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   8628篇
  免费   308篇
  国内免费   6篇
化学   5404篇
晶体学   168篇
力学   173篇
数学   1531篇
物理学   1666篇
  2023年   50篇
  2022年   154篇
  2021年   153篇
  2020年   147篇
  2019年   185篇
  2018年   202篇
  2017年   245篇
  2016年   452篇
  2015年   346篇
  2014年   433篇
  2013年   770篇
  2012年   496篇
  2011年   516篇
  2010年   375篇
  2009年   308篇
  2008年   515篇
  2007年   451篇
  2006年   384篇
  2005年   312篇
  2004年   266篇
  2003年   200篇
  2002年   158篇
  2001年   118篇
  2000年   108篇
  1999年   66篇
  1998年   76篇
  1997年   51篇
  1996年   64篇
  1995年   59篇
  1994年   43篇
  1993年   33篇
  1992年   43篇
  1991年   61篇
  1990年   48篇
  1989年   56篇
  1988年   59篇
  1987年   51篇
  1986年   49篇
  1985年   103篇
  1984年   99篇
  1983年   68篇
  1982年   62篇
  1981年   69篇
  1980年   64篇
  1979年   53篇
  1978年   40篇
  1977年   34篇
  1976年   43篇
  1975年   38篇
  1974年   32篇
排序方式: 共有8942条查询结果,搜索用时 46 毫秒
141.
142.
The uniqueness and the global Markov property for the regular Gibbs measure corresponding to the interaction $$U_\Lambda (\varphi ): = \lambda \int\limits_\Lambda {d_2 x\int {d\varrho (\alpha ):e^{\alpha \varphi } :_0 (x)} } $$ [forλ>0,d?(α) a probability measure with support in \(( - 2\sqrt {\pi ,} 2\sqrt \pi )\) ] is proved.  相似文献   
143.
144.
Adsorption of water vapour from humid air by selected carbon adsorbents   总被引:2,自引:0,他引:2  
The water uptake by carbon molecular sieves (CMS) and graphitized carbons, all of which are used to determine volatile organic compounds in air, was investigated using a direct experimental approach. CMS, e.g. Carboxen 1002, Carboxen 1003 and Anasorb CMS adsorb substantial amounts of water, in the range 400 to 450 mg per gram of adsorbent. Graphitized carbons, e.g. Carbrogaph 5TD and Carbopack X show low water trapping, less than 30 mg g(-1) and Carbopack Y as little as 5 mg g(-1) or less. The water sorption capacity for graphitized carbons is strongly dependent on the relative humidity (RH). The change of RH from 95 to 90% decreases the amount of adsorbed water by more than a factor of 2. Two different water adsorption mechanisms are operative: adsorption on polar centers and micropore volume filling. For graphitized carbons and CMS at low RH, adsorption on polar centers is involved. For CMS, once the threshold value of relative humidity (RHth) is surpassed, micropore volume filling becomes predominant. RHth is 44 +/- 3 and 42 +/- 3% for Carboxen 1002 and 1003, respectively, and 32 +/- 3% for Anasorb CMS. The CMS mass in the trap was found not to affect the mass of retained water under condition of incomplete saturation of adsorbent bed with water. Thus, the restrictions commonly imposed on the CMS mass are not necessary. The dry purging technique is suggested to remove adsorbed water. Carbograph 5TD and Carbopack X require only a few hundred ml of dry air to remove adsorbed water entirely. Water can also be purged out from CMS; however, much larger volumes of dry air are needed.  相似文献   
145.
Coordination of sigma-aryl carbanions by chloroiron(II) 5,20-ditolyl-10,15-diphenyl-21-oxaporphyrin (ODTDPP)Fe(II)Cl has been followed by (1)H NMR spectroscopy. Addition of pentafluorophenyl Grignard reagent (C(6)F(5))MgBr to the toluene solution of (ODTDPP)Fe(II)Cl in the absence of dioxygen at 205 K resulted in the formation of the high-spin (ODTDPP)Fe(II)(C(6)F(5)). The titration of (ODTDPP)Fe(II)Cl with a solution of (C(6)H(5))MgBr carried at 205 K yields a rare six-coordinate species which binds two sigma-aryl ligands [(ODTDPP)Fe(II)(C(6)H(5))(2)](-). Warming of the [(ODTDPP)Fe(II)(C(6)H(5))(2)](-) solution above 270 K results in the decomposition to mono-sigma-phenyliron species (ODTDPP)Fe(II)(C(6)H(5)). Controlled oxidation of [(ODTDPP)Fe(II)(C(6)H(5))(2)](-) with Br(2) affords (ODTDPP)Fe(III)(C(6)H(5))Br, which demonstrates a typical (1)H NMR pattern of low-spin sigma-aryl iron(III) porphyrin. The considered oxidation mechanism involves the (ODTDPP)Fe(III)(C(6)H(5))(2) species, which is readily reduced to the iron(I) 21-oxaporphyrin, followed by oxidation with Br(2) and replacement of one bromide anion by aryl substituent. The (1)H NMR spectra of paramagnetic iron complexes have been examined in detail. Functional group assignments have been made with the use of selective deuteration. The peculiar (1)H NMR spectral features of [(ODTDPP)Fe(II)(p-CH(3)C(6)H(4))(2)](-) (sigma-p-tolyl: ortho, 30.8; meta, 53.6; para-CH(3), 42.1; furan: -16.0; beta-H pyrrole: -27.5, -34.3, -41.8 ppm, at 205 K) are without a parallel to any iron(II) porphyrin or heteroporphyrin and indicate a profound alteration of the electronic structure of iron(II) porphyrin upon the coordination of two sigma-aryls.  相似文献   
146.
The suitability of the differential thermal and thermogravimetric techniques for the determination of compositions of solid and soft drug formulations has been studied. A total of 117 pharmaceutical preparations have been examined, including powders, dusting powders, capsules, granulates, tablets, tablets for sucking, effervescent tablets, dragees, suppositories and ointments. Both techniques have been shown to be applicable for identification of pharmaceutical preparations. A specification has been made of thermal processes which can be employed for assaying the main components of the preparations. A rough estimate of the relative errors has been given.
Zusammenfassung Die Eignung der differentialthermoanalytischen und thermogravimetrischen Methoden zur Bestimmung der Zusammensetzung fester und weicher ArzeneiprÄparate wurde studiert. Eine Gesamtzahl von 117 pharmazeutischen PrÄparaten, einschliesslich Pulver, Streupulver, Kapseln, Granulate, Tabletten, Saugtabletten, Brausetabletten, Dragees, Suppositorien und Salben wurde untersucht. Beide Methoden erwiesen sich als anwendbar zur Identifizierung pharmazeutischer PrÄparate. Eine Spezifizierung der zur Bestimmung der Hauptkomponenten der PrÄparate geeigneten thermischen Prozesse wurde durchgeführt. Eine grobe SchÄtzung der relativen Fehler wurde gegeben.

Résumé On a étudié l'utilité des techniques ATD et TG pour déterminer la composition des préparations pharmaceutiques solides et molles. On a soumis à l'examen un total de 117 préparations pharmaceutiques comprenant des poudres, des talcs, des capsules, des granulats, des tablettes, des tablettes à sucer, des tablettes effervescentes, des dragées, des suppositoires et des onguents. Les deux techniques se sont montrées applicables à l'identification des préparations pharmaceutiques. On a effectué une spécification des processus thermiques qui se prÊtent au dosage des composants essentiels des préparations. On a donné une évaluation approximative des erreurs relatives.

- - . 117 , , - , , , , , , , . , - . , . - .
  相似文献   
147.
In terms of the Avrami equation, the effects of selected organic pigments on the kinetic parameters of the isothermal crystallization of coloured polyamide 6 were examined in the temperature range 468–478 K. It was found that B Blue and BB Red are active nucleating agents, which is acribed to their crystalline structure. G Orange slightly accelerates the crystallization of PA 6 at lower temperatures and concentrations, whereas in other conditions its effect is the opposite.The isothermal data were confirmed by the results of non-isothermal crystallization. The degree of supercooling corresponding to the peak of the DSC curve was the measured feature in this case.It was found that B Blue and BB Red reduce the free energy of formation of criticalsize nuclei by 20 and 12%, respectively, which is reflected in the rates of crystallization.
Zusammenfassung Mit Hilfe der Avrami Gleichung wurde der Einfluß einiger ausgewählter organischer Pigmente auf die kinetischen Parameter der isothermen Kristallisation gefärbter Polyamide 6 im Temperaturbereich 468–478 K untersucht. B Blue und BB Red erwiesen sich als aktive Keimbilder, was mit Hilfe ihrer kristallinen Struktur erklärt werden kann. Bei niedrigeren Temperaturen und geringeren Konzentrationen wird die Kristallisation von PA 6 durch G Orange leicht beschleunigt, unter anderen Bedingungen kann das Gegenteil beobachtet werden. Die isothermen Daten wurden durch die Ergebnisse bei nichtisothermer Kristallisation bekräftigt. In diesem Falle wurde das dem DSC Peak entsprechende Maß an Unterkühlung untersucht. Es wurde gefunden, daß B Blue und BB Red die freie Energie zur Bildung von Keimen kritischer Größe um 20 bzw. 12% herabsetzen, was sich in den Kristallisationsgeschwindigkeiten widerspiegelt.

, -6 468–478 . , , . , . . . , , , 20 12%, .


This work was supported by the Polish Academy of Science, goal 01.14.  相似文献   
148.
The kinetics of the oxidation of promazine and chlorpromazine by hexaimidazolcobalt(III) were studied in the presence of a large excess of cobalt(III) and H+ ions using u.v.–vis. spectroscopy ([CoIII] = (1–6) × 10−3 m, [ptz] = (2.5–10) × 10−5 m, [H+] = 0.05–0.8 m, I = 1.0 m (H+, Na+, Cl), T = 333–353 K, l = 1 cm). In each case, the reversible reaction leads to formation of cobalt(II) species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (kobs) on [CoIII] with a non-zero intercept was established for both phenothiazine derivatives. A marked difference in the observed reaction rate for promazine and chlorpromazine is associated with the difference in its ability to undergo oxidation and is consistent with a trend in the redox potential changes for these reductants. The activation parameters for reactions studied were determined. Mechanistic consequences of all the results are discussed.  相似文献   
149.
Small angle X-ray scattering and differential scanning calorimetry methods were employed to characterise the internal order of structural phases present in polyurethane coatings obtained as a result of water evaporation from anionomer dispersions. Those anionomers were produced in the reaction of aromatic, cycloaliphatic and aliphatic diisocyanates with polyoxyethylene glycol, 2,2-bis-(hydroxymethyl)propionic acid and 1,6-hexamethylenediamine. The decisive effects were found from ionic and polar structures within the rigid urethane and urea segments on the ordered arrangement degree of the supermolecular structures in the obtained anionomers. That becomes apparent in differential scanning calorimetry thermograms and contributes to improved thermal stability of the produced polyurethane coatings.  相似文献   
150.
The interaction energy between two hydrogen atoms in the D and D′ 1Πu states of the hydrogen molecule has been calculated for large internuclear distances (12 ? R ? 25 bohr). The variational method and a very flexible trial wave-function were used. The results indicate that for the states under consideration the Rayleigh-Schrödinger perturbation theory with the multipole expansion of the interaction hamiltonian gives reliable results only for R > 25 bohr i.e. in the region where the interaction energies are practically negligible.  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