Boundary Scattering Research Articles

Effects of vacancy defects location on thermal conductivity of silicon nanowire: a molecular dynamics study

The improvement of thermoelectric figure of merit of silicon nanowire (SiNW) can be achieved by lowering its thermal conductivity. In this work, non-equilibrium molecular dynamics method was used to demonstrate that the thermal conductivity of bulk silicon crystal is drastically reduced when it is crafted as SiNW and that it can be reduced remarkably by including vacancy defects. It has been found that ‘centre vacancy defect’ contributes much more in reducing the thermal conductance than ‘surface vacancy defect’. The lowest thermal conductivity that occurs is about 52.1% of that of pristine SiNW, when 2% vacancy defect is introduced in the nanowire. The vibrational density of states analysis was performed to understand the nature of this reduction and it has been found that the various boundary scatterings of phonon significantly reduce the thermal conductivity. Also, larger mass difference due to voids induces smaller thermal conductivity values. These results indicate that the inclusion of vacancy defects can enhance the thermoelectric performance of SiNWs.

Dynamically Rough Boundary Scattering Effect on a Propagating Continuous Acoustical Wave in a Circular Pipe with Flow.

The pattern of the free surface of the turbulent flow in a partially filled circular pipe contains information on the underlying hydraulic processes. However, the roughness of the free surface of flow and its temporal variation in a pipe is a dynamic and non-stationary process that is difficult to measure directly. This work examines a new acoustic method that is used to study the characteristics of the free surface roughness under controlled laboratory conditions. The acoustic method makes use of a continuous sine wave that is transmitted through the air above the turbulent flow of water over a section of the pipe instrumented with an array of wave probes and microphones. The results obtained for a representative range of flow regimes and variety of pipe bed conditions illustrate that it is possible to unambiguously relate variations in the recorded acoustic field to the standard deviation in the free surface roughness and mean flow depth. These variations are clearly linked to the hydraulic friction factor of the pipe, which is shown to be related to airborne acoustic data obtained non-invasively.

Suppression of Grain Boundary Scattering in Multifunctional p‐Type Transparent γ‐CuI Thin Films due to Interface Tunneling Currents

AbstractTransparent p‐type conductive γ‐CuI thin films typically exhibit unexpectedly high hole mobilities in the range of 10 cm2 V−1 s−1 even when heavily textured. To explain this phenomenon, the transport properties of such thin films are investigated. The temperature‐dependent resistivities of the textured (111)‐oriented films with different carrier concentration are fitted using the fluctuation‐induced tunneling conductivity (FITC) model in series with a power law. The FITC model describes barriers at the grain boundaries whereas the power law considers the scattering in the metallic interior of the grains. Magnetoresistance measurements performed on a reactively DC‐sputtered thin film at low temperatures (T < 8 K) suggest a 2D weak antilocalization effect with phase coherence lengths of about 50 nm. This is corroborated by a typical logarithmic temperature dependence of the zero‐field conductance. An n‐type inversion layer or a defect band at the interfaces of the grains as origin of the 2D carrier system and the barriers at the grain boundaries is proposed. This leads to a conclusive description of the electrical transport properties of γ‐CuI thin films and explains the high hole mobilities which are due to a suppressed backscattering at the grain boundaries in the presence of tunneling channels.

Colloidal Synthesis of Te-Doped Bi Nanoparticles: Low-Temperature Charge Transport and Thermoelectric Properties.

Electronically doped nanoparticles formed by incorporation of impurities have been of great interest because of their controllable electrical properties. However, the development of a strategy for n-type or p-type doping on sub-10 nm-sized nanoparticles under the quantum confinement regime is very challenging using conventional processes, owing to the difficulty in synthesis. Herein, we report the colloidal chemical synthesis of sub-10 nm-sized tellurium (Te)-doped Bismuth (Bi) nanoparticles with precisely controlled Te content from 0 to 5% and systematically investigate their low-temperature charge transport and thermoelectric properties. Microstructural characterization of nanoparticles demonstrates that Te ions are successfully incorporated into Bi nanoparticles rather than remaining on the nanoparticle surfaces. Low-temperature Hall measurement results of the hot-pressed Te-doped Bi-nanostructured materials, with grain sizes ranging from 30 to 60 nm, show that the charge transport properties are governed by the doping content and the related impurity and nanoscale grain boundary scatterings. Furthermore, the low-temperature thermoelectric properties reveal that the electrical conductivity and Seebeck coefficient expectedly change with the Te content, whereas the thermal conductivity is significantly reduced by Te doping because of phonon scattering at the sites arising from impurities and nanoscale grain boundaries. Accordingly, the 1% Te-doped Bi sample exhibits a higher figure-of-merit ZT by ∼10% than that of the undoped sample. The synthetic strategy demonstrated in this study offers the possibility of electronic doping of various quantum-confined nanoparticles for diverse applications.

Impact of grain boundary characteristics on lattice thermal conductivity: A kinetic theory study on ZnO

Grain boundaries are natural interfaces present in polycrystalline materials and have an important role in transport properties. In this work, the impact of grain boundary crystallographic mismatch, local impurity modulation, and spacing on lattice thermal conductivity is examined from the kinetic theory approach, with ZnO as a case study. We employ a dislocation-based model to describe the grain boundary scatterings of phonons, in which structural characteristics of grain boundaries are explicitly built-in and grain boundary scattering time depends on phonon frequency. This is in contrast to the gray model or the commonly used Casimir limit, which is blind to both grain boundary features and phonon frequency. We show that the lattice thermal conductivity generally decreases with grain boundary misorientation angle, and this dependence is significant for small grain boundary spacing while it tends to diminish for a large one. Intriguingly, our results show that local grain boundary chemistry can affect even more substantially than the crystallographic misfit on phonon relaxation time and interfacial thermal (Kapitza) resistance. Our results suggest new opportunities in tuning lattice thermal conductivity besides the nanostructure engineering approach, and demonstrates the synergetic effects of grain boundary characteristics on phonon conduction in polycrystals.

Structure dependent negative magnetoresistance of amorphous carbon thin films

Negative magnetoresistance (MR) up to 13% was observed at 2K under the magnetic field of 7T in amorphous carbon thin films synthesized by chemical vapor deposition. No tendency of saturation of MR is observed up to the magnetic field of 7T. Shift of structural morphology from disordered amorphous carbon to ordered graphitic like carbon was studied by using Raman, X-ray Diffraction, High Resolution Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. Negative MR effect increases with the increase in structural order. The mechanism of this negative MR could be ascribed to the Weak Localization and Grain Boundary Scattering Conduction depending on the different temperature regimes.

Thermal and thermoelectric transport measurements of an individual boron arsenide microstructure

Recent first principles calculations have predicted that boron arsenide (BAs) can possess an unexpectedly high thermal conductivity that depends sensitively on the crystal size and defect concentration. However, few experimental results have been obtained to verify these predictions. In the present work, we report four-probe thermal and thermoelectric transport measurements of an individual BAs microstructure that was synthesized via a vapor transport method. The measured thermal conductivity was found to decrease slightly with temperature in the range between 250 K and 350 K. The temperature dependence suggests that the extrinsic phonon scattering processes play an important role in addition to intrinsic phonon-phonon scattering. The room temperature value of (186 ± 46) W m−1 K−1 is higher than that of bulk silicon but still a factor of four lower than the calculated result for a defect-free, non-degenerate BAs rod with a similar diameter of 1.15 μm. The measured p-type Seebeck coefficient and thermoelectric power factor are comparable to those of bismuth telluride, which is a commonly used thermoelectric material. The foregoing results also suggest that it is necessary to not only reduce defect and boundary scatterings but also to better understand and control the electron scattering of phonons in order to achieve the predicted ultrahigh intrinsic lattice thermal conductivity of BAs.

First-principles simulation of electron mean-free-path spectra and thermoelectric properties in silicon

The mean free paths (MFPs) of energy carriers are of critical importance to the nano-engineering of better thermoelectric materials. Despite significant progress in the first-principles–based understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy-dependent electron scatterings and MFPs in silicon from first principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in a higher than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.

Phonon scattering due to van der Waals forces in the lattice thermal conductivity of Bi2Te3 thin films

In this work, we calculate the thermal conductivity of layered bismuth telluride (Bi2Te3) thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion and compare our results with recently published experimental data and molecular dynamics simulation. The group velocity of each phonon mode is readily extracted from the full phonon dispersion obtained from first-principle density-functional theory calculation and is used along with the phonon frequency to compute the various scattering terms. Our model incorporates the typical interactions impeding thermal transport (e.g., umklapp, isotope, and boundary scatterings) and introduces a new interaction capturing the reduction of phonon transmission through van der Waals interfaces of adjacent Bi2Te3 quintuple layers forming the virtual superlattice thin film. We find that this novel approach extends the empirical Klemens-Callaway relaxation model in such anisotropic materials and recovers the experimental anisotropy while using a minimal set of parameters.

Robust topological surface state in Kondo insulator SmB6 thin films

Topological insulators are a class of materials with insulating bulk but protected conducting surfaces due to the combination of spin-orbit interactions and time-reversal symmetry. The surface states are topologically non-trivial and robust against non-magnetic backscattering, leading to interesting physics and potential quantum computing applications1, 2. Recently there has been a fast growing interest in samarium hexboride (SmB6), a Kondo insulator predicted to be the first example of a correlated topological insulator3, 4. Here we fabricated smooth thin films of nanocrystalline SmB6 films. Their transport behavior indeed shows that SmB6 is a bulk insulator with topological surface states. Upon decreasing the temperature, the resistivity \r{ho} of Sm0.14B0.86 (SmB6) films display significant increase below 50 K due to hybridization gap formation, and it shows a saturation behavior below 10 K. The saturated resistance of our textured films is similar to that of the single crystals, suggesting that this conduction is from the surface and robust against grain boundary scatterings. Point contact spectroscopy (PCS) of the film using a superconducting tip displays both a Kondo Fano resonance and Andreev reflection, suggesting the existence of both an insulating Kondo lattice and metallic surface states.

Thermal conductivity prediction of a Cu circular nanowire

A theoretical calculation mean-free-path (MFP) model was built for circular nanowires for the first time, taking both the surface scattering and the grain boundary scattering into account. The developed MFP model can be integrated into the kinetic theory to predict the axial electronic and lattice thermal conductivities in nanowires. Based on the model, a discussion was made of the size effects produced by both the surface scatterings and the grain boundary scatterings of the electrons. The size dependences of the phonon-produced specific heat and the phonon group velocity were also modeled and discussed. The model developed in this work gave a good prediction of the thermal conductivity. Generally, the grain boundary is more significant than the circular nanowire surface in influencing the electronic thermal conductivity. Enlarging the grain size can improve both the electronic/lattice thermal conductivity and the electrical conductivity of a Cu circular nanowire. The electronic thermal conductivity and electrical conductivity of a circular Cu nanowire were both higher than those of a rectangular one.

Numerical Analysis of the Boundary Scattering Effect on Transport Properties in Bi-Sb Nanowires

In this study, we have numerically analyzed the transport properties of Bi-Sb nanowires, taking into account wire boundary scattering. Wire boundary scattering slightly decreased the Seebeck coefficient of Bi-Sb nanowires. This effect is due to the observation that boundary scattering and the mobility ratio of L-point electrons to T-point holes in the nanowires are smaller than those in bulk Bi-Sb because the wire boundary scattering suppresses the mobilities of L-point electrons and heavy holes. The largest Seebeck coefficient for all wire diameters was obtained when the Sb concentration was 5 at.%. The effective mass approached zero near 5 at.% Sb, and the small effective mass led to a large subband shift in each band. Thus, a small effective mass enhances the quantum effect at a fixed wire diameter, even if wire boundary scattering is taken into account.

The Influence of Grain Boundary Scattering on Thermoelectric Properties of Mg2Si and Mg2Si0.8Sn0.2

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg2Si and Mg2Si0.8Sn0.2 samples were measured in the temperature range from 300 K to 850 K together with the Hall coefficients at room temperature. The features of the complex band structure and scattering mechanisms were analyzed based on experimental data within the relaxation-time approximation. Based on the obtained model parameters, the possibility of improvement of the thermoelectric figure of merit due to nanostructuring and grain boundary scattering was theoretically analyzed for both Mg2Si and the solid solution.

Thermal conductivities of metallic nanowires with considering surface and grain boundary scattering

The contributions of phonon and electron transport to the thermal conductivities of Cu and Ag nanowires are studied theoretically. The effects of surface and grain boundary scatterings are involved. The embeded atom method is employed to express the interatomic potential of nanowires. While the molecular dynamic simulation and Green-Kubo formulation are used to obtain the lattice thermal conductivity, a model derived from Boltzmann transport equation and the Wiedemann-Franz relation are used to calculate electronic thermal conductivity. In addition, diffuse mismatch model is used to calculate thermal resistance of grain boundary to modify the lattice thermal conductivity, meanwhile, Mayadas-Shatzkes model is used to consider the influence of grain boundary scattering on the electronic thermal conductivity. By coupling the lattice and electronic thermal conductivity, the effective thermal conductivity of nanowire is obtained. On this base, the influences of size and temperature are analyzed. It turns out that Cu and Ag nanowires have a similar tendency in the thermal conductivity. The contribution of electron transport to the thermal conductivity of nanowire is dominated, but the contribution of phonon transport cannot be ignored on the nanoscale. The thermal conductivity of nanowire decreases due to the grain boundary scattering. And it decreases with temperature increasing or size decreasing. The contribution of phonon transport becomes more important in the case of smaller size.

Large reduction in thermal conductivity for Ge quantum dots embedded in SiO2 system

Thermal conductivity (k(T)) of Ge quantum dots (QDs) embedded in SiO2 was investigated at T = 100–400 K. The Ge QD/SiO2 system appears to have much lower k(T) than their counterparts of bulk Ge and SiO2, and the reduction factor increases with the surface-to-volume ratio of the QD in SiO2. Attendant to reduced magnitude includes delayed Umklapp decline and weaker dependence on temperature for k(T). Effective medium analysis suggests the reduction in k primarily comes from the decreased group velocity thanks to the QD inclusion that induces interfacial stress on SiO2, phonon confinement, and boundary scatterings.

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

The electrical and optical properties of TaxSiyNz thin films deposited by reactive magnetron sputtering from individual Ta and Si targets were studied in order to investigate the effects of nitrogen and silicon contents on both properties and their correlation to the film microstructure. Three sets of fcc-TaxSiyNz thin films were prepared: sub-stoichiometric TaxSiyN0.44, nearly stoichiometric TaxSiyN0.5, and over-stoichiometric TaxSiyN0.56. The optical properties were investigated by near-normal-incidence reflectivity and ellipsometric measurements in the optical energy range from 0.375 eV to 6.8 eV, while the d.c. electrical resistivity was measured in the van der Pauw configuration from 20 K to 300 K. The optical and electrical measurements were interpreted using the standard Drude-Lorentz model and the so-called grain boundary scattering model, respectively. The electronic properties were closely correlated with the compositional and structural modifications of the TaxSiyNz films due to variations in the stoichiometry of the fcc-TaNz system and the addition of Si atoms. According to the nitrogen and silicon contents, fcc-TaxSiyNz films can exhibit room temperature resistivity values ranging from 102 μΩ cm to about 6 × 104 μΩ cm. The interpretation of the experimental temperature-dependent resistivity data within the Grain Boundary Scattering model, combined with the results from optical investigations, showed that the mean electron transmission probability G and the free carriers concentration, N, are the main parameters that control the transport properties of these films. The results indicated that the correlation between electrical and optical measurements with the chemical composition and the nanostructure of the TaxSiyNz thin films provides a pertinent and consistent description of the evolution of the Ta-Si-N system from a solid solution to a nanocomposite material due to the addition of Si atoms.

Boundary Scattering in Ballistic Graphene

We report magnetotransport measurements in ballistic graphene mesoscopic wires where the charge carrier mean free path is comparable to the wire width W. Magnetoresistance curves show characteristic peak structures where the peak field scales with the ratio of cyclotron radius R(c) and wire width W as W/R(c)=0.9±0.1, due to diffusive boundary scattering. The obtained proportionality constant between R(c) and W differs from that of a classical semiconductor two-dimensional electron system in which W/R(c)=0.55.

Observing Altshuler–Aronov–Spivak Oscillation in a Hexagonal Antidot Array of Monolayer Graphene

We show that hexagonal antidot lattices of monolayer graphene exhibited the Altshuler–Aronov–Spivak (AAS) effect in low field magnetoresistance. In higher magnetic fields, Aharonov–Bohm-type oscillations were visible. The phase of AAS oscillation indicated that the chirality effect of graphene is suppressed because of inter-valley scattering due to boundary scatterings.

SU-E-T-425: Impact of Model Based Dose Calculation Algorithm for Ir-192 Intracavitary Brachytherapy with Shielded Applicator.

To determine the impact of a grid based Boltzmann solver (GBBS) on a cohort of cervical cancer patients treated with Ir-192 intracavitary brachytherapy with shielded applicators. Retrospective plans were generated using BrachyVision v8.8 (TPS) with GBBS Acuros v1.3.1. The study includes 24 patients that had CT planning images acquired with CT/MR compatible applicators. Using the TPS applicator library, shielded colpostats and tandem (#AL13122005) were virtually positioned to replace the applicators seen on CT. Dwell weights were based on TG43 delivering 6 Gy to point A. Four GBBS calculations were performed to assess differences from the standard of practice TG43. The different GBBS calculations were: 1) no applicator modeled, body= 1 g/cc muscle, 2) applicator modeled, body=1g/cc muscle, 3) applicator modeled, CT-to-material mapping with contrast (vaginal packing, rectal, Foley balloon) = 1g/cc muscle, and 4) applicator model, CT-to-material mapping without material overrides. The multiple GBBS calculations allow differences from TG43 to be attributed to factors representing the modeling of source and patient boundaries (scatter conditions), tissue heterogeneities, and applicators. Differences between GBBS4 and TG43 at clinical dosimetric points were as follows: [mean ± standard (min, max)], Point A: - 2.5% ± 0.5% (-3.8%, -1.2%), Point B: -1.5% ± 1.0% (-3.2%, 1.1%), ICRU rectum: -8.4% ± 2.5% (-14.0%, -4.1%), D2cc rectum: -6.2% ± 2.6% (- 11.9%, -0.8%), ICRU bladder: -7.2% ± 3.6% (-15.7%, -2.1%); D2cc bladder: -3.4% ± 1.8% (-7.2%, -1.1%). Bar plots comparing the modeling factors previously listed show that applicator modeling is the largest contributor to differences from TG43. Clinically significant dose differences (>5%) relative to TG43 exist when using a model-based dose calculation algorithm such as the GBBS with shielded applicators. Differences were largely due to applicator modeling, not tissue heterogeneities, source modeling, or patient boundary modeling.

Quantum Transport in MesoscopicHe3Films: Experimental Study of the Interference of Bulk and Boundary Scattering

We discuss the mass transport of a degenerate Fermi liquid ^{3}He film over a rough surface, and the film momentum relaxation time, in the framework of theoretical predictions. In the mesoscopic regime, the anomalous temperature dependence of the relaxation time is explained in terms of the interference between elastic boundary scattering and inelastic quasiparticle-quasiparticle scattering within the film. We exploit a quasiclassical treatment of quantum size effects in the film in which the surface roughness, whose power spectrum is experimentally determined, is mapped into an effective disorder potential within a film of uniform thickness. Confirmation is provided by the introduction of elastic scattering centers within the film. The improved understanding of surface roughness scattering may impact on enhancing the conductivity in thin metallic films.