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排序方式: 共有255条查询结果,搜索用时 195 毫秒
71.
Baby AR Lacerda AC Velasco MV Lopes PS Kawano Y Kaneko TM 《Colloids and surfaces. B, Biointerfaces》2006,50(1):61-65
This research employed FT-Raman and PAS-FTIR spectroscopic techniques to evaluate the interaction of cetyl trimethyl ammonium chloride (CTAC), a cationic surfactant, on the stratum corneum (SC) of shed snake skins from Bothrops jararaca, used as model membranes. Surfactant aqueous solutions (50.0 and 0.78 gl(-1)) with neutral pH were applied on the samples with intervals of 4, 8 (whole SC) and 12h (SC tape-stripped). Samples presented modifications of the topography for all conditions of the assays and the monomers of the surfactant, instead of the micelles, seemed to interact with the keratin. The SC model membranes treated with CTAC have had an augment of water content (except for whole SC treated for 8h) indicated by the expansion of the band 3600-3300 cm(-1), mainly for the tape-stripped samples after 12h treatment. Concentration appeared to be an important factor related to an increase of the tissue hydration. 相似文献
72.
Korn Md Andrade JB Jesus DS Lemos VA Bandeira ML Santos WN Bezerra MA Amorim FA Souza AS Ferreira SL 《Talanta》2006,69(1):16-24
Lead is recognized worldwide as a poisonous metal. Thus, the determination of this element is often required in environmental, biological, food and geological samples. However, these analyses are difficult because such samples contain relatively low concentrations of lead, which fall below the detection limit of conventional analytical techniques such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry. Several preconcentration procedures to determine lead have therefore been devised, involving separation techniques such as liquid-liquid extraction, solid phase extraction, coprecipitation and cloud point extraction. Citing 160 references, this paper offers a critical review of preconcentration procedures for determining lead using spectroanalytical techniques. 相似文献
73.
Dias JM Lemos PC Serafim LS Oliveira C Eiroa M Albuquerque MG Ramos AM Oliveira R Reis MA 《Macromolecular bioscience》2006,6(11):885-906
Numerous bacteria have been found to exhibit the capacity for intracellular polyhydroxyalkanoates (PHA) accumulation. Current methods for PHA production at the industrial scale are based on their synthesis from microbial isolates in either their wild form or by recombinant strains. High production costs are associated with these methods; thus, attempts have been made to develop more cost-effective processes. Reducing the cost of the carbon substrates (e.g., through feeding renewable wastes) and increasing the efficiency of production technologies (including both fermentation and downstream extraction and recovery) are two such examples of these attempts. PHA production processes based on mixed microbial cultures are being investigated as a possible technology to decrease production costs, since no sterilization is required and bacteria can adapt quite well to the complex substrates that may be present in waste material. PHA accumulation by mixed cultures has been found under various operational conditions and configurations at both bench-scale and full-scale production. The process known as "feast and famine" or as "aerobic dynamic feeding" seems to have a high potential for PHA production by mixed cultures. Enriched cultures submitted to a transient carbon supply can synthesize PHA at levels comparable to those of pure cultures. Indeed, the intracellular PHA content can reach around 70% of the cell dry weight, suggesting that this process could be competitive with pure culture PHA production when fully developed. Basic and applied research of the PHA production process by mixed cultures has been carried out in the past decade, focusing on areas such as microbial characterization, process configuration, reactor operational strategies, process modeling and control, and polymer characterization. This paper presents a review of the PHA production process with mixed cultures, encompassing the findings reported in the literature as well as our own experimental results in relation to each of these areas. 相似文献
74.
For a prime l, let Φ l be the classical modular polynomial, and let h(Φ l ) denote its logarithmic height. By specializing a theorem of Cohen, we prove that \(h(\Phi_{l})\le 6l\log l+16l+14\sqrt{l}\log l\). As a corollary, we find that h(Φ l )≤6llog?l+18l also holds. A table of h(Φ l ) values is provided for l≤3600. 相似文献
75.
Aguiar JC Santiago GM Lavor PL Veras HN Ferreira YS Lima MA Arriaga AM Lemos TL Lima JQ de Jesus HC Alves PB Braz-Filho R 《Natural product communications》2010,5(12):1977-1980
The chemical compositions of the essential oils from the peel of ripe and unripe fruits of Hymenaea courbaril L., obtained by hydrodistillation, were analyzed by GC and GC-MS. The main constituents of the essential oil from the peel of the ripe fruits were the sesquiterpenes alpha-copaene (11.1%), spathulenol (10.1%) and beta-selinene (8.2%), while germacrene-D (31.9%), beta-caryophyllene (27.1%) and bicyclogermacrene (6.5%) were the major compounds in the oil from unripe fruits. The essential oils were tested against Aedes aegypti larvae and showed LC50 values of 14.8 +/- 0.4 microg/mL and 28.4 +/- 0.3 microg/mL for the ripe and unripe fruit peel oils, respectively. From the peel of the ripe fruits, the diterpenes zanzibaric acid and isoozic acid were isolated, along with the sesquiterpene caryolane-1,9beta-diol. To the best of our knowledge, this is the first report of this sesquiterpene in the genus. The structures of all compounds isolated were identified on the basis of their spectral data (IR, MS, 1D- and 2D-NMR) and by comparison with literature spectral data. 相似文献
76.
By regarding the vacuum as a perfect fluid with equation of state p = -, de Sitter's cosmological model is quantized. Our treatment differs from previous ones in that it endows the vacuum with dynamical degrees of freedom, following modern ideas that the cosmological term is a manifestation of the vacuum energy. Instead of being postulated from the start, the cosmological constant arises from the degrees of freedom of the vacuum regarded as a dynamical entity, and a time variable can be naturally introduced. Taking the scale factor as the sole degree of freedom of the gravitational field, stationary and wave-packet solutions to the Wheeler-DeWitt equation are found, whose properties are studied. It is found that states of the Universe with a definite value of the cosmological constant do not exist. For the wave packets investigated, quantum effects are noticeable only for small values of the scale factor, a classical regime being attained at asymptotically large times. 相似文献
77.
78.
Santos RP Silveira ER Lemos TL Viana FA Braz-Filho R Pessoa OD 《Magnetic resonance in chemistry : MRC》2005,43(6):494-496
A careful NMR analysis with full assignment of the 1H and 13C spectral data for two minor saponins isolated from stems of Cordia piauhiensis is reported. These saponins were isolated by high-performance liquid chromatography and characterized as 3beta-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-glucopyranosyl]pomolic acid 28-O-[beta-D-glucopyranosyl-(1 --> 6)-beta-D-glucopyranosyl] ester (1) and 3beta-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-glucopyranosyl]oleanolic acid 28-O-[beta-D-xylopyranosyl-(1 --> 2)-beta-D-glucopyranosyl-(1 --> 6)-beta-D-glucopyranosyl] ester (2). Their structures were established using a combination of 1D and 2D (1H, 1H-COSY, TOCSY, NOESY, gs-HMQC and gs-HMBC) NMR techniques, electrospray ionization mass spectrometry and chemical evidence. 相似文献
79.
In this work, Amberlite XAD-2 resin functionalized with 4,5-dihydroxy-1,3-benzenedisulfonic acid was synthesized, characterized and applied as a new packing material for an on-line system to nickel preconcentration. The method is based on the sorption of Ni(II) ions in a minicolumn containing the synthesized resin, posterior desorption using an acid solution and measurement of the nickel by spectrophotometry (PAR method). The optimization of the system was performed using factorial design and Doehlert matrix considering five variables: eluent concentration, PAR solution pH, sample flow rate, PAR solution concentration and sample pH. Signals were measured as peak height by using an instrument software. Using the experimental conditions defined in the optimization, the method allowed nickel determination with achieved sampling rate of 25 samples per hour, detection limit (3 s) of 2 μg l−1 and precision (assessed as the relative standard deviation) of 8.2-2.6%, for nickel solutions of 10.0-200.0 μg l−1 concentration, respectively. The experimental enrichment factor of the proposed system was 46, for 120 s preconcentration time. The proposed procedure was applied for nickel determination in food samples. Recoveries of spike additions (5 or 10 μg g−1) to food samples were quantitative (94-110%). 相似文献
80.
Preparation and Crystal Structures of Neutral and Cationic Copper(I) Mixed Ligand Complexes with Triphenylphosphane and Derivatives of Biimidazole Eight triphenylphosphanecopper(I) complexes with bibenzimidazole, tetramethylbiimidazole or tetrahydrobiimidazole were prepared and characterized so far as possible by elemental analysis, IR, 1H-NMR and 31P-NMR spectra. The crystal structures of two complexes with bibenzimidazole were determined. [Cu(bbimH2)(PPh3)2]Cl · CH2Cl2: Reaction of CuCl with bibenzimidazole in fused triphenylphosphane or [CuCl(PPh3)3] with bibenzimidazole in CH2Cl2. Space group P 1, Z = 2, 6440 observed independent reflections, R = 0.064 for refletions with I > 2σ(I). Lattice parameters at 203 K: a = 983.6; b = 1348.9; c = 1805.5 pm; α = 77.24; β = 80.90; γ = 85.81°. The crystal structure is built up by monomeric molecules with distorted tetrahedral coordination of the copper atom (CuN2P2) and bibenzimidazole as bidentate ligand. The chloride ion is linked by H-bonds with the NH groups of the bibenzimidazole. [{Cu(PPh3)2}2(μ-bbim)] · 2 CH2Cl2: Reaction of [CuCl(PPh3)3] with the dipotassium salt of bibenzimidazole in CH3OH/CH2Cl2. Space group P 1, Z = 1, 7192 observed independent reflections, R = 0.057 for reflections with I > 2σ(I). Lattice parameters at 203 K: a = 1334.1; b = 1386.8; c = 1443.7 pm; α = 107.51; β = 103.35; γ = 113.74°. The crystal structure is built up by centrosymmetric molecules with distorted tetrahedral coordination of the copper atoms (CuN2P2) and bibenzimidazolate(2–) as tetradentate bridging ligand. 相似文献