Oxygen diffusion in uranium dioxide in the temperature range of phase transitions

The structure of and oxygen diffusion in UO₂ are studied by the molecular dynamics method in the range of transition to the superionic state (melting of the oxygen sublattice) and near the melting point of UO₂. The temperature dependence of the diffusion coefficient of a doubly charged oxygen ion in UO₂ is constructed. In the crystalline state at temperatures between 1800 and 2600 K, this dependence is described by an exponential dependence with a diffusion activation energy of 2.6±0.2 eV. In the superionic state (2600–3100 K), the activation energy of diffusion of an oxygen anion decreases to 1.88±0.13 eV. In melt (3100–3600 K), the exponential dependence of the diffusion coefficient of O^2- persists but the activation energy of diffusion decreases still further, to 0.8±0.2 eV. Our experimental results agree (within the limits of experimental error) with data on oxygen diffusion in the crystalline phase obtained by other researchers.     

Self-diffusion of nematic liquid crystals in the isotropic phase near the nematic-isotropic transition

Self-diffusion coefficients of two nematic-liquid crystals, PAA and MBBA, were measured by the spin-echo technique over a temperature-range ≈30°C above the nematic-isotropic transition. For both systems, activation energies derived from such measurements agree fairly well those determined earlier from viscosity data.     

High-temperature transition of uranium dioxide to the super-ion state

A microscopic model of the high-temperature (T≈2670 K) phase transition of uranium dioxide to the super-ion state is developed. It is shown that accounting for the interaction of the point defect subsystem with the electron subsystem in the mean-field approximation (where this interaction leads to significant additional screening of the charge of some of the defects) and then calculating the configurational entropy of the point defects with allowance for the actual symmetry of the UO₂ crystalline lattice affords satisfactory agreement with the available experimental data on the degree of disorder of the anion sublattice and the behavior of the specific heat of uranium dioxide in the given temperature range. Zh. éksp. Teor. Fiz. 111, 585–599 (February 1997)     

Self-diffusion of uranium in uranium dicarbide UC2

The self-diffusion of uranium in uranium dicarbide, UC₂, was measured between 1670 and 2500 K using the α-energy degradation method with the thin layer condition and U-233 as tracer. The activation enthalpy was 5.7 ± 0.2 eV. A discontinuity was indicated at ≈ 2050 K, the transition temperature of α-UC₂ to β-UC₂.     

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

On the nature of metal-semiconductor phase transition in vanadium dioxide

The method of coercive temperature distribution function is used to prove that the metal-semiconductor temperature-induced phase transition in vanadium dioxide manifests itself as a Mott electronic transition.     

Self-diffusion of tris-naphthylbenzene near the glass transition temperature

We present a direct measurement of self-diffusion of a single-component glass-forming liquid at the glass transition temperature. Forward recoil spectrometry is used to measure the concentration profiles of deuterio and protio 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) following annealing-induced diffusion in a vapor-deposited bilayer. These experiments extend the range of measured diffusion coefficients in TNB by 6 orders of magnitude. The results indicate a decoupling of translational diffusion coefficients from viscosity or rotation. At T(g), D(T) is 400 times larger than expected from the Stokes-Einstein equation.     

Photoinduced semiconductor-metal phase transition in the surface layer of vanadium dioxide

The photoinduced semiconductor-metal phase transition occurring for a time Δt < 1 ps in the surface layer of vanadium dioxide is studied theoretically. A nonthermal mechanism of instability development is considered. An equation for the order parameter ξ of the photoinduced semiconductor-metal phase transition is derived. It is shown that the transition of the surface layer of VO₂ to the metallic state requires irradiation by a laser pulse whose energy density W exceeds a critical value W _c. The phase transition is initiated at the surface, after which the interface propagates deep into the sample. The critical energy density W _c, the velocity of propagation of the metal-semiconductor interface, the thickness z ₀, and the characteristic time Δt of formation of the metal layer are calculated. The theoretical results obtained are in agreement with the experimental data on irradiation of vanadium dioxide single crystals by high-intensity laser pulses.     

Metal-insulator phase transition and electrical switching in manganese dioxide

A complex investigation of the ac and dc electrical conductivities of the β phase of manganese dioxide has been performed in relation to its atomic structure. The change of conductivity mechanisms depending on temperature in a range of 450–25 K has been shown and the metal-insulator phase transition at low (<90 K) temperatures has been described. The specific feature of this transition by the Mott mechanism is that the metal phase exists at low temperatures, while at higher temperatures, the material manifests semiconductor properties. The low-temperature switching effect with a negative differential conductivity and an N-like current-voltage characteristic associated with the phase transition in manganese dioxide has been found.     

掺钨VO_2薄膜的电致相变特性

采用直流磁控溅射与后退火工艺相结合的方法,在掺氟SnO_2(FTO)导电玻璃基底上制备了高质量的掺钨VO_2薄膜,对薄膜的结构、表面形貌和光电特性进行测试,分析了钨掺杂对其相变性能的影响.结果表明,室温下掺钨VO_2薄膜的阈值电压为4.2 V,观察到阈值电压下约有两个数量级的电流突变.随着温度升高,相变的阈值电压降低,且电流突变幅度减小.当施加8 V电压时,分别在不同温度下测试了掺钨VO_2薄膜的透过率.温度为20和50℃时,掺钨VO_2薄膜相变前后的红外透过率差量分别为23%和27%.与未掺杂的VO_2薄膜相比,掺钨VO_2薄膜具有相变温度低、阈值电压低和电阻率小的特点,在高速光电器件中有广阔的应用前景.     

Single-particle plasmon resonance spectroscopy of phase transition in vanadium dioxide

We demonstrate thermally controlled plasmon resonance modulation of single gold nanoparticles on vanadium dioxide thin films by performing dark-field spectroscopy measurements at different temperatures. The plasmon resonance of the nanoparticles exhibits a significant blueshift in the visible range when the vanadium dioxide film undergoes its insulator-to-metal phase transition around 67 °C. More importantly, the resonance shift shows a clear hysteresis, mirroring the behavior of the vanadium dioxide film. At a fixed wavelength, the scattering intensity of Au particles also shows a hysteretic behavior decorated with an overshoot before (after) the insulator-metal (metal-insulator) phase transition of the vanadium dioxide film, suggesting that the nanoparticle is probing local variations in the phase transition.     

Role of adhesion in the metal-semiconductor phase transition in polycrystalline vanadium dioxide films

Etching of thin polycrystalline films of vanadium dioxide with hydrofluoric acid vapor has offered a possibility to reveal a significant influence of the extent of adhesion on the temperature position and shape of the hysteresis loop of the reflectivity. It has been established that, in the cases where silicon is used as a substrate, etching at room temperature is accompanied by incorporation of hydrogen into thin films of vanadium dioxide.     

Hysteresis loop construction for the metal-semiconductor phase transition in vanadium dioxide films

Thermal hysteresis of the reflectivity of vanadium dioxide films observed upon the metal-semiconductor phase transition is studied. The major hysteresis loop is assumed to form when the phase equilibrium temperature in film grains and the grain size vary and correlate with each other. Within the suggested concept of hysteresis loop formation, it is demonstrated that the major loop may be asymmetric, i.e., broadened (shifted) toward lower temperatures. Unlike hysteresis branches for VO₂ bulk single crystal, those for VO₂ films are extended along the temperature axis and may exhibit a step if the grain size distribution has several maxima. The validity of the concept is verified experimentally. It is also shown that atomic force microscopy (AFM) data for the grain size distribution can serve to determine the distribution parameters from the phase equilibrium temperatures without constructing a complete set of minor hysteresis loops, as was required before.     

Photoinduced semiconductor-metal phase transition and switching wave in the vanadium dioxide film

A switching wave has been studied in the VO₂ film during the photoinduced semiconductor-metal phase transition. The dependences of the critical radiation intensities corresponding to the direct and reverse phase transitions on the ambient temperature have been obtained in the framework of the thermal model. The profile and velocity of the switching wave have been calculated. The calculated data are compared with the experimental data.     

铀的结构相变及力学性能的第一性原理计算

基于密度泛函理论, 分别计算了α, γ铀的晶格常数、平衡态体积、体弹模量及其导数等, 与实验和其他第一性原理计算结果符合较好; 并根据焓-压强曲线得到了两相的相变压强～111GPa. 通过体心立方结构理想拉伸强度的计算, 分析其在极端加载条件下的结构行为. 另外, 计算了小应变情况下U-Nb (6.25at.%) 的能量-应变关系, 发现对应于剪切模量c’的应变会使得该结构的能量降低, 揭示了该结构的力学不稳定性. 关键词： 铀 相变 理想强度 结构稳定性     

电触发二氧化钒纳米线发生金属-绝缘体转变的机理

二氧化钒(VO_2)是一种强关联相变材料,在341 K下发生金属-绝缘体转变.尽管对于VO_2相变的物理机理进行了大量研究,但科学家仍未形成统一认识.与热致VO_2相变相比,电触发VO_2相变应用前景更为广阔,但其机理也更为复杂.本文利用原位通电杆和超快相机技术,在透射电镜下原位观察了单晶VO_2纳米线通电时的相转变过程,记录了相变过程中对应的电压-电流值,并在毫秒尺度下捕捉到了VO_2的过渡相态.发现VO_2电致相变并非由焦耳热引起,推断其机理是载流子注入.同时观察到电子结构相变和晶体结构相变存在解耦现象,进一步支持了上述推断.将VO_2纳米线两端施加非接触式电场,观察到VO_2纳米线在电场中的极化偏移,而未观察到相变发生,该现象同样支持相变的载流子注入机理.研究表明VO_2的金属-绝缘体转变遵循电子-电子关联机理,即根据电子关联的Mott转变进行.     

Effect of hydrogenation on the metal-semiconductor phase transition in vanadium dioxide thin films

Hydrogen penetration into thin films of vanadium dioxide from aqueous solutions of mono-, di-, and tribasic alcohols is studied. It is shown that this process is reversible, has a catalytic character, and brings about a lowering of the metal-semiconductor phase transition temperature. It is established that the phase transition in a hydrogenated H_xVO₂ film is completely suppressed for x> 0.04.     

Effect of deformation on the metal-semiconductor phase transition in vanadium dioxide thin films

It has been demonstrated that a small plastic deformation of aluminum substrates with vanadium dioxide thin films deposited on the substrate surfaces is accompanied by the appearance of elastic stresses in the films. Depending on the deformation technique, the elastic stresses can have different signs and the range of the metal-semiconductor phase transition in VO₂ shifts toward higher or lower temperatures as compared to the equilibrium phase temperature.     

Caloric characteristics of PbTiO3 in the temperature range of the ferroelectric phase transition

The heat capacity and thermal expansion of the PbTiO₃ ceramic sample have been measured in the temperature range 80?C970 K. The electrocaloric and barocaloric efficiencies of lead titanate in the ferroelectric phase transition range have been investigated by analyzing the experimental data in terms of the thermodynamic theory of phase transitions, the electrical equation of state P(T, E), the Pippard equation, and the S(T, p) diagram.