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   3790篇
  免费   139篇
  国内免费   20篇
化学   2849篇
晶体学   62篇
力学   36篇
数学   161篇
物理学   841篇
  2023年   15篇
  2022年   28篇
  2021年   33篇
  2020年   38篇
  2019年   36篇
  2018年   35篇
  2017年   27篇
  2016年   69篇
  2015年   70篇
  2014年   73篇
  2013年   185篇
  2012年   216篇
  2011年   229篇
  2010年   149篇
  2009年   147篇
  2008年   210篇
  2007年   243篇
  2006年   247篇
  2005年   263篇
  2004年   187篇
  2003年   204篇
  2002年   175篇
  2001年   65篇
  2000年   56篇
  1999年   60篇
  1998年   27篇
  1997年   42篇
  1996年   55篇
  1995年   21篇
  1994年   24篇
  1993年   26篇
  1992年   44篇
  1991年   31篇
  1990年   31篇
  1989年   27篇
  1988年   20篇
  1987年   18篇
  1986年   28篇
  1985年   59篇
  1984年   60篇
  1983年   19篇
  1982年   51篇
  1981年   50篇
  1980年   47篇
  1979年   37篇
  1978年   29篇
  1977年   25篇
  1976年   22篇
  1975年   18篇
  1974年   16篇
排序方式: 共有3949条查询结果,搜索用时 281 毫秒
991.
Patients with hyperthyroidism sometimes take much time to receive the final diagnosis. To improve patient QOL, simple screening for hyperthyroidism by thyroid non-specialists at the physical check-up is highly expected. Therefore, we applied both Bayesian-type and SOM-type neural networks since we assured the approach useful in analysing thyroid function diagnosis in the previous work. Routine test (14 parameters) data from 66 subjects with a known diagnosis (18 patients with hyperthyroidism and 48 healthy volunteers) were adopted as learning data, and then 142 individuals who also received the same routine tests at the Tohoku University Hospital were screened to predict patients with hyperthyroidism. Both neural networks using 14 parameters predicted several patients as having hyperthyroidism with high probability, including all three hyperthyroid patients diagnosed later by the physician. Further detailed analysis of the routine test parameters that were important for classification found that screening with a set of three parameters (alkaline phosphatase, serum creatinine and total cholesterol) or plus aspartate aminotransferase allowed for quite accurate screening. These results showed that the same neural networks as previous work allows simple screening of patients for hyperthyroidism on the basis of routine test data, and that physicians not specializing in the thyroid can rapidly identify individuals suspected of having hyperthyroidism, to permit a rapid referral for examination and treatment by thyroid specialists.  相似文献   
992.
We have developed a unique multistep film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate epitaxial films of superconducting sodium-cobalt oxyhydrate, Na(0.3)CoO2.1.3H2O. An epitaxial film of Na(0.8)CoO2 grown on an alpha-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na(0.3)CoO2.1.3H2O epitaxial film. The film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the epitaxial growth of superconducting Na(0.3)CoO2.1.3H2O.  相似文献   
993.
We have developed the E-selective olefination of ester carbonyls to afford tetrasubstituted, functionalized olefins and the C-S insertion of thiol esters to give beta-keto thiol esters via ynolates.  相似文献   
994.
We numerically propose a way to perform quantum computations by combining an ensemble of molecular states and weak laser pulses. A logical input state is expressed as a superposition state (a wave packet) of molecular states, which is initially prepared by a designed femtosecond laser pulse. The free propagation of the wave packet for a specified time interval leads to the specified change in the relative phases among the molecular basis states, which corresponds to a computational result. The computational results are retrieved by means of quantum interferometry. Numerical tests are implemented in the vibrational states of the B state of I2 employing controlled-NOT gate, and 2 and 3 qubits Fourier transforms. All the steps involved in the computational scheme, i.e., the initial preparation, gate operation, and detection steps, are achieved with extremely high precision.  相似文献   
995.
A molecular-dynamics (MD) simulation of structure-H hydrates was performed under constant pressure and temperature with 6120 TIP4P water molecules, 900 OPLS-UA methane molecules, and 180 large molecular guest substance (LMGS) molecules. The LMGS molecules were represented in the form of a one-site Lennard-Jones (LJ) model using the LJ parameters sigma and epsilon. In order to clarify the thermodynamic stability of structure-H hydrates, we calculated the free-energy difference, changing on the sigma and epsilon only of the LMGS molecules. In this simulation, stable crystals of structure-H hydrates and a minimum value of DeltaG were obtained at sigma approximately 6.2 A and large values of epsilon. All simulations were performed using the special-purpose computer hardware MDGRAPE-2.  相似文献   
996.
The dynamics of the reaction, V(a 4FJ)+NO-->VO(X 4Sigma-)+N was studied by using a crossed-beam technique at 16.4 kJ/mol of collision energy. The V atomic beam was generated by laser vaporization and crossed with the O2 beam at a right angle. The laser-induced fluorescence (LIF) for the transition of VO(B 4Pi-X 4Sigma) was used to determine the rotational state distribution of the reaction product in the vibrational ground state. Almost pure V(a 4FJ) beam was obtained by using the mixture of NH3 with N2 as a carrier gas. Comparing the LIF spectra of VO measured for two carrier gases, i.e., NH3N2 and pure N2, it was concluded that the vibrational ground state of VO(X 4Sigma-) is formed almost entirely from the reaction of V(a 4FJ) and the contribution of the metastable V(a 6DJ) is negligible. The observed rotational distribution was similar to a statistical prior prediction, and suggested that the title reaction proceeds via a long-lived intermediate, which is consistent with an electron transfer mechanism.  相似文献   
997.
We propose a statistical and macroscopic analysis to estimate the catalyst activity of water-assisted growth (super-growth) of single-walled nanotubes (SWNT) and to characterize SWNT forests. The catalyst activity was estimated to be 84% (+/-6%), the highest ever reported. The SWNT forest was found to be a very sparse material where SWNTs represent only 3.6% of the total volume. This structural sparseness is believed to play a critical role in achieving highly efficient growth.  相似文献   
998.
The structure of the vapor/ethanol-aqueous-solution interface has been carefully investigated focusing on an intermolecular hydrogen bond (HB) and molecular clusters bound by HBs. This paper is a continuation of our previous molecular dynamics (MD) study (Langmuir 2005, 21, 10885), and all analysis was performed based on five independent adsorption-equilibrated configurations of a slab of ethanol solution at 298.15 K, where the ethanol mole fraction of the solution, chi(e), is 0.0052, 0.012, 0.024, 0.057, and 0.12, respectively. The geometrical definition of HB enabled the detection of the HB between ethanol-ethanol, ethanol-water, and water-water molecules. The variations of the density of HB and the coordination number of HB across the vapor/solution interface were analyzed. Analysis on the density of HB reveals that a monolayer of adsorbed ethanol can be classified into two parts where ethanol molecules prefer to form HBs with each other and ethanol molecules prefer to form HBs with water molecules. Despite chi(e), the coordination number of ethanol-ethanol HB monotonically increases toward the vapor region, while those of ethanol-water and water-water HBs monotonically decrease. In addition, the variation of the mean size of both ethanol one-component clusters and ethanol/water binary clusters across the interface were analyzed. The mean size of an ethanol one-component cluster and that of an ethanol/water binary cluster are expressed as a maximum at the interface. These behaviors are linked with the size distributions of both one-component and binary clusters. A relatively large system in this calculation also enables detailed discussion about the molar dependency of the bulk structural properties of an ethanol solution.  相似文献   
999.
[Structure: see text] A 1,4-bis(phenyl)-1,4-dihydro[60]fullerene resulting from an efficient nucleophilic substitution has been obtained by reaction of a fullerene epoxide, C60O, with nucleophilic aromatic compounds in the presence of boron trifluoride etherate as a Lewis acid.  相似文献   
1000.
We consider the Cauchy problem for the nonlinear dissipative evolution system with ellipticity on one dimensional space
with S. Q. Tang and H. Zhao [4] have considered the problem and obtained the optimal decay property for suitably small data. In this paper we derive the asymptotic profile using the Gauss kernel G(t, x), which shows the precise behavior of solution as time tends to infinity. In fact, we will show that the asymptotic formula
holds, where D0, β0 are determined by the data. It is the key point to reformulate the system to the nonlinear parabolic one by suitable changing variables. (Received: January 8, 2005)  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