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91.
A new aromatic diamine monomer, N-(4-(9H-carbazol-9-yl)phenyl)-3,5-diaminobenzamide, was successfully prepared in four steps using carbazole as starting material and polymerized with three aromatic tetracaboxylic acid dianhydrides via the conventional two-stage synthesis including the polyaddition and chemical cyclodehydration to produce a series of the aromatic polyimides. The polyimides were characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy, differential scanning calorimetric (DSC) and thermo gravimetric analysis (TGA) analysis. The polyimides with inherent viscosities in the range of 0.38–0.46 dL/g showed excellent solubility in various solvents such as N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), pyridine and dioxane. DSC showed the glass transition temperatures (Tg) in the range of 277–288 °C. TGA showed that all polymers were stable, with 10% weights loss recorded above 524 °C in air atmosphere. Preliminary tests on films of the polyimides indicate that the materials are brittle. 相似文献
92.
93.
In this paper, a new analytical method for vibration analysis of a cracked simply supported beam is investigated. By considering a nonlinear model for the fatigue crack, the governing equation of motion of the cracked beam is solved using perturbation method. The solution of the governing equation reveals the superhaxmonics of the fundamental frequency due to the nonlinear effects in the dynamic response of the cracked beam. Furthermore, considering such a solution, an explicit expression is also derived for the system damping changes due to the changes in the crack parameters, geometric dimensions and mechanical properties of the cracked beam. The results show that an increase in the crack severity and approaching the crack location to the middle of the beam increase the system damping. In order to validate the results, changes in the fundamental frequency ratios against the fatigue crack severities are compared with those of experimental results available in the literature. Also, a comparison is made between the free response of the cracked beam with a given crack depth and location obtained by the proposed analytical solution and that of the numerical method. The results of the proposed method agree with the experimental and numerical results. 相似文献
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96.
Summary Let X ∼ Np(μ,σ2Ip) and let s/σ2 ∼ χ
n
2
, independent ofX, where μ and σ2 are unknown. This paper considers the estimation of μ (by δ) relative to a convex loss function given by (δ−μ)′[(1−α)Ip/σ2+αQ](δ−μ)/[(1−α)p/σ2+α tr (Q)], whereQ is a knownp×p diagonal matrix and 0≦α≦1. Two classes of minimax estimators are obtained for μ whenp≦3; the first is a new result and the second is a generalization of a result of Strawderman (1973,Ann. Statist.,1, 1189–1194). A proper Bayes estimator is also obtained which is shown to satisfy the conditions of the second class of minimax
estimators. The paper concludes by discussing the estimation of μ relative to another convex loss function.
This work was supported by the Army, Navy and Air Force under Office of Naval Research Contract No. N00014-80-C-0093. Reproduction
in whole or in part is permitted for any purpose of the United States Government. 相似文献
97.
A.H.N. Mousa 《Journal of fluorine chemistry》1976,8(6):467-469
The critical properties of hexafluorobenzene were measured, and compared with the most reliable values reported in literature. The vapour pressure was determined from 226°C to the critical point and correlated by an equation of state. 相似文献
98.
99.
100.
Al-Smadi Mousa El-khateeb Mohammad Moriyama Hayato Yoshida Yukihiro Kitagawa Hiroshi 《Transition Metal Chemistry》2021,46(6):465-470
Transition Metal Chemistry - Reactions of (dppe)NiCl2 (dppe?=?1,2-bis(diphenylphosphino)ethane) with chlorido-substituted phenyl thiolates (RClS?) produced the corresponding... 相似文献