Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi₂O₆, (M=actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, ΔH⁰_f at 298 K, for three brannerite compositions (kJ/mol): CeTi₂O₆ (−2948.8 ± 4.3), U_0.97Ti_2.03O₆ (−2977.9 ± 3.5) and ThTi₂O₆ (−3096.5 ± 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na₂O · 4MoO₃ solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (ΔH⁰_f-ox at 298 K): MO₂ + 2TiO₂=MTi₂O₆. Only UTi₂O₆ is energetically stable with respect to an oxide assemblage: U_0.97Ti_2.03O₆ (ΔH⁰_f-ox=−7.7±2.8 kJ/mol). Both CeTi₂O₆ and ThTi₂O₆ are higher in enthalpy with respect to their oxide assemblages with (ΔH⁰_f-ox=+29.4±3.6 kJ/mol) and (ΔH⁰_f-ox=+19.4±1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature.     

Irradiation effects of swift heavy ions in actinide oxides and actinide nitrides: Structure and optical properties

Actinide oxides have been used as nuclear fuels in the majority of power reactors working in the world and actinide nitrides are under investigation for the fuels of the future fast neutron fission reactors developed in Forum Generation IV. Radiation damage in actinide oxides UO₂, (U_0.92Ce_0.08)O₂, and actinide nitride UN has been characterized after irradiation with swift heavy ions. Fluences up to 3 × 10¹³ ions/cm² of heavy ions (Kr 740 Mev, Cd 1 GeV) available at the CIRIL/GANIL facility were used to simulate irradiation in reactors by fission products and by neutrons. The macroscopic effects of irradiation remains very weak compared with those seen in other ceramic oxides irradiated in the same conditions: practically no swelling can be measured and no change in colour can be observed on the irradiated part of a polished face of sintered disks. The point defects in irradiated actinide compounds have been characterized by optical absorption spectroscopy in the UV–Vis–NIR wavelength range. The absorption spectra before and after irradiation are compared, and unexpected stability of optical properties during irradiation is shown. This result confirms the low rate of formation of point defects in actinide oxides and actinide nitrides under irradiation. Actinide oxides and nitrides studied are >40% ionic, and oxidation state of the actinides seems to be stable during irradiation. The small amount of point defects produced by radiation (<10¹⁶ cm⁻²) has been identified from differences between the absorption spectrum before irradiation and the one after irradiation: point defects in oxygen or nitrogen lattices can be observed respectively in oxides and nitrides (F centres), and small amounts of U⁵⁺ would be present in all compounds.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.     

Surface modification of MgAl2O4 and oxides with heavy ions of fission fragments energy

In this paper we report on results of surface modification in several candidate materials for inert matrix fuel hosts (MgAl₂O₄, MgO and Al₂O₃) induced by (0.5–5) MeV/amu Kr, Xe and Bi ion bombardment in the fluence range of 2 × 10¹⁰–10¹² ions/cm². The size and shape of nanoscale hillock-like defects, each of which was created by the impact of a single ion, have been studied by using atomic force microscopy (AFM) technique. It was found that mean hillock height on sapphire and spinel surfaces depends linearly on the incident electronic stopping power. The hillocks are highest in MgAl₂O₄, having a lower threshold for the lattice disorder in the bulk material via relaxation of electronic excitations. As a possible reason for the hillocks formation, the plastic deformation due to the defects created by the Coulomb explosion mechanism in the target subsurface layer is suggested.     

The excitation power density effect on the Si nanocrystals photoluminescence

In the present work we have studied the photoluminescence (PL) behavior from Si nanocrystals (NCs) as a function of the excitation power density and annealing time. The NCs were produced in a SiO₂ matrix by Si implantations from room temperature (RT) up to 700 °C, followed by post-annealing in N₂ atmosphere at high temperature. With this aim we have changed the excitation power density (from 2 × 10⁻³ W/cm² up to 15 W/cm²) and the annealing time (from 10 min up to 15 h). The strong PL signal, which at 15 W/cm² is composed by a single-peak structure (650–1000 nm) centered at around 780 nm, expands up to 1200 nm showing a two-peak structure when measured at 20 × 10⁻³ W/cm². The peak structure located at the short wavelength side is kept at 780 nm, while the second peak, starting at around 900 nm, redshifts and increases its intensity with the implantation temperature and annealing time. The effect of the annealing time on the PL spectra behavior measured at low excitation power agrees by the first time with the Si NC growth according to quantum confinement effects.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

铀在北山地下水中的种态分布及溶解度分析

为了解铀酰离子在北山地下水中的吸附、扩散和迁移行为,利用地球化学计算软件PHREEQC,采用由OECD/NEA发布的最新铀的热力学数据,计算了铀在我国高放废物地质处置库重点研究区甘肃北山地下水中的种态分布,并分析了围岩中存在的方解石对铀溶解度的影响。计算结果表明,在北山地下水组成不变的前提下,在偏酸性条件下,铀主要以UO₂F⁺、UO₂SO₄、UO₂²⁺、UO₂F₂和UO₂(SO₄)₂^2-的形式存在,而在中性至弱碱性条件下,主要以 UO₂(CO₃)^4-₃、UO₂(CO₃)₂^2-、UO₂(OH)₃^-和UO₂(OH)₄^2-的形式存在。我国计划建造的高放废物处置库的设计深度为地下500～1000m,其水岩体系一般呈弱碱性。在这样的弱碱性水岩体系中,以阴离子形式存在的铀酰配合物具有较强的可移动性。当地下水的pH=7.56时,在Eh<24mV的条件下,铀主要以沥青铀矿的形式存在,而在更高的Eh条件下,则主要以UO₂²⁺与CO₃^2-和OH^-形成的阴离子配合物的形式存在。当地下水与空气接触时,O₂的存在会使Eh升高,此时铀的主要存在种态为UO₂²⁺及其各种配合物。当围岩体系中存在方解石时,在pH<8.0的条件下,铀在地下水中的溶解度会显著提高,而在更高pH条件下,方解石对铀的溶解度无明显影响。     

阳离子有序相Ce2Zr2O7.97的石墨还原制备及其结构表征

采用石墨还原法探索了CeO₂-ZrO₂二元体系在不同还原温度、还原时间条件下的物相组成及结构,借助X射线衍射、红外振动光谱等手段对还原样品进行了物相表征及结构分析。实验结果表明：通过石墨还原能在较低温度（T_red＝1050 ℃）、较短时间（t＝24 h）内合成纯相Ce₂Zr₂O_7.97,较传统先还原（95%Ar+5%H₂,T_red≥1400℃,t＞48 h）后氧化（O₂或空气气氛）工艺更简单经济;Ce₂Zr₂O_7.97相能在1050～1300 ℃、24～72 h还原条件下稳定存在并保留至室温而不发生相分解,其机制可能归因于石墨所形成的弱还原环境;Ce₂Zr₂O_7.97相拥有与烧绿石Ce₂Zr₂O₇相近的有序阳离子亚晶胞结构,但由于其氧空位（O3、O4、O11）被大部分氧离子所填充,导致该相结构对称性降低。本文提出的石墨还原法制备富氧相Ce₂Zr₂O_7.97有望成为一种较传统制备工艺更简单经济的新方法,而Ce₂Zr₂O_7.97相能在还原气氛中稳定存在,可为An₂Zr₂O_7+x固化体的存放环境提供借鉴和参考。     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Fundamental studies on extraction of actinides from spent fuels using liquefied gases – Conversion of copper into nitrate with NO2 and extraction of Nd(III) nitrate by CO2 with TBP

The nitrate conversion with liquefied NO₂ and the nitrate extraction with SC-CO₂ were demonstrated for copper powder and neodymium nitrate instead of actinide compounds. Copper contacted with NO₂ in an autoclave at 353 K changed to water-soluble compounds. By XRD analysis of the product in the atmospheric condition, the formation of copper nitrate hydrate (Cu(NO₃)₂·2.5H₂O) was confirmed. As for the extraction, neodymium nitrate loaded in an autoclave was successfully extracted with the fluid of TBP and SC-CO₂ at 15 MPa, 313 K.     

Standard molar Gibbs free energy of formation of PbO(s) over a wide temperature range from EMF measurements

The EMF of the following galvanic cells,

(render)

Kanthal,Re,Pb,PbOCSZO₂ (1 atm.),Pt

(render)

Kanthal,Re,Pb,PbOCSZO₂(1 atm.),RuO₂,Pt

were measured as a function of temperature. With O₂ (1 atm.), RuO₂ as the reference electrode, measurements were possible at low temperatures close to the melting point of Pb. Standard Gibbs energy of formation, Δ_fG⁰_mβ-PbO was calculated from the emf measurements made over a wide range of temperature (612–1111 K) and is given by the expression: Δ_fG⁰_mβ-PbO±0.10 kJ=−218.98+0.09963T. A third law treatment of the data yielded a value of −218.08 ± 0.07 kJ mol⁻¹ for the enthalpy of formation of PbO(s) at 298.15 K, Δ_fH⁰_mβ-PbO which is in excellent agreement with second law estimate of −218.07 ± 0.07 kJ mol⁻¹.     

High resolution channeling contrast microscopy and channeling analysis of SiGe quantum well structures

High resolution channeling contrast microscopy (CCM) and channeling measurements were carried out to characterize SiGe quantum well structures on micron thick graded layers (i.e. virtual substrates). The virtual substrates were grown by gas source molecular beam epitaxy at a pressure of 10⁻⁵ mbar and low pressure chemical vapor deposition at 10⁻² mbar on boron doped Si(0 0 1) substrates respectively. A homoepitaxial silicon buffer layer was grown prior to the deposition. The nominal structure is a 20 nm Si_0.75Ge_0.25 layer at the surface, followed by 10 nm pure Si, 500 nm Si_0.75Ge_0.25 and a 1000 nm thick graded SiGe (0–26%) layer. RBS was used to measure the depth profiles, and angular scans around the (1 0 0) axis were carried out to assess crystal and interface quality. CCM was used to acquire depth resolved images of micron-sized lateral inhomogenities (‘cross-hatch') present on both samples.     

Characterization of swift heavy ion tracks in CaF2 by scanning force and transmission electron microscopy

Single crystals of fluorite (CaF₂) were exposed to various swift heavy ions (Ca up to U) of energy 1–11.1 MeV per nucleon, covering a large range of electronic stopping power S_e between 4.6 and 35.5 keV/nm. The irradiated (1 1 1) cleaved surfaces were investigated by means of scanning force microscopy in tapping mode. Nanometric hillocks produced by the ion projectiles were analyzed in terms of creation efficiency E_eff, diameter and height values, and diameter–height correlation. Hillock formation appears with a low efficiency above a S_e threshold of 5 keV/nm. The mean height of these hillocks is approximately constant (1 nm) between 5 and 10 keV/nm and increases linearly with S_e above 10 keV/nm reaching 12.5 nm for the largest S_e value investigated. Similarly, the efficiency grows versus S_e achieving 100% for S_e > 13 keV/nm where each projectile produces an individual hillock. Above 13 keV/nm, the hillock height and diameter are strongly correlated. The diameter was deduced by graphical deconvolution of the scanning-tip curvature that is determined experimentally for each set of measurements. In the entire S_e regime, the mean diameter exhibits a constant value of 13 nm, which is significantly larger than 6 nm wide tracks observed by transmission electron microscopy.     

An experimental study on linear differential scattering coefficients of the radioactive compounds UO2(C2H3O2)2 · 2H2O and Th(NO3)4 · 5H2O at 60 keV

The linear differential scattering coefficients at 60 keV have been measured for UO₂(C₂H₃O₂)₂ · 2H₂O (uranyl-acetate) and Th(NO₃)₄ · 5H₂O (thorium-nitrate) radioactive compounds at seven angles ranging from 60° to 120° at intervals 10°. The obtained results have been compared with relativistic and non-relativistic theoretical values.     

Medium temperature carbon dioxide gas turbine reactor

A carbon dioxide (CO₂) gas turbine reactor with a partial pre-cooling cycle attains comparable cycle efficiencies of 45.8% at medium temperature of 650 °C and pressure of 7 MPa with a typical helium (He) gas turbine reactor of GT-MHR (47.7%) at high temperature of 850 °C. This higher efficiency is ascribed to: reduced compression work around the critical point of CO₂; and consideration of variation in CO₂ specific heat at constant pressure, C_p, with pressure and temperature into cycle configuration. Lowering temperature to 650 °C provides flexibility in choosing materials and eases maintenance through the lower diffusion leak rate of fission products from coated particle fuel by about two orders of magnitude. At medium temperature of 650 °C, less expensive corrosion resistant materials such as type 316 stainless steel are applicable and their performance in CO₂ have been proven during extensive operation in AGRs. In the previous study, the CO₂ cycle gas turbomachinery weight was estimated to be about one-fifth compared with He cycles. The proposed medium temperature CO₂ gas turbine reactor is expected to be an alternative solution to current high-temperature He gas turbine reactors.     

Low-temperature irradiation effects in lithium orthosilicates

The emission spectra of lithium orthosilicates (Li₄SiO₄₎ ceramics have been measured in the range of 1.8–5.8 eV under irradiation by 6–30 eV photons or 1–30 keV electrons at 6–300 K. The tunnel recombination phosphorescence, as well as luminescence, stimulated by 1.5–2.5 eV photons has been detected in the sample preliminarily irradiated at 6 or 80 K. The main peaks of thermally stimulated luminescence (TSL) in the irradiated ceramics have been observed at 72, 118 and 265 K. The creation spectra of the 118 K TSL peak, as well as the excitation spectrum of photostimulated luminescence (PSL) span the region of the intrinsic absorption of a lithium orthosilicate (9–30 eV). The intensity of PSL and the TSL peaks in Li₄SiO₄ ceramics prepared in hydrogen/argon atmosphere is several times lower than that in the mainly investigated Li₄SiO₄ ceramics prepared in the atmosphere of dry argon. The optical characteristics of Li₄SiO₄ are compared with the ones known for Li₂O and SiO₂. Low-temperature luminescent methods are promising for the investigation of electron–hole processes and radiation defects serving as the traps for tritium released in D–T fusion reactor blanket systems.     

The influence of calcium ions on the development of acidity in corrosion product deposits on SIMFUEL, UO2

In order to clarify the influence of groundwater constituents on the formation of corrosion products and secondary phase deposits on corroding/dissolving nuclear fuel surfaces under waste disposal conditions we have investigated the influence of Ca²⁺, present as CaCl₂. The influence of calcium ions on the anodic dissolution of SIMFUEL (doped uranium dioxide) has been characterized over the potential range 0–500 mV (vs. SCE). Through the use of X-ray photoelectron spectroscopy (XPS) the surface composition over this potential range has been determined. Ca²⁺ was found not to influence the conversion of U^IVO₂ to , but to suppress the subsequent formation of a U^VI surface species which lead to the formation of a hydrated deposit, UO₃ · yH₂O. The adsorption of Ca²⁺ on the UO₂ surface is believed to inhibit fuel dissolution either via inhibiting the stabilization of the cation precursor (UO₂(OH)₂)_ads or by blocking the O²⁻ anion transfer reaction from the fuel surface.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.