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Thermoelectric materials have long been proven to be effective in converting heat energy into electricity and vice versa. Since semiconductors have been used in the thermoelectric field, much work has been done to improve their efficiency. The interrelation between their thermoelectric physical parameters (Seebeck coefficient, electrical conductivity, and thermal conductivity) required special tailoring in order to get the maximum improvement in their performance. Various approaches have been reported in the research for developing thermoelectric performance, including doping and alloying, nanostructuring, and nanocompositing. Among different types of thermoelectric materials, layered chalcogenide materials are unique materials with distinctive properties. They have low self-thermal conductivity, and their layered structure allows them to be modified easily to improve their thermoelectric performance. In this review, basic knowledge of thermoelectric concepts and challenges for enhancing the figure of merit is provided. It discusses briefly different groups of layered chalcogenide thermoelectric materials with their structure and thermoelectric properties. It also reports different approaches in the literature for improving their performance and the recent progress done in this field. It highlights graphene as a promising nano additive to layered chalcogenide materials’ matrix and shows its effect on enhancing their figure of merit.

Hot pressing represents a promising consolidation technique for ball-milled bismuth telluride alloys, yet deep investigations are needed to understand its effect on the thermoelectric properties. This paper studies the effect of hot-pressing parameters (temperature and pressure) on the thermoelectric properties of the n-type Gr-Bi2Te2.55Se0.45 nanocomposite. Ultra-high pressure, up to 1.5 GPa, is considered for the first time for consolidating Bi2(Te,Se)3 alloys. Results from this study show that increasing the temperature leads to changes in chemical composition and causes noticeable grain growth. On the contrary, increasing pressure mainly causes improvements in densification. Overall, increments in these two parameters improve the ZT values, with the temperature parameter having a higher influence. The highest ZT of 0.69 at 160 °C was obtained for the sample hot-pressed at 350 °C and 1 GPa for 5 min, which is indeed an excellent and competitive value when compared with results reported for this n-type Bi2Te2.55Se0.45 composition. [ABSTRACT FROM AUTHOR]

Mechanical attrition—the mechanical alloying or milling of powders—is a very versatile and potent method of obtaining nanocrystalline or ultrafine grain structures with enhanced properties. This article presents three examples of enhanced properties obtained by materials in which the grain size has been reduced to the nanoscale or ultrafine scale by ball milling and consolidation of powders. Very high strength/hardness—the highest hardness yet reported for crystalline Mg alloys—for a ball milled Mg97Y2Zn1 alloy is due in part to the nanocrystalline grain structure, along with nanoscale precipitates. A ternary Cu-base alloy with a low stacking fault energy was found to have both high strength and good ductility in a nanocrystalline material synthesized by the in situ ball milling consolidation method. This is another example that shows nanocrystalline materials need not be brittle. It is shown that bulk thermoelectric materials with superior properties can be produced by the ball milling and consolidation of powders to provide an ultrafine grain structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanofiller dimensionality governs the thermoelectric properties of Bi 2 Te 3 composites. • The ZT enhancement of these composites is mostly preeminent to 2D nanofillers. • Superior thermoelectric properties are attained using doped Bi 2 Te 3 matrixes. This review paper summarizes and discusses the effect of nanosized fillers on bismuth telluride nanocomposites for thermoelectric applications. Classified into various dimensions based on electron confinement in space, the nanofiller effect on the figure-of-merit value is studied. By combining experimental data with theoretical models, the mechanisms for enhancing the thermoelectric properties were proposed. The outcomes of this review paper suggest that doped bismuth telluride composites (doped with selenium for n-type or antinomy for p-type) offer better thermoelectric properties compared to undoped composites. Moreover, improvements in undoped bismuth telluride composites are exclusive for the n-type only. The figure-of-merit value for bismuth telluride composites is less than two, where the 2D nanofillers show optimum ZT improvements. Nevertheless, the inconsistency of reported data in the thermoelectric area is a problem that must be addressed to have more control over the precision of the reported results for a better understanding of the concepts in this field. [ABSTRACT FROM AUTHOR]

• Different milling times affected the structural defects in graphene and its distribution in the n-type Bi 2 Te 2.7 Se 0.3 matrix. • Long milling time induced higher levels of structural defects in graphene and reduced the composite's electrical conductivity • Ten minutes of mechanical milling was the optimum time to enhance the thermoelectric properties of the composite. • The improved ZT is attributed to the low level of structural defects and reduced agglomeration of graphene in the composite. This study examines the effects of the structural integrity of graphene on the thermoelectric properties of n-type bismuth telluride alloy. Graphene/Bi 2 Te 2.7 Se 0.3 composites were prepared via mechanical alloying and spark plasma sintering techniques. Different graphene concentrations (0.05 and 0.5 wt%) and addition times (20 hrs, 10 mins, and 1 min) were considered. The thermoelectric properties were measured, and the results showed that the milling time affects graphene structure as well as its agglomeration. It is revealed that the optimum time to add the two-dimensional filler is during the last phase of mechanical milling as it will preserve graphene's structure and boost the electrical conductivity. It is also shown that as the milling time of graphene increases, the Seebeck coefficient improves. Even though an increase in the thermal conductivity is expected due to the high electrical conductivity, a clear reduction in the lattice thermal conductivity part was obtained due to the increased scattering at the new interfaces. The figure-of-merit for the optimum sample with 0.05 wt% graphene added in the last 10 mins of milling had an improvement of 19% at room temperature reaching a value of 0.5, and 25% at 160 °C achieving a final value of 0.81. [ABSTRACT FROM AUTHOR]

• Tin Selenide/Graphene nanoplatelets (SnSe/GNPs) nanocomposite is synthesized by ball milling and hot pressing. • Milling time has a significant effect on enhancing the distribution of graphene within the matrix. • Homogeneous distribution of graphene enable decoupling the Seebeck coefficient and the electrical conductivity. • Homogeneously distributed graphene provides a continuous path for carriers to move, hence enhancing the carrier mobility. • Optimum milling time has a significant effect on enhancing the power factor of SnSe/GNPs nanocomposite. [Display omitted] Tin selenide graphene nanocomposites (SnSe/GNPs) were fabricated with high-energy ball milling and hot pressing by varying the milling time of graphene. The effect of ball milling time on the graphene integrity and the dispersion homogeneity was investigated and the consequential variation in electrical properties of SnSe/GNPs were analyzed. The evolution of graphene sheets during milling as well as the crystal structure of SnSe/GNPs nanocomposites were systematically studied by X-ray diffraction, Raman analysis, scanning electron microscopy, and transmission electron microscopy. It has been proven that graphene was able to keep its crystallinity at short milling times, but it exhibits agglomeration and poor dispersion within the matrix. However, long milling time has a significant effect on increasing the disorders on graphene structure while it provides well dispersion of graphene. The calculated power factor increases with the addition of graphene and with increasing graphene milling time. The increased power factor is attributed to the homogeneous distribution of graphene, which results in a significant increase in electrical conductivity. At 773 K, the lowest power factor value was reported for the 1-min graphene-milled sample, whereas a 40% enhancement was reported for the 2-h graphene-milled sample. Across a wide temperature range (298–720 K), the 12-h graphene-milled sample shows the best performance owing to the simultaneous increase of electrical conductivity and Seebeck coefficient. These findings indicate the positive effect of milling time on the distribution of graphene, which in turn enables graphene to form a continuous net for carriers to move. This study could provide a greater understanding of the control factors of the mechanical milling process for preparing SnSe/GNPs nanocomposites in order to take full advantage of graphene's extraordinary properties by improving its distribution within the tin-selenide- based composite. [ABSTRACT FROM AUTHOR]

Numerous efforts have been made to understand and enhance the thermoelectric properties of bismuth telluride alloys. However, limited studies have been reported to investigate the corrosion behavior of these alloys and their suitability for industrial applications. This paper examines the corrosion behavior of three different bismuth telluride alloys prepared by induction melting: pristine Bi 2 Te 3 , n-type Bi 2 Te 2.55 Se 0.45 , and p-type Bi 0.6 Sb 1.4 Te 3. The electrochemical response of the samples is tested in 3.5 wt% NaCl solution at room temperature. Potentiodynamic polarization curves (Tafel plots) revealed that the n-type sample formed a passive layer compared to the pristine and the p-type samples, which showed pitting corrosion. The different trends observed in the samples were related to their evolved microstructures, where fingerprint-like features in the pristine and the p-type samples showed high susceptibility to pitting corrosion. In contrast, a unique needle-like microstructure of the n-type samples acted as nucleation sites for passive layer formation causing higher corrosion resistance in this sample. • Electrochemical response of undoped and doped Bi 2 Te 3 produced by induction melting was studied in 3.5 wt% NaCl solution. • The electrochemical response significantly depends on the microstructural features of the Bi 2 Te 3 alloys. • Fingerprint-like features in undoped Bi 2 Te 3 and Sb-doped alloys showed high susceptibility to pitting corrosion. • The needle-like microstructure of the Se-doped Bi 2 Te 3 acted as nucleation sites for passive layer formation, causing high corrosion resistance. [ABSTRACT FROM AUTHOR]