全文获取类型
收费全文 | 105877篇 |
免费 | 9535篇 |
国内免费 | 7111篇 |
专业分类
化学 | 46854篇 |
晶体学 | 895篇 |
力学 | 4362篇 |
综合类 | 340篇 |
数学 | 14820篇 |
物理学 | 27084篇 |
综合类 | 28168篇 |
出版年
2024年 | 287篇 |
2023年 | 1424篇 |
2022年 | 2302篇 |
2021年 | 2613篇 |
2020年 | 2624篇 |
2019年 | 2296篇 |
2018年 | 3161篇 |
2017年 | 3451篇 |
2016年 | 3581篇 |
2015年 | 3753篇 |
2014年 | 4493篇 |
2013年 | 5448篇 |
2012年 | 8871篇 |
2011年 | 8096篇 |
2010年 | 6104篇 |
2009年 | 5777篇 |
2008年 | 5066篇 |
2007年 | 4733篇 |
2006年 | 4184篇 |
2005年 | 7663篇 |
2004年 | 6735篇 |
2003年 | 4608篇 |
2002年 | 2408篇 |
2001年 | 1877篇 |
2000年 | 1724篇 |
1999年 | 2338篇 |
1998年 | 1983篇 |
1997年 | 1928篇 |
1996年 | 1773篇 |
1995年 | 1536篇 |
1994年 | 1389篇 |
1993年 | 1198篇 |
1992年 | 1129篇 |
1991年 | 1103篇 |
1990年 | 931篇 |
1989年 | 763篇 |
1988年 | 644篇 |
1987年 | 436篇 |
1986年 | 332篇 |
1985年 | 228篇 |
1984年 | 139篇 |
1983年 | 134篇 |
1982年 | 85篇 |
1981年 | 67篇 |
1979年 | 68篇 |
1976年 | 109篇 |
1975年 | 71篇 |
1973年 | 68篇 |
1972年 | 63篇 |
1966年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 828 毫秒
971.
Ren Yanliang Cheng Lin Wan Jian Li Yongjian Liu Junjun Yang Guangfu Zhang Lihua Yang Song 《中国科学B辑(英文版)》2006,49(1):88-96
The electronic singlet vertical excited states of photosynthetic reaction center (PSRC) in Rhodopseudomonas (Rps.) viridis were investigated by ZINDO and INDO/S methods. The effects of the interactions of pigment-pigment and pigment-protein on
the electronic excitations were examined. The calculation results showed that the interactions of pigment-pigment and pigment-protein
play an important role in reasonably assigning the experimental absorption and circular dichroism (CD) spectra of PSRC in
Rps. virids. By comparing the theoretically computed excited states with the experimental absorption and CD spectra, satisfactory assignments
of the experimental spectroscopic peaks were achieved. 相似文献
972.
Silica xerogels with different structures and morphology, synthesized using a sol-gel procedure, were used as a carrier of vanadium catalysts (VOCl3/AlEt2Cl) for ethylene polymerization. Two techniques of catalyst synthesis were applied: slurry impregnation and gas-phase adsorption and the relevant polymerization methods were then employed. The effect of the carrier structure and morphology on the vanadium loading in the catalysts, the catalyst’s activity and kinetic stability were investigated. 相似文献
973.
EDTA—Cu配合物用于还原糖的测定 总被引:2,自引:0,他引:2
首次报道了应用EDTA(乙二胺四乙酸二钠盐)-铜配合物(EC)测定还原糖的分光光度法,结果表明,pH4~11时EC的最大吸收波长为732nm,在HCO-3CO2-3缓冲介质中,配合物EC的吸光度与还原糖在077~161mg(六碳糖)范围内有很好的线性关系,相关系数大于0998,样品测定回收率在95%~105%之间。该法试剂易得、性质稳定,操作方便,灵敏度较高,取样量少于5mL。 相似文献
974.
合成了稀土铕、镧与吡啶-2,6二甲酸形成的二元配合物及铕与邻菲罗啉和吡啶-2,6二甲酸形成的三元配合物。经元素分析确定该配合物的组成分别为二元:Na3[Eu(DPC)3]·2H2O,Na3[La(DPC)3]·2H2O;三元:NaEu(DPC)2·phen·4H2O。用核磁共振研究了配体与稀土的配位方式,讨论了诱导效应、屏蔽效应及稀土离子的顺磁性对配合物化学位移和NMR谱图的影响。NMR研究表明,三种配合物具有相似的对称结构和相同的化学位移变化规律。吡啶-2,6二甲酸中的羧酸以单齿配位(整个分子为三齿配位),二元配合物中,铕和镧的配位数均为9,三元配合物中,铕的配位数最低为8。 相似文献
975.
976.
I Szumiel D W?odek E Niepokojczycka K J Johanson 《Journal of photochemistry and photobiology. B, Biology》1989,3(4):483-496
L5178Y-R and L5178Y-S cells differ in sensitivity to UVC radiation (D0 values: 2.8 and 9.0 J m-2 respectively, exposure in Fischer's medium). The UVC sensitivity is related to the excision repair ability. Benzamide (Bz), an inhibitor of adenosine diphosphoribosyl transferase (ADPRT), does not modify the lethal effect of UVC radiation in L5178Y-R cells, whereas it sensitizes L5178Y-S cells. The content of NAD+ after irradiation decreases only in the latter cells and this decrease can be prevented by 2 mM Bz treatment. In agreement with the survival data, in L5178Y-R cells neither the proportion of abnormal cells nor the frequency of chromatid aberration are affected by 2 mM Bz treatment, in contrast with L5178Y-S cells. Bz slightly reverses inhibition of 3H-thymidine incorporation only in L5178Y-S cells, but it does not affect the proportions of cells in the different phases of the cell cycle in either cell strain after UVC exposure. These data could be taken as an indirect indication of the involvement of ADPRT in DNA repair in UVC-irradiated L5178Y-S cells. However, the increase in the number of DNA strand breaks in UVC-exposed, Bz-treated cells compared with UVC-exposed untreated cells is the same in both L5178Y strains. 相似文献
977.
本文采用家兔肺泡巨噬细胞(AM)存活率和细胞过氧化脂质(LPO)值为测定指标,研究了硅氧的聚合度对其细胞毒性的影响。结果表明:各种聚合度的硅氧都有一定的细胞毒性,聚合度愈大,细胞毒性也愈大。胶体SiO_2的粒径增大,其细胞毒性降低。十硅酸盐及粒径小于5nm的硅溶胶的细胞毒性大于α-石英。聚合度小于6的低聚硅酸及其盐、粒径18nm以上的硅溶胶以及硅胶H的毒性皆小于α-石英。本实验细胞存活率降低和过氧化脂质值升高的趋势基本一致,这表明硅氧可能主要与细胞表面膜作用,膜上磷脂等表面活性物质被氧化和变性,从而导致细胞损伤。 相似文献
978.
Ana?C.?F.?RibeiroEmail author Victor?M.?M.?Lobo Artur?J.?M.?Valente Eduarda?F.?G.?Azevedo M.??da?G.?Miguel H.?D.?Burrows 《Colloid and polymer science》2004,283(3):277-283
Differential mutual diffusion coefficients of n-alkyltrimethylammonium bromides [CH3(CH2)n–1N(CH3)3Br, CnTAB] (n=10, 12, 14, 16) have been measured in aqueous solutions at 298.15 K using a conductimetric cell and an automatic apparatus to follow diffusion. The cell is based on an open-ended capillary, and the technique follows the diffusion process by measuring the resistance of a solution inside the capillaries at various times. The electrical conductances of those solutions have also been measured to calculate the critical micellar concentration (cmc). Thermodynamic analysis of the data suggests that the free ion concentration decreases at concentrations above the cmc, in agreement with theoretical predictions. The obtained values of the micellization parameters were used to model the mutual diffusion coefficients of CnTAB aqueous solutions. 相似文献
979.
A theory is developed for the potential distribution around a charged spherical colloidal particle carrying ionized groups on the particle surface in a medium containing its counterions (i.e., counterions produced from dissociation of the particle surface groups) and a small amount of added salts on the basis of the theory of Imai and Oosawa. Numerical solutions to the Poisson–Boltzmann equation for the potential distribution are obtained for the case of dilute (but not infinitely dilute) particle suspensions of volume fraction 1 for
a1 (where is the Debye–Hückel parameter and a is the particle radius). Here we have taken into account the effects of (i) counterions from the particle surface groups, and (ii) the finite particle volume fraction. These effects, which are usually neglected in the conventional Poisson–Boltzmann equation, are found to be important. It is found that, as in the case of completely salt-free media, there is a certain critical value of the particle charge (which is the same as that for the completely salt-free case). When the particle charge is lower than the critical value, the potential is given by a Coulomb potential. If the particle charge is higher than the critical value, then counterions are accumulated in the vicinity of the particle surface (counterion condensation) and the potential becomes less dependent on the particle charge. The above behaviors can be observed even for the case where the electrolyte concentration is higher than the concentration of counterions from the particle surface groups, if the conditions
1 and a1are both satisfied. 相似文献
980.
Thermolysis of benzannulated enyne-isocyanates 13 and enyne-isocyanates 36 and 37 promoted the cycloaromatization reactions to generate in situ O,4-didehydro-2-hydroxyquinolines and O,4-didehydro-2-hydroxypyridines, respectively, as reactive intermediates. These cycloaromatized intermediates could be captured either as biradicals and/or as zwitterions depending on the nature of the substituent at the alkynyl terminus. The intermediate derived from cycloaromatization of 13a bearing a phenyl substituent could be regarded as biradical 14, which then abstracts hydrogen atoms from gamma-terpinene leading to 2(1H)-quinolinone 15. Alternatively, the same intermediate could also be regarded as zwitterion 14', which then undergoes an initial hydride abstraction from gamma-terpinene followed by protonation to produce 15. The presence of a 2-phenylethyl substituent in 13b and 37a or a 2-methylphenyl substituent in 37b also allowed the resulting intermediates to be captured intramolecularly either as biradicals or as zwitterions, producing 2(1H)-quinolinone 19, 2(1H)-pyridone 39, and benzopyranopyridine 43, respectively. On the other hand, with a 2-methoxyphenyl, a 2-(dimethylamino)phenyl, or a 3-methoxypropyl substituent, the chemical behavior of the cycloaromatized adduct could be best accounted for in terms of a zwitterionic intermediate leading to benzofuro[3,2-c]quinolin-6(5H)-one (20), 5,11-dihydro-11-methyl-6H-indolo[3,2-c]quinolin-6-one (25), benzofuro[3,2-c]pyridin-1(2H)-one 44, 2,5-dihydro-2,5-dimethyl-1H-pyrido[4,3-b]indol-1-one 46, and related compounds. Interestingly, thermolysis of 37f bearing a 2-(methoxymethyl)phenyl substituent at the alkynyl terminus produced the unexpected benzopyranopyridine 56 as the major product in a process involving the cleavage of the bond between the methoxyl oxygen and the adjacent methylene carbon. The efficiency and selectivity of the cycloaromatization reaction could also be enhanced by the introduction of 1.1 to 10 equiv of dimethylphenylsilyl chloride to the reaction mixture to capture the resulting zwitterion. 相似文献