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   6197篇
  免费   239篇
  国内免费   401篇
化学   4573篇
晶体学   93篇
力学   16篇
综合类   5篇
数学   8篇
物理学   2142篇
  2023年   153篇
  2022年   74篇
  2021年   73篇
  2020年   94篇
  2019年   85篇
  2018年   79篇
  2017年   72篇
  2016年   156篇
  2015年   155篇
  2014年   189篇
  2013年   511篇
  2012年   339篇
  2011年   262篇
  2010年   279篇
  2009年   358篇
  2008年   326篇
  2007年   421篇
  2006年   323篇
  2005年   332篇
  2004年   292篇
  2003年   320篇
  2002年   256篇
  2001年   158篇
  2000年   192篇
  1999年   185篇
  1998年   146篇
  1997年   121篇
  1996年   131篇
  1995年   143篇
  1994年   119篇
  1993年   73篇
  1992年   104篇
  1991年   40篇
  1990年   30篇
  1989年   27篇
  1988年   40篇
  1987年   26篇
  1986年   14篇
  1985年   14篇
  1984年   15篇
  1983年   8篇
  1981年   11篇
  1980年   19篇
  1979年   7篇
  1978年   10篇
  1977年   7篇
  1976年   11篇
  1975年   5篇
  1974年   5篇
  1973年   13篇
排序方式: 共有6837条查询结果,搜索用时 0 毫秒
61.
Continuous furnaces are commonly used for steel billet reheating before a rolling operation. It is necessary to perform a number of measurements to set-up and operate the optimization system of the furnaces correctly. A charge temperature measurement using infrared detectors can be one of the usable measurement techniques. This non-contact measurement method is based on the detection of infrared radiation emitted from a measured surface. The radiation intensity depends on the surface temperature and emissivity, which is one of the most important parameters for infrared measurements. Advantages of the non-contact temperature measurement, as well as some problems regarding the surface emissivity, are presented. The direct steel billet temperature measurement procedure using infra-red detectors, emissivity determination procedures, and example results are introduced. It is shown that steel emissivity can vary from approx. 0.17 to 0.8, depending on the surface state, scale formation, and wavelength interval. These problems are critical for the charge temperature measurement using the infra-red detectors, and are discussed in this paper.  相似文献   
62.
Three new carbohydrates were isolated from the acidic hydrolysis part of the ethyl acetate extract of Cynanchum otophyllum Schneid (Asclepiadaceae) and one new carbohydrate from the ethyl acetate extract of Cynanchum paniculatum Kitagawa. Their structures were determined as methyl 2,6-dideoxy-3-O-methyl-α-D-arabino-hexopyranosyl-(1 → 4)-2,6-deoxy-3-O-methyl-β-D-arabino-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-α-D-arabino-hexopyranoside (1), ethyl 2,6-dideoxy-3-O-methyl-β-D-ribo-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-α-l-lyxo-hexopyranoside (2), met hyl 2,6-dideoxy-3-O-methyl-α-l-ribo-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-β-D-lyxo-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-α-D-arabino-hexopyranoside (3), and 2,6-dideoxy-3-O-methyl-β-D-ribo-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-α-d-arabino-hexopyranosyl-(1 → 4)-2,6-dideoxy-3-O-methyl-α -d-arabino-hexopyranose (4), respectively, by spectral methods.  相似文献   
63.
可见/近红外光谱漫透射技术检测西瓜坚实度的研究   总被引:3,自引:3,他引:3  
西瓜是一种广受世界各国消费者喜爱的水果,坚实度是西瓜的一个重要品质指标,文章利用可见/近红外漫透射光谱技术进行了西瓜坚实度(FM)的无损检测研究。采用偏最小二乘法(PLS)和主成分回归法(PCR)建立了FM与漫透射光谱的无损检测数学模型,对比分析了不同光谱预处理方法(原始光谱%T,一阶微分处理光谱D1(%T ),二阶微分处理光谱D2(%T )以及光谱的Savitsky-Golay法滤波)对模型预测性能的影响。根据模型相关系数(r)及预测平方根标准偏差(RMSEP)进行了不同模型的预测性能对比,结果表明:光谱经二阶微分处理并使用Savitsky-Golay法滤波后,采用PLS法可以得到最好的FM建模结果(r=0.974,RMSEP=0.589 N)。研究表明:应用可见/近红外漫透射光谱技术检测西瓜的坚实度是可行的,为今后快速无损评价大果形厚果皮类水果坚实度提供了理论依据。  相似文献   
64.
The synthesis of vanadyl phosphate by reaction of an alcoholic solution of V2O5 and o-H3PO4 has been studied. The solids obtained were investigated by various physico-chemical techniques as in situ X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential thermal analysis (DTA) under inert atmosphere. The compounds have been further characterized by infrared and UV–Visible spectroscopies. The isopropanol conversion is carried out to evaluate the catalytic activity of the samples. The isopropanol conversion increases with the reaction temperature from 433 to 503 K for vanadyl phosphorus oxides calcined at a different temperature. However, the conversion attains 100% at 503 K, and the solid calcined at 973 K exhibits high selectivity towards propene (100% at 503 K).  相似文献   
65.

Silver(I) complexes of selenones, [LAgNO3] and [AgL2]NO3 (where L is imidazolidine-2-selenone or diazinane-2-selenone and their derivatives) have been prepared and characterized by elemental analysis, IR and NMR (1H, 13C and 107Ag) spectroscopy. An upfield shift in the C=Se resonance of selenones in 13C NMR and a downfield shift in N-H resonance in 1H NMR are consistent with selenium coordination to silver(I). In 107Ag NMR, the AgNO3signal is deshielded by 450-650 ppm on coordination to selenones. Greater upfield shifts in 13C NMR were observed for [LAgNO3] compared to [AgL2]NO3complexes, whereas the opposite trend was observed for 1H and107Ag NMR chemical shifts.  相似文献   
66.
Seven new 1-fluorinated-4-phosphopyrazoles 2 have been prepared in 60–94% yield by cyclization of N-(2′,2′,2′-trifluoroethyl) phosphonyl β-hydrazones 1 with triethyl orthoformate in the presence of acetic acid. The IR data, the 1H, 13C, 19F, 31P chemical shifts and coupling constants of the compounds 2 are reported.  相似文献   
67.
Six new substituted diphenyltin(IV) O,O′-alkylene dithiophosphates, (C6H5)2Sn(X)S(S) POGO [G = —CH2C(CH3)2CH2—, X = Cl (1), SCN (3), ClO4 (5); G = —CH2C (C4H9)(C2H5)CH2—, X = Cl (2), SCN (4), ClO4 (6)], were synthesized by the reaction of the corresponding ammonium salts of the O,O’-alkylene dithiophosphates with an appropriate organotin(IV) chloride. The compounds were characterized on the basis of elemental and spectral analyses (ESI mass spectrometry, IR, 1H, 13C, 31P, and 119Sn NMR). The presence of a four-coordinated Sn atom and monodentate O,O’-alkylene dithiophosphate moiety in compounds 1–4 as well as bidentate O,O’-alkylene dithiophosphate unit in compounds 5,6 is established.  相似文献   
68.
In condensed phases, a highly symmetric gas‐phase molecule lowers its symmetry under perturbation of the solvent, which is vital to a variety of structural chemistry related processes. However, the dynamical aspects of solvent‐mediated symmetry‐breaking events remain largely unknown. Herein, direct evidence for two types of solvent‐mediated symmetry‐breaking events that coexist on the picosecond timescale in a highly symmetric anion, namely, hexacyanocobaltate, is presented: 1) an equilibrium symmetry‐breaking event in which a solvent‐bound species having lowered symmetry undergoes a population exchange reaction with the symmetry‐retaining species; 2) a dynamic symmetry‐breaking event that is composed of many dynamic population‐exchange reactions under fluctuating solvent interactions. Ultrafast two‐dimensional infrared spectroscopy is used to simultaneously observe and dynamically characterize these two events. This work opens a new window into molecular symmetry and structural dynamics under equilibrium and non‐equilibrium conditions.  相似文献   
69.
70.
Raman and IR spectroscopy were used to investigate the changes induced in yak hair keratin by the straightening treatment based on glyoxylic acid. The amino acidic residues that appeared involved in the reaction with glyoxylic acid were serine and lysine; the involvement of the latter was deduced by the spectroscopic detection of iminic species, resulting from the reaction between the aminic group of lysine and the carbonyl group of glyoxylic acid. The reaction with glyoxylic acid induced conformational rearrangements that mainly involved the fibre bulk rather than the cuticle. Changes in the average tyrosine environment and its hydrogen‐bonding state were detected: at increasing glyoxylic acid incorporation, the tyrosine residues appeared more exposed, probably because of H‐bond interactions with the COOH group. The distribution of the disulfide bridge conformation was also affected, although no cleavage of the S–S bond was detected, in agreement with the shiny and healthy appearance of the fibres. Copyright © 2014 John Wiley & Sons, Ltd.  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