C.T. Chang

Synthesis and Characterization of High-Tc Superconducting Oxides by the Modified Citrate Gel Process

High-Tc superconducting Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and Tl-Ca-Ba-Cu-O materials were synthesized using a modified citrate precursor process. Ethylenediamine was used to neutralize the metal citrate solution and facilitate the complex formation. Highly homogeneous and viscous gel without any precipitation was obtained. In the Tl-Ca-Ba-Cu-O system, the Ca-Ba-Cu-O precursor was first prepared by the sol-gel method, then mixed with thallium oxide for final sintering. The complex formation was monitored by UV/VIS spectrophotometry. The thermal decomposition of gels and the formation of superconducting phases had been studied using IR, DTA, TGA, and XRD. This process is highly reproducible and leads to powders with excellent homogeneity and small particle size for easy sinterability.

Synthesis and characterization of high-Tc Y-Ba-Cu-O superconducting oxides by coprecipitation from triethylamine-oxalate media

Abstract A modification of the oxalate coprecipitation process for preparing high-Tc superconducting oxides is reported. The superconducting Y-Ba-Cu-O powders with Tc up to 95.5 K have been prepared from an aqueous solution which contains Y, Ba and Cu salts via homogeneous coprecipitation using triethylamine and oxalic acid solution. The dried precipitate was found to have a specific surface area of 38.4 m2/g. Characterizations of the YBa2Cu3Oy powder by X-ray diffraction (XRD), thermal analysis, scanning electron microscope (SEM), electron microprobe analysis (EMPA), electrical resistivity and magnetic susceptibility measurements are reported. This process might be employed for large quantity preparation of the YBa2Cu3Oy system.

The synthesis, by triethylammoniumoxalate coprecipitation, and superconducting properties of Y(Ba1−xSrx)2Cu4O8

Superconductivity has been investigated in the system Y(Ba{sub 1-x}Sr{sub x}){sub 2}Cu{sub 4}O{sub 8}(O{le}x{le}0.5). The samples were prepared by triethylammoniaoxalate coprecipitation at ambient oxygen pressure. The superconducting transition temperature, T{sub c}, increased from a value of 85 K (x=0) to 90 K (x=0.1-0.2) and then decreased to 64 K at x=0.5. For compositions beyond the solubility limit (x=0.3), the magnitude of dT{sub c}/dx was high, ca. 45 K from 0.3 to 0.5. The maximum T{sub c}, 90 K, recorded from samples in the range x=0.1{approximately}0.2, were confirmed by magnetization measurements. These results are in contrast to those obtained from the Y(Ba{sub 1-x}Sr{sub x}){sub 2}Cu{sub 3}O{sub 7} system, in which T{sub c} decreased monotonically from 93 to 82 K for 0{le}x{le}0.5.

Improvement of CMOS-MEMS accelerometer using the symmetric layers stacking design

This study presents a novel CMOS-MEMS capacitive type accelerometer design which consists of symmetric layers (4 metal and 3 dielectric layers) stacking to reduce the bending of suspended structures due to thin film residual stresses. Thus, the capacitance loss caused by the mismatch of sensing electrodes is reduced. Moreover, structures with symmetric layers stacking have less thermal deformation by temperature variation. A simple post-CMOS process including oxide wet-etching and dry XeF2 etching is established to fabricate the device. Measurement shows maximum bending deformation of a suspended 390µm×430µm structure is only 1µm, and mismatch of fixed and movable sensing electrodes is reduced to 1µm. The bending curvature has only ∼2% change as temperature increased 80°C. The sensitivity of this accelerometer is 1.46mV/G (in comparison, the accelerometer with asymmetric layers stacking structure has sensitivity of 0.07mV/G), and the noise level is 0.35mG/√Hz.

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.

Coprecipitation process for the preparation of superconductive BiSrCaCu oxides

Abstract BiSrCaCu superconducting oxide powders were prepared from an aqueous solution containing Bi, Sr, Ca, and Cu salts and oxalic acid via coprecipitation using triethyl amine as a base. The dried precipitate was found to have a satisfactory stoichiometry and a particle size of 0.3 μm. Subsequent calcination at 800°C for 10 h and sintering at 870°C for 12 h yielded a superconducting phase with T c above 80 K. This process can be employed for large-scale preparation of BiSrCaCuO superconducting

Synthesis under ambient pressure of 80 K superconducting YBa2Cu4O8 powders via coprecipitation

Sub‐micron homogeneous powders of YBa2Cu4O8 having Tc∼80 K were synthesised from aqueous solutions containing Y, Ba, Cu salts, and oxalic acid as a precipitant via a chemical coprecipitation method using triethylamine as a base. An accelerated formation−within 24 h−of nearly single phase YBa2Cu4O8 powders was achieved. The intragrain critical current density at 5 K and H=1 T was determined to be 3.9×107 A/cm2. This process may have considerable potential in the large‐scale preparation of YBa2Cu4O8.

Synthesis, transport, magnetization and structural characterizations of Tl-Ca-Ba-Cu-O specimens with T0=123 K and Tonset=155 K

Abstract A stable, reproducible and bulk superconductor of Tl-Ca-Ba-Cu-O (TCBCO) compound with T 0 above 120 K and T onset =155 K has been prepared. Measurements of electrical and magnetic properties are reported. On the basis of the transmission electron microscopy and X-ray diffraction analyses, the predominant phase was determined to be a body-centered tetragonal structure ( a = b =0.557 nm, c =3.77 nm) with a space group of I4/mcm. The composition ratios of Tl, Ca, Ba and Cu of the phase were determined by energy dispersive spectrometry to be 1:2:3:4. A probable structure model involving four layers of Cu-O interposed between Tl-O layers is proposed.

Mechanical force-displacement transduction structure for performance enhancement of CMOS-MEMS pressure sensor

This study implements a mechanical force-displacement transduction structure in Fig.1a using the TSMC 0.18μm 1P6M CMOS process to improve CMOS-MEMS capacitive pressure sensor. The membrane will be deformed by pressure and cause the sensing-gap change between undeformed movable-electrode and fixed-electrode. Feature of this study is CMOS-MEMS deformed membrane and undeformed movable-electrode to enable the parallel-plate gap-closing pressure detection. Thus, the performance of pressure sensor can be improved. In comparison, the design with mechanical force-displacement transduction structure will increase sensitivity for 61% in pressure range (100kPa-60kPa). Moreover, sensitivity (non-linearity) of proposed design changes from 2.1fF/kPa (1.7%) to 1.9fF/kPa (2.5%) as pressure range changed from 100~60kPa to 100~20kPa. However, sensitivity (non-linearity) of existing design significantly drops from 1.3fF/kPa (1.5%) to 1.0fF/kPa (13.7%) as pressure range changed.

A sol-gel route to prepare TlCaBaCuO superconductor with Tc above 120 K

Abstract The preparation of Tl Ca Ba Cu O superconductor with Tc above 120 K from Ca Ba Cu O precursor mixed with thallium oxide was obtained. The Ca Ba Cu O precursor was prepared by sol-gel method from the proper complexation and polymerzation of the metal nitrates by citric acid and ethylene diamine. The excellent homogeneity and small particle size make the sol-gel precursor a potential candidate for Tl Ca Ba Cu O film preparation by screen-printing or spin-coating and thallium vapor diffusion.