Asymptotically linear elliptic boundary value problems

We study semilinear problems in which the nonlinear term has different asymptotic behavior at ± with the limits (1.2) spanning a finite number of eigenvalues of the linear operator.Research supported in part by an NSF grant.     

Natural radioactivity traces in South-Brazilian cereal flours by gamma-ray spectrometry

Summary Cereal flours are the major component of the Brazilian diet and are also important exportation products. Radioactivity concentrations of ²³² Th, ²²⁶Ra, ⁴⁰K and ¹³⁷Cs were determined in commercial samples of South-Brazilian cereal flours (soy, wheat, corn, manioc, rye and oat flour) to verify the radiological security of these foodstuffs. The measurements were carried out by gamma-ray spectrometry using a 66% relative efficiency HPGe detector. The ⁴⁰K flour activities, at 95% of confidence level were in: soy 474±3 Bq ^. kg^-1; corn 30.0±0.3 Bq ^. kg^-1; rye 94±1 Bq ^. kg^-1; manioc 67±1 Bq ^. kg^-1; oat 76±1 Bq ^. kg^-1 and wheat 36.2±0.4 Bq ^. kg^-1. The lower limit of detection for ⁴⁰K ranged from 0.54 to 1.43 Bq ^. kg^-1. The ¹³⁷Cs activities in flour samples were: soy ￡0.07 Bq ^. kg^-1, corn ￡0.01 Bq ^. kg^-1, oat ￡0.03 Bq ^. kg^-1 and in wheat, manioc and rye ￡0.02 Bq ^. kg^-1. The highest concentrations levels of ²³² Th and ²²⁶Ra were 0.69±0.04 Bq ^. kg^-1 and 0.44±0.03 Bq ^. kg^-1, respectively, in soy flour.     

MoP-NC纳米球负载Pt纳米粒子用于高效甲醇电解

实现绿色甲醇电解制氢需要高效的双功能催化剂。本文采用热处理结合乙二醇还原法成功制备了MoP-NC纳米球负载的超细Pt纳米粒子(平均粒径为2.53 nm)复合催化剂(Pt/MoP-NC)用于高效甲醇电解制氢。MoP-NC纳米球不仅能提高Pt纳米粒子的分散性并且增强Pt的抗中毒能力。电化学测试表明Pt/MoP-NC催化剂在酸性甲醇氧化反应(MOR)和析氢反应(HER)中具有较高的催化性能;其中,MOR的正向扫描峰值电流密度为90.7 mA∙cm⁻²,是商业Pt/C催化剂的3.2倍,在10 mA∙cm⁻²的电流密度下,HER的过电位低至30 mV,与商业Pt/C接近。由Pt/MoP-NC||Pt/MoP-NC组装的两电极电解槽驱动10 mA∙cm⁻²的电流密度仅需要0.67 V的电压,比相同条件下电解水的电压低1.02 V,大大降低了能量输入。Pt/MoP-NC的高催化性能主要来源于Pt活性中心与相邻层状多孔球形结构的MoP-NC载体之间电子效应及配体效应引起的抗一氧化碳中毒能力的提升和含氧物种的容易生成。     

Effect of He pre-plating on third sound in superfluid He

We recently reported observation of third sound resonances in superfluid films of pure ³He on a disk-shaped polished copper substrate. From the observed spectra of resonant modes, the third sound speed, the average superfluid density, and information on dissipation were obtained. The film thicknesses studied were larger than or comparable to the superfluid coherence length, which is near 100 nm. We describe preliminary results from efforts aimed at using third sound in thinner films on a substrate which has been coated with a few atomic layers of ⁴He.