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   23235篇
  免费   1462篇
  国内免费   3452篇
化学   18924篇
晶体学   1057篇
力学   216篇
综合类   192篇
数学   3686篇
物理学   4074篇
  2023年   169篇
  2022年   243篇
  2021年   738篇
  2020年   542篇
  2019年   536篇
  2018年   423篇
  2017年   558篇
  2016年   590篇
  2015年   551篇
  2014年   816篇
  2013年   1778篇
  2012年   1291篇
  2011年   1487篇
  2010年   1483篇
  2009年   1679篇
  2008年   1625篇
  2007年   1643篇
  2006年   1555篇
  2005年   1341篇
  2004年   1307篇
  2003年   1107篇
  2002年   945篇
  2001年   709篇
  2000年   695篇
  1999年   530篇
  1998年   519篇
  1997年   425篇
  1996年   325篇
  1995年   367篇
  1994年   323篇
  1993年   303篇
  1992年   284篇
  1991年   213篇
  1990年   123篇
  1989年   153篇
  1988年   136篇
  1987年   89篇
  1986年   61篇
  1985年   71篇
  1984年   90篇
  1983年   23篇
  1982年   44篇
  1981年   60篇
  1980年   51篇
  1979年   38篇
  1978年   27篇
  1977年   22篇
  1976年   11篇
  1975年   10篇
  1973年   9篇
排序方式: 共有10000条查询结果,搜索用时 588 毫秒
81.
After establishing hemi-Parkinsonian rat models, the relationships between neuron death and the expression of several proteins, such as c-Fos, GFAP, GDNF, NF-κB and some cytokines were determined. Therapeutics experiments with notoginsenoside-Rg1 were carried out. The research results show that the expressions of GFAP, NF-Kκ and c-Fos will obviously increase in the lesion side of the striatum and the expression of GDNF will decrease, which implies that the signal transduction pathway may participate in the apoptosis in neurons. The levels of some cytokines such as TNF-α, IL-1β in the striatum of PD rat models increased compared to those of normal rats. The results of the therapeutics experiments show that notoginsenoside-Rg1 may repress the immune inflammation response and regulate the immune function through the neuro-immune molecular network. Therefore, notoginsenoside-Rg1 can be used as an effective drug for anti-oxidation and anti-inflammation, and can be used in the therapy of Parkinson's disease(PD).  相似文献   
82.
本文用COSY,CHCOR等二维核磁共振技术对TRANILAST,2[]3-(3.4-二甲基苯基)1-氧代-2-丙烯基]氨基]苯甲到进行了~1H、~(13)C NMR谱数据分析及归属,并结合~(13)C弛豫时间T_1 及变温实验对合成产品进行了结构鉴定.  相似文献   
83.
运用Lyapunov函数方法,讨论了一维细胞神经网络模型的完全稳定性问题,给出了四组使模型具有完全稳定的充分条件.  相似文献   
84.
An investigation of the reaction of 1-lithio-1,3-diynes, generated in situ, with nitriles has been carried out. In the case of aromatic nitriles 1-arylalk-1-ene-3,5-diynylamines are formed, which undergo dimerization and cyclization on isolation, giving 3-(alka-1,3-diynyl)-4-(alk-2-ynyl)-2,6-diarylpyridines. The effect of the nature of the substituent in the benzonitrile molecule on the selectivity of the reaction and the yield of the products has been determined. A scheme is proposed for the conversions and the structures of the intermediates have been established. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 701–710, May, 2006.  相似文献   
85.
The bond random mixed compound Fe(Br0.9I0.1)2 has been studied by magnetization and Mössbauer measurements. Although the zero-field cooled and field-cooled magnetization variations are not like a typical spin glass one, the Mössbauer spectrum below Néel temperature shows a hyperfine field distribution. It implies that the 10% FeI2 mixed in FeBr2 can be induced by the bond random effect which causes the sample to exhibit a spin glass-like behavior.  相似文献   
86.
X. Yang 《Applied Surface Science》2006,252(10):3647-3657
The room-temperature adsorption and thermally induced processes of propionic acid and pyruvic acid on Ni(1 0 0) have been investigated by electron energy loss spectroscopy (EELS). Computational vibrational analysis of the optimized bidentate structures for acid-Ni model complexes (involving the organic acid and a Ni atom) has been performed by using the two-layer ONIOM method with the Density Functional Theory and used to interpret the vibrational EELS data. Dehydrogenation of the hydroxyl group is found to result in bonding of the carboxylate group in the propionate and pyruvate adspecies to either a single Ni surface atom in a bidentate configuration or two neighbouring Ni atoms in a bridge configuration. Given the similarities in the total energies and related vibrational frequencies obtained by the calculations in the case of pyruvate adspecies, it is difficult to differentiate the alternate adsorption structure, in which the keto O and hydroxyl O atoms are bonded to a Ni atom in a five-member chelate ring configuration. Furthermore, temperature-dependent EELS studies show that both the propionate and pyruvate adspecies could decompose upon annealing to above 400 K and further dissociate to CO adspecies above 550 K and to C and/or O above 600 K.  相似文献   
87.
We present the theoretical study of the effect of external random field characterized by a Gaussian probability distribution function on the continuous phonon spectrum of one-dimensional (1D) chain, based on the Jacobian matrix method. The cumulative effect of the random field and simple isotopic defect is studied analytically and numerically. The Gaussian random field removes a square-root divergence appearing in the phonon spectrum of ideal 1D chain. The impurity phonon DOS shows strong dependence on the variance and the mean of the random field and exhibits very different behavior from the non-random case: the continuous spectrum is expanded and the δ-peak, describing discrete impurity vibrations in the non-random chain with the impurity, falls into a continuous zone.  相似文献   
88.
The controlled reaction of equimolar quantities of maleic anhydride and glycidol in dimethoxyethane gives soluble polyesters with one hydroxyl group in each repeating unit. The reaction proceeds with stepwise ring opening of the components and gives highly viscous clear solutions in relatively short periods. In the first step, monomaleate ester formation takes place around 80 °C. The ring opening of the oxirane group is the second step, and it occurs at 120 °C. The overall reaction is the formation of soluble polyesters with moderate molecular weights (6000–18,000), without the elimination of water. The soluble polyesters can be crosslinked tightly by direct heating at 190 °C without additional vinyl monomer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2549–2555, 2003  相似文献   
89.
The spin polarized β-emitting nuclei12B (I π=1+,T 1/2=20.18 ms) were produced by the nuclear reaction11B(d, p)12B and by the selection technique of the incident deuteron energy and the12B recoil angle following the nuclear reaction. The nuclear magnetic moment of the short-lived nuclei12B was measured by β-NMR with the β-NMR and β-NQR setup established for the first time in China. The nuclear magnetic moment of12B was determined to be μ=0.99993±0.00048 nm org=0.99993±0.00048 after the precise correction of the Knight shift.  相似文献   
90.
To find an effective extraction and removal method for palladium(II), which is one of the main fission products from an acidic nuclear spent fuel solution, the extraction behavior of palladium(II) from a nitric acid medium by an acidic chelating extractant, 1-phenyl-3-methyl-4-trifluoroacetylpyrazolone-5-one (HPMTP) and a tertiary amine of high molecular weight, tri-n-octylamine (TOA), has been studied by spectrophotometry. A noticeable antagonistic extraction effect was observed in the extraction system under the given conditions. To understand this phenomenon, a preliminary investigation was performed to explain the mechanism of this reaction. According to the theory of corresponding solutions (TCS), the association reaction between HPMTP and TOA is proposed in the organic phase. An associated species, HPMTP·TOA, formed through hydrogen bonding in a chloroform medium might be the main reason why an antagonistic extraction effect occurred. The association constant between HPMTP and TOA was calculated to be 2.86±0.05.  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