Error correction techniques for high-performance differential A/Dconverters

Error correction techniques that overcome several error mechanism that can affect the accuracy of charge-redistribution analog-to-digital converters (ADCs) are described. A correction circuit and a self-calibration algorithm are used to improve the common-mode rejection of the differential ADC. A modified technique is used to self-calibrate the capacitor ratio errors and obtain higher linearity. The residual error of the ADC due to capacitor voltage dependence is minimized using a quadratic voltage coefficient (QVC) self-calibration scheme. A dual-comparator topology with digital error correction circuitry is used to avoid errors due to comparator threshold hysteresis. A fully differential charge-redistribution ADC implemented with these techniques was fabricated in a 5-V 1-μm CMOS process using metal-to-polysilicide capacitors. The successive-approximation converter achieves 16-b accuracy with more than 90 dB of common-mode rejection while converting at a 200-kHz rate     

Effects of the hob cutter regrinding and setting on ZE-type worm gear manufacture

A mathematical model for a ZE-type worm gear set is proposed. The characteristics of a ZE-type worm gear cut with an oversize hob cutter are investigated. Bearing contacts and kinematic errors in the ZE-type worm gear set due to regrinding of oversize hob cutters are also studied. Bearing contacts in the worm gear set were obtained using surface separation topology techniques and tooth contact analysis. It was found that bearing contact locations are sensitive to the hob cutter setting angle and setting distance. Kinematic error minimization was achieved modifying worm normal tooth angle. By applying the optimization theorem and computer simulation programs, optimum hob cutter settings and worm normal tooth angle were determined to improve bearing contact location and minimize the worm gear set kinematic errors. A numerical example is given to demonstrate the results of the investigation. The developed computer-aided manufacture program has successfully been applied to the worm gear manufacture and hob cutter design.     

Structural and optical studies on sol–gel derived ZnO thin films by excimer laser annealing

High-quality polycrystalline ZnO thin films were deposited onto alkali-free glasses at a temperature of 300°C in air ambience by combining sol–gel spin coating and KrF excimer laser annealing. The effects of laser irradiation energy density on the crystallization, microstructure, surface morphology, and optical transmittance of as-prepared ZnO thin films were investigated and compared to the results of thermally annealed ZnO thin films. The crystallinity level and average crystallite size of laser annealed ZnO thin films increased as laser energy density increased. The crystallinity levels and average crystallite size of excimer laser annealed (ELA) thin films were greater than those of the thermally annealed (TA) thin films. However, laser annealed thin films had abnormal grain growth when irradiation energy density was 175 mJ/cm². Experimental results indicated that the optimum irradiation energy density for excimer laser annealing of ZnO sol–gel films was 150 mJ/cm². The ELA 150 thin films had a dense microstructure, an RMS roughness value of 5.30 nm, and an optical band gap of 3.38 eV, close to the band gap of a ZnO crystal (3.4 eV).     

Synthesis and properties of liquid crystalline polymers with low Tm and broad mesophase temperature ranges

Novel thermotropic liquid crystalline polymers (TLCP) with improved processability and lower cost than Vectra^® were synthesized by esterification and melt polycondensation. Aromatic compounds, 6-hydroxyl-2-naphthoic acid, 2,6-naphthlenedicarboxylic acid, 4-hydroxybenzoic acid or terephthalic acid, were used as polycyclic stiff rod-like mesogenic groups. Aliphatic diol, ethylene glycol or 1,4-butanediol was used as flexible spacers to form semi-flexible main chain TLCP. The LCP products were characterized by FT-IR and ¹H NMR to identify their chemical structures. The analytical results showed the polymers had inherent viscosities of 0.35-0.54 dL/g. The crank shaft structure of the naphthalene decreased the melting temperature (T_m) of LCP (181-272 °C) to facilitate extrusion or injection molding, and maintained relatively high heat deflection temperature for high performance engineering plastics applications. The 5% weight loss decomposition temperatures were above 400 °C. LCPs have good solvent resistance and low hygroscopicity. The SEM morphology showed strong orientation on the surface in the flow direction and many micro-fibers structure in a sectional drawing. The optical textures of polymers observed by POM revealed a strong birefringence in the melts and indicated that they form nematic mesophase. All polymers have broad mesophase temperature (ΔT_meso) ranges.     

Synthesis of copolyimide containing fluorine and naphthalene with synergizing effect on dielectric constants

Summary Novel copolyimides containing fluorine and naphthalene groups were synthesized from 4,4-(Hexafluoroisopropylidene)diphthalic dianhydride (6FDA) and two diamines, 1,4-Bis(4-amino-2-trifluoromethylphenoxy) benzene (BATB) and 2,7-bis(4-aminophenoxy) naphthalene (BAPN) in various ratios via thermal imidization. The introduction of fluorinated groups into dianhydride improved transparency of copolyimides. All films are transparent. The structures and physical properties of copolyimides were characterized by FT-IR, DSC, TGA, XRD, UV-Vis and dielectric analyzer to study the effect of diamines on the physical properties of modified polymers. Copolyimides containing a higher percentage of BAPN have higher glass transition temperature and are less soluble in organic solvents. The copolyimide consisting of 50/50 mole% of BATB/BAPN has the lowest dielectric constant (2.60 at 1 KHz), reflecting the synergizing effect from both diamines.     

Fe–Nx/C electrocatalysts synthesized by pyrolysis of Fe(II)–2,3,5,6-tetra(2-pyridyl)pyrazine complex for PEM fuel cell oxygen reduction reaction

2,3,5,6-Tetra(2-pyridyl)pyrazine (TPPZ) was employed as a ligand to prepare an iron(II) complex (Fe–TPPZ) that served as a precursor to synthesize carbon-supported catalysts (Fe–N_x/C) through heat-treatment at 600, 700, 800 and 900 °C under N₂ atmosphere. Both the structure and composition of the synthesized Fe–N_x/C were analyzed by X-ray diffraction and energy-dispersive X-ray microanalysis, respectively. The rotating disk and ring-disk electrode measurements showed that these catalysts have strong ORR activity with an overall 4-electron transfer process through a (2 + 2)-electron transfer mechanism, which was assigned to the catalytic function of the Fe–N_x center. A study on the heat-treatment temperature on the ORR activity showed that 800 °C is the optimal temperature for the synthesized catalysts. Furthermore, the effect of both catalyst and Nafion^® ionomer loadings in the catalyst layer on the corresponding ORR activity was also investigated. The kinetic parameters such as the chemical reaction rate between O₂ and Fe–N_x/C (adduct formation reaction), the rate constant for the rate-determining step (RDS), and the electron numbers in the ORR, were obtained. The methanol tolerance of the catalyst was also tested. To validate the ORR activity, a membrane electrode assembly in which the cathode catalyst layer contained Fe–N_x/C was constructed and tested in a real fuel cell. The results obtained are encouraging when compared with similar non-noble catalysts.     

Equation‐oriented simulation and optimization of process flowsheets incorporating detailed spiral‐wound multistream heat exchanger models

Multiple chemical processes rely on multistream heat exchangers (MHEXs) for heat integration, particularly at cryogenic temperatures. Owing to their geometric complexity, the detailed design of MHEXs is typically iterative: the exchanger geometric parameters are selected to match process specifications resulting from a flowsheet optimization step; then, the flowsheet is reoptimized with the predictions of the MHEX model, and these steps are repeated until a convergence criterion is met. This paper presents a novel framework that allows—for the first time, to our knowledge—for the simultaneous optimization of the process flowsheet and the detailed MHEX design. Focusing on spiral‐wound MHEXs, we develop an equation‐oriented exchanger model using industry‐accepted heat transfer and pressure drop correlations for single‐phase and multiphase streams. We embed this model in our previously developed pseudo‐transient equation‐oriented process simulation and optimization framework. We demonstrate our approach on an industrial case study, the PRICO^® natural gas liquefaction process. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3778–3789, 2017     

Synthesis and characterization of sol-gel derived gallium-doped zinc oxide thin films

Gallium-doped ZnO (GZO) semiconductor thin films were prepared by a sol-gel spin coating process. The effects of Ga dopant concentrations on the microstructure, electrical resistivity, optical properties, and photoluminescence (PL) were studied. XRD results showed that all the as-prepared GZO films had a wurtzite phase and a preferred orientation along the [0 0 2] direction. ZnO thin films doped with Ga had lower electrical resistivity, lower RMS roughness, and improved optical transmittance in the visible region. The lowest average electrical resistivity value, 2.8 × 10² Ω cm, was achieved in the ZnO thin films doped with 2% Ga, which exhibited an average transmittance of 91.5%. This study also found that the optical band gap of Ga-doped films was 3.25 eV, slightly higher than that of undoped samples (3.23 eV), and the PL spectra of GZO films showed strong violet-light emission centers at about 2.86 eV (the corresponding wavelength of which is about 434 nm).     

Surface aerosol radiative forcing derived from collocated ground-based radiometric observations during PRIDE, SAFARI, and ACE-Asia

An approach is presented to estimate the surface aerosol radiative forcing by use of collocated cloud-screened narrowband spectral and thermal-offset-corrected radiometric observations during the Puerto Rico Dust Experiment 2000, South African Fire Atmosphere Research Initiative (SAFARI) 2000, and Aerosol Characterization Experiment-Asia 2001. We show that aerosol optical depths from the Multiple-Filter Rotating Shadowband Radiometer data match closely with those from the Cimel sunphotometer data for two SAFARI-2000 dates. The observed aerosol radiative forcings were interpreted on the basis of results from the Fu-Liou radiative transfer model, and, in some cases, cross checked with satellite-derived forcing parameters. Values of the aerosol radiative forcing and forcing efficiency, which quantifies the sensitivity of the surface fluxes to the aerosol optical depth, were generated on the basis of a differential technique for all three campaigns, and their scientific significance is discussed.