The influence of autoclave steam on polymer and organic fibre modified ceramic shells

The removal of wax from an unfired ceramic shell system without cracking or dimensional alterations is a key stage within the investment casting process. The effect of autoclave steam on polymer and fibre modified investment casting mould behaviour has been investigated. The polymer modified system exhibited a higher mechanical strength in the green dry state, but that strength significantly reduced when subjected to a simulated autoclave “wet” condition, giving a 38% reduction in a flat bar section and a 45% reduction in an edge test. It is suggested that this is related to the softening of the latex particles when they are in contact with steam. In comparison the fibre modified system showed a much lower reduction in strength when subjected to “wet” conditions. Calculating the adjusted fracture load (AFL) bearing capacity for the extra shell thickness of the fibre system showed that the fibre system outperforms the polymer system when the samples were tested “wet”, showing a 33% increase in a flat bar section and an increase of over 150% in the more vulnerable edge region. The results suggest that the effect of moisture must be taken into account when studying the shell behaviour under autoclave conditions.     

Dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing

The dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests, using a torsional Kolsky bar, were conducted on four steel specimens, two of which were annealed at 480 °C after ECAP, and then the test data were compared in terms of microstructures, tensile properties, and adiabatic shear-band formation. The equal-channel angular pressed specimen consisted of very fine, equiaxed grains of 0.2 to 0.3 μm in size, which were slightly coarsened after annealing. The dynamic torsional test results indicated that maximum shear stress decreased with increasing annealing time, whereas fracture shear strain increased. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen. Their width was smaller in the equal-channel angular pressed specimen than in the 1-hour-annealed specimen, but they were not found in the 24-hour-annealed specimen. Ultrafine, equiaxed grains of 0.05 to 0.2 μm in size were formed inside the adiabatic shear band, and their boundaries had characteristics of high-angle grain boundaries. These phenomena were explained by dynamic recrystallization due to a highly localized plastic strain and temperature rise during dynamic deformation.     

Bearing capacity of isolated square footing resting on contaminated sandy soil with crude oil

The main objective of this paper is the evaluation of bearing capacity of isolated square footing resting on clean and oil-contaminated sand. The study is aimed at determining the effect of percentage of oil content in sand and the depth of the contaminated sand layer under the footing on the bearing capacity of footing. A series of routine laboratory tests were carried out on sand to obtain the physical, mechanical, and chemical properties. Crude oil is used as a contamination material. The research-included the completion of experimental work that consists of a series of loading tests carried on model of isolated square footing resting on clean and oil-contaminated sand. The contaminated sand layers were prepared by mixing the sand with crude oil at percent (10 and 20) % by weight of dry sand to simulate the field conditions. The depth of contaminated sand layer (d) under the footing was chosen depending on the footing width (B) as expressed by (d/B) ratio and varied as (0.5, 1, 1.5 and 2) to simulate the field conditions. The results showed that the contamination influences the load-settlement curves and leads to a reduction in its bearing capacity. The bearing capacity of the footing decreases significantly with increasing both the percentage of oil content and the depth of the contaminated sand layer. The results also showed that the presence of oil in sand changes the mode of shear failure in sand under the footing from local shear failure to the punching shear failure.     

Fracture toughness and brittle-ductile transition controlled by grain boundary character distribution (GBCD) in polycrystals

The structural dependence of intergranular fracture processes in bicrystals and polycrystals of metals and alloys is first reviewed. It is shown that even in polycrystals, grain boundary structure plays a significant role in controlling the fracture properties of the material. Next, we evaluate quantitatively the effect of different types, frequencies and configurations of grain boundaries, so-called the grain boundary character distribution (GBCD), on the toughness of a three-dimensional (3D) polycrystals. The results show that the toughness of a polycrystals increases monotonically with increasing overall fraction of fracture-resistant low-energy boundaries in the material. A brittle-ductile transition, corresponding to a change of fracture mode from predominantly intergranular with low toughness to predominantly transgranular with high toughness, is observed when the overall fraction of low-energy boundaries reaches a critical value. For a 3D polycrystals with a non-random GBCD such that the fraction of low-energy boundaries on the inclined boundary facets is maximised, a smaller critical overall fraction of low-energy boundaries is needed to bring about the brittle-ductile transition. Similar effect is also found if the grains are made elongated and aligned with the stress axis. The results are discussed in relation to the concept of grain boundary design for strong and tough polycrystals proposed by one of the present authors (T.W.).     

Influence of chemical trace additives on the future ageing of the UA1 central detector

A number of systematic tests investigating the effect of various chemical additives on the ageing of prototypes of the UA1 Central Detector have been performed. In addition to the classical substances of water, ethanol and isopropanol emphasis is also put upon more uncommon chemicals, usually required as doping seeds for UV laser calibration. Chamber lifetime can be strongly influenced by them, depending on their concentration. Evidence is presented that proper choice and dosage can lead to a significant extension of normal wire chamber lifetime.     

Thermal Hydrolysis Synthesis and Characterization of Monoclinic Metahewettite CaV_6O_(16)·3H_2O

Monoclinic metahewettite CaV 6 O 16 ·3H 2 O has been fabricated via thermal hydrolysis of calcium vanadate(Ca 10 V 6 O 25). High purity calcium vanadate precipitate, featuring column structure with surface area of 8.61 m2/g, can be obtained by reacting sodium orthovanadate(Na 3 VO 4) with calcium oxide at 90 ℃ for 2 h. By acidifi cation of calcium vanadate in hot water at pH of 1.0-3.0, the monoclinic metahewettite crystals with uniform particle distribution, layered structure and nonporous structure can be fabricated. With the well crystallized layered structure, CaV 6 O 16 ·3H 2 O may be a potential cathode material for secondary batteries as well as super capacitor materials.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Iron oxide colouring of highly-translucent 3Y-TZP ceramics for dental restorations

The influence of 0.01–2 mol% Fe₂O₃ powder addition on the microstructure, mechanical and optical properties, and hydrothermal stability of highly-translucent 3Y-TZP ceramics is assessed and compared with commercially available co-precipitated 0.18 mol% Fe₂O₃ doped ZrO₂ powder-based ceramics. Only those ceramics with up to 0.1 mol% Fe₂O₃ resulted in a proper shade for dental zirconia ceramics, with a typical composition of 87 vol% t-ZrO₂ and 13 vol% c-ZrO₂. The amount of cubic phase increased at higher Fe₂O₃ content. The hardness (∼13 GPa) and fracture toughness (∼3.6 MPa m^1/2) of the 0.01 mol% - 0.1 mol% Fe₂O₃ doped 3Y-TZP was comparable, whereas the hardness decreased above 0.5 mol% Fe₂O₃ and the fracture toughness decreased above 2 mol% Fe₂O₃. The hydrothermal ageing resistance slightly increased for Fe₂O₃ concentrations up to 1 mol%, whereas the translucency slightly decreased with increasing Fe₂O₃ content.     

Effects of SDBS and ZrO2 additives on the microstructure and properties of silicon-bonded SiC porous ceramics

Porous silicon-bonded silicon carbide (SBSC) ceramics were prepared under argon atmosphere, with silicon as pore former and bonding material, simultaneously, sodium dodecyl benzene sulfonate (SDBS) and ZrO₂ as sintering additives, the effects of SDBS and ZrO₂ on the porosity, pore size, mechanical, physical and thermal properties and microstructures were investigated. The results suggested that suitable content of SDBS and ZrO₂ could not only effectively lower the sintering temperature to 1450 °C due to the sticky flow of molten silicon, but also increase the pore structure and improve the bending strength. The reason for this is that SDBS decomposed into Na₂O which reacted with ZrO₂ and impurity SiO₂, which was the native oxide film on the surface of SiC particles, to form a bonding phase between SiC particles to improve the bending strength; meanwhile, the disappearances of impurity SiO₂ would benefit the bond of molten silicon and silicon carbide particles, and silicon melt leaving pores in its original position to increase the pore structure. The optimal apparent porosity, bending strength, average pore size, gas permeance and residual bending strength after thermal shock cycles of SBSC porous ceramic sintered at 1450 °C with 5 wt% SDBS and 6 wt% ZrO₂ were 38.33%, 55.4 MPa, 11.3 μm, 106.4 m³/m²·h·kPa and 28.2 MPa, respectively.