Investigation into the microstructure and dynamic compressive properties of selective laser melted Ti–6Al–4V alloy with different heating treatments

As a commonly used engineering material, the mechanical properties of titanium alloy under dynamic loads are closely related to their microstructure. In this work, the effects of solution treatment (ST) and solution and aging treatment (SAT) on the microstructure and dynamic compressive properties of Ti–6Al–4V alloy manufactured by selective laser melting were studied. The results showed that the microstructure of selective laser melted Ti–6Al–4V consisted of nearly full acicular α′ martensite, then the acicular α′ martensite was decomposed into α+β phase with basket-weave morphology with solution treatment. Clusters of α2 particles with size of several hundred nanometers were precipitated in the α plates further with solution and aging treatment. The ultimate compressive strength (UCS) of selective laser melted TC4 alloy was increased with the increasing strain rate, showing strong strain rate hardening effect. Stress collapse happened once the strain exceeded 1500/s, which is the dominant failure model of selective laser melted TC4 under impacting load. As expected, the UCS of the ST sample decreased, but the ductility increased compared with the as-built sample; however, both the UCS and ductility of the SAT samples were enhanced synergistically due to the widely distributed α2 precipitates. Besides, the SAT samples had the highest energy absorption compared with the as-built and ST counterparts under the same conditions, indicating that the SAT samples had better load-bearing capacities

Modelling and analysis of TCP network dynamics

This thesis focuses on the application of feedback control theory to the study of data communication over the Internet. At the heart of this communication lies the transmission control protocol (TCP) which is responsible for reliable and efficient data transfer. Using recently developed fluid models of TCP, we treat its congestion control phase as a feedback system and, through analysis, provide insight on its performance. The contribution of the thesis is twofold. First, it introduces a matrix analysis tool, the matrix field of values, to the stability analysis of congested networks involving arbitrary numbers of heterogeneous TCP-controlled sources and congested links. This tool enables us to derive stability results for buffer-based active queue management (AQM) schemes, revealing the impact that routing plays on stability robustness. This matrix field of values also proves valuable in synthesizing stabilizing source controllers when one considers the possibility of TCP sources sending data over multiple paths. The second contribution of the thesis is to study the scenario where the fluid models predict congestion-control instability and hence predicts that the average behavior of TCP traffic is oscillatory. Using the theory of weakly-coupled oscillators, we formulate a network problem wherein oscillating traffic from multiple sources traverses a core congested link, and analyze the impact that packet loss at this core link has on the synchronization between the oscillating sources. This formulation allows one to then make connection between traffic throughput and the mechanisms contributing to packet loss, such as the congested router\u27s buffer size

Multi-path TCP: A joint congestion control and routing scheme to exploit path diversity in the Internet

We consider the problem of congestion-aware multi-path routing in the Internet. Currently, Internet routing protocols select only a single path between a source and a destination. However, due to many policy routing decisions, single-path routing may limit the achievable throughput. In this paper, we envision a scenario where multi-path routing is enabled in the Internet to take advantage of path diversity. Using minimal congestion feedback signals from the routers, we present a class of algorithms that can be implemented at the sources to stably and optimally split the flow between each source-destination pair. We then show that the connection-level throughput region of such multi-path routing/congestion control algorithms can be larger than that of a single-path congestion control scheme.

Active Tuning of Midinfrared Surface Plasmon Resonance and Its Hybridization in Black Phosphorus Sheet Array

The behaviors of anisotropic plasmons in black phosphorus have been fully exploited across a complete variety of system including individual nanoribbon, vertically offset paired ones, and nanoribbon/sheet hybrid system. Benefiting from its two-dimensional nature, plasmons can be actively controlled by either geometrical parameters or the carrier doping in black phosphorus, which allows for the emerging phenomenon of strong light–matter interaction at mid-infrared region. Remarkably, Rabi splitting over 17.3 meV is observed in the plasmonic spectra of the nanoribbon/sheet hybrid structure as harnessed by the strong interaction of coupled plasmons. We also propose a scheme that supports plasmon on a continuous monolayer black phosphorus by using a diffraction grating. By adjusting the geometrical parameters of the grating and the Fermi level of the black phosphorus slightly, the resonance wavelength can be changed notably. The results appealing open up possibilities of devising black phosphorus-based plasmons in the application of optical detection and sensing within the deep subwavelength regime at mid-infrared region

Enhancing Self‐Powered Terahertz Photodetection with VSe2 and Van Der Waals Heterostructure Integration via Photothermoelectric Effect

Abstract Owning unique optical and electronic properties, two dimensional (2D) materials have made remarkable strides in the field of photodetection applications. However, achieving highly sensitive and ultra‐broadband detection from microwave to terahertz (THz) range (0.02–0.54 THz) remains a significant challenge for photodetectors. This study presents a self‐powered THz photodetector based on VSe2 and its van der Waals heterostructure. The photoresponse of the photodetector is primarily attributed to the photothermoelectric effect. At room temperature, the device exhibits lower noise equivalent power values of 21 pW Hz−1/2 at 0.28 THz. This work has achieved ultra‐broadband detection and demonstrated the potential for large‐area imaging, providing a new avenue for the application of THz technology in nondestructive testing and biometric identification fields

Enhancement of terahertz response in a microstructure-integrated-type-II Dirac semimetal

Terahertz detection technology has been confronted with formidable impediments, notably the paucity of sensitivity and operating temperature for photodetectors based on traditional bulk materials. In an attempt to surmount the difficulties, we propose an innovative terahertz detector based on a PtSe2 (type-II Dirac semimetallic material) integrated asymmetric antenna structure that can enhance the terahertz photoresponse by capitalizing on meticulous fabrication procedures. Experimental outcomes demonstrate the remarkable characteristics of the photodetector in the terahertz band, encompassing fast response time (7 µs), large responsivity (3.267 A/W), and low noise equivalent power (3.96 pW/Hz0.5). These accomplishments can be ascribed to the incorporation of the asymmetric metal contact of the four-leaf clover antenna structure and the excellent thermoelectric characteristics of PtSe2. This pioneering investigation consequently unveils a novel methodology for the creation of proficient PtSe2-based terahertz detectors and serves as a catalyst for the promotion of applications and further research within the terahertz sphere

Determination of Epicenters before Earthquakes Utilizing Far Seismic and GNSS Data: Insights from Ground Vibrations

Broadband seismometers, ground-based Global Navigation Satellite Systems (GNSS), and magnetometers that were located within an epicentral distance of approximately 150 km consistently observed the novel anomalous behaviors of the common-mode ground vibrations approximately 5–10 days before the M6.6 Meinong earthquake in Taiwan. The common-mode ground vibrations with amplitudes near 0.1 m at frequencies ranging from 8 × 10−5 to 2 × 10−4 Hz were generated near the region close to the epicenter of the impending earthquake. The common-mode vibrations were consistently observed in seismic and GNSS data associated with five other earthquakes in four distinct areas. The results reveal that the common-mode vibrations could be a typical behavior before earthquakes. The causal mechanism of common-mode vibrations can be attributed to crustal resonance excitations before fault dislocations occur. Potential relationships with other pre-earthquake anomalies suggest that the common-mode vibrations could be ground motion before earthquakes, which was investigated for a significant length of time