全文获取类型
收费全文 | 1208篇 |
免费 | 45篇 |
国内免费 | 16篇 |
专业分类
化学 | 849篇 |
晶体学 | 4篇 |
力学 | 40篇 |
数学 | 168篇 |
物理学 | 208篇 |
出版年
2022年 | 57篇 |
2021年 | 48篇 |
2020年 | 43篇 |
2019年 | 43篇 |
2018年 | 31篇 |
2017年 | 33篇 |
2016年 | 73篇 |
2015年 | 34篇 |
2014年 | 43篇 |
2013年 | 122篇 |
2012年 | 85篇 |
2011年 | 70篇 |
2010年 | 65篇 |
2009年 | 42篇 |
2008年 | 48篇 |
2007年 | 53篇 |
2006年 | 50篇 |
2005年 | 44篇 |
2004年 | 33篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 15篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 2篇 |
1967年 | 2篇 |
1938年 | 3篇 |
排序方式: 共有1269条查询结果,搜索用时 15 毫秒
31.
32.
33.
In this study the geometry optimization of monomeric and dimeric forms (D1, D2, and D3) of hydantoin molecule were done using DFT method employing 6?C31++G(d, p) basis set. Harmonic and anharmonic wavenumbers and infrared intensities were computed at the same theory level. Experimental IR spectrum was recorded in the region 400?C4000 cm?1. It has also been characterized by 1H and 13C NMR spectrum. The hydrogen bond (HB) interaction of hydantoin was analyzed via dimers of hydantoin. Detailed vibrational wavenumber shifts and all vibrational mode analyses were reported. Total energy distributions (TED, %) calculations were done to characterize the fundamentals. 相似文献
34.
Simultaneous determination of dextromethorphan hydrobromide (DEX), phenylephrine hydrochloride (PHEN), and carbinoxamine maleate (CAR) in pharmaceutical preparations was performed by using liquid chromatograpy (LC) and spectrophotometry. In LC, the separation
was achieved on a C18 column and the optimal mobile phase for satisfactory separation in a gradient elution program was found
to be acetonitrile-sodium perchlorate solution (5: 95, v/v) initially, then a linear gradient up to 60% acetonitrile in 8
min. In spectrophotometric method, a chemometric technique, principal component regression (PCR), was used. In the method,
the absorbance data matrix corresponding to the concentration data matrix was obtained by the measurement of absorbances in
their zero order spectra by Δλ = 1 nm in the 210–300 nm range. Then, the calibration was obtained by using this data matrix
for the prediction of unknown concentrations of DEX, PHEN, and CAR in their ternary mixture. The methods proposed were validated
and successfully applied to a pharmaceutical preparation, capsule, and the results were compared. 相似文献
35.
Z. H. Ismail M. M. Ghorab E. M. A. Mohamed H. M. Aly M. S. A. El-Gaby 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2541-2554
Some novel thiourea,1,2,4-triazole, quinazoline, thieno[2,3-d]pyrimi-dine, and thiazolidine derivatives were synthesized to evaluate their antitumor activity. Compound (3f) is nearly as active as reference drug, (Doxorubicin) as positive control. 相似文献
36.
I. Spǎnulescu I. Dima Mohamed Ismail Abd El-Ati N. Bǎlţǎţeanu Talal Khalass 《辐射效应与固体损伤》2013,168(3-4):299-303
The influence of 8 MeV electrons on the crystalline structure of HgTe and Hg1-xCdxTe thin films was studied. HgTe and Hg1-xCdxTe layers were obtained by thermal evaporation and condensation in vacuum on optically flat silica glass substrates heated at different temperatures. One finds that the results of irradiation of HgTe and Hg1-xCdxTe thin films with 8 MeV electrons depend on the preparation conditions of the samples, and therefore on the level of perfection of the crystalline structure and the quantity of nonstoichiometric atoms. 相似文献
37.
Symmetrical and unsymmetrical bithiophene-substituted heterocycles bearing carbonitriles including imidazo[1,2-a]pyridine, benzimidazole, and pyridine derivatives have been synthesized via different synthetic protocols. The bithiophene bis-imidazo[1,2-a]pyridine derivatives 3a,b were achieved in three steps starting from 2-acetyl-5-bromothiophene. Suzuki coupling reaction of 2a with 5-formylthiophen-2-ylboronic acid forms the formyl derivative 5, which by condensation with 3,4-diaminobenzonitrile in the presence of sodium bisulfite furnishes the unsymmetrical bithiophene derivative6. The bis-benzimidazole derivative 8 was obtained via hexabutylditin-mediated homocoupling of 5-bromothiophene-2-carboxaldehyde, while the benzimidazole derivatives 12a,b were prepared via the formyl derivatives 11a,b, a product of Velsmier formylation reaction of 10a,b. Two synthetic protocols for the aryl/hetaryl-2,2′-bithiophene derivative 14 have also been presented. In addition, the guanyl hydrazones of bithiophenes, 16 and 17, were prepared from bis(tri-n-butylstannyl)-2,2′-bithiophene through a Stille coupling reaction followed by a condensation step. 相似文献
38.
Zahariah Ismail Mohd Suria Affandi Yusoff Khairudin Hashim 《Journal of Dispersion Science and Technology》2013,34(1):68-71
A stable palm-based anti-wrinkle lotion enriched with tocotrienols was successfully prepared. The stability of the product was due to the presence of liquid crystalline structure in the emulsion system. It helps in bringing the active to the dermis layer. Functionality active ingredients such as tocotrienols and anti-wrinkle were added in the formulation. A commercial plant extract was used to strengthen the collagen seated at the dermis layer. The product was formulated to pH 5.5 which is suitable for the skin. The mean droplet size is 10.2 µm with a monomodal distribution pattern which prevents agglomeration. This will decrease Brownian motion, thus prolonging its stability. Long-term acute moisturizing tests show a significant increase of between 24% and 17% hydration for five and six weeks respectively. The efficacy testing on 12 subjects confirmed a reduction in natural fine wrinkles on faces during aging or premature aging at ?7.6%. 相似文献
39.
Lukman Hakim Zahira Yaakob Manal Ismail Wan Ramli Wan Daud Ratna Sari 《Chemical Papers》2013,67(7):703-712
Hydroxyapatite-supported Ni-Ce-Cu catalysts were synthesised and tested to study their potential for use in the steam reforming of glycerol to produce hydrogen. The catalysts were prepared by the deposition-precipitation method with variable nickel, cerium, and copper loadings. The performance of the catalysts was evaluated in terms of hydrogen yield at 600°C in a tubular fixed-bed microreactor. All catalysts were characterised by the BET surface area, XRD, TPR, TEM, and FE-SEM techniques. The reaction time was 240 min in a fixed-bed reactor at 600°C and atmospheric pressure with a water-to-glycerol feed molar ratio of 8: 1. It was found that the Ni-Ce-Cu (3 mass %-7.5 mass %-7.5 mass %) hydroxyapatite-supported catalyst afforded the highest hydrogen yield (57.5 %), with a glycerol conversion rate of 97.3 %. The results indicate that Ni/Ce/Cu/hydroxyapatite has great potential as a catalyst for hydrogen production by steam reforming of glycerol. 相似文献
40.
A. Zarrouk I. El Ouali M. Bouachrine B. Hammouti Y. Ramli E. M. Essassi I. Warad A. Aouniti R. Salghi 《Research on Chemical Intermediates》2013,39(3):1125-1133
Corrosion inhibition efficiencies of 1,4-dihydroquinoxaline-2,3-dione (Q1) and 2-phenylthieno[2,3-b]quinoxaline (Q2) as corrosion inhibitors against the corrosion of steel surface in hydrochloric acid is studied by means of density functional approach B3LYP/6-31G calculations. Quantum chemical parameters such as highest occupied molecular orbital energy (E HOMO), lowest unoccupied molecular orbital energy (E LUMO), energy gap (ΔE), dipole moment (μ), electronegativity (χ), electron affinity (A), global hardness (η), softness (σ), ionization potential (I), the fraction of electrons transferred (?N), the global electrophilicity ω, and the total energy were calculated. All calculations have been performed by considering density functional theory using the GAUSSIAN03W suite of programs. 相似文献