ALD of Hydrogen Diffusion Barriers

Sarah Bull – University of Colorado Boulder


Tungsten nitride and boron nitride were analyzed experimentally and computationally as barriers for hydrogen diffusion using ALD and density functional theory.


0:00:02   Hello, everyone, and thank you for attending my talk on Atomic Layer deposition or a L. D of boron
0:00:07   nitride as a hydrogen environmental barrier coating. My name is terrible and I'm coming to you from
0:00:12   Alan Wimmer's group at the University of Colorado, Boulder. I want to focus on one application, in
0:00:18   particular of these ale, the environmental barrier coatings on that's for use in nuclear thermal
0:00:22   propulsion or NTP engines. These engines are of interest because they can achieve twice the specific
0:00:28   impulse of our best chemical combustion engines, thereby having trends at times and making further
0:00:33   destinations such as a journey to Mars. More feasible possibility. These engines work by pumping
0:00:39   liquid hydrogen into a nuclear reactor core where energy released from vision is used to heat that
0:00:45   hydrogen to 2700 Calvin before pumping it out of a supersonic nozzle to generate thrust. Inside the
0:00:53   nuclear reactor core house thes sir met fuel elements. Sermet refers the fact that they're both a
0:00:57   ceramic uranium oxide and a metallic the tungsten matrix. These Dermot fuel elements are meter in
0:01:04   length and contain 123 millimeter hydrogen flow channels throughout the length. As you would imagine,
0:01:10   when you subject these materials to hot hydrogen flow issues arise as you can see from the image on
0:01:15   the left after just three hours of testing in hot hydrogen flow. But 2000 Kelvin the outside coding
0:01:21   His craft on the fuel, um in itself is fractured, and this is due to multiple causes, including
0:01:26   chemical reaction of uranium and hydrogen to form uranium hydroids thes hydride. They're less dense
0:01:32   than the surrounding material, so they will expand volumetric Lee and exert pressure on the
0:01:36   surrounding matrix that's releases plastic deformation and fracture. In addition, vacancies in the
0:01:43   tungsten lattice contract up to 12 hydrogen atoms each. These hydrogen atoms will interact and
0:01:48   recombined for bubbles, which, much like the hydroids, will expand and exert pressure on the
0:01:54   surrounding material. Other defects, such as grain boundaries and dislocation cores, will also lower
0:01:59   the ideal fracture energy and make thes substrates more susceptible to hydrogen embroilment. In
0:02:05   addition, high concentrations of hydrogen grain boundaries can stabilize surfaces and lead to extra
0:02:11   thermic cleavage by lowering the inter facial grain boundary strength.
0:02:18   One possible solution is to prevent the interaction of hydrogen with the sermon do. Actually, this
0:02:24   idea is not new. However, it has previously been investigated the use of chemical vapor deposited,
0:02:28   or CBD, taxed in cote coated particles. However, as you can see here, the CBD tungsten coding is
0:02:37   nonconforming and contains low energy diffusion pathways for hydrogen still to defuse into an attack
0:02:43   the substrate. In addition, CVD is not self limited, so it cannot be used on the outside of the Sir
0:02:49   Matt fuel elements, um, as it would clog the entrances to these hydrogen flow channels without
0:02:55   coding the length. However, we hypothesize that switching away from CBD and to L. D will allow us to
0:03:02   deposit can formal and pinhole free environmental barrier cove ing on alleviate hydrogen in brittle
0:03:07   mint in addition to coating around the field particles before they're in bed in the Matrix. We can
0:03:13   also use a lady to coat the outside of the sermon elements within the hydrogen flow channels. And
0:03:18   this is due to the self limited nature of ale de where we can see here from the image on the right,
0:03:23   uh, FNL decoding within that high aspect ratio poor shown by Becker at all.
0:03:32   We wanted to first investigate this hypothesis through use of depositing theory legacy material of
0:03:37   tungsten. And to do this, we used by turbulent, you know, by dimethyl amino, tungsten and ammonia.
0:03:43   This deposits tungsten nitride. However, because tungsten night dried is not a thermodynamic lee
0:03:48   stable material, we can easily and yield the coding in order to remove nitrogen on leave behind a
0:03:54   crystalline tungsten coding. To do this, we use the food eyes bed. It will be set up shown here
0:03:59   where we have our tungsten containing precursor in a bubble ammonia in a lecture bottle on. We dosed
0:04:06   these alternating into the fluid eyes bed where we, um where we contain our Petraeus stabilizer
0:04:13   konia substrate 300 degrees Celsius and use a mass spectrometer in order to analyze the species in
0:04:19   the outlet stream. Using the set up, we coated the Y C microspheres the 65 and 100 l d cycles. After
0:04:29   coding we used f i b milling of the thickest coding of 100 l d cycles followed by E. D. S, in order
0:04:36   confirmed the presence of tungsten within that coating. After confirming the deposition of a
0:04:42   tungsten code containing coding, we use differential thermal analysis in order to investigate the
0:04:49   effect of hydrogen on our sample. So shown here is the DT a result with exo thermic peaks shown in
0:04:55   the downward direction, and I've highlighted the three exo thermic peaks that are present within
0:04:59   this temperature range from 0 to 1600 Celsius we tested are three samples uncoated 65 100 ale
0:05:07   decoded. You dress stabilizer Kony and 6% hydrogen and argon, as well as our thickest coding of 100
0:05:14   cycles and 100% and are are gone environment in order to determine which keeps air due to hydrogen
0:05:20   versus increased thermal energy. The peaks, highlighted in pink and blue, are due thio um,
0:05:28   decomposition and crystallization of our coding and these values of 550 around 800 degrees Celsius
0:05:35   correlate well with literature where we see an initial decomposition of the tungsten nitride in
0:05:40   orderto leave behind a 2 to 1 Texans nitrogen strike geometry around 800 degrees Celsius. We have a
0:05:46   final decomposition in order to leave behind a pure crystalline tungsten coding beyond 800 degrees
0:05:53   Celsius. Our environmental barrier coating is no longer tungsten nitride but pure tungsten.
0:05:59   Highlighted in green is our hydrogen reaction with the sample we see here that as we increase our
0:06:06   cycle coding are the code the cycle count for our coding from 0 to 6500 l D cycles. We also increase
0:06:14   that peak reaction temperature from 11 99 to 12. 14 to 12 37 degrees Celsius. So we have confirmed
0:06:21   our hypothesis that we can deposit a coding with L. D in order to reduce some of the hydrogen
0:06:26   embroilment. However, because the NDP temperature is closer to 2500 degrees Celsius, we're still far
0:06:33   from that operation temperature, so two possible solutions exist. One is to increase film thickness
0:06:39   on the others. To change ABC material toe one with a lower hydrogen, diffuse it Vitti and thereby
0:06:45   still stay within those low cycle accounts. Um, in order to take advantage of the higher activation
0:06:50   energy for hydrogen diffusion,
0:06:54   we chose to look into the second possible. The second solution and that was because this ale de
0:07:00   chemistry deposit, tungsten nitride, is extremely slow. It's about two hours per cycle as well as a
0:07:06   slow growth rate per cycle. A swell.
0:07:11   Therefore, in order to positive thicker film, we would have to switch our ale de chemistry, so we
0:07:16   decided instead to look into changing ABC material. To do this, we use density functional theory in
0:07:23   order. Thio predicts the activation energy for hydrogen diffusion into the various or factory
0:07:29   materials shown here. Activation energy can easily be calculated with density. Functional theory is
0:07:35   the difference in energy between the transition state and the relaxed initial state along the
0:07:39   hydrogen diffusion pathway. As you can see here, many of the night rides that we looked at our
0:07:47   predicted to perform better than the pure metallics, tungsten and molybdenum in particular. Boron
0:07:53   nitride is predicted to perform best, and we chose to investigate further hexagonal boron nitride
0:07:59   using density functional theory. Although cubic boron nitride is predicted to perform better, um, it
0:08:05   requires high pressures and temperatures in order for crystallization, whereas with atomic layer
0:08:10   deposition, it's more likely that we'll see that hexagonal crystal structure because hexagonal boron
0:08:17   nitride is asymmetric. I wanted to look further into the various diffusion pathways for hydrogen in
0:08:22   this material, keeping in mind that the lowest energy surface is that 001 surface and therefore the
0:08:29   most likely to be present on the surface of our coated sample. we can see here that as we compared
0:08:35   to fusion through a sheet versus between these hexagonal boron nitride sheets that we see a much
0:08:42   lower diffusion activation energy down to one e V for diffusion for surface diffusion on the
0:08:50   underside of the top sheet. 1.11 for surface diffusion on the second sheet and all the way down 2.5
0:08:57   eveyone diffusing from a boron atom to a boron atom between sheets. So this means that micro
0:09:03   structure is extremely important and thes boron nitride coatings must be deposited with sheets
0:09:11   parallel to the substrate surface. In order to take advantage of that three and a quarter evey
0:09:16   activation energy and depositing codings perpendicular to the surface will allow for essentially
0:09:23   channels to be formed of for low energy diffusion pathways. Done. 2.5 TV When looking into what
0:09:32   causes the high activation energy for diffusion through a sheet versus between these streets, my
0:09:38   first conclusion was to look into Starik hindrance where we see that when a hydrogen has to diffuse
0:09:44   through the surface, it has to push out the born in the nitrogen lattice ions from its diffusion
0:09:49   pathway. However, with hydrogen diffusion between sheets, we can see that there's very little
0:09:54   movement of the brawn and the nitrogen atoms in order to compensate for the fusing hydrogen. In
0:10:01   order to look into this derrick hindrance, I calculated reconfiguration energies for these various
0:10:07   diffusion pathways at the initial. The transition in the final state on this reconfiguration energy
0:10:12   was calculated as the difference of energy between the born and the nitrogen atoms at those states
0:10:18   versus the relaxed initial structure before being in contact with hydrogen so we can see those
0:10:25   reconfiguration. Energy is shown in orange across each bar. Thes bars correlate with activation
0:10:31   energy, the same as was reported on the previous slide. So we can see here that the highest
0:10:37   reconfiguration energy correlates with the highest activation energy. Um, and from this, this
0:10:43   suggests that there's a strong correlation between reconfiguration, energy and activation energy on
0:10:49   a strong energy penalty for this increased reconfiguration energy for after looking into boron
0:10:57   nitride, computational e, I looked into it experimentally through deposition using atomic layer
0:11:03   deposition with boron, try chloride and ammonia. Shown here is my mass spec trace where we can see
0:11:08   the self limited nature of this chemistry, where we see hydrogen chloride is a by product for both
0:11:13   the boron and the ammonia dose. Um, and we can see that as that hydrogen chloride signal falls, we
0:11:20   see the increase in the precursor signal, indicating a self limited chemistry and saturation of the
0:11:26   substrate. This chemistry had a much faster deposition rate than with Thompson. I tried off about
0:11:32   three nanometers per cycle after characterization on zirconia. We used this chemistry to deposit 115
0:11:40   165 and 310 foreign nitride LD cycles on the issue a stabilizer konia the same substrate as was
0:11:48   previously used for the tungsten nitride chemistry. We again used F i b milling followed by BDs in
0:11:54   order to confirm the presence of Born and I, John and Nitrogen. Within our film shown here is the E.
0:12:02   D s results, where we can see the peak due to boron as well as a slight impurity of iron within our
0:12:07   film. In addition, we used XPS in order to analyze the elemental composition of the surface of these
0:12:14   samples. And we see here again the presence of born and nitrogen, as well as a slight chlorine
0:12:20   impurity on these iron and chlorine contaminants within the film are likely dio to that boron. Try
0:12:26   chloride precursor. Possible corroding inside of that precursor bottle or potentially picking up
0:12:33   iron from the steel fluid eyes bed during deposition. After confirming the presence of boron and
0:12:40   nitrogen within the film, we again use differential thermal analysis in order to extract the
0:12:45   behavior of these samples in a 6% hydrogen and argon environment, or 100% are gone environment. So
0:12:54   we tested all three film thicknesses 101 115 1, 65 310 as compared to that uncoated control again
0:13:02   highlighted in blue. We see here born nitride crystallization, we tried to confirm with X rd.
0:13:09   However, these films are too thin compared to the y si substrate. However, the temperatures for
0:13:16   these peaks correlate well with previous reports and literature of chemical vapor deposit, boron
0:13:22   nitride films crystallizing below 1000 degrees Celsius and chemical vapor infiltration or on nitride
0:13:28   films crystallizing between 194 and 644 degrees Celsius. Finally, the peak highlighted in green eyes
0:13:37   due to hydrogen reaction with the substrate, and it's actually only present within our uncoated y si
0:13:43   substrate on this indicates that we have quenched that peak with our with our boron nitride film on.
0:13:50   This confirms our hypothesis that we can use boron nitride l d. In order to decrease hydrogen
0:13:57   diffusion and its associate ID and brittle mint of our ailed of our NTP substrates eso we've been
0:14:04   able to push that hydrogen peak reaction temperature above the temperature range shown here from 0
0:14:10   to 1400 degrees Celsius. So this warrants further testing at higher temperatures closer to that of
0:14:16   an NDP engineer 2500 degrees Celsius. We've also confirmed our hypothesis from density functional
0:14:23   theory that boron nitrite is predicted to perform much better than tungsten nitride where I've show
0:14:29   here the a sample from that previous slide from the DT analysis of tungsten nine child where we
0:14:35   still see those peaks from the tux tonight. Drug coated samples. However, we do not see those peaks
0:14:41   in the boron nitride coated sample. As a summary, we first deposited the legacy material of tungsten
0:14:48   using L. D of tungsten. I tried and decomposition in order to form a pure tungsten coding. We then
0:14:56   use Dita and saw that the hydrogen peak reaction temperature increases with increased cycle account.
0:15:01   However, it's still far below the NTP right operating temperature. We use density functional theory
0:15:07   to predict better barrier materials, and we saw the boron nitride has shown experiment is shown to
0:15:13   perform best using dft on experimentally. Also confirmed to perform better than for than tungsten. I
0:15:20   tried. We also showed using density functional theory that diffuse city of boron nitride is
0:15:27   extremely dependent on micro structure. On. In order, Thio realize that high activation energy boron
0:15:33   nitride sheets must be peril to the substrate surface. We also saw that the large activation energy
0:15:40   of hydrogen diffusion through a born I judge she likely rises from Starik. Hindrance, a za part of
0:15:46   feature work we look into. We want to look into testing boron nitride above 1400 degrees Celsius on
0:15:53   closer to that 2500 degrees Celsius operating temperature and compare these boron nitride codings to
0:15:59   thicker tungsten codings, um used deposited from a different body chemistry than that used for
0:16:06   tungsten nightshade. With that, I would like to acknowledge both of my advisers, Alan Weimer and
0:16:12   Charles Musgrave, as well as my lab mates within those groups and funding sources, and I look
0:16:17   forward to hearing your questions