Polyacrylamide gels formed with Cr+3 ion and Cr(acetate)3: Thermodynamically and kinetically controlled crosslinking reactions

The gelation of buffered aqueous Cr⁺³/polyacrylamide (PAAm) solutions has been studied over the pH range 2-11. With Cr⁺³ supplied as Cr(NO₃)₃ and a 7.5% hydrolyzed PAAm the upper limit for gelation was about pH 7, but Cr(acetate)₃/PAAm solutions gelled as high as pH 9. Evidence is presented that the formation of unreactive colloidal Cr(OH)₃(H₂O)₃, rather than polymer crosslinking, is favored thermodynamically for Cr⁺³/PAAm solutions at pH > 7. The crosslinks formed in Cr(acetate)₃/PAAm solutions at pH 7–9 are deduced to be kinetically controlled products. Independent evidence for the kinetic stability of Cr⁺³/PAAm gels above pH 7 has also been obtained. Other observations relevant to the mechanism of crosslinking of PAAm with Cr(acetate)₃ are described.     

Envelope detection and correction of SSB

Envelope detection is significantly less complex to implement and use than coherent detection if applied to SSB results in gross distortion unless the carrier component is very large. A method of correcting this distortion is discussed and a variant which achieves partial correction with some hardware simplification is investigated. Computer simulation shows that a reduction of 15 dB in second-harmonic distortion can be achieved, and this is confirmed by tests on a prototype system.     

Four-membered ring cyclic ketene –O,O–, –O,S–, –O,N–, –S,S–, –S,N–, and –N,N–acetals and their corresponding cations: a computational study

A systematic computational study of four-membered cyclic ketene –O,O–, –O,S–, –O,N–, –S,N– and –N,N-acetals as well as their protonated analogs have been performed at the second order M?ller Plesset level with a polarized triple zeta basis set. The main purpose of this study was to make predictions about the nucleophilicity of these systems and the variations in nucleophilicity with the hetero atoms. Our calculations suggest that all six target molecules are good nucleophiles, and that the N,N analog is the strongest and the S,S analog the weakest nucleophile. Our results include molecular geometries, bond lengths, proton affinities, vibrational frequencies, and calculated charges.     

“最低”有多低?——FPGA技术满足移动产品低功耗要求

电池供电和功率敏感应用的大幅增长（更遑论当今快速掘起中的高能效设计热潮），催生出了一股全球对低功耗半导体技术的强劲需求。的确，它确是当前业界发展的推动因素之一，但同时也为系统工程师带来了莫大的压力。这些工程师面临到对整体系统功耗有更严格限制之更严苛的设计规范与标准，而对于便携设备设计人员而言，即使只多出区区数毫瓦甚至微瓦的功耗，也可能导致重大的功率预算问题。     

Embedding data in DSB-AM using zero-synchronous modulation

The approximations involved in implementing a spectrally efficient hybrid modulation scheme known as zero-synchronous modulation (ZSM) are discussed. An experimental transmitter/receiver system for generating real ZSM signals is described and some results presented.     

Lasers in chemistry

This article reviews the programme of laser research and development being carried out in the Chemistry Division of the US Naval Research Laboratory. It discusses its impact on the overall laser field, particularly in research on chemical lasers, dynamics of laser processes, and laser-promoted chemical reactions. The future direction of laser research and laser applications in the Chemistry Division is outlined and a few specific experiments either under way or planned are described.