Electrical properties of plasma-sprayed yttria-stabilized zirconia films

Yttria-stabilized zirconia films, 100 to 200 m thick, were prepared by plasma spraying. The electrical properties were investigated by complex impedance spectroscopy. The results are compared with those obtained on sintered pellets prepared with the same powders used for spraying and on commercial single crystals. The ionic conductivity and the activation energies of sprayed films and single crystals are found to be very similar, and no grain-boundary effect is observed in the film complex impedance plots. These results are explained by the high density and purity of the sprayed films.     

Role of grain boundaries in determining strength and plastic deformation of yttria-stabilized tetragonal zirconia bicrystals

Mechanical properties of yttria-stabilized tetragonal zirconia (YSTZ) bicrystals under compressive loading are investigated by atomistic simulations. Previous studies on deformation of single-crystal YSTZ showed that some specific orientations promote dislocation emission, tetragonal to monoclinic phase transformation, or both. In this work, nanograins with different orientations are selectively combined to generate bicrystals with various grain boundaries (GBs). Simulation results show that regardless of orientation of nanograins, the strength of YSTZ bicrystals is higher when the GB plane is parallel to the loading direction, and in the case of [011]/\( \left[ {01\bar{1}} \right] \)-oriented YSTZ bicrystal, the strength even exceeds that of the single crystal. Independent plastic deformation of individual grains and their interactions at the GB plane are believed to be responsible for the observed increase in strength. GB plane inhibits the volume expansion of transformed monoclinic phase and therefore serves as a source of strengthening. In contrast, YSTZ bicrystal displays softer behavior when GB plane is perpendicular to the loading direction. GB plane acts as the source of softening by initiating local amorphous phase formation.     

Effect of grain size on the electrical properties of samaria-doped ceria solid electrolyte

Samaria-doped ceria (SDC) ceramic nanopowders have been synthesized by three different methods: co-precipitation, combustion, and hydrothermal. The Ce0.8Sm0.2O1.9 nanopowder with an average aggregate size of 47 nm and crystallite size of 7-11 nm prepared by the hydrothermal method has been used for further consolidation by pressureless (PLS) and spark plasma (SPS) consolidation methods at 1000-1400 degrees C to obtain dense ceramic samples with different grain sizes in the wide range of 80 to 1500 nm. The total electrical conductivities of 0.010-0.015 S/cm at 600 degrees C were obtained for the PLS-derived samples. The dense SDC nanostructured ceramics consolidated by SPS showed the very low activation energy of only 0.66 eV at temperatures of 300-400 degrees C, and the total electrical conductivity of 0.0056 S/cm. The enhancement of the grain-boundary framework quality by annealing of the nano-SDC electrolyte at 1000 degrees C for 20 hours almost doubled the average grain size. In addition the electrical conductivity was enhanced almost 40%, while keeping the activation energy unchanged.     

Field ion microscope studies of the propagation of substrate grain boundaries into an overgrowth

The roles of substrate grain boundaries and substrate surface topography in the nucleation and growth characteristics of thin and thick overgrowths were evaluated through field ion and transmission electron microscopy techniques. The results of these studies, utilizing both electrodeposited and vapor-deposited overgrowths, indicate that the substrate surface topography is generally continued in the overgrowths at thin coverages. Nucleation and growth characteristics are seen to be influenced more by surface asperities than by the existence of grain boundaries on substrate surfaces. Grain boundaries (or the interaction of grain boundaries with the substrate surface) are not observed to be sites for preferential nucleation and growth of thin films although they can act as a source for recrystallization and grain growth in thicker overgrowths (greater than 20μm).     

Study of grain boundaries influence on electrical properties of nitrides

The electronic conduction mechanism through grain boundaries in heteroepitaxial gallium nitride layers was explained by applying the model, which included three effects: thermionic emission over potential barrier, thermionic field emission through potential barrier and thermionic field emission through scattering barrier. Space charge potential barriers height at the grain boundary layer was estimated to be 80 meV from the measurement of the temperature dependence of layer resistivity. Influence of the deep traps location in the different regions of active layers of the MSM detector on the device performance was evaluated by 2D numerical simulation.     

High-temperature structure and properties of grain boundaries: long-range vs. short-range structural effects

Recent molecular-dynamics simulations of the high-temperature structure of high-energy grain boundaries in silicon and in f.c.c. metals have revealed a universal, highly confined, liquid-like structure. As a consequence, at elevated temperatures grain-boundary properties, such as the diffusivity and mobility, involve liquid-like mechanisms and are independent of the boundary misorientation, with activation energies related to the liquid state. At lower temperatures these boundaries undergo a reversible structural and dynamical transition from a confined liquid to a more or less disordered solid. Consistent with experiments, the temperature for this transition decreases with increasing grain-boundary energy, suggesting an important role of the degree of short-range grain-boundary structural disorder in key GB properties.     

Synthesis, microstructure and mechanical properties of ceria stabilized tetragonal zirconia prepared by spray drying technique

Ceria stabilized zirconia powders with ceria concentration varying from 6 to 16 mol% were synthesized using spray drying technique. Powders were characterized for their particle size distribution and specific surface area. The dense sintered ceramics fabricated using these powders were characterized for their microstructure, crystallite size and phase composition. The flexural strength, fracture toughness and microhardness of sintered ceramics were measured. High fracture toughness and flexural strength were obtained for sintered bodies with 12 mol% of CeO₂. Flexural strength and fracture toughness were dependent on CeO₂ concentration, crystallite size and phase composition of sintered bodies. Correlation of data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics. It has been demonstrated that the synthesis method is effective to prepare nanocrystalline tetragonal ceria stabilized zirconia powders with improved mechanical properties. Ce-ZrO₂ with 20 wt% alumina was also prepared with flexural strength, 1200 MPa and fracture toughness, 9.2 MPa√m.     

Molecular dynamics simulations of oxygen ion diffusion and superionic conduction in ytterbia-stabilized zirconia

Superionic conduction of oxygen ions in 10 mol% ytterbia-stabilized zirconia (YbSZ) at different temperatures is studied employing molecular dynamics simulations. Eventhough discrete hopping of one or two oxygen ions starts at about 675 K, onset of superionic conduction occurs at about 1200 K when almost all the oxygen ions participate in the hopping process. The activation energy for oxygen ion diffusion is found to be 53.25 kJ mol⁻¹. At this temperature and above, oxygen ionic conductivity exceeds 0.1 Ω⁻¹ cm⁻¹ thereby confirming superionic conduction in the YbSZ material. For 675 K < T < 1200 K, the material acts as a normal ionic conductor. The ionic conductivity values, obtained through our simulation compare well with experimental results. But activation energy for oxygen ion conduction, found from the Arrhenius plot of our simulation is 52.71 kJ mol⁻¹ which is 35% less than experimental value. Radial distribution functions, g(r) show that there is no sharp structural phase transition and no oxygen ion sub-lattice melting in YbSZ material at superionic transition. However, the reduction in, broadening and shifting of the peaks in g(r) for all ionic pairs, at higher temperatures, indicate a volume expansion of the crystal.     

Electrical properties of ion irradiated polypropylene films

The effect of high-energy (50 MeV) Li³⁺ ion beam irradiation on polypropylene (PP) film has been studied in the fluence range 2.4 × 10¹²−l.5 × 10¹⁴ ions/cm². The a.c. electrical properties of PP films were measured in the frequency range from 0.05– 100 kHz, and at temperature range between 30 and 140°C. This study indicates two peaks at 60°C and 120°C with comparatively high magnitudes. There is an exponential increase in conductivity with log of frequency and the effect is significant at higher fluences. The loss factor (tan δ) vs frequency plot suggests that PP film based capacitors may be useful below 10 kHz. The capacitance is constant over a wide temperature range up to 130°C. FTIR spectra of the PP films before and after irradiation indicate that intensity of C-H stretching vibration at 2900 cm⁻¹ is modified. The presence of many new peaks with the increase of fluence suggests the formation of alkanes and alkynes which might be responsible for the observed changes in the dielectric and electrical properties of PP films.     

Computing the mobility of grain boundaries

As current experimental and simulation methods cannot determine the mobility of flat boundaries across the large misorientation phase space, we have developed a computational method for imposing an artificial driving force on boundaries. In a molecular dynamics simulation, this allows us to go beyond the inherent timescale restrictions of the technique and induce non-negligible motion in flat boundaries of arbitrary misorientation. For different series of symmetric boundaries, we find both expected and unexpected results. In general, mobility increases as the grain boundary plane deviates from (111), but high-coincidence and low-angle boundaries represent special cases. These results agree with and enrich experimental observations.     

Effect of deformation-induced vacancies on diffusion properties of nonequilibrium grain boundaries

Technical Physics Letters - The kinetics of variation of the diffusion properties of nonequilibrium grain boundaries (GBs) related to the absorption of vacancies generated in the course of plastic...     

Effects of low- to medium-angle grain boundaries on creep properties of superalloy

The creep properties of Ni-based bicrystal specimens with low- to medium-angle grain boundaries are investigated at 1100°C and 130?MPa with two misorientation angles prepared by double-seed solidification. Experimental results show that the angle of the grain boundary with respect to the stretching direction has a strong effect on creep life. Molecular dynamics simulations show that the effect of the grain boundary on the neighbouring dislocation density is related to the grain boundary angle. A creep constitutive model with the grain boundary angle and the initial damage rate can be adopted to describe the creep damage evolution. The model is then modified according to the molecular dynamics simulation and experimental results to give more accurate predictions of creep life.     

驻留滑移带与晶界和孪晶界的交互作用

对含有退火孪晶的多晶铜进行了不同塑性应变幅下的应变疲劳实验,利用扫描电镜及其电子通道衬度技术（SEM-ECC）观察了表面滑移形貌、疲劳裂纹和位错组态,研究了驻留滑移带与晶界和孪晶界的交互作用．结果表明,在晶界附近和远离晶界处观察到位错组态分布的不均匀现象．这种不均匀性导致多晶铜中疲劳裂纹首先沿着普通大角晶界开裂,在孪晶界处由于应变相容性较好而难以产生疲劳裂纹．     

晶粒尺寸对CVD金刚石膜电学特性的影响

通过改变生长参数,采用热丝化学气相沉积(HFCVD)法制备了从10μm到90nm四种晶粒尺寸的金刚石膜,并制作了三明治结构的光电导探测器.采用原子力显微镜和拉曼光谱仪研究了薄膜的结构和表面形貌:表面粗糙度从423nm变化到15nm;晶粒越大,金刚石膜的质量越好.I-V特性测试结果表明随着晶粒尺寸的减小,金刚石膜的电阻率从1011Ω·cm减小到106Ω·cm.在5.9 keV的55Fe X射线辐照下,随着晶粒尺寸的减小,探测器的信噪比(SNR)呈减小趋势.     

Study of grain boundaries in polycrystalline Cr,Ta and W using nuclear gamma-resonance spectroscopy: Formation enthalpies of vacancy-oxygen complexes and single vacancies in grain boundaries,and dynamic properties of the grain boundary core in polycrystals of BCC metals

The formation of complexes of vacancies with oxygen atoms (complexes-VacO) in the core of grain-boundary (GB) leads to a dependency of the isomer shifts δ₁ of components-1 in NGR emission spectra of ⁵⁷Co(⁵⁷Fe) atomic probes localized in the GB core on the annealing temperature of polycrystalline metals in a technical vacuum. The formation enthalpies of complexes-VacO, Q_{cmpl, 1}, and single vacancies, Q_{Vac, 1}, in the GB core (states-1) in Cr, Ta and W polycrystals have been measured for the first time. Dynamic contributions to formation enthalpies Q_{Vac, 1} of single vacancies localized in the GB core in the state-1 and dynamic contributions to formation enthalpies Q_{cmpl, i} of complexes-VacO, which were localized in the lattice regions adjacent to GB’s (Adjacent Lattice Regions, ALR’s), states-2, and in the GB core, states-1, have been separated. The dynamic contribution to the formation enthalpy Q_Vac,1 of vacancies and Q_{cmpl, 1} of complexes-VacO in the GB core was several times smaller than the contribution to the formation enthalpy Q_{Vac, vol} of vacancies in the bulk of the crystallites because of low-frequency resonance modes of local collective vibrations of intrinsic atoms in the GB core.     

Further refinements in the energy of grain boundaries

Earlier surface dislocation analysis of a grain boundary recognized the tendency of the grain-boundary surfaces to coalesce in order to reduce surface energy. The coalescence process is described by a distribution of surface dislocations on the grain-boundary surfaces. In the present paper, previous analysis is further refined. In particular, the sum of the Burgers vectors of the surface array of grain-boundary dislocations is not equal to the Burgers vector of the grain-boundary lattice dislocation. Instead, the Burgers vector of the surface array is determined as a function of the coalescence of the grain-boundary surfaces. The conservation of Burgers vectors of dislocations is used to predict the presence of a screening array of dislocations. The screening array of dislocations is determined by minimization of the total energy of the configuration. The distortion around the boundary is relaxed by the screening array. In general, the distribution of the screening array is two dimensional. This result has been proved by the presence of a minimum energy configuration for two sets of screening arrays of dislocations situated at different distances from the boundary.