全文获取类型
收费全文 | 3117篇 |
免费 | 84篇 |
国内免费 | 9篇 |
专业分类
化学 | 2345篇 |
晶体学 | 36篇 |
力学 | 47篇 |
数学 | 425篇 |
物理学 | 357篇 |
出版年
2022年 | 31篇 |
2021年 | 31篇 |
2020年 | 48篇 |
2019年 | 39篇 |
2018年 | 37篇 |
2017年 | 31篇 |
2016年 | 58篇 |
2015年 | 52篇 |
2014年 | 73篇 |
2013年 | 151篇 |
2012年 | 162篇 |
2011年 | 206篇 |
2010年 | 117篇 |
2009年 | 94篇 |
2008年 | 174篇 |
2007年 | 179篇 |
2006年 | 176篇 |
2005年 | 196篇 |
2004年 | 164篇 |
2003年 | 141篇 |
2002年 | 139篇 |
2001年 | 44篇 |
2000年 | 31篇 |
1999年 | 39篇 |
1998年 | 35篇 |
1997年 | 33篇 |
1996年 | 44篇 |
1995年 | 22篇 |
1994年 | 32篇 |
1993年 | 38篇 |
1992年 | 25篇 |
1991年 | 37篇 |
1990年 | 26篇 |
1989年 | 16篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 15篇 |
1985年 | 27篇 |
1984年 | 36篇 |
1983年 | 17篇 |
1982年 | 36篇 |
1981年 | 24篇 |
1980年 | 32篇 |
1979年 | 28篇 |
1978年 | 21篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 22篇 |
1974年 | 17篇 |
1973年 | 16篇 |
排序方式: 共有3210条查询结果,搜索用时 31 毫秒
961.
Finite size effects on the structural and magnetic properties of sol–gel synthesized NiFe2O4 powders
Mathew George Asha Mary John Swapna S. Nair P.A. Joy M.R. Anantharaman 《Journal of magnetism and magnetic materials》2006
Nanoparticles of nickel ferrite have been synthesized by the sol–gel method and the effect of grain size on its structural and magnetic properties have been studied in detail. X-ray diffraction (XRD) studies revealed that all the samples are single phasic possessing the inverse spinel structure. Grain size of the sol–gel synthesized powders has been determined from the XRD data and the strain graph. A grain size of 9 nm was observed for the as prepared powders of NiFe2O4 obtained through the sol–gel method. It was also observed that strain was induced during the firing process. Magnetization measurements have been carried out on all the samples prepared in the present series. It was found that the specific magnetization of the nanosized NiFe2O4 powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size. The coercivity of the sol–gel synthesized NiFe2O4 nanoparticles attained a maximum value when the grain size was 15 nm and then decreased as the grain size was increased further. 相似文献
962.
Héctor Rosas Leidy Erika Sarmiento Vito Lubes María Goncalves Mary Lorena Araujo Felipe Brito 《Journal of solution chemistry》2008,37(5):701-711
The complex species formed between vanadium(III) cysteine and small blood serum bioligands: lactic, oxalic, citric and phosphoric
acids were studied in aqueous solution by means of electromotive force measurements, Emf(H), at 25 °C and 3.0 mol⋅dm−3 KCl as the ionic medium. The ternary complexes were studied by potentiometric measurements and the data analyzed using the
least-squares computational program LETAGROP to obtain the respective stability constants and stoichiometric coefficients.
The UV-Vis spectrophotometric measurements were done in order to make a qualitative characterization of the complexes formed
in aqueous solution. 相似文献
963.
Laurence A. Belfiore Mary Pat McCurdie 《Journal of Polymer Science.Polymer Physics》1995,33(1):105-124
d-Block transition-metal-containing polymer blends which form coordination complexes are described in this treatise. The model compounds are zinc acetate dihydrate, copper acetate dihydrate, nickel acetate tetrahydrate, cobalt chloride hexahydrate, palladium chloride bis (acetonitrile), and the dimer of dichlorotricarbonylruthenium (II). Two classes of ligands are of interest. Poly (4-vinylpyridine), P4VP, and copolymers that contain 4-vinylpyridine repeat units form complexes with zinc, copper, nickel, cobalt, and ruthenium salts. Atactic 1,2-polybutadiene contains olefinic sidegroups that displace weakly bound acetonitrile ligands and coordinate to palladium chloride. Thermal analysis via differential scanning calorimetry suggests that the glass transition temperature of the polymeric ligand is enhanced by these low-molecular-weight transition-metal salts in binary and ternary blends. In some cases, d-block salts function as transition-metal compatibilizers for copolymers that would otherwise be immiscible. The isothermal ternary phase diagram for polybutadiene with palladium chloride highlights regions of gelation, precipitation, and transparent solutions during blend preparation in tetrahydrofuran. Fourier transform infrared spectroscopy provides molecular-level data that support the concept of polymeric coordination complexes. High-resolution carbon-13 solid-state NMR spectroscopy identifies (1) near-neighbor interactions between polymeric pyridine ligands and the ruthenium salt, and (2) a considerable reduction in the molecular mobility of the polybutadiene chain backbone when it forms a coordination complex with palladium chloride. The elastic modulus of polybutadiene increases by three orders of magnitude when the palladium salt concentration is 4 mol % in a solid-state glassy film. A thermodynamic interpretation of ligand field stabilization energies appropriate to tetrahedral cobalt and octahedral nickel complexes is employed to estimate the synergistic enhancement of the glass transition temperature, particularly when coordination crosslinks are present. ©1995 John Wiley & Sons, Inc. 相似文献
964.
965.
This article evaluates the influence of temperature on the sorption of gases in two isomers of polynorbornene. The subject polymers were stereoisomers with nearly identical bulk density and total free volume. Because of differences in the mobility of the polymer backbone, the isomers packed differently resulting in differences in the average free‐volume element size within the matrix. The influence of these differences on free‐volume element size was characterized by the heat of sorption of gases in the matrix. The most pronounced differences were observed in the isosteric heats of sorption of condensable carbon dioxide and methane in the polymer isomers. This analysis suggests that the relative space available for sorption into free‐volume elements is higher in the methyl II isomer relative to methyl III. These conclusions support the physical characterizations reported in Part I of this series suggesting that the methyl II isomer has larger average free‐volume elements but fewer of them than the methyl III isomer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1939–1946, 2003 相似文献
966.
Beck Mary J. Johnson Robert D. Baker Chiquita S. 《Applied biochemistry and biotechnology》1990,24(1):407-414
Applied Biochemistry and Biotechnology - 相似文献
967.
The ion-pair solid-phase extraction (SPE) of 4-alkylphenols followed by derivatization with pentafluoropyridine is demonstrated. Under alkaline conditions, the 4-alkylphenols could be efficiently adsorbed on a C18 SPE cartridge conditioned with an ion-pair reagent, tetra-n-hexylammonium bromide. The ion pairs, ammonium phenolates, formed on the C18 solid phase, were eluted with a solvent containing the derivatizing reagent, pentafluoropyridine, and completely derivatized during the elution. After optimization of the adsorption and derivatization, we established a method for the determination of the 4-alkylphenols in water samples. The method showed good linearity between 20 and 1000 ng (200-10,000 ng for nonylphenol). By processing 20-ml samples, the method detection limits (MDL) were in the range of 5.2-8.9 ng/l for the 4-alkylphenols (76 ng/l for nonylphenol). To evaluate its applicability to a real aqueous matrix, several river water samples were analyzed. 相似文献
968.
Guo J Colgan TJ DeSouza LV Rodrigues MJ Romaschin AD Siu KW 《Rapid communications in mass spectrometry : RCM》2005,19(19):2762-2766
Direct analysis of laser capture microdissected malignant and normal endometrial epithelium using matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (MS) was able to detect a number of proteins that are overexpressed in malignant epithelial cells. A total of 16 physiologic and malignant endometrial samples were laser capture microdissected, including four proliferative and four secretory endometria, and eight endometrioid adenocarcinomas. Two of these proteins, at 10,834 and 10,843 Da, likely correspond to calgranulin A and chaperonin 10, two proteins that had previously been identified in endometrioid adenocarcinoma in whole tissue homogenate by MS analysis. Direct analysis by MALDI-MS not only confirms that these proteins are overexpressed in endometrial carcinoma, but also localizes them to the epithelial cells, the expected cancer site. 相似文献
969.
Functional dissection of sRNA translational regulators by nonhomologous random recombination and in vivo selection 总被引:3,自引:0,他引:3
Small nontranslated RNAs (sRNAs) regulate a variety of biological processes. DsrA and OxyS are two E. coli sRNAs that regulate the translation of rpoS, which encodes a protein sigma factor. Due to their structural complexity, the functional dissection of sRNAs solely by designing and assaying mutants can be challenging. Here, we present a complementary approach to the study of functional RNAs, in which highly diversified RNA libraries are generated by nonhomologous random recombination (NRR) and processed efficiently by in vivo selections that link RNA activities to cell survival. When applied to DsrA and OxyS, this approach rapidly identified essential and nonessential regions of both sRNAs. Resulting hypotheses about DsrA and OxyS structure-function relationships were tested and further refined experimentally. Our findings demonstrate an efficient, unbiased approach to the functional dissection of nucleic acids. 相似文献
970.
Krusic PJ Marchione AA Davidson F Kaiser MA Kao CP Richardson RE Botelho M Waterland RL Buck RC 《The journal of physical chemistry. A》2005,109(28):6232-6241
Vapor pressure and aqueous solubility are important parameters used to estimate the potential for transport of chemical substances in the atmosphere. For fluorotelomer alcohols (FTOHs), currently under scrutiny by environmental scientists as potential precursors of persistent perfluorocarboxylates (PFCAs), vapor pressure is the more significant property since these compounds are only very sparingly soluble in water. We have measured the vapor pressures of a homologous series of fluorotelomer alcohols, F(CF2CF2)nCH2CH2OH (n = 2-5), in the temperature range 21-250 degrees C by three independent methods: (a) a method suitable for very low vapor pressures at ambient temperatures (gas-saturation method), (b) an improved boiling point method at controlled pressures (Scott method), and (c) a novel method, requiring milligram quantities of substance, based on gas-phase NMR, a technique largely unfamiliar to chemists and holding promise for studies of relevance to environmental chemistry. The concordant values obtained indicate that recently published vapor pressure data overestimate the vapor pressure at ambient temperature, and therefore the volatility, of this series of fluorinated compounds. It was suggested that substantial intramolecular -O-H...F- hydrogen bonding between the hydroxylic proton and the two fluorines next to the ethanol moiety was responsible for their putative high volatility. Therefore, we have used gas-phase NMR, gas-phase FTIR, 2D NMR heteronuclear Overhauser effect measurements, and high-level ab initio computations to investigate the intramolecular hydrogen bonding in fluorotelomer alcohols. Our studies unequivocally show that hydrogen bonding of this type is not significant and cannot contribute to and cause unusual volatility. The substantially lower vapor pressure at ambient temperatures than previously reported resulting from our work is important in developing a valid understanding of the environmental transport behavior of this class of compounds. 相似文献