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   662928篇
  免费   7970篇
  国内免费   2154篇
化学   358771篇
晶体学   9633篇
力学   28565篇
综合类   19篇
数学   80499篇
物理学   195565篇
  2021年   5697篇
  2020年   6257篇
  2019年   6043篇
  2018年   7348篇
  2017年   7141篇
  2016年   11728篇
  2015年   8003篇
  2014年   11783篇
  2013年   30526篇
  2012年   23322篇
  2011年   28839篇
  2010年   19482篇
  2009年   19129篇
  2008年   26159篇
  2007年   26305篇
  2006年   24723篇
  2005年   22304篇
  2004年   20502篇
  2003年   18176篇
  2002年   17765篇
  2001年   19611篇
  2000年   14975篇
  1999年   11919篇
  1998年   9826篇
  1997年   9477篇
  1996年   9418篇
  1995年   8447篇
  1994年   8230篇
  1993年   7905篇
  1992年   8881篇
  1991年   8806篇
  1990年   8387篇
  1989年   8158篇
  1988年   8271篇
  1987年   8009篇
  1986年   7676篇
  1985年   10326篇
  1984年   10522篇
  1983年   8497篇
  1982年   8894篇
  1981年   8887篇
  1980年   8390篇
  1979年   8852篇
  1978年   8985篇
  1977年   8947篇
  1976年   8800篇
  1975年   8360篇
  1974年   8205篇
  1973年   8365篇
  1972年   5545篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The organo-inorganic composites based on a strongly acid gel resin including zirconium hydro-phosphate nanoparticles and their aggregates were studied by impedance spectroscopy, electron microscopy, and standard contact porosimetry. The porous structure of the polymer was transformed under the action of the inorganic filler. The nanoparticles in the transport pores provided a three- to fivefold increase in the electric conductivity of the nanocomposites compared with the conductivity of the nonmodified ionite and a decrease in the percolation threshold. The nanocomposite ionites demonstrated stability against the accumulation of organic substances during electrodeionization to extract Na+ from low-concentrated solutions.  相似文献   
992.
993.
Two iron(I) porphyrins were electrogenerated and then utilized as catalysts for the reductive dechlorination of 1,1‐bis(4‐chlorophenyl)‐2,2,2‐trichloroethane (DDT) in N,N′‐dimethylformamide. No reaction is observed between DDT and the Fe(III) or Fe(II) forms of the porphyrin, but the electrogenerated Fe(I) porphyrin efficiently catalyzes the electroreduction of DDT to give (1,1‐bis(4‐chlorophenyl)‐2,2‐dichloroethane) DDD, (1,1‐bis(4‐chlorophenyl)‐2,2‐dichloroethylene) DDE and (1,1‐bis(4‐chlorophenyl)‐2‐dichloroethane) DDMU as determined by GC‐MS analysis. The reductive dechlorination was monitored by electrochemistry, controlled potential electrolysis and spectroelectrochemistry and a mechanism for the reaction involving the reduced porphyrins and DDT is proposed. Comparisons are also made between the catalytic properties of metalloporphyrins containing iron, cobalt and manganese central metal ions under the same solution conditions.  相似文献   
994.
Abstract

The problem of charge localization in phosphothioates got more attention since thioanaloges proved useful for stereochemical investigntinns of biochemical processes. On the basis of the existing evidence the structure 1c has been confirmed for aqueous solutions.  相似文献   
995.
Abstract

Macrocyclic molecules capable of selective recognition of ionic or neutral guests are of considerable current interest. The design and synthesis of host molecules that contain phosphorus have been developed recently. Typically, the model compound A is modified to the more rigid molecule B which can exist as two exo/endo isomers. B represents a new family of preorganized ligands that can involve the phosphorus group during complexation of various guests. The exo/endo isomerisni was investigated by NMR studies. The complexing properties of B were studied by NMR and extraction method with alkaline and ammonium cations. The complexation constants mainly depend on the molecular geometry and the size of the cavity. The highest Ka constants are obtained with the endo isomer where the P=O bond is directed toward the center of the macrocycle. Complex formation and detailed stereochemical data for A and B will be presented.  相似文献   
996.
997.
Diphenyl diselenide reacts with O,O-dialkyl and alkylene dithiophosphoric acids in equimolar amounts in refluxing benzene to yield Ph 2 Se 2 HS 2 P(OR) 2 , R = Et, Pr-n, Pr-i, Bu-i and Ph, and Ph 2 Se 2 HS 2 POGO where G = CH 2 CMe 2 CH 2 m , --CH 2 CEt 2 CH 2 m , and --CMe 2 CMe 2 m . The complexes are sticky solids, soluble in common organic solvents and monomeric in nature. These have been characterized on the basis of elemental analysis, molecular weight determinations, UV, IR, and NMR ( 1 H, 13 C, and 31 P NMR). Spectral data reveal addition of dithiophosphate moieties to Ph 2 Se 2 .  相似文献   
998.
Adsorption of Ethoxylated Nonyl Phenol (ENP) surfactant from its aqueous solution onto a commercial tire cutting from different sites. Outer Peristalitic (O.P.), Outer Smouth (O.S.), and Internal Tire (I.T.) is studied after different immersion times, 24, 48, 72, and 144 hrs at 30° C. The adsorption isotherms are determined by a surface tension method. A two stepped L-type isotherm is characteristic below the CMC of ENP; above this concentration the isotherm deviates from the L-type and the slope of the isotherm increases sharply. A Langmuir fit has been found below the CMC of ENP. From the maximum amounts adsorbed at pseudo-plateau (Tmax of the tested samples it was found that the efficiency of adsorption onto the rubber surface decreases in the order, O.P. > O.S. > I.T.  相似文献   
999.
In order to clarify the roles of lipids in the water-holding property of stratum corneum, the forearm skin of healthy male volunteers was treated with acetone/ether (1/1) or sodium dodecyl sulfate (5%) for 1-30 min. A prolonged treatment period of 5-30 min produced a chapped and scaly appearance of the stratum corneum without any inflammatory reactions. Under these conditions, there was a marked decrease in the water-holding capacity of the stratum corneum accompanied by a selective loss of stratum corneum lipids such as cholesterol, cholesterol esters, and sphingolipids. Two daily applications of the isolated stratum corneum lipids to experimentally induced dry skins caused a significant increase of conductance, accompanied by a marked improvement in the level of scaling. Meanwhile, the isolated sebaceous lipids exhibited no significant recovery in both the conductance value and the scaling. Out of chroma-tographically separated fractions of the stratum corneum lipids, topical applications of ceramide fraction induced the highest increase in the conductance value. Topical applications of synthesized pseudo-ceramides also showed a significant recovery of the water-retaining properties accompanied by an improvement in the scaling only when the polar group has an amide bond in the major linkage. Analysis of the alkyl chain structures has revealed that a structural requirement for the recovery of the water retaining capacity is the presence of saturated-straight alkyl chains, not unsaturated or branched alkyl chains. These structural characteristics required for water-retaining function also paralleled their capacity to form multiconcentric lamellae vesicles in vitro which is also capable of acquiring bound water as shown by DSC thermograms. The present study suggests that ceramides with relatively shorter alkyl chain length serve as a water modulator in the multi-lipid bilayers through the stratum corneum.  相似文献   
1000.
The influence of water soluble meat protein (WSMP) /glyceryl monostearate/glyceryl distearate ratio on the time-dependent reduction of interfacial tension at the corn oil-water Interface has been investigated at pH's 2.5, 3.5 and 9.5.The total WSMP-glycerides content was kept constant at 1.0% wt/wt. Both variables had an effect, with maximum reduction in the steady state interfacial tension occurring when using 0.5% WSMP in conjunction with 0.5% glycerides (monoglyceride/diglyceride ratio 3/1) at pH 3.5.Under these conditions the WSMP and glycerides associate to form a complex at the interface. At other WSMP/glycerides ratios the interfacial film has a more heterogeneous composition,.  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