The crystallization mechanism of superconducting phases in the (Bi,Pb)−Sr−Ca−Cu−O system was determined on the basis of the
results of DTA, DTG and TG studies, supplemented by X-ray examination of ceramic powders obtained by the sol-gel method.
It has been demonstrated that the factor determining the formation of superconducting phases: Bi2Sr2CaCu2Ox (low-Tc) and Bi2Sr2Ca2Cu3Ox (high-Tc) is the kinetics of reaction of calcium and strontium carbonates with molten CuBi2O4. As a result of the reaction of the bimetallic compound CuBi2O4 with SrCO3 in the liquid phase the compound Bi2Sr2CuO6 is formed. This compound, reacting with calcium and copper oxides, yields superconducting phases: the low-Tc and the high-Tc phase. It has been also observed that an increase in the volume fraction of high-Tc phase in powder subjected to thermal treatment takes place probably due to the repeated disproportionation of low-Tc phase and its repeated synthesis from Bi2Sr2CuO6, CuO and CaO.
Differential thermal analysis shows many interesting properties that allow for prompt finding the place (p,T) and characteristics (endothermic, exothermic) of a phase transition or chemical reaction. In application to reactive sintering
under active gas atmosphere at high pressure despite of numerous technical problems encountered during DTA measurements we
found its unique possibilities. That allows for quantitative estimation of nitrogen involved in reaction of phase transition
from the hexagonal phase to the cubic phase of MoN, and vice versa, in high gas pressure condition. DTA high gas pressure
measurement system has the maximum operate temperature 2000Cat pressure 2 GPa.
Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a
view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in
the Y−Ba−Cu−O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the
decomposition kinetics of BaCO3 determines the formation rate of the superconducting YBa2Cu3O7−x (‘123’) phase. It follows from the decomposition kinetics of BaCO3 that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition
analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned
changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted
at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples
with advantageous functional parameters.
Kinetics of oxidation of Fe-Cr steel containing 25 wt.-percent Cr was studied as a function of temperature (1023–1173 K) for
up to 480 h in flowing air, which corresponds to SOFC cathode environment operating conditions. The oxidation process was
found to be a parabolic, suggesting that the diffusion of ionic defects in the scale is the slowest, rate determining step
and it occurs predominantly by short-circuit diffusion paths. Comparison of the determined activation energy of oxidation
of the studied steel with literature data indicates that at 1098–1173 K the chromia scale grows by the outward solid-state
diffusion of chromium interstitials, whereas at 1023–1098 K — through a significant contribution of counter-current oxygen/chromium
diffusion along Cr2O3 grain boundaries. The oxide scales were composed mainly of Cr2O3 with a continuous thin Mn1.5Cr1.5O4 spinel layer on top of the chromia scale. The oxidation test results on Fe-25Cr steel demonstrate the applicability of the
commercial type DIN 50049 stainless steel as interconnect for SOFC.
Authors:T. Brylewski, T. Maruyama, M. Nanko, and K. Przybylski
The high-temperature oxidation behavior of a ferritic alloy (SUS 430) in a SOFC environment, corresponding to the anode (H2/H2O gas mixture) and cathode (air) operating conditions, was determined with regard to application of the alloy as a metallic
separator material in SOFC. The oxidation kinetics of Fe-16Cr alloy (SUS 430), was studied by thermogravimetry in H2/H2O gas mixtures with pH/pHO=94/6 and 97/3 and in air, in the temperature range 1023-1223 K, for 3.6 up to 1080 ks. It was found that the protective oxide
scale, composed mainly of Cr2O3 with uniform thickness and excellent adhesion to the metal substrate, grows in accordance with the parabolic rate law. The
dependence of the parabolic rate constant, kp, of the scale on temperature obeys the Arrhenius equation: kp=6.810-4 exp (-202.3 kJ mol-1R-1T-1) for H2/H2O gas mixtures with pH/pHO=94/6. The determined kp was independent of the oxygen partial pressure in the range from 5.210-22 to 0.21 atm at 1073 K, which means that the rates of growth of the scale on Fe-16Cr alloy in the above-mentioned atmospheres
are comparable. The oxidation test results on Fe-16Cr alloy in H2/H2O gas mixtures and air demonstrate the applicability of SUS 430 alloys as a separator for SOFC.
Authors:K. Jerie, A. Baranowski, J. Gliński, and J. Przybylski
The results of positron annihilation experiments in aqueous solutions of 1,2- and 1,6-hexanediols were compared to those of ultrasonic velocity. For the former, the positron annihilation measurement shows strong hydrophobic interactions and formation of clathrate-like hydrates in solution, although their stoichiometries as well as architecture are untypical. Ultrasonic data for this system seem to be strongly influenced by a relaxation process and do not allow to conclude about formation of open-work structures in this system. For the system water + 1,6-hexanediol both the ultrasonic and annihilation methods suggest formation of labile, water-rich hydrates. They are very weak compared to those existing in the water + 1,2-hexanediol, what results from the hydrophilic hydration competing the hydrophobic one.
Authors:K. Jerie, A. Baranowski, J. Gliński, and J. Przybylski
Experimental results of positron annihilation experiments in solutions of n-butanol in three diols are compared to those of
ultrasonic velocity and absorption. Variations of the annihilation parameters with composition show that the effect of the
hydrophobic solute on the original diol structure is rather limited and occurs only in the system where ethylene glycol is
the solvent. Subtle similarities were found when compared it to the respective results for similar aqueous systems, suggesting
that crystalline-like solvates are forming in ethylene glycol. However, no such evidences were found for the systems where
butanediols are the solvents. Ultrasonic investigations do not confirm formation of any specific structures. This is caused,
most possibly, by the fact that compressibility of solvates is close to those of the pure components and/or that of unstructured
medium (regular solution).
Authors:T. Łada, K. Przybylski, A. Morawski, J. Prażuch, and T. Brylewski
The fabrication method of superconducting thin films of compositions HgBa2Ca2Cu3O8+δ (Hg-1223) and Tl2Ba2CuOy (2201) on single-crystalline SrTiO3 and LaAlO3 substrates is reported. The highest obtained Tc was 134 K and Jc over 106 A cm–2 at 77 K. High pressure DTA(HP-DTA) was applied to grow mercury- and thallium-based high-temperature superconducting crystals
and thin films, to identify melting points of particular phases within these oxide systems and determine suitable processing
conditions. The DTA system operates at the: maximum temperature of 1200C, volume up to 5 cm3, working pressure up to 1.5 GPa and at a working atmosphere — inert gas with up to 25% oxygen.
Authors:K. Przybylski, T. Brylewski, A. Morawski, and T. Łada
The influence of Ba2Ca2Cu3Ox precursor on the synthesis and properties of (Hg,Pb)Ba2Ca2Cu3O8+δ has been examined. Fine homogeneous Hg-free precursor powder of Ba2Ca2Cu3Ox of desirable phase composition was prepared by sol-gel method using EDTA acid as a complexing agent. A reproducible superconducting
sample of Hg0.8Pb0.2Ba2Ca2Cu3O8+δ with fine-grained, dense microstructure, composed predominantly of (Hg,Pb)-1223 phase and with advantageous magnetic properties,
was synthesized by high pressure crystallization in mercury environment of well-calcined Ba2Ca2Cu3Ox precursor.