首页 | 官方网站   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   3820篇
  免费   485篇
  国内免费   483篇
数理化   4788篇
  2024年   4篇
  2023年   58篇
  2022年   78篇
  2021年   101篇
  2020年   134篇
  2019年   138篇
  2018年   115篇
  2017年   81篇
  2016年   162篇
  2015年   135篇
  2014年   196篇
  2013年   251篇
  2012年   331篇
  2011年   324篇
  2010年   221篇
  2009年   193篇
  2008年   229篇
  2007年   215篇
  2006年   210篇
  2005年   183篇
  2004年   156篇
  2003年   158篇
  2002年   174篇
  2001年   136篇
  2000年   109篇
  1999年   132篇
  1998年   79篇
  1997年   62篇
  1996年   89篇
  1995年   70篇
  1994年   34篇
  1993年   40篇
  1992年   26篇
  1991年   31篇
  1990年   30篇
  1989年   22篇
  1988年   18篇
  1987年   17篇
  1986年   14篇
  1985年   12篇
  1984年   6篇
  1983年   5篇
  1982年   2篇
  1981年   1篇
  1980年   3篇
  1970年   1篇
  1963年   1篇
  1936年   1篇
排序方式: 共有4788条查询结果,搜索用时 703 毫秒
991.
建立了同时测定脂肪中11种苯并咪唑类药物残留标志物的高效液相色谱法。样品采用正己烷溶解后,加入抗氧化剂,用0.1 mol/L盐酸-乙腈(1∶1,体积比)提取,经正己烷脱脂后再次加入抗氧化剂,以MCX固相萃取柱净化。净化液浓缩处理后进行高效液相色谱分离,以甲醇-1%乙酸为流动相梯度洗脱,流速为1.0 mL/min,紫外检测波长为292 nm,采用外标法以峰面积定量检测。11种目标物均在25.0~1 000μg/L范围内线性良好,方法定量下限为50μg/kg,在3个加标水平下的回收率为82.0%~109%,相对标准偏差(RSD)为0.71%~9.9%。该方法能够满足脂肪中苯并咪唑类药物残留标志物的检测要求。  相似文献   
992.
This study investigates the recovery of oily sludge using ultrasound-assisted rhamnolipids and uses oil recovery yield as an evaluation index. The Box–Behnken response surface method was employed to investigate the individual and interactive effects of four different operating factors: frequency, dosage, liquid–solid ratio, and stirring speed. The model optimization results showed that the order of degree of influence of these four factors is frequency > liquid–solid ratio > dosage > stirring speed. The mathematical model predicted the highest oil recovery yield as 92.27%, which was highly accurate (in the 95% confidence interval) as from the experimental results, the highest oil recovery yield was 89.95% under optimal reaction conditions (frequency = 25.58 kHz, dosage = 150.45 mg/L, liquid–solid ratio = 4.1:1 mL/g, and stirring speed = 407 rpm). Thus, the deviation from the prediction model was only 2.32%, indicating that this method provides a theoretical basis for the treatment of oily sludge and can be implemented for practical application in Huaidong in the Xinjiang Province.  相似文献   
993.
高分子负载锰卟啉的合成及其催化烯烃环氧化反应研究   总被引:8,自引:0,他引:8  
本文用羟基取代四苯基卟啉锰通过醚键与聚苯乙烯树脂键联,合成了一类高分子负载化锰卟啉化合物。经ESR、IR、UV-vis散射光谱等测定证实其结构。本文同时考察了它们对烯烃环氧化反应的催化活性,结果表明,负载化金属卟啉在催化烯烃环氧化反应中能集均相和多相催化的优点于一体,不仅催化剂稳定性增强,而且易于分离和重复使用。  相似文献   
994.
Three new N,N′-dialkylimidazolium salts of cadmium-thiocyanate, [EtMeIm]2[Cd2(SCN)6] (2), [C6H4(CH2ImMe)2][Cd(SCN)4] (3), [C6Me3(CH2ImMe)3][CdBr3(SCN)](NO3) (4) have been prepared, and their crystal structures have been determined by X-ray diffraction. Crystal data: 2, monoclinic, C2/c, a=18.349(4) Å, b=7.8667(18) Å, c=21.399(5) Å, β=110.346(4)°, V=2896.1(11) Å3, Z=4, and R1=0.0561; 3, monoclinic, C2/c, a=20.347(7) Å, b=14.029(5) Å, c=9.380(3) Å, β=112.034(6)°, V=2482.1(15) Å3, Z=4, and R1=0.0397; 4, hexagonal, P63, a=b=10.7634(8) Å, c=16.0315(17) Å, V=1608.4(2) Å3, Z=2, and R1=0.0569. Compound 2 consists of triply bridged infinite one-dimensional cadmium-thiocyanate chains, and two independent cadmium atoms are octahedrally coordinated in 2N4S and 4N2S geometry, respectively. In 3, the cadmium atom is octahedrally coordinated with two cis N-bonded monodentate NCS ligands and four bridging SCN in a S trans to S, and N trans to N coordination fashion, and thus form doubly bridged infinite one-dimensional chains. Whereas 4 is mononuclear, consisting of a discrete [C6Me3(CH2ImMe)3]3+ cation, a nitrate, and [CdBr3(SCN)] ion, and each cadmium(II) ion is coordinated to three bromide and one nitrogen atom of SCN ion. The structures of these compounds are dictated by the imidazolium cations.  相似文献   
995.
A layer silicate‐like zinc(II) benzimidazolate polymer {[Zn2(Bim)3(OH)(H2O)]·(DMF)(H2O)3} ( 1 ) was synthesized at room temperature and characterized with X‐ray single‐crystallography: Monoclinic, space group C2/m (No.12), a = 10.423(3) Å, b = 17.416(6) Å, c = 16.583(5) Å, β = 92.698(4), V = 3006.8(17) Å3.  相似文献   
996.
The thio­semicarbazone mol­ecule in C10H12N6S2·2C3H7NO has symmetry. The thio­semicarbazone moiety and the benzene ring are essentially coplanar, with mean and maximum deviations of 0.03 and 0.11 Å, respectively. The di­methyl­form­amide mol­ecules bridge the thio­semicarbazone moieties, forming two‐dimensional sheets through N—H?O hydrogen bonds.  相似文献   
997.
A series of statistical copolymers (poly[(9,9‐di‐n‐hexylfluorene)‐co‐2‐{2,6‐bis‐[2‐(4‐diphenylaminophenyl)vinyl]pyran‐4‐ylidene}malononitrile) were synthesized by the Suzuki coupling reaction. The copolymers showed absorption bands at 379 and 483–489 nm, which were attributed to the oligofluorene segments and the segments containing 2‐[2,6‐bis(2‐{4‐[(4‐bromophenyl)phenylamino]phenyl}vinyl)pyran‐4‐ylidene]malononitrile ( 3 ), respectively. The absorption band around 483–489 nm increased with the feed ratio of 3 . The photoluminescence (PL) spectra of the copolymers showed emission bands at 420 and 573–620 nm. As the feed ratio of 3 increased, the PL emission in the longer wavelength region redshifted, and the intensity increased as well. The electroluminescence (EL) spectrum of the copolymers showed a very weak emission at 420 nm. The PL and EL emission colors redshifted dramatically with the increase in the feed ratio of 3 . The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the model compound (2‐{2,6‐bis[2‐(4‐diphenylaminophenyl)vinyl]pyran‐4‐ylidene}malononitrile) were determined to be ?5.34 and ?3.14 eV, respectively. It was concluded that energy transfer took place from the oligofluorene blocks to the segments containing 3 and that direct charge trapping occurred in the segments containing 3 during the EL operation. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the copolymer (x = 0.63, y = 0.37) containing 10 mol % 3 were very close to those (x = 0.67, y = 0.33) for National Television System Committee (NTSC) red with a maximum photometric power efficiency of 0.27 cd/A. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3729–3737, 2006  相似文献   
998.
The monomeric octa-aza bis-α-diimine macrocyclic complex [CoII(C10H20N8)(H2O)](ClO4)2 I, undergoes various reactions on the macrocyclic ligand. Reaction of complex I with triethylamine in double molar proportions, followed by slow aerial oxidation, produces a molecular dimeric complex [CoII(C10H14N8)]2, III, and a novel Co(I) complex [CoI(C10H19N8)], IV. Complex III is a staggered cofacial dimer with a cobalt-cobalt bond length 2.86(1) Å. The macrocyclic ligand of the complex contains an a-diimine function in each five-membered chelate ring, and a three-atom N-C-N? delocalized system in each six-membered chelate ring. Complex IV has the 5-5-6-6 chelate arrangement because one α-diimine moiety is rearranged to a syn-anti configuration. In the structure, the two fused six-membered chelate rings are fully conjugated and the two fused five-membered rings are saturated. However, when complex I reacts with excess triethylamine under the similar conditions, a dimeric complex of another type, [CoII(C10Hl6N8)]2, II, was generated, in which one N-N bond of the macrocyclic ligand is broken. Complex IV can be isolated also from the reaction of complex I with excess hydrazine, followed by slow aerial oxidation. When hydrazine in double molar proportions was used, complex [CoI(C10H17N8)(NHNH)] V, which contains a coordinated diazene ligand, was obtained. Only one six-membered chelate ring of complex V is deprotonated and oxidized to form a three-atom N-C-N? delocalized system. The structures of octa-aza complexes I-V are determined by X-ray crystallography: I, orthorhombic, C mca, a = 11.646(4), b = 17.049(3), c = 10.706(3) Å, Z = 4, R = 0.045, Rw = 0.047, based on 1024 reflections with I > 2σ(I); II, monoclinic, P 21/c, a = 9.814(3), b = 22.583(6). c = 14.632(9) Å, β = 98.90(5)°, Z = 4, R = 0.085, Rw = 0.101, based on 2033 reflections with I > 2σ(I); III, tetragonal, P 4/nmm, a = 15.614(3), c = 6.498(2) Å, Z = 4, R = 0.081, Rw = 0.115, based on 340 reflections with I > 2σ(I); IV, orthorhombic, P bca, a = 8.484(1), b = 16.662(3), c = 18.760(2) Å, Z = 8, R = 0.029, Rw = 0.024, based on 1441 reflections with I > 2σ(I); V, monoclinic, P 21/m, a = 7.892(3), b = 11.713(6), c = 9.326(4) Å, β = 108.03(3), Z = 2, R = 0.047, Rw = 0.056, based on 948 reflections with I > 2σ(I).  相似文献   
999.
The diorganotin(Ⅳ) complexes of N-(3,5-dibromosalicylidene)-α-amino acid, R2Sn(2-O-3,5-Br2C6H2CH= NCHRCOO)(where R=H, Me, i-Pr, Bz; R'=n-Bu, Cy), were synthesized by the reactions of diorganotin dichlorides with in situ formed potassium salt of N-(3,5-dibromosalicylidene)-α-amino acid and characterized by elemental analysis, IR and NMR (^1H, ^13C and ^119Sn) spectra. The crystal structures of n-Bu2Sn(2-O-3,5-Br2C6H2CH= NCHRCOO)(R=i-Pr, Bz) and Cy2Sn(2-O-3,5-Br2C6H2CH=NCHRCOO)(R=Me, Bz) were determined by X-ray single crystal diffraction and showed that the tin atoms are in a distorted trigonal bipyramidal geometry to form five- and six-membered chelate rings with the tridentate ligand. Bioassay results indicated that the compounds possess better in vitro antitumour activity against three human tumour cell lines, HeLa, CoLo205 and MCF-7, than cis-platin and moderate anti-bacterial activity against two bacteria, E. coli and S. aureus.  相似文献   
1000.
用溴化四正丁基铵作为相转移催化剂,苯甲醛、氯仿、α-苯基乙胺(la~c)和固体氢氧化钾在-5~0℃于无水乙腈中反应。反应产物2a~c经水解,氢解得到相应的光学活性苯甘氨酸(4a~c)。由1-苯基-2,2,2-三氯乙醇与1a反应也可制得2a,并对反应机理进行了讨论。  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-23

京公网安备 11010802026262号