Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Effect of spraying power on microstructure and bonding strength of MoSi2-based coatings prepared by supersonic plasma spraying

MoSi₂-based oxidation protective coatings for SiC-coated carbon/carbon composites were prepared using a supersonic plasma spraying at the power of 40 kW, 45 kW, 50 kW and 55 kW, respectively. Effect of spraying power on the microstructure and bonding strength of MoSi₂-based coatings was studied. The results show that coatings become more and more compact and the bonding strength increases when the spraying power increases from 40 kW to 50 kW. At the power of 50 kW, the coatings were dense and the bonding strength reached a maximum value of 14.5 MPa. As the spraying power is of sufficient magnitude, many cracks and pores reappaer in coatings and the bonding strength between coating and substrate also decreases.     

Influence of initial surface reconstruction on the interface structure of HfO2/GaAs

We show that the bonding structures and electrical properties of the HfO₂/GaAs interface can be controlled by a choice of the reconstruction on the initial GaAs surface. Electron-beam evaporation of HfO₂ onto the c(4 × 4) surface yielded As-O bonds at the interface, while Ga-O bonds were dominant at the interfaces formed on the (2 × 4) and (4 × 6) surfaces. Influences of the initial surface reconstruction on the interface structure persisted even after annealing at 673 K. Electrical characterization of Ir/HfO₂/GaAs capacitors indicated that the interfacial As-O bonds cause weak Fermi level pinning. It was also suggested that the interfaces dominated by the Ga-O bonds have trapping states in the upper half of the GaAs bandgap.     

Estimation of surface energy and bonding between AlMgB14 and TiB2

Composites of AlMgB₁₄ with TiB₂ display a positive deviation from the rule-of-mixtures for hardness and wear resistance [1] (Ahmed et al., 2006). This suggests exceptionally strong bonding between the two boride phases. A contributing factor to the strong bonding may be a close matching of surface energy in the two phases. A calculation was performed to estimate the surface energy and its temperature dependence in AlMgB₁₄, based on the critical crack energy release rate and assuming covalent bonding. Results predict that the surface energy of the two phases differs by about 0.4 J/m². Similar results were found for other transition metal diborides, and the surface energy of AlMgB₁₄ was close to that of TiC and TiN, two materials commonly used as sintering aids for TiB₂.     

Effect of O-vacancy defects on the Schottky barrier heights in Ni/SiO2 and Ni/HfO2 interfaces

We perform first-principles density functional calculations to study the electronic structure of Ni/HfO₂ and Ni/SiO₂ interfaces and the effect of O-vacancy (V_O) defects on the Schottky barrier height and the effective work function. We generate two interface models in which Ni is placed on O-terminated HfO₂ (1 0 0) and α-quartz (1 0 0) surfaces. As the concentration of V_O defects at the interface increases, the p-type Schottky barrier height tends to increase in the Ni/HfO₂ interface, due to the reduction of interface dipoles, whereas it is less affected in the Ni/SiO₂ interface.     

MoSi2 laser cladding—a comparison between two experimental procedures: Mo–Si online combination and direct use of MoSi2

There are very strong interests in developing low density advanced material systems for service at temperatures up to 1300°C. These materials should mainly have moderate fracture toughness at low and intermediate temperatures and should exhibit oxidation resistant behaviour. The intermetallic compound, MoSi₂ has been considered to be an attractive candidate due to its melting point (2030°C) and excellent oxidation resistance at high temperatures. In this paper, we compare the results obtained with two different techniques for laser cladding, one using an online combination between Mo and Si powders, the second using direct injection of the MoSi₂ powder.     

Improvement of interfacial adhesion in vertical GaN-based LEDs by introducing O2 plasma cleaning and intermediate layers

Interfacial adhesion between an indium tin oxide (ITO)/Ni/Ag/Ni/Au p-electrode, and Au and Ni/Au seeds in vertical GaN-based light emitting diodes (LEDs) was enhanced by O₂ plasma cleaning treatment of the Au surface in the p-electrode. However, AES and REELS analyses of the Au surface in the p-electrode detected surface damage to the p-electrode and photoresist (PR) passivation structure from O₂ plasma cleaning. W/Ni and Al/Ni adhesion layers were introduced in the Au seed to increase interfacial adhesion between Au seed and untreated PR passivation. Forward leakage current as low as 0.91 nA at 2 V was observed for the vertical LED with the Al/Ni/Au seed, for which adhesion strength to O₂ plasma-cleaned Au and untreated PR was 141.2 MPa and 62.8 MPa, respectively.     

Optical properties of sol-gel fabricated Ni/SiO2 glass nanocomposites

Nickel nanoparticles were grown in silica glass by annealing of the sol-gel prepared silicate matrices doped with nickel nitrate. TEM characterization of Ni/SiO₂ glass proves the formation of isolated spherical nickel nanoparticles with mean sizes 6.7 and 20 nm depending on annealing conditions. The absorption and photoluminescence spectra of Ni/SiO₂ glasses were measured. In the absorption spectra, we observed the band related to the surface plasmon resonance (SPR) in Ni nanoparticles. The broadening of SPR was observed with decrease of Ni nanoparticle size. The width of the surface plasmon band decreases 1.5 times at the lowering of temperature from 293 to 2 K because of strong electron-phonon interaction. The spectra proved the creation of nickel oxide NiO clusters and Ni²⁺ ions in silica glass as well.     

DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

A new quasi-one-dimensional molecular solid based on Ni(mnt)2 anion: Crystal structure and spin-gap transition

A new molecular solid, [1-(4′-bromo-2′-fluorobenzyl)-4-dimetylaminopyridinium]-bis(maleonitriledithiolato)nickel(III), (BrFBzPyN(CH₃)₂(Ni(mnt)₂)(1), has been prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. Compound 1 crystallizes in the orthorhombic space group Pnma, a=20.579(4) Å, b=7.078(1) Å, c=17.942(4) Å, α=β=γ=90°, V=2613.3(9) Å³, Z=4. The Ni(III) ions of 1 form a quasi-one-dimensional Zigzag magnetic chain within a Ni(mnt)₂⁻ column through Ni?S, S?S, Ni?Ni, or π?π interactions with an Ni?Ni distance of 4.227 Å. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 200 K, and antiferromagnetic interaction in the high-temperature phase (HT) and spin gap in the low-temperature phase (LT). The transition for 1 is second-order phase transition as determined by DSC analyses.     

Effects of substrates on the structure and dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films

Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films were grown on Pt(1 1 1)/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt(1 1 1)/Ti/SiO₂/Si substrates by a sol-gel processing technique, respectively. The BTS thin films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si and annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates exhibited strong (1 1 1) and perfect (1 0 0) orientations, respectively. The BTS thin films grown on un-annealed Pt(1 1 1)/Ti/SiO₂/Si substrates showed random orientation with intense (1 1 0) peak, while the films deposited on un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate exhibited random orientation with intense (1 0 0) peak, respectively. The dielectric constant of the BTS films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si, annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si, un-annealed Pt(1 1 1)/Ti/SiO₂/Si and un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates was 512, 565, 386 and 437, respectively, measured at a frequency of 100 kHz. A high tunability of 49.7% was obtained for the films deposited on annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate, measured at the frequency of 100 kHz with an applied electric field of 200 kV/cm. The high tunability has been attributed to the (1 0 0) texture of the films and larger grain sizes.     

DFT study of Ni-CeO2 interaction: Adsorption and insertion

DFT-GGA calculations are used to investigate interaction of atomic nickel with ceria. Nickel adsorption on surfaces is compared with insertion into the bulk and subsurfaces using VASP calculations. The adsorption is considered upon the (1 1 1) and (1 1 0) surfaces of ceria since these surfaces are the most stable ones being formally generated from the least number of bond cleavages (one or two Ce-O bonds per Ce, respectively). When Ni atom is adsorbed on the (1 1 1) surface, it occupies a position atop whereas over the (1 1 0) surface it occupies a bridging position. The adsorption quenches the spin. Results for insertion both in the bulk material and in the (1 1 1) and (1 1 0) subsurfaces are presented. For the bulk, it is shown that an increase of Ni amount from 1/4 to 1 makes insertion more exothermic. The later is accompanied by a lattice expansion and a reduction of symmetry. For an amount 1/4, the Ni is inserted to a tetrahedral site. At larger concentration, it is in trigonal environment of three oxygen atoms, additional oxygen ligands being less tightly bound. For insertion in the sublayers of the (1 1 1) subsurface, the nickel atom occupies a similar position, also a ternary site. The interaction energy for nickel atom insertion is comparable to that for nickel adsorption, slightly larger for (1 1 1), slightly smaller for (1 1 0) surface. Diffusion into the bulk is thus likely. When inserted, the distance Ni-Ce is becomes short, 2.70 Å, in agreement with experiment and the system may evolve with the formation of a Ni₂Ce alloy.     

Preparation and characterization of nickel nano-Al2O3 composite coatings by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique and conventional electroplating (CEP) technique from Watt's type electrolyte without any additives. The microstructure, hardness, and wear resistance of resulting composites were investigated. The results show that the incorporation of nano-Al₂O₃ particles changes the surface morphology of nickel matrix. The preferential orientation is modified from (2 0 0) plane to (1 1 1) plane. The microhardness of Ni-Al₂O₃ composite coatings in the SCD technique are higher than that of the CEP technique and pure Ni coating and increase with the increasing of the nano-Al₂O₃ particles concentration in plating solution. The wear rate of the Ni-Al₂O₃ composite coating fabricated via SCD technique with 10 g/l nano-Al₂O₃ particles in plating bath is approximately one order of magnitude lower than that of pure Ni coating. Wear resistance for SCD obtained composite coatings is superior to that obtained by the CEP technique. The wear mechanism of pure Ni and nickel nano-Al₂O₃ composite coatings are adhesive wear and abrasive wear, respectively.     

Ab initio study of phase stability,thermodynamic and elastic properties of C3N2 derived from cubic C20

In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that β-C₃N₂ is the only phase under high pressure, and α-C₃N₂ does not exist. β-C₃N₂ is a covalent crystal composed of strong CC and CN covalent bonds. Band gap of β-C₃N₂ increases with pressure. The width of antibonding state, shown in partial density of states (PDOS), keeps about 5 eV with rising pressures, which brings stable CN or CC covalent bonds. At sufficiently low temperatures, heat capacity (C_v) is proportional to T³; and at intermediate temperatures, C_v is governed by the details of vibrations of the atoms; finally, C_v reaches to β-C₃N₂'s Dulong–Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (α) increases with temperature, α is less than 1×10⁻⁵ K⁻¹. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Observation of large magnetic resistance in weak magnetic fields using CuPt/SiO2/Si/SiO2/CuPt structure

The magnetoresistive effect of CuPt(8 nm)/SiO₂(5 nm)/Si(50,000 nm)/SiO₂(5 nm)/CuPt(8 nm) structure made by e-beam evaporation technique is studied in this work. Variation in magnetoresistance obtained by I-V measurements at 77 K and in the presence of less than 5 mT magnetic field applied in parallel to the surface is investigated. We have found that this structure exhibit large magnetoresistance in low magnetic fields (i.e. <5 mT). Our results also indicate that the variation in magnetoresistance in the presence of external magnetic field has oscillatory behavior and has the maximum value of 3295%. This structure due to its high sensitivity to low magnetic fields can also be used as an active element in magnetic field sensor devices.     

Influence of MgO and ZrO2 buffer layers on dielectric properties of Ba(Zr0.20Ti0.80)O3 thin films prepared by sol-gel processing

Ba(Zr_0.20Ti_0.80)O₃ (BZT) thin films are deposited on Pt(1 1 1)/Ti/SiO₂/Si, MgO and ZrO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si substrates by a sol-gel process. The BZT thin films directly grown on Pt(1 1 1)/Ti/SiO₂/Si substrates exhibit highly (1 1 1) preferred orientation, while the films deposited on Pt(1 1 1)/Ti/SiO₂/Si substrates with MgO and ZrO₂ buffer layers show highly (1 1 0) preferred orientation. At 100 kHz, dielectric constants are 417, 311 and 321 for the BZT thin films grown on Pt(1 1 1)/Ti/SiO₂/Si, MgO and ZrO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si substrates, respectively. The difference in dielectric properties of three BZT films can be attributed to the series capacitance effect, interface conditions and their orientations.     

First-principles calculations on Al/AlB2 interfaces

The AlB₂ (1 1 1) surfaces and Al (1 1 1)/AlB₂ (0 0 0 1) interface were studied by first-principles calculations to clarify the heterogeneous nucleation potential of α-Al grains on AlB₂ particles in purity aluminium and hypoeutectic Al-Si alloys. It is demonstrated that the AlB₂ (0 0 0 1) surface models with more than nine atomic layers exhibit bulk-like interior, wherein the interlayer relaxations localized within the top three layers are well converged. The outmost layer of AlB₂ free surface having a preference of metal atom termination is evidenced by surface energy calculations. With Al atoms continuing the natural stacking sequence of bulk AlB₂, Al-Al metallic bonds are formed across interface during the combination of Al atoms with Al-terminated AlB₂ surface. The calculated interfacial energy of the Al/AlB₂ interface is much larger than that between the α-Al and aluminium melts, elucidating the poor nucleation potency of α-Al grains on AlB₂ particles from thermodynamic considerations.     

Chloromethane surface chemistry on Fe3O4(1 1 1)-(2 × 2): A thermal desorption spectrometry comparison of CCl4, CBr2Cl2, and CH2Cl2

The surface chemistry of CBr₂Cl₂ on the Fe₃O₄(1 1 1)-(2 × 2) selvedge of single-crystal α-Fe₂O₃(0 0 0 1) has been investigated using temperature programmed reaction and desorption (TPR/D) measurements. The spectra obtained in this case show that strong chemisorption occurs and that a series of adsorbed halogenated reaction products are present. By comparison, studies of the adsorbed phase of CH₂Cl₂ show that only physisorption occurs. The TPR/D spectra of CBr₂Cl₂ show that dissociative formation of CCl₂ followed by its reaction with lattice oxygen is central to the monolayer reaction chemistry in this chloromethane. The branching ratios of the various desorbed products are compared with those obtained from CCl₄ adsorbed on the same (2 × 2) surface.     

A study of the properties and microstructure of Ni81Fe19 ultrathin films with MgO

The anisotropic magnetoresistance (AMR) of a Ta (5 nm)/MgO (3 nm)/Ni₈₁Fe₁₉ (10 nm)/MgO (2 nm)/Ta (3 nm) film with MgO-Nano Oxide Layer (NOL) increases dramatically from 1.05% to 3.24% compared with a Ta (5 nm)/Ni₈₁Fe₁₉ (10 nm)/Ta (3 nm) film without the MgO-NOL layer after annealing at 380 °C for 2 h. Although the MgO destroys the NiFe (1 1 1) texture, it enhances the specular electron scattering of the conduction electrons at the NOL interface and suppresses the interface reactions and diffusion at the Ta/NiFe and NiFe/Ta interfaces. The NiFe (1 1 1) texture was formed after the annealing, resulting in a higher AMR ratio. X-ray photoelectron spectroscope results show that Mg and Mg²⁺ were present in the MgO_x films.