SECAT Newsletter, Vol. 10, Issue 2

Paul F. Rottman began as a student at the University of Kentucky, receiving his B.S. in Materials Science and Engineering in May 2010. He proceeded to Johns Hopkins University, Baltimore, MD where he received an M.S. in Materials Science and Engineering in March 2012 and a Ph. D. in May 2017.

Paul was a Postdoctoral Research Assistant in the Materials Department at the University of California, Santa Barbara from 2017 – 2019. He returned to the University of Kentucky in 2019.

I am an assistant professor of materials engineering at the University of Kentucky. My research background and current scope of active projects focus on the mechanical and microstructural characterization of metals. The range of metals I study is diverse, ranging from sputter deposited high entropy alloys to additively manufactured superalloys, but I approach problems with the diverse suite of characterization tools we’ve put together in my lab and available in shared facilities at UK. I also teach various courses in our Materials Engineering curriculum each semester.

Each day is typically a blend of my teaching and research responsibilities. Currently I teach a Materials Lab course to our 2nd year students in the fall and a Metals Processing course (mostly to graduating seniors) in the spring. During a semester I’ll typically spend some time each morning preparing or giving a lecture, grading assignments, or meeting with students. Most of my time on the research side is spent on reading, writing, and in meetings. Working in the field of additive manufacturing and high entropy alloys, there is a constant flood of new studies and journal articles each week—staying up to date with current research is both necessary and time-intensive! Most days I’ll meet with a graduate student or research collaborator to discuss recent progress and results. Being surrounded by such motivated and talented students, every day is different and brings with it new and exciting challenges!

This is an incredibly exciting time to work with and research metals. When I teach students and discuss potential career paths with them, I strive to dispel the thought that metals is a mature (i.e. boring) industry in comparison with other fields of materials. There are many challenges and interesting problems to the metals community waiting to be met with a fresh perspective from the next generation of engineers that are equipped with numerous experimental and computational tools that have been developed in the last few decades.

A main focus of my research is the characterization of additively manufactured alloys. Driven by the numerous benefits that are promised by additive manufacturing, we must better understand and, ideally, control the microstructures and properties of additive parts. This is especially true for alloys whose properties are strongly contingent on the controlled precipitation and distribution of phases and defect throughout the microstructure, as is the case for many aluminum alloys. My research group combines mechanical testing and electron microscopy with a foundational understanding of metallurgical phenomena to approach these problems.

The University of Kentucky resides right in the heart of a region in which metals processing and fabrication has a history that stretches far into the past. There are numerous companies that exemplify that history by being on the forefront of the metals industry. I arrived as an assistant professor to UK aware of that history but without the knowledge of how I can link with the existing regional expertise in metals, learn from industrial partners, and ideally contribute something from my experience in researching metals. Secat, Inc. has provided critical assistance in this endeavor, since they act as a nexus that draws the vast network of the regional aluminum industry in and connects them directly to the research infrastructure at UK so that I can learn from and engage with industrial partners directly.

Furthermore, Secat, Inc. has demonstrated a strong desire to invest in the education of materials engineers at UK. For the Metals Processing course that I teach, they worked with me and provided the material and intellectual support to cast a AA3104 ingot for the students to investigate. They were excited by the opportunity to directly observe some of the concepts taught about in lecture, and I am appreciative of Secat, Inc. for making that possible.

My family has a great love for the outdoors and nature. During my time as a postdoc at UC, Santa Barbara my wife and I would take every opportunity we could to explore the national parks that were in driving distance, often toting along our infant daughter along for the hikes. Now our kids are in the “too big to carry, too small to hike long distances” stage at 1 and 3 years old, but that doesn’t keep us from puttering around the many local parks and trails throughout central Kentucky.

Huanhuan “Hannah” Bai is currently in the final year of her PhD studies. In addition to working on her Dissertation, she is working at Secat part-time. This Tech-Talk is focused on the work Hannah is conducting for her PhD.

Nano scale tungsten (W) has various medical and industrial applications and has attracted attention from academia for decades. Hannah’s PhD research includes fundamental studies of nano scale tungsten generated by Physical Vapor Deposition (PVD).

Physical Vapor Deposition is one of the most common techniques used to generate nano scale metal materials. To fabricate tungsten nano particles, a custom-built nanoparticle generator (Figure 1) was used during PVD coating of a Silicon wafer, Sapphire (Al2O3), or Copper foil substrate. The source target was 99.95% pure tungsten, with diameter of 38.1 mm (1.50 inch) and thickness of 3.18 mm (0.125 inch).

Figure 1 shows a schematic and picture of the nanoparticle generator used for deposition. The main parameters that influence nanoparticle size are aggregation length (distance between sputter target and nozzle plate) and the ratio of pressures between the two chambers. Deposition was performed at ambient temperature with a base pressure before deposition of 10-7 Torr. During deposition, the pressures in the condensation chamber and deposition chamber were 0.9 Torr and 10-3 Torr, respectively, using argon as the process gas. Sputtering power of 125 W was applied for 40 min. The aggregation length was 15 cm.

Once the fabrication procedure is completed, sample morphology and composition were examined with advanced characterization techniques at the University of Kentucky’s Electron Microscopy Center (EMC). Analysis techniques included: Focused Ion Beam Scanning Electron Microscopy (FIB-SEM), x-ray Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscope (TEM). Figure 2 shows TEM results of deposited nano scale tungsten.

Heating experiments were performed on the samples that exhibited the desired thickness and structure. The heating experiments were operated in a Quanta SEM chamber and in a custom-built Ultra High Vacuum (UHV) Chamber.

During heating, nano scale tungsten (W) reacts with oxygen (O2) as following:

W(s) + O2(g) ↔ WO2 (s) (1)

WO2 (s) + ½ O2(g) ↔ WO3 (s) (2)

3WO3 (s) ↔ (WO3)3 (g) (3)

Interestingly, when tungsten nanoparticles were heated at 1100 ℃, under a pressure of 10-7 torr for 1-hour, continuous nano porous structure particles grew into separated, larger particles, As shown in Figure 3. On the other hand, when they were preheated at 700 ℃, 0.5 ~ 1 torr for 20 mins, then were annealed at 1100 ℃, under a pressure of 10-7 torr for 1 hour, tungsten particles became highly faceted.

Although highly faceted particles were obtained after the heating procedure, there were still some un-faceted nano particles remaining on the substrate, which implies that additional research is needed to fully convert the morphology of the particles. Therefore, future work will focus on optimizing parameters to improve the uniformity of the tungsten nanoparticles.

In conclusion, the result above revealed that morphology of the tungsten nano particles vary as heating condition (pressure, oxygen partial pressure, and temperature) changes. This provides an effective way of growing and tailoring the shape of tungsten nano particles without the use of chemical reactions. Moreover, since morphology plays a critical role in the properties of materials, such as catalytic activity, thermal stability, and surface energy, the results herein are meaningful for the design of surface sensitive materials.

Figure 1 schematic of nanoparticle generator

Figure 1 picture of nanoparticle generator

Figure 2 TEM results of deposited materials which showed a nano porous structure.

Figure 3 Morphologies of nano tungsten particles under different heating condition