Spark plasma sintered BaTiO3/graphene composites for thermoelectric applications

The viability of spark plasma sintered graphene/barium titanate ceramic matrix composites as thermoelectric materials is investigated. The temperature dependence of electrical conductivity, thermal conductivity and Seebeck coefficient was analyzed. The addition of low amounts of graphene oxide combined with the spark plasma sintering process increases electrical conductivity of pure BaTiO₃ several orders of magnitude, whereas the thermal conductivity shows only a moderate enhancement. The composites display a semiconducting behaviour, with the resistivity decreasing with increasing temperature and following a thermally activated temperature dependence at high T. A strong dependence of ZT figure of merit with the graphene concentration and the measurement temperature was found. Optimal values are found for 1.7 wt% graphene oxide at the maximum experimental temperature (600 K).     

Synergetic tuning of electrical/thermal transport via dual-doping in Bi0.96−xMgxPb0.06CuSeO

The layered oxyselenides BiCuSeO was recently discovered as potential thermoelectric. Our result reveals that the substitute for atom Bi³⁺ by Pb²⁺ & Mg²⁺ in (Bi₂O₂)²⁻ play an important role in electrical transport properties. The maximum electrical conductivity obtained is 460 Scm⁻¹ for Bi_0.84Mg_0.10Pb_0.06CuSeO at RT, highly above the 15 Sm⁻¹ for BiCuSeO. In synergy with low thermal conductivity (0.6-0.4 Wm⁻¹ K⁻¹) and large thermopower (300-500 μVK⁻¹), the highest ZT is achieved about 0.80 at 873 K for Bi_0.88Mg_0.06Pb_0.06CuSeO.     

Enhancement of Thermoelectric Performance in Hierarchical Mesoscopic Oxide Composites of Ca3Co4O9 and La0.8Sr0.2CoO3

The natural contradiction in enhancing electrical conductivity and thermopower in thermoelectric oxides makes it hard to improve the performance of a single thermoelectric oxide material. We report a facile method to construct a unique architecture of thermoelectric oxides that is promising to realize a simultaneous improvement of overall electrical conductivity and thermopower. Here, a series of two‐phase nanocomposites comprising of Ca₃Co₄O₉ (CCO) and La_0.8Sr_0.2CoO₃ (LSCO) has been synthesized through ball milling followed by spark plasma sintering (SPS) method. The electron microscope images reveal that the two constituents form the unique composites while retaining their individual crystalline and morphological identities. Owing to the hierarchical mesoscopic structure with nanoscale particles and submicrometer scale grain boundaries, an external strain is induced into the CCO grains by the LSCO nanoparticles to enhance the thermopower. The mesoscopic structure is also favorable for improving the electrical conductivity. Moreover, the long‐wavelength phonons can be scattered effectively from LSCO nanoparticles and the thermal conductivity is further suppressed. With compromises between power factor and thermal conductivity, the largest ZT achieved is up to 0.41 at 1000 K for the composites with 25 wt% of LSCO.     

Enhanced thermoelectric performance based on special micro-configuration of graphene-ZnO induced by high-pressure and high-temperature

Zinc oxide (ZnO) is a promising high-temperature thermoelectric material. Graphene is typically a two-dimensional material, and its development and application have attracted wide attention due to its excellent thermal stability and mechanical properties. To the best of our knowledge, the graphene-ZnO (C–ZnO) composite has never been studied in the field of thermoelectric conversion. The high-pressure and high-temperature (HPHT) technique has unique advantages in improving the thermoelectric properties of ZnO. In this study, for the first time, C–ZnO bulk energy materials with novel micro-configuration were prepared by rapid sintering using the HPHT method. Observation under a microscope revealed that as the doping amount of graphene increased, a large number of graphene nanowires formed connected between the ZnO grains, and with the excess amount of graphene introduced the morphology of the ZnO grains changed and their size became smaller. This novel micro-configuration of the 0.1C–ZnO sample showed an ultrahigh electrical conductivity of 2.8 × 10⁴ S/m with a significantly lower lattice thermal conductivity of 4.3 Wm^?1K^?1 at 973 K. Ultimately, at 973 K, the zT value of the 0.1C–ZnO sample was 129 times higher than that of pure ZnO. Therefore, the high-temperature thermoelectric material C–ZnO prepared by the HPHT method can be used in automobile exhaust systems and industrial boilers to effectively recover and reuse the waste heat.     

Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites

The Graphene/Mn_0.7Zn_0.3Fe₂O₄ composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343–973 K. The effect of graphene on the thermoelectrical properties of Mn_0.7Zn_0.3Fe₂O₄ was investigated. The dispersion of 2 wt% graphene in Mn_0.7Zn_0.3Fe₂O₄ effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973 K was obtained in the composite with 2 wt% graphene.     

Crystal structure modulation of SnSe thermoelectric material by AgBiSe2 solid solution

Although low thermal conductivity and high band degeneracy bring promising thermoelectric performance to cubic SnSe, its preparation strategies remain elusive. Here, a series of Sn_1–2x(AgBi)_xSe samples are synthesized using the vacuum melting (1173 K) and spark plasma sintering (723 K) methods. Owing to the increased configurational entropy caused by AgBiSe₂ solid solution, the cubic structure is obtained when x exceeds 0.2. The optimized carrier concentration significantly enhances electrical conductivity (σ), and maximal σ of 350 Scm⁻¹ is obtained in Sn_0.5(AgBi)_0.25Se. Combined with the low lattice thermal conductivity caused by the small sound velocity and strong anharmonicity, the figure of merit of 0.08 is reached in Sn_0.6(AgBi)_0.2Se at 500 K. The mechanical performances are also improved, and a high Vicker hardness of 1.5 GPa is obtained in Sn_0.4(AgBi)_0.3Se. This work demonstrates the importance of the configurational entropy in phase regulation and provides insights into the design of new thermoelectrics.     

Part-A: Synthesis of polyaniline and carboxylic acid functionalized SWCNT composites for electromagnetic interference shielding coatings

Polyaniline (PANI) was synthesized by chemical oxidation process by using Ammonium persulphate (APS) as an oxidizer and HCl as a dopant. The effects of altering the stoichiometric ratio of monomer to oxidizer, addition time, reaction temperature and dopant concentration on the electrical conductivity of PANI were studied in detail. The synthesis procedure was optimized to yield the PANI with maximum electrical conductivity. The pure PANI thus synthesized exhibited the maximum electrical conductivity 5.6 Scm⁻¹. Different PANI–cSWCNT composites were prepared by ex-situ and in-situ methods and a comparative evaluation of electrical conductivity was carried out. From the electrical conductivity measurements, it was seen that maximum conductivity 27.12 Scm⁻¹ was achieved for 20% cSWCNT loaded PANI composite prepared by in-situ method and 12 Scm⁻¹ for the same composite prepared by ex-situ method. The efficacy of in-situ method, for conductivity enhancement was attributed to the formation of PANI coating over cSWCNT during the synthesis. This coating formation was further substantiated by FTIR, XRD, DSC, TGA, FESEM, and HRTEM analysis. The results of these studies confirmed that the in-situ prepared 20% cSWCNT loaded PANI composite is the most preferred filler for developing polyurethane based EMI shielding coatings.     

Fabrication of a transversal multilayer thermoelectric generator with substituted calcium manganite

The sintering behavior and thermoelectric performance of Ca_0.99Gd_0.01Mn_0.99W_0.01O₃ was studied, and a multilayer thermoelectric generator was fabricated. The addition of CuO as sintering additive was found to be effective for the reduction in the sintering temperature from 1300°C to about 1000°C‐1050°C. Dense samples were obtained after firing at 1050°C, whereas some porosity remained after firing at 1000°C. Samples sintered at reduced temperature exhibit lower electrical conductivity, whereas the Seebeck coefficient S = ?150 μV/K at 100°C is not affected by lowering the sintering temperature. The figure of merit is ZT = 0.12 at 700°C for samples sintered at 1300°C; ZT = 0.08 and 0.03 were obtained for multilayer laminates sintered at 1050°C and 1000°C, respectively. A transversal multilayer thermoelectric generator (TMLTEG) was built by stacking layers of substituted CaMnO₃ green tapes, and printing AgPd conductor stripes onto the thermoelectric layers at an angle of 30° relative to the direction of the heat flow. The multilayer stack was co‐fired at 1000°C. The TMLTEG has a power output of 2.5 mW at ?T= 200 K in the temperature interval of 25°C‐300°C. A meander‐like generator with larger power output comprising six TMTEGs is also presented.     

Rapid fabrication of Se-modified skutterudites obtained via self-propagating high-temperature synthesis and pulse plasma sintering route

Selenium is an effective dopant in skutterudite-based thermoelectric materials. It strongly influences thermal transport properties due to effective phonon scattering. This study proposes a short-term fabrication route to Se-modified CoSb₃-based materials. Alloy synthesis was conducted via self-propagating high-temperature synthesis. Subsequently, pulse plasma sintering consolidated all materials. As a result, thermoelectric materials with high electrical properties homogeneity were obtained. Seebeck potential mapping showed the measured deviation of the Seebeck coefficient for all fabricated samples was between 5 and 7%. A very low thermal conductivity (1.59 W m^?1 K^?1, at 573 K) was achieved for the highest doped sample, and one of the lowest reported results obtained for bulk skutterudite-based thermoelectric materials ever. This resulted in a low lattice thermal conductivity (1.51 W m^?1 K^?1, at 573 K). This led to the highest ZT (0.27 at 623 K) for the highest doped sample.     

Enhanced thermoelectric properties and electronic structures of p-type BiCu1−xAgxSeO ceramics

The thermoelectric properties and electronic structures were investigated on p-type BiCu_1-xAg_xSeO (x=0, 0.02, 0.05, 0.08) ceramics prepared using a two-step solid state reaction followed by inductively hot pressing. All the samples consist of single BiCuSeO phase with lamella structure and no preferential orientation exists in the crystallites. Upon replacing Cu⁺ by Ag⁺, maximum values of electrical conductivity of 36.6 S cm⁻¹ and Seebeck coefficient of 350 μV K⁻¹ are obtained in BiCu_0.98Ag_0.02SeO and BiCu_0.92Ag_0.08SeO, respectively. Nevertheless, a maximum power factor of 3.67 μW cm⁻¹K⁻² is achieved for BiCu_0.95Ag_0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient. Simultaneously, this oxyselenide exhibits a thermal conductivity as low as 0.38 W m⁻¹ K⁻¹ and a high ZT value of 0.72 at 750 K, which is nearly 1.85 times as large as that of the pristine BiCuSeO. The enhancement of thermoelectric performance is mainly attributed to the increased density of states near the Fermi level as indicated by the calculated results.     

Mechanochemically Carboxylated Multilayer Graphene Nanoplatelets as Functionalized Carbon Nanofillers for Electrically Conductive Epoxy Spray Coatings

Dry ball milling of graphite under CO₂ pressure in a planetary ball mill affords carboxylated multilayer graphene nanoplatelets as carbon nanofillers (MFG‐CO₂) for carbon/epoxy spray coatings combining electrical conductivity up to 0.09 S cm^?1 with excellent adhesion and improved toughness. As confirmed by µCT‐imaging, the two‐stage homogenization by means of a speed mixer with subsequent shearing in a three‐roll mill uniformly disperses up to 60 wt% MFG‐CO₂ in aqueous emulsions of epoxy resins and hardener without impairing spray coating. The MFG‐CO₂ content governs surface roughness, as determined by 3D laser microscopy, gloss, electrical conductivity, and toughness without adversely affecting excellent adhesion. Mechanochemical tailoring MFG nanofillers holds great promise for the development of advanced epoxy spray coatings exhibiting an improved balance of thermooxidative, chemical and environmental stability, electrical and thermal conductivity, toughness, corrosion, and barrier resistance.     

Electrical Conductivity of Mullite Ceramics

The electrical conductivity of a lab‐produced homogeneous mullite ceramic sintered at 1625°C for 10 h with low porosity was measured by impedance spectroscopy in the 0.01 Hz to 1MHz frequency range at temperatures between 300°C and 1400°C in air. The electrical conductivity of the mullite ceramic is low at 300°C (≈0.5 × 10^?9 Scm^?1), typical for a ceramic insulator. Up to ≈ 800°C, the conductivity only slightly increases (≈0.5 × 10^?6 Scm^?1 at 800°C) corresponding to a relatively low activation energy (0.68eV) of the process. Above ≈ 800°C, the temperature‐dependent increase in the electrical conductivity is higher (≈10^?5 Scm^?1 at 1400°C), which goes along with a higher activation energy (1.14 eV). The electrical conductivity of the mullite ceramic and its temperature‐dependence are compared with prior studies. The conductivity of polycrystalline mullite is found to lie in‐between those of the strong insulator α‐alumina and the excellent ion conductor Y‐doped zirconia. The electrical conductivity of the mullite ceramic in the low‐temperature field (< ≈800°C) is approximately one order of magnitude higher than that of the mullite single crystals. This difference is essentially attributed to electronic grain‐boundary conductivity in the polycrystalline ceramic material. The electronic grain‐boundary conductivity may be triggered by defects at grain boundaries. At high temperatures, above ≈ 800°C, and up to 1400°C gradually increasing ionic oxygen conductivity dominates.     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.     

Highly conductive interconnected graphene foam based polymer composite

In this study, the impact of graphene sheet size on the electrical conductivity of interconnected graphene foam polymer composite is thoroughly investigated. Graphene oxide solution is produced from small flake graphite (SFG) (2–15 μm) and large flake graphite (LFG) (>100 μm), respectively. Each solution is used to produce three-dimensional GO foam, which is subsequently heat-treated to produce reduced graphene oxide (RGO) foam. The RGO foams are then infiltrated with poly(dimethylsiloxane) (PDMS) to produce graphene-PDMS (G-PDMS) composites. The in-plane electrical conductivity of the G-PDMS composite (0.4 wt%) from LFG reaches ∼3.2 S/m, which is more than two orders of magnitude greater than that of G-PDMS (1.9 wt%, 1.4 × 10⁻² S/m) from SFG. This value is also four orders of magnitude higher than that of the G-PDMS composite prepared from mechanical mixing of 4 wt% RGO powder made from SFG with PDMS (4.2 × 10⁻⁵ S/m). The though-plane electrical conductivity followed the same trend for SFG and LFG. This reveals that the interconnected graphene foam supplies more efficient paths for electron transfer inside the polymer than conventional graphene powder and the use of large sized graphene sheets can significantly improve the electrical properties of G-PDMS.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

Filtering low-energy carriers to enhance thermoelectric performance of Cd-doped SnSe via incorporation of MXene sheets

SnSe-based materials have attracted widespread attention in thermoelectrics due to their outstanding thermoelectric performance. However, the pristine and unmodified polycrystalline SnSe reveals poor electrical properties. Doping and constructing nanostructured composite architectures to produce energy filtering effect proved to be an effective method to strengthen thermoelectric performance. In this study, Ti₃C₂/Sn_0.98Cd_0.02Se composites are successfully fabricated by the solvothermal method combined with the electrostatic self-assembly method and spark plasma sintering. The phase interface introduced by incorporating Ti₃C₂ into Sn_0.98Cd_0.02Se can effectively filter low-energy carriers due to its generation of energy barriers, thereby the Seebeck coefficient of x wt% Ti₃C₂/Sn_0.98Cd_0.02Se x = (0.05, 0.5, 1) samples is better than that of the pristine Sn_0.98Cd_0.02Se over the whole temperature range. Meanwhile, high conductivity was also obtained in 1 wt% Ti₃C₂/Sn_0.98Cd_0.02Se sample so that the high power factor of 3.31 μWcm⁻¹K⁻² was acquired at 773 K. Ultimately, a peak ZT value of 0.41 was obtained at 773 K, compared with pristine Sn_0.98Cd_0.02Se, and the thermoelectric performance improved by 24%. This study offers an available approach to efficiently enhance the thermoelectric properties of polycrystalline SnSe-based materials.     

Semiconductor‐Insulator Transition in Undoped Rutile,TiO2, Ceramics

The electrical conductivity of undoped rutile ceramics is very dependent on sample processing conditions, especially the temperature and atmosphere during sintering and the subsequent cooling rate. Samples become increasingly semiconducting when quenched from temperatures above ~700°C without the need for a reducing atmosphere. Thus, samples quenched from 1400°C in air have conductivity ~1 × 10^?2 Scm^?1with activation energy ~0.01(1) eV over the temperature range 10–100 K, whereas similar samples that are slow cooled or annealed in air at 300°C–500°C are insulating with activation energy 1.67(2) eV and conductivity, e.g., 1 × 10^?7 Scm^?1 at 400°C. The very wide range of electrical properties is attributed to variations in oxygen content which are too small to be detected using thermogravimetry. Impedance analysis shows that, depending on cooling rate, partially oxidized samples may be prepared in which samples retain a semiconducting core, but have an oxidized outer layer.     

Effect of graphite on the power density of selenium doped polyaniline ink based hybrid screen-printed flexible thermoelectric generator

Enhancement in the thermoelectric performance of inorganic/organic hybrid composites and the need for low-cost, flexible thermoelectric generators have motivated this work. The thermoelectric effect on the addition of amorphous polyaniline, crystalline selenium, and layer-structured graphite, with different concentrations on the thermoelectric properties of selenium-doped polyaniline, is reported. Tuning of microstrain, dislocation density, and carrier concentration has improved the Seebeck coefficient by 39.10% and electrical conductivity by 60.22%. The maximum power output and power factor exhibited by the hybrid device are 1.89 nW and 0.42 nW/m²K² at a temperature difference of 100 °C. Replacing 90 wt% of Selenium-doped polyaniline with graphite resulted in a power density of 0.65 mW/m² under external load conditions.     

Enhanced Thermoelectric Properties of Bi2O2Se Ceramics by Bi Deficiencies

Polycrystalline Bi_2?xO₂Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773 K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change of effective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is ?568.8 μV/K for Bi_1.9O₂Se at 773 K, much larger than ?445.6 μV/K for pristine Bi₂O₂Se. Featured with very low thermal conductivity [~0.6 W·(m·K)^?1] and an optimized electrical conductivity, ZT at 773 K is significantly increased from 0.05 for pristine Bi₂O₂Se to 0.12 for Bi_1.9O₂Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.     

Improved thermoelectric properties of SiC with TiC segregated network structure

A TiC segregated network structure (SNS) approach was utilised to improve the thermoelectric properties of SiC. Different amounts of TiC particles were dry coated on SiC granules to form electrically conductive SNS; then the powder mixtures were spark plasma sintered at 2200°C. The TiC-SNS simultaneously increased the electrical and decreased thermal conductivity of SiC but adversely affected the Seebeck coefficient. By adding 10 vol% TiC, an ≈ 800% increase in electrical conductivity and a ≈ 50% decrease in thermal conductivity were achieved, but the Seebeck coefficient deteriorated due to the metallic nature of the material. A maximum ZT of 5.04 × 10⁻³ was achieved at 923 K, by limiting the Seebeck coefficient's reduction by optimising TiC content to 1.5 vol% while simultaneously increasing the electrical conductivity by ≈ 100% and reducing thermal conductivity by ≈ 40%. This ZT value is almost 90% higher than any value recorded in the literature for SiC.