Sputtering Rate Research Articles

The microstructural and stress evolution in sputter deposited Ni thin films

We report the stress and microstructural evolution for a series of Ni thin films sputter deposited over a range of rates (0.076 and 0.250 nm/s), pressures (0.27, 0.67 and 1.33 Pa), and substrate temperatures (ambient, 100, and 200 °C). In general, as the sputtering pressure increased, the stress-thickness product, measured by wafer curvature, became tensile and trended with an increase in pressure regardless of the deposition rate. However, at the lowest sputtering rate and highest substrate temperature, the films exhibited a compressive growth stress. The collective data was then fitted to a kinetic model that accounts for the stress generation at the grain boundaries, from grain growth, and from the creation of defects within the film. The model's predicted fitted parameters matched well to the experimental measurements except for films deposited at the lowest deposition pressure. These particular films exhibited a bimodal grain size distribution which could not be accommodated by the model's use of a singular grain diameter. Nevertheless, in monomodal grain sizes, the results do provide support for the kinetic model in helping to ascertain the various contributing factors for the intrinsic stress development and their microstructural relationship in context to the Thornton zonal morphology descriptors for thin films and coatings. • First use of a kinetic stress model to Ni sputter deposition • Kinetic model fits yield good agreement to experimental monomodal grain sizes. • Ascertaining energetic-based contributions to stress with varied process parameters.

Calibration capacity of hot-pressed hydrogen standards for glow discharge optical emission and mass spectrometry

Mixed copper and titanium hydride powder was hot-pressed and characterized by Carrier Gas Hot Extraction, X-Ray Diffraction, Thermal Gravimetric Analysis coupled with Mass Spectrometry, and Scanning Electron Microscopy. The hot-pressed and five conventional samples were applied for calibration of hydrogen in Glow Discharge Optical Emission and Mass Spectrometry. Up to the introduction of 15 ng/s hydrogen the emission yield model is useful in Glow Discharge Optical Emission Spectrometry. A correlation between saturation and even reversal of the emission yield of the spectral lines H121, H486 and H656 and low sputtering rates was found. Hydrogen effects exist for the spectral lines of Cu(II) 219 and Ti(I) 399. In Glow Discharge Mass Spectrometry, a linear dependency of the 1H ion current on the sputtered mass per time exists over the total range of hydrogen content investigated. Hydrogen effects also exist for the sensitivity of 48Ti and 63Cu. The sputtering rate of two-phase materials depends linearly on the sputtered mass per time of one phase, which allows the sputtering rate of two-phase materials with known composition to be predicted.

The Topography of Surface and Light Transmission by Quartz Windows after Exposure in a High-Frequency Discharge in Deuterium and Mixture of Deuterium with Nitrogen

The effect of a high-frequency discharge on the change in the topography of the surface of KU-1 optical quartz and transmission of visible light (400–1000 nm) is studied. The working gases of the discharge are D2 and a D2/N2 mixture, in which the fraction of N2 is 25 mol %. The addition of nitrogen increases the rate of sputtering from 60 to 300 nm/h without changing the stoichiometry of the surface layers. After the exposure in plasma, the root-mean-square roughness of the surface decreases from 1.3 to 0.6 nm. The transparence of quartz remains unchanged. The analysis of the roughness and calculation of the diffuse light scattering with a wavelength of 400 nm after passing through quartz are executed using the power spectral density functions.

Atomic layer deposition of hafnium and zirconium oxyfluoride thin films

Hafnium and zirconium oxyfluoride films may act as effective protective coatings during plasma processing. The low molar volume expansion/contraction ratios and the small estimated strain values versus fluorination/oxidation suggest that hafnium and zirconium oxyfluorides can serve as protective coatings in both fluorine and oxygen plasma environments. To demonstrate the procedures for depositing these films, hafnium and zirconium oxyfluorides with tunable stoichiometry were grown using atomic layer deposition (ALD) at 150 °C. Tetrakis(dimethylamido)hafnium and tetrakis(ethylmethylamido)zirconium were used as the metal precursors. H2O and HF were employed as the oxygen and fluorine precursors, respectively. MOxFy (M = Hf and Zr) films were grown using two deposition mechanisms: the nanolaminate method and the HF exchange method. In situ quartz crystal microbalance studies were employed to monitor the MOxFy growth. Both deposition methods observed a linear MOxFy growth at 150 °C. The nanolaminate method is defined by the sequential deposition of MOx ALD and MFy ALD layers. Compositional tunability was achieved by varying the ratio of the number of MOx ALD cycles to the number of MFy ALD cycles in the nanolaminate. The HF exchange method is based on the thermodynamically favorable fluorination reaction of MOx by HF. Variable oxygen-to-fluorine concentrations in these films were obtained either by changing the HF pressure or by varying the thickness of the underlying MOx ALD layers. Ex situ Rutherford backscattering spectroscopy measurements were utilized to determine the composition of the various MOxFy thin films. Both deposition techniques displayed a wide range of compositional tunability from HfO2 to HfF4 and ZrO2 to ZrF4. In addition, the physical sputtering rates of MOxFy films were estimated from the film removal rates during ex situ x-ray photoelectron spectroscopy depth profiling. The physical sputtering rates increased with F concentration in the MOxFy films.

Preferential sputtering in quantitative sputter depth profiling of multi-element thin films

Preferential sputtering plays an important role in the quantification of measured depth profiles. The distortion of the measured depth profile as compared to the original one has to be evaluated upon preferential sputtering in depth profiling. In the framework of the mixing-roughness-information depth (MRI) model, the influences of preferential sputtering on the depth profile of multi-element thin films are quantitatively evaluated as demonstrated for a layered structure of three elements with different sputtering rates and subsequent interfaces as A/BC/A. Moreover, the mass conservation upon Auger electron spectroscopy/X-ray photoelectron spectroscopy depth profiling of multi-element structure is discussed with respect to the preferential effect. Finally, as an example, the measured glow discharge optical emission spectroscope depth profiling data of 60x(3.0 nm Mo/0.3 nm B4C/3.7 nm Si)/Si(111) multilayer structure are quantitatively evaluated using the extended MRI model.

Application of Taguchi arrays using DOE for optimizing processing and understanding sputtered coated ZnO TFTs

This study applies the Taguchi orthogonal array (OA) to improve and optimize the performance of ZnO thin-film transistors (TFT). Radiofrequency (RF) sputtering method was used to deposit the active layer of TFTs. The annealing temperature, environmental conditions, sputter rate, and thin-film thickness were the parameters whose impact on output parameters like mobility was analyzed in the process optimization using design of experiment (DOE). The ZnO TFTs show state-of-the-art performance features, including high saturation mobility (0.83c), high Ion/Ioff ratio (104). The optimal configuration was found to be high annealing temperature 450 °C under N2 atmosphere, low sputter rate. This paper presents a method that can empower the fast optimization of metal oxide TFTs for future developments in the manufacturing process. If a full factorial design had been implemented, 64 tests would have been necessary, however in this work, we have reduced the number of tests to 9 only using Taguchi method.

A Hollow-Cathode Ion Source in Glow Discharge Mass Spectrometry

The paper proposes a new glow discharge ion source based on a hollow cathode. The proposed source produces a denser plasma than the previously used glow discharge sources. It is shown experimentally that the maximum rate of cathode sputtering of a conducting solid sample in a hollow cathode ~25 mm in diameter is achieved when the sample has the shape of a rod with a diameter of 2–4 mm and a length of 10–15 mm and is placed along the axis of the cathode cavity. If a cover with a hole is placed on the cathode cavity, a plasma bunch is formed near the hole (a “plasma plug”), which resists the flow of argon from the hollow cathode. The pressure in the discharge chamber decreases, and the glow discharge is concentrated in the cathode cavity. This results in a decrease in the background level in the glow discharge source by approximately two orders of magnitude. The mechanism of formation and extraction of ions from a glow discharge ion source is considered. The ions formed in the glow discharge plasma form two oppositely directed ion flows in the source, one of them with an ion energy of 100 eV or higher. These ions, accelerating in the region of the cathode potential drop, are directed to the cathode, bombard the analyzed sample, and spray it. The second flow of low-energy ions is drawn from the same region of negative luminescence and is transported in the opposite direction, that is, to the surface of the discharge chamber-anode due to ambipolar diffusion. These ions are extracted from a hole in the anode chamber of a conventional ion source by an accelerating voltage and used for mass spectral analysis. The use of a hollow cathode with a plasma plug, the effect of ambipolar diffusion, and a modified Pierce lens makes it possible to increase the luminosity of the glow discharge source by more than one order of magnitude.

Material Sputtering with a Multi‐Ion Species Plasma Focused Ion Beam

Focused ion beams are an essential tool for cross‐sectional material analysis at the microscale, preparing TEM samples, and much more. New plasma ion sources allow for higher beam currents and options to use unconventional ion species, resulting in increased versatility over a broader range of substrate materials. In this paper, we present the results of a four‐material study from five different ion species at varying beam energies. This, of course, is a small sampling of the enormous variety of potential specimen and ion species combinations. We show that milling rates and texturing artifacts are quite varied. Therefore, there is a need for a systematic exploration of how different ion species mill different materials. There is so much to be done that it should be a community effort. Here, we present a publicly available automation script used to both measure sputter rates and characterize texturing artifacts as well as a collaborative database to which anyone may contribute. We also put forth some ideas for new applications of focused ion beams with novel ion species.

ВЛИЯНИЕ СОСТОЯНИЯ МИШЕНИ СИСТЕМЫ Zr–Y–РЗМ НА СКОРОСТЬ ЕЕ РАСПЫЛЕНИЯ ПРИ НАНЕСЕНИИ КЕРАМИЧЕСКОГО СЛОЯ ТЗП С ПОМОЩЬЮ УСТАНОВКИ УОКС-3

The paper considers the operational features of the UOKS-3 magnetron sputtering system. The experimentally obtained data indicate that the sputtering rate of the substrate depends on a large number of factors, such as partial pressure of the working and reaction gases, distance between the target and the workpieces, and depth of the groove made on the working surface of the target. Using a comprehensive study of the influence of each of these factors, it is possible to more accurately predict the deposition rate of a ceramic layer of a thermal barrier coating on the surface of the processed products.

SiOxCyHz-TiO2 Nanocomposite Films Prepared by a Novel PECVD-Sputtering Process

Recently, there has been growing interest in the incorporation of particles in plasma deposited thin films to creation of multifunctional surfaces. In this work a new hybrid methodology based on the plasma enhanced chemical vapor deposition (PECVD) of hexamethyldisiloxane combined to the reactive sputtering of TiO2 is proposed for the preparation of SiOxCyHz-TiO2 composite films. Specifically, the effect of the proportion of O2 in the plasma environment on the morphology, chemical structure, elemental composition, wettability, thickness and surface roughness, of the films was studied. Agglomerates of TiO2 (16-83 μm) were detected into the organosilicon matrix with the concentration of particulates growing with the percentage of oxygen in the feed. In general, there was elevation in the angle of contact of the surfaces as the oxygen supply increased. Interpretation is proposed in terms of the influence of the oxygen supply on the TiO2 sputtering rate and in the oxidation of plasma species.

ВПЛИВ ЛОКАЛЬНИХ МЕХАНІЧНИХ НАПРУЖЕНЬ НА КОЕФІЦІЄНТ РОЗПИЛЕННЯ КРЕМНІЮ ІОННИМ ПУЧКОМ

A review of scientific publications and modeling of the effect of mechanical stresses on the sputtering yield of silicon by an ion beam is carried out. It is shown that the flux of atoms (from the depth to the surface) through interstitial or vacancy mechanisms due to the stress gradient caused by the limiting bending of the plate is insufficient to explain the increase in the sputtering coefficient. Calculations show that even the limiting elastic deformations do not significantly change the energy of atom detachment from the site, and an increase in the drift velocity of atoms due to the enrichment of the near-surface region with vacancies is insufficient to increase the sputtering rate. Consequently, it is necessary that the elastic deformation is transformed into plastic with the formation of mobile weakly bound atoms. The calculated stress distribution in a loaded silicon wafer using the COMSOL Multiphysics software package showed that the key driving force behind the increase in the silicon sputtering coefficient is the concentration of compressive and tensile stresses in the vicinity of the simulated crater during sputtering. The created crater is a stress concentrator, the gradients of which significantly exceed the values obtained by bending a plate without a crater. It is demonstrated that the generated stresses exceed the ultimate strength of the material in the vicinity of the crater, which begins to relax due to the expulsion of "excess" atoms in the tension region. The appearance of additional deformation-stimulated fluxes of weakly bound surface atoms at the bottom and walls of the crater provides an increase in the concentration of knocked-out atoms in the process of ion sputtering. Simulations predict an increase in sputtering yield of up to 40%. It is also shown that closely spaced craters, due to elastic interaction with each other, compensate each other's elastic fields, which has an effect on the value of the sputtering coefficient.

ION-PLASMA SYNTHESIS, STRUCTURE AND PROPERTIES OF Hf-C ALLOYS

In this work, studies have been carried out on the synthesis of coatings based on hafnium and carbon, using the method of ion-plasma spraying of coatings from alternating layers of hafnium and carbon with a thickness of each layer less than 1 nm. When performing the work, we used a four-channel magnetron setup. In which two oppositely located magnetrons were used. One magnetron is charged with a hafnium target, and the other magnetron is charged with a carbon target. The rate and optimal modes of sputtering of hafnium and carbon have been determined. The dependences of the amount of atomized hafnium and carbon on the power applied to the magnetron are obtained. The average atomization power of hafnium has been determined, which corresponds to the removal of the substance with the removal of carbon at its maximum possible atomization power. The synthesis of hafnium-carbon coatings in the concentration range from 59.8 to 5.3 at.%. Carbon was carried out, and X-ray studies of carbide coatings were performed. Determined in the hafnium-carbon system formed by magnetron deposition, at concentrations from 59.8 to 26.1 at. % carbon in the coating, the cubic phase of hafnium carbide is formed, with a decrease in the lattice parameter. At a concentration of 16.2 at. % carbon, along with the cubic phase of hafnium carbide, hafnium with a hexagonal structure begins to precipitate in the coating. With a further decrease in the carbon concentration in the coating, the hexagonal phase of hafnium becomes predominant.

Sputtering of the beryllium tungsten alloy Be2W by deuterium atoms: molecular dynamics simulations using machine learned forces

Material erosion and fuel retention will limit the life and the performance of thermonuclear fusion reactors. In this work, sputtering, reflection and retention processes are atomistically modeled by simulating the non-cumulative sputtering by deuterium projectiles on a beryllium–tungsten alloy surface. The forces for the molecular dynamics trajectories were machine learned from density functional theory with a neural network architecture. Our data confirms and supplements previous results for simulated sputtering rates. In the non-cumulative scenario we simulate, we did not observe reaction mechanisms leading to swift chemical sputtering. Thus, our sputtering rates at low impact energies are smaller than in comparable non-cumulative studies. The sputtering yields of the Be2W alloy are generally lower than those of pure beryllium. We found a strong dependence of the sputtering yield on the incident angle with an increase by about a factor of 3 for larger incident angles at 100 eV impact energy. In the pristine surface, a large majority of the impacting hydrogen projectiles at perpendicular impact remains in the surface.

Validation of EDGE2D-EIRENE and DIVIMP for W SOL transport in JET

Abstract Tungsten sputtering rates and density profiles predicted using the edge plasma codes EDGE2D-EIRENE and DIVIMP are found to agree within a factor of 4 with measurements of neutral and singly-ionized W spectral line emission in the JET low-field side (LFS) divertor, and within a factor of 2 with SXR, VUV, and bolometric calculations of the W density in the main plasma. The edge plasma W predictions are extended to the core plasma using JINTRAC integrated core-edge modelling. Prompt redeposition of W is identified as the primary reason for the discrepancy between predicted and measured W emission in the divertor. The studied plasmas include attached divertor conditions in L-mode and type-I ELMy H-mode plasmas typical for JET. To more accurately reproduce the spectroscopically inferred W sputtering rates in EDGE2D-EIRENE, imposing the experimentally observed Be concentration of order 0.5% in the divertor is necessary. However, the W density in the main plasma is predicted to be insensitive to whether or not W is sputtered by Be at the divertor targets. Instead, the majority of the predicted core W originated in L-mode from sputtering due to fast D charge-exchange atoms at the W-coated tiles above the LFS divertor, and in H-mode due to D and W ions at the targets during ELMs.

Sputtering of Presolar Grains via Galactic Cosmic Rays in the Interstellar Medium

Abstract The sputtering rate of presolar silicon carbide grains due to galactic cosmic rays has been computed for their experimentally deduced lifetimes (∼1 Gyr) in the interstellar medium. An ion target simulator, SDTrimSP, was used to model the sputtering of interstellar grains with varying sizes and thicknesses of the ice mantle formed around the grain during their journey through the interstellar medium. Temperature, composition, and density for four different types of molecular cloud environments (quiescent, low-mass young stellar objects (YSOs), intermediate-mass YSOs, and high-mass YSO weak processing) considered indicate the sputtering rate on the mantle ice composition depends on water composition to a certain extent. The model simulations indicate galactic cosmic ray(s) with an energy range from 10 MeV to 1 GeV are just capable of sputtering/destructing ∼13%–15% of the grain itself. This value, stretched over 1 Gyr is not as significant as the other destruction processes and therefore can be classified as a minor destruction process. The effect of galactic cosmic rays on the ice mantle and core is also noted with particular emphasis on amorphization/recoils generated inside the SiC core and their distribution within the grain.

Kinetic Modeling of Spacecraft Surfaces in a Plume Backflow Region

Plasma-surface interactions caused by electric propulsion devices are an important spacecraft aspect of the design that is difficult to measure in ground-based facilities. The negatively biased solar panel surfaces attract the slow-moving charge exchange (CEX) ions generated inside an ion core plume, which can cause surface sputtering on the protective coatings of the solar panels. We use a fully kinetic particle-in-cell direct simulation Monte Carlo (PIC-DSMC) approach that models both electron and ion trajectories to allow us to fully characterize the plasma sheath formed near these surfaces and to understand how the plasma sheath affects the trajectories of CEX ions, their incident energies and angles, and surface sputtering rates. We find that, outside the plasma core, the ion and electron distribution functions are highly non-Maxwellian, and the assumption of electron temperatures is questionable. We introduce a novel floating potential ground boundary condition that enables us to emulate the spacecraft ground for a high range of plasma number densities and surface charging conditions. Finally, we estimate the erosion of the surface using the kinetic results and surface yield empirical relations.

Optical and structural characterization of Tm2O3, TmN, and TmOxNy thin films grown by direct-current reactive magnetron sputtering

This paper reports the fabrication and characterization of several thulium oxide and nitride thin films grown by reactive magnetron direct-current sputtering. Hysteresis curves of the Tm emission spectra of the sputtering plasma versus the flow of N2 or O2 around the Tm-metal target were monitored. Emission spectra of atomic transition lines in the region between 370 and 420 nm were identified to be of neutral thulium. The plasma emission was compared to the hysteresis curves generated by monitoring the sputter rate and target voltage. The nitride films' composition and optical properties were determined by X-Ray Diffraction, and optical transmission spectroscopy. The composition of the oxide films was determined by energy dispersive X-ray spectroscopy. The films are initially amorphous but crystallize after thermal treatment at 800°C. The optical bandgap values obtained using the Tauc method are consistent with what has been previously reported for both Tm2O3 and TmN prepared by other methods.

In Situ Atomic Force Microscopy Depth-Corrected Three-Dimensional Focused Ion Beam Based Time-of-Flight Secondary Ion Mass Spectroscopy: Spatial Resolution, Surface Roughness, Oxidation.

Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect depth information about features on the top atomic layers of samples. By combining secondary ion mass spectroscopy (SIMS) with focused ion beam (FIB) milling in a scanning electron microscope (SEM), chemical information of sputtered structures can be visualized and located with high lateral and depth resolution. In this paper, a high vacuum (HV) compatible AFM was installed in a TESCAN FIB-SEM instrument that was equipped with a time-of-flight SIMS (ToF-SIMS) detector. To calibrate the sputtering rate and measure the induced roughness caused by the ToF-SIMS analysis, subsequent AFM measurements were performed on an inorganic multilayer vertical cavity surface-emitting laser sample. Normalized sputtering rates were used to aid the accurate three-dimensional reconstruction of the sputtered volume's chemical composition. Achievable resolution, surface roughness during sputtering, and surface oxidation issues were analyzed. The integration of complementary detectors opens up the ability to determine the sample properties as well as to understand the influence of the Ga+ ion sputtering method on the sample surface during the analysis.

A Study of Lithium Silicon Oxynitride (LiSiON) Deposited By RF Sputtering for All-Solid-State Thin Film Batteries

At present, we suffer from various environmental issues such as air pollution and rapid weather change. Air pollution is usually caused by the use of fossil fuels. To overcome the environmental issues, LIBs are reasonable candidates. In these days, lithium ion batteries (LIB) are very promising power suppliers for electronic devices, electrical vehicles (EV), and energy storage system (ESS) because of their high power densities.[1] Although the LIB has advantages compared to other power sources, LIBs with a liquid electrolyte have safety issues such as explosion and fire contributed by thermal or chemical instability. All-solid-state batteries are the solution to the problem of LIBs with liquid electrolytes. All-solid-state batteries have many advantages such as high energy densities, high stability, and applying high voltages compared to conventional LIBs.[2]As the portable and miniaturization of electronic devices and the development of wearable devices are to be in the spotlight, the development of a power supply for drive them is indispensable. The type of batteries that can meet this demand is an all-solid-state thin-film battery. Thickness of thin-film batteries is about 10 μm, which makes it suitable for power source of miniaturized electronic devices such as smart cards, RFID tags and medical devices. All-solid-state thin-film batteries also have better thermal stability than conventional Li-ion batteries. Higher capacity of thin-film batteries can be realized by applying high voltages. The key to the performance of all-solid-state thin-film batteries is a solid electrolyte.In order to deposit thin-film electrolytes, there are various available techniques such as a sputter, plasma laser deposition (PLD), e-beam evaporator, and so on. Among these techniques, the sputtering technique has an advantage compared to other deposition techniques. The sputtering techniques could deposit oxide and nitride materials. In addition, sputtering techniques are simple process and can be deposition with thin films uniformly. Comparing PLD and molecular beam epitaxy (MBE), the sputtering techniques are lower cost and low temperature for deposition. Therefore, sputters are suitable for commercialization.Generally, solid electrolytes are classified under oxide system and sulfide system. While sulfide electrolytes have high ionic conductivity and instability, oxide electrolytes have low ionic conductivity and high stability. A lot of oxide based solid state electrolytes were researched. The oxide-based solid electrolytes include LiPON, Li7La3Zr2O12 (LLZO), Li1+xAlxTi2-x(PO4)3(LATP), and lithium boron oxynitride. Among them, LiPON thin-film electrolytes are representative and commonly used as a thin-film electrolyte. However, since the ionic conductivity of LiPON is relatively low, it is necessary to improve the low ionic conductivity.In this study, a new lithium silicon oxynitride (LiSiON) thin-film electrolyte was deposited by RF sputtering technique. Surface morphologies and cross-sectional views of the thin-film electrolyte were characterized by field emission scanning electron microscope (FESEM). The thin film showed smooth surface without any cracks and pinholes. It can be thought that the smooth surface could decrease interfacial resistance between electrolyte and electrodes. In addition, surface morphologies were also characterized by atomic force microscopy (AFM). The sputtering rates were calculated by thickness of thin films on cross sectional views. Structural properties of the thin films were characterized by x-ray diffraction (XRD). The thin film showed amorphous properties compared to the target material which is a crystalline material. In addition, structural properties of the thin film were also characterized by transmission electron microscope (TEM). The thin film showed also amorphous properties with partially crystalline in LiSiON structures deposited by RF sputtering. Ionic conductivity of LiSiON was measured by electrochemical impedance spectroscopy (EIS).Cu thin films used as blocking electrodes with 150nm thickness were deposited by a direct current (DC) magnetron sputtering using a target with 2-inch diameter. The DC power and working pressure were set on 30W and 7 mTorr in Ar atmosphere, respectively. LiSiON thin film was deposited by an RF magnetron sputter at 200W power using Li4SiO4 target with 4-inch diameter in Ar/N2 (2:8) atmosphere. Ionic conductivity of the LiSiON thin film showed 2.47 ´ 10-6 (S/cm) which is similar to other lithium oxide thin films. For this reason, LiSiON thin-film electrolytes are research-worthy materials for use in all-solid-state thin-film batteries.Reference[1] M. Armand, J. Tarascon, Nature, 451, 652 (2008).[2] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, et al., J. Power Sources, 208, 210 (2012).

Electrochemical Behavior of Dysprosium Complex in Potassium Bis(trifluoromethylsulfonyl)Amide Melts

1. Introduction From the standpoint of energy conservation, the development of a recovery process for rare earth metals with reduced energy consumption is desired. In previous investigations, we demonstrated the recovery of Nd metal using low-temperature molten salts (LTMSs) [1,2], because an LTMS has many useful physicochemical properties [3] such as a wide electrochemical window, low liquid-phase temperature, and high ionic conductivity. In our findings, the reduction process of Dy(III) proceeded in two steps by way of Dy(II), [Dy(III)+e-→Dy(II), Dy(II)+2e-→Dy(0)] in [NTf2 -] based ionic liquids (ILs), because [Dy(III)(NTf2)5]2- and [Dy(II)(NTf2)4]2- in ILs were confirmed by Raman spectroscopy and DFT calculations [4]. However, there were no information about the detailed solvation conformations of Dy(II) and Dy(III) in K[NTf2] melts. Therefore, we have investigated the electrochemical behavior of Dy(II) and Dy(III) in K[NTf2] melts. 2. Experimental The K[NTf2] melts including Dy(III) (molar fraction: x Dy=0.1) were applied as the electrolytic for the electrochemical measurement and this solution was dried at 373 K in a vacuum chamber (<0.1 MPa) for 24 h. Cyclic voltammetry (CV) at 493±1.0 K was carried out using a cylindrical cell constituted from three-electrode system under Ar flow. The Pt electrode was used as a working electrode that was mirror polished using alumina paste. Platinum wires (φ=0.7 mm) were employed as a counter electrode and a quasi-reference electrode (Q.R.E), because the potential obtained using a Pt Q.R.E was stable and exhibited a good reproducibility at medium temperature. All the potentials were compensated for the redox reaction of K/K+ couple. As for the electrodeposition process, a prismatic Dy rod and a Cu substrate were employed as an anode and a cathode, respectively. The anode was surrounded by a soda lime tube with a Vycor glass filter at the bottom to prevent the diffusion of dissolution components from the anode into electrolyte. The potentiostatic electrolysis was carried out while stirring the electrolyte at 500 rpm in order to increase the current density of the electrodeposition process. An in-depth analysis of the electrodeposits was conducted with Al-Kα radiation by XPS. The sputtering rate was 27.2 nm min-1 estimated from the sputtering rate of the Si standard. 3. Results and discussion From the voltammetric analysis of Dy(III) in K[NTf2] melts, there were no apparent anodic peaks corresponding to an oxidation of Dy(0) in voltammogram. This result indicated that the reduction of Dy(III) was an irreversible process. It was also consistent with the reference [5], which was reported that no oxidation peak of Dy(0) in DMF and DMPT. There were two reduction peaks around +1.0 V and +2.3 V vs. K/K+ in voltammogram. The reduction peak around +2.3 V would be based on Dy(III)/Dy(II) charge transfer reaction. The reduction peak around +1.0 V would be corresponded to Dy(II)/Dy(0) deposition process. Regarding the charge transfer reaction, it was confirmed that the plot of the cathodic peak of the current density vs. the square root of the scan rate showed a good linear relation. It indicated that the charge transfer reaction was controlled by the diffusion process.In the electrodeposition process, the blackish-brown electrodeposits had a relatively strong adhesion on the Cu substrate. The Dy3d 5/2 peak of the electrodeposits was shown in Fig. 1. The Dy3d 5/2 peaks for Dy metal and Dy2O3 were theoretically positioned at 1295.8 eV and 1289.0 eV, respectively [6]. The Dy3d 5/2 spectra of the top surface and the middle layer showed a relatively good agreement with the theoretical value. This study enabled us to conclude that Dy metal was able to electrodeposit in a metallic state from the K[NTf2] melts. Acknowledgement This research was partially supported by the Grant-in-Aid for Scientific Research (No. 18H03404) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.