Direct fabrication of highly-dense Cu2ZnSnSe4 bulk materials by combustion synthesis for enhanced thermoelectric properties

    Direct fabrication of highly-dense Cu 2 ZnSnSe4 bulk materials by combustion synthesis for enhanced thermoelectric properties Guanghua Liu, Jiangtao Li, Kexin Chen, Yuyang Li, Min Zhou, Yemao Han, Laifeng Li PII: DOI: Reference:

S0264-1275(15)31030-3 doi: 10.1016/j.matdes.2015.12.172 JMADE 1196

To appear in: Received date: Revised date: Accepted date:

23 November 2015 28 December 2015 30 December 2015

Please cite this article as: Guanghua Liu, Jiangtao Li, Kexin Chen, Yuyang Li, Min Zhou, Yemao Han, Laifeng Li, Direct fabrication of highly-dense Cu2 ZnSnSe4 bulk materials by combustion synthesis for enhanced thermoelectric properties, (2015), doi: 10.1016/j.matdes.2015.12.172

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ACCEPTED MANUSCRIPT Direct fabrication of highly-dense Cu2ZnSnSe4 bulk materials by

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combustion synthesis for enhanced thermoelectric properties

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Guanghua Liu1,*, Jiangtao Li1,*, Kexin Chen2,*, Yuyang Li1, Min Zhou1, Yemao Han1, Laifeng Li1

1. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of

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Sciences, Beijing 100190, P. R. China

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2. State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and

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Engineering, Tsinghua University, Beijing 100084, P. R. China

*Corresponding author:

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Guanghua Liu, E-mail: [email protected] Jiangtao Li, E-mail: [email protected] Kexin Chen, E-mail: [email protected]

ACCEPTED MANUSCRIPT Abstract Thermoelectric materials are attractive for solar thermal energy conversion and waste heat

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recovery. The preparation of bulk thermoelectric materials usually involves multi-step processes

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with considerable time and energy consumption. Here we report an alternative way called combustion synthesis to realize one-step and fast fabrication of bulk Cu2ZnSnSe4 thermoelectric materials. The combustion synthesis was carried out in 2 MPa Ar gas atmosphere or in a

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high-gravity field in order to reduce the porosity in samples and complete simultaneous

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densification during synthesis. Nearly full-dense Cu2ZnSnSe4 samples with a porosity of <1% were prepared, showing thermoelectric properties similar to those of Cu2ZnSnSe4 materials prepared by

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other methods. The ZT of the Cu2ZnSnSe4 samples was clearly improved by partial substitution of Sn

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with In, and reached 0.59 at 773 K for the composition of Cu2ZnSn0.9In0.1Se4. Compared with the

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conventional melt growth and powder sintering methods, combustion synthesis offers a fast, one-step, and furnace-free way for directly producing bulk Cu2ZnSnSe4 samples, which may open up

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new possibilities for synthesis and applications of Cu2ZnSnSe4-based thermoelectric materials.

Keywords: combustion synthesis, thermoelectric materials

ACCEPTED MANUSCRIPT Introduction Thermoelectric materials can convert heat into electricity with no fossil fuel usage and may be used

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for power generation from solar thermal energy and waste heat.1,2 The efficiency of thermoelectric

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materials is determined by the dimensionless figure of merit (ZT), defined as ZT=(S 2σ/κ)T, where S, σ, κ and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. In the past decade, various approaches have been reported to

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increase ZT.3-8 In addition to ZT, another issue should be considered for thermoelectric materials is

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the cost, which is especially important for industrial application with massive use of materials. An undesired fact is that most thermoelectric materials with high ZT contain scarce, expensive, or toxic

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elements such as Te and Pb.9 In this case, the discovery of new thermoelectric materials composed

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of earth-abundant, less expensive, and nontoxic elements becomes the focus of attention, where an

conductivities.10-12

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example is the copper-based selenides (e.g. Cu2Se, Cu2SnSe3, Cu2ZnSnSe4) with low lattice thermal

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Besides the discovery of new materials, the development of novel fabrication techniques may provide another solution to the cost issue. Recently, a new method to synthesize thermoelectric materials has been reported, which is known as combustion synthesis or self-propagating high-temperature synthesis.13-18 Compared with the conventional approaches to produce thermoelectric materials such as growth from the melt and powder sintering, combustion synthesis appears to be more efficient with much reduced time and energy consumption. However, a major drawback of combustion synthesis is that, the synthesized products are usually porous bodies with poor strength, which need to be pulverized into powders and then densified by sintering to form usable bulk materials.13,15,16 The combustion synthesis method will be more promising if

ACCEPTED MANUSCRIPT densification can be finished simultaneously with synthesis and bulk materials are directly obtained in one step. To realize simultaneous densification during combustion synthesis, a mechanical

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pressure can be applied on the sample to cause a fast plastic deformation at high temperatures. By

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this means, a variety of refractory materials such as intermetallics, ceramics, and cermets have been prepared.18-21 In mechanical pressure assisted combustion synthesis, however, a very careful control of the whole processing is necessary to produce dense samples with uniform microstructure and

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complicated equipment is usually required. Moreover, this method has been limited to the

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fabrication of structural materials and not used for preparing functional materials like thermoelectric samples.

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Herein, we report a modified combustion synthesis method to realize one-step and direct

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fabrication of bulk thermoelectric materials, in which a gas pressure or high gravity is applied for

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achieving simultaneous densification during the synthesis. By this method, nearly full-dense Cu2ZnSnSe4 samples are prepared and their thermoelectric properties are investigated in the

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temperature range of 323-773 K. The effects of chemical composition on the electrical and thermal transport properties of the samples are discussed.

Experimental Materials synthesis: High-purity element powders of Cu (3N), Zn (4N), Sn (4N), Se (4N), and In (4N) were weighed and mixed in an agate mortar according to the stoichiometry. The powder mixture was cold pressed into a compact for combustion synthesis, which was carried out in two ways, as illustrated in Figure 1. (1) Combustion synthesis in Ar atmosphere. The reactant compact was loaded in a quartz crucible

ACCEPTED MANUSCRIPT on a graphite substrate and placed into a closed reaction chamber. A tungsten coil was fixed above the top surface of the compact. The reaction chamber was first evacuated and subsequently filled

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with Ar gas up to a pressure of 2 MPa. The compact was ignited by passing an electric current in the

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tungsten coil and then continued to burn in a self-sustained way, finally burning off in a few seconds.

(2) Combustion synthesis in a high-gravity field. The reactant compact was loaded in a quartz

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crucible, which was then wrapped with carbon felt and put into a larger graphite crucible. A

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graphite cap was used to close the quartz and graphite crucibles. The graphite crucible was wrapped with carbon felt and placed in a steel cup, and the cup was horizontally mounted at one side of a

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rotator in the reaction chamber. A counterweight was mounted at the other side of the rotator to

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keep balance. After the chamber was evacuated, the rotator was started, and an equivalent high

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gravity (G) was induced by high-speed rotation and expressed as G=ω2L, where ω is the angle velocity and L is the distance from the axis of rotation to the point of interest. When the high gravity

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reached G=800g (g=9.8 m/s2), the reactant compact was ignited.

Figure 1. An illustration of combustion synthesis of Cu2ZnSnSe4: (a) combustion synthesis in Ar atmosphere; (b) combustion synthesis in a high-gravity field.

Characterization and measurements: The bulk densities of the samples were measured according to the Archimedes principle. The phase assemblage was identified by powder X-ray diffraction (XRD;

ACCEPTED MANUSCRIPT D8 Focus, Bruker, Germany) using Cu-Kα radiation, with a scanning step of 0.02o and scanning rate of 4o/min. The microstructure was examined by scanning electron microscopy (SEM; S-4800, Hitachi,

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Japan), and energy dispersive spectroscopy (EDS; INCA, Oxford Instrument, UK) was used for

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chemical composition analysis. X-ray photoelectron spectra (XPS) were recorded with an ESCALAB 250Xi (ThermoFisher Scientific, USA) system using the monochromated Al Kα line as the X-ray source.

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The electrical conductivity (σ) and the Seebeck coefficient (S) were measured using LSR-3 (Linseis,

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Germany) in a static He atmosphere. The thermal diffusivity (λ) was measured by a laser flash method (LFA 457, Netzsch, Germany), and the thermal conductivity (κ) was calculated according to

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the relationship of κ=λCpρ, where Cp is the specific heat and ρ is the density. The lattice thermal

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factor (L=2.45×10-8 V2K-2).

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conductivity (κL) is calculated based on the Wiedemann-Franz law κ=κL+LσT, where L is the Lorentz

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Results and discussion

The reaction of 2Cu+Zn+Sn+4Se=Cu2ZnSnSe4 is exothermic, and the enthalpy change equals the standard formation enthalpy of Cu2ZnSnSe4, which is reported to be ΔfH=-312.2 kJ/mol.22 From the relevant thermodynamic data,22-25 the heat energy required for bringing Cu2ZnSnSe4 from room temperature to its melting point (805oC) can be calculated to be Q=155.6 kJ/mol. Because |ΔfH| is much larger than Q and the loss of heat should be limited in a short time during the fast reaction, it is expected that the reaction temperature reaches the melting point of Cu 2ZnSnSe4 and the synthesized Cu2ZnSnSe4 is fully or at least partially molten. In this case, the finally-obtained Cu2ZnSnSe4 bulk samples are produced by melt solidification instead of powder sintering. To prepare dense bulk samples with a low porosity, it is necessary to reduce the volume fraction of gas bubbles in the Cu2ZnSnSe4 melt or accelerate the removal of bubbles from the melt.

ACCEPTED MANUSCRIPT In combustion synthesis of Cu2ZnSnSe4, the maximum reaction temperature (805oC) is much higher than the boiling point of Se (685oC), and thus strong evaporation of Se is expected. The drastic

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evaporation of Se in a short time will induce an overflow phenomenon (the Cu2ZnSnSe4 melt

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containing many gas bubbles overflows out of the crucible), which was actually observed for combustion experiments in open air or in vacuum. As a result, highly porous samples with an irregular shape were obtained. According to the thermodynamics,26 the boiling point of a substance

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depends on the ambient pressure, and increases with increasing pressure. For example, when the

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ambient pressure is enhanced from 1 atm to 2 MPa, the boiling point of Se will greatly increase from 685oC to 1004oC. In other words, the evaporation of Se can be depressed by increasing the ambient

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pressure. Following this idea, combustion synthesis of Cu2ZnSnSe4 was carried out in 2 MPa Ar gas

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atmosphere, and bulk samples with regular cylindrical shape were obtained, as shown in Figure 2.

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Nevertheless, the samples were not entirely dense but consisted of two parts, where the upper part is porous and the lower part is highly dense with a bulk density very close to the theoretical value

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(Table 1), and the volume fraction of the dense part in the whole sample was about 50%. Bulk Cu2ZnSnSe4 samples with a regular cylindrical shape were also produced by combustion synthesis in a high-gravity field (Figure 2). In the samples, the volume fraction of dense part was about 85%, which was much larger than that in the sample synthesized in 2 MPa Ar atmosphere. It is likely that the high-gravity field is more effective than the gas pressure in reducing the porosity and increasing the volume fraction of dense part. As discussed before, the synthesized Cu2ZnSnSe4 is mostly or fully molten, in which gas bubbles are usually entrapped. The gas bubbles will rise upwards in the Cu2ZnSnSe4 melt because of the buoyancy, and the rising velocity is proportional to the gravity.27 In a high-gravity field with an acceleration of 800g (g=9.8 m/s2), the rising velocity of

ACCEPTED MANUSCRIPT gas bubbles should be 800 times of that under normal gravity of g. That is to say, the removal of gas bubbles from the Cu2ZnSnSe4 melt can be much accelerated in a high-gravity field, finally resulting

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in lower porosity and larger fraction of dense part in the samples.

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Figure 2. Photos of Cu2ZnSnSe4 samples prepared by combustion synthesis: (a) synthesized in 2 MPa

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Ar atmosphere (after machining); (b) synthesized in a high-gravity field.

Table 1. Bulk densities of the synthesized Cu2ZnSnSe4 samples Synthesis condition

Density (g/cm3)

in 2 MPa Ar atmosphere

5.67

in high-gravity field

5.68

Cu2.1Zn0.9SnSe4

in high-gravity field

5.64

Cu2ZnSn0.9In0.1Se4

in high-gravity field

5.62

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Sample

Cu2ZnSnSe4

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Cu2ZnSnSe4

After combustion synthesis, Cu2ZnSnSe4 was synthesized as the major phase in all the samples, except for minor impurity phases such as SnSe, Cu2Se, and CuSe (Figure 3). No significant difference was observed for different parts (e.g. the upper porous part and the lower dense part) in one sample. From the XRD results, the reaction among the element powders (Cu, Zn, Sn, and Se) was almost complete although the combustion reaction was very fast and lasted for only a few seconds

ACCEPTED MANUSCRIPT during combustion synthesis. A possible reason for the fast reaction is the melting of reactant powders. During the combustion synthesis, the maximum reaction temperature (805 oC) is much

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higher than the melting points of Zn (420), Sn (232), and Se (221). In this case, most reactants occur

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transportation, finally increasing the reaction rate.

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as liquid, which increases the contacting area among different reactants and facilitates the mass

Figure 3. XRD patterns of the synthesized Cu2ZnSnSe4 samples: (a) and (b) synthesized in 2 MPa Ar atmosphere, (a) for the lower dense part and (b) for the upper porous part; (c)-(f) synthesized in a high-gravity field, (c) Cu2ZnSnSe4, (d) Cu2.1Zn0.9SnSe4, (e) Cu2ZnSn0.9In0.1Se4, lower dense part, and (f) Cu2ZnSn0.9In0.1Se4, upper porous part. The diffraction peaks of the impurity phases are more clearly visible in the right figure by magnification in the intensity axis.

ACCEPTED MANUSCRIPT Figure 4 shows the XPS spectra of the Cu2ZnSnSe4 sample synthesized in 2 MPa Ar atmosphere. The Cu 2p spectrum shows two narrow symmetric peaks at 932.6 eV (2p3/2) and 952.4 eV (2p1/2) with a

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peak splitting of 19.8 eV, which is characteristic of Cu (I). The Zn 2p peaks appearing at 1022.2 eV

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(2p3/2) and 1045.2 eV (2p1/2) with a peak splitting of 23 eV correspond to Zn (II). The Sn 3d peaks appearing at 486.8 eV (3d5/2) and 495.2 eV (3d3/2) with a peak splitting of 8.4 eV can be assigned to Sn (IV). The Se 3d5/2 peak located at 54.6 eV indicates Se with a valence of -2. Similar XPS spectra

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were observed for the other samples, confirming the chemical valence of +1, +2, +4, and -2 for Cu,

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Zn, Sn, and Se, respectively.

Figure 4. XPS spectra of the Cu2ZnSnSe4 sample synthesized in 2 MPa Ar atmosphere.

ACCEPTED MANUSCRIPT Figure 5 shows SEM images of the Cu2ZnSnSe4 sample synthesized in 2 MPa Ar atmosphere. The upper part of the sample is highly porous, in which large pores with a pore size of >100 μm are

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visible. The lower part of the sample is dense with few pores observed. The back-scattered electron

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image clearly shows the presence of secondary phase, which looks bright and is homogeneously distributed at the grain boundaries of the major phase of Cu2ZnSnSe4. The grain size of Cu2ZnSnSe4

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ranges from tens of μm to larger than 100 μm.

Figure 5. SEM images of the Cu2ZnSnSe4 sample synthesized in 2 MPa Ar atmosphere: (a) and (b) upper porous part; (c) and (d) lower dense part.

Similar microstructure is observed in the sample synthesized in a high-gravity field (Figure 6), in which the bright secondary phase is located at the grain boundaries of the grey matrix of Cu2ZnSnSe4. By EDS analysis, the bright phase is rich in Sn and Se with a molar ratio of Sn/Se=1.04,

ACCEPTED MANUSCRIPT and thus assigned as SnSe by also considering the XRD results (Figure 3). Besides the bright SnSe phase, some bi-phase areas are observed at the grain boundaries of the matrix phase, showing a

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eutectic-like structure characterized by intersection of a bright phase and a dark phase with

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interphase spacing on a submicron scale (Figure 6 (e) and (f)). EDS analysis for the bi-phase area gives a composition of Cu:Sn:Se=3.0:1.0:2.1 in molar ratio. By considering both EDS and XRD results, the bright and dark phases are expected to be SnSe and binary Cu-Se compound (CuxSe),

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respectively.

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Figure 6. SEM images and EDS results of the Cu2ZnSnSe4 sample synthesized in a high-gravity field: (a) upper porous part; (b)-(f) lower dense part with EDS spectra. (a) and (b) are secondary electron images for fracture surface, and (c)-(f) are back-scattered electron images for polished surface.

ACCEPTED MANUSCRIPT The electrical and thermal transport properties of the synthesized Cu2ZnSnSe4 samples are measured in the temperature range of 323-773 K. The electrical conductivities of the samples fall in

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the ranges of 10000-19000 S m-1 and 6000-10000 S m-1 at 323 K and 773 K, respectively. For all the

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samples, the electrical conductivity decreases at first and then slightly increases with increasing temperature, and the exact reason for this transition of the σ-T dependence is unknown at present. In the whole temperature range, the Cu-excessive sample (Cu2.1Zn0.9SnSe4) shows the largest

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electrical conductivity, which may be attributed to the creation of holes (Cu 2+ 3d9 vs. Cu+ 3d10) and

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the conversion of electrically insulating paths ([ZnQ4]) to electrically conducting paths ([CuQ4]).28-30 It is noted but not well explained that the electrical conductivity is little changed by In-doping

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(Cu2ZnSn0.9In0.1Se4), which is not consistent with previous reports where In-doping results in a

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significant increase in electrical conductivity of Cu2ZnSnSe4.31

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The synthesized Cu2ZnSnSe4 samples exhibit positive Seebeck coefficients, confirming their p-type character. The Seebeck coefficients of the samples increase with increasing temperature, falling in

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the ranges of 64-110 μV K-1 and 195-241 μV K-1 at 323 K and 773 K, respectively. By comparing the three samples synthesized in a high-gravity field, it is found that both Cu-excess and In-doping lead to an enhancement of Seebeck coefficient. With high electrical conductivity and large Seebeck coefficient, the Cu-excessive sample (Cu2.1Zn0.9SnSe4) exhibits the maximum power factor in the whole temperature range, which reaches 0.50 mW m-1 K-2 at 773 K. Except for the stoichiometric sample synthesized in a high-gravity field, the other three samples show similar values in electrical conductivity, Seebeck coefficient, and powder factor at 773 K. It is also noticed that, for the stoichiometric composition (Cu2ZnSnSe4), the sample synthesized in a high-gravity field shows better electrical transport properties than those of the sample synthesized in 2 MPa Ar atmosphere

ACCEPTED MANUSCRIPT at lower temperatures. At higher temperatures, however, the sample synthesized in Ar atmosphere exhibits higher electrical conductivity and larger Seebeck coefficient. The difference in electrical

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may be related with their different structural characters.

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properties of the two samples is not well understood now, which requires further investigation and

Figure 7. Electrical transport properties of the Cu2ZnSnSe4 samples. σ: electrical conductivity, S: Seebeck coefficient, PF: power factor, PF= S2σ.

The temperature dependence of thermal conductivity and ZT of the Cu2ZnSnSe4 samples is plotted in Figure 8, where the lattice thermal conductivity is also calculated according to the Wiedemann-Franz law κ=κL+LσT (L=2.45×10-8 V2K-2). For all the samples, the thermal conductivity

ACCEPTED MANUSCRIPT (and also the lattice thermal conductivity) decreases with increasing temperature. At 773 K, the lattice thermal conductivities of all the samples are below 1.0 W m -1 K-1. The low thermal

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conductivities of the Cu2ZnSnSe4 samples are related with the complex crystallographic structure of

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the quaternary compound, which can be derived from the analog of binary II-VI zincblende with cross substitution of other atoms and exhibits a natural distorted structure that is effective for scattering phonons.29-31 The thermal conductivity of Cu2ZnSnSe4 can be further reduced by partial

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substitution of Sn with In, which will create atomic mass fluctuations and enhance phonon

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scattering. For this reason, the In-doped sample (Cu2ZnSn0.9In0.1Se4) shows the lowest thermal conductivity in the whole temperature range, and its lattice thermal conductivity is only 0.45 W m -1

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K-1 at 773 K. In contrast to In-doping, partial substitution of Zn with Cu does not decrease the

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thermal conductivity, and in fact the Cu-excessive sample (Cu2.1Zn0.9SnSe4) exhibits the highest

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thermal conductivity among the four samples. The high thermal conductivity of the Cu-excessive sample cannot be well explained at present, and a possible reason may be the reduction of the

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fraction of [ZnSnSe4] layers by substitution of Zn with Cu, because the [ZnSnSe4] layers are thought to be responsible for the low thermal conductivity of Cu 2ZnSnSe4 according to the PGEC theory.32,33 It is also noticed that, for the stoichiometric composition (Cu2ZnSnSe4), the sample synthesized in 2 MPa Ar atmosphere shows higher thermal conductivity than that of the sample synthesized in a high-gravity field.

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Figure 8. Temperature dependence of thermal conductivity (κ) and ZT of the Cu 2ZnSnSe4 samples. The inset shows the temperature dependence of the lattice thermal conductivity (κ L).

The ZT values of the Cu2ZnSnSe4 samples increase with increasing temperature and reach the maximum at 773 K. For the stoichiometric composition (Cu2ZnSnSe4), the two samples synthesized in 2 MPa Ar atmosphere and in a high-gravity field show similar ZT values, despite the clear difference in their electrical and thermal transport properties. Among the four samples, the In-doped sample (Cu2.1Zn0.9SnSe4) has the largest ZT of 0.59 at 773 K, which almost doubles those of

ACCEPTED MANUSCRIPT the other three samples (ZT=0.30-0.33) and is comparable to the best results reported for Cu2ZnSnSe4 materials.12,28,29,31,32 Compared with the conventional powder sintering approach, the

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combustion synthesis method is a fast and one-step way for directly preparing bulk Cu2ZnSnSe4

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samples, in which synthesis and consolidation are finished simultaneously and subsequent sintering is not necessary. In this case, combustion synthesis may offer an alternative method for producing

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Cu2ZnSnSe4-based thermoelectric materials with much-reduced time and energy consumption.

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Conclusion

Nearly full-dense Cu2ZnSnSe4 samples with a porosity of <1% were prepared by combustion

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synthesis in 2 MPa Ar atmosphere or in a high-gravity field. The gas pressure and high-gravity field

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were applied to reduce the porosity in samples and realize simultaneous densification during

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synthesis. The electrical and thermal transport properties of the prepared Cu 2ZnSnSe4 samples were investigated in the temperature range of 323-773 K. For the stoichiometric composition

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(Cu2ZnSnSe4), the two samples synthesized in 2 MPa Ar atmosphere and in a high-gravity field showed similar ZT values, despite the difference in their electrical and thermal transport properties. The substitution of Zn with Cu (Cu2.1Zn0.9SnSe4) enhanced the electrical transport properties (electrical conductivity, Seebeck coefficient, and power factor), but the ZT was not much improved because of increase in thermal conductivity. The In-doped sample (Cu2ZnSn0.9In0.1Se4) exhibited relatively high power factor and low thermal conductivity and thus showed the largest ZT of 0.59 at 773 K, which was comparable to the best results reported for Cu2ZnSnSe4-based materials. Compared with the conventional powder sintering approach, combustion synthesis offers a fast, one-step, and furnace-free way for directly producing bulk Cu2ZnSnSe4 samples, which may open up

ACCEPTED MANUSCRIPT new possibilities for synthesis and applications of Cu2ZnSnSe4 and other similar thermoelectric

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materials.

Acknowledgements

Financial supports from National Natural Science Foundation of China (Grant No. 51422211),

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National Magnetic Confinement Fusion Science Program of China (Grant No. 2014GB125000 and

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2014GB125005), Instrument Developing Project of the Chinese Academy of Sciences (Grant No.

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YZ201322), and Beijing Nova Program (Grant No. Z131103000413053) are acknowledged.

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ACCEPTED MANUSCRIPT Phys Status Solidi RRL, 2014, 8, 61-64. 29. Min-Ling Liu, Fu-Qiang Huang, Li-Dong Chen, I-Wei Chen. A wide-band-gap p-type

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thermoelectric material based on quaternary chalcogenides of Cu2ZnSnQ4 (Q=S,Se). Appl Phys

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30. Min-Ling Liu, I-Wei Chen, Fu-Qiang Huang, Li-Dong Chen. Improved Thermoelectric Properties of Cu-Doped Quaternary Chalcogenides of Cu2CdSnSe4. Adv Mater, 2009, 21, 3808–3812

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wide-gap stannite compounds Cu2ZnSn1-xInxSe4. Appl Phys Lett, 94, 122103, 2009. 32. Ch. Raju, M. Falmbigl, P. Rogl, X. Yan, E. Bauer, J. Horky, M. Zehetbauer, R. C. Mallik.

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33. Slack, G. A. in CRC Handbook of Thermoelectrics (ed. Rowe, D. M.) 407-440 (CRC, 1995).

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Graphical abstract

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Bulk Cu2ZnSnSe4 materials are directly produced by combustion synthesis under gas pressure or

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high gravity.

Simultaneous densification is finished during synthesis to realize one-step preparation of dense samples.

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By In-doping the Cu2ZnSn0.9In0.1Se4 sample shows an enhanced ZT of 0.59 at 773K.

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