Nanostructured Pt is the most efficient single-metal catalyst for fuel cell technology. Great efforts have been devoted to optimizing the Pt-based alloy nanocrystals with desired structure, composition, and shape for boosting the electrocatalytic activity. However, these well-known controls still show the limited ability in maximizing the Pt utilization efficiency for achieving more efficient fuel cell catalysis. Herein, a new strategy for maximizing the fuel cell catalysis by controlling/tuning the defects and interfaces of PtPb nanoplates using ion irradiation technique is reported. The defects and interfaces on PtPb nanoplates, controlled by the fluence of incident C ions, make them exhibit the volcano-like electrocatalytic activity for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and oxygen reduction reaction (ORR) as a function of ion irradiation fluence. The optimized PtPb nanoplates with the mixed structure of dislocations, subgrain boundaries, and small amorphous domains are the most active for MOR, EOR, and ORR. They can also maintain high catalytic stability in acid solution. This work highlights the impact and significance of inducing/controlling the defects and interfaces on Pt-based nanocrystals toward maximizing the catalytic performance by advanced ion irradiation strategy.
Distributed denial of service (DDoS) attack is a continuous critical threat to the Internet. Derived from the low layers, new application-layer-based DDoS attacks utilizing legitimate HTTP requests to overwhelm victim resources are more undetectable. The case may be more serious when such attacks mimic or occur during the flash crowd event of a popular Website. In this paper, we present the design and implementation of CALD, an architectural extension to protect Web servers against various DDoS attacks that masquerade as flash crowds. CALD provides real-time detection using mess tests but is different from other systems that use resembling methods. First, CALD uses a front-end sensor to monitor the traffic that may contain various DDoS attacks or flash crowds. Intense pulse in the traffic means possible existence of anomalies because this is the basic property of DDoS attacks and flash crowds. Once abnormal traffic is identified, the sensor sends ATTENTION signal to activate the attack detection module. Second, CALD dynamically records the average frequency of each source IP and check the total mess extent. Theoretically, the mess extent of DDoS attacks is larger than the one of flash crowds. Thus, with some parameters from the attack detection module, the filter is capable of letting the legitimate requests through but the attack traffic stopped. Third, CALD may divide the security modules away from the Web servers. As a result, it keeps maximum performance on the kernel web services, regardless of the harassment from DDoS. In the experiments, the records from www.sina.com and www.taobao.com have proved the value of CALD.
A test structure, which is called the ridge-furrow structure, is used to evaluate the leakage current of the Schottky contact on the mesa edge of an AlGaN/GaN heterostructure. The mesa edge leakage currents were measured at different temperatures from 300 to 500 K and analyzed. The conduction-band-edge energy distribution at the mesa edge is simulated by the Integrated Systems Engineering Technology Computer-Aided Design. Based on the simulation results, the electric field strength can be obtained as a function of reverse bias voltage. The mesa edge leakage current is found to agree with the predicted characteristics, which is based on the Frenkel-Poole emission model. Therefore, we believe that the Frenkel-Poole emission dominates the mesa edge leakage at temperatures between 300 and 500 K.Index Terms-AlGaN/GaN, characteristic electric field strength, Frenkel-Poole, leakage current, mesa edge, ridge-furrow structure.
The tungsten inert gas (TIG) welding-brazing technology using Mg based filler was developed to join AZ31B Mg alloy to TA2 pure Ti in a lap configuration. The results indicate that robust joints can be obtained with welding current in the range of 60-70 A and welding speed of 0?2 m min 21 . The joints were found to be composed of the coarse grained fusion zone accompanied with the precipitated phase of Mg 17 Al 12 , and a distributed Mg-Ti solid solution zone at the interface of Mg/ Ti, indicating that metallurgical bonding was achieved. The maximum tensile-shear strength of 193?5 N mm 21 , representing 82?3% joint efficiency relative to the Mg alloy base metal, was attained. The optimised Mg/Ti joint fractured at Mg fusion zone upon tensile-shear loading, mainly caused by grain coarsening. Moreover, the fracture surface practically consisted of scraggly areas, which was characterised by equiaxed dimple patterns accompanied with a few lamellar tearing.
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