John G. Fisher

Mass-scalable synthesis of 3D porous germanium–carbon composite particles as an ultra-high rate anode for lithium ion batteries

Electrode materials with three-dimensional (3D) mesoporous structures possess superior features, such as a shortened solid-phase lithium diffusion distance, a large pore volume, full lithium ion accessibility, and a high specific area, which can facilitate fast lithium ion transport and electron transfer between solid/electrolyte interfaces. In this work, we introduce a facile synthesis route for the preparation of a 3D nanoarchitecture of Ge coated with carbon (3D-Ge/C) via a carbothermal reduction method in an inert atmosphere. 3D-Ge/C showed excellent cyclability: almost 86.8% capacity retention, corresponding to a charge capacity of 1216 mA h g−1 even after 1000 cycles at a 2C-rate. Surprisingly, the high average reversible capacity of 1122 mA h g−1 was maintained at a high charge rate of 100C (160 A g−1). Even at an ultrahigh charge rate of 400C (640 A g−1), an average capacity of 429 mA h g−1 was attained. Further, the full cell composed of a 3D-Ge/C anode and an LiCoO2 cathode exhibited excellent rate capability and cyclability with 94.7% capacity retention over 50 cycles. 3D-Ge/C, which offers a high energy density like batteries as well as a high power density like supercapacitors, is expected to be used in a wide range of electrochemical devices.

Compositional and Structural Study of a (K 0.5 Na 0.5 )NbO 3 Single Crystal Prepared by Solid State Crystal Growth

In this work we investigated the chemical composition and structure of (K0.5Na0.5)NbO3 (KNN) single crystals grown by the solid state crystal growth method. The optical, scanning, and transmission electron microscopies were employed for the analysis of the chemical homogeneity and domain structure of the KNN crystal. No compositional inhomogeneities within experimental error were encountered in the KNN single crystals. The domain structure of the KNN single crystal, with a monoclinic unit cell, is composed of large 90 degrees domains of up to 100 microm width, which further consist of smaller 180 degrees domains with widths from 50 to 300 nm.

Simple synthesis of highly catalytic carbon-free MnCo2O4@Ni as an oxygen electrode for rechargeable Li-O2 batteries with long-term stability.

An effective integrated design with a free standing and carbon-free architecture of spinel MnCo2O4 oxide prepared using facile and cost effective hydrothermal method as the oxygen electrode for the Li–O2 battery, is introduced to avoid the parasitic reactions of carbon and binder with discharge products and reaction intermediates, respectively. The highly porous structure of the electrode allows the electrolyte and oxygen to diffuse effectively into the catalytically active sites and hence improve the cell performance. The amorphous Li2O2 will then precipitate and decompose on the surface of free-standing catalyst nanorods. Electrochemical examination demonstrates that the free-standing electrode without carbon support gives the highest specific capacity and the minimum capacity fading among the rechargeable Li–O2 batteries tested. The Li-O2 cell has demonstrated a cyclability of 119 cycles while maintaining a moderate specific capacity of 1000 mAh g−1. Furthermore, the synergistic effect of the fast kinetics of electron transport provided by the free-standing structure and the high electro-catalytic activity of the spinel oxide enables excellent performance of the oxygen electrode for Li-O2 cells.

Effect of metal chloride solutions on coloration and biaxial flexural strength of yttria-stabilized zirconia

The effect of three kinds of transition metal dopants on the color and biaxial flexural strength of zirconia ceramics for dental applications was evaluated. Presintered zirconia discs were colored through immersion in aqueous chromium, molybdenum and vanadium chloride solutions and then sintered at 1450 °C. The color of the doped specimens was measured using a digital spectrophotometer. For biaxial flexural strength measurements, specimens infiltrated with 0.3 wt% of each aqueous chloride solution were used. Uncolored discs were used as a control. Zirconia specimens infiltrated with chromium, molybdenum and vanadium chloride solutions were dark brown, light yellow and dark yellow, respectively. CIE L*, a*, and b* values of all the chromium-doped specimens and the specimens infiltrated with 0.1 wt% molybdenum chloride solution were in the range of values for natural teeth. The biaxial flexural strengths of the three kinds of metal chloride groups were similar to the uncolored group. These results suggest that chromium and molybdenum dopants can be used as colorants to fabricate tooth colored zirconia ceramic restorations.

Growth and Characterization of Single Crystals of Potassium Sodium Niobate by Solid State Crystal Growth

Due to the toxic nature of PbO in Pb(Zr,Ti)O3 (PZT), the most common type of piezoceramic, many studies on lead-free materials are being conducted worldwide (Roedel et al., 2009). One of the most studied groups of lead-free ferroelectric materials is based on a solid solution of potassium sodium niobate, K0.5Na0.5NbO3 (KNN) (Jaffe et al., 1971; Kosec et al., 2008). However, activity in attempts to find a lead-free replacement for PZT really accelerated with the discovery of textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics, with properties comparable to those of PZT (d33> 300 pC/N, relative permittivity > 1500, planar coupling coefficient kp> 60%)(Saito et al., 2004). One way to enhance the piezoelectric properties of KNN is to grow KNN-based single crystals along certain crystallographic directions, as has been demonstrated for the case of KNbO3 (Wada et al., 2004) and relaxor-based ferroelectric single crystals (Park & Shrout, 1997). The most frequently used methods for growing alkali niobate based single crystals are top seeded solution growth and the flux methods (Chen et al. , 2007; Davis et al., 2007; Kizaki et al., 2007; Lin et al., 2009). However, these methods are not yet fully commercialized due to high costs and poor reproducibility related to compositional inhomogeneity within the crystals. A possible way to improve the homogeneity of crystals with complex composition is to grow them by the low cost Solid State Single Crystal Growth (SSCG) method. The SSCG method is essentially a form of induced abnormal grain growth, a phenomenon which is very well known in the solid state sintering community. A significant breakthrough in the solid state synthesis of lead-free materials has been achieved in recent years (Kosec at al., 2010; Malic et al., 2008a). However, due to the strongly hygroscopic nature of alkaline carbonates usually used in solid state synthesis, different diffusion rates of involved ionic species during annealing, and the high sublimation rates of the alkaline species at high temperatures (Bomlai et al., 2007; Jenko et al., 2005; Kosec & Kolar, 1975; Malic et al., 2008b), it may be rather difficult to obtain compositionally homogeneous alkali niobate-based single crystals by SSCG.

CuO-based sintering aids for low temperature sintering of BaFe12O19 ceramics

Abstract This paper describes the effect of addition of 2 wt% of CuO, 30 mol% BaO–70 mol% CuO and BaCuO2 liquid phase sintering aids on the densification, microstructure and magnetic properties of BaFe12O19 ceramics. Addition of the sintering aids enabled reduction of the sintering temperature from 1250 °C to 1100 °C. The sintering aids caused abnormal grain growth in ceramics sintered at 1100 °C, with CuO having the strongest effect. All samples sintered at 1250 °C showed abnormal grain growth. Addition of CuO and BaO–CuO caused the grain size distribution to shift to larger values compared to the sample without sintering aid. The effect of the sintering aids on grain growth behavior is explained using the interface-reaction control theory of grain growth. The increase in grain size caused a reduction in coercivity of the samples with sintering aid addition, particularly in the samples sintered at 1100 °C.

Nonlinear Migration of Faceted Boundaries and Nonstationary Grain Growth in Ceramics

Recent investigations suggest that general grain boundaries can be categorized into two types: rough (atomically disordered) and faceted (atomically ordered). This paper reports our recent investigations on the migration behaviour of faceted boundaries and its effect on grain growth in polycrystalline ceramics. A model experiment has been performed using bi-layer samples of polycrystals with different average grain sizes and single crystals of BaTiO3 to study the migration behaviour of faceted boundaries. A non-linear relationship between grain boundary migration and the driving force for migration is revealed. Grain growth behaviour with respect to boundary faceting has also been studied in perovskites. The structural transition of boundaries between rough and faceted can be induced by changing oxygen partial pressure, adding dopants and changing temperature. The fraction of faceted boundaries was changed by changing oxygen partial pressure and donor doping. As the facet fraction decreased, the grain growth behaviour changed from stagnant and abnormal to normal. The different types of growth behaviour observed can be explained by the coupling effect of the maximum driving force for the boundary migration and the critical driving force for appreciable migration of faceted boundaries.

Growth of (Na0.5Bi0.5)TiO3-Ba(Ti1-xZrx)O3 single crystals by solid state single crystal growth

Single crystals of (Na0.5Bi0.5)TiO3-Ba(Ti1-xZrx)O3 are of interest for use in actuator applications due to their high electrostrictive strain. Growth of single crystals by traditional techniques requires specialised equipment and it is difficult to control chemical homogeneity in crystals with complex composition. In the present work, single crystals of 0.95(Na0.5Bi0.5)TiO3-0.05Ba(Ti1-xZrx)O3 have been grown for the first time by the solid state single crystal growth technique. <001 > -oriented SrTiO3 seed crystals were encased in powders of 0.95(Na0.5Bi0.5)TiO3-0.05Ba(Ti1-xZrx)O3 (x = 0.00, 0.01, 0.05, 0.10) and sintered. Single crystals of 0.95(Na0.5Bi0.5)TiO3-0.05Ba(Ti1-xZrx)O3 grew epitaxially on the seed crystals. Doping with Zr drastically reduced both the single crystal and matrix grain growth rates. Electron Probe Micro Analysis revealed the single crystals to be Na and Bi-deficient. Micro-Raman scattering showed that the single crystals and matrix grains have the same rhombohedral (Na0.5Bi0.5)TiO3 structure. The Raman modes of both single crystal and matrix broaden with increasing Zr content.

Effect of SrTiO3 content on the growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3 lead-free piezoelectric single crystals grown by the solid-state crystal growth method

In the present work, the single crystal growth by solid-state crystal growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3, where x = 0,1,2,3 mol-%, has been examined in order to study the effect of SrTiO3 content on single crystal growth. Powders were prepared by the conventional mixed oxide method. <001> KTaO3 seed crystals were buried in the powders, pressed into pellets and sintered at 1100°C for 1, 3, 5 and 10 h. Single crystals of the ceramic compositions grew onto the seeds. For the (K0.5Na0.5)NbO3 sample, both single crystal growth and abnormal grain growth in the matrix began to take place within 1 h. As the amount of SrTiO3 increased, the onset of both single crystal growth and abnormal grain growth were delayed. The effect of SrTiO3 addition on the single crystal and matrix grain growth behaviour is explained in terms of the mixed control theory of grain growth.

The Effect of Niobium Doping on the Electrical Properties of 0.4(Bi0.5K0.5)TiO3-0.6BiFeO3 Lead-Free Piezoelectric Ceramics

Ceramics in the system (Bi0.5K0.5)TiO3-BiFeO3 have good electromechanical properties and temperature stability. However, the high conductivity inherent in BiFeO3-based ceramics complicates measurement of the ferroelectric properties. In the present work, doping with niobium (Nb) is carried out to reduce the conductivity of (Bi0.5K0.5)TiO3-BiFeO3. Powders of composition 0.4(K0.5Bi0.5)Ti1−xNbxO3-0.6BiFe1−xNbxO3 (x = 0, 0.01 and 0.03) are prepared by the mixed oxide method and sintered at 1050 °C for 1 h. The effect of Nb doping on the structure is examined by X-ray diffraction. The microstructure is examined by scanning electron microscopy. The variation in relative permittivity with temperature is measured using an impedance analyzer. Ferroelectric properties are measured at room temperature using a Sawyer Tower circuit. Piezoelectric properties are measured using a d33 meter and a contact type displacement sensor. All the samples have high density, a rhombohedral unit cell and equiaxed, micron-sized grains. All the samples show relaxor-like behavior. Nb doping causes a reduction in conductivity by one to two orders of magnitude at 200 °C. The samples have narrow P-E loops reminiscent of a linear dielectric. The samples all possess bipolar butterfly S-E loops characteristic of a classic ferroelectric material. Nb doping causes a decrease in d33 and Smax/Emax.