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Papers by M. Mintz

Research paper thumbnail of The interaction of water vapor and hydrogen water mixtures with a polycrystalline uranium surface

Annales UMCS, Chemistry, 2008

Research paper thumbnail of STM imaging of electrically floating islands

Surface Science, 2006

Appearances and disappearances of Gd islands grown on top of a W(1 1 0) substrate were observed i... more Appearances and disappearances of Gd islands grown on top of a W(1 1 0) substrate were observed in time scales of hours after exposing the surface to a few Langmuirs of hydrogen. The phenomenon is presented and explained in terms of (temporary) creation of electrically floating islands, due to electrical decoupling of the island and substrate by the hydrogen that diffuses into the island/substrate interface. The disappearance of such an island is explained by forming a double barrier junction consisting of two tunneling barriers in series, causing, by charging, the potential of the island to become equal to that of the tip. The island then becomes ''invisible'' and the tip follows the corrugation of the surface under the substrate. The reappearance follows hydrogen mobility that retains the electrical conductivity of the island-substrate interface.

Research paper thumbnail of Interactions of oxygen and uranium studied by combined AES, XPS and DRS techniques

Surface Science, 1989

The room-temperature kinetics of oxygen accumulation on the sputter-cleaned U-O.lwt'%Cr alloy sur... more The room-temperature kinetics of oxygen accumulation on the sputter-cleaned U-O.lwt'%Cr alloy surface were followed by combined measurements utilizing Auger electron spectroscopy (AES) and direct recoil spectrometry (DRS). The similarity between the coverage versus exposure curves as obtained by the two techniques point to the mechanism of island growth spreading over the surface. At about 20 L exposure a complete coverage of the surface is attained and a protective layer with a depth of abou: 15 A (as obtained from XPS) is formed.

Research paper thumbnail of The Interpretation of Hydrogen Isotope Effects and their Relation to Microscopic Energy Related Parameters by Simplified Statistical Thermodynamic Models

Physica Scripta, 2001

The quantum physics of light is a most fascinating field. Here I present a very personal viewpoin... more The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single-and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Research paper thumbnail of Heat-induced redistribution of surface oxide in uranium

Journal of Nuclear Materials, 1990

ABSTRACT The redistribution of oxygen and uranium metal at the vicinity of the metal-oxide interf... more ABSTRACT The redistribution of oxygen and uranium metal at the vicinity of the metal-oxide interface of native and grown oxides due to vacuum thermal annealing was studied for uranium and uranium-chromium alloy using Auger depth profiling and metallographic techniques. It was found that uranium metal is segregating out through the uranium oxide layer for annealing temperatures above 450°C. At the same time the oxide is redistributed in the metal below the oxide-metal interface in a diffusion like process. By applying a diffusion equation of a finite source, the diffusion coefficients for the process were obtained from the oxygen depth profiles measured for different annealing times. An Arrhenius like behavior was found for the diffusion coefficient between 400 and 800°C. The activation energy obtained was Ea = 15.4 ± 1.9 kcal/mole and the pre-exponential factor, D0 = 1.1 × 10−8cm2/s. An internal oxidation mechanism is proposed to explain the results.

Research paper thumbnail of The kinetics and mechanism of cerium hydride formation

Journal of Alloys and Compounds, 1993

The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the te... more The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the temperature range 0-100 °C and the pressure range 0.1-32 arm. Combined kinetic rate measurements and metallographic observations enabled the evaluation of the intrinsic kinetic parameters of the reaction, i.e. the velocities of the hydride front progression and their temperature-pressure dependence. Arrhenius temperature plots of the velocities yielded a pressure-independent apparent activation barrier of about 0.2 +0.05 eV per H atom. For each temperature an apparent power pressure dependence of about ] was displayed. Different possible mechanisms are discussed. It has been concluded that the rate-limiting step for this reaction is the penetration of hydrogen from the surface (of the hydride layer) into the subsurface region. The activation energies for this penetration process and for the desorption of chemisorbed hydrogen (from the hydride surface) were estimated to be 0.24 and 0.09 eV respectively. conditions (e.g. about 0.1 /xm min ~ for uranium [8], 2×10-3 p.m min-1 for titanium* and 4×10-:~ jzm rnin-1 for hafnium**). According to a systematic scheme proposed for kinetic studies of hydriding reactions [7], kinetic measurements performed on samples with well-defined geometries, *Extrapolated from the data of Efron et al. [9] for the linear stage of the reaction (which follows the initial parabolic stage). **Extrapolated from the data of Levitin et al. [10].

Research paper thumbnail of High-pressure studies of the TiCr1.8–H2 system Statistical thermodynamics above the critical temperature

Journal of Alloys and Compounds, 1998

Pressure-composition isotherms of the TiCr-H system were measured within the temperature range 29... more Pressure-composition isotherms of the TiCr-H system were measured within the temperature range 298-4338K and over a wide 1.8 2 pressure range up to 1000 atm H. The above temperature range is well above the critical temperature, T , of the system. Hence, partial 2 C molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H / M atomic composition ratio (with M5Ti1Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg-Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H-H interaction parameter, h, was evaluated for each isotherm. For both approximations a similar temperature dependence of h(T) was obtained, with h changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T values (derived from the h(T) parameters) C and the experimental observations.

Research paper thumbnail of Investigation of Debye temperatures in hydrides using nuclear resonant scattering of γ rays

Journal of the Less Common Metals, 1984

Nuclear resonant scattering of 7 rays from an (n, 7) source can be used for deducing the Debye te... more Nuclear resonant scattering of 7 rays from an (n, 7) source can be used for deducing the Debye temperatures 6n of certain elemental components in any chemical compound in general and in metal hydrides in particular. The scattering process is described briefly and available resonance scatterers are tabulated. The advantages and disadvantages of this method with respect to its application to metal hydrides are discussed. The information obtained so far for metal hydrides by utilizing nuclear resonances in 62Ni and 48Ti is presented. Some suggestions for utilizing the technique described here in future investigations of Debye temperatures in hydrides are proposed.

Research paper thumbnail of Mixed mechanisms controlling hydrogen-metal reactions under steady state conditions: the diffusion-interface mechanism

Journal of Alloys and Compounds, 1991

ABSTRACT Hydrogen-metal reactions (forming metal hydrides) which progress by a “contractingenvelo... more ABSTRACT Hydrogen-metal reactions (forming metal hydrides) which progress by a “contractingenvelope” morphology under steady state conditions (i.e. at a constant velocity of the reaction front) may be controlled by different rate-limiting steps. Usually, a single-step mechanism (e.g. diffusion, interface processes, etc.) is considered for the interpretation of experimental kinetic data. In the present work a more generalized mixed mechanism which involves simultaneous contributions of two microscopic processes (i.e. diffusion and interface emission) is treated. The single-step mechanisms are derived as particular extreme cases of the generalized formalism. The requirements which establish the existence of either a single or a mixed mechanism are evaluated and the experimental kinetic behaviour is discussed.

Research paper thumbnail of Effects of gas phase impurities on the topochemical-kinetic behaviour of uranium hydride development

Journal of the Less Common Metals, 1988

ABSTRACT The hydriding kinetics of bulk uranium and U-0.1wt.%Cr, in the presence of oxidizing gas... more ABSTRACT The hydriding kinetics of bulk uranium and U-0.1wt.%Cr, in the presence of oxidizing gaseous impurities (oxygen and CO), were studied by combined rate measurements and metallographic examinations of partially reacted samples. The effect of the gaseous impurity (type and concentration) was examined metallographically, and the kinetic data were discussed in relation to these examinations.Below about 100 °C the reaction of uranium with pure hydrogen consists of the following sequence of steps: 1.(i) surface nucleation;2.(ii) homogeneous growth (pitting);3.(iii) relatively fast lateral growth leading to the formation of a reaction front which penetrates into the sample at a constant rate.The effects of oxygen and CO on the hydriding kinetics were related to their abilities to block hydrogen penetration into the uranium. Thus, it was found that oxygen affects only the penetration through the oxide layer, whereas CO affects the penetration through both the oxide and hydride layers.

Research paper thumbnail of Thermodynamic study of the neptunium-hydrogen system

Journal of the Less Common Metals, 1976

Research paper thumbnail of The reaction of hydrogen with magnesium alloys and magnesium intermetallic compounds

Journal of the Less Common Metals, 1980

... l. Schematic line diagrams for X-ray diffraction patterns obtained for the two allo-tropie fo... more ... l. Schematic line diagrams for X-ray diffraction patterns obtained for the two allo-tropie forms of Mg2NiH4 : (a) the high temperature cubic structure; (b) the low tempera-ture orthorhombic structure; (c) the calculated pattern assuming a tetragonal symmetry as reported in r f. 2 ...

Research paper thumbnail of Phase transitions in Lnx(1) Ln1−x(2) H2-H3 alloy-hydrogen systems

Journal of the Less Common Metals, 1977

Thermal h.c.p.-f.c.c. phase transitions for some Ln$')Ln$?, Ha-Ha systems were studied using comb... more Thermal h.c.p.-f.c.c. phase transitions for some Ln$')Ln$?, Ha-Ha systems were studied using combined TG-DTA techniques. Transition temperatures, transition enthalpy changes and composition limits were evaluated as a function of alloy composition, i.e. of 3~. These quantities were found to be influenced not only by the mean atomic number of the alloys but also by the nature of the elements. Irreversible h.c.p. + f.c.c. transitions occurred when the corresponding enthalpy changes were less than 15 kcal (mol Hz)-'. A linear relation was obtained between differences in composition limits and reciprocal transition temperatures. This relation has been accounted for by a simplified statistical model based on the Bragg-Williams approximation.

Research paper thumbnail of Heat pretreatment-induced activation of gadolinium surfaces towards the initial precipitation of hydrides

Journal of Alloys and Compounds, 2010

... (iv), Cooling down to the ambient temperature. The reactivity of either activated or deactiva... more ... (iv), Cooling down to the ambient temperature. The reactivity of either activated or deactivated (see Section 2.4) samples was determined by reference hydriding test which has been performed by exposing the sample to 600 Pa H 2 at 370 K for 480 s. ...

Research paper thumbnail of Factors affecting the hydriding properties of CaxEu1−xNi5 (0 ⩽ x ⩽ 1) compounds

Journal of the Less Common Metals, 1984

Electron spectroscopy studies (Auger electron spectroscopy, X-ray photoelectron spectroscopy and ... more Electron spectroscopy studies (Auger electron spectroscopy, X-ray photoelectron spectroscopy and UV photoemission spectroscopy) were performed on the Haucke compounds Ca,Eu,_,Ni, (3~ = 0, 0.17 and 0.75). These compounds display widely differing hydriding characteristics for the various x values, with regard both to activation kinetics and to equilibrium properties. The analysis of the valence band structures of these compounds reveals a striking similarity. Hence the differing equilibrium properties are probably not dominated by electronic contributions. Interstitial hole size calculations indicate a slight increase in the hole radii with increasing calcium content of the compound. This may account for the trend in the stabilities of the corresponding hydrides. Surface analysis of the argon-handled asinserted samples points to the presence of very thin (i.e. a few atomic layers) passivation films on the compounds. The variation in the reactivities of the as-inserted compounds towards hydrogen (during the first activation cycle) is attributed to the variation in the permeability characteristics of these oxidized overlayers. *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides IV, Eilat, Israel, April 9-13, 1984.

Research paper thumbnail of Initial adsorption kinetics of oxygen on polycrystalline copper

Surface Science, 1987

The initial adsorption kinetics of oxygen on a sputter-cleaned surface of polycrystalline copper ... more The initial adsorption kinetics of oxygen on a sputter-cleaned surface of polycrystalline copper was studied at room-temperature utilizing combined measurements with Auger electron spectroscopy (AES) and time-of-flight spectrometry of direct recoils (TOF-DR). These combined measurements enable one to distinguish between adsorption processes occurring simultaneously on different structural arrays present on the polycrystalline surface. Modeling the surface as composed of two types of arrays, i.e. "rough" (e.g. ridged planes, defects and irradiation damaged areas) and "smooth", it is possible to obtain quantitative kinetic data regarding adsorption on these local arrays. The "rough" arrays on polycrystalline copper display kinetic behavior similar to the ridged (110) plane of copper single-crystal. The "smooth" arrays on the other hand display initial sticking probabilities higher than obtained for single-crystals. This difference has been accounted for by an "equilibrium spillover" model assuming surface migration of adsorbed oxygen from the "rough" to the "smooth" arrays.

Research paper thumbnail of Quantification of surface Li in 16.4% CuLi alloys by direct recoil and Auger analysis of element specific chemisorption complexes

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1988

Methoxyl carbon and oxygen are shown to specifically add to the lithium fraction of a CuLi surfac... more Methoxyl carbon and oxygen are shown to specifically add to the lithium fraction of a CuLi surface exposed to methanol. The lithium surface fraction isdetermined by direct recoil spectroscopic (DRS) measurement of the H, C, and O added to the surface by methanol exposure and by subsequent comparison to the amounts of C, O, and metal determined by auger electron

Research paper thumbnail of The effect of vacuum annealing on the hydriding kinetics of zirconium

Journal of Alloys and Compounds, 1993

The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 a... more The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 atm hydrogen pressure were studied using combined metallographic and kinetic measurements. Two stages were identified in the reaction kinetics: a fast initial stage in which the apparent front velocity was 70 /zm min-1 and a second stage in which the front velocity gradually decreased until it stabilized around 8 p~m min-1. During the fast stage a special topochemical development of the hydride was observed, with the hydride progressing along grain boundaries which were previously oxidized during the vacuum annealing. This relatively fast process continued until a hydride overlayer was formed on the sample surface. The spatial extent of this stage was thus of the order of the grain size (around 40 /~m). During the second stage of the hydriding the rate-controlling step was the diffusion of hydrogen through the product layer leading to a decrease in the apparent reaction rate. Hence, the main effect of the vacuum preannealing on the hydriding rate was introduced through grain boundary oxidation, affecting the initial rapid stage.

Research paper thumbnail of Elastic moduli of polycrystalline TiCr1.8 over the temperature range of 3–410K

Journal of Alloys and Compounds, 2004

The elastic moduli of polycrystalline TiCr 1.8 have been measured over the temperature range of 3... more The elastic moduli of polycrystalline TiCr 1.8 have been measured over the temperature range of 3-410 K using the technique of resonant ultrasound spectroscopy (RUS). The reported results are for the C15 Laves-phase structure. The measured moduli display a normal temperature dependence, approaching 0 K with zero slope and decreasing linearly with increasing temperature at the higher temperatures. The Debye temperature, calculated from the measured moduli, was found to be 500 K. This material can absorb considerable amounts of hydrogen, but is unstable at ambient temperatures. The TiCr 1.8-H 2 system has a relatively low critical temperature, T c , which has previously enabled thermodynamic measurements to be made above T c. An elastic energy contribution to the enthalpy of formation has been calculated from the present measurements and compared to the thermodynamic results. The comparison suggests that the electronic contribution to the enthalpy of formation is comparable in magnitude to the elastic contribution, but opposite in sign.

Research paper thumbnail of Surface spectroscopy studies of the oxidation behavior of uranium

Journal of Nuclear Materials, 1982

... Company SURFACE SPECTROSCOPY STUDIES OF THE OXIDATION BEHAVIOR OF URANIUM J. BLOCH, U. ATZMON... more ... Company SURFACE SPECTROSCOPY STUDIES OF THE OXIDATION BEHAVIOR OF URANIUM J. BLOCH, U. ATZMONY, MP DARIEL, MH MINTZ and N. SHAMIR Nuclear Research ... The specimen chamber was evacuated to a 2 X 10~8 Pa base pressure without bak-ing. ...

Research paper thumbnail of The interaction of water vapor and hydrogen water mixtures with a polycrystalline uranium surface

Annales UMCS, Chemistry, 2008

Research paper thumbnail of STM imaging of electrically floating islands

Surface Science, 2006

Appearances and disappearances of Gd islands grown on top of a W(1 1 0) substrate were observed i... more Appearances and disappearances of Gd islands grown on top of a W(1 1 0) substrate were observed in time scales of hours after exposing the surface to a few Langmuirs of hydrogen. The phenomenon is presented and explained in terms of (temporary) creation of electrically floating islands, due to electrical decoupling of the island and substrate by the hydrogen that diffuses into the island/substrate interface. The disappearance of such an island is explained by forming a double barrier junction consisting of two tunneling barriers in series, causing, by charging, the potential of the island to become equal to that of the tip. The island then becomes ''invisible'' and the tip follows the corrugation of the surface under the substrate. The reappearance follows hydrogen mobility that retains the electrical conductivity of the island-substrate interface.

Research paper thumbnail of Interactions of oxygen and uranium studied by combined AES, XPS and DRS techniques

Surface Science, 1989

The room-temperature kinetics of oxygen accumulation on the sputter-cleaned U-O.lwt'%Cr alloy sur... more The room-temperature kinetics of oxygen accumulation on the sputter-cleaned U-O.lwt'%Cr alloy surface were followed by combined measurements utilizing Auger electron spectroscopy (AES) and direct recoil spectrometry (DRS). The similarity between the coverage versus exposure curves as obtained by the two techniques point to the mechanism of island growth spreading over the surface. At about 20 L exposure a complete coverage of the surface is attained and a protective layer with a depth of abou: 15 A (as obtained from XPS) is formed.

Research paper thumbnail of The Interpretation of Hydrogen Isotope Effects and their Relation to Microscopic Energy Related Parameters by Simplified Statistical Thermodynamic Models

Physica Scripta, 2001

The quantum physics of light is a most fascinating field. Here I present a very personal viewpoin... more The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single-and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Research paper thumbnail of Heat-induced redistribution of surface oxide in uranium

Journal of Nuclear Materials, 1990

ABSTRACT The redistribution of oxygen and uranium metal at the vicinity of the metal-oxide interf... more ABSTRACT The redistribution of oxygen and uranium metal at the vicinity of the metal-oxide interface of native and grown oxides due to vacuum thermal annealing was studied for uranium and uranium-chromium alloy using Auger depth profiling and metallographic techniques. It was found that uranium metal is segregating out through the uranium oxide layer for annealing temperatures above 450°C. At the same time the oxide is redistributed in the metal below the oxide-metal interface in a diffusion like process. By applying a diffusion equation of a finite source, the diffusion coefficients for the process were obtained from the oxygen depth profiles measured for different annealing times. An Arrhenius like behavior was found for the diffusion coefficient between 400 and 800°C. The activation energy obtained was Ea = 15.4 ± 1.9 kcal/mole and the pre-exponential factor, D0 = 1.1 × 10−8cm2/s. An internal oxidation mechanism is proposed to explain the results.

Research paper thumbnail of The kinetics and mechanism of cerium hydride formation

Journal of Alloys and Compounds, 1993

The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the te... more The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the temperature range 0-100 °C and the pressure range 0.1-32 arm. Combined kinetic rate measurements and metallographic observations enabled the evaluation of the intrinsic kinetic parameters of the reaction, i.e. the velocities of the hydride front progression and their temperature-pressure dependence. Arrhenius temperature plots of the velocities yielded a pressure-independent apparent activation barrier of about 0.2 +0.05 eV per H atom. For each temperature an apparent power pressure dependence of about ] was displayed. Different possible mechanisms are discussed. It has been concluded that the rate-limiting step for this reaction is the penetration of hydrogen from the surface (of the hydride layer) into the subsurface region. The activation energies for this penetration process and for the desorption of chemisorbed hydrogen (from the hydride surface) were estimated to be 0.24 and 0.09 eV respectively. conditions (e.g. about 0.1 /xm min ~ for uranium [8], 2×10-3 p.m min-1 for titanium* and 4×10-:~ jzm rnin-1 for hafnium**). According to a systematic scheme proposed for kinetic studies of hydriding reactions [7], kinetic measurements performed on samples with well-defined geometries, *Extrapolated from the data of Efron et al. [9] for the linear stage of the reaction (which follows the initial parabolic stage). **Extrapolated from the data of Levitin et al. [10].

Research paper thumbnail of High-pressure studies of the TiCr1.8–H2 system Statistical thermodynamics above the critical temperature

Journal of Alloys and Compounds, 1998

Pressure-composition isotherms of the TiCr-H system were measured within the temperature range 29... more Pressure-composition isotherms of the TiCr-H system were measured within the temperature range 298-4338K and over a wide 1.8 2 pressure range up to 1000 atm H. The above temperature range is well above the critical temperature, T , of the system. Hence, partial 2 C molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H / M atomic composition ratio (with M5Ti1Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg-Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H-H interaction parameter, h, was evaluated for each isotherm. For both approximations a similar temperature dependence of h(T) was obtained, with h changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T values (derived from the h(T) parameters) C and the experimental observations.

Research paper thumbnail of Investigation of Debye temperatures in hydrides using nuclear resonant scattering of γ rays

Journal of the Less Common Metals, 1984

Nuclear resonant scattering of 7 rays from an (n, 7) source can be used for deducing the Debye te... more Nuclear resonant scattering of 7 rays from an (n, 7) source can be used for deducing the Debye temperatures 6n of certain elemental components in any chemical compound in general and in metal hydrides in particular. The scattering process is described briefly and available resonance scatterers are tabulated. The advantages and disadvantages of this method with respect to its application to metal hydrides are discussed. The information obtained so far for metal hydrides by utilizing nuclear resonances in 62Ni and 48Ti is presented. Some suggestions for utilizing the technique described here in future investigations of Debye temperatures in hydrides are proposed.

Research paper thumbnail of Mixed mechanisms controlling hydrogen-metal reactions under steady state conditions: the diffusion-interface mechanism

Journal of Alloys and Compounds, 1991

ABSTRACT Hydrogen-metal reactions (forming metal hydrides) which progress by a “contractingenvelo... more ABSTRACT Hydrogen-metal reactions (forming metal hydrides) which progress by a “contractingenvelope” morphology under steady state conditions (i.e. at a constant velocity of the reaction front) may be controlled by different rate-limiting steps. Usually, a single-step mechanism (e.g. diffusion, interface processes, etc.) is considered for the interpretation of experimental kinetic data. In the present work a more generalized mixed mechanism which involves simultaneous contributions of two microscopic processes (i.e. diffusion and interface emission) is treated. The single-step mechanisms are derived as particular extreme cases of the generalized formalism. The requirements which establish the existence of either a single or a mixed mechanism are evaluated and the experimental kinetic behaviour is discussed.

Research paper thumbnail of Effects of gas phase impurities on the topochemical-kinetic behaviour of uranium hydride development

Journal of the Less Common Metals, 1988

ABSTRACT The hydriding kinetics of bulk uranium and U-0.1wt.%Cr, in the presence of oxidizing gas... more ABSTRACT The hydriding kinetics of bulk uranium and U-0.1wt.%Cr, in the presence of oxidizing gaseous impurities (oxygen and CO), were studied by combined rate measurements and metallographic examinations of partially reacted samples. The effect of the gaseous impurity (type and concentration) was examined metallographically, and the kinetic data were discussed in relation to these examinations.Below about 100 °C the reaction of uranium with pure hydrogen consists of the following sequence of steps: 1.(i) surface nucleation;2.(ii) homogeneous growth (pitting);3.(iii) relatively fast lateral growth leading to the formation of a reaction front which penetrates into the sample at a constant rate.The effects of oxygen and CO on the hydriding kinetics were related to their abilities to block hydrogen penetration into the uranium. Thus, it was found that oxygen affects only the penetration through the oxide layer, whereas CO affects the penetration through both the oxide and hydride layers.

Research paper thumbnail of Thermodynamic study of the neptunium-hydrogen system

Journal of the Less Common Metals, 1976

Research paper thumbnail of The reaction of hydrogen with magnesium alloys and magnesium intermetallic compounds

Journal of the Less Common Metals, 1980

... l. Schematic line diagrams for X-ray diffraction patterns obtained for the two allo-tropie fo... more ... l. Schematic line diagrams for X-ray diffraction patterns obtained for the two allo-tropie forms of Mg2NiH4 : (a) the high temperature cubic structure; (b) the low tempera-ture orthorhombic structure; (c) the calculated pattern assuming a tetragonal symmetry as reported in r f. 2 ...

Research paper thumbnail of Phase transitions in Lnx(1) Ln1−x(2) H2-H3 alloy-hydrogen systems

Journal of the Less Common Metals, 1977

Thermal h.c.p.-f.c.c. phase transitions for some Ln$')Ln$?, Ha-Ha systems were studied using comb... more Thermal h.c.p.-f.c.c. phase transitions for some Ln$')Ln$?, Ha-Ha systems were studied using combined TG-DTA techniques. Transition temperatures, transition enthalpy changes and composition limits were evaluated as a function of alloy composition, i.e. of 3~. These quantities were found to be influenced not only by the mean atomic number of the alloys but also by the nature of the elements. Irreversible h.c.p. + f.c.c. transitions occurred when the corresponding enthalpy changes were less than 15 kcal (mol Hz)-'. A linear relation was obtained between differences in composition limits and reciprocal transition temperatures. This relation has been accounted for by a simplified statistical model based on the Bragg-Williams approximation.

Research paper thumbnail of Heat pretreatment-induced activation of gadolinium surfaces towards the initial precipitation of hydrides

Journal of Alloys and Compounds, 2010

... (iv), Cooling down to the ambient temperature. The reactivity of either activated or deactiva... more ... (iv), Cooling down to the ambient temperature. The reactivity of either activated or deactivated (see Section 2.4) samples was determined by reference hydriding test which has been performed by exposing the sample to 600 Pa H 2 at 370 K for 480 s. ...

Research paper thumbnail of Factors affecting the hydriding properties of CaxEu1−xNi5 (0 ⩽ x ⩽ 1) compounds

Journal of the Less Common Metals, 1984

Electron spectroscopy studies (Auger electron spectroscopy, X-ray photoelectron spectroscopy and ... more Electron spectroscopy studies (Auger electron spectroscopy, X-ray photoelectron spectroscopy and UV photoemission spectroscopy) were performed on the Haucke compounds Ca,Eu,_,Ni, (3~ = 0, 0.17 and 0.75). These compounds display widely differing hydriding characteristics for the various x values, with regard both to activation kinetics and to equilibrium properties. The analysis of the valence band structures of these compounds reveals a striking similarity. Hence the differing equilibrium properties are probably not dominated by electronic contributions. Interstitial hole size calculations indicate a slight increase in the hole radii with increasing calcium content of the compound. This may account for the trend in the stabilities of the corresponding hydrides. Surface analysis of the argon-handled asinserted samples points to the presence of very thin (i.e. a few atomic layers) passivation films on the compounds. The variation in the reactivities of the as-inserted compounds towards hydrogen (during the first activation cycle) is attributed to the variation in the permeability characteristics of these oxidized overlayers. *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides IV, Eilat, Israel, April 9-13, 1984.

Research paper thumbnail of Initial adsorption kinetics of oxygen on polycrystalline copper

Surface Science, 1987

The initial adsorption kinetics of oxygen on a sputter-cleaned surface of polycrystalline copper ... more The initial adsorption kinetics of oxygen on a sputter-cleaned surface of polycrystalline copper was studied at room-temperature utilizing combined measurements with Auger electron spectroscopy (AES) and time-of-flight spectrometry of direct recoils (TOF-DR). These combined measurements enable one to distinguish between adsorption processes occurring simultaneously on different structural arrays present on the polycrystalline surface. Modeling the surface as composed of two types of arrays, i.e. "rough" (e.g. ridged planes, defects and irradiation damaged areas) and "smooth", it is possible to obtain quantitative kinetic data regarding adsorption on these local arrays. The "rough" arrays on polycrystalline copper display kinetic behavior similar to the ridged (110) plane of copper single-crystal. The "smooth" arrays on the other hand display initial sticking probabilities higher than obtained for single-crystals. This difference has been accounted for by an "equilibrium spillover" model assuming surface migration of adsorbed oxygen from the "rough" to the "smooth" arrays.

Research paper thumbnail of Quantification of surface Li in 16.4% CuLi alloys by direct recoil and Auger analysis of element specific chemisorption complexes

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1988

Methoxyl carbon and oxygen are shown to specifically add to the lithium fraction of a CuLi surfac... more Methoxyl carbon and oxygen are shown to specifically add to the lithium fraction of a CuLi surface exposed to methanol. The lithium surface fraction isdetermined by direct recoil spectroscopic (DRS) measurement of the H, C, and O added to the surface by methanol exposure and by subsequent comparison to the amounts of C, O, and metal determined by auger electron

Research paper thumbnail of The effect of vacuum annealing on the hydriding kinetics of zirconium

Journal of Alloys and Compounds, 1993

The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 a... more The effects of vacuum preannealing on the hydriding kinetics of zirconium metal at 400 °C and 1 atm hydrogen pressure were studied using combined metallographic and kinetic measurements. Two stages were identified in the reaction kinetics: a fast initial stage in which the apparent front velocity was 70 /zm min-1 and a second stage in which the front velocity gradually decreased until it stabilized around 8 p~m min-1. During the fast stage a special topochemical development of the hydride was observed, with the hydride progressing along grain boundaries which were previously oxidized during the vacuum annealing. This relatively fast process continued until a hydride overlayer was formed on the sample surface. The spatial extent of this stage was thus of the order of the grain size (around 40 /~m). During the second stage of the hydriding the rate-controlling step was the diffusion of hydrogen through the product layer leading to a decrease in the apparent reaction rate. Hence, the main effect of the vacuum preannealing on the hydriding rate was introduced through grain boundary oxidation, affecting the initial rapid stage.

Research paper thumbnail of Elastic moduli of polycrystalline TiCr1.8 over the temperature range of 3–410K

Journal of Alloys and Compounds, 2004

The elastic moduli of polycrystalline TiCr 1.8 have been measured over the temperature range of 3... more The elastic moduli of polycrystalline TiCr 1.8 have been measured over the temperature range of 3-410 K using the technique of resonant ultrasound spectroscopy (RUS). The reported results are for the C15 Laves-phase structure. The measured moduli display a normal temperature dependence, approaching 0 K with zero slope and decreasing linearly with increasing temperature at the higher temperatures. The Debye temperature, calculated from the measured moduli, was found to be 500 K. This material can absorb considerable amounts of hydrogen, but is unstable at ambient temperatures. The TiCr 1.8-H 2 system has a relatively low critical temperature, T c , which has previously enabled thermodynamic measurements to be made above T c. An elastic energy contribution to the enthalpy of formation has been calculated from the present measurements and compared to the thermodynamic results. The comparison suggests that the electronic contribution to the enthalpy of formation is comparable in magnitude to the elastic contribution, but opposite in sign.

Research paper thumbnail of Surface spectroscopy studies of the oxidation behavior of uranium

Journal of Nuclear Materials, 1982

... Company SURFACE SPECTROSCOPY STUDIES OF THE OXIDATION BEHAVIOR OF URANIUM J. BLOCH, U. ATZMON... more ... Company SURFACE SPECTROSCOPY STUDIES OF THE OXIDATION BEHAVIOR OF URANIUM J. BLOCH, U. ATZMONY, MP DARIEL, MH MINTZ and N. SHAMIR Nuclear Research ... The specimen chamber was evacuated to a 2 X 10~8 Pa base pressure without bak-ing. ...