Local Structure of Alkaline-Earth Boroaluminate Crystals and Glasses: II, 11B and 27Al MAS NMR Spectroscopy of Alkaline-Earth Boroaluminate Glasses

¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of alkaline-earth boroaluminate glasses show that the structure of these glasses are far more complicated than previously thought. The relative concentrations of 3- and 4-coordinated boron vary as found by previous investigators using continuous-wave NMR methods, but the ²⁷Al NMR spectra indicate the presence of Al in 4-, 5-, and 6-fold coordination, in contrast to previous assignments. Analysis of the data based on local charge balance considerations provides a qualitative model that correctly predicts compositional variations of the NMR peak intensities and ²⁷Al chemical shifts for a wide range of boroaluminate glass compositions.     

Theory and Practice in Metropolitan Strategy: Situating Recent Australian Planning

Over the last several years, metropolitan strategies have been produced for the five mainland state capital cities of Sydney, Melbourne, Brisbane, Adelaide and Perth. The distinctive characteristics of Australian urban planning mean that they can be shown to reflect an Australian paradigm of planning. However, this paradigm is rewritten in different ways in each strategy reflecting the particular institutional circumstances and political culture of each state. In doing so there is some strong reaffirmation of this paradigm, but also indications of a more relational and transactive manner of planning adding further process to product. Here there is some resonance with recent developments in planning theory and to some extent in practice—largely in Europe.

     

Determination of radionuclide exchangeability in freshwater systems

Two freshwater sediments were spiked with 57Co, 85Sr and 134Cs and left for adsorption times ranging from 1 day to over 60 days. Following adsorption, the 'exchangeable' pool of each radionuclide was measured using ammonium acetate extractions and a sequential leach procedure (Tessier et al., 1979), and the results were compared. Exchangeability was found to depend upon the sediment, radionuclide, sorption time and the identity of extracting agent. All three radionuclides showed a shift with increasing adsorption time from regular exchange sites to sites which are sterically hindered, but a fixation within the sediment was only observed for 57Co and 134Cs, with similar ammonium acetate extraction yields for both radionuclides. Misleading results were obtained during the sequential leach procedure due to redistribution and, therefore, the inclusion of a NH4+ leach after the MgCl2 extraction step was suggested. A mathematical model of element speciation was fairly successful at defining the 'exchangeable' fraction, suggesting that the chemically- and mathematically-defined fractions were similar.     

The Changing Role Of The City Centre In Adelaide And Melbourne

It is not too often that sufficient similarities and comparable data enable us to make significant comparisons between the central areas of Australian capital cities. This article presents such material about Adelaide and Melbourne. In particular, it takes advantage of floorspace and employment surveys carried out in these central areas in 1987 and 1992 to identify not only trends but important points of similarity and difference. Some of the more important reasons for these changes are canvassed in the context of current literature. The different policy responses of the two state governments and central city authorities are also examined as there has been a change in both cities to a state Liberal government following Labor governments which have incurred heavy financial losses and increased state debt.     

Rust,Dust and Bust: Planning Issues Arising from Replacement of Public Infrastructure

Recent reports by the South Australian Parliamentary Public Accounts Committee on the cost and timing of replacement of the State's major public infrastructure are analyzed. From them three implications are drawn from which three general conclusions concerning urban policy are derived.     

Systematic colonization and town planning in Australia and New Zealand

The theory and principles of systematic colonization, propounded most strongly by Edward Gibbon Wakefield, set new rules for British expansion overseas after 1830. This was particularly apparent in the foundation years of South Australia after 1836, and in the early settlement of New Zealand after 1840 in which Wakefield played a personal part. These outworkings developed strong and distinctive town planning themes in which two South Australian Surveyors‐General, William Light and George Goyder figured prominently. The annexation of the Northern Territory of Australia in the 1860s and 1870s, and the expansion of the frontier of settlement in inland South Australia from 1860 to 1890 displayed these themes.

This history demonstrates a breadth of concerns in the settlement of these territories which covers political economy; capitalist enterprise; the machinery of government and public policy concerning the survey, sale and use of land; the development of an urban system; and the planning of towns. This has not only left a strong imprint on the landscape but has left some influence on attitudes regarding land and property and on the style of government administration.     

Local Structure of Alkaline-Earth Boroaluminate Crystals and Glasses: I, Crystal Chemical Concepts—Structural Predictions and Comparisons to Known Crystal Structures

This paper presents a model which predicts glass structures based on local charge balance considerations. The model is shown to be consistent with known structures of borate, aluminate, and boroaluminate crystals and predicts that boroaluminate glasses should contain 3- and 4-coordinate boron, 4-, 5-, and 6-coordinate aluminum, and oxygens coordinated to one, two, three, and perhaps even four network-forming cations. The predicted glass structures are more complex than previous models for boroaluminate glasses, explaining the solid-state NMR data for the boroaluminate glasses discussed in Part II.     

Fatigue Mechanisms in High-Strength Silica-Glass Fibers

We use experimentally determined crack growth data for silica glass and a fracture mechanics model for delayed failure to predict the fatigue behavior for high-strength silica-glass fibers. The results of this model indicate that fracture mechanics methods can be used to adequately describe the fatigue behavior observed for high-strength silica-glass fibers at room temperature in humid conditions. The key feature to properly interpreting the fatigue of high-strength fibers is the use of a fracture-rate law in which the crack extension rate increases exponentially with applied stress. We show that a fracture mechanics approach to highstrength fiber fatigue can provide the basis for identifying additional fatigue mechanisms that may control failure in more aggressive fatigue environments.