全文获取类型
收费全文 | 469篇 |
免费 | 33篇 |
国内免费 | 3篇 |
专业分类
化学 | 340篇 |
晶体学 | 12篇 |
力学 | 14篇 |
综合类 | 2篇 |
数学 | 44篇 |
物理学 | 93篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 14篇 |
2021年 | 36篇 |
2020年 | 25篇 |
2019年 | 22篇 |
2018年 | 23篇 |
2017年 | 23篇 |
2016年 | 25篇 |
2015年 | 25篇 |
2014年 | 18篇 |
2013年 | 24篇 |
2012年 | 30篇 |
2011年 | 47篇 |
2010年 | 21篇 |
2009年 | 13篇 |
2008年 | 25篇 |
2007年 | 17篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 11篇 |
2003年 | 8篇 |
2002年 | 5篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有505条查询结果,搜索用时 31 毫秒
1.
Chetan Bhadaliya Bala Krishna Panigrahy Dipanjan Goswami Sovan Maiti Sanjay Jagannath Gurule Arshad H. Khuroo 《Biomedical chromatography : BMC》2022,36(4):e5337
A selective, highly sensitive, precise, and novel bioanalytical method has been developed and validated to quantify sinococuline, an active constituent present in the phytopharmaceutical drug product containing Cocculus hirsutus plant extract, in vivo. Chromatographic separation was achieved on a Luna Omega Polar-C18 bonded analytical column maintained at 45°C. The isocratic mobile phase consisted of methanol and ammonium formate buffer (60:40, v/v) at acidic pH with a low flow rate of 0.250 mL/min. Detection was performed on an API 4000 mass spectrometer using electrospray ionization in positive polarity and multiple reaction monitoring mode to achieve a lower limit of quantification of 1.50 ng/mL. Excellent accuracy and precision were obtained after extracting the analyte from plasma samples using a chemical analogue as an internal standard in the absence of an isotope-labeled compound. The extraction efficacy was evidenced from recovery study, and the analyte was found to be stable in plasma. Validation study demonstrated linearity with coefficient of correlation, r ≥ 0.99, and minimal matrix effect. This bioanalytical method was successfully applied to evaluate pharmacokinetic parameters of sinococuline from a phase I clinical trial of an aqueous extract of C. hirsutus in healthy human volunteers. 相似文献
2.
MUHAMMAD Naeem Ahmed KHAWAJA Ansar Yasin MUHAMMAD Nawaz Tahir IFZAN Arshad MURTAZA Madni 《结构化学》2015,34(1):26-32
Two new 1,4-disubstituted 1,2,3-triazoles-4-carboxylates were synthesized via click reaction. Compound 1a was synthesized by the interaction of 6-nitro-tetrazolo[1.5-a]-pyridine with ethyl propynoate at room temperature in the presence of Cu(OAc)2 as a catalyst and THF as solvent. Compound 1b was also synthesized by the same manner except that tert-butyl propionate, instead of ethyl propynoate, was used. The compounds were characterized by IR, 1H-NMR, 13C-NMR and single-crystal X-ray diffraction analysis. Compound 1a(C10H9N5O4) crystallizes in the triclinic system, space group P1 with a = 5.0894(9), b = 8.9834(13), c = 13.089(2) ?, α= 83.041(7), β= 80.256(7), γ=87.296(8)°, V = 585.24(16)?3, Z = 2, Mr = 263.22, crystal size(mm) = 0.37 × 0.20 ×0.18,(I 2σ(I)) = 8557, 2493, 1229, Rint = 0.057. Compound 1b(C12H13N5O4) crystallizes in the monoclinic system, space group P21/c with a = 6.8854(5), b = 21.783(2), c = 9.3986(8) ?,β = 93.239(4)°, V = 1407.4(2)?3, Z = 4, Mr = 291.27, crystal size(mm) = 0.38 × 0.22 × 0.20,(I 2σ(I)) = 11842, 3172, 1866, Rint = 0.047. Antimicrobial assay results showed that the title compounds display excellent activities to different bacterial and fungal strains. 相似文献
3.
Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations 总被引:1,自引:0,他引:1
Sinani VA Gheith MK Yaroslavov AA Rakhnyanskaya AA Sun K Mamedov AA Wicksted JP Kotov NA 《Journal of the American Chemical Society》2005,127(10):3463-3472
Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting. 相似文献
4.
William H. Daly Arshad Munir 《Journal of polymer science. Part A, Polymer chemistry》1984,22(4):975-984
The preparation of 3-aminopropylcellulose from cyanoethylcellulose is readily achieved. Reduction of the cyano groups with borane-dimethyl sulfide in tetrahydrofuran or a borane-tetrahy-drofuran complex proceeds quantitatively in 3 h to a corresponding 3-aminopropylcellulose. The presence of primary amine functions is confirmed by spectroscopy and a positive ninhydrin test; the concentration of amino substituents, as ascertained by titration, ranged from 1.2 to 6.4 meq/g. Because the derivatives are neither soluble nor excessively swollen in water, applications as ion-exchange resins or chromatographic supports can be envisioned. Treatment of 3-aminopropyl-cellulose with acetyl chloride, phenyl isocyanate, or p-toluenesulfonyl isocyanate produced 3-acetamido-, 3-(N′-phenyluredo)-, or 3-(N′-p-toluenesulfonyluredo)-N-propylcellulose. Alkylation with methyl chloride yielded a water-soluble quaternary ammonium salt. 相似文献
5.
Heather Davis Arshad Khan 《Journal of polymer science. Part A, Polymer chemistry》1994,32(12):2257-2265
A partial hydrolysis of amylose followed by the addition of iodine provides a spectrum almost identical to that of the amylopectin–iodine (API) complex suggesting the involvement of smaller “amylose-like” units in the API complex. Our theoretical studies on different polyiodine and polyiodide species suggest that a nearly linear I4 unit stabilized within the cavity of a small “amylose-like” helix is responsible for the characteristic API spectrum. Since there are 2.75 anhydroglucose residues (AGU) for every iodine atom in the amylose–iodine (AI) complex and a structural similarity exists between the API and the AI (amylose–iodine) complexes, we identify (C6H10O5)11I4 to be the chromophore in the API complex. © 1994 John Wiley & Sons, Inc. 相似文献
6.
The high TC
superconducting phase Bi2Sr2Ca2Cu3Ox
(2223) in the Pb-BSCCO system has been produced by EDTA-gel processing
using nitrate solutions. The precursor has heated in two stages, at 300 and
800°C each for 2 h, to avoid the burning of the important species involved
in the final product. The effects of time (6 to 48 h) and temperature (845
and 855°C) on the formation of the 2223 phase have been studied by sintering
the samples in air. Thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning
electron microscopy (SEM) and a vibrating sample magnetometer (VSM) have been
employed to investigate the powder produced at different stages of decomposition,
oxidation and formation of sintered materials from the powders. The volume-fraction
of the 2223 phase at 845°C increases with time, the maximum value of the
2223 phase was obtained at 120 h. It has been observed that the formation
of the high TC phase
is remarkably enhanced at the temperature of the endothermic peak of the DTA
curve. The best result has been obtained in the sample sintered for 24 h at
the temperature 855°C (endothermic peak). This also indicated that at
855°C, the large volume-fraction of 2223 phase with TC
113 K grew in short time and as the sintering time increased, it decomposed
into the Bi2Sr2CaCu2Ox
(2212) phase and other phases. 相似文献
7.
Mehmood A Malik A Anis I Khan PM Riaz M Makhmoor T Choudhary MI 《Natural product letters》2002,16(6):371-376
Two new oleanane triterpenes; 2alpha,3alpha,24-trihydroxyolean-12-ene-28,30-dioic acid ([structure: see text]) and 2alpha,3alpha,24,28-tetrahydroxyolean-12-ene ([structure: see text]) have been isolated from the roots of Atropa acuminata. Anti-oxidant p-hydroxyphenethyl trans-ferulate ([structure: see text]), beta-sitosterol-3-O-beta-D-glucopyranoside ([structure: see text]) and oleanolic acid ([structure: see text]) have also been reported for the first time from this species. The structures were determined by spectroscopic studies including 2D-NMR. 相似文献
8.
9.
10.