全文获取类型
收费全文 | 1086篇 |
免费 | 129篇 |
国内免费 | 157篇 |
专业分类
化学 | 879篇 |
晶体学 | 13篇 |
力学 | 66篇 |
综合类 | 5篇 |
数学 | 28篇 |
物理学 | 381篇 |
出版年
2023年 | 12篇 |
2022年 | 33篇 |
2021年 | 36篇 |
2020年 | 40篇 |
2019年 | 29篇 |
2018年 | 33篇 |
2017年 | 56篇 |
2016年 | 37篇 |
2015年 | 47篇 |
2014年 | 52篇 |
2013年 | 79篇 |
2012年 | 64篇 |
2011年 | 56篇 |
2010年 | 42篇 |
2009年 | 59篇 |
2008年 | 79篇 |
2007年 | 68篇 |
2006年 | 70篇 |
2005年 | 51篇 |
2004年 | 41篇 |
2003年 | 45篇 |
2002年 | 34篇 |
2001年 | 37篇 |
2000年 | 43篇 |
1999年 | 31篇 |
1998年 | 24篇 |
1997年 | 27篇 |
1996年 | 14篇 |
1995年 | 21篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1977年 | 1篇 |
排序方式: 共有1372条查询结果,搜索用时 15 毫秒
121.
I. I. Bashter 《辐射效应与固体损伤》2013,168(3-4):351-364
Abstract The fast neutron and gamma ray spectra measured behind different thickness of steel scrap concrete with density of 4 g/cm3 have been studied. The mix proportions by weight of this type of concrete were 1 cement: 6.89 steel scrap: 2.9 sand and 0.5 Water. Comparison with a standard ordinary concrete of density 2.3 g/cm3 have been carried out. The measurements were made using a collimated beam of both gamma rays and neutrons emitted from one of the horizontal channel of the Egyptian Research Reactor-1. A fast neutron and gamma ray spectrometer with a stilbene crystal was used to measure the spectra of fast neutrons and gamma rays. Pulse shape discrimination using the zero cross over technique was used to separate the photon pulses from the electron pulses. The equation due to Schmidt has been modified and applied for determining the neutron effective removal cross sections (~R) for steel scrap, ordinary, hematite–serpentine, ilmenite–limonite and ilmenite concretes. This equation gives results which are in good agreement with the measured values. The derived empirical equation in a previous work to calculate the neutron integral flux behind different thicknesses of different types of concretes, gives good results for steel scrap concrete under investigation comparing with the corresponding experimental data. Total neutron macroscopic cross sections, linear attenuation coefficients for gamma rays and the half-value layers for both radiations at different energies have been obtained for steel scrap concrete and comparing with the corresponding values of ordinary concrete. The results show that steel scrap concrete is better than ordinary, hematite–serpentine, ilmenite–limonite and ilmenite concretes from the radiation shielding point of view. 相似文献
122.
A kinetic investigation of removal of chromium from aqueous solutions with a strong cation exchange resin 总被引:1,自引:0,他引:1
A kinetic investigation was performed with an ion exchange resin for chromium. A strong cation exchange resin (Amberlite IR
120) was used for removal of chromium. The effects of concentration, resin amount, and stirring speed on kinetics were investigated.
The metal concentration range studied was between 5 to 160 mg/dm3 (the amount of solution was 4 dm3), the resin amount range was between 5 to 20 mg, and the stirring speed range was between 1000 to 3500 rpm. Equilibrium experiments
were performed for calculation of separation factor. Kinetic studies were done using a Kressman-Kitchener stirrer reactor system and the results were compared with existing kinetic models. Two models, Nernst-Plank film diffusion control model (fdc) and solid phase diffusion control model (pdc), respectively were identified, and the dependence
of the rate on parameters such as solution concentration, resin amount, stirring speed, etc. was examined for each of them. The interpretation of these data shows that the system is probably controlled by both film
and particle diffusion.
Correspondence: Sevgi Kocaoba, Department of Chemistry, Faculty of Art and Science, Yildiz Technical University, Davutpasa
Cad. No: 127, 34210 Davutpasa-Istanbul, Turkey. 相似文献
123.
A reusable and sensitive immunoassay based on phenylboronic acid immunoaffinity reactor in combination with flow injection chemiluminescence (CL) for determination of glycoprotein was described. The reactor was fabricated by immobilizing 3-aminophenylboronic acid (APBA) on glass microbeads with γ-glycidoxypropyltrimethoxysilane (GPMS) as linkage. The α-fetoprotein (AFP) could be easily immobilized on the APBA coated beads through sugar-boronic interaction. After an off-line incubation, the mixture of the analyte AFP with horseradish peroxidase-labeled AFP antibody (HRP-anti-AFP) was injected into the reactor. This led the trapping of free HRP-anti-AFP by the surface coated AFP on glass beads. The trapped HRP-anti-AFP was detected by chemiluminescence due to its sensitizing effect on the reaction of luminol and hydrogen peroxide. Under optimal conditions, the chemiluminescent signal was proportional to AFP concentration in the range of 10-100 ng mL−1. The whole assay process including regeneration of the reactor could be completed within 31 min. The proposed system showed acceptable detection and fabrication reproducibility, and the results obtained with the present method were in acceptable agreement with those from parallel single-analyte test of practical clinical sera. The described method enabled a low-cost, time saving and was potential to detect the serum AFP level in clinical diagnosis. 相似文献
124.
Visible‐light‐driven photocatalytic degradation of fenpyroximate in rotating packed bed reactor using Fe3O4@PbS@Ni2P magnetic nanocomposite photocatalyst: Response surface modelling and optimization 下载免费PDF全文
125.
In this communication, a porous stainless steel (PSS) tube was electrolessly plated into Pd–Ag membrane reactor which was used for separating hydrogen produced in an ethanol steam reforming reaction with the addition of oxygen, which has not been reported before. Palladium and silver were deposited on porous stainless steel tube via the sequential electroless plating procedure with an overall film thickness of 20 μm and Pd/Ag weight ratio of 78/22. Ethanol–water mixture (nwater/nethanol = 1 or 3) and oxygen (noxygen/nethanol = 0.2 or 0.7) were fed concurrently into the membrane reactor packed with MDC-3. The reaction temperatures were set at 593–723 K and the pressures 3–10 atm. The effect of oxygen addition plays a vital role on the ethanol steam reforming reaction, especially for the Pd–Ag membrane reactor in which a higher flux of hydrogen is required. If oxygen in the feed is not sufficient, it would be possible that steam reforming reaction prevails. Inversely, high O2 addition will shift the reaction scenario to be partial oxidation dominating, and selectivity of CO2 increases with increasing oxygen feed. At high pressure, autothermal reaction of ethanol would be easily reached. 相似文献
126.
S. Freni N. Mondello S. Cavallaro G. Cacciola V.N. Parmon V.A. Sobyanin 《Reaction Kinetics and Catalysis Letters》2000,71(1):143-152
A two-layer fixed-bed catalytic reactor for hydrogen production by steam reforming of ethanol is proposed. In this reactor ethanol is first converted to acetaldehyde over a Cu-based catalyst and then acetaldehyde is converted to a hydrogen-rich mixture over a Ni-based catalyst. It is shown that the use of such type of reactor prevents coke formation and provides hydrogen yields closed to equilibrium. 相似文献
127.
Ying Zhao Ying‐Wu Luo Changhuai Ye Bo‐Geng Li Shiping Zhu 《Journal of polymer science. Part A, Polymer chemistry》2009,47(1):69-79
Methyl methacrylate (MMA)/tert‐butyl methacrylate (tBMA) gradient copolymers having linear and hyperbolic composition profiles were synthesized. These special copolymer products were achieved via a model‐based computer‐controlled semibatch atom transfer radical copolymerization (ATRcoP) process. A simple ATRcoP model was developed based on the terminal model. The equilibrium constants in the ATRP of MMA and tBMA were estimated by the data correlation. The model was verified by batch experiments and was found to give good correlation for the polymerization rate, molecular weight, and copolymer composition data. The model coupled with a reactor model was then applied to the semibatch ATRcoP and was used to calculate comonomer feeding rates for the targeted gradient composition profiles. It was found that the experimental monomer conversion, molecular weight, and cumulative copolymer composition were in good agreement with their targeted theoretical values. The gradient copolymers had low polydispersities close to 1.1. This work demonstrated the feasibility of the model‐based semibatch ATRcoP in fine‐tuning gradient copolymer composition profiles. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 69–79, 2009 相似文献
128.
A bienzyme reactor sensor system with amperometric detection was developed for the determination of ornithine. The system based on the immobilized enzymes (ornithine carbamyl transferase and pyruvate oxidase) consisted of a buffer tank, a peristaltic pump, an enzyme reactor, an oxygen electrode and a recorder. Then, 0.1 M MOPS buffer, containing pyruvic acid (0.5 mM) and carbamyl phosphate (0.5 mM), was continuously transferred into the system at 35 °C. Phosphate ion was formed enzymatically by transformation of ornithine in the presence of carbamyl phosphate. Pyruvate oxidase is activated by the presence of phosphate. Therefore, ornithine was determined from the oxygen consumed upon oxidation of pyruvic acid catalyzed by pyruvate oxidase in the presence of phosphate ion. The limit of detection was 0.05 mM and the response was linear to 3 mM (R2=0.9905). The variation coefficients were 4.9 (n=15) and 3.9% (n=15) for 1.1 and 3.0 mM standard ornithine, respectively. Good comparative results (R2=0.9238) were observed between ornithine contents in prawn muscle determined by the proposed system and by the HPLC. One assay was completed within 4 min. The immobilized enzymes were stable for 2 months at 4 °C and more than 150 samples could be continually determined using this enzyme reactor. 相似文献
129.
Caiyun Xiong Xiaolin Nie Yixue Peng Xun Zhou Yangtao Fan Hu Chen Yanhui Liu 《理论物理通讯》2021,73(7):75602
Knots are discovered in biophysical systems, such as DNA and proteins. Knotted portions in knotted DNA are significantly bent and their corresponding bending angles are comparable with or larger than the sharp bending angle resulting in flexible defects. The role of flexible defects in the interplay of supercoiling and knotting of circular DNA were predicted by a Monte Carlo simulation. In knotted DNA with a particular knot type, a flexible defect noticeably enhances the supercoiling of the knotted DNA and the decreasing excitation energy makes the knotted portion more compact. A reduction in twist rigidity and unwinding of flexible defects are incorporated into the numerical simulations, so that interplay of supercoiling and knotting of circular DNA is studied under torsional conditions. Increasing unwinding not only results in a wider linking number distribution, but also leads to a drift of the distribution to lower values. A flexible defect has obvious effects on knotting probability. The summation of equilibrium distribution probability for nontrivial knotted DNA with different contour length does not change with excitation energy monotonically and has a maximum at an intermediate value of excitation energy around 5 kBT. In the phase space of knot length and gyration radius of knotted DNA, knot length does not anticorrelate with its gyration radius, which is attributed to the flexible defect in the knotted portion, which leads to the release of bending energy and inhibited the competition between entropy and bending energy. 相似文献
130.
Oxidative dehydrogenation of propane under steady-state and transient regimes over alumina-supported catalysts prepared from mixed V_2W_4O_(19)~(4-) hexametalate precursors 下载免费PDF全文
Sébastien Paul Elise Berrier Marcia Cristina Kaezer Frana Jean-Guillaume Eon 《天然气化学杂志》2010,19(2):123-133
An integrated approach combining the development of an innovative catalyst and the research of a set of adequate operating conditions for the propane oxidative dehydrogenation (ODH) is described. The experimental set-up, specially designed for steady-state and transient studies is presented. The preparation method, the characterization and the performances in steady-state and transient regimes of catalysts based on V_2W_4O_(19)~(4-) Lindqvist isopolyanion used as a precursor and supported on alumina are reported. The influence of the preparation method of the catalyst and the role of water in the feed gas are more particularly discussed. 相似文献