Cross-plane Direction Research Articles

Magnetic and thermal transport properties of SrFe12O19 permanent magnets with anisotropic grain structure

Permanent magnets are gaining increasing interest and importance for applications such as generators and motors. Thermal management is a key concern since performance of magnets decreases with temperature. We investigate the magnetic and thermal transport properties of rare earth-free, fine-grained SrFe12O19 magnets produced by the current activated pressure assisted densification. We propose a cooling scheme based on an anisotropic grain structure that can help retain magnetic performance under high temperature conditions. The synthesized magnets have aligned grains such that their magnetic easy axis is perpendicular to their largest surface area to maximize their magnetic performance. The SrFe12O19 magnets have fine grain sizes in the cross-plane direction and substantially larger grain sizes in the in-plane direction. This microstructure results in approximately a factor of two higher thermal conductivity in the in-plane direction, providing an opportunity for effective cooling. The phonons are the dominant heat carriers near room temperature. Temperature and direction dependent thermal conductivity measurements indicate that both Umklapp and grain boundary scattering are important in the in-plane direction, while grain boundary scattering dominates the cross-plane thermal transport. The proposed design strategy should translate well to other material systems and has important implications for thermal management of nanostructured permanent magnets.

Interface thermal conductance of van der Waals monolayers on amorphous substrates

Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant Ka, but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.

Anisotropic Heat Conduction in Two-Dimensional Periodic Silicon Nanoporous Films

Research on heat conduction in periodic nanoporous silicon films has drawn much attention due to its importance for developing highly efficient thermoelectric devices. Here, the thermal transport in two-dimensional (2D) periodic silicon nanoporous films is studied by a phonon Monte Carlo (MC) method and the theoretical analyses based on the Boltzmann transport equation (BTE). It is found that both the cross-plane and the in-plane effective thermal conductivities are significantly reduced when compared to those in the diffusive limit, and decrease with the increasing porosity or the decreasing period length. Importantly, our work reveals a strong anisotropy of the effective thermal conductivity of the 2D periodic nanoporous films; that is, the effective thermal conductivity in the in-plane direction is significantly less than that in the cross-plane direction, due to the anisotropic effects of material removal and pore boundary scattering. Interestingly, even the influence of the specular parameter that de...

Anisotropic temperature-dependent thermal conductivity by an Al2O3 interlayer in Al2O3/ZnO superlattice films

The thermal conductivity of superlattice films is generally anisotropic and should be studied separately in the in-plane and cross-plane directions of the films. However, previous works have mostly focused on the cross-plane thermal conductivity because the electrons and phonons in the cross-plane direction of superlattice films may result in much stronger interface scattering than that in the in-plane direction. Nevertheless, it is highly desirable to perform systematic studies on the effect of interface formation in semiconducting superlattice films on both in-plane and cross-plane thermal conductivities. In this study, we determine both the in-plane and cross-plane thermal conductivities of Al2O3 (AO)/ZnO superlattice films grown by atomic layer deposition (ALD) on SiO2/Si substrates in the temperature range of 50–300 K by the four-point-probe 3-ω method. Our experimental results indicate that the formation of an atomic AO layer (0.82 nm) significantly contributes to the decrease of the cross-plane thermal conductivity of the AO/ZnO superlattice films compared with that of AO/ZnO thin films. The cross-plane thermal conductivity (0.26–0.63 W m−1 K−1 of the AO/ZnO superlattice films (with an AO layer of ∼0.82 nm thickness) is approximately ∼150%–370% less than the in-plane thermal conductivity (0.96–1.19 W m−1 K−1) of the corresponding film, implying significant anisotropy. This indicates that the suppression of the cross-plane thermal conductivity is mainly attributed to the superlattice, rather than the nanograin columnar structure in the films. In addition, we theoretically analyzed strong anisotropic behavior of the in-plane and cross-plane thermal conductivities of the AO/ZnO superlattice films in terms of temperature dependence.

Dramatically enhanced thermoelectric performance of MoS2 by introducing MoO2 nanoinclusions

Adopting an oxygen doping strategy, the thermoelectric efficiency of MoS2 can be enhanced up to 50 times with the best performance appearing along the cross-plane direction.

The role of phonon-phonon and electron-phonon scattering in thermal transport in PdCoO2.

The layered metal oxide PdCoO2 is of fundamental interest in solid-state physics due to its unique nature as a two-dimensional electron gas. It is well known that electron-phonon scattering plays the leading role in the electrical transport of lots of materials; however, the influence of electron-phonon coupling on the thermal transport in PdCoO2 is rarely studied. Herewith we employ ab initio calculations to study the phonon lifetimes of PdCoO2 due to electron-phonon scattering and anharmonic phonon-phonon interactions. It is found that electron-phonon scattering has a large correction to the lattice thermal conductivity of PdCoO2 for both the in-plane and cross-plane directions, which is reduced by 21% and 27%, respectively. Moreover, the correction becomes larger and is more significant for the in-plane direction with decreasing temperature. As a result, the cross-plane lattice thermal transport overwhelms the in-plane value at low temperatures. This study provides useful guidance for the development of relevant devices involving two-dimensional electron gas systems.

Penumbral Dose Characteristics of Physical and Virtual Wedge Profiles

Purpose: Both physical and virtual wedges are used in radiotherapy to get uniform and desired dose distribution in clinical setting. All linear accelerators of different venders have computer controlled dynamic wedges called virtual wedge filters. Penumbra is one of the important photon beam characteristics needed to be understood in radiation therapy at the time of commissioning of Treatment Planning system (TPS) as well as applying various treatment planning algorithms in clinical applications. In this study we measured the dose profiles of open field, physical wedges (PW) and virtual wedges (VW) for energies (6 MV & 15 MV), various field sizes (10 × 10, 15 × 15 & 20 × 20 cm2), depths (dmax, 10 cm, 20 cm) and wedge angles (15°, 30°, 45° and 60°). From beam profile we calculated the penumbral width for open and wedged fields. The study was carried out on Siemens ONCOR IMRT Plus linear accelerator. The obtained penumbral width of PW and VW of all wedge angles was subtracted from the penumbral width of open field. The deviations in penumbral width were compared and statistically analyzed as a function of energy, depth, field size and wedge angles. Material and Method: The penumbral width was measured using IBA CC13 ion chamber in IBA Blue phantom (a 3D water phantom). The source to surface distance (SSD) during our study was kept 100cm and measurement was taken for 10 × 10, 15 × 15, 20 × 20 cm2 field sizes and for 15°, 30°, 45°, 60° wedges. These measurements were taken for both 6 MV and 15 MV photon energies. Virtual wedge profiles were acquired using LDA-99 linear detector array (IBA, Germany). The deviations in penumbral width for both PW and VW were calculated by subtracting the penumbral width from open field penumbral width in gun direction (in-plane) and deviation in VW penumbral width, and were obtained by subtracting the open field penumbral width in left-right direction (cross-plane) direction. The measured deviations were plotted for both PW and VW. Statistics on the measured deviations was performed by using SPSS Version 15. Results & Conclusion: The results of one way ANOVA (Analysis of Variance) show that the deviations are significant with energy and the deviations are higher in lower energy than higher energy. The deviations increase as depth increases, the deviations are also significant with depth. The deviations increase with field sizes; the deviations as a function of field size are highly significant. The deviations are higher in PW than VW but the deviations with wedge type are in-significant. As wedge angle increases, deviations also increase and the effect of wedge angle is highly significant on deviations.

Evidence of ballistic thermal transport in lithium niobate at room temperature

In ballistic transport, heat carriers such as phonons travel through the solid without any scattering or interaction. Therefore, there is no temperature gradient in the solid, suggesting zero thermal conductivity by Fourier's law. Ballistic transport is typically seen in high purity crystals at either temperatures below ~10K, or physical size below ~100nm, where the mean free path of the carrier is larger than the solid itself. In this letter, we show evidence of ballistic transport at room temperature in lithium niobate wafers in the in-plane and cross-plane directions under both steady state and high frequency heating that are monitored using both infrared and resistance thermometry. We report phonon mean free path in lithium niobate around 425μm, which is about 50 times higher than the largest phonon mean free path in the literature at room temperature.

P12. Commissioning and evaluation of a new commercial small animal radiation research platform Xstrahl XenX irradiator

Introduction The aim of this study was to perform the commissioning and acceptance tests of the commercial small animal radiation research platforms Xsthral XenX irradiator. Methods The Xstrahl-Xenx irradiator contains an X-ray tube with potentials of 20–220 kVp range. The inherent filtration of X-ray tube is 0.8 mm (Be). An additional filtration of 0.15 mm (Cu) is used in irradiation mode, and additional filtration of 1 mm (Al) is used for imaging. The source-to-axis (SAD) distance is 35 cm. Absolute dose were determined for the largest field without collimator (10 × 10 cm 2 ) following the recommendation of AAPM TG61 protocol using in air method to determine the dose-to-water at the surface of a water phantom for orthovoltage X-ray tubes. These measures were performed with an ion chamber and a calibrated dosimeter (PTW − 31013). Absolute dose for small field (10 × 10 mm 2 ) was determined using radiochromic films (Gafchromic EBT3). A calibration curve was fit to characterize the change in optical density (OD) in function of dose using mean intensities read from the red color channel. Films were irradiated with dose range from 0.5 Gy to 9 Gy. Then, films were scanned in transmission mode (positive mode), 48 bits RGB (16 bits per channel color), and 72 dpi resolution. In addition of dosimetric acceptance, other physics measurements were performed. These include measuring the beam homogeneity in the in-plan and cross-plane direction using film for large and small field. Other measurement such as reproducibility of tube voltage, dose output, accuracy of kVp, mA, time, and half value layer have been carried out. Results X-ray tube output and reproducibility for kilovoltage (difference between the measured and displayed value), dose output were performed five times. Reproducibility of kiloVoltage was ranged from −1.7% to 1%, of dose was lower than 0.1%. The maximum dose rate at SAD was 278 cGy/min for large filed without collimator and determined using an ion chamber, for small field it was 256 cGy/min and determined using gafchromic films after calibration. Homogeneity of radiation field was 6.5% and 5.4% in cross plan and in plan profile for large field. Homogeneity is better for small field with a collimator, 2.5% and 2.4% in cross plan and in plan profile respectively. Conclusions The acceptance tests and calibration of Xsthral XenX platform provide to researchers a tools to estimate doses delivered during irradiation of cells and mice in order to advance research in radiobiology. The dose calculation tool allowed us to performed RILA (Radiation-induced lymphocyte apoptosis) test from linac to Xenx irradiator.

Thermoelectric properties of topological insulator BaSn2

Recently, has been predicted to be a strong topological insulator by the first-principle calculations. It is well known that topological insulators have a close connection to thermoelectric materials, such as the family. In this work, we investigate thermoelectric properties of by the first-principles calculations combined with the Boltzmann transport theory. The electronic part is carried out by a modified Becke and Johnson (mBJ) exchange potential, including spin–orbit coupling (SOC), while the phonon part is performed using a generalized gradient approximation (GGA). It was found that the electronic transport coefficients between the in-plane and cross-plane directions showed strong anisotropy, while lattice–lattice thermal conductivities demonstrated almost complete isotropy. Calculated results revealed a very low lattice thermal conductivity for , and the corresponding average lattice thermal conductivity at room temperature is 1.69 , which is comparable or lower than those of lead chalcogenides and bismuth-tellurium systems as classic thermoelectric materials. Due to the complicated scattering mechanism, calculating the scattering time τ is challenging. By using an empirical s, the n-type figure of merit ZT is greater than 0.40 in wide temperature ranges. Experimentally, it is possible to attain better thermoelectric performance by strain or tuning size parameters. This work indicates that may be a potential thermoelectric material, which can stimulate further theoretical and experimental work.

Super-stretchable borophene

Recent success in the synthesis of the two-dimensional borophene on silver substrates has attracted strong interest in exploring its extraordinary properties for potential technological applications. The single-layer borophene has a buckled structure with atomic ridges. By using the first-principles density functional theory calculations, we show that the two-dimensional borophene is highly stretchable with strong anisotropy The strain-to-failure in the direction along the atomic ridges is nearly twice as large as that across the atomic ridges. The straining-induced flattening and the subsequent stretch of the flat borophene are accounted for the large strain-to-failure for tension along the atomic ridges. We also investigated the mechanics of monolayer borophene under biaxial tension and we found that the biaxial tension increases the strength across the atomic ridges but decreases the failure strain along the atomic ridges. Furthermore, when the bilayer borophene is stretched along the cross-plane direction, the strength and failure strain of the bilalyer borophene are much higher than those of the bilayer graphene due to the very strong inter-layer chemical bonding.

Thermal Conductance of Poly(3-methylthiophene) Brushes.

A wide variety of recent work has demonstrated that the thermal conductivity of polymers can be improved dramatically through the alignment of polymer chains in the direction of heat transfer. Most of the polymeric samples exhibit high conductivity in either the axial direction of a fiber or in the in-plane direction of a thin film, while the most useful direction for thermal management is often the cross-plane direction of a film. Here we show poly(3-methylthiophene) brushes grafted from phosphonic acid monolayers using surface initiated polymerization can exhibit through-plane thermal conductivity greater than 2 W/(m K), a 6-fold increase compared to spin-coated poly(3-hexylthiophene) samples. The thickness of these films (10-40 nm) is somewhat less than that required in most applications, but the method demonstrates a route toward higher thermal conductivity in covalently grafted, aligned polymer films.

Ultralow lattice thermal conductivity in topological insulator TlBiSe2

We present ab-initio calculations of the phonon thermal transport properties of topological insulator TlBiSe2. Our results point to a very low lattice thermal conductivity, comparable or lower than those of some popular good thermoelectric materials. Furthermore, we find a slight thermal anisotropy between the in-plane and cross-plane directions in TlBiSe2, markedly smaller than those of van-der-Waals topological insulators explored so far. These conclusions are confirmed and explained by comprehensive analysis of the phonon spectrum of TlBiSe2. The combination of ultralow lattice thermal conductivity and small anisotropy makes TlBiSe2 a promising candidate for thermoelectric applications.

SU-G-BRC-10: Feasibility of a Web-Based Monte Carlo Simulation Tool for Dynamic Electron Arc Radiotherapy (DEAR)

Purpose: DEAR is a radiation therapy technique utilizing synchronized motion of gantry and couch during delivery to optimize dose distribution homogeneity and penumbra for treatment of superficial disease. Dose calculation for DEAR is not yet supported by commercial TPSs. The purpose of this study is to demonstrate the feasibility of using a web-based Monte Carlo (MC) simulation tool (VirtuaLinac) to calculate dose distributions for a DEAR delivery. Methods: MC simulations were run through VirtuaLinac, which is based on the GEANT4 platform. VirtuaLinac utilizes detailed linac head geometry and material models, validated phase space files, and a voxelized phantom. The input was expanded to include an XML file for simulation of varying mechanical axes as a function of MU. A DEAR XML plan was generated and used in the MC simulation and delivered on a TrueBeam in Developer Mode. Radiographic film wrapped on a cylindrical phantom (12.5 cm radius) measured dose at a depth of 1.5 cm and compared to the simulation results. Results: A DEAR plan was simulated using an energy of 6 MeV and a 3×10 cm2 cut-out in a 15×15 cm2 applicator for a delivery of a 90° arc. The resulting data were found to provide qualitative and quantitative evidence that the simulation platform could be used as the basis for DEAR dose calculations. The resulting unwrapped 2D dose distributions agreed well in the cross-plane direction along the arc, with field sizes of 18.4 and 18.2 cm and penumbrae of 1.9 and 2.0 cm for measurements and simulations, respectively. Conclusion: Preliminary feasibility of a DEAR delivery using a web-based MC simulation platform has been demonstrated. This tool will benefit treatment planning for DEAR as a benchmark for developing other model based algorithms, allowing efficient optimization of trajectories, and quality assurance of plans without the need for extensive measurements.

Enhancement of cross-plane thermal conductivity and mechanical strength via vertical aligned carbon nanotube@graphite architecture

A three-dimensional (3D) carbon nanotube/exfoliated graphite block (CNT/EGB) was prepared by growing vertical aligned carbon nanotube (VACNT) at the surface of SiO2-coated exfoliated graphite plate (EGP) through chemical vapor deposition followed by hot-pressing. In such 3D CNT/EGB, EGPs were bridged by the VACNTs in the cross-plane direction, and the interface between EGPs and VACNTs was covalently bonded by SiC which formed by reaction of SiO2 and the adjacent carbon of EGPs and VACNTs. The length and growth density of VACNTs were adjusted by the growth time and concentration of catalysts. Thermal conductivity and mechanical strength of CNT/EGB were controlled by the growth states of VACNTs and hot-pressing. CNT/EGB showed a maximum cross-plane thermal conductivity (k⊥) of 38 W/mK, which is more than twice as much as that of EGB (14 W/mK). A remarkable increase in k⊥ was attributed to the efficient heat flow of VACNTs bridging EGPs in the cross-plane direction and the thermal conductive SiC interface between VACNTs and EGPs. Additionally, the increased bending (76 MPa) and compressive strength (59 MPa) of CNT/EGB was due to the combination of strong pull-out effect of high-density nanotubes and the strong covalent interconnections between VACNTs and EGPs.

Temperature-Dependent Mean Free Path Spectra of Thermal Phonons Along the c-Axis of Graphite.

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well-known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation. Our study provides important new insights into heat transport and phonon scattering mechanisms in graphite and other anisotropic van der Waals solids.

Thermal conductivities of single- and multi-layer phosphorene: a molecular dynamics study.

As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.

リチウムイオン電池用の電解液を含浸した電極シートの熱伝導率測定

We developed a thermal conductivity measurement system using a high-stiffness linear-motion guide and measured Li-ion battery electrodes which sealed within aluminum-laminated films. The apparent thermal conductivity of the battery electrode, which was measured at a temperature from minus several degrees to approximately 60°C, was from 1.1 to 1.4 W/(m･K) in the cross-plane direction, and from 37 to 38 W/(m･K) in the in-plane direction.

Thermoelectric properties of solution-synthesized n-type Bi2Te3 nanocomposites modulated by Se: An experimental and theoretical study

We report the investigation of the thermoelectric properties of large-scale solution-synthesized Bi2Te3 nanocomposites prepared from nanowires hotpressed into bulk pellets. A third element, Se, is introduced to tune the carrier concentration of the nanocomposites. Due to the Se doping, the thermoelectric figure of merit (ZT) of the nanocomposites is significantly enhanced due to the increased power factor and reduced thermal conductivity. We also find that thermal transport in our hot-pressed pellets is anisotropic, which results in different thermal conductivities along the in-plane and cross-plane directions. Theoretical calculations for both electronic and thermal transport are carried out to establish fundamental understanding of the material system and provide directions for further ZT optimization with adjustments to carrier concentration and mobility.

Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows tuning electrical resistivity over a very wide range (10−1−106Ωcm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests.