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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Observation of tantalum deposition and growth on TiB2 and ZrB2 from PISCES-RF deuterium and helium plasma exposures

Deuterium and helium plasma exposures on bulk TiB2 and ZrB2 samples were performed using the PISCES-RF linear plasma device. 40 and 90 eV deuterium ion plasma exposures were performed at 240 and 800 °C sample temperatures, and 80 eV helium ion plasma exposures were performed at 800 °C sample temperatures. Following plasma exposures, it was discovered that two plasma conditions (90 eV deuterium and 80 eV helium at 800 °C) resulted in thick (>200 nm) tantalum-rich (>10 at%) surface features on the targets, presumably from tantalum sourced from a tantalum adapter mask or cap used as part of the target holder. This work aims to characterize these tantalum-rich features and examine the mechanisms of impurity deposition.Plasma-induced surface morphology of the tantalum-rich surface layers depends on plasma properties and target temperature and chemistry. Greater titanium sputtering compared to zirconium resulted in more distinct surface features in the TiB2 samples compared to the ZrB2 samples via increased, prompt deposition onto tantalum surface impurities. There is still uncertainty as to why thick tantalum deposition only occurred under some plasma exposure conditions but not others; it is likely due to tantalum sputtering by a combination of boron molecules from the targets and carbon-impurities in the tantalum mask or targets. Impurity driven surface features are a well-documented phenomena in samples exposed to plasma from linear plasma device facilities—this work confirms the occurrence of this and emphasizes the need for chemistry characterization of isolated post-mortem surface features in plasma-exposed samples.

Nuckols, Lauren↗
DOE OSTI2024

High-pressure synthesis and neutron scattering study of tantalum hydride TaH 1.23(5) and a tantalum polymorph with A15-type structure

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group , Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a = 5.150(5) Å at T = 85 K. The hydrogen content determined by hot extraction was H/Ta - 1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, Δ⁢H⁡/Ta=0.2 was desorbed at around -70 °⁢C, and then the rest of the hydrogen was desorbed between 100 ⁢°C and 390 ⁢°⁢C. The A⁢15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A⁢15-TaH 1.23⁢(5) demonstrated that hydrogen atoms occupy the 24⁢k and 16⁢i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24⁢k and an increase of the 16⁢i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A⁢15-TaH 1.23⁢(5) at 72, 135, 145, and 166 meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24⁢k and 16⁢i sites, respectively. No superconductivity was found in A⁢15-TaH 1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructurally-Inspired Strategies to Print Tantalum and Tantalum-Tungsten Alloys

The goal of this project is to investigate strategies to print tantalum and tantalum- tungsten alloys, which are notoriously difficult to print with consistent results because of the sensitivity of the properties to small concentrations of interstitial impurities (particularly oxygen) and microstructure, and hence to processing conditions. The ultimate tensile stress (UTS) for non-additively manufactured Ta as a function of temperature shows a stunning variation. In direct metal laser sintering (DMLS) Ta, a strong dependence of porosity, grain morphology and texture on processing conditions was found.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High-rate strength response of tantalum from dynamic hole closure experiments

The science and engineering communities have significant interest in experimental platforms to evaluate and improve models for dynamic material deformation. While well-developed platforms exist, there are still gaps to fill for strain and strain rate conditions accessed during impact and other high-rate loading scenarios. To fill one such gap for strength measurements, a platform was recently developed that accesses high strain rate (≥ 10 5 /s) and large strain (≥ 50%) conditions by measuring the transient closure of a cylindrical hole using in situ x-ray imaging. In the work reported here, further refinement of the platform is performed to reduce the potential effects of porosity and anelasticity on the measurement. This helps us to isolate the strength effects that are the focus of the experiment. The updated experimental configuration employs a two-layer flyer design and elongated target to reduce the magnitude of the tensile excursions associated with rarefaction wave interactions. This allows for a more direct assessment of strength models commonly used for dynamic simulations of metals. Here we apply the new technique to well-characterized tantalum material, allowing for a robust connection to other experimental techniques. Deformation localization can be a concern in large strain experiments, and to help inform future use of the experimental platform, we use simulations with a sub-zone treatment of shear banding to explore potential localization behavior. Here we develop and utilize an experimental configuration with improved isolation of strength effects that can be applied to an expanded range of materials.

42 ENGINEERING↗
DOE OSTI2022

Microstructurally-Inspired Strategies to Print Tantalum and Tantalum-Tungsten Alloys

The goal of this project is to investigate strategies to print tantalum and tantalumtungsten alloys, which are notoriously difficult to print with consistent results because of the sensitivity of the properties to small concentrations of interstitial impurities (particularly oxygen) and microstructure, and hence to processing conditions. In direct metal laser sintering (DMLS) Ta, a strong dependence of porosity, grain morphology and texture on processing conditions was found.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Polypropylene Degradation Catalyzed by Tantalum Hydrides Supported on Sulfated Alumina

Ta–H + sites supported on sulfated aluminum oxide (SAO), [Ta(H) 2 (O–) 2 ][SAO] (1), catalyze the hydrogenolysis of isotactic polypropylene (iPP, M n = 13.3 kDa; D = 2.4; mmmm = 94%) to form low-molecular-weight branched alkanes (C 11 –C 30 ) in good yields (70%). Here, the alkanes formed lose stereochemical information originating from iPP, but residual iPP remains highly isotactic. In the presence of D 2 , similar mixtures of alkanes are formed containing –CH 3–x D x , –CHD, and –CD–. Residual iPP maintains high tacticity and incorporates deuterium primarily into –CH 3 groups of the polymer (–CH 3–x D x /–CHD–/–CD– ~10:1:0). 1 reacts with pinacolborane to form [TaH(κ 2 -H 2 BPin)(O–) 2 ][SAO] (2). At 200 °C in an iPP melt, 2 reacts to form products indicative of C–H bond activation at –CH 3 groups and internal –CH 2 – groups in iPP in a ~1:3 ratio, indicating a slight kinetic preference for C–H bond activation at internal –CH 2 – groups. 1 is more reactive toward iPP than high-density polyethylene in hydrogenolysis reactions

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Cationic Tantalum Hydrides Catalyze Hydrogenolysis and Alkane Metathesis Reactions of Paraffins and Polyethylene

Sulfated aluminum oxide (SAO), a high surface area material containing sulfate anions that behave like weakly coordinating anions, reacts with Ta(=CH t Bu)(CH 2 t Bu) 3 to form [Ta(CH 2 t Bu) 2 (O–) 2 ][SAO] (1). Subsequent treatment with H 2 forms Ta–H + sites supported on SAO that are active in hydrogenolysis and alkane metathesis reactions. In both reactions Ta–H + is more active than related neutral Ta–H sites supported on silica. This reaction chemistry extends to melts of high-density polyethylene (HDPE), where Ta–H + converts 30% of a low molecular weight HDPE (M n = 2.5 kg mol –1 ; D = 3.6) to low molecular weight paraffins under hydrogenolysis conditions. Under alkane metathesis conditions Ta–H + converts this HDPE to a high MW fraction (M n = 6.2 kDa; D = 2.3) and low molecular weight alkane products (C 13 –C 32 ). Furthermore, these results show that incorporating charge as a design element in supported d 0 metal hydrides is a viable strategy to increase the reaction rate in challenging reactions involving reorganization of C–C bonds in alkanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Tantalum Experiments in Fully-Reflected Water-Moderated Triangular-Pitched U(6.90)O 2 Fuel Rod Lattices (1.02 cm Pitch)

The United States (US) Department of Energy (DOE) Nuclear Energy Research Initiative funded the design and construction of the Seven Percent Critical Experiment (7uPCX) at Sandia National Laboratories (SNL). The start-up of the experiment facility and the execution of the experiments described here were funded by the DOE Nuclear Criticality Safety Program. The 7uPCX is designed to investigate critical systems with fuel for light water reactors in the enrichment range above 5 % 235 U. The 7uPCX assembly is a water moderated and — reflected array of aluminum-clad U(6.90 %)O 2 fuel rods. Other critical experiments performed in the 7uPCX assembly are documented in LEU-COMP-THERM-078, LEU-COMP-THERM-080, LEU-COMP-THERM 096, LEU- COMP-THERM-097, LEU-COMP-THERM-101, LEU-COMP-THERM-102, and LEU-COMP-THERM-111. Applied interests in neutron-absorbing elements at epithermal and intermediate neutron energy ranges at DOE’s Hanford Site and Savannah River Site were identified through DOE Office of Environmental Management (EM) workshops and needs surveys performed in conjunction with the National Nuclear Security Administration/Nuclear Criticality Safety Program. In response, a team lead by Oak Ridge National Laboratory (ORNL) performed a study evaluating the use of 7uPCX for testing the epithermal cross sections of materials at SNL. This marked the inception of the set of experiments described here. The purpose of these experiments was to measure the effects of tantalum in nearly-critical systems. The tantalum was introduced into the fuel arrays as experiment rods within a central test region. The central test region was designed to target the epithermal neutron energy range by providing a dry cylindrical cavity constructed of aluminum and lined with 0.04 in (0.1016 cm) thick cadmium sheet. The central test region has an outer diameter of 3.75 in (9.525 cm) length of 31 in (78.74 cm) and can hold 85 tantalum rods nominally 0.25 in (0.635 cm) outside diameter and 31.25 in (79.375 cm) long. The critical experiments were done using a set of triangular-pitched grid plates fabricated for these experiments. The grid plate set accommodated a fuel array of a total of 2016 fuel rod positions on a pitch of 0.4 in (1.016 cm) in a series of 21 hexagonal rings positioned around the central test region. The fuel used in these experiments was fabricated using unirradiated 6.90 % enriched UO 2 fuel pellets from fuel elements designed to be used in the internal nuclear superheater section of the Pathfinder boiling water reactor operated in South Dakota by the Northern States Power Company in the 1960s. The fuel elements were obtained from The Pennsylvania State University where they had been stored for many years. The fuel pellets in those fuel elements were removed from the original Incoloy cladding and reclad in 3003 aluminum tubes and end caps for use in the experiments reported here. The eight cases in this experiment series were performed in 2024 in the Sandia Critical Experiments (SCX) at the Sandia Pulsed Reactor Facility by varying the number of fuel rods in the critical assembly. The personnel involved in conducting these experiments included David Ames, Elijah Lutz, Beth Hanson, Patrick Ward, Jason Soares, and Augie Chapa from SNL, as well as Mathieu Dupont and B.J. Marshall from ORNL. Case 1 had no tantalum experiment rods. Case 2 had seven tantalum rods in the center of the central test region. Case 3 had 18 tantalum rods in the third hexagonal ring of the central test region. Case 4 had 19 tantalum rods in the center of the central test region. Case 5 had 30 tantalum rods in the outer most positions of the central test region. Case 6 had 37 tantalum rods in the center of the central test region. Case 7 had 61 tantalum rods in the center of the central test region. Case 8 had 85 tantalum rods filling all positions in the central test region. All eight critical experiments are judged to be acceptable as benchmark experiments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Chemical Analysis of Tantalum Thin Films compared to Niobium Thin Films using SIMS and XPS

Superconducting qubits are a leading platform for quantum computation. These circuits are typically made from superconducting materials like aluminum or niobium. However, the amorphous niobium oxide and aluminum oxide on the surface of these circuits introduce considerable RF loss due to the presence of two-level systems (TLS), which limits the maximum coherence times T1 to ~100 μs. Capping the niobium qubits with a tantalum layer leads to a 3- 5x improvement [Bal et al., 2024]. But even in this case, tantalum forms an amorphous surface oxide that introduces loss. In an effort to devise strategies to eliminate the presence of this oxide, we present a comprehensive study on the nature of Ta oxide using x-ray photoemission spectroscopy (XPS) and secondary ion mass spectrometry (ToF-SIMS) as a function of heat treatment. The thin films were annealed in ultra-high vacuum conditions and analyzed in situ to characterize the composition and evolution of the native tantalum oxide layer and oxide-metal interface. Our analysis reveals two critical differences between tantalum and niobium oxides: Nb2O5 completely dissolves at 400°C, while Ta2O5 persists even at 800°C. Additionally, tantalum oxide contains only a single suboxide (TaO), in contrast to niobium's two suboxides (NbO and NbO2). The suboxide of tantalum contributes minimally to the total oxide content and shows a relative increase with temperature. Understanding the oxide’s behavior will open new pathways for optimizing coherence times in tantalum qubits.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Supply Chain Improvement & Process Modification Printing in Tantalum

Refractory metals and alloys are distinguished by their exceptional thermophysical properties, including high melting and recrystallization temperatures, remarkable strength, and superior corrosion resistance, all of which surpass those of conventional alloys. These unique characteristics position these materials as ideal candidates for applications in extreme environments. However, their potential has historically been underexploited due to limitations in processing capabilities. Recent advancements in melt-based additive manufacturing (AM) processes present opportunities to overcome these limitations. Previous Sandia studies have successfully characterized pure tantalum produced through laser powder bed fusion (LPBF) externally at Castheon, revealing that LPBF-fabricated tantalum exhibits properties exceeding those of wrought materials. This promising outcome sparked increased interest in the internal additive manufacturing of tantalum. This project aimed to establish the new SLM 280 machine at SNL-CA to successfully produce the first tantalum prints and characterize the material. Additionally, efforts were made to enhance the machine by incorporating Inert equipment to minimize oxygen content within the print volume. The results demonstrated that internally manufactured LPBF tantalum not only met but exceeded the standards of wrought materials, even prior to the integration of the additional equipment. The inert equipment is almost successfully integrated and ready for use. Future research should focus on understanding how this equipment influences the process-structure-property relationships, as well as further optimizing the printing parameters for tantalum.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Chemical Profiles of the Oxides on Tantalum in State of the Art Superconducting Circuits

Abstract Over the past decades, superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of superconducting qubits. A major source of loss has been attributed to two level systems that are present at the material interfaces. It is recently shown that replacing the metal in the capacitor of a transmon with tantalum yields record relaxation and coherence times for superconducting qubits, motivating a detailed study of the tantalum surface. In this work, the chemical profile of the surface of tantalum films grown on c‐plane sapphire using variable energy X‐ray photoelectron spectroscopy (VEXPS) is studied. The different oxidation states of tantalum that are present in the native oxide resulting from exposure to air are identified, and their distribution through the depth of the film is measured. Furthermore, it is shown how the volume and depth distribution of these tantalum oxidation states can be altered by various chemical treatments. Correlating these measurements with detailed measurements of quantum devices may elucidate the underlying microscopic sources of loss.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Low-loss superconducting resonators fabricated from tantalum films grown at room temperature

The use of α-tantalum in superconducting circuits has enabled a considerable improvement in the coherence time of transmon qubits. The standard approach to grow α-tantalum thin films on silicon involves heating the substrate, which takes several hours per deposition and prevents the integration of this material with wafers containing temperature-sensitive components. We report a detailed experimental study of an alternative growth method of α-tantalum on silicon, which is achieved at room temperature through the use of a niobium seed layer. Despite a substantially higher density of oxygen-rich grain boundaries in the films sputtered at room temperature, resonators made from these films are found to have state-of-the-art quality factors, comparable to resonators fabricated from tantalum grown at high temperature. This finding challenges previous assumptions about correlations between material properties and microwave loss of superconducting thin films, and opens a new avenue for the integration of tantalum into fabrication flows with limited thermal budget.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Impact of Annealing on Superconducting Properties of Tantalum Thin Films

The pursuit for higher-coherence superconducting qubits has led to the adoption of new materials and innovative fabrication techniques. Among the commonly used superconducting materials, tantalum-based or tantalum-capped superconducting qubits have demonstrated the longest lifetimes, outperforming niobium devices even in the presence of their native oxides. Given the losses introduced by the surface oxide, one method to further improve performance is to reduce or eliminate the native oxide on tantalum through UHV annealing. In this work, we examine the effect of dissolved oxygen as a function of annealing temperature on the superconducting properties of tantalum thin films. Dissolved oxygen is quantified using atom probe tomography (APT) and secondary ion mass spectrometry (TOF-SIMS) and correlated with electromagnetic transport measurements performed using a Physical Property Measurement System (PPMS). This study observes that annealing decomposes the native oxide and drives oxygen diffusion into the film matrix. Increasing oxygen content decreases the superconducting transition temperature (Tc) and the residual resistivity ratio (RRR). The critical field decreases with the incorporation of oxygen, while the film’s coherence length decreases relatively by 5%. These results, as well as findings from ongoing studies linking fabrication processes to the structural and chemical properties of the superconducting film and Josephson junction, provide insight into effective strategies for improving the performance of superconducting devices.

Shinde, Maithilee [IIT, Chicago; Fermilab]↗
DOE OSTI2026

Effect of directed energy deposition process parameter on build quality of tantalum

Tantalum is a refractory metal used in a variety of harsh environment applications. Additive manufacturing of tantalum is limited based on its high melting point and affinity for oxygen. Directed Energy Deposition is an additive manufacturing technique with rapid deposition time and compositional flexibility within builds. Additively manufactured tantalum is susceptible to a variety of material and process-based defects. Various lack-of-fusion defects were identified resulting from excessive powder feed rates or insufficient laser power. Oxygen impurities in some samples caused cracking and increased material hardness. Precipitates were identified in the highly oxidized samples which were printed immediately after the build chamber was opened. High-density, low-defect parts were successfully produced. The effects of scan speed, laser power, and powder feed rate on density and defects were analyzed. A processing window was identified for producing high-quality parts which requires adequately high laser power and lower powder feed rate.

DED↗
DOE OSTI2023

Deformation, dislocation evolution and the non-Schmid effect in body-centered-cubic single- and polycrystal tantalum

A physically-informed continuum crystal plasticity model is presented to elucidate deformation mechanisms, dislocation evolution and the non-Schmid effect in body-centered-cubic (bcc) tantalum widely used as a key structural material for mechanical and thermal extremes. We show the unified structural modeling framework informed by mesoscopic dislocation dynamics simulations is capable of capturing salient features of the large inelastic behavior of tantalum at quasi-static (10 -3 s -1 ) to extreme strain rates (5000 s -1 ) and at low (77 K) to high temperatures (873 K) at both single- and polycrystal levels. Here we also present predictive capabilities of the model for microstructural evolution in the material. To this end, we investigate the effects of dislocation interactions on slip activities, instability and the non-Schmid behavior at the single crystal level. Furthermore, ex situ measurements on crystallographic texture evolution and dislocation density growth are carried out for polycrystal tantalum specimens at increasing strains. Numerical simulation results also support that the modeling framework is capable of capturing the main features of the polycrystal behavior over a wide range of strains, strain rates and temperatures. The theoretical, experimental and numerical results at both single- and polycrystal levels provide critical insight into the underlying physical pictures for micro- and macroscopic responses and their relations in this important class of refractory bcc materials undergoing large inelastic deformations.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effect of an oxide layer on high velocity impact of tantalum particles characterized using molecular dynamics

Micro-cold spray, also commonly referred to as the aerosol deposition method or vacuum kinetic spraying, is a process for depositing nanostructured films of metals and ceramics by impacting solid nanoparticles at high velocity onto a substrate at room temperature and at low pressures. Tantalum films deposited by micro-cold spray are desirable because tantalum is non-reactive to certain molten metals and can therefore serve as a non-reactive barrier coating. However, unlike most other metals deposited by micro-cold spray, tantalum quickly forms an oxide layer when exposed to air. In the deposition of films using traditional cold spray with much larger particles, native oxides must fracture for particle–substrate bonding to occur. Furthermore, the fraction of the particle that is oxidized in micro-cold spray and the strain rates experienced by the particles upon impact are both much larger than in cold spray, suggesting that the role of the oxide on film deposition may be distinct. The effect of this oxide layer on particle deformation and adherence to a substrate under high strain rates is studied using molecular dynamics simulations. The effect of oxide layer thickness, particle diameter, and impact velocity are examined.

42 ENGINEERING↗
DOE OSTI2023

Direct Visualization of Charge Migration in Bilayer Tantalum Oxide Films by Multimodal Imaging

Inspired by biological neuromorphic computing, artificial neural networks based on crossbar arrays of bilayer tantalum oxide memristors have shown to be promising alternatives to conventional complementary metal-oxide-semiconductor (CMOS) architectures. In order to understand the driving mechanism in these oxide systems, tantalum oxide films are resistively switched by conductive atomic force microscopy (C-AFM), and subsequently imaged by kelvin probe force microscopy (KPFM) and spatially resolved time-of-flight secondary ion mass spectrometry (ToF-SIMS). These workflows enable induction and analysis of the resistive switching mechanism as well as control over the resistively switched region of the film. In this work it is shown that the resistive switching mechanism is driven by both current and electric field effects. Reversible oxygen motion is enabled by applying low (<1 V) electric fields, while high electric fields generate irreversible breakdown of the material (>1 V). Fully understanding oxygen motion and electrical effects in bilayer oxide memristor systems is a fundamental step toward the adoption of memristors as a neuromorphic computing technology.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Anomalous elastic behavior of tantalum at high pressures: Experimental and theoretical studies

Elastic wave velocities of polycrystalline tantalum have been measured up to 13.6 GPa at room temperature using ultrasonic interferometry technique in a multi-anvil apparatus. The bulk and shear moduli, as well as their pressure derivatives are obtained using Eulerian 3rd order finite strain equations, yielding K S0 = 193.9(29) GPa, G 0 = 69.3(10) GPa, K S0' = 3.18(5) and G 0' = 0.88(1). Here the Steinberg-Guinan yield stress model is examined using the current shear modulus and its pressure derivative, where a slightly lower A 0 = 0.0127 GPa –1 compared to previous studies is obtained. First-principles calculations on the elasticity of tantalum have also been performed up to 200 GPa. Anomalous softening of the elastic shear constant C 44 and shear wave velocity V S is observed between 80– 180 GPa, whereas the anisotropy shows a softening-stiffening behavior with increasing pressure and reaches a minimum at around 150 GPa. Such anomalies might commonly originate from the nesting of Fermi surface for Group VB metals.

36 MATERIALS SCIENCE↗