C. H. Kao

Effect of temperature on the oxidation of Fe-7.5A1-o.65C alloy

An alloy with the chemical composition Fe-7.5A1-0.65C was employed to investigate the effect of temperature on oxidation between 600 and 900° C in dry air. Kinetic curves were determined by thermogravimetry analyses (TGA). Optical metallography and electron probe microanalysis (EPMA) were used to examine the oxide scales. At 600°C, the initial stage of oxidation followed a parabolic rate law, and oxidation subsequently, increased dramatically. Internal oxidation occurred beneath the nodule formed on the present alloy at 600°C. In contrast, no internal oxidation could be found in specimens of the Fe-7.5A1-0.65C alloy after oxidizing at 700, 800 and 900°C. The kinetic results have two distinct parabolic rates for the present alloy.

Preparation of high-Tc superconducting (Bi,Pb)2Sr2Ca2Cu3Oy phase via an oxalate route

Abstract (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y (2-2-2-3) (above 90%) has been prepared at a sintering temperature of 852°C within 10 h by sol-gel processing. Plate-like fine crystalline (Bi,Pb) 2 Sr 2 CaCu 2 O z (2-2-1-2) ( a − b plane diameter 1 μm, c direction thickness 0.1 μm) powder was used to prepare this 2-2-2-3 phase material. The 2-2-1-2 powder was prepared by a metal oxalate reaction using metal nitrate as metal sources, ethanol as solvent, and organic compounds as chelating agents to prepare the sol, and followed by a series of drying and calcination processes. The fine crystalline 2-2-1-2 powder was obtained at a temperature of 800°C for 10 h. T c (onset) of this 2-2-2-3 phase material was 105 K and T c (zero) was 92 K, which was obtained by pelleting the powder and measuring the resistivity by the four-probe method.

Effect of carbon on the oxidation of Fe-5.5 Al-0.55 C alloy

An alloy with the chemical composition Fe-5.5 wt% Al-0.55 wt% C is employed to investigate the effect of carbon on the oxidation at 600, 800 and 1000° C in dry air. Kinetic curves were determined by thermogravimetry analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales formed on the alloy surfaces. The kinetic curves observed at 600, 800 and 1000° C had simple, three-stage and two-stage parabolic rate laws, respectively. No carbide-free layer could be observed in the alloy which was oxidized at 600° C. In contrast, a carbide-free zone was found in specimens of the alloy after oxidizing at 800 and 1000°C.

Effects of sintering temperature on the superconducting and microstructural properties of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6OY/Ag2O composites

The effects of sintering temperature on the superconducting and microstructure properties of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6OY (BPSCCO)/Ag2O (0–50 wt%) superconductors were investigated. Based on the differential thermal analysis data, it was found that the addition of Ag2O to the BPSCCO system lowered the partial melting temperature (peritectic point), thereby promoting extra liquid formation in this system and affecting the stability of 2223 high-Tc phase of these composites. For example, the Tc (zero) of the BPSCCO/Ag2O (10 wt%) composite which was sintered at 843 °C in air was depressed by as much as 52 K. However, the addition of Ag2O (10–50 wt%) in the BPSCCO samples resulted in no significant effect on Tc when samples were treated under a lower sintering temperature (827 °C) in air. The correlations of superconducting properties with microstructures of these materials are discussed.

Effect of manganese on the oxidation of Fe-Mn-Al-C alloys

The effects of manganese on the oxidation of alloys with the chemical composition (wt%) Fe-5AAl-1.5Mn-0.58C and Fe-5.3Al-3.5Mn-0.53C at 600, 800 and 1000° C in dry air were investigated. Kinetic curves were determined by thermogravimetric analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales. The kinetic curves of Fe-5.4Al-1.5Mn-0.58C alloy oxidized at 600, 800 and 1000° C had simple, three- and two-stage parabolic rate laws, respectively. On the other hand, two stages of linear rate law were observed in Fe-53Al-3ZMn-0.53C alloy when oxidized at 600° C, while two distinct parabolic rate laws were found in the same alloy oxidized at 800 and 1000° C. Oxidation behaviours and the oxide formation mechanisms of the alloys at different temperatures are discussed in this paper.

Effect of Ag2O on the formation of high-Tc phase in the BPSCCO/Ag2O composites

Abstract The effects of Ag 2 O on the formation of the high-Tc BiPbSrCaCuO phase were investigated. The slopes for the formation of high-Tc phase are about 0.39 and 1.56 for BPSCCO and BPSCCO/Ag 2 O pellets, respectively. On the other hand, the results also indicated that the addition of Ag 2 O in the samples reduced the partial melting temperature and strongly enhanced the formation of high-Tc phase in a lower sintering temperature (831°C).

Oxidation and decarburization of an Fe-AI-C alloy

The oxidation behaviour of the Fe-5.5A1-0.55C (wt%) alloy was studied in air at 500 to 950° C. Specimens of two types of preparation were used: (i) homogenized for 4 h at 1100° C under an argon protective atmosphere to form carbide particles in the ferrite matrix (referred to Type 1 alloy); and (ii) homogenized for 96 h at 1100° C under an air atmosphere to form a ferrite matrix with full decarburization (Type 2 alloy). Type 1 alloy with or without a decarburized zone after oxidation had a higher oxidation rate than Type 2 alloy. The oxidation kinetics of Type 1 alloy are controlled by the stability of carbide particles. The interference of CO or CO2 formation influences vitally the formation of protective aluminium oxide. During the oxidation of Type 2 alloy, two oxidation periods appear with slightly different parabolic constant values. The oxidation rate increased with temperature to a maximum at 600° C and then decreased to a minimum at 800° C.

Rapid formation of the high-Tc phase in (Bi,Pb)-Sr-Ca-Cu-O superconductor via the sol-gel method

Bi-Pb-Sr-Ca-Cu-O powder was synthesized by the oxalate gel method. A sample with the composition of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy was used in this study. After pyrolysis of the gel precursor at 500 °C for 5 h, the resulting powder was calcined at 850 °C for another 5 h. The black powder was pressed into pellets and sintered at 852 °C for 5 h. The high-Tc phase was formed more easily in the sample with excess calcium and copper than in the theoretical composition. (Bi,Pb)2Sr2Ca2Cu3Oy (above 90%) was prepared as above within a relatively short time. Characterization of (Bi,Pb)2Sr2Ca2Cu3Oy superconductor by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, resistivity measurement and magnetic measurement, is reported.

A study of Bi-Pb-Sr-Ca-Cu-O/Ag tape prepared by the jelly-roll process

We combined the electrophoretic deposition technique and the jelly-roll process to fabricate Bi-Pb-Sr-Ca-Cu-O (BPSCCO)/Ag tapes. A thick film of BPSCCO on an Ag substrate was prepared by electrophoretic deposition. The BPSCCO/Ag sheet was rolled up to the jelly-roll configuration and then cold rolled to form tape with six superconducting layers. The critical current density of the tape determined by magnetic measurement at 5 K in a 0.2 T field was about 6.5*104 A cm-2 and decreased to 1.3*104 A cm-2 near 5 T.

Effects of starting powder with various high Tc phase contents on the expansion of BiPbSrCaCuO pellets

Abstract We investigate the effects of starting powder on the expansion of BiPbSrCaCuO pellets with various high Tc phase contents and sintered at 853 °C. The starting BiPbSrCaCuO powders, with various high Tc phase contents and prepared by the different calcined conditions, strongly influenced the expansion behaviours of the BiPbSrCaCuO pellets. The expansion of these pellets decreased with increasing high Tc contents. The relationship among the expansion behaviours, formation of high Tc phase and gas release from the pellet during sintering are discussed in this paper.