Deposition of Gd2Zr2O7 single buffer layers with different thickness for YBa2Cu3O7−δ coated conductors on metallic substrates

A series of Gd₂Zr₂O₇ (GZO) single buffer layers with different thicknesses were epitaxially grown on highly textured Ni–5 at.% W tapes using pulsed laser deposition. These allow the subsequent growth of high-quality superconducting YBa₂Cu₃O_7?δ layers. The superconducting transition temperature T_c reaches a maximum value of 92.4 K as well as a narrow transition width of 0.8 K for the optimized GZO layer thickness. The inductive measurements show the critical current density as high as 1.2 MA/cm² at 77 K in self-field, indicating that a GZO single buffer layer is a suitable alternative for simplifying the second generation high T_c superconducting coated conductors architecture.     

Electrical and structural investigations of perovskite structure reactively sputter deposited coatings

Sr(Zr_0.84Y_0.16)_0.91O_3 ? δ (SZY) and Ba(Zr_0.84Y_0.16)_0.96O_3 ? δ (BZY) protonic conductor coatings were co-sputter deposited from metallic targets in argon–oxygen reactive gas mixtures. The chemical and structural features were investigated by energy dispersive X-ray spectroscopy and X-ray diffraction, and their morphology was assessed by scanning electron microscopy of the surface and of brittle fracture cross sections. The electrical properties of the coating were determined by complex impedance spectroscopy as a function of temperature in air. Relationships are established between the electrical properties and the morphology of the coatings. The SZY as deposited coatings is amorphous and crystallises under the convenient perovskite structure after annealing treatment at 873 K under air. The BZY as deposited coatings is crystallised at 523 K in situ under perovskite structure and a further annealing treatment increases the grain size. Conductivities and activation energies of crystallised coatings were 3.1 · 10^? 5 S cm^? 1/2 · 10^? 5 S cm^? 1 and 0.65 eV/0.71 eV after stabilization at 773 K for strontium and barium zirconate, respectively.     

Dissociation of dimethyl ether at high temperatures

The decomposition of dimethyl ether (CH₃OCH₃) has been investigated behind incident shock waves in a diaphragmless shock tube using laser schlieren densitometry, LS (T = 1500–2450 K, P = 57 ± 4, 125 ± 5 and 253 ± 12 Torr). The LS density gradient profiles were simulated and excellent agreement was found between the simulations and experimental profiles. Rate coefficients for CH₃OCH₃ → CH₃O + CH₃ were obtained. They showed strong fall-off, and at the lower end of the experimental temperature range are close to the low pressure limit. First order rate coefficient expressions were determined over 1500 < T < 2450 K. k_57Torr = (3.10 ± 1.0) × 10⁷⁹T^?19.03 exp(?54417/T) s^?1, k_125Torr = (1.12 ± 0.3) × 10⁸³T^?19.94 exp(?55554/T) s^?1and k_253Torr = (1.02 ± 0.3) × 10⁷³T^?17.09 exp(?51500/T) s^?1. The effect of a roaming channel for decomposition of dimethyl ether was assessed and the best agreement was obtained with 1% dissociation of DME via the roaming path.     

Stabilization of high spin FCC Fe in (Fe/Pd)n multilayers

Polycrystalline (Fe/Pd)_n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t_Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d_Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d_Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd)_n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ_B, which is about 20% larger compared to μ=2.2 μ_B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.     

Effect of B4C diffusion barriers on the thermal stability of Sc/Si periodic multilayers

The optical properties of Sc/Si periodic multilayers are analyzed at three wavelengths in the X-ray range: 0.154, 0.712 and 12.7 nm. Fitting the reflectivity curves obtained at these three wavelengths enable us to constrain the parameters, thickness, density and roughness of the various layers, of the studied multilayers. Scattering curves were also measured at 12.7 nm on some samples to obtain an estimate of the correlation length of the roughness. Two sets of multilayers are used, with and without B₄C diffusion barrier at the interfaces. To see the efficiency of the B₄C layers the measures are performed after annealing up to 400 °C. A dramatic change of the structure of the Sc/Si multilayer is observed between 100 and 200 °C leading to a strong loss of reflectivity. For the Sc/B₄C/Si/B₄C multilayer the structure is stable up to 200 °C after which a progressive evolution of the stack occurs.     

Synthesis and evaluation of TiN–WC/TiN nanocomposite by the hybrid technique with arc ion plating and magnetron sputtering

By adjusting the power of WC target, novel TiN–WC films with different proportions of WC phase were prepared on TiN interlayer using the hybrid technique of arc ion plating (AIP) and magnetron sputtering (MS). The TiN–WC films were characterized by XRD, XPS, AFM, FESEM, Nano-indenter, and UMT-2MT tribometer. The TiN–WC film that is composed with TiN and WC phases was grown by 15–25 nm nanodotes along the primarily growth direction TiN (1 1 1) in AIP interlayer. Among the three TiN–WC films deposited at various powers of WC target, the TiN–WC2 (500 W) has the highest deposition rate of 1.4 nm/min. The content of WC phase increases as WC target power increases in the TiN–WC films. However, the deposition rate of TiN–WC film gains at first and then declines when WC target power exceeds 500 W because of the addictive poisoning of Ti target. In the present case, the incorporation of WC into TiN is found to result in a slight decrease in friction coefficient. Furthermore, the wear mechanism of multilayer AIP TiN films and MS TiN–WC/AIP TiN films was transformed from “severe wear” to “mild wear”. As a result, the 48TiN52WC film of 35 GPa hardness exhibits better tribological performance.     

Characterization of TiN thin films grown by low-frequency (60 Hz) plasma enhanced chemical vapor deposition

The characteristics of TiN thin films grown on glass substrates by very low frequency (60 Hz) PECVD were investigated along with the reactive plasma generated using a 60 Hz power source. The TiN film depositions were performed using a gaseous mixture of H₂, N₂ and TiCl₄ onto a substrate positioned between two electrodes using a floating substrate holder with a heating unit. The substrate is electrically floated to avoid sample damages due to ion bombardment. As-grown TiN films showed a NaCl-type fcc structure with a (200) crystallographic plane, low resistivity (～60 μΩ cm) and gold-like color. Crystallinity was improved, impurities such as O and Cl were reduced, and the atomic ratio of N/Ti became stoichiometric with the increase of substrate temperature. Particularly, no chlorine component was detected above 500 °C. Also, the N₂ partial pressure strongly affected the deposition rate and ratio of N/Ti. Otherwise, impurities and crystallinity barely changed with the change of N₂ pressure. The atomic ratio of N/Ti, impurities, and crystallinity of the films significantly affected the optical and electrical properties. Consequently, we produced stoichiometric Cl-free TiN films with golden color above 500 °C at 60 mTorr. The effects of temperature played an important role in controlling the film properties compared to the N₂ partial pressure.     

In situ characterization of transport properties of superconducting (Cu, C)-system thin films

Transport properties of (Cu, C)Ba₂CuO_x [(Cu, C)-1201] thin films have been characterized by in situ temperature dependence of resistivity, without breaking vacuum from the deposition to the measurement. In in situ transport properties measurements, the obtained results reveal that (Cu, C)Ba₂CuO_x films exhibit T_c > 20 K on the cased of conductivity at 290 K (σ_[290 K]) > 4 × 10² S/cm and temperature coefficient of resistivity (TCR) > 1.5 × 10^?3 K^?1, and doping level of them should be in between under-doped and optimally-doped states. Their results suggest that there would be possible to further increases of T_c, and XPS data suggest that (Cu, C)-system should have the excellent dopability in their charge reservoir and the possibility of low anisotropy.     

Influence of the antireflection,window, and active layers on optical properties of exponential-doping transmission-mode GaAs photocathode modules

In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si₃N₄/Ga_1 ? xAl_xAs/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si₃N₄ antireflection layer of 0.1 μm, the Ga_1 ? xAl_xAs window layer of more than 0.4 μm, and the GaAs active layer of 1.5 μm–2 μm and low average doping concentration.     

Thermal stabilities of metal bottom electrodes for Ta2O5 metal-oxide-metal capacitor structure

Thermal stabilities of various metal bottom electrodes were examined by using a Ta₂O₅ metal-oxide-metal (MOM) capacitor structure. After depositing 10-nm thick Ta₂O₅ on metal-electrode/poly-Si, we performed rapid thermal oxidation (RTO) at 850 °C for 60 s in an O₂ ambient. A chemical-vapor-deposition (CVD) WSi₂ electrode showed satisfactory thermal stability after the RTO, while other examined electrode materials exhibited thermal degradation caused by oxidation failure or interfacial reaction between the substrate poly-Si and the Ta₂O₅. After post-annealing at 650 °C for 30 min (in N₂ condition) with CVD TiN top electrode, an effective oxide thickness (T_ox) of ∼32 Å and a leakage current density of ∼10⁷ A/cm² at 1.25 V were obtained from the MOM capacitor with the WSi₂ bottom electrode. Other electrode materials, such as TiN, TiSi_x, WN_x, W, and Ta, were severely oxidized during the RTO in the MOM structures, and very poor capacitor properties were obtained in terms of T_ox and leakage current.     

Aqueous polymer–nitrate solution deposition of YBCO films

High critical current density YBa₂Cu₃O_7?x (YBCO) films were prepared by solution deposition of aqueous non-fluorine precursors. Non-fluorine polymer-assisted deposition (PAD) processes utilizing rheology modifiers and chelating agents were used to produce 50 nm films with a critical current density (J_c) over 3 MA/cm² and 400 nm films with J_c > 1 MA/cm²_. T_c measurements indicated that films have T_c values near 90 K. The total heat treatment time to produce these high performance films was less than 4 h. Rheology modifiers such as polyvinyl alcohol (PVA) and hydroxyethyl cellulose (HEC) were used to increase the thickness of deposited films independent of the solution cation concentration. Chelating agents such as polyethylene glycol (PEG) and sucrose increased the barium ion solubility. Nitrate crystallization during deposition was controlled through rapid drying with vacuum and coating with hot solutions.     

Epitaxial growth of La2Zr2O7 buffer layers for YBa2Cu3O7−δ coated conductors on metallic substrates using pulsed laser deposition

In the present work, La₂Zr₂O₇ (LZO) buffer layers were deposited using pulsed laser deposition (PLD) on various metallic substrates including epitaxial pure Ni on a LaAlO₃ (LAO) substrate as well as highly textured Ni–5 at.%W tapes. It is shown that the LZO deposited on pure Ni-buffered LAO exhibits a mixed orientation while LZO on Ni–5 at.%W grows epitaxially. This difference may be explained by the existence of a sulphur superstructure on the surface of Ni–5 at.%W tapes, promoting the epitaxial (0 0 l) nucleation of seed layers. Highly textured YBa₂Cu₃O_7?δ layers were prepared either by using a single buffer layer of LZO or bilayer buffers of CeO₂/LZO on Ni–5 at.%W. The superconducting transition temperature (T_c) increases with the LZO thickness, reaching a value of 90 K with a very narrow transition width (1.5 K) for 240 nm thick LZO layers. Inductive J_c measurements at 77 K in self-field show a value of about 0.96 MA/cm² for the thickest LZO layers, which is comparable to the value observed on standard buffer architectures such as CeO₂/YSZ/Y₂O₃.     

Structure and magnetic properties of co-sputtered Co–C thin films

We studied the structure and magnetic properties of co-sputtered Co_1−xC_x thin films using a transmission electron microscope (TEM) and a SQUID magnetometer. These properties were found to depend critically on deposition temperature, T_S, and composition, x. Generally, phase separation into metallic Co and graphite-like carbon phases proceeds with increasing T_S and decreasing x. Plan view and cross-sectional TEM images of the films prepared showed that Co grains about 10–20 nm in diameter and 30–50 nm in height are three-dimensionally separated by graphite-like carbon layers 1–2 nm thick. Optimum magnetic properties with saturation magnetization of 380 emu/cc and coercivity of 400 Oe were obtained for a film with x=0.5 and T_S=350°C.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Heterostructures design for Hf-Nitride/V-Nitride system

The multilayered films were grown via reactive r.f. magnetron sputtering technique by systematically varying the bilayer period (Λ) and the bilayer number (n), while maintaining constant the total coating thickness (~2.4 μm) on silicon(100) substrates. The multilayers were characterized through high-angle X-ray diffraction (HA-XRD), low-angle X-ray diffraction (LA-XRD), HfN and VN layers were analyzed via X-ray Photoelectron Spectroscopy (XPS) and electron and transmission microscopy (TEM). The HA-XRD results showed preferential growth in the face-centered cubic (111) crystal structure for HfN/VN multilayer systems with the (111)[100]_HfN//(200)[100]_VN epitaxial relation. The maximum coherent assembly was observed with presence of satellite peaks.     

Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H₂C₄O₄) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures T_di=245±5 °C (β=5 °C min^?1), 262±5 °C (β=10 °C min^?1), and 275±5 °C (β=20 °C min^?1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 °C (T_c⁺). Further heating with β=10 °C min^?1 in DSC revealed deviation of the baseline around 310 °C (T_i), and a large unusual exothermic peak around 355 °C (T_p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.     

Characterization of Rh,Mo and Rh–Mo particles formed on TiO2(110) surface: A TDS,AES and RAIRS study

The structural and chemical characterization of Rh, Mo and Rh–Mo nanosized clusters formed by physical vapor deposition on TiO₂ single crystal was performed by Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS) and Reflection Absorption Infrared Spectroscopy (RAIRS), applying CO as test molecule. On a slightly reduced titania surface 2D-like growth of Rh was revealed at 300 K up to 0.23 ML coverage by AES and CO-desorption experiments. For CO-saturated Rh particles TDS showed molecular CO desorption in a broad temperature range with T_p = 400, 440, 490 and 540 K (α-states), the latter state appearing only on the smallest Rh particles. The population of γ-state (T_p = 780–820 K) originating from the recombination of C and O atoms on the support began at Θ_Rh = 0.23ML and was maximized at around 1–2 ML Rh coverage, corresponding to 30% dissociation of CO. A possible dissociation precursor on Rh particles is identified as linearly bonded CO on step sites characterized by ν(C–O) of 2017 cm^? 1. Deliberation of CO₂ could not be detected between 170 and 900 K, showing the absence of disproportionation reaction. Instead of oxidizing CO molecules, oxygen atoms stemming from the dissociation of CO attached to the reduced centers of titania, indicating the role of adsorption sites at the perimeter of Rh particles in the decomposition process. 2 ML of predeposited Mo enhanced markedly the dispersion of Rh particles as a result of strong Rh–Mo interaction, but it slightly reduced the molecular α-CO desorption possibly due to enhanced dissociation. The formation of γ-CO was suppressed considerably through elimination of adsorption centers by Mo on the TiO₂ substrate. The reactivity of Rh layers deposited on Mo-covered surface towards CO was reduced after repeated annealing to 600 K due to partial encapsulation of Rh by titania, manifesting in the suppression of the more strongly bonded α-state. Mo-deposits (up to 0.5ML) on Rh particles decreased the saturation coverage of α-CO through a site-blocking mechanism without detectable influence on the binding energy of CO to Rh, indicating Mo island formation. The carbon arising from the decomposition of CO dissolved in the Mo-containing particles formed a solid solution stable even at 900 K, suggesting a possible role of molybdenum carbide regarding the enhanced catalytic activity of Rh clusters.     

The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(1 1 1) substrates by MOCVD

In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si_xN_y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si_xN_y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si_xN_y layer) and B (with Si_xN_y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω-scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si_xN_y intermediate layer significantly improved the optical and structural properties. In sample C (with Si_xN_y layer on Al_0.11Ga_0.89N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.     

Synthesis of smooth and superconducting (Cu, C)–Ba–O/CaCuO2/(Cu, C)–Ba–O films using SrCuO2 buffer

Artificially stacked structures of [(infinite layer CaCuO₂)_k/(superconducting (Cu, C)Ba₂CuO_x:(Cu, C)-1201)_l]_m were fabricated on SrCuO₂ buffer layer by means of sequential deposition of each block using pulsed laser deposition. Smooth and epitaxial growth all over the deposition cycles of (Cu, C)-1201 and CaCuO₂ were confirmed by streak patterns of in situ RHEED. In comparison with (Cu, C)-1201 single layer films, the artificial stacking resulted in a rise of superconducting properties, T_c-onset and T_c(ρ=0) up to 95 K and 65 K, respectively.