全文获取类型
收费全文 | 32355篇 |
免费 | 894篇 |
国内免费 | 43篇 |
专业分类
电工技术 | 376篇 |
综合类 | 21篇 |
化学工业 | 7035篇 |
金属工艺 | 655篇 |
机械仪表 | 613篇 |
建筑科学 | 1360篇 |
矿业工程 | 348篇 |
能源动力 | 752篇 |
轻工业 | 2280篇 |
水利工程 | 256篇 |
石油天然气 | 249篇 |
无线电 | 2063篇 |
一般工业技术 | 5042篇 |
冶金工业 | 7612篇 |
原子能技术 | 307篇 |
自动化技术 | 4323篇 |
出版年
2022年 | 295篇 |
2021年 | 510篇 |
2020年 | 346篇 |
2019年 | 412篇 |
2018年 | 578篇 |
2017年 | 521篇 |
2016年 | 647篇 |
2015年 | 521篇 |
2014年 | 757篇 |
2013年 | 1999篇 |
2012年 | 1226篇 |
2011年 | 1516篇 |
2010年 | 1136篇 |
2009年 | 1177篇 |
2008年 | 1413篇 |
2007年 | 1263篇 |
2006年 | 1155篇 |
2005年 | 1059篇 |
2004年 | 895篇 |
2003年 | 872篇 |
2002年 | 813篇 |
2001年 | 567篇 |
2000年 | 497篇 |
1999年 | 536篇 |
1998年 | 932篇 |
1997年 | 715篇 |
1996年 | 661篇 |
1995年 | 549篇 |
1994年 | 546篇 |
1993年 | 539篇 |
1992年 | 455篇 |
1991年 | 308篇 |
1990年 | 432篇 |
1989年 | 419篇 |
1988年 | 369篇 |
1987年 | 428篇 |
1986年 | 365篇 |
1985年 | 473篇 |
1984年 | 482篇 |
1983年 | 418篇 |
1982年 | 403篇 |
1981年 | 335篇 |
1980年 | 320篇 |
1979年 | 373篇 |
1978年 | 323篇 |
1977年 | 277篇 |
1976年 | 258篇 |
1975年 | 264篇 |
1974年 | 222篇 |
1973年 | 236篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
V. Petrauskaitè W. F. De Greyt M. J. Kellens 《Journal of the American Oil Chemists' Society》2000,77(6):581-586
In the present study, neutral oil loss (distillative and mechanical carry-over) during physical refining of coconut oil was
quantified. Neutral oil loss seems to depend on both the crude oil quality and the process conditions during deodorization.
The distillation of volatile glyceridic components (monoand diglycerides), originally present in the crude oil, was confirmed
as the major cause for the neutral oil loss. The amount of these volatile components in crude coconut oils cannot be derived
as such from the initial free fatty acid content. A lower deodorization pressure with less sparge steam resulted in a larger
neutral oil loss than a higher pressure with more steam. A “deodorizability” test on a laboratory scale under standardized
conditions (temperature=230°C, pressure=3 mbar, time=60 min, sparge steam=1%), to evaluate crude oil quality and to obtain
a more accurate prediction of the expected neutral oil loss and free fatty acid content in the fatty acid distillate, is described. 相似文献
972.
The intact preen wax esters of the red knot Calidris canutus were studied with gas chromatography/mass spectrometry (GC/MS) and GC/MS/MS. In this latter technique, transitions from the
molecular ion to fragment ions representing the fatty acid moiety of the wax esters were measured, providing additional resolution
to the analysis of wax esters. The C21−C32 wax esters are composed of complex mixtures of hundreds of individual isomers. The odd carbon-numbered wax esters are predominantly
composed of even carbon-numbered n-alcohols (C14, C16, and C18) esterified predominantly with odd carbon-numbered 2-methyl fatty acids (C7, C9, C11, and C13), resulting in relatively simple distributions. The even carbon-numbered wax esters show a far more complex distribution
due to a number of factors: (i) Their n-alcohol moieties are not dominated by even carbon-numbered n-alcohol moieties are not dominated by even carbon-numbered n-alcohols esterified with odd carbon-numbered 2-methyl fatty acids, but odd and even carbon-numbered n-alcohols participate in approximately equal amounts; (ii) odd carbon-numbered methyl-branched alcohols participate abundantly
in these wax ester clusters; and (iii) with increasing molecular weight, various isomers of the 2,6-, 2,8-, and 2,10-dimethyl
branched fatty acids also participate in the even carbon-numbered wax esters. The data demonstrate that there is a clear biosynthetic
control on the wax ester composition although the reasons for the complex chemistry of the waxes are not yet understood. 相似文献
973.
Laurence Lucchese Christopher M. Liauw Norman S. Allen Michele Edge Frank Thompson Robert S. Whitehouse 《Polymer Bulletin》2000,44(2):187-194
Summary Microwave dielectric loss spectroscopy (MDS) has shown remarkable potential as a tool for the observation of changes in elastomer network structure within the interphase region of natural rubber/ carbon black composites. During crosslinking (non elemental sulphur system) the dielectric loss progressively increased. However, during ageing at high temperature the dielectric loss decreased. Parallel studies on the unfilled matrix yielded no such variation in dielectric loss. It was therefore concluded that the changes in microwave responses were related to the development of the interphase during curing and its subsequent destruction during ageing. The microwave response was related to the composite mechanical properties as a function of ageing time. The effect of stabiliser was also investigated. Received: 13 September 1999/Revised version: 7 February 2000/Accepted: 10 February 2000 相似文献
974.
Robert M. Boddey Mark B. Peoples Brian Palmer Peter J. Dart 《Nutrient Cycling in Agroecosystems》2000,57(3):235-270
Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N2 fixed. Some of the problems involved in measuring N2 fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N2-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N2 fixation to trees is the so-called `15N natural abundance' technique which exploits naturally occurring differences in 15N composition between plant-available N sources in the soil and that of atmospheric N2. In this review we discuss probable explanations for the origin of the small differences in 15N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N2 fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N2 (%Ndfa) using the natural abundance procedure requires that both the 15N natural abundance of the N derived from BNF and that derived from the soil by the target N2-fixing species be determined. It is then assumed that the 15N abundance of the N2-fixing species reflects the relative contributions of the N derived from these two sources. The 15N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the 15N abundance of the N acquired from other sources is not several 15N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the 15N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N2-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N2-fixing perennial. The compromise solution is to evaluate the 15N abundance of a diverse range of neighbouring non-N2-fixing plants and to compare these values with that of the N2-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems. 相似文献
975.
Paul A. Moore Jennifer L. Grills Robert W. S. Schneider 《Journal of chemical ecology》2000,26(2):565-584
Many animals use chemical signals to acquire information about their habitats. The structure of this information is dependent upon specific features within a habitat, and the information in signals can be habitat-specific. We quantified the spatial and temporal information in an aquatic odor plume in three different artificial stream habitats with different substrate types by measuring turbulent odor plumes with an electrochemical detection system. Streams had one of three substrate types that correlated with typical aquatic habitats: sand (4.2 × 10–2 cm diameter), gravel (2.5 cm), and cobble (4.5 cm). As predicted from the hydrodynamics, the spatial and temporal structures of the signals were different on different substrates. Spectral analysis showed that the sand and cobble substrates had signals that were dominated by lower frequency fluctuations, whereas gravel had the highest and broadest range of signal fluctuations. Cross- and autocorrelations showed that signals on the gravel substrate had the largest spatial and shortest temporal components. Our results imply that the information obtained from chemical signals may be limited in some habitats. These constraints on information may affect how organisms perform chemically mediated behaviors. 相似文献
976.
Sorghum bicolor is an allelopathic crop that reduces the yield of succeeding crops. We have assessed its effect on the germination, emergence, and seedling growth of Arachis hypogea sown in soil that had had a prior sorghum cropping. A. hypogea was sown on rows and interrows of a previous sorghum crop in 1997 and 1998 in Senegal. Seedling establishment (germination rate and seedling weight) was better between rows than on rows of the previous crop. The highest concentrations of phenolic compounds occurred in the rows in 1998, while contents of row and interrow soils were similar in 1997. Vanillic acid was the main component of the six chemicals found in 1997 soils, whereas the 1998 soil samples contained mainly p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic, and p-coumaric acids (10 phenolics identified). The germination of peanut seeds in water (control), soil water extracts, and mixtures of pure phenolics (equivalent to those in 1997 and 1998 soil samples) was tested. All extracts inhibited germination compared to controls, but there was no significant difference among treatments, i.e., the inhibition was the same for seeds in soil solutions and those in the respective phenolic mixtures. Similarly, there were no significant differences among the germination rates in soil water extracts of rows and interrows or in the pure phenolic mixtures of rows and interrows. We propose a geometrical sowing pattern for peanuts between the rows of the previous sorghum crop to escape the latter's "allelopathic heritage." 相似文献
977.
Besson Michèle Blackburn Andy Gallezot Pierre Kozynchenko Oleksander Pigamo Anne Tennison Steve 《Topics in Catalysis》2000,13(3):253-257
Carbons derived from phenolic resins were used as catalysts for the aqueous phase oxidation of cyclohexanone at 140°C under air pressure. The reaction yielded mainly adipic, glutaric and succinic acids. The samples were modified by heat treatments in CO2, air or N2 at different temperatures and characterized to determine their surface area and porosity and to evaluate the functional groups on the surface. Treatments under CO2 or air increased the oxidation activity. The selectivity to adipic acid was maximum (33%) after activation in air which greatly increased the surface concentration of oxygen-containing functional groups. However, it was not possible to attribute the selectivity to specific acidic, neutral or basic groups present at the surface of carbons. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
978.
A novel surfactant and hydrotrope, sodium diisopropylnaphthalene sulfonate (SDIPNS) has been developed. It contains about
92% diisopropylnaphthalene sulfonate, compared to other materials which are less than 50% diisopropylnaphthalene sulfonate.
Aqueous solutions of 34–36% active SDIPNS have dual functionality. They have excellent surface properties and are compatible
with conventional anionic, nonionic, and amphoteric surfactants. They demonstrate good laundering detergency in combination
with sodium lauryl ethoxy sulfate, with or without builder. They maintain surface activity in 150 ppm hard water (Ca2+/Mg2+=2∶1), 5% NaCl, pH 2, and pH 12. They are effective hydrotropes. They enhance surfactant solubility, raise the cloud point
of nonionic surfactants, and modify the viscosity of surfactant formulations. They are light in color and are low-foaming.
Presented as a Poster Session at the American Oil Chemists' Society Annual Meeting, May 9–12, 1999, Orlando, Florida. 相似文献
979.
Thomas Bürgi Fachri Atamny Axel Knop‐Gericke Michael Hävecker Thomas Schedel‐Niedrig Robert Schlögl Alfons Baiker 《Catalysis Letters》2000,66(3):109-112
The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of
hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular
dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via
the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate
also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π*
and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate
the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation
over cinchona‐alkaloid‐modified Pt.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
980.
A test system based around a thin‐walled extruded solid electrolyte tubular reactor has been developed, which enables the
fuel reforming catalysis and surface chemistry occurring within solid oxide fuel cells and the electrochemical performance
of the fuel cell to be studied under genuine operating conditions. It permits simultaneous monitoring of the catalytic chemistry
and the cell performance, allowing direct correlation between the fuel cell performance and the reforming characteristics
of the anode, as well as enabling the influence of drawing current on the catalysis and surface reaction pathways to be studied.
Temperature‐programmed reaction measurements can be carried out on anodes in an actual SOFC, and have been used to investigate
the reduction characteristics of different anode formulations, methane activation and methane steam reforming, and to evaluate
the nature and level of carbon deposition on the anode during reforming.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献