Thermal Annealing In Argon Research Articles

Phase formation, structure and properties of quaternary MAX phase thin films in the Cr-V-C-Al system: A combinatorial study

Tailoring of individual atomic layers via alloying to synthesize quaternary MAX phases allows for expanding their chemical diversity and fine-tuning of their properties. In this study, Cr-V/C/Al multilayered precursors were deposited via combinatorial magnetron sputtering, creating nanostructured architectures with periodic stacking of nanolayers and varying Cr:V ratios. Their phase transformation and underlying reaction mechanisms during subsequent thermal annealing in argon towards the prospective quaternary solid solution (CrV)n+1AlCn MAX phases formation was systematically studied. Crystallization of (CrV)2AlC starts at approximately 500°C, while growth of higher-ordered (CrV)4AlC3 is observed from 960°C. Notably, intergrowth of (CrV)2AlC and (CrV)4AlC3 structures with coherent interface suggests that (CrV)2AlC acts as template for nucleation of (CrV)4AlC3. Thermal stability and high-temperature oxidation tests found that (CrV)2AlC exhibits higher thermal stability compared to (CrV)4AlC3 and films with Cr72.7V27.3 displayed good oxidation resistance at 1000°C, forming a protective bilayer oxide scale consisting of (Cr,Al)2O3 and Al2O3.

Synthesis of V2AlC thin films by thermal annealing of nanoscale elemental multilayered precursors: Incorporation of layered Ar bubbles and impact on microstructure formation

Periodically stacked nanoscale V/C/Al multilayered thin film precursors were deposited by magnetron sputtering using elemental targets. Their temperature-dependent phase transformation during subsequent thermal annealing in argon towards V2AlC MAX phase formation was investigated. Compositional and microstructural analyses at the nanoscale revealed undulation growth of the nanolayers and preferential incorporation of Ar atoms into amorphous carbon nanolayers within the as-deposited multilayered precursors. Single-phase and basal-plane-oriented V2AlC films were formed at annealing temperatures above 560 °C. The incorporated Ar atoms migrated and aggregated into high-density nanoscale Ar clusters/bubbles located essentially at grain boundaries after annealing, displaying pronounced layered distribution characteristics especially adjacent to the substrate/film interface. The heterogeneous incorporation of argon atoms or clusters in heterostructured films composed of multicomponent sublayers likely represents a common phenomenon during thin film nucleation and growth, while its impact on the macroscopic properties of films remains to be explored.

Enhancing the luminescence yield of Cr3+ in β -Ga2O3 by proton irradiation

In situ ion-beam-induced luminescence measurements reveal a strong enhancement of the Cr3+ emission yield in electrically conductive chromium doped β-Ga2O3 single crystals upon proton irradiation. The observed effect can be explained based on the Fermi-level pinning caused by radiation defects. This pinning of the Fermi level activates deep carrier traps that can act as sensitizers of the Cr3+ emission. In agreement with this model, in semi-insulating samples, where the Fermi level lies deep in the bandgap, the Cr3+ emission is present already in as-grown samples, and no enhancement of its intensity is observed upon proton irradiation. The boost of the Cr3+ emission yield by irradiation, observed in conductive samples, is reversed by thermal annealing in argon at temperatures above 550 °C for 30 s. The results reveal a high potential of Cr-doped β-Ga2O3 for in situ and ex situ optical radiation detection and dosimetry.

(Invited) Oxygen Evolution and Reduction on La0.5Sr0.5Co0.8Fe0.2O3- Δ Perovskites with Tunable Structural Features

Oxygen evolution (OER) and oxygen reduction (ORR) reactions are the fundamental processes occurring at the air electrode of metal-air batteries and fuel cells. Presently, large overpotentials are required for these reactions to occur at an appreciable rate. Therefore, development of catalysts capable of reducing the activation losses at the oxygen electrode is highly desirable. Perovskite oxides are one of the most interesting low-cost electrocatalysts for air electrodes, because they can adopt a wide variety of chemical compositions and crystal structures, thereby enabling correlative studies between the electrocatalytic activity and their electronic structure (1, 2). Recently, we reported a possible relation between the cell volume of La0.6Sr0.4Fe0.6Mn0.4O3-δ perovskites and their ORR activity and selectivity (2 e- vs ‘’2+2’’ e- pathway), with lower cell volumes corresponding to higher number of exchanged electrons (3). It was also reported by others that some structural features in the perovskite influence the activity of these mixed oxides for both ORR and OER (4-6). In this presentation, we will discuss our recent studies on the ORR and OER activity of a series of La0.5Sr0.5Co0.8Fe0.2O3- δ electrocatalysts prepared by solution combustion synthesis. The choice of diverse experimental conditions allowed us to tune oxygen deficiency, microstrain and other structural properties. As exemplified in Figure 1, for one of the investigated electrocatalysts, thermal annealing in argon reduces the bond angle between the oxygen atom and the two B-site metal cations, resulting in a higher electrocatalytic activity for both OER and ORR. In addition to structure–electrocatalytic activity correlations, we will complement our comparative studies with analytical scanning electron microscopy and spatially resolved elemental mapping of selected electrode surfaces subjected to accelerated stability tests. Figure 1. Crystal structures of La0.5Sr0.5Co0.8Fe0.2O3- δ prepared from solution combustion using citrate as fuel and thermal annealed in Ar (top right) and in Air (bottom left). Differences in the Co-O-Co bond angle are highlighted. Oxygen evolution (OER, top left) and oxygen reduction (ORR, bottom right) polarization curves recorded in 0.1 M KOH at 5 mVs-1 and 1600 rpm.

Ascorbic acid-tailored synthesis of carbon-wrapped nanocobalt encapsulated in graphene aerogel as electrocatalysts for highly effective oxygen-reduction reaction

For the future of fuel cells, investigations on high activity, stable, and low-cost oxygen-reduction reaction (ORR) catalysts are critical. Transition metals have attracted particular research interest in pursuit of such efficient and sustainable ORR electrocatalysts. In this work, very tiny Co nanoparticles were assembled on 3D graphene (3DG) aerogels uniformly with the help of ascorbic acid (AA) under a facile hydrothermal process. After further thermal annealing in argon, a few carbon layers are formed and wrapped on the Co nanoparticles. The synthesized materials were primarily investigated for ORR, and the experimental results indicate that the as-prepared Co/3DG shows an obviously positive onset potential (−0.01 V) and a much higher limiting current density (5.75 mA/cm2) than Co/3DG prepared without using AA (−0.1 V, 4.6 mA/cm2). Compared with commercial 20%Pt/C, Co/3DG prepared by using AA also displayed excellent durability and superb methanol tolerance performance, which suggests that the special structure of cobalt nanoparticles wrapped with a few carbon layers and its porous 3DG aerogel support can prevent its expansion and dissolution, improve its stability largely, and provide a new way for searching excellent ORR electrocatalysts.

Photoelectric characteristics of metal-Ga2O3-GaAs structures

We investigate the effect of thermal annealing in argon and of oxygen plasma processing on the photoelectric properties of GaAs-Ga2O3-Me structures. Gallium-oxide films are fabricated by photostimulated electrochemical oxidation of epitaxial gallium-arsenide layers with n-type conductivity. The as-deposited films were amorphous, but their processing in oxygen plasma led to the nucleation of β-Ga2O3 crystallites. The unannealed films are nontransparent in the visible and ultraviolet (UV) ranges and there is no photocurrent in structures based on them. After annealing at 900°C for 30 min, the gallium-oxide films contain only β-Ga2O3 crystallites and become transparent. Under illumination of the Ga2O3-GaAs structures with visible light, the photocurrent appears. This effect can be attributed to radiation absorption in GaAs. The photocurrent and its voltage dependence are determined by the time of exposure to the oxygen plasma. In the UV range, the sensitivity of the structures increases with decreasing radiation wavelength, starting at λ ≤ 230 nm. This is due to absorption in the Ga2O3 film. Reduction in the structure sensitivity with an increase in the time of exposure to oxygen plasma can be caused by the incorporation of defects both at the Ga2O3-GaAs interface and in the Ga2O3 film.

Formation of W/HfO2/Si gate structures using in situ magnetron sputtering and rapid thermal annealing

The W(150 nm)/HfO2(5 nm)/Si(100) structures prepared in a single vacuum cycle by rf magnetron sputtering were subjected to rapid thermal annealing in argon. It is found that at an annealing temperature of 950°C, the tungsten oxide WOx phase and the hafnium silicate HfSixOy phase grow at the W/HfO2 and HfO2/Si(100) interfaces, respectively. Herewith, the total thickness of the oxide layeris 30% larger than that of the initial HfO2 film. In addition, a decrease in the specific capacitance in accumulation Cmax and in the dielectric constant k (from 27 to 23) is observed. At an annealing temperature of 980°C, intensive interaction between tungsten and HfO2 takes place, causing the formation of a compositionally inhomogeneous HfxSiyWzO oxide layer and further decrease in Cmax. It is shown that a considerable reduction in the leakage currents occurs in the W/HfO2/X/Si(100) structures, where X is a nitride barrier layer.

One-source PVD of n-CuIn5Se8 photoabsorber films for hybrid solar cells

Highly photosensitive CuIn5Se8 photoabsorber layers of n-type were deposited onto glass/ITO substrates from the bulk CuIn5Se8 source polycrystal by using the one-source physical vapour deposition (PVD) technique with following thermal annealing. The parameters of deposition process and thermal annealing were selected with the purpose to prepare photoabsorber layers for the hybrid photovoltaic structures based on inorganic and conductive polymer functional layers. As-deposited structures were annealed at the temperature range from 450 to 500°C in vacuum. Also, thermal annealing in argon and in vacuum in sequence was applied for the part of obtained structures according to conception of the sequential annealing. Obtained results show the chalcopyrite structure of prepared photoabsorber films and dominating presence of the CuIn5Se8 phase in the layers annealed in vacuum. It was found, that the CuIn5Se8 layers deposited at the temperature of substrate of 200°C and annealed at 500°C for 2h in vacuum show maximal photosensitivity under white light illumination with an intensity of 100mW/cm2. On the basis of the electrochemical impedance spectroscopy the order of charge carriers concentration in prepared photoabsorber layers was evaluated and n-type of conductivity confirmed.

Study of the Impact of Growth and Post-Growth Processes on the Surface Morphology of 4H Silicon Carbide Films

In this paper we study the surface morphology of <11-20> 4° degree off, silicon terminated, 4H Silicon Carbide (4H-SiC) in terms of growth parameters and post growth argon thermal annealing. We find that out-of-equilibrium conditions favor the reduction of the surface roughness. Furthermore, we find preliminary indications that the same growth parameters that lead to the reduction of the surface roughness promote also a reduction of (1,3) and (4,4) stacking faults density.

Thermal Stability and Tribological Performance of DLC-Si–O Films

The thermal stability and tribological performance of silicon- and oxygen-incorporated diamond-like carbon films were investigated. The DLC-Si-O films were deposited using plasma-based ion implantation (PBII) method. The deposited films were annealed at 400°C, 600°C, and 750°C for 1 hour in vacuum, in argon, and in air atmospheres. Film properties were investigated using the Fourier transforms infrared spectroscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, and a ball-on-disk friction tester. The structures of the DLC-Si-O films with a low Si content (25 at.%Si, 1 at.%O) and high Si content (25 at.%Si, 1 at.%O) were not affected by the thermal annealing in vacuum at 400°C and 600°C, respectively, while they were affected by thermal annealing in argon and in air at 400°C. Film with 34 at.%Si and 9 at.%O after annealing demonstrated almost constant atomic contents until annealing at 600°C in vacuum. The friction coefficient of DLC-Si–O films with 34 at.%Si and 9 at.%O was shown to be relatively stable, with a friction coefficient of 0.04 before annealing and 0.05 after annealing at 600°C in vacuum. Moreover, the low friction coefficient of film annealed at 600°C in vacuum with 34 at.%Si and 9 at.%O was corresponded with low wear rate of 1.85 10−7 mm3/Nm.

Silicon nanoparticles in thermally annealed thin silicon monoxide films

Thin vacuum-evaporated SiO x films with embedded Si nanoparticles are studied by applying spectral ellipsometry and transmission electron microscopy. The Si nano-inclusions in the oxide matrix were formed by thermal annealing in argon at 700 and 1000 °C for 5 and 30 min, respectively. The results have shown that during annealing the films undergo densification, and Si nano-clusters with averaged size of 3 nm are formed in amorphous phase at 700 °C and in crystalline phase at 1000 °C.

Interfacial reactions of Ti/n-GaN contacts at elevated temperature

Interfacial reactions in Ti/GaN contacts have been studied using conventional and high-resolution transmission electron microscopy (TEM), energy-filtered TEM (EFTEM), and x-ray diffraction. The thin film contacts were fabricated by evaporating Ti on n-GaN and subsequent rapid thermal annealing in argon. An x-ray result shows that the as-deposited Ti on GaN is an epitaxial hcp phase, whereas a fcc phase was identified from the as-deposited metal layer in cross-sectional TEM specimens. This phenomenon is interpreted as the transformation of hcp-Ti phase to fcc-TiHx phase by hydrogen incorporation during TEM specimen thinning. At elevated temperature, first gallium and then nitrogen diffused into the metal layer. The reaction front moved into the Ti layer, and after annealing at 700 °C many voids were formed along the Ti/GaN interface at the GaN side. A sequence of phases GaN/TiN/Ti2GaN/Ti3Ga/Ti was identified in annealed contacts, and corresponds to the expected diffusion path. The ternary phase Ti2GaN was confirmed with electron diffraction and EFTEM. A planar TiN layer, which formed in direct contact to the GaN, presumably governs the electrical properties of the alloyed contact.

Violet/blue emission from epitaxial cerium oxide films on silicon substrates

Violet/blue photoluminescence was observed from epitaxial cerium oxide films on silicon substrates. The films were deposited on silicon (111) substrates under ultrahigh vacuum conditions using pulsed laser ablation of a cerium oxide target and treated by rapid thermal annealing in argon. High resolution transmission electron microscopy and x-ray diffraction measurements indicated the formation of a single crystal cerium oxide phase Ce6O11 different from CeO2 in the annealed films. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide.

Interaction of Titanium with Silica After Rapid Thermal Annealing in Argon, Nitrogen, and Oxygen

The interaction of Ti with SiO2 as a result of rapid thermal annealing in argon, nitrogen, and oxygen over a tempetature range of 600° to 1000° has been studied using the analytical techniques of Auger depth profiling, Rutherford backscattering, and X‐ray diffraction. After annealing in argon and nitrogen, Ti5Si3 (with some oxygen) was formed at the Ti/SiO2 interface at all temperatures in this range. For temperatures below 800° in Ar, an outer uniform layer of α‐Ti with randomly dissolved oxygen or Ti2O was formed. At temperatures above 800°, a Ti oxide (O/Ti ratio 1.6) was formed. Annealing in an oxygen ambient showed the formation of rutile with very little reaction of the Ti with SiO2. After annealing in nitrogen, a three‐layered structure of TiN/TiOx/Ti5Si3 (with x ≤ 1) was formed, with the relative thickness of each layer dependent upon the annealing temperature. Discussion of the effect of the ambient on the various reactions and diffusion mechanisms is also presented.

ChemInform Abstract: Testing of Stability and Spectroscopic Response of NiO(OH)/Ni2Si/n+/n‐Si Photoanode.

AbstractSputtered or vacuum evaporated Ni films on the surface of an epitaxial silicon wafer with n+/n junction can diffuse into the bulk of silicon to form Ni2Si under thermal annealing in argon at 650 °C. Under operation in 1 M KOH solution a multilayer NiO(OH)/Ni2Si/n+/n‐Si photoanode is established, which possesses an absorption spectrum from 450‐800 nm.