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Impact of densification on microstructure and transport properties of CaFe5O7
Solid State Sciences 54 (2016) 54e58
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Impact of densification on microstructure and transport properties of CaFe5O7 bert a, V. Hardy a, Y. Bre ard a, R. Maki b, T. Mori b, D. Pelloquin a, * C. Delacotte a, S. He a b
Laboratoire CRISMAT ENSICAEN UMR CNRS 6508, 6 Boulevard du Mar echal Juin, Caen Cedex 14050, France National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2015 Received in revised form 6 November 2015 Accepted 12 November 2015 Available online 22 November 2015
Monophasic CaFe5O7 ceramic has been synthesized by solid state route. Its microstructural features have been studied by diffraction techniques and electron microscopy images before and after Spark Plasma Sintering (SPS) annealings. This work is completed by measurements of electrical and thermal properties. Especially, attention is focused around the structural and electronic transition at 360 K for which specific heat measurements have revealed a sharp peak. Densification by SPS techniques led to a significant improvement of electrical conductivity above 360 K. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Ferrite Transmission electron microscopy Spark plasma sintering Seebeck coefficient
1. Introduction Research of inexpensive oxide systems with interesting electronic properties around the room temperature is an exciting challenge for potential applications. The recent multiferroic properties reported in the iron based systems such as BiFeO3-d [1] or LnFe2O4 [2] are promising and encourage us to explore other iron based systems. Some recent structural investigations in the CaeFeeO system, especially about the CaFe5O7 ferrite, have highlighted some complex physical and structural properties depending on temperature [3,4]. Especially we have revealed a drastic change in the magnetic susceptibility and electric measurements around 360 K associated to a reversible monocliniceorthorhombic transi€ssbauer spectroscopy tion. In the CaFe5O7 structure, recent Mo measurements have demonstrated that three kinds of Fe species (Fe2þ/Fe2þε/Fe3þ) coexist in an ordered manner at RT [4] below 360 K. In parallel, the nanostructural analyses performed from HREM-HAADF techniques evidence numerous extended FeO defects inside the as-prepared CaFe5O7 bulk material [4]. The impact of this structural transition on transport has so far been investigated by resistivity measurements, but these measurements are highly sensitive to disorder, and the presence of FeO defects can give extrinsic contribution. The Seebeck coefficient is
* Corresponding author. E-mail address: [email protected] (D. Pelloquin). http://dx.doi.org/10.1016/j.solidstatesciences.2015.11.006 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
less sensitive to disorder and can be a good probe to analyze the impact of this charge ordering phenomena. We have thus decided to investigate the Seebeck coefficient in this oxide. In order to study the evolution of FeO defects and to be able to measure transport properties in samples as dense as possible, this sample has been annealed by various Spark Plasma Sintering (SPS) process. Furthermore, there has been increasing interest in research of thermoelectric materials, because of possibilities for useful energy conversion of waste heat [5]. There is a large incentive to develop thermoelectric materials which can function at mid to high temperatures for concentrated sources especially suitable for solid state conversion. To this end, high temperature compounds such as boron carbide [6], Si:Ge [7], borides [8e10], oxides [11e15], silicides [16,17] are being extensively studied. Oxides are attractive for their potential to be used in air without covers or coating necessary for protection from oxidation, and it is of high interest to characterize the thermoelectric properties of unexplored oxides. This paper deals with the microstructural evolution after postsynthesis densification process and the impact of the latter on the defects and the electronic properties of CaFe5O7. 2. Experimental details Polycrystalline sample of CaFe5O7 was obtained by solid state reaction. Starting materials CaO, Fe2O3 and Fe powders were mixed and grounded according to the 1:2:1 ratios. Then these mixtures were pressed at about 2 tons/cm2 using cold isostatic press (CIP)
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
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Table 1 Process parameters of CaFe5O7 synthesis. Experiments
Without SPS SPS 1 SPS 2
Induction furnace sintering
SPS
Atmosphere
Temperature ( C)
Hold time (h)
Temperature ( C)
Hold time (min)
Relative Density (%)
vacuum vacuum vacuum
1000 1000 1000
20 20 20
e 750 850
e 10 15
65 75 97
and sintered under vacuum for 20 h at 1000 C using an induction furnace and a thermal ramp of 100 C min1 on the way up and in room temperature return. After checking purity of the sample by Xray powder diffraction, these powders were pressed at 50 MPa by using spark plasma sintering (SPS) in order to produce dense samples for transport measurements. Samples as pellets with 10 mm diameter were pressed at different temperatures. Synthesis conditions are summarized in Table 1. The quality of the samples was checked at each step of the process by X-ray powder diffraction measurements using Rigaku Ultima-3 diffractometer working with Cu Ka radiation. Structural models were refined by Rietveld method using the FULLPROF software [18]. The cell parameters evolution around 360 K of SPSsamples has been refined from powder X-ray diffraction data collected using Panalytical XpertPro diffractometer equipped with TTK450 thermal chamber working with Co Ka radiation. Electrical resistivity and Seebeck coefficient were measured with ULVAC ZEM-2 by using the four probe method and differential method, respectively. These measurements have been carried out with contacts in plane. To determine thermal conductivity, the thermal
Fig. 1. Experimental powder XRD patterns recorded for (a) as-prepared and (b) SPS2 CaFe5O7 samples. First line is related to the indexed peaks in the monoclinic cell (Table 2) and the second one is related to cubic FeO impurity (Fm 3 m).
diffusivity coefficients and relative specific heat were measured by the laser flash method (ULVAC TC-7000). Low temperature thermal conductivity was measured with the Thermal Transport Option in a Quantum Design PPMS. Microstructural observations were carried out jointly by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This work has been performed with ZEISS Field Emission Gun and FEI TECNAI 30UT microscopes operating at 3 kV and 300 kV respectively and equipped with an energy dispersive X-ray spectroscopy (EDXS) analyzer. 3. Results and discussion Phase purities of the synthesized materials were first characterized by X-ray powder diffraction (XRPD). The samples treated with and without SPS processing exhibit similar XRPD patterns like shown in Fig. 1. Whatever the thermal treatment, cubic FeO phase is detected as impurity. So SPS densification seems to have no drastic impact on CaFe5O7 crystalline structure. This point is confirmed by the electron diffraction (E.D.) study since a similar monoclinic cell with extra dots [3] can be deduced from the E.D. patterns whereas the EDS measurements lead to an average cation composition in agreement with the nominal ones. However a weak cell parameters evolution can be observed. Lattice parameters obtained from structural refinements are given in Table 2. Values obtained for each samples show that SPS post-synthesis process induces small changes: a parameter decreases while the b stacking parameter increases. No significant deviation is observed between both samples “without SPS” and “SPS 1”. These results lead to a small diminution of the cell volume for an increase of densification which operates mainly along the channels axis a. In the same way, a precise comparison of FWHM (hkl) peaks shows homogeneous values as the index values increase for the SPS2 sample, whereas two (h0l) and (0k0) sets can be discriminated for the assynthesized sample. In the latter, this point can be directly correlated to numerous stacking faults detected on the as-prepared sample along b direction, corresponding to the stacking sequence, by HREM images analyses (Fig. 2a). Similar HREM observations have been performed on SPS 97% sample (Fig. 2b) and the disappearance of defects is in agreement with this interpretation. Note that the disappearance of FeO extended defects does not impact the structural monocliniceorthorhombic transition at T ¼ 360 K since similar lattice evolution versus T is observed as reported for the as prepared CaFe5O7 sample [4]. EDXS analyses performed on all samples evidenced a good agreement with the starting cation ratio.
Table 2 Lattice parameters of CaFe5O7 refined with the P21/m space group at RT. Cell parameters
Single crystal Cmcm [19]
Without Post-synthesis SPS
SPS 1 Densification 75%
SPS 2 Densification 97%
a (Å) b (Å) c (Å) b ( ) Volume (Å3)
3.052 10.041 17.966 90 550.57
3.0516 17.9596 5.2504 107.103 275.03
3.0503 17.9665 5.2519 107.074 275.13
3.0462 17.9709 5.2517 107.031 274.89
(1) (7) (2) (2) (2)
(1) (3) (1) (1) (1)
(1) (3) (1) (1) (1)
56
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
Fig. 2. (a) [101] HREM image, recorded on the as-prepared sample, evidences extended defects areas along the stacking sequence. (b) [001] HREM image with corresponding FFT, recorded on SPS 97% sample, shows missing of defects. Crystal structure of CaFe5O7 with the same [001] orientation is also shown.
However, experimental standard deviation (ESD) value is lower after the densification process which is in agreement with the significant decrease of defects deduced from HREM observations. Fig. 3 shows surfaces of samples and evolution of the CaFe5O7 microstructure depending on the densification quality. It evidences absence of porosity for the most densified compound (Fig. 3b) compared to the standard sintered one (Fig. 3a). The electrical resistivity r depending on temperature in the range 300e900 K is plotted in Fig. 4a. All samples exhibit a decrease of electrical resistivity with increasing temperature and a clear regime change around 350e360 K. A very dramatic drop in the resistivity is observed. Such transition in the electronic transport corroborates the first physical properties studies of CaFe5O7 compound [4]. In this recent paper, we have highlighted a structural transition from monoclinic to orthorhombic symmetry at 360 K associated to a new distribution of charges of iron species. Magnetic susceptibility c depending on temperature has been also characterized for all samples and confirms our previous analyses [4] by presence of a sharp peak of c at 360 K. With temperature increasing, electrical resistivity is reduced and the ratio between densified and pristine compounds is close to 3. Clearly the increase of densification has a positive impact on transport with a significant reduction of electrical resistivity. The decrease of FeO inclusions inside the CaFe5O7 matrix could be also considered since the transport properties of related Fe1-dO oxides exhibit insulating behaviour [20] but this effect is hard to quantify. Fig. 4b shows the temperature dependence of the Seebeck coefficient. The three samples show a positive value below T ¼ 500 K and a negative value above that temperature, that indicates a p/n crossover-type behavior with the dominant carriers shifting from holes to electrons. Note that the observed cusp value ranges from 360 to 325 K after SPS annealing. This feature can be induced by a texturation effect modifying the material anisotropy. However, an interesting behavior is once again seen at 350e360 K with a discontinuous variation in the Seebeck coefficient. This illustrates the abrupt change in the electronic structure around the structural transition. The magnitude of the Seebeck coefficient is relatively constant for all samples (Fig. 4b), showing again that the Seebeck coefficient is not too sensitive to disorder or porosity. Striking behavior is once again observed around the transition for the thermal properties also. As expected for a phase transition with entropy change, there is a peak in the specific heat with a maximum at the temperature of 360 K (Fig. 5a). At high
temperature, measured specific heat values tend to 365 J K1 mol1, in agreement with the Dulong-Petit law (Cp(calc) ¼ 324 J K1 mol1). The temperature dependence of the thermal diffusivity coefficient is very interesting (Fig. 5b) with an abrupt reduction like an inverted spike at 320 Ke360 K. It can be observed that the
Fig. 3. Scanning electron microscopy images showing microstructure of CaFe5O7 (a) without SPS processing, and (b) after densification by SPS with a relative density of 97%.
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
57
Fig. 4. (a) Resistivity and (b) Seebeck coefficient measurements of CaFe5O7 compounds.
Fig. 5. (a) Specific heat and (b) thermal diffusivity measurements versus temperature for the most densified sample (97% density).
magnitude of the thermal diffusivity appears to be similar at temperatures below and above this inverted spike, i.e. similar for the two different crystal structures. However, by measuring in detail close to the transition, we can observe the dynamics of the structural change with a softening occurring around the transition. The total thermal conductivity can be divided into two parts (Fig. 6), one originating from the electrical conductivity
-proportional to it through the Lorenz number- and the other one originating from the heat carrying phonons. Note that in our case, the main part of the thermal conductivity is due to phonons. Indeed, at 800 K, the electronic part is lower than 1% of the total thermal conductivity. There is a clear effect of the porosity on the values of thermal conductivity, with a strong increase of k in the most densified sample. At 800 K, thermoelectric figure of merit ZT values are 8.8$104, 1.9$103 and 1.1$103 for “without SPS”, “SPS 1” and “SPS 2” samples respectively. Surprisingly, the best value is obtained for sample SPS 1 with 75% density. This is due to the slightly larger Seebeck coefficients at high temperatures -assumedly due to minute composition changes- and lower thermal conductivity due to the low density, outweighing the reduction in electrical conductivity. In any case, the present material is not a high performance thermoelectric material since the best thermoelectric material oxide exhibits a ZT value close to 0.4 at 1273 K for n -type [21] and close to 1 at 800 K for p-type [22]. Anyway the p/n crossover etype behavior observed around the structural transition at 360 K is interesting. 4. Concluding remarks
Fig. 6. Thermal conductivity evolution depending on densification process.
Post-synthesis densification process has been performed on CaFe5O7 oxide with success. Using Spark Plasma Sintering (SPS) technique, a densification close to 97% is obtained and has allowed to improve the electrical conductivity by a magnitude of 3 above the structural transition at T ¼ 360 K. We have observed interesting electrical and thermal property changes around that structural
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C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
phase transition. Especially, a clear and sharp peak is observed in specific heat measurements recorded before and after SPS annealing. More interestingly, such an annealing sets to a decrease of FeO inclusions and lead to a homogeneous CaFe5O7 structure without modifying its structural transition temperature and its intrinsic physical properties. At the phonon scale, it is interesting to observe the behavior of the thermal diffusivity and its softening around the structural transition at 360 K. Previously, softening of the bulk modulus has been observed in barium titanate near the Curie point [23]. Acknowledgments The authors acknowledge the financial support of the French Agence Nationale de la Recherche (ANR), through the program “Investissements d’Avenir” (ANR-10-LABX-09-01), LabEx EMC3. The authors thank to Dr. S. Gascoin and X. Larose for technical support in SEM and TEM analyses respectively. References [1] A. Lubk, M.D. Rossell, J. Seidel, Y.H. Chu, R. Ramesh, M.J. Hytch, E. Snoeck, Nano Lett. 13 (2013) 1410e1415. [2] J. Van den Brink, D.I. Khomskii, J. Phys. Condens. Matter 20 (2008) 434217. ard, D. Pelloquin, Key Eng. Mater. 617 (2014) [3] C. Delacotte, F. Hüe, Y. Bre 237e240.
ard, S. He bert, O. Pe rez, V. Caignaert, J.M. Greneche, [4] C. Delacotte, F. Hüe, Y. Bre D. Pelloquin, Inorg. Chem. 53 (2014) 10171e10177. [5] Thermoelectric nanomaterials, in: K. Koumoto, T. Mori (Eds.), Springer Series in Materials Science, Springer, Heidelberg, 2013. [6] C. Wood, D. Emin, Phys. Rev. B 29 (1984) 4582. [7] D.M. Rowe, J. Power Sources 19 (1987) 247. [8] T. Mori, T. Nishimura, J. Solid State Chem. 179 (2006) 2908. [9] T. Mori, D. Berthebaud, T. Nishimura, A. Nomura, T. Shishido, K. Nakajima, Dalton Trans. 39 (2010) 1027. Selected as a Hot Article. [10] S. Maruyama, Y. Miyazaki, K. Hayashi, T. Kajitani, T. Mori, Appl. Phys. Lett. 101 (2012) 152101. [11] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [12] H. Ohta, W.S. Seo, K. Koumoto, J. Am. Ceram. Soc. 79 (1996) 2193. [13] M. Ohtaki, K. Araki, K. Yamamoto, J. Electron. Mat. 38 (2009) 1234. [14] I. Terasaki, in: K. Koumoto, T. Mori (Eds.), Thermoelectric Nanomaterials, Springer Series in Materials Science, Springer, Heidelberg, 2013. bert, V. Hardy, R. Fre sard, Ch Simon, A. Maignan, Phys. Rev. [15] P. Limelette, S. He Lett. 97 (2006) 046601. [16] E.N. Nikitin, V.G. Bazanov, V.I. Tarasov, Sov. Phys. Sol. State 3 (1961) 2648. [17] J.V. Zaikina, T. Mori, K.A. Kovnir, U. Schwarz, Yu Grin, A. Shevelkov, Chem. Eur. J. 16 (2010) 12582e12589. [18] J. Rodriguez-Carvajal, “FULLPROF: A Program for Rietveld Refinement and Pattern Matching Analysis”, Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the IUCr, 1990, p. 127. Toulouse, France. [19] O. Evrard, B. Malaman, F. Jeannot, A. Courtois, H. Alebouyeh, R.J. Gerardin, Solid State Chem. 35 (1980) 112e119. [20] H.C. Chen, et al., J. Phys. Chem. Solids 43 (1982) 991. [21] D. Berardan, E. Guilmeau, A. Maignan, B. Raveau, Solid State Commun. 146 (2008) 97e101. [22] K. Fujita, T. Mochida, K. Nakamura Jpn, J. Appl. Phys. 40 (2001) 4644. [23] L. Dong, D.S. Stone, R.S. Lakes, Phil. Mag. Lett. 90 (2010) 23e33.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Impact of densification on microstructure and transport properties of CaFe5O7 bert a, V. Hardy a, Y. Bre ard a, R. Maki b, T. Mori b, D. Pelloquin a, * C. Delacotte a, S. He a b
Laboratoire CRISMAT ENSICAEN UMR CNRS 6508, 6 Boulevard du Mar echal Juin, Caen Cedex 14050, France National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2015 Received in revised form 6 November 2015 Accepted 12 November 2015 Available online 22 November 2015
Monophasic CaFe5O7 ceramic has been synthesized by solid state route. Its microstructural features have been studied by diffraction techniques and electron microscopy images before and after Spark Plasma Sintering (SPS) annealings. This work is completed by measurements of electrical and thermal properties. Especially, attention is focused around the structural and electronic transition at 360 K for which specific heat measurements have revealed a sharp peak. Densification by SPS techniques led to a significant improvement of electrical conductivity above 360 K. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Ferrite Transmission electron microscopy Spark plasma sintering Seebeck coefficient
1. Introduction Research of inexpensive oxide systems with interesting electronic properties around the room temperature is an exciting challenge for potential applications. The recent multiferroic properties reported in the iron based systems such as BiFeO3-d [1] or LnFe2O4 [2] are promising and encourage us to explore other iron based systems. Some recent structural investigations in the CaeFeeO system, especially about the CaFe5O7 ferrite, have highlighted some complex physical and structural properties depending on temperature [3,4]. Especially we have revealed a drastic change in the magnetic susceptibility and electric measurements around 360 K associated to a reversible monocliniceorthorhombic transi€ssbauer spectroscopy tion. In the CaFe5O7 structure, recent Mo measurements have demonstrated that three kinds of Fe species (Fe2þ/Fe2þε/Fe3þ) coexist in an ordered manner at RT [4] below 360 K. In parallel, the nanostructural analyses performed from HREM-HAADF techniques evidence numerous extended FeO defects inside the as-prepared CaFe5O7 bulk material [4]. The impact of this structural transition on transport has so far been investigated by resistivity measurements, but these measurements are highly sensitive to disorder, and the presence of FeO defects can give extrinsic contribution. The Seebeck coefficient is
* Corresponding author. E-mail address: [email protected] (D. Pelloquin). http://dx.doi.org/10.1016/j.solidstatesciences.2015.11.006 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
less sensitive to disorder and can be a good probe to analyze the impact of this charge ordering phenomena. We have thus decided to investigate the Seebeck coefficient in this oxide. In order to study the evolution of FeO defects and to be able to measure transport properties in samples as dense as possible, this sample has been annealed by various Spark Plasma Sintering (SPS) process. Furthermore, there has been increasing interest in research of thermoelectric materials, because of possibilities for useful energy conversion of waste heat [5]. There is a large incentive to develop thermoelectric materials which can function at mid to high temperatures for concentrated sources especially suitable for solid state conversion. To this end, high temperature compounds such as boron carbide [6], Si:Ge [7], borides [8e10], oxides [11e15], silicides [16,17] are being extensively studied. Oxides are attractive for their potential to be used in air without covers or coating necessary for protection from oxidation, and it is of high interest to characterize the thermoelectric properties of unexplored oxides. This paper deals with the microstructural evolution after postsynthesis densification process and the impact of the latter on the defects and the electronic properties of CaFe5O7. 2. Experimental details Polycrystalline sample of CaFe5O7 was obtained by solid state reaction. Starting materials CaO, Fe2O3 and Fe powders were mixed and grounded according to the 1:2:1 ratios. Then these mixtures were pressed at about 2 tons/cm2 using cold isostatic press (CIP)
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
55
Table 1 Process parameters of CaFe5O7 synthesis. Experiments
Without SPS SPS 1 SPS 2
Induction furnace sintering
SPS
Atmosphere
Temperature ( C)
Hold time (h)
Temperature ( C)
Hold time (min)
Relative Density (%)
vacuum vacuum vacuum
1000 1000 1000
20 20 20
e 750 850
e 10 15
65 75 97
and sintered under vacuum for 20 h at 1000 C using an induction furnace and a thermal ramp of 100 C min1 on the way up and in room temperature return. After checking purity of the sample by Xray powder diffraction, these powders were pressed at 50 MPa by using spark plasma sintering (SPS) in order to produce dense samples for transport measurements. Samples as pellets with 10 mm diameter were pressed at different temperatures. Synthesis conditions are summarized in Table 1. The quality of the samples was checked at each step of the process by X-ray powder diffraction measurements using Rigaku Ultima-3 diffractometer working with Cu Ka radiation. Structural models were refined by Rietveld method using the FULLPROF software [18]. The cell parameters evolution around 360 K of SPSsamples has been refined from powder X-ray diffraction data collected using Panalytical XpertPro diffractometer equipped with TTK450 thermal chamber working with Co Ka radiation. Electrical resistivity and Seebeck coefficient were measured with ULVAC ZEM-2 by using the four probe method and differential method, respectively. These measurements have been carried out with contacts in plane. To determine thermal conductivity, the thermal
Fig. 1. Experimental powder XRD patterns recorded for (a) as-prepared and (b) SPS2 CaFe5O7 samples. First line is related to the indexed peaks in the monoclinic cell (Table 2) and the second one is related to cubic FeO impurity (Fm 3 m).
diffusivity coefficients and relative specific heat were measured by the laser flash method (ULVAC TC-7000). Low temperature thermal conductivity was measured with the Thermal Transport Option in a Quantum Design PPMS. Microstructural observations were carried out jointly by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This work has been performed with ZEISS Field Emission Gun and FEI TECNAI 30UT microscopes operating at 3 kV and 300 kV respectively and equipped with an energy dispersive X-ray spectroscopy (EDXS) analyzer. 3. Results and discussion Phase purities of the synthesized materials were first characterized by X-ray powder diffraction (XRPD). The samples treated with and without SPS processing exhibit similar XRPD patterns like shown in Fig. 1. Whatever the thermal treatment, cubic FeO phase is detected as impurity. So SPS densification seems to have no drastic impact on CaFe5O7 crystalline structure. This point is confirmed by the electron diffraction (E.D.) study since a similar monoclinic cell with extra dots [3] can be deduced from the E.D. patterns whereas the EDS measurements lead to an average cation composition in agreement with the nominal ones. However a weak cell parameters evolution can be observed. Lattice parameters obtained from structural refinements are given in Table 2. Values obtained for each samples show that SPS post-synthesis process induces small changes: a parameter decreases while the b stacking parameter increases. No significant deviation is observed between both samples “without SPS” and “SPS 1”. These results lead to a small diminution of the cell volume for an increase of densification which operates mainly along the channels axis a. In the same way, a precise comparison of FWHM (hkl) peaks shows homogeneous values as the index values increase for the SPS2 sample, whereas two (h0l) and (0k0) sets can be discriminated for the assynthesized sample. In the latter, this point can be directly correlated to numerous stacking faults detected on the as-prepared sample along b direction, corresponding to the stacking sequence, by HREM images analyses (Fig. 2a). Similar HREM observations have been performed on SPS 97% sample (Fig. 2b) and the disappearance of defects is in agreement with this interpretation. Note that the disappearance of FeO extended defects does not impact the structural monocliniceorthorhombic transition at T ¼ 360 K since similar lattice evolution versus T is observed as reported for the as prepared CaFe5O7 sample [4]. EDXS analyses performed on all samples evidenced a good agreement with the starting cation ratio.
Table 2 Lattice parameters of CaFe5O7 refined with the P21/m space group at RT. Cell parameters
Single crystal Cmcm [19]
Without Post-synthesis SPS
SPS 1 Densification 75%
SPS 2 Densification 97%
a (Å) b (Å) c (Å) b ( ) Volume (Å3)
3.052 10.041 17.966 90 550.57
3.0516 17.9596 5.2504 107.103 275.03
3.0503 17.9665 5.2519 107.074 275.13
3.0462 17.9709 5.2517 107.031 274.89
(1) (7) (2) (2) (2)
(1) (3) (1) (1) (1)
(1) (3) (1) (1) (1)
56
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
Fig. 2. (a) [101] HREM image, recorded on the as-prepared sample, evidences extended defects areas along the stacking sequence. (b) [001] HREM image with corresponding FFT, recorded on SPS 97% sample, shows missing of defects. Crystal structure of CaFe5O7 with the same [001] orientation is also shown.
However, experimental standard deviation (ESD) value is lower after the densification process which is in agreement with the significant decrease of defects deduced from HREM observations. Fig. 3 shows surfaces of samples and evolution of the CaFe5O7 microstructure depending on the densification quality. It evidences absence of porosity for the most densified compound (Fig. 3b) compared to the standard sintered one (Fig. 3a). The electrical resistivity r depending on temperature in the range 300e900 K is plotted in Fig. 4a. All samples exhibit a decrease of electrical resistivity with increasing temperature and a clear regime change around 350e360 K. A very dramatic drop in the resistivity is observed. Such transition in the electronic transport corroborates the first physical properties studies of CaFe5O7 compound [4]. In this recent paper, we have highlighted a structural transition from monoclinic to orthorhombic symmetry at 360 K associated to a new distribution of charges of iron species. Magnetic susceptibility c depending on temperature has been also characterized for all samples and confirms our previous analyses [4] by presence of a sharp peak of c at 360 K. With temperature increasing, electrical resistivity is reduced and the ratio between densified and pristine compounds is close to 3. Clearly the increase of densification has a positive impact on transport with a significant reduction of electrical resistivity. The decrease of FeO inclusions inside the CaFe5O7 matrix could be also considered since the transport properties of related Fe1-dO oxides exhibit insulating behaviour [20] but this effect is hard to quantify. Fig. 4b shows the temperature dependence of the Seebeck coefficient. The three samples show a positive value below T ¼ 500 K and a negative value above that temperature, that indicates a p/n crossover-type behavior with the dominant carriers shifting from holes to electrons. Note that the observed cusp value ranges from 360 to 325 K after SPS annealing. This feature can be induced by a texturation effect modifying the material anisotropy. However, an interesting behavior is once again seen at 350e360 K with a discontinuous variation in the Seebeck coefficient. This illustrates the abrupt change in the electronic structure around the structural transition. The magnitude of the Seebeck coefficient is relatively constant for all samples (Fig. 4b), showing again that the Seebeck coefficient is not too sensitive to disorder or porosity. Striking behavior is once again observed around the transition for the thermal properties also. As expected for a phase transition with entropy change, there is a peak in the specific heat with a maximum at the temperature of 360 K (Fig. 5a). At high
temperature, measured specific heat values tend to 365 J K1 mol1, in agreement with the Dulong-Petit law (Cp(calc) ¼ 324 J K1 mol1). The temperature dependence of the thermal diffusivity coefficient is very interesting (Fig. 5b) with an abrupt reduction like an inverted spike at 320 Ke360 K. It can be observed that the
Fig. 3. Scanning electron microscopy images showing microstructure of CaFe5O7 (a) without SPS processing, and (b) after densification by SPS with a relative density of 97%.
C. Delacotte et al. / Solid State Sciences 54 (2016) 54e58
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Fig. 4. (a) Resistivity and (b) Seebeck coefficient measurements of CaFe5O7 compounds.
Fig. 5. (a) Specific heat and (b) thermal diffusivity measurements versus temperature for the most densified sample (97% density).
magnitude of the thermal diffusivity appears to be similar at temperatures below and above this inverted spike, i.e. similar for the two different crystal structures. However, by measuring in detail close to the transition, we can observe the dynamics of the structural change with a softening occurring around the transition. The total thermal conductivity can be divided into two parts (Fig. 6), one originating from the electrical conductivity
-proportional to it through the Lorenz number- and the other one originating from the heat carrying phonons. Note that in our case, the main part of the thermal conductivity is due to phonons. Indeed, at 800 K, the electronic part is lower than 1% of the total thermal conductivity. There is a clear effect of the porosity on the values of thermal conductivity, with a strong increase of k in the most densified sample. At 800 K, thermoelectric figure of merit ZT values are 8.8$104, 1.9$103 and 1.1$103 for “without SPS”, “SPS 1” and “SPS 2” samples respectively. Surprisingly, the best value is obtained for sample SPS 1 with 75% density. This is due to the slightly larger Seebeck coefficients at high temperatures -assumedly due to minute composition changes- and lower thermal conductivity due to the low density, outweighing the reduction in electrical conductivity. In any case, the present material is not a high performance thermoelectric material since the best thermoelectric material oxide exhibits a ZT value close to 0.4 at 1273 K for n -type [21] and close to 1 at 800 K for p-type [22]. Anyway the p/n crossover etype behavior observed around the structural transition at 360 K is interesting. 4. Concluding remarks
Fig. 6. Thermal conductivity evolution depending on densification process.
Post-synthesis densification process has been performed on CaFe5O7 oxide with success. Using Spark Plasma Sintering (SPS) technique, a densification close to 97% is obtained and has allowed to improve the electrical conductivity by a magnitude of 3 above the structural transition at T ¼ 360 K. We have observed interesting electrical and thermal property changes around that structural
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phase transition. Especially, a clear and sharp peak is observed in specific heat measurements recorded before and after SPS annealing. More interestingly, such an annealing sets to a decrease of FeO inclusions and lead to a homogeneous CaFe5O7 structure without modifying its structural transition temperature and its intrinsic physical properties. At the phonon scale, it is interesting to observe the behavior of the thermal diffusivity and its softening around the structural transition at 360 K. Previously, softening of the bulk modulus has been observed in barium titanate near the Curie point [23]. Acknowledgments The authors acknowledge the financial support of the French Agence Nationale de la Recherche (ANR), through the program “Investissements d’Avenir” (ANR-10-LABX-09-01), LabEx EMC3. The authors thank to Dr. S. Gascoin and X. Larose for technical support in SEM and TEM analyses respectively. References [1] A. Lubk, M.D. Rossell, J. Seidel, Y.H. Chu, R. Ramesh, M.J. Hytch, E. Snoeck, Nano Lett. 13 (2013) 1410e1415. [2] J. Van den Brink, D.I. Khomskii, J. Phys. Condens. Matter 20 (2008) 434217. ard, D. Pelloquin, Key Eng. Mater. 617 (2014) [3] C. Delacotte, F. Hüe, Y. Bre 237e240.
ard, S. He bert, O. Pe rez, V. Caignaert, J.M. Greneche, [4] C. Delacotte, F. Hüe, Y. Bre D. Pelloquin, Inorg. Chem. 53 (2014) 10171e10177. [5] Thermoelectric nanomaterials, in: K. Koumoto, T. Mori (Eds.), Springer Series in Materials Science, Springer, Heidelberg, 2013. [6] C. Wood, D. Emin, Phys. Rev. B 29 (1984) 4582. [7] D.M. Rowe, J. Power Sources 19 (1987) 247. [8] T. Mori, T. Nishimura, J. Solid State Chem. 179 (2006) 2908. [9] T. Mori, D. Berthebaud, T. Nishimura, A. Nomura, T. Shishido, K. Nakajima, Dalton Trans. 39 (2010) 1027. Selected as a Hot Article. [10] S. Maruyama, Y. Miyazaki, K. Hayashi, T. Kajitani, T. Mori, Appl. Phys. Lett. 101 (2012) 152101. [11] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [12] H. Ohta, W.S. Seo, K. Koumoto, J. Am. Ceram. Soc. 79 (1996) 2193. [13] M. Ohtaki, K. Araki, K. Yamamoto, J. Electron. Mat. 38 (2009) 1234. [14] I. Terasaki, in: K. Koumoto, T. Mori (Eds.), Thermoelectric Nanomaterials, Springer Series in Materials Science, Springer, Heidelberg, 2013. bert, V. Hardy, R. Fre sard, Ch Simon, A. Maignan, Phys. Rev. [15] P. Limelette, S. He Lett. 97 (2006) 046601. [16] E.N. Nikitin, V.G. Bazanov, V.I. Tarasov, Sov. Phys. Sol. State 3 (1961) 2648. [17] J.V. Zaikina, T. Mori, K.A. Kovnir, U. Schwarz, Yu Grin, A. Shevelkov, Chem. Eur. J. 16 (2010) 12582e12589. [18] J. Rodriguez-Carvajal, “FULLPROF: A Program for Rietveld Refinement and Pattern Matching Analysis”, Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the IUCr, 1990, p. 127. Toulouse, France. [19] O. Evrard, B. Malaman, F. Jeannot, A. Courtois, H. Alebouyeh, R.J. Gerardin, Solid State Chem. 35 (1980) 112e119. [20] H.C. Chen, et al., J. Phys. Chem. Solids 43 (1982) 991. [21] D. Berardan, E. Guilmeau, A. Maignan, B. Raveau, Solid State Commun. 146 (2008) 97e101. [22] K. Fujita, T. Mochida, K. Nakamura Jpn, J. Appl. Phys. 40 (2001) 4644. [23] L. Dong, D.S. Stone, R.S. Lakes, Phil. Mag. Lett. 90 (2010) 23e33.