Search

When limit-state functions are highly nonlinear, traditional reliability methods, such as the first-order and second-order reliability methods, are not accurate. Monte Carlo simulation (MCS), on the other hand, is accurate if a sufficient sample size is used but is computationally intensive. This research proposes a new system reliability method that combines MCS and the Kriging method with improved accuracy and efficiency. Accurate surrogate models are created for limit-state functions with minimal variance in the estimate of the system reliability, thereby producing high accuracy for the system reliability prediction. Instead of employing global optimization, this method uses MCS samples from which training points for the surrogate models are selected. By considering the autocorrelation of a surrogate model, this method captures the more accurate contribution of each MCS sample to the uncertainty in the estimate of the serial system reliability and therefore chooses training points efficiently. Good accuracy and efficiency are demonstrated by four examples.

GaN-based pin alpha-particle detectors grown on sapphire substrates have been subjected to 10 MeV electron irradiation over a cumulative dose range of 0 to 200 kGy. The pre- and post-irradiation detectors have been characterized with current–voltage and capacitance–voltage measurements, charge collection efficiency ( CCE ), and alpha-particle pulse-height spectroscopy . The results show that the performance of the detectors underwent significant changes due to enhanced carrier-hopping conductivity through defect states and deep-level traps in the space-charge region induced by the 10 MeV electron irradiation. Such detectors can be used for alpha detection with confidence in an environment of background high energy electrons , up to a dose of about 200 kGy, and the response can degrade rapidly if the dose exceeds 200 kGy. In this work, the maximum CCE was achieved in a detectors irradiated with a cumulative dose of 100 kGy.

The temperature-dependent radiation-detection performance of an alpha-particle detector that was based on a gallium-nitride (GaN)-based pin structure was studied from 290 K to 450 K. Current–voltage–temperature measurements ( I – V – T ) of the reverse bias show the exponential dependence of leakage currents on the voltage and temperature. The current transport mechanism of the GaN-based pin diode from the reverse bias I – V fitting was analyzed. The temperature-dependent pulse-height spectra of the detectors were studied using an 241 Am alpha-particle source at a reverse bias of 10 V, and the peak positions shifted from 534 keV at 290 K to 490 keV at 450 K. The variation of full width at half maximum ( FWHM ) from 282 keV at 290 K to 292 keV at 450 K is almost negligible. The GaN-based pin detectors are highly promising for high-temperature environments up to 450 K.

High-quality VO 2 films have been successfully synthesized on SiO 2 /Si substrates by a simple and efficient method in a tube furnace. The as-synthesized VO 2 films were structurally uniform and single-crystalline. These films show a large resistance change of more than four orders of magnitude with a 5 K width of the hysteresis loop as the temperature is cycled through the phase transition. VO 2 thin-film devices have been fabricated by reactive ion etching (RIE) and deposition processes. The current response of devices under light illumination and bias exhibited a higher sensitivity and a much wider pressure range than that without light illumination. Experimental data also revealed that the current response of a VO 2 film device shows an approximately linear dependence on the logarithm of air pressure. These high-quality VO 2 thin films with excellent properties have potentially promising application in vacuum sensor.

The crystal quality and stress state of Al0.5Ga0.5N epitaxial layers on 6H-SiC wafers by introducing an in-situ deposited SiNx nanomask layer grown by metal-organic chemical vapor deposition (MOCVD) were investigated. A SiNx interlayer with various growth times was inserted to the Al0.5Ga0.5N epilayers. The full width at half maximum (FWHM) of X-ray diffraction peaks and the density of etch pits decreased dramatically by the SiNx interlayer, indicating an improved crystalline quality. Also, it was found that the crack density and biaxial tensile stress in the Al0.5Ga0.5N film was significantly reduced by in situ SiNx interlayer from optical microscopy, photoluminescence spectra and Raman spectra. Finally, a crack-free 1.8 μm thick Al0.5Ga0.5N epilayer grown on 6H-SiC substrate using the optimized SiNx interlayer growth time was obtained.

A GaN-based pin neutron detector with a 6LiF conversion layer was fabricated, and can be used to detect thermal neutrons. Measurement of the electrical characteristic of the GaN-based pin neutron detector showed that the reverse leakage current of the neutron detector was reduced significantly after deposition of a 6LiF conversion layer on the detector surface. The thermal neutrons used in this experiment were obtained from an 241Am–Be fast neutron source after being moderated by 100-mm-thick high-density polyethylene. The experimental results show that the detector with 16.9-μm thick 6LiF achieved a maximum neutron detection efficiency of 1.9% at a reverse bias of 0 V, which is less than the theoretical detection efficiency of 4.1% calculated for our GaN neutron detectors.

Schottky diode nuclear radiation detectors based on semi-insulating gallium-arsenide (GaAs) wafers were fabricated. The minimum leakage current and current density of the detectors were 7.8 nA and 2.5 × 10−7 A/cm2, respectively, at a reverse bias of 100 V. The Schottky GaAs detectors were subjected to 10 MeV electron irradiation with a radiation dose of 0 to 200 kGy. Pre- and post-irradiation detectors were characterized by current–voltage, charge collection efficiency (CCE), and alpha-particle pulse-height spectroscopy. The detection efficiency of all detectors improved after irradiation, which may be attributed to defects induced by high-energy electron irradiation increasing the inhomogeneity of semi-insulating GaAs wafers. However, the energy resolution and CCE of the post-irradiation detectors decreased. After electron irradiation, the post-irradiation detectors were annealed at 200 °C for 5 min in nitrogen. The CCE of the annealed detectors improved, which suggests that the damages caused by high-dose electron irradiation can be repaired partially by annealing at 200 °C.

Influence of alternating current (AC) on pitting corrosion and stress corrosion cracking (SCC) behavior of X70 pipeline steel in the near-neutral pH environment under cathodic protection (CP) was investigated. Both corrosion and SCC are inhibited by −0.775 VSCE CP without AC interference. With the superimposition of AC current (1–10 mA/cm2), the direct current (DC) potential shifts negatively under the CP of −0.775 VSCE and the cathodic DC current decreases and shifts to the anodic direction. Under the CP potential of −0.95 VSCE and −1.2 VSCE, the applied AC current promotes the cathodic reaction and leads to the positive shift of DC potential and increase of cathodic current. Local anodic dissolution occurs attributing to the generated anodic current transients in the positive half-cycle of the AC current, resulting in the initiation of corrosion pits (0.6–2 μm in diameter). AC enhances the SCC susceptibility of X70 steel under −0.775 VSCE CP, attributing to the promotion of anodic dissolution and hydrogen evolution. Even an AC current as low as 1 mA/cm2 can enhance the SCC susceptibility.

Time-dependent system reliability is computed as the probability that the responses of a system do not exceed prescribed failure thresholds over a time duration of interest. In this work, an efficient time-dependent reliability analysis method is proposed for systems with bivariate responses which are general functions of random variables and stochastic processes. Analytical expressions are derived first for the single and joint upcrossing rates based on the first-order reliability method (FORM). Time-dependent system failure probability is then estimated with the computed single and joint upcrossing rates. The method can efficiently and accurately estimate different types of upcrossing rates for the systems with bivariate responses when FORM is applicable. In addition, the developed method is applicable to general problems with random variables, stationary, and nonstationary stochastic processes. As the general system reliability can be approximated with the results from reliability analyses for individual responses and bivariate responses, the proposed method can be extended to reliability analysis of general systems with more than two responses. Three examples, including a parallel system, a series system, and a hydrokinetic turbine blade application, are used to demonstrate the effectiveness of the proposed method.

A photodetection model for AlGaAs/GaAs nanowire photodetectors with graded composition is developed based on the numerical solution of coupled Poisson and continuity equations. Photocurrents and spectral sensitivities of nanowire photodetectors have been simulated as a function of Al content range, the wavelength of the incident light, the length of the nanowire, and contact types. The results demonstrate Ohmic–Schottky devices have the highest sensitivities among the four types of nanowire devices, which can be attributed to the strong built-in electric field induced by the graded composition and the dominant unidirectional built-in electric field due to the Schottky barrier. Schottky–Schottky devices show the lowest sensitivities due to the additional blocking effect of the Schottky barrier. The optimum nanowire length in which an Ohmic–Schottky structured nanowire photodetector can achieve maximum sensitivity is 7.4 μm for a nanowire with a linearly graded Al content range of 0–0.4 (x Al = 0.4). The optimum nanowire lengths for Ohmic–Ohmic (x Al = 0.3), Schottky–Schottky (x Al = 0.1), and Schottky–Ohmic (x Al = 0.0) nanowire devices are 6.5, 3.5, and 11 μm, respectively.

We reported a method combining the gas–liquid interface method and inductively coupled plasma (ICP) to fabricate highly-ordered nanowire arrays on a 2 inch GaAs substrate. A large-area and highly-ordered SiO2 nanosphere monolayer was fabricated and the GaAs nanowire array was etched by ICP. The flow rates and ratios of the etching gases were studied. Finally, highly-ordered GaAs nanowire arrays with high quality surface morphology were obtained with the volumetric flow rates at 9 sccm and 21 sccm for BCl3 and N2 gases, respectively. Reflectance of the samples was found to be consistently below 10%, the lowest being 3%.

Reliability analysis is time consuming, and high efficiency could be maintained through the integration of the Kriging method and Monte Carlo simulation (MCS). This Kriging-based MCS reduces the computational cost by building a surrogate model to replace the original limit-state function through MCS. The objective of this research is to further improve the efficiency of reliability analysis with a new strategy for building the surrogate model. The major approach used in this research is to refine (update) the surrogate model by accounting for the full information available from the Kriging method. The existing Kriging-based MCS uses only partial information. Higher efficiency is achieved by the following strategies: (1) a new formulation defined by the expectation of the probability of failure at all the MCS sample points, (2) the use of a new learning function to choose training points (TPs). The learning function accounts for dependencies between Kriging predictions at all the MCS samples, thereby resulting in more effective TPs, and (3) the employment of a new convergence criterion. The new method is suitable for highly nonlinear limit-state functions for which the traditional first- and second-order reliability methods (FORM and SORM) are not accurate. Its performance is compared with that of existing Kriging-based MCS method through five examples.

When limit-state functions are highly nonlinear, traditional reliability methods, such as the first order and second order reliability methods, are not accurate. Monte Carlo simulation (MCS), on the other hand, is accurate if a sufficient sample size is used, but is computationally intensive. This research proposes a new system reliability method that combines MCS and the Kriging method with improved accuracy and efficiency. Cheaper surrogate models are created for limit-state functions with the minimal variance in the estimate of the system reliability, thereby producing high accuracy for the system reliability prediction. Instead of employing global optimization, this method uses MCS samples from which training points for the surrogate models are selected. By considering the dependence between responses from a surrogate model, this method captures the true contribution of each MCS sample to the uncertainty in the estimate of the system reliability and therefore chooses training points efficiently. Good accuracy and efficiency are demonstrated by three examples.

Time-dependent system reliability is measured by the probability that the responses of a system do not exceed prescribed failure thresholds over a period of time. In this work, an efficient time-dependent reliability analysis method is developed for bivariate responses that are general functions of random variables and stochastic processes. The proposed method is based on single and joint upcrossing rates, which are calculated by the First Order Reliability Method (FORM). The method can efficiently produce accurate upcrossing rates for the systems with two responses. The upcrossing rates can then be used for system reliability predictions with two responses. As the general system reliability may be approximated with the results from reliability analyses for individual responses and bivariate responses, the proposed method can be extended to reliability analysis for general systems with more than two responses. Two examples, including a parallel system and a series system, are presented.Copyright © 2015 by ASME

Three different inlet configurations, including a original straight pipe and two bend pipes with different axial length, for a high speed low mass flow centrifugal compressor were modeled with whole blade passages and simulated unsteadily by 3D viscous Navier-Stokes equations. The performance disparities of compressor stage were tested and verified by experiments in which dynamic pressure data acquisition of internal flow field was performed. As the result shows, the choke point decreases to lower mass flow rate due to the distortion caused by bend-pipe inlet and is aggravated as the rotation speed increases. The distortion effect spreads circumferentially in impeller and makes the flow structure varied. The longer axial distance bent inlet leads to larger radial distortion and heavy blockage at mid-span under large mass flow mainly causes compressor choke margin narrowed. Bend pipe distortion brings an impact up to diffuser on unsteady pressure pulsation caused by blades sweep and the impact appears more powerful when it is closer to volute tongue.

Temperature-dependent material parameters and device performances of Ga x In 1− x As 1− y Sb y TPV cells applied in low temperature (800–1200 °C) radiators are simulated using the PC-1D. As is well known, the optimum bandgap ( E g ) decreases towards lower radiator temperatures. So far, the lowest achievable E g of Ga x In 1− x As 1− y Sb y at 300 K is 0.5 eV. We mainly considering the Ga 0.8 In 0.2 As 0.18 Sb 0.82 ( E g = 0.5 eV) TPV cell. The effects of doping concentration and recombination mechanisms of the emitter layer on photovoltaic conversion efficiencies ( η cel ) are analyzed in detail, and η cel can be improved by optimizing doping concentration and suppressing carrier recombination. The effects of GaSb window layer on η cel are also presented. It shows the type-II energy-band alignment GaSb(window)/GaInAsSb(emitter) heterostructure affect η cel mainly through V oc . For the first time, the effects of operating temperatures on device performances are analyzed based on temperature-dependent material parameters, and the temperature coefficients of the device performances are presented.

To study the performance of high and low stage compressors and that of the system as a whole, a two-stage turbocharging system was matched, and a special two-stage turbocharging system test bench was built. For each test curve, the speeds of the two stage turbochargers were adjusted to the fixed data, and a compressor performance experiment was performed. The results showed many differences between the corrected mass flow and the actual mass flow of the high pressure (HP) stage compressor. To find out the actual supercharging effect of the two-stage turbocharging system, it is better to adopt the actual mass flow. The two-stage turbocharging system in this paper has much higher efficiency under most operating conditions if the pressure ratio assignment is 1:1. The system can get very high supercharging pressure when the speeds of the two stage turbochargers are rather low, which ensures the system’s security and reliability.

The reliability of a system is usually measured by the probability that the system performs its intended function in a given period of time. Estimating such reliability is a challenging task when the probability of failure is rare and the responses are nonlinear and time variant. The evaluation of the system reliability defined in a period of time requires the extreme values of the responses in the predefined period of time during which the system is supposed to function. This work builds surrogate models for the extreme values of responses with the Kriging method. For the sake of computational efficiency, the method creates Kriging models with high accuracy only in the region that has high contributions to the system failure; training points of random variables and time are sampled simultaneously so that their interactions could be considered automatically. The example of a mechanism system shows the effectiveness of the proposed method.Copyright © 2015 by ASME

The response of a component in a multidisciplinary system is affected by not only the discipline to which it belongs, but also by other disciplines of the system. If any components are subject to time-dependent uncertainties, responses of all the components and the system are also time dependent. Thus, time-dependent multidisciplinary reliability analysis is required. To extend the current time-dependent reliability analysis for a single component, this work develops a time-dependent multidisciplinary reliability method for components in a multidisciplinary system under stationary stochastic processes. The method modifies the First and Second Order Reliability Methods (FORM and SORM) so that the Multidisciplinary Analysis (MDA) is incorporated while approximating the limit-state function of the component under consideration. Then Monte Carlo simulation is used to calculate the reliability without calling the original limit-state function. Two examples are used to demonstrate and evaluate the proposed method.Copyright © 2015 by ASME

For the frequency range of 1 kHz–10 MHz, the interface state density of Ni contacts on p-GaN is studied using capacitance-voltage (C–V) and conductance-frequency-voltage (G–f–V) measurements at room temperature. To obtain the real capacitance and interface state density of the Ni/p-GaN structures, the effects of the series resistance ( ) on high-frequency (5 MHz) capacitance values measured at a reverse and a forward bias are investigated. The mean interface state densities obtained from the – capacitance and the conductance method are and , respectively. Furthermore, the interface state density derived from the conductance method is higher than that reported from the Ni/n-GaN in the literature, which is ascribed to a poor crystal quality and to a large defect density of the Mg-doped p-GaN.

Negative electron affinity GaAs wire-array photocathodes have been fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material followed by Cs-O activation. Scanning electron microscope has revealed that the thus obtained high-density GaAs wire arrays had high periodicity, large height, and good morphology. Photoluminescence spectra indicated the wire arrays were of good crystalline quality and free from any obvious damage. Compared to the original GaAs wafer, the photoluminescence peak positions of the wire arrays were somewhat red-shifted, which may be attributed to the temperature effect and strain relaxation. The wire-array structures showed significantly reduced light reflection compared with the original wafer due to the excellent light-trapping effect. Cs-O activation experiments of the GaAs wire arrays have been performed to reveal the effect of incident angle on quantum efficiency. The results show that maximum quantum efficiency was obtained at about 30°. Given these unique electrical and optical properties, a GaAs wire-array photocathode is an attractive alternative to its planar-structured counterpart.

Recently Wireless Local Area Network (WLAN) has become a major highlight of the communications industry, the application prospect of which is very broad, and Wireless Bridge (WB) is the core network equipment of WLAN; this paper firstly introduces the overall function of the Wireless Bridge, conducts a thorough analysis on the characteristics and relevant technical issues of WB, and proposes the design solution for WB in WLAN of 802.11b based on the VxWorks embedded operating system, implements two parts of hardware and software, as well as completes the main function provided in the protocol.

The embedded and field-bus technology have a profound impact on industrial control area. The paper design an embedded numerical control (NC) system based on ARM, the embedded NC system has the characteristics of high precision, low cost, good openness and so on. Through extend CAN bus interface circuit on ARM which can realize the network of embedded NC system. The network for embedded numerical control system based on CAN bus can realize one computer control more NC tools and resolve the problem of low real-time which result from network transmission. The realization of system can improve efficiency of NC process and enhance real-time ability.

With the development of computer and network technology and the application of machine tool, in order to resolve the traditional stand-alone mode, enhancing the efficiency of NC (numerical control) system. Through introduce CAN (Controller Area Network) bus, a new type of LAN (Local Area Network) was designed. The network system structure and the embedded NC system based on ARM9 and motor control chip MCX314As were given. This LAN embedded NC system can realize one computer control more NC tool and resolve the problem of low real-time which result from network transmission. The realization of whole system can improve efficiency of NC processing and enhance real-time ability.

The resolution model of a graded doping and graded composition transmission-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, we calculate the theoretical modulation transfer function (MTF) of different graded doping and graded composition structures. The simulation results show that both graded composition and graded doping structures can increase the resolution of the photocathode. The exponentially doping and linear composition photocathode has the maximum resolution among the possible graded doping and graded composition photocathodes. The resolution improvement is attributed to the built-in electric field induced by a graded composition or graded doping structure. The simulation results also show that the MTFs of AlGaAs/GaAs cathodes increase as the AlGaAs layer thickness decreases, or the incident light wavelength increases.