Tetragonal Zirconia Polycrystals Reinforced with SiC Whiskers

The microstructure and the mechanical properties of hot-pressed tetragonal ZrO₂ polycrystals (TZP) reinforced with up to 30 vol% SiC whiskers were studied. The SiC whisker-TZP composites were stable under the hot-pressing conditions at 1450°C. Annealing in an oxidizing atmosphere at ∼1000°C resulted in glass formation and microcracking caused by whisker oxidation and transformation of the ZrO₂ grains near the whiskers to monoclinic symmetry. The fracture toughness was markedly improved by the dispersed whiskers (∼12 Mpa·m^1/2 at 30 vol% SiC) compared to the values measured for the matrix (∼6 Mpa·m^1/2). The flexural strength of the hot-pressed TZP-30 vol% SiC whisker composite at 1000°C (∼400 MPa) was twice that of the TZP matrix.     

Insurgent Architecture: An Alternative Approach to Design-Build

This ongoing experiment in community-based, design-build pedagogy tests the deployment of direct action as a form of "insurgent architecture." Through this we are developing strategies for design practices to mesh more productively with the everyday life of a recovering New Orleans neighborhood. While addressing both social and educational demands, this project also explores an alternative approach to architectural practice.     

FREEZING OF COOKED MEAT: INFLUENCE OF FREEZING RATE AND RECONSTITUTION METHOD ON QUALITY AND YIELD

The combined effects of freezing rate, frozen storage and reconstitution method on meat quality and yield were studied in multifactorial experiments using pan-fried 1.5 cm slices of longissimus muscles and deep-fat fried meat patties. On the whole, a slight improvement in sensory quality and yield was seen with increasing freezing rate, except that liquid nitrogen immersion freezing influenced flavor negatively. In contrast with earlier results of ours with raw beef, an advantage was now seen for reconstitution after previous thawing over reheating directly from the frozen state, and pan frying gave better sensory quality but lower yield than continuous microwave reheating. Tenderness of sliced, cooked beef tended to increase during frozen storage, while the opposite was earlier observed for frozen raw beef. Otherwise, a general trend was seen towards decreasing yield and sensory quality during frozen storage. A number of significant interactions between variables was obtained, suggesting that their influence on quality and yield is interrelated in a rather complex manner.     

α-β Sialon Ceramics Made from Different Silicon Nitride Powders

The present work is concerned with the sintering of an α-β sialon ceramic using five different silicon nitride powders from a single source. The parameters varied in the silicon nitride were the amount of "free' silicon, iron content, α:β ratio, and grain size as measured by BET surface. The sintering atmosphere was varied by use of protective powder beds with passive (boron nitride) and active (SiO-generating) properties. Five sintering temperatures between 1600° and 1800°C were used. Microstructural characterization as well as density, hardness, and fracture toughness measurements were carried out. The sintering conditions were found to be critical for obtaining fully dense materials and low weight change. The optimum sintering temperature was 1750°C. The silicon nitride powder with a high content of free silicon resulted in a material which was more susceptible to the sintering atmosphere conditions. An α-β sialon made from a silicon nitride powder with a high β-α phase ratio resulted in a higher β-α ratio in the sintered material.     

Transformation and Microcrack Toughening as Complementary Processes in ZrO2-Toughened Al2O3

Both tetragonal ( t ) and monoclinic ( m ) ZrO₂ particles in ZrO₂-toughened Al₂O₃ can give rise to toughening. In the stress field of propagating cracks, the t -ZrO₂ particles can undergo the stress-induced t → m transformation, and the residual stresses around already-transformed m -ZrO₂ particles can cause microcracking. The t -ZrO₂ particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr0₂-toughened Al₂O₃'s should contain a mixture of t - and m -ZrO₂.     

Fracture Toughness of Al2O3 with an Unstabilized ZrO2Dispersed Phase

The fracture toughness of Al₂O₃ is considerably increased by the incorporation of fine monoclinic ZrO₂ particles. Hot-pressed composites containing 15 vol % ZrO₂ yield K_lcvalues of ∼ 10 MN/m^3/2, twice that of the A1₂O₃ matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO₂during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values.     

Effect of temperature and time on solvothermal synthesis of tetragonal BaTiO<Subscript>3</Subscript>

Tetragonal BaTiO₃ nanoparticles are synthesized via solvothermal route in an ethanol water mixture. Ba(OH)₂·8H₂O is used as Ba precursor and TiO₂ (P25 Degussa ∼25 nm, 30% anatase, 70% rutile) is used as Ti precursor in the Ba : Ti molar ratio 2 : 1. Effect of temperature and time study on solvothermal synthesis of BaTiO₃ revealed that a moderate reaction temperature i.e. 185°C and longer reaction time favour tetragonal phase stabilization. Dissolution–precipitation appears to be the transformation mechanism for the crystallization of BaTiO₃ from particulate TiO₂ precursor.     

Dependence of Fracture Toughness of Alumina on Grain Size and Test Technique

The fracture toughness ( K_Ic ) of sintered alumina was measured using notched beam (NB) and indentation/strength-in-bending (ISB) techniques. K_Ic (NB) decreases with increasing grain size. For fine-grained materials (<5 μ m ), NB results overestimate K_Ic , and exhibit a substantial notch-radius sensitivity. A stress-intensity-derived model is used to explain this notch sensitivity. The ISB results are very similar to those obtained using the double-cantilever-beam (DCB) method and show an increasing fracture toughness with increasing grain size. The differences between the NB and ISB (DCB) results for coarser-grained materials are thought to be related to R -curve behavior.     

The Aqueous and Surface Chemistry of Activation in the Flotation of Sulphide Minerals—A Review. Part II: A Surface Precipitation Model

The activation of sulphide minerals by heavy metal ions at medium to alkaline pH solutions is reviewed. The pH-dependent characteristics of adsorption are identically indicated by adsorption isotherms, surface potential and flotation responses. The mechanisms involved are explored. It is found that the initial adsorption process is controlled essentially by surface precipitation of the activator hydroxide. The real activation of flotation is obtained during the second stage—the surface conversion step. The industrial practice of activation and flotation with a highly alkaline solution highlights the proposed model and mechanisms. The mechanism underlying the intermediate pH depression of flotation of the activated mineral is discussed.     

Craft and Innovation

This paper looks at a way of studio teaching that emphasizes direct experience with real materials. It argues that innovation takes place when a student "plays" with real materials and adopts a critical attitude toward craft. Recent studios demonstrate a way of working that discovers forms, strategies, and techniques unlike those discovered by a student when working to scale in drawing or model. An essential aspect of this teaching is the emphasis revealing to the student the critical thinking inherent in the activity of making.