首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2832篇
  免费   111篇
  国内免费   11篇
化学   2248篇
晶体学   25篇
力学   43篇
数学   101篇
物理学   537篇
  2022年   14篇
  2021年   24篇
  2020年   38篇
  2019年   46篇
  2018年   36篇
  2017年   31篇
  2016年   63篇
  2015年   54篇
  2014年   81篇
  2013年   129篇
  2012年   129篇
  2011年   202篇
  2010年   87篇
  2009年   104篇
  2008年   203篇
  2007年   185篇
  2006年   177篇
  2005年   161篇
  2004年   160篇
  2003年   129篇
  2002年   114篇
  2001年   40篇
  2000年   64篇
  1999年   45篇
  1998年   36篇
  1997年   31篇
  1996年   29篇
  1995年   20篇
  1994年   33篇
  1993年   18篇
  1992年   27篇
  1991年   20篇
  1990年   18篇
  1989年   14篇
  1988年   12篇
  1987年   21篇
  1986年   16篇
  1985年   31篇
  1984年   38篇
  1983年   21篇
  1982年   30篇
  1981年   33篇
  1980年   23篇
  1979年   28篇
  1978年   20篇
  1977年   21篇
  1976年   17篇
  1975年   15篇
  1974年   11篇
  1973年   17篇
排序方式: 共有2954条查询结果,搜索用时 46 毫秒
991.
The sensing of vapor odorants is highly demanded in the field of life and medical sciences. Although olfactory receptors (ORs) have potentials to recognize volatile organic compounds, the interaction of ORs, chemical vapors, and peptide components in olfactory mucus has yet to be analyzed to develop OR‐based sensors. A bioinspired electrophysiology technique is shown to record the response of reconstituted insect ORs to chemical vapors. To mimic the interface between ORs and olfactory mucus, OR expressing spheroids were loaded into a hydrogel microchamber array. A negative extracellular field potential shift of spheroids was successfully observed by the stimulation of their vapor cognate ligand. Importantly, the ligand repertoire of the OR of malaria vector mosquito examined by this method differed from that of in vivo studies. Our method is useful to develop protein‐based gas sensing techniques and to examine the binding of ORs and chemical vapors.  相似文献   
992.
Small‐angle bent‐core liquid‐crystalline (LC) molecules based on a 1,2‐bis(phenylethynyl)benzene central core have been synthesized that form banana smectic phases with a ferroelectric B7–antiferroelectric B2 phase sequence upon cooling. The formation of polar, switchable ferro‐/antiferroelectric banana phases indicates that, despite the low core bend angle of approximately 60°, banana smectic phases are still formed with the bend direction parallel to the layer. This study offers significant evidence that shows bent‐core molecules with a 60° bend angle can form the well‐known B2 and B7 banana phases. Consequently, it may lead to the preparation of a wide variety of novel bent molecules with low bend angles that spontaneously form an LC phase with both polarization and chirality.  相似文献   
993.
Phosphorus analogues of pyrromellitic diimides (PyDIs), which represent a family of privileged electron‐accepting organic compounds, have been successfully synthesized as novel electron‐accepting π‐conjugated molecules. Investigation into their physicochemical properties uncovered their prominent electron‐accepting abilities over the corresponding PyDI. Furthermore, theoretical studies revealed the significant contribution of σ*–π* hyperconjugation in stabilizing the LUMO+1.  相似文献   
994.
To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)2, and a diol, H2L, in aqueous solution, the acid dissociation constants (${K{{{\rm BL}\hfill \atop {\rm a}\hfill}}}$ ) of boronic acid diol esters (HBLs) were determined based on the well‐established concept of conditional formation constants of metal complexes. The pKa values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4‐difluorophenylboronic acid and chromotropic acid, 3‐nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K′, based on the main reaction: RB(OH)2+H2L ${{\mathop \leftrightarrow \limits ^{K{_{1}}}_{}}}$ RB(L)(OH)?+H3O+ instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B′) with uncomplexed diol species (L′) to form boronic acid diol complex species (esters, BL′) in aqueous solution at pH 5–11: B′+L′ ${{\mathop \leftrightarrow \limits ^{K{^\prime}}_{}}}$ BL′. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.  相似文献   
995.
Bis(2‐thienyl)diketopyrrolopyrrole with two ZnII‐cyclens (ZnCyc‐DPP) was designed and synthesized to evaluate the selective binding of ZnII‐cyclen with thymine base in single‐strand DNA as a tool for the construction of a highly ordered multichromophore system on DNAs. Through UV/Vis titrations, gel filtration chromatography, and circular dichroism spectroscopy, ZnCyc‐DPP formed J‐type DPP aggregates with oligo‐dTn DNAs. The DPP aggregates absorbed on a gold electrode exhibited good photocurrent responses. The present results show that binding ZnII‐cyclen–chromophore conjugates and thymine bases together is a powerful tool for preparing DNA‐templated multichromophoric systems with specific functions.  相似文献   
996.
Mitochondrial frataxin is involved in various functions such as iron homeostasis, iron–sulfur cluster biogenesis, the protection from oxidative stress and apoptosis and acts as a tumor suppressor protein. We now show that the expression of frataxin is stimulated in a p53‐dependent manner and prove that frataxin is a direct p53 target gene by showing that the p53‐responsive element in the promoter of the mouse frataxin gene is bound by p53. The bacterial expression of human frataxin stimulated maturation of human ferrochelatase, which catalyzes the insertion of iron into protoporphyrin at the last step of heme biosynthesis. Overexpression of frataxin in human cancer A431 and HeLa cells lowered 5‐aminolevulinic acid(ALA)‐induced accumulation of protoporphyrin and induced resistance to ALA‐induced photo‐damage, whereas p53 silencing with siRNA in non tumor HEK293T cells down‐regulated the expression of frataxin and increased the accumulation of protoporphyrin. Thus, the decrease of the expression of frataxin unregulated by p53 in tumor cells enhances ALA‐induced photo‐damage, by down‐regulation of mitochondrial functions.  相似文献   
997.
Single-mode fibers with low attenuation and sufficient reliability are especially important for long-span optical transmission systems. Conventionally, GeO2 has been added to the fiber core region in order to make a refractive index difference between the core and cladding. However, the addition of GeO2 increases Rayleigh scattering loss [1], and deteriorates the chemical stability against hydrogen atmosphere and γ-ray irradiation to bring about an increase in attenuation. [2][3] Therefore, pure silica core single-mode fiber was considered to be an ideal type of fiber, and it was succeeded in being fabricated by utilizing fluorine containing SiO2 cladding.  相似文献   
998.
The direct arylation of 5-aryloxazoles, prepared by the van Leusen reaction, with various aryl iodides is effectively promoted by a system of CuI combined with PPh3 and Na2CO3 as a ligand and a base, respectively, in DMF to produce the corresponding 2,5-diaryloxazoles in good yields.  相似文献   
999.
A fluorescent probe, DPPEC (1,2-dipalmitoylglycerophosphorylethanolamine labeled with coumarin) was developed for detecting hydroxyl radical (*OH) in lipid membranes. The coumarin moiety contributes to the fluorescent detection of *OH and the phospholipids moiety gives a driving force to localize the probe in lipid membranes. DPPEC in liposomal membranes rapidly reacted with *OH and increased the fluorescence intensity, depending on the concentration of *OH. The increase in the fluorescence intensity induced by *OH was effectively suppressed by the addition of DMSO. The probe exhibited a higher fluorescence response to *OH over other reactive oxygen species, such as hydrogen peroxide, nitric oxide, peroxynitrite, alkylperoxyl radical, and hypochlorite. DPPEC would be useful as a new type of fluorescent probe that can localize in lipid membranes and detect *OH efficiently.  相似文献   
1000.
Koji Chinen 《Discrete Mathematics》2008,308(24):6426-6440
In 1999, Iwan Duursma defined the zeta function for a linear code as a generating function of its Hamming weight enumerator. It can also be defined for other homogeneous polynomials not corresponding to existing codes. If the homogeneous polynomial is invariant under the MacWilliams transform, then its zeta function satisfies a functional equation and we can formulate an analogue of the Riemann hypothesis. As far as existing codes are concerned, the Riemann hypothesis is believed to be closely related to the extremal property.In this article, we show there are abundant polynomials invariant by the MacWilliams transform which satisfy the Riemann hypothesis. The proof is carried out by explicit construction of such polynomials. To prove the Riemann hypothesis for a certain class of invariant polynomials, we establish an analogue of the Eneström-Kakeya theorem.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号