Nano Research 1 DOINano 10.1007/s12274Res -014-0485-0

Characterization of thermal conductivity of

La0.95Sr0.05CoO3 thermoelectric oxide nanofibers

Weihe Xu1, Evgeny Nazaretski1, Ming Lu1, Hamid Hadim2 and Yong. Shi2()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0485-0 http://www.thenanoresearch.com on April 24, 2014

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TABLE OF CONTENTS (TOC)

Characterization of thermal conductivity of

La0.95Sr0.05CoO3 thermoelectric oxide nanofibers

Weihe Xu1, Evgeny Nazaretski1, Ming Lu1, Hamid Hadim2 and Yong Shi2*

1. Brookhaven National Lab, USA; 2. Stevens Institute of Technology, USA

Page Numbers. The font is A novel method that can measure thermal conductivity of individual thermoelectric oxide nanofibers prepared by electrospinning was ArialMT 16 (automatically developed. La0.95Sr0.05CoO3 nanofibers with the diameter of 140 nm and 290 nm were studied using this approach at ambient temperature.

inserted by the publisher)

Provide the authors’ website if possible. Weihe Xu, http://staff.ps.bnl.gov/staff.aspx?id=84135 Yong Shi, http://www.stevens.edu/nanodevices/people/faculty_profile.php?faculty_id=73

1 Nano Res DOI (automatically inserted by the publisher) Research Article Please choose one

Characterization of thermal conductivity of La0.95Sr0.05CoO3 thermoelectric oxide nanofibers

Weihe Xu1, Evgeny Nazaretski1, Ming Lu1, Hamid Hadim2 and Yong. Shi2()

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-1K-1, and 2.1 Wm-1K-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

thermal energies. With an increasing demand for 1. Introduction sustainable energy sources and requirements to Thermoelectric effects, including Seebeck and provide inexpensive integrated circuit Peltier effects, are the most straight forward (IC)-compatible micro-sensors and micro-actuators, methods to make conversion between electrical and there has been considerable amount of research

———————————— Address correspondence to Yong Shi, [email protected];

2 carried out in the field of thermoelectric devices ambient conditions. When reduced to nanoscale, [1-7]. However, when compared to conventional surface area of materials increases drastically and refrigeration and power systems, the major makes the oxidization problem of most disadvantage of thermoelectric materials is their thermoelectric materials extremely important [13]. low efficiency. The effectiveness of thermoelectric Thermoelectric oxides, such as NaCoO3, materials can be defined as a dimensionless La0.95Sr0.05CoO3 and other do not experience surface thermoelectric figure of merit, ZT, as shown below: oxidization problems, therefore they are potentially S2  good candidates for nanoscale thermoelectric ZT = T modules. Until recently these materials were not  (1) extensively studied since measured values of ZT where T is temperature, S is the Seebeck coefficient, were low and no ZT enhancement was expected σ is the electrical conductivity and к is thermal due to short phonon mean free path. In recent years, conductivity respectively [8]. For traditional bulk however, more oxide thermoelectric materials with thermoelectric materials, the maximum ZT is 1. high ZTs have been discovered [22-25]. Moreover, However, in order to compete with conventional experimental studies indicated that decrease of mechanical refrigeration systems in terms of thermal conductivity in nanoscale materials is not efficiency, an increase of thermoelectric efficiency only determined by the phonon mean free path [18, ZT is required. It is proposed that by reducing 26]. The internal transport spectrum of phonons linear size of thermoelectric materials down to plays also an import role [18, 27]. Therefore, studies nanometer scale ZT can be further increased [9, 10]. of nanoscale thermoelectric oxides are becoming The ZT enhancement occurs due to two factors. attractive [28-34]. For example, Y. Wang et al., First, increase of the Seebeck coefficient S was reported the decrease of thermal conductivity in predicted by Dresselhaus et al in early 1990s [11] SrTiO3 nanocomposite when decreasing grain size and confirmed experimentally by Yuan et al. [12]. [28]. H. Ohta et al., observed the increase of Seebeck

The authors reported 1D electron transport induced coefficient of SrTiO3 nanofilms when thickness was an increase of the Seebeck coefficient. Second, ZT reduced [29]. L. Shi et al. measured thermal can be further increased by reducing thermal conductivities of individual tin dioxide nanobelts conductivity coefficient κ when characteristic length [30]. Y. Wang et al studied thermal conductivity of scale of thermoelectric material becomes La1-xSrxCoO3 nanofibers by measuring pellet which comparable (or smaller) than phonon mean free was composed by nanofibers [31]. path. Reduction of linear dimensions gives rise to 1D nanostructures (nanowires and nanofibers) phonons’ boundary scattering and consequently are ideal samples for evaluating thermoelectric causes decrease of thermal conductivity. This performance of materials. When compared with 2D prediction has been confirmed through nanostructures (nanofilm), it is easier to interpret experimental results reported by different groups their properties by reducing their size [35, 11, 36]. [13-19]. For example, thermal conductivity of a When compared with 0D samples, such as single Bi2Te3 nanowire (1D) was reduced by an assemblies of nanoparticles, they are easier to order of magnitude when compared to Bi2Te3 bulk characterize. While having obvious advantages, (3D) samples [20]. In spite of promising initial direct measurements of thermal conductivity in a results, performance of nanocomposites like Bi2Te3 single thermoelectric oxide nanowire are have certain limitations [21]. One of the major challenging due to difficulties associated with problems pertains to oxidization of materials at fabrication and transfer of individual nanofibers

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

4 together with the silicon template. Details of the the corresponding diameters of the La0.95Sr0.05CoO3 annealing process are described in [41]. The nanofibers after annealing are listed in Table 1. relationship between the PVP concentrations and

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 substrate, MEMS devices in our particular case. PVP ratio Figure 1(b) is a micrograph of a single nanofiber on top of a Si substrate after annealing process. PVP ratio Diameter of fiber after annealing (% wt/vol) (nm) 2.2. Loading Nanofiber onto the MEMS Tester In previous work, we reported on a MEMS 7 30~40 device that can be used to measure thermal conductivity of a single nanostructure e.g. 10 100~150 nanowire, nanofiber or nanofilm. This device is

composed of suspended SiO2 beams. The Al lead 14 250~350 wires, which also worked as thermometers, and the 21 1000~2000 Cr heater were integrated on each beam. Every four beams were arranged as a group. During thermal Figure 1(a) is the schematic view of a fin-shape conductivity measurements, the nanofiber was silicon template with a nanofiber attached. The bridged between any two of the four beams. By silicon template was prepared using two processes heating the Cr heater on one beam and measuring e.g. lithography first followed by DRIE etching. The the temperature change on both beams, through gap between two neighboring fins was 50 µm, the this procedure thermal conductivity of the test width of each fin was 5 µm with corresponding sample can be determined. Details of the MEMS height of approximately 10 µm. The template was structures used during these measurements can be used to collect and provide individual suspended found in [42]. nanofibers for thermal conductivity measurements. The challenging step pertains to loading of a Since the top of each fin was just 5 µm wide, nanofiber on top of MEMS devices without breaking both. Up-to-date few transfer methods adhesion force between La0.95Sr0.05CoO3 nanofibers and the template itself was insignificant and have been reported. Two most commonly used allowed easy transfer of nanofibers to another methods are utilizing a nanoprobe or a liquid

5 droplet with nanofibers [44]. Unfortunately, these lithography process [45]. This method is also not methods are not suitable for thermoelectric oxide suitable for our applications since it may introduce nanofibers prepared by electrospinning since these polymer contamination of nanofibers and nanofibers are too fragile to survive such ultimately will affect results of thermal conductivity manipulation techniques. Alternative method measurements. In addition, this method involves reported by M. Pettes et al., involves samples being expensive e-beam lithography process [45]. put on top of PMMA layer with subsequent e-beam

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 and thickness of the SiO2 bottom and Al layers. A procedure and further improved MEMS device. The thin SiO2 top layer can be also deposited on the Al new MEMS structure incorporates bended beams layer by PECVD for insulation (not shown in the and bump structures, which in combination with a schematic view). Figure 2(b) is an SEM image of a Si fin-shaped template described earlier enable safe MEMS tester with curved beams. The thicknesses of transfer of La0.95Sr0.05CoO3 nanofibers on top of a the SiO2 bottom layer, the Al lead wires, and the

MEMS device. Figure 2(a) is the schematic SiO2 top layer were 200 nm, 100 nm, and 100 nm representation of a MEMS device with bended respectively. The deposition temperatures of the beams. The fabrication process of it is similar to that SiO2 top layer and bottom layer were 165 ℃ and reported earlier [41]. The curvature of the beams 400 ℃ respectively. The elevation from the center of can be adjusted by controlling the residue stress the beams to the chip surface was ~13 μm. In the

6 present work, MEMS devices were cut into 6.5 mm bump structure was lower than the peak of the x 6.5 mm chips (4 x 4 cells). Polymer films with the beams, as shown in Figure 2(c). In addition to using thickness of ~10 um were attached to the chips a separate polymer film, the bump structure can manually to work as a bump structure prior to also be integrated into a tester during tester nanofiber loading step, as shown in Figure 2(a). The fabrication process.

Figure 3. The process of loading of La0.95Sr0.05CoO3 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 La0.95Sr0.05CoO3 nanofiber.

The process of transferring La0.95Sr0.05CoO3 testers with nanofibers at the desired location, as nanofiber onto a tester is shown in Figure 3. During shown in Figure 3(c). Subsequently, SEM imaging the process, the tester chip was stamped on a silicon was performed to ensure that nanofibers on testers template upside down, as shown in Figure 3(a). had good contact with the beams. Finally, Pt was Since nanofibers were in contact with silicon deposited at the contact area between nanofibers template only (at the top of the fins), some of the and SiO2 beams by SEM to ensure a reliable nanofibers adhered to the tester after stamping. In mechanical and thermal contact [42, 44]. SEM this process only the center of the beams, which was instead of Focused Ion Beam (FIB) was employed higher than the bump structure, touched the for the Pt deposition to minimize damage of nanofibers on the silicon template, as shown in nanofibers. Figure 3(d) shows an SEM image of a

Figure 3(b). After stamping procedure, a 140 nm diameter La0.95Sr0.05CoO3 nanofiber on the microscope surveying was performed to locate MEMS tester after Pt deposition.

7 information. The length of Pt-free area is defined as 3. Measurement and discussion ‘effective length’ and is shown in Figure 4. The

In this work, two La0.95Sr0.05CoO3 nanofibers effective lengths of La0.95Sr0.05CoO3 nanofibers with with the diameter of 140 nm and 290 nm were the diameter of 140 nm and 290 nm were 1.85 μm prepared and their thermal conductivities were and 5.50 μm respectively. 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 Figure 4. SEM image of the Φ140 nm La0.95Sr0.05CoO3 with exactly the same nitride component nanofiber used during thermal conductivity measurements. concentration but different viscosity has been obtained. After that, obtained sol-gel precursors After loading La0.95Sr0.05CoO3 nanofibers, their were electrospun and annealed using the same thermal conductivities were measured at room process to ensure identical structure of nanofibers temperature. The detailed measurement procedure with different diameters. is described elsewhere [41]. Measured thermal

Finally, the effective length of La0.95Sr0.05CoO3 conductivities of La0.95Sr0.05CoO3 yielded values of nanofibers with different diameters was adjusted to 0.7 Wm-1K-1, and 2.1 Wm-1K-1 for 140 nm and 290 nm ensure that nanofibers to be measured have similar diameter nanofibers respectively. This result thermal conductance. Therefore, the measurement matched the trend of the data measured on error can be reduced. This was achieved by La0.9Sr0.1CoO3 nanofibers by Y. Wang et al [31], and depositing Pt on certain parts of the nanofibers. is shown in Figure 5. Thermal conductivity of

Figure 4 is an SEM image of a 140 nm diameter La0.95Sr0.05CoO3 bulk samples measured by J.

La0.95Sr0.05CoO3 nanofiber ready for measurement. A Androulakis at room temperature was 3 Wm-1K-1 ~300 nm-wide and ~500 nm-thick Pt layer has been [43], in accord with the assumption that by deposited on the nanofiber from both sides. reducing characteristic length scale down to nm one Since thermal resistance of the area with Pt can vary thermal conductivity significantly. 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

8 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

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. Figure 5. Measurements of thermal conductivity of Individual nanofibers were collected from a

La0.95Sr0.05CoO3 nanofibers (dots are the data of La0.9Sr0.1CoO3 template by a specifically designed MEMS tester reported by Y. Wang et al., triangle – present work) capable of manipulation and characterization of individual nanostructures. By this method, thermal

Phonon mean free path lph in La0.95Sr0.05CoO3 can conductivities of La0.95Sr0.05CoO3 nanofibers with the be estimated using following equation: diameters of 140 nm and 290 nm were measured at room temperature. Measured thermal conductivity lph  3/ Cv (2) values were 0.7 Wm-1K-1, and 2.1 Wm-1K-1, respectively. Both thermal conductivity values are kBD 1 1/3 v  ()2 lower than that reported for bulk samples. 6 N (3) Obtained results further support the argument that where к is measured thermal conductivity, C is the thermal conductivity is not only related to phonon specific heat (we used the value of 2.7×106 J/Km3 for mean free path [18, 26, 27] but is also affected by

LaCoO3 [46]), v is the sound velocity ( ~4500 m/s linear dimensions of the sample. Further reduction when θD=380 K [31]). Based on these assumptions of La0.95Sr0.05CoO3 nanofiber diameter may yield we estimate the mean free path in La0.95Sr0.05CoO3 to further increase of ZT values. In our future work, be approximately 2 Å . precision of the MEMS tester will be increased to

Thermal conductivity of La0.95Sr0.05CoO3 showed provide reliable measurements of thermal an obvious reduction when the diameter of conductivities in even thinner La0.95Sr0.05CoO3 nanofiber was much larger than the estimated nanofibers. The temperature-thermal conductivity phonon mean free path. Similar findings were relationship of the La0.95Sr0.05CoO3 nanofibers will reported in silicon samples [18, 27, 47]. Two also be studied. Thermal conductivities of other reasons may lead to this phenomenon. First, it may thermoelectric oxide nanofibers, like the single be due to the material’s internal transport spectrum crystal LaCoO3, will also be studied. of phonon. The long wavelength phonons have been reported to have a more than expected Acknowledgements contribution during heat transportation, which in turn makes material’s thermal conductivity more This work was carried out in part at the Center for sensitive to size reduction [48]. Second, since Functional Nanomaterials, Brookhaven National La0.95Sr0.05CoO3 nanofibers we tested were prepared

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