全文获取类型
收费全文 | 4014篇 |
免费 | 597篇 |
国内免费 | 455篇 |
专业分类
化学 | 2919篇 |
晶体学 | 40篇 |
力学 | 230篇 |
综合类 | 35篇 |
数学 | 473篇 |
物理学 | 1369篇 |
出版年
2024年 | 10篇 |
2023年 | 99篇 |
2022年 | 83篇 |
2021年 | 132篇 |
2020年 | 190篇 |
2019年 | 165篇 |
2018年 | 142篇 |
2017年 | 110篇 |
2016年 | 174篇 |
2015年 | 184篇 |
2014年 | 172篇 |
2013年 | 285篇 |
2012年 | 292篇 |
2011年 | 301篇 |
2010年 | 215篇 |
2009年 | 215篇 |
2008年 | 244篇 |
2007年 | 196篇 |
2006年 | 194篇 |
2005年 | 195篇 |
2004年 | 179篇 |
2003年 | 120篇 |
2002年 | 116篇 |
2001年 | 111篇 |
2000年 | 113篇 |
1999年 | 101篇 |
1998年 | 86篇 |
1997年 | 81篇 |
1996年 | 84篇 |
1995年 | 62篇 |
1994年 | 72篇 |
1993年 | 50篇 |
1992年 | 44篇 |
1991年 | 34篇 |
1990年 | 41篇 |
1989年 | 25篇 |
1988年 | 24篇 |
1987年 | 25篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1969年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有5066条查询结果,搜索用时 140 毫秒
141.
Mechanical properties and stability measurement of cholesterol-containing liposome on mica by atomic force microscopy 总被引:1,自引:0,他引:1
The micromechanical properties of pure and cholesterol modified egg yolk phosphatidylcholine (EggPC) vesicles prepared by sonication were studied by atomic force microscopy (AFM) on mica surface. The force curves between an AFM tip and an unruptured vesicle were obtained by contact mode. During approach, two repulsion regions with two breaks were observed. The slopes of the two repulsive force regimes for the pure EggPC vesicles are determined to be several times lower than that of EggPC/cholesterol vesicles. The elastic properties from force plot analysis based on the Hertzian model showed that Young's modulus (E) and the bending modulus (kc) of cholesterol-modified vesicles increased several-fold compared with pure EggPC vesicles. The significant difference is attributed to the enhanced rigidity of the EggPC vesicles as a result of the incorporation of cholesterol molecules. The behavior of cholesterol-modified vesicles upon adsorption is different from that in solution as revealed by mechanical properties. The results indicate that AFM can provide a direct method to measure the mechanical properties of adsorbed small liposomes and to detect the stability change of liposomes. 相似文献
142.
Yu Mao Wenxuan Zhao Shuo Lu Lei Yu Yi Wang Yong Liang Shengyang Ni Yi Pan 《Chemical science》2020,11(19):4939
Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for C–C bond formation. A decarboxylative cross-coupling method is described to afford substituted alkynes from various carboxylic acids using copper catalysts CuCl and Cu(acac)2. The photoexcitation of copper acetylides with electron-rich NEt3 as a ligand provides a general strategy to generate a range of alkyl radicals from RAEs of carboxylic acids, which can be readily coupled with a variety of aromatic alkynes. The scope of this cross-coupling reaction can be further expanded to aliphatic alkynes and alkynyl silanes using a catalytic amount of preformed copper-phenylacetylide. In addition, DFT calculations revealed the favorable reaction pathway and that the bidentate acetylacetonate ligand of the copper intermediate plays an important role in inhibiting the homo-coupling of the alkyne.Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for Cu-catalysed C–C bond formation. 相似文献
143.
Jemal M Huang M Mao Y Whigan D Powell ML 《Rapid communications in mass spectrometry : RCM》2001,15(12):994-999
The feasibility of quantitative bioanalysis by parallel-column liquid chromatography in conjunction with a conventional single-source electrospray mass spectrometer has been investigated using plasma samples containing a drug and its three metabolites. Within a single chromatographic run time, sample injections were made alternately onto each of two analytical columns in parallel at specified intervals, with a mass spectrometer data file opened at every injection. Thus, the mass spectrometer collected data from two sample injections into separate data files within a single chromatographic run time. Therefore, without sacrificing the chromatographic separation or the selected reaction monitoring (SRM) dwell time, the sample throughput was increased by a factor of two. Comparing the method validation results obtained using the two-column system with those obtained using the corresponding conventional single-column approach, the methods on the two systems were found to be equivalent in terms of accuracy and precision. The parallel-column system is simple and can be implemented using existing laboratory equipment with no additional capital outlays. A parallel-column system configured in this manner can be used not only for the within-a-run analysis of two samples containing two different sets of chemical entities, but also for the within-a-run analysis of two samples containing the same set of chemical entities. 相似文献
144.
3-苯氧甲基-6-(2,4-二氟苯基)-7H-1,2,4-三唑并[3,4-b][1,3,4]噻二嗪的合成与晶体结构研究 总被引:2,自引:0,他引:2
由苯氧乙酸出发,经多步反应,制得3-苯氧甲基-4-氨基-5-巯基-1,2, 4-三唑,它与2-氯-2',4'-二氟苯乙酮进行环化反应,生成3-苯氧甲基-6-( 2,4-二氟苯基)-7H-1,2,4-三唑并[3,4-b][1,3,4]噻二嗪,通过元素分析、红外 光谱、核磁共振氢谱与碳谱、质谱进行表征,并利用单晶X射线衍射法测定其结构 。晶体属单斜晶系、P2_(1/c)空间群,a = 1.339(3) nm, b = 1.4683(4) nm, c = 0.8606(2) nm, β = 108.49(2)°, Z = 4, F(000) = 736, R_1 = 0.0509。还 对均三唑并噻二唑两类稠杂环的晶体结构作了比较。 相似文献
145.
1—酰基—4—氯环己烷与取代芳烃的区域与立体选择性反应 总被引:2,自引:0,他引:2
从环己烯合成了1-酰基-4-氯环己烷,发现在其在AlCl3催化与苯,氯苯,溴苯等优先生成反式1-芳基-4-酰基环己烷,与联苯反应时选择性较差。 相似文献
146.
研究了羟丙基纤维素和羟丙基醋酸纤维素的合成。制备羟丙基纤维素的较佳反应温度为40℃左右和2小时或稍长,羟丙基含量随环氧丙烷浓度的增加而增加。在制备羟丙基醋酸纤维素时,随羟丙基含量的增加,所需醋化和水解的时间减少。以甘油-正丙醇或磷酸为添加剂,丙酮为溶剂经30~60s蒸发后制得了羟丙基醋酸纤维素反渗透膜。后者在20kg/cm~2下对氯化钠脱盐率可达95~98%,水通量1.0~2.1mL/cm~2·h,其使用温度上限比醋酸纤维素反渗透膜提高10℃以上。 相似文献
147.
建立了加速溶剂萃取同步净化-同位素内标-气相色谱-高分辨质谱同时测定水产品中32种多氯联苯含量的方法。通过在加速溶剂萃取仪中加入2 g无水硫酸钠、1 g弗罗里硅土、50 g中性氧化铝作为吸附剂实现同步净化的效果,萃取溶剂为二氯甲烷-正己烷(1:1,v/v),萃取温度为100℃,循环2次。萃取结束后分别用0.5 mL浓硫酸净化两次,净化液浓缩定容后,采用气相色谱-高分辨质谱测定,同位素内标法定量。32种多氯联苯在0.1~20 μg/L范围内平均相对响应因子(RRF)的相对标准偏差(RSD)值(n=7)均小于15%,定量限(S/N=10)为0.3~1.9 ng/kg。在草鱼和海鲈鱼空白基质中做加标回收试验,添加水平为5、20和50 ng/kg,得到的平均回收率为71.9%~119.0%(n=6),RSD为3.5%~19.6%。该方法背景干扰低,灵敏度高,重现性好,回收率稳定,适用于水产品中多氯联苯的检测。 相似文献
148.
Mass-selected C60 beam produced by laser ablation was accelerated and bombarded the (0001) surface of highly oriented pyrolitic graphite and (111) surface of gold single crystal. The samples were characterized by STM. The STM images showed that, the deposited species collapsed and formed planar structure on the solid surface, but the collapsed species were not dissociated and well oriented on the surface. Both positive and negative C60 ions were observed in the desorption mass spectra, confirming that the species collapsed on the solid surface are still the C60 clusters. 相似文献
149.
Two new uncommon nitrogenous sesquiterpenes, 11‐ethoxy‐3‐formamidotheonellin ( 1 ) and 7‐ethoxy‐3‐formamidobisabolane‐8,10‐diene ( 2 ), together with two known related sesquiterpenes, 3‐formamidotheonellin (=theonellin formamide; 3 ) and theonellin isothiocyanate ( 4 ), were isolated from the Hainan sponge Axinyssa aff. variabilis. Their structures were determined on the basis of extensive spectroscopic analysis and by comparison of their NMR data with those of known compounds. 相似文献
150.
When DNA is mixed with the cationic polyelectrolyte poly(diallyldimethyl ammonium chloride) (PDDA), the DNA/PDDA complex is formed instantaneously at room temperature. This complex is much more efficient in enhancing the fluorescence of Hoechst 33258 (H 33258) than DNA alone. Based on the interaction of H 33258 with the DNA/PDDA complex, a new fluorescence assay for DNA is described. At pH 7.3 in Tris-HCl buffered solution, the DNA/PDDA complex causes a sharp enhancement in fluorescence intensity of H 33258. Simultanously, the emission maximum wavelength of H 33258 blueshifts from 490nm to 450nm, while the excitation redshifts from 345 to 350nm. The calibration graphs for calf thymus DNA (ctDNA) and herring sperm DNA (hsDNA) are both linear up to 5.0µgmL–1 when the concentration of H 33258 and PDDA are fixed at 1.5×10–6 and 1.6×10–5molL–1, respectively. The method is specific for native DNA. The 3 detection limits for ctDNA and hsDNA are 1.8 and 5.6ngmL–1, respectively, i.e. much lower than in the presence of H 33258 alone. Four synthetic samples were determined satisfactorily. This method can also be developed to investigate the formation and the nature of the complexes between DNA and polycations, which have recently been widely applied in some fields such as genetic engineering and gene therapy. 相似文献