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Computation of the effect of pH on spur chemistry in water radiolysis at elevated temperatures

100%

Swiatla-Wojcik D.

tom Vol. 53, suppl. 1

31-37

Diffusion-kinetic model has been employed to calculate the effect of pH and associated ionic strength on the primary yields in the radiolysis of water from ambient temperature to 200°C. Account has been taken of the effect of ionic strength, I, up to 0.1 molźdm-3 in both acidic and alkaline solutions resulting from the addition of H+ and OH-,assuming the counter ions have unit charge. The primary yields are essentially independent of pH for I ? 10-4. AboveI = 10-4 molźdm-3 the primary yields of e-aq and H2 in acidic solutions decrease whereas the primary yields of the H atom, hydroxyl radical and hydrogen peroxide increase. At I >10-3 molźdm-3 in alkaline solutions, the OH radical and hydrogen peroxide are partially converted into Oo- and HO-2 , respectively. Increases in the total yields GoOH + GOo- and Ge-aq + GHo and a decrease in GH2O2 + GHO-2 have been found with increasing pH. At elevated temperatures the effect of pH is diminished. The temperature effect on the primary yields in acidic and alkaline solutions is nearly the same as in neutral water.

Mechanical properties and structure of magnesium alloys with graduate content of aluminium at elevated temperatures

88%

Čížek L. , Hanus A. , Greger M. , Juřička I. , Rusz S. , Prażmowski M. , Tański T. , Hernas A.

2007

tom Vol. 321, z. 89

91-92

At the contemporary stage of the development of the engineering thought, and the product technology itself, material engineering has entered the period of new possibilities of designing and manufacturing of elements, introducing new methods of melting, casting, forming, and heat treatment of the casting materials, finding wider and wider applications in many industry branches. Therefore the development of engineering aims at designs optimizing, reducing dimensions, weight, and extending the life of devices as well as improving their reliability [1-3]. Contemporary materials should possess high mechanical properties, physical and chemical, as well as technological ones, to ensure long and reliable use. The above mentioned requirements and expectations regarding the contemporary materials are met by the non-ferrous metals alloys used nowadays, including the magnesium alloys. Magnesium alloys and their derivatives, characterize of low density (1.5-1.8 g/cm3) and high strength in relation to their weight [1,3]. Knowledge of the relaxation properties of metal materials at elevated temperatures is necessary for the verification of susceptibility of castings to the creation of defects during the production and forming processes [1,4]. Temperature limits of materials where highest tension values are generated may be detected with tensile tests under high temperatures. Experimental investigation was made on magnesium alloy AZ91 - samples A and AZ61 - samples B (after ASTM Standard) in initial state as cast. The purpose of the measurement was the study of deformation and tension changing with temperature at the tensile test and in time with simultaneous acoustic emission (AE) measurement (in the case of alloy AZ61). These dependencies were also monitored at various temperatures of sample heating from 15°C to 400°C with crosspiece shift of 6mm/min. The measurement included material sample stress at the given temperature by tension at the INOVA electro hydraulic loading machine with a loading force of 20 kN with possibility of the acoustic emission (AE) monitoring. The test bar with 0 4 mm was warmed up in a graphite furnace in inert atmosphere (argon). The AE scanner records released elastic waves (overshoots) in a frequency band between 30 kHz and 400 kHz. The output from the scanner is carried to the AE preamplifier where it is amplified and impedance-adjusted so it is possible to be transferred to more far-reaching places. The signal is further carried to the EMIS 01 system and to the PC's hard disk and they are processed in the EXCEL. Microstructure of the alloys in initial state is formed by solid solution and by minority phases Mgn(Al,Zn)i2 in massive and dispersion form and showed dendritic segregation. During heating magnesium alloy AZ91 at chosen temperatures there occurs partial dissolution of minority phases. Homogenisation of microstructure is, however, accompanied by simultaneous forming of inter-granular non-integrities, which is unfavourable from the viewpoint of strength and plastic properties, especially at higher temperatures. Failure occurs practically at all temperatures basically by inter-crystalline splitting along the boundaries of original dendrites. Trans-crystalline plastic character of fracture in small areas at 300°C was occurred. Similar temperature dependence was occurred in the case of alloy AZ61. In this case the plasticity properties were at high level. An acoustic emission method was used for a better analysis of the course of the deformation action at the tensile test. The AE method especially enables a study of dynamics of these processes at various temperatures. The opportunity to study deformation processes preceding initiation of cracks and monitoring of initiation and crack growth as up to the macroscopic scale is a big advantage of the AE. The method is therefore used in the technical diagnostics and at a check of technological operations in the production process.

Quantitative Rietveld analysis of the decomposition of hardened rapid sulphoaluminate cement after exposure to elevated temperatures

75%

Tchekwagep J. J. K. , Chen D. , Mukhopadhyay A. K. , Wang S. , Huang S. , Cheng X.

tom Vol. 21, no. 3

576--594

Concrete made from rapid sulphoaluminate cement is widely used today, especially in China. It is likely to continue gaining popularity since its manufacture produces less CO2 than the process of manufacturing ordinary Portland cement. Elevated temperatures are among the most serious threats to the structural stability of this product. In the present study, laboratory tests were carried out, through Rietveld analysis and other systematic testing, on samples of hardened rapid sulphoaluminate cement paste exposed to six different temperatures. As the temperature increased, the content of minerals that contribute to rapid sulphoaluminate cement strength was reduced. There was also an increase in porosity. The results show that the chemical dehydration of rapid sulphoaluminate cement after exposure to elevated temperatures is great enough to increase the local pores’ absorption, a change that can be fatal to rapid sulphoaluminate cement concrete’s strength. This understanding could help us characterize strength reduction in a more effective manner, not just in laboratory samples but also in actual structures containing rapid sulphoaluminate cement that has been exposed to elevated temperatures.