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121.
This work reports the potential use of high-overtone self-focusing acoustic transducers for high-frequency ultrasonic Doppler. By using harmonic frequencies of a thick bulk Lead Zirconate Titanate (PZT) transducer with a novel air-reflector Fresnel lens, we obtained strong ultrasound signals at 60 MHz (3rd harmonic) and 100 MHz (5th harmonic). Both experimental and theoretical analysis has demonstrated that the transducers can be applied to Doppler systems with high frequencies up to 100 MHz. 相似文献
122.
An efficient and general synthesis of 2-alkyl-3,4-dihydro-3-oxo-2H-1,4-benzoxazines under controlled microwave heating has been established. It consists of a microwave-assisted reductive N-arylmethylation of substituted 2-aminophenols with aromatic aldehydes followed by a one-pot base-mediated regioselective O-alkylation of the N-arylmethyl-2-aminophenols with 2-bromoalkanoates to give the acyclic intermediates, which cyclize spontaneously to furnish the benzoxazine scaffolds in good to excellent yields. It was found that microwave heating over 180 °C was necessary for ring closure of the acyclic intermediates possessing an electron-withdrawing group. 相似文献
123.
Gaofeng ZONG 《Frontiers of Mathematics in China》2010,5(4):801-809
In this paper, we establish an exact asymptotic formula for the finite-time ruin probability of a nonstandard compound renewal
risk model with constant force of interest in which claims arrive in groups, their sizes in one group are identically distributed
but negatively dependent, and the inter-arrival times between groups are negatively dependent too. 相似文献
124.
125.
在溶剂热条件下,合成了 2 个基于 V 型辅助配体(bipmo、bppmo)的镉配位聚合物{[Cd(bipmo)(NDC)]·1.75H2O}n (1)和{[Cd(bppmo)(NDC)(H2O)]·H2O}n (2),其中 H2NDC=2,6 萘二羧酸,bipmo=双(4 (1H 咪唑 1 基)苯基)甲酮,bppmo=双(4 (吡啶 4 基)苯基)甲酮。利用单晶X射线衍射、键价和分析、红外光谱和元素分析对其结构进行了表征。研究发现,配合物1具有二重互穿的{63}拓扑结构。配合物2同样是 3 连接的{63}拓扑,却存在三重穿插结构。分析表明,V型配体对最终结构的形成有很大影响。此外,对配合物1和2的发光性质也进行了详细研究。 相似文献
126.
127.
Gaofeng Wang Jiechang Hou Jicheng Hu Bing-Zhong Wang 《International Journal of Infrared and Millimeter Waves》2002,23(2):267-274
Simple formulae for calculating circuit parameters of metal-insulator-semiconductor (MIS) interconnects are obtained using the energy based approach. The energy based approach results in a new consistent equivalent transmission line model that exhibits power equivalence to the original MIS waveguide. This new model has solid physical foundation and is valid for a wide range of frequency and impurity doping rate. At the quasi-TEM limit, this new model reduces to the existing model that was obtained using the contour integral based approach. 相似文献
128.
Sudden jumps in the crack tip velocity were revealed by numerical simulation (in both continuum/cohesive element and molecular dynamics approaches) and experiments for rapid shear cracking. The cracking velocity may accelerate from a sub-Rayleigh speed to the intersonic range, or from an intersonic speed to a higher one, when the reflected impact wave reloads the crack tip. On the other hand, the cracking velocity may decelerate from an intersonic speed to a lower one or recede to the sub-Rayleigh range when the fracture driving force declines. The velocity change encountered during intersonic cracking plays a different role from that in the acceleration or deceleration of a subsonic crack. A crack propagating at an intersonic speed would leave a shear wave trailing behind. When the crack decelerates or accelerates, the effect of the trailing wave will lead to a transition period from one steady-state solution of crack tip singularity to another. This investigation aims at quantifying these processes. The full field solution of an intersonic mode II crack whose speed changed suddenly from one velocity (intersonic or subsonic) to another (intersonic or subsonic) is given in closed form. The solution is facilitated via superposing a series of propagating crack problems that are loaded by dislocations to seal the unwanted crack-face sliding or by concentrated forces moving at various speeds to negate the crack-face traction. In contrast to the subsonic solution, the results in the intersonic case indicate that the elastic fields around the crack tip depend on the deceleration or acceleration history that is traced back over a long time. Singularity matching dictates the jump that may actually take place. 相似文献
129.
本文通过高温固相反应成功制备了Sr3ZnNb2O9∶0.3Eu3+,xNa+(x=0,0.1,0.2,0.3,0.4,0.5)系列荧光粉。X射线衍射分析和精修结果表明,Eu3+和Na+成功掺杂到Sr3ZnNb2O9基质中,并部分取代了Zn2+。采用扫描电子显微镜测试了样品的微观形貌和元素分布。光谱特性和热稳定性研究表明,Na+的最佳掺杂浓度为x=0.2,Na+的引入提高了Sr3ZnNb2O9∶0.3Eu3+荧光粉的热稳定性,活化能为0.163 eV。计算出Sr3ZnNb2O9∶0.3Eu3+, 0.2Na+样品... 相似文献
130.
1前言目前广泛使用的制冷工质R12,由于其对臭氧层的破坏,从96年起已被禁用。因为现有成熟的优良制冷工质R22具有很低的ODP值,而被公认为是近期良好过渡替代工质,并已证实十分适用于我国航运业。作者在制冷工质试验装置上,基于采用变频调速技术以R22替代R12模拟运行,并同时采用自制的喷射液冷阀,进行压缩机降低排气温度的热力学特性试验,解决替代过程中压缩机排温过高,润滑恶化的困难,为工程实际应用创造了条件。2喷射液冷阀的结构及工作原理本试验是采用喷射液冷阀,进行其热力学特性试验。喷射液冷阀在结构上与热力膨胀阀很相… 相似文献