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条件价值评估法在森林生态旅游非使用价值评估中范围效应的研究 总被引:1,自引:0,他引:1
条件价值评估法(CVM)是目前最受欢迎的环境资源价值评估方法之一,但同时也是争议最多的方法,争议的焦点围绕该方法的有效性和可靠性。影响CVM结果有效性的因素很多,其中范围效应是最重要因素。笔者归纳了研究CVM范围效应有关理论,并通过1个实际案例来验证CVM评估环境资源非使用价值时范围效应是否存在。研究结果表明:范围效应不是CVM普遍现象,并非由CVM方法本身的缺陷造成。 相似文献
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Complete genome sequence of Salmonella enterica serovar Typhimurium LT2. 总被引:34,自引:0,他引:34
M McClelland K E Sanderson J Spieth S W Clifton P Latreille L Courtney S Porwollik J Ali M Dante F Du S Hou D Layman S Leonard C Nguyen K Scott A Holmes N Grewal E Mulvaney E Ryan H Sun L Florea W Miller T Stoneking M Nhan R Waterston R K Wilson 《Nature》2001,413(6858):852-856
Salmonella enterica subspecies I, serovar Typhimurium (S. typhimurium), is a leading cause of human gastroenteritis, and is used as a mouse model of human typhoid fever. The incidence of non-typhoid salmonellosis is increasing worldwide, causing millions of infections and many deaths in the human population each year. Here we sequenced the 4,857-kilobase (kb) chromosome and 94-kb virulence plasmid of S. typhimurium strain LT2. The distribution of close homologues of S. typhimurium LT2 genes in eight related enterobacteria was determined using previously completed genomes of three related bacteria, sample sequencing of both S. enterica serovar Paratyphi A (S. paratyphi A) and Klebsiella pneumoniae, and hybridization of three unsequenced genomes to a microarray of S. typhimurium LT2 genes. Lateral transfer of genes is frequent, with 11% of the S. typhimurium LT2 genes missing from S. enterica serovar Typhi (S. typhi), and 29% missing from Escherichia coli K12. The 352 gene homologues of S. typhimurium LT2 confined to subspecies I of S. enterica-containing most mammalian and bird pathogens-are useful for studies of epidemiology, host specificity and pathogenesis. Most of these homologues were previously unknown, and 50 may be exported to the periplasm or outer membrane, rendering them accessible as therapeutic or vaccine targets. 相似文献
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偶联剂对HDPE基竹塑复合材料性能的影响 总被引:1,自引:0,他引:1
以高密度聚乙烯(HDPE)、竹粉(BF)为原料,马来酸酐接枝聚丙烯(MAH-g-PP)作为偶联剂,通过混炼、平板热压成型制备竹粉/HDPE复合材料,研究了马来酸酐接枝聚丙烯添加量(0、2%、5%、8%)对竹粉/HDPE复合材料吸湿性、吸水性、弯曲强度、拉伸强度以及抗冲击强度等性能的影响。结果表明:MAH-g-PP能显著改善竹粉/HDPE复合材料的吸湿、吸水性能,明显提高复合材料的力学性能; 当MAH-g-PP添加量为5%时,复合材料的吸湿、吸水率最低,静曲强度与弹性模量分别提高68%和30.4%,拉伸强度与抗冲击强度分别增长53.7%和75%; 而当MAH-g-PP添加量为8%时,复合材料的力学性能在一定程度上有所降低。红外光谱(FTIR)分析显示,添加MAH-g-PP后竹粉的游离羟基与马来酸酐之间发生了酯化反应。 相似文献
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Thi Ngoc Tram Nguyen Diem-Huong Nguyen-Tran Long Giang Bach Tran Hoang Du Truong Ngoc Thuy Trang Le Dai Hai Nguyen 《自然科学进展(英文版)》2019,29(6):612-616
A hybrid of organic/inorganic system, hollow mesoporous silica nanoparticles functionalized with poly(ethylene glycol), was synthesized in the presence of coupling agent and aminated ends. Doxorubicin (DOX) was used as a model drug to evaluate the effect of PEGylation on the loading capacity and release pattern of the system. Characterization with transmission electron microscopy (TEM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption-desorption, thermogravimetric analysis (TGA) and Fourier transformed infrared (FTIR) confirmed the PEG conjugation, as well as the amorphous mesoporous nature of the synthesized particles. Due to the capping effect of PEG molecules over the pores in the silica structure, the synthesized system was able to retain higher amount of cargo and establish a controlled manner when releasing. 相似文献
26.
Gupta GP Nguyen DX Chiang AC Bos PD Kim JY Nadal C Gomis RR Manova-Todorova K Massagué J 《Nature》2007,446(7137):765-770
Metastasis entails numerous biological functions that collectively enable cancerous cells from a primary site to disseminate and overtake distant organs. Using genetic and pharmacological approaches, we show that the epidermal growth factor receptor ligand epiregulin, the cyclooxygenase COX2, and the matrix metalloproteinases 1 and 2, when expressed in human breast cancer cells, collectively facilitate the assembly of new tumour blood vessels, the release of tumour cells into the circulation, and the breaching of lung capillaries by circulating tumour cells to seed pulmonary metastasis. These findings reveal how aggressive primary tumorigenic functions can be mechanistically coupled to greater lung metastatic potential, and how such biological activities may be therapeutically targeted with specific drug combinations. 相似文献
27.
Wang K Diskin SJ Zhang H Attiyeh EF Winter C Hou C Schnepp RW Diamond M Bosse K Mayes PA Glessner J Kim C Frackelton E Garris M Wang Q Glaberson W Chiavacci R Nguyen L Jagannathan J Saeki N Sasaki H Grant SF Iolascon A Mosse YP Cole KA Li H Devoto M McGrady PW London WB Capasso M Rahman N Hakonarson H Maris JM 《Nature》2011,469(7329):216-220
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General linear non-autonomous functional differential equations are considered. Explicit criteria for exponential stability are given. Furthermore, the authors present an explicit robust stability bound for systems subject to time-varying perturbations. Two examples are given to illustrate the obtained results. 相似文献
30.
Graphene-based composite materials 总被引:31,自引:0,他引:31
Stankovich S Dikin DA Dommett GH Kohlhaas KM Zimney EJ Stach EA Piner RD Nguyen ST Ruoff RS 《Nature》2006,442(7100):282-286
Graphene sheets--one-atom-thick two-dimensional layers of sp2-bonded carbon--are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (approximately 3,000 W m(-1) K(-1) and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene-graphene composite formed by this route exhibits a percolation threshold of approximately 0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes; at only 1 volume per cent, this composite has a conductivity of approximately 0.1 S m(-1), sufficient for many electrical applications. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications. 相似文献