bluuebunny/crossref_metadata_2025 huggingface dataset:
Native Defect Formation and Ionization Energies in Cadmium Telluride
Authors: Jaffe John E. , Bliss Mary
Published: June 2019

Deep intrinsic energy levels near the middle of the band gap in CdTe have been reported in a number of experiments. Based on earlier defect-supercell electronic structure calculations, at least some of these features have been attributed to the second ionization level of the Cd vacancy, while the TeCd antisite, possibly complexed with a Cd vacancy, has also been suggested to account for some midgap levels. Using high-accuracy LDA calculations with full lattice relaxation out to third neighbors, we find that (i) both acceptor states of the Cd vacancy are shallow, (ii) the donor states of an isolated TeCd are both more than 1 eV above the valence band maximum, (iii) the TeCd-VCd complex does indeed have acceptor states near midgap in CdTe and probably accounts for the native defect states in that energy range.
Study of Er-related Defects in a-Si:H(Er) Films Used in Light Emitting Heterostructures
Authors: Nazarov A. , Vovk Ja. , Lysenko V. , Kon'kov O. , Terukov E. , Ashok S.
Published: June 2019

Defects relating to Er-doping of hydrogenated amorphous silicon films in amorphous/ crystalline (a-Si:H(Er) / n-Si) heterostructures used as light emitters were studied. Electrical parameters of the defects were evaluated by thermally activated current (TAC) and capacitance (TACap) spectroscopy, and high-frequency (1 MHz) capacitance-voltage (C-V) characteristics measured at 77 K. For charging of the traps, visible light illumination at low temperatures and thermal-bias stress at various temperatures were used. The amplitude of TAC was found to increase monotonically with increase of Er concentration in the a-Si:H film. The traps, with activation energies in the range 0.10 - 0.35 eV and 0.5 - 0.6 eV, were charged under both light and thermal-bias stress, and may be associated with the deep levels in the band-gap of a-Si:H. High-temperature current peaks with activation energies of 0.8-0.9 eV emerged only under thermal-bias stress and were strongly dependent on the charging temperature. It is suggested that these high-temperature thermally activated processes are related to polarization of the Er-O bonds in the amorphous matrix of a-Si:H. C-V measurements show that charging of the structure by negative substrate bias leads to net electron trapping in the a-Si:H(Er) film, whereas charging by positive bias result in net hole trapping. The model of Er-related electroluminescence in a-Si:H(Er) / n-Si heterostructures involving electron-hole recombination via deep levels located in the vicinity of [Er-O] inclusions is discussed.
Low-Temperature Properties of Compensated Ge Films Used for Cryogenic Thermometers
Authors: Mitin V. F. , Kholevchuk V. V. , Dugaev V. K. , Vieira M.
Published: June 2019

Low temperature microsensors are designed for cryogenic applications. As a material for the sensors we use heavily doped compensated Ge films deposited on the semi-insulating GaAs substrates. We present the results of experimental and theoretical study of the low temperature resistance as a function of temperature and magnetic field for some models of temperature sensors. The computer simulations show a good agreement with experimental data.
Fabrication of Copper-Indium-Disulfide Films onto Mo/Glass Substrates Using Pulsed Laser Deposition
Authors: Mu R. , Wu M.H. , Liu Y. C. , Ueda A. , Henderson D.O. , Hmelo A.B. , Feldman L.C. , Hepp A.
Published: June 2019

Pico-second pulsed laser deposition (PLD) was employed to fabricate copper indium disulfide (CIS) thin films onto pure silica and Mo coated glass substrates. By properly preparing the target materials and controlling the elemental ratio of the Cu, In and S in the deposited film followed by post-thermal annealing, good quality copper-indium-disulfide(CIS) films can be obtained. A series of characterizations were conducted including XRD, RBS, IR, UV-Vis, AFM and STM analyses.
Absorber Films of Ag2S and AgBiS2 prepared by Chemical Bath Deposition
Authors: Rodriguez A. Nuñez , Nair M.T.S. , Nair P.K.
Published: June 2019

Ag2S thin films of 90 nm to 300 nm in thickness were deposited at 70°C on glass substrates immersed in a bath mixture containing silver nitrate, sodium thiosulfate and dimethylthiourea. When the films are heated in nitrogen at temperatures 200°C to 400°C, crystallinity is improved and XRD pattern similar to that of acanthite is observed. These films possess electrical conductivity of 10-3 (ohm cm)-1, are photoconductive and exhibit an optical band gap of 1.36 eV. When Ag2S thin film is deposited over a thin film of Bi2S3, also obtained by chemical bath deposition from bismuth nitrate, triethanolamine and thioacetamide, and heated at 300°C to 400°C in nitrogen, a ternary compound, AgBiS2 is formed. This material has an electrical conductivity of 5x10-5 (ohm cm)-1, is photoconductive and possesses optical band gap 0.95 eV.
A Promising New Photo-Catalyst InVO4for Water Molecule Decomposition in the Visible Wavelength Region
Authors: Oshikiri M. , Seitsonen A. P. , Parrinello M. , Boero M. , Ye J. , Zou Z.
Published: June 2019

A promising vanadium-based photo-catalyst (InVO4) has been proposed recently by our group. This catalyst has been shown to be active in the visible light range, up to a wavelength of about 600 nm, and is able to promote the decomposition of water. In this paper, we focus on its electronic structure, computed via first principles calculations, in order to figure out analogies and differences with similar systems (InNbO4, InTaO4and BiVO4) that have already been reported to act as photo-catalysts. An attempt is made to address the problem of the wavelength dependency of the photo catalytic activity of the InMO4(M = V, Nb, Ta) family and how this is related to the crystal structure. Finally, by using anab initiomolecular dynamics approach, we inspect the relaxation/reconstruction phenomena occurring at the exposed surface of the InVO4catalyst induced by the absorption of a water molecule, that represents a crucial step in the catalysis reaction.
Anomaly of Thermal Properties in Thin Films of La1-x SrxCoO3 Series Fabricated as Thermoelectric Materials
Authors: Takata Yoshiaki , Haneda Hajime , Adachi Yutaka , Wada Yoshiki , Mitsuhashi Takefumi , Ohtani Makoto , Fukumura Tomoteru , Kawasaki Masashi , Koinuma Hideomi
Published: June 2019

Composition spreads of La1-xSrxCoO3 (LSCO) were synthesized as thermo-electric transducer material by means of combinatorial material synthesis, and information on their thermal diffusivity throughout the specimens was obtained. Meanwhile, it is anticipated that the composition spreads of LSCO have a variety of characteristic properties of light absorption corresponding to their composition, namely their compositional variable, x. Hence, we used transient optical pump-and-probe techniques in a reflection geometry to measure thermal diffusion times on LSCO. The results of signal analysis indicate that LSCO is a peculiar synthesized substance whose apparent thermal diffusivity, 1/ι, changes abruptly at the critical point, where the compositional variable x is equal to 0.3.
Challenges for High Temperature Silicon Carbide Electronics
Authors: Zetterling C.-M. , Koo S.-M. , Danielsson E. , Liu W. , Lee S.-K. , Domeij M. , Lee H.-S. , Ostling M.
Published: June 2019

Silicon carbide has been proposed as an excellent material for high-frequency, high-power and high-temperature electronics. High power and high frequency applications have been pursued for quite some time in SiC with a great deal of success in terms of demonstrated devices. However, self-heating problems due to the much higher power densities that result when ten times higher electrical fields are used inside the devices needs to be addressed. High-temperature electronics has not yet experienced as much attention and success, possibly because there is no immediate market. This paper will review some of the advances that have been made in high-temperature electronics using silicon carbide, starting from process technology, continuing with device design, and finishing with circuit examples. For process technology, one of the biggest obstacles is long-term stable contacts. Several device structures have been electrically characterized at high temperature (BJTs and FETs) and will be compared to surface temperature measurements and physical device simulation. Finally some proposed circuit topologies as well as novel solutions will be presented.
Statistical Analysis of Micropipe Defect Distributions in Silicon Carbide Crystals
Authors: Elkington Troy , Emorhokpor Ejiro , Kerr Tom , Chen John , Essary Kevin , Golab Mike , Hopkins Richard H.
Published: June 2019

The density and distribution of micropipes (MPD's), a screw dislocation with a hollow core, are significant criteria for SiC wafer selection in device fabrication. For this reason the measurement of micropipes in SiC is an important problem. The micropipe distributions of 2’’ diameter 6H SiC wafers from three different PVT furnace designs in three different laboratories were statistically characterized. The micropipe distributions were measured in two ways: by KOH etching the wafer and then counting the hexagonal pits formed on the wafer surface using a microscope and imaging software; and by manually counting with an optical transmission microscope the micropipes on a un-etched wafer within a grid of 0.5mm squares superimposed over the wafer. None of the wafers had a random distribution of micropipes i.e. they did not follow a Poisson distribution. The micropipes were found to associate with each other i.e. there is a higher probability of there being other micropipes near a location where a micropipe has been found. In the eight samples characterized, 1.9% of the wafer area contained 32% of the total micropipes. We have analyzed the repeatability of the optical transmission microscopy method of micropipe characterization and found that it is dependent on the micropipe density; the standard deviation of the measurement increases with increasing MPD. The number of micropipes counted increases by about 10% with increasing magnification and 15% when the wafer is double side polished.
Characterization of High Quality Continuous GaN Films Grown on Si-Doped Cracked GaN Template
Authors: Soh C. B. , Zhang J. , Chi D.Z. , Chua S. J.
Published: June 2019

In this paper, deep level defects in high quality continuous GaN films grown over a cracked Si-doped GaN template has been studied using digital deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). From TEM observation, it is found that the density of pure screw dislocations have been effectively suppressed while pure edge dislocations remained in substantial quantity. From DLTS measurement, trap levels at Ec -ET ∼ 0.11-0.12 eV, 0.24-0.27 eV, 0.60-0.63 eV were detected in the high quality GaN layer. DLTS measurement was also carried out on the underlying cracked Si-doped GaN template after the top high quality continuous GaN film was removed by plasma etching. An additional defect level at Ec-Et ∼ 0.37 eV was detected which we attributed to defect decoration at screw dislocation. Both the trap levels Ec-ET ∼ 0.24–0.27 eV, 0.60-0.63 eV are believed to originate from mixed screw/edge dislocation based on observation of the logarithmic capture behavior. Trap level at Ec -ET ∼ 0.24-0.27eV, however, experiences a more drastic increase in transient capacitance (i.e. in trap concentration) compared to that of Ec -ET ∼ 0.60-0.63 eV after plasma etching, illustrating that the latter is related to a higher proportion of edge dislocation. The 0.11-0.12 eV trap level, which exhibits an exponential capture kinetic, is believed to be related to nitrogen vacancies. This high quality continuous GaN layer can be used as a template to grow any device structure and the underneath cracked Si-doped GaN layer may help to release stress for the top continuous GaN layer. This can bring about a cracked free epilayer for subsequent device fabrication.
Modeling of Nitride Semiconductor Based Double Heterostructure Tunnel Diodes
Authors: Moret M. , Ruffenach S. , Briot O. , Aulombard R.L.
Published: June 2019

Double heterostructure diodes were widely studied in the 80's, based on the use of “classical” III-V materials, i.e. based on (Ga, Al, In, As, P). Negative differential conductivities were predicted in these systems, making them highly interesting for hyperfrequency applications, but experimental results were always far from theoretical predictions, which resulted in a loss of interest for these systems. Recently, there has been a lot of efforts devoted to Nitride semiconductors, based on (Al,Ga,In)N alloys. These materials have been regarded as potential candidates for high frequency, high power applications, since they both exhibit high electron saturation velocities and high thermal and chemical stability. Moreover, although they usually contain a lot of growth defects, device properties were surprisingly good with regards to defect densities. In this work, we have modeled double heterostructure tunnel diodes based on AlGaN/GaN system. The extremely high conduction band offset in these materials, along with the quite low dielectric constants, and built-in electric fields (originating from both the spontaneous and piezoelectric polarization), are extremely favorable parameters, which may renew the interest for such devices in this material system. It will be shown that the built-in electric field leads to a symmetric potential profile under a given external applied bias, which optimizes the transmission coefficient in the structure. We have calculated extremely high peak/valley ratios, which suggests that even mid quality samples could still exhibit interesting device properties.
ZnO Thin Films Grown at Various Substrate Angles by Pulsed Laser Deposition
Authors: Kang Jeong Seok , Kang Hong Seong , Kim Jae Won , Lee Sang Yeol
Published: June 2019

ZnO thin films were grown with different plume-substrate angles and growth times by pulsed laser deposition. The angles between plume propagation direction and substrate plane were changed from 0°, 45° to 90°. The growth time was changed in order to adjust film thickness. From the XRD pattern exhibiting dominant (002) XRD peak of ZnO, all films were found to be c-axis oriented. Optical property of ZnO thin films were investigated by PL(Photoluminescence). The ratio of UV/green luminescent intensity of ZnO thin film increased, as the angle between plume and substrate plane decrease from 90° to 0°.
Atomic and Electronic Structure of Boron-Doped Diamond Grain Boundaries Studied by Arhvtem and ab-Initio Calculation
Authors: Togawa Hiroyuki , Ichinose Hideki
Published: June 2019

Atomic resolution high-voltage transmission electron microscopy and electron energy loss spectroscopy were performed on grain boundaries of boron-doped diamond, cooperated with the ab-initio calculation. Segregated boron in the {112}∑3 boundary was caught by the EELS spectra. The change in atomic structure of the segregated boundary was successfully observed from the image by ARHVTEM. Based on the ARHVTEM image, a segregted structure model was proposed.
The Shallow Implantation of Bismuth During the Growth of Bismuth Nanocrystals in Al2O3 by Pulsed Laser Deposition
Authors: Suárez-García A. , Barnes J-P. , Serna R. , Petford-Long A. K. , Afonso C. N. , Hole D.
Published: June 2019

The effect of the laser energy density used to deposit Bi onto amorphous aluminum oxide (a-Al2O3) on the growth of Bi nanocrystals has been investigated using transmission electron microscopy of cross section samples. The laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm-2). Across the range of energy densities, in addition to the Bi nanocrystals nucleated on the a-Al2O3 surface, a dark and apparently continuous layer appears below the nanocrystals. Energy dispersive X-ray analysis on the layer have shown it is Bi rich. The separation from the Bi layer to the bottom of the nanocrystals on top is consistent with the implantation range of Bi species in a-Al2O3. As the laser energy density increases, the implantation range has been measured to increase. The early stages of the Bi growth have been analyzed in order to determine how the Bi implanted layer develops.
Unique Functional Micro/nano-structures Created by Femtosecond Laser Irradiation
Authors: Hong M.H. , Huang S.M. , Wang W.J. , Tiaw K.S. , Teoh S.H. , Luk'yanchuk B. , Chong T.C.
Published: June 2019

Femtosecond (fs) laser application in three-dimensional (3D) optical recording is introduced. The laser irradiation on transparent glass and polymer matrix doped with fluorescent material is carried out, which changes the physical or chemical properties of the recording media and records information bits. With the change of the focusing positions inside the transparent substrates, 3D optical recording can be available for ultrahigh capacity data storage. Feasibility on fs laser drilling of poly-caprolactone (PCL) thin films for tissue engineering is investigated. It is found that precisely defined micro-hole arrays can be formed on the sample surfaces. Hydrophilic property of the processed samples is much improved, which provides good conditions for tissue cells to anchor on the man-made skin. Fs laser applications to form nanostructures on substrate surfaces are studied. Fs laser combination with near-field scanning optical microscopy (NSOM) to induce surface property modification in the sub 50-nm under NSOM tip and nanoparticles is also discussed.
Nonlinear Optical Spectroscopy of Cadmium Chalcogenide Nanocrystals
Authors: Seo J.T. , Yang Q. , Creekmore S. , Mangana J. , Anderson J. , Pompey C. , Temple D. , Peng X. , Qu J. L. , Yu W. , Wang A. , Jung S. , Ruh H. , Mott A. , Namkung M.
Published: June 2019

We report on the nonlinear optical properties of cadmium telluride (CdTe) semiconductor colloidal quantum dots. Transmission electron microscopy measurements revealed that the size of CdTe nanocrystal quantum dots, dependent on the growth reaction time, was ∼2-10 nm or near the exciton Bohr radius. The strong blue-shifts of the CdTe, CdSe and CdS nanocrystal absorption spectra and the atomic-like discrete energy states of exciton indicate an exciton quantum confinement. These are completely different optical properties from the bulk crystals. The energy transition for exciton absorption was assigned as h1→e+, h2→e+, h1+→e-, and h2+→e- for the 1st, 2nd, 3rd and 4th exciton absorption peaks. Z-scan and I-scan nonlinear spectroscopy revealed that the CdTe nanocrystal quantum dot in toluene (∼8 × 10-5 mol/L) has the negative nonlinearity (self-defocusing) with ∼ -1 × 10-13 m2/W and a high nonlinear figure of merit of ∼200. For the optical power self-limiting experiment, the CdTe nanocrystal was almost opaque above ∼0.8 MW/cm2 at the position of z∼6.9 cm.
Oblique-incidence Reflectivity Difference (OI-RD) and Leed Studies of Adsorption and Growth of Xe on Nb(110)
Authors: Thomas P. , Nabighian E. , Bartelt M.C. , Fong C.Y. , Zhu X.D.
Published: June 2019

We studied adsorption, growth and desorption of Xe on Nb(110) using an in-situ obliqueincidence reflectivity difference (OI-RD) technique and low energy electron diffraction (LEED) from 32 K to 100 K. The results show that Xe grows a (111)-oriented film after a transition layer is formed on Nb(110). The transition layer consists of three layers. The first two layers are disordered with Xe-Xe separation significantly larger than the bulk value. The third monolayer forms a close packed (111) structure on top of the tensile-strained double layer and serves as a template for subsequent homoepitaxy. The adsorption of the first and the second layers are zeroth order with sticking coefficient close to one. Growth of the Xe(111) film on the transition layer proceeds in a step flow mode from 54K to 40K. At 40K, an incomplete layer-by-layer growth is observed while below 35K the growth proceeds in a multilayer mode.
Ultrafast Pulsed Laser Deposition of Chalcogenide Glass Films for Low-loss Optical Waveguides
Authors: Luther-Davies B. , Kolev V. Z. , Lederer M. J. , Yinlan R. , Samoc M. , Jarvis R. A. , Rode A. V. , Giesekus J. , Du K.-M. , Duering M.
Published: June 2019

Ultra-fast pulsed laser deposition using high-repetition-rate short-pulse lasers has been shown to provide high optical quality, super smooth thin films free of scattering centres. The optimized process conditions require short ps or sub-ps pulses with repetition rate in the range 1-100 MHz, depending on the target material. Ultra-fast pulsed laser deposition was used to successfully deposit atomically-smooth, 5micron thick As2S3 films. The as-deposited films were photosensitive at wavelengths close to the band edge (≈520 nm) and waveguides could be directly patterned into them by photo-darkening using an Argon ion or frequency doubled Nd:YAG laser. The linear and nonlinear optical properties of the films were measured as well as the photosensitivity of the material. The optical losses in photo-darkened waveguides were <0.2 dB/cm at wavelengths beyond 1200nm and <0.1 dB/cm in as-deposited films. The third order nonlinearity, n2,As2S3, was measured using both four-wave mixing and the z-scan technique and varied with wavelength from 100 to 200 times fused silica (n2,Silica ≈3×10-16 cm2/W) between 1500nm and 1100nm with low nonlinear absorption.

Encouraged by the Ultrafast laser deposition results, we have built a new specialized modelocked picosecond laser system for deposition of optical films and for laser formation of nanoclusters. The newly developed “state of the art” powerful Nd:YVO laser can operate over a wide range of wavelengths, intensities, and repetition rates in MHz range. A brief description of the 50W laser installation is presented.
Pulsed Laser Processing of Metals and Semiconductors in Reactive Atmospheres: Laser Nitriding and Carburizing
Authors: Carpene Ettore , Schaaf Peter
Published: June 2019

Hard surface coatings and thin layers can be synthesized by irradiating pure substrates with short laser pulses in reactive atmospheres. Various lasers with different pulse durations τ and wavelengths λ can be used for this purpose: XeCl excimer laser, Nd-YAG laser, free electron laser and Ti:sapphire laser. The irradiation of materials such as aluminum, iron and silicon in controlled nitrogen and methane atmospheres leads to surface modifications (e.g. hardness improvement) and to the formation of homogeneous coatings. The evolution of the surface properties (phase formation, hardness) as well as the mass transport mechanisms during the laser irradiation have been studied in detail as a function of the experimental parameters (laser fluence, ambient gas pressure, number of laser shots) for the Excimer Laser treatments. The formation of homogeneous AlN layers after irradiation of aluminum substrates in N2 atmosphere, the synthesis of homogeneous cementite coatings after irradiation of iron substrates in CH4 atmosphere and the successful incorporation of carbon after irradiation of (100) and (111) silicon single crystals in methane gas will be reported.
Self-Limited Photo-Assisted Synthesis of Silicon Nanocrystals
Authors: Chen C. Y. , Kimura S. , Sen S. , Nozaki S. , Ono H. , Uchida K. , Morisaki H.
Published: June 2019

Anomalous growth of silicon (Si) nanocrystals (NCs) was observed during Raman scattering measurements of nanosilica SiOx (x=0.9) powder with an average diameter of 40 nm. It was found that Si NCs were formed by exposure to the laser beam. This photo-assisted synthesis is similar to the thermal synthesis of Si NCs, which forms Si NCs by thermally decomposing SiOx into Si and SiO2. However, the photo-assisted synthesis is more effective in forming Si NCs than the thermal synthesis. Even one second after irradiation of the nanosilica SiOx powder with laser, Si NCs with an average size of 5 nm were formed. The Si NC size increases with increasing the laser power and exposure time. It is interesting to observe a self-limited size for higher laser power and prolonged exposure. The photo-assisted synthesis of Si NCs is proved to be a promising technique with a wide range of applications in nanotechnology.
Radiation Induced Dissolution of UO2
Authors: Christensen Hilbert , Bjergbakke Erling
Published: June 2019

A model for the radiation induced dissolution of U02has been developed and tested using experiments described in the literature. The model makes use of computer calculations based on a reaction mechanism in which the dissolution is initiated by reactions of H202, OH and H02 radicals with the U02surface. The model gave a fair agreement between calculated and experimental results: For a dose rate of 400 rad/s the calculated yields of oxidation were 2.6 and 2.1 % - h−1· m−2after 1 and 2 h, respectively. The corresponding experimental yields were 1.9 and 1.5 % ·h−1· m−2, when corrected for dissolution without irradiation. The effect of various initial concentrations of H202has been tested, and a reasonable rate constant for the reaction of U02and H202has been found. The requirement for the rate constant is that H202begins to be effective above concentrations of 10−3 mol · dm−3in accordance with experiments by L. Johnson.
The Determination of the Solubility of Amorphous UO2 (Sn and the Mononuclear Hydrolysis Constants of Uranium(IV) at 25° C.
Authors: Bruno Jordi , Casas Ignasi , Lagerman Bo , Munoz Maria
Published: June 2019

We have measured the solubility 8f amorphous UO2in a wide pH range (2 to 10.5), in 0.5 M NaClO, at T=25 C. The species responsible for the solubility are U(OH) + and U'0H)h, with the stability constants lo:1,3=-1.1± 0.1 and log, 4 =- 5.4_.O. 2, respectively. No evidence for U(OH) is f6und up to pH=10.5. The same model explains satisfactorily the solubility data obtained by Parks and Pohl(5) at T=1000 C., in the pH range 1 to 10. The measured solubility of amorphous UO2 (s) in the pH_ýange 6 to 10.5, under reducing conditions, is rather high,. U(IV) =5 10 mols/l. This is in good agreement with previous data of Gayer and Leider(6) and Galkin and Stepanov(9).
Carbon-14 in Waste Packages for Spent Fuel in a Tuff Repository
Authors: Konynenburg R. A. Van , Smith C. F. , Culham H. W. , Smith H. D.
Published: June 2019

Carbon-14 is produced naturally by cosmic ray neutrons in the upper atmosphere. It is also produced in nuclear reactors, in amounts much smaller than the global inventory. About one-third of this is released directly to the atmosphere, and the other two-thirds remains in the spent fuel. Both the Environmental Protection Agency and the Nuclear Regulatory Commission have established limits on release of the 14C in spent fuel. This is of particular concern for the proposed repository in tuff, because of the unsaturated conditions and the consequent possibility of gaseous tansport of 14C as CO2Existing measurements and calculations of the 14C inventory in spent fuel are reviewed. The physical distribution and Wemical forms of the 14C are discussed. Available data on the release of 14C from spent fuel in aqueous solutions and in gaseous environments of air, nitrogen, and helium are reviewed. Projected N behavior in a tuff repository is described. It is concluded that 14C release measurements from spent fuel into moist air at temperatures both above and below the in situ boiling point of water as well as detailed transport calculations for the tuff geological environment will be needed to determine whether the IOCFR60 and 40CFR191 requirements can be met.
Statistical issues in Pitting Corrosion of Carbon Steel
Authors: Mcneil M. B.
Published: June 2019

A review is given of the state of understanding of the statistical distributions of pit depths and pit populations in carbon steel, with special reference to underground carbon steel structures, which appear to bear significant similarities to the alloys and conditions expected in basalt and salt high level waste repositories.

The data base on pit depth distributions is reviewed and the various phenomenological equations which have been used to rationalize laboratory and field observations are discussed and classified in terms of the statis- tical models underlying them. The present state of understanding of the physical phenomena underlying these distributions is reviewed, and sugges- tions for appropriate future research are presented.
Metal-Matrix Corrosion Studies in Canadian Shield Granitic Groundwaters
Authors: Mathew P. Mani , Krueger Paul A.
Published: June 2019

The Canadian Nuclear Fuel Waste Management program is evaluating two basic supported-shell container concepts for used-fuel immobilization.

One concept uses a compacted particulate material within the container to sunport a thin, corroston-resistant shell. The other concept, called “the metal-matrix concept”, uses a cast-in-place metal matrix for shell support.

This paper describes corrosion studies on the candidate matrix materials, lead, zinc and aluminum-7 wt.7 Si (A1-7 wt.7 Si), in the temperature range 293 to 423 K and the dissolved oxygen content range 8 to 410 mg/L. Test specimens were analyzed for general and localized corrosion and the corrosion products were identified.

The results showed that, unlike lead, zinc and Al-7 wt.% Si are susceptible to localized corrosion. Lead showed only low general corrosion rates of 2 to 83 pm/a compared with zinc (36 to 15800 μm/a) and Al-7 wt.% Si (6 to 209 μm/a) in the above temperature and oxygen content ranges. Maximum pit depths of about 0.9 mm on Al-7 wt.% Si and 0.6 mm on zinc were observed. These results indicate that, of the candidate matrix metals studied, lead has the greatest potential to retard radionuclide release from used-fuel disposal containers.
Leaching of Actinide-Doped Nuclear Waste Glass in a Tuff-Dominated System
Authors: Bazan Fernando , Rego J. , Aines R. D.
Published: June 2019

A laboratory leaching test has been performed as part of a project to evaluate the suitability of tuff rocks at Yucca Mountain, Nevada, as a site for a high-level nuclear waste repository. Glass samples were placed in water inside tuff vessels, and then the tuff vessels were placed in water inside Teflon containers. Glass-component leach rates and migration through the tuff were measured for samples of the ATM- 9actinide glass, which is a PNL 76-68 based glass doped with low levels of 99Tc, 237Np, 238U and 239Pu to simulate wastes. Disc samples of this glass were leached at 90°C for 30, 90, and 183 days inside tuff vessels using a natural groundwater (J-13 well-water) as the leachant. Some samples were held by 304L stainless steel supports to evaluate the effect of this metal on the release rate of glass constituents. At the end of each leaching interval, the J-13 water present inside and outside the rock vessel was analyzed for glass components in solution.

On the basis of these analyses, boron, molybdenum, and technetium appear to migrate through the rock at rates that depend on the porosity of each vessel and the time of reaction. The actinide elements (uranium, neptunium, and plutonium) were found only in the inner leachate. Sodium, silicon, and strontium are present in the rock as well as in the J-13 water, and the addition of these elements from the glass could not be determined. Normalized elemental mass loss values for boron, molybdenum, and technetium were calculated using the combined concentrations of the inner and outer leachates and assuming a negligible retention on the rock. The maximum normalized release was 2.3 g/m for technetium. Boron, molybdenum, technetium, and neptunium were released linearly with respect to each other, with boron and molybdenum released at about 85% of the technetium rate, and neptunium at 5-10% of the technetium rate. Plutonium was found at low levels in the inner leachate but was strongly sorbed on the steel and Teflon supports. Neptunium was sorbed to a lesser extent. Future analysis of the tuff vessels will determine whether the actinides were strongly sorbed on the surface of the tuff rock.
Borosilicate Glass Corrosion in the Presence of Steel Corrosion Products
Authors: Bart G. , Zwicky H.U. , Aerne E.T. , Graber TH. , Z'berg D. , Tokiwai M.
Published: June 2019

Since 1982, the corrosion behaviour of radioactive high level waste glass and of inactive glasses of the same composition has been studied in the framework of a Japanese-Swedish-Swiss (JSS) project. In addition to granite and backfilling material, the influence of steel corrosion products on glass corrosion has been investigated in experiments of up to one year at 90°C with a sample surface area to leachant volume ratio of 10 m−1. Leachant and steel corrosion products were separated from each other by filtration and analyzed by atomic absorption spectroscopy and γ-spectroscopy. The glass corrosion layers were characterized by optical and scanning electron microscopy and by secondary ion mass spectrometry.

It could be shown, that glass corrosion is enhanced by steel corrosion products. The quantity of corroded glass seems to be dependent on the surface area and not on the total amount of steel corrosion products in the system, suggesting a mechanism controlled by silica sorption on the steel corrosion product. If lead oxide is added to the system, glass corrosion is almost to- tally suppressed.
Large Scale Leach Testing of Dwpf Canister Sections
Authors: Bickford Dennis F. , Pellarin Daniel J.
Published: June 2019

A Large-Scale Leach Test Facility (LSLTF) has been constructed at the U.S. Department of Energy's Savannah River Laboratory (SRL) to perform static leach tests on 24-inch (610 mm)-diameter canister sections cut from simulated (nonradioactive) waste forms cast under reference conditions. The equipment and test procedures are designed to closely correspond to MCC-l leach test criteria.

Less than a factor of 3 increase in leachability results from combined scale-up, glass-cracking, leached surface area estimation, and surface roughness effects. This factor is dominated by surface roughness of saw cut surfaces. The factor is negligible when compared to the 200,000/1 ratio of glass sample masses.

The MCC-l, and other small-scale leach tests have been valuable in determining the relative merits of alternative waste form compositions. However, the actual waste glass to be stored in repositories is subject to fracture, devitrification, and container/glass interactions, which are difficult to simulate on a laboratory scale. Large-scale leach tests integrate these and other possible waste form characteristics that are not represented in typical small samples.

The facility, equipment, test method and results of one year leach test- ing are discussed. These results substantiate the applicability of small scale test data which precede this work, and the use of small scale tests to simulate leaching of the Defense Waste Processing Facility's borosilicate glass product. Exceptionally good sampling statistics make the large scale data particularly well suited for verification of models of leachability rates.
Structural Changes Around Uranium at the Surface of Waste Glasses Caused by Leaching
Authors: Barrett N.T. , Antonini G.M. , Thornley F.R. , Greaves G.N. , Manara A.
Published: June 2019

Structural changes occurring at the surface of leached borosilicate glasses containing 3%wt of uranium have been investigated using fluores- cence EXAFS spectroscopy at grazing incidence. X-ray penetration depths between 30 Å and above 1000 Å are possible by varying the incidence angle of the X-ray monochromatic radiation in the vicinity of the critical angle for total external reflection, øc. This enables the surface and bulk structure to be compared. It is found that a local increase in uranium concentration takes place near the surface of the glass during the first 30 minutes of leaching time. Uranium is coordinated with oxygen and other uranium atoms. The local distribution can be modelled by a square planar island structure comprised of uranyl-type units. This structure changes with corrosion time. After 30 minutes of leaching there is evidence of partial removal of the surface layer.
Simulation of Uranium Transport with Variable Temperature and Oxidation Potential: The Computer Program Thcc
Authors: Carnahan C. L.
Published: June 2019

A simulator of reactive chemical transport has been constructed with the capabilities of treating variable temperatures and variable oxidation potentials within a single simulation. Homogeneous and heterogeneous chemical reactions are simulated at temperature-dependent equilibrium, and changes of oxidation states of multivalent elements can be simulated during transport. Chemical mass action relations for formation of complexes in the fluid phase are included explicitly within the partial differential equations of transport, and a special algorithm greatly simplifies treatment of reversible precipitation of solid phases. This approach allows direct solution of the complete set of governing equations for concentrations of all aqueous species and solids affected simultaneously by chemical and physical processes. Results of example simulations of transport, along a temperature gradient, of uranium solution species under conditions of varying pH and oxidation potential and with reversible precipitation of uraninite and coffinite are presented. The examples illustrate how inclusion of variable temperature and oxidation potential in numerical simulators can enhance understanding of the chemical mechanisms affecting migration of multivalent waste elements.
Permanent Crystal Lattice Contraction, a Primary Mechanism in Thermally Induced Alteration of Na Bentonite
Authors: Pusch Roland
Published: June 2019

The basic process in thermal transformation of smectite to hydrous mica is generally thought to be a lattice charge change caused by partial replacement of tetrahedral silica by aluminum. This is assumed to yield preferential uptake of potassium with concomitant contraction and loss of expandability of smectite aggregates as well as release of silica that migrates by diffusion from interlayer space into interaggregate voids, where it may form amorphous silica hydrogels and possibly quartz.

The latter process was investigated by use of hydrothermal tests of fully water saturated, as well as partly saturated Na-montmorillonite gels, the intention being to identify possible heat-induced changes in expandability on a molecular scale by applying electron microscopy. The gels were exposed to a temperature of 225°C for 18 days in a first test series and the microstructural patterns compared with those of non-heated material.

A clear tendency of lattice contraction was observed in the heated clay gels, particularly in the non-saturated one. The microstructure had the form of networks of virtually non-expandable, interwoven dense stacks.

A possible physical explanation of the contraction is that the heat caused instability of the interlayer water lattices, yielding dominant interatomic mass forces which caused contraction of the stacks. In connection herewith, silica was released from the smectite lattices and precipitated at the edges of the stacks, which reduced or eliminated the expandability. Minute, precipitated silicic bodies, amorphous as well as crystal-line, appeared in both clays.
Nanomechanical Quantification of Polymer Energy Absorption
Authors: Tweedie Catherine A. , Smith James F. , Van Vliet Krystyn J.
Published: June 2019

Mechanical characterization of polymeric thin films and small volume structures is critical to device development in industrial applications ranging from low-k dielectric microelectronic packaging films to engineered and natural biological substrata. Although nanoindentation has the potential to quantify mechanical properties of polymeric systems, the established analyses developed for metals and ceramics (eg, calculation of hardness and Young's modulus) do not capture key aspects of viscoelastoplastic deformation and are therefore not quantitatively applicable. Here, we present a set of complementary, nanoscale contact-based experimental approaches that together characterize specific energy absorption as a unique mechanical characteristic of polymers, and provide examples for a set of amorphous polymers.
Strain mapping on gold thin film buckling and silicon blistering
Authors: Goudeau P. , Tamura N. , Parry G. , Colin J. , Coupeau C. , Cleymand F. , Padmore H.
Published: June 2019

Stress/Strain fields associated with thin film buckling induced by compressive stresses or blistering due to the presence of gas bubbles underneath single crystal surfaces are difficult to measure owing to the microscale dimensions of these structures. In this work, we show that micro Scanning X-ray diffraction is a well suited technique for mapping the strain/stress tensor of these damaged structures.
The Effect of Microstructural Inhomogeneity on Grain Boundary Diffusion Creep
Authors: Rastogi Kanishk , Moldovan Dorel
Published: June 2019

Stress concentration at grain boundaries (GB), a phenomena arising from microstructural inhomogeneity, is an important factor in determining the mechanical properties of polycrystalline materials. In this study we use mesoscopic simulations to investigate characteristics of the deformation mechanism of grain-boundary diffusion creep (Coble creep) in a polycrystalline material. The stress distribution along the grain boundaries in a polycrystalline solid under externally applied stress is determined and the mechanism of how topological inhomogeneities introduce stress concentrations is investigated. Microstructures with inhomogeneities of various sizes and distributions are considered and their effect on the stress distribution and creep rate is quantified.
Structural Control of Lithium Fluoride Thin Films
Authors: Yazicigil O.G. , Rafik D. , Vorontsov V. , King A.H.
Published: June 2019

Polycrystalline lithium fluoride thin films have a number of existing and potential uses, but the optimization of their microstructure has not yet been addressed systematically. We have developed a means of measuring the porosity in LiF films, and a method for performing detailed electron-microscopical studies on this normally beam-sensitive material. These techniques have been applied to assess the structure of LiF films immediately after deposition from the vapor phase, and also after subsequent annealing.
Depth-Dependent Hardness Characterization by Nanoindentation using a Berkovich Indenter with a Rounded Tip
Authors: Kim Ju-Young , Read David T. , Kwon Dongil
Published: June 2019

The height difference Δhb between the ideally sharp Berkovich indenter tip and a Δhb rounded tip was measured by direct observation using atomic force microscopy (AFM). The accuracy of the indirect area function method for measuring h was confirmed. The Δhb indentation size effects (ISE) in (100) single crystal copper, (100) single crystal tungsten, and fused quartz were characterized by applying the ISE model considering the rounded tip effect. The model fits the data these materials well, even though fused quartz does not deform by dislocations. However, a very small value of the ISE characteristic length h' was obtained for fused quartz. The present h' value for (100) copper is 32% larger than a previously-measured value for polycrystalline copper. This may indicate that grain boundaries suppress the dislocation activity envisioned in the ISE model.
Influence of Gas Atmosphere on the Plasticity of Metal Thin Films
Authors: Wübben T. , Dehm G. , Arzt E.
Published: June 2019

Stresses in thin films are routinely measured by the so-called substrate curvature technique. These experiments are usually carried out in air or under a protective gas atmosphere. In this contribution we describe a new set-up capable of performing substrate curvature measurements under ultra-high vacuum conditions. The advantages are the absence of possible artifacts due to gas/film interactions, better control of gas composition, and the possibility to measure chemical effects on mechanical properties in a controlled way. We present first results that indicate an unexpected sensitivity even of polycrystalline Cu films to the gas environment.
Laser Lateral Crystallization of Thin Au and Cu Films on SiO2
Authors: Kline J.E. , Leonard J.P.
Published: June 2019

Rapid lateral solidification via excimer laser melt processing is demonstrated in 200 nm thick pure Cu and Au films, encapsulated above and below by amorphous SiO2. Mask projection irradiation is used to selectively melt lines 3 to 30 m wide in the metal films, with lateral solidification proceeding transversely from the edge to the middle of the line. Encapsulation with the SiO2 overlayer and control of the fluence are found to be crucial parameters necessary to prevent dewetting while the films are molten. Transmission electron microscopy reveals large columnar grains with twin structures and other defects typical of rapid solidification.
Thermal Expansion of Low Dielectric Constant Thin Films by High-Resolution X-Ray Reflectivity
Authors: Omote Kazuhiko , Ito Yoshiyasu
Published: June 2019

By introducing high precision sample alignment technique, repeatability of incident angle to the sample surface for x-ray reflectivity (XRR) measurement is achieved to be within 0.3 arcsec. As a result, film thickness and density are possible to be measured repeatability within 0.03% and density within 0.26%. This accuracy realized to detect very small change of thermal expansion of thin films. The coefficient of thermal expansions (CTE) for porous low-k films deposited by CVD method were measured up to 400°C. The obtained values are in the range from 40 to 80 x10-6 K-1 and they are very large compare to that of copper (16-20 x10-6 K-1).
Laser Development at Q-Peak for Remote Sensing
Authors: Moulton Peter F.
Published: June 2019

Q-Peak has been developing solid state laser and nonlinear optical systems applied to remote sensing for nearly 20 years. Our first devices were based on the use of gas-discharge lamps to optically pump the laser crystals, a standard technology that traces back to 1960 with the first laser demonstration, using ruby (Cr3+:Al2O3) as the laser material. In later and most current systems we have employed the newer and more efficient use of semiconductor diode lasers as pump sources, which has allowed operation at higher levels of performance and makes practical the deployment of satellite-based, active remote-sensing systems.
Development of an All-Fiber Coherent Laser Radar for Precision Range and Velocity Measurements
Authors: Pierrottet Diego , Amzajerdian Farzin , Peri Frank
Published: June 2019

An all-fiber coherent laser radar system capable of high-resolution range and line of sight velocity measurements is under development with a goal to aid NASA's new Space Exploration initiative for manned and robotic missions to the Moon and Mars. Precision range and velocity data are key parameters to navigating planetary landing pods to the pre-selected site and achieving autonomous safe soft-landing. By employing a combination of optical heterodyne and linear frequency modulation techniques [1-3] and utilizing state-of-the-art fiber optic technologies, highly efficient, compact and reliable laser radar suitable for operation in a space environment is being developed.

The all-fiber coherent laser radar has several important advantages over more conventional pulsed laser altimeters or range finders. One of the advantages of the coherent laser radar is its ability to directly measure the platform velocity by extracting the Doppler shift generated by the platform motion. The Doppler velocity measurement is about two orders of magnitude more accurate than the velocity estimates obtained by laser altimeters using the rate of change in range [4]. Another advantage is continuous-wave operation that allows the use of highly efficient and reliable commercial off-the-shelf fiber optic telecommunication components. This paper will describe the design and operation of this laser radar sensor and discuss its projected performance. A laboratory breadboard system has already been developed as a step toward a flight prototype. The experimental data representing the potentials of this laser radar system will be reported.