Thermal analysis study of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3-x</Subscript>Er<Subscript>x</Subscript>O<Subscript>10+δ</Subscript> glass-ceramic system

Summary We have fabricated glasses in the Bi-2223 HT_c superconductor system with Bi₂Sr₂Ca₂Cu_3-xEr_xO_{10+ δ} nominal composition, where x=0.5 and 1.0, by the glass-ceramic technique. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization temperature and thermal properties has been obtained. The crystallization studies were made using DTA with several uniform rates. The calculations of crystallization activation energies, E_a, and the Avrami parameters, n, were made based on the non-isothermal kinetic theory of Kissinger and the Ozawa’s equations. The DTA data of the samples showed that the first crystallization temperature, T_x1, increases and the second crystallization temperature, T_x2, decreases by increasing the Er concentration. This suggests that the Er substitution had significant effect on the glassification of the BSCCO material due to change on the surface nucleation and increased ionic activities at high temperature region. The activation energy for crystallization, E_a, of the samples was also showed an increase at high Er concentration case. However, the Avrami parameter, n, decreased from 2.5 to 1.7 for x=0.5 and 1.0 samples, respectively. This suggests that the growth mechanism is diffusion-controlled and three-dimensional parabolic growth takes place near the first crystallization temperature. The oxidization rates and the activation barrier for oxygen out-diffusion process, E, was calculated using the TG data. It was found that the total mass gain in the x=0.5 sample is comparably smaller than that of the x=1.0 sample. This shows that the oxygen absorption of the x=1.0 sample is faster than the x=0.5 sample, leading to increase in the oxidization rate in the x=1.0 material.     

A numerical solution of Burgers’ equation by finite element method constructed on the method of discretization in time

In this paper, the Galerkin finite element method constructed on the method of discretization in time was applied to solve the one-dimensional nonlinear Burgers’ equation. The system of nonlinear equations obtained for each time step was solved by using the Newton method. In order to show the efficiency of the presented method, the numerical solutions obtained for various values of viscosity were compared with the exact solutions. It was seen that they were in excellent agreement.     

Analysis of desiccation and vitrification characteristics of carbohydrate films by shear-wave resonators

Desiccated state preservation of mammalian cells and tissues in the presence of carbohydrates has started to show promise in the last two decades. Certain carbohydrates play a major role in preservation by reducing molecular mobility in the desiccated state. In this communication, the feasibility of utilizing shear-wave resonators to collect real-time molecular mobility information during desiccation and vitrification of carbohydrate based thin films was demonstrated. Simultaneous quartz crystal microbalance experimentation and optical imaging were utilized to determine the conditions for thin film formation and the vitrification kinetics of certain carbohydrate solutions of biological importance. Using the technique presented here, it was possible to gain insight into the vitrification characteristics of carbohydrate solutions establishing the basics for future research with quantitative analysis of film properties and experimentation with live mammalian cells.     

Isothermal desiccation and vitrification kinetics of trehalose-dextran solutions

The promise of dried state preservation is based on the hypothesis that lowering molecular mobility to halt chemical reaction and deterioration rates is the primary factor for the long-term stability of the dried specimen. In this research, the feasibility of utilizing isothermal, isobaric vitrification as an economical alternative to the preservation technologies currently in use (mainly, cryopreservation and lyophilization) is explored. Desiccation and vitrification kinetics of model trehalose and trehalose-dextran systems were examined using gravimetric analysis, modulated differential scanning calorimetry, and X-ray crystallography. It was shown that vitrification can be achieved isothermally without crystallization and that vitrification of trehalose solutions can be significantly accelerated by incorporating high-molecular-weight dextrans. Additionally, it was shown that, for the same water content, the glass transition temperature of the trehalose-dextran solution is significantly higher than that of the binary trehalose solution, making the glassy state achievable and storage feasible.     

Formation of a Cmcm phase in SnS at high pressure; an ab initio constant pressure study

The stability of SnS at high pressure is studied using a constant pressure ab initio technique. For the first time, a pressure-induced phase transformation from the Pnma structure to a Cmcm structure with the application of pressure is predicted through the simulations in this material. The Cmcm phase is still a layered structure, consisting of rocksalt-like bilayers, similar to that formed at high temperatures. The Cmcm structure is fivefold coordinated. This phase transformation gradually proceeds and is due to the significant decrease of the second neighbor distances. This phase change is also studied by total energy calculations.     

Fabrication of NdCeCuO and effects of binding agents on the growth,micro-structural and electrical properties

NdCeCuO superconducting samples were fabricated using ethyl alcohol, acetone and ethylenediaminetetraacetic acid (EDTA) as binding agents. For evaporation of binding agents, the samples were heat treated at 1050 °C for 24 h and then at 950 °C for 6–48 h under argon atmosphere to obtain the superconducting phase. The best superconducting performance was found in the sample heat treated at 1050 °C for 24 h and then 950 °C for 12 h which was fabricated by using acetone as binding agent. The T_c and T₀ value was found to be ～25 K and 23.4 K, respectively. Grain size in the samples fabricated was calculated using Scherer equation and SEM data. It was found that grain size strongly depends on the binding agents and heat treatment conditions. Some cracks and voids on the surface of the samples were observed, which influences the superconducting and electrical transport properties of the samples.     

Thermoelectric power and thermal conductivity study of the Y3Ba5Cu8Ox system

In the present study, the effect of heat treatment conditions on the structural/microstructural, electrical and transport properties, such as thermoelectric power and thermal conductivity, of the Y-358 system was investigated. The Y-123 phase instead of Y-358 were obtained with heat treatments. XRD and SEM analyses showed that the best structural formation was obtained for the sample prepared at 900 °C for 24 h. All the samples exhibited the T_c value around 91 K. Positive S(T) value was obtained in all the samples. S(T) data was analyzed by “Two band model with linear T-term”. In thermal conductivity of the samples, small peak with broad maximum was found just below T_c, unlike that of the HT_c cuprates.