全文获取类型
收费全文 | 55691篇 |
免费 | 6220篇 |
国内免费 | 11137篇 |
专业分类
化学 | 47677篇 |
晶体学 | 1915篇 |
力学 | 1132篇 |
综合类 | 472篇 |
数学 | 3478篇 |
物理学 | 18374篇 |
出版年
2024年 | 111篇 |
2023年 | 942篇 |
2022年 | 1154篇 |
2021年 | 1901篇 |
2020年 | 1972篇 |
2019年 | 1752篇 |
2018年 | 1499篇 |
2017年 | 1969篇 |
2016年 | 2094篇 |
2015年 | 1807篇 |
2014年 | 2576篇 |
2013年 | 5102篇 |
2012年 | 3552篇 |
2011年 | 3876篇 |
2010年 | 3268篇 |
2009年 | 3999篇 |
2008年 | 3912篇 |
2007年 | 3913篇 |
2006年 | 3689篇 |
2005年 | 3264篇 |
2004年 | 3107篇 |
2003年 | 2605篇 |
2002年 | 2173篇 |
2001年 | 1790篇 |
2000年 | 1681篇 |
1999年 | 1357篇 |
1998年 | 1158篇 |
1997年 | 1016篇 |
1996年 | 874篇 |
1995年 | 876篇 |
1994年 | 756篇 |
1993年 | 588篇 |
1992年 | 556篇 |
1991年 | 429篇 |
1990年 | 287篇 |
1989年 | 246篇 |
1988年 | 202篇 |
1987年 | 143篇 |
1986年 | 105篇 |
1985年 | 114篇 |
1984年 | 95篇 |
1983年 | 39篇 |
1982年 | 68篇 |
1981年 | 96篇 |
1980年 | 65篇 |
1979年 | 68篇 |
1978年 | 45篇 |
1977年 | 50篇 |
1976年 | 30篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
报道了四种不同P/Pd摩尔比的膦化聚2,6-二甲基1,4-苯醚负载把催化剂的加氢和异构化性能;通过XPS、电镜和远红外对催化剂进行了表征;并考察了溶剂和温度对催化剂活性的影响. 相似文献
992.
The title compound [Mn(phendione)(PDC)(H2O)2]·2H2O (H2PDC=pyridine-2,6-dicarboxylic acid) has been prepared in aqueous solution and characterized by single X-ray diffraction structure determination, elemental analysis, IR spectroscopy, and thermal analyses. The compound crystallizes in Monoclinic system, space group C2/c, a=1.017 51(11) nm, b=1.483 25(11) nm, c=1.461 21(13) nm, β=109.86(10)°, V=2.074 1(3) nm3, Z=4, F(000)=1 028, μ=0.701 mm-1, Dc=1.609 g·cm-3, R1=0.028 9, wR2=0.078 8 [I>2σ(I)]. Crystal structure reveals that complex consists of one-dimensional chain framework bridged by hydrogen bonds that formed by uncoordinated water and oxygen atom of carboxyl group in PDC2-. Furthermore, the complexes form a three-dimensional super-molecular structure through hydrogen bonds. CCDC: 648570. 相似文献
993.
Two ferrates, K2FeO4 and BaFeO4, had been prepared and characterized by XRD, IR and SEM. The electrochemical tests of the samples were carried out in the voltage range of 0.8~2.0 V and current density of 0.5~3.0 mA·cm-2. The results indicated that performance of Zn-BaFeO4 battery was superior to that of Zn-MnO2 and Zn-K2FeO4 batteries. Nanometer SrTiO3 prepared by Sol-gel methode with different ratio was added to the BaFeO4 cathode in order to improve the discharge performance. The discharge capacity of the BaFeO4 cathode was increased from 224 mAh·g-1 to 246 mAh·g-1 by addition of 5% nanometer SrTiO3. The reason of enhancing BaFeO4 electrochemical performance was discussed. 相似文献
994.
采用表面改性法制备了负载型光催化材料CeO2-TiO2/SiO2,并用X射线衍射,比表面积测定,红外光谱,程序升温还原和紫外可见漫反射光谱技术对固体材料的结构和光响应性能进行了表征.结果表明,CeO2与TiO2在材料表面存在相互修饰作用,CeO2能拓展TiO2的光响应范围,使TiO2吸光区域由紫外拓宽至可见光区,从而提高材料的光能利用率;Ti4 可以进入CeO2晶格,形成Ce-O-Ti键合,使得Ti4 平均配位数增加,同时提高了CeO2体相晶格氧的活性;TiO2有助于提高CeO2在载体表面的分散程度,减小CeO2的微晶尺寸,提高固体材料的能隙值和氧化还原能力. 相似文献
995.
采用共沉淀法制备了一系列Cu-Zr-Ce-O复合氧化物催化剂,考察了ZrO2加入量、不同再生方法对催化剂CO选择性氧化反应性能的影响,并通过DSC-TPR、XRD和SEM手段对催化剂进行了表征。结果表明,添加ZrO2的Cu1Zr1Ce9Oδ催化剂在160 ℃~200 ℃,具有99%以上的CO转化率,并且催化剂的选择性相对较高。适量ZrO2的加入能够细化催化剂的颗粒,提高催化剂的热稳定性,改变催化剂的聚结方式。经氮气、氢气及氧气再生处理后的Cu1Zr1Ce9Oδ催化剂,催化活性有所不同,其中经氧气处理后的催化剂,表面吸附氧体积分数较高,活性恢复较好。 相似文献
996.
Wanxing Wei Yuanjiang Pan Yaozu Chen Cuiwu Lin Tengyou Wei Shukai Zhao 《Chemistry of Natural Compounds》2005,41(1):17-21
Five compounds, terephthalic acid mono-[2-(4-carboxy-phenoxycarbonyl)-vinyl] ester (1), (E)-3-(5′-hydroperoxy-2,2′-dihydroxy[1,1′-biphenyl]-4-yl)-2-propenoic acid (2), 3,4,5-trihydroxybenzoic acid (3), succinic acid (or butanedioic acid) (4), and 2,3,4,5,6-pentahydroxybenzoic acid (5), were isolated from Phyllanthus urinaria. The structures of these compounds were elucidated by means of spectral techniques including IR, MS, and 1D/2D NMR. 1 and 2 are new compounds.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 14–17, January–February, 2005. 相似文献
997.
用双-(2-羟乙基)二硫代甲酸铵(HEDC)在反相液相色谱中作检测某些金属离子的衍生化试剂,HEDC的金属螯合物微溶于水,可直接水样注射于C18柱中进行检测,范围为0.006~10mg/L相对偏差1%~2%,检测波长254nm,金属汞的整合物在HPLC分析前进行浓缩富集检测限可低至0.06~25μg/L,相对偏差小于2%。 相似文献
998.
Using renewable green hydrogen and carbon dioxide (CO2) to produce methanol is one of the fundamental ways to reduce CO2 emissions in the future, and research and development related to catalysts for efficient and stable methanol synthesis is one of the key factors in determining the entire synthesis process. Metal nanoparticles stabilized on a support are frequently employed to catalyze the methanol synthesis reaction. Metal-support interactions (MSIs) in these supported catalysts can play a significant role in catalysis. Tuning the MSI is an effective strategy to modulate the activity, selectivity, and stability of heterogeneous catalysts. Numerous studies have been conducted on this topic; however, a systematic understanding of the role of various strengths of MSI is lacking. Herein, three Cu/ZnO-SiO2 catalysts with different strengths of MSI, namely, normal precipitation Cu/ZnO-SiO2 (Nor-CZS), co-precipitation Cu/ZnO-SiO2 (Co-CZS), and reverse precipitation Cu/ZnO-SiO2 (Re-CZS), were successfully prepared to determine the role of such interactions in the hydrogenation of CO2 to methanol. The results of temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization illustrated that the MSI of the catalysts was considerably affected by the precipitation sequence. Fourier transform infrared reflection spectroscopy (FT-IR) results indicated that the Cu species existed as CuO in all cases and that copper phyllosilicate was absent (except for strong Cu-SiO2 interaction). Transmission electron microscopy (TEM), X-ray diffraction (XRD), and N2O chemical titration results revealed that strong interactions between the Cu and Zn species would promote the dispersion of Cu species, thereby leading to a higher CO2 conversion rate and improved catalytic stability. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2 conversion, followed by the Co-CZS catalyst (12.1%), and the Nor-CZS catalyst (9.8%). After the same reaction time, the normalized CO2 conversion of the three catalysts decreased in the following order: Re-CZS (75%) > Co-CZS (70%) > Nor-CZS (65%). Notably, the methanol selectivity of the Re-CZS catalyst was found to level off after a prolonged period, in contrast to that of Co-CZS and Nor-CZS. Investigation of the structural evolution of the catalyst with time on stream revealed that the high methanol selectivity of the catalyst was caused by the reconstruction of the catalyst, which was induced by the strong MSI between the Cu and Zn species, and the migration of ZnO onto Cu species, which caused an enlargement of the Cu/ZnO interface. This work offers an alternative strategy for the rational and optimized design of efficient catalysts. 相似文献
999.
Conductance measurements for fifteen uni-univalent salts in 2-methylpyridine-N-oxide (2-MPO) were made over the concentration range of 2–120×10–4 mol-dm–3 at 55°C. All conductance data were evaluated by the Lee-Wheaton equation. The results indicate that all of the salts studied are slightly associated in 2-MPO, which is, nevertheless, a reasonably good ionizing solvent compared to other solvents having a similar value for the dielectric constant. The tetraalkylammonium halides are weaker electrolytes than the tetraphenylborate salts in 2-MPO. Ionic limiting molar conductances were obtained by using as a reference electrolyte tris(iso-pentyl)butylammonium tetraphenylborate. Anions appear to be poorly solvated in this solvent but certain cations, particularly Na+ and K+, show evidence of specific ion-solvent interaction.This study was presented, in part, at the 6th Central Regional Meeting of the American Chemical Society, Detroit, Michigan, April 1974. 相似文献
1000.