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Special Seminar

Al-Cu Approximates in the Al3Cu4 Alloy

by: Prof. Chuang Dong

Date: Thursday September 03, 1998

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In the binary alloy systems constituting the ternary Al-Cu-TM (transition metal) quasicrystalline systems, many metastable quasicrystals have been found in Al-rich Al-TM systems, but little has been done in the Al-Cu system. The latter system has been addressed only recently because the only Al-rich Al-Cu phase is the non-approximate Al2Cu phase. The present talk will report on an investigation of the Al3Cu4 alloy, with an e/a ratio of 1.86, close to ternary Al-Cu-TM quasicrystals, in the search for Al-Cu approximates. Principal results include: 1) two new phases, a face-centered orthorhombic oF-Al43.2Cu56.8 (a = 0.8166, b = 1.4149, c = 0.9995 nm) and a body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a = 0.4083, b = 0.7074, c = 0.9995 nm), were found; 2) the twinning modes of these phases suggest that they are the decomposition products of a high-temperature parent phase e2-Al2Cu3, the atomic structure of which contains pentagonal atomic arrangements; and 3) the chemical-twinning mode of their basic structure B2 is 180[deg]/<111>, which is responsible for the pentagonal atomic configuration in the Al-Cu approximates as well as for the pseudo 5-fold B2 twinning.

Some preliminary friction tests on such B2-based approximates show that these phases have friction behaviors similar to those of quasicrystals.

Special Seminar

Normal and Superconducting Properties of Fermi System with Non-Degenerate Component: A Model for High-[Tc] Cuprates

by: Dr. George A. Levin

Date: Wednesday August 19, 1998

Time: 10:30 am – 11:30 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Many of the anomalies of the normal and superconducting state properties of the cuprates and their evolution with doping, from underdoped to optimally to overdoped regime, can be understood within the framework of a two-component model, one of which is conventional degenerate and the other, non-degenerate. The non-degenerate component appears when the Fermi surface of a conventional band is brought by tuning the stoichiometry in the vicinity of the top of another, submerged below the Fermi level, quasi-two-dimensional band. In the underdoped regime the excitations in the submerged band manifest themselves as a pseudogap, evident in the spin susceptibility, specific heat, tunneling conductances, resistivity, and Hall coefficient. The pseudogap vanishes with increasing doping, but the presence of the submerged band strongly affects the properties of the optimally and overdoped systems as well.

Special Seminar

Large Increases in Jc of Textured HTS via Radiation Damage

Dr. Roy  Weinstein

by: Dr. Roy Weinstein

Date: Friday July 31, 1998

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Jc is limited by motion of magnetic fluxoids. Pinning these fluxoids in place greatly increases Jc. Successful pinning centers have been produced by chemical, mechanical, and radiation techniques. The radiation methods which have been used include high Z ion beams, protons, neutrons, fusion of 6Li with neutrons followed by 7Li decay, and fission of U by neutrons (the U/n method).

I will address the U/n method and results to date. A detailed study of YBCO is reported, plus introductory studies of NdBCO, SmBCO, and BiSCCO. In YBCO increases in Jc by factors of 15-40 are obtained. In BiSCCO, increases in Jc by a factor of 20 are seen (in the high field regime), the irreversibility is approximately doubled, and the anisotropy is decreased by a factor of 8.

Special Seminar

Status and Perspectives of HTS Superconducting Electronics

by: Dr. Alex Braginski

Date: Thursday July 30, 1998

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The purpose of this talk is to give a realistic assessment of the status of high-temperature superconducting (HTS) electronics R&D and of the market impact of HTS's. Cryogenics, with issues of handling, reliability, and cost, remain the prime obstacle to acceptance by potential users. The advent of HTS's stimulated exaggerated and unrealistic hopes for quick and major developments, including market penetration. Indeed, technical developments have been most impressive and I will highlight the most pertinent of these. However, realistic time scales for complete development cycles (including industrially viable prototypes) can be longer than a decade. For any market, an attractive cost/benefit ratio must be convincingly demonstrated first. Only (1) HTS filter systems integrated with cryocoolers for cellular telecommunication and (2) HTS SQUID magnetometers have been developed to the point of potential or existing commercial availability. Even here, further R&D activity is required, e.g. for better product acceptance. In the case of (1), economic factors will define the market future and size within the next few years, with relatively firm prospects for satellite implementation and good possibilities for terrestrial base stations. In the case of (2), the development of new applications, especially in medical diagnostics, nondestructive materials evaluation, geophysical exploration, etc., might cause growing user acceptance. Prototypes of precision measuring HTS instruments (e.g. programmable voltage standards or D/A converters) might also emerge soon. LTS and HTS digital signal processors and switches might represent the biggest long-term future, e.g. in telecommunication, but only after semiconductor electronics hit their intrinsic physical barriers. Also, the VLSI/ULSI level must first be attained by superconducting digital (e.g. RSFQ) circuit technology. Within the next decade, all of the above and other electronic applications might conquer market niches, some of them sizable.

Special Seminar

Hydrogen Induced Cleaving of 6H-SiC and Si

by: Dr. O. W. Holland

Date: Tuesday June 02, 1998

Time: 2:00 pm – 3:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The hydrogen cleaving process can transfer thin layers from a bulk SiC wafer onto an alternate substrate. It consists of H implantation followed by wafer bonding and annealing [1]. The ability to synthesize thin films of SiC on inexpensive substrates will substantially reduce the cost of fabricating SiC-based integrated circuits.

Results describing both the physical and chemical behavior of H implanted into 6H-SiC, as well as a comparison with results in Si, will be presented. Physical characterization was done using RBS channeling, NRA, optical microscopy, and AFM, while IR was used to determine the atomic bonding of the implanted H. Hydrogen and residual damage profiles were determined over an extended range of annealing temperatures.

We found a dose of 3x1016 H/cm2 to be near the minimum dose for forming blisters, the initial indicators of the cleaving process. Blisters first appear after annealing at 850 [deg]C, where significant changes in the H profile are observed. The depth of the craters left after exfoliation is shown to match the range of the implanted hydrogen. While much of the physical characterization indicates a behavior similar to that in Si, scaled with the appropriate temperatures, IR measurements suggest something much different. Si-H bonding, considered to be critical in affecting the cleaving process in Si, is surprisingly not observed in SiC. The reasons for and implications of this observation will be discussed. These results will provide the underpinning for the SiC cleavage process, and its possible application to SiC technology.

[1] L. Di Cioccio et al., Mat. Sci. Eng. B46, 349 (1997).

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