Effect Of Residual Gas Research Articles

Reactive formation of C3N4 as a by-product of AISI 1070 parts produced by laser powder bed fusion in N2 atmosphere

AbstractIn metal additive manufacturing (AM), inert gases are traditionally used to achieve a controlled atmosphere and mitigate the effects of residual reactive gases. However, the interaction between gases and laser processes, particularly in reactive laser powder bed fusion (RL-PBF) technology, offers the possibility of opening up new avenues for material synthesis. In this experimental work, the authors observed the presence of C3N4 in the residual powder during the manufacture of AISI 1070 steel parts by L-PBF, indicating a reactive process occurred during parts production. This investigation revealed the formation in the working chamber of a waste product containing C3N4 carbon nitride, due to the reaction between the carbon released from the steel and the nitrogen in the chamber. Remarkably, despite carbon depletion, the final product of AISI 1070 steel complies with the specifications of use. Hence, the L-PBF machine was modified to allow black powder sampling from various locations in the chamber. Authors attempted to enhance the production of the C3N4 material by increasing the SED up to 7143 J/mm2 to sublimate a pure graphite rod and concurrently manufacture parts in AISI 1070, in a nitrogen atmosphere. The results obtained at higher SED values showed that in both cases (graphite rod or AISI 1070 steel) a C3N4 compound in the black powder is formed in the investigated atmosphere by reaction of nitrogen atoms with the carbon atoms vaporized by the laser beam. Thus, the study highlights the novel achievement of synthesizing carbon nitride as a high-value by-product while producing functional AISI 1070 steel parts via L-PBF through reaction with nitrogen atmosphere.

Electronic structure and properties of activated Al0.5Ga0.5N thin films and nanowires photocathode surface adsorbed with residual gas

To study effect of residual gas in vacuum system on structure and electrical performance of activated AlGaN photocathodes, adsorption models for H2O, CO and CO2 adsorbed on Al0.5Ga0.5N activated surfaces are established and investigated. The results indicate that activated Al0.5Ga0.5N surfaces are not easily adsorbed by residual gases compared to pristine surface. For Al0.5Ga0.5N thin film, H2O and CO exhibit higher affinity for adsorption on Cs/NF3-activated surfaces, while CO2 shows greater tendency to adsorb on Cs-activated surface·H2O, CO and CO2 are most easily adsorbed on Cs/NF3-activated surface for Al0.5Ga0.5N nanowires. Work function of Cs/NF3-activated thin film surface is 2.34 eV, and after adsorbing H2O, CO and CO2, work functions are 1.84 eV, 2.37 eV, and 3.43 eV, respectively. This means that residual gas will increase work function and is harmful to quantum efficiency improvement.

Emission enhancement of GaN field emitter arrays in an N2 environment

Field emitter arrays (FEAs) have the potential to operate at high frequencies and in harsh environments. However, the vacuum packaging of these devices poses a challenge due to the sensitivity of the emission phenomena to the surface properties of the cathode. Studying the effect of different residual gases on FEAs can help to understand the interaction of the emission surface with the environment and identify the feasibility and requirements for vacuum packaging. In this work, the effect of N2 exposure on 150 × 150 gallium-nitride-field emitter arrays (GaN-FEAs) was studied. The GaN-FEA was first operated at 10−9 Torr with a 1000 V DC anode voltage and a 50 V DC gate voltage, where the anode current was 6 μA. The device was then exposed to 10 000 l N2 at 10−7 Torr, and the anode current increased by 2.7 times during N2 exposure. The increase in the current was not permanent, and the current gradually decreased to its pre-exposure level after the N2 source was cut off. The results of N2 exposure were compared to Ar and O2.

THE EFFECT OF SURFACE ROUGHNESS AND ELECTROSTATIC BONDS OF RESIDUAL GASES ON THE SETTLING OF THE GYROSCOPE ROTOR

Consider the rotor of an electrostatic gyroscope (ESG) for contactless operation. The rotors of the ESG are made in the shape of a ball and placed in an electric field. The gyroscope operates contactless between the rotor and the mutual oscillation force of the electrodes. Air is sucked out of the chamber in which the rotor is located. Due to the lack of air resistance and the lack of friction force, the rotor can rotate for a long time. But the rotor, located inside the vacuum, also slows down. The reason for this is the roughness and residual deposits of the rotor surface. This is the notorious effect of the rotor of these residual gases on braking. When calculating the effect of residual gases on the rotor, it is considered as separate gases, and not as a homogeneous medium. For this reason, the viscosity of the gas can be set without consideration. The effect of the action of individual particles on the rotor is determined by the roughness of its surface. That is, the momentum by which the particles somehow hit the "peaks" or "troughs" creates a torque that acts on the rotor.

Conversion of a Small-Size Passenger Car to Hydrogen Fueling: 0D/1D Simulation of EGR and Related Flow Limitations

Hydrogen is seen as a prime choice for complete replacement of gasoline so as to achieve zero-emissions energy and mobility. Combining the use of this alternative fuel with a circular economy approach for giving new life to the existing fleet of passenger cars ensures further benefits in terms of cost competitiveness. Transforming spark ignition (SI) engines to H2 power requires relatively minor changes and limited added components. Within this framework, the conversion of a small-size passenger car to hydrogen fueling was evaluated based on 0D/1D simulation. One of the methods to improve efficiency is to apply exhaust gas recirculation (EGR), which also lowers NOx emissions. Therefore, the previous version of the quasi-dimensional model was modified to include EGR and its effects on combustion. A dedicated laminar flame speed model was implemented for the specific properties of hydrogen, and a purpose-built sub-routine was implemented to correctly model the effects of residual gas at the start of combustion. Simulations were performed in several operating points representative of urban and highway driving. One of the main conclusions was that high-pressure recirculation was severely limited by the minimum flow requirements of the compressor. Low-pressure EGR ensured wider applicability and significant improvement of efficiency, especially during partial-load operation specific to urban use. Another benefit of recirculation was that pressure rise rates were predicted to be more contained and closer to the values expected for gasoline fueling. This was possible due to the high tolerance of H2 to the presence of residual gas.

Insights into performance enhancement mechanism of GaAs(100) photocathode by Cs/Li/O and Cs/Li/NF3 co-deposition

To verify the effects of additional Li introduction on the Cs-O and Cs-NF3 activation processes of GaAs(100) photocathodes, Cs-Li-O and Cs-Li-NF3 co-adsorption models of GaAs(100) β2(2 × 4) surfaces with different Cs/Li ratios are investigated, on the basis of pristine Cs-O and Cs-NF3 adsorption models. Residual gases including CO2, H2O, CO and H2 are introduced to the Cs-Li-O and Cs-Li-NF3 co-adsorption models to compare the damaging effects of typical residual gases. Simulation results show that the optimal Cs/Li ratio is 3:1, wherein 6Cs-2Li-1NF3 surface model exhibits smaller work function and 6Cs-2Li-1O surface model has better immunity to residual gases. Meanwhile, 6Cs-2Li-1O surface has better resistance abilities to CO2, CO and H2 than 6Cs-1O surface, while the resistances of 6Cs-2Li-1NF3 surface to H2O, CO and H2 are worse than those of 6Cs-1NF3 surface. From the perspective of charge transfer, a reverse dipole moment [Cs/Li-gas] and two positive dipole moments, i.e., [Cs/Li-surface] and [Cs/Li-O/NF3] are formed. Therefore, it is inferred that, the Cs-Li-O activation recipe, in comparison to the Cs-O activation recipe, can improve the photoemission stability of GaAs photocathodes, whereas the Cs-Li-NF3 activation recipe can increase quantum efficiency but is more sensitive to residual gases.

Temperature effects of aluminum melt during the infiltration process into porous graphite matrix

Aluminum/graphite composites are receiving much attention due to the unique features and diverse possibilities in different fields. Generally, the aluminum melt into the inner pores of the graphite matrix requires an ambient prompting with high temperature and gas pressure. The infiltration dynamics of aluminum melt, which highly depends on the ambient temperature, would have a significant effect on the final composite’s performance. In this work, we have studied the temperature effect of aluminum melt during the infiltration process into the porous graphite matrix based on numerical simulation and experimental verification. Results show that the moderate temperature increase would effectively promote the infiltration of aluminum melt, achieving a higher filling rate with a shorter filling time. However, the promotion effects of the temperature increase would come to a certain limit due to the opposing effects of capillary porous and residual gas. The detailed influences of the temperature on the infiltration would be presented, and the final composite performance, including mechanical strength and thermal management capability, would be examined and discussed.

Experimental Investigation on Combustion and Performance of Diesel Engine under High Exhaust Back Pressure

The use of exhaust gas recirculation, complex after-treatment systems, advanced technology of high-strength engines, and underwater exhaust will lead to increased diesel exhaust back pressure (EBP). This will increase the residual exhaust gas and the exchange temperature in the cylinder and reduce the fresh air charged in the next cycle. In this work, the effects of two high EBP conditions (10 kPa and 25 kPa) on the performance of medium-speed ship engines under different loads are explored through experiments. The results show that the increase in EBP from 10 kPa to 25 kPa has little effect on the heat release rate, engine power, and engine start-up time. However, it will lead to ignition advance and the maximum pressure rise rate, peak pressure, and exhaust temperature increase. The increase in EBP has a more significant impact on the small valve overlap angle. Because the reduction in the valve overlap angle has led to an increase in the residual exhaust gas, further increases in EBP causes residual exhaust gas effects to be more pronounced. The effect of increasing EBP on fuel consumption depends primarily on which effect of exhaust back pressure on temperature and fresh air intake dominates.

Exergy evaluation of equivalence ratio, compression ratio and residual gas effects in variable compression ratio spark-ignition engine using quasi-dimensional combustion modeling

This study is founded on previously validated, in-house, comprehensive, two-zone, quasi-dimensional combustion model, predicting performance and emissions in spark-ignition engines. The model is extended to include exergy terms complementing the first-law analysis results, which provide further useful information by implementation on test results acquired from an experimental, Ricardo E6, variable compression ratio (VCR) gasoline engine at the authors’ laboratory, operated under various CR and equivalence ratio (EQR) values at wide-open throttle. The model consists of unburned and burned zones simulating the combustion process, following strictly the flame-front movement in combustion chamber. The eventual combustion of the unburned mixture turbulent entrainment into burned zone through the flame-front area is followed closely. The exergy terms of each simulated zone are identified and computed, considering also chemical exergy components (diffusion plus reactive). The accurate account of temperature and species histories in burned zone after entrainment from the unburned one, should lead to more precise evaluation. This is important if irreversibility is computed from balance of all other exergy terms. The investigation examines and compares exergy-wise various EQR, CR and residual gas fractions. Presented diagrams of exergy terms for each zone and cylinder charge provide detailed information for chemical exergy, irreversibility and exergy losses.

Nonlinear dynamics of cycle-to-cycle variations in a lean-burn natural gas engine with a non-uniform pre-mixture

The cycle-to-cycle variations (CCVs) in reciprocating internal combustion engines may cause negative influence on diving performance, fuel economy, and emissions. Especially, lean-burn technology or exhaust gas recirculation (EGR) was used to improve engine combustion efficiency and reduce NOx, and the combustion boundary was limited by increased CCVs. Therefore, it was important to identify the complex dynamics of CCVs and to take measures for inhibiting them. The CCVs based on indicated mean effective pressure (IMEP) time series were examined in a lean-burn natural gas engine with a non-uniform pre-mixture based on statistical and multifractal theories. Tests were conducted at an engine speed of 1000 rpm and low loads of 10% and 25%, and combustion data at an engine load of 10% were analysed in detail because the CCVs are less sensitive to changes of gas injection timing (GIT) at the higher engine load. The nonlinear dynamics of the CCVs was revealed at the different GIT from 1° to 120°CA ATDC. The statistical properties of IMEP time series were characterised by distributions of probability density functions (PDF), the multifractal complexities of the combustion fluctuations were quantitatively analysed by the singularity spectra in terms of the Holder exponent based on the theory of wavelet transform modulus maxima, and the primary source leading to the increased CCVs and complex dynamics in a natural gas engine with a non-uniform pre-mixture was identified using 3D-computational fluid dynamics simulations. Results show that as the GIT increased, the kurtosis of the IMEP time series systematically decreased from 592 to 1.8, the fast dynamics transitions from super-Gaussian to quasi-Gaussian distributions in combustion system were revealed, and the lower value of kurtosis implied the lower degree of intermittency. Except for the GIT of 60°CA ATDC, the value of the Holder exponent $$h_{0} > 0.5$$ , which implies that the CCVs for the other GITs behaved like a persistent walk or positive correlation. For GITs of 60° and 90°CA ATDC, the narrow broadness of the singularity spectrum implicated a monofractality, while the obvious multifractal properties could be identified for the other GITs. The transitions of the dynamic behaviours may be caused by the degree of mixture in-homogeneity combined with a new mechanism of “prior-cycle effects” proposed in this paper, and the effect mechanism was not only associated with the residual gas in the cylinder but also with the residual gas fuel in the intake port, rather than independent effect of residual gas in cylinder reported in previous researches. Our research results provided the deeper understanding on the dynamics of combustion system in multi-point injection natural gas engines and may be beneficial to achieve nonlinear prediction and to develop improved control strategies for inhibiting the CCVs.

Effect of residual gas on the optoelectronic properties of Mg-doped Ga0.75Al0.25N (0 0 0 1) surface

In order to verify the effect of residual gas molecules (H2, CH4, CO, H2O and CO2) adsorbing on the Mg-doped Ga1-xAlxN (0 0 0 1) surface, the photoelectronic properties of the adsorption systems are calculated. Through the first-principles calculation, the surface adsorption energy, work function, dipole moment, the Fermi and vacuum levels, the energy bands, density of states and absorption coefficients are analyzed. The calculation results demonstrate that the adsorption models of CO, H2O, and CO2 molecules are all physically stable structures. However, the adsorption of H2 and CH4 may not be stable at room temperature. CO2 molecule is the most easily attached to the cathode surface and results in a significantly increasement of the work function. The residual gas captures part of the electrons generated by the cathode surface, and then produces a dipole moment directing from the cathode surface to the residual gas molecules. In addition, tiny alteration of energy band is observed on the residual gas adsorption system. Due to the orbital motion, some new energy levels are also generated in the deep energy band. Moreover, the adsorption of CO, H2O and CO2 molecules also exert a noticeable effect on the absorption coefficient.

Estimation of parameters affected in internal exhaust residual gases recirculation and the influence of exhaust residual gas on performance and emission of a spark ignition engine

In the effort of improving the internal combustion engine performance, the exhaust residual gases and effective release energy are sensitive factors, which effect on engine efficiency and emission formation. Herewith we estimated and summarized the parameters which affect the internal exhaust residual gases recirculation, and investigate the effect of internal exhaust residual gas on peak pressure rise, effective energy, and engine emissions, which weren’t presented yet in the previous articles. It is knotty to investigate the residual gas ratio, the effective energy under the various testing conditions from the experiments. Through the simulation and experiment methods approach we eliminated certain above drawbacks. From results of the research, we thoroughly investigated the effects of engine speed, air–fuel ratio, valve overlap, combustion duration, intake port diameter-bore ratio, and bore-stroke ratio on the internal exhaust residual gases recirculation. We also found that the increase in the internal exhaust residual gas from 1% to 5% was due to the peak firing temperature decrease from 2900 K to 1250 K, the peak pressure rise decrease from 8 to 5.5 bar/deg, the effective release energy decrease from 0.85 to 0.53 kJ, the NOx emission reduction from 11.3 to 2.12 g/kwh and the engine brake torque decrease from 20.3 to 9 Nm.

Microstructural origin of compressive in situ stresses in electron-gun-evaporated silica thin films

Room-temperature electron-beam deposition of transparent silica produces thin films that are typically under large compressive stresses (a few hundreds of megapascals), yet of smaller density than tempered glass. The physical origin of these stresses remains unknown as all prior experimental studies were performed in manufacturing conditions where it is impossible to disentangle effects arising from vaporization versus residual gases. Here, we tackle this problem using a combination of experimental and numerical approaches. Our key experiment is performed after having achieved a high vacuum that allows us to approach the ideal case where residual gas effects are negligible. It shows that, even in this case, films are strongly compressive, which rules out in situ reactive processes as a possible origin of these stresses. Another test allows us to show that collisional slowing down causes films to grow much less dense and less compressive (even tensile). Our numerical model, designed to represent deposition in the absence of reactive processes, produces films with reasonable stress and density values, yet only when assuming that vaporized particles reach the growing film with kinetic energies of order a few electron volts, which is an order of magnitude higher than literature estimates. We argue that such values are plausible because, silica being an electric insulator, its surface may concentrate charges when irradiated by electrons, and hence its e-beam vaporization, which is a much more rapid process than its evaporation, is likely to involve Coulombian effects. Finally, the microstructural analysis of numerical systems shows that these compressive stresses result primarily from the fact that, in usual deposition conditions, films tend to be oxygen-deficient, and thus contain a significant fraction of coordination defects akin to partial oxygen vacancies, which cause their Si-O covalent network to be less cohesive and tensile than that of relaxed silica.

Effect of residual gas on structural, electrical and mechanical properties of niobium films deposited by magnetron sputtering deposition

Magnetron sputtering is an important method in the superconducting thin films deposition. The residual gas inside the vacuum chamber will directly affect the quality of the superconducting films. In this paper, niobium films are deposited by magnetron sputtering under different chamber residual gas conditions. The influence of baking and sputtering process on residual gas are studied as well. Surface morphology, electrical and mechanical properties of the films are analysed. The residual gas analysis result before the sputtering process could be regarded as a reference condition to achieve high quality superconducting thin films.

CO2 storage in depleted gas reservoirs: A study on the effect of residual gas saturation

Abstract Depleted gas reservoirs are recognized as the most promising candidate for carbon dioxide storage. Primary gas production followed by injection of carbon dioxide after depletion is the strategy adopted for secondary gas recovery and storage practices. This strategy, however, depends on the injection strategy, reservoir characteristics and operational parameters. There have been many studies to-date discussing critical factors influencing the storage performance in depleted gas reservoirs while little attention was given to the effect of residual gas. In this paper, an attempt was made to highlight the importance of residual gas on the capacity, injectivity, reservoir pressurization, and trapping mechanisms of storage sites through the use of numerical simulation. The results obtained indicated that the storage performance is proportionally linked to the amount of residual gas in the medium and reservoirs with low residual fluids are a better choice for storage purposes. Therefore, it would be wise to perform the secondary recovery before storage in order to have the least amount of residual gas in the medium. Although the results of this study are useful to screen depleted gas reservoirs for the storage purpose, more studies are required to confirm the finding presented in this paper.

Initial vacuum effects on the properties of sputter deposited Ga-doped ZnO thin films

Abstract Initial vacuum effects on the structural, electrical, optical properties of Ga-doped ZnO (GZO) films were investigated systematically. GZO thin films were prepared on glass substrates by RF magnetron sputtering at room temperature. With decreasing the initial vacuum, the electrical conductivity is enhanced due to an increase in the carrier concentration and mobility. Low initial vacuum resulted in the improvement in crystallinity, the increase of oxygen-vacancy and the decrease of the surface bonding, because of minimization the effect of residual gases in the process chamber, leading to an increase in the carrier concentration and mobility together. Especially, the behavior of oxygen-vacancy and surface bonding evidently are promoted at below 2 × 10−6 Torr. The average transmittance of all the thin films deposited was above 85% in the visible spectrum. The figure of merit for the GZO thin film improved, as initial vacuum decreased. The control of initial vacuum plays an important role in the subsequent properties of sputtered GZO thin films.

Effect of background pressure on Co/C multilayers.

Co/C multilayers with a period thickness of 3.54 nm and 30 bilayers were deposited by direct current magnetron sputtering with different background pressures. The effects of residual background gases were investigated. The films were characterized by using grazing incidence hard x-ray reflectivity, soft x-ray reflectivity, and x-ray photoelectron spectroscopy. The results indicate that the x-ray reflectivity of Co/C multilayers decreases with increasing background pressure as well as the increasing interlayer roughness. The inclusion of more residual background air increases the interdiffusion of Co and C layers.

Dramatic effect of residual gas on dynamics of isobaric adsorption stage of an adsorptive chiller

This paper reports an experimental study aimed to elucidate the effect of non-adsorbable gases (air and hydrogen) on dynamics of isobaric adsorption stage of an adsorptive chiller (AC) cycle. This is the first study performed on real small scale adsorbers, based on commercial heat exchangers (HExs) filled with loose grains of the adsorbent AQSOA™-FAM-Z02. The adsorption dynamics was studied by a gravimetric large temperature jump method under conditions of typical AC cycle. Three notable observations are described in the paper: (1) at any gas pressure Pgas, the experimental uptake curves are exponential; (2) the adsorption rate is extremely sensitive to traces of residual air and depends on the HEx geometry and the nature of residual gas; (3) the effect of hydrogen is less dramatic as compared with air. The minimal amount of residual air necessary to form the blocking air-rich layer is evaluated by developing a one-dimensional stationary model of the vapour adsorption in the presence of gas. This amount is found to be highly sensitive to the total vapour pressure and the HEx mass transfer surface area. Practical recommendations on avoiding/dumping the effect of residual gas are made.

The effect of residual gas scattering on Ga ion beam patterning of graphene

The patterning of graphene by a 30 kV Ga+ focused ion beam (FIB) is studied by in-situ and ex-situ Raman spectroscopy. It is found that the graphene surrounding the patterned target area can be damaged at remarkably large distances of more than 10 μm. We show that scattering of the Ga ions in the residual gas of the vacuum system is the main cause of the large range of lateral damage, as the size and shape of the tail of the ion beam were strongly dependent on the system background pressure. The range of the damage was therefore greatly reduced by working at low pressures and limiting the total amount of ions used. This makes FIB patterning a feasible alternative to electron beam lithography as long as residual gas scattering is taken into account.

Vacuum lifetime and residual gas analysis of parabolic trough receiver

The vacuum characteristics and lifetime are the key problems of parabolic trough receiver. Heat loss of the receiver will greatly increase when the vacuum has been lost. Especially, if hydrogen is inside the annulus space of the receiver, heat loss at a level is approximately a factor of four higher than the loss for a receiver with good vacuum. Suitable vacuum levels and residual gases should be maintained in the receiver to ensure performances during its projected lifetime. In this paper, the variations of composition and partial pressure of residual gases with temperature in the receiver were measured by a high sensitivity quadrupole mass spectrometer gas analyzer. The effects of residual gas and getter on the vacuum lifetime of receiver were analyzed. The results showed that hydrogen was the main residual gas in the annular space of receiver without getter, and the nitrogen was the main gas released in the receiver with getter. It can be confirmed that the residual gas analysis was a very effective way to predict and evaluate the vacuum lifetime of the receiver.