To understand the recent dielectric measurements made on HF-doped ice single crystals requires a full knowledge of the concentration of electrical defects present in ice and their subsequent interactions. Previous interpretations of the behaviour of HF-doped ice have concentrated upon specific features in isolation, whereas this paper presents analyses of a data set of 139 temperature and impurity combinations from 17 HF-doped ice single crystals. The interpretation of the behaviour of these crystals is in terms of several possible theoretical models. All models are based upon the common assumptions that HF molecules enter the ice lattice substitutionally and that excess Bjerrum and ionic defects can be formed at the HF sites. They also use the theory of electrical conduction in ice by Jaccard (1959) and the defect equilibria analysis in ice by Kroger (1974). All models yield values for the concentration, mobility, energy of formation and charges for the different types of electrical defect considered to be generated. From the model which assumes that only three fluorine centres exist, the approximate derived values of the mobility and charge for the L-defect and positive ionic defect are as follows: μ L = 5 × 10-8 m2 V-1 s-1 at 273 K, eDL = 0.44e; μ + = 2.7 × 10-8 m2 V-1 s-1 at 273 K, e ± 0.73 e . Finally, using the derived defect conductivities and the Jaccard theory of electrical conduction, the relaxation tune of HF-doped ice has been successfully predicted over a wide range of temperature and fluoride concentration.
THE ABOLITION OF the air-travel tax hasn’t pleased everybody. An Taisce has predicted that it will soon have to be re-instated to combat climate change.When it was first introduced in 2009 the tax on flights out of the country was €10, it was then reduced to €3 before the current Government announced in the Budget earlier this month that it will be abolished from April 2014.But the independent environmental charity An Taisce has said that removal of the tax “doesn’t make long-term sense” because the global climate change situation is becoming “even more alarming”.The group cites a recent report from a UN intergovernmental panel which said that it was 95 per cent convinced that humans have caused over half the global warming observed over the past 60 years.An Taisce say that air travel is by far the most carbon intensive form of transport per passenger kilometre travelled: The recent IPCC report stated that there is now only a 50-50 chance of avoiding runaway temperature rise. Airline chiefs can try and dress this issue up whatever way they like, but there are choices to be made between foreign holidays today and a sustainable future for the next generation.An Taisce say that these choices must include becoming less dependent on air travel by increasing overland links with Britain with greater sail-and-rail options for Irish travellers.Read: With the Air Travel Tax gone, Ryanair commits to a million more passengers >Pic: What happens when Americans fly home for Thanksgiving >