全文获取类型
收费全文 | 6950篇 |
免费 | 987篇 |
国内免费 | 563篇 |
专业分类
化学 | 4880篇 |
晶体学 | 32篇 |
力学 | 379篇 |
综合类 | 18篇 |
数学 | 796篇 |
物理学 | 2395篇 |
出版年
2024年 | 23篇 |
2023年 | 157篇 |
2022年 | 219篇 |
2021年 | 295篇 |
2020年 | 293篇 |
2019年 | 248篇 |
2018年 | 225篇 |
2017年 | 180篇 |
2016年 | 294篇 |
2015年 | 286篇 |
2014年 | 375篇 |
2013年 | 461篇 |
2012年 | 613篇 |
2011年 | 637篇 |
2010年 | 420篇 |
2009年 | 359篇 |
2008年 | 432篇 |
2007年 | 378篇 |
2006年 | 359篇 |
2005年 | 269篇 |
2004年 | 206篇 |
2003年 | 158篇 |
2002年 | 131篇 |
2001年 | 131篇 |
2000年 | 124篇 |
1999年 | 134篇 |
1998年 | 107篇 |
1997年 | 92篇 |
1996年 | 122篇 |
1995年 | 103篇 |
1994年 | 70篇 |
1993年 | 63篇 |
1992年 | 69篇 |
1991年 | 58篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 34篇 |
1987年 | 29篇 |
1986年 | 24篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1979年 | 20篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1975年 | 13篇 |
1973年 | 13篇 |
排序方式: 共有8500条查询结果,搜索用时 0 毫秒
981.
982.
983.
A simple and effective method, using calcium nitrate and triammonium phosphate as starting materials, for the preparation of water-dispersible hydroxyapatite nanorods (HAp) was reported. The process primarily involves the preparation of HAp with the addition of sodium citrate (NaC) and the exchange of absorbed ions (NaC) with sodium hexame taphosphate (NaP). The end products were investigated using various means in order to confirm the particles’ crystal phase and morphology and to understand how to improve their stability. The results demonstrate that the resulting HAp at 90 °C is rod-like with length of 300-400 nm and width of 40-60 nm. The zeta potential values of pure HAp, HAp-NaC, HAp-NaC/NaP are from −15.20, −30.89 to −44.84 mV. The settling time test shows the HAp-NaC/NaP could keep stable above 7 months without any creaming or visible sedimentation. The amount of NaC and the reaction temperature play significant roles in the whole process due to the formation of Ca containing precipitates. 相似文献
984.
985.
Current models for pressure drop prediction of slug flow pneumatic conveying in a horizontal pipeline system assume some type of steady state conditions for prediction,which limits their capability for increased predictive accuracy relative to experimental data.This is partly because of the nature of slug flow pneumatic conveying system,which,as a dynamic system,never becomes stable.By utilising conservation of mass (airflow),a dynamic pressure analysis model is proposed on the basis of the derivative of the upstream pressure behaviour.The rate of air permeation through slug,one of the important factors in the conservation model,is expressed as a function of a slug permeability factor.Other factors such as slug velocity,slug length and the fraction of stationary layer were also considered.Several test materials were conveyed in single-slug tests to verify the proposed pressure drop model,showing good agreement between the model and experimental results. 相似文献
986.
This paper focuses on the group of metalloproteins/metalloenzymes in the acetyl-coenzyme A synthesis pathway of anaerobic
microbes called Wood-Ljungdahl pathway, including formate dehydrogenase (FDH), corrinoid iron sulfur protein (CoFeSP), acetyl-CoA
synthase (ACS) and CO dehydrogenase (CODH). FDH, a key metalloenzyme involved in the conversion of carbon dioxide to methyltetrahydrofolate,
catalyzes the reversible oxidation of formate to carbon dioxide. CoFeSP, as a methyl group transformer, accepts the methyl
group from CH3-H4 folate and then transfers it to ACS. CODH reversibly catalyzes the reduction of CO2 to CO and ACS functions for acetyl-coenzyme A synthesis through condensation of the methyl group, CO and coenzyme A, to finish
the whole pathway. This paper introduces the structure, function and reaction mechanisms of these enzymes. 相似文献
987.
A phase-inversion/sintering technique has been employed in the production of La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) hollow fibre membranes, a bundle of which has then been placed in a high-temperature furnace for production of high purity oxygen from air at temperatures between 980 °C and 1060 °C. By applying a vacuum in the hollow fibre lumens, a product stream containing oxygen purity of 97.15% has been obtained. The downstream vacuum degree higher than 99 kPa shows negligible effect on the oxygen production rate. Studies on long-term operation suggest that the LSCF hollow fibre membranes are less stable for the oxygen production due to the segregation of the constituent membrane elements and the formation of new phases on the outer membrane surfaces. The effect of the operating cycle on the retrogression of membrane performance is much larger than that of duration used in a single cycle. 相似文献
988.
A. V. Gusev A. V. Kanshu K. V. Shandarova V. M. Shandarov E. V. Smirnov D. Kip C. Rüter Y. Tan F. Chen 《Russian Physics Journal》2008,51(9):943-948
Experimental multielement planar and channel waveguide structures induced by coherent radiation in bulk samples of photorefractive
lithium niobate and planar waveguides on their basis including those formed in a projection scheme with scaling of image dimensions
are described. The possibility of linear and nonlinear light localization in optically-induced bulk and planar one-dimensional
photorefractive photonic superlattices is demonstrated.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 57–62, September, 2008. 相似文献
989.
990.
H. Hussain B. H. Tan C. S. Gudipati Y. Liu C. B. He T. P. Davis 《Journal of polymer science. Part A, Polymer chemistry》2008,46(16):5604-5615
Amphiphilic diblock copolymers of polystyrene (PS) and poly(N‐vinylpyrrolidone) (PNVP) were prepared by a combination of ATRP and MADIX. Well‐defined PS with bromine end group was synthesized by ATRP in bulk at 110 °C using (1‐bromoethyl) benzene as an initiator. The Br‐ end group was then converted to xanthate as verified by 1H NMR spectroscopy, elemental analysis, and UV‐spectroscopy. PS‐b‐PNVP copolymers were produced by MADIX of NVP in bulk at 60 °C using PS‐xanthate as a macro‐chain transfer agent and the kinetics of polymerization were investigated. The structures of PS‐b‐PNVP were characterized using GPC and 1H NMR. Amphiphilic PS‐b‐PNVP could form spherical micelles with PS cores and PNVP shells in aqueous solution as confirmed by 1H NMR and laser light scattering (LLS). The values of critical micelle concentration of PS‐b‐PNVP and the average aggregation number of PS‐b‐PNVP in the micelles were measured using pyrene as a probe and static LLS, respectively. The aggregation number increases concomitantly with temperature (10–50 °C), but the hydrodynamic radius of the micelles remains almost constant over the same temperature range, which may indicate shell dehydration at a higher temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5604–5615, 2008 相似文献