全文获取类型
收费全文 | 7082篇 |
免费 | 1421篇 |
国内免费 | 1714篇 |
专业分类
化学 | 2976篇 |
晶体学 | 263篇 |
力学 | 361篇 |
综合类 | 209篇 |
数学 | 582篇 |
物理学 | 2165篇 |
无线电 | 3661篇 |
出版年
2024年 | 32篇 |
2023年 | 112篇 |
2022年 | 241篇 |
2021年 | 235篇 |
2020年 | 203篇 |
2019年 | 160篇 |
2018年 | 196篇 |
2017年 | 243篇 |
2016年 | 200篇 |
2015年 | 332篇 |
2014年 | 425篇 |
2013年 | 542篇 |
2012年 | 528篇 |
2011年 | 626篇 |
2010年 | 638篇 |
2009年 | 601篇 |
2008年 | 660篇 |
2007年 | 707篇 |
2006年 | 597篇 |
2005年 | 528篇 |
2004年 | 409篇 |
2003年 | 308篇 |
2002年 | 307篇 |
2001年 | 291篇 |
2000年 | 331篇 |
1999年 | 162篇 |
1998年 | 77篇 |
1997年 | 66篇 |
1996年 | 54篇 |
1995年 | 34篇 |
1994年 | 48篇 |
1993年 | 62篇 |
1992年 | 41篇 |
1991年 | 32篇 |
1990年 | 28篇 |
1989年 | 22篇 |
1988年 | 20篇 |
1987年 | 21篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 8篇 |
1981年 | 13篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Two New C—21 Steroidal Glycosides from Cynanchum aurichulatum 总被引:2,自引:0,他引:2
Two new C-21 steroidal glycosides, cynanauriculoside I and cynanauriculoside Ⅱ, were isolated from the roots of Cynanchum aurichulatum. Their structures were established using spectroscopic methods including one and two-dimensional NMR. 相似文献
82.
A novel super molecule [Na(C60H80O12)]+[Au(SCN)4]- was obtained by extraction of calix[4] arene (L) for an aqueous solution containing [AuCl4]-, and NaCl into CH2Cl2. The crystal structure and properties of the title super molecule were characterized by single-crystal X-ray diffraction, IR and 1H NMR spectra. The X-ray single crystal structure analysis shows that the crystal was Tetragonal system with space group P4/n and the unit cell parameters were as follows: a=2.880 8(18) nm, b=2.880 8(18) nm, c=2.292 4(5) nm, and final R indices [I>2σ(I)]: R1=0.048 5, wR2=0.106 1. CCDC: 243229. 相似文献
83.
Polyurethanes are one kind of relic protection materials commonly used. During artificial photo-ageing, three polyurethanes, HDI-based polyurethane, MDI-based polyurethane and TDI-based polyurethane, have been considered to undergo UV radiation. Photochemical degradation of the polyurethanes has been monitored by means of Fourier transform infrared spectroscopy with attenuated total reflection accessory (FTIR-ATR). It was proved that the mechanism of the photochemical degradation of polyurethanes might be the scissions of carbamate (urethane) groups and the re-reactions of radical groups formed in the scission reactions. From the experiment results HDI-based polyurethane, an aliphatic diisocyanate, could be considered to be more suitably used as relic protection materials among these three polyurethanes for its ageing products with less color. 相似文献
84.
Complex films of crosslinked poly(methylsiloxane-co-ethylene oxide) and lithium perchlorate were prepared. These solid state polymeric electrolytes show a markedly higher ionic conductivity, and excellent flexibility. The ionic conductivity of the network films closed to 10~(-5) Scm~(-1) at room temperature. The effects of Li~+ content, species and contents of crosslinking agents, molecular weight of poly(ethylene oxide) and temperature on the ionic conductivity of the network films were also investigated. 相似文献
85.
A polystyrene-bonded stannic chloride catalyst was synthesized by the method of lithium polystyryl combined with stannic chloride. This catalyst is a polymeric organometallic compound containing 0.25 mmol Sn (Ⅳ)/g catalyst. The catalyst showed sufficient stability and catalytic activity in organic reaction such as esterification, acetalation and ketal formation, and it could be reused many times without losing its catalytic activity. 相似文献
86.
A star network polymer with a pentaerythritol core linking four PEG-block polymeric arms was synthesized, and its corresponding gel polymer electrolyte based on lithium perchlorate and plasticizers EC/PC with the character being colorless and highly transparent has been also prepared. The polymer host was characterized and confirmed to be of a star network and an amorphous structure by FTIR, ^1H NMR and XRD studies. The polymer host hold good mechanical properties for pentaerythritol cross-linking. Maximum ionic conductivity of the prepared polymer electrolyte has reached 8.83 × 10 ^-4 S·cm^-1 at room temperature. Thermogravimetry (TG) of the polymer electrolyte showed that the thermal stability was up to at least 150 ℃. The gel polymer electrolyte was further evaluated in electrochromic devices fabricated by transparent PET-ITO and electrochromically active viologen derivative films, and its excellent performance promised the usage of the gel polymer electrolyte as ionic conductor material in electrochrornic devices. 相似文献
87.
88.
与轻质油品(汽油、柴油等)燃烧特性不同,重质燃料油在燃烧时,首先发生燃料的热分解,分解的产物扩散到气相空间发生着火、燃烧,最后剩下固体残炭,然后残炭再着火、燃烧,由于残炭的燃烧属于异相反应,比燃料气与空气的同相反应慢,导致其燃尽时间明显变长,文献[1]给出了沥青和十二烷单滴的燃烧过程温度变化曲线,从曲线可以看出,残炭的燃烧阶段明显比气体的燃尽时间长,而残炭的质量只为沥青质量的10%~20%[2,3]。虽然残炭含量低,但它在整个燃烧过程中占有十分重要的地位;文献[4]利用热重系统研究了渣油热加工的副产物石油焦的燃烧特性,主要从… 相似文献
89.
90.
Classical antifolates containing L-glutamic acid moiety in molecule have shortcomings such as drug resistance which is originated from the defective cell transport by mutation, and toxicity to the host which is due to unnecessarily long retention inside normal cells.[1] One strategy to overcome these shortcomings is to design nonclassical lipophilic inhibitors of folate requiring enzymes by deleting or modifying L-glutamic acid component from the folate analogues. 相似文献