The role of aggregates in the thermal stability of Mg-PSZ refractories for vacuum induction melting

Mg-PSZ refractories used as vacuum induction melting crucibles are particle-reinforced composites with aggregate and matrix phases comprising fused zirconia. Three commercial varieties were cycled eight times to service temperatures and their microstructural and thermomechanical evolution investigated, with focus placed on the aggregate populations. Two refractories, with large aggregates of similar size, were found to retain stiffness after cycling but in the refractory containing aggregates with high stabiliser levels, reaction between the stabiliser and Al and Si impurities produced secondary phases. Volume changes accompanying formation of these phases, and subsequent thermal expansion mismatches, led to aggregate break-up with consequent reductions in refractory toughness and strength. Secondary phases developed only rarely in the aggregates (with lower levels of stabiliser) of the second refractory. These aggregates remained intact and the refractory retained its toughness and strength. A third refractory contained small, unstabilised aggregates in a stabilised matrix and the strain mismatches that ensued during polymorphic transformation damaged microstructural interfaces. Refractory stiffness halved within eight cycles and toughness and strength were lost. All three refractories displayed R-curve behaviour and quasi-stable fracture curves were observed during bend tests. The study shows that when using fused zirconia aggregates to design refractories, engineers need to i) limit stabiliser concentrations - a difference of just ±1 wt% Mg (in the presence of impurity elements) may determine whether secondary phase formation occurs and ii) eliminate alumina and silica impurities when possible through substitution of zircon sand with baddeleyite as the source for fused zirconia.Open Acces

Teleportal Face-To-Face System.

A teleportal system which provides remote communication between at least two users. A projective display and video capture system provides video images to the users. The video system obtains and transmits 3D images which are stereoscopic to remote users. The projective display unit provides an augmented reality environment to each user and allows users to view, unobstructed, the other local users, and view a local site in which they are located. A screen transmits to the user the images generated by the projective display via a retro-reflective fabric upon which images are projected and reflected back to the users eyes

'Cover story': a study in land management

This article summarises an environmental research project undertaken by pupils of Mondeor High School, Johannesburg. The project was entered for the Enviro '85 Competition where it won the Civic Awareness Section and was overall winner of the competition

Assessment of seedling recruitment under manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) plantings at Shakespear and Wenderholm regional parks

Exclosure plots were monitored to investigate the impact of browsing on seedling recruitment by Trichosurus vulpecula, Oryctolagus cuniculus and Rattus rattus on seedlings under Leptospermum scoparium and Kunzea ericoides plantings in two Auckland Regional Council Parks (ARC), Shakespear and Wenderholm. The number of woody seedlings that established over a 17-month period was recorded. Gaps within the same Leptospermum scoparium and Kunzea ericoides canopy were created to investigate the influence of the canopy on seedling recruitment. Soil samples were taken to investigate the existing seed bank beneath the same Leptospermum scoparium and Kunzea ericoides canopies.At Wenderholm, net change in seedling density differed among treatments (P=0.014). Seedling density increased within the plots that excluded Trichosurus vulpecula and Oryctolagus cuniculus and within plots that additionally excluded Rattus rattus, but declined in the control plots. In contrast at Shakespear, although seedling density increased more within both the exclosure plots than in the control plots, this result was not statistically significant (P=0.728). At Wenderholm, the average seedling height increased within both types of exclosure plots, but declined in the control plots. However, these differences among treatments were not statistically significant (P=0.204). At Shakespear, seedlings increased in height within the Trichosurus vulpecula, Oryctolagus cuniculus and Rattus rattus exclosures and declined marginally in the other two treatments. Again, differences in height change among treatments were not statistically significant (P=0.202).At both regional parks, the greatest cause of mortality within the exclosures excluding Trichosurus vulpecula and Oryctolagus cuniculus was desiccation. All of the mortalities within the Trichosurus vulpecula, Oryctolagus cuniculus and Rattus rattus exclosures was unidentifiable. However, within the control plots, at Wenderholm, the greatest identified cause of mortality was browsing and at Shakespear, the only cause of mortality within the control plots was browsing.Seedbanks at Wenderholm and Shakespear under the Leptospermum scoparium and Kunzea ericoides plantings were dominated by forb species. A total of 1308 seedlings germinated from soil taken from Wenderholm, with exotic species making up 99.4% of germinations, with exotic species making up 97.9% of germinations. Similarly a total of 801 seedlings germinated from soil samples taken from Shakespear.At Wenderholm, the number of native seedling germinations within the gaps created in the Leptospermum scoparium and Kunzea ericoides canopy, was more than twice the number that germinated under the closed canopy. However, this difference was marginally non-significant (P=0.065). At Shakespear, the number of native seedling germinations within gaps created in the Leptospermum scoparium and Kunzea ericoides canopy was similar to the number that germinated under the closed canopy (P=0.2603).The results suggest that at Wenderholm, despite ongoing predator control, Trichosurus vulpecula and/or Oryctolagus cuniculus have had an adverse effect on the survival and growth of seedlings. The results also suggest that at Shakespear, Rattus rattus have had an adverse effect on the survival and growth of seedlings under the Leptospermum scoparium and Kunzea ericoides canopy. The distance from mature forest may also have had an impact on the dispersal of native seeds within the Leptospermum scoparium and Kunzea ericoides canopy. The implication of these results for the future management of restoration plantings in regional parks is discussed

Frame modelling of dynammic ecosystems

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg. in fulfilment of the requirements tor the degree of Master of ScienceThis thesis develops the theoretical basis of the qualitative frame based modelling technique, a paradigm recently proposed by Starfield for the modelling of ecosystems with a multiplicity of stable states. This technique is a refinement of the State-and- Transition conceptual model of Westoby et al which involves the division of the ecosystem dynamics into a catelog of stable 'states' and a suite of transitions between these states. The frame models of Starfield associate with each stable configuration of the ecosystem a qualitative rule based model for the key processes in that stable configuration. The aims of this thesis are the following, 1. A rigorous definition of frame modelling of dynamic ecosystems is proposed, and this theoretical foundation is used to demonstrate that qualitative frame models may be used to mode! dynamic ecosystems to an arbitrary accuracy. 2. The development of implementation software. A qualitative rule based frame modelling environment is presented. and a specification for an improved environment is proposed based on the theoretical work.Andrew Chakane 201