Epitaxial growth of YSZ buffer layer for YBa2Cu3O7−δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO₂ buffered biaxially textured Ni-W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa₂Cu₃O_7−δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO₂/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3-4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.     

Preparation of YSZ films by magnetron sputtering for anode-supported SOFC

YSZ films for anode-supported SOFCs were prepared by reactive sputtering method. It was found that the surface morphology of anode substrate has a very important effect on the quality of sputtered films. By applying an anode functional layer and making the anode surface smooth, dense and uniform YSZ films of 10 µm in thickness were successfully fabricated. The sintering behaviors of the sputtered YSZ films were also discussed. It is suggested that the optimized densification condition for the deposited YSZ films is sintering at 1250 °C for 4 h. Single cells with sputtered YSZ film as electrolyte and LSM-YSZ as active cathode materials were tested. 1.08 V open circuit voltage and a 700 mW/cm² maximum power density were achieved at 750 °C by using humidified H₂ as fuel and air as oxidant.     

Porous electrolyte-supported tubular micro-SOFC design

Due to the poor redox cycling resistance of the second generation of μ-SOFCs, a new generation of SOFC has been recently developed using a porous electrolyte-supported structure to overcome this problem. In this research, the porous structure was successfully fabricated with slip casting using calcined YSZ (ZrO₂ + 8 mol% Y₂O₃) with or without graphite as a pore former. Calcination of YSZ powder at 1300-1500 °C prior to making the slip leads to growth of YSZ crystals and particle size which results in a decrease in surface area and powder sinterability. This was found to be an important criterion in developing the porous structure as, due to the high sinterability of non-calcined YSZ, even the addition of graphite is inadequate to generate sufficient open porosity. A dense YSZ electrolyte layer was immediately coated on the porous structure using YSZ calcined at 1300 °C with a sequential slip casting method. Sample thickness was found to be a function of both graphite content as well as YSZ calcination temperature. Physical properties of the porous YSZ supports and SEM analysis of the support and coated electrolyte are presented.     

Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 °C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 °C to 500 °C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of ?-Fe_2-3N and γ′-Fe₄N during plasma nitriding and iron oxide phases of hematite (Fe₂O₃) and magnetite (Fe₃O₄) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8-1.5 μm in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 °C and 450 °C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment.     

Investigation of thermal annealing effects on microstructural and optical properties of HfO2 thin films

In the present paper, we investigate the effect of thermal annealing on optical and microstructural properties of HfO₂ thin films (from 20 to 190 nm) obtained by plasma ion assisted deposition (PIAD). After deposition, the HfO₂ films were annealed in N₂ ambient for 3 h at 300, 350, 450, 500 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE), UV Raman and FTIR were used for the physical characterisation of the as-deposited and annealed HfO₂ thin films. The results indicate that as-deposited PIAD HfO₂ films are mainly amorphous and a transition to a crystalline phase occurs at a temperature higher than 450 °C depending on the layer thickness. The crystalline grains consist of cubic and monoclinic phases already classified in literature but this work provides the first evidence of amorphous-cubic phase transition at a temperature as low as 500 °C. According to SE, XRR and FTIR results, an increase in the interfacial layer thickness can be observed only for high temperature annealing. The SE results show that the amorphous phase of HfO₂ (in 20 nm thick samples) has an optical bandgap of 5.51 eV. Following its transition to a crystalline phase upon annealing at 750 °C, the optical bandgap increases to 5.85 eV.     

Defects in nanocrystalline Nb films: Effect of sputtering temperature

Thin niobium (Nb) films (thickness 350-400 nm) were prepared on (1 0 0)Si substrate in a UHV chamber using the cathode beam sputtering. The sputtering temperature T_s was varied from 40 up to 500 °C and the influence of the sputtering temperature on the microstructure of thin Nb films was investigated. Defect studies of the thin Nb films sputtered at various temperatures were performed by slow positron implantation spectroscopy (SPIS) with measurement of the Doppler broadening of the annihilation line. SPIS was combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have found that the films sputtered at T_s = 40 °C exhibit elongated, column-like nanocrystalline grains. No significant increase of grain size with T_s (up to 500 °C) was observed by TEM. The thin Nb films sputtered at T_s = 40 °C contain a high density of defects. It is demonstrated by shortened positron diffusion length and a high value of the S parameter for Nb layer compared to the well-annealed (defect-free) bulk Nb reference sample. A drastic decrease of defect density was found in the films sputtered at T_s ≥ 300 °C. It is reflected by a significant increase of the positron diffusion length and a decrease of the S parameter for the Nb layer. The defect density in the Nb layer is, however, still substantially higher than in the well-annealed reference bulk Nb sample. Moreover, there is a layer at the interface between the Nb film and the substrate with very high density of defects comparable to that in the films sputtered at T_s < 300 °C. All the Nb films studied exhibit a strong (1 1 0) texture. The films sputtered at T_s < 300 °C are characterized by a compressive macroscopic in-plane stress due to lattice mismatch between the film and the substrate. Relaxation of the in-plane stress was observed in the films sputtered at T_s ≥ 300 °C. The width of the XRD profiles of the films sputtered at T_s ≥ 300 °C is significantly smaller compared to the films sputtered at lower temperatures. This is most probably due to a lower defect density which results in reduced microstrains in the films sputtered at higher temperatures.     

Spray pyrolysis deposition of YSZ and YSZ-Pt composite films

In the present paper procedures are described for spray pyrolysis deposition of YSZ films (0.1-30 μm in thickness) with 8 or 15 mole % of YO_1.5 on different substrates. Aqueous or ethylene glycol solutions of Y-Zr-citrates were used as starting material and O₂ as carrier gas. The crystal structure and the morphology of the films were studied.The optimal deposition and post-deposition annealing conditions were defined, taking into account the desired film thickness and characteristics. Substrate temperatures of 250 °C during the deposition followed by heating for 10 min to 400 °C after every spraying and to 590 °C after every three sprayings with final annealing at 590 °C for 2 h in static air atmosphere were found to be suitable for the production of dense, uniform and cracks-free films.     

Structural characterization of La0.9Ba0.1MnO3/Y-ZrO2 film by X-ray diffraction

Perovskite manganite La_0.9Ba_0.1MnO₃(LBMO) films were deposited on (0 0 1)-oriented single crystal yttria-stabilized zirconia (YSZ) substrate by 90° off-axis radio frequency magnetron sputtering. The film thickness ranged from 10 nm to 100 nm. Grazing incidence X-ray diffraction technique and high resolution X-ray diffraction were applied to characterize the structure of LBMO films. The LBMO film mainly consisted of (0 0 1)-orientated grain as well as weakly textured (1 1 0)-orientated grain. The results indicated that an amorphous layer with thickness of about 4 nm was formed at the LBMO/YSZ interface. The strain in LBMO film was small and averaged to be about -0.14%. The strain in the film was not lattice mismatch-induced strain but residual strain due to the difference in thermal expansion coefficient between film and substrate.     

8YSZ/(La0.8Sr0.2)0.95MnO3-δ cathode performance at 1-3 bar oxygen pressures

Pressurised operation of solid oxide fuel cells (SOFC) has been shown to significantly improve their performance (Singhal, 2000) [1], however little work has been done on the effects of pressure on SOFC cathodes. The effect of pressurised oxygen on the area specific polarisation resistance (ASRp) of (La_0.8Sr_0.2)_0.95MnO_3-δ/8YSZ SOFC cathodes was determined by electrochemical impedance spectroscopy (EIS). Pellets of 8YSZ were pressed and sintered at 1350 °C, and screen printed layers of LSM/8YSZ cathode and LSM current collector were applied and sintered at 1300 °C and 1200 °C respectively. EIS was carried out between 1 and 3 bar oxygen at 800-1000 °C. One process dominated the spectra, and was identified as process C, (Jorgensen and Morgensen, 2001) [2] by comparison of measured and reference frequency maxima, the dependence of polarisation resistance on P_O2, the capacitance, and the activation energy. It is suggested that this represents the physical process of dissociative adsorption of oxygen at the triple phase boundaries of the electrode. A second process, with a magnitude almost independent of P_O2, is observed, which may be process B [2], related to transport of oxygen ions in the YSZ.     

Kinetics and mechanical study of plasma electrolytic carburizing for pure iron

In this work, plasma electrolytic surface carburizing of pure iron in aqueous solution consisting of water, glycerin and NH₄Cl was investigated. Surface carburizing was carried out in 20% glycerin solution treated at 750 °C, 800 °C, 900 °C and 950 °C temperatures for 5, 10 and 30 min. The formation of hard carbon-rich layer on the surface of pure iron was confirmed by XRD analysis. Metallographic and SEM studies revealed a rough and dense carburized layer on the surface of the pure iron. Experimental results showed that the thickness of the carburized layers changes with the time and temperature. The average thickness of the carburized layer ranged from 20 to 160 μm. The hardness of the carburized samples decreased with the distance from the surface to the interior of the test material. The average hardness values of the carburized layers on the substrate ranged 550-850 HV, while the hardness of the substrate ranged from 110 HV to 170 HV. The dominant phases formed on the pure iron were found to be a mixture of cementite (Fe₃C), martensite (Fe + C) and austenite (FCC iron) confirmed by XRD. Wear resistance in all plasma electrolytic carburized samples is considerably improved in relation to the untreated specimen. After carburizing, surface roughness of the samples was increased. Friction coefficients were also increased because of high surface roughness.     

Partial electronic conductivity and electrolytic domain of bilayer electrolyte Zr0.84Y0.16O1.92/Ce0.9Gd0.1O1.95

Partial electronic conductivity and total conductivity have been determined by Hebb-Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr_0.84Y_0.16O_1.92(YSZ)/Ce_0.9Gd_0.1O_1.95(GDC) with thickness ratios 10^− 3/1 and 10^− 4/1 at 800°, 900° and 1000 °C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the “continuity hypothesis” that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed.     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Structural and magnetic properties of RF sputtered FePt/Fe multilayers

Series of [FePt(4min)/Fe(t_Fe)]₁₀ multilayers have been prepared by RF magnetron sputtering and post-annealing in order to optimize their magnetic properties by structural designs. The structure, surface morphology, composition and magnetic properties of the deposited films have been characterized by X-ray diffractometer (XRD), Rutherford backscattering (RBS), scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDX) and vibrating sample magnetometer (VSM). It is found that after annealing at temperatures above 500 °C, FePt phase undergoes a phase transition from disordered FCC to ordered FCT structure, and becomes a hard magnetic phase. X-ray diffraction studies on the series of [FePt/Fe]_n multilayer with varying Fe layer thickness annealed at 500 and 600 °C show that lattice constants change with Fe layer thickness and annealing temperature. Both lattice constants a and c are smaller than those of standard ones, and lattice constant a decreases as Fe layer deposition time increases. Only a slight increase in grain size was observed as Fe layer decreased in samples annealed at 500 °C. However, the increase in grain size is large in samples annealed at 600 °C. The coercivities of [FePt/Fe]_n multilayers decrease with Fe layer deposition time, and the energy product (BH)_max reaches a maximum in the samples with Fe layer deposition time of 3 min. Comparison of magnetic properties with structure showed an almost linear relationship between the lattice constant a and the coercivities of the FePt phase.     

Surface characterization of Pd/Ag23 wt% membranes after different thermal treatments

Hydrogen permeation measurements of 1.5-10 μm thick Pd/Ag23 wt% membranes before and after thermal treatments at 300 °C in air (both sides) or in the temperature range 300-450 °C in N₂ (feed side) and Ar (permeate side) were performed. Accompanying changes in surface topography and chemical composition were subsequently investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling. For a 2 μm thick membrane, the surface roughness increased for all annealing temperatures applied, while a temperature of 450 °C was required for an increase in roughness of both membrane surfaces to occur for the 5 μm membrane. The thickest membrane, of 10 μm, showed changed surface roughness on one side of the membrane only and a slight decrease in hydrogen permeance after all heat treatments in N₂/Ar. X-ray photoelectron spectroscopy investigations performed after treatment and subsequent permeation measurements revealed segregation of silver to the membrane surfaces for all annealing temperatures applied. In comparison, heat treatment at 300 °C in air resulted in significantly increased hydrogen permeance accompanied by increasing surface roughness. Upon exposure to oxygen, Pd segregates to the surface to form a 2-3 nm thick oxide layer (PdO), with more complex segregation behavior after subsequent reduction and permeance measurements in pure hydrogen. The available permeance data for the Pd/Ag23 wt% membranes after heat treatment in air at 300 °C is found to depend linearly on the inverse membrane thickness, implying bulk limited hydrogen permeation for thicknesses down to 1.5-2.0 μm.     

Investigation of Ni/Ta contacts on 4H silicon carbide upon thermal annealing

Nickel and Tantalum thin films with 3:5 thickness ratios were deposited in succession onto 4H-SiC substrate at room temperature. The samples were then heated in situ in vacuum at 650, 800 or 950 °C for 30 min. Glancing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES) and current-voltage (I-V) technique were used for characterising the interfacial reactions and electrical properties. Amorphous Ni-Ta can be formed by solid-state reaction at 650 °C. The minor dissolved Ni in the Ta metal promotes the reaction between Ta and SiC. With increasing annealing temperature up to 950 °C, the dominant carbide changes from Ta₂C to TaC and a layer structure is developed. Electrical measurements show that ohmic contact is formed after annealing at or above 800 °C.     

Effects of post-thermal annealing temperature on the optical and structural properties of gold particles on silicon suboxide films

In this work, silicon suboxide (SiO_x) thin films were deposited using a RF magnetron sputtering system. A thin layer of gold (Au) with a thickness of about 10 nm was sputtered onto the surface of the deposited SiO_x films prior to the thermal annealing process at 400 °C, 600 °C, 800 °C and 1000 °C. The optical and structural properties of the samples were studied using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and optical transmission and reflection spectroscopy. SEM analyses demonstrated that the samples annealed at different temperatures produced different Au particle sizes and shapes. SiO_x nanowires were found in the sample annealed at 1000 °C. Au particles induce the crystallinity of SiO_x thin films in the post-thermal annealing process at different temperatures. These annealed samples produced silicon nanocrystallites with sizes of less than 4 nm, and the Au nanocrystallite sizes were in the range of 7-23 nm. With increased annealing temperature, the bond angle of the Si-O bond increased and the optical energy gap of the thin films decreased. The appearance of broad surface plasmon resonance absorption peaks in the region of 590-740 nm was observed due to the inclusion of Au particles in the samples. The results show that the position and intensity of the surface plasmon resonance peaks can be greatly influenced by the size, shape and distribution of Au particles.     

Structural and optical properties of YSZ thin films grown by PLD technique

We report on the structural and optical properties of yttria stabilized zirconia (YSZ) thin films grown by pulsed laser deposition (PLD) technique and in situ crystallized at different substrate temperatures (T_s = 400 °C, 500 °C and 600 °C). Yttria-stabilized zirconia target of ∼1 in. diameter (∼95% density) was fabricated by solid state reaction method for thin film deposition by PLD. The YSZ thin films were grown on an optically polished quartz substrates and the deposition time was 30 min for all the films. XRD analysis shows cubic crystalline phase of YSZ films with preferred orientation along 〈1 1 1〉. The surface roughness was determined by AFM for the films deposited at different substrate temperatures. The nano-sized surface roughness is found to increase with the increase of deposition temperatures. For the optical analysis, a UV-vis-NIR spectrophotometer was used and the optical band gap of ∼5.7 eV was calculated from transmittance curves.     

Thickness dependence on thermal stability of sputtered Ag nanolayer on Ti/Si(1 0 0)

Thermal stability of Ag layer on Ti coated Si substrate for different thicknesses of the Ag layer have been studied. To do this, after sputter-deposition of a 10 nm Ti buffer layer on the Si(1 0 0) substrate, an Ag layer with different thicknesses (150-5 nm) was sputtered on the buffer layer. Post annealing process of the samples was performed in an N₂ ambient at a flow rate of 200 ml/min in a temperature range from 500 to 700 °C for 30 min. The electrical property of the heat-treated multilayer with the different thicknesses of Ag layer was examined by four-point-probe sheet resistance measurement at the room temperature. Phase formation and crystallographic orientation of the silver layers were studied by θ-2θ X-ray diffraction analysis. The surface topography and morphology of the heat-treated films were determined by atomic force microscopy, and also, scanning electron microscopy. Four-point- probe electrical measurement showed no considerable variation of sheet resistance by reducing the thickness of the annealed Ag films down to 25 nm. Surface roughness of the Ag films with (1 1 1) preferred crystallographic orientation was much smaller than the film thickness, which is a necessary condition for nanometric contact layers. Therefore, we have shown that the Ag layers with suitable nano-thicknesses sputtered on 10 nm Ti buffer layer were thermally stable up to 700 °C.     

Thermal stability of nanoscale silver metallization in Ag/W/Co/Si(1 0 0) multilayer

In this work, we have studied thermal stability of nanoscale Ag metallization and its contact with CoSi₂ in heat-treated Ag(50 nm)/W(10 nm)/Co(10 nm)/Si(1 0 0) multilayer fabricated by sputtering method. To evaluate thermal stability of the systems, heat-treatment was performed from 300 to 900 °C in an N₂ ambient for 30 min. All the samples were analyzed by four-point-probe sheet resistance measurement (R_s), Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and atomic force microscopy (AFM). Based on our data analysis, no interdiffiusion, phase formation, and R_s variation was observed up to 500 °C in which the Ag layer showed a (1 1 1) preferred crystallographic orientation with a smooth surface and R_s of about 1 Ω/□. At 600 °C, a sharp increase of R_s value was occurred due to initiation of surface agglomeration, WSi₂ formation, and interdiffusion between the layers. Using XRD spectra, CoSi₂ formed at the Co/Si interface preventing W silicide formation at 750 and 800 °C. Meantime, RBS analysis showed that in this temperature range, the W acts as a cap layer, so that we have obtained a W encapsulated Ag/CoSi₂ contact with a smooth surface. At 900 °C, the CoSi₂ layer decomposed and the layers totally mixed. Therefore, we have shown that in Ag/W/Co/Si(1 0 0) multilayer, the Ag nano-layer is thermally stable up to 500 °C, and formation of W-capped Ag/CoSi₂ contact with R_s of 2 Ω/□ has been occurred at 750-800 °C.     

Influence of Si-N interlayer on the microstructure and magnetic properties of γ′-Fe4N films

The (γ′-Fe₄N/Si-N)_n (n: number of layers) multilayer films and γ′-Fe₄N single layer film synthesized on Si (1 0 0) substrates by direct current magnetron sputtering were annealed at different temperatures. The structures and magnetic properties of as-deposited films and films annealed at different temperatures were characterized using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The results showed that the insertion of Si-N layer had a significant influence on the structures and magnetic properties of γ′-Fe₄N film. Without the addition of Si-N lamination, the iron nitride γ′-Fe₄N tended to transform to α-Fe when annealed at the temperatures over 300 °C. However, the phase transition from γ′-Fe₄N to ?-Fe₃N occurred at annealing temperature of 300 °C for the multilayer films. Furthermore, with increasing annealing temperature up to 400 °C or above, ?-Fe₃N transformed back into γ′-Fe₄N. The magnetic investigations indicated that coercivity of magnetic phase γ′-Fe₄N for as-deposited films decreased from 152 Oe (for single layer) to 57.23 Oe with increasing n up to 30. For the annealed multilayer films, the coercivity values decreased with increasing annealing temperature, except that the film annealed at 300 °C due to the appearance of phase ?-Fe₃N.