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Bi2Te3-based thermoelectric (TE) materials have been demonstrated to be a potential candidate for mainly thermoelectric cooling/refrigeration applications. However, minority charge carriers excitation at high temperature reduces thermopower which restricts these materials for the use in power generation. In present work, substitution of Ni on Sb site in Bi0.5Sb1.5-xNixTe3 (x = 0, 0.01, 0.04 and 0.08) actuates the system to supress the intrinsic excitation leading to shift in highest ZT to higher temperature regime. The Density functional theory (DFT) calculations and experimental results reveal that Ni in Bi0.5Sb1.5Te3 provides the extra holes and slightly reduces the band gap Eg which enhances the σ of Ni-doped Bi0.5Sb1.5-xNixTe3 samples and α at elevated temperature. Moreover, Ni-doping in Bi0.5Sb1.5Te3 also reduces κL which is attributed to the phonon scattering due to mass fluctuations and microstructural features such as grain boundary and strain field domain observed from HRTEM investigation. These favourable condition leads to maximum ZT∼1.38 at 433K for Bi0.5Sb1.46Ni0.04Te3 and ZTavg ∼1.1 between 300K and 503K. Interestingly the calculated theoretical TE conversion device efficiency η of Bi0.5Sb1.46Ni0.04Te3 (η∼5.5%) was achieved to be nearly twice than the efficiency of matrix Bi0.5Sb1.5Te3 (η∼3%). Experimental electronic transport is well corroborated with theoretically estimated DFT results.

Accurate and precise measurement with authentic data dispersion can be considered as a prime tool to realize any technologies at large scale. In the context of thermoelectric technology, a combination of Seebeck coefficient (α), electrical conductivity (σ) and thermal conductivity (κ) are prominent physical parameters that dictate the performance of thermoelectric materials. In this review article, we have stressed the attention on accurate and precise measurement of Seebeck coefficient that includes various sources of errors from contact geometry, sensors, measurement techniques and thermocouple. In addition to this, the solution of minimizing the errors associated in Seebeck measurement has also been elaborated.

The scientific research and new innovations are associated with the accurate and precise measurements which are crucial for operation of recent civilization. In order to maintain the precision and accuracy in analytical measurements, Reference Materials and Certified Reference Materials (CRMs) play a vital role for improving the measurement capability to obtain the property value close to its true value which in turn revamps the overall growth of country for better quality of life. Certified Reference Materials are basically used to assess the performance of analytical procedures and laboratory apparatus for authentic measurement data. In recent scientific world, many sophisticated instruments are emerging with their various facets of measurement capabilities to cater the need of high end research in the field of academia and industries. In recent years, powder X-ray diffraction (PXRD) has been shown as the most powerful characterization and non-destructive technique to explore the structural investigation in terms of phase identification, size of a unit cell, volume fraction of phases, site occupancy of different elemental component, anti-site defects in materials and having many more other applications. The identification of mentioned information from PXRD instrument is a basic need in material science, geology, polymer, forensic science and environment etc. This article focuses on the importance of CRMs for calibrating the instruments and validation of analytical methods. Also the article explicitly discuss about the importance and applications of PXRD technique together with the role of National Physical Laboratory, India the national institute of measurement NMI of India.

We report the growth of Ni-doped Bi0.5Sb1.35Ni0.15Te3 single crystal via the self-flux method. The crystalline nature of a grown single crystal was confirmed by the X-ray diffraction technique (XRD). Interestingly, the XRD pattern shows a sharp reflections of type of planes, revealing the growth of the crystal in c-direction. The grown single crystal was subjected for measurement of field dependence magnetization at 300 K and temperature-dependent magnetic moment. The electronic transport property of bulk single crystal was also carried out in a wide range of temperatures from 150 to 450 K. Reasonably large electrical conductivity σ ~ 1584 S/cm at room temperature was observed which shows ~ 400% enhancement in σ than the electrical conductivity of bare Bi0.5Sb1.5Te3 single crystal (400 S/cm at 300 K). This enhanced electrical conductivity results to significant power factor ~ 1.68 × 10− 3 W/m K2 at 300K which is 163% larger than that of bare Bi0.5Sb1.5Te3 single crystal (6.45 × 10− 4 W/m K2). Magnetic properties of a single crystal of Bi0.5Sb1.35Ni0.15Te3 reveal ferromagnetic behavior at 300 K. The photoluminescence (PL) behavior of Bi0.5Sb1.35Ni0.15Te3 single-crystal was also scrutinized. The PL spectra of Bi0.5Sb1.35Ni0.15Te3 single crystal shows the strong red emission peak in the visible region from 600 to 690 nm upon excitation at 375 nm wavelength, which corresponds to the optical bandgap of 2.1 eV.

Variation in structural and magnetic properties with changing valence electron count (VEC) has been studied well in the family of Heusler compounds, while such changes in VEC resulting in half-Heusler (HH) and full-Heusler (FH) composites have not been reported to observe their effect on the magnetic properties. Herein, we have synthesized the composite of HH and FH phases in Ni1+x MnSb (x = 0.0, 0.3, and 0.6) via changing VEC from 22 to 28 in order to investigate the structural and magnetic properties. Interestingly, a transition from half-metallic ferromagnetic to normal ferromagnetic was revealed in Ni1+x MnSb (x = 0.0, 0.3, and 0.6) materials with increasing VEC. The structural investigations of these materials were performed using a X-ray diffraction technique and analyzed by Rietveld Refinement software for all the samples. Rietveld analysis reveals the presence of a significant amount of the NiSb paramagnetic impurity phase in the HH NiMnSb system while in the case of Ni1+x MnSb (x = 0.3 and 0.6), no such impurity phase was observed. Only FH and HH phases in Ni1+x MnSb (x = 0.3 and 0.6) samples were noticed. The magnetic measurement performed on samples employing a vibrating sample magnetometer reveals the ferromagnetic ordering in all samples. A weak hysteresis loop with saturated magnetic moments ∼2.99 and 2.98 μB at room temperature was observed for NiMnSb and Ni1.3MnSb, respectively, while a strong hysteresis loop with lower magnetic moment of 0.88 μB was observed in the Ni1.6MnSb composite. Furthermore, the observed magnetic moments for the composite Ni1.3MnSb have been explained on the basis of the Slater-Pauling rule in relation to VEC.

Several off-stoichiometric compositions Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) were deliberately synthesized to produce in-situ composites based on compositional engineering approach. The structural characterization of these materials employing XRD, SEM, and HR-TEM reveals the formation of in-situ-composites containing Bi0.5Sb1.5Te3 as matrix phase and minor phases of either Sb rich or Te rich in different compositions. Thermoelectric properties of these Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) composites were studied in a wide range of temperatures extending from room temperature to 500 K. The electronic transport of these composites exhibits p-type semiconducting materials. The lowest thermal conductivity of ~ 0.69 W/m K @310 K was observed for Bi0.5Sb1.7Te3.12 composite, which was noted to be 14% reduced thermal conductivity when compared with that of the state-of-the-art Bi0.5Sb1.5Te3 (κ$$=$$ 0.82 W/m K) material. In addition to this, an enhanced power factor was also observed in Bi0.5Sb1.7Te3.12 which is primarily due to increased electrical conductivity of these materials. This enhanced power factor of the composition of Bi0.5Sb1.7Te3.12 coupled with reduced thermal conductivity yields to high ZT ~ 1.13 at nearly room temperature, making these materials viable for large scale applications.

This work was financially supported by the EPSRC (EP/P510968/1). New nanocomposites have been prepared by combining tin selenide (SnSe) with graphene oxide (GO) in a simple aqueous solution process followed by ice templating (freeze casting). The resulting integration of SnSe within the GO matrix leads to modifications of electrical transport properties and the possibility of influencing the power factor (S2σ). Moreover, these transport properties can then be further improved (S, σ increased) by funtionalisation of the GO surface to form modified nanocomposites (SnSe/GOmod) with enhanced power factors in comparison to unmodified nanocomposites (SnSe/GO) and “bare” SnSe itself. Functionalising the GO by reaction with octadecyltrimethoxysilane (ODTS; C21H46O3Si) and triethylamine (TEA;(CH3CH2)3N) switches SnSe from p-type to n-type conductivity with an appreciable Seebeck coefficient and high electrical conductivity (1257 S·m-1 at 539 K); yielding a 20-fold increase in the power factor compared to SnSe itself, prepared by the same route. These findings present new possibilities to design inexpensive and porous nanocomposites based on metal chalcogenides and functionalized carbon-derived matrices. Postprint

Nanostructuring approach on TiNiSn-based half-Heusler (HH) thermoelectric materials (TE) has been well established as the most prominent paradigm for achieving high figure of merit (ZT). Herein, we have extended this approach on our previously reported bulk nanocomposite (BNC), containing HH and Full Heusler (FH) with little traces of Ti6Sn5 phase in a stoichiometric composition Ti9Ni7Sn8 for the optimization of high thermoelectric performance. A synergistic effect of nanostructuring of Ti9Ni7Sn8 bulk nanocomposite (BNC) on its thermoelectric properties was noticed, revealing an enhanced value of ZT ~ 0.83 at 773 K. This enhancement in ZT value is mainly ascribed to significant reduction in thermal conductivity (κ ~ 1.0 W/mK at 773 K), through modification in grain as well as phase boundary scattering. The marginal enhancement in Seebeck coefficient observed is attributed to charge carrier filtering effect at the interface of HH/FH phases.

Bharatiya Nirdeshak Dravya (BND®) is the trademark of Certified Reference Material (CRM) of CSIR-NPL. BNDs are the primary standards, which ensure reliability and comparability of the measurements as a benchmark for the quality assurance achieved through international networking. Networking between the NMIs through bilateral cooperation, participation with international bodies/organization establishes wide-ranging recognition of BND in India. BND is the heart of India’s scientific and industrial development by establishing quality control in measurements. A detailed discussion to overcome barriers in international trade and standardization of measurement with high metrological aspect has been emphasized. BNDs are produced by CSIR-NPL: (i) in-house and (ii) in association with Reference Material Producers (RMPs) across the country as per ISO 17034 standard. A detailed emphasis on production of BNDs are given in Chap. 19. The BNDs produced by CSIR-NPL in different sectors are uploaded in the international COMAR database, i.e. COde d'Indexation des Materiaux de Reference (COMAR) for its global visibility. To enhance the impact of reference materials in the global economy, Bureau of Indian Standard (BIS) has recently established national mirror committee on reference materials which is a part of ISO-REMCO (Committee on reference materials). The main aim of this chapter is to make the readers acquainted with process and guidelines who are working in reference standard and looking for certification of their products. This chapter describes the terminology/definition and international networking. The metrological traceability of BNDs established are linked to SI units which have been elaborated in detail. Further, the metrological perspective, such as testing, calibration, method of validation, repeatability, reproducibility studies, control chart preparation, inter-laboratory comparison, quality control and quality assurance are also discussed for a broader readership. A detailed emphasis has been given on international key comparison, proficiency/round-robin testing, pilot study in various sectors like physico-mechanical, physico-chemical, foods/feedstuffs, biomedical, environmental, health-care, agricultural etc. in this chapter.

We have successfully synthesized Bi0.5Sb1.7Te3+x (x = 0, 0.12, 0.14) composite thermoelectric materials via solid state reaction followed by Spark Plasma Sintering (SPS). The structural characterization of these materials carried out by X-ray diffraction reveals to be composite phase consisting of Bi2Te3-type phase with small amount of Te and Sb phase. The micro-hardness performed on samples Bi0.5Sb1.7Te3+x(x =0, 0.12, 0.14) reveals the reduction of VHN with increasing Te concentration as compared to that of single phase state –of-the-art Bi0.5Sb1.5Te3 thermoelectric materials.

The thermoelectric and mechanical properties of in-situ developed half-Heusler and full Heusler composites in ZrNi1+xSn have been investigated. The structural analysis investigated by X-ray diffraction (XRD) reveals the materials a nanocomposite of half-Heusler (HH) as matrix phase and full-Heusler (FH) as inclusion. These nanocomposites exhibit low Seebeck coefficient and reasonably good electrical conductivity compared to the bare ZrNiSn half-Heusler. Interestingly, drastic reduction in thermal conductivity in these composites were noted which can be ascribed to distinct microstructures resulting to an effective phonon scattering from several grain boundaries and interface boundaries of HH and FH phase. Further, Hardness of these nanocomposites was measured using micro-hardness testing machine at an indentation load of 300g on different compositions.

Investigation of intermediate compounds of Heusler alloys has been the prime focus to study the various magnetic properties. We have successfully synthesized non-stoichiometric full- Heusler intermediate compounds of Ni2-xMn1+x Sb (0.25≤x≤0.75) to investigate their magnetic properties. XRD analysis using Rietveld refinement reveals the presence of a single-phase of Cu 2 MnAl prototype full-Heusler phase. Transmission electron microscopy analysis also confirms the presence of a single phase of best optimized composition (Ni 1.5Mn1.5Sb) of full-Heusler alloy. Interestingly, the saturated magnetic moments obtained from the M-H curves were found to be 4.45 μ B/f.u., 3.16 μ B/f.u. and 4.65 μ B/f.u. for Ni2-xMn1+xSb (x= 0.25, 0.5, 0.75) respectively which is the highest reported value (~4.65 μ B/f.u. for Ni1.25Mn1.75Sb) so far in this family of the compounds. Further, the reversibility of zero-field cooled (ZFC) and field cooled (FC) curves indicate that samples are stabilized in a magnetically homogeneous state. The first principle calculation performed on these materials is well corroborated with the experimental results.

The current study reports a facile, single step microwave assisted synthesis of nitrogen, oxygen-doped carbon quantum dots (NCQDs) with high quantum yield (~36%). The synthesized NCQDs with an average size of 4 nm demonstrate many fold increase in the intensity of photoluminescence (PL) over the undoped carbon quantum dots (CQDs). UV–Vis absorption peak exhibits a red shift of 26 nm for NCQDs with respect to pristine CQDs. The spectral shift was further confirmed by density functional theory based calculations. X-ray photoelectron spectroscopy measurements reveal the graphitic nature and significantly high nitrogen doping of NCQDs (38.2%). The NCQDs exhibited excitation dependent PL behaviour in the visible region with maximum emission peak recorded at 415 nm with optimum excitation wavelength of 340 nm. The remarkably enhanced PL properties of NCQDs have been employed as fluorescent probe for sensitive and selective sensing of Cu2+ ions. The Cu2+ quenches the fluorescence intensity of NCQDs due to its high binding affinity towards N and O containing functional groups in quantum dots. The NCQDs render a simple, reliable and sensitive detection of Cu2+ ions with limit of detection as low as 1.8 µM.

Doping strategy has been shown as an effective paradigm in most of the thermoelectric materials for optimization of high ZT. Herein, we have synthesized several compositions of Te doped p-type ZrCoSb0.9-xSn0.1Tex (0 ≤ × ≤ 0.08) Half-Heusler (HH) alloys employing arc melting followed by spark plasma sintering (SPS). X-ray Diffraction (XRD) analysis reveals the formation of single phase of MgAgAs type HH phase. Thermoelectric properties were measured from room temperature to 773 K. Interestingly, Te doping on Sb site in ZrCoSb0.9-xSn0.1Tex leads to decrease of lattice thermal conductivity and a minimum lattice thermal conductivity~ 1.98 WK−1m−1 at 773 K for composition ZrCoSb0.9-xSn0.1Tex (x = 0.08) was optimized. The reduction in thermal conductivity is attributed to scattering of phonons by mass fluctuation and grain boundaries. In addition to this, significant improvement in power factor (~16.89 μWK−2cm−1at 773K) for ZrCoSb0.9-xSn0.1Tex (x = 0.06) was observed to be optimized. The enhancement in power factor is mainly attributed due to increased Seebeck coefficient manifested by reduction in effective hole carrier density occurred by Te substitution on Sb site. Thus, combination of reduced thermal conductivity and enhanced power factor obtained via Te substitution at Sb site leads to a high ZT~ 0.53 for ZrCoSb0.9-xSn0.1Tex (x = 0.06). The present ZT~ 0.53 optimized for ZrCoSb0.9-xSn0.1Tex (x = 0.06) is 150% larger than that of un-doped ZrCoSb0.9Sn0.1 compound.

Manganese (Mn) based full-Heusler alloys have been shown to be the most promising materials due to their excellent magnetic behaviors for applications in magneto resistance, magnetic tunnel devices, spin valve and spin injection devices. Here, we report the study of crystal structure, electronic structure and magnetic properties of Mn2NiSb full-Heusler alloy. XRD investigation of Mn2NiSb followed by Rietveld refinement reveals the material to be successfully crystallized in a well-ordered Cu2MnAl prototype structure manifested by the existence of super-lattice diffraction peaks of (1 1 1) and (2 2 0). The experimental lattice parameter is estimated to be 6.01 A which is larger than the available reports. Magnetic measurement (M-H) performed by applying external magnetic field up to 1.8 Tesla reveals the saturated magnetic moment (MS) of 4.07μB/formula unit and 2.50μB/formula unit at temperature 5 K and 300 K respectively which is the highest value reported so far at 5 K for Mn2NiSb stabilized in Cu2MnAl type structure. Further, magnetization versus temperature (M-T) curve with zero-field cooled (ZFC) and field-cooled (FC) up to 300 K indicates that Mn2NiSb crystallized in a single phase. However, the square of magnetization and temperature curve was best fitted with linear fitting supporting to Stoner like ferromagnetism. In order to have a better understanding of magnetic behavior, the First principal calculation has also been performed. Interestingly, our experimental observations were found to be well corroborated with the theoretically estimated magnetic behavior.

Several off-stoichiometric compositions Bi0.5Sb1.5Te3+x (x = 0, 0.01, 0.015, 0.02) with varying Te concentrations were synthesized via melting the elemental components of stoichiometric compositions in a furnace operating at temperature 800 °C to study the role of excess Te on enhancing the thermoelectric properties. The structural characterization performed by XRD and TEM reveals the formation of a matrix phase of Bi0.5Sb1.5Te3and a minor phase of Te. A systematic investigation of electronic and thermal transport behaviour of Bi0.5Sb1.5Te3+x (x = 0, 0.01, 0.015, 0.02) were performed in wide temperature range. Interestingly, enhanced power factor (~22.6 μW/cm K2 at 486K) was optimized for composition Bi0.5Sb1.5Te3.010which is18% enhanced power factor(PF) than that of pristine Bi0.5Sb1.5Te3 material (PF~19.1 μW/cm K2 at 486K). In addition to this, a drastic reduction in thermal conductivity (1.01 W/m K at 486K) was also observed which is 11% reduction when compared to that of bare state-of-art Bi0.5Sb1.5Te3 material. This reduction in thermal conductivity is explained in terms of the scattering of phonons due to mass fluctuation, strain field domains, grain boundaries, and multiple interfaces. An enhanced ZT~1.08 at 486 K for the composition Bi0.5Sb1.5Te3.010 was optimized which is primarily due to enhanced power factor and reduced thermal conductivity.

Zintl compounds are potential candidates for efficient thermoelectric materials, because typically they are small band gap semiconductors. In addition, such compounds allow fine tuning of the carrier concentration by chemical doping for the optimization of thermoelectric performance. Herein, such tunability is demonstrated in Mg3Sb2-based Zintl compounds via Zn(2+) doping at the Mg(2+) site of the anionic framework (Mg2Sb2)(2-), in the series Mg3-xZnxSb2 (0 ≤ x ≤ 0.1). The materials have been successfully synthesized using the spark plasma sintering (SPS) technique. X-ray diffraction (XRD) analysis confirms a single solid solution phase of Mg3-xZnxSb2 (0 ≤ x ≤ 0.1). The thermoelectric properties are characterized by the Seebeck coefficient, electrical conductivity, and thermal conductivity measurements from 323 K to 773 K. Isoelectronic Zn substitution at the Mg site presents the controlled variation in the carrier concentration for optimizing the high power factor and reduced thermal conductivity. These results lead to a substantial increase in ZT of 0.37 at 773 K for a composition with x = 0.10 which is ∼42% higher than undoped Mg3Sb2. The electronic transport data for the Mg3-xZnxSb2 (0 ≤ x ≤ 0.1) compound are analyzed using a single parabolic band model predicting that Mg2.9Zn0.1Sb2 exhibits a near-optimal carrier concentration for high ZT. The electronic structure of transport properties of these disordered Mg3-xZnxSb2 (0 ≤ x ≤ 0.1) is also studied using density functional theory and the results obtained are in good agreement with experimental results. The low cost, lightness and non-toxicity of the constituent elements make these materials ideal for mid-temperature thermoelectric applications.

The concept of nanocomposite/nanostructuring in thermoelectric materials has been proven to be an effective paradigm for optimizing the high thermoelectric performance primarily by reducing the thermal conductivity. In this work, we have studied the microstructure details of nanocomposites derived by incorporating a semi-metallic Bi nanoparticle phase in Bi2Te3 matrix and its correlation mainly with the reduction in the lattice thermal conductivity. Incorporating Bi inclusion in Bi2Te3 bulk thermoelectric material results in a substantial increase in the power factor and simultaneous reduction in the thermal conductivity. The main focus of this work is the correlation of the microstructure of the composite with the reduction in thermal conductivity. Thermal conductivity of the matrix and nanocomposites was derived from the thermal diffusivity measurements performed from room temperature to 150 °C. Interestingly, significant reduction in total thermal conductivity of the nanocomposite was achieved as compared to that of the matrix. A detailed analysis of high-resolution transmission electron microscope images reveals that this reduction in the thermal conductivity can be ascribed to the enhanced phonon scattering by distinct microstructure features such as interfaces, grain boundaries, edge dislocations with dipoles, and strain field domains.

Several nanostructuring methods have been demonstrated to produce a variety of nanostructured materials, and these methods are well recognized as effective paradigms for improving the performance of thermoelectric materials. Among the variety of nanostructured materials, bulk nanostructured materials have been shown to be the most promising because they not only have high ZT, but they can also be fabricated in large quantities, unlike many other nanostructured materials, making them desirable for large scale industrial application. In this study, the nanostructuring paradigm is extended for the first time to the bulk Mg3Sb2 and Mg3Sb1.8Bi0.2 Zintl phase compounds, which despite the advantages of price and abundance, so far have been disregarded for thermoelectric research due to low ZT relative to the available state-of-the-art thermoelectric materials. The nanostructuring of bulk Mg3Sb2 and Mg3Sb1.8Bi0.2, employing high energy ball milling followed by spark plasma sintering yields a ZT of ∼0.4 and ∼0.94 at 773 K, which are 54% and 56% higher values, respectively, than their respective bulk counterparts. The enhancement in the ZT of these materials is primarily due to the significant reduction in thermal conductivity caused by phonon scattering at numerous grain boundaries of nanostructured materials. The observed decrease in the thermal conductivity of these bulk nanostructured materials is quantified using a simple model that combines the macroscopic effective medium approach (EMA) with the concepts of Kapitza resistance. The microstructural investigation of these nanostructured materials was carried out employing high resolution transmission electron microscopy (HRTEM).

All scale hierarchical architecturing, matrix/inclusion band alignment and intra-matrix electronic structure engineering, the so called panoscopic approach for thermoelectric materials has been demonstrated to be an effective paradigm for optimizing high ZT. To achieve such hierarchically organized microstructures, composition engineering has been considered to be an efficient strategy. In this work, such a panoscopic concept has been extended to demonstrate for the first time in the case of half-Heusler based thermoelectric materials via a composition engineering route. A series of new off-stoichiometric n-type Zr0.7Hf0.3Ni1+xSn (0 ≤x≤ 0.10) HH compositions have been modified to derive HH(1 -x)/full-Heusler (FH)(x) composite with an all scale hierarchically modified microstructure with FH inclusions within the matrix to study the temperature dependent thermoelectric properties. The structural analysis employing XRD, FE-SEM and HR-TEM of these materials reveal a composite of HH and FH, with hierarchically organized microstructures. In such a submicron/nano-composite, the electronic properties are observed to be well optimized yielding a large power factor; α(2)σ (∼30.7 × 10(-4) W m(-1) K(-2) for Zr0.7Hf0.3Ni1.03Sn) and reduced thermal conductivity (∼2.4 W m(-1) K(-1) for Zr0.7Hf0.3Ni1.03Sn) yielding a high ZT∼ 0.96 at 773 K for composition Zr0.7Hf0.3Ni1.03Sn which is ∼250% larger than the normal HH Zr0.7Hf0.3NiSn (ZT∼ 0.27 at 773 K). The enhancement in ZT of these composites has been discussed in terms of primary electron filtering, electron injection and several phonon scattering mechanisms such as alloy scattering, point defect scattering, and grain boundary scattering. The Bergman and Fel model is used to calculate effective thermoelectric parameters of these composites for comparing the experimental results.

The concept of nanocomposites derived by incorporating a second minor phase in bulk thermoelectric materials has established itself as an effective paradigm for optimizing high thermoelectric performance. In this work, this paradigm is for the first time extended to bulk Zintl phase Mg3Sb2 and its isoelectronically Bi-doped derivative Mg3Sb1.8Bi0.2 system. Herein, we report the synthesis, microstructural details, electronic structure and thermoelectric properties of (Mg3Sb2, Mg3Sb1.8Bi0.2)/graphene nanosheet (GNS) nanocomposites with different mass ratios. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) investigation reveals that Mg3Sb2 nanoparticles are homogenously anchored on the surface of GNS. We demonstrate that Mg3Sb2-based materials incorporated with a small content of graphene outperform optimally, resulting in potential p-type thermoelectric materials. The present nanocomposite additive of GNS deriving such a novel nanocomposite of (Mg3Sb2, Mg3Sb1.8Bi0.2)/GNS, enhances the electrical conductivity significantly, thereby resulting in a substantially large increase in the power factor. The enhanced electrical conductivity of these nanocomposites is attributed to the increase in the carrier concentration and high carrier mobility owing to the ultra high mobility of graphene. X-ray photoelectron spectroscopy (XPS) core level spectra confirm weak bonding between GNS and Mg3Sb2. Increase in carrier concentration is reflected in XPS valence band spectra and change in spectral weight near valence band maxima is indicative of increased electrical conductivity in the nanocomposite material. The thermal conductivity of these nanocomposites is noted to be reduced at high temperature. These favorable conditions lead to enhanced thermoelectric figure-of-merit (ZT) ≈ 0.71 at 773 K for Mg3Sb2/GNS and a ZT ≈ 1.35 at 773 K for Mg3Sb1.8Bi0.2/GNS nanocomposites with the mass ratio of 80 : 1 which are ∼170% and ∼125% higher values compared to bare Mg3Sb2 and bare Mg3Sb1.8Bi0.2 respectively. We strongly believe that the present novel strategy of fabricating such a nanocomposite of a Zintl compound by utilizing GNS as a nanocomposite additive, may provide an emerging path for improving thermoelectric properties of various Zintl phase compounds.

Ti-based bulk metallic glass (BMGs) and their bimodal composites are linked with the pronounced strain hardening after yielding but with much low value of strength. Therefore, developing Ti-based alloys with high yield strength and high plasticity is the current challenge. Here, we report the synthesis of ultra-fine grained bulk (UFG) (Ti 0.705 Fe 0.295 ) 100− x Ga x (0 ⩽ x ⩽ 2) bimodal eutectic composites with not only high strength and larger plasticity but also with high yield strength which is one of the important mechanical property for structural application. Reasonably high strength, high yield strength, strain to failure ratio, and enhanced plasticity of ∼7 ± 0.8% was observed in (Ti 70.5 Fe 29.5 ) 98 Ga 2 composite which is superior than Ti-based BMGs and bimodal composites. Modification of degree of eutectic structure refinement and volume fraction of constituent phases with the addition of Ga are the crucial factors in enhancing the mechanical properties of Ti–Fi–(Ga) composites.

Varying the valence electron concentration per unit cell (VEC) in a half-Heusler (HH) material gives a large number of structures and substructures that can be exploited to improve the thermoelectric performance. Herein, we studied Zr9Ni7Sn8 with VEC = 17.25, which is smaller than 18 for normal ZrNiSn half-Heusler, to explore the structural modifications for improvement of thermoelectric performance. The structural analysis employing XRD, SEM and TEM confirms the resulting material to be a composite of HH and Ni3Sn4-type phases. Rietveld analysis estimates the volume fraction of HH to be 75.6 ± 1.2% and 24.6 ± 0.8% for Ni3Sn4 phase. Interestingly, the present composite results in a substantial increase in electrical conductivity (σ) by ∼75% and a drastic reduction in thermal conductivity (κ) by ∼56%, leading to a thermoelectric figure of merit (ZT) of 0.38 at 773 K, which is ∼85% higher than in normal HH ZrNiSn. Further, the nanostructuring of the composite, achieved by mechanical milling, derives a significantly reduced κ (i.e. from 4.56 W m−1 K−1 to 3.36 W m−1 K−1, at 323 K), yielding a ZT of 0.90 at 773 K, which is >300% enhancement over the normal HH. The experimental results have been compared with the Bergman and Fel model for calculating effective thermoelectric parameters in composites.

Mg3Sb2-based Zintl compounds have recently attracted attention as a potential candidate for thermoelectric applications due to their low thermal conductivity and promising thermoelectric performance (i.e. ZT = 0.6 at 773 K in Mg3Sb2−xBix). We have reported previously that isoelectronic Bi3− substitution of Sb3− leads to a moderate increase in the electrical conductivity, enhanced Seebeck coefficient and reduced thermal conductivity. Herein, we report a large enhancement of the electrical conductivity while maintaining the Seebeck coefficient by substituting Pb4− on Sb3− sites in Mg3Sb2−xPbx (0 ≤ x ≤ 0.3) alloys. Transport measurements reveal that optimum doping of 10 at% Pb4− on Sb3− enhances the ZT to 0.84 at 773 K which is comparable to bismuth telluride and selenide industrial materials which are toxic and expensive. The enhancement in ZT is attributed to a decrease in lattice thermal conductivity and simultaneously an increase in the power factor resulting from the significant increase in the electrical conductivity. We observe that Pb4− substitutions on Sb3− sites in Mg3Sb2−xPbx (0 ≤ x ≤ 0.3) increase the hole carrier concentration. Electronic transport data of Mg3Sb2−xPbx (0 ≤ x ≤ 0.3) alloys have been analyzed using a single parabolic band model and have been compared to Mg3Sb2. The relatively high figure of merit and affordable material ingredients coupled with one step synthesis process makes these materials a promising cost effective solution as thermoelectric materials.

The concept of a composite derived by incorporating a second minor phase in bulk thermoelectric materials has established itself as an effective paradigm for optimizing high thermoelectric performance. In this work, the incorporation of submicron lamellae eutectic phases into cheap, abundant and non-toxic TiNiSn half-Heusler is extended for the first time to optimize its thermoelectric performance. Half-Heusler (HH) TiNiSn/eutectic Ti70.5Fe29.5 composites were fabricated by employing the arc-melting route, followed by the spark plasma sintering (SPS) technique. Incorporating the metallic submicron lamellae eutectic phase of Ti70.5Fe29.5 into the HH TiNiSn matrix results in a substantial increase in the power factor (∼57% higher than TiNiSn HH) and simultaneous reduction (∼25% lower than TiNiSn HH) in the thermal conductivity, leading to an enhanced thermoelectric figure-of-merit (ZT) of 0.41 at 773 K for the half-Heusler (HH) TiNiSn/eutectic Ti70.5Fe29.5 composite with a mass ratio a 33 : 1, which is 105% higher than its counterpart TiNiSn HH. This enhancement in power factor is primarily due to an increase in electrical conductivity, resulting from the inclusion of the metallic Ti70.5Fe29.5 eutectic phase, while the reduction in thermal conductivity can be ascribed to the enhanced phonon scattering by numerous lamellae interfaces of β-Ti and TiFe of the eutectic phase and also their interfaces with the HH phase. The effective value of the thermal conductivity of HH TiNiSn/eutectic Ti70.5Fe29.5 composites, calculated by the effective medium theory in the light of Maxwell–Eucken approximations, matches well with the experimental value of thermal conductivity.

Five bulk samples of n-type Zr 0.25 Hf 0.75 NiSn 0.975 Sb 0.025 half-Heusler (HH) alloy were fabricated by reacting elemental powders via (1) high temperature solid state (SS) reaction and (2) mechanical alloying (MA), followed by densification using spark plasma sintering (SPS) and/or hot pressing (HP). A portion of the sample obtained by SS reaction was mechanically alloyed before consolidation by hot pressing (SS–MA–HP). X-ray powder diffraction and transmission electron microscopy studies revealed that all SS specimen (SS–SPS, SS–HP, SS–MA–HP) are single phase HH alloys, whereas the MA sample (MA–SPS) contains metallic nanoprecipitates. Electronic and thermal transport measurements showed that the embedded nanoprecipitates induce a drastic increase in the carrier concentration ( n ), a large decrease in the Seebeck coefficient (S) and a marginal decrease in the lattice thermal conductivity ( κ l ) of the MA–SPS sample leading to lower ZT values when compared to the SS–HP samples. Constant values of S are observed for the SS series regardless of the processing method. However, a strong dependence of the carrier mobility ( μ ), electrical conductivity ( σ ) and κ l on the processing and consolidation method is observed. For instance, mechanical alloying introduces additional structural defects which enhance electron and phonon scattering leading to moderately low values of μ and large reduction in κ l . This results in a net 20% enhancement in the figure of merit (ZT=0.6 at 775 K). HH specimen of the same nominal composition with higher ZT is anticipated from a combination of SS reaction, MA and SPS (SS–MA–SPS).

In the present study, we report the enhancement in figure-of-merit (ZT) of nanostructured n-type Silicon-Germanium (Si80Ge20) thermoelectric alloy synthesized using high energy ball milling followed by spark plasma sintering (SPS). After 90 h of ball milling of elemental powders of Si, Ge and P (2 at.%), a complete dissolution of Ge in Si matrix has been observed forming the nanostructured n-type Si80Ge20 alloy powder. X-ray diffraction analysis (XRD) confirmed the crystallite size of the host matrix (Si) to be ∼7 nm and also indicated the formation of an additional phase of SiP nano-precipitates after SPS. HR-TEM analysis revealed that the nano-grained network was retained post-sintering with a crystallite size of size of 9 nm and also confirmed the SiP precipitates formation with a size of 4 to 6 nm. As a result, a very low thermal conductivity of ∼2.3W/mK at 900°C has been observed for Si80Ge20 alloy primarily due to scattering of phonons by nanostructured grains and nano-scaled SiP precipitates which further contribute to this scattering mechanism. Electrical conductivity values of SiGe sintered alloy are slightly lower to that of reported values in literature. This was attributed to the formation of SiP which creates a compositional difference between the grain boundary region and the grain region, leading to a chemical potential difference at interface and the grain region. Figure-of-merit (ZT) of n-type Si80Ge20 nanostructured alloy was found to be ≈1.5 at 900°C, which is the highest reported so far at this temperature.

Developing thermoelectric materials with high ZT revolves around various processing techniques, such as doping, substitution chemistry, and band structure engineering near the Fermi level due to embedded nanoscale precipitates and/or quantum dots, and more recently introduced compositionally alloyed hierarchically organized microstructures. In this work, we have extended atomic scale structural engineering combined with an overarching panoscopic approach enabling the development of hierarchically organized microstructures at multiple length scales to Hf-free p-type half-Heusler thermoelectric materials for the first time. A series of compositions ZrCo1+xSb0.9Sn0.1 have been synthesized employing a fast processing technique of arc melting followed by conventional hot pressing revealing composites of half-Heusler (HH) and full-Heusler (FH) at multiple length scales. Such microstructural modifications at various length scales lead to controlled tuning of the hole density, effective mass and band engineering. Interestingly, a simultaneous large enhancement of the power factor (72% >) and a reduction in thermal conductivity (30% >) of the resulting ZrCo1+xSb0.9Sn0.1 composites were observed. The enhancement in the power factor was primarily due to the increased Seebeck coefficient which resulted from a reduction in the effective hole carrier density via low energy electron filtering coupled with an increase in the hole carrier's effective mass (m*) due to band off-set minimization. The reduction in thermal conductivity can be ascribed to the enhanced phonon scattering by different frequency heat-carrying phonons due to various grain sizes at multiple length scales, such unique specific design strategies of materials described here result in superior thermoelectric properties which have been compared with several state-of-the-art p-type half Heusler materials. The Bergman-Fel model is used to calculate the effective thermoelectric parameters of these composites for comparing the experimental results.

High purity, nanometer sized surfactant-free nickel oxide (NiO) particles were produced in gram scale using a solution combustion method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), gas pycnometry and gas adsorption analysis (BET). The average particle size of the as-synthesized NiO increases significantly with the preheating temperature of the furnace, while the specific surface area decreases. A BET specific surface area of ∼100 m2/g was obtained for NiO nanoparticles with size as small as 3 nm synthesized at 300 °C. The thermal conductivity (κ) of pressed pellets of the synthesized NiO nanoparticles obtained using spark plasma sintering (SPS) and uniaxial hot pressing is drastically decreased (∼60%) compared to that of NiO single crystal. This strong reduction in κ with particle size suggests the suitability of the synthesized surfactant-free NiO nanoparticles for use as nanoinclusions when designing high performance materials for waste heat recovery.

Bulk Zr0.25Hf075NiSn half-Heusler (HH) nanocomposites containing various mole fractions of full-Heusler (FH) inclusions were prepared by solid state reaction of pre-synthesized HH alloy with elemental Ni at 1073 K. The microstructures of spark plasma sintered specimens of the HH/FH nanocomposites were investigated using transmission electron microscopy and their thermoelectric properties were measured from 300 K to 775 K. The formation of coherent FH inclusions into the HH matrix arises from solid-state Ni diffusion into vacant sites of the HH structure. HH(1–y)/FH(y) composites with mole fraction of FH inclusions below the percolation threshold, y∼0.2, show increased electrical conductivity, reduced Seebeck coefficient and increased total thermal conductivity arising from gradual increase in the carrier concentration for composites. A drastic reduction (∼55%) in κl was observed for the composite with y=0.6 and is attributed to enhanced phonon scattering due to mass fluctuations between FH and HH, and high density of HH/FH interfaces.

Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demonstrated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these improvements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and electrical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the "effective" carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T(-α) law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 - x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 - x)/FH(x) nanocomposites.

The effect of Bi (semimetal) nanoinclusions in nanostructured Bi 2 Te 3 matrices is investigated. Bismuth nanoparticles synthesized by a low temperature solvothermal method are incorporated into Bi 2 Te 3 matrix phases, synthesized by planetary ball milling. High density pellets of the Bi nanoparticle/Bi 2 Te 3 nanocomposites are created by hot pressing the powders at 200 ° C and 100 MPa. The effect of different volume fractions (0–7%) of Bi semimetal nanoparticles on the Seebeck coeffi cient, electrical conductivity, thermal conductivity and carrier concentration is reported. Our results show that the incorporation of semimetal nanoparticles results in a reduction in the lattice thermal conductivity in all the samples. A signifi cant enhancement in power factor is observed for Bi nanoparticle volume fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal conductivity and increase the power factor resulting in an increase in fiof merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coeffi cient and electrical conductivity as a function of carrier concentration data are consistent with the electron fi ltering effect, where lowenergy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.

Bismuth–antimony alloy nanoparticles have been synthesized by a facile solvothermal method using N,N-dimethylformamide and ethylene glycol as solvent/reducing agent; BiCl 3 , SbCl 3 and Bi(NO 3 ) 3 as precursors; and citric acid as a surface modifier/stabilizing agent. The particle size and size distribution of Bi nanoparticles were analyzed as a function of the synthesis conditions: molar ratio of precursor to surfactant, precursor concentration and reducing agent. Synthesis of Sb and Bi 0.88 Sb 0.12 under similar conditions was also investigated. The phase purity of nanoparticles was confirmed from X-ray diffraction and thermogravimetry and the nanoparticle morphology was investigated by transmission electron microscopy. A case study of Bi nanoparticles with detailed analysis of the particle morphology and size distribution of the nanoparticles is reported.

A series of samples with the composition Zr0.5Hf0.5Co1−x Ir x Sb0.99Sn0.01 (x = 0.0 to 0.7) were synthesized by high-temperature solid-state reaction at 1173 K. High-density pellets of the powders were obtained using hot press (HP) and spark plasma sintering (SPS) techniques. The thermoelectric properties of the pellets were measured from 300 K to 750 K. Independently of the pressing conditions, all Ir-containing samples (x > 0) showed p-type semiconducting behavior. At 300 K, the electrical conductivity and thermopower of Zr0.5Hf0.5Co1−x Ir x Sb0.99Sn0.01 materials surprisingly increased with increasing Ir concentration. The largest electrical conductivity and thermopower values of 150 S/cm and 140 μV/K, respectively, were observed at 300 K for x = 0.7. The thermal conductivity of the synthesized materials decreased with increasing Ir content, went through a minimum value (x = 0.3), and increased thereafter with further addition of Ir. Pellets fabricated by SPS showed smaller thermal conductivity than pellets of the same composition obtained from uniaxial hot pressing. A thermal conductivity value of ∼2.0 W/m K was observed at 300 K for an SPS pellet with the com- position Zr0.5Hf0.5Co0.5Ir0.5Sb0.99Sn0.01. The thermal conductivity of Zr0.5Hf0.5- Co1−x Ir x Sb0.99Sn0.01 decreased with rising temperature, and the smallest value of ∼1.5 W/m K was observed at 750 K for the SPS specimen with x = 0.5.

We show that Rh substitution at the Co site in Zr0.5Hf0.5Co1−xRhxSb0.99Sn0.01 (0≤x≤1) half-Heusler alloys strongly reduces the thermal conductivity with a simultaneous, significant improvement of the power factor of the materials. Thermoelectric properties of hot-pressed pellets of several compositions with various Rh concentrations were investigated in the temperature range from 300 to 775 K. The Rh “free” composition shows n-type conduction, while Rh substitution at the Co site drives the system to p-type semiconducting behavior. The lattice thermal conductivity of Zr0.5Hf0.5Co1−xRhxSb0.99Sn0.01 alloys rapidly decreased with increasing Rh concentration and lattice thermal conductivity as low as 3.7 W/m*K was obtained at 300 K for Zr0.5Hf0.5RhSb0.99Sn0.01. The drastic reduction of the lattice thermal conductivity is attributed to mass fluctuation induced by the Rh substitution at the Co site, as well as enhanced phonon scattering at grain boundaries due to the small grain size of the synthesized materials.

Bulk Ti–Fe–(In, Nb) composites with high strength (∼2350 ± 50 MPa) and reasonably good ductility (plastic strain ∼4.5 ± 0.5%) have been produced in the form of 2 mm as-cast rods. Analysis of the microstructure by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy indicates that it consisted of β-Ti dendrites as a primary phase and Ti (Fe, In and/or Nb) (B2) + β-Ti (A2) as eutectic phase. Modification of atomic structure, degree of eutectic structure refinement and volume fraction of the constituent phases with the addition of In and Nb are the crucial factors in enhancing the mechanical properties of Ti–Fe–(In, Nb) composites.

Bulk-type (Ti0.65Fe0.35)100−xGax (x = 0, 2, 4 at.%) ultrafine composites with high strength and plasticity have been prepared. The composites have a hypereutectic microstructure with primary FeTi phase and (β-Ti + FeTi) eutectic matrix. The composites exhibit high fracture strength of 2290–2766 MPa and large plasticity of 4.8–6.5% under compression. Refinement of eutectic structure and control of relative areal fraction of the constituent phases are the crucial factors in determining the mechanical properties.

Introducing vacancies in oxychalcogenides is an effective paradigm for the improvement of thermoelectric properties by reducing thermal conductivity through phonon scattering as well as by decreasing electrical resistivity via incorporation of holes through vacancies. In this paper, we present thermoelectric properties of Bi1−x CuSeO1−y with y = 0 for x = 0, 0.04, and y = 0.02 for x = 0.04, 0.08, 0.12. X-ray diffraction studies reveal BiCuSeO as the main phase with trace amounts of Cu1.8Se in Bi0.92CuSeO0.98 and Bi0.88CuSeO0.98. The impurity phases of Cu1.8Se in Bi0.92CuSeO0.98 and Bi0.88CuSeO0.98 could be due to the presence of vacancies. The electrical resistivity of Bi0.96CuSeO0.98 is lower than BiCuSeO, but higher than Bi0.96CuSeO, since Bi vacancies produce holes that are partially compensated by O vacancies. Electrical resistivity decreases with an increase in Bi vacancy content for y = 0.02. The Seebeck coefficient of samples shows that the similar trend as in electrical resistivity, following Mott's formula. Total and lattice thermal conductivity of Bi0.96CuSeO is higher than BiCuSeO as well as Bi0.96CuSeO0.98. This increases with further increase in bismuth vacancy. Introduction of vacancies (Bi and O) in BiCuOSe lead to higher thermal conductivities and lower Seebeck coefficients, and result in adverse effect on zT.

Nanoindentation experiments of a Ti45Zr16Be20Cu10Ni9 bulk metallic glass and partially vitrified nano-composite metallic glass matrix have been performed under a constant maximum load of 10 mN and constant loading rate of 0.08 mN s−1 with the aim of comparative study of their micro-plastic deformation behavior. Remarkable difference in deformation behavior was found in load–displacement curves of nanoindentation and pile-up morphologies around the indents. The difference in shear banding behavior has been attributed to the presence of nanosized icosahedral particles in amorphous matrix.

The effect of large concentration of oxygen on the microstructural development of Zr–Cu–Al–Ni bulk metallic glass (BMG) alloys, prepared from commercially available Zr sponge, has been studied. Apart from promoting crystallization, increased concentration of oxygen (\sim 8000 ppm) spawns additional phases. In particular, we report the appearance of \alpha-Zr, dendritic $Zr_2Cu$ phase and the $Zr_2Ni$ type cubic phases. Addition of oxygen scavenger like Yttrium only partially solves the problem. Phase evolution was also found to be sensitive to the cooling rate and hence to the thickness of the cast sample. Thus it is possible to produce a gradient microstructure with predominantly amorphous phase at the outer layer.

A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium (In) alone and co-doped with both indium and iodine (I) has been done. X-ray diffraction results showed all the samples to be of single phase. Scanning electron microscopy (SEM) results revealed the particle sizes to be in the range of micrometers, while high resolution transmission electron microscopy was used to investigate distinct microstructural features such as interfaces, grain boundaries, and strain field domains. Hall measurement at 300K revealed the carrier concentration similar to 10(19) cm(-3) showing the degenerate nature which was further seen in the electrical resistivity of samples, which increased with rising temperature. Seebeck coefficient indicated that all samples were n-type semiconductors with electrons as the majority carriers throughout the temperature range. A maximum power factor similar to 25 mu W cm(-1) K-2 for all In doped samples and Pb0.998In0.003Te1.000I0.003 was observed at 700 K. Doping leads to a reduction in the total thermal conductivity due to enhanced phonon scattering by mass fluctuations and distinct microstructure features such as interfaces, grain boundaries, and strain field domains. The highest zT of 1.12 at 773K for In doped samples and a zT of 1.1 at 770K for In and I co-doped samples were obtained. Published by AIP Publishing.

We report the use of Boltzmann transport theory to investigate the electrical properties of thermoelectric Mg${}_{2}$Si, Mg${}_{2}$Sn, and a supercell model of the 50-50 alloy. The results are based on first-principles electronic structure calculations with the modified Becke-Johnson potential of Tran and Blaha, which yields band gaps in good accord with experiment. The calculated transport coefficients are discussed in relation to the thermoelectric performance of these materials. The results imply roughly symmetric behavior with respect to carrier type and the possibility of improvements in $ZT$, especially for $p$-type and lower temperatures.

Metal nanoinclusions inside the bulk thermoelectric matrix have the potential to increase the power factor and reduce the lattice thermal conductivity. We have synthesized Bi2-xTe3+x (x=0, 0.05)compositions, to achieve better tenability in Seebeck and electrical conductivity. In this matrix phase, different volume fractions of Bi metal nanoinclusions were incorporated and its effect on thermoelectric properties is discussed. Ag metal nanoinclusions were incorporated into Bi2Te3(2:3) composition, and its effect on power factor is discussed here.

The formation of nanostructures within the matrix of half-Heusler thermoelectric materials can be produced by spinodal decomposition of off-stoichiometric compositions. CoSb is insoluble at high temperatures in Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler phases. This phase can be solubilized into the half-Heusler matrix by the use of high energy ball milling at room temperature as the synthetic method of choice. The metastable half-Heusler material decomposes in-situ while hot-pressing the powder sample into a compact pellet. Despite the fact that the thermal conductivity of the inclusion material, CoSb, is very large, (>35W/m•K), we observed reduction in the lattice thermal conductivity of the composite material. Furthermore, the electrical resistivity of the specimen was also reduced due to the metallic nature of the CoSb inclusion phase. Addition of a large fraction of the metallic inclusion leads to a percolation network of the metallic phase, thus reducing the Seebeck coefficient of the composites. Electron microscopy is carried out in order to examine boundaries between the two. Changes in the thermoelectric properties of Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler matrix with increasing mass percent of CoSb inclusion will be discussed.

Monodispersed nickel oxide nanoparticles have been synthesized using solution combustion synthesis method. Size of the nanoparticles was controlled by varying different reaction parameters such as reaction temperature and reaction time. Structure and morphology of the nanoparticles were investigated using X-ray diffraction and transmission electron microscopy. BET surface area of 99.7 m2/g was obtained for the nanoparticles synthesized at 300 °C. A decrease in surface area was observed with increase in reaction temperature. The nanoparticles were compacted using spark plasma sintering technique at 950 °C and thermal conductivity was studied on compacted sample. Significant decrease in thermal conductivity was observed for nanoparticles in compared to their bulk counter-part.

Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 composites with various concentrations of WO3 inclusions were synthesized by mechanical alloying using high energy shaker mill. High density hot pressed pellets of the synthesized materials were characterized using powder X-ray diffraction and transmission electron microscopy and their thermoelectric properties were investigated in the temperature range from 300 to 750 K. The electrical conductivity of the composites at 300 K decreases from 2500 S/cm for 0 wt.% WO3 alloy to 2200 S/cm for the composite with 2 wt.% WO3 inclusion. The electrical conductivity of composites containing 5 wt.% and 10 wt.% WO3 inclusions showed sharp increases with increasing WO3 content. The electrical conductivity of the composites monotonically decreases with rising temperature. All samples showed n-type semiconducting behavior and the thermopower values decrease with increasing WO3 content. The lattice thermal conductivity of the composites increases with increasing WO3 content. However, these values are about 30% lower than that of Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 alloy prepared by high temperature solid-state techniques. The synthesized composites showed lower figure of merit than the half-Heusler matrix due to large reduction in the thermopower values with increasing WO3 content.