180 research outputs found
The role of stoichiometric vacancy periodicity in pressure-induced amorphization of the Ga2SeTe2 semiconductor alloy
We observe that pressure-induced amorphization of Ga2SeTe2 (a III-VI
semiconductor) is directly influenced by the periodicity of its intrinsic
defect structures. Specimens with periodic and semi-periodic two-dimensional
vacancy structures become amorphous around 10-11 GPa in contrast to those with
aperiodic structures, which amorphize around 7-8 GPa. The result is a notable
instance of altering material phase-change properties via rearrangement of
stoichiometric vacancies as opposed to adjusting their concentrations. Based on
our experimental findings, we posit that periodic two-dimensional vacancy
structures in Ga2SeTe2 provide an energetically preferred crystal lattice that
is less prone to collapse under applied pressure. This is corroborated through
first-principles electronic structure calculations, which demonstrate that the
energy stability of III-VI structures under hydrostatic pressure is highly
dependent on the configuration of intrinsic vacancies
Comparative long-term evaluation of tacrolimus and cyclosporine in pediatric liver transplantation
Background. In this report, we compare the long-term outcome of pediatric liver transplantation (LTx) patients maintained with tacrolimus-based and with cyclosporine (CsA)-based immunosuppressive therapy. We examine long-term patient and graft survival, the incidence of rejection, and immunosuppression-related complications. Method. There were 233 consecutive primary LTx in children (ages <18 years) performed between October 1989 and December 1994 with tacrolimus-based immunosuppressive therapy (Group I). These were compared with 120 consecutive primary LTx performed with CsA-based immunosuppressive therapy between January 1988 and October 1989(Group II). Children in both groups were followed until July 1999. Mean follow-up was 91.41±17.7 months (range 55.6-117.8) for Group I, and 128±6.1 months (range 116.7-138.6) for Group II. Results. At 9 years of follow-up, actuarial patient and graft survival were significantly improved (patient survival 85.4% in Group I vs. 63.8% in Group II, P=0.0001; graft survival Group I 78.9% vs. 60.8% Group II, P=0.0003) and the rate of re -transplantation was significantly lower among patients in Group I (12% in Group I vs. 22.5% in Group II P=0.01). Children in Group I also experienced a significantly reduced incidence of acute rejection (0.97 per patient Group I vs. 1.5 per patient Group II P=0.002) and significantly less steroid resistant acute rejection episodes (3.1% in Group I vs. 8.6% in Group II P=0.0001). The mean steroid dose was significantly lower in Group I compared with Group II at all time points (P=0.0001) after LTx. Freedom from steroid was also significantly higher in Group I compared with Group H at all time points after LTx (ranging from 78% to 84% in Group I and 9% to 32% in Group H during a 1- to 7-year posttransplant period P=0.0001). The rate of hypertension was significantly lower in Group I than Group II (P=0.0001), and the severity of hypertension (need for more than one anti-hypertensive medication) was also significantly lower in Group I than Group II (P=0.0001). Although the rate of posttransplant lymphoproliferative disorder (PTLD) was not significantly different (13.7% Group I vs.8.3% Group II, P=0.13), the survival after PTLD was significantly better for Group I at 81.2% than for Group II at 50% after 5 years (P=0.034). Conclusion. The results suggest that tacrolimus-based therapy provides significant long-term benefit to pediatric LTx patients, evidenced by significantly improved patient and graft survival, reduced rate of rejection, and hypertension with lower steroid doses
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Preparation of BiCaSrCuO Specimens for High-Resolution Transmission Electron Microscopy
Simulation of Multicomponent Thin Film Deposition and Growth
Results from a multicomponent Monte Carlo simulation of the deposition and growth of YBa{sub 2}Cu{sub 3}0{sub 7} are presented and discussed. In particular, a detailed examination of the growth modes active during different morphological growth conditions is performed. At higher deposition rates, both (001) and (100) epitaxial variants (`c` and `a` type growth, respectively) are observed to grow by modes attributed to the classic Volmer-Weber mechanism. At very low deposition rates, the film is observed to grow in a distinct, cyclic, multi-stage process. Small islands of (0011) epitaxy nucleate and grow to one unit cell height followed by primarily horizontal growth or ``ledge extension`` until one unit cell layer has formed. This process then repeats. Simulated RHEED amplitude data from this growth process compares favorably to experimentally obtained data
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In-Situ Transmission Electron Microscopy and Computer Simulation Study of the Kinetics of Oxygen Loss in YBa2Cu3Oz
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Structural and interfacial characteristics of thin (<10 nm) SiO sub 2 films grown by electron cyclotron resonance plasma oxidation on (100) Si substrates
The feasibility of fabricating ultra-thin SiO{sub 2} films on the order of a few nanometer thickness has been demonstrated. SiO{sub 2} thin films of approximately 7 nm thickness have been produced by ion flux-controlled Electron Cyclotron Resonance plasma oxidation at low temperature on (100) Si substrates, in reproducible fashion. Electrical measurements of these films indicate that they have characteristics comparable to those of thermally grown oxides. The thickness of the films was determined by ellipsometry, and further confirmed by cross-sectional High-Resolution Transmission Electron Microscopy. Comparison between the ECR and the thermal oxide films shows that the ECR films are uniform and continuous over at least a few microns in lateral direction, similar to the thermal oxide films grown at comparable thickness. In addition, HRTEM images reveal a thin (1--1.5 nm) crystalline interfacial layer between the ECR film and the (100) substrate. Thinner oxide films of approximately 5 nm thickness have also been attempted, but so far have resulted in nonuniform coverage. Reproducibility at this thickness is difficult to achieve
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HIGH RESOLUTION ELECTRON IMAGING: AN APPROACH TO THE ATOMIC CHARACTERIZATION OF MATERIALS
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TRANSMISSION ELECTRON MICROSCOPY STUDIES OF GRAIN BOUNDARY PRECIPITATION IN SPINODALLY DECOMPOSED CU-NI-FE ALLOYS
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