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81.
We consider a priority queue in steady state with N servers, two classes of customers, and a cutoff service discipline. Low priority arrivals are "cut off" (refused immediate service) and placed in a queue whenever N1 or more servers are busy, in order to keep N-N1 servers free for high priority arrivals. A Poisson arrival process for each class, and a common exponential service rate, are assumed. Two models are considered: one where high priority customers queue for service and one where they are lost if all servers are busy at an arrival epoch. Results are obtained for the probability of n servers busy, the expected low priority waiting time, and (in the case where high priority customers do not queue) the complete low priority waiting time distribution. The results are applied to determine the number of ambulances required in an urban fleet which serves both emergency calls and low priority patients transfers. 相似文献
82.
Marita Wasner Wolfgang Pfleiderer Robert J. Suhadolnik Susan E. Horvath Ning Kon Ming-Xu Guan Earl E. Henderson Robert J. Suhadolnik Earl E. Henderson Martin E. Adelson 《Helvetica chimica acta》1996,79(3):609-618
Monomeric 3′-deoxyadenosine (cordycepin) was modified at the 2′-O- ( 13–18 ) and 5′-O-position ( 25–29 ) by the vitamins E, D2, and A and by the two lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol via succinate or carbonate linkages. The base-labile conjugates afforded protection groups like the 2-(4-nitro-phenyl)ethoxycarbonyl (npeoc) and monomethoxytrityl group (MeOTr) that are cleavable without harming the ester and carbonate bonds, respectively. Monomeric conjugates of cordycepin and vitamin E, vitamin D2, 1,2-di-O-palmitoylglycerol, and 1,2-di-O-hexadecylglycerol (see 13, 14, 17, 18, 25, 26, 28 , and 29 ) inhibited HIV-1-induced syncytia formation 1.7 to 6.2 fold compared to 1.5-fold for cordycepin (see Table); IC50 values for 25 and 28 were 257 and 267 m?M , respectively. In addition, the monomeric cordycepin-vitamin and -lipid conjugates inhibited HIV-1 RT activity 28–49% which compares with a 13% inhibition of HIV-1 RT observed for cordycepin. The minimal inhibition of HIV-1-induced syncytia formation and HIV-1 RT activity did not proceed by the activation of RNase L. The monomeric conjugates tested ( 13, 14 ) increased PKR expression. 相似文献
83.
Supercritical fluid extraction (SFE) is combined with supercritical fluid chromatography (SFC) in an analytical mode to develop a system for fractionating and enriching high value ferulate-phytosterol esters (FPE) contained in corn bran oil. Corn bran is initially extracted with neat supercritical carbon dioxide (SC-CO2) at various pressures (13.8, 34.5, and 69 MPa) and temperatures (40, 60, and 80 degrees C) to see if the FPE can be enriched in the extracts. These initial studies show the greatest percentage of FPE could be extracted under two sets of conditions: 69 MPa at 80 degrees C and 34.5 MPa at 40 degrees C. Both sets of parameters yield an extract containing approximately 1.25% FPE. A stock supply of corn bran oil is then produced by scaled-up SFE at 34.5 MPa and 40 degrees C for subsequent chromatographic fractionation. The SFE-obtained corn bran oil is then applied to the head of a minichromatographic column containing an amino-propyl sorbent. SFC is than commenced using neat SC-CO2 at 69 MPa and 80 degrees C to remove the majority of the triglyceride-based oil. Pressure and temperature are then lowered to 34.5 MPa and 40 degrees C, respectively, and ethanol is added as a modifier. The modifier is added in an increasing stepwise gradient program, and fractions are collected at equal volume intervals. The resultant fractions are analyzed by analytical high-performance liquid chromatography with evaporative light-scattering detection and show that FPE could be enriched to a 14.5% (w) level. 相似文献
84.
Stable radical 2-(6-uradinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl shows antiferromagnetic spin pairing with 2J/k= -14 K, attributable to a close contact between unpaired spin density on the imidazole-type nitrogen atoms; hydrogen bonds aid dimer formation, but do not appear to play an eletronic role in the magnetic behaviour. 相似文献
85.
Lindsey?Gillian?MackayEmail author C.?Peter?Taylor Richard?Bruce?Myors Ruth?Hearn Bernard?King 《Accreditation and quality assurance》2003,8(5):191-194
The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes. 相似文献
86.
A comparative evaluation has been made of both proton and (13)C nuclear magnetic resonance techniques in the quantitative characterization of commercial urea-formaldehyde resins. There is good agreement between data derived from (13)C NMR spectra and from (1)H high-field continuous wave, or low-field (Fourier transform) NMR spectra. Low-field continuous wave proton spectra exhibit inferior resolution and provide inaccurate quantitative data. Combination of (13)C and proton NMR with nitrogen analysis gives a quantitative characterization technique for these resins. 相似文献
87.
Martin J. Mays Paul R. Raithby Philip L. Taylor K. Kim Henrick 《Journal of organometallic chemistry》1982,224(4):C45-C48
The synthesis of a (μ4-PPh) and some related (μ3-PPh) mixed metal clusters containing ruthenium is described together with the X-ray structures of [Ru3Rh2(CO)13(PEt3)(μ4-PPh)] and [Ru3Au(μ2-H)(CO)9(PMe2Ph)(μ3-PPh)]. 相似文献
88.
The lack of reference materials, accurately certified for nitrite, is a problem in view of the importance of this species for environmental and medical reasons. This work outlines a plan for the preparation of nitrite isotopic reference materials (IRMs) in the form of high purity solutions, certified for their nitrite-nitrogen isotopic composition and nitrite concentration. To achieve the desired accuracy (expanded uncertainty U with a coverage factor k=2 of =2%), primary methods of measurement such as isotope dilution mass spectrometry (IDMS), gravimetry, and titrimetry must be used. The main difficulty is the stability of nitrite. Other problems expected in the preparation and certification of nitrite IRMs are described. Results from long term stability studies (up to 1.5 years) and a procedure for the purification of the candidate nitrite IRMs are presented. The purpose is to use these IRMs for high accuracy method calibrations and as anchor points for SI-traceable nitrite concentrations. Reference values linked to the SI system are useful to demonstrate the degree of international comparability of nitrite measurements in intercomparison programmes such as the IRMM-International Measurement Evaluation Programme (IMEP). 相似文献
89.
The silver, lead and mercuric toluenedithiolates were synthesised and analysed by both conventional chemical methods and thermoanalytical methods. The thermal decomposition was studied by thermogravimetric analysis in air, nitrogen and vacuum. The formulae of the decomposition products were derived using the mole weights and i.r. absorption spectra. The activation energies for the first stage of decomposition were calculated. The volatile products contain mostly carbon and hydrogen while the residues contain the corresponding metals either free or combined with sulfur.
Abstracted in part from a thesis submitted by T. Ktenas to the Graduate School of Howard University, in partial fulfillment of the requirements for the degree of Master of Science. 相似文献
Zusammenfassung Die Silber-, Blei- und Quecksilber-Toluoldithionate wurden synthetisiert und sowohl mit konventionellen chemischen Methoden als auch thermoanalytisch analysiert. Die thermische Zersetzung wurde durch thermogravimetrische Analyse in Luft, Stickstoff und Vakuum untersucht. Die Formeln der Zersetzungsprodukte wurden aus den Molekulargewichten und den Infrarot-Absorptionsspektren abgeleitet. Die Aktivierungsenergien für den ersten Zersetzungsschritt wurden berechnet. Die flüchtigen Produkte enthalten hauptsächlich Kohlenstoff und Wasserstoff, während die Rückstände die entsprechenden Metalle in freier Form oder in Form von Schwefelverbindungen enthalten.
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Abstracted in part from a thesis submitted by T. Ktenas to the Graduate School of Howard University, in partial fulfillment of the requirements for the degree of Master of Science. 相似文献
90.
Traditionally, the application of supercritical fluid chromatography has been limited to compounds of relatively low polarity.
Thus, we report here that one secondary amine and two quaternary amine salts were successfully eluted from a Deltabond Cyano-bonded
silica column with the addition of sodium alkylsulfonate to the methanol-modified, carbon dioxide-based mobile phase. A possible
ion-pairing interaction between the positively charged analytes and the anionic part of the sulfonate additive has been proposed.
In another set of experiments, the three amine salts readily eluted from both Ethyl-pyridine-bonded silica and Amino-bonded
silica phases without the need of additive although the peak shapes were less than desirable. The addition of sulfonate salt
to the mobile phase again sharpened the peaks. In the presence of a mixture of methanol and CO2, we suggest that these stationary phases are positively charged. We describe here also for the first time the employment
of a strong silica-based anion exchange (SAX) column for supercritical fluid separation of cationic species. Two elution mechanisms
were proposed for the three amine salts depending upon whether the ionic additive was incorporated into the mobile phase.
Separation as the ion-pair was proposed with an ionic additive in the mobile phase; whereas separation of the intact amine
salt from the positively charged basic stationary phases was suggested to be operational without the ionic additive. 相似文献