Understanding the Metal-Insulator Transition in Certain Metal-Oxides

With the increasing interest in finding novel methods of computing comes a complementary interest in studying the potentially useful electronic properties of Transition Metal Oxides. Some such oxides of particular interest are those which undergo a temperature-dependent metal to insulator transition (MIT), where this change in resistivity allows these materials to exhibit a property known as negative differential resistance, which makes them useful for implementation into memristive devices. These memristors are integrally important to a new circuit design that is engineered to emulate the spike-based processing of biological neurons, known as a neuristor. One important element that must be understood before we are able to adequately scale these devices down to practical sizes is the underlying nature of this transition, whether it is a structural distortion or a purely electronic transition, or some combination of the two. To that end, we used various spectroscopic and computational techniques to study the MIT\u27s of two promising materials which are known to undergo this type of metal to insulator transition, VO2, and Ti2O3 to find the underlying mechanisms driving their electronic transitions.https://orb.binghamton.edu/research_days_posters_spring2020/1088/thumbnail.jp

Quantifying electrochemical degradation in single-crystalline LiNi0.8Mn0.1Co0.1O2–graphite pouch cells through operando X-ray and post-mortem investigations

Layered nickel-rich lithium transition metal oxides (LiNixMnyCo1−x−yO2; where x ≥ 0.8), with single-crystalline morphology, are promising future high-energy-density Li-ion battery cathodes due to their ability to mitigate particle-cracking-induced degradation. This is due to the absence of grain boundaries in these materials, which prevents the build-up of bulk crystallographic strain during electrochemical cycling. Compared to their polycrystalline counterparts, there is a need to study single-crystalline Ni-rich cathodes using operando X-ray methods in uncompromised machine-manufactured industry-like full cells to understand their bulk degradation mechanisms as a function of different electrochemical cycling protocols. This can help us identify factors to improve their long-term performance. Here, through in-house operando X-ray studies of pilot-line-built LiNi0.8Mn0.1Co0.1O2–Graphite A7 pouch cells, it is shown that their electrochemical capacity fade under harsh conditions (2.5–4.4 V and 40 °C for 100 cycles at C/3 rate) primarily stems from the high-voltage reconstruction of the cathode surface from a layered to a cubic (rock salt) phase that impedes Li+ kinetics and increases cell impedance. Post-mortem electron and X-ray microscopy show that these cathodes can withstand severe anisotropic structural changes and show no cracking when cycled under such conditions. Comparing these results to those from commercial Li-ion cells with surface-modified single-crystalline Ni-rich cathodes, it is identified that cathode surface passivation can mitigate this type of degradation and prolong cycle life. In addition to furthering our understanding of degradation in single-crystalline Ni-rich cathodes, this work also accentuates the need for practically relevant and reproducible fundamental investigations of Li-ion cells and presents a methodology for achieving this

Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO2

The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ~810⁰C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap

Normas de instalación de lineas de transmisión

Presenta un estudio sobre los métodos de instalación de lineas eléctricas de transmissión aérea en nuestro medio. Analiza los términos técnicos mas usados, los métodos de tensado de conductores eléctricos, las formas de puesta a tierra y la selección del sitio apropiado para la instalación.GuayaquilIngeniero en Electricida

Electronic characteristics of ultra-thin passivation layers for silicon photovoltaics

Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (<2 nm) dielectric layers can serve as interlayers in passivating contacts. Here, ultra-thin passivating films of SiO2, Al2O3, and HfO2 are created via plasma-enhanced atomic layer deposition and annealing. It is found that thin negatively charged HfO2 layers exhibit excellent passivation properties—exceeding those of SiO2 and Al2O3—with 0.9 nm HfO2 annealed at 450 °C providing a surface recombination velocity of 18.6 cm s−1. The passivation quality is dependent on annealing temperature and layer thickness, and optimum passivation is achieved with HfO2 layers annealed at 450 °C measured to be 2.2–3.3 nm thick which give surface recombination velocities ≤2.5 cm s−1 and J0 values of ≈14 fA cm−2. The superior passivation quality of HfO2 nanolayers makes them a promising candidate for future passivating contacts in high-efficiency silicon solar cells

Oxygen-redox activity in non-Li-excess W-doped LiNiO2 cathode

The desire to increase the energy density of stoichiometric layered LiTMO2 (TM = 3d transition metal) cathode materials has promoted investigation into their properties at high states of charge. Although there is increasing evidence for pronounced oxygen participation in the charge compensation mechanism, questions remain whether this is true O-redox, as observed in Li-excess cathodes. Through a high-resolution O K-edge resonant inelastic X-ray spectroscopy (RIXS) study of the Mn-free Ni-rich layered oxide, LiNi0.98W0.02O2, we demonstrate that the same oxidized oxygen environment exists in both Li-excess and non-Li-excess systems. The observation of identical RIXS loss features in both classes of compounds is remarkable given the differences in their crystallographic structure and delithiation pathways. This lack of a specific structural motif reveals the importance of electron correlation in the charge compensation mechanism for these systems and indicates how a better description of charge compensation in layered oxides is required to understand anionic redox for energy storage