Accelerated sintering in phase-separating nanostructured alloys

Sintering of powders is a common means of producing bulk materials when melt casting is impossible or does not achieve a desired microstructure, and has long been pursued for nanocrystalline materials in particular. Acceleration of sintering is desirable to lower processing temperatures and times, and thus to limit undesirable microstructure evolution. Here we show that markedly enhanced sintering is possible in some nanocrystalline alloys. In a nanostructured W–Cr alloy, sintering sets on at a very low temperature that is commensurate with phase separation to form a Cr-rich phase with a nanoscale arrangement that supports rapid diffusional transport. The method permits bulk full density specimens with nanoscale grains, produced during a sintering cycle involving no applied stress. We further show that such accelerated sintering can be evoked by design in other nanocrystalline alloys, opening the door to a variety of nanostructured bulk materials processed in arbitrary shapes from powder inputs.United States. Defense Threat Reduction Agency (Grant HDTRA1-11-1-0062)United States. Army Research Office (Grant W911NF-09-1-0422)United States. Army Research Office (Grant W911NF-14-1-0539)Kwanjeong Educational Foundation (Korea

Diffusion of tungsten in chromium: Experiments and atomistic modeling

The solute diffusion of tungsten at low concentrations in chromium has been investigated both by experiments and computational methods. From finite-source diffusion experiments measured with an Electron Probe Micro Analyzer at temperatures from 1526 to 1676 K, it was found that the diffusivity of tungsten in chromium follows the Arrhenius relationship D=D[subscript 0]exp(-Q[over]RT), where the activation energy was found to be Q = 386 ± 33 kJ/mol. Diffusion of tungsten in chromium was investigated computationally with both the activation–relaxation technique (ART) and molecular dynamics (MD) using a hybrid potential. From ART, the effective diffusion activation energy was determined to be Q = 315 ± 20 kJ/mol based on a multi-frequency model for a monovacancy mechanism. From MD, the square displacement of tungsten was analyzed at temperatures between 1200 and 1700 K, and the diffusion activation energy was determined to be Q = 310 ± 18 kJ/mol. In spite of possible complications arising due to experimental compositions away from the dilute limit, the agreement between experiments and simulations falls within the calculated uncertainties, supporting a monovacancy mechanism for diffusion of tungsten in chromium.United States. Defense Threat Reduction Agency (Grant No. HDTRA1-11-1-0062)United States. Army Research Office (Grant No. W911NF-09-1-0422)Kwanjeong Educational Foundation (Korea)United States. Dept. of Energy (DOE Computational Science Graduate Fellowship, Grant No. DE-FG02-97ER25308)Hertz Foundatio

Duplex nanocrystalline alloys: Entropic nanostructure stabilization and a case study on W–Cr

Grain boundary (GB) segregation can markedly improve the stability of nanostructured alloys, where the fraction of GB sites is inherently large. Here, we explore the concept of entropically supported GB segregation in alloys with a tendency to phase-separate and its role in stabilizing nanostructures therein. These duplex nanocrystalline alloys are notably different, both in a structural and thermodynamic sense, from the previously studied “classical” nanocrystalline alloys, which are solid solutions with GB segregation of solute. Experiments are conducted on the W–Cr system, in which nanoduplex structures are expected. Upon heating ball-milled W–15 at.% Cr up to 950 °C, a nanoscale Cr-rich phase was found along the GBs. These precipitates mostly dissolved into the W-rich grains leaving behind Cr-enriched GBs upon further heating to 1400 °C. The presence of Cr-rich nanoprecipitates and GB segregation of Cr is in line with prediction from our Monte Carlo simulation when GB states are incorporated into the alloy thermodynamics.United States. Army Research Office (Grant W911NF-09-1-0422)United States. Army Research Office (Grant W911NF-14-1-0539)United States. Defense Threat Reduction Agency (Grant HDTRA1-11-1-0062)Kwan-Jung Scholarshi

Satellite Laser Ranging System at Geochang Sta

Korea Astronomy and Space Science Institute (KASI) has been developing the space optical and laser tracking (SOLT) system for space geodesy, space situational awareness, and Korean space missions. The SOLT system comprises satellite laser ranging (SLR), adaptive optics (AO), and debris laser tracking (DLT) systems, which share numerous subsystems, such as an optical telescope and tracking mount. It is designed to be capable of laser ranging up to geosynchronous Earth orbit satellites with a laser retro-reflector array, space objects imaging brighter than magnitude 10, and laser tracking low Earth orbit space debris of uncooperative targets. For the realization of multiple functions in a novel configuration, the SOLT system employs a switching mirror that is installed inside the telescope pedestal and feeds the beam path to each system. The SLR and AO systems have already been established at the Geochang station, whereas the DLT system is currently under development and the AO system is being prepared for testing. In this study, the design and development of the SOLT system are addressed and the SLR data quality is evaluated compared to the International Laser Ranging Service (ILRS) tracking stations in terms of single-shot ranging precision. The analysis results indicate that the SLR system has a good ranging performance, to a few millimeters precision. Therefore, it is expected that the SLR system will not only play an important role as a member of the ILRS tracking network, but also contribute to future Korean space missions

Abnormal spatial heterogeneity governing the charge-carrier mechanism in efficient Ruddlesden-Popper perovskite solar cells

Layered Ruddlesden-Popper perovskite (RPP) photovoltaics have gained substantial attention owing to their excellent air stability. However, their photovoltaic performance is still limited by the unclear real-time charge-carrier mechanism of operating devices. Herein, we report the correlation between the charge-carrier mechanism and the spatially heterogeneous RPP bulks induced by distinct sublattice cations in the state-of-the-art antisolvent-driven RPP devices. In particular, abnormal heterogeneities ranging from the lateral long-range to local sub-grain scale and corresponding charge-carrier behaviours are visualized for triple-cation RPPs. We discovered that such heterogeneities with a unitary 2D/3D hybrid suppress lattice vibrations and reduce Frohlich interactions by about 2 times, significantly promoting charge-carrier dynamics. Consequently, optimized triple-cation RPP solar cells greatly outperform their mono-cation counterparts. Furthermore, this principle can be applicable irrespective of 2D layer thickness (n > 2) and substrate type. This work provides a rationale for leveraging a disordered structure to stimulate charge-carrier motion and suggests the design principle of low-dimensional perovskites.

Evaluation of a Laser Altimeter using the Pseudo-Random Noise Modulation Technique for Apophis Mission

Apophis is a near-Earth object with a diameter of approximately 340 m, which will come closer to the Earth than a geostationary orbit in 2029, offering a unique opportunity for characterizing the object during the upcoming encounter. Therefore, Korea Astronomy and Space Science Institute has a plan to propose a space mission to explore the Apophis asteroid using scientific instruments such as a laser altimeter. In this study, we evaluate the performance metrics of a laser altimeter using a pseudorandom noise modulation technique for the Apophis mission, in terms of detection probability and ranging accuracy. The closed-form expression of detection probability is provided using the cross correlation between the received pulse trains and pseudo-random binary sequence. And the new ranging accuracy model using Gaussian error propagation is also derived by considering the sampling rate. The operation range is significantly limited by thermal noise rather than background noise, owing to not only the low power laser but also the avalanche photodiode in the analog mode operation. However, it is demonstrated from the numerical simulation that the laser altimeter can achieve the ranging performance required for a proximity operation mode, which employs commercially available components onboard CubeSat-scale satellites for optical communications

A Generic Software Development Process Refined from Best Practices for Cloud Computing

Cloud computing has emerged as more than just a piece of technology, it is rather a new IT paradigm. The philosophy behind cloud computing shares its view with green computing where computing environments and resources are not as subjects to own but as subjects of sustained use. However, converting currently used IT services to Software as a Service (SaaS) cloud computing environments introduces several new risks. To mitigate such risks, existing software development processes must undergo significant remodeling. This study analyzes actual cases of SaaS cloud computing environment adoption as a way to derive four new best practices for software development and incorporates the identified best practices for currently-in-use processes. Furthermore, this study presents a design for generic software development processes that implement the proposed best practices. The design for the generic process has been applied to reinforce the weak points found in SaaS cloud service development practices used by eight enterprises currently developing or operating actual SaaS cloud computing services. Lastly, this study evaluates the applicability of the proposed SaaS cloud oriented development process through analyzing the feedback data collected from actual application to the development of a SaaS cloud service Astation

Design of bulk nanocrystalline tungsten alloys via nano-phase separation sintering

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 96-100).An accelerated sintering method called 'nano-phase separation sintering' is developed, with specific applicability to nanostructured tungsten alloys. Nanocrystalline tungsten alloys containing minority additions of chromium are produced by high-energy ball milling and then consolidated. Such alloys exhibit the onset of sintering at a very low temperature around 950 °C and a very rapid rate of densification. The mechanism of this accelerated sintering is established through understanding the role of nano-scale, solid second phase precipitation during the sintering cycle, as analyzed by thermomechanical analysis, electron microscopy and x-ray diffraction. In addition, control experiments are used to establish that the accelerated sintering is apparently accomplished from the combination of two features of the powders: (i) nanocrystallinity and (ii) alloy supersaturation. In addition to accelerating sintering, the incorporation of alloying elements and second phases are also beneficial for mitigating grain growth during a thermal cycle, so nanophase separation sintering is thus naturally appropriate to the production of fine-grained bulk materials. Sintered compacts achieved through nano-phase separation sintering display 10~30 times smaller grain sizes at comparable densities than those produced by conventional accelerated sintering methods such as solid-state activated sintering and liquid phase sintering. The thermodynamic features and conditions for nano-phase separation sintering are further explored based on the binary phase diagram in order to generalize the concept to other alloy systems. After presenting a series of proposed alloy design rules, the consolidation of chromium with an addition of nickel is accelerated. Prospects of the technique for the development of full density bulk products in more complex alloy systems are also discussed.by Mansoo Park.Ph. D