Assel Aitkaliyeva

Phonon transport assisted by inter-tube carbon displacements in carbon nanotube mats

Thermal transport in carbon nanotube (CNT) mats, consisting of randomly networked multi-walled carbon nanotubes (MWNTs), is not as efficient as in an individual CNT because of the constrained tube-to-tube phonon transport. Through experiments and modeling, we discover that phonon transport in CNT mats is significantly improved by ion irradiation, which introduces inter-tube displacements, acting as stable point contacts between neighboring tubes. Inter-tube displacement-mediated phonon transport enhances conductivity, while inter-tube phonon-defect scattering reduces conductivity. At low ion irradiation fluence, inter-tube thermal transport enhancement leads to thermal conductivity increase by factor > 5, while at high ion irradiation fluence point defects within tubes cause a decrease in thermal conductivity. Molecular dynamics simulations support the experimentally obtained results and the proposed mechanisms. Further conductivity enhancement in irradiated mats was obtained by post-irradiation heat treatment that removes majority of the defects within the tubes without affecting thermally stable inter-tube displacements.

The change of microstructure and thermal properties in ion irradiated carbon nanotube mats as a function of ion penetration depth

A stack of three carbon nanotube (CNT) mats was irradiated with 3 MeV He ions. The change in structural and thermal properties of individual mats as a function of ion penetration depth was characterized using electron microscopy and laser flash techniques. Ion irradiation can enhance thermal conductivity of the mats by introducing inter-tube displacements, which improve phonon transport across adjacent nanotubes. The enhancement, however, is reduced at higher damage levels due to the increasing phonon-defect scattering within the tubes. This study demonstrates the feasibility of using ion irradiation to manipulate thermal transport in carbon nanotubes.

Implementation of focused ion beam (FIB) system in characterization of nuclear fuels and materials

Beginning in 2007, a program was established at the Idaho National Laboratory to update key capabilities enabling microstructural and micro-chemical characterization of highly irradiated and/or radiologically contaminated nuclear fuels and materials at scales that previously had not been achieved for these types of materials. Such materials typically cannot be contact handled and pose unique hazards to instrument operators, facilities, and associated personnel. Over the ensuing years, techniques have been developed and operational experience gained that has enabled significant advancement in the ability to characterize a variety of fuel types including metallic, ceramic, and coated particle fuels, obtaining insights into in-reactor degradation phenomena not achievable by any other means. The following article describes insights gained, challenges encountered, and provides examples of unique results obtained in adapting dual beam FIB technology to nuclear fuels characterization.

Nanometer-scale tunnel formation in metallic glass by helium ion irradiation

We have shown that upon high fluence helium ion irradiation, metallic glass Cu50Zr45Ti5 becomes highly porous at the depth of the helium projected range. The resulting porous region is characterized by the formation of a tunnel like structure and self-linkage of nanometer size gas bubbles. Furthermore, the irradiation leads to the formation of nanometer size CuxZry crystals that are randomly distributed. The results of this study indicate that the He-filled bubbles have attractive interactions and experience considerable mobility. Movement of the bubbles is believed to be assisted by ballistic collisions.

A Study of Irradiation Stability of Carbon Nanotubes

We report preliminary results on studying the radiation stability of carbon nanotubes (CNs). Three different combinations of radiation and in situ characterization are used. Experiments include 30 keV electron bombardment with in situ characterization using a scanning electron microscope (SEM), 30 keV Ga ion bombardment with in situ SEM characterization, and 2 MeV proton irradiation with in situ electric resistance measurements. The study shows the degradation of CNs with dimensional shrinkage upon radiation, and the existence of a quasi steady state of defect creation and annealing.

Ion irradiation induced bubble relaxation in SiC

He ion irradiation at 140 keV and subsequent annealing at 1200 K were used to introduce voids in 4H-SiC. Then, 1 MeV Si ion irradiation at room temperature was used to study irradiation-induced athermal annealing. Transmission electron microscopy and channeling Rutherford backscattering spectrum analysis were used to characterize samples. Platelet-like voids were formed at the boundary of heavily damaged layer with Si ions implanted to a fluence of 1×1015 cm−2. When Si fluence was increased to 1×1016 cm−2, transition of platelet-like voids into large voids was observed. The study provides evidence of ion irradiation induced athermal annealing. Monte Carlo simulation of atomic displacement on the internal surface of the He bubble suggests that ballistic collision from damage cascades plays an important role in bubble relaxation.

Analysis and comparison of focused ion beam milling and vibratory polishing sample surface preparation methods for porosity study of U-Mo plate fuel for research and test reactors

Uranium-Molybdenum (U-Mo) low enriched uranium (LEU) fuels are a promising candidate for the replacement of high enriched uranium (HEU) fuels currently in use in a high power research and test reactors around the world. Contemporary U-Mo fuel sample preparation uses focused ion beam (FIB) methods for analysis of fission gas porosity. However, FIB possess several drawbacks, including reduced area of analysis, curtaining effects, and increased FIB operation time and cost. Vibratory polishing is a well understood method for preparing large sample surfaces with very high surface quality. In this research, fission gas porosity image analysis results are compared between samples prepared using vibratory polishing and FIB milling to assess the effectiveness of vibratory polishing for irradiated fuel sample preparation. Scanning electron microscopy (SEM) imaging was performed on sections of irradiated U-Mo fuel plates and the micrographs were analyzed using a fission gas pore identification and measurement script written in MatLab. Results showed that the vibratory polishing method is preferentially removing material around the edges of the pores, causing the pores to become larger and more rounded, leading to overestimation of the fission gas porosity size. Whereas, FIB preparation tends to underestimate due to poor micrograph quality and surface damage leading to inaccurate segmentations. Despite the aforementioned drawbacks, vibratory polishing remains a valid method for porosity analysis sample preparation, however, improvements should be made to reduce the preferential removal of material surrounding pores in order to minimize the error in the porosity measurements.

Microstructural characterization of Pu-Zr fuels

The development of fuels for minor actinide transmutation systems has numerous technical challenges. The fuel has to behave in benign manner during core off-normal events, maintain integrity to high burnup, has to have ease of operation, low minor actinide fabrication loss under remote handling conditions, and lend itself to low-loss recycling processes. High burn-up, high fissile and fertile density capability, inherent safety characteristics, and favorable thermal response of the metallic fuels make them an excellent candidate for next generation fast reactor applications [1]. Despite the work conducted to this date, further research is needed to develop and certify the fuel for applications in transmutation systems. Complete understanding of the irradiation behavior of Pu-based fuels cannot be achieved without the knowledge of the phases and microstructure of unirradiated fuels.

TEM Identification of Phases in Metallic Pu-based Fuels

Metallic fuels are considered for application in advanced fast reactors because of their high burn-up, high fissile and fertile density capability, and high thermal conductivity with significant safety benefits [1]. Metallic fuels have several potential advantages, such as simple fabrication, robust performance, benign response to reactor transients, and relatively easy recycling using compact molten salt electrochemical processing. The uranium-plutonium-zirconium (U-Pu-Zr) alloys are considered to be one of the most promising metallic fuels. The addition of Zr in U-Pu matrix was sought to increase the melting temperature of U-Pu alloys and to enhance compatibility between the fuel and stainless-steel cladding by suppressing the interdiffusion of fuel and cladding constituents during steady-state reactor operations.

Sample preparation artifacts in nuclear materials and mitigation strategies

Diverse microstructures form in nuclear materials upon exposure to radiation. The defects produced during irradiation of materials can alter their mechanical properties and lead to embrittlement of reactor structural materials during service life. Therefore, it is imperative to know various radiation effects in reactor materials since it can aid in understanding in-reactor degradation behavior, accounting for irradiation effects in design, and producing new generation radiation-tolerant materials. Characterization of radiation-induced changes in reactor materials at the nano and atomic scales is typically conducted in transmission electron microscopes (TEM). Three most commonly used sample preparation techniques include electro-polishing, broadbeam ion milling, and focused ion beam (FIB) approach. However, preparation of samples using conventional sample preparation techniques, such as electro-polishing and ion milling, requires close-in, hands-on manipulation of the sample for extended periods of time. This is not feasible with highly radioactive nuclear materials.