High performance Al0.35Ga0.65As/GaAs HBT's

AlGaAs emitter heterojunction bipolar transistors (HBTs) are demonstrated to have excellent dc and RF properties comparable to InGaP/GaAs HBTs by increasing the Al composition. Al_0.35Ga _0.65As/GaAs HBTs exhibit very high dc current gain at all bias levels, exceeding 140 at 25 A/cm² and reaching a maximum of 210 at 26 kA/cm² (L=1.4 μm×3 μm, R_sb=330 Ω/□). The temperature dependence of the peak dc current gain is also significantly improved by increasing the AlGaAs mole fraction of the emitter. Device analysis suggests that a larger emitter energy gap contributes to the improved device performance by both lowering space charge recombination and increasing the barrier to reverse hole injection     

Microbiological quality and safety of quahog clams, Mercenaria mercenaria, during refrigeration and at elevated storage temperatures

The effects of storage temperatures and times on the microbiological quality and safety of hard-shelled quahog clams (Mercenaria mercenaria) were examined. Samples were stored at four different incubation temperatures (3.3, 7.2, 10.0, and 12.8 degrees C) for a period of 3 weeks, following their harvest from summer growing waters (> or = 27 degrees C) and winter waters (< or = 4 degrees C). Clams were analyzed for two naturally occurring pathogens, Vibrio parahaemolyticus and Vibrio vulnificus. During the summer, V. parahaemolyticus was isolated from 56% of the stored samples, with the highest concentration, 6,100/g, occurring on day 12 at 12.8 degrees C. Also, during the summer, V. vulnificus was isolated from 11% of the stored samples, with the highest concentration of 1,500/g occurring on day 15 at 7.2 degrees C. No Vibrio spp. were detected during the winter. During summer storage, aerobic mesophilic counts on plate count agar (PCA) containing 2% NaCl ranged from 10(4) to 10(8) CFU/g, and during storage of the winter samples, aerobic mesophilic PCA (with added NaCl) counts ranged from <100 to 10(4) CFU/g. Comparatively, summer storage mesophilic counts on PCA containing no added NaCl ranged from <100 to 10(5) CFU/g, and for the winter samples the range was <100 to 10(2) CFU/g. Coliform and fecal coliform counts ranged from <0.3 to 61.1/g and <0.3 to 24.4/g, respectively. There was no statistical correlation between the length of storage or the temperature of incubation and the presence of V. parahaemolyticus, V. vulnificus, coliforms, or fecal coliforms. However, storage time and incubation temperature affected the PCA counts (P < or = 0.05) in quahog clams.     

Exhaust Particle Size Distribution Measurements at the Tuscarora Mountain Tunnel

On-road particle size distributions were measured at the Tuscarora Mountain tunnel on the Pennsylvania Turnpike in May 1999. The data were obtained using a scanning mobility particle sizer. The nucleation modes of the size distributions contained most of the particles on a number concentration basis and exhibited peak diameters ranging from 11 to 17 nm. This observation is consistent with previous calculations and measurements, indicating that significant numbers of ultrafine aerosol particles can be expected in close proximity to busy motorways. The experiment provided 4 case studies for which the tunnel inlet data could be used to correct data obtained at the outlet, allowing for estimates of particle production within the tunnel. Exhaust particle production rates per vehicle kilometer were estimated; the results are presented with the caveat that the measurements were affected by ambient dilution. The 4 case study nucleation mode sizes varied inversely with ambient temperature. The light-duty vehicle contributions to the ultrafine particle distributions were apparently dominated by the heavy-duty vehicle contributions.     

Oxidation resistance of decorative (Ti,Mg)N coatings deposited by hybrid cathodic arc evaporation-magnetron sputtering process

Titanium-magnesium nitride coatings (Ti,Mg)N were deposited on steels and silicon substrates by hybrid reactive arc evaporation-magnetron sputtering process from cathodic Ti and sputter Mg targets in an argon/nitrogen gas mixture. X-ray diffraction analyses (XRD) of as-deposited coatings with various Mg/Ti atomic ratios gave evidence of a fcc TiN-like structure strongly oriented in the [111] direction. The TiN lattice parameter increases with the addition of Mg resulting from the substitution of Ti atoms by Mg ones. Optical investigations by spectrophotometry revealed that Mg addition to TiN leads to a change in colour from golden through coppery and violet to grey. Nanoindentation measurements showed that increasing Mg content does not alter the hardness of coatings. As-deposited films were annealed in air from 450 to 750 °C with a 100 °C step. XRD and Raman analyses revealed the formation of rutile TiO₂ and MgTiO₃ phases. Secondary neutral mass spectrometry measurements were performed to study the elemental depth profiles after air annealing. A diffusion of Mg atoms towards the film surface was evidenced above 650 °C, leading to the formation of the MgTiO₃ phase. However, thermogravimetric measurements showed that this oxide phase did not protect the films against high temperature oxidation. On the contrary, below 650 °C Mg affords to TiN a beneficial protective effect, able to reduce the oxidation kinetics by half.     

Effect of the deposition process on the composition and structure of sputtered lanthanum cuprate films

La-Cu-O thin films were deposited by two different methods. As a first process, the films were deposited by co-sputtering of metallic copper and lanthanum targets in various Ar-O₂ reactive mixtures. The oxygen flow rate introduced into the deposition chamber was optimised to fully oxidise the sputtered atoms. The atomic ratio La/Cu in the film was adjusted by the variation of the pulsed DC current applied to the copper target. Within this process, the experimental window range for successful synthesis of films with the expected La/Cu ratio was found to be very narrow. The second process, namely, reactive sputtering of a composite La-Cu target, showed easier control of the film composition. The La/Cu atomic ratio was strongly dependent on the composite target that evolved during the use of target. Whatever the deposition process, an annealing step in air was necessary to obtain the crystalline La₂CuO₄ phase. To find the temperature of crystallisation, a sample was annealed successively from 100 to 600 °C by step of 100 °C, plus an annealing at 800 °C. The crystalline nature of the sample was ensured by X-ray diffraction measurement after each annealing.     

步入DirectX的圣殿（上）

本文讲解TDirectX的基本概念，并介绍了托管DirectX类库的构成。此外还介绍了3D编程中的一些基本概念，进而对前一篇文章给出的示例代码进行了充实。     

The public and private problem of euthanasia. A comparison of The Netherlands and the United States

We present a 24 case-study of intact ossicular chain cholesteatomas among 158 first-look procedures performed between august 93 and april 96 (15%). The eradication of the disease was achieved via canal wall up technique without disrupting the ossicular chain in 20 cases (84%). Technical procedures are described. This new surgical attitude has enabled us to achieve better functional results. Second look procedures have not been necessary in all cases and occurrence of residuals has not been increased on a 14 months follow up period (2 cases out of 24 patients: 8%). Therefore, a longer follow up period is necessary to validate the results obtained using this recent concept.