全文获取类型
收费全文 | 12065篇 |
免费 | 126篇 |
国内免费 | 61篇 |
专业分类
化学 | 5324篇 |
晶体学 | 15篇 |
力学 | 713篇 |
数学 | 4030篇 |
物理学 | 2170篇 |
出版年
2024年 | 117篇 |
2023年 | 690篇 |
2022年 | 423篇 |
2021年 | 414篇 |
2020年 | 1468篇 |
2019年 | 1087篇 |
2018年 | 945篇 |
2017年 | 775篇 |
2016年 | 757篇 |
2015年 | 595篇 |
2014年 | 780篇 |
2013年 | 2887篇 |
2012年 | 556篇 |
2011年 | 91篇 |
2010年 | 69篇 |
2009年 | 66篇 |
2008年 | 69篇 |
2007年 | 68篇 |
2006年 | 53篇 |
2005年 | 94篇 |
2004年 | 95篇 |
2003年 | 45篇 |
2002年 | 18篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1999年 | 1篇 |
1998年 | 9篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 10篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Maryam Taherzadeh Marek Nečas 《Phosphorus, sulfur, and silicon and the related elements》2019,194(1-2):39-46
The synthesis, spectral characteristics (IR and NMR), elemental analysis and X-ray crystal structure of phosphorothioic triamide SP(NC5H10)3 (1) and its dinuclear mercury(II) complex [Hg2(μ-Cl)2(Cl)2{SP(NC5H10)3}2] (2) were investigated. A survey using the Cambridge Structural Database (CSD, version 5.38, May 2017) shows structures of coordination compounds of Au, Ag, Cd, Cu, Li, Mo, Ni, Pd, Te, Ti, Zn, and Zr with sulfur-donor SP(N)3-based ligands; the complex 2 is the first example of a mercury complex with the SP(N)3-based ligand studied by X-ray crystallography. Valence bond calculation was performed for the Hg–S bond in 2 and compared with the Hg–O bond in the only structure with a Cl2Hg–OP(N)3 structural motive in the CSD. The calculation confirms a more covalent nature of the Hg–S bond with respect to the Hg–O bond made by the EP(N)3-based ligands (E?=?S, O). The supramolecular structures based on C–H···S?=?P contacts in 1 and C–H···S═P and C–H···Cl–Hg assemblies in 2 are discussed. 相似文献
992.
Dr. Long Jiao Dr. Weijie Yang Dr. Gang Wan Rui Zhang Dr. Xusheng Zheng Dr. Hua Zhou Prof. Dr. Shu-Hong Yu Prof. Dr. Hai-Long Jiang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(46):20770-20776
Single-atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single-atom catalysts is not straightforward. Herein, a general strategy for synthesis of single-atom metals implanted in N-doped carbon (M1-N-C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal–organic frameworks (MOFs). The M1-N-C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single-atom metal species. When employed in electrocatalytic CO2 reduction, Ni1-N-C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe1-, Co1- and Cu1-N-C. Remarkably, the best-performer, Ni1-N-C, even demonstrated excellent CO FE at low CO2 pressures, thereby representing a promising opportunity for the direct use of dilute CO2 feedstock. 相似文献
993.
Alakes Maiti Bibek Patra G.P. Samanta 《International Journal of Mathematical Education in Science & Technology》2013,44(8):1061-1076
The present article deals with the problem of combined harvesting of a
Michaelis–Menten-type ratio-dependent predator–prey system. The
problem of determining the optimal harvest policy is solved by invoking
Pontryagin's Maximum Principle. Dynamic optimization of the harvest policy is
studied by taking the combined harvest effort as a dynamic variable. Computer
simulations are carried out to illustrate our analytical findings. Biological
and bioeconomical interpretations of the results are explained critically. 相似文献
994.
995.
Ali F. Al-Ghamdi 《Journal of Saudi Chemical Society》2009,13(3):235-241
Square-wave voltammetry was used to explore the adsorption property of ofloxacine complex with iron ion on the hanging mercury drop electrode (HMDE). By employing the adsorptive stripping voltammetric approach, a sensitive electroanalytical method for the quantitative analysis of ofloxacine antibiotic was achieved. A well-developed voltammetric peak was obtained in pH 7.5 Britton–Robinson buffer (B–R buffer) at ?1400 mV. The cyclic voltammetric studies indicated that the reduction process was irreversible and primarily controlled by adsorption. An investigations of the variation of adsorptive voltammetric peak current with supporting electrolyte, pH, accumulation time, accumulation potential, ion concentration, scan rate, pulse amplitude, SW frequency, working electrode area and convection rate has resulted in the recognition of optimal experimental conditions for ofloxacine analysis. The studied electroanalytical signal showed a linear response for ofloxacine in the concentration range 5 × 10?7 to 1.7 × 10?6 mol l?1 (r = 0.999). A limit of detection of 1.1 × 10?8 mol l?1 (3.98 ppb) with relative standard deviation of 1.21 RSD% and mean recovery of 99.6% were obtained. Possible interferences by several substances usually present in pharmaceutical formulation were also evaluated. The analytical quantification of ofloxacine in commercially available pharmaceutical formulation was performed and compared with data from HPLC technique. 相似文献
996.
Bergen HR Abraham RS Johnson KL Bradwell AR Naylor S 《Biomedical chromatography : BMC》2004,18(3):191-201
Primary systemic amyloidosis (AL) is characterized by the overproduction of immunoglobulin light chain proteins by a monoclonal, terminally differentiated B-lymphocyte or plasma cell clone. The free immunoglobulin light chains are deposited in an abnormal conformation as amyloid in a variety of organs in the body. The mechanism of amyloid formation is not well understood, but appears to be associated with some form of cleavage of the immunoglobulin light chain with subsequent aggregate formation. In an attempt to characterize the structure of amyloid-forming light chain proteins we developed an on-line immunoaffinity purification and subsequent characterization of free kappa and free lambda immunoglobulin light chains by electrospray ionization mass spectrometry. The methodology is totally automated and requires 20 micro L of serum. Mass spectral analysis of Bence Jones proteins under non-denaturing conditions was also utilized to examine the tertiary and quaternary structure of light chain proteins and clearly shows covalent dimer formation of lambda type light chain. This type of on-line assay may prove helpful in elucidating distinguishing features capable of discriminating AL from benign monoclonal gammopathies of undetermined significance as well as diagnosing AL. 相似文献
997.
Discontinuous phenomena, in which objects may behave continuously and sometimes discretely are not only found in nature and under laboratory conditions but also in simple, familiar contexts. For example, this phenomenon is skillfully incorporated into the internal structure of mechanical wristwatches. Unless an extremely small amount of state-dependent impulse is applied intermittently, the reciprocating rotational movement of the balance and hairspring, which is the heart of the mechanical wristwatch, cannot be maintained. The small amount of state-dependent impulse, which is often overlooked, can make a significant difference; however, very few studies have examined this subject. This study assumes the underlying cause of discontinuous behaviors as impulses generated when an object reaches a particular state, assuming that the continuous behavior follows the Liénard system, which is widely studied in the field of electrical circuits. The main theorem provides the conditions under which the effect of the impulses causes a stable limit cycle in the Liénard system, even if no limit cycle exists when there are no impulses. The Poincaré–Bendixson theorem for discontinuous dynamical systems and phase plane analysis are used to prove the main theorem. Several examples and their simulations are provided to illustrate the main theorem. 相似文献
998.
《Helvetica chimica acta》2017,100(10)
Chemical cross‐linking combined with MALDI ‐MS was applied to structural analysis of a protein nanocontainer. Specifically, an engineered variant of lumazine synthase from Aquifex aeolicus (AaLS ‐13) was investigated that self‐assembles into a capsid‐like structure and is known to encapsulate other proteins by Coulombic attraction. Two complementary soft ionization techniques, MALDI ‐MS and native ESI ‐MS , were utilized to map the subunit stoichiometry of the high molecular weight capsid. In accordance with the previously reported cryo‐electron microscopy structure of this protein container, only pentameric subunits were detected. This study highlights the possibility to map subunit stoichiometry via chemical cross‐linking with glutaraldehyde followed by MALDI ‐MS . The same approach was used to study protein‐protein interactions during encapsulation of GFP (+36) by the AaLS ‐13 capsid. Heterocomplexes between GFP (+36) and AaLS ‐13 multimers were not observed when mixed at maximal loading capacity (AalS‐13 monomer:GFP (+36) 4:1). This is in agreement with the known fast encapsulation of GFP (+36) by the protein capsid, which essentially removes any free GFP (+36) from the solution. Exceeding the maximal loading capacity by addition of excess GFP (+36) results in aggregation. 相似文献
999.
《Tetrahedron letters》2014,55(51):7124-7129
Mono and di-heteroaryl-4,4′-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (BODIPy) (1–5) were synthesized using Suzuki–Miyaura couplings. Hetero aryl substitution on 3- or 3,5-positions caused large bathochromic shifts (up to ∼150 nm) in absorption (569–652 nm) and fluorescence maxima (586–679 nm) in comparison to classical BODIPy. Quantum yields were found to be as high as 0.65. Singlet oxygen production activities of these compounds were studied by monitoring the absorbance quenching of 1,3-diphenylisobenzofuran, on exposure to light (>600 nm). Cellular uptake of compound 4 was demonstrated using cervical cancer cells and fibroblast cell line and was confirmed by the images obtained using confocal microscope. 相似文献
1000.
Tamara Mengoni Dolores Vargas Peregrina Roberta Censi Manuela Cortese Massimo Ricciutelli 《Natural product research》2016,30(3):268-275
The aim of this work was to provide a characterisation of volatile constituents from different commercial batches of henna (Lawsonia inermis) leaves of different geographic origin. Headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography–mass spectrometry (GC–MS) was used for the purpose. A total of 72 components were identified by GC–MS in the headspace of different henna samples which proved to differ considerably from each other, because they were characterised by different classes of components, mainly aliphatic compounds (9.0–64.7%), terpenoids (5.8–45.5%) and aromatics (7.9–45.2%), with alkanes (0.9–18.5%), aldehydes (2.1–18.8%) and carboxylic acids (3.1–29.3%), monoterpenes (3.4–30.0%) and sesquiterpenes (0.8–23.7%) and phenyl propanoids (0.6–43.1%), being the most abundant, respectively. Major representatives of these groups were n-hexadecane (0.5–4.7%), (2E)-hexenal (0.5–11.7%) and acetic acid (2.8–24.5%), limonene (0.8–14.7%), carvol (3.8–7.1%), geranyl acetone (1.4–7.9%) and (E)-caryophyllene (3.3–8.4%), and (E)-anethole (0.6–35.0%), respectively. We assume that factors such as the manufacturing process, the storage conditions and the different geographic origin of the samples may contribute to such variability. 相似文献