Year 2007, Volume 27

Abstract

The paper presents results of investigations on the application of glass-ceramic to bonding the CBN grains into abrasive composite. The application of the glass with specific chemical composition makes it possible to generate the designed crystalline phase in it. A type of crystalline phase has a significant influence on the elastic modulus E.

Abstract

Al₂O₃ / TiC ceramic composites were produced by hot pressing for the use of wire-drawing dies. Performance of these ceramic drawing dies was tested with a universal tensile testing machine on 65Mn steel wire. The wear surface features of the ceramic drawing dies were examined by scanning electron microscopy. Finite element method was used as a means of numerically evaluating stress and its distribution inside the ceramic dies during wire-drawing processes. The wear mechanisms of the ceramic drawing dies were investigated. Results showed that drawing speeds had no profound effect on the drawing forces; while the lubrication conditions had great influence on the drawing forces. The ceramic dies exhibit brittle fracture with evidence of cracks and scale chipping on both sides of the corners at the approach zone. Greater compressive stresses were found to be the main reason that caused the failure at these areas. While at the centre bore area of the approach section, the ceramic dies showed plowing features, and the primary wear mechanism at this area was abrasive wear. Examination of the center surface at the bearing zone demonstrated that the wear occurred by small abrasive. Brittle fracture was found to be the primary wear mechanism at the exit corner of bearing section owing to the greater stress concentration.

Abstract

Non-oxide SiC ceramics can withstand high temperatures ~1400 °C in severe combustion environments. Therefore such ceramics are interesting candidates for advanced combustion technologies, e.g. sophisticated porous burners. For the fabrication of porous SiSiC ceramics the DLR developed a new technology based on carbon sheets and lamellae. These basic materials can be combined to lightweight 3D C/C stacks. Through the variation of the amplitude and number of lamellae per inch, the open porosity and orientation of the pore channels could be tailored in a wide range. By using the pyrolysis followed by the liquid silicon-melt infiltration process the carbon stack could be directly converted into SiC in one shot. The residual open porosity can easily be filled with pure silicon to obtain 3D SiSiC structures with adequate mechanical strength and sufficient damage tolerance. Best results from durability tests were obtained with structures which are composed of oriented pore channels. Suitable structures should have angles (α ) of about α =±60° or less. The results from burner rig tests at LSTM with improved components have been very promising, since a lifetime up to 500 hours and 2000 start-ups could be obtained with α =±50° as well as with α =±60° sample. So far, no significant oxidation or degradation could be observed after 1939 h/10800 start-ups with α =±45° sample. These proof tests are ongoing and show that these novel cardboard like structures have a high potential for industrial applications.

Abstract

In the article it has been emphasized that superconductivity is only the enabling technology for Super-MAGLEV and the superconducting transmission lines, because all the cost is in cryogenic engineering, electrotechnology and general engineering infrastructure. So if in the long term the hydrogen economy can be combined with superconducting cables and ultra fast trains running in the same vacuum tunnels network it will be the forerunner to a transportation and energy transfer revolution called SuperTransGrid. In such system hydrogen fuel could be provided for energy storage and transfer, very effectively cooling high power superconducting cables but would also feed low pollution Cryoplanes and fuel-cell powered cars transporting train passengers at speed ~2000 miles/hour.

Abstract

Sile Region, northeastern Istanbul, includes the most important underclay deposit of Turkey. In the area, Upper Cretaceous calc-alkaline volcanic rocks are the major parent rocks of the Neogene sequence composed of plastic clays, underclays, sands, gravels and Quaternary alluvial deposits. The Sile Region consists of discontinuous cyclic thin coal beds and industrial clay deposits accumulated in some lacustrine basins. Several coal seams occur within the Upper Miocene succession with gray to greenish underclays; they occured in swamp environments, coal-forming dysaerobic environment, due to fluctuation of lake water level that provided the necessary humic and fulvic acids for the post-deposition chemical leaching of highly altered andesitic rocks to kaolin in dysaerobic, relatively low pH, conditions in saturated groundwater zones. These clays are composed of disordered kaolinite, illite, mixed-layer illite-smectite, smectite, gibbsite, quartz, pyrite, anatase, K-feldspar, goethite and organic matter. The underclays have been studied because this area is the biggest sedimentary industrial clay deposit of Turkey and economic importance. Mineralogical, chemical and technological tests have been conducted to assess their potentials in the ceramics industry for the production of tiles. They were tested at 50 °C intervals from 1000 to 1200 °C. The technological properties of underclays are characterized by low values of shrinkage and flexural strength after pressing, drying and firing. Water absorption values decreased below 5 wt.% at 1200 °C and these underclays can be used in the manufacture of vitrified tiles or sanitary ware. The chemical analyses generally show average silica, high potassium and medium alumina contents in all samples, typically of kaolinized and illitized materials. The Sile underclays exhibit very low S and Fe contents, but low plasticity and excessive refractory properties. Firing shrinkage increases slightly with increasing temperature, but there is a systematic increase in flexural strength.

Abstract

The effect of shaping methods (extrusion and dry pressing) on the firing transformations of a clay raw material was investigated in the range 850-1150 °C by X-ray diffraction, scanning electron microscopy and by evaluating some ceramic properties. Results show that the forming method has a marked effect on reactions pathways: e.g, whereas mullite together with cristobalite developed in pressed samples, leucite occurred in extruded ones. As a result of the pressing operation, mullite formed at low firing temperature (1000 °C). In addition, plagioclase (labradorite), orthoclase, magnesian spinel and a glassy phase were formed at lower firing temperatures, in pressed samples. On the other hand, it was found that the ceramic technological properties of extruded and pressed samples were mainly controlled by the presence of a glassy phase.

Abstract

The densification behaviour of clay/waterworks waste mix sintered in air at various temperatures is reported. The used waterworks waste comes from a municipal waterworks from South-Eastern Brazil. The ceramic pieces containing up to 15 wt.% of waterworks waste were uniaxially pressed and fired at temperatures from 850 °C to 1050 °C. The densification behaviour of the pieces was evaluated by: linear shrinkage, apparent density, water absorption, apparent porosity and flexural strength. The development of the microstructure was followed by SEM/BSE. The results revealed the densification behaviour to be only slightly influenced by the waterworks waste added for the given testing conditions. The flexural strength test data resulted in Weibull distribution with Weibull moduli in the range of 4.4 to 10.2.

Abstract

Special waste incinerator ashes (fly and bottom) and steelmaking slag were attrition milled, separately or in blend to give powders that were pressed into specimens. Pressed samples were sintered in a muffle furnace to form monolithic ceramics that were characterized by density, shrinkage on firing, water absorption, bending strength, hardness and fracture toughness measurements. Crystal phases were investigated by XRD and microstructures were observed by SEM. Excluding materials prepared using fly ashes alone, the mechanical properties of the sintered specimens are fairly good thank to their fine microstructure. They contain equiaxial grains and a limited amount of vitreous phase. Shrinkage on firing is high for every composition and the residual porosity seems to be mainly on the surface since water absorption is low. Samples obtained by the various blends were aged in an acid (HCl) water solution to evaluate the elution of the components. Leaching test data show that the sintering procedure limits the release of most of heavy metals present in the starting waste.

Abstract

The role of three additives with different cationic valencies, Li₂CO₃ , H3BO3 and H₃PO₄ on the vitrification characteristics of a tri-axial ceramic body containing kaolinite, potash feldspar and silica sand was investigated. Pressed samples containing these additives were fired at different temperatures, then characterized by volume shrinkage, bulk density, water absorption and microstructure with phase analyses. H₃BO₃ (B₂O₃) was observed to exert the most beneficial influences on the vitrification characteristics.

Abstract

Al₂O₃/(W,Ti)C ceramic composites were prepared for use as nozzles in coalwaterslurry (CWS) industrial boilers. Effects of nozzle length, nozzle inlet angle, nozzle exit structure, atomizing air pressure and experimental temperature on the erosive wear of these ceramic nozzles have been investigated. Results showed that reasonable nozzle structure is beneficial to decrease nozzle wear. Lower atomizing air pressure can help to increase nozzle life. Thermal shock damage such as cracking and chipping was considered as the dominant reason for the failure of ceramic nozzles.

Abstract

After the emergence of AC NbTi strands, superconducting transformers were successfully built. But the very high cost of 4 K cryogenics made these transformers economically not attractive. The high Tc superconductors (HTS), operating at much higher temperatures, change these conclusions with low cost HTS conductors. The high cost of PIT tapes and the relatively large AC losses remain issues. The second generation HTS wires, the REBCO coated conductors, are under development and achieved substantial progress recently. They operate at higher temperatures and intrinsically show lower AC losses especially for transformers. MgB₂ is the third option. The magnetic flux density conditions make possible the operation at 27 K and they show low costs. This paper provides a preliminary design for an on-board 40 MVA transformer using REBCO coated conductors and MgB₂ wires. Both superconducting transformers show similar volumes and weights. The power density per unit mass and volume is improved by a factor about two, cryogenic included, compared to resistive systems. This makes them very attractive for on-board mobile systems. The economical point of view is discussed based on some targets price/performance for superconductors and cryocoolers. MgB₂ is penalized by its operation at lower temperature (27 K / 77 K), which makes cryogenics very expensive. The advantage of the low cost of MgB₂ compared to REBCO may be lost except with very low AC loss MgB₂ tapes.