Moderate Vacuum Conditions Research Articles

Identification of material by X-ray fluorescence analysis with a pyroelectric X-ray generator

By changing the temperature of Lithium Tantalate (LiTaO3) single crystal at moderate vacuum conditions leads to generation of strong electric field. The uncompensated polarization during the heating or cooling of the crystal causes the ejection of electrons from either the dielectric layer on the surface of the crystal or from a metal target depending on the polarity. The electrons are accelerated and gain energy of up to 100 keV. The energy of these electrons can be determined by measuring the end-point energy of the X-ray spectrum that resulted from the electron interactions with the target. The conception of a pyroelectric accelerator enabled us to develop compact (portable) electron source, which does not require an external high-voltage and the use of hazardous materials. The compact and portable nature of pyroelectric-driven particle sources holds significant promise for applications in materials science, particularly for materials analysis methodologies. The research demonstrates the feasibility of utilizing the X-ray signal generated by irradiation with electrons to identify elements in each sample. It is revealed that employing only the electron beam enables the successful acquisition of quantitative information regarding the sample structure through pyroelectric driven PD-PIXE analysis. These findings set the stage for the development of a compact and versatile apparatus for elemental analysis of materials based on a pyroelectric source.

Cu2O photocathodes prepared by thermal reduction of CuO: Influence of O2 concentration on photocurrent stability

Copper oxide (CuO and Cu2O) p-type semiconducting photocathodes have gained attention for hydrogen production (proton reduction), due to their appropriate conduction band potentials and light sensitivity in the visible. However, photocorrosion is the main handicap for both compounds. In this work, the transformation of CuO layers deposited via spray pyrolysis on conductive FTO (fluorine doped tin oxide on glass) into Cu2O via thermal reduction was studied. Optimal conditions for obtaining Cu2O were found by in situ temperature-controlled XRD. Synthesis of Cu2O films was finally carried out under moderate temperature and vacuum conditions (380 °C, 1·10−2 mbar). The influence of 3 different oxygen concentration in the electrolyte on the stability of the Cu2O electrodes under light and electrical bias was studied, and it was found that the O2 content had a high impact on the photoelectrochemical performance of Cu2O electrodes. The stability of the electrodes against self-reduction was demonstrated.

Stability of electrons and X-rays generated in a pyroelectric accelerator

Conventional X-ray sources are far too bulky and require a high-power DC voltage. The pyroelectric X-ray generator technology has enabled us to develop portable, low-power X-ray sources for use in materials analysis, imaging, and other applications. Changing the temperature of single crystal lithium tantalate (LiTaO3) at moderate vacuum conditions gives an attractive possibility to generate and accelerate electron up to 100 keV. The electrons are ejected either from the crystal or from the target (depending on polarity). The electrons then generate X-rays via bremsstrahlung and characteristic X-ray emission processes. The aim of this experimental investigation is to explore the interesting feature of the pyroelectric accelerator that generates a monoenergetic electron flux with a stable value of peak energy for a long time. Here we present studies of features of electron flux in pyroelectric accelerator depending on the pressure of residual gas and the distance between the crystal and the target-collimator. We examine the correlation between monoenergetic electron production and avalanche discharge. We also studied outgassing from some accelerator components. The pyroelectric X-ray generator technology is currently being developed is a reliable, compact, stable, and reproducible X-ray source with controllable parameters, which does not require a high-power DC voltage or the use of hazardous (radioactive) materials.

Penetration of a Silicon Vapor through Residual Gaseous Medium during Siliconizing of a Carbon Composite Material

A new physical and mathematical approach is proposed for the description of a silicon vapor high-temperature transfer from the melt mirror to the surface of absorbing carbon porous medium in an atmosphere of residual gas under moderate vacuum conditions. The general hydrodynamic equations are simplified as much as possible and lead to the model, which is intended for further numerical implementation in the course of simulation the non-stationary three-dimensional process of high-temperature siliconizing of porous carbon products of arbitrary shape and under the condition of complex distribution of silicon sources and sinks. The physical and mathematical model of transport consists of only one non-linear differential equation in partial derivatives for the silicon vapor concentration in the atmosphere of argon or any other residual gas. The main achievement consists of obtaining two stationary analytical solutions for plane and cylindrical geometry in one-dimensional formulation of the problem, which explain the anomalously large silicon vapor flux into the porous material in full-scale experiments. The exact solutions for the studied gas mixtures are expressed in terms of the well-known verified values of material parameters. The high degree of the model usability is supported by numerical simulation in a non-steady two-dimensional case. It has shown that despite the low saturation density of gaseous silicon, the vapor-liquid phase process of high-temperature siliconizing of the carbon material is physically possible in a reasonable time.

Spatially Resolved Temperature Measurements during Condensation in Presence of a Non‐Condensable Gas

AbstractAxial temperature profiles of the vapor phase can provide valuable indication of the condensation process along the heat transfer path and highlight the impact of non‐condensable gases on a unit's performance. The local measurements at moderate vacuum conditions for condensation of a binary mixture are presented alongside the implementation and assessment of optical sensors in a heat transfer process. The results show a good depiction of the characteristic temperature behavior during the condensation process. The relation between temperature, partial pressure and mole fractions are shown.

Complex mixture analysis by two-dimensional mass spectrometry using a miniature ion trap

Quadrupole ion trap mass spectrometers are robust instruments that can perform tandem mass spectrometry (MS/MS) while being operated under moderate vacuum conditions. These features make quadrupole ion traps an attractive option as miniature mass spectrometers, particularly because the sensitivity and selectivity gained from employing MS/MS does not decrease with reduced instrument size. Recent developments in scan methodology have allowed quadrupole ion traps to be used to perform two-dimensional MS/MS (2D MS/MS) experiments which identify all precursor ions and their dissociation products. This study describes the implementation of this single scan in a miniature mass spectrometer where sample amount is limited, and time is prioritized. Remarkably, the miniature instrument 2D MS/MS experiment demonstrated greater sensitivity than a benchtop ion trap mass spectrometer utilizing the 2D MS/MS scan. Even more notably, the 2D MS/MS experiment performed using the miniature instrument achieved higher mass resolution than the benchtop instrument in both precursor and product ion dimensions. The miniature instrument's performance was tested on mixtures of drugs of abuse in the presence of a matrix as well as on a mixture of phospholipids.

Work function performance of a C12A7 electride surface exposed to low pressure low temperature hydrogen plasmas

The calcium aluminate electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) is chemically and thermally stable in ambient atmosphere, offers strong electron donating capabilities and exhibits an intrinsic work function of 2.4 eV. Thus, it is an attractive material for electron emitter and low work function applications. For an implementation in negative hydrogen ion sources, where the electride could serve as a converter surface for the H− production, compatibility with moderate vacuum conditions and exposure to low pressure low temperature hydrogen plasmas must be ensured. Hence, the stability and work function performance of a polycrystalline C12A7:e− sample are investigated under ion source relevant vacuum (∼10−6 mbar) and plasma conditions (electron temperature ∼2 eV, electron density ∼1016m−3). The work function is measured via the photoelectric effect, showing that long-term hydrogen plasma exposure results in a steady-state surface work function of 2.75±0.10 eV. Since the electride is very sensitive to surface contaminations, an increase of the surface work function occurs as soon as the plasma is switched off. Vacuum heat treatment of the degraded C12A7:e− surface after plasma activation leads to a work function minimum of 2.9 eV for temperatures in the range of 670–770 K. Exceeding a temperature of ∼770 K leads to an increase of the C12A7:e− surface work function. The sample is stable during long-term hydrogen plasma exposure (up to 10 h total plasma-on time tested) and no plasma-induced erosion is observed.

Effect of vacuum conditions on stability and crystallization of cobalt based amorphous alloy

Abstract The aim of this work was to investigate the effects of moderate vacuum conditions (10 Pa, 1 × 103 Pa and 2 × 104 Pa) on glass forming ability, thermal stability and aging behavior of Co42Cu1Fe20Ta5:5B26:5Si5 amorphous samples in terms of size and distribution of crystalline precipitates. The thinnest parts (300 μm) of the wedge-shaped samples were amorphous for all vacuum conditions, and they had similar compositional, structural and thermal characteristics. However, they represented different microstructural features after heat treatments at 950 K and 1100 K. The same phases precipitated in all samples but the amount and the size of the precipitates increased as the pressure was raised to the normal atmospheric value. The differences in the glass forming ability and the microstructure due to aging are considered to originate from lower thermal characteristics, such as relaxation and reduced glass temperature as well as γ parameter, resulting from the poor vacuum pressure used in the production process. The differences in the microstructures and hardness values caused by heat treatments have been observed.

High power conditioning and benchmarking of planar nitrogen-incorporated ultrananocrystalline diamond field emission electron source

Planar nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, has emerged as a unique field emission source attractive for accelerator applications because of its capability to generate high charge beam and handle moderate vacuum conditions. Most importantly, (N)UNCD sources are simple to produce: conventional high aspect ratio isolated emitters are not required to be formed on the surface, and the actual emitter surface roughness is on the order of only 100~nm. Careful reliability assessment of (N)UNCD is required before it may find routine application in accelerator systems. In the present study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode has been conditioned to $\sim$42~MV/m in a well-controlled manner. It reached a maximum output charge of 15~nC corresponding to an average current of 6~mA during an emission period of 2.5~$\mu$s. Imaging of emission current revealed a large number of isolated emitters (density over 100/cm$^{2}$) distributed on the cathode, which is consistent with previous tests in dc environments. The performance metrics, the emission imaging, and the systematic study of emission properties during rf conditioning in a wide gradient range assert (N)UNCD as an enabling electron source for rf injector designs serving industrial and scientific applications. These studies also improve the fundamental knowledge of the practical conditioning procedure via better understanding of emission mechanisms.

Direct Growth of GaN Nanowires by Ga and N2 without Catalysis

Gallium nitride nanowires (GaN NWs) are commonly synthesized using harsh/toxic conditions (e.g., ultrahigh vacuum, high substrate temperature, toxic materials, etc.) or by catalytic conditions. In this work, GaN NWs were grown on graphite substrates by the direct reaction of Ga atoms with excited N plasma without the need for a catalyst. This was achieved using plasma enhanced chemical vapor deposition (PECVD) under moderate vacuum conditions and temperatures below 900 °C. The GaN NWs consisted of polycrystalline nanostructures that appeared as pyramidal islands, with diameters from 90 to 200 nm and lengths from 4 to 20 μm. A model for the mechanism of nucleation and growth of the GaN NWs is proposed. The morphology and quality of the GaN NWs varied significantly depending on the fabrication parameters that were used. Moreover, they exhibited desirable photoluminescence (PL) and field emission (FE) properties. This method shows great promise for the simple, low cost, and environmentally friendly fabricati...

Femtosecond-laser-induced spin-polarized electron emission from a GaAs tip

It is shown that focusing circularly polarized 800 nm light pulses of duration 100 fs on the tips of p-GaAs crystalline shards having no negative electron affinity (NEA) activation results in electron emission that is both fast and spin-polarized. The 400 fs duration of the emission process was determined by pump/probe measurements. The three samples we investigated produced electron polarizations of 13.1(0.9)%, 13.3(0.7)%, and 10.4(0.2)%. Emission currents ranged between 50 pA and 3 nA with a sample bias of −100 V and an average laser power of 100 mW. The electron emission exhibited linear dichroism and was obtained under moderate vacuum conditions, similar to that of metallic tips. This source of spin-polarized electron pulses is “fast” in the sense that the electron emission process is of comparable duration to the laser pulses that initiate it.

Determination of the Cs distribution along a line of sight by the Zeeman splitting in an inhomogeneous magnetic field

Large-scale sources for negative hydrogen ions are required for the neutral beam injection of the ITER fusion device. Caesium is used for the conversion of hydrogen atoms into negative ions. The combination of moderate background vacuum conditions (10−7–10−6 mbar), the high reactivity of Cs, continuous evaporation of Cs and plasma-enabled redistribution of Cs and Cs+ forms a complex dynamics. ELISE (extraction from a large ion source experiment) is a half ITER-source scale experiment. A tunable diode laser absorption spectroscopy (TDLAS) diagnostic at the resonant Cs 62S1/2–62P3/2 transition (852 nm) has been applied to the source in order to measure the density of neutral Cs close to the conversion surface and thus gaining a better insight into the Cs dynamics. An inhomogeneous magnetic field created by permanent magnets (≈25–320 G) is located along the line-of-sight (LOS) of the TDLAS, leading to a varying effect of the Zeeman splitting of the concerning Cs states and thus the absorption line profile along the LOS. In long plasma pulses at ELISE, a change of the Cs absorption line profile takes place, which can be attributed to a variation of the Cs density profile along the LOS. In order to attribute a measured line profile to a certain magnetic field strength and thus a certain position along the LOS, absorption spectra with a defined B-field at a Cs vapor cell using the same diagnostic system have been measured. The strong broadening of the measured line profile at ELISE after several 10 s of plasma indicates that neutral Cs is mainly located in the area of high magnetic field strength, i.e. close to the side wall. A correlation between the measured Cs density and the source performance exists during the beginning of the pulse and is lost in this later phase.

Dynamic nuclear polarization in solid samples by electrical-discharge-induced radicals

Dynamic nuclear polarization (DNP) is a method for enhancing nuclear magnetic resonance (NMR) signals that has many potential applications in chemistry and medicine. Traditionally, DNP signal enhancement is achieved through the use of exogenous radicals mixed in a solution with the molecules of interest. Here we show that proton DNP signal enhancements can be obtained for solid samples without the use of solvent and exogenous radicals. Radicals are generated primarily on the surface of a solid sample using electrical discharges. These radicals are found suitable for DNP. They are stable under moderate vacuum conditions, yet readily annihilate upon compound dissolution or air exposure. This feature makes them attractive for use in medical applications, where the current variety of radicals used for DNP faces regulatory problems. In addition, this solvent-free method may be found useful for analytical NMR of solid samples which cannot tolerate solvents, such as certain pharmaceutical products.

A low pressure recirculated sweep stream for energy efficient membrane facilitated humidity harvesting

Water vapor in the atmosphere can be used as a source of fresh water by means of condensation. The use of selective membranes, that separate the water vapor from the other gases, allows a specific cooling of the concentrated vapor and makes the process more energy efficient. In this paper the different driving forces for the vapor permeation across the membrane are analyzed. The advantages and disadvantages of using vacuum and a sweep stream are assessed and a combination of these two is introduced, which provides an optimal condition for humidity harvesting. When air is recirculated from the condenser to serve as a sweep stream while the total system pressure is regulated with a vacuum pump, the driving force can be uncoupled from the permeate side pressure. It is demonstrated that with such a configuration substantial water production rates can be achieved, even at higher vacuum pressures, which reduces the work requirement of the vacuum pump. At the same time these moderate vacuum conditions reduce the energy demand for cooling the sweep stream so that the energy efficiency can be significantly improved compared to systems without membranes.

Influence of TiOx and Ti cathode interlayers on the performance and stability of hybrid solar cells

The performance of organic and hybrid solar cells is, in addition to the properties of the absorber layer, also strongly dependent on electrodes and interlayers extracting the charges generated in the absorber layer. Here, we investigate Ti and TiOx interlayers regarding the performance and stability of polymer/copper indium sulfide hybrid solar cells with aluminum and silver electrodes. For this, TiOx interlayers were prepared via thermal evaporation of titanium under moderate vacuum conditions in oxygen containing atmosphere, whereas Ti interlayers were deposited under high vacuum conditions. The titanium-based interlayers improved the power conversion efficiencies of solar cells with Al as well as with Ag electrodes. Also the stability of solar cells with Al electrodes was improved; however, solar cells with Ag electrodes and TiOx as well as Ti interlayers turned out to be much more stable and showed lifetimes of several thousand hours under continuous illumination.

Impact of growth parameters on the formation of carbon nanostructures through thermal deposition of silicon carbide

ABSTRACTCarbon nanotube (CNT) and graphene films form on silicon carbon (SiC) using a metal-catalyst-free thermal decomposition approach. In this work, the growth conditions used in the decomposition process are varied to investigate their impact on the type and quality of carbon allotrope formed on the SiC substrate. The nanostructure growth is performed using two approaches, both of which involve intense heating (1250-1700oC) under moderate vacuum conditions (10-2 – 10-5 Torr) without the aid of carbon rich feed gases or metal catalysts commonly used in Chemical Vapor Deposition (CVD) growth approaches. The first growth method uses a graphite resistance furnace capable of annealing wafer-sized samples. The second approach uses a high-intensity laser to heat a micro-meter scale spot size. The high-intensity laser heats the illuminated area of the SiC substrate while under vacuum conditions, resulting in a small-scale growth process similar to the conventional resistance furnace technique. Unique to this micro-scale approach is that in situ Raman spectroscopy is performed yielding instantaneous characterization of the resultant carbon nanostructure as it is formed. The laser-induced growth mechanism enables the impact of varied background vacuum pressures and temperatures to be evaluated in situ. This work reports the findings for various parameter sets implemented during growth, and provides insight into the physical mechanism influencing the growth process.

Radiofrequency pulsed glow discharge-ToFMS depth profiling of a CdTe solar cell: A comparative study versus time of flight secondary ion mass spectrometry

In this manuscript, the authors compare and contrast depth profile data generated on the same commercially available CdTe cell using two analytical techniques, Time of flight secondary ion mass spectroscopy (ToF-SIMS), which is a well-established technique, and radiofrequency pulsed glow discharge (rf-PGD)-ToFMS, which is an emerging technique. The authors demonstrate that pulsed-rf-GD-ToFMS allows for a rapid analysis over a large (4 mm diameter) area of the sample at moderate vacuum conditions, while ToF-SIMS analyzes smaller regions (typically about 200 × 200 μm2) of the sample at ultrahigh vacuum conditions; the authors note that ToF-SIMS enables three dimensional analysis at micron or better lateral resolution. Both techniques show, in general, good agreement; however, some discrepancies are observed and discussed herein.

Fabrication of Patterned Carbon Nanotube Field Emission Surfaces on SiC Substrates

ABSTRACTThis work focuses on the patterning of SiC substrates prior to carbon nanotube (CNT) formation using the surface decomposition growth method for the purpose of improving the field emission capabilities of the resultant CNT film. The thermal decomposition of silicon carbide (SiC) substrates is an established approach to create highly dense arrays of vertically aligned CNTs. The attractiveness of this growth approach is that the CNTs form without the aid of a catalyst metal, yielding potentially defect free CNTs ideal for various applications. Due to the high temperature anneals (1400-1700oC) and moderate vacuum conditions (10−2 – 10−5 Torr) necessary for the thermal decomposition process to initiate on the SiC substrate, patterning CNT outcroppings ideal for enhancing the surface’s field emission properties is more difficult when compared to metal catalyst based chemical vapor deposition growth processes on silicon substrates. The intent of the SiC patterning is to reduce field screening effects between neighboring emission sites during field emission while maintaining a high emission site density. Specifically, the SiC substrate is etched to form μm scale pillars on the SiC surface. Experimental findings show that SiC substrates patterned with μm scale pillars can be decomposed to form CNT topped field emission sites, yielding a field emission substrate that outperforms a non-patterned SiC/CNT film. A turn-on electric field of 4.0 V/μm was measured.

Spectroscopic evaluation of electron density and temperature of a laser plasma source in the vacuum ultraviolet

A time-integrated space-resolved emission spectroscopic technique was developed in the VUV spectral range for the measurement of the electron number density and excitation temperature of laser-produced steel plasmas. 400 mJ pulses of Nd:YAG laser (7 ns duration) emitted at the fundamental wavelength of 1.06 μm are used to create the steel plasmas under moderate vacuum conditions (∼ 0.2 mbar). The electron number density was estimated from measurements of the FWHM of the Stark-broadened C II 68.73nm carbon spectral line. The excitation temperature was evaluated by the Boltzmann’s plot method for five selected Fe III iron spectral lines in the wavelength range 89-93nm.

Kelvin probe force gradient microscopy of charge dissipation in nano thin dielectric layers

Application of Kelvin probe force gradient microscopy (KPFGM) to visualize the local charge dissipation in thin dielectric layers is considered. By this method, the local charge behavior in nano thin SiO2, Si3N4, and LaScO3 dielectric layers has been studied. Local charging of the layers has been performed at the point contact with a conductive probe. KPFGM potential images reveal variations of the surface potential in the locally charged areas, which makes it possible to detect the injected charge and to study its behavior. Special experiments on the SiO2 layers with embedded Si-nanocrystals, when lateral spreading of injected charge had been suppressed, permitted to demonstrate high (better than 20 nm) lateral resolution of KPFGM observations. A simple electrostatic model has been developed to estimate the total amount of injected charge. The obtained estimations made it possible to control charge retention in the dielectric layer and possible leaks into the substrate. The studied dielectric layers demonstrate a broadening of the charged area with time t, proportionally to t1/2, what indicates the domination of the diffusion mechanism in charge lateral spreading on the large time scale. These observations permitted to determine the diffusion coefficients, mobilities, and diffusion activation energies for charges in the studied dielectric layers. To obtain the correct information on the injected charge behavior, the parasitic charge dissipation through the surface film of adsorbed water should be reduced to a negligible level. It was achieved by working in moderate vacuum conditions with an additional sample heating.