Valence Charge Transfer Research Articles

Exploiting Mixed Valence Charge Transfer for Electrochromic and Electrofluorochromic Use.

An asymmetric mixed valence fluorophore with two different electron rich termini was investigated as a dual-role active material for electrochromism and electrofluorochromism. The fluorescence quantum yield (Φfl) and emission wavelength of the fluorophore were dependent on solvent polarity. The quantum yield of the material in an electrolyte gel, on a glass substrate and in a device was 40 %, 20 % and 13 % respectively. The fluorophore further underwent two near-simultaneous electrochemical oxidations. The first oxidation resulted in a 1000 nm red shift in the absorption to broadly absorb in the NIR, corresponding to the intervalence charge transfer (IVCT). Whereas the second oxidation led to a perceived green color at 715 nm with the extinction of the NIR absorbing IVCT. Owing to the dissymmetry of the fluorophore along with its two unique oxidation sites, the IVCT gives rise to a mixed valence transfer charge (MVCT). The coloration efficiency of the fluorophore in both solution and a device was 1433 and 200 cm2 C-1, respectively. The fluorescence intensity could be reversibly modulated electrochemically. The photoemission intensity of the fluorophore was modulated with applied potential in an operating electrochromic/electrofluorochromic device. Both the dual electrochromic and the electrofluorochromic behavior of the fluorophore were demonstrated.

Experimental and theoretical investigation of silicon-based carbon composite electrode for high performance Li-ion capacitors

Lithium-ion capacitors (LICs) have garnered significant attention in recent years due to their ability to overcome the shortcomings of lithium-ion batteries (LIBs) and supercapacitors (SCs). Silicon (Si) stands out as a superior anode material for LICs due to its compelling attributes, including a high theoretical specific capacity (4200 mAh/g) and a low de-lithiation potential. Nevertheless, the inherent challenges of Si, such as low electrical conductivity and significant volume expansion (300 %), contribute to low electrochemical performance. To address this issue, a conductive carbon layer is introduced on Si using a simple and scalable approach. The resulting architecture, known as carbon-encapsulated Si (Si/C), not only improves electrical properties by enhancing Li+ diffusion but also mitigates volume expansion, leading to enhanced capacity and cyclic stability. Theoretical findings based on density functional theory (DFT) support these enhancements, confirming improved interactive properties at an atomic scale, including low binding energy and accelerated charge transfer kinetics (higher valence charge transfer) between the Si/C electrode and Li ions. As a result, the ‘binder-free’ and flexible Si/C electrode exhibits a notable initial discharge capacity (3450 mAh/g at 0.05 C) with improved rate capability (3010 mAh/g at 0.1 C). When employed as an anode in LICs, the Si/C electrode exhibits outstanding performance, boasting a large energy density (222.29 Wh/kg), high power density (25 kW/kg), and superior cyclic stability (81.3 % over 10,000 cycles). These findings highlight the potential of the Si/C electrode as a formidable candidate for advanced energy storage applications.

Role of Chalcogen Defect Introducing Metal-Induced Gap States and Its Implications for Metal-TMDs' Interface Chemistry.

The contact resistance of the transition metal dichalcogenide (TMD) devices is not comparable to that of their silicon counterparts, probably due to a lack of clarity in their interface chemistry. Looking beyond the conventional Schottky-Mott rule, the metal chalcogen orbital overlaps, tunnel barrier, and metal-induced gap states (MIGSs) are crucial factors determining different metals' contact properties with TMDs. Exploring their properties helps TMDs' contact resistance engineering, driven mainly by their orbital overlaps and perturbing parameters. This work presents the interface chemistry of TMDs (MoS2, MoSe2, WS2, and WSe2) with different metals (Au, Cr, Ni, and Pd) in detail using density functional theory computations. Additionally, the work discusses the role of the chalcogen vacancy and interstitial defects in the metal-TMD interactions and corresponding MIGS features. The investigations reveal that Au does not show any significant MIGS due to its weak interactions with all the TMDs. However, other investigated metals have a strong affinity with TMDs, making significant MIGS contributions. All the metals offer n-type doping characteristics to TMDs due to valence charge transfer from the metals toward TMDs. The chalcogen vacancy boosts the orbital overlaps of the TMDs with all the metals. The vacancy reduces metal-TMD interfacial distance, which can be a promising technique to reduce the tunnel barrier and contact resistance. The MIGS and defect-induced gap states (DIGSs) reflect the possibility of Fermi-level pinning in the TMDs' contacts with Cr, Ni, and Pd. Besides, the work discloses that the chalcogen vacancy converts an n-type Pd-TMD interface into p-type due to reverse charge transfer after the vacancy. Chalcogen interstitial impurity also helps with contact resistance engineering for some metal-TMD systems by reducing the bond distance of the metal TMDs. Our study highlights the possibility of defect-assisted and MIGS-based contact engineering at the metal-TMD interfaces.

Heterogeneous metal trimer catalysts on Mo2TiC2O2 MXene for highly active N2 conversion to NH3

Activating the inert basal plane of functional-group-terminated MXenes is highly desirable for non-noble metal MXenes applied for the N2 reduction reaction (NRR) to value-added ammonia (NH3), but remains a significant challenge. Herein, we report the coupled interactions and catalytic mechanism of the supported trimeric metal M3 single-cluster catalysts (SCCs) on oxygen-vacancy-rich Mo2TiC2O2 (VO−Mo2TiC2O2) MXene materials for catalyzing this difficult reaction via spin-polarized density functional theory (DFT) calculations. Energy and electronic property analysis showed that the robust structural stability of the M3/VO−Mo2TiC2O2 catalysts stemmed from the vacancy defects, electronegativity difference, and significant charge redistribution. Based on the unique bidirectional electron transfer activation mechanism, the N2 adsorption strength can be well represented by a simple descriptor of the valence electrons in the d orbitals of M3 trimers. Remarkably, these confined Rh3, Ru3, Ni3 and Co3 SCCs on VO−Mo2TiC2O2 were capable of providing superior electrocatalytic activity and selectivity toward the NRR with ultra-low limiting potentials of −0.23, −0.32, −0.42 and −0.44 V, respectively. Finally, a mechanistic investigation illustrated that the valence electrons, charge transfer, and d-band center of the M3 active sites could collectively act as good descriptors to correlate the structure and NRR activity.

Self-supported graphene oxide encapsulated chalcopyrite electrode for high-performance Li-ion capacitor

The combination of lithium-ion batteries and supercapacitors makes lithium-ion capacitors (LICs) the ideal energy storage device for commercial applications. However, the high performance of LICs is restricted due to various shortcomings such as low electrode specific capacity and sluggish reaction kinetics. Additionally, the undesirable ‘polysulfide-diffusion’ effect in the cost-effective metal sulfide-based electrodes results in high capacity degradation. Therefore, developing novel strategies to surmount these drawbacks is of prime importance for the development of high-performance LICs. Herein, a novel composite material with the unique architecture of graphene oxide (GO) encapsulation on the chalcopyrite (CuFeS2) microspheres is fabricated as an anode for LIC. Compared to the traditional binder-based approach for electrode fabrication, the anode is fabricated using a ‘binder-free’ approach to enable the exposure of high-density electroactive sites. As a result, the fabrication of a ‘self-standing’ flexible electrode is obtained that finds numerous applications in wearable technology systems. Detailed electrochemical analysis reveals the enhanced reaction kinetics of the composite electrode along with the improved specific capacities and rate capabilities. The unique architecture of the GO encapsulation not only prevents the structural collapse of the chalcopyrite microspheres but also suppresses the polysulfide diffusion, resulting in higher cyclic stability. The role of such carbon encapsulation is studied in detail using experimental studies and is strongly supported by theoretical calculations based on density functional theory (DFT) for the first time. The calculated binding energy and charge transfer confirmed the improved interactive features of Li ions with the composite electrode in terms of enhanced binding energy and higher valence charge transfer. Additionally, the favorable binding energy of Li2S and Li2S4 polysulfides to the composite electrode than the pristine CFS electrode reflects restricted polysulfide diffusion, resulting in outstanding cyclic stability. The assembled LIC device based on the composite electrode displays a large energy density of 192.33 Wh/kg, a high power density of 14,280 W/kg, and excellent cyclic stability of >90 % for 10,000 cycles, outperforming most of the reported metal sulfide-based LICs.

Temperature stimulated bandgap tailoring in the smart chromogenic WO30.33H2O

The orthorhombic form of WO30.33H2O is prepared hydrothermally and the thermochromic property of the material is identified using spectroradiometer with in a wavelength range of 325 nm to 1075 nm. The variation of reflectance with temperature is observed and it is accompanied with a change in bandgap. The absorbance relation with temperature is analyzed and the presence of Inter Valence Charge Transfer (IVCT) band is detected in the absorbance spectra. The IVCT bands illustrate the homovalent charge transfer mechanism behind the thermochromism taking place in the compound. The change in bandgap with temperature provides further supportive evidence to the thermochromic phenomenon observed in the compound.

Thermally and Optically Activated Migration of Charge Carriers in Alkali Metal Sesquioxides.

The alkali metal sesquioxides A4O6 (A=K, Rb, Cs) are mixed‐valent with respect to oxygen and display several degrees of electronic and structural freedom, which give rise to diverse transport and ordering processes. We report on analyses of the respective underlying excitations by diffuse reflectance spectroscopy and thermally activated electron transport. Backed by DFT based band structure calculations we identify three possible mechanisms, inter valence charge transfer from peroxide to superoxide, excitation across the Jahn‐Teller gap of tilted superoxide anions, and polaron migration. The activation energies as found by the three different approaches are in a rather narrow range of 0.62–0.89 eV for Rb4O6 and 0.49–0.65 eV for Cs4O6, confirming opacity in the full range of visible light. The effect of the phase transition from cubic to tetragonal as demonstrated for the caesium representative corresponds to a marginal shift to higher activation energy.

High performance Li-ion capacitor based on novel carbon-coated chalcogenide anode

Lithium-ion capacitors (LICs) exhibit the combined advantages of lithium-ion batteries and supercapacitors. Till now, the development of efficient LICs with high energy-power densities with high cyclic stability is restricted due to the limited performance of the electrode in terms of low capacity and sluggish reaction kinetics. Developing a novel electrode that overcomes these existing drawbacks is one of the possible strategies to enhance electrochemical performance. In this work, the superior Li-ion storage capability of highly interconnected Cu2SnS3 (CSS) nanospheres encapsulated in the carbon nanotubes (CNTs) framework as an efficient anode for LIC is demonstrated. Benefiting from the combined charge storage mechanisms of the CSS (alloying + conversion) and the CNTs (insertion), an exceptional high electrochemical performance in terms of high energy density (158.77 Wh/kg) and high power density (12,500 W/kg) with superior cyclic stability for 10,000 cycles is obtained from the fabricated LIC device. The key factors attributed to the improved electrochemical performance are studied in detail using experimental results, comprehensive characterization techniques and are strongly supported by the relevant simulated models based on theoretical calculations. The plausible binding characteristics of the Li-ion and the valence charge transfer in the electrodes are evaluated using first-principle density functional theory (DFT).

Magneto-electronic properties and manipulation effects of Fe-adsorbed Sb/WS<sub>2</sub> heterostructure

To study the induced magnetism mechanism and magneto-electronic properties of non-magnetic two-dimensional van der Waals heterostructure adsorbing magnetic atoms, we construct Sb/WS<sub>2</sub> heterostructure, and consider its adsorbed Fe atoms. The calculated adsorption energy shows that T<sub>W</sub>, V<sub>Sb</sub> adsorption are the most likely positions for Fe atom adsorbed below and above the heterostructure, respectively, and T<sub>S_M</sub> adsorption is the most likely position for Fe atom adsorbed between two monolayers. The induced magnetism is due to the electron-spin rearrangement caused by the expansion of valence electronic configuration (VEC) and charge transfer after Fe atoms have been adsorbed. The T<sub>W</sub> adsorption and the T<sub>S_M</sub> adsorption make the nonmagnetic semiconducting heterostructure become a half-semiconductor (HSC), while V<sub>Sb</sub> adsorption turns the heterostructure into a bipolar magnetic semiconductor (BMS). In particular, the calculated magnetized energy indicates that the interlayer T<sub>S_M</sub> adsorption leads the heterostructure to holding the highest magnetic stability, which is enough to resist the influence of thermal fluctuation at room temperature. Quantum manipulation can cause the heterostructure to produce abundant magnetism, especially the flexible change of magnetic phase. For example, the application of external electric field can give rise to the magnetic phase transition among HSC, HM (half-metal) and BMS for the heterostructure, and the vertical strain can make the heterostructure realize the magnetic phase transition among HSC, HM and MM (magnetic metal). This study shows that the heterostructure can increase the adsorption region of transition metal atoms (below, interlayer and above), so as to produce rich magnetism, especially for the interlayer adsorption of transition metals, its magnetic stability against temperature is significantly enhanced.

Photochromic property of cerium oxide nanoparticles prepared by dialysis of solution of cerium nitrate

CeO2 nanoparticles were obtained by heating 50 mL of 0.1 M Ce(NO3) 3·6H2O solution mixed with 5 mL of 1 M NH3 aq. at 368 K for 24 h. When H2O and ethanol were used as solvents for cerium nitrate hydrate, obtained CeO2 nanoparticles were aggregated. On the other hand, in case that ethylene glycol was used for the solvent, stable sol of CeO2 nanoparticles dispersed in ethylene glycol. Furthermore, dialysis of the sol with H2O successfully enabled the exchange of the dispersion solvent from ethylene glycol to H2O and the average particle size was 23 nm, which was less than those of the other solvents. UV and visible light were irradiated onto the obtained CeO2 powders to examine their photochromic characteristics. The CeO2 nanoparticles obtained with the ethylene glycol solution showed change of absorbance spectra after irradiation of light whose wavelength was less than 600 nm, that is, the visible light irradiation caused photochromism on the CeO2 nanoparticles. According to Ce 3d XPS spectra of the CeO2 nanoparticles, valence state of the Ce ions in the CeO2 nanoparticles was mixed valence state of Ce3+ and Ce4+, which causes inter valence charge transfer absorption. Therefore, the change of the optical absorption spectra, photochromism, occurred with the optical absorption due to the charge transfer absorption between Ce3+ and Ce4+. Furthermore, the photochromic characteristics depended on heat treatment of the CeO2 nanoparticles. Heating the nanoparticles at more than 473 K decreased amount of oxygen vacancies, which affected the mixed valence state of Ce ions so that the photochromism by irradiation of the light within the wavelength range from 400 nm to 600 nm was greatly affected.

UV facilitated synergistic effects of polymetals in ore catalyst on peroxymonosulfate activation: Implication for the degradation of bisphenol S

The use of natural ore, especially iron-based ore, in sulfate radical-advanced oxidation processes (SR-AOPs) for organic contaminant control has received increasing attention. However, the rapid exhaustion of Fe(II) in ore has become a bottleneck restricting persulfate activation and radical generation. Herein, a natural Ti- and V-enriched magnetite (Ti-V-M) was exploited both as an activator and catalyst for UV-facilitated peroxymonosulfate (PMS) degradation of bisphenol S (BPS) in a water treatment process. The results revealed superior performance in which 100% of BPS was removed within 20 min in the UV/PMS/Ti-V-M system, and its pseudo-first-order kinetic constant was almost 3.55 times, 42.73 times, and 52.22 times higher than that for the PMS/Ti-V-M, UV/PMS, and UV/Ti-V-M systems, respectively. Under the synergistic action of multi-metals, the photocatalytic activity of the Ti-V-M was greatly improved with relatively low valence band potential (1.88 eV) and charge transfer resistance (12.60 Ω). Meanwhile, distinct from conventional PMS processes activated by monometallic Fe, the Fe(III)/Fe(II) circulation was accelerated significantly, mainly due to the excited h+-e− pairs in photosensitive TiO2 and subsequent electron transfer by V(III)/V(IV)/V(V) redox cycles. The above advantages strongly enhanced radical generation, and SO4–·, ·OH, O2–·, 1O2 and h+ were confirmed to play vital roles in BPS degradation. Furthermore, this system displayed excellent recyclability and stability for water treatment. Our study highlights the mechanism of synergistic action of multi-metals in natural minerals for UV-facilitated PMS activation and sheds new light on using cost-effective ores for highly efficient treatment of highly acidic and alkaline organic wastewater.

Charge transfer transitions in glasses - Attempt of a systematic review

In glasses, even low levels of dopants or impurities can give rise to very intense and broad charge transfer transitions from ligands (e.g. oxygen or fluorine ions) to the metal ion (L→M), absorbing strongly in the short wavelength ultraviolet. In an attempt of a systematic review of charge transfer transitions, we compile data of various glass systems with high intrinsic transmission that allow the observation of charge transfer (CT) transitions involving cations of different electronic configurations. Various glasses of different composition were selected as matrices, including fluoroaluminate glasses with low P2O5 content (FP10 = 35AlF3–10MgF2–30CaF2–15SrF2–10Sr(PO3)2), phosphate [SrP = Sr(PO3)2, NSP = Na2O-40SrO-50P2O5], silicate (NS = 15Na2O–85SiO2, DS = 33Na2O–67SiO2), aluminosilicate (BCAS = 10BaO–10CaO–15Al2O3–65SiO2) and borosilicate (NBS1 = 16Na2O–10B2O3–74SiO2, NBS2 = 4Na2O–1Al2O3–21B2O3–74SiO2, Duran = 5Na2O/K2O–1Al2O3–12B2O3–82SiO2) compositions. All glasses were prepared from very high purity materials and doped with various metal ions. Charge transfer transitions of electrons to or from these cations induce absorption and photoluminescence phenomena in the ultraviolet and visible spectral region, which were recorded by optical spectroscopy.Charge transfer transitions were considered for empty valence orbitals, that is, for the high valent 3d, 4d and 5d ions, and for Zn2+, Ag+, Cu+ with full d10 orbitals. 3d, 4d and 5d ions with partially filled valence orbitals that could be stabilized in the named glasses are studied as well. Doping concentrations for these allowed transitions typically ranged from 5 to 5000 wt-ppm of metal ions, with some samples also displaying higher dopant levels.Inter valence charge transfer (IVCT) transitions directly from one metal ion to a neighboring metal ion (M→M) of the same element or metal to metal charge transfer (MM-CT) between ions of different elements can also induce strong visible absorption and deep coloring for which some examples will be discussed.

Link between Structural and Optical Properties of CoxFe3–xO4 Nanoparticles and Thin Films with Different Co/Fe Ratios

CoxFe3–xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3–xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles’ surface.

In Situ Synchrotron X‐Ray Absorption Spectroscopy Studies of Anode Materials for Rechargeable Batteries

AbstractTaking advantage of a high‐flux light source, synchrotron X‐ray absorption spectroscopy (XAS) beamline is able to perform in situ/ex situ, element‐selective, and qualitative/quantitative experiments to elucidate electrochemical reaction mechanisms of batteries accurately and efficiently. In situ synchrotron XAS probes dynamic electronic and local atomic structure information, including valence state, charge transfer, local geometry and symmetry, bond number/length/type and disorder degree, of target elements of significance during battery operation, which facilitates to promote the development of rechargeable batteries by building accurate structure‐performance relationships fundamentally. In this review, the basic principles for XAS are briefly introduced, design strategies for in situ XAS experiments are proposed, salient in situ XAS studies of battery anodes are summarized, and current challenges and future opportunities based on XAS measurements are also outlined.

Chalcopyrite based carbon composite electrodes for high performance symmetric supercapacitor

In the present work, we demonstrate a facile and single-step hydrothermal synthesis of chalcopyrite based carbon composite material (CuFeS2/carbon nanotubes) with the unique architecture in which the porous CuFeS2 (CFS) microflowers are encapsulated by the carbon nanotubes (CNT). The structural, morphological, compositional, chemical, vibrational and electrochemical properties of the fabricated CFS/CNT composite are analyzed in detail through comprehensive characterization techniques. When used as binder-free negative electrode material in the supercapacitor application, the CFS/CNT composite exhibits exceptional electrochemical performance than its pristine form (CFS). The electrochemical analysis clearly reveals the pseudocapacitive nature of composite electrode with improved electrical and charge-transport properties. In three-electrode configuration, the CFS/CNT composite electrode exhibits a high specific capacitance of 667F/g, a high columbic efficiency of 95% with 100% cyclic stability for 3000 cycles. The key factors influencing the supercapacitive performance of the electrodes are validated using extensive experimental results and are backed up with theoretical calculations of the relevant simulated models. The plausible storage sites of Na+ ions along with its valence charge transfer, electronic properties and binding characteristics on the composite electrode are identified through first-principles density functional theory (DFT) calculations. Furthermore, to evaluate its feasibility for practical application, a solid-state symmetric device (CFS/CNT//CFS/CNT) based on polymer gel electrolyte (PVA-Na2SO4) was fabricated. The symmetric device exhibited the highest specific capacitance of 128F/g, an energy density of 22 Wh/kg, a power density of 2083 W/kg and notable durability (94% cyclic stability for 10,000 cycles) reflecting its formidable candidature for future energy storage systems.

Laser induced broadband white emission of Y2Si2O7 nanocrystals

The generation of broadband white emission of the α-Y2Si2O7 nanocrystals was observed upon excitation by focused beam ofCW lasers operating in near infrared, visible and blue regions. The influence of excitation energy on order of process is presented. The temperature of sample emitting laser induced white emission (LIWE) was determined by luminescence nanothermometry technique. It is found that the temperature is much lower than that predicted by blackbody model characterized with the same energy. It is shown that the LIWE is accompanied by efficient photocurrent. An origin of white emission is discussed in terms of multiphoton ionization process leading to the inter valence charge transfer emission.

Uncovering the influence of metallic and non-metallic impurities on the ideal shear strength and ductility of Ti: An ab-initio study

The mechanical properties of Ti alloys are significantly affected by the introduction of solute or impurity elements. Thus, the role of impurities on the hardening or softening behavior of α-Ti was investigated through first principles calculations. In particularly, to provide a comprehensive electronic and atomic basis of solute addition in α-Ti, the effect of metallic (V and Al) and non-metallic (C and O) impurities on the ideal shear strength (ISS) and generalized stacking fault energy (GSFE) across different slip systems of Ti were probed. The results revealed that the addition of V atom reduces both ISS and unstable stacking fault energy across various slip systems of α-Ti, whereas Al addition increases the ISS of Ti. Further, the addition of O atom decreases ISS for most of the slip systems while C solute atom increases ISS across all slip systems of α-Ti. To illustrate the underlying factors influencing the observed softening/strengthening behavior, the electronic density of states and valence charge transfer were determined. The electronic density of states calculations showed that the contribution from the d states of V atom decreases the stability of Ti-V solid solution, thereby leading to a decrease in the plastic anisotropy of α-Ti. Finally, the shearability parameter and critical resolved shear stress (CRSS) ratios across different slip systems of Ti solid solutions were calculated to understand the macroscopic effects of impurity addition on the deformation behavior of α-Ti at ambient conditions. Overall, the first-principles study provides an insight into the electronic basis of strengthening/softening effect of solute addition in α-Ti and assesses their implications on the deformation behavior of α-Ti alloys.

Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known. Using first-principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.

Effect of solutes on ideal shear resistance and electronic properties of magnesium: A first-principles study

Solution strengthening or softening is an effective way to enhance mechanical properties, especially in magnesium based alloys due to their inability to activate adequate non-basal deformation mechanisms at the room temperature. Hence, using first-principles calculations, the effects of several different alloying elements on the ideal shear resistance across various slip systems of Mg were investigated. The results reveal that the addition of a Ce or Zr solute atom decreases the ideal shear resistance (softening); whereas, the substitution of a Sn, Li or Zn atom increases the ideal shear resistance of Mg (strengthening). The dominant slip system in Mg was found to change from the basal partial (0001)[101¯0] to prismatic (101¯0)[112¯0] with the addition of a Ce or Zr solute atom; whereas, the addition of a Sn, Li or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer, which provides a quantum mechanical insight into the underlying factors influencing the observed softening/strengthening behavior, was probed. For instance, the electronic density of states calculations show that the contribution from d states of Ce and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. In the end, theoretical analyses were performed and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys.

Electronic-structure, corrosion and mechanical properties of nc-CrC/a-C:H films deposited by multi-arc ion plating

Nanocomposite films consisting of a hard nano-crystalline chromium carbide phase and amorphous carbon matrix (nc-CrC/a-C:H) were fabricated using multi-arc ion plating system. The series of films were deposited in various acetylene (C2H2) flow rates ranging from 50 to 150 standard cubic centi-meter per minute (sccm). The ingredient contents, microstructures, corrosion properties, mechanical characteristics and tribological behaviors of the nc-CrC/a-C:H films were investigated carefully as a function of the C2H2 flow rate. X-ray and ultraviolet photoelectron spectroscopies were performed to explain the intrinsic origin of the excellent properties of nc-CrC/a-C:H films from the viewpoint of chemical bonding and electronic-structure. The results showed the films were formed as nano-scale metastable cubic CrC (grain size of 2–10 nm) embedded in the hydrogenated amorphous carbon (a-C:H) matrix. As the flow rate increased, the content of a-C sp2 increased while the content of CrC decreased. The valence charge transfer at the interface between a-C:H and nc-CrC makes parts of the C-Cr bonds more ionic. The density of such charge transfer was quantified by the C-Cr*/C-Cr ratio. The charge transfer was further verified by the valence band of the films. The results indicated an obvious impact of the density of valence charge transfer on corrosion and mechanical properties. A relatively low density of charge transfer and high work function could result in an enhanced corrosion resistance of the films. A lower vacancies density caused by valence charge transfer could also strengthen the hardness. The change of the density of charge transfer could be attributed to the metallic chromium content in the films. Thus, we can open up a new possible pathway to design the nc-CrC/a-C:H films for applications.