F. H. Chen

National Chiao Tung University, Hsinchu, Taiwan, Taiwan

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Publications (10)18.57 Total impact

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    ABSTRACT: The field-cooled magnetization of high-T c superconducting ceramics measured in low magnetic fields exhibits the paramagnetic Meissner effect (PME), i.e., the diamagnetic signal initially increases with decrease in temperature but reaches a maximum at temperatureT d and later decreases with decrease in temperature. Even in some samples the signal is ultimately able to transform inversely into a paramagnetic regime once the sample is cooled below a temperatureT p as long as the applied field is sufficiently small. This PME has been observed in various high-T c cuprates and is explained by disparate aspects. An anisotropic model, in which the granular superconductors are assumed to be ideally anisotropic, was first alternatively proposed in the present work so as to theoretically account for this effect. On the other hand, an isotropic model, suitable for granular superconductors with randomly oriented grains, was proposed to deal with the samples prepared by a conventional solid-state reaction method. The anomalous magnetization behavior in the present model was demonstrated to be the superposition of the diamagnetic signal, which occurs as a result of the intragranular shielding currents, over the paramagnetic one due to the induction of the intergranular component induced by these currents where the intergranular one behaved as the efective pinning centers. The PME was demonstrated by this model to exist parasitically in granular superconductors. This intergranular effect is therefore worthy of remark when evaluating the volume fraction of superconductivity for the samples from the Meissner signal, in particular, at a low magnetic field.
    Journal of Superconductivity 01/1995; 8(1):43-56.
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    ABSTRACT: High T c Tl–Ba–Ca–Cu–O superconducting thin films have been made by the dc magnetron sputtering followed by postannealing. In this study, a Tl‐based microstrip ring resonator was fabricated by using standard microelectronic photolithographic patterning and wet chemical etching. Experimental results indicated that these ring resonators exhibit the best unloaded Q of 3269 at its resonant frequency of 3.35 GHz and some other samples display the double resonant behavior at 77 K. The frequency difference of the double resonant peaks depends on the input microwave power, and the splitting is ascribed to spatial variations in the film microstructure. The physical properties of the films are about 1 μm thick, as high as 105 K transition temperature, and greater than 10<sup>5</sup> A/cm<sup>2</sup> current density at zero magnetic field and 77 K. © 1994 American Institute of Physics.
    Applied Physics Letters 01/1995; · 3.52 Impact Factor
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    ABSTRACT: High- T c YBa2Cu4O8 (124) thin films have been made by d.c. magnetron sputtering deposition on (100) MgO substrates. The effect of several processing variables, including the ratio of oxygen to argon, total pressure, and substrate temperature, on the superconducting properties of the thin films, were systematically investigated. The as-prepared films annealed in flowing oxygen at 800C for 4 h under ambient pressure obtained nearly phase-pure 124 and exhibited superconducting onset transition at 75 K.
    Journal of Materials Science 12/1993; 29(21):5593-5598. · 2.31 Impact Factor
  • FH Chen, MF Tai, WC Horng, TY Tseng
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    ABSTRACT: The field-cooled magnetization of high-[ital T][sub [ital c]] superconducting ceramics measured in low magnetic fields has exhibited an unusual phenomenon, i.e., the diamagnetic signal initially increases with a decrease in temperature but reaches a maximum at temperature [ital T][sub [ital d]] and subsequently decreases with a decrease in temperature. In some samples the signal could even transform inversely into a paramagnetic regime once the sample was cooled below a temperature [ital T][sub [ital p]], provided the applied field was sufficiently small. This phenomenon has been observed in various high-[ital T][sub [ital c]] cuprates and been explained with different viewpoints. An alternative theoretical model was proposed in this work to account for these phenomena. The anomalous magnetization behavior in the present model was demonstrated to be a superposition of the diamagnetic signal, which occurs as a result of the intragranular shielding currents, and the paramagnetic signal, due to the induction of the intergranular component induced by these currents. The intergranular effect was demonstrated by this model to exist in granular superconductors. This intergranular effect would therefore be considered in evaluating the volume fraction of superconductivity for the samples from the Meissner signal, in particular, at a low magnetic field.
    Physical review. B, Condensed matter 08/1993; 48(2):1258-1265. · 3.77 Impact Factor
  • F.H. Chen, H.S. Koo, T.Y. Tseng
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    ABSTRACT: In this paper the modified sol-gel method using ethylenediaminetetraacetic acid as a chelating agent to prepare high-T{sub c} superconducting Bi-Pb-Sr-Ca-Cu oxides is reported. Results revealed that the desirable Bi-based compounds can be acquired by such a process with a high percentage of the 110 K phase and a T{sub c} (zero) of 108 K. On the other hand, a new method is proposed to quantitatively evaluate the volume fraction of the 110 K phase in Bi-based superconducting materials, mainly composed of 85 and 110 K phases. That is a lever-rule type of relation, V = (Y {minus} 29.07{degrees})/ (28.81{degrees} {minus} 29.07{degrees}), where V is the volume fraction of the 110 K phase and Y = 20 for the X-ray diffraction peak emerging at about 29{degrees} with a CuK {alpha} radiation source and 28.81{degrees} {le} Y {le} 29.07{degrees}. The volume fraction of the 2223 phase estimated in this way is more reliable than that estimated by measuring the area ratios of XRD peaks and it is expected to be even more reliable than those derived from conventional methods based on perfect diamagnetism.
    Journal of the American Ceramic Society 01/1992; · 2.43 Impact Factor
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    ABSTRACT: Pb-doped Bi-Sr-Ca-Cu high-Tc superconducting oxides have been prepared by a modified sol-gel process via the EDTA route. Upon decomposing the X-ray photoemission spectrum of the Cu 2p3/2 core level of the superconducting oxide into three components Cu1+, Cu2+ and Cu3+, we have found an obvious trend: the corresponding [Cu1+]/[Cu2+] ratio increased as more 110 K phase was formed in the sample. According to this trend, a defect model was proposed; it can reasonably explain some previous experimental results, such as a reduced oxygen atmosphere sintering, which effectively facilitates the formation of the 110 K phase, and the coexistence of copper and oxygen vacancies in Bi-system superconductors, observed by high-resolution transmission electron microscopy. On the other hand, the dimerization of holes in oxygen sites was also suggested to be a possible mechanism responsible for the increase in the ratio of [Cu1+]/[Cu2+].
    Physica C Superconductivity 01/1992; 197:151-156. · 0.72 Impact Factor
  • F. H. Chen, H. S. Koo, T. Y. Tseng
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    ABSTRACT: The growth of the Bi 2 Sr 2 Ca 2 Cu 3 O x phase in the high T c superconducting Bi‐Pb‐Sr‐Ca‐Cu‐O system through reacting Bi 2 Sr 2 CaCu 2 O x with Ca‐ or Cu‐rich intermediate phases, Ca 2 CuO 3 , CuO, and Ca 2 PbO 4 , has been studied. X‐ray diffraction and electrical and diamagnetic analyses revealed that the interaction and interdiffusion of these intermediate phase led to the formation of a high percentage of the 2223 phase and a sharp single‐step superconducting transition at T c (R = 0) = 108 K. Also, Ca 2 PbO 4 proved essential for accelerating the growth rate of the 110 K phase from the 85 K phase.
    Applied Physics Letters 03/1991; · 3.52 Impact Factor
  • Journal of Materials Science Letters 01/1991; 10(1):58-60.
  • F. H. Chen, H. S. Koo, T. Y. Tseng
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    ABSTRACT: The oxalate precursor yielded by a sol-gel processing can be pyrolysed and oxidized at 840 C for 40h into the desired high-T c superconductor in the lead-doped Bi-Sr-Ca-Cu-O system withT c(onset) = 140 K andT c(zero) = 104K. The mechanisms of the sol-gel reaction were monitored by analysis of Fourier transformation infrared (FTIR) spectroscopy. Moreover, identification of the various phases existing on the specimens after firing at selected temperatures, was made by analysis by FTIR spectroscopy and X-ray diffractometry. Based on the experiment results, the mechanisms of forming Bi2Sr2CaCu2O8+ (phase 2212) might be identified as the reaction of Bi2Sr2CuO6+, Ca2CuO3 and CuO at 800 C. Furthermore, Bi2Sr2Ca2Cu3O10+ (phase 2223) might be synthesized by the reaction of phase 2212, Ca2CuO3 and CuO at 840 C. Ca2PbO4 found in lead-doped materials may catalyse the formation reaction of Ca2CuO3 at 650 C and play a role of a flux to fuse Bi2Sr2CaCu2O8+, CuO and residual Ca2CuO3 and then improve the further synthesis of Bi2Sr2Ca2Cu3O10+ at 840 C.
    Journal of Materials Science 06/1990; 25(7):3338-3346. · 2.31 Impact Factor
  • Journal of Materials Science Letters 01/1990; 9(7):807-809.