Vacuum Atmosphere Research Articles

Structure and Thermal Stability of ε/κ-Ga2O3 Films Deposited by Liquid-Injection MOCVD.

We report on crystal structure and thermal stability of epitaxial ε/κ-Ga2O3 thin films grown by liquid-injection metal-organic chemical vapor deposition (LI-MOCVD). Si-doped Ga2O3 films with a thickness of 120 nm and root mean square surface roughness of ~1 nm were grown using gallium-tetramethylheptanedionate (Ga(thd)3) and tetraethyl orthosilicate (TEOS) as Ga and Si precursor, respectively, on c-plane sapphire substrates at 600 °C. In particular, the possibility to discriminate between ε and κ-phase Ga2O3 using X-ray diffraction (XRD) φ-scan analysis or electron diffraction analysis using conventional TEM was investigated. It is shown that the hexagonal ε-phase can be unambiguously identified by XRD or TEM only in the case that the orthorhombic κ-phase is completely suppressed. Additionally, thermal stability of prepared ε/κ-Ga2O3 films was studied by in situ and ex situ XRD analysis and atomic force microscopy. The films were found to preserve their crystal structure at temperatures as high as 1100 °C for 5 min or annealing at 900 °C for 10 min in vacuum ambient (<1 mBar). Prolonged annealing at these temperatures led to partial transformation to β-phase Ga2O3 and possible amorphization of the films.

A novel CoCrNi/W/Agx medium-entropy alloy matrix composite reinforced by Ag nanoparticle and self-lubricating mechanism under ambient air and high vacuum conditions

The present work aims to synthesize the CoCrNi/W/Agx composites with the high wear-resistance under ambient air and vacuum conditions. The friction and wear properties of the CoCrNi/W/Agx composites and CoCrNi medium entropy alloy were fully evaluated, and the chemical components and microstructural evolution of the tribo-films were observed and analyzed by X-ray photoelectron spectroscopy and scanning electron microscopy. The self-lubricating mechanism of the CoCrNi/W/Agx composites under ambient air and vacuum conditions was investigated. Increasing the Ag content increased the thickness of the tribo-films improving the friction and wear properties of the alloys, and the thickness of the tribo-films formed on the worn surface of the CoCrNi/W/Ag1.0 composite was about 150 nm. The CoCrNi/W/Ag1.0 composite exhibited an evident improvement in tribological performance, with a high wear-resistance of 2.79 × 10−5 mm3/N·m under vacuum condition and a volumetric wear rate as low as 1.35 × 10−5 mm3/N·m under ambient air condition.

Investigating suitable medium for the long-duration storage of Ti2CTx MXene

Recently, 2D material MXene gained a significant attention due to its unique properties like chemical diversity, hydrophilicity, 2D morphology, metallic conductivity, etc. Despite the promising properties of MXenes, these materials are not gaining momentum in various applications because of their poor chemical or oxidation stability. Earlier reports stated that organic solvents with low temperature storage could prevent the oxidation of Ti3C2Tx MXene flakes due to less number of dissolved oxygen molecules in the solvents. Due to the lack of a suitable storage media, Ti2CTx MXene is still receiving less attention despite having equivalent physio-chemical properties to Ti3C2Tx. To address this issue, this paper reports a suitable medium for preventing or delaying the degradation or oxidation of Ti2CTx MXene. Herein, Ti2CTx MXene was dispersed in different solvents for a duration of 3 months at ambient (room temperature) and freezing conditions (−20 °C and −80 °C) and characterized thereafter using XRD, Raman, SEM, TEM and XPS techniques. The results are suggested that polar solvents with a high surface tension, high viscosity, high boiling point, and high dielectric constant and in case of “Dry MXene”, keep it in a vacuum or inert atmosphere, will be suitable for long duration storage of Ti2CTx MXene at room temperature. However, it is not the best option for storing the material for a long duration of time. Finally, we have successfully find out the suitable storage medium to avoid the degradation and oxidation of Ti2CTx MXene for a long period of time and it is recommended that it should be dispersed in polar solvents and stored in a frozen condition like − 20 °C temperature or below (−80 °C). In case of “Dry MXene”, the storage condition should be at subzero temperature (−20 °C), and/or freezing conditions should be maintained to avoid degradation as well as oxidation.

Synthesis of Vacancy-Controlled Copper Iodide Semiconductor for High-Performance p-Type Thin-Film Transistors.

Copper iodide (CuI) has emerged as a promising p-type semiconductor material owing to its excellent carrier mobility, high transparency, and solution processability. Although CuI has potential for numerous applications, including perovskite solar cells, photovoltaic devices, and thin-film transistors (TFTs), the close relationship between the anion vacancy generation and the charge transport mechanism in CuI-based devices is underexplored. In this study, we propose solution-processed p-type CuI TFTs which were subject to the thermal annealing process in air and vacuum atmospheres at temperatures of 100, 200, and 300 °C. The chemical states and surface morphologies of the CuI thin films were systematically investigated, revealing the generation of iodine vacancy states and the reduction of carrier concentration, as well as increased film density and grain size according to the annealing condition. Further, the effective role of the Al2O3 passivation layer on the electrical characteristics of the solution-processed CuI TFTs is demonstrated for the first time, where the Al2O3 precursor greatly enhanced the electrical performance of the CuI TFTs, exhibiting a field-effect mobility of 4.02 cm2/V·s, a subthreshold swing of 0.61 V/decade, and an on/off current ratio of 1.12 × 104, which exceed the values of CuI TFTs reported so far. Based on the synergistic effects of the annealing process and the passivation layer that engineered the iodine vacancy state and morphology of CuI, the proposed CuI TFTs with the Al2O3 passivation layer showed excellent reliability under 100 times repeated operation and long-term stability over 216 h, where the transfer curves slightly shifted in the positive direction of 1.36 and 1.88 V measured at a current level of 10-6 A for the reliability and stability tests, respectively. Thus, this work opens a new window for solution-processed p-type CuI TFTs with excellent stability for developing next-generation complementary logic circuits.

Investigation on Strain Sensitivity and Temperature Behaviour of Nitrogen Doped 3C-SiC Thin Films

Electrical resistance-strain behaviour of nitrogen doped 3C-SiC thin films was investigated to determine whether they could be used as strain gauges. Using a hot wall vertical low pressure chemical vapour deposition (LPCVD) reactor, these films were deposited on thermally oxidised Si (100) and alumina substrates at 2.5 mbar pressure and 1040°C temperature from methyltrichlorosilane (MTS) precursor. The nitrogen doping gas utilised was ammonia (NH3). Using four-point bending method, the gauge factor (GF) was determined for all thin films (0, 9, 17, and 30 atomic % nitrogen doped). Systematic annealing of films in a vacuum atmosphere was used to determine the TCR of nitrogen doped 3C-SiC (111). Film sheet resistance was evaluated using a four-probe approach, and it was observed to decrease as the temperature rises from 40 to 550°C. The resistivity, average TCR, and strain sensitivity of film doped with 17 atomic % nitrogen concentration were 0.14 cm, -103 ppm/°C, and -9.6, respectively, indicating that it can be employed as a strain gauge material in high temperature applications. However, film doped with 30 atomic % of nitrogen concentration showed an increase in the resistivity, TCR and strain sensitivity.

Modification of Electrospun CeO2 Nanofibers with CuCrO2 Particles Applied to Hydrogen Harvest from Steam Reforming of Methanol.

Hydrogen is the alternative renewable energy source for addressing the energy crisis, global warming, and climate change. Hydrogen is mostly obtained in the industrial process by steam reforming of natural gas. In the present work, CuCrO2 particles were attached to the surfaces of electrospun CeO2 nanofibers to form CeO2-CuCrO2 nanofibers. However, the CuCrO2 particles did not readily adhere to the surfaces of the CeO2 nanofibers, so a trace amount of SiO2 was added to the surfaces to make them hydrophilic. After the SiO2 modification, the CeO2 nanofibers were immersed in Cu-Cr-O precursor and annealed in a vacuum atmosphere to form CeO2-CuCrO2 nanofibers. The CuCrO2, CeO2, and CeO2-CuCrO2 nanofibers were examined by X-ray diffraction analysis, transmission electron microscopy, field emission scanning electron microscopy, scanning transmission electron microscope, thermogravimetric analysis, and Brunauer-Emmett-Teller studies (BET). The BET surface area of the CeO2-CuCrO2 nanofibers was 15.06 m2/g. The CeO2-CuCrO2 nanofibers exhibited hydrogen generation rates of up to 1335.16 mL min-1 g-cat-1 at 773 K. Furthermore, the CeO2-CuCrO2 nanofibers produced more hydrogen at lower temperatures. The hydrogen generation performance of these CeO2-CuCrO2 nanofibers could be of great importance in industry and have an economic impact.

Ceramic Matrix Composites Obtained by the Reactive Sintering of Boron Carbide with Intermetallic Compounds from the Ti-Si System

In this study, we investigated the effect of adding two different intermetallics, Ti5Si3 and TiSi2, for the preparation of TiB2-SiC-B4C composites. As part of the research, stoichiometric composites consisting only of two phases TiB2 and SiC were obtained. The TiB2-SiC-B4C composites were prepared via pressureless sintering. The presence of the phases in the sintered composites was confirmed using X-ray diffraction and scanning electron microscopy. The SEM-EDS examination revealed that the TiB2 and SiC phases were formed during the composite process synthesis and were distributed homogeneously in the B4C matrix. The obtained results allowed us to usually exceed 2000 °C and the use of specialized equipment for firing, that is, vacuum or protective atmosphere furnaces as well as control and measurement equipment. Such an approach generates high costs that are decisive for the economics of the technological processes. In the case of our compositions, it is possible to lower the temperature to 1650 °C. The TiB2-SiC-B4C composites were classified as UHTCs.

Atmosferik Koşullara Bağlı Olarak CH3NH3PbI3 İnce Filmlerin Karanlık ve Fotoiletkenlik Davranışı

Methylammonium lead iodide (MAPbI3) (CH3NH3PbI3) has great potential for several fields, especially for photovoltaic applications. Unfortunately, MAPbI3 can seriously suffer from degradation. Hence, the degradation problem limits the photovoltaic and other potential applications of this material. In order to produce a stable photovoltaic material, the degradation mechanism and electronic defect profile of MAPbI3 should be understood carefully, especially regarding its conductivity. In this study, electronic defect changes due to stress factors were evaluated. MAPbI3 films were deposited using thermal chemical vapor deposition (Thermal CVD) and spin coating techniques. Morphological differences due to deposition temperatures were defined by using scanning electron microscopy (SEM). Elemental analysis and structural analysis were conducted by energy-dispersive X-ray spectroscopy (EDS) and X- ray diffraction (XRD), respectively. Deposited MAPbI3 films were exposed to in-situ stress factors such as vacuum atmosphere, deionized water vapor (DIWV) atmosphere, and UV light soaking at constant temperature (300 K) in order to define degradation and electronic defect changes. The electronic changes in the samples were investigated by time-dependent dark conductivity, temperature-dependent dark conductivity, and flux-dependent photoconductivity. This study provides an in-depth understanding of the degradation mechanism and electronic defect profile of MAPbI3 regarding its conductivity.

Forming and heat resistance study of NiAl‒ZrO<sub>2</sub> gradient thermal barrier material for superalloys

In this work the spark plasma sintering (SPS) technique was used to forming a functionally gradient thermal barrier material based on nickel aluminide (VKNA) and zirconium oxide (YSZ). Layer-by-layer powders of VKNA / mixtures VKNA + 15 (30) % YSZ / YSZ were consolidated in a graphite matrix at 1070°C in a vacuum atmosphere for 5 minutes and at a pressure of 30 MPa. It is determined that the sintered material has good adhesion of layers, there are no interlayer boundaries. To check the functionality of the material, thermocyclic tests were carried out at 1100°C in the air. The results showed that the material containing the sublayer VKNA + 30 % YSZ has better heat resistance. Ill. 5. Ref. 28. Tab. 2.

Rare Earth Complexes of Europium(II) and Substituted Bis(pyrazolyl)borates with High Photoluminescence Efficiency.

Rare earth europium(II) complexes based on d-f transition luminescence have characteristics of broad emission spectra, tunable emission colors and short excited state lifetimes, showing great potential in display, lighting and other fields. In this work, four complexes of Eu(II) and bis(pyrazolyl)borate ligands, where pyrazolyl stands for pyrazolyl, 3-methylpyrazolyl, 3,5-dimethylpyrazolyl or 3-trifluoromethylpyrazole, were designed and synthesized. Due to the varied steric hindrance of the ligands, different numbers of solvent molecules (tetrahydrofuran) are participated to saturate the coordination structure. These complexes showed blue-green to yellow emissions with maximum wavelength in the range of 490-560 nm, and short excited state lifetimes of 30-540 ns. Among them, the highest photoluminescence quantum yield can reach 100%. In addition, when the complexes were heated under vacuum or nitrogen atmosphere, they finally transformed into the complexes of Eu(II) and corresponding tri(pyrazolyl)borate ligands and sublimated away.

Atomic-scale observation of strain-dependent reversible topotactic transition in La0.7Sr0.3MnOx films under an ultra-high vacuum environment

Reversible topotactic phase transition between perovskite (PV) and oxygen-vacancy-ordered brownmillerite (BM) structures provides an effective platform for realizing the control of physical properties in complex transition metal oxides. However, such reversibility always requires extreme external conditions, that is, a high temperature and vacuum environment during the PV-BM transition, while an oxidizing atmosphere and relatively low temperature vice versa. Here, we experimentally observe the reversible process in strained La0.7Sr0.3MnOx films at atomic scale by using in-situ heating aberration-corrected scanning transmission electron microscopy (STEM). Apart from the conventional reduction reaction of creating oxygen-vacancy-ordered frameworks after heating in the TEM, the inverse process of BM-to-PV transition is unexpectedly discovered under such an ultra-high vacuum atmosphere (∼10−9 Torr) at room temperature. Moreover, this abnormal behavior is strain-dependent. The large compressive strain is found to be detrimental to the inverse phase transition. The density functional theory (DFT) calculations show that the high oxygen affinity of La0.7Sr0.3MnO2.5 is responsible for the reversible transitions. Our findings provide a new insight into the redox reactions of manganite and might be further utilized for potential applications in solid fuel cells, oxygen sensors or resistive switching memories.

Alumina/molybdenum nanocomposites obtained by colloidal synthesis and spark plasma sintering

Alumina/molybdenum nanocomposites were prepared by colloidal synthesis from alumina powder and molybdenum (V) chloride using ethanol as dispersion medium. Modified alumina was calcined at 450 °C in air atmosphere to remove chlorides, and then treated in a tubular furnace at 850 °C under Ar/H2 to reduce the MoO3 formed in the previous stage and obtain Al2O3 with molybdenum nanoparticles on the surface. Three different molybdenum contents were proposed (1, 5 and 10 wt % Mo), and pure alumina was used as reference, that were sintered by spark plasma sintering (SPS) under vacuum atmosphere at 1400 °C for 3 min with an applied pressure of 80 MPa. Composites were characterized by microstructure, hardness, toughness, and three-point bending test. The presence of molybdenum nanoparticles resulted in a fine-grained structure promoted by the presence of molybdenum at grain boundaries and triple points, as well as by the utilization of the SPS equipment. Hardness is at least a 20% greater and fracture toughness 30% larger in the composites than in the monolithic alumina.

Development and Experimental Verification of a High-Temperature and In-Plane Biaxial Testing Apparatus

Given the lack of primary data on heat-resistant composites under high-temperature conditions, the focus of this paper is the development of an in-plane biaxial apparatus under high temperatures and complex loads. Besides loading complex loads up to 80 kN, the apparatus can load under high-temperatures up to 2500 °C. A C/C tensile/compression test at 1700 °C illustrates the successful use of high-temperature digital speckle pattern technology to evaluate the in-plane mechanical properties of heat-resistant composites at 1700 °C under biaxial stress. A high-temperature impact test of a graphite specimen at 2500 °C shows that this apparatus can load at a high temperature in a vacuum and inert gas atmosphere. The yield characteristics of the Q235 steel sheet under in-plane stress show that the apparatus can conduct various mechanical loads, including tension–tension, tension–compression, and compression–compression loads. The proposed equipment can measure the in-plane mechanical properties of composite materials, particularly heat-resistant composites. The obtained results can be applied to structural design, life prediction, and reliability evaluation, as well as for the development, research, and design of aerospace instruments and critical materials.

Effects of mechanical activation atmosphere on the co-liquefaction of Xigou coal and petroleum vacuum residue

The effects of mechanical activation (MA) atmosphere on the co-liquefaction of Xigou coal and Karamay petroleum vacuum residues were investigated. MA induces a smaller particle size of coal powders, which are more easily dissolved in wash oil during co-liquefaction. FT-IR and 13C NMR data of coal samples show that MA can partially break alkyl side chains. Especially, it is easier for MA under hydrogen and air atmosphere to shorten or break alkyl side chains and bridged bonds compared to vacuum and nitrogen atmosphere. MA enhances pyrolysis reaction activity of coal samples through decreasing the apparent activation energy, which is conductive for co-liquefaction. For MA with hydrogen atmosphere, the highest conversion rate and oil yield of co-liquefaction are obtained, which are 98.2% and 84.4%, respectively, followed by air atmosphere. More distillate oil and heavy oil are produced from co-liquefaction with MA under hydrogen atmosphere, but fewer gaseous products. There are 9.7% petrol generated from co-liquefaction with MA under air atmosphere, which are higher than that from other atmospheres. MA partially destroys the physicochemical structure of coal and increases the concentration of free radicals because of the energy generated by collision and friction, thereby enhancing the reactivity in co-liquefaction.

Vacuum Tribological Properties of W-S-N Coatings Synthesized by Direct Current Magnetron Sputtering

This work deals with the investigation of the tribological performance of DC magnetron sputtered W-S-N coatings under vacuum atmosphere, as part of the exploration of multi-environment sliding properties of W-S-N solid lubricants. This study is part of the systematic testing of W-S-N solid lubricants in different environments, especially vacuum, which is often ignored. The trend is to test sliding properties in dry N2 by considering it as replacement of vacuum environment testing. This approach is not appropriate. In this work, a set of coatings was synthesized with N-alloying content in the range of 0–25.5 at.%. A maximum S/W ratio of 1.47 was observed for the pure WSx coating. A maximum hardness of 8.0 GPa was observed for 23 at.% of N-alloying. The coating with the lowest N content (14.6 at.%) displayed the lowest friction, specific wear rate and wear scar depth under vacuum conditions. Despite superior sliding performance at room temperature (35% humidity), 200 °C and dry nitrogen conditions, the performance of the WSN12.5 coating deteriorated vacuum environment.

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al2O3 and Al2O3-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al2O3-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al2O3 (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.

Kinetics and Microstructures of Cellular Recrystallization of Single Crystal Superalloy DD6

The samples of single crystal superalloy DD6 were grit blasted and heat treated at 1100°C for 0.5h, 1h, 2h, 4h, 8h, 16h at vacuum atmosphere, respectively, then the recrystallization microstructure and kinetics of DD6 alloy was investigated. The results showed that the cellular recrystallization grains nucleated in grit blasted single crystal samples heat treated at 1100°Cfor 0.5h, 1h, 2h, 4h, 8h, 16h. With the increase of the duration of heat heating process, the configuration of cellular recrystallization cleared up, and the depth of cellular recrystallization increased. While the coarse γ phase formed in the cellular recrystallization, and the shape of γ phases in the cellular recrystallization was almost equiaxed near the surface of the grit blasted samples and lamellar at the interface between cellular recrystallization and the original zone, respectively. The lamellar γ phase in the cellular grains was radial, and perpendicular to the cellular grain boundary. The recrystallization kinetics of single crystal superalloy DD6 was disclosed, with the increase of the duration of heat heating process, the depth of cellular recrystallization increase quickly and then almost keeps stable at last. The velocity of growth of cellular recrystallization increase very quickly at first, and then decrease at some stage, at last, the velocity tends to zero.

The influence of vacuum heat treatment on the pore structure of earlywood and latewood of larch

Abstract To investigate the changes in the pore structure of earlywood and latewood (EW and LW) in larch during the heat treatment process, this study applied nitrogen adsorption and mercury intrusion porosimetry (MIP) to measure wood pore characteristics. Wood samples were heat treated within a vacuum atmosphere at temperatures between 180 and 220 °C for 6 h. Analysis conducted on nitrogen adsorption isotherms indicated that the micropores and mesopores in the cell walls were present in both untreated and heat-treated EW and LW, and that the mesopores appeared as slit-shaped pores. More pores were detected in EW compared to LW, and these primarily absorbed in the range of 1.5–4 nm. Compared with the untreated wood, the total pore volume of treated wood decreased with increasing temperatures, with decreased pore volumes at 220 °C of 63 and 42% for EW and LW, respectively. The MIP results showed a greater porosity of EW compared to LW. Compared with the untreated wood, the porosity of heat-treated EW and LW first increased with increasing temperature, and then decreased at 220 °C. For treatment temperatures lower than 200 °C, heat treatment increased the proportion of macropores with pore sizes in the ranges of 100–1000 nm and 10,000–40,000 nm.

Homogeneous dispersion and mechanical performance of aluminum reinforced with high graphene content

In this research, trials were performed to improve the mechanical properties of pure aluminum, maintain its lightweight, and enhance the distribution of graphene with high content up to 1.5 wt%. Aluminum composites reinforced with 10% copper, 2.5% alumina, and different ratios of graphene up to 1.5% were manufactured by powder metallurgy followed by hot rolling. The suitable sintering conditions were 565°C for 60 min under a vacuum atmosphere. The powder metallurgy method showed a general improvement in aluminum’s microstructure. An excellent distribution of the different reinforcements was achieved up to 1.5 wt% GNs due to the long mixing time in the hexane solution. However, some aggregations of the GNs layers were observed at 1.5 GNs percent. The mapping analysis detected the nano-alumina distribution. The 1 wt% GNs sample exhibit the greatest improvement in hardness with 2.4 times and yield strength increment with 76% compared to pure aluminum. Also, the wear rate decreased significantly at 1 wt% GNs percent. By conducting the hot rolling process, all results exceeded their counterparts by the powder metallurgy method up to 1.5% GNS content. The accumulated GNs at 1.5 GNs percent were spread due to slipping them over each other as a result of the rolling process. Eliminating the internal pores and improving the distribution of graphene in that sample improved its hardness, yield resistance, and mechanical wear. After the rolling process, the hardness at 1.5 wt% GNs increased from 218 HV to 389 HV, and the yield strength from 203 MPa to 280 MPa.

Influence of sintering atmosphere on phase, microstructure and mechanical properties of Li4Si0.7Ti0.3O4 tritium breeding ceramics

Li4Si1–xTixO4 ceramic solid solution was considered as a promising tritium breeding material for fusion reactor blanket. In this work, the effects of the sintering atmosphere on the phase composition, microstructure, and mechanical properties of Li4Si0.7Ti0.3O4 ceramic pebbles were investigated for the first time, aiming to explore the optimal sintering atmosphere for this solid solution. The results show that compared with air and argon atmosphere, the negative pressure of vacuum atmosphere was more favorable for Ti to enter the Li4SiO4 crystal structure as a substitute for Si atom to form a solid solution. More importantly, sintering in argon-vacuum atmosphere can ensure the formation of solid solution while fully exerting the liquid phase sintering effect. The crushing load of the Li4Si0.7Ti0.3O4 ceramic pebbles sintered at 800 °C in argon-vacuum atmosphere reaches 30.3 ± 3.2 N. In general, the appropriate sintering atmosphere was of great significance for obtaining Li4Si0.7Ti0.3O4 solid solution to meet the performance requirements of tritium breeding blanket.