Thermoelectric materials are a highly promising category of energy conversion materials. In this paper, we present a multitude of approaches to enhance the efficacy of these materials. The review begins with an introduction to the fundamental concept of the thermoelectric figure of merit (ZT), a key parameter for assessing the performance of thermoelectric materials, as well as theories of electrical and thermal transport, which lay the groundwork for understanding and improving the performance of thermoelectric materials. Subsequently, this paper delves into several typical optimization strategies, including the enhancement of material performance through low-dimensionalization and quantum confinement effects, with detailed discussions on two-dimensional, one-dimensional, and zero-dimensional materials. The role of point defect engineering in modulating material properties and the significance of nano-composite materials in enhancing thermoelectric performance are also explored. Band engineering, an effective optimization technique, offers multiple possibilities for enhancing thermoelectric performance through the adjustment of carrier effective mass, utilization of resonance states, band degeneracy, band convergence, and bandgap tuning. Additionally, the application of phonon engineering in reducing thermal conductivity and improving thermoelectric conversion efficiency is highlighted. Discussions on special structures such as textures, single crystals, core–shell structures, and porous structures, as well as symmetry control strategies, highlight the importance of microstructural control in optimizing thermal conductivity. Consequently, the review explores the significance of the synergistic effects of different strategies, noting that an integrated application of these strategies can maximize the performance of thermoelectric materials. The use of materials genomics and machine learning in screening highly potential thermoelectric materials is also highlighted. Finally, the paper addresses the challenges and developments related to the stability, scalability, sustainability, and integration of thermoelectric materials with other systems. Overall, this article summarizes a series of optimization strategies for thermoelectric materials, providing valuable references and inspiration for researchers in the field, with the aim of further advancing the science of thermoelectric materials.

Transition metal nitride (TMN) films with excellent hardness and lubricating performance are versatile low dimension materials, which are widely used in various fields including industries, transportation, aerospace, and so on. This paper introduces one film design strategy and provides a review of the mechanisms for strengthening and lubricating nitride films. The design strategy refers to two aspects which determine the structures, the performance, the components, and the chemical constitutions of nitride films The strengthening mechanisms of nitride films are then illuminated in detail, including the solid solution effect, the grain size effect, the secondary phase effect, the stress or stress field effect, the template effect, and the valence electron concentration effect. Five lubricating mechanisms are next summarized, including the easy-shear nature, the tribo-chemical reactions, the lubricious fluorides, the textured contact surface, and the synergistic effect. This paper aims to give a comprehensive introduction for understanding the mechanisms of strengthening and lubrication of nitride films for students and researchers, as well as to understand the current research progress in nitride films for exploring research gaps.

In this study, reactive magnetron sputtering was applied for preparing NbCN-Ag films with different Ag additions. Ag contents in the as-deposited NbCN-Ag films were achieved by adjusting Ag target power. The composition, microstructure, mechanical properties, and tribological properties were characterized using energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscopy (HRTEM), Raman spectrometry, nano-indentation, and high-temperature sliding wear tests. Results indicated that face-centered cubic (fcc) NbN, hexagonal close-packed (hcp) NbN and fcc Ag, amorphous C and amorphous CN_x phase co-existed in the as-deposited NbCN-Ag films. After doping with 2.0 at.% Ag, the hardness and elastic modulus reached a maximum value of 33 GPa and 340 GPa, respectively. Tribological properties were enhanced by adding Ag in NbCN-Ag films at room temperature. When the test temperature rose from 300 to 500 °C, the addition of Ag was found beneficial for the friction properties, showing a lowest friction coefficient of ~0.35 for NbCN-12.9 at.% Ag films at 500 °C. This was mainly attributed to the existence of AgO_x, NbO_x, and AgNbO_x lubrication phases that acted as solid lubricants to modify the wear mechanism.

Mo₂N films were synthesized using the reactive magnetron sputtering system in a mixture of argon and nitrogen, and the tribological properties were investigated at different testing temperatures against an Al₂O₃ counterpart. The relative intensity ratio (RIR) method was used to calculate the weight fraction of the tribo-film (MoO₃) on the wear tracks of the films. The results showed that the average friction coefficient first increased from 0.30 at 25 °C to 0.53 at 200 °C, and then decreased to 0.29 at 550 °C, while the wear rate decreased from 2.1 × 10⁻⁶ mm³/Nmm at 25 °C to 5.3 × 10⁻⁷ mm³/Nmm at 200 °C and then increased to 3.1 × 10⁻⁵ mm³/Nmm at 550 °C. The weight fraction of tribo-film was mainly attributed to changes in the average friction coefficient and the wear rate. Besides this, the relative humidity also influenced the tribological properties at 25–200 °C.

Composite Nb-V-C-N films with different carbon content were synthesized using reactive magnetron sputtering system, and the influence of carbon on the microstructure, mechanical, and tribological properties of niobium vanadium nitride films was investigated. The films with carbon content below 9.1 at. % exhibited a two-phase of solid solution face-centered cubic (fcc) (NbV)(CN) and hexagonal close-packed (hcp) (NbV)2(CN); increasing the carbon content further induced the formation of amorphous carbon and CNx, and the films consist of fcc-(NbV)(CN), hcp-(NbV)2(CN), amorphous carbon, and CNx. Solid solution strengthening and the heterostructure led to an increase in hardness to ∼32 GPa at 9.1 at. % carbon. A further increase in carbon content led to a gradual decrease in hardness due to the formation of soft amorphous phases of carbon and CNx. The tribological properties of the films against alumina counterpart at room temperature were significantly influenced by the carbon content. The incorporation of carbon into the films decreased the average friction coefficient (μ) from ∼0.61 at 0 at. % carbon to ∼0.25 at 21.6 at. % carbon. As the carbon content increased, the wear rate first dropped from ∼2.6 × 10−8 mm3/(N mm) at 0 at. % carbon to ∼1.2 × 10−8 mm3/(N mm) at 9.1 at. % carbon, and then rose gradually to ∼8.7 × 10−7 mm3/(N mm) at 21.6 at. % carbon.