全文获取类型
收费全文 | 2286篇 |
免费 | 90篇 |
国内免费 | 320篇 |
专业分类
化学 | 2143篇 |
晶体学 | 17篇 |
力学 | 5篇 |
综合类 | 16篇 |
数学 | 242篇 |
物理学 | 273篇 |
出版年
2024年 | 9篇 |
2023年 | 68篇 |
2022年 | 50篇 |
2021年 | 75篇 |
2020年 | 151篇 |
2019年 | 105篇 |
2018年 | 94篇 |
2017年 | 114篇 |
2016年 | 106篇 |
2015年 | 67篇 |
2014年 | 131篇 |
2013年 | 508篇 |
2012年 | 167篇 |
2011年 | 137篇 |
2010年 | 100篇 |
2009年 | 114篇 |
2008年 | 111篇 |
2007年 | 121篇 |
2006年 | 115篇 |
2005年 | 86篇 |
2004年 | 87篇 |
2003年 | 56篇 |
2002年 | 11篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 10篇 |
1997年 | 12篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有2696条查询结果,搜索用时 15 毫秒
1.
The reaction of various α-silyl-α-keto esters with thiosemicarbazide at 50 °C in ethyl acetate was found to give α-silyl-substituted thiosemicarbazone-acetic acid esters in good yield. These may then be converted to their corresponding silyl-substituted 1,2,4-triazin-5-ones by cyclization under basic conditions. 相似文献
2.
Antonella Leggio 《Tetrahedron》2007,63(34):8164-8173
A highly efficient and practical synthesis of peptides in solution phase has been developed. The procedure is based on the use of p-nitrobenzenesulfonyl (nosyl) group for the protection of the amino function of α-amino acids. Every step of the procedure, protection of the amino function by the nosyl group, formation of the peptide bond, and removal of the sulfonamide group, is characterized by high yields and excellent purity of the final products. The described strategy allows the preparation of short peptide sequences keeping the chiral integrity of amino acid precursors. Compatibility of nosyl group with the side-chain protecting groups used in Fmoc-based strategy is demonstrated. The method here presented is an alternative strategy that could provide advantages for future peptide synthesis. 相似文献
3.
《Arabian Journal of Chemistry》2022,15(12):104368
Protein hydrolysates have the potential to be natural and safer sources of bioactive peptides. In this study, two proteases were used to hydrolyze Chinese sturgeon (Acipenser sinensis) protein, and the hydrolysates were then purified to yield antioxidant peptides. The degree of hydrolysis of 23.56 % and 18.14 % was obtained using papain and alcalase 2.4L, respectivly, and hydrolysates had 96.80 % and 87.24 % total amino acid content, respectivly. The papain hydrolysate (PH) and alcalase 2.4L hydrolysate (AH) showed good antioxidant activity against DPPH? (IC50 of 3.64 and 3.15 mg/mL) and ABTS?+ (IC50 of 1.92 and 1.58 mg/mL), respectively. The low-molecular-weight (<1000 Da) fraction of both hydrolysates demonstrated the highest antiradical activity (IC50 of 2.59 and 2.31 mg/mL, DPPH) and (IC50 of 1.54 and 1.36 mg/mL, ABTS), respectively. Nine peptides were separated from both hydrolysates using reverse phase high performance liquid chromatography (RP-HPLC). The IC50 for ABTS?+ scavenging activity of peptide P5 with valine, glycine and asparagine (MW of 282.13 Da) from PH, and peptide P3 with histidine, glycine and alanine (MW of 302.74 Da) from AH was 0.89 and 0.72 mg/mL, respectively. The fractions and purified peptides obtained from Chinese sturgeon hydrolysates could be utilized as natural antioxidant substitutes in pharmaceuticals and food products. 相似文献
4.
Four 2-(α-hydroxyalkyl)-2-oxazolines and a 2-(α-hydroxy-α,α-diphenyl)-2-benzooxazole were synthesized from β-amimoalcohols and α-hydroxy carboxylic acids under microwave irradiation in the absence of a solvent. 相似文献
5.
Phthalimide derivatives as nitrogen nucleophiles with α,β‐unsaturated aldehydes for asymmetric aza‐Michael additions have been reported. The reactions proceed smoothly to afford corresponding Michael adducts in good yields (up to 98%) and enantioselectivities (up to 95% ee). 相似文献
6.
Mitsuhiro Yoshimatsu Fumihiro Hatanaka 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):1007-1010
Abstract β-Ethoxy-α-phenyltellanylvinyl lithiums were successfully generated in situ from phenyltellanylacetaldehyde diethyl acetal with lithium 2,2,6,6-tetrame-thylpiperidide (LTMP) and underwent methylation or nucleophilic addition with aldehydes and ketones. The successive dehydration of the allylic alcohols with trimethylsilyl trifluoromethanesulfonate (TMSOTf) gave α-phenyltellanyl α,β-unsaturated aldehydes, which transformed to some useful compounds. 相似文献
7.
《Analytical letters》2012,45(14):1185-1189
Abstract A specific and non-destructive test for basic esters of benzilic and α-glycolic acids in water is described. The compounds are collected on a glass fiber filter as water insoluble complexes with sodium tetraphenylboron. The filter is dried and sprayed with a 2-diphenylacetyl-l, 3-indandione-l-imine derivative. A positive test is the visual observation of fluorescence. 相似文献
8.
采用熔体快淬的方法制备Pr2Fe14B/α-Fe纳米晶复合永磁材料.使用振动样品磁强计(VSM)测量样品的室温磁性能.实验合金成分为(PrxFe94.3-xB5.7)0.99Zr1(其中x=8.2,8.6,9.0,9.4,9.8,10.2,10.6,11.0,11.4(原子分数,%)).系统地研究了辊速及合金成分对快淬带磁性能的影响,当Pr原子分数由8.
关键词:
纳米复合永磁材料
熔体快淬
2Fe14B/α-Fe')" href="#">Pr2Fe14B/α-Fe
磁性 相似文献
9.
采用金属有机物化学气相沉积(MOCVD)法在蓝宝石(0001)衬底上制备出了Ga2(1-x)In2xO3(x=01—09)薄膜,研究了薄膜的结构、电学和光学特性以及退火处理对薄膜性质的影响.测量结果表明:当In组分x=02时,样品为单斜β-Ga2O3结构;x=05的样品,薄膜呈现非晶结构,退火处理后薄膜结构得到明显的改善
关键词:
金属有机物化学气相沉积
2(1-x)In2xO3薄膜')" href="#">Ga2(1-x)In2xO3薄膜
蓝宝石衬底
退火 相似文献
10.
Tsutomu Asano Takuro Furusho Mohammad Mahay Alam Yukihiro Tamba Chie Sawatari Md. Forhad Mina 《Journal of Macromolecular Science: Physics》2013,52(4):774-788
Isotactic polypropylene (iPP) film was melt-crystallized in a temperature gradient. The iPP film showed well oriented α- and β-crystalline textures along the gradient. The crystalline structure, phase transition boundary and lamellar twisting were examined by X-ray diffraction and laser light diffraction (LLD). On the α-β boundary, LLD shows a sharp streak perpendicular to the boundary, where the a-axis of the β-crystal is oriented perpendicular to the temperature gradient. Apart from the boundary, the a-axis of the β-crystal becomes parallel to the gradient. The β-crystal shows lamellar twisting with a pitch of 200 μm at room temperature. When heated the β-crystal, the lamellar distance of 295Å at room temperature decreases to 285Å at 80–100°C and then increases to more than 300Å above 120°C. During the heating, the value of the twist period increases from 200 to 210 μm at 90–100°C, and then to above 224 μm at 140°C. The increase of the twist period is related to the increasing crystalline thickness of the β-lamellae. 相似文献