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21 Feb 2013

Volume 113, Issue 7, Articles (07xxxx)

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J. Appl. Phys. 113, 073506 (2013); http://dx.doi.org/10.1063/1.4790173 (6 pages)

Uwe Kaiser, Sebastian Gies, Sebastian Geburt, Franziska Riedel, Carsten Ronning, and Wolfram Heimbrodt
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back to top Interdisciplinary and General Physics

Reflection and transmission of Lamb waves at an imperfect joint of plates

Naoki Mori, Shiro Biwa, and Takahiro Hayashi

J. Appl. Phys. 113, 074901 (2013); http://dx.doi.org/10.1063/1.4791711 (10 pages)

Online Publication Date: 15 February 2013

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The reflection and transmission of Lamb waves at an imperfect joint of plates are analyzed numerically by the modal decomposition method and the hybrid finite element method. The joint is modeled as a spring-type interface characterized by distributed normal and tangential stiffnesses. The analysis is focused on a low-frequency range where the lowest-order symmetric and antisymmetric Lamb waves are the only propagating modes. The frequency-dependent reflection and transmission characteristics of these Lamb modes are computed for different interfacial stiffnesses, together with the generation behavior of localized, non-propagating higher-order Lamb modes. As a result, S0-mode Lamb wave is shown to exhibit the reflection and transmission characteristics which are monotonically frequency-dependent. On the other hand, A0-mode Lamb wave shows complicated and non-monotonic frequency dependence in the reflection and transmission characteristics. The obtained Lamb wave characteristics are discussed in the light of approximate one-dimensional models constructed based on classical plate theories. As a result, the reflection and transmission coefficients of S0-mode Lamb wave are accurately reproduced by a simple model of longitudinal wave in thin plates, while those of A0-mode Lamb wave are well described by the Mindlin plate model of flexural wave. It is also shown that stiffness reduction at the corners of the contacting edges of plates has only minor influence on the reflection and transmission characteristics.
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46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)
46.70.De Beams, plates, and shells
02.70.Dh Finite-element and Galerkin methods
43.35.Pt Surface waves in solids and liquids
46.25.-y Static elasticity

Conductivity and frequency dependent specific absorption rate

Xiaoming Liu, Hui-Jiuan Chen, Yasir Alfadhl, Xiaodong Chen, Clive Parini, and Dongsheng Wen

J. Appl. Phys. 113, 074902 (2013); http://dx.doi.org/10.1063/1.4791928 (10 pages)

Online Publication Date: 20 February 2013

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Recently, a number of contradicting results have been reported regarding the electromagnetic (EM) energy absorption by highly conductive materials in a liquid phase. The argument rises from the fact that higher conductive media absorb more electromagnetic energy; this however would be constrained by the localized field values that are dictated by the dielectric variations, which may reduce the absorption rate. Using salted water as an example, a systematic investigation of the mechanisms of EM absorption in the presence of highly conductive materials is conducted in this work. A theoretical model is developed, which is supported by both numerical and experimental studies. The influence of salt concentration, dielectric properties, boundary conditions, and EM frequency on the specific absorption rate (SAR) is carefully examined. The results show that the presence of salt in water modifies the dielectric properties significantly in the RF range, while the effect is less prominent in the microwave range. The SAR is highly dependent on the conductivity and frequency, as well as the employed instrument that dictates the surrounding boundary conditions. To suit different applications, the SAR can be optimized by proper consideration of the concentration of high conductivity material, operating frequency, and instruments.
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66.10.Ed Ionic conduction
77.22.Ch Permittivity (dielectric function)

Effects of Sr2+ or Sm3+ doping on electromagnetic and microwave absorption performance of LaMnO3

Shuyuan Zhang, Quanxi Cao, Maolin Zhang, and Xuefang Shi

J. Appl. Phys. 113, 074903 (2013); http://dx.doi.org/10.1063/1.4792471 (7 pages)

Online Publication Date: 20 February 2013

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The perovskite structure La1−xSr(or Sm)xMnO3±δ (x = 0, 0.25) powders were successfully fabricated by the traditional solid state reaction method. The influences of Sr2+ or Sm3+ doping on the static and dynamic magnetic performance were investigated. The resonance phenomena could be attributed to natural resonance and exchange resonance. The substitution of Sr2+ or Sm3+ into La3+ could greatly influence the resonance frequency. Furthermore, the microwave absorption performance of LaMnO3 was improved after the incorporation of Sr2+ or Sm3+. It is obtained that Sr2+ doped LaMnO3 had superior performance compared with that of Sm3+ doped LaMnO3. The maximum reflection loss was −30.0 dB at 13.968 GHz with a thickness of 1.6 mm. The microwave absorption performance was attributed to both the dielectric and magnetic losses.
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78.70.Gq Microwave and radio-frequency interactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
61.72.up Other materials
61.43.Gt Powders, porous materials
75.60.-d Domain effects, magnetization curves, and hysteresis
77.22.Gm Dielectric loss and relaxation

Suppression of secondary electron yield by micro-porous array structure

M. Ye (叶 鸣), Y. N. He (贺永宁), S. G. Hu (胡少光), R. Wang (王瑞), T. C. Hu (胡天存), J. Yang (杨晶), and W. Z. Cui (崔万照)

J. Appl. Phys. 113, 074904 (2013); http://dx.doi.org/10.1063/1.4792514 (8 pages)

Online Publication Date: 21 February 2013

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We study secondary electron yield (SEY) suppression for metal materials using a roughened surface with a micro-porous array. First, we perform a Monte Carlo simulation of the electron trajectory in a single cylindrical well using a phenomenological model of secondary electron emission and the SEY suppression efficiency of a micro-porous array. The simulation results show that the SEY of a roughened surface is affected significantly by the aspect ratio of the micro-pores and the surface porosity of the metal plate. Then, to verify the simulation results, we produce a micro-porous array on metal plates using photolithography and measure their SEYs. We show that the micro-porous array structure can efficiently suppress the SEY of metal materials, and the measurements agree quantitatively with the corresponding simulation results. Finally, we derive an analytical formula to evaluate easily the SEY suppression efficiency of the Ag micro-porous array. In total, the micro-porous array proposed in this paper offers an alternative to SEY suppression in related areas such as multipactor effects in satellite payloads or electron cloud effects in accelerators.
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81.05.Bx Metals, semimetals, and alloys
79.20.Hx Electron impact: secondary emission

Impact of random fabrication errors on backward-wave small-signal gain in traveling wave tubes with finite space charge electron beams

Sean Sengele, Marc L. Barsanti, Thomas A. Hargreaves, Carter M. Armstrong, John H. Booske, and Y. Y. Lau

J. Appl. Phys. 113, 074905 (2013); http://dx.doi.org/10.1063/1.4792666 (9 pages)

Online Publication Date: 21 February 2013

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A one-dimensional small-signal theory for the backward-wave mode in a traveling-wave tube (TWT) is developed, which includes the effects of random fabrication errors. This is of interest since the backward-wave mode is the spatial harmonic typically responsible for instability in a TWT. The described model examines how gain and instantaneous 1-dB bandwidth of the backward-wave mode is affected by random fabrication errors, which are modeled as random perturbations of the phase velocity, interaction impedance, and loss along the TWT's length. Random variation of the phase velocity is found to have the largest effect on both the backward-wave gain and the bandwidth while having only a minor effect on fundamental, forward-wave mode behavior.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
02.50.-r Probability theory, stochastic processes, and statistics
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