Jing-Feng Li

Nanostructured AgPbmSbTem+2 System Bulk Materials with Enhanced Thermoelectric Performance

Nanostructured Ag0.8Pbm+xSbTem+2 (m = 18, x = 4.5) system thermoelectric materials have been fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods followed by annealing for several days to investigate the effect on microstructure and thermoelectric performance. It was found that appropriate annealing treatment could reduce both the electrical resistivity and the thermal conductivity at the same time, consequently greatly enhancing the thermoelectric performance. A low electrical resistivity of 2 x 10-3 Ohm-cm and low thermal conductivity of 0.89 W m-1 K-1 were obtained for the sample annealed for 30 days at 700 K. The very low thermal conductivity is supposed to be due to the nanoscopic Ag/Sb-rich regions embedded in the matrix. A high ZT value of 1.5 at 700 K has been achieved for the sample annealed for 30 days.

BiCuSeO oxyselenides: new promising thermoelectric materials

BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

High piezoelectric d33 coefficient in Li-modified lead-free (Na,K)NbO3 ceramics sintered at optimal temperature

LiNbO3-doped (Na,K)NbO3 lead-free piezoelectric ceramics were prepared by normal sintering, and the electrical properties were investigated with a special emphasis on the influence of sintering temperature. The ceramics synthesized at 1020–1080°C showed a phase transition from orthorhombic to tetragonal symmetry, which is similar to the morphtropic phase boundary (MPB). Because of such MPB-like behavior, a high piezoelectric coefficient d33 (314pC∕N) was obtained in the nominal composition 0.058LiNbO3–0.942[(Na0.535K0.480)NbO3] ceramic sintered at 1060°C; however, this high d33 value was reported previously only in the Li-modified (Na,K)NbO3-based ceramics with codopants of Ta and Sb to B site.

Enhanced thermoelectric properties in CoSb3-xTex alloys prepared by mechanical alloying and spark plasma sintering

CoSb3-xTex(x=0.05−0.3) skutterudite polycrystals with an average grain size of 160 nm were fabricated by mechanical alloying combined with spark plasma sintering. The variation of lattice parameter with Te content indicates that the solution limit of Te was x=0.15, above which the impurity phases of Te, CoTe2, and CoSb2 appeared, and the matrix cracked above 500 °C. All samples behaved as degenerate semiconductors. The forbidden energy gap was estimated to be 0.047 eV from the temperature corresponding to the occurrence of intrinsic excitation, which is in good agreement with Singh’s theoretical calculation (0.05 eV) [D. J. Singh and W. E. Pickett, Phys. Rev. B 50, 11235 (1994)]. The CoSb2.85Te0.15 sample had the highest power factor and the lowest thermal conductivity, resulting in the highest thermoelectric figure of merit, ZT=0.93 at 547 °C. The role of Te substitution in enhancing thermoelectric properties is discussed in relation to the bipolar diffusion mechanism.

Analysis of crystallographic evolution in (Na,K)NbO3-based lead-free piezoceramics by x-ray diffraction

The crystallographic structure of (Na,K)NbO3-based compounds and phase transitional behavior in (1−x)(K0.5Na0.5)NbO3–xLiNbO3 (x=0, 3, 6, and 8mol%) ceramics were accurately determined using high resolution x-ray diffraction (XRD). The phase transition between orthorhombic and tetragonal was distinctly characterized focusing on the XRD peak changes in {222} and {400} diffraction planes, by which lattice parameters of the perovskite type subcell, a, b, and c as well as angle β can be calculated. The discontinuous change in angle β at x=6mol% is located at the phase transitional region, where enhanced piezoelectric constant d33=215pC∕N was obtained. The present study provides a precise as well as simple method to investigate crystallographic structures in (Na,K)NbO3-based ceramics.

Remarkable Enhancement in Thermoelectric Performance of BiCuSeO by Cu Deficiencies

A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (∼0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 μV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.

Piezoelectric properties of low-temperature sintered Li-modified (Na, K)NbO3 lead-free ceramics

LiNbO3-doped (Na, K)NbO3 lead-free piezoceramics were prepared by conventional sintering at a temperature as low as 950 °C using excess Na2O additives. The crystal structure changed from orthorhombic to tetragonal with increasing LiNbO3 amount since the phase transition temperature TO−T shifted downward. In the region of two-phase coexistence, enhanced piezoelectric constant d33 (280 pC/N) and electromechanical coupling factor kp (48.3%) with a high Curie temperature TC (475 °C) were obtained in the nominal composition 0.92(Na0.535K0.48)NbO3–0.08LiNbO3. Our results open up the way to low-temperature sintering of (Na, K)NbO3-based lead-free piezoceramics with high performance.

High-performance Ag0.8Pb18+xSbTe20 thermoelectric bulk materials fabricated by mechanical alloying and spark plasma sintering

Polycrystalline AgnPbmSbTem+2n thermoelectric materials, whose compositions can be described as Ag0.8Pb18+xSbTe20 were prepared using a combined process of mechanical alloying and spark plasma sintering. Electric properties of the sintered samples with different Pb contents were measured from room temperature to 700K. The maximum power factor of 1.766mW∕mK2 was obtained at 673K for the Ag0.8Pb22SbTe20 sample, which corresponds to a high dimensionless figure of merit, ZT=1.37. This best composition is different from that reported before.

Polycrystalline BiCuSeO oxide as a potential thermoelectric material

This work revealed that BiCuSeO oxyselenide is a potential oxide-based thermoelectric material, whose dimensionless figure of merit (ZT) reaches ∼0.70 at 773 K. High phase-purity BiCuSeO polycrystalline materials with fine grains were synthesized by a facile method combining a solid-state reaction and spark plasma sintering. Purifying the constitutive phase and reducing the grain sizes by introducing a high-energy ball milling process before spark plasma sintering were found to be effective in property enhancement. The resultant single-phased BiCuSeO sample derived from ball-milled powders shows good electrical conductivity above 4.0 × 103 S m−1 and a large Seebeck coefficient above 200 μV K−1. This compound has a low thermal conductivity (∼0.5 W m−1 K−1), which is associated with its low phonon transport speed and Youngs modulus. Results indicated that BiCuSeO-based materials are promising for energy conversion applications in the moderate temperature range.

Synthesis and transport property of Cu1.8S as a promising thermoelectric compound

Polycrystalline Cu(1.8)S compounds were fabricated by using a combined process of mechanical alloying and spark plasma sintering. The Cu(1.8)S sample with a second Cu(1.96)S phase and a lot of micro pores shows its maximum ZT value 0.5 at 673 K which is the highest value for p-type sulfide thermoelectric materials so far.