Transport Properties In Order Research Articles

Evaluation of Temperature Variation in the Debye-Waller Factor for Single Crystalline Bulk Silicon Germanium by XAFS

1. Background and purpose Silicon germanium (SiGe) is expected as a next-generation electronic and thermoelectric device materials because of its higher carrier mobility than Si and lower thermal conductivity than Si and Ge. Therefore, it is important to understand the carrier scattering and phonon transport properties in order to apply SiGe as a device. We have reported that the atomic vibration in Si-rich SiGe thin films on Si substrates may be suppressed by the lattice strain, based on X-ray absorption fine structure (XAFS) measurements [1]. In this study, we demonstrated XAFS spectra of strain-free single crystalline bulk SiGe and estimated the Einstein temperature (T E) which characterizes phonons, to investigate the influence of Ge fraction and lattice strain in SiGe at nanoscale. 2. Experimental method Strain-free single crystalline bulk SiGe were grown by the traveling liquidus zone method with Ge fractions of 32 and 45 % [2]. We measured the XAFS spectra of the Ge-K absorption edge for the bulk SiGe in the transmission mode at BL14B2 in the SPring-8 synchrotron facility. We also measured a single crystalline bulk Ge as a reference sample. The sample temperature was controlled between 10 - 600 K using a refrigerator or a heating cell during measurements. Then, we extracted the EXAFS oscillations from the obtained XAFS spectra in order to investigate the temperature variation of the Debye-Waller factor. 3. Results and discussion Figure 1 shows the temperature dependence of the change in the Debye-Waller factor with reference to 10 K (Δσ 2) for Ge, SiGe thin films, and bulk SiGe. From Fig. 1, the change in Δσ 2 with temperature is slower for SiGe than for Ge. The effect of Ge fraction on the change in Δσ 2 is negligible. Further, the changes in Δσ 2 with temperature for SiGe were similar regardless of the lattice strain. Therefore, it is confirmed that the Ge fraction and the lattice strain in SiGe have little effect on the atomic vibration.Here, since σ 2 is expressed by Eq. (1) [3], Δσ 2 can be expressed as Eq. (2), where T is the absolute temperature, σ s 2 is the static Debye-Waller factor, A is a constant, and T E is the Einstein temperature. σ 2(T) = σ s 2 + Acoth(T E / 2T), (1)Δσ 2(T) = A[coth(T E / 2T) – coth{T E / (2 × 10)}], (2)The relationship between T E and the Ge fraction obtained from Eq. (2) is shown in Fig. 2. Figure. 2 indicates that T E decreases with increasing Ge fraction in Si-rich SiGe, suggesting that phonons are easily excited in SiGe with a high Ge fraction. Thus, it is considered that the effect of Ge fraction on atomic vibration in SiGe is much larger than their inner strains. 4. Acknowledgment The XAFS measurements were performed at BL14B2 in SPring-8, approved by JASRI, under proposal numbers 2019A1773, 2019B1802, and 2021A1661.

Characteristics of Gas Permeation Behaviour in Multilayer Thin Film Composite Membranes for CO₂ Separation.

Porous, porous/gutter layer and porous/gutter layer/selective layer types of membranes were investigated for their gas transport properties in order to derive an improved description of the transport performance of thin film composite membranes (TFCM). A model describing the individual contributions of the different layers’ mass transfer resistances was developed. The proposed method allows for the prediction of permeation behaviour with standard deviations (SD) up to 10%. The porous support structures were described using the Dusty Gas Model (based on the Maxwell–Stefan multicomponent mass transfer approach) whilst the permeation in the dense gutter and separation layers was described by applicable models such as the Free-Volume model, using parameters derived from single gas time lag measurements. The model also accounts for the thermal expansion of the dense layers at pressure differences below 100 kPa. Using the model, the thickness of a silicone-based gutter layer was calculated from permeation measurements. The resulting value differed by a maximum of 30 nm to the thickness determined by scanning electron microscopy.

Modeling and Experimental Studies of Mesoscopic Morphologies and Proton Conductivities of Cathode Catalyst Layers with Different Carbon Supports

Introduction Polymer electrolyte fuel cells (PEFCs) are considered to be promising for automotive and stationary power applications. One of the most important issues for PEFCs is the reduction of the Pt loading in the cathode catalyst layers (CCLs), which generally consist of carbon-supported Pt catalysts and ionomers. Carbon blacks are widely used as the support materials. It is well known that the performance of the CCL is greatly influenced by the type of carbon black. We need to deeply understand the relationships between the microstructure and the transport properties in order to improve the performance of the low-Pt-loaded CCL. Uchida et al. investigated the relationships between the microscopic (< 10 nm) morphologies of CCLs with different carbon blacks and their performances [1]. They revealed that the existence of Pt particles on the exterior surfaces of the carbon black particles and the uniform coverage of the ionomer on the carbon surfaces had a great impact on the performance at high current density. In the present study, we investigated the mesoscopic (10–200 nm) morphologies and the proton conductivities of the CCLs with different carbon blacks using experimental and modeling approaches. Experimental We used 30 wt% Pt on Ketjen black (surface area ca. 800 m2 g−1, Pt/CB800), graphitized Ketjen black (surface area ca. 150 m2 g−1, Pt/GCB150), an acetylene black (surface area ca. 800 m2 g−1, Pt/AB800), and a second acetylene black (surface area ca. 250 m2 g−1, Pt/AB250) as cathode catalysts. CCLs were fabricated by spraying inks with the cathode catalysts and ionomers (equivalent weight = 909 g eq−1) on 30-μm thick membranes. The Pt loading and the ionomer/carbon (I/C) weight ratio were controlled to be 0.4 mgPt cm−2 and 0.8, respectively. The pore size distributions (PSDs) of the CCLs were evaluated by mercury intrusion porosimetry. Anode catalyst layers with 0.1 mgPt cm−2 Pt loading and 0.8 I/C weight ratio were fabricated by spraying inks with 50 wt% Pt on Ketjen black catalysts and the ionomers. AC impedance measurements were performed at 0.5 V with a perturbation of 2 mV from 100 kHz to 0.1 Hz at 80°C and a 100% RH hydrogen feed to the anode (reference, counter electrode) and nitrogen to the cathode (working electrode). The proton conductivities of the CCLs were calculated from the 45° line of Nyquist plots, according to the literature [2]. Modeling The mesoscopic structure models of the CCLs were generated by the following two steps. First, the porous structures were reconstructed from the FIB-SEM images with a resolution of 4 nm. Then, the solid surfaces were uniformly coated with ionomer. The amount of the coated ionomer was determined by converting the I/C weight ratio to the I/C volume ratio, assuming that the apparent volume of the carbon black was the total volume of the solid phase and nanopores less than 10 nm. The PSDs were calculated from the volume of a non-wetting fluid which was pressed into the medium. The proton conductivities were calculated by Ohm’s Law, assuming that the protons transport through the ionomer regions with a proton conductivity of 10 S m−1, which is the bulk proton conductivity of the ionomer at 80°C and 100% RH. Results and Discussion Figure 1 shows the experimental and modeling results of the PSDs. The modeling results for the Pt/CB800 and the Pt/AB800 corresponded well with the experimental results. These results suggest that the uniform-ionomer-coating model is a good approximation for the CCLs using Pt/CB800 and Pt/AB800, while there were differences between the modeling results and the experimental results for Pt/GCB150 and Pt/AB250. It is considered that the ionomer was unevenly distributed in the CCLs using Pt/GCB150 and Pt/AB250. Figure 2 shows the experimental and modeling results of the proton conductivities. The modeling results for the Ketjen black-supported catalysts underestimated the proton conductivities, while those for the acetylene black-supported catalysts were overestimated. In particular, the deviations for Pt/CB800 and Pt/AB800 were large, in spite of the correspondences in these PSDs. It was suggested that the proton conductivities were influenced not only by the mesoscopic morphologies of the CCLs but also other factors, such as the microscopic (< 10 nm) morphologies and the chemical properties of the carbon surfaces. Acknowledgements This work was partially supported by funds for the SPer-FC Project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Y. C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources, 315 (2016) 179.R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5) (2005) A970. Figure 1

Requirements and possible simplifications for multi-ionic transport models – Case of concrete subjected to wetting-drying cycles in marine environment

In this paper, a physically and chemically based model, which describes coupled ion-moisture transport, is used to simulate chloride ingress in concrete elements subjected to wetting-drying cycles in marine environment. Various assumptions are tested, in order to quantify the influence of taking into account dissolution/precipitation kinetics or to underline the differences between complex model and very simple one assuming saturated conditions in the latter case. Numerical simulations are compared to experimental chloride concentration profiles. Experimental data were obtained on OPC concrete (w/c = 0.43) specimens exposed to 6 h/6 h seawater wetting-drying cycles in lab (where RH and T are controlled).The results show that when the initial amounts of hydration products are known, the assessment of the chloride binding parameters is not needed. In addition, including reaction kinetics in the model is essential and improves the predictions compared to a pure thermodynamical approach. Moreover, according to sensitivity analysis the boundary layer thickness that reflects the evaporation kinetics and the “intrinsic” permeability are not important parameters for the studied concrete. Updating at each time step the transport properties in order to account for dissolution/ precipitation of mineral species does not change very much the chloride content profiles as well. Furthermore, simulations carried out in saturated conditions provide similar results to those obtained when accounting for wetting-drying cycles for the studied concrete. Then, it may be possible to use a simpler model (ie saturated conditions), and thus avoid the problematic assessment of the “intrinsic” permeability, to predict the service life of a RC structure cast with the concrete studied.

Electrode and electrolyte materials for electrochemical capacitors

Among different electric energy storage technologies electrochemical capacitors are used for energy storage applications when high power delivery or uptake is needed. Their energy and power densities, durability and efficiency are influenced by electrode and electrolyte materials however due to a high cost/performance ratio; their widespread use in energy storage systems has not been attained yet.Thanks to their properties such as high surface area, controllable pore size, low electrical resistance, good polarizability and inertness; activated carbons derived from polymeric precursors are the most used electrode materials in electrochemical capacitors at present. Other electrode materials such as shaped nano-carbons or metal oxides are also investigated as electrode materials in electrochemical capacitors, but only as useful research tools.Most commercially used electrochemical capacitors employ organic electrolytes when offering concomitant high energy and high power densities. The use of aqueous based electrolytes in electrochemical capacitor applications is mainly limited to research purposes as a result of their narrow operating voltage. Recent studies on room temperature ionic liquids to be employed as electrolyte for electrochemical capacitor applications are focused on fine tuning their physical and transport properties in order to bring the energy density of the device closer to that of batteries without compromising the power densities.In this paper a performance analysis, recent progress and the direction of future developments of various types of materials used in the fabrication of electrodes for electrochemical capacitors are presented. The influence of different types of electrolytes on the performance of electrochemical capacitors such as their output voltage and energy/power densities is also discussed.

Resistive Plate Chambers: electron transport and modeling

We study the electron transport in gas mixtures used by Resistive Plate Chambers (RPCs) in high energy physics experiments at CERN. Calculations are performed using a multi term theory for solving the Boltzmann equation. We identify the effects induced by non-conservative nature of electron attachment, including attachment heating of electrons and negative differential conductivity (NDC). NDC was observed only in the bulk component of drift velocity. Using our Monte Carlo technique, we calculate the spatially resolved transport properties in order to investigate the origin of these effects. We also present our microscopic approach to modeling of RPCs which is based on Monte Carlo method. Calculated results for a timing RPC show good agreement with an analytical model and experimental data. Different cross section sets for electron scattering in C2H2F4 are used for comparison and analysis.

Computation of mass transport properties of apple and rice from X-ray microtomography images

Computational simulations of food processes such as baking, frying, and cooling require transport properties in order to accurately model the process. Coupled with the increased usage of X-ray computed microtomography to better understand manufactured food microstructures other than simple observation, there is an avenue to improve transport models, safety, and process optimization. This work does not focus on the development of new software or simulations but on taking existing software available in other fields of research and applying it to published images of food products to show how it can be used to improve our understanding of food. The two examples of food systems are apple and rice. Using both foods, this work presents open source computational software to calculate intrinsic permeability using a lattice-Boltzmann simulation, pore-size distribution, pore connectivity using the Hoshen and Kopelman algorithm, and vapor diffusivity and tortuosity of water by a random-walk. Simulations are validated when available. This work is extremely relevant to industry in that it allows for more quantitative data from X-ray μCT. Therefore, the increased data allows for improved product design, process optimization, and food safety. Additionally, the quantitative transport properties garnered can help improve transport models. These simulations can provide useful information where conventional experimentation will not always work.

Cellular concrete properties and the effect of synthetic and protein foaming agents

Cellular concrete consists of cement, water, aggregate and air voids. Cellular concrete can have between 10% and 70% air, which results in a material that is lightweight but may compromise the compressive strength and durability properties. Although it has been shown in the literature that less connected air voids yield a lower reduction in compressive strength, few studies have reported a detailed characterization of the microstructure of cellular concrete. It is critical to understand the impact of the microstructure and its implications on the development of strength, elastic modulus and transport properties in order to fully benefit from the lightweight properties, and this is the focus of this paper. Thirteen batches of cellular concrete were designed, prepared and tested. The air content varied, ranging from 6% to 35%. Three different foaming agents were used, one of which was protein-based and the other two were synthetic. Fresh properties such as slump, plastic air content and plastic density were measure for each batch of concrete as well as other properties, including hardened density, compressive strength, static elastic modulus, sorptivity, hardened air void distribution and thermal resistance. Key outcomes revealed that cellular concrete has good potential to be used for lightweight structural applications owing to its evolution of mechanical properties, transport properties and thermal resistance, but it is very sensitive to the type of foaming agent that is used. Outcomes showed that the foaming agent type had a noticeable effect on the thermal resistance and sorptivity coefficient but less of an effect on the mechanical properties. This is significant because the type of foaming agent used will directly influence the applications for which cellular concrete can be used. For example, cellular concrete designed with foaming agent B, which yields the greatest capillary pore volume is least suited for outdoor applications where the concrete may be exposed to moisture and or ions.

Mechanical properties of BixSb2−xTe3 nanostructured thermoelectric material

Research on thermoelectric (TE) materials has been focused on their transport properties in order to maximize their overall performance. Mechanical properties, which are crucial for system reliability, are often overlooked. The recent development of a new class of high-performance, low-dimension thermoelectric materials calls for a better understanding of their mechanical behavior to achieve the desired system reliability. In the present study we investigate the mechanical behavior of nanostructure bulk TE material p-type BixSb2−xTe3 by means of nanoindentation and 3D finite element analysis. The Young’s modulus of the material was estimated by the Oliver–Pharr (OP) method and by means of numerically assisted nanoindentation analysis yielding comparable values about 40 GPa. Enhanced hardness and yield strength can be predicted for this nanostructured material. Microstructure is studied and correlation with mechanical properties is discussed.

Oxygen and Glucose Distribution After Intracorneal Lens Implantation

Insertion of an implant in the cornea to achieve corneal multifocality has been suggested as a solution for presbyopia. However, unresolved issues related to nutrient transport need to be resolved. Our aim was to find the best lens position and influence lens transport properties in order to optimize nutrient supply to corneal cells. An axisymmetric corneal model was built to simulate the nutrient transport in the cornea. Oxygen and glucose concentrations were calculated for normal cornea and intracorneal lens wearing conditions. The simulation considers the different tissue layers (epithelium, stroma, and endothelium) as well as layer and solute concentration dependent consumption. The minimum oxygen tension in the cornea was found to be higher when the lens was placed at 3/4 of the corneal thickness. Moreover, in this position, the influence of the inlay diffusivity was smaller than at more anterior or posterior placements. The diffusivity of the inlay affects the way nutrients will be transported through the cornea. The threshold where glucose may diffuse through or around the implant was found to be 1/100th of the stromal diffusivity. Computational methods are especially attractive to study nutrient transport in the cornea due to the difficulties associated with in vivo or in vitro measurements. The exact parameters that dictate the corneal metabolism are not known. However, the combined analysis of oxygen and glucose distribution is valuable in order to predict the complex physiological changes that arise under intracorneal lens implantation.

A comprehensive study of carrier velocity modulation in DGSOI transistors

Velocity modulation transistors (VMT) are proposed as a way to explode short transit time between two adjacent channels with different transport properties in order to obtain a fast switch. Originally proposed for III–V heterostructures, a Monte Carlo study of silicon-based VMTs is presented in this work showing that surface roughness in double-gate silicon-on-insulator devices can be used as a mobility degradation mechanism to obtain current ratios higher than 30 and therefore feasible devices. Transient simulations have been also carried out obtaining sub-picosecond switch times for 0.1 μm gate length. Switch time limitations are also discussed including both intrinsic and extrinsic factors.

Transport and magnetic properties of magnetic alloy atom bridge

We have investigated the transport and magnetic properties of the atom bridge made from magnetic materials, which is the atom-scale wire constructed between a scanning tunneling microscopy tip and a solid surface, with ab initio calculations. In the case of FeNi alloy bridges, we have found that the atoms in the bridges can be designed to have significantly larger magnetic moment than that in the corresponding alloy bulk. We discuss about the origin of their transport properties in order to design the spin-polarized electron current through the bridges.

Thickness dependent properties of La 0.67Ca 0.33MnO 3 thin films

Epitaxial thin films of La 0.67Ca 0.33MnO 3 (LCMO) were prepared by pulsed laser deposition (PLD) on SrTiO 3 (100) substrates. After deposition the films were in situ annealed for 1 h in oxygen (10 5 Pa, 780°C). We studied the thickness dependence of the transport properties in order to address the relation between the microscopic structure and the metal–insulator transition temperature T MI. It could be demonstrated that T MI decreases systematically from the bulk value T MI=276 K with decreasing film thickness. Details of the temperature dependence of ϱ and its magnetic field dependence are discussed.