Microstructure engineering is an effective avenue for tuning the thermal and electrical transports to op-timize thermoelectric (TE) properties. Thermoelectric composites with nano-particle dispersion have been success-fully developed by using extrinsic or in-situ formation methods. The lattice thermal conductivity can be depressed by the scattering effects of nano particles to the medium-long-wavelength phonons. The enhanced electron density of states at the Fermi level and the carrier filtering effects caused by the nano-sized grain boundary are also positive for enhancing Seebeck coefficients. The mixing, in-situ oxidation and phase-separation precipitation process supply possibility to realize the nano-particle dispersed structure for different material systems. This paper reviews the re-cent progress of the research on nano-structured and nano-composite thermoelectric materials. The effects of the nano-dispersion on the electrical and thermal transports will be also discussed.

Thermoelectric materials Zr(Hf)NiSn(Sb) alloys were prepared by levitation melting followed by melt- spinning to refine the microstructure, and then consolidated by spark plasma sintering for the property measurements. XRD analysis showed that the half-Heusler phases were obtained. The microstructures of the melt spun thin ribbons were studied by the scanning electron microscope and transmission electron microscope. The thin ribbons were in the size of a few hundreds nanometers which didn’t grow too much during the sintering process. Nanocrystals were found in the crystal grains. The carrier concentration increased for the melt-spinning samples compared with the levitation melting samples, indicating that the nanocrystals were metallic. The increasing boundary scattering after melt-spinning made the lattice thermal conductivity decrease.

Intermetallic compounds Zr_1-xLa_xNiSn (x = 0.05, 0.1, 0.15, 0.2, 0.3, 0.4) and Zr_0.98R_0.02NiSn_0.98X_0.02(R = La, Ce; X=Sb, Bi) were synthesized using arc melting and spark plasma sintering (SPS) techniques. The changes of their crystal structures were analyzed by using X-ray diffractometer. Thermoelectric properties were evaluated in the temperature range of 300 to 925 K. For x≤0.15, single-phase samples can be obtained. With x > 0.15, a non-half-heusler phase formed. The content of the second phase increases with x. For the samples with x≤0.15, rare-earth doping can effectively reduce the thermal conductivity while keeping good electrical transport. The maximum value of ZT was obtained in Zr_0.98La_0.02NiSn_0.98Sb_0.02, which reaches 0.5 at 575K.

Single-phase Ba_0.3In_0.2Ni_0.05Co_3.95Sb₁₂ bulk materials were synthesized by melting and spark plasma sintering (SPS) methods. Ba_0.3In_0.2Ni_0.05Co_3.95Sb₁₂/SiO₂ nanocomposite material with nano-SiO₂ coating was prepared by wet chemical coating processing. Emphases were given on the evolution of microstructure, chemical composition, and thermoelectric properties of two kinds of (Ba,In) double-atom filled skutterudite based bulk materials during periodic thermal cycling 2000 times in the range of 300-723-300 K. It was found that the thermal stability of (Ba,In) double-atom filled skutterudite was remarkably improved after nano-SiO₂ coated. The microstructure and chemical composition along the grain boundaries of single-phase Ba_0.3In_0.2Ni_0.05Co_3.95Sb₁₂bulk material obviously changed, while the thermal cycling had no effect on Ba_0.3In_0.2Ni_0.05Co_3.95Sb₁₂/SiO₂ nanocomposite. Thus the nano-SiO₂ stabilized the grain boundary and prevented the elements diffusing and volatilizing from intra-grain. The ZT values of two kinds of materials were very close and kept unchanged at 300 K and 500 K during the thermal cycling. The ZT values of single-phase bulk materials gradually decreased at 800 K. However, the ZT values of nanocomposite mainly depended on the complex changes in thermal conductivity and increased with prolonging of quenching time, then reached 1.12 at 800 K after quenching for 1800 times, which was even higher than those of unquenched Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ nanocomposite (ZT=1.08) and single-phase Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ bulk material(ZT=1.10) quenched for 1800 times.

(Ag₂Te)_x(Bi_0.5Sb_1.5Te₃)_1-x(x=0, 0.025, 0.05, 0.1) alloys were synthesized by “Melting-Ball Milling-Spark Plasma Sintering” method. Transport properties measurements indicate that the Ag₂Te-doping can affect the temperature dependence of thermoelectric properties of the samples significantly. Samples with Ag₂Te-doping have better thermoelectric performances in the temperature range from 450 K to 550 K. Appropriate amount of Ag₂Te can enhance the phonon scattering of the alloys effectively, which lead to the lower thermal conductivities for these samples. Over the entire temperature range, sample (Ag₂Te)_0.05(Bi_0.5Sb_1.5Te₃)_0.95 exhibits the lowest lattice thermal conductivities, ranging within 0.2-0.3 W/(m·K) from room temperature to 575 K. The maximum ZT value of 0.84  is obtained at 575 K for the sample (Ag₂Te)_0.05(Bi_0.5Sb_1.5Te₃)_0.95. Compared with the one without doping, the ZT value is increased by almost 20%.

A series of p type bulk (Bi_0.25Sb_0.75)₂Te₃ thermoelectric materials were prepared by melt spinning-mechanical milling (MM) and spark plasma sintering (SPS). Electrical conductivity, Seebeck coefficient and thermal conductivity of the sintered samples were measured in the temperature range from 300K to 523K. The effects of milling time on thermoelectric properties were investigated in system. The results show that, with the extension of milling time, the electrical conductivity of the sample increases and then decreases, Seebeck coefficient changes little, while the thermal conductivity decreases with the increase of milling time. As a result, the bulk milled for 20h has the highest ZT value 0.96 at room temperature, meanwhile the bending strength reaches 91MPa.

Single-phase AgSbTe₂ powder was synthesized by sonochemicial method combining with deoxidizing heat-treatment. The influences of heat-treatment temperature, time and the initial ratio of starting reactants on the phases of the final products were investigated. Near single-phase AgSbTe₂ ternary powder can be obtained after 2h heat treatment under 500℃, and single-phase AgSbTe₂ can be obtained by changing the initio ratio of the starting reactants. Microstructure analysis shows that nanodots with dimension of 20-50nm are uniformly distributed on the surface of the as-prepared powders with the average size of about 10?m. A dimensionless thermoelectric figure of merit ZT = 1.14 is obtained at 570K for the sample with single phase.

A series of In-doped β-Zn₄Sb₃-based materials with nominal compositions of Zn₄Sb_3-xInx (0-0.08, ?x=0.02) by substituting Sb with In were designed in the paper. The single-phase In-doped β-Zn₄Sb₃-based bulk materials with no cracks were prepared by the combination of vacuum melting, furnace cooling and spark plasma sintering techniques. The electrical and thermal transport properties of Zn₄Sb_3-xIn_x in the temperature range of 300-700 K indicate that the In substitution for Sb in Zn₄Sb₃ compound brought the remarkable enhancement in carrier concentration and electrical conductivity, the almost complete vanishing of instinct excitation under high temperature, and the significant reduction in the lattice thermal conductivity. The lattice thermal conductivity for x=0.04 and 0.08 samples is very low and only about 0.21 W/(m·K) at 700 K. All In-doped β-Zn₄Sb₃-based bulk materials had higher ZT values as compared to undoped beta-Zn4Sb3 bulk material. A large ZT value of 1.13 has been achieved for Zn₄Sb_2.94In_0.06 at 700 K that increased by 69%.

β-Zn₄Sb₃ is one of the most important thermoelectric materials in the intermediate temperature range, while the poor mechanical properties limit its commercial application. A series of β-Zn_4+xSb₃ bulk materials with high thermoelectric performance and high mechanical properties were fabricated by a melt spinning (MS) technique followed by a quick spark plasma sintering (SPS) procedure. By adjusting the stoichiometric ratio of Zn and Sb, we optimize the thermoelectric performance of this series of bulk materials. With increasing the amount of Zn, electrical and thermal conductivities of the sample increase, and the Seebeck coefficient declines. The ZT_max is 1.13 at 700K for the Zn_4.32Sb₃ sample, compared with the M-ingot sample it increases by 47% at the same temperature. The samples prepared by MS-SPS method have much better mechanical properties compared with the samples prepared by traditional melting and SPS method. The pressive strength of MS-SPS samples was nearly twice of that of the sample prepared by melting method. This kind of high performance and high mechanical strength β-Zn_4+xSb₃ bulk material has great potential for commercial application.

The zone-melted n-type Bi₂Te₃ ingots were chosen as the starting material to prepare the bulk samples by two different synthesis routes including hand grinding combined with spark plasma sintering process (ZM+SPS) and melt spinning (MS) technique combined with a subsequent spark plasma sintering process (MS+SPS). The microstructures, thermoelectric properties and mechanical properties of three samples prepared by different techniques were studied. The ZM samples show rough grain size and strong grain orientations. After hand grinded and SPS process, the crystalline grains are refined and grain orientations are remarkably decreased. While the MS+SPS samples with fine grains have no distinct grain orientations. The results of thermoelectric properties and pressive strength measurement show that the maximum figure of merit ZT value reaches 0.72 at 430 K for ZM starting materials and the pressive strength is only about 40MPa. The maximum figure of merit ZT value decreases to 0.68 at 440 K for the ZM+SPS samples but the pressive strength are increased to 110MPa, which is about 175% improvement compared with ZM samples. The maximum figure of merit ZT value and pressive strength are 0.96 at 320 K and 200MPa respectively for the MS+SPS samples, the room temperature ZT and pressive strength are about 64% and 400% improvement compared with ZM samples.

Sb₂Se₃ nanowires were synthesized by a hydrothermal method at 150℃ for 3, 6,12 and 24 h using SbCl₃ and Se powder as the precursors, N₂H₄·H₂O as reductant. X-ray diffraction (XRD), transmission electron microscope (TEM), field emission scanning electron microscope (FESEM) and high-resolution TEM (HRTEM) were applied to analyze the phase distributions, microstructures and grain sizes of the nanostructured Sb₂Se₃. It was found that the pure orthorhombic Sb₂Se₃ nanowires were formed at 150℃ for 24h by the hydrothermal synthesis method. The reaction mechanism and crystal growth mechanism of Sb₂Se₃ nanowires were investigated in the light of the experimental results. The Sb₂Se₃ nanowires grow along the [001] direction. The formation mechanism is mainly related to the special crystal structure of Sb2Se3. The TE properties of SPS nanocomposites of Bi₂Te₃ with different amount of Sb₂Se₃ nanowires were investigated. The addition of 1 at% Sb₂Se₃ nanowires can improve the electric properties of the Bi₂Te₃ nanopowders.

Single phase Bi₂Sr₂Co₂O_y compound was synthesized by Sol-Gel method, using PEG20000 and ultrasonic treatment to optimize microstructure of powders, and bulk materials were gained after SPS technique. The influence of microstructure on electrical properties of Bi₂Sr₂Co₂O_y compound was investigated. The results show that adding of PEG20000 and ultrasonic treatment can decrease the size of powders obviously, the grain size of bulk materials decreases greatly, so the electrical properties of material increased. Bulk materials synthesized from the powders prepared with adding PEG20000 and ultrasonic treatment gain the highest ZT value of 0.041 at 873K.

Pure β-MnO₂ oxide was synthesized by hydrothermal synthesis of MnSO₄H₂O and (NH₄)₂S₂O₈. Spinel-type Li₄Mn₅O₁₂ precursors were synthesized via low temperature solid-phase reaction. Furthermore, MnO₂ ion-sieves with Li⁺ selective adsorption property were prepared by the acid treatment process to completely extract Li⁺ from the spinel Li₄Mn₅O₁₂ precursor. The effects of hydrothermal and solid-phase reaction process on the nanostructure, chemical stability and ion-exchange property of the ion-sieve material were examined with XRD, HRTEM, SAED, and Li⁺ selective adsorption measurements. The results show that Li₄Mn₅O₁₂ precursor and final MnO₂ ion-sieve are effectively controlled within low-dimensional structure, indicating that low temperature solid-phase reaction is more favorable to control the nanocrystalline structure than traditional high-temperature calcination process. The Li⁺ selective adsorption capacity is improved remarkably to 6.6 mmol/g at equilibrium and about 5.0 mmol/g at the initial Li⁺ concentration of only 5.0 mmol/L, which is significant for lithium extraction from aqueous solutions with very low lithium content.

Dense crystalline Li₃VO₄/Li₄SiO₄ solid solution films were prepared on Pt substrate electrochemically in SiO₂, V₂O₅, LiOH, absolute ethyl alcohol solution. XRD, IR, and Raman analyses indicate that the films are well-crystallized with γ-Li₃PO₄ structure. No Li₂SiO₃ impurity is found in the sample. The chemical composition of the film is 0.4 (Li₄SiO₄)-0.6(Li₃VO₄). Using a low boiling point ethyl alcohol as solvent can improve the films properties due to the increased vapor pressure generated in the reaction. The absolute ethyl alcohol of the reaction system facilitated the formation of the Li_3+xV_1-xSi_xO₄ phase. The surface of the films is smooth and fine grained due to the low solubility of the constituent ions in the solvent and the low formation speed of the films.

High temperature electrical conductivity of perovskite-type mixed with ionic-electronic conductor La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ  (LSCF) and La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ- Ce_0.9Gd_0.1O_1.95(LSCF-GDC) composite material were studied by the DC four-terminal technique. The activation energies of pure LSCF and LSCF-GDC composite for small polaron conduction were E_a1=9.72kJ/mol and E_a2=10.64kJ/mol, respectively. Through electrical conductivity relaxation method, i.e. a continuously resistance measurement during the sudden change oxygen under partial pressure and the surface exchange property of the two samples were also investigated. In the temperature range from 600℃ to 800℃ and the oxygen partial pressure range from 21kPa to 34kPa, the oxygen surface exchange coefficients (k_chem) were determined as 2.87×10^-6-6.91×10^-6cm/s. It is the catalysis effects of GDC on oxygen surface exchange process and the microstructure effect of introducing GDC that promoted the oxygen transport process of composite materials jointly. Based on the relationship of k_chem and temperature, the activation energies for surface exchange process was also estimated.

Hexadecyl trimethyl ammonium bromide (CTAB) was used as the additive in electrolyte for vanadium redox flow battery. Its stability and electrochemical performance were investigated by UV-Vis absorption spectrophotometry, scanning electron microscope(SEM), square wave voltammetry(SWV), cyclic voltammetry(CV) and examination of stabilization. The results show that the quaternary ammonium headgroups of CTAB micelles interacting with V(V) ions in electrolyte prevents pentavalent vanadium from further polymerization, which leads to a good suppression of the crystallization. The stable hemispherical particles forming at the graphite-liquid interface catalyze the redox reaction of V(IV)/V(V), which is called Micellar catalysis. EIS and charge-discharge tests show that the adding of CTAB makes charge transfer resistance much smaller, and doubles double-layer capacitance, so that the electrochemical reaction activity of the electrolyte improved, which is consistent with CTAB micellar catalysis.

In order to improve cycling stability and overall electrochemical properties of La-Mg-Ni-based hydrogen storage alloys, an electroplating treatment was applied on La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. The effect of cobalt-nickel coating on the morphological and electrochemical properties was studied. FESEM results show that spherical nickel-cobalt alloy particles are deposited on the surface of the alloys. Electrochemical tests indicate that the maximum discharge capacity, the cycling stability and the high rate dischargeability (HRD) are remarkably improved. After 200 charge/discharge cycles, the capacity retention rate increases from 60% (uncoated) to 80% (nickel-cobalt coated). The HRD at 1080mA/g rises by 23% for the nickel-cobalt coated alloy electrodes. Linear polarization and Electrochemical Impedance Spectroscope (EIS) results reveal that the surface of alloy electrodes with cobalt-nickel coatings is more catalytic for the electrochemical charge transfer reactions.

PbO₂ electrodes modified with different concentrations of polyethylene glycol (PEG) were prepared by electrodeposition. SEM and XRD were employed to investigate the surface morphology and crystal structure of PbO₂ electrodes, and its electrocatalytic acitivity was analyzed through degradation experiments of phenol. Furthermore, electrochemical impedance spectroscope (EIS) and liner sweep voltammetry (VA) were carried out to get a better understanding of its electrochemical properties. SEM images and XRD patterns show that PEG can decrease the partical size of PbO₂ and improve the crystal purity of β-PbO₂. The modified electrodes exhibite better electrocatalytic activity, lower charge transfer resistance and higher oxygen evolution potential compared with unmodified electrodes in the electrochemical experiments. Additionally, five times degradation experiments of phenol and accelerated lifetime tests reveal that the optimal electrode modified with 6g/L PEG has excellent electrocatalytic stability, longer lifetime and better corrosion resistance.

Electrodes were prepared by mixture of graphite powder (GP) and multi-walled carbon nanotubes (MWNT) at various ratios for all-vanadium redox flow battery. In this studies, the surface morphologies of the composite electrodes were characterized by scanning electron microscope (SEM), and electrochemical behaviors were investigated by cyclic voltammograms, impedance spectroscope and charge-discharge technique. SEM observation shows that the electrode surface roughness increases after adding MWNT to GP. The research results indicate that the MWNT added into GP can provide good electron conductive network between the GP particles, which results in a shorter current conducting pathway in the sheet GP and also a lower internal resistance for the electrodes. The best composition for the positive electrode of all-vanadium redox flow battery (VRB) with different content of MWNT is 15wt%. The current efficiency of VRB using 15wt% MWNT-GP composite electrode is above 93% and the voltage efficiency decrease with current densities increasing under current densities of 20?80 mA/cm2. The improvement in the electrochemical activity of 15wt% MWNT-GP composite electrode is ascribed to the decrease in the total resistivity of vanadium ions adsorption and desorption from MWNT-GP composite electrode and the charge transfer resistivity of MWNT-GP electrode at the electrolyte/electrode interface.

One-dimensional semiconductor materials Bi₂Te₃-Te nanocomposites, with a sheet-rod nanostructure can be fabrication on a large scale using Te nanowires as the in-situ template under a simple reflux process, and the yield is high up to 90%. The product was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS) and transmission electron microscope (TEM). It is a promising way to fabricate one-dimensional nanocrystals of other metals and semiconductors. The influence of KOH, EDTA and reaction time are discussed. The formation mechanism of such a heterostructure is also proposed.

Plate-like particles of (Ca_0.85OH)_1.16CoO₂ were prepared in which Nd was partially substituted for Ca to improve the thermoelectric performance. The values of Seebeck coefficient of as-synthesized oxides are all positive, showing that these oxides are p-type materials. The electrical conductivity initially decreases then increases with increasing Nd-doped amount, which is the coeffect of carrier concentration and carrier mobility. The Seebeck coefficient increases with the increasing Nd-doped amount below 473K. The power factor reaches 7.06×10^-5 W/(m·K²) at 573K for (Ca_0.75Nd_0.1OH)_1.16CoO₂.