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   96723篇
  免费   3708篇
  国内免费   2645篇
化学   37922篇
晶体学   1033篇
力学   7841篇
综合类   191篇
数学   33800篇
物理学   22289篇
  2023年   446篇
  2022年   455篇
  2021年   658篇
  2020年   803篇
  2019年   774篇
  2018年   10897篇
  2017年   10701篇
  2016年   6893篇
  2015年   1738篇
  2014年   1343篇
  2013年   1668篇
  2012年   5519篇
  2011年   12243篇
  2010年   6795篇
  2009年   7075篇
  2008年   7799篇
  2007年   9918篇
  2006年   1329篇
  2005年   2279篇
  2004年   2219篇
  2003年   2567篇
  2002年   1562篇
  2001年   669篇
  2000年   693篇
  1999年   620篇
  1998年   543篇
  1997年   511篇
  1996年   503篇
  1995年   410篇
  1994年   376篇
  1993年   338篇
  1992年   248篇
  1991年   260篇
  1990年   226篇
  1989年   170篇
  1988年   144篇
  1987年   146篇
  1986年   116篇
  1985年   92篇
  1984年   83篇
  1983年   60篇
  1982年   66篇
  1981年   56篇
  1980年   59篇
  1979年   44篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
Single crystal tin nano-rod arrays were fabricated by the electrodeposition method when amphiphilic triblock copolymer P123 was used as a soft template at a concentration much lower than that for forming a liquid crystalline phase.  相似文献   
942.
在很高的温度和适宜的生长条件下,分别采用熔盐籽晶法和高温引上法生长了高质量的YAP、NAB、KTP、LN、BBO、SBN等多元氧化物单晶,它们具有优良的物理化学性能,严格的化学比,固定的组成与结构以及较好的化学均匀性和电子束轰击下的稳定性。广泛地用于激光和非线性光学领域。我们选用这些晶体为原材料研制电子探针定量分析的标准样品。经过测量和标定,这些单晶标样符合中华人民共和国国家标准GB 4930-85(电子探针显微分析标准样品通用技术条件)的规定。含有稀土元素的标样如NAB和YAP能发出绿色荧光,是电子显微术中理想的阴极发光材料。  相似文献   
943.
Circularly polarized luminescent(CPL) materials possess special dissymmetric optical property, i.e. luminescent light having different intensities for left (L) and right(R) circularly polarized components. Recently, these materials have been applied in colour-image projection, stereoscopic displays and light-emitting diodes(LEDS)1-7. We have synthesized a new porphyrin derivative, tetra-4[4'-(2-methylbutoxy)benzoyloxy] phenyl porphyrin [T(MBBP)P], by introducing a chiral group, which p…  相似文献   
944.
Indium dodecatungstosilicate of the composition [In(OH)⋅5H2O]2 [SiW12O40]⋅H2O is synthesized and studied by means of IR spectroscopy, thermogravimetry, and X-ray phase analysis. The crystals of this compound are triclinic, space group P1, a 13.079(3), b 13.795(3), c 13.967(3) Å, α 90.08(3)°, β 103.76(3)°, ψ107.76(3)°, Z 2, and πcalc 4.900 g cm−3.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 1, 2005, pp. 21–23.Original Russian Text Copyright © 2005 by Kaziev, Dutov, Quinones, Ita, Sychkin.  相似文献   
945.
以生物碱(咖啡因)为衍生化试剂,用浸渍法使其负载于活性炭(AC)上,制成一种新的金属阴离子吸附材料———负载咖啡因-活性炭(CAC)。用ICP-AES研究了该吸附材料对金属阴离子[Mo7O24]6-的吸附性能;考察了其化学稳定性;还研究了不同浓度硫酸和氨水对待测物吸附率和解脱率的影响及共存离子的干扰影响。  相似文献   
946.
Protein tyrosine phosphatase 1B (PTP1B) functions by removing the phosphoryl group from tyrosinephosphorylated proteins in insulin signaling and metabolism. The regeneration of the active site involves a sulphenylamide intermediate derived from the intrastrand cross-linking between the catalytic serine and the neighboring backbone nitrogen. Two mechanisms have been proposed for the formation of the sulphenylamide intermediate and the subsequent reactivation of the catalytic site. In the current work, the proposed mechanisms have been investigated by the use of density functional theory calculations. Our results suggest that these two mechanisms have similar overall energy barriers and that the preferred route will be determined by the availability of hydrogen peroxide or other oxidizing reagents.  相似文献   
947.
Summary. A highly efficient, selective, fast, and cheap protocol is developed for oxidation of aromatic amines and alcohols utilizing 34% hydrogen peroxide in water catalyzed by some W- and Mo-based heteropolyoxometalates. Findings showed that dodecatungstophosphoric acid, H3PW12O40, was the most efficient catalyst in the examined oxidation reactions. This methodology may prove to be a valuable alternative for eco-friendly green oxidation. Inherent simplicity, easy work up, and using regenerable catalysts were other key aspects of this oxidation protocol.  相似文献   
948.
Several aminoacylbenzylamines and their analogs were synthesized and analyzed by electrospray ionization mass spectrometry together with high-resolution and tandem mass spectrometric techniques. Fragment ions ([M + H - CH3NO](+)) were observed and attributed to a transfer of the benzyl group to the N-terminal amino group, leading to elimination of formamide. The proposed mechanism is supported by accurate mass measurements, and by experiments on deuterium labeling and variations of functional groups.  相似文献   
949.
采用化学溶液分解法(CSD)在p—Si(111)衬底上制备了(Bi0.9Nd0.1)2Ti2o7薄膜。用X射线衍射技术分析了薄膜的结构和结晶性,结果显示,在600℃退火10min得到了结晶性较好、表面致密的多晶薄膜;用原子力显微镜描述了薄膜的表面形貌,与在XRD中观察到的择优取向是一致的;同时还研究了薄膜的电学性能。结果表明,在0~6V范围内,薄膜的漏电流小于1.53×10^10A;在室温100kHz下,其介电常数为166,介电损耗因子为0.227,显示出薄膜具有较好的绝缘性和介电性能。  相似文献   
950.
Solid-phase microextraction (SPME) with a disposable ionic liquid (IL) coating was developed for headspace extraction of benzene, toluene, ethylbenzene, and xylenes (BTEX) in paints. The SPME fiber was coated with IL prior to every extraction, then the analytes were extracted and desorbed on the injection port of gas chromatography, and finally the IL coating on the fiber was washed out with solvents. The coating and washing out of IL from the fiber can be finished in a few minutes. This disposable IL-coated fiber was applied to determine BTEX in water-soluble paints with results in good agreement with that obtained by using commercially available SPME fibers. For all the four studied paints samples, the benzene contents were under the detection limits, but relatively high contents of toluene, ethylbenzene and xylenes (56-271 microg g(-1)) were detected with spiked recoveries in the range of 70-114%. Compared to the widely used commercially available SPME fibers, this proposed disposable IL-coated fiber has much lower cost per determination, comparable reproducibility (RSD < 11%), and no carryover between each determination. Considering that IL possess good extractability for various organic compounds and metals ions, and that task-specific IL can be designed and synthesized for selective extraction of target analytes, this disposable IL coating SPME might has great potential in sample preparation.  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