全文获取类型
收费全文 | 15376篇 |
免费 | 661篇 |
国内免费 | 43篇 |
专业分类
化学 | 10534篇 |
晶体学 | 240篇 |
力学 | 495篇 |
综合类 | 1篇 |
数学 | 1168篇 |
物理学 | 3642篇 |
出版年
2024年 | 49篇 |
2023年 | 169篇 |
2022年 | 391篇 |
2021年 | 440篇 |
2020年 | 489篇 |
2019年 | 540篇 |
2018年 | 498篇 |
2017年 | 486篇 |
2016年 | 704篇 |
2015年 | 505篇 |
2014年 | 791篇 |
2013年 | 1313篇 |
2012年 | 1174篇 |
2011年 | 1195篇 |
2010年 | 787篇 |
2009年 | 617篇 |
2008年 | 788篇 |
2007年 | 759篇 |
2006年 | 602篇 |
2005年 | 523篇 |
2004年 | 404篇 |
2003年 | 337篇 |
2002年 | 276篇 |
2001年 | 160篇 |
2000年 | 136篇 |
1999年 | 102篇 |
1998年 | 73篇 |
1997年 | 104篇 |
1996年 | 104篇 |
1995年 | 77篇 |
1994年 | 70篇 |
1993年 | 94篇 |
1992年 | 98篇 |
1991年 | 84篇 |
1990年 | 73篇 |
1989年 | 71篇 |
1988年 | 51篇 |
1987年 | 47篇 |
1986年 | 49篇 |
1985年 | 70篇 |
1984年 | 69篇 |
1983年 | 56篇 |
1982年 | 49篇 |
1981年 | 44篇 |
1980年 | 49篇 |
1979年 | 67篇 |
1978年 | 57篇 |
1977年 | 57篇 |
1976年 | 42篇 |
1975年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Porous poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) membranes were prepared by solvent–nonsolvent evaporation technique. Morphology and porosity of the membranes were varied with different nonsolvents and had an effect on electrochemical properties. The porous membranes were functionalized with different liquid electrolyte solutions such as p-toluene sulfonic acid/phosphoric acid/sulfuric acid. Maximum electrolyte uptake and minimal electrolyte leakage were tailored by the optimized porosity of the membranes. Thermal behavior obtained in this study ensures the complete evaporation of nonsolvents and ensures its thermal stability. The pTSA-activated PVdF-HFP/THF membrane exhibited high ionic conductivity of about 27.27 mS/cm and a lower methanol permeability in the range of 9.7 × 10−8 cm2/s. High compatibility between pTSA solution and porous PVdF-HFP polymer electrolyte membrane enhances its electro chemical behavior than that of conventional liquid electrolytes. 相似文献
992.
993.
Poonam Kumari Kumud Malika Tripathi Lokesh Kumar Jangir Ragini Gupta Kamlendra Awasthi 《Materials Today Chemistry》2021
Graphene is an atomic layer thick carbon-based material with unique two-dimensional architecture and extraordinary physiochemical, optical, electrical, and mechanical properties. Graphene and its derivatives show significant promises for the development of nanoporous ultrathin filtration membranes capable of molecular separation properties. Graphene-based nanofiltration membranes featuring distinct laminar structures can offer various novel mass-transport phenomena for purifying water, energy storage and separation, gas separation, and proton conductors. The latest developments in water purification techniques through graphene-based membranes including engineering, design, and fabrication of diverse graphene, graphene-oxide, and graphene-composite membranes are provided here in relation to their application paradigm for purifying water. The critical views on pollutant removal mechanisms for water purification along with optimization measures are specially highlighted. In addition, the challenges, shortcomings, and future prospects are pointed out. The green and large-scale synthesis technology of graphene coupling with advanced membrane fabrication techniques can promote these state-of-the-art nanofiltration membranes for a wide range of applications. 相似文献
994.
Vittal S Shitut NR Kumar TR Vinu MC Mullangi R Srinivas NR 《Biomedical chromatography : BMC》2006,20(11):1252-1259
A simple, sensitive and specific high-performance liquid chromatography method is described for simultaneous determination of rosuvastatin (RST) and gemfibrozil (GFZ) in human plasma using celecoxib as an internal standard (IS). The assay procedure involved extraction of RST, GFZ and IS from plasma into acetonitrile. Following separation and evaporation of the organic layer the residue was reconstituted in the mobile phase and injected onto an X-Terra C(18) column (4.6 x 150 mm, 5.0 microm). The chromatographic run time was less than 20 min using flow gradient (0.0-1.60 mL/min) with a mobile phase consisting of 0.01 M ammonium acetate:acetonitrile:methanol (50:40:10, v/v/v) and UV detection at 275 nm. Nominal retention times of RST, GFZ and IS were 6.7, 13.9 and 16.4 min, respectively. Absolute recovery of both analytes and IS was greater than 90%. The lower limit of quantification (LLOQ) of RST and GFZ was 0.03 and 0.30 microg/mL, respectively. Linearity was excellent (r(2) = 0.999) in the 0.03-10 microg/mL and 0.3-100 microg/mL ranges for RST and GFZ, respectively. The inter- and intra-day precisions in the measurement of RST quality control (QC) samples 0.03, 0.09, 2.50 and 8.00 microg/mL were in the range 2.37-9.78% relative standard deviation (RSD) and 0.92-10.08% RSD, respectively. Similarly, the inter- and intra-day precisions in the measurement of GFZ quality control (QC) samples 0.30, 0.90, 25.0 and 80.0 microg/mL were in the ranges 2.79-6.27 and 0.96-9.69% RSD, respectively. Accuracies in the measurement of QC samples for RST and GFZ were in the range 85.43-107.23 and 84.98-102.35% respectively, of the nominal values. RST and GFZ were stable in the array of stability studies viz., bench-top, auto-sampler and freeze-thaw cycles. Stability of RST and GFZ was established for 1 month at -80C. The application of the assay in an oral pharmacokinetic study in rats co-administered with RST and GFZ is described. 相似文献
995.
Desouza Arun H. G. Tanner Robert D. Effler W. T. 《Applied biochemistry and biotechnology》1991,(1):655-666
Studies were conducted to establish the effect of the variation of environmental factors on the separation occurring in protein
systems, resulting from bubble fractionation in a bioreactor. The measure of separation was selected to be the separation
ratio. This is defined to be the ratio of either the top or the middle position concentration in the vessel to the bottom
concentration of the vessel. Invertase and α-amylase were the two “model” enzymes considered. It was observed that, under
certain conditions, i.e., a combination of the nature of the sparging gas and the medium pH, varying degrees of protein separation
were achieved. The pH of the system dramatically influenced the separation. It was found that the best separation occurred
at a certain pH, assumed to be at or close to thepI of the protein in question. Furthermore, it was observed that systems sparged with CO2 exhibited greater separation than systems sparged with air. In fact, in the case of invertase, almost threefold separation
was observed at the top port when the solution was sparged with CO2. 相似文献
996.
Among physical and nutritional parameters optimized by “one variable at a time” approach, four cultural variables (sucrose,
MgSO4
.7H2O, inoculum size, and incubation period) significantly affected glucoamylase production. These variables were, therefore,
selected for optimization using response surface methodology. The p-values of the coefficients for linear effect of sucrose and inoculum size were less than 0.0001, suggesting them to be the
key experimental variables in glucoamylase production. The enzyme production (34 U/ml) attained under optimized conditions
(sucrose, 2%; MgSO4
.7H2O, 0.13%; yeast extract, 0.1%; inoculum size, 5 × 106 spores per 50 ml production medium; incubation time, 48 h; temperature, 40°C; and pH 7.0) was comparable with the value predicted
by polynomial model (34.2 U/ml). An over all 3.1-fold higher enzyme titers were attained due to response surface optimization.
The experimental model was validated by carrying out glucoamylase production in shake flasks of increasing capacity (0.25–2.0 l)
and 22-l laboratory bioreactors (stirred tank and airlift), where the enzyme production was sustainable. Furthermore, the
fermentation time was reduced from 48 h in shake flasks to 32 h in bioreactors. 相似文献
997.
Kariate Sudhakara Prasad A. B. Arun P. D. Rekha Chiu‐Chung Young Jen‐Lin Chang Jyh‐Myng Zen 《Electroanalysis》2009,21(14):1646-1650
Aerobically grown Shewanella sp. bacterial suspension drop‐coated on a disposable screen‐printed carbon electrode was found to possess electroactivity without the aid of redox mediator. Cyclic voltammetric studies revealed the characteristics of a mixed diffusion adsorption‐controlled electrochemical process for direct electron transfer at the bacteria‐modified electrode. Both FE‐SEM and ATR FT‐IR experiments were carried out to investigate the surface characteristics. The electroanalytical applicability was further demonstrated for electrocatalytic reduction of arsenite, hydrogen peroxide and nitrite. Low cost and very simple manufacturing procedure allow for the proposed bacterial sensor to be applied as disposable devices. 相似文献
998.
Effect of annealing on hydrophobic stability of plasma deposited fluoropolymer coatings 总被引:1,自引:0,他引:1
Arun Kumar Gnanappa Cian O'Murchu Frank Peters Syed A.M. Tofail 《Polymer Degradation and Stability》2008,93(12):2119-2126
Fluorinated amorphous carbon (a-C:F) films e.g. plasma polymerised perfluorocyclobutane have long attracted much consideration due to their low surface energy, hydrophobicity, low refractive index, good electrical and thermal insulation and good thermal stability. Although a-C:F films have many advantages, hydrophobic stability over time in air and water remains a major concern. In this study, the effects of weathering conditions on the hydrophobicity of fluorocarbon films prepared from perfluorocyclobutane precursors were examined using water contact angle measurements. It was found that the high initial hydrophobicity of as-deposited films degrades rapidly in humid conditions. The stability of hydrophobicity can be significantly improved when a suitable treatment such as annealing is employed. The mechanism of weathering was explained with the help of a number of morphological and chemical characterisation techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). In particular, XPS results demonstrated that a reduction in the overall amount of -CF3 radical, oxygenation of surface fluorides and the formation of an overlayer all influence the degradation of fluorocarbon in aquatic environment. 相似文献
999.
1000.