**Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy**

**Caitlin A. Taylor 1, Samuel Briggs 1,2, Graeme Greaves 3, Anthony Monterrosa 1, Emily Aradi 3, Joshua D. Sugar 4, David B. Robinson 4, Khalid Hattar 1,\* and Jonathan A. Hinks <sup>3</sup>**


Received: 2 July 2019; Accepted: 13 August 2019; Published: 16 August 2019

**Abstract:** Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, bubble diameter remained constant throughout the implantation, but bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the bubble nucleation and growth behavior.

**Keywords:** in-situ; helium implantation; environmental transmission electron microscopy; palladium tritide

### **1. Introduction**

Helium is insoluble in almost all solids and precipitates into nanometer-sized bubbles that can result in mechanical property degradation and eventually fracture. At very high He concentrations (tens of atomic percent), micrometer scale blisters can form, resulting in exfoliation, gas release, or both [1,2]. Bubble nucleation and growth are sensitive to most environmental conditions, including material composition, crystal structure, and temperature.

Palladium-based materials are under consideration for many applications [3], including H2 purification, storage, and detection, as well as fuel cell catalysis, due to its ability to easily dissociate molecular H2 on its surfaces and incorporate H atoms into octahedral sites in its face-centered cubic (fcc) crystal structure as a metal hydride [4–6]. One of these applications is solid-state tritium storage, where 3H will decay to 3He with a half-life of 12.3 years, causing rapid accumulation of 3He in the Pd lattice. Studying 3He evolution in PdTx is difficult due to the safety constraints of radiological work, though some microscopy has been performed investigating early stage 3He bubble formation (less than one year of aging) [7–9]. Over these short aging times, 3He bubbles reached 1–2 nm in diameter. While He ion implantation has been utilized as an accelerated aging method to study blister formation in Pd metal at high doses [1,2], He implantation into Pd hydride is difficult in most facilities because a constant H2 over-pressure is required to maintain the hydride structure. If the over-pressure is removed, most H will diffuse out of the material [4–6].

New capabilities in in-situ ion irradiation allow for direct observation of He bubble nucleation and growth as a function of He implantation dose and temperature [10]. In the new Microscope and Ion Accelerators for Materials Investigations (MIAMI-2) facility at the University of Huddersfield [11], in-situ He implantation has been combined with Environmental Transmission Electron Microscopy (ETEM), which allows for imaging in the presence of milliTorr H2 pressures. We have used this combination of capabilities to investigate whether the presence of H2 affects the nucleation and early growth of bubbles. We performed much of the work at sub-ambient sample temperatures to increase the H solubility in the sample, but did not observe formation of the concentrated, β-phase Pd hydride, which is difficult to characterize with electron diffraction techniques. α-phase Pd hydride is expected to have formed to some degree under the experimental conditions, but cannot be properly identified using electron diffraction due to its characteristic minute change in lattice parameter Thus, cryogenic temperature likely dominated the observed He bubble nucleation and growth kinetics.

### **2. Materials and Methods**

#### *Specimens and Irradiation Treatment*

Palladium wire was purchased from Alfa Aesar (Alfa Aesar, Haverhill, MA, USA) and was annealed prior to TEM sample preparation to cause pre-existing voids identified near the surface to coalesce into larger voids that could not be confused with He bubbles. The wire was annealed at 700 ◦C for 1.5 h in an evacuated quartz ampoule with a base pressure of 1 <sup>×</sup> 10−<sup>7</sup> Torr at the time of sealing. TEM sample preparation was done using the Focused Ion Beam (FIB) method with a FEI Helios Nanolab 660 (ThermoFisher Scientific, Hillsboro, OR, USA). The resulting lamellae were mounted on Mo FIB grids and thinned to electron transparency, with final cleaning steps utilizing a 5 kV accelerating voltage. Samples were then transported and imaged in the Hitachi H-9500 ETEM (Hitachi High-Technologies, Tokyo, Japan) at the MIAMI-2 facility (University of Huddersfield, West Yorkshire, UK) [11]. Unless otherwise stated, all TEM imaging was conducted in a Bright Field (BF) imaging condition with an accelerating voltage of 300 kV. Initial TEM imaging showed a high density of defects, likely resulting from either the FIB procedure or the original wire extrusion process. Since pre-existing defects will affect 4He bubble nucleation and hydride formation, the specimens were annealed at 400 ◦C for one hour in vacuum using a Gatan Model 652 double-tilt heating holder (Gatan, Pleasanton, CA, USA) in an attempt to reduce defect density.

Thermodynamic calculations [12], shown in Figure 1a, were used to estimate the temperature required to hydride Pd at ETEM relevant pressures (on the order of 10−<sup>2</sup> Torr). The α-phase has been characterized by a slight unit cell expansion from the fcc Pd lattice of 3.88 Å to 3.89 Å when H/Pd = 0.03. As H2 content increases, a new set of fcc lattice reflections form, corresponding to the β-phase, which has a cell constant of 4.02 Å when the α→β transformation is complete (H/Pd~0.57) [13]. This corresponds to a 10% volume expansion, or a 3.6% lattice parameter expansion, compared to Pd metal.

**Figure 1.** Experimental parameters, including: (**a**) thermodynamic calculations showing when H2 is expected to absorb and desorb from pure Pd as a function of temperature and pressure, and (**b**) SRIM prediction, shown for a fluence of 1017 ions/cm2, of implantation depth, damage dose, and 4He concentration for 10 keV 4He into Pd at 18.7◦. Lines are meant to guide the eye in (**b**).

We expect concentrated hydride, or β-phase, to form below the "absorption" line in Figure 1a, and the dilute, or α-phase, to form above the "desorption" line at a given pressure as the temperature is increased. The crystal structure is expected to remain fcc in all cases. The region between the "desorption" and "absorption" lines in Figure 1a consists of α + β-phases [5,6]. These calculations do not include kinetic aspects of hydride formation, which are not well documented for Pd at cryogenic temperatures and may influence the achievement of the hydride phase and final stoichiometry. Furthermore, the thermodynamic data are extrapolated, and are potentially sample-dependent, so we consider Figure 1a to be only an approximate guide.

Samples were cooled to −100 ◦C in the ETEM using a Gatan Model 636 (Gatan, Pleasanton, CA, USA) liquid nitrogen-cooled cryogenic holder. Thermodynamic calculations (Figure 1a) show that the pressure must be above 6.4 <sup>×</sup> 10−<sup>4</sup> Torr to form concentrated <sup>β</sup>-phase at <sup>−</sup>100 ◦C. However, experimental isotherms show that the concentrated <sup>β</sup>-phase forms above ~1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> Torr at <sup>−</sup><sup>196</sup> ◦C [13]. An H2 atmosphere was introduced locally to the specimen. Local specimen pressure was maintained at between 7.5 <sup>×</sup> 10−<sup>3</sup> and 2.3 <sup>×</sup> 10−<sup>2</sup> Torr while BF imaging video data were collected. Specimen pressure was actively throttled to prevent electron gun pressure from rising to the trip point for the gun valve to close (3.8 <sup>×</sup> 10−<sup>4</sup> Torr). Initial experiments were conducted without an ion beam to observe potential microstructural changes associated with the formation of Pd hydride. Palladium hydride formation is characterized by a unit cell expansion [13], so electron diffraction patterns were recorded and utilized to measure the lattice strain in an H2 environment at different temperatures. Gas pressure was maintained for 30 min at −100 ◦C before temperature was increased to −60 ◦C. Temperature was held there for approximately 20 min before being increased to −20 ◦C and held for an additional approximately 10 min. Temperature was then reduced back to −100 ◦C to maximize H2 solubility and held for 20 min before beginning He implantation.

Helium implantation was performed using a gas-fed Colutron G-2 ion source (Colutron, Boulder, CO, USA) operating at an accelerating voltage of 10 kV. The 4He ion beam had been aligned prior to Pd sample loading using a custom-built Faraday stage. Ion beam intensity was measured to be approximately 4.0 <sup>×</sup> 1013 ions/cm2/s at the beginning of irradiation. The Pd sample was irradiated in the H2 environment for 42 min to a final nominal 4He fluence of 1017 ions/cm2, all with concurrent collection of BF video data. Ion beam current was monitored throughout via a skimming cup and was observed to drop by only ~3% between the beginning and end of specimen irradiation. Once this final fluence was accomplished, BF images and diffraction patterns were collected from various locations on the specimen. The Monte-Carlo based SRIM code [14] was used to simulate material damage and ion implantation for the given irradiation conditions (Figure 1b). SRIM calculations were performed using an incident 4He ion beam at an incoming angle of 18.7◦, impinging on Pd metal following the procedure given by Stoller et al. [15]. Displacements per atom (dpa) was calculated using Quick Calculation mode and the phonon.txt output file. A threshold displacement energy of 34 eV [16] and a density of 11.9 g/cm3 were used.

Bubble sizes and density were determined, where possible, using ImageJ [17] analysis. Image resolution variations can affect the bubble density analysis by up to an order of magnitude. To maintain consistency, only images in the under-focus condition were used. Bubbles were confirmed using both under- and over-focus images. A sample set of under- and over-focus images is provided in a Supplemental file. Images were all converted to a 1712 × 1712 resolution (used for in-situ video) before analysis and the same procedure was utilized on all images. Image analysis procedure was as follows: (1) Gaussian Blur with radius of 3, (2) Normalize Local Contrast with radii of 20 pixels, (3) invert to make bubbles appear dark, (4) "Mexican Hat" Filter with radius of 4 or 5, (5) threshold the entire image, and finally (6) Analyze Particles of area 0–infinity and circularity set to 0.6–1. Data were exported and bubbles with less than 1 nm diameter, the approximate TEM resolution limit, were removed from the dataset. Only average bubble size is provided because the standard deviation is small, usually less than 0.2 nm.

### **3. Results**

#### *3.1. Exposure to H2 at Cryogenic Temperature*

To determine the effects of an H2 atmosphere on the Pd sample at cryogenic temperatures, a sample was subjected to an H2 environment at temperatures between −100 ◦C and −20 ◦C. As shown in Figure 2, no significant microstructural changes were observed due to H2 alone. Selected Area Electron Diffraction (SAED) patterns were taken after each step. Experimental error in the SAED was not explicitly quantified, but is expected to be large in these experiments due to slight variation in tilt angle and sample height with variation in H2 flow rate or temperature. The degree of hydride phase formation, which is characterized by a 3.6% lattice expansion in PdH0.57, was therefore unquantifiable in these experiments. Image contrast changes apparent in Figure 2 are due to the sample bending during the temperature cycles.

**Figure 2.** In-situ TEM images of the Pd sample during pre-implantation H2 exposure. Images are in sequential order and show the sample (**a**) initially, (**b**) after 25 min of H2 exposure at −100 ◦C, (**c**) during the 14 min of H2 exposure at −60 ◦C, (**d**) after 7 min of H2 flow at −20 ◦C, and (**e**) after cooling back to <sup>−</sup><sup>100</sup> ◦C in H2 for 4He implantation.
