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Materials | About the 2015 Journal of Materials Science Robert W. Cahn Best Paper Prize

About the 2015 Journal of Materials Science Robert W. Cahn Best Paper Prize

The Editors and Publisher are pleased to announce the monthly finalists for the 2015 Journal of Materials Science Robert W. Cahn Best Paper Prize (the “Cahn Prize”). The Cahn Prize has been named in honor of the Journal’s founding editor, the late Professor Robert Wolfgang Cahn.

Each month the Editors selected a paper published in that month's issues via a rigorous nomination and voting procedure. The winning paper will be selected from the twelve finalists by a separate panel of distinguished materials scientists. The authors of the winning paper will receive an award of $5,000.

Winning Article: December: Structure, composition, and defect control during plasma spray deposition of ytterbium silicate coatings 

Bradley T. Richards, Hengbei Zhao, Haydn N. G. Wadley

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Environmental barrier coatings (EBCs) are needed to protect SiC structures exposed to high temperatures in water vapor-rich environments. Recent studies of a tri-layer EBC system consisting of a silicon layer attached to the SiC, a mullite diffusion barrier and a low-steam volatility ytterbium silicate topcoat have shown some promise for use at temperatures up to 1316 °C. However, the performance of the coating system appeared to be dependent upon the manner of its deposition. Here, an air plasma spray method has been used to deposit this tri-layer EBC on α-SiC substrates, and the effects of the plasma arc current and hydrogen content upon the structure, composition, and defects in ytterbium monosilicate (Yb2SiO5) and disilicate (Yb2Si2O7) topcoats are investigated. Modification of spray parameters enabled the loss of SiO from the injected powder to be reduced, leading to partial control of coating stoichiometry and phase content. It also enabled significant control of the morphology of solidified droplets, the porosity, and the microcracking behavior within the coatings. Differences between the Yb2SiO5 and Yb2Si2O7 are discussed in the context of their EBC application.

January: Morphology and oxygen vacancy investigation of strontium titanate-based photo electrochemical cells 

D. Hertkorn, M. Benkler, U. Gleißner, F. Büker, C. Megnin, C. Müller, T. Hanemann, H. Reinecke

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In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO3) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO3 powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO3 lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively...

February: Phase transformation kinetics during continuous heating of a β-quenched Ti–10V–2Fe–3Al alloy 

Pere Barriobero-Vila, Guillermo Requena, Fernando Warchomicka, Andreas Stark, Norbert Schell, Thomas Buslaps

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The effect of heating rate on the phase transformation kinetics of a Ti–10V–2Fe–3Al metastable β titanium alloy quenched from the β field is investigated by fast in situ high energy synchrotron X-ray diffraction and differential scanning calorimetry. The initial microstructure is formed by α″ martensite and fine ωath particles distributed in the retained β-phase matrix. The phase transformation sequence varies with the heating rate as revealed by analysis of the continuous evolution of crystallographic relationships between phases. At low temperatures an athermal reversion of α″ martensite into β takes place. This reversion occurs to a larger extent with increasing heating rate. On the other hand, diffusion–driven precipitation and growth of the ω phase is observed for lower heating rates accompanying the reverse martensitic transformation. Furthermore, the results show that the stable α phase can form through three different paths: (a) from the ω phase, (b) from α″ martensite, and (c) from the β phase.

March: Topological and Euclidean metrics reveal spatially nonuniform structure in the entanglement of stochastic fiber bundles 

T. Fast, A. E. Scott, H. A. Bale, B. N. Cox

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...We propose a combination of topological and Euclidean metrics to quantify these and other stochastic bundle characteristics. Topological metrics are based on the neighbor map of fibers, which is constructed on cross-sections of the bundle by Delaunay triangulation (or Voronoi tessellation). Variations of the neighbor map along the fiber direction describe fiber meandering, twist, etc. Euclidean metrics include factors such as local fiber density and fiber orientation. The metrics distinguish bundle types, enable quantification of the effects of the manufacturing history of bundles, and provide target statistics to be matched by virtual specimens that might be generated for use in fiber-scale virtual tests.

April: Nanoscale morphologies at alloyed and irradiated metal-oxide bilayers 

J. A. Aguiar, O. Anderoglu, S. Choudhury, J. K. Baldwin, Y. Wang, A. Misra, B. P. Uberuaga

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...In particular, the chemical evolutions of as-deposited Fe/Cr and irradiated Fe thin films on TiO2 are characterized to reveal structural and chemical changes associated with physical interactions induced by either alloying or irradiation. The results of the study conclude by comparing the effects of alloying with radiation-induced intermixing. We find that the extent of Fe intermixing into the TiO2 substrate is similar for both irradiated and alloyed films, indicating that both can lead to the formation of similar complex nanoscale morphologies at the interface. Our results highlight the complex and competing phenomena that dictate the structure and chemistry at these interfaces.

May: First-principles-based kinetic Monte Carlo studies of diffusion of hydrogen in Ni–Al and Ni–Fe binary alloys 

De Nyago Tafen

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The diffusion of dilute hydrogen in fcc Ni–Al and Ni–Fe binary alloys was examined using kinetic Monte Carlo method with input kinetic parameters obtained from first-principles density functional theory. The simulation involves the implementation of computationally efficient energy barrier model that describes the configuration dependence of the hydrogen hopping. The predicted hydrogen diffusion coefficients in Ni and Ni89.4Fe10.6 are compared well with the available experimental data. In Ni–Al, the model predicts lower hydrogen diffusivity compared to that in Ni. Overall, diffusion prefactors and the effective activation energies of H in Ni–Fe and Ni–Al are concentration dependent of the alloying element. The changes in their values are the results of the short-range order (nearest-neighbor) effect on the interstitial diffusion of hydrogen in fcc Ni-based alloys.

June: Non-coherent Cu grain boundaries driven by continuous vacancy loading 

W. S. Yu, M. J. Demkowicz

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We use atomistic modeling to study the response of three non-coherent grain boundaries (GBs) in Cu to continuous loading with vacancies. Our simulations yield insights into the structure and properties of these boundaries both near and far from thermal equilibrium. We find that GB energies vary periodically as a function of the number of vacancies introduced. Each GB has a characteristic minimum energy state that recurs during continuous vacancy loading, but in general cannot be reached without removing atoms from the boundary. There is no clear correlation of GB energies with GB specific excess volumes or stresses during vacancy loading. However, GB stresses increase monotonically with specific excess volumes. Continuous vacancy loading gives rise to GB migration and shearing, despite the absence of applied loads. Successive vacancies introduced into some of the boundaries accumulate at the cores of what appear to be generalized vacancy dislocation loops. We discuss the implications of these findings for our understanding of grain boundary sink efficiencies under light ion irradiation.

July: Instrument for stable high temperature Seebeck coefficient and resistivity measurements under controlled oxygen partial pressure 

Harlan J. Brown-Shaklee, Peter A. Sharma, Jon F. Ihlefeld

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The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO2) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO2-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O2 sensor and the transient behavior of the resistance as a proxy. A pO2 range of 10−25–100 atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt–Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO2 on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. An approximately 11 % increase in power factor over a pO2 range of 10−19–10−8 atm at 973 K for the donor-doped single crystals is observed.

August: Comparative investigation of different silane surface functionalizations of fullerene-like WS2 

Dietmar Haba, Thomas Griesser, Ulrich Müller, Andreas J. Brunner

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...In this work, IF-WS2 are functionalized using three different silane modifiers and investigated with X-ray photoelectron spectrometry, infrared spectroscopy, titration, thermogravimetric analysis, and mass spectroscopy. Eventually, they are dispersed within ethanol by sonication to compare the dispersing behavior. The combination of the different analytical techniques revealed that the IF-WS2 surfaces can be functionalized with two of the used silane modifiers, while the third one was repeatedly unsuccessful. The amount of Si on the particles seems to be a fairly clear indication for the success of the functionalization reaction. The IF-WS2 seems to oxidize during the functionalization process, probably producing acidic SO2 or SO3, which can fully acidify a basic surface modifier. The executed treatment without any added silane modifier improved the dispersibility of the IF-WS2 within ethanol to some extent, but added modifiers deteriorated it significantly. TEM images indicate that IF-WS2 particles form aggregates, which might be the reason for the limited dispersibility.

September: Assessment of ultrathin yttria-stabilized zirconia foils for biomedical applications 

C. Le Coadou, N. Karst, F. Emieux, O. Sicardy, A. Montani, G. Bernard-Granger, J. Chevalier, L. Gremillard, J.-P. Simonato

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Bulk yttria-stabilized zirconia is a well-studied material with various applications, in particular for biomedical devices. Interestingly, it is now possible to obtain thin stand-alone sheets (40 µm thick) of this material. We present a preliminary study dealing with the aging of these foils to determine whether they would be suitable for implantable neurostimulator housing. Accelerated hydrothermal aging experiments were performed at 134 °C and 2 bar. Various analyses of the material were carried out, especially by atomic force microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV–visible-near-infrared spectroscopy. The results show that the foils maintain their integrity during simulated long-term in vivo experiments, which indicates that they are not prone to fast degradation.

October: Detecting rare, abnormally large grains by x-ray diffraction 

B. L. Boyce, T. A. Furnish, H. A. PadillaII, D. Van Campen, A. Mehta

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...The present study presents a new method for rapid detection of unusually large grains embedded in a sea of much finer grains. Traditional X-ray diffraction-based grain size measurement techniques such as Scherrer, Williamson–Hall, or Warren–Averbach rely on peak breadth and shape to extract information regarding the average crystallite size. However, these line broadening techniques are not well suited to identify a very small fraction of abnormally large grains. The present method utilizes statistically anomalous intensity spikes in the Bragg peak to identify regions where abnormally large grains are contributing to diffraction. This needle-in-a-haystack technique is demonstrated on a nanocrystalline Ni–Fe alloy which has undergone fatigue-induced abnormal grain growth. In this demonstration, the technique readily identifies a few large grains that occupy less than 0.00001 % of the interrogation volume. While the technique is demonstrated in the current study on nanocrystalline metal, it would likely apply to any bimodal polycrystal including ultrafine grained and fine microcrystalline materials with sufficiently distinct bimodal grain statistics.

November: Inferring grain boundary structure–property relations from effective property measurements 

Oliver K. Johnson, Lin Li, Michael J. Demkowicz, Christopher A. Schuh

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...In the present work, we infer grain boundary structure–property correlations from measurements of the effective properties of a polycrystal. We refer to this approach as grain boundary properties localization. We apply this technique to a simple model system of grain boundary diffusivity in a two-dimensional microstructure, and infer the properties of low- and high-angle grain boundaries from the effective diffusivity of the grain boundary network. The generalization and use of these methods could greatly reduce the computational and experimental effort required to establish structure–property correlations for grain boundaries. More broadly, the technique of properties localization could be used to infer the properties of many microstructural constituents in complex microstructures.