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In this study, Cu2Se1-xSx (x = 0.1, 0.3, and 0.5) alloy powders were prepared by the hydrothermal synthesis method. In the subsequent sintering process, the spontaneous in situ phase separation process of the sample forms a two-phase hybrid structure. The generated Cu2S precipitates in the Cu2Se matrix noticeably enhance phonon scattering, which is beneficial for low thermal conductivity without significantly affecting the electrical transport performance. Ultimately, an optimized thermoelectric performance was obtained in Cu2Se0.9S0.1, reaching a peak zT value of 1.43 at 773 K, the optimum value among the Cu-Se-S systems at this temperature.

Modulating microstructures in a wide range from atomic defects to microscale structures independently can partially decouple the transport of charge carriers and phonons and thus enhance the figure of merit (zT) of thermoelectric materials. High mobility requires atomic scale purity, while introducing nanoscopic inhomogeneities leads to low thermal conductivity. Through a two-step sintering process with excess Te, lower reduction of mobility and decreased thermal conductivity were simultaneously achieved in (Bi,Sb)2Te3. Grain boundaries and defects that strongly impede charge carrier transport are reduced by the two-step sintering process leading to a higher mobility compared to that of the one-step sintered bulk. At the same time, removal of Te as well as Sb-rich inhomogeneities with lattice misfit to the matrix decreased the thermal conductivity. In this way, simultaneous maintenance of high mobility and low lattice thermal conductivity was illustrated. By further optimization of carrier concentration, the produced material showed an encouraging zT value of 1.38 at 323 K. The present work demonstrates a method for synthesizing high-efficiency thermoelectric materials through simultaneous optimization of the electrical and thermal transport properties.

The understanding of certain basic issues in (K,Na)NbO3 (KNN)-based systems is still missing, one of which being the influence of ZrO2 addition on the behaviors of KNN-based solid solutions at trace levels possibly introduced during the process of material preparation. Herein, KNN solid solutions with trace ZrO2 addition (0, 0.01, 0.05, and 0.1 mol%) were prepared by a normal sintering method. The dielectric permittivity and the piezoelectric coefficient d33 reach the maximum (er = 497 and d33 = 139 pC N−1) with 0.1 mol% ZrO2 addition. Moreover, the microscopic results supported by PFM are in agreement with those from the macroscopic perspective. To establish the relationship among the ZrO2 addition contents, the induced defects and the properties of the solid solutions, a related defect model is proposed to support the experimental observation and the origin of induced defects is discussed. The results demonstrate that the properties of the KNN solid solutions can be closely affected by trace zirconia addition.

Bi2S3 is an eco-friendly alternative compound for thermoelectric devices. However, the low electrical conductivity of the pristine Bi2S3 hinders the improvement of its ZT value, which further restricts its application in the field of thermoelectricity. In this work, we report the first attempt to optimize the thermoelectric properties of Bi2S3 by electroless nickel plating. A nickel plated Bi2S3 powder sample was synthesized by electroless nickel plating on the precursor Bi2S3 powder prepared by mechanical alloying. Then, the powder was sintered to a bulk material by spark plasma sintering. The relationships between the composition, microstructure and thermoelectric properties of the bulk samples were investigated. The XRD results showed a AgBi3S5 second phase which was formed by the reaction of Ag residues with the Bi2S3 substrate during the sintering process. The nickel element and AgBi3S5 second phase introduced in the nickel plating process directly affect the electronic conductivity and Seebeck coefficient of the nickel plating sample, resulting in the relatively high power factor of 244 μW m−1 K−2 at 628 K. What's more, the thermal conductivity of the sample was also reduced moderately, obtaining a low value of 0.40 μW m−1 K−1 at 628 K. Therefore, a maximum ZT value of 0.38 was obtained at 628 K for the nickel plated sample, which is three times higher than that (0.12 at 628 K) of pristine Bi2S3 materials.

AgPb m SbTe m +2 (abbreviation as LAST) is a promising thermoelectric material based on PbTe for mid-temperature applications. The electrical and thermal transport properties of LAST alloys are greatly affected by the formation of nanostructured precipitation depending on the processing conditions, especially for spark plasma sintering (SPS) combined with mechanical alloying (MA). This study was focused on the optimization of SPS temperature ( T SPS ) and found that its variation in a narrow range (100 K) leads to apparent differences of thermoelectric performances. The sample sintered at an optimized temperature ( T SPS = 923 K) shows a low κ total (0.829 Wm −1 K −1 ) and a high PF (14.9 Wm −1 K −2 ), which is mainly attributed to the nanoscale composition segregation of Ag and Sb. The highest ZT value of 1.28 at 773 K was obtained when T SPS = 923 K, along with the highest ZT eng value of 0.26 and the corresponding η max value of 5.8% at the range of 323 K ( T cold )–773 K ( T hot ), which were increased almost twice by the optimization of sintering temperature.

Last two decades have witnessed significant progress in thermoelectric research, to which materials processing has crucial contributions. Compared with traditional zone-melting method used for fabricating bismuth telluride alloys, new powder-based processes have more freedom for manipulating nanostructures and nanocomposites. Thermoelectric performance enhancement is realized in most thermoelectric materials by introducing fine-grained and nano-composite structures with accurately controlled compositions. This review gives a comprehensive summary on the processing aspects of thermoelectric materials with three focuses on the powder synthesis, advanced sintering process and the formation of nanostructures in bulk materials.

Bi2Te3-based materials have been widely used in thermoelectric conversion technologies. Thermoelectric modules are fabricated from Bi2Te3-based ingots, but utilization of recycled scraps produced during the machining of thermoelectric elements has attracted attention because of limited material resources. In this work, we proposed a strategy for reusing ingot scraps to produce practical high-performance thermoelectric materials via a spark plasma sintering process by adjusting the composition of the scraps. It was found that the SPS process with a transient liquid phase resulting from excess added Te enhanced the thermoelectric performance, and a appropriately long soaking time was needed for SPS to maximize this enhancement. A longer sintering time helped improve the electrical transport properties, while simultaneously reducing the lattice thermal conductivity. Eventually, the highest ZT value of 1.33 and maximum conversion efficiency of 4.2% were obtained in the sample sintered for 15 min. This work provides an effective strategy for recycling industrial scraps of Bi2Te3-based thermoelectric materials and demonstrates the importance of soaking time optimization when spark plasma sintering is used for re-processing.

Strontium titanite (SrTiO3), which is an experimentally-friendly thermoelectric material, could be a promising candidate for thermoelectric power generation applications. The theorectical study indicates the co-doping of La and Nb could enhance the thermoelectric performance, however, the thermoelectric figure of merits (ZTs) of SrTiO3 are still low because the co-doping process at nano-scale is experimentally difficult to control. Here we report a high performance SrTiO3 with La-Nb co-doping, which are prepared by a combination of hydrothermal method and high-efficiency sintering. Nano-scale co-doping is successfully modulated by hydrothermal method, and nano-inclusions precipitate during sintering process, to form complex microstructures. In this case, the electrical and thermal transport properties are optimized simultaneously by doping concentration and dopants type, resulting in a record-high ZT >0.6 at 1000−1100 K in the 10 mol% La and 10 mol% Nb doped SrTiO3 bulk materials. The nano-scale modulation doping and microstructure controlling approach validated in the present study should be also applicable for other thermoelectric materials.

Bismuth sulfide (Bi2S3) has been of high interest for thermoelectric applications due to the high abundance of sulfur on Earth. However, the low electrical conductivity of pristine Bi2S3 results in a low figure of merit (ZT). In this work, Bi2S3@Bi core–shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The core–shell nanowires were densified to Bi/Bi2S3 nanocomposite by spark plasma sintering (SPS), and the structure of the nanowire was maintained as the nanocomposite due to rapid SPS processing and low sintering temperature. The thermoelectric properties of bulk samples were investigated. The electrical conductivity of a bulk sample after sintering at 673 K for 5 min using Bi2S3@Bi nanowire powders prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h is 3 orders of magnitude greater than that of a pristine Bi2S3 sample. The nanocomposite possessed the highest ZT value of 0.36 at 623 K. This represents a new strategy for densifying core–shell powde...

The tin dioxide ceramics were prepared by the traditional solid phase sintering method. Influence of different CeO2 doping amount on the microstructure, electrical resistivity and density of the tin dioxide ceramics by SEM and XRD and other analytical instrument. The relationship between composition, properties and structure were obtained. The results showed that when CeO2 doping quantity increased, the electrical resistivity of tin dioxide ceramics decreased firstly and then increased, the density of tin dioxide ceramics increased. When sintered temperature was 1430°C and the doping quantity of CeO2 was 0.7wt.%,the properties of tin dioxide ceramics was good, crystal growth was most complete,the electrical resistivity was 966.68mΩ·cm and the density was 5.72g/cm3.

Thermoelectric (TE) materials have a promising application as they can interconvert thermal energy to electrical energy directly. Oxide TE materials have attracted more attention for this application due to their high temperature stability. In this work, In2O3(ZnO)5 intrinsic superlattice ceramics with a layered structure were synthesized by reaction sintering a mixed powder of In2O3 and ZnO at 1200 °C for different holding times (t = 6, 8, 10, 12 and 15 h) in air. Their thermoelectric properties including the electrical conductivity, Seebeck coefficient and thermal conductivity, were measured from 323 to 973 K. The thermoelectric properties depended on the holding time. The highest ZT value of 0.19 at 973 K was obtained for the sample with a 12 h holding time.

High temperature gas turbine sealing can increase the thermal efficiency of a gas turbine. In this paper, monoclinic phase YTaO 4 ceramics were fabricated via solid-state reaction. Phase composition and microstructures of the high-temperature-sintered YTaO 4 ceramics were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman Spectroscopy. Specific heat capacity rose gradually as temperature increased, due to volumetric expansion and phonon excitations. The thermal diffusivities and conductivities decreased significantly due to the effects of the porosity and phonon scattering. However, the thermal conductivities of the specimens were lower than that of 7–8 wt% yttria-stabilized zirconia (7-8YSZ), and YTaO 4 ceramics have better thermal stability than current (TBCs) material. The Vickers hardnesses of YTaO 4 ceramics as a function of sintering temperature were lower than that of 8YSZ, indicating YTaO 4 has better fracture toughness and thermal tolerance. The results demonstrate that YTaO 4 ceramics would be an excellent candidate for use as a thermal barrier coating material for high temperature gas turbines.

[Formula: see text] is a [Formula: see text] compound (where Pn = Bi and Sb, Ch = Te, Se, and S), which has attracted increasing attention as a candidate for use in thermoelectric applications. Previous studies demonstrated the advantage of [Formula: see text] thermoelectric materials, despite an inferior thermoelectric performance. Herein, a series of [Formula: see text] ([Formula: see text], 0.10, 0.15, 0.20, and 0.25) thermoelectric materials were prepared by powder compaction sintering. The effects of phase structures and microstructure of the [Formula: see text] bulk material were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The thermoelectric properties, including Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured systematically. The results show that carrier concentration increased with decreasing Se content, which in turn affected the electrical transport properties. Low Se contents gave larger power factor (PF) values than the pristine [Formula: see text] sample, the maximum PF value being [Formula: see text] at 320 K for [Formula: see text]. The variation in PF was attributed to the variations in electrical conductivity [Formula: see text] and Seebeck coefficient [Formula: see text] upon optimizing Se content. The [Formula: see text] samples showed an enhanced thermoelectric figure of merit (ZT) with increasing measurement temperature, due to the increased [Formula: see text] value, [Formula: see text], and decreased [Formula: see text]. The [Formula: see text] sample exhibited the highest ZT (0.28) at 575 K, while [Formula: see text] exhibited the lowest ZT (0.14) at 325 K. This indicated that tuning Se content was an effective way to enhance carrier concentration.

This work investigated the enhanced sintering and electrical properties of (Ba 0.95 Ca 0.05 )(Ti 0.90 Sn 0.10 )O 3 - x Li 2 O (BCTSLi x , x = 0, 1, 2, 3, 4, 5 and 6 mol%) lead-free piezoelectric ceramics benefiting from the Li 2 O addition. Highly dense BCTS ceramics with good piezoelectric properties were fabricated at reduced sintering temperatures by adding a small amount of Li 2 O. It was found that the sintering densification is promoted mainly by reaction liquid phase at the early sintering stage and the formation of oxygen vacancies should be considered as an additional factor. The coexistence of R and T phases was confirmed by the Raman spectrum at a temperature range from−50 °C to 40 °C as x = 2 and at room temperature for all other Li 2 O added samples, while their specific ratio of R to T phase decreases with increasing Li 2 O addition. Better properties with d 33 = 457 pC/N, k p = 25.18% and Q m = 491 were obtained when adding 4 mol% Li 2 O, which are attributed dominantly to the appropriate specific ratio of R to T phase because of the similar grain size and density for samples at 3 ≤ x ≤ 6.

Large-scale Cu2S tetradecahedrons microcrystals and sheet-like Cu2S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu2S powders prepared by the HS method is a tetradecahedron with the size of 1–7 μm. The morphology of β-Cu2S is a hexagonal sheet-like structure with a thickness of 5–20 nm. The results indicate that the morphologies and phase structures of Cu2S are highly dependent on the reaction temperature and time, even though the precursors are the exact same. The polycrystalline copper sulfides bulk materials were obtained by densifying the as-prepared powders using the spark plasma sintering (SPS) technique. The electrical and thermal transport properties of all bulk samples were measured from 323 K to 773 K. The pure Cu2S bulk samples sintered by using the powders prepared via HS reached the highest thermoelectric figure of merit (ZT) value of 0.38 at 573 K. The main phase of the bulk sample sintered by using the powder prepared via WCM changed from β-Cu2S to Cu1.8S after sintering due to the instability of β-Cu2S during the sintering process. The Cu1.8S bulk sample with a Cu1.96S impurity achieved the highest ZT value of 0.62 at 773 K.

Dense Li/Ta-codoped KNN-based piezoceramics with d(33)* up to 375 pm/V were successfully fabricated by conventional sintering at a temperature as low as 900 degrees C by using LiF as a sintering additive. The reduction of densification temperature up to 200 degrees C was realized by a transient liquid phase sintering mechanism, consequently no grain boundary phase was observed in the sintered samples. It was found that the addition of LiF could further shift down the tetragonal orthorhombic transition point (TT-O), indicating that a small amount of Li+ could diffuse into the A-site of KNN matrix. The introduction of LiF enhanced the linearity of strain curves of the ceramics, which is unambiguously in favor for the actuator application. The present work reveals that low-temperature sintered LiF-doped KNN-based piezoceramics demonstrates promising potential in multilayer-structured actuator applications. (C) 2013 Elsevier Ltd. All rights reserved.

To study the possibility of SnS as an earth-abundant and environmentally friendly thermoelectric material, the electrical and thermal transport properties of bulk materials prepared by combining mechanical alloying and spark plasma sintering were investigated. It was revealed that SnS has potential as a good thermoelectric material, benefiting from its intrinsically low thermal conductivity below 1.0 W/m/K above 400 K and its high Seebeck coefficient over 500 μV/K. Although the highest ZT value was 0.16 at 823 K in the pristine sample, further enhancement can be expected through chemical doping to increase the electrical conductivity. It was also revealed that changing the stoichiometric ratio and sintering temperature had less apparent influence on the microstructure and thermoelectric properties of SnS because redundant S in the powders decomposed during the sintering process.

0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O-3-AgSbO3 lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 degrees C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 degrees C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal-orthorhombic phase transition point (TT-O) but a lower Curie point (T-c), compared to undoped ones. The KNN-based piezoceramics became "hard" as CuO was added, supported by an increase of Q(m). Fairly good electrical properties of d(33)*=383 pm/V, epsilon(r)=860, Q(m)=188 and T-c=215 degrees C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 degrees C, demonstrating promising potential in practical applications. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Ternary sulfide AgBi 3 S 5 compound is expected to be a promising thermoelectric material because of its merits such as low electrical resistivity and environmentally compatible composition. In this study, a combined process of mechanical alloying and spark plasma sintering was applied to the fabrication of AgBi 3 S 5 bulk materials, and nearly single phase was obtained by optimizing the sintering conditions, although the mill-derived powders consisted of AgBiS 2 and Bi 2 S 3 phases. The electrical conductivity of the AgBi 3 S 5 bulk material decreases from 178 to 79 Scm −1 with raising temperature from 323 to 573 K, while the Seebeck coefficient ranges from −83 to −167 μV K −1 , thereby a maximum power factor 221 μW m −1 K −2 achieved at 573 K. It is notable that AgBi 3 S 5 has a low thermal conductivity, ranging from 0.70 to 0.64 W m −1 K −1 from room temperature to 573 K, which is the lowest value reported so far for this material. A peak thermoelectric figure of merit value of 0.20 was obtained at 573 K in the present pristine AgBi 3 S 5 sample.

Fine-grained Ti-doped FeVSb half-Heusler alloys were synthesized by combining mechanical alloying and spark plasma sintering and their thermoelectric properties were investigated with an emphasis on the influences of Ti doping and phase purity. It was found that substituting V with Ti can change the electrical transport behavior from n-type to p-type due to one less valence electron of Ti than V, and the sample with nominal composition FeV0.8Ti0.4Sb exhibits the largest Seebeck coefficient and the maximum power factor. By optimizing the sintering temperature and applying annealing treatment, the power factor is significantly improved and the thermal conductivity is reduced simultaneously, resulting in a ZT value of 0.43 at 500 °C, which is relatively high as for p-type half-Heusler alloys containing earth-abundant elements.

A small amount of BiFeO3 (BF) was incorporated into lead-free Li0.03(Na0.53K0.48)0.97Nb0.8Ta0.2O3 (LKNNT) piezoceramics for the purpose of property improvement via phase structure control. It was found that the addition of BF not only induced a series of phase transformations from orthorhombic to tetragonal and further to pseudocubic in LKNNT ceramics, but also enhanced its sintering densification behavior. Even tiny amount of BF addition down to 0.5 mol% could change the room-temperature phase structure from a dominant orthorhombic state to the coexistence of orthorhombic and tetragonal phases, resulting in peak piezoelectric properties of d 33 * ∼ 340 pm/V , kp ∼ 47.0%. When the amount of BF was increased to 0.75 mol%, the tetragonal phase became dominant in the modified LKNNT ceramics, which exhibited excellent temperature stability, despite its slightly lowered piezoelectric coefficients.

A composite structure with two separately doped phases was introduced into Ni-doped LaCoO3 ceramics to enhance its electrical conductivity while keeping low thermal conductivity. LaCo0.9Ni0.1O3−δ nanoparticles were dispersed in the sol solution of La0.9Sr0.1Co0.9Ni0.1O3−δ and separated from the solution by centrifugation, which were then calcined and consolidated using normal sintering in air. The electrical conductivity of the coated samples was obviously increased, although the Seebeck coefficient was slightly decreased by the above particle- coating method, resulting in a great improvement of power factor. The Seebeck coefficient and electrical conductivity are strongly dependent on the concentration of La0.9Sr0.1Co0.9Ni0.1O3−δ sol, which affects the coating content, but the thermal conductivity is less affected by its concentration especially at room temperature. The power factor was increased to 2.58 × 10−4 Wm−1 K−2 at room temperature, resulting in a twofold enhancement in a peak ZT value at 373 K for the coated sample as compared with that without coating.

Volatilization of alkali elements during sintering process is one of the major factors influencing the processing stability of lead-free (Na,K)NbO3 (KNN) piezoceramics. With a purpose to reduce sintering temperature by refining particle sizes, an extensive ball-milling step was intentionally introduced to the fabrication process of KNN-based ceramics with a nominal composition of Li0.058(Na0.52K0.48)0.942NbO3 by a solid-state powder synthesis and normal sintering method. It was revealed that prolonging the period for milling calcined powders before sintering process significantly reduced the particle sizes and hence sintering temperature. As a result, sintering at a reduced temperature of 1,025 °C resulted in a highly dense material with good piezoelectric and ferroelectric properties with converse piezoelectric coefficient d* 33 and remnant polarization P r reaching 266 pm/V and 15.7 μC/cm2, respectively. This study provided a facile and effective method to enhance the piezoelectric properties in KNN-based ceramics with an optimized composition by reducing the particle sizes.

Nonstoichiometric lead-antimony-silver-tellurium (LAST) system thermoelectric bulk materials Ag0.8Pb22.5SbTe20−x S x (x = 0 to 8.0) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The electrical and thermal transport properties were investigated in the temperature range of 300 K to 700 K. The x-ray diffraction (XRD) results indicated that sulfur entered the PbTe during MA process, but gradually precipitated in the sintering process in the form of PbS from the PbTe matrix when the sulfur content was changed from x = 2.0 to x = 8.0. It was confirmed that the addition of sulfur effectively reduced the lattice thermal conductivity. A low thermal conductivity of 0.83 W m−1 K−1 at 673 K was obtained for the Ag0.8Pb22.5SbTe12S8 sample. Benefiting from the high electrical conductivity, the Ag0.8Pb22.5SbTe18S2 sample reached a maximum ZT value of ∼0.97 at 673 K, which is 32% higher than its counterpart without sulfur substitution.

Bismuth sulfide (Bi2S3) polycrystalline bulks with a high relative density (>90%) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The microstructural and thermoelectric (TE) properties were investigated with a special emphasis on the influence of SPS temperature. The components (Bi/S ratio), grain size, and relative density of Bi2S3 bulk are sensitive to the SPS temperature, which all affect the electrical transport properties of samples. Elevating SPS temperature results in grain growth, densification, and spontaneous texturing, which benefit to enhance TE properties, whereas overheating leads to severe volatilization of both S and Bi and formation of porous microstructure. The highest figure of merit value reaches 0.22 at 573 K for the Bi2S3 sample obtained by applying SPS at 673 K for 5 min, which is the maximum value reported so far in bulk Bi2S3 system. The enhanced TE property of cheap and environmental friendly Bi2S3 material indicates a great promise in TE devices.

BiFeO3-modified lead-free piezoceramics Li0.05(Na0.515K0.485)0.95Nb0.8Ta0.2O3 (LKNNT) were successfully fabricated by conventional method to investigate its influences on phase structure and electricity as well as mechanical properties. A tiny amount of BiFeO3 (BF) changes the phase structure of LKNNT to tetragonal and further to pseudocubic, and also effectively improves the sintering ability and densification of LKNNT which enhances the mechanical property. The LKNNT piezoceramics with the addition of 1 mol% BF has the bulk density of 5.02 g/cm3 and an enhanced fracture strength of 142 MPa which is even higher than that of the sample of similar composition prepared by spark plasma sintering, also possesses room temperature electric properties of d 33* ~ 225 pm/V, k p ~ 36.3%, e r ~ 1140, tgδ ~ 0.018, T c ~ 305 °C and Q m ~ 80. The d 33* remains constant up to 150 °C and the change ratio of dielectric property largely decreases over the temperature range of −50–150 °C, which can be attributed to the temperature-independent phase structure of LKNNT piezoceramics, making the ceramics as a promising candidate for actual applications.

Silver antimonate (AgSbO 3 ) is a potential high-temperature thermoelectric oxide with a low thermal conductivity, but it is difficult to fabricate dense bulk material with high phase purity. Well-dispersed AgSbO 3 nanopowder with an average diameter of 50 nm was synthesized by an ion-exchange process in this study, the sintering density of which was apparently enhanced. Nearly single-phase AgSbO 3 ceramic samples with a relative density close to 90% were obtained when sintered at a relatively low temperature (1273 K). The electrical conductivity of AgSbO 3 increased greatly with increasing sintering temperature, probably because of defects originating from the compositional deviation caused by sintering, in addition to increased density. This study also confirmed that AgSbO 3 had a low thermal conductivity, from 1.1 W m −1 K −1 at room temperature to 0.8 W m −1 K −1 at 673 K.

Single-phase polycrystalline La0.08Sr0.92TiO3 ceramics were prepared by spark plasma sintering (SPS) using sol–gel-derived powders. The La0.08Sr0.92TiO3 bulks had highly relative density (∼92%), and the grain size of the samples obviously grew from 300 nm to 3 μm when the SPS temperature increased from 1203 to 1473 K. The electrical conductivity increased with SPS-processing temperature owing to the increase of relative density, grain size, and oxygen vacancies due to the reducing atmosphere of SPS. The absolute value of Seebeck coefficient increased as the SPS temperature increased from 1203 to 1373 K, and then decreased by further increasing SPS temperature to 1473 K. The sample SPSed at 1373 K had the largest absolute value of Seebeck coefficient of 196 μVK−1 at 679 K. Sol–gel process and SPS can significantly decrease the thermal conductivity of SrTiO3 based ceramics, whose lowest value was reduced to 1.19 Wm−1k−1 at 773 K in the sample SPSed at 1203 K. The maximum ZT value of 0.08 was obtained at 679 K for the sample SPSed at 1473 K.

Co-firing of piezoceramics with metals is drawing ever-increasing attention. This article reports Ni-particle-dispersed [Li 0.06 (K 0.5 Na 0.5 ) 0.94 ]NbO 3 (LKNN/Ni) lead-free piezoelectric composites that were sintered in a protective atmosphere of nitrogen gas. The base metal Ni was not oxidized and the piezoelectric LKNN preserved the perovskite structure. The microstructure observations show that Ni particles were uniformly dispersed in LKNN ceramics matrix. With increasing Ni content from 0 to 20 vol%, the piezoelectric constant d 33 of the LKNN/Ni composites decreased from 165 to 23 pC/N, and the corresponding dielectric constant ɛ r greatly increased. The 95%LKNN-5%Ni composite still exhibited a typical ferroelectric loop; however, the P - E curves measured for Ni > 5 vol% composites demonstrated some metallic characteristics. The LKNN/Ni composites could find applications in piezoelectric actuators with functionally graded microstructure (FGM).

With the assistance of excess Na2O additives, LiNbO3-doped (K, Na)NbO3 lead-free piezoceramics were successfully prepared by conventional sintering at a temperature as low as 950°C. Shrinkage curve measurement and micrograph observation elucidate that the nature of the densification behavior at 950°C is liquid phase sintering, during which the liquid phase penetrated between initial small particles and large quadrate grains of 20–40 μm formed in the solution–precipitation process. Although the density and grain size show inappreciable changes during the final stage of the coalescence process, the piezoelectric properties of the sintered samples are greatly influenced by the soaking time, depending on the optimization of which the piezoelectric constant d33 and the planar mode of the electromechanical coupling factor kp reached 280 pC/N and 48.3%, respectively. The present study revealed that good piezoelectric properties can be obtained in the Li-doped KNN ceramics even by normal sintering at a reduced temperature (950°C).

LaCoO3 ceramics were prepared by conventional solid state reaction and normal sintering at the temperatures ranging from 1373 to 1523 K. The sintered densities increased with increasing sintering temperature and exceeded 90 % of the theoretical values when sintered above 1473 K. The thermoelectric properties of the samples sintered at different temperatures were investigated from 323 to 673 K. The LaCoO3 samples showed a negative Seebeck coefficient, whose absolute values decreased dramatically with increasing temperature in the range of 323 to 460 K, then changed to a positive value and lightly decreased above 460 K. The electrical conductivity increased with increasing temperature, indicating a semiconducting behavior. The Seebeck coefficients showed little difference between the samples sintered at different temperatures, but the power factor of the sample sintered at a higher temperature was larger because of the higher electrical conductivity.

Cofiring of electroceramics with metals is receiving ever-increasing attention. In this paper, Cu-particle-dispersed [(Na 0.5 K 0.5 ) 0.94 Li 0.06 ]NbO 3 (NKLN/Cu) piezoelectric composites were fabricated by protective sintering in a nitrogen gas. The X-ray diffraction (XRD) results show that only perovskite NKLN and metallic Cu phases were detected in the composites. With increasing Cu content up to 30 vol%, the relative dielectric constant ɛ r markedly increased from 121 for monolithic NKLN ceramics to 3938 for NKLN–30 vol% Cu composite. The piezoelectric constant d 33 decreased from 171 pC/N to undetected and electromechanical coupling factor k p decreased from 26% to undetected, showing a gradually changed profile of piezoelectric property. The decrease in piezoelectric properties with increasing Cu content was ascribed to non-piezoelectric effect of metallic phases and poling characteristics of piezoceramics/metal composites. The NKLN/Cu composites could be utilized to provide a central electrode of functionally graded microstructure (FGM) to modify conventional bimorph-type piezoelectric actuators.

Recently, apatite-type lanthanum silicate, La9.33(SiO4)6O2, and its analogues with cationic doping in the La/Si site have been attracting significant interests due to their high oxide ion conduction. However, the well-known synthesis method is solid-state reaction, which suffers from problems such as the high sintering temperatures for dense pellets and the impurity phases in the final ceramics. In this paper apatite-type lanthanum silicates were prepared by precipitate method combining with an azeotropic-distillation process. The influences of powder calcination temperature on the relative density were investigated and the optimal calcination temperature was proved to be 1100 °C. The main phase of the powder calcined at 1100 °C was lanthanum silicate La9.33(SiO4)6O2 with trace impurities La2O3 and La2SiO5. However, dense pellet (96.63%) with pure lanthanum silicate phase was obtained at relatively low temperature (1550 °C). Electrical measurements have shown that conductivity behaviors of the sintered pellets were also calcination-temperature dependent. The highest overall conductivity values, σ500 °C = 1.58 × 10− 3 S cm− 1 and σ800 °C = 2.21 × 10− 2 S cm− 1, were obtained for pellet prepared from powder calcined at 1100 °C, which was much higher than that synthesized by other methods. The samples presented activation energies of 0.66 ± 0.01 eV. The results in the present study demonstrate that the precipitate-azeotropic-distillation approach is an alternative method to synthesize lanthanum silicate powders.

BaTiO3 ceramics with different domain sizes were prepared by spark plasma sintering (SPS) and normal sintering (NS) using hydrothermally synthesized 100 nm and 500 nm powders. It was found that their piezoelectric constant d33 shows great dependence on the size of ferroelectric domains. A high d33 up to 416 pC/N was obtained in the SPSed ceramics derived from 100 nm powders, whose domain width was identified

Alkaline volatilization during sintering in (K, Na)NbO3 (KNN)-based lead-free piezoceramics is a problem that causes non-stoichiometry and hence affects the phase structures and properties. This work investigated the depth-dependent distributions of alkaline elements in Li-modified KNN ceramics using X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that Li evaporated more severely than Na and K, consequently almost no Li elements remained on the as-sintered surface, while its content became stable at the depth over 55 μm from the outmost surface. In addition, X-ray diffraction results suggested the phase structure might show discrepancy at the pellet surface and the interior part due to the textured surface grains.

Microstructure characteristics, phase transition, and electrical properties of (Na 0 . 535 K 0 . 485 ) 0 . 926 Li 0 . 074 (Nb 0 . 942 Ta 0 . 058 )O 3 (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20-30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°-1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d 33 is enhanced to 276 pC/N, which is a high value for the Li-and Ta-modified (Na,K)Nb0 3 ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k p = 46.2%, thickness electromechanical coupling factor k t = 36%, mechanical quality factor Q m = 18, remnant polarization P r = 21.1 μC/cm 2 , and coercive field E c = 1.85 kV/mm.

Crystallographic structure, phase transition and electrical properties of lead-free (Na0.535K0.485)1−x Li x (Nb0.942Ta0.058)O3 (x=0.042–0.098) (NKL x NT) piezoelectric ceramics were investigated. The experimental results show that both Li content and sintering temperature strongly affect the orthorhombic–tetragonal polymorphic phase boundary (PPB), which results in remarkable differences of the piezoelectric property and its temperature stability in the NKL x NT ceramics. Chemical analysis indicates that sodium volatilizes more seriously than potassium and lithium with increasing sintering temperature. Due to the comprehensively optimized effects of Li content and sintering temperature, an enhanced piezoelectric constant d 33 (276 pC/N) was obtained at room temperature in the ceramics with x=0.074 sintered at 1000°C. In the same composition, a further high d 33 up to 354 pC/N was obtained at 43°C, which is close to its T o−t temperature. Furthermore, better temperature stability can be obtained when x=0.082 sintered at 1000°C, whose piezoelectric constant d 33 (236 pC/N) keeps almost constant from room temperature to 100°C. Such a temperature-independent piezoelectric property is available in the NKL x NT ceramics with high Li content because its T o−t was moved below room temperature.

Bismuth sulfide powders were synthesized by mechanical alloying (MA) and then consolidated by spark plasma sintering (SPS) technique. In order to improve the electrical transport properties of bismuth sulfides, the carrier concentration was optimized by modifying chemical composition of sulfur through producing sulfur vacancies, and the carrier mobility was enhanced by a two-step SPS as a hot-forging process through increasing grain orientation. The electrical resistivity of bismuth sulfides was reduced to 10−4 from 10−2 Ω m by optimizing sulfur content, and further lowered by hot-forging, whereby the power factor was significantly increased from 91 to 254 μW/mK2. The hot-forged Bi2S2.90 sample showed the highest ZT=0.11 (at 523 K), which is higher than the reported value. The present work revealed that bismuth sulfide compounds as a promising candidate of thermoelectric materials can be synthesized by a simple process.

Conventional sintering of (Na1−xKx)0.5Bi0.5TiO3 (abbreviated as NKBTx, x=18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant (d33) of about 192 pC/N with a high electromechanical coupling factor (kp) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C.

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO 3 -based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO 3 ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.

This paper reports the enhanced piezoelectric properties in [(Na0.535K0.480)0.966Li0.058] (Nb0.90Ta0.10)O3 ceramics sintered at 1075–1120 °C. Because of morphotropic phase boundary-like behavior achieved by altering the sintering temperature from 1090 to 1105 °C, the piezoelectric coefficient is enhanced to 268 pC/N, the highest value reported for the LiTaO3-doped (Na,K)NbO3 ceramics. The corresponding Curie temperature and remanent polarization reached 445 °C and 11.5 μC cm−2, respectively.

n-Type Bi2Te3 thermoelectric materials were prepared by spark plasma sintering (SPS) using mechanically alloyed powders. Seebeck coefficient and electrical conductivity of the resultant materials were measured, with emphasis on the effects of sintering process parameters on the electrical properties. The power factor was improved from 1.5 to 1.6 mW/mK2 when the SPS pressure was increased from 20 to 50 MPa at 623 K, and was further increased to 2.1 mW/mK2 under 50 MPa by raising the SPS temperature from 623 to 673 K. The maximum power factor was obtained for the sample sintered under optimized SPS process parameters, in which the sintering temperature and the sintering pressure are 673 K and 50 MPa, respectively.

A unique core–shell structure was observed in coarse grains in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity-induced core–shell grain structure reported previously in BaTiO3-based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger-sized but similar self-assembled subgrains. The electron-backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single-crystalline-like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN-based systems showed a similar grain growth transformation: from self-assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline.

The effects of sintering temperature on microstructure, density and electrical properties of Na0.5K0.5NbO3 prepared by normal sintering at 1020–1100°C were investigated. All the Na0.5K0.5NbO3 ceramics showed an orthorhombic structure similar to that of KNbO3 ceramics, while raising sintering temperature resulted in the shifting of diffraction peaks to a lower value and the formation of a second phase due to the excess volatilization of Na. Both density and d33 of the sintered samples increase with increasing sintering temperature. The density of the sample sintered at 1080°C reaches a maximum value (4.22 g/cm3, 92% of the theoretical density). The sample sintered at 1060°C showed a maximum d33 value of about 122 pC/N and the highest T c of about 409°C, and the corresponding E c and P r were about 13 kV/cm and 15 μ C/cm2, respectively.

Niobate ceramics such as NaNbO3 and KNbO3 have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na0.5K0.5NbO3 ceramics with submicron grains were prepared using SPS, whose density was raised to 4.47 g/cm3 (>99% of the theoretical density) at 920 °C. Reasonably good ferroelectric and piezoelectric properties were obtained in the SPSed Na0.5K0.5NbO3 ceramics after annealing in air. The effect of annealing time on the electrical properties was investigated to determine optimal processing condition. The piezoelectric parameter (d 33) of the Na0.5K0.5NbO3 ceramics annealed properly reached 148 pC/N.

Lanthanum-modified lead zirconate titanate (PLZT) ceramics possess the photostrictive effect applicable to photo-driven and wireless actuators. The piezoelectric and anomalous photovoltaic (APV) properties of PLZT ceramics with different compositions but the same grain size were evaluated to investigate the intrinsic compositional effects. After the adjustment of the sintering temperature and soaking time, the grain sizes of PLZT ceramics with different compositions were controlled in a narrow range of 0.7–0.8 μm. Both the maximum photocurrent and photo-EMF after grain size control were found at PLZT 4/52/48.

Lead lanthanum zirconate titanate, PLZT ceramics have drawn great attention in microactuation because the Photostrictive Effect in it. In this study, compositional dependence of photostrictive effect in PLZT was investigated. The factual compositions were configurated quantitatively by XRF to characterize the deviation in the preparation process. Different sintering periods were designed in an attempt to obtain PLZT ceramics with the same grain size but different compositions to explore the intrinsic photostrictive effect in the PLZT system. With this method, the maximum intrinsic photocurrent was obtained at 3.48/50.7/49.3, while the maximum intrinsic photovoltage was obtained at 3.44/52.73/47.27.

Lanthanum-modified lead zirconate-titanate (PLZT5/54/46) powders were prepared from the commercial PbO, La2O3, TiO2 and ZrO2 powders by high-energy ball milling process. The synthesized powders milled for different hours were characterized using XRD and SEM. Thedesigned PLZT perovskite phase was formed from the mixture of the starting materials after milling for 20 hours. PLZT(5/54/46) ceramics were obtained using the produced powders milled for 20 h followed by sintering at temperatures ranging from 1000 to1200°C. Ferroelectric property of the PLZT ceramics was also measured.