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   57392篇
  免费   329篇
  国内免费   732篇
系统科学   1414篇
丛书文集   452篇
教育与普及   317篇
理论与方法论   531篇
现状及发展   31738篇
研究方法   794篇
综合类   21015篇
自然研究   2192篇
  2014年   498篇
  2013年   1009篇
  2012年   862篇
  2011年   2908篇
  2010年   696篇
  2009年   869篇
  2008年   1061篇
  2007年   1207篇
  2006年   1294篇
  2005年   1369篇
  2004年   2352篇
  2003年   1899篇
  2002年   1592篇
  2001年   1360篇
  2000年   961篇
  1999年   1007篇
  1998年   669篇
  1997年   787篇
  1996年   538篇
  1994年   688篇
  1993年   692篇
  1992年   867篇
  1991年   767篇
  1990年   823篇
  1989年   642篇
  1988年   593篇
  1987年   630篇
  1986年   683篇
  1985年   815篇
  1984年   759篇
  1983年   640篇
  1982年   783篇
  1981年   814篇
  1980年   911篇
  1979年   1342篇
  1978年   1228篇
  1977年   1215篇
  1976年   1085篇
  1975年   1072篇
  1974年   960篇
  1973年   1185篇
  1972年   1249篇
  1971年   1240篇
  1970年   1292篇
  1969年   1215篇
  1968年   1164篇
  1967年   1076篇
  1966年   886篇
  1965年   730篇
  1958年   526篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
191.
Summary (1) No double refractile Iipoid droplets can be obtained by alcohol ether extraction from urine or from urinary protein as it is possible from blood serum.(2) The conclusion is drawn that the double refractile lipoids droplets in urinary sediment do not pass through the glomeruli and therefore are not reabsorbed by the tubuli, but that they come from the own lipoproteins of the epithelial cells of renal tubuli and pass into urine by dissolution and desquamation of these cells.  相似文献   
192.
Summary Five species ofChiroptera were studied and the following numbers were found:Rhinolophus hipposideros Bechst.N=27;Barbastella barbastellus Schr.N=16;Plecotus auritus L.N=16;Myotis myotis Bork.N=21;Miniopterus schreibersii Kuhl,N=23. Centric fusions occur in the chromosomal evolution ofChiroptera. Sex-chromosomes ofX-Y type.  相似文献   
193.
1 Results In addition to single wall and multiwall carbon nanotubes[1], several structures,which are more or less related to fullerenes,including carbon nanohorns[2a], carbon nanospheres[2b] and onion like carbon structures[2c] have been reported.A new simple straight forward method to access some of these structures is the solid-state pyrolysis of different organometallic complexes in a sealed vessel,which led so far to carbon nanotubes[3a,b], carbon nanocables[3c] and onions[3d].  相似文献   
194.
针对ADO.NET在处理"丢失的修改"问题时,不支持"关键字和已修改字段"的并发控制问题提出了相应的解决办法;同时对ADO.NET架构本身带来的异常更新问题提供了解决思路.  相似文献   
195.
196.
1 Results Strongly disordered metals with the high chemical and catalytic activity are often called as skeletal metals.Usually for their preparation the metallides of d-metal (which afterwards will be left in the “skeleton“) and chemically active component(s) are firstly synthesized.Then the last one(s) is (are) removed by the leaching with aqueous solutions of alkalis or carbonates.However,this method sometime fails,first of all,for the reactions,which should be realized in non-aqueous conditions.In th...  相似文献   
197.
1 Results Classic oxidants require rigorous control of the experimental conditions added with the problem of lack of selectivity. Catalysis by transition metals with environmentally safe oxidants provides synthetic routes to minimize pollution by giving environmental benign by-products. Fe (Ⅵ) is a powerful and a selective oxidant with Fe(Ⅲ) as a by-product, while hydrogen peroxide is clean with water as the only by-product. Separation of sodium or potassium ferrates requires tedious processes. Associat...  相似文献   
198.
1 Results The photosynthetic bacterial reaction center (RC) is a membrane protein complex.The RC is composed of three protein subunits and redox components such as bacteriochlorophylls, bacteriopheophytins,and quinones.The RC performs the photochemical electron transfer from the bacteriochlorophyll dimer through a series of electron donor and acceptor molecules to a secondary quinone,QB.QB accepts electrons from a primary quinone,QA,in two sequential electron transfer reactions.The second electron trans...  相似文献   
199.
1 Results The entrapment of biopolymers into silica by the sol-gel technique meets with incompatibility of inorganic and bioorganic components. The aim was to develop a compatible procedure biomimicking the biomineralization processes in the living nature. A suggested solution in Ref.[1-2] for the biopolymer entrapment into silica matrix is based on a novel silica precursor. The developed approach is distinguished from the common technique for fabrication of hybrid biopolymer-silica nanocomposite materi...  相似文献   
200.
It is well known that considering a non-Euclidean Minkowski metric in Multidimensional Scaling, either for the distance model or for the loss function, increases the computational problem of local minima considerably. In this paper, we propose an algorithm in which both the loss function and the composition rule can be considered in any Minkowski metric, using a multivariate randomly alternating Simulated Annealing procedure with permutation and translation phases. The algorithm has been implemented in Fortran and tested over classical and simulated data matrices with sizes up to 200 objects. A study has been carried out with some of the common loss functions to determine the most suitable values for the main parameters. The experimental results confirm the theoretical expectation that Simulated Annealing is a suitable strategy to deal by itself with the optimization problems in Multidimensional Scaling, in particular for City-Block, Euclidean and Infinity metrics.  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