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Structure determination of a calcium ɑ'-sialon containing 1.8 calcium atoms per unit cell shows that these atoms occupy the two large interstitial sites in the structure. The carbothermal reduction of ɑ'-sialons results in silicon carhide-aluminium nitride alloys with the wurtzite-type structure. A more direct method of firing powder mixtures of silicon nitride, aluminium nitride, carhon and calcium oxide in the temperature range 1650-2100 C° is explored. Phase relationships in the silicon carbide-aluminium nitride system have been determined up to 2100°C and contrary to previous reports there is a miscihility gap extending from about 40 m/o to 84 m/o aluminium nitride at 1800°C. 2H-SiC formed at the silicon carbide-rich end of the system by the reaction of silicon nitride with carbon at 1650°C contains up to 12 a/o nitrogen which stabilizes it relative to other polytypes. At higher temperatures, 2H transforms to 3C which is itself stabilized by nitrogen and, unlike nitrogen-free 3C, does not transform to 6H below 2l00°C. The phases produced by high-temperature reaction and heat-treatment are characterized by X-ray diffraction, scanning electron microscopy and chemical analysis. High resolution electron microscopy is used to examine structural details and particularly to follow the 2H→3C transformation in silicon carbide alloys.

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Randomly constructed networks of N elements governed by piecewise linear differential equations have been proposed as models for neural and genetic networks. In this model an element is labelled "on" if it is above a threshold, and "off" otherwise. The logical state of the network at any time is thus a Boolean vector. For each element, there is a rule (truth table) specified by the values of K input elements that determines whether it will switch its state (from 1 to 0 or from 0 to 1) at some future time. Parameter p biases the output state of each automata and influences the global dynamics of the system. Discrete switching network analogues of the piecewise linear differential equations show a transition from ordered to disordered dynamics as p and K are varied. The transition is analyzed in continuous switching networks. Previous studies of these networks have demonstrated the existence of steady state, periodic, and chaotic attractors. Numerical studies analyze the dynamics of a randomly chosen ensemble of networks as a function of N, K, and p. Theoretical insight into the change in dynamics is gained through the derivation of a Markov process for the number of elements at time t that are approaching their threshold. Analysis of this Markov model yields a critical relation between p and K that separates ordered dynamics from deterministic chaos.

A powerful new class of microscope, the Atomic Force Microscope, has led to a deeper understanding of the nanoscale world. The Scanning Probe Microscopy group at McGill University has developed an AFM whose general design lends itself readily to imaging, spectroscopy, and manipulation on an atomic scale. / There are many challenges to building such a system. A major concern is the isolation of the microscope from external vibration sources, such as building vibration, people walking and talking next to the microscope, etc. These vibrations are orders of magnitude larger than the precision needed to detect the small variations of an atomic surface. A number of measures were developed to provide the isolation needed, including a series damping plates that support the microscope. / An electromagnet was developed as an addition to the microscope as an aid to investigate the magnetic properties of samples. The magnet design combines a maximum magnetic field of over 1 kiloGauss with a simple construction and wire winding. Using this magnet, we will be able to rotate sample magnetizations in situ to observe switching times and profiles.