极限量规设计中的公差原则适用问题

在极限量规设计中应遵循两个原则,一是泰勒原则,二是内缩原则。由于设计时未考虑公差原则的影响,如果用相同方法设计出来的极限量规来检测遵循不同公差原则的孔和轴,就可能发生误判报废现象。例如图１所示三种不同标注方式的轴,在不考虑公差原则时设计出来的极限量规是相同的,用于检测图１ｂ、ｃ所示轴时就会出现误判报废。图１　三种不同标注方式的轴在图１ａ中,尺寸公差和形位公差之间遵循包容原则。包容原则是使实际要素（单一的或关联的）处处不得超越由其最大实体尺寸确定的理想形状的包容面,同时使其局部实际尺寸不得超越其最…     

Electrical conductivity of poly(2-ethynylthiophene) and poly(2-ethynylfuran) doped with electron acceptors

The electrical conductivity of poly(2-ethynylthiophene) (P2ET) and poly(2-ethynylfuran) (P2EF) doped with electron acceptors such as iodine, bromine, and ferric chloride was investigated. The maximum electrical conductivities of P2ET and P2EF doped with iodine were 3 × 10^?4 Ω^?1 cm^?1 and 5 × 10^?3 Ω^?1 cm^?1, respectively. The electrical conductivity was nearly independent with increasing molecular weight. The spectral measurements such as UV-visible, infrared, electron paramagnetic resonance, thermogravimetric analysis, and X-ray diffraction were also carried out.     

Fire-resistant polymer nanocomposites based on metal oxides and hydroxides

The fire-resistant properties of polymer nanocomposites based on polyvinylchloride and nanoparticles of metal compounds were studied. The investigation of nanoparticles and the determination of their sizes were carried out using scanning electron microscopy (SEM). The size of the nanoparticles of magnesium hydroxide was equal to 50–100 nm; iron oxide, 25–50 nm; and zinc oxide, 50–100 nm. Another type of fire retardant used in the work was aminotrimethylenephosphonic acid ammonium salt (AMPAS); however, the total content of the different additives did not exceed 15%. The nanocomposites obtained were investigated by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC); in addition, the parameters of the oxygen index (OI) and carbon residue (CR) were measured for them. The lowest combustibility was exemplified by the composite material containing as the fire retardants a mixture of AMPAS and ZnO.     

Efficient elections in chordal ring networks

We study the message complexity of the problem of distributively electing a leader in chordal rings. Such networks consist of a basic ring with additional links, the extreme cases being the oriented ring and the complete graph with a full sense of direction. We present a general election algorithm for these networks, and prove its optimality. As a corollary, we show thatO(logn) chords at each processor suffice to obtain an algorithm that uses at mostO(n) messages; this improves and extends a previous work, where an algorithm, also usingO(n) messages, was suggested for the case where alln-1 chords exist (the oriented complete network).     

Functional conservation among members of the Salmonella typhimurium InvA family of proteins

InvA, which is essential for Salmonella spp. to enter cultured epithelial cells, is a member of a family of proteins involved in either flagellar biosynthesis or the secretion of virulence determinants by a number of plant and mammalian pathogens. The predicted overall secondary structures of these proteins show significant similarities and indicate a modular construction with a hydrophobic amino-terminal half, consisting of six to eight potential transmembrane domains, and a hydrophilic carboxy terminus which is predicted to reside in the cytoplasm. These proteins can be aligned over the entire length of their polypeptide sequences, with the highest degree of homology found in the amino terminus and clusters of conserved residues in the carboxy terminus. We examined the functional conservation among members of this protein family by assessing the ability of MxiA of Shigella flexneri and LcrD of Yersinia pseudotuberculosis to restore invasiveness to an invA mutant of Salmonella typhimurium. We found that MxiA was able to complement the entry defect of the invA mutant strain of S. typhimurium. In contrast, LcrD failed to complement the same strain. However, a plasmid carrying a gene encoding a chimeric protein consisting of the amino terminus of LcrD and the carboxy terminus of InvA complemented the defect of the Salmonella invA mutant. These results indicate that the secretory systems in which these proteins participate are functionally similar and that the Salmonella and Shigella systems are very closely related. These data also suggest that determinants of specificity may be located at the carboxy termini of these proteins.     

Calculus of the modulus of continuity for nonconcave functions and applications

For a continuous nonconcave function on [a,b] we prove that the modulus of continuity of f satisfies ω(f;δ)=max{|f(a+δ)−f(a)|; |f(b−δ)−f(b)|}, for δ sufficiently small. Some applications in approximation theory are given.     

Synthesis and characterization of TiO2/poly(methyl methacrylate) nanocomposites via surface thiol-lactam initiated radical polymerization

An approach to the surface modification of TiO2 nanoparticles was described based on the thiol functionalization of TiO2 followed by thiol-lactam initiated radical polymerization (TLIRP) of methyl methacrylate (MMA). FT-IR, XRD and XPS analyses confirmed the grafting of the polymer on the TiO2 surface. TGA analysis revealed superior thermal stability of PMMA-g-TiO2 compared with PMMA. TEM measurements and time-dependent phase monitoring suggested much higher colloidal stability of PMMA-g-TiO2 than TiO2 in toluene. The controlled nature of the TLIRP of MMA from the surface of TiO2 was determined by GPC analysis.     

Effect of hydrogen on the stability of isovalently doped ZnSe crystals

We present thermodynamic analysis of the relative stability of V _ZnZn_iX_Se defect complexes (radioluminescence centers in ZnSe(X_Se) crystals) toward reactions with hydrogen in the temperature range 300 to 1300 K and analysis of reactions between the surface of ZnSe(X_Se) crystals and air that lead to hydrogen diffusion into the bulk of the crystal. The results, supported by experimental data, indicate that the stability of the complexes and the chemical stability of ZnSe(X_Se) crystals toward reactions with hydrogen increase in the order O_Se < S_Se < Te_Se.     

Modulating self-assembly of amyloidogenic proteins as a therapeutic approach for neurodegenerative diseases: strategies and mechanisms

Abnormal protein assembly causes multiple devastating disorders in the central nervous system (CNS), such as Alzheimer's, Parkinson's, Huntington's, and prion diseases. Due to the now extended human lifespan, these diseases have been increasing in prevalence, resulting in major public health problems and the associated financial difficulties worldwide. The wayward proteins that lead to disease self-associate into neurotoxic oligomers and go on to form fibrillar polymers through multiple pathways. Thus, a range of possible targets for pharmacotherapeutic intervention exists along these pathways. Many compounds have shown different levels of effectiveness in inhibiting aberrant self-assembly, dissociating existing aggregates, protecting cells against neurotoxic insults, and in some cases ameliorating disease symptoms in vivo, yet achieving efficient, disease-modifying therapy in humans remains a major unattained goal. To a large degree, this is because the mechanisms of action for these drugs are essentially unknown. For successful design of new effective drugs, it is crucial to elucidate the mechanistic details of their action, including the actual target(s) along the protein aggregation pathways, how the compounds modulate these pathways, and their effect at the cellular, tissue, organ, and organism level. Here, the current knowledge of major mechanisms by which some of the more extensively explored drug candidates work are discussed. In particular, we focus on three prominent strategies: 1)?stabilizing the native fold of amyloidogenic proteins, 2)?accelerating the aggregation pathways towards the fibrillar endpoint thereby reducing accumulation of toxic oligomers, and 3)?modulating the assembly process towards nontoxic oligomers/aggregates. The merit of each strategy is assessed, and the key points to consider when analyzing the efficacy of possible drug candidates and their mechanism of action are discussed.