Structural and Magnetic Investigations of Cs\u3csub\u3e2\u3c/sub\u3eAgF\u3csub\u3e4\u3c/sub\u3e and Its Doped Derivatives

This thesis describes the structural and magnetic properties of solid state silver fluorides such as Cs2AgF4 and its doped derivatives. A wide range of techniques such as X-ray and neutron diffraction, susceptibility measurements and inelastic neutron scattering experiments are used to characterize these materials. Silver fluorides are important given their strongly correlated magnetic behavior and the potential to discover a new class of high TC materials. Few studies have been done on silver (II) fluorides as their characterization has been problematic due to their inherent air and water sensitivity. This study is the first to accurately characterize two of these materials

STEM Graduate Students’ Development at the Intersection of Research, Leadership, and Innovation

Researcher innovation and leadership skills are fundamental to create implementable solutions to pressing societal- and market-based global problems. The Research to Innovation to Society (R2I2S) program is a transformative approach to graduate education, training students at the intersection of research, innovation, and leadership. We detail the design of the program, and a three-year exploratory investigation of its impact at one research university in the western United States. We found that, overall, students who participated in the program realized the value of thinking about their scientific research from a market-need perspective. Students perceived enhanced interest in and understanding of societal and market insights related to their own and other’s research. As well, students developed professional skills in communication, team collaboration, innovation, and entrepreneurial skills. We situate our findings in frameworks concerning the development of emerging professionals and argue for programming for STEM graduate students that extends the deep discipline knowledge-based model of professional development into one inclusive of leadership, communication, and innovation goals

Synthesis and Characterization of a New Ferroelectric with Low Lead Content, a High Curie Temperature, and a High Piezoelectric Response

Abstract A new solid solution (1−x)Bi(Fe2/8Ti3/8Mg3/8)O3–(x)PbTiO3 (BFTM‐PT) is synthesized and the electromechanical properties are measured. This system is defined as a low‐lead material with ferroelectric/piezoelectric behavior and a morphotropic phase boundary (MPB) that leads to enhanced properties. The MPB is located between x = 0.30 and 0.35 and coincides with a structural phase transition and a sharp increase in the piezoelectric response. The system demonstrates ferroelectric hysteresis where x = 0.325 displays the best properties with a maximum polarization of 39 µC cm−2 and a remnant polarization of 26 µC cm−2. The range of compositions has high Curie temperature (Tc), ranging from 625–650 °C. Materials with a Tc above 400 °C typically have a low d33 of <50 pC N−1 at room temperature. However, BFTM‐PT has a higher d33 that most other compositions with a Tc in this range, with the highest being 145 pC N−1 for x = 0.375. The d33 drops off above 100 °C, but doping studies can be done in the future to stabilize the piezoelectric response at higher temperatures. These outstanding properties open the possibility of new transducer applications, in particular ones requiring high temperature and high power

Water-Dispersible and Ferroelectric PEGylated Barium Titanate Nanoparticles

Dispersions of ferroelectric nanoparticles in aqueous medium can find promising applications in electro-optical, medical, and smart fluid technologies. In this report, we show the development of highly dispersed nano-sized ferroelectric barium titanate (BaTiO3) powders with high dielectric constant prepared using a simple, one-step low temperature solution method. The surface of these tetragonal-structured nanoparticles were modified with polyethylene glycol as a stabilizer and dispersant. The crystal structure, morphology and dielectric constant of samples are discussed in detail. The colloidal stability and surface behavior of these PEGylated barium titanate nanoparticles are studied by means of z -potential and dynamic light scattering measurements. We show changing the reaction conditions allows to tune the nanoparticle size. This research promotes a pathway to develop advanced ferroelectric nanomaterials with engineered properties in a simple way.</div

Isolation and Chemical Transformations Involving a Reactive Intermediate of MOF‑5

We report the isolation of a nonporous plate-like intermediate species (MOF-<i>i</i>) obtained during the synthesis of MOF-5 and the testing of this intermediate’s reactivity toward three metal ions (Zn^II, Cu^II, and Mn^II) in <i>N</i>,<i>N</i>-dimethylformamide at 120 °C. We obtained interpenetrated MOF-5 crystals from the reaction between MOF-<i>i</i> and Zn­(NO₃)₂·6H₂O, accompanied by a change in morphology from a plate to a cube. Reaction with CuCl₂·2H₂O did not disrupt the plate-like morphology of MOF-<i>i</i>, but it did result in the replacement of Zn^II by Cu^II and formation of a novel porous copper MOF. MOF-<i>i</i> showed no reactivity toward MnCl₂. Our results demonstrate that MOF-<i>i</i> imparts a selective reactivity that is different from the individual metal ions employed in conventional synthesis of MOFs and suggests that reactive intermediates may be useful in extending the diversity of metal–organic frameworks

New Mechanistic Insights on Na-Ion Storage in Nongraphitizable Carbon

Nongraphitizable carbon, also known as hard carbon, is considered one of the most promising anodes for the emerging Na-ion batteries. The current mechanistic understanding of Na-ion storage in hard carbon is based on the “card-house” model first raised in the early 2000s. This model describes that Na-ion insertion occurs first through intercalation between graphene sheets in turbostratic nanodomains, followed by Na filling of the pores in the carbon structure. We tried to test this model by tuning the sizes of turbostratic nanodomains but revealed a correlation between the structural defects and Na-ion storage. Based on our experimental data, we propose an alternative perspective for sodiation of hard carbon that consists of Na-ion storage at defect sites, by intercalation and last via pore-filling

Synthesis and Systematic Trends in Structure and Electrical Properties of [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, <i>m</i> = 1, 2, 3, and 4

Four compounds [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, where <i>m</i> = 1–4, were synthesized to explore the effect of increasing the distance between Se–V–Se dichalcogenide layers on electrical transport properties. These kinetically stable compounds were prepared using designed precursors that contained a repeating pattern of elemental layers with the nanoarchitecture of the desired product. XRD and STEM data revealed that the precursors self-assembled into the desired compounds containing a Se–V–Se dichalcogenide layer precisely separated by a SnSe layer. The 00<i>l</i> diffraction data are used to determine the position of the Sn, Se, and V planes along the <i>c</i>-axis, confirming that the average structure is similar to that observed in the STEM images, and the resulting data agrees well with results obtained from calculations based on density functional theory and a semiempirical description of van der Waals interactions. The in-plane diffraction data contains reflections that can be indexed as <i>hk</i>0 reflections coming from the two independent constituents. The SnSe layers diffract independently from one another and are distorted from the bulk structure to lower the surface free energy. All of the samples showed metallic-like behavior in temperature-dependent resistivity between room temperature and about 150 K. The electrical resistivity systematically increases as <i>m</i> increases. Below 150 K the transport data strongly indicates a charge density wave transition whose onset temperature systematically increases as <i>m</i> increases. This suggests increasing quasi-two-dimensional behavior as increasingly thick layers of SnSe separate the Se–V–Se layers. This is supported by electronic structure calculations

Dielectric and Ferroelectric Properties in Highly Substituted Bi₂Sr(A)TiNb₂O₁₂ (A = Ca²⁺, Sr²⁺, Ba²⁺) Aurivillius Phases

Structure–property relationships were determined for the family of three-layer Aurivillius materials Bi₂Sr­(A)­TiNb₂O₁₂ (A = Ca²⁺, Sr²⁺, Ba²⁺). X-ray and neutron diffraction along with selected area electron diffraction indicate that Bi₂SrBaTiNb₂O₁₂ crystallizes in the nonpolar <i>I</i>4/<i>mmm</i> space group, whereas the polar <i>B</i>2<i>cb</i> space group best describes Bi₂SrCaTiNb₂O₁₂ and Bi₂Sr₂TiNb₂O₁₂. Despite the different space groups, all three compositions show relaxor behavior as evidenced through <i>P</i>(<i>E</i>) and dielectric measurements. These relaxor properties are derived from the extensive amount of disorder in each composition that is found at every cationic crystallographic site and do not depend on the space group. This disorder is so extensive that it disrupts the ferroelectric properties allowed by symmetry in the <i>B</i>2<i>cb</i> space group. This work demonstrates the important role of cation substitution and site disorder in these three-layered Aurivillius materials and its significant effect on both ferroelectric and dielectric properties