Increase In Lattice Strain Research Articles

A new layered phase in the alumina-terephthalic acid-water system

Reactions between aqueous solutions of aluminum salts and ammonium terephthalate at pH 6–8 lead to the formation of a previously unknown, weakly ordered phase. It has signs of a layered compound with a unit cell parameter of 1.072–1.073 nm. Its empirical composition can be represented by the formula Al2O3 · xC8H6O4 · yH2O · δ(NH3), where x = 0.7–1.2, y = 4.4–6.0, and δ = 0–0.4. It has good structural order across the layers, whereas the order along the layers is weaker and depends on synthesis conditions. Calcination at 120°C leads to partial dehydration and changes the order along the layers, whereas the order across the layers changes little, with a slight increase in lattice strain. Heat treatment at 400°C leads to almost complete dehydration and fully eliminates the order along the layers, whereas the order across the layers is partially retained. Firing at 800°C leads to the formation of γ-Al2O3. A structural model has been proposed for the new phase.

Stripe order and superconductivity in the mechanically milled La1.6−xNd0.4SrxCuO4

We examine the effect of mechanical milling on the superconducting transition temperature Tc in stripe-ordered La1.6−xNd0.4SrxCuO4 with x∼1/8. The X-ray diffraction patterns of samples milled for 24h show that the orthorhombicity is slightly reduced, in addition to the reduction of crystallite size and the increase of lattice strain, by mechanical milling. The Tc is enhanced by the mechanical milling, although the magnitude of the diamagnetic signal is largely suppressed. The enhancement of Tc suggests that the static stripe is suppressed in the mechanically milled sample.

Synthesis of nanostructured Al–Mg–SiO2 metal matrix composites using high-energy ball milling and spark plasma sintering

Mechanical alloying by high-energy ball milling is successfully used to produce a metal matrix composite of Al–Mg reinforced with amorphous silica particulate. Four different compositions are chosen with varying Mg content (0.5, 1, 2.5 and 5 by wt.%) by keeping SiO2 content constant at 5wt.% to make nanocomposites by high energy ball milling and microcomposites by mechanical mixing. No new phases are found in 20h mechanically alloyed Al–Mg–SiO2 metal matrix composite. XRD study showed Mg is completely dissolved into the Al matrix. XRD observation also showed decrease in crystallite size and increase in lattice strain with progress of mechanical alloying. SEM micrographs indicate decrease in particle size via fracture and cold welding phenomena. The powders are made in the form of cylindrical pellets of 20mm diameter by Spark Plasma Sintering. X-ray diffraction analysis of the pellets obtained after sintering indicates the evolution of MgAl2O4 spinel structure along with Al2O3. Vickers hardness values observed for nanocomposites are more than twice as high as that of microcomposites.

Formation mechanism of nanocrystalline tungsten–titanium aluminides by ball milling of Ti(C,N)–W powders at subzero temperature

The effects of Al addition and milling at subzero temperatures on the transformation of Ti(C 0.5N 0.05)–W powder mixtures were investigated using XRD, SEM and TEM. Addition of Al powder improved the kinetics of solid solution between powder constituents. The effect of Al was ascribed to the increase of lattice strain at first stage milling process followed by the fast diffusion of atoms. The refinement of powder particles was not the determinant criteria for solid state dissolution. Multiple ternary compounds could be formed. Aluminium preferably reacted with tungsten. The W(Al,C) and W(Al,Ti) formed were stable, thus solubility of W in Ti(C 0.5N 0.05) in the presence of Al was limited.

The Effect of Reinforcement Phase on the Microstructure of Al-SiC Nanocomposite Powder Prepared via Mechanical Alloying

Mechanical alloying is one of the most successful methods for the manufacturing of metal matrix nanocomposite powders. In this study, Al/SiC metal matrix composite (MMCp) powders with volume fractions of 5, 10, and 15 percent SiC were successfully obtained after milling the powder for a period of 25 hours at a ball to powder ratio of 15:1 using high energy planetary milling. The Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were conducted to investigate the lattice strain of the matrix phase and the microstructure of the nanocomposite powders after 1, 10, and 25 hours of milling time. Also, the morphology of the Al-5%SiC nanocomposite powder was investigated using transmission electron microscopy (TEM). The results show that with the increase of both milling time and the reinforcement phase volume fraction, the lattice strain increases and the average size of aluminum phase crystallites decreases. Eventually, after 25 hours of milling, the nanocomposite powders show a spherical-like morphology and SiC particles were distributed in an aluminum matrix with appropriate order.

Mechanically induced fcc phase formation in nanocrystalline hafnium

A face-centered-cubic (fcc) phase was obtained in high-purity hafnium (Hf) metal powders subjected to mechanical milling in a high-energy SPEX shaker mill. X-ray diffraction and electron microscopy techniques were employed to evaluate the structural changes in the milled powder as a function of milling time. The effects of mechanical milling included a reduction in grain size, an increase in lattice strain, and formation of an fcc phase instead of an equilibrium hexagonal-close-packed (hcp) phase. During milling, the grain size of Hf decreased to below about 7 nm. Additionally, there was approximately 6% increase in atomic volume during the formation of the fcc phase. Chemical analysis of the milled powder indicated the presence of significant amounts of interstitial impurities. Even though any or all of the above factors could contribute to the formation of the fcc phase in the milled powder, it appears that the high level of interstitial impurities is at least partially responsible for the formation of the fcc phase.

Effects of mechanical milling on hydrogen storage properties of [formula omitted] alloy

The hydrogen storage alloy of Ti 0.32 Cr 0.43 V 0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.

Structural, Electrical and Magnetic Properties of Sb‐Doped Pr2/3Ba1/3MnO3 Perovskite Manganites.

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Structural, magnetotransport and morphological studies of Sb-doped La 2/3Ba 1/3MnO 3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La 2/3Ba 1/3Mn 1− x Sb x O 3 perovskite manganites. Pristine material La 2/3Ba 1/3MnO 3 (LBMO) shows two insulator–metal (I–M) transitions in the electrical resistivity–temperature ( ρ– T) behavior. While the higher temperature transition ( T P1) at ∼340 K is reminiscent of the usual I–M transition in manganites, the lower temperature transition ( T P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La 3+ and Ba 2+). With Sb-doping T P1 shifts to lower temperatures while T P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump ( T P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T> T P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron–magnon scattering is found to dominate in the metallic regime ( T< T P1).

Structural, electrical and magnetic properties of Sb-doped Pr 2/3Ba 1/3MnO 3 perovskite manganites

We report here the electrical, magnetic and micro-structural features of the Sb-doped (nominally at Mn-sites) Pr 2/3Ba 1/3MnO 3 perovskite manganites. Pristine material shows two insulator–metal (I–M) like transitions in the resistivity–temperature ( ρ– T) behaviour. While the higher temperature transition ( T P1) at ∼195 K is reminiscent of the usual metal–insulator transition, the lower temperature transition ( T P2) at ∼160 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (Pr and Ba). With Sb doping at the Mn-site, both the resistivity peaks are seen to shift to lower temperatures. Room temperature resistivity and the peak values are also successively increasing with Sb doping. Scanning electron micrographs of the samples indicate a gradual increase in their grain sizes with Sb which indicates a gradual decrease in the GB density. The higher temperature insulator–metal transition ( T P1) shift is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The observed overall resistivity increase and the shift in the resistivity hump ( T P2) with Sb are found related to the gradually decreasing GB density and the ensuing lattice strain increase at the grain boundaries. The intrinsic MR gets suppressed and the extrinsic MR gets enhanced with Sb doping. The observed low temperature resistivity upturn related to the localization of carriers, is also seen to increase with Sb.

Strain and composition profiles of self-assembled Ge∕Si(001) islands

Epitaxial growth of Ge∕Si(001) in the Stranski-Krastanow regime results in the formation of island ensembles with various sizes and morphologies. During formation there is generally a strain-driven Si diffusion into the Ge islands. We investigate this issue in an epilayer grown by molecular-beam epitaxy containing pyramids, domes, and superdomes. A series of samples obtained by wet chemical etching of the original layer for different times in diluted hydrogen peroxide was evaluated by atomic force microscopy, spectroscopic ellipsometry, and Raman scattering. The average island composition as etching proceeds becomes Si richer, changing from about Si0.2Ge0.8 to Si0.35Ge0.65, whereas the lattice strain increases, in particular, the material at the island summits is essentially relaxed. The composition of the wetting layer is nearly Si0.45Ge0.55. The results also reveal relatively Si-rich nuclei of a uniform size of ≃100nm for all domes and superdomes, in accordance with a dislocation-induced growth mechanism of superdomes.

Mechanical milling and subsequent annealing effects on the microstructural and hydrogenation properties of multisubstituted LaNi 5 alloy

The influence of milling and subsequent annealing on the microstructural properties of commercial-type Mm(Ni, Mn, Al, Co) 5.2 alloy have been investigated. On milling, a two stage process is observed from X-ray diffraction (XRD) profile analysis. At short milling times ( t < 200 min), ball collisions lead to crystallite-size decrease by dislocation formation together with a related increase of lattice strain. Later ( t > 200 min), further milling causes partial amorphous phase formation, while crystallite size as well as lattice strain of the crystalline phase remain constant (50 Å and 0.6%, respectively). At this stage, a volume cell expansion occurs which has been attributed to atomic site-interchange between A and B-type elements. The same two milling stages are observed by scanning electron microscopy regarding particle morphology: a fracture stage governed by plastic deformation (short milling times) and an agglomeration stage governed by cold-welding (long milling times). Microstructural properties of alloy take initial values on annealing at moderate temperatures, i.e., improvement of crystallization and recovery of cell volume. The relationship between alloy microstructure and hydrogenation properties for multisubstituted LaNi 5 alloy is outlined. The occurrence of amorphous phase produces detrimental effects in both reversible H-capacity as well as H-kinetics as compared to the nanocrystalline phase.

NiAl-Al2O3intermetallic matrix composite prepared by reactive milling and consolidation of powders

Reactive milling of nickel oxide and aluminium powders corresponding to the stoichiometric reaction 3NiO + 5Al resulted in the formation of intermetallic matrix composite NiAl-Al2O3, with 28 wt% of alumina. Prolongation of the milling process allowed obtaining the microstructure with nanosize range of crystallites of both phases, as showed XRD measurements and TEM observations. The refinement of microstructure was accompanied with an increase of lattice strain as a result of ball milling. The particles size and morphology changed from several tens of micrometers and polyhedron shape observed immediately after the reaction took place, to several micrometers and spherulitic shape after long-term milling.

Correlation between hydrogen storage properties and structural characteristics in mechanically milled magnesium hydride MgH 2

In this work, we clarified the correlation between hydrogen storage and crystallographic properties in nanostructural magnesium hydride MgH 2 prepared by mechanical milling under hydrogen gaseous atmosphere. At the early stage within 2 h milling, the amount of desorbed hydrogen decreases ∼16% from 7.3 to 6.1 wt.% and the onset temperature of dehydrogenation decreases by 70 K from 670 K, while both the powder size and the crystallite size in powder decrease with increasing the milling time down to 1 μm and 15 nm, respectively, and the lattice strain of 0.3% is rapidly introduced. At the middle stage with longer milling time than 2 h, however, the crystallite size hardly change, but the lattice strain is once released at 2–5 h milling and again increases for longer milling time than 5 h. On the other hand, the amount of desorbed hydrogen suddenly increases from 2 to 5 h, and again decreases with a little increase of lattice strain during 5–80 h milling. At the final stage, the hydrogen capacity and desorption temperature reach to saturation of, respectively, 6.5 wt.% and 600 K, whereas the crystallite size and lattice strain reach to saturation of ∼7 nm and 0.2%, respectively. The results obtained indicate that the reduction of crystallite size as well as the introduction of lattice strain in MgH 2 during milling gives rise to the decrease in hydrogen storage capacity.

The effect of lattice strain on step edge diffusion

The effects of lattice strain on the step edge diffusion of Ag(111) and Ag(100) surfaces are analyzed using molecular statics (MS) based on the semi-empirical embedded atom method (EAM) potential. The calculation of activation barriers of adatom diffusion on surfaces, diffusion over step edge, island corner diffusion, and diffusion along step under strained conditions shows that lattice strain greatly affects the energy barriers of various diffusion processes. For example, comparison shows that the Ehrlich-Schwoebel (ES) barrier decreases as lattice strain increases, which is opposite with strain dependency of surface diffusion barrier, and the diffusions along two different types of steps on Ag(111) surface have a lattice strain dependency different from each other. These different diffusion barrier behavior dependencies on lattice strain have interesting implications on the morphological evolution of strained thin film.

Microstructure characterization of high energy ball-milled nanocrystalline V 2O 5 by Rietveld analysis

Nanocrystalline vanadium pentoxide has been processed with high energy planetary ball-mill at two different ball-to-powder mass ratio (BPMR) for different lengths of time (0–35 h). Microstructure characterization of ball-milled samples is made by employing Rietveld’s X-ray powder structure refinement analysis in terms of lattice imperfections. The particle sizes and lattice strain values are found to be anisotropic. The plastic deformation induced in the process of ball-milling caused a massive decrease in particle size and increase in lattice strain within 15 min of milling, and a saturation of particle size of ∼6.5 nm is reached within 10 h of milling. The shape of milled particle is presumed to be parallelepiped and the influence of BPMR predominated in initial size reduction. The initial change in intensities of two major X-ray reflections is found to be due to insignificant change in atomic coordinates on the event of severe high energy impact. The peak broadening at initial stage of milling is due to fracture of particles, but later (after particle size saturation), dominated by increase in anisotropic lattice strains particularly along [0 0 1].

Preparation, characterization and the study of the thermal strain in Ni–W gradient deposits with nanostructure

In this paper the Ni–W gradient deposit with nanostructure is prepared by controlling the bath temperature. The plating bath contained sodium tungsten, nickel sulfate and citrate, and the preparation was operated at various current densities. During the course of electrodepositing, the bath was heated gradually from 298 to 333 K, and the corresponding current density ( D K) changed from 5 to 12 A/dm 2. X-ray diffraction and energy dispersive X-ray Analysis indicate that crystallite size of the deposit decreases from 10.89 to 1.54 nm and lattice strain increases with the increase of W content in the direction of deposition. The structure of the deposit changes from crystalline to amorphous stepwise with associated increase in lattice strain. These show that the deposit is gradient with nanostructure. Furthermore, the strain distribution in the Ni–W gradient deposit studied by the digital-image correlation method can eliminate the mismatch of expansion coefficients at the interface, thus relaxing the thermal stresses of the materials.

Effects of the low energy-ion beam assistance in the properties of TiC xN y thin films

The stoichiometry, structure and mechanical properties of ion beam sputtered TiC x N y thin films have been studied as a function of the assistance conditions. Tuning the ratio between the current density of assisting N 2 + ions and the target sputtering rate allows us to achieve films with stoichiometries in the range of 0.6> x>0.25, keeping x+ y≈1. The surface topography is also sensitive to the assisting conditions where the RMS roughness is an increasing parameter as the ion beam bombardment increases. In terms of the Ar + ion beam assistance, the films undergo a significant amorphisation and a steep increase in lattice strain as the current density and energy of the impinging ions are increased. The observed changes are discussed in terms of the energy transfer of the bombarding Ar + ions to the layers below the surface of the growing films, leading to vacancies and defects and to a sharp decrease in film crystallinity.

Influence of microstructure on the hydrogen permeability of 9%Cr–1%Mo ferritic steel

The influence of microstructure of 9%Cr–1%Mo steel on the hydrogen diffusivity, solubility and hence the permeability was investigated using electrochemical permeation technique. This steel was austenitised and cooled at various cooling rates to produce different microstructures. Tempering behaviour was also studied by heat treating for different durations at 1023 K. Characterisation of microstructures was carried out using scanning electron microscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid ferrite and martensite during slow cooling. Tempering the normalised steel resulted in the formation of fine intragranular M 2X precipitates and M 23C 6 on the boundaries. The hydrogen diffusivity and solubility showed a regular trend with the amount of strain in the lattice. Lattice defects and precipitates act as trap sites for hydrogen. Increase in lattice strain either due to increase in defect density, substructure or coherent precipitates resulted in decrease in diffusivity due to increase in trap sites. Martensite structure offered the maximum resistance to hydrogen diffusivity and tempered martensite the least resistance due to the annihilation of defects during tempering.

Recrystallization behaviour and electrical properties of germanium ion implanted polycrystalline silicon films

The recrystallization behaviour of undoped and phosphorus-doped polycrystalline silicon films amorphized by germanium ion implantation at doses ranging from 1 × 1015 to 1 × 1016cm-2 are investigated, and the electrical properties of phosphorus-doped films after recrystallization are studied. The phosphorus doping concentration ranges from 3 × 1018 to 1 × 1020cm-3. It is found that the nucleation rate decreases for undoped films and increases for phosphorus-doped films with increasing germanium dose; the growth rates decrease for both doped and undoped films. The decrease in nucleation rate is caused by the increase in implantation damage. The decrease in growth rate is considered to be due to the increase in lattice strain. The grain size increases with germanium dose for undoped films, but decreases for phosphorus-doped films. The dependence of the electrical properties of the recrystallized films as a function of phosphorus doping concentration with different germanium doses can be explained in terms of the grain size, crystallinity and grain boundary barrier height.