全文获取类型
收费全文 | 3335篇 |
免费 | 198篇 |
国内免费 | 47篇 |
专业分类
化学 | 2399篇 |
晶体学 | 39篇 |
力学 | 123篇 |
综合类 | 2篇 |
数学 | 388篇 |
物理学 | 629篇 |
出版年
2024年 | 39篇 |
2023年 | 68篇 |
2022年 | 362篇 |
2021年 | 310篇 |
2020年 | 194篇 |
2019年 | 176篇 |
2018年 | 163篇 |
2017年 | 129篇 |
2016年 | 205篇 |
2015年 | 139篇 |
2014年 | 153篇 |
2013年 | 269篇 |
2012年 | 220篇 |
2011年 | 251篇 |
2010年 | 141篇 |
2009年 | 116篇 |
2008年 | 133篇 |
2007年 | 117篇 |
2006年 | 83篇 |
2005年 | 65篇 |
2004年 | 36篇 |
2003年 | 35篇 |
2002年 | 36篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 9篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1964年 | 1篇 |
排序方式: 共有3580条查询结果,搜索用时 0 毫秒
121.
Muhammad Raza Shah Mehdi Hassan Said Nadeem Muhammad Iqbal Bhanger 《Supramolecular chemistry》2013,25(10-12):777-782
A novel rhodium biphenylic imidazole phenanthroline metal-organic complex (BIP-MC) has been synthesised and characterised as a stable supramolecule. The structure of compound was established on the basis ESI, 1H NMR and UV–vis spectroscopic data. The selectivity of BIP-MC as a new fluorescent chemosensor for various antibiotics has been explored. The supramolecular interaction of amoxicillin with BIP-MC enhanced the fluorescence activity of BIP-MC. A linear response of the sensor was observed in the measuring ranges of excitation 240–298 nm and emission 290–360 nm with detection limits of up to 10 μg/ml at an optimum pH 8.0. Based on the observations made here, a new quantitative method for the determination of this drug in synthetic samples without the use of separation of matrix is developed. It is also inferred that the possible fluorescence enhancement is due to the formation of exciplex between the BIP-MC and amoxicillin. These finding will thus help in pharmacokinetics studies of drugs. The sensor was used for the direct assay of amoxicillin antibiotic in commercial pharmaceutical preparations. 相似文献
122.
Efficient oxidation of alcohols with tert-butyl hydroperoxide catalyzed by Mo(CO)6 supported on multiwall carbon nanotubes modified with 4-aminopyridine is reported. The effect of various parameters such as catalyst amount, solvent and oxidant was studied. The catalyst, [Mo(CO)5@APy-MWCNT], showed high activity not only in the oxidation of benzylic and linear alcohols but also in the oxidation of secondary alcohols. The catalyst can be reused several times without significant loss of its activity. 相似文献
123.
CuO‐CeO2 nanocomposite is reported as a highly efficient and recyclable catalyst for the green synthesis of 1,8‐dioxooctahydroxanthenes in water. This catalyst can be recovered by simple filtration and recycled up to 8 consecutive runs without any losing of its efficiency. 相似文献
124.
Sebastian Reimann Alina Bunescu Andranik Petrosyan Muhammad Sharif Silke Erfle Constantin Mamat Tariel V. Ghochikyan Ashot S. Saghyan Anke Spannenberg Alexander Villinger Peter Langer 《Helvetica chimica acta》2013,96(10):1955-1967
A variety of 6‐(trichloromethyl)salicylates (=2‐hydroxy‐6‐(trichloromethyl)benzoates) were prepared by TiCl4‐mediated cyclization of 1,3‐bis(trimethylsilyloxy)buta‐1,3‐dienes with 1,1,1‐trichloro‐4,4‐dimethoxybut‐3‐en‐2‐one. The employment of trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) as Lewis acid resulted in the formation of trichloromethyl‐substituted cyclohexenones. The cyclizations proceeded with good‐to‐very‐good regioselectivities. 相似文献
125.
Muhammad A. Khosa Syed Sakhawaat Shah 《Journal of Dispersion Science and Technology》2013,34(7):1002-1007
Reactive black 5 (RB-5) dye was removed from a water stream using two cationic surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), via micellar enhanced ultrafiltration. Three membranes with different pore size were used for the determination of rejection coefficient and permeate flux of the solution at 1.5 bar trans-membrane pressure (TMP). The two surfactants (CPC and CTAB) played an almost negligible role in rejection efficiency with 5000 and 10,000 molecular weight cut-off membrane (MWCO), respectively. In this case, high rejection and low permeate flux was the result of a larger molecular size of RB-5 DYE being retained by comparatively smaller sized pores of membrane via ultrafiltration. However, CPC and CTAB surfactants showed 83% and 98% rejection coefficient, respectively, at a concentration greater than their CMC values against 30,000 MWCO. Permeate flux remained low and constant in presence of 5000 and 10,000 MWCO with a small variation against 30,000 MWCO for the two surfactants, thereby no appreciable effect on both surfactant concentrations on concentration polarization was estimated. Thus, RB-5 dye alone was determined to be responsible for membrane plugging or concentration polarization and ultimately for low permeate flux. The effect of trans-membrane pressure was also investigated during this study. 相似文献
126.
Solid state bio-processing of wheat straw was carried out through an indigenous fungal strain Pleurotus ostreatus IBL-02 under pre-optimized fermentation conditions. The maximum activity, 692±12 U/mL, of the industrially important manganese peroxidase (MnP) enzyme was recorded after five days of still culture incubation. The crude MnP was 2.1-fold purified with a specific activity of 860 U/mg after purification on a Sephadex-G-100 gel column. On native and SDS-PAGE electrophoresis gels, the purified MnP fraction was a single homogenous band of 45 kDa. An active fraction of MnP was immobilized using hydrophobic sol-gel entrapment comprising tetramethoxysilane (T) and propyltrimethoxysilane (P) at different T:P molar ratios. Characterization revealed that after 24 h incubation at varying pH and temperatures, the MnP fraction immobilized at a T:P ratio of 1:2 in the sol-gel retained 82% and 75% of its original activity at pH4 and 70 ℃, respectively. The optimally active fraction at a 1:2 T:P ratio was tested against MnSO4 as a substrate to determine the kinetic catalytic constants KM and Vmax . To explore the industrial applicability of P. ostreatus IBL-02 MnP, both the free and immobilized MnP were used for the decolorization of four different textile industrial effluents. A maximum of 100% decolorization was achieved for the different textile effluents within the shortest time period. A lower KM , higher Vmax , hyper-activation, and enhanced acidic and thermal resistance up to 70 ℃ were the novel catalytic features of the sol-gel immobilized MnP, suggesting that it may be a potential candidate for biotechnological applications particularly for textile bioremediation purposes. 相似文献
127.
A new synthetic method leading to N-heterocyclic carbene–silver(I) complexes [(R2-NHC)2Ag]+ [AgX2]? is developed by using benzimidazolium compounds, NaOH (as a base), silver salts and water (as the reaction medium). Single-crystal X-ray structure revealed that compound 1 comprises a linear [Ag-(Et2-Bimy)2]+ cation and a linear [AgBr2]? anion. These two ions are linked through an AgI–AgI association and staggered at an angle of 90.3°. 相似文献
128.
Kyung Ju Lee Muhammad Salman Maqbool Pullur Anil Kumar Kwang Ho Song Heon Phil Ha 《Research on Chemical Intermediates》2013,39(7):3265-3277
Economic production of titanium dioxide (yield >98 %) from ilmenite has been achieved by use of a modified sulfate reduction process. A series of samples were prepared by varying the concentration of titanium dioxide nuclei (0.2, 0.3, and 0.6 %) and further impregnation with antimony and vanadia. The structural and acidic properties of the samples were comprehensively studied by X-ray diffraction (XRD), transmission electron microscopy, BJH pore size distribution, and temperature-programmed desorption of NH3. The XRD results revealed the presence of intense peaks from anatase titanium dioxide. Enhancement of surface area was observed for second-time filtered samples, possibly because of loss of iron from the bulk. As a result, formation of additional micropores was apparent from N2 adsorption and desorption isotherms. Among all the antimony and vanadia-doped samples, the first-time filtered sample with the low concentration of nuclei (0.2 %) had the highest catalytic activity at low temperatures, owing to its larger pore size and abundant acidic species. 相似文献
129.
Bin Li Haoyu Liu Muhammad Amin Lindsay A. Wegiel Lynne S. Taylor Kevin J. Edgar 《Cellulose (London, England)》2013,20(4):2137-2149
Naringenin (Nar) is an important bioactive flavonoid with poor organic solubility and oral bioavailability. It is highly promising for treatment of conditions including diabetes, hyperlipidemia, and hepatitis C infection. Amorphous solid dispersion (ASD) of Nar is an appealing way to enhance its solubility, and carboxylated cellulose esters are attractive polymers for this purpose because of their ability to stabilize drugs against crystallization in both solid and solution phases, while restricting drug release to the pH of the small intestine (ca. 6.8). We demonstrate that ASDs of Nar can be formed using such carboxylated cellulose derivatives as cellulose acetate adipate propionate (CAAdP), carboxymethylcellulose acetate butyrate (CMCAB) and hydroxypropylmethylcellulose acetate succinate (HPMCAS). We compare Nar solution concentrations and release profiles from these cellulosic ASDs to those from pure crystalline Nar, and to Nar ASD in poly(vinylpyrrolidinone) (PVP). We show that all polymers in this study form ASDs with Nar, that the PVP ASDs release Nar at both gastric (1.2) and small intestine (6.8) pH, and that the cellulosic polymers release Nar selectively at neutral pH. Solution concentrations of Nar are significantly enhanced from these ASDs. These preliminary studies indicate that HPMCAS, CAAdP, and CMCAB are practical ASD polymers for Nar due to their ability to generate and stabilize high solution concentrations, and their pH-triggered drug release. 相似文献
130.
Solvent-free reactions were used for the synthesis of a series of imidazolium-based ionic salts: 3,3′-[pyridine-2,6-diylbis(methylene)]bis(1-R-1H-imidazol-3-ium)chloride; (R = methyl, ethyl, butyl, isobutyl, hexyl, and benzyl). A simple and effective filtration process was used to isolate all the products in high purity and with yields ?93% within a 24 h period. The highly pure ionic compounds which are precursors to N-heterocyclic carbene ligands used in catalysis were fully characterized as gray-white hygroscopic salts. 相似文献