Materials For Practical Devices Research Articles

Progress and prospects of plastic thermoelectric materials

In recent years, significant progress has been made in the research of plastic thermoelectric materials, exemplified by Ag<sub>2</sub>S-based alloys. These materials exhibit excellent room-temperature plasticity due to their low slipping barrier energy and high cleavage energy, with synergistic enhancements in plasticity and thermoelectric properties achievable through alloying and doping strategies. The latest study on Mg<sub>3</sub>Bi<sub>2</sub>-based single crystals demonstrated superior performance in terms of plastic deformation capability and room-temperature thermoelectric properties. Microstructural characterization and theoretical calculation have revealed the crucial role of dislocation glide in the plastic deformation process of Mg<sub>3</sub>Bi<sub>2</sub> single crystals, particularly the low slipping barrier energies observed in multiple slip systems. Importantly, the Te-doped single-crystalline Mg<sub>3</sub>Bi<sub>2</sub> shows a power factor of ~55 μW cm<sup>-1</sup> K<sup>-2</sup> and <i>zT</i> of ~0.65 at room temperature along the <i>ab</i> plane, which outperforms the existing ductile thermoelectric materials. These findings not only deepen the understanding of microscopic deformation mechanisms in plastic thermoelectric materials but also establish an important foundation for optimizing material properties and developing novel flexible thermoelectric devices. Future applications of these materials in practical devices still face challenges in thermal stability, chemical stability, and interfacial contact. Addressing these issues will promote the application of plastic thermoelectric materials in the field of flexible electronics.

Interplay between Thermal Stress and Interface Binding on Fracture of WS2 Monolayer with Triangular Voids.

The defect engineering of two-dimensional (2D) materials has become a pivotal strategy for tuning the electrical and optical properties of the material. However, the reliable application of these atomically thin materials in practical devices require careful control of structural defects to avoid premature failure. Herein, a systematic investigation is presented to delineate the complex interactions among structural defects, the role of thermal mismatch between WS2 monolayer and different substrates, and their consequent effect on the fracture behavior of the monolayer. Detailed microscopic and Raman/PL spectroscopic observations enabled a direct correlation between thermal mismatch stress and crack patterns originating from the corner of faceted voids in the WS2 monolayer. Aberration-corrected STEM-HAADF imaging reveals the tensile strain localization around the faceted void corners. Density functional theory (DFT) simulations on interfacial interaction between the substrate (Silicon and sapphire -Al2O3) and monolayer WS2 revealed a binding energy between WS2 and Si substrate is 20 times higher than that with a sapphire substrate. This increased interfacial interaction in WS2 and substrate-aided thermal mismatch stress arising due to difference in thermal expansion coefficient to a maximum extent leading to fracture in monolayer WS2. Finite element simulations revealed the stress distribution near the void in the WS2 monolayer, where the maximum stress was concentrated at the void tip.

Stable Formamidinium‐Based Centimeter Long Two‐Dimensional Lead Halide Perovskite Single‐Crystal for Long‐Life Optoelectronic Applications

AbstractSolution‐processable 2D metal‐halide perovskites are highly promising for cost‐effective optoelectronic applications due to their intrinsic multiquantum well structure. However, the lack of stability is still a major obstacle in the use of this class of materials in practical devices. Here, the authors demonstrate the stable optoelectronic properties using formamidinium (FA)‐based centimeter‐long 2D perovskite (BA)2FAPb2I7 high‐quality single‐crystal controlled by the thickness of two perovskite layers. The large area single‐crystal exhibits good crystallinity, phase purity, and spectral uniformity. Moreover, the (BA)2FAPb2I7 single‐crystal shows excellent stability at open atmospheric conditions when compared to methylammonium (MA)‐based (BA)2MAPb2I7 counterparts. The photodetectors fabricated using 2D perovskite single‐crystal on the rigid Si/SiO2 substrate reveal high photoresponsivity (Rλ)(≈5 A W−1), the fast response time (<20 ms), specific detectivity (D*) (≈3.5 × 1011 Jones), and excellent durability under 488 nm laser illumination. The Rλ and D* values are obtained from the (BA)2FAPb2I7 single‐crystal 25 times and three orders magnitudes, respectively, higher than the (BA)2MAPb2I7 single‐crystal. Additionally, the perovskite material on flexible polymer substrate reveals good photo‐sensing properties in both bending and nonbending states.

An Energy-Power Balancing Act: Balancing the High Capacity and Rate of LiMn2O4-Carbon Nanofoam Cathodes with Solution-Processable Conducting Polymer Anodes

As the energy and power demands of current and next-generation devices continually strain the capabilities of present power sources, there is a push to develop materials with high capacity and electrode architectures to facilitate delivering that capacity at high rates. En route to addressing this energy/power issue, we have shown that with the appropriate ultraporous electrode architecture, it is possible to extract full theoretical capacity from LiMn2O4, a nominal battery material, in as little as 18 seconds.[i] While representing a significant advancement at the single-electrode level, the implementation of these advanced electrode materials in practical devices has been hampered by the lack of suitable negative electrode materials with sufficient capacity to balance that of the LiMn2O4. A conducting polymer-based negative electrode would be ideal to pair with the LiMn2O4 in terms rate capability, but issues with solution processability have typically limited these materials to thin film-based electrodes, which will not provide adequate capacity for the LiMn2O4. Recently, the Reynolds group at Georgia Tech has developed synthetic protocols to overcome this roadblock for ProDOT-biEDOT-based polymers.[ii],[iii] This advancement makes it possible to incorporate such conducting polymers as thin, conformal coatings on the interior and exterior surfaces of conductive, porous carbon nanofoam-based electrodes. The nanoscale nature of the coating coupled with the intrinsically fast charge-discharge of the polymer supports high rate and the 3D projection of the conducting polymer via the macroscopically-thick carbon nanofoam provides high area-normalized capacitance to balance that of the LiMn2O4. [i]. Sassin, M.B., Long, J.W., Greenbaum, S.G., Stallworth, P.E., Mansour, A.N., Hahn, B.P., and Pettigrew, K.A., “Achieving electrochemical capacitor functionality from a traditional battery material: Conformal, nanoscale LiMn2O4 coatings on 3-D, device-ready carbon nanoarchitectures,” J. Mater. Chem. 1, 2431 (2013). [ii]. Ponder Jr., J. F.; Österholm, A. M.; Reynolds, J. R. “Designing a Soluble PEDOT Analogue without Surfactants or Dispersants,” Macromolecules 49, 2106 (2016). [iii]. Österholm, A. M.; Ponder Jr., J. F.; Kerszulis, J. A.; Reynolds, J. R., “Solution Processed PEDOT Analogues in Electrochemical Supercapacitors,” ACS Appl. Mater. Interfaces 8, 13492 (2016).

An Organolanthanide Building Block Approach to Single-Molecule Magnets.

Single-molecule magnets (SMMs) are highly sought after for their potential application in high-density information storage, spintronics, and quantum computing. SMMs exhibit slow relaxation of the magnetization of purely molecular origin, thus making them excellent candidates towards the aforementioned applications. In recent years, significant focus has been placed on the rare earth elements due to their large intrinsic magnetic anisotropy arising from the near degeneracy of the 4f orbitals. Traditionally, coordination chemistry has been utilized to fabricate lanthanide-based SMMs; however, heteroatomic donor atoms such as oxygen and nitrogen have limited orbital overlap with the shielded 4f orbitals. Thus, control over the anisotropic axis and induction of f-f interactions are limited, meaning that the performance of these systems can only extend so far. To this end, we have placed considerable attention on the development of novel SMMs whose donor atoms are conjugated hydrocarbons, thereby allowing us to perturb the crystal field of lanthanide ions through the use of an electronic π-cloud. This approach allows for fine tuning of the anisotropic axis of the molecule, allowing this method the potential to elicit SMMs capable of reaching much larger values for the two vital performance measurements of an SMM, the energy barrier to spin reversal (Ueff), and the blocking temperature of the magnetization (TB). In this Account, we describe our efforts to exploit the inherent anisotropy of the late 4f elements; namely, Dy(III) and Er(III), through the use of cyclooctatetraenyl (COT) metallocenes. With respect to the Er(III) derivatives, we have seen record breaking success, reaching blocking temperatures as high as 14 K with frozen solution magnetometry. These results represent the first example of such a high TB being observed for a system with only a single spin center, formally known as a single-ion magnet (SIM). Our continued interrelationship between theoretical and experimental chemistry allows us to shed light on the mechanisms and electronic properties that govern the slow relaxation dynamics inherent to this unique set of SMMs, thus providing insight into the role by which both symmetry and crystal field effects contribute to the magnetic properties. As we look to the future success of such materials in practical devices, we must gain an understanding of how the 4f elements communicate magnetically, a subject upon which there is still limited knowledge. As such, we have described our work on coupling mononuclear metallocenes to generate new dinuclear SMMs. Through a building block approach, we have been able to gain access to new double,- triple- and quadruple-decker complexes that possess remarkable properties; exhibiting TB of 12 K and Ueff above 300 K. Our goal is to develop a fundamental platform from which to study 4f coupling, while maintaining and enhancing the strict axiality of the anisotropy of the 4f ions. This Account will present a successful strategy employed in the production of novel and high-performing SMMs, as well as a clear overview of the lessons learned throughout.

Electrochemical Degradation Mechanism of Nano-Sized Sn-C Composite Negative Electrodes for Libs

Lithium-ion batteries (LIBs) are an important power source for both portable devices and electric vehicles. In order for electrical vehicles with LIBs to be embraced more fully, the specific energy of LIBs needs to be improved without sacrificing safety, material availability, and cost. Tin (Sn), as a negative electrode material with various morphologies or nanostructures, is one possible solution because tin not only provides high specific energy (994 mAhg-1) compared to graphite (~374 mAh g-1), but is also inexpensive and abundant [1]. Moreover, tin can be used for the negative electrode of a sodium ion battery, which have recently received much attention as an alternative of LiBs due to much lower cost due to the natural abundance of Na [2]. In spite of the aforementioned advantages of tin as negative electrode materials, tin based electrodes are still difficult to commercialize due to their severe capacity fading during battery cycling. The fade has been associated with the large volume expansion during lithiation, but the mechanism is not yet fully understood. In order to use tin materials in practical devices, the first step is to elucidate the cause of the degradation behavior, and next is to improve the cyclability of tin based electrodes through design of material, coating, and electrolyte additive, etc. Herein, we investigated the degradation behavior of the Sn-C composite negative electrodes, using electrochemical and micro-structural methods.The Sn material used in this study was synthesized by a chemical reducing method [3]. The Sn is spherical-shaped with diameter of 80 ~120 nm, as shown in Fig. a. To fabricate negative electrodes using the prepared nano-sized Sn particles, carbon black as conducting agent and PVdF binder were added and mixed well together in NMP solvent. The ink was applied to a copper foil, dried in a vacuum oven, and calendered. Fig. b shows the XRD pattern of the prepared Sn-C electrode, where Sn peak was mainly detected. Fig.c-d shows the lithiation and delithiation curves and capacity fade of the half cell employing the Sn-C negative electrode. The cell was lithiated and delithiated at 40 mAg-1 (20 mAg-1 for the 1st cycle) between 0.01 and 1.5 VLi/Li+. As shown in Fig. c, the first irreversible capacity loss (ICL) is large (~60%) presumably due to formation of a thick SEI layer, thus reducing the cyclable Li ions and active material during the 1st lithiation. During subsequent cycles (Fig.d), the capacity decreased gradually by about 1.2 % cycle-1 from 2nd to 50thcycles probably due to loss of conductivity and active materials. To reveal more clearly this degradation behavior, we analyze the initial cycle and subsequent cycles separately. Specific methods used in each cycle include: differential capacity (dC/dV) curve, electrochemical impedance spectroscopy (EIS), Focus Ion Beam (FIB)-Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS).

Long-range order of Ni2+ and Mn4+ and ferromagnetism in multiferroic (Bi0.9La0.1)2NiMnO6 thin films

Epitaxial thin films of biferroic (Bi1−xLax)2NiMnO6 have been grown on SrTiO3 (001) substrates. High resolution electron microscopy, energy-loss spectroscopy and synchrotron radiation have been used to demonstrate that, under appropriate growth conditions, stoichiometric, and fully oxidized thin films with long-range order of Ni2+ and Mn4+ ions can be obtained, despite the presence of randomly distributed dissimilar cations (Bi, La) at the A-site. This ordering leads to Ni2+–O–Mn4+ ferromagnetic interactions and its preservation in thin films is key for implementation of these biferroic materials in practical devices.

Texture, mechanical and thermoelectric properties of Ca3Co4O9 ceramics

Abstract Ca3Co4O9 thermoelectric (TE) ceramics have been processed using conventional sintering (CS), hot-pressing (HP) and spark plasma sintering (SPS). Microstructure investigations have revealed that HP processing is effective for obtaining highly textured 349 ceramics, with a degree of crystallite orientation showing a texture index of 6 mrd2, and that SPS affords strong densified 349 ceramics, typically 98% of the theoretical X-ray density. Investigations of mechanical and thermal properties were undertaken for a reliable integration of these materials in practical devices. In detail, these properties were assessed by microindentation, nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness of the HP and SPS samples were drastically improved, compared with the CS specimen. The thermal expansions were measured and shown, for the samples processed by HP and SPS, different between the directions parallel and perpendicular to the stress axis. Thermoelectric properties were investigated from 5 to 850 K and a remarkable power factor value of 550 μW m−1 K−2 was obtained at 850 K. Mechanical and TE properties were correlated to the microstructure and texture features.

Mechanical Aging Behavior of Pb(Zn 1/3 Nb 2/3 )O 3 -PbTiO 3 and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 Single Crystals

Single crystals of complex lead perovskite have attained great importance in recent times due to their exceptionally high piezoelectric and electromechanical constants. The application of these materials in practical devices will to a large extent depend on the aging and fatigue characteristics of these crystals. With this aim, the aging characteristics of <001>-oriented Pb(Zn 1/3 Nb 2/3 )O 3 -PbTiO 3 (PZN-PT) and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) single crystals have been studied under electrical and mechanical drive. Under continuous bipolar drive, a significant increment in the magnitude of hysteretic loss and decrement in the piezoelectric constant was observed. Subsequently, a novel experimental setup was fabricated to characterize the mechanical fatigue behavior of the piezo-single crystals. In this method, the crystals are placed between the two magnetostrictive plates and clamped. The cyclic mechanical stress is applied using strong magnetic fields. It was found that the piezoelectric constants decreased slowly at both short and long times but there is a pronounced decrement at intermediate times. This behavior is detrimental in designing reliable devices. The aging behavior of the piezocrystals improved significantly by doping. This improvement is suggested to be due to the restriction of relaxation processes and ordering of the system.

Electrochromism in anodic WO 3 films II: Optical and electrochromic properties of coloured “distorted” hexagonal films

The coloration absorption band in lithium-coloured WO 3 anodic films with different coloration rates has been determined by computations from reflectance data. The electrochromic characteristics directly deduced from multichannel optical analysis which, in addition to the spectral data, also give access to the coloration kinetics, are used as well. Different electrochromism theories are examined with regard to these results. The availability of anodic films as materials for practical devices is briefly described, with the focus on endurance tests during long cycling processes.

Piezoelectric Materials, A Review of Progress

This paper is a review of the properties of the more important piezoelectric materials. Section I discusses the basic principles of the phenomena of piezoelectricity and elasticity from the standpoint of crystal physics. Section II is a brief account of the electrical measurements of piezoelectric vibrators which are required to determine the basic parameters defining the piezoelectric, elastic and dielectric properties of the material. Sections III and IV, respectively, give the properties of a range of piezoelectric crystals and ceramics including, in the most important cases, some of the derived properties which affect the use of the materials in practical devices. It is concluded that it is unlikely that new piezoelectric crystals will be found to replace those in use at present for frequency control and selection but that there is greater scope for new piezoelectric ceramics for use as electromechanical transducers or as frequency selective elements where the highest stability is not required.