全文获取类型
收费全文 | 2391篇 |
免费 | 74篇 |
国内免费 | 65篇 |
专业分类
化学 | 1523篇 |
晶体学 | 18篇 |
力学 | 109篇 |
综合类 | 1篇 |
数学 | 309篇 |
物理学 | 570篇 |
出版年
2023年 | 24篇 |
2022年 | 42篇 |
2021年 | 45篇 |
2020年 | 28篇 |
2019年 | 38篇 |
2018年 | 18篇 |
2017年 | 18篇 |
2016年 | 44篇 |
2015年 | 38篇 |
2014年 | 84篇 |
2013年 | 116篇 |
2012年 | 172篇 |
2011年 | 180篇 |
2010年 | 114篇 |
2009年 | 114篇 |
2008年 | 117篇 |
2007年 | 130篇 |
2006年 | 134篇 |
2005年 | 116篇 |
2004年 | 99篇 |
2003年 | 64篇 |
2002年 | 71篇 |
2001年 | 56篇 |
2000年 | 42篇 |
1999年 | 34篇 |
1998年 | 33篇 |
1997年 | 27篇 |
1996年 | 19篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 29篇 |
1992年 | 26篇 |
1991年 | 21篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 21篇 |
1985年 | 31篇 |
1984年 | 36篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 21篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 17篇 |
1977年 | 16篇 |
1976年 | 18篇 |
1975年 | 17篇 |
1974年 | 16篇 |
1973年 | 11篇 |
排序方式: 共有2530条查询结果,搜索用时 0 毫秒
961.
Sol-gel synthesis and morphological control of nanocrystalline TiO(2) via urea treatment 总被引:1,自引:0,他引:1
Nanocrystalline TiO(2) rods and hollow tubes with an engraved pattern on the surface have been prepared by a novel anionic template-assisted sol-gel synthesis via urea treatment and under hydrothermal condition. X-ray diffractometry (XRD) results indicate that these nanocrystallines consist predominantly of anatase TiO(2), with minor amounts of rutile and brookite. Scanning and transmission electron microscopy (SEM and TEM) analyses reveal these rods and hollow tubes may result from the aggregates of nanorods of approximately 10 nm in diameter. The crystallographic faceting found from TEM further reveals the polymorphic nature of the nanocrystalline TiO(2) thus prepared. A "reverse micelle" formation mechanism taking into account the hydrothermal temperature, the pH effect of the sol-gel system, the isoelectric point, the formation of micelles, and the electrostatic interaction between the anionic surfactant and the growing TiO(2) particulates is proposed to illustrate the competition between the physical micelle assembly of the ionic surfactants and the chemical hydrolysis and condensation reactions of the Ti precursors. 相似文献
962.
The boundary effect on electrophoresis is investigated by considering a spherical particle at an arbitrary position in a spherical cavity. Our previous analysis is extended to the case where the effect of double-layer polarization can be significant. Also, the effect of a charged boundary, which yields an electroosmotic flow and a pressure gradient, thereby making the problem under consideration more complicated, is investigated. The influences of the level of the surface potential, the thickness of double layer, the relative size of a sphere, and its position in a cavity on the electrophoretic behavior of the sphere are discussed. Some results that are of practical significance are observed. For example, if a positively charged sphere is placed in an uncharged cavity, its mobility may have a local minimum as the thickness of the double layer varies. If an uncharged sphere is placed in a positively charged cavity, the mobility may have a local minimum as the position of the sphere varies. Also, if the size of a sphere is fixed, its mobility may have a local minimum as the size of a cavity varies. These provide useful information for the design of an electrophoresis apparatus. 相似文献
963.
Hsu FF Turk J Owens RM Rhoades ER Russell DG 《Journal of the American Society for Mass Spectrometry》2007,18(3):479-492
The multiple-stage ion-trap mass spectrometric approaches towards to the structural characterization of the monoacyl-PIM (triacylated PIM) and the diacyl-PIM (tetracylated PIM), namely, the PIM (diacylated PIM) consisting of one or two additional fatty acid substituents attached to the glycoside, respectively, were described. While the assignment and confirmation of the fatty acid substituents on the glycerol backbone can be easily achieved by the methods described in the previous article, the identity of the glycoside moiety and its acylation state can be determined by the observation of a prominent acylglycoside ion arising from cleavage of the diacylglycerol moiety ([M - H - diacylglycerol](-)) in the MS(2)-spectra of monoacyl-PIM and diacyl-PIM. The distinction of the fatty acid substituents on the 2-O-mannoside (i.e., R(3)CO(2)H) from that on the inositol (i.e., R(4)CO(2)H) is based on the findings that the MS(3)-spectrum of [M - H - diacylglycerol](-) contains a prominent ion arising from further loss of the fatty acid at the 2-O-mannoside (i.e., the [M - H - diacylglycerol - R(3)CO(2)H](-) ion), while the ion arising from loss of the fatty acid substituent at the inositol (i.e., the [M - H - diacylglycerol - R(4)CO(2)H](-) ion) is of low abundance. The fatty acyl moiety on the inositol can also be identified by the product-ion spectrum from MS(4) of the [M - H - diacylglycerol - R(3)CO(2)H](-) ion, which gives rise to a prominent ion corresponding to loss of R(4)CO(2)H. An [M - H - acylmannose](-) ion was also observed in the MS(2)-spectra and, thus, the identity of the fatty acid substituent attached to 2-O-mannoside can be confirmed. The combined information obtained from the multiple-stage product-ion spectra from MS(2), MS(3), and MS(4) permit the assignment of the complex structures of monoacyl-PIMs and diacyl-PIMs in a mixture isolated from M. bovis Bacillus Calmette Guérin. 相似文献
964.
We report a study of excited-state dynamics of trans,trans-distyrylbenzene in hexane solution with femtosecond two-color transient absorption spectroscopy. A consistent model of two distinct excited states, S1 and X, connected by the 10 ps dynamics is proposed with the support from the analysis of excited-state anisotropy. An investigation on the 10 ps dynamics with varying excitation energy has been also conducted. In the assumption of fast intramolecular vibrational redistribution, a dependence of this nonradiative 10 ps process on intramolecular temperature in the S1 state has been analyzed. We have found that an effective mode of approximately 1270 cm-1 is responsible for the 10 ps dynamics. The analysis of both anisotropy and pump-dependent results further implies that a distribution of rotamers may exist in the X state. The result indicates a drastic different excited-state relaxation pathway than that of trans-stilbene. 相似文献
965.
Wan-Hsin Chang Pei-Yi Liu Min-Hsuan Lin Chien-Ju Lu Hsuan-Yi Chou Chih-Yu Nian Yuan-Ting Jiang Yuan-Hao Howard Hsu 《Molecules (Basel, Switzerland)》2021,26(9)
Hyaluronic acid (HA) is a glycosaminoglycan that was first isolated and identified from the vitreous body of a bull’s eye. HA is ubiquitous in the soft connective tissues of animals and therefore has high tissue compatibility for use in medication. Because of HA’s biological safety and water retention properties, it has many ophthalmology-related applications, such as in intravitreal injection, dry eye treatment, and contact lenses. Due to its broad range of applications, the identification and quantification of HA is a critical topic. This review article discusses current methods for analyzing HA. Contact lenses have become a widely used medical device, with HA commonly used as an additive to their production material, surface coating, and multipurpose solution. HA molecules on contact lenses retain moisture and increase the wearer’s comfort. HA absorbed by contact lenses can also gradually release to the anterior segment of the eyes to treat dry eye. This review discusses applications of HA in ophthalmology. 相似文献
966.
Han-Ping Huang Chang Francis Hsu Yi-Chih Mao Long Hsu Sien Chi 《Entropy (Basel, Switzerland)》2021,23(4)
Gait stability has been measured by using many entropy-based methods. However, the relation between the entropy values and gait stability is worth further investigation. A research reported that average entropy (AE), a measure of disorder, could measure the static standing postural stability better than multiscale entropy and entropy of entropy (EoE), two measures of complexity. This study tested the validity of AE in gait stability measurement from the viewpoint of the disorder. For comparison, another five disorders, the EoE, and two traditional metrics methods were, respectively, used to measure the degrees of disorder and complexity of 10 step interval (SPI) and 79 stride interval (SI) time series, individually. As a result, every one of the 10 participants exhibited a relatively high AE value of the SPI when walking with eyes closed and a relatively low AE value when walking with eyes open. Most of the AE values of the SI of the 53 diseased subjects were greater than those of the 26 healthy subjects. A maximal overall accuracy of AE in differentiating the healthy from the diseased was 91.1%. Similar features also exists on those 5 disorder measurements but do not exist on the EoE values. Nevertheless, the EoE versus AE plot of the SI also exhibits an inverted U relation, consistent with the hypothesis for physiologic signals. 相似文献
967.
Hung-Jen?Cheng Chang-Long?KaoEmail author Yan-Fu?Chen Ping-Chih?Huang Ching-Yun?Hsu Chun-Hsiung?KueiEmail author 《Mikrochimica acta》2017,184(9):3179-3187
Carbon dots (CDs) modified with ethylene diamine (EDA) and the amino acids (AAs) Cys, His, Lys or Arg were synthesized, and their structures were confirmed by high resolution transmission electron microscopy, Raman spectrometry and X-ray photoelectron spectrometry. It is found that derivatization of the CDs with various AAs systemically modulates their electronic properties, and this results in a tunable selectivity in detection of metal cations via fluorescence quenching. The probes can be performed in aqueous solutions around pH 7. CDs can be excited under 345 nm excitation at room temperature and exhibit fluorescent peak at 450 nm. The decreasing fluorescence intensity is directly proportional to the concentration of metal cations. The limits of detection is 8.8 μg L?1 for Pb(II), 20 μg L?1 for Hg(II), 3.7 μg L?1 for Cu(II), 5.3 μg L?1 for Zn(II), 16 μg L?1 for Fe(III), and 7.2 μg L?1 for Cr(III), respectively. The different fluorescence response of the AA-modified CDs can be converted to logic gates and applied to photoelectronic nanoprobes by using microprocessors. In our perception, this assay has a large potential in terms of high-throughput screening for trace amounts of metal ions. 相似文献
968.
A facile route to the synthesis of γ-sulfonyl lactones 5 and β-sulfonyl styrenes 6 has been developed, achieving moderate to good yields via the (1) NaH mediated Michael addition of β-ketosulfones 3 and methyl acrylate in refluxing THF and (2) NaBH4 mediated stereoselective reduction/lactonization of δ-ketoesters 4 in boiling MeOH, or (3) boron trifluoride etherate mediated ring-opening of lactones 5 in MeOH at reflux. 相似文献
969.
We demonstrate high-efficiency, wideband-tunable, laser-ablated long-period fiber gratings that use an optical polymer overlay. Portions of the fiber cladding are periodically removed by CO(2) laser pulses to induce periodic index changes for coupling the core mode into cladding modes. An optical polymer with a high thermo-optic coefficient with a dispersion distinct from that of silica is used on a deep-ablated cladding structure so that the effective indices of cladding modes become dispersive and the resonant wavelengths can be efficiently tuned. The tuning efficiency can be as high as 15.8 nm/ degrees C, and the tuning range can be wider than 105 nm (1545-1650 nm). 相似文献
970.
The nanostructure of a biomaterial surface has strong influence on cell behavior. The migration of cells on nanostructured surfaces, however, has not been investigated so far. In this study, we used PCL/PEG diblock copolymers as model surfaces to examine the effect of nanoislands on migration of different cells, including fibroblasts and endothelial cells. The water sliding angle of the substrates was measured. The cell migration rate was examined under a real-time optical microscope. It was found that a greater cell migration rate correlated with the smaller sliding angle of the substrate. 相似文献