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71.
Gambi CM Giordano R Chittofrati A Pieri R Laurati M Baglioni P Teixeira J 《The journal of physical chemistry. B》2007,111(6):1348-1353
Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios. 相似文献
72.
Adriano Francisco Barbosa Arnaldo César Pereira Lauro Tatsuo Kubota César Ricardo Teixeira Tarley 《Talanta》2007,71(4):1512-1519
The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30 mg of MWCNTs previously oxidized with concentrated HNO3. The elution step is carried out with 1.0 mol L−1 HNO3. The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 24 full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6 μg L−1, respectively. A wide linear range varying from 8.6 up to 775 μg L−1 (r > 0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200 μg L−1 levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11 min−1, consumptive index of 0.45 mL and sampling frequency estimated as 14 h−1. Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples. 相似文献
73.
Marthyna P. Souza Miguel A. Cerqueira Bartolomeu W. S. Souza José A. Teixeira Ana L. F. Porto António A. Vicente Maria G. Carneiro-da-Cunha 《Chemical Papers》2010,64(4):475-481
Policaju-based coatings were applied on “Tommy Atkins” mangoes and the effects of four different treatments on mango shelf-life
were evaluated under storage condition at 4°C and 82 % of relative humidity over 28 days. The surface tension of mangoes was
found to be 29.04 mN m−1; the dispersive and polar components were 27.57 mN m−1 and 1.47 mN m−1, respectively, and the critical surface tension was 22.7 mN m−1. A significantly lower mass loss was observed in all mangoes treated with Policaju-based coatings. For all applied treatments,
no significant variation in the total soluble solids and pH was detected over the experimental storage time. The results show
that Policaju-based coatings have a positive effect on the shelf-life extension of mangoes at low storage temperatures (4°C). 相似文献
74.
Daniel P. Silva Tomáš Brányik Giuliano Dragone António A. Vicente José A. Teixeira João B. Almeida e Silva 《Chemical Papers》2008,62(1):34-41
This study deals with two innovative brewing processes, high gravity batch and complete continuous beer fermentation systems.
The results show a significant influence of the variables such as concentration and temperature on the yield factor of the
substrate into ethanol and consequently on the productivity of the high gravity batch process. The technological feasibility
of continuous production of beer based on yeast immobilization on cheap alternative carriers was also demonstrated. The influence
of process parameters on fermentation performance and quality of the obtained beers was studied by sensorial analysis. No
significant difference in the degree of acceptance between the obtained products and some traditional market brands was found.
Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May
2007. 相似文献
75.
Each alternative for a repair contract implies a specific responsetime and related cost. The response time is associated withthe commitment of repair time, based on the contract. A decisionmaker chooses the best alternative taking into account the systemperformance and the cost of the contract. This problem has beenanalysed through a multicriteria decision model. This decisionmodel supports decision makers in the determination of the bestcombination of contracting conditions. The decision model proposedallows the decision maker to quantify the consequences of anaction taking into account two basic criteria: the cost of thecontract and the system performance. Two different decisionmodels have been built to support decision makers. These decisionmodels are based on different multicriteria approaches. Thefirst, reported in a previous paper, is based on the multiattributeutility theory (MAUT). The model presented in this paper isbased on the ELECTRE I method combined with utility functions.The paper presents the main theoretical aspects related to bothapproaches and practical implications related to model building.A numerical application is presented in order to illustratethe use of the decision model. 相似文献
76.
Jaume Llibre Paulo R. da Silva Marco A. Teixeira 《Journal of Dynamics and Differential Equations》2007,19(2):309-331
Singular perturbations problems in dimension three which are approximations of discontinuous vector fields are studied in
this paper. The main result states that the regularization process developed by Sotomayor and Teixeira produces a singular
problem for which the discontinuous set is a center manifold. Moreover, the definition of sliding vector field coincides with
the reduced problem of the corresponding singular problem for a class of vector fields.
相似文献
77.
78.
Lucimara Mendonça CostaEmerson Schwingel Ribeiro Mariana Gava SegatelliDanielle Raphael do Nascimento Fernanda Midori de OliveiraCésar Ricardo Teixeira Tarley 《Spectrochimica Acta Part B: Atomic Spectroscopy》2011,66(5):329-337
The present study describes the adsorption characteristic of Cd(II) onto Nb2O5/Al2O3 mixed oxide dispersed on silica matrix. The characterization of the adsorbent has been carried out by infrared spectroscopy (IR), scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), energy dispersive X-ray fluorescence analysis (EDXRF) and specific surface area (SBET). From batch experiments, adsorption kinetic of Cd(II) was described by a pseudo-second-order kinetic model. The Langmuir linear isotherm fitted to the experimental adsorption isotherm very well, and the maximum adsorption capacity was found to be 17.88 mg g−1. Using the effective material, a method for Cd(II) preconcentration at trace level was developed. The method was based on on-line adsorption of Cd(II) onto SiO2/Al2O3/Nb2O5 at pH 8.64, in which the quantitative desorption occurs with 1.0 mol L−1 hydrochloric acid towards FAAS detector. The experimental parameters related to the system were studied by means of multivariate analysis, using 24 full factorial design and Doehlert matrix. The effect of SO42−, Cu2+, Zn2+ and Ni2+ foreign ions showed no interference at 1:100 analyte:interferent proportion. Under the most favorable experimental conditions, the preconcentration system provided a preconcentration factor of 18.4 times, consumption index of 1.08 mL, sample throughput of 14 h−1, concentration efficiency of 4.35 min−1, linear range from 5.0 up to 35.0 μg L−1 and limits of detection and quantification of 0.19 and 0.65 μg L−1 respectively. The feasibility of the proposed method for Cd(II) determination was assessed by analysis of water samples, cigarette sample and certified reference materials TORT-2 (Lobster hepatopancreas) and DOLT-4 (Dogfish liver). 相似文献
79.
Teixeira C Gomes JR Couesnon T Gomes P 《Journal of computer-aided molecular design》2011,25(8):763-775
Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) based on three-dimensional
quantitative structure–activity relationship (3D-QSAR) studies were conducted on a series (39 molecules) of peptidyl vinyl
sulfone derivatives as potential Plasmodium Falciparum cysteine proteases inhibitors. Two different methods of alignment were employed: (i) a receptor-docked alignment derived
from the structure-based docking algorithm GOLD and (ii) a ligand-based alignment using the structure of one of the ligands
derived from a crystal structure from the PDB databank. The best predictions were obtained for the receptor-docked alignment
with a CoMFA standard model (q
2 = 0.696 and r
2 = 0.980) and with CoMSIA combined electrostatic, and hydrophobic fields (q
2 = 0.711 and r
2 = 0.992). Both models were validated by a test set of nine compounds and gave satisfactory predictive r
2
pred values of 0.76 and 0.74, respectively. CoMFA and CoMSIA contour maps were used to identify critical regions where any change
in the steric, electrostatic, and hydrophobic fields may affect the inhibitory activity, and to highlight the key structural
features required for biological activity. Moreover, the results obtained from 3D-QSAR analyses were superimposed on the Plasmodium Falciparum cysteine proteases active site and the main interactions were studied. The present work provides extremely useful guidelines
for future structural modifications of this class of compounds towards the development of superior antimalarials. 相似文献
80.
Ferreres F Taveira M Gil-Izquierdo A Oliveira L Teixeira T Valentão P Simões N Andrade PB 《Rapid communications in mass spectrometry : RCM》2011,25(14):1972-1980
High-performance liquid chromatography-diode array detection-electrospray ionization multi-stage mass spectrometry (HPLC-DAD-ESI-MS(n)) is considered to be a very valuable tool for the characterization of compounds found in trace amounts in natural matrices, as their previous isolation and clean-up steps can be avoided. Micro-scale separation increases the potential of this analytical technique, allowing the determination of compounds in reduced samples. Spodoptera littoralis represents a major challenge to Solanaceae plants, as it is one of the most deleterious pests. The S. littoralis/Lycopersicon esculentum system was studied for the first time concerning glycoalkaloids and phenolics. Using HPLC-DAD-ESI-MS(n) we were able to characterize 15 phenolic compounds in L. esculentum leaves. Nine of them are reported for the first time. Some differences were found between leaves of cerasiforme and 'Bull's heart' varieties. However, in the materials of S. littoralis (larvae, adults, exuviae and excrements) reared in both L. esculentum leaves no phenolics were identified. α-Tomatine was the main glycoalkaloid in the host plant. The glycoalkaloid composition of the different S. littoralis materials was distinct, with α-tomatine and dehydrotomatine being the main detected compounds in larvae and excrements. These results add knowledge to the ecological interaction in this insect/plant duo, for which it is hard to obtain considerable sample amounts. 相似文献