Showing papers by "Daryoosh Vashaee published in 2017"

Phonon dynamics in type-VIII silicon clathrates: Beyond the rattler concept

Abstract: Clathrates can form a type of guest-host solid structures that, unlike most crystalline solids, have very low thermal conductivity. It is generally thought that the guest atoms caged inside the host framework act as ``rattlers'' and induce lattice dynamics disorders responsible for the small thermal conductivity. We performed a systematic study of the lattice dynamical properties of type-VIII clathrates with alkali and alkaline-earth guests, i.e., ${X}_{8}\mathrm{S}{\mathrm{i}}_{46}$ ($X=\mathrm{Na}$, K, Rb, Cs, Ca, Sr, and Ba). The energy dependent participation ratio (PR) and the atomic participation ratio of phonon modes extracted from density functional theory calculations revealed that the rattler concept is not adequate to describe the effect of fillers as they manifest strong hybridization with the framework. For the case of heavy fillers, such as Rb, Sr, Cs, and Ba, a phonon band gap was formed between the acoustic and optical branches. The calculated PR indicated that the fillers suppress the acoustic phonon modes and change the energy transport mechanism from propagative to diffusive or localized resulting in ``phonon-glass'' characteristics. This effect is stronger for the heavy fillers. Furthermore, in all cases, the guest insertion depressed the phonon bandwidth, reduced the Debye temperature, and reduced the phonon group velocity, all of which should lead to reduction of the thermal conductivity.

Characterization of biomaterials

Abstract: Recent advances in the development of the characterization tools have remarkably facilitated the biomaterials research. However, recognizing the correct set of tools from the bewildering number of the existing instruments may indeed be another challenge. Since these various instruments and techniques can be utilized for variety of different applications, appropriate selection of the characterization tools can spare significantly both time and resources. In fact, characterization of biomaterials can be quite multidisciplinary relying on chemical, physical, mechanical, or biological tools, and techniques. This chapter is prepared to help the interdisciplinary oral and dental researchers in recognizing the existing instruments and techniques and to facilitate the development and discovery of new materials and extending their applications in engineering of oral and dental tissues.

Effect of the Fabrication Technique on the Thermoelectric Performance of Mg-Based Compounds—A Case Study of n-Type Mg2Ge

Abstract: High performance, low cost, and low toxicity have been the main characteristics associated with magnesium-based thermoelectric materials. Nevertheless, the high volatility of magnesium creates challenges in the synthesis of these materials. In this work, n-type Mg2Ge is synthesized using a solid-state technique, fully characterized, and compared with Mg2Ge fabricated through different processes. We have found that Bi is an ineffective dopant in Mg2Ge and precipitates into Mg2Bi3. Regardless of the technique used, the loss of Mg by evaporation and formation of precipitates in Bi-doped samples resulted in a low charge carrier concentration and, consequently, a low power factor. The precipitates significantly reduced the lattice thermal conductivity, however, leading to a figure-of-merit, zT, of 0.4 at 725 K, improving the previously reported figure-of-merit, zT, of 0.2 for Sb-doped Mg2Ge. This work highlights the impact of the fabrication technique on the thermoelectric performance of Mg-based compounds.

Valleytronics of III–V solid solutions for thermoelectric application

Abstract: It is shown that the degeneracy of the bandstructure has different impacts on thermoelectric properties of III–V materials. A comprehensive list of III–V solid solution alloys that can make multi-valley bandstructures is presented and the transport parameters affecting their thermoelectric power factor are discussed. It is shown that in some cases, such as GaxIn1−xP ternary alloy, the power factor is enhanced significantly near the valley convergence point, but in some other cases, such as AlxGa1−xAs, the enhancement is marginal and away from the valley convergence point. Therefore, in addition to the bandstructure engineering, detailed calculation of the transport parameters is also required to determine the optimum alloy composition that can improve the thermoelectric properties.

Doping induced enhanced density of states in bismuth telluride

Abstract: Power factor enhancement through resonant doping is explored in Bi2Te3 based on a detailed first-principles study. Of the dopant atoms investigated, it is found that the formation of resonant states may be achieved with In, Po, and Na, leading potentially to a significant increase in the thermoelectric efficiency at room temperature. While doping with Po forms twin resonant state peaks in the valence and conduction bands, the incorporation of Na or In results in the resonant states close to the valence band edge. Further analysis reveals the origin of these resonant states. Transport calculations are also carried out to estimate the anticipated level of enhancement.

Growth factors for oral and maxillofacial regeneration applications

Abstract: A literature survey was conducted to examine the available information on the potential of growth factors (GFs) for maxillofacial regeneration applications. The goal of this chapter is to evaluate the biological nature of the GFs, their molecular level of activity and their osteogenic potential in maxillofacial regeneration. All the GFs examined in this study have a crucial role in human growth and development. Insulin-like growth factors (IGFs) have a vital role in general growth and maintenance of the body keleton. In postnatal skeletal regeneration, platelet-derived growth factor (PDGF) plays a fundamental role in the inducing proliferation of the undifferentiated mesenchymal cells. It is also an effective mediator for bone healing and regeneration during trauma and infection. The cooperation of IGF-I with PDGF has been beneficial in encouraging bone regeneration in maxillofacial defects. Transforming growth factor beta (TGF-β) in maxillofacial reconstruction, however, seems to be associated with uncertain results. The presence of committed cells is needed for stimulating bone creation by TGF-β. It has a biphasic influence, which represses proliferation and osteoblastic differentiation at high concentrations. Bone morphogenic proteins (BMPs) seem to be the most impactful GFs with regard to maxillofacial defect repair. The effectiveness of BMPs for maxillofacial defect repair is greatly dependent on the type of carrier, and because of this, has been subjected to unknown factors in the clinical possibility trials, leading to unclear results.