Hongyao Xie

Phase Segregation and Superior Thermoelectric Properties of Mg2Si1–xSbx (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

High thermoelectric performance of p-BiSbTe compounds prepared by ultra-fast thermally induced reaction

The traditional zone melting (ZM) method for the fabrication of state of the art Bi2Te3-based thermoelectric materials has long been considered a time and energy intensive process. Herein, a combustion synthesis known as the thermally induced flash synthesis (TIFS) is employed to synthesize high performance p-type BiSbTe alloys within 20 min compared to tens of hours for the ZM samples. The thermodynamic parameters and phase transformation mechanism during the TIFS process were systematically studied for the first time. TIFS combined with plasma activated sintering (PAS) results in a single phase homogeneous material with excellent repeatability, high thermoelectric performance (maximum ZT ∼ 1.2 at 373 K) and robust mechanical properties in a very short time of less than 20 min. The technologically relevant average ZT value of TIFS-PAS fabricated Bi0.5Sb1.5Te3 from 298 K to 523 K is 0.86, about a 46% improvement over the ZM sample. The compressive and bending strength of TIFS-PAS Bi0.5Sb1.5Te3 are also improved by about 5 fold compared with those of the ZM samples. Thermoelectric power generation modules assembled using the TIFS-based high performance n and p type materials show the largest thermoelectric conversion efficiency of 5.2% when subjected to a temperature gradient of 250 K, representing about 42% enhancement compared with the commercial ZM-based module. Because of the simplicity and scalability of the process and short synthesis time, the TIFS-PAS technology provides a new and efficient way for large-scale, economical fabrication of Bi2Te3-based thermoelectrics.

Enhanced ZT and attempts to chemically stabilize Cu2Se via Sn doping

Cu2Se is a p-type semiconducting compound that possesses excellent thermoelectric properties but degrades at elevated temperatures under large currents, precluding it from applications in harvesting waste heat. In this study, we make use of a doping approach to attempt to chemically stabilize Cu2Se while maintaining its superior thermoelectric properties. Specifically, we synthesized Cu2(1−x)SnxSe (x = 0, 0.01, 0.02 and 0.05) via melting, annealing and spark plasma sintering. We found that the ZT was enhanced the most in the x = 0.01 sample, averaging approximately a 15% increase over the pure Cu2Se throughout a broad temperature range of 473–823 K, and achieving a maximum ZT = 1 at T = 823 K. The enhancement is due to an increased power factor and a reduced thermal conductivity, which is a result of point defect scattering from Sn atoms in the Cu2Se matrix and grain boundary scattering from a micron-size secondary phase of SnSe. We further tested the ability of the Sn dopant to prevent material degradation at elevated temperatures under large currents. Increasing the Sn dopant content does indeed decrease the solid Cu precipitation but not enough to resolve the issue of material degradation. As a result, despite its improved ZT, Cu1.98Sn0.01Se is not yet ready for thermoelectric applications, and requires further effort to stabilize the structure.

Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance

In this study, a series of Ge1-xMnxTe (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge1-xMnxTe is profound. With increasing content of MnTe, the structure of the Ge1-xMnxTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m0 (GeTe) to 6.15 m0 (Ge0.85Mn0.15Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m-1 K-1 at 823 K. Subsequent doping of the Ge0.9Mn0.1Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge0.86Mn0.10Sb0.04Te). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.

Non-equilibrium synthesis and characterization of n-type Bi2Te2.7Se0.3 thermoelectric material prepared by rapid laser melting and solidification

Commercial production of thermoelectric (TE) modules features energy-intensive and time-consuming processes. Here, we propose a rapid, facile and low cost fabrication process for n-type single phase Bi2Te2.7Se0.3 that combines self-propagating high-temperature synthesis (SHS) with the laser non-equilibrium 3D printing method based on selective laser melting (SLM). The optimal SLM processing window for high quality single layers has been determined. Results show that the chemical composition of the sample is very sensitive to the laser energy density (EV) due to the selective vaporization of Se and Te. For energy densities EV of less than 33.3 J mm−3, the composition of the SLM-processed samples is relatively stable. However, as EV exceeds 33.3 J mm−3 and increases further, the vaporization rate of Te and Se significantly increases and is much higher than that of Bi. Empirical formulae relating the chemical composition of the resulting materials with the values of EV are obtained and are used to predict the composition of the SLM-processed material. Most importantly, the temperature dependent TE properties of the SLM-fabricated bulk sample result in a maximum ZT value of 0.84 at 400 K, which is comparable to that of the commercially available material. The work has laid a foundation for the future utilization of this technique for the fabrication of Bi2Te3-based thermoelectric modules.

Protein engineering of a new recombinant peptide to increase the surface contact angle of stainless steel

Biofouling seriously affects the properties and service life of metal materials. A number of studies have shown that the initial bacterial attachment to the metal surface and the subsequent formation of biofilm are dependent on the surface characteristics of the substratum, including metal surface free energy, roughness and metallurgical features. In this study, a novel recombinant fusion protein, which consists of receptor binding domain protein (RBD), truncated protein fragment of MrpF and alkaline phosphatase (PhoA) domains, has been constructed in an attempt to increase the surface contact angle of stainless steel. It has been confirmed that RBD has a strong affinity to 304 stainless steel; the truncated protein fragment of MrpF has high hydrophobicity and anchoring features, which can improve the contact angle of the material surface, whilst PhoA is an effective detection tool to monitor the expression and secretion of fusion protein. Multiple assays including FTIR, XPS, SEM-EDS and contact angle measurement revealed the existence of nitrogen and sulfur elements, binding energy shifts of nitrogen, carbon and oxygen atoms, and new FTIR peaks in treated stainless steel samples with increased contact angles at about 50°, confirming that a new organic steel material has been produced responding to these surface property changes. Using novel recombinant peptides to react with steel could become a new technique to increase the surface contact angle of the stainless steel for diverse applications.

Preparation and analysis of a new bioorganic metallic material

Biofouling on metal surfaces is one of the main reasons for increased ship drag. Many methods have already been used to reduce or remove it with moderate success. In this study, a synthetic peptide has been utilized to react with 304 stainless steel aiming to generate a bioorganic stainless steel using a facile technique. After the reaction, white matter was found on the surface of the treated stainless steel via SEM, whilst the nontreated stainless steel had none. Elemental analysis confirmed that excessive N existed on the surface of the treated samples using an integrated SEM-EDS instrument, implying the presence of peptides binding on the surface of the bioorganic stainless steel. The FTIR spectra showed amide A and II peaks on the surface of the bioorganic stainless steel suggesting that either the peptides grafted onto the steel surface or the polypeptide composition accumulated on the steel samples. XPS analysis of the treated steel demonstrated that there was nitrogen bonding on the surface and it was a chemical bond via a previously unreported chemical interaction. The treated steel has a markedly increased contact angle (water contact angle of 65.7 ± 4.7° for nontreated steel in comparison to treated, 96.4 ± 2.1°), which supported the observation of the wettability change of the surface, i.e. the decrease of the surface energy value after peptide treatment. The changes of the surface parameters (such as, Sa, Sq, Ssk and Sku) of the treated steel by surface analysis were observed.

High thermoelectric performance in Bi 0.46 Sb 1.54 Te 3 nanostructured with ZnTe

Defect engineering and nano-structuring are the core stratagems for improving thermoelectric properties. In bismuth telluride alloys nanosizing individual crystallites has been extensively studied in efforts to reduce the thermal conductivity, but nanostructuring with second phases has been more challenging. In this study, we demonstrate a thermoelectric figure of merit ZT of 1.4 at 400 K, realized in Zn-containing BiSbTe alloys (specifically Bi0.46Sb1.54Te3) by integrating defect complexity with nanostructuring. We have succeeded in creating nanostructured BiSbTe alloys containing ZnTe nanoprecipitates. We present a melt-spinning-based synthesis that forms in situ ZnTe nanoprecipitates to produce an extremely low lattice thermal conductivity of ∼0.35 W m−1 K−1 at 400 K, approaching the amorphous limit in the Bi2−xSbxTe3 system, while preserving the high power factor of Bi0.46Sb1.54Te3. These samples show excellent repeatability and thermal stability at temperatures up to 523 K. DFT calculations and experimental results show that Zn is inclined to form dual site defects, including two substitutional defects ZnBi/Sb′ and a Te vacancy, to achieve full charge compensation, which was further explicitly corroborated by Positron annihilation measurement. The strong enhancement of thermoelectric properties was validated in a thermoelectric module fabricated with the melt-spun p-legs (ZnTe-nanostructured BiSbTe) and zone-melt n-legs (conventional BiTeSe) which achieved a thermoelectric conversion efficiency of 5.0% when subjected to a temperature gradient of 250 K, representing about 40% improvement compared with a commercial zone-melt-based module. The results presented here represent a significant step forward for applications in thermoelectric power generation.

Structure and thermoelectric properties of 2D Cr2Se3−3xS3x solid solutions

Chromium selenide (Cr2Se3), consisting of earth-abundant elements, is a new cost-efficient thermoelectric material. In this study, a series of Cr2Se3−3xS3x (x = 0–0.1) solid solutions was synthesized by solid-state reaction combined with the spark plasma sintering (SPS) process. The correlation between sulphur substituted on selenium sites, the structure, and the thermoelectric properties of Cr2Se3−3xS3x solid solutions was systematically investigated. The solubility limit of S in Cr2Se3−3xS3x is about 10%. Through S substitutions, the band gap has been widened, the Seebeck coefficient has been effectively increased, and the lattice thermal conductivity has been substantially decreased. Mainly due to the remarkable decrease in the lattice thermal conductivity, the ZT values of Cr2Se3−3xS3x (x = 0–0.1) solid solutions have been increased. The maximum ZT value of 0.29 has been achieved at 623 K for the Cr2Se2.7S0.3 compound, which is 32% higher than the ZT value of pure Cr2Se3.

Thermoelectric Properties of Ga/Ag Codoped Type-III Ba24Ge100 Clathrates with in Situ Nanostructures

Because of the low thermal conductivity and high electrical conductivity, type-III Ba24Ge100 clathrates are potentially of interest as power generation thermoelectric materials for midto-high temperature operations. Unfortunately, their too high intrinsic carrier concentration results in a quite low Seebeck coefficient. To reduce the carrier concentration, we prepared a series of Ga/Ag codoped type-III Ba24Ge100 clathrate specimens by vacuum melting and subsequently compacted by spark plasma sintering (SPS). Doping Ga-Ag on the sites of Ge reduces the concentration of electrons and, at higher concentrations, also leads to the in situ formation of BaGe2 nanoprecipitates detected by the microstructural analysis. As a result of doping, the Seebeck coefficient increases, the thermal conductivity decreases, and the dimensionless figure of merit ZT reaches a value of 0.34 at 873 K, more than three times the value obtained with undoped Ba24Ge100.