全文获取类型
收费全文 | 1186篇 |
免费 | 53篇 |
国内免费 | 3篇 |
专业分类
化学 | 735篇 |
晶体学 | 25篇 |
力学 | 49篇 |
数学 | 211篇 |
物理学 | 222篇 |
出版年
2024年 | 2篇 |
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 41篇 |
2020年 | 39篇 |
2019年 | 34篇 |
2018年 | 29篇 |
2017年 | 49篇 |
2016年 | 53篇 |
2015年 | 53篇 |
2014年 | 46篇 |
2013年 | 115篇 |
2012年 | 101篇 |
2011年 | 103篇 |
2010年 | 77篇 |
2009年 | 64篇 |
2008年 | 73篇 |
2007年 | 64篇 |
2006年 | 40篇 |
2005年 | 43篇 |
2004年 | 51篇 |
2003年 | 25篇 |
2002年 | 24篇 |
2001年 | 8篇 |
2000年 | 13篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 7篇 |
1993年 | 1篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1972年 | 2篇 |
1907年 | 1篇 |
排序方式: 共有1242条查询结果,搜索用时 31 毫秒
11.
The monthly variation of selenium concentration in atmospheric particulate material of Ankara was investigated. The selenium
concentrations in possible pollution source materials like coal, fuel oil and their bottom and fly ashes were determined to
obtain the percent transference of selenium into the atmosphere. Instrumental thermal neutron activation analysis and atomic
absorption spectroscopy were applied for the analysis of selenium in the samples. Selenium enrichment factors with respect
to the fuels, soil of Ankara and crustal material were also calculated. Atmospheric selenium concentration is found to increase
during winter months and the main cause of this increase is the emission of selenium into the atmosphere due to fuel combustion. 相似文献
12.
Dai H Choe WS Thai CK Sarikaya M Traxler BA Baneyx F Schwartz DT 《Journal of the American Chemical Society》2005,127(44):15637-15643
We show that a protein with no intrinsic inorganic synthesis activity can be endowed with the ability to control the formation of inorganic nanostructures under thermodynamically unfavorable (nonequilibrium) conditions, reproducing a key feature of biological hard-tissue growth and assembly. The nonequilibrium synthesis of Cu(2)O nanoparticles is accomplished using an engineered derivative of the DNA-binding protein TraI in a room-temperature precursor electrolyte. The functional TraI derivative (TraIi1753::CN225) is engineered to possess a cysteine-constrained 12-residue Cu(2)O binding sequence, designated CN225, that is inserted into a permissive site in TraI. When TraIi1753::CN225 is included in the precursor electrolyte, stable Cu(2)O nanoparticles form, even though the concentrations of [Cu(+)] and [OH(-)] are at 5% of the solubility product (K(sp,Cu2O)). Negative control experiments verify that Cu(2)O formation is controlled by inclusion of the CN225 binding sequence. Transmission electron microscopy and electron diffraction reveal a core-shell structure for the nonequilibrium nanoparticles: a 2 nm Cu(2)O core is surrounded by an adsorbed protein shell. Quantitative protein adsorption studies show that the unexpected stability of Cu(2)O is imparted by the nanomolar surface binding affinity of TraIi1753::CN225 for Cu(2)O (K(d) = 1.2 x 10(-)(8) M), which provides favorable interfacial energetics (-45 kJ/mol) for the core-shell configuration. The protein shell retains the DNA-binding traits of TraI, as evidenced by the spontaneous organization of nanoparticles onto circular double-stranded DNA. 相似文献
13.
14.
Removal of methyl violet from aqueous solution by perlite 总被引:2,自引:0,他引:2
The use of perlite for the removal of methyl violet from aqueous solutions at different concentration, pH, and temperature has been investigated. Adsorption equilibrium is reached within 1 h. The capacity of perlite samples for the adsorption of methyl violet was found to increase with increasing pH and temperature and decrease with expansion and increasing acid-activation. The adsorption isotherms are described by means of the Langmuir and Freundlich isotherms. The adsorption isotherm was measured experimentally at different conditions and the experimental data were correlated reasonably well by the adsorption isotherm of Langmuir. The order of heat of adsorption corresponds to a physical reaction. It is concluded that the methyl violet is physically adsorbed onto the perlite. The removal efficiency (P) and dimensionless separation factor (R) have shown that perlite can be used for removal of methyl violet from aqueous solutions, but unexpanded perlite is more effective. 相似文献
15.
Weil Joseph Brewer Mark Hendrickson Richard Sarikaya Ayda Ladisch Michael R. 《Applied biochemistry and biotechnology》1998,(1):99-111
Yellow poplar wood sawdust consists of 41% cellulose and 19% hemicellulose. The goal of pressure cooking this material in
water is to hydrate the more chemically resistive regions of cellulose in order to enhance enzymatic conversion to glucose.
Pretreatment can generate organic acids through acid-catalyzed degradation of monosaccharides formed because of acids released
from the biomass material or the inherent acidity of the water at temperatures above 160°C. The resulting acids will further
promote the acid-catalyzed degradation of monomers that cause both a reduction in the yield and the formation of fermentation
inhibitors such as hydroxymethyl furfural and furfural. A continuous pH-monitoring system was developed to help characterize
the trends in pH during pretreatment and to assist in the development of a base (2.0 M KOH) addition profile to help keep
the pH within a specified range in order to reduce any catalytic degradation and the formation of any monosac-charide degradation
products during pretreatment. The results of this work are discussed. 相似文献
16.
In this study, the Schiff base ligands H2L1–H2L3 and their CuII, CoII, NiII, FeIII RuIII and VOIV complexes have been prepared and characterized by spectroscopic and analytical techniques. All the complexes are mononuclear.
Keto-enol tautomeric forms of the ligands have been investigated in polar and apolar solvents. The ligands favor the keto-form
in the C7H8 and C6H14. The C–C coupling reaction of the 2,6-di-t-butylphenol has been investigated by the CoII and CuII complexes. Thermal properties of the complexes have been assessed using thermal techniques and similar properties were found.
In the FeIII and RuIII complexes, firstly, the coordinated water molecule is lost from the complex; in the second step, the chloride ion leaves
the molecule in the 300–350 °C temperature range. Finally, the complexes decompose to the appropriate metal oxide at the higher
temperature ranges. The electrochemical properties of the complexes have been studied in the two different solvents (DMF and
CH3CN). 相似文献
17.
Time series models, which are constructed from the projections of the molecular-dynamics (MD) runs on principal components (modes), are used to mimic the dynamics of two proteins: tendamistat and immunity protein of colicin E7 (ImmE7). Four independent MD runs of tendamistat and three independent runs of ImmE7 protein in vacuum are used to investigate the energy landscapes of these proteins. It is found that mean-square displacements of residues along the modes in different time scales can be mimicked by time series models, which are utilized in dividing protein dynamics into different regimes with respect to the dominating motion type. The first two regimes constitute the dominance of intraminimum motions during the first 5 ps and the random walk motion in a hierarchically higher-level energy minimum, which comprise the initial time period of the trajectories up to 20-40 ps for tendamistat and 80-120 ps for ImmE7. These are also the time ranges within which the linear nonstationary time series are completely satisfactory in explaining protein dynamics. Encountering energy barriers enclosing higher-level energy minima constrains the random walk motion of the proteins, and pseudorelaxation processes at different levels of minima are detected in tendamistat, depending on the sampling window size. Correlation (relaxation) times of 30-40 ps and 150-200 ps are detected for two energy envelopes of successive levels for tendamistat, which gives an overall idea about the hierarchical structure of the energy landscape. However, it should be stressed that correlation times of the modes are highly variable with respect to conformational subspaces and sampling window sizes, indicating the absence of an actual relaxation. The random-walk step sizes and the time length of the second regime are used to illuminate an important difference between the dynamics of the two proteins, which cannot be clarified by the investigation of relaxation times alone: ImmE7 has lower-energy barriers enclosing the higher-level energy minimum, preventing the protein to relax and letting it move in a random-walk fashion for a longer period of time. 相似文献
18.
Atom transfer radical polymerization conditions with copper(I) bromide/2,2′-bipyridine (Cu/2,2′-bpy) as the catalyst system were employed for the homopolymerization and random copolymerization of 1-phenoxycarbonyl ethyl methacrylate (PCMA) with methyl methacrylate (MMA). Temperature studies indicated that the polymerizations occurred smoothly in bulk at 110 °C. Poly(PCMA)(polydispersity index=1.27) homopolymer was characterized and then used as macroinitiator for increasing its molecular weight. The homopolymerization of PCMA was also carried out under free radical conditions using 2,2′-azobisisobutyronitrile as an initiator.The monomer and polymers were characterized by FT-IR and 1H and 13C-NMR techniques. The glass transition temperatures, the solubility parameters and average-molecular weights of the polymers were determined. Thermal stabilities of the polymers were given as compared with each other by using TGA curves. Thermal degradation products of poly(PCMA)s obtained by ATRP and free radical polymerization were compared with each other by using 1H-NMR technique. 相似文献
19.
For the determination of vanadium in biological materials by flame AAS an enrichment is described which comprises chelation with oxine and adsorption on activated carbon: The relative standard deviation for 10 g V/L was found to be 6% (n=15).Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday 相似文献
20.
A large amount of inedible plant material, generated as a result of plant growth in a Controlled Ecological Life Support System
(CELSS), should be pretreated and converted into forms that can be recycled on earth as well as in space. The main portion
of the inedible biomass is lignocellulosic material. Enzymatic hydrolysis of this cellulose would provide sugars for many
other uses by recycling carbon, hydrogen, oxygen, and nitrogen through formation of carbon dioxide, heat, and sugars, which
are potential foodstuffs. To obtain monosaccharides from cellulose, the protective effect of lignin should be removed. White-rot
fungi degrade lignin more extensively and rapidly than other microorganisms.Pleurotus ostreatus degrades lignin effectively, and produces edible and flavorful mushrooms that increase the quality and nutritional value
of the diet. This mushroom is also capable of metabolizing hemicellulose, thereby providing a food use of this pentose containing
polysaccharide. This study presents the current knowledge of physiology and biochemistry of primary and secondary metabolisms
of basidiomycetes, and degradation mechanism of lignin. A better understanding of the ligninolytic activity of white-rot fungi
will impact the CELSS Program by providing insights on how edible fungi might be used to recycle the inedible portions of
the crops. 相似文献