Sandia’s Explosives Technology Group discovers key detonation behavior in common explosive
The explosive PETN (pentaerythritol tetranitrate) has been around for a century and is used by everyone from miners to the military, but it took new research by Sandia to begin to discover key mechanisms behind what causes it to fail at very small scales.
“Despite the fact explosives are in widespread use, there’s still a lot to learn about how detonation begins, and what properties of the explosive define the key detonation phenomena,” says Alex Tappan (2554), who has been with Sandia for 14 years, all of it in the Explosives Technology Group.
Explosives are typically studied by pressing powders into pellets; tests are then done to determine bulk properties. To create precise samples to characterize PETN at the mesoscale, the researchers developed a novel technique based on physical vapor deposition to create samples with varying thicknesses. That allowed them to study detonation behavior at the sub-millimeter scale and to determine that PETN detonation fails at a thickness roughly the width of a human hair. This provided a clue into what physical processes at the sub-millimeter level might dominate the performance of PETN.
Years of work went into the process, says Alex.
The idea is that by understanding the fundamental physical behavior of an explosive and the detonation process, researchers will be able to improve predictive models of how explosives will behave under a variety of conditions.
Getting a handle on the variables
Right now, “if we want to model the performance of an explosive, it requires parameters determined from experiments under a particular set of test conditions. If you change any of the conditions, those models we have for predictions don’t hold up any more,” says Rob Knepper (2554).
Physical vapor deposition works like this: Researchers put PETN powder in a crucible inside a vacuum chamber and heat it so the PETN sublimes or evaporates. Above the crucible is a flat substrate of plastic, ceramic, or metal, and the PETN vapor deposits on that, producing explosive films.
Such pristine samples allow the team to study the initiation and detonation behavior of explosives, Alex says.
“By varying deposition conditions, we’re starting to get a handle on how the deposition conditions affect the microstructure and how microstructure affects detonation behavior,” Rob adds.
The tests use less explosive than what’s inside a .22-caliber bullet, and researchers wearing safety glasses and ear protection can stand next to the experiment in a protective enclosure, Alex says.
“A typical experiment weighs about a tenth of an aspirin tablet,” he says. “If that tablet is 325 milligrams, we’re shooting about 32.5 milligrams. These are not huge.”
The team did multiple shots to determine at what point detonation fails.
“As size [thickness] decreases further and further, at some point the detonation will slow down and eventually fail,” Alex says.
His interest in the subject goes back to when he and his brother as kids fostered each other’s interest in fireworks and explosives. Alex, a chemist, became involved in Sandia’s projects through an interest in collaborations and because of a mentor, Anita Renlund, a senior scientist with the Explosives Technology Group who retired in 2008. Rob, whose background is in materials science, began working for Sandia in 2009 as a postdoctoral appointee working with Alex. Rob later moved to the Labs’ regular staff, continuing on many of the same projects.
Alex, Rob, and co-authors Ryan R. Wixom, Jill C. Miller, Michael P. Marquez, and J. Patrick Ball presented a paper at the 14th International Detonation Symposium in Coeur d’Alene, Idaho, in 2010. They wrote in the paper, “Critical Thickness Measurements in Vapor-Deposited Pentaerythritol Tetranitrate Films,” that the work represented the first highly resolved measurements of detonation failure in high-density PETN.
Work began as an LDRD project
It adds new information for a very old explosive.
“What we brought to the table is a new experiment that allowed samples to be made that are small enough to measure this critical thickness property,” Alex says. “Other research has been done on PETN in a different form or when it had a binder added to it. This is the first time these data have been done on the critical detonation geometry for pure, high-density PETN.”
In the past, diameter information was obtained through experiments using high-aspect-ratio cylinders of pressed pellets of differing diameters. But it’s difficult to press pellets with diameters smaller than 1 to 2 mm with precise density.
The work began under a three-year Laboratory Directed Research and Development project that ended in 2001. It’s now funded largely through a combination of internal and external programs.
The research falls under the umbrella of Sandia’s Microenergetics Program, which Alex says uses novel techniques to produce small-scale explosive samples to study ignition, combustion, and detonation phenomena. It began as a collaboration among researchers in the Explosives Technology Group 2550, Manufacturing Process Science and Technology Group 1830, Engineering Sciences Center 1500, and Microsystems Science and Technology and Components Center 1700.-- Sue Major Holmes
Greg Nielson named to Popular Science Brilliant 10 list
by Neal Singer
Greg Nielson (1719) has been selected by Popular Science magazine for one of its 2012 “Brilliant 10” awards — “a roundup of the 10 most promising young scientists working today [in North America].” Winners of this designation have gone on to win the Fields Medal (considered the Nobel prize of mathematics) and MacArthur ‘genius’ awards, according to a congratulatory note from the magazine’s editor.
Greg, a former Truman Fellow, was selected for helping lead the Sandia effort to create solar cells the size of glitter. Said Sandia Labs Director Paul Hommert, “This recognition of Greg’s groundbreaking contributions is testimony to his innovative spirit. It also reflects our broader Laboratory commitment to nurturing outstanding scientific achievement.”
Said Steve Rottler, Science and Technology VP 1000 and Chief Technology Officer, “This award confirms what those of us who work with Greg already know — he is an incredibly gifted engineer who is developing an innovative solution to a complex challenge facing society. We are very proud of Greg and the accomplishments of his team.”
The Sandia application sent in support of Greg and his team pulled no punches: “Greg Nielson and his team have created a new class of photovoltaic technology. The tiny pieces, each the size of a piece of glitter, sharply contrast with the so-called ‘bricks’ used by the photovoltaic industry. The microscale nature of the solar cells — each about the width of a human hair, and easily formed in the hundreds of thousands by widely used computer-chip fabrication techniques — offer significant benefits not available with traditional large-scale solar cells. The unique approach converts sunlight to electricity more efficiently. It increases the total power output available per unit area. It significantly lowers the cost for solar power. Most remarkably, the cells can be built into flexible products like tents, bags, or clothing, or embedded directly into more sturdy structures to become the outer shell of cell phones, tablets, or laptops. This is because the tiny units can be formed into three-dimensional structures with very sharp curves and corners, yet still be made of high-efficiency photovoltaic cells. No other PV technology possesses this capability.”
A paradigm-shifting success?
Support for Greg’s nomination came from a variety of sources external to Sandia.
Joseph H. Simmons, professor of Optical Sciences and of Materials Science and Engineering at the University of Arizona, wrote, “I have seen many photovoltaic . . . technologies. Sandia’s microscale photovoltaics are one with the most innovative approach and the best chance for a paradigm-shifting success.”
Wrote Stephen J. Fonash, Kunkle Chair Professor of Engineering Sciences at Pennsylvania State University, “I have seen few truly new visions for improving solar cell costs and efficiency; Sandia’s microscale photovoltaics is the most recent one I place into this special category.”
Wrote Jeffrey H. Hunt, an American Physical Society and Boeing Technical Fellow, “[Solar] applications to mobile ground units, airborne platforms, and space assets continue to depend on engineering the power to fit the system, rather than logically fitting the power to the application requirements. . . . The glitter cells are the only technology capable of bridging this important technical gap.”
Indications that Greg was a high achiever came early. At the age of 7, he had read all the children’s books in the Bountiful, Utah, public library. His mother, a strong supporter of education, talked the librarian into granting Greg an adult card so he could get more information on the insects and birds he saw around him.
His dad, now retired and working five hours a week at the Ace Hardware in Bountiful, is so proud of his son’s achievements that he relays news of them to store customers.
Supportive parents were supplemented by one of Greg’s high school classes, taught by a millionaire physics teacher who had made his money by inventing a machine that heated pop bottles and stretched them at state fairs. “He taught for the fun of it,” Greg says, “and every day or so he’d make up some experiment.”
The experiments included blowing up a car battery, filling a 15- foot-diameter weather balloon with helium and letting it fly upward, and rolling bowling balls off the school roof in attempts to hit a target below.
“The experiments wouldn’t have been OSHA-approved, but they influenced me significantly,” says Greg.
‘I love coming up with a solution’
Because of the influence of his physics teacher, Greg signed up for mechanical engineering when he went to college at Utah State. “I’m not that interested in discovering new scientific theory,” he says. “It’s when I hear of a problem that needs an engineering solution that my mind goes crazy. I love coming up with a solution.”
At Utah State, a telecommuting Sandian suggested Greg apply to Sandia for a summer internship. He worked for two summers under Rob Leland in the CUBIT group, doing software for mesh generation. It didn’t hurt his application that he was already — though still an undergraduate — working as a Utah State teaching assistant and also as a research assistant, testing rockets that combined solid fuel with liquid/gas oxidizers. The hybrid process enabled rocket propulsion mechanisms to be turned on and off, rather than burn without pause to the end of their solidfuel lives.
“The college didn’t have a lot of graduates, so they took advantage of undergraduates,” Greg says modestly.
He travelled to the Massachusetts Institute of Technology for his master’s and doctoral degrees. The degrees were in mechanical engineering, but because he was working under a thesis advisor interested in volume holography (creating a holographic lens to provide more information than could a physical lens under certain circumstances), he took courses in physics, mechanical and electrical engineering, and materials science, and ended by doing a thesis on optical micro- and nanostructures combined with MEMS (microelectromechanical systems). After completing a PhD, Greg was selected in the first crop of Sandia Truman Fellows.
“I had ideas about optical switching for MEMS devices I was excited about,” he says. “Sandia’s MEMS facilities seemed a fine site to work out the technology.”
But an unexpected conversation with Vipin Gupta (6124) changed Greg’s direction. Vipin had called Greg by mistake, looking for another Truman Fellow. “Vipin is willing to listen to people with crazy ideas,” Greg said. “It was extraordinary luck that we crossed paths.”
Greg at the time had envisioned a nonphotovoltaic way of converting sunlight into electricity through use of the piezoelectric effect. (When a piezoelectric material changes size, it creates a voltage.) Though he still thinks the premise could work, after many conversations with Vipin and others he settled on the more immediately practical idea that current solar photovoltaic generators “were using a lot of silicon they didn’t need to. With my background in microsystems, I saw they could save by a factor of 10 in materials cost.”
He credits his volunteer experience leading youth groups, Boy Scouts, and a church congregation for his ability to provide “steady pressure,” as he puts it, to move his projects forward.
“Another help,” he says, “is that many people feel strongly about solar and gravitate toward our project. They believe they are making a difference and I believe they are.”
Among those he cites for their helping in the earliest stage of the project, in addition to Vipin, are Jeff Nelson (1131), Murat Okandan, and Jose Luis Cruz-Campa (both 1719). “But there are many more on the team today.” Popular Science is expected to release its list of its 2012 selectees in its October issue, available in late September, along with sketches of the winners and their achievements.
Greg’s photovoltaic work has been supported by DOE’s Solar Energy Technology Program and Sandia’s Laboratory Directed Research & Development program.-- Neal Singer
Steve Castillo named HENAAC’s 2012 Engineer of the Year
by Bill Murphy
It wasn’t inevitable that Steve Castillo would become an engineer. But it wasn’t surprising, either, given his upbringing. It’s not surprising, either, given his upbringing, that he has moved to the very top of his chosen profession. Steve has been named the HENAAC 2012 Engineer of the Year, the highest accolade presented by HENAAC, the Hispanic Engineering National Achievement Awards Conference.
Steve joined Sandia in 2011 as manager of ISR Systems Engineering & Decision Support Dept. 5346. His immediate previous job was as executive vice president of the Colorado School of Mines. Before signing on at the Labs, Steve spent most of his accomplished 24-year career in academia, in jobs of increasing responsibility. While Sandia has been a new and different professional environment for Steve, he has found he likes the change.
“I really enjoy the demanding, fast pace of the work in [Airborne ISR Systems Group] 5340, the high quality of the technical staff I get to work with, and the tremendous national security mission impact our systems have,” he says.
Steve grew up in Belen, N.M., where the roots from both sides of his family reach back 300 years. For most of those centuries, his forebears made their living by farming and ranching the rich, wellwatered soil of the Middle Rio Grande Valley.
Steve’s grandfather, Alejandro Castillo, was the first in that long line to go to college, graduating with a degree in education, after which he spent 30 years as a teacher in the small community of Casa Colorada, south of Belen. Alejandro passed on his love of education to his own children; it was a lesson that stuck. Alejandro’s son — and Steve’s father — Philip, spent a long and successful career as a PhD electrical engineer in Los Angeles and Albuquerque. Philip’s influence, anchored by the support of Steve’s mother, clearly rubbed off on Steve and his five siblings, all of whom have gone on to successful professional careers.
College was not optional
In the Castillo household of Steve’s youth, discipline was strict but fair and education was not just valued; it was revered. In his home, Steve recalls, “going to college was not an option; it was a requirement.”
It was during his senior year in high school that Steve definitively resolved to become an engineer, a decision his parents very much supported. Steve knew the academic demands would be great but he was wellprepared; his family had moved back onto a farm in Belen and all the Castillo kids had chores to do, and not easy ones either. They all learned the meaning and value of hard work.
Steve credits his parents with being the biggest positive influence in his life, but there were other influential and inspirational adults in his life, too.
“In addition to my parents,” Steve says, “there were a few teachers that inspired me — my 5th grade school teacher, Leona Brown, a 7th grade civics teacher, Steven Prentice, and my high school college algebra teacher, Mr. Zamora.”
In his later career, Steve would remember — and act on — how important those positive role models were at a formative age.
After graduating from Belen High School in 1977 as a National Merit Scholar finalist and a member of the National Honor Society, Steve was accepted to several good engineering schools, but it was a personal letter from a college close to home that sealed the deal for him. The dean of the engineering school at New Mexico State University in Las Cruces, John Hernandez, heard about Steve and recruited him the way big time sport schools recruit top athletes.
Steve’s decision to go to Las Cruces right out of high school was the beginning of a lifelong relationship with NMSU. After earning his Bachelor of Science degree in electrical engineering at the school, he spent a year and a half at the AT&T Bell Lab facility in Denver. He subsequently moved on to earn a Master of Science degree and doctorate in electrical engineering at the University of Illinois at Urbana.
A consequential phone call
With his PhD in hand, Steve had a wide-open field of options. He interviewed for research positions at several government and private sector laboratories, but again, it was a personal intervention — a phone call this time — from Las Cruces that settled the issue for him. Steve’s NMSU undergraduate mentor, professor Gerry Flachs, suggested that he consider a teaching position at the university. It was something that had never really occurred to Steve, but he became intrigued by the idea and took a tenure-track position that set the direction of his career for the next 24 years.
First as a professor, then as an administrator, Steve became a major influence at NMSU’s engineering school. He taught more than 3,000 students, and graduated eight PhD and 22 MS electrical engineers, all while remaining deeply involved in research. He was lead author or contributor on scores of technical papers, focusing on the areas of his technical interests, which include electromagnetic theory, electromagnetic interference problems, numerical solution of electromagnetic problems, high performance computing, and computational linear algebra.
After years in the classroom as professor and department head, Steve became dean of the NMSU College of Engineering in 2004. In that role, he was successful in helping shape the school’s program direction and in raising more than $120 million in cash and in-kind gifts. Those resources helped establish several new endowed faculty positions and increased scholarship opportunities.
Steve is the recipient of many honors and awards for his professional accomplishments and his community service. His greatest reward, he says, is the opportunity he has had to work with and be a positive influence for his students. “I am very proud of the many young people I touched throughout my career that have gone on to become outstanding professionals and citizens,” Steve says. “Whether it was through a talk at a local high school, a student in a class or a student organization that I taught or mentored, or graduate students that I advised, I feel like I played at least some role in helping to shape their future. Even here at Sandia, I have run into many former students who are doing very well.”
Although he has left the classroom, Steve has not left behind his desire to be a positive role model and mentor. When he meets with young people today through community service, he encourages them to consider careers in STEM — science, technology, engineering, and math.
Says Steve, “I tell any young person that a career in STEM will give them the opportunity to be involved in the creation of wealth and a better standard of living for our society or even provide for the security of our country, and at the same time, pay them well enough to enjoy a comfortable lifestyle.
“A STEM career has the potential for making their daily work lives enjoyable because of the many ‘geewhiz’ moments that occur on the job in engineering and science professions. I would also tell them that the keys to a successful career in STEM are the mathematics, science, and communication skills they must obtain before they go on to college.”
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Steve will be presented HENAAC’s Engineer of the Year award at the organization’s annual conference in Orlando Oct. 11-13.-- Bill Murphy