Xianli Su

Stretchable nanoparticle conductors with self-organized conductive pathways

Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 1, 2, 3). Furthermore, implantable devices or stretchable displays need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals. This conductivity–stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent. Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition, or with specific alignment (the way the fillers are arranged in the matrix). However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment. Pre-strained substrates, buckled microwires and three-dimensional microfluidic polymer networks have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.

Self-propagating high-temperature synthesis for compound thermoelectrics and new criterion for combustion processing.

The existing methods of synthesis of thermoelectric (TE) materials remain constrained to multi-step processes that are time and energy intensive. Here we demonstrate that essentially all compound thermoelectrics can be synthesized in a single-phase form at a minimal cost and on the timescale of seconds using a combustion process called self-propagating high-temperature synthesis. We illustrate this method on Cu2Se and summarize key reaction parameters for other materials. We propose a new empirically based criterion for sustainability of the combustion reaction, where the adiabatic temperature that represents the maximum temperature to which the reacting compact is raised as the combustion wave passes through, must be high enough to melt the lower melting point component. Our work opens a new avenue for ultra-fast, low-cost, large-scale production of TE materials, and provides new insights into combustion process, which greatly broaden the scope of materials that can be successfully synthesized by this technique.

Preparation and thermoelectric properties of high-performance Sb additional Yb0.2Co4Sb12+y bulk materials with nanostructure

High-performance Sb excess Yb0.2Co4Sb12+y (y=0,0.3,0.6,1.0) bulk materials with nanostructure have been prepared by combining melt spinning technique with spark plasma sintering. Average grain size of the bulk samples is about 150nm when y=0 and 0.3, and the grain size increases with the increase of y. Moderately superfluous Sb may improve the electrical transport properties effectively and the thermal conductivity of the bulk samples decreases markedly due to the nanostructure. The thermoelectric performance of the samples is significantly improved, and the maximum figure of the merit reaches 1.26 for the Yb0.2Co4Sb12.3 compound at 800K.

Multi-Scale Microstructural Thermoelectric Materials: Transport Behavior, Non-Equilibrium Preparation, and Applications

Considering only about one third of the worlds energy consumption is effectively utilized for functional uses, and the remaining is dissipated as waste heat, thermoelectric (TE) materials, which offer a direct and clean thermal-to-electric conversion pathway, have generated a tremendous worldwide interest. The last two decades have witnessed a remarkable development in TE materials. This Review summarizes the efforts devoted to the study of non-equilibrium synthesis of TE materials with multi-scale structures, their transport behavior, and areas of applications. Studies that work towards the ultimate goal of developing highly efficient TE materials possessing multi-scale architectures are highlighted, encompassing the optimization of TE performance via engineering the structures with different dimensional aspects spanning from the atomic and molecular scales, to nanometer sizes, and to the mesoscale. In consideration of the practical applications of high-performance TE materials, the non-equilibrium approaches offer a fast and controllable fabrication of multi-scale microstructures, and their scale up to industrial-size manufacturing is emphasized here. Finally, the design of two integrated power generating TE systems are described-a solar thermoelectric-photovoltaic hybrid system and a vehicle waste heat harvesting system-that represent perhaps the most important applications of thermoelectricity in the energy conversion area.

Enhanced thermoelectric properties of Ba-filled skutterudites by grain size reduction and Ag nanoparticle inclusion

Ba-filled skutterudite compounds, Ba0.3Co4Sb12, with dispersed Ag nanoparticles have been synthesized by ball milling followed by hot pressing. The influence of Ag nanoparticles and their size distribution on electrical and thermal transport properties has been investigated in the temperature range from room temperature to 823 K. It was found that Ag nanoparticles in the Ba0.3Co4Sb12 matrix drastically enhance the electric conductivity and slightly increase the Seebeck coefficient. Surprisingly, Ag nanoparticles do not alter the carrier density of Ag/Ba0.3Co4Sb12 nanocomposites. This large improvement in the electrical conductivity as well as the corresponding power factor is assumed to come primarily from the enhanced mobility in Ag/Ba0.3Co4Sb12 nanocomposites. In addition, a large reduction in thermal conductivity is achieved by grain size reduction resulting from ball milling processing and as a result of the embedded Ag nanoparticles that scatter a wider range of phonon frequencies. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT some 30% higher in comparison to the parent Ba0.3Co4Sb12. Moreover, we found it is advantageous to employ a wider size distribution of Ag nanoparticles to reduce the thermal conductivity to a large degree by enhancing phonon scattering. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites.

Superparamagnetic enhancement of thermoelectric performance

The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal conductivity of the lattice through the design of either interface structures at nanometre or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelectric performance. Here we show that by embedding nanoparticles of a soft magnetic material in a thermoelectric matrix we achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles—in particular, their superparamagnetic behaviour (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)—lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. We show that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.

Ultra-fast synthesis and thermoelectric properties of Te doped skutterudites

The self-propagating-high-temperature-synthesis (SHS) technique is applied here for the first time to synthesize CoSb3 thermoelectric materials. Mixtures of Co and Sb powders were compacted into pellets which were ignited from one end. A single-phase skutterudite material was obtained in a very short period of time using the SHS process which is maintained by the heat released from the chemical reaction of Co with Sb. Thermodynamic parameters and kinetics of the SHS reaction are investigated. The ignition temperature, adiabatic temperature, and the propagation speed of the combustion wave in the synthesis of CoSb3 are 723 K, 861 K, and 1.25 mm s−1, respectively. Using the SHS technique followed by Plasma Activated Sintering (PAS), we synthesized high performance bulk skutterudites of composition CoSb2.85 Te0.15 with a ZT of 0.98 at 820 K, one of the highest ZT values for an unfilled form of skutterudites. Compared with the samples synthesized by the traditional methods, the synthesis time is shortened from the typical several days to less than 20 minutes. Our work opens a new avenue for ultra-fast, low cost, mass production fabrication of skutterudite-based materials, which may also be universally applicable for the synthesis of other thermoelectric materials.

High thermoelectric performance of mechanically robust n-type Bi2Te3−xSex prepared by combustion synthesis

The traditional zone melting (ZM) method for fabricating Bi2Te3-based thermoelectric materials has long been considered a time and energy intensive process. Here, a combustion synthesis called the self-propagating high-temperature synthesis (SHS) is employed to synthesize Bi2Te3-based thermoelectric materials. Thermodynamic and kinetic parameters of the SHS process relevant to Bi2Te3 and Bi2Se3 were systematically studied for the first time. SHS combined with plasma activated sintering (PAS) results in a single-phase homogeneous material with precisely controlled composition, no preferential orientation, high thermoelectric performance, and excellent mechanical properties. The technologically relevant average ZT value of SHS–PAS Bi2Te2.4Se0.6 from 298 to 523 K is 0.84, which is an increase of about 25% compared with the ZM sample. The compressive strength and the bending strength of SHS–PAS Bi2Te2.4Se0.6 are increased by nearly 250% and 30%, respectively, compared with those of the ZM samples, measured perpendicular to the c-axis. Moreover, the SHS–PAS process is very fast and shortens the synthesis time from tens of hours to 20 min. On account of the simplicity of the process, short synthesis time, minimal use of energy, and the scalability of the method, SHS–PAS technology provides a new and efficient method for large-scale, economical fabrication of Bi2Te3-based compounds.

In situ synthesis and thermoelectric properties of PbTe–graphene nanocomposites by utilizing a facile and novel wet chemical method

In this work, a facile and novel wet chemical method is adopted to synthesize PbTe–graphene nanocomposites, and the thermoelectric properties of the sintered bulk materials are discussed in detail. An intercalative nanostructure is formed by using commercial graphene oxide nanosheets as both the dispersant and the two-dimensional growth template for PbTe nanoparticles in the synthesis process, where PbTe nanoparticles are in situ synthesized and graphene oxide nanosheets are reduced to graphene at the same time. FESEM and TEM measurements indicate that PbTe nanoparticles with sizes of 20–60 nm are uniformly anchored on the surface of graphene, and the nanostructure is retained in the bulk. These novel nanocomposites show enhanced thermoelectric properties as compared to bare PbTe prepared by the same route, as well as to samples prepared by traditional methods. The ultra-high electron mobility of graphene improves the electrical conductivity of the PbTe–graphene nanocomposites, and their conductivity exceeds not only that of bare PbTe but also the samples prepared by the traditional melt-quenching and melt-cooling processing techniques. Moreover, the much-decreased size of the PbTe particles in the bulk material, caused by the intercalative structure, increases the concentration of interfaces which results in the thermal conductivity of the nanocomposite being lower compared with the bare PbTe sample. Therefore, a much higher ZT value of the PbTe–graphene nanocomposites is obtained, reaching a value of 0.7 at 670 K. This is 6 times the value of the bare PbTe sample and a significantly higher ZT than for any n-type PbTe samples prepared by any of the traditional synthesis routes.

Nanostructured bulk YbxCo4Sb12 with high thermoelectric performance prepared by the rapid solidification method

High performance nanostructured bulk skutterudites YbxCo4Sb12 (x = 0.1, 0.2, 0.3) have been prepared by combining the melt spinning technique with spark plasma sintering (SPS). The effects of different linear speeds V of the spinning copper wheel on the microstructure and phase composition of ribbon samples and bulk materials are studied, and the influence of the nanostructure and Yb filling fraction on thermoelectric (TE) properties is investigated. The microstructure characterization results show that with increasing speed V, the ribbons possess finer nanostructure and a more homogeneous component distribution. As a result, the formation of the skutterudite phase during a brief SPS processing of the ribbons is very rapid. Moreover, the resulting bulk samples are single-phase, high density skutterudites with nano-scale size grains. The results of TE property measurements show that with the increasing linear speed V, i.e. with the formation of a finer nanostructure, the thermal conductivity of bulk samples decreases markedly without any significant effect on the electronic transport properties. In fact, we observe an enhancement in the Seebeck coefficient as the grain size decreases. The Yb0.3Co4Sb12 compound reaches its highest ZT of 1.22 at 800 K.