Low Reactor Pressure Research Articles

Thermal evolution of graptolite and solid bitumen properties at high maturity under natural and artificial conditions

To refine the use of graptolite and solid bitumen as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared (IR) spectroscopies, reflectance, and geochemical screening using high-temperature pyrolysis experiments in comparison to naturally matured samples. Naturally matured samples included marine shales from the overmature Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations (herein referred to as Wufeng-Longmaxi) of the Sichuan Basin, China. Immature samples for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden. Pyrolysis experiments at 360 °C for 10 days and 27 days (hydrous conditions), and 450 °C 3 days, 550 °C 3 days, and 550 °C 10 days (all anhydrous) created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from IR spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. RBS values were systematically lower in artificially matured samples versus naturally matured samples with equivalent reflectance values, whereas G-FWHM values were systematically higher. This observation indicates that the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment or ordering of aromatic carbon arrays in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was typically higher than co-occurring solid bitumen in Wufeng-Longmaxi samples and the untreated Alum sample, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress.

An insight into growth transition in AlN epitaxial films produced by metal-organic chemical vapour deposition at different growth temperatures

This work demonstrates a clear picture growth transition of aluminium nitride (AlN) films from the three-dimensional (3D) to the two-dimensional (2D) regime on the sapphire substrate at various temperatures using metal-organic chemical vapour deposition (MOCVD) under low reactor pressure. The high deposition rate of large 3D AlN islands that isolated each other change to 2D growth mode with a smoother surface as temperature increases from 800 °C to 1340 °C. From x-ray diffraction measurement , the AlN (100), AlN (002), and AlN (101) planes exhibit strong peak monocrystalline AlN (002) films as the temperature increase. It found that the AlN film grew at 1100 °C in the Frank–van der Merwe or 2D growth mode exhibits the highest crystalline quality with the threading dislocation density around 2.21 × 10 9 cm −2 . In addition, the lattice vibrational parameters of the AlN films at 1100 °C shows the lowest phonon damping from IR spectra results. Thus, this study details the AlN epitaxial films growth transition, which is crucial for growing high crystalline quality AlN layer using the MOCVD technique. • AlN thin films transition from 3D to 2D as the growth temperatures increase at lower reactor pressure. • The smoothest surface was observed at a higher growth rate but not at a higher growth temperature. • The intermediate growth temperature produced the high-quality AlN thin films under Frank–van der Merwe or 2D growth mode. • Theoretical modelling of the infrared reflection spectrum reveals a stressed layer between AlN film and sapphire substrate.

The effect of composition variations on the response of steels subjected to high fluence neutron irradiation

A set of low alloy model reactor pressure vessel steels, with systematic variations in their Mn, Ni, and Si contents, were neutron-irradiated to high fluence (1.4 × 1020 n/cm2) in the Advanced Test Reactor at Idaho at 290°C and a flux of 3.6 × 1012 n/cm2s. The alloys were analysed using atom probe tomography and solute clusters were observed in each alloy, including in one alloy that contained low nominal levels of Mn (0.04 at. %) and Si (0.06 at. %). Changes in the mechanical properties of the alloys were correlated with cluster volume fractions. Whilst the effect of nominal composition was observed to influence cluster composition, cluster nucleation site was not observed to affect composition. Several grain boundaries were also analysed and the segregation behaviour of certain elements is discussed.

The Growth of AlN Single Layer on Sapphire at Low Pressure using Metalorganic Chemical Vapor Deposition (MOCVD)

In this work, aluminium nitride (AlN) single layer has been successfully grown on c-plane sapphire using metalorganic chemical vapor deposition (MOCVD) at low reactor pressure. The effects of growth temperature, ammonia (NH3) flux and trimethylaluminum (TMAl) flux towards AlN growth were investigated. It was noted that the reaction between NH3 and TMAl has affected the growth rate across the growth temperature. Field emission scanning electron microscopy (FESEM) equipped with Energy-dispersive X-ray (EDX) have revealed the formation of AlN single layer on the sapphire substrate and elemental composition of the layer, respectively. The dependence of growth rate on growth temperature, TMAl flux and NH3 flux has been observed. A relationship was drawn, whereby an increase in TMAl flux and decrease in NH3 flux would lead to an increase in the AlN growth. In addition, a drastic increase in the AlN growth was observed at high growth temperature, which was more than 1100°C. Furthermore, observation from atomic force microscopy (AFM) indicated improvement on surface roughness as growth rate increases. Further characterization was carried out using phase analysis using high resolution X-ray diffraction (HRXRD).

Applications of plasma-enhanced metalorganic chemical vapor deposition

Abstract The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorganic chemical vapor deposition (MOCVD) is examined, to improve control of microstructure, develop understanding of the underlying growth mechanisms, and expand the range of materials combinations for photovoltaic and solid-state lighting applications. Whereas plasma is commonly used with group-IV materials, such as silicon, few studies have examined the impact of plasma-enhanced MOCVD (PE-MOCVD) for III-V materials growth. PE-MOCVD provides improved decomposition of metalorganic precursors, which is driven by hydrogen plasma to augment hydride reactions and thermal pyrolysis, but plasma generation requires low reactor pressure. We have demonstrated elemental Al films grown by PE-MOCVD, which show distinct crystallographic texturing, and assume epitaxial microstructure upon post-growth annealing. We also demonstrate PE-MOCVD of GaAs at temperatures as low as 300 °C. With increasing radio-frequency power, the GaAs growth rate shows a transition from power limited to mass-transport limited. PE-MOCVD grown Ga0.5In0.5P shows no detectable spontaneous atomic ordering, which offers a mechanism to form order/disorder unicompositional heterostructures, without temperature change or impurity incorporation. In its current implementation, PE-MOCVD grown films have shown microstructural degradation, that we attribute to direct plasma exposure during growth. Improvements in the system design and growth sequences are at the focus of on-going efforts.

In-situ investigations of hydrogen influenced crack initiation and propagation under tensile and low cycle fatigue loadings in RPV steel

Present work aims to unveil the mechanism of hydrogen embrittlement (HE) in SA508 Grade 3 Class I low alloy reactor pressure vessel (RPV) steel. In-situ tensile and low cycle fatigue (LCF) tests are performed on specially designed specimens using tensile/fatigue testing stage under scanning electron microscope (SEM). Electrochemical hydrogen charging resulted in localized void formation at prior austenite grain boundaries (PAGBs) during tensile loading. Alongside the hydrogen induced weakening of PAGBs due to synergetic HELP (hydrogen enhanced localized plasticity) and HEDE (hydrogen enhanced decohesion) mechanisms of HE, fish-eyes formation around Al2O3–SiO2 type inclusions are the primary factors for hydrogen enhanced tensile properties degradation in subject RPV steel. During LCF loading, crack initiation and propagation is facilitated by long rod inter-lath cementite particles distributed along the bainitic ferrite lath boundaries in the un-charged specimen. In case of hydrogen charged specimen, the edge crack formed during LCF loading propagated through the specimen by cleavage. Predominantly plasticity (slip) driven transgranular crack propagation occurred in un-charged specimen. In contrary, hydrogen charging resulted in LCF crack to propagate in mixed intergranular and transgranular manner during early stages of propagation, whereas once the crack length exceeded 5 to 6 grains, cleavage type transgranular crack propagation was observed.

Metalorganic vapor phase epitaxy of pit-free AlN homoepitaxial films on various semipolar substrates

Semipolar AlN homoepitaxial films, which are expected to act as underlying layers of highly efficient light emitters, are fabricated on 15°-off (0001), (11¯02), and (112¯2) AlN substrates using the metalorganic vapor phase epitaxy method. In conventional (0001) AlN growth, low reactor pressures are preferred to enhance the migration of Al adatoms and to suppress parasitic reactions between trimethylaluminum and ammonia. In contrast, low-pressure growth generates numerous pits on the surface of semipolar AlN grown homoepitaxially, which are derived from defects formed in the initial growth stage. Herein we experimentally demonstrate that higher-pressure growth can drastically decrease the pit density. A higher-pressure growth realizes atomically smooth surfaces, strong near-band-edge emissions with narrow line widths (∼1–2 meV), and well-suppressed deep level emissions. The optimal reactor pressure to eliminate pits is 500 Torr in terms of the growth rate and nucleation density.

Phenomenological modelling of flow behaviour of 20MnMoNi55 reactor pressure vessel steel at cryogenic temperature with different strain rates

Abstract In the present study a phenomenological constitutive model is developed to describe the flow behaviour of 20MnMoNi55 low carbon reactor pressure vessel (RPV) steel at sub-zero temperature under different strain rates. A set of uniaxial tensile tests is done with the variation of strain rates and temperature ranging from 10−4 s−1 to 10−1 s−1 and -80 °C to −140 °C respectively. From the experimental data, family of flow curves at different temperatures and strain rates are generated and fitted exponentially. The strain rate and temperature dependence of the coefficients of the exponential flow curves are extracted from these curves and characterised through a general phenomenological constitutive coupled equation. The coefficients of this coupled equation are optimised using genetic algorithm. Finite element simulation of tensile tests at different strain rates and temperatures are done using this coupled equation in material model of Abaqus FEA software and validated with experimental results. The novelties of proposed model are: (a) it can predict precisely the flow behaviour of tensile tests (b) it is a simple form of equation where fitting parameters are both function of strain rate ratio and temperature ratio, (c) it has ability to characterize flow behaviour with decreasing subzero temperatures and increasing strain rates.

Effectiveness factor of two-dimensional ring-shaped catalyst pellets

Abstract The use of hollow catalytic support improves the utilization of the catalytic material because of the absence of the pellet core, and moreover ensures low reactor pressure drop owing to enhanced bed voidage. In this study, the expressions for the efficient computation of the effectiveness factor are derived for a ring-shaped catalyst pellet undergoing first-order irreversible reaction. The methodology consists of using solutions of one-dimensional problems to remove non-homogeneous boundary conditions. The expressions obtained exhibit significantly faster convergence behavior than those reported in literature. The shape parameters, namely, the height-to-diameter ratio and the inner-to-outer radii ratio, significantly affect the catalyst utilization, such that several-fold improvement in the effectiveness factor is achievable.

Correlating Irradiation-Induced Solute Clustering with Changes of Hardness in Low and High Flux Reactor Pressure Vessel Steels

Correlating Irradiation-Induced Solute Clustering with Changes of Hardness in Low and High Flux Reactor Pressure Vessel Steels

Magnesium incorporation efficiencies in MgxZn1−xO films on ZnO substrates grown by metalorganic chemical vapor deposition**Project supported by the National Natural Science Foundation of China (Grant No. 61474121) and the Major Scientific Project of Fujian Province, China (Grant No. 2014NZ0002-2).

We investigate the magnesium (Mg) incorporation efficiencies in MgxZn1−xO films on c-plane Zn-face ZnO substrates by using metalorganic chemical vapor deposition (MOCVD) technique. In order to deposit high quality MgxZn1−xO films, atomically smooth epi-ready surfaces of the hydrothermal grown ZnO substrates are achieved by thermal annealing in O2 atmosphere and characterized by atomic force microscope (AFM). The AFM, scanning electron microscope (SEM), and x-ray diffraction (XRD) studies demonstrate that the MgxZn1−xO films each have flat surface and hexagonal wurtzite structure without phase segregation at up to Mg content of 34.4%. The effects of the growth parameters including substrate temperature, reactor pressure and VI/II ratio on Mg content in the films are investigated by XRD analysis based on Vegard’s law, and confirmed by photo-luminescence spectra and x-ray photoelectron spectroscopy as well. It is indicated that high substrate temperature, low reactor pressure, and high VI/II ratio are good for obtaining high Mg content.

On the Analysis of Clustering in an Irradiated Low Alloy Reactor Pressure Vessel Steel Weld.

Radiation induced clustering affects the mechanical properties, that is the ductile to brittle transition temperature (DBTT), of reactor pressure vessel (RPV) steel of nuclear power plants. The combination of low Cu and high Ni used in some RPV welds is known to further enhance the DBTT shift during long time operation. In this study, RPV weld samples containing 0.04 at% Cu and 1.6 at% Ni were irradiated to 2.0 and 6.4×1023 n/m2 in the Halden test reactor. Atom probe tomography (APT) was applied to study clustering of Ni, Mn, Si, and Cu. As the clusters are in the nanometer-range, APT is a very suitable technique for this type of study. From APT analyses information about size distribution, number density, and composition of the clusters can be obtained. However, the quantification of these attributes is not trivial. The maximum separation method (MSM) has been used to characterize the clusters and a detailed study about the influence of the choice of MSM cluster parameters, primarily on the cluster number density, has been undertaken.

Effect of reactor pressure on the conformal coating inside porous substrates by atomic layer deposition

Atomic layer deposition (ALD) is renowned for its step coverage in porous substrates. Several emerging applications require a combination of this high step coverage with high throughput ALD, like spatial ALD. Often, high throughput ALD is performed at atmospheric pressure, and therefore, the effect of reactor pressure on the saturation dose is investigated. ALD inside porous substrates is governed by three key parameters: the reaction probability, the pore aspect ratio, and the precursor diffusion coefficient, of which the latter one contains the reactor pressure dependency. The effect of these parameters on the saturation dose is validated using Monte Carlo modeling, where the reactor pressure dependency is included through the mean free path. A reaction-limited and a diffusion-limited regime can be identified, and it is shown that for many realistic experimental conditions, even at low reactor pressures, the saturation dose is in the diffusion-limited regime. An expression for the pressure dependent saturation dose in the diffusion-limited regime is derived. For small pore diameters, the saturation dose is pressure independent, but for larger pores, higher saturation doses are required for atmospheric reactor pressures than for low reactor pressures. However, as high reactor pressures enable much higher precursor partial pressures than low reactor pressures, the resulting saturation times can be much shorter at atmospheric pressure than low pressure. Often, high surface area porous substrates will lead to supply limited conditions, and increased saturation times have to be taken into account. These results show that the atmospheric pressure ALD can be used for high throughput ALD inside porous substrates, as long as high precursor partial pressures and molar flows can be applied. This is experimentally demonstrated by a near 100% step coverage obtained by atmospheric spatial ALD of alumina in high aspect ratio pores.

Encoding abrupt and uniform dopant profiles in vapor-liquid-solid nanowires by suppressing the reservoir effect of the liquid catalyst.

Semiconductor nanowires (NWs) are often synthesized by the vapor-liquid-solid (VLS) mechanism, a process in which a liquid droplet-supplied with precursors in the vapor phase-catalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this "reservoir effect" is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (≤250 nm/min) and low reactor pressures (≤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures.

Competitive growth mechanisms of AlN on Si (111) by MOVPE.

To improve the growth rate and crystal quality of AlN, the competitive growth mechanisms of AlN under different parameters were studied. The mass transport limited mechanism was competed with the gas-phase parasitic reaction and became dominated at low reactor pressure. The mechanism of strain relaxation at the AlN/Si interface was studied by transmission electron microscopy (TEM). Improved deposition rate in the mass-transport-limit region and increased adatom mobility were realized under extremely low reactor pressure.

Critical cleavage fracture stress characterization of A508 nuclear pressure vessel steels

The critical cleavage fracture stress of SA508 Gr.4N and SA508 Gr.3 low alloy reactor pressure vessel (RPV) steels was studied through the combination of experiments and finite element method (FEM) analysis. The results showed that the value of the local cleavage fracture stress, σF, of SA508 Gr.4N steel was significantly higher than that of SA508 Gr.3 steel. Detailed microstructural analysis was carried out using FEGSEM which revealed much smaller grains, finer and more homogenous carbide particles formed in SA508 Gr.4N steel. Compared with the SA508 Gr.3 steel currently used in the nuclear industry, the SA508 Gr.4N steel possesses higher strength and notch toughness as well as improved cleavage fracture behavior, and is considered a better candidate RPV steel for the next generation nuclear reactors.

The optical and electrical properties of AP-MOVPE GaSb grown using TEGa and TMSb

High quality undoped metal-organic vapour phase epitaxial (MOVPE) GaSb layers have been grown on GaAs using the trimethylantimony/triethylgallium (TMSb/TEGa) precursor combination at 550°C and atmospheric pressure. The effects of the TMSb to TEGa flux ratio (i.e. V/III ratio) on the surface morphology and optical and transport properties of these layers have been investigated. The transport properties were found to be comparable to the best reported for un-doped GaSb grown using the same precursors at low reactor pressures. Two models (using either two monovalent acceptors or one divalent acceptor) were used to estimate the density of native acceptors and the ionization energy of the acceptors in GaSb. These models showed the shallow acceptor activation energy or first ionization energy to be ∼30 meV (falling within the reported range of 20–40 meV). The ionization energy of the deeper acceptor or second ionization of the level was determined to be ∼120 meV.

On the Cu precipitation behavior in thermo-mechanically embrittlement processed low copper reactor pressure vessel model steel

Cu precipitation behavior in 30% pre-deformed reactor pressure vessel model steel has been investigated during thermal ageing. In this work, a thermo-mechanical treatment based embrittlement process, including high temperature quenching, pre-deformation and thermal ageing, has been applied to a low-Cu (0.17wt.%) RPV model steel to produce Cu-rich precipitates. Ultrafine Cu-rich precipitates were observed by transmission electron microscope (TEM) and energy dispersive X-ray microanalysis following this embrittlement process. Moreover, differential scanning calorimetry was used to study the structure transformation of Cu precipitates during long-term thermal ageing. It is noted that for dilute Cu containing steel, high temperature quenching and pre-deformation was found to promote the production of high density of vacancies in calculation, which may facilitate the formation of the fine Cu-rich precipitation. As a result, it leads to a significant hardness peak in the initial stage of thermal ageing. The results were discussed to link with the thermo-mechanical embrittlement process, which provides a method for studying the Cu-rich precipitates formation mechanism in steels containing less concentration of impurity Cu.

GaN Layer Grown by Hydride Vapor Phase Epitaxy: Influence of the Reactor Pressure

In this paper, the influence of reactor pressure on the quality of GaN epilayer grown by Hydride vapor phase epitaxy (HVPE) is reported. A series of sample was fabricated at 0.2, 0.5, 0.7 and 1 atm. There were several samples at certain pressure in order to avoid the random affection. And the root mean square (RMS) is introduced to determine the repeatability. The rocking curve and photoluminescence spectrum are used to evaluate the quality of the grown epilayer. The reactor pressure has affection on the quality of GaN epilayer. Low reactor pressure is helpful to improving the quality of crystal structure.

Topography of [formula omitted] AlGaN, GaN and InGaN layers grown by metal-organic vapor phase epitaxy

The growth of AlGaN, GaN and InGaN layers on (202¯1) GaN substrates was investigated by metal-organic vapor phase epitaxy. All layers exhibit undulations along [101¯4] with a period length between 20nm and 45nm. Under certain growth conditions, GaN exhibits bunching of the undulations leading to an increase of the undulation period and amplitude. This is also observed in the ternary alloys InGaN and AlGaN. Strong impact of the growth temperature and reactor pressure on the surface roughness and photoluminescence properties is found for GaN layers, with the smoothest layers obtained at low growth temperatures and low reactor pressures. Atomic scale interpretation suggests that the undulations originate from (101¯0) and (101¯1) microfacets.