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Thermoelectric transport properties of polycrystalline titanium diselenide co-intercalated with nickel and titanium using spark plasma sintering
Journal of Solid State Chemistry 197 (2013) 273–278
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Thermoelectric transport properties of polycrystalline titanium diselenide co-intercalated with nickel and titanium using spark plasma sintering T.C. Holgate a, S. Zhu b, M. Zhou b, S. Bangarigadu-Sanasy c, H. Kleinke c, J. He b, T.M. Tritt b,n a
Department of Energy Storage and Conversion, The Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA c Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 b
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Received in revised form 20 July 2012 Accepted 28 July 2012 Available online 23 August 2012
Polycrystalline samples of nickel intercalated (0–5%) TiSe2 were attempted via solid-state reaction in evacuated quartz tubes followed by densification using a spark plasma sintering process. X-ray diffraction data indicated that mixed NiSe2 and TiSe2 phases were present after initial synthesis by solid-state reaction, but a pure TiSe2 phase was present after the spark plasma sintering. While EPMA data reveals the stoichiometry to be near 1:1.8 (Ti:Se) for all samples, comparisons of the measured bulk densities to the theoretical densities suggest that the off stoichiometry is a result of the co-intercalation of both Ni and Ti rather than Se vacancies. Due to the presence of excess Ti (0.085– 0.130 per formula) in the van der Waals gap of all the samples, the sensitive electron–hole balance is offset by the additional Ti-3d electrons, leading to an increase in the thermopower (n-type) over pristine, stoichiometric TiSe2. The effects of the co-intercalation of both Ni and Ti in TiSe2 on the structural, thermal, and electrical properties are discussed herein. & 2012 Elsevier Inc. All rights reserved.
Keywords: Thermoelectric materials Thermopower Thermal conductivity Spark plasma sintering Complex chalcogenides
1. Introduction Transition metal dichalcogenides make up a relatively large group of materials with several allotropes possible. This structurally well defined group contains representatives of many classes of electronic materials: insulators, semiconductors, semimetals, metals, superconductors, etc. [1]. Titanium diselenide is an interesting semimetallic material that exhibits a charge density wave transition near 200 K that is caused by electron–hole interactions arising from the overlap of Ti-3d electron bands at the L point and the spin-orbit-split Se-4p hole bands at the zone center rather than the usual electron–electron interactions [2,3]. Formed in the layered 1T polytype, TiSe2 consists of layers of TiSe6 octahedra where the layers are inter-bonded by weak van der Waals forces (Fig. 1) [4,5]. This structure can play host to numerous types of atomic and molecular intercalates that may reside in octahedrally coordinated sites at (0, 0, ½) in the van der Waals gap [5]. Many researchers have reported on the effects of transitional metal intercalates (i.e. Fe, Co, Cu, Ni) on the structural and electronic properties of TiSe2 single crystals, however, data on the thermal transport properties of 3 and 4d intercalated bulk samples is lacking [5–7]. One exception of note is a recent report
n Correspondence to: Department of Physics and Astronomy, Clemson University, 118 Kinard Lab, Clemson, SC 29634, USA. E-mail address: [email protected] (T.M. Tritt).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.07.057
by Hor and Cava that presents the effects of intercalating Cu on the thermoelectric properties of polycrystalline TiSe2 ySy [8]. In their work, they attempted to make use of the large thermopower peak associated with the charge density wave to optimize the low temperature thermoelectric performance of the material but concluded that the resistivity was too high. In most cases, single crystalline samples are prepared by chemical vapor transport while the polycrystalline samples of most studies were prepared by a multi-step firing process of raw elemental powders in an evacuated quartz tube with intermittent grinding and cold pressing. The use of spark plasma sintering as a preparation method has been widely overlooked previously due to the success of traditional methods in achieving high density polycrystalline samples (99% for cold pressing followed by annealing [8]). The results of this study show that spark plasma sintering (SPS) may prove to be an important process due to it being both a quicker route for synthesis and a possible tool for controlling self-intercalation of excess titanium into the material.
2. Material and methods All samples were prepared by sealing the appropriate amounts of Ni (99.996% Alfa Aesar-Puratronic), Ti (99.98% Strem), and Se (99.98% Alfa Aesar) powders in evacuated quartz tubes with a subsequent firing at 650 1C for 120 h. The resulting powders were ground and the phase checked by powder X-ray diffraction
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Fig. 1. Left: Unit cell of TiSe2 with intercalation sites shown in the van der Waals gap. Right: octahedral Se–Ti–Se layers with Ni intercalated at (00½) to form a NiSe6 octahedron.
(PXRD; Rigaku MiniFlex, Cu k-a radiation). Despite all samples containing a secondary NiSe2 phase in addition to the intended TiSe2 phase, the powders were processed using spark plasma sintering (SPS) at 800 1C for 6 min under 50 MPa of pressure. Bar shaped samples were cut from the center of the resulting pellets (12.7 mm diameter) with dimensions of about 2 2 7–8 mm3. Low temperature thermal conductivity, thermopower, and electrical resistivity were all measured on the same piece for each sample using custom designed systems that are described elsewhere [9,10]. Above room temperature, the electrical resistivity and thermopower were measured using a commercial measurement system (ZEM-2, Ulvac-Riko). Post-SPS structural analysis was performed using PXRD for qualitative analysis at Clemson, and then high resolution patterns were obtained at the University of Waterloo (INEL, monochromated Cu k-a1 radiation) with subsequent refinement performed at Clemson using the MDI Jade software package. Samples were sent to the University of Georgia’s Geology Department for electron probe microanalysis (EPMA) in order to determine the stoichiometry of the samples.
Table 1 Nominal compositions and compositions experimentally determined by electron probe microanalysis, normalized to two selenium atoms per formula. NixTi1 þ ySe2 Sample
X-Nominal
X-EPMA
Y-EPMA
1 2 3 4 5 6
0 0.007 0.01 0.02 0.04 0.05
0.000 0.009 0.012 0.022 0.045 0.053
0.130 0.115 0.099 0.109 0.085 0.098
Table 2 Comparison of theoretical densities based on EPMA data normalized assuming excess Ti (Se normalized) and Se vacancies (Ti normalized). Unit cell volume was calculated for each sample based on the experimentally determined lattice parameters of each sample. Theoretical density Sample Se2 normalized (Ti self-intercalation)
3. Results and discussion 3.1. Structure and composition Table 1 presents the EPMA results and reveals that both the Ni and Ti concentrations are about 10% greater than the nominal values. The amount of excess Ti only loosely correlates to the amount of Ni as there tends to be less excess titanium when more nickel is present. In all samples, the measured Ni concentration is greater than the nominal concentration because Se was lost during the SPS process resulting in a higher Ni:Se ratio, as well as the ratio of Ti:Se being greater than one half. If the EPMA data were normalized to one Ti atom per formula (which would be to assume Se vacancies instead of Ti co-intercalation) the resulting theoretical density would be less than that of pristine TiSe2, but in the case of excess Ti—presumably in the (0, 0, ½) sites in the van der Waals gap—the resulting theoretical density would of course be higher (see Table 2). Besides reports showing that even solid state reaction synthesis in evacuated quartz tubes at temperatures above 700 1C result in excess Ti [11], the added driving force from the pressure involved with the SPS process will seek to minimize the volume of the system. Excess Ti in the van der Waals gap is therefore more likely than selenium vacancies.
1 2 4 6
Ti normalized (Se vacancies)
Formula
g/cm3
Formula
g/cm3
Ti1.13Se2 Ni0.009Ti1.115Se2 Ni0.022Ti1.109Se2 Ni0.053Ti1.098Se2
5.4 5.32 5.17 5.23
TiSe1.79 Ni0.008TiSe1.79 Ni0.019TiSe1.80 Ni0.048TiSe1.82
4.82 4.75 4.62 4.67
The densities of the samples were determined using the Archimedes method with de-ionized water as the medium. The measured and theoretical (assuming excess Ti) densities are compared in Table 3. The results agree well for the samples with less than 1% Ni, but as the Ni concentration increases the measured density becomes higher than the theoretical. This unphysical result means that either the sample is not homogeneous or the calculated lattice parameters are incorrect. In a sense, both are likely to be true. While the high resolution PXRD patterns indicate that the samples are single phase, some peaks—especially the (001) peak—are somewhat diffuse and were difficult to profile using only one peak. Successful deconvolution using two peaks (Fig. 2) indicates that the layers may be corrugated or dimpled as the intercalates—being excess Ti only (Sample 1), or both Ti and Ni (Samples 2–6)—may not be evenly and randomly distributed in the van der Waals gap.
T.C. Holgate et al. / Journal of Solid State Chemistry 197 (2013) 273–278
Table 3 Comparison of densities measured by the Archimedes’ method and calculated densities assuming excess Ti rather than Se vacancies. The uncertainty of the measured density is estimated to be less than 70.01 g/cm3 for all samples.
275
Table 4 Comparison of the measured and estimated densities based on the assumption of a corrugated, two-phase structure. The uncertainty of the measured density is estimated to be less than 7 0.01 g/cm3 for all samples.
Sample
Measured (g/cm3)
Theoretical (g/cm3)
Percent difference
Sample
Measured (g/cm3)
Estimated (g/cm3)
Percent difference
1 2 4 6
5.36 5.32 5.32 5.47
5.404 5.318 5.175 5.232
0.75% 0.03% 2.79% 4.44%
1 2 3 4 5 6
5.36 5.32 5.35 5.32 5.37 5.47
5.346 5.353 5.346 5.371 5.390 5.402
0.3% 0.7% 0.1% 0.9% 0.3% 1.2%
Fig. 2. Experimental (001) peak of Sample 1 (Ti1.130Se2) fitted with one (left) and two peaks (right). The solid blue line near 2y ¼14.751 corresponds to the indexed (001) peak of pristine TiSe2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Of the two peaks that resulted from the fitting of the (001) peak of Sample 1, the less diffuse peak is centered at 2y ¼14.7521, ˚ which gives a d-spacing of 6.006 A—a value that corresponds closely to the literature value of the c-lattice parameter of TiSe2 ˚ [13]. The lower 2-theta value of the more diffuse peak (6.008 A) corresponds to a larger van der Waals gap (2y ¼14.6721, ˚ which is expected with excess Ti residing in the c¼ 6.038 A), gap since the isostructural (when the vacancies at z ¼0.5 are considered to be part of the basis) Ti2Se2 (c¼6.301) [12] structure is reached with full intercalation. If there does exist regimes of Ti self-intercalated and pristine, un-intercalated TiSe2, the size scale of the intercalated regimes are probably small enough that the volume of the boundary between the regimes is significant, as this will result in peak broadening. If it is assumed that Ti aggregates in the van der Waals gap and creates a corrugated or dimpled layered structure, then the crystal structure of Sample 1 (Ti1.130Se2) may be represented by a two phase system consisting of crystallites of Ti2Se2 embedded in a TiSe2 matrix. Using this scenario, a theoretical density may be estimated by an additive method where the density of each phase is calculated using the pristine reference lattice parameters and their contributions weighted by the EPMA data. For the nickel containing samples, the TiSe2 phase is replaced with NixTiSe2 and the appropriate lattice parameters were calculated using Vegard’s rule from Ref. [13] data. This representation assumes that in the regions where there is no excess Ti in the van der Waals gap, the Ni is randomly distributed and there is no intermixing of Ti and Ni intercalates. The density of NixTi1 þ dSe2 is therefore estimated using the atomic masses of the elements and the unit cell volume of the constituent phases (i.e. mTi and V Ti2 Se2 ) by
rest: ¼
xmNi þð1dÞðmTi þ2mSe Þ 2dðmTi þmSe Þ þ : V Nix TiSe2 , Vegard0 s Law V Ti2 Se2
Fig. 3. Lattice (solid lines) and total (dashed lines) thermal conductivity of Samples 1, 4, and 6.
Table 4 presents a comparison of the measured and estimated densities. The percent difference for all samples is on the order of the uncertainty of the density measurements using the Archimedes method. This comparison is valid only if the packing densities of the polycrystalline pellets are very high, but as mentioned in the introduction, this material is easy to densify with relative densities of 99% being obtained in cold pressed pellets. 3.2. Thermal conductivity The effect of nickel intercalation on the thermal conductivity can be seen in the systematic suppression of the ‘‘crystalline peak’’ of lattice thermal conductivity (klatt) with increasing nickel. This low temperature peak in the lattice thermal conductivity occurs in periodic systems where the low temperature T3 dependence due to the heat capacity starts to turn over and the phonon– phonon interactions start to dominate. Then as the temperature is further increased, then the increasing population of phonon states becomes more prominent with the T 1 dependence due to the smaller phonon–electron and then predominately phonon– phonon scattering that dominates at higher temperatures. When the periodicity of the lattice is disrupted by defects, this peak can be suppressed, and when the concentration of defects is very high, the lattice is very disordered and can exhibit amorphouslike thermal conductivity in which this peak is significantly
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suppressed. As can be seen in Fig. 3, with increasing Ni, the peak in the lattice thermal conductivity between 50 and 100 K is suppressed, indicating a more disordered lattice. Near room temperature, the sample without nickel (Sample 1) shows a total thermal conductivity (ktotal ¼ klatt þ kelec) that is still decreasing with temperature and appears to fall below that of the sample with the most nickel (Sample 6). The total thermal conductivity of both nickel containing samples appear to approach saturation as the lattice approaches its minimum and the T dependence of the electronic contribution—from the Wiedemann–Franz relation:kelec ¼ L0sT, (L0 ¼ 2.44 10 8 V2/K2)—is countered by the T 1 dependence of the electrical conductivity, s ¼1/r. However, the total thermal conductivity may increase at higher temperatures due to an increase in the electronic contribution as bi-polar conduction becomes an issue (due to involvement of the minority carriers in the conduction process) and the Weidemann–Franz relation requires an additional term. While the addition of Ni may first increase the total and lattice thermal conductivity at room temperature as the (110) planes are more strongly coupled to their neighbors and the anisotropy of the grains reduced, further addition of Ni will lower the lattice thermal conductivity as the system becomes more disordered. This explanation is corroborated by the observed reduction in the crystalline peak of the lattice contribution between 50 and 100 K. It is difficult to discern without further experiments whether the reduction of the lattice thermal conductivity is due to a reduction in the phonon density of states, the mean-free-path of the phonons, or both. 3.3. Electrical transport properties Approaching room temperature, the total thermal conductivity values of both Ni containing samples rise above that of Sample 1 because of the decreased electrical resistivity and its subsequent higher electronic contribution to the thermal conductivity. Fig. 4 presents the thermopower (a), electrical resistivity (r), and power factor (PF ¼ a2T/r). The effects of intercalation on the electrical transport can be explained by its effects on the relative concentrations of both holes and electrons, as well as by the disorder it introduces. For 0% Ni, the sample shows a peak in the thermopower at about 150 K that arises from the localization of carriers in the charge density wave (CDW) state. Di Salvo et al. showed that for pristine TiSe2, this peak is much larger ( 145 mV/K) and the thermopower becomes positive near room temperature as the number of holes and electrons are nearly the same, but the mobility of the holes increases more than that of the electrons [11]. The peak and turnover in the thermopower around 500 K is additional evidence that bi-polar conduction is taking place in this system. The excess Ti in the van der Waals gap inhibits the CDW state by offsetting the electron–hole ratio in terms of numbers as well as mobilities as the interstitial Ti atoms may add states near the Fermi level (initial electronic structure calculations using the TB-LMTO-ASA [14] method indicate this to be so, but spin-orbit coupling was not taken into account). As Ni is introduced into the van der Waals gap, the low temperature thermopower is reduced. The effective carrier concentration as a function of temperature between 5 and 300 K is presented in Fig. 5, which includes reference data on pristine TiSe2. The carrier concentration is systematically shifted towards positive values with increasing Ni at low temperatures. It is possible that the effect of the Ni on the crystal field results in localization of both the conduction electrons of the Ni as well as those of nearby Ti atoms. At higher temperatures, the electrons may become itinerate and the total effective carrier concentration becomes more negative.
Fig. 4. Electronic transport properties of NixTi1 þ dSe2.
The systematic reduction of the charge density wave anomaly in the thermopower as well as the shift towards lower temperature with increasing Ni may also be explained by the disorder introduced by intercalation. Since the CDW state in TiSe2 is a result of the pairing between Ti-3d electrons and Se-4p holes,
T.C. Holgate et al. / Journal of Solid State Chemistry 197 (2013) 273–278
277
increased the power factor to more than 0.2 Wm 1 K 1 at 300 K. With the temperature dependence of the thermopower becoming more linear and the electrical resistivity decreasing with increasing Ni, the Tmax of the power factor has been shifted towards higher temperatures and the maximum power factor (PF ¼0.38 at 550 K) has been achieved in Ni0.045Ti1.085Se2 due to having one of the lowest electrical resistivity values while being able to maintain moderate thermopower values with a nearly linear temperature dependence. Based on trends in the thermal conductivity values (at 300 K) of the samples with 2.2 and 5.3% Ni, the room temperature value of the 4.5% Ni sample ought be near 4 Wm 1 K 1, and would be expected to decrease at higher temperatures due to increased phonon–phonon scattering and little evidence of bipolar conduction at high temperatures. With this data, one may estimate that the ZT at 550 K would be on the order of 0.1. Though much lower than the ZT of state of the art thermoelectric materials of this temperature regime (ZT41), the low cost, high availability and relatively low toxicity of the constituent elements may validate further investigations of intercalating multiple species into the van der Waals gap of transition metal dichalcogenides for thermoelectric applications.
4. Conclusion Fig. 5. Effective carrier concentration of Samples 1, 2, and 6 with that of pristine TiSe2 from Di Salvo et al. [11].
strains in the lattice may easily disrupt the formation of electron– hole pairs. The carrier-offset description is supported by the high temperature (above 300 K) thermopower, which shows a systematic decrease of the bipolar effect with increasing Ni. With even 5% Ni the thermopower exhibits a metallic-like trend as the CDW is almost entirely suppressed and there is no overturn in the thermopower. Above 500 K, all samples containing Ni have a higher absolute thermopower than the sample without Ni due to the compensation of holes at higher temperatures, as the trends of the effective carrier concentrations becoming more prevalent in the regime above 300 K would suggest. The resistivity curves in Fig. 4 indicate that the addition of Ni generally increases the electronic conduction in the system—though not as monotonically as would be expected when simply counting electrons per formula unit or considering the systematic dependence of the thermopower on the Ni concentration. Although, the low temperature CDW anomalies in the resistivity curves generally follow the anomalies in the thermopower curves. The temperature at which the anomalies are centered is shifted towards lower temperatures with increasing Ni, and no anomaly is observed in the sample with the most Ni. In terms of understanding the dependence of the magnitudes of the resistance anomalies on Ni concentration and the relative magnitudes of the resistance before and after the CDW transition, the determination of the carrier concentration as a function of temperature for all samples would be helpful. In general, the thermopower of a sample is more sensitive to the carrier concentration and corresponding mobility than it is to structural defects and strains, whereas the resistivity is sensitive to both the carrier concentration and the presence of scattering centers. The non-monotonic dependence of the resistivity on the concentration of Ni may be a result of the competition between the addition of disorder and defects with the increase in carriers by the addition of more Ni. The thermopower, and thus the power factor (as defined earlier), of pristine TiSe2 is roughly zero near room temperature. As can be seen in Fig. 4, the excess Ti in the van der Waals gap has
Co-intercalated NixTi1 þ ySe2 samples were prepared using a solid state reaction followed by spark plasma sintering. The result of the SPS process at such an elevated temperature (800 1C) resulted in off stoichiometry between Ti and Se, which has been interpreted as co-intercalation of the excess Ti with Ni in the van der Waals gap between the a and b planes. The introduction of excess Ti in the van der Waals gap disrupts the structural order and offsets the balance of electron and hole states in the system. The addition of Ni into the van der Waals gap compounds these effects and shifts the system from a semimetal to one exhibiting a degenerate semiconductor-like thermopower. While the ZT is still too low for thermoelectric applications, other considerations such as cost and availability, in addition to the freedom afforded by the ability to intercalate many different species, results in the validation of the study of transition metal dichalcogenides intercalated with multiple species. However, it is important to consider the reproducibility of using spark plasma sintering in achieving the desired compositions. At least one sample was duplicated in an attempt to verify the reducibility and the percent differences of the nickel and excess titanium between the two samples as measured by EMPA were 10% and 6%, respectively. While this may be sufficient, post-SPS processing in an oven with a carefully controlled atmosphere of vacuum or low pressure selenium vapor may be a way to further control the stoichiometry as the selenium content will determine the amount of self-intercalation of the titanium and the relative concentration of intercalated nickel. In the future, a more careful study of the self-intercalation of Ti by varying the SPS parameters and the subsequent characterization of the electronic transport properties would be of value. Additionally, the co-intercalation of Ni and Ti in TiSe2 with sulfur substitution on the selenium sites may be of interest as a positive bandgap may be opened and an additional control parameter introduced for potential optimization of the thermoelectric properties.
Acknowledgments The authors would like to thank Chris Fleisher at the University of Georgia for EPMA the work and Matthew Hendrix at Clemson University for assistance in sample preparation. The
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work at Clemson University is supported by DOE/EPSCoR Implementation Grant (#DE-FG02-04ER-46139), and the SC EPSCoR cost-sharing program. Financial support from NSERC, CFI, and OIT for the work at the University of Waterloo is appreciated. References [1] J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193–335. [2] O. Anderson, R. Manzke, M. Skibowski, Phys. Rev. Lett. 55 (1985) 2188–2191. [3] G. Li, W.Z. Hu, D. Qian, D. Hseih, M.Z. Hasan, E. Morosan, R.J. Cava, N.L. Wang, Phys. Rev. Lett. 99 (2007) 027404–027407. [4] J.A. Wilson, F.J. Di Salvo, S. Mahajan, Adv. Phys. 24 (1975) 117–201.
[5] R.H. Friend, A.D. Yoffe, Adv. Phys. 36 (1987) 1–94. [6] G. Wu, H.X. Yang, L. Zhao, X.G. Luo, T. Wu, G.Y. Wang, X.H. Cheng, Phys. Rev. B. 76 (2007) 024513–024517. [7] M. Sasaki, A. Ohnishi, T. Kikuchi, M. Kitaura, K. Shimada, H.-J. Kim, J. Low Temp. Phys. 161 (2010) 375–386. [8] Y.S. Hor, R.J. Cava, Mater. Res. Bull. 44 (2009) 1375–1378. [9] A.L. Pope, R.T. Littleton IV, T.M. Tritt, Rev. Sci. Instrum. 72 (2001) 3129–3131. [10] A.L. Pope, B. Zalwilski, T.M. Tritt, Cryogenics 41 (2001) 725–731. [11] F.J. Di Salvo, D.E. Moncton, J.V. Waszczak, Phys. Rev. B. 14 (1976) 4321–4328. [12] F. Gronvold, F.J. Langmyhr, Acta Chem. Scand. 15 (1961) 1949–1962. [13] Y. Arnaud, M. Chevreton, A. Ahouandjinou, M. Danot, J. Rouxel, J. Solid State Chem. 18 (1976) 9–15. [14] O.K. Andersen, O. Jepsen, Phys. Rev. Lett. 53 (1984) 2571–2574.
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Thermoelectric transport properties of polycrystalline titanium diselenide co-intercalated with nickel and titanium using spark plasma sintering T.C. Holgate a, S. Zhu b, M. Zhou b, S. Bangarigadu-Sanasy c, H. Kleinke c, J. He b, T.M. Tritt b,n a
Department of Energy Storage and Conversion, The Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA c Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 b
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Received in revised form 20 July 2012 Accepted 28 July 2012 Available online 23 August 2012
Polycrystalline samples of nickel intercalated (0–5%) TiSe2 were attempted via solid-state reaction in evacuated quartz tubes followed by densification using a spark plasma sintering process. X-ray diffraction data indicated that mixed NiSe2 and TiSe2 phases were present after initial synthesis by solid-state reaction, but a pure TiSe2 phase was present after the spark plasma sintering. While EPMA data reveals the stoichiometry to be near 1:1.8 (Ti:Se) for all samples, comparisons of the measured bulk densities to the theoretical densities suggest that the off stoichiometry is a result of the co-intercalation of both Ni and Ti rather than Se vacancies. Due to the presence of excess Ti (0.085– 0.130 per formula) in the van der Waals gap of all the samples, the sensitive electron–hole balance is offset by the additional Ti-3d electrons, leading to an increase in the thermopower (n-type) over pristine, stoichiometric TiSe2. The effects of the co-intercalation of both Ni and Ti in TiSe2 on the structural, thermal, and electrical properties are discussed herein. & 2012 Elsevier Inc. All rights reserved.
Keywords: Thermoelectric materials Thermopower Thermal conductivity Spark plasma sintering Complex chalcogenides
1. Introduction Transition metal dichalcogenides make up a relatively large group of materials with several allotropes possible. This structurally well defined group contains representatives of many classes of electronic materials: insulators, semiconductors, semimetals, metals, superconductors, etc. [1]. Titanium diselenide is an interesting semimetallic material that exhibits a charge density wave transition near 200 K that is caused by electron–hole interactions arising from the overlap of Ti-3d electron bands at the L point and the spin-orbit-split Se-4p hole bands at the zone center rather than the usual electron–electron interactions [2,3]. Formed in the layered 1T polytype, TiSe2 consists of layers of TiSe6 octahedra where the layers are inter-bonded by weak van der Waals forces (Fig. 1) [4,5]. This structure can play host to numerous types of atomic and molecular intercalates that may reside in octahedrally coordinated sites at (0, 0, ½) in the van der Waals gap [5]. Many researchers have reported on the effects of transitional metal intercalates (i.e. Fe, Co, Cu, Ni) on the structural and electronic properties of TiSe2 single crystals, however, data on the thermal transport properties of 3 and 4d intercalated bulk samples is lacking [5–7]. One exception of note is a recent report
n Correspondence to: Department of Physics and Astronomy, Clemson University, 118 Kinard Lab, Clemson, SC 29634, USA. E-mail address: [email protected] (T.M. Tritt).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.07.057
by Hor and Cava that presents the effects of intercalating Cu on the thermoelectric properties of polycrystalline TiSe2 ySy [8]. In their work, they attempted to make use of the large thermopower peak associated with the charge density wave to optimize the low temperature thermoelectric performance of the material but concluded that the resistivity was too high. In most cases, single crystalline samples are prepared by chemical vapor transport while the polycrystalline samples of most studies were prepared by a multi-step firing process of raw elemental powders in an evacuated quartz tube with intermittent grinding and cold pressing. The use of spark plasma sintering as a preparation method has been widely overlooked previously due to the success of traditional methods in achieving high density polycrystalline samples (99% for cold pressing followed by annealing [8]). The results of this study show that spark plasma sintering (SPS) may prove to be an important process due to it being both a quicker route for synthesis and a possible tool for controlling self-intercalation of excess titanium into the material.
2. Material and methods All samples were prepared by sealing the appropriate amounts of Ni (99.996% Alfa Aesar-Puratronic), Ti (99.98% Strem), and Se (99.98% Alfa Aesar) powders in evacuated quartz tubes with a subsequent firing at 650 1C for 120 h. The resulting powders were ground and the phase checked by powder X-ray diffraction
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T.C. Holgate et al. / Journal of Solid State Chemistry 197 (2013) 273–278
Fig. 1. Left: Unit cell of TiSe2 with intercalation sites shown in the van der Waals gap. Right: octahedral Se–Ti–Se layers with Ni intercalated at (00½) to form a NiSe6 octahedron.
(PXRD; Rigaku MiniFlex, Cu k-a radiation). Despite all samples containing a secondary NiSe2 phase in addition to the intended TiSe2 phase, the powders were processed using spark plasma sintering (SPS) at 800 1C for 6 min under 50 MPa of pressure. Bar shaped samples were cut from the center of the resulting pellets (12.7 mm diameter) with dimensions of about 2 2 7–8 mm3. Low temperature thermal conductivity, thermopower, and electrical resistivity were all measured on the same piece for each sample using custom designed systems that are described elsewhere [9,10]. Above room temperature, the electrical resistivity and thermopower were measured using a commercial measurement system (ZEM-2, Ulvac-Riko). Post-SPS structural analysis was performed using PXRD for qualitative analysis at Clemson, and then high resolution patterns were obtained at the University of Waterloo (INEL, monochromated Cu k-a1 radiation) with subsequent refinement performed at Clemson using the MDI Jade software package. Samples were sent to the University of Georgia’s Geology Department for electron probe microanalysis (EPMA) in order to determine the stoichiometry of the samples.
Table 1 Nominal compositions and compositions experimentally determined by electron probe microanalysis, normalized to two selenium atoms per formula. NixTi1 þ ySe2 Sample
X-Nominal
X-EPMA
Y-EPMA
1 2 3 4 5 6
0 0.007 0.01 0.02 0.04 0.05
0.000 0.009 0.012 0.022 0.045 0.053
0.130 0.115 0.099 0.109 0.085 0.098
Table 2 Comparison of theoretical densities based on EPMA data normalized assuming excess Ti (Se normalized) and Se vacancies (Ti normalized). Unit cell volume was calculated for each sample based on the experimentally determined lattice parameters of each sample. Theoretical density Sample Se2 normalized (Ti self-intercalation)
3. Results and discussion 3.1. Structure and composition Table 1 presents the EPMA results and reveals that both the Ni and Ti concentrations are about 10% greater than the nominal values. The amount of excess Ti only loosely correlates to the amount of Ni as there tends to be less excess titanium when more nickel is present. In all samples, the measured Ni concentration is greater than the nominal concentration because Se was lost during the SPS process resulting in a higher Ni:Se ratio, as well as the ratio of Ti:Se being greater than one half. If the EPMA data were normalized to one Ti atom per formula (which would be to assume Se vacancies instead of Ti co-intercalation) the resulting theoretical density would be less than that of pristine TiSe2, but in the case of excess Ti—presumably in the (0, 0, ½) sites in the van der Waals gap—the resulting theoretical density would of course be higher (see Table 2). Besides reports showing that even solid state reaction synthesis in evacuated quartz tubes at temperatures above 700 1C result in excess Ti [11], the added driving force from the pressure involved with the SPS process will seek to minimize the volume of the system. Excess Ti in the van der Waals gap is therefore more likely than selenium vacancies.
1 2 4 6
Ti normalized (Se vacancies)
Formula
g/cm3
Formula
g/cm3
Ti1.13Se2 Ni0.009Ti1.115Se2 Ni0.022Ti1.109Se2 Ni0.053Ti1.098Se2
5.4 5.32 5.17 5.23
TiSe1.79 Ni0.008TiSe1.79 Ni0.019TiSe1.80 Ni0.048TiSe1.82
4.82 4.75 4.62 4.67
The densities of the samples were determined using the Archimedes method with de-ionized water as the medium. The measured and theoretical (assuming excess Ti) densities are compared in Table 3. The results agree well for the samples with less than 1% Ni, but as the Ni concentration increases the measured density becomes higher than the theoretical. This unphysical result means that either the sample is not homogeneous or the calculated lattice parameters are incorrect. In a sense, both are likely to be true. While the high resolution PXRD patterns indicate that the samples are single phase, some peaks—especially the (001) peak—are somewhat diffuse and were difficult to profile using only one peak. Successful deconvolution using two peaks (Fig. 2) indicates that the layers may be corrugated or dimpled as the intercalates—being excess Ti only (Sample 1), or both Ti and Ni (Samples 2–6)—may not be evenly and randomly distributed in the van der Waals gap.
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Table 3 Comparison of densities measured by the Archimedes’ method and calculated densities assuming excess Ti rather than Se vacancies. The uncertainty of the measured density is estimated to be less than 70.01 g/cm3 for all samples.
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Table 4 Comparison of the measured and estimated densities based on the assumption of a corrugated, two-phase structure. The uncertainty of the measured density is estimated to be less than 7 0.01 g/cm3 for all samples.
Sample
Measured (g/cm3)
Theoretical (g/cm3)
Percent difference
Sample
Measured (g/cm3)
Estimated (g/cm3)
Percent difference
1 2 4 6
5.36 5.32 5.32 5.47
5.404 5.318 5.175 5.232
0.75% 0.03% 2.79% 4.44%
1 2 3 4 5 6
5.36 5.32 5.35 5.32 5.37 5.47
5.346 5.353 5.346 5.371 5.390 5.402
0.3% 0.7% 0.1% 0.9% 0.3% 1.2%
Fig. 2. Experimental (001) peak of Sample 1 (Ti1.130Se2) fitted with one (left) and two peaks (right). The solid blue line near 2y ¼14.751 corresponds to the indexed (001) peak of pristine TiSe2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Of the two peaks that resulted from the fitting of the (001) peak of Sample 1, the less diffuse peak is centered at 2y ¼14.7521, ˚ which gives a d-spacing of 6.006 A—a value that corresponds closely to the literature value of the c-lattice parameter of TiSe2 ˚ [13]. The lower 2-theta value of the more diffuse peak (6.008 A) corresponds to a larger van der Waals gap (2y ¼14.6721, ˚ which is expected with excess Ti residing in the c¼ 6.038 A), gap since the isostructural (when the vacancies at z ¼0.5 are considered to be part of the basis) Ti2Se2 (c¼6.301) [12] structure is reached with full intercalation. If there does exist regimes of Ti self-intercalated and pristine, un-intercalated TiSe2, the size scale of the intercalated regimes are probably small enough that the volume of the boundary between the regimes is significant, as this will result in peak broadening. If it is assumed that Ti aggregates in the van der Waals gap and creates a corrugated or dimpled layered structure, then the crystal structure of Sample 1 (Ti1.130Se2) may be represented by a two phase system consisting of crystallites of Ti2Se2 embedded in a TiSe2 matrix. Using this scenario, a theoretical density may be estimated by an additive method where the density of each phase is calculated using the pristine reference lattice parameters and their contributions weighted by the EPMA data. For the nickel containing samples, the TiSe2 phase is replaced with NixTiSe2 and the appropriate lattice parameters were calculated using Vegard’s rule from Ref. [13] data. This representation assumes that in the regions where there is no excess Ti in the van der Waals gap, the Ni is randomly distributed and there is no intermixing of Ti and Ni intercalates. The density of NixTi1 þ dSe2 is therefore estimated using the atomic masses of the elements and the unit cell volume of the constituent phases (i.e. mTi and V Ti2 Se2 ) by
rest: ¼
xmNi þð1dÞðmTi þ2mSe Þ 2dðmTi þmSe Þ þ : V Nix TiSe2 , Vegard0 s Law V Ti2 Se2
Fig. 3. Lattice (solid lines) and total (dashed lines) thermal conductivity of Samples 1, 4, and 6.
Table 4 presents a comparison of the measured and estimated densities. The percent difference for all samples is on the order of the uncertainty of the density measurements using the Archimedes method. This comparison is valid only if the packing densities of the polycrystalline pellets are very high, but as mentioned in the introduction, this material is easy to densify with relative densities of 99% being obtained in cold pressed pellets. 3.2. Thermal conductivity The effect of nickel intercalation on the thermal conductivity can be seen in the systematic suppression of the ‘‘crystalline peak’’ of lattice thermal conductivity (klatt) with increasing nickel. This low temperature peak in the lattice thermal conductivity occurs in periodic systems where the low temperature T3 dependence due to the heat capacity starts to turn over and the phonon– phonon interactions start to dominate. Then as the temperature is further increased, then the increasing population of phonon states becomes more prominent with the T 1 dependence due to the smaller phonon–electron and then predominately phonon– phonon scattering that dominates at higher temperatures. When the periodicity of the lattice is disrupted by defects, this peak can be suppressed, and when the concentration of defects is very high, the lattice is very disordered and can exhibit amorphouslike thermal conductivity in which this peak is significantly
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suppressed. As can be seen in Fig. 3, with increasing Ni, the peak in the lattice thermal conductivity between 50 and 100 K is suppressed, indicating a more disordered lattice. Near room temperature, the sample without nickel (Sample 1) shows a total thermal conductivity (ktotal ¼ klatt þ kelec) that is still decreasing with temperature and appears to fall below that of the sample with the most nickel (Sample 6). The total thermal conductivity of both nickel containing samples appear to approach saturation as the lattice approaches its minimum and the T dependence of the electronic contribution—from the Wiedemann–Franz relation:kelec ¼ L0sT, (L0 ¼ 2.44 10 8 V2/K2)—is countered by the T 1 dependence of the electrical conductivity, s ¼1/r. However, the total thermal conductivity may increase at higher temperatures due to an increase in the electronic contribution as bi-polar conduction becomes an issue (due to involvement of the minority carriers in the conduction process) and the Weidemann–Franz relation requires an additional term. While the addition of Ni may first increase the total and lattice thermal conductivity at room temperature as the (110) planes are more strongly coupled to their neighbors and the anisotropy of the grains reduced, further addition of Ni will lower the lattice thermal conductivity as the system becomes more disordered. This explanation is corroborated by the observed reduction in the crystalline peak of the lattice contribution between 50 and 100 K. It is difficult to discern without further experiments whether the reduction of the lattice thermal conductivity is due to a reduction in the phonon density of states, the mean-free-path of the phonons, or both. 3.3. Electrical transport properties Approaching room temperature, the total thermal conductivity values of both Ni containing samples rise above that of Sample 1 because of the decreased electrical resistivity and its subsequent higher electronic contribution to the thermal conductivity. Fig. 4 presents the thermopower (a), electrical resistivity (r), and power factor (PF ¼ a2T/r). The effects of intercalation on the electrical transport can be explained by its effects on the relative concentrations of both holes and electrons, as well as by the disorder it introduces. For 0% Ni, the sample shows a peak in the thermopower at about 150 K that arises from the localization of carriers in the charge density wave (CDW) state. Di Salvo et al. showed that for pristine TiSe2, this peak is much larger ( 145 mV/K) and the thermopower becomes positive near room temperature as the number of holes and electrons are nearly the same, but the mobility of the holes increases more than that of the electrons [11]. The peak and turnover in the thermopower around 500 K is additional evidence that bi-polar conduction is taking place in this system. The excess Ti in the van der Waals gap inhibits the CDW state by offsetting the electron–hole ratio in terms of numbers as well as mobilities as the interstitial Ti atoms may add states near the Fermi level (initial electronic structure calculations using the TB-LMTO-ASA [14] method indicate this to be so, but spin-orbit coupling was not taken into account). As Ni is introduced into the van der Waals gap, the low temperature thermopower is reduced. The effective carrier concentration as a function of temperature between 5 and 300 K is presented in Fig. 5, which includes reference data on pristine TiSe2. The carrier concentration is systematically shifted towards positive values with increasing Ni at low temperatures. It is possible that the effect of the Ni on the crystal field results in localization of both the conduction electrons of the Ni as well as those of nearby Ti atoms. At higher temperatures, the electrons may become itinerate and the total effective carrier concentration becomes more negative.
Fig. 4. Electronic transport properties of NixTi1 þ dSe2.
The systematic reduction of the charge density wave anomaly in the thermopower as well as the shift towards lower temperature with increasing Ni may also be explained by the disorder introduced by intercalation. Since the CDW state in TiSe2 is a result of the pairing between Ti-3d electrons and Se-4p holes,
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increased the power factor to more than 0.2 Wm 1 K 1 at 300 K. With the temperature dependence of the thermopower becoming more linear and the electrical resistivity decreasing with increasing Ni, the Tmax of the power factor has been shifted towards higher temperatures and the maximum power factor (PF ¼0.38 at 550 K) has been achieved in Ni0.045Ti1.085Se2 due to having one of the lowest electrical resistivity values while being able to maintain moderate thermopower values with a nearly linear temperature dependence. Based on trends in the thermal conductivity values (at 300 K) of the samples with 2.2 and 5.3% Ni, the room temperature value of the 4.5% Ni sample ought be near 4 Wm 1 K 1, and would be expected to decrease at higher temperatures due to increased phonon–phonon scattering and little evidence of bipolar conduction at high temperatures. With this data, one may estimate that the ZT at 550 K would be on the order of 0.1. Though much lower than the ZT of state of the art thermoelectric materials of this temperature regime (ZT41), the low cost, high availability and relatively low toxicity of the constituent elements may validate further investigations of intercalating multiple species into the van der Waals gap of transition metal dichalcogenides for thermoelectric applications.
4. Conclusion Fig. 5. Effective carrier concentration of Samples 1, 2, and 6 with that of pristine TiSe2 from Di Salvo et al. [11].
strains in the lattice may easily disrupt the formation of electron– hole pairs. The carrier-offset description is supported by the high temperature (above 300 K) thermopower, which shows a systematic decrease of the bipolar effect with increasing Ni. With even 5% Ni the thermopower exhibits a metallic-like trend as the CDW is almost entirely suppressed and there is no overturn in the thermopower. Above 500 K, all samples containing Ni have a higher absolute thermopower than the sample without Ni due to the compensation of holes at higher temperatures, as the trends of the effective carrier concentrations becoming more prevalent in the regime above 300 K would suggest. The resistivity curves in Fig. 4 indicate that the addition of Ni generally increases the electronic conduction in the system—though not as monotonically as would be expected when simply counting electrons per formula unit or considering the systematic dependence of the thermopower on the Ni concentration. Although, the low temperature CDW anomalies in the resistivity curves generally follow the anomalies in the thermopower curves. The temperature at which the anomalies are centered is shifted towards lower temperatures with increasing Ni, and no anomaly is observed in the sample with the most Ni. In terms of understanding the dependence of the magnitudes of the resistance anomalies on Ni concentration and the relative magnitudes of the resistance before and after the CDW transition, the determination of the carrier concentration as a function of temperature for all samples would be helpful. In general, the thermopower of a sample is more sensitive to the carrier concentration and corresponding mobility than it is to structural defects and strains, whereas the resistivity is sensitive to both the carrier concentration and the presence of scattering centers. The non-monotonic dependence of the resistivity on the concentration of Ni may be a result of the competition between the addition of disorder and defects with the increase in carriers by the addition of more Ni. The thermopower, and thus the power factor (as defined earlier), of pristine TiSe2 is roughly zero near room temperature. As can be seen in Fig. 4, the excess Ti in the van der Waals gap has
Co-intercalated NixTi1 þ ySe2 samples were prepared using a solid state reaction followed by spark plasma sintering. The result of the SPS process at such an elevated temperature (800 1C) resulted in off stoichiometry between Ti and Se, which has been interpreted as co-intercalation of the excess Ti with Ni in the van der Waals gap between the a and b planes. The introduction of excess Ti in the van der Waals gap disrupts the structural order and offsets the balance of electron and hole states in the system. The addition of Ni into the van der Waals gap compounds these effects and shifts the system from a semimetal to one exhibiting a degenerate semiconductor-like thermopower. While the ZT is still too low for thermoelectric applications, other considerations such as cost and availability, in addition to the freedom afforded by the ability to intercalate many different species, results in the validation of the study of transition metal dichalcogenides intercalated with multiple species. However, it is important to consider the reproducibility of using spark plasma sintering in achieving the desired compositions. At least one sample was duplicated in an attempt to verify the reducibility and the percent differences of the nickel and excess titanium between the two samples as measured by EMPA were 10% and 6%, respectively. While this may be sufficient, post-SPS processing in an oven with a carefully controlled atmosphere of vacuum or low pressure selenium vapor may be a way to further control the stoichiometry as the selenium content will determine the amount of self-intercalation of the titanium and the relative concentration of intercalated nickel. In the future, a more careful study of the self-intercalation of Ti by varying the SPS parameters and the subsequent characterization of the electronic transport properties would be of value. Additionally, the co-intercalation of Ni and Ti in TiSe2 with sulfur substitution on the selenium sites may be of interest as a positive bandgap may be opened and an additional control parameter introduced for potential optimization of the thermoelectric properties.
Acknowledgments The authors would like to thank Chris Fleisher at the University of Georgia for EPMA the work and Matthew Hendrix at Clemson University for assistance in sample preparation. The
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work at Clemson University is supported by DOE/EPSCoR Implementation Grant (#DE-FG02-04ER-46139), and the SC EPSCoR cost-sharing program. Financial support from NSERC, CFI, and OIT for the work at the University of Waterloo is appreciated. References [1] J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193–335. [2] O. Anderson, R. Manzke, M. Skibowski, Phys. Rev. Lett. 55 (1985) 2188–2191. [3] G. Li, W.Z. Hu, D. Qian, D. Hseih, M.Z. Hasan, E. Morosan, R.J. Cava, N.L. Wang, Phys. Rev. Lett. 99 (2007) 027404–027407. [4] J.A. Wilson, F.J. Di Salvo, S. Mahajan, Adv. Phys. 24 (1975) 117–201.
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