficient and lower thermal conductivity than other compositions.
Madavali, Babu,Han, Seung-Tek,Shin, Dong-Won,Hong, Soon-Jik,Lee, Kap-Ho Materials Research Society of Korea 2017 한국재료학회지 Vol.27 No.8
In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properties of $(GeTe)_{0.85}(AgSbTe_2)_{0.15}$ (TAGS-85) were studied by a combination of gas atomization and spark plasma sintering. The crystal structure and microstructure of TAGS-85 were characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The oxygen content of both powders and bulk samples were found to decrease with increasing reduction temperature. The grain size gradually increased with increasing reduction temperature due to adhesion of fine grains in a temperature range of 350 to $450^{\circ}C$. The electrical resistivity was found to increase with reduction temperature due to a decrease in carrier concentration. The Seebeck coefficient decreased with increasing reduction temperature and was in good agreement with the carrier concentration and carrier mobility. The maximum power factor, $3.3{\times}10^{-3}W/mK^2$, was measured for the non-reduction bulk TAGS-85 at $450^{\circ}C$.
Madavali, Babu,Kim, Hyo-Seob,Lee, Kap-Ho,Hong, Soon-Jik Elsevier 2017 Intermetallics Vol.82 No.-
<P><B>Abstract</B></P> <P>The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new strategies to boost the Seebeck coefficient. In this research, different weight percentages of Y<SUB>2</SUB>O<SUB>3</SUB> (2, 4, and 6) nanoparticles (NPs) were incorporated into the pre-alloyed BiSbTe powder for making nanocomposites (NCs) by mechanical milling. The resultant NCs powders were subsequently consolidation by spark plasma sintering (SPS) at 450 °C. The existence of Y<SUB>2</SUB>O<SUB>3</SUB> nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The hardness of the nanocomposites was significantly improved (>49%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism. The electrical conductivity decreased while the Seebeck coefficient significantly improved (45%) at room temperature for the NCs to which 2 wt% Y<SUB>2</SUB>O<SUB>3</SUB> was added. This was due to the scattering of carriers through the energy filtering effect. The electronic component of the thermal conductivity greatly contributed to the reduction of total thermal conductivity (22%) in BiSbTe NCs to which 6 wt% Y<SUB>2</SUB>O<SUB>3</SUB> was added. A peak ZT of 1.24 was achieved for BiSbTe/(2 wt%) Y<SUB>2</SUB>O<SUB>3</SUB> NCs due to reduction in their thermal conductivity and improved Seebeck coefficient values.</P> <P><B>Highlights</B></P> <P> <UL> <LI> P-type BiSbTe/Y<SUB>2</SUB>O<SUB>3</SUB> nanocomposites have been fabricated by ball milling and Spark plasma sintering. </LI> <LI> Seebeck coefficient is eminently improved (45%) due to scattering of carriers through the energy filtering effect. </LI> <LI> Thermal conductivity is reduced by 22% in nanocomposites due to carrier/phonon scattering. </LI> <LI> Hardness of nanocomposites was significantly improved (>49%) compared to pure BiSbTe. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Significant improvement in Seebeck coefficient via energy filtering effect, and strong reduction in thermal conductivity from carrier/phonon scattering contributed to the enhanced ZT at room temperature.</P> <P>[DISPLAY OMISSION]</P>
Madavali, B.,Hong, S. J. Springer Science + Business Media 2016 Journal of electronic materials Vol.45 No.12
<P>In this research, the microstructure and transport properties of p-type Bi0.5Sb1.5Te3 thermoelectric materials were investigated as a function of milling time. The p-type Bi0.5Sb1.5Te3 alloys were fabricated by mechanical alloying of elemental chunks of bismuth, antimony, and tellurium. This was followed by plasma spark sintering at 673 K. The micro-Vickers hardness (98.7 Hv) was considerably improved in the 90-min sample due to the presence of fine grains in the matrix that prevented crack propagation via grain-boundary hardening. The lowest lattice thermal conductivity (0.63 W/mK) was obtained for the 90-min sample, a value slightly lower than the minimum total thermal conductivity (0.872 +/- 0.5 W/mK at 300 K) due to strong scattering of phonons and carriers owing to the completely randomness of the distribution of the fine-grain structure in the bulk samples. The maximum figure-of-merit (ZT = 0.98 +/- 0.5 at 300 K) was obtained for the 90-min sample due to its superior power factor values.</P>
Madavali, Babu,Kim, Hyo-Seob,Lee, Kap-Ho,Isoda, Yukihiro,Gascoin, Franck,Hong, Soon-Jik Elsevier 2016 Materials & Design Vol.112 No.-
<P><B>Abstract</B></P> <P>Development of large scale high performance thermoelectric materials is one of the challenges in thermoelectric energy conversion. We have successfully fabricated large scale production (3–5kg/min) of bismuth antimony telluride (Bi-Sb-Te) alloys by gas atomization (GA) and achieved a high figure of merit (<I>ZT</I>) value over unity (1) at 350K. To get more performance from thermoelectric materials, we controlled the grain size of GA powders via mechanical milling (GA+MA) and consolidated the resultant nano powder by spark plasma sintering. The <I>ZT</I> values of GA+MA samples were greatly improved (15%) over those of GA bulk due to shifting control of their grain size from micron order to submicron order. The improved <I>ZT</I> values are due to the suppression of lattice thermal conductivity, which was drastically decreased due high scattering of phonons at numerous grain boundaries and twin boundaries. The mechanical properties were greatly improved (GA+MA sample hardness improved >61% over the GA sample), which would provide impressive benefits for device fabrication and practical applications for thermoelectric energy conversion.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mass production (3–5kg/min) of high efficient <I>p</I>-type Bi-Sb-Te alloys were developed. </LI> <LI> Thermoelectric properties were enhanced by controlling microstructure via milling. </LI> <LI> The lattice thermal conductivity decreases by scattering of phonons at twin boundaries. </LI> <LI> Mechanical properties were significantly improved for fine grained compacted bulks. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Madavali, Babu,Shin, Dong-won,Kim, Dong-Soo,Hong, Soon-Jik Elsevier 2019 INTERMETALLICS Vol.105 No.-
<P><B>Abstract</B></P> <P>The decoupling between the electrical conductivity and thermal conductivity is a crucial challenge to elevate the figure of merit of a given thermoelectric material. In this research, thermoelectric properties of the rare-earths sesquioxides added Bi-Sb-Te material has been investigated systematically. The electrical conductivity (<I>σ</I>) is increased by increase in carrier concentration due to progressive movement of reduced Fermi level deeper into the valance band. Meanwhile, an effective mass of the rare earth added BiSbTe composites has been increased and contributed to the resonant distortion in density of states (DOS), that gives rises to the enhancement of Seebeck coefficient (<I>α</I>). Owing to the increment of both factors (<I>σ</I> &<I>α</I>), a considerable enhancement in power factor of 14% was achieved for the 2 wt% added BiSbTe/(<I>x</I>-wt%) Nd<SUB>2</SUB>O<SUB>3</SUB> composites. The low content addition of rare earth oxide particles are potentially acts as scattering centers for mid to long wavelength phonons and reduced the thermal conductivity by 7%. The effective decoupling of the electrical and thermal properties by rare earths sesquioxides (Nd<SUB>2</SUB>O<SUB>3</SUB>) provides a 15% enhancement in thermoelectric figure of merit, ZT for 2 wt% added BiSbTe/(<I>x</I>-wt%)Nd<SUB>2</SUB>O<SUB>3</SUB> composites.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Coupling between electrical& thermal conductivity, Seebeck coefficient are broken. </LI> <LI> Electrical conductivity is raised by alteration of Fermi level by rare-earth doping. </LI> <LI> Rare-earth in BSbTe causes resonant distortion in density of states (DOS). </LI> <LI> Dispersed rare-earth particles acts as scattering centers and reduced <I>κ</I> values. </LI> <LI> Dispersion of rare earths sesquioxides in BSbTe provides a 15% enhancement in ZT. </LI> </UL> </P>