全文获取类型
收费全文 | 334篇 |
免费 | 11篇 |
国内免费 | 10篇 |
专业分类
化学 | 147篇 |
力学 | 11篇 |
综合类 | 3篇 |
数学 | 46篇 |
物理学 | 71篇 |
综合类 | 77篇 |
出版年
2018年 | 3篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 7篇 |
2013年 | 41篇 |
2012年 | 14篇 |
2011年 | 11篇 |
2010年 | 3篇 |
2008年 | 7篇 |
2007年 | 9篇 |
2006年 | 11篇 |
2005年 | 5篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2002年 | 12篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 17篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1971年 | 3篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1959年 | 8篇 |
1958年 | 16篇 |
1957年 | 4篇 |
1956年 | 5篇 |
1955年 | 5篇 |
1954年 | 7篇 |
1948年 | 5篇 |
排序方式: 共有355条查询结果,搜索用时 0 毫秒
81.
龙爪芦荟和库拉索芦荟中微量锗的测定与红外光谱区别 总被引:1,自引:0,他引:1
采用吸光光度法对龙爪芦荟和库拉索芦荟中的微量锗进行了测定,并探讨了两种芦荟的红外光谱的区别。结果表明,龙爪芦荟中含锗量在26.1~28.6μg·g-1,回收率为91.6%~97 5%。库拉索芦荟中含锗量在16.8~20.0μg·g-1,回收率为87.1%~94.5%。两种芦荟的FT IR光谱图在2100cm-1处有明显差异,由此可区别两种芦荟。 相似文献
82.
LIGHT PROMOTION OF SEED GERMINATION IN Datura ferox IS MEDIATED BY A HIGHLY STABLE POOL OF PHYTOCHROME 总被引:3,自引:0,他引:3
J. J. CASAL R. A. SANCHEZ A. H. DI BENEDETTO LUCILA C. DE MIGUEL 《Photochemistry and photobiology》1991,53(2):249-254
Abstract— The duration of the far-red light-absorbing form of phytochrome (Pfr) of the photoreceptor pool involved in the control of seed germination was investigated for Datura ferox seeds. These seeds require both Pfr and alternating temperatures (20/30°C) to germinate. After 24 h imbibition (25°C), the seeds received pretreatment-light pulses providing different phytochrome photoequilibria (Pfr/P), followed by a 24 h dark incubation (25°C), and test-light pulses providing different Pfr/P immediately prior to transfer to alternating temperatures. Germination increased with increasing Pfr/P provided by the test-light pulses, but was unaffected by the pretreatment-light pulses. This suggests that phytochrome synthesis, phytochrome degradation and phytochrome-mediated changes in response to phytochrome were negligible. In other experiments, red light-pretreatment pulses were followed by dark incubations (25°C) of different duration before transfer to alternating temperatures. The proportion of Pfr remaining after the 25°C incubation period was estimated by comparing germination rates with those of seeds that received test-light pulses of known calculated Pfr/P immediately prior to the start of the cycles of alternating temperatures. More than 80% of the Pfr established by a Pfr/P= 0.87 light pulse was present and active even after 48 h dark incubation at 25°C. Surprisingly, when a pretreatmentlight pulse providing a Pfr/P= 0.70 was given, the reduction in [Pfr] was significantly faster.
Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light-absorbing form, the times to reach half the original Pfr pool being > 96 h or <14 h after light pulses providing Pfr/P= 0.87 or 0.70, respectively. 相似文献
Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light-absorbing form, the times to reach half the original Pfr pool being > 96 h or <14 h after light pulses providing Pfr/P= 0.87 or 0.70, respectively. 相似文献
83.
采用水热合成法制备了CaF2:xYb3+,yEr3+(x=0.1~0.8,y=0.01~0.08)纳米颗粒,利用X射线粉末衍射仪、透射电子显微镜和F-4600荧光分光光度计表征了样品的物相和形貌尺寸,并探究了Yb3+和Er3+掺杂浓度对样品的上转换发光性质的影响。结果表明,所合成的样品为立方相,球形颗粒,平均直径为 12 nm,敏化剂Yb3+的最佳掺杂摩尔分数为20%,而激活剂Er3+的最佳掺杂摩尔分数为6%。此时,绿光与红光的强度之比最大。 相似文献
84.
85.
A convenient synthesis of 3-acylamino-1,2-naphthoquinones (I) is presented. The addition of aromatic and aliphatic amines to I followed by exposure to oxygen gives the corresponding 4-arylamino- or 4-alkylamino-3-acylamino-1,2-naphthoquinones (II). The addition of 4-cyclo-hexylbutylamine to 3-trichloroacetamino-1,2-naphthoquinone took an anomalous course and 1-(4′-cyclohexylbutyl)-3(H)-naphth[1,2-d]imidazoline-2,4,5-trione (VII) was obtained. Treatment of II with refluxing acetic acid gave 1,2-disubstituted naphth[1,2-d]imidazole-4,5-diones (III). The reaction was successful with a variety of 4-substituted amino-3-acylamino-1,2-naphthoquinones (II) and usually occurred in excellent yield. However, the cyclization of II to III is subject to steric limitation and attempts to cyclize 4-tert-butylamino-3-acetamino-1,2-naphthoquinone to the corresponding imidazole derivative was unsuccessful. The infrared, ultraviolet and nuclear magnetic resonance spectra of I, II, and III are discussed in relation to their structures. 相似文献
86.
87.
C. Scott Blackwell Robert W. Broach Michael G. Gatter Jennifer S. Holmgren Deng‐Yang Jan Gregory J. Lewis Beckay J. Mezza Thomas M. Mezza Mark A. Miller Jaime G. Moscoso R. Lyle Patton Lisa M. Rohde Michael W. Schoonover Wharton Sinkler Ben A. Wilson Stephen T. Wilson 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2003,115(15):1779-1782
88.
89.
90.
Silva SA DE Castro JC Da Silva TG Da-Cunha EV Barbosa-Filho JM Da Silva MS 《Natural product letters》2001,15(5):323-329
A new phenylcoumarone type trinorlignan, krametosan (1), along with the known norlignans, ratanhiaphenol I (2) and 2-(2'-hydroxy-4',6'-dimethoxyphenyl)-5-(E)-propenylbenzofuran (3), the lignan conocarpan (4) and dinorlignan decurrenal (5), were isolated from the CHCl3 extract of the roots of Krameria tomentosa. The structure of these compounds were elucidated by the spectroscopic methods. 相似文献