Carbon nanotube thermal interface material for high-brightness light-emitting-diode cooling.

Aligned carbon nanotube (CNT) arrays were fabricated from a multilayer catalyst configuration by microwave plasma-enhanced chemical vapor deposition (PECVD). The effects of the thickness and annealing of the aluminum layer on the CNT synthesis and thermal performance were investigated. An experimental study of thermal resistance across the CNT array interface using the modified ASTM D5470 standard was conducted. It was demonstrated that the CNT-thermal interface material (CNT-TIM) reduced the thermal interfacial resistance significantly compared with the state-of-art commercial TIM. The optimized thermal resistance of the CNT arrays is as low as 7 mm(2) K W(-1). The light performance of high-brightness light-emitting diode (HB-LED) packages using the aligned CNT-TIM was tested. The results indicated that the light output power was greatly improved with the use of the CNT-TIM. The usage of the CNT-TIM can be also extended to other microelectronics thermal management applications.

Vibrational study of phosphate modes in GDP and GTP and their interaction with magnesium in aqueous solution.

Raman and infrared spectra were examined for guanosine 5'-diphosphate (GDP) and guanosine 5'-triphosphate (GTP) in aqueous solution. The vibrational modes were assigned on the basis of isotopic frequency shifts and relative intensities in the Raman and infrared spectra. The observed frequency shifts on 18O isotope labeling made it possible to identify the bands from each phosphate group (alpha, beta, gamma). Frequency shifts were observed as Mg2+ complexes with GDP and GTP. The results suggested that Mg2+ binds to GDP in a bidentate manner to the alpha, beta P[symbol: see text]O bonds and in a tridentate manner to the alpha, beta and gamma P[symbol: see text]O bonds of Mg.GTP. The results indicate that structure of Mg2+ coordinated to GTP in aqueous solution differs somewhat to that found for Mg.ATP.

Raman difference studies of GDP and GTP binding to c-Harvey ras.

The vibrational spectra of phosphate modes for GDP and GTP bound to the c-Harvey p21(ras) protein have been determined using 18O isotope edited Raman difference spectroscopy. A number of the phosphate stretch frequencies are changed upon GDP/GTP binding to ras, and the results are analyzed by ab initio calculations and through the use of empirical relationships that relate bond orders and bond lengths to vibrational frequencies. Bound GDP is found to be strongly stabilized by its interactions, mostly electrostatic, with the active site Mg2+. Bound GTP also interacts with the active site Mg2+ via its beta-phosphate group, as expected on the basis of crystallographic studies of bound GppNp. The angle between the nonbridging P&bondDot;O bonds of the gamma-phosphate of bound GTP increase by about 1-2 degrees compared to its solution value, thus bringing about a geometry that is closer to planar for these bonds as expected for the putative pentacoordinated transition state geometry of the phosphotransfer reaction. Modeling of the interactions at the nucleotide binding site suggests that the water molecule in-line with the P-O bond is positioned to bring about the change in bond angle. Moreover, a weak fifth bond (about 0.03 vu) appears to be formed between it and the gamma-phosphorus atom of bound GTP with a concomitant weakening of the O-P bond between the GDP leaving group and the gamma-phosphorus atom. Hence, an important role of the active site structure appears to be the strategic positioning of this in-line water. These structural results are consistent with a reaction pathway for GTP hydrolysis in ras of synchronous bond formation between the gamma-phosphorus of GTP and the attacking nucleophile and bond breaking between the gamma-phosphorus and the GDP leaving group.