A 40-GHz-bandwidth, 4-bit, time-interleaved A/D converter using photoconductive sampling

GaAs photoconductive switches have been integrated with two parallel 4-bit CMOS analog-to-digital (A/D) converter channels to demonstrate the time-interleaved sampling of wideband signals. The picosecond sampling aperture provided by low-temperature-grown-GaAs metal-semiconductor-metal switches, in combination with low-jitter short-pulse lasers, enables the optically-triggered sampling of electrical signals with tens of gigahertz bandwidth at low to medium resolution. A pair of parallel sampling paths, one for sampling and the second for feedthrough cancellation, generate a differential held signal that is quantized by a low-input capacitance, high-speed flash A/D converter. Dynamic offset averaging is employed to improve converter linearity. An experimental time-interleaved two-channel A/D converter provides about 3.5 effective bits of resolution for inputs up to 40 GHz when tested at an optically-triggered sampling rate of 160 MHz. The sampling rate was limited by the available optical source. Each A/D converter channel operates up to a 640-MHz conversion rate, dissipates 70 mW of power, and occupies an area of 150 /spl mu/m /spl times/ 450 /spl mu/m in a 2.5-V, 0.25-/spl mu/m CMOS technology.     

Studies on Chemical Modification of Porous Silicon‐Based Graded‐Index Optical Microcavities for Improved Stability Under Alkaline Conditions

Preparation of graded‐index optical microcavities based on porous silicon is demonstrated, and chemical modifications for obtaining improved stability under alkaline conditions are studied. Four surface modification methods for stabilizing the samples are examined, and the effects on the optical properties are verified. Two different thermal carbonization treatments resulting in hydrophilic and hydrophobic surfaces are employed. In addition, modification with undecylenic acid is performed on as‐prepared and thermally hydrocarbonized porous silicon surfaces. Stability and sensing capabilities of the modified samples are examined by subjecting them to different concentrations of methylamine and trimethylamine vapors. Vapor induced changes in the reflectance spectra are used for evaluating sensitivity and stability. Sensitivity towards ethanol vapor is also measured in order to compare the sensitivity to a normal organic solvent. The results show that the two carbonization treatments and the undecylenic acid functionalization of the hydrocarbonized surface result in greatly improved stability. In contrast, derivatization of as‐prepared porous silicon with undecylenic acid does not protect the surface sufficiently against oxidation under the highly basic conditions produced by the amine vapors. Surface chemistry is also shown to have a large effect on sensitivity towards the examined vapors. X‐ray photoelectron spectroscopy was used to assess changes in elemental composition of sample surface. The results suggest that thermally promoted addition of undecylenic acid on hydrocarbonized porous silicon is an effective method for producing highly stable optical structures with a carboxyl group functionalization.     

Self-assembly of compounds based on a glycerin skeleton as C-3 building block. Part II

Part I reviewed the properties of alkyl glyceryl ethers and their derivatives in forming self-assemblies such as liquid crystals. Part II covers strategies for the synthesis of several types of artificial ether lipids with a glycerin backbone, such as dicephalic surfactants, gemini surfactants, calamitic bola-amphiphiles, dendrimers, and membrane components of archaebacteria, which have been reported to constitute unique self-assemblies useful as materials for future nanochemistry.     

Morphology change in carbons prepared from pitch-polyvinylchloride mixtures under pressure

Mixtures of fractionated pitches with different molecular weights and polyvinylchloride (PVC) were carbonized at 650°C under a pressure of 30 MPa. The morphology of the carbons obtained was studied under a scanning electron microscope and the optical texture under a polarized light microscope. Gradual changes in morphology are observed, from lump-type to coralloidal through botryoidal with gradual diminution in the size of the primary particles. The content of PVC and also the number-averaged molecular weight were found to govern the morpohology of the resultant carbon.     

Uroporphyrinogen III cosynthetase in liver and blood in the Dubin-Johnson syndrome

The activities of uroporphyrinogen III cosynthetase in blood lysates from five patients with the Dubin-Johnson syndrome (DJS) and four control subjects and in liver homogenates from four patients and four control subjects were determined. No significant difference was found in enzyme activity between the two groups in either blood lysate or liver homogenate. These results indicate that low urinary coproporphyrin III output in the DJS is not due to deficiency of uroporphyrinogen III cosynthetase in the liver and the erythropoietic system.     

Studies on the binding of adenylyl-3', 5'-cytidine to ribonuclease

The interaction of adenylyl-3',5'-cytidine (ApC) with ribonuclease-A (RNAase-A) was studied by steady-state kinetics and ultraviolet difference spectroscopy. X-ray difference Fourier synthesis at 4 A resolution was also used to study the binding of ApC to RNAase-S. Unlike well-studied compounds like uridylyl-3',5'-adenosine, ApC binds in an unique way: (1) the cytidine moiety is bound to the B1 and R1 sites, (2) the adenosine moiety protrudes to the solution and is not fixed spatially and (3) the phosphate group is bound to the non-specific site (the "Po site") previously postulated (Sawada, F. and Irie, M. (1969) J. Biochem. (Tokyo) 66, 415--418) as the binding site for the 5'-phosphate of uridine 2',5'-diphosphate or uridine 3',5'-diphosphate. This conclusion is consistent with that derived for adenylyl-3',5' -4-thiouridine based on CD difference spectroscopy (White, M.D., Keren-Zur, M. and Lapidot, Y. (1977) Nucleic Acid Res. 4, 843--851). The "Po site" is most likely the epsilon-amino group of Lys 66.     

Cholesterol as synthetic building blocks for artificial lipids with characteristic physical,chemical and biological properties

Cholesterol is one of the most widely distributed natural materials and has a unique chemical structure such as a steroid skeleton. Many types of chemical transformations of cholesterol functional groups have been developed. There is an interest in the derivatization of cholesterol and to introduce alkyl branched fatty acids into the molecule. These have found applications in the formulation of cosmetics and toiletries over the past few decades. An extraordinary interesting case is related to cholesteryl esters and their use in gene therapy delivery systems. These results can be attributed to their potential for forming cell‐mimic membranes, because cholesterol is the most important building block of living cell membranes. In terms of organic synthesis, cholesterol is a strategically useful material. A typical case is remote functionalization by chemical reactions or by biocatalysis. In the future, cholesterol should be considered as a key compound, a building block for the construction of artificial lipid‐like membranes by self‐assembly. Also, as cholesterol is one of the members of the fat and oil family, fat and oil chemists should study and develop cholesterol chemistry even further.     

Pt-supported Spherical Mesoporous Silica as a Nanosized Catalyst for Efficient Liquid-Phase Hydrogenation

Mesoporous silica nanospheres with an average diameter of approximately 240 and 600 nm have been used as catalyst supports of active Pt nanoparticles, which were found to be highly efficient for liquid-phase hydrogenation under atmospheric H₂ compared to the micron-sized conventional mesoporous silica because of the suppression of mass-transport limitation toward active sites.     

Dissolution behavior of core structure materials by molten corium in boiling water reactor plants during severe accidents

For the decommissioning of the Fukushima Daiichi Nuclear Power Plant, it is necessary to consider the access route to the fuel debris for its removal, which can be determined by knowing the corruption situation of the core support structure. To predict the damage condition of reactor vessel, dissolution behavior of the core structure material should be understood. In this study, the dissolution behavior of core structure materials (stainless steel) by molten metallic corium (stainless steel + B₄C) originated from control rod and its cladding was investigated. As a result of immersion experiment, it was found that there were two types of dissolution mode in this system: (1) chemical dissolution by eutectic reaction between Fe and B and (2) physical dissolution caused by the grains falling off from solid steel due to infiltration of molten metal. Moreover, on the basis of kinetic analysis, it was considered that the chemical dissolution in this system was slow. Therefore, the dissolution is considered to mainly occur through the mechanism that physical dissolution precedes chemical dissolution.