烧绿石(La1-yNdy)2Zr2O7模拟固化241Am的晶体结构稳定性研究

为研究²⁴¹Am在La₂Zr₂O₇烧绿石中的固化行为及其对烧绿石晶体结构稳定性的影响,实验选用Nd作为²⁴¹Am的模拟物,采用Sol-喷雾热解法合成了(La_1-yNd_y)₂Zr₂O₇（0.0≤y≤1.0）系列样品,并借助X射线衍射和振动光谱手段对样品的晶体结构稳定性进行了研究。实验结果表明：随着Nd掺杂量的增加,O48f位置参数x_48f和I₍₁₁₁₎ /I₍₂₂₂₎均呈规律性增大,Raman谱逐渐展宽,IR谱发生蓝移,所有结果均证实用Nd不断替换La将导致烧绿石晶体结构有序化程度逐渐降低。另外,实验发现掺杂量y≈0.8是烧绿石晶体结构发生几何相变的逾渗阈值,超过该阈值有序的烧绿石结构将发生突变进而加速向无序萤石结构转变,该实验结果可为(La_1-yAm_y)2Zr2O7固溶体的结构稳定性研究提供参考。     

Light ion irradiation effects on stuffed Lu2(Ti2−xLux)O7−x/2 (x = 0, 0.4 and 0.67) structures

We have recently synthesized “stuffed” (i.e., excess Lu) Lu₂(Ti_2−xLu_x)O_7−x/2 (x = 0, 0.4 and 0.67) compounds using conventional ceramic processing. X-ray diffraction measurements indicate that stuffing more Lu³⁺ cations into the oxide structure leads eventually to an order-to-disorder (O-D) transition, from an ordered pyrochlore to a disordered fluorite crystal structure. At the maximum deviation in stoichiometry (x = 0.67), the Lu³⁺ and Ti⁴⁺ ions become completely randomized on the cation sublattices, and the oxygen “vacancies” are randomized on the anion sublattice. Samples were irradiated with 400 keV Ne²⁺ ions to fluences ranging from 1 × 10¹⁵ to 1 × 10¹⁶ ions/cm² at cryogenic temperatures (∼77 K). Ion irradiation effects in these samples were examined by using grazing incident X-ray diffraction. The results show that the ion irradiation tolerance increases with disordering extent in the non-stoichiometric Lu₂(Ti_2−xLu_x)O_7−x/2.     

Oxygen potential measurements of Am0.5Pu0.5O2−x by EMF method

The dependence of the oxygen potentials on oxygen non-stoichiometry and temperature of Am_0.5Pu_0.5O_2−x has been obtained by the electromotive force (EMF) method with the cell: (Pt) air |Zr(Ca)O_2−x| Am_0.5Pu_0.5O_2−x (Pt). The x value of Am_0.5Pu_0.5O_2−x was changed at 1333 K over 0.02 < x ? 0.25 by the coulomb titration method. The temperature dependence of the oxygen potential was also measured over the range of 1173-1333 K. It was found that the oxygen potential decreased from −80 to −360 kJ mol⁻¹ with increasing x from 0.021 to 0.22 at 1333 K and that it remained almost constant at −360 kJmol⁻¹ around x = 0.23. It was concluded that Am_0.5Pu_0.5O_2−x should be composed of the single fluorite-type phase over 0.02 < x ? 0.22 and the mixed phases of fluorite-type and (Am, Pu)₉O₁₆ at around x = 0.23.     

Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Oxygen potentials of mixed oxide fuels for fast reactors

Oxygen potentials of homogenous (Pu_0.2U_0.8)O_2−x and (Am_0.02Pu_0.30Np_0.02U_0.66)O_2−x which have been developed as fuels for fast breeder reactors were measured at temperatures of 1473-1623 K by a gas equilibrium method using an (Ar, H₂, H₂O) gas mixture. The measured oxygen potentials of (Pu_0.2U_0.8)O_2−x were about 25 kJ mol⁻¹ lower than those of (Pu_0.3U_0.7)O_2−x measured previously and were consistent with the values calculated by Besmann and Lindemer’s model. The measured oxygen potentials of (Am_0.02Pu_0.30Np_0.02U_0.66)O_2−x were slightly higher than those of MOX without minor actinides. No fuel-cladding chemical interaction is affected significantly by adding their minor actinides.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.     

Calorimetric measurements on (U,Th)O2 solid solutions

Enthalpy increments of urania - thoria solid solutions, (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ were measured by drop calorimetry in the temperature range 479 - 1805 K. Heat capacity, entropy and Gibbs energy function were computed. The heat capacity measurements were carried out also with differential scanning calorimetry in the temperature range 298 - 800 K. The heat capacity values of (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ at 298 K are 59.62, 61.02, 63.56 J K⁻¹ mol⁻¹, respectively. The results were compared with the data available in the literature. From the study, the heat capacity of (U,Th)O₂ solid solutions was shown to obey the Neumann - Kopp’s rule.     

Phasengleichgewichte in den systemen UO2-UO2,67-ThO2 und UO2 + x-NPO2

Solid-state chemical investigations have established that in the compositional range UO₂-UO_2.67-ThO₃ of the U-Th-O ternary system, the following single-phase domains exist: U₃O₈, which does not dissolve any ThO₂ in the solid state; an ordered M₄O₉ phase on the section between U₄O₉ and U₂Th₂O₉, below ≈ 1150 °C; and a phase with fluorite structure which occupies a large part of the system and which at 1250 °C is bounded by the compositions UO₂-UO_2.25 (U_0.43, ThO_0.57)O_0.25-ThO₃. The maximum O/M ratio of the “fluorite” phase is O:(U + Th) = 2.25. The highest oxidation valency of uranium is 5.30; this value falls as more thorium oxide is incorporated in the (U.Th)O_{2 + x} “fluorite” phase.     

Lattice parameters of (U, Pu, Am, Np)O2−x

The solid solutions of (U_{1−z−y’−y”}Pu_zAm_y’Np_y”)O_2−x (z = 0-1, y’ = 0-0.12, y” = 0-0.07) were investigated by X-ray diffraction measurements, and a database for the lattice parameters was updated. A model to calculate the lattice parameters was derived from the database. The radii of the ions present in the fluorite structure of (U, Pu, Am, Np)O_2−x were estimated from the lattice parameters measured in this work. The model represented the experimental data within a standard deviation of σ = ±0.025%.     

Vaporization behavior and Gibbs energy of formation of Rb2ThO3

The thermodynamic stability of rubidium thorate, Rb₂ThO₃(s), was determined from vaporization studies using the Knudsen effusion forward collection technique. Rb₂ThO₃(s) vaporized incongruently and predominantly as Rb₂ThO₃(s)=ThO₂(s) + 2Rb(g) + 1/2 O₂(g). The equilibrium constant K=p_Rb²·p_O2^1/2 was evaluated from the measurement of the effusive flux due to Rb vapor species under the oxygen potential governed by the stoichiometric loss of the chemical component Rb₂O from the thorate phase. The Gibbs energy of formation of Rb₂ThO₃ derived from the measurement and other auxiliary data could be given by the equation, Δ_fG°(Rb₂ThO₃,s)=−1794.7+0.42T ± .     

O NMR study in (Pu0.91Am0.09)O2

In order to investigate the magnetic properties of Am⁴⁺ ions in the cubic fluorite structure, we have performed ¹⁷O NMR measurements on (Pu_0.91Am_0.09)O₂. We have observed a temperature-dependent ¹⁷O NMR line broadening induced by classical dipolar hyperfine fields from the Am 5f moments. From NMR line simulations, the effective moment of the Am moments has been estimated to be μ_eff=1.38μ_B/Am. This value is comparable with μ_eff=1.32 or 1.53μ_B/Am reported for AmO₂. We have also carried out nuclear relaxation measurements for ¹⁷O nuclei. The magnetization recovery has been found to exhibit a nonexponential time dependence with an exponent β∼0.5. This result is well understood in terms of the nuclear relaxation mechanism induced by the Am 5f moment fluctuations via dipolar hyperfine fields.     

EXAFS study of the structural phase transition in the americium zirconate pyrochlore

The ²⁴¹Am₂Zr₂O₇ phase undergoes a structural transition from pyrochlore to defect fluorite driven by alpha self-irradiation. In an effort to better understand the underlying phenomena of this order-disorder transition, powder X-ray diffraction (PXRD) and X-ray absorption fine-structure (XAFS) spectroscopy experiments were conducted on two samples aged for 40 days (0.02 dpa) and 370 days (0.21 dpa), respectively. While the XAFS data support the phase transition observed by XRD, they reveal different local coordinations of americium and zirconium. The transition occurs through oxygen Frenkel and cation antisite formation. The XAFS, clearly showed that the ZrO polyhedron is stable against irradiation, probably a main factor explaining the excellent resistance to amorphization observed for americium zirconia defect fluorite structures.     

Thermal conductivity of (U,Pu,Np)O2 solid solutions

The thermal conductivities of (U_,Pu_,Np)O₂ solid solutions were studied at temperatures from 900 to 1770 K. Thermal conductivities were obtained from the thermal diffusivity measured by the laser flash method. The thermal conductivities obtained below 1400 K were analyzed with the data of (U,Pu,Am)O₂ obtained previously, assuming that the B-value was constant, and could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(z₁, z₂) = 3.583 × 10⁻¹ × z₁ + 6.317 × 10⁻² × z₂ + 1.595 × 10⁻² (m K/W) and B = 2.493 × 10⁻⁴ (m/W), where z₁ and z₂ are the contents of Am- and Np-oxides. It was found that the A-values increased linearly with increasing Np- and Am-oxide contents slightly, and the effect of Np-oxide content on A-values was smaller than that of Am-oxide content. The results obtained from the theoretical calculation based on the classical phonon transport model showed good agreement with the experimental results.     

Analysis of oxygen potential of (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x based on point defect chemistry

Stoichiometries in (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x were analyzed with the experimental data of oxygen potential based on point defect chemistry. The relationship between the deviation x of stoichiometric composition and the oxygen partial pressure P_O2 was evaluated using a Kröger-Vink diagram. The concentrations of the point defects in uranium and plutonium mixed oxide (MOX) were estimated from the measurement data of oxygen potentials as functions of temperature and P_O2. The analysis results showed that x was proportional to near the stoichiometric region of both (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x, which suggested that intrinsic ionization was the dominant defect. A model to calculate oxygen potential was derived and it represented the experimental data accurately. Further, the model estimated the thermodynamic data, and , of stoichiometric (U_0.7Pu_0.3)O_2.00 and (U_0.8Pu_0.2)O_2.00 as −552.5 kJ·mol⁻¹ and −149.7 J·mol⁻¹, and −674.0 kJ · mol^-1 and −219.4 J · mol⁻¹, respectively.     

Thermal studies on fluorite type ZryU1−yO2 solid solutions

Solid state reactions of UO₂ and ZrO₂ in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO₂ forming cubic fluorite type Zr_yU_1−yO₂ solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr_yU_1−yO_2+x, prepared in different gas streams were investigated. The lattice parameters of these solid solutions were expressed as a linear equation of x and y: a₀ (nm) = 0.54704 − 0.021x - 0.030y. The oxidation of these solid solutions for 0.1 ? y ? 0.2 resulted in cubic phase MO_2+x up to700 K and single orthorhombic zirconium substituted α-U₃O₈ phase at 1000 K. The kinetics of oxidation of Zr_yU_1−yO₂ in air for y = 0-0.35 were also studied using thermogravimetry. The specific heat capacities of Zr_yU_1−yO₂ (y = 0-0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334-860 K.     

Effects of Mg-ion implantation in α-Al2O3 and α-Al2O3:Mg crystals: Electrical conductivity and electronic structure changes

Undoped and Mg-doped α-Al₂O₃ single crystals were implanted with Mg ions, with an energy of 90 keV and a fluence of 10¹⁷ ions/cm². DC electrical measurements using the four-point probe method, between 295 and 428 K, were used to characterize the electrical conductivity of the implanted area. Measurements in this temperature range indicate that the electrical conductivity after implantation is thermally activated with an activation energy of about 0.03 eV both in undoped and in reduced Mg-doped α-Al₂O₃ crystals, whereas the activation energy in oxidized Mg-doped α-Al₂O₃ crystals remains close to that before implantation. The I-V characteristics of the latter samples reveal a blocking behavior of the electrical contacts on the implanted area in contrast to the ohmic contacts observed in α-Al₂O₃ single crystals with the c-axis perpendicular to the broad face, where the Mg ions were implanted. We conclude that the enhancement in conductivity observed in the implanted regions is related to the intrinsic defects created by the implantation, rather than to the implanted Mg ions. The relationship between the oxygen vacancy concentrations at different stages of etching and the changes in the electronic structure, the chemical bonding, and the Al³⁺(2p)/O²⁻(1s) and Mg²⁺(1s)/O²⁻(1s) relative intensities was studied by X-ray Photoemission Spectroscopy.     

Thermodynamic properties of ThxU1−xO2 (0 < x < 1) based on quantum-mechanical calculations and Monte-Carlo simulations

Th_xU_1−xO_2+y binary compositions occur in nature, uranothorianite, and as a mixed oxide nuclear fuel. As a nuclear fuel, important properties, such as the melting point, thermal conductivity, and the thermal expansion coefficient change as a function of composition. Additionally, for direct disposal of Th_xU_1−xO₂, the chemical durability changes as a function of composition, with the dissolution rate decreasing with increasing thoria content. UO₂ and ThO₂ have the same isometric structure, and the ionic radii of 8-fold coordinated U⁴⁺ and Th⁴⁺ are similar (1.14 nm and 1.19 nm, respectively). Thus, this binary is expected to form a complete solid solution. However, atomic-scale measurements or simulations of cation ordering and the associated thermodynamic properties of the Th_xU_1−xO₂ system have yet to be determined. A combination of density-functional theory, Monte-Carlo methods, and thermodynamic integration are used to calculate thermodynamic properties of the Th_xU_1−xO₂ binary (ΔH_mix, ΔG_mix, ΔS_mix, phase diagram). The Gibbs free energy of mixing (ΔG_mix) shows a miscibility gap at equilibration temperatures below 1000 K (e.g., E_exsoln = 0.13 kJ/(mol cations) at 750 K). Such a miscibility gap may indicate possible exsolution (i.e., phase separation upon cooling). A unique approach to evaluate the likelihood and kinetics of forming interfaces between U-rich and Th-rich has been chosen that compares the energy gain of forming separate phases with estimated energy losses of forming necessary interfaces. The result of such an approach is that the thermodynamic gain of phase separation does not overcome the increase in interface energy between exsolution lamellae for thin exsolution lamellae (10 Å). Lamella formation becomes energetically favorable with a reduction of the interface area and, thus, an increase in lamella thickness to >45 Å. However, this increase in lamellae thickness may be diffusion limited. Monte-Carlo simulations converge to an exsolved structure [lamellae || ] only for very low equilibration temperatures (below room temperature). In addition to the weak tendency to exsolve, there is an ordered arrangement of Th and U in the solid solution [alternating U and Th layers || {1 0 0}] that is energetically favored for the homogeneously mixed 50% Th configurations. Still, this tendency to order is so weak that ordering is seldom reached due to kinetic hindrances. The configurational entropy of mixing (ΔS_mix) is approximately equal to the point entropy at all temperatures, indicating that the system is not ordered.     

The standard molar enthalpy of formation of CdMoO4

The molar enthalpies of solution of CdMoO₄(s), CdO(s), Na₂ MoO₄(s) and NaF(s) in (10 mol HF(aq) + 4.41 mol H₂O₂(aq)) dm⁻³ have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of CdMoO₄(s) has been calculated to be Δ_fH°(298.15 K) = −(1034.3 ± 5.7) kJ mol⁻¹. This value of enthalpy of formation of CdMoO₄(s) agrees well with the estimated enthalpy of formation of this compound. There is no other report on the thermodynamic property measurements on this compound.     

Solid state synthesis of Mg-Ni ferrite and characterization by XRD and XPS

Single-phase magnesium-nickel ferrites with varying amounts of nickel and magnesium were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. A plot of lattice parameter versus composition of the ferrites (Mg_xNi_(1−x)Fe₂O₄, x?1) showed an abrupt deviation of lattice parameter linearity near MgFeO₄. The deviation was explained in terms of the distribution of Mg²⁺ in the octahedral and tetrahedral sites of the oxygen lattice. In XPS spectra, a broadening of the Mg 1s peak in Ni rich Mg-Ni ferrites from that observed in pure MgFe₂O₄, was explained by changes in the distribution of Mg²⁺ ion in tetrahedral and octahedral sites. A depth distribution of Mg in Ni_0.5Mg_0.5Fe₂O₄ showed an enrichment of Mg on surface.