全文获取类型
收费全文 | 1672篇 |
免费 | 58篇 |
国内免费 | 10篇 |
专业分类
电工技术 | 18篇 |
综合类 | 1篇 |
化学工业 | 480篇 |
金属工艺 | 32篇 |
机械仪表 | 9篇 |
建筑科学 | 63篇 |
矿业工程 | 2篇 |
能源动力 | 43篇 |
轻工业 | 169篇 |
水利工程 | 10篇 |
石油天然气 | 10篇 |
无线电 | 171篇 |
一般工业技术 | 307篇 |
冶金工业 | 188篇 |
原子能技术 | 13篇 |
自动化技术 | 224篇 |
出版年
2022年 | 8篇 |
2021年 | 21篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 20篇 |
2017年 | 25篇 |
2016年 | 26篇 |
2015年 | 28篇 |
2014年 | 42篇 |
2013年 | 89篇 |
2012年 | 58篇 |
2011年 | 103篇 |
2010年 | 68篇 |
2009年 | 78篇 |
2008年 | 94篇 |
2007年 | 67篇 |
2006年 | 76篇 |
2005年 | 50篇 |
2004年 | 63篇 |
2003年 | 52篇 |
2002年 | 71篇 |
2001年 | 35篇 |
2000年 | 36篇 |
1999年 | 40篇 |
1998年 | 51篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 24篇 |
1994年 | 29篇 |
1993年 | 26篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 17篇 |
1989年 | 23篇 |
1988年 | 19篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 29篇 |
1984年 | 13篇 |
1983年 | 22篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 10篇 |
1971年 | 8篇 |
排序方式: 共有1740条查询结果,搜索用时 31 毫秒
991.
The Δ5-unsaturated polymethylene-interrupted fatty acid (Δ5-UPIFA) contents and profiles of gymnosperm seeds are useful chemometric
data for the taxonomy and phylogeny of that division, and these acids may also have some biomedical or nutritional applications.
We recapitulate here all data available on pine (Pinus; the largest genus in the family Pinaceae) seed fatty acid (SFA) compositions, including 28 unpublished compositions. This
overview encompasses 76 species, subspecies, and varieties, which is approximately one-half of all extant pines officially
recognized at these taxon levels. Qualitatively, the SFA from all pine species analyzed so far are identical. The genus Pinus is coherently united—but this qualitative feature can be extended to the whole family Pinaceae—by the presence of Δ5-UPIFA
with C18 [taxoleic (5,9–18∶2) and pinolenic (5,9,12–18∶3) acids] and C20 chains [5,11–20∶2, and sciadonic (5,11,14–20∶3) acids]. Not a single pine species was found so far with any of these acids
missing. Linoleic acid is almost always, except in a few cases, the prominent SFA, in the range 40–60% of total fatty acids.
The second habitual SFA is oleic acid, from 12 to 30%. Exceptions, however, occur, particularly in the Cembroides subsection, where oleic acid reaches ca. 45%, a value higher than that of linoleic acid. α-Linolenic acid, on the other hand, is a minor constituent of pine SFA,
almost always less than 1%, but that would reach 2.7% in one species (P. merkusii). The sum of saturated acids [16∶0 (major) and 18∶0 (minor) acids principally] is most often less than 10% of total SFA,
and anteiso-17∶0 acid is present in all species in amounts up to 0.3%. Regarding C18 Δ5-UPIFA, taxoleic acid reaches a maximum of 4.5% of total SFA, whereas pinolenic acid varies from 0.1 to 25.3%. The very
minor coniferonic (5,9,12,15–18∶4) acid is less than 0.2% in all species. The C20 elongation product of pinolenic acid, bishomo-pinolenic (7,11,14–20∶3) acid, is a frequent though minor SFA constituent (maximum,
0.7%). When considering C20 Δ5-UPIFA, a difference is noted between the subgenera Strobus and Pinus. In the former subgenus, 5,11–20∶2 and sciadonic acids are ≤0.3 and ≤1.9%, respectively, whereas in the latter subgenus,
they are most often ≥0.3 and ≥2.0%, respectively. The highest values for 5,11–20∶2 and sciadonic acids are 0.5% (many species)
and 7.0% (P. pinaster). The 5,11,14,17–20∶4 (juniperonic) acid is present occasionally in trace amounts. The highest level of total Δ5-UPIFA is
30–31% (P. sylvestris), and the lowest level is 0.6% (P. monophylla). Uniting as well as discriminating features that may complement the knowledge about the taxonomy and phylogeny of pines
are emphasized. 相似文献
992.
Yves Houtmann Benoit Delebecque Geoffroy Lauvaux Claude Barlier Gabriel Ris 《Virtual and Physical Prototyping》2007,2(2):127-133
Slicing represents an important step in all rapid prototyping processes. This paper proposes the use of critical points for adaptive local slicing for strataconception in particular and rapid prototyping technologies in general. The context of using critical points in rapid prototyping to optimise usual slicing is presented. Its definition on both surfaces and triangular meshings is given and extended to other geometric entities, such as edges, facets and faces. Finally, this new slicing approach is compared with the usual one on a few models manufactured through the stratoconception process. 相似文献
993.
Occurrence of perfluorooctane sulfonate and other perfluorinated alkylated substances in harbor porpoises from the Black Sea 总被引:1,自引:0,他引:1
Van de Vijver KI Holsbeek L Das K Blust R Joiris C De Coen W 《Environmental science & technology》2007,41(1):315-320
Perfluorooctane sulfonate (PFOS) and other perfluorinated alkylated substances (PFAS) were determined in liver, kidney, muscle, brain, and blubber samples of 31 harbor porpoises (Phocoena phocoena relicta) of different age and sex stranded along the Ukrainian coast of the Black Sea. In all individuals and in all tissues, PFOS was the predominant PFAS, accounting for on average 90% of the measured PFAS load. PFOS concentrations were the highest in liver (327+/-351 ng/g wet wt) and kidney (147 +/-262 ng/g wet wt) tissue, and lower in blubber (18+/-8 ng/g wet wt), muscle (41+/-50 ng/g wet wt), and brain (24 +/-23 ng/g wetwt). No significant differences could be determined between males and females, nor between juvenile and adult animals (p > 0.05). Perfluorononanoic acid, perfluorodecanoic acid, perfluoroundecanoic acid, and perfluorododecanoic acid could be detected in liver tissue of approximately 25% of the individuals. Perfluorobutane sulfonate, perfluorobutanoic acid, and perfluorooctanoic acid were not detected in any of the porpoise livers. Although we investigated a potential intraspecies segregation according to the source of prey, using stable isotopes, no statistically significant correlation between PFOS concentrations and stable isotopes could be determined. It is, however, noteworthy that the contamination by PFOS in the Black Sea harbor porpoises is comparable to levels found in porpoises from the German Baltic Sea and from coastal areas near Denmark and, therefore, might pose a threat to this population. 相似文献
994.
Farine JP Sirugue D Abed-Vieillard D Everaerts C Le Quéré JL Bonnard O Brossut R 《Journal of chemical ecology》2007,33(2):405-415
In Leucophaea maderae, male calling behavior involves the release of a sex pheromone from the abdominal sternal glands. An extract of sternal glands
attracted conspecific females over a distance. The compounds present were identified as hydroxy-3-butan-2-one, (2R, 3R)-butanediol, senecioic acid, and (E)-2-octenoic acid. The same components are also present in male tergal glands. The identified compounds were tested on their
own and in mixtures. Their biological function is discussed. 相似文献
995.
Ouimet MC Brown TG Nadeau L Lepage M Pelletier M Couture S Tremblay J Legault L Dongier M Gianoulakis C Ng Ying Kin NM 《Accident; analysis and prevention》2007,39(4):743-750
Individuals who drive under the influence (DUI) of alcohol may be at greater risk for neurocognitive impairment because of their exposure to multiple sources of neurological risk. This could contribute to the persistence of DUI behaviour and influence the effectiveness of remedial interventions. The objectives of this study were to clarify the neurocognitive characteristics of DUI recidivists and the nature of potential impairments, and to explore relationships between these characteristics and the frequency of past DUI convictions. One hundred male recidivists were evaluated for visuospatial constructional abilities and visual memory, verbal fluency, attention skills, cognitive flexibility, spatial planning, and verbal and movement inhibition. Results indicated that a majority of recidivists showed signs of neurocognitive impairment on several dimensions. Impairment was most marked on visuospatial constructional abilities and visual memory. In contrast to previous studies, no participants were found to have impulse control problems. Measures of memory and cognitive efficiency were significantly associated with the frequency of past convictions. Finally, exploratory analyses of two potential sources of impairment, alcohol exposure and head trauma, suggested the role of excessive alcohol use as the most obvious associated factor. Overall, the findings indicate that neurocognitive impairments are a common feature in recidivists and may contribute to DUI persistence. Development of a DUI-specific neurocognitive assessment and greater understanding of how neurocognitive status influences DUI risk could lead to remediation strategies better adapted to the individual characteristics of recidivists. 相似文献
996.
Maalouf R Fournier-Wirth C Coste J Chebib H Saïkali Y Vittori O Errachid A Cloarec JP Martelet C Jaffrezic-Renault N 《Analytical chemistry》2007,79(13):4879-4886
The low but known risk of bacterial contamination has emerged as the greatest residual threat of transfusion-transmitted diseases. Label-free detection of a bacterial model, Escherichia coli, is performed using nonfaradic electrochemical impedance spectroscopy (EIS). Biotinylated polyclonal anti-E. coli is linked to a mixed self-assembled monolayer (SAM) on a gold electrode through a strong biotin-neutravidin interaction. The binding of one antibody molecule for 3.6 neutravidin molecules is determined using the surface plasmon resonance (SPR). The detection limit of E. coli found by SPR is 10(7) cfu/mL. After modeling the impedance Nyquist plot of E. coli/anti-E. coli/mixed SAM/gold electrode for increasing concentrations of E. coli (whole bacteria or lysed bacteria), the main parameter that is modified is the polarization resistance RP. A sigmoid variation of RP is observed when the log concentration of bacteria (whole or lysed) increases. A concentration of 10 cfu/mL whole bacteria is detected by EIS measurements while 103 cfu/mL is detected for lysed E. coli. 相似文献
997.
Yu Zhang Jing Bai Ziqi Guan Xinzeng Liang Yansong Li Jianglong Gu Yudong Zhang Claude Esling Xiang Zhao Liang Zuo 《金属学报(英文版)》2023,36(3):513
The phase stability, magnetic properties, martensitic transformation, and electronic properties of the Ni2−xMn1+x+ySn1−y system with excess Mn have been systematically investigated by the first-principles calculations. Results indicate that the excess Mn atoms will directly occupy the sublattices of Ni (MnNi) or Sn (MnSn). The formation energy (Ef) of the austenite has a relationship with the Mn content: Ef = 135.27(1 + x + y) − 293.01, that is, the phase stability of the austenite decreases gradually with the increase in Mn content. According to the results of the formation energy of austenite, there is an antiparallel arrangement of the magnetic moment between the excess and normal Mn atoms in the Ni2−xMn1+x+ySn1−y (x = 0 or y = 0) system, while the magnetic moment direction of the normal Mn atoms arranges antiparallel to that of MnNi atoms and parallel to that of MnSn atoms in the Ni2−xMn1+x+ySn1−y (x, y ≠ 0) system. The martensitic transformation occurs in some Ni2−xMn1+x+ySn1−y (x, y ≠ 0) alloys with large magnetic moments of ferrimagnetic austenite. Besides, the valence electrons tend to distribute around the Ni or MnNi atoms and mainly bond with the normal Mn atoms. The results of this work can lay a theoretical foundation for further development of the Ni2−xMn1+x+ySn1−y system as the potential ferromagnetic shape memory alloys. 相似文献
998.
Guanghua Chi Jean‐Claude Thill Daoqin Tong Li Shi Yu Liu 《Papers in Regional Science》2016,95(3):613-631
We introduce network science methods to uncover inherent characteristics of functional regions. An aggregate spatial interaction network is constructed based on a large mobile phone data set including 431 million mobile calls made by 10 million anonymous customers over one month and the geographic locations of the mobile base towers involved in each call. We use Thiessen polygons (termed ‘cells’) as the unit of analysis to approximate the service area of each mobile base tower. Major findings encompass the following three aspects. First, cells with high betweenness centrality are linearly distributed in space, which closely aligns with major transportation corridors. We find that this pattern can be explained by analysing the characteristics of calling activities on transportation networks. Second, we detect a two‐level hierarchy of communities that correspond well to county and prefecture‐level administrative unit boundaries. Lastly, almost every community identified at the two hierarchical levels contains a cell with high betweenness. These cells are located near the political and economic centres and play the role of hubs in the regional socio‐economic system. This research demonstrates that networks built from mobile phone data provide new understandings of spatial interactions and regional structures. 相似文献
999.
The lipids of three gram-negative bacteria, Acineto-bacter calcoaceticus, Marinobacter aquaeolei, and Pseudomonas oleovorans grown on mineral media supplemented with ammonium acetate or hydrocarbons, were isolated, purified, and their structures
determined. Three pools of lipids were isolated according to a sequential procedure: unbound lipids extracted with organic
solvents, comprising metabolic lipids and the main part of membrane lipids, OH−-labile lipids (mainly ester-bound in the lipopolysaccharides, LPS) and H+-labile lipids (mainly amidebound in the LPS). Unsaturated FA composition gave evidence for an aerobic desaturation pathway
for the synthesis of these acids in A. calcoaceticus and M. aquaeolei, a nonclassic route in gram-negative bacteria. Surprisingly, both aerobic and anaerobic pathways are operating in the studied
strain of P. oleovorans. The increase of the proportion of saturated FA observed for the strain of P. oleovorans grown on light hydrocarbons would increase the temperature transition of the lipids for maintaining the inner membrane fluidity.
An opposite phenomenon occurs in A. calcoaceticus and M. aquaeolei grown on solid or highly viscous C19 hydrocarbons. The increases of FA<C18 when the bacteria were grown on n-nonadecane, or of iso-FA in cultures on iso-nonadecane would decrease the transition temperature of the lipids, to maintain the fluidity of the inner membranes. Moreover,
P. oleovorans grown on hydrocarbons greatly decreases the proportion of β-hydroxy acids of LPS, thus likely maintaining the physical properties
of the outer membrane. By contrast, no dramatic change in hydroxy acid composition occurred in the other two bacteria. 相似文献
1000.
Goldman N Leforestier C Saykally RJ 《Philosophical transactions. Series A, Mathematical, physical, and engineering sciences》2005,363(1827):493-508
We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface (the third fitting of the Anisotropic Site Potential with Woermer dispersion to vibration-rotation-tunnelling data). VRT(ASP-W)III is shown to not only be a model of high 'spectroscopic' accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared with those from ab initio molecular dynamics, other potentials of 'spectroscopic' accuracy and with experiment. The results herein represent the first time to the authors' knowledge that a 'spectroscopic' potential surface is able to correctly model condensed phase properties of water. 相似文献