Tuning local chemistry of P2 layered-oxide cathode for high energy and long cycles of sodium-ion battery

Layered transition-metal oxides have attracted intensive interest for cathode materials of sodium-ion batteries. However, they are hindered by the limited capacity and inferior phase transition due to the gliding of transition-metal layers upon Na+ extraction and insertion in the cathode materials. Here, we report that the large-sized K+ is riveted in the prismatic Na+ sites of P2-Na0.612K0.056MnO2 to enable more thermodynamically favorable Na+ vacancies. The Mn-O bonds are reinforced to reduce phase transition during charge and discharge. 0.901 Na+ per formula are reversibly extracted and inserted, in which only the two-phase transition of P2 ↔ P’2 occurs at low voltages. It exhibits the highest specific capacity of 240.5 mAh g−1 and energy density of 654 Wh kg−1 based on the redox of Mn3+/Mn4+, and a capacity retention of 98.2% after 100 cycles. This investigation will shed lights on the tuneable chemical environments of transition-metal oxides for advanced cathode materials and promote the development of sodium-ion batteries

Aluminium and tantalum doping of sputtered TiO2 thin films

TiO2 is a promising material for the generation of hydrogen fuel from the photoelectrochemical splitting of water using sunlight. In this application, performance is determined by the ability to generate electron-hole pairs during illumination, and keep them separated long enough to do useful work. A TiO2-based homojunction, consisting of a graded composition is considered promising for this role. The use of magnetron sputtering is considered promising for the fabrication of such a material. The aim of this project is to explore the deposition of Al and Ta doped TiO2 films and identify deposition parameters that facilitate control of film crystallinity, control of doping level and control film thickness. Meeting these aims will provide the basis for fabricating TiO2-based homojunctions

Hot corrosion of borate melt and interface chemistry of borate-coated steel under tribological stimulation

Hot corrosion mechanism of Sodium Borate was investigated on mild-carbon steel at 800 degrees C by multiple surface characterizations. In addition, effects of tribological exposure on the interface chemistry were also examined in details by mean of ball-on-disk testing. The occurrence of interfacial reaction is characterized by the formation of a Sodium-rich film on the borate melt/oxide phase boundary. The basic dissolution reaction is accompanied by fluxing action of the protective scale in the case of static oxidation. External stressed shearing profoundly modifies the interface chemistry by establishing a hierarchical boundary film with remarkably low Oxygen density which renders improved oxidation resistance

CuI-TiO composite thin film for flexible electronic applications

Copper iodide (CuI) is a p-type transparent conductor that can be synthesized and doped at low temperature (≤ 100 oC) while maintaining its high-conductivity and high optical transmittance (> 75 %). The realization of such simultaneously high conductivity and transparency makes CuI useful for applications in both active and passive flexible electronics. However, a few of the major disadvantages of CuI include its optical and electronic stability at ambient atmosphere and reduced transparency with iodine doping. In this report, instead of using pure CuI, we fabricate CuI-TiO2 composite thin films which are highly transparent and stable at ambient conditions whilst maintaining degenerate p-type conductivity. The CuI-TiO2 composite film is >80 % transparent (450– 2000 nm range), highly conducting (~ 77 S/cm), heavily doped (> 1.2 x 1020 /cm3), with a mobility of ~ 3.5 cm2V-1s-1

Chromium doping of Ta3N5 thin films via thermal nitridation of sputtered tantalum oxide films

The effects of chromium doping on magnetron sputtered and thermally annealed Ta3N5 thin films have been investigated for the first time. The structural and compositional evolution of these films were closely investigated using XRD, SEM and SIMS. A simple mechanism for the incorporation of chromium into the Ta3N5 lattice was described based on kinetic experiments and SIMS analysis. Some basic recommendations can be made regarding the optimal method of preparing Cr–Ta3N5, however there is still much work to be done in characterising Cr–Ta3N5 with appreciable properties. In particular, this investigation indicates poor Cr solubility in Ta3N5 in spite of the favourable ionic radius

Fluorine passivation of defects and interfaces in crystalline silicon

Defects and impurities in silicon limit carrier lifetimes and the performance of solar cells. This work explores the use of fluorine to passivate defects in silicon for solar cell applications. We present a simple method to incorporate fluorine atoms into the silicon bulk and interfaces by annealing samples coated with thin thermally evaporated fluoride overlayers. It is found that fluorine incorporation does not only improve interfaces but can also passivate bulk defects in silicon. The effect of fluorination is observed to be comparable to hydrogenation, in passivating grain boundaries in multicrystalline silicon, improving the surface passivation quality of phosphorus-doped poly-Si-based passivating contact structures, and recovering boron−oxygen-related light-induced degradation in borondoped Czochralski-grown silicon. Our results highlight the possibility to passivate defects in silicon without using hydrogen and to combine fluorination and hydrogenation to further improve the overall passivation effect, providing new opportunities to improve solar cell performance

Clustering-induced high magnetization in Co-doped TiO2

High magnetization materials are highly desirable for the development of advanced multifunctional magnetic applications. Co:TiO2 is a widely investigated diluted magnetic semiconductor (DMS) system which persists ferromagnetism to above room temperature. However, the magnetic moment observed in Co:TiO2 so far typically lays between 0.01 μB per Co atom to the Co bulk value, 1.7 μB/Co, while higher saturation magnetization (Ms) surpasses 2 μB/Co which has only occasionally been reported. The huge magnetization difference suggests formation of Co nanoclusters is poorly controlled and immensely dependent on growth parameters. In this work, 5 at% Co-doped TiO2 thin films on SrTiO3 substrate were deposited via pulsed laser deposition. By delicately choosing the deposition parameters, various sized Co nanoclusters are embedded into TiO2 thin films with unique surface nanostructures. The Co nanoclusters show metallic features, confirmed by X-ray photoelectron spectroscopy. Moreover, the thin films exhibit excellent cluster-dependent Ms, reaching 2.5 μB/Co under optimal parameters. This work provides an effective approach to design high magnetization thin films with various surface morphology

Improved p-type conductivity in Al-rich AlGaN using multidimensional Mg-doped superlattices

A novel multidimensional Mg-doped superlattice (SL) is proposed to enhance vertical hole conductivity in conventional Mg-doped AlGaN SL which generally suffers from large potential barrier for holes. Electronic structure calculations within the first-principle theoretical framework indicate that the densities of states (DOS) of the valence band nearby the Fermi level are more delocalized along the c-axis than that in conventional SL, and the potential barrier significantly decreases. Hole concentration is greatly enhanced in the barrier of multidimensional SL. Detailed comparisons of partial charges and decomposed DOS reveal that the improvement of vertical conductance may be ascribed to the stronger pz hybridization between Mg and N. Based on the theoretical analysis, highly conductive p-type multidimensional Al0.63Ga0.37N/Al0.51Ga0.49N SLs are grown with identified steps via metalorganic vapor-phase epitaxy. The hole concentration reaches up to 3.5 × 1018 cm-3, while the corresponding resistivity reduces to 0.7 Ω cm at room temperature, which is tens times improvement in conductivity compared with that of conventional SLs. High hole concentration can be maintained even at 100 K. High p-type conductivity in Al-rich structural material is an important step for the future design of superior AlGaN-based deep ultraviolet devices

Effect of oxygen activity on chromium segregation in Cr-doped TiO2 single crystal

The present work determined the effect of chromium segregation on surface and near-surface composition of Cr-doped TiO2 (0.05 at.% Cr) after annealing in oxidising and strongly reducing environments at 1,073 and 1,273 K. The segregation-induced depth profiles were determined by secondary ion mass spectrometry (SIMS). It was observed that annealing in oxidising conditions [p(O2) = 105 Pa] and reducing conditions [10−11 Pa > p(O2) > 10−15 Pa] results in an enrichment and depletion of the surface layer in chromium, respectively. The observed effects may be used for tailoring the surface and near-surface composition in order to impose desired properties

Influence of sodium incorporation on kesterite Cu2ZnSnS4 solar cells fabricated on stainless steel substrates

Kesterite Cu2ZnSnS4 (CZTS) thin film solar cells are fabricated based on sputtering and post sulfurization process on stainless steel substrates. NaF layers with different thickness are used as external sodium source to address the lack of Na issue in Na-free steel substrates and 10 nm of NaF layer is proved to be the optimal thickness. Sodium incorporation into CZTS induces the metal element redistribution, facilitates CZTS film recrystallization and promotes the phase evolution. Appropriate amount of sodium is evidenced to yield an enhancement in power conversion efficiency from 3.07% to 4.10% whilst further increase in sodium tends to deteriorate the device efficiency. Device performance deterioration is correlated to the rather small grain size accompanied with a large amount of grain boundaries and unfavorable SnS2 secondary phase for CZTS samples without sodium, while pinholes and lower carrier density in absorber for CZTS samples with over amount of sodium