High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure
来源期刊:Rare Metals2018年第4期
论文作者:Atsuko Kosuga Yosuke Fujii Akito Horie
文章页码:360 - 368
摘 要:In this study, we introduced the temperaturedependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe2 and copper gallium telluride CuGaTe2 with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hotpressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction(SXRD) measurements clarifies that impurity phases such as Te and Ag2 Te in the AgGaTe2 matrix and Te and CuTe in the CuGaTe2 matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K.The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe2 and CuGaTe2 were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe2 and CuGaTe2. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe2 and CuGaTe2, which should aid their development for thermoelectric applications.
稀有金属(英文版) 2018,37(04),360-368
Atsuko Kosuga Yosuke Fujii Akito Horie
Department of Physical Science, Graduate School of Science,Osaka Prefecture University
JST, PRESTO
Department of Electronics,Mathematics and Physics,Graduate School of Engineering,Osaka Prefecture University
收稿日期:1 December 2017
基金:financially supported by a Grantin-Aid for Young Scientists (A) (No. 15H05548) of Japan;JST PRESTO of Japan (No. JPMJPR17R4);the Program to Support Research Activities of Female Researchers in Osaka Prefecture University in Japan;
Atsuko Kosuga Yosuke Fujii Akito Horie
Department of Physical Science, Graduate School of Science,Osaka Prefecture University
JST, PRESTO
Department of Electronics,Mathematics and Physics,Graduate School of Engineering,Osaka Prefecture University
Abstract:
In this study, we introduced the temperaturedependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe2 and copper gallium telluride CuGaTe2 with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hotpressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction(SXRD) measurements clarifies that impurity phases such as Te and Ag2 Te in the AgGaTe2 matrix and Te and CuTe in the CuGaTe2 matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K.The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe2 and CuGaTe2 were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe2 and CuGaTe2. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe2 and CuGaTe2, which should aid their development for thermoelectric applications.
Keyword:
Chalcopyrite; Silver gallium telluride; Copper gallium telluride; Thermoelectric; Crystal structure;
Author: Atsuko Kosuga e-mail:a-kosuga@p.s.osakafu-u.ac.jp;
Received: 1 December 2017
1 Introduction
The ternary compounds with chalcopyrite-type structure denoted as [A=Cu or Ag (Group Ⅰ elements); B=Al,Ga or In (Group Ⅲ elements);and C=S,Se or Te (Group Ⅵ elements)] are currently of technological interest because they show promise for use in photovoltaics and optoelectronics
Fig.1 a Crystal structure of AⅠBⅢCⅥwith a chalcopyrite structure with four formula units per cell and b CⅤ-centered tetrahedron in which each CⅥatom being tetrahedrally coordinated to two AⅠand BⅢatoms
The degree of tetragonality of compounds with chalcopyrite-type structure depends on the kinds of elements and temperature,which thereby has a considerable effect on their physical properties.In particular,the thermoelectric (TE) transport properties of these compounds has attracted considerable attention recently because they have favorable electrical properties to result in high TE performance,as shown in Fig.2
Fig.2 Temperature dependence of dimensionless figure of merit(ZT) for representative compounds with chalcopyrite-type structure
In this study,we focused on polycrystalline AgGaTe2and CuGaTe2 consolidated by hot pressing,which is a typical method to prepare sintered TE materials,and their high-temperature formation phases and crystallographic parameters were examined.Hot-pressed samples are intentionally used with the purpose of evaluating the hightemperature behavior of samples intended for TE applications.This study enables understanding of the high-temperature behavior of hot-pressed AgGaTe2 and CuGaTe2and provides clues to improve the high-temperature TE properties and stability of these materials for use in real TE applications.
2 Experimental
The starting materials of Ag (99.99%,shot,Rare Metallic Co.,Ltd.,Japan),Ga2Te3 (99.999%,chunk,Furuuchi Chemical Co.,Ltd.,Japan) and Te (99.999%,shot,Rare Metallic Co.,Ltd.) were mixed to form AgGaTe2 and Cu(99.99%,shot,Rare Metallic Co.,Ltd.),Ga2Te3 (99.999%,chunk,Furuuchi Chemical Co.,Ltd.) and Te (99.999%,shot,Rare Metallic Co.,Ltd.) were mixed to form CuGaTe2 according to the corresponding stoichiometric compositions and placed in silica vacuum tubes.The mixture was melted at 1273 K for AgGaTe2 and 1173 K for CuGaTe2 and kept overnight in the furnace before being quenched in water.The obtained ingot was annealed at 873 K for 72 h for AgGaTe2 and 773 K for 72 h for CuGaTe2 to obtain a homogeneous ingot sample.The ingot was then crushed into a fine powder.The fine powder was sintered into a disc-like shape with a hot press using conditions reported previously
To characterize the crystal structural parameters and identify impurity phases of the hot-pressed samples,synchrotron X-ray diffraction (SXRD) was performed at BL02B2 beamline of the SPring-8 synchrotron radiation facility,Japan.At beamline BL02B2,a large DebyeScherer camera and imaging plate were used for the detector.The wavelength used in this study was0.0419712 nm refined with a CeO2 standard.Each hotpressed sample was crushed into a fine powder and then sealed in a silica capillary under vacuum.It was measured diffraction profiles of hot-pressed AgGaTe2 and CuGaTe2with temperature increasing from 300 to 800 K in increments of 50 K to observe changes in crystal structural parameters and detect impurity phases with temperature.Rietveld analyses were carried out for the observed SXRD profiles using the RIETAN-FP program
The electrical resistivity (ρ) and Seebeck coefficient(S) of the hot-pressed AgGaTe2 and CuGaTe2 samples were measured by the standard four-probe method using a commercially available apparatus (ZEM-3;ULVAC Technologies) at 300-800 K in helium atmosphere.To assess the high-temperature stability of the hot-pressed samples,ten cycles of temperature-dependent measurements of p and S were repeated.
3 Results and discussion
3.1 AgGaTe2
To evaluate the temperature-dependent phase formation of hot-pressed AgGaTe2,the impurity phases were identified.The chalcopyrite-type structure of AgGaTe2 is the main phase at all temperatures studied (300-800 K),as shown in Fig.3.Depending on the impurity phase(s) present,the profiles are classified into four regions:RegionⅠ(300-350 K),where only the chalcopyrite-type structure of AgGaTe2 exists;RegionⅡ(400-650 K),where a small amount of Te appear as a secondary phase;RegionⅢ(at700 K),where the Te disappears and again only the chalcopyrite-type structure of AgGaTe2 exist;and RegionⅣ(750-800 K),where small amounts of Ag2Te and unidentified materials appear as impurity phases.
By taking a closer look at each region,all of the peaks are fully indexed as those of AgGaTe2 with a chalcopyritetype structure
As for Region Ⅲ,the peaks from Te disappear and only the chalcopyrite-type structure of AgGaTe2 is present.This result implies that Te sublimated or melted as the temperature rises.The thermogravimetry-differential thermal analysis (TG-DTA) of hot-pressed AgGaTe2 reveals that there is no weight loss from the sample in this temperature range;therefore,the possibility of sublimation of Te could be disregarded.The possibility of Te melting could also be excluded because Te has a melting temperature higher than 700 K.Instead,the eutectic reaction between Te and AgGaTe2 occurs at 673 K
Fig.3 SXRD data for heating process over temperature range of 300-800 K in angle range of 2.5°-20.0°(filled triangles,open triangles and question marks denoting Te,Ag2Te,and unidentified phases,respectively;numbersⅠ-Ⅳreferring to temperature regions used to classify profiles
Fig.4 SXRD patterns and results of Rietveld refinement of hotpressed AgGaTe2 at a 350 K and b 600 K (dotted and solid lines representing observed data and fitted patterns,respectively;upper and lower vertical marks indicating Bragg peaks of AgGaTe2 with chalcopyrite structure and Te,respectively;blue curve at bottom being difference between observed and calculated patterns;GOF, goodness of fit;Rs and Rwp,reliability factors)
The temperature dependence of tetragonal lattice constants (a and c) and tetragonal lattice volume (V) of AgGaTe2 are shown in Fig.5.It is found that a increases and c decreases with temperature increasing,leading to monotonic expansion of V.The calculated thermal expansion coefficients ofαa=18.9×10-6 K-1,αc=-2.48×10-6 K-1,andαv=33.8×10-6 K-1 are slightly different from previously reported values ofαa=13.5×10-6 K-1,αc=-8.8×10-6 K-1
At high temperature (around 800 K),the hot-pressed AgGaTe2 exhibits high TE performance;therefore,values of transport properties S andρat 800 K were plotted as a function of the number of measurement cycles (Fig.7).The values of S andρwere normalized to the corresponding value obtained from the first measurement.Both S andρrise with temperature and increases by 7%and26%,respectively,after ten measurement cycles.It is presumed that these increases could originate from decrease in carrier concentration due to mainly loss of Te from the AgGaTe2 matrix phase caused by repeated temperature-dependent measurement cycles from 300 to800 K.The relative amounts of precipitations of the impurity phases of Ag2Te and Te to the matrix phase are subtle,therefore,metallic properties of such phases would not contribute to S andρof the sample.
Fig.5 Temperature dependence of a tetragonal lattice constants (a and c) and b tetragonal lattice volume (V) for hot-pressed AgGaTe2 (dotted red lines showing fitting lines used to estimate linear thermal expansion coefficients of a and b and volume thermal expansion coefficient of V)
Fig.6 Temperature dependence of a tetragonal distortion(η)and anion displacement|u-0.25|and b isotropic displacement factor (U) for Ag,Ga and Te
Fig.7 Normalized values of Seebeck coefficient (S) and electrical resistivity (ρ) of AgGaTe2 at 800 K based on value obtained in the first measurement against number of measurement cycles
3.2 CuGaTe2
Figure 8 shows the temperature-dependent SXRD profiles for the hot-pressed CuGaTe2 sample from 300 to 800 K.The profiles obtained at all the temperatures contain the chalcopyrite-type structure of CuGaTe2 as the main phase.The profiles are classified into three regions according to the phase(s) present:RegionⅠ(300-450 K),where only the chalcopyrite-type structure of CuGaTe2 exists;RegionⅡ(500-650 K),where small quantities of impurity phases corresponding to Te and CuTe appear;and RegionⅢ(700-800 K),where these impurity phases disappear and only the chalcopyrite-type structure of CuGaTe2 is observed.The detailed changes in each region are as follows.In RegionⅠ,all of the peaks are well fitted to those of CuGaTe2 with a chalcopyrite-type structure
Fig.8 SXRD data for heating process over temperature range of 300-800 K in angle range of 2.5°-15.0°(filled and open triangles denoting Te and CuTe,respectively;NumbersⅠ-Ⅲreferring to temperature regions used to classify profiles)
Fig.9 SXRD patterns and results of Rietveld refinement of hot-pressed CuGaTe2 at a 300 K and b 600 K (dotted and solid linesrepresenting observed data and fitted patterns,respectively;upper, middle and lower vertical marks indicating Bragg peaks of CuGaTe2 with chalcopyrite structure,Te and CuTe,respectively;blue curve at bottom being difference between observed and calculated patterns)
Figure 10 shows the temperature dependence of a,c,and V of hot-pressed CuGaTe2.Both a and c increase with temperature,leading to linear expansion of V.The calculated thermal expansion coefficients areαa=18.9×10-6 K-1,αc=8.15×10-6 K-1,andαv=34.3×10-6 K-1 are slightly different from previously reported values ofαa=11.7×10-6 K-1 andαc=6.6×10-6 K-1
Figure 11a shows thatηis almost independent of temperature,which is caused by the similar expansion along the a-axis (αa) and the c-axis (αc).Compared with the results for AgGaTe2 which exhibits a positiveαa and negativeαc,the thermal expansion anisotropy of CuGaTe2is apparently smaller than that of AgGaTe2.The anion displacement|u-0.25|of CuGaTe2 decreases with temperature increasing,meaning that the difference between Cu-Te and Ga-Te bond length in the CuGaTe2 structure becomes small.This result implies that some defects at the Cu/Ga cation sites are likely to occur along with slipping out of Cu from the CuGaTe2 matrix with elevated temperature.Considering the similarity of CuGaTe2 with CuGaSe2,in which formation energy of defects including CuGa and GaCu is estimated to be low
Fig.10 Temperature dependence of a tetragonal lattice constants (a and c) and b tetragonal lattice volume (V) for hot-pressed CuGaTe2 (dotted red lines showing fitting lines used to estimate linear thermal expansion coefficients of a and b and volume thermal expansion coefficient of V)
Fig.11 Temperature dependence of a tetragonal distortion (η) and anion displacement|u-0.25|and b isotropic displacement factor (U) for Cu,Ga and Te
The U values for Cu,Ga and Te in CuGaTe2 are shown in Fig.11b.These values increase with temperature,indicating enhanced thermal vibration of each atom.It is notable that U of Cu is considerably larger than those of Ga and Te within the temperature range examined.This is probably because Cu atoms deviates substantially from their equilibrium positions or Cu occupies the interstitial sites in the lattice,as is the case for AgGaTe2.
Fig.12 Normalized values of Seebeck coefficient (S) and electrical resistivity (ρ) of CuGaTe2 at 800 K based on value obtained for the first measurement against number of measurement cycles
Figure 12 shows the values of transport properties S andρfor CuGaTe2 at 800 K as a function of the number of measurement cycles.Both S andρincrease with temperature,by 2%and 20%,respectively,after ten measurement cycles.These increases could partially originate from the gradual change in ratio and/or type of impurity phases and loss of Cu and Te from the CuGaTe2 matrix phase caused by repeated temperature-dependent measurements from300 to 800 K.As is the case in AgGaTe2,it is believed that these increases could originate from the decrease in carrier concentration due to mainly loss of Te from the CuGaTe2matrix phase caused by repeated temperature-dependent measurement cycles from 300 to 800 K.The relative amounts of precipitations of the impurity phases of CuTe and Te to the matrix phase are subtle,therefore,metallic properties of such phases would not contribute to S andρof the sample.Furthermore,the reported anomalous decrease in thermal conductivity of CuGaTe2 in the intermediate temperature range
It is clarified that with elevated temperature,certain amounts of impurity phases separate from both the AgGaTe2 and CuGaTe2 matrixes in polycrystalline samples consolidated by hot pressing.It is currently unclear whether these phenomena occur only in hot-pressed samples.Further work is needed to clarify this point,such as high-temperature SXRD analysis of powder samples of AgGaTe2 and CuGaTe2 before hot pressing to compare with the results of the current study.However,the results do at least suggest that the high-temperature TE properties of hot-pressed samples reported previously could be affected by changes in temperature-dependent formation phases and crystallographic parameters
4 Conclusion
The temperature-dependent formation phases of hot-pressed AgGaTe2 and CuGaTe2 were investigated by collecting SXRD data over the temperature range of 300-800 K and analyzing their crystallographic parameters.Hot-pressed samples were intentionally used with the purpose of evaluating the high-temperature behavior of samples intended for TE applications.Separation of Te and Ag2Te from the AgGaTe2 matrix and Te and CuTe from the CuGaTe2matrix was observed at some temperatures between 300and 800 K.These phenomena could cause the gradual increases of S andρobserved over ten measurement cycles from 300 to 800 K.The U values of Ag and Cu increase with temperature and are much larger than those of the other elements in AgGaTe2 and CuGaTe2,respectively,within the temperature range examined,which is likely attributed to either deviation of such atoms from their equilibrium positions,or the loss of Ag or Cu atoms that occupy interstitial sites.Theαa,αc andαv values are slightly different from those reported previously.Analysis of c/2a and u indicate that the structure of AgGaTe2becomes more distorted with temperature increasing in terms of both the tetragonal lattice and Te-centered tetrahedron.For CuGaTe2,the distortion of the tetragonal lattice is almost constant with temperature.Additionally,Te moves to the equilibrium position with temperature increasing,probably caused by arising the defects of Cu/Ga cation sites.These crystallographic parameters could substantially affect the TE properties of materials;thus,the insight obtained here reveals the important parameters to evaluate the high-temperature transport properties of hotpressed AgGaTe2 and CuGaTe2.
Acknowledgements This work was financially supported by a Grantin-Aid for Young Scientists (A)(No.15H05548) of Japan,JST PRESTO of Japan (No.JPMJPR17R4) and the Program to Support Research Activities of Female Researchers in Osaka Prefecture University in Japan.Synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI;Proposal Nos.2014B1334,2015A1363 and 2015B1377).We thank Prof.S.Yamanaka's group at Osaka University,Japan for hot pressing AgGaTe2 and CuGaTe2.
参考文献
[26] Pashinkin A,Fedorov V.Phase equilibria in the Cu-Te system.Inorg Mater.2003;39(6):539.