Determination of optimal location to monitor temperature in low pressure die casting process

Abstract

Controlling and eliminating defects, such as macroporosity, in castings is a continuing challenge that manufacturers must continually address. Since the encapsulation of liquid regions by a solid shell and subsequent formation of macroporosity cannot be detected during casting, the die temperature, which is routinely measured, has been used as an indirect indicator of this defect. A finite element model has been developed to predict the evolution of temperature as well as the volume of encapsulated liquid in a casting with a high propensity to form macroporosity. The boundary conditions in the model were iteratively adjusted until the temperature predictions matched the experimental data for a variety of operational conditions. A model based methodology has been developed to analyse the correlation between the die temperature and the encapsulated liquid volume. This methodology is employed to assess the suitability of different in-cycle die temperatures for use as indicators of macroporosity formation, and to help determine the optimal location to monitor temperature for the purpose of minimising macroporosity.     

Production and characterization of ITO-Pt semiconductor powder containing nanoscale noble metal particles catalytically active in dye-sensitized solar cells

Doped indium tin oxide (ITO) samples of a surface area around 60 m²/g have been synthesized. As doping component platinum and gold atoms were utilized, respectively. The powders were produced by pyrolyzing mixed metal oxide precursors in a first zone in a flame consisting of hydrogen and air, metering a noble metal compound and reducing gas into a second zone of the flame and separating off the solid obtained in a third zone. Well-defined solid phases In₂Pt and In₂Au in ITO were obtained, respectively. The production of a mixture with discrete SnO₂ entities could be avoided. The mixed oxide powder doped with platinum can be used as a catalyst. The structure and composition of materials obtained at different levels of platinum dosing into a flame reactor has been determined. The size of the noble metal-containing entities ranged at about 3.5–5.5 nm. The catalytic properties of these oxides containing noble metal have been utilized in the fabrication of dye-sensitized solar cells (DSCs). Long term stability of DSCs containing liquid electrolytes requires a glass sealing of the two electrodes at a temperature of over 600 °C. Such high temperatures are detrimental to the catalytic activity of the conventional platinum layers in the counter electrode. Improved catalytic activity could be achieved by platinum entities bound to indium tin oxide. This catalytic material can effectively be used in the DSCs with glass sealing, the sealing temperature being over 600 °C. Spray pyrolysis in a flame reactor is highly suitable to produce noble metal-containing oxides in one single production step. Platinum-containing ITO particles from flame pyrolysis production exhibit superior catalytic activity in dye-sensitized solar cells compared to material from conventional thermal decomposition.     

Negative plate macropore surfaces in lead-acid batteries: Porosity, Brunauer-Emmett-Teller area, and capacity

We propose an explanation for the production of an electrochemically active area during the electrochemical formation of lead-acid battery negative plates based on solid-state reactions. Our proposal is supported by experimental data. This study includes a critical review of the literature on charge/discharge mechanisms, porosity, and BET area. The critical review, through the latter two parameters, indicates the existence of both macro and micropores in positive plates, but only macropores in negative plates, with characteristic surface roughness. In the present paper the surface sulfation of the precursor is controlled using various acidic, neutral and alkaline solutions during an electrochemical formation process that does not include soaking. Our results confirm that variable roughness can be produced at the negative plate macropore surfaces. The morphological changes produced by different formation conditions are assessed by measuring the macroporosity, BET area, and capacity of single negative plates. Based on these concepts, a method was developed and applied to measure independently the contributions of geometrical surface macroporosity and roughness to the negative plate capacity.     

New TiO2 monolithic supports based on the improvement of the porosity

Titania monolithic supports produced with an additional macroporosity were prepared and their behaviour compared with commercial materials of the same composition. Activated carbon used as a template for macropore generation, was included in the preparation process of the ceramic dough. Mercury intrusion porosimetry (MIP), nitrogen adsorption–desorption, X-ray diffraction (XRD), TGA–DSC analyses were employed to study the modifications of the support properties. The systems prepared with the template, which was subsequently eliminated by thermal treatment, presented a notable increase in the macropore volume with respect to samples prepared without this pore generating agent (PGA), displaying a narrow pore size distribution with a mean pore diameter of 0.13 μm. The template was chemically inert and no modification on the crystal phases or the BET area was observed. The optimum composition was achieved for a sample produced with 10 wt.% of template and subsequently heat-treated in air at 550 °C to remove the PGA. Vanadia catalysts based on these supports displayed higher NO_x conversions during the NO and NO_x elimination by the SCR process at low temperature than the standard ones. This behaviour was related to the improvement in the effectiveness factor due to the reduction of the internal mass transfer limitations.     

Tough organogels based on polyisobutylene with aligned porous structures

Macroporous gels with aligned porous structures were prepared by solution crosslinking of butyl rubber (PIB) in cyclohexane at subzero temperatures. Sulfur monochloride was used as a crosslinker in the organogel preparation. The reactions were carried out at various temperatures between 20 and −22 °C as well as at various freezing rates. The structure of the gel networks formed at −2 °C consists of pores of about 100 μm in length and 50 μm in width, separated by polymer domains of 10-20 μm in thickness. The aligned porous structure of PIB gels indicates directional freezing of the solvent crystals in the direction of the temperature gradient. The size of the pores in the organogels could be regulated by changing the freezing rate of the reaction solution. The results suggest that frozen cyclohexane templates are responsible for the porosity formation in cyclohexane. In contrast to the regular morphology of the gels formed in cyclohexane, benzene as a crosslinking solvent produces irregular pores with a broad size distribution from micrometer to millimeter sizes due to the phase separation of PIB chains at low temperatures. Macroporous organogels prepared at subzero temperatures are very tough and can be compressed up to about 100% strain without any crack development. The gels also exhibit superfast swelling and deswelling properties as well as reversible swelling-deswelling cycles in toluene and methanol, respectively.     

Generation of meso- and microporous structures by pyrolysis of polysiloxane microspheres and by HF etching of SiOC microspheres

The porosity of polysiloxane microspheres obtained by emulsion processing of variably modified polyhydromethylsiloxane (PHMS) and subjected to pyrolysis in an Ar atmosphere at 450–650 °C was studied. Materials having micro- and mesopores with specific surface areas (SSAs) of up to 786 m²/g and pore volumes of up to 0.35 cm³/g were obtained. A high porosity was displayed by the microspheres heated at 600 °C that underwent deep depolymerization processes. Some polysiloxane microspheres were ceramized at temperatures of 1200–1500 °C and were subjected to etching by 35% aqueous HF. The microspheres heated to 1200–1400 °C were free of microcracks, whereas those ceramized at 1500 °C showed microcracks and macropores, although they preserved their spherical structure well. All of the microspheres ceramized at temperatures of 1200–1400 °C had low porosity. HF etching granted high micro- and mesoporosity to the materials ceramized at 1300–1500 °C. Microspheres heated at 1500 °C showed specific surface areas above 1000 m²/g after etching. These microspheres had low oxygen contents and were mostly composed of silicon carbide. Since they also showed macroporosity, HF etching of the polysiloxane microspheres ceramized at 1500 °C could be used to obtain hierarchically mesoporous-macroporous ceramic microspheres.     

Analysis of catalyst particle strength by impact testing: The effect of manufacturing process parameters on the particle strength

The mechanical strength of porous alumina catalyst carrier beads, used in the reforming units with continuous catalytic regeneration, was measured by impact testing. With this testing method particle strength can be measured at higher strain rates than the traditional crushing test method, hence providing a better simulation of pneumatic conveying and chute flow conditions, and also a large number of particles can be tested quickly. This is important for particles with a brittle failure mode such as the alumina particles used in this work as a wide distribution of mechanical strength usually prevails. Extensive impact testing was carried out first with an industrial sample, in order to understand the failure mechanism of this type of particles and to develop a methodology for analysing the extent of breakage by impact. Then the method was used to analyse the effect of a number of process parameters, such as filler, macroporosity and drying procedure on the particle strength with the aim of optimising the manufacturing process. The impact test results were then used to test the model of breakage behaviour of particulate solids proposed by Vogel and Peukert [Vogel and Peukert, Breakage behaviour of different materials—construction of a mastercurve for the breakage probability. Powder Technol., 129 (2003) pp. 101-110].     

Review od ADVANCES IN DRYING,Vol. 3,

RF hydrogels were synthesized by the sol-gel polycondensation of resorcinol with formaldehyde and RF cryogels were prepared by freeze drying of the hydrogels with t-butanol. The cryogels were characterized by nitrogen adsorption, density measurements, and scanning electron microscope. Their porous properties were compared with those of the aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >5.8× 10^?4m³/kg. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Aerogel-like carbons (carbon cryogels) were obtained by pyrolyzing RF cryogels in an inert atmosphere. The carbon cryogels were mesoporous materials with high surface areas >8.0× 10⁵m²/kg and large mesopore volumes >5.5× 10^?4m³/kg. When pyrolyzed, micropores were formed inside the cryogels more easily than inside the aerogels.