The authors wish to acknowledge the competitive faculty-initiated collaboration (FIC) grant from KAUST.

1. Preparation of Pr-doped SrTiO3 Powder
<ol>
	<li>In order to prepare 10 g of Sr0.95Pr0.05TiO3 powder, weigh the stoichiometric amounts of SrCO3 powder (7.53407 g), TiO2 nanopowder (4.28983 g), and Pr2O3 sintered lump (0.44299 g) following the reaction for x = 0.05:</li>
</ol>
<img alt="Equation 1" fo:content-width="5in" src="/files/ftp_upload/52869/52869eq1.JPG" width="550" />
<ol start="2">
	<li>Grind the weighed Pr2O3 sintered lumps to fine particles using an agate mortar and pestle.</li>
	<li>Add the weighed SrCO3 powder and TiO2 nanopowder to the Pr2O3 and continue grinding and mixing using an agate mortar and pestle until a visually homogenous powder is achieved.</li>
	<li>Load the ground powder into a glass jar and mix using a turbulator for 30 min to homogenize the mixture.</li>
	<li>Load the resulting mixed powder into a meticulously cleaned and polished stainless steel die (1 inch in diameter) and sandwich it between two stainless steel plungers.</li>
	<li>Cold press the powder using a press under an approximately 1 metric ton load.</li>
	<li>Eject the cold-pressed pellet by placing the die on a hollowed stainless steel cylinder and pushing the plungers and the pellet out from the top using a pushing rod. Upon ejection of the cold pressed pellet (green body) from the die, clean any contamination on the circumferential surface of the pellet by gently covering the pellet with a small piece of scotch-tape and removing a thin layer by ripping the tape.</li>
	<li>Place the pellet vertically in an alumina boat filled with commercially purchased SrTiO3 powder as the barrier between the alumina boat and the cold press pellet.</li>
	<li>Place the boat in a tube furnace, heat up to 1,300 &#176;C in 3 hr and keep it at this temperature for 15 hr. Allow the pellet to cool down to RT inside the furnace when the calcination is over. This step is referred to as the &#8220;calcination process&#8221; thereafter.</li>
	<li>Grind the pellet using the agate mortar and pestle and load the resulting powder into a glass jar for further mixing using the turbulator.</li>
	<li>Load the powder into the stainless steel die and cold pressed under an approximately 3 metric tons of load.</li>
	<li>Repeat the step 1.9 one more time at 1,400 &#176;C in 3 hr and keep it at this temperature for 20 hr.</li>
	<li>Grind the pellet using the agate mortar and pestle.</li>
	<li>Repeat steps 1.11, 1.12, and 1.13 one more time for the solid state reaction to reach completion.</li>
</ol>
2. Preparation of Bulk Polycrystalline Pr-doped SrTiO3 Ceramic
<ol>
	<li>Weigh 1.6 g of the as-prepared powder (for a disk 2 mm thick and 12.7 mm in diameter).</li>
	<li>Prepare circular graphoil pieces to cover the top and bottom interface of the sandwiched powder and graphite plungers in the graphite die. Also, prepare another rectangular graphoil piece to cover the inner wall of the graphite die.</li>
	<li>Load the as-prepared powder into a graphite die (12.7 mm in inner diameter) and sandwich the powder between two graphite plungers of the same size. Drill a 2-mm hole in the middle of the length of the graphite die and from the outer surface of the die to about 2 mm of the inner surface for temperature reading. 
	Note: Adjust the length of the graphite plungers remaining outside the die and position the center of the sandwiched powder cylinder where the hole is placed to get an accurate temperature reading. The faces of the graphite plungers need to be meticulously leveled during the machining of the pieces. Any misalignment can result in the cracking of the sintered pellet during spark plasma sintering.</li>
	<li>Cold-press the powder gently (load &#60; 200 kg) using a press prior to mounting on the spark plasma sintering plate inside the chamber. Use flat polished stainless steel support plates between the top and bottom plungers and the press stage to avoid damaging the graphite plungers.</li>
	<li>Wrap a piece of graphite felt around the die for insulation and secure it with graphite yarn. Devise a window on the graphite felt by cutting a rectangular piece of the felt where the temperature reading hole is placed on the die.</li>
	<li>Place the loaded graphite die and plungers in the spark plasma sintering chamber. Move the stage to the final position.</li>
	<li>Focus and align the Pyrometer target circle on the temperature reading hole of the die. Make sure the emissivity setting of the pyrometer is set for graphite.</li>
	<li>Close the chamber and put a 7.7 kN load (approximately 60 MPa) on the sample. Vacuum and purge the chamber with Ar three times and leave the chamber under dynamic vacuum of 6 Pa.</li>
	<li>Increase the temperature by increasing the current (manually or using a program). Use 250 A min-1 (corresponding to approximately 300-400 &#176;C min-1) for the optimized samples. This is the most important step of the spark plasma sintering process.</li>
	<li>Keep the temperature at 1,500 &#176;C for 5 min by manually adjusting the current or using the program. At the end of the 5 min holding period, shut the current off and quickly release the 7.7 kN load to avoid cracking the sample during the cooling down. Let the sample cool to RT inside the chamber.</li>
	<li>Release the bulk pellet from the graphite die gently using the cold press. This is done by placing the graphite die on a hollowed steel cylinder and ejecting the pellet and the graphite plungers using a steel pushing rod from the top.</li>
	<li>Remove the graphoil on top and bottom faces of the pellet as well as the circumferential surface using a sharp razor blade.</li>
	<li>Polish the sample using a rough sand paper (400 grid) down for 0.3-0.5 mm from each side to assure the complete removal of the graphoil. Clean the sample with acetone.</li>
</ol>
3. Characterization of Electronic and Thermal Transport Properties of Bulk Ceramics
<ol>
	<li>Determine the density of the ceramic disk, &#961;, using the Archimedes method.</li>
	<li>Measure the weight of the sample, Wdry, and then the weight of the sample submerged in water, Wwet, on a stabilized density measurement system and calculate the Archimedes density from</li>
</ol>
<img alt="Equation 2" fo:content-width="2.1in" src="/files/ftp_upload/52869/52869eq2.JPG" width="260" />
where &#961;water is the density of the water at the measurement temperature (e.g., equals 1 g cm-3 at 300 K).14
<ol start="2">
	<li>Measure the thermal diffusivity of the sample, d, using the transient laser-flash technique under a 75 ml min-1 flow of Ar. Measure the thickness of the sample, L, accurately first using a digital micrometer. 
	Note: Parallel-faced samples with different sizes and shapes (e.g., round discs 12.7 mm in diameter or square 10 x 10 mm2 disks) and thicknesses between 0.5 and 5 mm can be easily measured.
	<ol>
		<li>In the laser-flash thermal diffusivity technique, irradiate one face of the sample by a short (~1 msec) laser pulse and record the temperature rise on the opposite face by an infra-red detector. Then calculate thermal diffusivity by the laser-flash interface software from the thickness of the sample and the temperature rise-time profile using the Parker equation15</li>
	</ol>
	</li>
</ol>
<img alt="Equation 3" fo:content-width="1.9in" src="/files/ftp_upload/52869/52869eq3.JPG" width="220" />
where L is the thickness of the disk and t1/2 is the half-time of the maximum temperature rise of the other side of the sample.
Note: The Parker model15 assumes ideal conditions of adiabatic sample and instantaneous pulse heating, other models have been proposed over the years, which account for various losses in the measurement such as heat losses, finite pulse duration, non-uniform pulse heating and nonhomogeneous structures. We have used the Cowan model16 with pulse correction which is one of the most advanced methods. It should be noted that in order to maximize the amount of thermal energy transmitted from the front surface and to maximize the signal observed by the IR detector, the sample surfaces must be highly emissive. Usually this requires the application of a thin coating of graphite to the sample surfaces. An uncertainty of 2%-5% in the measurement of thermal diffusivity exists, arising from the determination of dimension.17
<ol start="3">
	<li>Cut the disk pellet using a diamond saw into rectangular bars, 2 x 2 x 10 mm3, electrical conductivity and Seebeck coefficient measurements as well as a square disk, 4 x 4 x 1.5 mm3 for high-temperature specific heat and a thin rectangular piece, 8 x 5 x 1 mm3 for Hall measurements.</li>
	<li>Measure the specific heat, Cp, of the sample on the flat and mirror-polished square piece (4 x 4 x 1.5 mm3) using a differential scanning calorimetry (DSC) under argon flow.18
	<ol>
		<li>Use a heating rate of 5 K min-1 up to 40 &#176;C for an isothermal hold for 10 min to allow the sample to reach thermal equilibrium followed by 20 K min-1 heating rate up to 500 &#176;C, with an exact cooling rate which followed. Perform the measurement under the flow of argon (50 ml min-1 is suggested).</li>
	</ol>
	</li>
</ol>
Note: Due to the sensitivity of the method used for analysis, conduct three measurements to determine the heat capacity including (1) a baseline measurement to subtract the background, (2) measurement of the specific heat of a standard material (such as sapphire) with a known CP, and (3) measurement of the specific heat of the sample. Ensure that samples are flat and mirror-polished in order to make an ideal contact with the bottom of the measurement crucible (Pt/Rh pans with Al2O3 crucibles used in this work). More details on the exact structure of the DSC stage, a comparison of the DSC techniques to others, and exact instructions for measuring a sample can be found in various sources.19
<ol start="5">
	<li>Calculate the high-temperature thermal conductivity, &#954;, of the sample from the measured values of thermal diffusivity, d, the specific heat, CP, and the density, &#961; using20</li>
</ol>
<img alt="Equation 4" fo:content-width="1.4in" src="/files/ftp_upload/52869/52869eq4.JPG" width="200" />
<ol start="6">
	<li>Gold plate the probes contact points (4 contacts) on the 2 x 2 x 10 mm3 piece cut from the sample to alleviate the contact resistance issues.
	<ol>
		<li>In order to sputter gold only on the desired contact areas, wrap a scotch-tape around the 2 x 2 x 10 mm3 sample to use as a stencil. Leave the 2 x 2 mm2 faces un-covered. Using a razor blade, cut out 2 very small holes (approximately 1 mm in diameter) in the middle of the 2 x 10 mm2 face along a line separated by the probes distance.</li>
		<li>Sputter a ~ 200 nm-thick gold film using a bench-top gold sputtering unit.21</li>
	</ol>
	</li>
	<li>Measure the electrical transport properties, namely the electrical conductivity and Seebeck coefficient) of the sample as a function of temperature22,23.
	<ol>
		<li>Measure electrical conductivity using the four-terminal method. Measure the Seebeck coefficient on the same setup using the measurements of temperature and voltage via the two middle thermocouple &#8220;probes&#8221;. Measure the distance between these two probes using a digital microscope. More details on the electrical transport measurements can be found elsewhere.22,23</li>
	</ol>
	</li>
	<li>Measure the Hall carrier concentration as a function of temperature on the 8 x 5 x 1 mm3 sample using a Physical Properties Measurement System.24</li>
</ol>

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The authors have nothing to disclose.

In this protocol, we have presented the steps of the synthesis strategy in order to successfully prepare bulk polycrystalline Pr-doped SrTiO3 ceramics exhibiting improved electronic and thermoelectric properties. The main steps of the protocol include (i) the solid-state synthesis of the doped SrTiO3 powder in air under atmospheric pressure and (ii) taking advantage of the capabilities of spark plasma sintering technique to densify the as-prepared powder into high-density bulk ceramics and at the same time to further dope the grain boundaries of the sample with Pr. It was demonstrated that by applying a high SPS heating rate (300-400 &#176;C min-1) the reflections in the X-ray diffraction patterns corresponding to the secondary phase(s) are fully disappeared (Figure 1C, D). The high heating rate is one of the key differences of this synthesis strategy with previous reports in the literature.17 Complete incorporation of Pr dopants in Sr sites, which lead to measured carrier concentration values corresponding to the nominal doping values12, is one of the main accomplishments of this synthesis strategy. As a result, higher carrier concentration values were observed for the samples prepared in this work comparing to the values reported in the literature for the samples prepared using other methods with same nominal doping concentrations. Taking advantage of the highly-reducing atmosphere of spark plasma sintering chamber (graphite die under dynamic vacuum and high heating rate) to reduce the oxide powder in order to create oxygen vacancies as another source of electron doping also differentiates this protocol from other samples reported in the literature prepared under Ar29 or forming gas (5-10% H in Ar)29 atmospheres.
Furthermore, it was observed that by applying a high SPS heating rate, secondary phases, which are primarily praseodymium oxide, can locally dope the grain boundary regions. This non-uniform doping of the ceramic samples resulted in the observation of an unexpected marked improvement in the carrier mobility, thermoelectric power factor as well as a significant reduction in the total thermal conductivity. The experimental data suggest that the observed enhancement is correlated to the unique microstructure of the ceramics and the presence of Pr-rich boundary regions. Such boundaries were not observed for the SrTiO3 ceramics doped with other dopants such as La (Figure 2C, inset) or prepared with other synthesis methods reported in the literature. A recent theoretical study by Dawson and Tanaka attempts to explain this observation (i.e., why Pr-doping induces core&#8722;shell formation and La-doping does not) by investigating the local structure and energetics of Pr- and La-doped SrTiO3 grain boundaries.30 Their calculations show a far stronger energetic benefit for Pr-doping of grain boundaries than La-doping. Since the electronic transport results could not be explained by effective medium theory,12 it is believed that a charge transfer mechanism is likely to be involved in the carrier mobility improvement.
The results prove that the strategy can be applied as a method to synthesize in&#160;situ composite structures particularly of the core-shell type structure. However, the efficiency of the non-uniform doping depends on the nature of the constituent phases of the composite. This method is limited by the melting point of the phases present in the material. High heating rates of 300-400 &#176;C min-1 can locally melt the material under mechanical pressure and either crack the sample or change the properties. Therefore, this protocol is a good synthesis strategy to be implemented to other oxides due to their high-temperature stability. Caution should be exercised when the method is applied to other thermoelectric materials. The conditions of spark plasma sintering technique need to be optimized before applying the strategy to other materials systems. It should be noted that due to the high heating rates applied, significant grain growth is expected if the densification part of the protocol is employed on nanosctructured powders.
Future work to modify the current protocol in order to further improve the thermoelectric properties will focus on the simultaneous optimization of the effect of the mechanical load (60 MPa in this protocol), the SPS soaking temperature, and soaking time to further reduce the thermal conductivity, improve the power factor, and to improve the success rate of the preparation of crack-free samples.
In conclusion, we have demonstrated the synthesis approach for the preparation of bulk polycrystalline Pr-doped SrTiO3 ceramics with Pr-rich grain boundaries exhibiting significantly improved electronic and thermoelectric properties. The synthesis strategy utilized in this work may open new horizons and opportunities to other properties and applications of this broadly functional perovskite where higher carrier mobility is desired. Moreover, the grain boundary engineering using spark plasma sintering demonstrated in this work can be implemented in other oxide materials to modify the physical properties.

Oxide thermoelectrics were shown to be promising candidates for high-temperature thermoelectric applications, from stability and cost perspectives to electronic transport properties. Among the n-type oxide thermoelectrics, highly doped strontium titanate (STO) has attracted much attention due to its intriguing electronic properties. However, a large total thermal conductivity (&#954; ~ 12 W m-1 K-1 at 300 K for single crystals)1 and a low carrier mobility (&#181; ~ 6 cm2 V-1 sec-1 at 300 K for single crystals)1 detrimentally affect the thermoelectric performance which is evaluated by a dimensionless figure of merit, ZT = &#945;2&#963;T/&#954;, where &#945; is the Seebeck coefficient, &#963; the electrical conductivity, T the absolute temperature in Kelvin, and &#954; the total thermal conductivity. We herein define the numerator as the power factor, PF = &#945;2&#963;T. In order for this oxide thermoelectric material to compete with other high-temperature thermoelectrics (such as SiGe alloys), a more pronounced increase in the power factor and/or decrease in lattice thermal conductivity are required.
The majority of the experimental studies in order to improve the thermoelectric properties of STO have mainly focused on the reduction of thermal conductivity through strain-field and mass fluctuation scattering of phonons. These attempts include: (i) Single- or double-doping of the Sr2+ and/or Ti4+ sites, as the main efforts with respect to this direction,2,3 (ii) Synthesis of natural superlattice Ruddlesden&#8211;Popper structures in order to further reduce the thermal conductivity through insulating SrO layers,4 and (iii) Composite engineering by addition of a nanosized second phase.5 However, up until recently, no enhancement strategy has been reported to substantially increase the thermoelectric power factor in these oxides. The reported maximum power factor (PF) values in bulk single- and poly-crystalline STO have been confined to an upper limit of PF &#60; 1.0 W m-1 K-1.
A variety of synthesis approaches and processing techniques have been employed to implement the ideas attempted above. The powder synthesis routes include conventional solid-state reaction,6 sol-gel,7 hydrothermal,8 and combustion synthesis,9 whereas conventional sintering,6 hot pressing10 and recently spark plasma sintering12 are among the common techniques used to densify the powders into bulk ceramics. However, for a similar dopant (e.g., La) and doping concentration, the resulting bulk ceramics exhibit a range of electronic and thermal transport properties. This is in large due to the strongly process-dependent defect chemistry of SrTiO3 which results in synthesis-dependent properties. There is only a handful of reports optimizing the synthesis and processing parameters to benefit thermoelectric transport. It is worth mentioning that due to the very small phonon mean free path in SrTiO3 (lph~2 nm at 300 K),11 nanostructuring is not a viable option for the improvement of the TE performance of bulk STO ceramics primarily through the reduction of the lattice thermal conductivity.
Recently, we reported more than 30% improvement in the thermoelectric figure of merit in non-uniformly Pr-doped SrTiO3 ceramics originating from a simultaneously enhanced thermoelectric power factor and reduced thermal conductivity.12,13 In this detailed video protocol, we present and discuss the steps of our synthesis strategy for the preparation of these Pr-doped STO ceramics exhibiting improved electronic and thermoelectric properties.

<table><tbody><tr><td>SrCO&lt;sub&gt;3&lt;/sub&gt; Powder, 99.9%</td><td>Sigma Aldrich</td><td>472018</td><td></td></tr><tr><td>TiO&lt;sub&gt;2&lt;/sub&gt; Nanopowder, 99.5%</td><td>Sigma Aldrich</td><td>718467</td><td></td></tr><tr><td>Pr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Sintered Lumps, 99.9%</td><td>Alfa Aesar</td><td>35663&amp;nbsp;</td><td></td></tr><tr><td>Spark Plasma Sintering&amp;nbsp;</td><td>Dr. Sinter Lab</td><td>SPS-515S</td><td></td></tr><tr><td>Resistivity/Seebeck Coefficient Measurement System</td><td>Ulvac-Riko</td><td>ZEM-2</td><td></td></tr><tr><td>Laser Flash Thermal Diffusivity Measurement System</td><td>Netzsch</td><td>LFA-457 Microflash</td><td></td></tr><tr><td>Differential Scanning Calorimetry (DSC) System</td><td>Netzsch</td><td>404C Pegasus</td><td></td></tr><tr><td>Physical Property Measurement system (PPMS)</td><td>Quantum Design</td><td></td><td></td></tr><tr><td>Field Emission Scanning Electron Microscope (FE-SEM)</td><td>Hitachi</td><td>SU-6600</td><td></td></tr><tr><td>Energyy-dispersive X-ray Spectroscopy (EDS)</td><td>Oxford Instruments</td><td></td><td></td></tr><tr><td>X-ray Diffractometer</td><td>Rigaku</td><td>Ultima IV</td><td></td></tr><tr><td>Bench-top Sputter Coater</td><td>Denton Vacuum</td><td>Desk II</td><td></td></tr><tr><td>Diamond&amp;nbsp; Wheel Saw</td><td>South Bay Technology</td><td></td><td></td></tr></tbody></table>

synthesis of non-uniformly pr-doped srtio3 ceramics and their thermoelectric properties

We demonstrate a novel synthesis strategy for the preparation of Pr-doped SrTiO3 ceramics via a combination of solid state reaction and spark plasma sintering techniques. Polycrystalline ceramics possessing a unique morphology can be achieved by optimizing the process parameters, particularly spark plasma sintering heating rate. The phase and morphology of the synthesized ceramics were investigated in detail using X-ray diffraction, scanning electron microcopy and energy-dispersive X-ray spectroscopy. It was observed that the grains of these bulk Pr-doped SrTiO3 ceramics were enhanced with Pr-rich grain boundaries. Electronic and thermal transport properties were also investigated as a function of temperature and doping concentration. Such a microstructure was found to give rise to improved thermoelectric properties. Specifically, it resulted in a significant improvement in carrier mobility and the thermoelectric power factor. Simultaneously, it also led to a marked reduction in the thermal conductivity. As a result, a significant improvement (> 30%) in the thermoelectric figure of merit was achieved for the whole temperature range over all previously reported maximum values for SrTiO3-based ceramics. This synthesis demonstrates the steps for the preparation of bulk polycrystalline ceramics of non-uniformly Pr-doped SrTiO3.

X-ray diffractions patterns were collected for the as-prepared powders and the corresponding bulk ceramics as a function of Pr-content (Figure 1) in order to study the effect of Pr-doping on the SrTiO3 lattice, solubility of Pr in SrTiO3 and the formation of secondary phase(s). The patterns confirm the formation of SrTiO3 phase in all the as-prepared powders where the reflections can be indexed to a cubic lattice with space group (Figure 1A). The monotonic change in the indexed lattice parameter from a = 3.906 for x = 0 (undoped) with increasing Pr content confirms the distortion of the lattice upon incorporation of smaller Pr3+ ions in Sr2+ sites. Weak reflections were also observed for x &#62; 0.05, corresponding to the intermediate praseodymium oxide phase (e.g., Pr5O9), which increase intensity with increasing nominal Pr concentration (Figure 1B). It was found that by optimizing the SPS heating rate the reflections corresponding to the secondary phase(s) are fully disappeared (Figure 1C, D).
A synthesis-structure-property relationship study was conducted in order to understand the effect of the SPS heating rate and the observed secondary phase on the electronic transport properties. Scanning electron micrographs were acquired in conjunction with energy-dispersive X-ray spectra to investigate the effect of the synthesis and densification parameters, particularly spark plasma sintering heating rate, on the morphology and the chemistry of the grains and grain boundaries (Figure 2). Synthesis-(micro) structure-property relationship was investigated by monitoring the temperature-dependence of electronic transport properties of the bulk ceramics (Figure 2). It was found that electrical conductivity can be significantly increased through the optimization of the SPS heating rate (Figure 2A). This improvement was attributed to a marked enhancement in the carrier mobility since similar Seebeck coefficient and carrier concentration values were obtained for samples densified under different heating rates (Figure 2A, inset). Scanning electron micrographs have shown that the Pr-rich secondary phase present in the as prepared powder (Figure 2B, inset) can partially dope the grain boundary region during the SPS process (Figure 2B). By the appropriate optimization of the SPS heating rate, the grain boundary region can be fully doped with Pr, in which case an enhancement in the carrier mobility is observed (Figure 2C).
Electronic and thermal transport properties of the bulk ceramics prepared with a high SPS heating rate of 300 &#176;C min&#8722;1 were measured as a function of temperature and Pr content in order to calculate the overall thermoelectric figure of merit (Figure 3). All samples exhibit a degenerate semiconducting behavior (i.e., metallic-like) for electrical conductivity (Figure 3A) and a corresponding diffusive-like thermopower (Figure 3B). Large thermoelectric power factor &#62; 1 W m-1 K-1 was observed for ceramics with x &#62; 0.075 in a broad temperature range reaching a maximum of 1.3 W m-1 K-1 for x = 0.15, corresponding to 3 at % Pr (Figure 3C). Simultaneously, a monotonic reduction in thermal conductivity was observed with increasing Pr up to x = 0.15 (Figure 3D). The optimum nominal Pr concentration was found to be x = 0.15 for these samples.13 More than 30% improvement in the dimensionless thermoelectric figure of merit (ZT) for the whole temperature range over all previously reported maximum values were achieved as a result of the simultaneous enhancement in the thermoelectric power factor and reduction in the thermal conductivity (Figure 3E).13 Maximum ZT value of 0.35 was obtained at 500 &#176;C. If the measurements are to be performed under a highly reducing atmosphere, maximum ZT values above 0.6 are predicted at 1,000 &#176;C by fitting the experimental electronic and thermal transport data. The possibility of further improvement of the power factor at these temperatures, and hence ZT also exist if the carrier concentration can be further increased.30
<img alt="Figure 1" src="/files/ftp_upload/52869/52869fig1.jpg" /> 
Figure 1. X-ray diffraction profiles. (A) X-ray diffraction (XRD) profiles of Sr1&#8722;xPrxTiO3&#8722;&#948; powders before SPS as a function of nominal Pr content. (B) Magnified view of the dashed rectangle in (a), (C) XRD profiles of Sr1&#8722;xPrxTiO3&#8722;&#948; with x = 0.075 before SPS (Powder) and after high-heating-rate SPS (Bulk Ceramic). Photographs of cold-pressed powder after solid-state reaction and the corresponding SPSed ceramic are shown. (D) Magnified view of the dashed rectangle in (C). Reprinted with permission.12. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52869/52869fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/52869/52869fig2.jpg" /> 
Figure 2. Effect of SPS heating rate. (A) Temperature dependence of electrical conductivity and Seebeck coefficient (inset) for Sr1&#8722;xPrxTiO3&#8722;&#948; ceramics with x = 0.075 processed using two different SPS heating rates of 100 &#176;C min&#8722;1 and 300 &#176;C min&#8722;1, respectively. (B) Backscattered electron (BSE) micrograph of the ceramic made under 100 &#176;C min&#8722;1 SPS. A typical Pr spectrum of the EDS line scan across a PrOy particle is shown. (C) Backscattered electron micrograph of the ceramic made under a 300 &#176;C min&#8722;1 SPS heating rate. A typical Pr spectrum of EDS line scan across two grains, grain 1 and grain 2, is shown. The inset depicts the BSE micrograph of the Sr0.95La0.05TiO3 ceramic prepared following the same recipe. Reprinted with permission.12 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52869/52869fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/52869/52869fig3.jpg" /> 
Figure 3. Thermoelectric transport properties. Temperature dependence of (A) electrical conductivity (&#963;), (B) Seebeck coefficient (&#945;), (C) power factor (defined as PF= &#945;2&#963;T), (D) total thermal conductivity and (E) figure-of-merit, ZT for Sr1&#8722;xPrxTiO3&#8722;&#948; ceramics as a function of Pr content. Temperature dependence of reported maximum ZT values in the literature is shown for comparison.2,25-28 Reproduced with permission.12,13 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52869/52869fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties at JoVE.com

Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

A protocol for the synthesis and processing of polycrystalline SrTiO3 ceramics doped non-uniformly with Pr is presented along with the investigation of their thermoelectric properties.

Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

We demonstrate a novel synthesis strategy for the preparation of Pr-doped SrTiO3 ceramics via a combination of solid state reaction and spark plasma ...

synthesis-non-uniformly-pr-doped-srtio3-ceramics-their-thermoelectric

Nanyang Technological University

Research

JoVE Journal

Chemistry

9.8K Views.  Clemson University. A protocol for the synthesis and processing of polycrystalline SrTiO3 ceramics doped non-uniformly with Pr is presented along with the investigation of their thermoelectric properties.

Video: Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the degradation of device performance. In order to evaluate such degradations using computer simulations, full matrix material properties at elevated temperatures are needed as inputs. It is extremely difficult to measure such data for ferroelectric materials due to their strong anisotropic nature and property variation among samples of different geometries. Because the degree of depolarization is boundary condition dependent, data obtained by the IEEE (Institute of Electrical and Electronics Engineers) impedance resonance technique, which requires several samples with drastically different geometries, usually lack self-consistency. The resonant ultrasound spectroscopy (RUS) technique allows the full set material constants to be measured using only one sample, which can eliminate errors caused by sample to sample variation. A detailed RUS procedure is demonstrated here using a lead zirconate titanate (PZT-4) piezoceramic sample. In the example, the complete set of material constants was measured from room temperature to 120 &#176;C. Measured free dielectric constants <img alt="Equation 1" src="/files/ftp_upload/53461/image001.jpg" /> and&#160;<img alt="Equation 2" src="/files/ftp_upload/53461/image002.jpg" /> were compared with calculated ones based on the measured full set data, and piezoelectric constants d15 and d33 were also calculated using different formulas. Excellent agreement was found in the entire range of temperatures, which confirmed the self-consistency of the data set obtained by the RUS.

This protocol describes the procedure of measuring the temperature dependence of the full set material constants of piezoelectric materials using resonant ultrasound spectroscopy (RUS).

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the ...

Engineering

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

Barium titanate (BaTiO3, hereafter BT) is an established ferroelectric material first discovered in the 1940s and still widely used because of its well-balanced ferroelectricity, piezoelectricity, and dielectric constant. In addition, BT does not contain any toxic elements. Therefore, it is considered to be an eco-friendly material, which has attracted considerable interest as a replacement for lead zirconate titanate (PZT). However, bulk BT loses its ferroelectricity at approximately 130 &#176;C, thus, it cannot be used at high temperatures. Because of the growing demand for high-temperature ferroelectric materials, it is important to enhance the thermal stability of ferroelectricity in BT. In previous studies, strain originating from the lattice mismatch at hetero-interfaces has been used. However, the sample preparation in this approach requires complicated and expensive physical processes, which are undesirable for practical applications.
In this study, we propose a chemical synthesis of a porous material as an alternative means of introducing strain. We synthesized a porous BT thin film using a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles were used as an organic template. Through a series of studies, we clarified that the introduction of pores had a similar effect on distorting the BT crystal lattice, to that of a hetero-interface, leading to the enhancement and stabilization of ferroelectricity. Owing to its simplicity and cost effectiveness, this fabrication process has considerable advantages over conventional methods.

Here, we present a protocol for the synthesis of porous barium titanate (BaTiO3) thin film by a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles are used as an organic template.

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Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

Sb2S3 is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical properties. Recently, we demonstrated its potential as next-generation solar cells by achieving a high photovoltaic efficiency of &#62; 6% in Sb2S3-sensitized solar cells using a simple thiourea (TU)-based complex solution method. Here, we describe the key experimental procedures for the deposition of Sb2S3 on a mesoporous TiO2 (mp-TiO2) layer using a SbCl3-TU complex solution in the fabrication of solar cells. First, the SbCl3-TU solution is synthesized by dissolving SbCl3 and TU in N,N-dimethylformamide at different molar ratios of SbCl3:TU. Then, the solution is deposited on as-prepared substrates consisting of mp-TiO2/TiO2-blocking layer/F-doped SnO2 glass by spin coating. Finally, to form crystalline Sb2S3, the samples are annealed in an N2-filled glove box at 300 &#176;C. The effects of the experimental parameters on the photovoltaic device performance are also discussed.

Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

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Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO3-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state.
The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO3-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Oxide materials show many exotic properties that can be controlled by tuning the oxygen content. Here, we demonstrate the tuning of oxygen content in oxides by varying the pulsed laser deposition parameters and by performing postannealing. As an example, electronic properties of SrTiO3-based heterostructures are tuned by growth modifications and annealing.

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Deposition of Bi2Te3 and Sb2Te3 Thin Films on Glass Substrates via RF Sputtering

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Through various studies on thermoelectric (TE) materials, thin film configuration gives superior advantages over conventional bulk TEs, including adaptability to curved and flexible substrates. Several different thin film deposition methods have been explored, yet magnetron sputtering is still favorable due to its high deposition efficiency and scalability. Therefore, this study aims to fabricate a bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin film via the radio frequency (RF) magnetron sputtering method. The thin films were deposited on soda lime glass substrates at ambient temperature. The substrates were first washed using water and soap, ultrasonically cleaned with methanol, acetone, ethanol, and deionized water for 10 min, dried with nitrogen gas and hot plate, and finally treated under UV ozone for 10 min to remove residues before the coating process. A sputter target of Bi2Te3 and Sb2Te3 with Argon gas was used, and pre-sputtering was done to clean the target&#39;s surface. Then, a few clean substrates were loaded into the sputtering chamber, and the chamber was vacuumed until the pressure reached 2 x 10-5 Torr. The thin films were deposited for 60 min with Argon flow of 4 sccm and RF power at 75 W and 30 W for Bi2Te3 and Sb2Te3, respectively. This method resulted in highly uniform n-type Bi2Te3 and p-type Sb2Te3 thin films.

Author Spotlight: Advancements in High-Performance Thermoelectric Thin Films Through Radio Frequency Magnetron Sputtering

The manuscript describes a protocol for radio frequency magnetron sputtering of Bi2Te3 and Sb2Te3 thermoelectric thin films on glass substrates, which represents a reliable deposition method that provides a wide range of applications with the potential for further development.

Through various studies on thermoelectric (TE) materials, thin film configuration gives superior advantages over conventional bulk TEs, including ...

Production of CdSe&#45;SnSe Nanocomposites from Consolidated SnSe Particles

Solution&#45;Processed, Surface&#45;Engineered, Polycrystalline CdSe&#45;SnSe Exhibiting Low Thermal Conductivity

In recent years, solution processes have gained considerable traction as a cost-effective and scalable method to produce high-performance thermoelectric materials. The process entails a series of critical steps: synthesis, purification, thermal treatments, and consolidation, each playing a pivotal role in determining performance, stability, and reproducibility. We have noticed a need for more comprehensive details for each of the described steps in most published works. Recognizing the significance of detailed synthetic protocols, we describe here the approach used to synthesize and characterize one of the highest-performing polycrystalline p-type SnSe. In particular, we report the synthesis of SnSe particles in water and the subsequent surface treatment with CdSe molecular complexes that yields CdSe-SnSe nanocomposites upon consolidation. Moreover, the surface treatment inhibits grain growth through Zenner pinning of secondary phase CdSe nanoparticles and enhances defect formation at different length scales. The enhanced complexity in the CdSe-SnSe nanocomposite microstructure with respect to SnSe promotes phonon scattering and thereby significantly reduces the thermal conductivity. Such surface engineering provides opportunities in solution processing for introducing and controlling defects, making it possible to optimize the transport properties and attain a high thermoelectric figure of merit.

Author Spotlight: Advancing Energy Solutions Using Nanocomposites as Processed Thermoelectric Materials

CdSe-SnSe nanocomposites are produced by consolidating surface-engineered SnSe particles. A simple aqueous synthesis is employed to produce SnSe particles. Coating SnSe particles with CdSe molecular complexes allows for controlling grain size and increasing the number of defects present in the nanocomposite, thus lowering the thermal conductivity.

In recent years, solution processes have gained considerable traction as a cost-effective and scalable method to produce high-performance thermoelectric ...

Synthesis of Non-uniformly Pr-doped SrTiO₃ Ceramics and Their Thermoelectric Properties

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