Daryoosh Vashaee

Bio: Daryoosh Vashaee is an academic researcher from North Carolina State University. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 48, co-authored 225 publications receiving 15724 citations. Previous affiliations of Daryoosh Vashaee include University of California, Santa Cruz & Oklahoma State University–Tulsa.

Thermal Stability of Lead Sulfide Nanocrystals Synthesized through Green Chemical Route

Abstract: In this research, lead sulfide (PbS) nanocrystals were synthesized via a simple, effective and green method. Then, the effects of heat-treatment at different temperatures on the PbS nanocrystals were investigated. In addition, the average crystallite size using Scherrer's formula, and lattice constant using Bragg's equation were calculated and compared with the standard value. The obtained results showed that an increase in the heat-treatment temperature from 250 to 450 °C brought a significant increase in the average crystallite size D of PbS nanocrystals from 13.16 nm to 32.90 nm. The thermal stability of nanocrystals determines the possibility of using these materials in devices operating under conditions above room temperature. It suggests that an increase in the thermal stability extends the temperature range of practical applications. However, an increase in the operating temperature can lead to structural and phase transformations.

The Effect of Grain Size and Volume Fraction on Charge Transport in Thermoelectric Nanocomposite of Bi2Te3-Sb2Te3

Abstract: It was shown by D. J. Bergman and L. J. Fel (J. Appl. Phys. 85, 8205, 1999) that in a composite material thermoelectric power factor, the product of the square of the Seebeck coefficient and electrical conductivity, can be enhanced over that of the individual constituents, but the figure-of- merit cannot. It is expected that this predication fails in nanocomposites due to the size effects which are ignored in this theory. In order to study the charge carrier transport in nanocomposites, we have applied a method based on Coherent Potential Approximation within effective mass approach. The method takes into account the average grain size as well as the grain size distribution and volume fraction of the different constituent in the nanocomposite material. We have applied this method to hole transport in nanocomposite of Bi2Te3-Sb2Te3 and showed the dependency of hole scattering rate as a function of the grain size.

Enhancement of Doping Concentration in P-Type SiGe Thermoelectric Alloy with the Addition of CrSi2

Abstract: We report enhancement in doping concentration of p- type SiGe by small addition of CrSi2 into the matrix. P-type SiGe thermoelectric alloy was synthesized by melting the elemental powders with and without addition of CrSi2. The transport properties of both ingots were measured in temperature range of 25- 850 C. The results showed that the addition of CrSi2 to SiGe enhances its electrical conductivity which is explained by enhancement in carrier concentration. Power factor of the composite sample was improved. Consequently, its figure-of-merit, ZT, increased compared with that of SiGe. In this study, the effect of nanostructuring on the SiGe : CrSi2 composite was further studied and its transport property were compared with those of crystalline composite sample. The DTA thermograph of SiGe : CrSi2 suggested the possibility of reaction between Cr and Si during sample synthesis which may resulted in formation of small amount of Cr1-xGex (x=0.02- 0.03) alloy in the matrix.

ElATools: A tool for analyzing anisotropic elastic properties of the 2D and 3D materials

Abstract: We introduce a computational method and a user-friendly code, named ElATools, developed for the analysis of anisotropic elastic properties. ElATools enables facile analysis of the second-order elastic stiffness tensor of two-dimensional (2D) and three-dimensional (3D) crystal systems. It computes and displays the main mechanical properties including the bulk modulus, Young's modulus, shear modulus, p-wave modulus, universal anisotropy index, Chung-Buessem anisotropy index, log-Euclidean anisotropy parameter, Cauchy pressure, Poisson's ratio, and Pugh's ratio, using three averaging schemes of Voigt, Reuss, and Hill. ElATools has a database with more than 13000 elastic stiffness constants for 3D materials available to the user. The program supports output files of the well-known computational codes IRelat, ElaStic, and AELAS. Three types of plotting and visualization tools are integrated to conveniently interface with GNUPLOT, XMGRACE, and view3dscene, offering immediate post-processing of the results. ElATools provides reliable means to investigate the mechanical stability based on the calculation of six (three) eigenvalues of the elastic tensor in 3D (2D) materials. It can efficiently identify anomalous mechanical properties, such as negative linear compressibility, negative Poisson's ratio, and highly-anisotropic elastic modulus in 2D and 3D materials, which are central properties to design and develop high-performance nanoscale electromechanical devices. Four case studies on selected material systems, ZnAu$_2$(CN)$_4$, CrB$_2$, and $\delta$-phosphorene, and a hypothetical set of systems with cubic symmetry are presented to demonstrate the descriptive and predictive capabilities of ElATools.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Complex thermoelectric materials.

Abstract: Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.