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   1168篇
  免费   140篇
  国内免费   120篇
化学   939篇
晶体学   40篇
力学   48篇
综合类   4篇
数学   145篇
物理学   252篇
  2023年   12篇
  2022年   15篇
  2021年   28篇
  2020年   17篇
  2019年   31篇
  2018年   21篇
  2017年   25篇
  2016年   50篇
  2015年   56篇
  2014年   62篇
  2013年   86篇
  2012年   81篇
  2011年   92篇
  2010年   48篇
  2009年   40篇
  2008年   76篇
  2007年   58篇
  2006年   60篇
  2005年   49篇
  2004年   56篇
  2003年   44篇
  2002年   61篇
  2001年   47篇
  2000年   30篇
  1999年   28篇
  1998年   22篇
  1997年   12篇
  1996年   15篇
  1995年   11篇
  1994年   17篇
  1993年   16篇
  1992年   12篇
  1991年   8篇
  1990年   14篇
  1989年   7篇
  1988年   4篇
  1987年   7篇
  1986年   5篇
  1985年   8篇
  1984年   10篇
  1982年   15篇
  1981年   9篇
  1979年   8篇
  1978年   4篇
  1976年   4篇
  1974年   7篇
  1971年   6篇
  1970年   4篇
  1968年   4篇
  1967年   4篇
排序方式: 共有1428条查询结果,搜索用时 15 毫秒
981.
建立了双频超声辅助酶解提取(DUED)对玉米中Cd(Ⅱ)和Pb(Ⅱ)进行提取的方法,并结合石墨炉原子吸收光谱法(GFAAS)对其检测.结果表明,双频超声辅助酶解提取方法的最佳酶解条件为:0.1 mLα-淀粉酶+0.1 g中性蛋白酶+0.2 g木瓜蛋白酶、2.5 mL提取剂(pH 5.0)和60℃的提取温度.最佳超声参数...  相似文献   
982.
The X‐site ion in organic–inorganic hybrid ABX3 perovskites (OHPs) varies from halide ion to bridging linkers like HCOO?, N3?, NO2?, and CN?. However, no nitrite‐based OHP ferroelectrics have been reported so far. Now, based on non‐ferroelectric [(CH3)4N][Ni(NO2)3], through the combined methodologies of quasi‐spherical shape, hydrogen bonding functionality, and H/F substitution, we have successfully synthesized an OHP ferroelectric, [FMeTP][Ni(NO2)3] (FMeTP=N‐fluoromethyl tropine). As an unprecedented nitrite‐based OHP ferroelectric, the well‐designed [FMeTP][Ni(NO2)3] undergoes the ferroelectric phase transition at 400 K with an Aizu notation of 6/mmmFm, showing multiaxial ferroelectric characteristics. This work is a great step towards not only enriching the molecular ferroelectric families but also accelerating the potential practical applications.  相似文献   
983.
Phenothiazine derivatives based on the 10‐phenyl‐10H‐phenothiazine ( NAS ) chromophore, namely 7‐phenyl‐7H‐benzo[c]phenothiazine ( NAS‐1 ) and 12‐phenyl‐12H‐benzo[a]phenothiazine ( NAS‐2 ), were designed and synthesized. NAS‐1 and NAS‐2 are constitutional isomers with different steric strains imposed on the phenothiazine core moiety. In solution, the more‐strained NAS‐2 possesses a bent structure and undergoes photoinduced structural planarization (PISP). In the crystal, despite the absence of PISP, bent NAS‐2 exhibits prominent excimer emission as well as emission mechanochromism, which is not observed in the planar‐like NAS and NAS‐1 . This unconventional observation results from the bent core structure facilitating π–π stacking of the peripheral naphthalene moieties. Two‐photon‐coupled depth‐dependent emission shows spectral differences between the surface and kernel of the NAS‐2 crystal, and is believed to be a general phenomenon, at least in part, for materials exhibiting emission mechanochromism.  相似文献   
984.
A ring is said to be right (resp., left) regular-duo if every right (resp., left) regular element is regular. The structure of one-sided regular elements is studied in various kinds of rings, especially, upper triangular matrix rings over one-sided Ore domains. We study the structure of (one-sided) regular-duo rings, and the relations between one-sided regular-duo rings and related ring theoretic properties.  相似文献   
985.
Two new sesquiterpenoids, castanin A (= 4R*,5R*,5aS*,8S*,8aR*,8bR*)‐8‐formyl‐4,5,5a,6,7,8,8a,8b‐octahydro‐5,8b‐dihydroxy‐3,5a,8‐trimethyl‐2‐oxo‐2H‐indeno[4,5‐b]furan‐4‐yl acetate; 1 ) and castanin B (= 1aS*,6R*,6aS*,7aR*,9aR*)‐1a,2,6,6a,7a,8,9,9a‐octahydro‐1a,5,7a‐trimethylbisoxireno[4,5 : 8,9]cyclodeca[1,2‐b]furan‐6‐yl acetate; 2 ) were isolated from the aerial parts of Salvia castanea Diels f. tomentosa, together with two known compounds, oplophanone and 1β,6α‐dihydroxyeudesm‐4(14)‐ene. Their structures were elucidated by spectroscopic analyses and, in the case of 2 , also by X‐ray crystallography. Castanin A ( 1 ) represents a novel sesquiterpenoid, with a contracted ring A, derived from eudesmanolide. A possible biogenetic pathway is proposed.  相似文献   
986.
制备了一种分子印迹荧光传感器用于菊酯类农药代谢物间苯氧基苯甲醛(3-PBD)的选择性测定。以3-PBD为模板分子,3-氨丙基三乙氧基硅烷为功能单体,四乙氧基硅烷为交联剂,采用溶胶-凝胶法在Mn掺杂ZnS量子点表面成功合成了荧光分子印迹聚合物。采用红外光谱、X射线衍射及X射线光电子能谱对聚合物进行表征,并对检测条件进行了优化。在最佳条件下,该荧光传感器对3-PBD的响应线性范围为0.3~5μmol/L,检出限为0.267μmol/L。此分子印迹荧光传感器可与农药的酶解过程相结合,用于菊酯类农药的快速检测。对实际样品的测定结果显示,3-PBD的回收率为84.0%~119%,相对标准偏差不超过5.0%。结果表明,该分子印迹荧光传感器可以用于食品中3-PBD的测定。  相似文献   
987.
Indeno[1,2‐b]fluorene‐based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π‐electron systems were prepared, and their structures were identified by X‐ray crystallography. With short π–π distances around 3.0 Å, [2.2](5,11)indeno[1,2‐b]fluorenophane and its precursor [2.2](5,11)indeno[1,2‐b]fluorene‐6,12‐dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H‐type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady‐state absorption spectra induced by charge‐transfer‐mediated superexchange interaction.  相似文献   
988.
A novel approach for in situ generation of AgI quantum dots by the confinement of a pillar[5]arene‐based supramolecular polymer network has been successfully developed. The supramolecular polymer network ( SPN‐QP ) was constructed by using a bis‐8‐hydroxyquinoline‐modified pillar[5]arene derivative as a host ( H‐QP ) and a bis‐pyridinium‐modified decane as guest ( G‐PD ). The SPN‐QP shows ultrasensitive response for Ag+. The limit of detection is about 7.44×10?9 M..Interestingly, when I? was added to the SPN‐QP +Ag+ system, an unexpected strong warm‐white fluorescence emission was observed. After carefu investigation, we found that the strong warm‐white fluorescence emission could be attributed to the in situ formation of AgI quantum dots under the confinement of the supramolecular polymer network ( SPN‐QP ). Based on this approach, ultrasensitive detection of I? was realized. The limit of detection for I? is 4.40×10?9 M. This study provides a new way for the preparation of quantum dots under the confinement of supramolecular polymer network as well as ultrasensitive detection of ions by in situ formation of quantum dots.  相似文献   
989.
Summary Commercially available equipment from two manufacturers served to set up an automated system for the precolumn phase-transfer-catalyzed dansylation of phenolic steroids, using ethynyl estradiol (EE) and estradiol (E) as model compounds. Using different mixing techniques, the on-line determination of EE and E in 200l untreated urine samples was achieved at a rate of 3–8 analyses per hour. Detection limits were calculated to be 3–5 ng/ml. Calibration curves in urine were linear over two orders of magnitude with r=0.999 (n=5) for EE and r=0.999 (n=6) for E. The repeatability of the determination of EE in urine (1g/ml) was 3.9% (RSD; n=20) and of E (1.5l/ml) 3.8% (RDS; n=10). The use of plasma instead of urine in the on-line procedures was not possible due to rapid formation of emulsions, but E and EE were determined in 100l plasma samples using a mild off-line mixing procedure in 10min. Detection limits were calculated to be ca 10ng/ml.A reaction detector, based on a solvent-segmented system, was developed for the on-line post-column dansylation of phenols and was coupled with a reversed-phase LC system. The highly selective system showed excellent linearity over at least two orders of magnitude with r=0.9999 (n=6) for both phenol and 2,5-dimethylphenol. The reproducibility was good with RSD values of around 2%. Detection limits for loop injections from standard solutions were calculated to be between 4 and 11ng.  相似文献   
990.
A four nanoprobe system has been installed inside a FEI XL30 F scanning electron microscope (SEM), and shown to be fully compatible with the normal functions of the SEM and also a Gatan cold stage (model C1003, -185-400 degrees C). With some selected examples of applications, we have shown that this nanoprobe system may be used effectively for gripping, moving and manipulating nanoobjects, e.g. carbon nanotubes, setting up electric contacts for electronic measurements, tailoring the structure of the nanoobject by cutting, etc. and even for making unexpected nanostructures, e.g. a nanohook. Applications in other areas have also been speculated, limitations or disadvantages of the current design of the probe system were discussed, and methods for possible improvement were suggested.  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