Hans-Conrad zur Loye

Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage

This review summarizes the current state of polymer composites used as dielectric materials for energy storage. The particular focus is on materials: polymers serving as the matrix, inorganic fillers used to increase the effective dielectric constant, and various recent investigations of functionalization of metal oxide fillers to improve compatibility with polymers. We review the recent literature focused on the dielectric characterization of composites, specifically the measurement of dielectric permittivity and breakdown field strength. Special attention is given to the analysis of the energy density of polymer composite materials and how the functionalization of the inorganic filler affects the energy density of polymer composite dielectric materials.

Recent developments in perovskite-based oxide ion conductors

Recent advances in the study of oxygen-deficient perovskites and brownmillerite phases as oxygen ion conductors are reviewed. The structures of these phases containing oxygen vacancies are surveyed and the role of the oxygen vacancies with respect to the ionic conductivity in these materials is discussed in detail.

Lattice and energy band engineering in AlInGaN/GaN heterostructures

We report on structural, optical, and electrical properties of AlxInyGa1−x−yNGaN heterostructures grown on sapphire and 6H–SiC substrates. Our results demonstrate that incorporation of In reduces the lattice mismatch, Δa, between AlInGaN and GaN, and that an In to Al ratio of close to 1:5 results in nearly strain-free heterostructures. The observed reduction in band gap, ΔEg, determined from photoluminescence measurements, is more than 1.5 times higher than estimated from the linear dependencies of Δa and ΔEg on the In molar fraction. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional electron gas at the heterointerface. The obtained results demonstrate the potential of strain energy band engineering for GaN-based electronic applications.

Design of a Random Quantum Spin Chain Paramagnet: Sr3CuPt0.5Ir0.5O6

A new class of magnetic behavior, random quantum spin chain paramagnetism, has been observed in the one-dimensional compound Sr3CuPt1-xIrxO6. A random quantum spin chain system has S = spins coupled by Heisenberg exchange interactions that are randomly ferromagnetic or antiferromagnetic between neighbors along the chain. This condition was fulfilled by members of the solid solution Sr3CuPt1-xIrxO6 (x = 0, 0.25, 0.50, 0.75, and 1), whose end-members, Sr3CuPtO6 and Sr3CuIrO6, are antiferromagnetic and ferromagnetic, respectively. Magnetic susceptibility data for the solid solution Sr3CuPt1-xIrxO6 (x = 0, 0.25, 0.50, 0.75, and 1) were collected and were found to be in excellent agreement with a theoretical model.

Crystal growth of Ba2MOsO6 (M=Li, Na) from reactive hydroxide fluxes

Abstract Single crystals of Ba2LiOsO6 and Ba2NaOsO6 were grown from molten hydroxide fluxes. The oxides form in the double perovskite structure and contain heptavalent osmium. The structures crystallize in space group Fm 3 m with lattice parameters of 8.1046(2) A and 8.2870(3) A for the lithium and sodium containing compounds, respectively. Magnetic susceptibility measurements show evidence of antiferromagnetic correlations below 8 K for Ba2LiOsO6 and ferromagnetic-like ordering below 8 K for Ba2NaOsO6. The measured magnetic moment is unusually small for both materials, μeff=0.733 BM and μeff=0.677 BM for the lithium and sodium containing compounds, respectively.

The first ‘two-over/two-under’(2O/2U) 2D weave structure assembled from Hg-containing 1D coordination polymer chains

Self-assembly of HgI2 with a semirigid ditopic ‘Z’ type ligand affords the zigzag chains that interweave into a clothlike 2D network in a ‘two-over/two-under’ (2O/2U) fashion.

Iron Oxide Coated Gold Nanorods: Synthesis, Characterization, and Magnetic Manipulation

We report a simple process to generate iron oxide coated gold nanorods. Gold nanorods, synthesized by our three-step seed mediated protocol, were coated with a layer of polymer, poly(sodium 4-styrenesulfonate). The negatively charged polymer on the nanorod surface electrostatically attracted a mixture of aqueous iron(II) and iron(III) ions. Base-mediated coprecipitation of iron salts was used to form uniform coatings of iron oxide nanoparticles onto the surface of gold nanorods. The magnetic properties were studied using a superconducting quantum interference device (SQUID) magnetometer, which indicated superparamagnetic behavior of the composites. These iron oxide coated gold nanorods were studied for macroscopic magnetic manipulation and were found to be weakly magnetic. For comparison, premade iron oxide nanoparticles, attached to gold nanorods by electrostatic interactions, were also studied. Although control over uniform coating of the nanorods was difficult to achieve, magnetic manipulation was improved in the latter case. The products of both synthetic methods were monitored by UV-vis spectroscopy, zeta potential measurements, and transmission electron microscopy. X-ray photoelectron spectroscopy was used to determine the oxidation state of iron in the gold nanorod-iron oxide composites, which is consistent with Fe2O3 rather than Fe3O4. The simple method of iron oxide coating is general and applicable to different nanoparticles, and it enables magnetic field-assisted ordering of assemblies of nanoparticles for different applications.

EuKNaTaO5: crystal growth, structure and photoluminescence property.

The synthesis, structure determination, and intrinsic luminescent properties of a new tantalate, EuKNaTaO(5), are reported. Single crystals of the title compound were grown out of a reactive KOH/NaOH eutectic melt. Luminescence in many materials is activated by doping a rare earth cation into the solid matrix; we report undoped, all-rare earth EuKNaTaO(5), which exhibits bright orange room temperature photoluminescence.

Oxygen Isotope Effect in High-Temperature Oxide Superconductors

The effect of oxygen isotope substitution on the superconducting transition temperature, Tc, has been measured for BaBi0.25Pb0.75O3 (Tc, ≈ 11 K) and Lal1.85 Ca0.15CuO4 (Tc ≈ 20 K), and is compared to the shifts observed for La1.85Sr0.15CuO4 (Tc ≈ 37 K) and YBa2Cu3O7 (Tc ≈ 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors.

New 3D bismuth-oxo coordination polymers containing terephthalate-based ligands: observation of Bi2O2-layer and Bi4O3-chain motifs

Two new 3D framework materials containing Bi2O2 layers and Bi4O3 chains have been obtained hydrothermally via the reactions between bismuth nitrate with terephthalic acid (TA) and bismuth nitrate with nitro-terephthalic acid (NO2TA), respectively. The Bi2O2 layers are connected into a 3D structure via bridging TA ligands to form Bi2O2(TA) (1), while the novel Bi4O3 chains are connected into a 3D framework via linking NO2TA ligands to form Bi4O3(NO2TA)3(H2O)·(H2O)0.68 (2). Compound 1 exhibits photoluminescence that is red-shifted compared to that of the TA ligand, while the photoluminescence is quenched in both compound 2 and the NO2TA ligand.