全文获取类型
收费全文 | 5105篇 |
免费 | 187篇 |
国内免费 | 16篇 |
专业分类
化学 | 3364篇 |
晶体学 | 54篇 |
力学 | 109篇 |
数学 | 555篇 |
物理学 | 1226篇 |
出版年
2023年 | 47篇 |
2022年 | 151篇 |
2021年 | 154篇 |
2020年 | 123篇 |
2019年 | 147篇 |
2018年 | 123篇 |
2017年 | 116篇 |
2016年 | 195篇 |
2015年 | 162篇 |
2014年 | 158篇 |
2013年 | 329篇 |
2012年 | 353篇 |
2011年 | 413篇 |
2010年 | 261篇 |
2009年 | 260篇 |
2008年 | 349篇 |
2007年 | 313篇 |
2006年 | 251篇 |
2005年 | 223篇 |
2004年 | 212篇 |
2003年 | 143篇 |
2002年 | 115篇 |
2001年 | 88篇 |
2000年 | 83篇 |
1999年 | 61篇 |
1998年 | 34篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 42篇 |
1994年 | 34篇 |
1993年 | 37篇 |
1992年 | 46篇 |
1991年 | 25篇 |
1990年 | 16篇 |
1989年 | 16篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1968年 | 4篇 |
排序方式: 共有5308条查询结果,搜索用时 31 毫秒
61.
A pressure-assisted capillary chromatography-mass spectrometry method based on the use of a monolithic octadecylsilica (ODS) capillary is proposed for the determination of aliphatic amines. A 25 mM citric acid buffer containing 10% methanol is used as running electrolyte. Separation is achieved by simultaneously applying a capillary electrophoresis (CE) voltage of 13 kV and an overimposed pressure of 8 bar. The use of pressure is required to ensure stable electrospray conditions. Analysis times are reduced by using a capillary column consisting of a 30 cm long monolithic silica capillary column bound with ODS and a fused-silica capillary column also 30 cm long. The proposed method was successfully applied to the determination of low-molecular-weight aliphatic amines in tap and river water. The analysis of real samples requires cleanup and preconcentration, which can be performed automatically by inserting a minicolumn in the replenishment system of the commercial instrument. 相似文献
62.
Jo?o L M Santos Arnaud Clausse José L F C Lima M L M F S Saraiva António O S Rangel 《Analytical sciences》2005,21(4):461-464
A new flow methodology exploiting the multi-pumping approach was developed for the spectrophotometric determination of ambroxol hydrochloride in pharmaceutical preparations. The flow manifold was implemented by using, exclusively, multiple solenoid-actuated micro-pumps, which acted simultaneously as sample insertion, solutions propelling and reagents commutation units. Linear calibration plots were obtained over an ambroxol concentration ranging from 10 to 200 mg l(-1) (r.s.d. < 0.5%, n = 15) and a sampling rate of about 60 samples per hour (flow rate = 1.92 ml min(-1), sample volume = 80 microl). 相似文献
63.
Cestari AR Vieira EF Dos Santos AG Mota JA de Almeida VP 《Journal of colloid and interface science》2004,280(2):380-386
In this work, chitosan beads were synthesized in acidic medium and cross-linked in 1% glutaraldehyde solution. The characterization of the materials using TG/DTG, XRD, and BET surface areas showed that the beads did not modify their characteristics after the cross-linking reaction. The cross-linked beads were utilized as adsorbents for the removal of the yellow-, blue-, and red-anionic reactive dyes from aqueous solutions at pH 2.0. Adsorption of the yellow-dye increased from 25 to 50 degrees C. However, adsorption of the blue-dye decreased from 25 to 50 degrees C. Interestingly, the adsorption of the red-dye decreased from 25 to 35 degrees C and increased from 45 to 50 degrees C. The kinetic data were evaluated using an Avrami kinetic model, where the parameter n was related to the determination of changes in the adsorption mechanisms. Adsorption data of the dyes in relation to the contact time, the chemical structures of the dyes, and temperature were presented and were discussed. 相似文献
64.
A preconcentration procedure was established for sulphate determination in rain waters at the mg/l level, employing a small column packed with the AG1-X8 (200-400 mesh) anionic resin inserted into a flow injection system. Sulphate determination was performed by using the turbidimetric method based on reaction with barium. For concentrations within 0.10 and 2.0 MgSO(2-)(4)/l, a throughput of 50 determinations/hr was achieved, and the relative standard deviation of results was better than 2%. 相似文献
65.
J. D. Santos E. Longo M. E. Banja V. A. A. Espinoza C. A. Taft 《International journal of quantum chemistry》2005,102(3):302-312
We studied the interaction between H, Li, Na, and K with one and two C60 molecules using unrestricted Hartree–Fock (UHF) methods. We investigated the effects of distances between the doping atoms and the C60 clusters, total charges, interaction energies, stabilities, HOMO‐LUMO energy differences, charge distribution, and potential energy surfaces. The effect of each doping atom was analyzed and potential technological applications discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
66.
L.M. Ilharco A.M. Santos M.J. Silva J.M.G. Martinho 《Journal of Sol-Gel Science and Technology》1997,8(1-3):877-882
The influence of the acid catalyst concentration on the structural evolution of a sol-gel system was studied by doping TEOS
based starting solutions with two fluorescent probes: a polystyrene chain (M
n
=1700) and a much shorter alkane chain (M=172), both of them labeled at both ends with 1-pyrenyl. For this purpose, each probe was incorporated in two TEOS∶H2O∶C2H5OH mixtures (molar ratios 1∶4∶1), one at pH 1.2 and the other at pH 2.5 (respectively below and above the isoelectric point
of silica). Very low concentrations of the probes were used (≤10−6
M), so the pyrene dimmers and excimers were formed only intramolecularly. The ratios of excimer to monomer fluorescence intensities
at excitation wavelength of 360 nm (where mainly the ground state pyrene dimmers are excited) were studied as a function of
time. Different evolutions of these ratios were observed, which allowed us to predict that the silica structure developsvia the formation of primary particles, even at pH values below the isoelectric point of silica, where it is not possible to
directly detect their formation. 相似文献
67.
Jörg Fridgen Georg Eickerling Ana M. Santos 《Journal of organometallic chemistry》2004,689(17):2752-2761
Novel chiral 2′-pyridinyl alcohols derived from isopropylidene-protected carbohydrates are reported. They show different characteristics at the hydroxy group, but are all suitable ligands for chiral molybdenum(VI) complexes of the type MoO2L2 (L = chiral 2′-pyridinyl alcoholate). MoO2(acac)2 served as starting material in the complex syntheses. The structure of one ligand and one dioxo complex were exemplary established by X-ray crystallography. For catalytic runs in the enantioselective epoxidation catalysis trans-methylstyrene was used as model substrate, tert-butylhydroperoxide and cumolhydroperoxide, resp., as the oxidant. 相似文献
68.
Huenerbein A Sipoli Marques MA Pereira Ados S de Aquino Neto FR 《Journal of chromatography. A》2003,985(1-2):375-386
The Medical Commission of the International Olympic Committee forbids the use of anabolic androgenic steroids and beta2-agonists to improve athletic performance. In this work we have selected examples of anabolic androgenic compounds and their metabolites to evaluate the GC-MS analysis of some trimethylsilyl derivatives. The aim is to set the best GC conditions to improve the detection within the whole range of analyte elution temperatures. The initial column temperature was changed to 105 or 140 degrees C followed by 40 degrees C min(-1) to 200 degrees C and then 15 degrees C min(-1) to 300 degrees C. Using 140 degrees C as the initial oven temperature it was possible to obtain narrower initial analyte distributions for the compounds that elutes at the beginning of the chromatogram as clenbuterol, mabuterol, epimethylenediol and norandrosterone, without loss of derivatized metabolites signal. Later. eluting analytes, such as the stanozolol metabolites, furazabol and oxandrolone were not affected. Temperatures below 140 degrees C. resulted in partial derivatization for some analytes mainly stanozolol related structures. Therefore evaluation of derivatization conditions as occurring in three steps, the vial, vaporization chamber and capillary column, was thoroughly assessed. The new program temperature improves the signal-to-noise ratio for some compounds and shows adequate resolution for endogenous compounds. Some of the difficult key separations necessary for doping control enforcement were also obtained with the proposed method. 相似文献
69.
On the Stability of the Schiff Bases of Pyridoxal 5′-Phosphate with Polypeptides Containing L-Lysine
M. Angeles García del Vado Gerardo R. Echevarria Francisco García Blanco Jos G. Santos Blanco Jos Laynez Vallejo Jos L. García de Paz 《Helvetica chimica acta》1991,74(8):1749-1756
We determined the apparent equilibrium constant of formation, KpH, of the Schiff bases of pyridoxal 5′-phosphate (PLP) and poly- and copolymers containing L -lysine, as a function of pH at 25° and a constant ionic strength of 0.1 M . The KpH values obtained at acidic and neutral pH were larger that those reported for Schiff bases of PLP and hexylamine. We determined calorimetrically ΔH of formation of Schiff bases of PLP and poly(L -lysine) (?4.5′kcal/mol), and PLP and hexylamine (?3.4 kcal/mol) at pH 7.00. Semi-empirical theoretical calculations (INDO and AMI methods) of a model compound of Schiff base of PLP and polypeptide containing L -lysine show the capability of specific interactions between groups of PLP and the peptide skeleton. 相似文献
70.
Hair color changes and protein damage caused by ultraviolet radiation 总被引:12,自引:0,他引:12
Santos Nogueira AC Joekes I 《Journal of photochemistry and photobiology. B, Biology》2004,74(2-3):109-117
Ultraviolet and visible radiations are known to damage hair. However, quantitative data relating damage to hair type, proteins and color to the radiation wavelength are missing. We studied the effect of UV plus visible, UVA plus visible, visible mercury-vapor lamp radiation and sunlight on (blended) virgin dark-brown, blond and red hair and (one head) virgin black and curly dark-brown hair. All hair types showed a substantial increase in protein loss in water after lamp and sun irradiation. The damaging effect of UVB was about 2-5 times higher than that of UVA plus visible radiation, depending on the hair type. Significant color changes were also observed in every hair type, after lamp and sun irradiation, being more pronounced for the light colored hairs. The luminosity difference parameter was the major contributor to the hair color changes, but significant changes in the red-green and yellow-blue parameters of every hair were observed. In this case, the damaging effect is ascribable mainly to UVA radiation. No significant changes in the mechanical properties or topography were observed in any case. We discuss these results in terms of hair type and composition and melanin types. 相似文献