全文获取类型
收费全文 | 363篇 |
免费 | 12篇 |
国内免费 | 3篇 |
专业分类
化学 | 215篇 |
晶体学 | 4篇 |
力学 | 10篇 |
数学 | 37篇 |
物理学 | 112篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 15篇 |
2020年 | 14篇 |
2019年 | 11篇 |
2018年 | 20篇 |
2017年 | 24篇 |
2016年 | 25篇 |
2015年 | 16篇 |
2014年 | 14篇 |
2013年 | 20篇 |
2012年 | 18篇 |
2011年 | 18篇 |
2010年 | 26篇 |
2009年 | 28篇 |
2008年 | 19篇 |
2007年 | 21篇 |
2006年 | 17篇 |
2005年 | 18篇 |
2004年 | 6篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1993年 | 1篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有378条查询结果,搜索用时 609 毫秒
191.
Kinetic studies on the reactions of O‐methyl ( 2 ) and O‐ethyl ( 3 ) phenyl phosphonochloridothioates with X‐pyridines have been carried out in acetonitrile at 35.0°C. The pyridinolysis rates of 2 are slightly faster than those of 3 . The substituent effects of X on the pyridinolysis rates of 2 and 3 are similar. The Hammett and Brönsted plots with X in the nucleophiles are biphasic concave upward with a break point at X = 3‐Ph for both substrates. The stepwise mechanism with rate‐limiting leaving group departure from the intermediate is proposed based on the βX values and biphasic concave upward free energy relationship for both substrates. The biphasic concave upward free energy relationships are rationalized by a frontside nucleophilic attack TSf with more basic pyridines and backside attack TSb with less basic pyridines for both substrates. 相似文献
192.
N Tibrewal TE Downey SG Van Lanen E Ul Sharif GA O'Doherty J Rohr 《Journal of the American Chemical Society》2012,134(30):12402-12405
Two enzymes of the gilvocarcin biosynthetic pathway, GilMT and GilM, with unclear functions were investigated by in vitro studies using purified, recombinant enzymes along with synthetically prepared intermediates. The studies revealed GilMT as a typical S-adenosylmethionine (SAM) dependent O-methyltransferase, but GilM was identified as a pivotal enzyme in the pathway that exhibits dual functionality in that it catalyzes a reduction of a quinone intermediate to a hydroquinone, which goes hand-in-hand with a stabilizing O-methylation and a hemiacetal formation. GilM mediates its reductive catalysis through the aid of GilR that provides FADH(2) for the GilM reaction, through which FAD is regenerated for the next catalytic cycle. This unusual synergy eventually completes the biosynthesis of the polyketide-derived defuco-gilvocarcin chromphore. 相似文献
193.
Mohammad Wasim Nasir Khalid Azeem Asif Mohammad Arif Jamshed Hussain Zaidi 《Journal of Radioanalytical and Nuclear Chemistry》2012,292(3):1153-1159
Mineral contents of strawberry, collected from different farms of Islamabad were analysed by semi-absolute k
0-instrumental neutron activation analysis and atomic absorption spectrophotometry. The samples were irradiated at two research
reactors located in Pakistan Institute of Nuclear Science & Technology (PINSTECH), Islamabad. The analytical methodologies
were validated by analysing reference materials, IAEA-336 (lichen) and IAEA-V-10 (hay powder). In all the samples, a total
of 26 elements were quantified, among them 16 elements (Ca, Cd, Cl, Co, Cr, Cs, Fe, K, Mg, Mn, Na, Pb, Ru, Sc, Sr and Zn)
were found in all the samples. The determined elemental concentrations in strawberry were compared with the reported values
from other countries. In comparison with the mineral contents of other fruits, strawberry stands best source of Mn and the
second most important source of K after banana. Intake of trace metals through this source was calculated and it was found
that strawberry provides Mn (1.95–3.68 mg/kg), Cr (19.2–46.3 × 10−3 mg/kg), Fe (3.45–8.72 mg/kg), K (1,520–1,670 mg/kg) and Mg (100–220 mg/kg), which forms 26, 19, 14, 7 and 7% of the recommended
dietary allowances for the respective metals. The daily intake of Cd and Pb were compared with the provisional tolerable weekly
intake defined by FAO/WHO. 相似文献
194.
Rizwan Ul Haq Sohail Nadeem Zafar Hayyat Khan Toyin Gideon Okedayo 《Central European Journal of Physics》2014,12(12):862-871
Current study examines the magnetohydrodynamic (MHD) boundary layer flow of a Casson nanofluid over an exponentially permeable shrinking sheet with convective boundary condition. Moreover, we have considered the suction/injection effects on the wall. By applying the appropriate transformations, system of non-linear partial differential equation along with the boundary conditions are transformed to couple non-linear ordinary differential equations. The resulting systems of non-linear ordinary differential equations are solved numerically using Runge-Kutta method. Numerical results for velocity, temperature and nanoparticle volume concentration are presented through graphs for various values of dimensionless parameters. Effects of parameters for heat transfer at wall and nanoparticle volume concentration are also presented through graphs and tables. At the end, fluid flow behavior is examined through stream lines. Concluding remarks are provided for the whole analysis. 相似文献
195.
M. R. Predtechenskii Yu. D. Varlamov S. N. Ul’yankin 《Russian Journal of Electrochemistry》2010,46(8):871-876
The test object was a fuel cell with free molten carbonate electrolyte providing realization of direct electrochemical oxidation
of solid hydrocarbons. This study involved the effect of the fuel type and dispersion as well as cathode reagent gas mixture
composition to the fuel cell functional parameters. The used fuels were dispersed and monolithic graphite, anthracite, and
jet coal specimens. The effect of oxygen/carbon dioxide ratio on the mixture fed to cathode to the open circuit cell voltage
and achieved current density levels was studied with respect to interrelation of the processes taking place in the cathode
and anode units of the fuel cell. A correlation was noted between the specific fuel cell characteristics and hydrogen content
in the fuel material. The highest level of current density and specific power was recorded for jet coal characterized with
high hydrogen content. The different characteristics of monolithic and dispersed fuel specimens were accounted for by the
effect of losses at contacts between particles. Achievement of high current density and specific power was demonstrated by
using dispersed coal fuel. 相似文献
196.
Validated HPLC–MS–MS method for simultaneous determination of atorvastatin and 2-hydroxyatorvastatin in human plasma—pharmacokinetic study 总被引:1,自引:0,他引:1
Borek-Dohalský V Huclová J Barrett B Nemec B Ulc I Jelínek I 《Analytical and bioanalytical chemistry》2006,386(2):275-285
Cholesterol-reducing statin drugs are the most frequently prescribed agents for reducing morbidity and mortality related to
coronary heart disease. In this publication a validated, highly sensitive, and selective isocratic HPLC method is reported
for quantitative determination of the major statin drug atorvastatin (ATV) and its metabolite 2-hydroxyatorvastatin (HATV).
Detection was performed with an electrospray ionization triple-quadrupole mass spectrometer equipped with an ESI interface
operating in positive-ionization mode. Multiple reaction monitoring (MRM) was used for MS–MS detection. The calibration plot
was linear in the concentration range 0.10–40.00 ng mL−1 for both ATV and HATV. Inter-day and intra-day precision and accuracy of the proposed method were characterized by measurement
of relative standard deviation (RSD) and percentage deviation, respectively; both were less than 8% for both analytes. The
limit of quantitation was 0.02 ng mL−1 for ATV and 0.07 ng mL−1 for HATV. The method was used for pharmacokinetic study of ATV and HATV. Pharmacokinetic data for all analytes are also reported. 相似文献
197.
T. M. Ul’yanova N. P. Krut’ko L. V. Titova Yu. G. Zonov 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2010,4(4):603-608
The correlation between temperature treatment conditions and the ratio of components in nanostructured fibrous powders with
a composition of ZrO2-Y2O3-Al2O3 and their porous crystal structure and physicochemical properties is studied. The dependences of the ratio between zirconia
tetragonal and monoclynic phases on the treatment temperature and the alumina content are found to have a nonmonotonic character.
The growth of zirconia crystallite size is suppressed by introduced nanocrystalline alumina in a temperature range of 600–1200°C,
which is caused by the processes of ternary solid solution formation. The bulk and picnometric density values of materials
are proportional to the temperature of heat treatment. The temperature dependence of the specific surface and the size of
oxide grain particles has an inversely proportional character. With increasing alumina content in the powders, the specific
surface increases, while the picnometric and bulk densities decrease. 相似文献
198.
The system of nonlinear equations describing a surface-stabilized ferroelectric smectic C* liquid crystal in the chevron geometry has been investigated by numerical methods in the framework of the continuum model
of liquid crystals. Stable orientational and structural configurations have been studied, and the results obtained have been
compared with those derived using simplified models. The height of the potential barrier separating two stable configurations
has been determined, and the transition between them under the action of the external electric field has been examined. It
has been demonstrated that this effect has a threshold character. The dependence of the threshold field on the film thickness
has been analyzed. 相似文献
199.
M. I. Yalandin A. G. Reutova M. R. Ul’maskulov K. A. Sharypov S. A. Shunailov N. S. Ginzburg I. V. Zotova E. R. Kocharovskaya A. S. Sergeev 《JETP Letters》2010,91(11):553-557
An effect of the nonlinear compression of ultrashort microwave pulses has been observed in the process of the amplification
of quasistationary electron beams. The Cherenkov mechanism of the interaction of a rectangular electron beam with a decelerated
wave in a waveguide partially filled with an insulator is used. The experiment has been conducted on a setup consisting of
two synchronized RADAN high-current accelerators. The first accelerator supplied a generator of 37-GHz superradiance pulses
with a duration of about 300 ps. The second accelerator with a beam current of up to 1.2 kA and an electron energy of about
300 keV was used in an amplifying section. The theoretical analysis shows that the amplification of the electromagnetic pulses
(at least by a factor of 4 in the power) is accompanied by a strong decrease in their duration (down to 100 ps). 相似文献
200.
A. A. Abramov V. I. Ul’yanova L. F. Yukhno 《Computational Mathematics and Mathematical Physics》2010,50(2):238-243
The general nonlinear self-adjoint eigenvalue problem for a differential algebraic system of equations on a half-line is examined.
The boundary conditions are chosen so that the solution to this system is bounded at infinity. Under certain assumptions,
the original problem can be reduced to a self-adjoint system of differential equations. After certain transformations, this
system, combined with the boundary conditions, forms a nonlinear self-adjoint eigenvalue problem. Requirements for the appropriate
boundary conditions are clarified. Under the additional assumption that the initial data are monotone functions of the spectral
parameter, a method is proposed for calculating the number of eigenvalues of the original problem that lie on a prescribed
interval of this parameter. 相似文献