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排序方式: 共有302条查询结果,搜索用时 203 毫秒
71.
72.
Steinmetz K Noterdaeme J Wagner F Wesner F Bäumler J Becker G Bosch HS Brambilla M Braun F Brocken H Eberhagen A Fritsch R Fussmann G Gehre O Gernhardt J v Gierke G Glock E Gruber O Haas G Hofmann J Hofmeister F Izvozchikov A Janeschitz G Karger F Keilhacker M Klüber O Kornherr M Lackner K Lisitano G van Mark E Mast F Mayer HM McCormick K Meisel D Mertens V Müller ER Murmann H Niedermeyer H Poschenrieder W Puri S Rapp H Röhr H Ryter F Schmitter K Schneider F Setzensack C Siller G Smeulders P 《Physical review letters》1987,58(2):124-127
73.
We have used the single‐pulse shock tube technique with postshock GC/MS product analysis to investigate the mechanism and kinetics of the unimolecular decomposition of isopropanol, a potential biofuel, and of its reaction with H atoms at 918‐1212 K and 183‐484 kPa. Experiments employed dilute mixtures in argon of isopropanol, a radical scavenger, and, for H‐atom studies, two different thermal precursors of H. Without an added H source, isopropanol decomposes in our studies predominantly by molecular dehydration. Added H atoms significantly augment decomposition, mainly by abstraction of the tertiary and primary hydrogens, reactions that, respectively, lead to acetone and propene as stable organic products. Traces of acetaldehyde were observed in some experiments above ≈ 1100 K and establish branching limits for minor decomposition pathways. To quantitatively account for secondary chemistry and optimize rate constants of interest, we employed the method of uncertainty minimization using polynomial chaos expansions (MUM‐PCE) to carry out a unified analysis of all datasets using a chemical model–based originally on JetSurF 2.0. We find: k(isopropanol → propene + H2O) = 10(13.87 ± 0.69) exp(?(33 099 ± 979) K/ T) s?1 at 979‐1212 K and 286‐484 kPa, with a factor of two uncertainty (2σ), including systematic errors. For H atom reactions, optimization yields: k(H + isopropanol → H2 + p‐C3H6OH) = 10(6.25 ± 0.42) T2.54 exp(?(3993 ± 1028) K /T) cm3 mol?1 s?1 and k(H + isopropanol → H2 + t‐C3H6OH) = 10(5.83 ± 0.37) T2.40 exp(?(1507 ± 957) K /T) cm3 mol?1 s?1 at 918‐1142 K and 183‐323 kPa. We compare our measured rate constants with estimates used in current combustion models and discuss how hydrocarbon functionalization with an OH group affects H abstraction rates. 相似文献
74.
F. Mertens 《Monatshefte für Mathematik》1892,3(1):293-308
Ohne Zusammenfassung 相似文献
75.
76.
77.
Jeffrey A. Mertens Jay D. Braker Douglas B. Jordan 《Applied biochemistry and biotechnology》2010,162(8):2415-2213
Catalytic properties of two glucoamylases, AmyC and AmyD, without starch binding domains from Rhizopus oryzae strain 99-880 are determined using heterologously expressed enzyme purified to homogeneity. AmyC and AmyD demonstrate pH
optima of 5.5 and 6.0, respectively, nearly one unit higher than the Rhizopus AmyA glucoamylase enzyme. Optimal initial activities are at 60 and 50 °C for AmyC and AmyD, respectively. Inactivation of
both enzymes occurs at 50 °C following 30 min pre-incubation. The two enzymes demonstrate substantially slower catalytic rates
toward soluble starch relative to AmyA. AmyC has similar k
cat and K
m for oligosaccharides to other Rhizopus and Aspergillus glucoamylases; however, the enzyme has a 2-fold lower K
mmaltose. AmyD has a 3-fold higher K
m and lower k
cat for maltooligosaccharides than AmyC and other glucoamylases. AmyC (but not AmyD) exhibits substrate inhibition. K
i for substrate inhibition decreases with increasing length of the oligosaccharides. Data from pre-steady-state binding of
AmyC to maltose and maltotriose and pre-steady-state to steady-state catalytic turnover experiments of AmyC acting on maltotriose
were used to interrogate models of substrate inhibition. In the preferred model, AmyC accumulates an enzyme-maltose-maltotriose
dead-end complex in the steady state. 相似文献
78.
The pozzolanic reaction between natural zeolite tuffs, portlandite and water was investigated over the course of the early
reaction period up to 3 days. Isothermal conduction calorimetry experiments supplemented by TG/DTG and XRD analyses assisted
in the elucidation of the sequence of reaction processes taking place. The calorimetry experiments clearly showed the dependence
of the pozzolanic reaction rate and associated heat release on the fineness of the zeolite tuff. Higher external surface areas
of pozzolans yield higher total heat releases. Also the exchangeable cation content of the zeolites influences the reaction
rate. Release of exchangeable alkalis into solution promotes the pozzolanic reaction by raising the pH and zeolite solubility.
The appearance of an exotherm after approximately 3 h of reaction is more conspicuous when alkali-rich zeolites are reacted.
This exotherm is conceived to be related to a transformation or rupture of initially formed reaction products covering the
zeolite grains. The formation of substantial amounts of ‘stable’ calcium silicate hydrate (C–S–H) and calcium aluminate hydrate
(C–A–H) reaction took place after an induction period of more than 6 h. The openness of the zeolite framework affects the
proneness of the zeolite to dissolution and thus its reactivity. Open framework zeolites such as chabazite were observed to
react much more rapidly than closed framework zeolites such as analcime. 相似文献
79.
80.
Bester K Beernaert H Bernreuther A Harbeck S Konieczka P Linsinger TP Mertens I Schimmel H Sejerøe-Olsen B Pauwels J De Poorter G 《Analytical and bioanalytical chemistry》2002,374(2):305-313
During the Belgian PCB crisis (1999/2000) the quality of the data from the PCB monitoring were studied with a proficiency testing experiment. Pork fat that was spiked at the Institute for Reference Materials and Measurements (EC-JRC-IRMM) was sent out as unknown to all laboratories that participated in the monitoring. In parallel, the material was certified on the basis of the results of several leading PCB laboratories throughout Europe that did not participate in the monitoring. During the first round 15 to 25% deviation was experienced, while in the end of this multistep intercalibration procedure the average deviation for the respective PCBs was 10.9% (PCB 28), 13.1% (PCB 52), 10.1% (PCB 101), 10.7% (PCB 118), 10.7% (PCB 138), 9.1% (PCB 153), 8.1% (PCB 180) and 8.2% for the sum of the 7 PCBs. The concentrations measured for the higher volatile PCB 28 had a tendency to show lower levels, while for PCB 180 this was less pronounced. On the other hand, PCB 153 showed results rather to the higher side. During this exercise the material was also tested for stability by one of the certifying laboratories. 相似文献