Indentation studies on 〈1 0 0〉, 〈0 0 1〉 and 〈1 0 1〉 face of ammonium dihydrogen phosphate (ADP) single crystals: A novel nonlinear optical material

In the present work, Vickers microhardness measurements were carried out on different faces i.e. (1 0 0), (0 0 1) and (1 0 1) of ammonium dihydrogen phosphate (ADP) single crystals grown by slow evaporation solution technique at room temperature in the load ranging from 0.2 to 2 N. The obtained results show that the load independent hardness values are 0.8, 0.7 and 0.66 GPa, respectively at different faces of ADP crystal. From the crack length measurements, the fracture toughness values (K_c) was estimated using Evans and Anstis model and the present studies suggest that Evans model is more relevant when compared to Anstis model. The load variation of some mechanical properties viz. brittle index number (B_i) and yield strength (σ_y) for ADP have also been calculated for the first time.     

DFT study on dissociative adsorption of SiH4 and GeH4 on SiGe(1 0 0)-2 × 1 surface

SiH₄ and GeH₄ dissociative adsorptions on a buckled SiGe(1 0 0)-2 × 1 surface have been analyzed using density functional theory (DFT) at the B3LYP level. The Ge alloying in the Si(1 0 0)-2 × 1 surface affects the dimer buckling and its surface reactivity. Systematic Ge influences on the reaction energetics are found in SiH₄ and GeH₄ reactions with four dimers of Si^∗-Si, Ge^∗-Si, Ge^∗-Ge, and Si^∗-Ge (∗ denotes the protruded atom). On a half H-covered surface, the energy barriers for silane and germane adsorption are higher than those on the pristine surface. The energy barrier for silane adsorption is higher than the corresponding barrier for germane adsorption. Rate constants are also calculated using the transition-state theory. We conclude that the SiGe surface reactivity in adsorption reaction depends on the Ge presence in dimer form. If the surface Ge is present in form of Ge^∗-Ge, the surface reactivity decreases as the Ge^∗-Ge content increases. If the surface Ge prefers to be in form of Ge^∗-Si at low Ge contents, the surface reactivity increases first, then decreases at high Ge surface contents when Ge^∗-Ge prevails. The calculated rate constant ratio of GeH₄ adsorption on Si^∗-Si over Ge^∗-Ge at 650 °C is 2.1, which agrees with the experimental ratio of GeH₄ adsorption probability on Si(1 0 0) over Si(1 0 0) covered by one monolayer Ge. The experimental ratio is 1.7 measured through supersonic molecular beam techniques. This consistency between calculation and experimental results supports that one monolayer of Ge on Si(1 0 0) exists in form of Ge^∗-Ge dimer.     

Micromechanical and thermal behaviour of gel grown pure- and sodium-modified copper tartrate crystals

Results regarding micromechanical characteristics of gel grown pure- and sodium-modified copper tartrate crystals, bearing composition CuC₄H₄O₆·3H₂O, (Cu)_0.77(Na)_0.23C₄H₄O₆·3H₂O and (Cu)_0.65(Na)_0.35C₄H₄O₆·H₂O, as obtained on using indentation induced hardness testing technique are reported. Thermal behaviour of these crystals in the temperature ranging from room temperature (∼25 °C) to about 600 °C is also reported. Pure copper tartrate crystals are found to be thermally more stable than the sodium-modified ones. Dependence of Vickers’ hardness number H_v on load ranging from 0.049 to 2.94 N on two different planes for all the three compositions is analyzed. It is shown that after initial rise in the value of H_v, the same achieves saturation at a load of 0.49 N. Modification of copper tartrate crystal by introducing sodium in its lattice brings about a change in the micromechanical characteristics. The saturation value of H_v decreases with increase in the concentration of sodium ions. The results on (0 0 1) and (1 1 1) planes for both pure and modified copper tartrate crystals suggest hardness anisotropy. Relative difference of hardness between the two planes and yield strength for both pure and modified copper tartrate crystals is worked out. The experimental results are analyzed for applicability of Meyer’s law and Proportional Specimen Resistance Model. It is suggested that the experimental results indicating reverse ISE phenomenon may be explained in terms of the existence of a distorted zone near the crystal-medium interface. The integral method of Coats and Redfern approximation applied to the thermoanalytical data suggests “Random Nucleation Model” for the reaction kinetics of these crystals. Non-isothermal kinetic parameters such as activation energy, frequency factor and order of reaction are calculated.     

Thermal, mechanical, and electronic properties of glycine-sodium nitrate crystal

Glycine-sodium nitrate, C₂H₅N₂NaO₅ (GSN), crystals were grown from aqueous solutions by slow cooling with a temperature lowering rate of 1 °C/day in the range of 40-22 °C. These crystals were analyzed by differential thermal and thermogravimetric analysis (DTA-TGA) and mechanical hardness tester in order to obtain their thermal and mechanical properties. Mechanical characterization was done by studying the variation of microhardness with applied load. The dielectric properties of GSN were calculated by using the CASTEP code within the framework of the generalized gradient approximation (GGA). For better understanding of the optical properties of GSN, the second derivative of ε₂(E) was evaluated. DTA-TGA analysis showed that the material has a thermal stability up to 198 °C. The microhardness test was carried out for several faces of GSN crystals, and the tests revealed a load dependence to hardness. Analysis of the second derivative of ε₂(E) allowed to obtain better resolution of the electronic transitions involving the energy bands. Besides, a theoretical representation of the orbitals’ energy diagram was obtained. A discussion about the relation of structure-properties and molecular character of GSN is presented here.     

Dislocation structure and microhardness of L-arginine perchlorate single crystal

This paper presents the chemical etching and microhardness studies carried out on L-arginine perchlorate (LAPCl), a non-linear optical crystal (NLO) with a view to characterize the dislocation structure and mechanical strength of the grown crystals. LAPCl crystals employed in this investigation were grown from aqueous solution by solvent evaporation method. Optical microscopic studies of as grown habit faces revealed formation of macrosteps and valley on (100) habit face. Chemical etching of (100) face with impurity added organic solvents played an important role in the formation of etch pits. Selective etching of matched pairs on opposite faces of the same plane confirms that the pits are formed at the dislocation sites. Microhardness measurement by indentation method shows decrease of microhardness with increasing load. Nature of hardness profile is explained with the help of Meyer’s law. The work hardening index value indicates that LAPCl belongs to hard crystal category.     

Effect of the spiral phase element on the radial-polarization (0, 1) LG beam

Radially-polarized beams can be strongly amplified without significant birefringent-induced aberrations. However, radially-polarized beam is a high-order beam, and therefore has to be transformed into a fundamental Gaussian beam for reduction the beam-propagation factor M². In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0, 1)^∗ beam with radial polarization, and compare this with the case when the input beam is a LG (0, 1)^∗ beam with spiral phase and uniform or random polarization. The LG (0, 1)^∗ beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0, 1)^∗ mode with M² = 2.8 to a nearly-Gaussian beam with M² = 1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M²)² was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.     

Spatially-variable retardation plate for efficient generation of radially- and azimuthally-polarized beams

We investigate conversion of a linearly-polarized Gaussian beam to a radially- or an azimuthally-polarized doughnut (0, 1)^∗ Laguerre-Gaussian (LG) beams, performed with a spatially-variable retardation (SVR) plate. The SVR plate is composed of eight sectors of a λ/2 retardation plate, each one with different orientation of the to crystal’s slow axis. The analysis reveal that nearly-pure radially- or azimuthally-polarized LG(01)^∗ beam with M² = 2.2 can be obtained, while the transformation efficiency is 89.6%. In the experiments, performed with Nd:YAG laser, we transformed a Gaussian beam with M² = 1.3 to a radially- and azimuthally-polarized (0, 1)^∗ Laguerre-Gaussian beams with M² = 2.5. We carefully characterized the polarization state of the obtained radially- and azimuthally-polarized beams, measuring Stokes parameters. The polarization purity of the obtained beams, calculated from the measured data, was as high as 96%.     

Fatigue properties of a S45C steel subjected to ultrasonic nanocrystal surface modification

An ultrasonic nanocrystal surface modification (UNSM) technique, at 3 different vibration strike numbers (34,000 times/mm², 45,000 times/mm², 68,000 times/mm²) was used to modify the surface structure and properties of S45C. These three process conditions respectively produced 2 μm, 12 μm and 30 μm nanocrystal layers. UNSM technique improves the following mechanical properties: microhardness, surface roughness, and compressive residual stress. Also, fatigue life increased with the vibration strike number. UNSM C3 (with the vibration strike number of 68,000 times/mm²) has improved the fatigue strength by as much as 33% for S45C. Optical microscope pictures show that cracks usually initiate from intergranular microcracks on the surface and then extend along the tip traces of UNSM which are considered as process defects. A simple math model (tearing adhesive plaster model) has been made to analyze the initiation and growth of cracks. Though most of the cracks initiate at the surface of specimens, surface nanocrystal layers can help to retard crack initiation. In S45C, the efficiency of crack resistance is more than 48%.the vibration strike number     

Initial stages of hydroxide formation and its reduction on Co(0 0 0 1) studied by in situ STM and XPS in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the initial stages of hydroxide formation and its reduction on Co(0 0 0 1) in 0.1 M NaOH. XPS investigations give chemical information about the adlayer composition after oxidation and at the different reduction stages. In the subpotential range of oxidation at E<−0.55 V (SHE) the formation of a Co(OH)₂(0 0 0 1) superstructure is observed. It shows a hexagonal symmetry with an average periodicity of P=1.25±0.20 nm. The coincidence cell of the observed structure consists of 16 unit cells Co(OH)₂(0 0 0 1) showing an average lattice parameter of a=0.33±0.05 nm and thus the Co(OH)₂ monolayer forms a 5 × 5 superstructure with respect to the underlying metallic Co(0 0 0 1) substrate. XPS results clearly prove the presence of hydroxide and exclude the formation of CoO in the subpotential range.At the very beginning of the reduction process small two-dimensional metal clusters and islands can be observed. It is assumed that they are crystallization nuclei for metal formation. They enlarge and grow together with other islands or larger terraces. During this reduction process two-dimensional adatomic arrays consisting of OH⁻-Co²⁺- OH⁻ trimers appear on the surface. Some of these trimers accumulate at step edges, and finally decorate them. This decoration builds up an energy barrier for further metal incorporation and prevents further growth of the terraces with remaining metal clusters on their surfaces. The reduction of the Co(OH)₂ layer is found to be not completed which is confirmed by XPS results.     

Microhardness and fracture mechanics of flux grown samarium doped gadolinium vanadate (Sm:GdVO4) single crystal

Indentation induced hardness studies on flux grown samarium doped gadolinium vanadate (Sm:GdVO₄) crystals are reported. Mechanical characteristics including Vickers microhardness, fracture mechanics, crack propagation, brittleness index and yield strength are assessed. The mechanical behaviour of the crystal is explained using various theoretical models viz., Meyer’s law, proportional specimen resistance (PSR) and Hays and Kendall’s approach. Variation of microhardness with load is found to be non-linear and is best explained by the concept of Newtonian resistance pressure as proposed by Hays and Kendall’s law. Classification of cracks as median type is explained. The values of fracture toughness (K_c), brittleness index (B_i) and yield strength (σ_y) have also been calculated.     

Mechanical characteristics of solution grown potassium zinc chloride crystals doped with lithium ions

Results of indentation-induced hardness testing studies on potassium zinc chloride crystals doped with Li⁺ ions, leading to an understanding of their mechanical behaviour, are presented. The Vickers hardness of these crystals for (1 0 0), (0 1 0) and (0 0 1) planes in the load range 20–160 g were studied. Load-independent values of hardness are estimated for the three crystallographic planes by applying Hays-Kendall’s and Li-Bradt models. The results showed that: (1) for the three crystallographic planes the load-independent hardness obtained by Li-Bradt model is higher than that predicted by Hays-Kendall’s, approach; (2) the load independent hardness of the (0 0 1) plane is higher than that of both (1 0 0) and (0 1 0) planes, (3) the values of load-independent hardness depend on Li⁺ concentrations in the K₂ZnCl₄ crystals, (4) the variations of crack length and crack morphology are described for studied crystal planes.     

Nanoindentation measurements on modified diamond-like carbon thin films

In the present study, we explored the effect of metallic interlayers (Cu and Ti) and indentation loads (5-20 mN) on the mechanical properties of plasma produced diamond-like carbon (DLC) thin films. Also a comparison has been made for mechanical properties of these films with pure DLC and nitrogen incorporated DLC films. Introduction of N in DLC led to a drastic decrease in residual stress (S) from 1.8 to 0.7 GPa, but with expenses of hardness (H) and other mechanical properties. In contrast, addition of Cu and Ti interlayers between substrate Si and DLC, results in significant decrease in S with little enhancement of hardness and other mechanical properties. Among various DLC films, maximum hardness 30.8 GPa is observed in Ti-DLC film. Besides hardness and elastic modulus, various other mechanical parameters have also been estimated using load versus displacement curves.     

Microstructure and mechanical properties of reactive magnetron sputtered Ti-B-C-N nanocomposite coatings

Ti-B-C-N nanocomposite coatings with different C contents were deposited on Si (1 0 0) and high speed steel (W18Cr4V) substrates by closed-field unbalanced reactive magnetron sputtering in the mixture of argon, nitrogen and acetylene gases. These films were subsequently characterized ex situ in terms of their microstructures by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), their nanohardness/elastic modulus and facture toughness by nano-indention and Vickers indentation methods, and their surface morphology using atomic force microscopy (AFM). The results indicated that, in the studied composition range, the deposited Ti-B-C-N coatings exhibit nanocomposite based on TiN nanocrystallites. When the C₂H₂ flow rate is small, incorporation of small amount of C promoted crystallization of Ti-B-C-N nanocomposite coatings, which resulted in increase of nano-grain size and mechanical properties of coatings. A maximum grain size of about 8 nm was found at a C₂H₂ flux rate of 1 sccm. However, the hardness, elastic modulus and fracture toughness values were not consistent with the grain size. They got to their maximum of 35.7 GPa, 363.1 GPa and 2.46 MPa m^1/2, respectively, at a C₂H₂ flow rate of 2 sccm (corresponding to about 6 nm in nano-grain size). Further increase of C content dramatically decreased not only grain size but also the mechanical properties of coatings. The presently deposited Ti-B-C-N coatings had a smooth surface. The roughness value was consistent with that of grain size.     

Effects of peak current density on the mechanical properties of nanocrystalline Ni-Co alloys produced by pulse electrodeposition

Cobalt content, grain size, microhardness and tensile strength of nanocrystalline Ni-Co deposits produced from a solution containing saccharin and cobalt sulfate at constant electrodeposition conditions (pulse on-time T_on at 1 ms and pulse off-time T_off at 15 ms) but varying the peak current density J_p were investigated. It is found that an increase in J_p makes the deposit Co content lower, colony-like morphology more obvious, grain size smaller, and hardness and tensile strength higher. All of the facts are believed to result from the higher overpotential and nucleation rates caused by the J_p increase. But its further increase could lead to reduction in the hardness and tensile strength. Peak current densities in the range of 100-120 A dm⁻² are recommended for the preparation of nanostructured Ni-Co alloy deposits with grain sizes in the range of 15-20 nm, containing 7-8% Co, possessing hardness of 590-600 kg mm⁻² and tensile strength of 1180-1200 MPa—significantly higher than the strength of pure nickel deposit which is produced by the similar method and gets similar grain size.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

Vortex structure of Gross-Pitaevskii model

The vortex line of the Gross-Pitaevskii model is studied. The kinetic helicity of the vortex is discussed, and vortex structure is classified by the Hopf index, linking number in geometry. A mechanism of generation and annihilation of vortex lines is given by the method of phase singularity theory. The dynamic behavior of the vortex at the critical points is discussed in detail, and three kinds of length approximation relations at the neighborhood of a critical point are given: l ∝ (t − t^∗)^1/2, l ∝ t − t^∗, l = const.     

A theoretical and matrix-isolation FT-IR investigation of the conformational landscape of N-acetylcysteine

The conformational landscape of N-acetylcysteine (NAC) has been investigated by a combined experimental matrix-isolation FT-IR and theoretical methodology. This combination is a powerful tool to study the conformational behavior of relatively small molecules. Geometry optimizations at the HF/3-21 level resulted in 438 different geometries with an energy difference smaller than 22 kJ mol⁻¹. Among these, six conformations were detected with a relative energy difference smaller than 10 kJ mol⁻¹ at the DFT(B3LYP)/6-31++G∗∗ level of theory. These were finally subjected to MP2/6-31++G∗∗ optimizations which resulted in five minima. The vibrational and thermodynamical properties of these conformations were calculated at both the DFT and MP2 methodologies. Experimentally NAC was isolated in an argon matrix at 16 K after being sublimated at 323 K. The most stable MP2 form appeared to be dominant in the experimental spectra but the presence of three other conformations with ΔE_MP2 < 10 kJ mol⁻¹ was also demonstrated. The experimentally observed abundance of the H-bond containing conformations appeared to be in good accordance with the predicted MP2 value.     

Grazing incidence impact of ions on an adatom-covered surface: Molecular-dynamics study of sputtering, surface-damage formation and ion-induced adatom mobility

Using molecular-dynamics simulation, we study the exemplary case of 5 keV Ar impact onto a Pt(1 1 1) surface at 83° incidence angle towards the surface normal. Under these conditions, on a flat terrace, the ions are specularly reflected, inducing neither damage nor sputtering of the surface. This situation changes drastically, if the surface is covered randomly with a coverage Θ of adatoms. For 0.1 < Θ < 0.5, average sputter yields and damage of up to a value of 5 and 9, respectively, are achieved. For coverages around Θ = 0.3-0.4, 50 adatoms are relocated on the average on the surface, affecting an area of around 1000 Å². This ion-induced mobility promotes adatom clustering, which typically leads to a rim-like structure around the ion impact point.     

Direct enrichment of olive oil in oleuropein by ultrasound-assisted maceration at laboratory and pilot plant scale

The possibility to improve the nutritional value of olive oil by enriching it in phenolic compounds from olive leaves (e.g., oleuropein) by ultrasonic maceration was studied. The experimental design used led to the following optimal extraction conditions: ultrasonic power of 60 W, temperature of 16 °C and sonication duration of 45 min. The high total phenolic content (414.3 ± 3.2 mg of oleuropein equivalent/kg of oil), oleuropein (111.0 ± 2.2 mg/kg of oil) and α-tocopherol (55.0 ± 2.1 g/kg of oil) concentrations obtained by optimized ultrasound-assisted extraction (UAE) proved the efficiency of this process when compared with the conventional solid-liquid extraction. Histochemical analyses showed that this efficiency is due to specific alteration of the phenol-containing leaf structures. Furthermore, the radical-scavenging activity of the processed oil (DPPH test) and its stability toward lipid autoxidation (heating test) confirmed its enrichment in antioxidants. Sensory evaluation of the enriched olive oil showed a slight increase in bitterness but an overall acceptability. Finally, the enriched olive oil was characterized by clear green color (L^∗, a^∗, b^∗ parameters).     

Mechanical behaviour and fracture mechanism of flux grown holmium orthoferrite single crystals

Results of indentation induced microhardness and fracture studies on (110) and (001) planes of flux grown HoFeO₃ single crystals using a Vicker's hardness tester, in the load range of 10-100 g are presented. The variation of microhardness with load is found to be non-linear and is explained by using Hays and Kendall's law. The hardness results and the indentation-induced cracking yields the values of the yield strength, fracture toughness and brittleness index for (110) and (001) planes of HoFeO₃ crystals. Transition from Palmqvist to median crack systems takes place at higher loads for (110) plane of the crystal whereas (001) plane is characterized by Palmqvist cracks only.