Microperforated insertion units: An alternative strategy to design microperforated panels

Microperforated panels (MPPs) coupled to a rigid wall have been proposed recently as an alternative to porous absorbers in situations having concerns with bacterial contamination and small particles discharge, like food, pharmaceutical and microelectronic industries. There exists also an increasing interest for MPP absorbers in the transportation industry and civil engineering. In general, an optimally designed MPP with good broadband absorption requires many submillimetric holes distributed over a panel of also submillimetric thickness. Such thin plates or foils become so fragile that they need to be protected from mechanical damage. In this paper, an alternative strategy is investigated which allows the design of MPPs with panels of millimetric thickness while maintaining their acoustic performance. These absorbers, named microperforated insertion units (MIUs), avoid the structural problems of the classical MPPs. An assessment of the sound absorption properties of these structures is presented. Comparisons between calculations and measurements are also made under two experimental conditions: plane waves at normal incidence (impedance tube) and free field (anechoic room).     

声流现象的研究及其应用

本文从声流产生的机理出发，综合了影响声流的诸因素，介绍了声流现象的检测方法及声流效应的应用。     

双轴晶体Nd^3＋：KGd（WO4）2的非偏振吸收光谱研究

报道了用三个方向相互垂直的非偏振吸收光谱测量计算Ｊｕｄｄ－Ｏｆｅｌｔ参数从而算出双轴晶体Ｎｄ３＋ＫＧｄ（ＷＯ４）２的荧光寿命、荧光分支比和发射截面的方法。得到的荧光寿命为１１９μｓ，平均发射截面为２．３×１０－１９ｃｍ２。     

Amorphous Fe–Mg alloy thin films: magnetic properties and atomic vibrational dynamics

Amorphous (a-) Fe _x Mg_1?x alloys are interesting materials for the investigation of non-Debye-like low-energy vibrational excitations. We have prepared a-Fe _x Mg_1?x alloy thin films (0.3 ≤ × ≤0.7) by vapour quenching. The amorphous state was confirmed by conversion electron Mössbauer spectroscopy between 4.2–300 K, and the x- and temperature-dependence of the isomer shift and hyperfine magnetic field was measured. For x= 0.6 and 0.7, magnetic ordering occurs below ~150 K. The atomic vibrational density of states, g(E), was determined by nuclear resonant inelastic scattering, providing clear evidence for the non-Debye-like low-energy vibrational excitations.     

Metallic radii of nonmetals

Experimental data are used to determine the atomic volumes and radii of the elements of subgroups IV–VIIA in the structures of metallic phases at high pressures. Metallic radii of nonmetals are compared with values based on Pauling and Goldschmidt calculations.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 220–222, February, 1994.     

K2O-ZnO-Al2O3-B2O3-SiO2系掺Cr3+透明莫来石微晶玻璃的研究

采用了X射线衍射、透射电镜和吸收光谱及荧光光谱技术研究了K2O－ZnO-Al2O3-B2O3－SiO2系掺Cr^3 玻璃的析晶性能和光谱。研究结果表明：在K2O－Al2O3-B2O3-SiO2系掺Cr^3 玻璃的基础上引入ZnO后，析晶性能明显改善；析晶温度降低，析出的纳米莫来石微晶均匀、规则；微晶玻璃的发光强度明显增强。     

Electron excitation cross section of atomic transitions in the two‐state approximation

The cross section of the 3s → 3p transition of sodium produced by electron impact has been calculated by performing a numerical integration of the coupled differential equations. The potential functions have been calculated exactly using the hydrogen‐like wave functions for the valence electron of the sodium with an effective charge adjusted to fit the experimental 3s → 3p line strength. The results compare very well with experimental data and with those obtained using more elaborate and sophisticated models.     

无原子布氏代数理论的计算复杂性

研究无原子布氏代数的计算复杂性 .得到了下面的新定理 :定理 1　无原子布氏代数理论Δ具有完全的量词消去法 ,也就是说每一个式子都Δ等价于一个开式子 .定理 2　无原子布氏代数的初等型Γ (x1,… ,xn)是由型内的不含量词的全体开式子所唯一决定 .定理 3　无原子布氏代数的一个长度为 n的语句的判断过程所消耗的 Turing时间和空间都是属于 2 2 cn指数级 .     

X-ray damage characterization in BaLiF3,KMgF3 and LiCaAlF6 complex fluorides

Two-inch sized KMgF₃,BaLiF₃ and LiCaAlF₆ (LiCAF) single crystals were grown by the Czochralski method under a CF₄ atmosphere. X-ray irradiation was used to carry out a comparative study of induced optical absorption phenomena and colour centre creation in the ultra-violet and visible spectral regions. The integral of the induced absorption spectra is significantly lower in LiCAF with respect to the other studied materials. It is found that the amplitude of the F-absorption band is suppressed more than a factor of 3 by Mg-doping. For Mg-doped crystals, the optimum doping concentration is about 0.2 mol% of Mg²⁺.     

Anomalies in the Absorption and Reflection of a High-Power Ultrashort Laser Pulse by a Plasma with a Solid-State Density

The patterns of absorption and reflection of an ultrashort laser pulse by a plasma with a solid-state density, which are realized in the mode of the normal skin effect, are studied. It is shown that a decrease in low-power pulse duration shorter than the period corresponding to the fundamental frequency of radiation leads to an increase in the absorption coefficient. If the pulse power is so high as to provide electron heating during a time shorter than the reciprocal fundamental frequency, nonlinear suppression of absorption takes place and the spectrum of the reflected radiation contains odd harmonics of the fundamental frequency.