Bridging the gap to mesoscale radiation materials science with transient grating spectroscopy

Direct mesoscale measurements of radiation-induced changes in the mechanical properties of bulk materials remain difficult to perform. Most widely used characterization techniques are either macro- or microscale in nature, focusing on overall properties or overly small areas for analysis. Linking the atomic structure of irradiated materials directly with their radiation-affected properties remains one of the largest unmet challenges in radiation materials science. By measuring the change in surface acoustic wave speed as a function of relative orientation on metallic single crystals, we demonstrate that transient grating (TG) spectroscopy experiments have the sensitivity necessary to detect radiation-induced material property changes. We also show that classical molecular dynamics (MD) simulations can be used to accurately simulate orientation-based changes in surface acoustic wave speed in TG experiments, by comparing with experimental measurements and theoretical predictions. The agreement between theory, simulation, and experiment gives confidence in classical MD as a predictive tool to simulate defect-based changes in elastic properties, which cannot yet be fully treated by theory. This ability is of critical importance for the informed use of TG spectroscopy to measure material property changes induced by radiation damage, which may vary by amounts formerly too small for reliable in situ detection. Finally, our MD simulation framework is used to study the effect of an imposed vacancy population on the acoustic response of several materials. The results of these studies indicate that TG experiments are well suited to the ex situ and in situ study of radiation-induced material property changes.National Science Foundation (U.S.) (Grant 1122374)National Science Foundation (U.S.) (Grant CHE-1111557)Transatomic Power (Award 023875-001)U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program. Grant NRC-HQ- 84-15-G-0045

In-situ investigation of the oxidation kinetics of Fe-12Cr-2Si using time-resolved transient grating spectroscopy

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 95-99).The design and validation of new alloys for engineering applications is limited by the speed at which materials may be tested. In particular, there exist few methods by which the thermal, mechanical, and structural properties of materials may be monitored in conditions that are dynamically changing their microstructure. These conditions, such as heat treatments, radiation exposure, or corrosive environments, are common when material performance needs to be validated. To offset this lack of capability, new non-destructive experimental tools must be developed to facilitate on-line, realtime testing of materials undergoing some type of evolution. In this thesis, a flexible, all-optical methodology known as dual heterodyne phase collection transient grating spectroscopy is developed for this purpose. This method adapts a traditional spectroscopic technique sensitive to thermal and mechanical properties for real-time use. A formalism is also developed to quantify both elastic and thermal transport properties of materials with second-scale resolution. These new tools are then used to study the short-timescale oxidation kinetics of Fe-12Cr-2Si, a model alloy with oxide layer formation properties similar to large classes of Fr-Cr alloys. By monitoring the effect of thin oxide layers on surface thermal transport, there exists a pathway to continuously determine the thickness of a tens of nanometers thick growing oxide layer in real-time. Despite the lack of clarity in the particular set of experimental results presented here, the potential for the methods developed in this thesis is large. In-situ materials testing of this type may allow for a drastic increase in the pace of materials development by reducing the need for post-evolution, destructive materials testing between each design iteration.by Cody Andrew Dennett.S.M

Time-resolved, dual heterodyne phase collection transient grating spectroscopy

The application of optical heterodyne detection for transient grating spectroscopy (TGS) using a fixed, binary phase mask often relies on taking the difference between signals captured at multiple heterodyne phases. To date, this has been accomplished by manually controlling the heterodyne phase between measurements with an optical flat. In this letter, an optical configuration is presented which allows for collection of TGS measurements at two heterodyne phases concurrently through the use of two independently phase controlled interrogation paths. This arrangement allows for complete, heterodyne amplified TGS measurements to be made in a manner not constrained by a mechanical actuation time. Measurements are instead constrained only by the desired signal-to-noise ratio. A temporal resolution of between 1 and 10 s, demonstrated here on single crystal metallic samples, will allow TGS experiments to be used as an in-situ, time-resolved monitoring technique for many material processing applications.National Science Foundation (U.S.) (Graduate Research Fellowship under Grant No. 1122374)United States. National Nuclear Security Administration (Stewardship Science Graduate Fellowship under cooperative agreement No. DE-NA000213)SUTD-MIT International Design Centre (IDC)U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program under Grant No. NRC-HQ-84-15-G-0045

Thermal diffusivity determination using heterodyne phase insensitive transient grating spectroscopy

The elastic and thermal transport properties of opaque materials may be measured using transient grating spectroscopy (TGS) by inducing and monitoring periodic excitations in both reflectivity and surface displacement. The “phase grating” response encodes both properties of interest, but complicates quantitative analysis by convolving temperature dynamics with surface displacement dynamics. Thus, thermal transport characteristics are typically determined using the “amplitude grating” response to isolate the surface temperature dynamics. However, this signal character requires absolute heterodyne phase calibration and contains no elastic property information. Here, a method is developed by which phase grating TGS measurements may be consistently analyzed to determine thermal diffusivity with no prior knowledge of the expected properties. To demonstrate this ability, the wavelength-dependent 1D effective thermal diffusivity of pure germanium is measured using this type of response and found to be consistent with theoretical predictions made by solving the Boltzmann transport equation. This ability to determine the elastic and thermal properties from a single set of TGS measurements will be particularly advantageous for new in situ implementations of the technique being used to study dynamic materials systems.United States. National Nuclear Security Administration. Stewardship Science Graduate Fellowship (cooperative Agreement No. DE-NA0002135)SUTD-MIT International Design Centre (IDC)U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program

Thermal diffusivity determination using heterodyne phase insensitive transient grating spectroscopy

The elastic and thermal transport properties of opaque materials may be measured using transient grating spectroscopy (TGS) by inducing and monitoring periodic excitations in both reflectivity and surface displacement. The “phase grating” response encodes both properties of interest, but complicates quantitative analysis by convolving temperature dynamics with surface displacement dynamics. Thus, thermal transport characteristics are typically determined using the “amplitude grating” response to isolate the surface temperature dynamics. However, this signal character requires absolute heterodyne phase calibration and contains no elastic property information. Here, a method is developed by which phase grating TGS measurements may be consistently analyzed to determine thermal diffusivity with no prior knowledge of the expected properties. To demonstrate this ability, the wavelength-dependent 1D effective thermal diffusivity of pure germanium is measured using this type of response and found to be consistent with theoretical predictions made by solving the Boltzmann transport equation. This ability to determine the elastic and thermal properties from a single set of TGS measurements will be particularly advantageous for new in situ implementations of the technique being used to study dynamic materials systems.United States. National Nuclear Security Administration. Stewardship Science Graduate Fellowship (cooperative Agreement No. DE-NA0002135)SUTD-MIT International Design Centre (IDC)U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program

The In Situ Ion Irradiation Toolbox: Time-Resolved Structure and Property Measurements

Abstract The dynamic interactions of ions with matter drive a host of complex evolution mechanisms, requiring monitoring on short spatial and temporal scales to gain a full picture of a material response. Understanding the evolution of materials under ion irradiation and displacement damage is vital for many fields, including semiconductor processing, nuclear reactors, and space systems. Despite materials in service having a dynamic response to radiation damage, typical characterization is performed post-irradiation, washing out all information from transient processes. Characterizing active processes in situ during irradiation allows the mechanisms at play during the dynamic ion-material interaction process to be deciphered. In this review, we examine the in situ characterization techniques utilized for examining material structure, composition, and property evolution under ion irradiation. Covering analyses of microstructure, surface composition, and material properties, this work offers a perspective on the recent advances in methods for in situ monitoring of materials under ion irradiation, including a future outlook examining the role of complementary and combined characterization techniques in understanding dynamic materials evolution.U.S. Department of Energy Idaho Operations Office (Contract DE-AC07-05ID14517

A thermal study of amorphous and textured carbon and carbon nitride thin films via transient grating spectroscopy

In this study, we are considering a novel way of growing carbon nitride (CN) films by using High Power Impulse Magnetron Sputtering (HiPIMS). Carbon and CN thin films were grown on silicon substrate with varying nanocrystalline texturing: some samples were amorphous while others were either nanocrystalline graphite (for the carbon sample) or fullerene-like (for the CN sample), with both samples having a graphitic nanostructure vertically ordered throughout the film. Their thermal diffusivity was computed using transient grating spectroscopy in order to compare the impact of the material's nanostructure on its thermal property as well as benchmarking the performance of CN. It was found that the thermal properties of carbon thin films were decreased when doped with nitrogen, which is attributed to the increased atomic disorder introduced by the nitrogen cross-linking, impacting the phonon propagation. The impact of nitrogen doping on thermal properties opens new avenues in engineering materials with tailored and varying thermal properties at the microscale. Keywords: Carbon nitride; HiPIMS; Transient grating spectroscop

Detecting self-ion irradiation-induced void swelling in pure copper using transient grating spectroscopy

Irradiation-induced void swelling remains a major challenge to nuclear reactor operation. Swelling may take years to initiate and often results in rapid material property degradation once started. Alloy development for advanced nuclear systems will require rapid characterization of the swelling breakaway dose in new alloys, yet this capability does not yet exist. We demonstrate that transient grating spectroscopy (TGS) can detect void swelling in single crystal copper via changes in surface acoustic wave (SAW) velocity. Scanning transmission electron microscopy (STEM) links the TGS-observed changes with void swelling-induced microstructural evolution. These results are considered in the context of previous work to suggest that in situ TGS will be able to rapidly determine when new bulk materials begin void swelling, shortening alloy development and testing times.United States. Department of Energy (Agreement DE-NA0002135

Applications of Transient Grating Spectroscopy to Radiation Materials Science

The ability to study radiation damage in situ will directly enable the rapid innovation and qualification of materials for nuclear applications by allowing direct observation of the effects of radiation damage accumulation. This is a challenging task, as the measurement technique must be noncontact, nondestructive, rapid, and still allow for online irradiation without interference. Applicable methods of mechanical spectroscopy are surveyed, noting their potential usefulness for characterizing radiation-induced microstructural changes in situ. The transient grating (TG) spectroscopy technique appears most suited for these studies, due to its noncontact, nondestructive nature, its ability to rapidly probe materials to the depth of ion irradiation, and the large number of deconvolvable components extractable from its signal. Work is proposed to separate the individual mechanisms of irradiation damage using in situ and ex situ TG spectroscopy, through a suite of single-effect and integrated experiments.National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant No. 1122374)National Science Foundation (U.S.) (Grant No. CHE- 1111557

Conceptual design study for heat exhaust management in the ARC fusion pilot plant

The ARC pilot plant conceptual design study has been extended beyond its initial scope [B. N. Sorbom et al., FED 100 (2015) 378] to explore options for managing ∼525 MW of fusion power generated in a compact, high field (B0 = 9.2 T) tokamak that is approximately the size of JET (R0 = 3.3 m). Taking advantage of ARC's novel design – demountable high temperature superconductor toroidal field (TF) magnets, poloidal magnetic field coils located inside the TF, and vacuum vessel (VV) immersed in molten salt FLiBe blanket – this follow-on study has identified innovative and potentially robust power exhaust management solutions. The superconducting poloidal field coil set has been reconfigured to produce double-null plasma equilibria with a long-leg X-point target divertor geometry. This design choice is motivated by recent modeling which indicates that such configurations enhance power handling and may attain a passively-stable detachment front that stays in the divertor leg over a wide power exhaust window. A modified VV accommodates the divertor legs while retaining the original core plasma volume and TF magnet size. The molten salt FLiBe blanket adequately shields all superconductors, functions as an efficient tritium breeder, and, with augmented forced flow loops, serves as an effective single-phase, low-pressure coolant for the divertor, VV, and breeding blanket. Advanced neutron transport calculations (MCNP) indicate a tritium breeding ratio of ∼1.08. The neutron damage rate (DPA/year) of the remote divertor targets is ∼3–30 times lower than that of the first wall. The entire VV (including divertor and first wall) can tolerate high damage rates since the demountable TF magnets allow the VV to be replaced every 1–2 years as a single unit, employing a vertical maintenance scheme. A tungsten swirl tube FLiBe coolant channel design, similar in geometry to that used by ITER, is considered for the divertor heat removal and shown capable of exhausting divertor heat flux levels of up to 12 MW/m2. Several novel, neutron tolerant diagnostics are explored for sensing power exhaust and for providing feedback control of divertor conditions over long time scales. These include measurement of Cherenkov radiation emitted in FLiBe to infer DT fusion reaction rate, measurement of divertor detachment front locations in the divertor legs with microwave interferometry, and monitoring “hotspots” on the divertor chamber walls via IR imaging through the FLiBe blanket. ©2018DOE NNSA Stewardship Science Graduate Fellowship (No. DE-NA0002135)National Science Foundation Graduate Research Fellowship (Grant No. DGE1122374