全文获取类型
收费全文 | 86587篇 |
免费 | 594篇 |
国内免费 | 416篇 |
专业分类
化学 | 29479篇 |
晶体学 | 864篇 |
力学 | 6862篇 |
数学 | 33704篇 |
物理学 | 16688篇 |
出版年
2023年 | 56篇 |
2022年 | 91篇 |
2021年 | 122篇 |
2020年 | 129篇 |
2019年 | 129篇 |
2018年 | 10542篇 |
2017年 | 10365篇 |
2016年 | 6290篇 |
2015年 | 1086篇 |
2014年 | 583篇 |
2013年 | 871篇 |
2012年 | 4274篇 |
2011年 | 10973篇 |
2010年 | 5983篇 |
2009年 | 6363篇 |
2008年 | 7010篇 |
2007年 | 9181篇 |
2006年 | 667篇 |
2005年 | 1685篇 |
2004年 | 1869篇 |
2003年 | 2222篇 |
2002年 | 1266篇 |
2001年 | 395篇 |
2000年 | 456篇 |
1999年 | 278篇 |
1998年 | 300篇 |
1997年 | 237篇 |
1996年 | 300篇 |
1995年 | 179篇 |
1994年 | 156篇 |
1993年 | 166篇 |
1992年 | 131篇 |
1991年 | 140篇 |
1990年 | 116篇 |
1989年 | 137篇 |
1988年 | 120篇 |
1987年 | 118篇 |
1986年 | 116篇 |
1985年 | 104篇 |
1984年 | 107篇 |
1983年 | 83篇 |
1982年 | 93篇 |
1981年 | 102篇 |
1980年 | 78篇 |
1979年 | 99篇 |
1978年 | 87篇 |
1977年 | 61篇 |
1976年 | 74篇 |
1974年 | 55篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
951.
Su-Hong Ge Xin-Lu Cheng Xin-Xing Wang Guang-xing Dong Gui-hua Sun 《Structural chemistry》2007,18(6):985-991
Some explosives are stable molecules with large energy barriers to chemical reaction, and in shock or impact initiation, a
sizable amount of phonon energy must be converted to the molecular internal higher vibrations by multiphonon up pumping. To
investigate the relationship between impact sensitivities and energy transfer rates, the number of doorway modes of explosive
molecules is estimated by a simple theory in which the rate is proportional to the number of normal mode vibrations. We evaluated
frequencies of normal mode vibrations of 13 explosive molecules which are CHNO nitramine-contained and have not been analyzed
previously. The number of doorway modes in the regions of 200–700 cm−1 was evaluated by the direct counting method. For more clear investigation of the relationship we have classified these 13
nitramine explosive molecules, by the number of nitramine group they contained, into two groups. There are eight molecules
that contained one nitramine group and five molecules that contained poly-nitramine groups. It is found that the number of
doorway modes shows a linearly correlation to the impact sensitivities derived from drop hammer tests. This result is in agreement
with that of several previous works. Besides, it is also noted in our study that in those nitramine explosives molecules with
similar molecular structure (similar number nitramine group they contained) and similar molecular weight, the correlation
between the sensitivity and the number of doorway modes is higher. We found that the vibrational frequency of ω corresponds
to nitro group motions of every molecule is contributed to the number of doorway modes in the regions of 200–700 cm−1. 相似文献
952.
Zusammenfassung Die Oxydation von Tiron durch H2O2 in basischem Milieu wird durch Kobalt stark katalysiert. Die katalysierten Vorgänge sind pH-abhängig. Im pH-Bereich 9 bis 11 entsteht ein rotes Oxydationsprodukt unbekannter Struktur; wahrscheinlich handelt es sich um ein o-Chinonderivat. Mit Hilfe der katalytischen Reaktion können 10–4
g Kobalt/5 ml nachgewiesen werden. In diesem pH-Bereiche stabilisiert Tiron H2O2 bei 100° C, bei 40° C jedoch katalysiert es — in Abhängigkeit von der Konzentration des Tirons — den Zerfall des H2O2. Der Effekt spielt im Mechanismus der katalysierten Reaktion wahrscheinlich eine Rolle, doch können die Zusammenhänge an Hand der bisher veröffentlichten Angaben nicht klar gedeutet werden. Der die Reaktion begleitende bzw. ihr folgende Nebenvorgang kann auf den katalytischen Zerfall des H2O2 · OOH-Komplexes zurückgeführt werden. Zwischen pH 7 und 9 entsteht bei der katalysierten Reaktion Semichinon, über pH 11 oxydiert Tiron das Semichinon unter Ringspaltung und Abspaltung von 1 Mol Sulfat zu Verbindungen, die Säurecharakter tragen.
Summary The oxidation of Tiron by H2O2 in basic milieu is strongly catalyzed by cobalt. The catalyzed reactions are pH-dependent. A red oxidation product of unknown structure results in the pH range 9 to 11; probably it is ano-quinone product. As little as 10–4 g cobalt/5ml can be detected by this catalytic action. In this pH region, Tiron stabilizes H2O2 at 100° C, but at 45° C it catalyzes the decomposition of H2O2, in relation to the concentration of the Tiron. The effect probably plays a rôle in the mechanism of the catalyzed reaction, but the up to now published informations are not sufficient to clarify the relationships. The auxiliary process, accompanying or following the reaction, can be attributed to the catalytic decomposition of the H2O2-OOH– complex. Semiquinone is produced by the catalytic reaction between pH 7 and 9, above pH 11, the Tiron oxidizes the semiquinone with opening of the ring and splitting off of 1 mol of sulfate yielding compounds which have an acidic character.
Résumé L'oxydation du Tiron par H2O2 en milieu basique est fortement catalysée par le cobalt. Les processus de catalyse dépendent du pH. Dans le domaine de pH de 9 à 11, il apparaît un produit d'oxydation rouge, de structure inconnue; il s'agit probablement d'un dérivé de l'o-quinone. La réaction catalytique permet de déceler 10–4 g cobalt/5 ml. Dans ce domaine de pH, à 100° C le Tiron stabilise l'eau oxygénée et à 45° C, suivant sa concentration, il catalyse la décomposition de l'eau oxygénée. L'effet joue probablement un rôle sur le mécanisme de la réaction catalysée; on ne peut pourtant pas interpréter clairement ces relations au moyen des indications publiées jusqu'ici. Le processus secondaire qui accompagne la réaction ou qui la suit, peut être ramené à la décomposition catalytique du complexe H2O2-OOH–. Entre pH 7 et 9, la réaction catalysée donne naissance à une semiquinone; au-dessus de pH 11, le Tiron oxyde la semi-quinone avec ouverture du cycle et élimination d'une mole de sulfate ce qui donne ensuite des composés qui portent le caractère acide.相似文献
953.
Zaharescu M. Jitianu A. Brăileanu A. Madarász J. Novák CS. Pokol G. 《Journal of Thermal Analysis and Calorimetry》2003,71(2):421-428
Hybrid materials with different amounts of organics permanently bound on the inorganic network obtained in the TEOS-MTEOS
(tetraethoxysilan-methyltriethoxysilan) system are used for obtaining coatings with different optical and mechanical properties.
To study the thermal stability of the mentioned materials, compositions with different molar ratios of the precursors were
prepared. The influence of the solvent and water amounts on the gelation process was also investigated. The gels obtained
were characterised by IR spectrometry and their decomposition temperatures were determined by DTA/TG. Thermal stability of
the gels is rather influenced by their composition than the conditions of the gelation process.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
954.
T. Kántor L. Bezúr J. Sztatisz E. Pungor 《Journal of Thermal Analysis and Calorimetry》1981,22(2):179-197
An atomic absorption spectrophotometer is coupled to a conventional thermoanalytical quartz furnace as used for TG and DTG to detect the thermally evolved products. In this combined system, the dry aerosol (smoke) obtained by cooling the vapour evolved is transported from the furnace to the flame for metal-specific atomic absorption detection. The particular design of the furnace outlet promotes the formation of stable aerosols. Optimum experimental conditions were determined, using zinc chloride solution, by varying the specimen mass, the heating rate and the flow rate of the furnace gas at a linear temperature program. The absorbancevs. temperature curves obtained with this method for various zinc compounds are compared with the corresponding DTG curves. The applicability of the technique for studying heterogeneous reactions with carbon tetrachloride and hexane vapours is presented. The utilization of an atomic absorption spectrophotometer equipped with a quartz cuvette for detecting the thermal evolution of mercury vapours is described, as well as detection potentials by molecular absorption (for NO and NH3) and light scattering (for smoke evolved from organic matter). The results obtained with the suggested methods may, in some respects, valuably complement the results achieved with DTG and with flame ionization detection.
Presented in part at the 24th Hungarian Conference on Analytical Sptecroscopy, Miskolc, June 15–18, 1981. Abstracts pp. 159–162.
The authors wish to express their thanks to S. Gál for his assistance in the present application of the temperature programmer developed by him and his group, and for the valuable discussions on the subject. Thanks are also due to K. Tomor and J. Kmives who participated in the comparative thermoanalytical measurements and their analysis. 相似文献
Zusammenfassung Ein Atomabsorptionsspektrometer wurde mit einem konventionellen thermoanalytischen Quarzofen gekoppelt um thermische Abspaltprodukte nachzuweisen. In diesem kombinierten System wird das durch Kühlung des entwickelten Dampfes erhaltene Aerosol (Rauch) vom Ofen in die Flamme für den metallspezifischen Atomabsorptionsnachweis übergeleitet. Die spezielle Ausbildung der Austrittsöffnung gewährleistet die Bildung eines stabilen Aerosols. Die optimalen Versuchsbedingungen wurden durch Zinkchloridlösungen bei Anderung der Probenmasse, der Aufheizgeschwindigkeit und der Strömungsgeschwindigkeit des Ofengases im linearen Temperaturprogramm ermittelt. Die mit dieser Methode für verschiedene Zinkverbindungen erhaltenen Absorptions—Temperatur-Kurven wurden mit den entsprechenden DTG-Kurven verglichen. Die Anwendbarkeit dieser Technik bei dem Studium heterogener Reaktionen mit Kohlenstofftetrachlorid und Hexandämpfen wird gezeigt. Der Einsatz eines mit einer Quarzküvette zum Nachweis der thermischen Entwicklung von Quecksilberdampf versehenen Atomabsorptionsspektrometers wird beschrieben, sowie die Nachweisgrenze durch molekulare Absorption (für NO und NH3) und Lichtstreuung (für aus organischem Material entwickelten Rauch). Die bei den beschriebenen Methoden erhaltenen Ergebnisse können, in mancher Hinsicht, die durch DTG und Flammenionisationsnachweis erhaltenen Ergebnisse wertvoll ergänzen.
- , , . (), , . . , . — , , -. . - , , , ( NO NH3) ( , ). , , , - .
Presented in part at the 24th Hungarian Conference on Analytical Sptecroscopy, Miskolc, June 15–18, 1981. Abstracts pp. 159–162.
The authors wish to express their thanks to S. Gál for his assistance in the present application of the temperature programmer developed by him and his group, and for the valuable discussions on the subject. Thanks are also due to K. Tomor and J. Kmives who participated in the comparative thermoanalytical measurements and their analysis. 相似文献
955.
The inducing method for preparing Ag-micelle solution with the use of mixed solvent/nonsolvent, and the morphological characterization of the generated metal–micelles were investigated and reported in this paper. In this method, an Ag containing metal chelate polymer (MCP) raw solution was preprepared by dissolving poly(vinyl acetate) (PVAc)–silver nitrate (AgNO3) MCP in conc. formic acid, and a mixed solvent of HCOOH/H2O with specific water composition was then added to induce the micellization of the MCP chain. The critical water concentration (CWC) that was needed for inducing the formation of the Ag-micelles, and the water concentration at which the flocculation of the Ag-micelles occurred in micellar solution, were studied by measuring the transmittance of the dilute MCP solution; the results showed that a long-lasting MCP solution with stable micelles might be prepared by using a H2O/HCOOH solvent of specific weight ratio 1:1.2. The effect of the AgNO3 concentration on the morphology of the Ag-micelles was also investigated by transmission electron microscopy (TEM). At AgNO3 concentration below 0.5 wt%, the Ag-micelles displayed a variety of core-shell structure; but as the AgNO3 concentration was increased to 1.0–2.0 wt%, micelles that had Ag-solid embedded in the micellar core were observed. 相似文献
956.
Thorsteinn Loftsson Ína B. Össurardóttir Thorsteinn Thorsteinsson Matt Duan Már Másson 《Journal of inclusion phenomena and macrocyclic chemistry》2005,52(1-2):109-117
The natural β-cyclodextrin (βCD) and its complexes have limited solubility in aqueous solutions. This low aqueous solubility, as well as low aqueous solubility of the guest molecule (i.e. triclosan or triclocarban (TCC)), can result in low complexation efficiency (CE). The purpose of this study was to enhance the apparent intrinsic solubility (S
0) of the guest molecule and its βCD complexes through ionization and addition of auxiliary compounds such as polymers, amino acids and metal ions. Both triclosan (pK
a
7.9) and TCC (pK
a
12.7) are weak acids. Addition of ethanol to the complexation medium enhanced S
0 of both triclosan and TCC but at the same time ethanol lowered the stability constant (K
c
) of their βCD complexes resulting in overall lowering of CE. Addition of small amount of water-soluble polymers enhanced the βCD solubilization of both guests, and addition lysine enhanced the solubilization of TCC. Ionization of triclosan resulted in significant enhancement of CE and enhanced triclosan release from tablets containing triclosan/βCD complex. The effect of ionization was not as pronounced in the case of TCC.This revised version was published online in July 2005 with a corrected issue number. 相似文献
957.
Stroch M Cajánek M Kalina J Spunda V 《Journal of photochemistry and photobiology. B, Biology》2004,75(1-2):41-50
Acclimation of the photosynthetic apparatus of chlorophyll b-less barley mutant chlorina f2 to low light (100 micromolm(-2)s(-1); LL) and extremely high light level (1000 micromolm(-2)s(-1); HL) was examined using techniques of pigment analysis and chlorophyll a fluorescence measurements at room temperature and at 77 K. The absence of chlorophyll b in LL-grown chlorina f2 resulted in the reduction of functional antenna size of both photosystem II (by 67%) and photosystem I (by 21%). Chlorophyll fluorescence characteristics of the LL-grown mutant indicated no impairment of the utilization of absorbed light energy in photosystem II photochemistry. Thermal dissipation of excitation energy estimated as non-photochemical quenching of minimal fluorescence (SV(0)) was significantly higher as compared to the wild-type barley grown under LL. Despite impaired assembly of pigment-protein complexes, chlorina f2 was able to efficiently acclimate to HL. In comparison with chlorina f2 grown under LL, HL-grown chlorina f2 was characterized by unaffected maximal photochemical efficiency of photosystem II (F(V)/F(M), doubled content of both beta-carotene and the xanthophyll cycle pigments and considerably reduced efficiency of excitation energy transfer from carotenoids to chlorophyll a. The enormous xanthophyll cycle pool size was however associated with reduced SV(0) capacity. We suggest that the substantial part of the xanthophyll cycle pigments is not bound to the remaining pigment-protein complexes and acts as filter for excitation energy, thereby contributing to the efficient photoprotection of chlorina f2 grown under HL. 相似文献
958.
Binary excess molar volumes, V
m
E, have been evaluated from density measurements, using a vibrating tube densimeter over the entire composition range for binary
liquid mixtures of ionic liquids 1-ethyl-3-methyl-imidazolium diethyleneglycol monomethylethersulphate [EMIM]+[CH3(OCH2CH2)2OSO3]− or 1-butyl-3-methyl-imidazolium diethyleneglycol monomethylethersulphate [BMIM]+[CH3(OCH2CH2)2OSO3]− or 1-methyl-3-octyl-imidazolium diethyleneglycol monomethylethersulphate [MOIM]+[CH3(OCH2CH2)2OSO3]−+methanol and [EMIM]+[CH3(OCH2CH2)2OSO3]−+water at 298.15, 303.15 and 313.15 K. The V
m
E values were found to be negative for all systems studied. The V
m
E results are explained in terms of intermolecular interactions and packing effects. The experimental data were fitted by the
Redlich-Kister polynomial. 相似文献
959.
Coordination reactions of copper(II) ions and their effect on non-covalent interactions in uridine (Urd) or uridine 5′-monophosphate (UMP) systems with nucleosides (Ado, Cyd, Thd) and nucleotides (AMP and CMP) in aqueous solutions have been studied. At high pH the effective coordination centers are deprotonated N(3) atoms from Urd and Thd, whereas at low pH, the N(3) atoms of pyrimidine nucleosides are blocked for coordination and the metallation sites are endocyclic
nitrogen atoms from Ado, Cyd, AMP and CMP. Moreover, at low pH, the main reaction center in nucleotide solutions is the phosphate group. The NMR study has proven the
occurrence of non-covalent ion-dipole interactions and stacking interactions in the systems considered. Introduction of a
copper ion in the majority of systems causes the disappearance of weak interactions between ligands. The structures of the
complexes in solution have been inferred from the equilibrium study: an analysis of the pH range of their occurrence with
respect to the pH range of deprotonation of particular groups in the compounds studied, using Vis, EPR and 13C as well as 31P NMR spectral analysis. 相似文献
960.
We present a method for computing classical Newtonian trajectories that minimize the path length or transit time from reactant
to product. Our approach is based on a generalization of the fast-marching method, which allows us to construct the solution
of the Hamilton-Jacobi equation for the action that optimizes the desired quantity. The resulting “reactive paths” can be
interpreted as reaction coordinates but, unlike more conventional choices, they contain dynamical information about the chemical
system of interest. 相似文献