Hall–Petch description of nanopolycrystalline Cu,Ni and Al strength levels and strain rate sensitivities

Strength and strain rate sensitivity measurements for nanopolycrystalline copper, nickel and aluminium materials are assessed on the basis of a Hall–Petch method of analysis. The H–P k_ε values are attributed to the need for cross-slip in the grain boundary regions. Transition of the H–P dependence to the stress for a single dislocation loop expanding against the grain boundary resistance provides an explanation of higher measurements for nanoscale aluminium material.     

Multiple antiferromagnet/ferromagnet interfaces as a probe of grain-size-dependent exchange bias in polycrystalline Co/Fe50Mn50

We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/Fe₅₀Mn₅₀. X-ray diffraction and transmission electron microscopy show that the Fe₅₀Mn₅₀ (FeMn) grain size increases with increasing FeMn thickness in the Co (30 Å)/FeMn system. Hence, in Co(30 Å)/FeMn(t_AF Å)/Co(30 Å) trilayers the two Co layers sample different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above 100 Å, where simple bilayers have a thickness-independent exchange bias, we are therefore able to deduce the influence of FeMn grain size on the exchange bias and coercivity (and their temperature dependence) simply by measuring trilayer and multilayer samples with varying FeMn thicknesses. This can be done while maintaining the (1 1 1) orientation, and with little variation in interface roughness. Increasing the average grain size from 90 to 135 Å results in a fourfold decrease in exchange bias, following an inverse grain size dependence. We interpret the results as being due to a decrease in uncompensated spin density with increasing antiferromagnet grain size, further evidence for the importance of defect-generated uncompensated spins.     

Effect of grain boundaries on the conductivity of high-purity ZrO2Y2O3 ceramics

Grain boundary conductivities are determined by complex impedance measurements (1–10⁶ Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure ceramics obtained from a commercial powder. The grain size was varied systematically in the region 0.36–55 μm. The grain boundary conductivity is strongly influenced by the grain size, impurities and cooling procedure. The grain boundary conductivity increases linearly with the grain size for small grain sizes (0.3 to 2–4 μm) and is constant for larger grain sizes. The calculated specific conductivity of the grain boundary for pure materials is about 100 times smaller than that of the bulk. The grain boundary thickness was estimated to be 5.4 nm. The activation energy of the grain boundary conductivity is 7 kJ mole^?1 higher than of the bulk.     

Relation between the microstructural properties of electrical steels and the Preisach modelling

In this paper, the relation of the microstructural properties of electrical steels and the material parameters in the dynamic Preisach model is established. Particular experimental work was performed in order to separate the influence of grain size from the crystallographic texture. First, the parameters defined in the dynamic Preisach model has been fitted using the hysteresis loss characteristics. Next, we identified and separated the effect of average grain size from the crystallographic texture on each material parameter defined in the Lorentzian–Preisach distribution function. Here, the same texture dependence was identified for all kinds of magnetisation processes, i.e. for the irreversible and reversible parts of the quasi-static magnetisation and for the AC excess loss behaviour. Finally, the Preisach distribution function is rewritten explicitly including the grain size and crystallographic texture dependence.     

具有不同颗粒尺寸的La2/3Ca1/3MnO3多晶样品阻温关系的Monte Carlo模拟

用溶胶-凝胶法制备了具有不同颗粒尺寸的La2/3Ca1/3MnO3多晶样品,并测量了该系列样品的电阻随温度的变化关系.借助Monte Carlo方法随机生成的无规电阻网络模型,对具有不同颗粒尺寸的La2/3Ca1/3MnO3多晶样品的电阻温度关系进行了模拟,模拟的结果同实验有非常好的一致性,在此基础上就颗粒边界效应对其电输运性质的影响进行了讨论.     

High-rate deformation of nanocrystalline iron and copper

Stress–strain curves are recorded during a high-speed impact and slow loading for nanocrystalline and coarse-grained iron and copper. The strain-rate sensitivity is determined as a function of the grain size and the strain. It is shown that the well-known difference between the variations of the strain-rate sensitivity of the yield strength with the grain size in fcc and bcc metals can be extended to other strain dependences: the strain-rate sensitivity of flow stresses in iron decreases with increasing strain, and that in copper increases. This difference also manifests itself in different slopes of the dependence of the strain-rate sensitivity on the grain size when the strain changes.     

Elastic moduli and internal friction of nanocrystalline Pd and PdSi as a function of temperature

Resonant ultrasound spectroscopy was used to study the elastic constants and internal friction of two nanocrystalline palladium samples over the temperature range 3–300 K. The first material, nc-Pd, had a grain size of 80–100 nm and a density 93% of that of single-crystal bulk palladium. The second material, nc-PdSi containing 0.5 at.% Si, had a grain size of 15–22 nm and a density 97% of the single-crystal value. The bulk and shear moduli were significantly reduced in the nc-Pd material from that expected based on single-crystal data, the effect being greater for the bulk modulus. The moduli of nc-PdSi were reduced 4–5% from that based on crystalline Pd. As compared to previous reports of the elastic moduli of nanocrystalline palladium (grain size 5–15 nm) the present values for the larger-grained nc-Pd are comparable, but the present values for the smaller-grained nc-PdSi are considerably higher. An internal friction peak and a modulus defect were found in the nc-Pd material, but not in the nc-PdSi material. These effects are attributed to a relaxation process at the grain boundaries. The temperature dependence of the moduli is similar to that of crystalline palladium and is strongly influenced by electronic effects.     

Grain size effect on twin density in as-deposited nanocrystalline Cu film

Here, we report the formation of twins and grain size dependence of twin density in nanocrystalline (NC) copper films fabricated by pulsed laser deposition. It is found that the percentage of grains containing twins decreases with decreasing grain size in the grain size range of 2–10?nm. Surprisingly, although the twins were formed during the deposition process without mechanical deformation, our analysis suggests that they are most likely deformation twins formed under high internal stress existing in the NC Cu films. This phenomenon may also happen in other NC metallic thin films where internal stresses are high.     

晶粒尺寸对Ba$lt;sub$gt;0.70$lt;/sub$gt;Sr$lt;sub$gt;0.30$lt;/sub$gt;TiO$lt;sub$gt;3$lt;/sub$gt;陶瓷介电性能的影响规律及机理研究

通过制备晶粒尺寸处于0.1—10 μm之间的致密Ba_0.70Sr_0.30TiO₃陶瓷,系统研究了晶粒尺寸对居里温度T_C、铁电相介电常数ε_F、峰值介电常数ε_M的影响规律,并深入分析了其内在的影响机理.研究表明：晶粒尺寸减小时,T_C刚开始基本不变,直到晶粒尺寸小到一定程度时才开始降低,此变化规律可由Buesseum的内应力模型解释;随晶粒尺寸的增加,ε_F先增加后减小,此变化规律可由Shaikh的串并联模型来解释,主要影响因素有内应力、畴、晶界;ε_M随晶粒尺寸的增加,在晶粒尺寸较小时先增加后减小,晶粒尺寸较大时略有增加,此变化规律可由弥散相变理论和串并联模型共同解释,在晶粒尺寸较小时主要影响因素为内应力、微畴和晶界,晶粒尺寸较大时主要影响因素为晶界. 关键词： 0.70Sr_0.30TiO₃陶瓷')" href="#">Ba_0.70Sr_0.30TiO₃陶瓷 介电常数 居里温度 晶粒尺寸     

相分布和晶粒尺寸对Nd2Fe14B/ɑ-Fe纳米复合永磁材料矫顽力的影响

本文采用统计平均方法研究了软、硬磁性晶粒尺寸及相分布对Nd2Fe14B/α-Fe纳米复合永磁材料矫顽力的影响。计算结果表明：对于单相纳米硬磁材料，磁体矫顽力随着硬磁性晶粒尺寸的减小而降低；对于软、硬两磁性相组成的Nd2Fe14B/a-Fe纳米复合永磁材料，两相的随机分布将导致磁体矫顽力随硬磁性晶粒尺寸的减小呈现极大值。本文的计算结果还表明当硬磁性晶粒尺寸大于软磁性晶粒的最佳尺寸时(15nm)，具有多层膜结构的Nd2Fe14B/a-Fe纳米复合永磁材料将比两相随机分布时具有更大的矫顽力。     

Grain-size effect in BaTiO3 ceramics: study by far infrared spectroscopy

The article offers comparative study of two nanocrystalline (50 and 100?nm averaged grain size) and two coarse-grain (1.2 and 10?µm averaged grain size) ceramics by means of Fourier transform infrared spectroscopy in a broad temperature range (10–570?K). Far infrared reflectivity spectra were fitted with the factorized model of the dielectric function and the evaluated dielectric function was compared with the results of low-frequency dielectric measurements. It appears that the stiffening and weakening of the overdamped soft mode is the only reason for the reduced dielectric permittivity of nanocrystalline ceramics in the paraelectric phase, but a strong grain-size dependence of the dielectric constant in the ferroelectric phase is not connected with the changes in lattice dynamics. All single-crystal symmetry changes were detected in all of the samples, but no phase-transition discontinuities was seen in nanocrystalline ceramics. A coexistence of more than one phase is suggested. Strong dependence of the Curie-Weiss temperature on the grain size, earlier revealed by others, is confirmed.     

Size-dependent grain-growth kinetics observed in nanocrystalline Fe

Measurements of grain growth in nanocrystalline Fe reveal a linear dependence of the grain size on annealing time, contradicting studies in coarser-grained materials, which find a parabolic (or power-law) dependence. When the grain size exceeds approximately 150 nm, a smooth transition from linear to nonlinear growth kinetics occurs, suggesting that the rate-controlling mechanism for grain growth depends on the grain size. The linear-stage growth rate agrees quantitatively with a model in which boundary migration is controlled by the redistribution of excess volume localized in the boundary cores.     

Influence of substrate bias voltage on the microstructure of nc-SiOx:H film

Amorphous silicon oxide containing nanocrystalline silicon grain(nc-SiO_x:H) films are prepared by a plasmaenhanced chemical vapor deposition technique at different negative substrate bias voltages.The influence of the bias voltage applied to the substrate on the microstructure is investigated.The analysis of x-ray diffraction spectra evidences the in situ growth of nanocrystalline Si.The grain size can be well controlled by varying the substrate bias voltage,and the largest size is obtained at 60 V.Fourier transform infrared spectra studies on the microstructure evolutions of the nc-SiO_x:H films suggest that the absorption peak intensities,which are related to the defect densities,can be well controlled.It can be attributed to the fact that the negative bias voltage provides a useful way to change the energies of the particles in the deposition process,which can provide sufficient driving force for the diffusion and movement for the species on the growing surface and effectively passivate the dangling bonds.Also the larger grain size and lower band gap,which will result in better photosensitivity,can also be obtained with a moderate substrate bias voltage of 60 V.     

Yield stress of nanocrystalline materials: role of grain-boundary dislocations,triple junctions and Coble creep

A theoretical model is suggested which describes the strengthening of nanocrystalline materials due to the effects of triple junctions of grain boundaries as obstacles for grain-boundary sliding. In the framework of the model, a dependence of the yield stress characterizing grain-boundary sliding on grain size and triple-junction angles is revealed. With this dependence we have found that, in as-fabricated nanocrystalline materials, the yield stress depends upon a competition between conventional dislocation slip and grain-boundary sliding. On the other hand, yield stress dependence on grain size in heat-treated nanocrystalline materials is described as that caused by a competition between conventional dislocation slip and Coble creep. Grain-size and triple-junction angle distributions are incorporated into the consideration to account for distributions of grain size and triple-junction angles, occurring in real specimens. The results of the model are compared with experimental data from as-fabricated and heat-treated nanocrystalline materials and shown to be in good agreement.     

Effects of grain size distribution on coercivity and permeability of ferromagnets

Grain size dependence of coercivity and permeability (GSDCP) theory is extended to include grain size distribution in ferromagnets. It is found that the experimental data do not agree with the GSDCP theory on the transition location of different grain size ranges (The GSDCP theory has three different grain size ranges for different magnetization processes.). Correspondingly, including the grain size distribution the GSDCP theory fits the experimental data very well. These results prove that the grain size distribution indeed affects the magnetic properties of nanocrystalline ferromagnets.     

晶化对A-Si:H膜中氢含量及键合形式的作用

本文研究了A-Si:H薄膜中氢原子总含量及硅-氢键合形式随薄膜中晶化程度的变化规律。指出,当晶粒大小为200±50?范围氢含量陡然下降,硅-氢键合由Si-H明显地向Si-H₂转化。认为,200±50?是微晶硅膜向多晶硅膜转化的一个相交点。 关键词：     

Structural phase transitions of the B1–B2 type in small-sized ionic crystals

The features of structural phase transitions that occur under high pressure are studied. The density functional theory is used to calculate the B1–B2 phase transition pressure as a function of the crystal size for small-sized alkali halide crystals. A size effect (an increase in the phase transition pressure as the crystal grain radius decreases) is revealed for the B1–B2 transitions in all halogen compounds except for lithium fluoride, for which the dependence is inverse.     

The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties

Barium strontium titanate (BST) thin films were prepared by RF magnetron sputtering. The dielectric constant-voltage curves and the hysteresis loops of BST thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the BST films exhibited size effects, i.e. a decrease in maximal polarization (P_m) and an increase in coercive electric field (E_c) with reduction in grain size. In our investigation, the dielectric constant, tunability and maximal polarization increased as the film thickness increased. Furthermore, the size dependence of the dielectric constant and tunability of Ba_0.6Sr_0.4TiO₃ thin films is determined by that of the maximal polarization and the coercive electric field.     

Microstructure and spin wave scattering in polycrystalline yttrium iron garnet

Polycrystalline ceramic YIG has been synthesized with submicron microstructure. The log-log plot of FMR linewidth versus average grain size gives a power law dependence α^-1₀ where α₀ is the average grain size. The instability threshold h_crit for increasing magnetic field vector of the microwave parallel pump shows a linear log-log dependence with average grain size.     

The resistivity of grain boundary of K-doped ruthenates in percolative conduction regime

Percolation theory has been involved to explain the temperature dependence of conductivity in the K-doped perovskite ruthenates and to estimate the resistivity of grain boundary in the percolative conduction regime. Using the two-layer simple effective medium model [A. Gupta, G.Q. Gong, G. Xiao, P.R. Duncombe, P. Lecoeur, P. Trouilloud, Y.Y. Wang, V.P. Dravis, J.Z. Sun, Phys. Rev. B 54 (1996) R15629] and assuming the scaling property of grain boundary system, we have obtained the new formula for grain boundary resistivity, which contains important factors for the grain size, boundary thickness, and boundary fractal dimension. The numerical results for the system A_0.5K_0.5RuO₃ (A=La, Y, Nd, Pr) are in very good agreement with the experiment. Importantly, it reveals that the percolative conduction plays a significant role in ceramic compounds containing polycrystalline grains and grain boundaries.