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   3979篇
  免费   196篇
  国内免费   49篇
化学   2707篇
晶体学   54篇
力学   93篇
数学   527篇
物理学   843篇
  2023年   23篇
  2022年   43篇
  2021年   73篇
  2020年   66篇
  2019年   75篇
  2018年   48篇
  2017年   51篇
  2016年   131篇
  2015年   103篇
  2014年   121篇
  2013年   251篇
  2012年   287篇
  2011年   328篇
  2010年   210篇
  2009年   160篇
  2008年   257篇
  2007年   219篇
  2006年   236篇
  2005年   195篇
  2004年   168篇
  2003年   160篇
  2002年   134篇
  2001年   73篇
  2000年   79篇
  1999年   68篇
  1998年   27篇
  1997年   53篇
  1996年   42篇
  1995年   40篇
  1994年   31篇
  1993年   45篇
  1992年   51篇
  1991年   41篇
  1990年   36篇
  1989年   32篇
  1988年   17篇
  1987年   19篇
  1986年   18篇
  1985年   33篇
  1984年   25篇
  1983年   6篇
  1982年   17篇
  1981年   15篇
  1980年   11篇
  1979年   20篇
  1978年   14篇
  1977年   9篇
  1976年   6篇
  1974年   13篇
  1973年   10篇
排序方式: 共有4224条查询结果,搜索用时 279 毫秒
91.
The urinary codeine-to-morphine ratios in fifteen volunteers administered codeine tablets at intervals were studied by gas chromatography (GC) and compared with one month's GC results for enzyme multiplied immunoassay technique (EMIT)-screened urine specimens in a mass-screening programme for abuse of opiate drugs, particularly heroin. It appears that when M less than 2 and C/M greater than 0 or when M greater than 2 and C/M greater than 0.5, where C and M are codeine and morphine concentrations in micrograms per 10 ml of urine, codeine consumption has to be presumed.  相似文献   
92.
This article describes the structure determination of five homoleptic d(10) metal-aryl/alkylacetylides [RC triple bond CM] (M=Cu, R=tBu 1, nPr 2, Ph 3; R=Ph, M=Ag 4; Au 5) by using X-ray single-crystal and powder diffraction. Complex 1.C6H6 reveals an unusual Cu20 catenane cluster structure that has various types of tBuC triple bond C-->Cu coordination modes. By using this single-crystal structure as a starting model for subsequent Rietveld refinement of X-ray powder diffraction data, the structure of the powder synthesized from CuI and tBuC triple bond CH was found to have the same structure as 1. Complex 2 has an extended sheet structure consisting of discrete zig-zag Cu4 subunits connected through bridging nPrC triple bond C groups. Complex 3 forms an infinite chain structure with extended Cu-Cu ladders (Cu-Cu=2.49(4)-2.83(2) A). The silver(I) congener 4 is iso-structural to 3 (average Ag-Ag distance 3.11 A), whereas the gold(I) analogue 5 forms a Au...Au honeycomb network with PhC triple bond C pillars (Au-Au=2.98(1)-3.26(1) A). Solid-state properties including photoluminescence, nu(C triple bond C) stretching frequencies and thermal stability of these polymeric systems are discussed in the context of the determined structures.  相似文献   
93.
Silver ions dissolved in a polymer matrix are additionally coordinated by carbonyl oxygens of asparagines and their counter anions interact with cationic sites, resulting in the enhanced activity of the silver ion as an olefin carrier for facilitated olefin transport.  相似文献   
94.
95.
96.
Styrene was polymerized in the presence of benzoyl peroxide (BPO) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) to yield polystyrene-TEMPO (PS-TEMPO) macroinitiator. The PS-TEMPO macroinitiator with different molecular weight was reacted with 4-vinyl pyridine (4-VP) to synthesize polystyrene-block-poly(4-vinyl pyridine) (PS-b-PVP), which was then quaternized with n-octyl iodide. Antibacterial activity of the quaternized copolymers was assessed against a gram negative bacterium (Pseudomonas aeruginosa) and a gram positive one (Staphylococcus aureus) by using the shake flask test method, and the results were compared with those of poly(styrene-random-4-vinyl pyridine) (P(ST-r--VP)). The quaternized copolymers inhibited greatly the growth of the bacteria, and PS-b-PVP was more active than P(ST-r-VP), which was ascribed to the fact that the content of quaternized 4-VP units on the surface of the particles of the former copolymer was higher than that corresponding to the latter one.  相似文献   
97.
The epoxy-impregnated graphite tube electrode bulk modified with 2-mercaptobenzoxazole, employed in a wall-jet configuration, was found to be useful for the continuous flow and flow injection stripping voltammetric determinations of AgI, HgII and BiIII. For continuous flow, detection limits for AgI, HgII and BiIII were 1.8 × 10−10 M, 1.9 × 10−9 M and 9.5 × 10−9 M, respectively (10 min accumulation, S/N = 3). Precisions for 5.00 × 10−9 M AgI, 1.00 × 10−8 M HgII and 1.00 × 10−7 M BiIII were 10.5%, 5.77 % and 7.90% (relative standard deviations, n = 6), respectively. In the case of flow injection stripping, with a 500 μL injection loop, detection limits of 0.59 ng, 2.0 ng and 120 ng were obtained for AgI, HgII and BiIII, respectively (S/N = 3). Selected metal ions, inorganic and organic substances were investigated for interferences. The electrode was tested with a certified sample and then applied to the determinations of the metal ions in a urine and a sea-water sample.  相似文献   
98.
99.
(+)-SCH 351448 (Na+ salt A) was synthesized employing ring-closing olefin metathesis reaction of an open diene diester intermediate for construction of the 28-membered macrodiolide structure. The open diene diester was prepared from the monomeric hydroxy carboxylic acid and two different olefin fragments. The monomeric hydroxy acid was synthesized via Julia-Julia coupling reaction of intermediates derived from the same olefinic fragments. Oxane units in these fragments were prepared by radical cyclization reactions of beta-alkoxyacrylates. Analogous SCH 351448 salts incorporating other mono- and divalent cations may be prepared. Under acidic conditions, SCH 351448 (Na+ salt A) was the most stable complex, but SCH 351448 (Ca2+ salt) and (Na+ salt B) appear to be physiologically important species.  相似文献   
100.
New diamino monomers IIa – IIg were synthesized in a two-step reaction sequence starting from p-acetamidobenzenesulfonyl chloride. Solution polymerization of these monomers in DMAC with terephthaloyl or isophthaloyl chloride resulted in the formation of a series of 14 poly(amide sulfonamide)s (PASAs) in excellent yield (> 95%). The polymers have in-trinsic viscosities of 0.32–1.11 dL g?1. Except for polymers IIIa ? p and IIId - p , all other PASAs were readily soluble in aprotic polar solvents including DMAC, DMF, and DMSO. Thermogravimetric analyses of the polymers showed moderate thermal stability with 10% weight loss being recorded in the range of 325–408°C. In addition, these polymers exhibit moderate chemical stabilities toward alkali, acidic, and chromic acid solution. © 1995 John Wiley & Sons, Inc.  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