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   72279篇
  免费   20738篇
  国内免费   3476篇
化学   76903篇
晶体学   350篇
力学   3698篇
综合类   258篇
数学   5532篇
物理学   9752篇
  2024年   519篇
  2023年   4990篇
  2022年   2443篇
  2021年   3584篇
  2020年   6005篇
  2019年   3538篇
  2018年   3330篇
  2017年   1476篇
  2016年   7114篇
  2015年   7190篇
  2014年   6803篇
  2013年   7295篇
  2012年   5457篇
  2011年   3148篇
  2010年   5177篇
  2009年   5167篇
  2008年   2749篇
  2007年   2159篇
  2006年   1399篇
  2005年   1172篇
  2004年   972篇
  2003年   752篇
  2002年   774篇
  2001年   662篇
  2000年   516篇
  1999年   462篇
  1998年   327篇
  1997年   352篇
  1996年   331篇
  1995年   400篇
  1994年   265篇
  1993年   405篇
  1992年   253篇
  1991年   207篇
  1988年   195篇
  1987年   159篇
  1981年   178篇
  1980年   211篇
  1979年   215篇
  1978年   222篇
  1977年   338篇
  1976年   395篇
  1975年   483篇
  1974年   504篇
  1973年   324篇
  1972年   456篇
  1971年   420篇
  1970年   625篇
  1969年   460篇
  1968年   502篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
131.
Summary The depressions in freezing point measurements in quaternary solubilized solutions or micro-emulsion in sodium oleate + phenol + benzene + water are measured. Maxima and minima were observed, Gibbs' phase rule was applicable, eutectic points were detected, congruent melting points were obtained, loose combination of molecules or clusters, were formed, phases of the system transferred and finally unstable emulsion separated out. This is contrary to the views ofMcBain who called these solutions as thermodynamically stable. The results were compared with the conductivity, stability and heat of solubilization-concentration measurements.
Zusammenfassung Es wird die Gefrierpunktserniedrigung von quaternären solubilisierten Lösungen im System Natriumoleat-Phenol-Benzol-Wasser gemessen. Die verschiedenen Befunde (Maxima und Minima des Gefrierpunktes in Abhängigkeit von der Zusammensetzung, Anwendbarkeit der Gibbs'schen Phasenregel, Auftreten von Eutektika, kongruente Schmelzpunkte etc.) dienen zum Beweis, daß diese Lösungen thermodynamisch instabil sind.


With 5 figures and 1 table  相似文献   
132.
Summary A thermodynamic treatment of homo-polymer systems out of linear chains with folded chain crystals is developed outgoing from appropriate models for single component systems. An expansion of thermodynamics to multi-micro-phase systems the structure of which is partially or totaly frozen is indispensable. General properties of melt crystallized homopolymers with folded chain crystals can be recognized indeed when the thermodynamic formalisms developed are applied.
Zusammenfassung Das Schmelzen in polymeren Einteilchensystemen mit Faltungskristallen einheitlicher Dicke kann thermodynamisch als Umwandlung 1. Ordnung in einer Richtung behandelt werden, wenn die Faltungslänge bis zur Umwandlungstemperatur konstant bleibt (Faltungslänge als innerer Zusatzparameter). Eine wesentliche begriffliche Erweiterung ist für eine phänomenologische Beschreibung mit den Mitteln der Thermodynamik unumgänglich, wenn eine Faltungskristallit-Dickenverteilung existiert, weil dann prinzipiell nur noch partielle Koexistenz bestimmter Fraktionen metastabiler autonomer Mikrophasen mit der Schmelze möglich ist. Partielles Aufschmelzen und Rektistallisation können so dann auch in Betracht genommen werden. Die entwickelten Konzeptionen bewähren sich in der Anwendung auf bekannte Experimente.

Notation g c (y);g m (Y) molar Gibbs-free energy of a chain of a lengthy within an extended chain crystal and the melt rsp - g o c ;g o m molar free enthalpy of the unit in the crystal lattice and the melt rsp - g(y,y, f) molar Gibbs-function of an ideally folded chain crystal with the fold heighty f - gco(y, y ef,y f) molar free enthalpy of the crystal corey co - g 0 ex ((yef) excess free enthalpy of the longitudinal layers of folded chain crystals - g f(yef,g o ex ) molar free enthalpy of the longitudinal layers of the folded chain crystals - g tot molar free enthalpy of a chain of the lengthy within a folded chain crystal with longitudinal layers - h o 1c ,h o m molar enthalpy of the chain unit within the crystal lattice and the melt rsp - h =h o m -h o c molar heat of fusion of the unit - C p=C p m -C p c difference of the molar specific heat of a unit within the melt and within the chain crystal - h D molar defect enthalpy of local defects within the crystal lattice - h D molar defect enthalpy of the unit - s o c ,s o m molar entropy of the chain unit within the crystal lattice and the melt rsp - s c m conformational entropy of a chain in the melt - s gk conformational entropy of a chain of lengthy within a super-lattice as indicated in figure 5, - s molar entropy of fusion of the melt - s n c nematic configurational entropy - T absolute temperature - T M melting temperature of extended chain crystals of infinite size - T M(y) melting temperature of extended chain crystals containing only chains of the lengthy - T M (y, y f) melting temperatureof folded chain crystals of the thicknessy f composed of chains of the lengthy - T M(y f) melting temperature of folded chain crystals of the thicknessy fy - eh excess free enthalpy of the chain ends occupying crystallographic places - ef excess free enthalpy of a single fold loop - z coordination number of the lattice - 7 Euler's constant - R Boltzmann's constant - y number of chain units - y f height of lamelliform folded chain crystals - f=(y/y f - 1) number of fold loops of a chain of a lengthy when being built into a folded chain crystal of the thicknessy f - y co thickness of the crystal core of the simplified twophase model - y et average thickness of the surface layers of folded chain crystals - N c number of crystallized units of a chain of the lengthy - x c molar number of crystallized units of a chain of the lengthy - x nc molar number of noncrystallized units - excess free enthalpy parameter - (y f) thickness distribution of the fold heightsy f With 15 figures and 2 tables  相似文献   
133.
The title compounds7 are formed in a general reaction by heating β-isothiocyanoketones3 with primary amines in inert solvents, or by thermal elimination of water from tetrahydro-6-hydroxy-6-methyl-2(1H)-pyrimidinethiones5, also in inert solvents. The 1-alkyl compounds can also be prepared under similar conditions from α,β-unsaturated ketones by reaction with alkylammonium rhodanides. The NMR-spectra show that the 1-substituted dihydro-6-methyl-2(1H)-pyrimidinethiones are in tautomeric equilibrium with the tetrahydro-6-methylene-2(1H)-pyrimidinethiones13. The reactivity of 1-alkyl and 1-aryldihydro-6-methyl-2(1H)-pyrimidinethiones is similar to that of dihydro-4,4,6-trimethyl-2(1H)-pyrimidinethione7 j, although their ring stability is certainly less.  相似文献   
134.
According to spectroscopic (IR, broadline proton NMR) and chemical (alkylation) investigations of disilver sulphamide the following molecular structure is assumed: $$\begin{gathered} O \hfill \\ || \hfill \\ H_2 N\_\_S\_\_NAg| \hfill \\ OAg \hfill \\ \end{gathered}$$ From the IR and NMR data deduction concerning the nature of the chemical bonds in this compound is possible. The instability of the still unknown mono-and trisilver sulphamide is discussed with regard to the structure of disilver sulphamide.  相似文献   
135.
The annealing induced morphological changes in isotropic block copoly(etherester)s are investigated by small angle X-ray scattering methods. The observed results are consistent with a lamellar model which is comprised of a crystalline core of thickness 35 to 45 Å, a diffuse boundary zone of width 9 Å, and an amorphous layer which is varying strongly in thickness. The enhanced crystallinity of the samples annealed at higher temperatures is mainly due the lateral growth of the lamellae; the long period increases by not more than 20%. The scattering power of the samples is explained by means of an equivalent two-phase model in which the crystalline phase has the same structure as theα-form of poly(butylene terephthalate), whereas the amorphous phase is a mixture of the uncrystallized ester segments and the ether segments. The crystallinity determined from the scattering power is very much higher than that determined from thermoanalytical investigations.  相似文献   
136.
The crystal structure of Ru2Si3 has been refined by leastsquares using three-dimensional X-ray data from a twinned crystal (1355 reflections,R=5.8%). Ru2Si3 is isostructural with Ru2Ge3 and like this compound a member of the Mn11Si19 structure family. The lattice parameters of the orthorhombic unit cell (Pnca—D 14 2h ) are:a=5.530 (1),b=11.060 (2) andc=8.952 (2) Å.

Mit 1 Abbildung  相似文献   
137.
It is well known that in the ergot alkaloids of the peptide type the proline appears in thel-configuration. Different methods of acid hydrolysis may lead to various cleavage products; hydrolysis by HCl yieldsd-proline, on the other hand hydrolytic cleavage by means of strongly acid cation exchange resin preserved the orginall-configuration.

Teilveröffentlichung der Dissertation vonH. Kolassa, Univ. Wien, 1973.  相似文献   
138.
The structure of squaric acid derivatives was investigated on the basis of the PPP—SCF—LCAO—MO—CI method utilizing the π-electron densities and bond orders. The parameters used in the calculations were modified to fit the experimental data for representative compounds. The data were obtained by X-ray photoelectron spectrometry and electron absorption spectroscopy (including the polarization of the bands as determined by liquid crystal induced circular dichroism measurements).  相似文献   
139.
Zusammenfassung Acenaphthylen wurde durch Anpolymerisation und zweifache Hochvakuumsublimation gereinigt. Die Reinigungsmethode wird ausführlich beschrieben.Bei verschiedenen Temperaturen wurde mit einer dilatometrischen Methode die thermische Bruttogeschwindigkeit gemessen. Zwischen 95 und 105 °C giltk br=7,08·1010 exp (– 30,9 kcal/RT).Die beträchtliche Aktivierungsenergie der Bruttoreaktion ist sowohl auf eine große Aktivierungsenergie der thermischen Startreaktion als auch auf eine relativ hohe Wachstumsaktivierungsenergie zurückzuführen.Das Polymerisationsverhalten von Acenaphthylen kann vermutlich aufgrund seiner sterischen Eigenschaften erklärt werden.
Summary Acenaphthylene was purified by partial polymerisation, followed by double stage high vacuum sublimation. The method of purification is described in detail.The thermal overall rate was measured at different temperatures by a dilatometric method. Between 95 and 105 °C the following relation holds:k th=7,08 · 1010 exp (– 30,9 kcal/RT).The considerable energy of activation of the overall reaction depends on a relatively high energy of activation of both the thermal initiation and the propagation reaction.It is supposed, that the polymerisation behaviour of acenaphthylene depends mainly on its steric properties.
  相似文献   
140.
IntroductionNano-sized semiconductors are of great interestdue to their special optical and electronic proper-ties[1,2].As one of the mostimportantⅡ—Ⅵsemicon-ductors,CdS has importantapplications in many fields,such as the solar cell field,non linear o…  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