全文获取类型
收费全文 | 3531篇 |
免费 | 118篇 |
国内免费 | 33篇 |
专业分类
化学 | 2039篇 |
晶体学 | 32篇 |
力学 | 77篇 |
数学 | 467篇 |
物理学 | 602篇 |
无线电 | 465篇 |
出版年
2022年 | 46篇 |
2021年 | 41篇 |
2020年 | 60篇 |
2019年 | 56篇 |
2018年 | 42篇 |
2017年 | 37篇 |
2016年 | 64篇 |
2015年 | 56篇 |
2014年 | 92篇 |
2013年 | 136篇 |
2012年 | 171篇 |
2011年 | 228篇 |
2010年 | 118篇 |
2009年 | 123篇 |
2008年 | 163篇 |
2007年 | 204篇 |
2006年 | 198篇 |
2005年 | 174篇 |
2004年 | 159篇 |
2003年 | 138篇 |
2002年 | 129篇 |
2001年 | 55篇 |
2000年 | 62篇 |
1999年 | 42篇 |
1998年 | 44篇 |
1997年 | 56篇 |
1996年 | 48篇 |
1995年 | 58篇 |
1994年 | 48篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 37篇 |
1990年 | 40篇 |
1989年 | 22篇 |
1988年 | 32篇 |
1987年 | 33篇 |
1986年 | 32篇 |
1985年 | 38篇 |
1984年 | 30篇 |
1983年 | 39篇 |
1982年 | 41篇 |
1981年 | 41篇 |
1980年 | 29篇 |
1979年 | 28篇 |
1978年 | 33篇 |
1977年 | 34篇 |
1976年 | 41篇 |
1975年 | 46篇 |
1974年 | 32篇 |
1973年 | 25篇 |
排序方式: 共有3682条查询结果,搜索用时 15 毫秒
891.
Automotive shredders need a reverse production planning strategy that includes determining at what price to purchase vehicle hulks from different sources. In this paper, we formulate the automotive reverse production planning and pricing problem in a nonlinear programming model, develop an approximate supply function for hulks when adjacent shredders price independently, and compare two hulk pricing strategies in three trends for ferrous metal and hulk prices: constant, increasing and decreasing. The case study results indicate that adjusting purchase price based on hulk composition in coordination with planning for purchasing, storing and processing can increase net revenue by 7–15%. 相似文献
892.
Matija Strli? Irena Kralj Cigi? Jana Kolar May Cassar 《Polymer Degradation and Stability》2009,94(6):886-890
Historic parchment is a macromolecular material, which is complex due to its natural origin, inhomogeneity of the skin structure, unknown environmental history and potential localised degradation. Most research into its stability has so far focussed on thermal and structural methods of analyses.Using gas chromatographic analysis of the atmosphere surrounding parchment during oxidation, we provide the experimental evidence on the production of volatile aldehydes, which can be the products of lipid autoxidation. Oxidation of parchment with different aldehyde emissions was additionally followed in situ using chemiluminometry and the same techniques were used to evaluate the oxidation of differently delipidised parchment. It was shown that the production of peroxides and the emission of aldehydes from the material decrease with lower lipid content. Building on this evidence, we can conclude that the presence of lipids (either initially present in the skin or resulting from conservation intervention) leads to oxidative degradation of collagen and that the non-destructive analysis of the emission of volatiles could be used as a quick tool for evaluation of parchment stability. 相似文献
893.
A passive RFID tag antenna in the UHF band using two bent dipoles and a modified double T-matching network is proposed. The antenna was fabricated with a thin copper layer printed on a 50 /spl mu/m-thick PET substrate for low-cost production. The proposed antenna provided a quasi-isotropic radiation pattern and a fairly broad bandwidth S/sub 11/<-10 dB, 8.5%) for conjugate-matching with a commercial tag chip. The efficiency of the antenna was about 90% in the operating frequency band. The measured readable range was between 170 -240 cm for an arbitrary rotation angle of the tag. 相似文献
894.
895.
896.
897.
Chien‐Yu Li Wen‐Yen Chiu Trong‐Ming Don 《Journal of polymer science. Part A, Polymer chemistry》2007,45(15):3359-3369
The morphology of PU/PMMA hybrid particles prepared by miniemulsion polymerization was predicted through the consideration of their Gibbs free energy changes. Five morphological states of PU/PMMA hybrid particles were proposed and their Gibbs free energy changes were calculated. Before the formation of hybrid particles, the initial state included a monomer mixture of PU prepolymer, MMA, a chain extender, TMP, and an initiator, which was in droplets suspended in water containing SDS. Two assumptions were made. First, the densities of all states were the same. Secondly, secondary nucleation of particles was negligible. Thus the size of initial droplet and final particle was unchanged through miniemulsion polymerization. The interfacial tensions were measured by a pendant drop method and were used for calculation. The preferred morphology of PU/PMMA hybrid particle had the minimum value of ΔGphase. Different NCO/OH ratios of PU and initiators of MMA were used to study the morphological change of PU/PMMA hybrid particles. When BD was used as the chain extender of PU, the hybrid particles showed the PU‐rich phase as the shell and PMMA‐rich as the core. When incorporating bisphenol A into PU polymer, the homogeneous structure of hybrid particle was preferred. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3359–3369, 2007 相似文献
898.
Alaa S. Abd‐El‐Aziz Rawda M. Okasha Leslie J. May Jeff Hurd 《Journal of polymer science. Part A, Polymer chemistry》2006,44(9):3053-3070
A number of classes of polynorbornenes containing cationic iron moieties within their side chains were prepared via ring‐opening metathesis polymerization with a ruthenium‐based catalyst. The iron‐containing polymers displayed excellent solubility in polar organic solvents. The weight‐average molecular weights of these polymeric materials were estimated to be in the range of 18,000–48,000. Thermogravimetric analysis of these polymers showed two distinct weight losses. The first weight loss was in the range of 204–260 °C and was due to the loss of the metallic moieties, whereas the second weight loss was observed at 368–512 °C and was due to the degradation of the polymer backbone. Cyclic voltammetry studies of the iron‐containing polymers showed that the 18 e? cationic iron centers underwent a reduction to give the neutral 19 e? complexes at half‐wave potential (E1/2) = ?1.105 V. Photolysis of the metallated polymers led to the isolation of the norbornene polymers in very good yields. Differential scanning calorimetry studies showed a sharp increase in the glass‐transition temperatures up to 91 °C when rigid aromatic side chains were incorporated into the norbornene polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3053–3070, 2006 相似文献
899.
Ben Brüggemann Tnu Pullerits Volkhard May 《Journal of photochemistry and photobiology. A, Chemistry》2007,190(2-3):372-377
Femtosecond laser pulse control of exciton dynamics in a biological chromophore complex is studied theoretically. The computations use the optimal control theory specified to open quantum systems and formulated in the framework of the rotating wave approximation. Based on the laser pulse induced formation of an excitonic wave packet the possibility to localize excitation energy at a certain chromophore within a photosynthetic antenna system (FMO complex of green bacteria) is investigated. Details of exciton dynamics driven by a polarization shaped pulse are discussed. 相似文献
900.
The enormous potential of biosensors in medical diagnostics has motivated scientists to develop newer innovative tools and advance biosensing technologies. The use of cell, organelles, nucleotides, aptamers, antibodies, affibodies, proteins, peptides, molecules, and printed polymers, merged with nanotechnology, offers excellent tools to prepare highly sensitive and advanced biosensors. Therefore, the current decade has witnessed a rapid surge in the fabrication of different nanomaterial-based biosensors. Among them, carbon nanomaterials (CNMs) have emerged highly attractive in the fabrication of both electrochemical and electrochemiluminescence (ECL) biosensors. On one hand, CNMs bear prominent electrical conductivity, large surface area to immobilize adequate amount of biomolecules, an enhanced loading capacity, improved biocompatibility, and active site for electrochemical reaction. Additionally, CNMs could be chemically modified for the covalent coupling with the biomolecules. On the other hand, both electrochemical and ECL biosensors allow for cost-effective, rapid, and real-time detection with excellent sensitivity and selectivity, with the capability of integrating different biomolecules and CNMs on the same chip. However, currently there is not a single review, which includes CNM-based electrochemical and ECL biosensors' current progress and trends. Therefore, this review intends to survey the current progress and future trends in CNM-based electrochemical and ECL biosensors. 相似文献