Polar Covalent Bonds Research Articles

Cu-bearing HHCCI interface combined with first principles calculation and corrosive wear resistance

The corrosive wear resistance of copper-bearing HHCCI was tested through experiments and HRTEM, combined with first-principles calculations to study the atomic structure, interface fracture work, thermodynamic stability, electronic structure and bonding structure of six Fe3Cr4C3(01 1‾ 0)/γ-Fe(101) interface models with different termination methods. The HRTEM results show that the interface formed by (01 1‾ 0) of M7C3-type carbide and (101) of the austenite matrix is a coherent interface. The first principles calculation results show that the interface formed by the Fe3Cr4C3(01 1‾ 0)-Cr termination model and the γ-Fe (101)-2 termination model has the highest interface bonding strength. The Fe-end/7Fe-2 interface model is the most stable. Both the interfacial chemical energy and the interfacial elastic energy will affect the overall thermodynamic stability of the Fe3Cr4C3(01 1‾ 0)/γ-Fe (101) interface. The fracture work of Fe3Cr4C3(01 1‾ 0) and γ-Fe (101) is greater than the corresponding interface adhesion work. Moreover, the fracture work on each terminal end of M7C3 along the (01 1‾ 0) surface is higher than that on each terminal end of γ-Fe along the (101) surface. The failure of HHCCI during corrosive wear may mainly occur in the interface area or the side close to the γ-Fe matrix. The chemical bonds in the Fe-end/7Fe interface are mainly Fe-Fe metal bonds, Fe-Cr metal bonds and some Fe-C polar covalent bonds. The chemical bonds in the Cr-end/7Fe interface are mainly Cr-Fe metal bonds and some Cr-C polar covalent bonds. The chemical bonds in the C-end/7Fe interface are mainly composed of Cr-Fe metal bonds, C-Fe polar covalent bonds and part of C-Cr polar covalent bonds, as a result, each interface exhibits different corrosive wear resistance potentials.

Si-C atomic line achieving efficient electrocatalytic nitrogen reduction reactions

In this study, the concept of a Si-C atomic line (SCAL) featuring fully exposed atomic sites is introduced for the first time, and the SCAL serves as a highly efficient electrocatalyst for nitrogen reduction reactions (NRR). Thanks to the strong binding strength of the Si-C bonds (polar covalent bonds), the SCAL exhibits exceptional structural stability at 298.15 K. With a band gap of 1.86 eV characterized by a semiconductor, SCAL exhibits relatively high conductivity which is advantageous for electrocatalytic processes. N2 is adsorbed on the C sites in an end-on mode due to the most negative adsorption energy (–0.39 eV), and the nature of adsorption is s-p orbital hybridization. Based on the end-on N2 adsorption, the entire NRR pathway is simulated and shows a relatively low energy barrier of 0.50 eV, which is relatively low compared to reported metal-free NRR electrocatalysts. The charge transfer mechanism reveals that the active sites (C atoms) are responsible for charge transfer, while other parts of SCAL act as storage warehouses for charge. Moreover, SCAL demonstrates a preference for NRR over hydrogen evolution reaction (HER), underscoring its excellent electrocatalytic selectivity. This study opens new avenues for designing high-performance catalysts.

Indirect Voltammetry Detection of Non-Electroactive Neurotransmitters Using Glassy Carbon Microelectrodes: The Case of Glutamate

Glassy carbon (GC) microelectrodes have been successfully used for the detection of electroactive neurotransmitters such as dopamine and serotonin through voltammetry. However, non-electroactive neurotransmitters such as glutamate, lactate, and gamma-aminobutyric acid (GABA) are inherently unsuitable for detection through voltammetry techniques without functionalizing the surface of the microelectrodes. To this end, we present here the immobilization of the L-glutamate oxidase (GluOx) enzyme on the surface of GC microelectrodes to enable the catalysis of a chemical reaction between L-glutamate, oxygen, and water to produce H2O2, an electroactive byproduct that is readily detectable through voltammetry. This immobilization of GluOx on the surface of bare GC microelectrodes and the subsequent catalytic reduction in H2O2 through fast-scan cyclic voltammetry (FSCV) helped demonstrate the indirect in vitro detection of glutamate, a non-electroactive molecule, at concentrations as low as 10 nM. The functionalized microelectrodes formed part of a four-channel array of microelectrodes (30 μm × 60 μm) on a 1.6 cm long neural probe that was supported on a flexible polymer, with potential for in vivo applications. The types and strengths of the bond between the GC microelectrode surface and its functional groups, on one hand, and glutamate and the immobilized functionalization matrix, on the other hand, were investigated through molecular dynamic (MD) modeling and Fourier transform infrared spectroscopy (FTIR). Both MD modeling and FTIR demonstrated the presence of several covalent bonds in the form of C-O (carbon–oxygen polar covalent bond), C=O (carbonyl), C-H (alkenyl), N-H (hydrogen bond), C-N (carbon–nitrogen single bond), and C≡N (triple carbon–nitrogen bond). Further, penetration tests on an agarose hydrogel model confirmed that the probes are mechanically robust, with their penetrating forces being much lower than the fracture force of the probe material.

Nature of the bond in intermetallic compounds

By studying 398 binary compounds crystallizing in five different crystal structures, it is shown that atomic radii of the elements RA and RB change to reach a constant radius ratio rA/rB(≡ϒ) in a cubic system. In systems of lower symmetry, a range of ϒ values, each corresponding to a certain axial ratio, is possible. Accurate charge values on atoms in solids are derived from the experimental internal radii rA and rB, using Shannon’s compilation of radii versus charge states for elements. It is found, for the first time, that charge transfer and radii change reverse their direction at the ϒ point. Electronegativity difference relative to its value at the ϒ point is recognized to correctly predict the charge transfer and radii changes with RA/RB. The deduced electronic structure of solids, based on spherical ions carrying partial charges, confirms that the metallic atom-pair bond is a resonating polar covalent bond. Identification of the number of electrons that contribute to cohesion and electrical conduction individually, allows successful prediction of electrical conductivity variation for transition metals and p-elements in the periodic table. From a knowledge of accurate valence of elements and of charge on the atoms, variation in the 197Au isomer shift of the gold compounds of CsCl type structure is predicted. The charges on atoms computed using the Quantum Theory of Atoms in Molecules (QTAIM) technique agree with those obtained in our work for compounds belonging to two of the structure types. For compounds belonging to the other three, the charge values obtained by the two methods do not agree. The disagreement correlates to not reckoning the role of the ϒ point. The effect of ϒ point is so fundamental and universal that most physical properties obtained by the secondary processing of primary DFT data, can be influenced by it.

Investigation of the effect of temperature and applied electric field on the aluminum-water reaction by ReaxFF molecular dynamics

Hydrogen energy is gaining attention as a non-polluting source. Aluminum’s reaction with water for propulsion and hydrogen production is widely used, but its nanoscale characteristics are unexplored. This study investigated aluminum hydrolysis in water vapor by ReaxFF-MD, examining effects of temperature and electric field. The oxidation of the aluminum surface is accompanied by aluminum dissolution, hydrogen generation and H3O+ formation, Formed hydrides and oxides were AlH3 and Al2O3. Hydrogen disappearance at 700 K and 900 K was due to conversion to AlH3. The diffusion of hydrogen and oxygen atoms in the Z direction was enhanced at increasing temperatures, Outer aluminum atoms preferentially reacted with oxygen to form Al2O3, while inner atoms reacted with hydrogen to form AlH3, the loosely packed outer oxide layer contains inclusions of AlH3. Studies at room temperature under different electric fields showed −Z field promoted the reaction while + Z field inhibited it, attributed to different electric field strengths’ effect on OH– ion electron-sharing Coulomb equilibrium, resulting in varying rates of hydroxide polar covalent bond cleavage. Our results clarify the atomic-scale effects of temperature and electric field on the aluminum-water reaction, providing guidance for optimizing experimental conditions and enhancing the efficiency of hydrogen production.

Achieving hardness‐strength‐toughness synergy in (Ti, W, Mo, Cr)(C, N)‐based cermets

AbstractTi(C, N)‐based cermets have been considered to be the most potential candidates for WC‐Co cemented carbides as tool material due to their various advantages. However, the trade‐off between hardness/strength and toughness limits their further application. Herein, we present new (Ti, W, Mo, Cr)(C, N)‐based cermets showing superior mechanical properties with hardness of 1525 MPa, transverse rupture strength of 2428 MPa, and fracture toughness of 11.44 MPa·m1/2 by compositional and interfacial modification. The strengthening and toughening mechanisms were revealed by experimental observation and theory calculation. It could be clarified that the high elastic modulus caused by a polar covalent bond in the hard phase and solid solution strengthening of the binder phase attributed to the hardness. The strong interface bonding between the core/rim, inner/outer rim, and rim/binder phases stemming from the composition optimization contributed to super crack resistance. Intergranular fracture in submicron‐scaled hard phases led to the crack deflection, transgranular fracture in the micron‐scaled hard phases consumed more energy due to their high intrinsic hardness and excellent interface coordination. The synergy of multi‐scaled hard particles brought about excellent comprehensive mechanical properties.

The characteristics and mechanical properties of Mo/VC interface structures via first-principles calculations

In the present work, 15 Mo/VC interfaces were investigated using first-principles calculations based on density functional theory. Four possible interface orientations, two terminations, and three stacking sites were considered. The adhesion energy (Wad) and interfacial energy (Eint) of these interface models were computed. The results indicate that the C-terminated hollow-site Mo(110)/VC(111) interface exhibits the highest stability with a larger Wad value of 10.64 J m−2 and the lowest Eint value of 2.98 J m−2, followed by the V-terminated central-site Mo(211)/VC(220) interface. Analysis of the electronic structure reveals the formation of strong polar covalent bonds at these interfaces. Additionally, simulations of tensile fracture processes were performed, demonstrating that at strains reaching 22% and 32%, respectively, the ideal tensile strengths for the C-terminated hollow-site Mo(110)/VC(111) interface and V-terminated central-site Mo(211)/VC(220) interface are ∼26.01 and 35.53 GPa. In particular, in the C-terminated hollow-site Mo(110)/VC(111) interface, fracture occurs in the Mo slabs due to concentrated strain when external strain is applied; meanwhile, uniform strain is observed in both Mo(211) and VC(200) slabs within this system. Notably, the V-terminated central-site Mo(211)/VC (220) interface demonstrates excellent tensile strength as well as toughness. These findings suggest that explaining solely based on adhesion work is insufficient to account for the observed tensile strength at these interfaces.

Ferromagnetic ordering in Fe: TiO2 solid solutions: A visual electron density mapping by maximum entropy method using powder X-ray data

Magnetic semiconducting FeTiO2 solid solutions with Fe compositions of 6 %, 9 %, and 12 % were analyzed using multi-phase Rietveld refinement analysis for average structure and maximum entropy for electronic structure. VSM analysis showed that the 9 % and 12 % Fe doped TiO2 compositions had a maximum magnetization of 0.3128 emu/g and coercivity of 574 Oe and 568 Oe, respectively. This confirms ferromagnetic tuning with Fe doping at or above 9 %. The MEM electron density at the bond critical points (BCP) of the apical and equatorial (Ti/Fe)–O bonds in 12 % Fe-doped TiO2 is 1.088 e/Å3 and 0.8652 e/Å3, indicating polar covalent bond. This work primarily examines a new empirical correlation between MEM electron density and magnetic parameters. Fe: TiO2 solid solutions have high magnetization and coercivity due to strong and identical (Ti/Fe) – O apical and equatorial polar covalent bonding, with low interstitial charge buildup at non-regular tetrahedral rutile lattice sites.

Tailoring intrinsic properties for tetrahedral carbon/copper composites construction via electronic interface engineering

The electronic structure induced interface properties are important for active control of the advanced equipment performance. Nevertheless, it is difficult to reveal the intrinsic mechanism by experiments. In this paper, the interfacial properties and electronic characteristics of eight different stacking ta-C/Cu interfaces were studied via first principles method. The work of adhesion of ta-C/Cu(110)-2nd interface the largest (6.19 J/m2) and its interfacial energy is the smallest (3.74 J/m2), which indicates that it is the most stable structure with the smallest interface formation resistance. In addition, the polar covalent bonds dominated by electronic structures are major contributor of the ta-C/Cu interface properties. During tensile process, the critical strain of ta-C/Cu(110)-2nd is the largest (17.0 %), and the tensile strength of ta-C/Cu(100)-2nd is the largest (61.59 GPa). When all interfaces are fractured under tensile force, the atoms on Cu surfaces are transferred to ta-C surfaces and separated into two free parts. When the energy of one of the wave valleys first reaches the threshold of the lowest plane-averaged potential, the interfacial structure breaks at the interfacial gap corresponding to the wave valley. Changes in interfacial bonding properties can therefore be judged by changes in the magnitude of the electrostatic potential.

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds.

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

Halogen-assisted octet binding electrons construction of pnictogens towards wide-bandgap nonlinear optical pnictides

The design of pnictide nonlinear optical crystals is quite different from chalcogenide and oxide those, in which a new paradigm need be developed to regulate the band gap, one of key optical parameters. In this work, two non-centrosymmetric halidepnictides, [Cd2P]2[CdBr4] (CPB) and [Cd2As]2[CdBr4] (CAB) were reported. The complete octet binding electrons of pnictogens were constructed by four Cd-P polar covalent bonds under the anchoring effect of halogens, creating an extremely flat valence band maximum with band dispersion of only 0.17 eV. As expected, the balance of the covalency and ionicity in CPB and CAB was successfully realized, leading to a wide band gap of 2.58 eV and 1.88 eV. Remarkably, CPB not only has a widest band gap among Cd-containing pnictides, but also exhibits a SHG effect of 1.2 × AgGaS2, moderate birefringence (0.088@visible light and calcd. 0.043@2050 nm) and a wide IR transmission range. This is the first time that the octet binding electrons construction strategy was utilized to design non-diamond like NLO pnictides with excellent performances.

Unusual Raman Enhancement Effect of Ultrathin Copper Sulfide.

In surface-enhanced Raman spectroscopy (SERS), 2D materials are explored as substrates owing to their chemical stability and reproducibility. However, they exhibit lower enhancement factors (EFs) compared to noble metal-based SERS substrates. This study demonstrates the application of ultrathin covellite copper sulfide (CuS) as a cost-effective SERS substrate with a high EF value of 7.2×104 . The CuS substrate is readily synthesized by sulfurizing a Cu thin film at room temperature, exhibiting a Raman signal enhancement comparable to that of an Au noble metal substrate of similar thickness. Furthermore, computational simulations using the density functional theory are employed and time-resolved photoluminescence measurements are performed to investigate the enhancement mechanisms. The results indicate that polar covalent bonds (Cu─S) and strong interlayer interactions in the ultrathin CuS substrate increase the probability of charge transfer between the analyte molecules and the CuS surface, thereby producing enhanced SERS signals. The CuS SERS substrate demonstrates the selective detection of various dye molecules, including rhodamine 6G, methylene blue, and safranine O. Furthermore, the simplicity of CuS synthesis facilitates large-scale production of SERS substrates with high spatial uniformity, exhibiting a signal variation of less than 5% on a 4-inch wafer.

Unveiling the sp2 ─sp3 C─C Polar Bond Induced Electromagnetic Responding Behaviors by a 2D N-doped Carbon Nanosheet Absorber.

The infertile electromagnetic (EM) attenuating behavior of carbon material makes the improvement of its performance remain a significant challenge. Herein, a facile and low-cost strategy radically distinct from the prevalent approaches by constructing polar covalent bonds between sp2 -hybridized and sp3 -hybridized carbon atoms to introduce strong dipolar polarization is proposed. Through customizing and selectively engineering the N moieties conjugated with carbon rings, the microstructure of the as-synthesized 2D nanosheet is gradually converted with the partial transition from sp3 carbons to sp2 carbons, where the electric dipoles between them are also tuned. Supported by the DFT calculations, a progressively enhanced sp2 ─sp3 C─C dipolar polarization is caused by this controllable structure evolution, which is demonstrated to contribute dominantly to the total dielectric loss. By virtue of this unduplicated loss behavior, a remarkable effective absorption bandwidth (EAB) beyond -10dB of 8.28GHz (2.33mm) and an ultrawide EAB beyond -5dB of 13.72GHz (4.93mm) are delivered, which upgrade the EM performance of carbon material to a higher level. This study not only demonstrates the huge perspective of sp2 ─sp3 -hybridized carbon in EM elimination but also gives pioneering insights into the carbon-carbon polarization mechanism for guiding the development of advanced EM absorption materials.

Enhancing electron transport of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene through polarity reduction of polar covalent bonds using organic molecular additives

In the electron-transporting organic semiconductor, polar covalent bonds are a necessary design to deepen the Lowest Occupied Molecular Orbitals for effective charge injection. However, these bonds can increase molecular dipole moment, which can impede charge transport. In this study, molecular additives including Tetrathiafulvalene (TTF), 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), and tetrakis(dimethylamno)ethylene (TDAE) were investigated. Despite being electron donors, no charge transfer occurred between the molecular additives and the π conjugated core of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13). However, the addition of 2% TTF resulted in a higher current density, while N-DMBI and TDAE led to a decrease. X-ray photoelectron spectroscopy, density functional theory, and reduced density gradient analysis indicated that a marginal charge transfer occurred from phosphorus atoms to TTF, forming a strong attractive bond between P = O and TTF, which was absent in N-DMBI and TDAE. The low molecular polarity and lack of polar covalent bonds are prerequisites for polarity reduction of the P = O bond in SPPO13, as demonstrated by the 2% addition of TTF resulting in higher electron transport current.

Adsorption behavior of monacolin K onto B12N12 and B12P12 fullerenes as Cardioprotective agent: Spectroscopic and DFT study

This study identified the adsorption behavior of monacolin K (MK) onto B12N12 and B12P12 fullerenes in both vacuum and water environments and determines the structural and electronic properties via the density functional theory (DFT) method. The adsorption energy of MK onto B12N12 and B12P12 fullerenes are calculated to be −0.92(A), −1.01 (B), −0.73 (C), −0.68 eV (D), and −0.89 eV (E). Complex B in comparison with other complexes has a strong hybridization between the drug (through the oxygen head of the lactone ring) and the B12N12 surface (boron atom) because of the formation of a polar covalent bond. Changes in the dipole moment of the studied structures demonstrate enhanced solubility. According to the thermodynamic parameters, the Gibbs free energy (ΔGads) values for complexes A, B, and C are found to be −0.21, −0.30, and −0.08 eV, whereas the enthalpy (ΔHads) values for these complexes are found to be −0.86, −0.96, and −0.74 eV, respectively. After MK adsorption onto the B12N12 and B12P12 fullerenes, the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are remarkably shifted, leading to a reduction in the energy gap (Eg) values and improved their electrical conductivity. Therefore, it is predicted that MK drug onto the B12N12 fullerene might be a good delivery carrier that will be an excellent vehicle to help save people.

First-principles study on surface corrosion of 6082 aluminum alloy in H+ and Cl− medium

In order to explore the corrosion mechanism of the 6082-aluminum alloy (Al alloy) in environments containing H+ and Cl−, we employ first-principle calculations to study the adsorption behaviors of H+ and Cl− on the surfaces of the Al matrix and its precipitated phases. This analysis is complemented with electrochemical corrosion tests to investigate the corrosion mechanism and the nature of the passivation film (Al2O3) formed on the 6082 Al alloy in the presence of H+ and Cl− ions. Our findings show that the adsorption energy of H+ on the Al (111) surface (-4.911 eV) is lower than that of Cl− (1.436 eV), indicating a stronger adsorption affinity for H+. Both H-Al and Cl-Al formed polar covalent bonds. For the Mg2Si (111) surface adsorption, the adsorption energies for Mg interaction with both ions (-4.829 eV for H+ and -4.893 eV for Cl−) are lower than those for Si, suggesting a stronger bond between H+ and Mg. Notably, the surface of the sample is preferentially corroded to the adsorbed Mg2Si phase, inferring its higher activity. In addition, when compared to the HCl solution, the samples in the NaCl solution exhibit a substantially lower corrosion current density(icorr) and corrosion rate. In contrast to the NaCl medium, the samples immersed in the HCl solution did not form a passivation film, leading to more active corrosion behavior with deeper corrosion pits. It is worth mentioning that the interfacial bonding between the passivation film and the underlying substrate is stronger, with an interfacial adhesion work of Al2O3 (0001)/Al (111) of 0.654298 J/m2, wherein the Al2O3 is well bonded to the surface of the 6082 Al alloy to inhibit corrosion. This study contributes a deeper understanding of the corrosion behavior exhibited by Al, thereby offering valuable insights for the design and application of high-performance corrosion-resistant Al alloys.

Designing Zr12O12 nanocage for the delivery of anticancer drugs: a theoretical study

A novel superatom-assembled Zr12O12 nanocage has been theoretically designed and characterized to investigate its potential application as a novel delivery carrier for 5-fluorouracil (5-Fu), mercaptopurine (MP), and thioguanine (TG) via density functional theory calculations in this work. The designed Zr12O12 nanocage possesses high stability in view of its large binding energy (E b), atomic cohesion energy (E col), and highest occupied molecular orbital-lowest unoccupied molecular orbital gap. Our results reveal that Zr12O12 tends to bind with 5-Fu via a single Zr–O bond and combine with MP and TG through multidentate chelate modes with the adsorption energies of −22.27 to −55.19 kcal mol−1. The Wiberg bond index, atoms in molecules theory, and localized molecular orbitals analyses demonstrate that all the newly formed linkage bonds between Zr12O12 and drugs are polar covalent bonds. In particular, among these studied drugs, the recovery time for the near-infrared light-triggered release of TG drug from Zr12O12 surface is the shortest, indicating that Zr12O12 can serve as an excellent candidate for the delivery of TG. This study not only offers a new member to enrich the inorganic nanocage family but also provides a potential carrier for the delivery of anticancer drugs.

Electronic characteristic, tensile cracking behavior and potential energy surface of TiC(111)/Ti(0001) interface: A first‑principles study

The preparation of TiC coating on Ti alloy surface to improve the wear resistance has attracted attention from researchers in aerospace field. The service life of TiC coating is related to the interfacial adhesion properties between TiC coating and Ti alloy substrate. However, it is difficult to explain its interfacial adhesion mechanism by experimental methods. Based on the termination of atoms on the TiC surface, two TiC/Ti interface models named as the C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface were constructed by first-principles. The interfacial electronic characteristic of C—Ti bond is a mixture of polar covalent and metal bonds, and that of Ti—Ti bond is the metal bond. The tensile strains of both C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface in the fracture stage are ranged from 12% to 14%. Their maximum tensile stresses are 16.201 GPa and 15.590 GPa, respectively. The sliding potential energy surface maximum of C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface are 5.387 J/m2 and 0.271 J/m2, respectively. And the sliding potential barriers on the minimum energy path are 2.094 J/m2 and 0.136 J/m2 with an ideal shear strength of 20.32 GPa and 1.61 GPa, respectively. In summary, the interfacial adhesion property of C-terminated-TiC(111)/Ti(0001) interface is better than that of Ti-terminated-TiC(111)/Ti(0001) interface.

Bonding and Tunneling.

Quantum mechanical electron tunneling is proposed as the mediator of chemical bonding. Covalent, ionic, and polar covalent bonds all rely on quantum mechanical tunneling, but the nature of tunneling differs for each bond type. Covalent bonding involves bidirectional tunneling across a symmetric energy barrier. Ionic bonding occurs by unidirectional tunneling from the cation to the anion across an asymmetric energy barrier. Polar covalent bonding is a more complicated type of bidirectional tunneling, consisting of both cation-to-anion and anion-to-cation tunneling across asymmetric energy barriers. Tunneling considerations suggest the possibility of another type of bond-denoted polar ionic-in which tunneling involves two electrons across asymmetric barriers.

Adhesion and mechanical properties of Zr/SiC interfaces: Insight from characteristics of structure and bonding by first-principles calculations

The structural, electronic, adherent and mechanical properties of the Zr/3C-, 4H-, and 6H-SiC interfaces are systematically investigated by first-principles calculations, where four types of C(I, II)- and Si(I, II)-terminated interfaces are focused. The Zr/SiC interface prefers to form C-termination, owing to stronger covalent bonding of Zr-C across the interface. The active C(I)-termination favors to form a more stable interface than the C(II)-termination. Intriguingly, the sequence is inverse for Si-termination although processing similar topological structures. Electronic analysis reveals that ionic attraction of C-Zr plays a crucial role in promoting the adhesion of the Si(II)-terminated interface although covalent, metallic and ionic bonds coexist in the interfaces. When imposing tension, cleavage takes place from Zr-Zr interlays for all SiC-Zr interfaces studied here. The resulting ideal strengths are very close. Coating SiC slightly enhances SiC-Zr claddings, but at the expense of ductility of Zr claddings. This owes to weakening the nature of metallic bonding through polar covalent bonds. We expect that the results provide theoretical guidance for the development of excellent corrosion-resistant claddings by means of combining merits of metals and ceramics in engineering.