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排序方式: 共有433条查询结果,搜索用时 15 毫秒
101.
YK Peng V Hight-Laukaran AE Peterson R Pérez-Escamilla 《Canadian Metallurgical Quarterly》1998,128(10):1672-1680
Breast-feeding is positively associated with the duration of postpartum amenorrhea; thus it is a major determinant of fertility in countries where effective contraceptive methods are not widely available. The objective of these analyses was to examine the association between maternal nutritional status and lactational amenorrhea (LA) among breast-feeding women. Women who were not pregnant, who were breast-feeding, who were not using hormonal contraceptives and who had a child =2 y old at the time of survey were included in the analyses. Multivariate logistic regression was used to examine this association with the use of Demographic and Health Surveys data collected in seven Sub-Saharan African countries between 1990 and 1994. Analyses were adjusted for nine confounders, including breast-feeding behavior, child nutritional status and child age. Within-country analyses consistently showed the trend of low maternal body mass index (BMI) associated with a higher likelihood of being amenorrheic. Pooled analyses (n = 9839) were performed using two child age groups (<9 mo and 9-24 mo). The <9 mo pooled analysis showed that women with BMI < 18.5 kg/m2 (odds ratio; 95% confidence interval: 1.6; 1.2-2.3) were more likely to remain amenorrheic at the time of the survey than their "better-nourished" counterparts. The 9-24 mo pooled analysis showed that the differential probability of being amenorrheic between undernourished and "better-nourished" women increased with time postpartum, where the adjusted difference in the median duration of amenorrhea associated with maternal nutritional status among breast-feeding women was 1.4 mo. These results suggest that maternal nutritional status plays an independent role in the return of ovulation after delivery. 相似文献
102.
The destruction of the carcinogen benzo[a]pyrene (BaP) by light is well known; laboratory workers are routinely advised to cover fluorescent lamps with yellow filters while treating samples containing BaP. However until recently the mechanism of oxidation by sunlight and ozone had not been studied in detail. Concentrations of benzo[a]pyrene in urban air are in the range of 5–10 μg/1000 m3. Oxidant concentrations (predominantly in the form of ozone) are reported to be in the range of 0.01 ppm (22 μg per m3). Thus a sampling system with a filter paper would filter about 22,000 μg of ozone passing through and collect about 5 μg of BaP for analysis. The effect of interactions of such large ozone concentrations with BaP deposited on the filter paper is reported; data for the oxidation rates for benzo[a]pyrene coated on quartz surface and exposed to ozone or sunlight are presented. The oxidation products were analysed by thin-layer chromatography and highpressure liquid chromatography. From about eight products detected in these experiments, three have been identified as quinones based on UV-absorption spectrometry and mass spectrometry. Oxidation rates as high as 100% per hour of exposure are observed when less than 0.1 μg of BaP is coated inside the quartz tubes and exposed to ozone or sunlight. Oxidation rates for benzo[a]pyrene (using tritiated BaP) were determined for two types of experimental conditions. In the first set, tritiated BaP was spotted onto a glass fibre paper and sampling continued for 24 h; the loss of BaP was found to be 88%. In the second set tritiated BaP was spotted at intervals of one hour, for eight hours, while sampling is being carried out and the loss of BaP during the period was estimated to be 50%. 相似文献
103.
Akhil Vohra Heather L. Filiatrault Stanley D. Amyotte R. Stephen Carmichael Natalie D. Suhan Conrad Siegers Lorenzo Ferrari Gregory J. E. Davidson Tricia Breen Carmichael 《Advanced functional materials》2016,26(29):5222-5229
The development of stretchable electronic devices that are soft and conformable has relied heavily on a single material—polydimethylsiloxane—as the elastomeric substrate. Although polydimethylsiloxane has a number of advantageous characteristics, its high gas permeability is detrimental to stretchable devices that use materials sensitive to oxygen and water vapor, such as organic semiconductors and oxidizable metals. Failing to protect these materials from atmosphere‐induced decomposition leads to premature device failure; therefore, it is imperative to develop elastomers with gas barrier properties that enable stretchable electronics with practical lifetimes. Here, butyl rubber—a material with an intrinsically low gas permeability traditionally used in the innerliners of tires to maintain air pressure—is reinvented for stretchable electronics. This new material is smooth and optically transparent, possesses the low gas permeability typical of butyl rubber, and vastly outperforms polydimethylsiloxane as an encapsulating barrier to prevent the atmospheric degradation of sensitive electronic materials and the premature failure of functioning organic devices. The merits of transparent butyl rubber presented here position this material as an important counterpart to polydimethylsiloxane that will enable future generation stretchable electronics. 相似文献
104.
Electrospun bioactive nanocomposite scaffolds of polycaprolactone and nanohydroxyapatite for bone tissue engineering 总被引:3,自引:0,他引:3
Thomas V Jagani S Johnson K Jose MV Dean DR Vohra YK Nyairo E 《Journal of nanoscience and nanotechnology》2006,6(2):487-493
Nanocomposite scaffolds based on nanofibrous poly(epsilon-caprolactone) (PCL) and nanohydroxyapatite (nanoHA) with different compositions (wt%) were prepared by electrostatic co-spinning to mimic the nano-features of the natural extracellular matrix (ECM). NanoHA was found to be well dispersed in polymers up to the addition of 20 wt%, after ultrasonication. The composite scaffolds were characterized for structure and morphology using XRD, EDX, SEM, and DSC. The scaffolds have a porous nanofibrous morphology with fibers (majority) having diameters in the range of 450-650 nm, depending on composition, and interconnected pore structures. SEM, EDX, and XRD analyses have confirmed the presence of nanoHA in the fibers. As the nanoHA content in the fibers increases, the surface of fibers becomes rougher. The mechanical (tensile) property measurement of the electrospun composites reveals that as the nanoHA content increases, the ultimate strength increases from 1.68 MPa for pure PCL to 2.17, 2.65, 3.91, and 5.49 MPa for PCL/nanoHA composites with the addition of 5, 10, 15, and 20 wt% nanoHA, respectively. Similarly the tensile modulus also increases gradually from 6.12 MPa to 21.05 MPa with the increase of nanoHA content in the PCL/nanoHA fibers, revealing an increase in stiffness of the fibers due to the presence of HA. DSC analysis reveals that as nanoHA in the composite scaffolds increases, the melting point slightly increases due to the good dispersion and interface bonding between PCL and nanoHA. 相似文献
105.
Photocatalytic degradation (PCD) of aqueous paraquat was accelerated by the addition of either phosphate or sulfate salt. Attachment of these anions to the TiO2 surface possibly results in increased adsorption of the cationic paraquat species and in turn its photocatalysis rate. The same effect was obtained more consistently using the Nafion (an anionic polymer)-coated TiO2. Enhanced PCD of paraquat and some amine compounds was noted. However the anionic and neutral compounds were not affected significantly. Nafion proved to be stable against photocatalysis. It has been suggested that the degradation rate is larger for the cationic compounds with higher pK(B). For a phenol-paraquat-TiO2 system, paraquat degradation did not begin till near-complete phenol removal. Using the Nafion-coated TiO2, both phenol and paraquat degradations started simultaneously. Nevertheless, complete paraquat removal still took longer than phenol. 相似文献
106.
A fibre optic strain sensor based on Rayleigh backscatter using a time division multiplexing scheme with an optical pulse is presented. The sensor has a gauge length of <0.5 m and a strain sensitivity of <1 nϵ/√Hz at 2 kHz 相似文献
107.
108.
109.
Molecular structure of the oxidized, recombinant, heterocyst [2Fe-2S] ferredoxin from Anabaena 7120 determined to 1.7-A resolution 总被引:3,自引:0,他引:3
BL Jacobson YK Chae JL Markley I Rayment HM Holden 《Canadian Metallurgical Quarterly》1993,32(26):6788-6793
The [2Fe-2S] ferredoxin produced in the heterocyst cells of Anabaena 7120 plays a key role in nitrogen fixation, where it serves as an electron acceptor from various sources and an electron donor to nitrogenase. The three-dimensional structure of this ferredoxin has now been determined and refined to a crystallographic R value of 16.7%, with all measured X-ray data from 30.0 to 1.7 A. The molecular motif of this ferredoxin is similar to that of other plant-type ferredoxins with the iron-sulfur cluster located toward the outer edge of the molecule and the irons tetrahedrally coordinated by both inorganic sulfurs and sulfurs provided by protein cysteinyl residues. The overall secondary structure of the molecule consists of seven strands of beta-pleated sheet, two alpha-helices, and seven type I turns. It is of special interest that 4 of the 22 amino acid positions thought to be absolutely conserved in nonhalophilic ferredoxins are different in the heterocyst form of the protein. Three of these positions are located in the metal-cluster binding loop. 相似文献
110.