High Base Pressure Research Articles

Scale-adaptive simulation of the separated flow past a 90°-inclined prolate spheroid

The separated flow past a 6:1:1 spheroid is numerically investigated by means of the scale-adaptive simulation technique. The Reynolds number based on the free-stream velocity and the diameter at middle-section of the spheroid is located in the subcritical regime, i.e., Re = 3900. In comparison with the circular cylinder at the same Reynolds number, about 35% drag reduction is acquired by the spheroid, and the fluctuations of lift and drag are suppressed effectively. According to the detailed comparison, the satisfactory drag reduction and suppression of fluctuating force obtained by the spheroid are closely associated with the higher base-pressure and lower turbulent fluctuations in the near wake. Abundant contrasts of the different spanwise sections are presented to reveal the mechanism of constrained flow and apex effect of the spheroid. In addition, in order to provide reliable data for testing and developing turbulence models, a large number of turbulence statistics are computed and compared with previous data of the circular cylinder and sphere at comparable Reynolds numbers. Lower Reynolds stress peaks and less vigorous coherent structures indicate that the three-dimensional force and constrained flow caused by the spheroid can lead to the formation of steady shear layer and vortex separation. Furthermore, proper orthogonal decomposition and dynamic mode decomposition are employed to understand the large-scale wake flow structures behind the spheroid. The modal analysis results confirm that the wake of the spheroid is more stable than the circular cylinder, reconfirming the effective flow control.

Large-eddy simulation of the compressible flows around a wavy-axis square cylinder

The compressible flows past a wavy-axis square cylinder are numerically carried out by means of the large-eddy simulation technique for two different free-stream Mach numbers (M∞=0.71 and 0.85), which are less than the critical Mach number Mcr (≈0.9). The Reynolds number based on the side-length of the wavy-axis square cylinder is chosen as 4×105. For comparison, the compressible flows around the corresponding normal square cylinder are also calculated. The control effects and mechanisms are systematically analyzed. Comparing the wavy-axis square cylinder with a normal square cylinder for M∞=0.71 and 0.85, about 23.5% and 8.1% drag reductions are acquired, respectively, and the fluctuating forces are suppressed significantly. Based on the analysis of drag decomposition, when M∞<Mcr, the drag reduction related to vortex force prevails over that relevant to compressible effect. Moreover, the wavy-axis square cylinder can also provide the effective control for type C moving shock. The effective drag reduction and suppression of fluctuating force obtained by the wavy-axis square cylinder are closely associated with the higher base-pressure and lower turbulent fluctuations in the near wake, which can be achieved by the strengthened compressibility and waviness effect of shear-layer. However, when M∞ approaches Mcr, the effective flow control from the wavy-axis square cylinder is attenuated due to the competition between strengthened compressibility and the waviness effect of the shear-layer.

Design and fabrication of a micro-opto-mechanical-systems accelerometer based on intensity modulation of light fabricated by a modified deep-reactive-ion-etching process using silicon-on-insulator wafer

Accelerometers that work based on intensity modulation of light are more sensitive, economically feasible, and have a simpler fabrication process compared to wavelength modulation. A micro-opto-electro-mechanical-system accelerometer based on intensity modulation of light is designed and fabricated. A movable shutter that is attached to the proof mass is designed to change the intensity of light. Moreover, the mechanical part is designed to improve the overall sensitivity and linear behavior in the measurement range. The designed accelerometer is fabricated by a deep-reactive-ion-etching (DRIE) process. The DRIE process used in this report is based on a Bosch-like process, which uses SF6 and a mixture of H2, O2, and SF6 gases in etching and passivation subsequences, respectively. This method has a lower plasma density and a higher base pressure and causes higher verticality, which results in lower optical efficiency loss in comparison with the Bosch process. Furthermore, the functional characteristics of the accelerometer are derived with analytical and numerical methods, and the results are compared. The functional characteristics of the accelerometer are as follows: a resonant frequency of 0.56 kHz, a mechanical sensitivity of 0.6μm/g, an optical sensitivity of 16%/nm, an overall sensitivity of 9.6%/g, a footprint of 2000×2000μm2, a measurement range of 3g, a mechanical cross axis sensitivity of 0.058μm/g, and an overall cross axis sensitivity of 0.00029%/g. These functional characteristics make the design appropriate for a large range of applications.

Air-based deposition of titanium‑aluminum oxynitride thin films by reactive magnetron sputtering

Quaternary titanium‑aluminum oxynitride (Ti,Al)NxOy films with various nitrogen and oxygen compositions can be applied to many interesting mechanical and technological applications. Conventionally, pure nitrogen and oxygen were required as reactive gases for producing the films by sputtering under a high vacuum environment. In this work, air was employed as a reactive gas instead. Thus, a low vacuum environment was sufficient, which could save a tremendous amount of processing time and cost. (Ti,Al)NxOy films were produced by dc magnetron sputtering at a high base pressure. By merely varying the air/Ar flow ratio in a suitable processing window, X-ray diffraction patterns show that obtained films could turn from a crystalline rock-salt structure to an amorphous phase. X-ray photoelectron spectroscopy revealed the compositions that could be tailored from N-rich to O-rich (Ti,Al)NxOy thin films. With increasing O/N ratios, as-deposited (Ti,Al)NxOy films could turn from metallic-like, semi-conductive, to dielectric-like behaviors. Moreover, the hardness and reduced Young's modulus of obtained (Ti,Al)NxOy films could also be tailored and achieved as high as 39.1 GPa and 331 GPa, respectively. This may be due to solid solution hardening effects.

NANOSTRUCTURED Ti-C THIN FILMS DEPOSITED BY THERMIONIC VACUUM ARC (TVA) TECHNOLOGY

Nanostructured titanium-carbon nanostructured thin films were prepared using the Thermionic Vacuum Arc (TVA) technology in different configurations under a varied number of Ti/C combinations at high base pressure of 1 x 10-6Torr with and without graded compositions. The layers consisting of about lOOnm Carbon base layer and seven 40nm alternatively Ti and C layers were deposited on Silicon substrates. On the other hand, in order to obtain C-Ti multilayer structures with variable thickness and different percentages in C and Ti oflayers, a 20nm thick C layer was first deposed on Si substrate and then seven Ti-C layers, each ofthese having thickness of up to 40nm were deposed. To perform the successively layers with various thickness were changed the discharge parameters for C and Ti plasma sources to obtain the desirable thickness. By changing ofsubstrate temperature between room temperature and 300°C and on the other hand the bias voltage up to -700V, different batches of samples were obtained for this study. The films were characterized by surface morphology, and microstructure, through Rutherford Backscattering Spectrometry (RBS), Raman Spectroscopy, Transmission Electron Microscopy (TEM), Grazing Incidence X-ray diffraction (GIXRD). Tribological and electrical measurements are also presented.

Experimental study on the performance of ultra high pressure common rail system

In order to overcome the difficulties of high pressure source design and parts integration in the injector, realizing the ultra high pressure injection and controllable fuel injection rate, an ultra high pressure common rail system based on domestic basic materials and manufacturing technology level was proposed and designed. The working principle of this system was first introduced; the performance test bench of ultra high pressure common rail system was built. Then, the influence of pressure-amplifier device structure parameters on the pressurization pressure peak was analyzed quantitatively, and on the basis of selecting the most appropriate combination of parameters, the pressure and fuel injection rate control characteristics were conducted. The results show that ultra high pressure common rail system can magnify fuel pressure to ultra high pressure state (more than 200 MPa) and by changing the control signal timing of pressure-amplifier device and injector solenoid valve, the flexible and controllable fuel injection rate can be achieved. Under the condition of the same pressurization ratio, the peak value of pressurization pressure increases gradually, and with the increase of pressurization ratio, the increasing trend of the pressurization pressure peak value is nonlinear. At the same time, under the same condition of spring preload, the greater of the spring stiffness, the higher of the rail base pressure can bear, that means the pressure-amplifier device can achieve pressurization at a higher base pressure. But if the spring stiffness is too large, the solenoid valve of pressure-amplifier device will not be opened due to insufficient electromagnetic force, so the specific selection should be considered in a compromise.

Anomalous characteristics of nanostructured hydrogenated carbon thin films

Hydrogenated diamond-like carbon (DLC) films are sought for several technological applications including electronics, optics, mechanics, and tribology. However, conventional hydrogenated DLC films, that commonly deposit at low base pressure (~10−5 to 10−8 Torr), have many intrinsic limitations in terms of moderate hardness (15–25 GPa), low electrical conductivity (in insulating regime), and they display amorphous morphology. Here we develop high-performance hydrogenated carbon-based films using a cost-effective and fast deposition approach. We report the room temperature synthesis of nitrogen incorporated nanostructured hydrogenated carbon films (n-C:N:H) at a high base pressure of ~5 × 10−3 Torr, employing a non-conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system equipped with only a primary pump. The n-C:N:H films show appealing properties such as wide band gap (2.35–2.9 eV), high optical transparency, high hardness (up to ~ 40 GPa), ultrahigh elasticity (elastic recovery ~95%), reasonably good electrical conductivity, and diode-like behavior when examined in n-C:N:H/Si heterojunction device configuration. Interestingly, some of n-C:N:H films revealed the multifunctional activities with properties surpassing to those of many low base pressure grown traditional amorphous DLC films. This discovery solves many fundamental concerns of hydrogenated DLC films and opens new paths for their enhanced commercial applications.

Influence of the hydrogen content on the optical properties of TiOx thin films

In this work, dark Ti(OxH)y films were obtained by DC magnetron reactive sputtering under different oxygen partial pressures (PO2) starting from a high base pressure chamber atmosphere (10−5 mbar) with the purpose of investigating the visible light absorption dependence of the hydrogen content and stoichiometry. The TiOx film compositions, thicknesses, and elemental depth profiles were measured by ion beam analysis (IBA) techniques in a self-consistent approach. The optical properties were investigated by visible and near-infrared spectrophotometry, and the crystallography was examined by small-angle X-ray diffraction. The films deposited at a low PO2 were dark, and as the partial oxygen pressure increases, they became transparent. The IBA showed that the hydrogen content exponentially decreased as the PO2 increased. The optical constants for the films, n and k, are dependent on oxygen and hydrogen ratio as well as the optical band gap that can be modulated from 0.40 to 3.25 eV, and thus a significant improved PEC activity. These modulations may be useful for optical or electronic devices as a variable semiconductor or dielectric layer.

The strong effect on MEMS switch reliability of film deposition conditions and electrode geometry

Microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) switches are of interest for applications ranging from radio frequency (RF) communication circuitry to power surge protection to low power computing. Reliable operation of the switch contacts is critical. Many researchers have investigated the effect of different materials on reliability, but little work has been reported on the effects of process conditions. In this study, the effects of the angle of deposition incidence, the microstructure, the interface adhesion, the base pressure, and the process transferability have been evaluated in Pt-coated MEMS switches. Among them, base pressure attained prior to deposition was most important, while an interaction between electrode geometry and film microstructure was also identified to play a role. A lower base pressure (2•10−6 Torr) improved cycle count significantly while a higher base pressure (8•10−6 Torr) resulted in low cycle counts (106 or less). Failure analysis indicated surface wear due to damage at a corner junction, where films deposited at different angles of deposition incidence meet. Mass spectroscopy analysis detected higher carbon content in Pt films with high base pressure. This suggests that grain boundaries weakened by impurity segregation exacerbated the geometric effect. Switches tested in a follow-up experiment, in which the base pressure was 5•10−7 Torr, again resulted in high cycle counts (330•106), reinforcing the importance of achieving low base pressure in improving the mechanical reliability of the corner junction.

Air-based sputtering deposition of TiNxOy films for solar selective absorber coatings applications

Titanium oxynitride (TiNxOy) thin films with tunable electrical and optical properties were produced by sputtering using air as a reactive gas at a high base pressure of 1.3 × 10−2 Pa. Pure N2 and O2 with a fixed ratio of 79/21 was also used as a reference reactive gas (denoted as S-air) at a low base pressure of 6.7 × 10−4 Pa to investigate the influence of trace components in air on the characteristics of the films. By merely changing either the air/Ar or the S-air/Ar flow ratio within suitable processing windows, crystal phases, microstructures, electrical resistivities, and optical properties could be tailored. In practice, the TiNxOy thin films produced by such air-based sputtering could turn from conductive, semiconductive, to dielectric and would correspondingly change from opaque, translucent, to transparent. For solar selective absorber (SSA) coatings evaluations, bilayer and gradient TiNxOy-based SSA structures were tested, respectively. The gradient TiNxOy SSA coatings exhibited the best performance with solar absorptance of 86.3% and thermal emittance of 11.3%. This facile air-based sputtering technique with such a gradient coating design has great potential for solar selective absorber coatings applications.

Cluster-based reduced-order modelling of the wake stabilization mechanism behind a twisted cylinder

The flow around a twisted elliptic cylinder is numerically calculated using large eddy simulation (LES) at the Reynolds number of 2 × 104, along with a circular cylinder for comparison. The mean drag coefficient and the fluctuating lift coefficient of the twisted cylinder are 27.99% and 94.01% lower, respectively, than that of the circular cylinder. The twisted surface leads to more stable wake, longer vortex formation length, higher base pressure and three-dimensional separation. In addition, cluster-based reduced-order modelling (CROM) is performed to analyze phase-dependent variations of the wake flow and evaluate the relationship between derived characteristic flow structures and their impact on forces. It captures the oscillatory behavior of Kármán vortex shedding in the wake of the circular cylinder. Meanwhile, two flow regimes, anti-phased and in-phase-dominated vortex shedding, generated by the twisted cylinder are extracted and distinguished. The two regimes prevail alternately with changing probability of occurrence along the spanwise of the twisted cylinder, which can be attributed to the different separation angles. The in-phase-dominated vortex shedding inhibits the effect of Kármán vortex shedding and is subjected to a lower force compared with the anti-phased vortex shedding.

Room temperature deposition of perpendicular magnetic anisotropic Co3Pt thin films on glass substrate

Co3Pt alloy thin films were deposited on the glass substrate at room temperature (RT) and 300°C, which showed high perpendicular magnetic anisotropy (PMA) and isotropy magnetic behaviors, respectively. Co3Pt HCP (0002) planes grew along the substrate plane for the films deposited at RT. The easy axis [0001] was consequently vertical to the substrate surface and obtained the predominant PMA. Large magnetic domains and sharp boundary also supported high PMA in RT-deposited samples. On the other hand, the PMA was significantly decreased with increasing the deposition temperature from RT to 300°C. Hard HCP(0002) and soft A1(111) co-existed in the film and the magnetic exchanged coupling between these two phases induced isotropy magnetic behavior. In addition, the various thicknesses (t) of the RT-deposited Co3Pt films were deposited with different base pressures prior to sputtering. The Kerr rotation loops showed high PMA and out-of-plane squareness (S⊥) of ~0.9 were found in low base pressure chamber. Within high base pressure chamber, Co3Pt films just show magnetic isotropy behaviors. This study provides a fabrication method for the preparation of high PMA HCP-type Co3Pt films on the glass substrate without any underlayer at RT. The results could be the base for future development of RT-deposited magnetic alloy thin film with high PMA.

Numerical investigation on body-wake flow interaction over rod–airfoil configuration

Numerical investigations of body-wake interactions were carried out by simulating the flow over a rod–airfoil configuration using high-order implicit large eddy simulation (HILES) for the incoming velocity$U_{\infty }=72~\text{m}~\text{s}^{-1}$and a Reynolds number based on the airfoil chord$4.8\times 10^{5}$. The flow over five different rod–airfoil configurations with different distances of$L/d=2$, 4, 6, 8 and 10, respectively, were calculated for the analysis of body-wake interaction phenomena. Various fundamental mechanisms dictating the intricate flow phenomena including force varying regulation, flow structures and flow patterns in the interaction region, turbulent fluctuations and their suppression, noise radiation and fluid resonant oscillation, have been studied systematically. Due to the airfoil downstream, a relatively higher base pressure is exerted on the surface of the cylinder upstream, and the pressure fluctuation on the surface of the rod–airfoil configuration with$L/d=2$is significantly suppressed, resulting in a reduction of the fluctuating lift. Following the distance between the cylinder and airfoil strongly decreases, Kármán-street shedding is suppressed due to the blocking effect. The flow in this interaction region has two opposite tendencies: the influence of the airfoil on the steady flow is to accelerate it and the counter-rotating vortices connecting with the leading edge of the airfoil tend to slow the flow down. There may be two flow patterns associated with the interference region, i.e. the Kármán-street suppressing mode and the Kármán-street shedding mode. The primary vortex shedding behind the cylinder upstream, and the shedding wake impingement onto the airfoil downstream, play a dominant role in the production of turbulent fluctuations. When primary vortex shedding is suppressed, the intensity of impingement is weakened, resulting in a significant suppression of the turbulent fluctuations. Due to these factors, a special broadband noise without a manifestly distinguishable peak is radiated by the rod–airfoil configuration with$L/d=2$. The fluid resonant oscillation within the flow interaction between the turbulent wake and the bodies was further investigated by adopting a feedback model, which confirmed that the effect of fluid resonant oscillation becomes stronger when$L/d=6$and 10. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the body-wake interaction.

Discrete Transition of Flow Over a Circular Cylinder at Precritical Reynolds Numbers

Experiments were made for flow over a smooth circular cylinder at Reynolds numbers of , relevant to the initial transition from the subcritical to critical regimes. Pressure measurements on the circular cylinder show that at a fixed flow speed, two bistable states are observed. Further data analysis indicates that the characteristics of these two states, named states A and B, respectively, are distinctly different. Whereas state B is identified as the subcritical state, state A shows flow characteristics toward the critical regime, namely, a higher base pressure, a more unsteady vortex-shedding frequency, a lower Strouhal number, and a greater fluctuating energy in low-frequency components. The microelectrical mechanical system thermal-tuft measurements on the circular cylinder show that state A represents a situation in which separation bubbles are situated symmetrically on the two sides of the circular cylinder. The separation bubbles induce pronounced unsteadiness in the flow and tend to move upstream as the Reynolds number increases.

The Effects of Trailing Edge Thickness on the Losses of Ultrahigh Lift Low Pressure Turbine Blades

Experimental, numerical and analytical methods were used to investigate the effects of the blade trailing edge thickness on the profile loss of ultrahigh-lift low-pressure turbine blades. Two cascades, the T106C and the T2, were studied. The loss obtained based on the data at the blade trailing edge plane and the plane 0.3 Chord downstream of the trailing edge agree with each other for T106C blade with and without upstream wakes at different Reynolds numbers. The blade profile losses were broken down as the suction surface boundary loss, the pressure side boundary loss and the mixing loss downstream of the trailing edge for six Reynolds numbers. Trailing edge thicknesses varying from 1.4% to 4.7% pitch were investigated at a Reynolds number of 210,000. It was found that the flow distributions across the passage at the trailing edge planes were highly nonuniform. In particular, and as a result, the trailing edge base pressure was higher than the mixed-out static pressure, so the contribution of the base pressure to the mixing loss downstream of the trailing edge plane was to reduce the loss. When the trailing edge thickness increases, there are three main effects: (1) the area with high base pressure region increases, which tends to reduce the downstream mixing loss; (2) the base pressure reduces, which tends to increase the loss; and (3) the flow diffusion downstream of the trailing edge, which tends to increase the loss. The overall result is the combined effect. For the T106C cascade, increasing the trailing edge thickness from 1.9% pitch to 2.8% pitch has a small effect on the loss. Further increasing the trailing edge thickness increases the loss. The T2 blade has a higher lift than the T106C blade, so the effects of the base pressure in reducing the mixing loss downstream of the trailing edge is more evident. The experimental results show that the profile loss first decreases and then increases as the trailing edge thickness increases. CFD, using the transition k-ω SST model and the k-ω SST model, provides good predictions of the aerodynamic performance. It was used to study the cases with trailing edge thicknesses of 1.4% pitch and 2.9% pitch. The profile loss is almost the same for these two trailing edge thickness. The results show that it is possible to use thicker blade trailing edges in low pressure turbines without aerodynamic penalty. This can lead to benefits in terms of mechanical integrity and manufacturing cost reductions.

Effect of Vacuum System Base Pressure on Corrosion Resistance of Sputtered Al Thin Films

Pure aluminum (Al) thin films were sputter deposited under conditions of variable base pressure of the deposition tool. The purpose of the study was to identify if the base pressure had a key influence on the electrochemical properties, namely corrosion resistance, of the Al deposits. The corrosion resistance was found to increase with increasingly high base pressure (i.e. lower quality vacuum). Potentiodynamic polarization experiments, pit growth studies, TEM and XPS were employed. From the results is posited that improved corrosion resistance is due to the presence of oxygen (O) being incorporated into the films during deposition resulting an O-containing (i.e. Al-O) alloy. This effect is important to identify because of the many reports in the literature of high pitting potentials for Al thin films, from studies with widely varying base pressure. Vacuum system base pressure was found to have a large effect on the corrosion resistance. A maximum base pressure in the low 10−7 Torr range is recommended to minimize oxygen uptake into the film.

Role of base pressure on the structural and nano-mechanical properties of metal/diamond-like carbon bilayers

The influence of base pressure on the structural and nano-mechanical properties of plasma produced metal (Cu and Ti)/diamond-like carbon (DLC) bilayers is reported. All of these samples show the formation of micro and nano-structured morphology with very low average roughness (less than 1nm). The Cu/DLC and Ti/DLC bilayers deposited at low base pressure exhibit comparatively higher hardness and better nano-mechanical properties than the ones grown at higher base pressure. Highest hardness as ~49GPa is obtained in Ti/DLC bilayer grown at low base pressure condition. Several other nano-mechanical parameters are also examined and correlated with each other. Present investigation will be useful to understand the role of base pressure on the properties of DLC coatings and may enhance its industrial application in term of hard and protective coatings.

Photoconductivity and characterization of nitrogen incorporated hydrogenated amorphous carbon thin films

The observation and origin of photoconductivity in high base pressure (∼10−3 Torr) grown nitrogen incorporated hydrogenated amorphous carbon (a-C:H:N) thin films is reported. The magnitude of conductivity at room temperature was measured to increase by nearly two orders of magnitude and exhibits a maximum ratio of photoconductivity to dark conductivity of 1.5 as the nitrogen content increased to 15.1 at. %. X-ray photoelectron spectroscopy, micro-Raman spectroscopy, and Fourier transform infrared spectroscopy reveal enhanced sp2 bonding at higher nitrogen contents. Residual film stress, Tauc band gap, hardness, and elastic modulus are all found to decrease with addition of nitrogen. The electrical characteristics suggest the creation of a-C:H:N/p-Si heterojunction diodes having rectifying behavior. The conductivity and electrical characteristics are discussed in term of band model, and the results show that high quality a-C:H:N films can be grown at high base pressures with properties comparable to those grown at low base pressures.

Influence of the sputtering reactive gas on the oxide and oxynitride La[sbnd]Ti[sbnd]O[sbnd]N deposition by RF magnetron sputtering

Perovskite LaTiON thin films have been grown by radio frequency magnetron sputtering from a LaTiO2N target. With a very low base pressure in the deposition chamber, two types of films can be obtained: colored oxynitride LaTiO2N films when nitrogen gas is introduced during sputtering or black N-doped LaTiO3 films when deposition is performed in pure argon. On SrTiO3 (001) substrate heated at 750°C, LaTiO2N films are epitaxially grown, while N:LaTiO3 films are poorly crystallized. With a higher base pressure in the deposition chamber, transparent La2Ti2O7 films are produced. They are (012) textured on (001) SrTiO3 substrate. As observed during the reactive sputtering of metallic targets, the evolution of the deposition rate and the nitrogen content in films according to the N2 percentage in the plasma is abrupt.

Low Vacuum Deposition of Aluminum Nitride Thin Films by Sputtering

Conventionally, sputtering deposition of thin films requires a low base pressure (high vacuum) to minimize influences of residual gases. Here, a high base pressure (low vacuum) was used, which can reduce significantly the overall processing time. Aluminum nitride ( AlN ) was selected as a model system of dielectric nitrides. All the analyses revealed that the obtained films under a low vacuum environment within specific processing windows exhibited characteristics similar to those of high‐vacuum made AlN films. Such a low vacuum deposition technique takes advantages of kinetically favorable formation of the nitride films and hence, has great potentials in many more technological applications.