全文获取类型
收费全文 | 1513篇 |
免费 | 76篇 |
国内免费 | 12篇 |
专业分类
化学 | 1030篇 |
晶体学 | 10篇 |
力学 | 72篇 |
综合类 | 4篇 |
数学 | 225篇 |
物理学 | 260篇 |
出版年
2024年 | 3篇 |
2023年 | 10篇 |
2022年 | 57篇 |
2021年 | 65篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 59篇 |
2017年 | 44篇 |
2016年 | 84篇 |
2015年 | 72篇 |
2014年 | 67篇 |
2013年 | 158篇 |
2012年 | 99篇 |
2011年 | 110篇 |
2010年 | 95篇 |
2009年 | 79篇 |
2008年 | 80篇 |
2007年 | 74篇 |
2006年 | 47篇 |
2005年 | 47篇 |
2004年 | 43篇 |
2003年 | 28篇 |
2002年 | 19篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 4篇 |
1998年 | 12篇 |
1996年 | 12篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 11篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有1601条查询结果,搜索用时 15 毫秒
91.
92.
Growth orientation of fluorapatite–zirconia nanopowders was investigated after mechanical activation and thermal annealing process in the range of 600–1,300 °C for 1 h. Results revealed that during heating of the composite nanopowders the transition of the monoclinic zirconia to tetragonal form and its stabilization by calcium fluoride originating from the decomposition of fluorapatite as well as the formation of a solid solution of calcium fluoride in zirconia occurred. The influence of annealing on the growth orientation of fluorapatite–zirconia composite nanopowders indicated that the crystal growth of fluorapatite was preferentially accentuated on the (002) face in the direction of the crystallographic c-axis after heat treatment. Based on FE–SEM observations, the experimental outcome was composed of both agglomerates and fine particles (~33 nm) after 600 °C, while annealing of the sample at 1,300 °C demonstrated the occurrence of abnormal grain growth. 相似文献
93.
Dementin S Burlat B Fourmond V Leroux F Liebgott PP Abou Hamdan A Léger C Rousset M Guigliarelli B Bertrand P 《Journal of the American Chemical Society》2011,133(26):10211-10221
Electrons are transferred over long distances along chains of FeS clusters in hydrogenases, mitochondrial complexes, and many other respiratory enzymes. It is usually presumed that electron transfer is fast in these systems, despite the fact that there has been no direct measurement of rates of FeS-to-FeS electron transfer in any respiratory enzyme. In this context, we propose and apply to NiFe hydrogenase an original strategy that consists of quantitatively interpreting the variations of steady-state activity that result from changing the nature of the FeS clusters which connect the active site to the redox partner, and/or the nature of the redox partner. Rates of intra- and intermolecular electron transfer are deduced from such large data sets. The mutation-induced variations of electron transfer rates cannot be explained by changes in intercenter distances and reduction potentials. This establishes that FeS-to-FeS rate constants are extremely sensitive to the nature and coordination of the centers. 相似文献
94.
Eshtiagh-Hosseini H Housaindokht MR Beyramabadi SA Tabatabaei SH Esmaeili AA Khoshkholgh MJ 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(3):1046-1050
A new N,N'-dipyridoxyl(1,4-butanediamine) [=H(2)BS] Schiff-base ligand and its Cu(II) salen complex, [Cu(BS)(H(2)O)(CH(3)OH)], were synthesized and characterized by IR, UV-vis, (1)H NMR, mass spectrometry and elemental analysis. Also, full optimization of the geometries, (1)H NMR chemical shifts (for the H(2)BS) and vibrational frequencies were calculated by using density functional theory (DFT) method. Structure of the H(2)BS ligand is not planar, i.e. two pyridine rings are not in the same plane. In the structure of the Cu complex, the Schiff-base ligand acts as a dianionic tetradentate ligand in N, N, O(-), O(-) manner. The coordinating atoms of BS(2-) occupy equatorial positions of the octahedral complex, where the H(2)O and CH(3)OH ligands locate at axial positions. The calculated results are in good agreement with the experimental data, confirming the suitability of the proposed and optimized structures for the H(2)BS ligand and its Cu complex. 相似文献
95.
Ebrahim Abouzari-Lotf Abbas Shockravi Ali Javadi 《Polymer Degradation and Stability》2011,96(5):1022-1028
A series of novel fluorinated heat-resistant poly(amide imide)s (PAIS) based on non-coplanar diimide-diacid monomer (DIDA) were synthesized and characterized. The poly(amide imide)s were obtained in high yields and possessed inherent viscosities in the range of 0.47-0.91 dL g−1. All of the polymers were amorphous in nature, showed outstanding solubility and could be readily soluble in common organic solvents such as N,N-dimethylacetamide, N-methyl-2-pyrrolidinoned, dimethyl sulfoxide, N,N-dimethylformamide, pyridine and tetrahydrofuran. Glass transition temperatures were in the range of 221-263 °C, as determined by differential scanning calorimetry. Degradation temperatures for 10% weight loss occurred all above 520 °C and char yields was more than 50% at 700 °C in nitrogen atmosphere. Moreover, these PAIs possessed low refractive indexes (n = 1.58-1.59), birefringence (Δn = 0.008-0.013), and dielectric constants (? ≈ 2.5) due to the trifluoromethyl pendent groups and ortho-catenated aromatic rings that interrupt chain packing and increase free volume. 相似文献
96.
Maha A. Hegazy Hassan A. M. Hendawy Soheir A. Weshahy Samah S. Abbas 《液相色谱法及相关技术杂志》2018,41(5):203-222
Fluorometholone (FLM) and Sodium Cromoglycate (CMG) are co-formulated in ophthalmic preparation and showed marked instability under different conditions. Two specific, sensitive and precise stability-indicating chromatographic methods have been developed and validated for their determination in the presence of their degradation products and FLM impurity. Ten components were efficiently separated by them. The first method was HPTLC-spectrodensitometry, where the separation was achieved using silica gel 60?F254 HPTLC plates and developing system of ethyl acetate: methanol (9:1, v/v). The second method was a reversed phase HPLC associated with kinetic study of the degradation process and was successfully applied for determination of the studied compounds in spiked rabbit aqueous humor. The mobile phase was acetonitrile: methanol: 0.05?M potassium dihydrogenphosphate (0.1% trimethylamine); pH 2.5, adjusted with orthophosphoric acid (20: 30: 50, by volume). In both methods, the separated components were detected at 240?nm and system suitability was checked. Good correlation was obtained in the range of 0.10–24.00 and 0.20–48.00?µg band?1, for FLM and CMG by HPTLC. While for HPLC, the linearity ranges from 0.01–50.00 and 0.05–50.00?µg?mL?1 for both drugs. The methods were applied in pharmaceutical formulation, where they were compared to the reported method with no significant difference. 相似文献
97.
Dr. Mahnaz Najafi Dr. Sara Abednatanzi Abbas Yousefi Prof. Mehrorang Ghaedi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(72):17999-18014
Photocatalysis has been known as one of the promising technologies due to its eco-friendly nature. However, the potential application of many photocatalysts is limited owing to their large bandgaps and inefficient use of the solar spectrum. One strategy to overcome this problem is to combine the advantages of heteroatom-containing supports with active metal centers to accurately adjust the structural parameters. Metal nanoparticles (MNPs) and single atom catalysts (SACs) are excellent candidates due to their distinctive coordination environment which enhances photocatalytic activity. Metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and carbon nitride (g-C3N4) have shown great potential as catalyst support for SACs and MNPs. The numerous combinations of organic linkers with various heteroatoms and metal ions provide unique structural characteristics to achieve advanced materials. This review describes the recent advancement of the modified MOFs, COFs and g-C3N4 with SACs and NPs for enhanced photocatalytic applications with emphasis on environmental remediation. 相似文献
98.
R. P. Singh R. P. Khatri E. R. Pambid N. M. Abbas 《Journal of Radioanalytical and Nuclear Chemistry》1990,142(2):499-504
A relatively less expensive Phillips PV 9500/SY 585 automatic energy dispersive X-ray fluorescence (EDXFR) spectrometer with rhodium target, was modified using a commercial kitchen aluminium wrapping foil filter, for the determination of chloride in bifunctional reforming catalysts. Quantitation of chloride was carried out by the Claisse-Quintin method with a catalyst sample of known chloride content as an external standard. The results of EDXRF chloride analysis of different catalyst samples, containing varying amounts of percent carbon and chloride, compared well (p>0.75) with those obtained by silver chloride precipitation method. The small values of percent relative standard deviations at 0.4 to 4.5% showed a good reproducibility of the method. With the total analysis time of a few minutes combined with no sample preparation, the present method seems to be superior as compared to presently employed methods of chloride analysis in reforming catalysts. 相似文献
99.
M. A. A. Al-Janabi H. A. Kaleefa M. G. Jalhoom E. H. Abbas A. S. Al-Warith 《Journal of Radioanalytical and Nuclear Chemistry》1987,111(1):165-175
99Mo was separated from uranium and insoluble fission product hydroxides. More than 98% of99Mo radioactivity was extracted with bis (2-ethylhexyl)phosphoric acid. The organic phase was washed and99Mo was back-extracted from the organic phase with NH4OH solution. The percent recovery from the organic phase was 91% and the purity of99Mo was more than 99%. Pure99mTc was also extracted from the organic phase with a saline solution. Reversed-phase partition chromatography was used for the purification of99Mo from131I and other fission products (10% HDEHP on kieselguhr bed).131I and other isotopes were quantitatively eluted with 0.1M H2SO4,99Mo was eluted using a mixture of 0.5 M HCl and 30% H2O2. 相似文献
100.