Sn Vapor Research Articles

Promoting subsurface Sn incorporation at Nb(100) oxide surface sites leading to homogeneous Nb3Sn film growth for superconducting radiofrequency applications

For next-generation superconducting radiofrequency (SRF) cavities, the interior walls of existing Nb SRF cavities are coated with a thin Nb3Sn film to improve the superconducting properties for more efficient, powerful accelerators. The superconducting properties of these Nb3Sn coatings are limited due to inhomogeneous growth resulting from poor nucleation during the Sn vapor diffusion procedure. To develop a predictive growth model for Nb3Sn grown via Sn vapor diffusion, we aim to understand the interplay between the underlying Nb oxide morphology, Sn coverage, and Nb substrate heating conditions on Sn wettability, intermediate surface phases, and eventual Nb3Sn nucleation. In this work, Nb-Sn intermetallic species are grown on a single crystal Nb(100) in an ultrahigh vacuum chamber equipped with in situ surface characterization techniques including scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Sn adsorbate behavior on oxidized Nb was examined by depositing Sn with submonolayer precision on a Nb substrate held at varying deposition temperatures (Tdep). Experimental data of annealed intermetallic adlayers provide evidence of how Nb substrate oxidization and Tdep impact Nb-Sn intermetallic coordination. The presented experimental data contextualize how vapor and substrate conditions, such as the Sn flux and Nb surface oxidation, drive homogeneous Nb3Sn film growth during the Sn vapor diffusion procedure on Nb SRF cavity surfaces. This work, as well as concurrent growth studies of Nb3Sn formation that focus on the initial Sn nucleation events on Nb surfaces, will contribute to the future experimental realization of optimal, homogeneous Nb3Sn SRF films.

Influence of Nb substrate morphology and atomic structure on Sn nucleation and early Nb3Sn growth

The enhanced accelerating performance of Nb3Sn-coated Nb superconducting radio frequency (SRF) cavities is severely limited by quenching sites at material defects formed during the Nb3Sn film growth procedure. In this work, we aimed to understand the complex surface-mediated interactions that drive initial Nb3Sn formation during the Sn vapor deposition procedure used to fabricate Nb3Sn coated SRF cavities. Nb substrates were modified, coated with Sn, and then characterized using an ultra-high vacuum (UHV) chamber equipped with metal deposition and in situ surface analysis capabilities. The atomic structure of the Nb oxide surface was modified to assess how the Nb surface defect density and crystallographic orientation impact the size of nucleated Sn islands and relative Nb3Sn growth rates. Formed Nb3Sn/Nb surfaces were visualized using ex situ scanning electron microscopy (SEM) and post-deposition annealing revealed Sn desorption and Nb3Sn degradation pathways. Finally, we showed that the Sn deposition conditions can be modified to overcome the growth barriers that are imposed by the Nb morphology. This study provides a unique bridge between fundamental growth studies in pristine conditions with observed phenomena of Nb3Sn grown on realistic cavity surfaces.

Understanding and optimization of the coating process of the radio-frequency Nb3Sn thin film superconducting cavities using tin vapor diffusion method

A study was conducted to understand and optimize the coating process of Nb3Sn thin film superconducting radio-frequency (SRF) cavities using the tin (Sn) vapor diffusion method. The characterization of nanometer-scale Sn droplets, millimeter-scale Sn spots, and locally extremely thin patchy areas was carried out. The causes of their occurrence were analyzed, and their influence on RF performance was distinguished and clarified. Furthermore, a method of achieving high-quality Nb3Sn films was explored by increasing the adsorption of residual Sn vapor to suppress the generation of nanometer-scale Sn droplets while under the premise of ensuring uniform film growth by increasing the Sn vapor flux. It was seen that the performance of the 1.3 GHz single cell Nb3Sn SRF cavity coated at IMP was significantly enhanced after the optimization of the coating process with a maximum accelerating gradient (Eacc,max) of over 18 MV/m and an unloaded quality factor (Q0) of more than 1 × 1010 at Eacc = 12 MV/m and 4.2 K. This study provides quantitative insights for understanding the coating process and provides an essential reference for coating high-performance Nb3Sn SRF cavities using the Sn vapor diffusion method.

Microstructural and Superconducting Radiofrequency Properties of Multilayer Sequentially Sputtered Nb3Sn films

Nb3Sn is considered as a potential candidate for superconducting radiofrequency cavities for particle acceleration due to its higher transition temperature of 18.3 K and higher superheating field of 400 mT. Nb3Sn films can be grown inside the surface of a Nb cavity by sequentially sputtering multiple layers of Nb and Sn thin films followed by annealing at 950 °C for 3 h. We report on the properties of Nb3Sn films grown on Nb substrates by magnetron sputtering. The films' crystal structure, surface morphology, and composition were characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The films had a polycrystalline Nb3Sn structure with a fine-grain surface and an atomic Sn composition of ~23%. The RF surface resistance of the films was measured for different temperatures at 7.4 GHz to understand the feasibility of this method for the SRF application. The RF surface resistance of the films was 5 mΩ at 12 K, which is about 2 orders of magnitude higher than 60 μΩ previously measured in Nb3Sn films grown by Sn vapor diffusion. The sputtered film had a superconducting transition at 17.2 K, which is also lower than 17.9 K observed in Nb3Sn film prepared by vapor diffusion.

Nb3Sn multicell cavity coating system at Jefferson Lab.

Superconducting radio frequency niobium cavities are the building blocks of modern accelerators for scientific applications. Lower surface resistance, higher fields, and high operating temperatures advance the reach of the future accelerators for scientific discovery as well as potentially enabling cost-effective industrial solutions. We describe the design and performance of an Nb3Sn coating system that converts the inner surface of niobium cavities to an Nb3Sn film. The niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl2 vapor during the heat cycle, which results in about 2 μm Nb3Sn film on the niobium surface. Film composition and structure as well as radio frequency properties with 1-cell R&D cavities and 5-cell practical accelerator cavities are presented.

Structure and surface morphology of GeSn/Si(001) layers grown by HW CVD with co-evaporation of Sn

This paper presents the results of the investigation of Ge1-xSnx epitaxial layers grown by the hot wire chemical vapor deposition (HW CVD) method with simultaneous evaporation of Sn from a standard effusion cell. The Ge1-xSnx with a Sn molar fraction of 7.2% and a full width at half maximum (FWHM) of the rocking curve of 7.6` demonstrated intense photoluminescence at room temperature. The peaks in the energy bands 0.70 – 0.73 eV and 0.63 – 0.65 eV have been observed in the photoluminescence spectra. These peaks were related to the direct and indirect interband radiative optical transitions in GeSn, respectively.

Analysis of RF losses and material characterization of samples removed from a Nb3Sn-coated superconducting RF cavity

Nb3Sn (TC ≈ 18 K and HSh ≈ 400 mT) is a prospective material to replace Nb (TC ≈ 9 K and Hsh ≈ 200 mT) in SRF accelerator cavities for significant cost reduction and performance enhancement. Because of its material properties, Nb3Sn is best employed as a thin film (coating) inside an already built RF cavity structure. A particular test cavity noted as C3C4 was a 1.5 GHz single-cell Nb cavity, coated with Nb3Sn using Sn vapor diffusion process at Jefferson Lab. Cold measurements of the coated cavity indicated the superconducting transition temperature of about 18 K. Subsequent RF measurements indicated field-dependent surface resistance both at 4.3 and 2.0 K. After initial cold measurements, the cavity RF loss distribution was studied with a thermometry mapping system. Loss regions were identified with thermometry and were cut out for material analysis. The presence of significantly thin patchy regions and other carbon-rich defects is associated with strong local field-dependent surface resistance. This paper summarizes RF and thermometry results correlated with material science findings.

Synthesis of three-dimensional cactus-shaped SnO2 crystals via thermal evaporation of Sn

Synthesis of three-dimensional cactus-shaped SnO2 crystals via thermal evaporation of Sn

Sn isotope fractionation during volatilization of Sn(IV) chloride: Laboratory experiments and quantum mechanical calculations

Volatilization is an important pathway of element transport in nature, and this process may be associated with stable isotope fractionation, which could be used to understand the elemental volatilization mechanisms. In this study, we report that evaporation of Sn(IV) chloride solution under experimental conditions (96 °C, 1 atmospheric pressure) results in significant loss of aqueous Sn(IV) and Sn stable isotope fractionation. The δ122/116Sn of the residue solution can increase by up to 0.50‰ (or a 0.33‰ increase in δ122/118Sn) after repeated evaporation, indicating preferential partitioning of isotopically light Sn species into the vapor phase during evaporation. The observed Sn loss and associated Sn isotope fractionation during the evaporation experiments can be described using a Rayleigh fractionation function, with a best-fitting isotope fractionation factor of −0.36‰ in Δ122/116Snvapor-aq (or −0.24‰ in Δ122/118Snvapor-aq). We also performed quantum mechanical calculations to assess the stability of different potential Sn(IV) species in aqueous and vapor phases, and derived the equilibrium Sn isotope fractionation factors between these Sn(IV) species. The calculation results suggest that the dominant gaseous species of Sn(IV), SnCl4, is isotopically heavier than the aqueous Sn(IV) species by 1.24‰–0.35‰ in δ122/116Sn (or 0.82‰–0.19‰ in δ122/118Sn) under equilibrium at 96 °C, which is opposite to the experimental results. Such contrast in the direction of Sn isotope fractionation implies that kinetic isotope fractionation, rather than thermodynamic equilibrium isotope fractionation, took place for SnCl4 in the evaporation experiments at 96 °C. The observed experimental data can be explained by a kinetic isotope fractionation model involving backward reaction of SnCl4 vaporization at the solution-vapor boundary. This study, in combination with a recent report of positive Δ122/116Snvapor-aq (or Δ122/118Snvapor-aq) factor during evaporation of SnCl4 at 150 °C (Wang et al., 2019a), suggests that Sn(IV) volatilization mechanisms may be different with and without fluid boiling. Combined laboratory experiments and quantum mechanical calculations on the isotopic effects of Sn(IV) chloride solution evaporation provide important constraints for understanding the rapidly accumulating Sn isotopes data from studies on Sn ore-forming processes, bronze metallurgy and archeology, and volatile elements in planetary processes.

Structural and superconducting properties of Nb3Sn films grown by multilayer sequential magnetron sputtering

Nb3Sn thin films are fabricated by sequential sputtering followed by annealing at 850–1200 °C in vacuum. The effects of the annealing temperature and time on the formation of Nb3Sn were examined. The film crystal structure, morphology, surface topography and composition were characterized by X-ray diffraction, field-emission scanning electron microscopy, atomic force microscopy, and energy dispersive X-ray spectroscopy analysis. The superconducting properties of the films were characterized by measuring the surface resistivity by the four-point probe method. The highest critical temperature Tc = 17.86 K was observed for the Nb3Sn films annealed at 950 °C for 3 h. When the films were annealed above 1000 °C for 3 h, Sn evaporation resulted in a Nb-rich film, degrading the superconducting performance. Annealing at 1200 °C for 3 h resulted in evaporation of Sn leaving less than 1% Sn on the surface, as detected by energy dispersive spectroscopy, and for that film a Tc of 7.9 K was observed.

Asymmetry of interface reactions in Ag-Sn thin film couples—In-situ synchrotron radiation study

Synchrotron radiation grazing incidence X-ray diffraction was used to study in situ the asymmetry of the interface reactions and the first phase formation at interfaces prepared by Ag evaporation onto Sn thin films (Ag/Sn case) and by Sn evaporation onto Ag thin films (Sn/Ag case). The ordered Ag3Sn phase forms first on both types of interfaces, in agreement with empirical thermodynamic criteria. Interface asymmetry is observed in the growth mode of the Ag3Sn phase, the reaction constant for Ag3Sn formation, the delay time for the appearance of the Ag3Sn phase, and in the grain-size growth kinetics. At the Ag/Sn interfaces, Ag3Sn growth is predominantly interface controlled and forms almost instantaneously, whereas at the Sn/Ag interfaces, it is diffusion controlled and appears 2 min after the beginning of the Sn deposition. The reaction constant for Ag3Sn formation at the Ag/Sn interface (k =0.22 ± 0.02 nm/s) is much larger than the reaction constant at the Sn/Ag interface (k ≪ 0.09 nm/s). The differences in the interface reactions are attributed to the larger diffusion coefficient of Ag in Ag3Sn (DAg = 3.9 ± 0.2 × 10−17 m2/s), compared to the diffusion coefficient of Sn in Ag3Sn (DSn = 6.2 ± 0.1 × 10−19 m2/s), estimated from the in situ X-ray diffraction data. The second intermetallic phase (Ag4Sn), existing in the Ag-Sn system, forms only in the presence of excess Ag atoms at the Sn/Ag interface (realized only after the complete consumption of the Sn bottom layers), because the Ag4Sn phase has a higher nucleation barrier ΔG* and a lower growth rate than the Ag3Sn phase.

Development of SnS0.4Se0.6 Ternary Alloy on Annealing of Thermally Deposited Films

The effect of annealing temperature on the evolution of different phases and morphological changes of thermally evaporated SnS1−xSex thin films was investigated. Thermal evaporation of Sn, S and Se constituent elements was performed under vacuum pressure ∼ 2 × 10−6 mbar in a vacuum coating unit (VCU) at room temperature. Annealing of the samples was carried out at a base vacuum pressure ∼ 1 × 10−3 mbar in a tubular furnace. Composition of the annealed samples was established as SnS0.4Se0.6 on the basis of energy dispersive x-ray analysis (EDAX). The films acquired alloy formation of SnS0.4Se0.6 with Sn at 473 K; SnS at 523 K; SnS and SnSe at 573 K; and SnSe at 623 K, respectively. The evolutions of the mixed phases of SnS0.4Se0.6 along with Sn, SnS or SnSe phases at different annealing temperatures were confirmed on the basis of x-ray diffraction (XRD). Moreover, the alterations in the morphological patterns of scanning electron microscopy (SEM) and variations in the parameters of Vis-NIR spectroscopic analysis corroborate to the results of x-ray diffraction (XRD). As the annealing temperature increased from 473 K to 623 K, a shift was observed in the peaks of different phases of SnS0.4Se0.6, causing variations in the unit cell parameters of constituent materials. Further, there was an improvement in the stoichiometry values of the constituent elements at higher annealing temperatures. The optical bandgap (Eg) values of SnS0.4Se0.6 films vary from 1.25 eV to 1.01 eV while absorption coefficient (α) values are ≥ 104 cm−1 for all annealing temperatures (i.e. 473–623 K), thereby making these materials, suitable candidates for the solar cell structures.

Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices

Providing an efficacious plasma facing surface between the extreme plasma heat exhaust and the structural materials of nuclear fusion devices is a major challenge on the road to electricity production by fusion power plants. The performance of solid plasma facing surfaces may become critically reduced over time due to progressing damage accumulation. Liquid metals, however, are now gaining interest in solving the challenge of extreme heat flux hitting the reactor walls. A key advantage of liquid metals is the use of vapour shielding to reduce the plasma exhaust. Here we demonstrate that this phenomenon is oscillatory by nature. The dynamics of a Sn vapour cloud are investigated by exposing liquid Sn targets to H and He plasmas at heat fluxes greater than 5 MW m−2. The observations indicate the presence of a dynamic equilibrium between the plasma and liquid target ruled by recombinatory processes in the plasma, leading to an approximately stable surface temperature.

Preparation and Measurement of Cassiterite for Sn Isotope Analysis

Increased interest in the fractionation of Sn isotopes has led to the development of several techniques for preparing cassiterite (SnO2, the primary ore of Sn) for isotopic analysis. Two distinct methods have been applied in recent isotopic studies of cassiterite: (a) reduction to tin metal with potassium cyanide (KCN) at high temperature (800 °C), with subsequent dissolution in HCl, and (b) reduction to a Sn solution with hydriodic acid (HI) at low temperature (100 °C). This study compares the effectiveness and accuracy of these two methods and contributes additional methodological details. The KCN method consistently yielded more Sn (> 70% in comparison with < 5%), does not appear to fractionate Sn isotopes at high temperatures over a 2‐hour period and produced consistent Sn isotope values at flux mass ratios of ≥ 4:1 (flux to mineral) with a minimum reduction time of 40 min. By means of a distillation experiment, it was demonstrated that HI could volatilise Sn, explaining the consistently low yields by this method. Furthermore, the distillation generated Sn vapour, which is up to 0.38‰ per mass unit different from the starting material, the largest induced Sn fractionation reported to date. Accordingly, the HI method is not recommended for cassiterite preparation for Sn isotopic analysis.

Preparation of (Cu,Ag)2SnS3 Thin Films by Sulfurization and Their Application to Solar Cells

(Cu,Ag)2SnS3 (CATS) thin films and solar cells have been fabricated by sulfurization process for the first time and their properties have been investigated. In first experiment, the stacked NaF/(Ag+Cu)/Sn precursor deposited by sequential evaporation of Sn, Ag and Cu elements and NaF followed by crystallization in selenium/tin mixing atmosphere for 30 min at 570°C. The Ag/(Ag+Cu) mole ratio and the grain sizes in the thin films increased with increasing the Ag/(Ag+Cu) mole ratio in the evaporation materials. In second experiment, two kinds of precursors constructed by NaF/(Ag+Cu)/Sn (Precursor A) and NaF/Sn/(Ag+Cu) (Precursor B) were crystallized by annealing in selenium/tin mixing atmosphere for 30 min at the various temperature. The open circuit voltage Voc in the CATS solar cells prepared from precursor B were larger than those from precursor A. The grain growth in CATS thin films was observed with increasing the sulfurization temperature. The best performance of the CATS solar cell fabricated from precursor B at the sulfurization temperature of 530°C corresponded to an open circuit voltage of 257mV, a short circuit current density of 9.31 mA/cm2, a fill factor of 0.26, and an efficiency of 0.61%, which was the first report of the photovoltaic effect for CATS thin film.

Self-Regulated Plasma Heat Flux Mitigation Due to Liquid Sn Vapor Shielding.

A steady-state high-flux H or He plasma beam was balanced against the pressure of a Sn vapor cloud for the first time, resulting in a self-regulated heat flux intensity near the liquid surface. A temperature response of the liquid surface characterized by a decoupling from the received heating power and significant cooling of the plasma in the neutral Sn cloud were observed. The plasma heat flux impinging on the target was found to be mitigated, as heat was partially dissipated by volumetric processes in the vapor cloud rather than wholly by surface effects. These results motivate further exploration of liquid metal solutions to the critical challenge of heat and particle flux handling in fusion power plants.

Evaporation Mechanism of Cu from Liquid Fe Containing C and S

A number of liquid–gas experiments were carried out in order to elucidate evaporation mechanism of Cu from liquid Fe containing C and S. Rate of Cu evaporation in liquid Fe droplets at 1873 K (1600 °C) was determined using electromagnetic levitation equipment. Evaporation rate of the Cu under various conditions (flow rate of gas mixtures, initial C, and S concentrations) was examined. It was found from a series of kinetic analyses of the experimental data that Cu evaporates in forms of Cu(g) and CuS(g). As was reported for the Sn evaporation from liquid iron (Jung et al. Met. Mater. Trans. 46B, 250–258, 2014), S plays two roles for the evaporation of Cu: accelerating the rate by forming CuS(g) and decelerating the rate by blocking evaporation sites. As a result of these combinatorial effects, the evaporation of Cu is decelerated at low S content, but is accelerated at high S content. Based on the elucidated mechanism, an evaporation model equation for Cu was developed in the present study, which takes into account (1) evaporation of Cu in the two forms (Cu(g) and CuS(g)), (2) surface blocking by S using ideal Langmuir adsorption, and (3) effect of C. The obtained rate constant of a reaction Cui + Si = CuSi(g), kCuSR, is 1.37 × 10−9 m4 mol−1 s−1, and the residual rate constant, kCuSr, is 4.11 × 10−10 m4 mol−1 s−1 at 1873 K (1600 °C). Both of them were found to be one order lower than those for Sn evaporation.

Effect of the N2/O2 ratio on the morphology of SnO2 crystals synthesized through the thermal evaporation of Sn

SnO2 crystals were synthesized on sapphire substrate through the thermal evaporation of Sn powder in an oxygen and nitrogen gas environment at 1000°C. No catalysts were used for the formation of the SnO2 crystals. One-dimensional structures such as wires and belts were formed in an N2 environment. With increasing the ratio of O2 to N2 in an ambient gas, the morphology of the SnO2 crystals changed from wire and belt to particle. This provides an evidence that the ratio of N2/O2 in the ambient gas is critical to the change in the morphology of SnO2 crystals. The X-ray diffraction analysis indicated that all the SnO2 crystals had a rutile tetragonal crystal structure. The cathodoluminescence spectra of the SnO2 crystals showed that N2/O2 ratio in the ambient gas had an influence on the emission properties.

Dark Conductivity in Semi-Insulating Crystals of CdTe:Sn

We prepared semi-insulating CdTe for radiation detectors by isothermal annealing of single crystals grown by Bridgeman technique in a sealed quartz container filled with Sn vapor. The resistivity of CdTe:Sn samples thus obtained was of order of 10SUP10/SUP Ohm·cm at room temperature with electrons lifetime of 2´10SUP-8/SUP s, which is appropriate for the applications desired. Analysis of electric transport characteristics depending on temperature, sample thickness and voltage applied revealed the presence of traps with concentration of about (4-5)´10SUP12/SUP cmSUP-3/SUP with the corresponding energy level at 0.8 ? 0.9 eV counted from the bottom of conduction band. The conductivity was determined by electron injection from electrodes in space charge limited current mode.

Influence of Surfactant and Ionic Liquid Media on Vapor Generation and Determination of Ag, Au, Cd, Cu, Ni, Sn, and Zn Using ICP-OES

In this study, the main parameters affecting Ag, Au, Cd, Cu, Ni, Sn, and Zn vapor generation in a surfactant reaction media of cetyl trimethyl ammonium bromide (CTAB) were evaluated. Detection using inductively coupled plasma optical emission spectrometry has been optimized. Improvements in the analytical performance for Ag and Ni were also obtained when CTAB was combined with the ionic liquid l-butyl-3-methylimidazolium. Standards (1 mg L1) were prepared in acid media. The optimum concentrations of the reagents in the standards were: 2.5×10-5 M (CTAB) and 0.6 M (nitric acid), when only CTAB was used. When both modifiers were combined, the concentrations were: 2.5 ×10-3 M of 1-butyl-3-methylimidazolium bromide and 1.5 ×10-5 M CTAB. In this case, 0.3 M of nitric acid was added. The optimum concentration of sodium borohydride to generate the vapors was 2.2% (w/v) in 0.05% (w/v) NaOH. These vapors were swept from the forced outlet of a lab-made gas-liquid separator to the torch with an argon flow of O.7 mL min-1. The limits of detection ranged from 0.4 μg L-1 Cu to 6.9 μg L-1 Au. The detection limit for silver improved three-fold when CTAB was used together with l-butyl-3-methylimidazolium bromide (from 14.3 to 4.9 μg L-1). Relative standard deviations were below 5% (n=l 1) for all of the elements, except for Cd (11.3 μg L-1). Since best results were obtained for most elements using only CTAB, a study of interferences for this modifier was performed. It was observed that a surfactant agent, such as CTAB, was a better reagent to avoid interferences than a complexing agent, such as 8-hydroxyquinoline. Moreover, the method showed good accuracy when determining Cd, Cu, Ni, and Zn in a wastewater reference material.