Scholarly Works, Materials Science and Engineering (MSE)

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Research articles, presentations, and other scholarship


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  • Understanding the Impact of Fuel on Surfactant Microstructure of Firefighting Foam
    Islam, Rezawana; Lattimer, Brian Y. (Springer Link, 2024-05-01)
    Aqueous film-forming foam is being phased out due to the environmental impacts of fluorinated surfactants contained in the firefighting foams. To develop an environmentally friendly firefighting foam, it is important to understand the factors controlling the firefighting performance of surfactants. Fuel transport through foam has been considered as a dominant mechanism for foam collapse. Therefore, the impact of fuels (heptane, octane and trimethylbenzene (TMB)) on surfactant microstructure was studied for three different types of surfactants (Capstone, Glucopon, and siloxane) that have applications in firefighting foam. Multiple techniques were used to identify the microstructure and interfacial properties of surfactants with and without exposure to liquid fuel. The ignition time of fuel vapor through foam and solubility of fuel through liquid surfactant solution were measured as well. This work shows fuel solubility has an impact on the surfactant microstructure and interfacial properties. In addition, fuel solubility and vapor pressure affect the ignition time of fuel vapors.
  • Magnetic Field Sensing via Acoustic Sensing Fiber with Metglas® 2605SC Cladding Wires
    Dejneka, Zach; Homa, Daniel; Buontempo, Joshua; Crawford, Gideon; Martin, Eileen; Theis, Logan; Wang, Anbo; Pickrell, Gary R. (MDPI, 2024-04-10)
    Magnetic field sensing has the potential to become necessary as a critical tool for long-term subsurface geophysical monitoring. The success of distributed fiber optic sensing for geophysical characterization provides a template for the development of next generation downhole magnetic sensors. In this study, Sentek Instrument’s picoDAS is coupled with a multi-material single mode optical fiber with Metglas® 2605SC cladding wire inclusions for magnetic field detection. The response of acoustic sensing fibers with one and two Metglas® 2605SC cladding wires was evaluated upon exposure to lateral AC magnetic fields. An improved response was demonstrated for a sensing fiber with in-cladding wire following thermal magnetic annealing (~400 °C) under a constant static transverse magnetic field (~200 μT). A minimal detectable magnetic field of ~500 nT was confirmed for a sensing fiber with two 10 μm cladding wires. The successful demonstration of a magnetic field sensing fiber with Metglas® cladding wires fabricated via traditional draw processes sets the stage for distributed measurements and joint inversion as a compliment to distributed fiber optic acoustic sensors.
  • A Meta-Analysis of the Effect of Moisture Content of Recycled Concrete Aggregate on the Compressive Strength of Concrete
    Cho, Sung-Won; Cho, Sung Eun; Brand, Alexander S. (MDPI, 2024-04-22)
    To reduce the environmental impact of concrete, recycled aggregates are of significant interest. Recycled concrete aggregate (RCA) presents a significant resource opportunity, although its performance as an aggregate in concrete is variable. This study presents a meta-analysis of the published literature to refine the understanding of how the moisture content of RCA, as well as other parameters, affects the compressive strength of concrete. Seven machine learning models were used to predict the compressive strength of concrete with RCA, including linear regression, support vector regression (SVR), and k-nearest neighbors (KNN) as single models, and decision tree, random forest, XGBoost, and LightGBM as ensemble models. The results of this study demonstrate that ensemble models, particularly the LightGBM model, exhibited superior prediction accuracy compared to single models. The LightGBM model yielded the highest prediction accuracy with R2 = 0.94, RMSE = 4.16 MPa, MAE = 3.03 MPa, and Delta RMSE = 1.4 MPa, making it the selected final model. The study, employing feature importance with LightGBM as the final model, identified age, water/cement ratio, and fine RCA aggregate content as key factors influencing compressive strength in concrete with RCA. In an interaction plot analysis using the final model, lowering the water–cement ratio consistently improved compressive strength, especially between 0.3 and 0.4, while increasing the fine RCA ratio decreased compressive strength, particularly in the range of 0.4 to 0.6. Additionally, it was found that maintaining moisture conditions of RCA typically between 0.0 and 0.8 was crucial for maximizing strength, whereas extreme moisture conditions, like fully saturated surface dry (SSD) state, negatively impacted strength.
  • Noncovalently particle-anchored cytokines with prolonged tumor retention safely elicit potent antitumor immunity
    Niu, Liqian; Jang, Eungyo; Chin, Ai Lin; Huo, Ziyu; Wang, Wenbo; Cai, Wenjun; Rong, Tong (American Association for the Advancement of Science, 2024-04-19)
    Preclinical studies have shown that immunostimulatory cytokines elicit antitumor immune responses but their clinical use is limited by severe immune-related adverse events upon systemic administration. Here, we report a facile and versatile strategy for noncovalently anchoring potent Fc-fused cytokine molecules to the surface of size-discrete particles decorated with Fc-binding peptide for local administration. Following intratumoral injection, particle-anchored Fc cytokines exhibit size-dependent intratumoral retention. The 1-micrometer particle prolongs intratumoral retention of Fc cytokine for over a week and has minimal systemic exposure, thereby eliciting antitumor immunity while eliminating systemic toxicity caused by circulating cytokines. In addition, the combination of these particle-anchored cytokines with immune checkpoint blockade antibodies safely promotes tumor regression in various syngeneic tumor models and genetically engineered murine tumor models and elicits systemic antitumor immunity against tumor rechallenge. Our formulation strategy renders a safe and tumor-agnostic approach that uncouples cytokines’ immunostimulatory properties from their systemic toxicities for potential clinical application.
  • Predicting Ion Sequestration in Charged Polymers with the Steepest-Entropy-Ascent Quantum Thermodynamic Framework
    McDonald, Jared; von Spakovsky, Michael R.; Reynolds, William T. (MDPI, 2024-03-01)
    The steepest-entropy-ascent quantum thermodynamic framework is used to investigate the effectiveness of multi-chain polyethyleneimine-methylenephosphonic acid in sequestering rare-earth ions (Eu3+) from aqueous solutions. The framework applies a thermodynamic equation of motion to a discrete energy eigenstructure to model the binding kinetics of europium ions to reactive sites of the polymer chains. The energy eigenstructure is generated using a non-Markovian Monte Carlo model that estimates energy level degeneracies. The equation of motion is used to determine the occupation probability of each energy level, describing the unique path through thermodynamic state space by which the polymer system sequesters rare-earth ions from solution. A second Monte Carlo simulation is conducted to relate the kinetic path in state space to physical descriptors associated with the polymer, including the radius of gyration, tortuosity, and Eu-neighbor distribution functions. These descriptors are used to visualize the evolution of the polymer during the sequestration process. The fraction of sequestered Eu3+ ions depends upon the total energy of the system, with lower energy resulting in greater sequestration. The kinetics of the overall sequestration are dependent on the steepest-entropy-ascent principle used by the equation of motion to generate a unique kinetic path from an initial non-equilibrium state.
  • Microstructures and Corrosion Properties of Wire Arc Additive Manufactured Copper–Nickel Alloys
    Song, Jie; Jimenez, Xavier A.; To, Albert C.; Fu, Yao (MDPI, 2024-02-14)
    The 70/30 copper–nickel alloy is used mainly in critical parts with more demanding conditions in marine settings. There is a need for innovative methods that offer fast production and cost-effectiveness in order to supplement current copper–nickel alloy manufacturing processes. In this study, we employ wire arc additive manufacturing (WAAM) to fabricate the 70/30 copper–nickel alloy. The as-built microstructure is characterized by columnar grains with prominent dendrites and chemical segregation in the inter-dendritic area. The aspect ratio of the columnar grain increases with increasing travel speed (TS) at the same wire feed speed (WFS). This is in contrast with the equiaxed grain structure, with a more random orientation, of the conventional sample. The sample built with a WFS of 8 m/min, TS of 1000 mm/min, and a track distance of 3.85 mm exhibits superior corrosion properties in the 3.5 wt% NaCl solution when compared with the conventional sample, as evidenced by a higher film resistance and breakdown potential, along with a lower passive current density of the WAAM sample. The corrosion morphology reveals the critical roles played by the nickel element that is unevenly distributed between the dendrite core and inter-dendritic area.
  • A Review on Tribocorrosion Behavior of Aluminum Alloys: From Fundamental Mechanisms to Alloy Design Strategies
    Zhang, Zhengyu; Dandu, Raja Shekar Bhupal; Klu, Edwin Eyram; Cai, Wenjun (MDPI, 2023-10-18)
    Tribocorrosion, a research field that has been evolving for decades, has gained renewed attention in recent years, driven by increased demand for wear- and corrosion-resistant materials from biomedical implants, nuclear power generation, advanced manufacturing, batteries, marine and offshore industries, etc. In the United States, wear and corrosion are estimated to cost nearly USD 300 billion per year. Among various important structural materials, passive metals such as aluminum alloys are most vulnerable to tribocorrosion due to the wear-accelerated corrosion as a result of passive film removal. Thus, designing aluminum alloys with better tribocorrosion performance is of both scientific and practical importance. This article reviews five decades of research on the tribocorrosion of aluminum alloys, from experimental to computational studies. Special focus is placed on two aspects: (1) The effects of alloying and grain size on the fundamental wear, corrosion, and tribocorrosion mechanisms; and (2) Alloy design strategies to improve the tribocorrosion resistance of aluminum alloys. Finally, the paper sheds light on the current challenges faced and outlines a few future research directions in the field of tribocorrosion of aluminum alloys.
  • Operando characterization and regulation of metal dissolution and redeposition dynamics near battery electrode surface
    Zhang, Yuxin; Hu, Anyang; Xia, Dawei; Hwang, Sooyeon; Sainio, Sami; Nordlund, Dennis; Michel, F. Marc; Moore, Robert B.; Li, Luxi; Lin, Feng (Nature Portfolio, 2023-07)
    Mn dissolution has been a long-standing, ubiquitous issue that negatively impacts the performance of Mn-based battery materials. Mn dissolution involves complex chemical and structural transformations at the electrode–electrolyte interface. The continuously evolving electrode–electrolyte interface has posed great challenges for characterizing the dynamic interfacial process and quantitatively establishing the correlation with battery performance. In this study, we visualize and quantify the temporally and spatially resolved Mn dissolution/redeposition (D/R) dynamics of electrochemically operating Mn-containing cathodes. The particle-level and electrode-level analyses reveal that the D/R dynamics is associated with distinct interfacial degradation mechanisms at different states of charge. Our results statistically differentiate the contributions of surface reconstruction and Jahn–Teller distortion to the Mn dissolution at different operating voltages. Introducing sulfonated polymers (Nafion) into composite electrodes can modulate the D/R dynamics by trapping the dissolved Mn species and rapidly establishing local Mn D/R equilibrium. This work represents an inaugural effort to pinpoint the chemical and structural transformations responsible for Mn dissolution via an operando synchrotron study and develops an effective method to regulate Mn interfacial dynamics for improving battery performance.
  • Studies on the structure and the magnetic properties of high-entropy spinel oxide (MgMnFeCoNi)Al2O4
    Krysko, Evan; Min, Lujin; Wang, Yu; Zhang, Na; Barber, John P.; Niculescu, Gabriela E.; Wright, Joshua T.; Li, Fankang; Burrage, Kaleb; Matsuda, Masaaki; Robinson, Robert A.; Zhang, Qiang; Katzbaer, Rowan; Schaak, Raymond; Terrones, Mauricio; Rost, Christina M.; Mao, Zhiqiang (AIP Publishing, 2023-10-20)
    The study of high-entropy materials has attracted enormous interest since they could show new functional properties that are not observed in their related parent phases. Here, we report single crystal growth, structure, thermal transport, and magnetic property studies on a novel high-entropy oxide with the spinel structure (MgMnFeCoNi)Al2O4. We have successfully grown high-quality single crystals of this high-entropy oxide using the optical floating zone growth technique for the first time. The sample was confirmed to be a phase pure high-entropy oxide using x-ray diffraction and energy-dispersive spectroscopy. Through magnetization measurements, we found (MgMnFeCoNi)Al2O4 exhibits a cluster spin glass state, though the parent phases show either antiferromagnetic ordering or spin glass states. Furthermore, we also found that (MgMnFeCoNi)Al2O4 has much greater thermal expansion than its CoAl2O4 parent compound using high resolution neutron Larmor diffraction. We further investigated the structure of this high-entropy material via Raman spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. From Raman spectroscopy measurements, we observed (MgMnFeCoNi)Al2O4 to display a combination of the active Raman modes in its parent compounds with the modes shifted and significantly broadened. This result, together with the varying bond lengths probed by EXAFS, reveals severe local lattice distortions in this high-entropy phase. Additionally, we found a substantial decrease in thermal conductivity and suppression of the low temperature thermal conductivity peak in (MgMnFeCoNi)Al2O4, consistent with the increased lattice defects and strain. These findings advance the understanding of the dependence of thermal expansion and transport on the lattice distortions in high-entropy materials.
  • High-entropy oxides: Harnessing crystalline disorder for emergent functionality
    Kotsonis, G. N.; Almishal, S. S. I.; Marques dos Santos Vieira, F.; Crespi, V. H.; Dabo, I.; Rost, Christina M.; Maria, J. P. (Wiley, 2023-06-24)
    High-entropy materials defy historical materials design paradigms by leveraging chemical disorder to kinetically stabilize novel crystalline solid solutions comprised of many end-members. Formulational diversity results in local crystal structures that are seldom found in conventional materials and can strongly influence macroscopic physical properties. Thermodynamically prescribed chemical flexibility provides a means to tune such properties. Additionally, kinetic metastability results in many possible atomic arrangements, including both solid-solution configurations and heterogeneous phase assemblies, depending on synthesis conditions. Local disorder induced by metastability, and extensive cation solubilities allowed by thermodynamics combine to give many high-entropy oxide systems utility as electrochemical, magnetic, thermal, dielectric, and optical materials. Though high-entropy materials research is maturing rapidly, much remains to be understood and many compositions still await discovery, exploration, and implementation.
  • On the thermal and mechanical properties of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O across the high-entropy to entropy-stabilized transition
    Rost, Christina M.; Schmuckler, Daniel L.; Bumgardner, Clifton; Bin Hoque, Md Shafkat; Diercks, David R.; Gaskins, John T.; Maria, Jon-Paul; Brennecka, Geoffrey L.; Li, Xiadong; Hopkins, Patrick E. (AIP Publishing, 2022-12-16)
    As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between "high-entropy"and "entropy-stabilized"phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10-6 K-1. Mechanical softening is also observed across the transition.
  • Long-term in situ ruminal degradation of biodegradable polymers in Holstein dairy cattle
    Galyon, Hailey; Vibostok, Samuel; Duncan, Jane; Ferreira, Gonzalo; Whittington, Abby; Cockrum, Rebecca R. (American Dairy Science Association, 2022-12-22)
    Using biodegradable materials such as polyhydroxyalkanoates (PHA) and poly(butylene succinate-co-adipate) (PBSA) to develop single-use agricultural plastics like bale netting may reduce the negative effects of plastic accumulation in the rumens of cattle. The objective of this research was to assess the long-term degradation of PHA, PBSA, and a PBSA:PHA blend (Blend) compared with a low-density polyethylene (LDPE) control. Polyhydroxyalkanoate, PBSA, Blend, and LDPE films were incubated in the rumens of 3 cannulated, nonlactating Holsteins for up to 150 d. In situ disappearance (ISD) and residue length were assessed after every incubation time. Data were analyzed with PROC MIXED in SAS and adjusted by Tukey's method to determine least squares differences between polymer treatments, incubation time, and their interaction. By 30 d, PHA achieved 100% degradation, with initiation occurring at 14 d indicated by ISD and a reduction in residue length. Poly(butylene succinate-co-adipate) and Blend did not achieve any significant ISD, but fragmentation of PBSA occurred at 60 d and fragmentation of Blend at just 1 d, likely due to abiotic hydrolysis. Low-density polyethylene achieved no ISD, and residue length did not change over incubation time. We propose that a PBSA:PHA blend is a valid alternative to polyethylene single-use agricultural plastic products based on its fragmentation within 1 d of incubation.
  • A topological kagome magnet in high entropy form
    Min, L.; Sretenovic, M.; Heitmann, T. W.; Valentine, T. W.; Zu, R.; Gopalan, V.; Rost, Christina M.; Ke, X.; Mao, Z. (Springer, 2022-03-18)
    Topological kagome magnets RMn6Sn6 (R = rare earth element) attract numerous interests due to their non-trivial band topology and room-temperature magnetism. Here, we report a high entropy version of kagome magnet, (Gd0.38Tb0.27Dy0.20Ho0.15)Mn6Sn6. Such a high entropy material exhibits multiple spin reorientation transitions, which is not seen in all the related parent compounds and can be understood in terms of competing magnetic interactions enabled by high entropy. Furthermore, we also observed an intrinsic anomalous Hall effect, indicating that the high entropy phase preserves the non-trivial band topology. These results suggest that high entropy may provide a route to engineer the magnetic structure and expand the horizon of topological materials.
  • Sensing performance of sub-100-nm vanadium oxide films for room temperature thermal detection applications
    Scott, E. A.; Singh, M. K.; Barber, J. P.; Rost, Christina M.; Ivanov, S.; Watt, J.; Pete, D.; Sharma, P.; Lu, T. M.; Harris, C. T. (AIP Publishing, 2022-11-14)
    Vanadium oxide films are widely employed as thermal detectors in uncooled infrared detection systems due to their high temperature coefficient of resistance near room temperature. One strategy toward maximizing detectivity and reducing the thermal time constant in these systems is to minimize the system platform dimensions. This approach necessitates thinner film thicknesses (≪100 nm), for which there is little information regarding thermal sensing performance. Herein, we report on the sensitivity of reactively sputtered vanadium oxide thin film resistive thermometers nominally ranging from 100 to 25 nm and assess the influence of thermal annealing. We demonstrate that films in this minimum limit of thickness maintain a high temperature coefficient while additionally providing an enhancement in characteristics of the noise equivalent power.
  • Size Effects on the Cross-Plane Thermal Conductivity of Transparent Conducting Indium Tin Oxide and Fluorine Tin Oxide Thin Films
    Olson, David H.; Rost, Christina M.; Gaskins, John T.; Szwejkowski, Chester J.; Braun, Jeffrey L.; Hopkins, Patrick E. (IEEE, 2018-08-06)
    Visibly transparent and electrically conductive oxides are attractive for a wide array of applications. Indium tin oxide (ITO) and fluorine tin oxide (FTO) are the subset of the larger transparent conducting oxide family and possess transmittance in the visible spectrum as well as high electrical conductivity. Even though their unique optical and electrical properties have been thoroughly examined, the thermal transport properties, namely, thermal conductivity in the cross-plane direction, have received much less attention. In this paper, using a series of ITO and FTO thin films comprising a range of thicknesses and grain sizes, we characterize the cross-plane thermal conductivity using time-domain thermoreflectance. We determine the heat capacity of the FTO films from simultaneous measurements of volumetric heat capacity and thermal conductivity on an 396-nm-thick FTO film. We show that the size effects have a considerable influence on the thermal conductivity from both the perspective of grain boundary and thin film scattering.
  • What is in a name: Defining "high entropy" oxides
    Brahlek, Matthew; Gazda, Maria; Keppens, Veerle; Mazza, Alessandro R.; McCormack, Scott J.; Mielewczyk-Gryń, Aleksandra; Musico, Brianna; Page, Katharine; Rost, Christina M.; Sinnott, Susan B.; Toher, Cormac; Ward, Thomas Z.; Yamamoto, Ayako (AIP Publishing, 2022-11-04)
    High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material's crystallinity, composition, and mixing length scales in determining its true definition.
  • Searching for superconductivity in high entropy oxide Ruddlesden-Popper cuprate films
    Mazza, Alessandro R.; Gao, Xingyao; Rossi, Daniel J.; Musico, Brianna L.; Valentine, Tyler W.; Kennedy, Zachary; Zhang, Jie; Lapano, Jason; Keppens, Veerle; Moore, Robert G.; Brahlek, Matthew; Rost, Christina M.; Ward, Thomas Z. (American Vacuum Society, 2021-11-29)
    In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results, in turn, are connected to extended x-ray absorption fine structure results, indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane (σ2 > 0.015 Å2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.
  • Power device breakdown mechanism and characterization: review and perspective
    Zhang, Ruizhe; Zhang, Yuhao (IOP Publishing, 2023-04)
    Breakdown voltage (BV) is arguably one of the most critical parameters for power devices. While avalanche breakdown is prevailing in silicon and silicon carbide devices, it is lacking in many wide bandgap (WBG) and ultra-wide bandgap (UWBG) devices, such as the gallium nitride high electron mobility transistor and existing UWBG devices, due to the deployment of junction-less device structures or the inherent material challenges of forming p-n junctions. This paper starts with a survey of avalanche and non-avalanche breakdown mechanisms in WBG and UWBG devices, followed by the distinction between the static and dynamic BV. Various BV characterization methods, including the static and pulse I-V sweep, unclamped and clamped inductive switching, as well as continuous overvoltage switching, are comparatively introduced. The device physics behind the time- and frequency-dependent BV as well as the enabling device structures for avalanche breakdown are also discussed. The paper concludes by identifying research gaps for understanding the breakdown of WBG and UWBG power devices.
  • Efficient Activation and High Mobility of Ion-Implanted Silicon for Next-Generation GaN Devices
    Jacobs, Alan G.; Feigelson, Boris N.; Spencer, Joseph A.; Tadjer, Marko J.; Hite, Jennifer K.; Hobart, Karl D.; Anderson, Travis J. (MDPI, 2023-04)
    Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 ?, producing a mobility of up to 137 cm(2)/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 x 10(19) cm(-3) box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices.
  • In situ electron tomography for the thermally activated solid reaction of anaerobic nanoparticles
    Ihara, Shiro; Yoshinaga, Mizumo; Miyazaki, Hiroya; Wada, Kota; Hata, Satoshi; Saito, Hikaru; Murayama, Mitsuhiro (Royal Society Chemistry, 2023-06)
    The nanoscale characterization of thermally activated solid reactions plays a pivotal role in products manufactured by nanotechnology. Recently, in situ observation in transmission electron microscopy combined with electron tomography, namely four-dimensional observation for heat treatment of nanomaterials, has attracted great interest. However, because most nanomaterials are highly reactive, i.e., oxidation during transfer and electron beam irradiation would likely cause fatal artefacts; it is challenging to perform the artifact-free four-dimensional observation. Herein, we demonstrate our development of a novel in situ three-dimensional electron microscopy technique for thermally activated solid-state reaction processes in nanoparticles (NPs). The sintering behaviour of Cu NPs was successfully visualized and analyzed in four-dimensional space-time. An advanced image processing protocol and a newly designed state-of-the-art MEMS-based heating holder enable the implementation of considerably low electron dose imaging and prevent air exposure, which is of central importance in this type of observation. The total amount of electron dose for a single set of tilt-series images was reduced to 250 e(-) nm(-2), which is the lowest level for inorganic materials electron tomography experiments. This study evaluated the sintering behaviour of Cu NPs in terms of variations in neck growth and particle distance. A negative correlation between the two parameters is shown, except for the particle pair bound by neighbouring NPs. The nanoscale characteristic sintering behavior of neck growth was also captured in this study.