Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme

Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme

TEM studies of multilayer ohmic contacts to n-type AlGaN/GaN

Ti and Pd barrier layers between the Al/Ti diffusion couple and the Au capping layer of multilayer ohmic contacts to n-type AlGaN/GaN field effect transistors were found to be ineffective in preventing the diffusion of Au to the AlGaN following high temperature rapid thermal annealing. The formation of a band of TiN grains at the contact/AlGaN interface is responsible for the activation of the contact. The presence of interfacial Au and threading dislocations are implicated in the formation of additional Ti-nitride inclusions into the AlGaN, although these do not appear to disrupt the Ti-nitride layer at the original contact/nitride interface, nor significantly influence the contact resistance

Predicting radioactive waste glass dissolution with machine learning

The vitrification of high-level nuclear waste within borosilicate glass and its disposition within a multi-barrier repository deep underground is accepted as the best form of disposal. Here, the ability of machine learning to predict both static and dynamic glass leaching behavior is analysed using large-scale unstructured multi-source data, covering a diverse range of experimental conditions and glass compositions. Machine learning can accurately predict leaching behavior, predict missing data, and time forecast. Accuracy depends upon the type of learning algorithm, model input variables, and diversity or size of the underlying dataset. For static leaching, the bagged random forest method predicts well, even when either pH or glass composition are neglected as input variables, additionally showing potential in predicting independent glass dissolution data. For dynamic leaching, accuracy improves if replacing final pH with a species dissolution rate as an input variable, although results show no preferred output species (Si, Na, or Al)

Assessing static glass leaching predictions from large datasets using machine learning

Radioactive waste vitrified within glass is planned to be ultimately disposed of within a geological disposal facility. This study has applied machine learning to predict static glass leaching using an international experimental database of approximately 450 glasses to train/test various algorithms. Machine learning can accurately predict B, Li, Na, and Si releases for this complex database with Tree-based algorithms (notably ‘BaggingRegressor’ and ‘RandomForestRegressor’ in Python). This is provided that leaching experiment results, including elemental releases, are incorporated within the algorithm training variables, given that this study finds inaccurate prediction solely using initial test parameters as features. The trained algorithms underwent additional testing using an external database with prediction showing worse performance, likely due to substantial MgO and Na2O pristine glass oxide compositional variations across databases, with B releases generally being overestimated and Na underestimated. The use of molar oxide content performed significantly better than weight-fraction oxide for learning

Evidence for Dirac nodes from quantum oscillations in SrFe2As2

We present a detailed study of quantum oscillations in the antiferromagnetically ordered pnictide compound SrFe2As2 as the angle between the applied magnetic field and crystalline axes is varied. Our measurements were performed on high-quality single crystals in a superconducting magnet, and in pulsed magnetic fields up to 60 T, allowing us to observe orbits from several small Fermi-surface pockets. We extract the cyclotron effective mass m☆ and frequency F for these orbits and track their values as the field is rotated away from the c axis. While a constant ratio of m☆/F is expected for a parabolic band, we observe deviations from this behavior. We conclude that this observation points to orbits derived from a band with Dirac dispersion near the Fermi level

Puffins, Pigs, Cod, and Barley: Palaeoeconomy at Undir Junkarinsfløtti, Sandoy, Faroe Islands

This paper reports on the zooarchaeological and archaeobotanical remains from the initial season of excavations at the Norse period site at Undir Junkarinsfløtti in the Faroe islands. These remains represent the first zooarchaeological analysis undertaken for the Faroes and only the third archaeobotanical assemblage published from the islands. The excavated deposits are described and the key findings from the palaeoenvironmental remains highlighted within the context of the wider North Atlantic environmental archaeology of the Norse period

Puffins, Pigs, Cod, and Barley: Palaeoeconomy at Undir Junkarinsfløtti, Sandoy, Faroe Islands

This paper reports on the zooarchaeological and archaeobotanical remains from the initial season of excavations at the Norse period site at Undir Junkarinsfløtti in the Faroe islands. These remains represent the first zooarchaeological analysis undertaken for the Faroes and only the third archaeobotanical assemblage published from the islands. The excavated deposits are described and the key findings from the palaeoenvironmental remains highlighted within the context of the wider North Atlantic environmental archaeology of the Norse period

Assessing the effect of radioactive waste glass dissolution on early-stage radionuclide migration using simplified geological repository Monte Carlo transport models

Funder: Engineering and Physical Sciences Research Council; doi: http://dx.doi.org/10.13039/501100000266; Grant(s): EPSRC Grant Number: EP/L015900/1Abstract: The vitrification of radioactive waste within glass and subsequent disposal within a geological disposal facility (GDF) requires a comprehensive understanding of the effect of glass dissolution on GDF performance. This paper aims to analyse the effect of both high-level and intermediate-level waste (HLW and ILW) glass dissolution source terms on radionuclide release into the geosphere just above the disposal vault (the ‘crown’). Radionuclide migration was simulated in GoldSim for HLW in either granite or clay host rocks with a bentonite buffer using carbon steel or copper canisters, whereas ILW simulations considered either granite or clay host rocks, in either bentonite buffer or cement backfill, using concrete or cast-iron canisters. Glass dissolution source terms were varied by coupling GoldSim and MATLAB to modify the initial, residual, and resumption dissolution rates of the glass or by applying the analytical GRAAL model to glass dissolution. HLW glass results indicate no preference of granite over clay host rocks for a given canister type but that a copper canister is preferable to steel. ILW results suggest that a granite–bentonite–cast-iron environment yields lowest crown activities with cast-iron preferable to concrete as the canister, bentonite preferable to cement as the buffer/backfill, and granite preferable to clay as the host rock. Varying glass dissolution source terms (initial, residual, and resumption dissolution rates) had an understood effect on radionuclide migration, although changes were arguably insignificant considering peak crown activity for both HLW and ILW

Reviews

George MacDonald: Divine Carelessness and Fairytale Levity. Daniel Gabelman. Reviewed by Bonnie Gaarden. The Gender Dance: Ironic Subversion in C.S. Lewis\u27s Cosmic Trilogy. Monika B. Hilder. Preface by Matthew Dickerson. Reviewed by Joe R. Christopher. Sir Gawain and the Green Knight: In a Modern English Version with a Critical Introduction. John Gardner. Reviewed by Perry Neil Harrison. Myths of Light: Eastern Metaphors of the Eternal. Joseph Campbell. Reviewed by Christopher Tuthill. The Riddles of the Hobbit. Adam Roberts. Reviewed by Jon Garrad. The Modern Literary Werewolf: A Critical Study of the Mutable Motif. Brent A. Stypczynski. Reviewed by Sharon L. Bolding. Fairy Tales Reimagined: Essays on New Retellings. Ed. by Susan Redington Bobby. Reviewed by Kazia Estrada. C.S. Lewis\u27s Perelandra: Reshaping the Image of the Cosmos. Ed. Judith Wolfe and Brendan Wolfe. Reviewed by Holly Ordway. The Ideal of Kingship in the Writings of Charles Williams, C.S. Lewis, and J.R.R. Tolkien: Divine Kingship is Reflected in Middle-Earth. Christopher Scarf. Reviewed by Melody Green. The Ring of Words: Tolkien and the Oxford English Dictionary. By Peter Gilliver, Jeremy Marshall, and Edmund Weiner. Reviewed by Mike Foster. Tolkien: The Forest and the City. Ed. Helen Conrad-O\u27Briain and Gerard Hynes. Reviewed by T.S. Miller. Tolkien Studies X. Edited by Michael D.C. Drout, Verlyn Flieger, and David Bratman. Reviewed by Janet Brennan Croft. Seven: An Anglo-American Literary Review 30. Edited by Marjorie Lamp Mead. Reviewed by Janet Brennan Croft