Michael S. Haluska

Phosphor Microparticles of Controlled Three-Dimensional Shape from Phytoplankton

We demonstrate how the precise three-dimensional 3D assembly characteristics of biomineralizing micro-organisms may be combined with synthetic chemical processing to generate photoluminescent microparticles with specific 3D shapes and tailored chemistries. Silica-based microshells with a rich variety of controlled shapes are assembled by a type of unicellular algal phytoplankton known as diatoms Bacillariophyceae. Each of the tens of thousands of diatom species generates a microshell with a particular 3D morphology that can be used as a shape-dictating particle template. In this demonstration, the microshells of Aulacoseira diatoms were converted into Eu 3+ -doped BaTiO3-bearing microparticles. The silica-based microshells were first converted into magnesia-based replicas via a gas/solid displacement reaction the silica of native diatom microshells is not chemically compatible with barium titanate. A conformal, sol-gel-derived coating of europium-doped barium titanate was then applied to the chemically compatible magnesia replicas to yield photoluminescent particles that retained the starting microshell shape. Upon stimulation with 337 nm UV light, the 3D microparticle replicas exhibited a bright red emission associated with the 5 D 0 → 7 F 2 transition of Eu+3.

X-ray diffraction analyses of 3D MgO-based replicas of diatom microshells synthesized by a low-temperature gas/solid displacement reaction

The nanostructural features of the gas-phase displacement reaction 2Mg( g )+SiO 2 →2MgO( s )+{Si}, where SiO 2 is in the form of diatom shells were studied via X-ray diffraction and Fourier methods. Diatomaceous powder heated to 700 °C in a sealed graphite cell in the presence of Mg vapor formed MgO via a displacement reaction. Warren-Averbach analysis performed on samples reacted for different times showed an initial sharp MgO grain size distribution which broadened with time. New MgO crystallization was shown to occur until about 60 min, whereafter only MgO grain growth occurred. Median MgO crystallite size increased from 7.5 to 24.9 nm during this period, whereas microstrain decreased dramatically past 60 min annealing time.

Closed, heated reaction chamber design for dynamic high-temperature x-ray-diffraction analyses of gas/solid displacement reactions

A closed, x-ray transparent chamber for containing a hot reactive gas (generated from an internal condensed source) has been designed and evaluated for use in dynamic x-ray-diffraction analysis of a gas/solid displacement reaction. The chamber consisted of a square-bottom base and lid machined from dense pyrolytic graphite. The base contained a flat pedestal, upon which SiO2 microshells (the reactant oxide) were placed, raised above adjacent cavities holding Mg flakes (the condensed precursor to the reactive gas). Upon heating to 650 °C, the Mg evaporated and reacted with the SiO2 inside the sealed chamber. By passing incident and diffracted x rays through the vertical side walls of the chamber and by blocking undesired graphite-diffracted x rays with platinum, the Mg(g)∕SiO2(s) displacement reaction could be tracked with time. This is the first use of dynamic high-temperature x-ray diffraction analysis to monitor the progress of a displacement reaction involving a reactant gas that was generated and conf...