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Zhang-Jie Wang Qing-Jie Li Yi-Nan Cui Zhan-Li Liu Evan Ma Ju Li Jun Sun Zhuo Zhuang Ming Dao Zhi-Wei Shan Subra Suresh 《Proceedings of the National Academy of Sciences of the United States of America》2015,112(44):13502-13507
When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.Metal single crystals of submicrometer dimensions with very low initial defect density often exhibit ultrahigh strength (1, 2) and large elastic strain, which offer opportunities for obtaining unprecedented physical and chemical characteristics by elastic strain engineering (3). High-temperature crystal growth (4, 5) or thermal annealing to eliminate structural defects such as dislocations (6) serves as a traditional route to produce such nearly pristine materials that are essentially free of defects.Research in recent years has shown that monotonic loading can reduce or even eliminate dislocations in submicroscale single crystals (1, 7, 8). Such “mechanical annealing,” however, comes at the expense of severe plastic deformation and significant changes in shape and/or dimensions of the crystal (9–12). Mechanical annealing could potentially serve as an attractive pathway for tailoring the properties of nanoscale devices in such applications as nanoprint embossing (13, 14) and electronics (15, 16), if defects could be eliminated by recourse to nondestructive means that do not introduce changes in crystal shape and/or dimensions.A dislocation in the vicinity of a free surface experiences an image force that tends to draw the defect toward the surface, with the magnitude of the force increasing as the dislocation moves closer to the free surface with an attendant reduction in its stored elastic energy (17). Here we hypothesize that one possible route to achieve mechanical healing in a metal single crystal would, therefore, entail the application of an external stress that is large enough to induce preexisting dislocations to move from their initial positions so that the image forces from the free surfaces of the small crystal could then attract them to the surface to facilitate their egress from the crystal. At the same time, we also envision that the externally imposed stress should not be large enough to introduce new dislocations either by heterogeneous surface nucleation or by dislocation interactions. The challenge in addressing these competing effects stems from the fact that the configuration of the preexisting dislocations could be extremely complex and that the critical stress necessary to activate or annihilate them may be high enough to generate new dislocations. Inspired by the common observation that it is much easier to pull out a partly buried object by shaking it first, we hypothesize that imposition of low amplitudes of cyclic straining could serve as an effective means to disengage those complex preexisting dislocation configurations in a crystal. Such fatigue-induced mechanical annealing would entail repeated low-stress amplitude loading that would cause “healing” by inducing pinned dislocations to move by dislodging them to be drawn toward the nearest free surface; the stress amplitude should not be large enough to facilitate the nucleation of new dislocations at surface heterogeneities or to foster the onset and growth of fatigue cracks (18).We demonstrate the validity of our hypothesis here by presenting unique experimental results of mechanical healing in pure aluminum single crystals, which are subjected to low-amplitude cyclic straining inside a transmission electron microscope (TEM). The experimental setup is shown in Fig. 1A, where the diamond grip used to impose a tensile strain on the dog-bone–shaped specimen is shown. Both the grip and the specimen were fabricated using the focused ion beam (FIB) technique. Two reference markers were purposely fabricated on the specimen to allow for accurate measurement of strain in the gauge section of an aluminum single crystal, 300 nm thick and 500 nm wide (Fig. 1 A–C). All of the tests were carried out at room temperature, under displacement rate control at a cyclic frequency of 0.45 cycle/s. The loading axis was along the crystallographic direction of the face-centered cubic (FCC) Al crystal, and the beam direction was close to the [110] zone axis, as confirmed by the selected area electron diffraction pattern. Two types of dislocations were identified in the as-fabricated aluminum crystal: dislocation lines inherited from the bulk crystal or generated during the fabrication process and Frank dislocation loops introduced by ion beam irradiation during FIB cutting, which usually occur in the regions close to the free surface (19). The initial densities of dislocation lines and dislocation loops in the as-fabricated crystal specimen were measured to be ∼2.0 × 1013 m–2 and 4.6 × 1013 m–2, respectively (Fig. 1D).Open in a separate windowFig. 1.Cyclic mechanical healing of the aluminum single crystal. (A) Experimental setup for quantitative real-time observations of cyclic tensile loading of the aluminum single crystal inside a TEM. (B and C) Dark-field TEM images of the single crystal before and after the cyclic mechanical healing treatment, respectively. C shows an essentially dislocation-free crystal at the scale of observation, whereas B shows preexisting dislocations. (D) Changes in the density of dislocation lines and dislocation loops after each cyclic straining sequence.Six groups of cyclic loading, G1–G6, were imposed on the aluminum crystal, with images of the crystal before cyclic loading and after completion of the G6 loading sequence shown in Fig. 1 B and C, respectively. The maximum nominal tensile strain applied on the crystal in this test was 0.006 (Fig. 1 B and C, it is seen that at the end of loading sequence G6, essentially all dislocations in the gauge length in between the two markers were driven out of the crystal. This observation was also confirmed by tilt examination in the TEM (details in SI Text, Fig. S1). After about 700 loading cycles with a maximum imposed nominal tensile strain of 0.006 ± 0.002, the total accumulated tensile plastic strain in the gauge length of the specimen was assessed to be only 0.002 ± 0.001. This demonstrates that low-amplitude cyclic deformation can indeed result in mechanical healing, consistent with our hypothesis. The last (G6) loading led to failure outside the gauge section (at the location marked by the red arrow in the bottom left corner of Fig. 1C), where abundant dislocation populations were preserved. Note that the larger cross-section area outside of the gauge section implies a lower nominal stress than within the gauge section. The fact that plastic yielding is observed at the lower-stress area outside the gauge section also supports the notion that the gauge length of the crystal has developed a higher strength due to mechanical healing through defect removal.
Open in a separate windowNote that the maximum strain values for G1–G4 and the minimum strain values for all six loading sequences were too small to be measured. They are indicated here as “—’’.
Open in a separate windowThe symbols “√” and “×” denote visible and invisible, respectively. Bold text indicates a vector.Open in a separate windowFig. S1.Dark-field TEM images of the Al sample shown in Fig. 1 after mechanical healing in the main text. (A) g = ; (B) g = . (Scale bars, 500 nm.) Bold text g denotes a reflection vector.Dislocation lines in the crystal behaved differently from dislocation loops under repeated cyclic straining, as shown in Fig. 1D where the dislocation density is plotted as a function of loading sequence through the six groups of low-amplitude fatigue. The density of dislocation lines (shown by red bars in Fig. 1D) decreased gradually with cyclic straining. Here, change from one loading sequence to another, from G1 to G6, resulted in transient effects that were “shaken down” to a lower density state at the conclusion of each loading group except G4. The extent of dislocation density drop in response to each loading group is determined by the peak stress differential with respect to the previous loading group. For example, if the nominal maximum tensile stress at the beginning of a loading sequence in a group was higher than that at the previous group (e.g., G1 to G2, G2 to G3, G4 to G5, and G5 to G6), extensive dislocation escape occurred at the free surface that gradually stabilized to a lower dislocation density by the time the end of the loading sequence in that group was reached. However, if the maximum stress level was lowered compared with that in the previous loading group (e.g., G3 to G4), little change could be detected in the density of dislocation lines. Specifically, when the stress was lowered from 170 MPa (loading sequence G3) to 90 MPa (loading sequence G4), the density of dislocation lines remain unchanged (Fig. 1D, blue bars) decreased only slightly. However, when the nominal stress was increased to 250 MPa, from G4 to G5, the density of dislocation loops dropped dramatically. This suggests that the nominal value of critical stress necessary to annihilate the dislocation loops is between 170 MPa and 250 MPa for the particular geometrical conditions of the experiment.Detailed analysis of real-time, in situ video images from the TEM experiments revealed that in response to cyclic loading, pinned dislocation lines first experienced oligodynamic motion over many cycles before exhibiting a sudden and dramatic change in morphology. One typical example is shown in Fig. 2. The highlighted dislocation line in Fig. 2 is the same one framed by the white rectangle in Fig. 1B. The nominal engineering stress of cycles 10 and 11 in loading sequence G4 is shown in Fig. 2A. The still frames extracted from Movie S1 corresponding to the stress states marked b–f in Fig. 2A are shown in Fig. 2 B–F, respectively. The profile of the curved dislocation chosen in this illustrative example is delineated in turn with white (Fig. 2B), red (Fig. 2C), light blue (Fig. 2D), green (Fig. 2E), and brown (Fig. 2F) dashed lines, with a different color chosen to represent the (changing) dislocation positions corresponding to those points marked in Fig. 2A. At the end of loading cycle 10, denoted as position d (Fig. 2A), the profile of the curved dislocation was only slightly different from that at the start of this loading cycle, denoted as b (Fig. 2D). A similar scenario was observed during the loading cycles before cycle 10 in loading sequence G4. However, the dislocation broke away from its original pinned position during loading cycle 11 (Fig. 2E) and changed its profile significantly from d to f (compare Fig. 2 D and F). Our observations from video images reveal that dislocation loops frequently served as pinning points for dislocation lines. Incremental changes in dislocation position and morphology in response to cyclic straining were often followed by an abrupt change; this pattern repeated until the dislocation was driven out of the crystal.Open in a separate windowFig. 2.Abrupt depinning and destabilization of a dislocation line due to repeated cyclic loading. (A) Evolution of nominal tensile stress during two selected cycles: 10 and 11 in loading sequence G4, described in Movie S1 that corresponds to the stress state marked in A. The direction of tensile straining is indicated by the arrows in C and E. (Scale bars in B–F, 200 nm.)In the first set of experiments described thus far, protruding markers were introduced in the single crystal, as noted earlier, to delineate clearly the gauge length of the specimen within which dislocation density and cyclic strain could be quantitatively determined. Although the presence of these markers provided reproducible trends in the overall mechanistic response, these protrusions could also serve as local stress concentrators that influence local dislocation density evolution (Movie S2). To avoid this artifact and to demonstrate further that mechanical healing occurs without the presence of these markers, aluminum single-crystal specimens without markers were fabricated using FIB processing.In all, we tested three specimens without protruding markers and six specimens with protruding markers. Fig. 1 provides a typical example of results obtained for specimens with protruding markers. All of the tests reported in this work were carried out under displacement rate control. To avoid introducing additional plastic deformation and transient cyclic deformation behavior, we purposely started to load all of the samples gradually from small displacement amplitudes. The normal stress was calculated to be the engineering stress, which is the readout force divided by the cross-sectional area.One typical example from a specimen without the protruding reference markers is shown in Fig. 3, where the specimen was 350 nm in thickness and 387 nm wide. The loading axis was along the direction and the beam direction was close to the [110] zone axis. Preexisting dislocations could be seen in the as-fabricated sample (Fig. 3A, Top). Three groups of cyclic straining were carried out to eliminate defects in the specimen. The corresponding stress vs. number of cycles is plotted in Fig. 3B. The nominal strain amplitudes used in these three groups of cyclic loadings were 0.64% (black), 1.29% (red) and 1.29% (blue), respectively. The abnormal stress evolution observed in the second group of cyclic loading (between cycles 28 and 40) was due to the uneven plastic deformation and a transient bending load resulted from unsymmetrically escaped dislocations. After repositioning the grip to compensate for the small tensile plastic strain generated during the second group of cyclic straining (Fig. S2), the third group of cyclic straining yielded an apparently pristine crystal. As shown in Fig. 3A, Bottom, no dislocations could be detected in the gauge section after 128 loading cycles. The specimen width was altered by less than 0.52% during cyclic loading that resulted in mechanical healing. Fig. 3C shows a plot of the nominal tensile stress as a function of the nominal tensile displacement of the specimen in 77, 93, and 128 cycles. Note that the area of the hysteresis loop exhibited by the stress–displacement curve diminishes with increasing number of cycles, as the crystal is mechanically healed through low-amplitude fatigue loading by eliminating dislocations. For this set of experiments, the dislocation density in the gauge section of the as-fabricated specimen was measured to be 5.47 × 1013 m−2. After 77 cycles, this dislocation density decreased to 6.49 × 1012 m−2. For n = 93 and n = 128, the density of visible dislocations dropped to essentially zero. When the gauge section of the crystal visibly becomes free of dislocations after 128 cycles, the associated stress–displacement also becomes free of hysteresis, thereby indicating a nearly fully reversible elastic deformation response during cyclic loading. The negative nominal stress upon unloading (Fig. 3 B and C) and the small mismatch between the loading and unloading segments of the curve are a consequence of adhesion between the grip and the specimen as well as plastic deformation outside of the gauge section (SI Text and Fig. S2). Subsequent monotonic loading in the strain-to-failure test demonstrated that this specimen yielded at a stress as high as 800 MPa followed by an abrupt stress drop and a large strain burst (red curve in Fig. 4).Open in a separate windowFig. 3.Changes in deformation characteristics of Al single crystal from mechanical healing. (A) Bright-field images of aluminum single crystal before (Top) and after (Bottom) mechanical healing. (B) Detailed loading parameters and the stress vs. number of cycles. (C) The force vs. displacement curves during different loading cycles, n = 77, n = 93, and n = 128.Open in a separate windowFig. 4.Engineering stress plotted as a function of plastic strain from strain-to-failure experiments on four specimens with different degrees of cyclic mechanical healing. The cyclic loading parameters are described in terms of maximum tensile stress (in megapascals) and number of cycles within each loading sequence (cited in parentheses).Open in a separate windowFig. S2.Schematic illustration of the origin of the compressive stress upon unloading. (A) Programmed specimen setup. (B) Once plastic deformation sets in, the zero stress position would be shifted by a distance that is equal to the plastic elongation of the sample (ΔL).Three key parameters dominate the healing effect: stress amplitude, number of cycles, and the loading sequence. Fig. 4 is a plot of the nominal maximum tensile stress as a function of nominal plastic strain during the strain-to-failure test of four specimens that were subjected to different mechanical fatigue healing treatments. The red curve in Fig. 4 (specimen S1) shows the data points for the specimen without protruding markers, for which results are presented in Fig. 3. The other three sets of data in Fig. 4 are from specimens that contained protruding markers. Specimen S2 was cycled 150 times under 200 MPa and 150 times under 550 MPa. Specimen S3 was cycled 50 times under 200 MPa and 50 times under 425 MPa. Specimen S4 is the as-fabricated specimen without any cyclic loading treatment. These results indicate that the mechanical properties of submicron-sized samples could be tailored through cyclic mechanical healing by the appropriate selection of stress amplitude, number of cycles, and the loading sequence.These results conclusively show that mechanical healing by defect elimination can be achieved by recourse to low-amplitude cyclic deformation, which leads to significantly improved apparent yield strength. The method demonstrated here thus offers a unique means to tune mechanical properties of submicron-sized metal crystals through cyclic loading while at the same time preserving its overall geometry. Microscale and macroscale FCC metal crystals subjected to low-amplitude cyclic loading generally exhibit easier nucleation and growth of fatigue cracks concomitantly with the development of unique dislocation patterns (such as edge dislocation dipoles forming vein-like structures as well as ladder-like structures known as persistent slip bands) than monotonic loading of the same maximum nominal tensile stress (18). The continual increases in dislocation density during cyclic loading, concomitantly with interactions among dislocations that produce geometric patterns that are unique to fatigue deformation, are known to be precursors for surface features such as intrusions and extrusions. These surface features are known to be sites at which fatigue cracks are formed in single-crystalline and polycrystalline metals and alloys of microscopic and macroscopic dimensions. These mechanisms of dislocation generation and rearrangement in conventional fatigue processes also influence cyclic strain hardening or cyclic strain softening in metals and alloys (18). However, our results presented here show that submicroscale pure Al crystals, when subjected to cyclic loading, exhibit strengthening through elimination of preexisting dislocations under appropriate conditions, without developing fatigue cracks.Our experiments also indicate that even specimens that undergo full mechanical healing exhibit posthealing yield strength well below the theoretical strength. For example, the yield strength of the crystal in Fig. 3 is only ∼800 MPa after mechanical healing (red curve in Fig. 4), whereas its theoretical strength is on the order of several gigapascals. This can be rationalized by noting that, although dislocations are removed during cyclic loading, slip steps will be created on sample surfaces (20). Consequently, the specimen surfaces are still susceptible to dislocation nucleation compared with an atomically smooth surface.To verify whether slip steps at surfaces, of atomic spacing dimensions, of the aluminum crystal can lead to significant flow strength reduction, we conducted atomistic simulations, using quasistatic athermal loading to model athermal yield strength and the free-end nudged elastic band (FENEB) method to assess the activation energy of dislocation nucleation as a function of applied stress level. See SI Text for details of the procedures used to perform atomistic simulations. The results of atomistic simulations of the mechanical behavior of pristine aluminum single crystals without any surface steps and those with 1b height (where b is the magnitude of the Burgers vector) surface steps are shown in Fig. S3. These results provide a mechanistic rationale for the experimental observations that cyclically healed Al single crystals exhibit flow strengths that are well below the theoretical limit values.Open in a separate windowFig. S3.Atomistic simulations of the mechanical behavior of pristine aluminum pillars without and with a surface step. (A) Strain–stress curves (from quasistatic athermal loading) for a perfect pillar and a pillar with a 1b step. Inset shows the simulation setup for these two cases. (B) Activation energy as a function of axial stress computed using the free-end nudged elastic band (FENEB) method.In summary, the in situ experiments described here demonstrate that cyclic healing of the small-volume Al single crystals could be achieved through low-amplitude cyclic deformation. By performing these unique experiments inside a TEM, we have observed the forward and reverse movements of dislocations during the loading and unloading cycles. The irreversibility of such dislocation motion leads to the unpinning of a population of dislocations that are pulled to the free surfaces of the crystal by image forces, which induce the dislocations to exit the crystal. The attendant drastic reduction in dislocation density of the crystal causes cyclic healing, which, in turn, results in a marked increase in the strength of the crystal.These results of cyclic-strain–induced healing and strengthening of the nanoscale, small-volume single crystals are in distinct contrast to traditional processes associated with cyclic strain hardening or softening of micrograined, large-volume metals and alloys of monocrystalline and polycrystalline structures (18). Consider, for example, the specific case of copper, where rapid increases in cyclic yield strength are commonly observed as a consequence of continuous generation and reorganization of dislocation structures during cyclic loading, which lead to unique geometric patterns such as dislocation vein structures, ladder structures composed of edge dislocation dipoles, and persistent slip bands (18). It is also known that specimens of copper with high initial dislocation density, such as those initially subjected to severe cold work, usually cyclically strain soften as the repeated cycling of the crystal leads to rearrangements of dislocations that seek a lower-energy equilibrium state (21, 22). Conversely, well-annealed copper specimens with low initial dislocation density cyclically strain harden as a result of fatigue-induced generation of dislocations whose interactions significantly strengthen the crystal and lead to an equilibrium state that remarkably has the same steady-state strength characteristics as the initially cold-worked copper subjected to cyclic loading (21, 22). Similar results were also reported in macroscopic specimens of polycrystalline copper, containing an initially low or high density of nanoscale twins, which were subjected to repeated frictional sliding (23). In this case, copper specimens with both a high and a low density of nanotwins eventually converged to a similar structure and local strength characteristic after repeated tribological deformation (23).The results presented in this paper, by contrast, point to a different and unique mechanism of healing of a submicroscale metal crystal in response to cyclic deformation, whereby defect elimination and strengthening are achieved. Our work thus demonstrates unique avenues for tailoring the defect structure and strengthening of nanoscale metal crystals, thereby circumventing the need for thermal treatments or large plastic deformation that could lead to significant dimensional and shape changes. 相似文献
Table 1.
Details of cyclic loading sequence for the Al crystal shown in Fig. 1| Cyclic loading sequence group no. | N | εmax | εmin | σmax, MPa | σmin, MPa | ρtype 1, 1012 m−2 | ρtype II, 1012 m−2 | Image |
| As fabricated | 0 | 20.4 | 46.1 | Fig. 1B | ||||
| G1 | 80 | — | — | 38 | 12 | 14.8 | 42.5 | |
| G2 | 175 | — | — | 44 | −23 | 11.1 | 38.8 | |
| G3 | 80 | — | — | 170 | 16 | 8.5 | 39.5 | |
| G4 | 65 | — | — | 90 | 0 | 8.7 | 37.0 | |
| G5 | 300 | 0.004 ± 0.002 | — | 250 | −92 | 4.1 | 0.13 | |
| G6 | 1 | 0.006 ± 0.002 | — | 420 | 14 | 1.1 | 0.13 | Fig. 1C |
Table S1.
Visibility of dislocations with various Burgers vectors (2b) under different reflection vectors (g) in the TEM| 2b | ||||||
| ±[110] | ±[101] | ±[011] | ||||
| g = | × | √ | √ | × | × | × |
| g = | × | √ | √ | √ | √ | √ |
| g = | √ | √ | × | √ | √ | √ |
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Stephan C. Schrader Rafael Beyar 《Catheterization and cardiovascular interventions》1998,44(2):179-187
The mechanical properties of metal stents are important parameters in the consideration of stent design, matched to resist arterial recoil and vascular spasm. The purpose of this study was to develop a system for a standardized quantitative evaluation of the mechanical characteristics of various coronary stents. Several types of stents were compressed by external hydrostatic pressure. The stent diameter was assessed by placing a pair of small ultrasonic sono-crystals on the stent. From pressure-strain diagrams the ultimate strength and radial stiffness for each stent were determined. For all stents, except the MICRO-II and the Wiktor stent, the diameter decreased homogeneously until an ultimate compressive strength was exceeded, causing an abrupt collapse. Expanded to 3 mm, the mechanical behavior of the beStent, the Crown and the Palmaz-Schatz stent (PS153-series) were comparable. The spiral articulated Palmaz-Schatz stent showed twice the strength (1.26 atm) of the PS-153 (0.65 atm). The NIR stent yielded a maximum strength of 1.05 atm. The MICRO-II and the Wiktor stent did not collapse abruptly but rather showed a continuous decline of diameter with increasing external pressure. The Cardiocoil stent behaved in a fully elastic manner and showed the largest radial stiffness. Difference in mechanical properties between stents were documented using a new device specifically developed for that purpose. These mechanical stent parameters may have important clinical implications. Cathet. Cardiovasc. Diagn. 44:179–187, 1998. © 1998 Wiley-Liss, Inc. 相似文献
4.
Linan Zhou Minhan Lou Junwei Lucas Bao Chao Zhang Jun G. Liu John Mark P. Martirez Shu Tian Lin Yuan Dayne F. Swearer Hossein Robatjazi Emily A. Carter Peter Nordlander Naomi J. Halas 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(20)
Light-induced hot carriers derived from the surface plasmons of metal nanostructures have been shown to be highly promising agents for photocatalysis. While both nonthermal and thermalized hot carriers can potentially contribute to this process, their specific role in any given chemical reaction has generally not been identified. Here, we report the observation that the H2–D2 exchange reaction photocatalyzed by Cu nanoparticles is driven primarily by thermalized hot carriers. The external quantum yield shows an intriguing S-shaped intensity dependence and exceeds 100% for high light intensities, suggesting that hot carrier multiplication plays a role. A simplified model for the quantum yield of thermalized hot carriers reproduces the observed kinetic features of the reaction, validating our hypothesis of a thermalized hot carrier mechanism. A quantum mechanical study reveals that vibrational excitations of the surface Cu–H bond is the likely activation mechanism, further supporting the effectiveness of low-energy thermalized hot carriers in photocatalyzing this reaction.Metal nanostructures that couple strongly to light and exhibit a driven, resonant oscillation of their free electrons, known as a localized surface plasmon resonance (LSPR), have attracted considerable recent interest for their role in light-driven chemical transformations (1). Both monometallic (2–5) and “antenna-reactor” multimetallic (6–9) plasmonic photocatalysts have been shown to be superior to traditional thermocatalysts in terms of activity, selectivity, and stability for a number of fundamental and industrially significant reactions. Chemical reactions involving adsorbates electronically (1) or vibrationally (10) activated by the energetic electrons or holes resulting from LSPR decay provide a widely accepted mechanism in plasmon-induced photochemistry (11, 12). Hot carriers (HCs) have been shown to reduce activation barriers and facilitate molecular rearrangements.The nonradiative decay of a surface plasmon results in the generation of two types of HCs: 1) the initial nonthermal HCs first generated by Landau-like damping, and 2) thermalized HCs, i.e., lower energy HCs created during relaxation and carrier multiplication through electron-electron interaction (13). The distribution of these relaxed HCs can be described phenomenologically by a high-effective-temperature Fermi-Dirac distribution (14), which is why they are referred to as thermalized HCs. Nonthermal HCs are typically regarded as the primary mediator of most HC-induced chemical processes, primarily because of their relatively long lifetime compared to HCs in extended metals and their higher energies relative to thermalized HCs. Nevertheless, the potential contribution of thermalized HCs is never excluded. Avanesian and Christopher have investigated the effect of thermalized HCs theoretically and shown that the contribution of thermalized HCs increases with the decrease of charge transfer barrier (15). However, experimental research highlighting the contribution of thermalized HC and exploring the corresponding kinetic behavior is still lacking.Here, we report a plasmonic photocatalysis system showing singular behavior where high–effective-temperature HCs appear to be the dominant contributor. Specifically, Cu nanoparticles grown onto a mixed metal oxide support photocatalyze H2 and D2 dissociation, detected by the formation of HD under laser illumination. The contribution of HCs was extracted from the total photocatalytic rate by subtracting the photothermally induced reaction rate, determined from the measured thermocatalytic rate together with simulated temperature distributions. The HC-mediated reaction rate exhibits a unique S-shape intensity dependence and an external quantum yield (EQY) greater than 1. An adiabatic model was applied to calculate the quantum yield of the thermalized HCs, which reproduces the quantum yield of the HC-mediated reaction rate and supports the observed above-unity quantum yield. First-principles quantum mechanical calculations identified the rate-determining step (the desorption of the diatomic molecule) and showed that HC activation to enable excited-state, lower-barrier HD desorption is likely achieved through sequential vibrational excitations of the surface Cu–H bond, which can be driven readily by low-energy HCs. 相似文献
5.
This paper shows the results of a study focused on the evolution and properties of mortars made with a mixture of portland cement (PC) and natural mordenite (Mor). To begin, samples of mordenite, cement and sand were studied with X-ray diffraction (XRD), X-ray fluorescence (XRF) and granulometric analysis (GA). Next, mortars with a ratio of 75% PC and 25% mordenite were prepared to determine their initial and final setting times, consistency and density. Continuing, the density, weight and compressive strength of the specimens were determined at 2, 7, 28, 90 and 365 days. Finally, the specimens were studied using SEM, XRD and XRF. The results of the study of the mordenite sample showed a complex constitution where the major mineral component is mordenite, and to a lesser degree smectite (montmorillonite), halloysite, illite, mica, quartz, plagioclase and feldspar, in addition to altered volcanic glass. Tests with fresh cement/mordenite mortar (CMM) showed an initial setting time of 320 min and a final setting time of 420 min, much longer than the 212–310 min of portland cement mortar (PCM). It was established that the consistency of the cement/mordenite mortar (CMM) was greater than that of the PCM. The results of the density study showed that the CMM has a lower density than the PCM. On the other hand, the density of cement/mordenite specimens (CMS) was lower than that of portland cement specimens (PCS). The CMS compressive strength studies showed a significant increase from 18.2 MPa, at 2 days, to 72 MPa, at 365 days, with better strength than PCS at 28 and 365 days, respectively. XRD, XRF and SEM studies conducted on CMS showed a good development of primary and secondary tobermorite, the latter formed at the expense of portlandite; also, ettringite developed normally. This work proves that the partial replacement of PC by mordenite does not have a negative effect on the increase in the mechanical strength of CMS. It indicates that the presence of mordenite inhibits the spontaneous hydration of C3A and controls the anomalous formation of ettringite (Ett). All this, together with the mechanical strength reported, indicates that mordenite has a deep and positive influence on the evolution of the mortar setting and is an efficient pozzolan, meaning it can be used in the manufacture of mortars and highly resistant pozzolanic cement, with low hydration heat, low density, stability in extremely aggressive places and a low impact on the environment. 相似文献
6.
This study investigates the interface bond strength and anchorage performance of steel bars within prefabricated concrete. Twenty-two specimens were designed and manufactured to study the interface bond behavior of deformed and plain steel bars under a larger cover thickness. Diameter of steel bars, strength grade of concrete, and anchorage length were considered influential factors. The finite element method (ABAQUS) was used for the validation of experimental results. The interface bond’s failure mechanism and the anchorage length in the prefabricated concrete under different concrete strength levels were explored and compared to national and international codes. A suitable value of the basic anchoring length for the prefabricated structure was recommended. The results show that the interface bond strength of prefabricated bridge members is directly proportional to the strength grade of the concrete, inversely proportional to the reinforcement diameter, and less related to anchorage length. The effect of the cover thickness of the surrounding concrete is negligible. Conversely, the bearing capacity of prefabricated bridge members depends on the strength of the concrete, the diameter of the steel bar, and the anchorage length. Furthermore, it is concluded that the mechanical bond strength accounts for 88% of the bond strength within prefabricated concrete. 相似文献
7.
Jan Valí?ek Marta Harni?árová Andreas ?chsner Zuzana Hutyrová Milena Ku?nerová Hakan Tozan Vít Michenka Vladimír ?epelák Du?an Mita? Jozef Zajac 《Materials》2015,8(11):7401-7422
The paper solves the problem of the nonexistence of a new method for calculation of dynamics of stress-deformation states of deformation tool-material systems including the construction of stress-strain diagrams. The presented solution focuses on explaining the mechanical behavior of materials after cutting by abrasive waterjet technology (AWJ), especially from the point of view of generated surface topography. AWJ is a flexible tool accurately responding to the mechanical resistance of the material according to the accurately determined shape and roughness of machined surfaces. From the surface topography, it is possible to resolve the transition from ideally elastic to quasi-elastic and plastic stress-strain states. For detecting the surface structure, an optical profilometer was used. Based on the analysis of experimental measurements and the results of analytical studies, a mathematical-physical model was created and an exact method of acquiring the equivalents of mechanical parameters from the topography of surfaces generated by abrasive waterjet cutting and external stress in general was determined. The results of the new approach to the construction of stress-strain diagrams are presented. The calculated values agreed very well with those obtained by a certified laboratory VÚHŽ. 相似文献
8.
Jawad Ahmad Zhiguang Zhou Rebeca Martínez-García Nikolai Ivanovich Vatin Jesús de-Prado-Gil Mohammed A. El-Shorbagy 《Materials》2022,15(7)
The by-product of the foundry industry is waste foundry sand (WFS). The use of WFS in building materials will safeguard the ecosystem and environmental assets while also durable construction. The use of industrial waste in concrete offsets a shortage of environmental sources, solves the waste dumping trouble and provides another method of protecting the environment. Several researchers have investigated the suitability of WFS in concrete production instead of natural river sand in the last few decades to discover a way out of the trouble of WFS in the foundry region and accomplish its recycling in concrete production. However, a lack of knowledge about the progress of WFS in concrete production is observed and compressive review is required. The current paper examines several properties, such as the physical and chemical composition of WFS, fresh properties, mechanical and durability performance of concrete with partially substituting WFS. The findings from various studies show that replacing WFS up to 30% enhanced the durability and mechanical strength of concrete to some extent, but at the same time reduced the workability of fresh concrete as the replacement level of WFS increased. In addition, this review recommended pozzolanic material or fibre reinforcement in combination with WFS for future research. 相似文献
9.
Mechanical surface treatments are needed to perform on components for fatigue life enhancement by introducing beneficial compressive residual stress and material strengthening. In this study, the combined turning with low plasticity burnishing (LPB) surface modification process was performed for the sake of improving mechanical properties of Inconel 718. Firstly, the evolution of microstructure and residual stress after the LPB process were analyzed with the aid of electron backscatter diffraction (EBSD) and X-ray diffraction (XRD), respectively. Secondly, the tensile behavior of treated samples was investigated through tension tests. Finally, the micro-strengthening mechanism of Inconel 718, induced by the LPB process, was revealed. The results show that the peak compressive stress is increased by a factor of 4.2 after the LPB process. The grain refinement induced by the LPB process is attributed to the increase of average misorientation and the formation of high angle grain boundaries (HAGBs). The enhanced yield strength depends on the decreased average spacing and the increased HAGBs. 相似文献
10.
Meijun Shang Hejun Li Ayaz Ahmad Waqas Ahmad Krzysztof Adam Ostrowski Fahid Aslam Panuwat Joyklad Tomasz M. Majka 《Materials》2022,15(2)
Environment-friendly concrete is gaining popularity these days because it consumes less energy and causes less damage to the environment. Rapid increases in the population and demand for construction throughout the world lead to a significant deterioration or reduction in natural resources. Meanwhile, construction waste continues to grow at a high rate as older buildings are destroyed and demolished. As a result, the use of recycled materials may contribute to improving the quality of life and preventing environmental damage. Additionally, the application of recycled coarse aggregate (RCA) in concrete is essential for minimizing environmental issues. The compressive strength (CS) and splitting tensile strength (STS) of concrete containing RCA are predicted in this article using decision tree (DT) and AdaBoost machine learning (ML) techniques. A total of 344 data points with nine input variables (water, cement, fine aggregate, natural coarse aggregate, RCA, superplasticizers, water absorption of RCA and maximum size of RCA, density of RCA) were used to run the models. The data was validated using k-fold cross-validation and the coefficient correlation coefficient (R2), mean square error (MSE), mean absolute error (MAE), and root mean square error values (RMSE). However, the model’s performance was assessed using statistical checks. Additionally, sensitivity analysis was used to determine the impact of each variable on the forecasting of mechanical properties. 相似文献
11.
The paper summarizes the outcomes of surgical treatment in 6 patients with nonspecific and tuberculous spondylitis of the craniovertebral area. These patients underwent decompressively stabilizing operations that consists of two steps: at first occipitospondylodesis was performed with a wire and protacryl then via transpharyngeal access with sanitation of the abscess cavity, by removing necrotic tissues--a saving resection of bony tissue sites within the healthy tissues, anterior stabilization of an affected part with an osseous autograft. The above procedure of surgical treatment along with bactericidal therapy yielded positive results by recovering spinal cord function, by forming a bony unit at the site of spinal inflammatory lesion. 相似文献
12.
Sadaqat Ali Muhammad Irfan Usama Muhammad Niazi Ahmad Majdi Abdul Rani Ahmad Rashedi Saifur Rahman Muhammad Kamal Asif Khan Mabkhoot A. Alsaiari Stanislaw Legutko Jana Petr Antonin Trefil 《Materials》2022,15(8)
AISI 316L stainless steel (SS) is one of the extensively used biomaterials to produce implants and medical devices. It provides a low-cost solution with ample mechanical properties, corrosion resistance, and biocompatibility compared to its counterpart materials. However, the implants made of this material are subjected to a short life span in human physiological conditions leading to the leaching of metal ions, thus limiting its use as a biomaterial. In this research, the addition of boron, titanium, and niobium with varying concentrations in the SS matrix has been explored. This paper explores the impact of material composition on modified SS alloy’s physical and mechanical properties. The study’s outcomes specify that the microhardness increases for all the alloy compositions, with a maximum increase of 64.68% for the 2 wt.% niobium added SS alloy. On the other hand, the tensile strength decreased to 297.40 MPa for the alloy containing 0.25 wt.% boron and 2 wt.% titanium additions compared to a tensile strength of 572.50 MPa for pure SS. The compression strength increased from 776 MPa for pure SS to 1408 MPa for the alloy containing niobium and titanium additions in equal concentrations. 相似文献
13.
Won-Kyung Kim Gigwon Hong Young-Ho Kim Jong-Min Kim Jin Kim Jung-Geun Han Jong-Young Lee 《Materials》2021,14(11)
In this study, highly concentrated hydrogen nanobubble water was utilized as the blending water for cement mortar to improve its compressive and flexural strengths. Highly concentrated nanobubbles can be obtained through osmosis. This concentration was maintained by sustaining the osmotic time. The mortar specimens were cured for 28 days, in which the nanobubble concentration was increased. This improved their flexural strength by 2.25–13.48% and compressive strength by 6.41–11.22%, as compared to those afforded by plain water. The nanobubbles were densified at high concentrations, which caused a decrease in their diameter. This increased the probability of collisions with the cement particles and accelerated the hydration and pozzolanic reactions, which facilitated an increase in the strength of cement. Thermogravimetric analysis and scanning electron microscopy were used to confirm the development of calcium silicate hydrate (C-S-H) and hydration products with an increase in the nanobubble concentration. Quantitative analysis of the hydration products and the degree of hydration were calculated by mineralogical analysis. 相似文献
14.
Finite element numerical progressive damage modelling and simulations applied to the strength prediction of airframe bolted joints on composite laminates can lead to shorter and more efficient product cycles in terms of design, analysis and certification, while benefiting the economic manufacturing of composite structures. In the study herein, experimental bolted joint bearing tests were carried out to study the strength and failure modes of fastened composite plates under static tensile loads. The experimental results were subsequently benchmarked against various progressive damage numerical modelling simulations where the effects of different failure criteria, damage variables and subroutines were considered. Evidence was produced that indicated that both the accuracy of the simulation results and the speed of calculation were affected by the choice of user input and numerical scheme. 相似文献
15.
This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume. 相似文献
16.
17.
Noor Md. Sadiqul Hasan Md. Habibur Rahman Sobuz Md. Munir Hayet Khan Nusrat Jahan Mim Md. Montaseer Meraz Shuvo Dip Datta Md. Jewel Rana Ayan Saha Abu Sayed Mohammad Akid Md. Tanjid Mehedi Moustafa Houda Norsuzailina Mohamed Sutan 《Materials》2022,15(22)
The incorporation of waste materials generated in many industries has been actively advocated for in the construction industry, since they have the capacity to lessen the pollution on dumpsites, mitigate environmental resource consumption, and establish a sustainable environment. This research has been conducted to determine the influence of different rice husk ash (RHA) concentrations on the fresh and mechanical properties of high-strength concrete. RHA was employed to partially replace the cement at 5%, 10%, 15%, and 20% by weight. Fresh properties, such as slump, compacting factor, density, and surface absorption, were determined. In contrast, its mechanical properties, such as compressive strength, splitting tensile strength and flexural strength, were assessed after 7, 28, and 60 days. In addition, the microstructural evaluation, initial surface absorption test, = environmental impact, and cost–benefit analysis were evaluated. The results show that the incorporation of RHA reduces the workability of fresh mixes, while enhancing their compressive, splitting, and flexural strength up to 7.16%, 7.03%, and 3.82%, respectively. Moreover, incorporating 10% of RHA provides the highest compressive strength, splitting tensile, and flexural strength, with an improved initial surface absorption and microstructural evaluation and greater eco-strength efficiencies. Finally, a relatively lower CO2-eq (equivalent to kg CO2) per MPa for RHA concrete indicates the significant positive impact due to the reduced Global Warming Potential (GWP). Thus, the current findings demonstrated that RHA can be used in the concrete industry as a possible revenue source for developing sustainable concretes with high performance. 相似文献
18.
Rados&#x;aw Mirski &#x;ukasz Matwiej Dorota Dziurka Monika Chuda-Kowalska Maciej Marecki Bartosz Pa&#x;ubicki Tomasz Rogozi&#x;ski 《Materials》2021,14(19)
This paper presents the strength properties of wooden trusses. The proposed solutions may constitute an alternative to currently produced trusses, in cases when posts and cross braces are joined with flanges using punched metal plate fasteners. Glued carpentry joints, although requiring a more complicated manufacturing process, on the one hand promote a more rational utilisation of available structural timber resources, while on the other hand they restrict the use of metal fasteners. The results of the conducted analyses show that the proposed solutions at the current stage of research are characterised by an approx. 30% lower static bending strength compared to trusses manufactured using punched metal plate fasteners. However, these solutions make it possible to produce trusses with load-bearing capacities comparable to that of structural timber of grade C24 and stiffness slightly higher than that of lattice beams manufactured using punched metal plate fasteners. The strength of wooden trusses manufactured in the laboratory ranged from nearly 20 N/mm2 to over 32 N/mm2. Thus, satisfactory primary values for further work were obtained. 相似文献
19.
The deformation structure and its contribution to strain hardening of a high manganese austenitic steel were investigated after tensile deformation at 298 K, 77 K and 4 K by means of electron backscatter diffraction and transmission electron microscopy, exhibiting a strong dependence of strain hardening and deformation structure on deformation temperature. It was demonstrated that sufficient twinning indeed provides a high and stable strain hardening capacity, leading to a simultaneous increase in strength and ductility at 77 K compared with the tensile deformation at 298 K. Moreover, although the SFE of the steel is ~34.4 mJ/m2 at 4 K, sufficient twinning was not observed, indicating that the mechanical twinning is hard to activate at 4 K. However, numerous planar dislocation arrays and microbands can be observed, and these substructures may be a reason for multi-peak strain hardening behaviors at 4 K. They can also provide certain strain hardening capacity, and a relatively high total elongation of ~48% can be obtained at 4 K. In addition, it was found that the yield strength (YS) and ultimate tensile strength (UTS) linearly increases with the lowering of the deformation temperature from 298 K to 4 K, and the increment in YS and UTS was estimated to be 2.13 and 2.43 MPa per 1 K reduction, respectively. 相似文献
20.
Improving the Mechanical Properties of Fly Ash-Based Geopolymer Composites with PVA Fiber and Powder
In this work, polyvinyl alcohol (PVA) fiber and powder were added to geopolymer composites to toughen fly ash-based geopolymer, and their different toughening mechanisms were revealed. Firstly, different contents of active granulated blast furnace slag (GBFS) were added to the geopolymer to improve the reactivity of the GBFS/fly ash-based geopolymer, and the best ratio of GBFS and fly ash was determined through experiments testing the mechanical properties. Different contents of PVA powders and fibers were utilized to toughen the geopolymer composites. The effect of the addition forms and contents of PVA on the mechanical properties, freeze–thaw cycle resistance, and thermal decomposition properties of geopolymer composites were systematically studied. The results showed that the toughening effect of PVA fiber was better than that of PVA powder. The best compressive strength and flexural strength of geopolymer composites toughened by PVA fiber were 41.11 MPa and 8.43 MPa, respectively. In addition, the composition of geopolymer composites was explored through microstructure analysis, and the toughening mechanisms of different forms of PVA were explained. This study provided a new strategy for the toughening of geopolymer composites, which can promote the low-cost and efficient application of geopolymer composites in the field of building materials. 相似文献