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44 records · Page 3

All sources NASA NTRS DOE OSTI

NASA NTRS1981

On the cause of the flat spot phenomenon observed in silicon solar cells at low temperatures and low intensities

The results of an effort to determine the mechanisms involved in the flat spot (FS) effect are given. It is suggested that the FS effect is due to a resistive metal-semiconductor-like (MSL) interface in parallel with the cell PN junction. Regions responsible for the FS effect lie under the front surface metallization in these cells, where the PN junction has been destroyed and replaced with a metal silicide-semiconductor interface. Such structural changes, which appear to be due to the thermally activated dissolution of the silicon, have been induced in cells as a result of isochronal heat treatments at temperatures between 450 C and 560 C. It has been found that a 650 A layer of Ta2O5 evaporated over the metallization is sufficient to prevent the underlying silicon from pitting during the subsequent heat treatment, although pitting at the metal silicon ambient interface could still be observed.

Weizer, V. G.↗

NASA NTRS1985

Oxidation Behavior of Glassy Alloys

The oxidation behavior of high temperature glassy alloys produced by rapid solidification processing is investigated and the effects of processing and composition on oxidation behavior is studied. Glassy Ta-44.5at%Ir, Ta-40at%Ir-10at%B and Nb-45at%Ir oxidized rapidly at 700 to 800 C at an oxygen partial pressure of .001 atm. The alloys were embrittled during the oxidation process. No apparent oxidation or embrittlement of the Ta-Ir alloy occurred after oxidation for 4h at 500 C at an oxygen partial pressure of .001 atm. Embrittlement occurred, however, after 100h of exposure under the latter conditions. Alloy embrittlement is associated with the partial or full conversion of the metallic glass to a mixture of crystalline beta-Ta2O5 and metallic iridium. Hot compaction of glassy alloys of this type must be limited to relatively low temperatures (approx. 500 C) and short times at the low temperatures unless extremely low oxygen partial pressures can be achieved during the compaction process.

Yurek, G.↗

NASA NTRS1987

A sputtering derived atomic oxygen source for studying fast atom reactions

A technique for the generation of fast atomic oxygen was developed. These atoms are created by ion beam sputtering from metal oxide surfaces. Mass resolved ion beams at energies up to 60 KeV are produced for this purpose using a 150 cm isotope separator. Studies have shown that particles sputtered with 40 KeV Ar(+) on Ta2O5 were dominantly neutral and exclusively atomic. The atomic oxygen also resided exclusively in its 3P ground state. The translational energy distribution for these atoms peaked at ca 7 eV (the metal-oxygen bond energy). Additional measurements on V2O5 yielded a bimodal distribution with the lower energy peak at ca 5 eV coinciding reasonably well with the metal-oxygen bond energy. The 7 eV source was used to investigate fast oxygen atom reactions with the 2-butene stereoisomers. Relative excitation functions for H-abstraction and pi-bond reaction were measured with trans-2-butene. The abstraction channel, although of minor relative importance at thermal energy, becomes comparable to the addition channel at 0.9 eV and dominates the high-energy regime. Structural effects on the specific channels were also found to be important at high energy.

Ferrieri, Richard A.↗

NASA NTRS1989

Metal-silicon reaction rates - The effects of capping

Evidence is presented showing that the presence of the commonly used anti-reflection coating material Ta2O5 on the free surface of contact metallization can either suppress or enhance, depending on the system, the interaction that takes place at elevated temperatures between the metallization and the underlying Si. The cap layer is shown to suppress both the generation and annihilation of vacancies at the free surface of the metal which are necessary to support metal-Si interactons. Evidence is also presented indicating that the mechanical condition of the free metal surface has a significant effect on the metal-silicon reaction rate.

Weizer, Victor G.↗

NASA NTRS1979

A low-cost solar-cell front contact using trapped silver mesh and electrostatic bonding

A new system to provide a contact with a silver mesh trapped between the cell and an encapsulating glass cover is described. Borosilicate glass is joined to the cell by electrostatic bonding during which the glass is deformed around the silver mesh to form a permanent optically coupled integral bond to the cell. The hermetic seal prevents the silver from oxidizing and destroying the electrical contact formed during the bonding process. It is demonstrated that electrostatic glass/silicon bonds have excellent stability under a variety of accelerated environmental tests. Electrostatic bonds to thin films of AL2O3, Ta2O5, and ZrO2 on glass can be used to make antireflective coatings for solar cells manufactured by this process. Test solar cells were manufactured by electrostatic bonding with good curve fill factors and efficiencies of 10 percent.

Landis, Geoffrey A.↗

NASA NTRS1997

Single Crystal Fibers of Yttria-Stabilized Cubic Zirconia with Ternary Oxide Additions

Single crystal fibers of yttria (Y2O3)-stabilized cubic zirconia, (ZrO2) with ternary oxide additions were grown using the laser float zone fiber processing technique. Ternary additions to the ZrO2-Y2O3 binary system were studied aimed at increasing strength while maintaining the high coefficient of thermal expansion of the binary system. Statistical methods aided in identifying the most promising ternary oxide candidate (Ta2O5, Sc2O3, and HfO2) and optimum composition. The yttria, range investigated was 14 to 24 mol % and the ternary oxide component ranged from 1 to 5 mol %. Hafnium oxide was the most promising ternary oxide component based on 816 C tensile strength results and ease of fabrication. The optimum composition for development was 81 ZrO2-14 Y203-5 HfO2 based upon the same elevated temperature strength tests. Preliminary results indicate process improvements could improve the fiber performance. We also investigated the effect of crystal orientation on strength.

Ritzert, F. J.↗

NASA NTRS2008

Oxidation of ZrB2 SiC TaSi2 Materials at Ultra High Temperatures

ZrB2 - 20v% SiC - 20v% TaSi2 was oxidized in stagnant air for ten minute cycles for times up to 100 minutes at 1627 C and 1927 C. The sample oxidized at 1627 C showed oxidation resistance better than that of the standard ZrB2 - 20v% SiC. The sample oxidized at 1927 C, however, showed evidence of liquid phase formation and complex oxidation products. The sample exposed at 1927 C was analyzed in detail by scanning electron microprobe and wavelength dispersive spectroscopy to understand the complex oxidation and melting reactions occurring during exposure. The as hot-pressed material shows the formation of a Zr(Ta)B2 phase in addition to the three phases in the nominal composition already noted. After oxidation, the TaSi2 in the matrix was completely reacted to form Ta(Zr)C. The layered oxidation products included SiO2, ZrO2, Ta2O5, and a complex oxide containing both Zr and Ta. Likely reactions are proposed based on thermodynamic phase stability and phase morphology.

Opila, E.↗

DOE OSTI2024

Phase comparison and equation of state for Ta 2 O 5

Abstract Tantalum pentoxide (Ta 2 O 5 ) is among the most technologically useful heavy transition metal oxides. Unfortunately, its crystal structure is the subject of long-standing and unresolved disagreement. Among other consequences, this uncertainty has made it impossible to formulate a robust high pressure equation of state for Ta 2 O 5 . Here, we report the results of high pressure x-ray diffraction experiments indexed against a comprehensive list of proposed Ta 2 O 5 phases. Five of the proposed phases produce good matches to experimental observations, and the compressibility parameters for these phases are all consistent within uncertainty. This means that regardless of the particular phase chosen, the Ta 2 O 5 equation of state can be described with bulk modulus K 0 = 138 ± 3.68 GPa and pressure derivative K 0 ′ = 1.82 ± 0.45 . Combining these experimental results with new density-functional theory calculations allows us to identify the λ phase as the best structural model of Ta 2 O 5 at ambient conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗