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Influence of Oxygen Partial Pressure during Processing on the Thermoelectric Properties of Aerosol-Deposited CuFeO2 Thomas Stöcker *, Jörg Exner, Michael Schubert, Maximilian Streibl and Ralf Moos Department of Functional Materials, Zentrum für Energietechnik (ZET), University of Bayreuth, Bayreuth 95440, Germany * Correspondence: [email protected]; Tel.: +49-921-557401 Academic Editor: Anke Weidenkaff Received: 11 January 2016; Accepted: 17 March 2016; Published: 24 March 2016

Abstract: In the field of thermoelectric energy conversion, oxide materials show promising potential due to their good stability in oxidizing environments. Hence, the influence of oxygen partial pressure during synthesis on the thermoelectric properties of Cu-Delafossites at high temperatures was investigated in this study. For these purposes, CuFeO2 powders were synthetized using a conventional mixed-oxide technique. X-ray diffraction (XRD) studies were conducted to determine the crystal structures of the delafossites associated with the oxygen content during the synthesis. Out of these powders, films with a thickness of about 25 µm were prepared by the relatively new aerosol-deposition (AD) coating technique. It is based on a room temperature impact consolidation process (RTIC) to deposit dense solid films of ceramic materials on various substrates without using a high-temperature step during the coating process. On these dense CuFeO2 films deposited on alumina substrates with electrode structures, the Seebeck coefficient and the electrical conductivity were measured as a function of temperature and oxygen partial pressure. We compared the thermoelectric properties of both standard processed and aerosol deposited CuFeO2 up to 900 ˝ C and investigated the influence of oxygen partial pressure on the electrical conductivity, on the Seebeck coefficient and on the high temperature stability of CuFeO2 . These studies may not only help to improve the thermoelectric material in the high-temperature case, but may also serve as an initial basis to establish a defect chemical model. Keywords: delafossite; thermoelectric properties; aerosol deposition method (ADM); room temperature impact consolidation (RTIC)

1. Introduction With thermoelectric generators, thermal energy can be directly converted into electrical energy. Great efforts have been undertaken in the past few decades to increase the efficiency-characterizing figure of merit (ZT) S2 σ ZT “ T (1) κ which depends on the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ). If one considers only the electrical parameters, the power factor (PF) is an established parameter of thermoelectric materials: PF “ S2 σ (2) ZT values above 1 were reported for semiconductors like Bi2´x Sbx Te3 and filled skudderudites like Ba0.3 Ni0.05 Co3.95 Sb12 or SnSe [1–7], and can be further enhanced when optimizing the thermoelectric properties through nanostructuring [8–11]. However, the commercial application of these materials Materials 2016, 9, 227; doi:10.3390/ma9040227

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is limited due to their high synthesis and production costs. In addition, several high-ZT-materials contain elements that are not abundant and most materials cannot be processed in an environmentally friendly manner. Stability in oxidizing atmospheres, particularly at elevated temperatures, is another serious issue. Hence, it is a research trend today to substitute those costly and less abundant thermoelectric materials with inexpensive materials, while sustaining acceptable figures of merit. Whereas thermoelectric materials based on conductive polymers or ceramic-polymer hybrids were investigated for room-temperature applications [12–17], oxides are especially attractive at elevated temperatures due to their chemical and high-temperature stability, with limitations to some special oxide classes like doped ZnO [18,19], while not having a negative impact on the environment [20–24]. The key challenge when using oxides as materials in thermoelectric generators is the enhancement of their usually low electrical conductivity. In recent years, considerable good figures of merit for layered p-type cobaltites were reported [25,26]. Even though these materials exhibit good thermoelectric properties, NaCo2 O4 for example is not supposed to be stable against temperature cycling and requires a complex synthesis route due to its highly anisotropic behavior [27]. Concerning n-type thermoelectric oxides, SrTiO3 exhibits the best properties, even though it has a low mobility (compared to classical semiconductors); the effective mass is particularly high resulting in a power-factor comparable with Bi2 Te3 at room temperature [28,29]. However, ZT is rather low owing to a very high thermal conductivity. Another promising n-type semiconducting oxide is Al-doped ZnO, with a reported ZT of 0.24 at 1000 ˝ C, and ZT = 0.47 at 975 ˝ C for Al-Ga doped ZnO [24,30]. Keeping this in mind, SrTiO3 and ZnO are the only n-type oxide materials reported with reasonably high ZT values [31]. In the past, some studies have described copper-iron-oxides and claimed them as promising thermoelectric materials due to their high Seebeck coefficient while sustaining a high electrical conductivity and thermal stability [32–35]. In this work, we evaluated the thermoelectric performance and the electrical conductivity of the delafossite-type oxide CuFeO2 , as it depends on the oxygen partial pressure at high temperatures, and found some interesting properties, especially an as yet unknown p-n-transition. Cu+ Fe3+ O2 delafossite type oxides belong to the R3;´ m space group and have a layered crystal structure. The Cu+ ions are coordinated by two O2´ ions and form O-Cu-O layers parallel to the c-axis, whereas the Fe3+ ions are coordinated by six O2´ ions in an octahedron [36]. By doping the Fe3+ site with divalent 3d cations, the electrical conductivity of the intrinsically p-type CuFeO2 can be enormously enhanced [37]. Consequently, doping with tetravalent 3d cations leads to n-type semiconductors [38]. Even though this behavior has been published earlier, the fundamental understanding of the electronic conduction mechanism has still not been fully elucidated. In this study, the novel aerosol deposition method (often abbreviated as AD method or ADM) is used to obtain dense ceramic thin-films of CuFeO2 . The AD is based on room temperature impact consolidation (RTIC) of ceramic powders and uses a pressure gradient to accelerate an aerosol of submicron particles through a nozzle to the substrate [39,40]. As the particles impact on the substrate, a dense layer forms by fracture and plastic deformation of the particles on the surface of the substrate [41–44]. Using this method, thin CuFeO2 and dense layers were prepared to study the oxygen partial pressure dependence on the thermoelectric properties, to compare aerosol deposited CuFeO2 with conventionally solid-state prepared CuFeO2 , and to deepen the understanding of their electrical conduction mechanism. 2. Experimental Ceramic CuFeO2 powders were synthesized in a conventional mixed-oxide technique using copper(I)-oxide (99.9%, Alfa-Aesar, Karlsruhe, Germany) and iron(III)-oxide (99%, Alfa-Aesar, Karlsruhe, Germany)). These starting materials were processed in a wet planetary ball mill (FRITSCH, Idar-Oberstein, Germany) with cyclohexane as solvent. After milling the powders for 4 h, the solvent

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was removed Idar-Oberstein, in a rotary evaporator (Heidolph Instruments, Schwabach, Germany). To elucidate (FRITSCH, Germany) with cyclohexane as solvent. After milling the powders for 4 h,the influence of the oxygen content of the gas atmosphere during the solid state reaction, CuFeO fired the solvent was removed in a rotary evaporator (Heidolph Instruments, Schwabach, Germany).2 To in elucidate 100% N2 the andinfluence CuFeO2 of fired 1% Ocontent in a high temperature furnace at 1050 ˝ C the in oxygen of the gas atmosphere during the solid state reaction, 2 were synthesized forCuFeO 12 h. The obtained powders were reground in a planetary balltemperature mill using the above 2 fired in 100% delafossite N2 and CuFeO 2 fired in 1% O 2 were synthesized in a high furnace mentioned with a delafossite 90 µm screen in order toreground reduce agglomerates dried at 1050 °Cmethod, for 12 h.sieved The obtained powders were in a planetaryand ball finally mill using ˝ above mentioned sieved with 90 µm screen in order to reduce (SEM, agglomerates and finally in the a furnace at 200 C method, for at least 24 h. A ascanning electron microscope Zeiss, Oberkochen, dried in aimage furnace 200calcined °C for at and least milled 24 h. A scanning electron microscope Zeiss, Oberkochen, Germany) ofatthe delafossite powder used for(SEM, the AD process is shown Germany) image of the calcined and milled delafossite powder used for the AD process is shown in Figure 1. It can be seen that there is a broad particle size distribution ranging from 0.1 to 30inµm Figure 1. It can be seen there is a broad distribution ranging from 0.1 to 30 µm which which is uncommon for that AD processes. Bulkparticle CuFeOsize 2 samples were formed into brick shaped pellets, ˝ is uncommon for ADand processes. CuFeO 2 samples were formed into brick shaped pellets, uniaxially cold pressed, sinteredBulk at 1050 C under the same gas atmosphere as the corresponding uniaxially cold pressed, and sintered at 1050 °C under the gas atmosphere as the corresponding starting powder. In order to determine the thermoelectricsame properties, platinum/gold thermocouples starting powder. In order to determine the thermoelectric properties, platinum/gold and platinum wires were attached to the sintered samples with platinum conductorthermocouples paste. Details of and platinum wires were attached to the sintered samples with platinum conductor paste. Details of the setup are shown in Figure 2. the setup are shown in Figure 2.

Materials 2016, 9, 227 4 of 16 Figure 1. 1. Scanning electron processedstarting startingpowder powderfor foraerosol aerosol deposition. Figure Scanning electronmicroscope microscope image image of a processed deposition.

The AD films were processed in a setup similar to previously published works [45–48]. It generally contains an aerosol generator, a deposition chamber and a vacuum pump (Edwards Germany, Kirchheim, Germany). In the deposition chamber and in the aerosol generator, a vacuum of 8 mbar is induced. Oxygen serves as a carrier gas at a flow rate of 6 L/min in the aerosol generator where an aerosol is created from the ceramic particles. These particles are transported through a slitnozzle with an orifice size of 10 × 0.5 mm² and accelerated up to several hundred m/s due to the pressure drop from the aerosol generator into the deposition chamber. The streaming aerosol is ejected on the target at a distance of 3 mm from the nozzle to the substrate and forms dense ceramic layers of several microns. For electrical measurements, AD films were deposited on alumina substrates (CeramTec, Marktredwitz, Germany) of a thickness of 635 µm, a length of 25 mm, and a Figure thermoelectric propertiesof ofbulk bulk sampleshave (a);and and aerosol-deposited Figure 2.Setup Setup todetermine determine thermoelectricplatinum/gold properties samples (a); aerosol-deposited width of 2. 12.5 mm,toon which screen-printed electrodes been applied before. To samples (b). samples (b). obtain XRD patterns, silicon was used as substrate material (CrysTec, Berlin, Germany). The silicon wafers had an orientation of (911), exhibiting no silicon reflexes in the measured XRD diffraction Figure 2avoiding depicts setup to the diffraction thermoelectric properties works of bulk[45–48]. (a) andItaerosolThe AD films were the processed indetermine a setup to similar to previously published generally angle range, substrate influences the pattern. deposited CuFeO 2 (b). In both cases, the resistance is measured by a four probe technique with offset Toan verify the phase composition of the starting powders to elucidate the (Edwards effect of theGermany, AD on contains aerosol generator, a deposition chamber and and a vacuum pump compensation (digital multimeter Keithley 2700). Byofknowing the geometry, the the electrical the crystallography of CuFeO 2 , X-Ray diffraction patterns both the calcined powder and aerosol Kirchheim, Germany). In the deposition chamber and in the aerosol generator, a vacuum of 8 mbar conductivity canwere be calculated: depositedOxygen films takenasataroom temperature using a PANalytical systemgenerator (PANalytical, is induced. serves carrier gas at a flow rate of 6 L/minXpert in thePro aerosol where 1particles Netherlands) operating withparticles. CuK radiation (1.541874are Å).transported The intensities werearecorded an Almelo, aerosol is created from the ceramic These through slit-nozzle (3) = ∙ = ∙several ∙ within 2 = 25° .. 60° step of 0.02°. The morphology the ADhundred films wasm/s examined scanning with an orifice size ofat 10a ˆ 0.5size mm² and accelerated up toof due tobythe pressure electron microscopy of both section and fracture pattern of the AD samples. In Equation (3), simages is the spacing between the inner Pt the electrodes, is the measured resistance, drop from the aerosol generator intothe thecross deposition chamber. The Rstreaming aerosol is ejectedb on widthatofa the sample, d thefrom thickness of the pellet the AD film, latter was thethe target distance ofand 3 mm the nozzle to theorsubstrate andrespectively. forms denseThe ceramic layers measured by a stylus profilometer (PGK/S2, Mahr, Göttingen, Germany). of several microns. For electrical measurements, AD films were deposited on alumina substrates To determine the Seebeck coefficient, S, an additional modulation heater in front of the samples generated an alternating temperature gradient over the specimens. The temperature difference between the thermocouples TC1 and TC2 was determined via the Au and Pt thermocouple tracks and contact pads, while the thermovoltage Umeas of the film was measured between the Pt contacts. Since the Seebeck coefficients of Pt and Au, SPt and SAu, respectively, are known, the Seebeck

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(CeramTec, Marktredwitz, Germany) of a thickness of 635 µm, a length of 25 mm, and a width of 12.5 mm, on which screen-printed platinum/gold electrodes have been applied before. To obtain XRD patterns, silicon was used as substrate material (CrysTec, Berlin, Germany). The silicon wafers had an orientation of (911), exhibiting no silicon reflexes in the measured XRD diffraction angle range, avoiding substrate influences to the diffraction pattern. To verify the phase composition of the starting powders and to elucidate the effect of the AD on the crystallography of CuFeO2 , X-Ray diffraction patterns of both the calcined powder and the aerosol deposited films were taken at room temperature using a PANalytical Xpert Pro system (PANalytical, Almelo, Netherlands) operating with CuKα radiation (1.541874 Å). The intensities were recorded within 2θ = 25˝ .. 60˝ at a step size of 0.02˝ . The morphology of the AD films was examined by scanning electron microscopy images of both the cross section and the fracture pattern of the AD samples. Figure 2 depicts the setup to determine the thermoelectric properties of bulk (a) and aerosol-deposited CuFeO2 (b). In both cases, the resistance is measured by a four probe technique with offset compensation (digital multimeter Keithley 2700). By knowing the geometry, the electrical conductivity can be calculated: 1 s s “ ¨ (3) σ“ R¨A R b¨d In Equation (3), s is the spacing between the inner Pt electrodes, R is the measured resistance, b the width of the sample, and d the thickness of the pellet or the AD film, respectively. The latter was measured by a stylus profilometer (PGK/S2, Mahr, Göttingen, Germany). To determine the Seebeck coefficient, S, an additional modulation heater in front of the samples generated an alternating temperature gradient over the specimens. The temperature difference between the thermocouples TC1 and TC2 was determined via the Au and Pt thermocouple tracks and contact pads, while the thermovoltage Umeas of the film was measured between the Pt contacts. Since the Seebeck coefficients of Pt and Au, SPt and SAu , respectively, are known, the Seebeck coefficient S of the delafossite film versus Pt can be determined from Umeas . It has to be corrected by the known Seebeck coefficient of platinum, SPt . Details of the evaluation of S can be found in [49] S “ SPt ´

Umeas ∆T

(4)

A periodic voltage, Uheater = U0 ¨cos(2π¨ƒmod,heater ¨t)was applied to the modulation heater. It generated the temperature difference ∆T = TTC2 ´ TTC1 with the frequency f mod : ∆T “ ∆T0 ¨ cos p2π ¨ f mod ¨ tq

(5)

Since heater power and applied modulation heater voltage show a quadratic relation, the temperature difference is modulated with the double frequency as the modulation heater voltage, i.e., f mod = 2f mod,heater [49]. In Equation (5), ∆T0 is the amplitude of the temperature modulation, f mod stands for the frequency of the temperature modulation, and t is the time. Umeas /∆T is determined by a regression analysis of many measured data pairs of the two signals ∆Tj and Umeas,i . They are plotted according to the following linear equation: Umeas,j “ a ¨ ∆Tj ` b

(6)

The slope, a, represents the quotient Umeas /∆T for Equation (6). This method allows elimination of interfering offset voltages. Further details of the data evaluation procedure and accuracies are given in [50]. For our experiments, f mod = 12.5 mHz was used, being low enough for our aerosol-deposited specimen to sustain a frequency-independent temperature gradient over the sample [51]. To circumvent interferences between the measurement of the electrical conductivity and the thermopower measurement, a custom-made switching device was used, enabling the automatic alternate measurement of both and electrically insulating them from each other.

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In order to determine the influence of the oxygen partial pressure on the thermoelectric properties, the transducers were placed in a tube furnace and gas mixtures of oxygen and nitrogen Materials 2016, 9, 227 5 of 16 In order to determine the influence of the oxygen partial pressure on the thermoelectric properties, were applied. The oxygen partial pressure was increased stepwise from 10−2.6 bar, being the lower the transducers were placed in a tube furnace and gas mixtures of oxygen and nitrogen were applied. order to determine the influence to of 1the partial on conductivity the thermoelectric limit of theInemployed mass-flow-controller, baroxygen and both thepressure electrical and the ´2.6 properties, the were transducers were increased placed astepwise tubepO furnace and mixtures of oxygen and nitrogen The oxygen partial pressure was from 10gas bar,pO being the lower limit of the Seebeck coefficient measured duringineach -step while each measurement cycle was 2 2 were applied. The oxygen partial pressure was the increased stepwise from 10−2.6 bar, the lower employed mass-flow-controller, to 1 bar and both electrical conductivity and thebeing Seebeck coefficient conducted at 700 °C, 800 °C, and 900 °C. of the during employed mass-flow-controller, to 1 pO bar and both the electrical conductivity and the ˝ werelimit measured each pO2 -step while each 2 measurement cycle was conducted at 700 C, Seebeck coefficient were measured during each pO -step while each pO2 measurement cycle was ˝ ˝ 2 C, and C. 3.800 Results and900 Discussion conducted at 700 °C, 800 °C, and 900 °C.

3. Results and Discussion 3.1. Characterization the Synthesized CuFeO2 Powders and AD Films 3. Results and of Discussion 3.1.The Characterization of the of Synthesized CuFeO andXRD AD Films 2 Powders by crystal structure CuFeO2 was determined from the calcined powders. Figure 3 3.1. Characterization of the Synthesized CuFeO2 Powders and AD Films showsThe the crystal patternstructure of CuFeOof 2 fired in 0% O2 (pure N2) and the pattern of the 1% O2 (1% O2, 99% N2) CuFeO2 was determined by XRD from the calcined powders. Figure 3 Thetogether crystal structure ofreference CuFeO2 was determined by XRD from calcined powders. Figurepeaks 3 fired powder with the (JPCD Thethe characteristic shows the pattern of CuFeO2 fired in 0%pattern O2 (pure N2 ) 39-0246). and the pattern of the 1% Odiffraction 2 (1% O2 , 99% N2 ) shows the pattern of CuFeO 2 fired in 0% O2 (pure N2) and the pattern of the 1% O2 (1% O2, 99% N2) offired CuFeO 2 can be observed in the pattern, indicating the rhombohedral 3R type with the R3;¯m space powder together with the reference pattern (JPCD 39-0246). The characteristic diffraction peaks fired powder[52]. together with the reference pattern (JPCD 39-0246). The characteristic diffraction peaks group symmetry of CuFeO can be observed in the pattern, indicating the rhombohedral 3R type with the R3;´ m space 2 of CuFeO 2 can be observed in the pattern, indicating the rhombohedral 3R type with the R3;¯m space

groupgroup symmetry [52].[52]. symmetry

018

#

018

CuFeO2 fired at 0 % O2

#

009 009

104

CuFeO2 fired at 0 % O2 104

101

101

006

006

relative intensity (a.u.)

relative intensity (a.u.)

# = Cu

012

012

# = Cu

CuFeO2 fired at 1 % O2 CuFeO2 fired at 1 % O2

CuFeO22Ref. CuFeO Ref.39-0246 39-0246

30

30

40

40

50

50

60

60

2/ ° 2/ ° Figure 3. XRD pattern of CuFeO 2 calcined under under pure oxygen mixed in nitrogen and the Figure 3. XRD pattern of CuFeO purenitrogen, nitrogen,1%1% oxygen mixed in nitrogen and the 2 calcined Figure 3. XRD pattern CuFeO under pure nitrogen, 1% oxygen mixed in nitrogen and the reference spectrum of CuFeO2JPCD 2calcined JPCD39-0246). 39-0246). reference spectrum ofofCuFeO 2 reference spectrum of CuFeO2 JPCD 39-0246). While the XRD pattern of CuFeO2 fired in 1% O2 appears free from secondary phases, a While the XRD CuFeO 1% OCuFeO free= from secondary secondary 2 fired 2 appears secondary phasepattern can be of seen for the Oin 2-fired at 2θ 43° (indicated inphases, Figure 3aphases, by #), a While the XRD pattern of CuFeO 20% fired in 1% O2˝ 2appears free from secondary phaseaccounting can be seen for the 0%copper. O2 -fired CuFeO at 2θ = 43 (indicated in Figure 3 by #), accounting for for elemental We assume this metal impurity is related to the low oxygen content 2 secondary phase can be seen for the 0% O2-fired CuFeO2 at 2θ = 43° (indicated in Figure 3 by #), of N2 in the alumina tube furnace, resulting in a reduction of Cu to Cu: elemental copper. We assume this metal impurity is related to2Othe low oxygen content of N2 in the

accounting for elemental copper. We assume this metal impurity is related to the low oxygen content alumina tube furnace, resulting in a reduction of Cu O to Cu: Cua2O 4 2Cu +O ↑ 2O to Cu: (7) of N2 in the alumina tube furnace, resulting2 in reduction of2 Cu

A similar behavior has already been by 22 Cureported 22O CuZhao + OO22et↑Òal. [53] for delafossites calcined under (7)(7) O  44 Cu ` Ar atmosphere. For further aerosol deposition of powders and for the measurements of the Athermoelectric similar behavior has already reported by Zhao et Oal.2 in[53] for delafossites calcined under properties, CuFeO2been calcined in a mixture of 1% nitrogen was used to avoid traces A similar behavior has already been reported by Zhao et al. [53] for delafossites calcined under Ar Ar atmosphere. further copper aerosolimpurities. deposition powders and for thecalculated measurements of the of the aboveFor mentioned Theoflattice parameters were by Rietveld atmosphere. For further aerosol deposition of powders and for the measurements of the thermoelectric analyses for Cu-delafossites fired in 1% oxygen to be a =of 3.0341 c = 17.169was Å, which thermoelectric properties, CuFeO 2 calcined in a mixture 1% OÅ2 and in nitrogen used corresponds to avoid traces properties, CuFeO2 calcined in a mixture of 1% O2 in nitrogen was used to avoid traces of the to pure CuFeO 2 data reported earlier [54]. of the above mentioned copper impurities. The lattice parameters were calculated by Rietveld

above mentioned copper impurities. The lattice parameters were calculated by Rietveld analyses for analyses for Cu-delafossites fired in 1% oxygen to be a = 3.0341 Å and c = 17.169 Å, which corresponds Cu-delafossites fired in 1% oxygen to be a = 3.0341 Å and c = 17.169 Å, which corresponds to pure to pure CuFeO2 data reported earlier [54]. CuFeO2 data reported earlier [54].

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012

006

Figure thethe XRD patterns of aerosol deposited CuFeO2CuFeO on silicon substrates. No secondary Figure 44depicts depicts XRD patterns of aerosol deposited 2 on silicon substrates. No phases or impurities were observed but the peaks got broader compared to the powder to measurements secondary phases or impurities were observed but the peaks got broader compared the powder arising from the reduction of the grain sizes during deposition. Based on the Rietveld refinement, measurements arising from the reduction of the grain sizes during deposition. Based on the Rietveld the calculated mean grain size of AD CuFeO was 90 nm compared to 300 nm for the 2 refinement, the calculated mean grain size of AD CuFeO2 was 90 nm compared to 300 nmcalcined for the powder. is a well-known effect in AD films and films has been many aerosol-deposited calcined This powder. This is a well-known effect in AD and observed has been for observed for many aerosolmaterials In addition, relative peak intensities from the pattern ofthe bulk and reference deposited[47,55]. materials [47,55]. Inthe addition, the relative peakdiffer intensities differ from pattern of bulk CuFeO indicating high lattice strain of aerosol-deposited films, which is also a known 2 and reference CuFeO2 indicating high lattice strain of aerosol-deposited films, which is phenomenon also a known for aerosol-deposited materials [56,57]. phenomenon for aerosol-deposited materials [56,57].

018

009

104

relative intensity (a.u.)

101

AD - CuFeO2

Bulk CuFeO2

CuFeO2 Ref. 39-0246

30

40

50

60

2/ ° Figure 4.4.XRD of aerosol-deposited CuFeO substrate compared to bulk and reference XRDpattern pattern of aerosol-deposited CuFeO 2 on silicon substrate compared to bulk and 2 on silicon CuFeO 39-0246). 2 (JPCD 39-0246). reference CuFeO 2 (JPCD

SEM cross-sectional cross-sectional images images shown shown in in Figure Figure 5a 5a and and bb indicate indicate crack-free crack-free bulk bulk CuFeO CuFeO2 and and dense dense SEM 2 layers of of aerosol aerosol deposited deposited CuFeO CuFeO2 on alumina, respectively. The film thickness is around 25 µm. layers 2 on alumina, respectively. The film thickness is around 25 µm. From the scanning electron microscope images the nano-sized microstructure of From the scanning electron microscope imagesshown shownininFigure Figure5c,5c, the nano-sized microstructure the aerosol deposited films becomes obvious. The primary particle size ranges from 50 nm to 100 nm, of the aerosol deposited films becomes obvious. The primary particle size ranges from 50 nm to being consistent with the XRD agglomerates 400 nm in size the nano100 nm, being consistent with analysis, the XRDwhile analysis, while agglomerates 400are nmembedded in size areinembedded sized matrix. This inhomogeneous distribution of grain sizes is due to the particle size distribution of in the nano-sized matrix. This inhomogeneous distribution of grain sizes is due to the particle size the starting powder for the ADM. While the CuFeO 2 powders exhibit a d50 = 6.5 µm (the medium distribution of the starting powder for the ADM. While the CuFeO2 powders exhibit a d50 = 6.5 µm value of the particle distribution), the d90 valuethe (90dpercent of the distribution lies below this (the medium value ofsize the particle size distribution), 90 value (90 percent of the distribution lies value) of the particles is much larger (d 90 = 16.1 µm). The film forming mechanism for AD layers is below this value) of the particles is much larger (d90 = 16.1 µm). The film forming mechanism for AD supposed to favor to mid-range particlesparticles aroundaround 1 µm, so mainly these particles contribute to theto layer layers is supposed favor mid-range 1 µm, so mainly these particles contribute the formation. The larger particles of the aerosol stream may have less energy to form new ceramic layers layer formation. The larger particles of the aerosol stream may have less energy to form new ceramic and are therefore intercalated between the AD-formed ceramic planes. This This phenomenon has also layers and are therefore intercalated between the AD-formed ceramic planes. phenomenon has been observed for other aerosol-deposited materials [39,58,59]. also been observed for other aerosol-deposited materials [39,58,59].

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Figure 5. (a) Polished cross sectional SEM image of bulk CuFeO2; (b) Polished cross sectional SEM Figure 5. (a) Polished cross sectional SEM image of bulk CuFeO2 ; (b) Polished cross sectional SEM image of aerosol deposited CuFeO2. The inset shows the boundary surface between substrate and film image of aerosol deposited CuFeO shows the boundary surface between 2 . The inset (a) Polished cross image of bulk CuFeO2; (b) Polished crosssubstrate sectionaland SEMfilm 2 with the inset showing details of the dense inFigure detail;5.(c) Fractography ofsectional aerosol SEM deposited CuFeO inimage detail;of(c) Fractography aerosol deposited CuFeO inset showing details ofand thefilm dense 2 with thesurface aerosol depositedof CuFeO 2. The inset shows the boundary between substrate CuFeO2 film. CuFeO film. 2 in detail; (c) Fractography of aerosol deposited CuFeO2 with the inset showing details of the dense CuFeO2 film. 3.2. Electrical Conductivity of Aerosol-Deposited and Bulk CuFeO2 3.2. Electrical Conductivity of Aerosol-Deposited and Bulk CuFeO2 In order Conductivity to compareofthe power factors, (s. CuFeO Equation (2)), of AD-processed CuFeO2 and 3.2. Electrical Aerosol-Deposited andPFBulk 2 In order to compare the power factors, PF (s. Equation (2)), of AD-processed CuFeO2 and standard ceramic-processed delafossites, both the electrical conductivity and the Seebeck coefficient In ceramic-processed order to compare the power factors, PF (s. Equation (2)), of AD-processed CuFeO 2 and standard delafossites, both the electrical conductivity and the Seebeck coefficient were determined. Figure 6 shows the temperature dependency of the electrical conductivity of ADstandard ceramic-processed delafossites, both the electrical conductivity and the Seebeck coefficient were determined. Figure 6 shows the temperature dependency of the electrical conductivity of CuFeO 2 and bulk CuFeO2 as well as the activation energy of conduction. were determined. Figure 6 shows the temperature dependency of the electrical conductivity of ADAD-CuFeO2 and bulk CuFeO2 as well as the activation energy of conduction. CuFeO2 and bulk CuFeO2 as well as the activation energy of conduction.

Figure 6. Electrical conductivity of bulk and aerosol deposited CuFeO2 and calculated activation Figure aerosol deposited depositedCuFeO CuFeO andcalculated calculated activation Figure6. 6.Electrical Electricalconductivity conductivity of bulk and and aerosol 2 2 and activation energies. The inset displays the abruptly decreasing electrical conductivity at 900 °C in detail. energies.The Theinset insetdisplays displaysthe the abruptly abruptly decreasing decreasing electrical °C˝ C in in detail. energies. electricalconductivity conductivityatat900 900 detail.

AD-processed samples show an offset in the electrical conductivity compared to bulk samples AD-processed samples show an offset in the electrical conductivity compared to bulk samples AD-processed samples show an offset in the electrical conductivity compared to bulk samples of almost one decade at room temperature, getting smaller with increasing temperature. This effect of almost one decade at room temperature, getting smaller with increasing temperature. This effect ofcan almost one decade at room temperature, getting smaller with2 films. increasing effect be attributed to the microstructure of the deposited CuFeO Whiletemperature. sintered bulkThis samples can be attributed to the microstructure of the deposited CuFeO2 films. While sintered bulk samples can be attributed to the grain microstructure of the deposited CuFeO films. While sintered bulk samples exhibit almost perfect interconnections, AD samples show regions of less densely connected exhibit almost perfect grain interconnections, AD samples show2 regions of less densely connected exhibit almost perfect grain interconnections, AD samples regions of less densely connected grains. In addition, high strains, as they are common for the show room temperature impact consolidation grains. In addition, high strains, as they are common for the room temperature impact consolidation grains. In addition, high strains, as they are common for the room temperature impact consolidation process, impede movements of the charge carriers and diminish the electrical conductivity [47]. With process, impede movements of the charge carriers and diminish the electrical conductivity [47]. With

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process, impede movements of the charge carriers and diminish the electrical conductivity [47]. With increasing temperature, the grains sinter as well as the microstrain releases, thus enhancing the electrical conductivity, a mechanism observed, e.g., for aerosol deposited MgB2 [60]. Since both aerosol-deposited CuFeO2 and bulk CuFeO2 behave as though thermally activated, the electrical conductivity increases exponentially and can be described by Equation (8); hence, Ea can be derived from the slope of the Arrhenius-like plot of the electrical conductivity as a function of the inverse temperature. ˆ ˙ ´Ea Materials 2016, 9, 227 8 of(8) 16 σ “ σ0 exp kB T

increasing temperature, the CuFeO grains 2sinter as well as the microstrain releases, thus enhancing the Both aerosol-deposited and bulk CuFeO a change in the activation energy. 2 indicate electrical a mechanism for aerosol While theconductivity, aerosol-processed sample observed, exhibits a e.g., change from Eadeposited = 0.28 eVMgB to E2a [60]. = 0.38 eV at 200 ˝ C, Sincesample both aerosol-deposited CuFeObehavior 2 and bulk CuFeO as though activated, the bulk shows this transition from Ea 2 =behave 0.24 eV to Ea =thermally 0.35 eV at 400 ˝ C. the electrical increases exponentially and can be described by Equation hence, Ea can The differentconductivity transition temperature may be attributed to the microstructure of AD (8); films mentioned be derived the of the the change Arrhenius-like plot of the electrical conductivity as a function of the above. The from values asslope well as of activation energy are consistent with previously published inversefrom temperature. work Dordor et al. [61], where both single-crystals and polycrystalline samples of CuFeO2 were investigated. − exp = (8) At temperatures above 800 ˝ C, the electrical conductivity of both samples decreases abruptly, supposedly induced by a certain oxygen loss [37]. To study the origin of this conductivity decrease, the Both aerosol-deposited CuFeO2 and bulk CuFeO 2 indicate a change in the activation energy. dependency of the electrical transport parameters conductivity (σ) and Seebeck coefficient (S) on the While the aerosol-processed sample exhibits a change from E a = 0.28 eV to Ea = 0.38 eV at 200 °C, the ˝ oxygen partial pressure (pO2 ) was investigated at 900 C for both aerosol deposited and bulk CuFeO2 . bulk Figure sample7 shows transition behavior from cycle. Ea = 0.24 eV towith Ea = a0.35 eVnitrogen at 400 °C. different shows this a characteristic measurement Starting pure gasThe atmosphere, transition temperature may be attributed to the microstructure of AD films mentioned above. The the oxygen partial pressure, pO2 , was increased stepwise. Compared to bulk CuFeO2 , aerosol deposited values asrespond well as the change energy are consistent with previously published from samples much fasteroftoactivation pO2 steps, promptly reaching an equilibrium state. Below work an oxygen Dordor et al. [61], where both single-crystals and polycrystalline samples of CuFeO 2 were partial pressure of 31 mbar (3.1% oxygen), CuFeO2 shows a p-type conduction behavior, as can be investigated. seen by the increasing conductivity with pO2 . With increasing pO2 , more oxygen is incorporated At temperatures abovein 800 the electrical conductivity of both samples decreases abruptly, into the material, resulting an°C, increased hole concentration, resulting in an increasing electrical supposedly induced by a certain oxygen loss [37]. To study the origin of this conductivity decrease, conductivity. Thus the σ (pO2 ) measurement supports the assumption that the abrupt decrease of the the dependency of the electrical transport parameters conductivity (  ) and Seebeck coefficient (S) ˝ electrical conductivity that occurs at 900 C (displayed in the inset in Figure 6) may be attributedon to the oxygen partial pressure (pO2) was investigated at 900 °C for both aerosol deposited and bulk a loss in oxygen. CuFeO2.

Figure 7. with varying Figure 7. Electrical Electrical conductivity conductivity of of bulk bulk and and aerosol aerosol deposited deposited CuFeO CuFeO22 with varying oxygen oxygen partial partial ˝ pressure at 900 C. The dotted line represents the oxygen partial pressure. pressure at 900 °C. The dotted line represents the oxygen partial pressure.

Figure 7 shows a characteristic measurement cycle. Starting with a pure nitrogen gas atmosphere, the oxygen partial pressure, pO2, was increased stepwise. Compared to bulk CuFeO2, aerosol deposited samples respond much faster to pO2 steps, promptly reaching an equilibrium state. Below an oxygen partial pressure of 31 mbar (3.1% oxygen), CuFeO2 shows a p-type conduction

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Astonishingly, the the conduction conduction mechanism mechanism changes changes from from p-type p-type to to n-type n-type behavior behavior at at an an oxygen oxygen Astonishingly, partial pressure of 31 mbar, i.e., with increasing pO the electrical conductivity decreases first sharply partial pressure of 31 mbar, i.e., with increasing pO22 the electrical conductivity decreases first sharply with aa huge hugeconductivity conductivitydecrease decreasebyby more than a half decade slightly at higher pO2 . with more than a half decade andand thenthen slightly at higher pO2. This This effect is more distinctive for aerosol deposited samples, since the response time for the change effect is more distinctive for aerosol deposited samples, since the response time for the change in pOin2 pOlarger compared to bulk samples, reaching stateofofequilibrium. equilibrium. The The double-logarithmic double-logarithmic 2 is larger is compared to bulk samples, notnot reaching a astate representation of of the the final final values values in in Figure Figure 88 accentuates representation accentuates this. this.

Figure representationof ofthe theelectrical electricalconductivity conductivityvs.vs.oxygen oxygenpartial partial pressure Figure 8. 8. Double Double logarithmic logarithmic representation pressure at ˝ at 900 °C for aerosol-deposited CuFeO 2 and bulk CuFeO2. 900 C for aerosol-deposited CuFeO2 and bulk CuFeO2 .

For typical semiconducting oxides, the electrical conductivity depends on the oxygen partial For typical semiconducting oxides, the electrical conductivity depends on the oxygen partial pressure acc. to Equation (9): pressure acc. to Equation (9): (9) const.∙ σ “ =const. ¨ pOO2 m (9) In mechanism may bebe deduced from thethe slope m. In aadouble-logarithmic double-logarithmicplot, plot,the theprevalent prevalentdefect defect mechanism may deduced from slope While typically slopes of m or mor= m −1/6, as they appear for the sample, can m. While typically slopes of=m+1/4 = +1/4 = ´1/6, as they appear foraerosol-deposited the aerosol-deposited sample, be by classical defect chemical means, seesee forfor instance [62–64], canexplained be explained by classical defect chemical means, instance [62–64],the theslope slopefor forthe the bulk bulk CuFeO CuFeO22 samples samplescan canonly only be be explained explained ifif one one assumes assumes that that no no equilibration equilibration has has been been settled, settled, i.e., i.e., the the final values are not equilibrium values. final values are not equilibrium values. The The abrupt abrupt change change of of the the conductivity conductivity at at around around 31 31 mbar mbar cannot cannot be be explained explained by by classical classical defect defect chemistry. chemistry. Instead, Instead, we we suggest suggest aa decomposition decomposition of of delafossite-type delafossite-type CuFeO CuFeO22 to to the the corresponding corresponding spinel spinel phase phase CuFe CuFe22O O44and andCuO, CuO,following followingEquation Equation(10) (10) 2 CuFeO2 + 1/2 O2  CuFe2O4 + CuO (10) 2 CuFeO2 ` 1{2 O2 Ñ CuFe2 O4 ` CuO (10) According to the Ellingham diagram of CuFeO2, this phase change occurs at a pO2 = 30 mbar at According to CuFeO the Ellingham diagram of CuFeO2 ,CuFe this phase change being occursinatagreement a pO2 = 30with mbar at 2 is a p-type semiconductor, 2O4 is n-type, our 900 °C [65]. While ˝ C [65]. While CuFeO is a p-type semiconductor, CuFe O is n-type, being in agreement with 900 conductivity vs. pO2 data [66]. Such a decomposition reaction 2could also explain the different distinct 2 4 our conductivity vs. pO reaction the different conductivity changes between bulkSuch anda decomposition aerosol deposited films.could Sincealso theexplain bulk samples are 2 data [66]. distinct conductivity changesdiffusion betweenisbulk andslower, aerosoland deposited the bulk samples are considerably thicker, oxygen by far a mixedfilms. phaseSince consisting of CuFeO 2 and considerably thicker, oxygen is simultaneously. by far slower, and a mixed phase consisting of CuFeO and CuFe 2O4 as well as CuO maydiffusion be present XRD measurements on samples that 2have CuFe2processed O4 as wellunder as CuO be present simultaneously. XRD measurements samples that have been 5%may oxygen also support these assumptions since the on XRD pattern clearly showed a mixed phase consisting of both CuFe2O4 and CuO (Figure 9). No CuFeO2 was found since the sample was exposed to the 5% O2 atmosphere for a long time (over several hours), so no evidence

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been processed under 5% oxygen also support these assumptions since the XRD pattern clearly showed a mixed2016, phase consisting of both CuFe2 O4 and CuO (Figure 9). No CuFeO2 was found since the sample Materials 9, 227 10 of 16 was exposed to the 5% O2 atmosphere for a long time (over several hours), so no evidence on the transition phase could be could obtained. order toIn elucidate thiselucidate mechanism particular, measurements on the transition phase be In obtained. order to thisinmechanism in particular, of the Seebeck coefficient were conducted. measurements of the Seebeck coefficient were conducted.

Figure 9. XRD XRD pattern pattern of of aa sample sample measured measured under under 5% 5% oxygen oxygen for for several several hours hours with with a reference O44(JPCD (JPCD34-0425) 34-0425)and andCuO CuO(JPCD (JPCD39-0629). 39-0629). pattern of CuFe22O

3.3. Thermoelectric Thermoelectric Properties Properties of Aerosol Deposited and Bulk CuFeO22 Astonishingly, the Seebeck coefficient of bulk CuFeO22 is is inferior inferior compared compared to aerosol deposited CuFeO22 at low pO 2 . This discrepancy cannot be explained in the manner at low pO2 . This discrepancy cannot be explained in the manner described described for for the electrical conductivity, sincethe thethermopower thermopower is independent of geometry the geometry the interconnection of conductivity, since is independent of the (here(here the interconnection of grains grains and ceramic layers) and the reduced mobility caused by the high microstrains. Since this and ceramic layers) and the reduced mobility caused by the high microstrains. Since this behavior behavior is not fully understood, and to elucidate the of change of the conduction mechanism from pis not fully understood, and to elucidate the change the conduction mechanism from p-type to type toatn-type 2 > 31.6 mbar, detailed measurements of dependency the oxygen of dependency of the n-type pO2 > at 31.6pO mbar, detailed measurements of the oxygen the thermopower ˝ C of both aerosol thermopower were conducted. 10 shows the Seebeck at 900 °C of both CuFeO aerosol2 were conducted. Figure 10 showsFigure the Seebeck coefficient at 900 coefficient deposited deposited CuFeO22as and bulk CuFeO 2 as a function of oxygen partial pressure. and bulk CuFeO a function of oxygen partial pressure.

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500

aerosol deposited sample 400

S / µV/K

300 200

bulk sample

100 0 -100 -200

-3

-2

-1

0

log(pO2 / bar) Figure 10. Seebeck Seebeck coefficient of aerosol-deposited with varying varying oxygen oxygen Figure aerosol-deposited CuFeO22 and bulk CuFeO22 with ˝ C. partial pressure at 900 °C. partial

With With increasing increasing oxygen oxygen partial partial pressure, pressure, the the Seebeck Seebeck coefficient coefficient of of bulk bulk CuFeO CuFeO22 declines declines slightly slightly up to a pO 2 of 31.6 mbar, thereafter dropping faster with a slope of −115 µV/K per decade pO2. It up to a pO2 of 31.6 mbar, thereafter dropping faster with a slope of ´115 µV/K per decade pO2 . becomes even negative atatpO 2 = 1 bar (100% O2 in the gas), indicating n-type conductivity. Aerosol It becomes even negative pO 2 = 1 bar (100% O2 in the gas), indicating n-type conductivity. Aerosol deposited coefficient of of +425 +425 µV/K µV/K up deposited CuFeO CuFeO22 shows shows an an always always constant constant Seebeck Seebeck coefficient upto toan anoxygen oxygen partial partial pressure of 31.6 mbar. However, in contrast to bulk CuFeO 2, the transition from the p-type to n-type pressure of 31.6 mbar. However, in contrast to bulk CuFeO2 , the transition from the p-type to n-type conductivity mechanismoccurs occurs sharper aerosol-deposited CuFeO 2, resulting in a negative conductivity mechanism sharper for for aerosol-deposited CuFeO 2 , resulting in a negative Seebeck Seebeck coefficient of S µV/K = −100atµV/K at 0.1 pObar, 2 = 0.1 bar, which persists at this value up to an oxygen coefficient of S = ´100 pO2 = which persists at this value up to an oxygen partial partial pressure of 1 bar. pressure of 1 bar. The The changing changing sign sign of of the the Seebeck Seebeck coefficient coefficient supports supports our our assumption assumption of of aa phase phase transition transition of of CuFeO 2 to CuFe2O4 and CuO with increasing oxygen partial pressure. Since oxygen equilibration CuFeO2 to CuFe2 O4 and CuO with increasing oxygen partial pressure. Since oxygen equilibration kinetics kinetics of of aerosol aerosol deposited deposited CuFeO CuFeO22 samples samples is is much much faster faster compared compared to to bulk bulk CuFeO CuFeO22,, the the transition transition appears measurement cycle, whereas bulk bulk CuFeO CuFeO2 appears more more pronounced, pronounced, being being completed completed within within one one measurement cycle, whereas 2 supposedly exhibits a phase mixture of both CuFeO 2 (p-type), CuFe2O4 (n-type) and CuO (p-type), supposedly exhibits a phase mixture of both CuFeO2 (p-type), CuFe2 O4 (n-type) and CuO (p-type), resulting resulting in in ambiguous, ambiguous, bipolar bipolar thermoelectric thermoelectric effects. effects. With With two two types types of of charge charge carriers carriers present, present, the the Seebeck coefficient of the material is the weighted average of the Seebeck coefficients associated to Seebeck coefficient of the material is the weighted average of the Seebeck coefficients associated to the the different charge carriers as described by Equation (11): different charge carriers as described by Equation (11): σn Sn +σp Sp S =σn Sn ` σp Sp (11) σn +σp S“ (11) σn ` σp with the Seebeck coefficients of the materials with different charge carrier types, Sn,p, and their with the Seebeck coefficients of the materials with different chargeincarrier Sn,pSeebeck , and their electrical electrical partial conductivities, σn,p , respectively [31]. Keeping mind types, that the coefficients partial conductivities, σ , respectively [31]. Keeping in mind that the Seebeck coefficients of the n-type n,pphases have opposite signs, the weighted Seebeck coefficient of a bipolar of the n-type and p-type and p-type phases opposite signs, the weighted coefficient of a bipolar materials. thermoelectric thermoelectric can have be small compared to the purelySeebeck n-type or p-type conducting The can be small compared to the purely n-type or p-type conducting materials. The measurements of the measurements of the Seebeck coefficient of aerosol deposited CuFeO2 indicate that at a pO2 < 31.6 Seebeck coefficient of aerosol deposited CuFeO indicate that at a pO < 31.6 mbar the prevailing phase 2 mbar the prevailing phase is CuFeO2 with a high thermopower of 2+425 µV/K. When increasing the is CuFeO with a high thermopower of +425 µV/K. When increasing thedecomposing pO2 , bipolar effects 2 pO2, bipolar effects occur in the transition region, due to the mixture of the CuFeOoccur 2 and the emerging CuFe2O4 and CuO phases. At high pO2, the transformation ends and the thermoelectric

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in the transition region, due to the mixture of the decomposing CuFeO2 and the emerging CuFe2 O4 and CuO phases. At high pO2 , the transformation ends and the thermoelectric measurements Materials 2016, 9, 227 12 ofindicate 16 the prevailing n-type CuFe2 O4 phase. For bulk CuFeO2 , this effect arises much more slowly, resulting indicate the prevailing CuFetransformation 2O4 phase. For bulk CuFeO2, this effect arises much in a measurements broader bipolar transition region,n-type and the is not finished at high pO2 within more slowly, resulting in a broader bipolar transition region, and transformation is notof finished the measurement cycle, resulting in a bipolar thermopower andthe a Seebeck coefficient ´15 µV/K at high pO2 within the measurement cycle, resulting in a bipolar thermopower and a Seebeck compared to ´120 µV/K for aerosol-deposited CuFeO2 at pO2 = 1 bar. In fact, it is believed that the bulk coefficient of −15 µV/K compared to −120 µV/K for aerosol-deposited CuFeO2 at pO2 = 1 bar. In fact, sample with a thickness of 500 µm does not reach an equilibrium within half an hour. If one assumes it is believed that the bulk sample with a thickness of 500 µm does not reach an equilibrium within an oxygen diffusion-controlled, equilibration kinetics to beequilibration proportional to half ankinetic hour. Ifthat one is assumes an oxygen kineticone thatfinds is diffusion-controlled, one finds the square of the thickness of the smallest geometry. In other words, the equilibration kinetics of kinetics to be proportional to the square of the thickness of the smallest geometry. In other words, 2 « 202 « 400. Hence, both the the AD sample should be faster by a factor of (d /d ) the equilibration kinetics of the AD sample should be faster AD by asample factor of (dbulk,sample/dAD sample)² ≈ 20² ≈ bulk,sample 400. Hence,and both theconductivity thermopowervalues and the values appear of the bulk samples appear to be thermopower the ofconductivity the bulk samples to be nonequilibrium values nonequilibrium and therefore always lie “between” the AD curves. sinceofthe and therefore alwaysvalues lie “between” the AD curves. Nevertheless, since the Nevertheless, detailed process aerosol detailedhas process of aerosol deposition has not yet fully understood, consequences of the deposition not yet been fully understood, thebeen consequences of the the room temperature impact room temperature impact consolidation on the thermoelectric properties, especially the diverging consolidation on the thermoelectric properties, especially the diverging Seebeck coefficient of bulk and Seebeck coefficient of bulk and AD processed samples at a pO2 < 31.6 mbar, remains an open-ended AD processed samples at a pO2 < 31.6 mbar, remains an open-ended question for further investigations. question for further investigations. Being of interest as high temperature thermoelectric material, the electrical conductivity and Being of interest as high temperature thermoelectric material, the electrical conductivity and ˝ C. Figure 11 shows the power factor Seebeck coefficients were investigated uptoto900 900°C. Seebeck coefficients were investigatedatattemperatures temperatures up Figure 11 shows the power factor (PF) of both aerosol deposited CuFeO processedbulk bulk CuFeO a maximum 2 2and 2 , exhibiting (PF) of both aerosol deposited CuFeO and standard standard processed CuFeO 2, exhibiting a maximum ˝ of PFof=PF59= µW/(K²¨m) 800°CCfor for aerosol deposited CuFeO and PFµW/(K²∙m) = 130 µW/(K²¨m) 59 µW/(K²∙m)atatTT = = 800 aerosol deposited CuFeO 2 and 2PF = 130 for bulk for 2, 2featuring the the same magnitude like other e.g., Ca3Co4e.g., O9 with bulk CuFeO CuFeO , featuring same magnitude likeoxide otherthermoelectrics, oxide thermoelectrics, Ca3 Co4 O9 with 225 µW/(K²∙m) 810µW/(K²¨m) µW/(K²∙m) for NaNa xCoO 2 [20].[20]. PF = PF 225= µW/(K²¨m) oror PFPF= =810 fordoped doped x CoO 2 140

bulk sample

PF / (µW/K²m)

120 100 80 60 40

aerosol deposited sample

20 0

200

300

400

500

600

700

800

900

T / °C Figure 11. Power factor (PF) ofof aerosol-deposited andbulk bulk CuFeO are guides Figure 11. Power factor (PF) aerosol-deposited CuFeO CuFeO22and CuFeO 2. The lineslines are guides for for 2 . The the eye the only. eye only.

4. Conclusions 4. Conclusions In the present study, the novel aerosol deposition method (ADM) was successfully employed to In the present study, the novel aerosol deposition method (ADM) was successfully employed fabricate dense and crack-free ceramic layers of several microns from the undoped p-type to fabricate dense and crack-free ceramic layers of several microns from the undoped p-type thermoelectric CuFeO2 at room temperature with no further heat treatment, thus avoiding thermoelectric at room temperature with nooffurther treatment, thus avoiding interactions interactionsCuFeO with 2the substrate or the influence sinter heat additives. By employing the aerosol with deposition the substrate or the influence of sinter additives. By employing the aerosol deposition method, method, measurements could be performed on very thin films enabling very fast

measurements could be performed on very thin films enabling very fast responses. Since the oxygen

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partial pressure plays a decisive role during the synthesis and application of Delafossites, XRD studies confirmed that a lowly oxidizing calcination atmosphere is essential for the preparation of single phase CuFeO2 . The process window, however, is small since at higher oxygen partial pressures, pO2 > 30 mbar at 900 ˝ C, a phase transition from CuFeO2 to the spinel-type CuFe2 O4 and CuO occurs. Astonishingly, we observed a sudden change of conduction from p-type to n-type at an oxygen partial pressure of pO2 = 30 mbar. While the electronic structure of CuFeO2 can be calculated by an enhanced local spin density approximation [67], this change in the conduction mechanism at a defined oxygen partial pressure has not been observed yet. Investigations on changing valance states of the copper and iron sites in CuFeO2 were also conducted in order to establish a defect chemical model [68]. However, we propose that the change is based, for instance, (at least partly) upon a phase transition from p-type semiconducting CuFeO2 to n-type CuFe2 O4 and CuO, resulting in a bipolar thermoelectric material. While the thermoelectric properties of the n-type phase are inferior to the p-type CuFeO2 , this material system can be of interest for use in thermoelectric generators, since both p-type and n-type materials can be precisely tailored only by defined process conditions based on the identical starting thermoelectric material. Nevertheless, detailed defect chemical investigations, particularly more measurements of electric transport parameters combined with other non-electrical analytical means, need to be conducted at defined and especially low-oxygen partial pressures in order to develop a comprehensive defect model of CuFeO2 . The measurements shown in this study may serve as an initial basis. Furthermore, the influence of dopants needs to be studied to tailor the thermoelectric properties, and detailed measurements on the thermal conductivity of thin aerosol-deposited films deserve further investigation since the reduction in grain size, resulting from the room temperature impact consolidation effect, could lead to a reduction of the thermal conductivity of CuFeO2 , probably due to increasing phonon scattering at grain boundaries thereby increasing the thermoelectric performance of delafossites. Acknowledgments: The authors are indebted to the following persons and organizations for supporting this work: A. Mergner (Department for Functional Materials) and M. Heider (BIMF) for SEM sample preparation and characterization. This publication was funded by the German Research Foundation (DFG) and the University of Bayreuth in the funding program “Open Access Publishing”. Author Contributions: Ralf Moos, Jörg Exner, Michael Schubert and Thomas Stöcker planned the experiments. Maximilian Streibl prepared the aerosol deposited samples. Ralf Moos supervised the study. Thomas Stöcker conducted the experiments and evaluated the data. All authors contributed to the article. Conflicts of Interest: The authors declare no conflict of interest.

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