Fabrication of alumina ceramics from powders made by sol–gel type hydrolysis in microemulsions

The engineering aspects of the preparation of nanostructured alumina ceramic precursors by alcoholate hydrolysis using microemulsions as reaction media are investigated here. The precipitate was subjected to several treatment steps. Although the properties of the primary precipitated powders are independent of the chemical or reaction engineering parameters of the precipitation procedure, the structure of treated powders and sintered, dense ceramics strongly depends on thermal and mechanical handling like crystallization or grinding of the alumina ceramic precursor. Strong differences are manifested in relative densities and sintering kinetics and can be observed by SEM analysis.     

Piercing in delicate materials with abrasive-waterjets

Abrasive waterjets (AWJs) have proven to be a versatile tool for precision machining of a variety of materials and are applicable to virtually any material; there is no heat-affected zone, no tooling, fast turnaround, and is cost-effective for large and small lot sizes. Nevertheless, damage to delicate material may occur during the initial piercing stage. The large buildup of piercing pressure (p _p) inside blind holes is responsible for the damage. Laboratory results show that p _p decreases with the hole depth. At 345 MPa, waterjet-induced p _p in an AWJ-pierced blind hole was measured and extrapolated to be about 180 MPa at the target surface. Such a large p _p exceeds the ultimate strength of many delicate materials such as laminates, composites, glass, and other brittle materials. We have discovered that the phase change of the working fluid of liquefied nitrogen (LN₂) in an abrasive cryogenic jet (ACJ) is an effective means to mitigate piercing damage. Most of the LN₂ evaporates before entering the blind hole, significantly reducing the piercing pressure. This paper investigates the causes and extent of piercing damage induced by AWJs. Understanding such causes has led to the development of a cost-effective flash abrasive waterjet (FAWJ) to emulate the phase change of the ACJ for mitigating piercing damage. The mechanisms leading to the mitigation of piercing damage by the FAWJ are described qualitatively and quantitatively. Comparison of the visual results of holes pierced in several delicate materials with AWJs and FAWJs is presented to correlate the extent and the mitigation of damage with the piercing pressure.     

Microstructure-Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia-Toughened Alumina

The rates of densification and the mechanical properties of pure Al₂O₃ and ZrO₂-toughened Al₂O₃ (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding-induced and fracture-induced monoclinic ZrO₂ formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four-point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m^1/2.     

Design of a Microstructured System for the Homogenization of Dairy Products at High Fat Content Part II: Influence of Process Parameters

Partial homogenization using a microstructured SHM (Simultaneous Homogenizing and Mixing) valve significantly reduces aggregation of fat globules within their homogenization by feeding the continuous phase directly into the droplet disruption zone (as discussed in part I of this work). It allows homogenization of cream containing up to at least 42 vol.‐% fat, and thus, significantly reduces processing costs without loss in product quality, i.e., overall process intensification. The present article details current results on the influence of material parameters, i.e., emulsifier system and fat content, and process parameters, i.e., homogenizing pressure and the temperature of both streams, on the fat globule size distribution. In contrast to conventional technology, SHM valve technology generates decreasing droplet sizes with increasing pressure or with increasing temperature of the homogenizing stream. This novel technique breaks new ground in dairy product design.     

Publication dynamics: Models and indicators

Models and indicators characterizing the dynamics of national publication productivity distributions are presented. The indicator triplet: transience, renewal, and dynamism is used to describe the «physical shape» of a national scientific community.