全文获取类型
收费全文 | 8917篇 |
免费 | 682篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 49篇 |
综合类 | 3篇 |
化学工业 | 2149篇 |
金属工艺 | 135篇 |
机械仪表 | 185篇 |
建筑科学 | 357篇 |
矿业工程 | 24篇 |
能源动力 | 236篇 |
轻工业 | 2354篇 |
水利工程 | 87篇 |
石油天然气 | 44篇 |
无线电 | 416篇 |
一般工业技术 | 1649篇 |
冶金工业 | 544篇 |
原子能技术 | 37篇 |
自动化技术 | 1336篇 |
出版年
2024年 | 12篇 |
2023年 | 95篇 |
2022年 | 162篇 |
2021年 | 318篇 |
2020年 | 231篇 |
2019年 | 262篇 |
2018年 | 378篇 |
2017年 | 409篇 |
2016年 | 383篇 |
2015年 | 296篇 |
2014年 | 406篇 |
2013年 | 828篇 |
2012年 | 585篇 |
2011年 | 629篇 |
2010年 | 494篇 |
2009年 | 473篇 |
2008年 | 495篇 |
2007年 | 457篇 |
2006年 | 376篇 |
2005年 | 237篇 |
2004年 | 224篇 |
2003年 | 224篇 |
2002年 | 198篇 |
2001年 | 151篇 |
2000年 | 130篇 |
1999年 | 121篇 |
1998年 | 218篇 |
1997年 | 157篇 |
1996年 | 102篇 |
1995年 | 89篇 |
1994年 | 75篇 |
1993年 | 59篇 |
1992年 | 33篇 |
1991年 | 29篇 |
1990年 | 22篇 |
1989年 | 33篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 23篇 |
1984年 | 26篇 |
1983年 | 25篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 12篇 |
1973年 | 12篇 |
排序方式: 共有9605条查询结果,搜索用时 31 毫秒
991.
992.
Vargas-Murga L Garcia-Alvarez A Roman-Viñas B Ngo J Ribas-Barba L van den Berg SJ Williamson G Serra-Majem L 《Food & function》2011,2(12):731-739
The popularity of herbal products, especially plant food supplements (PFS) and herbal medicine is on the rise in Europe and other parts of the world, with increased use in the general population as well as among specific subgroups encompassing children, women or those suffering from diseases such as cancer. The aim of this paper is to examine the PFS market structures in European Community (EC) Member States as well as to examine issues addressing methodologies and consumption data relating to PFS use in Europe. A revision of recent reports on market data, trends and main distribution channels, in addition an example of the consumption of PFS in Spain, is presented. An overview of the methods and administration techniques used to assess individual food consumption as a starting point, including their uses and limitations, as well as some examples of studies that collect Food Supplement (FS) information, including herbal/botanical/plant-derived products are also discussed. Additionally, the intake estimation process of food nutrients is described and used to propose the PFS ingredients intake estimation process. Nationally representative PFS consumption data is scarce in Europe. The majority of studies have been conducted in Scandinavia and the UK. However the heterogeneity of definitions, study design and objectives make it difficult to compare results and extrapolate conclusions. 相似文献
993.
The gas-phase kinetic coefficients of OH radicals with two primary fluorinated alcohols, CF(3)CH(2)CH(2)OH (k(1)) and CF(3)(CH(2))(2)CH(2)OH (k(2)), potential replacements of hydrofluorocarbons (HFCs), are reported here as a function of temperature (T = 263-358 K) for the first time. k(1) and k(2) (together referred as k(i)) were measured under pseudo-first-order conditions with respect to the initial OH concentration using the pulsed laser photolysis/laser induced fluorescence technique. The observed temperature dependence of k(i) (in cm(3) molecule(-1) s(-1)) is described by the following Arrhenius expressions: k(1)(T) = (2.82 ± 1.28) × 10(-12) exp{-(302 ± 139)/T} cm(3) molecule(-1) s(-1) and k(2)(T) = (1.20 ± 0.73) × 10(-11) exp{-(425 ± 188)/T} cm(3) molecule(-1) s(-1).The uncertainties in the Arrhenius parameters are at a 95% confidence level (± 2σ). Uncertainties in k(i)(T) include both statistical and systematic errors. Activation energies were (2.5 ± 1.2) kJ/mol and (3.6 ± 1.6) kJ/mol for the OH-reaction with CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH, respectively. The global lifetime (τ) at 275 K for CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH due to the OH-reaction was estimated to be ca. 2 weeks and 5 days, respectively. The reported Arrhenius parameters can be used in 3D models that take into account the geographical region and season of emissions for estimating a matrix of instantaneous lifetimes. As a consequence of the substitution of the -CH(3) group by a -CH(2)OH group in HFCs, such as CF(3)CH(2)CH(3) and CF(3)(CH(2))(2)CH(3), the tropospheric lifetime with respect to the OH reaction is significantly shorter and, since their radiative forcing is similar, global warming potentials of CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH are negligible. Therefore, CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH seem to be suitable alternatives to HFCs. 相似文献
994.
Mellisho CD González-Barrio R Ferreres F Ortuño MF Conejero W Torrecillas A García-Mina JM Medina S Gil-Izquierdo A 《Journal of the science of food and agriculture》2011,91(12):2132-2139
BACKGROUND This study was designed to describe the phenolic status of lemon juice obtained from fruits of lemon trees differing in iron (Fe) nutritional status. Three types of Fe(III) compound were used in the experiment, namely a synthetic chelate and two complexes derived from natural polymers of humic and lignine nature. RESULTS All three Fe(III) compounds were able to improve the Fe nutritional status of lemon trees, though to different degrees. This Fe(III) compound effect led to changes in the polyphenol content of lemon juice. Total phenolics were decreased (~33% average decrease) and, in particular, flavanones, flavones and flavonols were affected similarly. CONCLUSION Iron‐deficient trees showed higher phenolic contents than Fe(III) compound‐treated trees, though Fe deficiency had negative effects on the yield and visual quality of fruits. However, from a human nutritional point of view and owing to the health‐beneficial properties of their bioavailable phenolic compounds, the nutritional quality of fruits of Fe‐deficient lemon trees in terms of phenolics was higher than that of fruits of Fe(III) compound‐treated lemon trees. Moreover, diosmetin‐6,8‐di‐C‐glucoside in lemon juice can be used as a marker for correction of Fe deficiency in lemon trees. Copyright © 2011 Society of Chemical Industry 相似文献
995.
Sheyla Rehecho Mikel García-Iñiguez de Cirano Iciar Astiasarán Rita Yolanda Cavero 《LWT》2011,44(4):875-882
Aqueous and hydroalcoholic extracts from Verbena officinalis L. were obtained and characterised. The analysis by HPLC-DAD and LC-MS allowed the detection and identification of three iridoids, fifteen flavonoids and four phenolic acid derivatives. Four flavonoids, scutellarein 7-diglucuronide (9), scutellarein 7-glucuronide (13), pedalitin 6-galactoside (15) and scutellarein 7-glucoside (19) are reported for the first time from this plant. In addition, three new flavonoids have been isolated: scutellarein 7-O-(2-O-feruloyl)-diglucuronide (5), pedalitin 6-O-diglucuronide (6) and pedalitin 6-O-(2-O-feruloyl)-diglucuronide (13). To our knowledge, these flavonoids have not been reported as natural products. Both extracts showed significant antioxidant activity using three in vitro model systems and the results have been correlated with total phenolic and total flavonoid contents. The results have allowed establishing an important relation structure-activity and significant correlations have also been found between the mineral content and the flavonoids present in both extracts. 相似文献
996.
Curiel JA Ruiz-Capillas C de Las Rivas B Carrascosa AV Jiménez-Colmenero F Muñoz R 《Meat science》2011,88(3):368-373
The occurrence of in vitro amino acid activity in bacterial strains associated with fresh pork sausages packaged in different atmospheres and kept in refrigeration was studied. The presence of biogenic amines in decarboxylase broth was confirmed by ion-exchange chromatography and by the presence of the corresponding decarboxylase genes by PCR. From the 93 lactic acid bacteria and 100 enterobacteria strains analysed, the decarboxylase medium underestimates the number of biogenic amine-producer strains. 28% of the lactic acid bacteria produced tyramine and presented the tdc gene. All the tyramine-producer strains were molecularly identified as Carnobacterium divergens. Differences on the relative abundance of C. divergens were observed among the different packaging atmospheres assayed. After 28 days of storage, the presence of argon seems to inhibit C. divergens growth, while packing under vacuum seems to favour it. Among enterobacteria, putrescine was the amine more frequently produced (87%), followed by cadaverine (85%); agmatine and tyramine were only produced by 13 and 1%, respectively, of the strains analysed. Packing under vacuum or in an atmosphere containing nitrogen seems to inhibit the growth of enterobacteria which produce simultaneously putrescine, cadaverine, and agmatine. Contrarily, over-wrapping or packing in an atmosphere containing argon seems to favour the growth of agmatine producer-enterobacteria. The production of putrescine and cadaverine was associated with the presence of the corresponding amino acid decarboxylase genes. The biogenic amine-producer strains were included in a wide range of enterobacterial species, including Kluyvera intermedia, Enterobacter aerogenes, Yersinia kristensenii, Serratia grimesii, Serratia ficaria, Yersinia rodhei, Providencia vermicola and Obesumbacterium proteus. 相似文献
997.
Iris C. Zampini Roxana Ordoñez Norberto P. Giannini Pedro G. Blendinger María Inés Isla 《Food research international (Ottawa, Ont.)》2011,44(7):2345-2351
Fruits are known as good sources of phytochemicals, essential to prevent degenerative diseases like cancer and cardiovascular diseases. They contain a variety of antioxidants, which are useful to scavenge radical oxygen species (ROS). Considering the importance of natural products as a functional food, comparative studies between the antioxidant activity (AOA) and antimutagenic properties as well as phytochemical profile of Cactaceae fruits (Lepismium lorentzianum, Lepismium lumbricoides, Rhipsalis floccosa, and Pfeiffera ianthothele) from Argentinean Yungas, were performed. Different assays were applied: ABTS radical scavenging capacity, inhibition of lipid peroxidation using ??-carotene-linoleate model system and mutagenicity/antimutagenicity by Ames test. A pigment like betalain was detected in fruits of both Lepismium species; L. lorentzianum aqueous extracts showed a higher pigment content (60.6 mg BE/100 g FM) than L. lumbricoides (9.2 mg BE/100 g FM). Fruit preparations of L. lorentzianum (aqueous and ethanolic extractions) showed significantly higher total phenolic compound and pigments content than the other berries. In all preparations, high antioxidant activity was demonstrated. Lepismium species were more active than Rhipsalis and Pfeiffera (SC50ABTS values between 1.3 and 4.5 ??g/ml and IC50 ??-carotene-linoleate values between 9 and 45.8 ??g/ml). A significant correlation between pigment and phenolic compounds content and AOA was observed. The preparations showed no sign of mutagenicity at tested concentrations (until 50 ??g/plate). A significant antimutagenic effect was observed for L. lumbricoides, and a weak effect was shown for P. ianthothele and R. floccosa. Because of the diversity and abundance of bioactive phytochemicals found in these species, food, cosmetic, and pharmaceutical applications could be proposed. 相似文献
998.
Iñiguez-de-la-Torre I Mateos J Roelens Y Gardès C Bollaert S González T 《Nanotechnology》2011,22(44):445203
The understanding of the influence of surface charge effects on the electrical properties of nanostructures is a key aspect for the forthcoming generations of electronic devices. In this paper, by using an ultrafast electrical pulse characterization technique, we report on the room-temperature time response of a T-branch nanojunction which allows us to identify the signature of surface states. Different pulse widths from 500 ns to 100 μs were applied to the device. For a given pulse width, the stem voltage is measured and compared with the DC result. The output value in the stem is found to depend on the pulse width and to be related to the characteristic charging time of the interface states. As expected, the results show that the well-known nonlinear response of T-branch junctions is more pronounced for long pulses, beyond such a characteristic time. 相似文献
999.
Meseguer-Dueñas JM Más-Estellés J Castilla-Cortázar I Escobar Ivirico JL Vidaurre A 《Journal of materials science. Materials in medicine》2011,22(1):11-18
Alkaline hydrolysis of a polycaprolactone (PCL) network obtained by photopolymerization of a PCL macromer was investigated.
The PCL macromer was obtained by the reaction of PCL diol with methacrylic anhydride. Degradation of PCL network is much faster
than linear PCL; the weight loss rate is approximately constant until it reaches around 70%, which happens after approximately
60 h in PCL network and 600 h in linear PCL. Calorimetric results show no changes in crystallinity throughout degradation,
suggesting that it takes place in the crystalline and amorphous phases simultaneously. Scanning electron microscopy microphotographs
indicate that degradation is produced by a different erosion mechanism in both kinds of samples. The more hydrophilic network
PCL would follow a bulk-erosion mechanism, whereas linear PCL would follow a surface-erosion mechanism. Mechanical testing
of degraded samples shows a decline in mechanical properties due to changes in sample porosity as a consequence of the degradation
process. 相似文献
1000.
Tamariz E Wan AC Pek YS Giordano M Hernández-Padrón G Varela-Echavarría A Velasco I Castaño VM 《Journal of materials science. Materials in medicine》2011,22(9):2097-2109
Chemotropic proteins guide neuronal projections to their final target during embryo development and are useful to guide axons
of neurons used in transplantation therapies. Site-specific delivery of the proteins however is needed for their application
in the brain to avoid degradation and pleiotropic affects. In the present study we report the use of Poly (ethylene glycol)-Silica
(PEG-Si) nanocomposite gel with thixotropic properties that make it injectable and suitable for delivery of the chemotropic
protein semaphorin 3A. PEG-Si gel forms a functional gradient of semaphorin that enhances axon outgrowth of dopaminergic neurons
from rat embryos or differentiated from stem cells in culture. It is not cytotoxic and its properties allowed its injection
into the striatum without inflammatory response in the short term. Long term implantation however led to an increase in macrophages
and glial cells. The inflammatory response could have resulted from non-degraded silica particles, as observed in biodegradation
assays. 相似文献