Concentration Of Single Vacancies Research Articles

Coupling field optimization to improve the thermal transport of Gr/h-BN heterostructure

We perform a numerical evaluation on the multi-physical field coupling effect to improve the interfacial thermal conductivity (ITC) of Gr/h-BN (Graphene/Hexagonal boron nitride) planar heterostructure. The model of Gr/h-BN planar heterostructure (C-BN bonded configuration) is built to investigate the effect of the multifactor coupling effect of different temperatures, N-doping concentration, and single vacancy concentration on the ITC. Finally, the Box-Behnken orthogonal model is established to determine the optimal value of ITC under the coupling effect of multi-physical fields by response surface analysis, and the optimal value of ITC is 14.456 GW m−2 K−1 when the temperature is 673 K, the single vacancy is 2.57 %, and the N-doping is 18.9 %. It implies that the proper coupling configuration is found effective in improving the interfacial thermal conductivity of the Gr/h-BN heterostructure. This work is hoped to help promote the thermal reliability design of graphene heterogeneous interface for related micro/nano semiconductor devices.

The Effect of Single Vacancy Defects on Graphene Nanoresonators

We utilized molecular dynamics simulations to investigate the effects of single vacancy defects on the oscillation property of graphene nano-resonators. Single vacancies, representative point defects, are generated and unavoidable in the structure of graphene resonator during its synthesis process. We found that single vacancies deteriorate significantly the oscillation quality of the graphene resonator because of the softening of the local stiffness near single vacancies. The quality (Q) factor decreases monotonously as the concentration of single vacancy defects increases. We also found that thermal fluctuation at higher temperature accelerates the reduction of the Q factor of graphene resonators at the presence of single vacancies. At low temperature the softening effect due to single vacancies is dominant for the reduction of the Q factor of graphene resonators, while the thermal fluctuation in the out-of-plane direction becomes more dominant at high temperature. Moreover, the reduction rate of the Q factor with the increase of single vacancy concentration becomes smaller and thus less significant as temperature increases.

Ordering processes in Fe-Ga alloys studied by positron annihilation lifetime spectroscopy

To investigate the nature of defects in Galfenols (Fe100−xGax) with content near the local minimum (x=20.5 and x=22.4) and the local maximum of magnetostriction (x=26.3), the positron annihilation lifetime spectroscopy (PALS) and the X-ray diffraction analysis (XRD) are applied. It is found that the behavior of vacancies during ordering processes in the alloy with content higher than in stoichiometric composition of Fe3Ga (x>25) differs significantly from the other two alloys (x<25). All the studied alloys quenched from 1000°C exhibit a positron lifetime of about 175ps, which corresponds to high concentration of single vacancies (>2·10−4at−1). The growth of the bcc-born D03 ordered phase with relatively low concentration of vacancies in Fe79.5Ga20.5 and Fe77.6Ga22.4 samples causes a decrease of the average positron lifetime. The formation of the D03 phase in Fe73.7Ga26.3 does not reduce average positron lifetime, which indicates a high concentration of constitutional vacancies in the ordered D03 phase of Fe73.7Ga26.3. The D03→L12 phase transition is always accompanied by additional vacancy formation.

Control of Thermal and Electronic Transport in Defect-Engineered Graphene Nanoribbons

The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and non-equilibrium Green's function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone-Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone-Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance.

Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite

Irradiation-induced increase in the low-temperature specific heat has been measured in the temperature range of 1.9--43 K in graphite neutron-irradiated to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{n}/{\text{cm}}^{2}$ $(E&gt;1\text{ }\text{MeV})$ around 333 K. The increase of the lattice specific heat is interpreted as due to the hindered rotation of interstitial ${\text{C}}_{2}$ molecules in the periodic potential of $V(\ensuremath{\varphi})=({V}_{0}/2)(1+\text{cos}\text{ }2\ensuremath{\varphi})$, where $\ensuremath{\varphi}$ is the angle of rotation and ${V}_{0}$ is 0.040 eV. The first-excited rotational level is 0.0058 eV above the ground state and the rotational frequency is $1.39\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{s}}^{\ensuremath{-}1}$. This result shows that ${\text{C}}_{2}$ molecules do not form covalent bonds with atoms in the surrounding graphite layers. It is in marked contrast with the result of recent first-principles theoretical calculations that interstitial atoms form strong covalent bonds with atoms in the graphite layers. The concentration of ${\text{C}}_{2}$ molecules is estimated to be $f=1.16%$. The hole concentration, deduced from the electronic specific heat and the SWMcC band model, suggests that one single vacancy creates one hole and the single vacancy concentration is $2f$. Since the measurement of the lattice specific heat gave the concentration of the defects directly, we could evaluate some physical properties for a unit concentration of the defects. The volume changes by a single interstitial atom, an interstitial ${\text{C}}_{2}$ molecule, and a single vacancy are deduced to be $2.6\ifmmode\pm\else\textpm\fi{}0.3$, $9.3\ifmmode\pm\else\textpm\fi{}1.4$, and $\ensuremath{-}0.46\ifmmode\pm\else\textpm\fi{}0.07$ atomic volumes, respectively. The formation energy of a Frenkel pair is estimated to be $12.6\ifmmode\pm\else\textpm\fi{}2.5\text{ }\text{eV}$. The phonon scattering with a reciprocal relaxation time proportional to ${\ensuremath{\omega}}^{1.5}$, where $\ensuremath{\omega}$ is the angular frequency of phonons, is attributed to the scattering of phonons by the disklike strain around interstitial ${\text{C}}_{2}$ molecule clusters. A broad dip in the $a$-axis thermal conductivity observed below room temperature is attributed to the resonance scattering of phonons by the hindered rotation of interstitial ${\text{C}}_{2}$ molecules as well as by the vibration of these molecules as rigid units. The positron lifetime of 350 ps is suggested to be the lifetime of positrons trapped in an open space on the periphery of the interstitial clusters of ${\text{C}}_{2}$ molecules. The well-known Wigner energy is stored mainly as interstitial ${\text{C}}_{2}$ molecules and single vacancies.

Single vacancies in slowly cooled silicon crystals

To estimate the concentration of single vacancies in slowly cooled Si crystals, we calculated the diffusion length and pair formation of vacancies during cooling. The results showed that the concentration of quenched-in single vacancies at room temperature was much lower than that at high temperature.

Quenched-in defects and martensitic transformation in CuAlBe shape memory alloys

From positron lifetime and calorimetric measurements, the concentration and nature of the defects introduced by quenching from different T q temperatures, and their influence on the martensitic transformation undergone by a CuAlBe shape memory alloy are studied. The main effect of quenches is to modify the concentration of single vacancies in the system. The obtained dependence of the thermodynamic properties (transition temperatures and latent heat) of the martensitic transition upon T q, is explained in terms of the effective relative change of Be concentration due to quenched-in vacancies. Results are compared with published data for other Cu-based shape memory alloys.

Low-temperature radiation damage in silicon: I. Annealing studies on n-type material

The presence of electrically active defects in electron irradiated P-doped n-type silicon was monitored using capacitance and loss factor measurements. Irradiations were performed at temperatures ⪅12 K with electrons of energies ⪅1.50MeV. Seemingly, the concentration of single vacancies on the irradiated samples is low. This is deduced from the small recovery in the samples properties observed upon annealing at ∼75 K. Also two annealing stages observed at ∼140–150 K and ∼240 K are attributed to the self-interstitial. Electron traps observed at ∼( E c − 0.14) eV and ( E − 0.24) eV in the ga p are ascribed to the carbon interstitial and to the divacancy, respectively.

A stochastic theory for clustering of quenched-in vacancies-V. Temperature dependence of cluster density

It is known that the density of vacancy loops in quenched aluminium and stacking fault tetrahedra in quenched gold show a saturation for low ageing temperatures. The physical mechanism leading to this effect is not well understood. In this paper we consider a simple model which allows us to obtain the temperature dependence of total density. The analysis shows that the plateau region arises due to the fact that the number of absorption sites of a cluster is larger than the number of emission sites. The temperature dependence of the average number of vacancies in a cluster and the single vacancy concentration in equilibrium with the clusters are discussed.

Fim observation of defect clusters in tungsten irradiated with 200 KeV C+ion

Abstract Dose dependence of the size distribution of vacancy clusters in tungsten irradiated with 200 KeV C+ ions at 65 and 800°C was obtained with Field Ion Microscopy (FIM). At 65°C all of the vacancy clusters were below 15 A in diameter. At 800°C the density of vacancy clusters below 15 A was saturated, but the density of those above 15 A increased with dose. The results at both temperatures imply that the nuclei of voids would be the vacancy clusters below 15 A which come from depleted zones and that depleted zones play an important role in the genesis of voids in ion and neutron irradiation, kinetic analysis of the point defect behaviour based on rate equation theory has explained the experimental results at 65°C that total vacancy concentration and single vacancy concentration is proportional to t 1/2 (t is irradiation time).

The loss of single vacancies in metals during quenching

The Lomer equation, describing the rate of change of single vacancy concentration in a metal specimen during exponential quenching, was numerically solved for different quenching temperatures and different quenching rates. A relation has been found between the constantb, which characterizes the initial quenching rate, and the quenching temperature leading to the same percentual loss of vacancies. This relation enables us to determine the conditions of quenching, yielding the predetermined percentage of retained vacancies.

Kinetic Study of Vacancy Clustering and Annealing in Metals

The rate equations governing the clustering and annealing of vacancies in metals have been studied. Single vacancies were considered to be immobile at temperatures at which small clusters migrate. Clusters with up to seven vacancies were explicitly treated, and approximations were made to account for larger clusters. A variety of cases for different cluster binding and migration energies were run. Since the annealing curves generally gave little indication of the actual processes occurring, the input data could not be inferred from standard analyses of isothermal and isochronal data. For small binding energies the annealing was small, as some clusters reached sinks, and the single-vacancy concentration increased. For large binding energies the annealing was also small, as clustering dominated the recovery and few clusters reached sinks. Between these limits, complex results were obtained; the total annealing could be quite large with an appreciable decrease in concentration of single vacancies, and anomalously low activation energies were often found.

Observation of vacancies in the field-ion microscope

Measurements of the single vacancy concentration in well-annealed platinum, as revealed by field ion microscopy, are abnormally high (from 1 to 20×10 −3). An investigation indicates that such vacancies are introduced by the field evaporation process.