Rate Of Bond Rupture Research Articles

The impact of self-heating and its implications on hot-carrier degradation – A modeling study

A combination of hot-carrier degradation (HCD) and self-heating (SH) was acknowledged to be the most detrimental reliability issue in ultra-scaled field-effect-transistors (FETs) with confined architectures, such as fin and nanowire (NW) FETs. Although the view on whether SH accelerates or inhibits HCD in n-channel devices is controversial, it is commonly accepted that in p-channel FETs HCD becomes more pronounced due to SH. Therefore, we develop a framework for modeling coupled HCD and SH and validate it against experimental data acquired in p-channel Si NWFETs. This framework incorporates a carrier transport solver linked to a solver for the lattice heat flow equation; the latter solver allows one to evaluate a non-uniform lattice temperature distribution in the device. Carrier energy distribution functions obtained considering coordinate-dependent lattice temperature are used to calculate the rates of the multiple- and single-carrier processes of SiH bond dissociation. In addition to perturbation of carrier transport by SH, elevated lattice temperature results in enhancement of the thermal contribution to the bond rupture rate and shortens vibrational lifetime of the SiH bond, thereby reducing the multiple-carrier process rate. All these three aspects are captured by our framework, importance of each of them is analyzed, and our approach was shown to accurately reproduce experimental data.

Direct Evidence for a Peroxide Intermediate and a Reactive Enzyme–Substrate–Dioxygen Configuration in a Cofactor‐free Oxidase

AbstractCofactor‐free oxidases and oxygenases promote and control the reactivity of O2 with limited chemical tools at their disposal. Their mechanism of action is not completely understood and structural information is not available for any of the reaction intermediates. Near‐atomic resolution crystallography supported by in crystallo Raman spectroscopy and QM/MM calculations showed unambiguously that the archetypical cofactor‐free uricase catalyzes uric acid degradation via a C5(S)‐(hydro)peroxide intermediate. Low X‐ray doses break specifically the intermediate C5OO(H) bond at 100 K, thus releasing O2 in situ, which is trapped above the substrate radical. The dose‐dependent rate of bond rupture followed by combined crystallographic and Raman analysis indicates that ionizing radiation kick‐starts both peroxide decomposition and its regeneration. Peroxidation can be explained by a mechanism in which the substrate radical recombines with superoxide transiently produced in the active site.

Dynamic behavior of dual cross-linked nanoparticle networks under oscillatory shear

Via computer simulations, we investigate the linear and nonlinear viscoelastic response of polymer grafted nanoparticle networks subject to oscillatory shear at different amplitudes and frequencies. The individual nanoparticles are composed of a rigid spherical core and a corona of grafted polymers that encompass reactive end groups. With the overlap of the coronas on adjacent particles, the reactive end groups form permanent or labile bonds, and thus form a ‘dual cross-linked’ network. The existing labile bonds between particles can break and reform depending on the bond rupture rate, extent of deformation and the frequency of oscillation. We study how the viscoelastic behavior of the material depends on the energy of the labile bonds and identify the network characteristics that give rise to the observed viscoelastic response. We observe that with an increase in labile bond energy, the storage modulus increases while the loss modulus shows a more complex response depending on the labile bond energy. Specifically, in the case of the samples with the weaker labile bonds, the loss modulus increases monotonically, while for the samples with the stronger labile bonds, the loss modulus exhibits a minimum with an increase in frequency. We show that an increase in the storage modulus corresponds to an enhancement in the average number of bonds in the samples and the characteristics of the loss modulus depend on both the bond kinetics and the mobility of the particles in the network. Furthermore, we determine that the effective contribution of the bonds to the storage modulus decreases with increase in strain amplitude. In particular, while bond formation at small amplitude drives an increase in storage modulus, at large amplitudes it promotes clustering and formation of voids leading to strain softening. Our simulations provide a mesoscopic picture of how the nature of labile bonds affects the performance of cross-linked polymer-grafted nanoparticle networks.

Crucial roles of holes in electronic bond rupture on semiconductor surfaces

Structural changes on cleaved (110) surfaces of InP, induced by tunneling carriers from the tip of a scanning tunneling microscope, were studied. Bond rupture takes place at intrinsic P-sites only at negative sample voltages (i.e., under hole-injection conditions), resulting in the formation of P-vacancies, while injected electrons induce no structural change. The rate of bond rupture, showing a prominent threshold sample voltage at −2.2V, is a quadratic function of the tunneling current. Nonlinear localization of two holes injected into surface bands is crucial as the primary step in the electronic bond rupture on semiconductor surfaces.

Modeling the response of dual cross-linked nanoparticle networks to mechanical deformation

We develop a hybrid computational model for the behavior of a network of cross-linked polymer-grafted nanoparticles (PGNs). The individual nanoparticles are composed of a rigid core and a corona of grafted polymers that encompass reactive end groups. With the overlap of the coronas on adjacent particles, the reactive end groups can form permanent or labile bonds, which lead to the formation of a “dual cross-linked” network. To capture these multi-scale interactions, our approach integrates the essential structural features of the polymer grafted nanoparticles, the interactions between the overlapping coronas, and the kinetics of bond formation and rupture between the reactive groups on the chain ends. Via this model, we determine the tensile properties of the dual cross-linked samples. We find that the mechanical behavior of the network can be tailored by altering the bond energies of the labile bonds, the fraction of permanent bonds in the network and the thickness of the polymer corona. In particular, for a network with weaker labile bonds, an increase in fraction of permanent bonds and the contour length of the chain can yield a tough network that behaves like a polymeric material, which exhibits cold drawing/necking. On the other hand, similar changes to the network with stronger labile bonds lead to an increase in toughness, with the network characteristics being similar to that of a purely ductile material. Variations in the ratio between the strain rate and the bond rupture rate are also found to affect the response of the networks. Our model provides a powerful approach for predicting how critical features of the system affect the performance of cross-linked polymer-grafted nanoparticles.

Modeling the making and breaking of bonds as an elastic microcapsule moves over a compliant substrate

By integrating the lattice Boltzmann model for hydrodynamics, the lattice spring model for micromechanics of elastic solids, and the Bell model for bond formation and rupture, we examine the fluid driven motion of elastic microcapsules on compliant surfaces. The capsules, modeled as three-dimensional fluid-filled elastic shells, represent polymeric microcapsules or biological cells. We observed three regimes of capsule motion. Namely, the capsule rolls steadily along the substrate at a sufficiently high shear rate, it is stationary at a low shear rate, and exhibits an intermittent motion (saltation) at intermediate shear rates. At a given shear rate, the regime of capsule motion was found to depend on the substrate stiffness, and on the rate of rupture of the adhesive bonds. The capsule was observed to roll steadily on a sufficiently stiff substrate, and at a high rate of bond rupture. In the opposite limit of a soft substrate and low rate of bond rupture, the system was localized in the stationary regime. The findings provide guidelines for creating smart surfaces that could regulate the motion of the microcapsules.

Excitation-induced atomic desorption and structural instability of III–V compound semiconductor surfaces

We discuss the laser-induced structural instability of III–V compound semiconductor surfaces. The electronic instability is characterized by local bond rupture of both metallic and nonmetallic atoms at intrinsic surface sites, with the bond-rupture rate super linearly dependent on the excitation density. Spectroscopic studies show that bulk-valence excitation triggers surface bond rupture, and that valence holes are the responsible species. From our results, we propose a mechanistic model based on the two-hole localization induced by a high density of non-equilibrated valence holes, and demonstrate that the model describes all the important features quantitatively and consistently. Additional evidence that further supports the validity of the two-hole mechanism is presented.

Electronic bond rupture of Si atoms onSi(111)−(2×1)induced by valence excitation

Scanning tunneling microscopy study reveals the electronic local bond rupture of threefold-coordinated Si atoms at intrinsic sites on $\mathrm{Si}(111)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ under nanosecond-laser excitation at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The rate of bond rupture leading to monovacancy formation on the surface depends superlinearly on the excitation intensity. This primary step of surface-structural change is followed by efficient formation of vacancy clusters with two distinctive morphologies: vacancy strings aligned along the Si-atom chain in one dimension and vacancy islands developed across the chains in two dimensions. Quantitative analysis of the vacancy clustering process shows that the bond-rupture rate at sites nearest to pre-existing surface defects is enhanced more than a factor of 1000 relative to perfect sites. We also studied effects of different excitation wavelength and of Fermi-level positions on the bond-rupture process. The laser-induced surface bond-rupture mechanism is discussed in terms of two-hole localization of optically generated nonequilibrated valence holes on the surface sites.

Rupture of an extended object: A many-body Kramers calculation

We show how an extended object's strain field is redistributed when the material ruptures under by thermal activation. Through analytical calculations and molecular dynamics simulations, we show that in a polymer chain the distribution is exponentially localized around the point of rupture. The length scale of localization is determined by the strain and microscopic parameters of the interaction potential. We also derive an analytic expression for the rate of bond rupture by consistently treating the collective modes of the chain and the effect of dissipation on those modes. Our theoretical estimates are of the same order of magnitude as those obtained by simulations, as compared to earlier theories which had overestimated the rate of rupture by approximately two orders of magnitude. It is also noteworthy that the correction comes about through the effective attempt frequency rather than the effective barrier height.

Electronic bond rupture of Si-Dimers on Si(001)-(2x1) induced by pulsed laser excitation

A scanning tunneling microscopy study has revealed that 532 nm laser pulses of fluences well below melt and ablation thresholds induce electronic bond rupture of Si-dimers on the Si(001)-(2x1), resulting in the formation of single dimer-vacancies followed by progressive growth into vacancy clusters. The rate of bond rupture on the intrinsic (2x1) structure shows super-linearity with respect to excitation intensity, and saturates as the number of vacancies reaches a few percent, relative to total dimer sites. The mechanism of laser-induced bond rupture is discussed based on these results.

Carbon isotope effect studies in the mechanism of reactions of alkynes with formic acid

Carbon-13 fractionation observed in the course of carbon monoxide formation in the reaction of phenylacetylene with the large excess of liquid formic acid in the temperature interval 20–100°C has been investigated and compared with the13C fractionation in the dehydration of pure liquid formic acid. The anomalous temperature dependence of the13C fractionation has been interpreted as caused by the change of the kinetics and of the mechanism of CO formation from the one involving13C−H bond rupture rate determining step (operating in the presence of phenylacetylene) to the mechanism according to which HCOOH decarbonylates in liquid state. No large increase of the13C fractionation with rising of the reaction temperature from 70 to 134°C has been found in the case of decarbonylation of F.A. in the presence of large excess of phenylacetylene. The13C KIE was of 1.020 in the temperature interval 90–133.7°C in this case.

The Rate of Collagen Maturation in Rat and Human Skin

Hydrothermal Isometric Tension (HIT) relaxation was observed at the boiling point in rat, cat and human skin samples of different ages, after raising the temperature from 37 degrees to 100 degrees C at a rate of 1.15 degrees C/min. A Maxwellian relaxation to nearly zero was observed at the boiling point. A given velocity constant characterized a given tissue whether experiments were performed under pressure or under tensile stress. However, the velocity constant of the relaxation decreased as a function of age for all three species during the period studied. Moreover, the velocity constant decreased at the same constant rate for skin samples of cats aged 1 to nearly 5 years, of rats aged from 9 to 28 months and of children from birth to 10 years. We propose a model, based on the well known rubber-elastic properties of the denatured collagen network. In this model (1) HIT decreases as an exponential function of time during bond scission along the polymeric chains; (2) the velocity constant of the relaxation process is proportional to the rate of bond rupture and inversely proportional to the number of stable polymeric chains originally present per unit volume; (3) the evolution of the velocity constant as a function of aging finds an explanation. The HIT test should find useful applications in pathology and pharmacology, since it provides rapid, precise information on the stable state of collagen reticulation in small biopsies.

Bond Rupture during Ozone Cracking of Torsional Loaded Styrene-Butadiene Rubber

Abstract The molecular bond rupture rate during ozone attack of torsionally loaded rubber was determined from electron paramagnetic resonance (EPR) measurements. The rupture rate was successfully correlated by a Griffith-type energy balance to the strain-energy release rate in the samples. These observations substantiate the results from a similar study on tensile loading previously reported. In both cases there is a one-to-one correspondence between the rate of bond rupture (or crack growth) and the rate of energy release from the strain field and external work. A fracture energy, γn, of approximately 5×10−12 (α20 per cent) ergs per free radical formed during the cracking was experimentally determined.

Fracture behavior in nylon 6 fibers

AbstractA reaction rate model of fracture in polymer fibers is described. This model assumes that bond rupture is governed by absolute reaction rate theory with a stress‐aided activation energy. It is demonstrated that the key in obtaining good agreement between the model and experiment lies in taking proper account of the variation of stress on the tie‐chain molecules. The more taut chains rupture first, and the load is redistributed among the remaining unruptured tie chains. The effect of varying the temperature both in the model and in experiments on fracture in fibers is explored. Good agreement between predictions of the model and experiment is possible only with an undeterstanding of the distribution in stress on the tie chains. The distribution in stress on the chains was experimentally determined by monitoring the kinetics of bond rupture with electron paramagnetic resonance (EPR) spectroscopy. Temperature is found to have two effects on macroscopic strength. (1) The thermal energy aids the atomic stress in breaking the atomic bonds; as a consequence the rate of bond rupture of a family of bonds under a given molecular stress is increased. In this respect temperature might be viewed as decreasing the “strength” of a bond. (2) Temperature also serves to “loosen” the molecular structure and in this way modify the distribution in stress on the tie chains. To explain bond rupture and macroscopic fracture behavior quantitatively, account must be taken of both effects.

Bond rupture during ozone cracking of torsional loaded styrene–butadiene

AbstractThe molecular bond rupture rate during ozone attack of torsionally loaded rubber was determined from electron paramagnetic resonance (EPR) measurements. The rupture rate was successfully correlated by a Griffith‐type energy balance to the strain‐energy release rate in the samples. These observations substantiate the results from a similar study on tensile loading previously reported. In both cases there is a one‐to‐one correspondence between the rate of bond rupture (or crack growth) and the rate of energy release from the strain field and external work. A fracture energy, γn, of approximately 5×10−12 (±20%) ergs per free radical formed during the cracking was experimentally determined.

A micro‐macro correlation of ozone‐induced fracture in rubber

AbstractRate of molecular bond rupture is successfully correlated by a Griffith‐type energy balance to the strain energy release rate during ozone cracking of rubber. Rate of bond rupture is determined from electron paramagnetic resonance (EPR) measurements. The rate of strain energy release is determined from stress–elongation measurements during stress relaxation, creep, and cyclic loading tests. To compare with macroscopic crack studies, it was assumed that each ruptured bond created a given amount of fracture surface. Numerical agreement could be obtained by assuming each broken bond results in the production of an area of approximately 10−13 cm2. Using the surface energy density determined from stress relaxation tests in an energy balance gives surprisingly accurate predictions of expected behavior in creep and cyclic loading tests. There is a one‐to‐one correspondence between the rate of crack growth (bond rupture) and rate of energy release from strain and external work in all cases. It is proposed that such correlations give credence to a Griffith‐type approach to environmental cracking which it did not have previously.

Electron Paramagnetic Resonance Measurements of Strain Induced Ozone Cracking in Rubber

An electron paramagnetic resonance (EPR) method for monitoring the rate of atomic bond rupture in rubber subjected to uniaxial tensile strain in an ozone environment is described. The free radicals so formed are found to be quite stable and present in large, easily detectable concentrations. The initial rate of bond rupture was found to be approximately proportional to ozone concentration at fixed strain. Below a prestrain of three percent, no effect of the ozone environment was detected; above this threshold, the rate of rupture increases with strain up to strains of approximately 20 percent. It appears that this EPR technique is very sensitive and can yield information not readily obtainable by more conventional techniques.

Thermal degradation of polypropylene

AbstractThe molecular weight distributions of polypropylene samples subjected to various degrees of thermal degradation were experimentally determined and compared with those expected theoretically for random scission of the polymer chains. The comparison demonstrated that the chain breakage is predominately random and also indicates that current methods of determining molecular weight distributions are adequate for use in evaluating hypothesized degradation mechanisms. In terms of the free radical depolymerization theory, intermolecular transfer of the radical is the dominant process. A curve relating the degree of degradation to the intrinsic viscosity was obtained from calculations based upon a random degradation. With this curve, intrinsic viscosities measured as a function of degradation time could be used to obtain the time dependence of the degree of degradation. The observed degradation rates could not be described by a simple process in which the rate of bond rupture was proportional to the number of bonds present. Neither a similar process involving two bond types nor a dependence of the free radical diffusion rate upon the degree of degradation was by itself a satisfactory basis for interpreting the kinetics. The activation energy estimated for the polypropylene degradation at 250–300°C. was not significantly different from that determined by volatilization measurements at higher temperatures.

The photolysis of polyethylene terephthalate

AbstractThe photolysis of films of polyethylene terephthalate has been studied over a wide variation of exposure conditions. The rate of polymer bond rupture was measured by the change in molecular weight. The rate of energy absorption upon exposure to near ultraviolet light was determined from an analysis of the emission characteristics of the light sources used and the absorption characteristics of the polymer. The overall quantum yield for the photolysis was found to be 5 × 10−4 bonds ruptured per photon. This quantum yield is shown to be independent of the wave length of the light absorbed in the spectral region studied. Further, analysis of the effect of film thickness on the rate of bond rupture indicates that the reactions are initiated only by the light absorbed in the strong ultraviolet absorption band characteristic of the pure polymer. The quantum yields for the photolysis of this polymer and others reported in the literature are compared and related to the effect of changes in physical state on the reactions of simple monomeric systems. It is concluded that the low values typical of many polymers are a result of rapid recombination of free radicals due to a low mobility in the solid state.