Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)₂Te₃ composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)₂Te₃ porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)₂Te₃ porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm^?1K^?1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)₂Te₃ bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.     

Optimization of chemical and electrochemical parameters for the preparation of n-type Bi2Te2.7Se0.3 thin films by electrodeposition

A 2^3–1 fractional factorial design comprising four runs and three centre points was applied in order to optimize the electrodeposition process to find a compound with the best stoichiometry leading to a Bi₂Te_2.7Se_0.3 thin film suitable for thermoelectric applications. The key factors considered were the deposition potential, the percentage of bismuth and the percentage of selenium in the solution. The Bi^III, Se^IV, Te^IV electrolyte mixtures in 1 M HNO₃ (pH 0), allowed deposition of ternary alloys to be achieved at room temperature on stainless steel substrates. The deposition mechanism was investigated by linear voltammetry. The films were characterized by micropobe analysis, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The XRD patterns of the film show that the as-deposited are polycrystalline and isostructural to Bi₂Te₃. The SEM study shows that the film is covered by crystallites while the AFM image reveals a low level of roughness.     

Recent progress in electrodeposition of thermoelectric thin films and nanostructures

Thermoelectric power generators and coolers have many advantages over conventional refrigerators and power generators such as solid-state operation, compact design, vast scalability, zero-emissions and long operating lifetime with no maintenance. However, the applications of thermoelectric devices are limited to where their unique advantages outweigh their low efficiency. Despite this practical confine, there has been a reinvigorated interest in the field of thermoelectrics through identification of classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials. Although, there are a few reports which demonstrated the improvement of efficiency through nanoengineering, the successful application of these nanostructures will be determined by a cost-effective and high through-put fabrication method. Electrodeposition is the method of choice to synthesize nanoengineered thermoelectric materials because of low operating and capital cost, high deposition rates, near room temperature operation, and the ability to tailor the properties of materials by adjusting deposition conditions. In this paper, we reviewed the recent progress of the electrodeposition of thermoelectric thin films and nanostructures including Bi, Bi_1−xSb_x, Bi₂Te₃, Sb₂Te₃, (Bi_1−xSb_x)₂Te₃, Bi₂Se₃, Bi₂Te_3−ySe_y, PbTe, PbSe, PbSe_1−xTe_x and CoSb₃.     

Hydrothermal method for the synthesis of Sb2Te3, and Bi0.5Sb1.5Te3 nanoplates and their thermoelectric properties

In this study, a modified hydrothermal method is reported for the preparation of Sb₂Te₃ and Bi_0.5Sb_1.5Te₃ nanoplates and their bulk samples was prepared by spark plasma sintering (SPS). The crystal structure, morphology, and thermoelectric properties were investigated. The microstructure results indicate that the bulk samples consisted nanograins after SPS. The presence of nanograins, high Seebeck coefficient (181 μV/K), high electrical conductivity (763 Ω^?1 cm^?1), and low thermal conductivity (1.15 W/mK) has been achieved in Sb₂Te₃ nanoplate bulk samples. As a result, the dimensionless thermoelectric figure of merit (ZT) of 0.55 at 400 K was achieved. Moreover, the peak ZT shifted to higher temperature compared with other reported results found in literature.     

Texture prediction of Bi2Te3 electroplated layers using Hartman's theory of crystal growth

An experimental study of electroplated thin films of bismuth telluride, R3m crystal structure, shows that fibre textures are observed during the growth of the layers. Several orientations 00.1, 10.10, 11.0, 10.4, 10.8, 01.5 and 10.0 are developed during the deposition process, then reach a final orientation. Hartman's theory of crystal growth based on the periodic bond chain (PBC) in a crystal lattice is usually applied. This explains the equilibrium shape of a crystal defined by F faces and the faces observed during the growth (S and K faces). It also successfully predicts the orientations observed in bismuth telluride from the beginning to the end of the layer deposition. The theory may also be applied to other deposition processes.     

Thermoelectric transport and magnetoresistance of electrochemical deposited Bi2Te3 films at micrometer thickness

Bismuth telluride (Bi₂Te₃) is so far the best thermoelectric material for applications near room temperature, and also exhibits large magnetoresistance. While the electrochemical deposition approach can achieve effective growth of the Bi₂Te₃ films at micrometer thickness, the magnetoresistance transportation behavior of the electrochemically deposited Bi₂Te₃ films is yet not clear. In this work, we demonstrate the thermoelectric and magnetoresistance behaviors of the micrometer thick Bi₂Te₃ films deposited via electrochemical deposition approach. The optimum thermoelectric power factor is observed in the Bi₂Te₃ sample with electrochemical deposition thickness of ~6 μm followed by rapid photon annealing treatment, reaching the magnitude of ~1 μWcm⁻¹K⁻² that is similar to the previous reports. In contrast to the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃ films, the electronic transportations of the electrochemically deposited Bi₂Te₃ are more influenced by the carrier scatterings by the grain boundaries and lattice defect. As a result, their magnetoresistance (MR) shows a distinguished non-monotonic behavior when varying the magnetic field, while the magnitude of their MR exhibits a positive temperature dependence. These MR behaviors largely differ to the previously reported ones from the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃, in which cases their MR monotonically increases with the magnetic field and exhibits negative temperature dependence. This work reveals the previously overlooked role of grain boundary that also regulates the transportation properties of bismuth chalcogenides in the presence of magnetic field.     

Enhanced dielectric and energy-storage properties in BiFeO3-modified Bi0.5(Na0.8K0.2)0.5TiO3 thin films

Lead free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ thin films doped with BiFeO₃ (abbreviated as BNKT-xBFO) (x = 0, 0.02, 0.04, 0.08, 0.10) were deposited on Pt(111)/Ti/SiO₂/Si substrates by sol-gel/spin coating technique and the effects of BiFeO₃ content on the crystal structure and electrical properties were investigated in detail. The results showed that all the BNKT-xBFO thin films exhibited a single perovskite phase structure and high-dense surface. Reduced leakage current density, enhanced dielectric and ferroelectric properties were achieved at the optimal composition of BNKT-0.10BFO thin films, with a leakage current density, dielectric constant, dielectric loss and maximum polarization of < 2 × 10⁻⁴ A/cm³, ~ 978^, ~ 0.028 and ~ 74.13 μC/cm² at room temperature, respectively. Moreover, the BNKT-0.10BFO thin films possessed superior energy storage properties due to their slim P-E loops and large maximum polarization, with an energy storage density of 22.12 J/cm³ and an energy conversion efficiency of 60.85% under a relatively low electric field of 1200 kV/cm. Furthermore, the first half period of the BNKT-0.10BFO thin film capacitor was about 0.15 μs, during which most charges and energy were released. The large recoverable energy density and the fast discharge process indicated the potential application of the BNKT-0.10BFO thin films in electrostatic capacitors and embedded devices.     

Growth and electrical properties of high-Curie point rhombohedral Mn-Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films

High-quality ternary relaxor ferroelectric (100)-oriented Mn-doped 0.36Pb(In_1/2Nb_1/2)O₃-0.36Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (Mn-PIMNT) thin films were grown on SrRuO₃-buffered SrTiO₃ single-crystal substrate in a wide deposition temperature range of 550-620°C using the pulsed laser deposition method. The phase structure, ferroelectric, dielectric, piezoelectric properties, and nanoscale domain evolution were studied. Under the deposition temperature of 620°C, the ferroelectric hysteresis loops and current-voltage curves showed that the film owned significantly enhanced remnant ferroelectric polarization of 34.5 μC/cm² and low leakage current density of 2.7 × 10⁻¹⁰ A/cm². Moreover fingerprint-type nanosized domain patterns with polydomain structures and well-defined macroscopic piezoelectric properties with a high normalized strain constant of 40 pm/V was obtained. Under in situ DC electric field, the domain evolution was investigated and 180° domain reversal was observed through piezoelectric force microscope. These global electrical properties make the current Mn-PIMNT thin films very promising in piezoelectric MEMS applications.     

Pulsed electrodeposition of p-type CuInSe2 thin films

CuInSe₂ thin films have been electrodeposited on conductive glass using cyclic pulse electrodeposition. One cycle consists of consequtively applying potentials E₁ and E₂, each during 10 s and a total of 90 cycles are applied. E₁ is chosen between −0.7 and −0.9 V_SCE while E₂ is fixed at −0.1 V_SCE. The films are annealed in argon and then etched in KCN solution to eliminate remnant secondary phases. The material is characterized employing grazing incident X-rays diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive scanning spectroscopy. The presence of secondary phases seems to be reduced when compared to films prepared at fixed potentials. The films are crystalline and the overall quality improves by annealing in Ar. Photoelectrochemical tests, Mott–Schottky plots and I–V curves confirm p-type conduction. The diffusion regime imposed by the potential pulses could be responsible for the different morphology and composition of samples prepared with pulsed and potentiostatic electrodeposition.     

Optimized weighted mobility induced high thermoelectric performance of ZnO-based multilayered thin films

As a transparent thermoelectric oxide, gallium-doped zinc oxide (GZO) has the potential to power wearable or portable electronics and may be used in the integrated circuits industry for chip cooling. Constructing ZnO–GZO interfaces has been proposed as an effective strategy for improving thermoelectric performance of GZO thin films. However, without the aid of band structure calculation for multilayered films, it is hard to directly elucidate the underlying mechanisms of carrier transport. Weighted mobility is an indicator that reveals the inherent electronic transport properties like carrier scattering, electronic band structure, and so on. Thus, to further investigate the effects of ZnO–GZO interfaces on electrical properties of GZO thin films, the structures containing different numbers of ZnO–GZO interfaces were designed and the correlations among numbers of ZnO–GZO interfaces, weighted mobility, and electrical properties were explored. It was found that with more ZnO–GZO interfaces, the weighted mobility increased, and the power factor values also improved as well. Consequently, an enhanced power factor value reached 439 μW m⁻¹ K⁻² at 623 K. This work demonstrated the beneficial effects of multiple interfaces on the improvements of electrical transport performance through analyzing weighted mobility, which laid a foundation for further optimization of thermoelectric performance.     

Heat treatment effects on the structural and optical properties of thin Bi2(Se1-xTex)3 films

Thin layers of Bi₂-chalcogenides, in the form of Bi₂(Se_1-xTe_x)₃ films, were evaporated on glass substrates by means of the vacuum thermal evaporation. Microstructure of the as prepared layers was investigated by x-ray diffraction (XRD) analysis. Identifications of the surface morphology and roughness were determined via scanning electron microscope (SEM). Optical transmissivity spectra proved that the as prepared films have low transparency with growing trend upon increasing the wavelength beyond the infra-red region. Low transmittance was observed for the as prepared films. Heat treatment, in the form of temperature annealing, was carried out aiming at boosting the structural features and the materials transmissivity. Structural properties and surface features of the annealed films were probed also via XRD and SEM analyses. It was found that the crystal size increases while the micro-strain and the dislocation density decrease obviously due to annealing. It was also observed that the annealing process significantly enhances the materials transmission especially in the range of higher wavelengths. Optical band gap was studied after annealing at various temperatures. Notable change in the band gap value was observed as a result of annealing. The band gap of the undoped (Bi₂Se₃) materials showed significant rise from 0.14 to 1.79 eV due to annealing. Similarly, the Te-doped samples exhibited notable increase in their band gap values after annealing. For example, the optical band gap of the sample doped at x = 0.20 increased from 0.03 to 0.41 eV by annealing. On the other hand, transmittance was also enhanced by annealing. For samples treated at 250 °C for 3 h, their optical transmissivity is enhanced to over 99% at the visible near-IR range. Such significant enhancement can be ascribed to structural enhancements. With such enhancement in the optical transmissivity, optoelectronic applications including transparent electrode can be met.     

High thermoelectric performance of (Bi1-xPrx)2(Te0.9Se0.1)3 alloys prepared by high-pressure sintering method

In this article, n-type (Bi_1-xPr_x)₂(Te_0.9Se_0.1)₃ (x = 0, .002, .004, .008) alloys were fabricated by high-pressure sintering (HPS) method together with annealing. The effect of high pressure and Pr contents on the microstructure and thermoelectric performance of samples were explored in detail. The results show that the HPS samples are composed of nanoparticles. Pr doping has significant impacts on the electrical and thermal transport properties of the Bi₂Te_2.7Se_0.3 alloys. The HPS sample with x = .004 shows the maximum ZT value of .31 at 473 K, which is enhanced by 41% to compare with the Pr-free sample. Annealing can improve the thermoelectric properties by increasing the electrical transport properties and decreasing the thermal conductivity simultaneously. As a result, the highest ZT value of 1.06 is achieved for the annealed sample with x = .004 at 373 K, which is beneficial to the thermoelectric power generation.     

Large Piezoelectricity and Ferroelectricity in Mn‐Doped (Bi0.5Na0.5)TiO3‐BaTiO3 Thin Film Prepared by Pulsed Laser Deposition

Mn‐doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (MnBNBT) thin films were prepared on SrRuO₃ (SRO)‐coated (001) SrTiO₃ (STO) single crystal substrates by pulsed laser deposition under different processing conditions. Structural characterization (i.e., XRD and TEM) confirms the epitaxial growth of STO/SRO/MnBNBT heterostructures. Through the judicious control of deposition temperature, the defect level within the films can be finely tuned. The MnBNBT thin film deposited at the optimized temperature exhibits superior ferroelectric and piezoelectric responses with remanent polarization P_r of 33.0 μC/cm² and piezoelectric coefficient d₃₃ of 120.0 ± 20 pm/V.     

(1-x)(Bi1/2Na1/2)0.900Ba0.088Sr0.012TiO3-x(Bi1/2K1/2)TiO3无铅压电陶瓷的电学和力学性能

利用常规烧结方法制备出了多种A位离子掺杂的钛酸铋纳[(Bi1/2Na1/2)TiO3,BNT]无铅压电陶瓷.对BNT基陶瓷的电学性能和力学性能进行了研究.在(1-x)(Bi1/2Na1/2)0.900Ba0.088Sr0.012TiO3-x(Bi1/2K1/2)TiO3(x=0-0.14)陶瓷体系中,当x=0.10时,可获得最大压电常数(168pC/N).在1 kHz,这种陶瓷的介电常数、介电损耗和平面机电耦合系数分别为1 221,0.0361和0.2281.Curie温度随x的增加先增加,当x=0.12时,达到最高值(300℃),随后,当x值进一步增加,Curie温度降低.该种无铅压电陶瓷的Vickers硬度和断裂韧性分别为5.0GPa和2.0MP·m1/2,均高于Pb(Zr,Ti)O3陶瓷.     

Sintering temperature effect on thermoelectric properties and microstructure of the grained Bi1.9Gd0.1Te3 compound

Elemental composition, crystal and grain structures, specific electrical resistivity, thermopower, thermal conductivity, and thermoelectric figure-of-merit of grained Bi_1.9Gd_0.1Te₃ compounds sintered at T_S = 690, 720, 735, 750, 780 and 810 K have been studied. Strong T_S – effect on the grain structure was found. Fine-grained samples with average grain size of ˜500 nm were prepared at T_S = 690, 720 and 735 K, whereas coarse-grained samples with average grain size above 1100 nm were sintered at T_S = 750, 780 and 810 K. Evaporation of Te takes place at high temperatures, which results in forming of anti-site defects of Bi in Te-sites. Electrical and thermal properties of the fine-grained and coarse-grained samples happened to be rather different. The highest value of the thermoelectric figure-of-merit equal to ˜ 0.55 was observed for the sample sintered at T_S = 750 K. This temperature corresponds to transition from the fine-grained to coarse-grained samples.     

High thermoelectric performance of high-mobility Ga-doped ZnO films via homogenous interface design

Ga-doped ZnO (GZO) thin films grown on sapphire substrates have been widely investigated as a promising transparent thermoelectric (TE) material. However, due to the large lattice mismatch and thermal expansion between the sapphire substrate and GZO film, strain-induced lattice distortion impedes the transport of electrons, leading to low carrier mobility. In this study, ZnO homo-buffer layers with different thicknesses were inserted between sapphire substrates and GZO films, and their effect on the TE properties was investigated. A thin ZnO interlayer (10 nm) effectively reduced the lattice mismatch of the GZO film and improved the carrier mobility, which contributed to the large enhancement in the electrical conductivity. Simultaneously, energy filtering occurred at the interface between GZO and ZnO, resulting in a relatively high density of states (DOS) effective mass and maintaining a high Seebeck coefficient compared to that of the unbuffered GZO films. Consequently, the GZO film with a 10 nm thick ZnO buffer layer possessed a high power factor value of 449 μW m⁻¹ K⁻² at 623 K. This study provides a facile and effective method for optimizing the TE performance of oxide thin films by synergistically improving their carrier mobility and enhancing their effective mass.     

Preparation of SnTe thin films on Au(1 1 1) by electrodeposition route

Tin telluride (SnTe) thin films were deposited onto Au(1 1 1) substrates from an aqueous solution containing SnCl₂, TeO₂, and C₆H₅Na₃ at room temperature (25 °C) for the first time via electrodeposition route. The electrodeposition of the thin films was studied using cyclic voltammetry, compositional, structural, optical measurements and surface morphology. It was found that the stoichiometric SnTe thin films could be obtained at −0.50 V. The as-deposited thin films were crystallized in the preferential orientation along the (2 2 0) plane. SEM investigations indicated that the shape of thin films could be altered from a spherical particle to a dendritic crystal by increasing the deposition potential. The growth of the dendritic films proceeds via formation of nanoparticles and growth of dendritic crystals on these nanoparticles. The optical absorption studies as a function of deposition time indicated that the band gap of the SnTe thin film increases as the deposition time decreases.     

AgSbTe2 semi-nanocrystalline thin films as a multifunctional platform for optoelectronic and diode applications

The synthesis of the nanosized multifunctional thin film provides new solutions for many technological issues and consider a great step for miniaturized technology. Toward these goals, AgSbTe₂ semi-nanocrystalline thin films of different thicknesses were synthesized by the thermal evaporation technique. The structural features were investigated by X-ray diffraction, and selected area electron diffraction (SAED) yielding a semi-nanocrystalline thin film of grain size ranging from 9.98 to 21.38 nm. The energy-dispersive X-ray spectroscopy (EDAX) verified the high purity and stoichiometry of the deposited films. For optoelectronic application, many optical parameters, including band gap (E_g), Urbach energy (E_u), Refractive index (n), dispersion energy (E_d), electronic polarizability (α_e), and interband transition strength (J_CV) were extensively discussed. The optical band gap reduced from 1.41 to 1.04 eV upon increasing the thickness from 150 to 550 nm. The temperature dependence of the electrical resistivity (ρ) of nanosized thin film was measured and the activation energy was estimated and it was found that the resistivity increased up to 450 K asserting the semiconductor behavior of the films. As for diode application, The Ag/2D-MoS₂/p-AgSbTe₂ (550 nm)/n-Si/Al heterostructure diode was constructed by thermal evaporation and all the diode parameters alongside conduction mechanism were studied in detail. AgSbTe₂-based diode showed a low rectification ratio; however, the ideality factor (n) and zero bias barrier height (Φ_b) had optimal values of about 1.40 and 0.75 at room temperature, respectively.     

High recoverable energy storage density in (1-x)Bi0.5(Na0.8K0.2)0.5TiO3-xSrZrO3 thin films prepared by a sol-gel method

Lead-free (1-x)Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-xSrZrO₃ (abbreviated as BNKT-100xSZ) thin films were deposited on Pt(111)/Ti/SiO₂/Si using sol-gel/spin coating method. With the addition of SZ, the long-range ferroelectric order dominant in BNKT is disrupted, which boosts the ferroelectric relaxor behavior. Consequently, a high recoverable energy density of 34.69?J/cm³ combined with an efficiency of 59.32% was achieved at the optimal composition of BNKT-15SZ under a high electric field of 2100?kV/cm, which can be ascribed to the slim P-E loops induced by strong relaxor behavior (γ?=?1.93) and the enhanced breakdown strength. Moreover, BNKT-15SZ thin film capacitor presents a good thermal stability with minimal variations of the energy density (<10%) and the energy storage efficiency (<5%) over a wide temperature range of 30–100?°C. The results indicated that the BNKT-100xSZ thin films may be a promising environmental-friendly material for energy storage applications.     

Poly(ether imide)s containing cyano substituents and thin films made from them

A series of poly(ether imide)s containing functional cyano groups has been prepared by polycondensation reaction of 2,6‐bis(m‐aminophenoxy)benzonitrile with different bis(ether dianhydride)s, such as 2,2‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]isopropane dianhydride, 2,2‐bis[4‐(3,4‐dicarboxyphenoxy)‐phenyl]hexafluoroisopropane dianhydride, 1,1‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]cyclohexane dianhydride, and 3,3‐bis[4‐(3,4‐dicarboxyphenoxy) phenyl]phthalide dianhydride. The polymers were soluble in polar aprotic solvents and even in less polar solvents and were processed into thin films by casting their solutions. The properties of the polymers, particularly the crosslinking of macromolecular chains through cyano groups, were studied by using dynamic mechanical analysis (DMA), thermal stability, glass transition, and solubility measurements. Also, the molecular relaxations were evidenced by DMA and dielectric spectroscopy. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012