Af型自由基均聚反应的固化理论(Ⅲ)──高分子凝胶网络缺陷

应用高分子反应统计理论,分析了高分子网络的形成过程,讨论了A_f型自由基均聚反应凝胶网络中的缺陷悬吊环的数目及其形成几率.     

fac,trans‐[(2,2′‐Bipyridine‐N,N′)tricarbonylrhenium(I)]‐μ‐(4,4′‐bipyridine)‐N:N′‐{chlorobis[1,2‐phenylenebis(dimethylarsine)]ruthenium(II)} bis(hexafluorophosphate) diacetone solvate

The crystal structure of the heterobimetallic title compound, [ReRuCl(C₁₀H₁₆As₂)₂(C₁₀H₈N₂)₂(CO)₃](PF₆)₂·2C₃H₆O, is described. Both metal centres have pseudo‐octahedral coordination geometries, with a facial arrangement of the carbonyl ligands at Re^I and a trans disposition of the diarsine ligands at Ru^II. The dihedral angle between the pyridyl rings of the 4,4′‐bi­pyridine bridging ligand is 13.9 (6)°.     

NEEDING TO USE ALGEBRA

We have completed a Teacher Training Agency (TTA) funded project looking at the teaching and learning of algebra with one mixed-ability year 7 class (11–12 year-olds in the UK). We take Kieran's (quoted in Sutherland, 1997) definition of algebra and see ourselves as developing a'community of practice’ (Lave and Wenger, 1991) in the classroom where the practice is not that of mathematician but of inquirer (Schoenfeld, 1996) into mathematics. The teacher in this community acts as role model for inquirer and metacomments (Bateson, 1972) on the practice of inquiry. In this paper we present evidence for how such metacommenting supports the development of a'community of inquiry’ which leads to the'need’ for algebraic activity. We conclude with a case study, illustrating one student's need for algebra.     

Comparison of tensile and bulge tests for thin-film silicon nitride

The mechanical properties of thin-film, low-pressure chemical vapor deposited silicon nitride were measured in uniaxial tension and by a bulge test method suitable for wafer-level testing. This research compares the two approaches and presents additional data on silicon nitride. The common property from the two test methods is the Young's modulus. Tensile tests performed at the Johns Hopkins University provided a value of 257±5 GPa. Bulge tests conducted by Exponent, Inc., an engineering and scientific consulting firm, yielded a value of 258±1 GPa. It is concluded that this bulge test is a valid wafer-level test method. These tensile results, when added to earlier results, yield the following properties for low-stress silicon nitride: Young's modulus =255±5 GPa, Poisson ratio=0.23±0.02, and fracture strength=5.87±0.62 GPa.     

Mathematics teachers learning with video: the role,for the didactician,of a heightened listening

This article addresses two main questions, how do mathematics teachers learn from using video? and, what is the role of the didactician? A common problem is reported in the difficulty of keeping teacher discussion of video away from judgment and evaluation. A review of mathematics education literature revealed four existing models for the use of video with teachers. Drawing on enactivist ideas (Rosch, Concepts: core readings, pp 189–206, 1999), there are reasons why these models are likely to be productive and therefore suggestions for how teachers can learn from video. However, little is known about the role of the didactician in supporting learning. From empirical data, there is evidence that didacticians need to engage in a particular form of attention that I label a ‘heightened listening’ since there are simultaneous foci (on the content of teachers’ contributions and what kind of a comment is being made), in order to establish discussion norms and to support the development of new ways of seeing and acting in the classroom.     

Electroclinic behaviour of polymer doped low molar mass ferroelectric liquid crystals

Abstract

Recently there has been much interest in the electroclinic effect due to its potential application in high speed optical modulators and grey scale displays. In present materials, however, the electroclinic effect is very dependent on temperature and falls off rapidly with increasing temperature. This limits the useful device operating range. In this paper the electroclinic behaviour of new, side chain polymer doped, low molar mass ferroelectic liquid crystal mixtures is reported. By measuring the rate of change of the electroclinic coefficient with temperature it is shown that the temperature range over which the electroclinic effect exists increases with polymer concentration. Data are also presented on electroclinic response times.