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Publications (15)14.39 Total impact

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    ABSTRACT: This paper proposes an efficient bandwidth allocation scheme to fully utilize the multi-access ability of multimode terminals in heterogeneous networks. To model the fact that the suitability of different networks to different traffics varies, which is seldom considered in previous literature, the term “Match-Degree” is introduced and calculated using grey relational analysis. Afterwards, two methods, namely weighted bandwidth factor and cumulative utility function are proposed to integrate the influence of “Match-Degree” into the conventional utility-functions. Moreover, energy consumption and network price are also considered to make our model more comprehensive. The bandwidth allocation problem is formulated as maximizing the overall utility by adjusting the bandwidth allocated to each user. Based on optimization theory, an iterative algorithm is constructed to solve this problem. And numerical results validate the performance enhancement of our scheme compared with the existing ones.
    ICC 2014 - 2014 IEEE International Conference on Communications; 06/2014
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    ABSTRACT: The fast and accurate spectrum sensing over an ultra-wide bandwidth is a big challenge for the radio environment cognition. Traditional spectrum sensing technique is neither efficient nor necessary, wasting the spectrum access opportunities on the vacant spectrum holes of primary users (PUs). Considering the sparse signal feature, a novel compressed sensing technique is proposed by using the minimal sampling rate to detect spectrum holes, which is more efficient than the Nyquist sampling rate and traditional compressed sampling rate that is required to reconstruct the original signal. The proposed compressed sensing process is divided into two stages called approaching stage and monitoring stage. The first stage is to gradually approach the minimal sampling rate required to achieve the spectrum detection performance by using the feedback mechanism. And the second stage is to monitor the status of PU according to the threshold using the sampling rate from the first stage. Therefore, the overall sampling rate can be dramatically reduced without spectrum detection performance deterioration compared to the conventional static sampling algorithm. Numerous results show that the proposed compressed sensing technique can reduce the sampling rate to 35%, with acceptable detection probability over 0.9.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
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    ABSTRACT: Skutterudites are promising thermoelectric materials because of their high figure of merit, ZT, and good thermomechanical properties. This work reports the effective figure of merit, ZTeff, and the efficiency of skutterudite legs and a unicouple working under a large temperature difference. The p- and n-type legs are fabricated with electrodes sintered directly to the skutterudite during a hot pressing process. CoSi2 is used as the electrode for the n-type skutterudite (Yb0.35Co4Sb12) and Co2Si for the p-type skutterudite (NdFe3.5Co0.5Sb12). A technique is developed to measure the ZTeff of individual legs and the efficiency of a unicouple. An ZTeff of 0.74 is determined for the n-type legs operating between 52 and 595 °C, and an ZTeff of 0.51 for the p-type legs operating between 77 and 600 °C. The efficiency of the p–n unicouple is determined to be 9.1% operating between ∼70 and 550 °C.
    Advanced Energy Materials 01/2013; 3:245. · 14.39 Impact Factor
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    ABSTRACT: The thermoelectric properties of nanostructured silicon (Si) with a low concentration of germanium (Ge) are investigated. A low concentration of Ge leads to a significant cost reduction of the final product since Ge is at least 100 times more expensive than Si. By using only 5 atomic % Ge (Si95Ge5), we have achieved a thermoelectric figure-of-merit (ZT) of 0.95, similar to the ZT in the large grained Si80Ge20 alloy that is three times more expensive, and is almost four times that of the large grained bulk Si. The enhancement in the thermoelectric ZT for the nanostructured Si95Ge5 is mostly due to the reduced thermal conductivity caused by phonon scattering at the increased grain boundaries and the Ge alloying effect.
    03/2009;
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    ABSTRACT: The dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been successfully applied to a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient.
    03/2009;
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    ABSTRACT: Silicon-Germanium (SiGe) alloys have been the main thermoelectric materials in power generation devices operating from 500. C to 1000 . C. The main challenge for enhancement of thermal performance of Si-Ge system is the reduction of thermal conductivity. Here we report that by creating nano-pores, the thermal conductivity can be reduced to around 2 Wm-1K-1 with a little lower power factor. The nano-structured bulk alloy was made by first forming alloyed nano powders from commercial grade Si and Ge chunks with the dopant phosphorous (P) powder and sulfur powder and then by hot pressing the powders for their compaction. Followed by annealing at 1050 . C, nano pores were created inside the bulk disc. Our results showed that nano pores are very effective to scatter phonons, thus reduce the thermal conductivity.
    03/2009;
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    ABSTRACT: Skutterudites are good examples of phonon glass electron crystal (PGEC), which is proposed to be one of the most desirable materials to maximize the thermoelectric figure of merit. The skutterudite structure has two voids in each unit cell that are large enough to accommodate a variety of atoms, such as La, Ce, Nd, Sm, Yb, etc. These atomic void-fillers rattle about in their oversized cages, thereby drastically reducing thermal conductivity and maximizing ZT. My work on p-type skutterudites is based on compounds of a general formula RFe3.5 Co0.5Sb12 where R stands for a void filler. Besides the influence of rattling of the void fillers, thermal conductivity can be further depressed by the increased phonon scattering at the increased grain boundaries due to nano size grains.
    03/2009;
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    ABSTRACT: Bi2Te3-xSex is a classic room temperature n-type thermoelectric material. In spite of the long history of research, its ZT is still below 1. By directly making nano sized particles using mechanical alloy from element, then pressing the nanoparticles into 100% dense bulk sample with nano-structures by hot press, we expect to decrease the thermal conductivity by the increased grain boundary scattering of phonons so to improve the ZT above 1. The ratio of Te/Se was varied systematically to investigate its effect on thermal conductivity.
    03/2008;
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    ABSTRACT: Themoelectric materials with high dimensionless figure-of-merit (ZT) are greatly demanded in energy industry, among which bismuth telluride (Bi2Te3) exhibits decent ZT around room temperature. However, thermal conductivity of Bi2Te3 is still high which limits its wider use for low temperature cooling devices. Here we investigate nanostructured bulk n-type Bi2Te3-xSex by reducing the thermal conductivity via increased phonon scattering of the significantly increased grain boundaries due to nano size grains. We first make alloyed nanopowders by mechanical alloying a mixture of elements with the right ratio and then 100% nanostructured samples by hot press.
    03/2008;
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    ABSTRACT: Bismuth telluride alloys and their derivatives are the most important thermoelectric materials used in refrigeration devices around room temperature. Using mechanical alloying and hot press, we have achieved 100% dense nano-structured p-type BixSb2-xTe3 samples. We demonstrated here that the enhanced dimensionless figure-of-merit (ZT) are due to enhanced phonon-scattering, and the ZT peak could be easily shifted to higher temperature by varying the composition and processing conditions.
    03/2008;
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    ABSTRACT: Alloy nanoparticles BixSb2-xTe3 (x = 0.2, 0.3, 0.4, 0.5 and 0.6) were synthesized from bismuth, antimony, and tellurium and compacted into nanostructured bulk samples with full density. The thermoelectric properties were measured in the temperature range of 25 to 250^oC. We showed that a maximum dimensionless figure-of-merit (ZT) reached 1.27 at about 100^oC for Bi0.4Sb1.6Te3(x = 0.4), a significant improvement over the state-of-the-art p-type Bi2Te3 alloys. It is demonstrated that the enhanced ZT mainly comes from a reduced thermal conductivity due to the increased phonon scattering due to the nano size of the grains. Also, the ZT peaks shift from low temperature to high temperature with the decreasing of x, which indicates the less Bi content would benefit the potential power generation application while the more Bi content materials could be used as a potential cooling device. This new route of enhancing ZT value can be applied to other materials.
    03/2008;
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    ABSTRACT: Nanostructured Ni-doped skutterudites Co1-xNixSb3 (with x ranging from 0.01 to 0.09) have been prepared by ball-milling and direct-current induced hot press. It was found that the thermal conductivity was much reduced due to strong electrons-phonons scaterring from Ni-doping and grain boundary phonons scattering from nano-structures in the samples. A maximum dimensionless figure-of-merit of 0.71 has been obtained in Co0.91Ni0.09Sb3 at 525 C.
    03/2008;
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    ABSTRACT: Table salt was found to be very helpful in producing single crystal zinc oxide (ZnO) nanowires in grams quantity with a conversion efficiency of about 60-80%. The growth process involves heating the mixture of zinc powder and table salt at about 600-700ºC in flowing gases of oxygen and argon. A conversion efficiency of only 5-10% was achieved when salt was not used. The salt was completely removed by a few times water rinse. The as-grown ZnO nanowires are about 40 -- 100 nm in diameter and 5 -- 10 micrometer in length, and are grown on either zinc oxide particles or on the surfaces of the table salt crystals. Transmission electron microscope studies showed that these ZnO nanowires are highly crystallized single crystals. Photoluminescence spectra of the as-grown and salt-free ZnO nanowires using excitation of 325 nm showed that the ZnO nanowires have a very strong emission in the visible frequency without any emission from the band edge, meaning that surface states dominate the emission. The important role of the salt plays in the high conversion efficiency is discussed.
    01/2006;
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    ABSTRACT: Nanostructured single phase of Yb filled skutterudites CoSb3 with a nominal composition of YbxCo4Sb12 (X = 0.3, 0.35, 0.4, and 0.5) have been synthesized by ball milling and direct current induced hot press. Thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity from room temperature to 550^o were measured and discussed. It was found that Yb0.35Co4Sb12 has the optimal dimensionless figure of merit of 1.2 at 550^o.