Search

The aim of the present investigation is to develop mechanochemically synthesized nano-crystalline hydroxyapatite (nHAp) bioceramics using clam seashells and phosphoric acid by varying the CaO:H3PO4...

Transient liquid phase-like soldering technique has been successfully applied to prepare lead-free solder joints at 230 °C. Multiple interlayers of tin thin films in combination with a Sn-3.0Ag-0.5Cu (SAC305) solder paste have been used as filler for joining. Microstructural analysis was carried out by electron microscopy and energy-dispersive spectroscopy. For interlayer containing samples (Cu-Sn-SAC305-Sn-Cu), interfacial intermetallics layers found to be more homogeneous compared to the samples without interlayers (Cu-SAC305-Cu). Quality of the joints has been investigated nondestructively by ultrasonic method. On measuring the electrical resistivity by four-probe method, Cu-Sn-SAC305-Sn-Cu solder joint shows lower electrical resistivity compared to that of the Cu-SAC305-Cu solder joint, wherein nonlinear ultrasonic parameters confirm the superiority of former over later.

ZnO-silicon heterojunction solar cell having high efficiency and high fill factor is structured and simulated to study its photovoltaic properties under different device temperature. Current-voltage measurement of dimensionally optimised device structure revealed up to 20.44% efficiency and 83.66% fill factor at 1 Ω-cm 2 external series resistance at 300 K. Measured current-voltage shows decrease of open-circuit voltage from 659.13 to 109.1 mV and short-circuit current from 40.65 to 39.5 mA/cm 2 with increase of device temperature. Quasi-steady-state photoconductance measurement reveals small recombination current and long Shockley Read Hall lifetime under the selected temperature range indicating superior performance of the device. Adverse effect of surface recombination velocity at rear surface of the device is observed at lower temperature (100-600 K) range indicating better majority carrier collection at very high temperature 700 K and above.

This work demonstrates the use of mixing of ultrasonic Lamb waves to characterize the tempering behaviour in metallic plate. Lamb wave mixing has been used to measure the generated second harmonic during tempering of mod.9Cr-1Mo steel plate. As-received material is normalized at 1080°C and then tempered in the temperature range of 600 - 850°C with a step size of 50°C for 1.5hrs and followed by furnace cooling. Lamb wave mixing technique has been used to assess the tempering behaviour of this material. Nonlinear ultrasonic parameter β which is the ratio of 2nd harmonic amplitude to the multiplication of the fundamental amplitudes is determined from the mixing wave at each temperature and correlated with microstructural characteristics. It is seen that this nonlinear acoustic parameter (β) is sensitive towards coherency strain generated between precipitate and matrix during tempering.

Nonlinear ultrasonic is known to be a promising technique to characterize the microstructural degradation in engineering materials. This work demonstrates the use of nonlinear Lamb wave mixing technique to assess the localized deformation in modified 9Cr 1Mo steel during creep. Two Lamb wave modes of different frequencies (ω1 & ω2) are allowed to mix within the material under certain resonant condition to generate third type of harmonic waves of frequencies (ω1±ω2). This new generated wave carries the information of material nonlinearity from the mixing site and independent of the other extraneous nonlinear factors. Amplitude of the generated third wave depends on the third order elastic constants of the material. This study reveals that nonlinear Lamb wave mixing technique could be used to assess the localized deformation much prior to its failure.

Ice formation causes numerous problems in many industrial fields as well as in our daily life. The control of ice nucleation and rational design of anti-icing surface with low ice adhesion are desi...

In this work, we have carried out a systematic study of nucleation of a supercooled nanofluid droplet on a graphite substrate using molecular dynamics simulations. In particular, the effect of nano...

This paper highlights the evolution of different microstructures during short-term overheating failure of a boiler water wall tube made of SA210 Grade C steel. Short-term overheating failure of the boiler tube caused fish mouth rupture with thin lips, along the longitudinal direction of the tube with extensive tube bulging. Microstructure examination revealed elongation of grains at the fracture signifying plastic deformation due to hoop stress becoming equal to the flow stress of the tube at the elevated temperature. Overheating caused degeneration of the initial ferrite–pearlite microstructure. Spheroidization and coalescence of carbides occurred in the ferrite matrix from the initial ferrite–pearlite microstructure of the boiler tube near the ruptured region. Moreover, the formation of bainite microstructure at the fracture fish mouth opening indicates heating above the eutectoid temperature (lower critical temperature) of the tube material.

Non-collinear nonlinear ultrasonic (NCNLU) wave mixing technique has been established to study the localized plastic deformation at the crack tip during fatigue. A pair of ultrasonic shear wave was mixed non-collinearly to obtain a longitudinal wave of frequency equal to the sum of the two shear wave frequencies under a resonant condition. Experiments were carried out on notched 9Cr-1Mo 3-point bend specimen during high-cycle fatigue. The variation of the NCNLU parameter with the stress accumulation at the crack tip during the fatigue crack initiation and propagation and mapping of the deformation zone around the crack tip are described in this paper.

The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid/water and air/water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid/water and air/water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three-coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at ∼50 cm−1 is enhanced, whereas those at ∼200 and ∼500 cm−1 are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single-molecule and collective orientation relaxations are also examined. Single-molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water–graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water/solid and water/air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.

Being one of the most important joints in a pressurized heavy water reactor (PHWR), pressure tube rolled joint is critical during the construction and operation. This joint is formed through the roll expansion process that leads to an interference fit between the Zr 2.5%Nb pressure tube and stainless-steel end fitting. For integrity of this joint, a sufficient pull-out strength and leak tightness must be maintained, while having lower undesirable residual stresses, especially in the transition zone. The roll expansion process parameters are controlled to achieve optimum contact pressure and limit the tensile residual stress. The parameters of this roll expansion process are designed either through experiments or through numerical simulations. One important aspect that is often ignored in the numerical simulations is the effect of material constitutive behavior of the Zr 2.5%Nb alloy, particularly the anisotropic yield behavior and the cyclic stress-strain response. In this paper, these two aspects of constitutive behavior are investigated, using three-dimensional finite element simulations, to understand their influence on the residual stress development in the roll expansion process. The cyclic behavior was modelled using Chaboche nonlinear kinematic hardening model. The model parameters were extracted from low cycle fatigue experiments with the strain amplitude equivalent to that seen in roll expansion process. The results show that constitutive behavior, especially the cyclic behavior, has a significant impact on the residual stress. Anisotropic yielding of the pressure tube was also seen to affect the tensile residual stress and the contact pressure, although to a lesser extent. Finally, after comparison between four combinations of constitutive behavior, a simple isotropic yield with isotropic hardening model is suggested to predict the tensile residual hoop stress in the rolled joint, conservatively.

One mounded LPG storage vessel of petrochemical plant got suddenly exposed to cryogenic nitrogenand developed cracks in the shell plate adjacent to the bottom nozzle and was subsequently repair welded.The structural integrity of the repair weld region was assessed by NDT examinations. The integrity of there paired weld region was conrmed by conducting hydrotest. Acoustic emission (AE) monitoring during thehydrotest was also carried out. The acoustic emission signals generated during the hydrotest indicated thatthe signals were due to structural noise and microyielding of the vessel material. During the repressurizingcycle of the hydrotest, negligible AE signals were generated and this conrmed the structural integrity of therepair welded region of the vessel monitored by acoustic emission technique (AET).

Carbon dots (CDs) are known to have a wide range of applications, yet our understanding of their structures and chemistry remains uncertain because of their highly complex nanostructured framework. Here we attempt to elucidate the molecular structure and intrinsic mechanisms governing photoluminescence (PL) of CDs by trapping seven visibly distinct colored intermediates that evolved during pyrolytic metamorphosis of citric acid with dopant Ru(III). The “excitation-dependent” PL of doped CDs, Ru:CDs, can be tuned by ethylenediamine (EDA), yielding “excitation-independent” highly fluorescent nanodots, Ru:CNDEDAs. To mimic the optical and chemical properties of CDs, we devise a unique model cocktail comprising multiple fluorogenic molecules that truly supports the existence of chemically switchable conjugated moieties in CDs. We propose a plausible molecular level framework of CDs on the basis of spectroscopic findings and existing literature regarding thermal decomposition of CA. The PL of chemically engine...

Age-hardenable Al–Mg–Si alloys can be strengthened by precipitation hardening along with grain refinement and dislocation density. Low-temperature deformation not only increases the dislocation density but also arrests grain recovery and promotes precipitation hardening. In this investigation, sub-zero temperature rolling, in combination with artificial aging, was performed to achieve strengthening as well as substantial amount of plasticity. The thermomechanically treated alloys were subjected to nanoscratch measurements to study the tribological behavior in terms of plastic energy, recovery index, penetration depth and residual depth. Scratch hardness was measured and utilized to calculate the wear rate, recovery resistance, wear resistance and the wear coefficients using Archard׳s equation.

Recognition and binding of ice by proteins, crystals, and other surfaces is key for their control of the nucleation and growth of ice. Docking is the state-of-the-art computational method to identify ice-binding surfaces (IBS). However, docking methods require a priori knowledge of the ice plane to which the molecules bind and either neglect the competition of ice and water for the IBS or are computationally expensive. Here we present and validate a robust methodology for the identification of the IBS of molecules and crystals that is easy to implement and a hundred times computationally more efficient than the most advanced ice-docking approaches. The methodology is based on biased sampling with an order parameter that drives the formation of ice. We validate the method using all-atom and coarse-grained models of organic crystals and proteins. To our knowledge, this approach is the first to simultaneously identify the ice-binding surface as well as the plane of ice to which it binds, without the use of structure search algorithms. We show that biased simulations even identify surfaces that are too small or too weak to heterogeneously nucleate ice. The biasing simulations can be used to identify of IBS of antifreeze and ice nucleating proteins and to equilibrate ice seeds bound to an IBS for the calculation of heterogeneous ice nucleation rates using classical nucleation theory.

In this work, a simulation approach to improve the contribution of light generated current by the emitter layer is presented. Single emitter layer made of nanocrystalline carbide of silicon is replaced with a double layer of the same compound material but different elemental composition and optical properties in a microcrystalline silicon-based solar cell. Each layer of the emitter is made of carbides of silicon with different carbon to silicon fractions. The simulated study results showcase short circuit current and efficiency up to 37.86 mAcm−2 and 20.56% respectively for the device with a double emitter layer. The quantum efficiency also justifies an increase in photocurrent in the double layer device. This increase is mainly observed in the wavelength range of photons absorbed by the emitter layer. The calculated emitter saturation current density for the same device is as small as 1.7854 × 10−17 Acm−2. Furthermore, a strong dependence of device parameters on the physical properties of the intrinsic layer is observed. The overall results introduce double emitter as a new approach to improve the photocurrent of heterojunction solar cells.

In this work, we have considered the crystallisation behaviour of supercooled water in the presence of surface defects of varying size (surface fraction, α from 1 to 0.5). Ice nucleation on Ag exposed β-AgI (0001 plane) surface is investigated by molecular dynamics simulation at a temperature of 240 K. For systems with α>0.67, the surface layers crystallise within 150 ns. In the system with defects, we observe two distinct stacking patterns in the layers near the surface and find that systems with AA stacking cause a monotonic decrease in the early nucleation dynamics with an increase in defect size. Where AB stacking (α=0.833) is observed, the effect of the defect is diminished and the dynamics are similar to the plain AgI surface. This is supported by the variation in the orientational dynamics, hydrogen bond network stability, and tetrahedrality with respect to the defects. We quantify results in terms of the network topology using double-diamond cages (DDCs) and hexagonal cages (HCs). The configurations of the initially formed layers of ice strongly affect the subsequent growth even at long timescales. We assert that the retarded ice growth due to defects can be explained by the relative increase in DDCs with respect to HCs.

SA213-T22 steel had been widely used in petrochemical and power generation plants. But due to prolonged service exposure in various conditions, these tubes have been ageing continuously and deteriorations of the materials cannot be avoided. This paper presents a case study of SA213 type-T22 alloy steel reheater tube that failed in a boiler of a power plant. The failed tube was investigated through visual examination, mechanical properties evaluation, and microstructural analysis to find the root cause of failure. The tube was found to fail by long term over-heating due to formation of thick oxide scale formation leading to creep.

Nonlinear Lamb wave was used to analyse the effect of tempering temperature on modified 9Cr–1Mo (P91) steel. As-received material was normalized at 1080 °C and then tempered in temperature range of 600–850 °C with a step size of 50 °C for 1.5 h and followed by furnace cooling. Nonlinear ultrasonic parameter β was determined at each temperature using Lamb wave and correlated with microstructural characteristics like size of precipitate and density of dislocation. It was seen that acoustic nonlinearity parameter (β) was sensitive towards precipitate-matrix coherency strain, generated during tempering.

Overall series resistance of the contact increases due to unwanted pores created during the time of contact formation in solar cell. Light induced plating is an industrial established process which is used to reduce the series resistance and to improve the contact resistance resulting in increase of fill factor of solar cell through modification of front side contact. In this paper a theoretical modeling is done to show how distributed pores affect the series resistance and solar cell performance. Here we explored several parameters like: series resistance, shunt resistance, fill factor, short circuit current, and external quantum efficiency. Through our experimental setup it is found that the short circuit current improves significantly and also it is observed that during the time of experiment due to unintentional deposition of silver nanoparticles, external quantum efficiency improved by 5.249 % and a reduction of reflectance by 2.03 %. An overall enhancement of solar cell efficiency of 1.65 % is obtained during the process.

Present paper discusses about a new methodology to quantify cyclic damage through ultrasonic measurement. Based on experimental results, correlations have been made between damage accumulated inside the material due to strain excursions and corresponding ultrasonic parameters. It has been proposed that based on the existing correlation between attenuation coefficient and number of cycles to failure, fatigue failure characteristics can be partitioned into two regimes (a) failures due to early nucleation and rapid propagation of cracks and (b) failures due to delayed propagation and crack coalescence. Plastic strain accumulation and surface crack density have been chosen as two physical parameters directly influencing attenuation coefficient and it has been observed that with increasing plastic strain accumulation, ultrasonic attenuation increases. Between two primary echoes of ultrasonic spectra, some secondary defect echoes have been found. A damage descriptor has been introduced by taking difference between the bandwidth of defect echo and backwall echo normalized by overall amplitude frequency distributions of backwall echo. It has been found to bear sensitivity towards surface crack density. This quantitative estimation differentiates between the classical descriptions of damage due to dislocation mediated plasticity, micro crack generation and coalescence.

Higher harmonic generation using nonlinear ultrasonic is known to be very sensitive to microstructural degradation. This technique relies on nonlinear stress-strain relation i.e. nonlinear Hooke's law. The quantitative measurement to characterize the materials degradation is done by the nonlinear ultrasonic parameter, β. The objective of this work is to use nonlinear ultrasonic using Lamb wave to evaluate creep damage in modified 9Cr–1Mo steel. Relative change in β was seen to be dependent on the higher order elastic constants that evolve with microstructure as creep progresses as well as sub-structural changes. The extent of damage in the material due to creep exposure was evaluated by Kernel Average Misorientation (KAM) from EBSD and co-related with nonlinear acoustic parameter.

Unidirectionally aligned 1D wires of an organogelator ranging over 25 μm2 are fabricated at an air–water interface at a significantly low critical gelation concentration, which merge into 2D sheets with the aid of surface pressure.

In this paper a segmented photonic crystal fiber with segments of varying effective refractive index in the cladding has been proposed. The designs have been analyzed in view of the effect of change in their cladding parameters, such as refractive index contrast, air filling fraction (denoted by normalized air hole diameter), duty cycle of the segments, and core–cladding interface. The periodically varying high and low index regions have been created by considering variations in the air hole size. High index core region has been created by omitting a single air hole from the centre of the lattice. Phosphate glass (n=1.56) has been used as base material. The various designs obtained by varying the controlling parameters, have been investigated for properties such as the effective core and cladding indices, confinement of fundamental mode and higher order mode, and the V parameter values. These parameters have been investigated over near infrared (0.75 –1.4 μm) to short wavelength infrared (1.4–2.3 μm) spectral range using finite difference time domain method. The expression for V parameter for conventional photonic crystal fiber has been extended to the proposed design and single mode cutoff value for the design has been calculated. The proposed design offers robust single mode confinement over specified infrared (0.75–2.3 μm) wavelength range and a broad confinement peak has been observed containing the third optical communication window at 1.55 μm.

We have observed micron size pit formation on Ge surface due to bombardment of 26 keV Si− ion at normal incidence in the fluence range 1 × 1018 and 7 × 1018 ions/cm2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to follow the evolution of the surface morphology. The pits are of various shapes, e.g., crescent-shaped, kidney-like or circular structures. The two-field continuum model developed for small slope approximations can describe the pit formation and growth at the very beginning of ion bombardment. The growth of the pits at late times (high fluence) can be explained by the gradient dependent erosion mechanisms due to primary ion beam as well by secondary flux of particles originating from steep slopes. Energy dispersive X-ray analysis attached to SEM is employed to obtain the chemical information of the pitted surface. The depletion of Si at the bottom of the pits is explained due to lower diffusivity of Si in Ge.

Influence of microstructural changes on the second harmonics of sinusoidal ultrasonic wave during low cycle fatigue (LCF) deformation in pearlitic steel was studied. Fatigue tests were interrupted and at every interruption, non-linear ultrasonic (NW) parameter (beta) was determined. Microstructures of cyclically deformed specimens at various cycles were examined by transmission electron microscopy (TEM). The variation of beta with fatigue cycles was correlated with the microstructural changes and the results were explained through the variation in dislocation morphology and carbon content of the steel. (C) 2010 Elsevier B.V. All rights reserved.

Higher order harmonic analysis of ultrasonic signal has been applied to study the precipitation kinetics of 18Ni maraging steel. The material has been solution annealed (SA) at a temperature of 1088 K for 1 h and then air quenched. Ageing of SA materials have been done at 728 K for different durations of 10 min to 100 h and air cooled. It has been found that the measurement parameter of this new technique increases with the formation of finer precipitates at the initial stage of ageing till 12 h and then decreases at longer holding time with the coarsening of precipitates.

Nonlinear ultrasonic (NLU) and conventional ultrasonic techniques have been applied to assess the localized plastic deformation in polycrystalline copper during high-cycle fatigue (HCF). The measurement parameter of NLU, i.e., the NLU parameter, β, has been calculated from the power spectrum of the filtered and amplified received signal. For conventional ultrasonic, longitudinal velocity (V l) and attenuation coefficients (α) have been determined. It has been observed that the position of maximum localized plastic deformation cannot be detected by V l and α, whereas β detects the same at the stage of 30 pct of damage corresponding to the total fatigue life of the material. The result of NLU has also been correlated with the hardness during fatigue. This work reveals the potential of NLU to assess the localized deformation during fatigue much earlier to the failure.

This paper presents the design and simulation of a reconfigurable stop-band filter on a silicon substrate based on the combination of RF microelectromechanical system and metamaterial-based technologies. The device is implemented on coplanar waveguide structure by embedding complementary split-ring resonators on the central line and an RF MEMS varactor bridge supporting the neighboring ground planes. The response characteristics of this metamaterial-based filter can be dynamically tuned, thus enhancing its usefulness. The device operates within a frequency range of 16.5–19.5 GHz, giving a tuning range of 15%, and can be tuned from Ku-frequency band to K-frequency band. It works with a comparative low pull-in voltage of 17.42 V and a faster switching time of 0.138 µs. A thorough electromechanical analysis has been done by varying various structural and material parameters. Moreover, a comparative electrical performance of silicon and glass has been shown to overcome the cons of silicon by high-resistivity glass.

In this research paper, in total 212 individual leakage-free Polymethylmethacrylate (PMMA) microfluidic devices are fabricated by maskless lithography, hot embossing lithography and direct bonding technique. The effect of channel aspect ratio on dyed water flow is investigated using these microfluidic devices. Experimental studies show that the dyed water flow is faster on the surface of higher wettability. The effect of capillary pressure on dyed water flow is studied in the fabricated PMMA microfluidic devices. According to the experimental observations, the centrifugal force has prominent effect on the dyed water flow. Also, the effect of bend angle is investigated on the surface-driven capillary flow of water. The polystyrene microparticles have been separated in the microfluidic lab-on-a-chip systems using the investigated flow features. A 100% separation efficiency is achieved in these lab-on-a-chip systems. These microfluidic lab-on-a-chip systems can be used to separate blood cells from human whole blood for further clinical tests. These experimental studies are important in bioengineering applications. The effect of bend angle as channel geometry to control the surface-driven capillary flow is investigated as a novel approach to control the separation time in microfluidic lab-on-a-chip systems. Also, the effect of surface wettability as surface property to control the surface-driven capillary flow is investigated as a novel approach to control the separation time in microfluidic lab-on-a-chip systems.

We have studied scaling behavior of ripple structured mica surfaces. Clean mica (001) surface is sputtered by 500 eV Ar+ ion beam at 40° incidence angle for different time ranging from 28 minutes to 245 minutes to form ripples on it. The scaling of roughness of sputtered surface characterized by AFM is observed into two regime here; one is super roughening which is for above the crossover bombardment time (i.e, tx ≥ 105 min) with the scaling exponents α = αs = 1.45 ± 0.03, αlocal = 0.87 ± 0.03, β = 1.81 ± 0.01, βlocal = 1.67 ± 0.07 and another is a new type of scaling dynamics for tx ≤ 105 min with the scaling exponents α = 0.95 (calculated), αs = 1.45 ± 0.03, αlocal = 0.87 ± 0.03, β = 1.81 ± 0.01, βlocal = 1.67 ± 0.07. In the super roughening scaling dynamics, two types of power law dependency is observed on spatial frequency of morphology (k): for higher k values PSD ∼ k−4 describing diffusion controlled smoothening and for lower k values PSD ∼ k−2 reflecting kinetic roughening.

We propose and evaluate a 2-D photonic crystal waveguide with a single line defect that uses rectangular holes adjacent to waveguide, arranged in a hexagonal geometry. With an aspect ratio of 2:1 in rectangular holes, we obtained transmission efficiency of 94% which is larger than previously reported for its circular and elliptical counterpart. By controlling the single parameter of rectangular air hole, the slow light performance of the improved efficiency structure is analyzed. In the irregular waveguide, slow light is achieved with low group velocity and very low group velocity dispersion over large signal bandwidth of 23 GHz. The normalized delay bandwidth product (NDBP) for the proposed design is also measured and compared with NDBP of waveguide when instead of rectangular holes, elliptical holes is used adjacent to waveguide with same aspect ratio 2:1. Improvement in normalized delay bandwidth product is obtained for our proposed design when elliptical air holes of aspect ratio 2:1 adjacent to waveguide are replaced by rectangular holes of same aspect ratio.

Nonlinear ultrasonic is the new approach for the effective evaluation of material degradation in the area of non‐destructive evaluation (NDE). Fatigue damage of a material produces a substantial distortion of ultrasonic waves propagating through the degraded material. The wave distortion is quantified by means of a material nonlinearity parameter β that is defined as the ratio of the amplitude of the 2nd; harmonic to the square of the amplitude of the fundamental. This nonlinear parameter changes when, for instance, a distribution of microcracks appears inside the material. This feature has recently been found as a new potential application in the characterization of fatigued and degraded materials. In this present work, an attempt has been made to assess the localized plastic deformation during high cycle fatigue to locate the position of crack initiation much before the failure along the gage length of hourglass type specimen in polycrystalline copper by nonlinear ultrasonic technique.

The nanoscale ripple formation on mica surface is studied at off-normal ion incidence angles θ under Ar+ bombardment at energies E close to or below the threshold energy for physical sputtering. A phase diagram for domains of pattern formation is presented as a function of θ and E, which shows the stability/instability bifurcation angle close to 40°. The instability grows as ripple-like structures at lower angles, while at grazing angles the pattern emerges as dense array of needles. The results support the pure mass redistribution based paradigm for surface patterning as the erosion due to sputtering essentially tends to zero.

Irradiation of crystalline muscovite mica samples by 500 eV Ar+ ions at different incident angles can induce significant surface morphological variations. A periodic ripple pattern of nano-dimensions forms in the angle window 47°-70°. On the other hand, tilted conical protrusions develop on the surface at grazing incidence angles around 80°. From the derivative of the topographic images the distribution of the side-facet slopes in the ion incidence plane are measured, which is found to be strongly related to the pattern morphology. Additionally, it has been shown that, for the ripple structures, the base angles can be tuned by changing the ion fluence. An asymmetric sawtooth profile of the ripples obtained at low fluence is transformed to a symmetrical triangular profile at high fluence. As the slopes are found to be small, the pattern formation is not provoked by the gradient-dependent erosion mechanism rather it is the general effect of the curvature-dependent sputtering phenomena.