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991.
Ureas characteristically form one-dimensional hydrogen-bonded alpha-networks with a repeat distance of about 4.60 A. Oxamides form similar alpha-networks with a longer 5.05 A repeat distance. The urea of glycine and the oxamide of glycine were each cocrystallized with a series of four bipyridines, including two urea derivatives and two oxamide derivatives. This series of eight cocrystals was studied by X-ray diffraction in order to see what would happen when molecules that would normally form alpha-networks with incommensurate distances were forced into the same crystal. The two all-urea crystals and the two all-oxamide crystals contained the expected alpha-networks with repeat distances in accordance with normal urea or oxamide values. Four of the crystals were mixed, containing both oxamide and urea molecules. Three consisted of two-dimensional beta-networks with alternating parallel urea and oxamide subnetworks. The repeat distances averaged 4.87 A, a value close to the value expected for oxamides, but shorter than any previously observed examples. In the fourth mixed crystal, the urea alpha-network formed with a normal urea repeat distance, but the oxamide network did not form, the oxamide adopting an unusual molecular conformation that maximizes intramolecular hydrogen bonds instead. 相似文献
992.
Nguyen N. Son Y. Nagame I. Nishinaka 《Journal of Radioanalytical and Nuclear Chemistry》2001,250(2):255-261
Data on fragment mass yields in spontaneous and thermal neutron induced fission of wide range of nuclides from light actinides through heavy actinides were analyzed and completely described as mass distribution parameters, based on the concept of multimodal fission. The evidence that supports the existence of multi-components in the mass distribution of fission fragments, bimodal asymmetric mode in light actinide fission, bimodal symmetric mode in heavy actinide fission, and the existence of the boundaries of fission asymmetry has been obtained. 相似文献
993.
Kim Kyoung Heon Tucker Melvin P. Keller Fred A. Aden Andy Nguyen Quang A. 《Applied biochemistry and biotechnology》2001,91(1-9):253-267
Two-stage dilute acid pretreatment followed by enzymatic cellulose hydrolysisis an effectivemethod for obtaining high sugar
yields from wood residuessuchassoftwood forest thinnings. In the first-stage hydrolysis step, most of the hemicellulose is
solubilized using relatively mild conditions. The soluble hemicellu losic sugars are recovered from the hydrolysateslurry
by washing with water. The washed solids are then subjected tomoresevere hydrolysis conditions to hydrolyze approx 50% of
the cellulose to glucose. The remaining cellulose can further be hydrolyzed with cellulase enzyme. Our process simulation
indicates that the amount of water used in the hemicellulose recovery step has a significan tim pact on the cost of ethanol
production. It is important to keep water usage as low as possible while mainta ining relatively high recovery of solublesugars.
To achieve this objective, a prototype pilot-scale continuous countercurrent screw extractor was evaluated for the recovery
of hemicellulose from pretreated forest thinnings. Using the 274-cm (9-ft) long extractor, solubles recoveries of 98, 91,
and 77% were obtained with liquid-to-insoluble solids (L/1S) ratios of 5.6, 3.4, and 2.1, respectively. An empirical equation
was developed to predict the performance of the screwextractor. This equation predicts that soluble sugar recovery above 95%
can be obtained with an L/IS ratio as low as 3.0. 相似文献
994.
The capillary viscometer is used to measure the shear stress-shear rate relationship of a wide range of purely viscous fluids.
It is however not considered as an appropriate instrument for obtaining the yield stress and the post-yield behaviour of fluids
that have a yield stress. This is partly because conventional methods of processing the capillary viscometry data of purely
viscous fluids cannot be applied to similar data of fluids with yield stress. The unavoidable experimental noise in the capillary
data, particularly at low shear rates, also makes it difficult to obtain a reliable estimate of the yield stress from capillary
data. In this investigation the problem of converting the capillary viscometry data of yield stress fluids into a shear stress-shear
rate curve and a yield stress is formulated as a Volterra integral equation of the first kind. This is an ill-posed problem
i.e. noise in the data will be amplified by inappropriate methods of data processing. A method, based on Tikhonov regularisation
that takes into account the ill-posed nature of the problem, is then developed to solve this problem for fluids with yield
stress. The performance of this method is assessed by applying it to a set of “synthetic” capillary viscometry data with added
random noise and to a set of experimental data for a concentrated suspension of TiO2 taken from the literature. In both cases Tikhonov regularisation was able to extract the complete shear properties of these
fluids from capillary viscometry data alone.
Received: 22 November 1999/Accepted: 17 December 1999 相似文献
995.
Tucker Melvin P. Mitri Ragheed K. Eddy Fannie P. Nguyen Q. A. Gedvilas Lynn M. Webb John D. 《Applied biochemistry and biotechnology》2000,84(1-9):39-50
The process of converting renewable lignocellulosic biomass to ethanol requires a number of steps, and pretreatment is one
of the most important. Pretreatment usually in volves a hydrolysis of the easily hydrolyzed hemicellulosic component of biomass
using some form of thermal/chemical/mechanical action that results in a product that can be further hydrolyzed by cellulase
enzymes (the cellulosic portion). The sugars produced can then befermented to ethanol by fermentative microorganisms. If the
pretreatment step is not severe enough, the resultant residue is not as easily hydrolyzed by the cellulase enzyme. More severe
pretreatment conditions result in the production of degradation products that are toxic to the fermentative microorgan ism.
In this article, wereport the quantitative analysis of glucose, mannose, xylose, and acetic acid using Fourier transform infrared
(FTIR) spectroscopy on liquors from dilute-acid-pretreated softwood and hard wood slurries. Comparison of FTIR and high-performance
liquid chromatography quantitative analyses of these liquorsare reported. Recent developments in infrared probe technology
has enabled the rapid quantification of these sugars by FTIR spectroscopy in the batch reactor during optimization of the
pretreatment conditions, or interfaced to the computer controlling a continuous reactor for on-line monitoring and control. 相似文献
996.
Nguyen Quang A. Tucker Melvin P. Keller Fred A. Eddy Fannie P. 《Applied biochemistry and biotechnology》2000,84(1-9):561-576
Whole treechips obtained from softwood forest thinnings were pretreated via single-and two-stage dilute-sulfuric acid pretreatment.
Whole-tree chips were impregnated with dilute sulfuric acid and steam treated in a 4-L steam explosion reactor. In single-stage
pretreatment, wood chips were treated using a wide range of severity. In two-stage pretreatment, the first stage was carried
out at low severity tomaximize hemicellulose recovery. Solubilized sugars were recovered from the first-stage prehydrolysate
by washing with water. In the second stage, water-insoluble solids from first-stage prehydrolysate were impregnated with dilute
sulfuric acid, then steam treated at more severe conditions to hydrolyze a portion of the remaining cellulose to glucose and
to improve the enzyme digestibility. The total sugar yields obtained after enzymatic hydrolysis of two-stage dilute acid-pretreated
samples were compared with sugar yields from single-stage pretreatment. The overall sugar yield from two-stage dilute-acid
pretreatment was approx 10% higher, and the net enzyme requirement was reduced by about 50%. Simultaneous saccharification
and fermentation using an adapted Saccharomyces cerevisiae yeast strain further improved cellulose conversion yield and lowered the enzyme requirement. 相似文献
997.
Hong Kui Chen Guo Qiang Fu Yu Peter Hoi Zhang Guang Liu Ye Chua Hong 《Applied biochemistry and biotechnology》2000,84(1-9):971-980
Polyhydroxyalkanoates (PHAs) are biodegradable polymers produced by bacteria. In this study, the effect of C∶N molar ratio
on the monomer composition of PHAs was investigated, including medium chain length PHA produced by Pseudomonas mendocina 0806 and PHA blends consisting of monomers of 3-hydroxybutyrate and medium chain length hydroxyalkan⇘te produced by Pseudomonas pseudoalkaligenus YS1. It was observed that there were some fixed ranges of C∶N molar ratio that affect the monomer composition of PHA independently
of the substrate. For strain 0806, the ranges were C∶N <20, 20<C∶N<200, and C∶N>200. The monomer composition was constant
among these ranges when using glucose and octanoate as the sole substrate. For strain YS1, the ranges were C∶N<20, 20<C∶N<45,
and C∶N>45. These results are useful for controlling monomer composition in PHA production. 相似文献
998.
Nguyen V. Thoai 《Journal of Global Optimization》2000,18(4):321-336
The problem of optimizing some contiuous function over the efficient set of a multiple objective programming problem can be formulated as a nonconvex global optimization problem with special structure. Based on the conical branch and bound algorithm in global optimization, we establish an algorithm for optimizing over efficient sets and discuss about the implementation of this algorithm for some interesting special cases including the case of biobjective programming problems. 相似文献
999.
N. Antoni Q.S. Nguyen P. Ragot 《International Journal of Solids and Structures》2008,45(18-19):5189-5203
In automotive components, the cumulative microslip phenomenon is often observed for engine assemblies. This phenomenon results in an accumulation of the relative slips in a preferred tangential direction on the contact interface of two solids under cyclic loadings. A significant relative displacement may occur and leads to the assembly failure. In particular, a global rotation of the bearing shell may result from this mechanism of cumulated slips in conrod big end systems. To discuss this rotation problem, a model of two circular beams in frictional contact and submitted to a periodical rotating load is considered here. The aim is to give some simplified estimates of the critical rotation load based on a slip-shakedown analysis. The discussion holds for Tresca friction and can be extended to Coulomb friction under the assumption of small coupling. The static and kinematic slip-shakedown approaches are discussed. The obtained analytical results are shown to be in agreement with the finite element computations. 相似文献
1000.