Showing papers by "Daryoosh Vashaee published in 2012"
TL;DR: Quantitative analysis of real-time dynamical experiments reveals that there is an interplay between intralayer domain growth and interlayer coupling, suggesting the existence of cooperative multilayer epitaxy, and it is postulated that such long-range epitaxy is solvent-assisted.
Abstract: Liquid-crystalline phases of stacked lipid bilayers represent a pervasive motif in biomolecular assemblies. Here we report that, in addition to the usual smectic order, multicomponent multilayer membranes can exhibit columnar order arising from the coupling of two-dimensional intralayer phase separation and interlayer smectic ordering. This coupling propagates across hundreds of membrane lamellae, producing long-range alignment of phase-separated domains. Quantitative analysis of real-time dynamical experiments reveals that there is an interplay between intralayer domain growth and interlayer coupling, suggesting the existence of cooperative multilayer epitaxy. We postulate that such long-range epitaxy is solvent-assisted, and that it originates from the surface tension associated with differences in the network of hydrogen-bonded water molecules at the hydrated interfaces between the domains and the surrounding phase. Our findings might inspire the development of self-assembly-based strategies for the long-range alignment of functional lipid domains.
110 citations
TL;DR: In this paper, the Boltzmann transport equation for charge carriers and phonons is solved for Mg2Si and it is shown that bulk nanostructuring is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.
Abstract: In nanostructured bulk materials, the additional interfaces in the material enhance phonon scattering and reduce the thermal conductivity. However, interfaces also scatter electrons and deteriorate charge carrier transport. In order to benefit from the interfacial effects, the crystallite size in the material must be small compared with phonon mean free path (PMFP) and large compared with the charge carrier mean free path (CMFP). In this paper, we solve the Boltzmann transport equation for charge carriers and phonons. We show that bulk nanostructuring of Mg2Si is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.
70 citations
TL;DR: In this paper, the Debye model was used to calculate the lattice thermal conductivity of bulk magnesium silicide (Mg2Si), and a unique set of semi-empirical material parameters was obtained for both n-type and p-type materials through simulation testing.
Abstract: Thermoelectric (TE) materials based on alloys of magnesium (Mg) and silicon (Si) possess favorable properties such as high electrical conductivity and low thermal conductivity. Additionally, their abundance in nature and lack of toxicity make them even more attractive. To better understand the electronic transport and thermal characteristics of bulk magnesium silicide (Mg2Si), we solve the multiband Boltzmann transport equation within the relaxation-time approximation to calculate the TE properties of n-type and p-type Mg2Si. The dominant scattering mechanisms due to acoustic phonons and ionized impurities were accounted for in the calculations. The Debye model was used to calculate the lattice thermal conductivity. A unique set of semiempirical material parameters was obtained for both n-type and p-type materials through simulation testing. The model was optimized to fit different sets of experimental data from recently reported literature. The model shows consistent agreement with experimental characteristics for both n-type and p-type Mg2Si versus temperature and doping concentration. A systematic study of the effect of dopant concentration on the electrical and thermal conductivity of Mg2Si was also performed. The model predicts a maximum dimensionless figure of merit of about 0.8 when the doping concentration is increased to approximately 1020 cm–3 for both n-type and p-type devices.
56 citations
TL;DR: In this paper, the multiband Boltzmann transport equation was used to solve the relaxation time approximation for charge carriers and phonons in a bulk Si0.8Ge0.2 alloy.
Abstract: P-type nanostructured bulk Si0.8Ge0.2 and Si0.8Ge0.2 composites with CrSi2 nano-crystallite inclusions were synthesized via sintering approach. The composite structure showed power factor enhancement compared with nanostructured Si0.8Ge0.2 alloy. The experimental data for both structures were modeled with solving the multiband Boltzmann transport equation in the relaxation time approximation for charge carriers and phonons. The Si0.8Ge0.2 crystallite boundary scattering was modeled by a cylindrical potential barrier at the interfaces and the effects of CrSi2 nano-inclusions were modeled by spherical potential barriers in the Si0.8Ge0.2 lattice. The model calculations revealed that the enhancement in power factor is not an effect of hot carrier energy filtering, but it is due to the enhancement in charge carrier mobility in the composite structure. The analysis of charge carrier mobility components showed that while in nanostructured Si0.8Ge0.2 the ionize impurities and acoustic phonons are dominant scatte...
54 citations
TL;DR: In this article, the effect of bulk nanostructuring in magnesium silicide (Mg2Si) through simulation of thermoelectric properties using a multi-band semi-classical approach is presented.
Abstract: Nanostructuring techniques have steered the performance of many thermoelectric (TE) compounds towards significant improvement in performance in the last two decades. In this paper, we present a comprehensive study on the effect of bulk nanostructuring in magnesium silicide (Mg2Si) through simulation of thermoelectric properties using a multi-band semi-classical approach. It is shown that the magnitude of reduction in lattice thermal conductivity in nanostructured Mg2Si is comparable to that of reduction in charge carrier mobility for any chosen range of the grain sizes. The results are justified through a comparison with experimental data for both n-type and p-type Mg2Si characteristics versus temperature as well as doping concentration. In order to understand the underlying reasons for the detrimental effect of nanostructuring in Mg2Si, analogous calculations were performed on the well-known TE system of nanostructured Si0.8Ge0.2 and the results are compared. Model calculations show that in nanostructured Mg2Si a grain size of 20 nm results in approximately 40% reduction in lattice thermal conductivity, whereas the reduction in electrical conductivity is nearly 50% of its value in crystalline structures. For the case of nanostructured Si0.8Ge0.2, the loss in electrical conductivity was found to be a mere 20% of its magnitude in crystalline structures. The differential electrical and thermal conductivities versus charge carrier and phonon energies were calculated, respectively, and it was shown that the enhancement in Seebeck coefficient due to the energy filtering effect is also marginal. Therefore, it is conclusively shown that bulk nanostructuring in Mg2Si is not an efficient method to enhance ZT.
53 citations
TL;DR: In this article, the Boltzmann transport equation was used to calculate the electrical and thermal properties of nanostructured p-type SiGe alloys and the effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied.
Abstract: Nanostructured silicon germanium thermoelectric materials prepared by mechanical alloying and sintering method have recently shown large enhancement in figure-of-merit, ZT. The fabrication of these structures often involves many parameters whose understanding and precise control is required to attain large ZT. In order to find the optimum parameters for further enhancing the ZT of this material, we have grown and studied both experimentally and theoretically different nanostructured p-type SiGe alloys. The effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied. The electrical and thermal properties were calculated using Boltzmann transport equation and were compared with the data of nanostructured and crystalline SiGe. It was found that the thermal conductivity not only depends on the average crystallite size in the bulk material, but also it is a strong function of alloying, porosity, and doping concentration. The Seebeck coefficient showed weak dependency on average crystallite size. The electrical conductivity changed strongly with synthesis parameters. Therefore, depending on the synthesis parameters the figure-of-merit reduced or increased by ∼60% compared with that of the crystalline SiGe. The model calculation showed that the lattice part of thermal conductivity in the nanostructured sample makes ∼80% of the total thermal conductivity. In addition, the model calculation showed that while the room temperature hole mean free path (MFP) in the nanostructured sample is dominated by the crystallite boundary scattering, at high temperature the MFP is dominated by acoustic phonon scattering. Therefore, the thermal conductivity can be further reduced by smaller crystallite size without significantly affecting the electrical conductivity in order to further enhance ZT.
51 citations
27 Sep 2012
TL;DR: Among different materials used in preparation of nanocomposits, conducting polymers are one of the effective materials that can be employed to facilitate communication with neural system for regenerative purposes.
Abstract: Nanocomposites are a combination of a matrix and a filler, where at least one dimension of the system is on the nanoscale being less than or equal to 100 nm. Much work has focused on the construction of nanocomposites due to the structural enhancements in physico-chem‐ ical properties, and functionality for any given system [1-6]. The physico-chemical enhance‐ ments result from the interaction between the elements being near the molecular scale. Nanocomposite materials have also received interest for tissue engineering scaffolds by be‐ ing able to replicate the extracellular matrix found in vivo. Currently, researchers have creat‐ ed composite materials for scaffold formation which incorporate two or more materials. Some of these materials consist of minerals for bone tissue engineering including calcium, hydroxyapatite, phosphate, or combinations of different polymers, such as poly (lactic acid), poly (ε-caprolactone), collagen and chitosan, and many other different combinations [7-9]. Other work has focused on doping the polymer scaffolds with specific growth hormones or adhesion sequences to influence how cells attach to the scaffold and cause the scaffold to be‐ come a drug delivery vehicle for different kind of tissue engineering applications [10]. Among different materials used in preparation of nanocomposits, conducting polymers are one of the effective materials that can be employed to facilitate communication with neural system for regenerative purposes.
50 citations
TL;DR: In this paper, a semi-classical two-band model was proposed to describe the charge carrier and phonon transport properties of p-type manganese silicide alloys in crystalline and bulk nanostructured forms.
Abstract: Higher manganese silicide (HMS) alloys have a complex band structure with multiple valleys close to the conduction and valence band edges, which complicates the analysis of their electronic transport properties. We present a semi-classical two-band model that can describe the charge carrier and phonon transport properties of p-type HMS in crystalline and bulk nanostructured forms. The effect of grain boundaries is modeled with an interface potential scattering for charge carriers and diffusive and refractive scattering for phonons. A unique set of effective masses and acoustic phonon deformation potentials are introduced that can explain both electrical and thermal transport properties versus temperature. The acoustic phonon and ionized impurity scatterings for charge carriers and phonon-phonon, point defect, and electronphonon scattering mechanisms for phonons are included in the model. The simplicity of the presented model would be valuable especially for practical purposes. The thermoelectric transport properties of nanostructured HMS were calculated versus grain size and it was shown that even though bulk nanostructuring of HMS enhances thermoelectric performance, it is not sufficient to enhance considerably the figure-of-merit.
48 citations
TL;DR: In this paper, Zirconium titanate powders were synthesized by a straightforward sol-gel method using zirconia and titanium chlorides as metal precursors, deionized water as solvent and oxygen donor, and a NaOH solution for adjusting pH to 7.
43 citations
TL;DR: In this article, the preparation of ZrTiO4 thin film by a novel aqueous particulate sol-gel deposition method using carboxymethyl cellulose as dispersant was dealt with.
42 citations
TL;DR: These energy diagrams describe the ranges of core radius and number of lamellae that result in the formation of stable vesicle under specific conditions, thus providing prescriptions for the design of vesicles tailored for specific properties, including stability, cargo capacity, and resistance to deformation by osmotic stress.
Abstract: Lipid molecules in water form uni- or multilamellar vesicles in polydisperse form. Herein, we present energetic considerations for their equilibrium morphological organization. Our formulation provides elemental energy diagrams, which explain the polydispersity and account for the structural diversity. These energy diagrams describe the ranges of core radius (r(c)) and number of lamellae (N) that result in the formation of stable vesicles under specific conditions, thus providing prescriptions for the design of vesicles tailored for specific properties, including stability, cargo capacity, and resistance to deformation by osmotic stress. We deduced key design criteria as follows: 1) designing highly stable unilamellar vesicles requires low bending rigidity lipids and dimensions exceeding a few hundred nm in radii; 2) very large unilamellar vesicles (r(c)>several tens of microns) are not stable for typical lipids; lipids with higher bending rigidity are required; 3) the distribution of the stable size of vesicles is proportional to the bending rigidity; 4) for the case of multilamellar vesicles, vesicles with more than a few hundred layers usually exhibit greater structural integrity than those with lower degrees of lamellarity, especially when the core radii are small ( mM) is the most dominant factor in the free energy, suggesting active response by vesicles (e.g., poration) to release osmotic stress; and 6) vesicles with a core radius of a few hundred nm and more than hundred lamellae are more resistant to deformation by osmotic stress, thus making them more suited to applications involving osmotic pressure gradients, such as in drug delivery.
TL;DR: An efficient method for growing ordered bulk polycrystalline Bi0.5Sb1.5Te3 was used and the enhancement of thermoelectric power factor was demonstrated as discussed by the authors.
Abstract: It is known that the random orientation of crystallites in polycrystalline Bi0.5Sb1.5Te3 reduces its thermoelectric power factor. An efficient method for growing ordered bulk polycrystalline Bi0.5Sb1.5Te3 was used and the enhancement of thermoelectric power factor was demonstrated. For comparison, samples with randomly and preferentially oriented crystallites were sintered from similar powder of Bi0.5Sb1.5Te3. A press process similar to extrusion was used to align the crystallites during the hot press. The crystallites alignment was verified by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The dominant orientation in the aligned sample was in c-plane (0,0,15). The aligned sample demonstrated significant increase in electrical conductivity while the Seebeck coefficient remained unchanged. As a result, the thermoelectric power factor of the aligned sample was improved by ∼50%.
TL;DR: In this article, three-layer structures of higher manganese silicide (HMS) were prepared in a sintering process and the contact resistance was measured versus temperature.
Abstract: Five metals with large work functions including Co, Ni, Cr, Ti, and Mo and two silicides including MnSi and TiSi2 were examined to determine the best contact material for the thermoelectric material higher manganese silicide (HMS). Three-layer structures of HMS/contact/HMS were prepared in a sintering process. The contact resistance was measured versus temperature. The structures were subjected to x-ray diffraction and energy-dispersive x-ray spectroscopy examination. Thermal stability of the structures was determined by heating the samples to 700°C for different time intervals. The pure metals failed to make reliable contacts due to poor mechanical and chemical stability at high temperatures. In contrast, the metal silicides (MnSi and TiSi2) showed superior chemical and mechanical stability after the thermal stability test. The observed contact resistance of MnSi and TiSi2 was within the range of practical interest (10−5 Ω cm2 to 10−4 Ω cm2) over the entire range of investigated temperatures (20°C to 700°C). The best properties were found for the nanograined MnSi, for which the resistance of the contact was as low as 10−6 Ω cm2.
TL;DR: In this paper, the configuration of myelin figures that are formed upon hydration of lipid stack was studied qualitatively using epifluorescence microscopy, which indicated a direct relationship between local characteristics of parent lipid stack and the diameter of the myelin figure.
Abstract: Using epifluorescence microscopy, the configuration of myelin figures that are formed upon hydration of lipid stack was studied qualitatively. Little knowledge is currently available for conditions that determine the diameter of myelin figures and their degree of multilamellarity. Examining more than 300 samples, we realized that there are distinct populations of myelin figures protruding from discrete regions of lipid stack. Each population contains myelin figures with similar diameters. This indicates a direct relationship between local characteristics of parent lipid stack and the diameter of myelin figures. Evidenced by fluorescent images, we classified all the observed myelin figures into three major groups of (1) solid tubes, (2) thin tethers, and (3) hollow tubes. Solid tubes are the most common structure of myelin figures which appeared as dense shiny cylinders. Thin tethers, with long hair-shaped structure, were observed protruding from part of lipid plaque which is likely to be under tension. Hollow tubes were protruded from the parts that are unpinned from the substrate and possibly under low or no tension. The abrupt change in the configuration of myelin figures from solid tubes to hollow ones was described in a reproducible experiment where the pinned region of the parent stack became unpinned. Our observations can indicate a relation between the membrane tension of the source material and the diameter of the myelin figures.
19 Apr 2012
TL;DR: In this paper, the thermal and thermoelectric properties of p-type nanostructured silicon germanium (SiGe) as an important high-temperature high-energetic material was studied and compared with those of crystalline SiGe.
Abstract: Nearly 60% of the world's energy is wasted as heat. Thermoelectric materials can play an important role in green energy harvesting with their ability to convert waste heat into electricity. In this report, thermal and thermoelectric properties of p- type nanostructured silicon germanium (SiGe) as an important high temperature thermoelectric material was studied and compared with those of crystalline SiGe. The materials were synthesized via mechanical alloying and sintering approach. The different synthesis procedures resulted in two different conformation of SiGe. The first one was in nanostructure configuration and the other was in crystalline configuration containing large grains. Thermal and thermoelectric properties of both configurations were investigated in this manuscript. Although, differential thermal analysis (DTA) did not show significant differences between the thermal characteristics of nanostructured and crystalline SiGe, there were major changes in their thermoelectric properties. The nanostructured SiGe had lower electrical conductivity owing to the large scattering rate of electron at the grain boundaries. However, the lower mobility was accompanied by small thermal conductivity in nanostructured SiGe. The Seeback coefficient was grown in nanostructured SiGe as a result of lower carrier concentration. Considering the influence of all these factors, the nanostructured SiGe was thermoelectrically preferred as the figure-of-merit was increased specially at high temperatures.
TL;DR: By considering this fact that at the single-cell level, interactions with the extracellular environment occur on a nanometer length scale, the value of critical stress was estimated by considering the length variation and moving velocity of ameboid cells.
Abstract: A detailed theoretical model that combines the conventional viscoelastic continuum description of cell motion with a dynamic active stress is presented. The model describes the ameboid cells movement comprising of protrusion and adhesion of the front edge followed by detachment and movement of the tail. Unlike the previous viscoelastic descriptions in which the cell movement is steady, the presented model describes the “walking” of the cell in response to specific active stress components acting separately on the front and rear of the cell. In this locomotive model first the tail of the cell is attached to the substrate and active stress is applied to the front of the cell. Consequently, the stress in the tail increases. When the stress in the tail exceeds a critical value, namely critical stress, the conditions are updated so that the front is fixed and the tail of the cell is detached from the substrate and moves towards the front. Consequently, the stress in the tail decreases. When the stress goes to zero, the starting conditions become active and the process continues. At start the cell is stretched and its length is increased as the front of cell migrates more than the rear. However, after several steps the front and rear move equally and the cell length stays constant during the movement. In this manuscript we analyzed such cell dynamics including the length variation and moving velocity. Finally, by considering this fact that at the single-cell level, interactions with the extracellular environment occur on a nanometer length scale, the value of critical stress was estimated.
19 Apr 2012
TL;DR: In this article, a low-cost, scalable and quick method of synthesizing bulk thermoelectric higher manganese silicide is proposed for its industrial manufacturing for waste heat recovery in medium to high temperature range.
Abstract: Higher manganese silicide (HMS) is a useful thermoelectric material for waste heat recovery in medium to high temperature range. It is made from two of the most abundant materials on earth. Moreover, it is non-toxic and environmentally responsible. A low-cost, scalable, and quick method of synthesizing bulk thermoelectric higher manganese silicide is proposed for its industrial manufacturing. Heat treatment alloying of higher manganese silicide powder is proposed for producing its large quantity in a cost effective way. The process involves heat treatment of the elemental powders at 1050 °C for 1 hour. Thermoelectric properties of the resultant samples were studied and compared with the samples made by mechanical alloying. In comparison, both methods result in similar trends in thermoelectric properties however with significant cost reduction and ease of fabrication for the proposed method.
19 Apr 2012
TL;DR: In this article, a composite of SiGe-CrSi2 was synthesized by mechanical alloying and sintering method to achieve nanocrystalline structure composite powder by high energy ball milling.
Abstract: We report the enhancement of thermoelectric power factor in composite of SiGe-CrSi2. P-type SiGe- CrSi2 was synthesized by mechanical alloying and sintering method. In order to achieve nanocrystalline structure composite powder was prepared by high energy ball milling. Prepared powders were sintered at different press conditions to optimize for maximum power factor. The crystal structure and phase formation of SiGe and CrSi2 alloys in the composite were investigated using x- ray diffraction analysis. The electrical conductivity, Seebeck coefficient and thermal conductivity of sintered samples were measured from room temperature to 850 C. The result shows about 50% improvement in thermoelectric power factor of SiGe-CrSi2 compared to SiGe alloy.
19 Apr 2012
TL;DR: In this paper, the effect of inclusion of a minuscule quantity (0.25vol%) of Mg-Si-B-R based conductive glass-frit in order to reduce the brittleness of samples has been investigated on the Al-doped Mg2Si samples.
Abstract: The thermoelectric (TE) properties of magnesium silicide (Mg2Si) fabricated through mechanical alloying and hot-pressing have been characterized by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) between 300 K and 970 K. TE samples of 2-at% Bi doped and 2-at% Al doped specimens were sintered at 1173 K and 1123 K, respectively. Structural analysis of the sintered samples was performed by scanning electron microscopy. The effect of inclusion of a minuscule quantity (0.25-vol%) of Mg-Si-B-R based conductive glass-frit in order to reduce the brittleness of samples has been investigated on the Al-doped Mg2Si samples. Power factors (S2σT) of greater than 2 W/mK were obtained from samples doped with Al. The samples mixed with conductive glass frit have also exhibited higher stability at temperatures greater than 900 K when compared to samples doped with Bi. The Mg2Si:Bi and Mg2Si:Al samples showed a maximum figure-of- merit (ZT) of 0.53 and 0.66 at 970 K, respectively.
19 Apr 2012
TL;DR: In this paper, the authors studied differential thermal analysis (DTA) of Si0.80Ge0.20 thermoelectric alloy in detail and revealed that the phase impurity can hardly be seen in X-ray diffraction patterns and is often neglected.
Abstract: Thermoelectric effect becomes one of the important elements in sustainable energy due to its capability in green conversion of waste heat into electrical energy. Among various thermoelectric materials, nanostructured-Si0.80Ge0.20 is being widely investigated owing to its efficient thermoelectric effect at high temperature. In this manuscript, we studied differential thermal analysis (DTA) of Si0.80Ge0.20 thermoelectric alloy in detail. Our DTA study revealed the fact that in almost all alloys of nanostrcutured Si0.80Ge0.20 prepared with mechanical ball milling, the sample is not in Si0.80Ge0.20 phase but is in composite mixed phases of Si0.88Ge0.12 and small amount of Si0.55Ge0.45. This phase impurity can hardly be seen in X-ray diffraction patterns and is often neglected.
19 Apr 2012
TL;DR: In this article, an improvement on thermoelectric performance of higher manganese silicide MnSi1.75 is introduced by externally mixing 1 at% nanostructured MnSi to MnSi 1.75.
Abstract: Higher manganese silicide is one of the promising thermoelectric materials for waste heat recovery at medium temperature (500-700 °C). Improvement on thermoelectric performance of bulk thermoelectric higher manganese silicide MnSi1.75 is introduced by externally mixing 1 at% nanostructured MnSi to MnSi1.75. This method can reduce the thermal conductivity above 400 °C more than reducing the power factor of the higher manganese silicide. This would enhance the figure-of-merit ZT of MnSi1.75 to ZT0.5 without any doping at 570 °C. In comparison, the figure-of-merit of conventional MnSi1.75 is ZT0.3. This method can be easily applied to industrial manufacturing of this material to enhance its efficiency.
19 Apr 2012
TL;DR: In this paper, FeSi2 and SiGe thermoelectric materials were used to make a new composite with enhanced efficiency/cost factor relative to pure FeSi 2 and siGe compounds.
Abstract: Clean energy production and its efficient usage is one of the most important issues in the world today. Thermoelectric materials, which can convert heat into electricity, have drawn significant attention for waste-heat harvesting. In this report, FeSi2 and SiGe thermoelectric materials were used to make a new composite with enhanced efficiency/cost factor relative to pure FeSi2 and SiGe compounds.
19 Apr 2012
TL;DR: In this article, the authors investigated the phenomenon responsible for the electrically inactive Boron in nanostructured Silicon-Germanium alloy (Si1-xGex) and showed that SiB3 and SiB6 are not responsible for inactive boron as confirmed by X-ray diffraction analysis.
Abstract: Precipitation as the phenomenon responsible for the electrically inactive Boron in nanostructured Silicon-Germanium alloy (Si1-xGex) is reported and investigated. It is shown that SiB3 and SiB6 are not responsible for the inactive boron as confirmed by X-ray diffraction analysis. The increase of the change of thermoelectric properties of Si1-xGex alloy with thermal cycling accompanies the decrease of the crystallite size as confirmed by our experiments. The dependence of the carrier concentration on temperature was obtained from detailed theoretical modeling.
19 Apr 2012
TL;DR: In this paper, the electrical conductivity and thermal conductivity of BiSeTe and BiSbTe were investigated with the addition of 5% arsenic-doped SiGe.
Abstract: BiSeTe and BiSbTe have been two of the most efficient thermoelectric materials near room temperature applications for many years. In spite of recent progress in enhancement of the efficiency of BiSbTe thermoelectric materials, there has been little progress in developing efficient BiSeTe alloys. BiSeTe is an n-type thermoelectric material with negative Seebeck value and BiSbTe is p-type with positive Seebeck. We observed BiSeTe changes to p-type with the addition of 5% arsenic doped SiGe. After annealing process the Seebeck value changed sign again resulting in n-type BiSeTe. The electrical conductivity and thermal conductivity also changed during the course of annealing. Interestingly the minimum thermal conductivity corresponded to the maximum electrical conductivity and power factor of the p-type mode. This effect may prove to be a cornerstone in the enhancement and fabrication of thermoelectric devices based on bismuth telluride based alloys.
19 Apr 2012
TL;DR: In this article, lead sulfide (PbS) nanocrystals were synthesized via a simple, effective and green method and the effects of heat treatment at different temperatures on the PbS nanocrystal were investigated.
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.
19 Apr 2012
TL;DR: In this paper, a Coherent Potential Approximation within effective mass approach was used to study the charge carrier transport in nanocomposites and showed the dependency of hole scattering rate as a function of the grain size.
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.
19 Apr 2012
TL;DR: In this article, the effect of nanostructuring on the SiGe : CrSi2 composite was further studied and its transport property were compared with those of crystalline composite sample.
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.
19 Apr 2012
TL;DR: In this article, the authors present a method for accurate measurement of the thermal conductivity of a thermoelectric nanowire structure, taking into account the heat loss to the substrate and the supporting materials which are the main parasitic heat conduction paths.
Abstract: With the progress in nanoscale thermoelectric materials and devices the need for accurate characterization of these structures has become more apparent. Among the main three thermoelectric material properties of electrical conductivity, Seebeck coefficient, and thermal conductivity accurate measurement of thermal conductivity seems to be the most challenging due to its strong sensitivity to parasitic heat transfer paths. In this report we present a method for accurate measurement of the thermal conductivity of a thermoelectric nanowire structure. The method takes into account the heat loss to the substrate and the supporting materials which are the main parasitic heat conduction paths. The method can be extended to measure the thermal conductivity of other thermoelectric structures such as superlattice thin films and nanoparticles.
19 Apr 2012
TL;DR: In this article, the authors compared three different methods to prepare powders of (BixSb1-x)2Te3 binary alloys: conventional high energy mechanical milling and two alternative methods namely induction melting and thermomechanical method.
Abstract: Thermoelectric technology is becoming one of the important elements in sustainable energy due to its capability in green conversion of waste heat into electricity Solid solution alloys based on Bi2Te3 have been some of the most efficient thermoelectric materials near the room temperature for many years Recently there have been advances in the thermoelectric efficiency of (BixSb1-x)2Te3 alloy via bulk nanostructuring The fabrication process of the alloy from elements of Bi, Sb, and Te often involves mechanical alloying with extensive milling time This process is slow and consumes significant electric power Here we report and compare three different methods to prepare powders of Bi2Te3 and Sb2Te3 binary alloys and (BixSb1-x)2Te3 ternary alloys: conventional high energy mechanical milling and two alternative methods namely induction melting and thermomechanical method In all three methods the obtained powders are single phase and possess good homogeneity Efficiency and advantages of different methods have been discussed Alternative techniques introduced in this work provide a more efficient approach with higher yield for large scale preparation of (BixSb1-x)2Te3 thermoelectric structures