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81.
用原子捕获技术提高火焰原子吸收光谱法的灵敏度 总被引:1,自引:0,他引:1
采用简易的石英原子捕获器,使Cu,Ag,Au,Zn,Cd,Pb,Sb,Bi等易挥发元素在空气-乙炔火焰原子吸收光谱法中的灵敏度提高了3~5倍,并有较好的精密度,相对标准偏差在1%左右,方法中用此技术测定了标准物质中Pb、Cd的含量,获得了满意的结果,它较之石墨炉原子吸收光地具有快速,简便的特点,具有较大的实用价值。 相似文献
82.
Potential energy functions suitable for the (Cs1–xNaxT2O4)-A type zeolite family were obtained from the known crystal structure of Cs7Na5-A zeolite. Using these potential functions, several other crystal structures in these family were obtained by molecular mechanical calculations. The activation energy - to -cage transmission of the H2 molecule and the void volume of Cs3Na9-A zeolite were calculated. 相似文献
83.
Synthesis and characterization of block copolymers from 2‐vinylnaphthalene by anionic polymerization
Faquan Zeng Mu Yang Jianxin Zhang Sunil K. Varshney 《Journal of polymer science. Part A, Polymer chemistry》2002,40(24):4387-4397
The anionic polymerization of 2‐vinylnaphthalene (2VN) has been studied in tetrahydrofuran (THF) at ?78 °C and in toluene at 40 °C. 2VN polymerization in THF, toluene, or toluene/THF (99:1 v/v) initiated by sec‐butyllithium (sBuLi) indicates living characteristics, affording polymers with predefined molecular weights and narrow molecular weight distributions. Block copolymers of 2VN with methyl methacrylate (MMA) and tert‐butyl acrylate (tBA) have been synthesized successfully by sequential monomer addition in THF at ?78 °C initiated by an adduct of sBuLi–LiCl. The crossover propagation from poly(2‐vinylnaphthyllithium) (P2VN) macroanions to MMA and tBA appears to be living, the molecular weight and composition can be predicted, and the molecular weight distribution of the resulting block copolymer is narrow (weight‐average molecular/number‐average molecular weight < 1.3). Block copolymers with different chain lengths for the P2VN segment can easily be prepared by variations in the monomer ratios. The block copolymerization of 2VN with hexamethylcyclotrisiloxane also results in a block copolymer of P2VN and poly(dimethylsiloxane) (PDMS) contaminated with a significant amount of homo‐PDMS. Poly(2VN‐b‐nBA) (where nBA is n‐butyl acrylate) has also been prepared by the transesterification reaction of the poly(2VN‐b‐tBA) block copolymer. Size exclusion chromatography, Fourier transform infrared, and 1H NMR measurements indicate that the resulting polymers have the required architecture. The corresponding amphiphilic block copolymer of poly(2VN‐b‐AA) (where AA is acrylic acid) has been synthesized by acidic hydrolysis of the ester group of tert‐butyl from the poly(2VN‐b‐tBA) copolymer. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4387–4397, 2002 相似文献
84.
E. Cortés N. Gras L. Muñoz V. Cassorla 《Journal of Radioanalytical and Nuclear Chemistry》1982,69(1-2):401-415
Seven trace elements were determined in milk formulas, powder milk and infant food. Instrumental neutron activation analysis
was used for the analysis of Fe, Zn, Co, Cr and Rb, while As and Cu content was determined using radiochemical neutron activation
analysis. Differences were found in the trace element content of foods of the same type. Milk formulas are comparable with
human milk according to the results obtained for the elements analyzed. The samples of powder cow's milk presented similar
concentrations of the elements studied. Liver with vegetables was found to have the highest Cu content. A meal prepared at
home, made of beef and fresh vegetables showed a higher trace element content than its similar commercial one. An estimation
of the daily intake of the elements analyzed was made and compared with minimum recommended daily ingestion. 相似文献
85.
Zhang LJ Liu HG Feng XS Zhang RJ Zhang L Mu YD Hao JC Qian DJ Lou YF 《Langmuir : the ACS journal of surfaces and colloids》2004,20(6):2243-2249
Three kinds of Langmuir monolayers formed by dipalmitoylphosphatidylcholine (DPPC), arachidic acid (AA), and octadecylamine (ODA) were used as templates to study the initial stage of nucleation and crystallization of calcium phosphates. It was demonstrated that the combination of calcium ions (or phosphates) to the monolayer/subphase interface is a prerequisite for subsequent nucleation. It was found that calcium phosphate dihydrate (DPCD) formed at 25.0 degrees C for 12 h has a biphasic structure containing both amorphous and crystalline phases. These results showed that calcium phosphates were formed through a multistage assembly process, during which an initial amorphous phase DPCD was followed by a phase transformation into a crystalline phase and then the most stable hydroxyapatite (HAp). This provided new insights into the template-biomineral interaction and a mechanism for biomineralization. 相似文献
86.
Treatment of halides 5 with electrophilic alkenes 2 afforded the corresponding dihydrofurans 3 and 4 in the presence of 1, 4-diazabicyclo[2.2.2]octane (DABCO) with good to excellent yields and in a stereoselective manner in most cases. Moreover, the stereoisomers 3 and 4 could be easily transformed each other in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). 相似文献
87.
Qi Zhang Yunlong Shao Boye Li Yuanyuan Wu Jingying Dong Dongtang Zhang Yanan Wang Yong Yan Xiayan Wang Qiaosheng Pu Guangsheng Guo 《Chemical science》2021,12(11):4111
The analysis of single living cells, including intracellular delivery and extraction, is essential for monitoring their dynamic biochemical processes and exploring intracellular heterogeneity. However, owing to the 2D view in bright-field microscopy and optical distortions caused by the cell shape and the variation in the refractive index both inside and around the cells, achieving spatially undistorted imaging for high-precision manipulation within a cell is challenging. Here, an accurate and visual system is developed for single-cell spatial manipulation by correcting the aberration for simultaneous bright-field triple-view imaging. Stereo information from the triple view enables higher spatial resolution that facilitates the precise manipulation of single cells. In the bright field, we resolved the spatial locations of subcellular structures of a single cell suspended in a medium and measured the random spatial rotation angle of the cell with a precision of ±5°. Furthermore, we demonstrated the visual manipulation of a probe to an arbitrary spatial point of a cell with an accuracy of <1 pixel. This novel system is more accurate and less destructive for subcellular content extraction and drug delivery.We achieved the low-damage spatial puncture of single cells at specific visual points with an accuracy of <65 nm. 相似文献
88.
R. Mu?oz Olivas Philippe Quevauviller O. F. X. Donard 《Fresenius' Journal of Analytical Chemistry》1998,360(5):512-519
Long term stability of organic selenium compounds (selenocystine, selenomethionine, trimethylselenonium ion) has been studied
over a one year period for 2 analyte concentrations: 25 and 150 μg/L Se, at pH 4.5 in the dark, under different storage conditions:
temperature of –20°C, 4°C, 20°C, 40°C; in Pyrex, Teflon, or polyethylene containers; in an aqueous matrix or in the presence
of a chromatographic counter ion (pentyl sulfonate at 10–4 mol/L concentration). Light effects have also been tested. The stability of the selenium species was monitored by HPLC-ICP/MS.
Storage conditions can drastically alter the stability of organic selenium species. Organoselenium compounds were shown to
be stable in the dark over a one year period in an aqueous matrix at pH 4.5 in Pyrex containers at both 4°C and 20°C. Pyrex
vials exposed to natural sunlight at room temperature resulted in a steady decrease of the selenoamino acid concentration.
Teflon containers caused losses of less than 25% at both 4° C and 20° C in the dark. However, polyethylene vials presented,
at all temperatures tested, a rapid decrease of the TMSe+ concentration. The stability of the Se species studied did not show significant differences between 4° C and 20° C in any
container material used. Storage of solutions at 40° C led to slight differences between the Pyrex and Teflon containers.
However, polyethylene presented a drastic decrease of the three species over time at this higher temperature. Solutions frozen
at –20° C in polyethylene vials did not stabilize the TMSe+ signal. Finally, concentrations and matrices of the samples did not significantly affect the stability of the species.
Received: 15 July 1996 / Revised: 14 July 1997 / Accepted: 18 July 1997 相似文献
89.
Bolm C Hildebrand JP Muñiz K Hermanns N 《Angewandte Chemie (International ed. in English)》2001,40(18):3284-3308
Addition and substitution reactions with carbon nucleophiles are fundamental processes in organic synthesis, and the development of general catalytic asymmetric variants thereof is still a major challenge today. In contrast to enantioselective alkyl transfer reactions, the corresponding arylations have not yet reached a high level of maturity. The existing protocols are either of no general applicability or are limited in terms of selectivity. This article summarizes established routes for catalytic asymmetric aryl transfer together with the latest developments in this area. The scope and limitations of this reaction are discussed. 相似文献
90.
Maria Zaharescu Maria Crisan I. MuŠeviČ 《Journal of Sol-Gel Science and Technology》1998,13(1-3):769-773
Atomic Force Microscopy (AFM) was used to study the influence of thermal treatments on the structural and textural properties of the sol-gel TiO2 films obtained from Ti(OC3H7i)4. X-ray diffraction (XRD), ellipsometric and porosity measurements have also been made.The TiO2 sol-gel films were homogeneous, transparent and amorphous. Heat treatments in the 400–600°C range indicate that the films have a strong tendency to crystallization. The high initial homogeneity of the TiO2 films was preserved during the crystallization process. AFM shows that the thermally treated films exhibit uniform, monodispersed crystals. 相似文献