Trans. Nonferrous Met. Soc. China 25(2015) 3279-3285

Identification and characterization of single crystal Bi₂Te_3-xSe_x alloy

Emina ¹, Svetlana IVANOV², Zoran  ², Ljiljana  ³, Rudolf TOMANEC³, Lidija  ¹, Ana KOSTOV¹

1. Mining and Metallurgy Institute Bor, Zeleni bulevar 35, Bor 19210, Serbia;

2. Technical Faculty Bor, University of Belgrade, VJ 12, Bor 19210, Serbia;

3. Faculty of Mining and Geology, University of Belgrade,  7, Belgrade 11000, Serbia

Received 6 November 2014; accepted 6 April 2015

Abstract: The results of experimental investigation of n-type semiconductor based on Bi₂Te₃ alloy were presented. This material is used in manufacture of thermoelectric coolers and electrical power generation devices. Bi₂Te_2.88Se_0.12 solid solution single crystal has been grown using the Czochralski method. Monitoring of structure changes of the sample was carried out by electron microscope. The elemental composition of the studied alloy was obtained by energy dispersive spectrometry (EDS) analysis and empirical formula of the compound was established. X-ray diffraction analysis confirmed that the Bi₂Te_2.88Se_0.12 sample was a single phase with rhombohedral structure. The behavior upon heating was studied using differential thermal analysis (DTA) technique. Changes in physical and chemical properties of materials were measured as a function of increasing temperature by thermogravimetric analysis (TGA). The lattice parameters values obtained by X-ray powder diffraction analyses of Bi₂Te_2.88Se_0.12 are very similar to Bi₂Te₃ lattice constants, indicating that a small portion of tellurium is replaced with selenium. The obtained values for specific electrical and thermal conductivities are in correlation with available literature data. The Vickers microhardness values are in range between HV 187 and HV 39.02 and decrease with load increasing. It is shown that very complex process of infrared thermography can be applied for characterization of thermoelectric elements and modules.

Key words: Bi₂Te₃; Bi₂Te_3-xSe_x; single crystal; semiconductor; thermoelectrical properties; hardness; thermovision imaging

1 Introduction

Semiconductors are materials whose electronic properties depend on the energy gap (E_g) width or band gap and dopant concentration. It is well known that semiconductors with a relatively small band gap exhibit very good thermoelectric performance [1-3]. The E_g of semiconductors is relatively small [4] and at the room temperature (300 K) it is 0.66 eV. The energy gap of Bi₂Te₃ single phase varies with temperature and it is 0.13 [5], 0.15 [6,7], 0.21 eV [8] at room temperature.

Tellurium (Te) and selenium (Se) belong to twelve elementary semiconductors. Elementary semiconductors are those in which the properties are dependent on the semiconductor electronic structure. The presence of impurities even in very small proportions can have large effects on the properties of the thermoelectric material [9]. Therefore, the conductivity of semiconductors may easily be modified by introducing dopants into their crystal structure.

The n-type Bi₂Te_3-xSe_x semiconductors are well known to have excellent thermoelectric properties near room temperature, and are used in thermoelectric cooling devices and electrical power generation devices. Many studies on these materials have been performed in order to increase the figure of merit, Z, the parameter which evaluates the quality of thermoelectric material [10]. The figure of merit for the undoped Bi₂Te₃ at room temperature is 0.6 [11]. Thermal conductivity can provide information about the temperature range of operation [12]. Typical values of elemental semiconductors for the Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (λ) and figure of merit (Z_T) at room temperature are: S=200×10^-6 V·K^-1, σ=10⁵ Ω^-1·m^-1, λ=1-100 W·m^-1·K^-1, Z_T=0.1-1.0, where T is the temperature [13].

The electric conductivity is a characteristic of each material [14]. Only semiconductors offer the right combination of high Seebeck coefficient, high electrical conductivity and low thermal conductivity.

The change in Z_T with temperature for most of the known thermoelectric n-type materials studied to date is shown in Fig. 1.

The values of Bi₂Te_3-xSe_x alloy for electrical resistivity (ρ), the Seebeck coefficient (S), power factor (F_P) and thermal conductivity (λ) at 250-350 K are: ρ=10×10^-6 Ω·m, S=-200×10^-6 V·K^-1, F_P=40×10^-4 W·m^-1·K^-2, λ=1.4 W·m^-1·K^-1 [13].

The negative value of the Seebeck coefficient means that the major charge carriers in the sample are electrons.

Bi₂Te₃ has the tetradymite-type rhombohedral structure [14-18]. Bi₂Te₃ melting point is 585 °C [18,19].

The objective of this work is to supplement knowledge of the structure, mechanical and thermoelectrical properties of Bi₂Te_3-xSe_x alloy.

Fig. 1 Value of Z_T for most of known thermoelectric n-type semiconductors [10]

2 Experimental

Bi₂Te_2.88Se_0.12 sample was synthesized using the Czochralski method. Single crystals of 20 mm in length and 8 mm in diameter were obtained by this method. The obtained sample of semiconductor was examined using SEM-EDS, XRD, four probe method for measuring electrical conductivity, Hartman method for measuring specific heat conductivity, DTA, TGA and microhardness measurements. Thermovision imaging was carried out. Structural analysis was done [20] using a Reichert MeF₂ microscope (magnification up to 200) and SEM-EDS. Surfaces of the polished sample for light optical microscopy (LOM) and SEM-EDS prior to metallographic analysis, were etched with hydrofluoric, nitric and acetic acid etching solution to reveal the structure of the investigated sample [21]. EDS analysis was done to determine chemical composition of the studied sample as well as to check sample homogeneity.

The sample was further powdered for XRD analyses. X-ray intensity measurement was carried out on a PHILIPS PW 1710 automated diffractometer using monochromatized Cu K_a radiation (l=1.54178 ) and step-scan mode (2q range was from 4° to 90°, step 0.02°, time 0.5 s). The specific electrical conductivity of the sample d8 mm × 20 mm was measured at 25 °C. Four probe method was used.

The specific thermal conductivity was measured on the same sample and at the same temperature. Harman method was applied.

Phase transition temperatures and the physical property of the sample were analyzed using TG–DTA technique. Differential thermal analysis (DTA) and TG measurements were carried out using SDT Q60 (firm TA instruments) apparatus.

Microhardness was measured using instrument PTM-3 with Vicker’s diamond pyramid (Vickers indenter). All the measurements were made at room temperature, and the indentation time was kept at 15 s. Microhardness values vary with load and work- hardening effects of materials. The applied indenter load was varied from 15 to 150 g, and the diagonal of the impression was measured. Hardness of the crystals was calculated using the relation:

H_DP=1.8544 P/d²                                             (1)

where H_DP is the Vickers microhardness value; P is the indenter load in kg; and d is the diagonal length of the impression in mm. On each surface, several trials of indentation are taken and the average value of hardness is found.

In these researches, the digital thermal imaging camera Wohler IK 21, whose operation is based on a uncooled germanium thermoelectrical linear detector, has been used.

Thermovision imaging is a non-contact method within real-time register emission of heat or infrared radiation. Since all the bodies emit heat in this way, we can get a clear idea about the temperature of a body in relation to the environment in which the body is. There is almost no area in which this powerful technique is not applied. It is simple to use environmentally and friendly [22,23].

3 Results and discussion

3.1 SEM-EDS analysis

Chemical analysis was done using SEM-EDS giving empirical formula of the compound as Bi₂Te_2.88Se_0.12. SEM image with marked points of EDS chemical analysis of the investigated sample is presented in Fig. 2, while the results of experimental determination of composition by EDS are presented in Table 1.

Figure 2 shows the cracks which appear in the crystal. Special growth conditions are necessary for avoiding these cracks. A low temperature gradient in the melt and solid is necessary for achieving low dislocation density or strain in the crystal. The role of melt stirring during crystal growth is also crucial.

Fig. 2 SEM image (with investigated points) of longitudinal section of Bi₂Te_2.88Se_0.12 sample

Table 1 Results of SEM-EDS analysis of investigated sample Bi₂Te_2.88Se_0.12

Concentrations of elements in studied points are function of the peak areas in EDS diagram (Fig. 3).

The obtained chemical compositions in the investigated points are similar, which means that the sample has uniform chemical composition (Table 1).

3.2 XRD analysis

Lattice constants of powdered Bi₂Te_2.88Se_0.12 were determined by XRD analysis. The powder XRD patterns of Bi₂Te_2.88Se_0.12 given in Table 2 and Fig. 4, show the presence of only one crystalline phase of a relatively high degree of crystallinity. All reflections in the XRD pattern of Bi₂Te_2.88Se_0.12 were indexed to be a hexagonal unit cell: a=4.368(2), c=30.418(11) , V= 502.5(4)  ³, which is very similar to a=4.3896(2), c= 30.5019(10) , V=508.99(4)  ³ for Bi₂Te₃ [24] and shows that a small portion of Te is replaced with Se. The incorporation of smaller Se leads to the lattice contraction and therefore all diffraction lines were slightly shifted toward bigger 2q angles with respect to Bi₂Te₃ lines (Fig. 4). Diffraction peaks of basal reflections (specially (0 0 3), (0 0 6) and (0 0 15)) are much more intense than the calculated ones, indicating the existence of preferred orientation due to the layered structure and perfect basal cleavage (Fig. 4).

Fig. 3 EDS spectrum 1 (a) and spectrum 2 (b) of Bi₂Te_2.88Se_0.12

Fig. 4 Calculated XRD pattern of Bi₂Te₃ (a) using structural data given in Ref. [24] and measured powder XRD pattern of Bi₂Te_2.88Se_0.12 (b)

Table 2 XRD powder diffraction data for Bi₂Te_2.88Se_0.12

All semiconductors of the general formula Bi₂Te_3-xSe_x are solid solutions, which are isostructural with the Bi₂Te₃. They crystallize in the rhombohedral space group  (No. 166), and have layered structure of Bi₂Te₃ type. Chalcogens Ha in the structure are placed in two different Wyckoff positions: 3a and 6c, while the Bi atoms are found in 6c. Each chalcogen in the 3a position, Ha(3a), has 6 neighbouring Bi atoms, while chalcogen in the 6a position, Ha(6c), has 3 neighbours. This difference in coordination causes different lengths of Ha—Bi bonds: Ha(3a)—Bi bonds are longer and Ha(6c))—Bi bonds are shorter.

The basic part of the structure consists of five layers —[Ha(3a)]—Bi(6c)—[Ha(6c)]—Bi(6c)—[Ha(3a)]—. Layers of the Bi and chalcogens are distributed alternately along the hexagonal c axis and they are linked by the weak van der Waals bonds.

3.3 Thermoelectric properties

Electrical conductivity σ, Seebeck coefficient S and thermal conductivity λ have been measured to define quality. These properties strongly depend on carrier concentration, mobility, crystal structure and defects in the crystal structure. Values of Bi₂Te_2.88Se_0.12 crystal for electrical conductivity (σ), the Seebeck coefficient (S) and thermal conductivity (λ) at 298 K are: σ=1.03×10⁵ S·m^-1, S=207×10^-6 V·K^-1, λ=1.77 W·m^-1·K^-1.

ZHANG at al [25] have investigated 1-D/3-D structured AgNWs/Bi₂Te₃ nanocomposites and found that thermoelectric properties have been dramatically enhanced. Different researches have studied improvement of thermoelectric performance of nanocomposites, too [26-28].

3.4 Differential thermal analysis (DTA)

DTA was carried out under following conditions: air atmosphere, heating rate 10 °C/min, and T_max=800 °C. Al₂O₃ was used as a referent material during the measurements. The precision of the measurement in the investigated temperature interval was 2.5 °C. The total mass of the material was 1 g.

Obtained liquidus temperature (613.7 °C) in Fig. 5 is close to melting temperature of Bi₂Te₃ compound, but slightly increased, which can be contributed to the presence of Se. The obtained DTA results imply that the tested material is stable to about 400 °C. The first endothermic reaction at 409.7 °C corresponds to the formation of γ phase (Bi₂Te₂Se) which is affirmed in the studies reported by SOKOLOV et al [29] and LAHALLE et al [30]. The second reaction obtained at temperature of 491.3 °C does not correspond to any phase transformation which may be related to the binary phase diagram of Bi-Te or with phase transformations on Bi-Se binary diagram. We assumed that the second obtained temperature on the DTA curve refers to reaction of two phases: a Bi₂Te₃-based solid solution (β₁) and a Bi₂Te₂Se (γ) [26]. In this work, the range of Bi₂Te_3-xSe_x solid solution was determined.

Fig. 5 DTA curve of Bi₂Te_2.88Se_0.12 crystal

TGA was carried out under following conditions: heating rate, 10 °C/min, sample mass 0.182 g, T_max= 500 °C in air medium. The curves obtained from thermogravimetric analysis and thermogravimetric (TG) curve show the sample change in mass depending on temperature (Fig. 6). The compound is stable up to 350 °C and, after that, its mass increases which may be due to the undergoing reaction of an intermediate product with nitrogen. This is affirmed in the study reported by DHEEPA et al [31]. HANEMAN [32] has noted that the absorption of gases (O_2, N₂, CO₂) on the obtained bismuth telluride samples is small.

Fig. 6 TG curve of Bi₂Te_2.88Se_0.12 crystal

3.5 Microindentation hardness studies

Structural defects in tested material must affect the results of microhardness measurements. Hardness measurements quantify the resistance of a material to plastic deformation. Microhardness and dislocation density in the case of semiconducting crystals are interrelated.

Table 3 Microhardness of n-type investigated Bi₂Te_2.88Se_0.12 crystal

Figure 7 shows the variation of hardness with load for bismuth telluride, doped with Se. Hardness decreases with load.

According to literature, undoped single crystals of Bi₂Te₃ have Knoop of 155 N/mm² [33]. Since the paper presents different methods of measuring microhardness, the comparative analysis of the obtained results and available literature data is not possible.

Fig. 7 Variation of microhardness with load for single crystal Bi₂Te_2.88Se_0.12 at room temperature

3.6 Thermovision imaging of TE module

In order to test the efficiency of thermoelectric elements based on Bi₂Te₃ module, 27 elements of the p type measuring of 3 mm × 3 mm × 5 mm and 27 elements of the n type of the same size were developed (Fig. 8). Thermoelectric material of the p-type for the TE modules is Bi_0.5Sb_1.50Te_2.98Se_0.02 doped with zirconium.

Fig. 8 Thermoelectric module

Thermovision imaging of the module is shown in Fig. 9. One side of the module is heated up to 80 °C and then allowed to naturally cool down. The other side is passively cooled to about 22 °C. The module is loaded by thermogenic consumer of 2.2 Ω resistivity in order to obtain the maximum usable power since in the observing temperature range the internal resistance of the module is approximately constant with amount of 2.2 Ω resistivity. The resulting dependence of the output (electricity) power from the temperature difference at the hot and cold sides is shown in Fig. 10.

Fig. 9 Thermovision record of thermoelectric module

Fig. 10 Dependence of 30 W thermoelectric module output power from temperature difference of hot and cold sides

Figure 10 shows that the temperature difference between the hot and the cold sides is crucial for obtaining significant power at the output of the thermoelectric module.

4 Conclusions

1) A single crystal Bi₂Te_2.88Se_0.12 was investigated using different experimental methods, such as light optical microscopy, SEM-EDS, microhardness analysis, XRD, Four Probe Method, Harman method, TG-DTA and thermovision.

2) EDS analysis was done to determine chemical composition of the studied alloy. The results of the analysis are presented in this work together with the mole ratios of Bi to Te to Se.

3) By methods of differential thermal analysis and X-ray phase analysis, it is proved that the Bi₂Te₃-Bi₂Se₃ section is a system with a continuous series of solid solutions, with the formation of Bi₂Te_2.88Se_0.12 compound below the solidus line. DTA of the material shows the endothermic heat transfer during the process.

4) Microhardness of investigated Bi₂Te_2.88Se_0.12 single crystal alloy in the range of HV 39 to HV 187 is examined using different loads and obtained dependence is presented graphically. In order to test the efficiency of thermoelectric elements based on Bi₂Te₃ module, thermovision imaging is done. It is shown that thermal imaging research can be a simple tool for analyzing the heating uniformity of thermoelectric module.

Acknowledgement

The authors are grateful to the Ministry of Education, Science and Technological Development of the Republic of Serbia for financial support of projects: “Development of ecological knowledge-based advanced materials and technologies for multifunctional application” (Grant No. TR34005) and “New approach to designing materials for energy conversion and storage” (Grant No. OI172060) and “0-3D nanostructures for application in electronics and renewable energy sources: synthesis, characterisation and processing” (Grant No. III45007).

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单晶Bi₂Te_3-xSe_x合金的鉴定与表征

Emina  ¹, Svetlana IVANOV², Zoran  ², Ljiljana  ³, Rudolf TOMANEC³, Lidija  ¹

1. Mining and Metallurgy Institute Bor, Zeleni bulevar 35, Bor 19210, Serbia;

2. Technical Faculty Bor, University of Belgrade, VJ 12, Bor 19210, Serbia;

3. Faculty of Mining and Geology, University of Belgrade,  7, 11000 Belgrade, Serbia

摘  要：研究基于Bi₂Te₃合金的n型半导体的实验制备。该材料可用于制备热电散热器和发电设备。采用Czochralski法制备Bi₂Te_2.88Se_0.12固溶单晶体。采用电子显微镜研究样品的组织变化。利用EDS分析实验合金的成分，并确立化合物的经验分子式。XRD表明Bi₂Te_2.88Se_0.12样品为具有斜方六面体结构的单晶。利用差热分析研究合金的加热行为。利用热重分析研究材料的物理和化学性能随温度的变化。从XRD分析得到的Bi₂Te_2.88Se_0.12晶格参数与Bi₂Te₃的非常相近，表明只有少部分Te被Se取代。所得的单晶的比电导率和比热导率与已有数据相符。所得维氏显微硬度为HV 187~39.02，且随着载荷增加，硬度降低。结果表明可以采用红外热成像法对热电元件和模块进行表征。

关键词：Bi₂Te₃；Bi₂Te_3-xSe_x；单晶；半导体；热电性能；硬度；热视成像

(Edited by Yun-bin HE)

Corresponding author: Emina ; Tel: +381-30-454258; Fax: +381-30-435175; E-mail: emina.pozega@irm.bor.co.rs

DOI: 10.1016/S1003-6326(15)63964-4