Showing papers on "Grain size published in 2005"

Intrinsic and Extrinsic Size Effects in Fine-Grained Morphotropic-Phase-Boundary Lead Zirconate Titanate Ceramics

Abstract: The processing, electromechanical properties, and microstructure of lead zirconate titanate (PZT) ceramics over the grain-size range of 0.1-10 μm were studied. Using measurements over a large temperature range (15-600 K), the relative role of extrinsic contribution (i.e., domain-wall motion) was deduced to be influenced strongly by the grain size, particularly for donor-doped PZT. Analytical transmission electron microscopy studies were conducted to investigate the trend in domain configurations with the reduction of grain size. The correlations between domain density, domain variants, domain configurations (before and after poling), spontaneous deformation, and the elastodielectric properties were qualitatively discussed, leading to new insights into the intrinsic and extrinsic effects and relevant size effects in ferroelectric polycrystalline materials.

Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina

Abstract: The real part of the permittivity (E') and the tan δ of sintered alumina (Al 2 O 3 ) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan δ is found to depend very strongly on the pore volume, purity, and grain size. e' is far less sensitive to impurities and grain size. The dependence of e' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and e' of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near-theoretical density ranges between 9.1 x 10 -5 and 2.4 x 10 -5 depending on grain size.

High Purity Anatase TiO2 Nanocrystals: Near Room-Temperature Synthesis, Grain Growth Kinetics, and Surface Hydration Chemistry

Abstract: High purity, spherical anatase nanocrystals were prepared by a modified sol−gel method. Mixing of anhydrous TiCl4 with ethanol at about 0 °C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 °C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 °C leads to an increase in grain size of the anatase nanoparticles to 32 nm. By varying the calcination time from 2 to 48 h at 300 °C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D2 − D02 = k0tme(-Ea/RT) with a time exponent m = 0.286(±9) and an activation energy of Ea = 32 ± 2 kJ·mol-1. Thermogravimetric analysis in combination with infrared an...

Comprehensive study of the resistivity of copper wires with lateral dimensions of 100 nm and smaller

Abstract: Copper wires were prepared in a silicon oxide matrix using the methods of semiconductor manufacturing and were electrically characterized. The width of the smallest structure was 40 nm and of the largest, 1000 nm; the heights were 50, 155, and 230 nm. Many samples of each size have been measured in order to perform a systematic investigation. The resistivity of the sample was extracted using the temperature coefficient of resistance. A significant increase in the resistivity was found for the small structures (roughly a factor 2 for 50-nm width). A model based on physical parameters was used in the analysis of the electrical data and very good agreement was obtained. The sensitivity of the various model parameters obtained by a best-fit procedure to the experimental data has been investigated. The impact of width and height on the resistivity, the influence of electron scattering at grain boundaries compared to surface scattering, and the impact of grain sizes and impurities will be discussed in detail.

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

Abstract: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPa{center_dot}m{sup 1/2} and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPa{center_dot}m{sup 1/2} coupled with a reduced fracture strength.

Grain refinement and superplastic flow in an aluminum alloy processed by high-pressure torsion

Abstract: Disks of an Al–3% Mg–0.2% Sc alloy were processed by high-pressure torsion (HPT) to refine the grain size to ∼0.15 μm. Inspection of the disks after processing revealed a central core region having a relatively coarse and ill-defined microstructure. The size of this core region decreased with increasing numbers of turns in HPT. Measurements showed the hardness increased with increasing applied pressure and/or increasing numbers of turns. In addition, the hardness increased with increasing distance from the center of the disk and stabilized at distances greater than ∼2–3 mm. The values of the saturation hardness in the outer regions of the disks were similar at higher applied pressures and after larger numbers of turns. This saturation hardness was ∼3× the hardness in the solution-treated condition. Within the region of hardness saturation, the microstructure was reasonably homogeneous and consisted of ultrafine grains separated by high-angle grain boundaries. Tensile testing demonstrated the occurrence of high strain rate superplasticity after HPT with elongations to failure that were similar to those obtained in samples of the same alloy processed by equal-channel angular pressing (ECAP).

Ultrahigh strength and high ductility of bulk nanocrystalline copper

Abstract: We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

Nanoporous Au: A high yield strength material

Abstract: The plastic deformation of nanoporous Au under compressive stress was studied by depth-sensing nanoindentation combined with scanning electron microscope characterization. The nanoporous Au investigated in the current study exhibits a relative density of 42%, and a spongelike morphology of interconnecting ligaments on a length scale of ∼100nm. The material is polycrystalline with a grain size on the order of 10–60nm. Microstructural characterization of residual indentation impressions reveals a localized densification via ductile (plastic) deformation under compressive stress and demonstrates the ductile behavior of Au ligaments. A mean hardness of 145(±11)MPa and a Young’s modulus of 11.1(±0.9)GPa was obtained from the analysis of the load-displacement curves. The hardness of investigated np‐Au is ∼10 times higher than the hardness predicted by scaling laws of open-cell foams thus potentially opening a door to a class of high yield strength—low-density materials.

An Analysis of the Relationship between Grain Size, Solute Content, and the Potency and Number Density of Nucleant Particles

Abstract: To be able to determine the grain size obtained from the addition of a grain refining master alloy, the relationship between grain size (d), solute content (defined by the growth restriction factor Q), and the potency and number density of nucleant particles needs to be understood. A study was undertaken on aluminium alloys where additions of TiB2 and Ti were made to eight wrought aluminum alloys covering a range of alloying elements and compositions. It was found from analysis of the data that $$d = \frac{a}{{\sqrt[3]{{pct TiB_2 }}}} + \frac{b}{Q}$$ . From consideration of the experimental data and from further analysis of previously published data, it is shown that the coefficients a and b relate to characteristics of the nucleant particles added by a grain refiner. The term a is related to the maximum density of active TiB2 nucleant particles within the melt, while b is related to their potency. By using the analysis methodology presented in this article, the performance characteristics of different master alloys were defined and the effects of Zr and Si on the poisoning of grain refinement were illustrated.

Grain size effect on the semiconductor-metal phase transition characteristics of magnetron-sputtered VO2 thin films

Abstract: Single-phase vanadium dioxide (VO2) thin films have been grown on Si3N4∕Si substrates by means of a well-controlled magnetron sputtering process. The deposited VO2 films were found to exhibit a semiconductor-to-metal transition (SMT) at ∼69°C with a resistivity change as high as 3.2 decades. A direct and clear-cut correlation is established between the SMT characteristics (both amplitude and abruptness of the transition) of the VO2 films and their crystallite size.

Mechanism for Developing the Boundary Barrier Layers of CaCu3Ti4O12

Abstract: CaCu 3 Ti 4 O 12 (CCTO) was prepared by solid-state reaction and identified by X-ray diffractometry. The evolution of the microstructure was observed by scanning electron microscopy (SEM). It was found that discontinuous grain growth developed during sintering, and large abnormal grains played a very significant role in the dielectric behavior. Cu ions segregated to the boundaries and CuO located at the triple-point sites of the abnormal large grains were observed by electron energy loss and energy-dispersive X-ray spectroscopy, respectively. Moreover, two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under the thermal etching conditions at 960°C for 72 min, which solves the contradiction related to the interpretation of the giant dielectric response between ceramic and single-crystal CCTO. Complex impedance spectroscopy was used to analyze the conductivity of ceramic CCTO, which suggests that it consists of conducting domains with two kinds of insulating barrier layers of domain and grain boundaries. The insulating domain and grain boundaries were attributed to orderly arranged dislocations and segregation of Cu ions, respectively. The conduction of CCTO was found to be related to the porosity, the grain size, and the thickness of the insulating boundary layers. For a sample sintered at 1065°C for 3 h, the anomalously low resistivity and temperature-dependent dielectric constant were due to the fact that domain boundaries were not substantially formed. The possible reasons for the development of barrier layers and the variation of the dielectric constant with the sintering time are discussed. A barrier-layer model with dielectric response based on the Maxwell-Wagner type of relaxation for ceramic CCTO is proposed. Two kinds of dielectric responses occur, depending on the microstructure: they are dominated by the domain and domain boundary and by the grain and grain boundary for large grains and fine grains, respectively.

Li6ALa2Nb2O12 (A=Ca, Sr, Ba): A New Class of Fast Lithium Ion Conductors with Garnet-Like Structure

Abstract: Garnet-like structured metal oxides with the general formula Li6ALa2Nb2O12 (A=Ca, Sr, Ba) have been prepared by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X-ray diffraction data reveal that Li6ALa2Nb2O12 compounds are isostructural with the parent compound Li5La3Nb2O12. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca-substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr- and Ba-substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (∼14% of the total resistance at 20°C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba-compound Li6BaLa2Nb2O12 shows the highest ionic conductivity of 6 × 10−6 S/cm at room temperature (22°C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca- and Sr- compounds, respectively. The ionic conductivity is comparable with that of parent Li5La3Nb2O12 and other fast lithium ion conductors known so far.

Effect of Austenite Grain Size on the Morphology and Crystallography of Lath Martensite in Low Carbon Steels

Abstract: The crystallography, microstructure and mechanical property of as-quenched martensite of Fe-0.2C-Mn(-V) alloys of which the prior austenite grain sizes are 370-2 μm were studied. The prior austenite grain, whose size is larger than 28 μm, is divided by several packets. Those packets are subdivided by blocks containing sub-blocks, each of which corresponds to the Kurdjumov-Sachs variant. When the prior austenite grain size is about 2 μm, one packet tends to grow predominantly. Each packet is divided by blocks containing sub-blocks.

Deformation Behavior of Bimodal Nanostructured 5083 Al Alloys

Abstract: Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 µm. Compression loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation associated with the occurrence of a Luders band was observed in the bimodal alloys. As the volume fraction of coarse grains was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation.

Aging response of coarse- and fine-grained β titanium alloys

Abstract: The effect of heating rate to aging temperature and β grain size on the aging behavior of three metastable β titanium alloys, TIMETAL-LCB, VT22 and Ti-15-3-3-3 (‘Ti-15-3’), was established using in situ resistivity measurements, X-ray diffraction, optical microscopy, SEM, TEM and STEM characterization. The results revealed the alloys could be divided into two classes based on their aging behavior. TIMETAL-LCB and VT-22 formed fine plate-like α at slow heating rates to the aging temperature. This behavior was determined to be due to the precipitation of isothermal ω at low temperatures, which serves as nucleation sites for α. The slow heating rate yielded the best balance of strength and ductility, particularly in alloys with a fine (∼10 μm) β grain size. At high heating rates, the formation of isothermal ω was avoided, leading to coarse, plate-like α microstructures with less desirable properties. Ti-15-3, on the other hand, exhibited β phase separation during isothermal aging rather than isothermal ω formation. Much slower cooling rates were required to form fine α laths in Ti-15-3 compared to the other two alloys. The importance of specifying heating rate and aging temperature for the industrial heat treatment of β titanium alloys was thus established.

Influence of grain sizes on the mobility of organic thin-film transistors

Abstract: A mobility model for organic thin-film transistors (OTFTs) has been considered that fully accounts for the effect of grains and grain boundaries of the organic layer. The model has been applied to a top contact pentacene OTFT. Comparison between simulation results and experimental data shows a strong dependence of mobility as a function of grain size. The field-effect-extracted mobility is not linearly related to the grain size, but presents a rather abrupt reduction for a grain size smaller than 2μm.

Continuous dynamic recrystallization in an Al–Li–Mg–Sc alloy during equal-channel angular extrusion

Abstract: An Al–Li–Mg–Sc alloy with an initial grain size of ∼60 μm was processed by equal-channel angular extrusion (ECAE) at 300 °C up to a total strain of 12. Transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) were employed to establish the mechanism of grain refinement. It was found that new ultrafine grains evolved by a strain-induced continuous process, which is termed continuous dynamic recrystallization (CDRX). At ɛ ∼ 1, a well-defined subgrain structure had developed. Upon further straining the average mis-orientation of deformation-induced boundaries increased; low-angle boundaries (LAB) gradually converted into true high-angle boundaries (≥15°) (HAB). At ɛ ∼ 4, arrays of boundaries with low and high angle mis-orientations were observed. At ɛ ∼ 12, a structure dominated by HAB with an average grain size of ∼0.9 μm was formed. This size is roughly similar to that for subgrains developed at preceding strains. It was shown that CDRX occurs homogeneously; the formation of new grains takes place both along initial boundaries and within interiors of original grains as well.