全文获取类型
收费全文 | 1890篇 |
免费 | 43篇 |
国内免费 | 4篇 |
专业分类
化学 | 1061篇 |
晶体学 | 20篇 |
力学 | 67篇 |
数学 | 195篇 |
物理学 | 594篇 |
出版年
2022年 | 35篇 |
2021年 | 38篇 |
2020年 | 31篇 |
2019年 | 49篇 |
2018年 | 52篇 |
2017年 | 53篇 |
2016年 | 45篇 |
2015年 | 41篇 |
2014年 | 69篇 |
2013年 | 150篇 |
2012年 | 95篇 |
2011年 | 98篇 |
2010年 | 70篇 |
2009年 | 76篇 |
2008年 | 87篇 |
2007年 | 76篇 |
2006年 | 52篇 |
2005年 | 53篇 |
2004年 | 43篇 |
2003年 | 32篇 |
2002年 | 40篇 |
2001年 | 22篇 |
2000年 | 16篇 |
1999年 | 14篇 |
1998年 | 13篇 |
1996年 | 15篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 22篇 |
1984年 | 26篇 |
1982年 | 11篇 |
1981年 | 22篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 22篇 |
1977年 | 29篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 26篇 |
1973年 | 23篇 |
1972年 | 12篇 |
1971年 | 15篇 |
1957年 | 11篇 |
排序方式: 共有1937条查询结果,搜索用时 15 毫秒
931.
Mani E Sanz E Roy S Dijkstra M Groenewold J Kegel WK 《The Journal of chemical physics》2012,136(14):144706
We report a computational study on the spontaneous self-assembly of spherical particles into two-dimensional crystals. The experimental observation of such structures stabilized by spherical objects appeared paradoxical so far. We implement patchy interactions with the patches point-symmetrically (icosahedral and cubic) arranged on the surface of the particle. In these conditions, preference for self-assembly into sheet-like structures is observed. We explain our findings in terms of the inherent symmetry of the patches and the competition between binding energy and vibrational entropy. The simulation results explain why hollow spherical shells observed in some Keplerate-type polyoxometalates (POM) appear. Our results also provide an explanation for the experimentally observed layer-by-layer growth of apoferritin--a quasi-spherical protein. 相似文献
932.
Deepak K. Yadav Arvind K. Yadav Vishnu P. Srivastava Geeta Watal Lal Dhar S. Yadav 《Tetrahedron letters》2012,53(23):2890-2893
Bromodimethylsulfonium bromide (BDMS) was found to be a very efficient reagent for Lossen rearrangement of hydroxamic acids to the corresponding isocyanates which were subsequently trapped in situ with various amines to afford unsymmetrical ureas in good to excellent yields (64–89%). The protocol is experimentally simple, mild, and represents valuable alternative to the existing methods for in situ activation of hydroxamic acids promoting Lossen rearrangement. 相似文献
933.
A convenient synthetic protocol for the synthesis of imidazo[1,2-a]pyridine has been developed by employing one-pot three-component Ugi reaction by employing aromatic amidine, aromatic aldehyde, and isocyanide using 5 mol % of bromodimethylsulfonium bromide (BDMS) at room temperature. In addition, they also exhibit interesting fluorescence properties, which may be useful for fluorescent probe. Mild reaction conditions, non-aqueous work-up procedure, good yields, short reaction time, and no need of chromatographic separation are some of the salient features of the present protocol. 相似文献
934.
Glareh Azadi Matthew Seward Mona Utne Larsen Nina C. Shapley Anubhav Tripathi 《Applied biochemistry and biotechnology》2012,168(3):531-541
Techniques to inhibit gram-negative bacteria such as Shiga toxin-producing Escherichia coli are valuable as the prevalence of large-scale industrial food preparation increases the likelihood of contamination. Chitosan, the deacetylated derivative of chitin, has been demonstrated to inhibit bacteria growth in acidic environments, but is significantly less effective in preventing bacteria grown at pH?>7.0. Pulsed electric fields, constituting another method of bacteria inhibition, are difficult to generate at sufficient strength due to the high electric potentials required. This study utilizes adsorption of particulate chitosan in a very low electric field for an increased inhibition of gram-negative bacteria in neutral or alkaline pH conditions. Chitosan microparticles are demonstrated to flocculate E. coli, inhibit growth, and exhibit increased efficacy when combined with a low voltage electric field applied over 2-min intervals. Using sustained pulses of approximately 100?V/cm, it is demonstrated that bacteria viability is reduced by several orders of magnitude. The degree of bacterial inhibition is increased when chitosan microparticles are introduced to the system prior to imposing a small electric field. 相似文献
935.
Bifurcation phenomena and control for magnetohydrodynamic flows in a smooth expanded channel
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This work reports the effects of magnetic field on an electrically conducting fluid with low electrical conductivity flowing in a smooth expanded channel. The governing nonlinear magnetohydrodynamic (MHD) equations in induction- free situations are derived in the framework of MHD approximations and solved numerically using the finite-difference technique. The critical values of Reynolds number (based on upstream mean velocity and channel height) for symmetry breaking bifurcation for a sudden expansion channel (1:4) is about 36, whereas the value in the case of the smooth expansion geometry used in this work is obtained as 298, approximately (non-magnetic case). The flow of an electrically conducting fluid in the presence of an externally applied constant magnetic field perpendicular to the plane of the flow is reduced significantly depending on the magnetic parameter (M). It is expansion (1:4) is about 475 for the magnetic parameter M found that the critical value of Reynolds number for smooth = 2. The separating regions developed behind the smooth symmetric expansion are decreased in length for increasing values of the magnetic parameter. The bifurcation diagram is shown for a symmetric smoothly expanding channel. It is noted that the critical values of Reynolds number increase with increasing magnetic field strength. 相似文献
936.
937.
If $P(z) = \sum\limits_{\nu = 0}^n {c_\nu z^\nu } $ is a polynomial of degree n, then for |β| ≤ 1, it was proved in [4] that $\left| {zP'(z) + n\frac{\beta } {2}P(z)} \right| \leqslant n\left| {1 + \frac{\beta } {2}} \right|\mathop {\max }\limits_{|z| = 1} |P(z)|,|z| = 1 $ In this paper, first we generalize the above result for the s th derivative of polynomials and next we improve the above inequality for polynomials with restricted zeros. 相似文献
938.
Veena Dhayal Mukesh Kumar Atal Banwari Lal Choudhary Meena Nagar Rakesh Bohra 《Journal of Sol-Gel Science and Technology》2009,52(1):97-108
Ti(OPri)4 reacts with HOSi(OtBu)3 in anhydrous benzene in 1:1 and 1:2 molar ratios to afford alkoxy titanosiloxane precursors, [Ti(OPri)3{OSi(OtBu)3}] (A) and [Ti(OPri)2{OSi(OtBu)3}2] (B), respectively. Further reactions of (A) or (B) with glycols in 1:1 molar ratio afforded six complexes of the types [Ti(OPri)(O–G–O){OSi(OtBu)3}] (1A–3A) and [Ti(O–G–O){OSi(OtBu)3}2] (1B–3B), respectively [where G = (CH2)2 (1A, 1B); (CH2)3 (2A, 2B) and {CH2CH2CH(CH3)} (3A, 3B)]. Both (A) and (B) are liquids while all the other products are viscous liquids which get solidified on ageing. Cryoscopic molecular weight
measurements of the fresh products indicate their monomeric nature. FAB mass studies of (A) and (B) also indicate monomeric nature. However, FAB mass spectra of the two representative solids (1A) and (2B) suggest dimeric behavior of the glycolato derivatives. (A) distills at 85 °C/5 mm while other products get decomposed even under reduced pressure. TG analyses of (A), (B), (1A), and (1B) suggest formation of titania–silica materials at 200 °C for (A) and (B) and 350 °C for (1A) and (1B). The products have been characterized by elemental analyses, FTIR and 1H, 13C & 29Si-NMR techniques. All these products are soluble in common organic solvents indicating a homogenous distribution of the components
on the molecular scale. The Si/Ti ratio of the oxide may be controlled easily by the composition of the starting precursors.
Hydrolysis of the glycol modified derivative, (1A) by the Sol–Gel technique affords the desired homogenous titania–silica material, TiO2·SiO2 in nano-size while, the precursor (A) yields a non-stiochiometric silica doped titania material. However, pyrolysis of (A) yields nano-sized crystallites of TiO2·SiO2. All these materials were characterized by FTIR, powder XRD patterns, SEM images, and EDX analyses. 相似文献
939.
Paul BT Babu MS Santhoshkumar TR Karunagaran D Selvam GS Brown K Woo T Sharma S Naicker S Murugesan R 《Journal of photochemistry and photobiology. B, Biology》2009,94(1):38-44
Two novel cyclohexane-1,2-diamino and N,N dimethyl amino-propyl substituted hypocrellin B derivatives, abbreviated as CHA2HB and DMAHB, respectively were synthesized. These derivatives exhibited enhanced absorption in phototherapeutic window. Photodynamic action of these derivatives, investigated using optical and electron spin resonance methods, depended on both Type I and Type II mechanisms. Gel electrophoresis indicated 1O2/O2(.-) mediated DNA damage. CHA2HB displayed 20 fold increase in light dependent cytotoxicity on colon cancer cell line (HCT 116) than the well-known hypocrellin B (HB). The light induced, LD(50) values for CHA2HB and DMAHB were found to be 0.1 microM and 1.5 microM, respectively. The singlet oxygen generating efficiency followed the order HB>CHA(2)HB>DMAHB. But, the enhanced red absorption as well as the hydrophilicity renders the CHA2HB a better photodynamic therapeutic agent. 相似文献
940.
Building on the observation that metal complexation facilitates azomethine ylide formation, we report that chelating aldehydes participate in metal-templated, one-pot reactions with unprotected amino acid esters and activated olefins to provide highly substituted pyrrolidines. The high yields, broad substrate scope, excellent diastereoselectivities, functional group tolerance, and incorporation of commercially available materials in this reaction simplifies access to medicinally relevant proline derivatives. 相似文献