Titanate Ceramics with Pyrochlore Structure as a Matrix for Immobilization of Excess Weapons-Grade Plutonium: I. Radiation Resistance

Radiation resistance of titanate ceramics of a complex cationic composition with the pyrochlore structure, containing U, Pu, Ca, Gd, and Hf, is studied from the standpoint of its use for immobilization of excess weapons-grade plutonium. To induce radiation damage of the host matrix, isomorphic incorporation of ²³⁸Pu (t _{1 / 2} = 87.7 years) at the ceramics preparation stage was used. Radiation-induced structural transformations in the ceramics were monitored by X-ray diffraction up to the radiation doses providing full amorphization of the structure. The critical damaging (amorphization) dose for the ceramics studied is estimated to be about 120 ×10^{2 3} -events m^{- 3}. Taking into account the planned concentration of incorporated ²³⁹Pu in the ceramics at a level of 10 wt %, the lower limit of amorphization time of the actual ceramics under conditions of a waste disposal site is estimated to be about 360 years.     

Characterization of striations in polyethylene single crystals

New observations of polyethylene single crystals by electron microscopy are reported. Systematic studies of the contrast of striations and bend contours in bright and dark fields have been carried out. It is confirmed that bend contours are related to corrugations of the crystal. When rolled, these corrugations are changed into pleats. The quantity of extramaterial out of the substrate plane has been evaluated with the aid of the dynamical theory of contrast. Striations appear in the flat area and are proved to be due to local rotation and shearing of crystallographic planes. The change of contrast due to radiation damage is depicted and assigned to cross-linking in preferential planes which glide. The previous model of Bassett et al. of an initially non-flat-based pyramid-shaped crystal that coalesces to form corrugations is appropriate but the crystal lographic characteristics must be changed to fit our observations and we propose another structure.Laboratoire associé à l'Université Paul Sabatier.     

Crystalline to amorphous transformation during irradiation

Amorphization is often observed during irradiation of intermetallic compounds with energetic charged particles or neutrons. This paper discusses various mechanisms of radiation induced amorphization and also presents the results of amorphization in Al-Mn alloys.     

Radiation and chemical stability of a polyphase crystalline matrix based on synthetic murataite for incorporation of actinide wastes

The dose of X-ray amorphization of crystalline phases with murataite structure upon decay of incorporated ²⁴⁴Cm nuclide in ceramics Ca_2.5Mn₂Th_0.41Cm_0.12Ti_7.5Zr_0.5AlFeO_24.5 was determined. The average radiation dose for three samples prepared by fusion or sintering of the initial charge was 2.60 × 10¹⁸ α-decay g^?1, or 0.20 dpa (displacements per atom). Heating of the amorphized ceramics restored the crystalline phase structure. The leaching rates of Zr, Ti, Fe, and Al (MCC-1 test) from the amorphized ceramics remained essentially constant, whereas for Cm, Ca, and Mn they decreased by 1–2 orders of magnitude as compared to the initial sample. This fact was attributed to the presence of impurity phases of these elements with increased solubility in the freshly prepared sample.     

Computer simulation of amorphization phenomena in metals under irradiation

In this paper, we use a simplified model to simulate the amorphization phenomenon in metals under irradiation. The model has few parameters but can reflect the essential characteristics of the processes. Our simulation results show that the relaxation of collision cascades forms small crystal grains and the amorphization phenomenon is a result of total overlap of different grains, which depends on the irradiation temperature.     

Radiation and chemical resistance of synthetic ceramics based on ferritic garnet

Critical dose of X-ray amorphization of the garnet phase Ca_1.5Gd_0.908Cm_0.092Th_0.5ZrFe₄O₁₂, which is the base of the synthetic ceramics doped with ²⁴¹Cm (～2 wt %), was determined. Amorphization occurs at a dose of 1.6 × 10¹⁸ α-decay g^?1 or 0.17 dpa (displacements per atom). The chemical resistance of the amorphized ceramics to leaching (MCC-1 test) is comparable to that of the initial ceramics. The rates of Cm leaching from the freshly prepared and amorphized ceramics decrease with time and after storage for two weeks amount to 1.4 × 10^?3 and 5.8 × 10^?3 g m^?2 day^?1, respectively. Data on thermal restoration of the crystal structure of the ceramics are also presented.     

Synthesis and Determination of the Crystal Structure of Hydrides of Er(M,V)<Subscript>2</Subscript>, where M = Fe or Co,Compounds

We study the process of formation of the Laves phases AB₂ in Er(M, V)₂ (M = Fe or Co) systems and determine their hydrogen-sorption properties. The crystal structure of parent compounds and their saturated hydrides is analyzed by the X-ray diffraction method. It is shown that these compounds absorb hydrogen at pressures of 0.1– 0.12 MPa without amorphization. The formation of the ErFe₂ hydride is accompanied by the transformation of a cubic MgCu₂-type structure into a trigonal TbFe₂-type structure. Even an insignificant substitution of Fe with V or Co leads to an increase in the hydrogen-sorption capacity and prevents the changes in the crystal structure.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 62–66, November–December, 2004.     

Linking Interfacial Step Structure and Chemistry with Locally Enhanced Radiation‐Induced Amorphization at Oxide Heterointerfaces

Nanostructured materials and their interfaces have attracted recent interest for their functionality in a wide variety of different applications. However, the origins of these properties in several instances remain unknown. One promising aspect of nanomaterials is their role in materials design for mitigating radiation damage. In particular, engineered radiation tolerant materials would exploit the presence of internal interfaces to act as recombination centers and suppress damage accumulation. Realizing this promise, however, requires a fundamental understanding of how radiation‐induced defects interact with interfaces. Thus, studying the interfacial atomic structure and chemistry before and after irradiation is critical. In this study, we have performed transmission electron microscopy on a series of pristine and ion‐irradiated oxide interfaces to probe radiation‐induced effects. The CeO₂/SrTiO₃ interface, chosen as a model system for these studies, is characterized by differences in SrTiO₃ terminations or steps. Our salient result is that steps are centers for preferential amorphization in SrTiO₃, which we attribute to defect flow across the interface induced by non‐stoichiometry in CeO₂. The study concludes the interfacial atomic ordering in the form of steps thereby modifies the response to ion irradiation and suggests interface patterning as another parameter to functionalize radiation tolerant materials.     

Radiation and thermal effects on porous and layer structured materials as getters of radionuclides

The long term radiation and thermal effects on porous and layer structured materials that may function as getters for radionuclides have been evaluated using accelerated laboratory experiments including energetic electron, ion or neutron irradiation, as well as high-temperature thermal annealing. The materials studied include: zeolites, layered silicates (mica and smectite clays), open framework structured apatite and crystalline silicotitanate (CST) which is an important synthetic ion-exchange material for the chemical separation of high-level liquid radioactive wastes.In situ transmission electron microscopy during irradiation by energetic electrons and ions has shown that all the studied materials are susceptible to irradiation-induced amorphization. Amorphization can be induced by ionization and/or direct displacement processes. Amorphization may be preceded or accompanied with dehydration, layer spacing reduction and gas bubble formation. In the case of zeolites, CST and some layer silicates, radiation effects are significantly enhanced at higher temperatures. In fact, thermal annealing at high temperatures alone can cause complete amorphization of zeolites. Our experiments have shown that amorphization or even partial amorphization will cause a dramatic reduction (up to 95%) in ion-exchange and sorption/desorption capacities of zeolite for radionuclides, such as Cs and Sr. Because the near-field or chemical processing materials (e.g., zeolites or CST) will receive a substantial radiation dose after they have incorporated radionuclides, our results suggest that radiation effects may, in some cases, retard the release rate of sorbed or ion-exchanged radionuclides.     

砷化镓单晶中微晶和非晶的形成机制

利用高分辨电子显微镜对0.0049N和0.049N荷载Vickers压痕锈发砷化镓单晶的相转变进行了观察和研究,结果表明,在大小压痕作用下分别发生了单晶向和微晶的转变,微晶的结构由小于10nm,取向各异的纳米晶和非晶组成,在完全非晶化的结构中存在少量由几个原子组成的原子簇,在非晶和晶体的交界区能观察到许多晶体缺陷以及沿这些缺陷产生的晶格扭曲和非晶相岛,对这种非晶化现象提出了两种可能的诱发机制,高压力诱导非晶化和剪切诱导非晶化。     

Structural Effects on the Strength of Ceramics

The key features of the processes underlying the failure of ceramics are considered for a wide temperature range. The brittleness and high-temperature plasticity of ceramics are correlated with their crystal chemistry. The general issues related to the strength of ceramics are treated in terms of synergetics, which deals with the spatiotemporal ordering and self-organization in nonequilibrium systems. The strength of ceramics is shown to be governed by its structure on different scales—from atomic to macroscopic. The conclusion is drawn that, in the strict sense, strength is not a property of the material; rather it characterizes its quality.     

Structural trends and dielectric properties of Bi-based pyrochlores

The crystal structure and dielectric properties of Bi-based pyrochlore ceramics with formula Bi_1.5ZnM_1.5O₇ (M=Nb, Ta or Sb) were investigated. The three composition ceramics crystallize in the same pyrochlore structure, but the lattice parameters decrease with decreasing B-site ionic radius. Raman spectra analyses were performed to look for short-range structural differences and indicate that the Ta and Sb incorporation into B-site induce the A-site cations array to be more disordered. Ta or Sb replacing Nb lead to smaller dielectric constant and higher temperature coefficient values. The common characteristics for the three compositions are that they have rather low dielectric loss and high resistivity. The dielectric behavior with temperature for BZS and BZT samples were more flat than that of the BZN ceramics. Dielectric abnormal behavior at high temperature for all samples is typically associated with losses by conduction.     

Radiation stability of ceramics: Test cases of zirconia and spinel

The study of the effects of radiation in ceramics of potential use in electronic, space and nuclear industries appears to be a major challenge in the next decades. The collect and analysis of data dealing with the production and recovery of radiation damage in this type of materials are thus tasks of prime interest. In this article, we present a review of the main structural and chemical modifications observed in test case ceramics (yttria-stabilized zirconia and magnesium-aluminate spinel) submitted to ion bombardment and thermal treatments. We show that the stability under irradiation depends on the intrinsic properties of the materials and on irradiation parameters such as the ion energy, fluence and temperature. We also demonstrate that the recovery of damage upon annealing at elevated temperatures induces drastic physico-chemical modifications of the matrix.     

High-dose ion implantation of ceramics: benefits and limitations for tribology

Ion implantation is known to be capable of modifying the surface and near-surface physical, chemical and mechanical properties of solids pertaining to hardness and wear. This paper is concerned with such effects of ion implantation into sapphire and soda-lime-silica glass. It establishes the complex interplay between radiation damage, hardness, surface stress and, for the first time, friction. For sapphire, both the shallow indentation hardness response and the integrated near-surface stress increase with damage and exhibit maxima as the surface eventually amorphizes. For the glass, initial damage is shown to result in structural softening before rehardening at higher doses. The radiation-induced stress in the glass is a complex function of dose and seems partly linked to electronic rather than displacement processes. Some structural change also eventually occurs in the glass akin to amorphization in crystals and is accompanied by changes in hardness and surface stress. Superimposed on these patterns of behaviour are changes in the friction behaviour, part of which is ascribed to increased adhesion presumed due to implantation changing the surface affinity for water adsorption. These effects are demonstrated and discussed in the context of ion-implanted ceramics finding application as controlled friction and/or wear components in engineering applications. A number of caveats are established for such applications. Other effects such as gas bubble formation, crazing and sputtering are shown to lead to surface microstructures which can also play a deleterious role in tribological behaviour.     

三元层状碳化物Ti3SiC2的研究进展

本文综合介绍Ti3SiC2的最新研究进展.三元碳化物Ti3SiC2属于层状六方晶体结构,空间群为P63/mmC;它同时具有金属和陶瓷的优良性能,有良好的导电和导热能力,高弹性模量和低维氏显微硬度,在室温下可切削加工,在高温下能产生塑性变形,良好的高温热稳定性和优秀的抗氧化性能;应用CVD、SHS、HP/HIP等方法可制备该化合物,用HIP方法能制备高纯、致密的Ti3SiC2陶瓷;Ti3SiC2陶瓷材料自身有抵抗损伤的机理.     

X-ray diffraction effects in self-irradiation of intermetallic compounds of Cm

Self-irradiation strongly affects the structure of intermetallic compounds of Cm. An X-ray diffraction analysis revealed diverse phenomena associated with an increase or decrease in the unit cell volume of curium compounds in the course of self-irradiation, amorphization and restoration of the long-range order in the course of annealing, and disordering and ordering of the structure.     

Hydrogenation of the Laves Phases of Gd(Mn,Al)2, Tb(Mn,Al)2, and Tb(Fe,Al)2 Compounds

We study the formation of the AB₂ Laves phases in RE–M–Al (where RE is Gd or Tb and M is Fe or Mn) systems and determine their hydrogen-sorption properties. The crystal structures of the original compounds and their saturated hydrides are studied by the X-ray powder diffraction method. It is shown that these compounds absorb hydrogen at pressures of 0.1–0.12 MPa without amorphization and their hydrogen-sorption ability decreases as the amount of aluminum in the compound increases.     

Mn、Sb掺杂NBT—KBT—BT无铅压电陶瓷的研究

采用传统陶瓷制备工艺,利用XRD、SEM等测试分析方法,研究了MnCO3、Sb2O3掺杂对压电陶瓷晶体结构、表面形貌以及性能的影响。研究结果表明：所有组成均呈单一钙钛矿型固溶体特征,无其它晶相生成。掺杂陶瓷在1160℃左右烧结比较合适。MnCO3表现为典型的“受主”添加物特征。Sb2O3的掺杂对陶瓷性能的影响受多种因素共同作用,当Sb2O3的掺杂量为0.1％（质量分数）时,d33=148pC／N、tanδ=4.2％、εr=1516。MnCO3的掺杂可以促进晶粒生长,Sb2O3的掺杂使晶粒尺寸的均匀性降低。     

On amorphization as a deformation mechanism under high stresses

In this paper we review the work related to amorphization under mechanical stress. Beyond pressure, we highlight the role of deviatoric or shear stresses. We show that the most recent works make amorphization appear as a deformation mechanism in its own right, in particular under extreme conditions (shocks, deformations under high stresses, high strain-rates).