全文获取类型
收费全文 | 2822篇 |
免费 | 200篇 |
国内免费 | 596篇 |
专业分类
化学 | 3041篇 |
晶体学 | 135篇 |
力学 | 24篇 |
综合类 | 23篇 |
数学 | 3篇 |
物理学 | 392篇 |
出版年
2024年 | 7篇 |
2023年 | 21篇 |
2022年 | 49篇 |
2021年 | 72篇 |
2020年 | 106篇 |
2019年 | 74篇 |
2018年 | 65篇 |
2017年 | 91篇 |
2016年 | 106篇 |
2015年 | 117篇 |
2014年 | 147篇 |
2013年 | 317篇 |
2012年 | 187篇 |
2011年 | 143篇 |
2010年 | 134篇 |
2009年 | 151篇 |
2008年 | 155篇 |
2007年 | 165篇 |
2006年 | 170篇 |
2005年 | 154篇 |
2004年 | 138篇 |
2003年 | 154篇 |
2002年 | 116篇 |
2001年 | 88篇 |
2000年 | 82篇 |
1999年 | 91篇 |
1998年 | 70篇 |
1997年 | 47篇 |
1996年 | 60篇 |
1995年 | 54篇 |
1994年 | 42篇 |
1993年 | 32篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 18篇 |
1988年 | 20篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有3618条查询结果,搜索用时 46 毫秒
201.
Imidazole, 1,2,4-triazole, indole, and benzotriazole undergo conjugate additions with α, β-unsaturated carbonyl compounds in the presence of anhydrous potassium phosphate at ambient temperature to afford the corresponding Michael adducts in excellent yields. 相似文献
202.
A new and efficient method to synthesize a 3,3,6,6,9‐aryl‐1,8‐dioxo‐octahydroxanthene derivative using diammonium hydrogen phosphate as catalyst was performed in water at room temperature in a short periods. 相似文献
203.
Saeed Balalaie Morteza Bararjanian Masoumeh Sheikh‐Ahmadi Shohreh Hekmat Peyman Salehi 《合成通讯》2013,43(7):1097-1108
Diammonium hydrogen phosphate was used as a mild, efficient, neutral, and cheap catalyst for the synthesis of various 4H‐benzo[b]pyran derivatives via a one‐pot, three‐component condensation of aromatic aldehydes, active methylene compounds, and dimedone in aqueous media. 相似文献
204.
An efficient and environmentally benign method for the synthesis of organic carbamates was developed. Amines, CO2, and alkyl halides underwent a three-component reaction with the aid of K2CO3 and polyethylene glycol (PEG, MW = 400), affording the organic carbamates under ambient conditions. PEG could presumably act as a solvent and phase-transfer catalyst (PTC). Notably, the presence of PEG could also depress the alkylation of both the amine and the carbamate, thus resulting in enhanced selectivity toward the target carbamate. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
205.
Palladium supported on carbon (Pd/C) has been shown to be an effective catalyst for the cross-coupling of potassium p-tolyltrifluoroborate with a variety of aryl bromides and iodides. Yields ranging from moderate to good were obtained using Pd/C in ethanol/water mixtures with potassium carbonate as base at 50 °C under an air atmosphere. 相似文献
206.
A novel functionalized α-zirconium phosphate (F-ZrP) modified with intumescent flame retardant was synthesized by co-precipitation method and characterized. Poly (lactic acid) (PLA)/F-ZrP nanocomposites were prepared by melt blending method. The thermal stability and combustion behavior of PLA/F-ZrP nanocomposites were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL-94), scanning electronic microscopy (SEM), and cone calorimeter test (CCT). The results showed that the addition of flame retardant F-ZrP slightly affect PLA's thermal stability, but significantly improve the flame retardancy of PLA composites. In comparison with neat PLA, the LOI value of PLA/F-ZrP was increased from 19.0 to 26.5, and the UL-94 rating was enhanced to V-0 as the loading of F-ZrP at 10%. SEM results suggested the introduction of F-ZrP in the PLA system can form compact intumescent char layer during burning. All these results showed that the F-ZrP performed good flame retardancy for PLA. 相似文献
207.
《Journal of Dispersion Science and Technology》2013,34(5):683-689
Abstract The effects of two metal complexes of 2,2′‐dipyridylamine (bpya) ligand, [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2, in promoting the hydrolysis of bis(4‐nitrophenyl) phosphate (BNPP) have been kinetically investigated in Brij35 micellar solution and at 298 K, pH ranging from 6.41 to 8.6. In neutral micellar solution at 298 K, pH 7.02, the rate constants for the catalytic hydrolysis of BNPP by [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2 are 1.2 × 106 times and 1.5 × 105 times higher than those for the spontaneous hydrolysis, respectively. Kinetic studies show that the active species in the catalytic hydrolysis of BNPP is the aquo‐hydroxy form, and the relative kinetic and thermodynamic parameters indicate that the mechanism of the reaction involves intramolecular nucleophilic attack on the metal center‐bound diester. 相似文献
208.
为掌握生物质中钾对生物质超临界水降解过程的影响,选择生物质气化转化过程中生成的重要小分子中间产物甲醛作为研究对象,研究不同工艺条件下(反应温度400~650℃、压力23~29 MPa、停留时间4~12 s),单一钾成分(KHCO3/K2CO3/KCl)或混合钾成分(KHCO3、K2CO3、KCl)对甲醛超临界水降解气体产物的影响。结果表明,KHCO3、K2CO3、KCl、混合钾均降低了气体产物中CO的体积分数,提高了CO2的体积分数,但KCl的影响程度弱于其他三种钾成分。此外,由于不同钾成分均不利于气体产物中CO和H2体积分数的提高,从而使得气体产物的热值降低。KHCO3、K2CO3、KCl、混合钾均降低了H2、CO、CO2的产率以及气化率,其对H2产率以及气化率的抑制程度从大到小依次为:混合钾、KHCO3、K2CO3、KCl,且提高反应温度、延长反应停留时间时抑制气体生成的作用相对越明显。而当反应压力改变时,钾成分对H2产率及气化率的影响程度变化较小。较高反应温度、较高反应压力、较长停留时间时,混合钾中各成分出现协同作用,明显抑制了气体产物的生成。 相似文献
209.
WEN Lei HUANG Ya-Xi SUN Wei LIU Biaoa HUANG Chun-Zuo MI Jin-Xiao ZHUANG Rong-Chuan TANG Mei-Bo ZHAO Jing-Tai 《结构化学》2013,(12):1821-1828
A vanadyl phosphate containing a new member of tancoite-like single chain, (DAPH2)[VIVO(HPO4)2]·xH20 (x ≈ 0.2, DAP = 1,3-diaminopropane, C3H10N2), has been synthesized under hydro(solvo)thermal conditions. It crystallizes in orthorhombic space group P21212 (No. 18) with a = 7.1730(14), b = 19.252(4), c = 8.6557(17) A, Z= 4, V= 1195.3(4)A3, C3H14.38N2P2VO9.19, Mr = 338.47, Dc = 1.881 g/cm3,μ(MoKa) = 1.138 mm-1 and F(000) = 692. The final full-matrix least-squares refinement converged to R = 0.0408, wR = 0.1046 for 2498 observed reflections with I 〉 2σ(I) and R = 0.0456 and wR = 0.1080 for all data (2750) and S = 1.001. Its one-dimensional 1 structure consists of tancoite-like ∞1 {vIVO(HPO4)2}2- single chains surrounded by DAPH22+ ions and water molecules. The single chain is built from trans-corner-sharing octahedral {VIV= O…VIV} backbone loop-branched by HPO4 groups like staple forming a new member of tancoite single chain. Due to the special coordination of VIVO6, the ∞1 {VO(HPO4)2-} chain adopts a larger M-O-M angle (V-O-V = 135°) than those of tancoite chains reported before. The corner-sharing linear {VIV = O…VIV} chain structure also leads to a one-dimensional weak antiferromagnetic interaction at low temperature. The magnetic measurements confirm the 4+ valence state of vanadium. IR and TG results of the title compound are also discussed. 相似文献
210.
Attiq-ur-Rehman Amir Waseem Abdul Nabi Maqsood A. Khan 《International journal of environmental analytical chemistry》2013,93(14-15):1119-1129
Lucigenin chemiluminescence (CL) in conjunction with flow-injection analysis (FIA) is used for the determination of phosphate in freshwater samples. The procedure is based on the formation of molybdophosphoric heteropoly acid (MoP–HPA) by the reaction of phosphate and ammonium molybdate under acidic conditions. CL emission was observed as a result of oxidation of lucigenin in aqueous sodium hydroxide solution in the presence of MoP–HPA. Calibration was linear up to 500?µg?L?1 (r 2?=?0.9998; n?=?8), with a detection limit (S/N?=?3) of 0.95?µg?L?1. An injection throughput of 120 h?1, and relative standard deviation (RSD; n?=?4) of 1.3–3.2% were achieved in the concentration range studied. An on-line chelating column was used to remove interfering cations. The method was applied to freshwater samples, and the results (51?±?1.0 – 107?±?2.0?µg?L?1) did not differ significantly from results obtained using a spectrophotometric method (52.5?±?1.0 – 102?±?2.0?µg?L?1) at 95% confidence level (t-test). 相似文献