Synthesis and characterization of a pyrochlore solid solution in the Na2O-Bi2O3-TiO2 system

The compositional limits of a previously reported (J. Am. Ceram. Soc., 61, 5-8. (1978)) but relatively unstudied sodium-bismuth titanate pyrochlore solid solution are revised and their electrical properties presented. The pyrochlore solid solution we report forms via a different mechanism to that originally reported and occurs in a different location within the Na₂O-Bi₂O₃-TiO₂ ternary system. In both cases, relatively large amounts of vacancies are required on the A-sites and on the oxygen sites, similar to that reported for undoped ‘Bi₂Ti₂O₇’ pyrochlore. In contrast to ‘Bi₂Ti₂O₇’, this ternary pyrochlore solid solution can be prepared and ceramics sintered using conventional solid-state methods; however, the processing requires several challenges to be overcome to obtain dense ceramics. This cubic pyrochlore series has low electrical conductivity (and does not exhibit any evidence of oxide-ion conduction) and exhibits relaxor ferroelectric behavior with a broad permittivity maximum of ~100 near room temperature. Variable temperature neutron diffraction data do not provide any conclusive evidence for a phase transition in the pyrochlore solid solution between ~4 and 873 K.     

Finite element modeling of resistive surface layers by micro-contact impedance spectroscopy

Micro-contact impedance spectroscopy (MCIS) is potentially a powerful tool for the exploration of resistive surface layers on top of a conductive bulk or substrate material. MCIS employs micro-contacts in contrast to conventional IS where macroscopic electrodes are used. To extract the conductivity of each region accurately using MCIS requires the data to be corrected for geometry. Using finite element modeling on a system where the resistivity of the surface layer is at least a factor of ten greater than the bulk/substrate, we show how current flows through the two layers using two typical micro-contact configurations. This allows us to establish if and what is the most accurate and reliable method for extracting conductivity values for both regions. For a top circular micro-contact and a full bottom counter electrode, the surface layer conductivity (σ_s) can be accurately extracted using a spreading resistance equation if the thickness is ~10 times the micro-contact radius; however, bulk conductivity (σ_b) values can not be accurately determined. If the contact radius is 10 times the thickness of the resistive surface, a geometrical factor using the micro-contact area provides accurate σ_s values. In this case, a spreading resistance equation also provides a good approximation for σ_b. For two top circular micro-contacts on thin resistive surface layers, the MCIS response from the surface layer is independent of the contact separation; however, the bulk response is dependent on the contact separation and at small separations contact interference occurs. As a consequence, there is not a single ideal experimental setup that works; to obtain accurate σ_s and σ_b values the micro-contact radius, surface layer thickness and the contact separation must all be considered together. Here we provide scenarios where accurate σ_s and σ_b values can be obtained that highlight the importance of experimental design and where appropriate equations can be employed for thin and thick resistive surface layers.     

Assessing Grey Squirrel Dispersal Patterns within the Landscape Using Sequence Variation

Abstract

The grey squirrel Sciurus carolinensis is thought to have contributed to the decline of red squirrel S. vulgaris populations in the UK through resource competition and disease spread. This study used mtDNA sequencing to assess patterns of grey squirrel dispersal in the UK. Patterns of genetic variation within the dloop sequence were characterised for seven grey squirrel populations. Infiltration directions and potential barriers to dispersal are identified and discussed, with a focus on Cumbria, a county at the forefront of grey squirrel expansion. Understanding the dynamics of grey squirrel dispersal will aid their management at a landscape scale and enhance the conservation of red squirrels.     

Impedance Spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 Ceramics Modified with (K0.5Na0.5)NbO3

The electrical and dielectric properties of (1 ? x)(0.94Bi_1/2Na_1/2TiO₃–0.06BaTiO₃)–x(K_0.5Na_0.5NbO₃) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.     

Measurement of Nanometer Aerosols

This paper is a review of my work during the past 18 years on nanometer, i.e., submicrometer, aerosols. These aerosols scatter negligible light so they are difficult to study and must be observed by indirect methods such as diffusion batteries and condensation nucleus counters. Several diffusion batteries are described: “cluster tube” batteries, 5.5 km of 1-mm-diam. straight tubing mounted in clusters; collimated holes structures containing 5.1 km of holes, 1/4 mm in diameter; honeycomb structures containing 3.5 km of holes 1/3 mm in diameter; screen batteries containing 55 stainless steel screens in 10 sections; reticulated vitreous carbon batteries containing 60 k interconnected pores per cm5. From theory, the diffusion battery is shown to be only slowly discriminating, so a series of batteries and measurements is required for particle size analysis. Measurements were made with a continuous flow condensation nucleus counter developed to provide the steady flow required by diffusion battery theory. The measurements were found to agree with those made with an electrical aerosol generator. Particle size was analyzed by a “graphical stripping” method developed in this laboratory and by two computer programs described in the literature. Standard sampling methods such as the thermal precipitator and the electrostatic precipitator were tried but found to be inadequate. An induction furnace and a tube furnace were used to generate silver and gold, as well as NaCl aerosol. The furnaces were found to be superior to the more common hot-wire or exploding-wire methods, and heating the dry NaCl was preferred to spray-drying a suspension. Carbon aerosols of a large range of particle size and concentration were conveniently generated by the incomplete combustion of methane. A tube bridge, following Pollak's design, was built and used to test the “intrinsic” calibration of his counter. Good agreement was found with his calibration table up to a concentration of 300 k/cm³. Above that value, however, the tube bridge showed a progressive undercount so that the maximum value of 641 k given in Pollak's table was, according to our measurements, about 1200 k. The temperature drop during adiabatic expansion in the Pollak counter was measured with the aid of a resistance wire 12.7 um in diameter mounted along the axis of the fog tube. It was found both theoretically and experimentally that the dry adiabatic temperature drop is about 16 °C, in agreement with the literature. However, we found that the wet temperature drop is about 8°C, both experimentally and theoretically. It is frequently stated in the literature that the wet and adiabatic temperature drops are the same. The use of the above-described diffusion batteries in the laboratory and field is described. The collimated holes and honeycomb structures are suitable for aerosols of high concentration in the laboratory and uranium mine atmospheres. The carbon batteries are more suitable for radioactive aerosols of low concentration since their flow rate is 280 liters/min.     

The practice of vegetation survey and its relevance to landscape study in the uplands

The Canadian conservative ideology of an ordered controlled environment distinguished it from its revolutionary American neighbour. The ideology frequently involved Pastoral attitudes to nature. Only occasionally before the twentieth century did Canadian writers reveal negative views of their country's terrain.     

Mobilising community action towards a low-carbon future: Opportunities and challenges for local government in the UK

Over the last decade the important role that local authorities can play in catalyzing community action on climate change has been repeatedly emphasised by the UK Government. The paper examines this policy context and explores the options available to local authorities in terms of reaching and engaging their communities. The type of progressive response shown by some UK local authorities is illustrated with empirical evidence gathered through a study conducted in the London Borough of Islington focusing on their recently established ‘Green Living Centre’. The results confirm interest in this major council-led community initiative, with positive attitudes expressed by the majority of those questioned in terms of the advice and information available. However, it is also clear that many participants had preexisting pro-environmental attitudes and behavioural routines. Results from a broader sample of Islington residents indicate a substantial challenge in reaching the wider community, where enthusiasm for sustainability change and interest in this type of scheme were more mixed. The prospect for local government in addressing this challenge – and their ability to trigger and capitalize upon concepts of social change at the community level towards a lower carbon future – is discussed in the final part of the paper.     

Recovery kinetics in the presence of precipitates: The softening response of an Al–Mg–Sc alloy

Recovery of an Al–Mg–Sc alloy in the presence of Al₃Sc precipitates has been studied. The results demonstrate that recovery can be separated into two stages. The initial stage is precipitate-independent and consistent with the recovery kinetics of binary Al–Mg alloys. The second is strongly influenced by the presence of precipitates. Precipitates slow recovery, allowing recovered microstructures to be maintained over long annealing times to high homologous temperatures without recrystallization. A physically based model that accounts for the precipitate dependence of recovery is presented and compared to experimental results.