N-type Bi2Te3 Research Articles

Improved thermoelectric properties of n-type Bi2Te3 alloy deriving from two-phased heterostructure by the reduction of CuI with Sn

In this report, CuI and Sn co-doped n-type Bi2Te3 samples have been prepared by a high-temperature solid-state reaction, and the effect of co-doping on the thermoelectric properties was investigated from room temperature to 525 K. Sn single-doped and undoped Bi2Te3 were prepared for comparison. Detailed charge transport data including electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity are presented. Microscopic observation of CuI/Sn co-doped samples revealed that numerous distinctive microstructures such as nanoprecipitates of the Cu and SnI-rich phase were generated in the matrix. The lattice thermal conductivity of CuI/Sn co-doped Bi2Te3 was substantially reduced compared to those of undoped and single doped Bi2Te3. Benefiting from the improved electrical transport properties by doping and the reduced lattice thermal conductivity by numerous microstructures, the ZT value of the Bi2Te3 doped with 1 at.% CuI/Sn is distinctly enhanced to 1.24 at 425 K. The average ZT value (ZTave ~ 1.02) at 300–525 K was clearly higher than those of Sn-doped Bi2Te3 (ZTave ~ 0.54) and CuI-doped Bi2Te3 (ZTave ~ 0.98). This work indicates that the average ZT can be improved over a broad temperature range using a co-doping approach.

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi2Te3, we prepared Bi2Te3 nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi2Te3 samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3 μW cm−1 K−2 at 300 K) than that along the out-of-plane direction (8.1 μW cm−1 K−2 at 300 K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440 K for the nano-bulk n-type Bi2Te3 sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi2Te3 nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.

Decoupling effect of electrical and thermal properties of Bi2Te3-polypyrrole hybrid material causing remarkable enhancement in thermoelectric performance

We provided a Bi2Te3-polypyrrole hybrid material, in which energy band junction and phonon scattering effects should appear at the interface of the two components. The hybrid material exhibited increases in electrical resistivity and the Seebeck coefficient due the energy band junction, thus retaining the power factor without loss, whereas showing a great reduction in thermal conductivity because of the phonon scattering at the interface. This significant decoupling of electrical and thermal properties resulted in predominant figures of merit (ZTmax∼1.21 at 100°C and ZTave∼1.18 at 50–150°C) among n-type Bi2Te3 or Bi2(Te,Se)3 materials previously reported.

All Cross-Plane Thermoelectric Properties of n-Type Bi2Te3 Thin Films in the Temperature Range from 77 to 500 K

All Cross-Plane Thermoelectric Properties of <i>n</i>-Type Bi₂Te₃ Thin Films in the Temperature Range from 77 to 500 K

Interfacial energy band and phonon scattering effect in Bi2Te3-polypyrrole hybrid thermoelectric material

We hybridized n-type Bi2Te3 with an inexpensive and abundantly available conducting polymer, polypyrrole, to obtain a bulk-structured hybrid material in which the interfacial energy band and the phonon scattering effects should occur at the interface of the two components. The obtained hybrid material inevitably exhibited a lower electrical conductivity than pristine Bi2Te3, which may be attributable to carrier scattering at the interfacial energy barrier. However, the hybrid material completely compensated for this loss in electrical conductivity with a significant increase in the Seebeck coefficient, and thus it retained the power factor with no loss. In addition, the hybrid material displayed a much lower thermal conductivity than pristine Bi2Te3 owing to the phonon scattering effect. The hybrid material exhibited significant decoupling of the electrical and thermal properties, thus affording state-of-the-art figures of merit (ZT ∼ 0.98 at 25 °C, ZTmax ∼ 1.21 at 100 °C, and ZTave ∼ 1.18 at 50–150 °C) that exceed those of most of the previously reported n-type Bi2Te3 or Bi2(Te,Se)3 materials.

Simultaneous stoichiometric composition and highly (00l) orientation of flexible Bi2Te3thin films via optimising the DC magnetron sputter-deposition process

Simultaneous stoichiometric composition and highly (00l) orientation of flexible Bi2Te3 thin films were investigated under the DC magnetron sputtering parameters. Stoichiometric Bi2Te3 and highly (00l) orientation structure was obtained by sputtering conditions, preheat temperature at 350 °C, and working pressure of 1.8 × 10−3 mbar. This designed structure of layered compact feature with stoichiometry provide the relatively high mobility. The maximum carrier mobility of 118 cm2/V was observed for highly (00l) film. The electrical conductivity of thin film has been greatly enhanced, to a maximum of about 14.90 × 103 S/cm at 50 °C. This value is higher than those of hot-pressed or spark plasma sintering n-type Bi2Te3 bulk alloys. The maximum power factor of 12.5 × 10−3 W/m.K2 was obtained at 300 °C.

Transition from n- to p-type conduction concomitant with enhancement of figure-of-merit in Pb doped bismuth telluride: Material to device development

The majority of industrial, automobile processes, electrical appliances emit waste heat in the low-temperature range (<573 K), hence efficient thermoelectric materials operating in this range are highly needed. Bismuth telluride (Bi2Te3) based alloys are conventional thermoelectric material for the low-temperature application. The pure Bi2Te3 sample synthesized in this work exhibits n-type conduction. We demonstrate that by small doping of Pb at Bi site a transition in electrical transport form n- to p-type is observed. The figure-of-merit (ZT) of n-type Bi2Te3 is ~0.47 and optimized Bi1.95Pb0.05Te3 exhibit p-type conduction with enhanced ZT of ~0.63 at 386 K. The conversion efficiency of Bi1.95Pb0.05Te3 based single thermoelement with hot pressed Ni/Ag electrical contacts was found to be ~4.9% for a temperature difference (ΔT) of 200 K. The efficiency was further enhanced to ~12% (at ΔT ~ 494 K) in the segmented thermoelement consisting of Bi1.95Pb0.05Te3 and (AgSbTe2)0.15(GeTe) 0.85 (i.e. TAGS-85).

Control of oxygen content of n-type Bi2Te3 based compounds by sintering process and their thermoelectric properties

Oxygen content in n-type Bi2Te3-based alloys plays a crucial role in determining the thermoelectric properties. We aimed to control the residual oxygen that originated from the ball milling process; for this, sintering processes such as spark plasma sintering (SPS) and plasma activated sintering (PAS) were utilized. The reduction of oxygen content was successfully achieved with PAS, leading to improved electrical properties. With the removal of residual oxygen, the carrier concentration increased, resulting in reduced electrical resistivity. The phonon thermal conductivity was reduced due to the grain size of this sample, which was smaller than that of the hot-pressed (HP) samples. An enhanced value of the dimensionless figure of merit (ZT) of 0.83 at 473 K, an increase of 25.2% compared with that of the HP samples, was achieved. The selection of the appropriate sintering process should be considered to remove oxygen content, as well as to minimize grain growth.

Solution-processed n-type Bi2Te3−xSex nanocomposites with enhanced thermoelectric performance via liquid-phase sintering

The much slower progress in enhancing the thermoelectric performance of n-type Bi2Te3 than that of p-type Bi2Te3 based materials in the past decade hinders the widespread use in power generation and refrigeration. Here, a facile bottom-up solution-synthesis with spark plasma sintering (SPS) process has been developed to build n-type Bi2Te3− x Se x bulk nanocomposites, which substantially improves the power factor and decreases the lattice thermal conductivity by tuning the interface scattering of phonons and electrons. The stoichiometric composition in ternary Bi2Te3− x Se x nanocomposites is also tuned to optimize the carrier concentration and lattice thermal conductivity. The optimized bulk nanocomposite Bi2Te2.7Se0.3 exhibits a ZT of 1.1 at ~371 K, which is comparable to the corresponding commercially available ingots. Our results demonstrate the great potential of the solution-processed n-type Bi2Te3− x Se x nanocomposites for cost-effective thermoelectric applications.

High Thermoelectric Performance in Crystallographically Textured n-Type Bi2Te3- xSe x Produced from Asymmetric Colloidal Nanocrystals.

In the present work, we demonstrate crystallographically textured n-type Bi2Te3- xSe x nanomaterials with exceptional thermoelectric figures of merit produced by consolidating disk-shaped Bi2Te3- xSe x colloidal nanocrystals (NCs). Crystallographic texture was achieved by hot pressing the asymmetric NCs in the presence of an excess of tellurium. During the hot press, tellurium acted both as lubricant to facilitate the rotation of NCs lying close to normal to the pressure axis and as solvent to dissolve the NCs approximately aligned with the pressing direction, which afterward recrystallize with a preferential orientation. NC-based Bi2Te3- xSe x nanomaterials showed very high electrical conductivities associated with large charge carrier concentrations, n. We hypothesize that such large n resulted from the presence of an excess of tellurium during processing, which introduced a high density of donor TeBi antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semiconductor with a work function well below Bi2Te3- xSe x, might further contribute to increase n through spillover of electrons, while at the same time blocking phonon propagation and hole transport through the nanomaterial. NC-based Bi2Te3- xSe x nanomaterials were characterized by very low thermal conductivities in the pressing direction, which resulted in ZT values up to 1.31 at 438 K in this direction. This corresponds to a ca. 40% ZT enhancement from commercial ingots. Additionally, high ZT values were extended over wider temperature ranges due to reduced bipolar contribution to the Seebeck coefficient and the thermal conductivity. Average ZT values up to 1.15 over a wide temperature range, 320 to 500 K, were measured, which corresponds to a ca. 50% increase over commercial materials in the same temperature range. Contrary to most previous works, highest ZT values were obtained in the pressing direction, corresponding to the c crystallographic axis, due to the predominance of the thermal conductivity reduction over the electrical conductivity difference when comparing the two crystal directions.

Fabrication and characterization of roll-type thin-film thermoelectric generators

We propose and demonstrate a facile fabrication method of roll-type thin-film thermoelectric generators to conveniently use a low electric power converted from ambient heat sources. The generators consisted of n-type Bi2Te3 and p-type Sb2Te3 thin films, which were deposited on a polyimide sheet by radio-frequency magnetron sputtering. Both types of thin films were thermally annealed to increase the thermoelectric properties. The annealed thin films were connected to each other using silver paste, with the p-n junctions connected electrically in series. The completed generator consisted of five p-n pairs was 625 mm long and 26 mm wide. To form the roll-type structure, the generator was bent so that the fold lines were located at the p-n junctions. The performance of the generators, such as the open circuit voltage (Voc) and maximum output power (Pmax), were measured by applying a temperature difference (ΔT) between the ends of the generator. As a result, the generator exhibited a Voc of 20.1 mV and a Pmax of 48.5 nW at a ΔT of 30 K. In addition, we calculated the performances of generators with different film thickness or number of p-n pairs based on the experimental results.

Enhanced Thermoelectric Performance in n-Type Bi2Te3-Based Alloys via Suppressing Intrinsic Excitation.

Currently, the application of thermoelectric power generators based on Bi2Te3-based alloys for the recovery of low-quality waste heat is still limited because of the aggravated intrinsic excitation of the material at elevated temperatures. In this study, excessive Te and dopant I are introduced to the n-type Bi2Te2.4Se0.6 material with the purpose of suppressing its intrinsic excitation and improving the thermoelectric performance at elevated temperatures. These Te and I atoms act as electron donors to effectively reduce the density of minority carriers (holes) and weaken their negative contribution to the Seebeck coefficient. Likewise, the initial band structure and the carrier scattering mechanism are scarcely altered. Similar to the p-type Bi2Te3-based alloys, we found the "conductivity-limiting" mechanism is also well obeyed in the present n-type Bi2Te2.4Se0.6-based materials. The reduced minority carrier partial electrical conductivity in these Te-excessive and I-doped Bi2Te2.4Se0.6 samples significantly decreases the bipolar thermal conductivity, leading to lowered total thermal conductivity at elevated temperatures. Finally, the peak zT is successfully shifted up to higher temperatures for these Te-excessive and I-doped Bi2Te2.4Se0.6 samples. A maximum zT of 1.0 at 400 K and an average zT of 0.8 at 300-600 K have been realized in Te-excessive Bi2Te2.41Se0.6.

Screen-printed radial structure micro radioisotope thermoelectric generator

The micro radioisotope thermoelectric generator can be invoked as a long-life power supply in low-power device applications. Improving the current, voltage and power of power sensors by enhancing the properties of thermoelectric material composites matters in low-power device applications. The micro radioisotope thermoelectric generator driven by the temperature difference between radial thermoelectric legs printed on polyimide substrate and the loaded central heat source is reported in this study. The electrical conductivity of n-type Bi2Te2.7Se0.3, p-type Bi0.5Sb1.5Te3, and p-type Sb2Te3 radial thermoelectric legs are 24.57–165.8 S·cm−1, with Seebeck coefficients of −176.6, 223.3 and 139.7 µV·K−1 respectively. Thermoelectric legs are prepared by screen printing with a paste consisting of epoxy resin and BiTe-based powders. The generator has five couples of radial thermoelectric legs, and their material properties are optimized through selecting the preliminary curing temperature. The electrical conductivity of n-type Bi2Te2.7Se0.3, p-type Bi0.5Sb1.5Te3, and p-type Sb2Te3 thermoelectric legs are 24.57–165.8 S·cm−1, with Seebeck coefficients of −176.6, 223.3 and 139.7 µV·K−1 respectively. When loaded with 1.5 W isotope heat sources, the prototype generator would generate an open-circuit voltage of 68.41 mV, a short-circuit current of 329.0 µA, and an output power of 5.81 µW at 39.20 mV. Stacking and series-parallel can harvest considerable energy.

Control of Carrier Concentration by Ag Doping in N-Type Bi2Te3 Based Compounds

Many elements have been used as dopants to enhance the thermoelectric performance of Bi2Te3-related materials. Among them, Ag’s effect on thermoelectric properties, where Ag acts as a donor or acceptor, remains unclear. To elucidate the role of Ag in n-type Bi2Te3 based compounds, Ag was added to n-type (Bi0.9Sb0.1)2(Te0.85Se0.15)3. As the amount of Ag was increased, the electron concentration decreased, which means Ag acted as an acceptor. The added Ag atoms were found to occupy interstitial sites in the hexagonal lattices, as confirmed by X-ray analysis and first principles calculations. The reduction in electron concentration was attributed to the interaction between the interstitial Ag and intrinsic defects.

Pressure induced electronic phase transitions and superconductivity in n-type Bi2Te3

Temperature dependent-electrical resistance of n-type Bi2Te3 was investigated under high pressure. Superconductivity was detected at 4.9 GPa with Tconset = 2.8 K. Resistance and Tc suggest that there are two electronic phase transitions below 10 GPa. We conjecture that the bulk insulating phase first changes to semimetal and then to metal. The evolution of the Hall coefficient is qualitatively consistent with the proposed electronic phase transition. The origin of superconductivity and topological properties are also discussed.

Impurity-free, mechanical doping for the reproducible fabrication of the reliable n-type Bi2Te3-based thermoelectric alloys

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi2Te3 alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of ∼1019/cm3 via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5 × 10−3 W-m−1-K−2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high-temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

Effect of reducing agent strength on the growth and thermoelectric performance of nanocrystalline bismuth telluride

A novel combination of Trizma, as an environmentally friendly chelating agent, with either weak or strong reducing agent was used to produce n-type bismuth telluride (Bi2Te3) nanocrystals via water-based chemical route. The synthesized powders were consolidated into pellets utilizing spark plasma sintering (SPS). The sintered n-type pellets exhibited potentially high electrical conductivities (5.29 × 105 and 5.23 × 105 S.m−1) and low lattice thermal conductivities (0.12 and 0.25 Wm−1K−1); respectively. These thermoelectric (TE) properties suggested that the partially coherent boundaries permitted significant phonons scattering and electrons transfer. These led to an enhanced figure-of-merit (ZT) values (0.52 and 0.97), which are considered to be significant among the reported ZT values at room-temperature for the undoped synthesized n-type Bi2Te3 nanoparticles. Therefore, the current investigation displayed an efficient method to improve ZT of TE materials via nanostructure orchestrating, resulting in a worthy candidate n-type nanostructured Bi2Te3 for room-temperature TE applications.

Highly Efficient Thermoelectric Microgenerators Using Nearly Room Temperature Pulsed Laser Deposition.

Thermoelectric Bi2Te3 and Sb2Te3 thin films with high power factor were successfully obtained by pulsed laser deposition (PLD). Here, we demonstrate a well-controlled deposition of Bi2Te3/Sb2Te3 structures on glass substrates, through a shadow mask with micrometer-scale features. We establish an optimal growth temperature of 45 °C to attain compounds with suitable stoichiometric composition, as well as structural and electrical properties, to achieve high thermoelectric power factor. These films are produced without additional postannealing treatment or added gases. Indeed, crystalline films with Seebeck coefficients of 624 and -78 μV K-1 are obtained for Sb2Te3 and Bi2Te3, respectively. Microgenerators consisting of four pairs of n-type Bi2Te3 and p-type Sb2Te3 legs connected in series generate a maximum voltage of 50 mV and a power density of around 120 μW cm-2 for a temperature difference of 30 K across the hot and cold ends of the device. This low-temperature and simple PLD-deposited device represents an important step toward practical thermoelectric materials as well as efficient and compact microgenerators for low-temperature energy-harvesting applications.

Thermo-element geometry optimization for high thermoelectric efficiency

The figure of merit of thermoelectric materials is temperature dependent, and thus the local compatibility factor changes significantly along the thermo-element length. A local optimization method to maximize the efficiency of a function graded thermoelectric generator was proposed and discussed in this paper. By adjusting the cross-sectional area and segment's thickness, the reduced current equaled the compatibility factor of the material at every local thermo-element layer. This method can use the full potential of existing materials by maximizing the efficiency at every local thermo-element segment. For such a TEG working in a temperature range of 300–1100 K, the efficiencies of P-type segmented Bi0.5Sb1.5Te3/BiSbTe/-PbTe/FeNbSb thermo-element and a N-type segmented Bi2Te2.79Se0.21/Bi2Te2.9Se1.1/SnSe/SiGe thermo-element were 25.70% and 21.73%, respectively, much higher than the conventional segmented thermo-elements. The overall efficiency of the device was more than 23.72%, making it a promising technology to harvest energy from medium and high-temperature industrial components. The optimized TEG can be fabricated by SLS/SLM technology.

Combination of Electrodeposition and Transfer Processes for Flexible Thin-Film Thermoelectric Generators

To reduce consumption for ambient assisted living (AAL) applications, we propose the design and fabrication of flexible thin-film thermoelectric generators at a low manufacturing cost. The generators were fabricated using a combination of electrodeposition and transfer processes. N-type Bi2Te3 films and p-type Sb2Te3 films were formed on a stainless-steel substrate employing potentiostatic electrodeposition using a nitric acid-based bath, followed by a transfer process. Three types of flexible thin-film thermoelectric generators were fabricated. The open circuit voltage (Voc) and maximum output power (Pmax) were measured by applying a temperature difference between the ends of the generator. The thin-film generators obtained using thermoplastic sheets with epoxy resin exhibited a Voc that was tens of millivolts. In particular, the contact resistance of the thin-film generator decreased when silver paste was inserted at the junctions between the n- and p-type films. The most flexible thin-film generator fabricated in this study exhibited a Pmax of 10.4 nW at a temperature difference of 60 K. The current performance of the generators was too low, but we innovated a combination process to prepare them. It is expected to increase the performance by further decreasing the micro-cracks and contact resistance in the generators.