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581.
In-Jin Shon Seung-Mi Kwak Jung-Mann Doh Bang-Ju Park Jin-Kook Yoon 《Research on Chemical Intermediates》2013,39(3):1291-1299
The rapid sintering of a nanostuctured MgAl2O4 compound in a short time was investigated with high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostuctured materials. A highly dense nanostructured MgAl2O4 compound was produced with simultaneous application of 80 MPa pressure and induced current with output of total power capacity (15 kW) within 2 min. The sintering behavior, grain size, and mechanical properties of the MgAl2O4 compound were investigated. 相似文献
582.
Wonbaek Kim Seung-Mi Kwak Chang-Yul Suh Jae-Won Lim Sung-Wook Cho In-Jin Shon 《Research on Chemical Intermediates》2013,39(6):2339-2349
Pure Ti and Si powders were milled in a horizontal-rotation ball mill and in a high-energy ball mill to synthesize Ti5Si3 powders. The high-energy ball milling produced nanosized single-phase Ti5Si3 particles. Meanwhile, no reaction occurred during the horizontal milling. The two milled powders were consolidated using the high-frequency induction heated sintering method. A dense nanostructured Ti5Si3 compact was consolidated within 2 min using the mechanically synthesized Ti5Si3 powder. The retainment of nanoscale structure during sintering is believed to be the reason for the good mechanical properties of the Ti5Si3 compact. In comparison, the horizontally milled powder reacted to form Ti5Si3 partially on sintering. It is believed that the enhanced toughness of the horizontally milled samples may be due to the crack-deterring effect of softer Si/Ti grains. 相似文献
583.
Seung-Yeon Kwak SeungCheol Yang Na Ree Kim Jae Hong Kim Byeong-Soo Bae 《Journal of Sol-Gel Science and Technology》2013,65(1):46-51
Novel yellow and blue emissive dyes have been synthesized using 2,5-diamino-3,6-dicyanopyrazine and various alkoxysilanes and they are covalently bridged to cycloaliphatic epoxy functional oligosiloxane via non-hydrolytic sol–gel reaction. Dye-bridged hybrid materials (DBH) are fabricated by thermal curing the dye-bridged oligosiloxane. Structure and formation of dyes and siloxane network is studied using analysis method. Four components of red, yellow, green and blue emitting DBH cover entire visible range and white luminescence with high color rendering index is realized by controlling their combinations. We have ensured superior thermal stability DBH at 120 °C for 200 h caused by covalently bridged structure and robust siloxane matrix. 相似文献
584.
Dr. Atanu Jana Dr. Masatoshi Ishida Kyuju Kwak Young Mo Sung Dong Sub Kim Dr. Vincent M. Lynch Prof. Dr. Dongil Lee Prof. Dr. Dongho Kim Prof. Dr. Jonathan L. Sessler 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(1):338-349
A series of tetrathiafulvalene (TTF)‐annulated porphyrins, and their corresponding ZnII complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge‐transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge‐transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square‐wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis‐substituted TTF–porphyrin (both free‐base and ZnII complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent‐dependent charge‐transfer transitions, it is the π‐acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square‐wave voltammograms. In the case of the free‐base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF ? + generated through one‐electron oxidation, can interact with each other; this gives rise to multiple signals in the square‐wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF . + and TTF2+, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited‐state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited‐state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge‐transfer effects inferred from the ground‐state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge‐transfer interactions and π‐acceptor effects. 相似文献
585.
The direct functionalization of heterocyclic compounds has emerged as one of the most important topics in the field of metal-catalyzed C-H bond activation due to the fact that products are an important synthetic motif in organic synthesis, the pharmaceutical industry, and materials science. This critical review covers the recent progresses on the regioselective dehydrogenative direct coupling reaction of heteroarenes, including arylation, olefination, alkynylation, and amination/amidation mainly utilizing transition metal catalysts (113 references). 相似文献
586.
Giseop Kwak Ga Hyoung Lee Shang‐hun Shim Keun‐Byoung Yoon 《Macromolecular rapid communications》2008,29(10):815-820
Light‐guiding core/sheath fibers are fabricated from two different refractive index (RI) polymers by coaxial electrospinning. The morphology and fiber diameter are analyzed with scanning electron and fluorescence microscopies. It is found that the diameter of the core and the thickness of the sheath could be varied from 100 to 400 nm by changing the concentration of the outer solution. The incorporation of a chromophore into the inner material confirmed the uniformity of the core/sheath structure in long segments of the fibers. The boundary is clearly seen: the core diameter and the thickness of the sheath are constant along the fiber axis in fluorescence images. The ejected beam is circular and light‐guided along the fiber axis as in an optical fiber.
587.
In Tae Kim Sang Woo Lee Tae Hyoung Kwak Jung Youl Lee Hyang Suk Park So Young Kim Chang Min Lee Hoi Eun Jung Jun‐Gill Kang Tack‐Jin Kim Hee‐Jung Kang ChangMoon Park Ronald L. Elsenbaumer 《Macromolecular rapid communications》2002,23(9):551-554
Oxidative coupling reaction of 1‐hexyl–3,4‐dimethylpyrrole affords a conjugated conducting poly(1‐hexyl‐3,4‐dimethyl‐2,5‐pyrrolylene) (PHDP), which is completely soluble in common organic solvents. The luminescence of PHDP is comparable to that of poly(N‐vinylcarbazole) (PVK), which has been widely used in electroluminescence devices. The quantum efficiency of PHDP is 2.5 times higher than that of PVK. A rationalization is presented relating the conductivity of PHDP to its polymer structure. 相似文献
588.
The photoirradiation of trans‐ and cis‐poly(dimethylsilylenephenylenevinylene)s gave cis‐rich mixtures at equilibrium states. The degree of the photoisomerization could be exactly evaluated by comparing the UV spectra of the photoirradiated solutions with those of the trans and cis polymers. The geometric configuration of the trans and cis polymers was thermally stable and hardly changed even though they were heated. The trans and cis polymers exhibited different emission properties; e.g., trans polymer: λmax = 400 nm, quantum yield = 3.4×10–3; cis polymer: λmax = 380 nm, quantum yield = 1.5×10–3. 相似文献
589.
Dr. Ze-Hua Wu Dr. Artem Skabeev Prof. Dr. Yulian Zagranyarski Dr. Ruomeng Duan Prof. Dr. Jun-O Jin Prof. Dr. Minseok Kwak Prof. Dr. Thomas Basché Prof. Dr. Klaus Müllen Dr. Chen Li 《Angewandte Chemie (International ed. in English)》2023,62(52):e202315156
A new class of near-infrared (NIR) fluorophores, PAI , is obtained by consecutive C−N/C−C bond formation between diphenylamines and 9,10-dibromoperylenecarboximide. Owing to the rigid structure, extended π-conjugation and pronounced push-pull substitution, these fluorophores show emission maxima up to 804 nm and large Stokes shifts. The extraordinarily high fluorescence quantum yields from 47 % to 70 % are attributed to chloro substitution in the bay positions of the perylene core. These characteristics, together with high photostability, qualify them as useful NIR emitters for applications as biomarkers and security inks. 相似文献
590.
Jong-Pil Jeon Yu Jin Kim Dr. Se Hun Joo Dr. Hyuk-Jun Noh Prof. Sang Kyu Kwak Prof. Jong-Beom Baek 《Angewandte Chemie (International ed. in English)》2023,62(12):e202217416
Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts. 相似文献