Resistance Pressing Sintering Research Articles

Enhancement of thermoelectric performances of BiCuSeO through Y doping and grain refining

Thermoelectric properties of BiCuSeO oxyselenide ceramics were improved combining Y doping and grain refinement. Bi1−xYxCuSeO (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) (called as BYCSO below) samples were prepared by mechanical alloying and resistance pressing sintering (RPS). The testing results indicate that Y doping can increase the carrier concentration and electrical conductivity, increase the thermal conductivity. On the basis of Y doping, grain refinement by increasing ball milling strength was indicated to increase the concentration of Cu vacancies, which enhanced the electrical conductivity and power factor, and reduce the thermal conductivity. Finally, the ZT value is increased to 0.93, which is 98% higher than that of the pure sample.

Artificial porous structure: An effective method to improve thermoelectric performance of Bi2Te3 based alloys

Introduction of porous structure is a feasible and promising method for improving thermoelectric performances for thermoelectric materials. Meanwhile, the uniformity of pores distribution plays a decisive role in the objective evaluation of thermoelectric properties for porous structures. In this work, extremely uniform porous structures in p-type Bi0.4Sb1.6Te3 bulks were obtained by pre-mixing with NH4HCO3 combined with resistance pressing sintering (RPS) process. The thermoelectric performance was obviously affected by the porous structure. The increasing porosity resulted in a significant decrease in electrical and thermal conductivity. It was inspiring that the reduction of thermal conductivity could compensate for the deterioration of electrical conductivity, leading to the enhancement in ZT value. The maximum ZT value of 1.11 was obtained for the sample prepared by pre-mixing with 5 ​wt % NH4HCO3 at 343 ​K, which was 20% higher than that of the fully dense sample. Finally, the advantages on the homogeneity of pores, thermoelectric properties and mechanical properties of the sample prepared by this method are confirmed through comparing with the sample prepared without NH4HCO3, when their relative densities both are 90%.

Enhanced thermoelectric properties in BiCuSeO ceramics by Pb/Ni dual doping and 3D modulation doping

Bi1-xPbxCu1-xNixSeO (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) BiCuSeO based ceramics have been prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process. The effects of Pb/Ni doping and 3D modulation doping on the thermoelectric properties have been investigated systematically. For Pb doping, it can tune the Fermi level and promote the band convergence, decreasing the band gap and significantly enhancing the carrier concentration. Besides, it contributes to higher effective mass and Seebeck coefficient due to the delocalized 6s orbitals from the lone pair electrons. For Ni doping, Ni ions could enhance Seebeck coefficient. The results indicates that the substitution of Pb at Bi site and Ni at Cu site resulted in the significantly increase of electrical conductivity and power factor. Based on this, 3D modulation doping led to the simultaneous increase of the electrical conductivity and power factor. The maximum power factor of the modulation doped sample (Bi0.90Pb0.10Cu0.90Ni0.10SeO) reached 0.64 mW m−1 K−2, and the maximum ZT value reached 1.08, which is 1.3 and 1.0 times higher than those of the undoped sample, respectively.

Enhanced thermoelectric performance in BiCuSeO oxyselenides via Ba/Te dual-site substitution and 3D modulation doping

Here, Bi1-xBaxCuSe1-xTexO (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics have been prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process. The effects of Ba/Te doping on the thermoelectric properties have been investigated systematically. For Ba doping, it can tune the Fermi level and promote the band convergence, decreasing the band gap and significantly enhancing the carrier concentration; for the Te doping based on Ba doping, it can profoundly reduce the difference of electronegativity in (Cu2Se2)2- layer, increase bond covalency, and then increase the electrical conductivity. The results indicate that the substitution of Ba/Te resulted in significant increases of electrical conductivity and power factor. Based on this, modulation doping leads to the simultaneous increase of the electrical conductivity and Seebeck coefficient. Finally, the maximum power factor of 0.98 mW m−1 K−2 and ZT value of 1.17 were obtained for the modulation doped Bi0.90Ba0.10CuSe0.90Te0.10O bulk, which were ~ 3.5 and ~ 2.2 times respectively as those of the pristine BiCuSeO ceramic.

Enhancing thermoelectric and mechanical performances in BiCuSeO by increasing bond covalency and nanostructuring

The layered oxyselenide BiCuSeO thermoelectric ceramics are attracting much scientific attention for its excellent thermoelectric properties. Herein, Bi1-xSbxCuSe1-xTexO (x = 0, 0.01, 0.02, 0.04, 0.06, 0.08) ceramics have been prepared by ball milling (BM) and resistance pressing sintering (RPS) process. The effects of Sb/Te doping and grain size on the thermoelectric/mechanical properties are investigated systematically. For the former, it can tune the Fermi level and promote the band convergence, decreasing the band gap and concurrently enhancing the carrier concentration; for the latter, it can profoundly modify microstructure into an all-scale hierarchical architecture to scatter phonons with broad range of mean free paths, effectively reducing the lattice thermal conductivity. The results indicates that the substitution of Sb/Te resulted in the significantly increase of electrical conductivity and power factor, and BM effectively reduces the grain size to ~ 387 nm in the sample after sintering, which leads to a decrease in the thermal conductivity and an increase in the electrical conductivity. The maximum power factor of 0.72 m Wm−1 K−2 and ZT value of 1.19 were obtained for Bi0.92Sb0.08CuSe0.92Te0.08O with BM time = 16 h at 873 K, which was ~ 2.7 and ~ 2.2 times as that of the pristine BiCuSeO ceramic.

Effect of synthesis processes on the thermoelectric properties of BiCuSeO oxyselenides

The BiCuSeO based ceramics have been prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process. The effects of the parameters of annealing, ball milling and sintering on the microstructure, thermoelectric properties, and mechanical properties are investigated systematically. The results indicates that the samples prepared by MA exhibit higher carrier concentration due to higher Cu vacancies concentration, smaller grain size resulting in stronger phonon scattering, and correspondingly higher thermoelectric properties than those prepared by annealing. By prolonging milling time and increasing ball milling strength, it can further refine grains, increase the carrier concentration, enhance phonon scattering and further improve thermoelectric properties. Coupled with Ca/Pb doping, the electrical conductivity at room temperature increased to 445 Scm−1. The total thermal conductivity is below 0.90 Wm−1K−1 in the whole measured temperature range, which is attributable to the increased defects and gain boundaries. The maximum power factor of 0.75 mWm−1K−2 and ZT value of 1.15 were obtained for the Bi0.84Ca0.08Pb0.08CuSeO with BM time (t) = 16 h at 873 K.

Effect of Ba and Pb dual doping on the thermoelectric properties of BiCuSeO ceramics

The layered oxyselenide BiCuSeO thermoelectric ceramics are attracting more and more attention due to intrinsically low thermal conductivity. In this work, Bi1−x−yBaxPbyCuSeO (x = y = 0, 0.01, 0.02, 0.04, 0.06, 0.08; x = 0, y = 0.08; x = 0.08, y = 0) bulks have been prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process, and the effects of Ba/Pb doping on the thermoelectric properties and Vickers hardness of p-type BiCuSeO ceramics have been investigated systematically. The results indicated that the substitution of Bi3+ by Ba2+/Pb2+ resulted in the significantly increase of electrical conductivity and power factor due to the introduction of the carriers into the conductive (Cu2Se2)2− layers as the result of the introduction of negative charge into the carrier-storaging (Bi2O2)2+ layers. Specifically, the room temperature electrical conductivity significantly increases from ∼8 Scm−1 for pristine BiCuSeO to ∼244 Scm−1 for Bi0.92Ba0.04Pb0.04CuSeO, and further up to ∼443 Scm−1 for Bi0.84Ba0.08Pb0.08CuSeO. The maximum power factor of 0.66 mWm−1K−2 and dimensionless figure of merit (ZT) value of 1.01 were obtained for the Bi0.88Ba0.06Pb0.06CuSeO at 873 K.

Bi2Te3 based bulks with high figure of merit obtained from cut waste fragments of cooling crystal rods

Bi2Te3 based cutting waste fragments were recycled by vacuum metallurgical technology to prepare p-type 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.

Preparation and optimization of thermoelectric properties of Bi2Te3 based alloys using the waste particles as raw materials from the cutting process of the zone melting crystal rods

The p-type Bi2Te3 alloys were prepared using the waste particles from the cutting process of the zone melting crystal rods as the main raw materials by impurity removal process including washing, carbon monoxide reduction and vacuum metallurgical process. The thermoelectric properties of the Bi2Te3 based bulk materials were optimized by component adjustment, second smelting and resistance pressing sintering (RPS) process. All evidences confirmed that most of impurities from the line cutting process and the oxidation such as Sb2O3, Bi2O3 and Bi2Te4O11 could be removed by carbon monoxide reduction and vacuum metallurgical process adopted in this work, and the recycling yield was higher than 97%. Appropriate component adjustment treatment was used to optimize the carrier content and corresponding thermoelectric properties. Lastly, a Bi0.36Sb1.64Te3 bulk was obtained and its power factor (PF) could reach 4.24 mW m−1 K−2 at 300 K and the average PF value was over 3.2 mW m−1 K−2 from 300 K to 470 K, which was equivalent with the thermoelectric performance of the zone melting products from high purity elements Bi, Te and Sb. It was worth mentioning that the recovery process introduced here was a simple, low-cost, high recovery rate and green recycling technology.

The initial powder-refinement-induced donor-like effect and nonlinear change of thermoelectric performance for Bi2Te3-based polycrystalline bulks

p-type Bi0.5Sb1.5Te3 bulks were prepared from different original sizes of 36–203 μm by the resistance pressing sintering (RPS) method. It was demonstrated that the original powder refinement introduces lattice defects and facilitates a donor-like effect, resulting in a decrease of the carrier concentration and an increase of the effective mass of the carrier. The finer the original powder is, the higher the electrical resistivity and Seebeck coefficient are, and the lower the thermal conductivity is. Although the sintered sample from the smallest sized powders possessed the minimum thermal conductivity, its figure of merit was unsatisfactory due to the low electrical transport performance, and the sintered sample from the 200 ∼ 300 meshed original powders obtained a maximum figure-of-merit of 0.95 at 340 K. In addition, it was observed that the phenomenon of nonlinear change in electrical transport performance with initial powder refinement was due to the percolation effect. In short, the present work confirms that the initial powder size has a great influence on the thermoelectric properties of Bi2Te3-based materials, which should be taken seriously in our follow-up work.

Enhanced thermoelectric properties of Sb-doped BiCuSeO due to decreased band gap

As the best oxide based thermoelectric materials, the BiCuSeO compounds with very low lattice thermal conductivity and high figure of merit have been widely investigated because of the good thermal/chemical stability, low price and environmental friendliness. In this paper, the Bi1-xSbxCuSeO (x = 0, 0.005, 0.01, 0.02, 0.05, 0.08) oxychalcogenides were prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process in order to improve the thermoelectric properties. The effect of Sb doping on the microstructure and thermoelectric properties of p-type BiCuSeO ceramic has been investigated in detail. To analysis the change rule of the thermoelectric properties of Bi1-xSbxCuSeO due to Sb replacing Bi site, the lattice structure and electronic behavior of BsCuSeO (Bs = Bi or Sb) were calculated and simulated, respectively. The band gap of BsCuSeO has been reduced from 0.478 eV to 0.329 eV, and the bond length of Cu/Bi-Se has been decreased, the hole pocket in the band structure has been deeper, and the peaks of Cu/Se orbitals in the decomposed partial density of states have been broadened after Bi site was replaced by element Sb, which resulted in the significant increase of carrier concentration and electrical conductivity, while still keep a relatively high Seebeck coefficient and low thermal conductivity. The maximum power factor of 0.36 mWm−1K−2 and ZT value of 0.73 were obtained for the Bi0.95Sb0.05CuSeO at 873 K, which was 65% higher than that of the undoped stoichiometry BiCuSeO ceramic.

Evolution of thermoelectric performance for (Bi,Sb)2Te3 alloys from cutting waste powders to bulks with high figure of merit

Bi2Te3 based cutting waste powders from cutting wafers were firstly selected as raw materials to prepare p-type Bi2Te3 based thermoelectric (TE) materials. Through washing, reducing, composition correction, smelting and resistance pressing sintering (RPS) process, p-type (Bi,Sb)2Te3 alloy bulks with different nominal stoichiometries were successfully obtained. The evolution of microstructure and TE performance for (Bi,Sb)2Te3 alloys were investigated in detail. All evidences confirmed that most of contaminants from line cutting process such as cutting fluid and oxides of Bi, Sb or Te could be removed by washing, reducing and smelting process used in this work. The carrier content and corresponding TE properties could be adjusted effectively by appropriate composition correction treatment. At lastly, a bulk with a nominal stoichiometry of Bi0.44Sb1.56Te3 was obtained and its' dimensionless figure of merit (ZT) was about 1.16 at 90°C. The ZT values of Bi0.36Sb1.64Te3 and Bi0.4Sb1.6Te3 alloy bulks could also reach 0.98 and 1.08, respectively. Different from the conventional recycling technology such as hydrometallurgy extraction methods, the separation and extraction of beneficial elements such as Bi, Sb and Te did not need to be performed and the Bi2Te3 based bulks with high TE properties could be directly obtained from the cutting waste powders. In addition, the recycling technology introduced here was green and more suitable for practical industrial application. It can improve material utilization and lower raw material costs of manufacturers.

Resistance pressing sintering: A simple, economical and practical technique and its application to p-type (Bi,Sb)2Te3 thermoelectric materials

In the present work, starting from bismuth, antimony and tellurium granules, p-type (Bi,Sb)2Te3 alloys were successfully obtained via melt spinning (MS) combining with resistance pressing sintering (RPS) technique. The phase, microstructure and composition of the samples were evaluated by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive X-ray spectroscopy during each step in the preparation process, respectively. The electrical resistivity, seebeck coefficient and thermal conductivity of the samples were measured in the temperature range of 323–473K. The effects of the substituting of antimony by bismuth on the thermoelectric properties of p-type (Bi,Sb)2Te3 alloys were analyzed in detail. The relative densities of the samples prepared by RPS technique were all more than 98%. The partially oriented lamellar structure could be observed at some local regions of the samples prepared by MS–RPS process and the monolayer thickness of the lamellar structure was smaller than that of the zone melting (ZM) sample. All evidences about electrical and thermal transport properties suggested that suitable decreasing Bi content could effectively improve the ZT value of (Bi,Sb)2Te3 alloys. The maximum ZT value of 1.17 was obtained for the Bi0.44Sb1.56Te3 alloys at 323K. Different from the conventional hot pressing and spark plasma sintering, the RPS method introduced in this paper was more suitable for industrial application due to its simplicity, economy and high yields.

Improved thermoelectric properties of Bi2Te3−xSex alloys by melt spinning and resistance pressing sintering

Starting from bismuth, tellurium and selenium chunks, n-type Bi2Te3−xSex (x ⩽ 0.3) alloys were obtained by melt spinning (MS) combined with a resistance pressing sintering (RPS) process. The phases, microstructures and compositions of the samples were evaluated by x-ray diffraction, field emission scanning electron microscopy, and energy dispersive x-ray spectroscopy during each step in the preparation process, respectively. The influences of Se doping, MS and RPS processes on the thermoelectric (TE) properties of Bi2Te3−xSex alloys were investigated in detail. The Bi2Te3−xSex powders could be well compacted by the RPS process and the relative densities of the bulks prepared by RPS were all higher than 96%. The partially oriented lamellar structure could be observed at some regions of the samples prepared by RPS, and the monolayer thickness of the lamellar structure in the MS-RPS samples was smaller than that in the smelting-RPS sample. The MS process was confirmed as an excellent method to obtain fine microstructures and low lattice thermal conductivity for the TE materials. All evidence about electrical and thermal transport properties suggested that suitably increasing the Se content could effectively improve the ZT value. The maximum ZT value of 0.84 was obtained for the Bi2Te2.7Se0.3 prepared by MS-RPS at 423 K. As opposed to the conventional hot pressing and spark plasma sintering, the RPS method introduced here is more suitable for practical industrial application due to its cost saving and high efficiency.