Exploration of Ternary Intermetallic Materials Using Tin and Gallium Flux

The focus of this dissertation is the structure-property relationships of ternary intermetallic stannides and gallides. We are interested in these compounds because of the wide range of physical properties they possess. While investigating the Ln-Co-Sn system (Ln = lanthanide), we have synthesized single crystals that crystallize in the Yb3Rh4Sn13, Ho7Co6Sn23 and the Tb5Rh6Sn18 structure types. We observe the formation of a particular structure type depending on the size of the lanthanide present. The aforementioned compounds all contain CoSn6 trigonal prisms, which create voids occupied by Ln and Sn polyhedral units. We have also investigated the physical properties of these compounds to correlate the magnetic and transport phenomena observed. To determine the role of magnetic transition metals in lanthanide gallium compounds we have explored the Ln-Fe-Ga system and synthesized single crystals of Ln4FeGa12 (Ln = Y, Tb, Dy, Ho and Er). The structure consists of iron octahedra and face sharing rare-earth cuboctahedra (LnGa3). Magnetic susceptibility measurements performed on Yb4FeGa12 show magnetic ordering in the Fe octahedra, a feature not observed in the isostructural analogues with Tb, Dy, Ho and Er. We have also synthesized single crystals of YbCoGa5 using gallium flux. YbCoGa5 adopts the HoCoGa5 structure type which is made up of CoGa2 and YbGa3 structural units. The synthesis of YbCoGa5 has filled a void in the LnCoGa5 compounds by reported Yuri Grin. We have also studied the single crystal structures of Ni1-xCoxGa2S4 (x = 0, 0.2, 0.3, 0.5) and Ni0.9Mn0.1Ga2S4 to confirm composition and to study the role of doped Co and Mn in the two-dimensional antiferromagnet NiGa2S4

Ghana must go: modernity, memory and material culture in post-independence West Africa

This dissertation investigates the capacity for objects and material culture to embody opposing or multiple versions of the past, focusing on the post-independence upheavals of Ghana. The work combines historical research–examination of historical documents, news media, and objects themselves, as well as personal interviews and fieldwork–with a theoretical investigation of how we understand history to be constituted in material culture. One of the intentions of this investigation is to examine the ways in which national identities are formed in direct relation to material culture. Obliterated or nearly forgotten histories are traced and articulated through everyday things; the built environment; and structures both found and made. Some of the artefacts examined in this thesis include a plastic bag; early through late 20thcentury architectural forms, such as colonial and post-independence monuments; and items within the landscape and environment. These artefacts help to underscore the genesis of contemporary Ghana as a nation. The country’s cultural and political history are used to explain how identity and identifications are constructed in relation to ideas of modernity. Many of the artefacts under review are linked to Ghana’s complicated history as a slave trading nation; therefore a part of this dissertation includes an analysis of existing scholarship relating to a slave monument in the Volta Region. This part of the analysis show show artefacts can easily produce polyvalent readings that can undermine the reliability of source materials in the field of research. The dissertation’s central themes are framed by ideas drawn from a number of important thinkers, including Paul Gilroy, whose notion of the The Black Atlantic (1993) examines links between the cultures within the so-called ‘Black Triangle’ (the USA, Europe and Africa) in the context of the legacy of the Trans-Atlantic Slave Trade

The currency of practice: Reclaiming autonomy for the MFA

The roundtable panel met in February 2007 at the CAA Annual Conference in New York. Andrew E. Hershberger and Jon Rubin were among the original invitees but were unable to fly to New York because of inclement weather

Magnetic properties of the single crystal stannides Ln7Co6Sn23 (Ln=Dy, Ho) and Ln5Co6Sn18 (Ln=Er, Tm)

Single crystals of Ln7Co6Sn23 (Ln=Dy, Ho) and Ln5Co6Sn18 (Ln=Er, Tm) have been grown using tin-flux. Ln7Co6Sn23 (Ln=Dy, Ho) adopts the trigonal Ho7Co6Sn23 structure type in space group P3̄m1 (No. 164) with a=9.6420(4) Å, c=9.8590(5) Å, and V=793.77(6) Å3 for Dy7Co6Sn23 and a=9.6210(3) Å, c=9.8210(4) Å, and V=787.27(5) Å3 for Ho7Co6Sn23 (Z=1). Whilst Ln5Co6Sn18 (Ln=Er, Tm) adopts the tetragonal Sn1-xErxEr4Rh6Sn18 structure (I41/acd, No. 142) with a=13.5310(2) Å, c=26.9970(4) Å, V=4942.83(13) Å3 for Er5Co6Sn18 and a=13.5190(6) Å, c=26.9760(9) Å, V=4930.2(4) Å3 for Tm5Co6Sn18 (Z=8). We report the low-temperature magnetic behavior of the above compounds. Dy7Co6Sn23, Ho7Co6Sn2, Er5Co6Sn18 and Tm5Co6Sn18 show weak magnetic interactions. A signature of magnetic ordering is only observed in Dy7Co6Sn23. © 2007 Elsevier B.V. All rights reserved

Critical current behavior of superconducting MoN and Mo3 Sb7 microfibers

We present the transport and critical current measurements on carbon microfibers coated with 40-nm of polycrystalline δ-MoN and on carbon microfibers coated with 160 nm of Mo3 Sb7. In each case the coatings where synthesized directly onto the 6-μm diameter carbon fibers. The superconducting transition temperature of the MoN microfibers was Tc ∼13.1K, which is the highest temperature reported in any thin film form and is higher than what was reported for stoichiometric δ-MoN, produced by high pressure synthesis. The transition temperature of the Mo3 Sb7 -coated microfibers Tc =7.5K was significantly higher than that observed in powder samples. Near the transition temperature Tc, the critical current density Jc for both systems was well described by the power-law form [1- (T/ Tc) 2] α, where α=1.5 is the Ginzburg-Landau exponent. We extrapolated Jc (0) ≈5× 107 A/ cm2 for the MoN microfibers and Jc (0) ≈2.5× 105 A/ cm2 for the Mo3 Sb7 fibers. © 2008 The American Physical Society