Danilov, D.

Gleiter, H.

Hahn, H.

Wenzel, W.

English

KITopen

increase of the potential energy is observed. This increase isaccompanied by a compositional change. In fact, the fractioncA(r) of A atoms approaches 100% at the free surface. We usethe onset of the increase of the potential energy to define aradius Rc that divides the nanoparticle into a core region with r< Rc and a surface region with r > Rc. As may be seen fromFigure 3, the increase of cA(r) consists of two slowly varyingregions with a crossover in the first half of the surface region (r> Rc.) followed by a saturation in the outermost shell.In the subsequent step of the simulation, the nanoparticles(prepared at the deposition temperature T = 0.32) wereconsolidated at a pressure of p = 1.5 into a nanoglass, whichcorresponds to the consolidation procedure used in theexperimental studies to prepare PVD nanoglasses. Thesimulation box before and after consolidation is shown inFigure 4. For visualization of the simulation results, the atomsin the core and the surface regions of the nanoparticles arelabeled in red (core atoms in Figure 4) and blue or green(atoms in the surface regions, Figure 4). As may be seen fromFigure 4, no significant diffusion occurs between the core andsurface regions during or after the consolidation process.The density and the volume fraction of the nanoparticlecores and of the interfacial regions formed during consolidationwere computed by means of a Voronoi tessellation. Figure 4shows the density within the cores and within the interfacesduring the consolidation process. In fact, as far as the evolutionof the densities in the core and the interfacial regions duringconsolidation are concerned, three stages (labeled s1, s2, s3 inFigure 4) may be distinguished. In the first stage, s1, when thepressure increases linearly from p = 0 to p = 1.5, the outer shellsof the nanoparticles start to form amorphous interfaces. Theaverage density of these interfaces rapidly increases, while thedensity of the cores increases far less. In the second stage, s2,when the system is equilibrated at a pressure of p = 1.5, thedensity of cores remains unchanged, and the density of theinterfaces increases slightly by atomic rearrangements that closethe small holes that have remained after the stage s1. However,at all times, the density of the interfacial regions remained lowerthat the density of the cores. In stage s3, the pressure is relaxedlinearly to p = 0 and the densities of cores and the interfacesdecrease uniformly by approximately the same amount. Finally(after the relaxation of the stage s3), the system was equilibratedfor 10 times the simulation time of the entire consolidationprocess. The difference in the density between the amorphousgrains and the amorphous interfaces remains stable throughoutthis phase of the simulation.To investigate the thermal stability of the bulk glass incomparison to the PVD nanoglass, both structures wereincrementally heated mimicking the experimental differentialscanning calorimetry process. The resulting enthalpy data werefitted to an analytical expression (see Supporting Information)to obtain the heat capacity shown in Figure 5. When thetemperature was increased, a phase transition was observed, asindicated by a peak in the heat capacity (Figure 5). Table 1shows the onset temperature Tr, the peak temperature Tp, thewidth σp of the temperature range in which the phase transitiontakes place, and the enthalpy ΔH, defined as the integratedspecific heat over the peak region, of the phase transition.Clearly, the onset temperatures and the temperatures of thepeaks of the heat capacity of the bulk glass and of the PVDnanoglass differ. In fact, the bulk glass is found to be less stablethan the PVD nanoglass. At the same time, the highest value ofΔH , accompanied by the largest width σp of the transitiontemperature, of the PVD nanoglass is almost two times widerthan the one of the bulk glass. To unequivocally demonstrateFigure 2. Potential energy for bulk glass at different finaltemperatures of the cooling process in comparison to PVDnanoparticles at different deposition temperatures.Figure 3. Radial profiles of potential energy and composition in a nanoparticle prepared by vapor deposition (in the right panel: blue circles/green triangles correspond to the concentration of A/B atoms, respectively). Inset in the left panel shows a cross section of the nanoparticlewith red atoms representing the core and blue atoms representing the shell.that these features arise as a result of the PVD process, we haveprepared nanoparticles of the same size and composition bycutting the bulk glass into fragments, which were thensubjected to the same consolidation process as the PVDnanoparticles. Figure S2 shows the evolution of the densitiesduring the consolidation of amorphous particles derived fromthe bulk. The resulting material, called bulk derived nanoglass,features a variation in density of 0.86% that is more than 10times smaller than in the case of PVD nanoparticles where thevariation is 10.9%. The existence of the small density variationin the bulk derived nanoglass can be attributed to a limitedreorganization of atoms at the surface of the nanoparticlesderived from the bulk glass. Along with the small densityvariation, the reorganization has an effect of decreasing theenthalpy of the bulk derived nanoglass compared to the bulkglass. We have investigated the bulk derived nanoglass only todemonstrate that the enthalpy of the PVD nanoglass cannot bereached when bulk derived glass fragments undergo the sameprocess that we used to simulate the consolidation of the PVDglass particles to the bulk material (see Figure 5). To ourknowledge, there is no experimental protocol to break the bulkglass into nanoparticles while preserving its structure, andtherefore, the bulk derived nanoglass has no experimentalcounterpart. Table 1 shows that the consolidation at the highpressure leads to an increase in the heat of transition ΔH, asdefined above, but does not lead to the thermal ultrastability asthe transition temperature Tp only slightly differs from thevalue of the bulk glass. These finding demonstrate that both theinterior of the cores and the interfacial regions in PVDnanoglasses differ from their bulk glass counterparts.Using molecular dynamics simulations, we have investigated(1) the mechanism by which nanoparticles are formed during aPVD process and (2) the processes that result in the formationof a PVD nanoglass when these nanoparticles are consolidated.Figure 4. Consolidation of nanoparticles into a nanoglass and evolution of the density of core (grains) and shell (interfaces) groups of atomsduring consolidation and during the subsequent equilibration. The red atoms show both A and B atoms in cores. The blue color indicates theA atoms within the shells, and green color corresponds to the B atoms within the shells. The right panel shows the density of the centralregion and interface region in red/blue, respectively. The density of the bulk glass is given by the dashed line for reference.Figure 5. Left: enthalpy of the glass cooled from the melt (red), the bulk derived nanoglass (green), and the PVD nanoglass (blue). The insetshows the deviation of the ratio of the enthalpy of the bulk derived nanoglass and the PVD nanoglass relative to the bulk glass (positivedeviations indicate a lower energy). Right: heat capacity of bulk glass, PVD nanoglass, and bulk derived nanoglass as a function oftemperature.Table 1. Transition Temperature Tp, Heat of Transition ΔH(See Definition in Text), Width σp of the TemperatureRange, and the Onset Temperature TrTp ΔH σp Trbulk glass 0.418 0.066 0.017 0.343bulk-derived nanoglass 0.425 0.103 0.015 0.361PVD-nanoglass 0.442 0.194 0.027 0.371REFERENCES(1) Gleiter, H.; Schimmel, T.; Hahn, H. Nanostructured Solids −from Nano Glasses to Quantum Transistors. Nano Today 2014, 9,17−68.(2) Gleiter, H. Our Thoughts Are Ours, Their Ends None of OurOwn: Are There Ways to Synthesize Materials Beyond the Limitationsof Today? 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Mechanisms of Nanoglass Ultrastability

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Mechanisms of Nanoglass Ultrastability

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