Characterization of thermal conductivity of La 0.95 Sr 0.05 CoO 3 thermoelectric oxide nanofibers

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1 Nano Research Nano Res 1 DOI /s Characterization of thermal conductivity of La 0.95 Sr 0.05 CoO 3 thermoelectric oxide nanofibers Weihe Xu 1, Evgeny Nazaretski 1, Ming Lu 1, Hamid Hadim 2 and Yong. Shi 2 ( ) Nano Res., Just Accepted Manuscript DOI: /s on April 24, 2014 Tsinghua University Press 2014 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 TABLE OF CONTENTS (TOC) Characterization of thermal conductivity of La 0.95 Sr 0.05 CoO 3 thermoelectric oxide nanofibers Weihe Xu 1, Evgeny Nazaretski 1, Ming Lu 1, Hamid Hadim 2 and Yong Shi 2 * 1. Brookhaven National Lab, USA; 2. Stevens Institute of Technology, USA Page Numbers. The font is ArialMT 16 (automatically inserted by the publisher) A novel method that can measure thermal conductivity of individual thermoelectric oxide nanofibers prepared by electrospinning was developed. La 0.95 Sr 0.05 CoO 3 nanofibers with the diameter of 140 nm and 290 nm were studied using this approach at ambient temperature. Provide the authors website if possible. Weihe Xu, Yong Shi, 1

3 Nano Res DOI (automatically inserted by the publisher) Research Article Please choose one Characterization of thermal conductivity of La 0.95 Sr 0.05 CoO 3 thermoelectric oxide nanofibers Weihe Xu 1, Evgeny Nazaretski 1, Ming Lu 1, Hamid Hadim 2 and Yong. Shi 2 ( ) 1 Brookhaven National Lab, Upton, NY, 11973, USA 2 Department of Mechanical Engineering, Hoboken NJ, 07030, USA Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT Thermoelectric oxide nanofibers prepared by electrospinning are expected to have reduced thermal conductivity when compared to bulk samples. Measurements of nanofibers thermal conductivity is challenging since it involves sophisticated sample preparation methods. In this work, we present a novel method suitable for measurements of thermal conductivity of a single nanofiber. A microelectro-mechanical (MEMS) device has been designed and fabricated to perform thermal conductivity measurements on a single nanofiber. A special Si template was designed to collect and transfer individual nanofibers onto a MEMS device. Pt was deposited by Focused Ion Beam to reduce the effective length of a prepared nanofiber. La0.95Sr0.05CoO3 nanofibers with the diameter of 140 nm and 290 nm were studied and characterized using this approach at room temperature. Measured thermal conductivities yielded values of 0.7 Wm -1 K -1, and 2.1 Wm -1 K -1, respectively. Our measurements in La0.95Sr0.05CoO3 nanofibers confirmed that decrease of liner dimensions has a profound effect on its thermal conductivity. KEYWORDS Thermoelectric, Heat Transfer, Thermal Conductivity, Nanoscale, MEMS 1. Introduction Thermoelectric effects, including Seebeck and Peltier effects, are the most straight forward methods to make conversion between electrical and thermal energies. With an increasing demand for sustainable energy sources and requirements to provide inexpensive integrated circuit (IC)-compatible micro-sensors and micro-actuators, there has been considerable amount of research Address correspondence to Yong Shi, yong.shi@stevens.edu; 2

4 carried out in the field of thermoelectric devices [1-7]. However, when compared to conventional refrigeration and power systems, the major disadvantage of thermoelectric materials is their low efficiency. The effectiveness of thermoelectric materials can be defined as a dimensionless thermoelectric figure of merit, ZT, as shown below: ZT = 2 S T (1) where T is temperature, S is the Seebeck coefficient, σ is the electrical conductivity and к is thermal conductivity respectively [8]. For traditional bulk thermoelectric materials, the maximum ZT is 1. However, in order to compete with conventional mechanical refrigeration systems in terms of efficiency, an increase of thermoelectric efficiency ZT is required. It is proposed that by reducing linear size of thermoelectric materials down to nanometer scale ZT can be further increased [9, 10]. The ZT enhancement occurs due to two factors. First, increase of the Seebeck coefficient S was predicted by Dresselhaus et al in early 1990s [11] and confirmed experimentally by Yuan et al. [12]. The authors reported 1D electron transport induced an increase of the Seebeck coefficient. Second, ZT can be further increased by reducing thermal conductivity coefficient κ when characteristic length scale of thermoelectric material becomes comparable (or smaller) than phonon mean free path. Reduction of linear dimensions gives rise to phonons boundary scattering and consequently causes decrease of thermal conductivity. This prediction has been confirmed through experimental results reported by different groups [13-19]. For example, thermal conductivity of a single Bi2Te3 nanowire (1D) was reduced by an order of magnitude when compared to Bi2Te3 bulk (3D) samples [20]. In spite of promising initial results, performance of nanocomposites like Bi2Te3 have certain limitations [21]. One of the major problems pertains to oxidization of materials at ambient conditions. When reduced to nanoscale, surface area of materials increases drastically and makes the oxidization problem of most thermoelectric materials extremely important [13]. Thermoelectric oxides, such as NaCoO3, La0.95Sr0.05CoO3 and other do not experience surface oxidization problems, therefore they are potentially good candidates for nanoscale thermoelectric modules. Until recently these materials were not extensively studied since measured values of ZT were low and no ZT enhancement was expected due to short phonon mean free path. In recent years, however, more oxide thermoelectric materials with high ZTs have been discovered [22-25]. Moreover, experimental studies indicated that decrease of thermal conductivity in nanoscale materials is not only determined by the phonon mean free path [18, 26]. The internal transport spectrum of phonons plays also an import role [18, 27]. Therefore, studies of nanoscale thermoelectric oxides are becoming attractive [28-34]. For example, Y. Wang et al., reported the decrease of thermal conductivity in SrTiO3 nanocomposite when decreasing grain size [28]. H. Ohta et al., observed the increase of Seebeck coefficient of SrTiO3 nanofilms when thickness was reduced [29]. L. Shi et al. measured thermal conductivities of individual tin dioxide nanobelts [30]. Y. Wang et al studied thermal conductivity of La1-xSrxCoO3 nanofibers by measuring pellet which was composed by nanofibers [31]. 1D nanostructures (nanowires and nanofibers) are ideal samples for evaluating thermoelectric performance of materials. When compared with 2D nanostructures (nanofilm), it is easier to interpret their properties by reducing their size [35, 11, 36]. When compared with 0D samples, such as assemblies of nanoparticles, they are easier to characterize. While having obvious advantages, direct measurements of thermal conductivity in a single thermoelectric oxide nanowire are challenging due to difficulties associated with fabrication and transfer of individual nanofibers 3

5 into a characterization system. In spite of experimental difficulties certain progress has been recently reported. Thermal conductivity of a single Si nanofiber has been measured utilizing laser heating and Raman thermography [37, 38]. Since this is a non-contact method, the process can be rapid and is free of contact problems. However, it is difficult to have accurate estimates of laser power absorption thus making results susceptible to systematic errors. Another technique involves 3-ω method, it is widely used to measure thermal conductivity of thin films. This method has been further developed and applied to measurements of thermal conductivity of individual nanofibers and nano-ribbons [36, 39, 40]. In our previous works, we reported on a method for fabrication and Seebeck coefficient measurements of thermoelectric oxide nanofibers. La0.95Sr0.05CrO3 nanofibers have been prepared and characterized by this method [41]. We also reported on MEMS devices that can be used to measure thermal conductivity of a single carbon nanofiber [42]. In this work, we focused on studies of thermal conductivity of various La0.95Sr0.05CoO3 nanofibers with different diameters. At the beginning of this work, the fabrication process of La0.95Sr0.05CoO3 nanofibers was improved and single La0.95Sr0.05CoO3 nanofibers with different diameters were prepared. Then a MEMS device used in our previous work has been further optimized and a new sampler preparation method has been developed. Finally, thermal conductivity of La0.95Sr0.05CrO3 nanofibers with different diameters has been measured utilizing this MEMS device. The measurement showed that thermal conductivity of La0.95Sr0.05CoO3 nanofibers decreases with the reduction of their diameter. Thermal conductivity of La0.95Sr0.05CoO3 nanofiber with a diameter of 140 nm was ~33% of the thermal conductivity of a La0.95Sr0.05CoO3 nanofiber with the diameter of 290 nm and ~23% of the value reported for bulk La0.95Sr0.05CoO3 samples [43]. This result demonstrated that thermal conductivity of oxide thermoelectric materials, especially La0.95Sr0.05CoO3, can also be decreased by reducing the size down to nanoscale. Our experimental approach can be further applied to studies of thermal conductivity of most thermoelectric oxide nanofibers as well as other fragile nanostructures. 2. Experimental Details 2.1. Nanofiber preparation In our previous work, we reported the fabrication of La0.95Sr0.05CoO3 nanofibers with the diameter of 35 nm by electrospinning technique [41]. In this work, we prepared nanofibers with different diameters and collected them with a special template. The diameter of prepared La0.95Sr0.05CoO3 nanofibers was controlled by changing the viscosity of the sol-gel precursor prepared for electrospinning. When compared to optimizing other fabrication parameters (for example distance between the nozzle and collection substrate), changing the viscosity of the sol-gel precursor provides better control of nanofibers diameter. The sol gel precursor for electrospinning was prepared by mixing nitride and polyvinylpyrrolidone (PVP) solutions. The nitride solution was prepared by dissolving 50.4% wt/vol La(NO3)3 6H2O, 33.2% wt/vol Co(NO3)3 6H2O and 1.2% wt/vol Sr(NO3) in distilled water. The PVP solution consisted of 7~21% wt/vol PVP in ethanol. The two solutions were mixed with 10:7 volume ratios and stirred for 24 hours at room temperature. The PVP solutions with different PVP concentrations helped to modify the viscosity of the sol gel precursor. Those as-prepared sol-gel precursors were electrospun by the process reported elsewhere [41]. A fin-shape silicon template was used as a substrate to collect and hold fabricated nanofibers. After electrospinning process, nanofibers were annealed 4

6 together with the silicon template. Details of the annealing process are described in [41]. The relationship between the PVP concentrations and the corresponding diameters of the La0.95Sr0.05CoO3 nanofibers after annealing are listed in Table 1. Figure 1. (a) Schematic view of a fin-shaped silicon template with a nanofiber; b) SEM image of a nanofiber on top of a Si template after annealing. Table 1. The relationship between nanofiber s diameter and PVP ratio PVP ratio Diameter of fiber after annealing (% wt/vol) (nm) 7 30~ ~ ~ ~2000 Figure 1(a) is the schematic view of a fin-shape silicon template with a nanofiber attached. The silicon template was prepared using two processes e.g. lithography first followed by DRIE etching. The gap between two neighboring fins was 50 µm, the width of each fin was 5 µm with corresponding height of approximately 10 µm. The template was used to collect and provide individual suspended nanofibers for thermal conductivity measurements. Since the top of each fin was just 5 µm wide, adhesion force between La0.95Sr0.05CoO3 nanofibers and the template itself was insignificant and allowed easy transfer of nanofibers to another substrate, MEMS devices in our particular case. Figure 1(b) is a micrograph of a single nanofiber on top of a Si substrate after annealing process Loading Nanofiber onto the MEMS Tester In previous work, we reported on a MEMS device that can be used to measure thermal conductivity of a single nanostructure e.g. nanowire, nanofiber or nanofilm. This device is composed of suspended SiO2 beams. The Al lead wires, which also worked as thermometers, and the Cr heater were integrated on each beam. Every four beams were arranged as a group. During thermal conductivity measurements, the nanofiber was bridged between any two of the four beams. By heating the Cr heater on one beam and measuring the temperature change on both beams, through this procedure thermal conductivity of the test sample can be determined. Details of the MEMS structures used during these measurements can be found in [42]. The challenging step pertains to loading of a nanofiber on top of MEMS devices without breaking both. Up-to-date few transfer methods have been reported. Two most commonly used methods are utilizing a nanoprobe or a liquid 5

7 droplet with nanofibers [44]. Unfortunately, these methods are not suitable for thermoelectric oxide nanofibers prepared by electrospinning since these nanofibers are too fragile to survive such manipulation techniques. Alternative method reported by M. Pettes et al., involves samples being put on top of PMMA layer with subsequent e-beam lithography process [45]. This method is also not suitable for our applications since it may introduce polymer contamination of nanofibers and ultimately will affect results of thermal conductivity measurements. In addition, this method involves expensive e-beam lithography process [45]. Figure 2. MEMS tester with the bended beam and bump structure: (a) Schematic view of a tester with the bended beam and bump structure; (b) SEM image of a tester with curved beam; (c) Side view of the MEMS tester (figure not drawn to scale); In this work we developed new loading procedure and further improved MEMS device. The new MEMS structure incorporates bended beams and bump structures, which in combination with a Si fin-shaped template described earlier enable safe transfer of La0.95Sr0.05CoO3 nanofibers on top of a MEMS device. Figure 2(a) is the schematic representation of a MEMS device with bended beams. The fabrication process of it is similar to that reported earlier [41]. The curvature of the beams can be adjusted by controlling the residue stress and thickness of the SiO2 bottom and Al layers. A thin SiO2 top layer can be also deposited on the Al layer by PECVD for insulation (not shown in the schematic view). Figure 2(b) is an SEM image of a MEMS tester with curved beams. The thicknesses of the SiO2 bottom layer, the Al lead wires, and the SiO2 top layer were 200 nm, 100 nm, and 100 nm respectively. The deposition temperatures of the SiO2 top layer and bottom layer were 165 and 400 respectively. The elevation from the center of the beams to the chip surface was ~13 μm. In the 6

8 present work, MEMS devices were cut into 6.5 mm x 6.5 mm chips (4 x 4 cells). Polymer films with the thickness of ~10 um were attached to the chips manually to work as a bump structure prior to nanofiber loading step, as shown in Figure 2(a). The bump structure was lower than the peak of the beams, as shown in Figure 2(c). In addition to using a separate polymer film, the bump structure can also be integrated into a tester during tester fabrication process. Figure 3. The process of loading of La 0.95 Sr 0.05 CoO 3 nanofiber: (a) Stamp the MEMS tester chip on a silicon template; (b) schematic of a nanofiber being in contact with the MEMS tester; (c) schematic of a nanofiber at the desired location; (d) SEM image of a loaded 140 nm diameter La 0.95 Sr 0.05 CoO 3 nanofiber. The process of transferring La0.95Sr0.05CoO3 nanofiber onto a tester is shown in Figure 3. During the process, the tester chip was stamped on a silicon template upside down, as shown in Figure 3(a). Since nanofibers were in contact with silicon template only (at the top of the fins), some of the nanofibers adhered to the tester after stamping. In this process only the center of the beams, which was higher than the bump structure, touched the nanofibers on the silicon template, as shown in Figure 3(b). After stamping procedure, a microscope surveying was performed to locate testers with nanofibers at the desired location, as shown in Figure 3(c). Subsequently, SEM imaging was performed to ensure that nanofibers on testers had good contact with the beams. Finally, Pt was deposited at the contact area between nanofibers and SiO2 beams by SEM to ensure a reliable mechanical and thermal contact [42, 44]. SEM instead of Focused Ion Beam (FIB) was employed for the Pt deposition to minimize damage of nanofibers. Figure 3(d) shows an SEM image of a 140 nm diameter La0.95Sr0.05CoO3 nanofiber on the MEMS tester after Pt deposition. 7

9 3. Measurement and discussion In this work, two La0.95Sr0.05CoO3 nanofibers with the diameter of 140 nm and 290 nm were prepared and their thermal conductivities were measured. In order to minimize the uncertainty of results caused by the sample preparation and measurement processes, these La0.95Sr0.05CoO3 nanofibers were prepared and measured under similar conditions. To eliminate influence of possible nitride variation in different batches of sol-gel solution used for fabrication of nanofibers with different diameters, the following procedure has been followed. A large volume of nitride solution was prepared, next, it was separated into several equal parts. Each part of nitride solution was mixed with PVP/ethanol solution with different volume ratios. Therefore, the sol-gel precursors with exactly the same nitride component concentration but different viscosity has been obtained. After that, obtained sol-gel precursors were electrospun and annealed using the same process to ensure identical structure of nanofibers with different diameters. Finally, the effective length of La0.95Sr0.05CoO3 nanofibers with different diameters was adjusted to ensure that nanofibers to be measured have similar thermal conductance. Therefore, the measurement error can be reduced. This was achieved by depositing Pt on certain parts of the nanofibers. Figure 4 is an SEM image of a 140 nm diameter La0.95Sr0.05CoO3 nanofiber ready for measurement. A ~300 nm-wide and ~500 nm-thick Pt layer has been deposited on the nanofiber from both sides. Since thermal resistance of the area with Pt deposited is much smaller than that of the area without Pt, it is legitimate to assume that measured thermal resistance is dominated by bare nanofiber. Thermal resistance of the area with Pt deposited can be estimated using Wiedemann Franz law, details of calculations can be found in the supporting information. The length of Pt-free area is defined as effective length and is shown in Figure 4. The effective lengths of La0.95Sr0.05CoO3 nanofibers with the diameter of 140 nm and 290 nm were 1.85 μm and 5.50 μm respectively. Figure 4. SEM image of the Φ140 nm La 0.95 Sr 0.05 CoO 3 nanofiber used during thermal conductivity measurements. After loading La0.95Sr0.05CoO3 nanofibers, their thermal conductivities were measured at room temperature. The detailed measurement procedure is described elsewhere [41]. Measured thermal conductivities of La0.95Sr0.05CoO3 yielded values of 0.7 Wm -1 K -1, and 2.1 Wm -1 K -1 for 140 nm and 290 nm diameter nanofibers respectively. This result matched the trend of the data measured on La0.9Sr0.1CoO3 nanofibers by Y. Wang et al [31], and is shown in Figure 5. Thermal conductivity of La0.95Sr0.05CoO3 bulk samples measured by J. Androulakis at room temperature was 3 Wm -1 K -1 [43], in accord with the assumption that by reducing characteristic length scale down to nm one can vary thermal conductivity significantly. 8

10 by electrospinning method, their rough surfaces may lead to the enhanced phonon scattering on the boundaries thus causing a reduction of thermal conductivity coefficient [17]. 4. Conclusion Figure 5. Measurements of thermal conductivity of La 0.95 Sr 0.05 CoO 3 nanofibers (dots are the data of La 0.9 Sr 0.1 CoO 3 reported by Y. Wang et al., triangle present work) Phonon mean free path lph in La0.95Sr0.05CoO3 can be estimated using following equation: v lph k B D 3 / Cv (2) 1 ( ) 2 6 N 1/3 (3) where к is measured thermal conductivity, C is the specific heat (we used the value of J/Km 3 for LaCoO3 [46]), v is the sound velocity ( ~4500 m/s when θd=380 K [31]). Based on these assumptions we estimate the mean free path in La0.95Sr0.05CoO3 to be approximately 2 Å. Thermal conductivity of La0.95Sr0.05CoO3 showed an obvious reduction when the diameter of nanofiber was much larger than the estimated phonon mean free path. Similar findings were reported in silicon samples [18, 27, 47]. Two reasons may lead to this phenomenon. First, it may be due to the material s internal transport spectrum of phonon. The long wavelength phonons have been reported to have a more than expected contribution during heat transportation, which in turn makes material s thermal conductivity more sensitive to size reduction [48]. Second, since La0.95Sr0.05CoO3 nanofibers we tested were prepared In conclusion, we have developed technique to measure thermal conductivity of individual thermoelectric oxide nanofibers using MEMS devices. Nanofibers were prepared by electrospinning and deposited onto a Si template. Individual nanofibers were collected from a template by a specifically designed MEMS tester capable of manipulation and characterization of individual nanostructures. By this method, thermal conductivities of La0.95Sr0.05CoO3 nanofibers with the diameters of 140 nm and 290 nm were measured at room temperature. Measured thermal conductivity values were 0.7 Wm -1 K -1, and 2.1 Wm -1 K -1, respectively. Both thermal conductivity values are lower than that reported for bulk samples. Obtained results further support the argument that thermal conductivity is not only related to phonon mean free path [18, 26, 27] but is also affected by linear dimensions of the sample. Further reduction of La0.95Sr0.05CoO3 nanofiber diameter may yield further increase of ZT values. In our future work, precision of the MEMS tester will be increased to provide reliable measurements of thermal conductivities in even thinner La0.95Sr0.05CoO3 nanofibers. The temperature-thermal conductivity relationship of the La0.95Sr0.05CoO3 nanofibers will also be studied. Thermal conductivities of other thermoelectric oxide nanofibers, like the single crystal LaCoO3, will also be studied. Acknowledgements This work was carried out in part at the Center for Functional Nanomaterials, Brookhaven National 9

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