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The roles of atomic-scale dynamics and structure in the brittle fracture of silica

Description: We modeled the initiation of fracture in vitreous silica at various strain rates using molecular dynamics simulations. We avoided biasing the location for fracture initiation within the sample so that we could study the effects of dynamics and structure on determining the path to fracture, defined as the particular bonds that break during the course of fracture. We sought to show that the path to fracture would be primarily determined by the local variations in the structure of the vitreous phase at low strain rates, with diminished sensitivity on structural variations at higher strain rates. However, the results of our model indicate that the path to fracture is dependent not only on the initial structure of the system and the applied strain rate, but also on the initial phase of the thermal vibrations. This underscores the importance of atomic dynamics in determining the path to fracture in brittle materials and provides a justification for extending the analysis of fracture surfaces to the near-atomic scale.
Date: December 31, 1995
Creator: Swiler, T.P.; Varghese, T. & Simmons, J.H.
Partner: UNT Libraries Government Documents Department

The interpretation of HRTEM images of partially amorphized pyrochlore structure types

Description: Results of image simulations on partially amorphous microlite (Ca{sub 2}Ta{sub 2}O{sub 7}, pyrochlore structure) are presented. Results indicate that HRTEM images are not sensitive to position of amorphous layers within a crystalline matrix. It is also observed that the limit of detection of amorphous material within a crystalline matrix is dependent on total thickness. In thin crystals (<150{Angstrom}), up to 75 vol% crystalline material can give rise to aperiodic images, yet addition of a small amount of crystalline material (80 vol% crystalline) produces a periodic image. Images calculated for isolated spheres of amorphous material distributed within crystalline microlite suggest that isolated domains of amorphization are observable at sample thicknesses less than 3x the diameter of the feature. Image contrast of amorphized domains is enhanced by imaging at defocus settings significantly different than Scherzer focus. This indicate that interpretation of HRTEM images of partially amorphized crystalline materials should be undertaken with caution, and estimates of the volume of damage considered only qualitative.
Date: December 31, 1990
Creator: Miller, M.L. & Ewing, R.C.
Partner: UNT Libraries Government Documents Department

Properties of vacuum arc deposited amorphous hard carbon films

Description: Amorphous hard carbon films formed by vacuum arc deposition are hydrogen-free, dense, and very hard. The properties of amorphous hard carbon films depend strongly on the energy of the incident ions. A technique which is called Plasma Immersion Ion Implantation can be applied to vacuum arc deposition of amorphous hard carbon films to influence the ion energy. The authors have studied the influence of the ion energy on the elastic modulus determined by an ultrasonic method, and have measured the optical gap for films with the highest sp{sup 3} content they have obtained so far with this deposition technique. The results show an elastic modulus close to that of diamond, and an optical gap of 2.1 eV which is much greater than for amorphous hard carbon films deposited by other techniques.
Date: April 1, 1995
Creator: Anders, S.; Anders, A. & Raoux, S.
Partner: UNT Libraries Government Documents Department

Temperature-modulated calorimetry of the frequency dependence of the glass transition of poly(ethylene terephthalate) and ....

Description: Temperature-modulated differential scanning calorimetry, TMDSC, is new technique that permits to measure the apparent heat capacity vs modulation frequency. The method is briefly described and a quasi- isothermal measurement method is used to derive the kinetic parameters for PET and PS. A first-order kinetics expression was used to describe the approach to equilibrium and point out the limits caused by asymmetry and cooperativity of the kinetics. Use of a complex description of heat capacity and entropy is discussed. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment and the preexponential factor is correlated with the activation energy.
Date: March 1, 1997
Creator: Wunderlich, B. & Okazaki, I.
Partner: UNT Libraries Government Documents Department

Photovoltaic manufacturing technology monolithic amorphous silicon modules on continuous polymer substrates. Annual technical progress report, 5 July 1995--4 June 1996

Description: Iowa Thin Film Technologies` goal is to develop the most cost-effective photovoltaic manufacturing process possible. During the first year, they developed the capability of sputtering a high-quality (Zn(Al)O) successfully implemented increased deposition rates for the ZnO top contact deposition; improved registration and ink-line width to reduce area loss due to interconnects; developed a new alignment process and sensor to improve the speed and accuracy of registration for the patterning processes; developed a new Silver ink composition that allows finer print lines and lower series resistance; demonstrated an 8% overall improvement in area utilization; evaluated water-based insulator inks for compatibility with their processes; investigated and tested the use of roll-based lamination as a means to reduce the cost of assembly; developed straight roll lamination capability using pressure-sensitive adhesives and thermally activated bonding; and evaluated the use of the standard EVA/Tefzel encapsulant with a roll laminator.
Date: February 1, 1997
Creator: Jeffrey, F.
Partner: UNT Libraries Government Documents Department

In situ TEM study of ion-beam-induced amorphization of complex silicate structures

Description: In-situ TEM with ion irradiation was used to study the radiation- induced amorphization (metamictization) of naturally occurring silicates: neptunite [Na2KLi(Fe,Mn)2Ti2(SiO3)8], titanite (CaTiSiO5), gadolinite (REE2FeBe2Si2O10), zircon (ZrSiO4), and olivine [(Mg, Fe)2SiO4]. They were irradiated with 1.5 MeV Kr{sup +} ions the Argonne HVEM-Tandem Facility at room temperature with electron diffraction pattern monitored in situ. Critical doses required for amorphization of the electron transparent thickness of neptunite, titanite, gadolinite, zircon, and olivine are 1.7, 2.0, 2.3, 4.8, and 6.0x10{sup 14} ions/cm{sup 2}, respectively. Results show a correlation between amorphization dose and the chemical/structural complexity of these 5 minerals. The most complex, neptunite, becomes amorphous at the lowest critical dose. The critical amorphization dose also increases with the melting point.
Date: December 31, 1990
Creator: Wang, L.M.; Eby, R.K.; Janeczek, J. & Ewing, R.C.
Partner: UNT Libraries Government Documents Department

Neutron irradiation induced amorphization of uranium silicides

Description: Time-of-flight neutron powder diffraction studies of incrementally neutron-irradiated U{sub 3}Si and U{sub 3}Si{sub 2} have revealed details of progressive amorphization of bulk materials, crystalline transformation prior to amorphization, elastic strain and diffuse scattering resulting from scattering interference between crystalline and amorphous fractions. Density differences between amorphous and crystalline fractions give rise, respectively, to tensile and compressive strain in U{sub 3}Si and U{sub 3}Si{sub 2}. Diffuse scattering associated with each Bragg peak shows crystallographic direction-dependent variation both in magnitude and displacement (relative to the Bragg position). A theoretical model describing this behavior relates the size of the amorphous zones, and the magnitude and displacement of the diffuse scattering contribution. After complete amorphization of U{sub 3}Si and U{sub 3}Si{sub 2}, anneals to progressively higher temperatures generate gradual evolution of the short- to intermediate-range amorphous structures prior to re-crystallization.
Date: October 1, 1997
Creator: Richardson, J.W. Jr.; Birtcher, R.C. & Chan, S.K.
Partner: UNT Libraries Government Documents Department

Electronic transport in amorphous carbon

Description: Electronic transport in a-C films has been the subject of considerable debate. In this study, combined stress relaxation and electrical transport studies were used to identify the transport mechanism in a-C films prepared by pulsed-laser deposition. The stress relaxation was modeled by a first-order kinetic reaction involving transformation of 4-fold coordinated carbon atoms to 3-fold coordinated carbon atoms, and the distribution of activation energies for this process was determined. The activation energies were found to range from about 1 eV to over 2 eV, and using these activation energies, the increase in 3-fold carbon concentration with time-temperature annealing was obtained. Conductivity measurements were also performed as a function of time-temperature annealing. It was found that the conductivity of a-C films is exponentially proportional to increases in 3-fold carbon concentration. This result can be explained by thermally activated hopping along carbon 3-fold chains combined with chain-to-chain tunneling. From the data, a typical chain length was estimated to consist of 13 carbon atoms. The heterogeneous nature of the conductivity may explain the spatially localized electron emission which is observed in a-C assuming a tunnel barrier emission model.
Date: December 1, 1997
Creator: Sullivan, J.P. & Friedmann, T.A.
Partner: UNT Libraries Government Documents Department

Amorphous silicon research. Phase III technical progress report, August 1, 1996--July 31, 1997

Description: The principal objective of this R&D program is to expand, enhance and accelerate knowledge and capabilities for the development of high-performance, two-terminal multijunction hydrogenated amorphous silicon (a-Si) alloy cells and modules. The near-term goal of the program is to achieve 12% stable active-area efficiency using the multijunction approach. The long-term goal is to achieve 15% stable efficiency multijunction modules. The major effort of this program is to develop high efficiency component cells and incorporate them in the triple-junction structure to obtain the highest stable efficiency. New and improved deposition regimes were investigated to obtain better cell performance. Fundamental studies to obtain better understanding of material and cell performance were undertaken.
Date: November 1, 1997
Creator: Guha, S.
Partner: UNT Libraries Government Documents Department

Energy dependence of Ge amorphization by Ne, Ar or Kr ion irradiation

Description: Ge has been irradiated at RT by Ne, Ar, or Kr ions of different energies, and the doses required for complete amorphization determined by in situ TEM and electron diffraction. Onset of amorphization was detected after the lowest ion doses reflecting amorphization by individual ions. The ion dose required for complete amorphization increased nearly linearly with ion energy over the range 0.5 to 3.5 MeV for all ions. Amorphization cross sections have been determined for all ions and energies used. The displacements per atom required for complete amorphization decreased with ion energy or mass, owing to decrease in radiation annealing of amorphous volumes as a result of a decrease in fraction of low energy transfers to Ge atoms. Increasing the relative fraction low energy transfers to Ge atoms by simultaneous 1 MeV electron irradiation increased the Kr ion dose required for complete amorphization by as much as a factor of 2.5. The effect is believed to be due to an increase in the fraction of freely migrating defects produced by low energy transfers to Ge atoms.
Date: October 1, 1994
Creator: Birtcher, R.C.
Partner: UNT Libraries Government Documents Department

Simulation of mechanical deformation and tribology of nano-thin amorphous hydrogenated carbon (a:CH) films using molecular dynamics

Description: Molecular dynamics computer simulations are used to study the effect of substrate temperature on microstructure of deposited amorphous hydrogenated carbon (a:CH) films. A transition from dense diamond- like films to porous graphite-like films is observed between substrate temperatures of 400 and 600 K for a deposition energy of 20 eV. The dense a:CH film grown at 300 K and 20 eV has a hardness ({similar_to}50 GPa) about half that of a pure carbon (a:C) film grown under the same conditions.
Date: April 1, 1996
Creator: Glosli, J.N.; Belak, J. & Philpott, M.R.
Partner: UNT Libraries Government Documents Department

Short pulse laser-induced optical damage and fracto-emission of amorphous, diamond-like carbon

Description: Short pulse laser damage and ablation of amorphous, diamond-like carbon films is investigated. Material removal is due to fracture of the film and ejection of large fragments, which exhibit a broadband emission of microsecond duration.
Date: February 7, 2000
Partner: UNT Libraries Government Documents Department

Increased medium-range order in amorphous silicon with increased substrate temperature

Description: Using fluctuation electron microscopy, the authors have measured the medium-range order of magnetron sputtered silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. They find a smooth increase in the medium-range order of the samples, which they interpret in the context of the paracrystalline structural model as an increase in the size of and/or volume fraction occupied by the paracrystalline grains. These data are counter to the long-standing belief that there is a sharp transition between amorphous and polycrystalline structures as a function of substrate temperature.
Date: August 15, 2000
Creator: Voyles, P. M.; Gerbi, J. E.; Treacy, M. M. J.; Gibson, J. M. & Aberlson, J. R.
Partner: UNT Libraries Government Documents Department

Medium-range order in hydrogenated amorphous silicon measured by fluctuation microscopy

Description: The authors have characterized with fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H{sub 2} dilution of the SiH{sub 4} precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition methods and H{sub 2} content. The PECVD film grown with high H{sub 2} dilution contains Si crystals {approximately} 5 nm in diameter at a density of {approximately} 10{sup 9} cm{sup 2}. The amorphous matrix surrounding these crystals shows no difference in medium-range order from the standard PECVD film.
Date: April 17, 2000
Creator: Voyles, P. M.; Treacy, M. M. J.; Jin, H.-C.; Abelson, J. R.; Gibson, J. M.; Guha, S. et al.
Partner: UNT Libraries Government Documents Department

Bonding and hardness in nonhydrogenated carbon films with moderate sp(3) content

Description: Amorphous carbon films with an s p{sup 3} content up to 25% and a negligible amount of hydrogen have been grown by evaporation of graphite and concurrent Ar{sup +} ion bombardment. The s p{sup 3} content is maximized for Ar{sup +} energies between 200 and 300 eV following a subplantation mechanism. Higher ion energies deteriorate the film due to sputtering and heating processes. The hardness of the films increases in the optimal assisting range from 8 to 18 GPa, and is explained by the crosslinking of graphitic planes through s p {sup 3} connecting site.
Date: April 17, 2000
Partner: UNT Libraries Government Documents Department

The electronic transport mechanism in amorphous tetrahedrally-coordinated carbon films

Description: The electronic transport mechanism in tetrahedrally coordinated amorphous carbon was investigated using measurements of stress relaxation, thermal evolution of electrical conductivity, and temperature dependent conductivity measurements. Stress relaxation measurements were used to determine the change in 3-fold coordinated carbon concentration, and the electrical conductivity was correlated to this change. It was found that the conductivity was exponentially proportional to the change in 3-fold concentration, indicating a tunneling or hopping transport mechanism. It was also found that the activation energy for transport decreased with increasing anneal temperature. The decrease in activation energy was responsible for the observed increase in electrical conductivity. A model is described wherein the transport in this material is described by thermally activated conduction along 3-fold linkages or chains with variable range and variable orientation hopping. Thermal annealing leads to chain ripening and a reduction in the activation energy for transport.
Date: February 1, 1998
Creator: Sullivan, J.P.; Friedmann, T.A.; Dunn, R.G.; Stechel, E.B. & Schultz, P.A.
Partner: UNT Libraries Government Documents Department

Thermodynamics of paracrystalline silicon

Description: Fluctuation microscopy experiments have shown that the as-deposited structure of amorphous silicon thin films is paracrystalline. A paracrystal consists of small (&lt; 3 nm in diameter) topologically crystalline grains separated by a disordered matrix. Here the authors consider the thermodynamics of paracrystalline silicon as a function of the grain size and the temperature. They offer a simple model that qualitatively explains the observed metastability of the ordered structure at low temperature (300 K), the relaxation towards a more disordered structure at intermediate temperatures (600 K), and the recrystallization at high temperatures (1,000 K).
Date: May 9, 2000
Creator: Voyles, P. M.; Treacy, M. M. J. & Gibson, J. M.
Partner: UNT Libraries Government Documents Department

Bonding topologies in diamondlike amorphous-carbon films

Description: The carbon ion energy used during filtered cathodic vacuum arc deposition determines the bonding topologies of amorphous-carbon (a-C) films. Regions of relatively low density occur near the substrate/film and film/surface interfaces and their thicknesses increase with increasing deposition energy. The ion subplantation growth results in mass density gradients in the bulk portion of a-C in the growth direction; density decreases with distance from the substrate for films grown using ion energies &lt; 60 eV and increases for films grown using ion energies &gt; 160 eV. Films grown between these energies are the most diamondlike with relatively uniform bulk density and the highest optical transparencies. Bonding topologies evolve with increasing growth energy consistent with the propagation of subplanted carbon ions inducing a partial transformation of 4-fold to 3-fold coordinated carbon atoms.
Date: January 27, 2000
Partner: UNT Libraries Government Documents Department