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Bi2Te3 based bulks with high figure of merit obtained from cut waste fragments of cooling crystal rods
Materials Chemistry and Physics 201 (2017) 57e62
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Bi2Te3 based bulks with high figure of merit obtained from cut waste fragments of cooling crystal rods Qiusheng Xiang a, b, Xi'an Fan a, b, *, Xuewu Han a, b, Chengcheng Zhang a, b, Jie Hu a, b, Bo Feng a, b, Chengpeng Jiang a, b, Guangqiang Li a, b, Yawei Li a, b, Zhu He a, b a
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Waste fragments of cooling rods were recycled by vacuum metallurgy. The recycling technology is accordance with the principle of 3-R. The recycling yield of the Bi2Te3 based waste was over 98%. The ZT value of Bi0.4Sb1.6Te3 bulk from wastes could reach 1.07 at 379 K. The average ZT value of Bi0.4Sb1.6Te3 bulk from wastes was 1.0 from 299 K to 419 K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2016 Received in revised form 21 July 2017 Accepted 14 August 2017 Available online 25 August 2017
Bi2Te3 based cutting waste fragments were recycled by vacuum metallurgical technology to prepare ptype Bi2Te3 based thermoelectric materials. Through washing, vacuum metallurgical, component adjustment, second smelting and resistance pressing sintering (RPS) process, Bi0.4Sb1.6Te3 bulks were successfully obtained. All evidences confirmed that most of impurities from line cutting process such as Sb2O3 could be removed by washing, vacuum metallurgical process used in this work, and the recycling yield is up to 98%. The carrier content and corresponding thermoelectric properties could be adjusted effectively by appropriate component adjustment treatment. At lastly, a Bi0.4Sb1.6Te3 bulk was obtained and its' dimensionless figure of merit (ZT) could reach 1.07 at 379 K and the average ZT value was over 1.0 from 299 K to 419 K. Compared with the traditional process of recycling the single elemental Bi, Sb, Te by wet chemical metallurgical method, the vacuum metallurgical recycling technology introduced here is accordance with the principles of 3-R (reduce, reuse and recycle), and characterized by low cost, low degree of difficulty, low emission, high environmental protection and high efficiency, and the purified ingots can be directly used as raw material for the production of high-performance thermoelectric materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric performance Bi2Te3 Cutting waste Recycling Resistance pressing sintering
* Corresponding author. 185#, School of materials and metallurgy, Wuhan University of Science and Technology, 947 Heping Road, Qingshan District, Wuhan, 430081, China. E-mail address: [email protected] (X. Fan). http://dx.doi.org/10.1016/j.matchemphys.2017.08.037 0254-0584/© 2017 Elsevier B.V. All rights reserved.
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1. Introduction
2. Experimental
Recently the development of novel energy conversion [1e4] materials (such as photoelectric materials [5,6], thermoelectric materials [7,8] etc) has been received a great research interest to achieve a clean and sustainable world due to the limitation of fuels and environmental issues. And thermoelectric materials have the advantages of no noise, leakage and green to the environment etc in direct heat to electricity conversion and refrigeration. As one of the best thermoelectric materials near room temperature, the zonemelted Bi2Te3 have been commercialized for several decades [7e10]. However, the poor mechanical properties along crystal growth direction limit their application scopes, also lead to low material utilization rate of less than 50% [11]. A large number of thermoelectric waste materials which contains about ~48 tons of Te are generated in the semiconductor refrigeration industry every year [12]. Meanwhile thermoelectric waste also contains Bi, Sb, Se, Pb, Ni, Sn and other useful elements. It is worth nothing that the mineable time of Sb, Pb, Bi, Te in the earth (if sustained annual global primary production growth of 2%) are only 14 years, 18 years, 38 years, 72 years, respectively [13]. More important, the Sb, Te, Pb, Se and their compounds are toxic and dangerous [14]. In generally, the hydrometallurgical technology is used to extract the single element such as Bi, Te, Sb and Se from the thermoelectric waste materials. These useful elements are individually extracted by multi-step, high cost, long cycle, complicated and high-pollution process. Now, the traditional hydrometallurgical recycling technology mainly fall into two categories [12]. (1) The thermoelectric waste is oxidized by adding alkali in the smelting process, and then Te, Bi are separated by hydrometallurgical technology. It's mainly used to deal with the waste materials of high purity such as the head and tail of the crystal rod and block waste. (2) The thermoelectric waste is oxidized by oxidant in hydrochloric acid solution, then Te is retained in the residue, finally Te is obtained by refining the residue. It's mainly used to deal with the waste materials of low purity such as various chips and cutting powders. Unfortunately, both of them have the shortcoming such as long cycle, high cost and large environmental load because of wet chemical metallurgy process. So a simple, low cost, high utilization rate, no secondary pollution and green recycling technology is urgently needed. In this paper, Bi2Te3 based waste fragments were recycled by vacuum metallurgical technology. And we calculated and analyzed the saturated vapor pressure of bismuth, antimony, tellurium and their oxides, and demonstrated the feasibility of the whole vacuum metallurgical recycling process in theory. Furthermore, Bi2Te3 based cutting waste fragments were chosen as raw materials to prepare p-type Bi2Te3 based thermoelectric materials by washing, vacuum metallurgical removing impurity, component adjustment, second smelting and resistance pressing sintering (RPS) process. The overall recycling technology is a green process with no secondary pollution, low cost and high utilization rate (above 98%). Especially, the ZT value of the sample obtained by this series of processes is better than that of the corresponding zone-melting monocrystalline products in the whole temperature range. And the goal of this paper is to find a greener process to replace the traditional single element recovery process to reduce environmental pollution, shorten the material preparation cycle and save materials costs. However, the recovery of volatile elements in vacuum metallurgy had not been taken into consideration, which needs further improvement in future works.
In this work, the initial cutting waste fragments were obtained from Xianghe Huabei refrigeration equipment Co., LTD (Hebei, China) and used as received. These cutting waste fragments were contaminated by linear cutting liquid and their own oxidations. A typical vacuum metallurgical procedure was performed. Firstly, the initial cutting waste fragments were rinsed with deionized water and absolute ethyl alcohol to remove the contaminants from linear cutting liquid. Then, the waste fragments were dried in a vacuum drying oven for 24 h at 373 K, weighed, loaded in a unsealed quartz glass tube, and placed in a vacuum well-like melting furnace. Subsequently, the furnace was heated to 923 K for 30 min, and the degree of vacuum was held at 100 Pa. Lastly, the purified ingot was obtained. In order to adjust carrier concentrate and obtain the alloys with nominal component of Bi0.4Sb1.6Te3, appropriate amount of high purity (>99.99 wt %) Sb and Te granules were added into the purified ingot and then sealed into quartz glass tubes under vacuum. The raw materials sealed in quartz glass tubes were second melted in a rocking furnace and the temperature was fixed at 983 K and the soaking time was 120 min. Then, the sample was cooled to room temperature in furnace to get the Bi0.4Sb1.6Te3 ingot. Subsequently, the ingot was ground into powders with size of ~38 mm. Finally, the grinded powders were sintered by resistance pressing sintering (RPS) process at 683 K for 15 min under an axis pressure of 38 MPa in argon atmosphere [15,16]. The heating rate was fixed at 50 K/min. For comparison, purified crystal bar without component adjustment was also ground and sintered in the same condition. The phase structure of the bulk samples were characterized by X-ray diffraction (XRD, Philips) with an X'Per pro diffractometer by using Cu Ka radiation (l ¼ 1.5418 Å). The morphology and composition of bulk samples were observed by field-emission scanning electron microscopy (FESEM) (FEI, Nova400 NanoSEM) and energy dispersive X-ray spectroscopy (EDS). The electrical conductivity (s) were measured by four terminal method. The seebeck coefficient (S) were obtained by applying a dynamic temperature gradient (DT) of 4e8 K between both ends of the bars and measuring the output voltage (DV) between them, and the size of measured samples are 3 3 15 mm3. The seebeck coefficient was obtained by calculating the slope of the linear regression line through the least square method. The thermal conductivity (k) was calculated from the measured thermal diffusivity (l), specific heat (Cp) and volume density (d) by using the relationship k ¼ lCpd. The thermal diffusivity and specific heat of bulk samples were measured by a laser flash apparatus (LFA457, Netzsch, German). The volume densities (d) of the bulk samples were measured by the Archimedes method. The electrical properties and thermal properties of all bulk samples were measured in a direction perpendicular to the press direction during RPS process. Fig. 1 depicts the photos, SEM and the corresponding EDS, and XRD results of initial waste fragments, respectively. Fig. 1 (a) shows the surface of initial waste fragments is gray, no metallic luster, indicating that there are impurities on their surface such as oxides. From the EDS results (figure (c)), the presence of element O can be confirmed. Further, the XRD results (Fig. 1 (d)) indicate that there are Sb2O3 apart from the main phase of Bi0.5Sb1.5Te3. So the main impurities of the waste fragments is Sb2O3 from the oxidation of elements Sb. There are usually two ways to remove oxide without add new impurities: (1) using H2 or CO to restore the oxide, resulting in the
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Fig. 3. Real photo of quartz tube after vacuum melting process (a), SEM of area A (b) and corresponding EDS results (c), SEM of area B (d) and corresponding EDS results (e).
Fig. 1. Photos (a), SEM (b) and corresponding EDS results (c), and XRD pattern (d) of the initial waste fragments.
formation of the gaseous H2O or CO2 to leave system [17]; (2) volatilization of the oxide under high vacuum based on vacuum metallurgical recycle process [18]. In the work, the vacuum metallurgical recycle process is performed to remove the Sb2O3 impurity due to its high saturated vapor pressure. Fig. 2 shows the photos, SEM and corresponding EDS, and XRD results of the purified crystal bar. The surface of purified crystal bar is silvery white, with metallic luster (Fig. 2 (a)), and the recycling yield is up to 98%. In addition, the corresponding EDS results in Fig. 2 (c) indicates that there is no element O, confirming the purification effect of vacuum metallurgical process. Further, the XRD results (Fig. 2 (d)) indicate that there is no Sb2O3 apart from the main phase of Bi0.5Sb1.5Te3, further confirming the purification effect of vacuum metallurgical process. Fig. 3 shows the real photo of quartz tube after vacuum melting process, the SEM and corresponding EDS results of the two areas of
Fig. 2. Photos (a), SEM (b) and corresponding EDS results(c), and XRD pattern (d) of the purified crystal bar.
A and B on the inner wall of quartz tube. The volatiles deposited on the inner wall of quartz tube after vacuum melting process are obviously separated in two areas: pale yellow area (A) and black area (B). According to the result of EDS, it can be confirmed that there are elements O, Te and Sb in the two areas. Combined with the XRD results (Fig. 1 (d)), these volatiles should be mixture of Sb2O3 and Te. Possible explanations are shown in the following Fig. 4. Fig. 4 (a) shows the saturated vapor pressure of Sb, Bi, Te and Sb2O3 [19,20]. The saturated vapor pressures of Te and Sb2O3 are large enough at 923 K, and their value are 1.34 kPa and 1.06 kPa (Fig. 4 (a)), respectively, so their volatilization of Te and Sb2O3 must be considered. The saturated vapor pressure of Sb and Bi are very small and the value are only 45 Pa and 1 Pa (Fig. 4 (b)), respectively, so their volatilization of Sb and Bi can be ignored. In addition, the saturated vapor pressure of Te is much larger than that of Sb2O3 in whole temperature range. So the element Te volatilize preferentially with increasing temperature, but the Sb2O3 solidify preferentially with decreasing temperature. According to the result of EDS in the area of A, it can be found that there are lots of Sb2O3 and small amount of Te, indicating that the volatilized Sb2O3 mainly deposited on the area A. In addition, the yellow appearance at the area A also confirmed the presences of Sb2O3. According to the result of EDS in the area of B, it can be found that there are lots of Te and trace of Sb2O3, indicating that the volatilized Te mainly deposited on the area B. The temperature is decreasing from the center of vacuum furnace up to furnace mouth, and the saturated pressure of Te and Sb2O3 also decrease. When the temperature is reduced to 781 K, the saturated pressure of Te is 117 Pa, but the saturated pressure of Sb2O3 is only 13 Pa, namely most of Sb2O3 and small amount of Te condensed in the area of A, meanwhile most of Te is still in the gaseous state. When the temperature dropped to 647 K, the saturated vapor pressure of Te is only 1 Pa, namely most
Fig. 4. Temperature dependence of the saturated vapor pressure of Sb, Bi, Te and Sb2O3.
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of Te and trace of Sb2O3 condensed in the area of B. Fig. 5 (a) shows the XRD patterns of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3. It can be found that all elements Bi, Te and Sb are completely combined into a single phase with a rhombohedral structure and the impurity phase such as Sb2O3 are not observed, further confirming the purification effect of vacuum metallurgical recycle process. Fig. 5 (b, c) show the typical SEM of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3. It is clear that there is no large-scale preferred orientation in the sintered samples. However, partially oriented lamellar structure can be observed everywhere. In addition, there are some of fine pores with nano or submicron size distributed randomly in the as-RPSed samples, which should result from the volatilization of elements Te due to its high saturated vapor pressure. In order to adjust carrier concentrate of sample and obtain the alloys with nominal components of Bi0.4Sb1.6Te3, appropriate amount of high purity (99.99 wt %) Sb and Te granules were added into the purified ingots. Fig. 6 (a) shows the electrical conductivity based on temperature for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. The negative temperature dependence of the electrical conductivity
Fig. 5. XRD patterns and SEM fractographs of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 after component adjustment.
indicates a metallic conduction behavior due to the enhanced lattice scattering to carriers. In addition, it can be found that the electrical conductivity of the sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is higher than that of the sample without component adjustment. According to the definition equation, the electrical conductivity is closely related to the carrier concentration n and mobility m by the relationship: s ¼ nem [21,22]. As we all know, the dominative carriers (holes) are determined by antistructure defects due to the occupation of Te sites by Bi (Bi0 Te) and Sb (Sb0 Te) atoms for the p-type Bi-Sb-Te alloys. The small electronegativities difference between atoms Sb, Bi and Te (The electronegativities of Sb, Bi and Te are cSb ¼ 1.9, cBi ¼ 1.8 and cTe ¼ 2.1, respectively) easily lead to the formation of anti-site defects of Bi0 Te (Sb0 Te). Meanwhile, the polarity of the Sb-Te bond (Dc ¼ 0.2) is weaker than that of the Bi-Te bond (Dc ¼ 0.3), namely the Sb0 Te is more easily generated comparing with Bi0 Te. So, the content of antisite defects will increase the supplement of Sb, which results in the increase of carrier concentration [23,24] and electrical conductivity. Fig. 6 (b) shows Seebeck coefficient base on temperature for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. With increasing testing temperature, the Seebeck coefficient first increase and then decrease due to intrinsic excitation. In addition, the Seebeck coefficient of the sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is lower than that of the sample without component adjustment at low testing temperature. The Seebeck coefficient of thermoelectric materials in the intrinsic regime can usually be expressed as S z g ln n, where n is carrier concentration and g is scattering parameter [11,23,25]. So the higher carrier content due to the supplement of Sb result in the decrease of Seebeck coefficient [23]. Fig. 7 shows the behavior of Power factor, thermal conductivity and ZT for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. The value of power factor decreased with the increase of temperature as shown in Fig. 7 (a), because the decrease of the electrical conductivity (the maximum is about 100%) was significantly higher than that of the Seebeck coefficient (the maximum is about 20%). The maximum power factor value of 3.98 is obtained for the Bi0.4Sb1.6Te3 sample at 299 K. These results are similar with our previous work [18,25,26] and other literature reports [22,27,28]. The thermal conductivity is a contribution from electronic thermal conductivity, lattice thermal conductivity and bipolar thermal conductivity. The electronic thermal conductivity ke is related to electrical conductivity (s) and can be estimate by WiedemanneFranz law (ke ¼ L0sT), where L0 is Lorenz constant, L0 ¼ 2.0 108 V2/K2 for a heavily doped semiconductor, T is Kelvin temperature [15,27]. With the increase of temperature, the thermal
Fig. 6. Temperature dependence of electrical conductivity (a) and Seebeck coefficient (b) for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment.
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consumption, low emission and high efficiency. In the near future, n-typed Bi2Te3 based cutting waste fragments will be chosen as raw materials to prepare n-type Bi2Te3 based materials. Acknowledgement This work was supported by the National Natural Science Foundation of China [grand numbers 11074195, 51674181]; and the Key Projects of Hubei Provincial Department of Education [grand number D20151103]. References
Fig. 7. Temperature dependence of Power factor (a), thermal conductivity (b) and ZT (c) for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment.
conductivity decreased and then increased as shown in Fig. 7 (b). With the increase of testing temperature, the thermal conductivity first is dominated by the electronic thermal conductivity, so the thermal conductivity reduced, similar to the variation of the electrical conductivity. However, when the testing temperature reaches the intrinsic excitation point, the carrier content increase obviously, so the bipolar thermal conductivity dramatically increases, and result in the increase of the total thermal conductivity with increasing Further temperature. In addition, the thermal conductivity of the as-RPSed sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is higher than that of the sample without component adjustment at low testing temperature, which should ascribe to the increase of electronic of electronic thermal conductivity with the supplement of Sb and Te. The ZT values of the as-RPSed samples are shown in Fig. 7 (c). It can be seen that the ZT values have been significantly improved after composition adjustment. The highest ZT value of 1.07 is obtained at 379 K for the bulk with a nominal stoichiometry of Bi0.4Sb1.6Te3. These results are similar with our previous work [18,25,26] and other literature reports [22,27], which start from high purity elements Bi, Sb and Te. Besides, the average ZT value of the Bi0.4Sb1.6Te3 bulk obtained in this work is all over 1.0 from 299 K to 419 K, which is beneficial to expand the applied temperature range and achieve a stable service property for the thermoelectric power generation devices.
3. Conclusions In summary, starting from Bi2Te3 based cutting waste fragments, through a series of washing, vacuum metallurgical removing impurity, component adjustment, second smelting and RPS process, the p-type Bi0.4Sb1.6Te3 bulks were successfully obtained firstly. The maximum ZT value of 1.07 is obtained for the Bi0.4Sb1.6Te3 sample at 379 K, and it is greater than 1.0 from 299 K to 419 K. This result is comparable with our previous work and other literature reports which start from high purity elements Bi, Sb and Te. The recycle process introduced here is a green process with no secondary pollution, low cost, efficient, high utilization rate (above 98%) and high thermoelectric performance. Considered the three aspects of environmental and economic, whole recovery and reuse, the recycling technology introduced here is accordance with the principles of 3-R (reduce, reuse and recycle), and characterized by low
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Bi2Te3 based bulks with high figure of merit obtained from cut waste fragments of cooling crystal rods Qiusheng Xiang a, b, Xi'an Fan a, b, *, Xuewu Han a, b, Chengcheng Zhang a, b, Jie Hu a, b, Bo Feng a, b, Chengpeng Jiang a, b, Guangqiang Li a, b, Yawei Li a, b, Zhu He a, b a
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Waste fragments of cooling rods were recycled by vacuum metallurgy. The recycling technology is accordance with the principle of 3-R. The recycling yield of the Bi2Te3 based waste was over 98%. The ZT value of Bi0.4Sb1.6Te3 bulk from wastes could reach 1.07 at 379 K. The average ZT value of Bi0.4Sb1.6Te3 bulk from wastes was 1.0 from 299 K to 419 K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2016 Received in revised form 21 July 2017 Accepted 14 August 2017 Available online 25 August 2017
Bi2Te3 based cutting waste fragments were recycled by vacuum metallurgical technology to prepare ptype Bi2Te3 based thermoelectric materials. Through washing, vacuum metallurgical, component adjustment, second smelting and resistance pressing sintering (RPS) process, Bi0.4Sb1.6Te3 bulks were successfully obtained. All evidences confirmed that most of impurities from line cutting process such as Sb2O3 could be removed by washing, vacuum metallurgical process used in this work, and the recycling yield is up to 98%. The carrier content and corresponding thermoelectric properties could be adjusted effectively by appropriate component adjustment treatment. At lastly, a Bi0.4Sb1.6Te3 bulk was obtained and its' dimensionless figure of merit (ZT) could reach 1.07 at 379 K and the average ZT value was over 1.0 from 299 K to 419 K. Compared with the traditional process of recycling the single elemental Bi, Sb, Te by wet chemical metallurgical method, the vacuum metallurgical recycling technology introduced here is accordance with the principles of 3-R (reduce, reuse and recycle), and characterized by low cost, low degree of difficulty, low emission, high environmental protection and high efficiency, and the purified ingots can be directly used as raw material for the production of high-performance thermoelectric materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thermoelectric performance Bi2Te3 Cutting waste Recycling Resistance pressing sintering
* Corresponding author. 185#, School of materials and metallurgy, Wuhan University of Science and Technology, 947 Heping Road, Qingshan District, Wuhan, 430081, China. E-mail address: [email protected] (X. Fan). http://dx.doi.org/10.1016/j.matchemphys.2017.08.037 0254-0584/© 2017 Elsevier B.V. All rights reserved.
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Q. Xiang et al. / Materials Chemistry and Physics 201 (2017) 57e62
1. Introduction
2. Experimental
Recently the development of novel energy conversion [1e4] materials (such as photoelectric materials [5,6], thermoelectric materials [7,8] etc) has been received a great research interest to achieve a clean and sustainable world due to the limitation of fuels and environmental issues. And thermoelectric materials have the advantages of no noise, leakage and green to the environment etc in direct heat to electricity conversion and refrigeration. As one of the best thermoelectric materials near room temperature, the zonemelted Bi2Te3 have been commercialized for several decades [7e10]. However, the poor mechanical properties along crystal growth direction limit their application scopes, also lead to low material utilization rate of less than 50% [11]. A large number of thermoelectric waste materials which contains about ~48 tons of Te are generated in the semiconductor refrigeration industry every year [12]. Meanwhile thermoelectric waste also contains Bi, Sb, Se, Pb, Ni, Sn and other useful elements. It is worth nothing that the mineable time of Sb, Pb, Bi, Te in the earth (if sustained annual global primary production growth of 2%) are only 14 years, 18 years, 38 years, 72 years, respectively [13]. More important, the Sb, Te, Pb, Se and their compounds are toxic and dangerous [14]. In generally, the hydrometallurgical technology is used to extract the single element such as Bi, Te, Sb and Se from the thermoelectric waste materials. These useful elements are individually extracted by multi-step, high cost, long cycle, complicated and high-pollution process. Now, the traditional hydrometallurgical recycling technology mainly fall into two categories [12]. (1) The thermoelectric waste is oxidized by adding alkali in the smelting process, and then Te, Bi are separated by hydrometallurgical technology. It's mainly used to deal with the waste materials of high purity such as the head and tail of the crystal rod and block waste. (2) The thermoelectric waste is oxidized by oxidant in hydrochloric acid solution, then Te is retained in the residue, finally Te is obtained by refining the residue. It's mainly used to deal with the waste materials of low purity such as various chips and cutting powders. Unfortunately, both of them have the shortcoming such as long cycle, high cost and large environmental load because of wet chemical metallurgy process. So a simple, low cost, high utilization rate, no secondary pollution and green recycling technology is urgently needed. In this paper, Bi2Te3 based waste fragments were recycled by vacuum metallurgical technology. And we calculated and analyzed the saturated vapor pressure of bismuth, antimony, tellurium and their oxides, and demonstrated the feasibility of the whole vacuum metallurgical recycling process in theory. Furthermore, Bi2Te3 based cutting waste fragments were chosen as raw materials to prepare p-type Bi2Te3 based thermoelectric materials by washing, vacuum metallurgical removing impurity, component adjustment, second smelting and resistance pressing sintering (RPS) process. The overall recycling technology is a green process with no secondary pollution, low cost and high utilization rate (above 98%). Especially, the ZT value of the sample obtained by this series of processes is better than that of the corresponding zone-melting monocrystalline products in the whole temperature range. And the goal of this paper is to find a greener process to replace the traditional single element recovery process to reduce environmental pollution, shorten the material preparation cycle and save materials costs. However, the recovery of volatile elements in vacuum metallurgy had not been taken into consideration, which needs further improvement in future works.
In this work, the initial cutting waste fragments were obtained from Xianghe Huabei refrigeration equipment Co., LTD (Hebei, China) and used as received. These cutting waste fragments were contaminated by linear cutting liquid and their own oxidations. A typical vacuum metallurgical procedure was performed. Firstly, the initial cutting waste fragments were rinsed with deionized water and absolute ethyl alcohol to remove the contaminants from linear cutting liquid. Then, the waste fragments were dried in a vacuum drying oven for 24 h at 373 K, weighed, loaded in a unsealed quartz glass tube, and placed in a vacuum well-like melting furnace. Subsequently, the furnace was heated to 923 K for 30 min, and the degree of vacuum was held at 100 Pa. Lastly, the purified ingot was obtained. In order to adjust carrier concentrate and obtain the alloys with nominal component of Bi0.4Sb1.6Te3, appropriate amount of high purity (>99.99 wt %) Sb and Te granules were added into the purified ingot and then sealed into quartz glass tubes under vacuum. The raw materials sealed in quartz glass tubes were second melted in a rocking furnace and the temperature was fixed at 983 K and the soaking time was 120 min. Then, the sample was cooled to room temperature in furnace to get the Bi0.4Sb1.6Te3 ingot. Subsequently, the ingot was ground into powders with size of ~38 mm. Finally, the grinded powders were sintered by resistance pressing sintering (RPS) process at 683 K for 15 min under an axis pressure of 38 MPa in argon atmosphere [15,16]. The heating rate was fixed at 50 K/min. For comparison, purified crystal bar without component adjustment was also ground and sintered in the same condition. The phase structure of the bulk samples were characterized by X-ray diffraction (XRD, Philips) with an X'Per pro diffractometer by using Cu Ka radiation (l ¼ 1.5418 Å). The morphology and composition of bulk samples were observed by field-emission scanning electron microscopy (FESEM) (FEI, Nova400 NanoSEM) and energy dispersive X-ray spectroscopy (EDS). The electrical conductivity (s) were measured by four terminal method. The seebeck coefficient (S) were obtained by applying a dynamic temperature gradient (DT) of 4e8 K between both ends of the bars and measuring the output voltage (DV) between them, and the size of measured samples are 3 3 15 mm3. The seebeck coefficient was obtained by calculating the slope of the linear regression line through the least square method. The thermal conductivity (k) was calculated from the measured thermal diffusivity (l), specific heat (Cp) and volume density (d) by using the relationship k ¼ lCpd. The thermal diffusivity and specific heat of bulk samples were measured by a laser flash apparatus (LFA457, Netzsch, German). The volume densities (d) of the bulk samples were measured by the Archimedes method. The electrical properties and thermal properties of all bulk samples were measured in a direction perpendicular to the press direction during RPS process. Fig. 1 depicts the photos, SEM and the corresponding EDS, and XRD results of initial waste fragments, respectively. Fig. 1 (a) shows the surface of initial waste fragments is gray, no metallic luster, indicating that there are impurities on their surface such as oxides. From the EDS results (figure (c)), the presence of element O can be confirmed. Further, the XRD results (Fig. 1 (d)) indicate that there are Sb2O3 apart from the main phase of Bi0.5Sb1.5Te3. So the main impurities of the waste fragments is Sb2O3 from the oxidation of elements Sb. There are usually two ways to remove oxide without add new impurities: (1) using H2 or CO to restore the oxide, resulting in the
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Fig. 3. Real photo of quartz tube after vacuum melting process (a), SEM of area A (b) and corresponding EDS results (c), SEM of area B (d) and corresponding EDS results (e).
Fig. 1. Photos (a), SEM (b) and corresponding EDS results (c), and XRD pattern (d) of the initial waste fragments.
formation of the gaseous H2O or CO2 to leave system [17]; (2) volatilization of the oxide under high vacuum based on vacuum metallurgical recycle process [18]. In the work, the vacuum metallurgical recycle process is performed to remove the Sb2O3 impurity due to its high saturated vapor pressure. Fig. 2 shows the photos, SEM and corresponding EDS, and XRD results of the purified crystal bar. The surface of purified crystal bar is silvery white, with metallic luster (Fig. 2 (a)), and the recycling yield is up to 98%. In addition, the corresponding EDS results in Fig. 2 (c) indicates that there is no element O, confirming the purification effect of vacuum metallurgical process. Further, the XRD results (Fig. 2 (d)) indicate that there is no Sb2O3 apart from the main phase of Bi0.5Sb1.5Te3, further confirming the purification effect of vacuum metallurgical process. Fig. 3 shows the real photo of quartz tube after vacuum melting process, the SEM and corresponding EDS results of the two areas of
Fig. 2. Photos (a), SEM (b) and corresponding EDS results(c), and XRD pattern (d) of the purified crystal bar.
A and B on the inner wall of quartz tube. The volatiles deposited on the inner wall of quartz tube after vacuum melting process are obviously separated in two areas: pale yellow area (A) and black area (B). According to the result of EDS, it can be confirmed that there are elements O, Te and Sb in the two areas. Combined with the XRD results (Fig. 1 (d)), these volatiles should be mixture of Sb2O3 and Te. Possible explanations are shown in the following Fig. 4. Fig. 4 (a) shows the saturated vapor pressure of Sb, Bi, Te and Sb2O3 [19,20]. The saturated vapor pressures of Te and Sb2O3 are large enough at 923 K, and their value are 1.34 kPa and 1.06 kPa (Fig. 4 (a)), respectively, so their volatilization of Te and Sb2O3 must be considered. The saturated vapor pressure of Sb and Bi are very small and the value are only 45 Pa and 1 Pa (Fig. 4 (b)), respectively, so their volatilization of Sb and Bi can be ignored. In addition, the saturated vapor pressure of Te is much larger than that of Sb2O3 in whole temperature range. So the element Te volatilize preferentially with increasing temperature, but the Sb2O3 solidify preferentially with decreasing temperature. According to the result of EDS in the area of A, it can be found that there are lots of Sb2O3 and small amount of Te, indicating that the volatilized Sb2O3 mainly deposited on the area A. In addition, the yellow appearance at the area A also confirmed the presences of Sb2O3. According to the result of EDS in the area of B, it can be found that there are lots of Te and trace of Sb2O3, indicating that the volatilized Te mainly deposited on the area B. The temperature is decreasing from the center of vacuum furnace up to furnace mouth, and the saturated pressure of Te and Sb2O3 also decrease. When the temperature is reduced to 781 K, the saturated pressure of Te is 117 Pa, but the saturated pressure of Sb2O3 is only 13 Pa, namely most of Sb2O3 and small amount of Te condensed in the area of A, meanwhile most of Te is still in the gaseous state. When the temperature dropped to 647 K, the saturated vapor pressure of Te is only 1 Pa, namely most
Fig. 4. Temperature dependence of the saturated vapor pressure of Sb, Bi, Te and Sb2O3.
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of Te and trace of Sb2O3 condensed in the area of B. Fig. 5 (a) shows the XRD patterns of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3. It can be found that all elements Bi, Te and Sb are completely combined into a single phase with a rhombohedral structure and the impurity phase such as Sb2O3 are not observed, further confirming the purification effect of vacuum metallurgical recycle process. Fig. 5 (b, c) show the typical SEM of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3. It is clear that there is no large-scale preferred orientation in the sintered samples. However, partially oriented lamellar structure can be observed everywhere. In addition, there are some of fine pores with nano or submicron size distributed randomly in the as-RPSed samples, which should result from the volatilization of elements Te due to its high saturated vapor pressure. In order to adjust carrier concentrate of sample and obtain the alloys with nominal components of Bi0.4Sb1.6Te3, appropriate amount of high purity (99.99 wt %) Sb and Te granules were added into the purified ingots. Fig. 6 (a) shows the electrical conductivity based on temperature for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. The negative temperature dependence of the electrical conductivity
Fig. 5. XRD patterns and SEM fractographs of the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 after component adjustment.
indicates a metallic conduction behavior due to the enhanced lattice scattering to carriers. In addition, it can be found that the electrical conductivity of the sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is higher than that of the sample without component adjustment. According to the definition equation, the electrical conductivity is closely related to the carrier concentration n and mobility m by the relationship: s ¼ nem [21,22]. As we all know, the dominative carriers (holes) are determined by antistructure defects due to the occupation of Te sites by Bi (Bi0 Te) and Sb (Sb0 Te) atoms for the p-type Bi-Sb-Te alloys. The small electronegativities difference between atoms Sb, Bi and Te (The electronegativities of Sb, Bi and Te are cSb ¼ 1.9, cBi ¼ 1.8 and cTe ¼ 2.1, respectively) easily lead to the formation of anti-site defects of Bi0 Te (Sb0 Te). Meanwhile, the polarity of the Sb-Te bond (Dc ¼ 0.2) is weaker than that of the Bi-Te bond (Dc ¼ 0.3), namely the Sb0 Te is more easily generated comparing with Bi0 Te. So, the content of antisite defects will increase the supplement of Sb, which results in the increase of carrier concentration [23,24] and electrical conductivity. Fig. 6 (b) shows Seebeck coefficient base on temperature for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. With increasing testing temperature, the Seebeck coefficient first increase and then decrease due to intrinsic excitation. In addition, the Seebeck coefficient of the sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is lower than that of the sample without component adjustment at low testing temperature. The Seebeck coefficient of thermoelectric materials in the intrinsic regime can usually be expressed as S z g ln n, where n is carrier concentration and g is scattering parameter [11,23,25]. So the higher carrier content due to the supplement of Sb result in the decrease of Seebeck coefficient [23]. Fig. 7 shows the behavior of Power factor, thermal conductivity and ZT for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment. The value of power factor decreased with the increase of temperature as shown in Fig. 7 (a), because the decrease of the electrical conductivity (the maximum is about 100%) was significantly higher than that of the Seebeck coefficient (the maximum is about 20%). The maximum power factor value of 3.98 is obtained for the Bi0.4Sb1.6Te3 sample at 299 K. These results are similar with our previous work [18,25,26] and other literature reports [22,27,28]. The thermal conductivity is a contribution from electronic thermal conductivity, lattice thermal conductivity and bipolar thermal conductivity. The electronic thermal conductivity ke is related to electrical conductivity (s) and can be estimate by WiedemanneFranz law (ke ¼ L0sT), where L0 is Lorenz constant, L0 ¼ 2.0 108 V2/K2 for a heavily doped semiconductor, T is Kelvin temperature [15,27]. With the increase of temperature, the thermal
Fig. 6. Temperature dependence of electrical conductivity (a) and Seebeck coefficient (b) for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment.
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consumption, low emission and high efficiency. In the near future, n-typed Bi2Te3 based cutting waste fragments will be chosen as raw materials to prepare n-type Bi2Te3 based materials. Acknowledgement This work was supported by the National Natural Science Foundation of China [grand numbers 11074195, 51674181]; and the Key Projects of Hubei Provincial Department of Education [grand number D20151103]. References
Fig. 7. Temperature dependence of Power factor (a), thermal conductivity (b) and ZT (c) for the as-RPSed samples with nominal stoichiometry of Bi0.4Sb1.6Te3 and no component adjustment.
conductivity decreased and then increased as shown in Fig. 7 (b). With the increase of testing temperature, the thermal conductivity first is dominated by the electronic thermal conductivity, so the thermal conductivity reduced, similar to the variation of the electrical conductivity. However, when the testing temperature reaches the intrinsic excitation point, the carrier content increase obviously, so the bipolar thermal conductivity dramatically increases, and result in the increase of the total thermal conductivity with increasing Further temperature. In addition, the thermal conductivity of the as-RPSed sample with the nominal stoichiometry of Bi0.4Sb1.6Te3 is higher than that of the sample without component adjustment at low testing temperature, which should ascribe to the increase of electronic of electronic thermal conductivity with the supplement of Sb and Te. The ZT values of the as-RPSed samples are shown in Fig. 7 (c). It can be seen that the ZT values have been significantly improved after composition adjustment. The highest ZT value of 1.07 is obtained at 379 K for the bulk with a nominal stoichiometry of Bi0.4Sb1.6Te3. These results are similar with our previous work [18,25,26] and other literature reports [22,27], which start from high purity elements Bi, Sb and Te. Besides, the average ZT value of the Bi0.4Sb1.6Te3 bulk obtained in this work is all over 1.0 from 299 K to 419 K, which is beneficial to expand the applied temperature range and achieve a stable service property for the thermoelectric power generation devices.
3. Conclusions In summary, starting from Bi2Te3 based cutting waste fragments, through a series of washing, vacuum metallurgical removing impurity, component adjustment, second smelting and RPS process, the p-type Bi0.4Sb1.6Te3 bulks were successfully obtained firstly. The maximum ZT value of 1.07 is obtained for the Bi0.4Sb1.6Te3 sample at 379 K, and it is greater than 1.0 from 299 K to 419 K. This result is comparable with our previous work and other literature reports which start from high purity elements Bi, Sb and Te. The recycle process introduced here is a green process with no secondary pollution, low cost, efficient, high utilization rate (above 98%) and high thermoelectric performance. Considered the three aspects of environmental and economic, whole recovery and reuse, the recycling technology introduced here is accordance with the principles of 3-R (reduce, reuse and recycle), and characterized by low
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