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   4045篇
  免费   108篇
  国内免费   2篇
化学   2449篇
晶体学   28篇
力学   143篇
数学   643篇
物理学   892篇
  2023年   29篇
  2022年   45篇
  2021年   77篇
  2020年   70篇
  2019年   66篇
  2018年   119篇
  2017年   113篇
  2016年   185篇
  2015年   138篇
  2014年   152篇
  2013年   259篇
  2012年   292篇
  2011年   295篇
  2010年   182篇
  2009年   116篇
  2008年   233篇
  2007年   222篇
  2006年   170篇
  2005年   323篇
  2004年   282篇
  2003年   196篇
  2002年   82篇
  2001年   56篇
  2000年   50篇
  1999年   36篇
  1998年   25篇
  1997年   23篇
  1996年   21篇
  1995年   24篇
  1994年   29篇
  1993年   25篇
  1992年   15篇
  1991年   22篇
  1990年   10篇
  1989年   11篇
  1988年   5篇
  1987年   9篇
  1986年   11篇
  1985年   21篇
  1984年   15篇
  1983年   8篇
  1982年   8篇
  1981年   6篇
  1980年   12篇
  1979年   8篇
  1978年   5篇
  1976年   6篇
  1975年   4篇
  1974年   6篇
  1973年   7篇
排序方式: 共有4155条查询结果,搜索用时 406 毫秒
961.
Ubiquinone molecules have a high biological relevance due to their action as electron carriers in the mitochondrial electron transport chain. Here, we studied the dissociative interaction of free electrons with CoQ0, the smallest ubiquinone derivative with no isoprenyl units, and its fully reduced form, 2,3-dimethoxy-5-methylhydroquinone (CoQ0H2), an ubiquinol derivative. The anionic products produced upon dissociative electron attachment (DEA) were detected by quadrupole mass spectrometry and studied theoretically through quantum chemical and electron scattering calculations. Despite the structural similarity of the two studied molecules, remarkably only a few DEA reactions are present for both compounds, such as abstraction of a neutral hydrogen atom or the release of a negatively charged methyl group. While the loss of a neutral methyl group represents the most abundant reaction observed in DEA to CoQ0, this pathway is not observed for CoQ0H2. Instead, the loss of a neutral OH radical from the CoQ0H2 temporary negative ion is observed as the most abundant reaction channel. Overall, this study gives insights into electron attachment properties of simple derivatives of more complex molecules found in biochemical pathways.  相似文献   
962.
This study focused on the extraction, purification, and physicochemical characterization of γ-conglutin, a protein present in lupin seeds with properties of reducing blood glucose levels. Total protein was extracted with an alkaline-saline solvent, followed by isoelectric precipitation. Chromatographic purification of the precipitated fraction was performed using a cation exchange supermacroporous cryogel column. Electrophoresis of the eluted fraction from chromatography presented a single band of ∼48 kDa under non-reducing conditions (two bands of ∼30 and ∼17 kDa, under reducing conditions) confirming the success of the purification protocol. Liquid chromatography-tandem mass spectrometry analysis confirmed the identity of the protein as γ-conglutin. The purified γ-conglutin had an isoelectric point of 7.51, β-sheets prevailing as a secondary structure, and denaturation temperature close to 68°C. The outcome of this work showed that γ-conglutin was obtained with a high degree of purity. The proposed purification protocol is simple and can be easily scaled up.  相似文献   
963.
We give an algorithm to compute the ω-primality of finitely generated atomic monoids. Asymptotic ω-primality is also studied and a formula to obtain it in finitely generated quasi-Archimedean monoids is proven. The formulation is applied to numerical semigroups, obtaining an expression of this invariant in terms of its system of generators.  相似文献   
964.
Escalation of commitment, the tendency of decision makers to keep on investing in losing courses of action, has been shown to be a costly decision bias that affects many areas of decision making. Even though escalation is a widely studied phenomenon, there has been comparatively little research on how to avoid this bias. The present study focuses on de-escalation of commitment and proposes that causal loop diagrams (CLDs) can help to decrease escalating commitment to a failing course of action. By means of an experiment, this study shows that using a CLD decreases escalation tendencies.  相似文献   
965.
966.
The autoregressive Hilbertian process framework has been introduced in Bosq (2000). This book provides the nonparametric estimation of the autocorrelation and covariance operators of the autoregressive Hilbertian processes. The asymptotic properties of these estimators are also provided. The maximum likelihood approach still remains unexplored. This paper obtains the asymptotic distribution of the maximum likelihood (ML) estimators of the auto-covariance operator of the Hilbert-valued innovation process, and of the autocorrelation operator of a Gaussian autoregressive Hilbertian process of order one. A real data example is analyzed in the financial context for illustration of the performance of the projection maximum likelihood estimation methodology in the context of missing data.  相似文献   
967.
We develop and analyze a post processing technique for the family of low‐order mimetic discretizations based on vertex unknowns for the numerical treatment of diffusion problems on unstructured polygonal and polyhedral meshes. The post processing works in two steps. First, from the nodal degrees of freedom, we reconstruct an elemental‐based vector field that approximates the gradient of the exact solution. Second, we solve a local problem for each mesh vertex associated with a scheme degree of freedom to determine a post processed normal flux that is conservative and divergence preserving. Theoretical results and numerical experiments for two‐dimensional (2D) and 3D benchmark problems show optimal convergence rates. © 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 31: 336–363, 2015  相似文献   
968.
969.
Multi-parameter flow cytometry was used to monitor cell intrinsic light scatter, viability, and lipid content of Rhodotorula glutinis CCMI 145 cells grown in shake flasks. Changes in the side light scatter and forward light scatter were detected during the yeast batch growth, which were attributed to the different yeast growth phases. A progressive increase in the proportion of cells stained with PI (cells with permeabilized cytoplasmic membrane) was observed during the yeast growth, attaining 79% at the end of the fermentation. A high correlation between the Nile Red fluorescence intensity measured by flow cytometry and total lipid content assayed by the traditional gravimetric lipid analysis was found for this yeast, making this method a suitable and quick technique for the screening of yeast strains for lipid production and optimization of biofuel production bioprocesses. Medium growth optimization for enhancement of the yeast oil production is now in progress.  相似文献   
970.
The hydrolysis of triglycerides at the oil–water interface, synthesis of esters and transesterification in microaqueous conditions are catalysed by lipase. For its application, a proper purification method was necessary. This study examined the application of an aqueous two-phase system to partition porcine pancreatic lipase. The influence of molecular weight and concentration of polyethylene glycol (PEG), tie line length (TLL), potassium phosphate concentration, sodium chloride (NaCl) addition and temperature in the partition was studied. The enzyme was more efficiently purified in PEG 8,000 at 14.5 °C (PF?=?3.89-fold), presenting more recoveries at the top phase with shorter TLL and lower concentrations of PEG and potassium phosphate. Moreover, the increase of these variables repressed the purification and the further addition of NaCl did not promote the purification of the enzyme. These results demonstrated the efficiency of the aqueous two-phase system on lipase purification.  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