全文获取类型
收费全文 | 777篇 |
免费 | 41篇 |
国内免费 | 5篇 |
专业分类
化学 | 723篇 |
晶体学 | 1篇 |
力学 | 2篇 |
数学 | 54篇 |
物理学 | 43篇 |
出版年
2023年 | 3篇 |
2022年 | 14篇 |
2021年 | 32篇 |
2020年 | 23篇 |
2019年 | 29篇 |
2018年 | 12篇 |
2017年 | 12篇 |
2016年 | 26篇 |
2015年 | 38篇 |
2014年 | 25篇 |
2013年 | 49篇 |
2012年 | 62篇 |
2011年 | 71篇 |
2010年 | 50篇 |
2009年 | 36篇 |
2008年 | 57篇 |
2007年 | 61篇 |
2006年 | 45篇 |
2005年 | 53篇 |
2004年 | 33篇 |
2003年 | 32篇 |
2002年 | 20篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有823条查询结果,搜索用时 281 毫秒
801.
Continuous Production of Ethanol from Starch Using Glucoamylase and Yeast Co-Immobilized in Pectin Gel 总被引:1,自引:0,他引:1
This work presents a continuous simultaneous saccharification and fermentation (SSF) process to produce ethanol from starch
using glucoamylase and Saccharomyces cerevisiae co-immobilized in pectin gel. The enzyme was immobilized on macroporous silica, after silanization and activation of the
support with glutaraldehyde. The silica–enzyme derivative was co-immobilized with yeast in pectin gel. This biocatalyst was
used to produce ethanol from liquefied manioc root flour syrup, in three fixed bed reactors. The initial reactor yeast load
was 0.05 g wet yeast/ml of reactor (0.1 g wet yeast/g gel), used in all SSF experiments. The enzyme concentration in the reactor
was defined by running SSF batch assays, using different amount of silica–enzyme derivative, co-immobilized with yeast in
pectin gel. The chosen reactor enzyme concentration, 3.77 U/ml, allowed fermentation to be the rate-limiting step in the batch
experiment. In this condition, using initial substrate concentration of 166.0 g/l of total reducing sugars (TRS), 1 ml gel/1 ml
of medium, ethanol productivity of 8.3 g/l/h was achieved, for total conversion of starch to ethanol and 91% of the theoretical
yield. In the continuous runs, feeding 163.0 g/l of TRS and using the same enzyme and yeast concentrations used in the batch
run, ethanol productivity was 5.9 g ethanol/l/h, with 97% of substrate conversion and 81% of the ethanol theoretical yield.
Diffusion effects in the extra-biocatalyst film seemed to be reduced when operating at superficial velocities above 3.7 × 10−4 cm/s. 相似文献
802.
Vicente E Villar R Burguete A Solano B Ancizu S Pérez-Silanes S Aldana I Monge A 《Molecules (Basel, Switzerland)》2008,13(1):86-95
The unexpected substitution of fluorine atoms and phenoxy groups attached to quinoxaline or benzofuroxan rings is described. The synthesis of 2-benzyl- and 2-phenoxy-3-methylquinoxaline 1,4-di-N-oxide derivatives was based on the classical Beirut reaction. The tendency of fluorine atoms linked to quinoxaline or benzofuroxan rings to be replaced by a methoxy group when dissolved in an ammonia saturated solution of methanol was clearly demonstrated. In addition, 2-phenoxyquinoxaline 1,4-di-N-oxide derivatives became 2-aminoquinoxaline 1,4-di-N-oxide derivatives in the presence of gaseous ammonia. 相似文献
803.
Diastereomerically pure cationic Rh(I) complexes derived from phosphinite thioglycosides I were used as catalysts in highly enantioselective hydrogenations of enamides. The conformational similarity of alpha-D-arabinopyranose with beta-L-galactopyranose allows the synthesis of both enantiomers of alpha-amino acid derivatives such as D- and L-DOPA in excellent ee (97% and 98%), using derivatives of the former sugar as catalyst precursors. 相似文献
804.
Muñiz-Valencia R Ceballos-Magaña SG Gonzalo-Lumbreras R Santos-Montes A Izquierdo-Hornillos RC 《Journal of separation science》2008,31(12):2303-2309
An improved sample preparation procedure for the determination of 17 steroids (corticoids (CC) and androgenic anabolic steroids (AAS)), used potentially as growth promoters, in feed samples has been developed. This procedure is based on two reported LC-UV methods. The improved procedure includes a leaching process using ACN, saponification, and SPE using polymeric cartridges. The proposed method was validated according to the EU criteria established for quantitative screening methods in PFS. The extraction efficiencies, decision limits (CCalpha) and detection capabilities (CCbeta), for these compounds were in the ranges of 82-100%, 19-40, and 24-53 microg/kg, respectively. The repeatability and the within-laboratory reproducibility at 1.0, 1.5, and 2.0 CCbeta levels were smaller than 10%. Accuracy was in the 97-101% range. The robustness was evaluated using the Youden robustness test. This method was applied to the analysis of steroids in different kinds of FS with satisfactory results. 相似文献
805.
Muñiz-Valencia R Ceballos-Magaña SG Rosales-Martinez D Gonzalo-Lumbreras R Santos-Montes A Cubedo-Fernandez-Trapiella A Izquierdo-Hornillos RC 《Analytical and bioanalytical chemistry》2008,392(3):523-531
An isocratic LC method for the determination of melamine and its degradation products (ammelide, ammeline, and cyanuric acid),
used to increase the apparent protein content of rice protein concentrate, has been developed. Method development involved
optimization of different RP columns, aqueous mobile phases, pH, phosphate concentration, and temperature. The optimum separation
of these compounds was achieved using a Luna CN column (30 °C), 5 mmol L−1 sodium phosphate (pH 5.0) as mobile phase, 1 mL min−1 flow-rate, UV absorbance-DAD detection at 220 nm, and resorcine as internal standard; this enabled separation of these compounds
with baseline resolution (values in the 2.1–10.1 range) in about 8 min. Prior to HPLC, the developed sample preparation procedure
consisted in a leaching process using the above mentioned mobile phase. Method validation was carried out in rice protein
concentrates in accordance with the European Commission decision 2002/657/EC criteria. For this purpose, eight mandatory performance
characteristics for the conventional validation approach were determined: calibration graphs, extraction efficiencies, decision
limits, detection capabilities, precision (repeatability and within-laboratory reproducibility), accuracy, selectivity, and
robustness. The extraction efficiencies for these compounds were in the range 99–100% and the within-laboratory reproducibility
at 1.0, 1.5, and 2.0 detection capabilities concentration levels were smaller than 5, 4, and 3%, respectively. Finally, the
proposed method was successfully applied to the analysis of other rice protein concentrates and several animal feed samples. 相似文献
806.
Leandro M. de Carvalho Paulo C. do Nascimento Denise Bohrer Raquel Stefanello Eduardo J. Pilau Marcelo B. da Rosa 《Electroanalysis》2008,20(7):776-781
This paper describes a new voltammetric procedure for the inorganic speciation of As(III) and As(V) in water samples. The procedure is based on the chemical reduction of arsenate [As(V)] to arsenite [As(III)] followed by the voltammetric determination of total arsenic as As(III) at the hanging mercury drop electrode (HMDE) by adsorptive cathodic stripping voltammetry (AdCSV) in the presence of sodium diethyl dithiocarbamate (SDDC). The reduction step involved the reaction with a mixture of Na2S2O5 and Na2S2O3 in the concentrations 2.5 and 0.5 mg mL?1, respectively, and the sample heating at 80 °C for 45 min. The linear range for the determination of total arsenic as As(III) in the presence of SDDC was between 5 and 150 μg L?1 for a deposition time of 60 s (r=0.992). A detection limit of 1.05 μg L?1 for total As was calculated for the method in water samples using a deposition time of 60 s. The detection limits of 4.2 μg L?1 and 15.0 μg L?1 for total As in seawater and dialysis concentrates, respectively, were calculated using a deposition time of 60 s. The relative standard deviations calculated were 2.5 and 4.0% for five measurements of 20 μg L?1 As(V) as As(III) in water and dialysis concentrates, respectively, after chemical reduction under optimized conditions. The method was applied for the determination of As(III) and total As in samples of dialysis water, mineral water, seawater and dialysis concentrates. Recovery values between 86.0 and 104.0% for As(III) and As(V) added to the samples prove the satisfactory accuracy and applicability of the procedure for the arsenic monitoring. 相似文献
807.
Sendón R Sanches-Silva A Bustos J Martín P Martínez N Cirugeda ME 《Journal of separation science》2012,35(10-11):1319-1326
Agglomerated stoppers are manufactured from natural cork granules and adhesives. Esters, such as phthalates and adipates, are commonly used in adhesives at concentrations of typically 2-5%. Because of this, and regarding consumer safety, it is necessary to ensure that these compounds do not migrate into the beverage where the cork stopper is used. A reversed-phase high performance liquid chromatography method with tandem mass spectrometry detection is developed for the separation of nine phthalates into 12% ethanol, used as simulant of alcoholic beverages. The chromatographic separation was carried out with a Luna C18 (2) HSTcolumn (50 × 3.0 mm, 2.5 μm) with a gradient elution of water/methanol with 0.1% acetic acid at 300 μL min(-1). The method was validated for four selected phthalates: di-butylphthalate, di-isononylphthalate, di-isodecylphthalate, and butyl-benzyl phthalate, with recoveries ranging between 95% and 112% and intralaboratory precision (RSD) between 5 and 14%, depending on the phthalate. The lowest quantification limit, 0.15 mg kg(-1), was achieved for di-butylphthalate. Nevertheless, in all cases, the limits obtained guarantee the method utility if restriction limits set in Commission Regulation No 10/2011 for plastic materials are taken into account. 相似文献
808.
Correia RF Viseu MI Prazeres TJ Martinho JM 《Journal of colloid and interface science》2012,368(1):56-63
Iron oxide/MCM-41 nanocomposites, Fe(2)O(3)/MCM-41, containing 5%, 10%, and 20% (w/w) iron oxide, were prepared via a direct nonhydrothermal method at room temperature. The preparations were preformed by using iron(III) nitrate, tetra-ethoxysilane (TEOS), and cetyltrimethylammonium bromide (CTAB) mixed or unmixed with dodecyltrimethylammonium bromide (DTAB). The produced materials were dried and calcined at 550 °C for 3 h. Test materials were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), N(2) gas adsorption/desorption isotherms, small angle and wide angle X-ray diffraction (XRD). Results indicate that mixing of CTAB with DTAB does not harm the formation of blank MCM-41 structure. For the composite Fe(2)O(3)/MCM-41 materials, results showed formation of more stable MCM-41 structure with higher surface area and improved porosity in the presence of mixed (CTAB+DTAB) than in the presence of single (CTAB) surfactants for up to 10% Fe(2)O(3)/MCM-41 (w/w). This was explained in terms of the effect DTAB on contraction of the template micellar size to compensate for the expected size expansion upon the addition of ionic iron(III) nitrate precursor. Highly dispersed Fe(2)O(3) nanoparticles were formed in all cases even with the highest iron oxide percentage. Formation of the nanocomposites was postulated to be determined by fast nucleation and slow growth of iron oxide species, which facilitated formation of well dispersed iron oxide nanoparticles inside and on the wall of the MCM-41 material. 相似文献
809.
Toledano M Yamauti M Osorio E Monticelli F Osorio R 《Microscopy and microanalysis》2012,18(2):279-288
The aim was to study the effect of solvents on the phase separation of four commercial dental adhesives. Four materials were tested: Clearfil? SE Bond (CSE), Clearfil Protect Bond (CPB), Clearfil S3 Bond (CS3), and One-Up Bond F Plus (OUB). Distilled water or ethanol was used as a solvent (30 vol%) for microphase separation studies, by stereoscopy. For nanophase images, the mixtures were formulated with two different solvent concentrations (2.5 versus 5 vol%) and observed by atomic force microscopy. Images were analyzed by using MacBiophotonics ImageJ to measure the area of bright domains. Macrophase separations, identified as a loss of clarity, were only observed after mixing the adhesives with water. Nanophase separations were detected with all adhesive combinations. The area of bright domains ranged from 132 to 1,145 nm2 for CSE, from 15 to 285 nm2 for CPB, from 149 to 380 nm2 for CS3, and from 26 to 157 nm2 for OUB. In water-resins mixtures, CPB was the most homogeneous and OUB showed the most heterogeneous phase formation. In ethanol-resin mixtures, CSE attained the most homogeneous structure and OUB showed the most heterogeneous phase. Addition of 5 vol% ethanol to resins decreased the nanophase separation when compared with the control materials. 相似文献
810.
Santamaría-Pérez D Marqués M Chuliá-Jordán R Menendez JM Gomis O Ruiz-Fuertes J Sans JA Errandonea D Recio JM 《Inorganic chemistry》2012,51(9):5289-5298
Angle-dispersive X-ray diffraction measurements have been performed in acanthite, Ag(2)S, up to 18 GPa in order to investigate its high-pressure structural behavior. They have been complemented by ab initio electronic structure calculations. From our experimental data, we have determined that two different high-pressure phase transitions take place at 5 and 10.5 GPa. The first pressure-induced transition is from the initial anti-PbCl(2)-like monoclinic structure (space group P2(1)/n) to an orthorhombic Ag(2)Se-type structure (space group P2(1)2(1)2(1)). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a P2(1)/n phase has been found, which is a slight modification of the low-pressure structure (Co(2)Si-related structure). The initial monoclinic phase was fully recovered after decompression. Density functional and, in particular, GGA+U calculations present an overall good agreement with the experimental results in terms of the high-pressure sequence, cell parameters, and their evolution with pressure. 相似文献