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   3074篇
  免费   99篇
  国内免费   518篇
化学   3244篇
晶体学   30篇
力学   12篇
综合类   7篇
数学   4篇
物理学   394篇
  2024年   2篇
  2023年   36篇
  2022年   45篇
  2021年   45篇
  2020年   56篇
  2019年   70篇
  2018年   69篇
  2017年   77篇
  2016年   82篇
  2015年   79篇
  2014年   127篇
  2013年   230篇
  2012年   167篇
  2011年   173篇
  2010年   168篇
  2009年   177篇
  2008年   190篇
  2007年   190篇
  2006年   179篇
  2005年   172篇
  2004年   179篇
  2003年   138篇
  2002年   183篇
  2001年   111篇
  2000年   97篇
  1999年   94篇
  1998年   79篇
  1997年   60篇
  1996年   67篇
  1995年   69篇
  1994年   49篇
  1993年   53篇
  1992年   38篇
  1991年   26篇
  1990年   22篇
  1989年   26篇
  1988年   15篇
  1987年   2篇
  1986年   10篇
  1985年   7篇
  1984年   6篇
  1983年   4篇
  1982年   3篇
  1981年   3篇
  1980年   5篇
  1978年   2篇
  1977年   2篇
  1974年   3篇
  1972年   1篇
  1971年   1篇
排序方式: 共有3691条查询结果,搜索用时 10 毫秒
91.
铂(Ⅳ)—钼酸盐—丁基罗丹明B缔合显色反应的研究   总被引:7,自引:0,他引:7  
本文研究在聚乙烯醇(PVA)存在下,铂与钼酸铵和丁基罗丹明B(BRB)形成离子缔合物,该缔合物的最大吸收位于570nm,摩尔吸光系数ε值为9.26×10~5L·mol~(-1)·cm~(-1),服从比耳定律范围0~3.0μg Pt/25ml,缔合物至少稳定3周。测定铂的条件为C_(HClO_4)=1.4mol/L,C_(MoO_4~(2-)=1.1×10~(-3)mol/L,C_(BRB)=3.8×10~(-5)mol/L,0.08%PVA。考察了40多种共存离子的影响,除形成杂多酸的元素外,大多数不干扰。本法已用于催化剂和某些铂矿中铂的测定,结果较满意。  相似文献   
92.
甲醇/甲基叔丁基醚混合物的膜法分离,大多采用渗透汽化方法,少有采用蒸汽渗透法。用聚酰亚胺中空纤维膜,对以蒸汽渗透和渗透汽化两种方式分离甲醇/甲基叔丁基醚混合物(甲醇质量分数为0.01-0.30)的效果进行了对比。结果显示,在甲醇质量分数低于0.05时,蒸汽渗透较渗透汽化法的分离性能优越。  相似文献   
93.
A very simple, selective and sensitive spectrophotometric method for the determination of dodecyl benzene sulfonate (DBS) has been investigated. In this method, the ion pair of DBS and methylene blue was adsorbed onto plasticized poly(vinylchloride) membrane as a solid phase extraction medium. The absorbance of the blue membrane was measured at 660nm against a blank membrane using a spectrophotometer. The influence of type and amount of plasticizer, pH, concentration of methylene blue, temperature of sample solution and time of extraction on the absorbance of membrane was studied. Linear calibration was obtained over a concentration range of 0.1 to 6.5µgmL–1 of DBS. The method was applied to the determination of DBS in different commercial detergents. The precision and accuracy of the method is comparable with the conventional standard method. This method is cost-effective, and the use of organic solvent such as chloroform is completely eliminated.  相似文献   
94.
单扫描示波极谱法测定扑热息痛   总被引:2,自引:0,他引:2  
扑热息痛与NaNO2在中性介质中发生反应,其亚硝化衍生物具有良好的电活性。在pH9.37的Britton-Robinson缓冲溶液中,于-0.56V(vs.SCE)产生灵敏的极谱还原峰,峰电流与扑热息痛质量浓度在0.007-0.4mg/L范围有良好的线性关系,检测下限为5μg/L。此法已用于测定小儿速效感冒灵。  相似文献   
95.
The synthesis and characterization by size exclusion chromatography, liquid chromatography, NMR, matrix‐assisted laser desorption/ionization, thermal analysis, and other techniques of well‐defined and narrow molecular weight distribution macrocyclic polystyrene (PS), poly(2‐vinylpyridine), poly(α‐methylstyrene), poly (2‐vinyl‐naphthalene) (P2VN), and poly(9,9‐dimethyl‐2‐vinylfluorene) (PDMVF) containing a single 1,4‐benzylidene, methylidene, or 9,10‐anthracenylidene unit are reviewed. The absorption and emission spectroscopy of PS, P2VN, and PDMVF is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2139–2155, 2006  相似文献   
96.
The bacterial degradation of naphthalene and other PAH in liquid media by pure cultures was investigated by an isothermic flow-calorimeter. Anabolic and catabolic processes were found to be separated distinctly. The phase of exponential heat production was developed only weakly. This was attributed to the low solubility and the dependence of the dissolving rate of substrate as the rate determining step on cristal surface and diffusion.Methodical problems based on the local separation of the fermenter and the measuring cell. Because of the prevented gas exchange in the flow tubes and the enrichment of carbon dioxide metabolic activity of the microorganisms was lowered for times. The differences between the heat production in the fermenter and the flow tubes were reduced by developing a mathematical model.

  相似文献   
97.
The reaction of Pt2Ru4(CO)18, 1 with 1,8-bis(phenylethynyl)naphthalene, 2 has yielded two metal carbonyl cluster complexes: Ru2(CO)6[- 2-C10H6C4Ph2], 3 (60% yield) and Ru2Pt(CO)6[- 2-C10H6C4Ph2]2, 4 (8% yield). Both compounds were characterized by a single crystal X-ray diffraction analysis. Both products were formed as a result of fragmentation of the Pt2Ru4 cluster of 1. Compound 3 contains two ruthenium atoms. They are bridged by a tricyclic C10H6C4Ph2 ligand formed by the coupling of the two -carbon atoms of the alkyne groups. The -carbon atoms of the alkynes are -bonded to one of the ruthenium atoms to form a metallacycle and this entire group is -bonded to the second ruthenium atom. Compound 4 contains two ruthenium atoms with a platinum atom between them. This molecule contains two tricyclic C10H6C4Ph2 ligands similar to that in 3, and two metallacycles formed by coordination of the -carbon atoms of both ligands to the platinum atom. One ligand is -bonded to each of the ruthenium atoms.  相似文献   
98.
A chemiluminescence method for the determination of folic acid by the sodium hypochlorite–folic acid–semicarbazide hydrochloride system with a new flow injection technique has been established. The new method can perform simple, sensitive and rapid determinations of folic acid. The response to the concentration of folic acid, in the range of 1.0×10−75.0×10−5 g/ml, is linear. The relative standard deviation of the method is 2.3% (Cs=4.0×10−6 g/ml, n=11). The detection limit is 2.7×10−8 g/ml. This method is suitable for automatic and continuous analysis, and has been successfully tested for the determination of folic acid in a folic acid tablet.  相似文献   
99.
A supported liquid and a poly(vinyl chloride) (PVC)-based membrane selective for dodecylsulfate (DS) ion are described. The active element is a membrane containing a dissolved ion association complex of DS with cetylpyridinium (CP+) cation. The supported liquid membrane electrode (acetophenone as solvent) showed a Nernstian response towards the DS anion over the concentration range of sodium dodecylsulfate (SDS) from 8.3×10−3 to 1.0×10−6 mol dm−3 at 25 °C. The proposed electrode also showed a super-Nernstian potential response (108±2 mV decade−1) at low concentrations (1.0×10−9 to 1.0×10−6 mol dm−3). Moreover, this electrode showed good selectivity and precision (R.S.D.?2.0%), and was usable within the pH range 4.0-6.8. The proposed electrode revealed a lower limit of detection of 6.3×10−7 mol dm−3 and improved selectivity in comparison with the some previously reported DS ion selective electrodes. The isothermal temperature coefficient of this electrode amounted to −0.001 V °C−1. The liquid membrane electrode may find application in the direct determination of SDS by the standard addition method at pH 5.0, and in the physicochemical studies of surfactant solutions.  相似文献   
100.
Boiling temperature measurements have been made at ambient pressure for saturated ternary solutions of NaCl + KNO3 + H2O, NaNO3 + KNO3 + H2O, and NaCl + Ca(NO3)2 + H2O over the full composition range, along with those of the single salt systems. Boiling temperatures were also measured for the four component NaCl + NaNO3 + KNO3 + H2O and five component NaCl + NaNO3 + KNO3 + Ca(NO3)2 + H2O mixtures, where the solute mole fraction of Ca(NO3)2, x{Ca(NO3)2}, was varied between 0 and 0.25. The maximum boiling temperature found for the NaCl + KNO3 + H2O system is ≈134.9 C; for the NaNO3 + KNO3 + H2O system is ≈165.1 C at x(NaNO3) ≈ 0.46 and x(KNO3) ≈ 0.54; and for the NaCl + Ca(NO3)2 + H2O system is 164.7 ± 0.6 C at x{NaCl} ≈ 0.25 and x{Ca(NO3)2} ≈ 0.75. The NaCl + NaNO3 + KNO3 + Ca(NO3)2 + H2O system forms molten salts below their maximum boiling temperatures and the temperatures corresponding to the cessation of boiling (dry-out temperatures) of these liquid mixtures were determined. These dry-out temperatures range from ≈300 C when x{Ca(NO3)2} = 0 to ≥ 400 C when x{Ca(NO3)2} = 0.20 and 0.25. Mutual deliquescence/efflorescence relative humidity (MDRH/MERH) measurements were also made for the NaNO3 + KNO3 and NaCl + NaNO3 + KNO3 salt mixture from 120 to 180 C at ambient pressure. The NaNO3 + KNO3 salt mixture has a MDRH of 26.4% at 120 C and 20.0% at 150 C. This salt mixture also absorbs water at 180 C, which is higher than expected from the boiling temperature experiments. The NaCl + NaNO3 + KNO3 salt mixture was found to have a MDRH of 25.9% at 120 C and 10.5% at 180 C. The investigated mixture compositions correspond to some of the major mineral assemblages that are predicted to control brine composition due to the deliquescence of salts formed in dust deposited on waste canisters in the proposed nuclear repository at Yucca Mountain, Nevada.  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