Thermal and chemical approaches for oxygen catalytic recombination evaluation on ceramic materials at high temperature

During the atmospheric entry phase, the physico-chemical phenomena taking place on space shuttle walls can lead to an important excess of heating and damage of the protective materials. The aim of this work is the study of the catalytic recombination of atomic oxygen under plasma conditions chosen to simulate the atmospheric reentry. To do that, we have developed an experimental set-up MESOX (Moyen d'Essai Solaire d'OXydation), which associates a solar radiation concentrator and a microwave generator to reach high temperature, low enthalpy flow and low pressure plasma with an air gas flow. The study of atomic oxygen recombination on silicon- or aluminum-based ceramic materials, at high temperature (1000–1800 K) has been done for different pressures (200–2000 Pa) by a thermal and a chemical understanding. The results give a catalycity scale of materials (thermal recombination flux, q_rec, and coefficient of atomic oxygen recombination, γ). The catalycity activity is weak for the sintered SiC target with atomic oxygen recombination flux reaching 35 kW/m², however, for a target of sintered Al₂O₃, catalytic effect is obtained with energy fluxes between 90 to 180 kW/m². The recombination coefficient γ confirms the catalycity scale of these ceramic materials.     

Effect of ZnO on phase emergence, microstructure and surface modifications of calcium phosphosilicate glass/glass-ceramics having iron oxide

The effect of ZnO on phase emergence and microstructure properties of glass and glass-ceramics with composition 25SiO₂-50CaO-15P₂O₅-(10 − x)Fe₂O₃-xZnO (where x = 0, 2, 5, 7 mol%) has been studied. They have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Surface modifications of these glass-ceramics in simulated body fluid have been studied using Fourier transform infrared reflection spectroscopy (FTIR), XPS and SEM. Results have shown a decrease in the fraction of non-bridging oxygen with increase in zinc oxide content. Emergence of crystalline phases in glass-ceramics at different heat treatment temperatures was studied using XRD. When glass is heat treated at 800 °C calcium phosphate, hematite and magnetite are developed as major phases in the glass-ceramics samples with ZnO up to 5 mol%. In addition to these, calcium silicate (Ca₃Si₂O₇) phase is also observed when glass is heat treated at 1000 °C. The microstructure of the glass-ceramics heat treated at 800 °C exhibits the formation of nano-size (40-50 nm) grains. On heat treatment at 1000 °C crystallites grow to above 50 nm size and more than one phase are observed in the microstructure. The formation of thin flake-like structure with coarse particles is observed at high zinc oxide concentration (x = 7 mol%). In vitro studies have shown the surface modifications and formation of Ca-P-rich layer on the glass-ceramics when immersed in simulated body fluids (SBF) for different durations. The bioactive response was found to depend on ZnO content.     

Oxygen exchange kinetics of La0.58Sr0.4Co0.2Fe0.8O3 at 600 °C in dry and humid atmospheres

The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La_0.58Sr_0.4Co_0.2Fe_0.8O_3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient k_chem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of k_chem by a factor of 2 during exposure of the sample to a dry O₂-Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30-35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La₂O₃ secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of k_chem by an additional factor of 10, followed by a time dependent decay of about 15% kh^− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of k_chem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.     

Closed model of oxygen recombination on an Al2O3 surface

On the basis of cluster-approximation quantum-chemical calculations of the interaction of an α-Al₂O₃ surface with oxygen, the rate coefficients for the elementary steps of the heterogeneous recombination of atomic oxygen are determined in the framework of the Eley-Rideal and Langmuir-Hinshelwood mechanisms. For the diffusion layer near the studied surface, these coefficients are used to calculate the probabilities of heterogeneous catalytic recombination, surface coverage, and heat flux to the surface at temperatures of 200–2000 K and pressures of 1000–7000 Pa. The results are compared to the results a solid-state periodic model, low-temperature plasma etching studies, and empirical models of recombination of atomic oxygen on a SiO₂ surface.     

The role of process parameters in plasma surface chromising of Ti2AlNb-based alloys

In order to enhance the wear resistance of Ti₂AlNb-based alloy (O-phase), surface chromising was performed by double glow plasma process in this study. The effect of process parameters, such as temperature, time and pressure, on the microstructure, thickness, and micro-hardness of the alloyed layers was investigated. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were employed to analyze the composition distribution and microstructure of the alloy. The results showed that the optimum process parameters were as follows: 970 °C for temperature, 4 h for chromising time, and 30 Pa for pressure. Following the optimization the thickness, microstructure and micro-hardness of the modified layer achieved the designed requirements. The results of tribological tests showed that the friction coefficient of the chromised layer was lower than that of the matrix at the room temperature or 500 °C, and the specific wear rate of samples with plasma chromising at either room temperature or high temperature was decreased markedly.     

Photosensitivity of nanocrystalline ZnO films grown by PLD

We have studied the properties of ZnO thin films grown by laser ablation of ZnO targets on (0 0 0 1) sapphire (Al₂O₃), under substrate temperatures around 400 °C. The films were characterized by different methods including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM). XPS analysis revealed that the films are oxygen deficient, and XRD analysis with θ-2θ scans and rocking curves indicate that the ZnO thin films are highly c-axis oriented. All the films are ultraviolet (UV) sensitive. Sensitivity is maximum for the films deposited at lower temperature. The films deposited at higher temperatures show crystallite sizes of typically 500 nm, a high dark current and minimum photoresponse. In all films we observe persistent photoconductivity decay. More densely packed crystallites and a faster decay in photocurrent is observed for films deposited at lower temperature.     

The effect of surface modification by nitrogen plasma on photocatalytic degradation of polyvinyl chloride films

The solid-phase photocatalytic degradation of poly(vinyl chloride) (PVC) films was investigated under the ambient air in order to assess the feasibility of developing photodegradable polymers. Nitrogen plasma was used to modify PVC films to enhance the photocatalytic degradation of PVC with nano-sized anatase TiO₂. The plasma parameter varied in this study is discharge power from 30 to 120 W for a constant treatment time of 60 s and a constant gas pressure of 10 Pa. The photodegradation of the plasma-treated PVC-TiO₂ films was compared with that of pure PVC films and PVC-TiO₂ films performing weight loss monitoring, scanning electron microscopy (SEM) analysis, contact angle measurements, electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The wettability of the plasma-treated PVC is improved significantly. ESR revealed that the signal of radicals on the surface of the plasma-treated PVC film was enhanced after the treatment. Furthermore, the weight loss indicated that TiO₂ speeds up the photocatalytic degradation of PVC chains. The SEM image of the plasma-treated PVC-TiO₂ film showed a lot of crack on the film surface after irradiation. XPS indicated that the C and Cl atomic concentration reached minimum values on the surface of plasma-treated PVC-TiO₂ under identical photocatalytic condition. The experimental results reveal that plasma treatment can obviously enhance the photocatalytic degradation of PVC.     

Deposition and characterization of low temperature silicon nitride films deposited by inductively coupled plasma CVD

Silicon nitride films have been deposited at a low temperature (70 °C) by inductively coupled plasma chemical vapor deposition (ICP-CVD) technique and their physical and chemical properties were studied. For a deposited SiN sample, β-phase was observed and refractive index of 2.1 at 13.18 nm/min deposition rate was obtained. The attained stress of 0.08 GPa is lower as compared to the reported value of 1.1 GPa for SiN thin films. To study the deposited film, characterization was performed using X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), micro Raman spectroscopy, Fourier transfer infrared spectroscopy (FTIR), cross-section scanning electron microscopy (SEM) and atomic force microscopy (AFM).     

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.     

Ternary semiconductor compounds CuInS2 (CIS) thin films synthesized by electrochemical atomic layer deposition (EC-ALD)

In this paper the formation and characterization of the I-III-VI₂ semiconductor compound CuInS₂ (CIS) on gold substrate at room temperature by electrochemical atomic layer deposition (EC-ALD) method are reported. Optimum deposition potentials for each element are determined using cyclic voltammetry (CV) technique and Amperometric I-t method is used to prepare the semiconductor compound. These thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared spectroscopy (FT-IR). XRD results indicate that the CIS thin films have a (1 1 2) preferred orientation. The XPS analyses of the films reveal that Cu, In and S are present in an atomic ratio of approximately 1:1:2. And their semiconductor band gaps are found to be 1.50 eV by FT-IR.     

Effect of RF power and sputtering pressure on the structural and optical properties of TiO2 thin films prepared by RF magnetron sputtering

TiO₂ thin films were deposited onto quartz substrates by RF magnetron sputtering. The samples deposited at various RF powers and sputtering pressures and post annealed at 873 K, were characterized using X-ray diffraction (XRD), micro Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-vis spectroscopy and photoluminescence (PL) spectroscopy. XRD spectrum indicates that the films are amorphous-like in nature. But micro-Raman analysis shows the presence of anatase phase in all the samples. At low sputtering pressure, increase in RF power favors the formation of rutile phase. Presence of oxygen defects, which can contribute to PL emission is evident in the XPS studies. Surface morphology is much affected by changes in sputtering pressure which is evident in the SEM images. A decrease in optical band gap from 3.65 to 3.58 eV is observed with increase in RF power whereas increase in sputtering pressure results in an increase in optical band gap from 3.58 to 3.75 eV. The blue shift of absorption edge in all the samples compared to that of solid anatase is attributed to quantum size effect. The very low value of extinction coefficient in the range 0.0544-0.1049 indicates the excellent optical quality of the samples. PL spectra of the films showed emissions in the UV and visible regions.     

Oxidation behavior and mechanism of powder metallurgy Rene95 nickel based superalloy between 800 and 1000 °C

The oxidation behaviors of powder metallurgy (PM) Rene95 Ni-based superalloy in the temperature range of 800-1000 °C are investigated in air by virtue of isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that the oxidation kinetics follows a square power law as the time extends at each temperature. The oxidation layers are detected to be composed of Cr₂O₃, TiO₂ and a small amount of NiCr₂O₄. The cross-sectional morphologies indicate that the oxidation layer consists of three parts: Cr-rich oxide layer, Cr and Ti duplex oxide layer, and oxidation affected zone. Theoretical analyses of oxidation kinetics and thicknesses of oxidation layers confirm that the activation energy of oxidation of PM Rene95 superalloy is 165.32 kJ mol⁻¹ and the oxidation process is controlled by diffusions of oxygen, Cr, and Ti. Accordingly, a diffusion-controlled mechanism is suggested to understand the oxidation behaviors of PM Rene95 superalloy at elevated temperatures.     

Effects of carbonization and substrate temperature on the growth of 3C-SiC on Si(1 1 1) by SSMBE

The growth of 3C-SiC on Si(1 1 1) substrate was performed at different carbonization temperatures and substrate temperatures by solid-source molecular beam epitaxy (SSMBE). The properties of SiC film were analyzed with in situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The best carbonization temperature of 810 °C was found to be optimal for the surface carbonization. The quality of SiC film grown on Si at substrate temperature of 1000 °C is best. The worse crystalline quality for the sample grown at higher temperature was attributed to the large mismatch of thermal expansion coefficient between SiC and Si which caused more dislocation when sample was cooled down to room temperature from higher substrate temperature. Furthermore, the larger size of single pit and the total area of the pits make the quality of SiC films grown at higher temperature worse. More Si atoms for the sample grown at lower temperature were responsible for the degradation of crystalline quality for the sample grown at lower temperature.     

Low energy repetitive miniature plasma focus device as high deposition rate facility for synthesis of DLC thin films

Diamond-like carbon (DLC) films were deposited on Si (1 0 0) substrate using a low energy (219 J) repetitive (1 Hz) miniature plasma focus device. DLC thin film samples were deposited using 10, 20, 50, 100 and 200 focus shots with hydrogen as filling gas at 0.25 mbar. The deposited samples were analyzed by XRD, Raman Spectroscopy, SEM and XPS. XRD results exhibited the diffraction peaks related to SiO₂, carbon and SiC. Raman studies verified the formation amorphous carbon with D and G peaks. Corresponding variation in the line width (FWHM) of the D and G positions along with change in intensity ratio (I_D/I_G) in DLC films was investigated as a function of number of deposition shots. XPS confirmed the formation sp² (graphite like) and sp³ (diamond like) carbon. The cross-sectional SEM images establish the 220 W repetitive miniature plasma focus device as the high deposition rate facility for DLC with average deposition rate of about 250 nm/min.     

Influence of heating rate on the crystalline properties of 0.7BiFeO3-0.3PbTiO3 thin films prepared by sol-gel process

0.7BiFeO₃-0.3PbTiO₃ (BFPT7030) thin films were deposited on SiO₂/Si substrates by sol-gel process. The influence of heating rate on the crystalline properties of BFPT7030 thin films were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). XRD patterns of the films showed that a pure perovskite phase exists in BFPT7030 films annealed by rapid thermal annealing (RTA) technique. SEM and AFM observations demonstrated that the BFPT7030 films annealed by RTA at 700 °C for 90 s with the heating rate of 1 °C s⁻¹ could show a dense, crack-free surface morphology, and the films’ grains grow better than those of the films annealed by RTA at the same temperature with other heating rates. XPS results of the films indicated that the ratio of Fe³⁺:Fe²⁺ is about 21:10 and 9:5 for the films annealed by RTA at 700 °C for 90 s with the heating rate of 1 and 20 °C s⁻¹, respectively. That means the higher the heating rate, the higher the concentration of Fe²⁺ in the BFPT7030 thin films.     

Heat capacity anomaly in UO2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO₂ were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO₂ and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.     

Structural and morphological characterization of TiO2 nanostructured films grown by nanosecond pulsed laser deposition

TiO₂ has attracted a lot of attention due to its photocatalytic properties and its potential applications in environmental purification and self cleaning coatings, as well as for its high optical transmittance in the visible-IR spectral range, high chemical stability and mechanical resistance. In this paper, we report on the growth of TiO₂ nanocrystalline films on Si (1 0 0) substrates by pulsed laser deposition (PLD). Rutile sintered targets were irradiated by KrF excimer laser (λ = 248 nm, pulse duration ∼30 ns) in a controlled oxygen environment and at constant substrate temperature of 650 °C. The structural and morphological properties of the films have been studied for different deposition parameters, such as oxygen partial pressure (0.05-5 Pa) and laser fluence (2- 4 J/cm²). X-ray diffraction (XRD) shows the formation of both rutile and anatase phases; however, it is observed that the anatase phase is suppressed at the highest laser fluences. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the stoichiometry of the grown films. The surface morphology of the deposits, studied by scanning electron (SEM) and atomic force (AFM) microscopies, has revealed nanostructured films. The dimensions and density of the nanoparticles observed at the surface depend on the partial pressure of oxygen during growth. The smallest particles of about 40 nm diameter were obtained for the highest pressures of inlet gas.     

Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co doped TiO2 nanoparticles

Co doped TiO₂ nanoparticles have been synthesized by a simple sol-gel route taking 7.5, 9.5 and 10.5 mol% of cobalt concentration. Formation of nanoparticles is confirmed by XRD and TEM. Increase in d-spacing occurs for (0 0 4) and (2 0 0) peak with increase in impurity content. Valence states of Co and its presence in the doped material is confirmed by XPS and EDX. The entire vacuum annealed samples show weak ferromagnetism. Increased magnetization is found for 9.5 mol% but this value again decreases for 10.5 mol% due to antiferromagnetic interactions. A blocking temperature of 37.9 K is obtained, which shows shifting to high temperature as the dopant concentration is increased. The air annealed sample shows only paramagnetic behavior. Temperature dependent magnetic measurements for the air annealed sample shows antiferromagnetic behavior with a Curie-Weiss temperature of −16 K. Here we report that oxygen vacancy and cobalt aggregates are a key factor for inducing ferromagnetism-superparamagnetism in the vacuum annealed sample. Appearance of negative Curie-Weiss temperature reveals the presence of antiferromagnetic Co₃O₄, which is the oxidation result of metallic Co or cobalt clusters present on the host TiO₂.     

Effect of Ar+ irradiation on the electrical conductivity of BaCe0.9Y0.1O3−δ

The effects of 10 keV Ar⁺ ion irradiation on the electrical characteristics of BaCe_0.9Y_0.1O_2.95 subject to fluences of 0, 1.0 × 10¹⁷, 5.0 × 10¹⁷ and 1.0 × 10¹⁸ ions/cm² at room temperature, has been investigated using elastic recoil detection analysis (ERDA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and alternating current (AC) impedance measurements. It was confirmed from the ERDA results that the hydrogen concentration near the surface increased with increase of Ar⁺ ion fluence. This increase may be associated with the increasing quantities of hydrogen generated by interaction between oxygen vacancies, formed by irradiation, and H₂O from exposure to air. SEM images showed clearly that the number of surface defects due to modification increased with increasing fluence. In addition, the size of the defects showed a tendency to increase with increasing fluence. From the results of XPS analyses, providing information on the electronic states on the surface, it was evident that with increase in the Ar⁺ ion fluence, the quantity of excess oxygen, such as hydroxide, increased in the oxygen 1s XPS spectrum. In addition, it was indirectly found, from decomposition of the Ce 3d, spectrum that the concentration of oxygen vacancies increased with fluence, since the percentage of Ce³⁺ also increased. Accordingly, the surface modification led to the formation of more oxygen vacancies and a greater hydrogen concentration on the surface, since the H₂O interacted with some of them. From the results of the DC conductivity and AC impedance measurements, the proton conductivity was shown to predominate over the temperature range from 473 K to 823 K. It was concluded that the increase in these protons and vacancies generated from surface modification contributed to the increase of proton conductivity.     

Generation of oxygen vacancies in the surface of ferroelectric Pb(Nb,Zr,Ti)O3

Controlled generation of oxygen vacancies in the surface of ferroelectric thin films is crucial to study how surface reduction affects molecular adsorption and catalysis of gas-surface phenomena. We demonstrate the effective reduction in the surface of 4% niobium doped 20/80 PZT (PNZT) thin films. The sample was characterized by X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and heated at 200, 250 and 300 °C in a high vacuum system at 10⁻⁵ T of H₂. Auger peak-to-peak intensities was used to study the elemental concentrations during the reduction experiment. High-resolution XPS spectra were acquired before and after reduction process for detecting the changes of the oxygen signal. Vacancies production rates as slow as 0.21% per minute were achieved and the temperature was not a key parameter in the process. Experiments at higher hydrogen pressures and lower temperatures might improve the control of the vacancies production.