Helium Gas Pressure Research Articles

Rapid dendrite growth mechanism and solute distribution in liquid ternary Fe-Cr-Ni alloys

Stainless steels with excellent hardness and corrosion resistance performance have been widely used in industrial production. Ternary Fe-Cr-Ni alloys, as a model alloy of nickel chromium stainless steels, are of great importance in the fields of material science. Under non-equilibrium solidification condition, alloys may have new microstructure and improved performance. In this paper, two liquid ternary Fe-Cr-Ni alloys are deeply undercooled and rapidly solidified in a 3-m drop tube to investigate the microstructure evolution and solute distribution of alloy droplets with different sizes. In the drop tube experiments, the Fe-Cr-Ni alloy samples with a mass of 1.5 g are placed in a φ16 m mm×150 mm quartz tube with a 0.5-mm-diameter orifice at its bottom and heated by induction heating device in a high vacuum chamber. Then the samples are melted and overheated to 200 K above their liquidus temperatures for several seconds. The alloy melt is ejected out of the small orifice and dispersed into numerous droplets after adding high pressure helium gas flow. The alloy droplets with diameters ranging from 68 μm to 1124 μm are achieved. After experiments, the alloy droplets with different sizes are mounted respectively. Then they are polished and etched. The drop tube technique provides an efficient way to study the rapid solidification mechanism of alloys. Besides the experiments, the dendrite growth velocities of primary phase in two Fe-Cr-Ni alloys are calculated theoretically using the modified LKT/BCT model. As droplet size decreases, both cooling rate and undercooling increase exponentially and the morphologies of two alloys become well refined. Under the near-equilibrium solidification condition with a cooling rate of 10 K/min, both alloys consist of coarse lath-like α phase. After rapid solidification of Fe81.4Cr13.9Ni4.7 alloy droplets during free fall, the microstructure presents a lath-like α phase, resulting from the solid-solid phase transition. As undercooling increases, the primary δ phase is converted from the coarse dendrite with long trunk into equiaxed grain. For Fe81.4Cr4.7Ni13.9 alloy, the microstructure is composed of α phase grains. The transition of primary γ phase from coarse dendrite with long trunk to refined equiaxed grain occurs as the undercooling increases. Meanwhile, both dendrite trunk length and secondary dendrite arm spacing decrease drastically, suggesting that the rapid solidification is the main reason for grain refinement. Moreover, the relative segregation degree of solute Cr and Ni inside α phase grain also decreases obviously with the increase of undercooling, and the microsegregation of Ni is more remarkable than that of Cr. This suggests that the high cooling rate and undercooling cause the solute to be distributed evenly. Compared with that of γ phase, the dendrite growth velocity of δ phase is large and its dendrite tip radius is small. The two phase transform from solute diffusion controlled growth into thermal diffusion controlled growth as undercooling increases to 8 K. When undercooling is larger than 8 K and within the experimental undercooling range, the dendrite growth of both Fe-Cr-Ni alloys is controlled by thermal diffusion.

Influence of hydrodynamic behavior of laser ablation plume on cluster formation

We conducted fast-photography of the ablation plume and mass analysis of generated clusters. Time-resolved observation of the ablation plumes showed that the behavior of the plume drastically changes depending on the background helium gas pressure. Particularly when the pressure was more than 100 Pa, a strong light emission from the front edge of the plume was observed, indicating that the strong compression of the plume vapor occurs and the kinetic energy of the drifting plume may be converted into the thermal energy of the compressed vapor. We found that this plume compression process could be enhanced by introducing an ellipsoidal cavity, which is due probably to converging waves in the background gas towards the focal point of the ellipsoid. A TOF mass spectroscopy indicated that clusters having a mass corresponding to Al, AlO, Al2, and Al2O were generated in the laser ablation cavity and transported downstream. However, large aluminum clusters consisting of more than a few tens of atoms were not observed in the present experimental condition.

Can Hail and Rain Nucleate Cloud Droplets?

We present results from moist convection in a mixture of pressurized sulfur hexafluoride (liquid and vapor), and helium (gas) to model the wet and dry components of the Earth's atmosphere. To allow for homogeneous nucleation, we operate the experiment close to critical conditions. We report on the nucleation of microdroplets in the wake of large cold liquid drops falling through the supersaturated atmosphere and show that the homogeneous nucleation is caused by isobaric cooling of the saturated sulfur hexafluoride vapor. Our results carry over to atmospheric clouds: falling hail and cold rain drops may enhance the heterogeneous nucleation of microdroplets in their wake under supersaturated atmospheric conditions. We also observed that under appropriate circumstances settling microdroplets form a rather stable horizontal cloud layer, which separates regions of super- and subcritical saturation.

Experimental study on breakdown voltage of high pressure and high temperature helium gas between parallel electrodes

The DC and 50Hz AC breakdown voltage of helium between two parallel copper electrodes were measured to obtain the base data for insulation design of the electrical equipment working in the high pressure and high temperature helium gas in High Temperature Reactor (HTR). The measurements were performed in the distance of 0.25–0.53mm, the temperature of 15–180°C, and the pressure of 0.05–7MPa. It has been proved that, for both DC and 50Hz AC, the breakdown voltage increases with the increasing of the pressure, with the increasing of the distance, and with the decreasing of the temperature. The DC breakdown voltage is about 1.4 times of the value of 50Hz AC or, in other words, is similar to the peak value of 50Hz AC. The Paschen law, describing the relationship between the breakdown voltage of helium (Ub) and the product of gas density and electrode distance (ρd), was presented for DC and 50Hz AC respectively, based on the experimental data at high pressure and high temperature. Good linearity is shown in the data. Furthermore, the linear fitting equation was also presented. The slope of Paschen curve about Ub-ρd at the range of large ρd is influenced by the collision cross section and the ionization energy of helium atom, and not influenced by the temperature, for DC. Both for DC and 50Hz AC, the slope of Paschen curve about Ub-ρd is about 6×105 times of the slope of Paschen curve about Ub-Pd, where Pd is the product of gas pressure and electrode distance. Both for DC and 50Hz AC, the breakdown voltage of helium is less than one fifth of the value of air, at 0.1MPa and room temperature. However, the breakdown voltage of helium at 180°C and 7MPa is larger than the value of air at 0.1MPa and room temperature.

Investigation of thermal acoustic oscillations in a superconducting linac cryogenic system

During the commissioning stage of a superconducting linear accelerator (linac) three major cryogenic systems were seen to generate thermal acoustic oscillations (TAO): the cryostat liquid helium level sensor interface, the pre-cooling line and the cryogenic distribution valve box with associated transfer tube. By using Rott’s stability curves, published by other researchers elsewhere, an approximate map of the oscillations in the cryogenic systems under investigation was created for both straight and U-shaped tubes. This investigation confirmed that all systems under consideration fall within the critical range and that damping of TAO would be needed to achieve stable operation conditions. The geometry and hence the temperature distribution within the cryogenic system of the accelerator facility could not be altered arbitrarily. Possible TAO damping techniques were narrowed to two options: (a) adding restrictions at the cold end and (b) control of the helium gas pressure. In all systems, before modifications, pressure oscillations were clearly observed within the frequency range 0.25–47Hz. In two systems TAO were observed with no liquid helium present at the cold end. The TAO investigations provided solutions that resulted in a substantial improvement in the performance of the linac cryogenic and RF stabilization systems, under test.

Mechanically Induced Graphite-Nanodiamonds-Phase Transformations During High-Energy Ball Milling

Due to their unusual mechanical, chemical, physical, optical, and biological properties, nearly spherical-like nanodiamonds have received much attention as desirable advanced nanomaterials for use in a wide spectrum of applications. Although, nanodiamonds can be successfully synthesized by several approaches, applications of high temperature and/or high pressure may restrict the real applications of such strategic nanomaterials. Distinct from the current preparation approaches used for nanodiamonds preparation, here we show a new process for preparing ultrafine nanodiamonds (3-5 nm) embedded in a homogeneous amorphous-carbon matrix. Our process started from high-energy ball milling of commercial graphite powders at ambient temperature under normal atmospheric helium gas pressure. The results have demonstrated graphite-single wall carbon nanotubes-amorphous-carbon-nanodiamonds phase transformations carried out through three subsequent stages of ball milling. Based on XRD and RAMAN analyses, the percentage of nanodiamond phase + C60 (crystalline phase) produced by ball milling was approximately 81%, while the amorphous phase amount was 19%. The pressure generated on the powder together the with temperature increase upon the ball-powder-ball collision is responsible for the phase transformations occurring in graphite powders.

Mechanical testing of the IFMIF HFTM-DC prototype during operation in the HELOKA-LP helium loop

The so called High Flux Test Module (HFTM) represents the component of IFMIF (International Fusion Irradiation Facility) in which material specimens are being placed that accumulate high neutron induced damage rates (≥20dpa/fpy). Damage rates of this magnitude are limited to a volume of ∼500cm3 (attenuation in beam direction) behind a beam footprint of 20×5cm2, so the high flux region of the module is contained in a flat faced, cuboid volume of 5.6cm depth. Efforts for a high spatial utilisation and the demand for a high neutron transmission lead to a thin-walled container design. As an efficient, space-saving method to cool the container and the material specimens, an array of mini-channels (1mm gaps) through which low pressure (0.3MPa) helium gas is flowing is established. Installed closely to the target and due to space constraints by other irradiation modules, the HFTM is implemented as a slender and tall construction with features that are challenging for pressure equipment. Experimental studies on a 1:1 prototype of the HFTM-DC (double compartment) have been made in the Helium Loop Karlsruhe – Low Pressure (HELOKA-LP) during 2015. The experiments also included intensive testing to demonstrate the mechanical reliability of the HFTM under IFMIF relevant operation conditions. Therefore, the module was instrumented with numerous sensors which measure displacement, deformation and mechanical strain. The reactions on temperature and pressure loads were studied. In this paper the experimental results will be presented and compared to the numerical (FEM) simulation studies.

Feasibility of primary thermometry using refractive index measurements at a single pressure

Abstract Refractive index gas thermometry (RIGT) uses the dependence of the relative dielectric permittivity on the density of a noble gas (He), typically measured with a temperature-controlled resonator. On an isotherm, to a good approximation, the slope of the dielectric permittivity versus pressure gives the thermodynamic temperature T. To measure T with a low uncertainty, one must measure the absolute value of the pressure with a low uncertainty and know the compressibility of the resonator wall. This article shows how to use RIGT in a novel way between 5 K and 25 K. Instead of changing the pressure on an isotherm (Schmidt et al., 2007), a constant pressure of pure helium gas is maintained at multiple temperatures. After calibration of the resonator under vacuum at different temperatures, all thermometry is performed at a single pressure and referred to a fixed point of the International Temperature Scale of 1990 (ITS-90) (here the neon triple point at 24.5561 K). The quantity that yields the temperature is the ratio of the resonance frequencies of a microwave mode in the resonator measured at the fixed point and at the unknown temperature. This paper describes the theoretical model of Single-Pressure Refractive Index Gas Thermometry (SPRIGT) and analyses the shifts and broadenings due to non-ideal behaviour. The technique should enable a helium-based measurement of thermodynamic temperature with a resolution better than 25 μK and an uncertainty of around 250 µK.

Measurement of ion beam angular distribution at different helium gas pressures in a plasma focus device by large-area polycarbonate detectors

The paper presents an experimental study and analysis of full helium ion density angular distributions in a 4-kJ plasma focus device (PFD) at pressures of 10, 15, 25, and 30 mbar using large-area polycarbonate track detectors (PCTDs) (15-cm etchable diameter) processed by 50-Hz-HV electrochemical etching (ECE). Helium ion track distributions at different pressures, in particular, at the main axis of the PFD are presented. Maximum ion track density of ~4.4 × 104 tracks/cm2 was obtained in the PCTD placed 6 cm from the anode. The ion distributions for all pressures applied are ring-shaped, which is possibly due to the hollow cylindrical copper anode used. The large-area PCTD processed by ECE proves, at the present state-of-theart, a superior method for direct observation and analysis of ion distributions at a glance with minimum efforts and time. Some observations of the ion density distributions at different pressures are reported and discussed.

Design of High Frequency Pulse Tube Cryocooler for Onboard Space Applications

To meet the growing demands of on-board applications such as cooling meteorological payloads and the satellite operational constraints like power, lower mass, reduced size and redundancy; a Pulse Tube Cryocooler (PTC) is designed by arriving at an operating frequency of 100 Hz and Helium gas pressure of 35 bar based on insights obtained from combination of phasor diagram, pulse tube and regenerator geometries with overall system mass of ≤ 2.0 kg. High frequency operation would allow reducing the size and mass of pressure wave modulator for a given input power. High Frequency also helps in reducing the volume of regenerator for a given cooling power, which increases the power density and leads to faster cool down. A component level modelling of the regenerator for optimising length and diameter for maximum Coefficient of Performance (COP) is carried out using REGEN3.3. The overall system level modelling of PTC is carried out using 1-D software SAGE. The cold end mass flow rate of the optimised regenerator is taken as reference for the system modelling. The performance achieved in REGEN3.3 is 2.15 W of net heat lift against the performance of 1.02 W of net heat lift at 80 K in SAGE.

Effects of cyclic mean pressure of helium gas on performance of integral crank driven stirling cryocooler

An integral crank driven Stirling cryocooler is solidly based on concepts of direct IR detector mounting on the cryocooler’s cold finger, and the integral construction of the cryocooler and Dewar envelope. Performance factors of the cryocooler depend on operating conditions of the cryocooler such as a cyclic mean pressure of the working fluid, a rotational speed of driving mechanism, a thermal environment, a targeted operation temperature and etc.. At given charging condition of helium gas, the cyclic mean pressure of helium gas in the cryocooler changes with temperatures of the cold end and the environment. In this study, effects of the cyclic mean pressure of helium gas on performances of the Stirling cryocooler were investigated by numerical analyses using the Sage software. The simulation model takes into account thermodynamic losses due to an inefficiency of regenerator, a pressure drop, a shuttle heat transfer and solid conductions. Simulations are performed for the performance variation according to the cyclic mean pressure induced by the temperature of the cold end and the environment. This paper presents P-V works in the compression and expansion space, cooling capacity, contribution of losses in the expansion space.

Transformasi Genetik Kedelai dengan Gen Proteinase Inhibitor II Menggunakan Teknik Penembakan Partikel

<p class="p1">An experiment was conducted at the Molecular Biology and Genetic Engineering Laboratory of BB-Biogen, Bogor with an objective to obtain transgenic soybean plants containing the <em>proteinase inhibitor </em>II (<em>pin</em>II) gene. The experiment consisted of three steps, i.e., optimalization of the soybean transformation technique using the <em>gus </em>gene; transformation of soybean using the <em>pin</em>II gene, and molecular analysis of the transformed soybean plants. Two type of explants (young embryo and cotyledon) were bombarded with <em>p</em>RQ6 plasmid containing the <em>gus </em>gene with the following treatment: Helium gas pressure (1100 psi and 1300 psi), shoot distance (5 and 7 cm), and number of bombardment (1x and 2x). The result of <em>gus </em>assay indicated that the best bombardment was done on young cotyledon explants with 1100 psi Helium pressure, shoot distance 5 cm, and 1x bombardment. Transformation of the soybean explant using the <em>pin</em>II gene (inside the <em>p</em>TWa plasmid) was conducted using the best bombardment treatment from the first activity. Two plants from c.v. Wilis (WP<span class="s1">1</span>, WP<span class="s1">2</span>) and three plants from c.v. Tidar (TP<span class="s1">1</span>, TP<span class="s1">2</span>, TP<span class="s1">3</span>) were recovered from regeneration and selection of the transformed explants. Molecular analysis of the regenerated plants using the PCR technique showed that only WP<span class="s1">2 </span>contained the <em>pin</em>II gene. This plant was fertile and will be used for further evaluation.</p>

A looped three-stage cascade traveling-wave thermoacoustically-driven cryocooler

This paper introduces a novel, looped, thermoacoustically-driven cryocooler, which is a promising candidate for natural gas liquefaction applications. Several thermoacoustic heat engines of different diameters are connected end-to-end by small-diameter resonance tubes. One pulse tube cooler is connected to the branch of the last-stage engine. This system is highly reliable, efficient, and compact for a looped configuration. In particular, the system realized cascade acoustic power amplification with only a single power output, which promises large capacity and structure simplicity. Here, a three-stage traveling-wave system is designed and numerically investigated. A comparison is made with the existing multiple pulse tube cryocoolers which indicates the superior performance of this novel system. The distributions of some key parameters and the exergy loss are presented and analyzed to better understand the energy conversion process. Two area ratios are found to be critical to the engine performance. According to the calculation, using 7 MPa pressurized helium gas, a cooling power of 1300 W and a total exergy efficiency of 15.7% is achieved at 110 K with 20.2 kW input heat at 923 K.

Characterization of RF He-N2/Ar mixture plasma via Langmuir probe and optical emission spectroscopy techniques

A Magnetic Pole Enhanced inductively coupled RF He- N2/ Ar plasma is characterized using a Langmuir probe and optical emission spectroscopy (OES) techniques. The effect of helium mixing on electron density (ne) and temperature (Te), electron energy probability functions (EEPFs), [N] atomic density, and N2 dissociation is investigated. A Langmuir probe and a zero slope method based on trace rare gas-optical emission spectroscopy (TRG-OES) are employed to measure the electron temperature. It is noted that the electron temperature shows an increasing trend for both methods. However, the temperature measured by a zero slope method Te(Z·S) approaches the temperature measured by a Langmuir probe; Te(L·P) at 56% and above helium concentration in the discharge. “Advance actinometry” is employed to monitor the variation in [N] atomic density with helium concentration and gas pressure. It is noted that [N] atomic density increases at 56% and above helium in the discharge, which is consistent with the trend of electron temperature and EEPFs. A drastic enhancement in N2 dissociation fraction D1 determined by “advance actinometry” is noted at 56% and above helium concentration in the mixture due to modifications in different population and depopulation mechanisms. However, it is also noted that the dissociation fraction D2 determined by intensity ratio method increases linearly with helium addition.

Nanomechanical properties and fracture toughness of Bi3Se2Te thin films grown using pulsed laser deposition

Mechanical properties of thermoelectric (TE) materials are crucial for fabricating efficient and endurable TE devices. In this study, polycrystalline Bi3Se2Te thin films are grown on c-plane sapphire substrates at 250 °C and helium gas pressure of 6.5 × 10−1 Torr using pulsed laser deposition (PLD). The structural, compositional, morphological and mechanical properties of Bi3Se2Te thin films are studied. The Bi3Se2Te thin films are highly c-axis oriented structure and exhibit the stoichiometric compositions of Bi3Se2Te phase. The hardness and Young’s modulus of the Bi3Se2Te thin films are 5.6 ± 0.2 GPa and 197.2 ± 5.4 GPa, respectively, which determined by nanoindentation tests with the continuous stiffness measurement (CSM) mode. The true hardness of the Bi3Se2Te thin films is also calculated using the energy principle of nanoindentation contact. The micro-Vicker indentation-induced fracture behavior of the Bi3Se2Te thin films is observed and discussed.

Optical diagnostics with radiation trapping effect in low density and low temperature helium plasma

Low density (ne < 1011 cm−3) and low temperature (Te < 10 eV) helium plasma was generated by hot filament discharge. Electron temperature and density of neutral helium plasma were measured by Langmuir probe and were determined by line intensity ratio method using optical emission spectroscopy with population modelings. Simple corona model and collisional-radiative (CR) model without consideration for radiation trapping effect are applied. In addition, CR model taking into account the radiation trapping effect (RTE) is adopted. The change of single line intensity ratio as a function of electron temperature and density were investigated when the RTE is included and excluded. The changes of multi line intensity ratios as a function of electron temperature were scanned for various radiative-excitation rate coefficients from the ground state and the helium gas pressures related with the RTE. Our CR modeling with RTE results in fairly better agreement of the spectroscopic diagnostics for the plasma temperature or density with the Langmuir probe measurements for various helium gas pressures than corona modeling and CR modeling without RTE.

Experimental investigation of the repelling force from RF carpets

The theoretical description for the ion motion above a radio-frequency carpet was proposed in [S. Schwarz, Int. J. Mass Spectrom. 299 (2011) 71], and using this description, the maximal repelling force created by the RF carpet that leads to stable ion motion can be determined. The predicted changes in the repelling force were experimentally tested for different RF amplitudes, helium gas pressures, and electric push field strength that the RF carpet is counteracting. We observed good overall agreement between the experimental and the theoretical descriptions.

Nanomechanical, structural, and transport properties of Bi3Se2Te thin films

New ternary phase of Bi3Se2Te thin films were successfully grown through pulsed laser deposition with a single Bi2Se2Te crystal as the target. Bi-rich and Se- and Te-deficient compositions deposited at a substrate temperature (Ts) of 250 °C and helium gas pressures (PHe) ranging from 2.0 × 10−5 to 6.5 × 10−1 Torr aided the formation of the dominant Bi-rich phase: Bi3Se2Te. The films were grown epitaxially on Al2O3 (001) substrates with Bi3Se2Te [001]//Al2O3 [001] and Bi3Se2Te [110]//Al2O3 [210]. An increase in PHe remarkably enhanced the hardness and Young's modulus of the films, primarily because of the decrease in nanograin size, following the Hall–Petch relationship. Moreover, the Bi3Se2Te films present linear magnetoresistance under a high perpendicular magnetic field (B ≥ 4 T) and two-dimensional weak antilocalization effect under a low B (±1 T) which may be attributed to topological insulator surface state (TSS). Clearly, further theoretical calculations and experiments are needed to confirm the suggested interpretation.

Thermoelectric properties of nanocrystalline Bi3Se2Te thin films grown using pulsed laser deposition

This study reports the material and thermoelectric properties of n-type nanocrystalline Bi-Se-Te thin films grown using pulse laser deposition (PLD) with a Bi2Se2Te single crystal as the target. In order to optimize the transport and thermoelectric properties, Bi-Se-Te thin films are fabricated by maintaining the substrate temperature (TS) between 200 and 350 °C and a helium gas pressure (PHe) from 0.027 to 86.7 Pa. Unlike the target, all deposited films unexpectedly exhibit Bi3Se2Te phase with highly c-axis orientation, because of the high re-evaporation rates of Se and Te at the elevated TS ≥ 200 °C. The estimated grain (crystalline) size ranges between 18.4 and 39.4 nm, and this monotonically increases as TS increases. The Bi3Se2Te films demonstrate excellent electrical conductivities owning to the high carrier concentration in the order of 1020 cm−3. A window of deposition conditions for high thermoelectric power factors (PF) is TS of 250–350 °C and PHe of 40 Pa. The optimal PF is 8.3 μW/cmK2 for the Bi3Se2Te film prepared at 250 °C and 40 Pa.

Prediction of thermal behavior and trajectory of stratospheric airships during ascent based on simulation

For designers, operators and users, the ability to accurately predict thermal behavior and trajectory of stratospheric airships is very important. Thermal models and dynamic models of stratospheric airships during ascent are developed, including solar radiation, infrared radiation, convection heat transfer and gas expulsion equation. Based on the model, performance parameters of a stratospheric airship during ascent are obtained, including film temperature, helium gas temperature, air temperature, pressure differential, altitude and ascent velocity, changing regulation for these parameters are discussed, and influence of initial helium gas volume and film radiation properties on thermal behavior is analyzed. Simulation results show that, (1) stratospheric airships experience supercooling during ascent, the maximum value is about 30K, supercooling causes loss of net buoyancy, and affects ascent velocity and trajectory in the end, (2) stratospheric airships experience superheating at the floating altitude, and the maximum value is about 51K, (3) initial volume ratio of helium gas and the solar radiation absorptivity of film have important effect on thermal behavior and trajectory during ascent, the larger the initial volume ratio is, the faster the ascent velocity will be, and the bigger the solar radiation absorptivity of film is, the smaller the temperature differential between helium gas and outside atmosphere will be.