Veronique Da Ros

Juliusz Leszczynski

Bertrand Lenoir

Anne Dauscher

Christophe Candolfi

Philippe Masschelein

Christine Bellouard

Christian Stiewe

Eckard Müller

Jiří Hejtmánek

Veronique Da

Eckard Mueller

Jiri Hejtmanek

Thermoelectric Properties of InxCo4-yNiySb12 Skutterudite Compounds

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INTRODUCTION

The CoSb 3 -based filled skutterudites continue to attract a lot of attention as potential thermoelectric materials for elevated temperature applications. Particular interest is focused on ntype skutterudites as their thermoelectric properties are still not as good as in p-type skutterudites. A variety of approaches were applied to improve skutterudite thermoelectric properties involving partial filling of the structural voids, formation of CoSb 3 isoelectronic solid solutions and doping by proper substitution at the metal or pnicogen sites. Among all these diverse methods void filling was found to be the most efficient. The presence of a filler element leads to a strong reduction of the thermal conductivity as compared to the parent composition and also affects its electrical properties by introducing additional electrons and modifying band structure yielding high power factors. The best improvement of the thermoelectric properties was achieved by partial filling with rare earth elements (Yb, Eu [1,2]) and alkaline metals (Ca, Ba [3][4][5][6]) and ZT values exceeding 1 at T ≥600K were reported. In case of alkaline filling elements it was shown that further improvement of thermoelectric figure of merit can be obtained by careful tuning of carrier concentration accomplished by Ni doping. Quite recently H. Lee et al. [7] published very promising results of thermoelectric performance of indium partially filled CoSb 3 . For composition In 0.25 Co 4 Sb 12 they have measured maximum ZT value of 1.2 at 600 K. The aim of this work was to synthesize indium partially filled CoSb 3 pure and Ni doped samples to study Ni influence on thermoelectric properties of In filled compounds.

EXPERIMENT

Polycrystalline In x Co 4-y Ni y Sb 12 samples were prepared by a combination of melting and sintering techniques. Stoichiometric amounts of high purity In shots 99.99%, Sb shots 99.999%, Co 99.998%, and Ni 99.995% powders were loaded into a quartz ampoule. Because it is known that even small Ni impurity has strong effect on Seebeck coefficient and electrical resistivity of skutterudites high purity cobalt (Ni free) powder was used. The ampoule was sealed under reducing He-10% H 2 atmosphere and placed in a vertical oscillating furnace. Following that it was heated to 950ºC at 10ºC/min, left at 950ºC for 4 hours and quenched in a water bath. The obtained ingot was ground into fine powder using an agate mortar, cold-pressed into pellets and once again loaded into quartz ampoule and annealed at 670ºC for several days. The completely reacted pellets were crushed, pulverized and sieved to obtain grain fraction <50 µm. Final densification was carried out in a graphite die by hot pressing in an argon atmosphere at 650ºC for 2 h under 50 MPa.

Structural and chemical characterization of all the polycrystalline samples was accomplished by x-ray diffraction XRD as well as electron probe micro analysis EPMA. XRD measurements were conducted with a Siemens d500 diffractometer using monochromatic CoK α radiation in the 2 Θ range 20-120º with 2Θ step 0.032º and silicon powder as an internal standard for lattice constant calculation. The real compositions presented herein were obtained from EPMA analysis as an average of measurements taken at 30 random points, and normalized to full occupancy at the cobalt site. Electrical resistivity measurements were made on square-shape samples 4x4x1 mm 3 between 300 and 800 K using a standard Van der Paw method. Seebeck coefficient was measured in the temperature range 320 K -800 K. The carrier concentration was calculated from the Hall coefficient R H , assuming a single-carrier model and unity as a Hall factor. Thermal conductivity was determined for temperatures between 300 and 810 K by measuring the thermal diffusivity on a 10 mm diameter disc by a laser flash technique, the specific heat values for thermal conductivity evaluation were calculated from Dulong-Petit law.

DISCUSSION

Based on XRD measurements and EPMA analysis, it was confirmed that the samples consists of one main phase with skutterudite structure and some secondary phases. Sb and InSb impurities were observed in XRD measurements and the amount of these phases was estimated to be below 2% vol.. Electron microprobe results revealed that for all samples In and Ni distribution was homogeneous with compositions for all elements close to the nominal except In, where the real content was only about 50% of the nominal composition (see Table I). Single grains of Sb and InSb were observed likewise in the X-ray diffraction measurements and the observed amount of the impurity phases was less than 1% vol.. Presence of indium-rich phases explains the deviation between real and nominal composition.

Fig. 1 shows the lattice constant a as a function of the real indium content x. The lattice parameter a is increasing with increasing x linearly and follows a Vegard's law. This was observed for all filling atoms. In the indium case, the a parameter is smaller than for Tl, Ba [8,3] and for most rare-earth filler elements but is comparable with Ca [5] and Yb [1] at the same filling fraction. Similar relation of lattice constant and composition was reported by He et al. [7]. However, they were able to introduce more In into the skutterudite structure and that was reflected in higher lattice parameters they obtained for the same nominal compositions. The addition of nickel seems to have a small influence on the lattice parameter.

Transport properties of In x Co 4 Sb 12 series

The Seebeck and resistivity data for indium partially filled CoSb 3 are shown in figures 2 and 3 respectively. The pure CoSb 3 exhibits positive sign of Seebeck coefficient. Introduction of small amount of indium into the structural cages switch the dominant type of carriers from holes to electrons resulting in a negative Seebeck coefficients. At room temperature absolute Seebeck coefficient decreases with In filling fraction. For the samples with the real composition x=0.1-0.18 the magnitude of the thermopower monotically increases with increasing temperature. This behavior is typical for heavily doped semiconductors. For the sample with the lowest indium content x=0.04 at temperature close to 500 K the Seebeck coefficient reaches its maximum and begins to decrease with temperature reflecting the influence of holes. Intrinsic conductivity tends to play a role also for other indium filled samples at highest temperatures leading to flattening of the temperature dependence of the thermopower. The temperature dependence of the CoSb 3 electrical resistivity is typical semiconducting behavior. Indium insertion into the voids decreases 18 respectively. This is a little bit higher than for polycrystalline Th-filled CoSb 3 [7]. The indium insertion cause an increase of the electronic carrier concentration but the number of electrons given to the structure is much lower then in case of other filler atoms. The highest carrier concentration n = 9.6•10 19 cm -3 is obtained for the sample with highest indium content x=0.18. The thermal conductivity temperature dependence is shown in Fig. 4. Thermal conductivity is significantly decreased by In filling. For In content higher than 0.1 it remains almost unchanged with further In introduction. Small deviations are probably the result of microstructural and density differences between samples. Such a strong effect on thermal conductivity reduction is probably due to combination of multiple phonon scattering mechanisms like point defect scattering by mass fluctuation, strain field scattering and certainly phonon resonance scattering what was suggested in case of Ca [9]. Figure 5 shows that thermoelectric performance expressed by ZT is increasing with temperature and presents the maximum close to the Seebeck coefficient maximum. ZT coefficient is improved by increasing the filling fraction of In and the highest value is obtained in In 0.18 Co 4 Sb 12 with ZT close to 0.85 at 700-780 K.

Transport properties of In x Ni y Co 4-y Sb 12 series

Studying the Ni influence on thermoelectric properties of In x Co 4 Sb 12 was not an easy task because of large deviations between nominal and real indium concentration. Finally a group of samples with indium content close to 0.1 was chosen for comparison but in some cases it was not possible to distinguish effect of In content variations and Ni concentration. As a reference sample In 0.1 Co 4 Sb 12 sample is used. The temperature dependence of the Seebeck coefficient of In x Ni y Co 4-y Sb 12 samples is shown in Fig. 6. The overall thermopower behavior is not changed by Ni doping. Introduction of Ni y = 0.02 shifts the Seebeck coefficient about -20 µV/K increasing the absolute value. For higher Ni content thermopower magnitude decreases to the values close to the reference sample (y = 0.05, 0.06) and then falls 20 µV/K down. Nickel provides additional electrons to the structure increasing charge carrier concentration from 7.5•10 19 cm -3 to 1.47•10 20 in In 0.13 Ni 0.05 Co 3.95 Sb 12 . However, it is not quite clear how many electrons is given by Ni atom as In content is also varying. More evident effects are observed for carrier mobility which is strongly depressed by Ni impurity and is about a half of the value for reference sample at T = 300 K(see table I). The mobility decrease is proportional to Ni content. The temperature dependence of the mobility is also altered and is less steeper than for the reference sample revealing that ionized impurity scattering has a great importance even at temperatures above 100K. As a result of the carrier concentration and mobility changes, the resistivity at room temperature is initially reduced by introducing Ni and then increases above the resistivity value of the reference sample. At T>300K the temperature dependence of the resistance (Fig. 7) is more or less metallic like. Increasing the Ni content reduce the TCR and makes resistivity less temperature sensitive that could be qualitatively explained by the change in carrier scattering

CONCLUSIONS

The influence of indium filling on thermoelectric properties of CoSb 3 and following that influence of Ni substitution for Co on the properties of In partially filled cobalt triantimonide were investigated. It was found that In plays the role of a n-type dopant yielding high electron mobility values at room temperature close to 100 cm 2 /V•s and reducing significantly the thermal conductivity. This results in strong improvement of the thermoelectric properties and the maximum ZT value of 0.85 is reached at 700K for the sample In 0.18 Co 4 Sb 12 . In the In x Ni y Co 4- y Sb 12 system Ni acts as the n-type dopant and affects mainly on the electronic properties. These changes can be explained in terms of enhanced carriers scattering on ionised impurities. Doping In x Co 4 Sb 12 by small nickel amounts (y≈0.02) seems to have the beneficial effect on the thermoelectric properties. The maximum ZT value for In 0.12 Ni 0.02 Co 3.98 Sb 12 sample was increased of 6% compared to the reference sample. It will be interesting to investigate the influence of Ni on samples with higher indium content as they give a prospect of achieving figures of merit higher than one.

Figure 2 .

 2 Figure 2. Seebeck coefficient vs. temperature in In x Co 4 Sb 12 samples .

Figure 4 .Figure 5 .

 45 Figure 4. Thermal conductivity vs. temperature in In-filled CoSb 3 samples .

Figure 6 .

 6 Figure 6. Seebeck coefficient vs. Temperature in In x Ni y Co 4-y Sb 12 samples .

Table I .

 I Nominal and real compositions and physical properties of the In x Ni y Co 4-y Sb 12 samples at room temperature.

	Nominal composition	Real composition from EPMA measurements	Lattice parameter a (Å)	Relative density (%)	Carrier concentration n (cm -3 )	Carrier mobility µ (cm 2 /Vs)
	In 0.1 Co 4 Sb 12 In 0.2 Co 4 Sb 12 In 0.3 Co 4 Sb 12 In 0.22 Ni 0.03 Co 3.97 Sb 12 In 0.22 Ni 0.05 Co 3.95 Sb 12 In 0.22 Ni 0.08 Co 3.92 Sb 12 In 0.3 Ni 0.03 Co 3.97 Sb 12	In 0.041 Co 4 Sb 12 In 0.1 Co 4 Sb 12 In 0.18 Co 4 Sb 12 In 0.12 Ni 0.02 Co 3.98 Sb 12 In 0.13 Ni 0.05 Co 3.95 Sb 12 In 0.11 Ni 0.06 Co 3.94 Sb 12 In 0.14 Ni 0.02 Co 3.98 Sb 12	9.0407 9.0442 9.0504 9.0474 9.0454 9.0457 9.046	98.8 92.4 98.7 96.5 97.2 93.1 98.9	2.3•10 19 7.5•10 19 9.6•10 19 1.25•10 20 1.47•10 20 1.23•10 20 1.25•10 20	107 70.2 83.7 41.5 30.1 29.6 44.6
	In 0.3 Ni 0.08 Co 3.92 Sb 12	In 0.11 Ni 0.07 Co 3.93 Sb 12	9.0437	87	-	-

  Figure 3. Resistivity vs. temperature in In x Co 4 Sb 12 samples .

				10
	Co 4 Sb 12 In 0.1 Co 4 Sb 12	In 0.04 Co 4 Sb 12 In 0.18 Co 4 Sb 12	Thermal conductivity (W/m•K)	0 1 2 3 4 5 6 7 8 9				In 0.04 Co 4 Sb 12 In 0.1 Co 4 Sb 12 In 0.18 Co 4 Sb 12 In 0.14 Ni 0.02 Co 3.98 Sb 12 In 0.11 Ni 0.07 Co 3.93 Sb 12 CoSb 3
				200	300	400	500	600	700	800	900
						Temperature (K)

  Figure 7. Resistivity vs. temperature in In x Ni y Co 4-y Sb 12 samples .Figure 8. ZT vs. temperature in In x Ni y Co 4-y Sb 12 samples.mechanism from acoustic phonons to both acoustic phonons and ionized impurities. The thermal conductivity of In x Ni y Co 4-y Sb 12 samples is not modified significantly with the introduction of Ni as exemplified in Fig.4.The ZT versus temperature plot for In x Ni y Co 4-y Sb 12 samples is presented in Fig.8. Depending of the Ni doping one can see that it is possible to control the thermoelectric properties. It can be observed that small amount of nickel enhances thermoelectric figure of merit and gives the maximum value of ZT 0.68 at 650 K compared to ZT 0.64 for the reference sample In 0.1 Co 4 Sb 12 .

			0.7
			0.6
			0.5
	In 0.11 Ni 0.07 Co 3.93 Sb 12 In 0.11 Ni 0.06 Co 3.94 Sb 12 In 0.13 Ni 0.05 Co 3.95 Sb 12 In 0.12 Ni 0.02 Co 3.98 Sb 12 In 0.1 Co 4 Sb 12	ZT	0.1 0.2 0.3 0.4			In 0.11 Ni 0.07 Co 3.93 Sb 12 In 0.11 Ni 0.06 Co 3.94 Sb 12 In 0.13 Ni 0.05 Co 3.95 Sb 12 In 0.12 Ni 0.02 Co 3.98 Sb 12 In 0.1 Co 4 Sb 12
			300	400	500	600	700	800
					Temperature (K)

ACKNOWLEDGMENTS

This research was supported by the European Space Agency.


Véronique Da Ros
email: veronique.daros@mines.inpl-nancy.fr

Bertrand Lenoir

Anne Dauscher

Christophe Candolfi

Christine Bellouard

Christian Stiewe

Eckard Müller

Jiří Hejtmánek

V Da

J Hejtmanek

Thermoelectric Properties of NdxCo4-yNiySb12 Skutterudite Compounds

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Introduction

Among narrow band gap semiconductors, skutterudites have received particular attention over the last ten years. This interest was motivated first by their outstanding properties as engineering materials for power generation applications, but also because they exhibit intriguing physical features from a fundamental point of view (e.g. Kondo behaviour or superconductivity). [START_REF] Uher | Semiconductors and Semimetals[END_REF] The potential of a material for thermoelectric applications is determined by the material's dimensionless figure of merit ZT = S 2 T/rl, where S is the thermoelectric power, T the absolute temperature, r the electrical resistivity and l the total thermal conductivity (l = lL + lE the lattice and electronic contributions, respectively).

Skutterudites based on CoSb3 have been identified as being particularly interesting for thermoelectric applications at moderated temperatures. [START_REF] Nolas | [END_REF] The skutterudite structure offers many possibilities to tune the electrical and thermal properties by appropriate filling of the constitutional voids and/or substitutions on the Co or Sb sites. [START_REF] Uher | Semiconductors and Semimetals[END_REF]3] N-type or ptype materials with ZT values exceeding unity have been obtained in this way making skutterudites outstanding materials. The development of segmented thermoelectric unicouples using advanced thermoelectric materials, e.g., is currently under investigation. [START_REF] El-Genk | Tests results and performances compatisons of coated and un-coated skutterudite based segmented unicouples[END_REF] Several rare-earth elements can be inserted into the cages of the CoSb3 structure. [START_REF] Uher | Semiconductors and Semimetals[END_REF][START_REF] Nolas | [END_REF] If many studies were performed with La and Ce, [START_REF] Morelli | Cerium filling and doping of cobalt triantimonide[END_REF][START_REF] Nolas | Effect of partial void filling on the lattice thermal conductivity of skutterudites[END_REF] the case of Nd remains little explored if we except the work of Kuznetsov et al. [START_REF] Kuznetsov | Effect of partial void filling on the transport properties of NdCo4Sb12 skutterudites[END_REF] Thus, no attempt was made to optimize this interesting material system through doping or compensation.

In this paper, we are reporting on the preparation and the characterization of polycrystalline NdxCo4-yNiySb12. The influence of the filler element Nd and of the doping atom Ni on the electrical and thermal properties is discussed.

Experimental

The synthesis of NdxCo4-yNiySb12 skutterudites was achieved by a metallurgical technique we developed. [START_REF] Da Ros | Low temperature transport properties of Co4Sb12 skutterudite compounds partially filled with neodymium[END_REF] High purity Nd powder (99.9 %), Co nickel-free powder (99.998 %), Sb shots (99.999 %) and Ni powder (99.995 %) were used as the starting materials. Stoichiometric amounts of these elements were loaded into a quartz ampoule in an argonatmosphere glovebox. The tube was sealed under a He/H2 atmosphere and transferred into an oscillating furnace. The ampoule was heated-up to a temperature T (750°C<T<1050°C) at a rate of 1°C/min and maintained at T for 84 h. The obtained solid was then ground in an agate mortar into fine powders (< 100 µm) that were further compacted into pellets without any lubricant in a steel die. To achieve completely the reaction and to obtain the expected crystallographic phase, the pellets were annealed in a quartz ampoule under a He/H2 atmosphere at 620°C for 3 to 10 days. These materials were powdered again and densified. The densification was accomplished by uniaxial hot-pressing using graphite dies in an argon atmosphere at 600°C under 51 MPa.

The crystallographic structure and the chemical composition of all samples were carefully checked using respectively a Siemens D-500 diffractometer (X-Ray diffraction) and a CAMECA SX 100 electron microprobe (EPMA analyses).

To perform the transport property measurements, samples of various shapes (parallelepiped and cylindrical) were cut with a diamond wire saw from the hot-pressed ingots.

The thermoelectric power was measured between 300 -800 K by a standard method using a home-made apparatus. The thermal conductivity was evaluated by the combination of the thermal diffusivity measured by a laser flash apparatus (Netzsch LFA 427) and the heat capacity measured by differential scanning calorimetry (Netzsch DSC 404) in the 300 -800 temperature range. The density of our samples was 2006 International Conference on Thermoelectrics determined by a measure of their mass and volume at ambient temperature. We made no corrections for thermal expansion at high temperature. Low temperature electrical measurements (4 -300 K) were performed using a PPMS (Quantum Design) in the AC mode.

Results and discussion

The X-ray diffraction patterns of all the samples can be indexed with a unit cubic cell corresponding to the skutterudite structure, space group Im3, as illustrated in figure 1. The results show that all the samples are predominantly single phase. A small amount of a Sb-rich secondary phase, that never exceeds 2 vol. %, detected at 2 θ = 33.4, may also be present.

the Nd content of the matrix in deficit relative to the nominal composition. In the next of the text, x and y in NdxCo4-yNiySb12 will always refer to the actual composition as determined by EPMA.

For the Ni-free samples, the variation of the lattice parameter versus the content of Nd, x, was find to be linear (see figure 3), suggesting a Vegard's law. This behaviour is consistent with the findings of Kuznetsov et al. [START_REF] Kuznetsov | Effect of partial void filling on the transport properties of NdCo4Sb12 skutterudites[END_REF] However, while these authors find a solubility limit of x ~ 0.1, our investigations concluded on x ~ 0.05. X-ray maps reveal that the major elements, Co and Sb, are homogeneously distributed in the samples while small Ndrich precipitates are systematically detected (figure 2) making The temperature dependence of the thermoelectric power, of the different NdxCo4-yNiySb12 samples is reported in figure 4. The CoSb3 binary compound exhibits a positive thermoelectric power that decreases with the enhancement of the temperature. As it can be seen, the introduction of neodymium in the cavities of the skutterudite structure has a strong effect on the temperature-dependent thermoelectric power behaviour. The presence of Nd at a content as low as x = 0.016 leads to a negative thermoelectric power, meaning that the electrical conduction goes from a conduction dominated by the holes in CoSb3 to a conduction dominated by the electrons in the NdxCo4Sb12 compounds. A higher content of Nd (x = 0.052), decreases the thermopower in absolute values as a result of an increase of the carrier concentration. It can be seen that all curves exhibit a more or less pronounced inversion of the slope between 450-550 K according to the compound, revealing the participation of the minority carriers to the conduction, i.e. electrons for the binary compounds and holes for samples containing Nd. The influence of the substitution of Co by Ni on the thermopower of NdxCo4Sb12 samples is also shown in figure 4. The presence of Ni results in an increase of the carrier concentration (as seen in table 1) since Ni has one electron more than Co. For this reason, it is expected that the thermopower (in absolute value) of the samples containing Ni will be lower in comparison that of samples without Ni. This is effectively what we observe but we can note that for x ~ 0.05, the doping does not drastically decrease S. This point ) has already been underlined by other authors when fillers other than Nd, for instance alkaline earths, have been used. [3,[START_REF] Dyck | Thermoelectric properties of the n-type filled skutterudite Ba0.3Co4Sb12 doped with Ni[END_REF] In figure 5 are plotted the temperature dependences of the electrical resistivity between 4 -800 K. The binary CoSb3 compound exhibits an electrical resistivity that diminishes as a function of temperature, characteristic of a semiconducting behaviour. These observations coincide with the results of Puyet et al. [START_REF] Puyet | Electronic, transport, and magnetic properties of CaxCo4Sb12 partially filled skutterudites[END_REF] obtained with polycrystalline p type CoSb3. Neodymium insertion in the cavities of the crystalline structure modifies somewhat this behaviour since the electrical resistivity decreases, exhibits a minimum and then increases slowly when the temperature increases to further decrease at high temperature. We can note that the values are quite comparable to the values obtained with other filler elements (alkaline earths or rare earths) for similar filling fractions. [START_REF] Uher | Semiconductors and Semimetals[END_REF][START_REF] Puyet | Electronic, transport, and magnetic properties of CaxCo4Sb12 partially filled skutterudites[END_REF] More spectacular is the effect of Ni, which decreases substantially the values of the electrical resistivity. The room temperature values of the electrical resistivity, Hall carrier concentration and Hall mobility are listed in table 1 for the NdxCo4-yNiySb12 compounds studied.

near 300 K while this contribution is less pronounced at high temperature. Samples with or without Ni display quite similar thermal conductivity. This trend seems reasonable since Ni and Co have similar atomic masses that should not conduct to strong point defect scattering. From the S(T), r(T) and l(T) temperature dependences, we have calculated the temperature dependence of the dimensionless figure of merit in the 300 -800 K range. An extrapolation of the electrical resistivity for the Ni samples at high temperatures has been performed based on previous results. [3] The results are reported in figure 7. For the NdxCo4Sb12 samples, ZT is maximum for the higher filling fraction. The value is about 0.28 near 800 K. The presence of Ni is expected to improve significantly this value as it is illustrated in figure 7 from the estimation of ZT. Looking now to the thermal conductivity (figure 6), we can see that the insertion of Nd has particularly a beneficial effect at room temperature, lowering the thermal conductivity by roughly a factor 2. The effect is less marked at high temperature. The phononic contribution plays a major role

Conclusion

We investigated the transport properties of NdxCo4-yNiySb12 polycrystalline samples obtained via a metallurgical route. It was found that the solubility limit of Nd in CoSb3 is relatively low (~ 0.05) relative to other filler elements. Moreover, neodymium plays the role of a dopant (n type). Due to the low filling fraction, the ZT values of the NdxCo4Sb12 remain moderate. Doping this system by Ni seems to be favourable. This investigation should be pursued in more detail to have a better insight about the role of Ni on the transport properties of the NdxCo4Sb12 samples.

Figure 1 :Figure 4 :

 14 Figure 1: Typical X-ray diffraction pattern of NdxCo4Sb12 skutterudite

Figure 2 :Figure 3 :

 23 Figure 2: Sb-Lb and Nd-La maps as obtained by EPMA showing Sb homogeneity and the presence of Nd-rich phases

Figure 6 :

 6 Figure 6: Temperature dependence of the thermal conductivity for the NdxCo4-yNiySb12 compounds.

Figure 5 :

 5 Figure 5: Temperature dependence of the electrical resistivity

Figure 7 :

 7 Figure 7: Temperature dependence of the dimensionless figure of merit ZT for the NdxCo4-yNiySb12 compounds.

Table 1 :

 1 Hall carrier concentration, Hall mobility and Hall resistivity measured at room temperature for the different NdxCo4-yNiySb12 skutterudites studied

Acknowledgments

Support from the PAI "Barrande" and the European NoE CMA ("Complex metallic alloys") are completely acknowledged. C.C. thanks DGA for financial support.