Answers and Solutions

Answers and solutions to the puzzles in this issue

Apatite electrolytes : design, synthesis and modeling

The crystal chemistries of a range of germanate, silicate and rhenate apatites were studied as potential electrolytes for solid oxide fuel cells (SOFCs). While these compounds are often described as complying with the composition A10(BO4)6O2 (A = rare earth, alkali earth, alkali and B = germanium, silicon, transition metals), the character of apatite electrolytes is far more interesting. Indeed, it is the underlying complexity and flexibility of these substances that endows them with such promise as ion conductors. In this thesis, the nature of “hybrid” apatites is examined, and the critical role of extrastoichiometric oxygen associated in BO5 units for facilitating high conductivity with low activation energy considered. Following this approach, a taxonomy of polymorphic and polysomatic apatites is devised that provides a basis for the rationale design of new electrolytes and the optimization of performance. Apatites can be synthesized in various ways, including solid state sintering, sol-gel methods and hydrothermal reactions; but the former is favored since large quantities are readily fabricated for division and multi-technique characterization. X-ray, neutron and electron diffraction were employed to refine crystal structures and locate “interstitial” oxygen, supported by spectroscopic and microscopic methods to identify defects and recognize superlattices. In this manner, correlations between ionic conductivity and crystal chemistry were established.DOCTOR OF PHILOSOPHY (MSE

Crystal Structure of Electroceramics

Electrical, optical, ionic and magnetic properties of ceramics are primarily dictated by their crystal structure. They can be improved by introducing impurities, creating long range order/short range order/disorder, engineering defects and utilising specific crystal anisotropy and orientation. This Special Issue is aimed at manuscripts focusing on the recent development of electroceramics and its relation to the crystallography, including the characterisation aspect. In particular, the topic of interest covers the review paper on principles, difficulties and progress of crystal structure determination and refinement from powder diffraction, and ferroelectricity in binary crystals. Other interesting piezoelectric, electrical conducting, dielectric and ferroelectric ceramics are reported with various crystal structural characterisation and microscopy techniques

Tuning the morphology of ZnMn2O4 lithium ion battery anodes by electrospinning and its effect on electrochemical performance

ZnMn2O4 structures of various morphologies (nanorods, nanofibers, nanowebs) have been prepared via a facile electrospinning technique by a simple variation of the sintering profile, and have subsequently been employed as anodes in lithium ion battery applications. After the sintering process, as-spun nanofibers with high aspect ratio have broken into short segments of ZnMn2O4 nanorods (ZMO-NR). Incorporating an intermediate carbonization step has strengthened the mechanical integrity of as-spun nanofibers, resulting in the formation of sintered nanofibers (ZMO-NF) and nanowebs (ZMO-NW). On the basis of FESEM, HRTEM and XRD studies, the formation mechanism of nanostructures consisting of hierarchically self-assembled ZnMn2O4 nanocrystals is discussed. Particle size distribution is computed by Rietveld refinement and HRTEM micrographs, while the valence states are confirmed by XPS. The initial discharge of ZMO-NF and ZMO-NW demonstrated a high capacity of ~1469 mA h g−1 and 1526 mA h g−1, respectively, in the voltage ranges of 0.005 V and 3.0 V versus Li/Li+ at 60 mA g−1, associated with reversible capacities of ~705 mA h g−1 and 530 mA h g−1 after 50 cycles. Morphology tuning of anodes and the importance of interconnected nanoparticulate pathways for lithium ion diffusion are elucidated

Facile photochemical synthesis of graphene-Pt nanoparticle composite for counter electrode in dye sensitized solar cell

A low temperature route to synthesize graphene oxide–Pt nanoparticle hybrid composite by light assisted spontaneous coreduction of graphene oxide and chloroplatinic acid without reducing agent is demonstrated. Analysis indicates the importance of light as energy provider and ethanol as hole scavenger in the formation of small Pt nanoparticles (3 nm) on graphene oxide as well as graphene oxide reduction. Spray coating was used to deposit the hybrid material as a counter electrode in dye sensitized solar cells (DSCs). An efficiency of 6.77% for the hybrid graphene counter electrode has been obtained, higher than the control device made by low temperature sputtered Pt as counter electrode. Compatibility of the hybrid material with flexible plastic substrates was demonstrated yielding DSCs of an efficiency of 4.05%

Characterisation of Intermetallic Phases in Fusion Welded Commercially Pure Titanium and Stainless Steel 304

A series of trials to fusion weld commercially pure titanium (CPTi) to stainless steel 304 (SS304) have been conducted using laser beam welding (LBW) and gas tungsten arc welding (GTAW). Neither technique produced adequate weld joints with LBW showing a more promising result, while GTAW yielded separation of the workpieces immediately after welding. Cracking and fracturing took place mainly on the SS304 side, which was explained by the differences in the materials’ thermal properties. Various intermetallic phases formed during welding that were identified using energy dispersive X-ray spectroscopy (EDS) and electron backscattered diffraction (EBSD) technique and were compared with an isothermal ternary phase diagram of Fe-Cr-Ti. Their corresponding hardness values are reported and correlated with alloy compositions

Effect of TiO2 mesoporous layer and surface treatments in determining efficiencies in antimony sulfide-(Sb2S3) sensitized solar cells

We have fabricated antimony sulfide (Sb2S3) sensitized nano-structured solar cells prepared by chemical bath deposition. By incorporating a surface treatment on the antimony sulfide films, an improvement in solar cell performance is observed. The influence of the TiO2 photoanode structure on solar cell efficiencies is also studied by varying the TiO2 nanoparticle size. Although smaller nanoparticle size provides a much larger effective surface area for sensitizer attachment, the smaller pore size lead to an over-aggregation of Sb2S3 particles on the surface pores. Devices fabricated on particles of size ∼100 nm exhibited an efficiency of 2.3%.Published versio

Analysis of the magnetic field dependence of the isothermal entropy change of inverse magnetocaloric materials

In this work, the magnetic field dependence of the inverse magnetocaloric (MC) effect is analyzed using a mean field approach for describing antiferromagnetic to ferromagnetic magnetoelastic transitions. The model is able to describe both second- and first-order transition through the introduction of a magnetovolume energy term. The power law dependence for the field dependence of the isothermal magnetic entropy change (ΔSiso∝ΔHn), has an exponent n with an overshoot above 2 for first-order transitions, while it is not present for the second-order case. This is in excellent agreement with previous phenomenological observations, supporting the validity of recently proposed criterion to distinguish between first- and second-order thermomagnetic transitions. A main difference with respect to direct MC effect is that negative values of the exponent n are obtained at temperatures close to the transition. This is ascribed to the reduction of the inverse MC response due to the influence of the unavoidable ferromagnetic to paramagnetic transition at higher temperatures. The obtained features are qualitatively compared to those of GdBaCo2O6 (antiferromagnetic to ferromagnetic magnetoelastic transition), showing a good agreement between both experiments and the model. The obtained information is extrapolated to understand the behavior of the exponent n for a Ni49Mn36In15 sample (magnetostructural transition).Agencia Estatal de Investigación PID2019-105720RB-I0Universidad de Sevilla US-126017Consejería de Economía, Conocimiento, Empresas y Universidad P18-RT-746US Army Research Laboratory W911NF-19-2-021

Surfactant-free sub-2 nm ultrathin triangular gold nanoframes

Ultrathin triangular gold nanoframes were synthesized in high yield through selective gold deposition on the edges of triangular silver nanoprisms and subsequent silver etching with mild wet etchants. These ultrathin gold nanoframes are surfactant-free with tailorable ridge thickness from 1.8 to 6 nm and exhibit adjustable and distinct surface plasmon resonance bands in the visible and near-IR region. In comparison, etching of the nanoprism template via galvanic replacement can only create frame structures with much thicker ridges, which have much lower catalytic activity for 4-nitrophenol reduction than the ultrathin gold nanoframes.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore

Electrospun Zn_1–<i>x</i>Mn_<i>x</i>Fe₂O₄ Nanofibers As Anodes for Lithium-Ion Batteries and the Impact of Mixed Transition Metallic Oxides on Battery Performance

The structural and electrochemical properties of the mixed transition metallic oxides Zn_1–<i>x</i>Mn_<i>x</i>Fe₂O₄ nanofibers, which crystallize in a cubic spinel AFe₂O₄ structure, are investigated systematically with a gradual substitution of Zn by Mn. The crystal structural information studied by X-ray diffraction (XRD) depicts the formation of single phase spinel structure, while electron-dispersive X-ray spectroscopy (EDS) reveals the stoichiometric ratio between Zn and Mn. ZnFe₂O₄ exhibits a good capacity of ∼532 mAh g^–1 at 50th cycle through the interbeneficial conversion reaction and alloy–dealloy mechanism, with a first discharge working voltage of ∼0.83 V. Subsequently, the characteristic redox potential of each spinel is gradually reduced with the replacement of Mn. Furthermore, Zn_0.3Mn_0.7Fe₂O₄ demonstrates the highest capacity of ∼612 mA h g^–1 at 50th cycle among the solid solution series. Ex situ characterization by high-resolution transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) is conducted to study the participation of Mn in the battery performance. This report represents an example of how the electrochemical performance could be flexibly adjusted by tuning the ratio of transition metals within the spinel