Low Lattice Strain Research Articles

Phase Structure, Electrochemical Properties and Cyclic Characteristic of a Rhombohedral-Type Single-Phase Nd2MgNi9 Hydrogen Storage Alloy

A single-phase PuNi3-type Nd2MgNi9 alloy was firstly prepared through induction melting followed by zoning annealing method, and the structural and electrochemical properties of the alloy were studied. Low-angle X-ray diffraction and Rietveld refinement analyses verify that the crystal structure of the Nd2MgNi9 alloy is the PuNi3-type super-stacking structure with a space group R-3m. It is found that the Nd2MgNi9 alloy electrode exhibits excellent electrochemical properties. The discharge capacity of the alloy electrode is 370.6 mAh g−1 and its high rate dischargeability reaches up to 49.6% at a 1440 mA g−1 discharge current density. Especially, the cyclic stability is heightened to 92.1% at the 100th charge/discharge cycle and retains 79% at the 300th charge/discharge cycle, which are significantly better than those of the PuNi3-type La2MgNi9 alloy. On the basis of the R-3m crystal structure calculation results, we found that the single-phase Nd2MgNi9 alloy has the smaller volume deformation rate ΔV/V (+1.19%) and lower lattice strain (0.3180(3)%) after hydrogenation/dehydrogenation, resulting in the weaker pulverization degree of the alloy particle and minor oxidization/corrosion of active material.

Effect of stoichiometry on the size of titanium monoxide nanoparticles produced by fragmentation

Coarse disordered and ordered titanium monoxide powders differing in composition—substoichiometric (TiO0.92), near-stoichiometric (TiO0.97 and TiO0.99), and superstoichiometric (TiO1.23)—have been disintegrated by milling. According to X-ray diffraction and scanning electron microscopy data, milling produced nanoparticles down to 20 ± 10 nm in size. The basic structure of the nanoparticles prepared from the disordered powders was identical to the parent basic structure B1. The structure of the nanoparticles prepared from the ordered powders with the C2/m structure also remained unchanged. Using the Williamson–Hall method, we assessed the effect of the stoichiometry of the starting powder on the size of the nanoparticles and found that an ordered state of near-stoichiometric titanium monoxide ensures a factor of 3 lower lattice strain in the nanoparticles.

Effect of crystal orientation and texture on fatigue crack evolution in high strength steel welds

In the present study, electron backscattered diffraction is used to analyze the fatigue crack evolution in a high strength steel weld that was loaded cyclically in the plastic regime. Three prominent regions of a fatigue crack are investigated separately: crack tip, crack trajectory and crack initiation. Taylor and Schmid factors are mapped with respect to the defined loading matrix. Possible effective mechanisms are proposed based on the local plasticity properties like lattice rotation and misorientation. The analyses of the crack tip and trajectory regions show that although the critical resolved shear stresses in some regions are low, small deformation resistance of these regions can compromise the dislocation immobility and cause local fracture. It is shown that if the crack grows transgranularly, at least one side of the crack may show low lattice rotation or strain equivalent values, which indicates the relaxation of elastic stresses after fracture. The crack initiation is determined to be dominantly controlled by transcrystalline mechanism of initiation that takes place under plastic loading conditions. It is also shown that the secondary 〈123〉{111¯} type of slip systems were the most activated under such loading conditions.

Bilayer Graphene Grown on 4H-SiC (0001) Step-Free Mesas

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

X-Ray Peak Profile Analysis of Nanostructured Hydroxyapatite and Fluorapatite

389  Abstract—In present study, X-ray peak profile analysis (XPPA) by modified Williamson-Hall (W-H) models, namely W-H-isotropic strain model (W-H-ISM), W-H-anisotropic strain model (W-H-ASM) and W-H-energy density model (W-H-EDM), was employed to estimate the microstructural parameters such as, crystallite size, lattice strain, lattice deformation stress and deformation energy density from the powder diffraction data obtained for the microwave synthesized hydroxyapatite (HA) and fluorapatite (FA) nanoparticles prepared under identical processing conditions of mixing and aging. The as-prepared powder particles were also characterized by transmission electron microscopy (TEM) method. The average crystallite size values estimated for HA and FA by XPPA were correlated to their respective transmission electron microscopy (TEM) analysis results. In addition, the estimated values of HA and FA were correlated to their in-vitro dissolution characteristics studied by ethylenediamine tetra-acetic acid (EDTA) titrimetric method. It is found that the average crystallite size estimated by W-H models is in good agreement with TEM results. The controlled in-vitro dissolution behavior of FA was found to be resulted out of its higher crystallite size, lower lattice strain and lower dislocation density compared to that of HA.

Microstructural, optical and quantum confinement effect study of mechanically synthesized ZnTe quantum dots

ZnTe quantum dots (QD) have been synthesized in a quick single-step process by mechanically alloying a stoichiometric mixture of elemental Zn and Te powders at room temperature under Ar with 1 h of milling. The detailed microstructure of these powdered QD has been characterized by both Rietveld analysis of X-ray powder diffraction data and high resolution transmission electron microscopy. The results reveal that almost monodispersed spherical QD of ∼5 nm size were synthesized after 15 h of milling. These QD all belong to the cubic (Zn blende) phase and contain different kinds of stacking faults but with low lattice strain. The UV–vis absorbance spectra of ZnTe QD depict a significant blue shift with decreasing size of QD and the band gap estimated taken from the sharp absorbance peak position is greater than that of the bulk counterpart. The band gap increases with increasing milling time up to 15 h with a continuous decrease in the size of these QD and, therefore, their optical properties can be fine tuned by varying the milling time.

ChemInform Abstract: Rechargeable Hydrogen Batteries Using Rare-Earth-Based Hydrogen Storage Alloys

Abstract Electrochemical properties of low cost MmNi 5 -based-hydrogen storage alloys (Mm ≡ mischmetal) were extensively examined. The alloy MmNi 3.5 Co 0.7 Al 0.8 showed a very long cycle life with reasonable discharge capacity (250 mA h g −1 ) and rate capability. An ingot with a very high endurance and low lattice strain was obtained under controlled conditions, including high rate cooling in the casting process for obtaining a columnar structure, no heat treatment and prevention of stoichiometric deviation. A columnar structure was formed so that the c -axis of the hexagonal structure was oriented parallel to the cooling plane. This alloy was significantly distinguished in crystal growth from a manganese-containing alloy (MmNl 3.5 Co 0.5 Al 0.3 Mn 0.4 ) which had an equiaxial structure with considerable lattice strain, which needed conventional heat treatment. Deviation from stoichiometric composition to the nickel-rich side caused a significant decrease in capacity and cycle life owing to the precipitation of AlNi 3 at grain boundaries. The decay in capacity of the MH electrode using MmNi 3.6 Co 0.7 Al 0.8 was only 10% after 2000 cycles. The cylindrical sealed cell also showed a very long cycle life (a capacity decay of 6% after 2000 cycles). The high capacity sealed cell had a 1.5–2 times higher energy density (210 W h dm −3 ; 65 W h kg −1 ) with a longer cycle life and better rate capability than the high capacity Ni-Cd battery.

Photocatalytic Activities and Crystal Structures of Titanium Dioxide by Anodization: Their Dependence upon Current Density

The crystal structure, crystallinity and surface area of anodized TiO2 were systematically investigated focusing on the current density in the anodization. Anatase phase was evolved with an increase of the current density for the anodic oxides prepared in the electrolyte of 0.1 molL � 1 sulfuric acid, and the surface area was almost constant against the current density. On the other hand, rutile phase was evolved with the same of 1.2 molL � 1 sulfuric acid, and the surface area increased with the current density. Photocatalytic degradation rates of methylene blue (MB) were normalized by surface area of TiO2, showing that the maximum value was found in the anodic oxide with approximately 60% of rutile in the fraction. The specific rate constant for MB degradation was the best in the anodic oxide with low lattice strain and the crystallite size of appropriately 40 nm. (doi:10.2320/matertrans.M2010106)

An Easy One‐step Route to Carbon‐encapsulated Magnetic Nanoparticles

An easy one‐step fast route that utilizes simple and low‐cost starting reactants for the synthesis of carbon‐encapsulated magnetic nanoparticles (CEMNPs) is presented. The synthesis process is based on the thermolysis of a NaN3/C6Cl6 mixture with the addition of a pure (elemental) metal to be encapsulated (Fe, Co and Ni). This autothermal process generates a few grams of product in a single run and is completed within 1–2 seconds. The product consists of CEMNPs with diameters between 30 and 80 nm and amorphous carbon nanoparticles. X‐ray diffraction (XRD) revealed that the encapsulated particles are crystalline and possess low lattice strain (less than 1%). The crystallinity of the carbon phase was evaluated by XRD, Raman spectroscopy and by investigating its resistance to thermal oxidation. Magnetic measurements showed that the as‐obtained CEMNPs have soft ferromagnetic properties with coercive forces ranging between 49 and 224 Gs. In addition, CEMNPs can be obtained at temperatures below the vaporization point of the metal in question.

Dielectric Constant Controlled Solvothermal Synthesis of a TiO2 Photocatalyst with Tunable Crystallinity: A Strategy for Solvent Selection

AbstractPhase‐pure anatase TiO2 nanocrystals with a tunable crystallinity have been prepared by a one‐step solvothermal method in the absence of any templates or additives. The crystallinity can be successfully controlled by varying the dielectric constant of the reaction solvent. 2‐Propanol, which has the lowest dielectric constant of those studied, favors the crystallization of TiO2 nanocrystals with the best crystallinity, in other words nano‐TiO2 crystals with the largest crystallite size (14.8 nm), lowest lattice strain (7.27 × 10–3), and highest photocatalytic efficiency (almost six times that obtained in methanol). The effect of the dielectric constant on the solubility of the precursor and subsequent nucleation and crystal growth is discussed in detail. These discussions shed some light on solvent selection for the designed synthesis of nano‐TiO2 with different crystallinity. The photocatalytic activity of the as‐prepared samples has also been investigated with respect to their crystallinity.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Strain-Driven Growth of Zinc Oxide Nanowires on Sapphire: Transition from Horizontal to Standing Growth

Recently, we showed large-scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. Here, in examining the cross sections of such nanodevices, we use high-resolution transmission electron microscopy (HRTEM) and large-angle, convergent-beam electron diffraction (LACBED). We show how horizontally grown ZnO NWs influence their underlying sapphire surface and how substrate influences the growth directionality of the NWs. As a NW grows on sapphire, the substrate experiences a compressive strain of ≈7% in its [0001]sap direction (along the width of a NW) to minimize its lattice mismatch with the ZnO NW. Accordingly, ZnO expands along its width to improve its lattice match with the sapphire. The growth direction of (11̅00) is suggested to be the direction that produces a lower lattice strain between ZnO and sapphire. Analyses of NW/sapphire interfaces show that single-crystal NWs grow epitaxially and semicoherently with many fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations at the interface to local relaxation of zinc oxide strain into the sapphire surface. This relaxation is in agreement with the observed deformation of the sapphire underneath the NWs. We also define a critical NW thickness beyond which the growth mode changes from horizontal to standing. Results indicate that below this thickness, gold nanodroplets partially wet both sapphire and ZnO crystals. Above the critical thickness, gold preferentially wets the ZnO nanocrystal, and formation of misfit dislocations at the interface becomes energetically favorable. Combination of these two effects is used to explain the observed change in the growth modes of the NWs.

Theoretical study of stability and electronic structure of Li(Mg,Zn)N alloys: A candidate for solid state lighting

Using selective chemical mutation, we propose and investigate the electronic structure of an alloy with the potential to fill the green gap left open by existing InGaN based emission devices. The small mismatch between LiMgN and LiZnN, along with electronic band gaps spanning the visible range, makes them good candidates. Calculations are performed using a first-principles band structure method, with the special quasirandom structure approach employed to generate the random alloys. Comparison of LiMgN and LiZnN with their binary nitride analogs is made, and the energetic and electronic effects of alloy ordering are investigated. These alloys exhibit negative mixing enthalpies atypical of traditional binary nitride systems, which is explained in terms of the low lattice strain and the chemical bonding effects of the interstitial Li ions.

Effect of Powder Properties of Dielectric Filler on the Dielectric Properties of a Ceramics-Polymer Composite

Effects of powder properties of the filler, including the particle size, the specific surface area and the lattice strain on the dielectric permittivity and Q of the ceramics-polymer composites were investigated in order to achieve composites with high dielectric permittivity and Q. The dielectric properties of the composites were strongly influenced by the powder properties of the filler, despite that the fillers have the same composition. Generally, lower specific surface area and lower lattice strain of the filler lead to higher Q of the composites.

Effects of strain on the optimal annealing temperature of GaInNAsSb quantum wells

Dilute nitride alloys have required postgrowth annealing to improve their luminescent efficiency. There exists an optimal annealing temperature that results in the highest photoluminescence intensity. Several series of GaInNAsSb samples with varying indium and antimony contents were grown to examine the effects of compositional and strain changes on this optimal annealing temperature. We show that a relationship exists between this temperature and the lattice strain of GaInNAsSb quantum wells. As strain increases in these alloys, the optimal annealing temperature decreases. Higher optimal anneal temperatures are found with lower lattice strain. This has important ramifications for the design of long-wavelength GaInNAsSb devices.

セラミックス‐樹脂複合誘電体材料の誘電特性に及ぼす誘電体フィラーの粉体特性の影響

Effects of powder properties of the filler, including the particle size, the specific surface area and the lattice strain, on the dielectric permittivity and Q of ceramics-polymer composites have been investigated in order to achieve composites with high e and Q.Ca0.65Sr0.35TiO3 was chosen as the material of the ceramics dielectric filler for its high e and Q at maicrowave frequencies. Dielectric fillers of Ca0.65Sr0.35TiO3 with various particle sizes, specific surface areas, and lattice strains were prepared.The ceramics-polymer composites were prepared using these fillers, and the dielectric properties of the composites were evaluated at 2 GHz. The dielectric properties of the composites were strongly influenced by the powder properties of the filler, despite that the fillers have the same composition. Generally, lower specific surface area and lower lattice strain of the filler lead to higher e and Q of the composites.

Diffusion creep and partial melting in high temperature mylonitic gneisses, Hope Valley shear zone, New England Appalachians, USA

AbstractField, petrographic, microstructural and isotopic studies of mylonitic gneisses and associated pegmatites along the Hope Valley shear zone in southern Rhode Island indicate that late Palaeozoic deformation (c. 275 Ma) in this zone occurred at very high temperatures (>650 °C). High‐energy cuspate/lobate phase boundary microstructures, a predominance of equant to sub‐equant grains with low internal lattice strain, and mixed phase distributions indicate that diffusion creep was an important and possibly predominant deformation mechanism. Field and petrographic evidence are consistent with the presence of an intergranular melt phase during deformation, some of which collected into syntectonic pegmatites. Rb/Sr isotopic analyses of tightly sampled pegmatites and wall rocks confirm that the pegmatites were derived as partial melts of the immediately adjacent, isotopically heterogeneous mylonitic gneisses. The presence of syntectonic interstitial melts is inferred to have permitted a switch from dislocation creep to melt‐enhanced diffusion creep as the dominant mechanism in these relatively coarse‐grained mylonitic gneisses (200–500 µm syn‐deformational grain size). A switch to diffusion creep would lead to significant weakening, and may explain why the Hope Valley shear zone evolved into a major regional tectonic boundary. This work identifies conditions under which diffusion creep operates in naturally deformed granitic rocks and illuminates the deformation processes involved in the development of a tectonic boundary between two distinct Late Proterozoic (Avalonian) basement terranes.

Influence of Ce and Nd contents of Ml(NiCoMnAl)5 alloy on the performances of Ni–MH batteries

The influence of the Ce and Nd contents of Ml(NiCoMnAl)5 alloy on the performances of Ni–MH batteries was studied. It was found that the high rate dischargeability increased with increase of the Ce content and decrease of the Nd content, while charge retention decreased. An increase in the Ce and Nd contents improved the Ni–MH battery cycle life, and increase of the Nd content increased the discharge capacity of the MH electrode. XRD and PCI analysis showed that heat treatment made the chemical composition of the hydrogen storage alloy more homogeneous with lower lattice strain, and the hydrogen desorption plateau was smooth, which obviously improved the Ni–MH battery cycle life.

Low temperature lattice strain in PrNi 2Si 2

The lattice constants, a and c, are reported for the alloys, PrNi 2Si 2 and GdNi 2Si 2 in the temperature interval 12–300K. The c parameter for PrNi 2Si 2 exhibits a distinct drop at 60K indicating the existence of a lattice strain. There is no observable anomaly at 60K in the heat-capacity and electrical resistivity. We attribute this lattice strain to 4f electric quadrupolar effects.

Rechargeable hydrogen batteries using rare-earth-based hydrogen storage alloys

Electrochemical properties of low cost MmNi 5-based-hydrogen storage alloys (Mm ≡ mischmetal) were extensively examined. The alloy MmNi 3.5Co 0.7Al 0.8 showed a very long cycle life with reasonable discharge capacity (250 mA h g −1) and rate capability. An ingot with a very high endurance and low lattice strain was obtained under controlled conditions, including high rate cooling in the casting process for obtaining a columnar structure, no heat treatment and prevention of stoichiometric deviation. A columnar structure was formed so that the c-axis of the hexagonal structure was oriented parallel to the cooling plane. This alloy was significantly distinguished in crystal growth from a manganese-containing alloy (MmNl 3.5Co 0.5Al 0.3Mn 0.4) which had an equiaxial structure with considerable lattice strain, which needed conventional heat treatment. Deviation from stoichiometric composition to the nickel-rich side caused a significant decrease in capacity and cycle life owing to the precipitation of AlNi 3 at grain boundaries. The decay in capacity of the MH electrode using MmNi 3.6Co 0.7Al 0.8 was only 10% after 2000 cycles. The cylindrical sealed cell also showed a very long cycle life (a capacity decay of 6% after 2000 cycles). The high capacity sealed cell had a 1.5–2 times higher energy density (210 W h dm −3; 65 W h kg −1) with a longer cycle life and better rate capability than the high capacity Ni-Cd battery.

Microstructure of yttria-stabilized zirconia overcoats for thin film recording media

The microstructure of reactively sputtered zirconia overcoats (50 nm thick) for magnetic recording media is examined by transmission electron microscopy (TEM). The micrographs show that the microstructure can be altered from a porous to a tightly packed, and then to a coarse-grained, structure depending on the oxygen partial pressure used during film deposition. The porous film deposited under oxygen-deficient conditions resulted in structural relaxation giving reduced film stress and hardness as the continuous voids developed along intergrain regions. Concurrently, the strength of this overcoat film against wear fracture deteriorated due to the presence of these voids. Increasing oxygen partial pressure during film deposition heals these intergrain voids and leads to the coarsening of the cubic-phase grains. At oxygen partial pressure of 0.04 Pa, a relaxed film structure is again observed. However, it results from the low defect density and lattice strain within individual zirconia grains as the film approaches ideal stoichiometry. This relaxation effect, in turn, reduces the stress, hardness, and wear resistance of the overcoat film. Between these two extremes, a tightly packed, fine-grained film microstructure with optimum hardness and wear strength can be obtained by deposition at oxygen partial pressure of 0.02 Pa.