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   987篇
  免费   10篇
  国内免费   28篇
系统科学   20篇
丛书文集   2篇
教育与普及   3篇
理论与方法论   12篇
现状及发展   206篇
研究方法   131篇
综合类   625篇
自然研究   26篇
  2020年   5篇
  2019年   5篇
  2018年   14篇
  2017年   12篇
  2016年   8篇
  2015年   13篇
  2014年   19篇
  2013年   18篇
  2012年   67篇
  2011年   95篇
  2010年   37篇
  2009年   24篇
  2008年   52篇
  2007年   66篇
  2006年   52篇
  2005年   63篇
  2004年   39篇
  2003年   64篇
  2002年   53篇
  2001年   40篇
  2000年   42篇
  1999年   20篇
  1996年   4篇
  1992年   10篇
  1991年   7篇
  1990年   4篇
  1989年   10篇
  1988年   9篇
  1987年   9篇
  1986年   5篇
  1985年   9篇
  1983年   3篇
  1982年   6篇
  1981年   7篇
  1980年   8篇
  1979年   8篇
  1978年   7篇
  1977年   10篇
  1976年   5篇
  1975年   9篇
  1974年   5篇
  1973年   9篇
  1972年   3篇
  1971年   9篇
  1970年   9篇
  1969年   7篇
  1968年   5篇
  1967年   7篇
  1966年   5篇
  1965年   7篇
排序方式: 共有1025条查询结果,搜索用时 93 毫秒
11.
近红外光谱技术分析速度快、效率高、成本低、重现性好,典型的无损分析;主要用于样品类别鉴定和纯度检验.近红外光谱技术结合模式识别方法,在纯品油种类的鉴别中已得到成功应用[1,2],本实验室也初步建立了纯品油近红外谱图数据库[3],但该法在实际海面溢油样品的鉴别中应用仍甚少.  相似文献   
12.
13.
求解线性微分方程组6UN,llH(z).U(t),U(0)ll1的方法,已经由JamesWei和EdwardNorman给出(1),他们的方法建立在李代数理论的基础之上。本文讨论解的结构。对于上述方程组,其中U是有限维空间中依赖于时间的线性算子,而H(=6t(Ht、(z)+......、,!(r),!+(z),如果Ht,Hz,…,,!。生成可解的李代数L,则它的解U(U=explgt(r)HJexpk2(…”exp“(z)1可以表示为一个矩阵,其所有的元素都是g(illl,2,…,m)的初等函数,并且只出现指数函数与乘幂。最后用两个例子说明具体的解法。  相似文献   
14.
Methods were developed to determine the mass ratios of carbon isotopes in trace amounts of aerosol carbonate. A Finnigan MAT 252 mass spectrometer fitted with an on-line Kiel device was to determine the ^13C/^12C ratio in CO2 produced from the carbonate. A study using these methods was conducted to characterize the carbonate carbon isotopes in aerosol samples collected in Xi‘an on dusty and normal days during March and April 2002. Results of the study demonstrate that insights into the origin of the dust can be deduced from its isotopic composition. That is, the δ^13C ofcarbonate for dust storm samples ranged from -1.4‰ to -4.2‰, and this is consistent with sandy materials in dust source regions upwind. In contrast, for non-dusty days δ^13C ranged from -7.5% to -9.3‰, which is more similar to fine particles emitted from local surface soils. Comparisons of dust storm aerosols with surface soils from source regions and with aerosol samples collected downwind indicate that the δ^13C values did not change appreciably during longrange transport. Therefore, carbon isotopes have the potential for distinguishing among source materials, and this approach provides a powerful new tool for identifying dust provenance.  相似文献   
15.
Books and archives experience aging which is affected by a variety of factors, such as microorganism, humidity, light and components of paper [1]. Thus, the studies concerning impacts of those factors on paper aging are required. In this paper, an accelerated paper aging was carried out to investigate the effects of acid and metals such as alum, copper ( Ⅱ ) sulfate, copper ( Ⅱ ) chloride, and iron (Ⅲ) chloride on paper aging. It was found that both acid and metals had impacts on paper aging. In particular, paper aging was far more accelerated in case that acid and metals were present in paper at the same time.  相似文献   
16.
17.
A 3D porous carbon-manganese oxide ([email protected]) nanocomposite is successfully synthesized via a thermal plasma deposition method. The chemical bonds and compositions, phase structures, surface morphologies, etc. of as-obtained [email protected] nanocomposite were characterized by the various equipment, such as X-ray diffractometer, X-ray photoelectron spectroscopy, and electron microscopes. The electrochemical performances of the [email protected] nanocomposite electrode showed a specific capacitance of 780 F g?1 at a current density of 2 A g?1 and a capacitance retention rate of 99% after 5000 charge-discharge cycles at a high current density of 10 A g?1. These excellent capacitive performances may be attributed to the encapsulation of MnO nanoparticles by porous carbon sheets in the [email protected] MnO nanocomposite structure. It is believed that the carbon-encapsulated MnO nanoparticles can be protected from a volume deformation during the charge adsorption/desorption cycle and can be electrically improved by the encapsulated carbon sheets, resulting in better overall capacitive performance. In addition, this study also demonstrates the practical applicability by assembling a supercapacitor using the as-obtained [email protected] nanocomposite to glow a light emitting diode.  相似文献   
18.
A taxonomic review of three Asian species of Hesperonoe (Annelida: Polynoidae) is presented. Hesperonoe hwanghaiensis Uschakov and Wu, 1959 Uschakov PV, Wu B. 1959. [The polychaete worms of the families Phyllodocidae and Aphroditidae (Polychaeta, errantia) from the Yellow Sea]. Arch Inst Oceanol Sinica. 1:1–40, pls. I–X. (In Chinese and Russian). [Google Scholar] (type locality: Qingdao, China) is redescribed based on five specimens collected from an intertidal site in the Korean coast of the Yellow Sea, as a new record of this species from Korea, which is the only known habitat outside the type locality. Hesperonoe coreensis sp. nov. is described based on nine specimens collected from two intertidal sites in the Korean coast of the Yellow Sea, including the same site as the habitat of H. hwanghaiensis. Hesperonoe japonensis sp. nov. is described by re-examination of many specimens collected from eight intertidal sites and a subtidal one in Japan, which was previously misidentified as H. hwanghaiensis. The three species are clearly distinguishable from one another by the species-specific morphology of the macrotubercles (or marked ridges) on the surface of elytra. All of the three species seemed to be commensal with the burrowing mud shrimp Upogebia major in intertidal flats, except for an additional probable host of another upogebid shrimp Austinogebia narutensis for H. japonensis sp. nov. in a subtidal habitat. The morphological characteristics and host species of the three Asian species are compared with all of the other five congeners known from the North American Pacific and Arctic Sea.

http://www.zoobank.org/urn:lsid:zoobank.org:pub:51CB5C5B-0838-48AC-A0BA-8E63AA7628CB  相似文献   
19.
Though Hongbaekjeong(HBJ),a hebal mixture of three medicinal plants,has been traditionally used for arthritis and muscular pain,its scientific evidence still remains unclear.Thus,in the present study,analgesic and anti-inflammatory mechanism of HBJ was evaluated in vitro and in vivo.HBJ significantly reduced NO production and prostaglandin E2(PGE2)release and also attenuated the expression of cyclooxygenase 2(COX-2)in lipopolysaccharides(LPS)and interferon(IFN)-c treated RAW 264.7 cells.Furthermore,HBJ abrogated the production of proinflammatory cytokines such as interleukin(IL)1b,IL-6,IL-8 and monocyte chemoattractant protein-1(MCP-1)in LPS and IFN-c treated RAW 264.7 cells.In addition,HBJ significantly decreased the number of writhing syndrome induced by acetic acid,and also increased latency in hot-plate method and tail flick test in mice.Consistently,HBJ significantly reduced the edema volume in the hind paw of the rats with arthritis induced by Freund’s complete adjuvant(FCA)compared to untreated control.Collectively,our findings demonstrate the antiinflammatory and analgesic potential of HBJ via inhibition of proinflammatory cytokines and PGE2 release for treatment of arthritis and muscular pain.  相似文献   
20.
A total of 12 species of Cnemaspis (N = 104) from Southeast Asia were examined for gastrointestinal helminths. Samples consisted of nine species (n = 86) from Peninsular Malaysia: Cnemaspis affinis (n = 4); Cnemaspis baueri (n = 17); Cnemaspis biocellata (n = 12); Cnemaspis grismeri (n = 8); Cnemaspis kumpoli (n = 11); Cnemaspis limi (n = 9): Cnemaspis monachorum (n = 7); Cnemaspis pemanggilensis (n = 10); Cnemaspis peninsularis (n = 8); one species (n = 5) from Cambodia and Thailand, Cnemaspis chanthaburiensis (n = 5); and two species (n = 13) from Vietnam: Cnemaspis nuicamensis (n = 6) and Cnemaspis tucdupensis (n = 7). The aggregate helminth community consisted of one species of Cestoda, Cylindrotaenia malayi and nine species of Nematoda: Bakeria schadi, Meteterakis singaporensis, Parapharyngodon maplestoni, Maxvachonia sp., Physalopteroides sp., Physalopteridae gen. sp., Riticulariidae gen. sp., Seuratoidea gen. sp., Ascaridoidea gen. sp. Meteterakis singaporensis had the largest number of individuals (457) and greatest prevalence (24%). Twenty-eight new host records are reported.  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