19 research outputs found
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
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STEM Graduate Studentsâ 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 otherâs 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 (1âx)Bi(Fe2/8Ti3/8Mg3/8)O3â(x)PbTiO3 (BFTMâPT) is synthesized and the electromechanical properties are measured. This system is defined as a lowâlead 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 ”C cmâ2 and a remnant polarization of 26 ”C cmâ2. The range of compositions has high Curie temperature (Tc), ranging from 625â650 °C. Materials with a Tc above 400 °C typically have a low d33 of <50 pC Nâ1 at room temperature. However, BFTMâPT has a higher d33 that most other compositions with a Tc in this range, with the highest being 145 pC Nâ1 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
Enhanced Piezoelectric Properties From the Electric Field-Induced Ferroelectric Transition at the MPB of BiGaOâ-Substitued Na 1/2
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 MOFâ5
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 intermediateâs reactivity toward
three metal ions (Zn<sup>II</sup>, Cu<sup>II</sup>, and Mn<sup>II</sup>) 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 ZnÂ(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O, accompanied by a change in morphology from a plate to a
cube. Reaction with CuCl<sub>2</sub>·2H<sub>2</sub>O did not
disrupt the plate-like morphology of MOF-<i>i</i>, but it
did result in the replacement of Zn<sup>II</sup> by Cu<sup>II</sup> and formation of a novel porous copper MOF. MOF-<i>i</i> showed no reactivity toward MnCl<sub>2</sub>. 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 metalâorganic 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 âcard-houseâ
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)<sub>1.15</sub>]<sub><i>m</i></sub>(VSe<sub>2</sub>)<sub>1</sub>, <i>m</i> = 1, 2, 3, and 4
Four compounds [(SnSe)<sub>1.15</sub>]<sub><i>m</i></sub>(VSe<sub>2</sub>)<sub>1</sub>, where <i>m</i> = 1â4,
were synthesized to explore the effect of increasing the distance
between SeâVâSe 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 SeâVâSe 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 SeâVâSe layers. This is supported by electronic
structure calculations
Dielectric and Ferroelectric Properties in Highly Substituted Bi<sub>2</sub>Sr(A)TiNb<sub>2</sub>O<sub>12</sub> (A = Ca<sup>2+</sup>, Sr<sup>2+</sup>, Ba<sup>2+</sup>) Aurivillius Phases
Structureâproperty
relationships were determined for the
family of three-layer Aurivillius materials Bi<sub>2</sub>SrÂ(A)ÂTiNb<sub>2</sub>O<sub>12</sub> (A = Ca<sup>2+</sup>, Sr<sup>2+</sup>, Ba<sup>2+</sup>). X-ray and neutron diffraction along with selected area
electron diffraction indicate that Bi<sub>2</sub>SrBaTiNb<sub>2</sub>O<sub>12</sub> 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 Bi<sub>2</sub>SrCaTiNb<sub>2</sub>O<sub>12</sub> and Bi<sub>2</sub>Sr<sub>2</sub>TiNb<sub>2</sub>O<sub>12</sub>. 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