Effects of transition metal element doping on the structural and thermoelectric properties of n-type Bi 2-x Ag x Se 3 alloys (original) (raw)
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Te 3 (x = 0, 0.01, 0.015, 0.02) were investigated in the temperature range from 300 K to 525 K. The results show that Ge doping brings about 13-52% increase in electrical conductivity due to the increase of hole concentration, and 5-11% decrease in thermal conductivity owing to enhanced phonon scattering by the dopant atoms. As a result, a maximum thermoelectric figure of merit (ZT) reaches 1.48 at 350 K for Bi 0.4 Sb 1.59 Ge 0.01 Te 3, which is ~25% larger than that of Bi 0.4 Sb 1.6 Te 3. Present results demonstrate that Ge doping is an effective way to enhance the thermoelectric performance of Bi 0.4 Sb 1.6 Te 3 alloy. In the past half century Bi 2 Te 3-based alloys Bi x Sb 2−x Te 3 (BST) have developed into the most important thermoelectric(TE) material at low-temperature [1], and it is widely used in the field of refrigeration and waste heat utilization due to the capability of directly converting between heat and electrical energy. The conversion efficiency of TE materials is determined by a dimensionless figure of merit ZT = α 2 Tσ / κ tot , where α, T, σ and κ tot are the thermopower, absolute temperature, electrical conductivity and total thermal conductivity, respectively [2]. To improve the TE properties of BST bulk materials, many efforts have been executed, such as introducing nanophase into matrix [3-6] or nanostructure engineering to reduce thermal conductivity [7,8]. Besides , it was reported that adjusting the carrier concentration by varying the stoichiometric ratio of Bi and Sb [9-11] can optimize thermoelectric performance and get the optimal ZT value under this stoichiometric ratio Bi 0.4 Sb 1.6 Te 3. Doping a tiny amount of In, Ga or Cu [12-14] is another effective way to adjust the carrier concentration. For instance, Xinbing Zhao et al. [15] successfully improved the performance of BST and obtained peak ZT value of ~1.4 at 500 K by introducing dopant In and hot deformation. However, in previous reports the dopants researchers used [14,15], such as In and Ga, have relatively smaller elec-tronegativity χ (χ(Ga) = 1.81, χ(In) = 1.78) than that of Sb (χ(Sb) = 2.0), which suggests that doping with these elements will cause large reduction in carrier mobility [14,15] and will cause a decrease in power factor PF (=σα 2) of BST. Hence, in order to maintain high carrier mobility and PF and elevate resultant ZT of BST it is vital that one chooses the dopants with similar electronegativity to that of the replaced elements. In this work, we use Ge as dopant to substitute Sb. Since the electro-negativity of Ge and Sb (χ(Ge) = 2.01 and χ(Sb) = 2.0) are very close to each other, one can expect that the substitution of Ge for Sb is in favor of enhancement of PF by both optimizing carrier concentration and reducing drop of carrier mobility, besides the lowing thermal conductivity via impurity scattering of phonons. Indeed, our obtained results show that a slight amount of Ge doping (x = 0.01) causes 11.6% increase in PF and ~29.2% decrease in lattice thermal conductivity (at 300 K), simultaneously. As a result, a peak ZT value of 1.48 was achieved for the Bi 0.4 Sb 1.59 Ge 0.01 Te 3 at 350 K, which is around 25% larger than that of the pristine material. High purity elements, Bi (99.997%), Sb (99.99%), Te (99.95%) and Ge (99.99%) were weighed by stoichiometry Bi 0.4 Sb 1.6−x Ge x Te 3 (x = 0, 0.01, 0.015, 0.02), and sealed in evacuated quartz ampoules under vacuum of 10 −3 Pa. The sealed ampoules were subsequently heated up to 973 K at a speed of 3 K per minute and then kept at this temperature for 10 h, naturally cooled down to room temperature. Then the obtained alloy ingots were grinded into powders. Finally, the bulk samples were obtained by hot pressing under a pressure of 600 MPa in a vacuum at 673 K for 1 h. The phase structure of the obtained samples was studied by X-ray diffraction (Philips diffractometer, Cu Kα radiation) at room temperature. The field emission scanning electron microscopy (FE-SEM) is employed to study the surface fractured surfaces of bulk samples. The elemental distributions and constituents of the bulk samples were analyzed by using Energy-dispersive X-ray spectroscopy (EDS). Electrical conductivity and the thermopower were measured by ZEM-3. The Scripta Materialia 154 (2018) 118-122
Journal of Alloys and Compounds, 2006
Effect of Ag-doping on thermoelectric properties of n-type Bi 2 (Te 0.94 ,Se 0.06 ) 3 is studied. Bulk Bi 2 (Te 0.94 ,Se 0.06 ) 3 sample with single solid solution phase and high relative density (over 95%) is obtained by mechanical alloying and subsequent hot press sintering. The as-sintered sample has a microstructure of randomly distributed plate grains and its size is about 1 m. A V-shaped variation for the electrical properties with Ag doping is found which corresponds to different Ag atomic position in lattice among the doping range studied. A maximum figure of merit of about 1.7 × 10 −3 /K is achieved.
The use of (Bi 1-x Sb x ) 2 (Te 1-y Se y ) 3 solid solution alloys instead of the Bi 2 Te 3 binary compound allows a significant increase of its thermoelectric performance. This improvement is mainly related to the reduction of thermal conductivity of the crystal lattice due to increased phonon scattering obtained from the alloying effect. However the optimum amount of Sb and Se resulting in higher alloy performance remains uncertain. Moreover, it is not clear if the reduction of mobility of free charge carriers in alloys containing large amounts of Sb and Se is the only cause of thermoelectric performance degradation of such alloys. Other phenomena have been suggested to explain this behavior. In this research project, a series of quaternary alloys of n-type (Bi 1-x Sb x ) 2 (Te 1-y Se y ) 3 have been characterized with Sb and Se content varying from 0 to 10%. The specimens were produced by mechanical alloying of powders and hot extrusion. The thermoelectric properties have been studied in the temperature range from 230 to 350 K. The mobility and concentration of charge carriers were evaluated as a function of temperature from 15 to 300 K. The results indicate that, for quaternary alloys in the higher end of the concentrations of Se and Sb studied here, the hypothesis of charge carrier mobility degradation due only to alloying effect is not sufficient to explain the data. We will discuss the effect of additional generation of defect complexes for these cases.
Enhancement in thermoelectric properties due to Ag nanoparticles incorporated in Bi2Te3 matrix
Beilstein Journal of Nanotechnology, 2019
The present study aims to see the enhancement in thermoelectric properties of bismuth telluride (Bi2Te3) annealed at different temperatures (573 and 773 K) through silver (Ag) nano-inclusions (0, 2, 5, 10, 15 and 20 wt %). Transmission electron microscopy (TEM) images of Ag incorporated in Bi2Te3 annealed at 573 K shows tubular, pentagonal, trigonal, circular and hexagonal nanoparticles with sizes of 6–25 nm (for 5 wt % Ag ) and 7–30 nm (for 20 wt % Ag). Ag incorporated in Bi2Te3 annealed at 773 K shows mainly hexagonally shaped structures with particle sizes of 2–20 nm and 40–80 nm (for 5 wt % Ag) and 10–60 nm (for 20 wt % Ag). Interestingly, the samples annealed at 573 K show the highest Seebeck coefficient (S, also called thermopower) at room temperature (p-type behavior) for 5% Ag which is increased ca. five-fold in comparison to Ag-free Bi2Te3, whereas for samples with the same content (5% Ag) annealed at 773 K the increment in thermopower is only about three-fold with a 6.9-fo...
Journal of Physics D: Applied Physics, 2011
The concentration and doping of n-type doped (Bi (1−x) Sb x ) 2 (Te (1−y) Se y ) 3 thermoelectric alloys produced by powder metallurgy followed by hot extrusion are varied in order to optimize their performance for the generation of electricity. The material is polycrystalline and strongly textured, with an undetermined volumetric fraction of nanoscale subgrains, and its thermoelectric properties are optimal along the extrusion direction. Within the composition range 0 x, y 0.1 the quaternary (Bi 0.97 Sb 0.03 ) 2 (Te 0.93 Se 0.07 ) 3 shows the highest temperature-averaged dimensionless figure of merit ZT for applications where T C = 295 K and T H = 420 K. This average ZT is further optimized for values of carrier concentrations close to n = 3.4 × 10 19 cm −3 . The introduction of substitution elements constituting these quaternary alloys leads to an increase in the electronic equivalent density of states compared with Bi 2 Te 3 . This increase has a direct impact on the Seebeck coefficient, the electronic contribution to the thermal conductivity and the carrier mobility.
Journal of Electronic Materials, 2018
Bi 2 Te 3) x À (Sb 2 Te 3) 1Àx alloys with high thermoelectric properties were fabricated for waste heat energy recovery by mechanical alloying followed by spark plasma sintering. The samples' diffraction peaks, such as the (015) positions, were slightly shifted from high to low 2h angles with decreasing Sb 2 Te 3 content due to the occupation of Sb sites by Bi atoms in the crystal lattice. The electrical conductivity increased with (Sb 2 Te 3) content due to an increase in carrier concentration. The sample with the nominal composition of (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 exhibited a maximum thermoelectric figure of merit, ZT of 1.3 ± 0.06 at 400 K, and 1.07 ± 0.06 at 300 K. This enhanced ZT was successfully achieved by increasing (Sb 2 Te 3) content, which reduces intrinsic conduction at higher temperatures by increasing carrier concentration and band gaps. The enhanced thermoelectric performance of the (Bi 2 Te 3) 0.15 + (Sb 2 Te 3) 0.85 TE materials can provide exceptional benefits for power generation and cooling applications around 400 K.
Improved thermoelectric performance of Ag 2-x Al x Se through formation of AgAl phase
Mixed ion-electron conductor Ag 2 Se is an n-type semiconductor owing to the intrinsic Se vacancies. By reducing Se vacancies, Ag 2 Se has been demonstrated as a potential environment-friendly thermoelectric material for near room temperature application. In the present work, Al addition was found to be highly beneficial for improving the thermoelectric properties of Ag 2 Se. In Ag 1.95 Al 0.05 Se, a large enhancement in the charge carrier mobility (1127 cm 2 /V s) is witnessed due to the formation of Se-rich Ag 2 Se 1.02 phase with improved grain connectivity through in situ formed AgAl phase. The synergetic effect of low carrier concentration and enhanced mobility in Al doped samples resulted in a high Seebeck coefficient and high electrical conductivity, leading to a high power factor of 3039 lW/m K 2 at 300 K. The figure-of-merit of Ag 1.95 Al 0.05 Se was found to be 1.1 at 300 K i.e., 57% higher than for pure Ag 2 Se. The uni-leg device fabricated using Ag 1.95 Al 0.05 Se with electroplated Ni/Ag contacts showed a conversion efficiency of $3.2% for a temperature difference of 93 K, i.e., comparable to the best reported value for conventional bismuth telluride.
Thermoelectric properties of Bi-Sb samples grown by mechanical alloy
physica status solidi (c), 2005
PACS 72.15.Eb, 72.15.Jf, 84.60.Bk, 84.60.Rb Seebeck coefficient S(T), electric resistivity ρ(T ) and thermal conductivity κ(T ) measurements are reported on polycrystalline Bi 0.88Sb0.12 samples grown by mechanical alloy (MA) by using different milling time. The electrical and thermal characterization was carried out in the temperature range between 77 K and 300 K. The Seebeck coefficient is negative in whole measurement range and its magnitude increases with the temperature, S(T) exhibit an enhancement with the milling time up to reach values close to −300µV /K in the samples milled through 45 hours. The electrical resistivity shows a weak semiconductor behavior with the temperature, ρ(T ) was increasing with the processing time, but in all cases it shown values close to 10 −3 Ω − cm. The thermal conductivity measurements were carried out by using a steady-state heat-flow method. κ(T ) show a linear behavior with the temperature and its magnitude decreases with processing time up to values smaller than 1W/mK. By using SEM analysis the behavior of the granularity of the samples was studied as a function of the milling time. The dimensionless thermoelectric figure of merit ZT, reaches values close to 0.3, which are characteristic values for single crystalline alloys of these kind compounds. These results show that the mechanical alloy could be a useful technique in the production of thermoelectric materials.
Phase separation and interface effect in pseudo-quaternary composites of Ag Bi0.5Sb1.5−Te3−
Journal of Alloys and Compounds, 2017
We investigated anisotropic thermoelectric properties of p-type Ag x Bi 0.5 Sb 1.5Àx Te 3Àx (x ¼ 0.0, 0.1, 0.2 and 0.3) compounds, prepared by melting and hot press sintering. During hot press sintering and annealing process, the eutectoid decomposition of Ag 2 Te from (Bi,Sb) 2 Te 3 matrix induces intrinsic thermoelectric composites Ag 2þd Te/(Bi,Sb) 2 Te 3. The phase separation increases the phonon scattering which lowered the lattice thermal conductivity. We also found the metallic interfacial layer in a grain boundary from the measurements of conducting atomic force microscopy (AFM) and scanning Seebeck-and scanning thermal-microscopy (SThM). Here we argue that the interfacial conductivity attributes to the increase of power factor due to increase of mobility. Thereby, the phase separation of Ag 2 Te from the (Bi,Sb) 2 Te 3 matrix gives rise to the enhanced thermoelectric properties at mild-temperature range, resulting in the enhancement of dimensionless thermoelectric figure-of-merit ZT value of 1.02 at 567 K for x ¼ 0.1 along the vertical direction.