全文获取类型
收费全文 | 2684篇 |
免费 | 62篇 |
国内免费 | 15篇 |
专业分类
化学 | 1267篇 |
晶体学 | 44篇 |
力学 | 53篇 |
数学 | 324篇 |
物理学 | 789篇 |
无线电 | 284篇 |
出版年
2022年 | 36篇 |
2021年 | 44篇 |
2020年 | 55篇 |
2019年 | 60篇 |
2018年 | 56篇 |
2017年 | 54篇 |
2016年 | 73篇 |
2015年 | 54篇 |
2014年 | 95篇 |
2013年 | 185篇 |
2012年 | 117篇 |
2011年 | 140篇 |
2010年 | 103篇 |
2009年 | 101篇 |
2008年 | 112篇 |
2007年 | 88篇 |
2006年 | 67篇 |
2005年 | 76篇 |
2004年 | 57篇 |
2003年 | 56篇 |
2002年 | 56篇 |
2001年 | 26篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 23篇 |
1995年 | 23篇 |
1994年 | 24篇 |
1993年 | 40篇 |
1992年 | 35篇 |
1990年 | 21篇 |
1989年 | 30篇 |
1988年 | 35篇 |
1987年 | 28篇 |
1986年 | 29篇 |
1985年 | 32篇 |
1984年 | 33篇 |
1983年 | 32篇 |
1982年 | 32篇 |
1981年 | 45篇 |
1980年 | 46篇 |
1979年 | 34篇 |
1978年 | 42篇 |
1977年 | 44篇 |
1976年 | 29篇 |
1975年 | 28篇 |
1974年 | 28篇 |
1973年 | 38篇 |
1972年 | 25篇 |
排序方式: 共有2761条查询结果,搜索用时 15 毫秒
81.
Chattopadhyay S Srivastava AK Bisaria VS 《Applied biochemistry and biotechnology》2002,102(1-6):381-393
The root explants of the germinated seedlings of Podophyllum hexandrum were grown in MS medium supplemented with indole acetic acid (IAA) (2 mg/L) and activated charcoal (0.5%), and healthy callus
culture was obtained after incubation for 3 wk at 20°C. The cultivation of plant cells in shake flask was associated with
problems such as clumping of cells and browning of media, which were solved by the addition of pectinase and polyvinylpyrrolidone.
The effect of major media components and carbon source was studied on the growth and podophyllotoxin production in suspension
culture. It was found that glucose was a better carbon source than sucrose and that NH4
+:NO3
− ratio (total nitrogen concentration of 60 mM) and PO4
3− did not have much effect on the growth and product formation. The relative effect of culture parameters (inoculum level,
pH, IAA, glucose, NH4
+:NO3
− ratio, and PO4
3−) on the overall growth and product response of the plant cell suspension culture was further investigated by Plackett-Burman
design. This indicated that inoculum level, glucose, IAA, and pH had significant effects on growth and production of podophyllotoxin.
To identify the exact optimum concentrations of these parameters on culture growth and podophyllotoxin production, central
composite design experiments were formulated. The overall response equations with respect to growth and podophyllotoxin production
as a function of these culture parameters were developed and used to determine the optimum concentrations of these parameters,
which were pH 6.0, 1.25 mg/L of IAA, 72 g/L of glucose, and inoculum level of 8 g/L. 相似文献
82.
Russell SC Czerwieniec G Lebrilla C Tobias H Fergenson DP Steele P Pitesky M Horn J Srivastava A Frank M Gard EE 《Journal of the American Society for Mass Spectrometry》2004,15(6):900-909
The appearance of informative signals in the mass spectra of laser-ablated bio-aerosol particles depends on the effective ionization probabilities (EIP) of individual components during the laser ionization process. This study investigates how bio-aerosol chemical composition governs the EIP values of specific components and the overall features of the spectra from the bio-aerosol mass spectrometry (BAMS). EIP values were determined for a series of amino acid, dipicolinic acid, and peptide aerosol particles to determine what chemical features aid in ionization. The spectra of individual amino acids and dipicolinic acid, as well as mixtures, were examined for extent of fragmentation and the presence of molecular ion dimers, which are indicative of ionization conditions. Standard mixtures yielded information with respect to the significance of secondary ion plume reactions on observed spectra. A greater understanding of how these parameters affect EIP and spectra characteristics of bio-aerosols will aid in the intelligent selection of viable future biomarkers for the identification of bio-terrorism agents. 相似文献
83.
M Wu TD Perroud N Srivastava CS Branda KL Sale BD Carson KD Patel SS Branda AK Singh 《Lab on a chip》2012,12(16):2823-2831
We have developed a microfluidic platform that enables, in one experiment, monitoring of signaling events spanning multiple time-scales and cellular locations through seamless integration of cell culture, stimulation and preparation with downstream analysis. A combination of two single-cell resolution techniques-on-chip multi-color flow cytometry and fluorescence imaging provides multiplexed and orthogonal data on cellular events. Automated, microfluidic operation allows quantitatively- and temporally-precise dosing leading to fine time-resolution and improved reproducibility of measurements. The platform was used to profile the toll-like receptor (TLR4) pathway in macrophages challenged with lipopolysaccharide (LPS)-beginning with TLR4 receptor activation by LPS, through intracellular MAPK signaling, RelA/p65 translocation in real time, to TNF-α cytokine production, all in one small macrophage population (< 5000 cells) while using minute reagent volume (540 nL/condition). The platform is easily adaptable to many cell types including primary cells and provides a generic platform for profiling signaling pathways. 相似文献
84.
Palavalli RR Srivastava S Srivastava AK 《Applied biochemistry and biotechnology》2012,167(6):1831-1844
Genetically transformed, "Hairy roots" once developed can serve as a stable parent culture for in vitro production of plant secondary metabolites. However, the major bottleneck in the commercial exploitation of hairy roots remains its successful scale-up due to oxygen transfer limitation in three-dimensionally growing hairy root mass. Mass transfer resistances near the gas-liquid and liquid-solid boundary layer affect the oxygen delivery to the growing hairy roots. In addition, the diffusional mass transfer limitation due to increasing size of the root ball (matrix) with growth also plays a limiting role in the oxygen transfer rate. In the present study, a mathematical model is developed which describes the oxygen transfer kinetics in the growing Azadirachta indica hairy root matrix as a case study for offline simulation of process control strategies ensuring non-limiting concentrations of oxygen in the medium throughout the hairy root cultivation period. The unstructured model simulates the effect of oxygen transfer limitation in terms of efficiency factor (η) on specific growth rate (μ) of the hairy root biomass. The model is able to predict effectively the onset of oxygen transfer limitation in the inner core of the growing hairy root matrix such that the bulk oxygen concentration can be increased so as to prevent the subsequent inhibition in growth of the hairy root biomass due to oxygen transfer (diffusional) limitation. 相似文献
85.
B. Biyani A. J. Campagna D. Daruwalla C. M. Srivastava P. Ehrlich 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(3):327-339
The molecular weight distribution (MWD) of crystallizable polyphenylacetylene prepared near room temperature in the presence of ferric acetylacetonate and triethylaluminum was determined through use of fractions characterized by vapor pressure osmometry and gel permeation chromatography (GPC). The number- and weight-average molecular weights (M n and M w) are both less than the molecular weight corresponding to the maximum of the weight distribution function, which lacks a high molecular weight tail. M wandM n is less than is consistent with models allowing for chain termination characteristic of vinyl polymers. GPC elution volumes are much less than those characteristic of polystyrene of similar molecular weight, and the Mark-Houwink exponent is high (2.4 for M v 4800 to 6800). These data indicate more rodlike behavior than for polystyrene of equivalent molecular weight. The MWD and other data suggest intramolecular chain termination, possibly associated with the molecule's tendency to form paramagnetic defect states. 相似文献
86.
The oxidative cyclisation of bsenzylideneamino anilines, phenols and thiophenols to benzimidazoles, benzoxazoles and benzthiazoles respectively using barium manganate is described. 相似文献
87.
Singh G. Kapoor I. P. S. Srivastava J. Kaur J. 《Journal of Thermal Analysis and Calorimetry》2002,69(2):681-691
Three dimethylanilinium sulfates (DMAS) have been prepared and characterised by elemental and spectral studies. Thermal decomposition of these salts has been studied by TG and simultaneous TG-DTG technique and kinetic parameters were evaluated from both dynamic and isothermal TG data using mechanism based kinetic equations. The thermal decomposition pathways have also been suggested and it has been found that DMAS salts give dimethyl aminobenzenesulfonic acids (DMABSA) via solid state reaction. The primary step in the thermal decomposition involves proton transfer followed by sulfonation.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
88.
The α-oxoketene dithioacetals are simple synthetic intermediates widely utilized and implicated for the synthesis of a variety of heterocyclic compounds other than alicyclic and aromatic compounds. They act as 1,3-electrophilic three-carbon synthons. The α-oxoketene dithioacetal of pyrazolone derivatives can be efficiently converted through a base-catalyzed alcoholysis into the corresponding ester in a single one-step reaction with good yield of pure products. In this article, we summarize recent direct conversion of α-oxoketene dithioacetals to highly desirable esters. The overall process is an example of intramolecular rearrangement of bonds. Characterization and identification of all synthesized compounds were assigned through 1H NMR and mass spectroscopy. 相似文献
89.
Side chain bromination of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was carried out by using N-bromosuccinimide followed by phosphorylation of the bromo derivative with triethyl phosphite. Optimum conditions for minimum gel formation have been established. The products have been characterized by 'H-NMR and IR studies. Thermal behavior was investigated by thermogravimetry and differential scanning calorimetry in an air nitrogen atmosphere. Phosphorylated PPO starts to lose weight at 200°C, but the char yield increases with an increase in the phosphorus content of the polymer. 相似文献
90.
Barium Manganate Oxidation in Organic Synthesis: Part—IV: Oxidation of Aldehydes to Carboxylic Acids
The oxidation of aldehydes to corresponding carboxylic acids by barium manganate under heterogeneous conditions is described. Aromatic aldehydes were smoothly converted to carboxylic acids while aliphatic aldehydes underwent polymerisation. 相似文献