•Both Yb2O3 nanoprecipitates and CNTs are used for grain boundary engineering.•A maximum zT of 1.4 at 875 K and a device ZT of approximately 1.0 are achieved.•A thermoelectric conversion efficiency of 9.3% is realized.There have been few demonstrations that grain boundary engineering results in higher power generation efficiencies despite serious interest in using the strategy to improve zT. Using both Yb2O3 nanoprecipitates that decorate grain boundaries and carbon nanotubes to further reduce thermal conductivity and improve mechanical strength, a maximum zT of 1.43 at 875 K and a device ZT of approximately 1.0 in Yb-filled CoSb3-based nanocomposites are achieved. The preparation procedure of these materials is repeatable in mass production. Thermoelectric power generation modules based on these high-performance materials demonstrate a thermoelectric conversion efficiency of 9.3% under a temperature difference of 558 K. The results highlight nanostructured grain boundary engineering as a strategy to improve conventional thermoelectric materials, and the realistic prospect of large-scale thermoelectric power generation using skutterudite-based nanocomposites is demonstrated.A thermoelectric conversion efficiency of 9.3% is realized in skutterudite-based thermoelectric power generation module via grain boundary engineering. By using both Yb2O3 nanoprecipitates and carbon nanotubes to decorate grain boundaries, we achieve a maximum zT of 1.4 at 875 K and a device ZT of approximately 1.0 in Yb-filled CoSb3-based nanocomposites, which demonstrates the realistic prospect of large-scale thermoelectric power generation using skutterudite-based nanocomposites.Download high-res image (227KB)Download full-size image

•Increased grain boundaries and dislocations introduced by solely doping Se.•Annealing treatment in hydrothermal synthesis results in relatively high power factor.•Multi-scale hierarchical architecture is established.•The low lattice thermoelectric conductivity with multi-wavelength phonon scattering.•The peak ZT of 0.98 is achieved in n-type PbTe1-xSex (x = 0.06) alloys.The combination of feasible bottom-up hydrothermal synthesis is demonstrated to notably enhance thermoelectric performance of the PbTe matrix with different Se-doped contents. The instinct electrical properties are improved by annealing with reducing gas to achieve relatively high power factor. Furthermore, multi-scale hierarchical architecture is established together with the point defects and dislocations introduced by doping Se and the formation of nanoscale participates, couple with increased boundaries and a large grain size scale from 20 nm to 1.5 μm and, resulting in scattering an even broader frequency spectrum of phonon mean free path to reduce the lattice thermal conductivity effectively. Consequently, it plays an essential synergistic optimization to achieve a maximum ZT value of 0.98 at 625 K from the as-prepared n-type PbTe1-xSex (x = 0.06) alloys. This study provides a facile and low-cost method to fabricate multi-scale hierarchical architecture materials on a large scale with significant enhancement of thermoelectric performance.Download high-res image (130KB)Download full-size image

Dispersing the Ce3+ doped yttrium aluminum garnet (Ce:YAG) phosphor in the glass matrix has been widely investigated to replace conventional organic resin or silicone packaging. However, the reaction layer formed between commercial phosphors and glass matrix severely degrades the optical performance of Ce:YAG phosphor in silica glass (PiSG) materials. This paper demonstrates an ultra-fast method for preparing high performance PiSG materials. Instead of traditional melting process, the highly transparent PiSG samples can be rapidly fabricated from mixtures of commercial Ce:YAG phosphor and mesoporous SiO2 (SBA-15) powders using spark plasma sintering (SPS) at relatively low temperature (1000 °C) within short time (10 min). Owing to the inhibition of the deleterious interface reactions between Ce:YAG phosphor and silica glass matrix, the phosphor has been perfectly preserved, and the internal relative quantum yield of the PiSG sample reaches as high as 93.5% when excited at 455 nm, which is the highest efficiency in current research. Furthermore, combining the PiSG sample, we successfully fabricate a light-emitting diode (LED) module exhibiting a superior performance with luminous efficacy of 127.9 lm/W, correlated color temperature of 5877 K and color rendering index of 69 at the operating current of 120 mA. This work on the high performance LED modules provides not only a new approach to fabricate the functional glass-based materials that is sensitive to the high temperature, but also a possibility to extend the lifetime and improve the optical performances of the glass based LEDs.Download high-res image (189KB)Download full-size image

Dense graphene nanosheets (GNSs)/titanium carbide (TiC) composites have been produced from graphene oxide (GO)/TiC composite powders by spark plasma sintering. It is unexpected to observe that an introduction of 1.0 vol% GNSs from GO completely stops TiC grain growth by pinning their grain boundaries and densification is completed under the confinement of the flexible GNSs. Such a mechanism assumedly comes from the ultra-thin structure of GNSs, which indicates a crucial role GNSs may play in ceramic processing and has not been reported previously. Compared with monolithic TiC, the flexural strength of GNSs/TiC composites is significantly improved as a result of the refinement of matrix grains and excellent strength of GNSs, while the fracture toughness is enhanced due mainly to crack deflection, GNSs bridging and pull-out.

•Monodisperse ordered mesoporous TiO2 microspheres with large mesopores, high surface areas and highly crystallized framework were synthesized through a novel confinement synthesis method.•The morphology of the mesoporous TiO2 materials can be well tuned from solid mesoporous microspheres to hemi-microspheres and even to hollow mesoporous microspheres.•High photoconversion efficiencies up to 8.5% have been achieved by using the obtained mesoporous titania microspheres.Uniform discrete mesoporous titania microspheres have been synthesized via a facile and controllable interface-directed co-assembly approach by using 3-dimensional macroporous carbon (3DOMC) as the nanoreactor for the confined co-assembly of template molecules and titania source. By adjusting the synthesis parameters, hollow mesoporous microspheres and hemi-microspheres can also be synthesized. The obtained mesoporous TiO2 microspheres possess a large pore size (4.7 nm), high accessible surface area (145 m2/g), large pore volume (0.26 cm3/g) and highly crystallized anatase pore walls. The dye-sensitized solar cell based on the mesoporous TiO2 microspheres exhibits high photoconversion efficiencies up to 8.5%, which are largely attributed to their intrinsic high surface area, high porosity and well-connected crystalline framework.Monodisperse mesoporous TiO2 microspheres: uniform discrete mesoporous titania microspheres with high surface area and high crystalline framework have been synthesized via a facile and controllable interface-directed coassembly (IDCA) approach for dye sensitized solar cells with a high photoconversion efficiency of 8.5%.

Ca3Co4O9 polycrystalline thermoelectric ceramics with small amounts of silver nanoparticles (AgNPs) have been fabricated by spark plasma sintering (SPS) directly from the mixture of AgNPs prepared by wet chemical method and Ca3Co4O9 powders synthesized by solid-state reaction. The effects of AgNPs introducing on the morphology and thermoelectric properties have been investigated in detail. By this route, we achieve a lower thermal conductivity due to the increased phonon scattering, decreasing from 3.1 W/m(Ca3Co4O9) to 1.6 W/m(AgNPs/Ca3Co4O9) at 700 K. Besides, the nano-sized AgNPs effectively strengthen the p-type Ca3Co4O9 grain orientation (from 0.5 to 0.7), leading to a significant enhancement of the electrical conductivity. As a result, the Ca3Co4O9 sample containing 2.0 vol% AgNPs exhibits an exceedingly enhanced ZT value at 700 K, which is about 5 times higher than that of pure bulk Ca3Co4O9. The results demonstrate that introducing nanoparticles as a second nano-phase is a promising route to optimize the thermoelectric performance of Ca3Co4O9.

The transition between transparent and opaque of as-sintered zeolite samples is employed to study the temperature distribution of non-conducting samples during spark plasma sintering (SPS) process in this paper. The results show that temperature gradients exist in both radial and axial directions, which provide direct evidence of temperature distribution in the SPS process. The sample prepared at 1325 °C is transparent in the center and opaque in the edge, which suggests that the temperature of the center is higher than the edge and the temperature differential is about 26 °C by the thermal analysis model based on the experimental data in ANSYS code. The axial temperature gradient is also present in the as-sintered sample obtained at 1315 °C. The transparent part of the upper surface is larger than the lower surface, which illustrates that the temperature of the upper surface was higher than the lower surface and the difference is about 5 °C.

A facile and effective approach is demonstrated to prepare high-performance bulk AgNWs/PEDOT:PSS thermoelectric composites. The thermoelectric properties of the samples with different AgNWs contents are investigated in detail. The results show that well-dispersed AgNWs in the bulk PEDOT:PSS can give rise to a much higher electrical conductivity without a noticeable decrease in Seebeck coefficient or increase in thermal conductivity, which suggests the achievement of decoupling and optimizing the electrical conductivity and Seebeck coefficient. Consequently, the maximum ZT is observed to be 340% larger than that of the pure PEDOT:PSS sample. Our work clearly proves that the introduction of AgNWs is a promising model to significantly improve the thermoelectric properties of bulk organic-based TE materials.

An alternative and facile strategy to fabricate conducting reduced graphene oxide/polyaniline (rGO/PANI) hybrid composites with highly enhanced thermoelectric properties is introduced. rGO and PANI were homogeneously mixed by cryogenic grinding and then consolidated via spark plasma sintering. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy were employed to evaluate the phase structure and microstructure of the as-prepared composites. The results show that the CG technique could not only effectively refine the grain size of PANI, but also could induce more dislocations. The refined PANI particles are homogeneously dispersed and orderly arranged on the rGO templates as a result of the strong π–π conjugated interactions between PANI and rGO. The thermoelectric properties of the PANI samples containing different rGO content were systematically investigated. Compared with pure bulk PANI, rGO/PANI hybrid composites exhibit a distinct enhancement in the thermoelectric performance. Both the Seebeck coefficient and the electric conductivity were found to increase remarkably, resulting from the increased carrier mobility. The maximum Seebeck coefficient and electric conductivity of the rGO/PANI hybrid composites amazingly reached 15.934 μV K−1 and 1858.775 S m−1, respectively, and the maximum ZT was up to 4.23 × 10−4.

Mesoporous silica SBA-15 encapsulated with Au nanoparticles was consolidated by spark plasma sintering. It turned out to be an original and facile preparation route for incorporating Au nanoparticles in silica glass. The incorporation of Au nanoparticles in a silica glass matrix made the obtained glass coloured dark wine-red due to the surface plasmon resonance effect of Au nanoparticles, although the concentration of Au nanoparticles was extremely low (about 0.05 wt%). Besides, the Z-scan method using a femtosecond laser pulse was employed to measure the nonlinear optical property of this mesoporous composite-derived glass, which exhibited self-focusing feature at 720 nm with saturable absorption. The third-order nonlinear refraction index n2 and absorption coefficient β were 1.74 × 10−18 m2 W−1 and −5.72 × 10−12 m W−1, respectively. We believe that this approach could be extended to disperse other metal or semiconductor nanoparticles in a glass matrix.

In this context, a one-pot and in situ strategy for fabrication of AgNPs (Ag nanoparticles)/PANI (polyaniline) nanocomposites in a micellar solution of dodecylbenzene sulfonic acid (DBSA, anionic surfactant) is introduced. Guided by this strategy, AgNPs were directly synthesized from silver nitrate. AgNPs/PANI hybrid nanocomposites with AgNPs were consolidated via spark plasma sintering (SPS). The phase structure and microstructure of the as-prepared composites were evaluated by several characterizations, and the growth mechanism of AgNPs was speculated. The thermoelectric properties of the samples with increasing silver nitrate content were systematically investigated. Compared with pure bulk PANI, the thermoelectric performance of AgNPs/PANI hybrid nanocomposites exhibits a distinct enhancement on the addition of AgNPs. The Seebeck coefficient (S) decreased slightly while the electric conductivity (σ) was found to increase remarkably. However, thermal conductivity (κ) remained unchanged with increasing silver nitrate content, which resulted in an obvious enhancement in the figure of merit (ZT) of the composites. Consequently, the maximum ZT of the AgNPs/PANI hybrid nanocomposites amazingly reached 5.73 × 10−5, which is about 3.8 times of the ZT of the pure PANI (1.503 × 10−5). This study suggests that the hybridization of organic/low-dimensional metal particles is promising to effectively improve the thermoelectric properties of conducting polymers.

•Well-dispersed PbS quantum dots (QDs) were synthesized by a facile, environmentally friendly and inexpensive two-phase approach.•Lead myristate (PbM2) served as lead source in the experiment.•PbS QDs exhibited high photoluminescence, a narrow size distribution and a small full width at half maximum (FWHM).Colloidal PbS quantum dots (QDs) with narrow size distribution were synthesized via a facile two-phase approach in this paper. By elaborate selection of solvent and protective agent, cubic PbS QDs with nearly spherical shape and average diameter of approximately 4.0 nm were synthesized. The as-prepared PbS QDs had a relatively narrow emission in the photoluminescence (PL) spectrum with an emission of maximum about 430 nm and full width at half maximum (FWHM) of about 80 nm. The PL peak red-shifted and the FWHM increased when reaction times were extended. However, improving synthesis temperatures, the PL peaks became more sharpening. Varying Pb/S molar ratios made no difference to PL peaks due to the protection of oleic acid.

In this work, a facile strategy for the fabrication of PANI/multi-walled carbon nanotube (MWCNT) nanocomposites without the assistance of a dispersant is introduced. MWCNTs and polyaniline were homogeneously mixed by cryogenic grinding (CG) and then consolidated via Spark Plasma Sintering (SPS). X-ray power diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and field-emission scanning electron microscopy (FESEM) were employed to characterize the as-prepared composites. The XRD results showed that cryogenic grinding can refine the grain size of PANI and induce more dislocations. The FTIR spectra data showed that the peaks of the PANI/MWNT composites displayed a red shift. In the high resolution FESEM image, the layer-by-layer structure and smooth surface can be observed. The thermoelectric properties of the as-prepared nanocomposites were investigated as a function of MWCNT content. The results showed that the electrical conductivity increased remarkably with the increasing MWCNT content, and the maximum power factor was 10.73 × 10−8 W mK−2, higher than pure PANI. Additionally, as the MWNT content increased from 10% to 30%, the electrical conductivity of the PANI/MWNT composite increased from 3.51 S m−1 to 1.59 × 102 S m−1. This work demonstrates a simple and effective method for improving the dispersity of carbon nanotubes and the thermoelectric properties of conducting polymers.

As a relatively novel sintering technique, spark plasma sintering (SPS) has been used extensively over the past decade to prepare a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys. Many applications of the SPS technique are the fabrication of nanostructured materials using nanosize powdered precursors as starting materials. This article provides a review of research activities that concentrate on the development of the SPS reaction sintering (SPS-RS) to produce dense nanostructured materials, which indicate that it is possible to synthesize and compact dense bulk materials with controlled sub-micron or even nanoscale grain sizes by the use of the SPS technique.•This is a review about fabrication of nanostructured materials using SPS reaction sintering.•Nano-structured bulk materials are prepared using micron-size powders via SPS.•The challenges in SPS reaction sintering for preparing nano-structured bulk materials are given.

A novel method for rapid preparation of Bi2Te3 nano-sized powders with an average particle size of about 70 nm was developed. A starting powder mixture consisting of Bi2Te3 coarse particles of ∼5 mm was ground using cryogenic grinding in the liquid nitrogen. For comparison, the conventional high-energy ball milling was used to prepare the Bi2Te3 nano-sized powders. Sintering properties of as-prepared powders was investigated by spark plasma sintering (SPS). The effects of the preparation procedure on the crystallinity, morphology and structure were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that Bi2Te3 was not transformed into a non-equilibrium amorphous phase or decomposed during the cryogenic grinding process, and as-prepared nano-powders possessed excellent sinterability. This technique might also be applicable to other thermoelectric materials.

Graphene nanoribbons (GNR) have been fabricated by a microexplosion method without severe oxidation – filling multi-walled carbon nanotubes (MWCNT) with potassium and then reacting with water vigorously. Transmission electron microscopy and scanning transmission electron microscopy have verified the synthesis mechanism: when MWCNTs are effectively filled with potassium, the microexplosion generated by reaction between water and potassium can split the MWCNTs to form GNRs. Most of the obtained GNRs have smooth edges and the maximum wall thickness of MWCNTs that can be split by this method is around 10 nm.

A novel method for rapid preparation of Bi2Te3 nano-sized powders with an average particle size of about 70 nm was developed. A starting powder mixture consisting of Bi2Te3 coarse particles of ∼5 mm was ground using cryogenic grinding in the liquid nitrogen. For comparison, the conventional high-energy ball milling was used to prepare the Bi2Te3 nano-sized powders. Sintering properties of as-prepared powders was investigated by spark plasma sintering (SPS). The effects of the preparation procedure on the crystallinity, morphology and structure were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that Bi2Te3 was not transformed into a non-equilibrium amorphous phase or decomposed during the cryogenic grinding process, and as-prepared nano-powders possessed excellent sinterability. This technique might also be applicable to other thermoelectric materials.

CuInZnS quantum dots (CIZS QDs) were prepared via reflux method in aqueous solution using CuCl2·2H2O, InCl3·4H2O, Zn(OAc)2·2H2O and Na2S·9H2O as raw materials, l-glutathione (GSH) and sodium citrate (SC) as stabilizing agents, respectively. The effects of off-stoichiometry (Cu/In and Zn/Cu ratios) on the crystal structure and morphology were systematically studied by means of X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM), and the relative optical properties were also investigated by absorption and fluorescence spectra. The as-prepared water-dispersible CIZS QDs were around 3–4 nm and possessed the tetragonal chalcopyrite crystal structure. The photoluminescence (PL) intensity of QDs was significantly increased with decreasing the Cu/In ratio. Compared with the Cu/In ratio variation, changing Zn/Cu ratio was an effective strategy to realize a more uniform irradiation and a wide range of emission wavelength tunability.

Mesoporous silica SBA-15 encapsulated with Au nanoparticles was consolidated by spark plasma sintering. It turned out to be an original and facile preparation route for incorporating Au nanoparticles in silica glass. The incorporation of Au nanoparticles in a silica glass matrix made the obtained glass coloured dark wine-red due to the surface plasmon resonance effect of Au nanoparticles, although the concentration of Au nanoparticles was extremely low (about 0.05 wt%). Besides, the Z-scan method using a femtosecond laser pulse was employed to measure the nonlinear optical property of this mesoporous composite-derived glass, which exhibited self-focusing feature at 720 nm with saturable absorption. The third-order nonlinear refraction index n2 and absorption coefficient β were 1.74 × 10−18 m2 W−1 and −5.72 × 10−12 m W−1, respectively. We believe that this approach could be extended to disperse other metal or semiconductor nanoparticles in a glass matrix.

In this work, a facile strategy for the fabrication of PANI/multi-walled carbon nanotube (MWCNT) nanocomposites without the assistance of a dispersant is introduced. MWCNTs and polyaniline were homogeneously mixed by cryogenic grinding (CG) and then consolidated via Spark Plasma Sintering (SPS). X-ray power diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and field-emission scanning electron microscopy (FESEM) were employed to characterize the as-prepared composites. The XRD results showed that cryogenic grinding can refine the grain size of PANI and induce more dislocations. The FTIR spectra data showed that the peaks of the PANI/MWNT composites displayed a red shift. In the high resolution FESEM image, the layer-by-layer structure and smooth surface can be observed. The thermoelectric properties of the as-prepared nanocomposites were investigated as a function of MWCNT content. The results showed that the electrical conductivity increased remarkably with the increasing MWCNT content, and the maximum power factor was 10.73 × 10−8 W mK−2, higher than pure PANI. Additionally, as the MWNT content increased from 10% to 30%, the electrical conductivity of the PANI/MWNT composite increased from 3.51 S m−1 to 1.59 × 102 S m−1. This work demonstrates a simple and effective method for improving the dispersity of carbon nanotubes and the thermoelectric properties of conducting polymers.