全文获取类型
收费全文 | 6446篇 |
免费 | 579篇 |
国内免费 | 3400篇 |
专业分类
化学 | 9957篇 |
晶体学 | 40篇 |
力学 | 21篇 |
综合类 | 85篇 |
数学 | 8篇 |
物理学 | 314篇 |
出版年
2024年 | 14篇 |
2023年 | 110篇 |
2022年 | 201篇 |
2021年 | 250篇 |
2020年 | 339篇 |
2019年 | 363篇 |
2018年 | 319篇 |
2017年 | 306篇 |
2016年 | 308篇 |
2015年 | 299篇 |
2014年 | 425篇 |
2013年 | 594篇 |
2012年 | 397篇 |
2011年 | 477篇 |
2010年 | 393篇 |
2009年 | 365篇 |
2008年 | 451篇 |
2007年 | 451篇 |
2006年 | 413篇 |
2005年 | 415篇 |
2004年 | 444篇 |
2003年 | 387篇 |
2002年 | 298篇 |
2001年 | 300篇 |
2000年 | 305篇 |
1999年 | 240篇 |
1998年 | 232篇 |
1997年 | 248篇 |
1996年 | 188篇 |
1995年 | 190篇 |
1994年 | 152篇 |
1993年 | 193篇 |
1992年 | 115篇 |
1991年 | 62篇 |
1990年 | 64篇 |
1989年 | 42篇 |
1988年 | 28篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
991.
纳米碳纤维可用在催化材料、储氢材料、及纳米电子器件等方面。本文对用泡沫镍及负载型镍催化剂催化分解乙烯或丙烯制备纳米碳纤维进行了研究。利用X射线衍射仪、物理吸附仪、扫描电镜进行了分析表征,并考察了催化剂、碳源、生长温度对纳米碳纤维生长量、形貌、结构的影响。结果表明:在生长温度450℃,乙烯流率30mL/m in的条件下,负载型镍催化剂纳米碳纤维的生长量要高出泡沫镍3~6倍,负载型镍催化剂制备的纳米碳纤维直径为40~60纳米,小于泡沫镍的情况。泡沫镍催化分解乙烯制备纳米碳纤维时,纳米碳纤维的生长量和平均直径随温度的降低而逐渐减小。纳米碳纤维在泡沫镍上的最低生长温度为420℃,在低于480℃生长纳米碳纤维时泡沫镍的骨架结构不会被破坏,由此制备的纳米碳纤维在新型结构催化材料中有很好的的应用前景。 相似文献
992.
M. S. Batista M. A. Morales E. Baggio-Saitovich E. A. Urquieta-González 《Hyperfine Interactions》2001,134(1):161-166
The Fe species present in Fe/ZSM-5 catalysts prepared by ionic exchange in aqueous solution or in the solid state are investigated. The samples were characterized by XRD, EPR, Mössbauer Spectroscopy (MOS-S) and chemical analysis. XRD showed that the peak intensity decreased with the increase in Fe content in the samples. EPR and MOS-S showed that the Fe atoms in all the activated samples appear as Fe+3, independently of the method used in the catalyst preparation. The MOS-S data also showed that Fe/ZSM-5, prepared by ion exchange in the solid state presented lower hematite content and more Fe species in charge-compensation sites, the responsible for the catalytic activity, as confirmed by the behavior in the propane oxidation. 相似文献
993.
The mechanism for the formation of C2+ paraffins differs from the mechanism for the formation of methane. While methane is obtained due to hydrogenation of methyl
groups, C2+ hydrocarbons are formed as the result of the conversion of alkyl intermediates into hydroalkene intermediates, which can
decompose to give olefins or undergo hydrogenation to give saturated hydrocarbons.
__________
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 175–179, May–June, 2008. 相似文献
994.
Asahi Motoshige Yasuteru Mawatari Yoshiaki Yoshida Chigusa Seki Haruo Matsuyama Masayoshi Tabata 《Journal of polymer science. Part A, Polymer chemistry》2012,50(15):3008-3015
Polymerization of p‐n‐hexyloxyphenylacetylene (pHPA) by using a [Rh(norbornadine)Cl]2‐triethylamine catalyst was carried out at room temperature to afford stereoregular helical poly(p‐n‐hexyloxyphenylacetylene)s (PpHPAs). When ethanol and n‐hexane were used as polymerization solvents, a bright yellow PpHPAs, poly( Y ) with Mn = 8.5 × 104 and its purple red polymer, poly( R ) with Mn = 5.3 × 104 were obtained in 95% yields and 84% yields, respectively. Diffuse reflective UV–vis spectra of poly( Y ) and poly( R ) in solid phase showed different broad absorption peaks at 445 and 575 nm, respectively. X‐Ray diffraction patterns of poly( Y ) and poly( R ) showed typical columnar structures assignable to cis‐transoid and cis‐cisoid structures, respectively, which were also supported by molecule mechanics calculation. Poly( Y ) was irreversibly transformed to a reddish‐black polymer, poly( Y‐B ), which columnar diameter was nearly the same as that of poly( R ). Further, poly( Y ) showed an exothermic peak in the differential scanning calorimetry trace at 80 °C for 1 h in N2 gas. Thus, these findings suggest a thermally irreversible rearrangement from an unstable cis‐transoid form, poly( Y ) with a stretched cis‐transoid helix to a stable cis‐cisoid form, poly( R ), with a contracted cis‐cisoid helix in the solid phase to give poly( Y → B ) with the cis‐cisoid form. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
995.
Zhenhua Li Ying Song Pa Du Xinbin Ma Baowei Wang Genhui Xu 《Reaction Kinetics and Catalysis Letters》2001,73(1):135-142
The kinetics of hydrolysis of tetrahydrozoline hydrochloride in aqueous solution at 353, 363 and 368 K in the pH-range from
2.00 to 12.20 has been investigated. The decomposition products have been investigated by HPLC.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
996.
G. Saracco F. Geobaldo D. Mazza G. Baldi 《Journal of Thermal Analysis and Calorimetry》1999,56(3):1435-1442
A new method for the synthesis of fine catalyst powders is presented. Catalyst precursors are dissolved in an ethanol-water
mixture which is burned through a nozzle-type burner. As a result, catalyst powders are formed and removed from flue gases
through a filter. LaMnO3 catalysts for the catalytic combustion of methane have been prepared, characterised (TEM, XRD) and compared with a reference
manfacturing method (the citrates one) showing promising, though improvable, results.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
997.
Kinetic
model of gas phase polymerization of 1,3-butadiene catalyzed by supported rare earth
coordination system 总被引:1,自引:0,他引:1
Gas phase polymerization of 1,3-butadiene (Bd) catalyzed by supported rare earth coordination system is studied and a new kinetic model is proposed. Four elementary reactions or processes: the process of exposure and activation of potential active catalytic center, propagation, deactivation and chain transfer reaction to alkyl aluminum, are considered in this model. Some important parameters, such as monomer-consuming rate, are well expressed as the functions of macroscopic polymerization conditions such as pressure, temperature, and duration. The model can simulate the whole polymerization procedure satisfactorily. 相似文献
998.
999.
研究了水/ 甲苯乳化液中二苯并噻吩( 硫芴) 在分散型钼酸、磷钼酸和四硫钼酸铵催化剂存在下的加氢脱硫反应. 反应在高压釜中于340 ℃及三种不同的气氛即H2 ,H2/H2 O和CO/H2O(CO 和H2O 经水煤气转换反应( WGSR) 产生原位氢) 的存在下进行. 用GC和GCMS鉴定、分析了气体和液体产物的组成. 结果表明: 对硫芴的加氢脱硫反应,在分散型四硫钼酸铵催化剂存在下,原位产生的氢的效果仅比加入的氢气稍好,而在分散型钼酸和磷钼酸催化剂存在下,原位产生的氢远比加入的氢气有效. 实验结果还表明: 硫化氢能显著提高分散型钼酸和磷钼酸催化剂的加氢脱硫活性,但在分散型四硫钼酸铵催化剂存在下,硫化氢能促进加氢反应而抑制氢解反应. 一氧化碳和水均选择性地抑制氢解反应. 相似文献
1000.
Co-salen functionalized on graphene as an efficient heterogeneous catalyst for cyclohexene oxidation
Jing Sun Jian Zhang Liang Wang Longfeng Zhu Xiangju Meng Feng-Shou Xiao 《Journal of Energy Chemistry》2013,22(1):48-51
Co-salen functionalized on graphene with an average pore size of 27.7 nm as a heterogeneous catalyst exhibited good catalytic activity and recyclability in cyclohexene oxidation. 相似文献