J.S. Luo

Fullerenes as precursors for diamond film growth without hydrogen or oxygen additions

Diamond films are predominantly grown using approximately 1% of a hydrocarbon precursor in hydrogen gas. Hydrogen is generally believed to be necessary for the diamond thin‐film growth process. However, hydrogen in varying amounts is inevitably incorporated in the growing diamond lattice, leading to structural defects. We report here the successful growth of diamond films using fullerene precursors in an argon microwave plasma, a unique development achieved without the addition of hydrogen or oxygen. We speculate that collisional fragmentation of C60 to give C2 could be responsible for the high growth rate of the very‐fine‐grained diamond films.

Deposition and characterization of nanocrystalline diamond films

Highly uniform, smooth nanocrystalline diamond films have been fabricated with a magnetoactive microwave chemical vapor deposition system. The top and bottom magnet currents were 145 and 60 A, respectively, while the microwave power and substrate temperature were controlled at 1500 W and 850 °C, respectively during deposition. The total processing pressure was regulated at 40 Pa (300 mTorr) with gas‐flow rates of 30 sccm of hydrogen, 2.4 sccm of methane, and 1 sccm of oxygen. Diamond films obtained under these conditions have grain sizes between 0.1 and 0.3 μm, and a mean roughness of 14.95 nm. The growth rate is 0.1 μm/h. Characterization techniques have involved x‐ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Both x‐ray and electron diffraction patterns show no evidence of graphitic phase. Although a high density of twins and stacking faults was revealed by high‐resolution electron microscopy, compact diamond grains, and...

Kinetics and mechanism of the (B1,Pb)2Sr2Ca2Cu3O10 formation reaction in silver-sheathed pires

Abstract A detailed kinetic and mechanistic analysis of the growth of the (Bi 2− x Pb x )Sr 2 Ca 2 Cu 3 O 10 phase in silver-sheathed wires has been performed by an isothermal equilibration method. Silver tubes loaded with precursor powders were processed into wires using established metallurgical techniques. The wire specimens were immersed in a preheated equilibration apparatus, heat-treated at the desired temperature in 7.5% O 2 , for varying periods of time, then quenched in a room-temperature silicone oil bath. The results indicated that the kinetic data followed a nucleation growth mode1 derived for a reaction at the interface between thin sheets and a fine powder or a fluid. Transmission electron microscopy confirmed the two-dimensional reaction geometry and revealed the presence of an amorphous phase at grain boundaries, where rapid transport diffusion appears to occur due to the absence of the stabilizing influence of the regular lattice. A reduction in activation energy was observed at temperatures 2819°C which is tentatively attributed to the onset of a liquid-phase-controlled reaction (i.e. a phase boundary crossing). The effects of powder processing parameters and precursor particle size on the kinetic behavior and the growth rate of the (Bi 2− Pb x )Sr 2 Ca 2 Cu 3 O 10 phase are also discussed.

Influence of silver cladding on the formation and alignment of the (Bi2−xPbx)Sr2Ca2Cu3O10+δ phase

Cross‐section transmission electron microscopy was used to investigate the interaction of silver with (Bi,Pb)‐Sr‐Ca‐Cu‐O phases during the formation of (Bi2−xPbx)Sr2Ca2Cu3O10+δ (Bi‐2223) in a silver‐sheathed wire containing a powder composed of (Bi,Pb)2Sr2CaCu2O8+δ (Bi‐2212) plus second phases. Observations of the interfacial regions of samples quenched at different stages of conversion revealed that (1) Bi‐2212 is initially in direct contact with silver, with the (001) planes parallel to the interface; (2) an amorphous layer between Bi‐2212 and silver appears during the induction period that precedes the conversion reaction; and (3) Bi‐2223 is detected at the silver/powder interface hundreds of minutes before it begins to appear in regions of the powder away from the interface. The implications of these results are presented and discussed.

Magneto‐optical imaging of flux patterns in multifilamentary (BiPb)2Sr2Ca2Cu3Ox composite conductors

We present a study of the superconducting morphology of the transport current carrying cross section of a 19‐filament (BiPb)2Ca2Cu3Ox (Bi‐2223) composite conductor using magneto‐optical imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this technique allows a unique correlation of superconducting and microstructural properties. Direct evidence for enhanced superconducting properties in platelike regions along the silver/Bi‐2223 interface and for weak properties near the core of the filaments is obtained. Misaligned grain colonies are found to cause an interruption of the superconducting continuity in the filaments.

Phase chemistry and microstructure evolution in silver-clad (Bi/sub 2-x/Pb/sub x/)Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ wires

The reaction kinetics and mechanism that control the conversion of (Bi,Pb)/sub 2/Sr/sub 2/CaCu/sub 2/O/sub z/ (Bi-2212)+alkaline earth cuprates to (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (Bi-2223) in silver-clad wires were investigated as a function of equilibration temperature and time at a fixed oxygen partial pressure (7.5% O/sub 2/). Measured values for the fractional conversion of Bi-2223 versus time were evaluated based on the Avrami equation. SEM studies of partially and fully converted wires suggest: (1) the transformation to Bi-2223 is two-dimensional and controlled by a diffusion process; (2) liquid phases are present during part of the Bi-2212 to Bi-2223 conversion; and (3) growth of the (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ phase accompanies Bi-2223 formation.<<ETX>>

Effects of oxygen pressure on phase evolution and microstructural development in silver-clad (Bi,Pb)2Sr2Ca2Cu3O10+δ composite conductors

Abstract The kinetics of phase evolution and the development of microstructure in silver-clad composite conductors containing a Bi-Pb-Sr-Ca-Cu-O powder with a nominal stoichiometry of Bi 1.7 Pb 0.4 Sr 2 Ca 2 -Cu 3 O 10+δ were studied as a function of oxygen partial pressure ( pO 2 ), temperature, powder stoichiometry, and powder pre-treatment. The optimum temperature for growth kinetics of the Bi-2223 phase increased as the oxygen partial pressure was increased from 0.02 to 0.21 atm. The width of the phase stability region was largest for pO 2 = 0.075 atm and smallest for pO 2 = 0.21 atm. It was found that for a 25-h heat treatment, the reaction kinetics were the fastest in 0.075 atm of oxygen at temperatures between 825° and 830°C. The secondary phase particles (observed by scanning electron microscopy) were smaller for lower pO 2 (0.02 and 0.075 atm) than for higher pO 2 (0.13 and 0.21 atm).

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped Bi-2223 and Ag/Bi-2223 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made in the lead-doped Bi-2223 superconducting system in the temperature range 700–815°C by means of an oxygen titration technique that employs an yttria-stabilized zirconia electrolyte. The results of our studies indicate that processing or annealing lead-doped Bi-2223 at temperatures ranging from 750 to 815°C and at oxygen partial pressures ranging from ∼ 0.02 to 0.2 atm should preserve Bi-2223 as essentially single-phase material. Thermodynamic assessments of the partial molar quantities Δ S ( O 2 ) and Δ H ( O 2 ) indicate that the plateau regions in the plot of oxygen partial pressure versus oxygen stoichiometry (x) can be represented by the diphasic CuOCu2O system. In accord with the EMF measurements, it was found that lead-doped Bi-2223 in a silver sheath is stable at 815°C for oxygen partial pressures between 0.02 and 0.13 atm.

Stability and growth of the (Bi,Pb)2Sr2Ca2Cu3Ox phase in a silver sheath

Abstract The chemical stability and growth kinetics of the (Bi,Pb)2Sr2Ca2Cu3Ox (2223) phase have been investigated as a function of temperature and composition using x-ray diffraction, scanning electron microscopy and wavelength dispersive x-ray spectroscopy. Silver tubes were filled with precursor 2223 powders and processed into tapes using conventional deformation techniques. The tapes were processed at temperatures ranging from 810 to 830°C for times ranging from 10 to 12,000 minutes in an oxygen partial pressure of 0.075 atm. The 2223 phase shows limited non-stoichiometry of Cu, (Ca+Sr), and (Bi+Pb), but relatively broad solid solubility within the alkaline earths and heavy metals. The kinetics of transformation from Bi2Sr2Ca1Cu2Ox to the 2223 phase appear to be controlled by a two dimensional diffusion process.

Microscopic effects of self-radiation damage in 244 Cm-doped LuPO 4 crystals

Microscopic effects of self-radiation damage in {sup 244}Cm-doped LuPO{sub 4} crystals were examined with transmission electron microscopy. These LuPO{sub 4} crystals had been doped with 1 wt% {sup 244}Cm and exposed to a radiation dose as high as 5 x {approx} 10{sup 16} f-decay events/mg over 18 years. The microscopic analysis revealed dense arrays of individual defect clusters and numerous bubbles. Whereas, the defect clusters may be interpreted as residuals of alpha-recoil tracks, the bubbles likely resulted from the f-particles generated during the decay events. The bubbles were found to coalesce under electron beam irradiation. Despite the high accumulated dose over the 18 years, the samples exhibited sharp diffraction patterns and periodic lattice spacings. This finding indicated that the samples remained largely crystalline and that the radiation-induced lattice damage was recovered at a rate comparable to that of damage production. This high recoverability is discussed with respect of various annealing processes that may have occurred in the samples.