Small Active Volume Research Articles

Strain rate sensitivity of a 1.5 GPa nanotwinned steel

Two distinct regimes of strain rate sensitivity on yield strength are found in a high-strength nantwinned steel. The yield strength increases from 1410 to 1776 MPa when the strain rate increases from 10–3 to 1400 s−1. It is proposed from the measured small activation volume that the yielding of the nanotwinned steel at higher strain rates is governed by the dislocation bowing out from the carbon atmosphere. At lower strain rates, however, the yielding is controlled by the continuous re-pinning of dislocations due to the fast diffused carbon atoms, which leads to the relative insensitivity of yield strength to the strain rate.

The Effect of Pressure on Grain‐Growth Kinetics in Olivine Aggregates With Some Geophysical Applications

AbstractThe effect of pressure on grain‐growth kinetics of olivine was investigated up to 10 GPa at 1773 K under relatively water‐poor conditions. The results are interpreted using a relation to obtain the activation volume = 5.0 ± 1.1 cm3/mol for n = 2 or = 5.2 ± 1.1 cm3/mol for n = 3. The small activation volume means that grain‐growth kinetics in pure olivine aggregates is fast even in the dry deep upper mantle, implying that grain‐size is controlled by the pinning by other phases or by dynamic recrystallization except for the early stage after the phase transformation from wadsleyite in upwelling materials. The present results are applied to seismic wave attenuation that is likely controlled by grain‐boundary processes. The inferred peak in attenuation just below the oceanic lithosphere‐asthenosphere boundary from the NoMelt array is difficult to be explained by the pressure effects assuming the absorption band behavior because such a model requires a much larger activation volume than determined in this work and it also fails to explain high attenuation in the deep asthenosphere. This suggests that either melt accumulation or other processes such as elastically accommodated grain‐boundary sliding (EAGBS) is responsible for the peak in attenuation. The present results are also applied to EAGBS. We suggest that the deep upper mantle is likely to be relaxed by EAGBS, which implies that the shear velocity of the deep upper mantle can be several percent smaller than that inferred from single crystal elasticity.

Selective laser melting of Cu–Ni–Sn: A comprehensive study on the microstructure, mechanical properties, and deformation behavior

A comprehensive understanding of the deformation behavior and strengthening mechanism of the selective laser melted (SLM) alloy is critical in reaping the complete potential of the SLM process. Accordingly, this work focuses on the influence of SLM on the microstructure, deformation behavior, and mechanical properties of the Cu–Ni–Sn alloy compared with their cast (CC) counterparts. An equiaxed-columnar bimodal structure with dispersed γ particles was observed due to the rapid solidification conditions, which helps in eliminating the otherwise formed brittle dendrite structure. The unique SLM microstructure with increased dislocation density offers a superior combination of tensile strength and ductility that is much higher than the CC samples. The SLM microstructure involving dislocation cells and fine precipitations led to a relatively high strain rate sensitivity (0.012 ± 0.002) and a small activation volume (~115 b3). It has been observed that in the as-prepared SLM material the strength contribution comes mainly from grain refinement, dispersed precipitation, and increased dislocation density.

Design of a single-mode directly modulated orbital angular momentum laser

We propose a design of single-mode orbital angular momentum (OAM) beam laser with high direct-modulation bandwidth. It is a microcylinder/microring cavity interacted with two types of second-order gratings: the complex top grating containing the real part and the imaginary part modulations and the side grating. The side grating etched on the periphery of the microcylinder/microring cavity can select a whispering gallery mode with a specific azimuthal mode number, while the complex top grating can scatter the lasing mode with travelling-wave pattern vertically. With the cooperation of the gratings, the laser works with a single mode and emits radially polarized OAM beams. With an asymmetrical pad metal on the top of the cavity, the OAM on-chip laser can firstly be directly modulated with electrical pumping. Due to the small active volume, the laser with low threshold current is predicted to have a high direct modulation bandwidth about 29 GHz with the bias current of ten times the threshold from the simulation. The semiconductor OAM laser can be rather easily realized at different wavelengths such as the O band, C band, and L band.

Plastic deformation of single crystals of the equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy in tension and compression from 10 K to 1273 K

The plastic deformation behavior of single crystals of the quinary, equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy (HEA) with the face-centered cubic structure has been investigated in tension and compression as a function of crystal orientation and temperature from 10 K to 1373 K. The critical resolved shear stress (CRSS) for {111}<110> slip at room temperature is 42−45 MPa. It does not depend much on crystal orientation (i.e., the Schmid law holds true) and the sense (tension vs. compression) of the applied load. The CRSS for {111}<110> slip increases with the decrease in temperature, without showing any significant inertia effects at cryogenic temperatures below 77 K. Extrapolation from the measured yield stresses down to 10 K yields a CRSS value at 0 K of 168 MPa. At cryogenic temperatures, the measured strain-rate sensitivity of flow stress is consistent with a very small activation volume. The concept of stress equivalence holds true both for the temperature dependence of CRSS and the stress dependence of activation volume, indicating that solid-solution hardening is the major strengthening mechanism. Deformation twinning occurs at 77 K but not at room temperature, resulting in higher tensile elongation to failure at 77 K than at room temperature. Deformation twinning at 77 K occurs at a shear stress of 378 MPa on conjugate (1¯1¯1) planes in the form of Lüders deformation after large plastic strain (about 85%) achieved by the stage I (easy glide) and stage II (linear work-hardening) deformation.

Suitability of Chambers in Relative Dose Measurement of Small Fields for Accelerated Photon Delivered by a Medical Linear Accelerator

Introduction: Using the small field in modern radiotherapy, the present study aimed at measuring the relative dosimetry (scattering factor, percentage depth dose (PDD), and profile of penumbra) with ionization (FC65-G, CC13, CC01) and diode (razor) chambers. Material and Methods: Applying TRS-398 in Varian Clinac™ IX-5982 for 6 MV photon beams, the conditions (pressure, temperature, direction, polarity) were kept the same for a set of field sizes (1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5, 7 × 7, and 10 × 10 cm2), and relative dosimetry was performed at the North-East Cancer Hospital, Sylhet, Bangladesh. Results: During the output factor measurement in small fields, the razor showed better results than CC13. Taking CC01 as a standard in small fields, the data obtained from the study showed a good agreement with those of the previously published works. Conclusion: Razor, with extremely small active volume, was very much suited for small field dosimetry, except for PDDs.

107 Gb/s Ultra-High Speed, Surface-Normal Electroabsorption Modulator Devices

Short reach communication systems, such as datacenters and access networks, exhibit steep capacity growth and require opto-electronic technologies with small size, low complexity, and low power consumption. Here, we demonstrate large-scale arrays of reflective surface normal electroabsorption modulators (SNEAMs). With very small active volumes, SNEAMs enable ultra-wide electro-optic bandwidth (>>65 GHz). We show modulation at 25 Gbit/s with 1 Vpp drive voltage on packaged SNEAMs and ultra-high bit-rate modulation at 107 Gbit/s with bare chips. These modulators are polarization-independent and have very low total input/output coupling loss of 0.7 dB to single-mode-fibers. We package SNEAM arrays with arrayed waveguide gratings into wavelength division multiplexing transmitters. Due to their broad wavelength range of modulation, SNEAMs do not need power hungry wavelength tuning or locking systems. Future co-integration of SNEAM arrays with low-power electronic driver arrays will enable high-capacity, low-power electro-optic engines.

λ-Scale Embedded Active Region Photonic Crystal (LEAP) Lasers for Optical Interconnects

The distances optical interconnects must cover are decreasing as Internet traffic continues to increase. Since short-reach interconnect applications require many transmitters, cost and power consumption are significant issues. Directly modulated lasers with a wavelength-scale active volume will be used as optical interconnects on boards and chips in the future because a small active volume is expected to reduce power consumption. We developed electrically driven photonic crystal (PhC) lasers with a wavelength-scale cavity in which the active region is embedded in a line-defect waveguide of an InP-based PhC slab. We call this a λ-scale embedded active region PhC laser, or a LEAP laser. The device, whose active region has six quantum wells with 2.5 × 0.3 × 0.15 μm3 active volume, exhibits a threshold current of 28 μA and provides 10 fJ/bit of operating energy to 25 Gbit/s NRZ (non-return-to-zero) signals. The fiber-coupled output power is 6.9 μW. We also demonstrate heterogeneous integration of LEAP lasers on a SiO2/Si substrate for low-cost photonic integrated circuits (PICs). The threshold current is 40.5 μA and the output power is 4.4 μW with a bias current of 200 μA. These results indicate the feasibility of using PhC lasers in very-short-distance optical communications.

Tensile properties, strain rate sensitivity, and activation volume of additively manufactured 316L stainless steels

The tensile properties of additively manufactured (AM) metals and alloys are among the most important variables that impact the potential applications of these materials. Here we examine and report on the tensile properties of AM 316L stainless steels fabricated by the laser powder-bed-fusion (L-PBF) technique, via twelve sets of optimized laser processing parameters that produce materials with density >98.8 ± 0.10%. A heterogeneous microstructure is observed in all L-PBF samples, including microscopic features such as dislocations, cellular walls, elemental segregations, local misorientations, impurities, precipitates, and a large fraction of low-angle grain boundaries (2-10°, ∼40–60%). The derived average grain size defined by high-angle grain boundaries (>10°) is ∼30–50 μm. Tensile testing reveals a yield strength ranging from 552 to 635 MPa and a tensile-elongation-to-failure (TEF) of 0.09–0.42 for directly-printed samples, whereas these values are 592–690 MPa and 0.29–0.50 for samples machined from the as-built rectangular thin plates. In all samples, we observe a variation of tensile yield strength within ∼15% but not the TEF, suggesting marginal microstructural changes despite a wide range of laser processing parameters. The large scatter of TEF in directly-printed samples originates from the sensitivity of thin gauge geometry (∼2 mm2 cross-section area) to the built-in flaws. We measured a substantially higher strain rate sensitivity (m∼0.02–0.03) of L-PBF 316L compared to the coarse-grained counterparts (∼0.006), together with a small activation volume of ∼20–30b3 (where b is the Burgers vector of 316L). These deformation kinetics parameters suggest that the tensile plasticity of L-PBF 316L is controlled by a much finer microstructural length scale than the measured grain size, consistent with the high strength and juxtaposed nano- to macro-structures seen in these materials. Strategies to optimize the tensile properties of AM materials are discussed.

The significant impact of grain structure on large strain-rate sensitivity of ultrafine-grained low alloy steel under nanoscale deformation: Experimental and theoretical analysis

Load-controlled nanoscale deformation experiments were carried out in conjunction with post-mortem electron microscopy of nanoscale deformation region involving the use of focused-ion beam, to obtain nanomechanical insights on the strain rate sensitivity of ultrafine grained structure (UFG) and compare with the coarse-grained (CG) counterpart. The UFG steel had a grain size of ∼530 nm and spherical precipitates of ∼80 nm. Both yield strength (σy) and ultimate tensile strength (σUTS) increased significantly with increased strain rate from 0.0015 to 0.15 s−1, with consequent decrease in elongation-to-failure (ɛf) during tensile straining. Nanoscale deformation experiments indicated an increase of hardness from 3.02 ± 0.05 GPa to 3.36 ± 0.05 GPa on increasing the strain rate from 0.05 to 0.5 s−1 with positive strain rate sensitivity (SRS) of 0.025, which was larger than the CG counterpart and is in striking contrast to the observed phenomena in bcc metals. From the theoretical analysis, it is envisaged that the small activation volume of ∼25 b3 (b-Burgers vector) is a consequence of combination of UFG and nanosized precipitates that prevented the movement of dislocations leading to a peculiar hardening behavior, which was responsible for the abnormal SRS and activation volume of UFG ferritic steel.

Highly Sensitive Resonators for EPR Spectroscopy of Submicroliter/Submicromolar Biomacromolecular Samples

We report ‘planar inverse anapole’ microresonators with high quality factors (Q-factors) and nanoliter active volumes, that increase both absolute sensitivity and concentration sensitivity of inductive-detection electron paramagnetic resonance (EPR) spectroscopy at room temperature. Due to its inherently high sensitivity, ability to interrogate specific sites by site-directed spin-labeling, and advances in pulsed measurements, EPR spectroscopy has gained importance as a technique to elucidate protein structure and dynamics. However, the limit of detection of state-of-the-art EPR spectrometers (approximately 50 uL of a 50 uM solution) is still too high compared to the amounts that are easily obtainable for some biomacromolecules - for example, membrane proteins, which are notoriously difficult to express and purify in sufficient amounts for macromolecular structural determination. To further decrease the detection limit, an advantageous approach is to miniaturize the resonator, thereby decreasing the interrogated sample volume. This miniaturization can be easily accomplished using standard photolithographic techniques to create resonant devices with micron-sized features and nanoliter active volumes.[1], [2] However, such miniaturization also increases radiation losses from the resonators, resulting in poor Q-factors and offsetting the gains in sensitivity achieved by decreasing the active volume. Here, we report an innovative planar microresonator that is designed to have minimal radiation losses at room temperature,[3] thereby ensuring high Q-factors without the need to operate at cryogenic temperatures. Using this combination of a small active volume and high Q-factor, we project an increase in the sensitivity of room-temperature EPR spectroscopy (and by extension, low-temperature EPR spectroscopy) of two orders of magnitude.

Indentation size effect and strain rate sensitivity of Ni3Ta high temperature shape memory alloy

The present work focuses on mechanical properties of Ni3Ta alloy. Reversible martensitic transformation is found in the temperature range of 520–600 K. At room temperature Ni3Ta alloy mainly consists of a monoclinic phase. Ni3Ta alloy exhibits a forward indentation size effect, the value of load independence hardness is equal to 5.3 GPa. The compressive behavior is dependent on strain rate, which is characterized by small activation volume (6.4 b3), corresponding to the grain boundary sliding as the dominant deformation mechanism.

Notch fracture toughness of body-centered-cubic (TiZrNbTa)[sbnd]Mo high-entropy alloys

The notch fracture toughness (KQ) and its dependence on Mo concentration in as-cast body-centered-cubic (TiZrNbTa)100-xMox high-entropy alloys have been measured at room temperature. It is shown that the increase of Mo concentration results in a significant reduction in fracture toughness, with the KQ decreasing from 28.5 MPa√m for the Mo-free TiZrNbTa quaternary alloy to 18.7 MPa√m for the TiZrNbTaMo quinary alloy. The KQ of these HEAs scales inversely with increasing (d + s) electrons per atom. The fracture mode under Mode I loading transists from monolithic intergranular fracture for Mo-free TiZrNbTa to completely transgranular cleavage for the TiZrNbTaMo alloy. The brittleness is consistent with the known effects of refractory solutes on increasing the brittle-to-ductile transition temperature in Nb-based solutions. The embrittlement effect with alloying (especially Mo) is also attributable to the elevation of the critical temperature (T0), making the activation to overcome lattice resistance to dislocation motion increasingly difficult. The low ratio T/T0 (T = 300 K in our case) can in fact be inferred from the very small activation volume (3b3) measured for TiZrNbTa and TiZrNbTaMo.

The diffusive character of extension twin boundary migration in magnesium

A theoretical study of the coherent boundary migration of extension twin in magnesium is performed focusing on twinning transformations of host lattices and homogeneous nucleation of twinning dislocations. The twinning transformation of host lattice has a small activation volume which usually characterizes a civilian/diffusive mechanism. The critical width of a twinning dislocation dipole is less than fifteen nanometers. The corresponding per-atom energy to activate a dipole is lower than the thermal energy kBT at room temperature. A highly adaptive migration behavior of coherent twin boundary is expected and in accordance with previous experiment observations.

OA237] The output factor measurements of cyberknife system using various detectors

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.

Suitability of microDiamond detectors for the determination of absorbed dose to water around high-dose-rate 192 Ir brachytherapy sources.

Experimental dosimetry of high-dose-rate (HDR) 192 Ir brachytherapy (BT) sources is complicated due to high dose and dose-rate gradients, and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector microDiamond (PTW 60019, Freiburg, Germany) has a small active volume, high sensitivity, direct readout, and nearly water-equivalent active volume. The purpose of this study was to evaluate the suitability of microDiamond detectors for the determination of absorbed dose to water around HDR 192 Ir BT sources. Three microDiamond detectors were used, allowing for the comparison of their properties. In-phantom measurements were performed using microSelectron and VariSource iX HDR 192 Ir BT treatment units. Their treatment planning systems (TPSs), Oncentra (v. 4.3) and BrachyVision (v. 13.6), respectively, were used to create irradiation plans for a cubic PMMA phantom with the microDiamond positioned at one of three source-to-detector distances (SDDs) (1.5, 2.5, and 5.5cm) at a time. The source was stepped in increments of 0.5cm over a total length of 6cm to yield absorbed dose of 2Gy at the nominal reference-point of the detector. Detectors were calibrated in 60 Co beam in terms of absorbed dose to water, and Monte Carlo (MC) calculated beam quality correction factors were applied to account for absorbed-dose energy dependence. Phantom correction factors were applied to account for differences in dimensions between the measurement phantom and a water phantom used for absorbed dose calculations made with a TPS. The same measurements were made with all three of the detectors. Additionally, dose-rate dependence and stability of the detectors were evaluated in 60 Co beam. The percentage differences between experimentally determined and TPS-calculated absorbed doses to water were from -1.3% to +2.9%. The values agreed to within experimental uncertainties, which were from 1.9% to 4.3% (k=2) depending on the detector, SDD and treatment delivery unit. No dose-rate or intrinsic energy dependence corrections were applied. All microDiamonds were comparable in terms of preirradiation dose, stability of the readings and energy response, and showed a good agreement. The results indicate that the microDiamond is potentially suitable for the determination of absorbed dose to water around HDR 192 Ir BT sources and may be used for independent verification of TPS's calculations, as well as for QA measurements of HDR 192 Ir BT treatment delivery units at clinical sites.

A High Power Density Series-Stacked Energy Buffer for Power Pulsation Decoupling in Single-Phase Converters

A high-efficiency, high-power-density buffer architecture is proposed for power pulsation decoupling in power conversion between dc and single-phase ac. We present an active decoupling solution that yields improved efficiency and reduced circuit complexity compared to existing solutions. In the proposed architecture, the main energy storage capacitor is connected in series with an active buffer converter across the dc bus. The series-stacked capacitor blocks the majority of the dc bus voltage to reduce the voltage stress on the buffer converter, such that fast, low-voltage transistors can be employed for the buffer converter. Moreover, the series capacitor provides the majority of the power pulsation decoupling through a wide voltage swing, and the buffer converter only needs to process a small fraction of the total power of the entire architecture, allowing a very small active circuit volume and very high system efficiency. The circuit operation and design constraints are analyzed in detail. In the proposed buffer architecture, the series stacking of a nearly lossless capacitor and a lossy converter presents a challenge of capacitor voltage balancing and power loss compensation. We propose a control scheme exploiting the small ripple in the bus voltage and dc input current to compensate for the power loss in the buffer converter while maintaining the voltage balance. Light-load techniques are also introduced to ensure that the buffer architecture meets strict ripple requirements while providing sufficient loss compensation. A 2-kW hardware prototype based on low-voltage GaN switches has been built to demonstrate the performance of the proposed solution. A power density of 25 W/cm $^3$ (410 W/in $^3$ ) by rectangular box volume and an efficiency above 98.9% across a wide load range has been experimentally verified.

Identical activation volumes of dislocation mobility in the [100](010) and [001](010) slip systems in natural olivine

AbstractDislocation recovery experiments were performed on predeformed olivine single crystals at pressures of 2, 7 and 12 GPa and a constant temperature of 1650 K to determine the pressure dependence of the annihilation rate constants for [100](010) edge dislocation (a dislocation) and [001](010) screw dislocation (c dislocation). The constants of both types of dislocations are comparable within 0.3 orders of magnitude. The activation volumes of a and c dislocations are small and identical within error: 2.7 ± 0.2 and 2.5 ± 0.9 cm3/mol, respectively. These values are slightly larger and smaller than those of Si lattice and grain‐boundary diffusions in olivine, respectively. The small and identical activation volumes for the a and c dislocations suggest that the pressure‐induced fabric transition is unlikely in the asthenosphere. The decrease in seismic anisotropy with depth down in the asthenosphere may be caused by the fabric transition from A type or B type to AG type with decreasing stress with depth.

Spectral pointillism of enhanced Raman scattering for accessing structural and conformational information on single protein.

In this contribution, we provide new insights on the temporal fluctuations of surface enhanced Raman spectra (SERS) of large single molecules such as proteins. Because they can only fit partly into small active volume, SERS analysis is referred to spectral pointillism where only protein subdomains are shined and the whole protein landscape is built from the dynamics of successive individual spectra. By applying our approach on bovine serum albumin, we show that single protein subdomains are mostly comprised of three distinct amino acids. Surface amino acids such as lysine are preferentially detected in the open form of the protein. The investigation of the tryptophan Fermi doublet in the single protein regime is highly instructive on the protein conformation. We finally demonstrate that spectral pointillism enables to correlate individual amino acids with structural information.

Thermally activated deformation and the rate controlling mechanism in CoCrFeMnNi high entropy alloy

The nature of obstacles to dislocation motion in CoCrFeMnNi alloy was analyzed using the thermally activated deformation analyses at low temperatures. The strong temperature dependence of yield stress and small activation volume in CoCrFeMnNi favor the dislocation glide over the obstacles with high friction stress. The activation volume of CoCrFeMnNi alloy (10–100 b3) in this study is much smaller than those of conventional FCC metals (102~103 b3), but close to those observed in BCC metals (8–100 b3) and HCP metals (5–100 b3). The increase of the activation volume with strain supports overcoming the nanoscale inhomogeneity such as co-clusters and/or short range orders as the rate controlling mechanism. The transition of dislocation structure from planar array to cell structure at 20% strain in CoCrFeMnNi reported in the literature can be attributed to the prevalent shearing of nanoscale inhomogeneity with strain.