全文获取类型
收费全文 | 3754篇 |
免费 | 305篇 |
国内免费 | 395篇 |
专业分类
化学 | 979篇 |
晶体学 | 13篇 |
力学 | 1033篇 |
综合类 | 77篇 |
数学 | 1190篇 |
物理学 | 1162篇 |
出版年
2024年 | 8篇 |
2023年 | 31篇 |
2022年 | 84篇 |
2021年 | 135篇 |
2020年 | 118篇 |
2019年 | 92篇 |
2018年 | 95篇 |
2017年 | 161篇 |
2016年 | 236篇 |
2015年 | 157篇 |
2014年 | 191篇 |
2013年 | 238篇 |
2012年 | 183篇 |
2011年 | 214篇 |
2010年 | 192篇 |
2009年 | 227篇 |
2008年 | 224篇 |
2007年 | 205篇 |
2006年 | 199篇 |
2005年 | 183篇 |
2004年 | 178篇 |
2003年 | 141篇 |
2002年 | 123篇 |
2001年 | 108篇 |
2000年 | 87篇 |
1999年 | 86篇 |
1998年 | 79篇 |
1997年 | 69篇 |
1996年 | 56篇 |
1995年 | 59篇 |
1994年 | 53篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 23篇 |
1990年 | 24篇 |
1989年 | 14篇 |
1988年 | 21篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1982年 | 11篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
排序方式: 共有4454条查询结果,搜索用时 187 毫秒
71.
The vanadium trapping effect of Mg and La containing additives in cracking catalyst contaminated with 2300 ppm Ni and 4700 ppm V has been analyzed by microactivity test (MAT) and imaging secondary ion mass spectrometry (SIMS). The results of SIMS imaging are consistent with cracking activity data and show that the La/spinel is a superior vanadium trap for the fluid catalytic cracking of hydrocarbons (FCC) operation. La/spinel serves as a dual function additive for both vanadium trap and SOx removal. The optimum amount of La/spinel added to the cracking catalyst is about 15% by weight. This results in an increased catalytic activity, an increase in gasoline yield, and a decrease in coke and gas factors. The MgAl2O4 phase of Mg/Al2O3 additive is found capable of trapping vanadium while its MgO of Mg/Al2O3 phase can migrate to zeolite particles (the active component of the FCC catalyst) that, in turn, causes a decline in the activity of that catalyst. 相似文献
72.
Antifouling coatings are the most reliable way to prevent biofouling of immersed surfaces. As concerning the high toxicity of organotin paints, the tendency is the development of coatings, which do not present environmental risks. In this work, we prepare binders from modification of acrylic copolymers containing free carboxylic acid groups. Biocides chosen are tertiary ammonium salts; alkyl chain substituents with different length are considered. The chemical modifications of resins are carried out via a single step reaction. Modification extents are monitored through proton nuclear magnetic resonance and thermogravimetric analysis and the modified resins are characterised by Fourier transform infrared spectroscopy. The glass transition temperature of the acrylate systems is assessed by dynamical mechanical analysis (pin point method) and compared with data obtained by differential scanning calorimetry. The erosion and antifouling properties of the binders are followed during an exposure to marine environment by a visual observation. 相似文献
73.
Marina DellaGreca Antonio FiorentinoMarina Isidori Lucio PreviteraFabio Temussi Armando Zarrelli 《Tetrahedron》2003,59(26):4821-4825
Investigation of the ethyl acetate extract of Juncus acutus rhizomes revealed seven benzocoumarins, probably derived from oxidation of phenanthrenes already isolated from the plant. The structures were determined by means of spectroscopic methods. The anti-algal activity of the isolated compounds tested on the green alga Pseudokirchneriella subcapitata are reported here. 相似文献
74.
Basinska T 《Macromolecular bioscience》2005,5(12):1145-1168
Functional hydrophilic microspheres (latex particles) have found various applications in life sciences and in medicine - particularly in latex diagnostic tests. This paper presents a comprehensive review of studies on latex particles with a hydrophilic interfacial layer composed of various hydrophilic polymers with reactive groups at the ends of macromolecules or at each monomeric unit along the chain. Typical examples of these hydrophilic polymers are poly(2-hydroxyethyl methyl methacrylate), poly(acrylic acid), poly(N,N-dimethylacrylamide), polysaccharides, poly(ethylene oxide) and polyglycidol. Hydrophilic microspheres with different morphologies (uniform or core-shell, see Figure) have been synthesized by emulsion and dispersion polymerizations. The chemical structure of polymers which constitute the interfacial layer of microspheres has been investigated using a variety of instrumental techniques (such as XPS, SSIMS and NMR) and analytical methods based on specific chemical reactions suitable for the determination of particular functional groups. Microspheres are exposed to contact with proteins in the majority of medical applications. This paper presents examples of studies on the attachment of these biomacromolecules to microspheres. The relation between the structure of the interfacial layer of microspheres and the ability of these particles for the covalent binding of proteins is discussed. Several examples of diagnostic tests, in which hydrophilic microspheres with adsorbed or covalently immobilized proteins were used as reagents, are presented. The paper also contains a short review of the application of magnetic hydrophilic particles for protein separation. Examples of hydrophilic latex particles used for hemoperfusion or heavy metal ion separation are presented. Hydrophilic microspheres with uniform or core-shell morphologies. 相似文献
75.
The principle of a novel latex diagnostic test for the determination of antibodies against Helicobacter pylori in blood sera is described. The test is based on the measurement of the electrophoretic mobility of the microspheres with immobilized H. pylori antigens. The electrophoretic mobility of these microspheres depends on the concentration of the antibodies against H. pylori in suspending medium. Particles with hydrophilic polyglycidol in the surface layer were used for the test. The microspheres were obtained by an emulsifier-free emulsion copolymerization of styrene and alpha-tert-butoxy-omega-vinylbenzyl-polyglycidol macromonomer (D(n) = 220 nm, diameter polydispersity factor D(w)//D(n) = 1.02). Activation of polyglycidol hydroxyl groups with cyanuric chloride allowed for covalent immobilization of H. pylori antigens. The fraction of H. pylori not specifically adsorbed onto the microspheres was negligible. Changes of the electrophoretic mobility of the microspheres with the surface concentration of the covalently immobilized H. pylori antigens Gamma = (1.6 +/- 0.3) . 10(-3) g m(-2) were suitable for the detection of the antibodies in the sera of patients with titer in the range (determined by the indirect ELISA test) from 1:500 to 1:32 000. 相似文献
76.
In the present work, corrosion resistance of surface-coated galvanized steel was quantitatively determined by an analysis
of the alternating current (AC) impedance spectra measured on the salt-spray-tested specimen. To evaluate the corrosion resistance
of the surface-coated galvanized steel, AC impedance spectroscopy was performed on the salt-spray-tested specimen previously
exposed to salt-sprayed corrosive environment. From the analysis of the impedance spectra, the area fraction transient of
white rust θ
2(t) was theoretically derived from the equivalent circuit equation by using two fitting parameters. The values of the two fitting
parameters were determined by fitting the empirical transient equations to the area fraction of the resin coating layer and
to the total resistance obtained from the impedance spectra measured, respectively. From the analyses of θ
2(t) for four kinds of surface-coated galvanized steels with various resin coating layers, it is indicated that as the values
of the two fitting parameters decrease in the order of CP, GI, OD and OM (commercial trade names) specimens, the corrosion
resistance increases in that order as well. Furthermore, from the quantitative comparison of the two fitting parameters with
the polarization resistance of the upper resin coating layer R
p determined from the potentiodynamic polarization curve, it is suggested that the two fitting parameters decrease in value
as well with increasing R
p. 相似文献
77.
Carbon dioxide fixation by microalgae photosynthesis using actual flue gas discharged from a boiler 总被引:2,自引:0,他引:2
Matsumoto Hiroyo Shioji Norio Hamasaki Akihiro Ikuta Yoshiaki Fukuda Yoshinori Sato Minoru Endo Noriyoshi Tsukamoto Toshiaki 《Applied biochemistry and biotechnology》1995,(1):681-692
To mitigate CO2 discharged from thermal power plants, studies on CO2 fixation by the photosynthesis of microalgae using actual exhaust gas have been carried out. The results are as follows.
相似文献
1. | A method is proposed for evaluating the maximum photosynthesis rate in the raceway cultivator using only the algal physical properties; |
2. | Outdoor cultivation tests taking actual flue gas were performed with no trouble or break throughout 1 yr using the strain collected in the test; |
3. | The produced microalgae is effective as solid fuel; and |
4. | The feasibility studies of this system were performed. The system required large land area, but the area is smaller than that required for other biomass systems, such as tree farms. |
78.
Arthur Cohen 《Journal of multivariate analysis》1977,7(3):454-460
Let Ui = (Xi, Yi), i = 1, 2,…, n, be a random sample from a bivariate normal distribution with mean μ = (μx, μy) and covariance matrix . Let Xi, i = n + 1,…, N represent additional independent observations on the X population. Consider the hypothesis testing problem H0 : μ = 0 vs. H1 : μ ≠ 0. We prove that Hotelling's T2 test, which uses (Xi, Yi), i = 1, 2,…, n (and discards Xi, i = n + 1,…, N) is an admissible test. In addition, and from a practical point of view, the proof will enable us to identify the region of the parameter space where the T2-test cannot be beaten. A similar result is also proved for the problem of testing μx ? μy = 0. A Bayes test and other competitors which are similar tests are discussed. 相似文献
79.
As an extention of previous reports, variables affecting the performance of columns modified by silicon deposition have been optimized. The aim of this work was to produce thermostable apolar glass capillary columns regardless of the glass type used and to maintain column performance under stressful conditions. The main parameters studied were the influence of glass type, leaching effects, and silicon layer thickness on column activity. Deactivation and stationary phase conditions were held constant. Both coated and uncoated columns were tested. The thickness of the silicon layer was found to be relatively unimportant. There was no difference whether soft glass or borosiiicate glass was used and leaching before silicon deposition did not influence column activity. Bare silicon surfaces showed considerable activity especially in respect to interactions with free acids and bases. There are indications that the surface consists of silicon oxide and other oxygenated forms of silicon rather than of the element. Treatment of the silicon layers with dilute hydrofluoric acid and the strict exclusion of traces of oxygen and water did not improve the situation. In spite of such specific interactions, silicon surfaces were easily deactivated by heat treatment with polysiloxanes. Silicon surfaces deactivated by baking with octamethyltetrasiloxane at 400°C are inert and temperature stable and show characteristics similar to persilylated surfaces. 相似文献
80.
The pretreatment of waste-activated sludge (WAS) by electron beam irradiation was studied in order to improve anaerobic sludge
digestion. The irradiation dose of the electron beam was varied from 0.5 to 10 kGy. Batch and continuous-flow stirred tank
reactors (CFSTRs) were operated to evaluate the effect of the electron beam pretreatment on anaerobic sludge digestion. Approximately
30–52% of the total chemical oxygen demand (COD) content of the WAS was solubilized within 24 h after electron beam irradiation.
A large quantity of soluble COD, protein, and carbohydrates leached out from cell ruptures caused by the electron beam irradiation.
Volatile fatty acids production from the irradiated sludge was approx 90% higher than that of the unirradiated sludge. The
degradation of irradiated sewage sludge was described by two distinct first-order decay rates (k
1 and k
2). Most initial decay reaction accelerated within 10 d, with an average k
1 of 0.06/d for sewage sludge irradiated at all dosages. The mean values for the long-term batch first-order decay coefficient
(k
2) were 0.025/d for irradiated sewage sludge and 0.007/d for unirradiated sludge. Volatile solids removal efficiency of the
control reactor fed with unirradiated sewage sludge at a hydraulic retention time (HRT) of 20 d was almost the same as that
of the CFSTRs fed with irradiated sludge at an HRT of 10 d. Therefore, disintegration of sewage sludge cells using electron
beam pretreatment could reduce the reactor solid retention time by half. 相似文献