Search

The inverse relationship between piezoelectric coefficient (d33) and depolarization temperature (Td) in Bi0.5Na0.5TiO3-based ceramics is a longstanding obstacle for their applications. In this work, synergistically enhanced d33 and Td is achieved in AlN-modified 0.84Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.05BaTiO3 ceramics. Addition of 1 mol% AlN, increases both d33 from 165 to 234 pC/N and Td by ∼50 °C. Rietveld analysis of X-ray powder diffraction (XRD) data reveals an increase in the proportion of the tetragonal phase at 1 mol% AlN incorporation. Moreover, at this composition the modified ceramics exhibit larger grains and high-density lamellar nanodomains with sizes of 30–50 nm. Polarization reversal and domain mobility are thus significantly enhanced, contributing to the large d33. Temperature-dependent dielectric and XRD data revealed that the delayed thermal depolarization is attributed to the improved and poling-field stabilized tetragonality in the modified ceramics.

The development of lead-free ferroelectric ceramics exhibiting a high Curie point is regarded as a key aspect of improving the overall performance of high temperature sensing devices. Two-layer Dion-Jacobson phases have great potential for these applications, but the dielectric properties of many of these compounds have not been investigated and several are still erroneously considered to be non-polar. RbPrNb2O7, a two-layer Dion-Jacobson phase, has previously been identified as being non-polar at room temperature, with an orthorhombic crystal structure in space group Imma. Here, the structure of RbPrNb2O7 is re-examined and its ferroelectric (FE) character confirmed through ferroelectric, dielectric and piezoelectric measurements, using dense bulk ceramics. In addition, the microstructure and phase evolution of the compound were analysed by transmission electron microscopy, variable temperature X-ray powder diffraction and differential scanning calorimetry. RbPrNb2O7 is a ferroelectric at room temperature, exhibiting orthorhombic symmetry in space group I2cm. It undergoes a reversible FE ↔ FE phase transition at around 260 °C, with evidence for a diffuse apolar transition between 800 and 900 °C. Details of the subtle structural changes occurring at these transitions are discussed. RbPrNb2O7 possesses a high Curie point of ~1090 °C, making it a potential candidate for use in high temperature piezoelectric sensing.

Tectonic processes from the breakup of Rodinia to Gondwana assembly have been studied on large continents. However, the role of micro-continental blocks in Neoproterozoic supercontinental cycle is barely known. Geochronological and geochemical studies on a combination of 57 (meta-) sedimentary and (meta-) igneous rocks (one igneous rock and four meta-sedimentary samples are from this study) in the Indochina block (including its north part, i.e. the Simao terrane) reveal the paleoposition and tectonic processes during Precambrian supercontinental cycle. A mylonitized monzogranite from the NW part of the Indochina block (the Chongshan area) shows zircon U Pb ages of 867 ± 19 Ma. Precambrian and Early Paleozoic strata share similar zircon U Pb age peaks at ca. 2.5 Ga and ca. 0.95 Ga. The ca. 1.75 Ga age peak in the Precambrian strata is absent in those Early Paleozoic strata. The newly discovered 867 ± 19 Ma felsic intrusion in the Chongshan Metamorphic Belt and the shift of detrital zircon age patterns from the Neoproterozoic to Early Paleozoic confirm the presence of Precambrian magmatic activities in the Indochina block. Paleoposition and magmatic records show the Indochina block should be part of an Andean-type orogeny along the NW margin of the Rodinia during the Neoproterozoic. During the Neoproterozoic to Early Paleozoic, the role of Indochina as a continental block coherent with the Indian Craton at least during the breakup of Rodinia and Gondwana assembly. After Gondwana assembly, the Indochina block, as a section of an Andean-type continental arc along the eastern margin, blocked the exotic inputs and changed the age distribution patterns dramatically.

Constraints on the timing and style of the Tibetan Plateau growth help spur new understanding of the tectonic evolution of the northern Tibetan Plateau and its relation to the India-Asia continental collision. In this regard, records of tectonic deformation with accurate ages are urgently needed, especially in regions without relevant studies. The Kumkol basin, located between two major intermontane basins (the Hoh Xil and Qaidam basins), may hold clues to how these major basins evolve during the Cenozoic. However, little has been known about the exact ages of the strata and tectonic deformation of the basin. Herein, detailed paleomagnetic and structural studies are conducted on the southern Baiquanhe section in the central Kumkol basin, northern Tibetan Plateau. The magnetostratigraphic study indicates that the southern Baiquanhe section spans a time interval of 8.2–4.2 Ma. Well-preserved growth strata date to 7.5 Ma, providing evidence for a significant thrust fault-related folding. This thrust-related folding has also been identified in the Tian Shan foreland and in the northern Tibetan Plateau, most likely implying a pulsed basinward deformation during the late Miocene.

Nowadays, it remains a challenge to achieve a simultaneous improvement of dielectric constant, energy density, and temperature stability in dielectric ceramics. In this work, it was found that such drawback could be effectively resolved by using KTaO3 to substitute Na0.5Bi0.5TiO3. In (1-x)Na0.5Bi0.5TiO3-xKTaO3 ceramics, the temperature coefficient of capacitance (TCC) of x = 0.10 composition is lower than 15% in the temperature range of 25–390 °C, with a high dielectric constant of 2008 and a low dielectric loss of 0.0044 at 150 °C. Meanwhile, a large discharged energy density of ~2.50 J/cm3 with the variation less than± 3% in 5–190 °C was obtained under a low electric field of 18 kV/mm in x = 0.20 composition, accompanied with a high energy efficiency of> 80%. These results reveal that this ceramic system is a promising candidate for temperature-stable dielectric and energy storage capacitors applications.

The lead-free sodium bismuth titanate (BNT) system has been extensively investigated in the past decade due to its multi-functional electro-active properties. Here, we present a comprehensive review that encompasses the fundamentals and state-of-the-art in the development of BNT-based ceramics, with attention to the underlying composition, microstructure, and macroscopic properties. The phase structure, phase transitions, and relaxor characteristics of BNT and BNT-based solid solutions are described carefully, with a series of proposed phase diagrams. The attractive functional properties of BNT-based ceramics include piezoelectricity, electric-field-induced strain, and energy storage performance for applications in sensors, actuators, and dielectric capacitors. The focus of this review is on the microscopic origin of the macroscopic behavior, the proposed strategies for optimization of functional properties, and current challenges. Moreover, the potential applications of BNT-based ceramics in the areas of electrocaloric, oxide-ion conduction, and luminescence are briefly introduced. Finally, future perspectives are provided to highlight new and emerging research directions in this growing area.

Strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses of different size-fractions of sediments collected from the Heihe River in the North Qilian Orogen (NQO) were carried out to trace the sediment sources and to determine the relationships between the Sr–Nd–Hf isotopic behaviors, and the grain-size and hydraulic sorting effects during fluvial transport. Our results demonstrate that the sand and suspended load samples collected from the same site have different Nd isotopic compositions, while their 87Sr/86Sr ratios are only slightly different. These features indicate that contributions to the sediments from different sources with different grain sizes vary greatly. Coarse-grained sand may more intuitively reflect the variations in local sources than the suspended load. The suspended load samples are distributed along the Clay Array on the ԐHf vs. ԐNd diagram due to zircon sorting. A similar Nd Hf decoupling phenomenon is widespread in the global oceans and large river systems, indicating that the Nd Hf isotopic behavior depends on the hydrodynamic sorting of minerals during fluvial transport. Moreover, the crustal accretion of the NQO from the depleted mantle occurred in the Proterozoic based on the constraints provided by the TDM values of the river sediments. The Nd isotopic compositional characteristics of the river sediments indicate that the NQO has an affinity with the Yangtze Craton. The determination of the basement tectonic affinity of the NQO significantly contributes to our understanding of the Neoproterozoic evolution of the Gondwana continental margin.

The coupling between piezoelectricity and photoexcitation has recently been emerged as an effective strategy to improve the catalytic performance of semiconductors. Lead-free Bi0.5Na0.5TiO3 (BNT) is a promising system for effective piezo-photocatalysis owing to its favorable piezoelectricity, photoexcited response, and electrical conductivity. In this work, one-dimensional BNT nanorods prepared by a high-temperature molten-salt method were developed for piezo-photocatalysis applications for the first time, which exhibited excellent catalytic activity. The degradation efficiency of RhB solution (initial concentration C0 = 5 mg/L) in the presence of BNT nanorods reached 95% in 30 min, leading to a high first-order rate constant k of 0.094 min−1. For reveal the underlying mechanisms, the piezoresponse characterization, piezoelectric potential simulation, band diagram analysis, and photoelectrochemical measurements were carried out, and the BNT nanospheres synthesized by hydrothermal method were simultaneously studied for comparison. A higher piezoelectric coefficient d33* of 200 pm/V and a larger potential output under pressure were achieved in the BNT nanorods. The BNT nanorods also displayed the narrower band gap, stronger photocurrent response, and lower charge transfer resistance. These merits enabled the BNT nanorods to exhibit strong piezo-photocatalytic activity. Moreover, it was found that the BNT nanorods showed better piezo-photocatalytic activity for cationic dyes (RhB and MB) than anionic dyes (MO). For RhB, MB, and MO solutions with C0 of 10 mg/L, k values of 0.041, 0.075, and 0.013 min−1 were achieved, respectively.

In a mobile crowd sensing system, a smartphone undertakes many different sensing tasks that demand data from various sensors. In this paper, we consider the problem of scheduling different sensing tasks assigned to a smartphone with the objective of minimizing sensing energy consumption while ensuring Quality of SenSing (QoSS). First, we consider a simple case in which each sensing task only requests data from a single sensor. We formally define the corresponding problem as the Minimum Energy Single-sensor task Scheduling (MESS) problem and present a polynomial-time optimal algorithm to solve it. Furthermore, we address a more general case in which some sensing tasks request multiple sensors to report their measurements simultaneously. We present an Integer Linear Programming (ILP) formulation as well as two effective polynomial-time heuristic algorithms, for the corresponding Minimum Energy Multi-sensor task Scheduling (MEMS) problem. Extensive simulation results show that the proposed algorithms achieve significant energy savings, compared to a widely-used baseline approach; moreover, the proposed heuristic algorithms produce close-to-optimal solutions.

Recently, Choi et al. designed the first practical full-duplex wireless system, which challenges the basic assumption in wireless communications that a radio cannot transmit and receive on the same frequency at the same time. In this paper, we study cross-layer optimization for full-duplex wireless networks, comprehensively considering various resource and social constraints. We focus on (1) the problem of allocating resources to maximize the total profit of multiple users subject to node constraints and (2) the problem of allocating resources to minimize the network power consumption subject to user rate demands and node constraints. We formulate these problems as convex programming systems. By combining Lagrangian decomposition and subgradient methods, we design distributed iterative algorithms to solve these problems, which compute the optimized user information flow (i.e. user behavior) for the network layer and the optimized node broadcast rate (i.e. node behavior) for the MAC layer. Our algorithms allow each user and each node to adjust its own behavior individually in each iteration. We analyze the convergence rate, the amount of feasibility violation, and the gap between the optimal solution and our solution in each iteration. We also use the dual space information to analyze node load constraint violation.

Given an unreliable communication network, we seek for a node which maximizes the expected number of nodes that are reachable from it. Such a node is called a most reliable source (MRS) of the network. In communication networks, failures may occur to both links and nodes. Previous studies have considered the case where each link has an independent operational probability, while the nodes are immune to failures. In practice, however, failures may happen to the nodes as well, including both transmitting fault and receiving fault. Recently, another variant of the MRS problem is studied, where all links are immune to failures and each node has an independent transmitting probability and receiving probability, and an O(n2) time algorithm is presented for computing an MRS on tree networks with n nodes. In this paper, we present a faster algorithm for this problem, with a time complexity of O(n).

The survivable logical topology mapping problem in an IP-over-WDM optical network is to map each link (u,v) in the logical topology (at the IP layer) into a lightpath between the nodes u and v in the physical topology (at the optical layer) such that failure of a single physical link does not cause the logical topology to become disconnected. Kurant and Thiran (2007) [8] presented an algorithmic framework called SMART that involves successive contracting of circuits in the logical topology and mapping the logical links in the circuits into edge-disjoint lightpaths in the physical topology. In a recent work from Thulasiraman et al. (2009) [11] a dual framework involving cutsets was presented and it was shown that both these frameworks possess the same algorithmic structure. Algorithms CIRCUIT-SMART, CUTSET-SMART and INCIDENCE-SMART were also presented in [11]. All these algorithms suffer from one important shortcoming, namely, disjoint lightpaths for certain groups of logical links may not exist in the physical topology. Therefore, in such cases, we will have to augment the logical topology with new logical links to guarantee survivability. In this paper we address this augmentation problem. We first identify a logical topology that admits a survivable mapping under a physical link failure as long as the physical topology is 3-edge connected. We show how to embed this logical topology on a given logical topology so that the augmented topology admits a survivability mapping as long as the physical topology is 3-edge connected. We then generalize these results to achieve augmentation for survivability of a given logical topology under multiple physical link failures. Finally, we define the concept of survivability index of a mapping. We provide simulation results to demonstrate that even when certain requirements of the generalized augmentation procedure are relaxed, our approach will result in mappings that achieve a high survivability index.

Cognitive radio and Dynamic Spectrum Access (DSA) enable wireless users to share a wide range of available spectrums. In this paper, we study joint spectrum allocation and scheduling problems in cognitive radio wireless networks with the objectives of achieving fair spectrum sharing. A novel Multi-Channel Contention Graph (MCCG) is proposed to characterize the impact of interference under the protocol model in such networks. Based on the MCCG, we present an optimal algorithm to compute maximum throughput solutions. As simply maximizing throughput may result in a severe bias on resource allocation, we take fairness into consideration by presenting optimal algorithms as well as fast heuristics to compute fair solutions based on a simplified max-min fairness model and the well-known proportional fairness model. Numerical results show that the performance given by our heuristic algorithms is very close to that of the optimal solution, and our proportional fair algorithms achieve a good tradeoff between throughput and fairness. In addition, we extend our research to the physical interference model, and propose effective heuristics for solving the corresponding problems.

Location tracking is one of the most important issues in providing real-time applications over wireless networks due to its effect to quality of service (QoS), such as end-to-end delay, bandwidth utilization, and connection dropping probability. In this paper, we study cost minimization for locating mobile users under delay constraints in mobile wireless networks. Specifically, a new location tracking algorithm is developed to determine the position of mobile terminals under delay constraints, while minimizing the average locating cost based on a unimodal property. We demonstrate that the new algorithm not only results in minimum locating cost, but also has a lower computational complexity compared to existing algorithms. Furthermore, detailed searching procedures are discussed under both deterministic and statistic delay bounds. Numerical results for a variety of location probability distributions show that our algorithm compares favorably with existing algorithms.

In this paper, we study the problem of computing a minimum cost Steiner tree subject to weight constraint in a series–parallel graph where each edge has a nonnegative integer cost and a nonnegative integer weight. We present a strongly polynomial time approximation scheme for this NP-complete problem.

We study end-to-end routing in a communication system where there is a bandwidth and a propagation delay associated with each link, as well as a queuing delay associated with each intermediate node. We present a polynomial time algorithm for computing an optimal multichannel routing to transmit a given message. Examples are also given to show that several previously published path-based algorithms for this problem are suboptimal. We then generalize the multichannel routing problem to delay-constrained multichannel routing problem and show that this generalized problem can also be solved in polynomial time.

We investigate a practical variant of the well-known graph Steiner tree problem. In this variant, every target vertex is required to be a leaf vertex in the solution Steiner tree. We present hardness results for this variant as well as a polynomial time approximation algorithm with performance ratio ρ + 2, where ρ is the best-known approximation ratio for the graph Steiner tree problem.

Given n terminals in the Euclidean plane and a positive constant, find a Steiner tree interconnecting all terminals with the minimum number of Steiner points such that the Euclidean length of each edge is no more than the given positive constant. This problem is NP-hard with applications in VLSI design, WDM optical networks and wireless communications. In this paper, we show that (a) the Steiner ratio is 1 4 , that is, the minimum spanning tree yields a polynomial-time approximation with performance ratio exactly 4, (b) there exists a polynomial-time approximation with performance ratio 3, and (c) there exists a polynomial-time approximation scheme under certain conditions.

An important problem in computational biology is the genome rearrangement using reversals and transpositions. Analysis of genome evolving by reversals and transpositions leads to a combinatorial optimization problem of sorting by reversals and transpositions, i.e., sorting of a permutation using reversals and transpositions of arbitrary fragments. The reversal operation works on a single segment of the genome by reversing the selected segment. Two kinds of transpositions have been studied in the literature. The first kind of transposition operations delete a segment of the genome and insert it into another position in the genome. The second kind of transposition operations delete a segment of the genome and insert its inverse into another position in the genome. Both transposition operations can be viewed as operations working on two consecutive segments. In this paper, we introduce a third transposition operation which works on two consecutive segments and study sorting of a signed permutation by reversals and transpositions. By allowing reversals and the first kind of transpositions, or reversals and the first two kinds of transpositions, or reversals and all three kinds of transpositions, we have three problem models. After establishing a common lower bound on the number of operations needed, we present a unified 2-approximation algorithm for all these problems. Finally, we present a better 1.75-approximation for the third problem.

In this paper, we study the Steiner tree problem with minimum number of Steiner points and bounded edge-length (STPMSPBEL), which asks for a tree interconnecting a given set of n terminal points and a minimum number of Steiner points such that the Euclidean length of each edge is no more than a given positive constant. This problem has applications in VLSI design, WDM optimal networks and wireless communications. We prove that this problem is NP-complete and present a polynomial time approximation algorithm whose worst-case performance ratio is 5.

Let N = (V, A, c, l) be a network with node set V, arc set A, non-negative arc capacity function c and non-negative arc delay function l. We are interested in the fastest way to transmit ƒ amount of data from a designated source node s ϵ V to a designated destination node d ϵ V. When the transmission is restricted to a single path, the problem is called the quickest path problem which has been well studied in the past few years and many efficient algorithms are known. When the transmission is not restricted to a single path, the problem is called the quickest flow problem or the minimum transmission time problem in the literature. Since data transmission is not restricted to a single path in the minimum transmission time problem, the problem was believed to be harder. In (Kagaris et al., 1995), it was proved that the minimum transmission time problem with integer capacities and real delays is NP-hard. In this paper, we show that the minimum transmission time problem with integer capacities and integer delays is equivalent to the maximal dynamic flow problem studied by Ford and Fulkerson in 1962 and therefore can be solved efficiently by their path decomposition algorithm.

The Euclidean multi-facility location (EMFL) problem is one of locating new facilities in the Euclidean space with respect to existing facilities. The problem consists of finding locations of new facilities which will minimize a total cost function which consists of a sum of costs directly proportional to the Euclidean distances between the new facilities, and costs directly proportional to the Euclidean distances between new and existing facilities. In this paper, it is shown that the dual of EMFL proposed by Francis and Cabot is equivalent to the maximization of a linear function subject to convex quadratic inequality constraints and therefore can be solved in polynomial time by interior point methods. We also establish a theorem on the duality gap and present a procedure for recovering the primal solution from an interior dual feasible solution.

Let N = (V, A, c, l) be an input network with node set V, arc set A, positive arc weight function c and nonnegative arc weight function l. Let σ be the amount of data to be transmitted. The quickest path problem is to find a routing path in N to transmit the given amount of data in minimum time. In a recent paper, Chen and Chin proposed this problem and developed algorithms for the single pair quickest path problem with time complexity O(re + rn log n), where n = ¦V¦ , e = ¦A¦ , and r is the number of distinct capacity values of N. In this paper, we first develop an alternative algorithm for the single pair quickest path problem with same time complexity and less space requirement. We then study the constrained quickest path problem and propose an O(re + rn log n) time algorithm. Finally, we develop an algorithm to enumerate the first m quickest paths to send a given amount of data from one node to another with time complexity O(rmne + rmn2 log n).