首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   26篇
  免费   3篇
  国内免费   4篇
系统科学   3篇
现状及发展   5篇
研究方法   3篇
综合类   22篇
  2021年   1篇
  2020年   1篇
  2019年   1篇
  2018年   1篇
  2017年   2篇
  2016年   1篇
  2015年   1篇
  2014年   2篇
  2013年   1篇
  2012年   2篇
  2011年   4篇
  2009年   2篇
  2008年   2篇
  2007年   2篇
  2006年   4篇
  2005年   1篇
  2004年   1篇
  2003年   1篇
  2002年   2篇
  1993年   1篇
排序方式: 共有33条查询结果,搜索用时 31 毫秒
21.
22.
This research work based on the development of 6σ capability indices in the Modern Quality Management Practices in the world level companies is briefly introduced. The process performance index (when a process indicated by control charts is under control) and the percentage of defective products are calculated and compared with kσ. Finally, compared 6σ with 3σ, when process is on nominal (nominal - mean), the process has 1.5σ shift from nominal.  相似文献   
23.
Long-period astronomical forcing of mammal turnover   总被引:1,自引:0,他引:1  
Mammals are among the fastest-radiating groups, being characterized by a mean species lifespan of the order of 2.5 million years (Myr). The basis for this characteristic timescale of origination, extinction and turnover is not well understood. Various studies have invoked climate change to explain mammalian species turnover, but other studies have either challenged or only partly confirmed the climate-turnover hypothesis. Here we use an exceptionally long (24.5-2.5 Myr ago), dense, and well-dated terrestrial record of rodent lineages from central Spain, and show the existence of turnover cycles with periods of 2.4-2.5 and 1.0 Myr. We link these cycles to low-frequency modulations of Milankovitch oscillations, and show that pulses of turnover occur at minima of the 2.37-Myr eccentricity cycle and nodes of the 1.2-Myr obliquity cycle. Because obliquity nodes and eccentricity minima are associated with ice sheet expansion and cooling and affect regional precipitation, we infer that long-period astronomical climate forcing is a major determinant of species turnover in small mammals and probably other groups as well.  相似文献   
24.
Incorporation of selenocysteine (Sec), through recoding of the UGA stop codon, creates a unique class of proteins. Mice lacking tRNA(Sec) die in utero, but the in vivo role of other components involved in selenoprotein synthesis is unknown, and Sec incorporation defects have not been described in humans. Deiodinases (DIOs) are selenoproteins involved in thyroid hormone metabolism. We identified three of seven siblings with clinical evidence of abnormal thyroid hormone metabolism. Their fibroblasts showed decreased DIO2 enzymatic activity not linked to the DIO2 locus. Systematic linkage analysis of genes involved in DIO2 synthesis and degradation led to the identification of an inherited Sec incorporation defect, caused by a homozygous missense mutation in SECISBP2 (also called SBP2). An unrelated child with a similar phenotype was compound heterozygous with respect to mutations in SECISBP2. Because SBP2 is epistatic to selenoprotein synthesis, these defects had a generalized effect on selenoproteins. Incomplete loss of SBP2 function probably causes the mild phenotype.  相似文献   
25.
Cu-P-silicon carbide (SiC) composite coatings were deposited by means of electroless plating. The effects of pH values, temperature, and different concentrations of sodium hypophosphite (NaH2PO2·H2O), nickel sulfate (NiSO4·6H2O), sodium citrate (C6H5Na3O7·2H2O) and SiC on the deposition rate and coating compositions were evaluated, and the bath formulation for Cu-P-SiC composite coatings was optimised. The coating compositions were determined using energy-dispersive X-ray analysis (EDX). The corresponding optimal operating parameters for depositing Cu-P-SiC are as follows: pH 9; temperature, 90℃; NaH2PO2·H2O concentration, 125 g/L; NiSO4·6H2O concentration, 3.125 g/L; SiC concentration, 5 g/L; and C6H5Na3O7·2H2O concentration, 50 g/L. The surface morphology of the coatings analysed by scanning electron microscopy (SEM) shows that Cu particles are uniformly distributed. The hardness and wear resistance of Cu-P composite coatings are improved with the addition of SiC particles and increase with the increase of SiC content.  相似文献   
26.
Series of(Ag)x/(Cu0.5Tl0.5Ba2Ca2Cu3O10-δ) {(Ag)x/Cu Tl-1223} nano-superconductor composites were synthesized with different concentrations(i.e. x ? 0 4.0 wt%) of silver(Ag) nanoparticles. Low anisotropic Cu Tl-1223 superconducting matrix was prepared by solid-state reaction and Ag nanoparticles were prepared by a sol–gel method separately. The required(Ag)x/Cu Tl-1223 composition was obtained by the inclusion of Ag nanoparticles in Cu Tl-1223 superconducting matrix. Structural, morphological, compositional and superconducting transport properties of these composites were investigated in detail by x-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive x-rays(EDX)spectroscopy and four-point probe electrical resistivity(ρ) measurements. The inclusion of Ag nanoparticles enhanced the superconducting properties without affecting the tetragonal structure of the host Cu Tl-1223 matrix. The improvement in superconducting properties of(Ag)x/Cu Tl-1223 composites is most likely due to enhanced inter-grains coupling and increased superconducting volume fraction after the addition of metallic Ag nanoparticles at the inter-crystallite sites in the samples. The presence of Ag nanoparticles at the grain-boundaries may increase the number of flux pinning centers, which were present in the form of weak-links in the pure Cu Tl-1223 superconducting matrix.  相似文献   
27.
1 Results Enzyme catalysis is most attractive for the synthesis and modification of biologically relevant classes of fine organic compounds, which are difficult to prepare and to handle by conventional means[1]. In this study, commercial immobilized lipase from Candida antarctica (Novozym 435) was used in the preparation of fine organic compound with excellent properties and application as raw material for cosmetic formulation - oleyl palmitate. The effect of various reaction parameters were optimized c...  相似文献   
28.
The alkaloid content of the leaves and stem bark of Xylopia ferruginea plant was isolated and tested for its anticorrosion potential on mild steel corrosion in a hydrochloric acid medium by using electrochemical impedance spectroscopy, potentiodynamic polarization measurement, scanning electron microscopy (SEM), and Fourier transform infra red (FTIR) analysis. The experimental results reveal the effective anticorrosion potential of the plant extract. The mixed mode of action exhibited by the plant extract is evidenced from the polarization study. SEM images proof the formation of a protective layer over the mild steel surface, and this is supported by the FTIR study. The possible mode of the corrosion inhibition mechanism has also been discussed.  相似文献   
29.
The syndrome of congenital hypoparathyroidism, mental retardation, facial dysmorphism and extreme growth failure (HRD or Sanjad-Sakati syndrome; OMIM 241410) is an autosomal recessive disorder reported almost exclusively in Middle Eastern populations. A similar syndrome with the additional features of osteosclerosis and recurrent bacterial infections has been classified as autosomal recessive Kenny-Caffey syndrome (AR-KCS; OMIM 244460). Both traits have previously been mapped to chromosome 1q43-44 (refs 5,6) and, despite the observed clinical variability, share an ancestral haplotype, suggesting a common founder mutation. We describe refinement of the critical region to an interval of roughly 230 kb and identification of deletion and truncation mutations of TBCE in affected individuals. The gene TBCE encodes one of several chaperone proteins required for the proper folding of alpha-tubulin subunits and the formation of alpha-beta-tubulin heterodimers. Analysis of diseased fibroblasts and lymphoblastoid cells showed lower microtubule density at the microtubule-organizing center (MTOC) and perturbed microtubule polarity in diseased cells. Immunofluorescence and ultrastructural studies showed disturbances in subcellular organelles that require microtubules for membrane trafficking, such as the Golgi and late endosomal compartments. These findings demonstrate that HRD and AR-KCS are chaperone diseases caused by a genetic defect in the tubulin assembly pathway, and establish a potential connection between tubulin physiology and the development of the parathyroid.  相似文献   
30.
1 Results The effect of ammonium nitrate (NH4NO3) content in 40 wt.% PEO and 60 wt.% chitosan blend has been analyzed in this study.The sample containing 40 wt.% NH4NO3 exhibited the highest room temperature conductivity.In order to ascertain that water does not influence the conductivity,the samples were dried in a dessicator and the conductivity determined daily until it shows a constant value.Results are as shown in Fig.1.Samples containing other salt concentrations were also kept in the dessicator f...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号